Gold as a promoter for the activity of palladium in carbon-supported catalysts for the liquid phase oxidation of glyoxal
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Catalysis Letters Vol. 99, Nos. 1–2, January 2005 ( 2005)
Gold as a promoter for the activity of palladium in carbon-supported catalysts for the liquid phase oxidation of glyoxal to glyoxalic acid Sophie Hermans and Michel Devillers* Catholic University of Louvain, Unite´ de Chimie des Mate´riaux Inorganiques et Organiques, Place L. Pasteur 1/3, B-1348 Louvain-la-Neuve, Belgium
Received 10 August 2004; accepted 14 October 2004
Bimetallic Pd–Au/C catalysts were prepared by various methods, including deposition–precipitation. It was found that these catalysts are highly active for the selective oxidation of glyoxal into glyoxalic acid in aqueous phase, while the corresponding monometallics displayed no (in the case of Au/C) or very little (in the case of Pd/C) activity. KEY WORDS: carbon; glyoxal; glyoxalic acid; gold; liquid phase; palladium; selective oxidation; supported catalysts.
1. Introduction Since the precursor work by Haruta et al. gold has found a new surge of interest for catalytic applications [1–4]. It was found that gold supported on titania (or other similar inorganic oxides) could be an active catalyst for CO oxidation at low temperature, but under strict control of the Au particle size:only nanoparticulate gold displayed the desired activity [5,6]. This has been explained in various terms, including relativistic effects, differences of electron density and number of surface atoms (in low coordination number) in nano-sized objects, or increased support/particle interface [7]. Since these early reports, various researchers have found an activity for monometallic supported gold catalysts, in a variety of oxidation and hydrogenation reactions [3], including liquid phase applications [8]. In particular, the group of Prati et al. has demonstrated the ability of Au/C catalysts to oxidise diols [9,10], carbohydrates [11] and aldehydes [12]. However, most of these studies rely on the ability of Au to act as an active element by itself, and bimetallic catalysts including gold remain scarce. In particular, the bimetallic Pd–Au association has been given little attention until very recently. The use of Pd–Au/SiO2 catalysts is a well-established commercial route for the synthesis of vinyl acetate by acetoxylation of ethylene since the 1960’s, but it is still poorly understood [13]. In recent studies, it has been shown that the addition of gold to a supported Pd catalyst does not improve its performance in some cases [14], but it has also been reported that bimetallic Pd–Au catalysts supported on various substrates (such as silica, alumina, ceria, titania, carbon, zeolites, magnesium fluoride, niobium oxide, etc.) give rise to * To whom correspondence should be addressed. E-mail: [email protected]
increased activity and/or selectivity when compared to their monometallic counterparts, and this for a wide variety of catalytic applications (hydrogenation of toluene, naphthalene, acetylene, 1,3-butadiene, or 2-hexyne; methanol or nitric oxide decomposition; hydrodechlorination; hydrodesulfurization; methylc
