Reduction of Nano-Cu₂O: Crystallite Size Dependent and the Effect of Nano-Ceria Support

Abstract

Copper(I) oxide (Cu2O) is an effective catalyst in the CO oxidation reaction. While high surface to volume ratio in nanoparticles will increase their catalytic efficiency, it posts a stability problem. Here we study the stability of nano-cuprite against reduction as a function of its crystallite size and upon interaction with a nano-ceria support. A systematic analysis of isothermal reduction of a series size of monodispersed Cu2O nanocrystals (±7%) with time-resolved X-ray diffraction (TR-XRD) provides the time-resolved phase fraction of Cu2O and the time when reduction product of Cu (fcc) first appears. The initial phase fraction of nano-Cu2O is less than one with the balance attributed to an amorphous CuO shell. Since no peaks of crystalline CuO (monoclinic) were observed, a core–shell structure with an amorphous CuO shell is proposed. From the analysis, Cu2+ content in corresponding to shell increases from 0 to 33% as Cu2O decreases to 8 nm from the bulk. Based on the reduction profiles, a time size reduction (TSR) diagram is constructed for the observed Cu2O phase behavior during reduction. The incorporation onto a nano-CeO2 support (7 nm) significantly stabilizes our nano-Cu2O in a reducing atmosphere. The oxygen supply propensity in terms of oxygen nonstoichiometry of CeO2–y is shown to be lower when a larger crystallite size CeO2 (20 nm) support is used. The larger oxygen capacity in smaller nano-CeO2 support is analyzed and explained by the “Madelung model” with size-dependent bulk modulus of nano-ceria.