Reaction mechanism of water gas shift over Aun clusters: A density functional theory study

The stability and catalytic activity of Au10, Au13 and Au20 clusters in water gas shift (WGS) reaction were investigated by density functional theory (DFT); the adsorption behavior of reaction species and the reaction mechanism of WGS on various Aun clusters were explored. The results indicated that the stability of three Aun clusters follows the order Au10 < Au13 < Au20, whereas their electron delocalization and adsorption capacity decreases in the sequence of Au13 > Au10 > Au20. Three Aun clusters exhibit the same rate-determining step for WGS, i.e. H2O dissociation; however, they are quite different in the actual reaction routes. Over Au10 cluster, the WGS reaction follows the carboxyl mechanism, characterized by the direct dissociation of COOH*; over Au13 cluster, the redox mechanism applies, suggested by the disproportionation of two OH*; over Au20 cluster, the WGS reaction proceeds via the carboxyl mechanism, represented by the disproportionation of COOH* and OH*. A comparison for the optimal reaction paths over three Aun clusters suggests that the Au13 cluster has the highest catalytic activity in the WGS reaction at low temperature.