Mechanism of Methanol Synthesis on Cu through CO2 and CO Hydrogenation

We present a comprehensive mean-field microkinetic model for the methanol synthesis and water-gas-shift (WGS) reactions that includes novel reaction intermediates, such as formic acid (HCOOH) and hydroxymethoxy (CH3O2) and allows for the formation of formic acid (HCOOH), formaldehyde (CH2O), and methyl formate (HCOOCH3) as byproducts. All input model parameters were initially derived from periodic, self-consistent, GGA-PW91 density functional theory calculations on the Cu(111) surface and subsequently fitted to published experimental methanol synthesis rate data, which were collected under realistic conditions on a commercial Cu/ZnO/Al2O3 catalyst. We find that the WGS reaction follows the carboxyl (COOH)-mediated path and that both CO and CO2 hydrogenation pathways are active for methanol synthesis. Under typical industrial methanol synthesis conditions, CO2 hydrogenation is responsible for ∼2/3 of the methanol produced. The intermediates of the CO2 pathway for methanol synthesis include HCOO*, HCOOH*, CH3O2*, CH2O*, and CH3O*. The formation of formate (HCOO*) from CO2* and H* on Cu(111) does not involve an intermediate carbonate (CO3*) species, and hydrogenation of HCOO* leads to HCOOH* instead of dioxymethylene (H2CO2*). The effect of CO is not only promotional; CO* is also hydrogenated in significant amounts to HCO*, CH2O*, CH3O*, and CH3OH*. We considered two possibilities for CO promotion: (a) removal of OH* via COOH* to form CO2 and hydrogen (WGS), and (b) CO-assisted hydrogenation of various surface intermediates, with HCO* being the H-donor. Only the former mechanism contributes to methanol formation, but its effect is small compared with that of direct CO hydrogenation to methanol. Overall, methanol synthesis rates are limited by methoxy (CH3O*) formation at low CO2/(CO + CO2) ratios and by CH3O* hydrogenation in CO2-rich feeds. CH3O* hydrogenation is the common slow step for both the CO and the CO2 methanol synthesis routes; the relative contribution of each route is determined by their respective slow steps HCO* + H* → CH2O* + * and HCOOH* + H* → CH3O2* + * as well as by feed composition and reaction conditions. An analysis of the fitted parameters for a commercial Cu/ZnO/Al2O3 catalyst suggests that a more open Cu surface, for example, Cu(110), Cu(100), and Cu(211) partially covered by oxygen, may provide a better model for the active site of methanol synthesis, but our studies cannot exclude a synergistic effect with the ZnO support.