Stable CuO/La2Sn2O7 catalysts for soot combustion: Study on the monolayer dispersion behavior of CuO over a La2Sn2O7 pyrochlore support

Abstract

To understand the dispersion behavior of metal oxides on composite oxide supports and with the expectation of developing more feasible catalysts for soot oxidation, CuO/La2Sn2O7 samples containing varied CuO loadings were fabricated and characterized by different techniques and density functional theory calculations. In these catalysts, a spontaneous dispersion of CuO on the La2Sn2O7 pyrochlore support formed, having a monolayer dispersion capacity of 1.90 mmol CuO/100 m2 La2Sn2O7 surface. When loaded below this capacity, CuO exists in a sub-monolayer or monolayer state. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and Bader charge and density of states analyses indicate that there are strong interactions between the sub-monolayer/monolayer CuO and the La2Sn2O7 support, mainly through the donation of electrons from Cu to Sn at the B-sites of the structure. In contrast, Cu has negligible interactions with La at the A-sites. This suggests that, in composite oxide supports containing multiple metals, the supported metal oxide interacts preferentially with one kind of metal cation in the support. The Raman, in situ diffuse reflectance infrared Fourier transform spectroscopy, and XPS results confirmed the formation of both O2– and O22– as the active sites on the surfaces of the CuO/La2Sn2O7 catalysts, and the concentration of these active species determines the soot combustion activity. The number of active oxygen anions increased with increase in CuO loading until the monolayer dispersion capacity was reached. Above the monolayer dispersion capacity, microsized CuO crystallites formed, and these had a negative effect on the generation of active surface oxygen sites. In summary, a highly active catalyst can be prepared by covering the surface of the La2Sn2O7 support with a CuO monolayer.