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The surface properties of vanadium compounds by X-ray photoelectron spectroscopy
Applied Surface Science 148 Ž1999. 64–72
The surface properties of vanadium compounds by X-ray photoelectron spectroscopy Jeong-Gil Choi
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Department of Chemical Engineering, Hannam UniÕersity, 133 Ojeong-dong, Taedug-gu, Taejon 300-791, South Korea Received 9 November 1998; accepted 3 February 1999
Abstract The electronic structures of vanadium carbides have been examined using X-ray photoelectron spectroscopy ŽXPS. before and after Ar sputtering. After Ar ion sputtering for 1 h, there was no significant variation observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and O 1s for vanadium carbides used in the present study. A species with a binding energy of 513.2 " 0.2 eV from vanadium carbides was newly observed, and was estimated to be V 1.3q using the plot of the V 2p 3r2 binding energies against the oxidation number. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4 " 0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. For all of 1-h sputtered vanadium carbides except for V8 C 7-4, the average ratio of C crV dq was nearly unity. Using NH 3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in this study. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Vanadium carbides
1. Introduction There is a spreading interest in the use of early transition metal carbides and nitrides as catalysts andror catalyst supports w1,2x. The primary reason for this is that these materials have unique properties that are characterized by the coexistence of metallic, covalent and ionic bondings w3x. Thus far, a great
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deal of attention has been particularly given to molybdenum and tungsten nitrides and carbides w2,4–8x, since the previous studies reported that there are remarkable similarities between the catalytic properties of these materials and platinum-group metals for hydrocarbon conversion reactions including dehydrogenation w9–11x, isomerization w7,8,11x and hydrogenolysis w8,10,11x. Since catalysis is a surface phenomenon, it is indispensable to fully understand the surface properties of these materials when used as catalysts. However, very little has been reported concerning the surface properties of these materials, and their effects on the catalytic function.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 3 2 - 4
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
In the present study, the electronic structures and the surface compositions of vanadium carbides have been elucidated using X-ray photoelectron spectroscopy ŽXPS.. To date, there are only a few publications for the vanadium carbides used as catalysts. Therefore, we also attempted to discuss the possible relationships between the surface properties and catalytic functions of vanadium carbides. For this purpose, the vanadium carbides were prepared by the temperature-programmed carburization of vanadium oxide precursors ŽV2 O5 . with pure CH 4 or a mixture of 49.9% CH 4 in H 2 . The X-ray diffraction ŽXRD. and Brunauer–Emmett–Teller ŽBET. total surface area measurements were employed to evaluate the structural and sorptive properties of these materials. For reference, the vanadium oxide precursors were also used for XPS studies.
2. Experimental Vanadium carbides used in the current study were prepared by the reaction of vanadium oxide ŽV2 O5 , 99.95%, Junsei Chemical. with pure CH 4 or a mixture of 49.9% CH 4 in H 2 ŽTaedug Gas. in a temperature-programmed manner. Different heating rates and molar hourly space velocities were used to obtain the final products with different structural and compositional properties: two heating rates of 120 and 240 K hy1 , and two CH 4 molar hourly space velocities of 20.4 and 10.2 hy1 . Here, the molar hourly space velocity was defined as the reactive gas molar flow rate ŽCH 4 . divided by the molar amount of precursor ŽV2 O5 .. More details about the synthesis of vanadium carbides were similar to that of vanadium nitrides and were described elsewhere w12,13x. After synthesis, the final products were passivated at room temperature in a mixture of 0.5% O 2 in He ŽTaedug Gas. flowing at 20 cm3 miny1 to prevent bulk oxidation. This passivation was continued for 2 h. The product was then removed from the reactor for XPS analysis. The flow method using a Quantasorb Model Chembet 3000 sorption analyzer was used for measurements of the BET surface areas of vanadium carbides. The bulk structures of the materials were evaluated using a computer controlled Rigaku Rotaflex DMAX-B rotating anode X-ray diffractometer with a Cu-K a radiation source. The
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brief bulk and sorptive properties of vanadium carbides are shown in Table 1. Particle and crystallite sizes were estimated using the BET surface areas and XRD results, respectively. The average size of particles in vanadium carbides, d p , was estimated using the following equation d p s 6rSg r , where Sg is the BET surface area, and r is the density of the primary bulk phase Ž r s 3.36 and 5.64 g cmy3 for V2 O5 and V8 C 7 , respectively..The average crystallite sizes, d c , were calculated using the Scherrer equation w14x, d c s K lrŽ B cos u ., where K is a constant Žhere taken to be 0.9., l is the wavelength of the ˚ ., B is the corrected peak X-radiation Ž1.5405 A width, and u is the Bragg angle of the diffraction peak. The peak width was taken to be the full-width at half-maximum intensity of the most intense peak in the pattern. The X-ray photoelectron spectroscopic experiments were carried out using a VG ESCA LAB 220 I spectrometer using the Mg-K a X-ray source. The XPS spectra of the samples were collected before and after Ar ion sputtering at 3 kV for 1 h. The total sputtered depth after 1 h sputtering was estimated to ˚ with a sputtering rate of 12 A˚ miny1 . be about 720 A For some selected samples, different Ar ion sputtering conditions were used for determining the influence of sputtering on the electronic structure of the samples. Vacuum in the test chamber during the collection of spectra was typically less than 4 = 10y9 Torr. The spectrometer energies were calibrated using the Au 4f 1r2 peak at 84.0 eV and the Cu 2p 3r2 peak at 932.6 eV. Gaussian andror Lorentzian peaks were deconvoluted using a non-linear least-squares algorithm. The atomic compositions were estimated
Table 1 Bulk and sorptive properties of vanadium carbides Specimen no.
Phase present
Crystallite size a ˚. ŽA
Particle size b Žnm.
Surface area Žm2 gy1 .
V8 C 7 -4 V8 C 7 -7 V8 C 7 -8 V8 C 7 -11 V8 C 7 -31
V8 C 7 V8 C 7 V8 C 7 V8 C 7 V8 C 7
359 176 251 206 240
256 148 137 99 34
4 7 8 11 31
a b
Measured using XRD peak. Estimated using surface area.
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J.-G. Choi r Applied Surface Science 148 (1999) 64–72
based on comparisons of the integrated peak areas normalized by the atomic sensitivity factors. Errors in the composition are expected to be less than "15% w15x.
3. Results and discussion 3.1. Electronic structures of Õanadium carbides Using XPS, the electronic structures of vanadium carbides have been examined before and after Ar sputtering. After Ar ion sputtering for 30 min or 1 h, no change in the surface electronic structure was observed for vanadium carbides used in the present study. However, the primary species for Ar-sputtered materials was V dq while that for as-prepared materials was V 4q or V 5q. The only exception to this trend was for V8 C 7-7 which contained V dq as the primary species both before and after Ar ion sputtering. Even though the sputtering time was continuously increased after Ar ion sputtering for 1 h, there was no significant variation observed in the amount Žas expressed in area. of core levels of V 2p, C 1s, and O 1s. Therefore, 1 h sputtering with Ar ion was used as a standard method to pretreat all the vanadium carbides. Fig. 1 shows typical XPS spectra of the V 2p doublet in vanadium oxides ŽV2 O5 and VO 2 ., and vanadium carbide ŽV8 C 7 . as a function of the binding energy. The spectrum for the vanadium carbide in the V 2p region showed the presence of two well-resolved spectral lines at 513.2 eV and 520.9 eV ŽFWHMs 1.7.. These were assigned to the V 2p 3r2 Žat 513.2 eV. and V 2p1r2 Žat 520.9 eV. spin–orbit components, respectively. A species with a V 2p 3r2 binding energy of 513.2 " 0.2 eV from vanadium carbides was newly observed so that it was temporarily connoted as V dq where 0 - d - 4. The binding energies of V 2p were shifted to lower ones Ž517.3 ™ 516.1 ™ 513.2 eV. as the oxides were transformed to carbide during carburization ŽV 5q™ V 4q™ V dq .. As discussed later, using V oxides the oxidation number of this species was attempted to be determined by the plot of the V 2p 3r2 binding energies against the oxidation number. It is noteworthy that with increasing Ar ion sputtering time, the
Fig. 1. Typical XPS spectra of V 2p 3r 2 –V 2p1r2 doublets for V 5q, V 4q, and V dq as a function of the binding energy.
area percent of V dq for all the vanadium carbides was increased while those of V 4q and V 5q were decreased ŽTable 2.. These results indicate that the portions of carbide are exposed at the surface while the oxide layers are removed from the surface. This is supported by the calculation for the relative intensity of VVC xrVVO t as shown in Table 3. Here, VC x and VOt indicate the carbide and the total oxides ŽV2 O5 and VO 2 ., respectively. The relative intensity of VVC xrVVO t increased with the increase of the sputtering time for vanadium carbides. Fig. 2 shows the C 1s XPS spectrum in the vanadium carbides consisting of three deconvoluted components. The corresponding binding energies of C 1s are shown in Table 4. The C 1s peak with a binding energy of 285.6 " 0.2 eV was considered to be associated with adsorbates on the surface such as CO, CO 2 , and hydrocarbons Žthe peak C a in Fig. 2.. The carbon peak ŽC f . with a binding energy of 284.3 " 0.2 eV corresponds to the free carbon con-
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
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Table 2 XPS binding energies ŽeV. of V 2p1r 2 and V 2p 3r2 core levels for vanadium carbides and oxides Specimena no.
V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2 V2 O5 V2 O5-s2
V 2p1r 2
V 2p 3r2
V d q ŽVC x .
V 4q ŽVO 2 .
V 5q ŽV2 O5 .
V d q ŽVC x .
V 4q ŽVO 2 .
V 5q ŽV2 O5 .
520.9Ž7. b 520.8Ž11. 520.9Ž21. 520.8Ž26. 520.7Ž27. 520.9Ž12. 520.8Ž19. 520.9Ž25. 520.9Ž7. 520.9Ž17. 521.0Ž14. 520.9Ž24. – –
523.6Ž8. 523.7Ž7. 523.6Ž4. 523.6Ž5. 523.7Ž3. 523.6Ž9. 523.5Ž6. 523.6Ž4. 523.7Ž15. 523.6Ž9. 523.7Ž10. 523.6Ž4. 523.6Ž6. 523.7Ž17.
524.8Ž18. 524.9Ž15. 524.8Ž7. 524.8Ž3. 524.9Ž3. 524.8Ž12. 524.8Ž9. 524.8Ž4. 524.9Ž12. 524.8Ž8. 524.7Ž9. 524.8Ž5. 524.8Ž24. 524.9Ž18.
513.2Ž15. 513.1Ž23. 513.2Ž43. 513.2Ž52. 513.2Ž54. 513.2Ž24. 513.1Ž38. 513.2Ž51. 513.2Ž14. 513.3Ž33. 513.3Ž28. 513.2Ž47. – –
516.1Ž15. 516.0Ž13. 516.1Ž10. 516.2Ž9. 515.9Ž7. 516.1Ž18. 516.2Ž12. 516.1Ž7. 516.1Ž29. 516.0Ž18. 515.9Ž20. 516.1Ž9. 516.1Ž13. 516.1Ž33.
517.3Ž37. 517.1Ž31. 517.3Ž15. 517.3Ž5. 517.1Ž6. 517.3Ž25. 517.2Ž16. 517.3Ž9. 517.3Ž23. 517.2Ž15. 517.2Ž19. 517.3Ž11. 517.3Ž57. 517.3Ž32.
a
The s1 and s2 indicate Ar sputtering time of 30 min and 1 h, respectively. Ž.: area percent. d : 0 - d - 4 in oxidation number. b
taminated on the surface of carbides. This result also suggested that this free carbon was graphite-like. The peak C c shows the carbidic carbon with 282.4 " 0.2 eV, which is ascribed to the photoelectrons ejected from the carbon in vanadium carbide lattice. Thus, these carbidic carbon peaks increased with the increase of the sputtering time as shown in Table 4.
This is also supported by the increase in the relative intensity of C crC t in Table 4. A similar binding energy for the carbidic carbon in vanadium carbide has been observed by Hakansson et al. w16x. They reported that using the VC Ž100., the deconvoluted peak that appeared at a binding energy of 282.2 eV was ascribed to the surface carbidic carbon.
Table 3 Atomic ratios of V 2p 3r 2 , C 1s, and O 1s core levels for vanadium carbides Specimen no.
C crV dq
C crVt
ŽOd q Op .rVt
VVC xrVVO t
ŽC c q Od q Op .rVt
V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2
0.12 0.10 0.10 0.99 0.96 1.54 1.07 0.76 1.22 0.79 1.32 1.19
0.08 0.06 0.05 0.78 0.76 0.56 0.35 0.22 0.29 0.30 0.87 0.73
1.28 1.17 1.18 0.80 0.72 1.99 1.70 1.36 2.16 1.83 2.42 1.11
0.28 0.52 1.78 3.71 4.26 0.56 1.33 3.17 0.27 1.00 0.72 2.45
1.36 1.23 1.23 1.58 1.48 2.55 2.05 1.58 2.45 2.13 3.29 1.84
The C c and Vt indicate the carbidic carbon and total vanadium, respectively. The VOt and C t represent the total amount of vanadium oxides ŽVO 2 and V2 O5 ., and the total carbon amount ŽC t s C c q C f q C a ., respectively. d : 0 - d - 4 in oxidation number.
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
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Fig. 2. Representative XPS spectrum of C 1s energy region for vanadium carbides. Three different carbons are described as carbidic ŽC c ., free ŽC f ., and adsorbed ŽC a . carbons.
A typical XPS spectrum in the O 1s energy region for vanadium carbide can be shown in Fig. 3 with three deconvoluted peaks ŽOa , Op , and Od .. Table 4 also shows the binding energy of O 1s with the sputtering time. Chemisorbed or lattice oxygen can
be identified with a binding energy of 532.1 " 0.2 eV, which was deconvoluted as a peak Oa in Fig. 3. The other two O 1s peaks with different binding energies of 530.1 " 0.2 eV Žpeak Od . and 530.8 " 0.2 eV Žpeak Op . were considered to be due to vanadium
Table 4 XPS binding energies ŽeV. and relative intensities of C 1s and O 1s core levels for vanadium carbides and oxides Specimena no. C 1s
O 1s
Relative intensity
C c Žcarbidic. C f Žfree. C a Žadsorbed. Od ŽVO 2 . Op ŽV2 O5 . Oa Žadsorbed. C crCq C crŽC f q C a . OarŽOd q Op . t V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2 V2 O5 V2 O5-s2 a
282.5Ž4. b 282.4Ž8. 282.5Ž15. 282.6Ž13. 282.6Ž17. 282.5Ž10. 282.4Ž14. 282.5Ž18. 282.5Ž7. 282.6Ž12. 282.6Ž11. 282.5Ž16. – –
284.3 284.4 284.3 284.4 284.5 284.3 284.4 284.3 284.3 284.4 284.5 284.3 284.3 284.3
285.8 285.6 285.8 285.7 285.6 285.8 285.6 285.8 285.8 285.7 285.6 285.8 285.8 285.8
530.2 530.1 530.2 530.2 530.3 530.2 530.1 530.2 530.2 530.1 530.2 530.3 530.3 530.3
530.8 530.9 530.8 530.9 530.8 530.9 530.8 530.9 530.9 530.8 530.9 530.9 530.9 530.9
The s1 and s2 indicate Ar sputtering time of 30 min and 1 h, respectively. Ž.: area percent. q The C t represent the total carbon amount ŽC t s C c q C f q C a .. b
532.2Ž8. 532.2Ž24. 532.1Ž26. 532.3Ž23. 532.1Ž46. 532.1Ž31. 532.3Ž40. 532.1Ž42. 532.1Ž19. 532.2Ž47. 532.3Ž12. 532.1Ž64. 532.1Ž12. 532.1Ž19.
0.04 0.08 0.15 0.13 0.17 0.10 0.14 0.18 0.07 0.12 0.11 0.16 – –
0.04 0.09 0.18 0.15 0.20 0.11 0.16 0.22 0.08 0.14 0.12 0.19 – –
0.09 0.32 0.35 0.30 0.85 0.45 0.67 0.72 0.23 0.89 0.14 1.78 0.13 0.23
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
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Fig. 3. Representative XPS spectrum of O 1s energy region for vanadium carbides. Peaks Od and Op are due to vanadium oxides, and peak Oa is ascribed to strongly adsorbed oxygen.
dioxide ŽV 4q . and vanadium pentoxide ŽV 5q ., respectively. Vanadium carbides used in this study had the same, or very similar O 1s binding energies. Notwithstanding the lack of well-assigned oxygen species, it is a general consensus that peaks at ; 530 eV and at ; 532 eV are ascribed to oxides and oxygen species dissolved in the metal or to adsorbed oxygen, respectively. Accordingly, the oxygen species with the higher binding energy was assigned to strongly adsorbed oxygen Žas Oy. , OHy, or H 2 O. The relative intensity of the lattice oxygen inserted or dissolved into the vanadium carbides Žexpressed as Oa . to the oxygen in vanadium oxides ŽVO 2 and V2 O5 . Žexpressed as Od q Op . increased significantly after ion sputtering ŽTable 4., showing that there is a relationship between relative intensity and surface area of vanadium carbides. These results indicate that the oxygen extent inserted in high surface area vanadium carbide is larger than that in the low surface area carbide. Similar observations have been observed for tantalum carbides w17x and molybdenum nitrides w18x. 3.2. Atomic ratios and relatiÕe intensities The atomic ratios ŽCrV, OrV. and relative intensities of V 2p 3r2 , C 1s, and O 1s core levels for
vanadium carbides are summarized in Tables 3 and 4. Quantitative relationships between carbon and vanadium ŽCrV atomic ratio. were derived from calculation of peak area ratios of the respective states in the deconvoluted C 1s spectra and V 2p 3r2 spectra ŽFigs. 2 and 4.. Similarly, the OrV atomic ratios were estimated using the V 2p 3r2 and O 1s peak areas. Here, V dq represents carbide portion while Vt shows the total vanadium metal consisting of the portions of carbides ŽV dq . and oxides ŽV 4q and V 5q .. These peak areas were normalized using the atomic sensitivity factors. The atomic sensitivity factors for the V 2p 3r2 , C 1s, and O 1s peaks were 1.3, 0.25, and 0.66, respectively w15x. Table 3 shows two different types of carbon– vanadium atomic ratios, C crV dq and C crVt . While the atomic ratio of C crV dq indicates the surface composition of vanadium carbide, the C crVt indicates a ratio of carbidic carbon number over total vanadium metal number at surface. With the exception of V8 C 7-4, the average ratio of C crV dq for all the 1 h sputtered vanadium carbides was nearly unity. This result suggested that the VC phase is present at the surface of vanadium carbides. However, the average bulk composition of carbonr vanadium obtained from a CHN analyzer was found to be ; 1.75. Here, the primary reason for the larger
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J.-G. Choi r Applied Surface Science 148 (1999) 64–72
Fig. 4. Typical deconvoluted XPS spectrum of V 2p energy region for vanadium carbides.
composition than 0.875 of carbonrvanadium for V8 C 7 was considered to be due to the presence of polymeric carbon produced during carbiding of the vanadium oxide. Accordingly, it can be seen that for vanadium carbides, the properties of surface are different from those of bulk. It is generally known that the smaller compositions of carbonrvanadium suggests the carbon-deficient vanadium carbides while the larger values indicate the vanadium carbides containing excess carbon atoms located on sites other than lattice carbons in the cubic structure, such as interstitial or substitutional of vanadium atoms. These excess carbon atoms may have the different bonding from V–C one in the cubic structure. A preliminary experiment was performed to understand the relationship between the carbonr vanadium and the catalytic activity. Using NH 3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in the current study. The ratios of C crV dq and C crVt generally decreased as the sputtering time increased. This behavior was also observed for the atomic ratios of oxygen to vanadium as shown in Table 3. These results indicate that the
effect of sputtering was larger on the light elements of C and O than on the heavy V metal. Similar results were also found for Mo 3p 3r2 , N 1s, and O 1s in the molybdenum nitrides w18,19x. Tables 2 and 3 show that from the peak deconvolutions of C 1s and V 2p, the binding energies for carbidic carbon of C 1s and V dq 2p 3r2 were estimated to be 282.4 " 0.2 and 513.2 " 0.2 eV, respectively. With reference energies for C 1s and V 2p, the shifts in the binding energy of the C 1s and V 2p levels were unambiguously estimated. Taking reference energies of 511.9 eV for V 2p and 284.4 eV for C 1s, the V 2p peak tended to shift to higher energies while the C 1s peak shifted to lower energies. This result indicated that the electron charge was transferred from the V 3d states to the C 2p states. This transfer of electron charge was evidenced by the difference of electronegativity between V Ž1.63. and C Ž2.55.. As discussed later, the direction of charge transfer from vanadium to carbon can be also confirmed by the estimation of positive oxidation state of vanadium in the current study. Accordingly, it was considered that an ionic character of bonding was induced in the V–C lattice while the covalency of the V–C bonding has been reduced w20,21x. The expected binding energy separation between elemental V and adsorbed carbon is 227.5 eV. This result
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
suggested that after carbiding, the increase of 3.3 eV in binding energy separation between the V dq 2p 3r2 and C 1s peaks, and the binding energy separation of ; 229.5 eV between V 0 and C 1s were attributed to the formation of vanadium carbides. In a separate study significant amount of carbon, residue from the synthesis of the vanadium carbides via a reaction of V2 O5 with CH 4 , and H 2 O have been observed on the surface of the as-prepared materials w12x. Therefore, it is not surprising that the atomic ratios of ŽC c q Od q Op .rVt were estimated to be in excess of unity. After 1 h sputtering the atomic ratios of ŽC c q Od q Op .rVt decreased by ; 20% for all the vanadium carbides. This result again indicates that the sputtering pretreatment used in the present study could influence the elements of C and O in the lattice. 3.3. Determination of Õanadium oxidation number As can be seen in Fig. 2, the deconvolution of vanadium 2p spectra was performed to obtain the distribution of vanadium oxidation states. The V 2p spectra typically consisted of two envelopes. The V 2p 3r2 –V 2p1r2 doublet was fitted so that each peak had the same Gaussian line shape and width ŽFWHM.. The relative intensities of spin–orbit doublet peaks are given by the ratio of their respective degeneracies and the intensity ratio for the V 2p 3r2 – V 2p1r2 doublet should be I Ž2p 3r2 .rI Ž2p1r2 . s 2r1. A splitting energy of ; 7.7 eV was expected for the V 2p 3r2 –V 2p 1r2 doublet. The deconvolution of V 2p spectra shows three species of V 5q, V 4q and V dq, and gave rise to a new species with a V 2p 3r2 binding energy of 513.2 " 0.2 eV ŽTable 2.. The binding energy of this new species is midway between those assigned to V 4q Ž516.0 " 0.2 eV. and V 0 Ž511.9 " 0.2 eV., and is about 2.8 eV lower than that for V 4q. As stated earlier, since the new species of vanadium carbide was temporarily denoted as V dq where 0 - d - 4, it is worthwhile to attempt to determine the oxidation number of this species. We tried to use the calibration method generally known in the determination of the oxidation number by using the plot of the V 2p 3r2 binding energies against the oxidation number. Here, the binding energies of V 0 , V 4q, and V 5q
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Fig. 5. Plot of V 2p 3r 2 binding energies as a function of the oxidation number.
which are generally accepted, were used in this study. Therefore, the result from Fig. 5 suggested that the binding energy of 513.2 eV might be ascribed to V 1.3q. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Subsequently, this charge transfer suggests an actual ionic contribution to the bonding of vanadium carbides. Thus far, no reliable standard for the binding energy of V 1.3q has ever been published. However, Chen et al. w22x reported that a similar vanadium oxidation state exists for vanadium carbide films. They used the near-edge X-ray absorption fine structure ŽNEXAFS. technique to investigate the electronic and structural properties of thin vanadium carbide films on a vanadium Ž110. single-crystal surface. It was reported that the oxidation state of vanadium carbide thin film was estimated to be in the range of 1.2 " 0.2 electrons per vanadium using the NEXAFS results of VŽ110., VCrVŽ110. and various model vanadium oxides.
4. Conclusions No significant variation was observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and 0 1s for vanadium carbides after Ar ion 1 h sputtering. A species with a binding
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J.-G. Choi r Applied Surface Science 148 (1999) 64–72
energy of 513.2 " 0.2 eV from vanadium carbides was newly observed, and was estimated to be V 1.3q. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. The binding energies of V 2p were shifted to lower ones Ž517.3 ™ 516.1 ™ 513.2 eV. as the oxides were transformed to carbide during carburization ŽV 5q™ V 4q™ V dq .. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4 " 0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. These carbidic carbon peaks increased with the increase of the sputtering time and this response was also supported by the increase in the relative intensity of C crC t . For all of the 1-h sputtered vanadium carbides except for V8 C 7-4, the average ratio of C crV dq was nearly unity. This result suggested that the VC phase is present at the surface of vanadium carbides. However, the catalytic activity measurements using NH 3 decomposition reaction exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at the surface of vanadium carbide. The surface composition was different from the average bulk composition of ; 1.75. The primary reason for the larger composition in bulk was considered to be due to the presence of polymeric carbon produced during carburization of the vanadium oxide.
Acknowledgements This work was financially supported by a 1998 Hannam University Grant. The author thanks YoungJin So for her assistance in obtaining the XPS data for the vanadium carbides.
References w1x L. Volpe, S.T. Oyama, M. Boudart, in: G. Poncelet, P. Grange, P.A. Jacobs ŽEds.., Preparation of Catalysts III, Elsevier, New York, 1983, p. 147. w2x J.S. Lee, S. Locatelli, S.T. Oyama, M. Boudart, J. Catal. 125 Ž1990. 157. w3x L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. w4x J.-G. Choi, J.R. Brenner, C.R. Colling, B.G. Demczyk, J.L. Dunning, L.T. Thompson, Catal. Today 15 Ž1992. 201. w5x J.-G. Choi, R.L. Curl, L.T. Thompson, J. Catal. 146 Ž1994. 218. w6x J.-G. Choi, J.R. Brenner, L.T. Thompson, J. Catal. 154 Ž1995. 33. w7x R.B. Levy, M. Boudart, Science 181 Ž1973. 547. w8x F.H. Ribeiro, R.A. Dalla Betta, M. Boudart, J. Baumgartner, E. Iglesia, J. Catal. 130 Ž1991. 86. w9x G.V. Samsonov, T.G. Bulankova, P.A. Khodak, E.M. Preshedromirskaya, V.S. Sinelnikova, V.M. Sleptsov, Kinet. Katal. 10 Ž1969. 863. w10x L. Leclercq, M. Provost, H. Pastor, G. Leclercq, J. Catal. 117 Ž1989. 384. w11x Z. Gao, D.Y. Qin, Chin. J. Chem. 3 Ž1990. 207. w12x J.-G. Choi, J. Ha, J.-W. Hong, Appl. Catal. 168 Ž1998. 47. w13x J.-G. Choi, M.-K. Jung, S. Choi, T.-K. Park, I.-H. Kuk, J.-H. Yoo, H.S. Park, H.-S. Lee, D.-H. Ahn, H. Chung, Bull. Chem. Soc. Jpn. 70 Ž1997. 1. w14x B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, MA, 1978. w15x D. Briggs, M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Wiley, New York, 1983. w16x K.L. Hakansson, L.I. Johansson, M. Hammar, M. Gothelid, Phys. Rev. B 47 Ž1993. 10769. w17x J.-G. Choi, accepted for Appl. Catal. Ž1999.. w18x J.-G. Choi, D. Choi, L.T. Thompson, Appl. Surf. Sci. 108 Ž1997. 103. w19x B.G. Demczyk, J.-G. Choi, L.T. Thompson, Appl. Surf. Sci. 78 Ž1994. 63. w20x L. Pauling, The Nature of the Chemical Bond, 3rd edn., Cornell Univ. Press, Ithaca, NY, 1960. w21x J.-G. Choi, L.T. Thompson, Appl. Surf. Sci. 93 Ž1996. 143. w22x J.G. Chen, C.M. Kim, B. Fruhberger, B.D. DeVries, M.S. Touvelle, Surf. Sci. 321 Ž1994. 145.
The surface properties of vanadium compounds by X-ray photoelectron spectroscopy Jeong-Gil Choi
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Department of Chemical Engineering, Hannam UniÕersity, 133 Ojeong-dong, Taedug-gu, Taejon 300-791, South Korea Received 9 November 1998; accepted 3 February 1999
Abstract The electronic structures of vanadium carbides have been examined using X-ray photoelectron spectroscopy ŽXPS. before and after Ar sputtering. After Ar ion sputtering for 1 h, there was no significant variation observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and O 1s for vanadium carbides used in the present study. A species with a binding energy of 513.2 " 0.2 eV from vanadium carbides was newly observed, and was estimated to be V 1.3q using the plot of the V 2p 3r2 binding energies against the oxidation number. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4 " 0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. For all of 1-h sputtered vanadium carbides except for V8 C 7-4, the average ratio of C crV dq was nearly unity. Using NH 3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in this study. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Vanadium carbides
1. Introduction There is a spreading interest in the use of early transition metal carbides and nitrides as catalysts andror catalyst supports w1,2x. The primary reason for this is that these materials have unique properties that are characterized by the coexistence of metallic, covalent and ionic bondings w3x. Thus far, a great
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Tel.: q82-42-629-7856; Fax: q82-42-623-9489; E-mail: [email protected]
deal of attention has been particularly given to molybdenum and tungsten nitrides and carbides w2,4–8x, since the previous studies reported that there are remarkable similarities between the catalytic properties of these materials and platinum-group metals for hydrocarbon conversion reactions including dehydrogenation w9–11x, isomerization w7,8,11x and hydrogenolysis w8,10,11x. Since catalysis is a surface phenomenon, it is indispensable to fully understand the surface properties of these materials when used as catalysts. However, very little has been reported concerning the surface properties of these materials, and their effects on the catalytic function.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 3 2 - 4
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
In the present study, the electronic structures and the surface compositions of vanadium carbides have been elucidated using X-ray photoelectron spectroscopy ŽXPS.. To date, there are only a few publications for the vanadium carbides used as catalysts. Therefore, we also attempted to discuss the possible relationships between the surface properties and catalytic functions of vanadium carbides. For this purpose, the vanadium carbides were prepared by the temperature-programmed carburization of vanadium oxide precursors ŽV2 O5 . with pure CH 4 or a mixture of 49.9% CH 4 in H 2 . The X-ray diffraction ŽXRD. and Brunauer–Emmett–Teller ŽBET. total surface area measurements were employed to evaluate the structural and sorptive properties of these materials. For reference, the vanadium oxide precursors were also used for XPS studies.
2. Experimental Vanadium carbides used in the current study were prepared by the reaction of vanadium oxide ŽV2 O5 , 99.95%, Junsei Chemical. with pure CH 4 or a mixture of 49.9% CH 4 in H 2 ŽTaedug Gas. in a temperature-programmed manner. Different heating rates and molar hourly space velocities were used to obtain the final products with different structural and compositional properties: two heating rates of 120 and 240 K hy1 , and two CH 4 molar hourly space velocities of 20.4 and 10.2 hy1 . Here, the molar hourly space velocity was defined as the reactive gas molar flow rate ŽCH 4 . divided by the molar amount of precursor ŽV2 O5 .. More details about the synthesis of vanadium carbides were similar to that of vanadium nitrides and were described elsewhere w12,13x. After synthesis, the final products were passivated at room temperature in a mixture of 0.5% O 2 in He ŽTaedug Gas. flowing at 20 cm3 miny1 to prevent bulk oxidation. This passivation was continued for 2 h. The product was then removed from the reactor for XPS analysis. The flow method using a Quantasorb Model Chembet 3000 sorption analyzer was used for measurements of the BET surface areas of vanadium carbides. The bulk structures of the materials were evaluated using a computer controlled Rigaku Rotaflex DMAX-B rotating anode X-ray diffractometer with a Cu-K a radiation source. The
65
brief bulk and sorptive properties of vanadium carbides are shown in Table 1. Particle and crystallite sizes were estimated using the BET surface areas and XRD results, respectively. The average size of particles in vanadium carbides, d p , was estimated using the following equation d p s 6rSg r , where Sg is the BET surface area, and r is the density of the primary bulk phase Ž r s 3.36 and 5.64 g cmy3 for V2 O5 and V8 C 7 , respectively..The average crystallite sizes, d c , were calculated using the Scherrer equation w14x, d c s K lrŽ B cos u ., where K is a constant Žhere taken to be 0.9., l is the wavelength of the ˚ ., B is the corrected peak X-radiation Ž1.5405 A width, and u is the Bragg angle of the diffraction peak. The peak width was taken to be the full-width at half-maximum intensity of the most intense peak in the pattern. The X-ray photoelectron spectroscopic experiments were carried out using a VG ESCA LAB 220 I spectrometer using the Mg-K a X-ray source. The XPS spectra of the samples were collected before and after Ar ion sputtering at 3 kV for 1 h. The total sputtered depth after 1 h sputtering was estimated to ˚ with a sputtering rate of 12 A˚ miny1 . be about 720 A For some selected samples, different Ar ion sputtering conditions were used for determining the influence of sputtering on the electronic structure of the samples. Vacuum in the test chamber during the collection of spectra was typically less than 4 = 10y9 Torr. The spectrometer energies were calibrated using the Au 4f 1r2 peak at 84.0 eV and the Cu 2p 3r2 peak at 932.6 eV. Gaussian andror Lorentzian peaks were deconvoluted using a non-linear least-squares algorithm. The atomic compositions were estimated
Table 1 Bulk and sorptive properties of vanadium carbides Specimen no.
Phase present
Crystallite size a ˚. ŽA
Particle size b Žnm.
Surface area Žm2 gy1 .
V8 C 7 -4 V8 C 7 -7 V8 C 7 -8 V8 C 7 -11 V8 C 7 -31
V8 C 7 V8 C 7 V8 C 7 V8 C 7 V8 C 7
359 176 251 206 240
256 148 137 99 34
4 7 8 11 31
a b
Measured using XRD peak. Estimated using surface area.
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J.-G. Choi r Applied Surface Science 148 (1999) 64–72
based on comparisons of the integrated peak areas normalized by the atomic sensitivity factors. Errors in the composition are expected to be less than "15% w15x.
3. Results and discussion 3.1. Electronic structures of Õanadium carbides Using XPS, the electronic structures of vanadium carbides have been examined before and after Ar sputtering. After Ar ion sputtering for 30 min or 1 h, no change in the surface electronic structure was observed for vanadium carbides used in the present study. However, the primary species for Ar-sputtered materials was V dq while that for as-prepared materials was V 4q or V 5q. The only exception to this trend was for V8 C 7-7 which contained V dq as the primary species both before and after Ar ion sputtering. Even though the sputtering time was continuously increased after Ar ion sputtering for 1 h, there was no significant variation observed in the amount Žas expressed in area. of core levels of V 2p, C 1s, and O 1s. Therefore, 1 h sputtering with Ar ion was used as a standard method to pretreat all the vanadium carbides. Fig. 1 shows typical XPS spectra of the V 2p doublet in vanadium oxides ŽV2 O5 and VO 2 ., and vanadium carbide ŽV8 C 7 . as a function of the binding energy. The spectrum for the vanadium carbide in the V 2p region showed the presence of two well-resolved spectral lines at 513.2 eV and 520.9 eV ŽFWHMs 1.7.. These were assigned to the V 2p 3r2 Žat 513.2 eV. and V 2p1r2 Žat 520.9 eV. spin–orbit components, respectively. A species with a V 2p 3r2 binding energy of 513.2 " 0.2 eV from vanadium carbides was newly observed so that it was temporarily connoted as V dq where 0 - d - 4. The binding energies of V 2p were shifted to lower ones Ž517.3 ™ 516.1 ™ 513.2 eV. as the oxides were transformed to carbide during carburization ŽV 5q™ V 4q™ V dq .. As discussed later, using V oxides the oxidation number of this species was attempted to be determined by the plot of the V 2p 3r2 binding energies against the oxidation number. It is noteworthy that with increasing Ar ion sputtering time, the
Fig. 1. Typical XPS spectra of V 2p 3r 2 –V 2p1r2 doublets for V 5q, V 4q, and V dq as a function of the binding energy.
area percent of V dq for all the vanadium carbides was increased while those of V 4q and V 5q were decreased ŽTable 2.. These results indicate that the portions of carbide are exposed at the surface while the oxide layers are removed from the surface. This is supported by the calculation for the relative intensity of VVC xrVVO t as shown in Table 3. Here, VC x and VOt indicate the carbide and the total oxides ŽV2 O5 and VO 2 ., respectively. The relative intensity of VVC xrVVO t increased with the increase of the sputtering time for vanadium carbides. Fig. 2 shows the C 1s XPS spectrum in the vanadium carbides consisting of three deconvoluted components. The corresponding binding energies of C 1s are shown in Table 4. The C 1s peak with a binding energy of 285.6 " 0.2 eV was considered to be associated with adsorbates on the surface such as CO, CO 2 , and hydrocarbons Žthe peak C a in Fig. 2.. The carbon peak ŽC f . with a binding energy of 284.3 " 0.2 eV corresponds to the free carbon con-
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
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Table 2 XPS binding energies ŽeV. of V 2p1r 2 and V 2p 3r2 core levels for vanadium carbides and oxides Specimena no.
V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2 V2 O5 V2 O5-s2
V 2p1r 2
V 2p 3r2
V d q ŽVC x .
V 4q ŽVO 2 .
V 5q ŽV2 O5 .
V d q ŽVC x .
V 4q ŽVO 2 .
V 5q ŽV2 O5 .
520.9Ž7. b 520.8Ž11. 520.9Ž21. 520.8Ž26. 520.7Ž27. 520.9Ž12. 520.8Ž19. 520.9Ž25. 520.9Ž7. 520.9Ž17. 521.0Ž14. 520.9Ž24. – –
523.6Ž8. 523.7Ž7. 523.6Ž4. 523.6Ž5. 523.7Ž3. 523.6Ž9. 523.5Ž6. 523.6Ž4. 523.7Ž15. 523.6Ž9. 523.7Ž10. 523.6Ž4. 523.6Ž6. 523.7Ž17.
524.8Ž18. 524.9Ž15. 524.8Ž7. 524.8Ž3. 524.9Ž3. 524.8Ž12. 524.8Ž9. 524.8Ž4. 524.9Ž12. 524.8Ž8. 524.7Ž9. 524.8Ž5. 524.8Ž24. 524.9Ž18.
513.2Ž15. 513.1Ž23. 513.2Ž43. 513.2Ž52. 513.2Ž54. 513.2Ž24. 513.1Ž38. 513.2Ž51. 513.2Ž14. 513.3Ž33. 513.3Ž28. 513.2Ž47. – –
516.1Ž15. 516.0Ž13. 516.1Ž10. 516.2Ž9. 515.9Ž7. 516.1Ž18. 516.2Ž12. 516.1Ž7. 516.1Ž29. 516.0Ž18. 515.9Ž20. 516.1Ž9. 516.1Ž13. 516.1Ž33.
517.3Ž37. 517.1Ž31. 517.3Ž15. 517.3Ž5. 517.1Ž6. 517.3Ž25. 517.2Ž16. 517.3Ž9. 517.3Ž23. 517.2Ž15. 517.2Ž19. 517.3Ž11. 517.3Ž57. 517.3Ž32.
a
The s1 and s2 indicate Ar sputtering time of 30 min and 1 h, respectively. Ž.: area percent. d : 0 - d - 4 in oxidation number. b
taminated on the surface of carbides. This result also suggested that this free carbon was graphite-like. The peak C c shows the carbidic carbon with 282.4 " 0.2 eV, which is ascribed to the photoelectrons ejected from the carbon in vanadium carbide lattice. Thus, these carbidic carbon peaks increased with the increase of the sputtering time as shown in Table 4.
This is also supported by the increase in the relative intensity of C crC t in Table 4. A similar binding energy for the carbidic carbon in vanadium carbide has been observed by Hakansson et al. w16x. They reported that using the VC Ž100., the deconvoluted peak that appeared at a binding energy of 282.2 eV was ascribed to the surface carbidic carbon.
Table 3 Atomic ratios of V 2p 3r 2 , C 1s, and O 1s core levels for vanadium carbides Specimen no.
C crV dq
C crVt
ŽOd q Op .rVt
VVC xrVVO t
ŽC c q Od q Op .rVt
V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2
0.12 0.10 0.10 0.99 0.96 1.54 1.07 0.76 1.22 0.79 1.32 1.19
0.08 0.06 0.05 0.78 0.76 0.56 0.35 0.22 0.29 0.30 0.87 0.73
1.28 1.17 1.18 0.80 0.72 1.99 1.70 1.36 2.16 1.83 2.42 1.11
0.28 0.52 1.78 3.71 4.26 0.56 1.33 3.17 0.27 1.00 0.72 2.45
1.36 1.23 1.23 1.58 1.48 2.55 2.05 1.58 2.45 2.13 3.29 1.84
The C c and Vt indicate the carbidic carbon and total vanadium, respectively. The VOt and C t represent the total amount of vanadium oxides ŽVO 2 and V2 O5 ., and the total carbon amount ŽC t s C c q C f q C a ., respectively. d : 0 - d - 4 in oxidation number.
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
68
Fig. 2. Representative XPS spectrum of C 1s energy region for vanadium carbides. Three different carbons are described as carbidic ŽC c ., free ŽC f ., and adsorbed ŽC a . carbons.
A typical XPS spectrum in the O 1s energy region for vanadium carbide can be shown in Fig. 3 with three deconvoluted peaks ŽOa , Op , and Od .. Table 4 also shows the binding energy of O 1s with the sputtering time. Chemisorbed or lattice oxygen can
be identified with a binding energy of 532.1 " 0.2 eV, which was deconvoluted as a peak Oa in Fig. 3. The other two O 1s peaks with different binding energies of 530.1 " 0.2 eV Žpeak Od . and 530.8 " 0.2 eV Žpeak Op . were considered to be due to vanadium
Table 4 XPS binding energies ŽeV. and relative intensities of C 1s and O 1s core levels for vanadium carbides and oxides Specimena no. C 1s
O 1s
Relative intensity
C c Žcarbidic. C f Žfree. C a Žadsorbed. Od ŽVO 2 . Op ŽV2 O5 . Oa Žadsorbed. C crCq C crŽC f q C a . OarŽOd q Op . t V8 C 7-4 V8 C 7-4s1 V8 C 7-4s2 V8 C 7-7 V8 C 7-7s2 V8 C 7-8 V8 C 7-8s1 V8 C 7-8s2 V8 C 7-11 V8 C 7-11s2 V8 C 7-31 V8 C 7-31s2 V2 O5 V2 O5-s2 a
282.5Ž4. b 282.4Ž8. 282.5Ž15. 282.6Ž13. 282.6Ž17. 282.5Ž10. 282.4Ž14. 282.5Ž18. 282.5Ž7. 282.6Ž12. 282.6Ž11. 282.5Ž16. – –
284.3 284.4 284.3 284.4 284.5 284.3 284.4 284.3 284.3 284.4 284.5 284.3 284.3 284.3
285.8 285.6 285.8 285.7 285.6 285.8 285.6 285.8 285.8 285.7 285.6 285.8 285.8 285.8
530.2 530.1 530.2 530.2 530.3 530.2 530.1 530.2 530.2 530.1 530.2 530.3 530.3 530.3
530.8 530.9 530.8 530.9 530.8 530.9 530.8 530.9 530.9 530.8 530.9 530.9 530.9 530.9
The s1 and s2 indicate Ar sputtering time of 30 min and 1 h, respectively. Ž.: area percent. q The C t represent the total carbon amount ŽC t s C c q C f q C a .. b
532.2Ž8. 532.2Ž24. 532.1Ž26. 532.3Ž23. 532.1Ž46. 532.1Ž31. 532.3Ž40. 532.1Ž42. 532.1Ž19. 532.2Ž47. 532.3Ž12. 532.1Ž64. 532.1Ž12. 532.1Ž19.
0.04 0.08 0.15 0.13 0.17 0.10 0.14 0.18 0.07 0.12 0.11 0.16 – –
0.04 0.09 0.18 0.15 0.20 0.11 0.16 0.22 0.08 0.14 0.12 0.19 – –
0.09 0.32 0.35 0.30 0.85 0.45 0.67 0.72 0.23 0.89 0.14 1.78 0.13 0.23
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
69
Fig. 3. Representative XPS spectrum of O 1s energy region for vanadium carbides. Peaks Od and Op are due to vanadium oxides, and peak Oa is ascribed to strongly adsorbed oxygen.
dioxide ŽV 4q . and vanadium pentoxide ŽV 5q ., respectively. Vanadium carbides used in this study had the same, or very similar O 1s binding energies. Notwithstanding the lack of well-assigned oxygen species, it is a general consensus that peaks at ; 530 eV and at ; 532 eV are ascribed to oxides and oxygen species dissolved in the metal or to adsorbed oxygen, respectively. Accordingly, the oxygen species with the higher binding energy was assigned to strongly adsorbed oxygen Žas Oy. , OHy, or H 2 O. The relative intensity of the lattice oxygen inserted or dissolved into the vanadium carbides Žexpressed as Oa . to the oxygen in vanadium oxides ŽVO 2 and V2 O5 . Žexpressed as Od q Op . increased significantly after ion sputtering ŽTable 4., showing that there is a relationship between relative intensity and surface area of vanadium carbides. These results indicate that the oxygen extent inserted in high surface area vanadium carbide is larger than that in the low surface area carbide. Similar observations have been observed for tantalum carbides w17x and molybdenum nitrides w18x. 3.2. Atomic ratios and relatiÕe intensities The atomic ratios ŽCrV, OrV. and relative intensities of V 2p 3r2 , C 1s, and O 1s core levels for
vanadium carbides are summarized in Tables 3 and 4. Quantitative relationships between carbon and vanadium ŽCrV atomic ratio. were derived from calculation of peak area ratios of the respective states in the deconvoluted C 1s spectra and V 2p 3r2 spectra ŽFigs. 2 and 4.. Similarly, the OrV atomic ratios were estimated using the V 2p 3r2 and O 1s peak areas. Here, V dq represents carbide portion while Vt shows the total vanadium metal consisting of the portions of carbides ŽV dq . and oxides ŽV 4q and V 5q .. These peak areas were normalized using the atomic sensitivity factors. The atomic sensitivity factors for the V 2p 3r2 , C 1s, and O 1s peaks were 1.3, 0.25, and 0.66, respectively w15x. Table 3 shows two different types of carbon– vanadium atomic ratios, C crV dq and C crVt . While the atomic ratio of C crV dq indicates the surface composition of vanadium carbide, the C crVt indicates a ratio of carbidic carbon number over total vanadium metal number at surface. With the exception of V8 C 7-4, the average ratio of C crV dq for all the 1 h sputtered vanadium carbides was nearly unity. This result suggested that the VC phase is present at the surface of vanadium carbides. However, the average bulk composition of carbonr vanadium obtained from a CHN analyzer was found to be ; 1.75. Here, the primary reason for the larger
70
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
Fig. 4. Typical deconvoluted XPS spectrum of V 2p energy region for vanadium carbides.
composition than 0.875 of carbonrvanadium for V8 C 7 was considered to be due to the presence of polymeric carbon produced during carbiding of the vanadium oxide. Accordingly, it can be seen that for vanadium carbides, the properties of surface are different from those of bulk. It is generally known that the smaller compositions of carbonrvanadium suggests the carbon-deficient vanadium carbides while the larger values indicate the vanadium carbides containing excess carbon atoms located on sites other than lattice carbons in the cubic structure, such as interstitial or substitutional of vanadium atoms. These excess carbon atoms may have the different bonding from V–C one in the cubic structure. A preliminary experiment was performed to understand the relationship between the carbonr vanadium and the catalytic activity. Using NH 3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in the current study. The ratios of C crV dq and C crVt generally decreased as the sputtering time increased. This behavior was also observed for the atomic ratios of oxygen to vanadium as shown in Table 3. These results indicate that the
effect of sputtering was larger on the light elements of C and O than on the heavy V metal. Similar results were also found for Mo 3p 3r2 , N 1s, and O 1s in the molybdenum nitrides w18,19x. Tables 2 and 3 show that from the peak deconvolutions of C 1s and V 2p, the binding energies for carbidic carbon of C 1s and V dq 2p 3r2 were estimated to be 282.4 " 0.2 and 513.2 " 0.2 eV, respectively. With reference energies for C 1s and V 2p, the shifts in the binding energy of the C 1s and V 2p levels were unambiguously estimated. Taking reference energies of 511.9 eV for V 2p and 284.4 eV for C 1s, the V 2p peak tended to shift to higher energies while the C 1s peak shifted to lower energies. This result indicated that the electron charge was transferred from the V 3d states to the C 2p states. This transfer of electron charge was evidenced by the difference of electronegativity between V Ž1.63. and C Ž2.55.. As discussed later, the direction of charge transfer from vanadium to carbon can be also confirmed by the estimation of positive oxidation state of vanadium in the current study. Accordingly, it was considered that an ionic character of bonding was induced in the V–C lattice while the covalency of the V–C bonding has been reduced w20,21x. The expected binding energy separation between elemental V and adsorbed carbon is 227.5 eV. This result
J.-G. Choi r Applied Surface Science 148 (1999) 64–72
suggested that after carbiding, the increase of 3.3 eV in binding energy separation between the V dq 2p 3r2 and C 1s peaks, and the binding energy separation of ; 229.5 eV between V 0 and C 1s were attributed to the formation of vanadium carbides. In a separate study significant amount of carbon, residue from the synthesis of the vanadium carbides via a reaction of V2 O5 with CH 4 , and H 2 O have been observed on the surface of the as-prepared materials w12x. Therefore, it is not surprising that the atomic ratios of ŽC c q Od q Op .rVt were estimated to be in excess of unity. After 1 h sputtering the atomic ratios of ŽC c q Od q Op .rVt decreased by ; 20% for all the vanadium carbides. This result again indicates that the sputtering pretreatment used in the present study could influence the elements of C and O in the lattice. 3.3. Determination of Õanadium oxidation number As can be seen in Fig. 2, the deconvolution of vanadium 2p spectra was performed to obtain the distribution of vanadium oxidation states. The V 2p spectra typically consisted of two envelopes. The V 2p 3r2 –V 2p1r2 doublet was fitted so that each peak had the same Gaussian line shape and width ŽFWHM.. The relative intensities of spin–orbit doublet peaks are given by the ratio of their respective degeneracies and the intensity ratio for the V 2p 3r2 – V 2p1r2 doublet should be I Ž2p 3r2 .rI Ž2p1r2 . s 2r1. A splitting energy of ; 7.7 eV was expected for the V 2p 3r2 –V 2p 1r2 doublet. The deconvolution of V 2p spectra shows three species of V 5q, V 4q and V dq, and gave rise to a new species with a V 2p 3r2 binding energy of 513.2 " 0.2 eV ŽTable 2.. The binding energy of this new species is midway between those assigned to V 4q Ž516.0 " 0.2 eV. and V 0 Ž511.9 " 0.2 eV., and is about 2.8 eV lower than that for V 4q. As stated earlier, since the new species of vanadium carbide was temporarily denoted as V dq where 0 - d - 4, it is worthwhile to attempt to determine the oxidation number of this species. We tried to use the calibration method generally known in the determination of the oxidation number by using the plot of the V 2p 3r2 binding energies against the oxidation number. Here, the binding energies of V 0 , V 4q, and V 5q
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Fig. 5. Plot of V 2p 3r 2 binding energies as a function of the oxidation number.
which are generally accepted, were used in this study. Therefore, the result from Fig. 5 suggested that the binding energy of 513.2 eV might be ascribed to V 1.3q. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Subsequently, this charge transfer suggests an actual ionic contribution to the bonding of vanadium carbides. Thus far, no reliable standard for the binding energy of V 1.3q has ever been published. However, Chen et al. w22x reported that a similar vanadium oxidation state exists for vanadium carbide films. They used the near-edge X-ray absorption fine structure ŽNEXAFS. technique to investigate the electronic and structural properties of thin vanadium carbide films on a vanadium Ž110. single-crystal surface. It was reported that the oxidation state of vanadium carbide thin film was estimated to be in the range of 1.2 " 0.2 electrons per vanadium using the NEXAFS results of VŽ110., VCrVŽ110. and various model vanadium oxides.
4. Conclusions No significant variation was observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and 0 1s for vanadium carbides after Ar ion 1 h sputtering. A species with a binding
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energy of 513.2 " 0.2 eV from vanadium carbides was newly observed, and was estimated to be V 1.3q. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. The binding energies of V 2p were shifted to lower ones Ž517.3 ™ 516.1 ™ 513.2 eV. as the oxides were transformed to carbide during carburization ŽV 5q™ V 4q™ V dq .. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4 " 0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. These carbidic carbon peaks increased with the increase of the sputtering time and this response was also supported by the increase in the relative intensity of C crC t . For all of the 1-h sputtered vanadium carbides except for V8 C 7-4, the average ratio of C crV dq was nearly unity. This result suggested that the VC phase is present at the surface of vanadium carbides. However, the catalytic activity measurements using NH 3 decomposition reaction exhibited that the vanadium carbide with the smallest C crV dq ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at the surface of vanadium carbide. The surface composition was different from the average bulk composition of ; 1.75. The primary reason for the larger composition in bulk was considered to be due to the presence of polymeric carbon produced during carburization of the vanadium oxide.
Acknowledgements This work was financially supported by a 1998 Hannam University Grant. The author thanks YoungJin So for her assistance in obtaining the XPS data for the vanadium carbides.
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