studies. However, while transmission experiments are the most frequently used for
the determination of the active sites using probe molecules, the question arises as to
whether they are pure in situ experiments according to the definition proposed by
Haw [56], because UHV or HV are recommended for obtaining reproducible FTIR
spectra of adsorbed probe molecules? Since the use of probe molecules in transmission mode to determine acidic properties is the most utilised method, this
question remains unanswered.
However, both transmission and DRIFT modes are also used to monitor the
surface-active species appearing at a catalyst’s surface during the catalytic reaction.
Transmission experimentation in FTIR is a direct method of measurement, as it
does not create as many secondary optical effects as reflectance-based methods.
However, if the simplicity of the catalyst preparation is considered, DRIFT appears
to be the method of choice for its ability to analyse the studied material directly, in
its original form. In the transmission mode, there is a limited selection of options as
KBr wafers, as the dispersal of solids practically prevents their interaction with
gaseous agents. The thin, self-supporting or Si wafers on which catalysts are
deposited are the method for transmission measurements. What is more, if we
consider the structure and mechanical stability of self-supported wafers, it has to be
taken into account that the pressure which secures the integrity of the pellets may
also change the material structure and properties [7]. To monitor a catalyst’s surface
in transmission-absorption special in situ cells were developed. Some details of
these can be found in several references [6, 7, 57, 58].
Transmission experiments are commonly used not only to determine the active
sites at a catalyst’s surface using probe molecules under HV/UHV conditions, but
also in transient-state measurements and examinations of surface intermediates
using operando methodology [57, 59–62]. However, the vibrational patterns of
surface intermediates are complex and often need additional analysis by complimentary methods to be properly resolved. In the study by Rasmussen et al. [57], the
surface properties during NH 3 -SCR over V 2 O 5 /WO 3 /TiO 2 /sepiolite catalyst were
characterised by operando IR/MS. IR measurements of the catalyst surface under
NO + O 2 dynamic conditions revealed several bands, including those from
adsorbed NH 4
+ appearing at 1435–1450 cm
−1 , those from ammonia coordinated at
the Lewis acid site at 1610–1620 cm
−1 and those of molecular water adsorbed at
the catalyst surface, at 1620 cm
−1 (Fig. 11.6). The assignment of both NH 3 and OH
bands from vibrating adsorbates may be problematic, since both bands overlap. To
solve this problem, the chemometric methods are more and more frequently
engaged, as Multivariate Curve Resolution Alternating Least Squares (MCR-ALS)
methods, for example, can be used to separate the overlapping bands [57].
Since the preparation procedures in classical transmission-absorption mode
require fastidious preparation of self-supporting diluted pellets, the DRIFT method
has become one of most utilised in situ method for the characterisation of catalysts.
The variety of applications of DRIFT analysis in catalyst characterisation, from
surface analysis to determination of surface intermediates, is a result of the simplicity of the method. Indeed, catalyst preparation is reduced to minimum, although
the impact of particle size and reflectivity of the sample on the spectra cannot be
11 In Situ and Operando Techniques in Catalyst Characterisation …
341
the determination of the active sites using probe molecules, the question arises as to
whether they are pure in situ experiments according to the definition proposed by
Haw [56], because UHV or HV are recommended for obtaining reproducible FTIR
spectra of adsorbed probe molecules? Since the use of probe molecules in transmission mode to determine acidic properties is the most utilised method, this
question remains unanswered.
However, both transmission and DRIFT modes are also used to monitor the
surface-active species appearing at a catalyst’s surface during the catalytic reaction.
Transmission experimentation in FTIR is a direct method of measurement, as it
does not create as many secondary optical effects as reflectance-based methods.
However, if the simplicity of the catalyst preparation is considered, DRIFT appears
to be the method of choice for its ability to analyse the studied material directly, in
its original form. In the transmission mode, there is a limited selection of options as
KBr wafers, as the dispersal of solids practically prevents their interaction with
gaseous agents. The thin, self-supporting or Si wafers on which catalysts are
deposited are the method for transmission measurements. What is more, if we
consider the structure and mechanical stability of self-supported wafers, it has to be
taken into account that the pressure which secures the integrity of the pellets may
also change the material structure and properties [7]. To monitor a catalyst’s surface
in transmission-absorption special in situ cells were developed. Some details of
these can be found in several references [6, 7, 57, 58].
Transmission experiments are commonly used not only to determine the active
sites at a catalyst’s surface using probe molecules under HV/UHV conditions, but
also in transient-state measurements and examinations of surface intermediates
using operando methodology [57, 59–62]. However, the vibrational patterns of
surface intermediates are complex and often need additional analysis by complimentary methods to be properly resolved. In the study by Rasmussen et al. [57], the
surface properties during NH 3 -SCR over V 2 O 5 /WO 3 /TiO 2 /sepiolite catalyst were
characterised by operando IR/MS. IR measurements of the catalyst surface under
NO + O 2 dynamic conditions revealed several bands, including those from
adsorbed NH 4
+ appearing at 1435–1450 cm
−1 , those from ammonia coordinated at
the Lewis acid site at 1610–1620 cm
−1 and those of molecular water adsorbed at
the catalyst surface, at 1620 cm
−1 (Fig. 11.6). The assignment of both NH 3 and OH
bands from vibrating adsorbates may be problematic, since both bands overlap. To
solve this problem, the chemometric methods are more and more frequently
engaged, as Multivariate Curve Resolution Alternating Least Squares (MCR-ALS)
methods, for example, can be used to separate the overlapping bands [57].
Since the preparation procedures in classical transmission-absorption mode
require fastidious preparation of self-supporting diluted pellets, the DRIFT method
has become one of most utilised in situ method for the characterisation of catalysts.
The variety of applications of DRIFT analysis in catalyst characterisation, from
surface analysis to determination of surface intermediates, is a result of the simplicity of the method. Indeed, catalyst preparation is reduced to minimum, although
the impact of particle size and reflectivity of the sample on the spectra cannot be
11 In Situ and Operando Techniques in Catalyst Characterisation …
341