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1.
We report the kinetic parameters for the water–gas shift (WGS) reaction on Pt catalysts supported on ceria and alumina under fuel reformer conditions for fuel cell applications (6.8% CO, 8.5% CO 2, 22% H 2O, 37.3% H 2, and 25.4% Ar) at a total pressure of 1 atm and in the temperature range of 180–345 °C. When ceria was used as a support, the turnover rate (TOR) for WGS was 30 times that on alumina supported Pt catalysts. The overall WGS reaction rate ( r) on Pt/alumina catalysts as a function of the forward rate ( rf) was found to be: r = rf(1 − β), where rf = kf[CO] 0.1[H 2O] 1.0[CO 2] −0.1[H 2] −0.5, kf is the forward rate constant, β = ([CO 2][H 2])/( Keq[CO][H 2O]) is the approach to equilibrium, and Keq is the equilibrium constant for the WGS reaction. The negative apparent reaction orders indicate inhibition of the forward rate by CO 2 and H 2. The surface is saturated with CO on Pt under reaction conditions as confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The small positive apparent reaction order for CO, in concert with the negative order for H 2 and the high CO coverage is explained by a decrease in the heat of adsorption as the CO coverage increases. Kinetic models based on redox-type mechanisms can explain the observed reaction kinetics and can qualitatively predict the changes in CO coverage observed in the DRIFTS study. 相似文献
2.
A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO 2 in the flue gases from fossil fuel-based power plants without CO 2 separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO 2 reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H 2) with desired H 2/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO 2 reforming of methane. These two advantages have been demonstrated by tri-reforming of CH 4 in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH 4 conversion and about 80% CO 2 conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO 2 conversion in the presence of H 2O and O 2 in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO 2 follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO 2 ≈ Ni/ZrO 2 ≈ Ni/Al 2O 3 > Ni/CeZrO. The higher CO 2 conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO 2 with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO 2 and H 2O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al 2O 3 catalysts for tri-reforming. 相似文献
3.
The direct synthesis of methanethiol, CH 3SH, from CO and H 2S was investigated using sulfided vanadium catalysts based on TiO 2 and Al 2O 3. These catalysts yield high activity and selectivity to methanethiol at an optimized temperature of 615 K. Carbonyl sulfide and hydrogen are predominant products below 615 K, whereas above this temperature methane becomes the preferred product. Methanethiol is formed by hydrogenation of COS, via surface thioformic acid and methylthiolate intermediates. Water produced in this reaction step is rapidly converted into CO 2 and H 2S by COS hydrolysis. Titania was found to be a good catalyst for methanethiol formation. The effect of vanadium addition was to increase CO and H2S conversion at the expense of methanethiol selectivity. High activities and selectivities to methanethiol were obtained using a sulfided vanadium catalyst supported on Al2O3. The TiO2, V2O5/TiO2 and V2O5/Al2O3 catalysts have been characterized by temperature programmed sulfidation (TPS). TPS profiles suggest a role of V2O5 in the sulfur exchange reactions taking place in the reaction network of H2S and CO. 相似文献
4.
MgO-promoted Ni/Al 2O 3 catalysts have been investigated with respect to catalytic activity and coke formation in combined steam and carbon dioxide reforming of methane (CSCRM) to develop a highly active and stable catalyst for gas to liquid (GTL) processes. Ni/Al 2O 3 catalysts were promoted through varying the MgO content by the incipient wetness method. X-ray diffraction (XRD), BET surface area, H 2-temperature programmed reduction (TPR), H 2-chemisorption and CO 2-temperature programmed desorption (TPD) were used to observe the characteristics of the prepared catalysts. The coke formation and amount in used catalysts were examined by SEM and TGA, respectively. H 2/CO ratio of 2 was achieved in CSCRM by controlling the feed H 2O/CO 2 ratio. The catalysts prepared with 20 wt.% MgO exhibit the highest catalytic performance and have high coke resistance in CSCRM. MgO promotion forms MgAl 2O 4 spinel phase, which is stable at high temperatures and effectively prevents coke formation by increasing the CO 2 adsorption due to the increase in base strength on the surface of catalyst. 相似文献
5.
SO 2, which is an air pollutant causing acid rain and smog, can be converted into elemental sulfur in direct sulfur recovery process (DSRP). SO 2 reduction was performed over catalyst in DSRP. In this study, SnO 2-ZrO 2 catalysts were prepared by a co-precipitation method, and CO and coal gas, which contains H 2, CO, CO 2 and H 2O, were used as reductants. The reactivity profile of the SO 2 reduction over the catalysts was investigated at the various reaction conditions as follows: reaction temperature of 300–550 °C, space velocity of 5000–30,000 cm 3/g -cat. h, [reductant]/[SO 2] molar ratio of 1.0–4.0 and Sn/Zr molar ratio of SnO 2-ZrO 2 catalysts 0/1, 2/8, 3/5, 5/5, 2/1, 3/1, 4/1 and 1/0. SnO 2-ZrO 2 (Sn/Zr = 2/1) catalyst showed the best performance for the SO 2 reduction in DSRP on the basis of our experimental results. The optimized reaction temperature and space velocity were 325 °C and 10,000 cm 3/g -cat. h, respectively. The optimal molar ratio of [reductant]/[SO 2] varied with the reductants, that is, 2.0 for CO and 2.5 for coal gas. SO 2 conversion of 98% and sulfur yield of 78% were achieved with the coal gas. 相似文献
6.
Gradient-corrected density functional theory was used to investigate the adsorption of H 2S on Pd(1 1 1) surface. Molecular adsorption was found to be stable with H 2S binding preferentially at top sites. In addition, the adsorption of other S moieties (SH and S) was investigated. SH and S were found to be preferentially bind at the bridge and fcc sites, respectively. The reaction pathways and energy profiles for H 2S decomposition giving rise to adsorbed S and H were determined. Both H 2S (ad) → SH (ad) + H (ad) and SH (ad) → S (ad) + H (ad) reactions were found to have low barriers and high exothermicities. This reveals that the decomposition of H 2S on Pd(1 1 1) surface is a facile process. 相似文献
7.
The oxidation and reforming kinetics of methane by O 2, CO 2 and H 2O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO 2 and H 2O reforming. Total oxidation of methane to CO 2 and H 2O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO 2 or H 2O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO 2 and H 2O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes. 相似文献
8.
The TiO 2 supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO 2 Rh/TiO 2 > Pd/TiO 2 > Au/TiO 2 TiO 2. HCHO could be completely oxidized into CO 2 and H 2O over Pt/TiO 2 in a gas hourly space velocity (GHSV) of 50,000 h −1 even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO 2 was only 20% over the Rh/TiO 2 at 20 °C. The Pd/TiO 2 and Au/TiO 2 showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO 2 supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed. 相似文献
9.
A nickel-based catalytic filter material for the use in integrated high temperature removal of tars and particles from biomass gasification gas was tested in a broad range of parameters allowing the identification of the operational region of such a filter. Small-scale porous alumina filter discs, loaded with approximately 2.5 wt% Al 2O 3, 1.0 wt% Ni and 0.5 wt% MgO were tested with a particle free synthetic gasification gas with 50 vol% N 2, 12 vol% CO, 10 vol% H 2, 11 vol% CO 2, 12 vol% H 2O, 5 vol% CH 4 and 0–200 ppm H 2S, and the selected model tar compounds: naphthalene and benzene. At a typical face velocity of 2.5 cm/s, in the presence of H 2S and at 900 °C, the conversion of naphthalene is almost complete and a 1000-fold reduction in tar content is obtained. Technically, it would be better to run the filter close to the exit temperature of the gasifier around 800–850 °C. At 850 °C, conversions of 99.0% could be achieved in typical conditions, but as expected, only 77% reduction in tars was achieved at 800 °C. Conversion data can be reasonably well described with first order kinetics and a dominant adsorption inhibition of the Ni sites by H2S. The apparent activation energies obtained are similar to those reported by other investigators: 177 kJ/mol for benzene and 92 kJ/mol for naphthalene. The estimated heat of adsorption of H2S is 71 kJ/mol in the benzene experiments and 182 kJ/mol in the naphthalene experiments, which points at very strong adsorption of H2S. Good operation of the present material can hence only be guaranteed at temperatures above 830 °C mainly due to the strong deactivation by H2S at lower temperatures. 相似文献
10.
Silicoaluminophosphate (SAPO) membranes with Si/Al gel ratios from 0.05 to 0.3 were synthesized by in situ crystallization onto porous, tubular stainless steel support. Pure SAPO-34 membranes were obtained when the Si/Al ratio was 0.15 or higher. The adsorbate polarizability correlated with the adsorption capacity on SAPO-34, and the amounts of gases adsorbed were in the order: CO 2 > CH 4 > N 2 > H 2. The Si/Al ratio did not affect the pore volume significantly, but it changed the CO 2 and CH 4 adsorption equilibrium constants. The SAPO-34 membranes effectively separated CO 2 from CH 4 for feed pressures up to 7 MPa. At 295 K, for a pressure drop of 138 kPa and a 50/50 feed, the CO 2/CH 4 selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. At 7 MPa, the CO 2/CH 4 selectivity was 100 and the CO 2 permeance was 4 × 10 −8 mol/(m 2 · s · Pa) at 295 K. This membrane was also separated CO 2/N 2 (selectivity = 21) and H 2/CH 4 (selectivity = 32) mixtures at 295 K and a pressure drop of 138 kPa. Competitive adsorption and difference in diffusivities are responsible for CO 2/CH 4 and CO 2/N 2 separations, whereas the H 2/CH 4 separation was due to diffusivity differences. For a membrane with Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed, and the CO 2/CH 4 selectivity was lower. 相似文献
11.
This study focuses on the direct sulfur recovery process (DSRP), in which SO 2 can be directly converted into elemental sulfur using a variety of reducing agents over Ce 1−xZr xO 2 catalysts. Ce 1−xZr xO 2 catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO 2 reduction were as follow: the space velocity (GHSV) was 30,000 ml/g -cat h and the ratio of [CO (or H 2, H 2 + CO)]/[SO 2] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce 0.5Zr 0.5O 2 catalyst and CO, respectively. In this case, the SO 2 conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO 2 removal and elemental sulfur recovery was achieved in the reduction of SO 2 with CO as a reducing agent than that with H 2. In the reduction of SO 2 by H 2 over the Ce 0.5Zr 0.5O 2 catalyst, SO 2 conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO 2 using synthetic gas with various [CO]/[H 2] molar ratios over the Ce 0.5Zr 0.5O 2 catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO 2 reduction using the synthetic gas with various [CO]/[H 2] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce 1−xZr xO 2 catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H 2. 相似文献
12.
Zn–Ti-based sorbents promoted with cobalt and nickel additive were prepared by simple physical mixing of single oxides. Their capacities for removing H 2S and NH 3 simultaneously, emitted from coal gasifiers, were investigated in a micro-reactor at 1 atm and 650 °C. NH 3 within the fuel gases did not affect the sulfur removing capacity of the Zn–Ti-based sorbent. The additives, cobalt and nickel, were found to be active components in NH 3 decomposition as well as H 2S absorption, while major components such as ZnO and TiO 2 did not show any activity in the NH 3 decomposition reaction. NH 3 was decomposed over both oxide and sulfide forms of the additives, even though the NH 3 decomposition ability of their sulfides dramatically decreased in the presence of H 2 gas owing to the equilibrium limitation of NH 3 decomposition. In the case of oxide forms, cobalt oxide showed excellent NH 3 decomposition capacity regardless of H 2 concentrations, while the capacity of nickel oxide depended on the H 2 concentrations. 相似文献
13.
将H 2S和CO 2混合酸气一步转化制合成气,既实现了二者无害化处理,又生产出合成气,是一条理想的废气资源化利用新路线。由于分子结构稳定,在常规条件下因受热力学平衡限制,二者转化率极低。而在低温等离子体中,H 2S和CO 2可被激发为高活性物种来参与反应。研究了具有不同Si/Al摩尔比的ZSM-5催化剂与低温等离子体结合实现H 2S-CO 2一步高选择性制合成气,显著提高了H 2S-CO 2转化性能。考察了ZSM-5催化剂中Si/Al比和低温等离子体放电条件等对反应的影响。其中,当Si/Al比为80时表现出最优催化性能,最高H 2和CO产率分别达到56.1%和10.0%。对常规条件和低温等离子体氛围下的不同ZSM-5催化剂上CO 2、H 2S、CO、H 2等化学吸脱附行为进行了对比研究,发现低温等离子体促进了催化剂对CO 2、H 2及CO分子的吸附活化,进而明显提升了H 2S和CO 2转化。 相似文献
14.
Pt-Fe/mordenite catalysts coated on ceramic straight-channel monoliths were evaluated for the preferential oxidation of carbon monoxide (PROX) in hydrogen-rich gas streams. In a feed gas containing 1% CO, 1% O 2, with the balance H 2, CO conversion reached almost 100% at temperatures ranging from 100 to 130 °C, i.e., an outlet CO concentration of less than 10 ppm. Even in a synthetic reformate gas (1% CO, 1% O 2, 15% H 2O, 20% CO 2, balance H 2), the monolithic catalyst exhibited excellent activity, reducing the CO concentration to less than 100 ppm. In particular, under optimized conditions, an outlet CO concentration of less than 10 ppm was realized. This is the first report that has demonstrated that monolithic catalysts could achieve the 10-ppm target level at a low O 2/CO ratio and a high space velocity in a single-stage reactor. Excellent durability of the monolithic catalyst is expected, based on a lack of deterioration in performance during 500 h of operation. 相似文献
15.
The conversion of CO/H 2, CO 2/H 2 and (CO+CO 2)/H 2 mixtures using cobalt catalysts under typical Fischer–Tropsch synthesis conditions has been carried out. The results show that in the presence of CO, CO 2 hydrogenation is slow. For the cases of only CO or only CO 2 hydrogenation, similar catalytic activities were obtained but the selectivities were very different. For CO hydrogenation, normal Fischer–Tropsch synthesis product distributions were observed with an of about 0.80; in contrast, the CO 2 hydrogenation products contained about 70% or more of methane. Thus, CO 2 and CO hydrogenation appears to follow different reaction pathways. The catalyst deactivates more rapidly for the conversion of CO than for CO 2 even though the H 2O/H 2 ratio is at least two times larger for the conversion of CO 2. Since the catalyst ages more slowly in the presence of the higher H 2O/H 2 conditions, it is concluded that water alone does not account for the deactivation and that there is a deactivation pathway that involves the assistance of CO. 相似文献
16.
Sharp NO and O 2 desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H 2 treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N 2O treatment. The amounts of O 2 and NO desorption were dependent on the pretreatment pressure of N 2O in the H 2 and N 2O treatment. The adsorbed species could be regenerated by the H 2 and N 2O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N 2O with CH 4. However, the reaction rate of CH 4 activation by the adsorbed species formed after the H 2 and N 2O or the HT and N 2O treatment was not so high as that of the CH 4 + N 2O reaction over the catalyst after O 2 treatment. The simultaneous presence of CH 4 and N 2O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N 2O dissociation can activate CH 4 in the SCR of N 2O with CH 4. 相似文献
17.
Attention has been increasingly paid to the partial oxidation of lower alkanes to synthesis gas, due to its intrinsic energy saving process. We studied the partial oxidation of ethane (POE) on Co loaded on various supports. The POE performance varied as follows: Y 2O 3, CeO 2, ZrO 2, La 2O 3 SiO 2, Al 2O 3, TiO 2 > MgO. Comparing Y 2O 3 and CeO 2, the carbon deposition during the POE was negligible on CeO 2 and therefore CeO 2 was the most preferable support. By changing space velocity and O 2 partial pressure, reaction mechanism of POE was studied and it was revealed that two-step mechanism was prevailing; combustion of ethane to H 2O and CO 2 and subsequent reforming of ethane with H 2O and CO 2 to synthesis gas. Co/CeO 2 catalyst exhibited high and stable catalytic activity for 10 h; high ethane conversion of 18% (maximum ethane conversion 20% at O 2/C 2H 6 = 0.2) with H 2 and CO selectivities of 93 and 84%, respectively. 相似文献
18.
On-board fuel processors are being developed to provide hydrogen-rich gas to the polymer electrolyte fuel cell automotive propulsion systems. Whereas the anode catalyst in the fuel cell has low tolerance for carbon monoxide, 10–100 ppm, reforming of gasoline and other hydrocarbon fuels generally produces 1–2% of CO. Of the many methods of removing CO from the reformer gas, preferential oxidation (PrOx) of CO over noble-metal catalysts is practiced most frequently. In this paper, we present experimental data for CO conversion on a Pt-based catalyst that is active at room temperature and was coated on a ceramic monolith. The data is used to develop an empirical correlation for selectivity for CO oxidation as a function of CO concentration and oxygen stoichiometry at 30,000–80,000/h space velocity. The selectivity correlation is used in a model to analyze the performance of multi-stage, adiabatic PrOx reactors with heat exchange between the stages to cool the reformate to 100 °C. An optimization algorithm is used to determine the operating conditions that can reduce CO concentration to 10 ppm while minimizing parasitic loss of H 2 in the reformate stream. It is found that the 10 ppm constraint limits the maximum inlet CO concentration to 1.05% in a single-stage reactor and to 3.1% in a two-stage reactor. The results clearly show the incremental reduction in parasitic H 2 loss by addition of second and third stages. 相似文献
19.
Oxidative reforming of biomass derived ethanol over an inexpensive Ni–Cu/SiO 2 catalyst has been carried out with respect to solid polymer fuel cell (SPFC) applications. Two types of runs were performed, either under diluted conditions (with helium as diluent) or under conditions corresponding to an on-board reformer. Selectivities of ethanol reforming have been analyzed as a function of operating parameters: reaction temperature, H 2O/EtOH molar ratio and O 2/EtOH molar ratio of the feed to the reformer. The hydrogen content and the CO 2/CO x molar ratio in the outlet gases were used as parameters to optimize the operating conditions in the reforming reactor. The tests carried out at on-board reformer conditions evidenced that an H 2O/EtOH molar ratio=1.6 and an O 2/EtOH molar ratio=0.68 at 973 K allow a hydrogen rich mixture (33%) that can be considered of high interest for SPFC. Furthermore, the use of oxygen decreases the production of methane and coke which increases in turn the lifetime of the catalyst. The stability of this catalyst has been fully demonstrated by long time runs. 相似文献
20.
Palm kernel shell (PKS) biochars with different levels of carbon conversion were initially prepared using a tube furnace, after which the reactivity of each sample was assessed with a thermogravimetric analyzer under a CO 2 atmosphere. The pore structure and carbon ordering of each biochar also examined, employing a surface area analyzer and a Raman spectroscopy. Thermogravimetric results showed that the gasification index R s of the PKS biochar decreased from 0.0305 min -1 at carbon conversion (x)=20% to 0.0278 min -1 at x=40%. The expansion of micropores was the dominant process during the pore structure evolution, ad mesopores with sizes ranging from 6 to 20, 48 to 50 nm were primarily generated during gasification under a CO 2/H 2O mixture. The proportion of amorphous carbon in the PKS biochar decreased significantly as x increased, suggesting that the proportion of ordered carbon was increased during the CO 2/H 2O mixed gasification. A significantly reduced total reaction time was observed when employing a CO 2/intermittent H 2O process along with an 83.46% reduction in the steam feed, compared with the amount required using a CO 2/H 2O atmosphere. 相似文献
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