Helena Kaper

Conversion to a circular economy needs efficient methods to recover critical metals from electronic waste. Here, we demonstrate a two‐step, purification‐free process to transform palladium contained in waste multilayer ceramic capacitors (MLCCs) into highly active heterogeneous catalysts. After acid leaching, Pd(II) was selectively extracted (97%–100% yield) using di‐ n ‐octyl sulfide (DOS) or N , N , N ′, N ′‐tetrahexylmalonamide (THMA), then directly impregnated onto γ ‐Al ₂ O ₃ . The resulting catalysts—containing residual Ag or Bi—showed activity at the level of a commercial reference (1 wt% Pd on γ ‐Al ₂ O ₃ ) in methane oxidation ( T ₅₀ = 325°C–327°C) and hindered olefin hydrogenation (quantitative conversion during the first cycle at 185°C/20 bar), suggesting reasonable tolerance to metallic impurities. Notably, DOS‐extracted catalysts retained full activity despite sulfur content (0.5–0.6 wt%), while THMA‐derived systems showed three times higher intrinsic activity than the reference catalyst at 350°C. HRTEM‐EDS revealed well‐dispersed Pd or Pd/Ag nanoparticles. This approach eliminates the need for complete Pd purification, offering a sustainable route to upcycle complex e‐waste into value‐added materials for industrial catalysis.

Through a combination of modulated chemical excitation and RPES analyses, we elucidated the dynamic behavior of oxygen species on the ceria surface, revealing how changes in CO/O₂ gas composition influence the electronic structure of CeO₂.

Electronic waste as an alternative source of palladium is used for the preparation of heterogeneous catalysts for methane oxidation. The strategy developed in this paper involves two steps: optimization of the leaching protocol to recover the Pd from the waste, and development of a suitable impregnation protocol directly from the leachate, which is a strongly acidic solution. The precipitation‐deposition technique is particularly suitable for the preparation of catalysts from acidic media. The impact of Ba, Bi, and Ag impurities on the catalytic performance is studied on model catalysts. The presence of these elements and their impact on the catalytic properties are analyzed and discussed using X‐ray diffraction, nitrogen physisorption, scanning and transmission electron microscopy. Through variation of the pH, the impurities co‐deposited with Pd can be limited and the catalytic performance of catalysts derived from real waste can be greatly enhanced.

In the present work, a series of CaTi_1-xFe_xO_3-δ (0 < x < 0.5) materials are prepared using a modified Pechini method based on citric acid and a polyol as chelating agents. The synthesis conditions are optimized with respect to the specific surface area and phase purity by varying polyols (ethylene glycol, glycerol, and 1.6-hexanediol) and the ratio between citric acid, polyols, and cations. The impact of the polyols and the iron content (up to 40 mol % on the B site) is studied with respect to the oxygen exchange rate, reducibility using H₂-TPR, and catalytic performance for methane total oxidation. A correlation between the oxygen exchange rate studied using ¹⁸O exchange in powdered samples of CaTi_1-xFe_xO_3-δ (0 < x < 0.5) and ferric sites determined using Mössbauer spectroscopy and H₂-TPR is established. The oxygen activation and diffusion in CaTi_1-xFe_xO_3-δ (0 < x < 0.5) continuously increase in the studied range of Ti substitution. The methane oxidation performance does not increase above x = 0.3, showing that methane oxidation is not limited by surface oxygen activation and CH₄ is activated by specific iron sites in Fe-doped perovskites.

A large signal of gas-phase CO overlapping with those of adsorbates is often present when investigating catalysts by operando diffuse reflectance FT-IR spectroscopy. Physically removing CO(g) from the IR cell may lead to a fast decay of adsorbate signals. Our work shows that carbonyls adsorbed on metallic Pt sites fully vanished in less than 10 min at 30 °C upon removing CO(g) when redox supports were used. In contrast, a broad band assigned to CO adsorbed on oxidized Pt sites was stable. It was concluded that physically removing CO(g) at room temperature during IR analyses will most likely lead to changes in the distribution of CO(ads) and a misrepresentation of the Pt site speciation, misguiding the development of efficient low-temperature CO oxidation catalysts. A tentative representation of the nature of the Pt phases present depending on the feed composition is also proposed.

The support material can play an important role in oxidation catalysis, notably for CO oxidation. Here, we study two materials of the Brownmillerite family, CaFeO_2.5 and SrFeO_2.5, as one example of a stoichiometric phase (CaFeO_2.5, CFO) and one existing in different modifications (SrFeO_2.75, SrFeO_2.875 and SrFeO₃, SFO). The two materials are synthesized using two synthesis methods, one bottom-up approach via a complexation route and one top-down method (electric arc fusion), allowing to study the impact of the specific surface area on the oxygen mobility and catalytic performance. CO oxidation on ¹⁸O-exchanged materials shows that oxygen from SFO participates in the reaction as soon as the reaction starts, while for CFO, this onset takes place 185 °C after reaction onset. This indicates that the structure of the support material has an impact on the catalytic performance. We report here on significant differences in the catalytic activity linked to long-term stability of CFO and SFO, which is an important parameter not only for possible applications, but equally to better understand the mechanism of the catalytic activity itself.

The design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides toward CH₄ and CO oxidation was studied here. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi_0.65Fe_0.35O_3−δ with specific surface areas (SSA) ranging from 45 to 80 m²/g and pore sizes from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron X-ray diffraction (XRD) and TG-MS combined with H₂-TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH₄ oxidation is influenced more by the dioxygen release. During isothermal long-term catalysis tests, however, the continuous oxygen release from large SSA materials promoted both CO and CH₄ conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties.

Pt‐basierte Materialien sind weitverbreitet in der heterogenen Katalyse für die katalytische Entfernung von Schadstoffen. Oft wird der Zustand des aktiven Platins als abhängig von den experimentellen Bedingungen beschrieben: bei niedrigen Temperaturen unterhalb der Light‐off‐Temperatur liegt mit CO vergiftetes metallisches Pt vor, bei höheren Temperaturen oberhalb der Light‐off‐Temperatur oxidiertes Pt. Im Gegensatz zu vorherigen Studien zeigen wir hier, dass metallisches wie auch oxidiertes Pt in ähnlichen Proportionen an der Oberfläche von hoch aktiven, ca. 1 nm großen Nanopartikeln unter Reaktionsbedingungen bei 30 °C vorliegen. Die simultane Gegenwart von metallischem und oxidiertem Pt ermöglicht eine Synergie zwischen den beiden Phasen. Dabei bietet das metallische Pt Adsorptionsstellen für CO, während das oxidierte Pt vermutlich reaktiven Sauerstoff zuführt. Unsere Ergebnisse zeigen die Komplexität von dualen oxidischen‐metallischen Pt‐Katalysatoren und ihre leicht veränderliche Natur unter den jeweiligen Reaktionsbedingungen.