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Fig. 1: CCD image of NO LIF signal (intensity rises from blue to white).

The primary focus of this project is to elucidate fundamental chemical processes occurring at catalyst surfaces during plasma-catalytic removal of nitrogen oxides (NO_x), 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.  In the final funding cycle, we continued our fundamental gas phase density studies using optical emission spectroscopy (OES) and laser-induced fluorescence (LIF, Figure 1) and extended our studies to include vibrational temperature studies of NO in various gas mixtures.  We have expanded 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 in this funding year focused on using platinum and gold with silicon wafers as controls.  Notably, Au-coated Si wafers were most efficient in removing NO from plasma mixtures (e.g. NO, NO/Ar, or N₂/O₂/Ar).  As seen from Fig. 2 OES spectra, the presence of Au significantly decreases the NO signal at the lowest applied rf power (P) of 50 W.  At P > 50 W, NO is effectively eliminated, most likely because higher P effectively dissociates molecules in the system, and even though recombination reactions can reform NO, dissociation can lead to formation of alternate gas-phase species.  Clearly control of the process gases and their relative concentrations, along with the overall power and catalyst type is required to achieve viable plasma processes for removal of NO.

Introduction of water vapor or methane introduces complexity and a wider range of plasma species that can affect [NO] through gas-phase reactions.  Although N and O can recombine to form NO, the rate constant for this reaction is lower than that for forming NO and H via reaction of N atoms with OH.  Additional reactions involving nitrogen atoms have been proposed as important to either NO destruction, or formation of NO under conditions high oxygen concentration.  Addition of water promotes formation of NO, perhaps via the loss of singlet oxygen atoms through reaction with H₂O to form OH radicals.  Without H₂O (or a hydrocarbon) in the system, NO can be removed via reactions with vibrationally excited singlet N₂.  Similar trends were observed with methane addition.

Rotational and Vibrational Temperatures. Knowledge of energy partitioning between different species is important to an overall understanding of the chemistry occurring in our plasmas.  Thus, this year we have expanded our rotational temperature (Θ_R) database, Table 1, and added vibrational temperature (Θ_V) measurement of NO in our gas mixtures, Table 1.  These data were collected utilizing LIF and OES spectroscopy.  For all systems, Θ_R does not change appreciably with P or with gas mixture. 

NO vibrational temperatures, Θ_V, were determined from OES spectra, and ranged from 1400-1700 K.  Θ_V is not dependent upon gas mixture or substrate, but values increasing slightly with P.  OES spectra were also used to determine Θ_V(N₂) and again, there was little dependence on plasma or substrate type, but Θ_V(N₂) values are much cooler, ~400-500 K, suggesting NO is rotationally thermalized, but that vibrationally “hot” NO persists.  The lower values for Θ_V(N₂) suggest vibrationally hot N₂ reacts or is quenched, most likely via reaction to form NO.  Notably, Θ_V(N₂) is lower than predicted, suggesting vibrational-translational energy transfer occurs  more rapidly than NO formation reactions in the systems studied. Further work currently underway focuses on the use of ceramic oxides as catalytic surfaces.

Organic Contaminant Detection and Removal.  We expanded our studies to include plasma pollution control, including detection and removal of organic contaminants from water sources.  This work has continued in the final year of the project. Although large volumes of water may not be treatable with plasma methods, the ability to detect contaminants in ultrapure water sources (e.g those used in the microelectronics industry) was also a goal.  Among the many potential contaminants of water sources, those associated with fuel oxygenate additives such as methyl tert-butyl ether (MTBE) are of significant concern as MTBE readily partitions into aqueous phases.  We used our inductively-coupled rf plasmas for detection of methanol and MTBE in water samples using OES as the detection method.  Using emission from CO*, a detection limit of 0.01 ppm was determined for each organic contaminant.  Complementary mass spectrometry data were also collected to explore decomposition mechanisms for both CH₃OH and MTBE.  Specifically, we found that CH₃OH decomposition is achieved primarily via an oxidative dehydrogenation mechanism, whereas MTBE abatement occurs via both decomposition and oxidation mechanisms.  This work has resulted in one manuscript that has been submitted for publication and was provisionally accepted with minor revisions.

Summary/Impact.  We have further explored plasma-catalytic processes involving NO_x removal, including additional gas-phase characterization of NO and expanded the scope to include other projects with an environmental focus.  Ms. Michelle Morgan, completed her M.S. degree in 2009 (Thesis title: “Gas-phase and Surface Analysis for Exploration of Plasma Catalytic Reduction of NO_x”).  Ms. Kristina Trevino, was responsible for the water remediation project.  This work was presented at National and Regional AVS meetings; two manuscripts (one published, one under review) have resulted with two more in preparation.