Urea adsorption at rhodium single-crystal electrodes

Structural aspects for the adsorption of urea at rhodium electrodes have be en studied in perchloric acid solutions by combining electrochemical and in -situ FTIR spectroscopy experiments with single-crystal surfaces. Urea mole cules adsorb at the rhodium electrode surface in competition with hydrogen and oxygenated species. Potential-dependent coverage changes, similar to th ose typically observed with specifically adsorbed anions, have been found t o be structure-sensitive. Strong adsorption of urea molecules at the Rh(100 ) electrode surface allows the isolation of an irreversibly adsorbed layer that can be characterized in urea-free solutions. The infrared spectra coll ected for each rhodium surface show different adsorption bands for the adso rbed urea molecules that can be related to different orientations and/or ad sorption sites. Urea molecules seem to adsorb always at the Rh(100) surface through the two nitrogen atoms with the C-O axis having a nonzero componen t perpendicular to the electrode surface. Only O-bonded urea molecules are detected at the Rh(lll) electrode surface. Potential-dependent changes for the intensity of the C-O stretching band can be understood on the basis of the tilting of the C-O axis at potentials higher than 0.50 V. In the case o f the Rh(110) electrode, O-bonded and N-bonded oriented molecules predomina te at low and high potentials, respectively. This change in the orientation of the adsorbed urea molecules can be related to the competitive adsorptio n of oxygenated species.