Browsing by Author "Rabeah, Jabor"
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- ItemCobalt Single-Atom Catalysts with High Stability for Selective Dehydrogenation of Formic Acid(Weinheim : Wiley-VCH, 2020) Li, Xiang; Surkus, Annette-Enrica; Rabeah, Jabor; Anwar, Muhammad; Dastigir, Sarim; Junge, Henrik; Brückner, Angelika; Beller, MatthiasMetal–organic framework (MOF)-derived Co-N-C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co-N-C catalyst achieves superior activity, better acid resistance, and improved long-term stability compared with nanoparticles synthesized by a similar route. High-angle annular dark-field–scanning transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance, and X-ray absorption fine structure characterizations reveal the formation of CoIINx centers as active sites. The optimal low-cost catalyst is a promising candidate for liquid H2 generation. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemThe Effect of Iron and Vanadium in VOy/Ce1-xFexO2-δ Catalysts in Low-Temperature Selective Catalytic Reduction of NOx by Ammonia(Weinheim : WileyY-VCH Verlag, 2020) Keller, Sonja; Agostini, Giovanni; Antoni, Hendrik; Kreyenschulte, Carsten R.; Atia, Hanan; Rabeah, Jabor; Bentrup, Ursula; Brückner, AngelikaSupported VOy/Ce1-xFexO2-δ catalysts (x=0, 0.5, 0.1, 0.2) and bare supports were prepared and tested in selective catalytic reduction (SCR) of NOx by NH3 between 150 and 300 °C with a GHSV of 70 000 h−1. Iron was found to be beneficial for the activity of the pristine supports, reaching 80 % conversion at 275 °C. When vanadium was additionally introduced into the system, iron was found to be detrimental for NOx-conversion. To derive structure-reactivity relationships, V-free supports and VOy/Ce1-xFexO2-δ catalysts were characterized by XRD, XPS, Raman spectroscopy and TEM. In situ XANES, as well as operando DRIFTS and EPR measurements were performed to study the behavior of the catalysts under reaction conditions. Up to an iron content of x=0.1, a solid Ce1-xFexO2-δ solution was formed. Higher iron contents led to formation of iron oxide agglomerates. These agglomerates, as well as an increased amount of surface oxygen species were found to be responsible for increased NOx-conversion over of pure supports. For V-containing catalysts, an interaction of Fe and V centers could be found. Under reaction conditions, Fe3+ was preferentially reduced instead of V5+, decreasing the catalytic activity of VOy/Ce1-xFexO2-δ. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemElectrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex(Weinheim : Wiley-VCH, 2020) De, Ratnadip; Gonglach, Sabrina; Paul, Shounik; Haas, Michael; Sreejith, S.S.; Gerschel, Philipp; Apfel, Ulf-Peter; Vuong, Thanh Huyen; Rabeah, Jabor; Roy, Soumyajit; Schöfberger, WolfgangThe controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular MnIII-corrole complex, which is modified on the three meso-positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO2. Electrochemical reduction of MnIII leads to an electroactive MnII species, which binds CO2 and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO2 to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h−1, when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the MnIII-corrole center. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemFacile Synthesis of a Stable Side-on Phosphinyne Complex by Redox Driven Intramolecular Cyclisation(Weinheim : Wiley-VCH, 2020) Lange, Helge; Schröder, Henning; Oberem, Elisabeth; Villinger, Alexander; Rabeah, Jabor; Ludwig, Ralf; Neymeyr, Klaus; Seidel, Wolfram W.Alkyne complexes with vicinal substitution by a Lewis acid and a Lewis base at the coordinated alkyne are prospective frustrated Lewis pairs exhibiting a particular mutual distance and, hence, a specific activation potential. In this contribution, investigations on the generation of a WII alkyne complex bearing a phosphine as Lewis base and a carbenium group as Lewis acid are presented. Independently on potential substrates added, an intramolecular cyclisation product was always isolated. A subsequent deprotonation step led to an unprecedented side-on λ5-phosphinyne complex, which is interpreted as highly zwitterionic according to visible absorption spectroscopy supported by TD-DFT. Low-temperature 31P NMR and EPR spectroscopic measurements combined with time-dependent IR-spectroscopic monitoring provided insights in the mechanism of the cyclisation reaction. Decomposition of the multicomponent IR spectra by multivariate curve resolution and a kinetic hard-modelling approach allowed the derivation of kinetic parameters. Assignment of the individual IR spectra to potential intermediates was provided by DFT calculations. © 2020 The Authors. Published by Wiley-VCH GmbH
- ItemImpact of Al Activators on Structure and Catalytic Performance of Cr Catalysts in Homogeneous Ethylene Oligomerization : A Multitechnique in situ/operando Study(Weinheim : Wiley-VCH Verlag, 2019) Grauke, Reni; Schepper, Rahel; Rabeah, Jabor; Schoch, Roland; Bentrup, Ursula; Bauer, Matthias; Brückner, AngelikaThe effect of different AlR3 activators (R=methyl, ethyl, isobutyl, n-octyl) has been studied in comparison to modified methylaluminoxane (MMAO) by operando EPR as well as by in situ UV-vis, ATR-IR and XANES/EXAFS spectroscopy during oligomerization of ethylene at 20 bar and 40 °C with a homogeneous Cr complex catalyst formed in situ upon mixing a Cr(acac)3 precursor, a Ph2PN(iPr)PPh2 ligand (PNP) and the activator. Coordination of PNP to Cr(acac)3 is initiated only in the presence of an activator. Highest 1-octene productivity (detected during operando EPR measurements) was obtained with MMAO which promotes bidentate coordination of the ligand to form an active (PNP)CrII(CH3)2 chelate complex. Rising bulkiness of R in AlR3 leads to only monodentate coordination of PNP to the Cr center by one P atom and increasing reduction to CrI to a maximum extend of around 30 % for AlOct3. This lowers the catalytic performance, which is mainly governed by the mode of PNP coordination rather than by the CrI content. ©2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemLigand electronic fine-tuning and its repercussion on the photocatalytic activity and mechanistic pathways of the copper-photocatalysed aza-Henry reaction(London : RSC Publ., 2020) Li, Chenfei; Dickson, Robert; Rockstroh, Nils; Rabeah, Jabor; Cordes, David B.; Slawin, Alexandra M.Z.; Hünemörder, Paul; Spannenberg, Anke; Bühl, Michael; Mejía, Esteban; Zysman-Colman, Eli; Kamer, Paul C.J.A family of six structurally related heteroleptic copper(i) complexes of the form of [Cu(N^N)(P^P)]+ bearing a 2,9-dimethyl-1,10-phenanthroline diimine (N^N) ligand and a series of electronically tunable xantphos (P^P) ligands have been synthesized and their optoelectronic properties characterized. The reactivity of these complexes in the copper-photocatalyzed aza-Henry reaction of N-phenyltetrahydroisoquinoline was evaluated, while the related excited state kinetics were comprehensively studied. By subtlety changing the electron-donating properties of the P^P ligands with negligible structural differences, we could tailor the photoredox properties and relate them to the reactivity. Moreover, depending on the exited-state redox potential of the catalysts, the preferred mechanism can shift between reductive quenching, energy transfer and oxidative quenching pathways. A combined study of the structural modulation of copper(i) photocatalysts, optoelectronic properties and photocatalytic reactivity resulted in a clearer understanding of both the rational design of the photocatalyst and the complexity of competing photoinduced electron and energy transfer mechanisms. © The Royal Society of Chemistry.
- ItemMetal/Metal Redox Isomerism Governed by Configuration(Weinheim : Wiley-VCH, 2020) Ludwig, Stephan; Helmdach, Kai; Hettenschmidt, Mareike; Oberem, Elisabeth; Rabeah, Jabor; Villinger, Alexander; Ludwig, Ralf; Seidel, Wolfram W.A pair of diastereomeric dinuclear complexes, [Tp′(CO)BrW{μ-η2-C,C′-κ2-S,P-C2(PPh2)S}Ru(η5-C5H5)(PPh3)], in which W and Ru are bridged by a phosphinyl(thiolato)alkyne in a side-on carbon P,S-chelate coordination mode, were synthesized, separated and fully characterized. Even though the isomers are similar in their spectroscopic properties and redox potentials, the like-isomer is oxidized at W while the unlike-isomer is oxidized at Ru, which is proven by IR, NIR and EPR-spectroscopy supported by spectro-electrochemistry and computational methods. The second oxidation of the complexes was shown to take place at the metal left unaffected in the first redox step. Finally, the tipping point could be realized in the unlike isomer of the electronically tuned thiophenolate congener [Tp′(CO)(PhS)W{μ-η2-C,C′-κ2-S,P-C2(PPh2)S}Ru(η5-C5H5)-(PPh3)], in which valence trapped WIII/RuII and WII/RuIII cationic species are at equilibrium. © 2020 The Authors. Published by Wiley-VCH GmbH
- ItemMultivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV–Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions(Weinheim : Wiley-VCH, 2020) Rabeah, Jabor; Briois, Valérie; Adomeit, Sven; La Fontaine, Camille; Bentrup, Ursula; Breckner, AngelikaOperando EPR, XANES/EXAFS, UV-Vis and ATR-IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least-squares fitting (MCR-ALS) of simultaneously recorded XAS and UV-Vis data sets, it was found that an initially formed (bpy)(NMI)CuI- complex (bpy=2,2′-bipyridine, NMI=N-methylimidazole) is converted to two different CuII species, a mononuclear (bpy)(NMI)(CH3CN)CuII-OOH species detectable by EPR and ESI-MS, and an EPR-silent dinuclear (CH3CN)(bpy)(NMI)CuII(μ-OH)2⋅CuII (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)CuII-TEMPO monomers that are also EPR-silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non-reducible CuII leads to slight deactivation at extended reaction times. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemNi-In Synergy in CO2Hydrogenation to Methanol(Washington, DC : ACS Publications, 2021) Zhu, Jiadong; Cannizzaro, Francesco; Liu, Liang; Zhang, Hao; Kosinov, Nikolay; Filot, Ivo A.W.; Rabeah, Jabor; Brückner, Angelika; Hensen, Emiel J.M.Indium oxide (In2O3) is a promising catalyst for selective CH3OH synthesis from CO2but displays insufficient activity at low reaction temperatures. By screening a range of promoters (Co, Ni, Cu, and Pd) in combination with In2O3using flame spray pyrolysis (FSP) synthesis, Ni is identified as the most suitable first-row transition-metal promoter with similar performance as Pd-In2O3. NiO-In2O3was optimized by varying the Ni/In ratio using FSP. The resulting catalysts including In2O3and NiO end members have similar high specific surface areas and morphology. The main products of CO2hydrogenation are CH3OH and CO with CH4being only observed at high NiO loading (≥75 wt %). The highest CH3OH rate (∼0.25 gMeOH/(gcath), 250 °C, and 30 bar) is obtained for a NiO loading of 6 wt %. Characterization of the as-prepared catalysts reveals a strong interaction between Ni cations and In2O3at low NiO loading (≤6 wt %). H2-TPR points to a higher surface density of oxygen vacancy (Ov) due to Ni substitution. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electron paramagnetic resonance analysis of the used catalysts suggest that Ni cations can be reduced to Ni as single atoms and very small clusters during CO2hydrogenation. Supportive density functional theory calculations indicate that Ni promotion of CH3OH synthesis from CO2is mainly due to low-barrier H2dissociation on the reduced Ni surface species, facilitating hydrogenation of adsorbed CO2on Ov © 2021 The Authors. Published by American Chemical Society
- ItemOrigins of high catalyst loading in copper(i)-catalysed Ullmann-Goldberg C-N coupling reactions(Cambridge : RSC, 2017) Sherborne, Grant J.; Adomeit, Sven; Menzel, Robert; Rabeah, Jabor; Brückner, Angelika; Fielding, Mark R.; Willans, Charlotte E.; Nguyen, Bao N.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.
- ItemPractical Catalytic Cleavage of C(sp3)−C(sp3) Bonds in Amines(Weinheim : Wiley-VCH, 2019) Li, Wu; Liu, Weiping; Leonard, David K.; Rabeah, Jabor; Junge, Kathrin; Brgckner, Angelika; Beller, MatthiasThe selective cleavage of thermodynamically stable C(sp3)−C(sp3) single bonds is rare compared to their ubiquitous formation. Herein, we describe a general methodology for such transformations using homogeneous copper-based catalysts in the presence of air. The utility of this novel methodology is demonstrated for Cα−Cβ bond scission in >70 amines with excellent functional group tolerance. This transformation establishes tertiary amines as a general synthon for amides and provides valuable possibilities for their scalable functionalization in, for example, natural products and bioactive molecules. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemRelations between Structure, Activity and Stability in C3N4 Based Photocatalysts Used for Solar Hydrogen Production(Basel : MDPI, 2018-1-29) Sivasankaran, Ramesh P.; Rockstroh, Nils; Hollmann, Dirk; Kreyenschulte, Carsten R.; Agostini, Giovanni; Lund, Henrik; Acharjya, Amitava; Rabeah, Jabor; Bentrup, Ursula; Junge, Henrik; Thomas, Arne; Brückner, AngelikaSolar hydrogen production from water could be a sustainable and environmentally friendly alternative to fossil energy carriers, yet so far photocatalysts active and stable enough for large-scale applications are not available, calling for advanced research efforts. In this work, H2 evolution rates of up to 1968 and 5188 μmol h−1 g−1 were obtained from aqueous solutions of triethanolamine (TEOA) and oxalic acid (OA), respectively, by irradiating composites of AgIn5S8 (AIS), mesoporous C3N4 (CN, surface area >150 m2/g) and ≤2 wt.% in-situ photodeposited Pt nanoparticles (NPs) with UV-vis (≥300 nm) and pure visible light (≥420 nm). Structural properties and electron transport in these materials were analyzed by XRD, STEM-HAADF, XPS, UV-vis-DRS, ATR-IR, photoluminescence and in situ-EPR spectroscopy. Initial H2 formation rates were highest for Pt/CN, yet with TEOA this catalyst deactivated by inclusion of Pt NPs in the matrix of CN (most pronounced at λ ≥ 300 nm) while it remained active with OA, since in this case Pt NPs were enriched on the outermost surface of CN. In Pt/AIS-CN catalysts, Pt NPs were preferentially deposited on the surface of the AIS phase which prevents them from inclusion in the CN phase but reduces simultaneously the initial H2 evolution rate. This suggests that AIS hinders transport of separated electrons from the CN conduction band to Pt NPs but retains the latter accessible by protons to produce H2.
- ItemRole of SrCO3 on Photocatalytic Performance of SrTiO3-SrCO3 Composites(Basel : MDPI, 2022) Boga, Bíborka; Steinfeldt, Norbert; Moustakas, Nikolaos G.; Peppel, Tim; Lund, Henrik; Rabeah, Jabor; Pap, Zsolt; Cristea, Vasile-Mircea; Strunk, JenniferPerovskites such as SrTiO3 are interesting for photocatalytic applications due to their structure-related and electronic properties. These properties are influenced by the presence of SrCO3 which is often formed simultaneously during the hydrothermal synthesis of SrTiO3. In this study, SrTiO3-SrCO3 composites with different contents of SrCO3 (5–24 wt%) were synthesized. Their morphological, structural, and optical properties were investigated using complementary methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen sorption, and diffuse reflectance spectroscopy (DRS). Their photocatalytic activity was assessed during the degradation of diclofenac (DCFNa) in aqueous solution and CO2 photoreduction under Xe lamp irradiation. Improved photocatalytic efficiency in DCFNa degradation was observed for all the studied composites in comparison with SrTiO3, and the highest mineralization efficiency was obtained for the sample with 21 wt% SrCO3 content. The presence of SrCO3 led to an increased concentration of active species, such as •OH radicals. Otherwise, its presence inhibits CH4 and C2H6 production during CO2 photoreduction compared with pure SrTiO3.
- ItemA selective route to aryl-triphosphiranes and their titanocene-induced fragmentation(Cambridge : RSC, 2019) Schumann, André; Reiß, Fabian; Jiao, Haijun; Rabeah, Jabor; Siewert, Jan-Erik; Krummenacher, Ivo; Braunschweig, Holger; Hering-Junghans, ChristianTriphosphiranes are three-membered phosphorus cycles and their fundamental reactivity has been studied in recent decades. We recently developed a high-yielding, selective synthesis for various aryl-substituted triphosphiranes. Variation of the reaction conditions in combination with theoretical studies helped to rationalize the formation of these homoleptic phosphorus ring systems and highly reactive intermediates could be isolated. In addition we showed that a titanocene synthon [Cp2Ti(btmsa)] facilitates the selective conversion of these triphosphiranes into titanocene diphosphene complexes. This unexpected reactivity mode was further studied theoretically and experimental evidence is presented for the proposed reaction mechanism. This journal is © The Royal Society of Chemistry.
- ItemSupported CuII Single-Ion Catalyst for Total Carbon Utilization of C2 and C3 Biomass-Based Platform Molecules in the N-Formylation of Amines(Weinheim : Wiley-VCH, 2021) Dai, Xingchao; Wang, Xinzhi; Rabeah, Jabor; Kreyenschulte, Carsten; Brückner, Angelika; Shi, FengThe shift from fossil carbon sources to renewable ones is vital for developing sustainable chemical processes to produce valuable chemicals. In this work, value-added formamides were synthesized in good yields by the reaction of amines with C2 and C3 biomass-based platform molecules such as glycolic acid, 1,3-dihydroxyacetone and glyceraldehyde. These feedstocks were selectively converted by catalysts based on Cu-containing zeolite 5A through the in situ formation of carbonyl-containing intermediates. To the best of our knowledge, this is the first example in which all the carbon atoms in biomass-based feedstocks could be amidated to produce formamide. Combined catalyst characterization results revealed preferably single CuII sites on the surface of Cu/5A, some of which form small clusters, but without direct linking via oxygen bridges. By combining the results of electron paramagnetic resonance (EPR) spin-trapping, operando attenuated total reflection (ATR) IR spectroscopy and control experiments, it was found that the formation of formamides might involve a HCOOH-like intermediate and .NHPh radicals, in which the selective formation of .OOH radicals might play a key role. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
- ItemTracing Active Sites in Supported Ni Catalysts during Butene Oligomerization by Operando Spectroscopy under Pressure(Washington, DC : ACS, 2016) Rabeah, Jabor; Radnik, Jörg; Briois, Valérie; Maschmeyer, Dietrich; Stochniol, Guido; Peitz, Stephan; Reeker, Helene; La Fontaine, Camille; Brückner, AngelikaSupported Ni catalysts have been studied during the dimerization of butenes by operando electron paramagnetic resonance (EPR) and in situ X-ray absorption spectroscopy (XAS) at 353 K and up to 16 bar. Single NiI/NiII shuttles were identified as active sites, whereby the conversion of initial NiI to NiII by oxidative addition of butene is obviously faster than the re-reduction of NiII to NiI by reductive elimination of the C8 product, rendering the equilibrium percentage of NiI small. At p ≤ 2 bar, NiI single sites form inactive Ni0 aggregates, while this is suppressed at higher pressure (∼12 bar). A reaction mechanism is proposed.