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Combining PM-IRRAS with optical imaging techniques for in situ studies of CO oxidation

  • Lisa Rämisch (Lund University, Sweden)
Date
Tuesday 31 March 2020
Time
Address
Hotel NH Noordwijk Conference Centre Leeuwenhorst

Lisa Rämischa, Sabrina Gerickea, Sebastian Pfaffa, Johan Zetterberga
aDivision of Combustion Physics, Lund University, SE-221 00 Lund, Sweden

Today, catalytic reactions are involved in roughly 90 % of all processes in the chemical industry [1]. Reactions at gas-surface interfaces, such as the cleaning of the car exhaust through CO oxidation, fall under the umbrella of heterogeneous catalysis. In order to improve the selectivity and performance of catalysts, an understanding of the intrinsic reaction mechanisms on an atomic scale is required. Electron based spectroscopy techniques suffer from pressure limitation due to scattering of electrons in the gas phase. This creates a pressure gap between the research and the catalytic practical conditions.

To bridge this pressure gap, we are developing a setup which combines polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) with optical imaging techniques, such as planar laser induced fluorescence (PLIF) and surface optical reflectance (SOR), to access information about the gas and the surface phase during catalytic reactions in situ. PM-IRRAS is based on photon excitation and collection and therefore beneficial for studies at higher pressures. Its main advantages are the simultaneous collection of gas and surface species, its large spectral range and the fact that it causes no radiation damage.

We have earlier shown that the combination of PLIF and SOR in situ measurements correlate well and provide new insights into the gas-surface interactions [2]. PLIF allows us to monitor the two-dimensional spatial distribution of the gas distribution above the metal catalyst surface in real-time. In case of CO oxidation for example, this is done by collecting the fluorescence signal of CO2 induced by a laser [3]. Simultaneously with gas phase, we can track structural changes of the surface of the entire catalyst or a region of interest with a compact setup based on surface optical reflectance (SOR) [2], where reflected red LED light from the catalyst is imaged in situ. During CO oxidation, the change in reflectivity is associated with the formation of an oxide on the surface or with roughening of the surface [4].

However, none of the above-mentioned techniques provide chemical information. Our most recent experimental development therefore combines PLIF and SOR with a surface sensitive infrared technique, PM-IRRAS, based on Fourier Transform Interferometry (FTIR), where the polarization of the infrared light is modulated. Since the vibrational modes of the surface and gas species couple to different polarization directions, it is possible to distinguish spectra from the two and thereby obtain in-situ spectroscopic information about the adsorbed molecules on the surface. Thanks to the FTIR, our PM-IRRAS setup allows for a good signal-to-noise ratio and spectral resolution. Here, we combine all three techniques in a compact and mobile system to extract spatial, temporal and chemical information about CO oxidation on a Pd(100) surface. Our system provides a unique and  practical new experimental setup to measure catalytic reactions.

Acknowledgements

This project is financially supported by the Knut and Alice Wallenberg foundation, the Swedish Research Council, and the Swedish Foundation for Strategic Research. The authors would like to express their gratitude for this generous financial support.

References

  1. Chorkendorff I. , Niemansverdriet J.W., “Concepts of Modern Catalysis and Kinetics”, Wiley VCH, 2003
  2. Zhou J. et al. “Simultaneous Imaging of Gas Phase over and Surface Reflection of a Pd(100) Single Crystal during CO oxidation”. The Journal of Physical Chemistry , 2017, 121
  3. Zetterberg, J. et al. “Spatially and temporally resolved gas distributions around heterogeneous catalysts using infrared planar laser-induced fluorescence”, Nat. Commun. 2015, 6:7076.
  4. Onderwaater,W.G. et al. “In Situ Optical Reflectance Difference Observations of CO Oxidation over Pd(100)”, J. Phys. Chem. C Nanometer Interfaces, 2017

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