Gold supported on ceria and ceria–alumina promoted by molybdena for complete benzene oxidation

New gold–molybdena catalysts supported on ceria and ceria–alumina in reaction of complete benzene oxidation were studied. The catalysts were characterized by means of XRD, TPR, XPS and Raman spectroscopy. High and stable catalytic activity was established in the temperature region 200–240 °C. The presence of gold causes a modification in ceria structure leading to an increase of Ce³⁺ and oxygen vacancies formation. The loading of Al³⁺ increases additionally the oxygen vacancies, while a tendency of decrease of Ce³⁺ amount was observed. The presence of alumina results also in a larger share of active oxygen species proved by analysis of O 1s XPS spectra. The differences in the activities within the starting temperature range (150–180 °C) and in the region of 100% conversion (200–240 °C) could be explained by supposing that in the LT region the electron transfer between nanosized gold and ceria particles via oxygen vacancies has a crucial role. In the HT region the oxygen mobility, provoked by the defective structure of ceria due to the presence of Al³⁺, becomes of prevailing importance. It was also concluded that alumina prevents the gold and ceria agglomeration, which is the main factor to avoid deactivation under extreme reaction conditions.     

Effects of the structure of ceria on the activity of gold/ceria catalysts for the oxidation of carbon monoxide and benzene

Using precipitated cerium hydroxide dried at 100 °C as the support, highly dispersed ceria-supported gold catalyst was prepared. Compared with similarly prepared catalysts supported on low-surface area ceria, gold on the high-surface area support showed more resistance to sintering and was more active toward the oxidation of CO and benzene. The oxidation of benzene was very dependent on the structure of ceria. Temperature-programmed reduction showed that the reducibility of surface oxygen was higher for high-surface area ceria. It is proposed that the creation of surface oxygen vacancies by ceria surface reduction promoted oxygen adsorption. Active oxygen formed by dissociation of the adsorbed oxygen is the active species for benzene oxidation, and the dissociation is promoted by gold nanoparticles. When present together in the reactant mixture, benzene inhibited the oxidation of CO, but CO enhanced the oxidation of benzene. Moisture had a promoting effect on both CO and benzene oxidation.     

Gold/hydroxyapatite catalysts: Synthesis, characterization and catalytic activity to CO oxidation

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.     

Effect of fluorination of alumina support on activity of platinum catalysts for complete oxidation of benzene

Platinum supported on fluorinated alumina is more active for the total oxidation of benzene than is the catalyst with the same Pt loading supported on hydrophilic unfluorinated alumina. The Pt-F/alumina catalyst contains well-dispersed small Pt particles, in contrast to Pt/alumina. The high dispersion is a consequence of a strong metal-support interaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gold based catalysts on ceria and ceria-alumina for WGS reaction (WGS Gold catalysts)

Gold catalysts supported on ceria-alumina have been studied in WGS reaction. A high and stable activity was established for the catalysts supported on ceria-alumina, prepared by mechanochemical activation in comparison to the corresponding samples, where ceria-alumina support was prepared by coprecipitation. The catalysts were characterized by means of XRD, TPR, Raman and XPS spectroscopy. A correlation between WGS activity and the redox activity was found. On the basis of the results obtained a model of the reaction mechanism and of active sites was proposed.     

Rh oxide reducibility and catalytic activity of model Pt–Rh catalysts

Pt and Rh were impregnated by different methods into the washcoat to investigate the differences in Rh oxidation state and catalytic activity of the samples. Both fresh and laboratory aged samples were studied. Clear differences in catalytic activity were noticed between the catalysts with different Pt and Rh addition methods. The best oxidation activity for fresh catalysts was achieved with the catalyst having both Pt and Rh deposited into the Ce–Zr mixed oxide. However, this state was observed to be unstable, and hence, this particular catalyst was dramatically deactivated in air ageing at high temperature. After ageing, the catalyst having both Pt and Rh impregnated lastly into the entire calcined washcoat matrix had the best activity in all three reactions, carbon monoxide and hydrocarbon oxidation and nitrogen oxide reduction. According to XPS studies, Rh was in easily reducible form in all the fresh samples. After ageing, the highest portion of reducible Rh was observed in the sample having also the best catalytic activity.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Correlation between metal oxidation state and catalytic activity: hydrogenation of crotonaldehyde over Rh catalysts

The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al₂O₃ and Rh/SiO₂ catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh³⁺ species. This behavior is not observed (as expected) for Rh/SiO₂ catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gold catalysts supported on titanium oxide for catalytic wet air oxidation of succinic acid

Catalytic wet air oxidation of a representative organic compound (aqueous solution of 5 g l⁻¹ succinic acid at 190 °C and 50 bar total air pressure) was investigated over gold on titania prepared from the deposition–precipitation method (with urea or NaOH) and compared to experiments performed over a Ru/TiO₂ catalyst. These preliminary results demonstrate that gold catalysts are efficient for the degradation of this organic acid. The catalytic activity is strongly dependent on the gold particle size characterized by transmission electron microscopy (TEM) with smaller particles producing higher turnover frequencies. Modification of metal dispersion occurs during reaction, leading to minor activity.     

Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation

In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.Three series of catalysts of varying Co content (up to 21 wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–pvi pvi edf, nta–pvi edf ≈ pvi and edf > nta–pvi > pvi.The catalysts prepared were characterized using various techniques (BET, UV–vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on Co₃O₄-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned Co₃O₄ crystallites: the ratio ‘amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step’ closely related with the ratio ‘amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step’ as well as the composition of the precipitated phase.The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H₂O)₆²⁺·2NO₃⁻. Upon calcination it is transformed into loosely bounded Co₃O₄ crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts.The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800 °C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content.The application of the nta–pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(II)–nta]⁻ and [Co(II)–2nta]⁴⁻ complexes, in which the Co(H₂O)₆²⁺ complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta–pvi catalysts a precipitated phase containing the [Co(II)–nta]⁻·NH₄⁺(or H⁺) and [Co(II)–2nta]⁴⁻·4NH₄⁺ (or 4H⁺) complex salts predominates. Upon calcination these are transformed into Co₃O₄ crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity.     

Gold supported CeO2/Al2O3 catalysts for CO oxidation: influence of the ceria phase

A series of low loading gold supported ceria/alumina catalysts have been prepared by the deposition–precipitation method, varying the pH of the synthesis. The catalysts were characterised by means of XRD, TEM, S_BET, XRF and UV–Vis techniques, and their catalytic activity towards CO oxidation in the absence and in presence of water in the stream, were tested. It has been found that in this low loading gold catalysts, where the metallic particles are far away one from another and the oxygen transportation is not the limiting step of the reaction, the electronic properties of the ceria phase and the structure of the metal-support perimeter more than the diameter of the gold nanoparticles is the determinant factor in the catalytic performances of the solid.     

Synthesis and catalytic activity of SBA-15 supported catalysts for styrene oxidation

Cu(II) and Mn(II) metals embedded on mesoporous SBA-15 were synthesized by co-precipitation technique.The support and catalysts were characterized by SEM–EDX,TEM,BET,XRD and ICP-AES methods.The catalytic activity of these catalysts was evaluated for styrene oxidation at various reaction conditions such as styrene to TBHP mole ratio,temperature,catalyst amount by using TBHP as an oxidizing agent.Major reaction products were styrene oxide and benzaldehyde and highest styrene conversion(97.3%) was observed at styrene to TBHP mole ratio of 1:4,temperature at 80 °C and 20 mg of catalyst.Further,the recyclability of the catalysts was observed and found that they can be recycled three times without major loss in their activity and selectivity.     

Correlation between activity and oxygen storage capacity of ceria on Pd-only three-way catalysts

One way of enhancing the thermal stability and NO_x, THC and CO conversion of a catalyst is to improve the thermal stability and oxygen storage capacity (OSC) of the ceria. The appropriate mixing ratios of bulk and stabilized ceria are especially very important for designing Pd-only three-way catalysts. In this paper, we discuss the surface phenomena of stabilized and unstabilized (bulk) ceria, the OSC of catalysts, and the correlation between activity and OSC of Pd-only catalysts with mixing ratios of bulk ceria and stabilized ceria.     

Design of new gold catalysts supported on mechanochemically activated ceria-alumina, promoted by molybdena for complete benzene oxidation

Gold catalysts, supported on mechanochemically activated ceria-alumina, nonpromoted and promoted by molybdena, were studied in the reaction of complete benzene oxidation. Higher activity of Au-Mo catalysts was established in the low temperature region, while high temperatures samples, containing only gold exhibit higher benzene conversion and the cross-point of the curves depends on alumina content. The addition of alumina by mechanochemical treatment leads to a surface modification of ceria, e.g. oxygen vacancies are formed prevailing on the ceria surface. The molybdena loading leads to the oxygen vacancies occupation. The calculated hydrogen consumption and the enhanced reduction of ceria surface layers in general correlate with the activities of the catalysts. The XPS data supported the role of Ce³⁺ and partially charged gold particles in the formation of the complex AuV₀Ce³⁺ as the active site for the redox processes. The modification of ceria in the presence of gold and the formation of oxygen vacancies in close contact with Ce³⁺ ensure the enhanced electron transfer.     

微孔-介孔分子筛负载氧化钴的苯催化完全氧化性能

用水热合成法合成了微孔-介孔分子筛MSZ,采用等体积浸渍法制备了Co_3O_4/微孔-介孔分子筛催化剂。用X射线衍射技术对材料进行表征,考察微孔-介孔分子筛的水热稳定性和催化剂对苯的催化完全氧化性能。研究发现,微孔-介孔分子筛水热稳定性较好,负载质量分数为35%Co_3O_4时,催化剂活性较高,活性组分晶粒的完整性和分散度是影响催化剂活性的重要因素,35%Co_3O_4/MSZ的活性高于35%Co_3O_4/MCM-41和35%Co_3O_4/ZSM-5。     

氯化氢氧化反应催化剂研究进展

大量副产氯化氢的资源化高效利用是涉氯行业亟需解决的共性难题。氯化氢催化氧化循环制氯气是一个低能耗、可持续发展的有效途径，而催化剂的设计与制备是该过程的核心。本文重点介绍了铜基、钌基和铈基等催化剂，并对各类催化剂的作用机理及其活性、稳定性等性能进行了归纳。不同于铜基催化剂，钌基和铈基等催化剂主要按照Langmuir-Hinshelwood路径催化反应，具有更佳的反应活性及稳定性。基于该反应为放热过程，指出降低反应温度是增强氯化氢转化的关键。此外，活性组分烧结导致分散性变差是钌基和铈基催化剂的主要失活因素。高低温活性、高稳定性的复合氧化物催化剂将是未来本领域重点研究的方向。     

添加Sm对Pt /SBA-15催化苯完全氧化反应活性和热稳定性的影响

以Pt/SBA-15为催化剂,考察催化剂载体中添加Sm对于苯的完全氧化反应活性和热稳定性影响。采用了一种简便的记录起燃温度曲线和催化剂热稳定性评价方法,即向装载好催化剂的固定床反应器中持续通入恒定流量的反应气,逐步阶段性升高反应温度,同时在线检测出口尾气的浓度变化, 得到起燃温度曲线后,继续提高反应温度,然后恒定在某一设定的温度(如550℃)持续运行较长时间, 期间定时在线取样分析,如果有必要还可以连续考察降温情况下催化剂的反应活性情况。研究结果表明,几种催化剂低温活性次序为：Pt/SBA-15≈Pt/4%Sm₂O₃/SBA-15> Pt/Sm₂O₃> 4%Sm₂O₃/SBA-15,而对于高温稳定性则是Pt/4%Sm₂O₃/SBA-15> Pt/1.2%Sm₂O₃/SBA-15> Pt/SBA-15,Pt/Sm-SBA-15(SG)> Pt/SBA-15(SG)。总之,Sm的添加虽然未能提高Pt/SBA-15的低温催化活性,但是能明显提高催化剂在高温情况下活性的稳定性。1%Pt/4%Sm₂O₃/SBA-15同时具备较好的低温催化活性和高温稳定性,具有较好的应用前景。     

Complete oxidation of benzene on Co---Cr and Co---Cr oxide catalysts

Supported mixture metal oxide systems, Cu---Cr and Co---Cr on γ-A1₂O₃ and γ-A1₂O₃+ SiO₂ were prepared and studied. They exhibited catalytic activity in the complete oxidation of benzene.     

苯直接催化氧化制苯酚的研究

一步法由苯直接催化氧化制取苯酚的反应具有潜在工业生产意义，其环境友好的特点有利于今后的发展。目前，采用沸石分子筛催化剂的多相催化反应在精细化学品合成中的应用得到广泛研究。评述了苯直接催化氧化制取苯酚反应中的沸石分子筛/ H₂O₂体系和沸石分子筛/ N₂O体系的研究进展。并对催化反应中的影响因素和反应机理进行了阐述。     

Vanadia supported on ceria: Characterization and activity in liquid-phase oxidation of ethylbenzene

Vanadia/ceria catalysts (2–10 wt% of V₂O₅) were prepared by wet impregnation of ammonium metavanadate in oxalic acid solution. Structural characterization was done with energy dispersive X-ray analysis (EDX), powder X-ray diffraction (XRD), BET surface area measurements, FT-IR spectroscopy and nuclear magnetic spectral analysis (⁵¹V MASNMR). XRD and ⁵¹V MASNMR results show highly dispersed vanadia species at lower loadings and the formation of CeVO₄ phase at higher V₂O₅ loading. The catalytic activity of catalysts was conducted in liquid phase oxidation of ethylbenzene with H₂O₂ as oxidant. The oxidation activity is increased with loading up to 8 wt% V₂O₅ and then decreased with further increase in V₂O₅ content to 10 wt%. Different vanadia species evidenced by various techniques were found to be selective towards ethylbenzene oxidation. The CeVO₄ formation associated with increased concentration of vanadia on ceria results the production of acetophenone along with 2-hydroxyacetophenone.