Impact of zeolite and γ-alumina intra-particle diffusion on the performance of a dual layer catalyst

Multi-layer catalytic converters reduce the cost and space of the system and improve the heat distribution for cold-start applications. However, the removal of pollutants can be limited by internal diffusion through several coated layers. In this paper we investigate the impact of diffusion limitation on the performance of a model dual-layer oxidation catalyst. The samples consist of bottom a Pt/gamma-Al2O3 layer further coated by a top layer of either zeolite or alumina - both materials are of general interest for automotive exhaust gas aftertreatment. Zeolites find applications in ammonia slip catalysts, selective catalytic reduction of NOx and hydrocarbon adsorbers, and alumina is typically used in oxidation catalysts, three-way catalysts and lean NOx traps. The samples are tested in a lab reactor for CO oxidation as a probe reaction and the extent of diffusion limitation on CO conversion is evaluated. The gas transport in dual layer systems is further investigated by a 1D+1D model combined with a 3D digital reconstruction of pore space, and the contribution of micro-, meso- and macro-pores to the overall transport and conversion is quantified. The internal pores of zeolites are an order of magnitude smaller than those found in gamma-Al2O3, which results in a substantially different intra-particle diffusivity. However, the analyzed samples and performed parametric study demonstrate that the macropores in the coated layers represent a key structural property that enable efficient transport. Relatively similar values of overall effective diffusivity are therefore achieved in macro porous zeolite and alumina layers, regardless of quite different sizes of small intra-particle pores. (C) 2016 Elsevier B.V. All rights reserved.