Browsing by Subject "Nitromethane"
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Item Open Access FT-IR Spectroscopic characterization of the intermediates in the selective catalytic reduction of NO with methane on Pd/ZrO(formula)-WO(formula) catalyst(Bilkent University, 2004) Çayırtepe, İlknurThis work involves in situ FT-IR spectroscopic study of the routes of formation, composition and thermal stability of strongly bound NOx complexes on the surface of Pd/tungstated zirconia, and transformation of the surface NOx complexes in the presence of methane in order to elucidate the mechanism of selective catalytic reduction of NO with methane. Sol-gel polymer-template synthesis was chosen to obtain high surface area in the preparation of the tungstated zirconia used as support (WO3 nominal content of 18.6 wt %). The Pd(II) ions (0.1 wt%) have been deposited by impregnation. PXRD characterization shows that the support and the catalyst are tetragonal and contain mesoporous phase. The adsorption of NO at room temperature on the tungstated zirconia shows presence of coordinatively unsaturated Zr(IV) ions. The spectrum of NO adsorbed on palladium modified on tungstated zirconia reveals the existence of two types of Pd(II) sites. No exposed Zr(IV) ions are observed. The surface NOx species (N2O3, nitro and nitrito ions) on both samples are produced at room temperature by oxidation of NO with the W6+=O species. In the case of the Pd/tungstated zirconia, palladium(II) can oxidize NO to NO2 at 623 K. The adsorption of NO/O2 mixture at room temperature on the samples studied leads to formation of various kinds of surface nitrates characterized by different modes of coordination. The thermal stability of the nitrate species formed on both samples is comparable: They disappear after dynamic evacuation at 673 K. However, lower concentration of the surface nitrates on the Pd/tungstated zirconia compared to that on the tungstated zirconia indicates that in the former case the nitrates are coordinated to the support. The experimental results show that methane interacts differently with the NOx-precovered tungstated zirconia and Pd/tungstated zirconia although both materials are able to activate methane at the same temperature in absence of adsorbed NOx species. In the case of the tungstated zirconia the surface nitrates suppress the oxidation of methane, whereas the NOx-precovered Pd/tungstated zirconia catalyzes the formation of nitromethane. The latter compound is considered as a key intermediate in the selective catalytic reduction of NO with methane in excess oxygen. A mechanism, which involves direct activation of methane by the catalyst, leading to the products of the selective reduction (N2, CO2 and H2O) is proposed.Item Open Access FT-IR spectroscopic investigation of the surface reaction of CH4 with NOx species adsorbed on Pd/WO3–ZrO2 catalyst(2007) Kantcheva, M.; Cayirtepe, I.The interaction of methane at various temperatures with NO x species formed by room temperature adsorption of NO + O2 mixture on tungstated zirconia (18.6 wt.% WO3) and palladium(II)-promoted tungstated zirconia (0.1 wt.% Pd) has been investigated using in situ FT-IR spectroscopy. A mechanism for the reduction of NO over the Pd-promoted tungstated zirconia is proposed, which involves a step consisting of thermal decomposition of the nitromethane to adsorbed NO and formates through the intermediacy of cis-methyl nitrite. The HCOO- formed acts as a reductant of the adsorbed NO producing nitrogen. © Springer Science+Business Media, LLC 2007.Item Open Access In-situ FT-IR spectroscopic investigation of NO(formula) + CH(formula) surface reactions on palladium promoted WO(formula)/TiO(formula)-ZrO(formula) mixed oxides(Bilkent University, 2005) Ağıral, AnılThe 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.