Alkoxide Synthesis from Methyl and tert-Butyl Nitrite on

A one-step synthesis of adsorbed alkoxides from alkyl nitrites dosed on Cu(100) is ... methyl nitrite (CH3ONO) to form methoxide (CH3O) and tert-butyl...
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Langmuir 2001, 17, 786-790

Alkoxide Synthesis from Methyl and tert-Butyl Nitrite on Cu(100) H. Ihm, K. Scheer, H. Celio, and J. M. White* Department of Chemistry and Biochemistry, Center for Materials Chemistry, University of Texas at Austin, Austin, Texas 78712 Received August 8, 2000. In Final Form: November 30, 2000 A one-step synthesis of adsorbed alkoxides from alkyl nitrites dosed on Cu(100) is illustrated using methyl nitrite (CH3ONO) to form methoxide (CH3O) and tert-butyl nitrite (t-C4H9ONO) to form tertbutoxide (t-C4H9O). Compared with an alternative, dosing CH3OH on an O-covered Cu surface, only minor differences in the concentration and local chemical environment of CH3O are evident. The potential energy profiles associated with the two synthesis methods are contrasted, and the key role played by the weak RO-NO bond is emphasized.

Introduction Methanol, CH3OH, is a widely used starting material in the chemical industry1 and is often produced from synthesis gas (CO and H2) interacting with a Cu-Zn oxide/ alumina catalyst. Methoxy, CH3O, is an important intermediate1,2 in these industrial processes and, alongside other adsorbed alkoxy moieties, is also of interest as a relatively simple model heteroatom species for investigating molecular-level surface phenomena involving oxidation. Although there are many theoretical and experimental studies of methoxy on surfaces,3-6 the surface chemistry of tertiary butoxy, t-C4H9O, has received much less attention.7,8 Single crystal copper is an interesting model substrate that typically has greater partial oxidation activity than silver9,10 and less than platinum.11,12 Because alcohols (ROH) do not readily thermally dissociate on Cu, adsorbed RO (alkoxide) has typically been prepared by dosing ROH (alcohol) on an oxygen-predosed surface.13 This reaction is thermally activated and can produce 2RO + H2O for each coadsorbed O. In this paper, we report an alternative synthesis of RO based on thermal activation of the weak RO-NO bond in alkyl nitrites, specifically synthesizing adsorbed CH3O and t-C4H9O by dosing CH3ONO and t-C4H9ONO on Cu(100). Although the conventional method works well, our method has some advantages; the process requires only * Corresponding author. E-mail: [email protected]. Fax: 512-471-9495. (1) Moulijn, J. A.; van Leeuwen, P. W. N. M.; van Santen, R. A. Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis; Elsevier Science: Amsterdam, The Netherlands, 1999; pp 51-54 and 174-175. (2) Twigg, M. V. Catalyst Handbook, 2nd ed.; Manson: U.K., 1996; pp 441-468. (3) Ryberg, R. Phys. Rev. B 1985, 31, 2545. (4) Ryberg, R. J. Chem. Phys. 1985, 82, 567. (5) Ryberg, R. Chem. Phys. Lett. 1981, 83, 423. (6) Mudalige, K.; Warren, S.; Trenary, M. J. Phys. Chem. B 2000, 104, 2448. (7) Camplin, J. P.; McCash, E. M. J. Chem. Soc., Faraday Trans. 1996, 92, 4695. (8) Brainard, R. L.; Madix, R. J. J. Am. Chem. Soc. 1989, 111, 3826. (9) Fieberg, J. E.; White, J. M. J. Vac. Sci. Technol., A 1997, 15, 1674. (10) Jenniskens, H. G.; van Essenberg, W.; Kadodwala, M.; Kleyn, A. W. Surf. Sci. 1998, 402-404, 140. (11) Hayden, B. E. Surf. Sci. 1983, 131, 419. (12) Peck, J. W.; Mahon, D. I.; Beck, D. E.; Bansenaur, B.; Koel, B. E. Surf. Sci. 1998, 410, 214. (13) Sexton, B. A. Surf. Sci. 1979, 88, 299.

one step (dosing RONO), and the RO coverage is easier to control (by simply changing the RONO exposure). Furthermore, alkyl nitrite synthesis is straightforward.14 On the other hand, care must be exercised to avoid, or deal with, NO dissociation. In this paper, alkoxides produced from CH3ONO and t-C4H9ONO dosed on Cu(100) at 100 and 300 K are examined, and for methoxide the results are compared with the method utilizing CH3OH + O. Finally, we compare the surface reaction paths, focusing on the role played by the relatively weak RO-NO bond. Experimental Section Experiments were performed in a two-level ultrahigh vacuum (UHV) chamber (base pressure of 2 × 10-10 Torr). The lower level is equipped with a single-pass cylindrical mirror analyzer (Physical Electronics) used for Auger electron spectroscopy (AES). A differentially pumped quadrupole mass spectrometer (Extrel C-50) was used for temperature-programmed desorption (TPD) and residual gas analysis (RGA) measurements. The upper level is coupled to a commercial Fourier transform infrared spectrometer (Nicolet, Magna-IR 860) and a mercury cadmium telluride (MCT) detector. The collimated IR beam from a SiC source was incident at a grazing angle (82°), chosen as a compromise between the theoretical absorption optimum (88°) and minimal light loss. The reflected light was p-polarized with a ZnSe wire grid polarizer and focused on the detector (frequency cutoff ∼ 725 cm-1). A spectrum comprised 1500 scans at 4 cm-1 resolution. The entire optical path outside UHV was purged with dry air to eliminate interference with CO2 and H2O. Excellent noise level performance was achieved (∼0.004% of ∆R/R peakto-peak noise). Further details are available elsewhere.15 The 13 mm dia Cu(100) crystal was cleaned by Ar+ sputtering at 850 K for 15 min followed by annealing in a vacuum at 980 K for 5 min. AES confirmed surface cleanliness, and the (100) crystal quality was checked occasionally using reflection/absorption infrared spectroscopy (RAIRS) of adsorbed CO. A chromelalumel thermocouple, inserted into a small hole drilled into the side of the Cu(100), was used to measure the substrate temperature. Thermal contact to liquid nitrogen yielded a base temperature between 95 and 100 K. For TPD, the ramp rate was 2.5 K s-1. CH3ONO was synthesized and purified using a standard protocol.14,16 CH3OH (99.8%, Mallinckrodt), t-C4H9ONO (90%, (14) Pressley, L. A.; Pylant, E. D.; White, J. M. Surf. Sci. 1996, 367, 1. (15) (a) White, J. M.; Ihm, H.; Smith, K. C.; Celio, H. Increasing the C-O Bond Anharmonicity of Methoxy on Cu(100) with Coadsorbates. American Vacuum Society 46th Symposium, 1999. (b) Celio, H.; Ihm, H.; Smith, K.; White, J. M. To be published.

10.1021/la001144u CCC: $20.00 © 2001 American Chemical Society Published on Web 01/06/2001

Alkoxide Synthesis on Cu(100)

Figure 1. RAIRS of CH3ONO (A) and t-C4H9ONO (B) dosed on Cu(100). Upper panel: (a) 1.5 langmuir of CH3ONO dosed on Cu(100) at 95 K, (b) 0.1 langmuir of CH3ONO dosed on Cu(100) at 95 K, and (c) 1.5 langmuir of CH3ONO dosed at 95 K, annealed to 200 K, and recooled to 95 K. Lower panel: (d) 0.5 langmuir of t-C4H9ONO dosed at 100 K, (e) 0.1 langmuir of t-C4H9ONO dosed at 100 K, and (f) 0.5 langmuir of t-C4H9ONO dosed at 100 K, annealed at 200 K, and recooled to 100 K. Dashed lines identify alkoxide, and solid lines identify alkyl nitrite. Curves (a) and (d) include multilayer contributions. There are no alkyl nitrite contributions to curves (c) and (f). Aldrich), and t-C4H9OH (99.0%, EM Scientific) were used after several freeze-pump-thaw cycles. To initiate a dose, a known pressure was established behind a preset leak valve connected to a 3 mm i.d. tube that ended 30 mm away from the face of the Cu(100). To terminate a dose, the volume behind the preset leak valve was evacuated. The leak valve was preset (and never changed) to give a background pressure rise to 5 × 10-9 Torr when Ar was dosed with the system at 300 K, that is, no cryogenic pumping due to liquid nitrogen cooled parts. Multiplying this pressure rise by the dosing time was used to calculate nominal nitrite and alcohol exposures in langmuir (1 langmuir ) 1 × 10-6 Torr s). O2 (>99%, Air Products) was used without further purification and dosed by backfilling through a standard leak valve.

Results and Discussion CH3ONO Dosed at 95 K. Figure 1A shows the RAIRS of (a) 1.5 langmuir of CH3ONO dosed on Cu(100) at 95 K, (b) 0.1 langmuir of CH3ONO dosed on Cu(100) at 95 K, and (c) 1.5 langmuir of CH3ONO dosed at 95 K, annealed to 200 K, and recooled to 95 K. On the basis of TPD data (not shown), dose a includes multilayer CH3ONO, whereas dose b is 70% of the first layer saturation. The absolute monolayer (ML) coverage is estimated below. Table 1 lists the peaks and their assignments along with data for CH3ONO in an Ar matrix and for 0.5 ML of CH3O on Cu(100) synthesized by reaction of CH3OH with O.6,17 (16) Tarte, P. J. Chem. Phys. 1952, 20, 1570.

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Except the peaks at 2808, 2880, and 2918 cm-1, which are from CH3O (Table 1), the features of spectrum a are due to CH3ONO. Spectrum (b) contains five peaks, 990, 1932, 2810, 2881, and 2920 cm-1, assigned to modes of CH3O (marked by dashed lines) and small signals from CH3ONO (marked by thin solid lines). The 990 cm-1 peak in (b) is relatively broad, suggesting contributions from both CH3ONO and CH3O. We conclude that during dosing at 95 K dissociative adsorption initially dominates, but with increasing exposure the contribution from molecular adsorption increases. The small signals at 1520 and 1558 cm-1 in spectrum (b) (marked by gray areas) are ascribed to NO adsorbed at bridge sites.18 After annealing (a) to 200 K and recooling, spectrum (c), all vibrational modes from molecular CH3ONO disappear, leaving only the five CH3O modes at 1003, 1942, 2808, 2880, and 2918 cm-1 and a peak at 1857 cm-1 ascribed to a stretching mode of dimerized nitric oxide (NO)2.18,19 The intensity and frequency increase in the ν(CO) mode tracks the CH3O coverage increase. Apparently, during annealing more CH3O-NO bonds dissociate, and the increased surface crowding induces NO dimerization. The fact that the symmetric mode (1857 cm-1) but not the asymmetric mode (∼1780 cm-1) is observed suggests that the N-N axis of (NO)2 is parallel to the surface.18 (NO)2 is observed at 200 K and desorbs as NO at 260 K (TPD not shown). Typically, (NO)2 on Cu either dissociates to form N2O or desorbs as NO below 200 K.19-25 To rationalize the unexpected thermal stability, we propose that CH3O blocks sites, thereby inhibiting NO dissociation and stabilizing (NO)2. This is reasonable because CH3O occupies 4-fold hollow sites26 and can, thus, block facile N-O bond dissociation.22 Consistent with N2O desorption,18,20 there is barely detectable NO dissociation evidenced by small O, and much smaller N, AES signals after heating to 550 K (not shown). In this context, it is interesting that the 1857 cm-1 peak does not appear when CH3ONO is dosed at 200 K (not shown). The absence of evidence for either NO or (NO)2 when dosing CH3ONO at 200 K is attributed to residence time effects; NO formed during dosing at 100 K is retained and accumulates, allowing formation and stabilization of (NO)2 during warming to 200 K, whereas NO formed during dosing at 200 K does not accumulate. Thus, at 200 K the instantaneous NO coverage is always very low and negligible (NO)2 forms. CH3O Surface Coverage Estimate. To estimate the monolayer (ML) CH3O surface coverage, defined as species per surface Cu atom, we used calibrated AES data. The ratios, O(510 eV)/Cu(920 eV), of AES signals measured at (17) Bodenbinder, M.; Ulic, S. E.; Willner, H. J. Phys. Chem. 1994, 98, 6441. (18) (a) Brown, W. A.; Sharma, R. K.; King, D. A.; Haq, S. J. J. Phys. Chem. B 1996, 100, 12559. (b) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 2578. (19) Dumas, P.; Suhren, M.; Chabal, Y. J.; Hirschmugl, C. J.; Williams, G. P. Surf. Sci. 1997, 371, 200. (20) (a) Johnson, D. W.; Matloob, M. H.; Roberts, M. W. J. Chem. Soc., Chem. Commun. 1978, 40. (b) Matloob, M. H.; Roberts, M. W. J. Chem. Soc., Faraday Trans. 1 1997, 73, 1393. (c) Johnson, D. W.; Matloob, M. H.; Roberts, M. W. J. Chem. Soc., Faraday Trans. 1 1979, 75, 2143. (21) Wendelken, J. F. Appl. Surf. Sci. 1982, 11/12, 172. (22) van Daelen, M. A.; Neurock, M.; van Santen, R. A. Surf. Sci. 1998, 417, 247. (23) Balkenende, A. R.; den Daas, H.; Huisman, M.; Gijzeman, O. L. J.; Geus, J. W. Appl. Surf. Sci. 1991, 47, 341. (24) Wee, A. T. S.; Lin, J.; Huan, A. C. H.; Loh, F. C.; Tan, K. L. Surf. Sci. 1994, 304, 145. (25) Dhesi, S. S.; Haq, S.; Barrett, S. D.; Leibsle, F. M. Surf. Sci. 1996, 365, 602. (26) Do¨bler, U.; Baberschke, K.; Sto¨hr, J.; Outka, D. A. Phys. Rev. B 1985, 31, 2532.

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Table 1. Vibrational Modes of Methoxy and Methyl Nitrite

assignment

CH3ONO/Cu(100) at 95 K multilayera

νa(C-H) 2δs′(CH3) 2δa(CH3) νs(C-H) 2ν(C-O) ν(NdO)

2999(c), 2880(t) 2918 2880 2949(c), 2918(t), 2808

δa′(CH3) δs(CH3) F(CH3) ν(C-O) ν(N-O)

1412(c) 1452(t), 1439(c) 1261(c), 1184(t) 1032(t), 999, 985(c) 837(c), 812(t)

1651(t), 1614(c)

CH3ONO/Cu(100) at 95 K 0.7 MLa

CH3ONO/ Cu(100) at 200 K

CH3ONO/ Cu(100) at 300 K

2918 2880 2808 1942 1857

2910 2872 2796 1954

cis-CH3ONO in Ar matrixb 3001

2920 2881 2810 1932 1651(t), 1632(c), 1558, 1520 1452(t), 1433(c) 1267(c) 990 (broad)

1435 1003

1018

0.5 ML of CH3O on Cu(100)c

trans-CH3ONO in Ar matrixb 2882

3031, 2951

2912, 2822

1613

1665

1454, 1408 1438 1230, 1142 990 (broad) 838

1467, 1424 1447 1180, 1118 1043 807

2911(2912) 2876(2876) 2800(2799) 1946 N/A (1433) 1014(1020) N/A

a The CH O signals and the molecular NO signals are shown in bold and italics, respectively; (c) cis isomer, (t) trans isomer. b Reference 3 17. c Reference 6.

Table 2. Vibrational Modes of tert-Butoxy, tert-Butyl, tert-Butyl Nitrite, and tert-Butyl Alcohol assignment ν(OH) νa(CH3) νs(CH3) ν(NdO) δa(CH3) δs(CH3) δ(OH) νa(CCC) ν(C-O) F(CH3) ν(N-O) νs(CCC) a

t-C4H9ONO/Cu(100) multilayer 0.7 ML 200 K N/A 2985, 2962 2942, 2913, 2845 1624 1477, 1462, 1397 1373 N/A 1268, 1251, 1195 1160 1041, 955, 928 875, 826, 784

t-C4H9•a Ar matrix

N/A 2962 2845, 2906

N/A 2958 2885 (weak)

N/A 2931 2825

1495, 1460, 1385 1369 (br)

1458, 1383

N/A 1455

N/A 1248, 1175, 1155 1155

N/A 1167

N/A

1167 931

N/A 1252, 1189, 1126, 992 N/A 733

? 872, 793 (br)

1370, 1279

885

References 28 and 29. b Reference 30. c Reference 7.

d

t-C4H9OHb Ar matrix 3626 2988, 2973 2943, 2908, 2888, 2876 N/A 1477, 1469, 1464 1392, 1372, 1367 1328 1240, 1214 1140 1013, 921, 915 N/A 746

t-C4H9OH/Cu(100)c multilayer t-butoxy

t-C4H9ONOd Ar matrix

unresolved 2976

2962e

3002, 2995, 2989 2945, 2914, 2883

N/A 1476

N/A 1462e

1638 1476, 1464, 1395

1386, 1364

1371, 1369

unresolved 1243

N/A

1209 1025, 923, 915

1158 872

N/A 752

N/A 1268, 1252, 1246, 1197, 1180 ? 1037, 957 ? 808, 769, 761

trans isomer. Reference 31. e Coupled.

Figure 2. RAIRS, all acquired at 200 K, of saturated ((5%) CH3O on Cu(100) prepared by two different methods: (a) by dosing 0.5 langmuir of CH3ONO on clean Cu(100) at 300 K and (b) and (c) by dosing CH3OH on O-covered Cu(100) at 200 K. The CH3O coverages are (b) 0.4 ML and (c) 0.42 ML, and based on calibrated AES data taken after dosing CH3OH, no O(a) remains for case (b) whereas 0.1 ML remains for case (c).

300 and 800 K after a 1.5 langmuir dose of CH3ONO at 100 K were compared with that measured for a standard, namely, 0.5 CO per surface Cu.27 At 300 K, the O(KVV) AES signal contains contributions from CH3O and a small

amount of dissociated NO. When the sample is heated to 800 K, the oxygen in CH3O desorbs, mainly as CH2O with a peak at 350 K (not shown), and leaves only the dissociated NO. Thus, the O/Cu AES ratio difference between 300 and 800 K is a measure of the O in CH3O. This O/Cu AES ratio is then compared to the O/Cu ratio obtained for 0.5 ML of CO and gives the CH3O coverage of this system. However, care must be exercised in the AES measurement, because the chemical states change and because the highenergy e-beam could induce some CH3O dissociation or desorption. Because changes of the AES peak structure were marginal, dissociation induced by the e-beam is the more troublesome problem. If it occurs, the CH3O will be overestimated. To minimize the effects of such damage, the e-beam dose was minimized and the 300 and 800 K AES spectra were measured independently, not consecutively. From the calibrated AES measurement, the calculated CH3O surface coverage was 0.15 ML (0.15 CH3O per surface Cu). This result is consistent with an estimate of 0.13 ML based on an independently determined RAIRS calibration of the ν(CO) intensity for adsorbed CH3O.15 t-C4H9ONO Dosed at 100 K. Figure 1B shows the RAIRS after dosing t-C4H9ONO on Cu(100): (d) 0.5 langmuir dosed at 100 K, (e) 0.1 langmuir dosed at 100 K, and (f) 0.5 langmuir dosed at 100 K, annealed at 200 K, and recooled to 100 K. As for CH3ONO, the 0.1 langmuir exposure corresponds to 70% of the first layer saturation. The peak assignments and energies from our spectra are (27) Ryberg, R. Surf. Sci. 1982, 114, 627.

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Figure 3. Schematic reaction path potential energy diagrams for reactions of RONO(g) and ROH(g) on Cu(100) to form adsorbed RO and, for RONO, either dissociated NO or NO(g) and, for ROH, either H(g) or H2(g). All numerical values have units of kJ mol-1.

listed in the first four columns of Table 2. For comparison, this table also includes data for t-C4H9, t-C4H9OH, and trans-t-C4H9ONO in Ar matrixes and t-C4H9OH and t-C4H9O on Cu(100).7,28-31 Consistent with CH3ON dosed at 95 K, t-C4H9O (marked by dashed lines) and t-C4H9ONO coexist at 100 K (Figure 1d,e). After the sample is heated to 200 K, spectrum (f), only seven peaks remain, each assignable to t-C4H9O. Interestingly, unlike CH3ONO there is no evidence for contributions from (NO)2 after annealing to 200 K. For comparison, when 0.5 langmuir of t-C4H9ONO was dosed at 200 or 300 K only signals attributed to t-C4H9O appear in RAIRS (not shown). In TPD, there is detectable NO desorption near 260 K but it is a small fraction of that found for CH3ONO. t-C4H9O starts to dissociate at 500 K (not shown). The desorption products are not fully identified by TPD but include H2, t-C4H9OH, and CH3COCH3.8 We now turn to a discussion of the local adsorption geometry of t-C4H9O. Keeping in mind that cross sections may preclude detection, there are 24 vibrational modes with energies in the range examined. Of these, 16 can be active for C3v symmetry and 19 for Cs symmetry. In other work,7 t-C4H9O was prepared from t-C4H9OH on O/Cu(100) and a C3v geometry was proposed on the basis of four peaks, corresponding to the four strongest peaks in spectrum (f), identified in the 750-3200 cm-1 range. Consistent with this work, our spectra contain these four peaks and, because of improved sensitivity, three small additional peaks at 2885, 1383, and 931 cm-1. To summarize, for C4H9ONO dosed on Cu(100) at 100 K and annealed to 200 K or dosed between 200 and 300 K, the evidence points to C4H9O accumulation in C3v geometry with negligible retention of NO or its dissociation products. Comparing Two Methods of Preparation. To compare, on the basis of RAIRS, methoxy synthesized from CH3ONO with that from reaction between CH3OH and O, (28) Pacansky, J.; Chang, J. S. J. Chem. Phys. 1981, 74, 5539. (29) Pacansky, J.; Koch, W.; Miller, M. D. J. Am. Chem. Soc. 1991, 113, 317. (30) Korppi-Tommola, J. Spectrochim. Acta 1978, 34A, 1077. (31) Barnes, A. J.; Hallam, H. E.; Waring, S.; Armstrong, J. R. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1.

we prepared O-covered Cu by dosing various amounts of O2 at 300 K and then exposing the O-covered surface to CH3OH. Figure 2 shows RAIRS spectra, all taken at 200 K, of CH3O prepared for three different initial conditions: (a) dosing sufficient CH3ONO on Cu(100) at 300 K to saturate CH3O and (b and c) dosing CH3OH on Oprecovered Cu(100) at 200 K. Using AES data as described earlier, the estimated CH3O and O coverages present after reaction at 200 K, that is, during RAIRS, are 0.40 ML of CH3O and 0.0 ML of O for (b) and 0.42 ML of CH3O and 0.1 ML of O for (c). A detailed analysis of the ν(CO), 2ν(CO), and νs(CH3) modes in (b) and (c) gives insight regarding how the presence of O perturbs the spectra of CH3O. Comparison with (a) then provides one measure of the amount of O present when CH3ONO is dosed to saturation at 300 K. For (a), (b), and (c), the positions of the ν(CO) peaks are 1018, 1014, and 1020 cm-1, the peak widths are 6.8, 6.5, and 6.2 cm-1, and the integrated peak intensities, ∆R/ R(%), are 2.6, 2.6, and 3.1, respectively. Similarly, the overtone, 2ν(CO), peaks are at 1952, 1954, and 1955 cm-1 and the widths are 7.9, 7.2, and 8.6 cm-1. For the νs(CH3) mode, the peak positions are the same in all three cases, the widths are 10.5, 9.3, and 11.7 cm-1, and the ∆R/R values are 0.58, 0.69, and 0.58%. From this analysis, it is clear that the saturation concentration of CH3O prepared from the nitrite (CH3ONO) is not measurably less than that prepared from the alcohol (CH3OH) and that the perturbation by coadsorbed O is small. In earlier work, O coadsorption was found to perturb the peak positions and widths of ν(CO) and 2ν(CO) modes in different ways.15 When the O co-coverage was increased to 0.10 ML with a constant CH3O coverage of 0.125 ML, the ν(CO) and the 2ν(CO) frequencies shifted linearly by 3 and 4 cm-1, respectively, and the ν(CO) and the 2ν(CO) peak widths increased linearly from 3-4 cm-1 to 5-6 cm-1. When the O co-coverage was increased above 0.10 ML, the 2ν(CO) increased nonlinearly, whereas the linear increases of the ν(CO) frequency and peak width continued. For example, the 2ν(CO) peak width became as large as 30 cm-1 at 0.25 O co-coverage. Considering the strong perturbation of coadsorbed oxygen when its coverage exceeds 0.10 ML,

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we infer that the coadsorbed oxygen for case (a) is less than 0.1 ML. In terms of absolute coverage, the position of the ν(CO) peak is in the range observed for 0.5 ML of CH3O on Cu(100) (Figure 2b and Table 1).3-6,15 We conclude that preparation either by dissociation of CH3ONO or by reaction of CH3OH with O forms methoxy in chemical environments indistinguishable by RAIRS with respect to sites occupied and neighboring species.15 Further, the saturation coverages of CH3O are comparable. As shown in Figure 1, O-NO bonds in CH3ONO and t-C4H9ONO break readily upon adsorption on Cu(100) at 95 or 100 K. The O-H bonds in CH3OH and t-C4H9OH do not cleave under these conditions.7 This is attributable mainly to the weakness of RO-NO bonds as compared to RO-H bonds. To explore this further, we develop (Figure 3) a reaction path potential energy diagram (kJ mol-1) that compares the energetics of forming adsorbed methoxy and tert-butoxy by dissociation of either nitrite or alcohol. Listed within parentheses in kJ mol-1, the known heats of formation are H2 (0), Cu (0), CH3ONO (-66), t-C4H9ONO (-172), CH3OH (-201.5), t-C4H9OH (-312.5), NO (90.3), and CH3O (15).32-36 For t-C4H9O, -91.2 kJ mol-1 was calculated using these data and the dissociation energy of t-C4H9O-NO (171.1 kJ mol-1).33,34 The dissociation energy of CH3O-NO is 174 kJ mol-1.33,34,37 On the basis of TPD data, calculated heats of adsorption (kJ mol-1) on Cu(100) are CH3ONO (31.4), t-C4H9ONO (40.5), CH3OH (37.9), t-C4H9OH (42.5), and NO (66.9).7,10 The NO adsorption energy determined by our experiment (66.9 kJ mol-1) lies in the unusually broad region reported for various copper-based catalysts, 30-110 kJ mol-1.38 The (32) (a) Ray, J. D.; Gershon, A. A. J. Phys. Chem. 1962, 66, 1750. (b) Geiseler, G.; Thierfelder, W. Z. Phys. Chem. (Neue Folge) 1961, 29, 248. (c) Gray, P.; Pratt, M. W. T. J. Chem. Soc. 1958, 3403. ∆fHg(methyl nitrite) ) -65.4, -70.3, and -62.5 kJ/mol, respectively. We used the average of the three values in this paper. (33) Batt, L.; Christie, K.; Milne, R. T.; Summers, A. J. Int. J. Chem. Kinet. 1974, 6, 877. (34) Access C4H9ONO at http://webbook.nist.gov.chemistry. (35) Chase, M. W., Jr. NIST-JANAF Thermochemical tables, 4th ed. J. Phys. Chem. Ref. Data, Monogr. 1998, 9, 1-1951. (36) Shustorovich, E. Metal-Surface Reaction Energetics; VCH Publishers: New York, 1991; pp 53-107 and references therein. (37) Effenhauser, C. S.; Felder, P.; Huber, J. R. J. Phys. Chem. 1990, 94, 296. (38) Carniti, P.; Gervasini, A.; Ragaini, V. J. Chem. Soc., Faraday Trans. 1997, 93, 8, 1641.

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adsorption bond energies (kJ mol-1) are H (