甲烷部分氧化及重整反应多相催化剂的研究进展

介绍国内外甲烷制合成气技术中多相催化剂的研究进展,包括催化剂在水蒸气重整、甲烷部分氧化和二氧化碳重整制合成气技术中的研究情况,重点评述了催化剂的活性组分、助剂和载体方面的研究进展及存在的问题。     

Partial oxidation of methane to synthesis gas. Behaviour of different Ni supported catalysts

The behaviour of Ni supported catalysts, obtained using Ni(NO₃)₂ and Ni-acetylacetonate as precursor compounds, is analyzed. It is observed that initial activities and selectivities are similar for both systems, but the stability differs significantly. The systems show different carbon structures and sintering rates, depending on the precursor compound employed.     

Reaction mechanism of partial oxidation of methane to synthesis gas over supported ni catalysts

A mechanistic study on the partial oxidation of methane to synthesis gas (H₂ and CO) was conducted with supported nickel catalysts. To investigate the reaction mechanism, pulse experiments, O₂-TPD, and a comparison of the moles of reactants and products were carried out. From the O₂-TPD experiment, it was observed that the active catalyst in the synthesis gas production desorbed oxygen at a lower temperature. In the pulse experiment, the temperature of the top of the catalyst bed increased with the pulses, whereas the temperature of the bottom decreased. This suggests that there are two kinds of reactions, that is, the total oxidation of methane (exothermic) at the top and reforming reactions (endothermic) at the bottom. From the comparison of the moles of reactants and products, it was found that the moles of CO₂, CH₄ and H₂O decreased as the moles of H₂ and CO increased. The results support the mechanism that synthesis gas is produced through a two-step reaction mechanism: the total oxidation of methane to CO₂ and H₂O takes place first, followed by the reforming reaction of the produced CO₂ and H₂O with residual CH₄ to form synthesis gas. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

天然气甲烷部分氧化制合成气的研究进展

甲烷部分氧化制合成气是高转化率、高选择性、高空速、低H2/CO、温和的放热反应,综述了近几年来甲烷部分氧化制合成气的催化剂、反应机理及活性中心的研究进展及反应中的存在问题。     

A study of carbon deposition on catalysts during the partial oxidation of methane to synthesis gas

The deposition of carbon on catalysts during the partial oxidation of methane to synthesis gas has been investigated and it has been found that the relative rate of carbon deposition follows the order Ni>Pd>Rh>Ir. Methane decomposition was found to be the principal route for carbon formation over a supported nickel catalyst, and electron micrographs showed that both whisker and encapsulate forms of carbon are present on the catalyst. Negligible carbon deposition occurred on iridium catalysts, even after 200 h.     

Influence of nickel distribution on properties of spherical catalysts for partial oxidation of methane to synthesis gas

Two kinds of Ni/Al₂O₃ catalysts, i.e., eggshell and uniform catalysts, were prepared by impregnation method using acetone and water solution of nickel nitrate, respectively, and their properties were investigated by means of EDS, XRD and TPR. Their catalytic performance in methane partial oxidation (POM) to synthesis gas was also evaluated. Because the partial oxidation of methane is a very fast reaction, the intra-particle diffusion was very remarkable. Due to nickel component distributing in the outer region of particles, the eggshell catalyst had higher effectiveness factor, which resulted in its better catalytic performance than the uniform one.     

Partial oxidation of methane to synthesis gas over Rh-hexaaluminate-based catalysts

The partial oxidation of methane to synthesis gas over catalysts consisting of Rh supported on hexaaluminates (BaAl₁₂O₁₉, CaAl₁₂O₁₉ and SrAl₁₂O₁₉) was investigated at atmospheric pressure and high reactant dilution in order to compare their performances within the kinetic-controlling regime. Comparison with the results obtained over a commercial Rh/-Al₂O₃ system indicates that hexaaluminate catalysts are active and selective in this reaction. Despite of the higher surface area of the support, hexaaluminate-supported catalysts were found less stable, active and selective than an -Al₂O₃-supported catalyst.     

Stabilisation of active nickel catalysts in partial oxidation of methane to synthesis gas by iron addition

Mixed LaNi_xFe_(1−x)O₃ perovskite oxides (0≤x≤1) have been prepared by a sol–gel related method, characterised by X-ray diffraction (XRD), specific surface area measurements, transmission electron microscopy (TEM) coupled to an energy dispersive X-ray spectrometer (EDS). These systems are the precursors of highly efficient catalysts in partial oxidation of methane to synthesis gas. Studies on the state of these systems after test show the stabilisation of active nickel by increasing the amount of iron. These systems permit to control the reversible migration of nickel from the structure to the surface. The best mixed perovskite for the partial oxidation of methane is LaNi_0.3Fe_0.7O₃.     

Mechanism of the partial oxidation of methane to synthesis gas over Pd

The partial oxidation of methane to synthesis gas has been studied in an isothermal continuous flow reactor operated with the phases in plug flow, using a silica-supported palladium catalyst. The reaction mechanism involves the sequential combustion and reforming of methane. The catalyst bed is not uniform in terms of the composition of the palladium phase. The implications for investigations using a pulse apparatus are discussed. Finally, large palladium crystallites readily grow carbon filaments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic partial oxidation of methane to synthesis gas using monolithic reactors

The partial oxidation of methane to synthesis gas has been studied in a monolithic type catalyst containing 5 wt.-% Ni or 1.5 wt.-% Pt. The experiments were carried out at 600–900°C, 1 bar and with space times in the range of 0.2–0.005 s. Comparisons between quartz and steel reactors demonstrated that steel reactors without any kind of lining are not suitable for these kind of experiments. The axial temperature profile in the monoliths indicates that the reaction consists of a total oxidation of some of the methane followed by the endothermic reforming reactions and the water-gas shift reaction. Equilibrium product distribution was obtained except at very low reaction times. A reduction in the metal dispersion by more than 50% did not have any significant influence on the conversion or the product distribution.     

Carbon dioxide reforming of methane to synthesis gas over supported Ni catalysts

Carbon dioxide reforming of methane to synthesis gas has been investigated over supported Ni catalysts in the temperature range of 500–850°C. Addition of CaO (10mol%) promoter to the Ni/γ-Al₂O₃ resulted in an increase of reaction rate and an improvement of catalyst stability, which may be related to enhanced reducibility of the promoted catalyst. The kinetic studies show that the overall reaction can be described by a Langmuir-Hinshelwood mechanistic scheme, assuming that methane dissociation is the rate determining step. In addition to adsorbed CO and formate species, three types of carbonaceous species, C, C_β and C_γ, were found to exist on the Ni catalyst. While the active C, species is suggested to be responsible for CO formation, the less active C_β and C_γ species are attributed to causing catalyst deactivation.     

A dual catalyst bed for the autothermal partial oxidation of methane to synthesis gas

Dual bed catalysts were found to produce high yields (>85%) of hydrogen from methane and air in a millisecond contact time reactor. The dual bed catalyst consisted of a 5 mm platinum combustion catalyst followed by a 5 mm nickel steam reforming catalyst. The platinum catalyst was used to totally oxidize approximately one-quarter of the methane feed to carbon dioxide and water. In the nickel catalyst, the carbon dioxide and water reformed the remaining methane to hydrogen and carbon monoxide. This process is favored at high flow rates, because the heat generated in the platinum catalyst is convected to the nickel catalyst at a higher rate. The heat delivered to the nickel catalyst favors the endothermic reforming reactions that generate the hydrogen and carbon monoxide.     

Heat transport limitations and reaction scheme of partial oxidation of methane to synthesis gas over supported rhodium catalysts

The partial oxidation of methane to synthesis gas over supported Rh catalysts is investigated, paying particular attention to removing heat transport limitations and identifying the reaction conditions within the kinetic-controlling regime. The results obtained suggest that the reaction follows the sequence of total oxidation to CO₂ and H₂O, followed by reforming reactions to synthesis gas.     

Supported nanocomposite catalysts for high-temperature partial oxidation of methane

In order to utilize the vast potential of nanoparticles for industrial catalysis, it is necessary to develop methods to stabilize these particles at realistic technical conditions and to formulate nanoparticle-based catalysts in a way that facilitates handling and reduces health and safety concerns. We have previously demonstrated that metal nanoparticles can be efficiently stabilized by embedding them into a high-temperature stable nanocomposite structure. Building onto these results, we report here on the next step towards a simple, hierarchically structured catalyst via supporting platinum barium-hexaaluminate (Pt-BHA) nanocomposites onto a range of different conventional and novel support structures (monoliths, foams, and felts). The catalysts were characterized via SEM, TEM, XRD, porosimetry, chemisorption, and reactive tests in catalytic partial oxidation of methane to synthesis gas (CPOM), and compared to conventionally prepared Pt-catalysts. In particular silica felt supported Pt-BHA showed excellent activity and selectivity combined with good stability and very low noble metal requirement at the demanding high-temperature conditions of short-contact time CPOM. Overall, we see great potential for these supported nanocomposite catalysts for use in demanding environments, such as high-temperature, high-throughput conditions in fuel processing and similar energy-related applications.     

Methane partial oxidation to synthesis gas using nickel on calcium aluminate catalysts

Nickel was supported on calcium aluminate carriers that were obtained with varying CaO to Al₂O₃ molar ratios and calcination temperatures. The variations of the supports lead to catalysts of different surface properties and catalytic performance. Metallic nickel (Ni⁰) was proven to be the active species for the methane partial oxidation reaction. The presence of filamentous carbon on used catalysts was also suggested. The differences in the catalytic activity and selectivity for the methane partial oxidation reaction was ascribed to a varying degree of reducibility of the surface nickel species.     

Mechanistic aspects of carbon dioxide reforming of methane to synthesis gas over Ni catalysts

A study of the kinetic isotope effect (CH₄/CO₂ CD₄/CO₂) for carbon dioxide reforming of methane to synthesis gas shows that an isotope effect exists with k_{CH 4}/k_{CD 4} ratio of 1.05–1.97, depending on reaction temperature and catalyst applied. The attainment of stable performance over Ni/La₂O₃ catalyst is found to be related to the strong chemisorption of CO₂, weak chemisorption of CH₄ and slow rate of CH_x formation, and fast rate for CH_x removal by oxidation.     

Partial oxidation of methane to synthesis gas

Partial oxidation of methane to synthesis gas has been carried out over a number of transition metal catalysts under a range of conditions. It is found that the metals Ni, Ru, Rh, Pd, Ir and Pt, either supported on alumina or present in mixed metal oxide precursors, will bring the system to equilibrium. The yield of CO and H2 improves with increasing temperature in the range 650–1050 K, and decreases with increasing pressure between 1 and 20 atm. An excellent yield (92%) is obtained with a 421 N₂CH₄O₂ ratio at 1050 K and atmospheric pressure, with a space velocity of 4×10⁴ hour^–1.     

Novel supports for nickel-based catalysts for the partial oxidation of methane

Zirconia-supported nickel catalysts with different amounts of aluminum (Al/Zr = 0.2, 1 and 2) were studied in this work in order to find alternative supports for nickel-based catalysts for the partial oxidation of methane. This reaction is a promising route for producing hydrogen and syngas for different applications. Samples were prepared by precipitation and impregnation techniques, characterized by several techniques and evaluated in the partial oxidation of methane in the range of 450–750 °C and 1 atm. It was found that aluminum affects the textural and catalytic properties of zirconia-supported nickel catalysts. The tetragonal phase of zirconia was stabilized by aluminum and gamma-alumina was also found in the aluminum-richest samples. Aluminum increased the porosity and the specific surface area of the solids. The catalytic activity also increased with the amount of aluminum in solids probably due to the stronger interaction of nickel with the support, which slowly generates active sites during the reduction step. The methane conversion and hydrogen selectivity increased with temperature, indicating no deactivation. The hydrogen to carbon monoxide molar ratio decreased due to aluminum but was not significantly affected by temperature. The coke produced was not harmful to the catalysts and aluminum affected its amount, although no simple relationship was found between these parameters. The most promising catalyst was the sample with aluminum to zirconium molar ratio of 2, which showed high activity and hydrogen selectivity and was stable under the reaction condition.     

Catalytic partial oxidation of methane to synthesis gas in a ceramic membrane reactor

Methane has been selectively converted to synthesis gas using a two-zone fixed bed of a Ni/Al₂O₃ catalyst inside a modified ceramic membrane. The first zone of the reactor was surrounded by an impervious wall, and therefore behaved as a conventional fixed bed reactor. In the second zone, some of the reaction products could preferentially diffuse out of the reactor, which yielded higher than equilibrium methane conversions. The influence of the different operating conditions has been studied, and the performance of the membrane reactor has been compared to that of a fixed bed reactor. The membrane reactor has also been used at pressures above atmospheric (2 bar), with good conversions and selectivities.     

甲烷与二氧化碳重整制合成气Ni与Co基催化剂的研究进展

综述了助剂、载体与催化剂制备方法对Ni基催化剂、Co基催化剂、Ni-Co基催化剂上甲烷与二氧化碳重整制合成气催化性能的影响,以及甲烷与二氧化碳重整催化反应机理的研究,展望了催化剂未来的发展方向。