Structural changes, alumina-supported

In situ dynamic surface structural changes of catalyst particles in response to variations in gas environments were examined by ETEM by Gai et al. (78,97). In studies of copper catalysts on alumina, which are of interest for the water gas shift reaction, bulk diffusion of metal particles through the support in oxygen atmospheres was shown (78). The discovery of this new catalyst diffusion process required a radical revision of the understanding of regeneration processes in catalysis. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Numerical simulations were conveniently used to describe complex fluid dynamic behavior in micro structures [36,101], Van der Linde et al. [101] solved the coupled diffusion equations for reacting species and compared the results with data from the oxidation of CO on alumina-supported Cr using the step-response method. Transient periodical concentration changes in micro channels were numerically calculated by various authors [34, 88, 124],... 

Monometallic catalysts on alumina supports are ideal cases for X-ray fluorescence analysis the matrix (alumina or alumina containing approximately 2% weight chlorine) remains " constant in terms of the matrix effects, and analysis can be conducted directly on finely crushed powder. Texture or grain effects must, however, be carefully avoided by using, in the preparation of standards, the same support as that of the catalysts. Any change in the nature of the support (granulometry, structural modifications, etc.) should be systematically indicated by the person requesting the analysis. 

DRS spectra indicated the change of promoter structure, with spinel formation at temperatures exceeding 700°C, i.e. incorporation of Ni(Co) in alumina support causing the destruction of previously formed active phases of the catalyst. 

The catalytic layer of monolithic automotive reactors usually consist of active metals (Pt, Pd, Rh) supported on alumina. One of the most important problems set by these catalysts is the decrease in their activity after thermal exposure to the exhaust gas itself (Ref.l). It is well known that this thermal deactivation is directly related to the sintering of the active components. Moreover, this modification of the supported metal is drastically enhanced by structural changes of the support. Thus using TEM experiments, Chu et al (Ref. 2) have reported rapid sintering of platinum during the structural transition Y-AI2O3 to (X-AI2O3. 

The reflection of y-alumina and PdO ciystals can be distinguished on the samples calcined at 500 °C. When the calcination temperature is increased fi om 500 "C to 1000 "C, alumina undergoes changes of crystal phase as reported in previous studies [17], This modification of the crystal structure is usually accompanied by a decrease of the surface area. In the case of Ba- and La-modified material, this modification of the crystal structure is noticeably diminished. On the samples calcined at 1000 C (right part of the figure), the increase in crystalline character affects the palladium more than the alumina support. The presence of La and Ba within the alumina structure effectively avoids these transformations. In contrast, the samples supported on pure alumina suffer from the formation of various phases, mostly 0-AI2O3. However, it is possible to observe traces of a-ALOs phases. 

It is well known that the Mn+ ion has approximately the same radius as the A1+ ion. It is suggested that the crystal lattice of the alumina support exerts an inductive action on the manganese sufficient to cause a change of oxidation state. It might be said that the manganese oxide attempts to imitate the crystal structure of the support, even to the extent of changing oxidation state. The effect is somewhat related to the phenomenon of oriented overgrowth, and is reminiscent of a current theory of nucleoprotein synthesis. 

Chang, J.R., Chang, S.L., Lin, T.B. (1997). y-Alumina-Supported Pt Catalysts for Aromatics Reduction A Structural Investigation of Sulfur Poisoning Catalyst Deactivation. Journal of Catalysis, Vol.169, No.l, 0uly 1997), pp. 338-346, ISSN 0021-9517... 

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