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

Abstract

Novel Pd sulfated ceria–zirconia catalysts for the SCR of NO x by methane were prepared by impregnation of a PdCl 2 precursor dissolved in ammonia solution on a sulfated Ce 0.21Zr 0.79O 2 support, itself obtained by direct sulfation of a ceria–zirconia nanopowder in 0.5 M H 2SO 4. Best CH 4-SCR performances under the conditions 150 ppm NO; 1500 ppm CH 4; 7 vol.% O 2 were achieved using Pd loadings in the range 0.24–0.53 wt.% with 30–35% NO conversion to N 2 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 CH 4 concentrations or adding H 2O 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.