Composition of Clusters Formed by Plasma Discharge in Simulated Engine Exhaust 2000-01-2967
Previously reported experiments revealed the presence of a small number of clusters or very small particles in the effluent of a nonthermal plasma reactor when treating a simulated engine exhaust mixture. These clusters are smaller than 7 nm. The quantity of clusters is orders of magnitude smaller than the particulate diesel or gasoline engine exhaust typically contains. In this report, we describe further experiments designed to determine the chemical composition of the clusters.
Clusters were collected on the surface of a silicon substrate by exposing it to the effluent flow for extended time periods. The resulting deposits were analyzed by high mass resolution SIMS and by XPS. The SIMS analysis reveals NH4+, CH6N+, SO-, SO2-, SO3- and HSO4- ions. XPS reveals the presence of N and S at binding energies consistent with that of ammonium sulfate. Although it is not possible to precisely determine the compounds present on the surface, the following conclusions can be drawn:
Absence of aluminum in the clusters confirms that the particles are not alumina sputtered from electrode surfaces by the plasma discharge
Sulfates are a likely product. Although the majority of the SO2 input passes through the plasma, a small fraction is converted and will react to form sulfates that readily condense
The largest peak in the SIMS positive ion data is NH4+. However, because the total number of particles is so small, this probably cannot account for the “missing NOx” reported in other plasma exhaust treatment investigations.
Large molecular weight organic compounds are present, which must be be synthesized from partial reaction of the input propene and propane.
Particles were not found to be present downstream of a catalyst substrate when it was used in conjunction with the plasma discharge.