15 Search Results

Here, this work investigates Ru-CeO₂-TiO₂ catalysts for the CO₂ methanation reaction and compares their performance with previously studied Ru-CeO₂ systems. Despite the lower Ru loading, the TiO₂-containing catalysts exhibit significantly higher activity. To understand this behavior, in situ X-ray absorption spectroscopy (XAS) was carried out at the Ru K-edge and Ce L₃-edge. Unlike Ru-CeO₂, which displays reversible redox behavior of Ru, the Ru-CeO₂-TiO₂ catalysts show irreversible Ru reduction and a substantially higher fraction of Ce³⁺ species under all tested conditions (H₂, CO₂, H₂/CO₂). The stabilization of metallic Ru during methanation, together with the enhanced formation of Ce³⁺ promoted by TiO₂more » through interfacial electronic transfer, accounts for their superior performance. Complementary in situ DRIFTS measurements reveal the formation and rapid consumption of bidentate carbonates and formates. These species act as a key intermediate in methane formation. Overall, these findings highlight the crucial role of the mixed CeO₂-TiO₂ oxide in tuning the surface chemistry of the catalysts by stabilizing metallic Ru, enhancing ceria reducibility, and promoting efficient reaction pathways for CO₂ methanation. The manipulation of metal↔oxide-oxide interactions can be a very useful tool when dealing with the valorization of CO₂.« less

Sodium-ion batteries are a promising lower-cost alternative to lithium-ion batteries, but further improvements in electrochemical performance are required. One strategy to increase capacity is to enable reversible high-valent cationic and anionic redox in layered cathode materials; however, this is typically accompanied by structural degradation. Here, in this study, we elucidate the mechanism by which Fe-doped Na₂Mn₃O₇, featuring ordered transition metal-vacancies, achieves reversible high-valent redox. Using Mössbauer spectroscopy, soft X-ray absorption spectroscopy (XAS), and in-situ hard XAS, we demonstrate reversible high-valent cationic redox involving both Fe and Mn while in-situ Raman confirms the absence of local structural degradation associated with oxygenmore » redox. Combining in-situ X-ray diffraction with theoretical calculations, we further identify a previously unreported global phase transition from the $$\bar{P1}$$ to the $$P2_1/c$$ space group during electrochemical cycling and develop a physical model describing this structural evolution. These results provide insights for structurally stable layered sodium transition metal oxide cathodes with reversible high-valent redox.« less

Metal-free heteroatom-doped carbocatalysts are promising alternatives to precious metals for alkane direct dehydrogenation/hydrogenation and reversible hydrogen storage, yet the nature of their active sites remains poorly understood. This study investigates a nitrogen assembly carbocatalyst (NAC) for efficient and selective hydrocarbon dehydrogenation. For ethylbenzene, NAC maintains a selectivity of >99% towards styrene at a conversion of >20% for 120 hours at a weight hourly space velocity of 0.4 h⁻¹. Theoretical studies suggest that closely spaced graphitic nitrogen sites are the active sites for the chemisorption and dehydrogenation of ethylbenzene, and the robustness of these sites is supported by ambient-pressure X-ray photoelectronmore » spectroscopy. Kinetic analysis reveals a temperature-dependent reaction profile, with distinct activation energies and reaction orders at 300 and 500 °C. Isotope-labeling studies indicate that dehydrogenation primarily proceeds via initial cleavage of the benzylic C–H bond, and the faster desorption of ethylbenzene at higher temperatures contributes to the difference in kinetic behavior. Importantly, the NAC catalyst also enables efficient hydrogenation of styrene back to ethylbenzene at 250 °C, allowing for reversible hydrogen storage using a single catalyst at moderate temperatures. These findings underscore the significance of constructing high densities of closely spaced graphitic nitrogen in carbocatalysts for enhanced activity and selectivity.« less

Experiments of in-situ DRIFTS combined with modulation excitation (ME) spectroscopy showed a rich surface chemistry associated with the adsorption of CO₂ on nanocubes and nanospheres of ceria. The nanocubes exposed faces with a (100) orientation, with the edges and corners displaying (110) and (111) orientations, respectively. Here, the nanospheres mainly contained ceria (111) and (110) planes. DFT calculations showed that CO₂ is a multidentate adsorbate on ceria that can undergo changes in its bonding configuration depending on the chemical environment. At 250 °C, a temperature typically used for the conversion of CO₂ into oxygenates, alkanes and olefins, CO₂ reacted withmore » O centers or OH groups present on the nanocubes and nanospheres to yield bi- and tri-dentate carbonates, hydroxycarbonates, and formates. Both nanostructures were highly reactive and a dynamic equilibrium was established: carbonate species were rapidly generated upon the injection of CO₂ and they decomposed upon the removal of CO₂ from the gas phase. In the case of the ceria nanocubes, the adsorption/desorption processes were essentially reversible, opening the door to catalytic transformations. A larger concentration of defects in the ceria nanospheres led to strongly bound carbonates and formates that may be spectators, site blockers, or surface modifiers in catalytic processes. In the ME studies, additional intermediates were detected, and it was clear that the response of surface species to the presence/absence of CO₂ was highly dependent on the morphology of the ceria nanostructures.« less

Insertion reactions play a central role in the catalytic synthesis of ethanol and higher alcohols. X-ray photoelectron and absorption spectroscopies have been used to follow migratory CO insertion and C─C coupling in a cis-[Ru(2,2′-bipyridine)₂(CO)(CH₃)]⁺ complex heated in a vacuum or exposed to CO. Heating of the Ru complex in a vacuum to temperatures above 50 °C induced spontaneous migration of CO into the Ru─CH₃ bond to yield a ─COCH₃ ligand. In conclusion, after adding CO to the background gas, the CO insertion reaction was seen at room temperature, opening the door for the synthesis of ethanol and more energy densemore » liquids.« less

It is generally accepted that metal–support interaction is very important for the hydrogenation of CO₂ to methanol, but little has been revealed about the feature of interfacial active sites under real reaction conditions since there are only limited techniques that can be applied under high-pressure conditions. Here, in this work, by combining multiple in situ and operando techniques on a model Cu/ceria catalyst, we have tracked Cu and ceria sites for methanol formation. Under the reaction condition, it is found that upon reaching the reaction temperature, oxidized Cu species in the as-synthesized catalyst immediately change into metallic Cu species. Followingmore » this, it is the gradual formation of methanol, the changing rate of which coincides with the formation of a unique Ce³⁺ species. The combined experimental results and density functional theory (DFT) calculations have determined that the formed Ce³⁺ sites driven by the reaction conditions are bound to hydrides, adsorbed carbonate species, and interfacial active Cu sites. The Cu-ceria interaction in this complex moiety is weak and can be easily disturbed with reaction environment variations, leading to dynamic changes at the interface upon the hydrogenation of active carbonate intermediates, which are precursors for the formation of methanol. The formation of this unique Cu–Ce³⁺ interface and its dynamicity lead to an increase of methanol selectivity from less than 20% to 60%. These results suggest that reactant-derived species (H^– and carbonate in this work) can be essential components of the active center with the functions of manipulating the metal−oxide interaction and directing reaction pathways.« less

The hydrogenolysis of plastics is limited by active-site inaccessibility and inefficient mass transport of bulky polymer chains. To overcome these challenges, this work developed two-dimensional MXene-supported Ru (Ru@MXene) catalysts. Lyophilization of a solution containing dispersed MXene sheets and Ru precursors enabled the confinement of Ru species within the MXene interlayers, which act as pillars to expand the interlayer spacing. Building on this, a silica-pillared MXene-supported Ru (Ru@P-MXene) with even larger interlayer spacing exhibited a reaction rate of 914.9 g_C5–C35 g_Ru ^–1 h^–1 for the hydrogenolysis of low-density polyethylene (LDPE) into valuable liquid chemicals (e.g., C₅–C₃₅). A comparison of product yieldsmore » between Ru@P-MXene and Ru@MXene suggests that elongated Ru particles confined within the MXene support expose their side facets for the reaction. In conclusion, this work demonstrates a new application of MXene in thermochemical catalysis, offering a solution to the challenges of active-site accessibility, mass transport, and reaction confinement in chemical plastic upcycling.« less

The performance of Ru/CeO₂ catalysts under CO₂ hydrogenation conditions was studied using in situ X-ray absorption fine structure (XAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to understand the structural evolution and chemical nature of each component under reaction conditions. The catalysts were prepared using a reverse microemulsion method that maximized the dispersion of RuOx particles on ceria. A RuO_x → Ru transformation was observed upon exposure of the RuO_x/CeO₂ systems to H₂ at 250 °C. For a sample with 5% molar Ru, two-dimensional clusters (2-4 atoms) of Ru formed on top of the ceria support. An increase inmore » the loading of ruthenium to 20% molar led to the formation of nanoplates of the metal 2-3 atomic layers thick. Both Ru structures were highly dispersed on ceria. They were oxidized after exposure to CO₂ at 250 °C. However, under CO₂/H₂ mixture, they remained in a metallic state as two-dimensional clusters and nanoplates. In situ DRIFTS studies, the 5% and 20% Ru/CeO₂ catalysts showed distinct reaction intermediates when exposed to CO₂ or CO₂/H₂ (1/4) reaction mixtures. Normalized to the Ru molar percentage, the 5% Ru/CeO₂ system was the most active catalyst exhibiting a selectivity of ~ 60% CH₄ and 40% CO at 250 °C. On the other hand, under the same reaction conditions, the 20% Ru/CeO₂ system was less active but had a CH₄ selectivity close to 80%. In conclusion, these results highlight the importance of the structure of the metallic Ru particles as a factor that determines the catalytic performance.« less

Dispersed metal species in siliceous zeolites have been actively studied for non-oxidative dehydrogenation of ethane (NDE). Fundamental insights into the dynamics of metal species in zeolites under reaction conditions have rarely been explored. Herein, we report an atomic level understanding of the dynamics and activity of cobalt (Co) sites in dealuminated Beta zeolite (DeAl-BEA) for NDE during induction and reaction conditions with extensive characterization techniques such as diffuse reflectance UV–vis, solid state nuclear magnetic resonance and X-ray photoelectron, X-ray diffraction along with in situ Fourier transform infrared and X-ray absorption spectroscopy. For a catalyst with 0.5 mass % Co loading,more » tetrahedral Co²⁺ mononuclear sites, di-coordinated to the zeolite framework and with two silanol groups in vicinity (i.e., (≡SiO)₂Co(HO–Si≡)₂), form upon exposure to hydrogen during induction and persist through the NDE reaction. Increasing the Co loading to 3.0 mass % yielded Co sites with similar electronic and coordination structures but slightly elongated Co–O bonds. Upon cooling to room temperature, the Co sites persisted in the same coordination environment, though the disappearance of a feature in the Co K-edge near-edge region revealed changes in the active site’s electronic structure coinciding with modest shifts in bond lengths. The electronic structure and activity of (≡SiO)₂Co(HO–Si≡)₂ sites were studied comparatively to a few other hypothetical Co²⁺ coordination structures, using electronic structure calculations and microkinetic simulations. The simulations showed that NDE is controlled by β-hydride elimination following C–H bond activation and that Co-sites possessing flexibility because of neighboring silanol defects are more active. Interestingly, dinuclear Co–O–Co sites (i.e., (≡SiO)Co(HO–Si≡)₂–O–(HO–Si≡)₂Co(≡SiO)) were more active than the mononuclear (≡SiO)₂Co(HO–Si≡)₂ sites because of favorable hydrogen bonding with the vicinal silanol groups. In conclusion, the present study bridges the gap between the knowledge acquired by ex-situ characterizations and the active sites under the reaction conditions in alkane dehydrogenation chemistry.« less

A combination of several in-situ techniques (XRD, XAS, AP-XPS, E-TEM) was used to explore links between the structural and chemical properties of a Cu@TiO_x catalyst under CO₂ hydrogenation conditions. The active phase of the catalyst involved an inverse oxide/metal configuration, but the initial core@shell motif was disrupted during the pre-treatment in H₂. As a consequence of strong metal-support interactions, the titania shell cracked and Cu particles migrated from the core to on top of the oxide with the simultaneous formation of a Cu-Ti-O_x phase. The generated Cu particles had a diameter of 20-40 nm and were decorated by small clustersmore » of TiO_x (< 5 nm in size). Results of in-situ XAS and XRD and images of E-TEM showed a very dynamic system, where the inverse oxide/metal configuration promoted the reactivity of the system towards CO₂ and H₂. At room temperature, CO₂ oxidized the Cu nanoparticles (CO_2,gas → CO_gas + O_oxide) inducing a redistribution of the TiO_x clusters and big modifications in catalyst surface morphology. The generated oxide overlayer disappeared at elevated temperatures (> 180 °C) upon exposure to H₂, producing a transient surface that was very active for the reverse water-gas shift reaction (CO₂ + H₂ → CO + H₂O) but was not stable at 250 °C. When oxidation and reduction occurred at the same time, under a mixture of CO₂ and H₂, the surface structure evolved toward a dynamic equilibrium that strongly depended on the temperature. Neither CO₂ nor H₂ can be considered as passive reactants. In the Cu@TiO_x system, morphological changes were linked to variations in the composition of metal-oxide interfaces which were reversible with temperature or chemical environment and affected the catalytic activity of the system. Finally, the present study illustrates the dynamic nature of phenomena associated with the trapping and conversion of CO₂.« less