Search

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 MOx(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 MOxnanoparticles at a content of ∼0.75 wt% through the deposition precipitation (DP) method. Except for FeOx, MOxloaded 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 MOx/SMO composites can be attributed to the improved reducibility of adsorbed oxygen (Oads) and the increased oxidation state of Mn cations at the interface. Such behavior accelerates the decomposition of carbonates, as indicated by the H2temperature-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 CuOx/SMO composite also stems from the additional CO adsorption on the CuOxsites. 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).

A series of mullite SmMn2O5oxides were prepared by citric acid (CA), hydrothermal (HT) and co-precipitation (CP) and combustion of ethylene glycol and methanol solutions (EG&M) methods, and tested for NOx-assisted soot combustion. The catalyst performance tests were conducted under “loose” contact conditions and the physical and chemical properties characterized by XRD, FTIR, BET, SEM, XPS, and O2-TPD and H2-TPR measurements. The SmMn2O5-EG&M catalyst possessed a distinct morphology with slabs separated by interconnected macropores, and exhibited the overall highest catalytic activity for soot combustion. The corresponding T10, T50, and T90were 283 °C, 368 °C, and 420 °C, respectively, and the CO2selectivity was 99.6%. The soot combustion activation energy for the SmMn2O5-EG&M catalyst is 65 kJ mol−1. O2-TPD and H2-TPR measurements demonstrated higher mobility and better reducibility of surface adsorbed oxygen for the EG&M sample. The soot combustion is greatly accelerated by the NO2-assisted mechanism under a NO + O2atmosphere and further enhanced by the interconnected macropores of the catalyst, facilitating an intimate contact between the soot and the catalyst. Our study reveals that SmMn2O5mullite is an effective catalyst for NOx-assisted soot combustion.

A series of LaxSm1−xMn2Oδ(x= 0, 0.1, 0.3, 0.5) catalysts were synthesized through a co-precipitation method. The catalytic activity for NO oxidation was enhanced with La substitution, and the maximum activity was achieved at x= 0.3. XRD and HRTEM results revealed the formation of a multiphase oxide as well as the interface structure between the mullite (SmMn2O5) phase and Mn-rich perovskite (La0.96MnO3.05) phase. The main impact of different La/Sm molar ratios is the amount of surface adsorbed oxygen (Oads) and surface Mn4+ions as revealed by XPS results. The NO oxidation performance was enhanced through La addition by promoting the decomposition of nitrate/nitrite species and desorption of NO2, improving the reducibility of surface adsorbed oxygen, as determined by H2-TPR, NO + O2-TPRD and in situDRIFTS studies. Mono-, bi-dentate and bridged nitrates formed on the surface were determined to be the primary reaction intermediates.