In the past, a majority of techniques relied on the use of the UHV conditions or
reaction “freezing” to obtain structural information on the catalyst surface or active
sites. However, that approach is far from the working conditions of the catalysts,
which for a vast majority of processes entail elevated temperature and pressure. The
necessity of observing catalyst structure under real working conditions was the
crucial point in the development of the in situ experiments. According to Haw [2],
an ideal in situ investigation should look at catalyst structure and morphology, and
the reaction intermediates under real working conditions. However, in the literature,
the term in situ is also used for the spectroscopic measurements of catalysts or metal
oxide surface upon contact with the adsorbate or reacting species [3]. The definition
proposed by Daydov [3] does not exclude a whole palette of measurements under
HV/UHV, utilising quantitative experiments with probe molecules such as
ammonia, pyridine or CO for the determination of surface acidity. Thus, taking into
account that there are two different ideas of in situ analysis, the question arises as to
whether the application of the UHV technique really reflects the structure of the
surface of a working catalytic material. Another question is presented by Daydov’s
attitude to combining HV/UHV with in situ examinations in order to collect as
much information about the catalytic system as possible.
On the other hand, when considering the analytical methods for catalyst characterisation, we have to take into account that the phenomenon of catalysis is
limited to the very surface (external surface) of a catalytic material, on which
reactants adsorb, form intermediates and then convert to products. Amongst the
analytical methods which are able to focus on this surface are the microscopic
methods (AFM and, to an extent, SEM), in situ spectroscopic methods (Raman,
FTIR and UV/VIS) and ion (LEIS) and light scattering (SAXS). Ambient XPS
(rarely available yet in laboratories) and Raman spectroscopy can aid surface
studies with only limited confidence, since the signal emanating from a sample,
depending on penetration depth, provides information on the external surface (a few
monolayers in XPS, a few microns in Raman) to only a limited extent. This
important drawback of a majority of the so-called surface methods can be overcome
by in situ and operando analysis utilising surface probe molecules.
Since in situ characterisation methods provide information on catalyst structure
under real reaction conditions, rather than yielding data on overall activity, the
development of a methodology able to monitor both parameters at the same time
was highly desirable for further assessment of structure–activity relationships. This
led to the development of the methodology of operando spectroscopic measurements. This term was originally proposed by Prof. Miguel Bañares during a discussion with Eric Gaigneaux, Gerhard Mestl and Bert Weckhuysen at the 220th
ACS National Meeting in Washington, in 2002. The term operando refers to the
Latin word describing working or operating conditions [4, 5]. Since in situ and
operando methodologies have been established as conventional methods, the
number of scientific papers on their use and development for catalyst studies has
been growing. According to the Web of Science (Fig. 11.1) both in situ and
operando techniques have become unwritten standards for studies dealing with the
preparation and characterisation of catalysts for various heterogeneous reactions.
11 In Situ and Operando Techniques in Catalyst Characterisation …
335
reaction “freezing” to obtain structural information on the catalyst surface or active
sites. However, that approach is far from the working conditions of the catalysts,
which for a vast majority of processes entail elevated temperature and pressure. The
necessity of observing catalyst structure under real working conditions was the
crucial point in the development of the in situ experiments. According to Haw [2],
an ideal in situ investigation should look at catalyst structure and morphology, and
the reaction intermediates under real working conditions. However, in the literature,
the term in situ is also used for the spectroscopic measurements of catalysts or metal
oxide surface upon contact with the adsorbate or reacting species [3]. The definition
proposed by Daydov [3] does not exclude a whole palette of measurements under
HV/UHV, utilising quantitative experiments with probe molecules such as
ammonia, pyridine or CO for the determination of surface acidity. Thus, taking into
account that there are two different ideas of in situ analysis, the question arises as to
whether the application of the UHV technique really reflects the structure of the
surface of a working catalytic material. Another question is presented by Daydov’s
attitude to combining HV/UHV with in situ examinations in order to collect as
much information about the catalytic system as possible.
On the other hand, when considering the analytical methods for catalyst characterisation, we have to take into account that the phenomenon of catalysis is
limited to the very surface (external surface) of a catalytic material, on which
reactants adsorb, form intermediates and then convert to products. Amongst the
analytical methods which are able to focus on this surface are the microscopic
methods (AFM and, to an extent, SEM), in situ spectroscopic methods (Raman,
FTIR and UV/VIS) and ion (LEIS) and light scattering (SAXS). Ambient XPS
(rarely available yet in laboratories) and Raman spectroscopy can aid surface
studies with only limited confidence, since the signal emanating from a sample,
depending on penetration depth, provides information on the external surface (a few
monolayers in XPS, a few microns in Raman) to only a limited extent. This
important drawback of a majority of the so-called surface methods can be overcome
by in situ and operando analysis utilising surface probe molecules.
Since in situ characterisation methods provide information on catalyst structure
under real reaction conditions, rather than yielding data on overall activity, the
development of a methodology able to monitor both parameters at the same time
was highly desirable for further assessment of structure–activity relationships. This
led to the development of the methodology of operando spectroscopic measurements. This term was originally proposed by Prof. Miguel Bañares during a discussion with Eric Gaigneaux, Gerhard Mestl and Bert Weckhuysen at the 220th
ACS National Meeting in Washington, in 2002. The term operando refers to the
Latin word describing working or operating conditions [4, 5]. Since in situ and
operando methodologies have been established as conventional methods, the
number of scientific papers on their use and development for catalyst studies has
been growing. According to the Web of Science (Fig. 11.1) both in situ and
operando techniques have become unwritten standards for studies dealing with the
preparation and characterisation of catalysts for various heterogeneous reactions.
11 In Situ and Operando Techniques in Catalyst Characterisation …
335