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1.
Bin Wen 《Fuel》2002,81(14):1841-1846
The NO SCR (selective catalytic reduction) activity with H2 in the presence of excess O2 was investigated over Pd/MFI catalyst prepared by sublimation method. With GHSV=90?000 h−1, a very high steady-state conversion of NO to N2 (70%) is achieved at 100 °C. Significant reorganizations take place inside the catalyst upon its first contact with all reactants and products at the reaction temperature. Pd0, which has a significant role in the NO-H2-O2 reaction, is possibly the active site for NO reduction. The formation of Pd-β hydride deactivates the catalyst for NO reduction. Throughout the entire NO-H2-O2 reaction, no N2O or NO2 is formed; N2 is the only N-containing product. The presence of O2 inhibits the formation of undesirable NH3. The rate of the NO+H2 reaction is fast or comparable to that of the H2+O2 reaction. The oxidation of Pd0 and subsequent agglomeration of PdO are responsible for the decreased NO reduction activity at high temperature.  相似文献   

2.
The rates and product selectivities of the C3H6-NO-O2 and NO-H2 reactions over a Pt/Al2O3 catalyst, and of the straight, NO decomposition reaction over the reduced catalyst have been compared at 240C. The rate of NO decomposition over the reduced catalyst is seven times greater than the rate of NO decomposition in the C3H6-NO-O2 reaction. This is consistent with a mechanism in which NO decomposition occurs on Pt sites reduced by the hydrocarbon, provided only that at steady state in the lean NO x reaction about 14% of the Pt sites are in the reduced form. However, the (extrapolated) rate of the NO-H2 reaction at 240C is about 104 times faster than the rate of the NO decomposition reaction thus raising the possibility that NO decomposition in the former reaction is assisted by Hads. It is suggested that adsorbate-assisted NO decomposition in the C3H6-NO-O2 reaction could be very important. This would mean that the proportion of reduced Pt sites required in the steady state would be extremely small. The NO decomposition and the NO-H2 reactions produce no N2O, unlike the C3H6-NO-O2 reaction, suggesting that adsorbed NO is completely dissociated in the first two cases, but only partially dissociated in the latter case. It is possible that some of the associatively adsorbed NO present during the C3H6-NO-O2 reaction may be adsorbed on oxidised Pt sites.  相似文献   

3.
The NO-H2 reaction has been studied over a Pt(100) single crystal surface as a function of temperature and partial pressures of the reactants. The activity as well as the selectivity, shows oscillatory behaviour under isothermal conditions from 420 K to 520 K. The oscillations observed for the formation rates of N2 and NH3 are out of phase with those found for the formation rate of N2O. These observations are in line with recently proposed mechanisms for the formation of N2, NH3 and N2O.  相似文献   

4.
The NO reduction by H2 on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andP NO = –1.5 ×10–1 Torr andP H2 =1×10-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H2 and NO-NH3 reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.  相似文献   

5.
Formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 have been studied in aqueous acidic medium over Pd/SiO2 catalyst in presence of different halide ions (viz. F, Cl and Br). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H2O2 formation selectivity in the H2 to H2O2 oxidation over the catalyst. The H2O2 destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br was found to selectivity promote the conversion of H2 to H2O2 by significantly reducing the H2O2 decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H2O2 formation by promoting the H2 combustion to water and/or by increasing the rate of decomposition/hydrogenation of H2O2 over the catalyst. An optimum concentration of Br ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H2O2 yield in the direct synthesis.  相似文献   

6.
The catalytic dehydro-aromatization reaction over Mo/HZSM-5 catalyst was drastically stabilized by the co-addition of 5.4% H2 and 1.8% H2O to methane feed at 750 °C, 0.3 MPa and methane space velocity of 3000 mL g−1 h−1, suppressing the coke formation effectively, compared with single hydrogen or steam addition.  相似文献   

7.
This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H2 over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H2 reaction Pt/Al2O3>Pd/Al2O3>Rh/Al2O3 and for the NO+CO reaction Rh/Al2O3>Pd/Al2O3> Pt/Al2O3. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H2 reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al2O3 during the NO+H2 reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N2O) irrespective of the nature of the reducing agent (CO or H2). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N2 via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H2+O2 reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.  相似文献   

8.
Rh-Sn/SiO2 catalysts prepared by the reaction of (CH3)4Sn with Rh metal particles supported on SiO2 have remarkably high activities for NO-H2 reaction and NO dissociation. The bimetallic surface structure of Rh-Sn/SiO2 composed of an isolated Rh atom surrounded by six Sn atoms, is presented by Rh K-edge and Sn K-edge EXAFS, FT-IR, TEM and CO adsorption.  相似文献   

9.
The reduction of NO by H2 was studied over three different Pt-Rh single crystal surfaces, i.e. Pt-Rh(111), (410) and (100). The adsorption and dissociation of NO was studied by HREELS, LEED, XPS, AES and TDS. It was found that the dissociation of NO and its reaction with H2 is very surface structure sensitive. The selectivity towards nitrogen and the dissociation activity increases in the same order, i.e. Pt-Rh(111) < (100) < 410). Nitrogen atoms were easily hydrogenated at 400 K in hydrogen to NH x (x = 1 or 2) on the surface. A model is proposed in which the selectivity of the NO-H2 reaction over Pt-Rh surfaces is determined by the relative amounts of hydrogen, NO and nitrogen adatoms on the surface.  相似文献   

10.
Formation of Coo phases with different surface structure over 10 wt% Co/Al2O3 and their catalytic properties were induced by pretreatments in H2 at 570 K for 1 h or 20 h. Electronic behaviour of the Coo phase, which consists of small (after 1 h reduction) or large bulk-like particles (after 20 h reduction), did not change during the CO hydrogenation after 5 h on stream as was determined by XPS. On the basis of the measured C2+ hydrocarbon selectivities the CO molecules are suggested to dissociate on small Co particles to a larger extent than on large cobalt particles. The slight decrease in the catalytic activity with increasing time on stream obtained for the long-term reduced sample is explained by the change in the surface Coo content detected by XPS. The increase in the catalytic activity along with the change in olefin selectivity, measured for the sample reduced for 1 h, is interpreted by the change of a reaction path involving the Coo-support interface during the initial period of the reaction.  相似文献   

11.
CO2 reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of 13C-labeled and unlabeled CO2 to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H2O and with adsorbed CO to form CO2. The H2O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H2O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO2 with increasing reactor residence time. The forward rate of CO2 formation increases with increasing residence time as H2O concentration increases, but the net CO2 formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO2 addition to synthesis gas does not influence CO2 forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO2 could lead to the minimization of CO2 formation during FTS and to the preferential removal of oxygen as H2O. This, in turn, leads to lower average H2/CO ratios throughout the catalyst bed and to higher olefin content and C5+ selectivity among reaction products. The addition of 13CO2 to H2/12CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO2 is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO2 to form hydrocarbons during reactions of H2/CO/CO2 mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO2 as the chemical interconversion of CO and CO2 becomes rapid at WGS reaction equilibrium.  相似文献   

12.
The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO2 catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO2 to CO in the pulse reaction study with 1 ml volume of He/CO2, but the rate of CO formation was significantly enhanced with H2 participating in the reaction. The TPD spectra of CO2 obtained by feeding H2/CO2 over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H2 and CO2 is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.  相似文献   

13.
This study focused on toluene photodecomposition in the presence of H2O over metal (Ba, Al, Si, V, and W)-incorporated TiO2. The nanometer-sized, metal-TiO2 photocatalyst samples, including Ba2+, Al3+, Si4+, V5+, and W6+ ions, were prepared by using the solvothermal method. The X-ray photoelectron spectroscopy (XPS) results showed that the Ti-OH peak, which indicates hydrophilicity, increased with increasing Al and Si ion components but decreased with increasing Ba, V, and W ion components. The contact angles were distributed over the range of 0–10° on almost all films (200-nm thick) after irradiation for 2 h, and in particular approached 0° on the Al-TiO2 and Si-TiO2 nanometer-sized films after just 30 min. The toluene (100 ppm) photodecomposition in the continuous system increased in the order of Al-TiO2>Si-TiO2>pure TiO2>W-TiO2>Ba-TiO2>V-TiO2, and the maximum toluene conversion rate achieved was 45% over Al-TiO2 film after 120 min. The toluene conversion remarkably increased; however, over all photocatalysts, with H2O addition during the toluene photo-decomposed reaction, and in particular, the conversion reached up to 90% after 120 min over Al-TiO2 and Si-TiO2 with increased hydrophilicity. After photoreaction for 24 h, minimal carbon was deposited on the photocatalyst under both reaction conditions, with and without H2O addition, although the deposited carbon amounts were smaller for the former. These results confirmed that the hydrophilicity of the photocatalyst had a greater effect on toluene decomposition, while the photocatalytic deactivation could be retarded by H2O supplementation during toluene decomposition.  相似文献   

14.
In this paper we describe the non-linearity of the NO-H2 reaction over Rh surfaces. Rate oscillations have been observed over a stepped (111) surface with (100) steps, (Rh(533) at low pressures (10?4 Pa) below 500 K, while no oscillations could be observed under these conditions over a Rh(100) surface and a stepped (100) surface with (111) steps, Rh(711). The thermal stability of the N atoms formed during the reaction explains the observed structure sensitivity. Moreover, the results suggest that diffusion of N atoms is needed to synchronise the rate oscillations, a process that is absent on Rh(100) and Rh(711).  相似文献   

15.
The complete photocatalytic oxidation of C2H4 with O2 into CO2 and H2O has been achieved on ultrafine powdered TiO2 photocatalysts and the addition of H2O was found to enhance the reaction. The photocatalytic reaction has been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C2H4 with O2 into CO2, CO, and H2O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C2H4. The photoformed OH species as well as O 2 and O 3 anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C2H4 with O2 into CO2 and H2O. Interestingly, small amount of Pt addition to the TiO2 photocatalyst increased the amount of selective formation of CO2 which was the oxidation product of C2H4 and O2.  相似文献   

16.
The reduction of lean NOx using ethanol in simulated diesel engine exhaust was carried out over Ag/Al2O3 catalysts in the presence of H2O and SO2. The Ag/Al2O3 catalysts are highly active for the reduction of lean NOx by ethanol but the reaction is accompanied by side reactions to form CH3CHO, CO along with small amounts of hydrocarbons (C3H6, C2H4, C2H2 and CH4) and nitrogen compounds such as NH3 and N2O. The presence of H2O enhances the NOx reduction while SO2 suppresses the reduction. The presence of SO2 along with H2O suppresses the formation of acetaldehyde and NH3. By infrared spectroscopy, it was revealed that the reactivity of NCO species formed in the course of the reaction was greatly enhanced in the presence of H2O. The NCO species readily reacts with NO in the presence of O2 and H2O at room temperature, being converted to N2 and CO2 (CO). Addition of SO2 suppresses the formation of NCO species and lowers the reactivity of the NCO species. However, the reduction of NOx is still kept at high conversion levels in the presence of H2O and SO2 over the present catalysts. About 80% of NOx in the simulated diesel engine exhaust was removed at 743 K. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

17.
Methoxy formed on Al2O3 from13CO and H2 coadsorption on Ni/Al2O3 was trapped by C2H5OH adsorption and temperature-programmed reaction (TPR). The presence of excess C2H5OH significantly increases the rate of13CH3OH and (13CH3)2O formation. The13CH3OH forms by the reaction of C2H5OH with13CH3O on Al2O3. In the absence of C2H5OH,TPR following13CO and H2 coadsorption did not produce significant amounts of13CH3OHor(13CH3)2O.  相似文献   

18.
In situ FTIR spectroscopy was used to characterize the oxygen adspecies and its reactivity with CH4 over LaOF and 15 mol% BaF2/LaOF catalysts at OCM temperature (750-800°C). It was found that gas-phase oxygen was activated on the surface of LaOF and 15 mol% BaF2/LaOF, which had been pretreated under vacuum at 750 or 800°C, forming O 2 - species at high temperature (750-800°C). At 750°C, the adsorbed O 2 - species can react with pure CH4 accompanied by formation of gas-phase C2H4 and CO2, and there is a good correlation between the rate of disappearance of surface O2and the rate of formation of gas-phase C2H4. The O 2 - species was also observed over the catalysts under working condition, and it reacted with CH4 in a manner that was consistent with its role in a catalytic cycle. These results suggest that O 2 - may be the active oxygen species for OCM reaction over these catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

19.
The effect of water on the sulphation of limestone   总被引:1,自引:0,他引:1  
Chunbo Wang  Yewen Tan 《Fuel》2010,89(9):2628-2632
A series of tests was conducted in a thermogravimetric analyzer (TGA) to study the sulphation behaviour of limestone in the presence of water over the temperature range of 800-850 °C. Four different Canadian limestones, all with a particle size range of 75-425 μm, were sulphated using a synthetic flue gas with a composition of 15% CO2, 3% O2, 0% or 10% H2O, 1750 ppm SO2 and the balance N2. Water was shown to have a significant promotional effect on sulphation, especially in the diffusion-controlled stage. However, the effect of water during the kinetic-controlled stage appeared to be much less pronounced. Based on these results, it is proposed that the presence of water leads to the transient formation of Ca(OH)2 as an intermediate, which in turn reacts with SO2 at a faster rate than CaO does. Alternatively stated, it appears that H2O acts as catalyst for the sulphation reaction of CaO.  相似文献   

20.
The kinetic behavior in the direct synthesis of H2O2 with Pd–Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation (DP) methods on α-Al2O3 asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br, pH) determine the selectivity in H2O2 formation, influencing the rate of the consecutive reduction of H2O2 (which is faster with respect to H2O2 decomposition on the metal surface) and/or of direct H2 + O2 conversion to H2O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel and consecutive to H2O2 formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics. Increasing the noble metal loading on the membrane not only increases the productivity to H2O2, but also the selectivity, due to the formation of larger, less defective, Pd particles.  相似文献   

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