Showing papers by "Angelika Brückner published in 2017"
TL;DR: In this article, the best V2O5/Ce1-xTixO2 (3, 5, and 7 wt % V; x = 0, 0.5, 1) and bare supports were tested in the selective catalytic reduction (SCR) of NO by NH3 at different gas hourly space velocities (GHSVs) and were comprehensively characterized using XRD, pseudo in situ XPS, and UV-vis DRS as well as EPR and DRIFTS in in situ and operando mode.
Abstract: Supported V2O5/Ce1–xTixO2 (3, 5, and 7 wt % V; x = 0, 0.1, 0.3, 0.5, 1) and bare supports have been tested in the selective catalytic reduction (SCR) of NO by NH3 at different gas hourly space velocities (GHSVs) and were comprehensively characterized using XRD, pseudo in situ XPS, and UV–vis DRS as well as EPR and DRIFTS in in situ and operando mode. The best V/Ce1–xTixO2 (x = 0.3, 0.5) catalysts showed almost 100% NO conversion and N2 selectivity already at 190 °C with a GHSV value of 70000 h–1, which belongs to the best performances observed so far in low-temperature NH3-SCR of NO. The corresponding bare supports still converted around 80% to N2 under the same conditions. On bare supports, SCR proceeds via a Langmuir–Hinshelwood mechanism comprising the reaction of adsorbed surface nitrates with adsorbed NH3. On V/Ce1–xTixO2, nitrate formation is not possible, and an Eley–Rideal mechanism is working in which gaseous NO reacts with adsorbed NH3 and NH4+. Lewis and Bronsted acid sites, though adsorption o...
152 citations
TL;DR: In this article, the photoexcitation of electrons to the conduction band (CB-e−), their further trapping by dissolved O2 and O3, and the evolution of reactive oxygen species (ROS) have been semiquantitatively visualized.
Abstract: Photocatalytic ozonation of wastewater pollutants by sunlight is a highly attractive technology close to real application. Understanding this process on the atomic scale and under realistic working conditions is challenging but vital for the rational design of catalysts and photocatalytic decontamination systems. Here we study two highly active C3N4 photocatalysts (bulk C3N4 and a nanosheet-structured C3N4) under simultaneous visible-light irradiation and O3 bubbling in water by in situ EPR spectroscopy coupled with an online spin-trapping technique. The photoexcitation of electrons to the conduction band (CB-e–), their further trapping by dissolved O2 and O3, and the evolution of reactive oxygen species (ROS) have been semiquantitatively visualized. A dual role of O3 in boosting the CB-e– to •OH conversion is confirmed: (i) an inlet 2.1 mol % O3/O2 gas mixture can trap about 2–3 times more CB-e– upon aqueous C3N4 suspension than pure O2 and further produce •OH by a robust •O3–-mediated one-electron-reduc...
127 citations
TL;DR: The first heterogeneous single atom catalysts (SACs) are described, conveniently prepared by decorating alumina nanorods with platinum atoms, which efficiently catalyzes hydrosilylation of industrially relevant olefins with high TON.
Abstract: Catalytic hydrosilylation represents a straightforward and atom-efficient methodology for the creation of C–Si bonds In general, the application of homogeneous platinum complexes prevails in industry and academia Herein, we describe the first heterogeneous single atom catalysts (SACs), which are conveniently prepared by decorating alumina nanorods with platinum atoms The resulting stable material efficiently catalyzes hydrosilylation of industrially relevant olefins with high TON (≈105) A variety of substrates is selectively hydrosilylated including compounds with sensitive reducible and other functional groups (N, B, F, Cl) The single atom based catalyst shows significantly higher activity compared to related Pt nanoparticles
109 citations
TL;DR: In this paper, a model was proposed to explain the first two activity maxima while the origin of the third one remains to be elucidated, assuming major contributions of dimeric vanadate to the catalytic performance.
Abstract: V2O5-WO3/TiO2 catalysts of compositions typically used in SCR applications were subjected to calcinations in air at temperatures between 873 K and 1023 K for different durations, starting from a mildly calcined state in which species structure originating from wet preparation had been fixed by calcination at 623 K for 1 h. After calcination, samples were examined with respect to their SCR activity and to structural changes by nitrogen physisorption, XRD, Raman spectroscopy, EPR, temperature-programmed reduction, XPS and Low-Energy Ion Scattering (LEIS). Driven by loss of BET surface area, samples exhibited a complex behavior with up to three well-separated maxima of SCR activity before final deactivation. While the first of these maxima depended on the calcination rate (temperature), the other two maxima occurred at specific ranges of the BET surface area. During calcination, tungstate was segregated from the support surface forming WO3 under more severe conditions. Observations in the EPR spectra are in conflict with assignments of observed SCR activities to isolated vanadate sites. Assuming major contributions of dimeric vanadate to the catalytic performance, a model was proposed, which explains the first two activity maxima while the origin of the third one remains to be elucidated. In this model, segregation of tungstate results in formation of active V-O-V structures from less active isolated vanadate species previously separated by excessive amounts of tungstate. The main role of tungstate is to provide optimum sizes of vanadate ensembles, but a direct favorable influence of tungstate on vanadate is not excluded.
56 citations
TL;DR: A mechanistic investigation of Ullmann–Goldberg reactions using common bases led to the identification of pathways for catalyst deactivation and the solid form of the inorganic phase was found to have critical influence on the mechanism of the reaction.
Abstract: A mechanistic investigation of Ullmann–Goldberg reactions using soluble and partially soluble bases led to the identification of various pathways for catalyst deactivation through (i) product inhibition with amine products, (ii) by-product inhibition with inorganic halide salts, and (iii) ligand exchange by soluble carboxylate bases. The reactions using partially soluble inorganic bases showed variable induction periods, which are responsible for the reproducibility issues in these reactions. Surprisingly, more finely milled Cs2CO3 resulted in a longer induction period due to the higher concentration of the deprotonated amine/amide, leading to suppressed catalytic activity. These results have significant implications on future ligand development for the Ullmann–Goldberg reaction and on the solid form of the inorganic base as an important variable with mechanistic ramifications in many catalytic reactions.
43 citations
09 Mar 2017
TL;DR: In this article, a brief summary of homogeneous proton reduction systems using sacrificial agents with special emphasis on non-noble metal systems applying convenient iron(0) sources is presented.
Abstract: Photocatalytic hydrogen generation is considered to be attractive due to its combination of solar energy conversion and storage. Currently-used systems are either based on homogeneous or on heterogeneous materials, which possess a light harvesting and a catalytic subunit. The subject of this review is a brief summary of homogeneous proton reduction systems using sacrificial agents with special emphasis on non-noble metal systems applying convenient iron(0) sources. Iridium photosensitizers, which were proven to have high quantum yields of up to 48% (415 nm), have been employed, as well as copper photosensitizers. In both cases, the addition or presence of a phosphine led to the transformation of the iron precursor with subsequently increased activities. Reaction pathways were investigated by photoluminescence, electron paramagnetic resonance (EPR), Raman, FTIR and mass spectroscopy, as well as time-dependent DFT-calculations. In the future, this knowledge will set the basis to design photo(electro)chemical devices with tailored electron transfer cascades and without the need for sacrificial agents.
32 citations
TL;DR: In this paper, the role of NO2 in fast NH3-SCR of NOx combined in situ FTIR and EPR spectroscopic studies were carried out to correlate adsorbate formation at the catalyst surface (FTIR) with changes of the different iron sites (EPR).
Abstract: For elucidating the role of NO2 in the fast NH3-SCR of NOx combined in situ FTIR and EPR spectroscopic studies were carried out to correlate adsorbate formation at the catalyst surface (FTIR) with changes of the different iron sites (EPR). For this purpose interactions of the feed components NO, NO2, and NH3 with Fe sites in a commercial Fe-ZSM-5 were studied and reactivities of preadsorbed species towards other reactants (nitrates created by NO2 adsorption towards NO and NH3, and ammonia/NH4
+ towards NO2 and NO/O2) were investigated. In addition, the behavior of this catalyst in standard and fast SCR was studied. NO2 in the feed gas mixture causes an accelerated formation of nitrates mainly adsorbed on β-Fe sites on which they are reduced by NH3 and NO, more effectively by the mixture of both. The adsorption of NO2 and NH3 at different but adjacent sites facilitate the formation of NH4NO2 which decomposes into N2 and H2O. The Fe sites in β and γ position are efficiently reoxidized by NO2 already at 150 °C and are thus kept in a redox-active state.
28 citations
TL;DR: In this article, a manganese-catalyzed carbonylative transformation of alkyl iodides and amides was developed, and a variety of imides were prepared in moderate to good yields.
Abstract: A selective manganese‐catalyzed carbonylative transformation of alkyl iodides and amides was developed. A variety of imides were prepared in moderate to good yields. Alkyl bromides could also be applied by in situ treatment with NaI to give the corresponding alkyl iodides. Notably, no additives or expensive ligands were required here. As the first example of the carbonylative coupling of alkyl iodides with amides, the simple reaction conditions and the advantages of a manganese catalyst make this new general procedure more attractive and practical than conventional techniques. Mechanistically, control experiments and electron paramagnetic resonance spectroscopy studies were also performed, and the radical nature of this new process was proven.
27 citations
TL;DR: In this paper, the impregnation (IM) and reductive precipitation (RP) methods were used for photocatalytic H2 generation from H2O/methanol mixtures with visible and UV/Vis light.
Abstract: Monometallic Cu and bimetallic Cu/Au-TiO2 catalysts were prepared by impregnation (IM) and reductive precipitation (RP) methods in sequential (SP) and simultaneous mode (CP) and tested for photocatalytic H2 generation from H2O/methanol mixtures with visible (400–700 nm) and UV/Vis light (320–500 nm). Comprehensive studies by high-resolution (HR)-STEM, X-ray photoelectron spectroscopy (XPS), and different in situ methods (X-ray absorption near-edge structure (XANES), UV/Vis, and EPR spectroscopy) revealed that IM leads to dispersed surface Cu species with no clear particle formation, which is poorly active under visible light, whereas plasmonic Cu0 nanoparticles formed by RP are about three times more active under the same conditions. In Cu/Au-TiO2 catalysts prepared by RP-SP, highly dispersed Cu surface species boost H2 production under UV/Vis light, owing to the effective separation within TiO2 and electron trapping by Cu, whereas small Cu0 and Au0 particles remain widely separated. When Cu/Au-TiO2 catalysts are prepared by RP-CP, mixed Cu/Au particles of uniform size (4–8 nm) provide the highest H2 evolution rates under visible light, owing to effective surface plasmon resonance absorption.
24 citations
TL;DR: Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)CuI(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV-vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of iminidazole in terms of ligand and/or base.
Abstract: Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)CuI(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV–vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)CuI/II(IM)a(IM)b complex is formed, in which the Cu–N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of CuI to CuII and, thus, helps to activate O2 by electron transfer from CuI to O2. The rates of CuI oxidation and CuII reduction and, thus, the rates of benzaldehyde formation depend on R of the R–N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The CuI/CuII interplay leads ...
19 citations
01 Aug 2017
TL;DR: Combined X-ray photoelectron and absorption fine spectroscopy (XAFS) investigations on 10 wt % Pd-16 wt% Sb/TiO2 catalyst allow new insight into the impact of the co-component Sb on the active Pd species, which catalyzes the gas-phase acetoxylation of toluene to benzyl acetate.
Abstract: Combined X-ray photoelectron and absorption fine spectroscopy (XAFS) investigations on 10 wt % Pd–16 wt % Sb/TiO2 catalyst allow new insight into the impact of the co-component Sb on the active Pd species, which catalyzes the gas-phase acetoxylation of toluene to benzyl acetate. Ex situ Pd 3d XPS and Pd L-edge XANES studies indicate the presence of an excess electron charge on metallic Pd species (more 4d electrons than in Pd metal) formed after several hours of stream. This observation was explained by the electron transfer from metallic Sb incorporated into the Pd bulk to the neighboring Pd atoms. TEM-EDX analysis confirms the presence of intermixed Pd–Sb particles with an atomic ratio of between 5 and 6 in the most active catalysts and of 3 in deactivated samples. In situ Sb-K-edge XAFS investigations provide evidence that the Pd–Sb interaction is more pronounced under reaction feed than in the ex situ samples.
TL;DR: In this article, the authors studied Fe-ZSM-5 catalysts prepared via solid-state ion exchange by using field dependent Mossbauer spectroscopy at low temperature in order to identify the different iron species present in this type of catalyst.
Abstract: Iron loaded zeolites like Fe-ZSM-5 are potent candidates for the catalytic abatement of nitrogen oxides from car exhaust, e.g. from Diesel engines. Recent problems in this field show that there is an urgent need in further improvement of such catalysts, for which a full analysis of Fe species present in them under different conditions is highly desirable. We have studied Fe-ZSM-5 catalysts prepared via solid-state ion exchange by using field dependent Mossbauer spectroscopy at low temperature in order to identify the different iron species present in this type of catalyst in the fresh state and after use in catalysis. Mossbauer spectroscopy proved to be the key technique for a full understanding of species structures, but due to the complexity of structures, guidance by parallel EPR experiments and control by SQUID magnetometry were essential to prove reliability of derived species distributions.