Chemical state

About: Chemical state is a research topic. Over the lifetime, 2378 publications have been published within this topic receiving 78183 citations.

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Abstract: Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO2-x/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H2 and H 2O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO2-x electrodes undergo Ce3+/Ce4+ oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H2 electro-oxidation and H2O electrolysis. The active regions extend ∼150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce3+/Ce 4+ shifts in the ∼150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Dynamic Changes in the Structure, Chemical State and Catalytic Selectivity of Cu Nanocubes during CO2 Electroreduction: Size and Support Effects

Abstract: In situ and operando spectroscopic and microscopic methods were used to gain insight into the correlation between the structure, chemical state, and reactivity of size- and shape-controlled ligand-free Cu nanocubes during CO2 electroreduction (CO2 RR). Dynamic changes in the morphology and composition of Cu cubes supported on carbon were monitored under potential control through electrochemical atomic force microscopy, X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy. Under reaction conditions, the roughening of the nanocube surface, disappearance of the (100) facets, formation of pores, loss of Cu and reduction of CuOx species observed were found to lead to a suppression of the selectivity for multi-carbon products (i.e. C2 H4 and ethanol) versus CH4 . A comparison with Cu cubes supported on Cu foils revealed an enhanced morphological stability and persistence of CuI species under CO2 RR in the former samples. Both factors are held responsible for the higher C2 /C1 product ratio observed for the Cu cubes/Cu as compared to Cu cubes/C. Our findings highlight the importance of the structure of the active nanocatalyst but also its interaction with the underlying substrate in CO2 RR selectivity.

Evolution of structure and chemistry of bimetallic nanoparticle catalysts under reaction conditions.

Abstract: Three series of bimetallic nanoparticle catalysts (RhxPd1−x, RhxPt1−x, and PdxPt1−x, x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized RhxPd1−x and PdxPt1−x nanoparticles have a core−shell structure whereas the RhxPt1−x alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The RhxPd1−x and RhxPt1−x nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O2 at 300 °C) to reducing (H2 or CO at 300 °C) or catalytic (mixture of NO and CO at 300 °C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the PdxPt1−x nanoparticles was observed. The difference in restructuring b...

High-Resolution X-ray Photoelectron Spectra of Organosulfur Monolayers on Au(111): S(2p) Spectral Dependence on Molecular Species

Abstract: We have used high-resolution X-ray photoelectron spectroscopy (XPS) to investigate the molecular dependence on sulfur chemical states of organosulfur monolayers of alkanethiol, dialkyl disulfide, m...

Monitoring the Chemical State of Catalysts for CO2 Electroreduction: An In Operando Study

Abstract: A major concern of electrocatalysis research is to assess the structural and chemical changes that a catalyst may itself undergo in the course of the catalyzed process. These changes can influence not only the activity of the studied catalyst but also its selectivity toward the formation of a certain product. An illustrative example is the electroreduction of carbon dioxide on tin oxide nanoparticles, where under the operating conditions of the electrolysis (that is, at cathodic potentials), the catalyst undergoes structural changes which, in an extreme case, involve its reduction to metallic tin. This results in a decreased Faradaic efficiency (FE) for the production of formate (HCOO–) that is otherwise the main product of CO2 reduction on SnOx surfaces. In this study, we utilized potential- and time-dependent in operando Raman spectroscopy in order to monitor the oxidation state changes of SnO2 that accompany CO2 reduction. Investigations were carried out at different alkaline pH levels, and a strong co...