Effect of Al source on the physicochemical properties of Cu-Al spinel catalysts and the catalytic performance for reverse water gas shift

The excessive of fossil fuels has caused a swift rise in global carbon dioxide levels, resulting in severe climate change and environmental pollution. The research on the conversion of CO₂ into high value-added chemicals is of great significance for CO₂ reduction. Due to the high chemical activity of CO, a first conversion of CO₂ to CO is meaningful, which makes the subsequent conversions become easier. Therefore, the reverse water gas shift reaction is considered to be an important intermediate step of CO₂ hydrogenation to methanol, ethanol and other carbon-containing high value-added industrial products. For the reverse water gas shift reaction, several catalyst systems were researched, including supported catalysts, mixed metal oxide catalysts and transition metal carbide catalysts. Among these catalysts, Cu-based catalysts were widely reported owing to the high activity and CO selectivity. Recently, we found that Cu-Al spinel catalysts can be used as the efficient sustained release catalysts for reverse water gas shift reaction. High surface area pseudo-boehmite acts as an appropriate Al source for the synthesis of Cu-Al spinel catalysts by the mechanochemical method. However, the impurity elements in pseudo-boehmites showed significant influence on the formation and properties of Cu-Al spinel, and the catalytic performance for reverse water gas shift reaction. To unravel this point, four pseudo-boehmites with unequal contents of impurity elements (Na, Fe, Si, and S) and copper hydroxide were used for the synthesis of Cu-Al spinel solid solution catalysts by both high-energy ball milling and solid-phase calcination procedures. The physicochemical properties of the catalysts were characterized by ICP-AES, TG, XRD, H₂-TPR, and BET methods, and the catalytic performances were investigated in reverse water gas shift reaction. The results showed that impurity elements in pseudo-boehmite samples had significant effects on the crystal property, reducibility, texture property and catalytic performance of the Cu-Al spinel catalysts. Specifically, Si facilitated the synthesize of high specific surface area catalysts but was detrimental to the formation of Cu-Al spinel, thus leading to a low catalytic activity. Cu-Al spinel catalysts with a small amount of Na and Fe also showed low catalytic activities. S species would be decomposed and removed during the precursor calcination step at high temperature of 950 ℃, thus giving little effect on the catalytic activity. Importantly, the catalyst synthesized based on the pseudo-boehmite with the lowest content of impurity elements had the highest content of hardly-reducible spinel, and exhibited the highest catalytic activity for CO₂ hydrogenation to CO. In addition, the Cu-Al spinel catalyst with the highest catalytic activity was selected for the in-situ DRIFTS and CO₂-TPD-MS characterizations. The results showed that the formate species, including monodentate formate on Al, bidentate formate on Al, and bidentate formate on Cu, were intermediate species of CO₂ hydrogenation to CO over Cu-Al spinel catalysts. Notably, low peak intensities were detected with monodentate formate on Al and bidentate formate on Cu, but the bidentate formate on Al showed higher peak intensity. Especially, the content of bidentate formate on Al was in line with the catalytic activity at different reaction time, implying that the bidentate formate on Al was the main intermediate. This work provides guidance to catalyst synthesis using pseudo-boehmite as raw material.