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B 62, R11921 (2000). We also thank A. Ng for initial discussions that led to this temperature. The increased lattice stability will 14. K. Widmann et al., Phys. Rev. Lett. 92, 125002 (2004). study. Funding was provided by the Natural Science and also increase the barrier to nucleation as the elec- 15. T. Ao et al., Phys. Rev. Lett. 96, 055001 (2006). Engineering Research Council. R.E. thanks the Alexander tron distribution and nuclear configurations are 16. S. Mazevet, J. Clérouin, V. Recoules, P. M. Anglade, von Humboldt Foundation for financial support. G. Zérah, Phys. Rev. Lett. 95, 085002 (2005). strongly coupled. Full-scale ab initio molecular 17. Y. Ping et al., Phys. Rev. Lett. 96, 255003 (2006). Supporting Online Material dynamics calculations are now needed to under- 18. J. R. Dwyer et al., Philos. Trans. R. Soc. London. Ser. A www.sciencemag.org/cgi/content/full/1162697/DC1 364, 741 (2006). SOM Text stand this phenomenon to which the experiment Figs. S1 and S2 provides rigorous benchmarks for comparison. 19. C.-Y. Ruan, Y. Murooka, R. K. Raman, R. A. Murdick, Nano Lett. 7, 1290 (2007). References 20. A. Plech, V. Kotaidis, S. Gresillon, C. Dahmen, G. von 3 July 2008; accepted 8 January 2009 References and Notes Plessen, Phys. Rev. B 70, 195423 (2004). Published online 22 January 2009; 1. B. J. Siwick, J. R. Dwyer, R. E. Jordan, R. J. D. Miller, 21. C. T. Hebeisen et al., Opt. Express 16, 3334 (2008). 10.1126/science.1162697 Science 302, 1382 (2003). 22. J. Hohlfeld et al., Chem. Phys. 251, 237 (2000). Include this information when citing this paper. being achieved even in the absence of halide and Switching Off Hydrogen Peroxide acid promoters (17). For commercial competi- tiveness with the indirect process, it is essential Hydrogenation in the Direct that H2 selectivity is increased to >95%. We have now addressed this problem and show that acid pretreatment of carbon support materials can Synthesis Process switch off the sequential hydrogenation and de- composition of H O , thereby achieving the target Jennifer K. Edwards,1 Benjamin Solsona,1 Edwin Ntainjua N,1 Albert F. Carley,1 2 2 2,3 3 1 selectivities and producing high rates of H2O2 Andrew A. Herzing, Christopher J. Kiely, Graham J. Hutchings * synthesis under intrinsically safe conditions. We have previously investigated a variety of Hydrogen peroxide (H2O2) is an important disinfectant and bleach and is currently manufactured support materials for Au-Pd catalysts (8, 13, 14, 16) from an indirect process involving sequential hydrogenation/oxidation of anthaquinones. However, and, specifically, the selectivities for H2 utiliza- a direct process in which H2 and O2 are reacted would be preferable. Unfortunately, catalysts for tion under our standard reaction conditions (18) the direct synthesis of H2O2 are also effective for its subsequent decomposition, and this has are carbon (80%) ~ SiO2 (80%) > TiO2 (70%) > limited their development. We show that acid pretreatment of a carbon support for gold-palladium Al2O3 (14%) (17). To understand these selectiv- alloy catalysts switches off the decomposition of H2O2. This treatment decreases the size of the ity differences, we investigated the hydrogena- alloy nanoparticles, and these smaller nanoparticles presumably decorate and inhibit the sites for tion and decomposition of H2O2 over these the decomposition reaction. Hence, when used in the direct synthesis of H2O2, the acid-pretreated catalysts and supports (Table 1). The oxide sup- catalysts give high yields of H2O2 with hydrogen selectivities greater than 95%. ports showed no activity in the absence of the metals, but the carbon support did show some ydrogen peroxide (H2O2) is an important and produces concentrated H2O2, although most reactivity, as noted previously (15). When the commodity chemical used primarily for applications require very dilute solutions. Direct metals are added, all show appreciable activity 1 Hdisinfection and bleaching ( )andwill processes that oxidize H2 have been investigated, for these nonselective reactions with the order of be used in the future for the manufacture of pro- and supported palladium catalysts are known to activity being broadly in line with the observed pylene oxide using the titanium silicalite TS-1 as be effective (3–5). However, as in any redox H2 selectivities in the synthesis reaction. Because a catalyst (2). It is currently produced by an in- process where a reactive intermediate is required these catalysts are prepared by wet impregnation direct process in which H2 and O2 are kept apart as the final product, the key problem is stabilizing of an acidic solution of the metal salts onto the using the sequential hydrogenation and oxidation the resulting H2O2 so that it does not decompose support (18), we investigated the effect of an ini- of an anthraquinone (2). For economic reasons, and form water. Indeed, all catalysts so far tial acid pretreatment step in which the support the indirect process is carried out on a large scale identified for direct H2O2 synthesis are equally effective for its sequential hydrogenation or de- 2 H O 6–12 2 1School of Chemistry, Cardiff University, Main Building, Park composition to water ( ) (Scheme 1), and H2 Place, Cardiff CF10 3AT, UK. 2National Institute of Standards acid and halides must be added to ensure that H + O H O and Technology, Surface and Microanalysis Science Division, some H2O2 is retained (6, 9–12). 2 2 2 2 100 Bureau Drive, Mailstop 8371, Gaithersburg, MD 20899– 3 To date, the best H2 selectivity reported for 8371, USA. CenterforAdvancedMaterialsandNano- 3 technology, Lehigh University, 5 East Packer Avenue, Bethle- the direct synthesis process using Pd is ~80% ( ). H O + 0.5 O hem, PA 18015­–3195, USA. We have shown (8, 13–16) that alloying gold 2 2 *To whom correspondence should be addressed. E-mail: with palladium markedly enhances the catalyst [email protected] activity and selectivity, with a selectivity of 80% Scheme 1. www.sciencemag.org SCIENCE VOL 323 20 FEBRUARY 2009 1037 REPORTS −1 −1 was treated with 2% aqueous HNO3 and dried H2O2 kgcat .h determined at 2 min reaction the acid pretreatment are critical; the effect can be −1 −1 before addition of the metals (18). For the Au-Pd time, and 160 mol H2O2 kgcat .h determined observed even when quite dilute acid solutions catalysts on oxide supports, this step led to a de- at 30 min reaction time, with 40% hydrogen (2 vol %) are used. crease in combined hydrogenation and decom- conversion. The addition of nitric acid to the Interestingly, both the untreated and the acid- position, but for the carbon support, the Au-Pd reaction mixture before H2O2 synthesis—which pretreated Au catalyst also show no activity for catalysts showed no such activity when reacting a is an established procedure for stabilizing H2O2 sequential hydrogenation (Fig. 1A); unfortunately, 4 weight percent (wt %) H2O2 solution (Table 1). (1, 2, 10–12) because H2O2 decomposition is both these catalysts also show remarkably little To investigate this effect, we examined the sta- known to be a base catalyzed process—also led activity for H2O2 synthesis (Table 2, experiments bility of higher concentration solutions of H2O2 to an enhancement in the yield of H2O2, but the 7 to 9). However, these results do show that the using the carbon-supported materials. Solutions effect was not sustained upon subsequent catalyst adsorption of Au onto the surface of the carbon of H2O2 were stirred with high pressure H2 (5% reuse (Fig. 1B). Treatment of any of the supports support blocks the sites responsible for loss of H2/CO2, 3 MPa) in the presence of the support or after metal deposition did not enhance catalyst H2O2 because the catalyst has no activity for the the catalyst (10 mg) for 30 min at 2°C (i.e., stan- performance, and addition of nitric or hydro- decomposition reaction.