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

Novel Pd sulfated ceria–zirconia catalysts for the SCR of NOx by methane were prepared by impregnation of a PdCl2 precursor dissolved in ammonia solution on a sulfated Ce0.21Zr0.79O2 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 PdOx 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 DeNOx, 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.