天然气催化燃烧催化剂的研究(Ⅱ）

介绍了天然气催化燃烧钙钛矿型氧化物催化剂、六铝酸盐型催化剂以及负载型非贵金属催化剂的研究现状。对于钙钛矿型氧化物催化剂,利用A位取代或调整B位元素的种类及配比、新的技术和方法制备高比表面积或具有纳米结构的钙钛矿型氧化物,是提高其甲烷催化燃烧活性的重要手段。六铝酸盐型催化剂具有很高的热稳定性和甲烷燃烧活性,但起燃温度较高,通过采用将金属Pd负载到六铝酸盐上或改变制备方法,提高其比表面积,以提高其低温反应活性。负载型非贵金属催化剂研究最多的是过渡族金属,其氧化活性、抗毒性能和耐久性都存在问题,需进一步研究。     

Surfactant-mediated synthesis of metal substituted hexaaluminate from alumina sol

Manganese substituted hexaaluminate has been prepared using environmentally benign surfactants such as Triton X-100, under ambient condition with a commercial alumina sol and metal acetate precursors. The surface area of the pure alumina can be controlled to 10–70 m² g⁻¹ using cetyltrimethylammonium chloride after heating in oxygen flow at 1200°C for 6 h. The crystal structure of the obtained alumina was high purity θ-Al₂O₃. Incorporation of La and Mn leads to the formation of the high purity manganese substituted hexaaluminate with a surface area of 30–40 m² g⁻¹ which is also controllable using organic additives such as urea. The catalytic activity of the manganese substituted hexaaluminate was comparable to the sol–gel derived hexaaluminate catalyst from metal alkoxides.     

Hydroformylation of mixed octenes catalyzed by supported rhodium-based catalyst

In order to solve the difficult separation between catalyst and products in homogeneous system, the activated carbon (AC)-supported rhodium-based catalyst (Rh/AC) was prepared. Hydroformylation of mixed octenes catalyzed by Rh/AC was studied, and compared to that catalyzed by RhCl(CO)(TPPTS)₂ [TPPTS: trisodium salt of tris(m-sulphonylphenyl) phosphine], and [Rh(CH₃COO)₂]₂-Ph₃PO (Ph₃PO: triphenyl phosphine oxide). The performance test of the catalysts showed the Rh/AC presented higher catalytic activity, selectivity and air-stability. During the recycle experiments Rh/AC could be used 4 times without significant loss of rhodium. The effects of the supports, rhodium loading and reaction conditions on the catalytic performance of Rh/AC were investigated. The results showed petroleum coke-based activated carbon with higher surface area and more basic groups was advantageous to the formation of aldehydes. The heterogeneous Rh/AC catalyst displayed higher catalytic activity and reusability.     

Thick-film coating of hexaaluminate catalyst on ceramic substrates for high-temperature combustion

Structural and chemical stabilities of substituted hexaaluminate catalyst films coated on some ceramic substrates were investigated for high-temperature combustion applications. The thermal stability of the hexaaluminate catalyst films on -SiC substrate was greatly enhanced by the insertion of both, a neat hexaaluminate and mullite intermediate layer. Pure alumina substrate was preferable in depositing the substituted hexaaluminate catalyst film to pure mullite, mullite-zirconia composite, or partially stabilized zirconia substrates. The thermal stability of the hexaaluminate catalyst films, coated on these oxide ceramic substrates, greatly depended on the extent of diffusion of components between the film and substrate at high temperatures. The substituted hexaaluminate microparticles in the film rearranged during the sintering process above 1400°C, and their (00l) plane oriented parallel to the surface of the substrate. Manganese introduced in the substituted hexaaluminate films or disks gradually decreased with an increase in the heat-treatment temperature because of its volatilization.     

制备方法对BaNi0.2Mn0.8Al11O19-δ催化燃烧二甲醚的催化活性的影响

Catalytic combustion of dimethyl ether (DME) over hexaaluminate catalyst BaNi0.2Mn0.8Al11O19-δ has been investigated. The catalysts were prepared with the sol-gel method and reverse microemulsion method respectively and characterized by thermogravimetry-differential thermal analysis, X-ray diffraction and transimission electron microscope. It was found that the formation of Mn, Ni modified hexaaluminate was a relatively slow process via two solid state reactions and spinel structure was a transition phase. At the same calcined temperature and time, the catalyst prepared with the reverse microemulsion method could form the hexaaluminate phase more easily than that prepared with the sol-gel method. The catalyst BaNi0.2Mn0.8Al11O19-δ prepared with the reverse micro-emulsion method appeared a plate-like morphology, while it appeared a needle-like morphology when using the sol-gel method. The catalytic activities of catalysts BaNi0.2Mn0.8Al11O19-δ prepared with two different methods for DME combustion were tested. It showed that catalyst prepared with the reverse microemulsion method had better catalytic activity, i. e. T10% of DME had decreased by 45℃, about 90% conversion of diemthyl ether at 380℃.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Mn、Fe取代六铝酸盐的结构和甲烷催化燃烧性能

采用化学可控共沉淀法制备了系列取代六铝酸盐LaMe_xAl_12-xO_19-δ（Me＝Fe、Mn）催化剂,研究了焙烧温度和Fe、Mn的离子取代量对催化剂比表面、结构及甲烷催化燃烧活性的影响.结果表明,催化剂前驱物经1000℃焙烧,催化剂中开始有六铝酸盐晶相生成; 当焙烧温度提高到1200℃时,样品主要以六铝酸盐晶相存在.增加Fe、Mn离子取代量可以提高六铝酸盐晶相的结晶度,但同时导致晶粒增大,引起比表面下降.由XPS和TPR分析表明,Mn在六铝酸盐结构中以+2价和+3价混合价态存在,而Fe以+3价形式存在.用Fe和Mn离子取代晶格中的Al³⁺大大提高了六铝酸盐对甲烷催化燃烧活性,当Mn离子的取代数为1,Fe离子的取代数为2时催化剂的活性最高.     

Fe、Ce掺杂六铝酸盐催化剂制备及脱硝抗硫性能

用共沉淀法制备六铝酸盐催化剂LaM_xAl_12-xO_19-δ(M=Fe、Ce、FeCe)。用XRD、H₂-TPR以及NO转化率对催化剂进行表征和活性验证,分析不同离子取代Al³⁺对六铝酸盐结构和脱硝抗硫活性的影响。结果表明,Fe离子易于进入六铝酸盐晶格,形成完整的六铝酸盐结构,LaFeAl₁₁O_19-δ催化剂具有良好的低温脱硝活性。Ce离子不易进入六铝酸盐晶格,主要以CeO₂的形式存在,LaCeAl₁₁O_19-δ催化剂有良好的高温脱硝活性。加入SO₂后,两种催化剂都有不同程度的失活现象,其中,LaFeAl₁₁O_19-δ催化剂失活较为明显。LaFeCeAl₁₀O_19-δ催化剂中,Fe离子和Ce离子间会产生协同作用,提高了催化剂的脱硝抗硫活性。     

Application of Ce0.33Zr0.63Pr0.04O2-supported noble metal catalysts in the catalytic wet air oxidation of 2-chlorophenol: Influence of the reaction conditions

In this work, different procedures, namely carbonate coprecipitation and modified solid–solid diffusion, were used to prepare hexaaluminate samples, unsupported or supported onto θ-Al₂O₃. These samples were used as catalyst for the methane total oxidation as synthesized or after impregnation of 1 wt% Pd. It was observed that the modified solid–solid diffusion procedure is an efficient method to obtain the hexaaluminate structure. At a theoretical ratio x of hexaaluminate onto Al₂O₃ less than 0.6 (xLa_0.2Sr_0.3Ba_0.5MnAl₁₁O₁₉ + (1−x)·Al₂O₃, with x = 0.25, 0.60), samples with high specific surface area and θ-Al₂O₃ structure are then obtained. Large differences in catalytic activity can be observed among the series of sample synthesized. All the pure oxide samples (i.e. without palladium) present low catalytic activity for methane total oxidation compared to a reference Pd/Al₂O₃ catalyst. The highest activity was obtained for the samples presenting a θ-Al₂O₃ structure (with x = 0.60) and a high surface area. Impregnation of 1 wt% palladium resulted in an increase in catalytic activity, for all the solids synthesized in this work. Even if the lowest light-off temperature was obtained on the reference sample, similar methane conversions at high temperature (700 °C) were obtained on the stabilized θ-Al₂O₃ solids (x = 0.25, 0.60). Moreover, the reference sample is found to strongly deactivate with reaction time at the temperature of test (700 °C), due to a progressive reduction of the PdO_x active phase into the less active Pd° phase, whereas excellent stabilities in reaction were obtained on the pure and palladium-doped hexaaluminate and supported θ-Al₂O₃ samples. This clearly showed the beneficial effect of the support for the stabilization of the PdO_x active phase at high reaction temperature. These properties are discussed in term of oxygen transfer from the support to the palladium particle. Oxygen transfer is directly related to the Mn³⁺/Mn²⁺ redox properties (in the case of the hexaaluminate and stabilized θ-Al₂O₃ samples), that allows a fast reoxidation of the metal palladium sites since palladium sites reoxidation cannot occur directly by gaseous dioxygen adsorption and dissociation on the surface.     

Mild hydrogenation of quinoline to decahydroquinoline over rhodium nanoparticles entrapped in aluminum oxy-hydroxide

An Rh/AlO(OH) catalyst was prepared by a sol–gel method. This catalyst showed an excellent catalytic performance for the complete hydrogenation of quinoline to decahydroquinoline at relatively mild conditions. The growth of Rh-particle size and the decrease in the number of surface hydroxyl groups during heat treatment resulted in a significant decrease in catalytic properties. The excellent catalytic performance of the fresh Rh/AlO(OH) was attributed to the cooperation between the hydroxyl groups on the support and on the active metal centers.     

The role of multiwalled carbon nanotubes in enhancing the catalytic activity of cobalt tetraaminophthalocyanine for oxidation of conjugated dyes

Multiwalled carbon nanotubes (MWCNTs) used to support a metallophthalocyanine catalyst (CoTAPc-MWCNTs) were prepared using covalent immobilization of cobalt tetraaminophthalocyanine (CoTAPc) on them, and characterized by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectra and thermogravimetric analysis. The oxidative decoloration of rhodamine 6G (Rh6G) in the presence of CoTAPc-MWCNTs and H₂O₂ was investigated by examination of UV-Vis absorption spectra. The results showed that Rh6G was oxidized efficiently in the CoTAPc-MWCNTs/H₂O₂ system. The introduction of MWCNTs resulted in a marked enhanced catalytic activity that CoTAPc does not have. Electron paramagnetic resonance spin-trap experiments indicated that CoTAPc-MWCNTs have a novel non-radical pathway, which is different from common CoTAPc catalytic systems. Furthermore, the result of online electrochemical measurement in the CoTAPc-MWCNTs/H₂O₂ system suggested that MWCNTs might participate directly in the electron transfer process in the catalytic oxidation of this conjugated dye. In this catalytic system, MWCNTs provide strong adsorption to conjugated Rh6G due to their special sp²-hybridized surface, and are able to rapidly oxidize the conjugated dye by a special electron transfer pathway.     

The Effect of Chelating Agent on the Catalytic and Structural Properties of Sm2Zr2O7 as a Methane Combustion Catalyst

The Sm₂Zr₂O₇ of pyrochlore structure was studied as a catalyst for high temperature combustion. It was prepared by the sol-gel method with and without acetylacetone as a chelating agent. The order of crystallinity and the surface area were improved when acetylacetone was used. The catalytic activity of CH₄ combustion was also enhanced and superior to that of Mn-substituted hexaaluminate above 550 °C.     

铜锌掺杂的六铝酸盐分解N2O催化性能研究

采用共沉淀法制备了Cu和Zn取代六铝酸盐（LaCu_xZn_1-xAl₁₁O_19-δ,x=0、0.2、0.4、0.5、0.6、0.8、1.0）催化剂,通过XRD和BET等技术对催化剂进行表征,利用微型固定床反应器考察催化剂对N₂O的分解活性。结果表明,以(NH₄)₂CO₃为沉淀剂制备的催化剂在1 200 ℃焙烧4 h,可以形成完整的六铝酸盐晶型,Cu和Zn能够取代Al³⁺进入六铝酸盐晶体结构。LaCu_xZn_1-xAl₁₁O_19-δ催化剂对N₂O分解有较好的催化活性,其中,LaCu_0.8Zn_0.2Al₁₁O_19-δ活性较好。在LaCu_xZn_1-xAl₁₁O_19-δ六铝酸盐中,Cu为N₂O催化分解的主要活性元素,Zn有助于提高催化剂的稳定性,但由于六铝酸盐的特殊结构,Zn在催化剂中的作用较小。     

Selective hydrogenation of oleic acid to fatty alcohols using Rh-Sn-B/TiO2 catalysts: Influence of Sn content

The influence of the catalyst Sn content on the production of fatty alcohol from oleic acid by selective hydrogenation was studied using Rh-Sn-B catalysts supported on TiO₂. The crystal phase of the support was analyzed by X-ray diffraction (XRD), the reduction state of the metal phase by temperature-programmed reduction (TPR), and the electronic state of surface species by X-ray photoelectron spectroscopy (XPS). The metal activity was evaluated by the dehydrogenation of cyclohexane. It was found that the increase in Sn content leads to a proportional drop in the catalytic activity, which could be related to a metallic interaction between Rh and Sn, as shown by TPR. Oxide and metallic Sn, as well as Rh⁰ and Rh³⁺, were found by XPS on the catalyst surface. Metallic Rh was, however, found in higher concentration than oxidized Rh in all cases. The yield to fatty alcohols increased with Sn content, and its maximum value for oleyl and stearyl alcohol was 96%. Furthermore, a higher yield (88.3%) was obtained out of unsaturated fatty alcohol (oleyl alcohol), which has proved to be more valuable than saturated alcohol. This was attributed to an adequate Rh/Sn ratio, which modulates the hydrogenating activity of Rh and makes the metal function more selective for hydrogenation of the carbonyl group. The influence of the support on the catalyst performance decreases as the Sn content increases. The support has a practically negligible influence on the catalyst activity for 4–5 wt.% of Sn content.     

二甲醚催化燃烧研究的进展

介绍了近年来二甲醚作为一种清洁能源的研究状况，进行了二甲醚的催化燃烧研究的初探。通过对贵金属、金属氧化物和六铝酸盐等催化剂应用于二甲醚催化燃烧的比较，发现六铝酸盐最具研究应用前景。     

Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion

The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al₂O₃, ZrO₂, FSM-16, CeO₂, La₂O₃) showed that the activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N₂O experiment over a Rh/USY catalyst suggested that the catalytic N₂O decomposition occurs on oxygen-covered surface and that O₂ may be freed on collision of N₂O molecules with the adsorbed oxygen atoms.     

Support-induced promotional effects on the activity of automotive exhaust catalysts 1. The case of oxidation of light hydrocarbons (C2H4)

The kinetics of oxidation of a light hydrocarbon (C₂H₄) were studied on catalysts comprising of combinations of one of three metals, Pt, Pd or Rh supported on five different supports, that is, SiO₂, γ-Al₂O₃, ZrO₂ (8% Y₂O₃), TiO₂ or TiO₂ (W⁶⁺). Significant variation of turnover frequency with the carrier was observed, which cannot be explained by structure sensitivity considerations and is attributed to interactions between the metal crystallites and the carrier. The catalytic activity of these metal-support combinations was investigated over a wide range of partial pressures of ethylene and oxygen. In a separate set of experiments, the kinetics of C₂H₄ oxidation were also investigated on polycrystalline Rh films interfaced with ZrO₂ (8 mol% Y₂O₃) solid electrolyte in a galvanic cell of the type: C₂H₄, O₂, Rh/YSZ/Pt, air, during regular open-circuit conditions as well as under Non-Faradic Electrochemical Modification of Catalytic Activity (NEMCA), that is, closed-circuit conditions. Up to 100-fold increase in catalytic activity was observed by supplying O²⁻ ions to the catalyst surface via positive potential application to the catalyst. The observed kinetic behavior upon increasing catalyst potential parallels qualitatively the observed alteration of turnover frequency with variation of the support of the Rh crystallites.     

Catalytic Purification of Exhaust Gases Over Pd–Rh Alloy Catalysts

Three-way catalysts with low content of Pd–Rh alloy particles used as active components were synthesized and studied. Monometallic and bimetallic mixed catalysts with corresponding precious metals loading were chosen as reference samples. The catalytic activity was tested in oxidation of carbon monoxide, hydrocarbons, and in nitrogen oxides reduction. The stability of the samples was estimated by prompt thermal aging in situ technique. Ethane hydrogenolysis testing reaction was used to determine the surface concentration of Pd and Rh, and to confirm the alloy formation. Photoluminescence and electron paramagnetic resonance spectroscopy were applied to clarify the possible reasons of deactivation and to elucidate the mechanism of stabilization. It was shown that Pd–Rh alloyed catalyst is characterized by comparable activity and enhanced stability. While the Pd and Rh particles of monometallic samples were found to interact with support at high temperatures resulting in sintering and bulk diffusion, the particles of alloy type kept their initial state of dispersion and catalytic activity.     

Stability Improvement of Rh/γ-Al2O3 Catalyst Layer by Ceria Doping for Steam Reforming in an Integrated Catalytic Membrane Reactor System

Significant improvement over the equilibrium methane conversion level was achieved by performing the reforming of methane in a catalytic membrane reactor, which was prepared by integrating a microporous silica membrane with a sandwiched-type Rh/γ-Al₂O₃ catalyst layer. However, the methane conversion activity decreased progressively owing to the deactivation of the intermediate catalyst layer under the reaction environments. On the other hand, addition of CeO₂ as a promoter for the Rh/γ-Al₂O₃ catalyst significantly improved the catalyst stability. The improvement was achieved probably by the kinetic and oxidative stabilization of the catalyst matrix with CeO₂. However, compared to the nonpromoted system, the ceria-promoted systems displayed lower catalytic activities based on the Rh/Ce ratios. The results led to the conclusion that a controlled interplay of the catalytic potentiality of Rh and the stabilization effect of Ce are essential to obtain an acceptable system. The membrane quality and its performance decreased especially with high Ce incorporation in the catalyst layer, possibly as a result of the observed microstructural variations in the catalyst layer with the Ce addition. Therefore, a consensus between the activity and stability of the material as a catalyst and the textural characteristics of the catalyst layer as a support layer for the silica membrane is considered to be an important factor that decides the success of the approach. A possible mechanism has been suggested to explain the role of ceria as a promoter in the Rh/γ-Al₂O₃ system.     

Structure - function relationships in heterogeneous catalysis: replacing the asterisk

A new concept for describing the active site in heterogeneous catalysis, called the embedded surface molecule, is defined. The surface molecule consists of the atoms from reagent, catalyst and product which participate directly in the catalytic act. Embedding considers the relationship of the surface molecule to the rest of the catalyst. Embedding modifies the electronic environment of the surface molecule and also provides reagent molecules to the site by surface diffusion. The concept is illustrated by reference to the catalytic oxidation of carbon monoxide at the Rh(110) surface. Two different types of kinetics are observed, depending on the oxygen coverage and it is argued that the changeover is largely due to a change in embedding conditions, while the reacting surface molecule is unchanged.