Investigation of diesel ash particulate matter: A scanning electron microscope and transmission electron microscope study

Investigation of ash PM deposited in a diesel particulate filter (DPF) operating on a light truck by means of SEM and TEM reveals the following: ash inside the DPF occurs in form of chemically very inhomogeneous, mostly brittle agglomerates accumulated at the plugged ends of inlet channels and deposited directly on the inlet channel walls all along the filter length. Ash agglomerates occur within pores of the channel walls. A minor part of ash PM may escape to the atmosphere. The individual ash phases are mostly crystalline with round outlines and sizes between ca. 170 and 60 nm, down to 7–12 nm, that is far below the breathable size range PM10. Aggregation of the predominantly finest fraction of ash particles leads to densification, which may translate to fewer breakouts from the DPF. EDX mapping and chemical analyses of the bulk ash reveal that ash consists mainly of Ca, Mg, P, Zn, S, O and minor Fe, Al and Si. Based on TEM diffraction data of ash single phases, combined with data on their chemistry, the diversity of ash phases is higher than previously presumed. Comparison with the ash particles of a heavily used DPF from a passenger car operating with Fe-based fuel-borne additives reveals characteristics very similar to those found for the light truck DPF with a tendency to generally lower sizes of the participating phases, mostly between 30 and 60 nm. 4–40 nm large, locally abundant Pt particles deriving from the coating material of the diesel oxidation catalyst (DOC) upstream the DPF occur within ash agglomerates of both DPFs. Ash collected from the exhaust gas at the exit of the light truck DPF under normal engine operation reveals that some fine particles, as well as a few of the larger (200–600 nm) ash-bearing agglomerates escape filtration. Very fine ash particles are reaching the atmosphere also attached onto soot agglomerates.

► Ash agglomerates, up to a few μm large, escape filtration and reach the ambient air.
► The size of individual ash phases goes down to a few nanometers, far below PM10.
► Part of the finest ash particulates form larger aggregates and may thus be trapped by DPFs.
► Ash microscopic characteristics are independent of aftertreatment conditions and engine loading.
► Pt particles (4–40 nm) deriving from the oxidation catalyst surface may reach ambient air.