Field desorption of intermediates in the rhodium carbonyl formation reaction

The formation of Rh carbonyl reaction intermediates resulting from the interaction of gas phase CO with Rh field emitter tip surfaces has been investigated with the imaging atom-probe. A Rh tip was placed in a background of ∼10-7 Torr CO and heated to temperatures between 400 and 500 K. As the carbonyl formation reaction proceeded, surface species were field desorbed and mass analyzed. The primary species detected were singly-charged CO and doubly-charged Rh, Rh(CO), Rh(CO)2, and Rh(CO)3. Measurement of the temperature dependence of the various ion signals indicated that all species except Rh(CO)2++ desorbed with approximately the same activation energy (1.0 to 1.3 eV). This result, combined with a consistent CO/Rh ratio of 3 for all mass scans, suggests that these signals were produced by fragmentation of a Rh(CO)3 parent molecule. The Rh(CO)2++ signal was found to disappear at higher temperatures suggesting that it was due to an intermediate in a stepwise reaction sequence. Over the narrow field range investigated (1.5 to 2.0 V/A), the activation energies for desorption were independent of the applied field. Time-gated images of the various desorbed ions indicated enhanced desorption around the (100) pole, but no real structural sensitivity in the formation of the reaction intermediates could be identified