surface sensitive molecules, providing information on real-time changes in the
structure of the catalyst surface. The in situ approach fills the so-called material and
process gaps in catalyst investigation when model simplified materials where
studied under conditions far removed from those inside industrial rectors.
In this chapter, the main spectroscopic techniques used for the in situ characterisation of catalyst materials are described. These include FTIR, Raman and
UV-Vis spectroscopies. Notwithstanding the spectroscopic method under scrutiny,
the strategy for choosing probe molecules is the same. Depending on the type of
active centre, different chemical agents can be used for both qualitative and
quantitative analysis. Alcohols, however, can be used as universal probes, as they
are able to convert to various products upon interaction with different active centres,
in brief: on acidic centres—to eters, on basic—to carbonates, on redox—to aldehydes or carboxylic acids. For analysis of Brønsted and Lewis acidic sites, molecules that are their basic pairs are used, and vice versa. Amongst the molecules with
basic properties, pyridine and ammonia are used most frequently, and amongst the
acidic molecules, carbon monoxide is most prevalent. Special devices are designed
for the titration of active sites on a catalyst surface, for the determination of their
number that is necessary for the derivation of turnover-frequency values (TOF).
Despite the fact that determination of TOF is not always obvious, especially if there
are many different kinds of active sites and if they are not fully accessible for a
given probe molecule, TOF is unquestionably the most universal gauge for measuring and comparing catalyst activity.
Different spectroscopic methods that are utilised for in situ catalyst characterisation are able to focus on different aspects of a catalyst surface during a chemical
process, and in this sense they are regarded as complementary. While IR can focus
on the structure of the probe molecule-active site complexes, Raman spots the
changes in the structure of metal oxides (the most commonly used catalytic
materials) upon these interactions, and UV-Vis captures the shifts in electronic
states that accompany the changes. For studies of reaction mechanisms, in situ
methods are used for tracking the above-mentioned changes during the catalytic
process. Additionally, operando techniques allow simultaneous analyses of reaction
products, giving insight into changes not only in the surface structure, but also in
catalyst activity. The spectroscopic in situ and operando methods are discussed in
terms of the used instrumentation, sampling and specific realms of their application,
and discussed using examples from the literature.
Finally, new trends in the development of in situ methods are presented, with
particular reference to:
• Time-resolved techniques for tracing fast reactions demonstrated for IR
spectroscopy.
• Coupled techniques, such as UV-Vis/Raman, used for imaging catalyst surfaces
at mesoscale and microscale, including in in situ mode. The AFM/Raman
technique is discussed in more detail.
• Fluorescence techniques at nanoscale, distinguishing changes on the catalyst
surface.
11 In Situ and Operando Techniques in Catalyst Characterisation …
353
structure of the catalyst surface. The in situ approach fills the so-called material and
process gaps in catalyst investigation when model simplified materials where
studied under conditions far removed from those inside industrial rectors.
In this chapter, the main spectroscopic techniques used for the in situ characterisation of catalyst materials are described. These include FTIR, Raman and
UV-Vis spectroscopies. Notwithstanding the spectroscopic method under scrutiny,
the strategy for choosing probe molecules is the same. Depending on the type of
active centre, different chemical agents can be used for both qualitative and
quantitative analysis. Alcohols, however, can be used as universal probes, as they
are able to convert to various products upon interaction with different active centres,
in brief: on acidic centres—to eters, on basic—to carbonates, on redox—to aldehydes or carboxylic acids. For analysis of Brønsted and Lewis acidic sites, molecules that are their basic pairs are used, and vice versa. Amongst the molecules with
basic properties, pyridine and ammonia are used most frequently, and amongst the
acidic molecules, carbon monoxide is most prevalent. Special devices are designed
for the titration of active sites on a catalyst surface, for the determination of their
number that is necessary for the derivation of turnover-frequency values (TOF).
Despite the fact that determination of TOF is not always obvious, especially if there
are many different kinds of active sites and if they are not fully accessible for a
given probe molecule, TOF is unquestionably the most universal gauge for measuring and comparing catalyst activity.
Different spectroscopic methods that are utilised for in situ catalyst characterisation are able to focus on different aspects of a catalyst surface during a chemical
process, and in this sense they are regarded as complementary. While IR can focus
on the structure of the probe molecule-active site complexes, Raman spots the
changes in the structure of metal oxides (the most commonly used catalytic
materials) upon these interactions, and UV-Vis captures the shifts in electronic
states that accompany the changes. For studies of reaction mechanisms, in situ
methods are used for tracking the above-mentioned changes during the catalytic
process. Additionally, operando techniques allow simultaneous analyses of reaction
products, giving insight into changes not only in the surface structure, but also in
catalyst activity. The spectroscopic in situ and operando methods are discussed in
terms of the used instrumentation, sampling and specific realms of their application,
and discussed using examples from the literature.
Finally, new trends in the development of in situ methods are presented, with
particular reference to:
• Time-resolved techniques for tracing fast reactions demonstrated for IR
spectroscopy.
• Coupled techniques, such as UV-Vis/Raman, used for imaging catalyst surfaces
at mesoscale and microscale, including in in situ mode. The AFM/Raman
technique is discussed in more detail.
• Fluorescence techniques at nanoscale, distinguishing changes on the catalyst
surface.
11 In Situ and Operando Techniques in Catalyst Characterisation …
353