Cobalt oxide surface reduction rate

Few studies have systematically examined how chemical characteristics of organic reductants influence rates of reductive dissolution. Oxidation of aliphatic alcohols and amines by iron, cobalt, and nickel oxide-coated electrodes was examined by Fleischman et al. (38). Experiments revealed that reductant molecules adsorb to the oxide surface, and that electron transfer within the surface complex is the rate-limiting step. It was also found that (i) amines are oxidized more quickly than corresponding alcohols, (ii) primary alcohols and amines are oxidized more quickly than secondary and tertiary analogs, and (iii) increased chain length and branching inhibit the reaction (38). The three different transition metal oxide surfaces exhibited different behavior as well. Rates of amine oxidation by the oxides considered decreased in the order Ni > Co >... 

Kinetic runs in step b in Fig. 8c started with a very fast reduction of approximately e per molecule, after which a slow reductioh took place, yielding sigmoidal reduction curves. This, indicates that reduction of Co2+ to Co° is controlled by the formation and slow growth of reduction nuclei of metallic cobalt on. the surface of the reduced phase in step a (nucleation model). Initially, the reduction rate increases because of the growth of nuclei already formed and the appearance of new ones. At a certain point the reduction nuclei start to overlap at the inflection point, the interface of. the oxidized and reduced phases and the reduction rate both begin to decrease. Reduction of this type is described by the Avrami-Erofeev equation (118)... 

To further test this hypothesis freshly-reduced catalysts were reacted with high-pressure steam (5 atm). A significant loss of BET surface area (from 215 to 188 mVg) is observed after Co/Davisil was reduced at 1 atm and reacted with 5 atm steam for 24 h (see Table 3). Increasing the space velocity by a factor of four also increases the rate of BET surface area loss (from 12.5 % / 24 h to 39.0 % / 24 h). Extents of reduction of cobalt oxide to cobalt metal before and after steam treatment are shown in Table 3. After steam treatment the cobalt oxide-support interaction is apparently substantially increased, i.e., the fraction of cobalt reduced to the metal at 400°C decreases from 89 to 4% moreover, the amount of cobalt-silicates (as inferred from TPR spectra shown elsewhere [22, 23]) also increases after steam treatment. This latter observation is consistent with the substantially higher extent of reduction of these catalysts (71-72%) at 750 C, a temperature at which a significant fraction of cobalt silicate can be reduced to the metal. 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

TPR profiles were also run on samples which were pre-reduced in situ for 2 hr at 400°C with a H2 flow rate of 5 ml/min. In general, cobalt associated with the reduction phases I and II was pre-reduced, while that associated with phase III reduction did not reduce. However, in addition, a substantial metal support interaction occurred to give n oxidic species especially on the lew surface area Si02 and Ti02 supports. 

The first surface challenge can be addressed by strong oxidation conditions [46] and careful hematite preparation, but the slow water oxidation kinetics are probably intrinsic to hematite. Nevertheless, methods have recently been found to increase the oxidation rate and thus reduce the overpotential. For example, the water oxidation by cobalt has been extensively studied and is known to be particularly rapid [114]. Indeed the treatment of Fe203 photoanodes (prepared by APCVD) with a monolayer of Co " resulted in a ca. 0.1 V reduction of the photocurrent onset potential [105]. Since this treatment also increased the plateau photocurrent it is good evidence that the reaction rate was increased, and the Co " did not just fill surface traps. Following the report of a remarkably effective cobalt-phosphate (Co-Pi)- based water oxidation catalyst [115], the overpotential was reduced even further on hematite photoanodes by Gamelin and coworkers [116]. Their results are shown in Fig. 4.11. 

In the first case, it is possible to identify a maximum rate this profile is typical of auto-catalysed reactions. In the second case, the rate of reaction decreases continuously until the reaction process is completed as there is a continuous decrease of the metal /oxide interface. It is common, in catalysis, to have a supported system that may exhibit a different reductive behaviour in comparison to unsupported metal oxides due to possible interactions between the metal and the support. The metal/support interactions may modify the reaction mechanism, promoting the atom diffusion on the surface of supported metal oxides or inhibiting the reduction process. This last is the case of cobalt supported on alumina, where cobalt aluminate, that is a system very difficult to be reduced. 

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