Iron—molybdenum—oxide catalysts for selective oxidation of hydrogen sulfide to sulfur

The selective oxidation of hydrogen sulfide to sulfur was studied over iron-molybdenum oxides with various Fe-Mo ratios. Strong synergistic phenomenon in catalytic activity was observed for the Fe-Mo-O binary oxides. Under identical reaction conditions, the areal rates of the binary oxides were superior to those of the corresponding single oxide catalysts, which suggest that the new compound Fe₂(MO₄)₃ formed in the binary oxide is more active than Fe₂O₃ and MoO₃. The oxidation rates of H₂S were found to exhibit first-order dependence on the hydrogen sulfide concentration, which implies that the activation of H₂S is the rate-limiting step.     

A study on selective oxidation of hydrogen sulfide over zeolite-NaX and-KX catalysts

Selective oxidation of hydrogen sulfide (H₂S) was studied on zeolite-NaX and zeolite-KX. Elemental sulfur yield over zeolite-NaX was achieved about 90% at 225 °C for the first 4 hours, but it gradually decreased to 55% at 40 hours after the reaction started. However, yield of elemental sulfur on zeolite-KX was obtained within the range of 86% at 250 °C after 40 hours. The deactivation of the zeolite-NaX and -KX catalysts was caused by the coverage of a sulfur compound, produced by the selective oxidation of H₂S over the catalysts. The coverage of a sulfur compound over the zeolite-NaX and -KX was confirmed by the TPD (temperature-programmed desorption) tests utilizing thermogravimetric analysis and FT-IR analysis. Even though high temperature was required to prevent the deactivation of zeolite-NaX, the temperature cannot be raised to 250 °C or above due to the SO₂ production and the decrease of thermodynamic equilibrium constant. Zeolite-KX was superior to the zeolite-NaX for both its selectivity to elemental sulfur and its resistance to deactivation in the selective oxidation of H₂S.     

New effective catalysts based on mesoporous nanofibrous carbon for selective oxidation of hydrogen sulfide

The systems based on granular mesoporous nanofibrous carbonaceous (NFC) materials synthesized by decomposition of hydrocarbons over nickel-containing catalysts are promising catalysts for selective oxidation of hydrogen sulfide. Sample series of nanofibrous carbon with three main types of their fiber structures and different contents of metal catalysts inherited from the catalysts for their synthesis were studied in this reaction. The correlation between NFC structure and its activity and selectivity in hydrogen sulfide oxidation was determined. The metal inherited from the initial catalysts for the synthesis of NFC influences the activity and selectivity of the resulting carbon catalysts. A particular influence is observed in the case of the catalyst withdrawn from the synthesis reactor at the stage of stationary operation of the metal catalyst (low specific carbon yields per unit weight of the catalyst). The presence of the metal phase results in an increase in the carbon catalyst activity and in a decrease in the selectivity to sulfur. NFC samples with the highest activity and selectivity are nanotubes and those with graphite planes perpendicular to the axis of the fibers. Carbon nanotubes have high selectivity, while samples obtained on copper–nickel catalysts also possess high activity. The promising NFC catalysts provide high conversion and selectivity (almost independent of the molar oxygen/hydrogen sulfide ratio) when a large excess of oxygen is contained in the reaction mixture.     

Cobalt molybdenum oxide catalysts for selective oxidation of cyclohexane

Oxidation of cyclohexane has been carried out using molecular oxygen over cobalt molybdenum oxide (CoMoO₄) catalysts in solvent free conditions. The catalysts were prepared using citrate method with three different molar ratios of Co:Mo, 1:1, 1:2, and 2:1 along with individual oxides for comparative studies. While all the catalysts showed significant activity and selectivity, CoMoO₄ with 1:1 ratio showed the best performance compared to the others with a conversion of 7.38%, with selectivity to cyclohexanol and cyclohexanone (KA oil) of 94.3%, in 1 h. The performance of the catalyst, has been studied as a function of oxygen pressure, reaction temperature, and catalyst loading. It was observed that the catalyst deactivates during the course of the reaction. The reasons for deactivation and methods for restoring the activity have been studied. A kinetic model is presented that captures the complex kinetics and matches well with the experimental data. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4384–4402, 2016     

Silver catalysts supported over activated carbons for the selective oxidation of CO in excess hydrogen: effects of different treatments on the supports

Commercial activated carbon (AC) was used as a support either in its original form or after various modifications, giving diverse textural and surface chemical characteristics. The changes of these properties were monitored by N₂ adsorption–desorption isotherms, temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Ag catalysts were prepared over the AC supports by a conventional wet impregnation method. The catalytic performances of supported Ag/AC catalysts in the selective oxidation of CO in excess H₂ were tested. The results indicate the textural and surface chemical characteristics are responsible for the different catalytic performances.     

Testing molten metal oxide catalysts over structured ceramic substrates for diesel soot oxidation

Onboard tests of an oxide (CoO_x) and a molten mixed oxide catalyst (CoPbO_x) supported on cordierite wall flow filters (Corning Durotrap CO EX80) were performed on rollers with a 1.9-l light duty vehicle for three different driving conditions including two constant speeds with different loadings and a standard European cycle (NEDC). The balance point temperatures obtained in these tests were used to compare the catalytic activity under onboard conditions with laboratory measurements. Onboard tests showed similar catalytic activities with the previous micro-reactor experiments performed in “loose contact” mode [D. Uner, M.K. Demirkol, B. Dernaika, Appl. Catal. B: Environ., in press]. The concentration profiles of the active layer determined by SEM-EDX analysis after use and after aging revealed that the mobile component, PbO_x, migrated in the flow direction and accumulated at the closed end of the channel. There is no direct evidence for the evaporative loss of PbO_x. Regions of high activity indicated by low carbon amounts with unique Pb–Co ratios were determined by SEM analysis.     

过氧化氢氧化苯酚铜锰复合氧化物催化剂的制备

采用固相法制备了新型催化剂铜锰复合氧化物催化剂。利用高效液相色谱法对苯酚过氧化氢氧化反应产物进行分析,以苯酚转化率和苯二酚收率为评价指标,对催化剂进行了评价。结果表明,催化剂性能随铜与锰物质的量比、焙烧温度、焙烧时间和研磨时间等因素的增大呈先升高再降低的趋势,并与络合剂的还原性有关。确定最佳工艺条件为:室温下,按锰与铜物质的量比1:2将Cu_2(OH)_2CO_3、MnCO_3和适量H_2C_2O_4·2H_2O混匀,置于研钵匀速研磨10 min,马弗炉400℃焙烧2 h。最佳条件下,苯酚转化率为63.7%,苯二酚收率为59.1%。     

Carbon catalyst supports for oxidative decomposition of hydrogen sulfide

Carbon supports having various porous structures and compositions of the mineral matter were obtained. On the basis of the carbon supports, a number of catalyst samples were prepared via the decomposition of an iron complex adsorbed in support pores. The effect of the treatment mode during catalyst preparation on the formation of iron clusters in the porous volume of the support has been studied by means of Mössbauer spectroscopy. The catalysts deposited on carbon supports were tested in the decomposition of hydrogen sulfide.     

SHS-produced β-sialons as supports for oxidation catalysts

SHS-produced β-sialons Si_6−z Al _z O _z N_8−z (z = 1, 3, 4) were tested as supports for oxidation catalysts comprising of unary, binary or ternary mixtures of transition metal (Cr, Mn, Fe, Co, Ni, Cu) oxides and also KMnO₄ and K₂Cr₂O₇. In oxidation of CO and propane (C₃H₈), the highest activity was exhibited by the catalysts based on cobalt oxides. Suggested are some methods for activation of the above catalysts.      

Lignocellulosic-derived catalysts for the selective oxidation of propane

The formation of dispersed VPO phases on a lignocellulosic-based activated carbon results in a catalyst that is selective for the partial oxidation of propane, and stable under oxidizing conditions. The use of a stable activated carbon as a catalytic support for active VOx species during the partial oxidation of propane is described for the first time.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over Bi-V-Sb-O catalysts

We investigated the selective oxidation of hydrogen sulfide to elemental sulfur and ammonium thiosulfate by using Bi₄V_2-xSb_xO_11-y catalysts. The catalysts were prepared by the calcination of a homogeneous mixture of Bi₂O₃, V₂O₅, and Sb₂O₃ obtained by ball-milling adequate amounts of the three oxides. The main phases detected by XRD analysis were Bi₄V₂O₁₁, Bi_1.33V₂O₆, BiSbO₄ and BiVO₄. They showed good H₂S conversion with less than 2% of SO₂ selectivity with a feed composition of H₂S/O₂/NH₃/H₂O/He=5/2.5/5/60/27.5 and GHSV=12,000 h^-1 in the temperature ranges of 220–260 ‡C. The highest H₂S conversion was obtained for x=0.2 in Bi₄V_2-xSb_xO_11-y catalyst. TPR/TPO results showed that this catalyst had the highest amount of oxygen consumption. XPS analysis before and after reaction confirmed the least reduction of vanadium oxide phase for this catalyst during the reaction. It means that the catalyst with x=0.2 had the highest reoxidation capacity among the Bi₄V_2-xSb_xO_11-y catalysts.     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.     

Novel supports for nickel-based catalysts for the partial oxidation of methane

Zirconia-supported nickel catalysts with different amounts of aluminum (Al/Zr = 0.2, 1 and 2) were studied in this work in order to find alternative supports for nickel-based catalysts for the partial oxidation of methane. This reaction is a promising route for producing hydrogen and syngas for different applications. Samples were prepared by precipitation and impregnation techniques, characterized by several techniques and evaluated in the partial oxidation of methane in the range of 450–750 °C and 1 atm. It was found that aluminum affects the textural and catalytic properties of zirconia-supported nickel catalysts. The tetragonal phase of zirconia was stabilized by aluminum and gamma-alumina was also found in the aluminum-richest samples. Aluminum increased the porosity and the specific surface area of the solids. The catalytic activity also increased with the amount of aluminum in solids probably due to the stronger interaction of nickel with the support, which slowly generates active sites during the reduction step. The methane conversion and hydrogen selectivity increased with temperature, indicating no deactivation. The hydrogen to carbon monoxide molar ratio decreased due to aluminum but was not significantly affected by temperature. The coke produced was not harmful to the catalysts and aluminum affected its amount, although no simple relationship was found between these parameters. The most promising catalyst was the sample with aluminum to zirconium molar ratio of 2, which showed high activity and hydrogen selectivity and was stable under the reaction condition.     

Novel efficient catalysts based on Ru or Pd oxide for selective liquid-phase oxidation of alcohols with nitrous oxide

Ru^IV–M and Pd^II–M (MCo^III, Fe^III and Mn^III) binary oxides supported on γ-alumina are demonstrated to be novel, active catalysts for selective liquid-phase oxidation of primary and secondary alcohols to, respectively, aldehydes and ketones with nitrous oxide under 10 bar N₂O pressure and 100 °C. The Ru and Pd catalysts have comparable activities but Ru ones are more selective, the Ru–Co system (Ru/Co=1:1) showing the best performance. Activated primary alcohols (benzyl and cinnamyl alcohol) and secondary alcohols give aldehydes or ketones with 100% selectivity at 95–100% conversion. Non-activated primary alcohols (e.g. 1-dodecanol) give aldehydes with some over-oxidation to acids; the latter is completely inhibited by adding a radical scavenger. The Ru–Co catalyst can be reused without loss of selectivity, although with a gradual decrease in its activity. Compared to the Ru–Co catalysed oxidation of alcohols with O₂ studied earlier, the oxidation with N₂O has the advantage of considerably higher selectivity, albeit occurring under higher pressures.     

Vanadium species in new catalysts for the selective oxidation of methane to formaldehyde: Activation of the catalytic sites

New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. In situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)₂VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 °C under vacuum, leading to a site giving a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state:

(V⁴⁺–O⁻)^*  (V⁵⁺O²⁻). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed.     

Iron-chromium-molybdenum oxide catalysts for methanol oxidation

The activity and selectivity of a precipitated iron-chromium-molybdenum oxide catalyst (Mo/(Fe + Cr) = 2.5/(0.5 + 0.5)) towards methanol mild oxidation have been studied by a flow-circulation method. Commensurable activity and selectivity with those of the industrial Fe₂ (MoO₄)₃-MoO₃ catalysts as well as an enhanced stability have been found. The Mössbauer spectra of fresh and tested catalysts show that during the catalytic reaction a partial reduction occurs and a steady state composition differing from the initial one is formed.     

SBA -15负载铁基氧化物催化剂上丙烷选择氧化催化性能的研究

用等体积浸渍法制备了SBA - 15负载铁基和钾修饰的Fe基氧化物催化剂,使用紫外-拉曼光谱对催化剂结构进行表征,并评价催化剂对丙烷的选择氧化活性与选择性.结果表明,介孔结构SBA - 15对丙烷选择氧化活性优于常规的SiO2;对于负载K- Fe/SBA - 15系列催化剂,低Fe负载量时,隔离的四配位Fe氧化物是丙烷...     

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。