A research team led by Prof. GAO Peng and Prof. LI Shenggang at Shanghai Advanced Research Institute (SARI) of the Chinese Academy of Sciences developed an efficient FeZn-based catalyst for methanol synthesis via CO₂ hydrogenation. After investigating the crystal structure engineering of FeZn catalysts, the research team reported for the first time that ZnFe₂O₄ spinel enables high methanol selectivity of 84.5% in CO₂ hydrogenation.

The research results were published in the latest issue of Chem.

The catalytic conversion of CO₂ into alcohols using low-cost green hydrogen is a promising solution for mitigating CO₂ emissions. Substantial progress has been made in developing efficient CO₂-to-methanol catalysts and revealing the reaction mechanisms, in which highly selective CO₂ hydrogenation into alcohols remains a great challenge, due to the difficulty in controlling the C-C coupling steps.

Noble metal-based catalysts were reported to exhibit high ethanol selectivity in CO₂ hydrogenation to higher alcohols containing two or more carbon atoms (C₂₊OH). The research team investigated how crystal structure engineering of FeZn catalysts transformed selective product formation from methanol to C₂₊OH, realizing unprecedented higher alcohol synthesis (HAS) performance from CO₂ hydrogenation.

Researchers first synthesized the single-phase crystal structure of ZnFe₂O₄ spinel. The ZnFe₂O₄ oxide sample was activated in situ prior to the CO₂ hydrogenation reaction.

The experimental observation showed that more than 97% of the reaction products consisted of methanol and ZnFe₂O₄ catalyst exhibited a high CH₃OH selectivity and CO₂ conversion. ZnO and Fe₂O₃ oxides were synthesized and tested for comparison study, which shown a much lower CO₂ conversion and CH₃OH selectivity.

The results indicated the ZnFe₂O₄ spinel phase enabled high methanol selectivity during CO₂ hydrogenation. The experimental analysis suggested the oxygen vacancy on the surface of ZnFe₂O₄ spinel is the active site for CO₂ and H₂ activation.

It was also found that spinel ZnFe₂O₄ plays a vital role in the formation of oxygenates from CO₂ hydrogenation, whereas the formation of the Fe₅C₂ phase facilitates both carbon chain growth and the insertion of C1 oxygen-containing intermediates to yield C₂₊OH after further hydrogenation, which greatly increases the selectivity of C₂₊OH in addition to olefins.

Moreover, introducing Fe₅C₂ phase to the formation of the ZnFe₂O₄/Fe₅C₂ interface greatly promotes C₂₊OH selectivity in oxygenates to 98.2%, giving a very high C₂₊OH productivity and an unprecedented C₃₊OH yield of 5.3%, which surpass the current reported maximum C₃₊OH yield of 2.1%.

Theoretical calculations further revealed the role of the ZnFe₂O₄/Fe₅C₂ interface in the C-C coupling. The ZnFe₂O₄/Fe₅C₂ interface drives selective transformation to C₂₊OH by promoting the facile migration of alkyl species to the interface and coupling with CHO* species, which contribute to the breaking of the Anderson-Schulz-Flory(ASF) distribution and result in the much lower methanol selectivity than expected.

This work demonstrates a new strategy for selectivity tuning in alcohol synthesis by crystal structure engineering.

Reaction networks for CO₂ hydrogenation to methanol over the ZnFe₂O₄ and CO₂ to C₂₊OH over the ZnFe₂O₄/Fe₅C₂ interface (Image by SARI)