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

Abstract

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., ∼−0.5 to ∼−1.0 vs ∼−0.1 to ∼−0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pdelectrocatalysts, previous studies disagree with regard to palladium's ability to form a hydride phase (PdH x ) under CO 2 reduction reaction (CO 2 RR) conditions, as well as on the influence of this PdH x on its CO 2 RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO 2 RR with adsorbed CO, whose precise influence on the formation of PdH x and corresponding effects on the CO 2 RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdH x formation at CO 2 RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO 2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO 2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdH x (with x ≈ 0.6) at all applied potentials under CO 2 RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.

In-Situ Liquid Phase Transmission Electron Microscopy and Electron Diffraction Provides Mechanistic Insight into Electrochemical CO2 Reduction on Palladium/Palladium Hydride Catalysts

“…When applying more negative electrode potentials during chronoamperometry, the subsequent linear sweep voltammetry measurements showed that the H desorption peaks shifted to higher potentials. This is likely due to the higher concentration of adsorbed *CO species accumulated at these more negative electrode potentials as claimed previously, 62 following the electrolysis hold (Figure 5c) showed the appearance of a *CO stripping peak between ca. 0.9 and 1.1 V vs RHE, with the magnitude of the peak increasing with increasing electrolysis hold times.…”

“…For each linear sweep voltammetry measurement that followed the chronoamperometric hold under CO2R conditions (3 min hold at potentials from -0.1 to -0.5 V vs RHE), two oxidation peaks were observed and likely attributed to the oxidation of adsorbed surface *CO species or the desorption of H species. [56][57][58][59][60][61] As could be observed in Figure 5b, holding the potential at -0.1 V vs. RHE contributed to buildup of a *CO covered surface, as indicated by the oxidation peak observed at ca. 0.9 V vs RHE in the subsequent linear sweep voltammetry (inset of Figure 5b).…”

Impact of Hydrogen Concentration for CO2 reduction on PdHx

“…The scaling relations between different adsorbates show there are still good linear relations, but the segregation of H may have an influence on their scaling relations. According to the activity volcano, Pd 64 H 64 , Pd 64 H 63 and Pd 64 H 39 corresponding H concentrations of 1, 0.97, and 0.60respectively, show high current densities at an overpotential of 0.5 V. The Pd 64 H 39 surface is stable when the surface is in equilbrium with H 2 gas, and the H concentration match well with the experimentally determined 60% H concentration of Pd aerogels 20. Due to the existence of the competitive reactions between CO 2 RR and HER, the selectivity plot toward CO and H 2 is calculated and illustrates these three candidates have better CO production compared to others, especially for Pd 64 H 64 .…”

“…Pd can be completely transformed to Pd hydride at a potential of about -0.6 V vs. RHE assuming the surface is in equilibrium with protons and electrons. However, characterization of Pd aerogels by Schmidt and co-workers suggests H concentration is saturated around 60% already at -0.1 V, 20 which suggests the surface can be in equilibrium with H 2 gas rather than protons and electrons. In that case, the Pd 64 H 39 surface is likely the active surface for CO production.…”

Preparation, Quantification, and Reaction of Pd Hydrides on Pd/Al2O3 in Liquid Environment

“…Palladium (Pd) is an established active metal for industrial-scale hydrogenation reactions because of its high reactivity and ability to split hydrogen . Pd forms an extensively studied hydride phase due to its hydrogen storage properties. , Together with the industrial relevance of Pd as a catalyst, the characterization of this hydride phase has been of increasing interest because it has also been identified as the reactive species in hydrogenation reactions , and electrochemical processes. , Most relevant characterization studies aiming at the identification and quantification of Pd hydrides (PdH) have been conducted in the gas phase, ,, while fine chemicals and active pharmaceutical ingredients often rely on liquid-phase catalytic processes . Pd hydrides have been identified in liquid phase under steady-state conditions, , but their utilization has not been studied so far under in situ or operando conditions in transient experiments.…”