Role of bimetallic Au-Ir subnanometer clusters mediating O₂ adsorption and dissociation on anatase TiO₂ (101)

A comprehensive computational study on the oxygen molecule (O₂) adsorption and activation on bimetallic Au-Ir subnanometer clusters supported on TiO₂(101)- up to five atoms in size-is performed. A global optimization density functional theory-based basin-hopping algorithm is used to determine putative global minima configurations of both mono- and bimetallic clusters supported on the metal oxide surface for all sizes and compositions. Our results indicate a strong cluster-oxide interaction for monometallic Ir clusters with calculated adsorption energy (E_ads) values ranging from -3.11 to -5.91 eV. Similar values are calculated for bimetallic Au-Ir clusters (-3.21 up to -5.69 eV). However, weaker E_ads values are calculated for Au clusters (ranging from -0.66 to -2.07 eV). As a general trend, we demonstrate that for supported Au-Ir clusters on TiO₂(101), those Ir atoms preferentially occupy cluster-oxide interface positions while acting as anchor sites for the Au atoms. The overall geometric arrangements of the putative global minima configurations define O₂ adsorption and dissociation, particularly involving the monometallic Au₅ and Ir₅ as well as the bimetallic Au₂Ir₃ and Au₃Ir₂ supported clusters. Spontaneous O₂ dissociation is observed on both Ir₅ and on the Ir-metallic part of Au₃Ir₂ and Au₂Ir₃ supported clusters. This is in sharp contrast with supported Au₅, where a large activation energy is needed (1.90 eV). Interestingly, for Au₅, we observe that molecular O₂ adsorption is favorable at the cluster/oxide interface, followed by a smaller dissociation barrier (0.71 eV). From a single cluster catalysis point of view, our results have strong implications in the ongoing understanding of oxide supported bimetallic while providing a useful first insight into the continuous in silico design of novel subnanometer catalysts.