Paladyum depolanmış WO3/TiO2-ZrO2 karışım oksit katalizörünün yüzeyinde gerçekleşen NOX+CH4 reaksiyonlarının in-situ FT-IR spektroskopisi ile incelenmesi

The interaction of methane at various temperatures with NOx species formed by roomtemperature adsorption of NO/O2 mixture on tungstated zirconia-titania (25 wt % of WO3,denoted as WZT) and palladium(II)-promoted (1.5 wt % of Pd) zirconia-titania (1.5Pd/ZT)and tungstated zirconia-titania (1.5Pd/WZT) is investigated using in situ FTIR spectroscopy.The structure and surface properties of ZT, WZT, 1.5Pd/ZT and 1.5Pd/WZT arestudied by XRD, DR-UV-Vis spectroscopy and FT-IR spectroscopy of adsorbed CO and NO.Zirconia-titania was prepared by a homogenous coprecipitation of urea at 70oC. Formation ofcrystalline ZrTiO4 compound at calcination temperature of 600oC is observed. Based on thedata of XRD and DR-UV-Vis spectra, very good mixing of oxides has been achieved withhigh surface area (118 m2/g) and small crystallite size (4.4 nm). The WZT sample hasparatungstate type polytungstate species forming intermediate WOx surface domains whichgive rise to strong Brønsted acidity. Pd-containing samples were prepared impregnating theZT and WZT samples with Pd(NO3)2.2H2O solution. The WZT support can stabilize isolatedPd2+ ions coordinated to surface oxygen atoms.The spectrum of CO adsorbed on the ZT sample reveals the presence of coordinativelyunsaturated (cus) Zr4+ and Ti4+ sites. Their amount decreases considerably after themodification of the sample with WO3. The adsorption of CO and NO on the 1.5Pd/ZT and1.5Pd/WZT samples indicates the presence of palladium ions in two different environments.The adsorption of NO at room temperature on the samples studied involves process ofdisproportionation of NO on surface oxide ions leading to formation of adsorbed anionicnitrosyl, NO , and NO2.The addition of molecular oxygen to the NO causes its oxidation to NO2/N2O4. Thesegases adsorb molecularly over the surface and undergo self-ionization and disproportionationwith the participation of surface hydroxyl groups. Introduction of WOx species and Pd2+ ionsto the zirconia-titania mixed oxide hinders the processes of NO2/N2O4 self-ionization anddisproportionation by elimination of the necessary active sites.NOx species formed at room temperature on the WZT and 1.5Pd/ZT samples suppressthe oxidation of the methane, whereas in the case of the 1.5Pd/WZT catalyst the surfacenitrates initiate the formation of nitromethane. Mechanism for the reduction of NO over the1.5Pd/WZT catalyst is proposed, which involves a step of thermal decomposition of thenitromethane to adsorbed NO and partially oxidized hydrocarbons (methoxy and/or formatespecies) through the intermediacy of cis-methyl nitrite. The reduction of the adsorbed NO bythe partially oxidized hydrocarbons leads to the products of the CH4-SCR, molecular nitrogenand carbon oxides. Under in situ conditions, nitromethane and cis-methyl nitrite are stabilizedon the surface of the 1.5Pd/WZT catalyst, whereas the adsorption of the authentic reagentresults in adsorbed nitromethane and its trans isomer. It is concluded that nitromethaneformed in situ and authentic nitromethane follow different decomposition routes.Keywords: In situ FT-IR; Pd promoted zirconia-titania; Tungstated zirconia titania; Mixedoxides; Reactivity of stable surface NOx compounds; NOx; Selective catalytic reduction bymethane; Mechanism; Nitromethane; Methyl nitrite.