(646d) Rates and Reversibility for CO2 Hydrogenation on Cu-Based Catalysts

Methanol synthesis via CO₂ hydrogenation, unlike CO hydrogenation, occurs concurrently with reverse water-gas shift (RWGS), thereby reducing methanol selectivity. We report that CO₂ hydrogenation rates and selectivity are single-valued functions of hydrogen pressure; while CO₂ partial pressure does not impact methanol selectivity during CO₂ hydrogenation on Cu/ZnO/Al₂O₃, increasing H₂ partial pressure enhances methanol selectivity as methanol synthesis exhibits an apparent first order dependence on P_H2 while RWGS is zeroth order in P_H2. These apparent reaction orders persist from CO₂:H₂ = 1:1 to 1:120 such that changes in P_H2 can increase methanol selectivity from 5 to 80%. Rate measurements on Cu/Al₂O₃ catalysts in combination with in situ chemical titrations using methylene chloride demonstrate that site density is invariant with H₂ partial pressure; the apparent first order dependence on P_H2 for methanol synthesis indicates a dearth of H* species at all process conditions during CO₂ hydrogenation on Cu/ZnO/Al₂O₃.

Partial pressures of products were varied both by purposeful addition of product species to the CO₂/H₂ feedstock and by operation of reaction under reversible regimes. These results consistently demonstrate H­₂O preferentially inhibits the unidirectional forward rate of methanol synthesis while CO inhibits methanol synthesis and RWGS equally such that methanol selectivity is unchanged. These differences in H₂O and H₂ dependencies suggest distinct pathways and intermediates for methanol synthesis and RWGS. Altogether, our work provides novel kinetic and mechanistic insights regarding rates and reversibility of elementary steps involved in CO₂ hydrogenation on Cu/ZnO/Al₂O₃ catalysts.