Automotive catalyst thermal deactivation

Lester, G.R., Brennan, J.F., Hoekstra, J., The Relative Resistance of Noble Metal Catalysts to Thermal Deactivation , Catalysts for the Control of Automotive Pollutants. ACS Advances in Chemistry Series 143. 1975, pp24-31. 

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. 

Three way catalysts are generally used today for controlling exhaust emissions from automotive Internal combustion engines. Cerium oxide is widely employed as an additive in three way catalysts because of its abilities to store oxygen and to improve dispersion of platinum.[1] One drawback of the three way catalyst is that it tends to give rise to thermal deactivation caused by crystallization of cerium oxide, sintering of platinum and... 

Most of the emphasis of this chapter is on the mixed-oxide solid solution oxygen storage materials that comprise the advanced catalyst formulations in use today and are still under development. In particular, we focus on their durability, both with respect to thermal and chemical deactivation, while also briefly reviewing special uses of these and other oxygen storage materials in automotive applications. 

In the past few years the tudy of morphology changes undergone by supported Rh particles during oxidation / reduction thermal treatments has attracted considerable attention (refs 1-4). These phenomena are of particular importance in the deactivation of automotive exhaust catalysts for which rhodium is the main active Ingredient for the reduction of NO. 

In order to suggest an efficient catalyst system for automotive emission control, Pd-W03 and Pd-La203 catalysts were characterized before and after thermal aging and phosphorous contact, li was found that deactivation of Pd-W03 involves severe metal vaporization during ihermal aging. On the other hand, Pd-La203 has exceptional thermal stability while it is poisoned by 1-5 wi. phosphorous,... 

Sintering is an important mode of deactivation in supported metals which find broad application as catalysts in automotive, chemical, gas, and petroleum industries. The high surface area support (carrier or substrate) in these catalysts serves several functions (1) to increase the dispersion and utilizaticm of the catalytic metal phase, (2) to separate rfiysically metal crystallites and to bind them to its surface, thereby enhancing their thermal stability toward agglomeration, and (3) in stxne cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

A very drastic and common deactivation phenomenon with automotive emission control catalysts is the irreversible mechanical destruction of the support during road use by breakage in the case of ceramic monoliths, by telescoping of the matrix or breakage of the foil in the case of some metallic substrates and, formerly, by attrition in the case of bead catalysts. With ceramic monoliths, sudden temperature changes can cause thermal stresses and consequent breakage. 

Find et al. [25] developed a nickel-based catalyst for methane steam reforming. As material for the microstructured plates, AluchromY steel, which is an FeCrAl alloy, was applied. This alloy forms a thin layer of alumina on its surface, which is less than 1 tm thick. This layer was used as an adhesion interface for the catalyst, a method which is also used in automotive exhaust systems based on metallic monoliths. Its formation was achieved by thermal treatment of microstructured plates for 4h at 1000 °C. The catalyst itself was based on a nickel spinel (NiAl204), which stabUizes the catalyst structure. The sol-gel technique was then used to coat the plates with the catalyst slurry. Good catalyst adhesion was proven by mechanical stress and thermal shock tests. Catalyst testing was performed in packed beds at a S/C ratio of 3 and reaction temperatures between 527 and 750 °C. The feed was composed of 12.5 vol.% methane and 37.5 vol.% steam balance argon. At a reaction temperature of 700°C and 32 h space velocity, conversion dose to the thermodynamic equilibrium could be achieved. During 96 h of operation the catalyst showed no detectable deactivation, which was not the case for a commercial nickel catalyst serving as a base for comparison. 

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