Mechanism and Micro Kinetic Model for Electroreduction of CO₂ on Pd/C: The Role of Different Palladium Hydride Phases

Abstract

We measured the reaction kinetics of the electrochemical reduction of CO2 to formate on Pd/C and evaluated several proposed mechanisms, by comparing model-described and observed rates of formation of HCOO– and H2 as a function of several parameters (p H2 , p CO2 , and potential). An α-/β-hydride mechanism, based on an α-hydride phase active for CO2 reduction and a β-hydride Pd-phase active for hydrogen evolution, described the experimental data of our and other laboratories reported in the literature satisfactorily. After parametrization, using a data set including only H2, this mechanism also predicted the outcome of D2 isotope labeling experiments correctly. Analyses of the results indicate that the α-/β-hydride ratio and hydride formation rate are key factors affecting the formate production rate and the selectivity, thereby identifying areas for further (spectroscopic) studies and mechanism validation.

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

“…Based on these results, it is generally accepted in the literature that CO 2 -electroreduction on Pd-based electrocatalysts proceeds on the different PdH x -phases formed across the relevant potential range. − ,, As an example of this, Gao et al hypothesized that, in the Pd-nanoparticle electrocatalysts employed in CO 2 -electroreduction, the pure α- or mixed α/β-phases formed at low overpotentials ( vide supra ) are the active species leading to high CO 2 -to-formate selectivities, while the pure β-phase formed at high overpotentials is responsible for the high CO-yields preponderantly found under these conditions. Along these lines, the α/β-hydride ratio (and formation rate thereof) has been shown to affect the product selectivity at low overpotentials . In contrast to this, Min et al and Rahaman et al have postulated that the β-phase already forms at low overpotentials and that the hydride in the nanoparticle’s bulk participates in the generation of formate via a hydrogenation mechanism similar to the one described in heterogeneous catalysis. − …”

Intercalated Gold Nanoparticle in 2D Palladium Nanosheet Avoiding CO Poisoning for Formate Production under a Wide Potential Window

“…40−44 α-PdH x , in which hydrogen is covalently bonded to palladium, is the specifically active phase for formate production via forming the HCOO* intermediate. 40,44,45 Unfortunately, the vulnerable surface of Pd due to CO poisoning 38,46,47 constrains the formate production within a narrow potential window. In detail, the strong affinity between Pd and CO, as well as poor CO desorption, could possibly lead to the destruction of the α-PdH x phase, giving rise to a shift in intermediate selectivity from HCOO* to COOH* and poor operating lifetime under an applied potential.…”

“…Until now, enormous efforts have been made toward designing metal electrocatalysts, such as Pd, , Bi, − Sn, , In, , Pb, , etc., for efficient formate/formic acid production. In particular, palladium-based nanomaterials are thought as promising candidates to produce formate through CO 2 RR at a near-equilibrium potential of 0 to −0.2 V versus reversible hydrogen electrode (RHE) ( V RHE ) with the selectivity reaching above 90%. ,− Recent studies show that palladium hydride (α-PdH x and β-PdH x ) is the active phase during the CO 2 RR electrolysis. − α-PdH x , in which hydrogen is covalently bonded to palladium, is the specifically active phase for formate production via forming the HCOO* intermediate. ,, Unfortunately, the vulnerable surface of Pd due to CO poisoning ,, constrains the formate production within a narrow potential window. In detail, the strong affinity between Pd and CO, as well as poor CO desorption, could possibly lead to the destruction of the α-PdH x phase, giving rise to a shift in intermediate selectivity from HCOO* to COOH* and poor operating lifetime under an applied potential. ,,, Therefore, avoiding strong CO adsorption or even CO production on the Pd surface is particularly important for developing an efficient Pd-based catalyst for the production of formate.…”

Application of machine-learning-based global optimization: potential-dependent co-electrosorbed structure and activity on the Pd(110) surface

“…The interaction and reaction of electrosorbates is of fundamental scientific and technological importance in electrochemistry. 1,2 Palladium, as a unique material for the hydrogen storage, [3][4][5] has long demonstrated that hydrogen can easily adsorb on the surface and then permeate into the metal sublayer. [6][7][8][9] It is naturally expected that H and reactive species could occur on the surface simultaneously at the reduction potential, where the co-electrosorbed species further regulate the electrocatalytic reaction, such as the N 2 reduction reaction (NRR).…”

“…[6][7][8][9] It is naturally expected that H and reactive species could occur on the surface simultaneously at the reduction potential, where the co-electrosorbed species further regulate the electrocatalytic reaction, such as the N 2 reduction reaction (NRR). 4,10,11 It was thus a longstanding challenge to probe the potential-dependent atomic structure at the co-electrosorbed species/electrode interface in order to shed light on the co-electrosorbed surface structures and their impact on the catalytic reactivity.…”