Role of Na in promoting n-butanol production via CO₂ hydrogenation over a Cu-Co catalyst

Alkali-metal-promoted Cu catalysts have significant potential for the direct conversion of CO₂ to alcohols. However, the effect of alkali metals on alcohol selectivity during CO₂ hydrogenation is poorly understood. Therefore, we aimed to understand the impact of Na on selective hydrogenation over a Cu-Co catalyst. We found that the catalyst without the Na promoter (UCuCo-9P) converted only 14.8% of CO₂ with a 7.3% alcohol selectivity at 330 °C, 4 MPa, and an H₂/CO₂ ratio of 1. In contrast, the addition of Na by coprecipitation (Na–CuCo-9P) increased the CO₂ conversion to 20.6%, coinciding with a dramatic increase in alcohol selectivity up to 27.9%. Further, over 70% of alcohols produced by Na–CuCo-9P were C₃₊ alcohols, with n-butanol having the highest composition of 51.5%. In contrast, the Na-promoted catalyst prepared by wet impregnation (Na–CuCo-9W) achieved only half the alcohol selectivity of Na–CuCo-9P, at a low CO₂ conversion of 12.8%. Crucially, Na–CuCo-9P was more stable than UCuCo-9P during a 200 h on-stream stability test. Further, we found that Na promotion lowers the activation energy barrier for the C₂₊ oxygenate intermediate, which favors n-butanol synthesis during tandem reactions. This work elucidates the role of Na in Cu-Co catalysts on direct CO₂-to-higher-alcohol synthesis and offers insights into the rational design of catalysts using alkali promoters.