Pd-based sulfated zirconia prepared by a single step sol–gel procedure for lean NOx reduction

Pd-based sulfated zirconia catalysts have been prepared through a single step (one-pot) sol–gel preparation technique, in which both sulfate and Pd precursors were dissolved in an organic solution before the gelation step. Observation of the calcination procedure through TGA/DSC and mass spectrometry revealed that the addition of increasing amounts of Pd resulted in the evolution of organic precursor species at lower temperatures. In situ XRD experiments showed that tetragonal zirconia is formed at lower temperatures and larger zirconia crystallites are formed when Pd is added to the gel. Although tetragonal zirconia was the only phase observed through XRD, Raman spectra of samples calcined at 700 °C showed the presence of both the tetragonal and the monoclinic phase, indicating a surface phase transition. DRIFTS experiments showed NO species adsorbed on Pd2+ cations. Pd/SZ catalysts prepared through this single step method were active for the reduction of NO2 with CH4 under lean conditions. Calcination temperature had a significant effect on this activity, with samples calcined at 700 °C being much more active than those calcined at 600 °C, despite the observed transition to the monoclinic phase. This activity may be linked to observed changes in the surface sulfate species at higher calcination temperatures.

Graphical abstractPd/sulfated zirconia catalysts were prepared through a single step sol–gel procedure. The catalysts were active for the reduction of NO2 with CH4 under lean conditions. Higher calcinations temperatures resulted in more active catalysts, corresponding to support phase changes and differences in the observed surface sulfate species. This figure shows Raman spectra revealing changes in the surface zirconia phase at different calcination temperatures. Figure optionsDownload full-size imageDownload as PowerPoint slide