First Principles Study of CO Adsorption-CO2 Desorption Mechanisms on Oxidized Doped-Gold Cationic Clusters MAunOm+ (M = Ti, Fe; n=1, 4-7; m=1-2)

A first principles study of the formation and desorption of CO2 on oxidized doped-gold complexes MAunOm+ m is presented for n = 1, 4-7, m = 1-2, and M = Ti, Fe. We start with the ground state geometry of MAunO2+ complexes obtained in a previous work. After the adsorption of a first CO molecule on MAunO2+, two intermediate complexes with small energy difference are preferably formed, one of them with a preformed CO3 carbonate and the other one with a preformed CO2. The former is more bound than the later only for n = 6-7, and for FeAu4O2+. Two mechanism for the desorption of CO2 are studied. In the first one, here called sequential mechanism, we choose the intermediate O-MAun+-CO2 complex with a preformed CO2 as the preferential to desorb a CO2 molecule leaving the MAunO+ cluster. A second CO molecule is then adsorbed on that residual aggregate forming, among other equilibrium compositions, a MAun+-CO2 complex from which a CO2 molecule can be desorbed to yield the deoxidized MAun+ bimetallic cluster. In the second CO2 desorption mechanism, called here simultaneous mechanism, we consider the adsorption of a second CO on each one of MAun+-CO3 and O-MAun+-CO2 intermediate complexes resulting from the first CO adsorption. Three types of new intermediate complexes are preferably formed: (a) MAun+-CO-CO3 with preformed CO3 and CO radicals; (b) MAun+-C2O4 with a preformed C2O4 oxalate; (c) MAun+-CO2-CO2 with two preformed CO2 molecules bonded on the M impurity. We calculate the simultaneous desorption of two CO2 molecules from the intermediate complex of case (c) leaving the deoxidized MAun+ cluster in the ground state, except for a few cases where FeAun+ structural deformation occurs following the optimization of intermediate complexes. The effects of cluster size and transition metal impurity on the CO adsorption and CO2 desorption energies are discussed. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 510-519, 2011