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1.
The complete catalytic oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over alumina supported noble metal catalysts (Pt and Pd) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppm) in air, between 250°C and 550°C in a conventional fixed bed reactor. The catalysts were prepared in a range of metal contents from 0.1 to 1 wt%. Palladium catalysts resulted to be more active than platinum catalysts in the oxidation of both chlorinated volatile organic compounds. DCE was completely destructed at 375°C, whereas TCE required 550°C. HCl was the only chlorine-containing product in the oxidation of DCE in the range of 250–400°C. Tetrachloroethylene was observed as an intermediate in the oxidation of TCE, being formed to a significant extent between 400°C and 525°C. CO was also detected in the oxidation of both DCE and TCE over Pd catalysts, though at temperatures of complete destruction, CO 2 was the only carbon-containing product. The Pt catalysts were selective to CO 2 at the studied conditions. 相似文献
2.
Activities of a series of metals (Pt, Pd, Rh, Cu, Mn) supported on TiO 2 were investigated for the catalytic oxidation of formaldehyde. Among them, Pt/TiO 2 was found to be the most promising catalyst. Nitrogen adsorption, hydrogen chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed reduction (TPR) by H 2 were used to characterize the platinum catalysts. Using Ce 0.8Zr 0.2O 2, Ce 0.2Zr 0.8O 2, SiO 2 as supports instead of TiO 2, the activity sequence of 0.6 wt.% platinum with respect to the supports is TiO 2 > SiO 2 > Ce 0.8Zr 0.2O 2 > Ce 0.2Zr 0.8O 2, and this appears to be correlated with the dispersion of platinum on supports rather than the specific surface areas of the catalysts. Platinum loading on TiO 2 has a great effect on the catalytic activity, and 0.6 wt.% Pt/TiO 2 catalyst was observed to be the most active, which could be attributed to the well-dispersed platinum surface phase. The reduction temperature greatly affects the particle size and, consequently, the catalytic activity. The smaller particle size of platinum, due to its high dispersion on support, has a positive effect on catalytic performance. Increasing formaldehyde concentration and space velocity exhibits an inhibiting effect on the catalytic activity. 相似文献
3.
Oxidation activity and stability under reaction was investigated for a series of mixed oxide catalysts, doped or not by a precious metal (Pd, Pt). The reaction feedstock, containing CO, H 2, CH 4, CO 2 and H 2O, simulated gases issued from H 2 production processes for fuel cells. Contrarily to conventional noble metal catalysts, mixed oxide samples present generally good stability under reaction at high temperature. The activities measured for the perovskite and hexaaluminate catalysts, are however largely lower than that of the reference Pd/Al 2O 3 catalyst. High activities were obtained after impregnation of 1.1 wt.% Pd or 0.8 wt.% Pt on the hexaaluminates samples. Even if Pd/Al 2O 3 was found to present a high activity, this sample suffered from drastic deactivation at 700 °C. Better stability were obtained on perovskite. Furthermore, doping hexaaluminate by Pt led to samples with good activities and high stability. Even if better activities were obtained by doping the hexaaluminate samples by Pd, the Pd/BaAl 12O 19 strongly deactivated, as it was previously observed for the reference catalyst. Interestingly, this Pd deactivation was not observed when Pd was impregnated on the Mn substituted hexaaluminate, leading to a stable and active catalyst. This suggests that it is possible to stabilize the palladium in its oxidized form at high temperature (700 °C) on the surface of some supports. 相似文献
4.
The photo-catalytic production of hydrogen from liquid ethanol, a renewable bio-fuel, over Rh/TiO 2, Pd/TiO 2 and Pt/TiO 2, anatase, has been studied. In the absence of the metal, TiO 2 shows negligible production of molecular hydrogen. The addition of Pd or Pt dramatically increases the production of hydrogen and a quantum yield of about 10% is reached at 350 K. On the contrary, the Rh doped TiO 2 is far less active. The low activity of Rh compared to that of Pd and Pt is not due to poor dispersion or low available Rh sites on the surface, as analyzed by XPS and TEM. For all three catalysts, TEM shows most particles with a size less than 10 nm. XPS results show that while the state of Pd and Pt particles in the as-prepared catalysts was mostly metallic that of the Rh was composed of non-negligible contribution of Rh cations. The extent of reaction of a series of alcohols was also studied, for comparison, on Pt/TiO 2. It was found that the reaction is governed by the solvation of the alcohol. In that regard, the production of molecular hydrogen over Pt/TiO 2 showed the following trend: methanol ≈ ethanol > propanol ≈ isopropanol > n-butanol. 相似文献
5.
The performance of four different alumina-supported noble metal catalysts (0.5% of Pd, Pt, Rh and Ru, respectively) for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h −1) in air was studied in this work. Experiments were carried out at both dry and wet (20,000 ppm of H 2O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO 2, HCl, Cl 2, C 2Cl 4, CCl 4 and CHCl 3), and stability at reaction conditions. As general trend, the activity of the catalysts decreases in the order Ru Pd > Rh > Pt. Concerning to the effect of the water addition, no important effect on the catalyst activity was observed, except in the case of Pt, for which an increase of the catalytic activity was observed. Reaction mechanism (and hence product distribution) is very similar for Rh, Pd and Pt, being in these cases C2Cl4 the only organochlorinated by-product detected. In the case of Ru, the reaction mechanism seems to be quite different, CCl4 and CHCl3 being the main organic by-products. Simple power-law kinetic expressions (first order on trichloroethene concentration for Pd, Rh and Ru, and zeroth order for Pt) provide fairly good fits for catalytic performance of the studied catalysts. Finally, deactivation studies show that both formation of active metal chlorides (especially in the case of Rh) and fouling (especially for Pd and Pt) are the main deactivation causes. 相似文献
6.
Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH 3OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe 2O 3, TiO 2, etc.). While the supported gold catalysts are less active in H 2 oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO 2, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol. 相似文献
7.
Operando DRIFTS was applied to the study of the evolution of surface species formed on a Pd (2 wt.%)/γ-Al 2O 3 catalyst in various conditions. No differences were observed as a function of the initial oxidation state of palladium. Formates/carbonates species were identified at low temperature (<400 °C) and disappeared when CO 2 production started. These species come from the Pd-catalyzed interaction of CO with the alumina support, while CO 2 induces hydrogenocarbonates formation at low temperature (<300 °C). Their presence does not explain the inhibiting effect of CO 2 observed in CCM on Pd/γ-Al 2O 3 catalysts. 相似文献
8.
Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800 °C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9 MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications. Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650 °C, with reactivation upon cooling. Rh catalyst also deactivated above 750 °C, but did not reactivate. Pt catalyst was active above 600 °C. Fuel-lean reaction products were CO2 and H2O for all three catalysts. The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600 °C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H2) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior. High-pressure (0.9 MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion. 相似文献
9.
The combined CO 2 reforming and partial oxidation (POX) of n-heptane was studied on various noble metal zirconia catalysts between 700 and 900 °C. The activity order of the metals was Rh > Pd > Ir > Pt. Selectivity to syngas increased with the activity of the catalysts but the H 2 to CO molar ratio decreased. The activity and selectivity of the 0.25 wt% Rh/ZrO 2 catalyst were close to the performance of a commercial 15 wt% NiO/Al 2O 3 catalyst. The conversions and product compositions were compared to the calculated thermodynamic equilibria. 相似文献
10.
A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO 2 or Ce 0.63Zr 0.37O 2, were prepared. Catalysts were fully characterized using XRD, N 2 adsorption at −196 °C, TEM and H 2 chemisorption. Oxygen storage processes were carefully investigated. The influence of temperature was checked and a key role of oxygen diffusion was further demonstrated. A review of the reactions involved in the CO transient oxidation reaction is finally proposed. 相似文献
11.
The catalytic performance of supported noble metal catalysts for the steam reforming (SR) of ethanol has been investigated in the temperature range of 600–850 °C with respect to the nature of the active metallic phase (Rh, Ru, Pt, Pd), the nature of the support (Al 2O 3, MgO, TiO 2) and the metal loading (0–5 wt.%). It is found that for low-loaded catalysts, Rh is significantly more active and selective toward hydrogen formation compared to Ru, Pt and Pd, which show a similar behavior. The catalytic performance of Rh and, particularly, Ru is significantly improved with increasing metal loading, leading to higher ethanol conversions and hydrogen selectivities at given reaction temperatures. The catalytic activity and selectivity of high-loaded Ru catalysts is comparable to that of Rh and, therefore, ruthenium was further investigated as a less costly alternative. It was found that, under certain reaction conditions, the 5% Ru/Al 2O 3 catalyst is able to completely convert ethanol with selectivities toward hydrogen above 95%, the only byproduct being methane. Long-term tests conducted under severe conditions showed that the catalyst is acceptably stable and could be a good candidate for the production of hydrogen by steam reforming of ethanol for fuel cell applications. 相似文献
12.
Selective production of hydrogen by partial oxidation of methanol (CH 3OH + (1/2)O 2 → 2H 2 + CO 2) over Au/TiO 2 catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO 2 catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au 0), non-metallic gold (Au δ+) and Au 2O 3, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O 2/CH 3OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming. 相似文献
13.
The oxidation of benzyl alcohol with molecular oxygen under solvent-free conditions has been investigated using a range of titania-supported Au–Pd alloy catalysts to examine the effect of the Au–Pd ratio on the conversion and selectivity. The catalysts have been compared at high reaction temperature (160 °C) as well as at 100 °C, to determine the effect on selectivity since at lower reaction temperature the range of by-products that are formed are limited. Under these conditions the 2.5 wt.% Au–2.5 wt.% Pd/TiO 2 was found to be the most active catalyst, whereas the Au/TiO 2 catalyst demonstrated the highest selectivity to benzaldehyde. Toluene, formed via either a hydrogen transfer process or an oxygen transfer process, was observed as a major by-product under these forcing conditions. 相似文献
14.
We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO 2, 22% H 2O, 37.3% H 2, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate ( r) on Pt/alumina catalysts as a function of the forward rate ( rf) was found to be: r = rf(1 − β), where rf = kf[CO] 0.1[H 2O] 1.0[CO 2] −0.1[H 2] −0.5, kf is the forward rate constant, β = ([CO 2][H 2])/( Keq[CO][H 2O]) is the approach to equilibrium, and Keq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO 2 and H 2. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H 2 and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study. 相似文献
15.
The activity of mixed steam and CO 2 reforming of CH 4 to produce synthesis gas was investigated and compared with those of steam reforming alone and CO 2 reforming alone at 600–900°C under atmosphere pressure on MgO-supported noble metals. Mixed reforming shows a far lower CH 4 conversion than the value for thermodynamic equilibrium. The activity decreases following the order Ru,Rh> Ir> Pt,Pd. Little deactivation was observed for Ru, Rh and Ir catalysts. An isotope labelled 13CO 2 experiment was carried out in situ for mixed reforming on Rh/MgO and the results suggest that CO 2 dissociates as CO-M and O-M. The results of the temperature program reaction (TPR) of mixed reforming shows that CH 4 adsorbs and dissociates before reaction starts and that CO 2 reforming and steam reforming start simultaneously. A possible reaction mechanism is discussed. 相似文献
16.
Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water–gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g −1) of active intermediate species found in the carbon-path from CO to the CO 2 product gas (use of 13CO), and in the hydrogen-path from H 2O to the H 2 product gas (use of D 2O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area ( θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g −1) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO 2 and H 2 products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction. 相似文献
17.
Perylene diimide based organic sensitizers capable of electron generation under illumination were used to initiate gas phase photo reduction reactions on TiO 2 thin and thick film surfaces. For comparison [Ru(Bpy) 3] 2+ dye sensitizers were also studied. The photo reduction of CO 2 was carried out under static conditions in the gas phase. TiO 2 films were coated on hollow glass beads via a sol–gel procedure. Pt was incorporated on the films either by adding the precursor salt in the sol, Pt(in), or by wet impregnation of calcined film with an aqueous solution of the precursor salt, Pt(on). Organic sensitizers were incorporated on the films by wet impregnation of the film from an aqueous solution. Under UV illumination, the methane yields of platinized TiO 2 thin films decreased in the following order: Pt(on)·TiO 2 > Pt(in)·TiO 2 > TiO 2. The presence of organic sensitizers inhibited the catalytic activity of pure and platinized TiO 2 thick films under UV illumination. The relative enhancement of the reaction yields in the presence of the organic sensitizer under visible light illumination depended on the presence of Pt as well as the incorporation method of Pt in the TiO 2 structure. The reaction yields were better when Pt was impregnated on the TiO 2 film than when Pt was incorporated in the Ti sol. On the other hand, pure or platinized TiO 2 under visible light illumination was totally inactive indicating the role of the organic dye in generating catalytically active electrons under visible light. 相似文献
18.
The catalytic activities of Pt and Au supported on TiO 2 were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO 2 was observed only over the TiO 2-supported Au catalyst, indicating that SO 2 can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts. 相似文献
19.
For thermodynamic reasons, CO 2 has always been considered as inert at mild reaction temperatures (300 °C). In this study, we show that CO 2 may be used as a valuable compound for the catalytic combustion of methane (CCM), if ceria-based materials are used as support for the palladium active phase. Adding CO 2 in the feed significantly improves performances of ceria-zirconia supported catalysts. On the contrary, catalytic performances are inhibited on Pd/γ-Al 2O 3. Inhibition can be avoided by mixing the Pd/γ-Al 2O 3 catalyst with some CeO 2 evidencing cooperation phenomena between both catalysts. In situ DRIFTS experiments show that the inhibition of the alumina-supported catalyst is not due to formation of carbonates species. After an in situ reducing pre-treatment, pure CO 2 is able to rapidly oxidize reduced Pd/Ce 0.21Zr 0.79O 2 catalyst at 300 °C. Dissociation of CO 2 on Ce 0.21Zr 0.79O 2 would be responsible for the oxidation process. Thus, CO 2 helps in replenishing the O reservoir (OSC) of the Ce-Zr-O support which is normally consumed by reductants such as CH 4, H 2 or other HC's. XPS experiments show enrichment in oxygen species bound to Ce (Low BE O1s) on the surface of ceria-zirconia when working in the presence of CO 2. Implications of these results on the behavior of ceria-containing catalysts can be important for practical applications, e.g., in automotive exhaust catalysis. 相似文献
20.
The selective catalytic reduction of NO x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH 4–O 2 reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO x reduction, while for the NO 2–CH 4–O 2 reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO x conversion in NO 2–CH 4–O 2 reaction was significantly higher than that in NO–CH 4–O 2 reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO 2]/([NO]+[NO 2]) in the effluent gases in NO–O 2 and NO 2–O 2 reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO 2, and high activity for NO 2 reduction to N 2 (Pd, Rh); (ii) high activity for NO oxidation to NO 2, and low activity for NO 2 reduction to N 2 (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO x by methane. 相似文献
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