Fundamental
chemistry in heterogeneous catalysis is increasingly
explored using operando techniques in order to address
the pressure gap between ultrahigh vacuum studies and practical operating
pressures. Because most operando experiments focus
on the surface and surface-bound species, there is a knowledge gap
of the near-surface gas phase and the fundamental information the
properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species
and temperature distributions in operando catalysis
experiments using complementary near-surface optical and mass spectrometry
techniques. The partial oxidation of methanol over a silver catalyst
demonstrates the value of these diagnostic techniques at 600 Torr
(800 mbar) pressure and temperatures from 150 to 410 °C. Planar
laser-induced fluorescence provides two-dimensional images of the
formaldehyde product distribution that show the development of the
boundary layer above the catalyst under different flow conditions.
Raman scattering imaging provides measurements of a wide range of
major species, such as methanol, oxygen, nitrogen, formaldehyde, and
water vapor. Near-surface molecular beam mass spectrometry enables
simultaneous detection of all species using a gas sampling probe.
Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether,
and methyl formate, provides insights into catalytic mechanisms of
the partial oxidation of methanol. The combination of these techniques
provides a detailed picture of the coupling between the gas phase
and surface in heterogeneous catalysis and enables parametric studies
under different operating conditions, which will enhance our ability
to constrain microkinetic models of heterogeneous catalysis.
The methyl radical plays a central role in plasma-assisted hydrocarbon chemistry but is challenging to detect due to its high reactivity and strongly pre-dissociative electronically excited states. We report the development of a photo-fragmentation laser-induced fluorescence (PF-LIF) diagnostic for quantitative 2D imaging of methyl profiles in a plasma. This technique provides temporally and spatially resolved measurements of local methyl distributions, including in near-surface regions that are important for plasma-surface interactions such as plasma-assisted catalysis. The technique relies on photo-dissociation of methyl by the fifth harmonic of a Nd:YAG laser at 212.8 nm to produce CH fragments. These photofragments are then detected with LIF imaging by exciting a transition in the B-X(0,0) band of CH with a second laser at 390 nm. Fluorescence from the overlapping A-X(0,0), A-X(1,1), and B-X(0,1) bands of CH is detected near 430 nm with the A-state populated by collisional B-A electronic energy transfer. This non-resonant detection scheme enables interrogation close to a surface. The PF-LIF diagnostic is calibrated by producing a known amount of methyl through photo-dissociation of acetone vapor in a calibration gas mixture. We demonstrate PF-LIF imaging of methyl production in methane-containing nanosecond pulsed plasmas impinging on grounded metallic or dielectric surfaces. Absolute calibration of the diagnostic is demonstrated in a diffuse, plane-to-plane discharge. Measured profiles show a relatively uniform distribution of up to 30 ppm of methyl. Relative methyl measurements in a filamentary plane-to-plane discharge and a plasma jet reveal highly localized intense production of methyl. The utility of the PF-LIF technique is further demonstrated by combining methyl measurements with formaldehyde LIF imaging to capture spatiotemporal correlations between methyl and formaldehyde, which is an important intermediate species in plasma-assisted oxidative coupling of methane.
Pulsed dielectric barrier discharges (DBD) in He-H2O and He-H2O-O2 mixtures are studied in near atmospheric conditions using temporally and spatially resolved quantitative 2-D imaging of the hydroxyl radical (OH) and hydrogen peroxide (H2O2). The primary goal was to detect and quantify the production of these strongly oxidative species in water-laden helium discharges in a DBD jet configuration, which is of interest for biomedical applications such as disinfection of surfaces and treatment of biological samples. Hydroxyl profiles are obtained by laser-induced fluorescence (LIF) measurements using 282 nm laser excitation. Hydrogen peroxide profiles are measured by photo-fragmentation LIF (PF-LIF), which involves photo-dissociating H2O2 into OH with a 212.8 nm laser sheet and detecting the OH fragments by LIF. The H2O2 profiles are calibrated by measuring PF-LIF profiles in a reference mixture of He seeded with a known amount of H2O2. OH profiles are calibrated by measuring OH-radical decay times and comparing these with predictions from a chemical kinetics model. Two different burst discharge modes with 5 and 10 pulses per burst are studied, both with a burst repetition rate of 50 Hz. In both cases, dynamics of OH and H2O2 distributions in the afterglow of the discharge are investigated. Gas temperatures determined from the OH-LIF spectra indicate that gas heating due to the plasma is insignificant. The addition of 5% O2 in the He admixture decreases the OH densities and increases the H2O2 densities. The increased coupled energy in the 10-pulse discharge increases OH and H2O2 mole fractions, except for the H2O2 in the He-H2O-O2 mixture which is relatively insensitive to the additional pulses.
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