(560bp) Influence of Coordination Environment Around Anchored Single-Site Cobalt Catalyst for CO2 Hydrogenation

As the field of carbon dioxide capture and storage is expanding as a means of mitigating CO₂ emissions, transforming carbon dioxide into value added chemicals and fuels represents a more economically viable approach. Recently, various technologies of upgrading CO₂ have been explored, most notably electrochemical conversion and thermo-catalytic conversion. Typical catalysts for CO₂ hydrogenation consist of supported cobalt nanoparticles, which exhibit optimal catalytic activity in the size range of 5-15nm^[1]. However, such catalysts have several factors that can affect catalytic activity, such as surface crystal faceting, particle size, or the relative ratio of exposed surface defects^[2]. As CO₂ hydrogenation has been shown to be a highly surface sensitive reaction, the surface structures of nanoscale catalyst can be prohibitively complex to establish a clear structure/activity relationship to guide the design of active catalyst.

In this study, cobalt single site catalyst supported on silica were explored due to their highly uniform active sites; allowing for definitive claims as to which surface species are responsible for specific reaction mechanisms. To characterize the structure and dispersion of the single-site catalysts, techniques such as UV-vis, XAFS, XPS, TPR, and Raman were utilized under ambient conditions as well as under reductive environments to simulate reaction conditions. This thorough understanding of the surface moieties under ambient and reductive environments coupled with their corresponding catalytic performance during CO₂ hydrogenation allows us to discern how the transition between isolated atoms to small nanoparticles affects the reaction mechanism. To understand the surface under a reductive environment, we performed in situ XAFS, and pretreatment studies via XPS, using hydrogen as our reductant to elucidate the nature of the active site. We have found that under reductive environments below 500^oC, the surface remains almost entirely in the Co²⁺ (Td) geometry. Our findings from our CO₂ hydrogenation studies have shown that isolated atoms promote the preferential formation of CO via the RWGS reaction while small ensembles between Co²⁺(Td) and Co⁰ form both methane and CO, suggesting a change in reaction mechanism; which will ultimately be elucidated by this work.

[1] a)V. Iablokov, S. K. Beaumont, S. Alayoglu, V. V. Pushkarev, C. Specht, J. Gao, A. P. Alivisatos, N. Kruse, G. A. Somorjai, Nano Letters 2012, 12, 3091-3096; b)Y. Zhu, S. Zhang, Y. Ye, X. Zhang, L. Wang, W. Zhu, F. Cheng, F. Tao, ACS Catalysis 2012, 2, 2403-2408.

[2] a)G. Melaet, W. T. Ralston, C. S. Li, S. Alayoglu, K. An, N. Musselwhite, B. Kalkan, G. A. Somorjai, Journal of the American Chemical Society 2014, 136, 2260-2263; b)J. Jimenez, A. Bird, M. Santos Santiago, C. Wen, J. Lauterbach, Energy Technology 2017, 5, 884-891.