H2 Adsorption on Cu4-xMx (M = Au, Pt; x=0-4) Clusters: Similarities and Differences As Predicted by Density Functional Theory

The active search for alternatives to current fossil fuels via energy conversion mechanisms has bolstered the study of the adsorption of molecules on surfaces and clusters as a fundamental step before moving into complex heterogeneous catalysis scenarios. In this respect, intensive investigations have been thoroughly performed to obtain cheaper and more efficient catalysts, such as in the hydrogen evolution reaction (HER) in the electrocatalysis field. In this paper, we present systematic density functional theory simulations of H-2 adsorption on bimetallic Cu4-xMx (M = Pt, Au; x = 0-4) neutral clusters. The molecular and dissociative adsorption mechanisms are explored, and the occurrence of one or the other is explained in terms of the reactivity of the host clusters. We begin with a global exploration of the potential energy surface to obtain the ground-state structures. The effects of H-2 adsorption and host cluster composition on the interatomic distances are investigated. A detailed analysis of H-2 adsorption on the Cu4-xPtx family shows that dissociative adsorption occurs for x = 1-4, that is, for all clusters containing Pt, while nondissociative molecular adsorption is obtained in Cu4-xAux clusters with x = 0-3, that is, for all clusters containing Cu. This is consistent with the values of the electronic HOMO-LUMO gap, which are predicted to be small (high reactivity) and large (small reactivity) for bimetallic Cu-Pt and Cu-Au clusters, respectively. The adsorption energy increases with the Pt load in Cu-Pt clusters, while it is almost constant in the case of Cu-Au clusters.