The role of a solid surface for initiating gas-phase reactions is still not well understood. The hydrogen atom (H) is an important intermediate in gas-phase ethane dehydrogenation and is known to interact with surface sites on catalysts. However, direct measurements of H near catalytic surfaces have not yet been reported. Here, we present the first H measurements by laser-induced fluorescence in the gas-phase above catalytic and noncatalytic surfaces. Measurements at temperatures up to 700 °C show H concentrations to be at the highest above inert quartz surfaces compared to stainless steel and a platinum-based catalyst. Additionally, H concentrations above the catalyst decreased rapidly with time on stream. These newly obtained observations are consistent with the recently reported differences in bulk ethane dehydrogenation reactivity of these materials, suggesting H may be a good reporter for dehydrogenation activity.
The Sandia-PRF has built a new capability for the low-temperature plasma community for the simultaneous imaging of molecular rotation/vibration nonequilibrium, electric field, and the distribution of OH radical and formaldehyde in reactive low temperature plasma systems. The system is currently investigating the plasma-assisted deflagration to detonation transition in a micro-combustor channel.
The predictive understanding of catalytic surface reactions requires accurate microkinetic models, and while decades of work has been devoted to the elucidation of the reaction steps in these models, many open questions remain. One key issue is a lack of approaches enabling the local spatially resolved assessment of catalytic activity over a surface. In this report, we detail efforts to develop a new diagnostic approach to solve this problem. The approach is based upon laser resonance enhanced multiphoton ionization of reaction products emitted into the gas phase followed by spatially resolved imaging of the resultant ions or electrons. Ion imaging is pursued with a velocity-selected spatially resolved ion imaging microscope, while electron imaging was attempted in a low energy electron microscope. Successful demonstration of the ion imaging microscope coupled with the development of transport simulations shows promise for a revolutionary new tool to assess local catalytic activity
Chen, Timothy Y.; Goldberg, Benjamin M.; Kolemen, Egemen; Ju, Yiguang; Kliewer, Christopher J.
With the increased interest in CH4 as a fuel for power generation, propulsion, and catalytic reforming, spatially and timeresolved quantitative measurements of CH4 are increasingly needed to advance these technologies. Hybrid fs/ps coherent anti-Stokes Raman scattering (fs/ps CARS) has been demonstrated to measure temperature and chemical species concentrations with tens of microns of spatial resolution on the picosecond time scale. However, accurate time-domain and frequency-domain models are necessary to understand the effect of probe delay on the fs/ps CARS signal. In this work, a time-domain model was developed for the CH4 11 vibrational Q-branch validated by delay scans across pressures ranging from 70 Torr to 600 Torr and furnace setpoint temperatures up to 1000 K. A simple modified exponential energy gap (MEG) law was implemented to fit to the room temperature delay scans to approximate the Q-branch linewidths. It was also found that changing the collisional partner did not influence the time-domain decay of the CH4 Q-branch signal prior to 100 picosecond probe delays. Comparison between simultaneously measured N2 Q-branch and CH4Qbranch spectra showed good agreement with evaluated temperatures.
Chen, Timothy Y.; Goldberg, Benjamin M.; Kliewer, Christopher J.; Kolemen, Egemen; Ju, Yiguang
Due to concerns about climate change, there is significant interest to establish CH4 lean burn engines or convert it to valuable industrial chemicals using non-equilibrium plasmas. To quantitatively understand the dynamics and chemistry of plasma discharge in CH4 fuel mixtures, it is necessary to obtain time and spatially resolved data of key parameters such as the CH4 concentration and degree of rotation-vibration non-equilibrium. Rotational fs/ps CARS was used to simultaneously measure rotational and vibrational temperatures of a pin-to-pin 40% CH4/60% N2 nanosecond-pulsed discharge at 60 Torr, while the CH4 concentration was measured by vibrational CARS. The measurement region was 2 mm along the electrode axis, within 150 μm of the cathode surface. Gradients in N2 rotational and vibrational temperature and CH4 number density were observed to evolve in time and space. The vibrational temperature peaked above 6000 K, 100 μs after the voltage pulse, and the majority of CH4 consumption occurred during the voltage pulse. Additional CH4 consumption along with rapid heating occurred during the first 2 μs of the afterglow, indicating a role of electronically excited N2 quenching in dissociation of CH4.
In many fields of study, from coherent Raman microscopy on living cells to time-resolved coherent Raman spectroscopy of gas-phase turbulence and combustion reaction dynamics, the need for the capability to time-resolve fast dynamical and nonrepetitive processes has led to the continued development of high-speed coherent Raman methods and new high-repetition rate laser sources, such as pulse-burst laser systems. However, much less emphasis has been placed on our ability to detect shot to shot coherent Raman spectra at equivalently high scan rates, across the kilohertz to megahertz regime. This is beyond the capability of modern scientific charge coupled device (CCD) cameras, for instance, as would be employed with a Czerny-Turner type spectrograph. As an alternative detection strategy with megahertz spectral detection rate, we demonstrate dispersive Fourier transformation detection of pulsed (~90 ps) coherent Raman signals in the time-domain. Instead of reading the frequency domain signal out using a spectrometer and CCD, the signal is transformed into a time-domain waveform through dispersive Fourier transformation in a long single-mode fiber and read-out with a fast sampling photodiode and oscilloscope. Molecular O- and S-branch rotational sideband spectra from both N2 and H2 were acquired employing this scheme, and the waveform is fitted to show highly quantitative agreement with a molecular model. The total detection time for the rotational spectrum was 20 ns, indicating an upper limit to the detection frequency of ~50 MHz, significantly faster than any other reported spectrally-resolved coherent anti-Stokes Raman detection strategy to date.
Coherent anti-Stokes Raman spectroscopy (CARS) is a sensitive technique for probing highly luminous flames in combustion applications to determine temperatures and species concentrations. CARS thermometry has been demonstrated for the vibrational Q-branch and pure-rotational S-branch of several small molecules. Practical advantages of pure-rotational CARS, such as multi-species detection, reduction of coherent line mixing and collisional narrowing even at high pressures, and the potential for more precise thermometry, have motivated experimental and theoretical advances in S-branch CARS of nitrogen (N2), for example, which is a dominant species in air-fed combustion processes. Although hydrogen (H2) is of interest given its prevalence as a reactant and product in many gas-phase reactions, laser bandwidth limitations have precluded the extension of CARS thermometry to the H2 S-branch. We demonstrate H2 thermometry using hybrid femtosecond/picosecond pure-rotational CARS, in which a broadband pump/Stokes pulse enables simultaneous excitation of the set of H2 S-branch transitions populated at flame temperatures over the spectral region of 0-2200 cm-1. We present a pure-rotational H2 CARS spectral model for data fitting and compare extracted temperatures to those from simultaneously collected N2 spectra in two systems of study: a heated flow and a diffusion flame on a Wolfhard-Parker slot burner. From 300 to 650 K in the heated flow, the H2 and N2 CARS extracted temperatures are, on average, within 2% of the set temperature. For flame measurements, the fitted H2 and N2 temperatures are, on average, within 5% of each other from 300 to 1600 K. Our results confirm the viability of pure-rotational H2 CARS thermometry for probing combustion reactions.
Ultrabroadband coherent anti-Stokes Raman spectroscopy (CARS) was employed for one-dimensional imaging of temperature and major species distributions simultaneously in the near-wall region of a CH4/air flame supported on a side-wall-quenching burner. Automatic temporal and spatial overlap of the approximetaly 7 fs pump and Stokes pulses was achieved through a two-beam CARS phase-matching scheme and the crossed approximately 75 ps probe beam provides excellent spatial sectioning of the probed location. Concurrent detection of N2 O2 H2 CO CO2 and CH4 was performed. A CH4/air premixed flame at lean stoichiometric and rich conditions and Reynolds number = 5000 was probed as it quenches against a cooled steel side-wall parallel to the flow providing a persistent flame-wall interaction. An imaging resolution of better than 40 μm was achieved across the field-of-view allowing thermochemical states of the thermal boundary layer to be resolved to within approximately 30 μm of the interface.