How to stabilize highly active Cu+ cations in a mixed-oxide catalyst

•A TiCuOx film can be formed exposing exclusively Cu+ sites.•The presence of Ti in the mixed-oxide film chemically stabilizes Cu+ sites.•CO adsorbs stronger on TiCuOx than in either metallic or oxidized Cu.

Mixed-metal oxides exhibit novel properties that are not present in their isolated constituent metal oxides and play a significant role in heterogeneous catalysis. In this study, a titanium-copper mixed-oxide (TiCuOx) film has been synthesized on Cu(1 1 1) and characterized by complementary experimental and theoretical methods. At sub-monolayer coverages of titanium, a Cu2O-like phase coexists with TiCuOx and TiOx domains. When the mixed-oxide surface is exposed at elevated temperatures (600–650 K) to oxygen, the formation of a well-ordered TiCuOx film occurs. Stepwise oxidation of TiCuOx shows that the formation of the mixed-oxide is faster than that of pure Cu2O. As the Ti coverage increases, Ti-rich islands (TiOx) form. The adsorption of CO has been used to probe the exposed surface sites on the TiOx–CuOx system, indicating the existence of a new Cu+ adsorption site that is not present on Cu2O/Cu(1 1 1). Adsorption of CO on Cu+ sites of TiCuOx is thermally more stable than on Cu(1 1 1), Cu2O/Cu(1 1 1) or TiO2(1 1 0). The Cu+ sites in TiCuOx domains are stable under both reducing and oxidizing conditions whereas the Cu2O domains present on sub-monolayer loads of Ti can be reduced or oxidized under mild conditions. The results presented here demonstrate novel properties of TiCuOx films, which are not present on Cu(1 1 1), Cu2O/Cu(1 1 1), or TiO2(1 1 0), and highlight the importance of the preparation and characterization of well-defined mixed-metal oxides in order to understand fundamental processes that could guide the design of new materials.

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