The surface chemistry of the Fischer-Tropsch catalytic reaction over Co has still several unknows. Here, we report an in-situ X-ray photoelectron spectroscopy study of Co(0001) and Co(), and in-situ high energy surface X-ray diffraction of Co(0001), during the Fischer-Tropsch reaction at 0.15 bar - 1 bar and 406 K - 548 K in a H₂/CO gas mixture. We find that these Co surfaces remain metallic under all conditions and that the coverage of chemisorbed species ranges from 0.4–1.7 monolayers depending on pressure and temperature. The adsorbates include CO on-top, C/-C_xH_y and various longer hydrocarbon molecules, indicating a rate-limiting direct CO dissociation pathway and that only hydrocarbon species participate in the chain growth. The accumulation of hydrocarbon species points to the termination step being rate-limiting also. Furthermore, we demonstrate that the intermediate surface species are highly dynamic, appearing and disappearing with time delays after rapid changes in the reactants’ composition.

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 Haber-Bosch process produces NH₃ from N₂ and H₂^1,2, typically with Fe and Ru³.  HB has been proposed as the most important scientific invention in the 20th century⁴. The chemical state during reaction has been proposed as oxides⁵, nitrides², metallic, or surface nitride⁶. The proposed rate-limiting step has been the dissociation of  N₂^7–9, reaction of adsorbed nitrogen¹⁰, or desorption of NH₃¹¹. Due to the vacuum requirement for surface-sensitive techniques, studies at reaction conditions are limited to theory computations^12–14. We determined the surface composition, during NH₃ production, at pressures up to 1 bar and temperatures as high as 723 K on flat, stepped Fe, and stepped Ru single crystal surfaces using operando X-ray Photoelectron Spectroscopy¹⁵. We found that all surfaces remain metallic. On Fe only a small amount of adsorbed N remains, yet Ru’s surface is almost adsorbate free. At 523 K, high amines (NH_x) coverages appear on the stepped Fe surface. The results show that the rate-limiting step on Ru is always N₂ dissociation. Still, on Fe the hydrogenation step involving adsorbed N atoms is essential for the total rate, as predicted by theory¹³. If the temperature is lowered on Fe, the rate-limiting steps switch and become surface species’ hydrogenation.

We investigated Cu nanoparticles (NPs) on vicinal and basal ZnO supports to obtain an atomistic picture of the catalyst’s structure under in situ oxidizing and reducing conditions. The Cu/ZnO model catalysts were investigated at elevated gas pressures by high energy grazing incidence X-ray diffraction and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We find that the Cu nanoparticles are fully oxidized to Cu₂O under atmospheric conditions at room temperature. As the nanoparticles swell during oxidation, they maintain their epitaxy on basal ZnO (000 ± 1) surfaces, whereas on the vicinal ZnO (101̅4) surface, the nanoparticles undergo a coherent tilt. We find that the oxidation process is fully reversible under H₂ flow at 500 K, resulting in predominantly well-aligned nanoparticles on the basal surfaces, whereas the orientation of Cu NPs on vicinal ZnO was only partially restored. The analysis of the substrate crystal truncation rods evidences the stability of basal ZnO surfaces under all gas conditions. No Cu–Zn bulk alloy formation is observed. Under CO₂ flow, no diffraction signal from the nanoparticles is detected, pointing to their completely disordered state. The AP-XPS results are in line with the formation of CuO. Scanning electron microscopy images show that massive mass transport has set in, leading to the formation of larger agglomerates. 

The state of the surface near-region during CO hydro- genation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C.

The electronic excitation occurring on adsorbates at ultrafast timescales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) of a simple well-known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel [Ni(100)] surface, following intense laser optical pumping at 400 nm. We observe ultrafast (∼100  fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few picoseconds timescale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation.

Subsurface oxygen has been proposed to be crucial in oxide-derived copper (OD-Cu) electrocatalysts for enhancing the binding of CO intermediates during CO₂ reduction reaction (CO₂RR). However, the presence of such oxygen species under reductive conditions still remains debated. In this work, the existence of subsurface oxygen is validated by grazing incident hard X-ray photoelectron spectroscopy, where OD-Cu was prepared by reduction of Cu oxide with H₂ without exposing to air. The results suggest two types of subsurface oxygen embedded between the fully reduced metallic surface and the Cu₂O buried beneath: (i) oxygen staying at lattice defects and/or vacancies in the surface-most region and (ii) interstitial oxygen intercalated in metal structure. This study adds convincing support to the presence of subsurface oxygen in OD-Cu, which previously has been suggested to play an important role to mitigate the σ-repulsion of Cu for CO intermediates in CO₂RR. 

Carbide formation on iron-based catalysts is an integral and, arguably, the most important part of the Fischer–Tropsch synthesis process, converting CO and H₂ into synthetic fuels and numerous valuable chemicals. Here, we report an in situ surface-sensitive study of the effect of pressure, temperature, time, and gas feed composition on the growth dynamics of two distinct iron–carbon phases with the octahedral and trigonal prismatic coordination of carbon sites on an Fe(110) single crystal acting as a model catalyst. Using a combination of state-of-the-art X-ray photoelectron spectroscopy at an unprecedentedly high pressure, high-energy surface X-ray diffraction, mass spectrometry, and theoretical calculations, we reveal the details of iron surface carburization and product formation under semirealistic conditions. We provide a detailed insight into the state of the catalyst’s surface in relation to the reaction.

The CO hydrogenation reaction over the Rh(111) and (211) surfaces has been investigated operando by X-ray photoelectron spectroscopy at a pressure of 150 mbar. Observations of the resting state of the catalyst give mechanistic insight into the selectivity of Rh for generating ethanol from CO hydrogenation. This study shows that the Rh(111) surface does not dissociate all CO molecules before hydrogenation of the O and C atoms, which allows methoxy and other both oxygenated and hydrogenated species to be visible in the photoelectron spectra.