In Situ XRD, XPS, TEM, and TPR Study of Highly Active in CO Oxidation CuO Nanopowders

Abstract

Copper(II) oxide nanopowders exhibit a high catalytic activity in CO oxidation at low temperatures. The combination of in situ XPS, XRD, and HRTEM methods was applied to investigate initial steps of CuO nanoparticles reduction, to identify oxygen and copper species and to revealed structural features in the dependence on reducing power of reaction medium. At the oxygen deficient surface of CuO nanopowders the metastable Cu 4 O 3 oxide was formed under the mild reducing conditions −10 –5 mbar CO or CO + O 2 mixture with oxygen excess. Destruction of Cu 4 O 3 structures in strong reducing medium ( P (CO) ≥ 10 –2 mbar) or under UHV conditions resulted in the formation of Cu 2 O which was epitaxially bounded with initial CuO particle. The reversible bulk reduction of CuO nanopowder to Cu 2 O at temperatures ∼150 °C can be explained by effortless propagation of Cu 2 O∥CuO epitaxial front inside the nanoparticle. The model of the surface restructuring along the {−111}CuO → {202}Cu 4 O 3 → {111}Cu 2 O planes under the reduction of CuO nanopowders is proposed. The initial surface of CuO nanopowders is probably distorted and resembles Cu 4 O 3 -like structures that facilitates the CuO x ↔ Cu 4 O 3 transition in mild reducing conditions. Such restructuring results in a unique electronic Cu 4 O 3 structure with high oxygen deficiency and low-valence Cu 1+ sites stimulating the formation of highly reactive CO and O 2 adsorbed species. It was shown that the most active oxygen species on the surface of CuO x is stabilized as O –, which was previously reported in papers by Roberts and Madix in their study of the copper–oxygen systems.