Carbon monoxide benefits catalysis with gold
A team of chemists at Leiden university have discovered that carbon monoxide can increase the activity of gold as a catalyst. This is surprising, because carbon monoxide is toxic and, generally speaking, inhibits catalytic reactions. The researchers published their remarkable findings in the online version of the prestigious journal Nature Chemistry.
Surprising results
Gold as an electrode, dangling in water. In this way, a surface of gold in that is in contact with water can be studied. (Photo by Marc de Haan, cover of Nature Chemistry)
‘Ordinarily, carbon monoxide is a toxin that prevents catalysts from working. However, it now appears that it in fact increases the activity of gold as a catalyst.’ A highly unexpected discovery, according to Marc Koper, Leiden Professor of Fundamental Surface Science. For him it marks new territory in our understanding of gold in its unusual role as a catalyst. ‘It shows that many of the existing rules that govern the catalysis of metals do not apply to gold. In the case of gold, it appears that there are subtle interactions involved which we do not expect under normal circumstances.’
Gold
Gold is not the first substance that comes to mind when one thinks of catalysts. Better known examples are metals like platinum, iridium or palladium. Gold is chemically inert, so it barely reacts with other substances. This is not a useful quality for a catalyst, which serves to speed up reactions between chemical substances. Even so, the last twenty years of research have proven that there are a number of reactions in which gold, under certain conditions, can act as a catalyst.
Carbon monoxide
One of these reactions is the oxidisation of the toxic gas carbon monoxide, turning it into harmless carbon dioxide. Gold will catalyse this chemical reaction in a watery solution, in which the solvent must be alkaline: in other words, it must have a high pH value. Such solvents are usually substances containing a hydroxyl group (an oxygen atom bonded to a hydrogen atom). Koper had already discovered that the carbon monoxide attaches itself to the gold during such reactions.
A new concept
This discovery led Koper’s team to the theory that not only the carbon monoxide, but also the hydroxyl groups attach themselves to the gold. ‘We came up with the concept that carbon monoxide promotes the adsorption of hydroxyl groups onto gold,’ Koper says. ‘The oxygen from the hydroxyl groups is subsequently used to convert carbon monoxide to carbon dioxide.’ His calculations were able to prove that these substances indeed influence each other in this manner.
Experiment with methanol
If that is the case, Koper realised, then the hydroxyl groups which have adsorbed onto the gold might also oxidise other substances in the solution besides carbon monoxide. The team has now tested this hypothesis with a number of alcohols because it is known that gold can catalyse the oxidation of alcohols. And indeed, in the case of methanol, for example, oxidisation proved to occur much more effectively with carbon monoxide than without.
New experiments
The results have paved the way for a large number of new experiments. ‘One wonders whether nitrogen, for example, would yield the same results,’ Koper speculates. ‘Or we might catalyse reduction reactions in the same way as we are now catalysing oxidation reactions. We hope to try that next.’ Eventually these new discoveries might be applied to fuel cells, where catalytic reactions also play a role.
Links
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P. Rodriguez, Y. Kwon & M. Koper, The promoting effect of adsorbed carbon monoxide on the oxidation of alcohols on a gold catalyst, Nature Chemistry (online, 11 december 2011)
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