Rates and reversibility of CO₂ hydrogenation on Cu-based catalysts

Kinetics of reaction pathways involved in the conversion of CO₂ to methanol and CO on Cu/ZnO/Al₂O₃ are resolved using in situ chemical titration, steady-state kinetic measurements, and mathematical formalisms for reversibility to probe salient species governing methanol selectivity and yield during CO₂ hydrogenation. Across a range of H₂:CO₂ = 1:1 to H₂:CO₂ = 80.5:1, active site density determined from in situ chlorine uptake remained invariant; hence, observed trends in rates can be interpreted as only arising from reaction kinetics and not from changing active site density. Kinetic and thermodynamic contributions to rates are decoupled to evaluate forward and reverse rates of methanol synthesis and reverse water-gas shift (RWGS) reactions. These kinetic analyses show that the forward rates of methanol synthesis exhibit persistent first order dependence on hydrogen pressure and are inhibited by water more significantly than the forward rates of RWGS. In contrast, the reverse rates of methanol synthesis and RWGS are both inhibited by H₂. Consequently, without any modifications to the Cu/ZnO/Al₂O₃ catalyst formulation, methanol selectivity can be increased to > 80 % by increasing inlet H₂ partial pressure and methanol yield can be enhanced by ∼ 20 % by adding water adsorbents even under conditions far from equilibrium. The kinetic treatments presented herein demonstrate a dearth of H* species during catalysis, provide thermodynamic constraints precluding sequential RWGS and CO hydrogenation as the pathway for methanol synthesis, reveal P_H2 and P_H2O as salient in determining methanol selectivity and yield by impacting both the forward and reverse rates of CO₂ hydrogenation on Cu/ZnO/Al₂O₃, and explicate the fundamentals of novel sorption-enhanced methanol synthesis, which not only alleviates equilibrium constraints but also alters the intrinsic rate at which the system approaches equilibrium.