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Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies.

Preferential oxidation of CO (COPrOx) is a catalytic reaction targeting the removal of trace amounts of CO from hydrogen-rich gas mixtures. Non-noble metal catalysts, such as Cu and Co, can be equally active to Pt for the reaction; however, their commercialization is limited by their poor stability. We have recently shown that CoO is the most active state of cobalt for COPrOx, but under certain reaction conditions, it is readily oxidized to Co3O4 and deactivates. Here, we report a simple method to stabilize the Co2+ state by vanadium addition. The V-promoted cobalt catalyst exhibits considerably higher activity and stability than pure cobalt. The nature of the catalytic active sites during COPrOx was established by operando NAP-XPS and NEXAFS, while the stability of the Co2+ state on the surface was verified by in situ NEXAFS at 1 bar pressure. The active phase consists of an ultra-thin cobalt-vanadate surface layer, containing tetrahedral V5+ and octahedral Co2+ cations, with an electronic and geometric structure that is deviating from the standard mixed bulk oxides. In addition, V addition helps to maintain the population of Co2+ species involved in the reaction, inhibiting carbonate species formation that are responsible for the deactivation. The promoting effect of V is discussed in terms of enhancement of CoO redox stability on the surface induced by electronic and structural modifications. These results demonstrate that V-promoted cobalt is a promising COPrOx catalyst and validate the application of in situ spectroscopy to provide the concept for designing better performing catalysts.

The solid oxide electrolysis cell (SOEC) technology has a huge potential for future mass production of hydrogen, mainly due to its high electrical-to-chemical energy conversion efficiency. However, the durability and the performance of SOEC devices are inferior to that of other competitive electrolysis technologies inhibiting the commercialization of SOECs. Despite the fact that Ni-based cermets are currently the most widely used cathode materials for SOEC, change of the nickel oxidation state has been accused as a major issue limiting the performance of these devices. In this work we provide operando experimental evidence of the active surface oxidation state and composition of nickel/doped-ceria cermets under water electrolysis conditions using ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, combined with quantitative spectra simulation. Remarkably under specific operational conditions, nickel is maintained in a partially oxidized state which, counterintuitive to the expected behavior, can be beneficial to the cell performance. This finding may initiate new improvement strategies for SOEC electrodes based on thorough optimization of the operational conditions, in order to engineer in situ the most propitious electrode configuration.

Graphical abstract PdIn intermetallic phases can be switched in methanol steam reforming between a CO2-selective multilayer and an In-diluted phase by annealing at 453 K or 623 K. Highlights ► A multilayer Pd1In1 phase in MSR is highly CO2-selective between 493 and 623 K. ► An In-diluted PdIn intermetallic phase yields CO formation via full methanol dehydrogenation. ► Higher reaction temperatures are needed for comparable CO2-TOF values as on supported PdIn/In2O3. ► A bimetal-oxide synergism for efficient CO2 formation is operative., In situ X-ray photoelectron spectroscopy (in situ XPS) was used to study the structural and catalytic properties of Pd–In near-surface intermetallic phases in correlation with previously studied PdZn and PdGa. Room temperature deposition of ∼4 monolayer equivalents (MLEs) of In metal on Pd foil and subsequent annealing to 453 K in vacuum yields a ∼1:1 Pd/In near-surface multilayer intermetallic phase. This Pd1In1 phase exhibits a similar “Cu-like” electronic structure and indium depth distribution as its methanol steam reforming (MSR)-selective multilayer Pd1Zn1 counterpart. Catalytic characterization of the multilayer Pd1In1 phase in MSR yielded a CO2-selectivity of almost 100% between 493 and 550 K. In contrast to previously studied In2O3-supported PdIn nanoparticles and pure In2O3, intermediate formaldehyde is only partially converted to CO2 using this Pd1In1 phase. Strongly correlated with PdZn, on an In-diluted PdIn intermetallic phase with “Pd-like” electronic structure, prepared by thermal annealing at 623 K, methanol steam reforming is suppressed and enhanced CO formation via full methanol dehydrogenation is observed. To achieve CO2-TOF values on the isolated Pd1In1 intermetallic phase as high as on supported PdIn/In2O3, at least 593 K reaction temperature is required. A bimetal-oxide synergism, with both bimetallic and oxide synergistically contributing to the observed catalytic activity and selectivity, manifests itself by accelerated formaldehyde-to-CO2 conversion at markedly lowered temperatures as compared to separate oxide and bimetal. Combination of suppression of full methanol dehydrogenation to CO on Pd1In1 inhibited inverse water–gas-shift reaction on In2O3 and fast water activation/conversion of formaldehyde is the key to the low-temperature activity and high CO2-selectivity of the supported catalyst.

The NH3 + O2 reaction on a Pt(533) surface has been studied in the 10−4 mbar range and close to 1 mbar pressure with in situ X-ray photoelectron spectroscopy using synchrotron radiation. The coverages of the various O- and N-containing surface species have been followed in T-cycling experiments with varying mixing ratios O2/NH3 and varying total pressure. In heating/cooling cycles hysteresis of ∼50−100 K width occur. Adsorbed NOad already decomposes at T > 350 K. Under stationary conditions, no adsorbed NO could be detected. At no time during the experiments were Pt bulk oxides formed. A shift in the surface core level component of the Pt 4f spectrum by more than 0.5 eV toward higher binding energy is attributed to Pt atoms of the (100) step edges which are coordinated to more than one oxygen atom similar to the model proposed by Wang et al. Phys. Rev. Lett. 2005, 95, 256102.

An unsupported Pd-rich GaPd2 sample in form of a thin film has been prepared by alternating layer deposition of Pd and Ga metal and was subsequently used as a structurally and chemically stable model system to clarify the catalytic properties of the unsupported intermetallic compound GaPd2 in methanol steam reforming (MSR). The sample revealed a slightly Pd-richer GaPd2 bulk composition of Ga28Pd72, as evidenced by EDX analysis, low-energy ion scattering, X-ray diffraction measurements, and depth profiling by in situ X-ray photoelectron spectroscopy. The latter additionally showed a high stability of GaPd2 both under methanol and oxidative methanol steam reforming conditions. No active redox chemistry of Ga species or other reaction-induced oxidative Ga surface segregation has been detected during catalytic MSR reaction. Corroborating these observations, corresponding catalytic experiments under methanol steam reforming conditions revealed only a, in comparison with elemental Pd, very small activity in methanol dehydrogenation (CO formation rate at maximum 0.019 mbar min-1 ; 0.08 site-1 s-1 ). Unsupported thin film Pd-rich GaPd2 with the given surface and bulk stoichiometry must therefore be considered a poor methanol steam reforming/dehydrogenation catalyst. In oxidative steam reforming experiments, only total oxidation without significant H2 formation has been observed.

Solid Oxide Fuel Cells (SOFCs) attract substantial commercial interest owing to their high efficiency in generating electrical power from fuels such as methane or hydrogen. In recent years, also the research on the inverse process – the conversion of excess electrical energy in chemical energy via Solid Oxide Electrolysis Cells (SOECs) – is gaining more and more attention. As electrode on the reducing side of both SOFCs and SOECs commonly Ni/yttria stabilized zirconia (YSZ) electrodes are used but they are known to suffer from several problems, like sulfur poisoning (in SOFC operation), sintering of Ni particles, poor redox cycling stability, and coking. In contrast, mixed conducting oxide anodes are expected to exhibit improved properties. In the present contribution the electrode kinetics of the perovskite-type mixed conductor La0.6Sr0.4FeO3-δ (LSF) is investigated in H2/H2O atmosphere by means of impedance spectroscopy and the results are compared to the polarization resistance of LSF in O2 atmosphere. To compensate for the relatively low electronic conductivity of LSF under reducing conditions a sputter deposited Pt thin film grid was used as a current collector. Interestingly, the surface resistances of these model-composite electrodes in H2/H2O and O2 at 600 °C were virtually the same indicating a high electrochemical activity of the LSF surface when operated as SOFC anode. Moreover, synchrotron-based near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and impedance spectroscopy experiments were conducted simultaneously on electrochemically polarized LSF electrodes. Of special interest was the effect of electrochemical polarization on the composition and valence states of near surface cations of the LSF electrodes. Under cathodic polarization the formation of near-surface metallic iron was observed, accompanied by a strong improvement of the electrode’s water splitting activity [1]. This correlation of metal exsolution and electrochemical performance suggests a fundamentally different water splitting mechanism in presence of the metallic iron species and may provide new directions in the quest for novel mixed conducting SOEC cathodes for high-temperature electrochemical water splitting. [1] A. K. Opitz et al., Angewandte Chemie International Edition 54 (2015) p. 2628-2632. Figure 1