Surface reaction temperature dependence

Preocanin, T., Krehula, S.. and Kallay, N., Enthalpy of surface reactions Temperature dependency of pH of acidic or basic concentrated hematite suspension, Appl. Surf. Sci., 196, 392, 2002. 

The apparent activation energy is then less than the actual one for the surface reaction per se by the heat of adsorption. Most of the algebraic forms cited are complicated by having a composite denominator, itself temperature dependent, which must be allowed for in obtaining k from the experimental data. However, Eq. XVIII-47 would apply directly to the low-pressure limiting form of Eq. XVIII-38. Another limiting form of interest results if one product dominates the adsorption so that the rate law becomes... 

Siace surface reactions iavolved with antiwear and EP additives depend not only on the type of mbbiag materials but also oa operatiag temperature, surface speed, and corrosion questions, selection should be carefully iategrated with the oil type, machine design, and operatiag coaditioas. 

Students of professor R. G. Anthony at College Station, TX used a mechanism identical (by chance) to that in UCKRON for derivation of the kinetics. Yet they assumed a model in which the surface reaction controls, and had two temperature dependent terms in the denominator as 13,723 and 18,3 16 cal/mol. Multiplying both the numerator and the denominator with exp(-15,000) would come close to the Ea,/R about 15,000 cal/mol, with a negative sign, and a denominator similar to that in the previously discussed models. 

Hicks (H6) and Frazer and Hicks (F3) considered the ignition model in which exothermic, exponentially temperature-dependent reactions occur within the solid phase. Assuming a uniformly mixed solid phase, the one-dimensional unsteady heat-flow equation relates the propellant temperature, depth from the surface, and time by the nonlinear equation ... 

A and E refer to the desorption, dissociation, decomposition or other surface reactions by which the reactant or reactants represented by M are converted into products. If [M] is constant within the temperature interval studied, then the values of A and E measured refer to this process. Alternatively, if the effective magnitude of [M] varies with temperature, the apparent Arrhenius parameters do not specifically refer to the product evolution step. This is demonstrated quantitatively by the following example [36]. When E = 100 kJmole-1 andA [M] = 3.2 X 1030 molecules sec-1, then rate coefficients at 400 and 500 K are 2.4 X 1017 and 1.0 X 1020 molecules sec-1, respectively. If, however, E is again 100 kJ mole-1 and A [M] varies between 3.2 X 1030 molecules sec-1 at 500 K and z X 3.2 X 1030 molecules sec-1 at 400 K, the measured values of A and E vary significantly, as shown in Fig. 7, when z ranges from 10-3 to 103. Thus, the measured value of E is not necessarily identifiable with the rate-limiting step if a concentration of a participant is temperature-dependent. This... 

In the case of control by surface reaction kinetics, the rate is dependent on the amount of reactant gases available. As an example, one can visualize a CVD system where the temperature and the pressure are low. This means that the reaction occurs slowly because of the low temperature and there is a surplus of reactants at the surface since, because of the low pressure, the boundary layer is thin, the diffusion coefficients are large, and the reactants reach the deposition surface with ease as shown in Fig. 2.8a. 

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care ... 

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. 

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