"Understanding the principles that govern nanocatalysis is key to developing more effective catalysts," said Uzi Landman of Georgia Tech. "Designing catalysts that are more efficient, more selective and more specific to a certain type of reaction can lead to significant savings in manufacturing expenses."

Landman and colleagues investigated charging in gold octamers - clusters of eight gold atoms - bound to oxygen-vacancy F-centre defects on a magnesium oxide (MgO) surface. (An F-centre is an anion vacancy in an ionic crystal that tends to be filled by electrons.)

An octamer is the smallest gold cluster that can act as a heterogeneous catalyst for the oxidation of carbon monoxide to carbon dioxide. It functions at temperatures as low as 140 K. If bound to a magnesia surface that doesn't contain oxygen vacancies, the gold clusters are unable to act as a catalyst for the reaction.

Studies have predicted that it's the partial transfer of negative charge from the substrate F-centre into the gold nanocluster that causes its catalytic activity. In this work, the researchers looked at the vibrational properties of carbon monoxide molecules adsorbed onto the gold clusters in order to study the tendency of the F-centres to charge the gold clusters.

"If carbon monoxide is a strong bond, then there is a certain frequency to this vibration," said Landman. "If the bond of the carbon monoxide becomes weaker, then the frequency becomes lower. That's exactly what we saw - when we had defects in the magnesium oxide, we had larger shifts than when we had magnesium oxide without defects."

The team says that this weakening of the bond is due to the negative charge that the gold nanocluster has received from the F-centre. The nanocluster transfers an electron to the carbon monoxide and oxygen molecules adsorbed onto it, weakening their bonds and ultimately enabling the oxidation reaction to occur.

"We knew the specific number of atoms in the catalyst and that defects in the catalytic beds were important," said Landman. "Now we know why those defects are so essential - because they allow the catalyst to be electrically charged."

The F centres also act to anchor the gold nanoclusters, preventing them from coalescing into larger structures.

"We hope these guidelines will lead to more research in search of nano-sized catalysts," said Landman. "It's possible that at the nanoscale you may find catalysts that can do things under gentler, cheaper conditions."

The researchers reported their work in Science.