A detailed investigation has been carried out into the effect of annealing RuO, xH,O at different temperatures on its ability to mediate the oxidation of water, and its stability towards anodic corrosion. The percentage corrosion of the heated samples when exposed to a strong oxidant such as CeIV ions decreased with increasing annealing temperature, and at temperatures >/ 140 "C no corrosion was observed, only 0,-catalytic activity. This effect was associated with a decrease in percentage H,O content of the samples owing to a reduction in the number of defect sites present. Raising the annealing temperature of the powders from 140 to 900 "C caused a concomitant drop in catalytic activity as a result of an observed decrease in crystallite size and surface area available for catalysis measured by X-ray diffraction and transmission electron microscopy-techniques. Thus an annealing temperature of 140-150 "C for the RuO;xH,O samples represented an optimum for catalytic activity where corrosion was absent. When this material was used to catalyse the oxidation of water by CerV ions, it retained its activity after repeated use and was able to generate an amount of 0, which was 16 times larger than that of the catalyst (in molar terms). In addition, this catalyst was found to mediate the oxidation of water by many other strong oxidants such as BrO;, MnO;, IO;, PbO, and MnO, (activated).

No abstract

A 'test system' has been developed in order to investigate the corrosion stabilities and 0,-catalytic activities exhibited by samples of RuO, * xH,O when exposed to a strong oxidant such as Cexv ions. The RuO;xH,O samples included commercial powders as well as one prepared in the laboratory. The majority of these samples, including the prepared samples, were unstable towards anodic corrosion ( x 86 YO corrosion) and showed little 0, catalytic activity (0, yields 8%). The small amount of 0, evolved was believed to originate from water bound in the oxide powder. These RuO, -xH,O samples appeared, from thermogravimetric analysis (t .g.a.) and differential gravimetric analysis (d. t .g.a.) measurements, to be highly hydrated (H,O content = 24-26 %). A minority of commercial RuO, .xH,O powders, when analysed by t.g.a. and d.t.g.a., were found to be in a partially dehydrated form (H,O content = 18%). These samples showed a much greater resistance towards corrosion (9-1 3 % corrosion) and were able to mediate the oxidation of water (0, yield = 92 YO) by CeIV ions. The corrosion of Ru0,-xH,O by other oxidants, e.g. BrO;, MnO; or PbO,, was also studied and the results are discussed. The implications of these findings are considered. Ruthenium dioxide hydrate (RuO, xH,O) has, for several years, found frequent use as a catalyst for the oxidation of water by a strong oxidant (S+), i.e. RuO,. zH,O 4S+ + 2H,O -4s +4H+ + 0,where S+ = Ce4+, Ru(bipy)i+ or Fe(bipy)i+. True catalysts for reaction (1) are rare, however, since most materials either (i) undergo some degree of anodic corrosion themselves when acting as a catalyst or (ii) are stable towards anodic corrosion but inactive as a catalyst. An '0,-catalyst ' would find application in many different areas, including industrial electrochemistry and solar-energy conversion.A major approach of solar-energy conversion is the development of a photochemical system capable of driving the cleavage of water1 into H, and O,, i.e. photochemical system 2H,O ' 2H,+0,. sunlight An integral part of an efficient photochemical system for reaction (2) would be a stable, fast-acting and specific O,-cataly~f.l-~ The recent of RuO,.xH,O in this role seemsto be due to a misplaced trust in its high corrosion stability and catalytic activity, and this trust appears to arise from two major sources.

No abstract

No abstract