11.2.2.2 2D Correlation Spectroscopy
Two-dimensional correlation spectroscopy (2D COS) was originally developed to
analyse NMR spectra and untangle the information derived from many overlapping
signals [86]. The possibility of analysing the spectra as a function of the two
variables represented a breakthrough in spectral analysis. Although the application
of 2D COS to the analysis of NMR spectra is still dominant, there is an increasing
interest in connecting it with other spectroscopic techniques, including FTIR or
Raman. The first application of 2D COS to vibrational spectroscopy was proposed
by Noda in 1988 [87]. Since then, 2D COS has attracted a wide group of
researchers from various fields such as catalysis and surface science. For the studies
of catalysis, the application of 2D COS to spectroscopic studies may provide
several advantages [86]:
• simplification of complex spectra by adding a second dimension;
• increase in spectral resolution of derived spectra and separation of overlapping
peaks;
• determination of the sequential order during analysis of the individual bands at
the asynchronous spectra;
• heterospectral correlation by the correlation of results obtained using different
spectroscopies (such as FTIR and Raman spectroscopy);
• correlation between different methods, such as mass spectrometry, gas chromatography, etc.
The use of 2D COS in in situ techniques has been reported in references [88–92].
In work by Chlebda et al. [69], 2D COS was used to improve in situ characterisation
and band assignment of surface intermediates appearing on a series of metal oxide
catalysts during methane catalytic combustion. During the reaction, oxidative and
non-oxidative conditions were applied. Exemplary results of in situ 2D FTIR COS
for mixed Co/Pd/c-Al 2 O 3 catalysts under non-oxidative conditions are presented in
Fig. 11.8. Synchronous spectrum analysis has revealed two groups of cross-peaks at
synchronous spectra (Fig. 11.9a), with positive and negative increase of bands at
increasing reaction temperature occurring for cross-peaks. Analysis of the corresponding asynchronous spectrum (Fig. 11.9b) allowed the sequence of the band
changes during the experiment to be determined as follows: 1590 ! 1390 !
1380 ! 1620 ! 1304 ! 1520 ! 1505 ! 1540 ! 1420 ! 1560 ! 1460 !
1350 ! 1330 ! 1340. Further analysis of both synchronous and asynchronous
spectra achieved by the application of 2D COS to the in situ FTIR spectroscopies
allows the reaction mechanism to be determined, based on in situ 2D COS DRIFT
experiments.
The 2D COS spectroscopy can also be successfully applied for in situ µRaman
structural studies of cobalt catalysts deposited on metallic supports [70]. The
methodology presented in work by Chlebda et al. [70] shows that the use of in situ
DRIFT, µRaman and DR-UV-Vis methods in parallel with the application of 2D
COS provides complimentary information on the catalysts. The 2D COS in situ
11 In Situ and Operando Techniques in Catalyst Characterisation …
345
Two-dimensional correlation spectroscopy (2D COS) was originally developed to
analyse NMR spectra and untangle the information derived from many overlapping
signals [86]. The possibility of analysing the spectra as a function of the two
variables represented a breakthrough in spectral analysis. Although the application
of 2D COS to the analysis of NMR spectra is still dominant, there is an increasing
interest in connecting it with other spectroscopic techniques, including FTIR or
Raman. The first application of 2D COS to vibrational spectroscopy was proposed
by Noda in 1988 [87]. Since then, 2D COS has attracted a wide group of
researchers from various fields such as catalysis and surface science. For the studies
of catalysis, the application of 2D COS to spectroscopic studies may provide
several advantages [86]:
• simplification of complex spectra by adding a second dimension;
• increase in spectral resolution of derived spectra and separation of overlapping
peaks;
• determination of the sequential order during analysis of the individual bands at
the asynchronous spectra;
• heterospectral correlation by the correlation of results obtained using different
spectroscopies (such as FTIR and Raman spectroscopy);
• correlation between different methods, such as mass spectrometry, gas chromatography, etc.
The use of 2D COS in in situ techniques has been reported in references [88–92].
In work by Chlebda et al. [69], 2D COS was used to improve in situ characterisation
and band assignment of surface intermediates appearing on a series of metal oxide
catalysts during methane catalytic combustion. During the reaction, oxidative and
non-oxidative conditions were applied. Exemplary results of in situ 2D FTIR COS
for mixed Co/Pd/c-Al 2 O 3 catalysts under non-oxidative conditions are presented in
Fig. 11.8. Synchronous spectrum analysis has revealed two groups of cross-peaks at
synchronous spectra (Fig. 11.9a), with positive and negative increase of bands at
increasing reaction temperature occurring for cross-peaks. Analysis of the corresponding asynchronous spectrum (Fig. 11.9b) allowed the sequence of the band
changes during the experiment to be determined as follows: 1590 ! 1390 !
1380 ! 1620 ! 1304 ! 1520 ! 1505 ! 1540 ! 1420 ! 1560 ! 1460 !
1350 ! 1330 ! 1340. Further analysis of both synchronous and asynchronous
spectra achieved by the application of 2D COS to the in situ FTIR spectroscopies
allows the reaction mechanism to be determined, based on in situ 2D COS DRIFT
experiments.
The 2D COS spectroscopy can also be successfully applied for in situ µRaman
structural studies of cobalt catalysts deposited on metallic supports [70]. The
methodology presented in work by Chlebda et al. [70] shows that the use of in situ
DRIFT, µRaman and DR-UV-Vis methods in parallel with the application of 2D
COS provides complimentary information on the catalysts. The 2D COS in situ
11 In Situ and Operando Techniques in Catalyst Characterisation …
345