Rates, reversibility, and site densities for CO2 hydrogenation on Cu-based catalysts

We show herein that while the reverse water-gas shift (RWGS) reaction is zeroth order in hydrogen partial pressure, methanol synthesis is first order in hydrogen partial pressure at H₂-to-CO₂ ratios up to 120 using a conventional Cu/ZnO/Al₂O₃ catalyst with a CO₂/H₂ feedstock. In situ chemical titration with methylene chloride reveals that chlorine uptake during catalysis remains constant across a wide range of H₂-to-CO₂ ratios, affirming that the observed hydrogen apparent reaction order reflects intrinsic kinetic behaviors devoid of contribution from changes in active site density due to reduction of the ZnO support. The persistent first order dependence on hydrogen partial pressure suggests a dearth of H* surface species and demonstrates that methanol selectivity can be modulated from 45 to 80% at P_CO2 = 4.7 bar by varying hydrogen partial pressure alone.
From a mechanistic standpoint, CO₂ hydrogenation and RWGS involve different surface intermediates, as evidenced by rate measurements under both reversible and irreversible conditions that show the forward unidirectional rate of methanol synthesis is inhibited by H₂O more so than that of RWGS. Co-processing CO, however, suggests the two reactions still occur on the same active site as selectivity is unchanged with CO partial pressure with rates of methanol synthesis and RWGS being inhibited proportionally. By leveraging both kinetic and thermodynamic arguments, the presented work provides key insights regarding surface coverage, active site density, and network connectivity of CO₂ hydrogenation on Cu/ZnO/Al₂O₃ catalysts.