Catalysis is generally thought of as a surface phenomenon, yet a new study demonstrates that what happens several atomic layers below the catalyst surface can have a big impact on catalytic reactions. Detre Teschner at the Fritz Haber Institute, in Berlin, and colleagues show that the atoms just beneath the surface of a palladium catalyst help determine if alkyne hydrogenation proceeds selectively to an alkene or proceeds nonselectively all the way to an alkane (Science 2008, 320, 86). The group had previously studied selective partial hydrogenation of alkynes, showing that carbon from 1-pentyne is incorporated into the first three atomic layers of a palladium catalyst and forms a Pd-C phase that affects the hydrogenation activity. Now, with both in situ X-ray photoelectron spectroscopy and gamma activation analysis, the group has monitored the carbon and hydrogen content in palladium during catalysis. They found that when palladium indiscriminately hydrogenates alkynes to alkanes, far fewer carbon atoms are dissolved in the catalyst's subsurface. "This result suggests that not only the surface but also the subsurface region is affected by the chemical potential of the reaction mixture," the authors write.