Correction: Improving time-resolution and sensitivity of in situ X-ray photoelectron spectroscopy of a powder catalyst by modulated excitation.

[This corrects the article DOI: 10.1039/D3SC01274C.].

Improving time-resolution and sensitivity of in situ X-ray photoelectron spectroscopy of a powder catalyst by modulated excitation.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful tool to characterize the surface structure of heterogeneous catalysts in situ. In order to improve the time resolution and the signal-to-noise (S/N) ratio of photoemission spectra, we collected consecutive APXP spectra during the periodic perturbation of a powder Pd/Al2O3 catalyst away from its equilibrium state according to the modulated excitation approach (ME). Averaging of the spectra along the alternate pulses of O2 and CO improved the S/N ratio demonstrating that the time resolution of the measurement can be limited solely to the acquisition time of one spectrum. Through phase sensitive analysis of the averaged time-resolved spectra, the formation/consumption dynamics of three oxidic species, two metal species, adsorbed CO on Pd0 as well as Pdn+ (n > 2) was followed along the gas switches. Pdn+ and 2-fold surface PdO species were recognised as most reactive to the gas switches. Our approach demonstrates that phase sensitive detection of time-resolved XPS data allows following the dynamics of reactive species at the solid-gas interface under different reaction environments with unprecedented precision.

Dynamic interplay between metal nanoparticles and oxide support under redox conditions.

The dynamic interactions between noble metal particles and reducible metal-oxide supports can depend on redox reactions with ambient gases. Transmission electron microscopy revealed that the strong metal-support interaction (SMSI)-induced encapsulation of platinum particles on titania observed under reducing conditions is lost once the system is exposed to a redox-reactive environment containing oxygen and hydrogen at a total pressure of ~1 bar. Destabilization of the metal-oxide interface and redox-mediated reconstructions of titania lead to particle dynamics and directed particle migration that depend on nanoparticle orientation. A static encapsulated SMSI state was reestablished when switching back to purely oxidizing conditions. This work highlights the difference between reactive and nonreactive states and demonstrates that manifestations of the metal-support interaction strongly depend on the chemical environment.

A DuMond-type crystal spectrometer for synchrotron-based X-ray emission studies in the energy range of 15-26 keV.

The design and performance of a high-resolution transmission-type X-ray spectrometer for use in the 15-26 keV energy range at synchrotron light sources is reported. Monte Carlo X-ray-tracing simulations were performed to optimize the performance of the transmission-type spectrometer, based on the DuMond geometry, for use at the Super X-ray absorption beamline of the Swiss Light Source at the Paul Scherrer Institute. This spectrometer provides an instrumental energy resolution of 3.5 eV for X-ray emission lines around 16 keV and 12.5 eV for emission lines at 26 keV, which is comparable to the natural linewidths of the K and L X-ray transitions in the covered energy range. First experimental data are presented and compared with results of the Monte Carlo X-ray simulations.

Increasing the activity of copper exchanged mordenite in the direct isothermal conversion of methane to methanol by Pt and Pd doping.

PtCu- and PdCu-mordenite allow for isothermal reaction at 200 °C for the stepwise methane to methanol conversion with comparably high yields. In contrast to traditional Cu-zeolites, these materials are more reactive under isothermal conditions than after high temperature activation.

Quantitative region-of-interest tomography using variable field of view.

In X-ray computed tomography, the task of imaging only a local region of interest (ROI) inside a larger sample is very important. However, without a priori information, this ROI cannot be exactly reconstructed using only the image data limited to the ROI. We propose here an approach of region-of-interest tomography, which reconstructs a ROI within an object from projections of different fields of view acquired on a specific angular sampling scheme in the same tomographic experiment. We present a stable procedure that not only yields high-quality images of the ROI but keeps as well the quantitative contrast on the reconstructed images. In addition, we analyze the minimum number of projections required for ROI tomography from the point of view of the band region of the Radon transform, which confirms this number must be estimated based on the size of the entire object and not only on the size of the ROI.

Identification of a new low energy 1u state in dicopper with resonant four-wave mixing.

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A' is identified with T0 = 20 100.4090(16) cm-1 (63Cu2). Rotational analysis of the A'-X (0,0) and (1,0) transitions leads to the assignment of A' 1u. Ab initio calculations present the first theoretical description of the low energy states of the copper dimer in Hund's case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state.

A three-dimensional view of structural changes caused by deactivation of fluid catalytic cracking catalysts.

Since its commercial introduction three-quarters of a century ago, fluid catalytic cracking has been one of the most important conversion processes in the petroleum industry. In this process, porous composites composed of zeolite and clay crack the heavy fractions in crude oil into transportation fuel and petrochemical feedstocks. Yet, over time the catalytic activity of these composite particles decreases. Here, we report on ptychographic tomography, diffraction, and fluorescence tomography, as well as electron microscopy measurements, which elucidate the structural changes that lead to catalyst deactivation. In combination, these measurements reveal zeolite amorphization and distinct structural changes on the particle exterior as the driving forces behind catalyst deactivation. Amorphization of zeolites, in particular, close to the particle exterior, results in a reduction of catalytic capacity. A concretion of the outermost particle layer into a dense amorphous silica-alumina shell further reduces the mass transport to the active sites within the composite.Catalyst deactivation in fluid catalytic cracking processes is unavoidably associated with structural changes. Here, the authors visualize the deactivation of zeolite catalysts by ptychography and other imaging techniques, showing pronounced amorphization of the outer layer of the catalyst particles.

Distribution of aluminum over different T-sites in ferrierite zeolites studied with aluminum valence to core X-ray emission spectroscopy.

The potential of valence to core Al X-ray emission spectroscopy to determine aluminum distribution in ferrierite zeolites was investigated. The recorded emission spectra of four samples prepared with different structure directing agents exhibit slight variations in the position of the main emission peak and the intensity of its low energy shoulder. Theoretical calculations indicate that an increased intensity of the Kßx shoulder in the Al emission spectra can be linked to a predominant occupation of the T3 site by a single aluminum atom. This study thus suggests that valence to core X-ray emission spectroscopy can be applied to help determine the occupation of aluminum at crystallographic T-sites in zeolites.

Rovibrational Characterization of High-Lying Electronic States of Cu2 by Double-Resonant Nonlinear Spectroscopy.

The available knowledge of the electronically excited states of the copper dimer is limited. This is common for transition metals, as the high density of states hinders both experimental assignment and computation. In this work, two-color resonant four-wave mixing spectroscopy was applied to neutral Cu2 in the gas phase. The method yielded accurate positions of individual rovibrational lines in the I-X and J-X electronic systems. This revealed the term symbols for the I and J states as 1Πu (1u) and 1Σu+ (0u+), respectively. For the 63Cu2 isotopologue, accurate molecular constants were obtained. The characterization of the J state finally allowed decisive determination of its electron configuration. The J state is obtained from the ground state by promotion of a 3dπg electron into the weakly bonding 4pπu molecular orbital. From the data analysis, lifetimes of the I state (between 10 ps and 5 ns) and J state (66 ns) were inferred.

Surface enhanced infrared absorption of chemisorbed carbon monoxide using plasmonic nanoantennas.

We report the enhancement of infrared absorption of chemisorbed carbon monoxide on platinum in the gap of plasmonic nanoantennas. Our method is based on the self-assembled formation of platinum nanoislands on nanoscopic dipole antenna arrays manufactured via electron beam lithography. We employ systematic variations of the plasmonic antenna resonance to precisely couple to the molecular stretch vibration of carbon monoxide adsorbed on the platinum nanoislands. Ultimately, we reach more than 1500-fold infrared absorption enhancements, allowing for an ultrasensitive detection of a monolayer of chemisorbed carbon monoxide. The developed procedure can be adapted to other metal adsorbents and molecular species and could be utilized for coverage sensing in surface catalytic reactions.

Experimental and theoretical investigation of the vibrational band structure of the 1 Πu5-1 Πg5 high-spin system of C2.

Vibrational levels of the recently observed high-spin transition (1 Πu5-1 Πg5) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear double-resonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d Πg3, υ = 8 and 1 Πg5, υ = 3 states results in accurate molecular constants for the υ = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d Πg3, υ = 8 are improved. The first excited vibrational state of 1 Πu5 is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d Πg3, υ=6∼1 Πg5,υ= 0 system. The rotationally resolved spectra yield 11 transitions to 1 Πu5, υ = 1 that include four spin-substates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, υ = 1 and 2 of the lower quintet state (1 Πg5) are measured by performing perturbation-assisted double-resonant excitation to the 1 Πu5, υ = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings.

X-ray emission spectroscopy: highly sensitive techniques for time-resolved probing of cerium reactivity under catalytic conditions.

Oxygen storage materials such as ceria are used in many catalytic applications because they can reversibly bind and release oxygen. Tools are needed to observe and quantify this activity which involves a change in the cerium oxidation state and to understand the involvement of cerium in catalytic processes. To prove that cerium changes its oxidation state in the catalytic cycle the transient rates of Ce3+ formation and decay should be compared to the overall reaction rate. For such mechanistic studies the time resolution is essential as the quantification of the Ce3+ species should be faster than the reaction rate. However, it is challenging to follow the dynamic changes of the cerium oxidation state under reaction conditions, especially when the concentration of cerium atoms involved in the reaction cycle is low. In this paper, we evaluate the sensitivity of high-resolution X-ray emission-based methods for the in situ time-resolved quantification of small concentrations of Ce3+ in ceria-based materials. We demonstrate that resonant X-ray emission spectroscopy (RXES) at optimal excitation energy is more sensitive than high energy resolution off-resonant spectroscopy (HEROS) and non-resonant X-ray emission spectroscopy (non-resonant XES) and that it can track the reactivity of less than 0.3% of cerium atoms in a 1% Pt/CeO2 catalyst in a plug-flow reactor with sub-second time resolution. These results demonstrate that X-ray emission-based methods can be used as very sensitive tools and provide new insights into dynamic changes of the oxidation state in reducible oxides in a variety of applications.

Synthesis of sub-nanometer gold particles on modified silica.

The deposition of gold on silica tends to give large particles when using conventional techniques. We report the preparation of 0.8 ± 0.2 nm particles on a modified SBA-15 support. The method involves the functionalization of silica with amine groups and deposition of gold at basic pH. These catalysts are highly active and selective in the dehydrogenation of formic acid.

Perturbation-facilitated detection of the first quintet-quintet band in C2.

The first high-spin transition in C2 (1 (5)Πu - 1 (5)Πg) is observed by perturbation-facilitated optical-optical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C2 radicals in the initial a (3)Πu, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate "gateway" states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 (5)Πg, v = 0 and the d (3)Πg, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 (5)Πu. In addition, satellite transitions (ΔJ ≠ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only.

Photoelectron spectrometer for attosecond spectroscopy of liquids and gases.

A new apparatus for attosecond time-resolved photoelectron spectroscopy of liquids and gases is described. It combines a liquid microjet source with a magnetic-bottle photoelectron spectrometer and an actively stabilized attosecond beamline. The photoelectron spectrometer permits venting and pumping of the interaction chamber without affecting the low pressure in the flight tube. This pressure separation has been realized through a sliding skimmer plate, which effectively seals the flight tube in its closed position and functions as a differential pumping stage in its open position. A high-harmonic photon spectrometer, attached to the photoelectron spectrometer, exit port is used to acquire photon spectra for calibration purposes. Attosecond pulse trains have been used to record photoelectron spectra of noble gases, water in the gas and liquid states as well as solvated species. RABBIT scans demonstrate the attosecond resolution of this setup.

Phosphine and phosphine oxide groups in metal-organic frameworks detected by P K-edge XAS.

Phosphine metal-organic frameworks (P-MOFs) are crystalline porous coordination polymers that contain phosphorus functional groups within their pores. We present the use of X-ray absorption spectroscopy (XAS) at the P K-edge to determine the phosphine to phosphine oxide ratio in two P-MOFs with MIL-101 topology. The phosphorus oxidation state is of particular interest as it strongly influences the coordination affinity of these materials for transition metals. This method can determine the oxidation state of phosphorus even when the material contains paramagnetic nuclei, differently from NMR spectroscopy. We observed that phosphine in LSK-15 accounts for 72 ± 4% of the total phosphorus groups and that LSK-12 contains only phosphine oxide.

Atomically dispersed rhodium on a support: the influence of a metal precursor and a support.

The influence of the support type and the metal precursor on the dispersion of rhodium after calcination and reduction was determined. The combination of electron microscopy and X-ray absorption analysis allowed the quantification of the amount of atomically dispersed rhodium in the samples. Higher amounts of atomically dispersed rhodium atoms are obtained when metal impregnation is performed with a rhodium acetate precursor in comparison to a rhodium chloride precursor over supports of the same composition. The stability of rhodium is improved with the addition of promoters; the co-presence of samaria and ceria in the support and metal impregnation with a rhodium acetate precursor leads to the highest amount of atomically dispersed rhodium remaining after reductive treatment at 773 K.

Communication: The electronic structure of matter probed with a single femtosecond hard x-ray pulse.

Physical, biological, and chemical transformations are initiated by changes in the electronic configuration of the species involved. These electronic changes occur on the timescales of attoseconds (10(-18) s) to femtoseconds (10(-15) s) and drive all subsequent electronic reorganization as the system moves to a new equilibrium or quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is crucial for understanding the potential energy surfaces upon which chemical and biological reactions take place. Here, we report on the determination of the electronic structure of matter using a single self-seeded femtosecond x-ray pulse from the Linac Coherent Light Source hard x-ray free electron laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we were able to obtain information about the electronic density of states with a single femtosecond x-ray pulse. We show that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis and that the measured spectral shape is independent of the large intensity fluctuations of the incoming x-ray beam. Moreover, we demonstrate the chemical sensitivity and single-shot capability and limitations of HEROS, which enables the technique to track the electronic structural dynamics in matter on femtosecond time scales, making it an ideal probe technique for time-resolved X-ray experiments.

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies.

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV-9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.