CATALYTIC PROPERTIES OF PD ATOM DEPOSIT ON NI(111)

Pd atom deposits are studied under UHV conditions and their catalytic properties are tested in the 1,3-butadiene hydrogenation reaction. Ni(111) surface activity is very poor (3.4 x 10(14) molecules converted/cm2.s at 300 K and under P(H2) = 35 Torr). After a 0.5 ML Pd deposit (at 300 K) a strong increase in activity is observed; it reaches an average value between Pd(111) activity (1.5 x 10(15) mol/cm2.s) and Pd(110) activity (7.4 x 10(15) mol/cm2.s). After 15 min annealing of a similar deposit at 475 K, a further increase is observed (2 x 10(16) mol/cm2.s) and the activity becomes greater than the activity of Pd monocrystalline faces. Selectivity towards butenes always remains near to unity. The Pd deposit is accompanied by an important increase of the LEED pattern background; the contrast is improved with annealing. Auger analysis indicates that the Pd surface concentration variation during the annealing has not varied and that no noticeable electronic change between the non-annealed and the annealed sample exists. From LEIS studies, a superficial surface rearrangement is presumed to occur leading to the formation of Pd islands, or alloying in the first layer.