Due to an increased demand for renewable energy, direct catalytic valorization of CO₂ into methanol (liquid fuel additive and precursor to other value added chemicals) has been receiving increasing attention over the past decade [1]. Among a variety of catalyst formulations that can produce methanol from the mixture of carbon dioxide and hydrogen - copper-zinc-alumina catalyst (CZA), industrially designed for methanol synthesis from syngas, shows the most promising results. In spite of extensive efforts, the activity and selectivity threshold for this well investigated material is still far from commercial utilization and further catalyst improvement is still required. Numerous studies, investigating mechanistic aspects of methanol synthesis over CZA material, are based on experiments usually made under conditions (low temperature, vacuum) that are far away from the real catalytic experiment (>15 bar; 220-280 ºC). Hence, even the structure of active sites and especially the role of copper-zinc alloys in this catalytic system, are subject of intense debate [2-3]. This situation has motivated us, to investigate the mechanistic role of copper-zinc alloy in carbon dioxide hydrogenation to methanol by using operando time-resolved XAS (at both Cu and Zn K-edges), SSITKA-FTIR and XRD under relevant catalytic conditions (15 bar; 260 ºC), supported by theoretical modelling and TEM [4].

Applying different in situ activation protocols by varying of reduction temperature and partial pressures of hydrogen and carbon dioxide during pretreatment steps, we were able to tune content of copper-zinc alloy in CZA catalyst. We found that CZA materials, containing significantly different amount of reduced zinc (49 and 16 mol. %), possess almost identical catalyst activity (0.38 and 0.44 mmol g^-1 min^-1, respectively), pointing out that no correlation exists between the observed methanol productivity and the amount of CuZn brass present in the system. To provide a more unambiguous understanding of the role of copper-zinc alloy in this reaction, operando XAS (time resolution 10 s) and XRD measurements during transient switches from hydrogen to CO₂/H₂ reaction mixture and back were performed. It was revealed that CuZn alloy, which was considered as an active site in this reaction, upon relevant catalytic conditions (CO₂/H₂ reaction mixture, 260 ºC, 15 bar) undergoes oxidation with the formation of wurtzite ZnO, which can activate CO₂ and form surface zinc formate species, which according to SSITKA-FTIR and qXAS is the reactive intermediate. We found that formation of the copper-zinc alloy followed by oxidation under reaction conditions forms a well-developed Cu/ZnO interface, which is where the reaction takes place. Metallic copper facilitates hydrogen splitting, while the zinc phase is responsible for CO₂ activation, which subsequently forms methanol.

[1] M.D. Porosoff et al., Energy Environ. Sci., 2016, 9, 62–73.
[2] S. Kattel et al., Science, 2017, 355, 1296–1299.
[3] J. Nakamura et al., Science, 2017, 357, eaan8074.
[4] M. Zabilskiy et al., Nat. Commun., 2020, 11, 2409