Effect of Drying Methods on the Structure and Combustion Activity of Mn—Substituted Hexaaluminate Catalysts

Conventional oven drying (COD) and supercritical drying (SCD) methods were applied to the preparation of Mn-substituted hexaaluminate (BaMnA111O19-α) catalysts. The effect of drying methods on phase composition, specific surface area, pore structure, reduction behavior of Mn^3 ions, and combus-tion activity of the samples was investigated. The homogenous mixing of the components in the sol-gel process could be maintained by SCD, and the hexaaluminate phase was almost the only phase of the resulting materials after calcination. H2-TPR revealed that the Mn^3 ions in the sample obtained by SCD were easier to be reduced than that by COD. Moreover, the samples obtained by SCD have higher surface area, narrower pore size distribution, and higher combustion activity than those obtained by COD.     

Ba、Mn对Al2O3热稳定性和甲烷催化燃烧活性的影响

采用溶胶-凝胶法制备BaO•6Al2O3系列催化剂.考察了Ba、Mn的引入对Al2O3的热稳定性及甲烷催化燃烧性能的影响.结果表明, Ba在高温下与Al2O3首先生成BaAl2O4，然后进一步与Al2O3反应生成具有β-Al2O3结构的BaAl12O19六铝酸盐，抑制了Al2O3进一步的烧结，提高了催化剂的热稳定性. Mn的引入也能够促进六铝酸盐的生成并提高催化剂的甲烷燃烧活性.采用超临界干燥法可抑制干燥过程中因毛细收缩引起的比表面积降低.所制备的BaMnAl11O19催化剂经1200 ℃焙烧4 h后，比表面积为35.1 m2•g－1，在空速40000 h－1条件下，10％甲烷转化温度(T10％)为500 ℃，催化活性明显高于常规干燥法制备的相应催化剂.     

Thermal Stability of Pt/Al2O3 Catalysts Prepared by Sol-Gel

A series of Pt/Al₂O₃ catalysts were prepared using a sol-gel method. The influence of several parameters used in the synthesis including: metal content, identity of the metal precursor, and the water/alkoxide ratio on the structural properties of the fresh (dried) and calcined samples were studied. It was found that the BET surface area decreased with an increase in the platinum content. A surface area of 500 m²/g was obtained following calcination at 773 K. The structure of fresh samples as determined by FTIR corresponded to that of a pseudoboehmite structure. Samples prepared using a water/alkoxide ratio (H₂O/ATB) of 9 showed a well-defined, uniform pore size distribution following calcination at 773 K. Metal dispersions comparable to those obtained using impregnation methods were obtained. Aging studies (calcination at 873 K for 24 h) performed on these catalysts, exhibited sintering behavior which were similar to Pt/Al₂O₃ catalysts prepared by other methods. The sample prepared using a H₂O/ATB ratio of 9 had the highest surface area and was more thermally resistant towards metal sintering. A bimodal metal particle size distribution was observed: some particles exhibited sintering while others of similar size showed a greater thermal stability to sintering. The sample having the largest surface area and the highest thermal stability following thermal treatment was a consequence of a more condensed structure and a higher pore roughness obtained after drying the gel. This enabled the formation of an alumina structure which was more amorphous and limited aggregation of platinum particles due to surface diffusion within the pore structure.     

Improvement of the Pd/Al2O3 Catalyst by the Control of the Sol-Gel Preparative Parameters

In this work, the metal dispersion of the Pd/Al₂O₃ catalyst prepared by sol-gel method is improved by an adequate optimisation of the preparative variables. First, the gelation temperature and the ageing time are selected, in order to avoid the reduction of the metal precursor (palladium acetylacetonate, Pd(acac)₂) by the solvent (sec-butanol, sB). The metal sintering effect on the catalysts treated in oxygen at 500°C is then minimized when the alumina pore size is controlled by the variation of the alumium alkoxide (AsB) concentration and the acetic acid amount ([AcA]/[AsB]). The appearance of new palladium particles on the alumina surface and the matching between the particle diameters and the pore sizes were also effective for the metallic surface area improvement on the samples treated in oxygen at 800°C. Compared to the reference catalysts, the higher metal dispersion obtained on the sol-gel ones was the determinant factor for their higher catalytic activity in methane combustion.     

超临界干燥方法对甲烷燃烧催化剂LaMnAl11O19结构及活性的影响

 摘要：用水热合成法制备了锰取代的六铝酸盐催化剂，并比较了超临界干燥法和普通烘箱干燥法对催化剂结构及甲烷燃烧反应活性的影响．DTA－MS结果表明，超临界干燥过程中，催化剂前驱物中的表面铝羟基部分被乙氧基取代．这种表面修饰作用可保持铝分散的均匀性，使催化剂前驱物中碳酸锰和碳酸镧的分解温度明显降低，且氢氧化铝的脱水温度维持在较适宜的范围内；焙烧后，易形成六铝酸盐相．甲烷燃烧反应结果表明，用超临界干燥方法制得的催化剂对甲烷燃烧反应的催化活性明显高于用普通烘箱干燥方法制得的催化剂．     

Supercritical Drying Applied to Natural “Gels”: Allophanic Soils

Allophanic soils behave as gels during a classical drying with an important irreversible shrinkage which can modify the soil physical properties. These soils are interesting in terms of mitigation of the greenhouse effect (C sequestration) and the knowledge of the true soils features are of importance. Like for silica gels, we use the CO₂ supercritical drying procedure (SCD) to control the drying step. The objective is to preserve the structural and textural properties of the soils, which can be affected by the classical drying. We show also that despite the necessary solvent exchange and high pressure of the supercritical drying, the chemical composition and the allophanic features are not strongly affected.The textural properties such as specific surface area are higher for the supercritical dried samples compared to the classical dried samples, indicating the preserving effect of the SCD and showing the interest of the application of this drying method to this natural and complex “gels”. With these data, we show possible effects of the specific surface area on the C and N content of the allophanic soils.     

SiO2气凝胶的制备及微孔分布

本文以正硅酸乙酯(TEOS)为原料，水和乙醇为溶剂，盐酸和氨水为催化剂，采用溶胶-凝胶，水热合成和低温冷冻干燥等工艺，制备了SiO₂气凝胶，研究了pH值、TEOS浓度及焙烧温度对气凝胶的比表面积、孔径分布的影响。结果表明：在碱性条件下，制得的气凝胶比表面积较大，孔径较窄。气凝胶在较高温度下，比表面积有所下降，孔径略微变粗。     

Preparation of a highly active nanocrystalline TiO2 photocatalyst from titanium oxo cluster precursor

Nanocrystalline TiO₂ (sample S1) was prepared from a titanium oxo cluster (Ti₇O₄(OEt)₂₀) precursor via a sol-gel route. This photocatalyst showed a higher photocatalytic activity than the TiO₂ (sample S2) obtained from titanium tetraisopropoxide. The samples were characterized by thermal analysis (TGA/DSC), X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy, N₂ adsorption (BET surface area), infrared absorption spectroscopy (FT-IR) and X-ray photoelectron spectroscopy. The characterization results show that both samples are anatase nanocrystals with particle sizes of about 12 nm, but the more photocatalytically active sample S1 has more surface hydroxyl groups and larger surface area and pore volume than sample S2.     

Combustion synthesis and characterization of porous perovskite catalysts

Porous perovskite-type complex oxides LaCoO₃ and La_0·95Sr_0·05Ni_0·05Co_0·95O₃ were produced by combustion method. The properties of these porous materials such as crystal structures, particle sizes, surface patterns, pore size, surface area and pore volume were characterized by X-ray diffraction( XRD), scanning electron microscopy(SEM) and BET measurements. The results indicated that all porous materials are of the perovskite-type complex oxides. Doping Sr²⁺ ions on site A and doping Ni²⁺ ions on site B entered the crystal lattices of LaCoO₃ in the place of La³⁺ and Co³⁺, respectively, and the maximum peak of XRD patterns of doping sample was weaken and broaden. Morphological microscopy demonstrated agglomerates involved mostly thin smooth flakes and layers perforated by a large number of pores and its lamella decreased with the introduction of Sr²⁺ and Ni²⁺. Hysteresis loop in the N₂ adsorption-desorption isotherm of samples indicated its porous structures and the doping effect on its pore size, surface area and pore volume were improved. The porous catalysts have been tested for methane catalytic combustion and the results showed that these catalysts possessed high catalytic activity.     

Rapid Synthesis of Nanocrystalline TiO2/SnO2 Binary Oxides and Their Photoinduced Decomposition of Methyl Orange

A rapid and simple method, the so-called stearic acid method (SAM) was developed to prepare nanostructured TiO₂/SnO₂ binary oxides by combustion of stearic acid precursors. The preparative process was studied by Fourier transform infrared spectroscopy (FT-IR). During the preparative process, metal precursors were dispersed in stearic acid at molecular level. Microstructure of the samples was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), BET specific surface area measurement and the results were compared with those obtained by conventional sol-gel method. The photocatalytic decomposition of methyl orange was used as a model system to determine the relative influences of the preparation method and the concentration of SnO₂ on the photocatalytic activities. It was found that preparative methods affected the crystalline structure of TiO₂/SnO₂ powders and the anatase phase of TiO₂ was stabilized by the addition of SnO₂ in SAM. The samples prepared by SAM showed better dispersity, larger specific surface area and the TiO₂/SnO₂ (r=0.15, SAM) catalyst showed higher photocatalytic activity than Degussa P25.     

以氨水和碳酸铵为沉淀剂制备氧化铝的对比研究

γ-Al2O3，是一种常用的催化剂载体，它具有比表面大和价廉易得等优点。但对于很多高温反应体系，如汽车尾气催化净化，其热稳定性在很大程度上影响了汽车尾气净化催化剂的活性和稳定性，因此提高γ-Al2O3的高温热稳定性对保持汽车尾气净化催化剂的反应活性、延长催化剂的使用寿命非常重要。     

Sol–gel synthesis,characterization and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods

In this study, boehmite sols were used as alumina precursors for preparing mesoporous γ-aluminas by two different methods. In one case polyethylenimine was used as a structure-directing agent, and in another case ultrasound treatment was applied. Nitrogen physisorption showed that aluminas that had been prepared by these methods demonstrated different porous structures. The sample obtained without additional treatment had closely packed spherical particles and pores had ink-bottle neck morphology. Ultrasound treatment led to the transformation of ink-bottle pores into cylindrical form and to the increase in surface area and pore volume. Aluminas prepared using polyethylenimine as a template showed larger cylindrical wormhole-like mesopores with a broader pore size distribution, high surface area and pore volume. Catalytic tests showed that textural properties as well as crystallite size were very important parameters of synthesized samples which affected the catalytic activity in the methanol dehydration reaction. It was found that γ-Al₂O₃ prepared by ultrasound treatment had large crystallite size and demonstrated high catalytic activity.     

Changes in coal char reactivity and texture during combustion in an entrained flow reactor

Char particle combustion is the slowest step in the combustion of coal, therefore char reactivity and texture have an important influence on this process. In this work, two coals were devolatilised in an entrained flow reactor and the chars obtained were burned under different experimental conditions in order to achieve various degrees of burnoff. Char reactivity was determined by means of non-isothermal thermogravimetric analysis, and the conversion-time data were evaluated by the random pore model proposed by Bhatia and Perlmutter. Char texture was characterised by means of N₂ and CO₂ adsorption isotherms. The surface areas obtained were used to calculate intrinsic reaction rate parameters. It was found that under chemical controlled conditions, the available surface area during combustion is best represented by the N₂ surface area.     

Ambient pressure dried tetrapropoxysilane-based silica aerogels with high specific surface area

In the present paper, we report the synthesis of tetrapropoxysilane (TPOS)-based silica aerogels with high surface area and large pore volume. The silica aerogels were prepared by a two-step sol-gel process followed by surface modification via a simple ambient pressure drying approach. In order to minimize drying shrinkage and obtain hydrophobic aerogels, the surface of the alcogels was modified using trichloromethylsilane as a silylating agent. The effect of the sol-gel compositional parameters on the polymerization of aerogels prepared by TPOS, one of the precursors belonging to the Si(OR)₄ family, was reported for the first time. The oxalic acid and NH₄OH concentrations were adjusted to achieve good-quality aerogels with high surface area, low density, and high transparency. Controlling the hydrolysis and condensation reactions of the TPOS precursor turned out to be the most important factor to determine the pore characteristics of the aerogel. Highly transparent aerogels with high specific surface area (938 m²/g) and low density (0.047 g/cm³) could be obtained using an optimized TPOS/MeOH molar ratio with appropriate concentrations of oxalic acid and NH₄OH.     

Thermal stabilities of hexagonal and orthorhombic YbFeO3 synthesized by solvothermal method and their catalytic activities for methane combustion

Thermal stabilities of hexagonal and orthorhombic YbFeO₃ samples synthesized by the solvothermal method were investigated. The morphology and BET surface area of hexagonal YbFeO₃ did not change by calcination, while orthorhombic YbFeO₃ easily sintered, resulting in a decrease of the BET surface area. The hexagonal YbFeO₃ sample, which had a high surface area (29 m²/g) after calcination at 800 °C, had higher catalytic activity for methane combustion than the orthorhombic YbFeO₃ samples calcined at 800 °C.     

Preparation and structure characterization of carbons prepared from resorcinol-formaldehyde resin by CO2 activation

In this work, carbon xerogels with a high pore volume and surface area (up to 2.58 cm³/g and 3200 m²/g respectively) have been synthesized using the sol-gel polycondensation of resorcinol (R) with formaldehyde (F) in a basic medium of monoethanolamine (MEA), followed by drying and pyrolysis. This medium (MEA) has not been used in previous investigations. The effect of activation with CO₂ on the pore size distribution and the chemical functional groups has been investigated using N₂ (77 K) adsorption, FTIR and elemental analysis techniques. A series of experiments has been conducted to investigate the effect of activation time and activation temperature. Activation of the samples was carried out at 850, 900 and 980 °C for times ranging from one to three hours. Within the range of activation conditions, an increase in activation time at 850 °C results in a continuous steady rise of the BET surface area and total pore volume. However, at the two higher temperatures, the surface area shows a maximum when plotted against activation time. FT-IR results show that the use of MEA as a catalyst leads to the formation of nitrogen functional groups in the surface of the resin.     

HY Zeolite Modified by Liquid Silane Deposition and Its Catalytic Activity in Selective tert-Butylation of Naphthalene

HY zeolites were modified by chemical liquid deposition with i-C₄H₉Si(OC₂H₅)₃, followed by hydrothermal treatment. The samples were characterized by X-ray diffraction, N₂ adsorption and pulse mass analysis to investigate the influence on framework structure, specific surface area, pore diameter and adsorption behaviors. The catalytic performances of HY zeolite and the modified samples in the alkylation of naphthalene with tert-butyl alcohol were also evaluated. The results showed the modification of HY zeolite did not change framework structure but increased specific surface area, decreased average pore diameter, and reduced the size of pore opening. Catalytic activity of the modified HY zeolite catalyst for tert-butylation of naphthalene was decreased compared with that of HY zeolite catalyst while shape-selectivity of 2,6-di-tert-butylnaphthalene (2,6-DTBN) was increased obviously, the highest 2,6-DTBN/2,7-DTBN ratio of 6.62 obtained.     

Silica supported sulfated zirconia prepared by a sol-gel process: Effect of solvent evacuation procedure on the structural, textural and catalytic properties

Sol-gel process was used to prepare silica supported sulfated zirconia catalysts. The main parameter studied in this work was the gel drying method through four different ways of solvent evacuation. Textural, structural as well as the acidic properties of the four samples were studied using N₂ physisorption, X-ray diffraction, mass spectrometry, sulfur chemical analysis and adsorption-desorption of pyridine. The isomerization of n-hexane was used as a catalytic test. The surface areas and the pore distributions are highly affected by the drying mode. One of the four drying methods leads to a solid having improved textural properties and presenting both the crystalline ZrO₂ tetragonal phase and a particular type of sulfate mode bond. It seems that these three conditions are necessary for achieving high catalytic activity.     

Catalytic activity of Mn-substituted barium hexaaluminates for methane combustion

The catalysts of hexaaluminate (BaMnxAl12-xO19-δ , x = 1.0, 2.0, 3.0, 4.0) to be used in methane combustion have been successfully synthesized by co-precipitation method and supercritical drying. The crystalline structure and surface area of catalyst were characterized by X-ray diffraction (XRD) and nitrogen adsorption analysis of BET method. BET analysis revealed that the preparing and drying method proposed here provides stable materials with higher surface area of 51.4 m2/g in comparison to materials prepared using conventional ambient drying method for BaMnxAl12?xO19-δ calcined at 1200℃ under oxygen. XRD analysis indicated that formation of a pure single phase BaMnxAl12-xO19-δ occurred up to x = 3 in the case of Mn-substituted barium hexaaluminates. Incorporation of Mn in excess leads to BaAl2O4 phase formation. As far as the valence state of Manganese ions was concerned, the introduced Mn ions were either divalent or trivalent. The first Mn ions were introduced in the matrix essentially as Mn2 and only for BaMn3Al9O19-δ does manganese exist exclusively as Mn3 ; the higher the Mn concen- tration, the higher the proportion of Mn3 . Catalytic activity for methane combustion has been measured for Mn-substituted barium hexaaluminates, light-off temperature was observed in the 512-624℃ range. The highest activity was obtained for catalysts containing 3 Mn ions per unit cell, which reveals that the BaMnxAl12-xO19-δ catalyst was a promising methane combustion catalyst with high activity and good thermal stability. Temperature programmed reduction (TPR) under hydrogen has been used to correlate the catalytic activity with the amount of easily reducible species.     

铁掺杂的高比表面二氧化硅的制备及其苯酚降解性能

研究了以聚乙二醇为模板剂、正硅酸乙酯(TEOS)为硅源制备铁掺杂的多孔二氧化硅的方法。开发了一步完成多孔材料制备和掺杂的新工艺。研究了不同的铁元素掺杂量对样品性能的影响。采用低温氮吸附、SEM、FTIR、XRD方法表征了样品的比表面、孔结构和表面基团等信息。最优样品比表面大于700 m²·g^-1、孔容大于1 mL·g^-1。研究了所制备的多孔材料和双氧水共同降解水中苯酚的能力,研究发现负载铁的催化剂可以在很宽的pH值范围内(3~8)和双氧水协同使用,这可能是因为铁元素被牢固负载于多孔二氧化硅的骨架上,避免了其在高pH值下发生的水解反应。