Kinetics of chemisorption. Deducing kinetic laws from experimental data

Abstract

The kinetics of chemisorption have been inadequately represented in the literature by second-order equations and by Elovich equations. The conditions at which non-valid equations become apparently applicable are discussed. Assumed isotherms are often congruent to the experimental one at a limited range of coverage and predict a correct value for the rate dq/dt and for the change of rate d²q/dt² in that range; however, they have been applied beyond the range of validity by using plotting procedures that allow the location of the region of congruency to be adjusted and the effect of data outside this region to be attenuated. The second-order rate equation q_∞/(q_∞–q)=αt+ 1 where q_∞ and α are constants, is congruent to the Elovich equation when q≈ 0.5q_∞. Reciprocally, the Elovich equation q=c+(1/b)ln t where b and c are constant, is indistinguishable from a second-order equation when q≈ 2/b. Both equations can be fitted at certain conditions to data obeying the equation q=kt^1/v where k and v are constants. Differential analysis shows that the experimental isotherms generally obey a complex function for which z=(dq/dt)^–1 is sigmoid.