| Popis: |
About ten years ago, some catalytic reactions were termed “demanding reactions” by Boudart who three years later also introduced the concept of “structure sensitive reactions”. As time passed, the two terms were confounded because the reactions chosen to exemplify the concept were both structure sensitive and demanding (like hydrogenolysis) or structure insensitive and facile (like hydrogenation and H2 - D2 exchange). However, discrimination between the terms must be made for the isotopic exchange reaction between alkanes and D2, which according to the original definition is demanding (very large differences in activity of different metals) but is structure insensitive (small changes in activity due to alloying and dispersion). The question arises whether the demanding character of a reaction is related simply to the surface structure of the catalyst. The kinetic equations derived from a model of “active site” for alkane chemisorption, that we have recently introduced, account quantitatively for the H2 pressure effect measured on the adsorption rate of alkanes. The large variations of the H2 pressure effect as well as the large variations of the temperature factor with pressure range and temperature range are also accounted for. These variations, associated with the surface coverage function G in the adsorption rate-equation, are similar in value and sign, to those observed for catalytic reactions like hydrogenolysis, isomerisation. Since a large number of potential sites is required to form the active site for alkane chemisorption, G is very sensitive to hydrogen coverage and varies over many orders of magnitude with experimental conditions. As G is dependent on the thermodynamics of adsorption, it contains a demanding character that is to be compared with the one observed when comparing the activity for exchange on different metals. A function of coverages by chemisorbed radicals in equilibrium with the gaseous reactants, at a very-high power, must be written in the kinetic rate equation of a catalytic reaction not only for kinetic reasons but also to express equilibrium requirements. These equilibrium requirements are related or to the degree of dehydrogenation of the most abundant surface intermediate or to the size of the chemisorbed radicals. Effects of structure should be differentiated from those of coverages as they may be of similar magnitude. But the surface structure of the metal is important for the thermodynamics of adsorption. The effects of structure and of coverages are thus interrelated. Demanding character and structure sensitivity as derived from the only catalytic activity measurements is not sufficient as information to make correlations with the metal structure. The thermodynamics of adsorption of the radicals in equilibrium with the gaseous reactants should also be determined (mainly the pressure and temperature dependence of the coverages). Since the literature does not describe any system for which all this information is simultaneously available, at this time we must restrict ourselves to a qualitative analysis of the effects of structure (alloying and dispersion) as compared with the effects of coverages. |
