More Surface, More Reactivity

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Science  26 Sep 2008:
Vol. 321, Issue 5897, pp. 1742
DOI: 10.1126/science.321.5897.1742b

To gain a better understanding of palladium's reactivity as a hydrogenation catalyst, many model studies that use well-defined single-crystal surfaces have focused on what should be the simplest substrate, ethylene. Although this reaction is facile for reactant pressures near ambient, at very low pressures (ultrahigh-vacuum conditions), the reactivity on close-packed surfaces is low (yields of ethane <1%), and not much greater on supported nanoparticles (<5%). This difference is attributed to a lack of surface hydrogen caused by absorption into the bulk. Dohnálek et al. have prepared model catalysts through ballistic deposition of Pd atoms at cryogenic conditions (22 K) and glancing angles such that one-quarter of the atoms are surface exposed. The as-prepared nanoporous films showed much higher reactivities (50%), which decreased when the films were densified by reaction cycles that went to room temperature (with ethane desorbing by 250 K) or after annealing to higher temperatures. The authors note that although surface roughening treatments can also create a large number of active sites, the low fraction of bulk atoms in the nanoporous films limits removal of hydrogen from the surface and boosts overall reaction rates. — PDS

J. Phys. Chem. C 112, 10.1021/jp803880x (2008).

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