Role of alkali metal additives on adsorption of CO on small copper clusters [CunAMm (m + n ⩽ 7, m < n)]: A DFT study

Abstract

In the present research, CO adsorption on alkali pre-adsorbed copper clusters [CunAMm clusters (m+n⩽7, m<n)] has been investigated and compared to corresponding adsorption on naked Cu clusters. We have followed the changes in adsorption energies (binding, interaction and Gibbs free energies) as well as restructuring and activation of CO adsorbate. Results show that CO tends to adsorb at on-top site in both naked and alloy clusters. Compared to naked Cu clusters, CO adsorption on all alloy clusters is consistent with more elongation of CO bond and dramatic redshift) up to 297cm−1) in stretching frequency. The mentioned effect is surprisingly obtained whether the adsorption energies are improved or not. In addition to wrapped charge transfers within the system that can rationalize the result, we obtained that when carbon atom applies higher percentage of its 2s orbital in Cu-C connection, the adsorption energies are more favorable. We found that in some cases, adding alkali metal atom leads to breaking of the π-bond of CO (at least in one spin part) which will be important in CO dissociation processes such as in Fischer-Tropsch synthesis. Density Functional Theory (DFT), TPSS meta-GGA exchange-correlation functional was used which has been picked up through DFT calibration.