CO oxidation on stepped Rh[ n (1 1 1) × (1 1 1)] single crystal electrodes: a voltammetric study

Abstract

The CO electrooxidation reaction has been studied on Rh[ n(1 1 1) × (1 1 1)]-type electrodes in 0.5 M H 2SO 4 using cyclic voltammetry. Examination of the blank cyclic voltammograms (BCVs) of the electrodes under investigation [Rh(1 1 1), Rh(5 5 4), Rh(5 5 3) and Rh(3 3 1)] shows that a distinction can be made between the electrodes on the basis of their voltammetric profiles. An increase in reversibility of the peaks in the hydrogen/(bi)sulfate region and an increase in the potential at which surface oxidation occurs, has been observed for decreasing step density. Higher reversibility of the peaks in the low-potential region therefore indicates more steps and/or defects on the surface. Cycling the potential to within the oxidation region leads to disordering of the surface, which, unlike results reported previously, can be identified in the BCV. Calculations on the hydrogen/(bi)sulfate adsorption/desorption charge seem to indicate that (bi)sulfate prefers adsorption on the terraces and not on the (1 1 0) steps. The fact that disordering of the surface leads to lower charges in the hydrogen/(bi)sulfate region, supports this notion. Both CO stripping and also bulk CO oxidation experiments showed a pronounced effect of the surface structure on the reaction rate. In general, the activity of the electrodes increases for increasing step density. Also disordering of the surface increases the activity, due to an increasing number of defects. For CO stripping experiments, on well-ordered electrodes a pre-shoulder is observed prior to the main oxidation peak, which can be ascribed to CO oxidation next to or at the steps. The presence of the pre-shoulder as well as a pronounced tailing of the main oxidation peak suggests that diffusion of CO on rhodium may be slow.