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1.
The effect of different reducing agents (H 2, CO, C 3H 6 and C 3H 8) on the reduction of stored NO x over PM/BaO/Al 2O 3 catalysts (PM = Pt, Pd or Rh) at 350, 250 and 150 °C was studied by the use of both NO 2-TPD and transient reactor experiments. With the aim of comparing the different reducing agents and precious metals, constant molar reduction capacity was used during the reduction period for samples with the same molar amount of precious metal. The results reveal that H 2 and CO have a relatively high NO x reduction efficiency compared to C 3H 6 and especially C 3H 8 that does not show any NO x reduction ability except at 350 °C over Pd/BaO/Al 2O 3. The type of precious metals affects the NO x storage-reduction properties, where the Pd/BaO/Al 2O 3 catalyst shows both a high storage and a high reduction ability. The Rh/BaO/Al 2O 3 catalyst shows a high reduction ability but a relatively low NO x storage capacity. 相似文献
2.
Catalytic performance of Sn/Al 2O 3 catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO 2 had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al 2O 3 (SG) catalyst, and the maximum NO 2 conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO x desorption accompanied with O 2 around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H 2O and SO 2 at low temperature, and the temperature window was also broadened in the presence of H 2O and SO 2, however the NO x desorption and NO conversion decreased sharply on the 300 ppm SO 2 treated catalyst, the catalytic activity was inhibited by the presence of SO 2 due to formation of sulfate species (SO 42−) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al 2O 3 (SG) was not decreased in the presence of large oxygen. 相似文献
3.
The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al 2O 3 and 1 wt.% Pd/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al 2O 3 catalyst exhibits higher overall NO x reduction activity. Limited amounts of N 2O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al 2O 3 and Pd/Al 2O 3 catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al 2O 3. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity ( i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity ( i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C. 相似文献
4.
The storage and release of NO 2 on alumina-supported barium oxide has been studied with particular attention to the stoichiometry of the two processes. At 400 °C the storage process is characterised by a short period of complete uptake, possibly as nitrito or nitro species, followed by a slower partial uptake in which approximately one NO is released for every three NO 2 lost. The latter reaction appears to supply the oxygen necessary to store NO 2 as nitrate ions. Molecular O 2 has little direct involvement even if in large excess. The second storage reaction also occurs, but to a much lesser extent, with Al 2O 3 alone. During temperature programmed desorption, release of NO x from Al 2O 3 peaks at 430 °C with evolution of NO 2 and some O 2. Release from BaO/Al 2O 3 exhibits an additional peak near 520 °C corresponding to formation of NO and a higher O 2 concentration. The NO may arise from NO 2 since BaO/Al 2O 3 has activity for NO 2 decomposition by 500 °C. Although CO 2 at low concentration is rapidly taken up by BaO/Al 2O 3 at 400 °C it is displaced by NO 2 and does not interfere with storage. Thermodynamic calculations show that the formation of Ba(NO 3) 2 by the reaction of NO 2 with bulk BaCO 3 under the conditions used here is more favourable above 380 °C if NO is evolved than if O 2 is consumed. 相似文献
5.
The reduction of NO x by hydrogen under lean burn conditions over Pt/Al 2O 3 is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al 2O 3. In this case, although pure H 2 and pure CO are ineffective for NO x reduction under lean burn conditions, H 2/CO mixtures are very effective. With a realistic (1:3) H 2:CO ratio, typical of actual exhaust gas, Pd/Al 2O 3 is significantly more active than Pt/Al 2O 3, delivering 45% NO x conversion at 160 °C, compared to >15% for Pt/Al 2O 3 under identical conditions. The nature of the support is also critically important, with Pd/Al 2O 3 being much more active than Pd/SiO 2. Possible mechanisms for the improved performance of Pd/Al 2O 3 in the presence of H 2+CO are discussed. 相似文献
6.
A multi-component NO x-trap catalyst consisting of Pt and K supported on γ-Al 2O 3 was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H 2O and CO 2 on NO x storage. The Al 2O 3 support was shown to have NO x trapping capability with and without Pt present (at 250 °C Pt/Al 2O 3 adsorbs 2.3 μmols NO x/m 2). NO x is primarily trapped on Al 2O 3 in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO x/m 2, and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al 2O 3 catalyst are coordinated on the Al 2O 3 support at saturation. When 5% CO2 was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H2O was included in a feed stream with 300 ppm NO and 12% O2, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al2O3-based nitrates decreased by 92%. Interestingly, with both 5% CO2 and 5% H2O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al2O3 destabilized the K–CO2 bond; specifically, H2O mitigates the NOx storage capacity losses associated with carboxylate competition. 相似文献
7.
The effect of the Pd addition method into the fresh Pd/(OSC + Al 2O 3) and (Pd + OSC)/Al 2O 3 catalysts (OSC material = Ce xZr 1−xO 2 mixed oxides) was investigated in this study. The CO + NO and CO + NO + O 2 model reactions were studied over fresh and aged catalysts. The differences in the fresh catalysts were insignificant compared to the aged catalysts. During the CO + NO reaction, only small differences were observed in the behaviour of the fresh catalysts. The light-off temperature of CO was about 20 °C lower for the fresh Pd/(OSC + Al 2O 3) catalyst than for the fresh (Pd + OSC)/Al 2O 3 catalyst during the CO + NO + O 2 reaction. For the aged catalysts lower NO reduction and CO oxidation activities were observed, as expected. Pd on OSC-containing alumina was more active than Pd on OSC material after the agings. The activity decline is due to a decrease in the number of active sites on the surface, which was observed as a larger Pd particle size for aged catalysts than for fresh catalysts. In addition, the oxygen storage capacity of the aged Pd/(OSC + Al 2O 3) catalyst was higher than that of the (Pd + OSC)/Al 2O 3 catalyst. 相似文献
8.
A mean field model, for storage and desorption of NO x in a Pt/BaO/Al 2O 3 catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al 2O 3; (ii) NO oxidation on Pt/BaO/Al 2O 3; (iii) NO x storage on BaO/Al 2O 3; (iv) NO x storage on Pt/BaO/Al 2O 3 with thermal regeneration and (v) NO x storage on Pt/BaO/Al 2O 3 with regeneration using C 3H 6. In this paper, we focus on the last sub-system. The kinetic model for NO x storage on Pt/BaO/Al 2O 3 was constructed with kinetic parameters obtained from the NO oxidation model together with a NO x storage model on BaO/Al 2O 3. This model was not sufficient to describe the NO x storage experiments for the Pt/BaO/Al 2O 3, because the NO x desorption in TPD experiments was larger for Pt/BaO/Al 2O 3, compared to BaO/Al 2O 3. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO x from Pt/BaO/Al 2O 3. To this NO x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO 2 and C 3H 6. The main reactions for continuous reduction of NO x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments. 相似文献
9.
Well crystallised aluminium borate Al 18B 4O 33 has been synthesised from alumina and boric acid with a BET area of 18 m 2/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al 18B 4O 33 was prepared by conventional impregnation of Pd(NO 3) 2 aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al 18B 4O 33 in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al 2O 3. Pd/Al 18B 4O 33 exhibited a much lower activity than Pd/Al 2O 3 when treated in hydrogen at 500 °C or aged in O 2/H 2O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al 18B 4O 33 which became as active as Pd/Al 2O 3. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al 2O 3 and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al 18B 4O 33. The existence of two distinct types of PdO species formed on Al 18B 4O 33 and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O 2. 相似文献
10.
A quaternary catalyst library of 56 samples comprising all combinations of four elements, viz. Ag, Co, Cu, In, with six equally spaced atomic fraction increments from 0 to 1 was prepared by impregnation of a proprietary mesoporous alumina support. Catalytic properties of the library were tested in the selective catalytic reduction (SCR) of NO x by propane under lean conditions in the temperature range 400–500 °C. The catalytic data acquired by a parallel 64-channel microreactor system with automated time-of-flight mass spectrometric analysis have been evaluated regarding selectivity–compositional relationships, synergistic effects for NO x conversion, and efficiency of propane utilization. Full conversion of NO x is achieved over Ag–Co combinations at 450 °C with N 2 selectivities of more than 90% and reductant utilization of 20% in a feed of 1500 ppm NO, 1500 ppm propane and 5 vol.% O 2 (space velocity of 36,000 cm 3 g cat−1 h −1). For the single-component catalysts Ag/Al 2O 3, Co/Al 2O 3, Cu/Al 2O 3, and In/Al 2O 3, the state of the elements on the mesoporous alumina was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Cobalt forms a spinel-like cobalt aluminate phase whereas copper and indium are present as oxides with small sizes not detectable by XRD. Silver occurs in both metallic state and as Ag 2O, and forms Ag n clusters of at least two different sizes, predominantly with diameters of about 30 nm. The conclusions are consistent with the reducibility of the single-component catalysts samples by H 2. Surface area measurements and pore size distributions revealed reasonable modifications of the textural properties. The main pore size of the alumina support is decreased from 7 to ca. 5 nm after loading of the active components. 相似文献
11.
NO conversion to N 2 in the presence of methane and oxygen over 0.03 at.%Rh/Al 2O 3, 0.51 at.%Pt/Al 2O 3 and 0.34 at.%Pt–0.03 at.%Rh/Al 2O 3 catalysts was investigated. δ-Alumina and precious metal–aluminum alloy phases were revealed by XRD and HRTEM in the catalysts. The results of the catalytic activity investigations, with temperature-programmed as well as steady-state methods, showed that NO decomposition occurs at a reasonable rate on the alloy surfaces at temperatures up to 623 K whereas some CH4 deNOx takes place on δ-alumina above this temperature. A mechanism for the NO decomposition is proposed herein. It is based on NO adsorption on the precious metal atoms followed by the transfer of electrons from alloy to antibonding π orbitals of NO(ads.) molecules. The CH4 deNOx was shown to occur according to an earlier proposed mechanism, via methane oxidation by NO2(ads.) to oxygenates and then NO reduction by oxygenates to N2. 相似文献
12.
For the first time, the coupling of fast transient kinetic switching and the use of an isotopically labelled reactant ( 15NO) has allowed detailed analysis of the evolution of all the products and reactants involved in the regeneration of a NO x storage reduction (NSR) material. Using realistic regeneration times (ca. 1 s) for Pt, Rh and Pt/Rh-containing Ba/Al 2O 3 catalysts we have revealed an unexpected double peak in the evolution of nitrogen. The first peak occurred immediately on switching from lean to rich conditions, while the second peak started at the point at which the gases switched from rich to lean. The first evolution of nitrogen occurs as a result of the fast reaction between H 2 and/or CO and NO on reduced Rh and/or Pt sites. The second N 2 peak which occurs upon removal of the rich phase can be explained by reaction of stored ammonia with stored NO x, gas phase NO x or O 2. The ammonia can be formed either by hydrolysis of isocyanates or by direct reaction of NO and H 2. The study highlights the importance of the relative rates of regeneration and storage in determining the overall performance of the catalysts. The performance of the monometallic 1.1%Rh/Ba/Al2O3 catalyst at 250 and 350 °C was found to be dependent on the rate of NOx storage, since the rate of regeneration was sufficient to remove the NOx stored in the lean phase. In contrast, for the monometallic 1.6%Pt/Ba/Al2O3 catalyst at 250 °C, the rate of regeneration was the determining factor with the result that the amount of NOx stored on the catalyst deteriorated from cycle to cycle until the amount of NOx stored in the lean phase matched the NOx reduced in the rich phase. On the basis of the ratio of exposed metal surface atoms to total Ba content, the monometallic 1.6%Pt/Ba/Al2O3 catalyst outperformed the Rh-containing catalysts at 250 and 350 °C even when CO was used as a reductant. 相似文献
13.
A comparison study was carried out on non-thermal plasma (NTP)-assisted selective catalytic reduction (SCR) of NO x by propene over Ag/USY and Ag/Al 2O 3 catalysts. Ag/USY was almost inactive in thermal SCR while it showed obvious activities in NTP-assisted SCR at 100 °C–200 °C. Although the NO x conversion over Ag/Al 2O 3 was also enhanced at 300 °C–400 °C by the assistance of NTP, it was ineffective below 250 °C. The intermediates over Ag/USY and Ag/Al 2O 3 were investigated using in situ DRIFTS method. It was found that key intermediates in HC-SCR, such as NCO, CN, oxygenates and some N-containing organic species were enriched after the assistance of NTP. The differences in the behaviors of above intermediates were not found between these two kinds of catalysts. However, some evidences suggested that different properties of the absorbed NO x species resulted in the distinction of SCR reactions over Ag/USY and Ag/Al 2O 3. TPD profiles of Ag/Al 2O 3 showed that nitrates formed over the catalyst were quite stable at low temperatures, which might occupy the active sites and were unfavorable to SCR reactions. The nitrates over Ag/USY were unstable, among which the unidentate nitrate species is probably contributed to the SCR reactions at low temperatures. 相似文献
14.
The NO x storage and reduction functions of a Pt–Ba/Al 2O 3 “NO x storage–reduction” catalyst has been investigated in the present work by applying the transient response and the temperature programmed reaction methods, by using propylene as the reducing agent. It is found that: (i) the storage of NO x occurs first at BaO and then at BaCO 3, which are the most abundant sites following regeneration of catalyst with propylene; (ii) the overall storage process at BaCO 3 is slower than at BaO; (iii) CO 2 inhibits the NO x storage at low temperatures; (iv) the amount of NO x stored up to catalyst saturation at 350 °C corresponds to 17.6% of Ba; (v) the reduction of stored NO x groups is fast and is limited by the concentration of propylene in the investigated T range (250–400 °C); (vi) selectivity to N 2 is almost complete at 400 °C but is significantly lower at 300 °C due to the formation of NO which can be tentatively ascribed to the presence of unselective Pt–O species. 相似文献
15.
Performance of NO x traps after high-temperature treatments in different redox environments was studied. Two types of treatments were considered: aging and pretreatment. Lean and rich agings were examined for a model NO x trap, Pt–Ba/Al 2O 3. These were done at 950 °C for 3 h, in air and in 1% H 2/N 2, respectively. Lean aging had a severe impact on NO x trap performance, including HC and CO oxidation, and NH 3 and N 2O formation. Rich aging had minimal impact on performance, compared to fresh/degreened performance. Deactivation from lean aging was essentially irreversible due to Pt sintering, but Pt remained dispersed with the rich aging. Pretreatments were examined for a commercially feasible fully formulated NO x trap and two model NO x traps, Pt–Ba/Al 2O 3 and Pt–Ba–Ce/Al 2O 3. Pretreatments were done at 600 °C for 10 min, and used feed gas that simulated diesel exhaust under several conditions. Lean pretreatment severely suppressed NO x, HC, CO, NH 3 and N 2O activities for the ceria-containing NO x traps, but had no impact on Pt–Ba/Al 2O 3. Subsequently, a relatively mild rich pretreatment reversed this deactivation, which appears to be due to a form of Pt–ceria interaction, an effect that is well known from early work on three-way catalysts. Practical applications of results of this work are discussed with respect to NO x traps for light-duty diesel vehicles. 相似文献
16.
The reaction between hydrogen and NO was studied over 1 wt.% Pd supported on NO x-sorbing material, MnO x–CeO 2, at low temperatures. The result of pulse mode reactions suggest that NO x adsorbed as nitrate and/or nitrite on MnO x–CeO 2 was reduced by hydrogen, which was spilt-over from Pd catalyst. The NO x storage and reduction (NSR) cycles were carried out over Pd/MnO x–CeO 2 in a conventional flow reactor at 150 °C. In a storage step, NO was removed by the oxidative adsorption from a stream of 0.04–0.08% NO, 5–10% O 2, and He balance. This was followed by a reducing step, where a stream of 1% H 2/He was supplied to ensure the conversion of nitrate/nitrite to N 2 and thus restore the adsorbability. It was revealed that the NSR cycle is much more suitable for the H 2–deNO x process in excess O 2, compared to a conventional steady state reaction mode. 相似文献
17.
A small single-cylinder diesel engine is used to thermally age model (Pt + Rh/Ba/γ-Al 2O 3) lean NO x traps (LNTs) under lean/rich cycling at target temperatures of 600 °C, 700 °C, and 800 °C. During an aging cycle, fuel is injected into the exhaust to achieve reproducible exotherms under lean and rich conditions with the average temperature approximating the target temperature. Aging is performed until the cycle-average NO x conversion measured at 400 °C is approximately constant. Engine-based NO x conversion decreased by 42% after 60 cycles at 600 °C, 36% after 76 cycles at 700 °C and 57% after 46 cycles at 800 °C. The catalyst samples were removed and characterized by XRD and using a microreactor that allowed controlled measurements of surface area, precious metal size, NO x storage, and reaction rates. Three aging mechanisms responsible for the deactivation of LNTs have been identified: (i) loss of dispersion of the precious metals, (ii) phase transitions in the washcoat materials, and (iii) loss of surface area of the storage component and support. These three mechanisms are accelerated when the aging temperature exceeds 850 °C—the γ to δ transition temperature of Al 2O 3. Normalization of rates of NO reacted at 400 °C to total surface area demonstrates the biggest impact on performance stems from surface area losses rather than from precious metal sintering. 相似文献
18.
The objective of this work was to study the promotional effect of Pt on Co-zeolite (viz. mordenite, ferrierite, ZSM-5 and Y-zeolite) and Co/Al 2O 3 on the selective catalytic reduction (SCR) of NO x with CH 4 under dry and wet reaction stream. After being reduced in H 2 at 350°C, the PtCo bimetallic zeolites showed higher NO to N 2 conversion and selectivity than the monometallic samples, as well as a combination of the latter samples such as mechanical mixtures or two-stage catalysts. After the same pretreatment, under wet reaction stream, the bimetallic samples were also more active. Among the other catalysts studied with 5% of water in the feed, (NO = CH 4 = 1000 ppm, O 2 = 2%), the NO conversion dropped to zero over Co 2.0Mor at 500°C and GHSV = 30,000 h −1, whereas it is 20% in Pt 0.5Co 2.0Mor. In Pt/Co/Al 2O 3 the NO x conversion dropped below 5% with only 2% of water under the same reaction conditions. The specific activity given as molecules of NO converted per total metal atom per second were 16.5 × 10 −4 s −1 for Pt 0.5Co 2.0Fer, 13 × 10 −4 s −1 for Pt 0.5Co 2.0Mor, 4.33 × 10 −4 s −1 for Pt 0.5Co 2.0ZSM-5 and 0.5 × 10 −4 s −1 for Pt/Co/Al 2O 3. The Y-zeolite-based samples were inactive in both mono and bimetallic samples. The species initially present in the solid were Pt° and Co°, together with Co 2+ and Pt 2+ at exchange positions. Co° seems not to participate as an active site in the SCR of NO x. Those species remained after the reaction but some reorganization occurred. A synergetic effect among the different species that enhances both the NO to NO 2 reaction, the activation of CH 4 and also the ability of the catalyst to adsorb NO, could be responsible for the high activity and selectivity of the bimetallic zeolites. 相似文献
19.
A catalytic deSoot–deNO x system, comprising Pt and Ce fuel additives, a Pt-impregnated wall-flow monolith soot filter and a vanadia-type monolithic NH 3-SCR catalyst, was tested with a two-cylinder DI diesel engine. The soot removal efficiency of the filter was 98–99 mass% with a balance temperature (stationary pressure drop) of 315 °C at an engine load of 55%. The NO x conversion ranged from 40 to 73%, at a NH 3/NO x molar ratio of 0.9. Both systems were measured at a GHSV of 52 000 l/(l h). The maximum NO x conversion was obtained at 400 °C. The reason for the moderate deNO x performance is discussed. No deactivation was observed after 380 h time on stream. The NO x emission at high engine loads is around 15% lower than that of engines running without fuel additives. 相似文献
20.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
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