甲醇合成催化剂的工艺参数优化及反应机理探讨

对自主研制的甲醇合成催化剂进行工艺参数优化,同时根据实验结果对合成气条件下甲醇合成的反应机理进行探讨。实验采用16通道反应器,考察反应温度和接触时间对合成甲醇反应速率的影响,确定适宜的催化剂工艺条件。结果表明,在较长接触时间下,随着温度的升高,CO转化率、H_2转化率和甲醇相对含量先升高后降低,CO_2转化率降低。在较低温度下,CO_2转化率随接触时间延长基本不变,表明甲醇中碳元素主要来自于CO,而CO_2浓度处于水汽变换反应与加氢形成甲醇反应之间的平衡状态。     

Ni-CeO2-K/γ-Al2O3催化剂上沼气联合重整制合成气

采用超声波辅助等体积浸渍法制备Ni-CeO₂-K/γ-Al₂O₃催化剂用于沼气联合重整反应,采用 BET、XRD、TG/DTG等技术对催化剂性质进行了表征,在微型固定床反应装置中研究了反应温度、体积空速、原料气组成等对沼气联合重整反应特性的影响,并对催化剂的稳定性进行了研究。结果表明,助剂CeO₂的加入,提高了催化剂中Ni的分散度,降低了催化剂还原温度。升高反应温度和减小体积空速,能够提高沼气中CH₄和CO₂的转化率;原料气中加入水蒸气,能够明显提高H₂/CO体积比;加入的O₂容易与H₂、CO发生反应,CH₄转化率稍有提高。在常压、反应温度850℃、体积空速为100000h^-1、摩尔比CH₄∶CO₂∶H₂O∶O₂∶Ar=1∶0.5∶0.5∶0.1∶0.01的优化条件下,沼气中CH₄转化率超过95%,CO₂转化率超过75%,生成合成气H₂/CO体积比约为1.6,反应48h后,催化剂未见积炭,保持稳定的活性。与沼气干重整相比,沼气联合重整不利于沼气中CO₂的转化。     

Partial oxidation and combined reforming of methane on Ce-promoted catalysts

Pt/ZrO₂ and Pt/Ce_0.14Zr_0.86O₂ catalysts containing 0.5 and 1.5 wt.% Pt were studied in order to evaluate the effect of the support reducibility and metal dispersion on the catalyst stability for the partial oxidation and the combined partial oxidation and CO₂ reforming of methane. The Pt/Ce_0.14Zr_0.86O₂ catalysts proved to be more active, stable and selective than Pt/ZrO₂ catalysts during the partial oxidation reaction. No increase in deactivation was observed when the CH₄:O₂ feed ratio was increased from 2:1 to 4:1. In addition, no water formation was observed at the high CH₄:O₂ ratios. The activity of the catalyst is dependent upon both the dispersion and the ability of the catalyst to resist carbon deposition.

The addition of CO₂ resulted in a decrease in the methane conversion and a decrease in the H₂/CO ratio for the Ce_0.14Zr_0.86O₂ and ZrO₂ supported catalysts. Small increases in the temperature of the bed have been recorded during the partial oxidation reaction. However, within a few minutes the temperature stabilizes below the furnace temperature providing indirect evidence for the combined combustion and reforming mechanisms previously proposed. The 1.5 wt.% Pt/CeZrO₂ catalyst shows promise for the autothermal reforming reaction based on the stability during transient operation.     

Designing the activity/selectivity of surface acidic, basic and redox active sites in the supported K2O–V2O5/Al2O3 catalytic system

The supported K₂O–V₂O₅/Al₂O₃ catalytic system was designed to create surfaces that were 100% acidic, 100% basic, 100% redox, mixed redox-acidic and mixed redox-basic. The resulting nature of the surface sites was controlled by the impregnation of the specific additives (K-basic or V-redox/acidic), their order of impregnation and their surface coverage. The exact locations of the surface methoxy intermediates (AlOCH₃, KOCH₃ or VOCH₃) on the mixed oxide catalyst surfaces during methanol oxidation were determined with in situ Raman spectroscopy. The surface chemistry of the various surface sites and their surface reaction intermediates were chemically probed by CH₃OH oxidation steady-state and temperature programmed surface reaction (TPSR) spectroscopy studies. The specific reactivity order and the product selectivity of the various surface sites were found to be: VOCH₃ (HCHO) AlOCH₃ (CH₃OCH₃) KOCH₃ (primarily CO₂ and minor amounts of HCHO). Formation of dimethoxy methane, (CH₃O)₂CH₂, required the presence of dual surface redox-acidic sites surface redox sites to yield H₂CO and surface acidic sites to insert the surface methoxy into H₂CO to form dimethoxy methane, (CH₃O)₂CH₂. The addition of basic surface potassium oxide to Al₂O₃ possessing surface acid sites completely suppressed reactions from the surface acidic sites and formed a surface with only basic characteristics. The addition of redox surface vanadia to the supported K₂O/Al₂O₃ catalyst was able to completely suppress reactions from surface basic sites and formed a surface with only redox characteristics. These studies demonstrate that it is possible to determine the specific surface site requirements for each reaction pathway for methanol oxidation to products, and that this informative approach should also be applicable to other reactant molecules.     

Selective formation of CH3OH in the photocatalytic reduction of CO2 with H2O on titanium oxides highly dispersed within zeolites and mesoporous molecular sieves

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 CO₂ with H₂O to produce CH₄ and CH₃OH 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 CO₂ with H₂O with a high selectivity for the formation of CH₃OH, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce CH₄, being similar to reactions on the powdered TiO₂ catalysts. Ti-mesoporous molecular sieves exhibit high photocatalytic reactivity for the formation of CH₃OH, its reactivity being much higher than the powdered TiO₂ 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 CH₄ in place of CH₃OH formation.     

Enthalpy recovery of gases issued from H2 production processes: Activity and stability of oxide and noble metal catalysts in oxidation reaction under highly severe conditions

Oxidation activity and stability under reaction was investigated for a series of mixed oxide catalysts, doped or not by a precious metal (Pd, Pt). The reaction feedstock, containing CO, H₂, CH₄, CO₂ and H₂O, simulated gases issued from H₂ production processes for fuel cells. Contrarily to conventional noble metal catalysts, mixed oxide samples present generally good stability under reaction at high temperature. The activities measured for the perovskite and hexaaluminate catalysts, are however largely lower than that of the reference Pd/Al₂O₃ catalyst. High activities were obtained after impregnation of 1.1 wt.% Pd or 0.8 wt.% Pt on the hexaaluminates samples. Even if Pd/Al₂O₃ was found to present a high activity, this sample suffered from drastic deactivation at 700 °C. Better stability were obtained on perovskite. Furthermore, doping hexaaluminate by Pt led to samples with good activities and high stability. Even if better activities were obtained by doping the hexaaluminate samples by Pd, the Pd/BaAl₁₂O₁₉ strongly deactivated, as it was previously observed for the reference catalyst. Interestingly, this Pd deactivation was not observed when Pd was impregnated on the Mn substituted hexaaluminate, leading to a stable and active catalyst. This suggests that it is possible to stabilize the palladium in its oxidized form at high temperature (700 °C) on the surface of some supports.     

Performance characteristics of fluidized bed syngas methanation over Ni-Mg/Al2O3 catalyst

The performance characteristics of isothermal fluidized bed syngas methanation for substitute natural gas are investigated over a self-made Ni–Mg/Al2O3 catalyst. Via atmospheric methanation in a laboratory fluidized bed reactor it was clarified that the CO conversion varied in 5% when changing the space velocity in 40–120 L·g?1·h?1 but the conversion increased obviously by raising the superficial gas velocity from 4 to 12.4 cm·s?1. The temperature at 823 K is suitable for syngas methanation while obvious deposition of uneasy-oxidizing Cγoccurs on the catalyst at temperatures around 873 K. From a kinetic aspect, the lowest reaction temperature is suggested to be 750 K when the space velocity is 60 L·g?1·h?1. Raising the H2/CO ratio of the syngas increased proportionally the CO conversion and CH4 selectivity, showing that at enough high H2/CO ratios the active sites on the catalyst are sufficient for CO adsorption and in turn the reaction with H2 for forming CH4. Introducing CO2 into the syngas feed suppresses the water gas shift and Boudouard reactions and thus increased H2 consumption. The ratio of CO2/CO in syngas should be better below 0.52 because varying the ratio from 0.52 to 0.92 resulted in negligible increases in the H2 conversion and CH4 selectivity but decreased the CH4 yield. Introducing steam into the feed gas affected little the CO conversion but decreased the selectivity to CH4. The tested Ni–Mg/Al2O3 catalyst manifested good stability in structure and activity even in syngas containing water vapor.     

Mechanistic study of partial oxidation of methane to synthesis gas over supported rhodium and ruthenium catalysts using in situ time-resolved FTIR spectroscopy

In situ time-resolved FTIR spectroscopy was used to study the reaction mechanism of partial oxidation of methane to synthesis gas and the interaction of CH₄/O₂/He (2/1/45) gas mixture with adsorbed CO species over SiO₂ and γ-Al₂O₃ supported Rh and Ru catalysts at 500–600°C. It was found that CO is the primary product for the reaction of CH₄/O₂/He (2/1/45) gas mixture over H₂ reduced and working state Rh/SiO₂ catalyst. Direct oxidation of methane is the main pathway of synthesis gas formation over Rh/SiO₂ catalyst. CO₂ is the primary product for the reaction of CH₄/O₂/He (2/1/45) gas mixture over Ru/γ-Al₂O₃ and Ru/SiO₂ catalysts. The dominant reaction pathway of CO formation over Ru/γ-Al₂O₃ and Ru/SiO₂ catalysts is via the reforming reactions of CH₄ with CO₂ and H₂O. The effect of space velocity on the partial oxidation of methane over SiO₂ and γ-Al₂O₃ supported Rh and Ru catalysts is consistent with the above mechanisms. It is also found that consecutive oxidation of surface CO species is an important pathway of CO₂ formation during the partial oxidation of methane to synthesis gas over Rh/SiO₂ and Ru/γ-Al₂O₃ catalysts.     

Characterization of self-standing Ti-containing porous silica thin films and their reactivity for the photocatalytic reduction of CO2 with H2O

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 CO₂ with H₂O at 323 K. UV irradiation of these Ti-containing porous silica thin films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH as well as CO and O₂ 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 H₂O 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 CH₃OH.     

Liquid phase methanol reforming with water over silica supported Pt–Ru catalysts

The mechanism of the liquid phase methanol reforming reaction over silica supported Pt–Ru catalyst was investigated by kinetic studies, employing a pyrex glass reactor with reflux condensers connected to a closed gas circulation system under ambient pressure. The rate of H₂ formation over Pt–Ru/SiO₂ catalysts was more than 20 times faster than that over Pt/SiO₂ catalysts with high selectivity for CO₂ (72.3%), indicating a marked addition effect of Ru. In the case of HCHO–H₂O reaction over Pt–Ru/SiO₂, the H₂ formation rate was five times larger than that in the CH₃OH–H₂O reaction but selectivity to CO₂ was only 4%. On the contrary, in the HCOOCH₃–H₂O and HCOOH–H₂O reactions, both high activity and selectivity were observed over Pt–Ru/SiO₂. These results clearly indicate that the CO₂ formation does not proceed via HCHO decomposition and following water gas shift reaction. We propose the following pathway for liquid phase methanol reforming reaction over Pt–Ru/SiO₂; a partly dehydrogenated methanol (CH₂OH^*) is the initial reaction intermediate, from which H₂ and CO₂ are formed through HCOOCH₃ and HCOOH as the successive reaction intermediates.     

锆基双金属氧化物催化剂硫中毒的研究

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...     

Preparation and gaseous photocatalytic activity of smooth potassium dititanate film

A new smooth potassium dititanate film was prepared by sol-gel method and characterized by thermogravimetry (TG) and differential scanning calorimetry (DSC), X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Visible diffuse reflectance and Raman spectroscopy. The gaseous photocatalytic activity of smooth K₂Ti₂O₅ films was studied using contact angle analysis from the photocatalytic decomposition of octadecyltrichlorosilane (OTS) based self-assembled monolayers (SAMs) formed on K₂Ti₂O₅ films. The photocurrent response of the film was determined by an electrochemical method. It was shown that the films were smooth, compact, and transparent when formed on glass. Compared with TiO₂ film, the K₂Ti₂O₅ film showed wide absorption in the ultraviolet and visible region. It was found that the monolayers on K₂Ti₂O₅ decomposed much faster than those on TiO₂ under UV irradiation of 254 nm in air. The film also exhibited a stronger photoresponse and a more stable anodic photocurrent. The K₂Ti₂O₅ film efficiently decomposes the alkylsiloxane monolayers under UV irradiation in air and it was found to be a good photocatalyst for gaseous organic pollutant treatment.     

Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts

Reforming of methane with carbon dioxide to synthesis gas (CO/H₂) has been investigated over rhodium supported on SiO₂, TiO₂, γ-Al₂O₃, MgO, CeO₂, and YSZ (ZrO₂ (8 mol% Y₂O₃)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using ¹³CH₄) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al₂O₃ catalyst during reforming reaction at 750°C are primarily derived from the CO₂ molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH₄ molecular route is found to be very small.     

MnOx对Pt/CeZrO2催化剂在水汽和CO2环境下CO氧化的影响

以CeZrO₂固溶体为载体,发现MnO_x的添加能促进Pt/CeZrO₂催化剂的CO氧化性能,并研究了MnO_x含量对催化剂CO氧化活性及抗H₂O和CO₂性能的影响。结果表明,随着MnO_x含量增加,催化剂活性呈现先升高后降低的趋势,在MnO_x含量为0.5%（质量分数）时活性最佳。MnO_x的添加降低了Pt颗粒尺寸并影响催化剂还原性能从而促进反应活性。水汽和CO₂对Pt/CeZrO₂催化剂的CO氧化活性有抑制作用,而MnO_x的加入能显著提高催化剂的抗水汽和CO₂的能力。反应动力学结果表明,在Pt/CeZrO₂催化剂上,反应气中引入H₂O和CO₂后,CO的反应级数有明显升高,说明H₂O和CO₂在催化剂表面与CO竞争吸附,导致CO反应活性下降;而在Pt/MnO_x/CeZrO₂催化剂上,CO的反应级数略有升高,说明MnO_x的添加能有效抑制H₂O和CO₂与CO的竞争吸附,从而改善了催化剂的抗H₂O和CO₂性能。     

Highly efficient K7NiV13O38-catalyzed hydroxylation of aromatics with aqueous hydrogen peroxide (30%)

K₇NiV₁₃O₃₈·16H₂O was found to be an efficient homogeneous catalyst in a mixture solvent of CH₃CN and acetic acid for the hydroxylation of aromatics with 30% aqueous H₂O₂. The new catalytic system showed an unprecedented catalytic activity and selectivity for the hydroxylation of aromatics bearing an electron-withdrawing group. In the case of methyl benzoate, the yield of hydroxylated products reached 73%.     

Catalytic synthesis of methanethiol from hydrogen sulfide and carbon monoxide over vanadium-based catalysts

The direct synthesis of methanethiol, CH₃SH, from CO and H₂S was investigated using sulfided vanadium catalysts based on TiO₂ and Al₂O₃. 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 CO₂ and H₂S 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.     

SCR of NOx with CH3OH on H-mordenite: mechanism and reaction intermediates

The selective reduction of NO_x over H-mordenite (H-m) was studied using CH₃OH as reducing agent. Results are compared with those obtained with other conventional reducing agents (ethylene and methane), with gas-phase reactions, and with other metal-exchanged mordenites (Cu-mordenite (Cu-m) and Co-mordenite (Co-m)). H-m was found to be an effective catalyst for the SCR of NO_x with CH₃OH. When different reducing agents were compared over H-m, CH₃OH > C₂H₄ > CH₄ was the order according to the maximum NO conversion obtained using 1% of oxygen in the feed. Instead, if selectivity is considered, the order results CH₄ > CH₃OH > C₂H₄. In reaction experiments, two distinct zones defined by two maxima with NO to N₂ conversion are obtained at two different temperatures. A correlation exists between the said zones and the CO : CO₂ ratio. At low temperatures, CO prevails whereas at high temperatures CO₂ prevails. These results indicate that there exist different reaction intermediates. Evidence from reaction experiments, FTIR results, and transient experiments suggest that the reaction mechanism involves formaldehyde and dimethyl ether (DME) as intermediates in the 200–500°C temperature range. The surface interaction between CH₃OH (or its decomposition products) and NO is negligible if compared with NO₂, indicating that the oxidation of NO to NO₂ on acid sites is a fundamental path in this system. Different from other non-oxygenated reductants (methane and ethylene), a gas-phase NO_x initiation effect on hydrocarbon combustion was not observed.     

High combustion activity of methane induced by reforming gas over Ni/Al2O3 catalysts

During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH₄/H₂O/O₂/Ar = 20/10/20/50 than under CH₄/H₂O/O₂/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH₄/H₂O/O₂/Ar = 20/10/20/50, although it is mainly oxidized to NiAl₂O₄ under CH₄/H₂O/O₂/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH₄/H₂O = 10/10) to the combustion gas (CH₄/O₂ = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity.     

PERMEATION THROUGH KAPTON® POLYIMIDE AT ELEVATED TEMPERATURES

The permeabilities of CH₄, CO₂, CH₃OH, H₂O, O₂, 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 CH₄ to nearly 0 for H₂O 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.     

CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion via cavity ring-down spectroscopic analysis

CH₄/CO₂ reforming over La₂NiO₄ and 10%NiO/CeO₂–La₂O₃ catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH₄, H₂O, CO₂ and CO molecules were recorded in the near infrared spectral region. Our results indicated that La₂NiO₄ is superior to 10%NiO/CeO₂–La₂O₃ in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CH_x⁺, OH⁺ and H⁺ species.