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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.