128 Search Results

Direct ethylene epoxidation is among the highest value processes in the chemical industry, yet the reaction mechanism remains debated. A central question is whether the unpromoted Ag catalyst is metallic or oxidized under reaction conditions, as this determines the active oxidant species. Using ambient pressure X-ray photoelectron spectroscopy at chemical potentials simulating industrial conditions, we find that under oxidizing environments, nucleophilic oxygen (∼80% surface coverage) and some carbonate impurities (∼20% coverage) form on Ag(111). Upon switching to an industrially relevant 5:2 ethylene-to-oxygen ratio at 433 K, nucleophilic oxygen is consumed, leaving mostly surface carbonate and bare Ag. The Ag(111) surfacemore » maintains ∼50% exposed metallic sites under these conditions. This indicates that proposed mechanisms involving a fully oxidized surface may not represent the state of the surface under relevant reaction conditions and that bare Ag sites, which are necessary to form the oxametallacycle intermediate thought to drive selective epoxidation, are available.« less

Here, we investigated the mechanisms underlying strong metal-support interactions in CO oxidation using model systems where noble metal crystals support reducible, monolayer-thick CoO_x films. The effect of the Co oxidation state, film thickness, and substrate identity were studied in varying reaction conditions using ambient pressure X-ray photoelectron spectroscopy. At low O₂ pressures, the same oxide phase forms on both Pt(111) and Au(111) surfaces. But when heated at higher O₂ pressures, the oxide phase depends on the substrate. We found that CoO_x/Pt is more active for the CO oxidation reaction than CoO_x/Au, even when both surfaces stabilize the same oxide phase.more » DFT calculations on these and related noble-metal-supported CoO_x films reveal an SMSI-induced reactivity enhancement that strongly depends on the oxide film thickness and which is mediated by charge transfer between the metal and oxide. Charge transfer is also found to correlate with the reaction energy and activation barrier for CO oxidation. This effect was found to be greatest for oxide films on Pt, decreasing on other noble metal supports in the order Pt > Pd > Au > Ag, in agreement with the experiments. The role of charge transfer in the activation barriers and reaction energies provides insight into the nature of SMSI-induced catalytic activity, and suggests that the noble metal work function can serve as an indicator for the strength of SMSI effects.« less

Palladium-based catalysts are often used in carbon dioxide (CO₂) hydrogenation reactions with different possible reaction pathways producing methanol, methane, formic acid, or carbon monoxide. We used ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to understand the surface reaction mechanism of CO₂ hydrogenation on Pd(111). AP-XPS results show that the order in which the reactants are introduced yields different initial surface species. In a CO₂ and H₂ mixture, carbonaceous species (CH_x, C*, PdC_x) are formed upon heating Pd(111) at 400 K and above. In addition, a Pd(111) sample pre-exposed to an elevated pressure of H₂ followed by evacuation to ultrahigh vacuum (UHV)more » can store enough hydrogen atoms in the subsurface and bulk to provide a hydrogen source for the CO₂ hydrogenation reaction to occur when exposed to CO₂ gas alone. In conclusion, the formation of carbonaceous species likely occurs through the decomposition of a transient CH_xO intermediate facilitated by low-coordinated sites and surface defects.« less

Methanol is a strategic energy vector for the storage and delivery of energy and is a widely used precursor for the synthesis of many high-value chemicals. The hydrogenation of carbon dioxide (CO₂) into methanol is a key process in industrial operations. Here, in this study, we show that an oxide-oxide interface generated by a low loading (0.15 ML) of In₂O_3-x on a TiO₂(110) substrate has a high activity and selectivity as a catalyst for the CO₂ + 3H₂ → CH₃OH + H₂O process. The properties of the In₂O_3-x-TiO₂ interface under reaction conditions were investigated using a combination of synchrotron-based ambientmore » pressure X-ray photoelectron spectroscopy (AP-XPS), temperature programmed desorption (TPD), and catalytic testing. The In₂O_3-x overlayer spread out on top of the titania and was rich in defects and O vacancies that activated CO₂ and H₂ as reactants, without destroying CH₃O and CH₃OH as reaction products. The In₂O_3-x/TiO₂(110) catalyst is at least one order of magnitude more active than bulk indium oxide while maintaining a very high selectivity (~80%) towards methanol production. Under the rich hydrogen environment of methanol synthesis, the oxide-oxide interactions allowed only a partial reduction of the In cations, preventing the formation of metal alloys as seen in the case of catalysts with metal-indium oxide interfaces. Thus, the dispersion of low loadings of In₂O_3-x on a stable oxide substrate is a valid and low-cost approach for generating efficient catalysts for CO₂ valorization.« less

The interaction of oxygen with silver is a key descriptor of the catalytic reactivity of silver nanoparticles which are ubiquitous in large-scale partial oxidation reactions like ethylene epoxidation. Despite Ag(100) being proposed as the most selective facet, it is less studied than (111) and (110) surfaces. Using scanning tunneling microscopy and synchrotron X-ray photoelectron spectroscopy, we report that, in addition to the well-known O adatoms formed from O₂ dissociation on Ag(100), hydroxyl groups (OH), at a binding energy of ∼ 531 eV, are also present. The O/OH ratio depends on exposure to water and surface temperature. These assignments are consistentmore » with our density functional theory calculations, which indicate that the formation of two OH groups from an O atom and H₂O molecule is exothermic. These results indicate that, in addition to O, OH is present even under ultrahigh vacuum conditions and therefore should be considered in proposed catalytic pathways.« 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

The selective conversion of methane (CH4) under mild conditions remains challenging due to strong C-H bonds and catalyst coking. We systematically investigated sub-monolayer MgO nanostructures on Au(111), where two-dimensional (2D) MgO islands with stable Mg-O-Au interfaces catalyze low-temperature CH4 activation and C-C coupling. Upon CH4 exposure at 300 K, surface-bound CHx and C2Hx intermediates formed and persisted post-evacuation, indicating robust CHx-O-Mg linkages. Temperature-programmed studies revealed that C-H activation and C-C coupling intensify with heat: the CHx signal grew continuously while the C2Hx signal reached a plateau at 400-500 K. O 1s and Mg 2p attenuation confirmed adsorption of the hydrocarbonsmore » on MgO. Catalytic tests at 500 K yielded C2H6 (70%) and C2H4 (30%) without coking, underscoring MgO's role as an active catalyst. These results offer new design principles for developing coke-resistant and low-temperature methane upgrading catalysts.« less

Integrated rocksalt‐polyanion cathodes (DRXPS) are promising candidates for next‐generation lithium‐ion battery cathode materials that combine high energy density, stable cycling performance, and reduced reliance on Co and Ni. In this work, we investigated Li₃Mn_1.6P_0.4O_5.4F_0.6, a new DRXPS cathode with fluoride incorporation. A pure spinel phase was formed and a discharge capacity retention of 84% was achieved after 200 cycles between 1.5 and 4.8 V versus Li/Li⁺. In comparison, the similarly synthesized Li₃Mn_1.6Nb_0.4O_5.4F_0.6, in which all P⁵⁺ was substituted by Nb⁵⁺ while maintaining the same stoichiometry for all other elements, crystallized in a disordered rocksalt structure, and exhibited inferior capacity retentionmore » and rate capability than the P⁵⁺ counterpart. Our findings expand the compositional space of DRXPS to include F⁻, justify the viability of integrating polyanion groups in rocksalt‐type cathodes, and highlight the superiority of P⁵⁺ as a cation charge compensator compared to the commonly used Nb⁵⁺. This work thereby advances the design of robust, high‐performance cathode materials for sustainable batteries.« less

Synchrotron-based ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to study the surface chemistry of methanol on TiO₂(110), examining the effects of methanol pressure, oxide temperature, and coadsorption with H₂. At 300 K, the adsorption of methanol on TiO₂(110) leads to the formation of CH₃O on the surface with a minor amount of CH₃OH present. The easy cleavage of the O–H bond in the alcohol agrees with the predictions of theoretical calculations, and most of the adsorbed CH₃O was not associated with the presence of Ti³⁺ sites in the oxide substrate. The adsorbed CH₃O was removed from the TiO₂(110) surfacemore » by heating to 600 K without the deposition of CH_x fragments or C on the oxide. The results of temperature-programmed desorption (TPD) showed the evolution of methanol and formaldehyde at 360 and 490 K as a result of a disproportionation reaction: 2CH₃O → CH₃OH + CH₂O. This surface chemistry, where there is no rupture of the C–O bond, and only selective cleavage of O–H and C–H bonds, is very different from that found on metals used in catalysts for methanol reforming, where massive conversion of the alcohol into CO, CH_x and C species is seen. Furthermore, the TPD data indicate that any CH₃O formed on the oxide surface can be hydrogenated and desorbed as CH₃OH at temperatures below 550 K. In this respect, titania is an ideal support for catalysts employed to achieve methanol synthesis through CO₂ hydrogenation.« less

All-solid-state batteries hold greater promise for improving safety and energy density over conventional battery technology employing organic liquid electrolytes. One of the required features of a Li⁺ conducting solid electrolyte is electrochemical stability, attained thermodynamically or kinetically, within the targeted operating voltage and temperature ranges. Therefore, understanding of the oxidative or reductive degradation mechanism is important to allow the design of stable solid electrolyte materials. This work contributes to building an understanding of the oxidative degradation mechanism in lithium solid electrolytes at cell operating conditions. Here, we have focused on resolving the oxidative decomposition mechanism of Al-doped lithium garnet Li_6.28Al_0.24La₃Zr₂O₁₂more » (LLZO) as a state-of-the-art inorganic ceramic electrolyte. By combining experimental and computational analyses, we show that oxidation of LLZO occurs by simultaneous loss of oxygen and lithium from the structure, resulting in substoichiometric LLZO, at a moderate temperature (80 °C) and a high electrode potential (4.3 V vs Li/Li⁺). Based on X-ray absorption and diffraction analyses, we find that the zirconium coordination shells in LLZO contract while the crystal structure experiences positive chemical strain upon electrochemical oxidation. The results from ex situ structural characterization of both the local structure and crystal symmetry are supported by a substoichiometric LLZO with lithium and oxygen vacancies, modeled by density functional theory (DFT) calculations. These chemical and structural changes in LLZO suppress effective lithium-ion conductivity by an order of magnitude. Formation of lithium and oxygen vacancies in LLZO upon electrochemical oxidation is different from prior thermodynamic predictions of phase decomposition of LLZO. The difference here is that the experiments were conducted at near-room temperature, which can hinder the kinetics of phase separation, and thus, the resultant LLZO solid electrolyte is still single-phase but substoichiometric in Li and O. In conclusion, these findings contribute an important degradation mechanism of the electrolyte, relevant for practical operational conditions of solid-state batteries.« less