The first stages of the anodic oxidation of polycrystalline copper electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Cu20 was found to be preceded by electrosorption of oxygen species, that occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. This surface oxide was formed via random electrodeposition. The width of the first current peak indicates the presence of lateral attractive interactions in the chemisorbed layer. The surface layer did not show any ageing effect.
The ~teractions of 02 and N20 in the low pressure range with a Cu(ll1) surface and of CO with adsorbed oxygen have been studied with ellipsometry, Auger electron spectroscopy and LEED. The adsorption of 02 was investigated in the 10-6-104Torr range and at crystal temperatures ranging from 23 to 4OO'C. 02 chemisorbs dissociatively with an initial reaction probability of about 10m3 and an apparent activation energy of 2-4 kcal/mol, which depends on the substrate temperature, up to a saturation coverage of 0.45. The probability of decomposition of N20 is 10ms at 300°C, and the activation energy is 10.4 kcal/mol For 250 < T< 4OO'C. The oxygen coverage saturates at B = 0.45 as well. For both oxidation reactions the kinetics can be described with a precursor state model. With LEED no superstructures were observed. The probability of the reaction of CO with adsorbed oxygen is 4 X lo-' at 250°C and is initially independent of the oxygen coverage. The reaction is assumed to proceed via a Langmuir-Hinshelwood mechanism. The activation energy for the reaction COad + oad -+ CO* is 18-20 kcal/mol.
The first stages of the oxidation of polycrystalline silver electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Ag20 was found to be preceded by dissolution of silver species and deposition of a surface oxide. The equilibrium oxide coverage depended on the electrode potential and occurred within a few seconds Surface oxide formation probably took place via a process of random electrodeposition. No ageing effect was observed in the chemisorbed layer.(1) INTRO DUCTION The use of silver oxide electrodes in storage batteries has stimulated many investigations of the anodic oxidation of silver in alkaline electrolytes. Nevertheless, much remains to be discovered about the oxidation mechanisms and the structure of the silver oxides formed. (See refs. 1--12 for reviews.)Cyclic voltammograms ( Fig. 1) generally show four peaks in the anodic direction (A1 to A4) and two in the cathodic direction (C3 and C4). A3 and A4 are related to the formation of Ag20 and AgO and C4 and C3 are assigned to reduction of AgO and Ag20, respectively [11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The small peak A2 has been variously attributed to the formation of AgOH [13,15], to oxidation of adsorbed hydrogen [ 18], to preferential oxidation of an activated lattice [19,25], and to the dissolution of silver as Ag(OH)~ with diffusion of the product into the solution [22,23].The minor peak A1 has attracted relatively little attention. Stonehart and Portante [15,16] took it to be an artefact arising from the method of metal fabrication, because this peak was not observed upon oxidation of silver, electrodeposited from a plating bath. B~ezina et al. [28], rather surprisingly, stated that at potentials which are 200 to 700 mV more negative than that of the A1 peak, surface oxides are formed. According to them the A1 peak is related to formation of a larger amount of Ag20. Giles et al. [29] concluded from impedance studies that the initial step in the oxidation, at potentials negative to the reversible Ag/Ag20 potential (140 mV vs. Argenthal, in 1 M NaOH) is the formation of a soluble species Ag(OH)~ which diffuses away from the electrode. They stated that this was followed by phase formation on the electrode which
Ellipsometry, LEED and Auger electron spectroscopy have been used to study the interactions of Oa and NzO with a clean annealed Cu(ll0) surface and the reaction of CO with adsorbed oxygen in the monolayer range. Gas pressures were in the range 1O-8-1O-4 Torr and crystal temperatures varied between 23-4OO'C. The changes in the ellipsometric angles A and $I per oxygen atom upon adsorption and removal of oxygen depend on the coverage 0, the temperature and on the azimuth of the plane of incidence of the light beam. The kinetics of the chemisorption of oxygen is independent of the crystal temperature, initial sticking probabiiity ~0.2. The LEED data and the adsorption kinetics indicate an attractive interaction in the adsorbed layer in the [OOl] direction. The initial decomposition probabi~~y of NaO at room temperature is 0.15 and decreases with increasing temperature; the maximum coverage is 0.5 monolayer. The LEED patterns observed were the same as those with 0s. The reaction probability of CO with adsorbed oxygen increases with decreasing oxygen coverage (order of mag nitude -1O-5, apparent activation energy -6 kcaI/mol). This increase has been attributed to the operation of the Langmuir-Hinshelwood mechanism.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.