Apparent activation energies on catalysts with heterogeneous surfaces: II. Dual-site catalysts and compensation effect

Abstract

The apparent activation energy has been calculated for reactions over a catalyst with two types of site each with true activation energy E 1 or E 2. These calculations have been made for different ratios of type-1/type-2 sites, temperature ranges from 323–1023 °K, values of E 2 − E 1 up to 20 kcal/mole, and several levels of experimental accuracy in the determination of log (apparent rate constant). The apparent activation energy is shown to be very dependent upon the temperature range used in its determination. For example, the apparent activation energies for the same reaction over the same catalyst determined at temperatures of 423–523 °K or 723–823 °K can differ by as much as 9 kcal/mole. Despite the presence of two types of site, the apparent rate constant will still appear to obey the Arrhenius equation under conditions typical of most catalytic measurements. Only under extreme conditions will curvature be detectable. If the true activation energies and preexponential factors are related in such a way as to exhibit the compensation effect, i.e., B 1 exp ( −E 1 RT c ) = B 2exp ( −E 2 RT c ) where T c = temperature at which the rates are equal on both types of site, then a series of catalysts in which the ratio of type-1/type-2 sites is varied will also show the compensation effect. The catalytic behaviour of some binary alloys is in agreement with this conclusion.