Reverse hydrogen spillover on and hydrogenation of supported metal clusters: insights from computational model studies.

"Reverse" spillover of hydrogen from hydroxyl groups of the support onto supported transition metal clusters, forming multiply hydrogenated metal species, is an essential aspect of various catalytic systems which comprise small, highly active transition metal particles on a support with a high surface area. We review and analyze the results of our computational model studies related to reverse hydrogen spillover, interpreting available structural and spectral data for the supported species and examining the relationship between metal-support and metal-hydrogen interactions. On the examples of small clusters of late transition metals, adsorbed in zeolite cavities, we showed with computational model studies that reverse spillover of hydrogen is energetically favorable for late transition metals, except for Au. This preference is crucial for the chemical reactivity of such bifunctional catalytic systems because both functions, of metal species and of acidic sites, are strongly modified, in some cases even suppressed - due to partial oxidation of the metal cluster and the conversion of protons from acidic hydroxyl groups to hydride ligands of the metal moiety. Modeling multiple hydrogen adsorption on metal clusters allowed us to quantify how (i) the support affects the adsorption capacity of the clusters and (ii) structure and oxidation state of the metal moiety changes upon adsorption. In all models of neutral systems we found that the metal atoms are partially positively charged, compensated by a negative charge of the adsorbed hydrogen ligands and of the support. In a case study we demonstrated with calculated thermodynamic parameters how to predict the average hydrogen coverage of the transition metal cluster at a given temperature and hydrogen pressure.

First Hybrid Embedding Scheme for Polar Covalent Materials Using an Extended Border Region To Minimize Boundary Effects on the Quantum Region.

We present an improved scheme for constructing the border region within a hybrid quantum mechanics/molecular mechanics (QM/MM) embedded cluster approach for zeolites and covalent oxides that ensures proper modeling of adsorption complexes with QM regions of moderate size. The procedure employs a flexible orbital basis set on monovalent oxygen pseudoatoms at the boundary of the QM cluster and introduces a pseudopotential description without explicit representation of valence electrons for their immediate Si neighbors in the MM region. This novel QM/MM border scheme, implemented in the elastic polarizable environment method for polar covalent materials (covEPE), provides an accurate description of the local structure of zeolites and other silica based materials. We assessed the performance of the novel border scheme by comparing calculated and experimental results for structures, vibrational frequencies, and binding energies of CO adsorption complexes at bridging OH groups in zeolites with FAU and MFI structures. In addition, when modeling zeolite-supported metal clusters, the new approach implies considerably reduced corrections due to the basis set superposition error, compared to our previous scheme for treating the border region of the QM partition [J. Phys. Chem. B 2003, 107, 2228].

Comparison of all sites for Ti substitution in zeolite TS-1 by an accurate embedded-cluster method.

We studied the preferential location of Ti centers in the framework of the Ti-containing MFI zeolite TS-1 using a hybrid DFT/MM embedding method developed recently. This "covalent elastic polarizable environment" (covEPE) cluster embedding allows a complete and self-consistent treatment of solid covalent systems such as zeolites. For the present study, we used a gradient-corrected density functional approach. The resulting structural features of both Si- and Ti-substituted forms of the zeolite framework fit well with available experimental information. The calculated substitution energy of Ti at the 12 crystallographically different tetrahedral sites of the MFI structure vary within 19 kJ/mol with T12 and T2 as most and least preferred sites, respectively. On the basis of these computational results and the preferential sites for Ti substitution reported from different experimental investigations, we concluded that the Ti distribution in the TS-1 framework is not governed by the thermodynamic stability of the pure material.

Effects of the Aluminum Content of a Zeolite Framework: A DFT/MM Hybrid Approach Based on Cluster Models Embedded in an Elastic Polarizable Environment.

We report the first computational study with a sophisticated quantum mechanics/molecular mechanics (QM/MM) technique that addresses the effect of the aluminum content on the properties of acidic zeolites. To account for both electrostatic and mechanical interaction between the QM cluster and its MM environment, we used cluster models embedded in the covalent variant of the elastic polarizable environment (covEPE) [Nasluzov, V. A.; Ivanova, E. A.; Shor, A. M.; Vayssilov, G. N.; Birkenheuer, U.; Rösch, N. J. Phys. Chem. B 2003, 107, 2228]. For the practical application of the covEPE method, it was necessary to develop a new force field for Al containing zeolites. Two types of zeolite materials, FAU and MFI, were employed as examples. We modeled the variation of the Al content both in the MM environment and in the QM cluster, and we studied pertinent properties of bridging OH groups of the zeolite frameworks, OH vibrational frequencies, and deprotonation energies. The computational results suggest that the local structure and the location of the OH groups exert a stronger effect than the variation of the Al content of the framework.