The adsorption and reaction of HCl on Pd(111)

HCl adsorbed onto clean Pd(111) produces three distinct HCl desorption states in thermal desorption spectroscopy (TDS), all of which are populated simultaneously, even at exposures as low as 0.33 L. One desorption state emanates from molecularly adsorbed HCL (220 K), while the other two (300 and 470 K) emanate from recombination of H plus Cl atoms. The lowest temperature recombinative peak (beta at similar to 300 K) belongs to recombination of chlorine adatoms with surface hydrogen. This reaction is competitive with the recombinative desorption of H-2. Finally, the highest temperature peak with an onset at 470 K (alpha) is the recombination of the remaining Cl with hydrogen dissolved in the Pd crystal, which emerges from the bulk onto the surface at approximately 470 K. Some of the chlorine adatoms order on the Pd surface to yield a root3 x root3 R 30 degrees overlayer structure at coverages as low as theta = 0.13 ML, This structure persists through a range of temperatures from 100 to 320 K, when initially adsorbed at 100 K. The order-disorder transition at similar to 320 K is reversible.