It has been known that ultra-fine, nanoscale gold particles exhibit specific physical and chemical properties different from those of the ordinary coarse gold grains (“Ultra-fine Particles” published by Agne Publishing Center in 1986). In particular, such ultrafine gold is catalytically active and can be used as a catalyst for oxidizing carbon monoxide to form carbon dioxide. The use of catalytically active gold also has been proposed to catalyze other oxidation reactions such as the oxidation of carbonaceous soot in diesel exhaust streams, oxidation of unsaturated and saturated hydrocarbons, and the like.
Generally, ultra-fine particles of gold are very mobile and possess large surface energies and, therefore, tend to coagulate easily. In fact, it has been difficult to prevent such coagulation from occurring, making ultrafine gold hard to handle. Such mobility is undesirable inasmuch as the catalytic activity of gold tends to fall off as its particle size increases. This problem is relatively unique to gold and is much less of an issue with other noble metals such as Pt and Pd. Therefore, the development of the methods to deposit and immobilize ultra-fine gold particles on a carrier in a uniformly dispersed state has been desired.
The primary methods known to date to deposit catalytically active gold on various supports recently have been summarized by Bond and Thompson (G. C. Bond and David T. Thompson, Gold Bulletin, 2000, 33(2) 41) as including (i) coprecipitation, in which the support and gold precursors are brought out of solution, perhaps as hydroxides, by adding a base such as sodium carbonate; (ii) deposition-precipitation, in which the gold precursor is precipitated onto a suspension of the pre-formed support by raising the pH, and (iii) Iwasawa's method in which a gold-phosphine complex (e.g., [Au(PPh3)]NO3) is made to react with a freshly precipitated support precursor. Other procedures such as the use of colloids, grafting and vapor deposition meet with varying degrees of success.
These methods, however, suffer from serious difficulties resulting in a situation as aptly described by Wolf and Schuth (Applied Catalysis A; General 226 (2002) 2): (hereinafter the Wolf et al. article). “Although rarely expressed in publications, it also is well known that the reproducibility of highly active gold catalysts is typically very low.” The reasons for this serious reproducibility problem with these methods include: the difficulty in controlling gold particle size, the poisoning of the catalyst by ions such as Cl, the inability of these methods to control nano-sized gold particle deposition, the loss of active gold in the pores of the substrate, the necessity in some cases of thermal treatments to activate the catalysts, inactivation of certain catalytic sites by thermal treatment, the lack of control of gold oxidation state, and the inhomogeneous nature of the hydrolysis of gold solutions by the addition of a base.
DE 10030637 A1 describes using PVD techniques to deposit gold onto support media. However, the support media exemplified in working examples are merely ceramic titanates made as described under conditions in which the media would lack nanoporosity. Thus, this documents fails to appreciate the importance of using nanoporous media to support catalytically active gold deposited using PVD techniques. WO 99/47726 and WO 97/43042 provide lists of support media, catalytically active metals, and/or methods for providing the catalytically active metals onto the support media. However, these two documents also fail to appreciate the benefits of using nanoporous media as a support for catalytically active gold deposited via PVD. Indeed, WO 99/47726 lists many supports as preferred that lack nanoporosity.
In short, gold offers great potential as a catalyst, but the difficulties involved with handling catalytically active gold have severely restricted the development of commercially feasible, gold-based, catalytic systems.