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

Abstract

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 (Eads) values ranging from -3.11 to -5.91eV. Similar values are calculated for bimetallic Au-Ir clusters (-3.21 up to -5.69eV). However, weaker Eads values are calculated for Au clusters (ranging from -0.66 to -2.07eV). 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 (1.90eV). Interestingly, for Au5, we observe that molecular O2 adsorption is favorable at the cluster/oxide interface, followed by a smaller dissociation barrier (0.71eV). 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.