Reduced surface excess enthalpy

Just as for gas adsorption, we can define a partial differential enthalpy of adsorption of a component, A ads/i(, which would correspond to the adsorption, from a solution of molality b, of an infinitesimal amount of component i, dn, on a solid surface already covered with solute at a reduced surface excess concentration J (n) ... 

Figure 3 shows the enthalpy reduction experimentally observed in three different powders made with MgHa nanoparticles [7]. It also shows the relative importance of the three different nanostructures studied in this paper. It is clear from the data that the particle size and size distribution are not enough to predict the enthalpy of formation and that deformed regions with excess enthalpy can explain the experimental observation. Figure 3 also illustrates that surface and grain boundary effects do not reduce the enthalpy sufficiently and that the excess volume effect is needed to explain the observed reduction in the enthalpy of formation. 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

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