Reverse hydrogen spillover in supported subnanosize clusters of the metals of groups 8 to 11. A computational model study

In a recent computational study [G. N. Vayssilov, B. C. Gates and N. Rösch, Angew. Chem., Int. Ed. Eng., 2003, 42, 1391], we found zeolite-supported Rh₆ clusters, interacting with hydroxyl groups of the support, to undergo partial oxidation due to reverse spillover of hydrogen onto the metal cluster. Now, we have extended this model study to transition metal clusters M₆ of the platinum and gold groups. According to the model calculations, reverse spillover of hydrogen onto the zeolite-supported metal clusters is energetically favored for all 12 metals. For most metals, the clusters M₆ exhibit a compact form in either of the two states - bare supported and with hydrogen impurities. However, for Cu and Ag, the structures of the clusters with H impurities, were determined to be more open, whereas Au₆ exhibited an almost planar structure in either state. The estimated energy for reverse hydrogen spillover is lowest for the clusters Au₆ and Ag₆, 18 and 52 kJ mol^-1 per transferred hydrogen, and highest for the clusters Ir₆ and Os₆, 229 and 247 kJ mol^-1, respectively. Because of these model results, one would expect small metal clusters, supported on OH covered surfaces, likely to be oxidized and partially covered by hydrogen, substantially affecting the electron distribution and the chemical reactivity of the clusters. To assist in the experimental discrimination of hydrogen impurities of adsorbed metal clusters, we propose two criteria: metal core levels are predicted to be stabilized in the case of reverse hydrogen spillover and the number of metal-oxygen contacts is calculated to be twice as large in clusters with hydrogen impurities.