Nanoscale composition inhomogeneity in silica-aluminas prepared by various methods, страница 5

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