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1.
Isotopic transient tracer techniques have been employed to study heterogeneous hydroformylation on Rh/SiO 2 and Mn–Rh/SiO 2. Pulse injection of D 2 and
allowed tracing of the deuterium and CO incorporation pathway into the aldehyde product. The d 1- and d 2-propionaldehyde responses showed a double-peak, or two-hump, response to the D 2 pulse, while
showed a single-hump response to the
pulse. Analysis of the product responses to the D 2 pulse in CO/H 2/C 2H 4 and CO/H 2/C 2H 4/C 2H 5CHO suggests that the first hump of the d 1- and d 2-propionaldehyde responses was due to rapid H/D exchange with adsorbed propionaldehyde via enol intermediates. The decay of the second hump was due to reaction of adsorbed acyl with spillover hydrogen/deuterium. The
response was due to CO insertion followed by acyl hydrogenation. Compartment modeling of the product responses from the
and D 2 pulse inputs allowed determination of residence times of adsorbed intermediates, surface coverages of adsorbed intermediates, and the elementary rate constants for acyl hydrogenation and CO insertion. Elementary rate constants for acyl hydrogenation determined from this study were consistent with the value calculated by transition state theory (TST). The addition of Mn promoter to Rh/SiO 2 increased coverages of
,
, and
and shifted the rate-limiting step for propionaldehyde formation. Acyl hydrogenation is the rate-limiting step on Rh/SiO 2 while CO insertion and acyl hydrogenation are both kinetically significant on Mn–Rh/SiO 2. 相似文献
2.
We examined the effect of the activation process on the structural and morphological characteristics of a cobalt-based catalyst for Fischer–Tropsch synthesis. A 10 wt.% Co/SiO 2 catalyst prepared by wet impregnation was separately activated under H 2, CO or a H 2/CO mixture. The structural changes during activation from 298 to 773 K were studied by in situ X-ray diffraction. Catalysts were examined by SEM, TEM, XPS and in situ DRIFT-MS. The H 2/CO activation produced redispersion of cobalt particles and simultaneous carbon nanostructures formation. The catalyst showed the highest performance in the Fischer–Tropsch synthesis after the H 2/CO activation. 相似文献
3.
The hydrogenation of CO over an Rh vanadate (RhVO 4) catalyst supported on SiO 2 (RhVO 4/SiO 2) has been investigated after H 2 reduction at 500°C, and the results are compared with those of vanadia-promoted (V 2O 5–Rh/SiO 2) and unpromoted Rh/SiO 2 catalysts. The mean size of Rh particles, which were dispersed by the decomposition of RhVO 4 after the H 2 reduction, was smaller (41 Å) than those (91–101 Å) of V 2O 5–Rh/SiO 2 and Rh/SiO 2 catalysts. The RhVO 4/SiO 2 catalyst showed higher activity and selectivity to C 2 oxygenates than the unpromoted Rh/SiO 2 catalyst after the H 2 pretreatment. The CO conversion of the RhVO 4/SiO 2 catalyst was much higher than that of V 2O 5–Rh/SiO 2 catalyst, and the yield of C 2 oxygenates increased. We also found that the RhVO 4/SiO 2 catalyst can be regenerated by calcination or O 2 treatment at high temperature after the reaction. 相似文献
4.
Concentration jump and isotopic transient experiments show that at H 2/CO = 3, one atmosphere pressure, and 523 K both unsupported cobalt and Rh/SiO 2, have near monolayer coverage of molecular CO but coverage of the primary reactive intermediate below 5% of a monolayer. Modeling the data for unsupported Co requires inclusion of a second reactive surface carbon pool about four times less active than the primary pool. Coverages of both reactive intermediates increase with temperature and with H 2/CO ratio, the latter suggesting that CO dissociation is assisted by hydrogen. On Rh/SiO 2, deactivation appears to be caused by growth of an inactive surface carbon species that blocks sites for the reactive intermediate. Initially, CO dissociation is rate determining over the Rh catalyst. Over the Co catalyst and over Rh after long reaction times or at lower temperature, hydrogcnation of the reactive intermediate produced by CO dissociation has a low enough rate constant to allow the coverage of the intermediate, relative to the rate of reaction, to attain a value which is small but sufficient to slow the dynamic response of 13CH 4 to a CO to 13CO switch. Thus, CO dissociation is not rate determining in these systems. 相似文献
5.
A series of CoO x/Al 2O 3 catalysts was prepared, characterized, and applied for the selective catalytic reduction (SCR) of NO by C 3H 8. The results of XRD, UV–vis, IR, Far-IR and ESR characterizations of the catalysts suggest that the predominant oxidation state of cobalt species is +2 for the catalysts with low cobalt loading (≤2 mol%) and for the catalysts with 4 mol% cobalt loading prepared by sol–gel and co-precipitation. Co 3O 4 crystallites or agglomerates are the predominant species in the catalysts with high cobalt loading prepared by incipient wetness impregnation and solid dispersion. An optimized CoO x/Al 2O 3 catalyst shows high activity in SCR of NO by C 3H 8 (100% conversion of NO at 723 K, GHSV: 10,000 h −1). The activity of the selective catalytic reduction of NO by C 3H 8 increases with the increase of cobalt–alumina interactions in the catalysts. The influences of cobalt loading and catalyst preparation method on the catalytic performance suggest that tiny CoAl 2O 4 crystallites highly dispersed on alumina are responsible for the efficient catalytic reduction of NO, whereas Co 3O 4 crystallites catalyze the combustion of C 3H 8 only. 相似文献
6.
Selective catalytic reduction of NO x by C 3H 6 in the presence of H 2 over Ag/Al 2O 3 was investigated using in situ DRIFTS and GC–MS measurements. The addition of H 2 promoted the partial oxidation of C 3H 6 to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NO x on Ag/Al 2O 3 surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H 2 on the SCR of NO x by C 3H 6 over Ag/Al 2O 3 catalyst. 相似文献
7.
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
8.
The effects of carbon dioxide on the dehydrogenation of C 3H 8 to produce C 3H 6 were investigated over several Cr 2O 3 catalysts supported on Al 2O 3, active carbon and SiO 2. Carbon dioxide exerted promoting effects only on SiO 2-supported Cr 2O 3 catalysts. The promoting effects of carbon dioxide over a Cr 2O 3/SiO 2 catalyst were to enhance the yield of C 3H 6 and to suppress the catalyst deactivation. 相似文献
9.
The decomposition of different hydrocarbons (CH 4, C 2H 6, C 2H 4, C 2H 2, C 3H 8, and C 3H 6) over Ni (5 wt.%)/SiO 2 catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO 2 catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenesacetylene. The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene. 相似文献
10.
Alkali halide added transition metal oxides produced ethylene selectively in oxidative coupling of methane. The role of alkali halides has been investigated for LiCl-added NiO (LiCl/NiO). In the absence of LiCl the reaction over NiO produced only carbon oxides (CO 2 + CO). However, addition of LiCl drastically improved the yield of C 2 compounds (C 2H 6 + C 2H 4). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH 4. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH 4 to C 2H 4 through C 2H 6 (2CH 4 → C 2H 6 → C 2H 4). The study on the oxidation of C 2H 6 over the LiCl/NiO showed that the oxidative dehydrogenation of C 2H 6 to C 2H 4 occurs very selectively, which is the main reason why partial oxidation of CH 4 over LiCl/NiO gives C 2H 4 quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH 4. The catalyst model under working conditions was suggested to be the NiO covered with molten LiCl. XPS studies suggested that the catalytically active species on the LiCl/NiO is a surface compound oxide which has higher valent nickel cations (Ni (2+δ)+ or Ni 3+). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH 4---CD 4 exchange studies suggested that the rate-determining step of the reaction is the abstraction of H from the vibrationally excited methane by the molecular oxygen adsorbed on the surface compound oxide. 相似文献
11.
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
12.
The activity and selectivity of rhenium promoted cobalt Fischer–Tropsch catalysts supported on Al 2O 3, TiO 2 and SiO 2 have been studied in a fixed-bed reactor at 483 K and 20 bar. Exposure of the catalysts to water added to the feed deactivates the Al 2O 3 supported catalyst, while the activity of the TiO 2 and SiO 2 supported catalysts increased. However, at high concentrations of water both the SiO 2 and TiO 2 supported catalyst deactivated. Common for all catalysts was an increase in C 5+ selectivity and a decrease in the CH 4 selectivity by increasing the water partial pressure. The catalysts have been characterized by scanning transmission electron microscope (STEM), BET, H 2 chemisorption and X-ray diffraction (XRD). 相似文献
13.
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
14.
The hydrogenation of CO over mixed oxides (RhVO 4, Rh 2MnO 4) supported on SiO 2 has been studied after H 2 reduction at 300°C and at 500°C, and the results compared with those of unpromoted Rh/SiO 2 catalysts. Rh was more highly dispersed (40 Å) after the decomposition of RhVO 4 by the H 2 reduction than those of Rh 2MnO 4/SiO 2 and unpromoted Rh/SiO 2 catalysts. The activity and the selectivity to C 2 oxygenates of the mixed-oxide catalysts after the H 2 reduction were higher than those of the unpromoted Rh/SiO 2 catalysts, but the activity of the RhVO 4/SiO 2 catalyst increased more dramatically after the decomposition by the H 2 reduction at 300°C, and hence the yield of C 2 oxygenates increased. These results suggest that a strong metal–oxide interaction (SMOI) was induced by the decomposition of the mixed oxides after the H 2 reduction. The catalytic activity and selectivity were reproduced repeatedly by the calcination and reduction treatments of the spent (used) catalyst because of the regeneration of RhVO 4 and redispersion of Rh metal. 相似文献
15.
This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO 2 with C 2H 4 as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C 2H 4/SO 2, and pretreatment are all important factors affecting catalyst activity. Fe 2O 3/γ-Al 2O 3 was found to be the most active catalyst among six γ-Al 2O 3-supported metal oxide catalysts tested. With Fe 2O 3 as the active species, of the supports tested, CeO 2 is the most suitable one. Using this Fe 2O 3/CeO 2 catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C 2H 4/SO 2 is 1:1, and the catalyst presulfidized by H 2+H 2S exhibits a higher performance than those pretreated with H 2 or He. Although the feed concentrations of C 2H 4:SO 2 being 3000:3000 ppm provide a higher conversion of SO 2, the sulfur yield decreases drastically at temperatures above 300 °C. With higher feed concentrations, maximum yield appears at higher temperatures. The C 2H 4 temperature-programmed desorption (C 2H 4-TPD) and SO 2-TPD desorption patterns illustrate that Fe 2O 3/CeO 2 can adsorb and desorb C 2H 4 and SO 2 more easily than can Fe 2O 3/γ-Al 2O 3. Moreover, the SO 2-TPD patterns further show that Fe 2O 3/γ-Al 2O 3 is more seriously inhibited by SO 2. These findings may properly explain why Fe 2O 3/CeO 2 has a higher activity for the reduction of SO 2. 相似文献
16.
In this study, a novel bifunctional catalyst IrFe/Al 2O 3, which is very active and selective for preferential oxidation of CO under H 2-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al 2O 3 catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO 2 formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al 2O 3 support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al 2O 3 > co-IrFe/Al 2O 3 > FeIr/Al 2O 3. The three catalysts were characterized by XRD, H 2-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al 2O 3, the resulting structure (IrFe/Al 2O 3) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O 2 activation on the FeO x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O 2 adsorbed on FeO x sites; the reaction may take place at the interface of Ir and FeO x or via a spill-over process. 相似文献
17.
A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na 3PO 4-Mn/SiO 2/cordierite monolithic catalyst, and an improved Stansch kinetic model was established to calculate the OCMreactions using the computational fluid dynamicsmethod and Fluent software. The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min -1 under standard state, the CH 4/O 2 ratio is 3 and the temperature and pressure is 800 ℃ and 1 atm, respectively. The contour of the characteristic parameters in the catalyst bed was analyzed, such as the species mass fractions, temperature, the heat flux on side wall surface, pressure, fluid density and velocity. The results showed that the calculated valuesmatchedwell with the experimental values on the conversion of CH 4 and the selectivity of products (C 2H 6, C 2H 4, CO,CO 2 and H 2) in the reactor outlet with an error range of ±4%. The mass fractions of CH 4 and O 2 decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and 0.120 at the outlet, where the mass fractions of C 2H 6, C 2H 4, CO and CO 2 were 0.0245, 0.0460, 0.0537 and 0.116, respectively. Due to the existence of laminar boundary layer, the mass fraction contours of each species bent upwards in the vicinity of the boundary layer. The volume of OCM reaction was changing with the proceeding of reaction, and the total moles of products were greater than reactants. The flow field in the catalyst bed maintained constant temperature and pressure. The fluid density decreased gradually from 2.28 kg·m -3 at the inlet of the catalyst bed to 2.18 kg·m -3 at the outlet of the catalyst bed, while the average velocity magnitude increased from 0.108 m·s -1 to 0.120 m·s -1. 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.
In this paper, results concerning the development of sulfur tolerant catalysts for Fischer–Tropsch synthesis (FTS), C 2+ alcohol synthesis, methanol and/or DME synthesis are presented. In the FTS reaction on Fe using H 2-rich syngas such as the biomass-derived syngas, the composition of catalyst pretreatment gas and the addition of MnO on Fe had strong impacts on its sulfur resistance as well as activity. Especially the Fe/MnO catalyst pretreated with CO showed a much lower deactivation rate and a higher FTS activity than an Fe/Cu/K catalyst in the presence of H 2S. For C 2+ alcohol synthesis a novel preparation method was developed for a highly active MoS 2-based catalyst that is well known as the sulfur tolerant catalyst. Besides some metal sulfides were found to show higher CO hydrogenation activities than MoS 2. In particular, both Rh and Pd sulfides were active and selective for the methanol synthesis. Modified Pd sulfide catalyst, i.e. sulfided Ca/Pd/SiO 2, showed an activity that was about 60% of that of a Cu/ZnO/Al 2O 3 catalyst in the absence of H 2S. This catalyst preserved 35% of the initial activity even in the presence of H 2S. The sulfided Ca/Pd/SiO 2 mixed with γ-Al 2O 3 was also available for in situ DME synthesis in the presence of H 2S. 相似文献
20.
Fifteen catalysts were prepared by intermittently impregnating alumina washcoats with water solutions containing La 3+, Co 2+ and PdCl 2−4 ions/complex and calcining them at 550–820°C. The catalysts were evaluated with respect to light-off performance, at stationary and transient feed gas stoichiometry, respectively, and redox characteristics, using NO/CO/C 3H 6/O 2/N 2 gas mixtures to simulate car exhaust. Alumina supported Pd exhibited three-way activity, i.e., simultaneous oxidation of CO and C 3H 6 and reduction of NO in a narrow interval around stoichiometric composition of the feed gas. Compared to Pd alone, addition of La or Co caused a widening of the interval under net reducing conditions. Addition of Co to Pd caused a significant increase in the activities for oxidation of CO and C 3H 6 under stoichiometric conditions. The conversions of CO and C 3H 6 started at about 100 degrees lower temperatures over Co-promoted Pd compared to unpromoted Pd. A marked increase in the activity for the reduction of NO at transient conditions was observed over Co-promoted Pd compared to unpromoted Pd. The catalysts were characterized by X-ray powder diffraction, scanning electron microscopy, and transmission electron microscopy combined with energy-dispersive spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), and specific surface area measurements. Only Co 2+ could be detected by XPS in the surface layers of the Co-containing sample. A significant part of the cobalt is present in forms which can be oxidized and reduced under synthetic car exhaust conditions. These oxidizable/reducible cobalt sites are predominantly active for oxidation of CO and C 3H 6, hence promoting the reduction of NO over Pd by initiating these exothermic reactions in the catalyst. 相似文献
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