Theoretical calculation of the interaction of hydrogen with models of coordinatively unsaturated centers on alumina

Ab initio calculations with large basis sets and electron correlation were conducted on the reaction of hydrogen molecules with (HO)(3)Al(OH2)(x) clus ters, where x = 0, 1, and 2. In this way, the reactivity of Al(III) species was studied as a function of their level of coordinative unsaturation. For the tricoordinated species, the geometry of the starting cluster was obtai ned by the optimization of the species (HO)(3)Al(OH2)(x+1) and removal of t he extra water molecule, on the basis of idea that aluminum oxide surfaces are formed by calcination of hydrated forms. Two models, one assuming rigid ity and the other allowing for flexibility of the tricoordinated aluminum c enter, were examined. The complexes with physisorbed and chemisorbed hydrog en were optimized in the same way. The reactions of tetra- and pentacoordin ated aluminum dusters were studied without any constraints on the geometry. The calculations predicted the hydrogen chemisorption to be endothermic in all cases, the order being E(x = 0) < E(x = 1) < E(x = 2). The chemisorpti on pathway was investigated and its transition structure and energy barrier were established. The energy barriers for chemisorption, determined as the relative energies of the transition structures E-TS varied with the coordi nation number of the aluminum atom as E-TS(x = 0) < E-TS(x = 1) < E-TS(x = 2) The barriers were similar for the rigid and for the flexible tricoordina ted aluminum clusters. A significant conclusion is that tetracoordinated si tes on alumina must be thought of as reactive (if not the reactive) sites. The literature description (on the basis of ab initio calculations with sma ll basis sets at the HF level) of hydrogen chemisorption as an acid-base re action, involving hydrogen heterolysis concerted with the attachment of the proton to oxygen (basic site) and the hydride to aluminum (acid site), is not substantiated by our calculations. Instead, the chemisorption occurs th rough the interaction of Hz with the aluminum (metal ion catalysis) until b oth hydrogen atoms are bonded to Al, after which one of the hydrogens migra tes to an adjacent oxygen atom. B3LYP calculations give results in reasonab le agreement with the MP2 calculations, attesting to the appropriateness of the density functional theory method for these types of structures.