Search

The addition of Re to Ni on TiO2 yields efficient catalysts for the hydrogenation of acids and esters to alcohols under mild conditions. Rhenium promotes the formation of atomically dispersed and sub-nanometre-sized bimetallic species interacting strongly with the oxide support.

The direct synthesis of hydrogen peroxide presents an attractive and atom efficient route to an important commodity chemical. The direct synthesis process has attracted considerable research interest into catalysts which could provide an alternative to the current industrial process. Many catalyst systems have been evaluated, most of which utilise Pd as the active component. The activity of the catalysts can be promoted in a number of ways, either by addition of a second metal, or by adding promoters into the reactant phase such as acid and halides. Acidic supports can increase the activity of catalysts without addition of aqueous acids by minimising the decomposition of hydrogen peroxide, and catalysts can be doped with halide ions to minimise the non-selective hydrogenation reaction. The addition of gold to form bimetallic particles has also been shown to increase hydrogen peroxide yield by increasing hydrogen selectivity without the need to add acid and halides. If the process was to ever be envisaged as a competitor to the anthraquinone process it would need to be operated in a continuous system. Recent research has focused on the use of continuous reactor systems such as membrane, fixed bed and microreactors. Recent literature regarding catalyst design and continuous process development as well as historical perspective of the process is discussed in this review.

The catalytic reduction of carboxylic acid derivatives has witnessed a rapid development in recent years. These reactions, involving molecular hydrogen as the reducing agent, can be promoted by heterogeneous and homogeneous catalysts. The milestone achievements and recent results by both approaches are discussed in this Review. In particular, we focus on the mechanistic aspects of the catalytic hydrogenation and highlight the bifunctional nature of the mechanism that is preferred for supported metal catalysts as well as homogeneous transition metal complexes.

The effect of Re-addition to Pt/TiO2catalysts for the hydrogenation of carboxylic acids and esters to their corresponding alcohols was investigated. The highest catalytic activity was observed for the materials with Pt:Re molar ratio of 1:2, which allowed for a complete conversion of hexanoic acid under mild conditions. The hydrogenation of esters over the same catalysts was much more difficult. The reactions showed only moderate alcohol yields which also depended strongly on the alkoxy moiety of the ester substrate. The highest yield of ca. 25% was achieved by the hydrogenation of ethyl hexanoate. Conversion of methyl hexanoate was much less efficient. This was attributed to the inhibiting effect of methanol by-product. In situ FTIR spectroscopy suggests that the decarbonylation of methanol and the resulting CO poisoning of the catalyst surface in the course of the reaction is the most likely cause of the low activity of Pt and Pt-Re/TiO2towards methyl ester hydrogenation.

The direct synthesis of hydrogen peroxide is investigated using silica supported Au, Pd and AuPd catalysts. Acid pre-treatment of the silica leads to an increase in the activity of the Pd-containing catalysts and also gives rise to a synergistic enhancement in activity when Au and Pd are combined that is absent in the untreated catalysts. The acid pre-treated mono and bi metallic catalysts show much reduced H2O2 hydrogenation activity compared to their untreated counterparts. The acid pre-treatment increases the concentration of the hydroxyl groups on the support, as demonstrated using inelastic neutron scattering, which helps to disperse the metals on the support. Pd can be dispersed effectively on silica, but Au is very poorly dispersed although this is enhanced marginally by the acid pre-treatment. The origin of the synergistic effect is discussed and is considered to be related to the incorporation of a small amount of Au within the Pd nanoparticles.

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.

A key discovery in the last two decades has been the realisation that gold, when prepared as supported nanoparticles, is exceptionally effective as a redox catalyst. The catalytic efficacy is enhanced further by the alloying of gold with palladium and this is particularly exemplified for the direct synthesis of hydrogen peroxide where supported gold palladium alloy nanoparticles are found to be highly active. In this paper we report a study of ceria-supported gold, palladium and gold–palladium nanoparticles for the direct synthesis of hydrogen peroxide and show that ceria can be a potentially interesting support for this reaction. However, with current methods of catalyst fabrication ceria-supported monometallic palladium catalysts have a superior performance to bimetallic gold–palladium catalysts.

We report the preparation of Au-Pd nanocrystalline catalysts supported on activated carbon prepared via a sol-immobilization technique and explore their use for the direct synthesis of hydrogen peroxide and the oxidation of benzyl alcohol. In particular, we examine the synthesis of a systematic set of Au-Pd colloidal nanoparticles having a range of Au/Pd ratios. The catalysts have been structurally characterized using a combination of UV-visible spectroscopy, transmission electron microscopy, STEM HAADF/XEDS, and X-ray photoelectron spectroscopy. The Au-Pd nanoparticles are found in the majority of cases to be homogeneous alloys, although some variation is observed in the AuPd composition at high Pd/Au ratios. The optimum performance for the synthesis of hydrogen peroxide is observed for a catalyst having a Au/Pd 1:2 molar ratio. However, the competing hydrogenation reaction of hydrogen peroxide increases with increasing Pd content, although Pd alone is less effective than when Au is also present. Investigation of the oxidation of benzyl alcohol using these materials also shows that the optimum selective oxidation to the aldehyde occurs for the Au/Pd 1:2 molar ratio catalyst. These measured activity trends are discussed in terms of the structure and composition of the supported Au-Pd nanoparticles. © 2010 American Chemical Society.

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.

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 catalytic reduction of carboxylic acid derivatives has witnessed a rapid development in recent years. These reactions, involving molecular hydrogen as the reducing agent, can be promoted by heterogeneous and homogeneous catalysts. The milestone achievements and recent results by both approaches are discussed in this Review. In particular, we focus on the mechanistic aspects of the catalytic hydrogenation and highlight the bifunctional nature of the mechanism that is preferred for supported metal catalysts as well as homogeneous transition metal complexes.

The addition of a small amount of Pt to a CeO2-supported AuPd (weight ratio of 1:1) catalyst significantly enhances the activity in the direct synthesis of H2O2 (autoclave, 5% H2/CO2 and 25% O2/CO2, 4.0 MPa, 2 °C, 30 min) and decreases the non-desired over-hydrogenation and decomposition reactions.

The direct synthesis of hydrogen peroxide offers a potentially green route to the production of this important commodity chemical. Early studies showed that Pd is a suitable catalyst, but recent work indicated that the addition of Au enhances the activity and selectivity significantly. The addition of a third metal using impregnation as a facile preparation method was thus investigated. The addition of a small amount of Pt to a CeO2-supported AuPd (weight ratio of 1:1) catalyst significantly enhanced the activity in the direct synthesis of H2O2 and decreased the non-desired over-hydrogenation and decomposition reactions. The addition of Pt to the AuPd nanoparticles influenced the surface composition, thus leading to the marked effects that were observed on the catalytic formation of hydrogen peroxide. In addition, an experimental approach that can help to identify the optimal nominal ternary alloy compositions for this reaction is demonstrated.

We have investigated the effect of heat treatment in air on Au-Pd nanoparticles supported on titania and activated carbon prepared via the immobilisation of PVA-stabilised alloy nanoparticles. The catalytic activity of the gold-palladium nanoparticles was affected by both metal and PVA loading, as well as the degree of interaction of the nanoparticles with the support. The turnover frequency numbers for benzyl alcohol and hydrogen peroxide synthesis were also sensitive to the calcination procedure employed and find a doubling of catalytic activity when using activated carbon as opposed to TiO2 as the support material. These results illustrate the importance of understanding the precise metal-support interaction of catalyst systems designed for benzyl alcohol oxidation and hydrogen peroxide synthesis. © The Royal Society of Chemistry 2013.

We report a convenient excess anion modification and post-reduction step to the impregnation method which permits the reproducible preparation of supported bimetallic AuPd nanoparticles having a tight particle size distribution comparable to that found for sol-immobilization materials but without the complication of ligands adsorbed on the particle surface. The advantageous features of the modified impregnation materials compared to those made by conventional impregnation include a smaller average particle size, an optimized random alloy composition, and improved compositional uniformity from particle-to-particle resulting in higher activity and stability compared to the catalysts prepared using both conventional impregnation and sol immobilization methods. Detailed STEM combined with EDX analyses of individual particles have revealed that an increase in anion concentration increases the gold content of individual particles in the resultant catalyst, thus providing a method to control/tune the composition of the nanoalloy particles. The improved activity and stability characteristics of these new catalysts are demonstrated using (i) the direct synthesis of hydrogen peroxide and (ii) the solvent-free aerobic oxidation of benzyl alcohol as case studies.

In this study, a systematic series of AuPd bimetallic particles were prepared by colloidal synthesis methods, in order to gain better control over the particle size distribution and structure. Particles having random alloy structures, as well as 'designer' particles with Pd-shell/Au-core and Au-shell/Pd-core morphologies, have been prepared and immobilized on both activated carbon and TiO2 supports. Aberration corrected analytical electron microscopy (ACEAM) has been extensively used to characterize these sol-immobilized materials. In particular, state-of-the-art z-contrast STEM-HAADF imaging and STEM-XEDS spectrum imaging has been employed. These techniques have provided invaluable new (and often unexpected) information on the atomic structure, elemental distribution within particles, and compositional variations between particles for these controlled catalyst preparations. In addition, we have been able to compare their differing thermal stability, sintering and wetting behaviors on activated carbon and TiO2 supports. These sol immobilized materials have also been compared as catalysts for (i) benzyl alcohol oxidation and (ii) the direct production of H2O2 in an attempt to elucidate the optimum particle morphology/support combination for each reaction. © 2011 The Royal Society of Chemistry.

Akeydiscoveryinthelasttwodecadeshasbeentheidentifica-tion that gold, when prepared as supported nanoparticles, isexceptionallyeffectiveasanoxidationcatalyst.Followingthisdiscovery, there has been an explosion of interest in gold as acatalyst and several hundred papers and tens of patents arebeing published annually. Gold catalysis is now a major topicforbothheterogeneousandhomogeneouscatalysisworldwide.Previouslywehaveshownthatsupportedgoldandgoldpalla-dium catalysts are very active and selective for the oxidationof alcohols and hydrocarbons and also the direct synthesis ofhydrogen peroxide

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

The recent availability of aberration corrected analytical electron microscopes (ACAEM) is revolutionizing our ability to characterize nanostructured catalyst materials. Some recent case studies are presented whereby the application of the high angle annular dark field (HAADF) imaging technique, coupled with STEM-XEDS analysis, has given us a more detailed and realistic view of the catalyst morphology. The examples chosen include supported Au catalysts for low temperature CO oxidation and supported AuPd bimetallic alloy catalysts for the direct production of H2O2.

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.O 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 using supported gold palladium catalysts prepared by incipient wetness impregnation is described and discussed. The effect of an acid pre-treatment step on the activated carbon support prior to the deposition of the metals, together with the effect of the calcination temperature, has been investigated. The acid pre-treated samples all show superior activity to those materials prepared with the omission of this acid pre-treatment stage. The calcination temperature affects both the re-usability and hydrogenation activity of the catalysts. Detailed characterisation using X-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy is described. The enhanced activity is associated with a higher surface concentration of palladium in the acid pre-treated samples which is principally present as Pd2+. Calcination of the catalysts at 400°C is required to achieve re-usable and stable catalysts, and this is associated with the morphology and dispersion of the metal nanoparticles. The surface ratio of Pd0/Pd2+ is found to be an important factor controlling the hydrogenation of hydrogen peroxide, and a series of controlled reduction and re-oxidation of a sample show how the Pd0/Pd2+ surface ratio can influence the relative rates of hydrogen peroxide synthesis and hydrogenation.