Interactions of sulfur with nickel surfaces: Adsorption, diffusion and desorption

Effects of surface impurities on surface diffusion of CO on Ni(110)

“…This is seen from the known activation energy barrier of 15-28 kcal/mol and diffusivity of ϳ10 Ϫ12 cm 2 /s at 500 K for S diffusion on Ni field emitters. 22 The maximum diffusivity is less than 10 Ϫ19 cm 2 /s at 240 K. The above picture is confirmed by scanning tunneling microscopy STM studies showing that S at low coverages diffuses readily on Ni͑110͒ even at room temperature and tends to end up at step sites. 23 Coadsorption of S and CO would have S adsorbed at the twofold hollow sites, and CO at the short-bridge sites.…”

“…15,16 No surface reconstruction induced by S has been observed. The binding between S and Ni is so strong that the S atoms do not desorb from the surface even at 1200 K. 22 However, they diffuse readily on the surface at sufficiently high temperatures. For example, under our sample preparation conditions, with the sample finally flash annealed at 570 K, S atoms should reach their thermal equilibrium positions on the surface and then remain stationary when the sample is cooled to 240 K or lower for CO diffusion measurements.…”

Impurity Effect on Surface Diffusion: CO/S/Ni(110)

“…A long-range interaction model also does not work because it would require a significant change in the potential of -30 CO adsorption sites by each S atom to explain the observed results, while all theoretical calculations predict that the effect of S on the substrate surface cannot extend beyond the next-nearest neighbors [11]. [We note that S atoms on Ni(110) are not highly mobile at our measurement temperatures [9]. ] An alternate plausible model assumes that the S impurities occupy the step sites and modify the step potentials, leading to step-controlled surface diffusion.…”

“…The concentration of S was measured by the intensity change of the 152 eV S peak in the Auger spectrum normalized to that of the saturation coverage of Os = 0.67 ML corresponding to a p(3 x 2) structure at room temperature [8]. The S atoms were known to desorb only at very high temperatures (above 1200 K) [9]. A brief Ilashing of the sample to 650 K was sufficient to desorb CO but did not affect the S coverage.…”

Thermal Desorption of Potassium from Clean and Sulfur Covered Nickel

“…The emitter temperature was determined measuring the changes of resistance of a loop segment. The sulfur and potassium sources used in these experiments were similar to those described elsewhere [3,4].…”