Catalyst carburization studies

Carburization Studies of Iron Fischer-Tropsch Catalysts... 

The effect of heating rate during carburization (H2/CO = 3/1) on the magnetic properties of the catalysts were studied at 1 atm in the magnetic susceptibility apparatus and in the tubular reactor (Table VII). The mass gain of the samples carburized at 1 atm in the susceptibility... 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Agrawal et al.33 performed studies of Co/A1203 catalysts using sulfur-free feed synthesis gas and reported a slow continual deactivation of Co/A1203 methanation catalysts at 300°C due to carbon deposition. They postulate that the deactivation could occur by carburization of bulk cobalt and formation of graphite deposits on the Co surface, which they observed by Auger spectroscopy. 

The conversion of iron catalysts into iron carbide under Fischer-Tropsch conditions is well known and has been the subject of several studies [20-23], A fundamentally intriguing question is why the active iron Fischer-Tropsch catalyst consists of iron carbide, while cobalt, nickel and ruthenium are active as a metal. Figure 5.9 (left) shows how metallic iron particles convert to carbides in a mixture of CO and H2 at 515 K. After 0.5 and 1.1 h of reaction, the sharp six-line pattern of metallic iron is still clearly visible in addition to the complicated carbide spectra, but after 2.5 h the metallic iron has disappeared. At short reaction times, a rather broad spectral component appears - better visible in carburization experiments at lower temperatures - indicated as FexC. The eventually remaining pattern can be understood as the combination of two different carbides -Fe2.2C and %-Fe5C2. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

The applications of IR spectroscopy in catalysis are many. For example, IR can be used to directly characterize the catalysts themselves. This is often done in the study of zeolites, metal oxides, and heteropolyacids, among other catalysts [77,78], To exemplify this type of application, Figure 1.11 displays transmission IR spectra for a number of Co Mo O (0 < x < 1) mixed metal oxides with various compositions [79]. In this study, a clear distinction could be made between pure Mo03, with its characteristic IR peaks at 993, 863, 820, and 563 cm-1, and the Mo04 tetrahedral units in the CoMo04 solid solutions formed upon Co304 incorporation, with its new bands at 946 and 662 cm-1. These properties could be correlated with the activity of the catalysts toward carburization and hy-drodenitrogenation reactions. 

In connection with the aromatization of methane induced by Mo-H-ZSM-5 catalysts (see Section 3.5.2), Mo2C on H-ZSM-5 prepared by carburation of M0O3 was studied in the aromatization of ethane,427 ethylene,428 and propane 429 The high dehydrogenation activity of Mo2C and the ability of H-ZSM-5 to induce aromatization of alkenes makes this an active and selective catalyst for aromatics production. 

In order to understand better these interesting systems without complications that might arise due to different preparation procedures, we compared oxygen-treated WC and Mo2C prepared by similar reduction/ carburization procedures from their respective oxides. The effects of pretreatment conditions were also studied. An attempt was made to correlate the kinetic behavior of these catalysts in n-hexane-H2 reactions with their physical properties obtained from X-ray diffraction (XRD), CO chemisorption, temperature-programed reaction (TPR) with flowing H2 or He, temperature programed desorption (TPD) of adsorbed NH3, and X-ray photoelectron spectroscopy (XPS). 

We did not observe two ferromagnetic phases, Fe2.2C (c phase) and Fes04, as previously observed by TMA in carburized iron (1,2). This diflFerence in the studies is probably attributable to differences in catalyst compositions and reaction conditions. In a few but not all instances of Ref. 2, the assignment of Fes04 of an inflection at about 825 K in the TMA curve appears to be incorrect because the inflection disappeared upon cooling. This disappearance is a strong indication of a thermal reaction between Fe and FesC. 

Studies of the clean and Pt-modified carburized W single crystals provided a fundamental understanding of how methanol decomposition proceeds under UHV conditions. These studies suggest that tungsten carbide-based electrocatalysts, especially metal-modified tungsten carbides, are promising anode catalyst materials to replace Pt in the DMFC. These conclusions are further examined on more applicable polycrystalUne foils. These foils are better representations of the complex morphology... 

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