operando

Introduction
The characterisation of catalyst in reaction condition is widely used at laboratory scale and
provides useful information that generally leads to the proposition of reaction mechanism.
Infrared spectroscopy is very powerful due to the high sensibility to adsorbed species detection.
Both transmission and DRIFT modes are involved and generally the specific geometry of the
sample in transmission mode (wafer of catalyst) appears far from ideal plug flow reactor
conditions. However few studies deal with the optimisation of this category of reactors [1]. The
present study combines response time distribution measurements and modelling of different
configuration of the geometry of the reactor. In particular, two types of reactors were compared
with two geometries: a circular pellet and a square pellet with specific sample holder.
Experimental
The -alumina obtained by sol–gel method using alkoxide precursors was used for the pellet.
The catalyst was loaded in the IR cell and connected to an automatic setup for the realisation of
a transient pulse of N2O in helium during 2 seconds at 25°C, 10 mL/min and atmospheric
pressure. Quadruple mass spectrometer was connected at the entrance of the cell or at the exit
of the cell.
Modelling of the cell was performed by means of Comsol software, version 3.5a with Chemical
Engineering Module, and version 4.0, using 3D geometry. Gas flow was treated first by steadystate incompressible Navier-Stokes equations, followed by transient modelling of convection
and diffusion.
Results
Two configuration of the sample holder were used in this study, Figure 1. The circular
geometry is similar to other operando
reactors using FTIR transmission
spectroscopy described in the
literature [2]. A cylindrical body
made of stainless steel maintained a
cylindrical wafer of catalyst, 16 mm
of diameter, about 40-60 μm of
thickness. Because of the symmetry
of the reactor, only ¼ of the actual
cell was represented. Alternately, we
developed a square geometry for the
Figure 1: Two configurations of sample holder and meshing:
sample holder. In this case, the selfCircular and square geometry
supported sample was square wafer
Circular geometry
0,030
Square geometry
0,03
Inlet
Inlet
0,025
Experimental
outlet
0,020
Theoretical
outlet
0,015
0,010
0,005
N2O concentration (mol/m3)
A. Carias Henriquez, S. Pietrzyk, F. Dhainaut, C.Dujardin*
Université Lille Nord de France, UCCS UMR CNRS 8181, 59650 Villeneuve d’Ascq, France
*christophe.dujardin@univ-lille1.fr
of catalyst with 12 mm edge. Because of the symmetry of the reactor, ½ of the wafer actual cell
was represented for the modelling. The Reynolds number was calculated to determine the
character of flow in the cell, by taking into account the places where the gas velocities were
highest (entry and exit holes). These values are lower than 1, therefore the consideration of
laminar flow remains valid for both geometries. Profiles of gas velocity obtained from
modelling with module CFD clearly discriminated both geometries. The distribution of gas
velocity near the circular wafer suggested that the construction of the circular reactor is far from
ideal. By contrast, this deviation was corrected when a square geometry was used.
The transient pulses of N2O was modelled and compared to the experimental signals (Figure 2).
As observed, a good agreement between experimental signals and theoretical signals was
obtained and was related with the volume of the sample holder, geometry and dispersion of the
gas into the cell. The existence of dead volumes was highlighted by the observation of tailing
peak in the experimental and modelled curves. This effect was a little bit more marked in the
circular geometry, which can be related to the presence of complex hydrodynamic phenomena.
The relative importance of dead volumes was also evaluated and suggested that the inlet and
outlet of square geometry could be improved. Further modelling of residence time distribution
would be discussed in order to qualify both geometries of the sample holder. Finally, the
consequences of the geometry to the amount of adsorbed species would be examined for further
applications in operando IR spectroscopy.
N2O concentration (mol/m3)
How to optimise operando reactors? Modelling of IR cell devoted to
in situ and operando characterisation of catalyst
0,025
Experimental
outlet
Theoretical
outlet
0,02
0,015
0,01
0,005
0,000
0
0
20
40
60
Time (s)
80
100
120
0
20
40
60
Time (s)
80
100
120
Figure 2: Modelling of transient pulse for two configurations of sample holder: Circular wafer
and square geometry
Conclusion
The design of operando reactors is a key point for further development of infrared spectroscopy
to kinetic measurements. The comparison of two geometries clearly underlines that a deviation
of gas velocity is evidenced near the catalyst surface when classical circular geometry is used.
The new design with square geometry leads to a significant improvement with homogeneous
gas velocity for example.
References
1. S. Pietrzyk A. Y. Khodakov, C. Dujardin, COMSOL conferences (2007) Grenoble
2. T. Lesage et al., Top. Catal. 30–31 (2004) 31.