Catalysis : an Integrated Approach to Homogeneous ...

10 - CATALYST CHARACTERIZATION WITH SPECTROSCOPIC TECHNIQUES
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Rh
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Fig. 10.5. TPR/TPO of silica-supported Rh, Fe, and Fe-Rh catalysts. The left TPR curves have been
measured from freshly impregnated catalysts, the right ones after complete oxidation in the TPO
experiment (from van't Blik and Niemantsverdriet [14]).
chloride. Note the difference in reduction temperature between the noble metal
rhodium and the non-noble metal iron. The bimetallic combination reduces
largely in the same temperature range as the rhodium catalyst does, indicating that
rhodium catalyzes the reduction of the iron. This forms evidence that rhodium and
iron are well mixed in the fresh catalyst. The TPR patterns of the freshly prepared
catalysts consist of two peaks, one coincides with that of the TPR pattern of the
fully oxidized catalyst (right panel of Fig. 10.5) and can thus be assigned to the
reduction of the metal oxide. The remaining peak of the first TPR is assigned to
the reduction of the metal salt precursors, iron nitrate and rhodium chloride.
The area under a TPR or TPO curve represents the total hydrogen consump-
tion, and is commonly expressed in terms of moles of H2 consumed per mol of
metal atoms, H*/M. The ratios of almost 1.5 for rhodium in Fig. 10.5 indicate that
rhodium was present in the form of Rh3'. For iron, the H2/M ratios are signifi-
cantly lower, indicating that this metal is only partially reduced.
TPR provides a quick characterization of metallic catalysts. It gives informa-
tion on the phases present after impregnation and on the eventual degree of
reduction. For bimetallic catalysts, TPR patterns often indicate if the two compo-
nents are mixed or not. A more detailed description of TPR and TPS is given in
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