Oxidation rate temperature dependence

Many inorganic ions are oxidized by FCIO3 in aqueous solution (112). The oxidation rate often depends on the pH of the solution and the temperature. For example the oxidation of KI in the presence of NaHCOs is barely detectable in caustic soda, a slow oxidation occurs and in 0.1 M mineral acid one observes (92) quantitative reaction within 4 hr according... 

It means that in reality the macroradicals are concentrated in a thin layer near the surface of polymer particles. Fig. 8 shows the temperature dependence of the rate constants of oxidation reactions for the three polymers investigated (curves b, c, d). One can see that in the temperature range investigated this dependence is in agreement with the Arrhenius equation. Let us examine the initial sections of oxidation curves. Analysis of the curves shows that they can be represented as a superposition of two different exponents corresponding to two different rate constants of radical oxidation. The temperature dependence of rate constants determined from the initial sections of oxidation curves of polymethyl-metacrylate is shown in Fig. 8 (curve a). Following fact is of interest ... 

Because the reaction takes place in the Hquid, the amount of Hquid held in the contacting vessel is important, as are the Hquid physical properties such as viscosity, density, and surface tension. These properties affect gas bubble size and therefore phase boundary area and diffusion properties for rate considerations. Chemically, the oxidation rate is also dependent on the concentration of the anthrahydroquinone, the actual oxygen concentration in the Hquid, and the system temperature (64). The oxidation reaction is also exothermic, releasing the remaining 45% of the heat of formation from the elements. Temperature can be controUed by the various options described under hydrogenation. Added heat release can result from decomposition of hydrogen peroxide or direct reaction of H2O2 and hydroquinone (HQ) at a catalytic site (eq. 19). 

Cementation coatings rely on diffusion to develop the desired surface aUoy layer. Not only does the coating continue to diffuse into the substrate during service, thereby depleting the surface coating, but often the substrate material diffuses into the surface where it can be oxidized. Because the diffusion rate is temperature dependent, this may occur slowly at lower service temperatures. 

The stmcture of residual char particles after devolatilization depends on the nature of the coal and the pyrolysis conditions such as heating rate, peak temperature, soak time at the peak temperature, gaseous environment, and the pressure of the system (72). The oxidation rate of the chat is primarily influenced by the physical and chemical nature of the chat, the rate of diffusion and the nature of the reactant and product gases, and the temperature and pressure of the operating system. The physical and chemical characteristics that influence the rate of oxidation ate chemical stmctural variations, such as the... 

Section 1.9 showed that as long as an oxide layer remains adherent and continuous it can be expected to increase in thickness in conformity with one of a number of possible rate laws. This qualification of continuity is most important the direct access of oxidant to the metal by way of pores and cracks inevitably means an increase in oxidation rate, and often in a manner in which the lower rate is not regained. In common with other phase change reactions the volume of the solid phase alters during the course of oxidation it is the manner in which this change is accommodated which frequently determines whether the oxide will develop discontinuities. It is found, for example, that oxidation behaviour depends not only on time and temperature but also on specimen geometry, oxide strength and plasticity or even on specific environmental interactions such as volatilisation or dissolution. 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

The oxidation of nitric oxide, NO, is a reaction involved in smog production. It is moderately rapid at normal temperatures. The oxidation of methane, CHt (household gas), however, occurs so slowly at room temperature that we may say that, for all practical purposes it doesn t react at all. Again, the difference in the reaction rates must depend upon specific characteristics of the reactants, NO and CH,. 

Conversely, the use of elevated temperatures will be most advantageous when the current is determined by the rate of a preceding chemical reaction or when the electron transfer occurs via an indirect route involving a rate-determining chemical process. An example of the latter is the oxidation of amines at a nickel anode where the limiting current shows marked temperature dependence (Fleischmann et al., 1972a). The complete anodic oxidation of organic compounds to carbon dioxide is favoured by an increase in temperature and much fuel cell research has been carried out at temperatures up to 700°C. 

Figure 10.7 shows the temperature dependence of CO oxidation rate on a rhodium surface, as reported by Bowker et al. It shows that the rate of reaction maximizes when both reactants, adsorbed CO and O, are present in comparable quantities at a temperature where the activation barrier of the reaction can be overcome. 

Part 1. Kinetics and Energetics of Dry Oxidation. The simplest approach to data analysis is to assume that only a single class of oxidation reactions is important and to make the related assumption that the temperature dependence of the single rate constant k can be represented by an Arrhenius equation. In this way we obtain... 

---