Surface and bulk transformations of Ag2CuMnO4 delafossite during the interaction with CO+O2 mixture

Abstract

Ternary oxide of silver, copper and manganese (Ag2CuMnO4) with delafossite-type structure demonstrates excellent catalytic activity in the reaction of CO oxidation at room temperature and even below. To prepare delafossite-based catalyst the hydrothermal approach using metal nitrates in alkaline solution with an excess of peroxodisulphate was applied. X-ray diffraction pattern of Ag2CuMnO4 particles was successfully simulated taking into an account crystallite shape anisotropy, particle size distribution, the presence of stacking faults, and the lattice expansion along c axis. As-prepared Ag2CuMnO4 sample was characterized by the presence of Ag1+-, Cu2+- and Mn4+-like surface species predominantly. In situ XRD data revealed the thermal stability of delafossite-type structure in catalytic CO+O2 mixture up to 500 °C, while ex situ XPS showed an evident reorganization of Ag2CuMnO4 surface at markedly lower temperatures (200–250 °C). Heating in CO+O2 medium at 150–400 °C also resulted in significant catalytic activation of Ag2CuMnO4 owing to the Mn enrichment and optimization of manganese and copper electronic exchange. The low-temperature activity of Ag2CuMnO4 catalyst in CO+O2 reaction was proposed to be mainly provided by redox transitions with the participation of Cu1+/Cu2+ and Mn3+/Mn4+ couples, while the catalytic role of silver species is considered as auxiliary only. The innovation point of this work is related to the investigation of the dynamics of surface and bulk structure transformations in connection with the catalytic activation of delafossite particles for the low-temperature oxidation.