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Olefin metathesis catalyst. Part I. Angle-resolved and depth profiling XPS study of tungsten oxide on silica
JOURNAL OF
MOLECULAR CATALYSIS Journal of Molecular Catalysis 90 ( 1994) 43-52
Olefin metathesis catalyst. Part I. Angle-resolved and depth profiling XPS study of tungsten oxide on silica F. Verpoort”,“, L. Fiermansb, A.R. Bossuyf,
L. Verdonck”
‘Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281. B-9000 Ghent, Belgium ‘Department ofSolid State Sciences, Surface Physics Division, Ghent University, Krijgslaan 281, B-9000 Ghent, Belgium
Abstract Model supports consisting of a thin layer of SiOz on a silicon single crystal have been used to study the WO,/SiO,/Si ( 100) catalyst precursor. Compared to the powder analogues, a drastic increase in spectral resolution and detailed band structure is observed in the XPS spectra. Conventional XPS, angle dependent and depth profiling X-ray photoelectron spectroscopy show the presence of two types of tungsten oxide on the surface: microcrystallites of W03 and a tungsten oxide monolayer chemically bonded to the silica matrix. Key words: depth profiling; silica; surface characterization; tungsten; XPS
1. Introduction X-ray photoelectron depth
profiling
spectroscopy (XPS) combined with angle dependent (AR) and measurements is a powerful tool for surface characterization of heteroge-
neous catalytic systems. However, XPS spectra of insulating materials, e.g. W03/Si02, suffer from signal distortion and broadening with loss of resolution due to inhomogeneous sample charging. This prevents a straightforward analysis of the signal envelopes whereby much of the hidden information is not available for interpretation. Several authors [ 1,2] have found that detrimental charging can be avoided by using a model support consisting of a silicon or aluminium crystal covered by an oxide layer typically a few nanometers thick. The active phase is usually deposited onto the support by evaporation, or by the decomposition of suitable metal carbonyls [ 21, *Corresponding author: fax. ( + 32.9)2644983. 03045102/94/$07.00 SSDIO304-5102(93)
0 1994 Elsevier Science B.V. All rights reserved E0302-W
44
F. Verpoort et al. /Journal
ofMolecular Catalysis
90 (1994) 43-52
In this study we use SiO*/Si ( 100) model supports [ 31 for the preparation of a WOs/ SiO,/Si( 100) precursor by way of incipient wetness impregnation from an aqueous solution of ammonium tungstate. Heating in air at 600°C forms the desired tungsten oxide. The conducting model support offers attractive advantages over the porous powder sample. Inhomogeneous sample charging is avoided producing spectra of much better resolution and reliable peak shape. Furthermore, angle-resolved XPS becomes meaningful since the direction perpendicular to the surface is defined. In this paper we report on the characterization of the W03/Si0, precursor by general, AR- and depth profiling XPS.
2. Experimental
2.1. Preparation
A 9 wt.% W03/Si0, precursor has been prepared by pore volume impregnation of 3 grams of silica (Polypor surface area 345 m’/g) with 25 ml ammonium tungstate (Aldrich) solution (0.6 g ( NH4)2W04 in 25 ml H20 and heating until the (NH,) 2WO4 is dissolved [ 41) . After the impregnation the sample is dried at 100°C and subsequently calcined in air at 600°C. The model support, a Si( 100) single crystal ( 10 X 10 X 1 mm), was oxidized at 500°C in air for 11 h. The SiO,/Si( 100) surface is impregnated by a drop of an aqueous solution of ammonium tungstate (0.0376 g/l). Drying the sample in an evacuated desiccator followed by calcination at 600°C results in a heavy loading of the edges while the center, which was the region chosen for study, shows a uniform distribution.
2.2. Spectroscopic
techniques
XPS spectra were obtained with a Perkin Elmer PHI 5500 ESCA spectrometer equipped with a monochromatized Al KQ source, a hemispherical analyzer connected to a multichannel detector and a manipulator which enables the variation of the take-off angle of the photoelectrons between 0 and 90”. XPS spectra of the model support and the model precursor were measured with 11.75 eV pass energy and those of the powder precursor, immobilised on an adhesive tape, at 48.50 eV pass energy. Charge correction for the powder samples is related to the C 1s peak at 284.6 eV. The spectra were accumulated, smoothed, and integrated using the PHI-ACCESS program. Constraints suited to impose the spin doublet character of the line pairs, e.g., ratios of linewidths and intensities, binding energy splitting, had to be implemented by the user. XPS depth profiling was performed in the same apparatus. The surface was bombarded with 4 keV Art ions and the argon beam was rastered over an area of 3 X 3 mm around the point of impact of the ion beam.
F. Verpoort et al. /Journal
45
of Molecular Catalysis 90 (1994)43-52
3. Results and discussion 3. I. Characterization
of the SiOz /Si( 100) model support
The Si 2p XPS spectrum of the model support is shown in Fig. 1. The experimental data are given in Table 1. Two peaks are clearly observed, one at 102.80 eV, originating from SiOZ [ 51, and one at 98.8 1 eV due to the silicon substrate. Both peaks are actually unresolved 2p,,,-2p,,, doublets. The known energy separation (eV) and intensity ratio between the Si 2p,,, and the Si 2p, ,2 are introduced in the curve fitting program producing the binding energy data in Table 1. Niemantsverdriet et al. [ 61 proved that the thin layer of SiO, on Si( 100) is an acceptable model for high surface area Si02 if the model support is oxidized at elevated temperatures. Fig. 2 shows the Si4+/Sio intensity as a function of the take-off angle, relative to the surface normal, (full line) for the angle-resolved XPS spectra of the SiO*/Si ( 100) model support after oxidation for 11 h at 500°C in air.
105
104
103
102
101
100
Binding
Energy
Fig. 1. XPS spectrum of the Si 2p region measured on the SiO,/Si( Table 1 XPS data of the SiO,/Si(
100) model support after calibration
99 (eV)
100) model support.
on C 1s ( Eb = 284.6 eV)
Element
Binding energy (eV)
FWHM (eV)
Assignment
c
1s
284.60
1.45
contamination
0 1s
532.30
1.62
O*- (SiO,)
Si ~PW 2P,/, 2P,,, 2P,/,
98.5 1 99.12 102.49 103.10
0.55 0.60 1.59 1.27
Si” (Silicon) Si’ (Silicon) Si4+ ( SiO,) Si4+ ( SiO,)
99
F. Verpoort
et al. /Journal
of Molecular
Catalysis
90 (1994) 43-52
!
I
6.o
, 0
20
40
Fig. 2. Angle-resolved (----).
Element
model catalyst
angle
XPS data of the SiO,/.Si( 100) model support together with theoretical,
Table 2 XPS data of the powder and model catalyst after calibration
powder catalyst
SO
60
take-off
on C
fitted profile!
1s (Eb= 284.6 eV)
Binding energy (eV)
FWHM (eV)
Assignment
c 1s
284.60
0 Is Si 2p w 4f
532.90 103.82 37.80
1.45 2.25 2.25 4.00
C of contamination O’- of SiO, Si”+of SiO z Wh+
c 1s 0 Is
284.60 534.00
1.71 1.44
C of contamination O”- adsorption by
Si 2~,,, 2P,/, 2P,,, 2P,/, W 4f,,* 4f,,2 4f,,, 4f,,*
532.88 53 1.27 102.93 103.54 98.35 98.96 36.79 38.90 35.84 37.95
1.43 1.58 1.36 1.38 0.52 0.58
1.87 1.60 1.54 I .60
H,O Ozm of SiO, 02- of WO, crystal Si4 + Si4+ SiO Si’ W6+Of-OW0 * W~+Of-oWOZ Wh+ of W0.t crys2tal w6’ of WO, crystal
F. Verpoort et al. /Journal
of Molecular Catalysis 90 (1994) 43-52
47
This compares well with the fitted theoretical curves (dashed lines) calculated ford = 3.2 and d = 3.4 nm using the following relation and assuming a homogeneous layer: zSiO*
-=
onS,02hSiOz(
1 - e-d’As’02cosO) -dlAsto2COSe
ISi
mSihSi
e
with Zsio:, and Zsi the XPS intensities of the Si 2p peaks for the SiOz and Si components, nSio, (0.036 mol/cm3) and ylsi (0.083 mol/cm3) the atomic densities, (Tthe cross section, equal for Si4+ and SiO, hsi (31.4 A) and hsio, (37 A) the inelastic mean free paths of electrons at the prevalent kinetic energy through Si and SiO*, respectively, with values taken from Tanuma et al. [ 71, 8 the take-off angle with respect to the surface normal and d the thickness of the SiOz layer. Recalculation with d = 3.3 nm produces a curve which agrees very well with the measured values. We can conclude that, in our experiment, the surface of the SiO,/Si( 100) model support is covered by a 3.3 nm SiOz layer following oxidation in air at 500°C for 11 h. 3.2. W03 /SiO, powder precursor In Fig. 3a,b the XPS spectra of the 0 1s and W 4f region of the WOJSi02 precursor are visualized. The XPS data are summarized in Table 2. The broad peaks in the spectra of the powder precursor exhibiting a charge shift of 5.10 eV have been charge corrected by using the C 1s peak at 284.6 eV as an internal reference [ 81. The two components of the W 4f doublet are not resolved due to inhomogeneous charge broadening (Fig. 3c), which results in line widths of the order of 4.00 eV. If the W 4f signal is fitted with a doublet using fixed 4f,,, : 4f,,, intensity ratio of 0.78 ( * 0.03) and an Eb splitting of 2.1 (kO.1) eV [9], a W 4f,,, binding energy of 36.66 eV is obtained, which is in agreement with the presence of WO, [ lo]. 3.3. W03 /SiO, /Si (ZOO) model precursor The spectra of the model precursor obtained using the same instrument settings, show a much better resolution (Fig. 3c,d). The XPS spectrum of 0 1s electrons (Fig. 3b) characterized by a weak asymmetry indicating multiple band structure, has a maximum at 532.88 eV. This is in agreement with the 0 1s electron binding energy for Si02 [ 1 l-141 since the oxygen atoms in the silica matrix produce the basic contribution to the spectrum. The 0 1s band was resolved and a satisfactory approximation of the signal envelope is obtained on superposition of a minimum of three bands: at 532.88 eV, having the maximum intensity (87.22% contribution), at 53 1.27 eV (5.79% contribution), and at 534.00 eV (6.99% contribution). The peak at 53 1.27 eV coincides with the 0 1s electron peak for free W03 [ 111. This band is assigned to W03 present on the surface of SiOz in the form of microcrystallites [ 151. The nature of this peak can be confirmed by angle-resolved XPS. Large take-off angles ( > 60”) favor the detection of real surface particles while at angles greater than 45”, the surface composition is obscured by the increasing importance of the in depth analysis of the matrix (Fig. 4). The 534.00 eV maximum, at higher binding energies, can be assigned to 0 1s oxygen electrons of adsorbed water [ 161.
48
F. Verpoort et al. /Journal of Molecular Catalysis 90 (1994) 43-52
The tungsten 4f doublet is readily resolved into its 712 and 5 /2 components. The linewidth of the W 4f of the model support is 1.54 eV compared to 4.00 eV in the spectrum of the powder precursor. Applying the Scofield W 4f,,, : W 4f,,, intensity ratio of 0.78, Eb splitting of 2.1 eV and Eb = 35.4 eV for W03 as constraints, curve fitting resolves the W signal envelope of the WO,/SiO,/Si ( 100) model support into two doublets (Fig. 3d). The low intensity doublet ( Eb W 4f,,, = 36.79 eV) is shifted to higher binding energies in comparisontoWO,andW0,2[17]. This band is tentatively assigned to the tungsten oxide linked by a chemical bond to the SiOz matrix.
10 9
Powder
precursor
(4
8 :
7
-8 =
5
532
531 Binding
530 Energy
529 (eV1
(b)
532
531 Binding
530 Energy
529 (eV1
F. Verpoort et al. /Journal of Molecular Catalysis 90 (1994) 43-52
9
Powder
49
(c)
precursor
%
31
36
35
Binding
Energy
36 IS+‘)
10 -
g :: * ::
Model
precursor
(4
y 1:: - 6:: = 5:: , :: 3 :: * ::
, :: 30
31
36 Binding
Fig. 3. XPS spectra of the 0 1s and the W 4f region of a calcined calcined WO,/SiO,/Si( 100) model precursor (c,d).
35 Energy
WOJSiO,
3h IcV)
powder precursor
(a,b)
and a
3.4. XPS depth projiling
By XPS depth profiling the relative concentration of the surface elements as a function of depth can be measured. In Fig. 5 the depth profile is shown for the WO,/SiO,/Si ( 100) model precursor. The simultaneous removal of W and 0 is consistent with the presence of poorly dispersed W03 particles which leave a considerable fraction of the support uncovered. The precursor shows no features characteristic for layered structures.
of h4olecular
F. Verpoort et al. /Journal
50
Catalysis
90 (1994)
/ i\ l!L,
43-52
f
f
1 \
I
\
ff
\
‘\,
/
, /’
01 536
535
‘-__
53L
533
532
Binding
Fig. 4. XPS spectra of the 0 1s region of a calcined WO,/SiO,/Si(
take-off angle, (-)
-_
Sk
GO Energy
529 (@VI
100) model precursor as a function of the
70”, (- - -) 30”.
0
1
2
3
4 Sputter
Fig. 5. XPS depth profile of the WO,/SiO,/Si(
(min)
100) model precursor.
Furthermore the XPS depth profiling data also confirm Eb = 53 1.27 eV to microcrystallites of W03.
3.5. Angle-resolved
5 Time
the assignment
of the peak with
XPS
The indications obtained from XPS depth profiles can be verified by take-off angle dependent XPS measurements. If a system has a layered structure, its XPS intensity should increase with respect to that of the substrate when the spectrum is recorded at grazing angles. Furthermore for a poorly dispersed phase the angle dependence will be less pronounced.
F. Verpoort et al. /Journal
of Molecular Catalysis 90 (1994) 43-52
51
W bt/Sib* 1.4 1.2 -
0.8 -
.
0.6 0.4 0.2 -
0
10
20
30
40
60
50 take
- off
70
80
90
angle
Fig. 6. W 4f/Si 2p XPS intensity ratios as a function of the take-off angle for a calcined WOJSiOJSi( model precursor.
100)
Fig. 6 clearly shows that the W 4f/Si”’ XPS intensity ratio for the precursor prepared by impregnation hardly depends on the take-off angle. This confirms the supposed low dispersion of the impregnation precursor. 4. Conclusion A flat, conducting model support consisting of a few nanometers of SiOZ on a Si ( 100) substrate, offers interesting possibilities for the application of XPS, and other surface techniques hampered by charging effects, for instance SIMS, AES, to study aspects of the surface chemistry involved in catalyst preparation. Electrical charging is almost completely avoided resulting in a spectacular improvement in spectral resolution. In addition the geometry of the support makes the application of angle-dependent XPS and sputter depth profiles meaningful. The results suggest the simultaneous presence of at least two types of tungsten oxide structures, microcrystallites of W03 and a small tungsten oxide monolayer on the surface of the W0,/Si02/Si (100) precursor.
Acknowledgements We thank T. Kuiper (Philips, Eindhoven) for providing Si( 100) crystals and the “Nationaal Fonds voor Wetenschappelijk Onderzoek” and the ’ ‘Onderzoeksfonds” of the Ghent University for financial support.
52
F. Verpoort et al. /Journal
ofMolecular Catalysis 90 (1994) 43-52
References [ 11 P.A. Spevack, S. McIntyre, Appl. Catal., 64 (1990) 191. [2] B.G. Frederick, G. Apai, T.N. Rhodin, J. Am. Chem. Sot., 109 ( 1987) 4797. [3] Samples provided by Philips, Eindhoven. [4] A.G. Basrur, S.R. Patwardhan, S.N. Vyas, J. Catal., 127 (1991) 86. [5] T.E. Madey, C.D. Wagner, A. Joshi, J. Electron Spectrosc., 10 (1977) 359. [6] A.M. de Jong, L.M. Eshelman, L.J. van IJzendoom, J.W. Niemantsverdriet, Surf. Interface Anal., 18 ( 1992) 412. [7] S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interface Anal., 11 ( 1988) 577. [8] W. Grtlnert, E.S. Shpiro, R. Feldhaus, K. Anders, G.V. Antoshin, Kh.M. Minachev, J. Catal., 107 (1987) 522. [9] J.H. Scofield, J. Electron Spectrosc. Relat. Phenom., 8 ( 1976) 129. [lo] P. Biloen, G.T. Pott, J. Catal., 30 ( 1973) 169. [ 111 G.B. Wills, J. Fathikalajahi, S.K. Gangwal, S. Tang, Reel. Trav. Chim. Pays-Bas, 96 ( 1977) Ml 10. [ 121 E. Gorlich, A. Stoch, J. Stoch, J. Solid State Chem., 33 (1980) 121. [ 131 A. Stoch, J. Stoch, Mater. Chem., 6 (1981) 335. [ 141 G. Hollinger, Y. Jugnett, P. Pertosa, et al., Chem. Phys. Lett., 36 (1975) 441. [ 151 L.L. Murrell, D.C. Grenoble, R.T.K. Baker, et al., J. Catal., 71 (1983) 203. [ 161 J.K. Gimzewski, B.D. Padalia, S. Frosman, et al,, Surf. Sci., 62 ( 1977) 386. [ 171 L. Salvati, L.E. Makowsky, J.M. Stencel, et al., J. Phys. Chem., 85 (1981) 3700.
MOLECULAR CATALYSIS Journal of Molecular Catalysis 90 ( 1994) 43-52
Olefin metathesis catalyst. Part I. Angle-resolved and depth profiling XPS study of tungsten oxide on silica F. Verpoort”,“, L. Fiermansb, A.R. Bossuyf,
L. Verdonck”
‘Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281. B-9000 Ghent, Belgium ‘Department ofSolid State Sciences, Surface Physics Division, Ghent University, Krijgslaan 281, B-9000 Ghent, Belgium
Abstract Model supports consisting of a thin layer of SiOz on a silicon single crystal have been used to study the WO,/SiO,/Si ( 100) catalyst precursor. Compared to the powder analogues, a drastic increase in spectral resolution and detailed band structure is observed in the XPS spectra. Conventional XPS, angle dependent and depth profiling X-ray photoelectron spectroscopy show the presence of two types of tungsten oxide on the surface: microcrystallites of W03 and a tungsten oxide monolayer chemically bonded to the silica matrix. Key words: depth profiling; silica; surface characterization; tungsten; XPS
1. Introduction X-ray photoelectron depth
profiling
spectroscopy (XPS) combined with angle dependent (AR) and measurements is a powerful tool for surface characterization of heteroge-
neous catalytic systems. However, XPS spectra of insulating materials, e.g. W03/Si02, suffer from signal distortion and broadening with loss of resolution due to inhomogeneous sample charging. This prevents a straightforward analysis of the signal envelopes whereby much of the hidden information is not available for interpretation. Several authors [ 1,2] have found that detrimental charging can be avoided by using a model support consisting of a silicon or aluminium crystal covered by an oxide layer typically a few nanometers thick. The active phase is usually deposited onto the support by evaporation, or by the decomposition of suitable metal carbonyls [ 21, *Corresponding author: fax. ( + 32.9)2644983. 03045102/94/$07.00 SSDIO304-5102(93)
0 1994 Elsevier Science B.V. All rights reserved E0302-W
44
F. Verpoort et al. /Journal
ofMolecular Catalysis
90 (1994) 43-52
In this study we use SiO*/Si ( 100) model supports [ 31 for the preparation of a WOs/ SiO,/Si( 100) precursor by way of incipient wetness impregnation from an aqueous solution of ammonium tungstate. Heating in air at 600°C forms the desired tungsten oxide. The conducting model support offers attractive advantages over the porous powder sample. Inhomogeneous sample charging is avoided producing spectra of much better resolution and reliable peak shape. Furthermore, angle-resolved XPS becomes meaningful since the direction perpendicular to the surface is defined. In this paper we report on the characterization of the W03/Si0, precursor by general, AR- and depth profiling XPS.
2. Experimental
2.1. Preparation
A 9 wt.% W03/Si0, precursor has been prepared by pore volume impregnation of 3 grams of silica (Polypor surface area 345 m’/g) with 25 ml ammonium tungstate (Aldrich) solution (0.6 g ( NH4)2W04 in 25 ml H20 and heating until the (NH,) 2WO4 is dissolved [ 41) . After the impregnation the sample is dried at 100°C and subsequently calcined in air at 600°C. The model support, a Si( 100) single crystal ( 10 X 10 X 1 mm), was oxidized at 500°C in air for 11 h. The SiO,/Si( 100) surface is impregnated by a drop of an aqueous solution of ammonium tungstate (0.0376 g/l). Drying the sample in an evacuated desiccator followed by calcination at 600°C results in a heavy loading of the edges while the center, which was the region chosen for study, shows a uniform distribution.
2.2. Spectroscopic
techniques
XPS spectra were obtained with a Perkin Elmer PHI 5500 ESCA spectrometer equipped with a monochromatized Al KQ source, a hemispherical analyzer connected to a multichannel detector and a manipulator which enables the variation of the take-off angle of the photoelectrons between 0 and 90”. XPS spectra of the model support and the model precursor were measured with 11.75 eV pass energy and those of the powder precursor, immobilised on an adhesive tape, at 48.50 eV pass energy. Charge correction for the powder samples is related to the C 1s peak at 284.6 eV. The spectra were accumulated, smoothed, and integrated using the PHI-ACCESS program. Constraints suited to impose the spin doublet character of the line pairs, e.g., ratios of linewidths and intensities, binding energy splitting, had to be implemented by the user. XPS depth profiling was performed in the same apparatus. The surface was bombarded with 4 keV Art ions and the argon beam was rastered over an area of 3 X 3 mm around the point of impact of the ion beam.
F. Verpoort et al. /Journal
45
of Molecular Catalysis 90 (1994)43-52
3. Results and discussion 3. I. Characterization
of the SiOz /Si( 100) model support
The Si 2p XPS spectrum of the model support is shown in Fig. 1. The experimental data are given in Table 1. Two peaks are clearly observed, one at 102.80 eV, originating from SiOZ [ 51, and one at 98.8 1 eV due to the silicon substrate. Both peaks are actually unresolved 2p,,,-2p,,, doublets. The known energy separation (eV) and intensity ratio between the Si 2p,,, and the Si 2p, ,2 are introduced in the curve fitting program producing the binding energy data in Table 1. Niemantsverdriet et al. [ 61 proved that the thin layer of SiO, on Si( 100) is an acceptable model for high surface area Si02 if the model support is oxidized at elevated temperatures. Fig. 2 shows the Si4+/Sio intensity as a function of the take-off angle, relative to the surface normal, (full line) for the angle-resolved XPS spectra of the SiO*/Si ( 100) model support after oxidation for 11 h at 500°C in air.
105
104
103
102
101
100
Binding
Energy
Fig. 1. XPS spectrum of the Si 2p region measured on the SiO,/Si( Table 1 XPS data of the SiO,/Si(
100) model support after calibration
99 (eV)
100) model support.
on C 1s ( Eb = 284.6 eV)
Element
Binding energy (eV)
FWHM (eV)
Assignment
c
1s
284.60
1.45
contamination
0 1s
532.30
1.62
O*- (SiO,)
Si ~PW 2P,/, 2P,,, 2P,/,
98.5 1 99.12 102.49 103.10
0.55 0.60 1.59 1.27
Si” (Silicon) Si’ (Silicon) Si4+ ( SiO,) Si4+ ( SiO,)
99
F. Verpoort
et al. /Journal
of Molecular
Catalysis
90 (1994) 43-52
!
I
6.o
, 0
20
40
Fig. 2. Angle-resolved (----).
Element
model catalyst
angle
XPS data of the SiO,/.Si( 100) model support together with theoretical,
Table 2 XPS data of the powder and model catalyst after calibration
powder catalyst
SO
60
take-off
on C
fitted profile!
1s (Eb= 284.6 eV)
Binding energy (eV)
FWHM (eV)
Assignment
c 1s
284.60
0 Is Si 2p w 4f
532.90 103.82 37.80
1.45 2.25 2.25 4.00
C of contamination O’- of SiO, Si”+of SiO z Wh+
c 1s 0 Is
284.60 534.00
1.71 1.44
C of contamination O”- adsorption by
Si 2~,,, 2P,/, 2P,,, 2P,/, W 4f,,* 4f,,2 4f,,, 4f,,*
532.88 53 1.27 102.93 103.54 98.35 98.96 36.79 38.90 35.84 37.95
1.43 1.58 1.36 1.38 0.52 0.58
1.87 1.60 1.54 I .60
H,O Ozm of SiO, 02- of WO, crystal Si4 + Si4+ SiO Si’ W6+Of-OW0 * W~+Of-oWOZ Wh+ of W0.t crys2tal w6’ of WO, crystal
F. Verpoort et al. /Journal
of Molecular Catalysis 90 (1994) 43-52
47
This compares well with the fitted theoretical curves (dashed lines) calculated ford = 3.2 and d = 3.4 nm using the following relation and assuming a homogeneous layer: zSiO*
-=
onS,02hSiOz(
1 - e-d’As’02cosO) -dlAsto2COSe
ISi
mSihSi
e
with Zsio:, and Zsi the XPS intensities of the Si 2p peaks for the SiOz and Si components, nSio, (0.036 mol/cm3) and ylsi (0.083 mol/cm3) the atomic densities, (Tthe cross section, equal for Si4+ and SiO, hsi (31.4 A) and hsio, (37 A) the inelastic mean free paths of electrons at the prevalent kinetic energy through Si and SiO*, respectively, with values taken from Tanuma et al. [ 71, 8 the take-off angle with respect to the surface normal and d the thickness of the SiOz layer. Recalculation with d = 3.3 nm produces a curve which agrees very well with the measured values. We can conclude that, in our experiment, the surface of the SiO,/Si( 100) model support is covered by a 3.3 nm SiOz layer following oxidation in air at 500°C for 11 h. 3.2. W03 /SiO, powder precursor In Fig. 3a,b the XPS spectra of the 0 1s and W 4f region of the WOJSi02 precursor are visualized. The XPS data are summarized in Table 2. The broad peaks in the spectra of the powder precursor exhibiting a charge shift of 5.10 eV have been charge corrected by using the C 1s peak at 284.6 eV as an internal reference [ 81. The two components of the W 4f doublet are not resolved due to inhomogeneous charge broadening (Fig. 3c), which results in line widths of the order of 4.00 eV. If the W 4f signal is fitted with a doublet using fixed 4f,,, : 4f,,, intensity ratio of 0.78 ( * 0.03) and an Eb splitting of 2.1 (kO.1) eV [9], a W 4f,,, binding energy of 36.66 eV is obtained, which is in agreement with the presence of WO, [ lo]. 3.3. W03 /SiO, /Si (ZOO) model precursor The spectra of the model precursor obtained using the same instrument settings, show a much better resolution (Fig. 3c,d). The XPS spectrum of 0 1s electrons (Fig. 3b) characterized by a weak asymmetry indicating multiple band structure, has a maximum at 532.88 eV. This is in agreement with the 0 1s electron binding energy for Si02 [ 1 l-141 since the oxygen atoms in the silica matrix produce the basic contribution to the spectrum. The 0 1s band was resolved and a satisfactory approximation of the signal envelope is obtained on superposition of a minimum of three bands: at 532.88 eV, having the maximum intensity (87.22% contribution), at 53 1.27 eV (5.79% contribution), and at 534.00 eV (6.99% contribution). The peak at 53 1.27 eV coincides with the 0 1s electron peak for free W03 [ 111. This band is assigned to W03 present on the surface of SiOz in the form of microcrystallites [ 151. The nature of this peak can be confirmed by angle-resolved XPS. Large take-off angles ( > 60”) favor the detection of real surface particles while at angles greater than 45”, the surface composition is obscured by the increasing importance of the in depth analysis of the matrix (Fig. 4). The 534.00 eV maximum, at higher binding energies, can be assigned to 0 1s oxygen electrons of adsorbed water [ 161.
48
F. Verpoort et al. /Journal of Molecular Catalysis 90 (1994) 43-52
The tungsten 4f doublet is readily resolved into its 712 and 5 /2 components. The linewidth of the W 4f of the model support is 1.54 eV compared to 4.00 eV in the spectrum of the powder precursor. Applying the Scofield W 4f,,, : W 4f,,, intensity ratio of 0.78, Eb splitting of 2.1 eV and Eb = 35.4 eV for W03 as constraints, curve fitting resolves the W signal envelope of the WO,/SiO,/Si ( 100) model support into two doublets (Fig. 3d). The low intensity doublet ( Eb W 4f,,, = 36.79 eV) is shifted to higher binding energies in comparisontoWO,andW0,2[17]. This band is tentatively assigned to the tungsten oxide linked by a chemical bond to the SiOz matrix.
10 9
Powder
precursor
(4
8 :
7
-8 =
5
532
531 Binding
530 Energy
529 (eV1
(b)
532
531 Binding
530 Energy
529 (eV1
F. Verpoort et al. /Journal of Molecular Catalysis 90 (1994) 43-52
9
Powder
49
(c)
precursor
%
31
36
35
Binding
Energy
36 IS+‘)
10 -
g :: * ::
Model
precursor
(4
y 1:: - 6:: = 5:: , :: 3 :: * ::
, :: 30
31
36 Binding
Fig. 3. XPS spectra of the 0 1s and the W 4f region of a calcined calcined WO,/SiO,/Si( 100) model precursor (c,d).
35 Energy
WOJSiO,
3h IcV)
powder precursor
(a,b)
and a
3.4. XPS depth projiling
By XPS depth profiling the relative concentration of the surface elements as a function of depth can be measured. In Fig. 5 the depth profile is shown for the WO,/SiO,/Si ( 100) model precursor. The simultaneous removal of W and 0 is consistent with the presence of poorly dispersed W03 particles which leave a considerable fraction of the support uncovered. The precursor shows no features characteristic for layered structures.
of h4olecular
F. Verpoort et al. /Journal
50
Catalysis
90 (1994)
/ i\ l!L,
43-52
f
f
1 \
I
\
ff
\
‘\,
/
, /’
01 536
535
‘-__
53L
533
532
Binding
Fig. 4. XPS spectra of the 0 1s region of a calcined WO,/SiO,/Si(
take-off angle, (-)
-_
Sk
GO Energy
529 (@VI
100) model precursor as a function of the
70”, (- - -) 30”.
0
1
2
3
4 Sputter
Fig. 5. XPS depth profile of the WO,/SiO,/Si(
(min)
100) model precursor.
Furthermore the XPS depth profiling data also confirm Eb = 53 1.27 eV to microcrystallites of W03.
3.5. Angle-resolved
5 Time
the assignment
of the peak with
XPS
The indications obtained from XPS depth profiles can be verified by take-off angle dependent XPS measurements. If a system has a layered structure, its XPS intensity should increase with respect to that of the substrate when the spectrum is recorded at grazing angles. Furthermore for a poorly dispersed phase the angle dependence will be less pronounced.
F. Verpoort et al. /Journal
of Molecular Catalysis 90 (1994) 43-52
51
W bt/Sib* 1.4 1.2 -
0.8 -
.
0.6 0.4 0.2 -
0
10
20
30
40
60
50 take
- off
70
80
90
angle
Fig. 6. W 4f/Si 2p XPS intensity ratios as a function of the take-off angle for a calcined WOJSiOJSi( model precursor.
100)
Fig. 6 clearly shows that the W 4f/Si”’ XPS intensity ratio for the precursor prepared by impregnation hardly depends on the take-off angle. This confirms the supposed low dispersion of the impregnation precursor. 4. Conclusion A flat, conducting model support consisting of a few nanometers of SiOZ on a Si ( 100) substrate, offers interesting possibilities for the application of XPS, and other surface techniques hampered by charging effects, for instance SIMS, AES, to study aspects of the surface chemistry involved in catalyst preparation. Electrical charging is almost completely avoided resulting in a spectacular improvement in spectral resolution. In addition the geometry of the support makes the application of angle-dependent XPS and sputter depth profiles meaningful. The results suggest the simultaneous presence of at least two types of tungsten oxide structures, microcrystallites of W03 and a small tungsten oxide monolayer on the surface of the W0,/Si02/Si (100) precursor.
Acknowledgements We thank T. Kuiper (Philips, Eindhoven) for providing Si( 100) crystals and the “Nationaal Fonds voor Wetenschappelijk Onderzoek” and the ’ ‘Onderzoeksfonds” of the Ghent University for financial support.
52
F. Verpoort et al. /Journal
ofMolecular Catalysis 90 (1994) 43-52
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