Single atom alloys catalyse reactions

In the field of heterogeneous catalysis, researchers have always tried to minimise the amount of active metal - often expensive noble metals like platinum and palladium - in catalysts to reduce costs. Because catalysis is a surface phenomenon, they have also tried to minimise the size of the deposited metal islands to ensure that most of the metal is present as surface atoms.

Now, researchers at Tufts University in Massachusetts, led by Charles Sykes, have achieved the ultimate in both cases by demonstrating that single palladium atoms in copper can catalyse industrially significant hydrogenation reactions, like the conversion of acetylene to ethylene.
 
The key is to create what Sykes and his colleagues call ''single atom alloys (SAAs).'' While this sounds like a misnomer, the term is meant to indicate a bimetallic alloy in which (a) the more catalytically active metal (Pd in this case) is present in a very small concentration - say, 0.01 monolayers - in the surface layer of the other component (Cu in this case), and (b) an atom of the more active metal is thermodynamically more stable when surrounded by the host metal (Cu) than by other atoms of the same element (Pd). The result is a single Pd atom in a sea of Cu - a single atom alloy
 
Through years of study of the geometric and electronic properties of metal alloys with scanning tunneling microscopy (STM), Sykes says, ''We have had tantalising glimpses that when catalytically active atoms are atomically dispersed they are still capable of activating molecules.'' In this latest research, which was reported recently in Science, they used low temperature STM (LT-STM) and temperature-programmed reaction analysis on the same samples to observe the mechanism of hydrogenation reactions.
 
What they saw using LT-STM at 5 K is that single Pd atoms in a Cu(111) plane were able to activate (dissociate) molecular H2 into H atoms, which then spilled over onto the Cu surface. Notably, pristine Cu(111) is unable to cause this H2 dissociation by itself. The H atoms bind weakly to the Cu(111) surface, react with acetylene to form ethylene with high selectivity, then leave the surface in the product molecule.

If the H atom was bound too tightly it would not be available for reaction with acetylene. ''The single Pd atoms have converted the otherwise entirely inactive Cu(111) surface into an effective and very selective hydrogenation catalyst by providing both a low-energy entrance route for Ha [adsorbed H atoms] and many Cu sites where it is weakly bound,'' the researchers note in the paper.
 
Sykes speculates that some industrial catalytic processes may already operate through this single atom alloy mechanism, but researchers had no way of knowing that until now. ''Our system offers the opportunity to discover what arrangement of surface atoms is ideal for a particular chemical reaction,'' he concludes.