Morphology effect of tungsten oxide on Ce/W catalyst for selective catalytic reduction of NO with NH3: Influence of structure-directing agents

Abstract

The morphology effect of tungsten oxide on the NH3-SCR activity of the supported CeO2 catalyst was experimentally investigated by comparing the catalytic performance of WO3 in different morphologies. Nanoparticles (W-NP), nanorods (W-NR) and microspheres self-assembled by nanorods (W-MP) of WO3 were synthesized by using different carboxylic acid structure-directing agents. Based on this, the impregnation method was adopted to form the composite catalysts including the Ce/W-NP, the Ce/W-NR and the Ce/W-MP. After the impregnation of Ce species, a new redox cycle of Ce4++W5+↔Ce3++W6+ is formed due to the interaction of W and Ce ions, which significantly facilitates the redox reaction on the composite catalyst. However, compared with nanorods (W-NR) and microspheres self-assembled by nanorods (W-MP), WO3 nanoparticles (W-NP) enclosed with (0 0 1) facets of the hexagonal WO3 shows the best redox ability due to the largest molar ratio of W5+/(W5++W6+) and the highest concentration of Ce4+ on the surface of Ce/W-NP, which was proven by the results of XPS. As a consequence, the Ce/W-NP presents the strongest surface redox properties and the largest molar ratio of Oα/(Oα+Oβ), which is beneficial to promote the NH3-SCR activity of the catalyst. It is worth mentioning that the morphology of tungsten oxide that remarkably affects the activity of the catalyst is determined by tartaric acid, citric acid and oxalic acid, indicating the significance of the structure-directing agents for improving the NH3-SCR activity of the impregnated Ce/W catalyst.