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Λ M°from # 1 M(A) H2 , M(A) reacts to form M°
^y
/««(A)! / M(B) M,c
-J -«
\
V
**
CO M(B) reacts
J
Binding energy
M(o
^ M° from M(B)
M(C) V /
Figure 2. Protocol for study of heterogeneous catalysts by reactive gas treatment combined with ESCA binding energies so that their compos ite ESCA spectrum is a broadened doublet, as shown on the left side of Figure 2. Theoretically it is possible to separate such species by curve resolu tion. However, our philosophy has been that measurement is superior to number crunching. Our protocol uses selective reactivity to produce changes in the spectra that can be measured and correlated with particular species. For example, in Figure 2 assume that M(A) reacts with hydrogen [M(A) + H 2 -* M°] but that M(B) and M(C) do not. After treatment of the catalyst with hydrogen, the spectrum in the upper right would result. The part of the ESCA spectrum due to the metal can be separated from the "oxidized" part of the spectrum, and the relative intensity of the metal peak can be measured. Similarly, if M(B) reacts with CO [M(B) + CO -> M°] but nei ther M(A) nor M(C) reacts, the rela tive intensity of the metal peak can be used to approximate M(B). This is shown in the lower right of Figure 2. Because M(C) does not react with ei ther H2 or CO, it can be calculated by difference. The above methodology is based on the assumption that the inte grated area from the metal peak pro duced by a treatment is equivalent to the fraction of a species initially present. Below we will examine the va
lidity of such an assumption and how we can compensate when this criterion is not met. Effect of sintering If an oxidic catalyst shows no change in dispersion on reduction or other chemical treatment, relatively simple data treatment can be applied. One can select an ESCA peak charac teristic of each catalyst species, and the fraction of the integrated area due to a species will correspond to its atomic fraction on the catalyst sur face. However, a common problem in chemical treatment of catalysts (par ticularly H 2 reductions) is sintering— the growth of particles on treatment. Assume that an oxidic catalyst con taining two types of particles initially of equal size is treated with hydrogen. Assume that one species does not re act, whereas the other reacts to pro duce metal particles much larger than the size of their precursors. Equal numbers of the two equal-sized parti cles will contribute equally to the ESCA intensity before reduction. Af ter reduction, however, calculations based on the intensity from the large metal particles will underestimate the amount of metal produced because of the smaller per atom contribution to ESCA intensity of the larger (i.e., sin tered) particles. Table I shows how
Table 1. ESCA intensity ratios for Ni/Al 2 0 3 catalysts
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Intensity ratio Ni/AI Catalyst 5% 7% 9% 11%
Ni0/AI 2 0 3 Ni0/AI 2 0 3 Ni0/AI 2 0 3 Ni0/AI 2 0 3
CIRCLE 230 ON READER SERVICE CARD 1184 A · ANALYTICAL CHEMISTRY, VOL. 5 8 , NO. 12, OCTOBER 1986
Oxidic
Reduced
0.15 0.18 0.22 0.28
0.10 0.11 0.12 0.12
