Gold Magic and the Black Art of Catalysis
What's Hot in Chemistry, July/August 2010
By Dr. John Emsley
Papers about graphene occupy six positions in the current Hot Ten, while two slots are devoted to the remarkable iron-arsenic superconductor. These topics have been covered in previous issues of Science Watch®. The two new papers in the list deal with very different topics: covalent radii (#6) and gold as a catalyst (#9).
Paper #6 gives revised values for covalent radii of the elements up to californium (atomic number 96). The report comes from the University of Barcelona, Spain, where theoreticians led by Santiago Alvarez used the radii of nitrogen, carbon, and oxygen, which were assessed in several ways, to calculate all the other radii, or, if this was not possible, to infer them from periodic trends. Clearly this data is proving useful, although the concept of covalent radius has been around for more than 75 years.
However, it is paper #9 from a group at the Department of Chemistry at the University of Cambridge led by Richard Lambert and Brian Johnson which reports some remarkable findings about gold particles as catalysts and which reveals just how important their size is if they are to be effective.
The chemical inertness of gold always seemed to exclude it as a catalyst until the late 1980s, when researchers in Japan showed that nanoparticles of the metal, supported on semiconducting transition-metal oxides, could catalyze the oxidation of CO to CO2 at low temperatures. Today gold is at the forefront of catalysis research, much of which has focused on selective oxidation reactions, which are of industrial importance.
"Many still regard catalysis as something of a black art. Research at Cambridge is now shining a bright light on this rather dark corner of chemistry."
The ideal oxidation process would use only molecular oxygen and a hydrocarbon as reactant, and without the need for other less desirable reagents like peroxides. This is exactly what #9 shows is possible. The Cambridge group studied styrene, which can be oxidized to benzaldehyde and styrene oxide. The reaction was done at 100° C with toluene as the solvent and the catalyst was 0.6% by weight of gold nanoparticles fixed to an inert substrate like boron nitride. The yields of benzaldehyde and styrene oxide were, respectively, about 80% and 20%.
As paper #9 reports, the size of the nanoparticles is the key to activity. Various sizes were tested, some as large as 30 nanometers (nm), and they were measured using high-resolution transmission electron microscopy. It would appear that only the smaller gold particles are able to effect the necessary pre-dissociation of O2 to O atoms for the reaction to proceed.
Particles of around 1.4 nm work best, and these consist of clusters of 55 gold atoms, a so-called "magic number" which ensures a particularly stable arrangement. Larger particles were ineffective. The Au55 clusters were produced by methods which involved triphenylphosine and chloride ions and it was necessary to remove traces of these by heating, otherwise product yields were reduced.
The Cambridge group are currently widening their research to other important synthetic organic chemistry reactions, such as Sonagashira coupling, hydroamination, and enantioselective hydrogenation. (Sonagashira coupling is the joining of alkynes to hydrocarbons.) They have also found that gold alloys are promising catalysts for NOx abatement.
Gold is increasingly useful as both a homogeneous and a heterogeneous catalyst, and some of the most recent advances in gold nanoparticle catalysis involve reactions carried out in solution. The team are hoping to answer the question of where the active site resides. Is it at the surface of the gold nanoparticles (heterogeneous catalysis) or with soluble species leached into solution (homogeneous catalysis)? Recently they have shown that heterogeneous catalysis is overwhelmingly predominant for Sonagashira coupling catalyzed by gold nanoparticles.
Lambert sees their work in a wider context: "Clearly, it’s important to address this question, and not just for catalysis by gold, in order to signpost promising future developments. The scope and importance of gold nanoparticle-catalyzed reactions seems likely to increase. When these are carried out in solution it’s essential, though not easy, to establish the relative importance of homogeneous and heterogeneous pathways."
Many still regard catalysis as something of a black art. Research at Cambridge is now shining a bright light on this rather dark corner of chemistry.
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.
What's Hot in Chemistry | |||
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Rank | Paper |
Cites This Period Jan-Feb 10 |
Rank Last Period Nov-Dec 09 |
1 | Y. Kamihara, et al., " Iron-based layered superconductor La[O1-xFx]FeAs (x = 0.05-0.12) with Tc = 26 K," J. Am. Chem. Soc., 130(11): 3296-7, 19 March 2008. [Tokyo Inst. Technol., Yokohama, Japan] *273SL | 77 | 1 |
2 | X.L. Li, et al., "Chemically derived, ultrasmooth graphene nanoribbon semiconductors," Science, 319(5867): 1229-32, 29 February 2008. [Stanford U., CA] *267SX | 41 | 2 |
3 | C. Lee, et al., "Measurement of the elastic properties and intrinsic strength of monolayer graphene," Science, 321(5887): 385-8, 18 July 2008. [Columbia U., New York, NY] *327FB | 36 | 5 |
4 | H.A. Becerrill, et al., "Evaluation of solution-processed reduced graphene oxide films as transparent conductors," ACS Nano, 2(3): 463-70, March 2008. [Stanford U., CA; Nankai U., Tianjin, China] *280EM | 25 | † |
5 | R.R. Nair, et al., "Fine structure constant defines visual transparency of graphene," Science, 320(5881): 1308, 6 June 2008. [U. Manchester, U.K.; U. Minho, Braga, Portugal] *309DO | 24 | † |
6 | B. Cordero, et al., "Covalent radii revisited," Dalton Trans., 21: 2832-8, 2008. [U. Barcelona, Spain] *301WW | 24 | † |
7 | A. Reina, et al., "Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition," Nano Letters, 9(1): 30-5, January 2009. [MIT, Cambridge] *395IZ | 24 | † |
8 | C. de la Cruz, et al., "Magnetic order close to superconductivity in the iron-based layered LaO1-xFx FeAs systems," Nature, 453(7197): 899-902, 12 June 2008. [6 U.S. and China institutions] *311WV | 21 | 3 |
9 | M. Turner, et al., "Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters," Nature, 454(7207): 981-3, 21 August 2008. [U. Cambridge, U.K.] *339QB | 21 | † |
10 | A.A. Balandin, et al., "Superior thermal conductivity of single-layer graphene," Nano Letters, 8(3): 902-7, March 2008. [U. Calif., Riverside] *273QS | 20 | † |
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend. |
KEYWORDS: GOLD CATALYSIS, SELECTIVE OXIDATION, RICHARD LAMBERT, BRIAN JOHNSON, STYRENE, GOLD NANOPARTICLES.
Citing URL: http://sciencewatch.com/ana/hot/che/10julaug-che/