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

Abstract

Copper (I) oxide Cu 2 O 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 nanoceria support. A systematic analysis of isothermal reduction of a series size of mono-dispersed Cu 2 O nanocrystals (±7%) with time-resolved X-ray diffraction (TR-XRD) provides the time-resolved phase fraction of Cu 2 O and the time when reduction product of Cu (f.c.c) first appears. The initial phase fraction of nano-Cu 2 O is less that 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, Cu 2+ content in corresponding to shell increases from zero to 33% as Cu 2 O decreases to 8nm from the bulk. Based on the reduction profiles, a Time-Size-Reduction (TSR) diagram is constructed for the observed Cu 2 O phase behavior during reduction. The incorporation onto a nano-CeO 2 support (7nm) significantly stabilizes our nano-Cu 2 O in a reducing atmosphere. The oxygen supply propensity in terms of oxygen non-stoichiometry of CeO 2-y is shown to be lower when a larger crystallite-size CeO 2 (20nm) support is used. The larger oxygen capacity in smaller nano-CeO 2 support is analyzed and explained by the "Madelung Model" with size-dependent bulk modulus of nano-ceria.

Lattice Expansion in Metal Oxide Nanoparticles: MgO, Co3O4, & Fe3O4

“…There is increasing experimental evidence that bulk modulus in fact changes as crystal size decreases below 50 nm . It is important to amass well‐characterized and nearly mono‐dispersed nanoparticles over a wide range of crystal sizes of an oxide‐system material so that changes in not only lattice parameter but also bulk modulus can be definitively investigated . The conclusions that we have carefully walked through with MgO, about surface stress, hydrostatic energy, and the importance of bulk modulus, apply to the Co 3 O 4 and Fe 3 O 4 that we have synthesized as well.…”

Morphology-Dependent CO Reduction Kinetics and Surface Copper Species Evolution of Cu2O Nanocrystals

“…This order is inconsistent with the morphology dependence of various Cu 2 O NCs reduced in high CO concentrations, indicating that the reduction is not determined by the surface reactivities of various Cu 2 O NCs. In addition to the reactivity of surface lattice O, the diffusivity of subsurface/bulk lattice O into the surface also influences the reducibility of Cu 2 O by limiting the continuous occurrence of CO reacting with Cu 2 O, − likely affecting the E a and eventually leading to the reducibility differences of various Cu 2 O NCs.…”

XAFS structural characterization of Cu vapour derived catalysts supported on poly‐4‐vinylpyridine and carbon

“…For this reason, whatever its origin, this contribution is surely related to a surface effect. It can probably come from the presence of a thin CuO surface layer around the Cu 2 O NPs because of the extra oxygen atoms present in the on‐surface sites that, can induce a stabilization of the formed NPs but also from other effects like interactions with the substrate . The near‐edge region of the XANES spectra, shown in Fig.…”