transfer metal complexes. The latter follows the definition of Brønsted (protonic)
and Lewis (non-protonic) acids and bases. Real industrial catalysts may contain the
whole palette of active sites. Furthermore, the two categories overlap if we consider, for example, metal cations which can be regarded as both redox and Lewis
non-protonic acidic sites.
The characterisation of active sites is performed at the molecular level. In order
to determine the acidic properties of the catalysts, several characterisation methods
have been developed. These include titration methods, sorption, adsorption and
temperature-programmed desorption of probe molecules, and spectroscopic techniques [11, 19]. Amongst the listed methods, the spectroscopic techniques are most
commonly used, due to their versatility and availability.
The selection principles for a probe molecule used in spectroscopic methods to
determine desired acidic properties was proposed by Knözinger [13]. The selected
probe molecule used in spectroscopic studies must conform to several criteria that
cover the following aspects [13, 14]:
• The spectra response of the interaction between probe molecule with active sites
must allow for the distinction between Brønsted and Lewis sites.
• Extinction factors are sufficiently high and known to allow for the determination
of quality and strength of measured centres.
• The acidic strength of the probe molecule should be selected to achieve the
optimum response in acid–base interaction
• The selected probe molecule should be specific to differentiate between the sites
of different strengths.
• The size of the probe molecule should be selected for the material under
scrutiny, so that it can diffuse through the pores to cover the maximum number
of the active sites.
• The measurement conditions (temperature) during surface probing should be
selected to avoid reactions other than those with active centres.
In fact, selecting an ideal probe molecule is often a difficult task, because it is
impossible in practice to meet all of the above-mentioned criteria completely.
The probe molecules that are commonly used to determine catalyst surface
acidity can be divided into two groups, with weak and strong bases. The group of
strong bases includes pyridine, ammonia, piperidine, trimethylamine and n-butyloamine [13, 20, 21]. The interaction of these probe molecules provides information
about both Lewis and Brønsted acid centres. Amongst the group of the strong bases,
ammonia and pyridine are the most frequently used [13, 22, 23]. The group of weak
bases, which includes benzene and substituted benzenes, CO, N 2 , O 2 and H 2 , is also
used to determine both Lewis and Brønsted acidity [14]. The principle that lies
behind the determination of the acidity by weak base probes is the formation of
weak hydrogen bonds with the hydroxyl group present on the catalyst surface [14].
An example of the application of the in situ FTIR method to studies of catalyst
active sites is a recent study by Ochońska et al. [22], devoted to preparation and
characterisation of Cu-exchanged Y, USY and ZSM-5 zeolites used as catalysts for
11 In Situ and Operando Techniques in Catalyst Characterisation …
337
and Lewis (non-protonic) acids and bases. Real industrial catalysts may contain the
whole palette of active sites. Furthermore, the two categories overlap if we consider, for example, metal cations which can be regarded as both redox and Lewis
non-protonic acidic sites.
The characterisation of active sites is performed at the molecular level. In order
to determine the acidic properties of the catalysts, several characterisation methods
have been developed. These include titration methods, sorption, adsorption and
temperature-programmed desorption of probe molecules, and spectroscopic techniques [11, 19]. Amongst the listed methods, the spectroscopic techniques are most
commonly used, due to their versatility and availability.
The selection principles for a probe molecule used in spectroscopic methods to
determine desired acidic properties was proposed by Knözinger [13]. The selected
probe molecule used in spectroscopic studies must conform to several criteria that
cover the following aspects [13, 14]:
• The spectra response of the interaction between probe molecule with active sites
must allow for the distinction between Brønsted and Lewis sites.
• Extinction factors are sufficiently high and known to allow for the determination
of quality and strength of measured centres.
• The acidic strength of the probe molecule should be selected to achieve the
optimum response in acid–base interaction
• The selected probe molecule should be specific to differentiate between the sites
of different strengths.
• The size of the probe molecule should be selected for the material under
scrutiny, so that it can diffuse through the pores to cover the maximum number
of the active sites.
• The measurement conditions (temperature) during surface probing should be
selected to avoid reactions other than those with active centres.
In fact, selecting an ideal probe molecule is often a difficult task, because it is
impossible in practice to meet all of the above-mentioned criteria completely.
The probe molecules that are commonly used to determine catalyst surface
acidity can be divided into two groups, with weak and strong bases. The group of
strong bases includes pyridine, ammonia, piperidine, trimethylamine and n-butyloamine [13, 20, 21]. The interaction of these probe molecules provides information
about both Lewis and Brønsted acid centres. Amongst the group of the strong bases,
ammonia and pyridine are the most frequently used [13, 22, 23]. The group of weak
bases, which includes benzene and substituted benzenes, CO, N 2 , O 2 and H 2 , is also
used to determine both Lewis and Brønsted acidity [14]. The principle that lies
behind the determination of the acidity by weak base probes is the formation of
weak hydrogen bonds with the hydroxyl group present on the catalyst surface [14].
An example of the application of the in situ FTIR method to studies of catalyst
active sites is a recent study by Ochońska et al. [22], devoted to preparation and
characterisation of Cu-exchanged Y, USY and ZSM-5 zeolites used as catalysts for
11 In Situ and Operando Techniques in Catalyst Characterisation …
337