The poisoning effects of phosphorus on CeO2-MoO3/TiO2 DeNOx catalysts: NH3-SCR activity and the formation of N2O

Abstract

In this paper, the poisoning effects of phosphorus on CeO2-MoO3/TiO2 catalysts were investigated for their role in selective catalytic reduction of NOx by NH3 (NH3-SCR) applications. A series of P-poisoned CeO2-MoO3/TiO2 catalysts were prepared by the impregnation method. To investigate the deactivation mechanism of NH3-SCR, the samples were characterized by BET, NH3-TPD, XPS, in situ DRIFTS studies and NH3 catalytic oxidation. The activity tests showed that the DeNOx activity declined significantly after phosphorus loading, and a large quantity of N2O formed over the poisoned catalysts. It was found that the active metal sites at the surface were covered by phosphate (mainly cerous phosphate). The chemical deactivation effects of phosphorus on CeO2-MoO3/TiO2 mainly resulted from decreases in the number of active sites and in the adsorption capacity for NOx species. As the loading of phosphorus reached 1.32%, physical deactivation effects, e.g., severe changes in thermal stability and surface structure, also accounted for the decline in activity. The formation of substantial amounts of N2O after phosphorus loading, despite heavily poisoned Ce-sites, might be attributed to Mo-sites being mildly poisoned across the CeO2-MoO3/TiO2 catalyst. Thus, after phosphorus loading, the NH3-SCR occurred mainly on Mo-sites. Lastly, the increase in N2 selectivity at high temperatures with the increasing phosphorus loadings was clarified by a kinetics study.