"Multi-Technique Problem Solving in an Industrial
Environment"
Shell, Thornton Laboratories, Chester-
Wednesday 6th of January 1999
"TEM/XPS Characterisation of Industrial
Catalysts"
A. Knoester, R. Haswell, P.M. Nuyten and G.C. Smith, Shell
Research & Technology Centre, Amsterdam
In the Petrochemical industry, heterogeneous catalytic processes play
an important role. Hence, in industrial laboratories much effort is put in the
development/improvement of catalysts with respect to their activity, selectivity,
lifetime, resistance to contamination, etc. In this context, analytical characterisation
of catalysts, both as synthesised and after use, is of prime importance. In the Shell
Research and Technology Centre Amsterdam, XPS and TEM, amongst other techniques, are often
used in a very complimentary way to unravel the morphology of complex multi-component
systems.
|
|
Figure 6: Possible Models of
A+B+Catalyst |
|
| |
|
In principle, XPS may directly reveal which components (i.e. active
species, promoters, support material) of a catalyst are accessible to the reactants. The
XPS signals can be converted to (average) particle sizes and coverage of the support
material. In practice, the interpretation of the signals is often more difficult, however.
The catalyst components may, for example, have a bimodal or a multi-modal size
distribution and/or be (partly) present in the form of alloys and/or have some other form
of intimate contact (e.g. particles of component A can be present on top of particles B or
vice versa). As a consequence, the final interpretation will often depend on the
morphological model which is expected to be the most relevant one.
| |
|
| |
Figure 7: TEM/EDX Analysis of a commercial
catalyst |
| |
|
Analytical TEM may provide direct bulk information on size
distributions and corresponding weight fractions of the components. In addition, local
TEM/EDX measurements may tell which components are in contact with each other. However,
except in exceptional situations, TEM cannot provide information on the nature of the
contact.
 |
|
| Figure 8: Commercial catalyst: How to
sub-divide the XPS Signals? |
|
| |
|
Combining TEM and XPS results, more detailed conclusions can be drawn.
To this end, use is made of home-developed software (Topol1) discriminating between different
morphological models, which may occur in catalysts consisting of one support material and
up to two other phases. Recently, this software package has been upgraded by including Cumpson
electron attenuation lengths in addition to the Seah and Dench factors which were used up
to know.
| |
 |
| |
Figure 9: A Final Model of a Commercial
Catalyst using Topol, TEM and XPS |
| |
|
Also, the programme has become available in the form of an Excel
worksheet. This multi-technique approach will be illustrated with results which were
recently obtained on a commercial Shell catalyst.
H.P.C.E. Kuipers et al, "The
Characterisation of Heterogeneous Catalysts by XPS Based on Geometrical Probability",
Surface and Interface Analysis, Vol. 8, 235-242 (1986).
|
