CO oxidation over MOx (M = Mn, Fe, Co, Ni, Cu) supported on SmMn2O5 composite catalysts

Abstract

The ability to oxidize CO at relatively low temperature is barely satisfactory for pure SmMn₂O₅ (SMO) catalysts. While loading precious metals like platinum nanoclusters on mullite SMO is an effective approach to enhance its catalytic performance, the high cost of noble metals has limited its large scale application. In this work, we systematically study an alternative approach to composite catalyst formation by loading non-precious metal oxides MO_x (M = Mn, Fe, Co, Ni, Cu) onto SMO mullite for CO oxidation. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) and high-resolution transmission electron microscopy (HRTEM) analyses show that composite catalysts are successfully formed by loading MO_x nanoparticles at a content of ∼0.75 wt% through the deposition precipitation (DP) method. Except for FeO_x, MO_x loaded samples in general exhibit much higher reaction rates of CO oxidation than bare SMO especially at the temperature range below 100 °C. The promoted activity for MO_x/SMO composites can be attributed to the improved reducibility of adsorbed oxygen (O_ads) and the increased oxidation state of Mn cations at the interface. Such behavior accelerates the decomposition of carbonates, as indicated by the H₂ temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy results. We demonstrate that the best catalytic performance of the CuO_x/SMO composite also stems from the additional CO adsorption on the CuO_x sites. The work presented herein provides a method to enhance the CO oxidation performance of SMO and helps in understanding the oxidation reaction mechanism involving mullites with loaded transition metal oxides (TMOs).