Size Effect of Cu Nanoparticles from Cu–Al Spinel Catalysts on Product Distribution via CO Hydrogenation

The intrinsic activity of active sites in catalysts remains a controversial issue, regarding structural sensitivity reactions. Herein, Cu nanoparticles with a broad size distribution were effectively synthesized by tuning the Cu/Al molar ratios of Cu–Al spinel catalysts. The product distribution was found to have a strong dependence on the size of the Cu nanoparticles. It was observed that large Cu nanoparticles were conducive to CO dissociation and CHx* formation, thereby promoting the insertion of CHx* into CHxO*/CO* species, which enhanced the carbon chain growth capacity for synthesizing ethanol. Conversely, the smaller Cu nanoparticles facilitated the HCOO* and CH3O* intermediates formation, and the CH3O* was hydrogenated to CH3OH, which is then dehydrated to produce dimethyl ether (DME). The results revealed that the Cu1Al1 catalyst achieved a superior ethanol fraction (24.86%) attributable to the presence of appropriately sized Cu nanoparticles (24.9 nm), which effectively boosted the C–C coupling. However, the Cu1Al3 catalyst demonstrated higher DME selectivity (43.98%) as a result of the stronger synergism between smaller Cu nanoparticle size (15.9 nm) and a Cu–Al spinel surface containing acidic sites. Additionally, the larger specific surface area improved the dispersion of Cu species, thereby enhancing the hydrogenation capacity of CO. This work greatly broadens understanding of the intrinsic activity of Cu nanoparticle size, providing an efficient strategy for improving the selectivity of desired products.