A New Catalyst Technology to Overcome Diffusion and Transport Limitations 2021-26-0210

The purification efficiency of exhaust gas catalysts depends on several factors. One of the most important factors is the diffusivity of the exhaust gases in the catalytic coating layer, especially at moderate to high temperature and space velocity conditions. Porous silica, γ-alumina, zirconia, carbons and many other porous crystalline materials that are commonly used as catalysts and catalyst supports are traversed by a labyrinth of tortuous micro and mesopores. If the connectivity is very low, the labyrinth of pores becomes more difficult to penetrate, increasing the overall “tortuosity” and slowing down the transportation of gas molecules within catalyst layers.

A new approach to overcome these diffusion and transport limitations in an exhaust gas catalyst is to create a continuous interconnected network of mesopores and macropores via increase in void fractions within the washcoat components and layers. In this study, various organic and inorganic pyrolyzable pore forming agents (PFAs) were added to the washcoat before coating on the substrate. After coating, the catalysts were calcined at a temperature corresponding to the thermal decomposition temperature of the PFA, during which these PFA’s burn out and leave randomly distributed large inter-particle spaces within the catalytic coating layer.

A typical TWC was selected for this study and evaluated using a ramped light off test operated from 80°C to 550°C in a simulated gasoline exhaust feed at two different space velocities. The powder catalyst was characterized using X-Ray Diffraction (XRD) and BET nitrogen adsorption isotherms to understand the effects of pore formers on crystal structure and the surface area of the catalyst. Overall, one organic PFA (PF1) based catalyst showed 10°C, 24°C and 8°C benefit in CO, HC and NOx T50 light off, respectively, and one inorganic PFA (PF2) based catalyst showed 36°C, 45°C and 45°C benefit, both tested at 60k space velocity. At 90k space velocity, a clear 40°C - 45°C benefit was observed for both PF1 and PF2 modified catalysts. A 10 to 30 m²/g increase in BET surface area was obtained in both cases. Identical XRD patterns for the reference and developed catalysts showed that addition of PFAs helped in improving catalytic efficiency without disturbing the crystalline nature of the catalyst.