distribution of the bosons from the excited to the ground state. It is obvious that the
described effects are analogical to the excitation and de-excitation induced by IR
radiation but occur for entirely different molecular structures. Stokes effects are
more intense under conditions of catalytic reaction, although they vanish in the
prevailing Rayleigh scattered light. For this reason, Raman spectroscopy was losing
favour for many years, until new instrumentation was developed around 30 years
ago, giving it new applications in the fields of catalysis.
Unlike IR spectroscopy, Raman does not require any optical arrangements to
collect light from the surface of the solids, or any special optical windows except
glass. The majority of set-ups use microscopes which are able to transfer laser
incident light and collect back-scattered light. The choice of incident laser light
wavelength depends on the catalytic system studied. The in situ Raman experiments
also require proper choice of the in situ cells. There are two temperature cells
available commercially that are designed to comply with Raman microscopes for
the purpose of catalyst surface investigation. The Linkam cell, although designed to
reach a temperature of 1000 °C, is equipped with a ceramic cup that glows at high
temperature, disturbing the weak Raman signal. The Harrick is made entirely of
steel, but allows a temperature no higher than 550 °C. Both are equipped with
heating and gas supplying systems.
In fact, both IR and Raman spectroscopy are perfect choices for the in situ
molecular characterisation of catalysts, as their selection rules depend on the
molecular structure of the studied material, and they can provide real-time information on its changes. Additionally, Raman is entirely complementary to the IR
method. This is not only because of the opposite selection rules, but also because of
the specific absorption of typical materials used in heterogeneous catalysis. IR is
especially suitable for the analysis of the adsorbed probe molecules as they absorb
light above 1000 cm
−1 , and while the typical support oxides such as Al 2 O 3 or SiO 2
do not give any signals, Raman spectroscopy compensates for this. In Raman
spectroscopy, the majority of the support oxides do not scatter light above
100 cm
−1 , which gives perfect circumstances for the analysis of the majority of the
metal oxides used in catalysis and which give Raman signals, typically in the range
200–1200 cm
−1 . An important advantage of Raman spectroscopy over IR is that
water, a ubiquitous molecule in a great many reactions, is practically invisible in
Raman. However, we have to take into account that the structure of a working
catalyst exposed to high temperatures is different from that which occurs under
ambient conditions in which water molecules are incorporated to the structure of the
metal oxides both of the catalyst and the support materials. This is another
important reason why in situ methods are the only adequate approach for assessing
catalyst structure under working conditions.
Amongst the catalytic systems most intensely studied by Raman spectroscopy
are bulk metal oxides and supported metal oxides. There are also examples of the
application of Raman spectroscopy to zeolite and metallic catalysts. The bulk metal
oxides can be studied in terms of the existence of different crystalline phases, their
transformation, and the extent of their crystallisation. If aided with a microscope,
11 In Situ and Operando Techniques in Catalyst Characterisation …
347
described effects are analogical to the excitation and de-excitation induced by IR
radiation but occur for entirely different molecular structures. Stokes effects are
more intense under conditions of catalytic reaction, although they vanish in the
prevailing Rayleigh scattered light. For this reason, Raman spectroscopy was losing
favour for many years, until new instrumentation was developed around 30 years
ago, giving it new applications in the fields of catalysis.
Unlike IR spectroscopy, Raman does not require any optical arrangements to
collect light from the surface of the solids, or any special optical windows except
glass. The majority of set-ups use microscopes which are able to transfer laser
incident light and collect back-scattered light. The choice of incident laser light
wavelength depends on the catalytic system studied. The in situ Raman experiments
also require proper choice of the in situ cells. There are two temperature cells
available commercially that are designed to comply with Raman microscopes for
the purpose of catalyst surface investigation. The Linkam cell, although designed to
reach a temperature of 1000 °C, is equipped with a ceramic cup that glows at high
temperature, disturbing the weak Raman signal. The Harrick is made entirely of
steel, but allows a temperature no higher than 550 °C. Both are equipped with
heating and gas supplying systems.
In fact, both IR and Raman spectroscopy are perfect choices for the in situ
molecular characterisation of catalysts, as their selection rules depend on the
molecular structure of the studied material, and they can provide real-time information on its changes. Additionally, Raman is entirely complementary to the IR
method. This is not only because of the opposite selection rules, but also because of
the specific absorption of typical materials used in heterogeneous catalysis. IR is
especially suitable for the analysis of the adsorbed probe molecules as they absorb
light above 1000 cm
−1 , and while the typical support oxides such as Al 2 O 3 or SiO 2
do not give any signals, Raman spectroscopy compensates for this. In Raman
spectroscopy, the majority of the support oxides do not scatter light above
100 cm
−1 , which gives perfect circumstances for the analysis of the majority of the
metal oxides used in catalysis and which give Raman signals, typically in the range
200–1200 cm
−1 . An important advantage of Raman spectroscopy over IR is that
water, a ubiquitous molecule in a great many reactions, is practically invisible in
Raman. However, we have to take into account that the structure of a working
catalyst exposed to high temperatures is different from that which occurs under
ambient conditions in which water molecules are incorporated to the structure of the
metal oxides both of the catalyst and the support materials. This is another
important reason why in situ methods are the only adequate approach for assessing
catalyst structure under working conditions.
Amongst the catalytic systems most intensely studied by Raman spectroscopy
are bulk metal oxides and supported metal oxides. There are also examples of the
application of Raman spectroscopy to zeolite and metallic catalysts. The bulk metal
oxides can be studied in terms of the existence of different crystalline phases, their
transformation, and the extent of their crystallisation. If aided with a microscope,
11 In Situ and Operando Techniques in Catalyst Characterisation …
347