Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium-exchanged insoluble heteropolyacid catalysts

Palladium-exchanged insoluble heteropolyacid (Pd_0.15Cs_xH_2.7?xPW₁₂O₄₀) catalysts were prepared with a variation of cesium content (x = 2.0, 2.2, 2.5, and 2.7), and were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Pd_0.15Cs_xH_2.7?xPW₁₂O₄₀ showed high catalytic performance even in the absence of H₂SO₄ additive, indicating that Pd_0.15Cs_xH_2.7?xPW₁₂O₄₀ acted as an efficient catalyst and served as an alternate acid source in the reaction. The catalytic performance of Pd_0.15Cs_xH_2.7?xPW₁₂O₄₀ increased with increasing surface acidity of the catalyst. Among the catalysts tested, Pd_0.15Cs_2.5H_0.2PW₁₂O₄₀ catalyst with the largest surface acidity showed the highest yield for hydrogen peroxide.     

Direct synthesis of hydrogen peroxide from H2 and O2 using zeolite-supported Au-Pd catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using zeolite-supported Au-Pd catalysts is described using two zeolites, ZSM-5 and zeolite Y, using an impregnation method of preparation. The addition of Pd to Au for these catalysts significantly enhances the productivity for hydrogen peroxide. The use of zeolites as a support for Au-Pd gives higher rates of hydrogen peroxide formation when compared with alumina-supported Au catalysts prepared using a similar method. The addition of metals other than Pd is also investigated, but generally Au-Pd catalysts give the highest activity for the synthesis of hydrogen peroxide. The addition of Ru and Rh have no significant effect, but the addition of Pt does enhance the activity for the selective formation of hydrogen peroxide.     

Direct synthesis of hydrogen peroxide from H2 and O2 using zeolite-supported Au catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using zeolite-supported Au catalysts is described and their activity is contrasted with silica- and alumina-supported Au catalysts. Two zeolites were investigated, ZSM-5 and zeolite Y. The effect of calcination of these catalysts is studied and it is found that for uncalcined catalysts high rates of hydrogen peroxide formation are observed, but these catalysts are unstable and lose Au during use. Consequently, reuse of these catalysts leads to lower rates of hydrogen peroxide formation. However, catalysts calcined at 400 °C are more stable and can be reused without loss of gold. The use of zeolites as a support for Au gives comparable rates of hydrogen peroxide formation to alumina-supported Au catalysts and higher rates when compared with silica-supported catalysts. prepared using a similar method. Zeolite Y-supported catalysts are more active than ZSM-5-supported catalysts for the stable calcined materials. It is considered that the overall activity of these supported catalysts may be related to the aluminium content as the activity increases with increasing aluminium content.     

Preferential CO oxidation over activated carbon supported catalysts in H2-rich gas streams containing CO2 and H2O

Selective CO oxidation (PROX) was studied at 423 K over 1% Pt–0.25% SnO_x and 1% Pt–1% CeO_x catalysts supported on un-oxidized and oxidized activated carbon (AC) using feed mixtures simulating the reformate coming from fuel processors. Effects of the addition of 15% CO₂ or (15% CO₂ + 10% H₂O) into feed mixtures containing 1% CO, 1% O₂, 60% H₂ and He were determined for nine different AC-supported catalysts, and the results were compared with those obtained with pure H₂-rich feed. Unlike other PROX catalysts having oxide supports, introduction of CO₂ into pure feed drastically increased CO conversion on all nine catalysts supported on oxidized or un-oxidized AC regardless of impregnation strategy.1% Pt–0.25% SnO_x supported on HNO₃-oxidized AC stands out as a potential candidate for commercial use in PROX since it yields 100% CO conversion under realistic feed conditions. 1% Pt–1% CeO_x catalysts prepared by sequential or co-impregnation and supported on air-oxidized AC also give 100% CO conversion in H₂-rich feed containing (CO₂ + H₂O) during extended run times and hence hold promise as PROX catalysts.     

Catalytic oxidation of hydrogen over Au/TiO2 catalysts

Selective catalytic oxidation of hydrogen in the presence of hydrocarbons was studied in a fixed bed quartz reactor, over 3 wt%Au/TiO₂ and 5 wt%Au/TiO₂ catalysts. This reaction can be utilised in the production of light alkenes via catalytic dehydrogenation, providing in situ heat to the endothermic dehydrogenation reaction and simultaneously removing a fraction of the produced hydrogen. It is important to avoid the non-selective combustion of the hydrocarbons in the mixture. Both 3 wt%Au/TiO₂ and 5 wt%Au/TiO₂ are active for the combustion of hydrogen, but in a gas mixture with propane and oxygen the selectivity is dependent upon the feed ratio of hydrogen and oxygen. At 550 °C, with propane present, no carbon oxides are formed when the H₂:O₂ ratio is four, but at lower ratios some CO₂ and some CO is formed.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over a Pd core-silica shell catalyst

Pd core–silica shell particles (Pd@SiO₂) were prepared by encapsulating Pd colloids with a silica shell through the Stöber method. The palladium core particles were well dispersed (Dispersion = 43%) and had uniform size (4 nm) and shape inside the porous silica shell. Pd@SiO₂ showed good catalytic activity (554 mmol H₂O₂/g Pd·h) for the direct synthesis of H₂O₂, which was better than those of impregnated Pd catalysts (Pd/SiO₂ and Pd/Al₂O₃). It is expected that the stabilization of less coordinated Pd crystals in a highly dispersed state by core-shell formation is effective for the improvement of H₂O₂ production.     

Naphthalene oxidation over vanadium-modified Pt catalysts supported on γ-Al2O3

Alumina-supported platinum catalysts modified by vanadium were synthesised and tested for the complete oxidation of naphthalene. The catalysts were characterised by BET, pulsed CO chemisorption, powder X-ray diffraction, laser Raman spectroscopy and temperature-programmed reduction. Whilst BET and CO chemisorption results showed that the addition of vanadium modified both the textural properties of the support and the distribution of Pt, XRD and TPR data suggested the presence of V₂O₅ on catalysts with higher V loadings. TPR data showed that the concentration of V₂O₅ and possibly some other vanadium species increased as the V loading increased. Only 0.5%V was found to promote the activity of the 0.5%Pt/γ-Al₂O₃ catalyst. The activity enhancement has been related to the presence of a more easily reducible vanadium species coupled with the enhanced number of surface Pt sites. On the other hand, the reduced activity demonstrated by catalysts with higher vanadium content (1 – 12%) has been attributed to the presence of crystalline V₂O₅.     

Selective partial oxidation of light paraffins with hydrogen peroxide on thin-layer supported Nafion-H catalysts

Thin-layer carbon supported Nafion-H catalysts were found to be active and highly selective (S>98%) for the partial oxidation of C₁-C₃ alkanes, in a three phase catalytic membrane reactor (3PCMR), under mild conditions and in the presence of H₂O₂. The influences of the catalyst teflon loading and H₂O₂ concentration on the reaction rate have been evaluated. A reaction pathway, based on the electrophilic hydroxylation of the C-H bond of alkanes with protonated hydrogen peroxide (H₃O ₂ ⁺ ), is discussed.     

Synergism in methanol synthesis from carbon dioxide over gold catalysts supported on metal oxides

Gold deposited on various oxides with high dispersion was found to be active for the hydrogenation of CO₂ at temperatures between 150 and 400°C. Product selectivities greatly depended on the nature of support oxide. Acidic oxides like TiO₂ gave higher CO₂ conversions but low methanol yields. Zinc oxide component was indispensable for selective methanol synthesis. Significantly, large particle size effect of gold was observed and smaller gold particles gave higher methanol productivity per exposed surface area of gold. This can be explained by an increase in the perimeter area of gold particles with a decrease in particle size. Methanol yield was greatly enhanced in a Au/ZnO---TiO₂ catalyst probably due to an increase in gold-zinc oxide interface.     

Catalytic oxidation of trichloroethylene over TiO2 supported ruthenium catalysts

Different types of TiO₂ (anatase, P25 and rutile) supported ruthenium catalysts were synthesized by wet impregnation and directly reduced in H₂. The distribution characteristics of ruthenium species were thoroughly studied before and after trichloroethylene oxidation. The results show that ruthenium oxide species are very unstable in the anatase phase, but quite stable in the rutile phase of TiO₂. This phenomenon results in different catalytic behaviors for the Ru/TiO₂ catalysts. The Ru/TiO₂ (P25) catalyst has the best catalytic performance among these catalysts. The complete conversion temperature of trichloroethylene is in the temperature range of 260–270 °C.     

催化湿式氧化中负载型催化剂制备

采用浸渍法制备了负载型催化剂CuO/γ-Al_2O_3和Fe_2O_3/γ—Al_2O_3,以甲基橙为代表化合物,考察了制备因素对催化活性的影响,结果表明,CuO/γ-Al_2O_3的活性高于Fe_2O_3/γ-Al_2O_3,催化湿式氧化甲基橙2 h,脱色率接近100%。正交试验和稳定性研究表明,焙烧温度对催化活性影响较大,350℃焙烧,催化剂活性组分Cu溶出较少,且重复使用情况较好。采用SEM和XRD等手段对CuO/γ-Al_2O_3进行表征,发现其活性组分分散度良好。     

Low-temperature CO oxidation over CuO/Fe2O3 catalysts

Copper iron composite oxide catalysts have been prepared by co-precipitation method. The catalytic activity and stability of the catalysts on CO oxidation were evaluated by using a microreactor-GC system. The results indicated that the copper iron composite oxide catalysts exhibited obviously high stability and catalytic activity on CO oxidation at low temperature. The effect of the calcination temperature, the molar ratios of copper to iron, the specific surface areas and the particle sizes on the catalytic activity of the catalysts was investigated in this paper.     

Preparation of supported gold catalysts from gold complexes and their catalytic activities for CO oxidation

A phosphine-stabilized mononuclear gold complex Au(PPh₃)(NO₃) (1) and a phosphine-stabilized gold cluster [Aug(PPh₃)₈](NO₃)₃ (2) were used as precursors for preparation of supported gold catalysts. Both complexes 1 and 2 supported on inorganic oxides such as -Fe₂O₃, TiO₂, and SiO₂ were inactive for CO oxidation, whereas the 1 or 2/ oxides treated under air or CO or 5% h₂/Ar atmosphere were found to be active for CO oxidation. The catalytic activity depended on not only the treatment conditions but also the kinds of the precursor and the supports used. The catalysts derived from 1 showed higher activity than those derived from 2. -Fe₂O₃ and TiO₂ were much more efficient supports than SiO₂ for the gold particles which were characterized by XRD and EXAFS.     

One-step synthesis of methyl isobutyl ketone from acetone and hydrogen over Pd/(Nb2O5/SiO2) catalysts

The surface-phase oxides of niobia on silica substrate ((Nb₂O₅/SiO₂), NS(x)) were prepared and characterized. Such oxides were used as supports for palladium catalysts. The one-step synthesis of methyl isobutyl ketone (MIBK) from acetone and hydrogen in liquid phase was also investigated over Pd/NS(x) catalysts. Experimental results indicate that these catalysts were effective for the formation of MIBK; since little of the parallel by-product of isopropanol (IPA) was formed, these catalysts reached selectivities of 88–92% MIBK and 2–3% IPA at 30–35% conversion. The reactivity of Pd/NS(x) declined obviously with an increase of water content that accumulated in a semi-batch reaction system. The water could be partially expelled and the deactivation of catalysts could be improved by using a fixed bed continuous flow reaction system.     

Oscillations of CO and CO + H2 oxidation in air over supported and unsupported Pt catalysts

Oscillation behaviour of the oxidation of CO (0.2–2.2%) in air over Pt wire coils and over Pt/Al₂O₃ catalysts deposited onto the coils (pellistors) has been investigated. The waveforms differ considerably between the two catalytic systems. Over unsupported Pt at 240–260° C, regular oscillations were accompanied by slowly declining activity and by deposition of carbon. Over supported Pt at 110–180°C, relatively complex but sustained oscillation occurred by a different mechanism. This oscillation was greatly enhanced by H₂ (0.25–1.0%), and may involve fluctuations in the concentrations of CO and H₂ around the supported catalyst.     

Low temperature oxidation of CO over supported PdCl2-CuCl2 catalysts

PdCl₂-CuCl₂ catalyst supported on activated carbon was examined for the low temperature oxidation of CO. The catalyst developed in the present study was active and stable at ambient conditions if water were existing in the feed gas stream. The addition of Cu(NO₃)₂ into the PdCl₂-CuCl₂ catalyst significantly enhanced the CO oxidation activity. X-ray diffraction study revealed that the role of Cu(NO₃)₂ was to stabilize active Cu(II) species, Cu₂Cl(OH)₃, on the catalyst surface which maintains the redox property of palladium. When HC1 and SO₂ were also existing in the feed, they easily inactivated the catalyst. It was found that HC1 and SO₂ inhibited the formation of active Cu(II) species on the catalyst surface.     

Partial methane oxidation into synthesis gas over catalysts supported on meshed metallic materials

Comparative tests of metal catalysts (Pd, Ni, Co, and Cu on a metal gauze and FeCrAl alloy foil) obtained in two ways—thermochemical and electrochemical metal deposition—have been carried out in order to develop efficient catalysts supported on meshed metallic materials for partial methane oxidation into synthesis gas. The tests have been performed in the 680–1000°C range in a flow reactor (10 mm i.d.) with a catalyst in the form of a rolled metal gauze or FeCrAl foil mounted inside. The metallic supports (gauze and foil) with metals (Pd, Ni) supported on them are promising as catalysts for producing synthesis gas by partial methane oxidation in a narrow range of oxygen : methane molar ratios (O₂ : CH₄ = 0.45–0.55). An SEM examination of the catalyst surfaces has demonstrated that the thermochemical deposition of Group VIII metals yields a more branched active metal surface that ensures almost 100% methane conversion at a CO selectivity of >90% even at 900°C.     

Direct epoxidation of propylene with hydrogen peroxide over TS-1 catalysts: Effect of hydrophobicity of the catalysts

TS-1 (titanium silicalite-1) catalysts were hydrothermally synthesized in the presence of polymethylmethacrylate bead (denoted as TS-1_PMMA) and polystyrene bead (denoted as TS-1_PS) for use in the direct epoxidation of propylene with hydrogen peroxide. TS-1 catalyst was also synthesized by a hydrothermal method in the absence of polymer bead. It was revealed that TS-1_PMMA and TS-1_PS catalysts showed a better catalytic performance than TS-1 catalyst in terms of conversion of hydrogen peroxide, selectivity for propylene oxide, and yield for propylene oxide, due to their enhanced hydrophobicity. Among three catalysts, TS-1_PS with the highest hydrophobicity showed the best catalytic performance.