PNNL

Abstract

The development and comprehension of supported metal catalysts for CO2 hydrogenation is of paramount importance in mitigating the net CO2 emissions. Supported Ru catalysts have been widely recognized in facilitating CO2 methanation, on which recent findings suggest that the CO2 hydrogenation process can be manipulated to favor the reverse water-gas shift (RWGS) pathway by precisely adjusting the size of Ru particles. However, the size-dependent impact of Ru remains a topic of lively debate. In this work, Ru/MgO catalysts with Ru in the form of single atoms (Ru1) and atomic single-layer (RuASL) structures were prepared for CO2 hydrogenation. The 1.0Ru/MgO catalyst (with 1 wt.% of Ru), featuring a mixture of Ru1 and RuASL with a size of 0.6-1.0 nm, showed the highest CO yield (38% at 500 °C) with balanced CO2 conversion and CO selectivity. Transient CO2 hydrogenation and temperature-programmed surface reaction (TPSR) studies suggested that the adsorbed CO2 species participated in CO2 hydrogenation. On Ru1 sites, CO2 hydrogenation followed the RWGS pathway, resulting in the production of CO. In contrast, on RuASL sites, the enhanced H2 dissociation ability, along with the presence of adsorbed bidentate and monodentate carbonate species at the Ru-MgO interfaces, facilitated the formation of CH4 through the CO2 methanation pathway. This study highlights the critical roles of Ru structure and local environment in defining the CO2 hydrogenation pathways and provides new design principles for highly active Ru-based catalysts.