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1.
The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MOx/Al2O3 and MOx/TiO2 supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H2O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MOx/Al2O3 and Pt/MOx/TiO2 catalysts, it is shown that the presence of MOx results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al2O3 or Pt/TiO2, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MOx. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H2O, 20% H2 and 6% CO2, showed that certain composite Pt/MOx/Al2O3 and Pt/MOx/TiO2 catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.  相似文献   

2.
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 (Al2O3, MgO, TiO2) 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/Al2O3 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   

3.
Noble metal (Rh, Pt, Pd, Ir, Ru, and Ag) and Ni catalysts supported on CeO2–Al2O3 were investigated for water gas shift reaction at ultrahigh temperatures. Pt/CeO2–Al2O3 and Ru/CeO2–Al2O3 demonstrated as the best catalysts in terms of activity, hydrogen yield and hydrogen selectivity. At 700 °C and steam to CO ratio of 5.2:1, Pt/CeO2–Al2O3 converted 76.3% of CO with 94.7% of hydrogen selectivity. At the same conditions, the activity and hydrogen selectivity for Ru/CeO2–Al2O3 were 63.9% and 85.6%, respectively. Both catalysts showed a good stability over 9 h of continuous operation. However, both catalysts showed slight deactivation during the test period. The study revealed that Pt/CeO2–Al2O3 and Ru/CeO2–Al2O3 were excellent ultrahigh temperature water gas shift catalysts, which can be coupled with biomass gasification in a downstream reactor.  相似文献   

4.
Changbin Zhang  Hong He   《Catalysis Today》2007,126(3-4):345-350
The TiO2 supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO2  Rh/TiO2 > Pd/TiO2 > Au/TiO2  TiO2. HCHO could be completely oxidized into CO2 and H2O over Pt/TiO2 in a gas hourly space velocity (GHSV) of 50,000 h−1 even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO2 was only 20% over the Rh/TiO2 at 20 °C. The Pd/TiO2 and Au/TiO2 showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO2 supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed.  相似文献   

5.
A total of 10 noble metal (Rh, Pt, Pd, Ru and Ir) catalysts, either supported on CeO2 or Ce0.63Zr0.37O2, were prepared. Catalysts were fully characterized using XRD, N2 adsorption at −196 °C, TEM and H2 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   

6.
Different γ-Al2O3 supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C1O1 carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al2O3 (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation.  相似文献   

7.
Activities of a series of metals (Pt, Pd, Rh, Cu, Mn) supported on TiO2 were investigated for the catalytic oxidation of formaldehyde. Among them, Pt/TiO2 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 H2 were used to characterize the platinum catalysts. Using Ce0.8Zr0.2O2, Ce0.2Zr0.8O2, SiO2 as supports instead of TiO2, the activity sequence of 0.6 wt.% platinum with respect to the supports is TiO2 > SiO2 > Ce0.8Zr0.2O2 > Ce0.2Zr0.8O2, 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 TiO2 has a great effect on the catalytic activity, and 0.6 wt.% Pt/TiO2 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   

8.
Pt supported on γ-Al2O3, TiO2 and ZrO2 are active catalysts for the CO2 reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/γ-A12O3 < Pt/TiO2 < Pt/ZrO2. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO2 and Pt/ZrO2, deactivation started immediately after the start of the reaction, while the Pt/γ-A12O3 catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.  相似文献   

9.
The catalytic performance of Pt and Rh catalysts for the selective catalytic reduction (SCR) of NO by propylene in the presence of excess oxygen has been investigated over catalysts supported on six different metal oxide carriers (CeO2, Al2O3, TiO2, YSZ, ZrO2 and W6+-doped TiO2). It has been found that the nature of the dispersed metal affects strongly the light-off temperature of propylene, the maximum NO conversion to reduction products and the selectivity towards nitrogen. For a given support, Pt catalysts are always more active for both NO reduction and propylene oxidation, but are much less selective towards N2, compared to Rh catalysts. Rhodium catalysts are able to selectively reduce NO even in the absence of oxygen in the feed. However, their activity is suppressed with increasing oxygen feed concentration possibly due to the formation of less reactive rhodium oxides. In contrast, oxygen promotes the de-NOx activity of platinum catalysts but decreases selectivity towards nitrogen. Results are explained by considering the effects of the nature of the metallic phase and the support on the elementary steps of the propylene-SCR reaction. It is concluded that the catalytic performance of both metals may be improved by proper selection of the support.  相似文献   

10.
Palladium (Pd) supported on CeO2-promoted γ-Al2O3 with various CeO2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO2 structure and dispersion on the γ-Al2O3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO2 structure. Pd is present on the oxidized CeO2-promoted catalysts as Pd0, Pd+ and Pd2+, at ratios strongly dependent on CeO2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H2 or in the presence of amorphous CeO2, partial encapsulation of Pd particles occurs. CeO2 structure influences the CH4 steam reforming reaction rates. Crystalline CeO2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al2O3 or on amorphous (reduced) CeO2/Al2O3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO2/γ-Al2O3 than on Pd/γ-Al2O3, which is attributed to a catalytic synergism between Pd and CeO2. The low rates on the reduced Pd/CeO2/Al2O3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.  相似文献   

11.
The kinetics of oxidation of a light hydrocarbon (C2H4) were studied on catalysts comprising of combinations of one of three metals, Pt, Pd or Rh supported on five different supports, that is, SiO2, γ-Al2O3, ZrO2 (8% Y2O3), TiO2 or TiO2 (W6+). Significant variation of turnover frequency with the carrier was observed, which cannot be explained by structure sensitivity considerations and is attributed to interactions between the metal crystallites and the carrier. The catalytic activity of these metal-support combinations was investigated over a wide range of partial pressures of ethylene and oxygen. In a separate set of experiments, the kinetics of C2H4 oxidation were also investigated on polycrystalline Rh films interfaced with ZrO2 (8 mol% Y2O3) solid electrolyte in a galvanic cell of the type: C2H4, O2, Rh/YSZ/Pt, air, during regular open-circuit conditions as well as under Non-Faradic Electrochemical Modification of Catalytic Activity (NEMCA), that is, closed-circuit conditions. Up to 100-fold increase in catalytic activity was observed by supplying O2− ions to the catalyst surface via positive potential application to the catalyst. The observed kinetic behavior upon increasing catalyst potential parallels qualitatively the observed alteration of turnover frequency with variation of the support of the Rh crystallites.  相似文献   

12.
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 H2O) conditions. Catalysts were compared in terms of activity, selectivity for the different reaction products (CO2, HCl, Cl2, C2Cl4, CCl4 and CHCl3), 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   


13.
A. Yee  S. J. Morrison  H. Idriss   《Catalysis Today》2000,63(2-4):327-335
The reactions of ethanol over Rh/CeO2 have been investigated using the techniques of temperature programmed desorption (TPD) and FT-IR spectroscopy, in addition to steady state catalytic tests. A comparison with previous studies of ethanol adsorption over Pd/CeO2 [J. Catal. 186 (1999) 279] and Pt/CeO2 [J. Catal. 191 (2000) 30] catalysts is presented. The apparent activation energy for the reaction was 49, 40, and 43 kJ mol−1 for Rh/CeO2, Pd/CeO2 and Pt/CeO2, respectively, while the turnover number (TON) at 400 K was 5.9, 8.6 and 2.6, respectively. Surface compositions of catalysts were characterised by XPS. A decrease of the atomic O(1s)/Ce(3d) ratio of the CeO2 support indicates its partial reduction upon addition of the noble metal. The extent of reduction per metal atom was in the following order: Pt>Pd>Rh. FT-IR and TPD studies have shown that dehydrogenation of ethanol to acetaldehyde occurred over Pd/CeO2, Pt/CeO2 and Rh/CeO2. Moreover, Rh/CeO2 readily dissociated the C–C bond of ethanol at room temperature to form adsorbed CO (IR bands at 1904–2091 cm−1). This was corroborated by the low desorption temperature of CH4 over Rh/CeO2 (450 K) when compared to that of Pd/CeO2 (550 K) or Pt/CeO2 (585 K).  相似文献   

14.
The photo-catalytic production of hydrogen from liquid ethanol, a renewable bio-fuel, over Rh/TiO2, Pd/TiO2 and Pt/TiO2, anatase, has been studied. In the absence of the metal, TiO2 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 TiO2 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/TiO2. It was found that the reaction is governed by the solvation of the alcohol. In that regard, the production of molecular hydrogen over Pt/TiO2 showed the following trend: methanol ≈ ethanol > propanol ≈ isopropanol > n-butanol.  相似文献   

15.
Ni catalysts supported on γ-Al2O3, CeO2 and CeO2–Al2O3 systems were tested for catalytic CO2 reforming of methane into synthesis gas. Ni/CeO2–Al2O3 catalysts showed much better catalytic performance than either CeO2- or γ-Al2O3-supported Ni catalysts. CeO2 as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al2O3 catalysts for this reaction. A weight loading of 1–5 wt% CeO2 was found to be the optimum. Ni catalysts with CeO2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO2-promoted catalysts are attributed to the oxidative properties of CeO2.  相似文献   

16.
Methane combustion over Pd/Al2O3 catalysts with and without added Pt and CeO2 in both oxygen-rich and methane-rich mixtures at temperatures in the range 250–520°C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al2O3, which was greatly enhanced over Pd–Pt/Al2O3. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd–Pt/Al2O3 and Pd/Al2O3. Such non-steady behaviour was almost eliminated over Pd/Al2O3–CeO2. Under a very narrow range of conditions and over a Pd/Al2O3 packed bed, oscillation of methane combustion was observed.  相似文献   

17.
Catalytic dechlorination of chlorotoluene to toluene was carried out using several supported Rh-based catalysts in a 2-propanol solution of NaOH at ambient temperature (27°C). A carbon-supported Rh catalyst (Rh/C) showed high catalytic activity, although an induction period was involved in the reaction and the activity of the catalyst reduced during storage in air. The existence of Pt on the Rh catalyst was effective in overcoming the activity reduction by exposure to air and gave the reaction without any induction period. The composite Rh–Pt catalyst supported on TiO2 as well as on carbon was much more active for the reaction than the catalysts supported on SiO2, MgO and Al2O3.  相似文献   

18.
In the steam gasification of biomass, the additive effect of noble metals such as Pt, Pd, Rh and Ru to the Ni/CeO2/Al2O3 catalyst was investigated. Among these noble metals, the addition of Pt was most effective even when the loading amount of added Pt was as small as 0.01 wt.%. In addition, the catalyst characterization suggests the formation of the Pt–Ni alloy over the Pt/Ni/CeO2/Al2O3.  相似文献   

19.
Catalytic combustion of ethyl acetate, acetaldehyde, and toluene was investigated on various supported Ru catalysts prepared by the impregnation method, and the effect of reduction treatment on the activity was examined. Among the as-calcined catalysts tested, Ru/CeO2 showed the highest activity for all tests regardless of the pre-treatment in hydrogen atmosphere. The catalytic activity of Ru/SnO2 was significantly degraded by the reduction treatment, whereas the activity of Ru/ZrO2 and Ru/γ-Al2O3 was enhanced. To reveal these phenomena, the as-calcined and reduced catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), and BET surface area. The dispersion of ruthenium on the supports was evaluated by chemisorption methods of carbon monoxide. The catalytic activity was strongly related to ruthenium species easily oxidizable and reducible at low temperatures. Such ruthenium species were loaded on CeO2 in a highly dispersed state, resulting in the highest activity.  相似文献   

20.
Pt-Pd bimetal catalysts were prepared in order to develop and investigate catalysts with excellent activity and stability for benzene destruction. In the reaction results, the addition of Pt to Pd/γ-Al2O3 catalyst brought about the increase of catalytic activity. Moreover, it was effective in preventing the deactivation of the catalysts in benzene combustion. The addition of some amount of Pt made Pd particles available for better benzene combustion. On the contrary, the addition of Pt beyond a certain amount decreases activity because of the Pd active sites overlapped with the Pt active sites. The activity of the catalysts is related to oxidation state of metal, Pd/Al ratio and particle size on γ-Al2O3. These effects of Pt addition to Pd catalysts were studied by XPS, XRD, and TEM analyses.  相似文献   

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