1,721,301 research outputs found
An in situ near-ambient pressure X-ray Photoelectron Spectroscopy study of Mn polarised anodically in a cell with solid oxide electrolyte
No full textThis paper reports an in situ study of the anodic behavior of a model solid oxide electrolysis cell (SOEC) by
means of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) combined with near edge X-ray
absorption
fine structure (NEXAFS) measurements. The focus is on the anodic surface chemistry of MnOx,
a model anodic material already considered in cognate SOFC-related studies, during electrochemical
operation in CO2, CO2/H2O and H2O ambients. The XPS and NEXAFS results we obtained, complemented
by electrochemical measurements and SEM characterisation, reveal the chemical evolution of Mn under
electrochemical control. MnO is the stable chemical form at open-circuit potential (OCP), while Mn3O4
forms under anodic polarisation in all the investigated gas ambients. Carbon deposits are present on the
Mn electrode at OCP, but they are readily oxidised under anodic conditions. Prolonged operation of the
MnOx anode leads to pitting of the Mn
films, damaging of the triple-phase boundary region and also to
formation of discontinuities in the Mn patch. This is accompanied by chemical transformations of the
electrolyte and formation of ZrC without impact on the surface chemistry of the Mn-based anode
An in situ near-ambient pressure X-ray Photoelectron Spectroscopy study of Mn polarised anodically in a cell with solid oxide electrolyte
Full text linkThis paper reports an in situ study of the anodic behavior of a model solid oxide electrolysis cell (SOEC) by means of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) combined with near edge X-ray absorption fine structure (NEXAFS) measurements. The focus is on the anodic surface chemistry of MnOx, a model anodic material already considered in cognate SOFC-related studies, during electrochemical operation in CO2, CO2/H2O and H2O ambients. The XPS and NEXAFS results we obtained, complemented by electrochemical measurements and SEM characterisation, reveal the chemical evolution of Mn under electrochemical control. MnO is the stable chemical form at open-circuit potential (OCP), while Mn3O4 forms under anodic polarisation in all the investigated gas ambients. Carbon deposits are present on the Mn electrode at OCP, but they are readily oxidised under anodic conditions. Prolonged operation of the MnOx anode leads to pitting of the Mn films, damaging of the triple-phase boundary region and also to formation of discontinuities in the Mn patch. This is accompanied by chemical transformations of the electrolyte and formation of ZrC without impact on the surface chemistry of the Mn-based anode
An in situ near-ambient pressure X-ray photoelectron spectroscopy study of CO2 reduction at Cu in a SOE cell
Full text linkThe cathodic behavior of a model solid oxide electrolysis cell (SOEC) has been studied by means of near-ambient pressure (NAP) X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), aiming at shedding light on the specific role of the metallic component in a class of cermets used as electrodes. The focus is on the surface chemistry and catalytic role of Cu, the increasingly popular metallic component in electrodes used in CO2 electrolysis and CO2/H2O co-electrolysis. The NAP-XPS and NEXAFS results, obtained in situ and operando conditions and under electrochemical control, have provided important insights about the evolution of the chemical composition of the Cu surface. We have found that in dry CO2 ambient carbon deposits are scavenged at low cathodic potential by the oxidising action of nascent O, while at high cathodic polarisations C grows due to activation of CO reduction. Instead, in CO2/H2O mixtures, surface deposit of C is steady over the whole investigated potential range. The presence of adsorbed CO has also been detected during electrolysis of CO2/H2O mixtures, while no CO is found in pure CO2 ambient
Operation of calcium-birnessite water-oxidation anodes: interactions of the catalyst with phosphate buffer anions
Full text linkInvestigating the interfaces between electrolytes and electrocatalysts during electrochemical water oxidation is of great importance for an understanding of the factors influencing catalytic activity and stability. Here, the interaction of a well-established, nanocrystalline and mesoporous Ca-birnessite catalyst material (initial composition K0.2Ca0.21MnO2.21·1.4H2O, initial Mn-oxidation state ∼+3.8) with an aqueous potassium phosphate buffer electrolyte at pH 7 was studied mainly by using various electron microscopy and X-ray spectroscopy techniques. In comparison to electrolyte solutions not containing phosphate, the investigated Ca-birnessite electrodes show especially high and stable oxygen evolution activity in phosphate buffer. During electrolysis, partial ion substitutions of Ca2+ by K+ and OH−/O2− by HnPO4(3−n)− were observed, leading to the formation of a stable, partially disordered Ca–K–Mn–HnPO4–H2O layer on the outer and the pore surfaces of the active electrocatalyst material. In this surface layer, Mn3+ ions are stabilized, which are often assumed to be of key importance for oxygen evolution catalysis. Furthermore, evidence for the formation of [Ca/PO4/H2O]− complexes located between the [MnO6] layers of the birnessite was found using the soft Ca 2p and Ca L-edge X-ray spectroscopy. A possible way to interpret the observed, obviously very favorable “special relationship” between (hydrogen)phosphates and Ca-birnessites in electrocatalytic water oxidation would be that HnPO4(3−n)− anions are incorporated into the catalyst material where they act as stabilizing units for Mn3+ highly active centers and also as “internal bases” for the protons released during the water-oxidation reaction
Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure
No full textThe interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations
In Situ XANES XPS Investigation of Doped Manganese Perovskite Catalysts
Full text linkStudying catalysts in situ is of high interest for understanding their surface structure and electronic states in operation. Herein, we present a study of epitaxial manganite perovskite thin films (Pr1-xCax MnO3) active for the oxygen evolution reaction (OER) from electro-catalytic water splitting. X-ray absorption near-edge spectroscopy (XANES) at the Mn L- and O K-edges, as well as X-ray photoemission spectroscopy (XPS) of the O 1s and Ca 2p states have been performed in ultra-high vacuum and in water vapor under positive applied bias at room temperature. It is shown that under the oxidizing conditions of the OER a reduced Mn2+ species is generated at the catalyst surface. The Mn valence shift is accompanied by the formation of surface oxygen vacancies. Annealing of the catalysts in O2 atmosphere at 120 °C restores the virgin surfaces
Subsurface Species in Heterogeneous Catalytic Reactions: Insights by in situ Photoelectron Spectroscopy
No full text
The influence of subsurface carbon on the selectivity in the hydrogenation reaction of 1-pentyne over Pd catalysts
No full textHigh Pressure X-Ray Photoelectron Spectroscopy: A Tool to Bridge the Pressure Gap in Heterogeneous Catalysis
No full text- …
