Understanding Heterogeneous Catalytic Reactions

A catalyst is a chemical substance that is capable of increasing the speed of a reaction. When multiple reactions take place, an appropriate catalyst may selectively boost the speed of a given reaction at the cost of the other reactions. Therefore, catalysts hold a significant interest since they allow to enhance yields and to fine tune the properties of the end product.

Vibrational Spectroscopic Characterization

Vibrational spectroscopic characterization can occur all along the life of the catalyst, from the control of the precursor synthesis to the control of the catalyst regeneration through the analysis of the surface reaction mechanism. To this end, Raman spectroscopy has been used in heterogeneous catalysis for characterizing massive and supported oxides. Raman has high spatial resolution and is capable of probing the low wavenumber spectral range where most of the active phases, i. e. metal sulfides or oxide, have their characteristic lines. As a complementary method, FTIR has been extensively utilized for characterizing the surface acidity of the catalysts via the adsorption of probe molecules.

FTIR/Raman Combination Microprobe System

The FTIR/Raman combination microprobe system allows concurrent analysis of a single sample area by both techniques. A microscope is directly combined to an infrared interferometer and a Raman spectrograph. As the spatial resolution is very different due to the diffraction limit caused by the wavelength of the probing radiation, the IR and laser beams coincide at the sample and offer information on the same area of material. By leveraging the different information obtained from the two techniques, this innovative technology allows the characterization of ongoing heterogeneous catalytic processes through a dedicated cell linked to the microscope stage. The capability to study the same reaction through both techniques offers complementary vibrational data about the mechanisms that occur at the surface of the catalyst during a catalytic reaction.

FTIR and Raman spectroscopic measurements were carried out on a LabRAM IR spectrometer, which is an integrated dispersive Raman and FT-IR microscope. Figure 1 shows an all-reflecting objective that allows both FTIR and Raman spectra to be obtained.

Figure 1. Picture of the in-situ device and schematic representation of the Cassegrain objective with both laser(He/Ne 633nm) and IR beams.

Dedicated Cell

Powdered catalyst was positioned in a Linkam stage that is directly adapted to the instrument’s microscope. The instrument offers controlled temperature and atmosphere. The cell’s window was made of ZnSe material, which is known for its total inertness under the working conditions and high transmission in the infrared. Although ZnSe contains a high Raman spectrum of less than 350cm^-1, potential interferences of these aspects with the signal of interest can be prevented by using the microscope’s confocality. In figure 2, no contribution of the ZnSe window is observed and the MoO₃ spectrum is well resolved.

Figure 2. Raman spectra of (A) ZnSe window material and of (B) molybdenum oxide introduced in the cell.

DeNO_x Reaction: A Heterogeneous Catalytic Process

Heterogeneous DeNO_x catalytic process is performed to prevent nitrogen oxide gas by reducing the surface of metal particles. A γ-alumina supported Pd/Al₂O₃ was used a catalyst. Raman and IR information on the catalytic system at constant state can be achieved concurrently on the LabRAM IR system. This helped in identifying the poisoning species and intermediate ones to better understand gas-surface interactions and to improve composition and morphology of catalysts.

Figure 3. In situ Raman spectra obtained on (A) a 1 wt. % Pd/γ-Al₂O₃ exposed to air at room temperature, (B)after reduction under H₂ at 573 K for 2h and after 5x10^-3 NO atm exposure at 473 K (C) for 30 mn.

Figure 4. In situ IR spectra obtained on (A) a 1 wt.% Pd/γ-Al₂O₃ exposed to air at room temperature, (B) after reduction under H₂ at 573 K for 2h and after 5x10^-3 NO atm exposure at 473 K (C) for 10mn,

During the nitrogen oxide gas reduction, both IR and Raman were recorded; figures 3 and 4 show the IR and Raman spectra. A small contribution from ZnSe occurs in the low wavenumber part of the Raman spectrum but because of the reduced size of the confocal hole, this line can be prevented, eliminating any interference with the Raman signal of interest.

Conclusion

Concurrent in situ characterizations of heterogeneous catalytic reactions by Raman and FTIR spectroscopies have been successfully carried out with the LabRAM IR using a dedicated cell. These measurements underline the advantages of both the techniques for studying catalytic reactions. Raman spectroscopy provides access to information located at low wavenumbers, thus allowing the characterization of the active phase during the reaction.

This information has been sourced, reviewed and adapted from materials provided by HORIBA.

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