Catalysis : an Integrated Approach to Homogeneous ...

370 10 - CATALYST CHARACTERIZATION W m SPECTROSCOPIC TECHNIQUES
nanometer resolution. A STEM instrument combines the two modes of electron
microscopy.
As illustrated by Fig. 10.4, an electron microscope offers additional possi-
bilities for analyzing the sample. Diffraction patterns (spots from a single-crystal
particle and rings from a collection of randomly oriented particles) enable one to
identify crystallographic phases as in XRD. Emitted X-rays are characteristic for
an element and allow for a determination of the chemical composition of a
selected part of the sample (typical dimension 10 nm). This technique is called
electron microprobe analysis (EMA, EPMA) or, referring to the usual mode of
detection, energy dispersive analysis of X-rays (EDAX or EDX). Also the Auger
electrons carry information on sample composition, as do the loss electrons. The
latter are potentially informative on the low Z elements, which have a low
efficiency for X-ray fluorescence.
For studying supported catalysts, TEM is the commonly applied form of
electron microscopy. Detection of supported particles is possible provided that
there is sufficient contrast between particles and support. This may impede
applications of TEM on supported oxides. Determination of particle sizes or of
distributions therein rests on the assumption that the size of the imaged particle
is truly proportional to the size of the actual particle and that the detection
probability is the same for all particles, independent of their dimensions. Semi in
situ studies of catalysts are possible by coupling the instrument to an external
reactor. After evacuation of the reactor, the catalyst can be transferred directly
into the analysis position, without seeing air [12]. For reviews of TEM on catalysts
we refer to refs. [11,12].
10.2.3 Temperature Programmed Techniques
Temperature programmed reduction (TPR), oxidation (TPO), desorption
(TPD), sulphidation (TI'S) and reaction spectroscopy (TPRS) form a class of
techniques in which a chemical reaction is monitored while the temperature
increases linearly in time. These techniques are applicable to real catalysts and
single crystals and have the advantage that they are experimentally simple and
inexpensive in comparison to many other spectroscopies. Interpretation on a
qualitative base is rather straightforward [13].
The basic set up for TPR, TPO and TPD consists of a reactor and a thermal
conductivity detector to measure the hydrogen content in TPR/TPD or the
oxygen content in TPO, of the gas mixture before and after reaction. More
sophisticated TP equipment contains a mass spectrometer for the detection of
reaction products.
TPR and TPO patterns of silica-supported rhodium, iron, and iron-rhodium
catalysts are shown in Fig. 10.5 [14]. These catalysts were prepared by pore
volume impregnation from aqueous solutions of iron nitrate and rhodium