Size-Dependent Active Site and Its Catalytic Mechanism for CO2 Hydrogenation Reactivity and Selectivity over Re/TiO2

The relationship of structure sensitivity between metal size and CO2 hydrogenation performance is still unclear. Herein, using a unique set of well-defined TiO2-supported rhenium (Re) catalysts and systematical characterizations, we clearly demonstrate that the active site and reaction mechanism are closely related to the size effect. A wave-like size-dependent activity of CO2 conversion is also identified. In the size range from single atom to 1.0 nm, the reverse water gas shift (RWGS) reaction controls CO2 hydrogenation, and its turnover frequency decreases with the size increasing. Conversely, for clusters over 1.0 nm, CO2 methanation becomes the main reaction and shows a volcano-like size-dependent performance. The mechanistic study reveals that the perimeter site in single-atom catalysts dictates the RWGS reaction via formate pathways. In contrast, the active site in the Re cluster is the edge site, where CO2 reduces to CO via a redox pathway and subsequently hydrogenates to methane over the edge site. This discovery may deepen the mechanistic understanding of structure sensitivity.