Adsorption and thermal behavior of CO and NO on Pd{110} and Pd{320}

Abstract

Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)×(100)]], have been investigated using molecular beam surface scattering (MBSS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temperature range of 250–500 K. No evidence for long-range ordering of CO adlayers on Pd{320} was observed. Saturation coverage for CO/Pd{320} at 300 K is 0.61 ML. The initial sticking probability of CO is 0.91 and is shown to depend only weakly on the surface temperature. It is also found that the initial sticking probability of CO on Pd{110} exhibits a strong dependence on the kinetic energy of CO molecules and the CO adsorption on Pd{320} may also exhibit this same behavior. CO desorbs molecularly from Pd{320} with desorption features near 390 and 500 K. NO is shown to adsorb molecularly below 420 K with a high initial sticking probability close to 0.9. The initial sticking probability is largely temperature independent up to 450 K. No long-range order was observed from NO/Pd{320} in the temperature range of 300–600 K. The saturation coverage of NO is 0.64 ML at 300 K. It is demonstrated that NO partially dissociates upon heating, yielding N2 and N2O. Desorption of molecular NO occurs on both surfaces between 300–600 K, with N2 and N2O desorbing above 450 K. In contrast to Pd{110}, a high temperature N2 peak near 600 K is seen during NO TPD from Pd{320}, which is attributed to tightly bound, adsorbed nitrogen atoms. The relative TPD yields of N2 and N2O from Pd{110} and Pd{320} are found to be strongly dependent on the initial NO coverage with enhanced dissociation at lower coverages. Pd{320} is found relatively unreactive towards the thermal dissociation of NO. The activity of Pd{110} and Pd{320} towards NO dissociation upon adsorption increases with surface temperature. The relative yields of N2 and N2O reach a maximum near 550 K of ∼40% and ∼20%, respectively. By direct comparison to NO/Pd{110}, the present study shows that the presence of (100) steps on the Pd{320} surface does not significantly enhance its reactive properties with respect to adsorption and thermal behavior of CO and NO.