J. Chem. SOC., Faraday Trans. 1, 1982, 78, 1451-1455 High-resolution Auger Electron Spectra of Adsorbed NO, NH, and N, on Sulphur-segregated and Oxidized Vanadium Surfaces BY KAZUNARI DOMEN, SHUICHI NAITO, MITSUYUKI SOMA,? TAKAHARU ONISHI* AND KENZI TAMARU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 11 3, Japan Received 22nd April, 198 1 The absorption of NO, NH, and N, on oxidized and sulphur-segregated surfaces of vanadium polycrystalline foil has been studied using high-resolution Auger electron spectroscopy (HRAES). On the oxidized surface, four different types of adsorbed nitrogen species were detected, i e . NO (a), NH, (a), N, (a) and N (a). On the other hand, on the sulphur-segregated surface, NO is adsorbed easily at room temperature, but NH, and N, were not adsorbed.When electron emission from a solid surface via an Auger process involves valence electrons of a surface element, high-resolution Auger electron spectroscopy (HRAES) gives useful information not only concerning species and abundances of elements (as in the case of conventional AES), but also concerning the valence-band structure, i.e. the chemical state of a particular element on the solid surface. So far chemisorbed species on polycrystalline Mo, W, Pd and Fe have been successfully studied by HRAES.f-5 In this report the adsorption of NO, NH, and N, was investigated on oxidized and sulphur-segregated surfaces of vanadium, which is of particular interest because of the possibility of studying chemical effects in the metal HRAES, as has been demonstrated by ESCA and AES meas~rements.~-~ EXPERIMENTAL The details of the apparatus employed in this study have been reported previously.1° The minimum pressure attainable in the vacuum chamber was 7 x lo-’ Pa, and the resolution of the analyser was ca.400 meV with an electron beam current of 1 x lop6 A and a primary energy of 1.5 keV. Vanadium foil (15 x 5 x 0.01 mm) of 99.8% purity was electrolytically etched in a solution of methanol and sulphuric acid, and then flashed in u.h.v. By this treatment the vanadium surface was covered with sulphur, which segregated from the bulk as shown in fig. 1 (a). Vanadium LMM and LMV spectra are in good agreement with the X-ray excited spectra of the clean evaporated film reported by Brundle et which indicates that the metallic state of the vanadium remains the same even though much sulphur may be segregated on the surface.The peak at ca. 508 eV was assigned to the vanadium LVV transition, according to Fiermans et a1.8 Fig. 1 (b) shows the vanadium surface oxidized by 0, (6.7 x lop4 Pa, 1 min) at room temperature. It is accordingly suggested that the surface sulphur may be removed by oxygen and the vanadium peaks are broadened by oxidation. The vanadium L VV peak is obscured by the oxygen KLL peak. There should be three peaks in the oxygen KLL Auger ~pectrum.~ The peak at lowest kinetic energy, which is assigned to the KL,L, transition, seems to be hidden by the vanadium LMV transition. When the 1- Present address: National Institute for Environmental Studies, Yatabe, Tsukuba, Ibaraki 305, Japan.14511452 NO, NH, AND N, ADSORBED ON v temperature of the foil was raised to 523 K under an oxygen atmosphere, the sulphur disappeared completely from the surface and the shoulder to the lower-energy side of the vanadium LM,,, V peak grew considerably, indicating the oxidation of vanadium metal under these conditions. When the temperature of the foil was raised to 673 K in u.h.v., the amount of surface oxygen decreased and that of sulphur increased again as shown in fig. 1 (c). I - I l l l l r C I I 1 120 170 380 5 50 FIG. I.-S(LMM), V(LMM, LMV, LVV) and O(KLL) Auger spectra of vanadium surfaces after various pretreatments: (a) flashed in u.h.v.; (b) oxidized by 0, (6.7 x Pa, 1 min) at room temperature; (c) the temperature of the foil was raised to 673 K in u.h.v., after the treatment of (6).kinetic energy/eV RESULTS AND DISCUSSIONS (I) ADSORPTION OF NO ON SULPHUR-SEGREGATED AND OXIDIZED VANADIUM SURFACES As is shown in fig. 2, upon introduction of NO onto the sulphur-segregated vanadium surface, the nitrogen KLL line emerged and sulphur LMM peak decreased. The shape of the nitrogen KLL spectrum is similar to that for the nitrogen molecule adsorbed on Fe or Mo, which is assigned to dissociatively adsorbed n i t r ~ g e n . ~ - ~ Accordingly the overall reaction of NO with the surface is considered to be the removal of sulphur by oxidation accompanied by nitrogen chemisorption. I t is notable that the sulphur-segregated vanadium surface can activate NO, in contrast to the palladium surface, where the segregated sulphur strongly poisons the dissociative adsorption of l2 Since oxygen is used for the oxidative removal of sulphur, the increase in the oxygen KLL peak is relatively small whereas the vanadium LMV peak broadens, as is the case for oxidation by O,, which is attributable to the formation of a strong V-N bond.The dissociatively adsorbed nitrogen is stable at 873 K and disappeared by flashing in uacuo. On the other hand, when NO was introduced at room temperature onto a surface oxidized by 6.7 x Pa 0, at 673 K for 2 min, no nitrogen peak was detected.DOMEN, NAITO, SOMA, ONISHI AND TAMARU 1453 However, when NO was introduced at room temperature onto a surface oxidized by 6.7 x Pa 0, at room temperature for 30 s, two peaks of the nitrogen KLL transition were observed, as shown in fig.2. By raising the temperature to 673 K, the peak to lower kinetic energy decreased and the resultant lineshape [fig. 2(d)] was the same as that for the sulphur-segregated surface [fig. 2(b)]. These results demonstrate that at a certain oxidation state the vanadium surface can accomodate both undissociated and dissociated NO. 120 170 \ 390 \ 4 90 5 40 340 440 kinetic energy/eV FIG. 2.--S(LMM), N(KLL), V(LMV, LVV) and O(KLL) Auger spectra: (a) flashed in u.h.v.; (b) NO (6.7 x Pa, 2 min) adsorbed on the sulphur-segregated surface (a) at room temperature; (c) NO (6.7 x Pa, 30 s) adsorbed on the oxidized surface (by 0, = 6.7 x lo-* Pa, 1 min, at room temperature) at room temperature; (d) temperature of the foil raised to 673 K in u.h.v., after the treatment of (c).(11) ADSORPTION OF NH, AND N, ON SULPHUR SEGREGATED AND OXIDIZED SURFACES When NH, was introduced onto the sulphur-segregated surface, only a small peak for the nitrogen KLL transition was detected initially, but this increased considerably with prolonged electron bombardment ( E = 1.5 keV). The nitrogen spectrum was similar to that of fig 2(b), suggesting the dissociative adsorption of NH, to form N. When NH, was introduced onto the oxidized surface at room temperature, a different type of nitrogen peak was detected [fig. 3(a) and (b)]. This is assignable to partially dehydrogenated NH, species, where x is 1 or 2, according to the literat~re.~ A similar spectrum was observed for the adsorption of NH, on Pd.5 When the temperature was1454 NO, NH, AND N, ADSORBED ON v raised to 873 K [fig.3(c)], the lineshape changed to the type shown in fig. 2(b); this change is probably due to the dehydrogenation of NH, to form chemisorbed nitrogen. When N, was introduced on the sulphur-segregated surface at room temperature, no nitrogen peak was detected and there was little effect produced by electron bombardment. On the other hand, by introducing a mixture of N, and 0, on the sulphur-segregated surface at room temperature, a nitrogen peak was observed [fig. 3 (d)] which is apparently similar to that of NH, adsorbed on the oxidized surface [fig. 3(b)]. However, this nitrogen KLL line disappeared completely at 523 K. This peak t 1 I I I I 3 40 3 90 kinetic energy/eV FIG.3.-N(KLL) Auger spectra: (a) NH, (6.7 x Pa, 30 s) was adsorbed on oxidized surface (by 0, = 6.7 x Pa, 1 min, at room temperature) at room temperature without electron bombardment. (b) NH, (6.7 x Pa) was adsorbed on the same surface of (a) with electron bombardment ( E = 1.5 keV) at room temperature. (c) The temperature of the foil was raised to 873 K in u.h.v., after (b). ( d ) N, + 0, Pa, 2 min) were adsorbed on sulphur-segregated surface at room temperature. (both 6.7 x is similar to that of N, adsorbed on Fe at 373 K or Mo at 83 K, and the shoulder to higher kinetic energy is more clear than that of N, on Fe and/or Mo. Consequently this species is considered to be non-dissociatively adsorbed nitrogen. N, adsorption on the sulphur-segregated surface from a mixture of N, and 0, is considered to occur on the exposed vanadium atom where sulphur has been removed by oxygen.Otherwise the sulphur atom prevents the adsorption of the nitrogen molecule. On the other hand, the adsorption of NO or NH, assisted by electron bombardment is not inhibited by sulphur on the vanadium surface. The sulphur present on the vanadium is very reactive for oxidation. Vanadium oxide is known to be a good catalyst for the oxidation of SO, to SO,. In this connection the reactivity of sulphur segregated on the vanadium surface toward oxidation isDOMEN, NAITO, SOMA, ONISHI A N D TAMARU 1455 considerable. On the oxidized surface three different nitrogen species are detected at room temperature, i.e. non-dissociatively adsorbed NO, NH, and weakly bonded N,.It is difficult to determine on which oxidation state of vanadium these species are adsorbed. Perhaps the surface is a mixture of various oxidation states, as shown in fig. 1 and pointed out by Brundle et aZ.9 It is of interest that adsorption of the test molecules on the oxidized surface at room temperature tends to be rather associative in comparison with that on the sulphur-segregated surface. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Surf. Sci., 1974, 46, 567. T. Kawai, K. Kunimori, T. Kondow, T. Onishi and K. Tamaru, Phys. Rev. Lett., 1974, 33, 533. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Chem. Lett., 1975, 12, 1303. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Surf Sci., 1976, 54, 525. K. Kunimori, T. Kawai, T. Kondow, T. Onishi and K. Tamaru, Sut$ Sci., 1976, 54, 302. F. J. Szalkowski and G. A. Somorjai, J. Chem. Phys., 1972, 56, 6097. L. Fiermans and J. Vennik, Surf. Sci., 1971, 24, 541. L. Fiermans and J. Vennik, Surf. Sci., 1973, 35, 42. C. R. Brundle, Surf. Sci., 1975, 52, 426. lo T. Kondow, T. Kawai, K. Kunimori, T. 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