J . Phys. Chem. 1987, 91, 3821-3827
3821
Alteration in Dissociation Energetics for 7r-Acceptor Adsorbates: N,O, NO, N,, and 0, on W (100)-p( 5X 1)-C E. K. Baldwin and C. M. Friend* Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: December 1 1 , 1986)
The adsorption and reaction of N20, NO, 02,and N2on W(lO0)-~(5x1)-Chave been investigatedby temperature programmed reaction spectroscopy and isotopic exchange methods. The energies required for N-N and N-0 bond scission are increased on the W(lOO)-p(5XI)-C surface relative to clean W(100). At exposures below 0.3 langmuir, N 2 0 undergoes N-0 bond cleavage below 170 K, producing adsorbed molecular nitrogen and atomic oxygen. Higher exposures result in reversible molecular N 2 0 desorption at 170 and 235 K, corresponding to approximate desorption energies of 9 and 13 kcal/mol, respectively. No N-N bond cleavage occurs for N 2 0 on the -p(SXl)-C surface. Likewise, no N-N bond scission is observed for N2: molecular N2 is reversibly desorbed from W(lOO)-p(5Xl)-C near 180 K. Nitric oxide undergoes competing dissociation and direct N-N bond formation reactions below 300 K. Dissociation results in formation of molecular nitrogen and carbon monoxide via atom recombination above 1250 and at 900 K, respectively. N-N bond formation yields nitrous oxide and molecular nitrogen at temperatures below 200 K. N 2 0 formation proceeds via direct N-N bond formation, rather than NO dissociation. Inhibition of NO and N 2 0 dissociation at 100 K is key to the N2 formation reaction. Site blocking is excluded as the basis for the inhibition of dissociation. An NO dimer intermediate is proposed for this process. Adsorbed NO, N 2 0 ,and 0, produce CO above 900 K. CO is formed from reaction of adsorbed atomic oxygen with surface carbon in a peak centered near 1000 K, denoted as pI-CO. If the surface carbon is depleted, excess adsorbed atomic oxygen reacts with carbon which diffuses from the subsurface region, to produce carbon monoxide at 1300 K, defined as P2-C0.
on W(110) demonstrate that dissociation occurs at 100 K for low Introduction N 2 0 exposures. Higher exposures result in molecular N 2 0 adThe adsorption and reaction of N O on transition-metal surfaces sorption. The ultraviolet photoemission data are consistent with has been extensively studied' due to its importance in heterogemolecular nitrous oxide bound parallel to the W ( 110) surface. neous catalysis and its relative chemical simplicity. Nitric oxide The N(1s) and O( 1s) X-ray photoemission spectra of this system is known to adsorb either dissociatively or molecularly on tranare essentially identical with those of gas-phase N 2 0 . Bonding sition-metal surfaces, with the extent of dissociation dependent configurations with N 2 0 perpendicular to the surface have been on the specific surface. Presumably, the amount of electron density observed on R u ( O O ~ and ) ~ Cu(1 lO).'O An alternative N 2 0 addonated into the 2 r * orbital by the transition-metal surface insorption configuration, identified on Pt(l1 l),Il is bound via the fluences the degree of N-0 bond weakening2 and therefore the terminal nitrogen and is inclined at approximately 35O to the relative amount of dissociation. This investigation is part of an surface. Molecular adsorption of N 2 0 at low surface temperatures ongoing effect to understand the effect of electronegative adlayers is observed on W(110)8 and W(100) at high exposures, and on on the chemistry of n-acceptor molecules on tungsten surfaces. oxygen pretreated W( 100) by using X-ray photoemission specThe W( 100)-p(5X 1)-C surface, studied previously, is considered a good model for the basal plane of bulk tungsten ~ a r b i d e . ~ . ~ troscopy. l 2 Likewise, earlier studies of N O on W(100)7,'2-'8W(l 10),'9-22 Energy-dependent ultraviolet photoemission studies have shown and polycrystalline t ~ n g s t e n ~ have ~ - ~ ' demonstrated that N O that the W(IOO)-p(5Xl)-C surface has a valence band structure almost identical with that of the basal plane of bulk WC.j The photon energy dependent photoemission data also showed that the W( 100)-p(5X 1)-C surface d-band occupancy decreases near (8) Fuggle, J. C.; Menzel, D. SurJ Sci. 1979, 79, 1 . the Fermi level compared to clean W(100).6 These data are (9) Umbach, E.; Menzel, D. Chem. Phys. Left. 1981, 8 4 , 491. (IO) Spitzer, A.; Liith, H. Phys. Rev. B 1984, 30, 3098. consistent with the W ( 100)-p(5Xl-C surface being electron de(11) Avery, N. R.; Surf. Sci. 1983, 131, 501. ficient in comparison to the clean W(100) surface. Tight binding (12) Baldwin, E. K.; Friend, C. M. J . Vue. Sei. Technol. A 1986, 4 , 1407. calculations for N O on Ni( 11 1)2show that the N O 2n* orbitals (13) Pelach, E.; Viturro, R.; Folman, M. Surf.Sei. 1985, 161, 553. interact with metal d-band states near EF Thus, the documented (14) Weinberg, W. H.; Merrill, R. P. Surf.Sei. 1972, 32, 317. depletion of electron density near EF on the W(lOO)-p(5Xl)-C (15) Shinar, R.; Maniv, T.; Folman, M. Surf. Sei. 1984, 141, 158. surface is expected to alter the dissociation energies for N O and (16) Bhattacharya, A. K.; Broughton, J. Q.; Perry, D. L. Surf.Sei. 1978, 78, L689. other r-acceptor molecules on the surface. (17) Usami, S.; Nakaaima, T. Jon. J . ADD^. Phvs. 1974. SuouI. 2, 237. The surface chemistry of N 2 0 on transition metals has not been (18) Ballu, Y.; Armstrong, R. A:; Lecank; J. C R. Acud. Sci.'furis Ser., extensively investigated. Previously reported investigations have B, 1972, 274, 718. shown that the presence of atomic oxygen and/or nitrogen inhibits (19) Masel, R. I.; Umbach, E.; Fuggle, C.; Menzel, D. Surf.S r i . 1979, 79. 26. N 2 0 dissociation on tungsten surfaces, favoring molecular ad(20) Klein, R.; Yates, J. T., Jpn. J . Appl. Phys. 1974, Suppl. 2, 461. sorption.' X-ray8 and ultraviolet9 photoemission studies of N 2 0 (21) Rawlings, K. J.; Foulias, S. D.; Hopkins, B. J. Surf.Sei. 1981, 108,
(1) See, for example: Joyner, R. W. Catalysis (London) 1982, 5, 33. (2) Sung, Shen-Shu; Hoffmann, R. S.; Theil, P. A. J. Phys. Chem. 1986, 90, 1380. (3) Benziger, J. B.; KO, E. I.; Madix, R. J. J . Card 1978, 54, 414. (4) Recent ion scattering results have called this proposed structural model into question. Overbury, S. H., private communication. (5) Stefan, P. M.; Shek, M. L.; Spicer, W. E. Surf.Sci. 149, 423. (6) Comparison of W(100) and W(lOO)-p(5Xl)-C valence band photoemission spectra as a function of energy demonstrates that the observed differences are not the result of electron scattering at the surface. (7) Baldwin, E. K.; Friend, C. M. J. Phys. Chem. 1985, 89, 2576.
0022-3654/87/2091-3821!$01.50/0
49. (22) L690. (23) (24) (25) (26) (27)
Rawlings, K. J.; Foulias, S. D.; Hopkins, B. J. Surf.Sci. 1978, 111,
Yates, J. T.; Madey, T. E. J . Chem. Phys. 1966, 45, 1623. Madey, T. E; Yates, J. T.; Erickson, N.E. Surf.Sci. 1974,43, 526. Tamura, T.; Hamamura, T. Bull. Chem. Soc. Jpn. 1976, 49, 1780. Sato, M. Jpn. J . Appl. Phys. 1977, 16, 653. Junimori, K.; Kawai, T.; Kondow, T.; Onishi, T.; Tamaru, K. Chem. Lett. 1975, 1303. (28) Sato, M. SurJ Sci. 1980, 95, 269. (29) Sato, M. Jpn. J. Appl. Phys. 1976, 15. 1995.
0 1987 American Chemical Society
3822 The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 undergoes facile dissociation to atomic nitrogen and oxygen at low exposures under ultra-high vacuum conditions and surface temperatures below 120 K. On both the W(l 10)l6and W(100)'2 surfaces, higher NO exposures result in N 2 0 formation at low temperatures with an activation energy of 1 1 3 kcal/mol. The presence of atomic oxygen and/or nitrogen on the W( 100) surface leads to an analogous N 2 0formation reaction. The mechanism of the N 2 0formation reaction on oxygen pretreated W( 100) was shown to involve direct N-N bond formation without involvement of atomic nitrogen or oxygen intermediates. No nitric oxide dissociation was observed on the W( 100)-p(2X2)-0 surface (eo 1.0).'* One key to N 2 0 formation from NO reaction on oxygen pretreated surfaces is that NO not dissociate at low surface temperatures. If we assume that NO dissociation is the result of donation of electron density from the metal d band to the 2 r * orbital of NO as discussed above, it is reasonable to expect that any electronegative adatom will render the surface less effective in activating the N-0 bond. In this work, the same general pattern of reactivity is observed for NO, N20,and N2on the W(lO0)-p(5Xl)-C surface as on the oxygen pretreated surfaces. Most importantly, the activation energies for N-0 and N-N bond scission are increased with respect to reaction on clean W(100). In fact, no N-N bond cleavage is observed on the W( lOO)-p(SXl)-C surface under the conditions of the experiment. In addition, both N 2 0and N2 are formed from NO at surface temperatures below 300 K via nondissociative reaction pathways. This study supports the contention that electronegative adlayers should generally limit NO and N 2 0 dissociation on tungsten and allow direct N-N bond formation. This investigation also rules out site blocking as a limiting factor in the dissociation of NO on this model carbide surface.
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Experimental Section All temperature programmed reaction data were obtained in an ultra-high vacuum system with an approximate working base Torr, described in detail elsewhere.' Gas pressure of 2 X was introduced with a calibrated directed dosing system. All temperature programmed reaction spectra were obtained with a quadrupole mass spectrometer interfaced to an IBM PC, allowing collection of data for up to 16 masses in a single experiment. Radiative heating (dT/dt = 15 K/s) was used to generate temperature programmed reaction data in the range of 120-800 K . For higher temperature data, electron bombardment heating was used. During electron bombardment heating, the crystal was biased positively by 110-140 V with respect to ground while the grounded filament was resistively heated. The electron bombardment induced heating rates varied from 80 K/s at 700 K to 12 K / s at 1400 K. Retarding field Auger spectroscopy and low-energy electron diffraction were used to monitor surface composition and order, respectively. Procedures for gas handling and crystal preparation were identical with those described previously? The W( lOO)-p(SXl)-C surface was prepared by exposing initially clean W( 100) to a dose of 50 langmuir of ethylene while the crystal was maintained at 1400 K, unless otherwise specified. Low-energy electron diffraction and Auger electron data were all obtained at crystal temperatures of less than 300 K. The data obtained for various annealing temperatures were obtained after flashing to the temperature indicated and subsequent cooling. LEED data were obtained at 130 K in cases where no annealing was required. Results Dinitrogen. Temperature programmed desorption of dinitrogen adsorbed on W(IOO)-p(SXl)-C exclusively yields N2 in a single peak near 180 K, defined as a. The temperature of the maximum N, desorption rate decreases by approximately 7 K as a function
Baldwin and Friend
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Figure 1. Temperature programmed desorption spectra of N2 (28 amu) from N2adsorbed on W(lOO)-p(5Xl)-C, with heating rates of dT/dr = 15 K/s.
1 OOK
p(sxl1
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700 900 1100 1300 1600 Temperature ( K )
Figure 2. LEED, Auger, and temperature programmed reaction spectra of C O desorbing from W(lO0)-p(SXl)-C saturated with 02.The LEED and Auger data were obtained after annealing to the given temperature and allowing the crystal to cool below 300 K.35 The temperature programmed reaction experiment used electron bombardment heating, with heating rates varying from 60 to 12 K/s.
of coverage up to saturation as shown in Figure 1. This effect was not investigated in detail. Coadsorption of a mixture of lsN2 and I4N, produces no I4NlSNin the desorption spectrum: only I5N, and I4N2are detected. These results unequivocally demonstrate that the 180 K peak results from reversible desorption of molecular nitrogen. The (5x1) LEED pattern remained intact and no change in the reactivity of the surface was observed subsequent to the temperature programmed desorption of dinitrogen. Dioxygen. Dioxygen adsorbed on W(lOO)-p(5Xl)-C at 120 K reacts with surface carbon to produce CO, which desorbs in two peaks, at -1000 and 1300 K, denoted as PI and p2 respectively. For ethylene exposures greater than = 12 langmuir, the carbon Auger signal and -p(SXl)-C LEED pattern after reaction are identical with those from the original surface. No desorption of atomic oxygen, dioxygen, or carbon dioxide is observed. The results for dioxygen adsorption at 300 K are identical. These results are in correspondence with earlier studies. The adsorption of O2 on W(IOO)-p(5Xl)-C has been investigated previously by temperature programmed reaction s p e c t r o ~ c o p yand ~ ~by ultra-
(30) Miki, H.; Inomata, H.; Kato, K.; Kioka, T.; Kawasaki, K. Surf. Sci. 1984, 141, 473. (31) Gasser, R. P. H. Lawrence, C P. Surf. Sci. 1968, 10, 91.
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( 3 2 ) KO, E. 1.; Madix, R. J. J . Phys. Chem. 1981, 85, 4019.
Dissociation Energetics for n-Acceptor Adsorbates
The Journal of Physical Chemistry, Vol. 91. No. 14, 1987 3823
TABLE I: Comparison of the Integrated Intensity of CO Desorption from B-CO on W(100) and W(lO0)-p(SX1)-C Saturated with 0 2 , NO, and N204 relative integral adsorption P, + P 2 9 adsorbate temp, K desorption intensity 0 2 120 2.0 NO 120 1.o 300 1.2 0.7 120 N2O 2.0 300 co 120 1.o Reference 34.
violet photoemission spe~troscopy.~~ Oxygen was found to adsorb readily at room temperature, recombining with carbon and desorbing as C O in a temperature range of 1000 to 1300 K . The reaction of saturation exposures (24 langmuirs) of 0, adsorbed on the standard (50 langmuirs of ethylene) W ( IOO)-p(5x1)-C surface at 120 K was investigated in detail. On this surface, C O desorbs in two peaks, a t 1000 (pl) and 1320 K (@,), as shown in Figure 2. The integrated CO desorption intensity34 is given in Table I. Auger35and LEED data, also shown in Figure 2, were obtained after annealing to three temperatures: below 800 K, beofre any CO desorption, at 1050-1200 K, between the desorption peaks, and above 1400 K, after reaction is complete. Below 800 K, LEED shows a diffuse 1 x 1 pattern, with faint 5x1 spots, and the Auger C / W ratio decreases to 80% of the bare and -c(2X2) carbide value. At 1050-1200 K, overlapping LEED patterns appear, and the Auger C / W ratio drops further, to 70% of the clean surface value. Above 1400 K, the -p(SXl)-C LEED pattern reappears, and the C / W Auger ratio returns to that of the original carbide surface. Repeated reaction of saturation oxygen exposures with the surface left after C O desorption produces temperature programmed reaction spectra essentially identical with those from the original -p(5Xl)-C surface. The 1300 K p2-C0desorption shifts to higher temperatures with each subsequent saturation oxygen exposure, i.e. as carbon is removed. Thus, the surface remaining after reaction has reactivity almost identical with that of the original -p(5X1) surface. Six saturation 0, exposures are required to remove all carbon. The C O reaction and desorption kinetics depend on the carbon and oxygen coverages. The low temperature p1-C0desorption maximum shifts from =920 to 1000 K as 0, exposure increases to ~2 langmuirs, but is unaffected by larger 0, exposures. For oxygen exposures less than ~ 0 . langmuir, 3 no p 2 - C 0 formation is observed at 1300 K, and the (5x1) LEED pattern remains unchanged throughout the reaction, regardless of the ethylene exposure used to make the -p(SXl)-C surface. The p 2 - C 0 desorption is also absent for ethylene exposures less than =20 langmuir, regardless of 0, exposure. W ( lOO)-p(5Xl)-C surfaces produced by exposure to less than -1 2 langmuirs of ethylene are not necessarily regenerated after reaction: the final surface state depends on the relative coverages of carbon and oxygen. For > &-, all carbon is removed in the reaction, leaving oxygen on the surface, and producing a -p(2X1) LEED pattern, which is characteristic of atomic oxygen on W ( 100) for 0.5 < 00 < 1 .O. For eo < eC,carbon remains on the surface. The C / W Auger ratio3s is smaller than that found for the - (5Xl)-C surface, and the surface displays a mixture of the !c and -c(2X2) LEED patterns, corresponding to unreconstructed tungsten atom positions and carbon coverages of 5.0 X 1014 and 6.7 X IOl4 atoms/cm2, (33) Stefan, P. M.; Helms, C. R.; Perino, J. T.; Spicer, W. E. J . Vuc. Sci. Technol. 1979, 16, 571. (34) The values for integrals were obtained by digitally integrating tem-
perature programmed reaction spectra with respect to time. Heating rates were identical t0 each desorption spectrum, but varied from 140 K/s at 900 K to 60 K/s at 1500 K within each spectrum. Integral values are normalized to the desorption of P-CO from W( 100). The mass spectrometer used detected CO and N, with approximately equal sensitivities. (35) The Auger C/W ratios reported are the ratio of the heights of the C KLL (271 eV) and W N O 0 (169 eV) peaks. LEED patterns were obtained with a 155-eV beam energy.
I
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600 700 900 1100 1300 1500 Temperature ( K )
Figure 3. Temperature programmed reaction spectrum of 1.6 langmuirs of 'SNzOadsorbed on W(lOO)-p(5Xl)-C. T h e IsN2and I5NzOdesorption spectra used radiative heating (dT/dr = 15 K/s), while the CO desorption utilized electron bombardment heating (dT/dt varied from 60 to 12 K/s). T h e data are not corrected for mass spectrometer ionization cross section or fragmentation. Less than 10% of the N2 desorption intensity results from N20 fragmentation.
re~pectively.~ If eo 9OOK) A
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Exposure (L) Figure 6. Integrated desorption intensityS5of the products of NO reaction on W(IOO)-p(5Xl)-C as a function of coverage. The low temperature desorption spectra used radiative heating (dT/dt = 15 K/s), while data obtained above 700 K ptilized electron bombardment heating (dT/dr varied from 60 to 12 K/s).
that from O2 adsorption (Table I), Le., triple that resulting from adsorption at 120 K. The (5x1) LEED pattern persists after N 2 0 adsorption at 120 K, with increased background intensity and diffuse diffraction spots. A [;
