Analytical relations between reaction order and tafel slope derivatives for electrocatalytic reactions involving chemisorbed intermediates

It is shown that characteristic relations between reaction order R and Tafel slope derivatives b can be deduced for the two principal desorption types of pathways, “electrochemical” and “recombination”, common in electrocatalytic reactions involving a single chemisorbed intermediate leading to dimeric product, eg H2, Cl2. The experimentally measurable R usually differs from the kinetically-significant surface reaction order in heterogeneous processes through the logarithmic derivative d ln θ/d ln c of the absorption isotherm function θ(c), where θ is the fractional coverage of the reaction surface by chemisorbed intermediates and/or reagent species. Cases where significant lateral interaction effects arise are considered and distinguished from those for which Langmuir adsorption behaviour obtains. Results based on the quasi-equilibrium hypothesis in the treatment of consecutive electrochemical reaction kinetics are distinguished from those derived by the steady-state method; simple factors arise which relate the results for R and for b derived by the two methods. The dependences of R and b values for the two desorption mechanisms on coverage, θ by the kinetically involved intermediate are evaluated and exhibit usefully distinguishable characteristics. Applications to some selected experimental results are discussed with regard to conclusions about reaction mechanisms based on evaluation of R in relation to b values.