Strong metal-support interaction hydrogen

The structure of supported rhodium catalysts has been the subject of intensive research during the last decade. Rhodium is the component of the automotive exhaust catalyst (the three-way catalyst) responsible for the reduction of NO by CO [1], In addition, it exhibits a number of fundamentally interesting phenomena, such as strong metal-support interaction after high temperature treatment in hydrogen [21, and particle disintegration under carbon monoxide [3]. In this section we illustrate how techniques such as XPS, STMS, EXAFS, TEM and infrared spectroscopy have led to a fairly detailed understanding of supported rhodium catalysts. 

Baker R.T.K., Kim K.S., Emerson A.B., Dumesic J.A. (1986) A Study of the Platinum-Titanium Dioxide System for the Hydrogenation of Graphite Ramifications of Strong Metal-Support Interactions, J. Phys. Chem. 90(5), 860-866. 

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

In addition, the present results clearly show that y-alumina improves the catalytic activity and selectivity to long chain hydrocarbons. Particularly, when the Fe-K is supported on y-alumina, the selectivity to olefins is improved as well as the activity and chain growth ability. It is likely that the marked improvements achieved by using both K and alumina is due to well dispersed active phase Fe-K. The better dispersion of Fe and K should provide the efficient interaction of Fe and K as well more active sites for the CO2 hydrogenation. The improved dispersion seems to be the result of strong metal-support interaction. 

Wang S-Y, Moon SH, Vannice MA (1981) The effect of SMSl (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts 11. Kinetic behavior in the methanation reaction. J Catal 71 167... 

Titania-supported Metals. - After reduction at 473 K, platinum-group metals supported on Ti02 chemisorbed both hydrogen and carbon monoxide in quantities indicative of moderate-to-high dispersion, but following reduction at 773 K chemisorption was drastically lowered e.g., H/Mt <0.01 for Pt, Ir, and Rh, 0.05-0.06 for Pd and Ru, and 0.11 for Os). Agglomeration, encapsulation, and impurities were eliminated as possible causes and a strong metal-support interaction (SMSI) was proposed. Titania is not unique in its SMSI properties and 11 oxides used to support iridium were classified as follows ... 

In a second set of experiments, overlapping thin films of nickel and titania were heated to 1100 K in the presence of hydrogen in a controlled atmosphere scanning transmission electron microscope (STEM). During reaction the metal film was observed to restructure into discrete particles which eventually moved and attacked the titania substrate. We believe that this behavior provides direct evidence that both the mobility of reduced titania and metal species are involved in the initiation of strong metal-support interactions. 

The results of the present study demonstrate that migration of both titania and metal species may be involved in the initiation of strong metal-support interactions during treatment in hydrogen at temperatures in excess of 770 K. 

The work of Tauster and coworkers (1,2) showed that hydrogen chemisorption is suppressed on group VIII metals supported on a series of oxides after these samples have been reduced at high temperatures. The term strong metal-support interactions (SMSI) was introduced to describe this behavior. A similar suppression in hydrogen chemisorption has since been reported for many other supported metal systems 0-5). However, the use of other chemical probes (4, 5) demonstrated that different mechanisms of metal-support interactions could exist for different types of oxides. Furthermore, even for a so-called SMSI oxide, the degree of interaction could be influenced by many parameters such as crystallite size and reduction temperature. It would thus be desirable to find an approach to systematically compare catalytic behavior of different systems. 

In the preparation of Ni/Hp catalysts by the deposition-precipitation method (DP), nickel hydrosilicates are formed mainly but not exclusively in the external surface of the Hp zeolite. The strong metal-support interaction induced by the DP preparation method prevents the Ni metal particles from sintering during the activation of the catalysts (calcination and reduction) and a homogeneous distribution of small nickel particles is obtained. The catalyst prepared by DP showed better catalytic activity in the hydrogenation of naphthalene than the catalyst prepared by cationic competitive exchange. 

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