Crystal Structure Studies of Co-based Catalysts: Operando Synchrotron X-Ray Absorption and Diffraction

Abstract

Crystal Structure Studies of Co-based Catalysts: Operando Synchrotron X-Ray Absorption and Diffraction Nevzat Yigit, Liliana Lukashuk, Karin Föttinger and Günther Rupprechter Institute of Materials Chemistry, Technische Universität Wien, Vienna, 1060, Austria Catalytic preferential CO oxidation (PrOx), i.e. CO+H2+0.5O2→CO2+H2, is a key reaction for removing traces of CO from the H2-rich stream. Transition metal oxides, especially cobalt-based materials, are perspective catalysts for PrOx. However, the current understanding of PrOx over these catalysts is mainly based on pre- and postcatalyst characterization, and open questions include the nature of active sites and reaction pathways. The ultimate goal of this work was to investigate the nature of the active sites/phases of Co-based catalysts in order to understand the reaction mechanism of PrOx. For this purpose, we compared the structural changes of Co in different chemical compounds and oxidation states (Co3O4, and LaCoO3 perovskite) using synchrotron-based operando X-ray absorption spectroscopy (XAS) (l811 beamline, Max-lab II, Lund, Sweden) and operando X-ray powder diffraction (XRD) (I11beamline, Diamond, UK and MS beamline, SLS, Switzerland). The results of our study illustrate that during PrOx the bulk structure of LaCoO3 and Co3O4 remain intact, when CO oxidation is dominant [1]. The fact that cobalt is preserved in a high oxidation state in the case of LaCoO3 during PrOx explains the wide temperature operation window for selective CO oxidation in excess of hydrogen in comparison to Co3O4. Incorporation of Co into the LaCoO3 structure leads to partial reduction of LaCoO3 (i.e., microstructural changes of LaCoO3 as revealed by XRD) in the PrOx mixture and formation of defects during the preferential oxidation of CO to CO2 in a wide temperature range of 100-300 °C. The knowledge obtained in this study could help in developing novel catalysts for PrOx applications via tuning structural and microstructural properties of cobalt-based materials. ____ [1] Lukashuk, L. et al., Operando XAS and NAP-XPS studies of preferential CO oxidation on Co3O4 and CeO2-Co3O4 catalysts., J. Catal. 2016, 344, 1-15.