The catalytic hydrogenation of CO2 with renewable H2 represents a promising pathway for reducing CO2 emissions. Among the various hydrogenation products, the formation of methanol (CO2 + 3H2 → CH3OH + H2O) is preferred because methanol can be used not only as a fuel but also as a reactant to produce olefins and other value-added chemicals . Due to the chemical inertness of CO2, its catalytic conversion to methanol encounters significant challenges, in particular in maintaining catalytic selectivity and stability at CO2 conversion levels that are acceptable for industrial processes. At present the most commonly used catalyst is Cu/ZnO/Al2O3 [2–4]. There are at least two main issues that are potentially hindering the large-scale application of this catalyst: (1) the lack of definitive understanding of the active sites, and (2) the instability of this catalyst at relatively high CO2 conversions. For example, currently there is debate about the role of ZnO in this catalyst. One possibility is that there is an intimate synergy between Cu and ZnO at the interface, where ZnO could act as a structural modifier, hydrogen reservoir or direct promoter for bond activation. The other possibility is that a highly active ZnCu alloy forms by partial reduction of ZnO or a decoration of Cu with metallic Zn. The inhomogeneous nature of the supported Cu/ZnO/Al2O3 catalyst in most cases prevents one from obtaining conclusive information regarding the active sites under reaction conditions, therefore making it difficult to develop strategies for significant enhancement of catalytic performance. A recent paper by Can Li's group  has demonstrated the possibility of significantly improving the catalytic selectivity and stability at reasonably high CO2 conversions. Using a catalyst consisting of a ZnO–ZrO2 solid solution, these authors achieved a methanol selectivity of 86%–91% at CO2 conversions greater than 10%. Equally important, this catalyst remains stable after 500 hours on stream. By using a combination of experimental evaluation, spectroscopic characterization and density functional theory (DFT) calculations, this paper also provides convincing evidence regarding the origin of the excellent performance of the ZnO–ZrO2 solid-solution catalyst. Furthermore, the ZnO–ZrO2 catalyst also shows different CO2 hydrogenation mechanisms to those proposed on Cu/ZnO/Al2O3. There are two potential advantages of this catalyst over Cu/ZnO/Al2O3: (1) the relatively more homogeneous nature, due to the solid solution, of ZnO–ZrO2 makes it easier to utilize in situ techniques and computational methods to unravel the active sites under reaction conditions to further improve the catalytic performance, and (2) the promising stability of this catalyst might provide opportunities for large-scale processes for CO2 hydrogenation. In summary, the work by Can Li's group has identified an alternative, solid-solution catalyst with promising catalytic performance in terms of conversion, selectivity and stability for CO2 hydrogenation to methanol. These promising results should provide opportunities for follow-up research for solid-solution catalysts, both for methanol synthesis and potentially for other applications in heterogeneous catalysis, such as the selective hydrogenation of molecules containing C=O bonds. REFERENCES 1. Porosoff MD , Yan BH , Chen JG . Energy Environ Sci 2016 ; 9 : 62 – 73 . https://doi.org/10.1039/C5EE02657A Crossref Search ADS 2. Behrens M , Studt F , Kasatkin I et al. Science 2012 ; 336 : 893 – 7 . https://doi.org/10.1126/science.1219831 Crossref Search ADS PubMed 3. Kuld S , Thorhauge M , Falsig H et al. Science 2016 ; 352 : 969 – 74 . https://doi.org/10.1126/science.aaf0718 Crossref Search ADS PubMed 4. Kattel S , Ramírez PJ , Chen JG et al. Science 2017 ; 355 : 1296 – 9 . https://doi.org/10.1126/science.aal3573 Crossref Search ADS PubMed 5. Wang J , Li G , Li Z et al. Sci Adv 2017 ; 3 : e1701290 . Crossref Search ADS PubMed © The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
National Science Review – Oxford University Press
Published: Sep 1, 2018
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