Publications

On the right hand side is a list of publications I authored or co-authored.

Publications per year

2020: 11
2019: 9
2018: 11
2017: 6
2016: 2
2015: 3
2014: 5
2013: 5
2012: 1
2011: 2
2010: 3
Dynamics of gold clusters on ceria during CO oxidation , Chang, M.W., Zhang, L., Davids, M.R., Filot, I.A.W., Hensen, E.J.M., J. Catal., 2020, 392, 39-47

Modeling the evolution of the active phase in terms of shape and composition in supported catalysts as function of the reaction conditions is an important challenge. Herein, we employ methods rooted in genetic algorithms and grand canonical Monte-Carlo simulations to determine relevant structures of a gold clusters placed on a ceria support in oxygen atmosphere. Based on optimized structures, microkinetics simulations identify relevant structures present under the out-of-equilibrium conditions of the ongoing CO oxidation reaction. The results emphasize the role of cationic gold atoms at the interface between gold and ceria, which bind oxygen atoms strongly. At low temperature and low CO/O2 ratio, the catalytic cycle for a 6-atom gold cluster mainly involves the Au6O3 and Au6O2 couple via the associative O2 mechanism of CO oxidation. At higher CO/O2 ratio and elevated temperature, the Au6O and Au6 couple are the dominant active species.

Stability of heterogeneous single-atom catalysts: a scaling law mapping thermodynamics to kinetics , Su, Y.Q., Zhang, L., Wang, Y., Liu, J.X., Muravev, V., Alexopoulos, K., Filot, I.A.W., Vlachos, D.G., Hensen, E.J.M., Npj Comput. Mater., 2020, 6, 144

Heterogeneous single-atom catalysts (SACs) hold the promise of combining high catalytic performance with maximum utilization of often precious metals. We extend the current thermodynamic view of SAC stability in terms of the binding energy (Ebind) of single-metal atoms on a support to a kinetic (transport) one by considering the activation barrier for metal atom diffusion. A rapid computational screening approach allows predicting diffusion barriers for metal–support pairs based on Ebind of a metal atom to the support and the cohesive energy of the bulk metal (Ec). Metal–support combinations relevant to contemporary catalysis are explored by density functional theory. Assisted by machine-learning methods, we find that the diffusion activation barrier correlates with (Ebind)2/Ec in the physical descriptor space. This diffusion scaling-law provides a simple model for screening thermodynamics to kinetics of metal adatom on a support.

A theoretical study of CO oxidation and O2 activation for transition metal overlayers on SrTiO3 perovskite , Zhang, L., Chang, M.W., Su, Y.Q., Filot, I.A.W., Hensen, E.J.M., J. Catal., 2020, 391, 229-240

Understanding the catalytic properties of metal/metal oxide interfaces is gaining importance in heterogeneous catalysis. Density functional theory calculations were employed to understand the mechanism of CO oxidation on TM/SrTiO3 catalysts (TM/STO, TM = Au, Ag, Pd, Pt, Rh, and Ir). Au benefits from O2 dissociation at the Au/STO interface with lower barrier than on corresponding closed-packed Au surface. Doping of STO with fluorine lowers the activation barriers for O2 dissociation. Brønsted-Evans-Polanyi relations are identified for O2 dissociation and a density of states analysis provides insight into the activation of O2 at TM/STO interfaces. Full catalytic cycles for CO oxidation are formulated including O2 dissociative and associative mechanisms. Microkinetics simulations show that an O2 associative mechanism at Au/STO and Ag/STO interfaces is dominant for CO oxidation. For a more reactive metal like Pd, CO2 formation involving interface sites present higher barriers than on the metal itself because of too strong binding of O at the Pd/STO interface. Pt/STO and Rh/STO also follow such a conventional Langmuir-Hinshelwood mechanism, while Ir is too reactive leading to a shift of the reaction at the Ir/STO interface involving dissociated O2. The mechanistic insights are discussed with respect to recent experimental literature on Pd/STO and Au/STO for which different structure-performance relationships were formulated. We predict that F-doping of STO can further improve the catalytic performance of TM/STO catalysts for CO oxidation.

Detailed chemomechanistic sensitivity study on the alkoxylation of fatty amines , Müller, P., van Eeten, K., Winkenwerder, W., van der Schaaf, J., Filot, I.A.W., Int. J. Chem. Kinet., 2020, 52 (11), 861-871

The mechanism of the autocatalytic alkoxylation of fatty amines was elucidated using a combined experimental and theoretical approach. The kinetic parameters of the elementary reaction steps are fitted to the experimental data gained in semibatch for propylene and butylene oxides with dodecylamine. A quality-of-fit sensitivity study was conducted to assess the robustness and accuracy of the model. Herein, we identified the kinetic parameters that are either crucial for a good fit or rate controlling in terms of the overall kinetics. It was found that the critical steps are the activation of the epoxide ring by a hydroxyl group present on either the intermediate or final product, highlighting the autocatalytic nature of the reaction. Furthermore, we introduced a so-called degree of conversion control to characterize the importance of each elementary reaction step toward the epoxide conversion. The degree of conversion control showed that the uncatalyzed route toward the secondary amine is only influential at low conversion. At higher conversion the route via the catalytic intermediate dominates. Alternative mechanisms were investigated as well but did not significantly improve the quality of fitting and were thus discarded.

The vital role of step-edge sites for both CO activation and chain growth on cobalt Fischer-Tropsch catalysts revealed through first-principles based microkinetic modeling including lateral interactions , Zijlstra, B., Broos, R.J.P., Chen, W., Bezemer, G.J., Filot, I.A.W., Hensen, E.J.M., ACS Catal., 2020, 10 (16), 9376-9400

Microkinetic modeling is a bottom-up approach that pinpoints activity and selectivity controlling elementary reaction steps. We have applied this method to the Fischer-Tropsch (FT) reaction by computing all relevant elementary reaction steps at Co(11"2" ̅1) step-edge and Co(0001) terrace sites. Our model includes important aspects such as the impact of coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. We found that CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers, while the overall barriers via CO insertion are much higher. A comparison with the CO dissociation barrier indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The simulations predict kinetic parameters that correspond well with experimental kinetic data. They show that the Co(11"2" ̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, which is caused by faster hydrogenation of the hydrocarbon chain. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4. We show that the much higher H/CO coverage ratio during CO2 hydrogenation causes this high CH4 selectivity. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint low-reactive terrace sites near step-edge sites as the origin of unwanted CH4.

Design of Nonideal Eutectic Mixtures Based on Correlations with Molecular Properties , Kollau, L.J.B.M., Tuinier, R., Verhaak, J., Den Doelder, J., Filot, I.A.W., Vis, M., J. Phys. Chem. B, 2020, 124 (25), 5209-5219

In this work, a statistical analysis was performed to reveal how the molecular properties are correlated with the nonideal behavior observed in eutectic mixtures. From this, a statistical model, combined with theory and experimental results, was developed to predict the nonideal behavior of a specific set of eutectic mixtures, consisting of quaternary ammonium bromides with dicarboxylic acids and polyols. The combination of this analysis and this model can be considered as a first step toward the a priori design of eutectic mixtures. The analysis performed is based on principal components. The descriptors used for this are molecular properties of the constituents of these mixtures. The molecular properties are a combination of experimental, theoretical, and computed properties. The analysis reveals that there are strong correlations between the nonideality of the mixtures and a measure of the acidity of the hydrogen bond donating protons, the displacement of the bromide anion, and the bulkiness of the quaternary ammonium salt. Our analysis highlights the design rules of deep eutectic systems (DES), enabling control over the extent of the liquid window. Our model enables prediction of the eutectic temperature for a range of related mixtures.

Bioorthogonal Tetrazine Carbamate Cleavage by Highly Reactive trans-Cyclooctene , van Onzen, A.H.A.M., Versteegen, R.M., Hoeben, F.J.M., Filot, I.A.W., Rossin, R., Zhu, T., Wu, J., Hudson, P.J., Janssen, H.M., Ten Hoeve, W., Robillard, M.S., J. Chem. Am. Soc., 2020, 142 (25), 10955-10963

The high rate of the ‘click-to-release’ reaction between an allylic substituted trans-cyclooctene linker and a tetrazine activator has enabled exceptional control over chemical and biological processes. Here we report the development of a new bioorthogonal cleavage reaction based on trans-cyclooctene and tetrazine, which allows the use of highly reactive trans-cyclooctenes, leading to 3 orders of magnitude higher click rates compared to the parent reaction, and 4 to 6 orders higher than other cleavage reactions. In this new pyridazine elimination mechanism, wherein the roles are reversed, a trans-cyclooctene activator reacts with a tetrazine linker that is substituted with a methylene-linked carbamate, leading to a 1,4-elimination of the carbamate and liberation of a secondary amine. Through a series of mechanistic studies, we identified the 2,5-dihydropyridazine tautomer as the releasing species and found factors that govern its formation and subsequent fragmentation. The bioorthogonal utility was demonstrated by the selective cleavage of a tetrazine-linked antibody–drug conjugate by trans-cyclooctenes, affording efficient drug liberation in plasma and cell culture. Finally, the parent and the new reaction were compared at low concentration, showing that the use of a highly reactive trans-cyclooctene as the activator leads to a complete cycloaddition reaction with the antibody–drug conjugate in seconds vs hours for the parent system. Although the subsequent release from the IEDDA adduct is slower, we believe that this new reaction may allow markedly reduced click-to-release reagent doses in vitro and in vivo and could expand the application scope to conditions wherein the trans-cyclooctene has limited stability.

A theoretical study of the reverse water-gas shift reaction on Ni(111) and Ni(311) surfaces , Zhang, M., Zijlstra, B., Filot, I.A.W., Li, F., Wang, H., Li, J., Hensen, E.J.M., Can. J. Chem. Eng., 2020, 98, 740-748

This paper presents a systematic comparison study of the surface redox reaction mechanism for reverse water-gas shift (RWGS) over Ni(111) and Ni(311) surfaces. Specifically, the most stable surface intermediates and the reaction kinetics involved in the direct CO2 activation and water formation steps are computed with density functional theory calculations and compared for the two different Ni surfaces. The results show that CO2, CO, O, H, OH, and H2O species adsorb stronger on Ni(311) than on Ni(111). Compared to Ni(111), the overall barriers for direct CO2 activation and water formation on Ni(311) are lower by 23 and 17 kJ/mol, respectively. These observations indicate that the RWGS reaction through the surface redox mechanism should be preferred on Ni(311).

First-principles microkinetics simulations of electrochemical reduction of CO2 over Cu catalysts , Zijlstra, B., Zhang, X., Liu, J.-X., Filot, I.A.W., Zhou, Z., Sun, S., Hensen, E.J.M., Electrochimica Acta., 2020, 335, 135665

Electrochemical reduction of CO2 can contribute to the storage of excess renewable electricity in chemical bonds. Here we incorporate reaction energetics for CO2 reduction on Cu(111) and Cu(211) determined by DFT calculations in microkinetics simulations to predict the influence of surface topology, the presence of water and possible diffusion limitations on current density-potential curves and Faradaic efficiencies. A reaction-diffusion model was used that takes into account the effect of electrochemical potential on the stability of intermediates and associated activation barriers in proton-coupled electron transfer steps as well as diffusion of protons and CO2 from the bulk electrolyte to the electrode surface. The basic model can well reproduce hydrogen evolution including the effect of proton diffusion limitations and a shift of proton reduction (low potential) to water reduction (high potential). Considering CO2 electro-reduction, the stepped Cu(211) surface is more active than the Cu(111) terrace towards HCOO(H), CO and CH4. The presence of a catalytic H2O molecule increases the overall rate and selectivity to products (CO and CH4) derived from dissociated CO2. A catalytic H2O molecule facilitates the difficult electrochemical CO2 activation step to COOH and suppresses the competing activation step towards HCOO, which mainly yields HCOO(H). In general, the current densities increase at higher negative potential and the products follow the sequence CO2 → CO → CH4. That is to say, CO2 is converted to CO via COOH dissociation, followed by CO hydrogenation. Trendwise, the simulated product distribution follows the potential-dependent distribution observed in experiment. The low selectivity to CH3OH can be understood from the fast electrochemical steps that lead to CHx-OH dissociation. At high overpotentials the hydrogenation step from CO2 to COOH controls both activity and selectivity towards CH4. At high potential CO2 reduction becomes increasingly diffusion-limited, thus limiting the selectivity of CO2 reduction vs. hydrogen evolution. This aspect supports the need for better design of mass transfer in electrochemical reactors, which operate at high current density.

A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of theta-Fe 3 C , Broos, R.J.P., Klumpers, B., Zijlstra, B., Filot, I.A.W., Hensen, E.J.M., Catal. Today, 2020, 342, 152-160

Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of ϴ-Fe 3 C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), 11¯1, and (010) terminations of ϴ-Fe 3 C. On the other hand, H-assisted CO dissociation is preferred on the 01¯1, (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (01¯1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of ϴ-Fe 3 C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature ϴ-Fe 3 C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than α-Fe (known to be unstable under Fischer-Tropsch conditions) and the χ-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.

Linear activation energy-reaction energy relations for LaBO3 (B = Mn, Fe, Co, Ni) supported single-atom platinum group metal catalysts for CO oxidation , Zhang, L., Su, Y-Q., Chang, M-W., Filot, I.A.W., Hensen, E.J.M., J. Phys. Chem. C, 2019, 123 (51), 31130-31141

Single-atom catalysts are at the center of the attention of the heterogeneous catalysis community, because they exhibit unique electronic structure distinct from nanoparticulate forms resulting in very different catalytic performance combined with increased usage of often costly transition metals. Proper selection of a support that can stably keep the metal in a high dispersion is crucial. Here, we employ spin-polarized density functional theory and microkinetics simulations to identify optimum LaBO3 (B = Mn, Fe, Co, Ni) supported catalysts dispersing platinum group metals as atoms on their surface. We identify a strong correlation between the CO adsorption energy and the d-band center of the doped metal atom. These CO adsorption strength differences are explained in terms of the electronic structure. In general, Pd-doped surfaces exhibit substantially lower activation barriers for CO2 formation than the Rh- and Pt-doped surfaces. Strong Brønsted-Evans-Polanyi correlations are found for CO oxidation on these single-atom catalysts, providing a tool to predict promising compositions. Microkinetics simulations show that Pd-doped LaCoO3 is the most active catalyst for low-temperature CO oxidation. Moderate CO adsorption strength and low reaction barriers explain the high activity of this composition. Our approach provides guidelines for the design of highly active and cost-effective perovskite supported single-atom catalysts.

Understanding carbon dioxide activation and carbon–carbon coupling over nickel , Vogt, C., Monai, M., Sterk, E.B., Palle, J., Melcherts, A.E.M., Zijltra, B., Groeneveld, S., Berben, P.H., Boereboom, J.M., Hensen, E.J.M., Meirer, F., Filot, I.A.W., Weckhuysen, B.M., Nature Comm., 2019, 10, 5330

Carbon dioxide is a desired feedstock for platform molecules, such as carbon monoxide or higher hydrocarbons, from which we will be able to make many different useful, value-added chemicals. Its catalytic hydrogenation over abundant metals requires the amalgamation of theoretical knowledge with materials design. Here we leverage a theoretical understanding of structure sensitivity, along with a library of different supports, to tune the selectivity of methanation in the Power-to-Gas concept over nickel. For example, we show that carbon dioxide hydrogenation over nickel can and does form propane, and that activity and selectivity can be tuned by supporting different nickel particle sizes on various oxides. This theoretical and experimental toolbox is not only useful for the highly selective production of methane, but also provides new insights for carbon dioxide activation and subsequent carbon–carbon coupling towards value-added products thereby reducing the deleterious effects of this environmentally harmful molecule.

A theoretical study of the reverse water-gas shift reaction on Ni(111) and Ni(311) surfaces , Zhang, M., Zijlstra, B., Filot, I.A.W., Li, F., Wang, H., Li, J., Hensen, E.J.M., Can. J. Chem. Eng., 2019, 98 (3), 740-748

In this study, a systematic comparison study of the surface redox reaction mechanism for reverse water‐gas shift (RWGS) over Ni(111) and Ni(311) surfaces is presented. Specifically, the most stable surface intermediates and the reaction kinetics involved in the direct CO2 activation and water formation steps are computed with density functional theory calculations and compared for the two different Ni surfaces. The results show that CO2, CO, O, H, OH, and H2O species adsorb stronger on Ni(311) than on Ni(111). Compared to Ni(111), the overall barriers for direct CO2 activation and water formation on Ni(311) are lower by 23 kJ/mol and 17 kJ/mol, respectively. These observations indicate that the RWGS reaction through the surface redox mechanism should be preferred on Ni(311).

Supramolecular interactions between catalytic species allow rational control over reaction kinetics , Teunissen, A.J.P., Paffen, T.F.E., Filot, I.A.W., Lanting, M.D., van der Haas, R.J.C., de Greef, T.F.A., Meijer, E.W., Chem. Sci., 2019, 10, 9115-9124

The adaptivity of biological reaction networks largely arises through non-covalent regulation of catalysts' activity. Such type of catalyst control is still nascent in synthetic chemical networks and thereby hampers their ability to display life-like behavior. Here, we report a bio-inspired system in which non-covalent interactions between two complementary phase-transfer catalysts are used to regulate reaction kinetics. While one catalyst gives bimolecular kinetics, the second displays autoinductive feedback, resulting in sigmoidal kinetics. When both catalysts are combined, the interactions between them allow rational control over the shape of the kinetic curves. Computational models are used to gain insight into the structure, interplay, and activity of each catalytic species, and the scope of the system is examined by optimizing the linearity of the kinetic curves. Combined, our findings highlight the effectiveness of regulating reaction kinetics using non-covalent catalyst interactions, but also emphasize the risk for unforeseen catalytic contributions in complex systems and the necessity to combine detailed experiments with kinetic modelling.

Efficient Base-Metal NiMn/TiO2 Catalyst for CO2 Methanation , Vrijburg, W.L., Moioli, E., Chen, W., Zhang, M., Terlingen, B.J.P., Zijlstra, B., Filot, I.A.W., Zuttel, A., Pidko, E.A., Hensen, E.J.M., ACS Catal., 2019, 9 (9), 7823-7839

Energy storage solutions are a vital component of the global transition toward renewable energy sources. The power-to-gas (PtG) concept, which stores surplus renewable energy in the form of methane, has therefore become increasingly relevant in recent years. At present, supported Ni nanoparticles are preferred as industrial catalysts for CO2 methanation due to their low cost and high methane selectivity. However, commercial Ni catalysts are not active enough in CO2 methanation to reach the high CO2 conversion (>99%) required by the specifications for injection in the natural gas grid. Herein we demonstrate the promise of promotion of Ni by Mn, another low-cost base metal, for obtaining very active CO2 methanation catalysts, with results comparable to more expensive precious metal-based catalysts. The origin of this improved performance is revealed by a combined approach of nanoscale characterization, mechanistic study, and density functional theory calculations. Nanoscale characterization with scanning transmission electron microscopy-energy- dispersive X-ray spectroscopy (STEM-EDX) and X-ray absorption spectroscopy shows that NiMn catalysts consist of metallic Ni particles decorated by oxidic Mn2+ species. A mechanistic study combining IR spectroscopy of surface adsorbates, transient kinetic analysis with isotopically labeled CO2, density functional theory calculations, and microkinetics simulations ascertains that the MnO clusters enhance CO2 adsorption and facilitate CO2 activation. A macroscale perspective was achieved by simulating the Ni and NiMn catalytic activity in a Sabatier reactor, which revealed that NiMn catalysts have the potential to meet the demanding PtG catalyst performance requirements and can largely replace the need for expensive and scarce noble metal catalysts.

Coverage Effects in CO Dissociation on Metallic Cobalt Nanoparticles , Zijlstra, B., Broos, R.J.P., Chen, W., Oosterbeek, H., Filot, I.A.W., Hensen, E.J.M., ACS Catal., 2019, 9 (8), 7365-7372

The active site of CO dissociation on a cobalt nanoparticle, relevant to the Fischer–Tropsch reaction, can be computed directly using density functional theory. We investigate how the activation barrier for direct CO dissociation depends on CO coverage for step-edge and terrace cobalt sites. Whereas on terrace sites increasing coverage results in a substantial increase of the direct CO dissociation barrier, we find that this barrier is nearly independent of CO coverage for the step-edge sites on corrugated surfaces. A detailed electronic analysis shows that this difference is due to the flexibility of the adsorbed layer, minimizing Pauli repulsion during the carbon–oxygen bond dissociation reaction on the step-edge site. We constructed a simple first-principles microkinetic model that not only reproduces experimentally observed rates but also shows how migration of carbon species between step-edge and terrace sites contributes to methane formation.

Understanding the impact of defects on catalytic CO oxidation of LaFeO3Supported Rh, Pd and Pt single atom catalysts , Zhang, L., Filot, I.A.W., Su, Y.-Q.., Liu, J.X., Hensen, E.J.M., J. Phys. Chem. C, 2019, 123 (12), 7290-7298

Understanding the intrinsic catalytic properties of perovskite materials can accelerate the development of highly active and abundant complex oxide catalysts. Here, we performed a first-principles density functional theory study combined with a microkinetics analysis to comprehensively investigate CO oxidation on promising LaFeO3 catalysts containing single atoms of Rh, Pd and Pt for CO oxidation. La defects and subsurface O vacancies considerably affect the local electronic structure of these single atoms adsorbed at the surface or replacing Fe in the surface of the perovskite. As a consequence, not only the stability of the introduced single atoms is enhanced but also the CO and O2 adsorption energies are modified. This also affects the barriers for CO oxidation. Uniquely, we find that the presence of La defects results in a much higher CO oxidation for the doped perovskite surface. A linear correlation between the activation barrier for CO oxidation and the surface O vacancy formation energy for these models is identified. Our results also indicate that the presence of subsurface O vacancies is capable of promoting CO oxidation on the LFO surface with an adsorbed Rh atom. The insights revealed herein guide the design of perovskite-based three-way catalyst through compositional variation.Understanding the intrinsic catalytic properties of perovskite materials can accelerate the development of highly active and abundant complex oxide catalysts. Here, we performed a first-principles density functional theory study combined with a microkinetics analysis to comprehensively investigate CO oxidation on promising LaFeO3 catalysts containing single atoms of Rh, Pd and Pt for CO oxidation. La defects and subsurface O vacancies considerably affect the local electronic structure of these single atoms adsorbed at the surface or replacing Fe in the surface of the perovskite. As a consequence, not only the stability of the introduced single atoms is enhanced but also the CO and O2 adsorption energies are modified. This also affects the barriers for CO oxidation. Uniquely, we find that the presence of La defects results in a much higher CO oxidation for the doped perovskite surface. A linear correlation between the activation barrier for CO oxidation and the surface O vacancy formation energy for these models is identified. Our results also indicate that the presence of subsurface O vacancies is capable of promoting CO oxidation on the LFO surface with an adsorbed Rh atom. The insights revealed herein guide the design of perovskite-based three-way catalyst through compositional variation.

Theoretical approach to predict stability of supported single-atom catalysts , Su, Y., Wang, Y., Liu, J.X., Filot, I.A.W., Alexopoulos, K., Zhang, L., Muravev, V., Zijlstra, B., Vlachos, D., Hensen, E.J.M., ACS Catal., 2019, 9 (4), 3289-3297

Heterogeneous single-atom catalysts involve isolated metal atoms anchored to a support, displaying high catalytic performance and stability in many important chemical reactions. We present a general theoretical framework to establish the thermodynamic stability of metal single atoms and metal nanoparticles on a support in the presence of adsorbates. As a case study, we establish for Pt–CeO2 the CO partial pressure and temperature range within which Pt single atoms are more stable than Pt nanoparticles. Density functional theory and kinetic Monte Carlo simulations demonstrate that Pt atoms doped into the CeO2 surface exhibit a very high CO oxidation activity and thermodynamic stability in comparison to models involving Pt single atoms on terraces and steps of CeO2. An intermediate CO adsorption strength is important to explain a high activity. Our work provides a systematic strategy to evaluate the stability and reactivity of single atoms on a support.

First-principles based microkinetic modeling of transient kinetics of CO hydrogenation on cobalt catalysts , Zijlstra, B., Broos, R.J.P., Chen, W., Filot, I.A.W., Hensen, E.J.M., Catal. Today, 2020, 342, 131-141

Computational efforts towards a fundamental understanding of the underlying mechanistic pathways in synthesis gas conversion processes such as Fischer-Tropsch synthesis are exemplary for the developments in heterogeneous catalysis. Advances in transient kinetic analysis methods contribute to unraveling complex reaction pathways over nanoparticle surfaces. Tracing the activity and selectivity of Fischer-Tropsch catalysts to the individual events occurring at the active site remains difficult with experimental techniques. Here we provide simulations of transient kinetics at the scale of the active site by making use of the reaction energetics for CO hydrogenation to methane on stepped and terrace cobalt surfaces that are suitable models for cobalt FT nanoparticle catalysts. We investigate the hydrogen-deuterium kinetic isotope effect and simulate common steady-state and chemical isotopic transients. Comparison to experimental literature leads to important mechanistic insights. Direct CO dissociation is the main pathway for breaking the Csingle bondO bond and it occurs exclusively on step-edge sites. While the experimentally observed hydrogen-deuterium kinetic isotopic effect is often used as evidence for H-assisted CO dissociation, we show that hydrogenation of C and O as partly rate-controlling steps provides an alternative explanation. The simulations of the chemical transients provide significant insight into the importance of the changing surface coverages that strongly affect the reaction rate. The reversibility of CO dissociation on cobalt step-edges is evident from simulations of 12C16O/13C18O scrambling being in good agreement with experimental data.

Reversible Restructuring of Silver Particles during Ethylene Epoxidation , van Hoof, A.J.F., Filot, I.A.W., Friedrich, H., Hensen, E.J.M., ACS Catal., 2018, 8 (12), 11794-11800

The restructuring of a silver catalyst during ethylene epoxidation under industrially relevant conditions was investigated without and with vinyl chloride (VC) promotion. During non-VC-promoted ethylene epoxidation, the silver particles grow and voids are formed at the surface and in the bulk. Electron tomography highlighted the presence of voids below the Ag surface. A mechanism is proposed involving reconstruction of the silver lattice and defect sites induced by oxygen adsorption on the external surface and grain boundaries, which finally create pores. Promotion of the catalytic reaction by VC suppresses to a significant extent void formation. The use of VC also redisperses silver particles, initially grown during ethylene epoxidation without VC. This process is rapid as the average size decreased from 172 to 136 nm within 2 h. These insights emphasize the dynamic nature of the silver particles during the ongoing ethylene epoxidation reaction and indicate that particle size and morphology strongly depend on reaction conditions.

Quantum-chemical-based microkinetics simulations of syngas conversion over MoS2(100) surface , Fariduddin, F., Filot, I.A.W., Zijlstra, B., Hensen, E.J.M., Chemical Engineering Science, 2019, 198, 166-183

MoS2 is a potential catalyst for the conversion of synthesis gas obtained from different carbon-containing feedstock into methane, hydrocarbons and alcohols. We performed a combined density functional theory and microkinetics simulation study of all relevant reaction pathways of CO and H2 into methane, ethylene, ethane, formaldehyde, methanol, carbon dioxide and water at the bare and partially sulfided Mo-edge of MoS2(1 0 0). Reaction barriers were substantially lower for the 25% sulfur-covered Mo-edge in comparison to the bare Mo-edge. H-assisted CO dissociation is preferred over direct CO dissociation for both surfaces. Microkinetics simulations predict a negligible methanation rate for the bare Mo-edge, which contradicts experiment. The discrepancy stems from oxygen poisoning of the surface. Oxygen removal barriers are substantially lowered at a sulfur coverage of 25%. The resulting CO conversion rate and product distribution are in good agreement with reported experimental data. These simulations show how density functional theory combined with microkinetics simulations can predict performance of catalytic surface used in complex chemical reactions.

Highly active and stable CH4 oxidation by substitution of Ce4+ by two Pd2+ ions in CeO2(111) , Su, Y., Liu, J., Filot, I.A.W., Zhang, L., Hensen, E.J.M., ACS Catal., 2018, 8 (7), 6552-6559

Methane (CH4) combustion is an increasingly important reaction for environmental protection, for which Pd/CeO2 has emerged as the preferred catalyst. There is a lack of understanding of the nature of the active site in these catalysts. Here, we use density functional theory to understand the role of doping of Pd in the ceria surface for generating sites highly active towards the C-H bonds in CH4. Specifically, we demonstrate that two Pd2+ ions can substitute one Ce4+ ion, resulting in a very stable structure containing a highly coordinatively unsaturated Pd cation that can strongly ad-sorb CH4 and dissociate the first C-H bond with a low energy barrier. An important aspect of the high activity of the stabilized isolated Pd cation is its ability to form a strong sigma-complex with CH4, which leads to effective activation of CH4. We show that also other transition metals like Pt, Rh and Ni can give rise to similar structures with high activity towards C-H bond dissociation. These insights provide us with a novel structural view of solid solutions of transition metals such as Pt, Pd, Ni and Rh in CeO2, known to exhibit high activity in CH4 combustion.

Potential enthalpic energy of water in oils exploited to control supramolecular structure , van Zee, N.J., Adelizzi, B., Mabesoone, M.F.J., Meng, X., Aloi, A., Zha, R.H., Lutz, M., Filot, I.A.W., Palmans, A.R.A., Meijer, E.W., Nature, 2018, 558 (7708), 100-103

Water directs the self-assembly of both natural and synthetic molecules to form precise yet dynamic structures. Nevertheless, our molecular understanding of the role of water in such systems is incomplete, which represents a fundamental constraint in the development of supramolecular materials for use in biomaterials, nanoelectronics and catalysis. In particular, despite the widespread use of alkanes as solvents in supramolecular chemistry, the role of water in the formation of aggregates in oils is not clear, probably because water is only sparingly miscible in these solvents - typical alkanes contain less than 0.01 per cent water by weight at room temperature. A notable and unused feature of this water is that it is essentially monomeric. It has been determined previously that the free energy cost of forming a cavity in alkanes that is large enough for a water molecule is only just compensated by its interaction with the interior of the cavity; this cost is therefore too high to accommodate clusters of water. As such, water molecules in alkanes possess potential enthalpic energy in the form of unrealized hydrogen bonds. Here we report that this energy is a thermodynamic driving force for water molecules to interact with co-dissolved hydrogen-bond-based aggregates in oils. By using a combination of spectroscopic, calorimetric, light-scattering and theoretical techniques, we demonstrate that this interaction can be exploited to modulate the structure of one-dimensional supramolecular polymers.

Transition metal doping of Pd(111) for the NO+CO reaction , Zhang, L., Filot, I.A.W., Su, Y.-Q., Liu, J.-X., Hensen, E.J.M., J. Catal., 2018, 363, 154-163

The replacement of platinum group metals by non-noble metals has attracted significant attention in the field of three-way catalysis. Here, we use DFT calculations to comprehensively study NO reduction by CO and CO oxidation on Pd(1 1 1) and transition metal doped Pd(1 1 1). Whilst direct NO dissociation is very difficult on metallic Pd(1 1 1), doping with transition metals can substantially lower the reaction barrier for NO dissociation. The lowest barrier is predicted for Ti-doped Pd(1 1 1). An electronic structure analysis shows that the low barrier is due to the strong adsorption of N and O on surface sites involving Ti atoms. It relates to strong hybridization of the N and O orbitals with the half-filled d-band of the metallic surface. At the same time, the anti-bonding states are shifted above the Fermi level, which further strengthens the adsorption of N and O. A Brønsted-Evans-Polanyi relation for NO dissociation on TM-doped Pd(1 1 1) surfaces is identified. The complete reaction pathway for N2, N2O and CO2 formation on Pd(1 1 1) and Ti-doped Pd(1 1 1) was considered. Besides more facile NO dissociation, the energy barrier for CO oxidation is decreased for the Ti-doped surface. Microkinetics simulations confirm that the activity and selectivity for NO reduction and CO oxidation are drastically improved after Ti doping. Our findings indicate that doping of Pd with non-noble metal can further improve the performance of three-way catalysts.

Optimum Particle Size for Gold-Catalyzed CO Oxidation , Liu, J.-X., Filot, I.A.W., Su, Y., Zijlstra, B., Hensen, E.J.M., J. Phys. Chem. C, 2018, 122 (5), 8327-8340

The structure sensitivity of gold-catalyzed CO oxidation is presented by analyzing in detail the dependence of CO oxidation rate on particle size. Clusters with less than 14 gold atoms adopt a planar structure, whereas larger ones adopt a three-dimensional structure. The CO and O2 adsorption properties depend strongly on particle structure and size. All of the reaction barriers relevant to CO oxidation display linear scaling relationships with CO and O2 binding strengths as main reactivity descriptors. Planar and three-dimensional gold clusters exhibit different linear scaling relationship due to different surface topologies and different coordination numbers of the surface atoms. On the basis of these linear scaling relationships, first-principles microkinetics simulations were conducted to determine CO oxidation rates and possible rate-determining step of Au particles. Planar Au9 and three-dimensional Au79 clusters present the highest CO oxidation rates for planar and three-dimensional clusters, respectively. The planar Au9 cluster is much more active than the optimum Au79 cluster. A common feature of optimum CO oxidation performance is the intermediate binding strengths of CO and O2, resulting in intermediate coverages of CO, O2, and O. Both these optimum particles present lower performance than maximum Sabatier performance, indicating that there is sufficient room for improvement of gold catalysts for CO oxidation.

Quantum-Chemical DFT Study of Direct and H- and C-Assisted CO Dissociation on the X-Fe5C2 Hägg Carbide , Broos, R.J.P., Zijlstra, B., Filot, I.A.W., Hensen, E.J.M., J. Phys. Chem. C, 2018, 122 (18), 9929-9938

The first step in the Fischer–Tropsch reaction is the production of C1 monomers by the dissociation of the C–O bond. Although Fe is the active metal, it is well known that under typical reaction conditions, it changes into various carbide phases. The Hägg carbide (χ-Fe5C2) phase is usually considered as the catalytically active phase. We carried out a comprehensive DFT study of CO dissociation on various surface terminations of the Hägg carbide, selected on their specific site topology and the presence of stepped sites. Based on the reaction energetics, we identified several feasible CO dissociation pathways over the Hägg carbide. In this study, we have compared the direct CO dissociation with H- and C-assisted CO dissociation mechanisms. We demonstrated that the reaction rate for CO dissociation critically depends on the presence and topology of interstitial C atoms close to the active site. Typically, the CO dissociation proceeds via a direct C–O bond scission mechanism on the stepped sites on the Fe carbide surface. We have shown a preference for the direct CO dissociation on the surfaces with a stepped character. The H-assisted CO dissociation, via a CHO intermediate, was preferred when the surface did not have a clear stepped character. We have also shown that activation barriers for dissociation are highly dependent on the surface termination. With a consistent data set and including migration corrections, we then compared the CO dissociation rates based on a simplified kinetic model. With this model, we showed that besides the activation energy, the adsorption energy of the CO, the C and the O species are important for the reaction rate as well. We found that the most active surface termination is a (111̅) surface cut in such a way that the surface exposes B5 sites that are not occupied by the C atoms. On these B5 sites, the direct CO dissociation presents the highest rate.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd , Liu, J.-X., Su, Y., Filot, I.A.W., Hensen, E.J.M., J. Am. Chem. Soc., 2018, 140 (13), 4580-4587

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

An active alkali-exchanged faujasite catalyst for para-xylene production via the one-pot Diels-Alder cycloaddition/dehydration reaction of 2,5-dimethylfuran with ethylene , Rohling, R., Uslamin, E.A., Zijlstra, B., Tranca, I., Filot, I.A.W., Hensen, E.J.M., Pidko, E.A., ACS Catal., 2018, 8 (2), 760-769

The one-pot Diels-Alder cycloaddition (DAC)/dehydration (D) tandem reaction between 2,5-dimethylfuran and ethylene is a potent pathway towards biomass-derived para-xylene. In this work, we present a new, cheap and active low-silica potassium exchanged faujasite (KY, Si/Al = 2.6) catalyst. Catalyst optimization was guided by a computational study of the DAC/D reaction mechanism over different alkali-exchanged faujasites using periodic DFT calculations complemented by microkinetic modelling. Two types of faujasite models were compared, i.e., a high-silica alkali-exchanged faujasite model representing isolated active cation sites and a low-silica alkali-exchanged faujasite in which the reaction involves several cations in close proximity. The mechanistic study points to a significant synergetic cooperative effect of the ensemble of cations in the faujasite supercage towards the DAC/D reaction. Alignment of the reactants by their interactions with the cationic sites and stabilization of reaction intermediates contribute to high catalytic performance. Experiments confirmed the prediction that KY is the most active catalyst among low-silica alkali-exchanged faujasites. This work is an example of how catalytic reactivity of zeolites depends on multiple interactions between the zeolite and reagents.

Stable Pd-Doped Ceria Structures for CH4 Activation and CO Oxidation , Su, Y.-Q., Filot, I.A.W., Liu, J.-X., Hensen, E.J.M., ACS Catal., 2018, 8 (1), 75-80

Doping CeO2 with Pd atoms has been associated with catalytic CO oxidation, but current surface models do not allow CO adsorption. Here, we report a new structure of Pd-doped CeO2(111), in which Pd adopts a square planar configuration instead of the previously assumed octahedral configuration. Oxygen removal from this doped structure is favorable. The resulting defective Pd-doped CeO2 surface is active for CO oxidation and is also able to cleave the first C–H bond in methane. We show how the moderate CO adsorption energy and dynamic features of the Pd atom upon CO adsorption and CO oxidation contribute to a low-barrier catalytic cycle for CO oxidation. These structures, which are also observed for Ni and Pt, can lead to a more open coordination environment around the doped-transition-metal center. These thermally stable structures are relevant to the development of single-atom catalysts.

Optimum Cu Nanoparticle Catalysts for CO2 Hydrogenation Towards Methanol , Zhang, X., Liu, J.-X., Zijlstra, B., Filot, I.A.W., Zhou, Z., Sun, S., Hensen, E.J.M., Nano Energy, 2018, 43, 200-209

Understanding the mechanism of CO2 hydrogenation to methanol is important in the context of renewable energy storage from societal and technological point of view. We use density functional theory calculations to study systematically the effect of the size of Cu clusters on the binding strengths of reactants and reaction intermediates as well as the activation barriers for the elementary reaction steps underlying CO2 hydrogenation. All the elementary reaction barriers exhibit linear scaling relationships with CO and O adsorption energies. Used in microkinetics simulations, we predict that medium-sized Cu19 clusters exhibit the highest CO2 hydrogenation activity which can be ascribed to a moderate CO2 coverage and a low CO2 dissociation barrier. The nanoscale effect is evident from the strong variation of CO and O adsorption energies for clusters with 55 or less Cu atoms. The reactivity of larger clusters and nanoparticles is predicted to depend on surface atoms with low coordination number. Optimum activity is correlated with the bond strength of reaction intermediates determined by the d-band center location of the Cu clusters and the extended surfaces. The presented size-activity relations provide useful insight for the design of better Cu catalysts with maximum mass-specific reactivity for CO2 hydrogenation performance.

Mechanism of Cobalt-Catalyzed CO Hydrogenation: 2. Fischer–Tropsch Synthesis , Chen, W., Filot, I.A.W., Pestman, R., Hensen, E.J.M., ACS Catal., 2017, 7 (12), 8061-8071

Fischer–Tropsch (FT) synthesis is one of the most complex catalyzed chemical reactions in which the chain-growth mechanism that leads to formation of long-chain hydrocarbons is not well understood yet. The present work provides deeper insight into the relation between the kinetics of the FT reaction on a silica-supported cobalt catalyst and the composition of the surface adsorbed layer. Cofeeding experiments of 12C3H6 with 13CO/H2 evidence that CHx surface intermediates are involved in chain growth and that chain growth is highly reversible. We present a model-based approach of steady-state isotopic transient kinetic analysis measurements at FT conditions involving hydrocarbon products containing up to five carbon atoms. Our data show that the rates of chain growth and chain decoupling are much higher than the rates of monomer formation and chain termination. An important corollary of the microkinetic model is that the fraction of free sites, which is mainly determined by CO pressure, has opposing effects on CO consumption rate and chain-growth probability. Lower CO pressure and more free sites leads to increased CO consumption rate but decreased chain-growth probability because of an increasing ratio of chain decoupling over chain growth. The preferred FT condition involves high CO pressure in which chain-growth probability is increased at the expense of the CO consumption rate.

Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation , Chen, W., Pestman, R., Zijlstra, B., Filot, I.A.W., Hensen E.J.M., ACS Catal., 2017, 7 (12), 8050–8060

The mechanism of CO hydrogenation to CH4 at 260 °C on a cobalt catalyst is investigated using steady-state isotopic transient kinetic analysis (SSITKA) and backward and forward chemical transient kinetic analysis (CTKA). The dependence of CHx residence time is determined by 12CO/H2 → 13CO/H2 SSITKA as a function of the CO and H2 partial pressure and shows that the CH4 formation rate is mainly controlled by CHx hydrogenation rather than CO dissociation. Backward CO/H2 → H2 CTKA emphasizes the importance of H coverage on the slow CHx hydrogenation step. The H coverage strongly depends on the CO coverage, which is directly related to CO partial pressure. Combining SSITKA and backward CTKA allows determining that the amount of additional CH4 obtained during CTKA is nearly equal to the amount of CO adsorbed to the cobalt surface. Thus, under the given conditions overall barrier for CO hydrogenation to CH4 under methanation condition is lower than the CO adsorption energy. Forward CTKA measurements reveal that O hydrogenation to H2O is also a relatively slow step compared to CO dissociation. The combined transient kinetic data are used to fit an explicit microkinetic model for the methanation reaction. The mechanism involving direct CO dissociation represents the data better than a mechanism in which H-assisted CO dissociation is assumed. Microkinetics simulations based on the fitted parameters confirms that under methanation conditions the overall CO consumption rate is mainly controlled by C hydrogenation and to a smaller degree by O hydrogenation and CO dissociation. These simulations are also used to explore the influence of CO and H2 partial pressure on possible rate-controlling steps.

Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria , Su, Y.-Q., Liu, J.-X., Filot, I.A.W., Hensen, E.J.M., Chem. Mater., 2017, 29 (21), 9456–9462

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO2(111). Starting from stable Pdn clusters (n = 1–21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pdn clusters on the CeO2(111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.

Mechanism of Carbon Monoxide Dissociation on a Cobalt Fischer-Tropsch Catalyst , Chen, W., Zijlstra, B., Filot, I.A.W., Pestman, R., Hensen, E.J.M., ChemCatChem, 2018, 10 (1), 136-140

The way in which the triple bond in CO dissociates, a key reaction step in the Fischer-Tropsch (FT) reaction, is a subject of intense debate. Direct CO dissociation on a Co catalyst was probed by (CO)-C-12-O-16/(CO)-C-13-O-18 scrambling in the absence and presence of H-2. The initial scrambling rate without H-2 was significantly higher than the rate of CO consumption under CO hydrogenation conditions, which indicated that the surface contained sites sufficiently reactive to dissociate CO without the assistance of H atoms. Only a small fraction of the surface was involved in CO scrambling. The minor influence of CO scrambling and CO residence time on the partial pressure of H-2 showed that CO dissociation was not affected by the presence of H-2. The positive H-2 reaction order was correlated to the fact that the hydrogenation of adsorbed C and O atoms was slower than CO dissociation. Temperature-programmed insitu IR spectroscopy underpinned the conclusion that CO dissociation does not require H atoms.

Unravelling the Pathway Complexity in Conformationally Flexible N-Centered Triarylamine Trisamides , Adelizzi, B., Filot, I.A.W., Palmans, A.R.A., Meijer, E.W., Chem. Eur. J., 2017, 23 (25), 6103-6110

Two families of C3-symmetrical triarylamine-trisamides comprising a triphenylamine- or a tri(pyrid-2-yl)amine core are presented. Both families self-assemble in apolar solvents via cooperative hydrogen-bonding interactions into helical supramolecular polymers as evidenced by a combination of spectroscopic measurements, and corroborated by DFT calculations. The introduction of a stereocenter in the side chains biases the helical sense of the supramolecular polymers formed. Compared to other C3-symmetrical compounds, a much richer self-assembly landscape is observed. Temperature-dependent spectroscopy measurements highlight the presence of two self-assembled states of opposite handedness. One state is formed at high temperature from a molecularly dissolved solution via a nucleation–elongation mechanism. The second state is formed below room temperature through a sharp transition from the first assembled state. The change in helicity is proposed to be related to a conformational switch of the triarylamine core due to an equilibrium between a 3:0 and a 2:1 conformation. Thus, within a limited temperature window, a small conformational twist results in an assembled state of opposite helicity.

CO oxidation on Rh-doped hexadecagold clusters , Liu, J.-X., Zhiling L., Filot, I.A.W., Su, Y., Tranca, I., Hensen, E.J.M., Catal. Sci. Technol., 2017, 7 (1), 75-78

Exploring the unique catalytic properties of gold clusters associated with specific nano-architectures is essential for designing improved catalysts with a high mass-specific activity. We investigate the geometric and electronic structure of hexadecagold clusters in which Rh was doped. Density functional theory calculations demonstrate that the resulting neutral and negatively charged Rh-doped Au16 clusters are stable and bind CO and O2 stronger than Au16. Consequently, activation barriers for CO oxidation are lowered. Microkinetics simulations predict especially negatively charged Rh-doped Au16 clusters to exhibit very high CO oxidation activity, already at sub-ambient temperature. Our findings highlight the promise of alloying gold clusters with more reactive transition metals and the importance of charge transfer from the support in heterogeneous gold systems in catalyzing CO oxidation.

Kinetic aspects of chain growth in Fischer-Tropsch synthesis , Filot, I.A.W., Zijlstra, B., Broos, R.J.P., Chen, W., Pestman, R., Hensen, E.J.M., Faraday Discuss., 2017, 197, 153-164

Microkinetics simulations are used to investigate the elementary reaction steps that control chain growth in the Fischer-Tropsch reaction. Chain growth in the FT reaction on stepped Ru surfaces proceeds via coupling of CH and CR surface intermediates. Essential to the growth mechanism are C-H dehydrogenation and C hydrogenation steps, whose kinetic consequences have been examined by formulating two novel kinetic concepts, the degree of chain-growth probability control and the thermodynamic degree of chain-growth probability control. For Ru the CO conversion rate is controlled by the removal of O atoms from the catalytic surface. The temperature of maximum CO conversion rate is higher than the temperature to obtain maximum chain-growth probability. Both maxima are determined by Sabatier behavior, but the steps that control chain-growth probability are different from those that control the overall rate. Below the optimum for obtaining long hydrocarbon chains, the reaction is limited by the high total surface coverage: in the absence of sufficient vacancies the CHCHR → CCHR + H reaction is slowed down. Beyond the optimum in chain-growth probability, CHCR + H → CHCHR and OH + H → H2O limit the chain-growth process. The thermodynamic degree of chain-growth probability control emphasizes the critical role of the H and free-site coverage and shows that at high temperature chain depolymerization contributes to the decreased chain-growth probability. That is to say, during the FT reaction chain growth is much faster than chain depolymerization, which ensures high chain-growth probability. The chain-growth rate is also fast compared to chain-growth termination and compared to the steps that control the overall CO conversion rate, which are O removal steps for Ru.

A quantum-chemical DFT study of CO dissociation on Fe-promoted stepped Rh surfaces , Filot, I.A.W., Fariduddin, F., Broos, R.J.P., Zijlstra, B., Hensen, E.J.M., Catal. Today, 2015, 275, 111–118

The present density functional theory study provides insight into the effect of Fe promotion on the CO dissociation reaction on a stepped Rh surface. On the basis of a density of states analysis we demonstrate that Fe is able to promote the CO dissociation reaction by stabilizing the oxygen atom in the transition state. This effect critically depends on the location of the Fe substitution in the Rh(211) surface and the pathway of the CO dissociation reaction. This work explains the higher activity and selectivity encountered in experimental studies during CO hydrogenation on Rh nanoparticles.

Charge Transport over the Defective CeO2(111) Surface , Su, Y.-Q., Filot, I.A.W., Liu, J.-X., Tranca, I., Hensen, E.J.M., Chem. Mater., 2016, 28 (16), 5652–5658

First-principles calculations have been performed to explore the charge transport process over defective CeO2(111). Charge transport can proceed either by direct migration of the oxygen anion (i.e., vacancy diffusion) or by a polaron-hopping-assisted mechanism. On the basis of DFT+U calculations, we found that the latter process is significantly more favorable than the former. The overall barrier for charge transport involving polaron migration, followed by oxygen diffusion, is determined by the barrier for polaron hopping, which amounts to 0.18 eV. This computed value is in good agreement with the experimental barrier for ceria with a low defect density. We have shown by a careful analysis of the magnetization density, the density of states, and the reaction pathway trajectory that this process is phonon induced. Our results provide valuable insights into carrier drift processes over defective metal oxide surfaces.

Identification of step-edge sites on Rh nanoparticles for facile CO dissociation , Ligthart, D.A.J.M., Filot, I.A.W., Almutairi, A.A.H., Hensen, E.J.M., Catalysis Communications, 2016, 77, 5-8

Understanding the dependence of the rate of catalytic reactions on metal nanoparticle size remains one of the great challenges in heterogeneous catalysis. Especially, methods to probe step-edge sites on technical supported nanoparticle catalysts are needed to put structure–activity relations on a surer footing. Herein, we demonstrate that N2 is a useful IR probe for the semi-quantitative identification of step-edge sites on zirconia-supported metallic Rh nanoparticles. The intensity of the strongly perturbed band at 2205 cm− 1 correlates with the CO bond dissociation rate under conditions relevant to the Fischer–Tropsch reaction. Due to the intermediate reactivity of Rh, step-edge sites are required to dissociate the strong CO bond. DFT calculations show that N2 prefers to adsorb on top of low-coordinated surface atoms such as steps, corners and edges. The occurrence of the intensity maximum at intermediate particle size is explained by the presence of surface overlayers on terraces that give rise to step-edges. These step-edge sites are important in the dissociation of di-atomic molecules such as CO, NO and N2.

First-Principles-Based Microkinetics Simulations of Synthesis Gas Conversion on a Stepped Rhodium Surface , Filot, I.A.W., Broos, R.J.P., van Rijn, J.P.M., van Heugten, G.J.H.A., van Santen, R.A., Hensen, E.J.M., ACS Catal., 2015, 5, 5453-5467

The kinetics of synthesis gas conversion on the stepped Rh(211) surface were investigated by computational methods. DFT calculations were performed to determine the reaction energetics for all elementary reaction steps relevant to the conversion of CO into methane, ethylene, ethane, formaldehyde, methanol, acetaldehyde, and ethanol. Microkinetics simulations were carried out on the basis of these first-principles data to predict the CO consumption rate and the product distribution as a function of temperature. The elementary reaction steps that control the CO consumption rate and the selectivity were analyzed in detail. Ethanol formation can only occur on the stepped surface, because the barrier for CO dissociation on Rh terraces is too high; step-edges are also required for the coupling reactions. The model predicts that formaldehyde is the dominant product at low temperature, ethanol at intermediate temperature, and methane at high temperature. The preference for ethanol over long hydrocarbon formation is due to the lower barrier for C(H) + CO coupling as compared with the barriers for CHx + CHy coupling reactions. The C(H)CO surface intermediate is hydrogenated to ethanol via a sequence of hydrogenation and dehydrogenation reactions. The simulations show that ethanol formation competes with methane formation at intermediate temperatures. The rate-controlling steps are CO removal as CO2 to create empty sites for the dehydrogenation steps in the reaction sequence leading to ethanol, CHxCHyO hydrogenation for ethanol formation, and CH2 and CH3 hydrogenation for methane formation. CO dissociation does not control the overall reaction rate on Rh. The most important reaction steps that control the selectivity of ethanol over methane are CH2 and CH3 hydrogenation as well as CHCH3 dehydrogenation.

Microkinetic Modeling of the Oxygen Reduction Reaction at the Pt(111)/Gas Interface , Donato, F., Zhu, T., Mueller, J.E., Filot, I.A.W., Hensen, E.J.M., Jacob, T., Catal. Lett., 2015, 145 (1), 451-457

A microkinetic model of the oxygen reduction reaction (ORR) on Pt(111) under a gaseous H2 and O2 atmosphere is used to predict and explain which compositions of H2 and O2 lead to the fastest rate of water formation for temperatures between 600 and 900 K. For a stoichiometric (2:1) mixture of H2 and O2 the rate-determing step is found to transition from O⋆ hydrogenation to O2⋆ dissociation over this same temperature range. These results are explained in terms of the temperature dependence of the surface coverages of O⋆ and H⋆ and are shown to be consistent with kinetic models aimed at understanding the ORR under electrochemical conditions.

The Optimally Performing Fischer-Tropsch Catalyst , Filot, I.A.W., van Santen, R.A., Hensen, E.J.M., Angew. Chem. Int. Ed., 2014, 53 (47), 12746–12750

Microkinetics simulations are presented based on DFT-determined elementary reaction steps of the Fischer–Tropsch (FT) reaction. The formation of long-chain hydrocarbons occurs on stepped Ru surfaces with CH as the inserting monomer, whereas planar Ru only produces methane because of slow CO activation. By varying the metal–carbon and metal–oxygen interaction energy, three reactivity regimes are identified with rates being controlled by CO dissociation, chain-growth termination, or water removal. Predicted surface coverages are dominated by CO, C, or O, respectively. Optimum FT performance occurs at the interphase of the regimes of limited CO dissociation and chain-growth termination. Current FT catalysts are suboptimal, as they are limited by CO activation and/or O removal.

Quantum chemistry of the Fischer–Tropsch reaction catalysed by a stepped ruthenium surface , Filot, I.A.W., van Santen, R.A., Hensen, E.J.M., Catal. Sci. Technol., 2014, 4, 3129-3140

A comprehensive density functional theory study of the Fischer–Tropsch mechanism on the corrugated Ru(1121) surface has been carried out. Elementary reaction steps relevant to the carbide mechanism and the CO insertion mechanism are considered. Activation barriers and reaction energies were determined for CO dissociation, C hydrogenation, CHx + CHy and CHx + CO coupling, CHxCHy–O bond scission and hydrogenation reactions, which lead to formation of methane and higher hydrocarbons. Water formation that removes O from the surface was studied as well. The overall barrier for chain growth in the carbide mechanism (preferred path CH + CH coupling) is lower than that for chain growth in the CO insertion mechanism (preferred path C + CO coupling). Kinetic analysis predicts that the chain-growth probability for the carbide mechanism is close to unity, whereas within the CO insertion mechanism methane will be the main hydrocarbon product. The main chain propagating surface intermediate is CH via CH + CH and CH + CR coupling (R = alkyl). A more detailed electronic analysis shows that CH + CH coupling is more difficult than coupling reactions of the type CH + CR because of the σ-donating effect of the alkyl substituent. These chain growth reaction steps are more facile on step-edge sites than on terrace sites. The carbide mechanism explains the formation of long hydrocarbon chains for stepped Ru surfaces in the Fischer–Tropsch reaction.

Correlating Fischer-Tropsch activity to Ru nanoparticle surface structure as probed by high-energy X-ray diffraction , Quek, X.Y., Filot, I.A.W., Pestman, R., van Santen, R.A., Petkov, V., Hensen, E.J.M., Chem. Comm., 2014, 50 (45), 6005-6008

Synchrotron X-ray diffraction coupled to atomic pair distribution function analysis and Reverse Monte Carlo simulations is used to determine the atomic-scale structure of Ru nanoparticle catalysts for the Fischer–Tropsch reaction. The rate of CO hydrogenation strongly correlates with the abundance of surface atoms with coordination numbers of 10 and 11. DFT calculations confirm that CO dissociation proceeds with a low barrier on these Ru surface atom ensembles.

Reactivity of CO on Carbon-Covered Cobalt Surfaces in Fischer-Tropsch Synthesis , Joos, L., Filot, I.A.W., Cottenier, S., Hensen, E.J.M., Waroquier, M., van Speybroeck, V., van Santen, R.A., J. Phys. Chem. C, 2014, 118 (10), 5317-5327

Fischer–Tropsch synthesis is an attractive process to convert alternative carbon sources, such as biomass, natural gas, or coal, to fuels and chemicals. Deactivation of the catalyst is obviously undesirable, and for a commercial plant it is of high importance to keep the catalyst active as long as possible during operating conditions. In this study, the reactivity of CO on carbon-covered cobalt surfaces has been investigated by means of density functional theory (DFT). An attempt is made to provide insight into the role of carbon deposition on the deactivation of two cobalt surfaces: the closed-packed Co(0001) surface and the corrugated Co(112̅1) surface. We also analyzed the adsorption and diffusion of carbon atoms on both surfaces and compared the mobility. Finally, the results for Co(0001) and Co(112̅1) are compared, and the influence of the surface topology is assessed.

Development of a benzimidazole-derived bidentate P,N-ligand for enantioselective iridium-catalyzed hydrogenations , Weemers, J.J.M., Sypaseuth, F.D., Bäuerlein, P.S., van der Graaff, W.N.P., Filot, I.A.W., Lutz, M., Müller, C., Eur. J. Org. Chem., 2014, 2, 350-362

The development of a novel benzimidazole-derived bidentate P,N-ligand and its application in Ir-catalyzed hydrogenation is described. The ligand backbone was obtained through a one-pot tandem hydroformylation–cyclization sequence and the enantiomers of the generated alcohol were separated by chiral HPLC. By comparing the experimentally obtained CD spectra of the enantiomers with the simulated spectra generated from time-dependent DFT calculations, the absolute configuration could be obtained. The chiral alcohols could further be isolated on a larger scale after transesterification by using Candida Antarctica lipase B (Novozym 435) and could subsequently be converted into the corresponding chiral P,N-ligand by reaction with ClPPh2. The coordination properties of the racemic P,N-ligand were investigated and the molecular structure of the RhI complex [(P,N)Rh(CO)Cl] was determined by X-ray crystal structure analysis. The corresponding chiral cationic IrI complex was used as catalyst for the enantioselective hydrogenation of prochiral N-phenyl-(1-phenylethylidene)amine and trans-α-methylstilbene. For the N-aryl-substituted imine, enantiomeric excesses of only 10 % were obtained, whereas the unfunctionalized olefin could be hydrogenated with enantiomeric excesses of up to 90 %. Interestingly, the modular synthetic access to the P,N-hybrid system described here allows facile modification of the ligand structure, which should extend the scope of such novel P,N-ligands for asymmetric catalytic conversions to a large extent in the future.

Conformational Analysis of Chiral Supramolecular Aggregates: Modeling the Subtle Difference between Hydrogen and Deuterium , Nakano, Y., Markvoort, A.J., Cantekin, S., Filot, I.A.W., ten Eikelder, H.M.M., Meijer, E.W., Palmans, A.R.A., J. Am. Chem. Soc., 2013, 135 (44), 16497-16506

A detailed analysis of the conformational states of self-assembled, stereoselectively deuterated benzene-1,3,5-tricarboxamides ((S,S,S)-D-BTAs) reveals four different conformers for the supramolecular polymers. The relative amount of the conformers depends on the solvent structure and the temperature. With the help of a model, the thermodynamic parameters that characterize the different conformational states were quantified as well as the amount of the species that occur at different stages of the polymerization process. The results show that small changes in the stability between different types of conformers formed by (S,S,S)-D-BTAs—in the order of a few J mol–1—arise from the combination of interactions between the solvent/supramolecular aggregate, temperature, and solvent structure. While the introduction of a deuterium label allows to sensitively probe the solvophobic effects in the supramolecular aggregation, a rationalization of the observed effects on a molecular level is not yet straightforward but is proposed to result from subtle effects in the vibrational enthalpy and entropy terms of the isotope effect.

Catalytic properties of extraframework iron-containing species in ZSM-5 for N2O decomposition , Li, G., Pidko, E.A., Filot, I.A.W., van Santen, R.A., Li, C., Hensen, E.J.M., J. Catal., 2013, 308, 386-397

The reactivity of mononuclear and binuclear iron-containing complexes in ZSM-5 zeolite for catalytic N2O decomposition has been investigated by periodic DFT calculations and microkinetic modeling. On mononuclear sites, the activation of a first N2O molecule is favorable. The rate of catalytic N2O decomposition over Fe2+ and [FeIIIO]+ sites is very low because of the very high barriers (>180 kJ/mol) for the activation of the second N2O molecule necessary to complete the catalytic cycle by O2 formation. The catalytic cycles for N2O decomposition over binuclear [FeII(μ-O)FeII]2+ and [FeIII(μ-O2)FeIII]2+ species are interconnected. The catalytic cycle involves the interconversion of these species upon dissociation of N2O on the former complex. As the coordination of reactive Fe centers changes along the reaction coordinate, there are changes in the spin state of the complexes, which affect the overall potential energy diagram. These changes in spin multiplicities facilitate O2 formation and desorption steps. Based on the DFT-computed potential energy diagrams, microkinetic model simulations were carried out to predict reaction rates and kinetic parameters. The rate of O2 formation is much higher on binuclear sites than on mononuclear sites. For mononuclear sites, the apparent activation energy is ∼180 kJ/mol, close to the barrier for dissociating a second N2O molecule. It is consistent with first-order behavior with respect to the partial pressure of N2O. Binuclear sites display much higher reactivity. At low temperature, O2 desorption is rate controlling, whereas at higher temperatures, the rate is controlled by the two N2O dissociation reactions on [FeII(μ-O)FeII]2+ and [FeIII(μ-O)2FeIII]2+. This leads to first-order behavior with respect to N2O. An alternative path involving N2O adsorption and dissociation on [OFe(μ-O)2Fe]2+ is energetically favorable but does not contribute to the catalytic cycle because O2 desorption from the [OFe(μ-O)2Fe]2+ intermediate is preferred over the activation of a third N2O molecule due to entropic reasons.

Mechanism and microkinetics of the Fischer-Tropsch reaction , van Santen, R.A., Markvoort, A.J., Filot, I.A.W., Ghouri, M.M. and Hensen, E.J.M., Phys. Chem. Chem. Phys., 2013, 15 (40), 17038-17063

The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer–Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer–Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer–Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C–O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer–Tropsch catalysis.

Self-healing systems based on disulfide-thiol exchange reactions , Pepels, M.P.F., Filot, I.A.W., Klumperman, L., Goossens, J.G.P., Pol. Chem., 2013, 4 (18), 4955-4965

New thermoset systems based on disulfide bonds were synthesized with self-healing capabilities. The self-healing mechanism is not related to disulfide–disulfide exchange reactions, but to thiol–disulfide exchange reactions that are pH-dependent. Stress relaxation experiments showed large relaxation for systems having PTM2 as a curing agent, which indicates that the system can rearrange its molecular structure as a mechanism to release stress. However, relaxation rates decreased for samples tested longer after production. This indicates the disappearance of thiol-groups probably caused by thiol–thiol oxidation.

Microkinetics of steam methane reforming on platinum and rhodium metal surfaces , Zhu, T., van Grootel, P.W., Filot, I.A.W., Sun, S-G., van Santen, R.A., Hensen, E.J.M., J. Catal., 2013, 297, 227-235

We have investigated the most important elementary reaction steps in the steam methane reforming (SMR) process for planar and stepped Pt surfaces (dissociative CH4 adsorption, CHads–Oads recombination, H2O activation) and compared activation barriers for Rh surfaces. Compared to Rh, the lower reactivity of Pt results in (i) higher barriers for dissociative CH4 adsorption and (ii) endothermic formation of OHads and Oads. Microkinetic simulations show that Rh nanoparticle catalysts will be more active than Pt ones. The rate-controlling step is dissociative CH4 adsorption occurring on low-coordinated surface atoms (edges, corners, step-edges). The stepped surfaces are much more reactive than planar surfaces of the corresponding metals. For stepped Pt surfaces, CO formation via recombination of Cads + OHads is favored because of the low Oads coverage. At higher temperatures, deactivation may occur due to poisoning by carbonaceous species because the rate of OHads/Oads formation becomes too low compared to the rate of CHads formation. This occurs at lower temperature for Pt than for Rh because of the lower Pt–O bond energy.

The origin of isotope-induced helical-sense bias in supramolecular polymers of benzene-1,3,5-tricarboxamides , Filot, I.A.W., Palmans, A.R.A., Hilbers, P.A.J., Hensen, E.J.M., de Greef, T.F.A., Pidko, E.A., Phys. Chem. Chem. Phys., 2012, 14, 13997-14002

The molecular origin of the isotope-induced diastereomeric enrichment in helical supramolecular polymers consisting of trialkylbenzene-1,3,5-tricarboxamides (BTAs) is studied using plane-wave DFT calculations. We demonstrate that the creation of a chiral center at the α-position of the alkyl chains of a BTA by H–D exchange leads to a small but notable preference for the formation of supramolecular hydrogen bonded structures with a particular helicity. The bias for one helical sense preference is caused by the orientation of the vibrational eigenmodes of the C–H and C–D stretching frequencies at the chiral center and by hyperconjugative destabilization of the anti C–H orbital.

Cooperative Two-Component Self-Assembly of Mono- And Ditopic Monomers , Smulders, M.M.J., Nieuwenhuizen, M.M.L., Grossman, M., Filot, I.A.W., Lee, C.C., de Greef, T.F.A., Schenning, A.P.H.J., Palmans, A.R.A., Meijer, E.W., Macromolecules, 2011, 44 (16), 6581-6587

A N-methylated benzene-1,3,5-tricarboxamide (BTA) was synthesized, characterized, and introduced as a monotopic BTA monomer capable of interacting with the supramolecular polymer formed via the cooperative self-assembly of the analogous ditopic BTA monomers. Using optical spectroscopy and viscometry, in combination with mathematical modeling and DFT calculations, we were able to understand in detail the consequence of introducing a second monotopic component in the self-assembly of BTA monomers into long supramolecular polymers, taking explicitly the cooperative nature of the self-assembly process into account. To this end, a binary self-assembly model that includes both the monotopic and ditopic BTA monomer and that addresses the presence of both monomers and polymers (characteristic of a cooperative supramolecular polymer) was developed and successfully applied to model the viscometry data. The binary self-assembly model presented herein can be more generally applied to other cooperative supramolecular polymers to which a second component is added that can interact with the monomers and/or polymers and thus can contribute to a better understanding of more complex self-assembling systems.

Size and Topological Effects of Rhodium Surfaces, Clusters and Nanoparticles on the Dissociation of CO , Filot, I.A.W., Shetty, S.G., Hensen, E.J.M., van Santen, R.A., J. Phys. Chem. C, 2011, 115 (29), 14204-14212

The present density functional theory study provides insight into the reactivity of the surface metal atoms of extended/periodic Rh surfaces, clusters, and nanoparticles toward CO adsorption and dissociation. Our results demonstrate that the defect site in a B5 configuration is the most active one for CO dissociation on all three considered systems. However, the reactivity of the B5 site for CO dissociation depends critically on the size of the system. The barrier for CO dissociation barrier on the B5 site increases for smaller particles. The lowest barrier is found for the B5 site of a stepped Rh (211) surface. CO dissociation on this site occurred with a barrier below the desorption energy of CO.

Understanding Cooperativity in Hydrogen-Bond-Induced Supramolecular Polymerization: A Density Functional Theory Study , Filot, I.A.W., Palmans, A.R.A., Hilbers, P.A.J, van Santen, R.A., Pidko, E.A., de Greef, T.F.A., J. Phys. Chem. B, 2010, 114 (43), 13667-13674

Understanding the molecular mechanism of cooperative self-assembly is a key component in the design of self-assembled supramolecular architectures across multiple length scales with defined function and composition. In this work, we use density functional theory to rationalize the experimentally observed cooperative growth of C3-symmetrical trialkylbenzene-1,3,5-tricarboxamide- (BTA-) based supramolecular polymers that self-assemble into ordered one-dimensional supramolecular structures through hydrogen bonding. Our analysis shows that the cooperative growth of these structures is caused by electrostatic interactions and nonadditive effects brought about by redistribution of the electron density with aggregate length.

Dynamic Supramolecular Polymers Based on Benzene-1,3,5-tricarboxamides: The Influence of Amide Connectivity on Aggregate Stability and Amplification of Chirality , Stals, P.J.M., Everts, J.C., de Bruijn, R., Filot, I.A.W., Smulders, M.M.J., Martín-Rapún, R., Pidko, E.A., de Greef, T.F.A., Palmans, A.R.A., Meijer, E.W., Chem. Eur. J., 2010, 16, 810-821

N-Centred benzene-1,3,5-tricarboxamides (N-BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid-state behaviour and self-assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X-ray diffraction revealed that the chiral N-BTA derivatives with branched 3,7-dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Colho. In contrast, N-BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable-temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N-BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N-BTAs revealed a cooperative self-assembly behaviour of the N-BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants-and-soldiers as well as the majority-rules principles were operative in stacks of N-BTAs. In fact, the self-assembly of N-BTAs resembles closely that of their carbonyl (C=O)-centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self-assembly of N- and C=O-BTAs were analysed by density functional theory (DFT) calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of N-BTAs. The lower interaction energy is due to the higher energy penalty for rotation around the Ph-NH bond compared to the Ph[BOND]CO bond and the diminished magnitude of dipole–dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N-BTAs and C=O BTAs.

Tuning the Extent of Chiral Amplification by Temperature in a Dynamic Supramolecular Polymer , Smulders, M.M.J., Filot, I.A.W., Leender, J.M.A., van der Schoot, P., Palmans, A.R.A., Schenning, A.P.H.J., Meijer, E.W., J. Am. Chem. Soc., 2010, 132 (2), 611-619

Here, we report on the strong amplification of chirality observed in supramolecular polymers consisting of benzene-1,3,5-tricarboxamide monomers and study the chiral amplification phenomena as a function of temperature. To quantify the two chiral amplification phenomena, i.e., the sergeants-and-soldiers principle and the majority-rules principle, we adapted the previously reported sergeants-and-soldiers model, which allowed us to describe both amplification phenomena in terms of two energy penalties: the helix reversal penalty and the mismatch penalty. The former was ascribed to the formation of intermolecular hydrogen bonds and was the larger of the two. The latter was related to steric interactions in the alkyl side chains due to the stereogenic center. With increasing temperature, the helix reversal penalty was little affected and remained rather constant, showing that the intermolecular hydrogen bonds remain intact and are directing the helicity in the stack. The mismatch penalty, however, was found to decrease when the temperature was increased, which resulted in opposite effects on the degree of chiral amplification when comparing the sergeants-and-soldiers and the majority-rules phenomena. While for the former a reduction in mismatch penalty resulted in a decrease in degree of chiral amplification, for the latter it resulted in a stronger chiral amplification effect. By combining the sergeants-and-soldiers and majority-rules phenomena in a diluted majority-rules experiment, we could further determine the effect of temperature on the degree of chiral amplification. Extending the experiments to different concentrations revealed that the relative temperature, i.e., the temperature relative to the critical temperature of elongation, controls the degree of chiral amplification. On the basis of these results, it was possible to generate a general “master curve” independent of concentration to describe the temperature-dependent majority-rules principle. As a result, unprecedented expressions of amplification of chirality are recorded.