CO ADSORPTION ON STEPPED PD(112) - STUDIES BY THERMAL AND ELECTRON-STIMULATED DESORPTION

The adsorption of CO on a stepped Pd(112) surface, Pd(S)[3(111)X(001)1, at 300 K has been studied by temperature programmed desorption (TPD), electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED) methods. CO adsorbs molecularly on Pd(112) at 300 K, reversibly desorbing near 465 K at low coverages with first-order desorption kinetics. The activation energy and pre-exponential factor of CO thermal desorption are observed to decrease strongly with increasing CO coverage, exhibiting a logarithmic compensation effect. Evidence is presented that CO preferentially adsorbs at the step sites of Pd(112) at low coverages, and that both terrace and step sites are occupied at higher coverages. The cross-section for production of positive ions during electron bombardment of adsorbed CO depends strongly on the CO coverage. No excited-state neutral particles were detected at any CO coverage during ESD measurements. In addition, no evidence for long-range ordering of the CO adlayers on Pd(112) was observed.