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1.
The composition and structure of the product of mixture CH 4+CD 4 oxidative coupling over natural manganese mineral catalyst at 3% and 25% methane conversion in redox mode at 850°C have been determined by IR-Absorption- Reflection spectroscopy technique. At low methane conversion there were ethanes: H 3OCH 3, H 3OCD 3, D 3CCD 3 and ethylenes: H 2CCH 2, H 2CCD 2, D 2CCD 2 only The data obtained showed that the reaction proceeds by gas-phase CH 3, CD 3 radicals coupling and ethane is the primary C 2-product and ethylene is produced by gas-phase conversion of ethane. 相似文献
2.
Selective production of hydrogen by partial oxidation of methanol (CH 3OH + (1/2)O 2 → 2H 2 + CO 2) over Au/TiO 2 catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO 2 catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au 0), non-metallic gold (Au δ+) and Au 2O 3, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O 2/CH 3OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming. 相似文献
3.
Self-standing porous silica thin films with different pore structures were synthesized by a solvent evaporation method and used as photocatalysts for the photocatalytic reduction of CO 2 with H 2O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO 2 and H 2O led to the formation of CH 4 and CH 3OH as well as CO and O 2 as minor products. Such thin films having hexagonal pore structure exhibited higher photocatalytic reactivity than the Ti-MCM-41 powder catalyst even with the same pore structure. From FTIR investigations, it was found that these Ti-containing porous silica thin films had different concentrations of surface OH groups and showed different adsorption properties for the H 2O molecules toward the catalyst surface. Furthermore, the concentration of the surface OH groups was found to play a role in the selectivity for the formation of CH 3OH. 相似文献
4.
Highly dispersed titanium oxide catalysts have been prepared within zeolite cavities as well as in the zeolite framework and utilized as photocatalysts for the reduction of CO 2 with H 2O to produce CH 4 and CH 3OH at 328 K. In situ photoluminescence, ESR, diffuse reflectance absorption and XAFS investigations indicate that the titanium oxide species are highly dispersed within the zeolite cavities and framework and exist in tetrahedral coordination. The charge transfer excited state of the highly dispersed titanium oxide species play a significant role in the reduction of CO 2 with H 2O with a high selectivity for the formation of CH 3OH, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce CH 4, being similar to reactions on the powdered TiO 2 catalysts. Ti-mesoporous molecular sieves exhibit high photocatalytic reactivity for the formation of CH 3OH, its reactivity being much higher than the powdered TiO 2 catalysts. The addition of Pt onto the highly dispersed titanium oxide catalysts promotes the charge separation which leads to an increase in the formation of CH 4 in place of CH 3OH formation. 相似文献
5.
Dispersing La 2O 3 on δ- or γ-Al 2O 3 significantly enhances the rate of NO reduction by CH 4 in 1% O 2, compared to unsupported La 2O 3. Typically, no bend-over in activity occurs between 500° and 700°C, and the rate at 700°C is 60% higher than that with a Co/ZSM-5 catalyst. The final activity was dependent upon the La 2O 3 precursor used, the pretreatment, and the La 2O 3 loading. The most active family of catalysts consisted of La 2O 3 on γ-Al 2O 3 prepared with lanthanum acetate and calcined at 750°C for 10 h. A maximum in rate (mol/s/g) and specific activity (mol/s/m 2) occurred between the addition of one and two theoretical monolayers of La 2O 3 on the γ-Al 2O 3 surface. The best catalyst, 40% La 2O 3/γ-Al 2O 3, had a turnover frequency at 700°C of 0.05 s −1, based on NO chemisorption at 25°C, which was 15 times higher than that for Co/ZSM-5. These La 2O 3/Al 2O 3 catalysts exhibited stable activity under high conversion conditions as well as high CH 4 selectivity (CH 4 + NO vs. CH 4 + O 2). The addition of Sr to a 20% La 2O 3/γ-Al 2O 3 sample increased activity, and a maximum rate enhancement of 45% was obtained at a SrO loading of 5%. In contrast, addition of SO =4 to the latter Sr-promoted La 2O 3/Al 2O 3 catalyst decreased activity although sulfate increased the activity of Sr-promoted La 2O 3. Dispersing La 2O 3 on SiO 2 produced catalysts with extremely low specific activities, and rates were even lower than with pure La 2O 3. This is presumably due to water sensitivity and silicate formation. The La 2O 3/Al 2O 3 catalysts are anticipated to show sufficient hydrothermal stability to allow their use in certain high-temperature applications. 相似文献
6.
Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH 4–O 2 reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N 2 formation in NO–CH 4–O 2 vs. NO–CD 4–O 2 reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH 4+CD 4)–O 2 reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5. A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions. Further, in contrast to Co-ZSM-5, NO x–CH 4–O 2 reaction was much slower than CH 4–O 2 reaction for Pd/H-ZSM-5; the presence of NO x retards the reaction of CH 4 over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH 4 is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO 2 strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH 4–O 2 reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5. 相似文献
7.
The permeabilities of CH 4, CO 2, CH 3OH, H 2O, O 2, and CO through films of Kapton® polyimide were measured at temperatures of 50, 100, 150, 200, and 250°C and pressures below 1 atm. Apparent activation energies for the permeation of the pure components ranged from 31.6kJ mol -1 for CH 4 to nearly 0 for H 2O under the conditions studied. The ideal permselectivity for methanol relative to methane decreased from over 100 to under 10 as the temperature was increased from 50 to 250°C. 相似文献
8.
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
9.
Different Pt and Pd catalysts supported on an activated carbon were prepared by using different metal precursors. Prepared catalysts were pretreated at 400 °C under different atmospheres to decompose the precursor compound and reduce the metal. After pretreatments, the supported catalysts were characterized by H 2 chemisorption, X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy to know their metal dispersion, particle size, distribution and oxidation state. Afterwards, the catalysts were tested in methanol partial oxidation with two different O 2/CH 3OH molar ratios. Results obtained in this reaction were compared with those obtained for methanol decomposition in inert atmosphere. For Pt catalysts, there was an increase in methanol conversion and hydrogen production and a decrease in carbon monoxide production under oxidizing conditions. Both methanol conversion and partial oxidation reactions appear to be sensitive to Pt particle structure in the particle size range studied. Results obtained under oxidizing conditions differed between Pd and Pt catalysts. Finally, catalytic activity in methanol partial oxidation was more affected by Pt than Pd particle size in the size range studied. 相似文献
10.
Regeneration of S-poisoned Pd/Al 2O 3 catalysts for the abatement of methane emissions from natural gas vehicles was addressed in this work. Investigations were devoted to determine the temperature threshold allowing for catalyst reactivation under different CH4 containing atmospheres. Under lean combustion conditions in the presence of excess O2, partial regeneration took place only above 750 °C after decomposition of stable sulphate species adsorbed on the support. Short CH4-reducing, O2-free pulses led to partial catalyst reactivation already at 550 °C and to practically complete regeneration at 600 °C. Also in this case reactivation was associated with SO2 release due to the decomposition of stable support sulphates likely promoted by CH4 activation onto the reduced metallic Pd surface. Rich combustion pulses with CH4/O2 = 2 were equally effective to CH4-reducing pulses in catalyst regeneration. These results suggest that a regeneration strategy based on periodical natural gas pulses fed to the catalyst by a by-pass line might be efficient in limiting the effects of S-poisoning of palladium catalysts for the abatement of CH4 emissions from natural gas engine. 相似文献
11.
制备不同负载量的Pd基和Pt基催化剂,建立催化剂活性评价装置,考察贵金属Pd和Pt负载量对甲烷转化率的影响,结果表明,甲烷转化率最高的Pd和Pt负载质量分数分别为1.25%和2%,相同负载质量分数0.1%时,Pd基催化剂的甲烷转化率优于Pt基催化剂。催化剂的BET比表面积大小不能反映催化剂的催化活性,二者之间无线性关系。 相似文献
12.
Ethanol steam reforming was studied over Ni/Al 2O 3 catalysts. The effect of support (- and γ-Al 2O 3), metal loading and a comparison between conventional H 2 reduction with an activation method employing a CH 4/O 2 mixture was investigated. The properties of catalysts were studied by N 2 physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). After activity tests, the catalysts were analyzed by scanning electron microscopy (SEM) and thermogravimetric analysis (TG/DTA). Ni supported on γ-Al 2O 3 was more active for H 2 production than the catalyst supported on -Al 2O 3. Metal loading did not affect the catalytic performance. The alternative activation method with CH 4/O 2 mixture affected differently the activity and stability of the Ni/γ-Al 2O 3 and the Ni/-Al 2O 3 catalyst. This activation method increased significantly the stability of Ni/-Al 2O 3 compared to H 2 reduction. SEM and TG/DTA analysis indicate the formation of filamentous carbon during the CH 4/O 2 activation step, which is associated with the increasing catalyst activity and stability. The effect of temperature on the type of carbon formed was investigated; indicating that filamentous coke increased activity while encapsulating coke promoted deactivation. A discussion about carbon formation and the influence on the activity is presented. 相似文献
13.
Distinction between geometric and ligand effects has been studied by the infrared study of CO chemisorption on four catalyst samples modified by niobia. According to earlier studies the metal-support interface plays a crucial role in the CH 3OH + D 2 reaction. Platinum and niobia content, oxidative-reductive treatment and chloride content has a great influence of the niobia decorating Pt surface. High temperature reduction does not effect niobia-free sample, while niobia addition decreases the bridged/linear ratio of the adsorbed CO, especially in the SMSI state. Simultaneously the peak positions are shifting towards higher frequencies. Both geometric effect of decoration and ligand effect are observed. The observed effect has been further supported by XRD data for the platinum crystallite sizes and a quantitative parameter for the characterization of SMSI is proposed 相似文献
14.
The effectiveness of Ag/Al 2O 3 catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al 2O 3, and Al(OH) 3 was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, O 2, NO + O 2-temperature programmed desorption (TPD), H 2-temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and activity test, with a focus on the correlation between their redox properties and catalytic behavior towards C 3H 6-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C 3H 6 conversion was achieved over Ag/Al 2O 3 (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al 2O 3 (I) is deemed to be crucial for NO selective reduction into N 2. By contrast, a high C 3H 6 conversion simultaneously with a moderate N 2 yield was observed over Ag/Al 2O 3 (II) prepared from a γ-Al 2O 3 source. The larger particles of Ag mO ( m > 2) crystallites were believed to facilitate the propene oxidation therefore leading to a scarcity of reductant for SCR of NO. An amorphous Ag/Al 2O 3 (III) was obtained via employing a Al(OH) 3 source and 500 °C calcination exhibiting a poor SCR performance similar to that for Ag-free Al 2O 3 (I). A subsequent calcination of Ag/Al 2O 3 (III) at 800 °C led to the generation of Ag/Al 2O 3 (IV) catalyst yielding a significant enhancement in both N 2 yield and C 3H 6 conversion, which was attributed to the appearance of γ-phase structure and an increase in surface area. Further thermo treatment at 950 °C for the preparation of Ag/Al 2O 3 (V) accelerated the sintering of Ag clusters resulting in a severe unselective combustion, which competes with SCR of NO reaction. In view of the transient studies, the redox properties of the prepared catalysts were investigated showing an oxidation capability of Ag/Al 2O 3 (II and V) > Ag/Al 2O 3 (IV) > Ag/Al 2O 3 (I) > Ag/Al 2O 3 (III) and Al 2O 3 (I). The formation of nitrate species is an important step for the deNO x process, which can be promoted by increasing O 2 feed concentration as evidenced by NO + O 2-TPD study for Ag/Al 2O 3 (I), achieving a better catalytic performance. 相似文献
15.
A study of CO oxidation by O 2 over Pt catalysts, promoted by MnO x and CoO x, is described. The activities of Pt/SiO 2, Pt/MnO x/SiO 2 and Pt/CoO x/SiO 2 are compared with commercial Pt/Al 2O 3, Pt/Rh/Al 2O 3 and Pt/CeO x/Al 2O 3 catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO x/SiO 2 > Pt/MnO x/SiO 2, Pt/CeO x/Al 2O 3 > Pt/Al 2O 3, Pt/Rh/Al 2O 3, Pt/SiO 2. Over Pt/CoO x/SiO 2 CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO x/SiO 2 in the CO/O 2 reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO x/SiO 2, Pt/MnO x/SiO 2 or Pt/CeO x/Al 2O 3 catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO x/SiO 2 catalyst in CO oxidation. 相似文献
16.
Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al 2O 3 catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH 4 and reaction products (CO, H 2, and CO 2) were observed. Comparative studies using H 2-TPR, O 2-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al 2O 3 with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO x), that intimately interacting with the Al 2O 3 surface (Rh iO x) and the Rh(AlO 2) y species formed by diffusion of rhodium oxides in to sublayers of Al 2O 3 [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H 2-TPR and O 2-TPD experiments. Among them, the first two species can be easily reduced by H 2 at temperature below 350 °C, while the last one can only be reduced by H 2 at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO x and Rh iO x species can be reduced by CH 4 in the reaction mixture. Compared to the Rh/Al 2O 3 with high Rh loadings, the catalysts with low Rh loadings contain more Rh iO x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al 2O 3 with low Rh loadings can be related to the behaviour of Rh(AlO 2) y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction. 相似文献
17.
The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH 4 traces under lean-burn conditions was studied in the presence or the absence of water or H 2S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H 2S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H 2S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H 2S-poisoned Pd/Al 2O 3 catalyst was studied by TPD in O 2/He. Poisoning species decompose above 873 K as SO 2 and O 2 in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O 2/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H 2S-poisoned Pd/Al 2O 3. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested. 相似文献
18.
The selective catalytic reduction (SCR) of NO by C 3H 6 in excess oxygen was evaluated and compared over Ag/Al 2O 3 and Cu/Al 2O 3 catalysts. Ag/Al 2O 3 showed a high activity for NO reduction. However, Cu/Al 2O 3 showed a high activity for C 3H 6 oxidation. The partial oxidation of C 3H 6 gave surface enolic species and acetate species on the Ag/Al 2O 3, but only an acetate species was clearly observed on the Cu/Al 2O 3. The enolic species is a more active intermediate towards NO + O 2 to yield—NCO species than the acetate species on the Ag/Al 2O 3 catalyst. The Ag and Cu metal loadings and phase changes on Al 2O 3 support can affect the activity and selectivity of Ag/Al 2O 3 and Cu/Al 2O 3 catalysts, but the formation of enolic species is the main reason why the activity of the Ag/Al 2O 3 catalyst for NO reduction is higher than that of the Cu/Al 2O 3 catalyst. 相似文献
19.
在工业二氧化碳加氢制甲醇过程中,硫化氢气体的引入将对该过程中使用的催化剂活性及稳定性带来负面的影响。基于此,采用微反应合成法成功制备了InZrO x和ZnZrO x锆基催化剂,并研究了在二氧化碳加氢反应中,硫化氢气体对锆基催化剂的结构性质及其催化性能的影响规律。结果表明,在T=573 K、p=3.0 MPa和GHSV=18 000 mL/(g cat·h)条件下,仅通入二氧化碳/氢气反应气时,InZrO x和ZnZrO x催化剂的二氧化碳转化率和甲醇选择性分别为7.2%、9.3%和93%、92%。在二氧化碳/氢气原料气中通入体积分数为5×10 -3硫化氢气体时,InZrO x和ZnZrO x催化剂的二氧化碳转化率和甲醇选择性都降为0,这主要是因为硫化氢气体占据了氧空位,导致锆基双金属氧化物催化剂硫中毒失活。当停止通硫化氢气体时,InZrO x和ZnZrO x催化剂的二氧化碳转化率和甲醇选择... 相似文献
20.
Catalysts prepared as bulk VSb 0.1O x and supported V 2O 5/Al 2O 3, V 2O 5-Sb 2O 3/Al 2O 3 and Sb 2O 3/Al 2O 3 (containing 0.5, 1 or 2 theoretical monolayers of V 2O 5 or Sb 2O 3) were tested in the oxidative dehydrogenation of iso-butane at 550°C in i-C 4H 10:O 2:He=20:10:70 gas mixture. Fresh and used catalysts were characterised by BET, XRD and XPS. Reactivity and thermochemistry of active oxygen taking part in the redox cycle with ethane and hydrogene were studied using in situ differential scanning calorimetry. Temperature-programmed desorption of O 2 in He flow was also investigated and in situ DRIFT was applied to investigate surface species of the catalysts in flows of i-C 4H 10, O 2 and i-C 4H 10/O 2 mixture. Supported VSb yO x catalysts are more active and selective than bulk one. V-only supported catalysts display a high efficiency due to the high reactivity of VOX-species. In bulk catalyst, the surface is enriched with antimony. In supported samples, the surfaces V/Sb are close to the calculated ones. In the presence of antimony, the amount of active oxygen species and their reactivity in redox transformation is improved. The rates of vanadium reduction and reoxidation are also higher. Compared to V-only catalysts, supported V-Sb-catalysts display a lower coking activity and higher on-stream stability. 相似文献
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