The Role of Reactive Reaction Intermediates in Two‐Step Heterogeneous Electrocatalytic Reactions: A Model Study

Abstract

This paper summarises the result of previous experimental investigations of heterogeneous electrocatalytic reactions performed in flow cells which provide an environment with controlled parameters. Measurements of the oxygen reduction reaction in a flow cell with an electrode consisting of an array of Pt nanodisks on a glassy carbon substrate exhibited a decreasing fraction of the intermediate H2O2 in the overall reaction products with increasing density of the nanodisks. A similar result is true for the dependence on the catalyst loading in the case of a supported Pt/C catalyst thin‐film electrode, where the fraction of the intermediate decreases with increasing catalyst loading. Similar effects have been detected for the methanol oxidation.In order to give a possible explanation to the observed effect, we present a model of multistep heterogeneous electrocatalytic oxidation and reduction reactions based on an adsorption‐reaction‐desorption scheme using the Langmuir assumption and macroscopic transport equations. A continuum based model problem in a vertical cross‐section of a rectangular flow cell is proposed in order to explain basic principles of the experimental situation. It includes three model species A, B, C, which undergo adsorption and desorption at a catalyst surface, as well as adsorbate reactions from A to B to C. These surface reactions are coupled with diffusion and advection in the Hagen Poiseuille flow in the flow chamber of the cell. High velocity asymptotic theory and a finite volume numerical method are used to obtain approximate solutions to the model. Both approaches show a behaviour similar to the experimentally observed. Working in more general situations, the finite volume scheme was applied to a catalyst layer consisting of a number of small catalytically active areas corresponding to nanodisks. Good qualitative agreement with the experimental findings is established for this case as well.

The impact of spectator species on the interaction of H2O2 with platinum – implications for the oxygen reduction reaction pathways

“…[26][27][28][29][30][31][32][33][34] An increased H 2 O 2 formation (or a lower ''effective'' number of electrons transferred) has been additionally observed either when the loading of high-surface-area Pt catalysts is substantially decreased or when oxygen mass transport rates are very high (e.g., by the use of microelectrodes). 7,12,[35][36][37][38][39][40][41][42] These studies indicate that under these conditions, the ORR proceeds (at least in part) through the formation and desorption of H 2 O 2 , a fraction of the latter being re-adsorbed and eventually further reduced to water. Proving or disproving that the desorption-re-adsorptiondissociation scheme 36,38 is universal for the ORR would certainly be a major milestone for the understanding of the ORR.…”

“…7,12,[35][36][37][38][39][40][41][42] These studies indicate that under these conditions, the ORR proceeds (at least in part) through the formation and desorption of H 2 O 2 , a fraction of the latter being re-adsorbed and eventually further reduced to water. Proving or disproving that the desorption-re-adsorptiondissociation scheme 36,38 is universal for the ORR would certainly be a major milestone for the understanding of the ORR.…”

Effect of mass transfer on the oxygen reduction reaction catalyzed by platinum dendrimer encapsulated nanoparticles

“…A "desorption + readsorption + reaction" mechanism was proposed to explain this effect (26,30). Under this assumption, more H 2 O 2 is obtained at high k t (low catalyst loading or high flow rates), due to lower probability for H 2 O 2 readsorption and further reaction on different Pt sites (Scheme 1) (26,30). Kucernak and coworkers studied the effect of high k t conditions (up to 2 cm s −1 ) on the ORR catalyzed by single, submicron Pt particles affixed to the ends of nanoelectrodes (3).…”

“…The key finding was that the amount of electrogenerated H 2 O 2 increased with decreasing catalyst coverage. A "desorption + readsorption + reaction" mechanism was proposed to explain this effect (26,30). Under this assumption, more H 2 O 2 is obtained at high k t (low catalyst loading or high flow rates), due to lower probability for H 2 O 2 readsorption and further reaction on different Pt sites (Scheme 1) (26,30).…”