Time-resolved reaction studies on the atomic scale: NO and CO adsorption on stepped Rh surfaces

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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 tR between the pulses has been varied in the range 100 μs≤tR≤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+, RhxNn+, and RhOn+ ions (x,n=1,2). This finding suggests that part of the adsorbed NO undergoes dissociation during tR. 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 tR 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 Ed=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 tR 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)2 formation 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.

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