Hydrogen Adsorption on Small Zeolite-Supported Rhodium Clusters. A Density Functional Study

Using a periodic density functional approach, we modeled the dissociative adsorption of hydrogen on small Rh clusters, supported inside the cavity of a faujasite-type zeolite. H spillover from a zeolite hydroxyl group to the metal cluster is favorable by at least 160 kJ/mol. Therefore, we used the complexes HRh₃ and HRh₄ in deprotonated zeolite as reference for H adsorption from the gas phase. For the most stable location of the complexes, at three four-membered rings, the average adsorption interaction of hydrogen decreases monotonously with increasing H loading. Adsorption energies are calculated from −70 to −52 kJ/mol for the complexes H₃Rh₃ and H₇Rh₃, respectively, and from −67 to −47 kJ/mol for the complexes H₃Rh₄ and H₉Rh₄, respectively. The preferred coordination of H ligands changes with the H loading, bridging at Rh–Rh bonds at low loading and terminal at Rh centers at high loading. Concomitantly the average Rh–Rh nearest-neighbor distances increase from 242 to 272 pm (Rh₃) and from 247 to 265 pm (Rh₄). A thermodynamic model based on the calculated Gibbs free energies of the structures studied suggests that complexes with maximum H loading, H₇Rh₃ and H₉Rh₄, dominate in a wide range of H₂ pressures and temperature. The calculated atomic charges suggest that the metal moieties are oxidized due to reverse hydrogen spillover and adsorption of hydrogen from the gas phase

Catalytic Transformations of 1-Butene over Palladium. A Combined Experimental and Theoretical Study

Applying a density functional approach to slab models of planar, (111), and rough, (110), Pd surfaces, we determined the isomerization free energy barriers of 1-butene to be significantly lower than the hydrogenation barriers. Microkinetic modeling allows one to mirror the kinetic experiments on conversions of 1-butene at the corresponding single-crystal surfaces in a qualitative fashion. Despite the inherent limitations of such kinetic modeling, theoretical predictions are fully supported by experimental data using Pd model catalysts: i.e., Pd(111) and Pd(110) surfaces. The isomerization mechanism was calculated to proceed via an initial dehydrogenation of 1-butene to 1-buten-3-yl as an intermediate—in contrast to the commonly proposed 2-butyl intermediate, associated with the Horiuti–Polanyi mechanism. Our modeling results rule out the original assumption that isomerization has to start with a hydrogenation step to rationalize the dependence of isomerization on hydrogen. However, this hydrogen dependence may arise in the second step, after an initial dehydrogenation, as suggested by the experimental data under hydrogen-deficient conditions.J.S.-A. acknowledges financial support by Generalitat Valenciana (BEST/2007/045) and MEC/OEAD (Acciones Integradas HU2006-002) supporting a research stay in Vienna. This work was supported by grant no. 1527700033 of the A*STAR Science and Engineering Research Council as well as generous allotments of computational resources at the National Supercomputing Center Singapore and the A*STAR Computational Resource Center