Catalyst ageing microscopy

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

When a catalyst is exposed to a stoichiometric gas mixture, whether oscillating or stationary, the resulting deactivation is very moderate when the temperature of the pretreatment is raised from 500°C to 900°C. This deactivation can only be proven by the values of the T50 recorded for the reaction between NO and HC and not for that between CO and O2. This confirms that the CO/O2 reaction is structure insensitive. Indeed, a moderated sintering of the metallic phase, as seen by microscopy, has no influence upon the reaction when compared to a catalyst aged from 500°C to 900°C. The implication is that the best way to observe an effect related to sintering is to consider the reactions involving HC or NO since they are usually structure sensitive [15,16]. 

Hydroxyl-Zr bentonite catalyst for esterification was prepared. Effects of the preparation conditions (calcination temperature, calcination time and ageing time) on interlayer distance, specific surface area and surface total acidity of catalyst have been studied. XRD, BET method and potentiometric titration were respectively used to detennine interlayer distance, specific surface area and surface total acidity of catalyst. Scanning Electron Microscopy was performed to observe the images of samples. 

By XAS investigations of model and automotive catalysts (Pt, Rh / Ce02 / AI2O3) associated with analytical electron microscopy, it is shown that for both model and aged catalysts treated in air at high temperatures, true alloyed phases were formed. 

Frequently exposed to very high temperatures, the catalysts used for depollution of motor vehicle exhaust gases are deactivated mainly by structural and textural evolution processes. This paper describes how the catalytic activity of a typical three-way catalyst (platinum-rhodium on a wash-coated cordierite monolith) was determined for the removal of hydrocarbons of various types before and after high-temperature treatments. Electron microscopy (CTEM and STEM) was used to determine metal particle size in fresh and aged catalysts. 

Thermal aging of 3-way catalysts based on Pt+Rh on alumina promoted by Ce and Fe oxides has been followed by examinations using electron microscopy (CTEM and STEM). Three particle populations exist in the wash-coat of catalysts of this type R, Pt/Rh and Ce/Fe. The rhodium that is not associated with the R is also detected for new catalyst, but in a form that is hyperdispersed in the atomic state or in the form of very small particles tliat are not visible by tlie CTEM technique. The heat treatments performed (16 h in wet nitrogen at 900 or 980°C) result in an increase in the size of the particles, whose mean diameter changes from 3 to 7.5 and then to 12.5 nm. The mixed Pt/Rh particles are also enriched in Rh at the expense of the very small Rh particles, which eventually disappear. 

B.R. Powell "Analytical Electron Microscopy of a Vehicle Aged Automotive Catalyst", Applied Catal., 53, 233-250,1989. 

By the 1980s most of the aluminosilicate zeolites currently used industrially were known, and the emphasis shifted to the study of these materials using a range of powerful new techniques that came of age at this time. These included, in particular, solid state NMR, X-ray and neutron powder diffraction analysis, high resolution electron microscopy and computational methods. All were ideal for the study of structural details of solids that were rarely available, and never used in industrial applications, other than as microcrystalline powders. All these techniques are applicable to the bulk of the solid - this in turn makes up the (internal) surface, which is accessible to adsorbed molecules. Since the techniques are able to operate under any conditions of gas pressure, they may be used to extract structural details in situ under the operating conditions of ion exchange, adsorption and catalysis. In particular, zeolitic systems have proved ideal for the study, understanding and subsequent improvement of solid acid catalysts. 

High resolution transmission electron microscopy (TEM) was further used to confirm the results described above. Figure 12.14 shows the TEM images of the catalyst after stable operation and after the aging stress test. While the catalyst after stable operation only shows the lattice fringes of anatase covered by a layer of amorphous vanadium oxide surface species, the stressed catalyst shows a more complex structure. The core of the support oxide particle still shows the lattice fringes of anatase, but lattice distances of rutile can be seen in a surface-near layer around... 

Both transmission electron microscopy (TEM) and X-ray diffraction (XRD) have complemented the kinetics evaluation with RDE, through particle size distribution information, to determine Pt particle size effect on the specific activity of Pt and Pt alloys. Fig. 10 shows the change in Pt particle size off-cell aging. The aged catalyst now can be applied in RDE and evaluated for kinetic constants. From XRD in particular, one can use the Scherrer s equation to determine the Pt crystalline size through the following equation ... 

Powell, B.R. and Chen, Y.-L. (1989) Analytical electron microscopy of a vehicle-aged automotive catalyst. Appl. Catal., 53, 233. 

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