The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol−1. H2 was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol−1. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol−1 per H2. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta3N5-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol−1. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta3N5. The studies offer significant new insight with respect to the chemistry of N2 and H2 with tantalum nitride surfaces in the presence of cobalt promoters.

We report a detailed density functional theory (DFT) study in conjunction with extended X-ray absorption fine structure (EXAFS) experiments on the geometrical and local electronic properties of Cu adatoms and Cu(II) ions in presence of water molecules and of CuO nanoclusters on the CeO2(110) surface. Our study of (CuO)n(=1,2&4) clusters on CeO2(110) shows that based on the Cu–O environment, the geometrical properties of these clusters may vary and their presence may lead to relatively high localization of charge on the exposed surfaces. We find that in the presence of an optimum concentration of water molecules, Cu has a square pyramidal geometry, which agrees well with our experimental findings; we also find that Cu(II) facilitates water adsorption on the CeO2(110) surface. We further show that a critical concentration of water molecules is required for the hydrolysis of water on Cu(OH)2/CeO2(110) and on pristine CeO2(110) surfaces.

Periodic and molecular density functional theory calculations have been applied to elucidate the associative mechanism for hydrazine and ammonia synthesis in the gas phase and hydrazine formation on Co3Mo3N. We find that there are two activation barriers for the associative gas phase mechanism with barriers of 730 and 658 kJ/mol, corresponding to a hydrogenation step from N2 to NNH2 and H2NNH2 to H3NNH3, respectively. The second step of the mechanism is barrierless and an important intermediate, NNH2, can also readily form on Co3Mo3N surfaces via the Eley–Rideal chemisorption of H2 on a pre-adsorbed N2 at nitrogen vacancies. Based on this intermediate a new heterogeneous mechanism for hydrazine synthesis is studied. The highest relative barrier for this heterogeneous catalysed process is 213 kJ/mol for Co3Mo3N containing nitrogen vacancies, clearly pointing towards a low-energy process for the synthesis of hydrazine via a heterogeneous catalysis route.

Neutron scattering methods observed complete room temperature conversion of methanol to framework methoxy in a commercial sample of methanol-to-hydrocarbons (MTH) catalyst H-ZSM-5, evidenced by methanol immobility and vibrational spectra matched by ab initio calculations. No methoxylation was observed in a commercial HY sample, attributed to the dealumination involved in high silica HY synthesis.

A graphical abstract is available for this content

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300–400 K. The experimental diffusion coefficients were measured in the range 2–5 × 10−10 m2 s−1 and simulated diffusion coefficients calculated in the range of 1.6–3.2 × 10−9 m2 s−1. Activation energies were measured as 8.8 and 6.9 kJ mol−1 using QENS and MD respectively. Differences may be attributed predominantly to the experimental use of a dealuminated HY sample, containing significant defects such as strongly adsorbing silanol nests, compared to a perfect simulated crystal containing only evenly distributed Brønsted acid sites. Experimental and simulated diffusivities measured in this study are lower than those obtained from those previously calculated in siliceous faujasite, due to methanol H-bonding to Brønsted acid sites as observed in the MD simulations. However, both experimental and simulated diffusivities were significantly higher than those obtained in NaX, due to the higher concentration of extraframework cations present in the previously studied structures.

Cobalt molybdenum nitride (Co3Mo3N) is one of the most active catalysts for ammonia synthesis, although the atomistic details of the reaction mechanism are currently unknown. We present a dispersion-corrected (D3) DFT study of the adsorption and activation of molecular nitrogen and hydrogen on Co3Mo3N-(111) surfaces to identify possible activation sites for ammonia synthesis. H2 was found to adsorb both molecularly on the Mo3N framework and dissociatively on Co8 clusters or Mo3 clusters that were exposed due to N-vacancies. We find that there are two possible activation sites for N2 where both N2 and H2 can coadsorb. The first is a Mo3 triangular cluster that resides at 3f nitrogen vacancies, and the second is a surface cavity where N2 is activated by a Co8 cluster, the second being a more efficient activation site. N2 was found to adsorb in three adsorption configurations: side-on, end-on, and an unusual tilt end-on (155°) configuration, and the existence of these three adsorption configurations is explained via MP2 calculations and the sphere-in-contact model.

Zeolites are versatile and fascinating materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities to obtain the best material for a given application. Over the last decades, theoretical modeling has matured to a level that model guided design has become within reach. Major hurdles have been overcome to reach this point and almost all contemporary methods in computational materials chemistry are actively used in the field of modeling zeolite chemistry and applications. Integration of complementary modeling approaches is necessary to obtain reliable predictions and rationalizations from theory. A close synergy between experimentalists and theoreticians has led to a deep understanding of the complexity of the system at hand, but also allowed the identification of shortcomings in current theoretical approaches. Inspired by the importance of zeolite characterization which can now be performed at the single atom and single molecule level from experiment, computational spectroscopy has grown in importance in the last decade. In this review most of the currently available modeling tools are introduced and illustrated on the most challenging problems in zeolite science. Directions for future model developments will be given.

The diffusion of longer n-alkanes (n-C8–n-C16) in silicalite was studied using molecular dynamics (MD) simulations at a temperature range of 300–400 K, with loadings appropriate for direct comparison with previously carried out quasielastic neutron scattering (QENS) studies. The calculated diffusion coefficients were in close agreement with experimental values, significantly closer than those calculated using more primitive framework and hydrocarbon models, and in the case of the longer alkanes, closer agreement than those calculated by MD studies using the same model, but not using experimental loadings. The calculated activation energies of diffusion agreed with experiment to within 1.5 kJ mol−1 for shorter alkanes of the range, but with a larger difference for tetra and hexadecane, due to factors which cannot be reproduced using periodic boundary conditions. Channel switching between the straight and sinusoidal channel system was found for octane at higher temperatures, where more than one octane molecule was located in the channel, which was attributed to the molecular size of octane, and the repulsion caused by the presence of the extra octane molecules in the channel system, allowing the potential barrier of channel switching at the junctions to be breached.

We employ hybrid density functional calculations to analyze the structure and stability of the (101̅0) and (112̅0) ZnO surfaces, confirming the relative stability of the two surfaces. We then examine morphological features, including steps, dimer vacancies, and grooves, at the main nonpolar ZnO surface using density functional methods. Calculations explain why steps are common on the (101̅0) surface even at room temperature, as seen in experiment. The surface structure established has been used to obtain the definitive ionization potential and electron affinity of ZnO in good agreement with experiment. The band bending across the surface is analyzed by the decomposition of the density of states for each atomic layer. The upward surface band bending at the (101̅0) surface affects mostly the valence band by 0.32 eV, which results in the surface band gap closing by 0.31 eV; at the (112̅0) surface, the valence band remains flat and the conduction band bends up by 0.18 eV opening the surface band gap by 0.12 eV.

Water adsorption on the cubic (111) surface of yttria-stabilized zirconia (YSZ) was investigated using density functional theory calculations. Relaxation of atomic positions away from ideal cubic lattice sites, in particular of the oxygen anion sublattice, is observed on including both phase-stabilizing dopants and water adsorption. A large slab model has been used to explore the effects of extended relaxation throughout the anionic sublattice and the role of vacancy–vacancy interactions on water adsorption. Dissociative adsorption of water to fill a surface vacancy site, accompanied by concerted oxygen movement in the vacancy cluster region of the slab, leads to a very strong adsorption of −2.20 eV, blocking surface sites for oxygen activation. We show that the use of larger slab models leads to a more detailed representation of the YSZ surface system.

The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore ‘breathing’ assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.

The nucleation mechanism of zeolite A is investigated by means of Density Functional Theory (DFT) calculations. We calculated the Gibbs free energy change for the polymerization and cyclization reactions involved in the nucleation of zeolite A in the gas phase and solution between 298 and 450 K. Our analysis reveals that the four-ring species formed could be the most likely to participate in the nucleation of zeolite A, and its nucleation mechanism could proceed by a reaction route which involves the formation of the double-four-ring.

Density functional theory (DFT) and the conductor like screening model (COSMO) are employed to investigate the condensation reactions involving key aluminosilicate species: the Si(OH)4 or Al(OH)4Na monomers and the AlSiO(OH)6Na dimer, where we investigate reactions with the four, six, double four, and double six rings to form a series of fused rings in the gas phase and solution. Our calculations suggest that the Al(OH)4Na monomer may not participate in these condensation reactions as such participation would generate structures that contradict Loewenstein’s rule; in contrast, on the basis of our results, we propose that condensation and growth may occur predominantly via the AlSiO(OH)6Na dimer.

We investigate the structures of stabilities of small aluminosilicate clusters using molecular simulations employing density functional theory (DFT) and the conductor like screening model (COSMO), which allow us to model clusters both in the gas phase as well as in solution. We report the relative structures and energies of clusters containing between one and six Si/Al atoms and the effect on them of the interaction with Na+ and of intramolecular hydrogen bonding. Our results reveal that with the exception of the dimer, “Lowenstein” clusters (without Al–O–Al linkage) are more energetically favorable than “non-Lowenstein” clusters (which contain such bridges) in the gas phase. The stability of aluminosilicate clusters is strongly affected by solvation, with the solvent influencing their conformations. In solution, all of the most stable clusters follow not only Lowenstein’s rule, but also Dempsey’s rule.

The adsorption of Ni, Pd, and Pt on the clean and hydroxylated (0001) and (1−102) α-alumina has been investigated using first principles methods. On both clean surfaces, the metals have very similar adsorption mechanisms where the metal promotes a charge transfer from a surface oxygen to a surface aluminum. The adsorption mechanism on the hydroxylated surfaces is different due to the unavailability of surface aluminum. Strong stabilization of the metal by the hydroxyl groups is observed for Pt and Ni due to the rupture of a surface hydroxyl group and the formation of a hydride species on the metal. The stabilization of Pd is found to be much less marked than Pt and Ni due to the lower strength of the Pd−H bond.

Density functional theory has been used to investigate propane oxidation over iron antimony oxide, an important process in the petrochemical industry. Detailed microscopic mechanisms have been revealed for the following three key reactions: (i) the initial hydrogen abstraction from propane, the rate-determining step in propane oxidation to acrolein (CH2═CH−CHO), (ii) the formation of propene, a possible intermediate in the process, and (iii) the production of acrolein via the economically valuable one-stage mechanism. We have found that in the initial C−H splitting, the most feasible cleavage takes place via a homolytic dissociation, involving two oxygen atoms along the [010] direction of the surface and occurring in the methylene group. The heterolytic dissociation paths are generally less favorable than the homolytic processes; and in the former, there is almost no preference in abstracting H from methylene or methyl group. Subsequent reactions, after the first H abstraction via homolytic splitting, cannot lead to propene formation due to the large energy barriers encountered. In contrast, when the first H abstraction takes place on the CH2 species via a heterolytic splitting path, propene can subsequently be readily formed. Moreover, following a heterolytic splitting path for the first H abstraction from a CH3 species, we have identified a facile one-stage mechanism for the direct conversion of propane to acrolein, without having to proceed via the propene intermediate. The mechanistic details presented in this work are consistent with experiment, and lay the foundations for an understanding of activity and selectivity of this catalyst toward propane oxidation.

The nature of glycine−glycine interactions in aqueous solution has been studied using molecular dynamics simulations at four different concentrations and, in each case, four different temperatures. Although evidence is found for formation of small, transient hydrogen-bonded clusters of glycine molecules, the main type of interaction between glycine molecules is found to be single N—H···O—C hydrogen bonds. Double-hydrogen-bonded “dimers”, which have often been cited as a significant species present in aqueous solutions of glycine, are only observed infrequently. When double-hydrogen-bonded dimers are formed, they dissociate quickly (typically within less than ca. 4 ps), although the broken hydrogen bonds have a higher than average probability of reforming. Several aspects of the clustering of glycine molecules are investigated as a function of both temperature and concentration, including the size distribution of glycine clusters, the radii of gyration of the clusters, and aspects of the lifetimes of glycine−glycine hydrogen bonding by means of hydrogen-bond correlation functions. Diffusion coefficients for the glycine clusters and water molecules are also investigated and provide results in realistic agreement with experimental results.

This paper reported results of in situ XANES/EXAFS studies of iron substituted AlPO-5. Fe-AlPO-5s were synthesized with different templates. These materials were also studied using XRD, SEM, EDX and in situ XRD/EXAFS. Here the aim is to investigate the influence of template on the state of iron in the as-synthesized and calcined form. It was found that, depending on the nature of template, the coordination of Fe(III) ions changes from octahedral to tetrahedral at different temperatures.Download full-size imageHighlights► Investigate the influence of template on the state of iron in the prepared samples. ► With different templates, coordination of Fe changes from VI to IV at different t°C. ► Different template doesn't affect the nature of Fe sites. ► Different template leads to different behaviors of Fe ions in redox chemistry. ► Different templates affect mechanism of crystal growth.

Density functional theory is used to investigate the microscopic mechanisms of oxidation of propene (CH3CHCH2) to acrolein (CH2CHCHO) over iron antimony oxide (FeSbO4). Two routes for acrolein formation are investigated. The first starts from a chemisorbed state, in which propene binds with the surface via the π orbitals; acrolein formation can be triggered first by the abstraction of an allylic H atom towards the active bridging O atom, followed by the abstraction of a second H atom toward either an O or an Sb atom and the subsequent desorption of the acrolein thereby formed. The second route starts from a direct dissociation of the propene molecule without the need to proceed through a chemisorbed precursor, which, however, is kinetically hindered. The first route is compared with the mechanisms proposed from experiment. We also discuss the mechanisms of propane oxidation to acrolein, in which propene oxidation is an important step.

The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol−1. H2 was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol−1. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol−1 per H2. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta3N5-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol−1. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta3N5. The studies offer significant new insight with respect to the chemistry of N2 and H2 with tantalum nitride surfaces in the presence of cobalt promoters.

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300–400 K. The experimental diffusion coefficients were measured in the range 2–5 × 10−10 m2 s−1 and simulated diffusion coefficients calculated in the range of 1.6–3.2 × 10−9 m2 s−1. Activation energies were measured as 8.8 and 6.9 kJ mol−1 using QENS and MD respectively. Differences may be attributed predominantly to the experimental use of a dealuminated HY sample, containing significant defects such as strongly adsorbing silanol nests, compared to a perfect simulated crystal containing only evenly distributed Brønsted acid sites. Experimental and simulated diffusivities measured in this study are lower than those obtained from those previously calculated in siliceous faujasite, due to methanol H-bonding to Brønsted acid sites as observed in the MD simulations. However, both experimental and simulated diffusivities were significantly higher than those obtained in NaX, due to the higher concentration of extraframework cations present in the previously studied structures.

A graphical abstract is available for this content

The diffusion of n-alkanes ranging from length n-C8 to n-C20 in the zeolite silicalite is studied using classical molecular dynamics simulations. Different simulations were performed using a united-atom hydrocarbon model with a rigid zeolite framework, an all-atom hydrocarbon model with a rigid zeolite framework, and an all-atom hydrocarbon model with a flexible zeolite framework, all at 300 K. The latter two models have never previously been used to simulate longer alkanes in silicalite. Diffusion coefficients measured using a rigid zeolite framework exhibited a periodic dependence on chain length in the [010] direction in line with the previously observed phenomenon of resonant diffusion, regardless of the hydrocarbon model used. Explanations are considered in terms of the location of low energy traps within the silicalite structure, presenting a diffusion barrier. A monotonic dependence on diffusivity with chain length was observed however, on using an all-atom hydrocarbon model and a flexible framework, which was attributed to the occurrence of pore ‘breathing’ assisting diffusion. It was also noted that the calculated diffusion coefficients were up to an order of magnitude lower, and experimental diffusion coefficients are in much closer agreement when the latter model is used.

Neutron scattering methods observed complete room temperature conversion of methanol to framework methoxy in a commercial sample of methanol-to-hydrocarbons (MTH) catalyst H-ZSM-5, evidenced by methanol immobility and vibrational spectra matched by ab initio calculations. No methoxylation was observed in a commercial HY sample, attributed to the dealumination involved in high silica HY synthesis.

The diffusion of longer n-alkanes (n-C8–n-C16) in silicalite was studied using molecular dynamics (MD) simulations at a temperature range of 300–400 K, with loadings appropriate for direct comparison with previously carried out quasielastic neutron scattering (QENS) studies. The calculated diffusion coefficients were in close agreement with experimental values, significantly closer than those calculated using more primitive framework and hydrocarbon models, and in the case of the longer alkanes, closer agreement than those calculated by MD studies using the same model, but not using experimental loadings. The calculated activation energies of diffusion agreed with experiment to within 1.5 kJ mol−1 for shorter alkanes of the range, but with a larger difference for tetra and hexadecane, due to factors which cannot be reproduced using periodic boundary conditions. Channel switching between the straight and sinusoidal channel system was found for octane at higher temperatures, where more than one octane molecule was located in the channel, which was attributed to the molecular size of octane, and the repulsion caused by the presence of the extra octane molecules in the channel system, allowing the potential barrier of channel switching at the junctions to be breached.

Zeolites are versatile and fascinating materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities to obtain the best material for a given application. Over the last decades, theoretical modeling has matured to a level that model guided design has become within reach. Major hurdles have been overcome to reach this point and almost all contemporary methods in computational materials chemistry are actively used in the field of modeling zeolite chemistry and applications. Integration of complementary modeling approaches is necessary to obtain reliable predictions and rationalizations from theory. A close synergy between experimentalists and theoreticians has led to a deep understanding of the complexity of the system at hand, but also allowed the identification of shortcomings in current theoretical approaches. Inspired by the importance of zeolite characterization which can now be performed at the single atom and single molecule level from experiment, computational spectroscopy has grown in importance in the last decade. In this review most of the currently available modeling tools are introduced and illustrated on the most challenging problems in zeolite science. Directions for future model developments will be given.