The XPS spectra of the metathesis catalyst tungsten oxide on silica gel

Journal of Electron Spectroscopy and Related Phenomena, 14 (1978) 453-466 0 Elsevler Sclentlflc Pubhshmg Company, Amsterdam - Prmted In The Netherlands

THE XPS SPECTRA OF THE METATHESIS OXIDE ON SILICA GEL

F P J M

KERKHOF

and J A

TUNGSTEN

MOULIJN

Instztute for Chemzcal Technology, Amsterdam (The Netherlands) A

CATALYST

Unwerszty

of Amsterdam,

Unrverslty

of Gronrngen,

HEERES

Laboratory (Received

for Physlcal

Chemrstry,

Gronmgen

(The Netherlands)

20 April 1978)

ABSTRACT XPS spectra of supported and unsupported tungsten oxides before and after use m the metathesis reactlon of propene are reported It IS shown that the broad peaks, usually measured for supported materials, are due to lme broadenmg caused by dlfferentlal chargmg A new sample preparation technique 1s described which leads to a considerable reduction of the Ime broadening Both XPS and X-ray diffraction show that in fresh catalysts a well-defmed phase of WO3 1s present The reductron of the supported trloxlde by propene to W2005s, as concluded from X-ray dlffractlon measurements, results m the formatlon of WV- and/or WxV-species m used catalysts For unsupported WO3 these lower valencles have been observed m the valence-band spectra

INTRODUCTION

To understand catalytic phenomena m heterogeneous catalysis, mformatlon 1s needed on the structure of the surface of the catalyst Catalysts often consist of a “promoter” and a “carrier” For this type of catalyst, basic properties are the degree of dlsperslon and the chemical composltlon of the promoter These propertles depend on the interaction between the earner and the promoter When the mteractlon 1s predominant rt 1s even possible that a dlstmctlon between promoter and support IS not meaningful In that case the catalytic actlvlty must be attributed to a newly formed compound of promoter and carrier It has been well documented that X-ray photoelectron spectroscopy (XPS) 1s a valuable tool to obtam mformatlon on the surface structure of supported metals and metal oxides’ In this paper a study 1s reported of an apphcatlon

454

of XPS to slhca promoted with tungsten oxide, which 1s an active catalyst for the metathesis of alkenes2 It 1s well known that, durmg the first operating hours, the actlvlty of the tungsten/slhca catalyst increases This increase 1s accompanied by a change m colour of the catalyst from yellow to blue, which 1s probably caused by reduction of the yellow trloxlde to one of the lower tungsten oxldes3 We studied the interaction of tungsten oxide and silica by measurmg the XPS spectra of core electrons (e g S1(2p) and W(4f)) and the valence-band spectra of several samples It ~111 be shown that reduction of the catalyst by propene results m the filling of the 5d orbltals of the tungsten atoln Durmg the study two major problems became apparent (1) the XPS peaks of Sl, 0 and W were rather broad, (2) the samples can be damaged by the lrradlatlon mslde the spectrometer - m the case of yellow non-reduced catalysts, this could be concluded from the appearance of a blue shade after lrradlatlon The first problem was attnbuted to differential chargmg of the sample and was solved by usmg a new sample preparation method The second problem could be satlsfactonly reduced by cooling the sample and simultaneously reducing the mtenslty of the X-ray source EXPERIMENTAL

Ma tenals and methods The catalysts were prepared by lmpregnatlon of silica gel (Damson-Grace 62, 180-212pm) with aqueous solutions of ammomum metatungstate (99 9%, Koch-Light laboratones) After 1mpregnatIon the catalysts were dmed overrnght at 120°C and fluldlzed for 2 h m air at 550°C The amount of W03 m these catalysts, the surface area and pore volume are aven m Table 1 TABLE

1

MORPHOLOGICAL

wt

%

of W03a

PROPERTIES Atomzc

OF THE CATALYSTS

ratzo W Sz

BET-surface (m2 g-’

16 34 58 12 3 19 1 41 8

1 1 1 1 1 15

240 110 63 28 16

areab

)

358 331 346 306 274 167

z Measured by actwatlon analysis at the Energle Centrum Calculated from Nz adsorptlon at 98 K

Average pore (cm3 g-’ ) 1 0 0 0 0 0

Nederland

01 95 94 89 76 44

volumeb

455

Two different techniques of sample preparation for XPS were applied One method was mounting the powdered samples on adhesive tape The second method IS not conventional and will be described 1n detail The catalyst particles are we11powdered and slightly wetted with demlnerallzed water In this way a paste 1s obtained which can be placed 1n the small trough, used as sample holder After drying for 2 h at 120°C a coherent structure 1s obtained After this procedure, which will be referred to as the “wet method”, the sample was put into the spectrometer for recording the spectra Spectrometer procedures The XPS spectra were obtamed with an AEI ES-200 spectrometer using an Mg Karsource (1253 6 eV) with a line wrdth of 0 7 eV The spectrometer was evacuated to better than 7 x 10m6N mm2 (5 x 10d8 torr) and data were collected with a PDP-8 computer Unless otherwise stated the spectra were recorded at a source power of 180 W, while the sample temperature ranged from - 15 to - 35°C To diminish radiation damage of the sample inside the spectrometer we applied also a lower X-ray source power (80 W) and lower sample temperature (- 100°C) for collecting the spectra of these samples The S1(2p) peak at 104 1 eV was used as the reference With the C(ls) peak at 285 0 eV the same results are obtained Whenever possible we preferred the S1(2p) peak because of 1ts higher intensity The binding energres of the unresolved W(4f) electrons were determined at the half-width of the peak measured at half-maxImum-height To avoid oxldatlon 1n a1r of propene-treated samples, two methods were used (1) 1n situ treatment with propene 1n the sample preparation chamber of the spectrometer, (2) treatment m a Pyrex reactor which could be sealed under vacuum -this seal was broken 1n a nitrogen-box attached to the spectrometer and the samples were only mounted on tape after careful removal of oxygen and water from the nitrogen atmosphere

RESULTS The X-ray dlffractlon patterns (XRD) of fresh catalysts are shown 1n F1g 1 From the literature 1t can be concluded that crystalline tungsten trloxlde 1s formed m samples with a content of tungsten oxide exceeding 12% It 1s obvious that the line width 1n the case of the catalysts 1slargerthan for the unsupported tungsten tnox1de We attnbute this to 11nebroadening caused by the small size of the crystallltes After reaction with propene the dlffractogram has changed to the pattern of W2,,05s as reported by Magneh4 So we conclude that, m the samples with a largerpromoter content, crystalline

456

%WO,

191 123 ha 34 16 00

60

50

40

30

20

10

-2e-

Figure

1 X-ray

dzffractlon

patterns

of pure tungsten trroxlde

and some fresh catalysts

W03 1s present which under the reaction condltlons (propene at 0 1 MPa, 720K) IS converted mto Wz005s By comparmg the areas of the peaks It can be concluded that a large part of the tungsten on these catalysts 1s present as crystallme tungsten trloxlde, whereas other compounds are not present or only to a non-detectable extent XPS results wrth catalysts mounted on tape The tungsten 4f peaks of some fresh catalysts are Dven m Fig 2 To make a compmson possible also a sample of unsupported tnoxlde was measured One of the stnkmg features of these spectra 1s the difference between the W(4f) doublet of pure W03 and the broad W(4f) peaks observed m the catalysts A slmllar difference between supported and unsupported tungsten tnoxlde on slhca has been reported m the literature’. Except for the catalyst with 41 8% WO, all the signals have the same shape and bmdmg energy Spectra of some used catalysts are Bven m Fig 3 Clearly, neither a varymg tungsten content nor a reactlon with propene mfluences the spectra One might conclude that tungsten atoms m fresh and used catalysts have the same ill-defined chemical environment As will be shown this conclusion 1s m fact based on an artefact Table 2 summarizes the results of fresh and used samples mountid on tape The results of W03 and S102 are compared with literature values m

457

418

-BBE(eV)Fqwre

2 XPS spectra of the W(4fl

electrons

m various fresh catalysts

Table 3 It can be concluded that our results agree well with the reported values When the gold decoration technique 1s used, lower values for the W(4f 7/Z) signal are obtamed A similar effect can be observed 1n the values of the b1ndmg ener@es of the S1(2p) peak 1n some of the gold-decorated samples reported by Madey et al l3 The intensities mentioned 1nTable 2 are peak areas referred to unltscannlng time and attenuation The intensity rat10 of the W(4f) doublet and the O(ls) peak 1n pure W03 was 1 25 The rat10 S1(2p)/O(ls) 1n S10, was 0 171 So the 1ntens1tles of W and S1 relative to one oxygen atom are 3 76 and 0 342 respectively From the calculated photoelectnc cross-sections, sven by Scof1eld**, theoretical values for these relative intensities can be deduced For Mg Ka! radiation this results 1n a(W4f)/a(Ols) = 3 44 and o(S12p)/o(Ols) = 0 290 It can be concluded that the calculated and expenmental values are 1n good agreement

20%

42

34

38

Figure 3 W(4f) signals from used catalysts

experimentat

.f resh A used

01

catalyst cataiyst

02

(w/sl)b,(k

Figure 4 ExperImental and calculated r$Jatlve mtensltles photoelectron cross-sections of ScofieldLL

The calculation IS based on the

In a recent pubhcatlon, however, Madey showed that a number of mvestlgators, usmg Mg Koc n-radiation, found values of 0 41-O 11 for the Sl/O ratio m the same silica sample (Damson C-950 silica ge1)13 Our value of 0 171 for the observed mtenslty ratio hes m this range From the large spread m these reported data one must conclude that the use

18 28 28 27 26 27 26 27 28 27

533 2

533 1 533 2 5332

SiOz (catalyst support)

Used catalyst 5 8% WO3 Used catalyst 12 3% WO3 Used catalyst 19 1% WO3

1041 104 1 1041 104 1

251 3 143 4 159 3 1518

104 1 1041 1041 1041 1041

73 239 152 156 102 205

44 2 4 8 3 1

BE (ev)

Intb

N2P)

27 27 27

25

27 27 26 27 25

(ev)

FWHM

244 274 260

430-

43 0 263 27 5 191 323

Znt

379 38 1 37 7

360 37 8 377 37 9 377 377

(ev)

BE

W(4tJ

45 46 4 8

-

16 4 5 46 45 45 44

(ev)

368 617 985

922289 388 634 624 33 52

FWHM Znt

OF THE SAMPLES MOUNTED ON TAPE

a FWHM = Full width at half maxlmum height b Int = Intensltles referred to unit attenuation and scanning time m arbitrary umts

catalyst 3 4% WO3 catalyst 5 8% WO3 catalyst 12 3% WO3 catalyst 19 1% WO3 catalyst 41 8% WO3

5310 533 1 5332 5332 5332 5331

Fresh Fresh Fresh Fresh Fresh

wo3

FWHMa (ev)

Peak O(ls)

BE (ev)

Sample

BINDING ENERGIES, HALF-WIDTHS AND INTENSITIES

TABLE 2

0151 0225 0379

-

0067 0148 0231 0327 1038

Int( W4f)/ Int(Sz2p)

18 17 18 19 16

0 17 0 17 0 17

0 17

0 0 0 0 0

Znt(Sr2p)l Int(Ols)

a b ’ d

t

+ t t -

-

Golddecoratlon technqueb 5310 Not gven 5310 Not given Not given 530 2 530 3 531 2 533 2 533 0 533 2 533 2 533 0 532 7 5329

O(ls)

104 1 1039*02 104 2 1040+05 Not given 103 5 103 5

-

SGP)

Values referred to C( 1s) = 285 0 eV i- Gold-decoration technique appbed -- gold-decoration technique not applied WO3 as an oxide layer on W foil Average value of 12 observations with different spectrometers

s102

s102

s102

s102

W03 WO3 W03 W03 W03 W03 W03 W03 s102 SlOz s102

Sample

Bmdmg energy

BINDING ENERGIES OF S102 AND WO3 AS GIVEN IN THE LITERATURE’

TABLE 3

-

36 36 36 36 36 35 35 35 -

0 0 3 0 8 7 3 2

W(4f 7/2) This work Blloen and Potts Nefodov et al ’ De Angehs and Schlavello8 Carlson and McGmreg ’ Nefodov et al 7 Colton and Rabelals” Ng and Hercules” This work Carriere et al l2 Escard et al a Madey et al l3 ’ Nefodov et al 6 Nefodov et al 7 Nefodov et al ’

Reference

461

of literature values for relative 1ntens1tles can lead to serious errors and therefore the use of reference samples must be strongly recommended The W/S1 1ntenslty ratio as found by XPS 1s plotted m F1g 4 The W/S1 ratio as would be predicted from the overall composlt1on of the sample and the relative photoelectric cross-sections 1s also given Figure 4 shows that at higher tungsten contents the experimental intensity 1s much lower than the value predicted by curve b This 1s 1n agreement with the fact that the crystallite size 1s20 nm which 1sten times the escape depth of W (4f) electrons in W03 We hope to gwe a further interpretation of the intensity ratio of promoter and caZTler signal 1n this and other catalysts 1n the near future21 XPS results for catalyst samples prepared by the wet method

Because 1n the case of catalyst samples mounted on adhesive tape broad peaks were observed, It was decided to check our results with the sample preparation technique described above It was hoped that this new method would d1mm1sh the hne broadenmg caused by dIfferentA chargmg Figure 5(I) and (II) show the W(4f) and S1(2p) peaks from the same catalyst and recorded with the same scanning time but with the use of two different sample preparation methods It 1s obvious that both the Sl and W peak mdths are smaller when the wet method 1s applied To confirm that the peak broadening 1n the case of

b I 42

I

I 38

I

I 34

-BEE(eV)-

I

I

108

-

(I)

Figure 5 (I) W(4f)

peak of a 19% WOj catalyst the wet method (b) (II) SI( 2p) peak of a 19% prepared by the wet method (b)

mounted

I

104

I

I

b

100

BE(eV)(IF1 on tape (a) and prepared by mounted on tape (a) and

WOJ catalyst

462

b I

I

I

42

a

1

38 -

34

BE(eV)

-

Figure 6 W(4# peak of a 19% WO3 catalyst, after (b) m s&u treatment with propene

I

I

I

I 4

8

-

1

1

prepared

by the wet method,

before

(a) and

I

0

BEkM-

Fqure 7 Valence-band spectra of tungsten trloxlde (a), oxldes, VIZ WOa 72 and WOzm (b) and of propene-treated

a mixture of two tungsten tungsten trloxrde (c)

sub-

463

“conventional” sample meparatlon IS, indeed, an artefact, a sample prepared according to the wet method was ground and mounted on adhesive tape This again resulted m relative broad peaks We further investigated the electrical contact between the slhca carmer and the supported tungsten by applying a voltage (from - 10 V up to + 10 V) to the sample holder This proved that the difference m bmdmg energy between the Sl(Zp) and W(4f) peaks was 66 6 ? 0 2 eV for fresh and used catalysts, independent of the applied voltage This experiment proves the good electrical contact between tungsten and slhca A plot of sample charge versus the probe bias voltage was slmllar to the plot of msulatmg materials as reported by McGllp and Main” These experlments show the vahdlty of a model where small crystals of tungsten tlnoxlde are supported on the large msulatmg surface of srhca With one sample prepared by the wet method we performed an m situ treatment with propene [ 16 h, 670 K, 1 torr (= 125 Nmw2 )] From Fig 6 It can be concluded that m this case the reduction of the supported material results m the appearance of one or more peaks from lower tungsten valencles

The valence-band

spectra

of supported

and unsupported

materzals

The reduction to W,,05s IS shown by the appearance of a shoulder on the low energy side of the W(4f) peak This reduction should also bevisible m the valence-band regon We therefore measured the valence-band spectra of some supported and unsupported samples Our results for some unsupported tungsten oxides mounted on tape are presented m Fig 7 The presence of the filled 5d and 6s orbltals m the samples of W03 _X and propene-treated W03 IS proven by the appearance of a peak at 1 eV next to the large peak of the O(2p) electrons at 7 eV We also tried to measure the valence-band spectra of a catalyst treated m situ These measurements were hampered by the fact that a long scannrng time was necessary to obtam a good signal/noise ratio Durmg the recordmg of the spectra a slight reduction always takes place, which of course also results m the flllmg of the outer orbltals This effect 1s shown m Fig 8 Figure 8 shows that, owing to the long scannmg time, the 5d (and 6s) orbltals become occupied This reduction IS also shown by the appearance of a blue shade on the samples which were u-radiated for several hours The effect was much smaller for the unsupported trloxlde which was stable under u-radlatlon (see Figs 2 and 7) This difference m stability can be caused by the low thermal conductlvlty of porous silica m vacuum, which results m a less effectlve sample coolmg m the case of supported W03

DISCUSSION

Two phenomena on adhesive tape

hampered

our experiments

mth catalyst samples mounted

464

J

I

I

I

I

I

6

4

2

0

-2

-4

-BE(&)-

Fgure 8 Valence-band spectra of a fresh 19% WO3 catalyst and 170 mm (b) X-ray source power 80 W

after

scannmgfor 40 min (a)

(1) reduction of the samples inside the spectrometer, (2) the appearance of broad W, 0 and 531peaks The first effect 1s well known and has been reported by others, e g Clmmo et al l6 for slhca-supported chromium oxide, and De Angehs and Schlavello8 for tungsten bronzes, it 1s also treated m a review of Jolly17 The effect can be decreased by using a lower source power, by strongly coolmg the samples and by measuring with short scanning time The second effect IS probably caused by dlfferentlal chargmg of the silica particles Differential chargmg IS a known cause of line broadening m the XPS spectra of solids as was shown, for example, by Bancroft et al l4 m their measurements for the solid- and gas-phase spectra of Sn-organometalhc compounds In the case of catalyst particles which are insulators mounted on adhesive tape, it might be possible that not all the particles get the same charge This results m different kinetic enerses of the electrons from, for example, the Sl atoms m different particles Even when the particles are pressed m conducting material like mdlum, we observed the occurrence of broad peaks owmg to differential chargmg With the samples prepared by the wet method a coherent structure 1s obtained and the chargmg of the sample will be more uniform We conclude that, when XPS ISused to study catalysts, great care has to be taken to avoid dlfferentlal chargmg of the samples It IS possible that earlier studies where relative broad peaks have been reported are m error owing to the dlfferentlal chargmg effects* 16*18*l9 The

465

peak width of the carrier material would gve an mdlcatlon of this effect and therefore it must be preferred that both bmdmg energes and peak widths are reported not only for the active species but also for the supporting matenal A prehmmary study with a catalyst sample prepared m such a way that these broadening effects could be avoided showed the presence of lower oxldatlon states of tungsten after propene treatment By companng Fig 6 with the results of De Angehs and Schlavello8 it can be concluded that m the used catalysts Wlv and WV species are present The valence-band spectra confirm the filling of the outer orbltals of the tungsten atom after propene treatment The shape and bmdmg energy of the W(4f) electrons m the fresh 19% WOJ catalyst, prepared according to the wet method, are slmllar to the values for unsupported tungsten trloxlde This confirms the presence of a well-defined crystallme phase of tungsten trloxlde m catalysts with higher tungsten contents The Interactions between support and promoter m catalysts with lower tungsten oxide contents were the subJect of further mvestlgatlons and are reported elsewhere23* 24

SUMMARY

The salient conclusions of this work are as follows (1) At higher tungsten contents a maJor part of tungsten m WO3/S102 catalysts IS present as a crystallme phase of tunsten trloxlde which after use as a catalyst 1sconverted to W20058 (2) In these used catalysts tungsten (V) and/or (IV) species are present The occupation of the 5d and 6s orbltals of the tungsten atom caused by this reduction could be shown m the valence-band XPS spectra of unsupported tnoxlde (3) Dlfferentlal chargmg of the samples mounted on adhesive tape caused a hne broadening of the SI, 0 and W peaks By using a different sample preparation method this could be avolded

ACKNOWLEDGEMENTS

This study was supported by the Netherlands Foundation for Chemical Research (S 0 N ) with financial ad from the Netherlands Orgamzatlon for the advancement of Pure Research (2 W 0 ) Thanks are due to Dr G Sawatzky from the laboratory for Physical Chemistry of the Umverslty of Gronmgen for his help m the Interpretation of the XPS spectra, and Dr. B Koch (Department of X-ray Spectrometry and Dlffractometry, Unlverslty of Amsterdam) for help m the mterpretatlon of the dlffractograms recorded by Mr W Molleman

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22 23 24

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96(11)