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

Abstract

The relationship of structure sensitivity between metal size and CO 2 hydrogenation performance is still unclear. Herein, using a unique set of well-defined TiO 2 -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 CO 2 conversion is also identified. In the size range from single atom to 1.0 nm, the reverse water gas shift (RWGS) reaction controls CO 2 hydrogenation, and its turnover frequency decreases with the size increasing. Conversely, for clusters over 1.0 nm, CO 2 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 CO 2 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.