Observation of Transient Iron(II) Formation in Dye-Sensitized Iron Oxide Nanoparticles by Time-Resolved X-ray Spectroscopy

The reduction of ferric iron in solid phase minerals leads to the mobilization of ferrous iron in the environment and is thus a crucial component of the global iron cycle. Despite the importance of this process, a mechanistic understanding of the structural and chemical changes that are caused by this electron transfer reaction is not established because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Ultrafast time-resolved X-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring during chemical reactions that can be initiated by a fast laser pulse. We use this approach with ∼100 ps resolution to monitor the speciation of Fe atoms in iron oxide nanoparticles following photoinduced electron transfer from a surface-bound photoactive dye molecule. These data represent the first direct real-time observation of the dynamics of ferrous ion formation and subsequent reoxidation in iron oxide

Tracking Electrons and Atoms in a Photoexcited Metalloporphyrin by X-ray Transient Absorption Spectroscopy

Simultaneously tracking electronic and molecular structures of a photoexcited metalloporphyrin, present for only 200 ps in a dilute solution, has been realized using X-ray transient absorption spectroscopy (XTA). Using laser pulses as excitation sources and delayed X-ray pulses as probes, we were able to identify the excited state electronic configuration of a nickel porphyrin as singly occupied 3dx2-y2 and 3dz2 molecular orbitals (MOs) with an energy gap of ∼2.2 eV, and energy shifts 4pz MOs to 1.5 eV higher relative to that of the ground state, and an expanded porphyrin ring characterized by lengthening of Ni−N and Ni−C bonds. Moreover, kinetic XTA signals at different X-ray photon energies demonstrate the capability for acquiring the correlation and coherence between different optically excited states with the same technique. These results provide guidance for theoretical calculations as well as insightful understanding of optically excited states that play important roles in photochemical processes

Elucidation of the Active Phase and Deactivation Mechanisms of Chromium Nitride in the Electrochemical Nitrogen Reduction Reaction

Metal nitrides have been suggested to be effective catalysts for the electrochemical nitrogen reduction reaction (ENRR) based on computational investigations; however, experimental verification has been scarce. In this work, we demonstrate that chromium nitride is an active and selective ENRR catalyst in a Nafion-based membrane electrode assembly. Both the specific ENRR rate (1.4 × 10–11 mol cm–2 s–1) and faradic efficiency (0.58%) on the chromium nitride catalyst are approximately 20 times higher than those on Pt at −0.2 V vs the reversible hydrogen electrode. Although the only bulk phase identified by X-ray diffraction of the chromium nitride catalyst is pure phase Cr2N, X-ray photoelectron spectroscopy (XPS) investigations reveal that CrN, CrNxOy, and CrOx species, in addition to Cr2N, are present on the surface of the catalyst. In contrast, a synthesized chromium nitride sample with a bulk CrN phase shows a negligible ENRR rate. XPS shows that the synthesized sample does not possess any Cr2N species on the surface, which leads to the identification of Cr2N as the active phase in ENRR. Batch cell testing with 15N2 as the feed forms both 14NH3 and 15NH3, indicating the involvement of surface N in the activation of dinitrogen, i.e., a Mars–van Krevelen mechanism. Two mechanisms of catalyst deactivation are identified: (1) leaching of surface N at lower potentials (<−0.4 V) and (2) slow conversion of the active Cr2N phase to the inactive CrN phase at −0.2 V

Visualizing Interfacial Charge Transfer in Ru-Dye-Sensitized TiO₂ Nanoparticles Using X-ray Transient Absorption Spectroscopy

A molecular level understanding of the structural reorganization accompanying interfacial electron transfer is important for rational design of solar cells. Here we have applied XTA (X-ray transient absorption) spectroscopy to study transient structures in a heterogeneous interfacial system mimicking the charge separation process in dye-sensitized solar cell (DSSC) with Ru(dcbpy)₂(NCS)₂ (RuN3) dye adsorbed to TiO₂ nanoparticle surfaces. The results show that the average Ru-NCS bond length reduces by 0.06 Å, whereas the average Ru−N(dcbpy) bond length remains nearly unchanged after the electron injection. The differences in bond-order change and steric hindrance between two types of ligands are attributed to their structural response in the charge separation. This study extends the application of XTA into optically opaque hybrid interfacial systems relevant to the solar energy conversion

Reduction of Propionic Acid over a Pd-Promoted ReO_<i>x</i>/SiO₂ Catalyst Probed by X‑ray Absorption Spectroscopy and Transient Kinetic Analysis

A Pd-promoted Re/SiO₂ catalyst was prepared by sequential impregnation and compared to monometallic Pd/SiO₂ and Re/SiO₂. All samples were characterized by electron microscopy, H₂ and CO chemisorption, H₂ temperature-programmed reduction, and <i>in situ</i> X-ray absorption spectroscopy at the Re L_III and Pd K-edges. The samples were also tested in the reduction of propionic acid to 1-propanol and propionaldehyde at 433 K in 0.1–0.2 MPa H₂. Whereas monometallic Pd was inactive for carboxylic acid reduction, monometallic Re catalyzed aldehyde formation but only after high-temperature prereduction that produced metallic Re. When Pd was present with Re in a bimetallic catalyst, Pd facilitated the reduction of Re in H₂ to ∼+4 oxidation state at modest temperatures, producing an active catalyst for the conversion of propionic acid to 1-propanol. Under the conditions of this study, the orders of reaction in propionic acid and H₂ were approximately zero and one, respectively. Transient kinetic analysis of the carboxylic acid reduction to alcohols revealed that at least 50% of the Re in the bimetallic catalyst participated in the catalytic reaction. The Pd is proposed to enhance the catalytic activity of the bimetallic catalyst by spilling over hydrogen that can partially reduce Re and react with surface intermediates

Adapting an Electron Microscopy Microheater for Correlated and Time-Resolved Ambient Pressure X‑ray Photoelectron Spectroscopy

Environmental transmission electron microscopy (ETEM) can follow structural and chemical changes in nanomaterials under reaction conditions, including at temperatures up to 1300 °C and pressures up to ∼20 mbar. However, ETEM studies are limited to localized areas of a sample and can benefit from correlative studies of a similar sample on larger length scales. Here, we describe the customization of a commercial microheater holder used in ETEM to anode-based ambient pressure X-ray photoelectron spectroscopy (APXPS), where chemical and electronic structure information is averaged over several hundred square micrometers. To benchmark heating capabilities in the APXPS instrument, core level binding energies were followed during surface chemical reactions of a Pd film, including CO adsorption–desorption, Pd oxidation, and PdO reduction. The fast heating feature of the microheaters advanced XPS experimental capabilities, including the collection of subsecond time-resolved spectra of the Pd 3d5/2 peak during the reduction of an oxidized Pd film. The potential to use these heaters in a closed gas cell was also demonstrated by oxidizing Pd in a partial pressure of air locally dosed to the sample surface. General tips and considerations for the application of commercial ETEM heaters to any XPS system with a focused X-ray beam are discussed

Highly Active Ceria-Supported Ru Catalyst for the Dry Reforming of Methane: In Situ Identification of Ru^δ+–Ce³⁺ Interactions for Enhanced Conversion

The metal–oxide interaction changes the surface electronic states of catalysts deployed for chemical conversion, yet details of its influence on the catalytic performance under reaction conditions remain obscure. In this work, we report the high activity/stability of a ceria-supported Ru–nanocluster (<1 nm) catalyst during the dry reforming of methane. To elucidate the structure–reactivity relationship underlying the remarkable catalytic performance, the active structure and chemical speciation of the catalyst was characterized using in situ X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS), while the surface chemistry and active intermediates were monitored by in situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Methane activates on the catalyst surface at temperatures as low as 150 °C. Under reaction conditions, the existence of metal–support interactions tunes the electronic properties of the Ru nanoclusters, giving rise to a partially oxidized state of ruthenium stabilized by reduced ceria (Ruδ+–CeO2–x) to sustain active chemistry, which is found to be very different from that of large Ru nanoparticles supported on ceria. The oxidation of surface carbon is also a crucial step for the completion of the catalytic cycle, and this is strongly correlated with the oxygen transfer governed by the Ruδ+–CeO2–x interactions at higher temperatures (>300 °C). The possible reaction pathways and stable surface intermediates were identified using DRIFTS including ruthenium carbonyls, carboxylate species, and surface −OH groups, while polydentate carbonates may be plain spectators at the measured reaction conditions

Suppression of Superconductivity and Nematic Order in Fe_1–<i>y</i>Se_1–<i>x</i>S_<i>x</i> (0 ≤ <i>x</i> ≤ 1; <i>y</i> ≤ 0.1) Crystals by Anion Height Disorder

Connections between crystal chemistry and critical temperature Tc have been in the focus of superconductivity, one of the most widely studied phenomena in physics, chemistry, and materials science alike. In most Fe-based superconductors, materials chemistry and physics conspire so that Tc correlates with the average anion height above the Fe plane, i.e., with the geometry of the FeAs4 or FeCh4 (Ch = Te, Se, or S) tetrahedron. By synthesizing Fe1–ySe1–xSx (0 ≤ x ≤ 1; y ≤ 0.1), we find that in alloyed crystals Tc is not correlated with the anion height like it is for most other Fe superconductors. Instead, changes in Tc(x) and tetragonal-to-orthorhombic (nematic) transition Ts(x) upon cooling are correlated with disorder in Fe vibrations in the direction orthogonal to Fe planes, along the crystallographic c-axis. The disorder stems from the random nature of S substitution, causing deformed Fe­(Se,S)4 tetrahedra with different Fe–Se and Fe–S bond distances. Our results provide evidence of Tc and Ts suppression by disorder in anion height. The connection to local crystal chemistry may be exploited in computational prediction of new superconducting materials with FeSe/S building blocks