Ceramic Foam Catalyst Substrates for Diesel Oxidation Catalysts: Pollutant Conversion and Operational Issues

摘要

In the field of automotive exhaust catalysts, foam-type substrates have been proposed as alternatives to the well-established honeycomb substrates. The ceramic foams developed and manufactured in our laboratory are capable of redistributing the flow of exhaust gases, enhancing turbulence, mass transfer and species mixing, without increasing flow resistance and pressure drop to prohibitive levels. Based on the characteristics of turbulent mass and heat transfer, ceramic foam based catalysts have the potential for achieving similar pollutant conversion performances as state of art honeycomb catalysts with substantially lower precious metal requirements. In this paper we demonstrate this potential with a small Diesel powered Heavy Duty truck with a ceramic foam Diesel Oxidation Catalyst (DOC). Given the substantial differences in geometrical properties between foams and honeycombs a direct comparison with equal coating thickness, amount and precious metal amount is not feasible. We present however systematic comparisons with known precious metal amounts while pointing out important differences in the wash coating characteristics. As strongly affected, crucial operational characteristics we comment on, are the pressure drop and the homogenizing properties of the substrates on the temperature distribution. For the DOC applications we compared catalysts with an overall volume ratio of 1.4:1 (honeycomb: foam). The comparisons involved the serial production honeycomb to specifically coated foams. Here the wash coat amounts have been chosen in order to achieve almost equal layer thicknesses (wash-coat amount ratio of 6.5:1, honeycomb: foam). The entire precious metal load on the foam was approx. 2.8 times less than the corresponding one on the compared honeycomb. The conversion performances achieved with the foam catalyst were almost equal to those achieved by the honeycomb in respect to CO and THC oxidation, although honeycombs had 2.8 times more precious metals. In addition, the foams exhibited substantially better particle oxidizing behavior, as particle number measurements have shown. Nevertheless, NO oxidation light-off performance of the foams was worse. This may be attributed to different wash coat compositions, the wash coat composition of the serial production honeycomb was not fully known. The comparison of two differently coated foam DOCs have given insights in the conversion dependencies on the coating parameters and have shown further optimization directions.