Surface reactions and adsorbate-induced reconstruction: CO and NO on Rh crystals

Field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS) have been used in order to study the interaction of CO and NO with Rh field emitter crystals. Strong morphological changes along with local plane reconstructions of the samples have been observed with atomic resolution in FIM. With both gases the originally hemispherical crystals have been found to transform into polyhedral shapes. Results are shown for reaction temperatures of 500 K (NO) and 520 K (CO). Interestingly, while NO adsorption causes (1 x 2) missing-rows on Rh{011}, the (1 x 1) bulk-truncation form is maintained during reaction with CO. Furthermore, the {012} planes show a tendency to dissolve in favor of growing {137} while no such phenomenon is observed with CO. A PFDMS analysis of the adsorbed layer has led to the detection of large amounts of NO+ at temperatures up to 570 K and reaction times t(R) less than or equal to 10 ms. Thus molecular adsorption is the dominating process under these conditions. From the temperature dependence of the Rh2N2+ intensities an activation energy E-dis = 27 kJ/mol has been determined for N-O bond breaking. The results are discussed in context with reaction studies on Rh single crystal surfaces and polycrystalline foils.