Browsing by Subject "Adsorption."
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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(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 Gold supported on tungstated zirconia : synthesis, characterization and in situ FT-IR investigation of NO(formula) + CH(formula) surface reactions(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(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.Item Open Access Item Open Access Sorption behavior of radioiodine on organic rich soil, alumina and clay minerals(1992) Assemi, ShoelehSorption behavior of radioiodine on organic rich soil, alumina, chlorite- illite clay mixture, kaolinite and bentonite have bc;en studied using the batch method. was used as tracer. Characterization of samples were done by FTIR, X-ray diffraction spectrometry and particle size distribution by Andreason pipette method. The grain size of the samples used were all below 38/xm and the experiments were performed at room temperature. The aqxreos phase used in all experiments were synthetic ground water with a composition similar to the underground waters of the regions where the samples were obtained. Stock solutions containing tracer, were prepared using synthetic ground- water. Initial concentration of inactive iodide ion in the solutions ranged from 10~®to 10“^ mmole/ml. The influence of contact time, solution/solid ratio, pH of synthetic ground- water, biomass of soil and I~ ion concentration on the sorption of radioiodine on the organic rich soil were investigated. The effect of I~ ion concentration and complexing agents on the sorption behavior of radioiodine on alumina and clay minerals were also studied. A rather slow kinetics was observed for tlie adsor])tion of radioiodine on the organic rich soil, tending saturation within 14 da.ys. It was observed that distribution ratio increased with increasing ratio of the volume of solution to the mass of soil(V/m ratio) and reached a platee.u after V/m=50. Distribution ratio did not change very much by changing the pH of the solution. Comparison of Eh-pH behavior of the solutions with the standard Eh-pH graph of iodine-water system indicated that in the whole pH rcinge, iodine is mostly preseirt in I~ form. To observe the effect of soil biomass soil samples were sterilized by heat and gamma-ray source and the results were found to be much lower than the control sample. Increasing the concentration of iodide ion in the .solution, caused a decrease in Rd. values. Desorption Wcvs ol)served only for low initial iodine concentrations < 1 X I0~^7nmol/ml), for organic rich soil which suggests that at least two sites and/or mechanisms are involved in the sorption. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. The slope of Freundlich isotherm was found to be less than 1 which indicates the non-linearity of the isotherm. The mean free energy of adsorption was calculated from the slope of Dubinin-Radushkevich isotherm and was found as about 11 k.J/mol. The distribution of sites on organic rich soil was calculated using the Freundlich and Dubinin-Radushkevich constants, considering Cl~ as the competing ion with I~ in the solution. The affinities of sites were found to be about three times higher for Cl~ than I~. Among the clay minerals, the highest Rd value was found for chlorite-illite cla}^ mixture but tlie Rd values found for alumina and the other sanrples were well below those found for the organic rich soil. The sorption data were fitted to Freundlich and Dubinin-Radushkevich isotherms. All the Freundlich isotherms were non-linear (slopeItem Open Access Sorption behaviour of Ba++, Co++ and Zn++ ions on alumina, kaolinite and magnesite(1991) Gökmenoğlu, ZelihaThe need for ultimate disposal of nuclear wastes has stimulated a renewal of interest in the adsorption behaviour of various nuclides on minerals of the type found in and around the various types of repositories that have been proposed. These adsorption studies are needed in order to estimate rates of transport of the nuclides in the event of water penetration into and through the repository. In this work the sorption-desorption behaviour of Ra·*”*·, and Zn'^'^ on minerals from different regions of Turkey has been investigated by means of a batch technique. The mineral samples used are mainly alumina, kaolinite and magnesite types. and were used as isotopic tracers. The distribution coefficients for the sorption of the three cations studied on different minerals were calculated. Alumina mineral was used for Co·*"*· and Zn'^'^ sorption experiments whereas kaolinite and magnesite were used to study the sorption behaviour of Cc»·*·*· and Zn++. The samples were seperated into different particle size ranges by AndrecLsen Pipette method. The particle size range used throughout the experiments was 20-38//m .The groundwaters used for different sorbing materials were from Beyşehir, Seydişehir and Mihalhçcık. The groundwaters used in the experiments were prepared synthetically in the laboratory. In the solutions prepared with ground- waters, initied concentrations ranged from 1.04x10“® to 1.04x10"® meq/ml for Co·*····, 7.67x10"® to 7.67x10"'* meq/ml for Zn'^'^ and 7.65x10"® to 7.65x10"® meq/ml for The samples were shaken with a shaker at 190 rpm and phase separations were carried out by centrifuging. The measurements were performed by a Nal(Tl) detector. Sorption and desorption kinetics were studied during 16 days except for the adsorption of C0++ on magnesite which was studied during 24 days, and adsorption rates were calculated from first order rate equation. Rapid adsorption was observed at high concentrations. It was observed that, about 2 days of contact was enough for values on kaolinite to reach steady-state whereas for Co^'^ on magnesite at least 25 days were needed. Rd values of on alumina mineral were quite high (2857 ml/g). Rd values for on minerals ranged from 15 (on kaolinite) to 4274 ml/g (on alumina) depending on the type of minéral and the Rd values of Zn'^'^ on minerals ranged from 26 (on magnesite) to 3800 ml/g (on alumina). It was observed that, alumina minerals adsorb ions more than kaolinite and magnesite type of minerals. Adsorption-desorption process was found to be reversible for Ba'^'^ and Co^'^ sorption on alumina and Co^'^ sorption on kaolinite. However, a paxtially reversible mechanism was observed for Zn'^'^ on.alumina and kaolinite and the sorption of (70++ and Zn++ ions on magnesite. Concentration dependent ion sorption isotherms were found to fit to Fre- undlich type of isotherms. The specific sorption concentration or the amount of radionuclide sorbed per gram of soil were calculated. It was seen that the sorbed concentration, Ca decreases with increasing mass M of the adsorbing solid. The change of the system parameter, 7 and specific sorbed concentration, Ca,o with the initial ion concentration is given by appropriate equations. The results obtained from V/M ratios indicate that in batch experiments the influence of the V/M ratio should be taken into account although according to the fundamental distribution law, distribution coefficients should be independent of V/M ratio. Distribution coeiRcients for adsorption of on binary mixtures of various minerals have been determined to see whether Rd of mixtures could be predicted from those of the pure minerals. It is shown that the system corresponds to a diluted experiment and the overall Rd is limited by the high Rd of alumina.Item Open Access Sorption studies of cesium and barium on magnesite using radiotracer and x-ray photoelectron spectroscopy(1997-06) Shahwan, TalalAs the consumption of the radioactive materials is continuously increasing, the problem of disposing the resulting v^^astes safely is becoming more challenging. One way through which these radioactive wastes could be isolated from the biological environment is by disposing them in deep geological formations. Clay minerals are proposed as backfill buffering materials that can delay the migration of the radionuclides and thus decrease the contamination of underground waters. The extent of retardation of the radionuclide migration is dependent on factors like time of contact, pH and Eh of groundwater, concentration, temperature and grain size of the mineral particles. Up to now, several studies were carried out to examine the effect of such parameters on the sorption behavior of different radionuclides on various kinds of minerals. This study was conducted to investigate the effects of time, concentration and temperature on the sorption behavior of cesium and barium ions on magnesite. Cesium 137 140 and barium have the radioactive isotopes Cs (t^^^ = 30.1 y) and Ba (t^^^ = 12.8 d) 1/2 produced in high yields during the fission process which are important in radioactive waste considerations. Magnesite is a mineral composed mainly of magnesium carbonate 2+ together with minor amounts of quartz and has a single exchangeable cation. Mg . The radiotracer method and x-ray photoelectron spectroscopy, which is a powerful surface sensitive tool, were used in this study. The results obtained from both methods complemented each others and were in good agreement. Kinetic studies of the sorption process show that equilibrium was approached within one day of contact for both of cesium and barium ions. The data of the sorption of both cations using different concentrations at various temperatures were most adequately described by the Freundlich type isotherms which correspond to multilayer adsorption on heterogeneous surfaces. The values of the Freundlich constants k and n imply that barium ions have slightly larger adsorption affinity and adsorption intensity than cesium ions. The adsorption data at low concentrations were also observed to obey the Dubinin-Radushkevich type isotherms which describe monolayer adsorption on heterogeneous or homogeneous surfaces. The adsorption data were very poorly described by the Langmuir type isotherms. Thermodynamic parameters such as enthalpy change, AH°, entropy change, AS° and free energy change of adsorption, AG°, were calculated from the sorption data of cesium and barium ions at different temperatures. The values obtained for AH° and AS° were -37 kJ/moL, -0.09 kJ/moL.K and -13 kJ/moL, -0.009 kJ/moL.K for cesium and barium ions respectively. The negative AH° values indicate the exothermic nature of adsorption which means that low temperatures are favored. The decrease in entropy upon adsorption implied by the negative AS° values is indicative of the stability of adsorption for both cations. The values of AG° at different temperatures were all negative indicating the spontaneity of the adsorption process for both cesium and barium ions. The magnitudes of AG° were seen to be within the 8-16 kJ/moL range which is the energy range of ion- exchange type processes.