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1.
The oxidation state of palladium on SiO 2–Al 2O 3 used for propane combustion was examined by XPS and XRD, and the correlation of the catalytic activity with the oxidation state of palladium was systematically studied. The propane conversion over 5 wt% Pd/SiO 2–Al 2O 3 was measured in the range 1.0≤ S≤7.2 ( S is defined as [O 2]/5[C 3H 8] based on stoichiometric ratio). The propane conversion strongly depended on the S value and reached the maximum at S=5.5. The oxidation state of palladium also changed with the S value; palladium particles were more oxidized under the reaction mixture of higher S value. On the sample used for the reaction at S=5.5, both of metallic palladium and palladium oxide were found. It is concluded that partially oxidized palladium which has optimum ratio of metallic palladium to palladium oxide shows the highest catalytic activity in propane combustion. 相似文献
2.
The selective catalytic reduction (SCR) of NO x assisted by propene is investigated on Pd/Ce 0.68Zr 0.32O 2 catalysts (Pd/CZ), and is compared, under identical experimental conditions, with that found on a Pd/SiO 2 reference catalyst. Physico-chemical characterisation of the studied catalysts along with their catalytic properties indicate that Pd is not fully reduced to metallic Pd for the Pd/CZ catalysts. This study shows that the incorporation of Pd to CZ greatly promotes the reduction of NO in the presence of C 3H 6. These catalysts display very stable deNO x activity even in the presence of 1.7% water, the addition of which induces a reversible deactivation of about 10%. The much higher N 2 selectivity obtained on Pd/CZ suggests that the lean deNO x mechanism occurring on these catalysts is different from that occurring on Pd 0/SiO 2. A detailed mechanism is proposed for which CZ achieves both NO oxidation to NO 2 and NO decomposition to N 2, whereas PdO x activates C 3H 6 via ad-NO 2 species, intermediately producing R-NO x compounds that further decompose to NO and C xH yO z. The role of the latter oxygenates is to reduce CZ to provide the catalytic sites responsible for NO decomposition. The proposed C 3H 6-assisted NO decomposition mechanism stresses the key role of NO 2, R-NO x and C xH yO z as intermediates of the SCR of NO x by hydrocarbons. 相似文献
3.
Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH 4 and C 3H 8, in order to investigate the effect of the concentration of the fuels, O 2 and H 2O on NO x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr 0.8La 0.2MnAl 11O 19− prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO x emission capability. NO x emission at 1500 °C was increased linearly with O 2 concentration, whereas water vapor addition decreased NO x emission in CH 4 combustion over the Sr 0.8La 0.2MnAl 11O 19− catalyst. In the catalytic combustion of C 3H 8 over the Sr 0.8La 0.2MnAl 11O 19− catalyst, the amount of NO x emitted was raised in the temperature range between 1000 and 1500 °C when the C 3H 8 concentration increased from 1 to 2 vol.%. It was found that NO x emission in this temperature range was reduced effectively by adding water vapor. 相似文献
4.
A series of La(Co, Mn, Fe) 1−x(Cu, Pd) xO 3 perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N 2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O 2, NO + O 2, C 3H 6, in the absence or presence of 5% H 2O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C 3H 6, 1% O 2, 0 or 10% H 2O, and 50,000 h −1 space velocity. The objective was to investigate the influence of H 2O addition on catalytic behavior. A good performance (100% NO conversion, 77% N 2 yield, and 90% C 3H 6 conversion) was achieved at 600 °C over LaFe 0.8Cu 0.2O 3 under a dry feed stream. With the exposure of LaFe 0.8Cu 0.2O 3 to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N 2 yield, and 86% C 3H 6 conversion. A competitive adsorption between H 2O vapor with O 2 and NO molecules at anion vacancies over LaFe 0.8Cu 0.2O 3 was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C 3H 6 + NO + O 2 reaction. This H 2O deactivation was also verified to be strictly reversible by removing steam from the feed. 相似文献
5.
The vapor-phase selective oxidation of propylene (H 2CCHCH 3) to acrolein (H 2CCHCHO) was investigated over supported V 2O 5/Nb 2O 5 catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO x species on the Nb 2O 5 support in the sub-monolayer region (<8.4 V/nm 2). Surface allyl species (H 2CCHCH 2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO x coverage. Two surface VO x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O 2 partial pressures. C 3H 6-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O 2 was critical for maintaining the surface VO x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond. 相似文献
6.
The effect of the nature and distribution of VO x species over amorphous and well-ordered (MCM-41) SiO 2 as well as over γ-Al 2O 3 on their performance in the oxidative dehydrogenation of propane with O 2 and N 2O was studied using in situ UV–vis, ex situ XRD and H 2-TPR analysis in combination with steady-state catalytic tests. As compared to the alumina support, differently structured SiO 2 supports stabilise highly dispersed surface VO x species at higher vanadium loading. These species are more selective over the latter materials than over V/γ-Al 2O 3 catalysts. This finding was explained by the difference in acidic properties of silica- and alumina-based supports. C 3H 6 selectivity over V/γ-Al 2O 3 materials is improved by covering the support fully with well-dispersed VO x species. Additionally, C 3H 6 selectivity over all materials studied can be tuned by using an alternative oxidising agent (N 2O). The improving effect of N 2O on C 3H 6 selectivity is related to the lower ability of N 2O for catalyst reoxidation resulting in an increase in the degree of catalyst reduction, i.e. spatial separation of active lattice oxygen in surface VO x species. Such separation favours selective oxidation over CO x formation. 相似文献
7.
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
8.
The performance of unpromoted and MO x-(M: alkali (earth), transition metal and cerium) promoted Au/Al 2O 3 catalysts have been studied for combustion of the saturated hydrocarbons methane and propane. As expected, higher temperatures are required to oxidize CH 4 (above 400 °C), compared with C 3H 8 (above 250 °C). The addition of various MO x to Au/Al 2O 3 improves the catalytic activity in both methane and propane oxidation. For methane oxidation, the most efficient promoters to enhance the catalytic performance of Au/Al 2O 3 are FeO x and MnO x. For C 3H 8 oxidation a direct relationship is found between the catalytic performance and the average size of the gold particles in the presence of alkali (earth) metal oxides. The effect of the gold particle size becomes less important for additives of the type of transition metal oxides and ceria. The results suggest that the role of the alkali (earth) metal oxides is related to the stabilization of the gold nanoparticles, whereas transition metal oxide and ceria additives may be involved in oxygen activation. 相似文献
9.
The performance of the active catalyst 5%V 2O 5-1.9%MgO/TiO 2 in propane oxidative dehydrogenation is investigated under various reactant contact modes: co-feed and redox decoupling using fixed bed and co-feed using fluid bed. Using fixed bed reactor under co-feed conditions, propane is activated easily on the catalyst surface with selectivities ranging from 30 to 75% depending on the degree of conversion. Under varying oxygen partial pressures, especially for higher than the stoichiometric ratio O 2/C 3H 8 = 1/2, nor the propane conversion or the selectivities to propene and COx are affected. The performance of the catalyst in the absence of gas phase oxygen was tested at 400 °C. It was confirmed that the catalyst surface oxygen participates to the activation of propane forming propene and oxidation products with similar selectivities as those obtained under co-feed conditions. The ability of the catalyst to fully restore its activity by oxygen treatment was checked in repetitive reduction–oxidation cycles. Fluid bed reactor using premixed propane–oxygen mixtures was also employed in the study. The catalyst was proved to be very active in the temperature range 300–450 °C attaining selectivities comparable to those of fixed bed. 相似文献
10.
Pd/Nb 2O 5/Al 2O 3 catalysts were investigated on propane oxidation. Diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analysis suggested that monolayer coverage was attained between 10 and 20 wt.% of Nb 2O 5. Temperature programmed reduction (TPR) evidenced the partial reduction of niobium oxide. The maximum propane conversion observed on the Pd/10% Nb 2O 5/Al 2O 3 corresponded to the maximum Nb/Al surface ratio. The presence of NbO x polymeric structures near to the monolayer could favor the ideal Pd 0/Pd 2+ surface ratio to the propane oxidation which could explain the promoting effect of niobium oxide. 相似文献
11.
The adsorption of HCN on, its catalytic oxidation with 6% O 2 over 0.5% Pt/Al 2O 3, and the subsequent oxidation of strongly bound chemisorbed species upon heating were investigated. The observed N-containing products were N 2O, NO and NO 2, and some residual adsorbed N-containing species were oxidized to NO and NO 2 during subsequent temperature programmed oxidation. Because N-atom balance could not be obtained after accounting for the quantities of each of these product species, we propose that N 2 and was formed. Both the HCN conversion and the selectivity towards different N-containing products depend strongly on the reaction temperature and the composition of the reactant gas mixture. In particular, total HCN conversion reaches 95% above 250 °C. Furthermore, the temperature of maximum HCN conversion to N 2O is located between 200 and 250 °C, while raising the reaction temperature increases the proportion of NO x in the products. The co-feeding of H 2O and C 3H 6 had little, if any effect on the total HCN conversion, but C 3H 6 addition did increase the conversion to NO and decrease the conversion to NO 2, perhaps due to the competing presence of adsorbed fragments of reductive C 3H 6. Evidence is also presented that introduction of NO and NO 2 into the reactant gas mixture resulted in additional reaction pathways between these NO x species and HCN that provide for lean-NO x reduction coincident with HCN oxidation. 相似文献
12.
The interaction of sulfur dioxide with a commercial NO x storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO 2 were identified. Under lean conditions (exposure to SO 2 and O 2) at 350 °C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NO x storage capacity. Under fuel rich conditions (SO 2/C 3H 6) at 350–500 °C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides. 相似文献
13.
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
14.
A methodical basis of the evolutionary method for selection and optimization of heterogeneous catalytic materials was developed. For validation, the oxidative dehydrogenation of propane was used as a model reaction. Various oxides (V 2O 5, MoO 3, MnO 2, Fe 2O 3, GaO, MgO, B 2O 3, La 2O 3) were chosen as primary components for the generation of catalytic materials. The first generation consisting of 56 catalytic materials was created by combination of the primary components in a stochastic manner. The materials of each preceding generation were selected based on the catalytic results obtained and subjected to an evolutionary procedure applying mutation and crossover operators to create further generations of catalytic materials of different qualitative and quantitative compositions. For illustration, four generations were created with a total number of tested catalytic materials of 224. As a result of the preliminary optimization procedure an increase in the propene yield was achieved with increasing number of generations; the results can be certainly improved by screening further generations of catalytic materials. Under standard conditions used for testing ( T=500°C, C 3H 8/O 2=3, p(C 3H 8)=30 Pa), the highest C 3H 6 yield amounted to 9.0% ( S=57.4%) in the 3rd generation on V 0.22Mg 0.47Mo 0.11Ga 0.20O x. 相似文献
15.
La 1−xSr xMnO 3 ( x=0, 0.1, 0.3, 0.5, 0.7) perovskite-type oxides (PTOs) were prepared by coprecipitation under various calcination temperature, and their performances for the NO reduction were evaluated under a simulated exhaust gas mixture. The X-ray diffraction (XRD) and thermogravimetric analysis were carried out to find the formation process of the perovskite. The NO reduction rate under different reaction temperature, the concentration of oxygen and the presence of hydrocarbon were observed by the input/output analysis. In the presence of 10% excess oxygen, the catalyst La 0.7Sr 0.3MnO 3 calcined at 900 °C showed a NO reduction rate of 61% at 380 °C. The study of the reaction curves showed that C 3H 8 could act as the reducer for the NO reduction below 400 °C. The NO reduction is highly affected by increasing the O 2 concentration from 0.5 to 10%, especially at high temperatures when oxygen becomes more competitive than NO on the oxidation of C 3H 8, leading to a decrease of the NO reduction from 100% to zero at 560 °C. 相似文献
16.
CeO 2 and CeReO x_ y catalysts are prepared by the calcination at different temperatures ( y = 500–1000 °C) and having a different composition (Re = La 3+ or Pr 3+/4+, 0–90 wt.%). The catalysts are characterised by XRD, H 2-TPR, Raman, and BET surface area. The soot oxidation is studied with O 2 and NO + O 2 in the tight and loose contact conditions, respectively. CeO 2 sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La 3+ or Pr 3+/4+ hinders the grain growth of CeO 2 and, thereby, improving the surface catalytic properties. Using O 2 as an oxidant, an improved soot oxidation is observed over CeLaO x_ y and CePrO x_ y in the whole dopant weight loading and calcination temperature range studied, compared with CeO 2. Using NO + O 2, the soot conversion decreased over CeLaO x_ y catalysts calcined below 800 °C compared with the soot oxidation over CeO 2_ y. CePrO x_ y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O 2. The improvement in the soot oxidation activity over the various catalysts with O 2 can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO x_ y with NO + O 2 is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO x_ y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO 2 oxidation activity, that determines soot oxidation activity, is improved over all CePrO x catalysts. 相似文献
17.
Nanoparticles of Ce xZr 1−xO 2 ( x = 0.75, 0.62) were prepared by the oxidation-coprecipitation method using H 2O 2 as an oxidant, and characterized by N 2 adsorption, XRD and H 2-TPR. Ce xZr 1−xO 2 prepared had single fluorite cubic structure, good thermal stability and reduction property. With the increasing of Ce/Zr ratio, the surface area of Ce xZr 1−xO 2 increased, but thermal stability of Ce xZr 1−xO 2 decreased. The surface area of Ce 0.62Zr 0.38O 2 was 41.2 m 2/g after calcination in air at 900 °C for 6 h. TPR results showed the formation of solid solution promoted the reduction of CeO 2, and the reduction properties of Ce xZr 1−xO 2 were enhanced by the cycle of TPR-reoxidation. The Pd-only three-way catalysts (TWC) were prepared by the impregnation method, in which Ce 0.75Zr 0.25O 2 was used as the active washcoat and Pd loading was 0.7 g/L. In the test of Air/Fuel, the conversion of C 3H 8 was close to 100% and NO was completely converted at λ < 1.025. The high conversion of C 3H 8 was induced by the steam reform and dissociation adsorption reaction of C 3H 8. Pd-only catalyst using Ce 0.75Zr 0.25O 2 as active washcoat showed high light off activity, the reaction temperatures ( T50) of 50% conversion of CO, C 3H 8 and NO were 180, 200 and 205 °C, respectively. However, the conversions of C 3H 8 and NO showed oscillation with continuously increasing the reaction temperature. The presence of La 2O 3 in washcoat decreased the light off activity and suppressed the oscillation of C 3H 8 and NO conversion. After being aged at 900 °C for 4 h, the operation windows of catalysts shifted slightly to rich burn. The presence of La 2O 3 in active washcoat can enhance the thermal stability of catalyst significantly. 相似文献
18.
The catalytic performance and the behavior of NO x storage and reduction (NSR) over a model catalyst for lean-burn gasoline engines have been mainly investigated and be discussed based on the temperature and reducing agents use in this study. The experimental results have shown that the NO x storage amount in the lean atmosphere was the same as the NO x reduction amount from the subsequent rich spike (RS) above the temperature of 400 °C, while the former was greater than the latter below the temperature of 400 °C. This indicated that when the temperature was below 400 °C compared with the NO x storage stage, the reduction of the stored NO x is somehow restricted. We found that the reduction efficiencies with the reducing agents decrease in the order H 2 > CO > C 3H 6 below 400 °C, thus not all of the NO x storage sites could be fully regenerated even using an excessive reducing agent of CO or C 3H 6, which was supplied to the NSR catalyst, while all the NO x storage sites could be fully regenerated if an adequate amount of H 2 was supplied. We also verified that the H 2 generation more favorably occurred through the water gas shift reaction than through the steam reforming reaction. This difference in the H 2 generation could reasonably explain why CO was more efficient for the reduction of the stored NO x than C 3H 6, and hinted as a promising approach to enhance the low-temperature performance of the current NSR catalysts though promoting the H 2 generation reaction. 相似文献
19.
The mechanism of the NO/C 3H 6/O 2 reaction has been studied on a Pt-beta catalyst using transient analysis techniques. This work has been designed to provide answers to the volcano-type activity behaviour of the catalytic system, for that reason, steady state transient switch (C 3H 6/NO/O 2 → C 3H 6/Ar/O 2, C 3H 6/Ar/O 2 → C 3H 6/NO/O 2, C 3H 6/NO/O 2 → Ar/NO/O 2, Ar/NO/O 2 → C 3H 6/NO/O 2, C 3H 6/NO/O 2 → C 3H 6/NO/Ar and C 3H 6/NO/Ar → C 3H 6/NO/O 2) and thermal programmed desorption (TPD) experiments were conducted below and above the temperature of the maximum activity ( Tmax). Below Tmax, at 200 °C, a high proportion of adsorbed hydrocarbon exists on the catalyst surface. There exists a direct competition between NO and O 2 for Pt free sites which is very much in favour of NO, and therefore, NO reduction selectively takes place over hydrocarbon combustion. NO and C 3H 6 are involved in the generation of partially oxidised hydrocarbon species. O 2 is essential for the oxidation of these intermediates closing the catalytic cycle. NO 2 is not observed in the gas phase. Above Tmax, at 230 °C, C 3H 6 ads coverage is negligible and the surface is mainly covered by O ads produced by the dissociative adsorption of O 2. NO 2 is observed in gas phase and carbon deposits are formed at the catalyst surface. From these results, the state of Pt-beta catalyst at Tmax is inferred. The reaction proceeds through the formation of partially oxidised active intermediates (CxHyOzNw) from C 3H 6 ads and NO ads. The combustion of the intermediates with O 2(g) frees the Pt active sites so the reaction can continue. Temperature has a positive effect on the surface reaction producing active intermediates. On the contrary, formation of NO ads and C 3H 6 ads are not favoured by an increase in temperature. Temperature has also a positive effect on the dissociation of O 2 to form O ads, consequently, the formation of NO 2 is favoured by temperature through the oxygen dissociation. NO 2 is very reactive and produces the propene combustion without NO reduction. These facts will determine the maximum concentration of active intermediates and consequently the maximum of activity. 相似文献
20.
The effect of the addition of a second fuel such as CO, C 3H 8 or H 2 on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO 3/La-γAl 2O 3 catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C 3H 8 and H 2. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H 2≥C 3H 8>CH 4. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH 4 when pure methane or CO/CH 4 mixtures are used. For H 2/CH 4 and C 3H 8/CH 4 mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH 4 homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures. Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners. 相似文献
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