Modeling Hydrogen Spillover on Oxide Catalytic Centers

The spillover of hydrogen on an inorganic oxide support is studied using DFT calculations. Quantum-chemical calculations for model catalytic sites with structure H(–Al–O–)3(OH)6 and HH+(‒Al–O–)3(OH)6 are performed using the DFT B3LYP/6-31G * energy functional versus electron density with full energy optimization and calculation of the frequencies of normal vibrations. Geometric parameters, electronic characteristics, distribution of electron density, total energies, energies and entropies of transformation, and frequencies of normal vibrations are calculated for the compounds. The geometric structure of molecules and frequencies of normal vibrations are determined using 6-31G* atomic basis sets. The activation energy of hydrogen transfer is determined for these complexes. It is found that an additional positive charge on a catalytic center HH+(–Al–O–)3(OH)6 sharply lowers the activation energy of hydrogen transfer, the value of which is 13.92 kcal/mol. It is shown that the formation of a complex of water with catalytic center HH+(–Al–O–)3(OH)6 further reduces the activation energy of hydrogen transfer to 5.45 kcal/mol, in agreement with the experimental values of the activation energy of spillover hydrogen on aluminum oxide. The mechanism of the hydrogen transfer reaction in a complex of adsorbed water with a charged catalytic center HH+(‒Al–O–)3(OH)6 adequately reflects the mechanism of hydrogen spillover over an inorganic oxide carrier.