Photocatalytic hydrogen (H2) production is a promising route for alternative energetics. Understanding structure–activity relationships is a crucial step towards the rational design of ...photocatalysts, which requires the application of operando spectroscopy under relevant working conditions. We performed an operando investigation on a catalytic system during the photodeposition of Pt on TiO2 and photostimulated H2 production, using simultaneous laboratory X-ray absorption spectroscopy (XAS), UV–Vis spectroscopy, and mass spectrometry. XAS showed a progressive increase in Pt fluorescence for Pt deposited on TiO2 for over an hour, which is correlated with the signal of the produced H2. The final Pt/TiO2 catalyst contained Pt(0) particles. The electronic features corresponding to the Pt4+ species in the UV–Vis spectrum of the solution disappear as soon as UV radiation is applied in the presence of formic acid, which acts as a hole scavenger, resulting in the presence of Pt(0) particles in solution.
A setup for fluorescence‐detected X‐ray absorption spectroscopy (XAS) with sub‐second time resolution has been developed. This technique allows chemical speciation of low‐concentrated materials ...embedded in highly absorbing matrices, which cannot be studied using transmission XAS. Using this setup, the reactivity of 1.5 wt% Pt/CeO2 catalyst was studied with 100 ms resolution during periodic cycling in CO‐ and oxygen‐containing atmospheres in a plug‐flow reactor. Measurements were performed at the Pt L3‐ and Ce L3‐edges. The reactivity of platinum and cerium demonstrated a strong correlation. The oxidation of the catalyst starts on the ceria support helping the oxidation of platinum nanoparticles. The new time‐resolved XAS setup can be applied to various systems, capable of reproducible cycling between different states triggered by gas atmosphere, light, temperature, etc. It opens up new perspectives for mechanistic studies on automotive catalysts, selective oxidation catalysts and photocatalysts.
A highly sensitive setup for measuring fluorescence‐detected X‐ray absorption spectroscopy with sub‐second time resolution has been developed at the SuperXAS beamline at the Swiss Light Source. This opens new opportunities for understanding the dynamic structure of heterogeneous catalysts.
Formation of gold nanoparticles (NPs) from the mixture containing NaAuCl4 as a precursor, octadecene as a solvent and oleylamine as a reducing agent was studied in situ by means of optical and X-ray ...spectroscopies. Dynamic light scattering (DLS) revealed the presence of initial aggregates of 500 nm average size which split into nanoparticles of about 8 nm width shortly after the reduction from Au3+ to Au+ has been completed. Based on Density Functional Theory (DFT) simulations and analysis of X-ray absorption spectra (XAS) we identified the structure of Au3+ and Au+ gold complexes. Quantitative analysis shows that Au NPs formation proceeds in following steps: substitution of chlorine ligands in Au3+ complex by four oleylamines; reduction of Au3+ to Au+ coordinated by two oleylamines. Latter process occurs in oleylamine micelles in octadecene. The third step is a fragmentation of large micelles into smaller ones shortly after reduction Au3+ to Au+, and subsequent slow growth of Au NPs via reduction of Au+ to Au0.
•Initial agglomerates are split into small 8 nm particles during the reduction of Au.•Au(III) complex is coordinated by four N located at 2.04 Å.•Au(I) complex is coordinated by two N atoms at 2.05 Å.•No Cl atoms are present in the first coordination of Au at all stages of nanoparticle formation.
The formation of palladium hydrides is a well-known phenomenon, observed for both bulk and nanosized samples. The kinetics of hydrogen adsorption/desorption strongly depends on the particle size and ...shape, as well as the type of support and/or coating of the particles. In addition, the structural properties of hydride phases and their distribution also depend on the particle size. In this work, we report on the in situ characterization of palladium nanocubes coated with HKUST-1 metal-organic framework (Pd@HKUST-1) during desorption of hydrogen by means of synchrotron-based time-resolved X-ray powder diffraction. A slower hydrogen desorption, compared to smaller sized Pd nanoparticles was observed. Rietveld refinement of the time-resolved data revealed the remarkable stability of the lattice parameters of α- and β-hydride phases of palladium during the α- to β- phase transition, denoting the behavior more similar to the bulk materials than nanoparticles. The stability in the crystal sizes for both α- and β-hydride phases during the phase transition indicates that no sub-domains are formed within a single particle during the phase transition.
Adsorption of ethylene on palladium, a key step in various catalytic reactions, may result in a variety of surface-adsorbed species and formation of palladium carbides, especially under industrially ...relevant pressures and temperatures. Therefore, the application of both surface and bulk sensitive techniques under reaction conditions is important for a comprehensive understanding of ethylene interaction with Pd-catalyst. In this work, we apply in situ X-ray absorption spectroscopy, X-ray diffraction and infrared spectroscopy to follow the evolution of the bulk and surface structure of an industrial catalysts consisting of 2.6 nm supported palladium nanoparticles upon exposure to ethylene under atmospheric pressure at 50 °C. Experimental results were complemented by ab initio simulations of atomic structure, X-ray absorption spectra and vibrational spectra. The adsorbed ethylene was shown to dehydrogenate to C
H
, C
H
and C
H species, and to finally decompose to palladium carbide. Thus, this study reveals the evolution pathway of ethylene on industrial Pd-catalyst under atmospheric pressure at moderate temperatures, and provides a conceptual framework for the experimental and theoretical investigation of palladium-based systems, in which both surface and bulk structures exhibit a dynamic nature under reaction conditions.
The size and shape distribution of metal nanoparticles (NPs) are important parameters that need to be tuned in order to achieve desired properties of materials for practical applications. In the ...current work, we present the synthesis of palladium NPs supported on silica by three different methods, applying reduction by sodium borohydride, hydrazine vapors, and polyethylene glycol (PEG). The synthesized materials were characterized by X-ray diffraction, X-ray fluorescence, transmission electron microscopy, surface area and porosity measurements, and thermogravimetric analysis. Similar nanoparticle sizes with narrow size distribution centered at 8 nm were obtained after reduction by sodium borohydride and hydrazine vapors, whereas the smallest particle size of about 4.8 nm was obtained after reduction by PEG. The effect of modification of the initial palladium chloride compound by ammonium hydroxide was found to lead to the formation of larger particles with average size of 15 nm and broader size distribution. In addition, the process of the reduction of palladium by PEG at different reduction stages was monitored by UV-Vis spectroscopy. CO-stripping voltammetry showed that reduction in hydrazine and in PEG allowed the preparation of Pd NPs with high electrochemically-active surface area. Such NPs are promising materials for electrocatalysis.
The stock‐specific distribution of maturing salmon in the North Pacific has been a persistent information gap that has prevented us from determining the ocean conditions experienced by individual ...stocks. This continues to impede understanding of the role of ocean conditions in stock‐specific population dynamics. We assessed scale archives for 17 sockeye salmon (Oncorhynchus nerka) stocks covering the entire North Pacific, from the Columbia River (Washington State and British Columbia) to Kamchatka Peninsula (Russia), to infer salmon locations during their last growing season before returning to their spawning grounds. The approach used, first pioneered in salmon stocks in the Atlantic, relies on the relationship between temporal changes in δ13C in salmon scales and sea surface temperature to estimate salmon distribution based on correlation strength. An advantage of this approach is that it does not require fish sampling at sea, but relies on existing fishery agency collections of salmon scales. Significant correlations were found for 7 of the stocks allowing us to propose plausible feeding grounds. Complementary information from δ15N, historical tagging studies, and connectivity analysis were used to further refine distribution estimates. This study is a first step toward estimating stock‐specific distributions of salmon in the North Pacific and provides a basis for the application of the approach to other salmon scale archives. This information has the potential to improve our ability to relate stock dynamics to ocean conditions, ultimately enabling improved stock management. For example, our estimated distributions of Bristol Bay and NE Pacific stocks demonstrated that they occupy different areas with a number of the former being distributed in the high productivity shelf waters of the Aleutian Islands and Bering Sea. This may explain why these stocks seem to have responded differently to changes in ocean conditions, and the long‐term trend of increased productivity of Bristol Bay sockeye.
Stock‐specific salmon high seas distribution is largely unknown. We used the correlation between sea temperature and stable isotope ratio in scales of 17 salmon stocks to estimate their distributions during their last year at sea. This has resulted in a better comprehension of salmon migration routes, differences in stock productivity trends over large areas (e.g., Bristol Bay vs Northeast Pacific) and differences in productivity of stocks with geographically close spawning grounds.
The thermal decomposition of manganese borohydride Mn(BH4)2 was studied by means of synchrotron-based X-ray absorption spectroscopy (XAS), X-ray powder diffraction (XRPD) and theoretical density ...functional (DFT) modeling aiming to elucidate changes of the local atomic structure upon hydrogen desorption and to determine possible decomposition reaction products. XRPD patterns indicate profound structural changes in the material above 120 °C with subsequent amorphization. DFT simulations predict the collapse of the highly porous framework structure upon hydrogen desorption and significant reduction of Mn-B and Mn-Mn interatomic distances by 19% and 41% respectively. These estimations are in a good agreement with the quantitative analysis of the X-ray absorption spectra above Mn K-edge. Based on XAS we derive possible decomposition products and reaction path. In particular, the amount of Mn metallic phase was estimated to be less than 5% after the heating up to 200 °C. Several structural models for the final state of manganese borohydride in a heating process are constructed by means of energy minimization in conjunction with evolutionary algorithms.
Display omitted
•XRPD reveal amorphization of Mn(BH4)2 above 120 °C both in a vacuum and H2 atmospheres.•Calculations predict the collapse of the porous Mn(BH4)2 framework upon hydrogen release.•The concentration of the metallic Mn does not exceed 5% in the reaction products.•Average Mn-B and Mn-Mn distances are 2.17 Å and 2,72 Å in amorphous Mn-B composite.
We genotyped Chinook salmon (Oncorhynchus tshawytscha) from the Bering Sea and North Pacific Ocean for 43 single-nucleotide polymorphisms (SNPs) to investigate seasonal distribution and migration ...patterns. We analyzed 3563 immature fish from 22 spatiotemporal strata; composition analyses were performed using genotype data from spawning stocks spanning the species range. Substantial variation in stock composition existed among spatial and seasonal strata. We inferred patterns of seasonal migration based upon these data along with data from previous tag, scale, and parasite studies. We found that stocks from western Alaska and Yukon River overwinter on the Alaska continental shelf then travel to the middle and western Bering Sea during spring-fall. Stocks from California to Southeast Alaska were distributed in Gulf of Alaska year-round, with a substantial portion of this group migrating northward to the eastern Bering Sea during spring-fall. Proportions of Russian stocks increase when moving east to west in both the Bering Sea and North Pacific Ocean. These data can be used to better understand the impacts of fisheries and climate change on this valuable resource.
In this paper, we investigated the influence of steam treatment on structural group composition of resins and asphaltenes of heavy oil. The object of investigation was oil-saturated rocks from ...Riphean-Vendian complex. The extracted crude oil was determined as a high-viscous fluid. The resins and asphaltenes destructed in a small extent due to thermal treatment. The oil-soluble iron-based catalyst intensified the destructive processes. The content of sulfur compounds (-SO) in resins and asphaltenes drastically decreased due to reduction reaction of sulfoxide to sulfide and hydrogen sulfide. The results showed that catalytic aquathermolysis, even at low temperature ranges, promoted the cracking reaction of most macromolecular components and increased the content of light fractions of heavy oil. Consequently, it reduced its viscosity.
PDF
