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

Cobalt (Co)-based catalysts exhibit exceptional versatility in CO2 hydrogenation, yielding valuable fuels and chemicals like methane, methanol, and ethanol. Generally, the Co-0-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 Co-0-Co2+ synergy is effectively quantified for CO2 hydrogenation. The balanced Co-0-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 Co-0-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 Co-0-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 Co-0-Co2+ synergy (precipitated at pH 8.5 and reduced at 750 degrees C) possesses a high CO2 conversion rate of 26.8 mmol(gcat)(-1) h(-1) at a low temperature (i.e., 180 degrees C).