Notable in situ surface transformation of Cu2O nanomaterials leads to dramatic activity enhancement for CO oxidation

Abstract

Cu₂O is an important compound for many promising applications; however, its real state under serious application conditions, such as redox atmosphere of catalysis reactions, has seldom been investigated. In this study, a dramatic and sustainable activity enhancement for CO oxidation on catalytically used Cu₂O nanoporous sphere samples was discovered as compared to that on their fresh counterparts. To illustrate the phenomenon, comprehensive characterizations such as XRD, H₂-TPR, TEM, XPS, and CO-TPD were performed on fresh and used Cu₂O samples. It was found that after one or several catalytic runs, the main phase among the used samples retained the crystalline feature of Cu₂O. However, the surface would transform into a stable multivalent composite interface with almost unchanged Cu(II) : Cu(I) ratio (around 4.0) and active oxygen distribution, even after only one catalytic run for CO oxidation; this indicated a notable reaction-induced or in situ surface restructuring effect of the Cu₂O substrates. Compared to that of pure Cu₂O, CuO, and reported Cu_xO samples, the active chemisorbed oxygen on these in situ restructured samples was observed in particularly higher surface composition (with the ratio of chemisorbed oxygen to lattice oxygen being around 5.5); this was further demonstrated to be highly efficient for the oxidization of CO in a relatively low temperature range even in the absence of O₂ in the atmosphere and could be the main contributor to speed up CO oxidation in these samples. A similar enhancement in the activity on the used samples for CO oxidation as compared to that on the fresh samples was further testified in Cu₂O nanomaterials with different morphologies such as cubic, octahedral, and 18-facet polyhedral nanoparticles; this implied that the surface restructuring effect under a redox reaction atmosphere was a common feature of the Cu₂O-based materials, and the transformed surface could act as a superior and stable interface for heterogeneous catalysis. These findings might help in recognizing the real state of Cu₂O-containing materials under serious application conditions or designing highly efficient Cu₂O-based materials via convenient atmosphere-controlled manipulation for advanced applications.