The molecular structures and reactivity of V2O5/TiO2/SiO2 catalysts

A series of V₂O₅/TiO₂/SiO₂ catalysts were structurally investigated byin situ Raman spectroscopy and chemically probed by methanol oxidation in order to determine the molecular structure-reactivity relationships of the V₂O₅/TiO₂/SiO₂ catalysts. Only surface TiO _x species are present on the 3% TiO₂/SiO₂ catalysts, and the surface TiO _x species as well as bulk TiO₂ (anatase) particles coexist on the 40% TiO₂/SiO₂ catalyst. The deposition of 1–3% vanadium oxide onto 3% TiO₂/SiO₂ and 4% vanadium oxide onto 40% TiO₂/SiO₂ forms only a surface vanadium oxide phase.In situ Raman studies reveal that the surface vanadium oxide species preferentially exist on the titania sites of the TiO₂/SiO₂ system. The interaction between the surface vanadia and the surface titania overlayer on SiO₂ increases the methanol oxidation reactivity by two orders of magnitude relative to V₂O₅/SiO₂. In the presence of bulk TiO₂ (anatase) particles on the SiO₇ support, the reactivity of the surface vanadia further increases by an order magnitude relative to the catalysts containing only surface titania, and is close to that of surface vanadia on bulk TiO₂. This suggests that the surface VO _x -TiO₂ (bulk) interactions results in a more active site than the surface VO _x -TiO _x -SiO₂ interactions. In addition, the V₂O₅/TiO₂/SiO₂ catalysts exhibit high selectivity towards HCHO because redox sites are predominant on the surface of these catalysts with essentially no acid site present.     

Pd/silica cluster catalysts: synthesis and reactivity with H2 and C2H4

Silica-supported Pd clusters were characterized by in situ EXAFS spectroscopy. Clusters with an average nuclearity of six atoms were derived from either an inorganic or an organometallic precursor by reduction at 100–150C. Despite the small size of the clusters, EXAFS contributions from the metal-support interface were not detected. These clusters and larger ones formed by reduction at 320C absorb hydrogen on cooling in H₂ to 30C; the resultant interstitial hydride species decompose in vacuo at 30C. Vacuum treatment at 300C removes chemisorbed H₂ yielding bare Pd clusters. In contrast to larger crystallites, the Pd clusters do not react with C₂H₄ at 150 to form interstitial carbide species.     

An in situ high pressure FT-IR study of CO2/H2 interactions with model ZnO/SiO2, Cu/SiO2 and Cu/ZnO/SiO2 methanol synthesis catalysts

In situ FT-IR spectroscopy allows the methanol synthesis reaction to be investigated under actual industrial conditions of 503 K and 10 MPa. On Cu/SiO₂ catalyst formate species were initially formed which were subsequently hydrogenated to methanol. During the reaction a steady state concentration of formate species persisted on the copper. Additionally, a small quantity of gaseous methane was produced. In contrast, the reaction of CO₂ and H₂ on ZnO/SiO₂ catalyst only resulted in the formation of zinc formate species: no methanol was detected. The interaction of CO₂ and H₂ with Cu/ZnO/SiO₂ catalyst gave formate species on both copper and zinc oxide. Methanol was again formed by the hydrogenation of copper formate species. Steady-state concentrations of copper formate existed under actual industrial reaction conditions, and copper formate is the pivotal intermediate for methanol synthesis. Collation of these results with previous data on copper-based methanol synthesis catalysts allowed the formulation of a reaction mechanism.     

V2O5负载量对V2O5／TiO2催化剂SCR法脱硝的影响

以TiO2载体,采用浸渍法制备了不同V2O5负载量的用于选择性催化还原NOx的V2O5／TiO2催化剂。利用BET,SEM和Ⅺ①,对不同V2O5负载量的催化剂组成、结构、形貌和性能进行了表征,考察不同V2O5负载量对催化剂制备的影响。结果表明制备的催化剂具有较多的中孔和微孔,催化剂中V2O5含量的增加,会降低催化剂的表面积;V2O5含量为2％的V2O5／TiO2催化剂样品比表面积最大,但是其活性非常低;V2O5含量为4％催化剂比表面积较大,NOx脱转化率高;V2O5的负载量小时,V2O5主要以等轴聚合的钒基型式（V3O7和V6O13）存在,这些钒基是催化剂的活性中心;当负载量超过6％,V2O5主要以结晶相存在,占据大量活性位,降低催化效果。     

Catalytic decomposition of ethanol on ­V2O5/AlPO4 catalysts

Different ratios of vanadium pentoxide supported on aluminum phosphate (1–30 mol%) have been prepared by an impregnation method. The original and calcined samples were characterized by TG, DTG, DSC, X‐ray diffraction, IR spectra, N₂ adsorption and electrical conductivity measurements. The catalytic decomposition of ethanol has been carried out at 210 °C in a flow system at 1 atm using air as a carrier gas. The results indicate that the catalysts calcined at 400 °C were active and selective towards ethene formation whereas the samples calcined at 600 °C showed a drastic reduction in both activity and selectivity. © 2000 Society of Chemical Industry     

Effect of alkali metals additives to V2O5/TiO2 catalyst on physicochemical properties and catalytic performance in oxidative dehydrogenation of propane

The effect of alkali metal additives Li, K, and Rb to V₂O₅/TiO₂ catalyst on the rate of catalyst reduction with propane and reoxidation with oxygen, sorption of propene, and the electron work function has been examined. The results have been correlated with the catalytic performance in oxidative dehydrogenation, ODH, of propane. It has been found that the rates of reduction, reoxidation and the ODH of propane decrease in the order: VTi>LiVTi>KVTi>RbVTi. The activation energies of the reduction and reoxidation are not, however, affected by the presence of the alkali metals. The same sequence has been observed for the work function values of the catalysts. It is argued that alkali metal poisons the centres of the hydrocarbon activation. The yield and selectivity to propene in the ODH of propane increase, however, for the promoted catalysts, following the above sequence. This effect is ascribed to the decrease in the heat of the propene adsorption, which is due to the increase in the basicity and decrease in acidity on the promoted catalysts.     

Supported binary oxide monolayers as selective oxidation catalysts: Bi2Mo3O12/TiO2

Monolayers of bismuth molybdates supported on low-area TiO₂ (anatase) catalyse the oxidation/isomerisation of 1-butene to butadiene and 2-butenes with combined selectivities of 80–100% and show significantly lower specific rates of deep oxidation than does -Bi₂Mo₃O₁₂. As loadings are increased above 2 wt% (equivalent to one monolayer), selectivities remain high and, although the butadiene yields are generally only moderate, in some cases they exceed that shown by the unsupported -phase. This revised version was published online in July 2006 with corrections to the Cover Date.     

不同方法负载钨钒钛催化剂对催化氧化邻二氯苯的影响

钨钒钛催化剂是消除钢铁冶炼废气中的二噁英最好的催化剂,为了降低工业生产成本,并获得良好的催化效果,将实验室筛选出的钨钒钛催化剂负载到不同载体上,研究催化剂负载量及不同负载方法对降解邻二氯苯催化效率的影响。采用溶胶-凝胶-浸渍法和直接浸渍法在堇青石蜂窝陶瓷上负载V_2O_5/WO_3/TiO_2催化剂,用X射线衍射、扫描电镜和X射线能量色散谱对制备的催化剂进行表征,结果表明,直接浸渍法在堇青石蜂窝陶瓷上负载的V_2O_5/WO_3/TiO_2催化剂含量比溶胶-凝胶-浸渍法负载的含量高,粒度较小,表面光滑无缝隙,活性成分V和W分散较好,催化活性高,在(250～350)℃邻二氯苯去除率大于95%,直接浸渍法负载的催化剂与堇青石的最佳质量比为1:100。     

Selective catalytic reduction of NO with NH3 over Nb2O5-promoted V2O5/TiO2 catalysts

In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH₃ only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst: calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings.     

Influence of the V2O5 Loading on the Structure and Activity of V2O5/TiO2 SCR Catalysts for Vehicle Application

In this paper the effect of the vanadium oxide loading on the surface vanadia structure and the activity as well as selectivity in the catalytic reduction of NO with NH₃ was studied for a V₂O₅/TiO₂ model system. A series of TiO₂ (WO _x stabilized anatase) supported vanadia catalysts with varying loadings were characterized by laser Raman spectroscopy, ⁵¹V MAS-NMR, V K XANES. To determine the acidic properties, DRIFTS measurements were done with pyridine adsorbed on the samples. The measurements indicate that with increasing active phase loading square pyramidal coordinated surface vanadia species are replaced by an amorphous highly dispersed vanadium oxide phase with a coordination like V₂O₅. In addition, the ratio of Brønsted to Lewis acid sites is shifted from a comparatively low to an equal level at high loadings. This structural change is accompanied by a clearly improved catalytic activity and selectivity.     

A5微孔分子筛负载V2O5催化氧化对硝基甲苯

将A₅微孔分子筛浸渍于偏钒酸铵溶液,过滤,120 ℃干燥2 h,550 ℃焙烧6 h,制备了含有V₂O₅的A5微孔分子筛催化剂。采用X射线衍射、N₂吸附-脱附和扫描电镜等进行表征,并将其用于以H₂O₂为氧化剂、乙酸酐为溶剂的对硝基甲苯催化氧化生成对硝基苯甲醇的反应。在反应温度40 ℃和反应时间4 h条件下,以质量分数35%的H₂O₂为氧化剂和V₂O₅质量分数8.5%的V₂O₅/A₅为催化剂,能够获得较好的对硝基甲苯转化率（35.54%）和较高的对硝基苯甲醇选择性(67.16%)。反应结束后,在母液中通过电感耦合等离子体原子光谱仪没有检测到V₂O₅。V₂O₅/A₅催化剂连续使用3次,对硝基甲苯转化率为34.21%,对硝基苯甲醇选择性为63.39%,催化剂活性无明显降低。     

Effect of vanadia loading in propane oxidative dehydrogenation on V2O5/SiO2 catalysts

The influence of V₂O₅ loading on the catalytic behaviour of V₂O₅/SiO2₂ catalysts in the oxidative dehydrogenation of propane to propylene (POD) has been investigated. The different activity-selectivity pattern of low (5 wt%) and highly (>10 wt%) loaded V₂O₅/SiO₂ catalysts is explained in terms of different surface vanadia species.     

3-甲基吡啶一步气相氧化合成烟酸的V2O5/TiO2催化剂研究进展

烟酸作为重要的化工和医药中间体广泛应用于不同领域.相较于传统工艺,3-甲基吡啶气相一步催化氧化合成烟酸的方法具有成本低、污染低、产品质量高等优点,展现出良好的应用前景.目前V₂O₅/TiO₂系列催化剂在3-甲基吡啶的氧化反应中表现出了良好的氧化活性.本文主要分析了催化剂表面的VO_x结构,并概述了载体、制备方法、负载量、焙烧过程和助剂掺杂对结构的影响,探讨催化剂中各组分之间的相互作用,根据相关文献推测3-甲基吡啶氧化的反应机理.但该类催化剂仍然存在反应速率和选择性较低、温度窗口窄、反应过程中放出的热量对催化剂的影响等一些技术问题.文章从催化剂的开发和反应器的设计两方面进行了展望.     

过渡金属单原子催化剂活化H2O2/PMS/PDS降解有机污染物的研究进展

单原子催化剂（SACs）是一种将金属以原子态负载于载体上的新型材料,具有原子利用率高、催化活性强和易回收等优点,使其在催化降解有机污染物方面备受关注。本文介绍了SACs的催化影响因素,总结了SACs催化降解有机污染物在环境领域中的应用。此外,着重综述了不同过渡金属（Fe、Co、Mn、Cu等）单原子催化剂在基于双氧水或过硫酸盐的高级氧化技术中的催化机理,单原子金属（M）一般与N键合形成活性位点M—N _x,活化氧化剂生成自由基或单线态氧,高效降解有机污染物。最后,提出未来SACs在催化降解有机污染物的研究方向是合成金属负载量高、稳定性高、pH适用范围更广的SACs,以及根据SACs的结构-性能关系和催化机理,对目标污染物设计特定催化剂。     

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

The influence of Lewis and Brønsted acid sites on the performance of V₂O₅/TiO₂ and V₂O₅–WO₃/TiO₂ catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Brønsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO₂, without any by-products desorption.

Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Brønsted acid sites and the hydrolysis of anhydride during wet tests.     

How oxide carriers affect the reactivity of V2O5 catalysts in the oxidative dehydrogenation of propane

The catalytic pattern of several oxide carriers (MgO, Al₂O₃, ZrO₂, TiO₂, SiO₂, HY zeolite) and supported V₂O₅ (4.7–5.3 wt%) catalysts in the oxidative dehydrogenation of propane to propylene (PODH) has been comparatively investigated. The fundamental role of the oxide support on both reducibility and reactivity of vanadia catalysts has been assessed. A direct relationship between the specific surface activity of oxide carriers and that of vanadia catalysts is discussed. The inverse relationship between the specific activity and the onset temperature of reduction marks the prevailing redox behaviour of V₂O₅ catalysts in the PODH reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

An FT-IR study of the conversion of 2-chloropropane, o-dichlorobenzene and dibenzofuran on V2O5-MoO3-TiO2 SCR-DeNOx catalysts

Spectra of the adsorbed species arising from contact of a V₂O₅-MoO₃-TiO₂ model SCR catalyst with ortho-dichlorobenzene (o-DCB) and dibenzofuran (DBF) and their evolution with the temperature are presented and discussed. Dichlorobenzene adsorbs weakly probably on Lewis acid sites through the chlorine atom. A very fast nucleophilic substitution on dichlorobenzene to a chlorophenate species occurs already at RT. On the contrary, adsorption of dibenzofuran is molecular, probably through the oxygen atom on Lewis sites. The aromatic rings of both molecules tend to be later oxidized to give carboxylate species. Parallel experiments with chloropropane show that dehydrochlorination occurs readily, hydrochloric acid is adsorbed quite weakly and that propene can be further oxidized. However, heavier oxidized species like cyclic anhydrides are also formed from chloropropane. The data suggest that the dechloration step of the phenyl ring could be not critical. On the contrary, the building-up of aromatic compounds from smaller molecules is possible and the oxidation of phenyl ring can be slow near reaction conditions. The possibility that the reaction between phenate species and chlorobenzenes give the “de novo” synthesis of dioxins is envisaged.     

V2O5-WO3/TiO2脱硝催化剂机械强度和孔隙率的响应曲面模型

氨气选择性催化还原技术(NH₃-SCR)是目前燃煤电厂消除氮氧化物(NO_x)最有效、最成熟的方法之一,其中催化剂作为技术体系的核心,其成型技术逐渐成为研究热点。本文针对V₂O₅-WO₃/TiO₂蜂窝式脱硝催化剂的成型工艺,通过响应曲面法对其机械强度和孔隙率进行研究,分别得出压制压力和聚甲基丙烯酸甲酯(PMMA)微球添加量对机械强度和孔隙率的显著回归模型。发现一定的压制压力和PMMA微球的添加可以使得压制而成的蜂窝式脱硝催化剂既能保持较高的机械强度,又能维持高的孔隙率。孔径分布及扫描电子显微镜(SEM)结果表明催化剂壁上存在明显的纳米-微米多级孔。活性测试结果显示:催化剂表现出了优良的NO_x转化率及N₂选择性,满足了目前工业脱硝领域的需求。     

助剂和制备方法对V2O5/Al2O3催化性能的影响

用浸渍法制备了负载型MxOy-V2O5/Al2O3催化剂,同时考察了不同的制备方法对MoO3-V2O5/Al2O3催化剂的性能影响。用BET、H2-TPR、TPD平衡法等考察了催化剂的比表面积、氧化还原性能、表面氧脱附性能,结果表明,添加变价助剂Mo、Fe和Cr氧化物会增加供氧数目,提高α-蒎烯转化率。MoO3-V2O5/Al2O3催化剂表面O 2-和O-物种较多,活性也较好,桃金娘烯醛收率较高。MoO3-V2O5/Al2O3催化剂的制备方法影响其催化性能,表面化学改性法制备的催化剂表面具有较多O2-和O-物种,有利于催化剂的活性提高,α-蒎烯的转化率达88.9%,桃金娘烯醛的收率为79.1%。