1998
J . Phys. Chem. 1989, 93, 1998-2004
Reactivity of Pyridine on Mo( IIO): C-H and C-N Bond Activation J. G. Serafin and C. M. Friend* Department of Chemistry, Haruard University, Cambridge, Massachusetts 021 38 (Received: April 1 1 , 1988; In Final Form: September 21, 1988)
Pyridine undergoes competing desorption (30 i 10%)and decomposition to surface carbon, nitrogen, and gaseous dihydrogen on Mo( 1 IO). Pyridine is shown to molecularly desorb, since no reversible C-H bond activation occurs in isotopic exchange experiments. The temperature at which total decomposition involving the cleavage of C-H, C-C, and C-N bonds occurs was found to increase with increasing coverage of pyridine and related surface fragments. Some C-H bond breaking occurs at temperatures less than 325 K as evidenced by isotopic labeling results. X-ray photoelectron spectroscopy results show that C-C bond activation occurs below 500 K. Notably, at least one of the C-N bonds in pyridine is intact at temperatures up to 500 K at high coverages on the basis of X-ray photoelectron spectroscopy results. An isotope effect in dihydrogen formation was observed in the reaction of a mixture of C,D,N and CSHSN. At high coverage and low temperature there is more than one species on the surface, one of which is assigned as molecular pyridine bound through nitrogen. The presence of surface hydrogen increases the amount of molecular desorption and, therefore, decreases the total amount of irreversible decomposition by -40%. As a result of the lower coverage of species on the surface after molecular desorption, total decomposition, and in particular C-N bond cleavage, occurs at lower temperature than on the initially clean surface.
Introduction The interaction of pyridine (c-C5HSN)with metal surfaces has been widely studied1-16largely due to the importance of pyridine as a model compound for catalytic hydrodenitrogenation reactions. In industry, hydrodenitrogenation is performed heterogeneously using either cobalt molybdenum oxides or nickel molybdenum oxides supported on alumina as the catalysts at temperatures of 600-800 K and under high pressures of hydrogen. Studies of model compounds, such as pyridine, indole, and quinoline, on nickel molybdenum catalysts suggest that ring hydrogenation is the first step in the reaction of the heterocycles, which then undergo C-N bond activation to form hydrocarbon p r o d ~ c t s . ' ~Two barriers that must be overcome in activating the C-N bond in pyridine and other aromatic heterocycles in order to form hydrocarbon products are the stability lost due to the breaking of the aromatic character of the ring and the relatively high strength of the C-N bond. The interaction of pyridine with single-crystal transition-metal surfaces has been investigated primarily by spectroscopic techniques and, in general, the emphasis in these studies has been on the nature of the pyridine-metal interaction rather than the mechanisms leading to decomposition and/or formation of products and the kinetics of C-H and C-N bond cleavage.
(1) Mate, C. M.; Somorjai, G. A.; Tom, H. W. K.; Zhu, X.D.; Shen, Y. R. J. Chem. Phys. 1988, 88, 441. (2) Schoofs, G R.; Benziger, J. B. J . Phys. Chem. 1988, 92, 741. (3) DiNardo, N. J.; Avouris, Ph.; Demuth, J. E. J. Chem. Phys. 1984, 81, 2169. (4) Wexler, R. M.; Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. J . Am. Chem. Soc. 1982, 104, 2034. ( 5 ) Grassian, V. H.; Muetterties, E. L. J . Phys. Chem. 1986, 90, 5900. ( 6 ) Johnson,A. J.; Muetterties, E. L.; Stohr, J.; Sette, F. J . Phys. Chem. 1985,89, 4071. (7) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Letf. 1980, 76, 201. (8) Bader, M.; Hasse, J.; Frank, K. H.; Puschmann, A,; Otto, A. Phys. Reu. Lett. 1986, 56, 1921. (9) Nyberg, G. L.; Bare, S. R.; Hoffman, P.; King, D. A,; Surman, M. Appl. Surf. Sci. 1985, 22/23, 392. (10) Surman, M.; Bare, S. R.; Hoffmann, P. Surf.Sci. 1987, 179, 243. (11) Grassian, V. H.; Muetterties, E. L. J. Phys. Chem. 1987, 91, 389. (12) Waddill, G. D.; Kesmodel, L. L. Chem. Phys. Lett. 1986, 128, 208. (13) Netzer, F. P.; Mack, J. U. J. Chem. Phys. 1983, 79, 1017. (14) Netzer, F. P.; Mack, J. U. Chem. Phys. Lett. 1983, 95, 492. (15) Mack, J. U.; Bertel, E.; Netzer, F. P. Surf. Sci. 1985, 159, 265. (16) Bandy, B. J.; Lloyd, D. R.; Richardson, N. V. Surf: Sci. 1979, 89, 344. (17) Laine, R. N. Catal. Reu.-Sci. Eng. 1983, 25, 459.
0022-3654/89/2093-1998$01.50/0
Pyridine has been found to interact with transition-metal surfaces primarily via three general types of bonding: through the nitrogen lone pair,7-10*'3-'5through the T-ring electrons in an $-type intera~tion,~-'* and through an a-pyridyl species in which the CY carbon has lost a hydrogen and both carbon and nitrogen interact with the surface.'" In this study, we investigate the factors controlling pyridine reactivity on the Mo( 110) surface using temperature-programmed reaction and X-ray photoelectron spectroscopy. Of specific interest are the relative kinetics for C-N and C-H bond activation and the effect of surface hydrogen on the reaction kinetics because of the importance of these processses in the related hydrodenitrogenation process. Molybdenum was chosen for these studies because of its use in commercial hydrodenitrogenation catalysis described above. The (1 10) face of molybdenum was used since it is the most thermodynamically stable face, and thus surface reconstruction will not play a significant role in the observed reaction kinetics.
Experimental Section All experiments were performed in two ultrahigh-vacuum Torr. The temchambers with base pressures of 1.5 X perature-programmed reaction spectra were collected in a chamber described elsewherels which contains a quadrupole mass spectrometer surrounded by a liquid nitrogen cooled shield. The chamber is also equipped to perform Auger electron spectroscopy, which was used to monitor crystal cleanness, and low-energy electron diffraction, which was used to check surface order. Preparation of the Mo( 110) single crystal was performed in the manner described previously.'* Pyridine was obtained from Aldrich (Gold Label 99+%), degassed, and used without further purification. Hydrogen (research purity) and deuterium (99.5%) were obtained from Matheson. Exposures are expressed in units of Torpsecond, which refer to the product of the backing pressure of the adsorbate in the doser reservoir (uncorrected) and the time in seconds the doser is open. The doses are referenced to saturation exposure, which is defined as the exposure after which there is no further increase in the amount of product formation. A crystal dosing temperature of 100 K was used in all experiments unless otherwise noted. The X-ray photoelectron spectra were collected in a chamber equipped similarly to the one described above, except for the addition of a Physical Electronics 5300 X-ray photoelectron spectroscopy system which has also been described previou~ly.'~ (18) Roberts, J. T.; Friend, C. M. J . Am. Chem. Soc. 1986, 108, 7204.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 1999
Reactivity of Pyridine on Mo( 1 10)
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Figure 1. Temperature-programmed desorption spectra of pyridine on Mo( 110) at saturation coverage for a dosing temperature of 100 K. The
cracking fraction at 52 amu was used to detect molecular pyridine. The only other gaseous product observed below 800 K is dihydrogen, which has the temperature-programmed desorption also shown in Figure 1. X-ray photoelectron data were analyzed and curve-fitted by using the computer and software provided with the Physical Electronics system. The C(ls), N(ls), and Mo(3d) regions were monitored for collection times of 8, 8, and 2 min, respectively. Multiple spectra were added together for the N ( 1s) region, and the clean Mo(l10) spectra subtracted from them because of the overlap of photoemission from the Mo(3p) peaks with the N(1s) region. Background subtraction and the overlap of photoemission peaks decreased the signal-to-noise ratio for the N ( 1s) region, making only a qualitative interpretation of the N( 1s) data possible. In particular, the areas of the N ( 1s) peaks for different annealing temperatures cannot be compared since the normalization used before background subtraction introduced large errors in the subtracted intensity. The clean background spectrum was also subtracted from the C( 1s) data; however, acquisition of multiple spectra was not necessary in the C(1s) region. The Mo(3d) peaks were monitored to normalize the number of counts in the spectra added together and to calibrate the binding energies in the N(1s) and C( Is) regions. Binding energies were found to be accurate to within f0.2eV. For the temperature annealing experiments, multilayers of pyridine were adsorbed on the crystal at 100 K, flashed to the temperature of interest twice, and recooled to 100 K before X-ray photoelectron data were collected. Each spectrum was collected after a new dose of pyridine, since sequential heating and X-ray photoelectron data acquisition of the same pyridine dose were found to cause beam damage to the surface species.
Results Temperature-Programmed Desorption Studies. Pyridine and dihydrogen are the only gaseous products detected in the temperature-programmed reaction spectra (Figure 1) obtained after exposure of multilayers of pyridine to Mo(l10) at a crystal temperature of 100 K. A comprehensive search of all masses in the range of 0-100 amu was performed, and it was determined that no other volatile products are formed. The mass 52 data shown correspond to pyridine desorption; however, both 79 amu (parent ion) and 52 amu were monitored to ensure that their intensities were in agreement with the fragmentation pattern of pyridine in our mass spectrometer. Analogous temperature-programmed reaction spectra, obtained for C5D5N and pyridine-2,5-d2 (data not shown), were essentially identical to the C5H5Nspectra shown in Figure 1. In particular, there are no significant differences in the peak temperatures, lineshapes or relative intensities in the dihydrogen and pyridine thermal reaction spectra for pyridine-do, pyridine-2,5-d2, and pyridine-d,. Surface carbon and nitrogen, apparent in the Auger electron spectrum after temperature-programmed reaction of pyridine to ~
(19) Friend, C. M.; Serafin, J. G.J . Chem. Phys. 1988, 88, 4037.
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Temperature ( K 1
Figure 2. Temperature-programmeddesorption spectrum of dihydrogen from the decomposition of pyridine on Mo( 110) as a function of exposure. The exposures used were as follows: (a) 2.4, (b) 4.5, (c) 9.0, (d) 11.4, and (e) 13.5 Torrss. The crystal dosing temperature was 100 K.
800 K, are formed as nonvolatile reaction products. Approximately 70% of the pyridine adsorbed at saturation decomposes to surface carbon on the basis of the relative carbon intensities measured before and after temperature-programmed reaction to 850 K.20 Pyridine desorption is first observed at a dose 0.85 of saturation. The first pyridine peak to appear as the coverage is increased is at 375 K (Figure l), observed for a dose of 11.4 Torres (0.85 of saturation). A second peak centered a t 190 K appears for an exposure of 13.5 Torr-s. Since the dihydrogen formation peaks are not affected after exposures greater than 13.5 T o w s this exposure is defined as saturation coverage. Increasing the pyridine exposure further leads to the observation of two pyridine peaks at 175 and 160 K. The 160 K peak indefinitely increases in intensity with exposure and is attributed to sublimation of multilayers of pyridine. We tentatively attribute the 175 K peak to desorption of a physisorbed second layer of pyridine since its appearance does not affect dihydrogen formation during temperature-programmed reaction. The temperatures of all pyridine desorption peaks are independent of coverage. Dihydrogen (2 amu) is formed in two unresolved peaks, designated & and p2, shown in Figure 1 for reaction of a saturation exposure of pyridine. The development of the dihydrogen formation spectrum as a function of pyridine exposure is shown in Figure 2. The desorption of dihydrogen from the recombination of hydrogen atoms occurs at 450 K in the high-coverage limit on Mo( 1 In the low-coverage limit dihydrogen desorbs at 550 K. Figure 2a-c corresponds to exposures where no pyridine desorption is detected, exposures 0.18,0.33, and 0.67 of saturation, respectively. As the pyridine exposure is increased, the H2 formation spectrum broadens and splits into at least two unresolved peaks. In Figure 2d, at a coverage where only the highest temperature (375 K) pyridine desorption peak is observed (0.85 of saturation), the P2-H2 peak is shifted to 595 K. At saturation coverage (Figure 2e), there is a slight increase in the amount of H2 produced in the range 400-500 K. Isotopic exchange experiments demonstrate that all the pyridine desorption features arise from wholly molecular states: there is no H-D exchange in any of the pyridine desorption peaks. N o mixed isotopes of pyridine are formed in the reaction of a mixture of CSHSNand C5D5Nwhen mixtures with compositions in the range of 0.3k0.65 to 0.50:0.50 of pyridine-do:pyridine-ds are studied at saturation exposures. Furthermore, there is no isotopic (20) The quantitative Auger electron measurements were made by measuring the relative peak heights of the C(KLL) to the Mo(LMM) transition. The amount of decomposition relative to desorption was estimated by taking the ratio of the two values measured at 100 and 850 K for doses that were of saturated exposure but were below that where multilayers formed. The carbon Auger ratio is a measure of the amount of decompositon relative to molecular desorption since pyridine is the only carbon-containing species that evolved from the surface. (21) Roberts, J. T.; Friend, C. M. Surf. Sci. 1987, 186, 201.
2000
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Serafin and Friend
577 K
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Figure 3. The 2, 3, and 4 amu temperature-programmed desorption spectrum for a saturation dose of a CSDJN and CSHsN mixture on Mo( 1 IO).
Binding Energy ( e V ) Figure 5. C(1s) X-ray photoelectron spectrum for a multilayer dose of pyridine on Mo( 110) annealed to (a) 200, (b) 500, and (c) 800 K.
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Temperature ( K ) Figure 4. Effect of hydrogen preadsorption on the temperature-programmed desorption of pyridine (52 amu) on Mo(ll0). Pyridine desorption is shown after exposure of multilayer doses of pyridine to (a) initially clean Mo(l10) and (b) Mo(l10) saturated with hydrogen atoms. The initial surface temperature was 100 K in both cases, and the heating rate identical.
mixing in the pyridine desorption from the reaction of pyridine2,5-d2 in a temperature-programmed reaction experiment performed after saturation exposure. Lastly, reaction of C5H5Nin the presence of a saturated layer of deuterium atoms or reaction of C5D5Nin the presence of a saturated layer of hydrogen atoms showed no isotopic mixing in the molecular desorption peaks. We note, however, that preadsorption of hydrogen or deuterium atoms qualitatively alters the pyridine temperature-programmed reaction spectra, as described below. While the reaction of either C5H5Nand C5D5Non Mo( 110) are qualitatively and quantitatively the same, an isotope effect is clearly evident when a mixture of C5H5N and C5D5N is simultaneously reacted on the surface (Figure 3). Specifically, the ratio of the dihydrogen peaks is different for the three dihydrogen isotopes Hz, D2, and HD. In this competitive reaction of pyridine-do and pyridine-d5, the peak heights of P2-D2> P1-D2, whereas the PI and p2 peaks heights for H D and H2 are nearly equal. The precise ratio of the PI:p2peaks for H,, HD, and D2 depends on the exact composition of the dosing mixture. Further, the temperature of the Pz desorption maximum is shifted to higher temperature by 8 K for H D and by 17 K for D, relative to H2. Preadsorption of hydrogen or deuterium atoms dramatically alters the temperature-programmed reaction spectra of pyridine on the Mo( 110) surface. Figure 4 compares the temperatureprogrammed desorption spectra of pyridine obtained after exposure of multilayer quantities of pyridine to clean Mo( 1 10) with spectra obtained from pyridine reaction in the presence of a saturation coverage of hydrogen atoms. A single low-temperature molecular
desorption is observed at 2 12 K for the hydrogen-precovered surface compared to the two peaks observed at 190 and 375 K in the case of no preadsorption. Analogous results are obtained for C5D5Nreaction in the presence of a saturation coverage of deuterium or hydrogen atoms. The amount of molecular desorption during the temperatureprogrammed reaction of pyridine on Mo( 110) also increases by approximately 40% when a saturation dose of hydrogen or deuterium atoms is preadsorbed. Integrating the area under the pyridine temperature-programmed reaction peaks for the two cases showed that approximately 40% more desorption was measured for the hydrogen-precovered Mo( 110) surface after a saturation exposure of pyridine. This result was also confirmed by integration of the carbon X-ray photoelectron spectroscopy peaks before and after molecular desorption on the two surfaces, discussed below. The temperature-programmed reaction spectra of H2, HD, and Dz produced from CsD5N decomposition are qualitatively and quantitatively altered by the presence of surface hydrogen (data not shown). When a saturation dose of hydrogen is preadsorbed with a 0.85 saturation exposure of CSDSN,there is a predominance of Hz and H D desorption in the PI temperature regime, 300-500 K. In contrast, both PI- and &D2 are observed in the temperature-programmed reaction of pyridine-d5 on the initially clean surface. On the basis of these observations, the &-dihydrogen formation peak must be due, in part, to atom recombination on the surface, and the &dihydrogen peak must arise from the decomposition of a surface fragment that has not undergone substantial reversible C-H bond cleavage. The temperature of the p2 desorption is dependent on the coverage of preadsorbed hydrogen. In the absence of preadsorbed hydrogen the P2-D2peak temperature is at 595 K, while the p2 temperature maximum shifts to 560 K when CSD5Nis reacted in the presence of a saturation coverage of hydrogen atoms. Further, the intensities of the PI- and &D2 peaks are changed by hydrogen atom preadsorption. While the PI absolute intensity is approximately the same with or without hydrogen preadsorption, the P2 peak intensity is reduced. This is consistent with the results discussed above, in which the absolute amount of deuterium produced from C5DsN decomposition is decreased by hydrogen preadsorption. The changes in the temperature of the PZmaximum of the D2 spectra and the decrease in hydrogen desorption are directly analogous to the differences observed in H, desorption from coverages of pyridine lower than 11.4 Toms on the initially clean Mo( 110) surface described above. Preadsorption of deuterium with pyridine+ yields analogous results. X-ray Photoelectron Studies. Figures 5 and 6 show the C ( 1s) and N( 1s) X-ray photoelectron spectra, respectively, of a saturation dose of pyridine on Mo( 110) obtained after annealing to
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2001
Reactivity of Pyridine on Mo( 110)
TABLE 11: Summary of X-ray Photoelectron Data for L o w Coverages of Pyridine on Mo( 110) and Pyridine on Hydrogen-PrecoveredMot 110) binding energy, eV annealing pyridine C( 1s) N ( 1s) dose, Toms temp, K
Low Coverage 9.0 (0.67) 11.4 (0.85)
100 500 100 500
284.4 286.2 285.4
286.0 398.4 283.1 396.7 284.6 399.7 398.3 284.1 283.1 397.1
-
Hydrogen Preadsorption
