A hreels, tpd study of nitric oxide adsorption, desorption and reaction on clean and sulfur covered palladium (100)

The effect of sulfur poisoning on the adsorption, desorption and reactions of NO on the Pd(100) surface was studied using TPD, EELS, and LEED at sulfur precoverages ranging from zero to saturation (0.5 monolayer) and for NO coverages between zero and saturation (0.65 ML). NO adsorption is primarily associative. Simple steric blocking accounts for the decrease in NO saturation coverage with adsorbed sulfur. Correspondingly, a 25% decrease in the saturation coverage of NO was observed at a sulfur coverage of 0.1 ML, as expected for simple atop site blockage by sulfur. Thereafter the NO coverage declined more slowly, with detectable NO adsorption at θS(sat). Three different vibrational modes were detected by EELS at 1510, 1720, and 1750 cm−1, which were assigned respectively to a bridge site, an atop site and a weakly bound NO. The 1720 cm−1 loss frequency is coverage dependent. NO adsorption on the clean surface gives rise to atop site occupation in a c(2 × 2) LEED structure at 300 K, and a p(4 × 2) structure at 410 K, corresponding to coverages of 0.5 and 0.25 ML, respectively. The p(4 × 2) structure was generally observed to form a single domain. In the presence of 0.03 ML of sulfur the p(4 × 2) LEED pattern is not observed at any temperature, but the c(2 × 2) LEED pattern persists up to at least 0.2 ML sulfur. Correspondingly, the atop to bridge site transfer was not observed, and the high temperature desorption state was eliminated by adsorbed sulfur, implying that the most strongly bound state is bridge bonded. Only small amounts of N2 and N2O are observed to desorb during heating of adsorbed NO, and oxygen is not observed as a product. At sulfur coverages above 0.10 ML these side reactions were suppressed.