Catalysts caught in the act: an operando investigation of copper during CO ₂ hydrogenation Elizabeth S. Jones, Robert S. Weatherup University of Oxford, UK Two foremost challenges in mitigating global warming are replacing fossil fuels with renewable alternatives, and storing/use of carbon captured from CO 2 -emitting processes. Methanol production by CO 2 hydrogenation promises a possible solution to both of these issues. 1 CO 2 hydrogenation is performed industrially using a CO 2 / H 2 mix with a Cu-based catalyst where the addition of CO is known to increase the methanol yield. 1 However mechanistic understanding of this reaction and the role played by CO remains limited. Soft X-ray spectroscopies can provide details on the chemical state of copper to uncover the chemistry behind this reaction, however the typical requirement for measurement under high vacuum (UHV) constrains how realistic these studies can be. There have been many recent developments improving operando techniques to enable studies under realistic pressure conditions. 2 A promising approach is to use an environmental cell which encloses the desired gas and separates it from the high vacuum environment, using an X-ray transparent window (SiN). 3 Using a custom-designed high pressure environmental cell we have studied model Cu catalysts using operando Near Edge X-ray Adsorption Fine Structure spectroscopy (NEXAFS) up to pressures of 1 bar and temperatures of 200 o C. 4 A thin Cu film was deposited onto a SiN membrane which sealed the environmental cell. The aim of the study was to investigate how the Cu oxidation states varied when exposed to H 2 and CO 2 in different sequential order and how the introduction of CO can further influence the chemical state of Cu. It was found that H 2 can provide a protective barrier to oxidation from CO 2 when dosed first, however if H 2 was added after CO 2 it is unable to return the surface to its metallic state where CO is then required for reduction (Fig. 1).
To improve this technique further, X-ray photoelectron spectroscopy (XPS) was recently performed over CuZn nanoparticles at 1 bar. This study benefited from a more realistic catalyst as well as enhanced surface sensitivity from XPS. This required an electron transparent window such as graphene. However, the manufacture of graphene windows is challenging. We have developed a method to reliably produce graphene windows via a Au and polydimethylsiloxane (PDMS) support frame. Preliminary results indicate the presence of Zn aids reduction of Cu in H 2 even without CO (Fig. 2).
References 1. S. Navarro-Jaén et al., Nat. Rev. Chem. , 2021, 5 , 564–579. 2. E. S. Jones et al., ACS Symp. Ser. , 2021, 1396 , 175–218. 3. R. S. Weatherup et al., J. Phys. Chem. Lett. , 2016, 7 , 1622–1627. 4. J. E. N. Swallow et al., J. Am. Chem. Soc. , 2023, 145 , 6730–6740.
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