Gold catalysis: helping create a sustainable future
- 1.3k Downloads
In recent years there has been a general realisation that supported gold and gold bimetallic nanoparticles can be very effective for a broad range of redox reactions. In this paper we review the preparation of gold palladium nanoparticles using a sol-immobilisation methodology and show their effectiveness for the oxidation of benzyl alcohol and the direct synthesis of hydrogen peroxide.
KeywordsGold catalysis Alcohol oxidation Hydrogen peroxide synthesis
Gold for many centuries has held a special place in the attention of the general public partly due its intrinsic value but also because of the beautiful jewelry and artwork that can be created from it. The reason for this is that gold is the most noble of metals and consequently is considered immutable. From a chemical perspective, gold has therefore relatively little inherent interest as it appears in its bulk form to have a very limited chemistry. However, a major discovery of the 1980s was that gold, when sub-divided down to the nanoscale, can be highly reactive and can activate small molecules. This has led to an explosion of interest in the chemistry of gold and, in particular, the synthesis and use of small gold-containing nanoparticles as redox catalysts.
On the basis of many studies, Au nanoparticles have been shown to be particularly effective for a broad range of oxidation and hydrogenation reactions as well as chemical synthesis [1, 2, 3, 4, 5, 6, 7, 8]. These include the low temperature oxidation of CO , especially when used as part of the processes for purifying a fuel cell feedstock [9, 10], the synthesis of vinyl chloride by the hydrochlorination of ethyne , the selective oxidation of alkenes to epoxides [12, 13] and alcohols to aldehydes  as well as selective hydrogenation . Recently, research in our groups has shown that the alloying of Pd with Au can enhance the activity of the nanoparticles, and such materials have been found to be particularly effective for the direct synthesis of hydrogen peroxide from its elemental constituents [16, 17, 18, 19, 20] and the oxidation of alcohols . In this paper, we review an aspect of our work concerning the synthesis of supported Au–Pd alloy nanoparticles prepared by sol-immobilisation for two important classes of redox reaction; namely the direct synthesis of hydrogen peroxide and the oxidation of benzyl alcohol. These two reactions have very broad interest in the chemical community as they both display key features concerning selectivity control. In the case of hydrogen peroxide synthesis, it is important that the non-selective formation of water is minimised, and in the case of alcohol oxidation to the corresponding aldehyde it is essential that the over-oxidation to the carboxylic acid is minimised. By way of background information, hydrogen peroxide (H2O2) is a major commodity chemical  and has significant uses in cleaning and bleaching. At present, H2O2 is produced using an indirect process which involves the sequential hydrogenation and oxidation of an alkyl anthraquinone, thereby avoiding the potential for explosive contact between hydrogen and oxygen to occur . The direct synthesis of hydrogen peroxide is not a reaction that is easy to control, since the formation of water is preferred under all conditions, and the quest to find a suitable catalyst for the direct reaction between H2 and O2 has remained a challenge for almost a century, and as such, it has attracted considerable attention. Supported Pd catalysts have been the focus of much of the research [24, 25, 26, 27, 28, 29, 30, 31, 32]. Our work on this topic [16, 17, 18, 19, 20] has shown that a combination of Pd with Au promotes both the activity and the selectivity for the direct H2O2 synthesis reaction. Furthermore, these catalysts are also very effective for the oxidation of alcohols, and once again the addition of Pd to Au increases both the selectivity and the activity . The link between these two reactions is considered to be at the level of the possible surface intermediates present during the reaction, and it is thought that a surface hydroperoxy species may be responsible for the selective chemistry observed in both processes. In our initial studies [16, 17, 18, 19, 20], we used a wet co-impregnation method to prepare gold–palladium nanoparticles and concluded that either homogeneous random alloys or core–shell structures  could be formed. Based on this work, we hypothesised that small homogeneous Au–Pd nanoparticles were likely to be the active species in these catalysts and we set out to utilise a sol-immobilisation method  in an attempt to ‘tailor-make’ these nanoparticles. We reasoned that since the volume fraction of these ultra-small alloy particles formed using the co-impregnation method was quite small [19, 20], pre-forming colloidal nanoparticles should be more material efficient and hence overcome this problem. We found that this latter method can indeed produce well-defined particle size distributions . However, the catalysts do not retain the selectivity displayed by the catalysts prepared by wet impregnation [16, 17, 18, 19, 20]. This paper reviews some of the essential structural features and reactivity traits of Au–Pd alloy catalysts prepared using a sol-immobilisation method.
A general preparation method for Au, Pd and Au–Pd nanoparticles using sol-immobilisation
Characterisation of sol-immobilised Au, Pd and Au–Pd nanoparticles
Hydrogen peroxide synthesis and hydrogenation
In an earlier study  we observed that in the catalysts prepared using the impregnation method, the very smallest particles were mainly Pd and that 3–5 nm Au–Pd nanoparticles comprised typically 98 % Pd to 2 % Au. As the activity was considered to be associated with these small particles, this led us to conclude that for optimum activity we needed to find a method to fabricate such particles. However, the results of the sol-immobilised catalysts might suggest that this hypothesis is not necessarily the case, since the Pd-rich Au–Pd colloidal nanoparticles (i.e. Pd:Au ≥ 3) actually display a lower productivity than those with a Au:Pd = 1:2 molar ratio. It must also be taken into account that this effect could also be explained on the basis that the hydrogenation activity of the catalysts is expected to increase with Pd content and this somewhat cancels out the positive effect of adding Pd to the Au nanoparticles for the direct synthesis.
Benzyl alcohol oxidation
The catalytic oxidation of benzyl alcohol under free solvent conditions is an appropriate model reaction for exploring the catalytic performance of bimetallic Au–Pd catalysts and the results obtained for the conversion of benzyl alcohol to benzaldehyde are shown in Fig. 6. It is found that the catalytic performance in terms of reaction specificity is not dependent on the composition and all catalysts give high selectivities to benzaldehyde which is the primary product. Hence, the main outcome of alloying Au with Pd is to enhance activity. The activity of the monometallic 1 wt% Au/C sample was very poor, but on increasing the Pd content we observed a progressive increase in catalytic activity. This increase of activity reached a broad maximum with a Au:Pd molar ratio between 1:1 and 1:3. A further increase in the Pd content resulted in a progressive decrease in catalytic activity. It is important to note that even in the presence of a minor amount of gold or palladium (i.e. in the case of 1:7 and 7:1 Au:Pd molar ratios) a significant increase in the catalytic activity is observed as compared to the mono-metallic counterparts. It is apparent that the activity profile for benzyl alcohol oxidation has the same dependency on the Au:Pd molar ratio as does the direct synthesis and hydrogenation of hydrogen peroxide.
It is apparent that carbon-supported Au–Pd nanoparticles prepared by sol-immobilisation are very effective for the three reactions; namely, the direct synthesis of H2O2 and its subsequent hydrogenation and the selective oxidation of benzyl alcohol. For all these reactions we show that the optimum ratio of Au:Pd is ca. 1:1.85. As shown in Fig. 6 the catalytic activity per mol of metal of the Pd:Au series of catalysts for hydrogen peroxide synthesis, hydrogen peroxide hydrogenation and benzyl alcohol oxidation also shows a similar dependence on the Au:Pd molar ratio. This suggests that the active sites for these reactions could be common which is an interesting observation. However, it is also clear that there is considerable variation in the composition of the individual nanoparticles prepared by sol-immobilisation and we are now exploring synthetic strategies that will help to overcome this problem since ideally we need to be able to synthesise bimetallic nanoparticles with both uniform particle size distributions and well-defined composition that does not vary from particle-to-particle.
- 9.Landon P, Ferguson J, Solsona BE, Garcia T, Carley AF, Herzing AA, Kiely CJ, Golunski SE, Hutchings GJ (2005) Selective oxidation of CO in the presence of H2, H2O and CO2 via gold for use in fuel cells. Chem Commun 3385–3387Google Scholar
- 10.Landon P, Ferguson J, Solsona BE, Garcia T, Al-Sayari S, Carley AF, Herzing A, Kiely CJ, Makkee M, Moulijn JA, Overweg A, Golunski SE, Hutchings GJ (2006) Selective oxidation of CO in the presence of H2, H2O and CO2 using Au/Fe2O3 catalysts for use in fuel cells. J Mat Chem 16:199–208CrossRefGoogle Scholar
- 16.Landon P, Collier PJ, Papworth AJ, Kiely CJ, Hutchings GJ (2002) Direct formation of hydrogen peroxide from H2/O2 using a gold catalysts. Chem Commun 2058–2059Google Scholar
- 19.Edwards JK, Carley AF, Herzing AA, Kiely CJ, Hutchings GJ (2008) Direct synthesis of hydrogen peroxide from H2 and O2 using supported Au-Pd catalysts. Faraday Disc 138:225Google Scholar
- 23.Hess HT (1995) Hydrogen peroxide. In: Kroschwitz I, Howe-Grant M (eds) Kirk-othmer encyclopaedia of chemical engineering, vol 13. Wiley, New York, p 961Google Scholar
- 24.Henkel H, Weber W (1914) Manufacture of hydrogen peroxide. US Patent 1108752Google Scholar
- 25.Van Weynbergh J, Schoebrechts JP, Colery JC (1995) Direct synthesis of hydrogen peroxide by heterogeneous catalysis, catalyst for the said synthesis and method of preparation for the said catalyst. US Patent 3265447706Google Scholar
- 26.Paparatto G, D’Aloisio R, De Alberti G, Furlan P, Arca V, Buzzoni R, Meda L (1999) New catalyst, process for the production of hydrogen peroxide and its use in oxidation processes. EP Patent 0978316A1Google Scholar
- 27.Zhou B, Lee L-K (2001) Catalyst and process for direct catalytic production of hydrogen peroxide. US Patent 6168775Google Scholar
- 28.Nystrom M, Wangard J, Herrmann W (2001) Process for producing hydrogen peroxide. US Patent 6210651Google Scholar
- 32.Choudhary VR, Samanta C, Gaikwad AG (2004) Drastic increase of selectivity for H2O2 formation in direct oxidation of H2 to H2O2 over supported Pd catalysts due to their bromination. Chem Commun 2054–2055Google Scholar
- 33.Pritchard J, Kesavan L, Piccinini M, He Q, Tiruvalam R, Dimitratos N, Lopez-Sanchez JA, Carley AF, Edwards JK, Kiely CJ, Hutchings G (2010) Direct synthesis of hydrogen peroxide and benzyl alcohol oxidation using Au-Pd catalysts prepared by sol immobilization. J Langmuir 26:16568–16577CrossRefGoogle Scholar
This article is published under license to BioMed Central Ltd. Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.