J . Phys. Chem. 1988, 92, 741-750
74 1
Reactions of Organonitrogen Molecules with Ni(100) Gregory R. Schoofs and Jay B. Benziger* Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 (Received: February 10, 1987; In Final Form: August 7 , 1987)
The adsorption and reaction of a variety of organonitrogen compounds on a Ni( 100) surface have been examined with temperature-programmed reaction, Auger electron spectroscopy,and infrared spectroscopy. Monomethylamine adsorbs via the nitrogen lone pair of electrons and then undergoes C-N bond scission yielding adsorbed carbon, dihydrogen, and ammonia. Aniline *-bonds to the surface and polymerizes to form a thermally stable poly(ani1ine) surface film. Pyridine undergoes a temperature-inducedorientational transformation. At low temperatures pyridine adsorbs with its ring parallel to the surface. At higher temperatures it appears to form an cy-pyridyl species with an activation barrier of 85 kJ/mol. Methyl groups on 2,6-lutidine sterically hinder this reaction. Methyl groups on 3,s-lutidine stabilize bonding via the nitrogen lone pair of electrons. The methyl groups on 3J-lutidine increase electrophilic addition activity relative to pyridine and lead to polymerization of 3,5-lutidine, forming a thermally stable polymer film. Pyrimidine reacted in almost identical fashion to pyridine, suggesting that increased nucleophilic activity had little effect on the reaction behavior of heterocyclic compounds and that electrophilic reactions predominate.
nickel,z632 iron,)3 and oxidized m o l y b d e n ~ m .Nearly ~~ all of these Introduction studies employed a single spectroscopy to ascertain the state and The reactions of organonitrogen compounds on transition-metal geometry of molecularly adsorbed pyridine. Adsorption through surfaces are of interest from the viewpoints of nitrogen removal the x-electrons would result in a flat configuration, with the in petroleum processing1+ and surface polymerization in making pyridine ring essentially parallel to the surface, whereas adsorption conducting There have been a number of studies via the lone pair of electrons on the nitrogen atom or as an aexamining the fundamental physical chemistry of organonitrogen pyridyl would result in a perpendicular or tilted configuration, compounds on transition-metal surfaces. Most of those studies with the pyridine ring nearly orthogonal to the plane of the surface. have been concerned with the nature of the adsorption bond to The results conflict, however, depending on the experimental the surface, and only a few have examined how the nitrogen atom conditions and ambiguities in the data. Bandy et al. interpreted affects the reactivity. their data to mean that pyridine adsorbed perpendicular tQ the Surface science experiments have provided some clues about copper surface,I2but Nyberg questioned their assignment, claiming the nature of adsorbed organonitrogen compounds on several the data could imply pyridine adsorbed in either perpendicular metals. Schmidt and co-workers examined the decomposition of or parallel geometries.I3 After noting the above argument, Netzer monomethylamine on a Pt( 111) surface.8 The amine was found and Mack postulated that pyridine adsorbed inclined to a Pd(ll1) to react by dehydrogenation to cyanogen species. Masel and surface at an unspecified angle between 0' and 90' with respect co-workers have suggested that C-N bond scission is sensitive to to the plane of the s ~ r f a c e . I ~Demuth * ~ ~ et al. found evidence of surface structure and found that monomethylamine decomposed a concentration-induced phase transformation on Ag( 11 I), in by C-N bond scission on Pt(100).9 Shirley and co-workers which pyridine adsorbed on Ag( 111) at 140 K with its ring parallel suggested that monomethylamine adsorbed on Ni( 100) molecuto the surface at low concentrations but with its ring perpendicular larly at 300 K, bonding through the electron lone pair on the to the surface at high concentration^.^^^'^ Keleman apd Kaldor17 nitrogen.I0 thought that pyridine physisorbed on Ag(ll0) at temperatures Aromatic nitrogen compounds have received much more attention. Sexton found that the first monolayer of pyrrole bound to Cu( 100) with the ring parallel to the surface." At low temperatures, a second monolayer of pyrrole can condense inclined (13) Nyberg, G. L. Surf. Sci. 1980, 95, L273. (14)Netzer, F. P.; Mack, J. U. Chem. Phys. Lett. 1983, 95, 492. relative to the first monolayer and the Cu( 100) surface. Tem(15) Netzer, F. P.; Mack, J. U. J . Chem. Phys. 1983, 79, 1017. perature-programmed reaction (TPR) of the multilayered surface (16) Grassian, V. H.;Muetterties, E. L. J . Phys. Chem. 1987, 91, 389. identified peaks at 170, 200, and 250 K as successive layers (17)Keleman, S. R.;Kaldor, A. Chem. Phys. Lert. 1980, 73, 205. evaporated or desorbed. N o evidence for any bond severance was (18) Demuth, J. E.;Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980, 76, 201. found. (19) Avouris, Ph.; Demuth, J. E. J . Chem. Phys. 1981, 75, 4783. The literature includes surface science studies of pyridine on (20) Netzer, F.P.;Bertel, E.; Matthew, J. A. D. Surf.Sci. 1980, 92, 43. copper,I2J3p a l l a d i ~ m , ' ~silver,l7-I9 -~~ iridium,20,21p l a t i n ~ m , ~ ~ - ~ ~(21) Mack, J. U.;Bertel, E.; Netzer, F. P. Surf.Sci. 1985, 159, 265. (22) Gland, J. L.; Somorjai, G. A. Surf.Sci. 1973, 38, 157. (23) Johnson, A.L.;Muetterties, E.L.;Stohr, J. J . Am. Chem. Soc. 1983, (1) Kellet, T. F.; Sartor, A. F.; Trevino, C. A. Hydrocarbon Process. 1980, 105, 7183.
59, 139.
(2) Satterfield, C. N.Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980. (3) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw-Hill: New York, 1979. (4) Ahuja, S . P.; Derrien, M. L.; LePage, J. F. Jnd. Eng. Chem. Prod. Res. Deu. 1970, 9, 272. (5) Skotheim, T. A., Ed. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986. (6) Diaz, A. F.; Logan, J. A. J . Electroanal. Chem. 1980, 11 1, 1 1 1. (7) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J . Phys. Chem. 1985, 89, 1441. ( 8 ) Hwang, S.; Seebauer, E. G.; Schmidt, L. D. Surf.Sci. 1987,188, 219. (9) Thomas, P. A,; Masel, R. I. J . Vac. Sci. Techno/., A 1987, 5, 1106. (10) Baca, A. G.; Schulz, M. A.; Shirley, D. A. J . Chem. Phys. 1985, 83, nnn1 (1 1) Sexton, B. A. Surf. Sci. 1985, 163, 99. (12) Bandy, B. J.; Lloyd, D. R.; Richardson, N. V. Surf. Sci. 1979, 89, 344.
0022-3654/88/2092-0741$01.50/0
(24) Johnson, A.L.; Muetterties, E. L.;Stohr, J.; Sette, F. J . Phys. Chem. 1985,89, 407 1.
(25) Kingsley, J. R.;Hemminger, J. C. Langmuir 1986, 2, 460. (26) Wexler, R.M.; Tsai, M.-C.; Friend, C. M.; Muetterties, E. L.J. Am. Chem. SOC.1982, 104, 2034. (27) DiNardo, N. J.; Avouris, Ph.; Demuth, J. E. J. Chem. Phys. 1984, 81, 2169. (28) Avouris, Ph.; DiNardo, N. J.; Demuth, J. E. J. Chem. Phys. 1984, 80, 49 1. (29) Robota, H.J.; Whitmore, P. M.; Harris, C. B. J. Chem. Phys. 1982, 76, 1692. (30) Kishi, K.;Ikeda, S. J. Phys. Chem. 1969, 73, 2559. (31) Kishi, K.; Chinomi, K.; Inoue, Y.;Ikeda, S. J. Catol. 1979,60, 228. (32) Harradine, D.; Campion, A. J . Vac. Sci. Technol., A 1986, 4, 1467. (33) Inoue, Y.;Kishi, K.; Ikeda, S. J . Electron Spectrosc. Relar. Phenom. 1983, 31, 109. (34) Henry, R. M. Ph.D. Thesis, Northwestern University, Evanston, IL, 1983.
0 1988 American Chemical Society
742
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Schoofs and Benziger
TABLE I: List of Reactants and Their Characteristics
monomethyl- basic, nonaromatic compound (Matheson Gas amine Products, 98% purity) aniline basic, aromatic, nitrogen not in aromatic ring (Aldrich Chemicals, 99.5% purity) piperidine basic, nonaromatic, cyclic compound, hydrogenated form of pyridine (Alfa Products, 98% purity) pyridine basic, aromatic, nitrogen part of aromatic structure (Fisher Scientific, 99% purity) 3,5-lutidine dimethylpyridine, ligand stabilized, enhanced electrophilic activity relative to pyridine (Aldrich Chemicals, 98% purity) 2,6-lutidine dimethylpyridine, ligand stabilized, decreased electrophilic activity relative to pyridine (Aldrich Chemicals, 99% purity) pyrimidine basic, aromatic, dual nitrogen activity, high nucleophilic activity (Aldrich Chemical, 99% purity)
below 190 K, which could explain the phase transformation claimed by Demuth et aL1**I9Pyridine adsorption on Ir( 11 1)20-21 and on Mo( 100)-p(2X1)034 have each been investigated by just one group, who found evidence for only the perpendicular state. On Pt( 11l ) , Johnson et al.23initially reported that pyridine adsorbed perpendicular to the surface at room temperature, in agreement with the results of Gland and Somorjai,,, but then revised their interpretation to suggest that pyridine underwent successive thermal transformations from a random state to a tilted state at 240 K, followed by the formation of an a-pyridyl at 410
K.24 Much confusion surrounds the behavior of pyridine on nickel surfaces. Robata et felt that pyridine probably adsorbed flat on Ni( 111) at room temperature, although they said they could not be sure. Demuth et al.27928found evidence for temperatureand concentration-induced phase transformations for pyridine on Ni(100), where the perpendicular state was favored at high concentrations and temperatures of 300-500 K. Kishi et al. found the opposite behavior.30 They claimed that pyridine adsorbed perpendicularly to nickel films at about 140 K, with the ring parallel to the surface at 290 K, and perpendicularly again via the formation of an a-pyridyl at about 400 K. The very limited temperature-programmed reaction experiments performed to data hardly clarify pyridine reactivity. Hydrogen evolution following pyridine adsorption on Ni(100) at room temperature indicated peaks at approximately 375,500, and 600 K.26 The first hydrogen peak at 375 K suggested the formation of an a-pyridyl intermediate. Similar hydrogen results and interpretations have been made for pyridine on Pt(l1 l).24 Pyridine has been observed to desorb molecularly from Pt( 111) primarily near 200 K with some also desorbing near 275 K24 and from Ag(ll0) at 215 K." Surface science experiments involving pyrazine and aniline have also been made. Pyrazine adhered to Ag( 11l)I9 and Ni( 11 1)29 in a flat fashion via the *-electrons. On iron and nickel films at room temperature, aniline appeared to adsorb both molecularly via the *-electrons and dissociatively via an anion formed by release of a proton from the amine group.31 Work function measurements suggested that aniline adsorbed via the *-electrons parallel or nearly parallel to Pt( 100) and Pt( 11 1) surfaces.35 Organonitrogen compounds may be classified into four general types-basic and nonbasic compounds and aromatic and nonaromatic compounds. To understand the role of the nitrogen atom in adsorption and reaction, we felt it was important to compare the reactions of a variety of organonitrogen compounds with nitrogen in a variety of forms. To this end the reactions of the compounds listed in Table I have been examined with Auger electron spectroscopy (AES), temperature-programmed reaction (TPR), and reflection absorption infrared spectroscopy (RAIS) on a clean Ni( 100) surface. The results presented here indicate ( 3 5 ) Gland, J. L.; Somorjai, G.A. Adu. Colloid InterfaceSci. 1916.5. 205.
Figure 1. Schematic of the apparatus.
a wide variety of reactivity of organonitrogen species that depend not only on the nitrogen atom but on other functional groups in the molecule as well. Experimental Section The experiments were carried out in an ion-pumped stainless steel vacuum chamber equipped with four-grid LEED optics (also used as a retarding field analyzer for AES), glancing incidence electron gun, ion gun, quadrupole mass spectrometer, CaF2 windows, and gas dosing system. The CaF, windows allowed focused infrared beams from the reflection absorption infrared spectrometer to enter the vacuum system and reflect off the Ni crystal. A schematic of the vacuum system with the infrared system in place is shown in Figure 1. A 13 mm X 0.5 mm thick Ni(100) crystal (Noah Chemical) was mounted on a rotatable sample manipulator. The sample was cooled by thermal conduction to ca. 160 K and was heated by radiation to 1200 K. The crystal was cleaned by repeated argon ion sputtering and annealing cycles. The clean surface was verified by a sharp p( 1X 1) LEED pattern and surface coverages of less than 0.01 monolayer of impurities as determined by AES (1 monolayer = 1.6 X lOI5 cm-2), Well-defined p(2X2)X adlayers of C, N, 0, S, and C1 were prepared as calibration standards to monitor surface coverages by AES.36,37 Prior to any experiment the crystal was cleaned and cooled to below 180 K. The gas manifold was filled to a pressure of ca. 300 mTorr, and the gas was admitted to the vacuum system through a 0.25-mm4.d. tube facing the crystal. The background Torr during pressure in the chamber never exceeded 5 X exposure to the various gases. The pumping characteristics of the gases varied widely so it was not possible to get accurate exposures. Exposure times were varied to determine the exposure conditions necessary to saturate the surface. Exposure times varied from 1 to 20 s; in all cases background adsorption was negligible. All the results reported here are for saturation coverages of the various reactants unless otherwise noted. After adsorption of the reactant the crystal temperature was ramped at 35 K/s to 1200 K and five masses were monitored with the mass spectrometer. One mass-to-charge was always reproduced as a standard. Products were identified by their mass fragmentation patterns. Mass fragmentation patterns were checked from m / q 2 through m / q 128. Specific masses checked for each reactant are listed in Tables 111-VII. Mass fragmentation patterns of all the reactants and some of the products were obtained and are listed in Table 11. The infrared spectrometer was a scanning wavelength ellipsometer, and its operation has been described in detail elsewhere.38 The CaF, windows limited its operation to the spectral region 1350-4000 cm-'. Infrared spectra were closely examined in the two ranges 1350-1700 and 2700-3700 cm-' for bending and stretching modes, respectively.
-
( 3 6 ) Benziger, J. B.; Myers, A. K.; Schoofs, G. R. Langmuir, in press. ( 3 7 ) Schoofs, G. R. Ph.D. Thesis, Princeton University, Princeton, NJ, 1986. (38) Benziger, J. B.; Schoofs, G. R.; Preston, R. E. Appl. Opt. 1987, 26, 343.
Reactions of Organonitrogen Molecules with Ni( 100)
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 743
TABLE 11: Fragmentation Patterns of the Reactants
m/q 14 15 16 17 25 26 27 28 29 30 31 37 38 39 40 41 42 43 44 49 50 51 52 53 54
ammonia
hydrogen cyanide
10 82 100
ethene
31 20
8
24 100
monomethylamine pyridine piperidine
8 55 58 100
48 15 2
16 87 16 100
2-picoline 4-picoline aniline 3,5-iutidine 2,6-lutidine
27 7
21 2
14 2
9 2
14
27 2
10 42
23 30 25
25 28 36
7 9 17
7 54 30
8 28 12
12 29 82 28 13 7
14 33 100 39 13
5 12 33 12 10
7 24 100 19 16 4
6 21 100 22 16 21
25 44 21 11
3 19 27 13 13 16
6 6 7 4 5
19 34 21 19 4
8 14 9 8
7 15 10 39 94 25
8 14 8 42 66 25
4 9 6 35 72
1 6 13 6 23 21
1 6 14 8 40 61
39 14 70 14
11 4 16 5
31 9 47 73
18
io0 65 53
60 7 9 20 4
9 41 52 100 9
55 56 57 61 62 63 64 65 66 70 77 78 79 80 84 85 92 93 106 107
19 6 21 35 26 34 2 2 2 3 5 11 45 27
7
6 35
23
5
27 100
23 77
42 20
TABLE 111: Monomethvlamine TPR Yields
rei yields, % 250 K
365 K
ml9
peak
peak
13 14 15 16 17 26 27 28 29
0 4 17 9 9 3 20 89 20
0 0 0 88 100 0 0 0 0
re1 yields, % 250K
365 K
mlq
peak
peak
30 31 31 39 41 43 44 45
100 59 1
0 0 0 0 0 0 0 0
0 0 0 0 0
31 74
+ z W
LL LL 3
0 E W
i-
W
I
0
E t-
o w
R
m / q 2 x5
0)
v,
a
-2 200
Monomethylamine. Adsorption of monomethylamine on Ni(100) at 175 K resulted in desorption of CH3NH2( m / q 31) at 250 K, N H 3 ( m / q 17) at 365 K, and H2 ( m / q 2 ) at 380 K. The TPR results for these products are shown in Figure 2, and the fragmentation patterns used for production identification are shown in Table 111. After heating to 600 K AES indicated 0.07 monolayer carbon and 0.08 monolayer nitrogen remained adsorbed on the surface. The desorption products suggested that the carbon residue should be greater than the nitrogen residue. Carbon diffusion into Ni becomes appreciable above 600 K so the carbon deficiency probably is due to carbon diffusion into the bulk of the crystal. Nitrogen does not diffuse into the Ni crystal as carbon
14 100
300
400
500
600
TEMPERATURE Y'K)
Figure 2. Temperature-programmedreaction spectra for monomethylamine on Ni(100).
does. The nitrogen desorbed at 6 7 5 , 9 0 0 , and 1100 K. No other reaction products were noted; in particular, neither methane nor H C N was observed as desorption products. The ammonia product desorbing at 365 K was formed by a surface reaction at that temperature. Ammonia desorption following the adsorption of ammonia occurred at 250 K.37 Experiments with CH,ND2 indicated the ammonia product was comprised primarily of ND2H and ND,. The H2 and D2 peaks from CH3ND2were at 370 and 385 K, respectively, suggesting that
Schoofs and Benziger
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
144
I-
z W a: a:
n
ixio-' A M P I
m/q
2 xi
3 0
z
E
v,
0
W
f
IW
z a
5a:
LL t-
+ 0 W
a m
v,
I
m/q 93 X I
m
2-
4
z
1
2900
3100
3500
3300
3700
I 200
400
600
800
I000
WAVENUMBER (cm-')
1200
TEMPERATURE FK) Figure 3. Temperature-programmed reaction spectra for aniline on
Ni( 100). TABLE I V Aniline TPR Yields re1 yields, % 260 K 860 and mlq peak 960 K peaks 13 15 16 17 26 27 28 29 30 31 32 39
83 97 100 95 72 74 65 34 31 35 34 60
14 15 17 42 36 100 30 4 2 2 1 0
re1 yields, % K 860 and peak 960 Kpeaks
260
mlq 40 41 42 43 44 45 55 56 57 66 93
38 33 23 19 24 24 15 21 14 54 65
0 0 0 0 0 0 0 0 0 0 0
C-H scission preceded N-H bond scission. The separation of the H2 and D2 peaks may also be due to an isotope effect if the desorption is limited by hydrogen recombination. Aniline. Aniline reacted in much different fashion than monomethylamine on Ni(100). After adsorption at 170 K, aniline, dihydrogen, and various hydrocarbons all desorbed at 260 K. The TPR results shown in Figure 3 and fragmentation patterns shown in Table IV indicate a very complex mixture of species at 260 K, with the distribution skewed in favor of light gases. The significant yields of m / q 39 and 66 at 260 K are not consistent with aniline but suggest the formation of a hydrocarbon or perhaps an aniline oligmer that fragments differently than aniline. From the mass spectral data done we are unable to define the set of desorption products. A second set of desorption peaks were observed at 960 K, with a shoulder at 860 K. These product peaks consisted of NH3 ( m l q 17), HCN ( m / q 271, C Z H(~m / q 281, and HZ(m/q 2). N o other reaction products were observed to desorb between 260 and 860 K. In addition, little or no N2 desorption was evolved at 1100 K from the interaction of aniline with Ni(100). The fate of the amine hydrogens and aromatic hydrogens was examined with aniline-2,3,4,5,6-d5. The H/D ratio in the dihydrogen desorption products was 1.6 at 260 K and 3.7 at 960 K. This indicates more of the amine hydrogens are lost at the lower temperature. Following aniline adsorption at 170 K AES exhibited a graphitic carbon structure, which persisted until temperatures exceeding 900 K were reached. After heating to 1100 K the carbon Auger peak showed a characteristic three-peak carbide s t r u c t ~ r eand ,~~~~ the carbon coverage was 0.25 monolayer. (39) Haas, T. W.; Grant, J. T.; Dooley, G. J. In Proceedings of the 2nd International Symposium on AdsorptionlDesorption Phenomena; Ricca, F., Ed.; Academic: London, 1973. (40) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G . E.; Weber, R. E. Handbook of Auger Electron Spectroscopy; Physical Electronics: Eden Prarie, MN, 1976.
1400
1450
1500
1550
1600
1650
1700
WAVENUMBER (cm-')
Figure 4. Infrared spectra of aniline on Ni(100): (a) saturation coverage; (b) multilayer coverage; (c) after heating to 450 K.
Figure 4 shows infrared spectra for aniline adsorbed on Ni( 100). The spectra shown in (a) and (b) represent increasing exposure at 170 K. Product yields at 960 K and the AES after heating to 960 K are the same after both (a) and (b). The desorption product yields at 260 K increased with increasing exposure of the surface to aniline at 170 K. Ring vibrations appear in the lowfrequency spectra, N-H stretches occur above 3100 cm-', and there is a weak C-H stretching vibration at 3040 ~ m - ' . The ~~ spectrum for the higher exposure shows more pronounced features than for the lower exposure, but the relative intensities of the peaks did not change appreciably. Aniline surface coverages less than that corresponding to spectrum a showed diminishing infrared intensities, but the relative intensities did not change. The TPR results showed the product yields at 260 K decreased more rapidly than the product yields at 960 K with a decreased exposure, but no change in the fragmentation patterns of the products was noted. After heating to 450 K the infrared spectra (Figure 4c) showed the ring vibrations and C-H stretching features had vanished while the N-H stretching feature narrowed but remained intense. These results were independent of aniline exposure. The infrared spectrum did not change between heating from 450 to 850 K. After heating to above 900 K the N-H stretching feature disappeared and the infrared spectrum resembled that of a clean surface. The RAIS data suggest that aniline condenses and probably solidifies following adsorption at 170 K. Extrapolating the vapor pressure equilibrium of aniline indicates that the dew point of aniline equals 170 K at a partial pressure of 1 X 1O-Io Torr.42 Any condensed aniline probably freezes at this temperature since the normal melting point of aniline is 267 K. Heating to 450 K causes some aniline to react while the multilayers appear to desorb at 260 K. (41) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Chemical: Milwaukee, WI, 1981. (42) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K . The Properties of Gases and Liquids; McCraw-Hill: New York, 1917.
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 745
Reactions of Organonitrogen Molecules with Ni( 100)
I 1400
I 1450
1500
1550
1600
1650
WAVENUMBER (cm-')
TEMPERATURE
Figure 6. Infrared spectra of pyridine on Ni(100): (a) gas-phase pyridine; (c) pyridine adsorbed at 200 K; (b) after heating to 375 K with initial saturation coverage.
VK)
Figure 5. Temperature-programmed reaction spectra for pyridine on Ni( 100).
TABLE V Pyridine TPR Yields re1 yields, 7% 490 K 650 K Deak peak Peak
m/a
100 93 81 68 58 70 64 51 31 27 15 28 22 23 24 32 29 28 29 30 32
50 51 52 53 54 55 56 57 58 67 68 69 70 78 79 81 83 84 85 112 128
280 K
m/a 13 15 16 17 25 26 27 28 29 30 31 32 37 38 39 40 41 42 43 44 45
90 87 75 68 77 100 56 75 34 26 35 38 11 11 16 37 30 28 30 39 38
11 23 27 10 14 25 53 100 8 3 3 0 0
0 0
0 0 0 0 0 0
re1 yields, % 490 K 650 K peak peak peak
280 K 54 59 60 8 5 5 10 9 9 11 13 15 22 34 27 4 2 2 2 0 0
69 69 67
28 16 11 10 8 7 6 7 7 11 21 28 6 3 2 2 0 0
0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
0 0 0 0
PiperidinelPyridine. The adsorption and reaction of pyridine and piperidine were compared to ascertain the effect of aromatic stabilization. The temperature-programmed reaction spectra after pyridine adsorption on Ni( 100) a t 180 K are shown in Figure 5; fragmentation patterns of the desorption product peaks are listed in Table V. Three sets of desorption products were observed at 280, 490, and 650 K. The fragmentation pattern a t 650 K is consistent with desorption of ethene ( m l q 28). Small amounts of other products such as monomethylamine ( m / q 31) and methane ( m / q 16) may also be desorbing at 650 K. The fragmentation patterns for desorption products at 280 and 490 K are both complex. At both 280 and 490 K desorption of pyridine ( m / q 79) and a small amount of piperidine (m/q 85) was evident. Pyridine desorption accounts in part for fragments at m / q 78, 50-54,37-41, and 26-27. Piperidine desorption accounts in part for fragments at m / q 70, 55-57, 41-44, and 26-30. However, neither of these species can account for the large amount of low molecular weight fragments ( m / q 13-16) and the very significant yields of fragments with m l q 26-32 occurring at 280 and 490 K. These results suggest some pyridine decomposed to CH4, HCN, and CH3NH2. Figure 5 also shows significant dihydrogen desorption at both 280 and 490 K, indicating C-H bonds were being broken at both those temperatures. Dihydrogen desorption
was also evident at 650 K, coincident with the ethene desorption. Lastly, N2 desorption was observed at 880 and 1100 K. The TPR results for piperidine were similar to those of pyridine. Three sets of product desorption peaks were observed at 250,485, and 660 K. The primary product at 660 K was ethene with some dihydrogen. The product yields at 485 K after piperidine adsorption were virtually identical with those after pyridine adsorption. The product yields at 250 K were again a complex mixture of pyridine, piperidine, CH4, HCN, and CH3NH2. The major difference between piperidine or pyridine adsorption was the product distribution at 250 K (280 K for pyridine). The dihydrogen yield at the low temperature was 3 times greater for piperidine than pyridine. Also, the piperidine/pyridine product ratio, as measured by the ( m / q 85)/(m/q 79) product yield, was 0.33 after piperidine adsorption and 0.07 after pyridine adsorption. These results indicated that C-H bond scission occurred facily a t or below 250 K, dehydrogenating piperidine to pyridine, and above 300 K the reaction events from piperidine and pyridine adsorption were identical. The Auger spectrum showed a C / N ratio of 4.8 after pyridine adsorption at 180 K. The carbon peak structure was similar to g r a ~ h i t e . Heating ~ ~ , ~ ~ to 350 K reduced the pyridine coverage from 0.21 to 0.18 monolayer (based on nitrogen coverage), but the C / N ratio remained constant. After heating to 550 K the nitrogen coverage decreased to 0.10 monolayer and the C / N ratio decreased to 3.2. Heating from 550 to 750 K resulted in no change in the nitrogen coverage while the carbon coverage decreased from 0.32 to 0.12 monolayer. This is consistent with the identification of the desorption product at 650 K as ethene. The carbon Auger peak at 750 K was a sharp three-peak structure characteristic of a metal carbide; at 550 K the carbon Auger peak was a combination of a graphite and carbide structure. The nature of the adsorbed species was examined with R A E . Results shown in Figure 6 indica,te no in-plane ring vibrations at 1597 cm-' for pyridine adsorbed a t 200 K. In addition, no C-H or N-H stretching features were observed in the region 2700-3700 cm-]. After heating to 375 K the infrared spectrum shows a distinct in-plane ring vibration at 1597 cm-'. This feature is almost identical in location to the in-plane vibration for gas-phase pyri d i r ~ e . The ~ ~ C-H bending modes that occur between 1425 and 1450 cm-I in gas-phase pyridine are very weak features in the spectrum for adsorbed pyridine. A C-H stretching mode was also observed at 3080 cm-l for adsorbed pyridine heated to 375 K. These infrared data show that pyridine undergoes a temperature-induced orientational transformation from a state where the ring is nearly parallel to the surface to a state where the ring is (43) The gas-phase pyridine spectrum was taken with our spectrometer with a gas cell in place and modifying the phase sensitive detection to measure gas-phase absorption spectra.
746 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Schoofs and Benziger
I
T
1
0.02%
u
= I597 cm"
I
I 200
I50
250
300
350
400
450
500
TEMPERATURE
TEMPERATURE P K )
Temperature-programmed reflection absorption infrared spectrum of pyridine on Ni(100) at Y = 1597 cm-I. Figure 7.
("K)
Figure 8. Temperature-programmed reaction spectra for 3,5-lutidine on Ni(100).
TABLE VI: High-Temperature 3,5-Lutidine TPR Yields re1 yields, % re1 yields, %
m/q 13
15 16 17 26 27 29
30 31 32
39
600 K peak 3 6 3 2 27
100 2 1 2 0 0
860 and 1160 K peaks 54 100
65 40 34 34 32
28 50 38
600 K
m/q
peak
40 41
0 0 0 0 0 0 0
42
43 55 57
78 79 93 107
9 0 0
860 and 1160 K peaks 4 4
8 5 3 3 0 0 0 0
2900
3100
3000
3200
WAVENUMBER (cm-')
0
bound at a substantial angle with respect to the Ni( 100) surface. (This will be referred to as the perpendicular state.) Cooling the sample from 375 to 200 K did not cause the 1597-cm-I feature to diminish, indicating that the orientational transformation is irreversible. The activation of this orientational phase transformation can Figure 7 be examined with temperature-programmed shows the absorption at 1597 cm-' as a function of temperature after pyridine adsorption at 180 K, with a heating rate of 1 K/s. The increased absorption at 255 K is coincident with pyridine desorption and suggests that pyridine orients itself perpendicular to the surface prior to desorbing. The rise in absorption between 300 and 350 K occurs as pyridine flips from a parallel to a perpendicular state. Assuming a frequency factor l o i 3s-', the activation energy for the process is 85 kJ/mol. The absorption feature decays above 400 K as the pyridine desorbed and reacted. Lutidines. The reactions of picolines and lutidines were examined to elucidate steric and electronic effects of methyl groups on the reactivity of pyridine. We report here the results for 2,6-lutidine and 3,5-lutidine as representative of the effects of methyl substituents. The TPR results for 3,5-lutidine adsorbed at 180 K are shown in Figure 8, and fragmentation patterns are summarized in Table VI. Desorption of lutidine was not observed at any temperature. Dihydrogen desorbed at 400 K; at 600 K both dihydrogen and H C N desorbed. A variety of light gases including CH4 ( m / q 15), H C N ( m / q 27), CH3NH2 ( m / q 31), NH3 ( m / q 17), and H, evolved in two desorption peaks at 860 and 1160 K. In addition, higher molecular weight hydrocarbon fragments C3 ( m / q 43) and C4 ( m / q 55, 57) were among the products at 860 and 1160 K. Dinitrogen desorbed at 890 and 11 10 K. Auger spectra were taken after adsorbing 3,Slutidine at 180 K and subsequently heating to 500,700, 900, and 1200 K. The AES results showed that no 3J-lutidine desorbed below 500 K as the C / N ratio and the nitrogen coverages did not change (44) Benziger, J.
I
1 2800
B.;Schoofs, G.R.Surf. Sci. 1986, 171, L401.
1350
1400
1450
1500
1550
1600
1650
1700
WAVENUMBER (cm-')
Figure 9. Infrared spectra of 3,5-lutidine on Ni(lO0): (a) adsorbed at 180 K; (b) heated to 700 K.
between 180 and 500 K. The carbon Auger peak had a graphitic structure over the temperature range 180-700 K, suggesting carbon-carbon bonds had not been broken. The C / N Auger ratio increased from 5.6 to 6.3 and the nitrogen coverage decreased from 0.17 to 0.13 monolayer between heating from 500 to 700 K, consisting with the observation of H C N desorption. After heating to 900 K the carbon Auger signature began to show a three-peak carbide structure, and after heating to 1200 K the Auger spectrum showed a distinctive nickel carbide structure. The carbon coverage after heating to 1200 K was 0.24 monolayer. The infrared spectrum of 3,5-lutidine adsorbed at 180 K shown in Figure 9 is similar to a liquid-phase ~ p e c t r u m . ~The ' ring vibrations, C - C and C-H bending modes in the 1350-170O-cm-' range, are indicative of 3,5-lutidine bonding perpendicular to the surface. No C-H stretching features were observed, probably as a result of the C-H bonds having a weak dipole component normal to the surface. There was no evidence for multilayer formation of 3,5-lutidine at 180 K. Infrared spectra taken after heating to 750 K show changes associated with losses of the C-C and C-H bending modes, while the in-plane ring vibration at ca. 1600 cm-' remains intense. The spectra also show C-H stretching
Reactions of Organonitrogen Molecules with Ni( 100)
-
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 147
m / q 27 x25
H 311 (--400
2800
2900
3000
3100
3200
WAVE NUMBE R ( c m-')
m / q 107 x5
600
800
1000
1200
TEMPERATURE (OK)
Figure 10. Temperature-programmed reaction spectra for 2,6-lutidine
on Ni(100). v)
v,
TABLE VII: 2,dLutidine TPR Yields
re1 yields, % 535 K 670 K peak peak peak 74 43 10 13 100 43 9 93 33 54 40 8 21 52 57 100 57 100 61 45 50 0 38 32 0 39 30
280 K
mlq 13
15 16 17
26 27 28
29 30 31 32
39
46
27 12
0 0
re1 yields, % 535 K 670 K peak peak peak 77 38 0 48 42 0 34 30 0
280 K
mlq
39 40 41 42
41
43
28
55 57
18 17 13 15 14 27
78 79 93 107
12 25 15 10 22
28 12 0
0 0 0 0 0 0 0 0
modes at 3030 and 3 130 cm-' characteristic of sp2 CH bonding. These infrared results suggest that 3,5-lutidine has reacted but the ring remains intact and normal to the surface. The reactions of 2,6-lutidine on Ni( 100) were dramatically different from those of 3,Slutidine. TPR of 2,6-lutidine adsorbed at 170 K shown in Figure 10 indicated most of the 2,6-lutidine ( m / q 107) desorbed intact at 280 K. The fragmentation patterns listed in Table VI1 suggested methane also evolved at 280 K evidenced by substantial m / q 15 and 16 products. Some of the 2,6-lutidine also reacted on the surface and resulted in the desorption of picoline ( m / q 93), pyridine ( m / q 79), and H C N ( m / q 27) at 535 K. H C N and C2H4 ( m / q 28) were minor desorption products at 670 K. Dihydrogen desorption was observed at 320 K, with lesser peaks at 535 and 670 K. Dinitrogen desorption was observed at 890 and 1150 K. AES indicated the saturation coverage of 2,6-lutidine at 170 K was 0.1 1 monolayer (based on nitrogen coverage). After heating to 300 K the coverage decreased to 0.03 monolayer, consistent with the observation that most of the 2,dlutidine desorbed at 280 K. The carbon Auger signature changed from a graphite structure to a carbide structure during heating to 700 K, indicating complete decomposition at 700 K. Infrared spectra for 2,6-lutidine adsorbed on Ni( 100) are shown in Figure 11. After adsorption at 170 K the RAIS results (Figure 1la) show no evidence for the in-plane ring vitrations at ca. 1600 cm-I. C-H stretching modes at 2900 and 2970 cm-I appear to result from the symmetric and asymmetric methyl group stretches. There was no evidence for the aromatic C-H stretching bonds near 3100 cm-I. These data indicate the 2,6-lutidine adsorbs with its ring nearly parallel to the Ni( 100) surface. The RAIS results after heating the adsorbed 2,6-lutidine to 500 K shown in Figure 1l b indicate the C-H stretching bands are greatly weakened due to desorption, and there is still no evidence of any in-plane ring vibration. Pyrimidine. The reactions of pyrimidine with Ni( 100) were investigated to see the effect of a second nitrogen atom in the
1400
1450
1500
1550
1600
I€ 80
WAVENUMBER (cm-')
Figure 11. Infrared spectra of 2,6-lutidine on Ni(100): (a) adsorbed at 180 K; (b) heated to 450 K.
pyridine ring. The 1,3-diazine was selected as it can bond to the surface through one nitrogen in a perpendicular configuration, and the second nitrogen makes the 2-carbon particularly vulnerable to nucleophilic attack. The TPR results for pyrimidine adsorption on Ni(100) at 180 K were very similar to the results obtained for pyridine. Pyrimidine desorbed at 255 and 475 K. H C N and H 2 were also observed as reaction products from pyrimidine decomposition at 475 and 650 K. AES revealed that the pyrimidine desorbed or decomposed to adsorbed atomic constituents by 700 K as the carbon Auger signal was characteristic of a metal carbide. The infrared spectra for pyrimidine showed no discernible feature in the 1300-1 700-cm-' range after pyrimidine adsorption at 180 K. After heating to 320 K the infrared spectrum showed a distinct absorption band at 1590 cm-I characteristic of the in-plane ring band, indicating that the pyrimidine reoriented. After heating to 500 K the infrared spectrum was again featureless, indicating decomposition.
Discussion Most of the findings reported here can be accounted for in the context of electrophilic surface reactions and frontier molecular orbital theory. The interaction of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals of the surface and organic reagents determines much of the chemistry. The H O M O and LUMO for the Ni(100) surface both lie at Fermi energy level, which is approximately equal to the work function. The local density of states at the surface exhibits a maximum at the Fermi level, around which the distribution of available levels is approximately symmetric.45 Relative to vacuum the work function of Ni(100) is at -510 k J / m 0 1 . ~ ~Figure 12 shows the HOMO'S and L U M O s for a nickel surface, monomethylamine, pyridine, and aniline. The HOMO(7A') of monomethylamine lies at -940 kJ/mol and is the lone electron pair on the nitrogen, and the LUMO lies at +1440 kJ/mol and is the antibonding orbital for C-N bonding. (45) Schrieffer, J. R.;Soven, P. Phys. Today 1976, 28(4), 24. (46) Albano, E. V. Surf. Sci. 1984, 141, 191.
748 The Journal of Physical Chemistry, Vol. 92, No. 3, 1988
Schoofs and Benziger
IO00
500
Vacuum = 0
E
h
- In
-500
\
3 &
Y
150
170
19C
210
230
250
TEMPERATURE PK) Figure 13. Piperidine/pyridineequilibrium. Equilibrium constant from ref 41.
-1000
U
W
t - I500
Hi
lionomethyl Amine
Aniline
Pyndine
Figure 12. Energy levels for frontier orbitals of the Ni(100) surface and organonitrogen compounds.
The most significant interaction of monomethylamine with the Ni(100) surface is anticipated between the H O M O of monomethylamine and the LUMO of the nickel surface, resulting in the surface making an electrophilic attack at the nitrogen lone electron pair. The interaction between the LUMO of monomethylamine and the H O M O of the nickel surface is expected ~ ~ is less. However, to be weaker as the Ni d-orbital, u * overlap this back-bonding interaction will shift electron density from the nickel d-orbitals to the antibonding C-N orbital, weakening the C-N bond leading to C-N bond rupture. This is consistent with the experiment results that showed C-N bond scission. The data show that the N H 2 group can be hydrogenated to NH,. From the data it is not possible to say whether this occurs concurrent with or subsequent to C N bond scission. Desorption of monomethylamine at 250 K appears to be the result of a decrease in the nickel d-orbital rCNinteraction. At initial coverages less than saturation the desorption of monomethylamine at 250 K decreased substantially more than the decrease in NH, and H2 products. This suggests that at high monomethylamine surface coverages the empty d-levels of the nickel at the surface are highly occupied due to the interaction with the nitrogen lone electron pair. While the back-bonding interaction between the filled nickel d-levels and the u * , - ~level should be unaffected, the interaction between the uCNlevel and the empty nickel d-levels will be curtailed. Hence, the C-N bond will not be weakened as much at high adsorbate coverages and the monomethylamine is more likely to desorb intact. Aniline displayed very unusual behavior. As discussed above, aniline apparently freezes on the surface at 170 K in a multilayer configuration. Because the infrared spectrum for the multilayer is similar to condensed aniline, there appears to be no strong ordering of the orientation of the aniline molecules in the multilayer. After heating to above 300 K a monolayer of an aniline derivative remained on the surface. Infrared spectra indicate that the N-H bonds remain intact and are oriented with a dipole contribution normal to the surface. AES indicates that the basic molecular structure remains intact to temperatures above 800 K as the carbon signature resembles that of a molecular structure and not a metal carbide. Furthermore, the infrared spectra indicate that if the aromatic ring structure remains intact, the ring must be parallel to the plane of the surface as it is not infrared active. Lastly, this aniline derivative is thermally very stable and does nut deumposc bclm SO0 K. Above that temperature it decomposes to light gases, primarily H2 and HCN, and adsorbed carbon atoms. The hydrogen evolved above 800 K comes from
both the ring C-H bonds and the amine hydrogens as evidenced by the results with C6DSNH2. One interpretation of these results is that aniline chemisorbed with its ring parallel to the surface, and the N-H bonds oriented almost normal to the surface. This bonding geometry implies that the lone electron pair on the nitrogen and the ?r-orbitals of the benzene ring both contribute to the bonding on the surface. We have discounted this interpretation for two reasons. First, there was substantial hydrogen evolved from aniline decomposition at 260 K. Second, all the nitrogen evolved as HCN; no NH, or N, products were observed. From a MO viewpoint electron donation from the H O M O of aniline (T,) to the nickel d-orbitals would weaken the C-N bond, facilitating C-N bond scission; also the LUMO, ?r*-level, for aniline is antibonding for the C-N bond so that the back-donation interaction should also facilitate C-N bond scission. As no C-N bond scission was observed, it appears probable that the adsorbed species was not aniline. An alternative explanation of these results is that the surface acts as an electrophile and forms aniline “cations”. These cations initiate polymerization of adsorbed aniline to form an extensive network structure such as aniline black. Aniline is readily polymerized electrochemically to poly(ani1ine) by oxidation$’ which is proposed to have a linear structure:48
Electrochemical oxidation clearly promotes the electrophilic interactions proposed above for the polymerization on the metal surface. Electrochemically, the reaction proceeds indefinitely due to a continual drain of electrons. On a metal surface without any applied potential the reaction is limited by the availability of surface states to act as electron acceptors. Poly(ani1ine) formed on the Ni( 100) surface would be expected to show great thermal stability and decompose by a pyrolysis process, consistent with the TPR results. The infrared spectra indicate that if poly(ani1ine) is formed, it forms with the phenyl ring parallel to the surface and the NH bond at a substantial angle relative to the surface. The complex mixture of desorption products at 260 K after aniline desorption may be the result of oligimerization occurring concurrently with desorption of the frozen multilayer. The aniline oligimers may decompose and fragment to give the broad distribution of fragments reported in Table IV. Mass spectra of pyrolyzed polymers are complex and difficult to d e c o n v o l ~ t e . ~ ~ The TPR results for piperidine on Ni(100) mimic those for pyridine with the exception that more piperidine and dihydrogen were evolved at low temperature. This suggests that piperidine (47) Hales, J. L.; Herington, E. F. G.Trans. Faraday SOC.1957, 53, 616. (48) Mohilner, D. M.; Adams, R. N.; Argersinger, Jr., W . J. J . Am. Chem. SOC.1962, 84, 3618. (49) Gardella, J. A,; Hercules, D. M. Anal. Chem. 1981, 308, 297.
Reactions of Organonitrogen Molecules with Ni( 100) dehydrogenated to pyridine at low temperature on Ni(100). Figure 13 shows the equilibria for the gas-phase reaction piperidine ~i pyridine 3H2
+
at low partial pressures of hydrogen. The equilibrium greatly favors pyridine relative to piperidine at temperatures above 200 K in ultrahigh vacuum. The equilibrium for adsorbed pyridine and piperidine would be expected to be even more favorable for piperidine dehydrogenation as pyridine appears to be more strongly adsorbed, resulting in a more favorable free energy of reaction for adsorbed the species. These results suggest that the Ni(100) surface is extremely active for dehydrogenation of piperidine. These results are not surprising as nickel is well-known as an active catalyst for hydrogenation/dehydrogenation.2 Subsequent to dehydrogenation of piperidine its reaction behavior parallels that of pyridine. Our infrared studies show that pyridine adsorbed with its ring parallel to the nickel surface at temperatures below 300 K and a temperature-induced orientational transformation occurred near 350 K. Demuth and co-workers previously reported a similar temperature-induced orientational tran~formation.~’ Demuth and co-workers also reported concentration-induced phase transformations of pyridine on Ni(100) and A g ( l l 1 ) at temperatures below 150 K. We were unable to observe similar concentration effects at temperatures of 170 K and greater. However, Demuth et al. may have witnessed formation of pyridine multilayers. The vapor pressure of pyridine at 140 K is predicted to be 1 X lo-’’ Torr,42and the normal melting point of pyridine is 232 K so at temperatures below 150 K it is quite probable that multilayers of pyridine froze on the surface, similar to the effect we observed with aniline. The molecular orbital picture of pyridine suggests why pyridine adsorbs flat. the H O M O S in pyridine are a levels associated with the ring. One would therefore expect the bonding of pyridine to result from the interaction of the a-electron cloud with the Ni(100) surface, similar to the case of b e n ~ e n e . ~We ~ , have ~ ~ examined the adsorption of pyridine on a 17-atom nickel cluster using extended Huckel theory; details of the technique may be found elsewhere.S0The results of these calculations show the most stable configuration for adsorbed pyridine is with the ring parallel to the surface over a fourfold hollow 2.10 8,above the plane of the surface. The most stable perpendicular configuration was with the nitrogen atom 1.75 8,directly above a nickel atom and was less stable than the parallel configuration by approximately 30 kJ/mol. At temperatures above 300 K the temperature-programmed reflection infrared results indicate that the pyridine ring went from a parallel to a perpendicular orientation and that this orientation change was irreversible. The TPRAIS results also suggest that as molecular pyridine desorbed at 280 K it reoriented prior to desorbing from a perpendicular orientation. One explanation for the reorientation of adsorbed pyridine is that the pyridine molecule undergoes electrophilic attack at the nitrogen, forming a u-type bond with the pyridine oriented normal to the surface. To be consistent with the data, this transformation must be activated and the u-bound pyridine must be thermodynamically more stable than the a-bound pyridine. This explanation is not easily reconciled with the observation from TPRAIS showing pyridine reorienting and desorbing at 280 K. An alternative explanation to these data, suggested by Muetterties and c o - w o r k e r ~ ,is~ ~ that . ~ ~an a-pyridyl intermediate is formed. The TPR results reported here are in close agreement with those previously published by Wexler et aLZ6 They used labeled pyridine to show the a-hydrogens were lost before the other hydrogens on pyridine. The a-pyridyl may be explained as resulting from an electrophilic attack at the nitrogen. If electrophilic attack is directed at the nitrogen atom at the n orbital, the surface HOMO’S can act as weak nucleophiles to attack the pyridine ring (50) Myers, A. K.; Benziger, J. B. Langmuir 1987, 3, 414. (51) Myers, A. K.; Schoofs, G. R.; Benziger, J. B. J . Phys. Chem. 1987, 91. 2230. (52) Grassian, V . H.; Muetterties, E. L J . Phys. Chem. 1986, 90, 5900
The Journal of Physical Chemistry, Vol. 92, No. 3, 1988 749 at the a-position. Our E H T calculations showed that the electrophilic attack at the nitrogen decreased the electron density at the a-carbon, making it more susceptible to nucleophilic attack. The a-pyridyl species decomposed at 490 K to a variety of products; in addition, some of the a-pyridyl species were hydrogenated and desorbed as pyridine. The variety of products observed at both 250 and 490 K suggests that a variety of C-H and N-H bonds are being formed to give rise to CH, and CH3NH2. The formation of these products from pyridine is complex, and the results presented here do not give any clear indication to the reaction paths forming these products. We attempted a few coadsorption experiments of deuterium and pyridine but found no evidence for deuterium incorporation into the products. This may be due to the deuterium desorbing before the a-pyridyl reacts. The decomposition also produced an adsorbed hydrocarbon species that reacted to yield CzH4and H2at 650 K. Similar results were previously reported by Muetterties and co-workers on both Niand Pt(1 l 1)52where a dihydrogen product was observed at 650 K. We have not been able to identify this surface species, but it may be a pyridine oligomer. Addition of methyl groups to the pyridine ring influences the interaction with the Ni( 100) surface in three ways: (i) the methyl groups sterically restrict certain bonding orientations; (ii) the methyl groups donate electrons to the ring, increasing the aromatic stabilization; (iii) the increased electron density increases the reactivity for electrophilic reactions. All three of these effects manifest themselves in the behavior of 3,5-lutidine. The methyl groups on 3,5-lutidine cause the parallel bonding orientation to be less favorable. The methyl groups push the ring away from the surface to reduce repulsive interactions; however, in the perpendicular orientation the steric repulsions are not a factor. Hence, in contrast to the result for pyridine, the perpendicular orientation is most stable for 3,5-lutidine. In addition, methyl groups act as electron donorss3 helping to stabilize the adsorbed lutidine, partially replenishing the depleted electron density after electrophilic attack at the nitrogen. The infrared spectra clearly show that 3,5-lutidine adsorbs in a perpendicular orientation, and the TPR results show no desorption of lutidine, suugesting a surface species more stable than pyridine. The AES, RAIS, and TPR results for 3,5-lutidine all indicate that a stable molecular species existed on the surface up to 1000 K with a ring vibration normal to the surface. The thermal stabilities of these species are similar to those observed for aniline and ascribed to a surface polymerization reaction. A possible explanation of these data is that 3,5-lutidine dimerizes and possibly polymerizes on the Ni( 100) surface. At higher pressures 3-alkylpyridines readily form 2,2’-bipyridyls and higher molecular weight species on Raney nickel catalysts.s4 Sasse and Whittle have proposed a mechanism for bipyridyl formation, which is basically an electrophilic addition at the 2-position activated by electron donors at the 3-positi0n.~~The oligomers of 3,5-lutidine are very stable on the surface and decompose by a high-temperature pyrolysis reaction. As the ring is oriented perpendicular to the surface, this decomposition yields C3 and C4hydrocarbon products. This differs from the behavior of aniline where the ring was parallel to the surface and only C, products were evolved during decomposition. The reaction behavior of 2b-lutidine contrasts sharply with that of 3,5-lutidine. The methyl groups at the 2- and 6-positions hinder bonding between the nitrogen’s lone pair of electrons and the surface, resulting in the parallel configuration being unstable. The infrared results indicate the molecule does indeed lie flat on the surface. TPR indicates most of the 2,6-lutidine desorbs at 280 K, in contrast to 3,s-lutidine where no desorption of the molecule was observed. We previously indicated that pyridine reoriented on the surface, forming an a-pyridyl species via electrophilic attack at the nitrogen (53) Streitweiser, A,; Heathcock, C. H. Introduction to Organic Chemistry; MacMillan:
New York, 1979. (54) Sasse, W. H. F.; Whittle, C. P. J . Chem. Soc. 1961, 1347. (55) Sasse, W. H. F.; Whittle, C. P. Aust. J . Chem. 1963, 16, 14.
750
J . Phys. Chem. 1988, 92, 750-753
and subsequent a-hydrogen bond scission as suggested by Muetterties and c o - ~ o r k e r s .A~ small ~ ~ ~ ~fraction of the 2,6lutidine appears to react by the same pathway. The evolution of methane at 280 K may correspond to a nucleophilic attack at the a-carbon and C-C bond scission. This leads to the formation of a methyl-substituted a-pyridyl species. The methyl group stabilizes this species relative to the a-pyridyl species so the reaction temperature increased to 535 K relative to 490 K for the hydrogensubstituted derivative. The methyl-a-pyridyl led to the formation of picoline, in analogy with the observation that the a-pyridyl led to pyridine formation. Reflection infrared spectroscopy did not detect the ring vibrations for the methyl-a-pyridyl species, but as most of the 2,6-lutidine desorbed intact the surface coverage may have been below the detection limit. The evolution of light gases at 635 K from 2,6-lutidine was similar to the results observed for pyridine, but the temperature was shifted higher by 35 K for the lutidine species. The methyl group stabilized the intermediate, leading to these reaction products. It is suspected that the intermediate accounting for these products is a oligimer of the a-pyridyl species, though more studies are needed to clarify this. The interactions of pyrimidine with Ni(100) were similar to the results obtained with pyridine. Molecular desorption of pyrimidine was observed at 255 K, a slightly lower temperature than observed for pyridine. Both pyrimidine and pyridine displayed orientation changes near 300 K as detected by R A E . The apyrimidyl species formed from pyrimidine reacted at 475 K, showing the second nitrogen destabilized the species slightly relative to pyridine. The second nitrogen atom also altered the TPR product distribution. Hydrogen cyanide evolved during pyrimidine TPR at 650 K, whereas ethene evolved from pyridine at this temperature.
adsorption of reagents as follows: (1) Monomethylamine adsorbed by bonding between the Ni and the lone electron pair on the nitrogen. (2) Aniline adsorbed with its ring parallel to the Ni surface, forming a a-bond between the ring and the surface and a bond between the nitrogen lone electron pair and the surface. (3) Pyridine adsorbed with the ring parallel to the surface, forming a a-bond. After the initial electrophilic interaction a nucleophilic interaction between the HOMO of the surface and the LUMO of the nitrogen compound proceeded. This resulted in C-N bond scission with monomethylamine, polymerization of aniline, and the formation of a-pyridyl species with pyridine. The bonding of pyridine to the surface could be influenced by methyl group substituents. Specifically, methyl groups in the 3,s-positions appeared to force pyridine to adsorb via the nitrogen lone pair of electrons. The methyl groups reduced the stability of a-bonded pyridine and increased the electron density in the ring stabilizing the nitrogen-bound pyridine. Methyl groups in the 2,6-positions reduced the stability of a-bonded pyridine but also sterically hindered adsorption at the nitrogen lone pair of electrons. Aniline and 3,s-lutidine were both activated for electrophilic addition reactions. This increased activity for electrophilic addition led to the surface polymerization of these species. The conformation of the surface polymers appeared to be controlled by the initial electrophilic interaction of the species with the surface; poly(aniline) had its ring parallel to the surface, whereas poly(lutidine) had its ring perpendicular to the surface. These surface polymers were thermally very stable, pyrolyzing above 900 K.
Acknowledgment. We thank the Air Force Office of Scientific Research (Grant 86-0050)and Chevron Research Co. for their financial support of this work. We also thank Andrea Myers for fruitful discussions regarding molecular orbital theory. Registry No. Ni, 7440-02-0; N2, 7727-37-9; C, 7440-44-0; H2, 1333-74-0; NH,, 7664-41-7; monomethylamine, 74-89-5; aniline, 6253-3; poly(aniline), 25233-30-1; pyridine, 110-86-1;2,6-lutidine, 10848-5; pyrimidine, 289-95-2.
Conclusions The reactions of a variety of organonitrogen compounds on a Ni( 100) surface have been examined. All the species appeared to interact initially with the surface via an electrophilic interaction between the H O M O of the organonitrogen reagent and the LUMO (empty d orbitals) of the metal. Specifically, this led to
Methane Activation by the Lanthanide Oxides K. D. Campbell, H. Zhang, and J. H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: February 20, 1987; I n Final Form: August 28, 1987)
The catalytic production of gas-phase methyl radicals from methane over the lanthanide oxides was measured by using a matrix isolation electron spin resonance (MIESR) technique. The results show that metal centers with multiple stable oxidation states are not a requirement for activity. The oxide with the greatest activity per gram was hydrothermally treated La203, which was ca. 5 times more active than the next highest oxide. Hydrothermally treated La203and Sm203have the largest activities per square meter. Results obtained in a conventional flow reactor for selected oxides indicate that the CHI conversions per square meter are in the order Nd,03 > L a 2 0 3> Sm2O3> Dy203> CeO,. Oxidative coupling of CH, results in C2H6 and C,H, (C, compounds), and the combined C2 yields follow the same order with Dy,O, >> CeO,. The MIESR and flow reactor results are in qualitative agreement, with the exception of Nd,O,.
Introduction Considerable progress has been made in understanding the mode by which methane is activated for the oxidative addition to metal centers and for the generation of methyl radicals.'-3 The latter (1) Bergman, R. G . SLitwr 1984, 223, 902. (2) Crabtree, R. H. Chem. Reu. 1985, 85, 245. (3) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J . Am. Chem. SOC.1985, 107, 58.
0022-3654/88/2092-0750$01.50/0
reaction forms the basis for the high-temperature catalytic oxidation of CH4, both to oxygenates (methanol and f~rmaldehyde)~ and to higher hydrocarbons (mainly ethane and (4) Liu, H.-F.; Liu, R.-S.; Liew, K.-Y.; Johnson, R. E.; Lunsford, J. H . J . Am. Chem. SOC.1984, 106, 4117. ( 5 ) Keller, G. E.; Bhasin, M. M. J . Coral. 1982, 73, 9. (6) Ito, T.; Lunsford, J. H. Nature (London) 1985, 314. (7) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. SOC. 1985, 107, 5062.
0 1988 American Chemical Society
