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.     

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.     

羟基磷灰石负载钯催化剂用于H2和O2直接合成H2O2的动力学

H₂和O₂直接合成H₂O₂过程绿色环保,反应具有原子经济性,是最有潜力的H₂O₂合成新方法之一。采用等量浸渍法,将Pd负载于羟基磷灰石（HAp）载体上,得到了高分散的Pd/HAp纳米催化剂,Pd平均粒径2.5 nm。运用幂指数模型,研究该催化剂在H₂O₂加氢及H₂和O₂直接合成反应中的动力学,计算得到H₂O₂加氢、H₂O₂和H₂O的生成反应的表观活化能及O₂、H₂表观反应级数。结果表明低温及高O₂分压有利于H₂O₂的生成,而高H₂分压则有利于H₂O的生成。     

Preparation, performances and reaction mechanism for the synthesis of H2O2 from H2 and O2 based on palladium membranes

A series of new tubular catalytic membranes (TCM's) have been prepared and tested in the direct synthesis of H₂O₂. Such TCM's are asymmetric -alumina mesoporous membranes supported on macroporous -alumina, either with a subsequent carbon coating (CAM) or without (AAM). Pd was introduced by two different impregnation techniques. Deposition–precipitation (DP) was applied to CAM's to obtain an even Pd particles distribution inside the membrane pore network, whereas electroless plating deposition (EPD) was successfully applied to AAM's to give a 1–10 μm thick nearly-dense Pd layer. Both type of membranes were active in the direct synthesis of H₂O₂. Catalytic tests were carried out in a semi-batch re-circulating reactor under very mild conditions. Concentrations as high as 250–300 ppm H₂O₂ were commonly achieved with both CAM's and AAM's after 6–7 h time on stream, whereas the decomposition rate was particularly high in the presence of H₂. Important features are the temperature control and pre-activation. In order to slow down the decomposition and favor the synthesis of H₂O₂ a smooth metal surface is needed.     

Direct synthesis of H2O2 acid solutions on carbon cathode prepared from activated carbon and vapor-growing-carbon-fiber by a H2/O2 fuel cell

Direct synthesis of H₂O₂ acid solutions was studied using a gas-diffusion cathode prepared from activated carbon (AC), vapor-growing-carbon-fiber (VGCF) and poly-tetra-fluoro-ethylene (PTFE) powders, with a new H₂/O₂ fuel cell reactor. O₂ reduction to H₂O₂ was remarkably enhanced at the three-phase boundary (O₂(g)-electrode(s)-acid(l)) at the [AC + VGCF] cathode. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk acid solutions were essential for H₂O₂ accumulation. Synergy of AC and VGCF was observed for the H₂O₂ formation. RRDE and cyclic voltammetry studies indicated that the surface of AC functioned as the active phase for O₂ reduction to HO₂, and VGCF functioned as an electron conductor and a promoter to convert HO₂ to H₂O₂. A maximum H₂O₂ concentration of 353 mM (1.2 wt%) was accomplished under short-circuit conditions (current density 12.7 mA cm⁻², current efficiency 40.1%, geometric area of cathode 1.3 cm², reaction time 6 h).     

直接合成过氧化氢Pd/TiO2催化剂的制备

常温和常压下,氢氧直接合成过氧化氢过程中,采用浸渍沉淀法制备Pd/TiO₂催化剂。考察了pH、搅拌时间t₁、静置时间t₂及焙烧温度等制备条件对催化剂活性的影响。正交实验结果表明,影响催化剂活性的顺序为：pH＞搅拌时间＞焙烧温度＞静置时间。在pH=8、t₁=0、t₂=0和（250～300）　℃焙烧3 h的条件下,制备的催化剂活性最好。     

Au–TiO2 catalysts stabilised by carbon nanofibres

The catalytic activity and stability in the water–gas shift reaction have been tested for Au-based catalysts prepared by deposition of Au from colloid solutions. The supports that have been used are TiO₂, TiO₂ supported on carbon nanofibres (CNF) and CNF. Thermal treatments of the samples show that the Au particle size depends on the support material and hence the interaction between the Au particles and the support. In situ X-ray absorption spectroscopic (XAS) measurements during the water–gas shift reaction show no changes in the first Au–Au coordination number for the catalysts containing CNF. Furthermore, improved short-time stability is obtained compared to the AuTiO₂ catalysts. The improved stability is achieved by the CNF stabilising small TiO₂ particles and hence prevent subsequent sintering of the Au particles.     

Generation of H2O2 from H2 and O2 over zeolite beta containing Pd and heterogenized organic compounds

The catalytic generation of H₂O₂ from H₂ and O₂ has been studied over zeolite beta-supported Pd and zeolite beta-adsorbed organic compounds such as 1,4-benzoquinone (BQ), hydroquinone (HQ), azobenzene (AB) and hydrazobenzene (HAB). According to catalytic results, zeolite beta-supported Pd catalysts display effective performance relative to those prepared from other types of zeolites reported and Pd-loaded zeolite beta-adsorbed HQ catalysts show enhanced activity compared to zeolite beta-supported Pd catalysts. In situ UV–Vis spectroscopic study indicates that HQ can readily be converted to BQ reversibly under H₂ and air inside zeolite beta only in the presence of Pd. The results suggest that HQ acts as a strong hydrogen transfer agent to promote the production of H₂O₂ from H₂ and O₂ in cooperation with a Pd catalyst. By contrast, adsorption of BQ, AB and HAB induces suppression of the catalytic properties of Pd/zeolite beta.     

An in situ study of the NO + H2 + O2 reaction on Pd/LaCoO3 based catalysts

In situ X-ray diffraction (XRD) and quasi in situ X-ray photoelectron spectroscopy (XPS) measurements were complementary used to investigate structural and surface modifications of a palladium-supported on LaCoO₃ perovskite catalyst under various controlled atmospheres, particularly during the reduction of NO by hydrogen under lean conditions, in the presence of a large excess of oxygen.

An extensive reduction of the perovskite was evidenced during the pre-activation thermal treatment of the palladium-supported catalyst under hydrogen at 773 K leading to the formation of Pd particles in contact with Co⁰ and La₂O₃. In the presence of an excess of oxygen, the catalyst structure changes during the reaction. The reduced solid is progressively transformed into LaCoO₃ in the range of 873–1173 K. However, such a bulk transformation probably occurs at lower temperatures at the surface of the solid according to XPS analyses. At the same time, the binding energy (BE) level of the Pd 3d_5/2 photopeak increases up to 337.5 eV which reveals the stabilisation of oxidic palladium species in a different chemical environment than that corresponding to PdO. Such changes induced different catalytic properties of the catalyst during the reduction of NO by H₂.     

Pd–Ag/SiO2 and Pd–Cu/SiO2 cogelled xerogel catalysts for selective hydrodechlorination of 1,2-dichloroethane into ethylene

While 1,2-dichloroethane hydrodechlorination over pure palladium mainly produces ethane, increasing silver or copper content in bimetallic catalysts results in an increase in ethylene selectivity. The specific consumption rate of 1,2-dichloroethane decreases when silver or copper loading increases. The turnover frequency, that is, the number of catalytic cycle per active site (palladium atom and its surrounding silver or copper atoms) and per second, seems to be independent of surface composition of alloy particles and 1,2-dichloroethane hydrodechlorination is insensitive to the atom's nature (silver or copper).     

Phenol photodegradation with oxygen and hydrogen peroxide over TiO2 and Fe-doped TiO2

Photodegradation of phenol was investigated with two types of oxidant agents in water, oxygen and hydrogen peroxide, at two different reaction pH with a series of nanosized iron-doped anatase TiO₂ catalysts with different iron contents. The catalysts have been prepared by a sol–gel/microemulsion method. Firstly, iron-doped titania catalysts were studied with respect to their activity behavior when oxygen was used as oxidant agent in the photocatalytic degradation of aqueous phenol in comparison with un-doped reference catalysts. Secondly, two catalysts (TiO₂ and 0.7 wt.% Fe-doped TiO₂) were selected to extend the study for the employment of hydrogen peroxide as oxidant at different concentrations and two initial reaction pHs. An enhancement of the photocatalytic activity is observed only for relatively low doping level (ca. 0.7 wt.%) in catalyst calcined at 450 °C preferably using hydrogen peroxide as oxidant agent which is attributable to the partial introduction of Fe³⁺ cations into the anatase structure. Nevertheless, it has been demonstrated that catalyst surface properties can play an important role during phenol photodegradation process on the basis of the analysis of differences found in the photoactivity as a function of reaction pH.     

Methane oxidation over vanadium-modified Pd/Al2O3 catalysts

Three different vanadium-modified Pd/Al₂O₃ catalysts were prepared and tested as catalysts for the deep oxidation of methane. Vanadium was added to the palladium catalyst by incipient wetness of palladium catalyst in order to modify its properties and improve its thermal stability and thioresistance. The behaviour of vanadium-modified catalysts depends on the concentration of this compound, being 0.5 wt.% the optimum amount. However, when strong catalyst poisons are present in the gas (SO₂), these modified catalysts do not show a better performance than unmodified catalyst. Bimetallic catalysts were tested with and without further reduction, being observed that reduced bimetallic catalysts perform worse than the non-reduced ones.     

氢氧直接合成过氧化氢贵金属催化剂的研究

采用浸渍法制备了一系列负载型钯催化剂，用于催化氢气和氧气直接合成过氧化氢的反应。分别考察了钯负载节、溶剂、载体预处理对反应的影响；结合XPS分析推断了催化剂活性组分价态。结果发现钯最佳负载量为1．88％（质量分数）；氢气在溶剂中的溶解度越大其反应转化率也越高，其中甲醇和丙酮都是良好的溶剂；载体经过卤化铵预处理可大幅度地提高催化剂的选择性；金属态钯为具有催化活性的价态。     

Supported gold and gold palladium catalysts for selective chemical synthesis

Catalysts based on gold are now well established as very active and selective for broad ranges of redox reactions. Although primarily known for selective and preferential oxidation reactions, gold catalysts are also highly effective for selective hydrogenation. Hydrogenation reactions provide the focus for this perspective paper that is based on a François Gault lecture given at the Sabatier Conference in 2007. In particular, two reactions will be discussed; namely, the use of supported gold catalysts for selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols, and the use of supported gold palladium alloys for the direct hydrogenation of molecular oxygen to form hydrogen peroxide in preference to water.     

Oxidation of dilute H2 and H2/O2 mixtures by hydrogenases and Pt

Hydrogenase enzymes that allow micro-organisms to gain energy from oxidation of H₂ undergo efficient electrocatalysis of H₂ oxidation or production when adsorbed on a graphite rotating disk electrode [K.A. Vincent, A. Parkin, F.A. Armstrong, Chem. Rev. 107 (2007) 4366]. Combining potential sweeps or steps with precisely controlled gas exchanges is enabling us to build up a detailed understanding of the many factors that control the chemistry of nickel-iron membrane-bound hydrogenase (MBH) enzymes. The observation that the MBH enzymes from Ralstonia strains have extremely high affinity for H₂ and continue oxidising H₂ in the presence of O₂ and CO has relevance for selective fuel cell catalysis [K.A. Vincent, J.A. Cracknell, J.R. Clark, M. Ludwig, O. Lenz, B. Friedrich, F.A. Armstrong, Chem. Commun. (2006) 5033; K.A. Vincent, J.A. Cracknell, O. Lenz, I. Zebger, B. Friedrich, F.A. Armstrong, Proc. Natl. Acad. Sci. U.S.A. 102 (2005) 16951], and this has led us to compare the ability of hydrogenases and platinum to oxidise low levels of H₂ and mixtures of H₂ and O₂. We show that Pt is a poor catalyst for oxidation of sub-atmospheric levels of H₂ compared to the MBH from Ralstonia eutropha H16, and that at a platinised electrode, H₂ oxidation competes less favourably with reduction of O₂ compared to the situation at hydrogenase-modified graphite. This should have implications for development of future selective energy catalysts able to concentrate the energy available from dilute H₂.     

Selective oxidation of hydrogen sulfide over mixture catalysts of V–Sb–O and Bi2O3

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium–antimony mixed oxide catalysts. The investigation was focused on the phase cooperation between V–Sb–O and Bi₂O₃ in this reaction. Strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V–Sb–O and Bi₂O₃. Temperature-programmed reduction (TPR) and oxidation (TPO), two separated bed reaction tests, and XPS analyses were carried out to explain this synergistic effect by the reoxidation ability of Bi₂O₃.     

Highly selective Pd/KX catalysts for low-pressure one-step synthesis of 2-ethylhexanal from n-butyraldehyde and hydrogen

2-ethylhexanal (2EH) was directly synthesized from n-butyraldehyde and hydrogen at 1 atm and 150°C using the catalysts prepared by supporting tetraamine palladium(II) chloride on the potassium ion-exchanged zeolite X (Pd/KXW) and on the potassium ion-added zeolite X (Pd/KXU). The latter catalyst contains a larger amount of potassium ion and exhibits higher activity and very high selectivity of 2EH (>93%) due to its greater basicity. The main reaction path possibly starts from self-condensation of n-butyraldehyde on the catalyst basic sites to form 2-ethyl-3-hydroxyhexanal followed by dehydration to 2-ethyl-2-hexenal which was then hydrogenated on the metallic Pd sites to 2EH. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of composition and promoters in Au/TS-1 catalysts for direct propylene epoxidation using H2 and O2

A series of Au/titanium silicalite-1 (TS-1) catalysts with different Si/Ti ratios and promoted with alkali and alkaline earth cations were prepared by deposition–precipitation (DP) and tested for direct propylene epoxidation. It was found that the gold loading and catalytic activity was highly dependent on the pH of the DP synthesis solution and the final composition of the catalyst. Addition of Group 1 metals such as K or Cs had little effect on the gold content, but increased activity, while Group 2 metals such as Mg, Ca, Sr, and Ba increased both the gold content and the catalytic activity. The highest improvement was provided by a Mg promoted catalyst, which at 443 K and 0.1 MPa with a H₂/O₂/C₃H₆/Ar = 1/1/1/7 feed mixture gave a propylene oxide (PO) formation rate of 88 gPO h⁻¹ kg_cat⁻¹, compared to 57 gPO h⁻¹ kg_cat⁻¹ for an unpromoted catalyst, corresponding to a 50% enhancement of activity. Ammonia temperature-programmed desorption (NH₃-TPD) measurements indicated little change in adsorption amount with promotion indicating that the yield increase was not due to the elimination of acidic sites on the catalyst. Instead, the improved catalytic performance was ascribed to increased Au capture efficiency and dispersion by the catalyst. The effect of Si/Ti ratio, pH of synthesis, and the promoter ions on the gold content could be understood from their effect on the surface charge of the support.     

Treatability studies on domestic wastewater using UV/H2O2 process

Advanced oxidation processes (AOPs) are emerging and promising technology both as an alternative treatment to conventional wastewater treatment methods and enhancement of current biological treatment methods especially dealing with highly toxic and low biodegradable wastes. In this paper, the results of domestic wastewater treatment using H₂O₂/UV process in both batch and continuous mode are presented. Over 95% reduction in COD was achieved in less than 60 min of reaction time. Optimum conditions for pH and H₂O₂ dosage for this process was found to be 3 and 50 mg L⁻¹, respectively. A pretreatment in the form of removal of turbidity is recommended for the success of the process in the long run. Electric energy required is estimated to be 10 kWh kg⁻¹ COD on the average.