Nanoscale composition inhomogeneity in silica-aluminas prepared by various methods, страница 3

accuracy of composition measurements (see [10]).

This allowed the use of the Cliff-Lorimer formula

for calculating the Si/Al weight concentration ratio

based on the simultaneously registered X-ray K

intensities for Al and Si:


CAl à





; (1)

where CSi and CAl are the weight concentrations of

the Si and Al elements in the volume excited by

the electron beam (see Fig. 1), ISi and IAl are the

intensities, (i.e. the integral counts of the respective

Fig. 1. Schematic representation of the beam±specimen±detector

geometry during microanalysis, corresponding to a single particle

(see the text for the discussion concerning the geometrical values).

C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97 87

peaks obtained after subtraction of the background

counts) of the respective X-ray K
 lines emitted by the

Si and Al atoms registered simultaneously, whereas

KAlSi is a speciÆc parameter characterising the spectrometer.

Fig. 1 shows the geometry used in our measurements.

We tried to fulÆl two conØicting conditions: to

explore the narrowest specimen surface area and

volume and, on the other hand, to get an acceptable

counting accumulation in a reasonable lapse of time.

The former condition requires a strongly focalised

electron beam, whereas the second one requires a

relatively high counting rate per investigated ``point''.

By working in the STEM-HR mode, using the smallest

condenser aperture which is recommended by the

manufacturer of the electron microscope for getting

the highest attainable STEM image resolution, these

conditions were satisfactorily matched when the accumulation

time was 300 s for all analysed points, except

for two specimens where the time was 600 s per point.

During that time the electron beam was in a Æxed

position under the control of the scanning device. The

particles subjected to analysis, as well as the explored

``points'', were selected in a random manner.

Table 1

Chemical composition data of comparable samples belonging to the two series of sol±gel silica±aluminas subjected to the microanalytical

nanoscale investigation by EDS X-ray spectroscopy

Sample name ([1]) Laboratory prepared samples Industrially prepared samples

Global chemical composition

(as established by laboratory





Global chemical

composition (as indicated

by the producer)




Percent Global Si/Al

weight ratio a

Percent Global Si/Al

weight ratioa

SA-95 4.5% Al2O3 18.74 Fig. 1(a) 6.5% Al2O3 12.70 Fig. 1(b)

95.5% SiO2 93.5% SiO2

SA-90 10.8% Al2O3 7.29 Fig. 1(c) 10% Al2O3 7.95 Fig. 1(d)

89.2% SiO2 90% SiO2

SA-85 14.8% Al2O3 5.08 Fig. 2(a) 12% Al2O2 6.48 Fig. 2(b)

85.2% SiO2 88% SiO2

13.3% Al2O3 5.76 Fig. 5(a)

86.7% SiO2


13.3% Al2O3 5.76 Fig. 5(b)

86.7% SiO2


SA-75 26.1% Al2O3 2.501 Fig. 2(c) 24±26% Al2O3 2.65e Fig. 2(d)

73.9% SiO2 76±74% SiO2


SA-50 50.6% Al2O3 0.862 Fig. 3(d)

49.4% SiO2

SA-40 62.2% Al2O3 0.536 Fig. 3(a) 60% Al2O3 0.588 Fig. 3(b)

37.8% SiO2 40% SiO2

SA-30 70.5% Al2O3 0.369 Fig. 3(c)

29.5% SiO2

SA-15 86.8% Al2O3 0.134 Fig. 4(a) 85% Al2O3 0.1558 Fig. 4(b)

13.2% SiO2 15% SiO2

a These values are marked in the graphical representations by (A).

b First delivery.

c Second delivery.

d The pro-ducer did not indicate a precise value for the chemical global composition.

e This value is calculated by considering an average composition of 25% Al2O3á75% SiO2

88 C. SaÃrbu, B. Delmon / Applied Catalysis A: General 185 (1999) 85±97

As shown in Fig. 1, because of the unavoidable drift

effect occurring always in the electron microscope

during long-lasting observation of an object, the electron

beam normally sweeps an area larger than the

electron beam cross-section during each ``point'' analysis.

The extension of this area depends on the beam

diameter and the drift length.We tried to minimise the

drift effect by keeping for some time a larger area of

the observed grid hole under the action of the unfocussed

electron beam, until the drift rate diminished

and some stabilisation was observed, and starting the