Catalytic reactor for operando spatially resolved structure–activity profiling using high-energy X-ray diffraction

In heterogeneous catalysis, operando measurements probe catalysts in their active state and are essential for revealing complex catalyst structure–activity relationships. The development of appropriate operando sample environments for spatially resolved studies has come strongly into focus in recent years, particularly when coupled to the powerful and multimodal characterization tools available at synchrotron light sources. However, most catalysis studies at synchrotron facilities only measure structural information about the catalyst in a spatially resolved manner, whereas gas analysis is restricted to the reactor outlet. Here, a fully automated and integrated catalytic profile reactor setup is shown for the combined measurement of temperature, gas composition and high-energy X-ray diffraction (XRD) profiles, using the oxidative de­hydrogenation of C$_{2}$H$_{6}$ to C$_{2}$H$_{4}$ over MoO$_{3}$/γ-Al$_{2}$O$_{3}$ as a test system. The profile reactor methodology was previously developed for X-ray absorption spectroscopy and is here extended for operando XRD. The profile reactor is a versatile and accessible research tool for combined spatially resolved structure–activity profiling, enabling the use of multiple synchrotron-based characterization methods to promote a knowledge-based optimization of a wide range of catalytic systems in a time- and resource-efficient wa

Investigation of heterogeneously catalyzed reactions using molecular beam sampling mass spectrometry with threshold ionization

Ziel der vorliegenden Arbeit war es zu prüfen, ob sich die Molekularstrahl-Massenspektrometrie mit Bestimmung von Ionisations- und Auftrittspotentialen eignet, in-situ die Gasphase über einem, unter realkatalytischen Bedingungen arbeitenden, Katalysator zu analysieren. Im Vordergrund stand der Nachweis reaktiver und daher kurzlebiger Gasphasenintermediate (z.B. Radikale) bei der Platin-katalysierten Bildung von Blausäure aus Methan und Ammoniak. Könnten solche Spezies nachgewiesen werden, wäre das ein Hinweis auf homogen ablaufende Reaktionsschritte in der Gasphase. Dazu wurde ein Labormodell eines technischen Blausäurereaktors über ein Molekularstrahl-Interface an ein Quadrupol-Massenspektrometer gekoppelt, welches neben der massenspektrometrischen Analyse die Bestimmung von Ionisations- und Auftrittspotentialen erlaubte. Der Reaktor bestand aus einem elektrisch beheizten, innen mit Pt beschichteten Keramikrohr und konnte unter Atmosphärendruck, Temperaturen bis 1300°C und Flüssen bis 4000ml/min betrieben werden. Er simulierte ein Einzelrohr des technischen Rohrbündelreaktors. Die Ankopplung an das im Hochvakuum arbeitende Massenspektrometer erfolgte über ein differentiell gepumptes Interface bestehend aus Düse, Skimmer und Kollimator. Die Probennahme mittels Molekularstrahl sollte reaktive Spezies einfrieren und einen schnellen und stoßfreien Transport in die Ionenquelle des Massenspektrometers gewährleisten. Neben der Blausäurebildung wurden mit dieser Anordnung weitere, in Bezug auf Gasphasenintermediate interessante, Pt-katalysierte Reaktionen untersucht. Das waren im Einzelnen die katalytische Methanverbrennung, die Ammoniakoxidation und die thermische Zersetzung von Ammoniak in die Elemente. Zunächst wurde ein theoretisches Modell an die für Helium gemessene Ionisations-Effektivitätskurve angepasst, um den mit den experimentell bestimmten Ionisations- und Auftrittspotentialen verbundenen Fehler zu ermitteln. Dieser war mit 0.6eV hinreichend klein, um alle erwarteten Intermediate eindeutig indentifizieren zu können. Durch Messung der Ionisations-Effektivitätskurve auf der Massenzahl m/z=29 amu war es in der Tat möglich, Methylenimin (CH2=NH) als Gasphasenintermediat bei der Bildung von Blausäure aus Methan und Ammoniak nachzuweisen. Die Identifizierung des Methylenimins erfolgte anhand seines, im Vergleich zu allen anderen überlagernden Molekülionen herausragend niedrigen Ionisationspotentials. Der experimentelle Nachweis dieses Moleküls bestätigt eine Vorhersage einer in der Literatur veröffentlichten theoretischen Arbeit, welche die Dehydrierung von Methylenimin zu Blausäure als homogenen Schlüsselschritt des Blausäurebildungsmechanismus postuliert. Kurzlebige Intermediate wie z.B. Radikale wurden bei keiner der untersuchten Reaktionen gefunden. Die Nichtnachweisbarkeit dieser Spezies kann jedoch nicht als Beweis ihres Nichtvorhandenseins dienen, da die durchgeführten Experimente und eine eingehende Analyse des apparativen Aufbaus zeigten, dass der in-situ Charakter der Methode im Hinblick auf sehr kurzlebige Intermediate nicht gegeben war. Die Detektionsmethode an sich erwies sich zwar als geeignet, aber es gelang nicht, eine auch für sehr reaktive Moleküle repräsentative Probe der Reaktionszone zu erhalten. Im letzten Teil der Arbeit wird daher ein überarbeitetes Konzept vorgestellt, welches sich derzeit in der Realisierungsphase befindet. Der Neuaufbau übernimmt die massenspektrometrische Detektion, arbeitet aber mit einer verbesserten Probennahme. Die Gasmischung expandiert hierbei aus der Reaktionszone durch eine sich direkt in der katalytisch aktiven Wand befindende Düse in eine Niederdruckumgebung ("Fenn"-Typ Interface). Reaktive Spezies werden so unmittelbar eingefroren und die Zahl ihrer Stöße mit anderen Molekülen und der Rohrwand minimiert.Goal of the present work was to verify if molecular beam sampling mass spectrometry with simultaneous determination of ionization- and appearance potentials can be used to study in-situ the gas phase above a catalyst, working under technical conditions. In the focus of interest was thereby the detection of reactive and consequently short-lived gas phase intermediates (e. g. radicals) in the platinum catalyzed formation of hydrocyanic acid from methane and ammonia. If such species were detected, it would be an indication of reaction steps proceeding homogeneously in the gas phase. To reach this goal a molecular beam sampling interface has been used to connect a bench-scale model of the technical reactor with a quadrupole mass spectrometer, which allowed in addition to its normal operation the determination of ionization- and appearance potentials. The reactor consisted of an electrically heated ceramic tube, covered on the inside with platinum, and could be operated at atmospheric pressure, temperatures up to 1300°C and gas flows up to 4000mlmin. This configuration simulated a single tube of the technical tube bundle reactor. The connection to the mass spectrometer operating under high vacuum was accomplished by means of a differentially pumped interface consisting of nozzle, skimmer and collimator. The purpose of such a molecular beam sampling interface is to quench reactive species and to provide their collisionless transport to the ion source of the mass spectrometer. Not only the hydrocyanic acid formation but also other Pt-catalyzed reactions have been investigated with this setup. These were the catalytic combustion of methane, the ammonia oxidation and the ammonia decomposition into the elements. First of all, a theoretical model has been fitted to the ionization efficiency curve of helium to determine the error connected with experimentally obtained ionization- and appearance potentials. The value of 0.6eV was low enough to allow for unambiguous identification of all of the expected intermediates. Indeed, by measuring the ionization-efficiency curve at the mass number m/z=29amu, it was possible to detect methylenimine (CH2=NH) as gas phase intermediate in the hydrocyanic acid formation from methane and ammonia. Methylenimine was identified by its ionization potential which is lower than those of any of the interfering ions. The experimental proof of this intermediate confirms a prediction of a published theoretical work which postulated the dehydrogenation of methylenimine to hydrocyanic acid as a homogeneous key step in the reaction mechanism. More short-lived intermediates such as radicals could not be detected in any of the investigated reactions. However, the non-detectability of these species can not prove their absence as the performed experiments and a careful investigation of the experimental setup revealed that the entire method could not be considered as in-situ with respect to real short-lived intermediates. Although the detection method proved suitable, it was not possible to draw a sample from the reaction zone that is representative even for very reactive molecules. For this reason, a revised experimental setup, which is currently under construction, is presented in the last part of this work. The new apparatus adopts the mass spectrometric detection but employs an improved sampling. Here, the gas mixture expands through a nozzle, which is integrated in the catalytically active wall, directly out of the reaction zone into a low pressure background ("Fenn"-type interface). Consequently, reactive intermediates are quenched instantaneously and the number of collisions that they perform with other gas molecules and with the tube wall is minimized

Blausäure aus Methan und Ammoniak - Untersuchungen mittels Molekularstrahl - Massenspektrometrie und Bestimmung von Ionisations- und Auftrittspotentialen

1. Einführung und Motivation 2. Experimenteller Aufbau, Vorgehen 3. Ionisations- und Auftrittspotentiale 4. Ergebnisse 5. Zusammenfassung, Diskussion 6. Ausblick 7. Danksagun

Effect of the catalyst pore structure on fixed-bed reactor performance of partial oxidation of n-butane: A simulation study

The effect of catalyst pore structure on n-butane oxidation to maleic anhydride in a fixed-bed reactor was investigated by numerical simulations. The micro- and macro- pore model of Wakao and Smith was applied to model the diffusion-reaction inside the catalyst pellet. The studied pore structure parameters were macro-pore porosity, mean macro-pore diameter and mean micro- pore diameter. A fixed-bed reactor was simulated with a detailed two-dimensional heterogeneous model under typical industrial conditions. Simulation results have demonstrated that the reactor performance is sensitive to the chosen pore structure parameters especially macro-pore porosity and mean micro-pore diameter. A bi-modal catalyst pellet with bigger macro-pores and smaller micro-pores is favored to achieve higher yields of maleic anhydride. This work highlights the potential of improving this process by pore structure optimization

Investigation of radial heat transfer in a fixed-bed reactor: CFD simulations and profile measurements

Three dimensional CFD simulations with computer generated packings by DEM simulations are becoming a powerful approach for simulating heat transfer in fixed-bed reactors with small D/d ratio. To put forward this approach to be a preferred design tool in industry and academia, experimental validations are needed. In this work, profile measurement techniques and the DEM-CFD simulation approach are applied to obtain radial temperature profiles in a thin tube fixed-bed reactor packed with glass spheres and steatite rings at different packing heights and different flow rates. Good agreement is found between measured and simulated results for both packing types. The high resolution of the profile measurement provides the possibility for critical judgement of the simulations

Mechanism of H₂ and CO formation in the catalytic partial oxidation of CH₄ on Rh probed by steady-state spatial profiles and spatially resolved transients

Spatially resolved species and temperature profiles have been measured for the catalytic partial oxidation of CH4 on autothermally operated Rh-coated α - Al₂ O₃ foams in both steady state and transient mode. A probe consisting of a thin quartz capillary and a thermocouple was moved with sub-mm resolution through the catalyst/heat shield stack to sample the gases into a mass spectrometer and measure temperature. The probe was also used to follow the relaxation of the system upon perturbation by stepping the stoichiometry of the reactants periodically. The spatial profiles in steady state show that H₂ and CO are formed in presence of gas phase oxygen (oxidation zone) and after total oxygen conversion by steam reforming (reforming zone). The stepwise change of the reactant stoichiometry in the transient experiments allowed decoupling chemistry (ms timescale) and temperature (s timescale). By exposing a catalyst, initially operating very hot at C / O = 0.6, suddenly to a CH₄ rich feed (C / O = 1.4), it was possible to follow how the integral H₂ and CO production rates decrease with decreasing temperature of the catalyst. The reverse switch from C / O = 1.4 to 0.6 showed how the integral H2 and CO production rates increased with increasing temperature of the catalyst. Differential reaction rates were obtained by performing these transients spatially resolved at two adjacent points in the catalyst. For C / O = 0.6 → 1.4, H2 and CO formation show a strict linear Arrhenius behavior over the entire temperature range from ∼ 1100 to ∼ 600 C. For C / O = 1.4 → 0.6, the Arrhenius plots show two straight segments, a steep increase in the formation rate from ∼ 630 to ∼ 710 C and a weak increase from ∼ 725 to ∼ 1000 C. In both cases, H2 formation is more activated than CO formation indicating two different rate determining steps