Density Functional Theory Study On Oxidation Of CO Catalyzed By Cu_nM (M=Al、Pt、Co) Clusters

In recent years, there are many investigations on Cu metal dueto its extensive applying in the field of nanotechnology and catalyst.Alloyed Cu material aroused the interest of scientists because thatalloy material has unique properties, which is distinct from thecharacter of elementary material. Full optimized structures andnormal-mode frequencies are found using the DFT/PBE method. Anall-electron basis set6–31G*is used for Al, H, O and C atoms. TheLANL2DZ pseudopotential is adopted for the Cu, Pt and Co atom.The contents are listed as follows:1. We have made an exhaustive study of the mechanism of COoxidation catalyzed by CunAl (n=1-3) clusters on gas phase. It isshown that mixing two different metals (Al and Cu) can havebeneficial effects on the catalytic activity than monometallicCun+1(n=1-3) cluster toward the reaction of CO oxidation. Thesecond CO molecule on the CunAl(n=1-3) clusters drives the reactionbetween CO and O2with a lowest barrier height of0.21,0.69, and 0.18eV to occur. These reaction barriers are comparable to thoseassociated with smaller-sized copper clusters (~0.90eV). Thealloyed Cu3Al cluster is proposed as the best effectivenanocatalysts.2. The mechanism of CO preferential oxidation reaction (PROX)catalyzed by CunPt (n=3-12) clusters has been studied. The resultsindicate that the Cu5Pt cluster are two-dimensional structure, theother are three-dimensional structure. The most favored adsorptionsite of H2for all clusters is on the Pt sites. And the adsorption of H2molecule on CunPt clusters can be performed with two kinds ofpatterns, such as top to top and side by side. The energy barrier ofhydrogen dissociation depends on the activation of the H2moleculewhich adsorbed on CunPt clusters. In other words, the largerdistance of H-H bond of the CunPtH2, the lower energy barrier ofhydrogen dissociation catalyzed by CunPt clusters. The lowestenergy barrier of hydrogen dissociation is0.02eV. The O-O bonddistances have a similar trend with the binding energies of the3O2tothe CunPtH-H (n=3-12), which is the more negative binding energiescorresponds to the larger O-O bond distances, but the cluster ofCu4Pt is abnormal. The trend of O-O bond distance and NBO chargeis similar, which is due to the fact that metal to oxygenback-donation increase the population of π*orbital leading to weakening of O-O bonds and thereby this bond gets lengthen. Thecalculated results demonstrate that the reaction of CO-PROXoccurs via the main intermediates of COOH and OH. All energybarriers of CO-PROX reaction catalyzed by CunPt (n=3-12) clustersrevealed that the Cu6Pt cluster is the best effective catalyst towardthis reaction. The lowest energy barrier of CO-PROX reaction is0.54eV (Cu6Pt).3. Investigated the mechanism of water-gas shift (WGS)reaction catalyzed by Cu12Co and Cu13clusters. Calculated resultsindicate that the lowest energy state of Cu12Co and Cu13clusters isIhsymmetry; the adsorption sites of H2O on Cu12Co and Cu13all is topsite, however, the adsorption sites of CO on Cu12Co and Cu13is topand hollow sites respectively; the rate-determine step isOH*+*→O*+H*for redox and formate mechanism,COOH*+*→CO2*+H*(Cu12Co) and2H*→H2*+*(Cu13) for carboxylmechanism; the carboxyl mechanism is optimal pathway comparedto the redox and formate mechanism; the catalytic activity ofCu12Co is excellent compared to Cu13.