Palladium-alumina Cryogel with High Thermal Stability and CO Oxidation Activity

Abstract  

Palladium-alumina cryogel was prepared from palladium nitrate and aluminum sec-butoxide through a sol–gel processing and subsequent freeze drying. The cryogel showed higher thermal stability of palladium and catalytic CO oxidation activity than the corresponding xerogel and impregnation catalysts. The superior stability and activity were ascribed to the uniform distribution of palladium ions in the boehmite gel by the one-pot preparation of palladium-boehmite co-gel, and also to the suppression of transfer and aggregation of the metal during the subsequent freeze drying. As a result palladium ions were finely distributed and stabilized throughout the alumina cryogel support, which consequently suppressed the sintering of palladium at elevated temperatures.     

Characterization of Al2O3 Supported Nickel Catalysts Derived from RF Non-thermal Plasma Technology

Catalysts derived from non-thermal plasma techniques have previously shown unusual and highly advantageous catalytic properties including room temperature reduction, unusual metal particle structure and metal-support interactions, and enhanced selectivity and stability. This study focuses on the characterization of Al₂O₃ supported Ni catalysts derived from the RF non-thermal plasma technique with in-situ XRD, TPR-MS and STEM and on relating the results to the enhanced activity and stability of benzene hydrogenation. The results suggest that catalysts with plasma treatments before impregnation are relatively easier to be reduced and result in better activities under mild reduction conditions. These plasma treatments stabilize the nickel particle sizes of air(B) and H₂(B) catalysts at 600 °C by slowing down the sintering process. Plasma treatments after the impregnation of precursors, on the other hand, tend to delay the growth of nickel particles below 600 °C, forming smaller Ni particles, but with a sudden increase in particle size near 600 °C. It suggests that the structure of Ni nitrate and the metal-support interaction have been altered by the plasma treatments. The reduction patterns of plasma treated catalysts are, therefore, changed. The catalyst with a combination plasma treatment demonstrates that the effect of a combination plasma treatment is larger than either the plasma treatment before or after the impregnation alone. Both plasma treatments before and after the impregnation of metal precursor play important roles in modifying supported metal catalysts.     

Influence of Pd Precursor and Method of Preparation on Hydrodechlorination Activity of Alumina Supported Palladium Catalysts

A series of alumina supported Pd catalysts were prepared by the novel deposition-precipitation method adopting the chloride precursor (DP-Cl) of Pd and varying the metal content from 0.25 to 1.0 wt%. The catalytic properties of prepared catalysts were studied by various characterization techniques such as N₂ adsorption, CO chemisorption, TPR, XRD, XPS, and TEM techniques. The activity and stability of the catalysts were evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene operating at atmospheric pressure. At 1 wt% of Pd the catalyst showed higher chlorobenzene conversion with good stability when tested for a period of 25 h, whereas the other catalysts exhibited a loss in activity with time. In order to elucidate the exceptional activity and stability of this catalyst, a few more catalysts with 1 wt% Pd were prepared by impregnation technique and also using a non-chloride precursor, palladium nitrate. The 1 wt% DP-Cl catalyst again was found to be the best among the others. The activity and stability of the DP-Cl catalyst was also found to be superior to two low-dispersed catalysts, each with 10 wt% Pd, prepared by conventional impregnation method using the chloride and nitrate as the precursors. The characterization results reveal that the high activity and stability of the DP-Cl catalyst is related to the formation of electron deficient Pd species and its stabilization in the octahedral vacancies of alumina.     

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al₂O₃ aerogels were prepared from cyclic nickel glycoxide, (CH₂O)₂Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO₂-reforming of methane was investigated. The NiO–Al₂O₃ aerogels showed higher activity than impregnation NiO/Al₂O₃ catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H₂ reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels. This revised version was published online in July 2006 with corrections to the Cover Date.     

One-Step Flame Synthesis of an Active Pt/TiO2 Catalyst for SO2 Oxidation—A Possible Alternative to Traditional Methods for Parallel Screening

Flame synthesis as a route for production of composite metal oxides has been employed for the one-step synthesis of a supported noble metal catalyst, i.e., a Pt/TiO₂ catalyst, by simultaneous combustion of Ti-isopropoxide and platinum acetylacetonate in a quench-cooled flame reactor. The average size of the platinum particles supported on aggregated nanoparticles of TiO₂ is approximately 2 nm. The high SO₂ oxidation activity of the catalyst proves that platinum is not “hidden” in the titania matrix. The flame-produced catalyst showed catalytic activity similar to that of samples prepared by wet platinum impregnation of pure titania.     

结构型LaMnO3钙钛矿催化剂的催化燃烧活性和热稳定性

为降低堇青石载体对钙钛矿催化剂活性和稳定性的影响,以堇青石蜂窝陶瓷为基材,采用原位沉淀和悬浮浸渍技术分别制备了SiO₂和La₂O₃为涂层的结构型LaMnO₃催化剂,通过甲苯催化燃烧反应考察了催化剂的活性和热稳定性。结果表明,原位沉淀技术虽然可以均匀和高强度地在载体表面负载La、Mn活性组分,但无法在表面形成LaMnO₃钙钛矿的活性相。悬浮浸渍技术则可以保持LaMnO₃催化剂的结构和活性,结构催化剂与粉末LaMnO₃表现出相似的活性规律。La₂O₃涂层比SiO₂涂层可以更有效地保持LaMnO₃在蜂窝陶瓷载体表面的高活性和热稳定性。     

Pt0.02Sn0.003Mg0.06 on γ-alumina: a stable catalyst for oxidative dehydrogenation of ethane

The advantages of two-step oxidative dehydrogenation as an alternative method for manufacturing small alkenes are outlined. In a nutshell, the process is based on separating the gaseous oxygen and hydrocarbon feeds in time. In the first step, alkanes are dehydrogenated in the presence of a solid oxygen carrier (without gaseous oxygen). Subsequently, the carrier is reoxidised using a gaseous feed. This process requires a dehydrogenation catalyst that is selective and stable under severe redox cycling. In search for such a catalyst, we prepare and study various platinum/tin catalysts supported on alumina. The catalysts are doped with either magnesia or potassium oxide. The activity, selectivity, and stability of these catalysts in the dehydrogenation of ethane to ethylene are then investigated under severe redox cycling conditions (600 °C and 10% (v/v) oxygen in the regeneration step). Pt_0.02Sn_0.003Mg_0.06 is found to be the most stable combination. The catalysts’ dispersion and the metal–support interactions are studied using transmission electron microscopy (TEM) and temperature-programmed hydrogen desorption (TPD). The effects of the (earth)alkali promoter and the interaction between the metal catalyst and support are discussed.     

Thermal stability of Pt particles of Pt/Al2O3 catalysts prepared using microemulsion and catalytic activity in NO–CO reaction

Sintering behaviors of the Pt particles of Pt/Al₂O₃ catalyst prepared using different preparation methods (microemulsion, sol–gel, and impregnation methods) were investigated. It was found that the catalyst prepared by microemulsion had a higher resistance to sintering than did the sol–gel and impregnation catalysts. To limit the sintering even more, the catalysts were pressed. The resistance to sintering in all the catalysts was improved by pressing. The pressed microemulsion catalyst was little deactivated in the NO–CO reaction by thermal treatment at 700 °C for 12 h, and had a high activity relative to that of the sol–gel and impregnation catalysts.     

Activity and thermal stability of sonochemically synthesized MoS2 and Ni-promoted MoS2 catalysts

Sonochemically synthesized MoS₂/Al₂O₃, which had a hydrodesulfurization (HDS) activity that was significantly greater than that of a catalyst prepared by impregnation, exhibited low thermal stability due to sintering of MoS₂ crystallites at high temperatures. The thermal stability was improved when the catalyst was promoted with Ni. In this study, we compared the activity and thermal stability of different Ni-promoted MoS₂ catalysts, which were prepared by addition of Ni to MoS₂ using either impregnation (IMP) or chemical vapor deposition (CVD). After use in the HDS of dibenzothiophene (DBT) at 673 K for 2 h, the initial activity of the un-promoted catalyst was partially lost, while that of the Ni-promoted catalysts was preserved. Ni added by CVD interacted more intimately with MoS₂ than Ni added by impregnation because CVD allowed selective deposition of Ni on the MoS₂ edge sites. Another advantage of the CVD method over the impregnation method is that Ni(CO)₄, which was used as the Ni precursor in the former method, could be decomposed at much lower temperatures than in the case of Ni(NO₃)₂, which was used in the impregnation method. As a result, Ni-promoted catalysts prepared using Ni-CVD showed superior HDS activity compared with those prepared using Ni-impregnation.     

制备方法对Pd燃烧催化剂性能的影响

采用浸渍法和溶胶 凝胶法制备了负载型单钯燃烧催化剂,考察了它们在CO、C3H6的氧化活性和抗烧结性能等方面的差异。发现用浸渍法制备的催化剂初活性较高,用溶胶 凝胶法制备的催化剂热稳定性得到了大幅度的提高。通过XRD、XPS、AAS和CO-TPD等表征手段,初步探讨了造成这些差异的原因。     

Catalytic selective reduction of NO with propylene over Cu-Al2O3 catalysts: influence of catalyst preparation method

NO reduction to N₂ by C₃H₆ was investigated and compared over Cu-Al₂O₃ catalysts prepared by four different methods, namely, the conventional impregnation, co-precipitation, evaporation of a mixed aqueous solution, and xerogel methods. It was found that the catalyst preparation method as well as the Cu content exerts a significant influence on catalyst activity. For the catalysts prepared by the first three preparation methods, with the increase of Cu content from 5 to 15 wt%, the maximum NO reduction conversion decreased slightly, but the temperature for the maximum NO reduction also decreased. For the xerogel Cu-Al₂O₃, there was a significant decrease in NO reduction conversion with the increase of Cu content from 5 to 10 wt%. In the absence of water vapour, the Cu-Al₂O₃ catalyst prepared by the impregnation method exhibited the highest activity toward NO reduction. The purity of alumina support was found to be a crucial factor to the activity of the Cu-Al₂O₃ catalyst prepared by impregnation. In the presence of water vapour, a substantial decrease in NO conversion was observed for the Cu-Al₂O₃ catalysts prepared by the first three methods, especially for the impregnated Cu-Al₂O₃ catalyst. In contrast, the presence of water vapour showed only a minor influence on the xerogel 5 wt% Cu-Al₂O₃ and it showed the highest activity for NO reduction in the presence of 20% water vapour. The xerogel 5 wt% Cu-Al₂O₃ catalyst was also found to be less affected by a 5 wt% sulfate deposition than the Cu-Al₂O₃ catalysts prepared by other methods.     

Characterization of aerogel Ni/Al2O3 catalysts and investigation on their stability for CH4-CO2 reforming in a fluidized bed

The CH₄-CO₂ reforming was investigated in a fluidized bed reactor using nano-sized aerogel Ni/Al₂O₃ catalysts, which were prepared via a sol–gel method combined with a supercritical drying process. The catalysts were characterized with BET, XRD, H₂-TPR and H₂-TPD techniques. Compared with the impregnation catalyst, aerogel catalysts exhibited higher specific surface areas, lower bulk density, smaller Ni particle sizes, stronger metal-support interaction and higher Ni dispersion degrees. All tested aerogel catalysts showed better catalytic activities and stability than the impregnation catalyst. Their catalytic stability tested during 48 h reforming was dependent on their Ni loadings. Characterizations of spent catalysts indicated that only limited graphitic carbon formed on the aerogel catalyst, while massive graphitic carbon with filamentous morphology was observed for the impregnation catalyst, leading to significant catalytic activity degradation. An aerogel catalyst containing 10% Ni showed the best catalytic stability and the lowest rate of carbon deposition among the aerogel catalysts due to its small Ni particle size and strong metal-support interaction.     

Silicon nitride supported platinum catalysts for the partial oxidation of methane at high temperatures

The catalytic partial oxidation of methane has been studied over platinum silicon nitride supported catalysts in the temperature range of 900–1100°C at a contact time of 0.35 ms at atmospheric pressures. The feed consisted of a mixture of CH₄/O₂/N₂≈2/1/10. The catalysts were prepared by impregnation of platinum bis-acetyl-acetonate on silicon nitride powder (Si₃N₄). The different catalysts were characterized by chemical analysis, XPS and TEM. Minor particle sintering occurs during reaction. Metal losses were observed at 900°C, for catalysts containing 1.0 and 2.2 wt.% of platinum. A catalyst with a low amount of platinum (0.045 wt.%) appeared to be stable at 900°C, as no platinum loss was observed. The high stability of the 0.045 wt.% Pt/Si₃N₄ catalyst could be attributed to particular interactions between the metal and the support.     

Gas phase hydrogenation of ortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) over unpromoted and alkali metal promoted-alumina supported palladium catalysts

A series of alkali metals (Li, Na, K and Cs) promoted alumina-supported palladium catalysts were prepared by a wet impregnation method and characterized by X-ray diffraction (XRD) and CO chemisorption measurements. The samples were tested for the gas phase hydrogenation of ortho-chloronitrobenzene (O-CNB) to ortho-chloroaniline (O-CAN) in a fixed-bed micro reactor at 250 °C under normal atmospheric pressure. The promoted-Pd/Al₂O₃ catalysts show higher conversion for O-CNB and the hydrogenation activity of O-CNB per site decreases with the increasing ionic radius of the alkali metal promoter ions. However, the selectivity for O-CAN remains more or less the same in both unpromoted and promoted catalysts and also irrespective of the nature of the alkali metal promoter ions used for promotion of alumina support. Despite, similar activity and selectivity observed between Li- and Na-promoted Pd/Al₂O₃ catalysts, the Na-promoted showed higher resistance for coke formation than a Li-promoted catalyst. The increase in the intrinsic activity of palladium site on alkali promotion has been attributed to the increase in hydrogenation activity over promoted catalysts.     

Influence of the Alkali Promoter on the Activity and Stability of Transition Metal (Cu, Co, Fe) Based Structured Catalysts for the Simultaneous Removal of Soot and NOx

Structured catalysts prepared by means of coating cordierite monoliths with alumina-based suspensions containing transition metals such as Cu, Co and Fe and alkali/alkali-earth promoters such as K and Ba. Textural and structural features of these catalysts were analyzed by means of N₂ adsorption and SEM. Their activity in the simultaneous removal of soot and NO_x was assayed in a lab-scale installation, using a carbon black as diesel surrogate. Catalysts exhibited significant activity in deNO_x and soot oxidation. K and Ba enhanced both NO_x adsorption and soot–catalyst contact. However Ba contributed to a greater extent to the adsorption of N-species, which moreover presented higher thermal stability than on K-catalysts, and K showed higher mobility than Ba. Thus, Ba-containing catalysts showed increased activity towards NO_x reduction but shifted to higher temperatures in comparison to K-catalysts, which on the other hand resulted more active towards soot oxidation than Ba-ones. Fe-based catalyst turned out to be less active both in soot oxidation and NO_x reduction than Co and Cu-based ones. Intensive calcination of the catalysts at 800 °C for 5 h resulted in substantial loss of K and Ba. Loss of promoter depends, however, on the metal contained in the catalyst. In this sense Fe-containing catalysts showed higher stability. Calcination has a substantial effect on catalytic activity. Catalyst significantly lost their NO_x adsorption capacity and showed similar activity than a catalyst prepared in absence of promoter, pointing to a substantial change in reaction mechanism and reaction predominantly occurring on metallic sites upon the loss of alkali/alkali-earth compound.     

Steam reforming of kerosene over a metal-monolithic alumina-supported Ru catalyst: Effect of preparation conditions and electrical-heating test

A plate-type anodic alumina support (γ-Al₂O₃/Fe–Cr–Ni alloy/γ-Al₂O₃) was used to prepare a series of Ru catalysts. The performance of these catalysts was investigated in the steam reforming of kerosene (SRK). Ethanol solution impregnation was used to enhance metal dispersion on the catalyst surface. The catalyst prepared by ethanol solution impregnation and dried at 120 °C (Ru/Al₂O₃-ED) gave a higher metal dispersion and more favorable activity and durability than that prepared in aqueous solution. However, owing to shrinking caused by the oxidation of ruthenium species, high temperature calcination in air after impregnation greatly decreased the metal dispersion on the catalyst surface, regardless of the impregnation solution type. In contrast to calcination in air, high temperature N₂ treatment could decompose the reducible ruthenium species on the Ru/Al₂O₃-ED completely. This indicates that H₂ pre-reduction is not an essential procedure for the SRK reactions over this catalyst. The experimental results also confirm this hypothesis. The effect of Ce addition was also investigated and was found to enhance significantly the catalyst tolerance to carbon deposition thereby improving the SRK durability of Ru/Al₂O₃-ED under a high space velocity. In addition, owing to the high electrical resistance of the Fe–Cr–Ni alloy, the anodic alumina catalyst itself can be used as a heater, when an electrical current is applied along the alloy layer. Under an electrical-heating pattern, the SRK reaction system reached stability within 15 min, offering a strong possibility for shortening the start-up time of conventional reformers from 1 to 2 h to a few minutes.     

Metal organic framework derived Ni/CeO2 catalyst with highly dispersed ultra-fine Ni nanoparticles: Impregnation synthesis and the application in CO2 methanation

CO₂ methanation is a promising strategy to convert the greenhouse gas into synthetic natural gas, which is known as a clean fuel with higher energy density. Ni/CeO₂ is one of the active catalysts for CO₂ methanation reaction. However, Ni/CeO₂ with conventional metal/support structure suffers from the problem of nanocrystal coarsening during high-temperature reaction and thus decreasing catalytic activity. In this work, a Ni/CeO₂ catalyst with novel structure was designed and synthesized through impregnation of Ce-based metal organic framework (MOF) with Ni precursor followed by calcination process. Due to the confinement effect of ultra-small pores derived from the MOF, Ni/CeO₂ catalysts with ultrafine Ni nanoparticles, high dispersion and good thermostability were resulted. Among all the samples, Ni/CeO₂ calcined at 600 °C showed the best catalytic performance due to the highest amount of oxygen vacancies. This work demonstrates a facile way to synthesize a broad range of ultra-fine metal/metal oxide nanocomposite catalysts with high catalytic activity and good stability for various applications.     

Microwave-Assisted Solution Combustion Synthesis of Spinel-Type Mixed Oxides for Esterification Reaction

Environmentally benign production of biodiesel using heterogeneous catalysts is an important issue that requires the preparation of catalysts with high activity and reusability by a simple procedure. In this study, pure alumina and zinc oxide- and copper oxide-modified alumina were tested for the esterification of oleic acid to its methyl ester. The microwave-assisted solution combustion (MSC) method was used to prepare the catalyst samples that were characterized by X-ray diffraction (XRD), Fourier transform infra-red (FTIR), Brunauer–Emmett–Teller (BET) surface area measurement, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The spinel-type zinc aluminate was successfully synthesized, but CuO-modified alumina particles were clearly observed. Whereas in the alumina, much lower activity was observed than the doped alumina, zinc aluminate showed higher activity than copper oxide-alumina due to its smaller particle size, higher BET surface area, larger average pore size, and higher acidity. Finally, after the reaction conditions for the conversion of oleic acid were optimized for the case of spinel ZnAl₂O₄ as nanocatalyst, a yield of higher than 94% was obtained at 180°C, 9 methanol/oleic acid molar ratio, 3?wt% of catalyst for 6?h. Furthermore, the catalyst deactivation was not observed after four reaction cycles at the optimal reaction conditions, indicating the feasibility of the proposed method for the preparation of industrial catalysts.     

Thermal stability improvement of hopcalite catalyst

The properties of hopcalite in alumina matrix formed by rehydration of transition aluminas have been investigated in comparison with conventional hopcalite catalyst. By using X-ray diffraction, thermal analysis and activity measurements in combustion ofn-butane and air mixture it was found that the presence of alumina matrix leads to improvement of thermal stability at elevated temperatures and to limitation of spinel phase recrystallization. The modified catalyst was found to show much higher activity after thermal treatment at temperatures above 773 K than pure hopcalite at the same conditions.     

Investigation of the thermal stability of the chromia-alumina catalysts for the process of the one-stage dehydrogenation of n-butane

The effect of the hugh-temperature (800–1000°C) treatment in air of the Cr₂O₃/Al₂O₃ catalyst, which contained 18Cr₂O₃ + 0.4Na⁺, wt % and was prepared under laboratory conditions with the use of Pural SB1 grade high-purity pseudoboehmite, on the variation in the phase composition of the catalyst, specific surface, and catalytic characteristics in the dehydrogenation reaction of n-butane (yield and selectivity by ΣC₄ olefins and 1,3-butadiene, conversion of n-butane) was investigated depending on the calcination temperature of the catalyst. It is shown that thermal stability depends on the following main factors: the method preparation of catalysts, the phase composition of the starting aluminum hydroxide, carrier texture, and the presence of modifying additions and impurities of other metals. In the case of the same chemical composition of the catalyst, the samples obtained by the wet mixing of pseudoboehmite with an aqueous solution of chromic anhydride are most thermally stable compared with the impregnation samples. It is established that the addition of cerium improves the thermal stability and activity of the impregnation Al-Cr catalyst, while the impurity of the Fe³⁺ ions (up to 0.1 wt %) does not worsen these characteristics. The investigated samples of the catalyst are more thermally stable than the imported industrial catalyst, which loses activity and specific surface after calcination at 900–1000°C. The determination of the thermal stability of fresh catalysts and the factors affecting it can be used as the preliminary evaluation of the future lifetime of catalysts.