How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Abstract

Development of reversible and stable catalysts for the electrochemical reduction of CO2 is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO2 reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

Fundamental Reaction Limitations for Formic Acid Oxidation

“…(iii) Pt( 111) is more active than Pd(100) in the low overpotential region; however, interestingly, this relationship shifts at higher potentials. (iv) As an observation it is known that there is a difference for Pt and Pd with respect to the CO poisoning during FAOR 8 .The ideal FAOR catalyst on the other hand should show reversible cyclic voltammetry (CV), high activity, low onset potential and stable currents, as illustrated in Figure S2.…”

“…the CO 2 reduction reaction (CO2RR). 7,8 Combining these two reactions allows for a closed carbonloop with formic acid working as a liquid energy storage media. CO2RR to formic acid and the formic acid oxidation reaction (FAOR) can be written in the form:…”

“…FAOR exhibits the highest intrinsic pure metal activity on Pt and Pd. 8,13,14 However, the reaction is affected by high overpotentials and formation of various poisoning intermediates. 15 The following observations can be made in the literature, as shown in Supporting information (SI)…”

Matching emerging formic acid synthesis processes with application requirements

“…Chen et al recently elucidated the mechanism behind this CO poisoning in detail. 6 On Pd, the reaction follows a direct dehydrogenation mechanism such that it does not suffer from CO poisoning. Bifunctional Pd x Pt 1−x or combinations of Pt with Au, Cu, Sn, Pb and Bi combine a high activity with a lower susceptibility to CO poisoning.…”

Pd-Au nanostructured electrocatalysts with tunable compositions for formic acid oxidation

“…On Pt, the FAOR typically undergoes the two oxidation pathways in parallel [16,[22][23][24]. In contrast, on Pd, it generally follows the first pathway, being fully oxidized to CO2 [17,[25][26][27][28]. Studies on different Pd facets and overlayers have revealed that FAOR on Pd is highly active and sensitive to both the catalyst structure and the electrolyte composition [29].…”