Iron-doped nickel oxyhydroxides, Nix(Fe1-x)OyHz, are among the most promising oxygen evolution reaction (OER) electrocatalysts in alkaline environments. Although iron (Fe) significantly enhances the catalytic activity, there is still no clear consensus on whether Fe directly participates in the reaction or merely acts as a promoter. To elucidate the Fe’s role, we performed operando X-ray spectroscopy studies supported by DFT on Nix(Fe1-x)OyHz electrocatalysts. We probed the reversible changes in the structure and electronic character of Nix(Fe1-x)OyHz as the electrode potential is cycled between the resting (here at 1.10 VRHE) and operational states (1.66 VRHE). DFT calculations and XAS simulations on a library of Fe structures in various NiOyHz environments are in favor of a distorted local octahedral Fe(III)O3(OH)3 configuration at the resting state with the NiOyHz scaffold going from α-Ni(OH)2 to γ-NiOOH as the potential is increased. Under catalytic conditions, EXAFS and HERFD spectra reveal changes in p-d mixing (covalency) relative to the resting state between O/OH ligands and Fe leading to a shift from octahedral to square pyramidal coordination at the Fe site. XES measurements and theoretical simulations further support that the Fe equilibrium structure remains in a formal Fe(III) state under both resting and operational conditions. These spectral changes are attributed to potential dependent structural rearrangements around Fe. The results suggest that ligand dissociation leads to the C4v symmetry as the most stable intermediate of the Fe during OER. This implies that Fe has a weakly coordinated or easily dissociable ligand that could serve to coordinate the O-O bond formation and, tentatively, play an active role in the Nix(Fe1-x)OyHz electrocatalyst.

We have used in situ X-ray photoelectron spectroscopy to obtain information about the chemical state of a Fe single-crystal catalyst upon addition of CO₂ in the syngas feed during Fischer–Tropsch synthesis (FTS) between 85 and 550 mbar. We found that at certain temperatures, the ternary mixture of CO, CO₂, and H₂ yields a chemical state which is resemblant of neither the CO hydrogenation nor the CO₂ hydrogenation reaction mixtures in isolation. The addition of CO₂ to a CO + H₂ reaction mixture mostly affects the chemical state at low-temperature FTS conditions (i.e., below 254 °C). In this temperature span, the ternary reaction mixture resulted in a carburized surface, whereas the CO + H₂ reaction led to surface oxidation. We propose a hypothesis, where a carbonate intermediate produced by CO₂ interaction with Fe oxide aids the reduction of the Fe oxide, paving the way for the carburization of the Fe by dissociated CO. Very small differences in the spectra of the CO + H₂ and the CO + CO₂ + H₂ reaction mixtures were observed above 254 °C, suggesting that the CO₂ is a spectator in these conditions. Changing the total pressure of both the CO hydrogenation and ternary reaction mixture causes quantitative changes in the spectra at both low- and high-temperature FTS conditions, the degree of oxidation/carburization was affected in the low-temperature-FTS regime, and the degree of hydrocarbon build-up was affected in the high-temperature-FTS.

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO₂/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO₂/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO₂ gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO₂ methanol synthesis.  

Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. These reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.

A femtosecond laser process is presented increasing the surface area of copper electrocatalysts for an electrochemical CO₂ reduction reaction (CO₂RR). The laser treatment allows us to tune the surface morphology and the chemical composition of the copper electrocatalysts. This tunability is used to correlate the role of the surface area and catalyst dopants with the selectivity of the CO₂RR. The liquid products of the CO₂RR are monitored through ex situ nuclear magnetic resonance spectroscopy. The products’ distribution shows that the electrode surface area plays a key role in the electrochemical conversion of CO₂ into multicarbon liquid products. We show that sulfur dopants boost the production of formate. Remarkably, by co-doping sulfur and fluoride, we show that the chalcogenide dopant counteracts the known boosting effect of fluoride to convert CO₂ into multicarbon products. Oxygen doping in the range of 2–19 atom % does not significantly affect the distribution of liquid products from CO₂ electroreduction. In a broad perspective, this work highlights the potential of the femtosecond laser process to fine-tune surfaces to produce photo- and electrocatalyst materials.