(513b) CO2 Hydrogenation on Cu-Based Catalysts

We ally steady-state kinetics, kinetic isotope effects, and density functional theory (DFT) calculations to elucidate that CO₂ hydrogenation on both Cu/ZnO/Al₂O₃ and Cu/Al₂O₃ catalysts can be described by a mechanistic sequence in which the surface is saturated with either H-adatoms (H*) or molecular formic acid (HCOOH**). Methanol synthesis and reverse water-gas shift (RWGS) exhibit persistent first and zeroth order P_H2 dependence at P_H2 0.5 bar, respectively, with RWGS transitioning to positive order P_H2 dependence only at P­_H2 0.5 bar, indicative of high H* coverage under conventional methanol synthesis conditions. Changes in P­_CO2 (0.25 – 4.7 bar) do not influence the methanol selectivity or the H₂ reaction orders but do impact reaction rates following a Langmuir-type dependence; such is consistent with methanol synthesis and RWGS sharing an active site on which P_CO2 governs the relative coverages of H* and HCOOH**, each having the same number of H per site. DFT-derived energies corroborate the observed high coverages of HCOOH** made possible through the destabilization of bidentate formates (HCOO**) by H*. Distinct H₂/D₂ isotope effects exhibited by methanol synthesis and RWGS then stipulate that the two reactions involve disparate reaction pathways with HCOOH** and COOH* yielding methanol and CO, respectively. Congruently, methanol synthesis rates are disproportionately inhibited by H₂O, which can be attributed to H₂O altering the rate determining step of methanol synthesis as co-processing H₂O/D₂O changes both the H₂ reaction order and the H₂/D₂ isotope effect. This work augments and rectifies previously purported reaction mechanisms for CO₂ hydrogenation and shows that a branching reaction mechanism involving molecular formic acid and carboxylate is in full accord with all observed kinetic trends of rate and selectivity without the need to invoke distinct active sites for methanol synthesis and RWGS on both Cu/ZnO/Al₂O₃ and Cu/Al₂O₃.