The direct synthesis of hydrogen peroxide from molecular H2 and O2 offers an attractive alternative to the current means of production of this powerful oxidant, on an industrial scale. Herein we investigate the role of nitric acid addition, during catalyst preparation as a means of improving catalytic performance, under reaction conditions that have previously been found to be optimal for H2O2 production. The addition of dilute nitric acid during catalyst preparation is found to lead to a significant improvement in H2O2 synthesis activity, through the modification of particle size and control of Pd oxidation state.

The direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 represents a potentially atom-efficient alternative to the current industrial indirect process. We show that the addition of tin to palladium catalysts coupled with an appropriate heat treatment cycle switches off the sequential hydrogenation and decomposition reactions, enabling selectivities of >95% toward H2O2. This effect arises from a tin oxide surface layer that encapsulates small Pd-rich particles while leaving larger Pd-Sn alloy particles exposed. We show that this effect is a general feature for oxide-supported Pd catalysts containing an appropriate second metal oxide component, and we set out the design principles for producing high-selectivity Pd-based catalysts for direct H2O2 production that do not contain gold.

The effect of halide and acid addition on the direct synthesis of hydrogen peroxide is studied for magnesium oxide- and carbon-supported bimetallic gold-palladium catalysts. The addition of acids decreases the hydrogenation/decomposition of hydrogen peroxide, and the effect is particularly pronounced for the magnesium oxide-supported catalysts whilst for carbon-supported catalysts the pH requires close control to optimize hydrogen peroxide synthesis. The addition of bromide leads to a marked decrease in the hydrogenation/decomposition of hydrogen peroxide with either catalyst. These effects are discussed in terms of the structure of the gold-palladium alloy nanoparticles and the isoelectric point of the support. We conclude that with the highly active carbon-supported gold-palladium catalysts these additives are not required and that therefore this system presents the potential for the direct synthesis of hydrogen peroxide to be operated using green process technology.

The direct synthesis of hydrogen peroxide (H2O2) represents a potential alternative to the currently industrially used anthraquinone process, and Au-Pd catalysts have been identified as effective catalysts. To obtain a direct process, a detailed understanding of the reaction conditions in a continuous flow system is needed. In this study, we use a flow reactor to study reaction conditions independently, including total gas flow rate, catalyst mass, reaction pressure, solvent flow rate, and H-2/O-2 molar ratio. The study was carried out without the addition of any halide or acid additives often used to suppress the sequential hydrogenation and decomposition reactions that allowed the kinetics of these reactions to be studied along with the synthesis reaction. A global kinetic model describing the net and gross synthesis rate is proposed, and on the basis of this model, we propose that the decomposition reaction suppresses the production of H2O2 to a greater extent than hydrogenation and that catalyst design studies should aim at blocking or generating catalysts without O-O dissociation sites.

The direct synthesis of hydrogen peroxide is investigated using ruthenium containing catalysts. Ruthenium is not soluble in Au but forms alloys with palladium. We have therefore investigated Ru–Au, Ru–Pd as well as trimetallic formulations supported on titania. The addition of ruthenium enhances the direct synthesis activity for all the catalysts studied and the effect is dependent on the amount of ruthenium added. The calcination conditions are shown to affect both activity and reusability.

The direct synthesis of hydrogen peroxide from molecular H2 and O2 represents a green and economic alternative to the current anthraquinone process used for the industrial production of H2O2. In order for the direct process to compete with the anthraquinone process, there is a need for enhanced H2O2 yields and H2 selectivity in the process. We show that Au–Pd-exchanged and supported Cs-containing heteropolyacid catalysts with the Keggin structure are considerably more effective in achieving high H2O2 yields in the absence of acid or halide additives than previously reported catalysts. The Au–Pd-exchanged Cs-heteropolyacid catalysts also show superior H2O2 synthesis activity under challenging conditions (ambient temperature, water-only solvent and CO2-free reaction gas). Au plays a crucial role in achieving the improved performance of these heteropolyacid-based catalysts. The heteropolyacid limits the subsequential hydrogenation/decomposition of H2O2.

The effect of adding Pt to a highly active ceria polycyclic aromatic hydrocarbon total oxidation catalyst has been investigated for the oxidation of naphthalene. The addition of Pt to ceria suppressed the performance of the catalyst for total oxidation. The addition of Pt reduced catalyst surface area, decreased the ceria crystallite size, decreased the concentration of ceria defects and increased the reducibility of the catalyst. The suppression of activity has been attributed to strong metal-support interaction between Pt and ceria, which limits the availability of lattice oxygen for the oxidation process, which follows the Mars-Van Krevelen redox mechanism in the absence of Pt. It is postulated that the presence of dispersed Pt, alters the mechanism of naphthalene oxidation over the ceria catalyst.

The direct synthesis of hydrogen peroxide from H2 and O2 offers the possibility of a new green production method for this important commodity chemical. Active catalysts for this reaction are typically prepared using an impregnation method and it is important to identify improvements in the preparation methodology that can result in more active catalysts that retain their stability. The effect of the precise procedure by which the metals are impregnated onto TiO2 and C supports during the preparation of supported Au–Pd catalysts has been investigated and it is shown that the two supports exhibit significant differences. The concentration of the solution of the mixed aqueous solution of HAuCl4 and PdCl2 immediately prior to the initial drying step has a profound effect on the structure and activity of the TiO2-supported catalysts. TiO2-supported catalysts prepared using impregnation with the minimal amount of added water whilst ensuring that the catalyst is not formed into a paste (i.e. still contains ca. 1.5–2 ml of H2O) prior to drying at 110 °C exhibit very high activity (ca. 120 mol H2O2 kgcat−1 h−1) which is equivalent to the corresponding carbon-supported catalyst. The presence of more water (ca. 2–28 ml) in the catalyst impregnation step prior to drying leads to a significant change in the particle size distribution and a bimodal distribution is observed for the TiO2-supported catalysts. These catalysts also show a change in the nature of the Au and Pd nanoparticles. Unfortunately, TiO2-supported catalysts prepared in this manner are not stable on re-use. However, catalysts prepared using a similar method, but with the removal of ca. 75% of the initial H2O ensuring that a paste is formed prior to drying, are found to be fully re-usable. In contrast, for carbon-supported catalysts dilution of the Au and Pd compounds during the initial impregnation step, coupled with subsequent removal of water to form paste with varying water content, did not affect the activity and these catalysts could be re-used without loss of catalyst performance. The effect of the catalyst structure on activity and re-usability is discussed.

The direct synthesis of hydrogen peroxide from H(2) and O(2) has been studied using a high activity AuPd/TiO(2) catalyst. In particular, the effect of variation in the reaction conditions on the productivity of hydrogen peroxide formation is investigated in detail. The effect of H(2)/O(2) molar ratio, temperature, total pressure and solvent composition has been studied and optimised conditions identified. In addition, the effect of carrying out the synthesis reaction in the presence of hydrogen peroxide is investigated and the competing reactions of hydrogen peroxide formation, decomposition and hydrogenation are discussed and optimal operating conditions are identified.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The effect of pretreating Au-Pd catalysts on MgO and C supports with aqueous bromide solution, prior to using them for the direct synthesis of hydrogen peroxide, has been investigated. These two supports were selected since the parent materials exhibit contrasting microstructures and activities. The carbon-supported catalysts comprise homogeneous Au-Pd alloy nanoparticles, which give high activity, whereas the MgO-supported catalyst has Au-Pd alloys with a Pd-rich surface and a Au-rich core, which result in lower activity. Pretreatment of these catalysts with bromide was found to enhance H2O2 productivity and the degree of enhancement was largely dependent on the nature of the Au-Pd nanoparticles. Whereas bromide pretreatment significantly enhanced H2O2 productivity over the MgO-based catalysts, the carbon-based catalyst only showed a subtle promotional effect. Very low loadings of bromide (0.00034-0.044 wt%) were required to yield a significant positive effect. Higher bromide loadings (0.5-8.3 wt%) proved deleterious. The promotional effect has been correlated to selective poisoning of sites responsible for H2O2 hydrogenation and decomposition. In view of the limited effect of bromide pretreatment on the yield of H2O2 coupled with the effective performance of the carbon supported Au-Pd catalysts in the absence of halides, for practical processes the addition of halides is not considered advisable with this catalyst system.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Hydrogen peroxide (H 2 O 2 ) is an important disinfectant and bleach and is currently manufactured from an indirect process involving sequential hydrogenation/oxidation of anthaquinones. However, a direct process in which H 2 and O 2 are reacted would be preferable. Unfortunately, catalysts for the direct synthesis of H 2 O 2 are also effective for its subsequent decomposition, and this has limited their development. We show that acid pretreatment of a carbon support for gold-palladium alloy catalysts switches off the decomposition of H 2 O 2 . This treatment decreases the size of the alloy nanoparticles, and these smaller nanoparticles presumably decorate and inhibit the sites for the decomposition reaction. Hence, when used in the direct synthesis of H 2 O 2 , the acid-pretreated catalysts give high yields of H 2 O 2 with hydrogen selectivities greater than 95%.

Polycyclic Aromatic Hydrocarbons (PAHs) are a group of Volatile Organic Compounds (VOCs), which have serious health problems associated with their emission into the atmosphere. Catalytic oxidation is an effective abatement process to control PAH emissions, and the types of catalysts investigated have been reviewed. The majority of studies have used naphthalene as a model PAH, and in particular, catalysts containing palladium and platinum have demonstrated high activity for total oxidation. Catalysts based on the precious metals include those supported on high surface area supports, which have also been modified by adding further components, and metal exchanged zeolites. Metal oxide catalysts have also been employed and the most active for total oxidation are ceria-based. Studies of PAH total oxidation have largely been reported only in the last 10 years, and there still remains wide scope to develop improved catalysts and understand their catalytic mechanisms.

The influence of cerium salt/urea ratio on the activity of nanocrystalline ceria catalysts prepared by homogeneous precipitation with urea for the complete oxidation of naphthalene has been evaluated. Ceria catalysts were prepared from five different cerium salt/urea ratios (2:1, 1:1, 1:2, 1:3 and 1:4). Catalyst characterization (by BET, XRD and TPR) only revealed subtle differences in the characteristics of these catalysts with cerium salt to urea ratio. However, Raman and scanning electron microscopy (SEM) results indicated differences in the oxygen defect concentration (FWHM of Raman band) and morphology of the catalysts with variation of the preparation ratio. Catalysts prepared with 2:1, 1:1 and 1:4 were significantly more active than those prepared from 1:2 and 1:3 ratios. A relationship between the concentration of oxygen defects and naphthalene oxidation activity has been established. The activity of the catalysts is thought to be related to a combination of oxygen defect concentration, surface reducibility and morphology.

Catalysis by gold and gold-palladium nanoparticles has attracted significant research attention in recent years. These nanocrystalline materials have been found to be highly effective for selective and total oxidation, but in most cases the catalysts are prepared using precipitation or impregnation. We report the preparation of Au-Pd nanocrystalline catalysts supported on carbon prepared via a sol-immobilisation technique and these have been compared with Au-Pd catalysts prepared via impregnation. The catalysts have been evaluated for two selective chemical syntheses, namely, oxidation of benzyl alcohol and the direct synthesis of hydrogen peroxide. The catalysts have been structurally characterised using a combination of scanning transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts prepared using the sol immobilisation technique show higher activity when compared with catalysts prepared by impregnation as they are more active for both hydrogen peroxide synthesis and hydrogenation, and also for benzyl alcohol oxidation. The method facilitates the use of much lower metal concentrations which is a key feature in catalyst design, particularly for the synthesis of hydrogen peroxide.

A range of Pt supported catalysts have been evaluated for the total oxidation of naphthalene. Catalysts contained 0.5wt% Pt on a range of supports (γ-Al2O3, TiO2, SiO2, SnO2, and CeO2). SiO2 was the best support, the 0.5%Pt/SiO2 catalyst showing a conversion to carbon dioxide of over 90% at 200°C (100vppm naphthalene, GHSV=45,000h−1). The catalyst also showed a considerably higher activity (in the temperature range 100–175°C) than a CeO2 catalyst recently reported to be one of the most effective catalysts for the total oxidation of naphthalene. The high activity of the 0.5%Pt/SiO2 catalyst has been attributed to the relatively low dispersion and relatively large size of Pt particles. Furthermore, due to the acidic and non-reducible nature of the SiO2, platinum is expected to have a weak interaction with the support. XPS data identified the presence of Pt0 on the surface and this contributes to the high activity.

Pd-only, Au-only and bimetallic AuPd catalysts supported on a range of supports (Al2O3, TiO2, MgO, and C) have been prepared by impregnation and tested for the hydrogenation and decomposition of hydrogen peroxide under conditions similar to those used in direct synthesis of hydrogen peroxide. Hydrogenation and decomposition are the main pathways for loss of selectivity and yield in the direct synthesis reaction, and the support is found to be a crucial parameter with respect to hydrogenation and decomposition activity. We show that by making the right choice of support for both the monometallic and bimetallic Au and Pd catalysts, it is possible to achieve very low hydrogen peroxide hydrogenation and decomposition activity, thus enhancing hydrogen peroxide productivity during synthesis. Carbon is found to be the optimal support for both monometallic Au and Pd catalysts as well as Au–Pd alloys, since carbon-supported catalysts gave the lowest hydrogenation and decomposition activities. Au-only catalysts were generally less active than Pd-only catalysts when utilizing the same support and metal loading. The addition of Au to Pd catalysts supported on TiO2 and carbon resulted in a decrease in both H2O2 hydrogenation and decomposition while the reverse effect was observed for the Al2O3 and MgO-supported catalysts. These effects are discussed in terms of the basicity of the support, and in particular the isoelectronic point of the support, which is a major factor in controlling the stability of hydrogen peroxide under reaction conditions.