Browsing by Subject "Surface chemistry."
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Item Open Access Finding an optimum surface chemistry for [Formula] systems as NOx storage materials(Bilkent University, 2010) Şentürk, Göksu SedaTitania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases, thermal stabilities and the dispersion of the surface oxide/nitrate domains were investigated. The structural characterization of the synthesized NOx storage materials were analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray (EDX) and transmission electron microscopy (TEM). Comparative analysis of the results showed that the TiO2/Al2O3 support material derived by the co-precipitation of the corresponding hydroxides via the sol-gel technique, exhibited distinctively more homogenous distribution of TiO2 domains. The functionality/performance of these materials upon NOx and SOx adsorptions were monitored by temperature programmed desorption (TPD) and insitu Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites functioned as strong anchoring sites for surface BaO domains and were tailored to enhance surface dispersion of BaO. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+ /N2O3 on alumina << nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary oxide materials showed that titania (TiO2) was a promising candidate for improving the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3 and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the presence of titania domains was seen to decrease the SOx desorption temperatures and enhance the sulfur-tolerance of these materials by destabilizing the accumulated sulfate species. SOx exposure on the synthesized materials led to a significant decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra showed that the sulfur deposition on the NOx storage materials promoted by TiItem Open Access Gold supported on tungstated zirconia : synthesis, characterization and in situ FT-IR investigation of NO(formula) + CH(formula) surface reactions(Bilkent University, 2012) Mametsheripov, SerdarThe potential of gold supported on tungstated zirconia as a catalyst for selective catalytic reduction of NOx with propene (C3H6-SCR) was investigated by in situ FT-IR spectroscopy. Samples of tungstated zirconia were prepared by both impregnation and coprecipitation methods using ammonium metatungstate (AMT) as a precursor. Gold was deposited on the supports via cationic adsorption from aqueous solution of [Au(en)2]Cl3 complex (en = ethylenediamine). The samples were characterized by XRD, XPS, BET, DRUV-vis, ICP-MS and FT-IR spectroscopy of adsorbed CO. The results show that the samples consist of tetragonal zirconia crystallites hosting uniform layer of polytungstate species. The gold particles occupy preferentially the WOx-free zirconia surface and the dispersion of gold depends on the amount of coodinatively unsaturated (cus) Zr4+ ions. The modification of zirconia by tungsten facilitates the gold uptake but at the same time causes decrease in the concentration of (cus) Zr4+ ions thus lowering the despersion of gold clusters. The interaction of gold supported on WOx-free (Au/ZrO2) and WOx-modified zirconia samples (Au/xWZ-I, where “I” denotes the incorporation of WO3 by impregnation and x = 5, 12, and 20 wt % of WO3) with NO+O2 gas mixture shows that the W-containing samples promote the formation of NO2 at room temperature. The FT-IR spectra obtained at room temperature during the contact of CO with gold samples containing pre-adsorbed NOx species reveal the formation of isocyanates (NCO) coordinated to gold sites. The generation of Au NCO species in the ad-NOx+CO reaction is confirmed by using 13CO and treatment of the adsorbed isocyanates with water vapor. The gold isocyanates display high thermal stability. However, they react readily with NO2 at room temperature. This finding suggests that gold supported on tungstated zirconia could be of interest as a low-temperature catalyst for COSCR of NOx. The FT-IR spectra recorded during the contact of C3H6 and O2 gas mixture with Au/ZrO2 and Au/xWZ-I samples lead to the conclusion that the WOx-free sample catalyzes the complete oxidation of propene. The Au-promoted tungstated samples, which contain redox (W=O groups) and Brønsted acid sites, favor the partial oxidation of the hydrocarbon. The results of a detailed mechanistic investigation show that the activation of propene in the presence of NOx species adsorbed on Au/xWZ-I samples takes place at room temperature producing surface isopropoxides. The interaction of the latter species with the surface nitrate complexes leads to the formation of nitroacetone [CH3C(O)CH2NO2]. It is proposed that at higher temperatures (e.g. 150oC) the nitroacetone coordinated to gold particles transforms through an internal redox process producing surface acetates and Au NCO species. The isocyanates react with the NO3 /NO2 surface complex formed by oxidation of NO yielding molecular nitrogen, N2O and COx as reaction products. The goldfree samples do not cause the formation of NCO species under the same experimental conditions. This experimental fact suggests that the Au particles play fundamental role in the formation of the NCO species. The amount of Au-NCO species produced is the highest on ZrO2-based catalyst containing 1.8 wt % of gold and 12 wt % of WO3 (Au/12WZ-I sample). This material combines better gold dispersion with sufficient amount of Brønsted acid sites necessary for the activation of propene to hydrocarbon oxygenates leading to the formation of nitroacetone. Based on the catalytic activity measurements, it is concluded that among the materials studied, the Au/12WZ-I catalyst could be promising in the C3H6-SCR of NOx.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.