Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

Iron Based Catalysts 39
and MgO (Di Felice,2011). Alternatively calcination can be applied in air at 700°C for 1 hour (Azhar
Uddin,2008).
Calcinated Scallop Shell:
An interesting alternative that was studied (Guan,2012) involved the wet impregnation of 2.5 wt %
iron on calcinated scallop shell, a material with porous structure that can recycled and is appropriate
for tar decomposition via steam reforming, as seen in
Figure 16 . Catalytic activity was observed only after a period of reactions, which indicates that in
the beginning the metal oxides were reduced to their metallic forms by the syngas, which was initially
produced from the pyrolysis of biomass. The catalysts obtained at 800 °C calcination temperature
were chosen for the steam reforming of tar, as they exhibited the best results.
Figure 16: Mechanism of decomposition of tar on a calcinated scallop shell
(Guan,2012)
4.9 Thermal Procedures for Catalyst Characterization
Analyses performed before the use of the catalysts:
Temperature Programmed Desorption (TPD) is a method to describe the energy of sorption, as
in the desorbed molecules from a surface when the surface temperature is increased. Usually catalysts
are subjected to thermal procedures prior to their use. When the iron catalysts comprise calcium
or magnesium oxides they can easily absorb steam and CO2 from air, as molecules come in
contact with a surface, minimizing their energy by forming a chemical bond with the surface. Therefore,
during thermal treatments it is likely that they desorb these compounds causing error in the
data that is about to be obtained. Hence, TPD has to be applied in order to remove the presorbed
CO2 and H2O from their surface (Di Felice,2011). The apparatus used for this procedure is usually
a thermal gravimetric apparatus. A high flow of inert gas, usually Helium (total flow 50ml/min), is
established over the shallow bed of sample and the temperature is increased at fixed rate. Typical,
rates are 15°C/min or 15°C/min and the sample is heated up to 900-1000° C. Then the sample is
cooled down and the procedure is being repeated for several times at different programming rates.
The amount of gas desorbed is recorded versus the sample temperature and the characterization
of the strength of the sorbent-surface bond is also possible (Polychronopoulou,2006),
(White,1990).
Temperature-programmed reduction (TPR) is a technique for the characterization of solid materials
and is often used in the field of heterogeneous catalysis to find the most efficient reduction
conditions. According to the procedure followed by Polychronopoulou et al. (Polychronopoulou,2006)
the initially oxidized catalyst sample is placed in a container (U-tube) and is submitted to
a programmed temperature rise ( 30 °C /min) while a reducing gas mixture is flows over it. This
sample vessel is positioned in a furnace with temperature control equipment. To remove the present
air, the container is filled with an inert gas (nitrogen, argon or helium). Flow controllers are
used to add hydrogen (2 vol% H2/He with a flow rate 50 NmL/min). The composition of the gaseous
mixture is measured at the exit of the sample container with appropriate detectors (thermal conductivity
detector, mass spectrometer) so the quantification and determination of H2 consumption is
possible. The sample in the oven is heated up to 900°C. If a reduction takes place at a certain
temperature, hydrogen is consumed. The most accurate way of measuring the reduction is by
measuring the vapor content in the outlet of the catalyst. This is due to the fact that in the inlet concentration
of water is zero, so any increase can be measured more accurately.