Cohesive strength measurement of catalyst layer : uniform drying and on-line monitoring

Catalysed chemical reactions usually only occur on the surface of the catalyst, therefore the rate of reaction is dependent on the amount of surface area available. In order to achieve this, the active phase is distributed over a high surface area support material.

In catalytic converters, automotive catalysts are deposited on the interior walls of a ceramic or metallic monolith in the form of a coating. The open structure of the monoliths allows the gases to pass through the catalyst without causing too much pressure drop. The strength of the coated layer is important for the life (and activity) of a catalytic converter. The coated catalyst layer can fail in both cohesive and adhesive manner, and the lack of methods to separately quantify the individual strength has become an issue. For this reason, the current paper proposes an improved method to measure the cohesive strength of a catalytic layer by implementing a uniform drying system for the preliminary method developed in a previous publication (Yang et al., 2016).

The cohesive strength obtained was found to vary with particle size, pH of catalyst suspension, and drying rate of the suspension. In short, a suspension with d90 approximately of 9 μm, pH = 4 and dried under a low drying rate led to a layer with high cohesive strength. Variation in the cohesive strength is explained based on particle mobility and packing behaviour obtained from on-line monitoring of the drying process of a washcoat layer. In addition the cohesive strength is also in close agreement with the DLVO theory except at very low and high pHs where dissolution of the particles may have an influence. A combination of low resistance for particle movement and longer exposure to a drying environment was found to improve the cohesive strength. Particle mobility and packing behaviour which was retrieved on a micrometre length scale from on-line monitoring is one step further to understand the origin of cohesive strength of a coated layer beyond the widely investigated macroscopic preparation conditions such as viscosity and drying rate.