11.2.2.1 Enhanced in Situ FTIR Techniques
The description of in situ FTIR spectroscopy as a tool for the characterisation of
catalysts would be incomplete without consideration of time-resolved techniques,
including step-scan and rapid-scan methods. Since the lifetime of the reacting
species at the catalyst surface in the form of transition state or intermediate species
varies 10
−12
–10
−13 s, the development of rapid-scan techniques was highly desired.
The rapid-scan method was developed in the 1970 s. Its application during catalyst
characterisation allows the collection of spectra in a time of 10
−3 s in modern FTIR
spectrometers. The step-scan method decreases the acquisition time to 10
−9 s. The
significant shortening of the spectra recording using both rapid-scan and step-scan
methods made possible experimental spectroscopic studies including transient
absorption experiments, pulsed sorption–desorption studies and kinetic studies
[75–82]. Nevertheless, the use of the rapid and step-scan techniques is very
demanding, requiring highly stable and reproducible conditions during the experiment. Additional factors such as pressure changes or temperature fluctuations may
also affect the experimental results [75]. The correct set-up of time-resolved techniques, their complexity and sensitivity to even millisecond condition fluctuations
mean that the literature does not contain a great many reports on this matter [75–78,
83–85].
Monitoring fast reactions can be achieved with FTIR analysis by using time or
phase-resolved techniques. In a study by Iwamoto and Hoshino [76], the isotopic
exchange of
14 NO and
15
NO species over CoZSM-5 catalysts was measured by the
application of rapid-scan FTIR spectroscopy. For the first time in the literature, it
was shown that
15 NO molecules are rapidly exchanged by
14 NO from the gaseous
phase until equilibrium in gaseous NO was achieved. Achieving the equilibrium
state for the
15 NO–
14 NO exchange was more rapid for mononitrosyl than dinitrosyl
adsorbates, and lasted 0.3 s.
In another study, by Thibault-Starzyk et al. [83], the deNO x process by CO over
Ag/Al 2 O 3 catalysts using the step-scan FTIR spectroscopy was investigated. The
pulse heating of the reactants was performed by using a femtosecond laser with a
10 Hz repetition rate. The authors concluded that the rate-limiting step in the CO
deNO x reaction is the flipping of cyanide groups from Ag nanoparticles to the
alumina support. The lifetime of the rate-limiting step was estimated to be as short
as 2 µs. This was evidenced by the gradual decrease in the 2130 cm
−1 band
attributed to AgCN, and the increase of the 2265 cm
−1 band, attributed to isocyanate species adsorbed on alumina.
Rapid recent development of synchrotron radiation has opened new opportunities to perform operando XANES/DRIFT analysis of heterogeneous catalysts. In
work by Brieger et al. [72], the structure and surface properties of the EuroPt-1
catalyst (6.3 wt% Pt/SiO 2 ) during CO sorption studies were measured using
operando XANES/DRIFT. Since XANES is not able to differentiate between the
adsorbed CO species over the EuroPt-1 catalyst, the use of DRIFT analysis was
demanded. Indeed, the characteristic bands at 2070, 1850 and 1720 cm
−1 assigned
to atop, bridge-bonded and triple-bonded CO species.
344
P. Jodłowski and J. Łojewska
The description of in situ FTIR spectroscopy as a tool for the characterisation of
catalysts would be incomplete without consideration of time-resolved techniques,
including step-scan and rapid-scan methods. Since the lifetime of the reacting
species at the catalyst surface in the form of transition state or intermediate species
varies 10
−12
–10
−13 s, the development of rapid-scan techniques was highly desired.
The rapid-scan method was developed in the 1970 s. Its application during catalyst
characterisation allows the collection of spectra in a time of 10
−3 s in modern FTIR
spectrometers. The step-scan method decreases the acquisition time to 10
−9 s. The
significant shortening of the spectra recording using both rapid-scan and step-scan
methods made possible experimental spectroscopic studies including transient
absorption experiments, pulsed sorption–desorption studies and kinetic studies
[75–82]. Nevertheless, the use of the rapid and step-scan techniques is very
demanding, requiring highly stable and reproducible conditions during the experiment. Additional factors such as pressure changes or temperature fluctuations may
also affect the experimental results [75]. The correct set-up of time-resolved techniques, their complexity and sensitivity to even millisecond condition fluctuations
mean that the literature does not contain a great many reports on this matter [75–78,
83–85].
Monitoring fast reactions can be achieved with FTIR analysis by using time or
phase-resolved techniques. In a study by Iwamoto and Hoshino [76], the isotopic
exchange of
14 NO and
15
NO species over CoZSM-5 catalysts was measured by the
application of rapid-scan FTIR spectroscopy. For the first time in the literature, it
was shown that
15 NO molecules are rapidly exchanged by
14 NO from the gaseous
phase until equilibrium in gaseous NO was achieved. Achieving the equilibrium
state for the
15 NO–
14 NO exchange was more rapid for mononitrosyl than dinitrosyl
adsorbates, and lasted 0.3 s.
In another study, by Thibault-Starzyk et al. [83], the deNO x process by CO over
Ag/Al 2 O 3 catalysts using the step-scan FTIR spectroscopy was investigated. The
pulse heating of the reactants was performed by using a femtosecond laser with a
10 Hz repetition rate. The authors concluded that the rate-limiting step in the CO
deNO x reaction is the flipping of cyanide groups from Ag nanoparticles to the
alumina support. The lifetime of the rate-limiting step was estimated to be as short
as 2 µs. This was evidenced by the gradual decrease in the 2130 cm
−1 band
attributed to AgCN, and the increase of the 2265 cm
−1 band, attributed to isocyanate species adsorbed on alumina.
Rapid recent development of synchrotron radiation has opened new opportunities to perform operando XANES/DRIFT analysis of heterogeneous catalysts. In
work by Brieger et al. [72], the structure and surface properties of the EuroPt-1
catalyst (6.3 wt% Pt/SiO 2 ) during CO sorption studies were measured using
operando XANES/DRIFT. Since XANES is not able to differentiate between the
adsorbed CO species over the EuroPt-1 catalyst, the use of DRIFT analysis was
demanded. Indeed, the characteristic bands at 2070, 1850 and 1720 cm
−1 assigned
to atop, bridge-bonded and triple-bonded CO species.
344
P. Jodłowski and J. Łojewska