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44510-AC5
Surface Reactivity of Radicals and Ions During Plasma-Catalytic Removal of Nitrogen Oxide (NOx) Pollutants
����������� The primary focus of this project is to elucidate fundamental chemical processes occurring at catalyst surfaces during plasma-catalytic removal of nitrogen oxides (NOx), beginning with nitric oxide (NO).� Mechanisms for NO removal were investigated using the imaging of radicals interacting with surfaces (IRIS) technique, which examines the steady-state reactivity of plasma-generated species at catalytic surfaces.� IRIS was also used to complete fundamental relative gas phase density studies and analyze the effects of gas composition, gas additives and applied radio frequency (rf) power.
����������� Gas Phase Density Studies.� Initial experiments focused on gas phase density profiles of NO in plasmas by changing gas composition to examine effects of gas additives on NO removal.� The relative gas-phase density data reported here represent the average laser-induced fluorescence (LIF) intensity from multiple charge-coupled device (CCD) images, Figure 1, acquired using the same plasma parameters.� Error bars represent one standard deviation of the mean.
����������� NOx are formed when fuel is burned at high temperatures, the combustion process.� However, NOx can also be produced through gas-phase plasma reactions.� Figure 2 contains NO LIF data for N2/O2 plasmas as a function of % N2 in the feed.� These data were collected at two different applied rf powers (P).� For both P, NO concentration decreases as the %N2 increases.� There is significantly more NO produced at higher P, however, clearly indicating P dissociates the feed gases, allowing formation of NO.
����������� The amount of N2 in exhaust fumes is ~70%, thus we used this concentration for most of our studies.� Because small amounts of NO are found in exhaust, 5 mTorr of NO (5% of total) were used with N2 andO2 (25%).� NO LIF signal decreases significantly as P increases, in contrast to the P dependence in Figure 2.� This suggests that additional reactions are occurring in the plasma when NO is added to the N2/O2 system.� One possibility is that nitrogen and oxygen react with NO instead of each other.� Future studies will explore this hypothesis using mass spectrometry and optical emission spectroscopy (OES).
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����������� Studies of exhaust systems have shown a water content dependence on NO removal.� Thus, we added H2O vapor to our system and monitored the effect on NO concentration.� In general, as the humidity increases, the amount of NO removed increases (LIF intensity decreases).� At the lowest [H2O], NO appears to be relatively constant with P, which may mean this level of humidity is not sufficient to significantly affect NO removal.�
����������� Hydrocarbons are also found in exhaust fumes, thus we also examined effects of methane addition (~3%) on removal of NO.� In addition to CH4, the gas mixture also contained N2, O2, and NO.� In CH4/N2/O2/NO plasmas, there is a significant reduction in NO as the P is increased, whereas the NO intensity in the (~4%) H2O system does not change significantly with P.� This suggests that hydrocarbons may be more efficient than H2O for NO removal.
Identification of excited state plasma species was performed using OES, with Ar added to NO plasmas as a reference.� Species identified include NO, N2, O, and Ar.� Dependence of OES signals on P was also examined, revealing the same trend as with the LIF studies.� Namely, NO density in the plasma decreases with increasing P, Figure 6.� Gold coated silicon wafers were placed in the reactor to examine catalytic behavior.� At P = 25 W, NO signals were significantly diminished, and at higher P, nearly all of the NO was removed.� We are currently exploring plasma-surface interactions of NO using IRIS, and preliminary results suggest NO is highly reactive on a variety of substrate materials.
This project represents a new area of study for the Fisher group as this is the first time we have explored plasma-catalytic processes.� Since initiating the project, one graduate student, Ms. Michelle Morgan, has joined the group and focused her studies on NO plasma-catalytic behavior.� Currently two new graduate students considering joining the group have expressed specific interest in these projects.� Potential additional projects for these students would focus on analysis of other pollutants such as SOx and NO2.� During the past year, we have established collaborations with two other groups to obtain catalytic materials.� In the future however, we will be synthesizing our own substrates to view the plasma-catalytic effects on a variety of substrates.� We would also note that this project has spawned two additional environmentally-related projects in different areas, focused on plasma remediation of dense media (e.g. water) and on modification of materials used in aqueous separations (e.g. desalination).
