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Abstract

The Interaction of NO and CO with stepped surfaces of Rh Field emitter tips has been studied by means of pulsed field desorption mass spectrometry (PFDMS) at temperature 300< T < 600 K and steady gas pressures p < 1.3×10⁻⁴ Pa. The repetition frequency of the pulses, i.e., the field free reaction time t_R between the pulses has been varied in the range 100 μs≤t_R≤1 s in order to obtain information about the kinetics of surface reactions. The pulsed field desorption (PFD) mass spectra obtained during NO adsorption always contain NO⁺, Rh_xNⁿ⁺, and RhOⁿ⁺ ions (x,n=1,2). This finding suggests that part of the adsorbed NO undergoes dissociation during t_R. For short reaction times, i.e., low surface coverages, NO adsorption is found to be predominantly molecular. Using a probe hole ∼150 atomic sites close to the (100) pole of the tip surface have been analyzed in detail. The equilibration between adsorption and thermal desorption of NO has been monitored by t_R variation. In this manner the mean lifetimes τ before first order thermal desorption can be determined. From the temperature dependence of the τ values the thermal desorption kinetics have been evaluated. An activation energy E_d=102 kJ/mol and a preexponential τ₀=3×10⁻¹⁴ s are found. The PFD mass spectra obtained during CO adsorption on stepped Rh planes contain CO⁺ and Rh(CO)ⁿ⁺_y ions (n=1,2; y=1–3). The t_R dependence of these species has been measured and found to be consistent with a consecutive surface reaction between chemisorbed CO and Rh step atoms leading to adsorbed Rh subcarbonyls. Possibly Rh–Rh bonds are broken during Rh(CO)₂formation so that this species is liberated and able to diffuse into the terrace regions of the surface. Only a small amount of adsorbed CO undergoes dissociation at long reaction times.