Inhomogeneity, catalyst

The development of a theory accounting for catalyst inhomogeneity is determined by the availability of a great number of experimental data (calorimetric, isotopic, etc.) that give evidence to this inhomogeneity. At present, numerous qualitatively new experimental data have been accumulated that are to be theoretically substantiated [66, 67]. 

However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

In the SCR process, ammonia, usually diluted with air or steam, is injected through a grid system into the flue/exhaust stream upstream of a catalyst bed (37). The effectiveness of the SCR process is also dependent on the NH to NO ratio. The ammonia injection rate and distribution must be controlled to yield an approximately 1 1 molar ratio. At a given temperature and space velocity, as the molar ratio increases to approximately 1 1, the NO reduction increases. At operations above 1 1, however, the amount of ammonia passing through the system increases (38). This ammonia sHp can be caused by catalyst deterioration, by poor velocity distribution, or inhomogeneous ammonia distribution in the bed. 

The present work demonstrates that the mixed oxide catalyst with inhomogeneous nanocrystalline MosOu-type oxide with minor amount of M0O3- and Mo02-type material. Thermal treatment of the catalyst shows a better performance in the formation of the crystals and the catalytic activity. The structural analysis suggests that the catalytic performance of the MoVW- mixed oxide catalyst in the partial oxidation of methanol is related to the formation of the M05O14 t3 e mixed oxide. 

Fig. 4 shows the SEM images of SWNTs purified by the thermal oxidation and acid-treated. Fig. 4(a) shows a SEM image of the raw soot. In addition to the bundle of SWNTs, carbonaceous particles are shown in the figure. These stractural features mi t be causal by various in the arcing process because of an inhomogeneous distribution of catalysts in the anodes [7]. It can be seen that the appearance of SWNTs was curled and quite different fiom that of MWNTs. Fig. 4(b) shows a decrease of amorphous carbons after oxidation. The basic idea of the selective etching is that amorphous carbons can be etched away more easily than SWNTs due to the faster oxidation reaction rate [2]. Since the CNTs are etched away at the same time, the yield is usually low. The transition metals can be etched away by an add treatment. Fig. 4(c) shows the SEM image of the acid-treated sample, where the annealed sample was immersed in 10 % HCl. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

This new design is sought to overcome the limits of conventional porous fixed-bed reactors using an electrode phase flowing through the pores [65]. The latter systems suffer from the low conductivity of the electrolyte phase. This generates electrical resistance and leads to accumulation of the electrical current in certain reactor zones and hence results in a spatially inhomogeneous reaction. This means poor exploitation of the catalyst and possible reductions in selectivity. 

The effect of mutation is different in case of stopped samples, but the phenomenon cannot be completely avoided. Here, the experimental time period At is determined by the poison diffusion. The catalyst poison solution is sprayed on top of a reacting sample and then diffuses into the core of the sample where it stops the reaction sequentially layer by layer. This leads to small inhomogeneity in the sample, since the reaction near the upper surface is stopped earlier than the reaction near the bottom of the mold. 

Figure 3.7 Comparison of the monochromatic Mo 3d XPS spectra of M0O3 in an insulating silica-supported catalyst and in a conducting, thin silica film-supported model catalyst, showing the effect of inhomogeneous charge broadening (courtesy of H. Korpik, Eindhoven).

Matrix effects and inhomogeneous sample charging seriously hinder quantitative analysis of SIMS on technical catalysts. Although full quantitation is almost impossible in this area, the interpretation of SIMS data on a more qualitative base nevertheless offers unique possibilities. Molecular cluster ions may be particularly informative about compounds present in a catalyst. 

The SCR process is highly sensitive to inhomogeneities in the ammonia distribution at the inlet of the catalyst layer [26, 27]. 

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