Effect of the catalyst supports on the removal of perchloroethylene (PCE) over chromium oxide catalysts

The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on TiO₂, Al₂O₃, SiO₂, SiO₂–Al₂O₃ and activated carbon. The phase of chromium oxide on the catalyst surface is critical for the oxidation of PCE. The catalytic activity of PCE removal enhances as the formation of Cr(VI) species on the catalyst surface increases. The surface area and the type of the catalyst supports were also essential for high performance in the PCE oxidation. In addition, the structure of Cr(VI) on the catalyst surface also plays an important role for the decomposition of PCE. The polymerized Cr(VI) mainly formed by the interaction of metals with the support is the active reaction site for the present reaction system. CrO_x/TiO₂ reveals the strongest PCE removal activity among the catalysts examined in the present study. This revised version was published online in July 2006 with corrections to the Cover Date.     

A pilot plant study for catalytic decomposition of PCDDs/PCDFs over supported chromium oxide catalysts

Chromium oxide catalysts supported on TiO₂ and Al₂O₃ were examined in a fixed-bed flow reactor system for the removal of PCE (perchloroethylene), a simulant of 2,3,7,8-TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), and in a pilot plant employing actual flue gas from a sintering plant for the removal of PCDDs/PCDFs (poly-chlorinated dibenzo-dioxin/poly-chlorinated dibenzo-furan). The 12.5 wt.% chromium oxides supported on TiO₂ and Al₂O₃ revealed excellent stability and performance of PCE removal in the feed gas stream containing water vapor. In a pilot plant study, the catalysts washcoated on the honeycomb reactor revealed 93–95% of PCDDs/PCDFs removal activity over CrO_x/Al₂O₃-HC20 (CrO_x/Al₂O₃ catalyst washcoated on 20 cell-honeycomb), and more than 99% of the decomposition activity over CrO_x/TiO₂-HC20 (CrO_x/TiO₂ catalyst washcoated on 20 cell-honeycomb) at 325 °C and 5000 h⁻¹ of reactor space velocity without the de novo synthesis of PCDDs/PCDFs. In particular, CrO_x/TiO₂-HC20 showed 94% of PCDDs/PCDFs decomposition activity even at 280 °C reaction temperature. The catalyst also exhibited significant NO removal activity. The chromium oxide seems to be a promising catalyst for the removal of PCDDs/PCDFs and NO_x contained in the flue gas.     

Mesoporous silica supported cobalt oxide catalysts for catalytic removal of benzene

Two types of mesoporous silica SBA-15 with different pore diameter were synthesized with an ageing temperature of 373 K and an ageing temperature of 308 K, respectively; in addition, mesoporous silica with amorphous structure was synthesized by adding organosiloxane as part of the silica source during the synthesis procedure. Mesoporous silica and conventional alumina supported cobalt oxide catalysts were prepared by incipient wetness impregnation method. These materials were characterized by FT-IR, nitrogen adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Temperature programmed reduction (TPR) techniques, and the activity of the supported cobalt oxide catalysts for deep oxidation of benzene were evaluated in a fixed-bed reactor. It seems that the pore diameter of the silica increase with the elevation of the ageing temperature. Mesoporous silica supported cobalt oxide catalysts are more active than conventional alumina supported ones. Cobalt oxide can be relatively better dispersed on the surface of mesoporous silica which has larger pore diameter and surface areas. Meanwhile, more silanol groups exist on the surface of amorphous silica, which could induce a strong interaction with the supported cobalt oxide species, leading to poor activity for benzene oxidation.     

Catalytic gas-phase fluorination of 1,1,1-trifluoro-2-chloroethane over chromium (III) oxide: Preparation of hydrofluoroalkanes

The transformation of CF₃CH₂Cl over bulk chromium(III) oxide, fluorinated or not, leads mainly to the fluorination product CF₃CH₂F and to the alkene CF₂=CHCl. In the presence of HF, CF₃CH₂F is the main product. In absence of HF this product is also obtained with HF formed in the elimination reaction of CF₃CH₂Cl to CF₂=CHCl. At the same time a part of HF is used for the fluorination of chromium oxide. The XPS analysis of fluorinated catalysts with HF gives surface fluorine atomic contents which correspond to a F/Cr ratio of 0.36. A survey of the effect of partial pressures of HF and of CF₃CH₂Cl shows a zero order for HF and a 0.6 order for CF₃CH₂Cl. The fluorination reaction could occur on a partially fluorinated chromium(III) oxide involving HF oligomer species previously postulated by Rowley et al. [L. Rowley, J. Thomson, G. Webb and J.M. Winfield, Appl. Catal. A, 79 (1991) 89–103].     

Effect of perchloroethylene (PCE) and hydraulic shock loads on a membrane‐aerated biofilm reactor (MABR) biodegrading PCE

BACKGROUND: A membrane‐aerated biofilm reactor (MABR) has previously been used to provide both anaerobic and aerobic conditions for mineralisation of perchloroethylene (PCE). However, very little is known about the stability of this reactor under hydraulic and PCE shock loads. An MABR was therefore subjected to sudden hydraulic and PCE shock loads in order to investigate its stability under such conditions. RESULTS: After each shock, the reactor responded with an increase of chemical oxygen demand (COD) and volatile fatty acids (VFA)s, a breakthrough of PCE and its biodegradation intermediates in the effluent, and a decrease in methane production. Although some PCE biodegradation intermediates were found in the effluent during each shock loading, the MABR performance recovered without the accumulation of any particular PCE biodegradation intermediates during PCE shock loads. During the hydraulic shock loads, the MABR was unstable at hydraulic retention times (HRTs) of 6 h with PCE and its biodegradation intermediates detected in the effluent. However, these intermediates were degraded when the HRT was reset to 48 h. CONCLUSIONS: This study suggests that MABRs can withstand fluctuations in influent strength and flows which occur in wastewater treatment works. Copyright © 2009 Society of Chemical Industry     

Catalytic nitroxidation of para-xylene over chromium oxide based catalysts

The synthesis of para-tolunitrile (PTN) and terephthalonitrile (TPN) by reacting nitrogen monoxide with para-xylene was studied in presence of three chromium oxide catalysts. The Cr₂O₃—Al₂O₃ catalyst was prepared under the form of aerogel or xerogel and shows excellent selectivities in nitriles along with good stability. However, the unsupported Cr₂O₃ catalyst showed much less performance and, in particular, produced no TPN. The beneficial action of alumina was explained in terms of stabilization of the Cr⁵⁺ species. Kinetic results seemed to indicate a redox-type mechanism of the reaction. Moreover the PTN was shown to be an intermediate in TPN formation.     

Photooxidation and biotrickling filtration for controlling industrial emissions of trichloroethylene and perchloroethylene

A two-stage process integrating ultraviolet oxidation and biotrickling filtration (UV-BTF) was developed for treatment of industrial air streams contaminated with trichloroethylene (TCE) and perchloroethylene (PCE). Laboratory-scale studies demonstrated that the UV-BTF system consistently achieved contaminant removal efficiencies of 99-100% under optimal conditions. The UV-BTF process employs an advanced oxidation process (AOP) such as UV photooxidation as pretreatment for alleviating biological toxicity attributed to high and/or fluctuating contaminant concentrations, and for transforming the contaminants to more easily biodegradable products. The UV-BTF process showed significantly better performance than the BTF process without UV photooxidation pretreatment. Free-radical reaction mechanisms, pathways, and product formations were proposed for the UV photolysis of TCE and PCE. The photooxidation was mediated by chlorine atom reactions, and the principal products for TCE were phosgene, dichloroacetyl chloride (DCAC), low levels of trichloroacetyl chloride (TCAC), carbon monoxide, and hydrochloric acid; while those for PCE were phosgene, TCAC, low levels of DCAC, carbon dioxide, and hydrochloric acid. These products were identified and quantified by a combination of techniques, namely, gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared (FTIR) spectroscopy. These photooxidation products were completely removed in the BTF stage of the UV-BTF process, as indicated by subsequent GC-MS analyses. The destruction of photolysis products in the BTF stage occurred by a combination of hydrolysis, adsorption and microbial degradation.     

负载型NiO和CoO催化剂上N2O分解研究

本研究在对多种金属氧化物催化剂进行初步筛选的基础上，研制出以莫来石为载体的负载型NiO、CoO催化剂，考察了分解温度、催化剂组成和负载量对N2O分解率的影响，并对其分解反应动力学进行了研究。结果表明莫来石负载NiO、CoO催化剂对N2O分解有良好的催化性能；其反应速度对N2O均为一级反应；同样负载量下NiO有更好的催化分解活性；这一研究为开发阻力低、催化性能好的工业用蜂窝型规整填料奠定了基础。     

Catalytic properties of supported tungsten oxide catalysts

Catalytic properties of the WO₃-MgF₂ preparations obtained by the impregnation (I-series) and coprecipitation (C-series) method have been determined in acid environment. Various tungsten complexes causing the Lewis and Broensted acidity have been observed by IR spectroscopy. For catalysts of the I-series the isolated tungsten species were responsible for the activity in acid reactions, whereas for the C-series preparations the polymerized oxide was also involved.     

Partial oxidation of methane over silica supported molybdenum oxide catalysts

Two kinds of MoO₃/SiO₂ catalysts, MoO₃-I and MoO₃-S, were prepared by impregnation and sol-gel method, respectively. When MoO₃ loading was increased, formation of MoO₃ crystals was observed to begin at a MoO₃ loading of 8 and 16 wt% with MoO₃-I and MoO₃-S, respectively. The highest yield of formaldehyde from methane oxidation was attained also at those critical values of MoO₃ loading of 8 and 16 wt% over MoO₃-I and MoO₃-S, respectively. It is suggested that the active species for formaldehyde formation is well dispersed molybdenum oxide clusters on SiO₂ support: the optimum dispersion of the clusters affords the highest activity for formaldehyde formation.     

Hydrogenation of carbon dioxide over alumina supported Fe-K catalysts

The hydrogenation of CO₂ has been studied over Fe/alumina and Fe-K/alumina catalysts. The addition of potassium increases the chemisorption ability of CO₂ but decreases that of H₂. The catalytic activity test at high pressure (20 atm) reveals that remarkably high activity and selectivity toward light olefins and C₂₊ hydrocarbons can be achieved with Fe-K/alumina catalysts containing high concentration of K (K/Fe molar ratio = 0.5, 1.0). In the reaction at atmospheric pressure, the highly K-promoted catalysts give much higher CO formation rate than the unpromoted catalyst. It is deduced that the remarkable catalytic properties in the presence of K are attributable to the increase in the ability of CO₂ chemisorption and the enhanced activity for CO formation, which is the preceding step of C₂₊ hydrocarbon formation.     

Catalytic decomposition of nitrous oxide over perovskite type solid oxide solutions and supported noble metal catalysts

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been investigated over various solid oxide solutions (SOS), La_0.8Sr_0.2MO_3– (M=Cr, Fe, Mn, Co or Y), La_1.8Sr_0.2CuO_4– and supported Pd, Pt catalysts. The reaction was carried out in a gradientless recycle reactor at 1 atm pressure with a feed gas containing about 0.5% N₂O (in helium). Among the various solid solutions, La_0.8Sr_0.2CoO_3– showed a maximum N₂O conversion of 90% at 600C. The order of activity observed for N₂O decomposition was La_0.8Sr_0.2CoO_3–>La_0.8Sr_0.2FeO_3–>La_1.8Sr_0.2CuO_4–> La_0.8Sr_0.2MnO_3–La_0.8Sr_0.2CrO_3–La_0.8Sr_0.2YO_3–. The activity of La_0.8Sr_0.2CoO_3– was compared with supported Pd, Pt and also with unsubstituted LaCoO₃ catalysts under similar reaction conditions. Among all the catalysts tested in this study, Pd/Al₂O₃ showed the lowest light-off temperature for N₂O decomposition. The activity of La_0.8Sr_0.2CoO_3– was found to be comparable to Pd/Al₂O₃ catalyst at temperatures above 500 C. The influence of added oxygen (about 4%) in the feed was examined over La_0.8Sr_0.2CoO_3– and Pd/Al₂O₃ catalysts and only in the case of cobalt catalyst was the conversion of N₂O decreased by 13%. By choosing varied sintering conditions, La_0.8Sr_0.2CoO_3– of different BET surface areas were prepared and the light-off temperature was found to decrease with increase in surface area. The results obtained over solid solutions are discussed on the basis of the cation mixed valency and oxygen properties of the catalyst.     

负载型钴锰复合金属氧化物催化剂催化分解N_2O

以ZrO_2为载体,采用浸渍法制备负载型钴锰复合金属氧化物催化剂,研究催化剂活性组分负载量、Co与Mn物质的量比、焙烧条件及含H_2O气氛对N_2O转化率的影响。结果表明,催化剂最佳制备条件为:活性组分Co负载质量分数3%,Co与Mn物质的量比为1∶1,焙烧升温速率2℃·min-1,焙烧温度900℃。该条件制备的负载型钴锰复合金属氧化物催化剂在反应温度850℃时,N_2O转化率达98.7%。当反应气氛中H_2O体积分数小于20%条件下,850℃时N_2O转化率高于90%,表明催化剂具有较强的抗水性能。     

Supported chromium oxide catalysts using metal carboxylate complexes: dehydrogenation of propane

The present paper constitutes the first stage of a systematic study of the influence of precursor nature and structure on the catalyst behavior in dehydrogenation of propane, by varying the nature of the ligands and the nuclearity of the starting compounds at fixed net surface potentials. We show results obtained by the use of different Cr carboxylates to design the Cr₂O₃/γ-Al₂O₃ catalyst at low loadings and these results are compared with those obtained for chromic acid. The following short chain chromium compounds were selected as precursors: citrate (Cr(III)), dimer monohydrate acetate (Cr(II)), acetyl acetonate (Cr(III)) and hydroxyacetate (Cr(III)). An exhaustive characterization by means of BET surface, XPS, XRD, X-ray fluorescence, laser Raman spectroscopy, EPR and catalytic test in propane dehydrogenation enabled us to draw relevant conclusions. Low metal–support interaction might be the major cause of polymerization in Carbox. The interaction between the chromia phase and the support surface, which stabilizes different oxidation stages and coordinations of the chromia species, defines the surface architecture. Part of the Cr³⁺ is incorporated in the vacant octahedral sites of the spinel surface. The larger extension of this phenomenon in Cral might be responsible for surface inactive species that could cause a loss of catalytic activity. Deactivation between cycles might depend on the catalyst stabilization by the incorporation of Cr³⁺ into the support structure. The surface array of monomer species in a polymer species environment stabilizes the catalyst architecture and confers characteristics of high stability between cycles. The above considerations permit to infer that chromium carboxylates are interesting precursors for the control of surface species in chromia/Al₂O₃ catalysts.     

负载型La2-xSrxCoO4类钙钛矿复合氧化物CO催化氧化性能

采用柠檬酸络合-浸渍法制备了以镁铝尖晶石MgAl2O4为载体、复合氧化物La2-xSrxCoO4(X=0.2、0.4、0.6和0.8)为活性组分的催化剂,通过XRD、H2-TPR和BET等分析方法对催化剂进行表征,利用固定床微型反应器测试催化剂对CO氧化的催化性能,并考察水蒸汽和SO2对其活性的影响.结果表明,X=0....     

Cordierite monolith supported perovskite-type oxides — catalysts for the total oxidation of chlorinated hydrocarbons

The catalytic behaviour of LaMnO₃, LaCoO₃ and (La_0.84Sr_0.16)(Mn_0.67Co_0.33)O₃ perovskites supported on cordierite monoliths was studied in the total oxidation of chlorinated hydrocarbons (CHC). By-products (higher chlorinated hydrocarbons, lower molecular coupling and cracking products) were formed in the low temperature range depending on the kind and concentration of CHC in the feed and the reaction conditions. The preparation method (impregnation or coating) influences catalytic activity and by-product formation. Addition of water, methane or propane to the feed diminishes the by-product formation significantly. In the case of monolith supported LaCoO₃ perovskite, an irreversible catalyst deactivation was observed.     

Oxidative decomposition of dichlorodifluoromethane (CFC-12) in the presence of butane over Tungsten(VI) oxide catalysts supported on alumina–zirconia

The oxidative decomposition of dichlorodifluoromethane (CFC-12) in the presence of butane was investigated with tungsten(VI) oxide catalyst supported on alumina–zirconia. In this reaction, it is considered that CFC-12 might react with water formed by the combustion of butane at high temperatures above 773 K, and that most of CFC-12 might react directly with oxygen at low temperatures below 723 K. The CFC-12 conversion over the tungsten(VI) oxide catalyst decreased with the passage of time on stream at low temperatures below 673 K. This deactivation of the catalyst was attributed to the coke deposition on the acid sites of the catalyst as well as to the fluorination of the acid sites. The CFC-12 conversion at 4 h on stream became higher when the catalyst was loaded with platinum or palladium, but the butane conversion was not changed by loading. The amount of the coke deposited on platinum- or palladium-loaded tungsten(VI) oxide catalysts, which exhibited a high CFC-12 conversion at 4 h on stream, was very small. Accordingly, the depression of the catalyst deactivation might be caused by the combustion of the coke deposited on the acid sites due to the catalysis of platinum and palladium.     

Cr-doped Zr, Si-mesoporous molecular sieves as catalysts of CH2Cl2 oxidation

Zirconium-containing mesoporous silica of nominal Si/Zr ratio in the range 40–5 have been synthesized using dodecylamine as a structure-directing surfactant. The samples were characterized with PXRD, SEM/EDX, BET, CEC measurement and chemical analysis. The materials were used as supports for chromium species incorporated by means of cation exchange procedure. Gradual substitution of Si by Zr in the mesoporous framework results in an increasing structural disorder, increasing cation exchange capacity and increasing capacity for incorporation of Cr. All Cr-doped ZrMMSx catalysts are active in the deep oxidation of methylene chloride reaching 100% conversion at temperature ≥400°C. The catalytic performance depends strongly on the catalyst composition. The role of particular components in determining the catalytic activity and selectivities to various products is discussed.     

负载Pt-B非晶态催化剂的氯代硝基苯加氢性能

分别以纳米碳管（CNTs）、SiO₂、γ-Al₂O₃和活性炭(AC)为载体,贵金属铂(Pt)为活性组分,用浸渍－化学还原法制备了负载型Pt-B非晶态催化剂,并以邻氯硝基苯的氢化为探针反应考察其催化性能。结果表明,Pt-B/CNTs催化剂具有较好的抑制脱卤的效果,分别较其他三种载体所得脱卤率降低了5.5倍、12.1倍和14.75倍。在SAED和TEM表征的基础上,从电子效应、催化剂粒子大小及载体的结构等方面解释了产生上述差别的原因。