Effects Of Solid Acid Modification On Pt/CeO₂-ZrO₂Catalyst For Propane Oxidation

In recent years, with the automobile exhaust pollution have being seriouslyincreased, more stringent regulations are presented for the purification ofhydrocarbon emissions （HCs）. It is therefore important to find routes to improve theactivity of Pt/CeO₂-ZrO₂catalyst for HCs oxidation.In this work, we introduced H₂SO4as a promoter to modify the Pt/CeO₂-ZrO₂catalyst for HCs oxidation. A series of sulfated CeO₂-ZrO₂with Pt supported andunsupported were prepared by impregnating with sulfuric acid solution and Pt（NO3）2solution （denoted as CZ and Pt/CZ, after sulfation denoted as SCZ and Pt/SCZ,respectively）, and then their corresponding activities for propane oxidation wereevaluated. The results show that the sulfation of CZ supports improved the Ptsupported catalysts significantly. And when the loading amount of sulfuric acidsolution was10wt.%, the Pt/SCZ catalyst shows the best catalytic activity. Comparedwith the T50of Pt/CZ catalyst, the Pt/SCZ catalyst decreased by about60oC.The prepared catalysts were further characterized by XRD, BET, Raman,H₂-TPR, NH₃-TPD, CO and NH₃adsorption, C3H8adsorption and in-situ FTIRoxidation. The nature of the sulfates, the influence of sulfation on the acidic sites, thesate of Pt, the reducibility of Pt^δ+, as well as their relationship with the promotionaleffect in propane oxidation were explored.The results revealed that the sulfation gave rise to the generation of sulfates,which can be presented successively as surface sulfates and bulk sulfates, respectively.The electronegative sulfates on CZ support interact with Pt and altered the metallicplatinum to more unsaturated coordination state (Pt^δ+), these metastable Pt^δ+species,which are liable to the atmosphere and have higher reducibility evidenced by our COadsorption experiments, may serve as the new active sites and thus appear to beresponsible for the enhanced activity on Pt/SCZ. By using NH₃adsorption anddesorption, we found that the surface sulfates can greatly improve the Br nsted aciddensity of the catalyst. The most active catalyst occurs at the intermediate loadingamount （10wt.%）, which corresponding to the maximum of surface sulfates and the maximum of Br nsted acidity density, as well as the largest amounts of Pt^δ+sites.By further investigating the propane oxidation with in-situ FTIR spectrometer,we found that the existence of surface sulfates on the CZ basic support inhibited thegeneration of surface carboxylates, bicarbonates, bidentate carbonates and unidentatecarbonates, all of which were generated in large amounts on the Pt/CZ catalyst. Bycovering the surface basic sites on CZ support, the surface sulfates on Pt/SCZ catalystprotect the active sites and therefore improved the activity of propane oxidation.