Palladium catalysts supported on sulfated ceria-zirconia for the selective catalytic reduction of NO_x by methane: Catalytic performances and nature of active Pd species

摘要

Novel Pd sulfated ceria-zirconia catalysts for the SCR of NO_x by methane were prepared by impregnation of a PdCl2 precursor dissolved in ammonia solution on a sulfated Ce_(0.21)Zr_(0.79)O2 support, itself obtained by direct sulfation of a ceria-zirconia nanopowder in 0.5 M H2SO4. Best CH4-SCR performances under the conditions 150 ppm NO; 1500 ppm CH4; 7 vol.% O2 were achieved using Pd loadings in the range 0.24-0.53 wt.% with 30-35% NO conversion to N2 around 350-400 °C. By contrast, the formation of a PdO_x phase at superior Pd loadings was found detrimental for the SCR activity because it promotes also the non-selective methane oxidation. Increasing CH4 concentrations or adding H2O to the feed have opposite trends on DeNO_x, activity, the latter being detrimental at low-medium temperatures due to inhibiting effects on adsorption. As evidenced by DRIFTS of CO and pyridine adsorption, UV-Vis, HRTEM-EDX and XRD, the nature of the Pd species active in SCR could be [Pd(O)-H]~(x+) isolated species and/or small [(PdO)_n-H]~(x+) clusters possessing a high Lewis acidity and a low reducibility.