Metal oxidation, kinetics

G I Golodets, Possible causes of nonstationary phenomena in gas phase radical chain oxidation in the presence of transition metal oxides, Kinet Catal (Engl) 28 1074 (1987)... 

Corrosion of310 SS was studied in the presence ofNaClandNaS04 salt mixtures at 750 C [28]. Five different mixtures ofNaCl/Na2S04 with the following wt% were used 100/0,75/ 25, 50/50,25/75, and 0/100. The salts were apphedto specimens of 310 SS and placed in air at 750 C. Figure 11.19 shows parabolic metal oxidation kinetics for the control specimen with no applied salt (labeled oxidation in the legend) after about 3 h [28]. The specimens that underwent hot corrosion exhibited various types of rate laws for their oxidation. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

The aforementioned inconsistencies between the paralinear model and actual observations point to the possibility that there is a different mechanism altogether. The common feature of these metals, and their distinction from cerium, is their facility for dissolving oxygen. The relationship between this process and an oxidation rate which changes from parabolic to a linear value was first established by Wallwork and Jenkins from work on the oxidation of titanium. These authors were able to determine the oxygen distribution in the metal phase by microhardness traverses across metallographic sections comparison of the results with the oxidation kinetics showed that the rate became linear when the metal surface reached oxygen... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

There are few studies in the literature on the kinetics and mechanism of oxidation over base metal oxides. Blumenthal and Nobe studied the oxidation of CO over copper oxide on alumina between 122 and 164°C. They reported that the kinetics is first order with respect to CO concentration, and the activation energy is 20 kcal/mole (77). Gravelle and Teichner studied CO oxidation on nickel oxide, and found that the kinetics is also first order with respect to CO concentration (78). They suggested that the mechanism of reaction is by the Eley-Rideal mechanism... 

The kinetics of oxidation over noble metals is dramatically different and much more complex. Every chemical species has an inhibiting effect on the rate of oxidation of another species. Carbon monoxide is a particularly strong self-poison, so that its oxidation kinetics usually proceeds at a negative order with respect to CO concentration. The kinetics also... 

The kinetic activities of noble metals and of base metal oxides are complementary, so that a mixture of the two would perform better than each class of material alone. We have already observed in Fig. 16 that noble metals have superior activity at high temperatures but base metal oxides have superior activity at low temperatures. Since the CO oxidation kinetics is negative first order with respect to CO over platinum but first order with respect to CO over copper chromite, the rates must be faster over platinum at low CO concentration but the reverse is true at high CO concentrations, as shown in Fig. 19. 

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

The properties of barrier layers, oxides in particular, and the kinetic characteristics of diffusion-controlled reactions have been extensively investigated, notably in the field of metal oxidation [31,38]. The concepts developed in these studies are undoubtedly capable of modification and application to kinetic studies of reactions between solids where the rate is determined by reactant diffusion across a barrier layer. 

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... 

There have been relatively few detailed kinetic studies of the decompositions of metal acetates, which usually react to yield [1046] either metal oxide and acetone or metal and acetic acid (+C02 + H2 + C). Copper(II) acetate resembles the formate in producing a volatile intermediate [copper(I) acetate] [152,1046,1047]. 

Decompositions of crystalline mixed hydroxides to mixed oxides often occur at temperatures lower than those required to produce the same phases through the direct interaction of metal oxides. This route thus offers an attractive approach for the preparation of catalysts of high area and activity [1147]. Detailed kinetic investigations comparable with those for the dehydroxylations of a number of pure hydroxides (Sect. 2.1) are not, however, available. 

Davies, S. and Morgan, J. J. (1989). Manganese (II) oxidation kinetics on metal oxide surfaces, /. Colloid... 

The complex Pd-(-)-sparteine was also used as catalyst in an important reaction. Two groups have simultaneously and independently reported a closely related aerobic oxidative kinetic resolution of secondary alcohols. The oxidation of secondary alcohols is one of the most common and well-studied reactions in chemistry. Although excellent catalytic enantioselective methods exist for a variety of oxidation processes, such as epoxidation, dihydroxy-lation, and aziridination, there are relatively few catalytic enantioselective examples of alcohol oxidation. The two research teams were interested in the metal-catalyzed aerobic oxidation of alcohols to aldehydes and ketones and became involved in extending the scopes of these oxidations to asymmetric catalysis. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Andersen et al. predicted that similar results would be expected for the corrosion of other multivalent metals oxidizing via lower oxidation states. They also pointed out that their interpretation was consistent with the kinetics of the corrosion of copper in oxygenated HCl solutions. Here the final product is Cu and thus there is no vulnerable intermediate. In consequence, the rate of copper dissolution from either Nj-saturated or 02-saturated HCl solutions was the same at a given potential in conformity with the additivity principle. 

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