Identifying Continuous Series of Elementary Steps in Electrocatalysis by a Novel Isotope Exchange Method
Authors: Riess Ilan 
Published: February 2020

Objective

There is no method to date for determining continuous series of elementary steps in an electrode reaction as well as in oxidation and hydration reactions and also in heterogeneous catalysis. The overall reaction, and in many cases also intermediate steps and species, can be identified but not a whole continuous series of elementary steps.
We suggest a novel method for determining series of elementary steps in surface reactions. It is based on a new way of applying and interpreting the isotope exchange technique.1,2 The method is applicable to the identification of series of elementary steps in electrocatalysis, heterogeneous catalysis and oxidation and hydration reactions.
An outstanding example of a problem for which no answer was found so far is the role of the oxygen vacancy in the cathodic reaction in a solid-oxide-fuel-cell (SOFC). Is the vacancy required for the early step of dissociation of an oxygen molecule or is it involved in the cathodic reaction only at a later step after the molecule is already dissociated?

New Results

We concentrate on the cathodic reaction in SOFCs. The novel method then requires that the isotope to be exchanged is 18O2. The identification is then based on the relation between the rate, r(16O18O), of the production of the mixed molecules 16O18O, the oxygen partial pressure, P(O2) and the concentration of acceptor doping, [A] (if any) in the oxide on which the reaction takes place. We evaluate the theoretical relations for ten plausible continuous reaction series at a cathode in an SOFC and show that all have different relation of the rate r(16O18O) vs. P(O2 and [A] and can thus be identified once the relation is determined experimentally.
Here are two examples of r(16O18O) vs. P(O2) and [A] relations calculated. The first reaction is: O2(gas) + Vad
x +VO,s
●● → O­ad
\ + OO,s
x +3h●(1)
where we have used the Kröger-Vink notation of point defects, Vad
x is a neutral vacant adsorption site, VO,s
●● a doubly charged oxygen vacancy in the surface layer s, O­ad
\ a singly charged, adsorbed oxygen ion, OO,s
x an oxygen ion in the lattice of the surface layer and h● a hole in the valence band. The oxygen molecule dissociates by interaction with an oxygen vacancy in the surface layer. I.e. the oxygen vacancy is imperative for the dissociation process. The rate of evaporation of the mixed molecules 16O18O when the oxide saturated with 16O is suddenly exposed to 18O2 is, in the short time approximation,
r(16O18O) α P(O2)7/4[A]5/2(2)
On the other hand in the following reaction the molecule dissociates on the surface without interaction with an oxygen vacancy:
O2(gas) + 2Vad
x → 2O­ad
\ + 2h●(3)
In this case,
r(16O18O) α P(O2)7/4[A]1/2(4)
Changing the source that provides electronic charges to the oxygen from the valence band to the conduction band changes the rate relation. Thus instead of reaction (1) let us consider the following reaction,
O2(gas) + Vad
x +VO,s
●● + 3e\ → O­ad
\ + OO,s
x(5)
In this case,
r(16O18O) α P(O2)-1/2[A]1/4(6)

Significance

The method is of fundamental significance. The r(16O18O) vs. P(O2) and [A] relations are specific to the reactions shown. This can be demonstrated for two consecutive reactions, as well.1,2 The latter allows to identify a whole series of elementary steps which include dissociation of the oxygen molecule and which ends by a slow step. The fast elementary steps are combined into a first fast reaction which is followed by a slow step as the second reaction. The corresponding r(16O18O) vs. P(O2) and [A] relations can serve to identify all the elementary steps involved in the two reactions.

Acknowledgement

This research was supported by Israel Science Foundation, ISF, under grant No. 699/11.

Reference

I. Riess, Solid State Ionics, 280, 51 (2015).

I. Riess, Solid State Ionics, (in press).