2576
J. Phys. Chem. 1985,89, 2576-2581
are explained in terms of coalescence of the liquid phase as the loading is raised, followed by pore filling of the support. The explanation presupposes, and helps to confirm, the existence of a large contribution to retention from adsorption at the liquid surface. In studying mixed retention phenomena it is necessary (i) to use low solute concentrations (as a rough rule of thumb, (ii) to measure retention mole fraction in liquid phase, x , at constant solute concentration, rather than constant sample size, independent of the liquid loading, and (iii) to use closely spaced liquid loadings to reveal the presence of a wetting transition. The choice of method of separating the partition and adsorption contributions to retention depends largely on the degree of peak asymmetry.
The phenomenon of coalescence in squalane, with a previously unsuspected discontinuity in retention behavior, makes previous work on this widely studied liquid potentially open to question. There is a further implication for the practice of using silanized supports: it cannot be assumed that this necessarily simplifies the adsorption behavior.
Acknowledgment. We thank the Science and Engineering Research Council of the United Kingdom for supporting this work. G.A.O. thanks the University of Petroleum and Minerals for sabbatical leave during which part of this work was done. Registry No. Squalane, 111-01-3; acetone, 67-64-1; butan-2-one, 78-93-3; ethyl acetate, 141-78-6.
Reaction of NO on W(lOO), W(IOO)-p(2X2)-0, and W(IOO)-p(2Xl)-O Nondissociative Pathway for N-N Bond Formation
Surfaces: A
E. K. Baldwin and C. M. Friend* Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: October 26, 1984) The chemistry of NO, N,, and N20 adsorbed at 120 K has been studied on W(lOO), W(lOO)-p(2xl)-O, and W(lOO)-p(2X2)-0 surfaces under ultrahigh vacuum conditions by using thermal desorption spectroscopy, isotope exchange experiments, Auger electron spectroscopy, and low energy electron diffraction. Dissociative adsorption of the adsorbates studied was inhibited on the -p(2X1)-0 and -(2X2)-0 surfaces. The inhibition of NO dissociation on the oxygen-pretreated surfaces gave rise to qualitatively different NO chemistry, yielding N 2 0and N, as reaction products. Isotopic exchange experiments establish that the mechanism for the reaction of NO to form N,O proceeds via a nondissociative pathway with NO dimer proposed as the reaction intermediate.
Introduction The chemistry associated with tungsten-based surfaces has been shown to be strongly dependent on surface composition and structure.I In particular, oxygen adlayers are known to dramatically change the types of surface intermediates and reaction products formed by a wide range of adsorbates. For example, oxygen pretreatment of W( 100) blocks the dissociative adsorption of C02and decreases the extent of NH3 decompo~ition.~In this work, the reaction and adsorption of nitric oxide on clean W ( 100) and two well-ordered, thermally stable oxygen overlayers, W ( 100)-p(2X2)-0 and W(lOO)-p(2Xl)-O, has been studied. The reaction and adsorption of NO on transition-metal surfaces has received particular attention in the surface science literature due to its relative simplicity and rich catalytic c h e m i ~ t r y . ~Of particular interest are catalytic oxidation and reduction reactions of nitric oxide to less environmentally deleterious molecules such as N, and N 2 0 . Two modes of adsorption have typically been considered in studies of NO: dissociative adsorption, which yields atomic nitrogen and oxygen, and molecular adsorption of monomeric NO. Well-documented examples of both types of adsorption have been reported in the literature. Quantitative molecular adsorption and Pt(100),6 and desorption of NO has been established on Pd(l1 F’t( 1 1l).’ Dissociative adsorption in varying degrees has also been documented on N i ( l 1 l ) , s R u ( O O ~ ) ,Ir(lOO),10 ~ I r ( l 1 1),11 (1) See, for example: Aronovich, R. A.; Fal’kov, I. G.; Yablonskaya, A. I.; Tuktarova, L. S.; Grigor’ev, V. F.; Bol’shakov, D. A,; Yablonskii, 0. P. K i m . Katal. 1982, 23, 1268. (2) KO, E. I.; Madix, R. J. J . Phys. Chem. 1981, 85, 4019. (3) Egawa, Chikashi; Shindo, Hitoshi; Onishi, Takaharu. J . Chem. SOC., Faraday Trans. 1 1981, 77, 927. (4) (a) See, for example: Joyner, R. W. Catalysis (London) 1982,5, 33. (b) Harrison, B.; Wyatt, M.; Gough, K.G. Ibid. 1982, 5, 127. (5) Conrad, H.; Ertl, G.; Kuppers, J.; Latta, E. E.Surf.Sci. 1977,65, 235. ( 6 ) Bonzel, H. P.; Pirug, G. Surf.Sci. 1977, 62, 45. Weinberg, W. H.; Lambert, R. M. Surf. Sci. 1976,57, (7) Comrie, C. M.;
619. (8) Conrad, H.; Ertl, G.; Kuppers, J.; Latta, E. E. Surf.Sci. 1975,50,296. ( 9 ) Thomas, G.E.; Weinberg, W. H. Phys. Reu. Lett. 1978, 41, 1181.
0022-3654/85/2089-2576$01.50/0
Rh( 11 1),l2 and polycrystalline Re.I3 Thermal reaction of molecularly adsorbed NO gives rise to competing desorption of gaseous N O and dissociation to adsorbed nitrogen and oxygen atoms. Atomic nitrogen and oxygen may undergo recombination at elevated temperatures to form gaseous NO, N2, or 02,with the ratio of products and reaction energetics being dependent on the specific transition-metal surface. Thus, catalytic nitrogen formation has been thought to proceed solely via a dissociative mechanism. Herein we present the results of investigations on clean and oxygen-pretreated W ( 100). The presence of surface oxygen decreases the probability of NO dissociation, resulting in a nondissociative pathway for N, and N 2 0 formation from nitric oxide.
Experimental Section Experiments were performed in a bakeable stainless steel ultrahigh vacuum chamber (Varian), with a base pressure of 1.5 X torr. The chamber is oil and vibration free, pumped by an integral 220 L/s ion pump and a Ti sublimation pump, and roughed with a liquid nitrogen cooled cryosorption pump. The W(100) crystals were oriented and polished by using standard methods by the Cornell Materials Research Center, with a final polishing using l-Hm diamond paste. The final crystal dimensions were 0.25 in. in diameter and 0.05-0.1 5411. thick. The crystal was mounted on a manipulator with x , y , z translation and 360° rotary motion with a temperature range of 100-2600 K. The crystal was heated either via electron bombardment or radiatively. The maximum temperature attainable via radiative heating is 1200 K. Higher temperatures were attained by using electron bombardment with 600-eV electrons 100 K/s). The temperature from the same filament (dT/dt
-
-
ly
(10) Kuppers, J.; Michel, H. Surf.Sci. 1979, 85, L201. (11) Kanski, H.; Rhodin, T. Surf.Sci. 1977, 65, 63. (12) (a) Root, T.W.; Schmidt, L. D.; Fischer, G. B. J. Vac. Sci. Technol. A 1984, 2, 885. (b) Root, T. W.; Schmidt, L. D.; Fischer, G. B. Surf.Sci. 1983, 134, 30. (13) Schulze, D.D.; Utley, D. L.; Hance, R. L. Surf.Sci. 1981, 102, L9.
0 1985 American Chemical Society
The Journal of Physical Chemistry. Vol. 89, No. 12, 1985 2577
Reaction of N O on Tungsten Surfaces was monitored with a W-5% Re/W-20% Re thermocouple spotwelded to the side of the crystal. The crystal was cleaned by repetitive cycles of heating to 1300 K in an O2 flux equivalent to 10” torr of O2 for 10-20 min and subsequent flashing to -2600 K for 15 s. Adsorbate concentration and surface cleanliness were monitored with retarding field Auger electron spectroscopy (Varian). LEED was used to check surface order and to identify ordered overlayer structures. Thermal desorption spectra were obtained by using a quadrupole mass spectrometer (UTI, Model 100C) mounted in a differentially pumped liquid nitrogen cooled cryoshield. The cryoshield effectively eliminated background desorption signal, so that only molecules desorbing from the W ( 100) crystal face were detected. The orifice of the mass spectrometer shield was 0.125 in. in diameter and was positioned -0.125 in. from the crystal. The mass spectrometer was interfaced with an IBM PC, which controlled the mass selected by the spectrometer and stored the spectrometer output. Approximately 20 ms are required to obtain desorption data for a given mass and temperature, allowing data collection for a maximum of eight masses in a single experiment. Thermal desorption spectra were usually obtained by using radiative heating, resulting in a heating rate of -25 K/s at temperatures below 700 K. Electron bombardment with 100-200-eV electrons was used when monitoring desorption above 800 K. The resulting maximum heating rate was 90 K/s and variable. Masses 44,28, and 30 were monitored for N 2 0 , N2, and NO, respectively, in experiments utilizing radiative heating. Mass 14 was monitored in high-temperature electron bombardment desorptions of N2due to a large background rise in mass 28. Masses 28 and 30 were significant cracking fractions of N 2 0 : all 28 and 30 amu coincident with N 2 0 desorption were accounted for as cracking fractions. Gases were introduced with a directed dosing system, which used a multicapillary array located 1.O in. from the crystal to provide a beam of molecules directed at the crystal. Three identical dosers were utilized in the experiments. This arrangement enhanced the molecular flux at the crystal by a factor of about 100 relative to the background pressure. The pressure behind the multicapillary array was about torr during dosing, while the background pressure in the chamber stayed below 5 X lo4 torr. Dosages were calibrated by calculating the molecular flux following measurement of the conductance of the gas introduction system.I4 The fluxes are converted to langmuirs (1 langmuir = 10” toms = 3 x IOi4 molecules = 1 monolayer for unit sticking probability). The gases used in the experiments were NO, 02,N 2 0 , and N2, all Matheson research purity grade, and were not purified further. Isotopically labeled compounds were supplied by Stohler Isotope Chemicals (I8O2,99%; lSN20,99%) and Cambridge Isotope Co. (I5N2,99%; 15N0, 99%). The O2 was introduced in a separate doser in all experiments described in order to eliminate reaction of O2and NO, reagents within the dosing system. Mass spectra of reactant gases dosed into the chamber were obtained subsequent to every thermal desorption spectrum in order to preclude reaction within the dosing system from contributing to the observed chemistry and to calibrate the mass spectral cracking pattern for the reactant molecules. Oxygen overlayers were characterized by Auger and LEED. Two oxygen overlayer structures were used; p[(2X1) (1X2)] structure with estimated coverage of 0.5 (hereafter referred to as W(lOO)-p(2Xl)-O), and a ~ ( 2 x 2 with ) 8 e 1.0, as measured by using Auger electron s p e c t r o ~ c o p y . ~The ~ overlayers were formed reproducibly by heating the crystal to 1050 K and dosing with O2 at a pressure of 1 X 10” torr. Exposure of 1-1.5 langmuirs yielded the p[(2X 1)-( 1X2)] surface, and 7-9-langmuirs exposures gave the ~ ( 2 x 2 ) .
-
Tad--
-
~~
~~
~~~
(14) (a) The conductance through a constriction tube 0.005 in. i.d. X -2.0 in. in length was measured. Assuming molecular flow and using the value obtained for the gas conductance, the flux at the crystal was calculated by the method of Stewart and Ehr1i~h.I‘~ (b) Stewart, C. N.; Ehrlich, G. “Studies on Activated Chemisorption”; Science Library, University of Illinois; Report R-6 12, 1975.
1
(“)
A + h 0 O L 1
14L
4 04J JL 0 ‘-
nn
UlL
EL
ulv
5L
0
I
3. 8L
b
3
A
L
1.7L
1.3L 100
300
500
700
900
I300
1100
1500
1700
Temperature (K)
Figure 1. Thermal desorption spectra of N2 (28 amu) produced from NO on W(100) are-shownas a function of NO expo~ure.’~ The desorption data obtained below 700 K utilized radiative heating (dT/dt 30 K/s) and that above 900 K utilized electron bombardment heating, resulting in a highly nonlinear heating profile (dT/dt, N 100 K/s). The 28-amu desorption near 200 K is the result of cracking of N20 in the mass
-
spectrometer.
-
(x4)
rads-1 2 0 K
L 14L
-
+
l2OK
8L
5L
3.6L 1. 31
1M
300
500
700
900
Temperature
1100
1300
1500
1700
(K)
Figure 2. Thermal desorption profiles of N 2 0 (mass 44) produced from NO on W(100) are depicted as a function of NO exposure.14 Radiative heating (dT/dt 30 K/s) was used in obtaining all spectra shown. The high-temperaturetail on the N20desorption peak is probably the result of slow pumping speed for N20.
Results and Discussion Clean W(100). Thermal desorption spectroscopy evidences two regimes of nitric oxide reactivity as a function of coverage, as shown in Figures 1 and 2. At low coverage, NO thermal desorption yields gaseous N 2 in two coverage-dependent thermal desorption peaks above 1100 K, and oxygen, which remains on the surface, in agreement with previous work. The N2desorption peaks appear sequentially as a function of N O coverage; the highest temperature peak, which shifts continuously to lower temperatures as a function of NO coverage, populates initially, with the maximum rate of desorption occurring at 1450 K at saturation NO exposures. Thermal desorption of a mixed 1 5 N 0 / 1 4 N 0adlayer resulted in a statistical distribution of 14N14N,lSN-l4N, and 15N-lSN in the molecular nitrogen desorption peaks above 1100 K. Thus, these two higher temperature N 2 desorption features result from the recombination of surface nitrogen atoms with energetics approximately the same as those observed for recombination of N atoms produced from N2 adsorption.16 This interpretation is consistent with earlier LEEDIS and UPS” data for NO adsorbed on W(100) at 300 K where (15) Usami, S . ; Toshinobu, N. Jpn. J. Appl. Phys. Suppl. 1974, 2, 237. (16) Clavenna, L. R.; Schmidt, L. D. Surf. Sci. 1970, 22, 365.
2578
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
Y x
2
fl
CVI VI,
e(Ye, , '4 -c 3s
191
- x O
L
eL , LO
L
!it no
LI)L
VI"
0 I
3L 1. 51.
LI,
I .EL '
100
300
500
700
800
1lW
1300
15M
1700
Temperature (K)
Figure 3. Thermal desorption spectra obtained on W(100) subsequent to adsorption of N 2 0 are shown as a function of N20exposure. At low temperature, N 2 0 desorption is observed whereas only N2 desorption is observed at temperatures above 1000 K. The N2desorption spectra were obtained by using electron bombardment heating.
dissociative adsorption was proposed. XPS and ELS studies of N O on clean W(110) also evidence complete dissociative adsorption at 300 K18a,band dissociation at low exposures at 100 K.18a Numerous studies on polycrystalline W also support a dissociative adsorption At high N O exposures (53.0 langmuirs, 8, 0.75) a second regime of reactivity is evident. Production of NzO at 200-220 K is observed, with the NzO desorption feature shifting to higher temperature as a function of N O exposure (Figure 2). N o nitric oxide desorption was evident up to exposures of 20 langmuirs. Consideration of the energetics of N 2 0 formation near 200 K suggests a mechanism involving direct N-N bond formation without proceeding through a nitrogen-oxygen bond scission step, since molecular nitrogen desorption is observed at 160 K (Edes E 9 kcal/mol)20 on clean W(100) following molecular adsorption of N 2 at high coverages while N 2 formation from nitrogen atom recombination occurs above 1000 K. Adsorption of NzO on W( 100) yields molecular nitrogen and NzO as gaseous products in a thermal desorption experiment. At low coverage, N, is produced in a coverage-dependent desorption peak at temperatures above 1200 K. At high N 2 0 exposures (21.0 langmuir), N 2 0 is desorbed in a coverage-dependent peak at 200-220 K as shown in Figure 3. Thermal desorption of a mixture of 15N20and 14N,0 yielded only 15N20,14N20in the 200 K desorption peaks. The absence of isotopic mixing establishes that the desorption feature centered at 200 K results from desorption of molecularly adsorbed N20. The gaseous molecular nitrogen formed at high temperature results from recombination of atomic nitrogen on the surface as evidenced by the production of I5N,, I4N,, and 15N14Nfor temperatures greater than 1000 K in the thermal desorption spectrum obtained subsequent to exposure of the W( 100) surface to an 14N20/15N20 mixture. These data are consistent with previous XPS studies of NzO adsorbed on W( 110) where dissociative adsorption of Nz@ was observed at low coverages with subsequent molecular N 2 0 adsorption at
-
(17) Bhattacharya, A. L.; Broughton, J. W.; Perry, D. L. Surf. Sci. 1978, 78, L689. (18) (a) Masel, R. I.; Umbach, E.; Fuggle, C.; Menzel, D. Sufi. Sci. 1979, 79, 26. (b) Rawlings. K. J.; Foulias, S. D.; Hopkins, B. J. Surf. Sci. 1981, 108, 49. (19) (a) Miki, H.; Inomata, H.; Kato, K.; Kioka, T.; Kawasaki, K. Surf. Sci. 1984, 141,473. (b) Yates, J. T., Jr.; Madey, T. E. J . Chem. Phys. 1966, 45, 1623. (c) Tamura, T.; Hanamura, T. Bull. Chem. SOC.Jpn. 1976, 49, 1780. (d) Kunimori, K.; Kawai, T.;Kondow, T.; Onishi, T.; Tamura, K. Chem. Lett. 1975, 1303. (e) Sato, M. Jpn. J. Appl. Phys. 1977, 16, 653; 1977, 15, 1995. (0 Sato, M. Surf.Sci. 1980, 95, 269. (8) Madey, T. E.; Yates, J. T., Jr.; Erickson, N . E. Surf.Sci. 1974, 43, 526. (h) Madey, T. E.; Yates, J. T., Jr. Jpn. J . Appl. Phys., Suppl. 1974, 2, 461. (20) (a) The estimate of .& assumes a simple Arrhenius desorption s-I. (b) See, for example: King, D. modelznband a preexponential of A . Surf.Sci. 1975, 47, 384.
-
Baldwin and Friend high exposures at 100 K.,l Dissociative adsorption above 300 K was also proposed on W(110)2za and polycrystalline Preadsorption of I5NO at low coverages (52.0 langmuirs) or at high coverages (55.0 langmuirs) and subsequent annealing to 300 K was utilized as a means of producing I5N atoms on the W(100) surface.23 Subsequent adsorption of I4NO at 120 K and thermal desorption resulted solely in the formation of I4N-l4N and 14N-'4N-0at 220 K, indicating that adsorbed N atoms formed from dissociated N O do not react to form N 2 0 at 200 K. The thermal desorption profile obtained from annealing, cooling, and subsequent adsorption of N O did not differ from those obtained from high exposures of NO. This sequence "saturated" the dissociative pathway, yielding the maximum attainable surface 15Nconcentrations. Analogous experiments were performed as a function of initial I5NO exposures. In all cases, no isotopically mixed 14N15N0was produced. These isotopic exchange experiments definitively establish that the N,O formed at low temperatures is a result of direct N-N bond formation.24 These results may be contrasted to the interpretation of earlier XPS work on W( 110)Isa where directly analogous behavior was seen as a function of nitric oxide coverage and substrate temperature. At high coverages and 100 K, the N(1s) and O(1s) regions of the XPS spectrum developed new features compared to the spectrum obtained at 300 K. The XPS data obtained for high exposures of N O at 100 K were consistent with coexisting The authors posadsorbed N, 0, transient NO, and N20.1sa~Z1 tulated that N,O formed via reaction of N O with surface nitrogen atoms. Although there may be a dependence on surface crystallography, comparison of the data obtained on the W(100) and W( 110) surfaces is indicative of qualitatively similar chemistry. The isotopic exchange experiments suggest a mechanism for low-temperature N 2 0 formation that proceeds via an N O dimer. However, the (NO), may not be surface stable, but may react rapidly to yield adsorbed N 2 0 . No new diffraction features were observed upon adsorption of N O or N 2 0 at 120 K. A (2x1) LEED pattern resulted subsequent to thermal desorption of N O or N 2 0 , and oxygen was observed in the Auger spectrum. The (2x1) LEED pattern exists over a broad coverage range and has been observed previously from N O thermal d e c o m p ~ s i t i o n . I ~ ~ ~ ~ - ~ ~ Oxygen-Pretreated W(lO0).The effect of oxygen adlayers on nitric oxide chemistry was investigated on two well-ordered, thermally stable surfaces, W(lOO)-p[(2X1) + (1 X2)]-0 and W( 100)-p(2X2)-0 with approximate oxygen coverages of -0.5 and 1.O, respe~tively.~~ Low-energy electron diffraction patterns of these surfaces are shown in Figure 4. These two structures have been studied extensively by using a variety of spectroscopic method^.,^-^^ The structure proposed for the -p(2X 1 ) - 0 allows
(21) (a) Fuggle, J. C.; Menzel, D. Surf.Sci. 1979, 79, 1~ (b) Umbach, E.; Menzel, D. Chem. Phys. Lett. 1981, 84, 491. (22) (a) Weinberg, W. H.; Merrill, R. P. Surf. Sci. 1972, 32, 317. (b) Gasser, R. P. H.; Lawrence, C. P. Surf.Sci. 1968, 10, 91. (23) Coadsorption and subsequent annealing to 300 K of a mixture of I4NO and ISNOfollowed by cooling and thermal desorption yielded 14N2e), 1SN2(g), and 14NIJN(g) at temperatures above 1000 K. No other products or low-temperature N2desorption were observed, thus establishing that all remaining nitrogen is atomically adsorbed following the annealing experiment. (24) The two possible mechanisms for low-temperature N, and NZOformation would lead to different isotopically labeled products. The dissociative pathway would lead to isotopically mixed products as shown in the scheme below. 1.
2. 3.
"NO(,,
-
lSNads+ Oedr preadsorption and dissociation
-
14NO(sd,) + lJNads
''N(,ds) + lsNada
-
i4N-1S-O(,d,)
-+
14N-"N-0 (8)
+
IsN-14N(,b) lsN-14N(8)
(25) (a) Coverages were estimated by Auger spectroscopy, using the cal(b) Pons, F.; Le Hericy, J.; Langeron, J. P. ibration method of Pons et Surf. Sci. 1977, 69, 565. (26) Bauer, E.; Poppa, H.; Viswanath, Y. Surf. Sci. 1976, 58, 517. (27) Kramer, H. M.; Bauer, E. Surf. Sci. 1980, 92, 53. (28) Feuerbacher, B.; Adriaens, Surf. Sci. 1974, 45, 553.
Reaction of NO on Tungsten Surfaces
The Journal of Physical Chemistry. Vol. 89, No. 12. 1985 2519
R lonproture 00
FEpw 5. Thermal desorption apecira obtained from reaction of NO on W(IOO)-p(2XI)-O. NlO (44amu) and N (14 amu) daorption spatra are shown in the top and in the bottom pOrtMn0 of the tigun. nspstivelv. Electron bombardment was used to h a t the crystal.
for place exchange of rows of W and 0 atoms; thus, the 0 atoms lie somewhat below the surface plane leaving W atoms exposed?)-'6 The -(2X2)-0 surface has not been as well characterized; however, there is agreement that the surface tungsten atoms 1eU)IlStNCt. In general, both the -p(2X I ) - 0 and -p(2X2)-0 surfaces were less effectivethan the clean surface in dissociating Nz, NzO.and NO. Dissociative adsorption of all three molecules studied was completely blocked on the -(2X2)-0 surface, with a limited amount of dissociation occurring on the -(2X1)-0 surface." The thermal desorption spectrum obtained following Nz adsorption at 120 K on both the -(2X1)-0 and -(2X2)-0 consisted of a single Nz desorption peak centered at -220 K. Thermal desorption of an "Nz/"Nz mixture yielded no "N-I5N establishing that this N, feature results from molecularly adsorbed nitrogen. Adsorption at 120 K and subsequent thermal reanion of NO yielded gaseous NzO at 200-220 K on both the -p(2XI)-O and -p(2X2)-0 surfaces and gaseous N, above loo0 K and at 280 K on the -(2Xl)-O and -(2X2)-0 surfaces, respectively. The mpeciive thermal desorption data obtained from reaction of NO on the -(2X1)-0 and -(2X2)-0 surfaces are shown in Figures 5 and 6. The amount of N2 produced above 1000 K on the p(2XI)-O surface was -2 times smaller than that obtained for similar NO coverages on clean W(100). At high NO exposures
4. Lov-energy electron diffmnion ptlems for the three suflaaca investi6aicdanshown: (a)clean W(1OO). (b) W(IOO)-P[(Zxl)+(lx 2)l-0. and (c) W(lOO)-P(2X2).0. The bcam energy vas 62 eV in all three cam.
(29) FmiWKim. H.;Ilmch. H.: I.ehwald. S. Phys. Rm.8.I97S.14,1362. (30) Hold. J.: Schafcr. J. Sur/. Sci. 1%1, 108. L387. (31) (a) Bradshaw. A. M.: Mcnzcl. D.;Stcinkilberg, M.Jpn. 1. Appl. Phys., Suppl. 1974.2.841. (b) Bradshaw, A. M.:M e n d . D.:Steinkilberg. M. Discuss. Fomdoy Sm.1974.46. (32) Carme. J . D.: Barikau. J. M. .Turf Sei. 1983. l34.886. (33) Yu. M. L. Suf. Sci. 1978. 71. 121. (34) Prime. S.: Nichus. H.:Baucr. E. Surf, Sei. 1577.65. 141. (35) Prime. S.; Nichus. H.:Baucr. E. Surf, Sd. 1978. 75.635, (36) Alnot. P.; k h m . R. J.; BNndk. C. R.. unpublished mului
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
r
1
A 1
.BL-
.
3
N,O
120K
=,T
r cc_
I
*I
100
200
300
400
500
600
700
(K) Figure 6. Desorption data obtained from thermal reaction of NO on a W(lOO)-(ZX2)-0 surface. The N20, N2, and NO desorption spectra were all obtained by using radiative heating. No desorption products were detected above 700 K in complementary experiments obtained with electron bombardment heating. Temperature
-
(58.0 langmuirs), NO desorbed from the W ( lOO)-p(2X2)-0 surface at 175 K. No nitric oxide desorption was detected on the -(2X1)-0 surface up to exposures of 30 langmuirs. As in the case of clean W( loo), the N 2 0 desorption spectrum obtained from thermal reaction of N O was the same as that obtained subsequent to N 2 0 adsorption on both the -p(2X 1 ) - 0 and -p(2X2)-0 surfaces. Isotopic exchange experiments of a coadsorbed layer of 15N20and 14N20on the oxygen pretreated surfaces yielded no 14N-15N-0, establishing that the N 2 0 desorption feature resulted from molecularly adsorbed N 2 0 . Thus, the N,O desorption obtained following adsorption of N O on the oxygen adlayers is desorption limited; i.e., N 2 0 is a surface stable intermediate. N 2 desorption features at 280 and >lo00 K on the p(2X2)-0 and -(2X1)-0 surfaces, respectively, were observed in the thermal desorption spectrum obtained subsequent to N 2 0 adsorption at 120 K.45Reaction of the 1 4 N 2 0 / 1 5 Y zmixture 0 yielded no isotopically mixed products at 280 K on le -(2X2)-0 surface and a statistical distribution of lSN2,14N2,m d 15N-14N above lo00 K on the -(2X1)-0 surface. Desorption of N 2 0 from 'sO-labeled overlayers yielded no 180-containing products. Thus, the 780 K N 2 desorption peak on the p-(2X2)-0 surface arises from the decomposition of molecularly adsorbed N 2 0 . The high-temperature production of N 2 from N 2 0 on W ( 100)-p(2X 1 ) - 0 results from surface nitrogen atom recombination, reflecting some dissociation of N 2 0 on the p(2X 1)-0 surface. Thus, the gaseous N 2 produced from reaction of NO on both oxygenpretreated surfaces resulted from decomposition of surface N 2 0 . The NO desorption feature observed at 175 K following large exposures of NO on the W( lOO)-p(2X2)-0 surface is attributable to reversible desorption of molecularly adsorbed NO. No N 2 0 desorption is coincident with this NO feature, precluding N O production as a gas-phase cracking fraction. No isotopic exchange experiments were performed to verify this, however. Reaction
Baldwin and Friend of NO on I80-labeled adlayer surfaces yielded no N I 8 0 or N,'*O. Adsorption of l5NO at high exposures (-2.0 langmuirs) and 120 K followed by annealing to 300 K was used to produce ISN atoms on the -(2X1)-0 surface. Subsequent adsorption of I4NO (-2.0 langmuirs) at 120 K did not yield any cross-labeled N 2 0 a t 200 K in the thermal desorption experiment. As on clean W ( loo), these experiments eliminate surface recombination of atomic nitrogen and NO as a mechanism for NzO formation from N O on the p-(2X1)-0 surface.24 Analogous experiments could not be performed on the -p(2X2)-0 surface due to the inability of this surface to produce atomic nitrogen via dissociation of N,, NO, or N,O. The absence of an N, desorption feature above 200 K and the lack of dependence of the reaction energetics for N,O formation on the detailed nature of the oxygen overlayer suggest that the mechanism operative on the -p(2X2)-0 surface is the same as that on the other surfaces studied, e.g., that N 2 0 production does not p r d via reaction of adsorbed NO and nitrogen atoms. No previous investigations of NO or N 2 0 on oxygen-pretreated tungsten surfaces have been reported. No new LEED features were observed at 120 K; an increase in the LEED background intensity was evident at high exposures of both NO and N20. The (2x1) and (2x2) LEED patterns remained subsequent to desorption.
Discussion On the basis of thermal desorption data and isotopic exchange experiments, oxygen pretreatment of the W( 100) generally inhibits the dissociation of species that have ll* orbitals that may accept electron density, e.g., COz, N,, NO, and N 2 0 . The effect of surface oxygen on NO and NzO chemistry is qualitatively the same on W( 100) surfaces,with oxygen produced from NO dissociation below 300 K and on the two overlayer structures produced from 0, adsorption at high temperatures. The quantitative aspects of dissociative adsorption do vary as illustrated by the partial dissociation of NO and N 2 0 on W(lOO)-p(2XI)-O compared to complete inhibition of NO and N 2 0 dissociation on the W( 100)-p(2X2)-0 surface during thermal desorption experiments with the same heating rate and vacuum conditions. No detailed kinetic data have been obtained for any of these reaction systems, to date. Thus, no detailed model of reaction kinetics is possible. Structural effects, e.g., changes in the geometry of available sites for adsorption or dissociation, probably do not cause qualitative changes in the observed chemistry. The qualitatively similar conclusions of previous XPS work studying NO and N 2 0 adsubstantiate this assertion. The relative sorption on W( 110)'8*,z1 insensitivity of NO and NzO chemistry to changes in surface geometry can be contrasted to the strong dependence of the dissociation probability of N2 on tungsten surface crystallography, which differs by 3 orders of magnitude on W( 100) compared to W ( I 101.37 Further investigation to establish whether electronic structural changes dominate the NO chemistry is currently underway. In particular, the effect of other electronegative spectator atoms such as N, S , or C1 will be studied. If withdrawal of electron density from surface tungsten atoms plays a key role, the N O and N 2 0 chemistry should be qualitatively similar for any electronegative adatom that does not directly participate in the surface reaction. The most striking chemistry is that of the reaction of nitric oxide on the oxygen-pretreated surfaces. Formation of an N-N bond without prior NO dissociation has been established on the oxygen pretreated W(100) surfaces studied herein. We propose that the reaction of NO to form NzO on oxygen-pretreated W(100) proceeds via an NO dimer, as shown schematically in eq 1-9. The
-
NO(,)
-+
[NO,a,I+
[NO(a)I+ Na + O(a) -+
2[NO(a,It
-+
[(N0)2(a)It
(1)
(2)
(3)
(37) (a) Madey, T. E.; Yates, J. T.; Nuovo Cimento N. 1967, 2, 483. (b) Tamm, P. W.; Schmidt, L. D. S u r - Sci. 1971, 26, 286.
J . Phys. Chem. 1985,89,2581-2585
species that are bracketed and denoted with a dagger superscript may or may not be surface stable. Competing dissociation and desorption pathways are available for both N O and N 2 0 in this mechanism. The relative importance of each step is dependent on the nature of the oxygen overlayer with steps 2 and 9 dominating clean W ( 100) chemistry. Dimer formation is predicated on the inhibition of NO dissociation via the presence of surface oxygen. Nitric oxide forms a centrosymmetric dimer in the solid state as established by x-ray d i f f r a c t i ~ n . More ~ ~ recently, dimeric N O has been studied by using X-ray and ultraviolet photoemission in condensed multilayer^^^ and in N O adsorbed on Ag(ll1) at 20 K." Previously reported infrared studies of nitric oxide adsorbed on supported molybdena catalysts also evidence dimer f ~ r m a t i o n . ~Formation ' of dimeric nitric oxide on P t ( l l 1 ) has also been previously proposed on the basis of vibrational although further investigation supported a model with monomeric N O with more than one adsorption (38) Dulmage, W. J.; Meyers, E. A.; Lipscomb, W. N. Acta Crystallogr. 1953, 5, 760. (39) Tonner, B. P.; Kao, C. M.; Plummer, E. W.; Caves, T. C.;Messmer, R. P.; Salaneck, W. R. Phys. Rev. Lett. 1983, 51, 1378. (40) (a) Behm, R. J.; Brundle, C. R. J . Vac. Sci. Technol., A 1984, 2, 1020. (b) Nelin, C . J.; Bagus, P. S.; Behm, J.; Brundle, C. R. Chem. Phys. Let. 1984, 105, 58. (41) Perl, J. B. J . Phys. Chem. 1982, 86, 1615. (42) Ibach, H.; Lehwald, S. Surf.Sci. 1978, 76, 1. (43) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (44) Severson, M. W.; Overend, J. J . Chem. Phys. 1982, 76, 1584. (45) The atomic nitrogen on the -(2X1)-0 which results in NZWdesorption above 1000 K may result from either direct NO or N 2 0 dissociation on the
surface. (46) The extent of dissociation to atomic nitrogen of N2, N20, and NO
were dependent on their initial coverage. Quantitative measurements of the dissociation probability could not be made due to the nonlinearity in the heating rate required to attain temperatures sufficient to desorb N, formed via atom recombination.
2581
The nitric oxide dimer formed may not be surface stable, but may react rapidly to form adsorbed N 2 0 . The similarity of the thermal desorption data for NzO produced from N O vs. N,O adsorption suggests that N 2 0 is produced from NO prior to desorption. This assertion is also consistent with the observation of production of adsorbed N z O on W( 110) following high N O exposures at 100 K.21 The nitric oxide dimer may be stable at lower temperatures or on the highest coverage oxygen adlayer, W( 100)-p(2X2)-0. The observation of molecular N O desorption at 175 K on the -p(2X2)-0 surface suggests that nitric oxide dimer may be present on this surface at low temperature. Further spectroscopic investigation is necessary to identify the surface stable intermediates. Conclusion Key features of the chemistry of N O and NzO on W( 100) and oxygen-pretreated W ( 100) surfaces have been identified. The presence of surface oxygen inhibits the dissociation of NO, NzO, and N,. As a result of the inhibition of N O dissociation, nitric oxide reacts to form N z O via a nondissociative pathway on the oxygen-pretreated surfaces. The production of N 2 0 is postulated to proceed via an N O dimer which reacts to form adsorbed nitrous oxide. We submit that the inhibition of nitric oxide dissociation is critical in the proposed dimer formation, as opposed to a drastic change in the N O bonding mode. The inhibition of dissociation would allow for the natural tendency of N O to form a dimeric species. Comparison of this work with previously reported studies of NO and N 2 0 adsorption on W(110) suggests a relative insensitivity to surface crystallography. Furthermore, qualitatively similar behavior is associated with all oxygen-pretreated surfaces studied.
Acknowledgment. We thank and acknowledge Prof. T. N. Rhodin and the Cornel1 Materials Research Laboratory for supplying the W(100) crystals used in these experiments. This work was supported in part by the Harvard Materials Research Laboratories (NSF DMR 80-20247), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Research Corporation (Cottrell Research Grant No. 9787). C.M.F. acknowledges the receipt of an IBM Faculty Development Award, 1983-85. Registry No. NO, 10102-43-9; N1, 7727-37-9; N,O, 10024-97-2; Olr 7782-44-7; W, 7440-33-7.
In-Crystal Polarizability of 02P. W. Fowler University Chemical Laboratory, Cambridge, CB2 1 E W England
and P. A. Madden* Physical Chemistry Laboratory, Oxford, OX1 3QZ England (Received: December 19, 1984)
An ab initio value for the polarizability of the 02-ion in the MgO crystal has been found by coupled Hartree-Fock theory with Mder-Plesset correlation corrections. The calculation includes the effects of the Madelung potential and nearest-neighbor overlap on the oxide polarizability. We find a = 1.83 A3, which is within 10% of an experimental value. The effects of the Madelung potential in CaO, SrO, and BaO on the 0,- polarizability are also calculated. The results of the calculation are used to appraise the treatment of the crystalline environment in other theoretical schemes.
1. Introduction The dielectric permittivity of an ionic crystal at optical fiequencies (em) is related to the "in-crystal" values of the ionic polarizabilities via the Clausius-Mossotti formula (€- - l ) / ( e m + 2) = (3Vt,)4Ca' (1) i
The sum contains the in-crystal polarizability ( a i )of each ion in the unit cell, which is of volume V. Tessman et al.' used this relationship and the assumption that the in-crystal polarizability was a constant, independent of the particular ionic environment (1) Tessman, J. R.; Kahn, A. H.; Shockley, W. Phys. Rev. 1953, 92, 890.
0022-3654/85/2089-2581.%01.50/0 0 1985 American Chemical Society
