Publications

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

Publications per year

2018: 7
2017: 7
2016: 2
2015: 3
2014: 5
2013: 5
2012: 1
2011: 2
2010: 3
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.

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.

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.

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.

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, 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.

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., 2017, 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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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, 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.

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.

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, 126, 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.

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, 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.