(781e) Investigation of Model Heterogeneous Catalysts Via Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRAS)

Ultra high vacuum (UHV) studies on metal single crystal surfaces have been used to explore the elemental steps of different catalytic reactions (1, 2). However, the adsorption structure and reaction pathways observed under UHV conditions may not necessarily be the same as those present at elevated pressures in realistic reactions, and the metal single crystal surfaces cannot explain the particle size effect or metal-support interactions of industrial catalysts. These two main discrepancies between UHV studies and technical heterogeneous catalysis are referred frequently to the “pressure gap” and “material gap” (3). To bridge these gaps, experimental systems are required to be able to run reactions from UHV to elevated pressure (~10³ Torr) conditions, and advanced in situcharacterization techniques that can handle high-pressure detection are needed, since most conventional surface science techniques (e.g. AES or LEED) cannot be used at reaction conditions due to the short mean free path of electrons.

In our work, a customized UHV-compatible high pressure cell combined with quadrupole mass spectrometer (QMS) and PM-IRAS is used to carry out in situ investigations of different heterogeneous catalytic reactions on model catalysts, ranging from single crystals, e.g. Pt(100), to model supported catalysts, e.g. Ru/Al₂O₃. The uniqueness of our system is that interested reactions can be investigated systematically from UHV to elevated pressure conditions with PM-IRAS, which can easily remove the gas phase interference and give us only the surface species vibrational information, allowing detailed understanding of the surface chemistry during reactions. In addition, model supported catalysts are synthesized to bridge the “material gap”.  For example, alumina thin film supported Ruthenium nanoparticles are synthesized and used to investigate ammonia decomposition, which is not only important for CO_x-free hydrogen production to feed proton exchange membrane fuel cells (4), but also an important step to understand the selective catalytic reduction of nitric oxide with ammonia for NO_x emission control in industry (5).

References

1.            G. A. Somorjai, Introduction to Surface Chemistry and Catalysis.  (John Wiley & Sons, Inc., 1994).

2.            K. Christann, Introduction to surface physical chemistry.  (Steinkopff Verlag Darmstadt Springer-Verlag New York, 1991).

3.            R. Günther, W. Christian, Spectroscopic studies of surface–gas interactions and catalyst restructuring at ambient pressure: mind the gap! Journal of Physics: Condensed Matter 20, 184019 (2008).

4.            F. Schuth, R. Palkovits, R. Schlogl, D. S. Su, Ammonia as a possible element in an energy infrastructure: catalysts for ammonia decomposition. Energy & Environmental Science 5, 6278 (2012).

5.            P. R. Ettireddy, N. Ettireddy, T. Boningari, R. Pardemann, P. G. Smirniotis, Investigation of the selective catalytic reduction of nitric oxide with ammonia over Mn/TiO2 catalysts through transient isotopic labeling and in situ FT-IR studies. Journal of Catalysis 292, 53 (2012).