Using Fundamental Spectroscopy to Elucidate Kinetic and Energetic Mechanisms within Environmentally Relevant Inductively Coupled Plasma Systems

Abstract

Understanding energy distributions and kinetic processes in NO plasma systems is vital to realizing their potential in a range of applications, including pollution abatement. Energy partitioning between degrees of freedom and multiple molecules formed within NO plasma systems (N, NO, N/O) was investigated using both optical emission and broadband absorption spectroscopies. Specifically, we determined electron temperatures (T) as well as rotational (T) and vibrational (T) temperatures for various N (BΠ and CΠ) and NO (XΠ and AΣ) states. T and T for both molecules (regardless of state) show a strong positive correlation with applied plasma power, as well as a negative correlation with system pressure. In all cases, T values are significantly higher than T for both species, suggesting vibrational modes are preferentially excited over rotational degrees of freedom. Time-resolved optical emission spectroscopy was utilized to determine rate constants, providing mechanistic insight and establishing the relationships between system parameters and plasma chemistry. Ultimately, the combination of these data allows us to glean information regarding both the kinetics and energetics of N and NO molecules formed within nitrogen- and oxygen-containing plasma systems for potential applications in gas remediation of pollutants.

Employing Optical Emission Spectroscopy to Elucidate the Impact of Titanium Dioxide in Plasma-Assisted Catalysis

“…Additionally, if the catalyst does indeed enhance the average electron energy, then this enhancement could depend on the amount of catalyst in the discharge. The electron temperature (T e ) measured from OES lines in our system (described previously 88 ) indicates that T e ranged from ∼2.4 to 2.7 eV in CH 4 plasmas, independent of pressure and power. Moreover, the presence of the TiO 2 catalyst does not appreciably affect T e , further suggesting the change in T V (CH) noted in our systems may not arise from a change to the plasma discharge, but rather may be related to vibrational quenching from plasma−surface interactions.…”

Efforts Toward Unraveling Plasma-Assisted Catalysis: Determination of Kinetics and Molecular Temperatures within N2O Discharges

“…We have previously reported vast differences in vibrational temperatures of excited-and ground-state molecules; therefore, it will be essential to probe the ground-state species in these N 2 O-catalyst systems via BAS. 28 As evidenced by the energy partitioning and kinetic studies here, it is necessary to consider both thermodynamic and mechanistic implications for the applicability of PAC technologies, nominally the fourth major observation of this work. The decomposition of N 2 O on metal oxide catalysts is often expressed using Langmuir−Hinshelwood (L-H) or Eley−Rideal (E-R) models, schematically represented in…”

“…This trend is not prevalent in the zeolite system, T R (N 2 ) values at P = 100 W and 125 W are within the experimental error, although T R generally increases within increasing rf power. Rotational energy distributions of absorbing and emitting N 2 molecules in a variety of discharges (e.g., N 2 , N 2 O, N 2 /O 2 ) have been previously studied via broadband absorption spectroscopy (BAS) and OES, respectively, where all T R (N 2 ) values were less than 440 K. 28 This suggests that the rotational populations within N 2 molecules readily thermalize to slightly above room temperature, an observation that holds here, as all calculated T R (N 2 ) values are less than 405 K (Figure 6a). Rotational temperatures for excited-state NO radicals are significantly elevated compared to N 2 .…”

“…The addition of an active material can clearly impact the plasma itself; therefore, it is crucial to characterize gas-phase species with and without a catalyst in the system. We have previously studied emitting and absorbing species in N 2 O discharges, 28 determining that rotational (T R ) and vibrational (T V ) temperatures for both N 2 and NO molecules increase with increasing applied rf power (P) and decrease with increasing system pressure (p). This suggests that as more energy is supplied to the discharge through increasing applied power, this energy is transferred to the gas-phase molecules, thereby populating higher vibrational states.…”

“…a Values in parentheses represent standard deviation calculated from the mean of n ≥ 3 trials. b Previously reported 28. …”