Unraveling the active site for CO2 hydrogenation to methanol over Cu/ZnO catalysts: insights from DFT and microkinetic modeling

Abstract

Extensive efforts have been devoted to the Cu/ZnO/Al₂O₃ catalyst system, which is widely applied in the industrial methanol synthesis from CO₂. However, the nature of active sites remains controversial due to challenges in experimentally isolating contributions from different surface structures and the absence of standardized theoretical comparisons. Here, we systematically study several representative active sites on Cu/ZnO catalysts using DFT calculations and microkinetic modeling. The Cu/ZnO interface exhibits the highest intrinsic activity, with a turnover frequency (TOF) of 1.4 × 10^–2 s⁻¹ under typical reaction conditions, which is over four times higher than that of the Cu–Zn alloys and consistent with experimental observations. In contrast, the ZnO/Cu(111) overlayer is inactive due to prohibitively high barriers along all pathways. Reaction pathway analysis indicates that methanol synthesis proceeds primarily via the HCOO pathway, rather than through direct CO₂ dissociation followed by CO hydrogenation. In addition, we find that a high H₂O pressure in the system may further increase the surface coverage of OH* or O*, thereby hindering methanol formation by blocking active sites. These findings highlight the importance of considering adsorbate–adsorbate interactions in microkinetic modeling. Furthermore, tuning the coverage of specific intermediate species or the concentration of product can effectively enhance the reaction activity. This work offers theoretical insights into the fundamental origins of catalytic activity in Cu/ZnO systems and may facilitate the rational design of next-generation catalysts for methanol synthesis.