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 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 (F_Co,r,s)", the Co⁰–Co²⁺ synergy is effectively quantified for CO₂ hydrogenation. The balanced Co⁰–Co²⁺ synergy in catalysts yields a moderate F_Co,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 CH_x intermediate followed by hydrogenation to form ethanol. Conversely, catalysts with excessively high or low F_Co,r,s display poor Co⁰–Co²⁺ synergy and consequently lower TOFs. A clear volcano-shaped relationship between F_Co,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).