In light of the current environmental crisis, the selective hydrogenation of CO₂ to methanol has been intensively studied to reduce greenhouse gas emissions. The benchmark catalyst for this process, Cu/ZnO/Al₂O₃ (CZA), has received considerable attention from the research community for its dynamic behavior under catalytic conditions (50-100 bar and 200-300 °C), and the mechanistic pathway remains a matter of debate. Previous studies have identified three possible reaction mechanisms: (i) the formate pathway, in which adsorbed and activated CO₂ binds a hydrogen atom via the carbon, (ii) the formyl pathway, in which CO₂ is first converted to CO, which in turn is hydrogenated, and (iii) the trans-COOH pathway, whereby the hydrogen atom is bound to the oxygen in CO₂. Out of these three proposed mechanisms, the latter is based on theoretical calculations calculations and has yet to be confirmed with experimental evidence.^[1,2]

In this work, we investigate the effect of pressure on the reaction mechanism of CO₂ hydrogenation over the industrial CZA catalyst using combined steady-state isotopic transient kinetic analysis-Fourier-transform infrared (SSITKA-FTIR) methodology. The reactions are performed at five different pressures and the changes are monitored via mass spectroscopy (MS, Fig. a), which reveals the quick and irreversible conversion of CO₂. This is supported by the lack of any carbonate species in the FTIR spectra. Formate is identified as the key intermediate across all pressures (Fig, b), confirming the formation of methanol via the above-mentioned formate pathway. No characteristic vibrational bands for any of the other proposed intermediates, responding to the isotopic switch from ¹²CO₂ to ¹³CO₂, could be observed. Even below 0.15 bar, where methanol can no longer be detected in the product stream, formate remains present on the surface of the catalyst and responsive to the isotopic switches. The calculated residence times of CO and formate reveal that, at low pressures, the CO₂ to CO conversion cannot follow through a formate intermediate, due to the noticeably shorter residence times of CO. At higher pressures, the similar retention times of CO and formate render it difficult to distinguish between parallel and consecutive pathways (Fig. c). The results of this mechanistic study bring us closer to the intentional design of optimized catalysts for the CO₂ to methanol conversion.

Figure: a. Normalized MS response and b. difference FTIR spectra acquired at 15 bar during the switch to labelled gas, focused on the formate region; c. schematic representation of the mechanistic findings.

[1] M. Zabilskiy, V. L. Sushkevich, D. Palagin, M. A. Newton, F. L. Krumeich, J. A. van Bokhoven, Nat. Commun., 2020, 11:2409

[2] G. Pacchioni, ACS Catal., 2024, 14, 2730−2745