44510-AC5
Surface Reactivity of Radicals and Ions During Plasma-Catalytic Removal of Nitrogen Oxide (NOx) Pollutants
Fig. 1: CCD image of NO LIF signal (intensity rises from blue to white). 
����������� This project's focus is elucidation
of fundamental chemical processes occurring at surfaces during plasma-catalytic
removal of nitrogen oxides (NOx).� Mechanisms for NO removal were investigated
using the imaging of radicals interacting with surfaces (IRIS) technique, which
examines steady-state reactivity of plasma-generated species at catalytic
surfaces.� The second funding cycle
focused on fundamental gas phase density studies using optical emission
spectroscopy (OES) and laser-induced fluorescence (LIF, Figure 1) in the
presence of catalysts.� We also expanded
the project to other areas of plasma pollution control, namely detection of
organic contaminants in water streams.
Fig. 2: Raw OES spectra of 95/5 NO/Ar plasmas (a) without and (b) with Au-coated Si wafer at applied rf power = 50 W.� Triangles = O; and diamonds = Ar emission lines. 
����������� OES Studies.� Experiments here focused on using three-way
catalytic converter surfaces of platinum and gold.� Notably, Au-coated Si wafers were most efficient
in removing NO from plasmas (either NO, NO/Ar, or N2/O2/Ar).� The Figure 2 OES
spectra show Au (Fig. 2b) significantly decreases the NO signal at the lowest applied
rf power (P), 50 W.� At P
> 50 W, NO is effectively eliminated.�
Over time, the Au loses its ability to remove NO from the plasma.� Current data collection efforts focus on understanding
the nature of Au surfaces and how they interact with the gases.�
����������� NOx are formed when fuel is burned at high temperatures, and can be produced through gas-phase reactions.� We have observed this in N2/O2 plasmas with and without additives that simulate the environment in exhaust fumes. Our results demonstrate the amount of H2O vapor added does change observed trends.� The additional source of oxygen results in competing reactions that occur primarily at elevated P.� At high P, the NO signal decreases due to increased dissociation of the NO formed by reaction of N + O.
����������� Rotational Temperatures. Knowledge of energy partitioning is important to an overall understanding of the chemistry occurring in our plasmas.� We characterized the rotational temperature (ΘR) of NO in our plasmas, Table 1.� These data were collected utilizing LIF spectroscopy.� For 100% NO and 90/10 N2/O2 plasmas, ΘR does not change appreciably with P.� Conversely, ΘR in the NO/Ar mixture appears to increase slightly with P, although it is within the experimental error.� Moreover, ΘR in the NO system is somewhat elevated relative to values found in NO/Ar and N2/O2 mixtures.� Thus, NO is rotationally cooled by collisions with Ar or when formed via bimolecular collisions in N2/O2.
Table 1.� Rotational Temperatures for NO in Different Plasmas (K)
| |||
Applied rf power (W)
| NO (100%)
| NO/Ar (12:88)
| N2/O2 (90/10)
|
25 | 356 � 12 | 317 � 10 | 317 � 36
|
50 | 342 � 12 | 312 �� 9 | 323 � 40 |
75 | 366 � 15 | 321 � 32 | 322 � 15 |
100 | 366 � 20 | 312 � 13 | 325 � 18 |
125 | 368 � 34 | 339 � 25 | 320 � 13 |
150 | 366 � 30 | 348 � 24 | 320 � 12 |
����������� Organic Contaminant Detection and Removal.� We expanded our studies to plasma pollution control, including detection and removal of organic contaminants from water sources.� Although large volumes of water may not be treatable with plasma methods, the ability to detect contaminants in ultrapure water (e.g those used in the microelectronics industry) was also a goal.� Among the potential contaminants, those associated with fuel oxygenate additives (methyl tert-butyl ether (MTBE)) are of concern as MTBE partitions into aqueous phases.� We used OES in our plasmas for detection of methanol and MTBE in water samples.� Using CO* emission, a detection limit of 0.01 ppm was determined for each organic contaminant.� Complementary mass spectrometry data were collected to explore decomposition mechanisms for both CH3OH and MTBE. �We found that CH3OH decomposition is achieved primarily via an oxidative dehydrogenation mechanism, whereas MTBE abatement occurs via both decomposition and oxidation.� This work has resulted in one submitted manuscript.
����������� Summary/Impact.� In the second year of this project we have further explored plasma-catalytic processes involving NOx removal, including gas-phase characterization of NO. �We have expanded the scope to include other projects that have an environmental focus. �Michelle Morgan is the graduate student working on the NOx aspects of the project.� Kristina Trevino was responsible for the MTBE and methanol water remediation project.� Both students have presented their work at three different conferences this year (National AVS, Regional ACS, GRC on Plasma Science).� In addition, a new graduate student who joined the group in the spring of 2008 will be working on analysis of other pollutants such as SOx and NO2.� We are continuing to build on our results via development of additional environmentally-related projects, including modification of materials used in aqueous separations (e.g. desalination).
