Leander Haug

For the direct reduction of CO₂ and H₂O in solid oxide electrolysis cells (SOECs) with cermet electrodes toward methane, a fundamental understanding of the role of elemental carbon as a key intermediate within the reaction pathway is of eminent interest. The present synchrotron-based in situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) study shows that alloying of Ni/yttria-stabilized-zirconia (YSZ) cermet electrodes with Cu can be used to control the electrochemical accumulation of interfacial carbon and to optimize its reactivity toward CO₂. In the presence of syngas, sufficiently high cathodic potentials induce excess methane on the studied Ni/yttria-stabilized-zirconia (YSZ)-, NiCu/YSZ- and Pt/gadolinium-doped-ceria (GDC) cermet systems. The hydrogenation of carbon, resulting from CO activation at the triple-phase boundary of Pt/GDC, is most efficient.

For the direct reduction of CO2 and H2O in solid oxide electrolysis cells (SOECs) with cermet electrodes toward methane, a fundamental understanding of the role of elemental carbon as a key intermediate within the reaction pathway is of eminent interest. The present synchrotron-based in situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) study shows that alloying of Ni/yttria-stabilized-zirconia (YSZ) cermet electrodes with Cu can be used to control the electrochemical accumulation of interfacial carbon, and to optimize its reactivity toward CO2. In the presence of syngas, sufficiently high cathodic potentials induce excess methane on the studied Ni/yttria stabilized zirconia (YSZ)-, NiCu/YSZ- and Pt/gadolinium doped ceria (GDC) cermet systems. The hydrogenation of carbon resulting from CO activation at the triple-phase boundary of Pt/GDC is most efficient.

A versatile multifunctional laboratory-based near ambient pressure x-ray photoelectron spectroscopy (XPS) instrument is presented. The entire device is highly customized regarding geometry, exchangeable manipulators and sample stages for liquid- and solid-state electrochemistry, cryochemistry, and heterogeneous catalysis. It therefore delivers novel and unique access to a variety of experimental approaches toward a broad choice of functional materials and their specific surface processes. The high-temperature (electro)catalysis manipulator is designed for probing solid state/gas phase interactions for heterogeneous catalysts including solid electrolyzer/fuel cell electrocatalysts at pressures up to 15 mbar and temperatures from room temperature to 1000 °C. The liquid electrochemistry manipulator is specifically designed for in situ spectroscopic investigations of polarized solid/liquid interfaces using aqueous electrolytes and the third one for experiments for ice and ice-like materials at cryogenic temperatures to approximately −190 °C. The flexible and modular combination of these setups provides the opportunity to address a broad spectrum of in situ and operando XPS experiments on a laboratory-based system, circumventing the limited accessibility of experiments at synchrotron facilities.

To identify the synergistic action of differently prepared Ni-ZrO₂ phase boundaries in methane dry reforming, we compared an “inverse” near-surface intermetallic NiZr catalyst precursor with the respective bulk-intermetallic Ni_xZr_y material and a supported Ni-ZrO₂ catalyst. In all three cases, stable and high methane dry reforming activity with enhanced anticoking properties can be assigned to the presence of extended Ni-ZrO₂ phase boundaries, which result from in situ activation of the intermetallic Ni-Zr model catalyst systems under DRM conditions. All three catalysts operate bifunctionally; methane is essentially decomposed to carbon at the metallic Ni⁰ surface sites, whereas CO₂ reacts to CO at reduced Zr centers induced by a spillover of carbon to the phase boundaries. On pure bulk Ni⁰, dissolved carbon accumulates in surface-near regions, leading to a sufficiently supersaturated state for completely surface-blocking graphitic carbon segregation. In strong contrast, surface-ZrO₂ modified bulk Ni⁰ exhibits virtually the best decoking and carbon conversion conditions due to the presence of highly dispersed ZrO₂ islands with a particularly large contribution of interfacial Ni⁰-ZrO₂ sites and short C-diffusion pathways to the latter.

Graphitic deposits anti‐segregate into Ni⁰ nanoparticles to provide restored CH₄ adsorption sites and near‐surface/dissolved C atoms, which migrate to the Ni⁰/ZrO₂ interface and induce local Zr_xC_y formation. The resulting oxygen‐deficient carbidic phase boundary sites assist in the kinetically enhanced CO₂ activation toward CO(g). This interface carbide mechanism allows for enhanced spillover of carbon to the ZrO₂ support, and represents an alternative catalyst regeneration pathway with respect to the reverse oxygen spillover on Ni‐CeZr_xO_y catalysts. It is therefore rather likely on supports with limited oxygen storage/exchange kinetics but significant carbothermal reducibility.

Graphitische Ablagerungen lösen sich in Ni⁰‐Nanopartikeln auf, um rezyklierte CH₄‐Adsorptionsstellen und oberflächennahe/gelöste C‐Atome bereitzustellen, die zur Ni⁰/ZrO₂‐Grenzfläche wandern und eine lokale Zr_xC_y‐Bildung induzieren. Die daraus resultierenden sauerstoffarmen karbidischen Phasengrenzflächen unterstützen die Kinetik der CO₂‐Aktivierung zu CO(g). Dieser Grenzflächen‐Karbid‐Mechanismus ermöglicht einen verstärkten Übertritt von Kohlenstoff auf den ZrO₂‐Träger und stellt einen alternativen Weg der Katalysatorregeneration im Vergleich zum umgekehrten Sauerstoffübertritt auf Ni/CeZr_xO_y‐Katalysatoren dar. Er ist daher eher auf Trägern mit begrenzter Sauerstoffspeicher‐ bzw. Austausch‐Kinetik, aber signifikanter karbothermischer Reduzierbarkeit, wahrscheinlich.

A series of ultra-clean, unsupported Cu-doped and Pd-doped Ni model catalysts was investigated to develop the fundamental concept of metal doping impact on the carbon tolerance and catalytic activity in the dry reforming of methane (DRM). Wet etching with concentrated HNO₃ and a subsequent single sputter–anneal cycle resulted in the full removal of an already existing oxidic passivation layer and segregated and/or ambient-deposited surface and bulk impurities to yield ultra-clean Ni substrates. Carbon solubility, support effects, segregation processes, cyclic operation temperatures, and electronic and ensemble effects were all found to play a crucial role in the catalytic activity and stability of these systems, as verified by X-ray photoelectron spectroscopy (XPS) surface and bulk characterization. Minor Cu promotion showed the almost complete suppression of coking with a moderate reduction in catalytic activity, while high Cu loadings facilitated carbon growth alongside severe catalytic deactivation. The improved carbon resistance stems from an increased CH₄ dissociation barrier, decreased carbon solubility in the bulk, good prevailing CO₂ activation properties and enhanced CO desorption. Cyclic DRM operation on surfaces with Cu content that is too high leads to impaired carbon oxidation kinetics by CO₂ and causes irreversible carbon deposition. Thus, an optimal and stable NiCu composition was found in the region of 70–90 atomic % Ni, which allows an appropriate high syngas production rate to be retained alongside a total coking suppression during DRM. In contrast, the more Cu-rich NiCu systems showed a limited stability under reaction conditions, leading to undesired surface and bulk segregation processes of Cu. The much higher carbon deposition rate and solubility of unsupported NiPd and Pd model catalysts results in severe carbon deposition and catalytic deactivation. To achieve enhanced carbon conversion and de-coking, an active metal oxide boundary is required, allowing for the increased clean-off of re-segregated carbon via the inverse Boudouard reaction. The carbon bulk diffusion on the investigated systems depends strongly on the composition and decreases in the following order: Pd > NiPd > Ni > NiCu > Cu.