ELECTROCHEMICAL-BEHAVIOR OF RHODIUM IN ALKALINE AND ACIDIC SOLUTIONS OF HEAVY AND REGULAR WATER

The behaviour of rhodium as an electrode for hydrogen (protium and deu terium) and oxygen evolution in both alkaline and acidic, heavy and re gular water solutions has been investigated primarily by cyclic voltam metry. The main features, such as adsorption and underpotential deposi tion of hydrogen (both protium and deuterium) as well as the specific charge capacities first for chemisorbed oxygen and, subsequently, furt her for monolayer (alpha-phase) with subsequent pronounced multilayer (beta-phase) oxide growth with successive increase in oxygen content, preceding hydrogen and oxygen evolution, respectively, with characteri stic desorption peaks, were more or less marked in both electrolytes. Some distinctly different behaviours, however, have been observed reve aling that heavy and regular water behave almost as different solvent ambients. In contrast to some other noble metals (Pt, Pd, Au, Re) and in common with Ir, the hydrogen and oxygen evolving limits for Rh keep their potential values unaltered in alkaline media of both heavy and regular water. Hydrogen absorption, besides adsorption, of both protiu m and deuterium has been clearly marked by the continuously growing ch arge capacity of the diffusional desorption peak, whose extent depends on the evolving rate and contact time of hydrogen evolution and disti nctly exceeds both one-to-one hydrogen to rhodium (H Rh or D/Rh) atom coverage on the exposed electrode surface, and relative to the corresp onding reversible adsorption wave charge area for its underpotential d eposition. In addition, the hydrogen oxidation peak. Immediately follo wing its desorption (in particular from acidic heavy water) has also b een clearly marked on voltammograms. A distinct merging and melding to gether of two initial deuterium reversible desorption peaks into the d iffusional desorption peak in acidic heavy water has also been discern ibly scanned. Oxide formation usually starts at more anodic potentials together with deuterium oxidation and, specifically in acidic media, proceeds vigorously with higher and continuously growing rates and mer ging together with evolving molecular oxygen, while the prevailing oxy gen evolution thereby becomes shifted to more positive potential value s. These features reveal that due to its distinctly different steric f actor, heavy water, in particular in acidic media, behaves as a strong er oxidizing agent than regular water. Some discernible properties of the interplay between hydrogen and oxygen on the rhodium electrode in both electrolytes along the potential axis have been clearly marked an d pointed out. The Rowland or EDTA effect on potentiodynamic and elect rocatalytic features of rhodium has also been scanned and displayed.