Alkali-Regulated Acid–Base Balance for Selective C1 Oxygenate Production over Oxide-Modified Copper Catalysts

Abstract

The selective hydrogenation of CO₂ into oxygenated C1 products requires a balanced distribution of basic CO₂ adsorption sites, hydrogen activation sites, and interfacial hydrogenation centers. In this work, alkali-assisted regulation of oxide-modified copper catalysts was investigated to clarify how surface acid–base properties influence methanol selectivity. A series of Na-, K-, and Cs-promoted Cu–ZrOx catalysts were synthesized by sequential impregnation, with alkali loadings ranging from 0.2 to 2.0 wt%. Catalytic performance was evaluated at 230–270 °C, 3.0 MPa, and H₂/CO₂ = 3. The 0.8 wt% K-promoted catalyst showed the best performance, achieving a CO₂ conversion of 14.7%, methanol selectivity of 78.3%, and methanol space–time yield of 468.5 gMeOH kgcat⁻¹ h⁻¹ at 250 °C. Compared with the unpromoted catalyst, the CO selectivity decreased from 28.6% to 15.9%, while the apparent activation energy for methanol formation decreased from 72.8 to 57.6 kJ mol⁻¹. CO₂-TPD results showed that moderate K addition increased the density of medium-strength basic sites from 186 to 312 μmol gcat⁻¹, enhancing CO₂ adsorption without causing excessive carbonate accumulation. In situ DRIFTS revealed that K promotion strengthened the formation of reactive formate species at 1582 and 1365 cm⁻¹, while suppressing linearly adsorbed CO on Cu sites. However, alkali loadings above 1.5 wt% reduced methanol productivity by blocking Cu surface sites required for H₂ dissociation. Kinetic analysis indicated that the optimized catalyst exhibited a CO₂ reaction order of 0.38 and an H₂ reaction order of 0.94, suggesting that hydrogen supply remained kinetically important after alkali modification. These results demonstrate that alkali promoters improve methanol selectivity mainly by tuning the acid–base balance at oxide–copper interfaces rather than simply increasing CO₂ uptake. This study provides a practical strategy for improving C1 oxygenate formation through controlled surface basicity.