laboratory prepared specimens, (a), (c) and (d), are compared with one industrial specimen (b). For A and B see Fig. 1 and text.
92 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97
Fig. 6. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in specimens with a high content of alumina. For A and B see
Fig. 1 and text.
Fig. 7. Nanoscale fluctuations of the Si/Al weight concentration ratio measured in two industrially prepared specimens with the same content
of alumina, in principle, but corresponding to different batches.
C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 93
Table 2
Chemical impurities detected by EDS X-ray spectrometry in the analysed samples
Sample name Laboratory prepared samples Industrially prepared samples
Global chemical
composition (%)
Detected
impurities
Corresponding graphical
representation
Global chemical composition
(as indicated by the producer)
Detected
impurities
Corresponding graphical
representation
SA-95 4.5% Al2O3
95.5% SiO2
Important traces
of S, Ca, K, Na, Cl
Fig. 1(a) 6.5% Al2O3
93.5% SiO2
None Fig. 1(b)
SA-90 10.8% Al2O3
89.2% SiO2
Important traces of
S, Na, Ca, K, Cl
Fig. 1(c) 10% Al2O3
90% SiO2
None Fig. 1(d)
SA-85 14.8% Al2O3
85.2% SiO2
Faint traces of S, Ca Fig. 2(a) 12% Al2O3
88% SiO2
Large quantity
of S; traces of Na
Fig. 2(b)
13.3% Al2O3 None Fig. 5(a) first delivery
86.7% SiO2
13.3% Al2O3 None Fig. 5(b) second delivery
86.7% SiO2
SA-75 26.1% Al2O3
73.9% SiO2
None Fig. 2(c) 24-26% Al2O3
76-74% SiO2
None Fig. 2(d)
SA-50 50.6% Al2O3
49.4% SiO2
Large quantity of
Na; traces of S,
K, Ca, Cl and P
Fig. 3(d)
SA-40 62.2% Al2O3 None Fig. 3(a) 60% Al2O3 None Fig. 3(b)
37.8% SiO2 40% SiO2
SA-30 70.5% Al2O3 None Fig. 3(c)
29.5% SiO2
SA-15 86.8% Al2O3
13.2% SiO2
None Fig. 4(a) 85% Al2O3
15% SiO2
Faint traces of S,
Na,Cl, K, Ca
Fig. 4(b)
94 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97
the data concerning the laboratory samples are more
numerous.
The zig-zagged curves in the Ægures have been
traced only for displaying more clearly the generally
high, and sometimes considerable local variation of
the Si/Al ratio measured at the nanoscale level. No
other signiÆcance should be attributed to these curves.
The errors affecting the measured values of the Si/
Al weight concentration ratio, (i.e. the values of the
CSi/CAl ratio) were calculated by the usual statistical
approach. It is generally admitted [14,15] and experimentally
veriÆed [16] that the counting of X-ray
photons emitted under excitation by any means
(including electron beam bombardment) obeys a
Gaussian distribution with a standard deviation
equal to the square root of the accumulated counts
at each analysis point. Therefore, the standard deviation
for each of the Al and Si peaks is calculated as the
square root of the peak integral à ÅÅI p where I is the
counted peak integral after background subtraction. In
our case, only a single measurement can be made for
each analysed ``point'', because it is impossible to
position again the electron beam on exactly the same
``point''. The statistical signiÆcance of calculated as
described above is that there is a 99.73% chance that
the measured value of the peak integral be within the
3 interval centred around the true integral value of
the registered peak.
According to Cliff-Lorimer Eq. (1), the Ænal relative
error of the CSi/CAl ratio, (i.e. the Si/Al weight
concentration ratio) measured on each analysed point
is the sum of the relative errors of IAl, ISi and KAlSi.
Our graphical representations show the error bars for
the Si/Al ratio calculated using the relative error of
the peak integral determination.
The meaning of the horizontal lines marked (A) and
(B) on all graphical representations is as follows. A is
the value of the Si/Al weight concentration ratio
calculated on the basis of the global silica/alumina
chemical content. This global chemical content is
accurately known for the laboratory prepared samples
and less accurately (sometimes only as a concentration
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