On the Comprehension of Mechanical, Thermal and Chemical Evolution of Exhaust Gases after Treatment Catalysts

Abstract

The stricter environmental regulations impose a drastic reduction of vehicles emissions and a longer durability of the automotive catalyst. The studies of the catalyst evolution in real conditions are very expensive. It is interesting to elaborate an ageing process to well simulate the real conditions of the automotive catalyst ageing at the laboratory scale. Thermal, chemical and mechanical effects can cause degradations of the catalytic performances. Thermal ageing is the result of high temperature that surges in the catalytic converters. Chemical ageing is due to phosphorus, zinc, magnesium, calcium or sulfur originating from either engine oil additives or fuel contaminants. Mechanical ageing (cracks and detachment) is the consequence of thermal and chemical ageing (thermal and textural stresses). Understanding the formation of crack patterns and spalling of thin films is challenging in fracture mechanics. The need for two conditions, one involving energy and the other one stresses, has recently been shown. The stress condition defines a threshold below which the pattern formation is inhibited. As this threshold is not reached, the energy accumulates. Then, at onset, depending on the strength and toughness of the material, the amount of energy can be sufficiently large to give rise to a more or less dense lattice of cracks. Following, after initiation, when the crack tips are close to the support, the newly created small cells tend to separate from it, thicker the film and more harmful the debonding.