Evaluation of an oxidation catalyst (“catalytic stripper”) in eliminating volatile material from combustion aerosol

Combustion aerosol is a mixture of solid and volatile particulate matter. Separation of solid particles for research or regulatory purposes is often conducted with thermal treatment of the aerosol. For example, European automotive emission regulations address solid particles above 23 nm, which are separated by dilution and heating in a volatile particle remover (VPR). This study evaluated an oxidation catalyst – often referred to as a “catalytic stripper” (CS) – as an alternative technique to remove volatile components. A version of the CS was examined in this paper, where the oxidation catalyst was combined with a sulphur trap in order to oxidise hydrocarbon species and to bind sulphates on the CS surface. In order to characterise the performance of the CS, the position of the sulphur trap upstream or downstream of the oxidation catalyst was examined in relation to the light-off temperature, hydrocarbon oxidation efficiency, and sulphur storage capacity, defined as the point where sulphate particles start to form downstream of the CS. With the best performance achieved when the trap was positioned downstream of the oxidation catalyst, the CS was then characterised in terms of particle losses in the range 6–100 nm. Losses were found rather independent of particle size above 30 nm but significantly increased below 23 nm. The efficiency in removing volatile particles was characterised using tetracontane particles. Furthermore, the overall performance of the CS was compared against the VPR by using diesel nucleation mode particles as the challenge aerosol. Results showed that the CS could directly be used as an alternative to VPR for combustion aerosol measurements if only particles above 23 nm were considered. Extending the measurement below this range would also be possible. This would however require an evaporation tube to vaporise material before this reached the CS and attention in addressing the rapidly increasing losses with decreasing particle size in this range.

► A catalytic stripper can replace the volatile particle remover of the PMP sampling system.
► Particle losses and volatile removal efficiency exceed PMP specifications.
► The sulphur storage capacity is sufficient for several thousands of hours of operation at typical sampling concentrations.
► Additional thermal treatment is required for extending the measurement below 23 nm.