DOI: 10.1002/cssc.200800257 Effect of Halide and Acid Additives on the Direct Synthesis of Hydrogen Peroxide using Supported Gold–Palladium Catalysts Edwin Ntainjua N., Marco Piccinini, James C. Pritchard, Jennifer K. Edwards, Albert F. Carley, Jacob A. Moulijn, and Graham J. Hutchings* [a] Introduction There is a growing research interest in the synthesis of hydro- gen peroxide, H 2 O 2 , because of its use as an environmentally benign oxidant for chemical synthesis. [1] However, H 2 O 2 is a commodity chemical produced at a very large scale (ca. 3 Mt annually) with principle uses as disinfectant and as bleach, and at present chemical synthesis utilizes only a small proportion of the H 2 O 2 produced annually. The need to reduce the use of chlorine in bleaching processes and waste-water treatment [2] necessitates a higher production of H 2 O 2 to meet applications in the textile, paper, and pulp industries. This partly accounts for the 10 % per annum increase in global H 2 O 2 production. [3] H 2 O 2 is currently produced by using the sequential hydrogena- tion and oxidation of alkyl anthraquinone. This process is con- sidered to be non-green for a number of reasons, and in addi- tion it is considered to be uneconomical on a small scale be- cause the optimization of this process technology that has oc- curred in the many decades since its first use have been based on large-scale manufacture. In order to meet the growing demand for H 2 O 2 , there is a current requirement for a new green and cost-effective production route. In 1914, Henkel and Weber [4] synthesized H 2 O 2 directly from H 2 and O 2 for the first time by using palladium-based catalysts and since then there has been a growing interest to develop the process. The direct process is currently regarded as a green and economically viable alternative to the indirect anthraquinone process, but as yet catalysts that can compete with the indirect process have not been identified . Until relatively recently, palladium-sup- ported materials were the most widely used catalysts for the direct synthesis of H 2 O 2 ; [4–31] however, we have recently shown [32–41] that the addition of Au to Pd significantly enhances the activity and selectivity of palladium-supported catalysts in direct H 2 O 2 synthesis. One of the most significant challenges in the direct process is the non-selective hydrogenation and decomposition of H 2 O 2 once it has been formed. Most catalysts that are effective for H 2 O 2 synthesis also tend to hydrogenate or decompose H 2 O 2 to H 2 O. This has been addressed for palladium-supported cata- lysts by the addition of halides to an aqueous acidic reaction medium, [22–31] and there are a number of industrial studies [10–19] that highlight the beneficial effect of halides in the direct pro- duction of H 2 O 2 . Acids and halides have been found to limit either H 2 O 2 hydrogenation/decomposition or the hydrogena- tion of O 2 to H 2 O with monometallic Pd catalysts, and hence enhanced H 2 O 2 yields/selectivities are observed. Indeed, in some cases, no selectivity is observed with Pd catalysts in the absence of halides. [42] In contrast, we have recently shown that the addition of acid and halide together to a gold–palladium- supported catalyst is deleterious for H 2 O 2 synthesis. [39] The in- fluence of halide-only or acid-only added to the reaction medium for H 2 O 2 synthesis over these bimetallic catalysts has, however, not yet been investigated. This study aims at investi- gating the individual effect of adding acid (H 3 PO 4 , HNO 3 ) or halide (Br  ) to the reaction mixture for H 2 O 2 synthesis using gold–palladium-supported catalysts. [a] Dr. E. Ntainjua N., M. Piccinini, J. C. Pritchard, Dr. J. K. Edwards, Dr. A. F. Carley, Prof. J. A. Moulijn, Prof. G. J. Hutchings Cardiff Catalysis Institute School of Chemistry, Cardiff University Main Building, Park Place, Cardiff, CF10 3AT (UK) Fax: (+ 44) 2-920-874-030 E-mail : hutch@cardiff.ac.uk 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 ad- dition 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 bro- mide leads to a marked decrease in the hydrogenation/decom- position of hydrogen peroxide with either catalyst. These ef- fects are discussed in terms of the structure of the gold–palla- dium alloy nanoparticles and the isoelectric point of the sup- port. We conclude that with the highly active carbon-support- ed 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. ChemSusChem 2009, 2, 575 – 580  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 575