Chemical bonding effects in Sc compounds studied using X-ray absorption and X-ray photoelectron spectroscopies

Advances on understanding the nature of the chemical bonding and electron correlation effects during the X-ray absorption process in ionic–covalent metal complexes has been achieved for most of the transition elements, except for scandium, due to the lack of a systematic series of spectroscopic reference spectra and the shortage of standard crystallographic data on scandium compounds. To close the gap, the chemical bonding effects in eight Sc compounds are studied using X-ray absorption spectroscopy (XAS) at Sc K and L2,3 absorption edges and X-ray photoelectron spectroscopy (XPS). Indeed, the fine structure of the XAS Sc K edge reflects the chemical sp3-like bond formed between scandium and the ligand while the L2,3 edge and the pre-edge features of the K-edge provide a direct insight into the crystal field parameters at the Sc site in the coordination compound. The XPS data provide the information on binding energies of the core electrons involved in the electron transitions caused by the absorption of high energy X-rays. XAS and XPS complement each other by accessing the information on Sc structure on bulk and the surface. Herein, comprehensive information on the electronic structure of well-known crystalline materials based on Sc is given with spectroscopic fingerprints X-ray data. This will help to predict the formation of chemical bonds in the unknown components via the systematic evaluation of the available spectroscopic fingerprints

Hydrocarbon and soot oxidation over cerium and iron doped vanadium SCR catalysts

V$_{2}$O$_{5}$−WO$_{3}$/TiO$_{2}$ (VWTi) catalysts are widely employed for selective catalytic reduction (SCR) of NO$_{x}$. However, due to their poor thermal stability the application in diesel particulate filters (DPFs), i. e. 2‐way SCRonDPF is limited. In this study, the potential of Ce‐ and Fe‐doped VWTi systems for hydrocarbon and soot oxidation in addition to the SCR activity was systematically investigated for fresh and thermally aged samples. The formation of metal vanadates upon thermal aging, as identified by X‐ray diffraction, Raman and X‐ray adsorption spectroscopy, prevents drastic sintering of the support and maintains a high NO$_{x}$−SCR and hydrocarbon oxidation activity. Additionally, the doped VWTi catalysts show a slight increase of the CO$_{2}$ selectivity during hydrocarbon oxidation, which represents an important aspect for such multifunctional catalysts. Despite of the advantages, the formation of metal vanadates hinders the mobility of vanadium species and decreases the soot oxidation ability of the doped catalysts. Interestingly, a promising soot oxidation activity was identified for the VWTi−Fe sample after aging at 650 °C, which resulted in decomposition of the iron vanadate and generation of highly dispersed and mobile V$_{2}$O$_{5}$

Mechanistic Insights into the Selective Oxidation of 5-(Hydroxymethyl)furfural over Silver-based Catalysts

Silver-catalyzed oxidation of 5-(hydroxymethyl)furfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was investigated using Ag/ZrO$_{2}$ and Ag/TiO$_{2}$ catalysts. The reaction proceeded very selectively without formation of the dicarboxylic acid in the presence of air and NaOH as a base. In situ X-ray absorption spectroscopy (XAS) performed systematically under varying reaction conditions in a specially designed cell evidenced that reduced silver particles are the catalytically active species in this reaction. Although an incomplete reduction of Ag/ZrO$_{2}$ and Ag/TiO$_{2}$ was observed after the catalyst preparation even after reduction in hydrogen, silver was reduced to the metallic state as soon as HMF was introduced to the reaction mixture at room temperature and stayed reduced throughout the reaction under conditions optimized for high HFCA yield. The degree of silver reduction and product formation differed for varying reaction conditions, indicating that reduced silver particles, a homogeneous base and oxygen are needed in order to achieve high HFCA yield. Based on the catalytic and spectroscopic experiments, a detailed reaction mechanism is proposed involving a dehydrogenation pathway of an intermediately formed geminal diol in basic aqueous solution

Liquid-phase Synthesis of Highly Oxophilic Zerovalent Niobium and Tantalum Nanoparticles

Zerovalent niobium, Nb(0), and tantalum, Ta(0), nanoparticles are prepared via a one-pot, liquid-phase synthesis. For this, NbCl$_{5}$/TaCl$_{5}$ are dissolved in pyridine and reduced by lithium pyridinyl. Deep black suspensions of very small, highly uniform nanoparticles are obtained with average diameters of 2.1 ± 0.4 nm (Nb(0)) and 1.9 ± 0.4 nm (Ta(0)). Whereas suspensions are chemically and colloidally stable, powder samples are very reactive. TEM/HRTEM, XRD, FT-IR, and XANES are used for characterization

Operando XAS Study of Pt-Doped CeO2 for the Nonoxidative Conversion of Methane

The methane to olefins, aromatics, and hydrogen (MTOAH) process via Pt/CeO$_2$ catalysts poses an attractive route to improve yield and stability for the direct catalytic conversion of methane. In this study, two sets of samples, one composed of PtO$_x$ single sites on ceria and the other with additional Pt agglomerates, were prepared. Both sets of samples showed enhanced catalytic activity for the direct conversion of methane exceeding the performance of pure ceria. Pulsed reaction studies unraveled three reaction stages: reduction of the ceria support during activation, an induction phase with increasing product formation, and finally, stable running of the catalytic reactions. The reduction of ceria was confirmed by X-ray absorption spectroscopy (XAS) after conducting the MTOAH reaction. Operando X-ray absorption spectroscopy at challenging reaction temperatures of up to 975 °C in combination with theoretical simulations further evidenced an increased Pt–Ce interaction upon reaction with CH$_4$. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra proved decoration and encapsulation of the Pt particles by the CeO$_2$/Ce$_2$O$_3$ support or a partial Ce–Pt alloy formation due to the strong metal–support interaction that developed under reaction conditions. Moreover, methyl radicals were detected as reaction intermediates indicating a reaction pathway through the gas-phase coupling of methyl radicals. The results indicate that apart from single-atom Pt sites reported in the literature, the observed Pt–Ce interface may have eased the activation of CH$_4$ by forming methyl radicals and suppressed coke formation, significantly improving the catalytic performance of the ceria-based catalysts in general

Hydrotreatment of Fast Pyrolysis Bio-oil Fractions Over Nickel-Based Catalyst

Residual biomass shows potential to be used as a feedstock for fast pyrolysis bio-oil production for energetic and chemical use. Although environmentally advantageous, further catalytic upgrading is required in order to increase the bio-oil stability, by reducing reactive compounds, functional oxygen-containing groups and water content. However, bio-oils may separate in fractions either spontaneously after ageing or by fractionated condensation. Therefore the effects of upgrading on different fast pyrolysis bio-oil (FPBO) fractions obtained from a commercially available FPBO were studied by elemental analysis, GC-MS and 1H-NMR. Not only the FPBO was upgraded by catalytic hydrotreatment, but also the heavy phase fraction formed after intentional aging and phase separation. The reactions were conducted between 175 and 325 °C and 80–100 bar by using a nickel–chromium catalyst in batch experiments. The influence of the hydrotreatment conditions correlated with the composition of the upgraded products. Higher oxygen removal was obtained at higher temperatures, whereas higher pressures resulted in higher hydrogen consumption with no significant influence on deoxygenation. At 325 °C and 80 bar 42% of the oxygen content was removed from the FPBO. Compounds attributed to pyrolysis oil instability, such as ketones and furfural were completely converted while the number of alcohols detected in the upgraded products increased. Coke formation was observed after all reactions, especially for the reaction with the fraction rich in lignin derivatives, likely formed by polymerization of phenolic compounds mainly concentrated in this phase. Independently of the feedstock used, the upgraded bio-oils were very similar in composition, with reduced oxygen and water content, higher energy density and higher carbon content