700
J. Phys. Chem. 1903, 87,788-793
in either the EPR or ENDOR spectra. Furthermore, the Hllz ENDOR spectrum is virtually identical with that of the pyridine complex. Consequently, we conclude that in solution the fifth ligand binds in the axial position as in the case of pyridine itself. A difference between the structure found in the crystalline state and in solution has been found previously for a number of VO(acac),/substituted pyridine The magnitudes of the hyperfine componenh of the OH proton (A, = 5.4, A , = 2.6 MHz) in methanol complexed with Cu(acad2 e s t a h s h that this ligand binds axially as well. OH proton hyperfine interactions of A, = 6.0 and A,, = 3.1 MHz were found for the VO(acac),/CH,OH adduct.' It is noteworthy that similar hyperfine components for axially coordinated OH have been found in the ENDOR spectra of a variety of systems. For instance, C U ( H ~ O )(A, ~ ~=+ 6.0, A, = 3.1 MHz)~,and VO(HzO)52+ (A, = 5.7,A, = 3.3 MHzf,lgaCu2+-CH30H(A, = 5.7, A,., = 3.0 MHz)? Moo3+in (frozen) 12 N HCl ( A , = 5.0, A,, = 2.4 M H z ) , ~VO(imidazole):+ ~ ( A , = 6.0, A,, = 3.1 M H z ) . ~Evidently, ~ the characteristic splittings and line shape of the ENDOR signals readily identify axially coordinated hydroxyl groups.
Conclusions It has been shown that proton hyperfine tensors of the Cu(acac), complex can be determined by ENDOR exper(31) Da Silva, J. J. R. F.; Wootton, R. Chem. Commun. 1969, 3175. Caria, M. R.; Haigh, J. M.; Nassimbeni, L. R. J.Inorg. Nucl. Chem. 1972, 34, 3171. (32) Kirste, B.; van Willigen, H., unpublished results. (33) Mulks, C. F.; Kirste, B.; van Willigen, H. J.Am. Chem. SOC. 1982, 104, 5906.
iments on frozen solutions,aided by the use of frozen liquid crystals and selectively deuterated Cu(aca& Signs of the hyperfine components were determined with TRIPLE resonance experiments. As a new application of this technique, it is shown that TRIPLE resonance performed on powder samples may serve to determine angles between hyperfine tensor axes. This possibility is based on the additional orientational selectivity provided by these experiments. Pumping of a specific transition in a powder ENDOR spectrum affects a narrow range of molecular orientations, and only the nuclei of these molecules can contribute to the TRIPLE spectrum. In agreement with the crystal structure of Pd(aca& as determined by X-ray analysis, we found a strong intermolecular hyperfine interaction of Cu(acac)zdoped into Pd(aca& with axially located protons. The ENDOR spectra of the adducts of Cu(aca& with methanol or pyridine prove that the fifth ligand is bound at a vacant axial position. The fact that structural information on adducts can be obtained easily by means of ENDOR experiments in frozen solution or powders suggests that this technique may be a valuable tool in investigations of catalysis by Cu(acac)z and similar transition-metal complexes. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Financial support by the National Science Foundation (PRM-8100525, PCM-7903440) and the Department of Energy (DE-AC02-81ER10911)for the purchase of equipment is gratefully acknowledged. Registry No. Cu(acac)z,13395-16-9; Cu(acac)-3-d),,18115-99-6; Cu(acac)2MeOH,76748-58-8;Cu(acac)*py,38867-27-5.
Chemisorption of CO and CO, on Gold-Supported Tho, Films W. McLean, C. A. Colmenares," R. L. Smlth, University of California, Lawrence Livermore National Laboratory, Livermore, California 94550
and G. A. Somorjal University of California, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received: March 8, 1982: In Final Form: Ju/y 9, 1982)
The chemisorption of CO and COPon gold-supported Thoz films has been studied by X-ray photoelectron spectroscopy (XPS). At pressures of Pa both gases were found to adsorb without dissociating at room temperature. The Tho2films were prepared by drying and calcining a Th(NOJ4-ThOzslurry which was painted onto a gold substrate. Films prepared in this manner were mechanically and chemically stable, and were characterized by Auger electron spectroscopy (AES),X-ray photoelectron spectroscopy (XPS), X-ray diffraction, and scanning electron microscopy (SEM). The calculated atomic sensitivity factor (ASF) for the Th(4f7/,) photoelectron peak (6.7) is in relatively good agreement with the value recently reported in the literature (7.8).
Introduction Previous studies of the adsorption of CO and COPonto partially oxidized Th(ll1)' suggested that at hgh coverage these gases adsorbed on the Th surface without dissociating; however, the nature of the adsorption on the clean (1) W. McLean, C. A. Colmenares, R. L. Smith, and G. A. Somorjai, Phys. Reu. B , 25, 8 (1982).
metal was complex and could be explained by various competing mechanisms. Thus we undertook this work to study the chemisorption of CO and COz on a well-defined Tho2surface at low pressures (-10" Pa) using XPS. This work is also part of a series of studies investigating the use of thoria as a Fischer-Tropsch ~ a t a l y s t . ~To , ~reach our (2) H. Pichler and K. H. Zieseke, Brennstoff Chem., 30, 13 (1949).
0022-3654/83/2087-0788$01.50/00 1983 American Chemical Society
Chemisorption of CO and CO, on Tho,
U
Figure 1. Sample heater assembly: (a) Tho, film on gold support; (b) gold-coated 316 stainless steel heater sheath; (c) heater element; (d) MgO packing; (e) vacuum/pressure fitting; (f) thermocouple feedtroughs; (9)heater leads.
goal required the development of a technique to produce thin, small-area Tho2 films supported on an inert metal substrate which would closely resemble a practical catalyst. We produced thin ( 10OOO A), mechanically stable Tho2 films with good adhesion to a gold substrate. When such films are used the problems associated with uniformly heating samples of a good insulator such as thoria could be avoided, and the severe charging problems usually encountered during Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) measurements with this material could be reduced. The alternate method of using oxidized thorium for our studies led to incomplete oxidation at room temperature and to diffusion of oxygen into the bulk metal as the temperature of the samples was raised.
-
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983
789
heater assembly shown in Figure 1allowed us to operate at high temperatures without biasing the sample, and has been used in ultrahigh vacuum (UVH) as well as in highpressure catalysis experiments. Surface analysis of the films was carried out in a UHV system operating at a base pressure of 66 nPa. The system was equipped with a double-pass cylindrical mirror analyzer (CMA) with an internal electron gun for AES,and a nonmonochromatized Mg K a X-ray source for XPS. Auger spectra were taken with a 3-kV, 20-pA electron beam, 6-V peak-to-peak modulation voltage, and a 0.03-9 time constant. Highresolution photoelectron spectra of regions of particular interest were accumulated in a pulse counting mode with the analyzer pass energy set at 25 eV (resolution 1 eV). Multiple passes were made through each of the spectral regimes in order to average out background noise. The valence band and C(1s) regions required 1024 scans, while for the Th(4f) and O(1s) 256 scans were sufficient to get good signal-to-noise ratios. Spectra displayed in Figures 5 and 6 were smoothed and normalized to the Th(4f7/,) peak of the clean film. Binding energies were measured relative to the Th(4f7,,) peak at 335.0 eV. Exposures to CO and C02 were made at Pa after cleaning the Tho2film by argon-ion bombardment (2 keV, -2 pA/cm2, 5 X 5 mm raster) for approximately 10 min. Sample charging was not found to be a significant problem during XPS measurements because the T h o 2 coating was thin enough to allow charges to drain through the gold substrate. Peak shifts of 0.4 eV with respect to an oxidized Th(ll1) surface were observed; these shifts were significantly smaller than the 2.8-eV difference between “carbidic”carbon and bound CO observed in ref l.
Results Characterization of Films. SEM photographs of a representative Tho2 film are reproduced in Figure 2. They show a macroscopically rough surface with a few randomly distributed holes -2 pm in diameter. X-ray diffraction analysis indicated that the films were stoichiometric Tho2 with a lattice constant of 5.597 A. XPS and AES measurements of the Tho2 surface prior to any treatment in the vacuum system revealed carbon and chlorine to be the principal contaminants; these were Experimental Section removed by argon-ion bombardment. The Auger spectrum of the resulting “clean” Tho2 surface is reproduced in The thoria films were prepared by metal nitrate calciFigure 3 and the 4f photoelectron levels in Figure 4d. In nation, a technique which has often been used as a first step in the manufacture of supported metal ~ a t a l y s t s . ~ Figure 4 we show the oxidation behavior of thorium to demonstrate that the oxidized metal is not suitable to The starting material consisted of a slurry of Th(NO,),/ study the adsorption of gases on Tho2. The incomplete Tho2 in H 2 0 prepared by adding 1 part by weight of oxidation of the metal at ambient temperature is revealed powdered Tho2to 3 parts by weight of a saturated solution by the presence of the metal 4f7/2line at -333.1 eV as of Th(N03)4in H20. Thin layers of this slurry were apshown in Figure 4a. Oxidation of the metal at 620 K plied to a gold foil after its surface had been roughened produces symmetrical Th(4f) levels immediately after by 5-grit bead blasting; excess material was removed with oxidation (Figure 4b). However, after 5 min at this tema clean cloth or a paper wipe to leave the thinnest possible perature, unoxidized metal peaks due to oxygen diffusion layer of solution on the surface. This specimen was airinto the bulk become distinctly detectable (Figure 4c). dried and then slowly heated in air to 870 K in -2.5 hs, The presence of gold on the “clean” surface was not held at temperature for -10 min, and then cooled to room immediately obvious because of the near coincidence of temperature. This process was repreated about 10 times. the major Auger peaks for thorium and gold. This was also The films were finally dehydrated by a vacuum bake at the case for the photoelectron spectrum where nearly all 1000 K for 1h. of the principal gold peaks overlap with either thorium Tho2samples prepared by this technique were mounted photoelectron peaks, shake-up transitions, oxygen Auger onto a gold-coated indirect heater (see Figure 1). The transitions, or plasmon loss peaks (Table I). In order to heater was capable of reaching temperatures up to 800 K determine the presence of gold on the surface we compared in hydrogen at 3.0 MPa. The temperature gradient from the intensity ratio of the [Th(4f7/,) + A ~ ( 4 d ~ /peak ~ ) ] at the heater face to the sample surface was -20 K. The 335 eV to the Th(4f5/,) peak at 344.3 eV to the ratio Th(4f7/,)/Th(4f52) found for single-crystal Tho2, Tho2 (3) C. A. Colmenares and W. McLean, Lawrence Livermore National powder imbedded in indium foil, and oxidized thorium Laboratory Report UCID-19286, Dec 1981. (4) J. H. Sinfelt, Prog. Solid State Chem., 10, 55 (1975). metal. The average ratio in these cases was 1.29 f 0.01,
790
he Journal of f%ysIcal Chemistry, Vol. 87, No. 5, 1983
McLean et al.
?
‘
I
Possiblegold Auger I tramitions
1
1 II I II
I II
I1 I
I
Possiblethorium Auga lransilionr
I
I 500
lo00
Electron energy (OW
Flgure 3. Auger spectrum of cleaned Thopcoating.
3500
3000 2500 W T
2000
” 350
340
330
Binding energy (eV)
Figure 4. Thorium 4f levels on (a) thorium metal oxidized at ambient temperature; (b) thorium metal oxidized at 620 K; (c) oxidized metal from (b), held at 620 K for 5 min; (d) Tho, from coating process. Dashed lines indicate approximate photoelectron background.
malized by Scofield’s photoionization cross s e ~ t i o n swe ,~ found that this corresponded to apparent gold concentrations of 10 and 20% for these two cases, respectively. The compition of the “clean” surface based on the atomic sensitivity factors (ASF’s) for CMA’s recently compiled by Wagner et al.9 and neglecting possible gold content was Th02.14C0.11. The remaining carbon could not be removed by additional ion bombardment because the ion beam hit the sample at an oblique angle so that portions of the rough surface were shadowed. This carbon could not be removed by heating to -750 K in 0.13 mPa of oxygen for several hours.
H
2EA Flgure 2. SEM micrographs of Tho, coating.
whereas for the supported films the ratio increased from 1.35 for the clean and CO-covered films to 1.42 for the C02-covered film. When the peak intensities were nor-
(5) J. H. Scofield, J. Electron Spectrosc., 8, 129 (1976). (6) C. D. Wagner, W. M. Riggs, L. E. Davis, and J. Moulder, “Handbook of X-ray Photoelectron Spectroscopy”,Perkin-ElmerCorp., Eden Prairie, MN, 1979. (7) J. C. Fuggle, A. F. Burr,L. M. Watson, D. J. Fabian, and W. Lang, J. Phys. F, 4, 335 (1974). (8) J. C. Fuggle, E. Kiilne, L. M. Watson, and D. J. Fabian, Phys. Rev. B, 16, 750 (1977). (9) C. D. Wagner, L. E. Davis, M. V. Zeller, J. A. Taylor, R. H. Raymond, and L. H. Gale, Surf. Interface Anal., 3, 211 (1981).
The Journal of Physical Chemistv, Vol. 87,No. 5, 1983 791
Chemisorption of CO and CO, on Tho,
TABLE I: Binding Energies (BE) and Photoemission Cross Sections ( u ) for Selected Thorium, Gold, and Oxygen Peaks for hv = 1254 eV thorium gold oxygen level BE, eV’ oa,’ level BE, eV8 oa,5 level BE, eV’ 3.3 6.1
5d5~z
6P3,Z
17.2
5ds~z
87.07 93.g7 335.0 344.3 351.7
4f-.. -~
5d3~z 4f7~z
4fs~, ( 4 f 5 ~ z 1, o(2P) shake-up
a
0.293
5d3~z
+ -
VB
1
2.52 1.70 23.30 1a. 21
0.87 0.62
4f:::
83.7 87.5
9.79 7.68
4d,,,
335.0
9.50
353.0 546.2 761.9
6.42 4.55 1.45
4d3~z
4P3,Z 4~112
oav5
0.0145 0.0073
2P3,Z
2PllZ
O( Is) plasmon loss 0 KLILZ,,
547 765.4
In units of 2200 barns. 500
°
1 900
1
1
I
400
--
300
W
2
700
200
600
100
500
< 295
285
275
Binding energy (eV)
+
Figure 5. Carbon (1s) region on Tho,: (a) Th 100 langmuir of 0,; (b) Tho, coating CO (exposures as marked): (c) Tho, coating
cop.
+
+
Absorption of CO and COz on Thoz. The XPS spectra of the valence band regions of the “clean” Tho, film and after saturation coverage with CO and COz did not reveal any significant changes in those spectral features that were associated with Thoz, such as the Th(6p1j2),O(2s1/J, and Th(Gp,,,) peaks. We did observe pronounced differences in the range 0-10 eV caused by the Au(5d) peaks from the substrate (see Table I). Since the sample was cleaned by argon-ion bombardment before exposure to gas, the gold became more visible as the experiments progressed. In agreement with the relative 4f intensities we estimate that -10-20 at. 7% gold was present in the near-surface region based on the ratio of the gold contribution to the peak between 0-10 eV and the Th(6p3/,) peak, corrected by Scofield’s cross section^.^ We did not observe any changes in the Th(4f) levels of T h o z peaks following adsorption of CO or COz;however, the fwhm of the film (4f7,2)(Figure 4d) peak was greater than that of the Th(ll1) oxidized at 620 K (Figure 4a) because of the rough, polycrystalline nature of the coated sample, and contributions from the Au(4d5/,) line of the substrate, also at 335 eV. In Figure 5 we have shown changes in the binding energy of the C(1s) peak and a function of CO coverage and for C02 saturation coverage. The binding energy for CO shifted from 284.6 to 285.4 eV for coverages from 1 lang-
‘(a)
0 5 0
Tho, on Th
530
I
!O
Binding energy (eV)
+
Figure 6. Oxygen (1s) region on Tho,: (a) Th 100 langmuir of 0,; (b) “clean” Tho, coating: (c) Tho, coating 10 langmuir of CO: (d) Tho, coating 2 langmulr of CO,.
+
+
muir (1 langmuir 0.13 mPa.s 3 1 X lo4 toms) to 2.5 langmuir (saturation coverage) indicating the chemisorption of molecular C04 (Figure 5b). A single, broad peak at 284.3 eV was observed for the chemisorption of COz on Thoz (Figure 5c) also indicating the presence of molecular COZ.l The contribution of the Th(5slj2)peak to curves b and c of Figure 5 was taken into account by substracting the Th(5s, 2) background for oxidized thorium (Figure 5a) from the [&s) Th(5s)I peak of curves b and c of Figure 5. The spectra of the O(1s) level for oxidized Th(ll1) and the T h o z coating after exposure to CO and COz are presented in Figure 6. The O(1s) peak for the Tho, on Th appeared at 531 eV (Figure 6a) and it is presented here for reference. A slight asymmetry was observed in the O(1s) peaks of Thoz and Tho2plus CO as shown in peaks b and c of Figure 6; we also observed an increase in the intensity of the O(ls) upon adsorption of CO. A second O(1s) peak appeared at -533 eV after C02 adsorption (Figure 6d). Quantitative analyses of the oxygen, carbon, and thorium content of the near-surface region of oxidized thorium and the Thoz films exposed to CO and C02 were initially performed with the atomic sensitivity factors (ASF) of ref 6. The O/Th results calculated with ASF’s from this
+
792
The Journal of Physical Chemistry, Vol. 87, No.
5, 1983
McLean et at.
TABLE 11: Quantitative Analysis of Elements in the Near-Surface Region of Oxidized Thorium and T h o , Coatings Plus CO and CO, .
at. substrate
gas
0
exposurea
%b
C
Th
formulab
formulaC
formulad
Th(ll1) 0, 40 0.62 0.37e Tho,.,, Tho,.,, ThO2.24 Tho, 0.56 0.04 0.40 ThOl.38CO.W Th02.14C0, I I Tho 1.X4c0.04 Tho, co 1.0 0.55 0.06 0.39 Th01.41C0.15 Th02.1qC0.20 ThO,.,,CO.,, Tho, co 2.5 0.56 0.07 0'37 Th01.51C0.19 Th02.34C0.25 ThoZ.01C0.21 Tho2 co 5.0 0.56 0.07 0.37 ThOl.5lCO.19 ThO2.34Co.25 ThO2.",C0.,2 0.08 0.59 co2 10.0 ThoT 0.34 ThOl.74CO.24 ThO2.,*C",2, ThO.3lC0.25 a Units of langmuir. 1 langmuir zs 0.13 mPa.s = 1 x torrs. Atomic sensitivity factors6 for Th(4f7,, 4.8;0(1s) = 0.63; and C(1s) = 0.205. Atomic sensitivity factors' for Th(4f,,,) = 7.8; O(1s) = 0.66 and, C(1s) = 0.25. Atomic sensitivity factors for Th(4f7,,) = 6.7 (this work); O(1s) = 0.66';and C(1s) = 0.25.9 e Based on samples 79% oxidized.
d=
TABLE 111: Atomic Sensitivity Factors for Th(4f7,,) Referred to F( 1s) and Calculated O/Th Ratios (Mg Kct X rays) O/Th sample T h o , film sputter cleaned ( 3 samples) T h o 2single-crystal (sputtered) Th t 40 langmuir of 02,20 "C (no sputtering) av value
ASFa 7.3
t
this work
0.1 1.84
F
ref gC
0.03 2.14
6.7
2.00
2.33
6.0
2. 24d
2.64
6.7 i 0.6 2.03
i-
0.20 2.37
+_
0.25
This work. Based on ASF for O(1s) = 0.66 of ref 9. Based on ASF's for O(1s) = 0.66 and Th(4f7,,) = 6.7 f Based on ref 9 values of O( Is) = 0.66 and 0.6. Calculated by taking into account the Th(4f,,,) - 7.8. oxide contribution to the Th(4f) only. Error bands shown are 1 a. a
reference, neglecting gold, are unexpectedly low and are summarized in Table 11. Using new relative atomic sensitivity factors reported by Wagner et al.9 for the O(1s) [0.66] and C(1s) [0.25] levels we have calculated an average value of 6.7 f 0.6 for the relative ASF of the Th(4f7I2)peak which is in relatively good agreement with their recently published value of 7.8. Our average value is based on measurements on thorium oxide films, on a T h o 2single crystal, and on clean T h + O2 as shown in Table 111. Stoichiometries of these samples calculated with our ASF's for Th(4fi 2) and those of Wagner et al.9 are also shown in Table If.
Discussion The observed binding energies for C(1s) (284.6-285.4 eV) compare well with the values obtained by McLean et al.' for molecular CO bound to partially oxidized T h ( 111). These authors found that atomic carbon on this crystal face had a distinctly different binding energy of 281.6 eV. The same is true for the chemisorption of C02where the experimentally measured binding energy of -284.3 eV was found to be characteristic of C 0 2bound to partially oxidized th0rium.l CO has been found to initially adsorb on Tho2 as a neutral species as shown by the electron paramagnetic resonance work of Brey et al.,'O while the infrared studies of Pichat and Mathieu" present evidence for an OCO complex according to the spectrum of thorium formate. CO acquires a slight positive charge (CO+) as its surface coverage on Tho2 increases as reported by Claudel et a1.I2in their kinetic and electrical conductivity studies (10) W. S. Brey, Jr., R. B. Gammage, and Y. P. Virmani, J. Phys. Chem. 75, 895 (1971). (11) P. Pichat and M. V. Mathieu, Proc. Int. Congr. Catal. 3rd, 1964, 1, 224 (1965). (12) B. Claudel, F. Juillet, Y. Trambouze, and J. Veron, Proc. Int. Congr. Catal., 3 4 1964, 1, 214 (1965).
of the catalytic oxidation of CO on thoria. This has also been shown to be the case by Breysse et al.13 in their calorimetric study of the same process, and by Meriaudeau et al.I4 in their electron spin resonance studies of the adsorption of CO on Tho2. These last authors postulated that adsorbed CO exists in the form of a positively charged axial radical with an unpaired electron located on a R orbital. Two adsorption species were detected by ESRI4 for which the unpaired electron was respectively 33 and 15% on the carbon atom. Our experimental results are consistent with the mechanism described above since we observed a slight increase in the C(1s) binding energy from 284.6 to 285.4 eV, indicating a slight amount of positive charge on the CO complex. A direct comparison of metal formate binding energies (-288 eV)6 with the experimentally determined values is not warranted in this case because of the differences between surface complexes and free-existing chemical compounds. Infrared studies by Pichat et al.15 for the chemisorption of CO and C02on thoria showed that CO was chemisorbed as a surface carboxylate and that C 0 2 was adsorbed as bidentate and monodentate carbonates at room temperature. The bidentate was not very stable and was removed simply by evacuation while the monodentate was found to be stable in vacuum up to 773 K. This behavior was further confirmed by the calorimetric work of Breysse et a l . I 3 who suggested that the monodentate carbonate form is likely to be negatively charged. The EPR studies of Brey et al.'O confirm the formation of a C02-radical formed by the oxidation of CO on Thoz. further evidence is provided by the infrared work of Colmenares16on the chemisorption of C 0 2 on p- and n-type uranium oxides, where this gas was found to chemisorb as a COz- ion on p-type UOz.ls. The binding energies of the C(1s) level for CO and COz in the gas phase are 295.9 and 297.2 eV, re~pectively,'~ while for carbonates and carboxylates they range from 287 to 289 eV;6 in contrast, for C02 adsorbed on T h o 2 we determined a binding energgy of 284.3 eV which is much lower than that measured for CO (284.6-285.4 eV) and for carbonates and carboxylates. The most plausible explanation for this behavior is that carbon dioxide is present as a C0,- ion on the surface of Tho2which is in agreement with the results from the authors reviewed so far. In our work both CO or COz were evacuated from the vacuum system at room temperature after exposure of the Thoz
-
(13) M. Breysse, B. Claudel, M. Prettre, and J. Veron, J . Catal., 24, 106 (1972). (14) P. Meriaudeau, M. Breysee, and B. Claudel, J . Catal., 35, 184 (1974). (15) P. Pichat, J. Veron, B. Claudel, and V. Mathieu, J. Chem. Phys., 63, 68 (1966). (16) C. Colmenares, J. Phys. Chem., 78, 2117 (1974). (17) K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P. F. Heden,
K. Hamrin, U. Gelius, T. Bergmark, L. 0. Wermer, R. Manne, and Y. Baer, "ESCA Applied to Free Molecules", North-Holland, Amsterdam, 1971.
Chemlsorptlon of CO and
CO, on Tho,
The Journal of Physical Chemlstry, Vol. 87, No. 5, 1983 793
TABLE IV: Relative Atomic Sensitivity Factors and Photoelectric Cross Sections for Th( 4f7,,) Referred to F(1s) (Mg Ka X rays) cross section ASF this worka
ref 6
ref 9
theor (ref 5)
*
4.8
7.8
5.47
6.7
0.6
exptl (ref 1 9 ) 6.2b
Based on ASF's for O(1s) = 0.66' and C(1s)= 0.25.9 This value was obtained from Evans' workI9 by taking their total experimental differential cross section for the 4f level ( 8 . 6 9 ) , removing the correction that they applied for 90" angular dependence, and taking the contribution for the 4f7,, level to be 0.58 (7/12)of the total cross section.
sample, thus we conclude from the results cited above that the C(1s) peak at -284.3 eV is due to a negatively charged monodentate carbonate. In addition, the 533-eV O(ls) signal observed when C02 is chemisorbed on T h o 2 films can then be assigned to the carbonyl oxygen of a charged monodentate carbonate species on T h o 2 as shown in the inset in Figure 6. Future studies at high temperature and pressure will be performed to look for a correlation between surface chemistry and bonding and catalytic product distribution. These will be particularly relevant because thoria Fischer-Tropsch catalysts2p3have been reported to produce methanol and branched C4's and unsaturated hydrocarbons. The presence of bound CO on the thoria surface suggests that a CO insertion mechanism similar to that outlined by Schulz and Zein el Deen18may be operative; this reaction scheme predicts a similar product distribution. Wagner et al.9 have done considerable work trying to unravel the problems associated with quantifying photoelectron spectroscopy data. They have critically analyzed experimental data and compared it with theoretically calculated values to come to the conclusion that many of the commonly accepted calculated cross-section data are in error by as much as 40% in some cases for strong photoelectron lines. Similar conclusions are reported in an earlier study by Evans, Pritchard, and Thomas.lQ Values of calculated cross sections and atomic sensitivity factors for Th(4f7/,) from the works cited a b o ~ e ,as ~ Jwell ~ as the values of our average ASF, are tabulated in Table IV. Our ASF value for the Th(4f7 2) level is -50% higher than that listed in ref 6 (4.8) and brings the O/Th ratio of our oxidized thorium metal, single-crystalThoz and the oxide films prepared in this work to 2.03 f 0.20 (Table 111). A revised analysis of the oxygen, carbon, and thorium content of the near-surface region based on ref 9 and on our work is presented in the right-hand columns of Table 11. We believe that we were dealing with stoichiometric Tho2 because of its thermodynamic stability and because this compound exhibits a very narrow range of stoichiometry (the x in Th02+xonly varies on the order of parts per million).20 Also, X-ray diffraction of the films showed the bulk to be stoichiometric Tho2, and the shapes and binding energies of the Th(4f) peaks and shake-up satellites were characteristic of Tho2. An excess of metal would have shown up as a knee on the low-binding energy side of the Th(4f712) peak, similar to that seen in Figure 4a. The (18)H.Schulz and A. Zein el Deen, Fuel R o c . Technol., 1,45(1977). (19)S. Evans, R. G. Pritchard, and J. M. Thomas, J . Electron Spectrosc. Relat. Phenom., 14, 341 (1978). (20)J. C. Rivigre, Brit. J . A p p l . Phys., 16,1507 (1965).
TABLE V : Identification of Major Peaks and Satellites Produced by Mg K a Radiation between 330 and 350 eV peakenergy, eV
peak identification
excitation source
334.0 335.0 335.9 343.4 344.3
Th(4f5,,) Th(4f,,,) Th(4f,,,) Th(Bf,,,)O( 2p)/VB Th(4f5,,)
Mg Ka, Mg Ka I,, Mg Ka, Mg K a , , , Mg Ka 1,2
results shown in Table 111, whether based on our calculated ASF or that of Wagner et a1.: confirm our belief that all of our samples, within experimental error, were stoichiometric thorium dioxide. Furthermore, LEED studies of the oxidation of clean (lOO)*land (111)22 faces of thorium have shown that stoichiometric Tho2 is produced. Since we obtained almost identical results from the oxidation of thorium metal, from sputtered oxide films, and from sputtered single-crystal Tho2 (Table 111),we conclude that sputtering did not significantly change the surface stoichiometry of the last two samples. The calculation of the thorium ASF is influenced by the measurement of the intensity of the Th(4f) levels which may be affected by the following factors (see Table V): (1)satellite peaks due to shake-up transitions (the Th(47/2), 0(2p)/VB shake-up peak appears at -343.4 eV and is difficult to separate from the Th(4f5/,) level at 344.3 eV); (2) ghost peaks due to satellite lines in the X-ray source (the Th(4fsj2)level excited by Mg K q 4 lines appear at 334 and 335.9 eV, respectively, and thus bracket the Th(4f7/,) peak at 335 eV); and (3) background substraction (The method used for background substraction in our case was arbitrary but gave reproducible results. A straight line was drawn from the background at 350 eV to the background level at 330 eV, and the peak areas were partitioned at a point midway between the maximum intensities of the Th(4f7/2)and Th(4f,i2) levels.) (1)and (2) partially offset each other and (1)has been shown not to be very significant by Evans et al.19 The background substraction method was valid as verified in our earlier work' where the intensity of the Th(4f7/,) peak (measured by the area under the peak) for clean, partially oxidized, and fully oxidized thorium was invariant within f3%.
Conclusions We have reported a technique for preparing mechanically stable T h o z films on chemically inert substrates. These films can be cleaned and characterized in UHV and are suitable for gas adsorption studies. They are also amenable for service in the high temperature, high pressure hydrogen environments needed to study the FischerTropsch synthesis. We have found that both CO and C02 adsorb on Tho2at room temperature and at low pressures without dissociating. Acknowledgment. We acknowledge T. McCreary for preparing the Tho2films. This work was performed under the auspices of the U S . Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. Registry No. CO, 630-08-0; COz, 124-38-9;Thoz, 1314-20-1; Au, 7440-57-5. (21)T.N.Taylor, C. A. Colmenares,R. L. Smith, and G. A. Somorjai, Surf. Sci., 5 , 317 (1976). (22)R. Bastasz, C. A. Colmenares, R. L. Smith, and G. A. Somorjai, Surf. Sci., 67,45 (1977).
