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

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

Platinum Black by XPS

XPS spectra of Pt black in the as received state showed O and C impurities along with Pt. An in situ treatment by O2 and H2 increased Pt intensity and removed a part of oxygen and carbon impurities. The quasihomogeneous model was used for quantitative evaluation applying atomic sensitivity factors published in the literature (Ref. 1). Decomposition of the O 1s region indicated the presence of adsorbed O, OH, and H2O as well as CO and CO species, whereas the C 1s region could be decomposed to give Pt–C, graphite, CxHy polymer, and oxidized C entitie

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

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 geträgerte, nano-skalige Edelmetall-Katalysatoren sind bekannt dafür, eine Reihe von Oberflächen-Veränderungen zu erfahren, wenn sie bei erhöhter Temperatur reduziert werden. Diese Prozesse beinhalten beispielsweise (Oberflächen-) Legierungsbildung und die Ausblidung von teilweise reduzierten, oxidischen Träger- Schichten, in beiden Fällen hervorgerufen durch Starke Metall-Träger Wechselwirkung (Strong Metal-Support Interaction, SMSI). Die vorliegende Arbeit untersuchte eine Reihe von oxid-geträgerten 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 oberflächen- und volumensensitive Methoden wie XPS, Chemisorption, TEM, DRIFTS oder XRD zum Einsatz, um den Einfluss von elektronischen und strukturellen Veränderungen auf die Aktivität bei katalytischer Oxidation von Kohlenmonoxid zu untersuchen, welche als wichtigste Testreaktion bei Normaldruck durchgeführt wurde. Die Katalysatoren wurden auf reproduzierbare Weise durch kontrollierte Ko-Fällung und durch Imprägnierung hergestellt. Das untersuchte Pd/Eisenoxid System zeigt Bedeckung der Pd Oberfläche nach Reduktion bei vergleichsweise niedrigen Temperaturen. Diese Bedeckung war instabil in sauerstoffhaltiger Umgebung und bildete sich reversibel aus. Abhängig von der Pd Partikelgröße erhöht sie die Aktivität bei der CO-Oxidation. Legierungsbildung findet bei höheren Reduktionstemperaturen statt. Im Falle von Pd/Zinkoxid findet reversible Legierungsbildung an der Oberfläche statt, die ebenfalls die CO-Oxidation begünstigt, aber ebenfalls schnell deaktiviert. Nach Reduktion bei noch höheren Temperaturen konnte die zusätzliche Ausbildung einer oxidischen Überschicht beobachtet werden, die das System nicht weiter aktivierte, sondern insgesamt die Zahl der aktiven Plätze reduzierte. Wegen Legierungsbildung zeigt das Zinkoxid-System bei höheren Umsätzen in der Acetylenhydrierung ein anderes Selektivitätsverhalten als das Eisenoxid-System. Im Fall von Pd/Titanoxid kommt es während der Reduktion ebenfalls zu reversibler Oberflächen- Bedeckung durch teilweise reduzierten Träger. Anders als in den beiden anderen Fällen verringert diese Schicht hier jedoch die Aktivität in der CO-Oxidation. Pd/Aluminiumoxid wurde als schwer reduzierbares Referenz-System untersucht. Wie erwartet zeigt es keine durch SMSI hervorgerufenen Veränderungen. Schlussendlich konnte in dieser Arbeit gezeigt werden, dass CO-Oxidation eine einfache und geeignete Methode ist, SMSI zu untersuchen und ihren Einfluss auf Aktivität und Stabilität von dem anderer Prozesse zu trennen. Die Effekte von Oberflächenveränderungen auf die katalytische Aktivität dieser Test-Reaktion hängen 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

The catalytic reduction of NO by H-2 on Ru(0001): Observation of NHads species

Adsorption of NO and the reaction between NO and H-2 were investigated on the Ru(0001) surface by X-ray photoelectron spectroscopy (XPS). Surface composition was measured after NO adsorption and after the selective catalytic reduction of nitric oxide with hydrogen in steady-state conditions at 320 K and 390 K in a 30:1 mixture of H-2 and NO (total pressure = 10(-4) mbar). After steady-state NO reduction, molecularly adsorbed NO in both the linear on-top and threefold coordinations, NHads and N-ads species were identified by XPS. The coverage of the NHads and N-ads species was higher after the reaction at 390 K than the corresponding values at 320 K Strong destabilisation of N-ads by O-ads was detected. A possible reaction mechanism is discussed. (c) 2005 Elsevier B.V. All rights reserved

Interaction of oxygen with silver at high temperature and atmospheric pressure: A spectroscopic and structural analysis of a strongly bound surface species

X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and ion scattering spectroscopy (ISS) have been used to study the Ag(111) single-crystal surface after exposure to O2 at high temperature and at atmospheric pressure. The activated formation of a strongly bound surface layer has been observed, as identified by an asymmetry of the Ag 3d5/2 core-level peak at 367.3 eV and an O 1s peak at 529.0 eV (Oγ). In addition, oxygen was found to be dissolved in the bulk (Oβ), exhibiting an O 1s binding energy between 531 and 530 eV depending on its abundance. X-ray-excited oxygen KVV Auger electron spectroscopy revealed the presence of Oγ by additional peaks at 514.8 and 494.7 eV. UPS displayed oxygen-derived bands located above the emission from the Ag 4d band at 3.2 and 2.5 eV. Oxygen-related peaks below the Ag 4d band were identified as resulting from OH groups formed by reaction of surface oxygen (Oα) with residual hydrogen. The incorporated oxygen caused a pronounced charge separation as reflected by a 1 eV increase in the work function. ISS measurements revealed that Oγ is incorporated in the topmost surface layer, shielding underlying Ag atoms from the He+ beam. All spectroscopic data point to the presence of one monolayer of silver-embedded oxygen, which is in dynamic equilibrium with surface atomic oxygen segregated from the bulk at high temperature. The oxygen embedded in the topmost silver layer is strongly bound to the metal, with its interaction being different from adsorbed atomic oxygen and bulk Ag2O. It is stable up to 900 K, in contrast to the binary silver oxides, and relevant for high-temperature oxidation reactions catalyzed by Ag. A qualitative analysis is presented of the chemical bonding of the different surface species in comparison to the situation of a complex silver oxide reference

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

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

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 (Oγ) or held in subsurface sites (Oβ), 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

Development of hydrotalcite-derived Ni catalysts for the dry reforming of methane at high temperatures

Catalytic dry reforming of methane (DRM) is an attractive technology for industrial production of synthesis gas, an important feedstock for the production of many basic chemicals. The endothermic reaction operates at high temperatures above 640 °C. On nickel based catalysts high syngas yields are obtained. However, catalyst deactivation by coke formation over Ni based catalysts is still challenging. Deeper understanding of the structure-performance-relationships is needed to integrate the DRM in the well-established downstream syngas chemistry. This thesis presents a systematic study on the development of a long-term active and thermally stable Ni/MgAl oxide catalyst for the DRM reaction by understanding and optimization of the catalyst synthesis. Regarding the active catalyst, particular emphasis was laid on the understanding of the formation of carbon deposits. By comprehensive structural characterizations of the material in all stages of the preparation, a synthesis route via a Ni,Mg,Al hydrotalcite-like precursor was developed that leads to nanostructuring of the catalytic material. This procedure was successfully applied to Ni/MgAl oxide catalysts with various compositions. Upon high-temperature reduction the catalysts form Ni nanoparticles which are embedded in an oxide matrix and covered by an overlayer. The nature of the overgrowth was investigated applying surface sensitive methods, revealing the presence of predominantly oxidic species. Interestingly, the overgrowth was found to effectively attenuate the carbon formation. Despite coke formation and high Ni loading up to 55 wt.-%, the CH4 conversion in the DRM at 900 °C was stable over 100 hours. The thermal stability of the Ni nanoparticles is attributed to the embedding nature of the oxide matrix. This allows the high-temperature operation without losing substantial active Ni surface area. Furthermore, the DRM activity, as well as the carbon formation, was strongly depending on the Ni content. The incorporation of a higher amount of Ni was found to increase the activity as well as the coking propensity. By analysis of the spent catalysts thermal and compositional dependencies on the formed carbon species were found. The amount of filamentous carbon decreases with higher reaction temperature and lower Ni content. The carbon formation was found to be a continuous process over the investigated time and caused mainly by methane pyrolysis. From the overall gained insights it can be concluded, that a good catalyst have to make a compromise between activity and coke resistance, which can be controlled by an interplay of Ni dispersion, embedment and metal-support-interactions. This work demonstrates the relevance of a detailed characterization at all stages of the catalyst preparation, as well as after the reaction, to understand and improve the catalytic performance by rational approaches. The experimental findings give new insights into the current state of reforming knowledge and coke formation and will contribute to the development of advanced catalysts for DR

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

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