In-situ FT-IR spectroscopic investigation of NO(formula) + CH(formula) surface reactions on palladium promoted WO(formula)/TiO(formula)-ZrO(formula) mixed oxides
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The interaction of methane at various temperatures with NOx species formed by room temperature 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 are studied 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 of crystalline ZrTiO4 compound at calcination temperature of 600oC is observed. Based on the data of XRD and DR-UV-Vis spectra, very good mixing of oxides has been achieved with high surface area (118 m2 /g) and small crystallite size (4.4 nm). The WZT sample has paratungstate type polytungstate species forming intermediate WOx surface domains which give rise to strong Brønsted acidity. Pd-containing samples were prepared impregnating the ZT and WZT samples with Pd(NO3)2.2H2O solution. The WZT support can stabilize isolated Pd2+ ions coordinated to surface oxygen atoms. The spectrum of CO adsorbed on the ZT sample reveals the presence of coordinatively unsaturated (cus) Zr4+ and Ti4+ sites. Their amount decreases considerably after the modification of the sample with WO3. The adsorption of CO and NO on the 1.5Pd/ZT and 1.5Pd/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 of disproportionation of NO on surface oxide ions leading to formation of adsorbed anionic nitrosyl, NOí , and NO2. The addition of molecular oxygen to the NO causes its oxidation to NO2/N2O4. These gases adsorb molecularly over the surface and undergo self-ionization and disproportionation with the participation of surface hydroxyl groups. Introduction of WOx species and Pd2+ ions to the zirconia-titania mixed oxide hinders the processes of NO2/N2O4 self-ionization and disproportionation by elimination of the necessary active sites. NOx species formed at room temperature on the WZT and 1.5Pd/ZT samples suppress the oxidation of the methane, whereas in the case of the 1.5Pd/WZT catalyst the surface nitrates initiate the formation of nitromethane. Mechanism for the reduction of NO over the 1.5Pd/WZT catalyst is proposed, which involves a step of thermal decomposition of the nitromethane to adsorbed NO and partially oxidized hydrocarbons (methoxy and/or formate species) through the intermediacy of cis-methyl nitrite. The reduction of the adsorbed NO by the partially oxidized hydrocarbons leads to the products of the CH4-SCR, molecular nitrogen and carbon oxides. Under in situ conditions, nitromethane and cis-methyl nitrite are stabilized on the surface of the 1.5Pd/WZT catalyst, whereas the adsorption of the authentic reagent results in adsorbed nitromethane and its trans isomer. It is concluded that nitromethane formed in situ and authentic nitromethane follow different decomposition routes.
KeywordsIn situ FT-IR
Pd promoted zirconia-titania
Tungstated zirconia titania
Reactivity of stable surface NOx compounds
Selective catalytic reduction by methane
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