•Operando photoelectron spectroscopy of Ir NPs under OER conditions in aqueous electrolyte.•Electron-hole shared between Ir 5d and O 2p as key in formation active species in the OER.•DFT calculations ...of the amount of hole character in Δp and Δd depending on the oxidation state.
An electrode for the oxygen evolution reaction based on a conductive bi-layered free standing graphene support functionalized with iridium nanoparticles was fabricated and characterized by means of potentiometric and advanced X-ray spectroscopic techniques. It was found that the electrocatalytic activity of iridium nanoparticles is associated to the formation of Ir 5d electron holes. Strong Ir 5d and O 2p hybridization, however, leads to a concomitant increase O 2p hole character, making oxygen electron deficient and susceptible to nucleophilic attack by water. Consequently, more efficient electrocatalysts can be synthesized by increasing the number of electron-holes shared between the metal d and oxygen 2p.
Display omitted
Electrochemistry is a promising building block for the global transition to a sustainable energy market. Particularly the electroreduction of CO2 and the electrolysis of water might be strategic ...elements for chemical energy conversion. The reactions of interest are inner-sphere reactions, which occur on the surface of the electrode, and the biased interface between the electrode surface and the electrolyte is of central importance to the reactivity of an electrode. However, a potential-dependent observation of this buried interface is challenging, which slows the development of catalyst materials. Here we describe a sample architecture using a graphene blanket that allows surface sensitive studies of biased electrochemical interfaces. At the examples of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and environmental scanning electron microscopy (ESEM), we show that the combination of a graphene blanket and a permeable membrane leads to the formation of a liquid thin film between them. This liquid thin film is stable against a water partial pressure below 1 mbar. These properties of the sample assembly extend the study of solid–liquid interfaces to highly surface sensitive techniques, such as electron spectroscopy/microscopy. In fact, photoelectrons with an effective attenuation length of only 10 Å can be detected, which is close to the absolute minimum possible in aqueous solutions. The in-situ cells and the sample preparation necessary to employ our method are comparatively simple. Transferring this approach to other surface sensitive measurement techniques should therefore be straightforward. We see our approach as a starting point for more studies on electrochemical interfaces and surface processes under applied potential. Such studies would be of high value for the rational design of electrocatalysts.
Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the ...chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X‐ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl‐like species, namely Co(CO)x, promoting the reduction of Co3+ to Co2+, which is believed to be the active site of the catalyst.
The electronic structure of cobalt on graphene was investigated with a new flow cell that provides electronic and chemical information on the active surfaces by means of X‐ray photoelectron spectroscopy. It is shown that cobalt is anchored on graphene via carbonyl‐like species, promoting the reduction of Co3+ to Co2+, which is believed to be the active site of the catalyst.
The selective hydrogenation of propyne over a Pd-black model catalyst was investigated under
operando
conditions at 1 bar making use of advanced X-ray diffraction (bulk sensitive) and photo-electron ...spectroscopy (surface sensitive) techniques. It was found that the population of subsurface species controls the selective catalytic semi-hydrogenation of propyne to propylene due to the formation of surface and near-surface PdC
x
that inhibits the participation of more reactive bulk hydrogen in the hydrogenation reaction. However, increasing the partial pressure of hydrogen reduces the population of PdC
x
with the concomitant formation of a β-PdH
x
phase up to the surface, which is accompanied by a lattice expansion, allowing the participation of more active bulk hydrogen which is responsible for the unselective total alkyne hydrogenation. Therefore, controlling the surface and subsurface catalyst chemistry is crucial to control the selective alkyne semi-hydrogenation.
Surface-sensitive ambient pressure X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy combined with an electrocatalytic reactivity study, multilength-scale ...electron microscopy, and theoretical modeling provide insights into the gas-phase selective reduction of carbon dioxide to isopropanol on a nitrogen-doped carbon-supported iron oxyhydroxide electrocatalyst. Dissolved atomic carbon forms at relevant potentials for carbon dioxide reduction from the reduction of carbon monoxide chemisorbed on the surface of the ferrihydrite-like phase. Theoretical modeling reveals that the ferrihydrite structure allows vicinal chemisorbed carbon monoxide in the appropriate geometrical arrangement for coupling. Based on our observations, we suggest a mechanism of three-carbon-atom product formation, which involves the intermediate formation of atomic carbon that undergoes hydrogenation in the presence of hydrogen cations upon cathodic polarization. This mechanism is effective only in the case of thin ferrihydrite-like nanostructures coordinated at the edge planes of the graphitic support, where nitrogen edge sites stabilize these species and lower the overpotential for the reaction. Larger ferrihydrite-like nanoparticles are ineffective for electron transport.
Water splitting is a promising technology in the path towards complete renewable energy within the hydrogen economy but overcoming the sluggishness of the oxygen evolution reaction (OER) is a major ...challenge. Iridium-based oxides remain the most attractive materials for the OER under acidic conditions since they offer the combination of activity and stability. Gaining knowledge about how these materials have such an ability is of great interest to develop improved electrocatalysts for the OER. Among the different iridium-based oxides the materials with high concentrations of electron deficient oxygen (OI−) have been shown to have higher OER activity, however, they also have high dissolution rates, seemingly due to the presence or formation of IrIII species. In contrast, rutile-type IrO2, which does not contain IrIII species, has high dissolution resistance but the OER activity remains comparatively low as only low coverages of OI− species are formed under OER. The apparent link between OI− and IrIII species that comes from these observations has yet to be proven. In this work, using ab initio thermodynamics and in situ X-ray photoelectron and absorption spectroscopy we show that the same electrophilic OI− species that appear on Ir-based oxides under OER can be formed on IrIV+δ by mild thermal oxidation of rutile-type IrO2, without the presence IrIII species.
The renewable electricity-driven electrocatalytic hydrogenation of biomass-derived furanic compounds produces biopolymer (polyurethane) precursors under mild reaction conditions. The widely used Ag ...and Cu electrocatalysts failed in the selective conversion of the aldehyde into the alcohol in concentrated electrolytes due to the contribution of the electrodimerization. Herein, we proposed 3D CeO2-based catalysts for the electrocatalytic hydrogenation of 5-hydroxymethylfurfural (HMF) electrolytes (0.02, 0.05, and 0.10 M) to 2,5-bishydroxymethylfuran (BHMF). An electrodeposition approach was adopted to coat CeO2 on Cu open-cell foams. The ex-situ characterization of electrocatalysts revealed that they were made of a CeO2 layer containing Cu species. The migration of Cu from the foam to the coating started during the electrodeposition, while the electroreduction conditions provoked the formation of Cu particles. The in situ characterization by X-ray absorption spectroscopy evidenced that the Ce4+ to Ce3+ reduction occurred just after the application of the cathodic potential; moreover, copper species were reduced to Cu0 during the experiments. The combination of partially reduced CeO2 and Cu particles not only provided selective reaction sites but also increased the electrical conductivity of the electrode. Consequently, the in situ-developed Cu-CeO2 electrocatalysts promoted the selective electrocatalytic hydrogenation of the more concentrated 0.10 M HMF electrolytes, overperforming previously reported AgCu materials at −0.51 V vs RHE.
PDF
