共查询到20条相似文献,搜索用时 31 毫秒
1.
Various partial oxidation products were identified on the surface of TiO 2 and an 8% SiO 2–TiO 2 binary catalyst used for the photocatalytic oxidation of gas-phase toluene. Using in situ FTIR spectroscopy, benzaldehyde and benzoic acid were identified on the surface of the deactivated photocatalysts. Additional GC/MS analysis of methanol-extracted surface species confirmed the presence of benzaldehyde and benzoic acid and detected small concentrations of benzyl alcohol. Apparently, benzaldehyde is the main partial oxidation product that is further oxidized to benzoic acid. Benzoic acid is strongly adsorbed on the surface of the catalyst. There seems to be a correlation between the accumulation of benzoic acid on the surface and catalyst deactivation. The presence of gas-phase water in the reactive mixture seems to retard the formation of benzoic acid. The SiO2–TiO2 photocatalyst is more active and appears to deactivate slower than TiO2. This binary oxide is photocatalytically active even in the absence of gas-phase oxygen. It also seems to have a higher toluene adsorption capacity than TiO2. The acidity of the different oxides was examined using FTIR spectroscopy of adsorbed pyridine. The results indicate that no pure metal oxide displays Brønsted acidity but when SiO2 is cofumed with TiO2, Brønsted acidity of intermediate strength is generated. The generation of new surface sites may be responsible for the increased activity. The mechanism of this promotion effect is not clearly understood and further studies are required to elucidate it. 相似文献
2.
To get the low temperature sulfur resistant V 2O 5/TiO 2 catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V 2O 5/TiO 2 catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V 2O 5/TiO 2 catalysts showed high catalytic activity for NH 3 selective catalytic reduction (NH 3-SCR) of NO x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V 2O 5/TiO 2 > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V 2O 5/TiO 2 for maximum NO x conversion, which also showed high resistance to SO 2 in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V 2O 5/TiO 2 and compared with commercial W(10%)V 2O 5/TiO 2 catalyst. High electrical conductivity difference (Δ G) for Sb(2%)/V 2O 5/TiO 2 catalyst with temperature was observed. SO 2 deactivation experiments were carried out for Sb(2%)/V 2O 5/TiO 2 and W(10%)/V 2O 5/TiO 2 at a temperature of 230 °C for 90 h, resulted Sb(2%)/V 2O 5/TiO 2 was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V 2O 5/TiO 2 than Sb(2%)/V 2O 5/TiO 2 catalyst. 相似文献
3.
A modified sol–gel process was used to prepare nanostructured TiO 2 catalysts of controlled particle size (i.e. 6, 11, 16 and 20 nm). The influence of the TiO 2 particle size in the gas phase photocatalytic oxidation of toluene was investigated under both dry and humid conditions. The main products of reaction were carbon dioxide and water, although small amounts of benzaldehyde were also detected. The smaller particle size (i.e. 6 nm) lead to higher conversion and complete mineralization of toluene into CO 2 and H 2O. Both electronic and structural effects (i.e. size and ensemble effects) are responsible for the excellent performance of 6 nm TiO 2 catalyst for toluene photo-degradation. The structural differences between 6 nm TiO 2 and larger catalysts were analyzed using EPR spectroscopy. 相似文献
4.
Possible application of the TiO 2/UV–VIS photocatalytic process in the destruction of nitrogen-containing malodorous compounds was evaluated. Pyridine (C 5H 5N), propylamine (C 3H 7NH 2) and diethylamine (C 4H 10NH) were photodegraded in the presence and in the absence of oxygen. Degradation of nitrogen-containing organic compounds was confirmed by mass balance taking into consideration NH 4+ and NO 3− ions trapped at the TiO 2 surface. Photocatalytic deactivation was observed in all cases. On-line mass spectrometry was used to identify byproducts in the gas phase formed during the degradation process. GC–MS analysis of the dichloromethane-extract of aqueous species leached from the surface of deactivated catalyst, as well as pre-concentration in a Tenax column were used to identify intermediates in the gas phase. These byproducts are considered to be the major ones responsible for deactivation of TiO 2. 相似文献
5.
Hydrotreating of Maya heavy crude oil over high specific surface area CoMo/TiO 2–Al 2O 3 oxide supported catalysts was studied in an integral reactor close to industrial practice. Activity studies were carried out with Maya crude hydrodesulfurization (HDS), hydrodemetallization (HDM), hydrodenitrogenation (HDN), and hydrodeasphaltenization (HDAs) reactions. The effect of support composition, the method of TiO 2 incorporation, and the catalyst deactivation are examined. Supported catalysts are characterized by BET specific surface area (SSA), pore volume (PV), pore size distribution (PSD), and atomic absorption. It has been found that sulfided catalysts showed a wide range of activity variation with TiO 2 incorporation into the alumina, which confirmed that molybdenum sulfided active phases strongly depend on the nature of support. The pore diameter and nature of the active site for HDS, HDM, HDN, and HDAs account for the influence of the large reactant molecules restricted diffusion into the pore, and/or the decrease in the number of active sites due to the MoS 2 phases buried with time-on-stream. The textural properties and hysteresis loop area of supported and spent catalysts indicated that catalysts were deactivated at the pore mouth due to the metal and carbon depositions. The atomic absorption results agreed well regarding the textural properties of spent catalysts. Thus, incorporation of TiO 2 with γ-Al 2O 3 alters the nature of active metal interaction with support, which may facilitate the dispersion of active phases on the support surface. Therefore, the TiO 2 counterpart plays a promoting role to HDS activity due to the favorable morphology of MoS 2 phases and metal support interaction. 相似文献
6.
Kinetic studies show deactivation of TiO 2 catalysts during aqueous-phase and gas-phase photooxidation of trichloroethene (TCE). Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were used to examine adsorbed species on TiO 2 photocatalyst surfaces after reaction, and TPD was used to determine how reactants and products adsorb on the TiO 2 surface. Used and deactivated catalysts were analyzed after participating in either aqueous-phase or gas-phase photooxidation of TCE. The XPS spectra showed little difference between the surface composition of fresh TiO 2 and that of a deactivated catalyst from the aqueous-phase photoreactor. Chlorine was observed only on catalysts used in the gas-phase photocatalytic decomposition of TCE. Differences due to photoreaction were observed in TPD spectra of water, carbon monoxide, and carbon dioxide. Both the total amount desorbed and the temperature of desorption of carbon monoxide and carbon dioxide were quite different for used and deactivated catalysts from the two photoreactions. Apparently strongly bound species, such as carbonates, accumulated on the surface and formed carbon monoxide upon high-temperature decomposition. Small amounts of chlorinated compounds desorbed from the used and deactivated catalysts following gas-phase photoreaction. Dichloroacetyl chloride (DCAC), a reaction intermediate, can adsorb strongly on TiO 2 and readily displaces TCE. Thermally decomposed DCAC reduces the number of available adsorption sites for DCAC and TCE. An interesting low-temperature oxygen desorption peak was observed from catalysts treated with H 2O 2, which improves catalytic activity. This feature indicates that H 2O 2 is stable on TiO 2 at room temperature and decomposes at 420 K. 相似文献
7.
Solar TiO 2-photocatalysis was applied to waters from a natural wastewater treatment plant located in the Universidad de Las Palmas de Gran Canaria. Degussa P-25 TiO 2 and its mixture with activated carbon (AC–TiO 2) were used as catalysts. The presence of ozone and certain ions such as phosphates on the photocatalytic degradation of organic matter was also studied. Disinfection experiments have provided interesting results, particularly when using the catalyst AC–TiO2 and ozone, since total disinfection was achieved in less than 60 min. No bacterial reappearance at 24 or 48 h was observed. Additionally, this catalyst gave important TOC and some ions concentrations reductions. Studies in catalyst reuse revealed that the catalyst AC–TiO2 showed almost no deactivation. 相似文献
8.
The deactivation of TiO 2 Degussa P25 during the gas-phase photocatalytic oxidation of ethanol has been studied. Water vapor plays a clear competitive role for surface sites adsorption, thus hampering the ethanol photo-oxidation. Dark adsorption of ethanol on a fresh catalyst shows a Langmuirian behavior with the formation of a monolayer of adsorbate. Dark adsorption in a TiO 2 surface that has been used in consecutive photocatalytic experiments of ethanol degradation gives non-Langmuirian isotherms, indicating the existence of noticeable changes of the catalyst surface structure. After several irradiations the catalyst activity decreases. Such deactivation has been investigated, observing that the rate constant of ethanol and acetaldehyde (its main degradation product) oxidation decreases with irradiation time. Several surface treatments have been studied in order to find suitable procedures for catalytic activity recovery, but regular decay of activity is always observed after every treatment. 相似文献
9.
The effect of the modification of vanadia catalysts supported on TiO 2/SiO 2 by the oxides of Al, Mg and Te, and K 2SO 4on the selective oxidation of toluene in the vapor phase has been studied. The catalysts were prepared by successive impregnation and characterized by BET surface measurements, XRD, XPS, and TPR. Addition of the second component decreased specific activity in all cases, except Al, mainly due to the decrease of surface area. Intrinsic activity was increased with addition of Te and Al, and decreased by that of Mg, while K 2SO 4 had little effect. These differences could be explained by the observed changes in either vanadium surface dispersion or reducibility. Selectivity to benzaldehyde increased markedly with addition of Te or K 2SO 4, that caused the formation of new oxide phases, V 3Ti 6O 17 and TiV 2O 6, in which vanadium is in a partially reduced state. 相似文献
10.
The relationships between morphology and Lewis acid and base character of surface sites of two types of titania powders (TiO 2 P25 and TiO 2 Merck) were studied by HRTEM and FTIR spectroscopy of adsorbed molecules. Electron micrographs revealed that TiO 2 P25 microcrystals have a prismatic shape, mainly exposing (0 0 1) and (0 1 0) surface planes. TiO 2 Merck powder, which exhibits a significantly lower specific surface area, appeared constituted by large roundish microcrystals. FTIR spectra of adsorbed CO indicated that Ti 4+ ions exposed on (0 0 1) and (0 1 0) faces of TiO 2 P25 particles are Lewis acid centres significantly stronger than those present on the surface of TiO 2 Merck microcrystals. As in both cases the exposed cations are coordinated to five oxygen anions, the observed differences in Lewis acidity are ascribed to some difference in the geometric arrangement of the O 2− ligands. Such difference in structure affects the basicity of these centres also. In fact, a fraction of O 2− ions on the surface of TiO 2 P25 behave as basic centres toward CO 2 linearly adsorbed on neighbour Ti 4+ centres, and then Lewis acid–base pairs can be recognised. By contrast, no basic activity towards CO 2 was detected for the TiO 2 Merck sample. The two titania powders exhibited different chemical behaviour in condition of high surface hydration also. Hydroxyl groups on the surface of hydrated TiO2 P25 are able to transform benzaldehyde molecules in hemiacetalic-like species, whereas C6H5CHO molecules are only weakly perturbed by interaction with the OH groups on TiO2 Merck particles. This feature could be related to the different photocatalytic behaviour in the oxidation of toluene in gas phase, where benzaldehyde was found as a relevant intermediate species. 相似文献
11.
It has been shown that CO 2 could be transformed into hydrocarbons when it is in contact with water vapour and catalysts under UV irradiation. This paper presents an experimental set-up to study the process employing a new approach of heterogeneous photocatalysis using pellet form of catalyst instead of immobilized catalysts on solid substrates. In the experiment, CO 2 mixed with water vapour in saturation state was discharged into a quartz reactor containing porous TiO 2 pellets and illuminated by various UV lamps of different wavelengths for 48 h continuously. The gaseous products extracted were identified using gas chromatography. The results confirmed that CO 2 could be reformed in the presence of water vapour and TiO 2 pellets into CH 4 under continuous UV irradiation at room conditions. It showed that when UVC (253.7 nm) light was used, total yield of methane was approximately 200 ppm which was a fairly good reduction yield as compared to those obtained from the processes using immobilized catalysts through thin-film technique and anchoring method. CO and H 2 were also detected. Switching from UVC to UVA (365 nm) resulted in significant decrease in the product yields. The pellet form of catalyst has been found to be attractive for use in further research on photocatalytic reduction of CO 2. 相似文献
12.
Various vanadium-containing catalysts were searched for the commercial application in the selective oxidation of H 2S to elemental sulfur at low temperatures (less than 250°C) in the presence of excess (more than 35 vol.%) water. In the test of binary oxides, it was found that TiVO x was the only catalyst that could sustain its activity without deactivation at 230°C. The best catalytic activity (85–90% sulfur yield) was obtained when VO x/TiO 2 was incorporated with other metals such as Fe, Cr and Mo. Reaction occurred via redox mechanism and the reoxidation of reduced vanadium was the rate-limiting step. A long-term deactivation observed during the reaction was due to slower reoxidation of reduced vanadium by oxygen than the reduction by H 2S. Catalytic activities of VO x/SiO 2, VO x/TiO 2 and V–Fe–Cr–Mo–O x/TiO 2 were well correlated with their redox properties that were observed by TPR/TPO and XPS measurements. 相似文献
13.
Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al 2O 3 as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO 2 formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO 2 formation are sensitive to the phenol and O 2 concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO 2 formation than did bulk MnO 2 or bulk TiO 2. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions. 相似文献
14.
Combined effect of H 2O and SO 2 on V 2O 5/AC the activity of catalyst for selective catalytic reduction (SCR) of NO with NH 3 at lower temperatures was studied. In the absence of SO 2, H 2O inhibits the catalytic activity, which may be attributed to competitive adsorption of H 2O and reactants (NO and/or NH 3). Although SO 2 promotes the SCR activity of the V 2O 5/AC catalyst in the absence of H 2O, it speeds the deactivation of the catalyst in the presence of H 2O. The dual effect of SO 2 is attributed to the SO 42− formed on the catalyst surface, which stays as ammonium-sulfate salts on the catalyst surface. In the absence of H 2O, a small amount of ammonium-sulfate salts deposits on the surface of the catalyst, which promote the SCR activity; in the presence of H 2O, however, the deposition rate of ammonium-sulfate salts is much greater, which results in blocking of the catalyst pores and deactivates the catalyst. Decreasing V 2O 5 loading decreases the deactivation rate of the catalyst. The catalyst can be used stably at a space velocity of 9000 h −1 and temperature of 250 °C. 相似文献
15.
TiO 2 nanoparticles have been prepared by a novel alkoxide sol–gel precipitation. The presence of active carbon in different percentages could act as an interesting template. Upon calcination, carbon is eliminated leaving surface features significantly different from TiO 2 prepared in the absence of carbon. Wide surface and structural characterisation of samples have been carried out. Correlations with carbon percentage is pointed out from this characterisation. Interesting spherical aggregates of nanosized TiO 2 are observed from TEM images probably stabilised by the presence of carbon. Physicochemical correlations made will be very useful in further application of these TiO 2 to be used as potential high surface area photocatalyst. 相似文献
16.
The effect of tungsten and barium on the thermal stability of V 2O 5/TiO 2 catalyst for NO reduction by NH 3 was examined over a fixed bed flow reactor system. The activity of V 2O 5/sulfated TiO 2 catalyst gradually decreased with respect to the thermal aging time at 600 °C. The addition of tungsten to the catalyst surface significantly enhanced the thermal stability of V 2O 5 catalyst supported on sulfated TiO 2. On the basis of Raman and XRD measurements, the tungsten on the catalyst surface was identified as suppressing the progressive transformation of monomeric vanadyl species into crystalline V 2O 5 and of anatase into rutile phase of TiO 2. However, the NO removal activity of V 2O 5/sulfated TiO 2 catalyst including barium markedly decreased after a short aging time, 6 h at 600 °C. This may be due to the transformation of vanadium species to inactive V–O–Ba compound by the interaction with BaO which was formed by the decomposition of BaSO 4 on the catalyst surface at high reaction temperature of 600 °C. The addition of SO 2 to the feed gas stream could partly restore the NO removal activity of thermally aged V 2O 5/sulfated TiO 2 catalyst containing barium. 相似文献
17.
A lost of culturability of bacteria Escherichia coli K12 was observed after exposition to a solar simulator (UV–vis) in a laboratory batch photoreactor. The bacterial inactivation reactions have been carried out using titanium dioxide (TiO 2) P25 Degussa and FeCl 3 as catalysts. At the starting of the treatment, the suspensions were at their “natural” pH. An increase in the efficiency in the water disinfection was obtained when some advanced oxidation processes such as UV–vis/TiO 2, UV–vis/TiO 2/H 2O 2, UV–vis/Fe 3+/H 2O 2, UV–vis/H 2O 2 were applied. The presence of H 2O 2 accelerates the rate of disinfection via TiO 2. The addition of Fe 3+ (0.3 mg/l) to photocatalytic system decreases the time required for total disinfection (<1 CFU/ml), for TiO 2 concentrations ranging between 0.05 and 0.5 g/l. At TiO 2 concentrations higher than 0.5 g/l the addition of Fe 3+ does not significantly increase the disinfection rate. The systems: Fenton (H 2O 2/Fe 3+/dark), H 2O 2/dark, H 2O 2/TiO 2/dark showed low disinfection rate. The effective disinfection time (EDT 24) was reached after 60 and 30 min of illumination for the Fe 3+ and TiO 2 photoassisted systems, respectively. EDT 24 was not reached for the system in the absence of catalyst (UV–vis). The effect on the bacterial inactivation of different mixture of chemical substance added to natural water was studied. 相似文献
18.
Deactivation kinetics of a V/Ti-oxide catalyst was studied in partial oxidation of toluene to benzaldehyde (BA) and benzoic acid (BAc) at 523–573 K. The catalyst consisted of 0.37 monolayer of VO x species and after oxidative pre-treatment contained isolated monomeric and polymeric metavanadate-like vanadia species under dehydrated conditions as was shown by FT Raman spectroscopy. Under the reaction conditions via in situ DRIFTS fast formation of adsorbed carboxylate and benzoate species was observed accompanied by disappearance of the band of the monomeric species (2038 cm −1) (polymeric species were not controlled). Slow accumulation of maleic anhydride, coupling products and/or BAc on the surface caused deactivation of the catalyst during the reaction. Temperature-programmed oxidation (TPO) after the reaction showed formation of high amounts of CO, CO 2 and water. Rate constants for the steps of the toluene oxidation were derived via mathematical modelling of reaction kinetics at low conversion and constant oxygen/toluene ratio of 20:1. The model allows predicting deactivation dynamics, steady-state rates and selectivity. The highest rate constant was found for the transformation of BA into BAc explaining a low BA yield in the reaction. 相似文献
19.
The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al 2O 3, TiO 2 and SiO 2 have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al 2O 3 supported catalyst, while the activity of the TiO 2 and SiO 2 supported catalysts increased. However, at high concentrations of water both the SiO 2 and TiO 2 supported catalyst deactivated. Common for all catalysts was an increase in C 5+ selectivity and a decrease in the CH 4 selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H 2 chemisorption and X-ray diffraction (XRD). 相似文献
20.
Anthraquinonic acid green 25 (AG 25) removal was investigated by plasmachemistry using non-thermal gliding arc at atmospheric pressure. The gaseous species formed in the discharge, and especially OH radicals, induce strong oxidizing effects in the target solution. The removal of the dye was carried out in the absence and presence of TiO 2 as photocatalyst. The decolourization of AG 25 was followed by UV–vis spectrometry (at 643 nm), while the degradation was followed by COD measurements. The effects of operating variables such as initial concentration of AG 25 and catalyst concentration were investigated. Experiments were carried out to optimise the amount of TiO 2. The results showed that maximum degradation was attained for 2 g L −1 TiO 2 concentration. At this optimum concentration, the dye (80 μM) was totally decolourized within 15 min of plasma-treatment time, and 93% removal of initial COD was attained after a 180-min plasma-treatment time. In the absence of catalyst, colour removal was 46% after 15 min, while COD abatement reached 84% after 180 min. The extent of degradation decreased with initial concentration and the time required for complete degradation increased. In all cases, the plasma-treated samples in the presence or absence of catalyst were found to follow pseudo-first order reaction kinetics. The TiO 2-mediated plasmachemical process showed potential application for the treatment of dye solutions, resulting in the mineralization of the dye confirmed by sulfate ion formation. 相似文献
|