310 research outputs found

    Kinetic studies of propane oxidation on Mo and V based mixed oxide catalysts

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    The present work concentrates on the systematic kinetic study of the one-step propane oxidation to acrylic acid over a well defined, phase-pure M1 MoVTeNbOx catalyst. The bulk structural stability of the catalyst is a key issue for kinetic studies. The stability of the phase-pure M1 MoVTeNbOx catalyst under various conditions (steam-containing, steam-free, net reducing, stoichiometric and net oxidizing feed compositions) was evidenced by an in-situ XRD experiment which suggested that the bulk structure is homogeneous and constant under reaction conditions. Thereby, the heterogeneously catalyzed reactivity is exclusively determined by the surface properties, which in turn, are controlled by the chemical potential of the gas phase. A kinetic study on the reaction variables (temperature, steam content and redox potential) was carried out. Stable catalytic performance was observed for all the conditions. Cycling experiments showed the reversibility of the conversion and selectivity decrease upon exposing the catalyst to dry and reducing feed, respectively. Further catalytic experiments revealed that the reactivity spans over 5 orders of magnitude in the order of acrolein oxidation>>propylene oxidation>propane oxidation>>carbon monoxide oxidation~water gas shift reaction. The negligible CO oxidation activity suggested that the CO and CO2 are formed via two independent pathways in propane oxidation over M1. The stage-wise addition of oxygen lead to an improvement of the catalytic performance by 5% compared to the conventional single-tube reactor. Further experiments in the two-stage reactor revealed that the phase-pure M1 is not reoxidized by N2O. The addition of propylene in the two-stage reactor revealed a slight competitive adsorption on the active sites with propane, which observation was supported by the results of microcalorimetric experiments. On the other hand, the addition of CO and CO2 in the two-stage reactor showed that these products do not adsorb competitively with the educt or intermediates. In the literature much of the kinetic data was reported for ill-defined catalyst surfaces. In contrast to that, the present work reports the kinetic study of propane selective oxidation to acrylic acid on a well defined phase-pure and structurally stable M1 MoVTeNbOx catalyst. This study may contribute to the better kinetic and mechanistic understanding of the propane selective oxidation reaction

    Structure-function relationship of Strong Metal-Support Interaction studied on supported Pd reference catalysts

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    Transition metal oxide supported, nano-scaled noble metal catalysts are known to show a variety of surface modifications when they are being reduced at increasing temperatures. Such processes involve for example (surface) alloying and the formation of partially reduced oxidic support overlayers that are both induced by the so-called strong metal-support interaction (SMSI). The present work investigated a series of oxide supported Palladium powder catalysts with a loading variation between 1-5 wt.-% on their structure-function relationship after reduction in different media and at different temperatures to create a reference system and explore the nature of SMSI. Hereby surface and bulk sensitive techniques like XPS, chemisorption, TEM, DRIFTS or XRD were applied to study the influence of electronic and structural modifications on the activity in catalytic oxidation of carbon monoxide which served as the main test reaction and was conducted at ambient pressure. The catalysts were synthesized reproducibly by a controlled co-precipitation approach and by impregnation. The investigated Pd/iron oxide system shows palladium surface decoration at comparably low reduction temperatures. The surface cover was found to be volatile in oxygen containing atmosphere and formed reversibly. Dependent on the Pd particle size it increases the CO oxidation activity. Alloy formation occurs at higher reduction temperatures. In case of the Pd/zinc oxide system reversible surface alloying takes place during reduction that is also beneficial for CO oxidation, but again deactivates fast. When being reduced at even higher temperatures the additional formation of an oxidic overlayer could be observed that does not further activate the system but leads to an overall reduction of active sites. Due to alloy formation, the zinc oxide system at higher conversions shows a different selectivity behavior in acetylene hydrogenation, compared to the iron oxide system. Also in case of the Pd/titania system, reversible surface decoration by partially reduced support happens during reduction. Different to the other investigated systems the surface-cover reversibly decreases CO oxidation activity however. The Pd/alumina system was studied as a less reducible reference. As expected, it does not show SMSI-induced modifications. In the end the work clearly shows that CO oxidation is a convenient method to study activity and stability of SMSI and decouple it from other involved processes. The effects of surface modification on the catalytic activity in this test reaction however strongly depend on the specific system and pre-conditioning and can either be of activating or deactivating nature. The basic principles involved in case of SMSI seem to apply both in UHV model systems and at powder systems at ambient pressure as found by the catalytic measurements.Übergangsmetalloxid getrĂ€gerte, nano-skalige Edelmetall-Katalysatoren sind bekannt dafĂŒr, eine Reihe von OberflĂ€chen-VerĂ€nderungen zu erfahren, wenn sie bei erhöhter Temperatur reduziert werden. Diese Prozesse beinhalten beispielsweise (OberflĂ€chen-) Legierungsbildung und die Ausblidung von teilweise reduzierten, oxidischen TrĂ€ger- Schichten, in beiden FĂ€llen hervorgerufen durch Starke Metall-TrĂ€ger Wechselwirkung (Strong Metal-Support Interaction, SMSI). Die vorliegende Arbeit untersuchte eine Reihe von oxid-getrĂ€gerten Palladium Pulverkatalysatoren mit einer Variation der Beladung von 1- 5 Gewichts-% auf ihre Struktur-Eigenschafts Beziehungen nach Reduktion in verschiedenen Medien und bei veschiedenen Temperaturen, um ein Referenzsystem zu entwickeln und der Natur von SMSI auf den Grund zu gehen. Dabei kamen oberflĂ€chen- und volumensensitive Methoden wie XPS, Chemisorption, TEM, DRIFTS oder XRD zum Einsatz, um den Einfluss von elektronischen und strukturellen VerĂ€nderungen auf die AktivitĂ€t bei katalytischer Oxidation von Kohlenmonoxid zu untersuchen, welche als wichtigste Testreaktion bei Normaldruck durchgefĂŒhrt wurde. Die Katalysatoren wurden auf reproduzierbare Weise durch kontrollierte Ko-FĂ€llung und durch ImprĂ€gnierung hergestellt. Das untersuchte Pd/Eisenoxid System zeigt Bedeckung der Pd OberflĂ€che nach Reduktion bei vergleichsweise niedrigen Temperaturen. Diese Bedeckung war instabil in sauerstoffhaltiger Umgebung und bildete sich reversibel aus. AbhĂ€ngig von der Pd PartikelgrĂ¶ĂŸe erhöht sie die AktivitĂ€t bei der CO-Oxidation. Legierungsbildung findet bei höheren Reduktionstemperaturen statt. Im Falle von Pd/Zinkoxid findet reversible Legierungsbildung an der OberflĂ€che statt, die ebenfalls die CO-Oxidation begĂŒnstigt, aber ebenfalls schnell deaktiviert. Nach Reduktion bei noch höheren Temperaturen konnte die zusĂ€tzliche Ausbildung einer oxidischen Überschicht beobachtet werden, die das System nicht weiter aktivierte, sondern insgesamt die Zahl der aktiven PlĂ€tze reduzierte. Wegen Legierungsbildung zeigt das Zinkoxid-System bei höheren UmsĂ€tzen in der Acetylenhydrierung ein anderes SelektivitĂ€tsverhalten als das Eisenoxid-System. Im Fall von Pd/Titanoxid kommt es wĂ€hrend der Reduktion ebenfalls zu reversibler OberflĂ€chen- Bedeckung durch teilweise reduzierten TrĂ€ger. Anders als in den beiden anderen FĂ€llen verringert diese Schicht hier jedoch die AktivitĂ€t in der CO-Oxidation. Pd/Aluminiumoxid wurde als schwer reduzierbares Referenz-System untersucht. Wie erwartet zeigt es keine durch SMSI hervorgerufenen VerĂ€nderungen. Schlussendlich konnte in dieser Arbeit gezeigt werden, dass CO-Oxidation eine einfache und geeignete Methode ist, SMSI zu untersuchen und ihren Einfluss auf AktivitĂ€t und StabilitĂ€t von dem anderer Prozesse zu trennen. Die Effekte von OberflĂ€chenverĂ€nderungen auf die katalytische AktivitĂ€t dieser Test-Reaktion hĂ€ngen jedoch stark vom entsprechenden System und der Vorbehandlung ab und können sowohl aktivierender als auch deaktivierender Natur sein. Die Grundlegenden Prinzipien, die bei SMSI eine Rolle spielen, scheinen sowohl im Fall von Modell-Systemen unter UHV-Bedingungen als auch bei Pulver-Systemen bei Normaldruck zu gelten, wie durch die katalytischen Messungen gezeigt wurde

    Iridium oxohydroxide electrocatalysts for the oxygen evolution reaction

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    Iron impregnation on the amorphous shell of vapor grown carbon fibers and the catalytic growth of secondary nanofibers

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    Vapor grown carbon fibers (VGCFs) with diameters of several microns were synthesized and investigated by high resolution transmission electron microscopy. It was found that the shell of the VGCFs consisted of densely-packed domains embedded in loosely-packed matrix, and both were highly amorphous. Regular edge planes as observed on the surface of fishbone nanofibers do not exist on VGCFs. Hence, surface treatment is more important for the deposition of catalysts. Ammonium ferric citrate (AFC) was employed for the impregnation of iron, where the high viscosity of the aqueous solution of AFC is beneficial. Calcination was found to be a key step to improve the dispersion of the iron particles, which can be attributed to enhanced interactions between iron and carbon due to the gasification of carbon occurring at the iron-carbon interface. Quantitative analysis by X-ray photoelectron spectroscopy showed that the calcination of the supported AFC led to a higher atomic concentration of iron on the surface, indicating smaller particle size and higher dispersion. Secondary carbon nanofibers were grown subsequently on the VGCFs from cyclohexane. The specific surface area was enhanced considerably, from less than 1 m2 g-1 to 106 m2 g-1 after the growth of the secondary nanofibers. The obtained composites are promising materials as structured support in heterogeneous catalysis

    Surface-enhanced Raman scattering from surface and subsurface oxygen species at microscopically well-defined Ag surfaces

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    Ag(111) and Ag(110) surfaces exposed to oxygen at elevated temperatures (∌800 K) exhibit strongly enhanced Raman bands at 803 and 627 cm−1 which are attributed to O atoms strongly chemisorbed on the surface (OÎł) or held in subsurface sites (OÎČ), respectively. In contrast to usual experience, surface-enhanced Raman scattering is occurring here under well-defined conditions up to temperatures of 900 K which is attributed to the joint operation of delocalized electromagnetic enhancement (caused by surface roughness provided by oxygen-induced faceting) and local resonance due to the particular electronic properties of the surface sites

    The role of carbonaceous deposits in the activity and stability of Ni-based catalysts applied in the dry reforming of methane

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    Highly stable Ni catalysts with varying Ni contents up to 50 mol% originating from hydrotalcite-like precursors were applied in the dry reforming of methane at 800 and 900 °C. The integral specific rate of methane conversion determined after 10 h on stream was 3.8 mmol s-1 gcat-1 at 900 °C. Due to the outstanding high activity, a catalyst mass of just 10 mg had to be used to avoid operating the reaction in thermodynamic equilibrium. The resulting WHSV was as high as 1.44 × 106 ml gcat-1 h-1. The observed axial temperature distribution with a pronounced cold spot was analyzed by computational fluid dynamics simulations to verify the strong influence of this highly endothermic reaction. Transmission electron microscopy and temperature-programmed oxidation experiments were used to probe the formation of different carbon species, which was found to depend on the catalyst composition and the reaction temperature. Among the formed carbon species, multi-walled carbon nanofibers were detrimental to the long-term stability at 800 °C, whereas their formation was suppressed at 900 °C. The formation of graphitic carbon at 900 °C originating from methane pyrolysis played a minor role. The methane conversion after 100 h of dry reforming at 900 °C compared to the initial one amounted to 98% for the 25 mol% Ni catalyst. The oxidative regeneration of the catalyst was achieved in the isothermal mode using only carbon dioxide in the feed

    Quantitative screening of an extended oxidative coupling of methane catalyst library

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    A comprehensive microkinetic model, including catalyst descriptors, that accounts for the homogeneous as well as heterogeneously catalyzed reaction steps in Oxidative Coupling of Methane (OCM) was used in the assessment of large kinetic datasets acquired on five different catalytic materials. The applicability of the model was extended from alkali magnesia catalysts represented by Li/MgO and Sn-Li/MgO and alkaline earth lanthana catalysts represented by Sr/La2O3 to rare earth-promoted alkaline earth calcium oxide catalysts, represented by LaSr/CaO, and to a Na-Mn-W/SiO2 catalyst. The model succeeded in adequately simulating the performance of all five investigated catalysts in terms of reactant conversion and product selectivities in the entire range of experimental conditions. It was found that the activity of Sr/La2O3, in terms of methane conversion, is approximately 2, 5, 30 and 33 times higher than over the La-Sr/CaO, Sn-Li/MgO, Na-Mn-W/SiO2 and Li/MgO catalysts, respectively, under identical operating conditions. This was attributed mainly to the high stability of adsorbed hydroxyls, the high stability of adsorbed oxygen and the high concentration of active sites of Sr/La2O3. The selectivity towards C2 products was found to depend on the methyl radical sticking coefficient and the stability of the adsorbed oxygen and was the highest on the Na-W-Mn/SiO2 catalyst, that is 75% at about 1% methane conversion and 1023 K, 190 kPa and inlet molar CH4/O2 ratio of 4

    Influence of Contaminants in Steel Mill Exhaust Gases on Cu/ZnO/Al<sub>2</sub>O<sub>3</sub> Catalysts Applied in Methanol Synthesis

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    The influence of impurities in steel mill exhaust gases on ternary Cu/ZnO/Al2O3 catalysts was studied for conventional methanol synthesis, which is one of the central reactions within the cross‐industrial approach of Carbon2Chem¼. A series of hydrocarbons was identified as inert spectators for methanol synthesis. Several catalyst poisons like N‐containing compounds or O2 show reversible characteristics at low pressure. However, by increasing the partial pressure of O2, poisoning becomes irreversible, indicating different poisoning mechanisms concerning the reversibility of deactivation

    Amine-based solvents for exfoliating graphite to graphene outperform the dispersing capacity of N-methyl-pyrrolidone and surfactants

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    Four organic amine-based solvents were discovered which enable direct exfoliation of graphite to produce high-quality and oxygen-free graphene nanosheets. These solvents outperform previously used solvents and additives such as N-methyl-pyrrolidone and surfactants in terms of their dispersing capacity. The resulting dispersions allow the facile fabrication of zeolitic imidazolate framework (ZIF)–graphene nanocomposites with remarkable CO2 storage capability

    Effects of Potassium and Manganese Promoters on Nitrogen-Doped Carbon Nanotube-Supported Iron Catalysts for CO₂ Hydrogenation

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    Nitrogen-doped carbon nanotubes (NCNTs) were used as a support for iron (Fe) nanoparticles applied in carbon dioxide (CO2) hydrogenation at 633 K and 25 bar (1 bar = 105 Pa). The Fe/NCNT catalyst promoted with both potassium (K) and manganese (Mn) showed high performance in CO2 hydrogenation, reaching 34.9% conversion with a gas hourly space velocity (GHSV) of 3.1 L·(g·h)−1. Product selectivities were high for olefin products and low for short-chain alkanes for the K-promoted catalysts. When Fe/NCNT catalyst was promoted with both K and Mn, the catalytic activity was stable for 60 h of reaction time. The structural effect of the Mn promoter was demonstrated by X-ray diffraction (XRD), temperature-programmed reduction (TPR) with molecular hydrogen (H2), and in situ X-ray absorption near-edge structure (XANES) analysis. The Mn promoter stabilized wĂŒstite (FeO) as an intermediate and lowered the TPR onset temperature. Catalytic ammonia (NH3) decomposition was used as an additional probe reaction for characterizing the promoter effects. The Fe/NCNT catalyst promoted with both K and Mn had the highest catalytic activity, and the Mn-promoted Fe/NCNT catalysts had the highest thermal stability under reducing conditions
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