Characterization of particulate matter deposited in diesel particulate filters: Visual and analytical approach in macro-, micro- and nano-scales

Abstract

Multi-scale analytical investigations of particulate matter (soot and ash) of two loaded diesel particulate filters (DPF) from (a) a truck (DPF1) and (b) a passenger car (DPF2) reveal the following: in DPF1 (without fuel-borne additives), soot aggregates form an approximately 130–270 μm thick, homogeneous porous cake with pronounced orientation. Soot aggregates consist of 15–30 nm large individual particles exhibiting relatively mature internal nanostructures, however, far from being graphite. Ash aggregates largely accumulate at the outlet part of DPF1, while minor amounts are deposited directly on the channel walls all along the filter length. They consist of crystalline phases with individual particles of sizes down to the nanoscale range. Chemically, the ash consists mainly of Mg, S, Ca, Zn and P, elements encountered in lubricating oil additives. In the passenger car DPF2 (with fuel-borne additives), soot aggregates form an approximately 200–500 μm thick, inhomogeneous porous cake consisting of several superposed layers corresponding to different soot generations. The largest part of the soot cake is composed of unburned, oriented soot aggregates left behind despite repeated regenerations, while a small part constitutes a loose layer with randomly oriented aggregates, which was deposited last and has not seen any regeneration. Fe-oxide particles of micro- to nano-scale sizes, originating from the fuel-borne additive, are often dispersed within the part of the soot cake composed of the unburned soot leftovers. The individual soot nanoparticles in DPF2 are approximately 15–40 nm large and generally less mature than in the truck DPF1. The presence of soot leftovers in DPF2 indicates that the addition of fuel-borne material does not fully compensate for the temperatures needed for complete soot removal. Ash in DPF2 is filling up more than half of the filter volume (at the downstream part) and is dominated by Fe-oxide aggregates, due to the Fe-based fuel-borne additive, but otherwise its chemical composition reflects compounds of lubricating oil additives.