Credit: © 2008 AAAS

Selective hydrogenation of alkynes to alkenes on the surface of palladium is a process that has been used industrially for many years. It sounds like a straightforward reaction, but why it stops after one hydrogenation step and does not further hydrogenate the products to alkanes has never been fully understood. Study of the mechanism has been hampered because there has been no way to examine the system under reaction conditions. Now, Detre Teschner at the Fritz Haber Institute in Berlin and co-workers1 have developed techniques to do just that.

In the classical catalysed hydrogenation reaction, surface-adsorbed hydrogen reacts with an adsorbed alkyne and does so selectively to produce alkenes. But this is not the only type of hydrogen that can, in principle, hydrogenate the alkyne. Hydrogen dissolved within the palladium can also take part in the reaction and, because it is much more energetic than surface-adsorbed hydrogen, it should lead to the total hydrogenation of alkynes to alkanes.

Using X-ray photoelectron spectroscopy and prompt gamma activation analysis Teschner and coworkers found that a subsurface layer of carbon is responsible for the selectivity observed in the hydrogenation by ensuring that only surface-adsorbed hydrogen takes part in the reaction. This carbon layer is formed from the fragmentation of alkynes that occurs during an initial activation process associated with low selectivity and high concentrations of dissolved hydrogen. The carbon atoms not only displace the subsurface hydrogen from the palladium catalyst, but also disrupt the transport of dissolved hydrogen to the surface, reducing the chances of total hydrogenation occurring.