Theoretical study of CO adsorption and oxidation on Au_3-5 clusters supported on silico-aluminophospates

The Au_3-5 clusters supported on silico-aluminophospates (SAPO-11) were studied using quantum chemistry calculations, by means of the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au₃/SAPO-11, Au₄/SAPO-11, Au₅/SAPO-11, CO-Au₃/SAPO-11, CO-Au₄/SAPO-11 and CO-Au₅/SAPO-11 aggregates to analyze the geometries of small clusters of Au₃, Au₄ and Au₅ on SAPO-11 support. Au₃ cluster presents a triangle structure in Au₃/SAPO-11. Au₄ cluster shows a "Y shaped" structure in Au₄/SAPO-11. Au₄ as a rhombus structure is also studied but it is a less stable structure. Au₅ cluster displays a "pentagon shaped" structure as the most stable. The CO interaction with Au₃, Au₄ and Au₅/SAPO-11 is studied; this CO adsorption is different from that reported in the literature. The formation energy δE_F of the aggregates, the CO adsorption energy δE_ads on them and the change of energy of the oxidation reaction are presented. Some relevant charges and bond indexes were calculated. On supporting all Au_3-5 there is a charge transfer from the Au cluster to the binding O atoms. On CO adsorption in all Au_3-5 clusters there is a charge transfer from the cluster to the CO.