Systematic Theoretical Study of CO Activation over Clean and Potassium-Modified Transition Metals

Abstract

By using the periodic density functional theory (DFT) method, the adsorption and activation of CO on clean and potassium-modified transition metals, which includes Fe(100), Co(0001), Ni(111)/(100), Cu(111), Ru(0001), Rh(111)/(100), Pd(111), and Pt(111) surfaces, have been investigated systematically. The calculation results show that the additive of potassium atoms greatly enhances the binding strength of CO and O while only slightly improving the adsorption of C atoms. Moreover, the activation energy of CO dissociation is decreased in the presence of potassium additive atoms. Our results indicate that the CO activity enhancement by the series of alkali metals (Li, Na, K, Rb, and Cs) on Ni(100) increases in the order of Li < Na < Cs < Rb = K. On the basis of the electronic structure and geometric analysis, the physical origin of the promotion effect has been clarified, and it can be attributed to the direct electron transfer between potassium and O atoms involved in CO. The effect of an external electric field on CO activation was also investigated, in which it was shown that a positive electric field accelerated CO activation by elongating the C–O bond and transferring electrons. It is obvious that CO adsorption is enhanced when the orientation of the external electric field is the same as the direction of charge transfer. At last, the promoting effect of potassium on CO activation drops as the oxide state of potassium increases due to the steric effect and direct chemical bonding.