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Single-atom speed-up

August 21, 2011

Picture from Wikipedia - Catalytic converter on a Dodge van (platinum is used as catalyst inside)

Catalysts are substances that speed up the rate of chemical reactions, without being consumed in the reaction themselves. Their influence extends over a wide range of areas, from reducing toxic exhaust in our cars (cataytic converter) to controlling metabolic processes inside our bodies (enzymes). Chemical reactions have an inertia or “activation energy”, a barrier that the reactants must climb over in order to form the products. Catalysts lower the height of this barrier by forming reaction intermediates, which enable faster conversion of reactants into products.

Noble metals like gold and platinum (Pt) are important heterogenous catalysts i.e. they are used in their solid form to speed up reactions in the gaseous state. The reacting gases “adsorb” (stick) onto active sites on the metal particles, where the activation energy is lowered. An important example is the oxidation of carbon monoxide (CO) into carbon dioxide (CO2). CO is a colorless, odourless, tasteless but extremely toxic gas that reacts with haemoglobin in the blood directly, compromising its capacity to carry oxygen. Concentrations as low as 500 parts per million in air can be fatal. 

CO in the presence of oxygen converts to CO2, which is non-toxic; however, the reaction rate is extremely small. For metal catalysts to work efficiently, it is crucial to have a large surface area (for a given mass) for the reacting gases to adsorb onto. It is only the atoms on the surface of the metal particles which catalyse the reaction; atoms in the bulk remain inactive. Imagine a spherical particle – the smaller the size, the higher the fraction of atoms on the surface. Nanometer (one billionth of a meter) sized particles, consisting of tens to hundreds of atoms, have proven to be good catalysts. The ultimate limit, however, would be having individual atoms of the metal separated from each other, dispersed on a given surface, to maximize catalytic efficiency.

Researchers at the Chinese Academy of Sciences in Dalian and the Tsinghua University in Beijing have now achieved that holy grail of catalysis. They developed a catalyst consisting of single Pt atoms on a surface of iron oxide, where one out of every 1430 atoms of iron, on average, was replaced by Pt. The sample precipitated out as a solid from a reaction between liquid solutions containing compounds of iron and Pt. The novel single-atom based catalyst was shown to have a record “turnover frequency” (TOF) of 1360 for CO oxidation, which means that on average about 1360 molecules of CO were converted to CO2 per second, for every Pt atom present. This speed-up is about 100 times better than those achieved by commercial Pt-based catalysts and about 3 times better than the best known gold-based catalysts. The catalytic performance was also found to be robust after hours of reaction time.

Extensive computer simulations were also performed to determine the mechanism of the catalysed oxidation reaction. First, an oxygen molecule (O2, containing two atoms) adsorbs onto a Pt atom, which is followed by the adsorption of a CO molecule onto the same Pt atom. The CO molecule reacts with one of the oxygen atoms at this low activation energy site to release a molecule of CO2. Then, another CO molecule sticks to the Pt atom and reacts with the remaining oxygen atom there to produce another CO2 molecule, after which the entire cycle repeats.

The authors believe that this single-atom speed-up can be extended to other precious-metal based catalytic systems, and greatly reduce the cost of commercial noble-metal catalysts.

 

Qiao, B., Wang, A., Yang, X., Allard, L., Jiang, Z., Cui, Y., Liu, J., Li, J., & Zhang, T. (2011). Single-atom catalysis of CO oxidation using Pt1/FeOx Nature Chemistry, 3 (8), 634-641 DOI: 10.1038/nchem.1095

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