Steam reforming of naphthalene as model biomass tar over iron-aluminum and iron-zirconium oxide catalyst catalysts

In this study the catalytic properties of iron-based mixed metal oxides such as iron-alumina (Fe-Al) and iron-zirconia (Fe-Zr) were investigated at 850 °C in a fixed bed reactor for the steam reforming of naphthalene as a model biomass tar compound. The effects of addition of copper species (CuO) to the iron-based mixed metal oxide catalysts were also examined. For Fe-Al catalysts, the catalytic activities for naphthalene conversion increased with increasing Fe content except for 100Fe-0Al. The catalytic activities of Fe-Al and Fe-Zr were comparable at steady state conditions. Compound oxides were formed in the cases of Fe-Al, but not in Fe-Zr. A strong peak in the vicinity of 2θ = 45° for metallic iron was observed after catalytic experiments in the XRD patterns of all catalysts, which could be related to the active sites of the catalysts. The addition of CuO increased the activities and stability of the Fe-Al catalysts. The reasons for catalytic activity enhancement due to CuO addition can be explained as follows: copper dispersed evenly in the compound oxides facilitate the reduction of iron oxides to metallic iron and prevent the catalytic deactivation due to decrease in surface area of the catalysts during the reaction.     

NiO-MgO固溶体蜂窝催化剂部分氧化重整净化生物质气特性

采用分步浸渍法制备了NiO-MgO固溶体蜂窝催化剂,以60 h连续重整实验,考察了不同临氧条件下MCM催化剂催化生物质气重整净化的性能。利用TG-DSC对催化剂的表面积炭进行了分析,采用GC、GC-MS等手段对生物质气组成及焦油转化产物进行了分析。结果表明：加入O₂后进行临氧重整,可改善出口合成气的品质,H₂/CO摩尔比较干重整时提高了10%左右。但当O₂/fuel摩尔比超过0.127时,H₂/CO摩尔比有所下降。O₂的加入可明显提高催化剂的催化活性和稳定性,随着O₂ 含量的增加出口合成气中的CH₄含量降低,临氧重整条件下焦油的转化率较无氧条件时提高了5%,转化率达到99%以上。热重分析表明,O₂ 含量的增加会减少催化剂表面积炭量。实验中添加从气化现场提取的生物质焦油,经检测焦油大部分转化为H₂、CO及痕量轻质组分,且对干重整中难于转化的含氧有机化合物等转化更为彻底。     

Partial oxidation (POX) reforming of gasoline for fuel-cell powered vehicles applications

As a part of the development of a gasoline processor for integration with a proton-exchanged membrane (PEM) fuel cell, we carried out the POX reforming reaction ofiso-octane, toluene and gasoline over a commercial methane reforming catalyst, and investigated the reaction conditions required to prevent the formation of carbon and the effect of fuel constituents and sulfur impurities in gasoline. The H₂ and CO compositions increased with increasing reaction temperature, while those of CO₂ and CH₄ decreased. It is desirable to maintain an O/C molar ratio of more than 0.6 and an H₂O/C molar ratio of 1.5 to 2.0 for vehicle applications. It has been found that carbon formation in the POX reforming ofiso-octane occurs below 620 °C, whereas in the case of toluene it occurs below 640 °C. POX reforming of gasoline constituents led to the conclusion that hydrogen production is directly related to the constituents of fuels and the operating conditions. It was also found that the coke formation on the surface of catalysts is promoted by sulfur impurities in fuels. For the integration of a fuel processor with PEM fuel cell, studies are needed on the development of new high-performance transition metal-based catalysts with sulfur and coke-resistance and the desulfurization of fuels before applying the POX reformer based on gasoline feed.     

Investigation of alumina-supported Ni and Ni-Pd catalysts by partial oxidation and steam reforming of n-octane

Aseries of nickel and nickel-palladium supported upon alumina catalysts were prepared in order to obtain a suitable catalyst that could be used in the process of producing hydrogen by partial oxidation and steam reforming of n-octane. Hydrogen production by partial oxidation and steam reforming (POSR) of n-octane was investigated over alumina-supported Ni and Ni-Pd catalysts. The process occurred by a combination of exothermic partial oxidation and endothermic steam reforming of n-octane. It was found that Ni/Al₂O₃ catalyst activity was high at high temperatures and increased with the Ni loadings. Its activity, however, was not obviously increased when Ni loadings were over 5.0 wt%. Compared with nickel catalyst, the bimetallic catalyst of Ni-Pd/A1₂O₃ showed markedly increased activity and hydrogen selectivity at experimental conditions. The catalytic performance also became more stable when the palladium was added, which indicated that palladium plays an essential role in the catalytic action. The used catalysts of Ni-Pd/A1₂O₃ were regenerated three times by using air at space velocity of 2,000 h⁻¹ to obtain a long duration catalyst. Also, the typical catalyst was characterized by using SEM, BET, TG and ICP methods in detail.     

Partial oxidation of ethane to synthesis gas over Co-loaded catalysts

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: Y₂O₃, CeO₂, ZrO₂, La₂O₃  SiO₂, Al₂O₃, TiO₂ > MgO. Comparing Y₂O₃ and CeO₂, the carbon deposition during the POE was negligible on CeO₂ and therefore CeO₂ was the most preferable support. By changing space velocity and O₂ partial pressure, reaction mechanism of POE was studied and it was revealed that two-step mechanism was prevailing; combustion of ethane to H₂O and CO₂ and subsequent reforming of ethane with H₂O and CO₂ to synthesis gas. Co/CeO₂ catalyst exhibited high and stable catalytic activity for 10 h; high ethane conversion of 18% (maximum ethane conversion 20% at O₂/C₂H₆ = 0.2) with H₂ and CO selectivities of 93 and 84%, respectively.     

Evaluation of different oxygen carriers for biomass tar reforming (I): Carbon deposition in experiments with toluene

In this work, an innovative method for gas conditioning in biomass gasification is analyzed. The objective is to remove tar by selectively reforming the unwanted hydrocarbons in the product gas with a chemical looping reformer (CLR), while minimizing the carbon formation during the process. Toluene, in a concentration of 600-2000 ppmv, was chosen as a tar model compound. Experiments were performed in a TGA apparatus and a fixed bed reactor. Four oxygen carriers (60% NiO/MgAl₂O₄ (Ni60), 40% NiO/NiAl₂O₄ (Ni40), 40% Mn₃O₄/Mg-ZrO₂ (Mn40) and FeTiO₃ (Fe)) were tested under alternating reducing/oxidizing cycles. Several variables affecting the reducing cycle were analyzed: temperature, time for the reduction step and H₂O/C₇H₈ molar ratio. Ni40 and Mn40 presented interesting characteristics for CLR of biomass tar. Both showed stable reactivity to C₇H₈ after a few cycles. Ni40 showed a high tendency to carbon deposition compared to Mn40, specially at high temperatures. Carbon deposition could be controlled by decreasing the temperature and the time for the reduction step. The addition of water also reduced the amount of carbon deposited, which was completely avoided working with a H₂O/C₇H₈ molar ratio of 26.4.     

Steam reforming of tar from a biomass gasification process over Ni/olivine catalyst using toluene as a model compound

A Ni/olivine catalyst, previously developed for biomass gasification and tar removal during fluidized bed steam gasification of biomass, was tested in a fixed bed reactor in toluene steam reforming as a tar destruction model reaction. The influence of the catalyst preparation parameters (nickel precursor, calcination temperature and nickel content) and operating parameters (reaction temperature, steam to carbon S/C ratio and space-time) on activity and selectivity was examined showing a high toluene conversion and a low carbon formation compared to olivine alone. The steam reforming of toluene was found to be of zero order for water and first order for toluene. Activation energy required for Ni/olivine was determined to be about 196 kJ mol⁻¹ in accordance with literature. Catalyst activity and stability and its resistance against carbon formation were discussed on the basis of X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature programmed oxidation (TPO) results. Characterization before test (XRD, temperature programmed reduction (TPR), Mössbauer spectroscopy) have shown the presence of NiO–MgO solid solution, formed on the surface of olivine support, which explains the efficiency of the catalyst calcined at 1100 °C. After test, Ni–Fe alloys were observed (TEM, Mössbauer spectroscopy). It was suggested that magnesium oxide enhanced steam adsorption, facilitating the gasification of surface carbon and that Ni–Fe alloys prevented carbon deposition by dilution effect.     

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.     

Catalytic performance of supported Ni catalysts in partial oxidation and steam reforming of tar derived from the pyrolysis of wood biomass

Activity test of Ni/Al₂O₃, Ni/ZrO₂, Ni/TiO₂, Ni/CeO₂ and Ni/MgO catalysts in the partial oxidation (POT) and steam reforming of tar (SRT) derived from the pyrolysis of cedar wood was performed. In these activity tests, the order of the performance in both reactions was similar. Catalyst characterization was also carried out by means of H₂ adsorption, TPR and XRD. From the combination of catalyst characterization with the results of the activity tests, it is suggested that the conversion of tar in POT and SRT is mainly controlled by the number of surface Ni metal. In addition, Ni/CeO₂ showed smaller amount of coke than other catalysts in the POT and SRT. From the TGA profiles of active carbon mixed with catalysts, it is found that Ni/CeO₂ promoted the reaction of active carbon with O₂ and steam. The function of the fluidized bed reactor in the POT with respect to coke and tar amount was discussed.     

NiO/MgO整体式催化剂上甲烷部分氧化制备合成气

甲烷部分氧化制备合成气反应过程具有反应速率快、能耗低和H₂与CO物质的量比适用于合成甲醇及F-T合成等优点,是一种有希望替代传统水蒸汽重整的方法。研究在NiO/MgO蜂窝陶瓷整体式催化剂上的甲烷部分氧化过程,主要考察涂层载体、活性组分Ni含量、涂层载体前驱体、焙烧温度和还原温度对催化剂反应性能的影响。采用XRD、H₂-TPR和N2吸附等表征前驱体及其负载活性组分NiO后的晶相、还原特性和吸附性能。结果表明,采用浸渍法制备催化剂时,Mg(NO₃)₂为涂层载体MgO前驱体,在NiO负载质量分数20%、焙烧温度（500~600） ℃和还原温度750 ℃条件下制备的催化剂NiO/MgO-N性能较好,活性较稳定;以NiO/MgO-N为催化剂,在反应温度800 ℃、n(O₂)∶n(CH₄)＝0.5和空速9 723 h^-1条件下,CH₄转化率94.4%,H₂选择性99.9%,CO选择性92.9%。     

Catalytic reforming of model tar compounds from hot coke oven gas with low steam/carbon ratio over Ni/MgO-Al2O3 catalysts

The catalytic reforming of toluene and naphthalene was performed to investigate the possibility for directly converting tar components from hot coke oven gas (COG) with lower steam/carbon (S/C) molar ratios to light fuel gases. The NiO/MgO-Al₂O₃ catalysts reduced exhibited excellent catalytic activity, stability and sulphur tolerance. The effects of various reaction conditions and S/C ratios on the catalytic performance were investigated in detail. Toluene and naphthalene were completely converted into small gas molecules at 700-800 °C and S/C = 0.28. An appropriate amount of steam benefited the methanation reaction of CO and H₂. The effects of N₂, CH₄ or CO in COG were also discussed. Relative to N₂, CO contributed to the conversion of toluene and the formation of CH₄, but the opposite was true for CH₄. The sulphur tolerance was tested by adding H₂S in the feed gas. The reaction results were explained by a water cycle mechanism.     

Ceramic foam as a potential molten salt oxidation catalyst support in the removal of soot from diesel exhaust gas

An exploratory study has been carried out to determine the potential of open-pore ceramic foam as support for molten-salt diesel soot oxidation catalysts. These catalysts are based on eutectic mixtures of Cs₂O, V₂O₅, MoO₃, and Cs₂SO₄. Open-pore ceramic foams are very interesting, since they can act as supports and as soot filters. 50 ppi -alumina foam has been tested as support substance. Attractive methods have been developed for depositing the molten salt on the external surface of the pore walls of the foam. The combustion of ‘loose-contact’ synthetic soot has been analysed with a micro-flow reactor. The oxidation rate is a function of the amount of molten salt deposited on the foam. Stable rates in the range from 5 to 25 μg_soot/g_soot,initial/s at 650 K have been observed. These rates are in the same order of magnitude as measured for the best catalytic fuel additives; and, therefore, catalytic foams are very promising. The initially minor ‘deactivation’ that occurs after the thermal treatment up to 100 h at 725 K probably has a physical explanation. During the treatment, the molten salt redistributes to a configuration that has less catalyst available for accelerating the soot oxidation.     

Evolution of biomass char structure during oxidation in O2 as revealed with FT-Raman spectroscopy

FT-Raman spectroscopy has been used to identify structural features and evaluate the structural evolution of biomass chars during gasification with air. Chars prepared from the pyrolysis of a cane trash sample with a fast particle heating rate in a novel fluidised-bed/fixed-bed reactor at 500, 700 and 900 °C were oxidised at 400 °C in air in a TGA. The data derived from the spectral deconvolution of Fourier Transform — Raman spectra suggest that the 500 °C char showed very different structural features after pyrolysis and during oxidation from the 700 and 900 °C chars, while the differences between the latter two chars were small. Preferential consumption by O₂ of smaller aromatic rings and structures of somewhat aliphatic characteristics left the char more enriched with larger aromatic ring systems. The changes in char structure are in agreement with the observed reactivity measured in O₂ in a thermogravimetric analyser.     

Partial oxidation of methane over nickel catalysts supported on various aluminas

Partial oxidation of methane (POM) was systematically investigated in a fixed bed reactor over 12 wt% Ni catalysts supported on α-A1₂O₃, γ-A1₂O₃ and θ-A1₂O₃ which were prepared at different conditions. Results indicate that the catalytic activity toward POM strongly depends on the BET surface area of the support. All the Ni/ θ-Al₂O₃ catalysts showed high activity toward POM due to the less formation of inactive NiAl₂O₄ species, the existence of NiO, species and stable θ-Al₂O₃ phase. Although Ni/γ-Al₂O₃ showed the highest activity toward POM, long-time stability cannot be expected as a result of the deterioration of the support at higher temperature, which is revealed from BET results. From the reaction and characterization results, it is inferred that the optimal conditions for the preparation of θ-Al₂O₃ are 1,173 K and 12 h.     

Partial oxidation of methane to CO and H2 over nickel and/or cobalt containing ZrO2, ThO2, UO2, TiO2 and SiO2 catalysts

The influence of different metal oxide supports (i.e. ZrO₂, ThO₂, UO₂, TiO₂ and SiO₂) on the performance of Ni- and/or Co-containing catalysts [Ni and/or Co/MO₂ mole ratio (where M=Zr, Th, U, Ti or Si)=1.0] in the oxidative methane-to-syngas conversion at very low contact time (GHSV=5.2×10⁵ cm³ g⁻¹ h⁻¹ at STP) was investigated. The nickel-containing ZrO₂, ThO₂ and UO₂ catalysts (with or without pre-reduction by hydrogen at 500°C) showed good performance in the process; the order of their performance is NiO–ThO₂>NiO–UO₂>NiO–ZrO₂. The NiO–TiO₂ showed appreciable catalytic activity only after its reduction at 800°C. However, this catalyst and the NiO–SiO₂ catalyst showed poor performance in the process. These two catalysts are also deactivated very fast, mostly because of sintering of Ni and/or formation of catalytically inactive binary metal oxide phases by solid–solid reaction at the high catalyst calcination and/or catalytic reaction temperature. Although the Ni-containing ThO₂, UO₂ and ZrO₂ catalysts showed good performance, carbon deposition on them during the process is fast. However, because of the addition of cobalt to these catalysts (with Co/Ni=1.0), the rate of carbon deposition on them in the process is drastically reduced. This Co addition however resulted in a significant decrease in both the conversion and selectivity; the decrease in the selectivity was small.     

Partial oxidation of methane over Ni/Ce-Zro<Subscript>2</Subscript>/θ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The catalytic behavior of Ni/Ce-ZrO₂/θ-Al₂O₃ has been investigated in the partial oxidation of methane (POM) toward synthesis gas. The catalyst showed high activity and selectivity due to the heat treatment of the support and the promotional effect of Ce-ZrO₂. It is suggested that the support was stabilized through the heat treatment of γ-Al₂O₃ and the precoating of Ce-ZrO₂, on which a protective layer was formed. Moreover, sintering of the catalyst was greatly suppressed for 24 h test. Pulse experiments of CH₄, O₂ and/or CH₄/O₂ with a molar ratio of 2 were systematically performed over fresh, partially reduced and well reduced catalyst. Results indicate that CH₄ can be partially oxidized to CO and H₂ by the reactive oxygen in complex NiO_x species existing over the fresh catalyst. It is demonstrated that POM over Ni/Ce-ZrO₂/θ-Al₂O₃ follows the pyrolysis mechanism, and both the carbonaceous materials from CH₄ decomposition over metallic nickel and the reactive oxygen species present on NiO_x and Ce-ZrO₂ are intermediates for POM.