Synergy of Co0-Co2+ in cobalt-based catalysts for CO2 hydrogenation: Quantifying via reduced and exposed atoms fraction

Abstract

Cobalt (Co)-based catalysts exhibit exceptional versatility in CO2 hydrogenation, yielding valuable fuels and chemicals like methane, methanol, and ethanol. Generally, the Co0–Co2+ synergy plays a vital role in promoting this process, yet a quantifying descriptor remains absent. This study presents a series of CoAl-CO32− layered double hydroxide (LDH) derived catalysts synthesized through coprecipitation. By tuning precipitation pH and reduction temperature, various Co-based catalysts with distinct reduction degrees and dispersions were obtained. By introducing the descriptor "fraction of reduced and surface exposed Co (FCo,r,s)", the Co0–Co2+ synergy is effectively quantified for CO2 hydrogenation. The balanced Co0–Co2+ synergy in catalysts yields a moderate FCo,r,s, resulting in high turnover frequencies (TOFs) for CO2 conversion via HCOO intermediate formed at the Co0–Co2+ interfaces. The HCOO species can be hydrogenated to form methane and methanol, as well as coupled with CHx intermediate followed by hydrogenation to form ethanol. Conversely, catalysts with excessively high or low FCo,r,s display poor Co0–Co2+ synergy and consequently lower TOFs. A clear volcano-shaped relationship between FCo,r,s and TOFs for CO2 conversion, methane, methanol, and ethanol formation underscores its significance as a key determinant of Co-based catalysts' CO2 hydrogenation performance. The catalyst with optimized Co0–Co2+ synergy (precipitated at pH 8.5 and reduced at 750 oC) possesses a high CO2 conversion rate of 26.8 mmol gcat−1 h−1 at a low temperature (i.e., 180 oC).