Applications to Heterogeneous Catalysis

In Section I, C the different Mossbauer parameters were individually discussed with reference to possible catalytic applications. The purpose of that discussion was to provide a physical feeling for the parameters and an appreciation of their possible uses in catalysis. In general, however, for the study of a particular catalytic phenomenon the decision whether also to employ Mossbauer spectroscopy is not based only on the consideration of a single Mossbauer parameter. Thus, in the next sections we discuss, based on a number of examples, the manner in which various catalytic phenomena can be investigated through the systematic employment of the Mossbauer parameters. 

In the preparation and stablization of small, supported-catalyst particles, the consideration of surface mobility is essential. If the active component is in a high state of dispersion, conditions under which high mobility is attained must be avoided, since these conditions lead to particle size growth. On the other hand, a poorly dispersed component may be partially redispersed by treatment in a more highly mobile state. In supported catalyst systems, the interaction between the dispersed species (the active component) and the support is always of important concern, and a measure of the mobility of the active component is an indirect measure of this important interaction. 

For unsupported catalysts, where particle sizes are typically an order of magnitude larger than those for supported catalysts, the mobility of various species in the bulk structure may be of interest when considering how the bulk structure and composition are reflected in the surface properties of the particle. In addition, bulk mobility is an important consideration in the understanding of solid state reactions and phenomena such as sintering. 

The manifestations of mobility in the Mossbauer parameters are perhaps best introduced with reference to a series of papers by the Russian Laboratory at the Institute of Chemical Physics in Moscow (115-120). Several examples from this series will demonstrate the underlying principles. One system studied was tin (using the 119Sn, 23.9-keV transition) on 300 m2gm-1 silica gel. The Mossbauer spectra of this sample are characteristic of both SnO and Sn02 H20 components being present in approximately equal proportions (115), and it is from the temperature dependences of the respective spectral areas that information about the mobilities of these two species 

In a subsequent publication (118), the SnO and Sn02nH20 interactions with the silica support were studied on samples with different pore diameters ranging from 0.5 to 27 nm, and on a synthetic mordenite of 0.6-nm pore diameter. From the temperature dependence of the respective spectral areas, it was concluded that both the SnO and the Sn02 H20 components were more strongly bonded to supports with smaller pore diameters. In addition to the spectral area, however, the spectral width is also expected to reflect 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

Preparation of Highly Active Monolayer-Dispersed Catalysts 

Rendering the active component into the monolayer-dispersed state is an important measure to undertake to enhance the activity of a catalyst. A striking example is the highly active catalyst system for 

Oxides or salts may play the part of promoters or additives in a heterogeneous catalyst. Their function in various catalyst systems can vary widely and is too complicated to have been adequately elucidated so far. However, we have found that they often operate as surface modifiers. For example, adding a small amount of rare earth oxide such as La203 to the methanation catalyst Ni/y-Al203 can significantly increase its activity and thermal stability (34). 

This catalyst was prepared by impregnating y-Al203 with a Ni(N03)2 and La(N03)3 solution. By drying and calcinating the impregnated support we then obtained a system of monolayer-dispersed NiO and La203 on 

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R. L. Moss and L. Whalley Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. Gravelle... 

Applications to Heterogeneous Catalysis. Ultrasonic irradiation can alter the reactivity observed during the heterogeneous catalysis of a variety of reactions. Sonication has shown such behavior 1) by... 

Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis... 

W. M. H. Sachtler and R. A. van Santen Mossbauer Spectroscopy Applications to Heterogeneous Catalysis James A. Dumesic and Henrik Tops0e Compensation Effect in Heterogeneous Catalysis... 

Concentration modulation experiments have been reported for applications to heterogeneous catalysis (48). The experimental implementation was accomplished by periodically flowing solutions with different (reactant) concentrations over the catalyst immobilized on the IRE. Fast concentration modulation in the liquid phase is limited by mass transport (diffusion and convection), and an appropriately designed cell is essential. The cell depicted in Fig. 12 has two tubes ending at the same inlet (65). This has the advantage that backmixing in the tubing upstream of the cell can be avoided. With this cell, concentration modulation periods of about 10 s were achieved (45,65). 

Mossbauer Spectroscopy Applications to Heterogeneous Catalysis James A. Dumesic and Henrik... 

The first article by Xie and Tang provides a review of Spontaneous Monolayer Dispersion of Oxides and Salt onto Surfaces of Supports Applications to Heterogeneous Catalysis. ... 

Shustorovich, E., Ed. In Metal-Surface Reaction Energetics Theory and Applications to Heterogeneous Catalysis, Chemisorption, and Surface Diffusion Wiley-VCH Weinheim, 1991. 

Xie, Y. Tang, Y. Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports applications to heterogeneous catalysis. Adv. Catal. 1990, 37, 1 3. 

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