Investigating the impact of catalysts on N₂ rotational and vibrational temperatures in low pressure plasmas

Abstract

Future application of plasma assisted catalysis (PAC) requires a thorough understanding of energy partitioning within the plasma-catalyst system. We have studied the impact of adding a catalyst (TiO2 and zeolites) on the plasma energetics within an N2 low-temperature, radio frequency (RF) plasma by measuring the vibrational and rotational temperatures of the gas-phase N2. The presence of either micro- or nano-structured materials (zeolite and TiO2 substrates, respectively) within the plasma significantly decreases N2 (g) vibrational temperature, suggesting these materials promote vibrational relaxation within the discharge upon interaction with a catalytic substrate. In addition to evaluating the spectroscopic characteristics of the N2 discharge, we have assessed material morphology and chemical composition before and after plasma exposure. The porous network of both TiO2 and zeolite substrates were maintained after plasma exposure, although small amounts of nitrogen incorporation occurred at the surface of both materials.

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

“…T V (CH) measured in our plasmas without and with a TiO 2 substrate in the discharge indicates T V (CH) is significantly decreased (∼2550–3100 K) upon addition of the catalyst. We have previously illustrated a similar trend in 100% N 2 plasmas, where T V (N 2 ) was measured without and with two different catalysts: TiO 2 and zeolite NaY . In these studies, T V (N 2 ) decreased by ∼400–1000 K with either catalyst in the discharge.…”

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

“…36 The primary oxygen binding environments include the Si−O−Si band at 531.7 eV and Si−O−Al band at 530.8 eV. 31,37 The single-stage system depicts an increase in the Si(−O 4 ) (Figure 11b) and Si−O−Al (Figure 11e) binding environments compared to the untreated or two-stage system. This suggests that some surface "cleaning" occurs to expose the underlying aluminosilicate bridges.…”

“…We have previously studied the impact of zeolite catalysts within a 100% N 2 plasma, where a decay to steady-state emission (similar to the NO within N 2 O herein) was documented at 125 and 150 W with zeolite(s) present. 31 Furthermore, the addition of a single zeolite pellet in the coil region of the reactor decreased T V (N 2 ) at P ≥ 75 W. Upon increasing the amount of zeolite in the system via lining the reactor shown in Figure 1a with zeolite pellets, enhanced N 2 vibrational cooling was measured at all applied rf powers studied. 31 To further assess these hypotheses of plasma−material interactions in a more environmentally relevant plasma system, a third major finding herein was the deciphering of energy partitioning between degrees of freedom and across multiple molecules (i.e., N 2 and NO) formed in N 2 O plasmas.…”

“…Molecular sieves were ground with a mortar and pestle into a fine powder with an experimentally determined specific surface area of 930 m 2 g −1 then placed in an 18,000 psi pellet press (Carver) to create 0.3 g of zeolite pellets (13 mm × 2 mm). 31 In specific studies, Pt nanopowder and zeolite pellets were oven-calcinated at 150 °C for 3 h. The precursor gases used were N 2 O (Airgas, >99%) and Ar (Airgas, >99.999%). Reported gas ratios are based on the partial pressure of each constituent of the feed gas; small amounts of Ar were added to the feed as an inert gas actinometer in specified gas-phase studies.…”

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

“…Reactors operating under radiofrequency (RF) plasma excitation have been researched to a lesser extent, as compared to DBD reactors, 261 but have recently gained substantial attention in several research groups. [190][191][192][193][194][195][196]298 In the late 1980s and early 1990s, Uyama et al and Tanaka et al used zeolite 182,187 as well as Fe 183,188 and Mo 183 wires as catalyst in the downstream of their low pressure (650 Pa) RF plasma apparatus. Fe catalysts resulted in the highest product yield.…”