Methane synthesis and sulfur removal over a Ru catalyst probed in situ with high sensitivity X-ray absorption spectroscopy

Tuning the electronic metal-carbon interactions in Lignin-based carbon-supported ruthenium-based electrocatalysts for enhanced hydrogen evolution reactions

Ruthenium (Ru) nanoparticles dispersed on carbon support are promising electrocatalysts for hydrogen evolution reaction (HER) due to strong electronic metal-carbon interactions (EMCIs). Defects engineering in carbon supports is an effective strategy to adjust EMCIs. We prepared nitrogen/sulfur co-doped carbon supported Ru nanoparticles (Ru@N/S-LC) using sodium lignosulfonate and urea as feedstocks. Intrinsic S dopants from sodium lignosulfonate create rich S defects, thus enhancing the EMCIs within Ru@N/S-LC, leading a faster electron transfer between Ru nanoparticles and N/S-LC compared with N-doped carbon supported Ru nanoparticles (Ru@N-CC). The resulting Ru@N/S-LC exhibits an enhanced work function and a down-shifted d-band center, inducing stronger electron capturing ability and weaker hydrogen desorption energy than Ru@N-CC. Ru@N/S-LC requires only 7 and 94 mV overpotential in acidic medium and alkaline medium to achieve a current density of 10 mA cm-2. Density Functional Theory (DFT) calculations were utilized to clarify the impact of sulfur (S) doping and the mechanism underlying the notable catalytic activity of Ru@N/S-LC. This study offers a perspective for utilizing the natural dopants of biomass to adjust the EMCIs for electrocatalysts.

Catalytic Descriptor Exploration for Ru-Based Fischer-Tropsch Catalysts: Effect of Chlorine and Sulfur Addition

As an important factor in the design of catalysts, catalytic descriptor exploration has emerged as a novel frontier in heterogeneous catalysis. Here, the underlying structure-activity relationships of Ru-based catalysts are theoretically studied to shed light on this area. Calculations of different competing reaction paths suggest that the HCO*-mediated path─because of two synergistic active sites─is more favorable than others. In addition, compared to unadulterated Ru catalysts, the presence of Cl enhances the hydrocarbon production, whereas the presence of S decreases it. After a systematic examination of a series of structure-activity relationships (42 in total), we found that both charge transfer and average charge difference of active Ru atoms are good descriptors for the binding stability of reactants. However, for reactivity the Gibbs free energy of the reactants performs better. More interestingly, due to the quite different catalytic processes of the dissociation and hydrogenation steps, their correlations have opposite slopes.

Unraveling the Active Vanadium Sites and Adsorbate Dynamics in VOx/CeO2 Oxidation Catalysts Using Transient IR Spectroscopy

The oxidative dehydrogenation (ODH) of propane over supported vanadia catalysts is an attractive route toward propene (propylene) with the potential of industrial application and has been extensively studied over decades. Despite numerous mechanistic studies, the active vanadyl site of the reaction has not been elucidated. In this work, we unravel the ODH reaction mechanism, including the nuclearity-dependent vanadyl and surface dynamics, over ceria-supported vanadia (VO_x/CeO₂) catalysts by applying (isotopic) modulation excitation IR spectroscopy supported by operando Raman and UV-vis spectroscopies. Based on our loading-dependent analysis, we were able to identify two different mechanisms leading to propylene, which are characterized by isopropyl- and acrylate-like intermediates. The modulation excitation IR approach also allows for the determination of the time evolution of the vanadia, hydroxyl, and adsorbate dynamics, underlining the intimate interplay between the surface vanadia species and the ceria support. Our results highlight the potential of transient IR spectroscopy to provide a detailed understanding of reaction mechanisms in oxidation catalysis and the dynamics of surface catalytic processes in general.

Ruthenium 4d-to-2p X-ray Emission Spectroscopy: A Simultaneous Probe of the Metal and the Bound Ligands

Ruthenium 4d-to-2p X-ray emission spectroscopy (XES) was systematically explored for a series of Ru²⁺ and Ru³⁺ species. Complementary density functional theory calculations were utilized to allow for a detailed assignment of the experimental spectra. The studied complexes have a range of different coordination spheres, which allows the influence of the ligand donor/acceptor properties on the spectra to be assessed. Similarly, the contributions of the site symmetry and the oxidation state of the metal were analyzed. Because the 4d-to-2p emission lines are dipole-allowed, the spectral features are intense. Furthermore, in contrast with K- or L-edge X-ray absorption of 4d transition metals, which probe the unoccupied levels, the observed 4p-to-2p XES arises from electrons in filled-ligand- and filled-metal-based orbitals, thus providing simultaneous access to the ligand and metal contributions to bonding. As such, 4d-to-2p XES should be a promising tool for the study of a wide range of 4d transition-metal compounds.

Ruthenium 4d-to-2p XES was applied to a series of molecular Ru complexes with varied coordination environment, oxidation state and site symmetry. Through correlations to calculations, it is demonstrated the Ru 4d-to-2p XES provides a unique probe of both the filled ligand np and filled metal 4d orbitals, providing a promising new tool for the study of a wide range of 4d transition metals.

Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNO x by modulation excitation IR spectroscopy

The small pore zeolite Cu-SSZ-13 is an efficient material for the standard selective catalytic reduction of nitrogen oxides (NO _x ) by ammonia (NH₃). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analysed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the experiments were performed under cyclic Cu⁺/Cu²⁺ redox, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments provide spectroscopic evidence for previously postulated intermediates Cu-N([double bond, length as m-dash]O)-NH₂ and Cu-NO₃ in the NH₃-SCR deNO _x mechanism and for the role of [Cu²⁺(OH^-)]⁺. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.

Investigation of periodically driven systems by x-ray absorption spectroscopy using asynchronous data collection mode

We report the development, testing, and demonstration of a setup for modulation excitation spectroscopy experiments at the Inner Shell Spectroscopy beamline of National Synchrotron Light Source - II. A computer algorithm and dedicated software were developed for asynchronous data processing and analysis. We demonstrate the reconstruction of X-ray absorption spectra for different time points within the modulation pulse using a model system. This setup and the software are intended for a broad range of functional materials which exhibit structural and/or electronic responses to the external stimulation, such as catalysts, energy and battery materials, and electromechanical devices.

An operando emission spectroscopy study of Pt/Al2O3 and Pt/CeO2/Al2O3

In situ time-resolved spectroscopic examination of catalysts based on well dispersed nanoparticles on metal oxides under transient conditions significantly facilitates the elucidation of reaction mechanisms. In this contribution, we demonstrate the level of structural information that can be obtained using high-energy resolution off-resonant spectroscopy (HEROS) to study 1.3 wt% Pt/Al₂O₃ and 1.3 wt% Pt/20 wt% CeO₂/Al₂O₃ catalysts subjected to redox pulsing. First, HEROS is compared with XANES in a temperature programmed reduction experiment to demonstrate the increased sensitivity and time resolution of HEROS. Second, modulation excitation spectroscopy is exploited by redox pulsing to enhance the sensitivity of HEROS to structural changes by the application of phase sensitive detection (PSD) to the time-resolved HEROS data set. The HEROS measurements were complemented by resonant X-ray emission (RXES) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy measurements performed under identical conditions and in a single reactor cell in order to probe different aspects of the catalyst materials under the selected experimental conditions.

Homogeneous and highly dispersed Ni–Ru on a silica support as an effective CO methanation catalyst

The highly dispersed SiO₂-supported nickel-based catalysts for CO methanation were prepared by an ethylene glycol (EG) modified wet-impregnation method. The results indicate that the highly dispersed 20Ni/SiO₂ (EG) catalyst realized good stability and higher catalytic activity than the catalyst obtained from a non-pretreated silica support (20Ni/SiO₂) in CO methanation, due to the smaller nickel particles and strong nickel–silica interaction. By the addition of a small amount of noble metal promoter (Ru, Pt, Pd), the catalytic activity for CO methanation was further improved dramatically and follows the order Ru > Pt > Pd. The added noble metal promoter enhanced the reduction of the nickel oxide by spill-over-hydrogen during reduction treatment, and provided more active species for the methanation reaction, resulting in 7 times higher CO conversion than the non-pretreated 20Ni/SiO₂ catalyst. The 20Ni–0.5Ru/SiO₂ (EG) catalyst presents superb catalytic performance in CO methanation with high activity (CO conv. 80.2%) as well as high methane selectivity (90.3%) at 275 °C without any deactivation during 50 h reaction. The obtained catalysts were characterized by XRD, TG/DTA, TEM, XPS, TPR, H₂ chemisorption, and in situ DRIFTS.

The highly dispersed SiO₂-supported nickel-based catalysts for CO methanation were prepared by an ethylene glycol (EG) modified wet-impregnation method.

Recent advances in methanation catalysts for the production of synthetic natural gas

This review summarizes the recent progress in methanation catalysts for SNG production, which will provide insights for future catalysts design.

Modulated excitation extended X-ray absorption fine structure spectroscopy

Modulated excitation improves the sensitivity of EXAFS by phase sensitive detection as demonstrated by simulated and experimental time-resolved FT-EXAFS spectra.

Adding diffuse reflectance infrared Fourier transform spectroscopy capability to extended x-ray-absorption fine structure in a new cell to study solid catalysts in combination with a modulation approach

We describe a novel cell used to combine in situ transmission X-ray absorption spectroscopy (XAS) with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in a single experiment. The novelty of the cell design compared to current examples is that both radiations are passed through an X-ray and IR transparent window in direct contact with the sample. This innovative geometry also offers a wide surface for IR collection. In order to avoid interference from the crystalline IR transparent materials (e.g., CaF2, MgF2, diamond) a 500 μm carbon filled hole is laser drilled in the center of a CaF2 window. The cell is designed to represent a plug flow reactor, has reduced dead volume in order to allow for fast exchange of gases and is therefore suitable for experiments under fast transients, e.g., according to the concentration modulation approach. High quality time-resolved XAS and DRIFTS data of a 2 wt.% Pt/Al2O3 catalyst are obtained in concentration modulation experiments where CO (or H2) pulses are alternated to O2 pulses at 150 °C. We show that additional information can be obtained on the Pt redox dynamic under working conditions thanks to the improved sensitivity given by the modulation approach followed by Phase Sensitive Detection (PSD) analysis. It is anticipated that the design of the novel cell is likely suitable for a number of other in situ spectroscopic and diffraction methods.

Illuminating surface atoms in nanoclusters by differential X-ray absorption spectroscopy

We use differential extended X-ray absorption fine structure (Δ-EXAFS) to monitor the Ar-induced surface restructuring of silica-supported Pd nanoclusters (1 nm diameter) at 77 K.