Improvement of Silica–alumina Supports for Diesel Oxidation Catalysts through Control of Both Composition and Pore Structure

Precious metals, primarily Pt and Pd, are indispensable as active components in industrial catalysts intended for practical applications. In particular, they are very commonly utilized in diesel exhaust gas purification catalysts, and hence an increase in the demand for these precious metals is expected as the number of vehicles in use worldwide continues to grow and as emission controls in various countries become increasingly stringent1). A typical diesel after-treatment system is shown in Fig. 1, in which the diesel oxidation catalyst (DOC) is situated on the inlet side of a series of units within the system. The DOC is necessary for the removal of the soluble organic fraction in the particulate matter (PM), as well as the removal of CO and hydrocarbons (HCs) present in the gas phase. Furthermore, to allow the regeneration of a diesel particulate filter (DPF) placed downstream of the DOC by burn-out of the filtered PM, DOCs with high catalytic activity for fuel oxidation are required so as to allow for active heating. This is performed by the catalytic combustion of fuel in the DOC, supplied by retarded combustion in the engine, by post injection into the engine cylinders, or by direct injection into the exhaust pipe. Additionally, the DOC works to oxidize NO to NO2 to promote the regeneration of the DPF through oxidation of the PM and the selective catalytic reduction of NOx using NH3 as a reductant2). In this catalytic system, precious metals are primarily employed in the DOC and DPF, in particular the DOC which may contain large quantities of such metals. Therefore, to reduce the amounts of such metals required in diesel exhaust systems, it is necessary to improve the efficiency with which they function in the DOC. A DOC is composed primarily of a support material such as Al2O3 and an active component such as Pt or Pd, the latter being dispersed as nano-scale particles over the interior pore surfaces of the former. In this study, we focused our attention on the support material. To date, extensive research has been carried out to improve DOC support materials with regard to their compositions and pore structures. In terms of the composition, the addition of secondary components such as Fe3), Mn4), W5), and Si6),7) to Al2O3 has been investigated as a means of improving thermal durability and controlling the acidity/basicity balance. With regard to pore structure, multidimensional-structured Al2O3 materials possessing pores of different size scales, such as micro-meso8),9), meso-macro10),11), and micro-meso9 Journal of the Japan Petroleum Institute, 58, (1), 9-19 (2015)