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

Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)x(100)]], have been investigated using molecu lar beam surface scattering (MBSS), temperature programmed desorption (TPD) , low energy electron diffraction (LEED), and ultraviolet photoelectron spe ctroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temper ature range of 250-500 K. No evidence for long-range ordering of CO adlayer s 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 depe nd only weakly on the surface temperature. It is also found that the initia l 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 e xhibit this same behavior. CO desorbs molecularly from Pd{320} with desorpt ion features near 390 and 500 K. NO is shown to adsorb molecularly below 42 0 K with a high initial sticking probability close to 0.9. The initial stic king probability is largely temperature independent up to 450 K. No long-ra nge 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 th at NO partially dissociates upon heating, yielding N-2 and N2O. Desorption of molecular NO occurs on both surfaces between 300-600 K, with N-2 and N2O desorbing above 450 K. In contrast to Pd{110}, a high temperature N-2 peak near 600 K is seen during NO TPD from Pd{320}, which is attributed to tigh tly bound, adsorbed nitrogen atoms. The relative TPD yields of N-2 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 w ith surface temperature. The relative yields of N-2 and N2O reach a maximum near 550 K of similar to 40% and similar to 20%, respectively. By direct c omparison 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 p roperties with respect to adsorption and thermal behavior of CO and NO. (C) 2001 American Institute of Physics.