J . Phys. Chem. 1988, 92, 6694-6700
6694
Evidence for Nitrile Hydrogenation on W( 100)-(5X 1)-C: Spectroscopic Studies of Surface Intermediates Derived from HCN J. G. Serafin and C. M. Friend* Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: December 29, 1987)
The reactions and structures of HCN and intermediates derived from its surface reactions on W( 100)-(5Xl)-C have been investigated by using high-resolution electron energy loss, X-ray photoelectron, and temperature-programmed reaction spectroscopies. A mixture of two species are present at 200 K: relatively unperturbed HCN, designated the a-HCN state, and a strongly perturbed nitrile, designated the y state. The a-HCN is proposed to bind through the nitrogen atom atop a single metal center. Competing reaction and molecular desorption occur in the range of 200-500 K. In this temperature range, a-HCN desorbs with the maximum rate of desorption observed at 450 K corresponding to a desorption energy of =27 kcal/mol. Nitrogen-hydrogen bond formation yielding adsorbed HCNH is a competing reaction. Surface hydrogen atoms from the low-temperature decomposition of HCN are proposed to participate in the N-H bond formation reaction. Hydrogen atom recombination to yield H2 also occurs to a limited extent in the 200-500 K temperature regime. Dissociation to form surface atomic carbon and nitrogen which is probably derived from the y state, proposed to be v2-CN, also occurs in this temperature regime. At a surface temperature of 500 K, only HCNH and atomic carbon and nitrogen are present on the surface. The C-N bond of the HCNH is rehybridized and has a bond order of approximately 1.8. The C-N and C-H stretches are observed at 1400 and 2940 cm-', respectively, consistent with an sp2 hybridization of the nitrile carbon center. HCNH undergoes N-H and C-H bond scission in the range of 500-800 K. The major reaction products are gaseous P-HCN and H2, both of which have a maximum rate of desorption at 650 K. Some competing complete dissociation of the HCNH to yield atomic carbon, nitrogen, and H2 also occurs. Heating to 1500 K regenerates the (5x1)-C surface with the same chemical activity, diffraction pattern, and X-ray photoelectron spectra. The atomic nitrogen is removed from the surface by N, formation at 1500 K.
Introduction The study of the reactions of hydrogen cyanide (HCN) and other nitriles on metal surfaces is of interest because nitriles are model n-acceptor adsorbates and their reactions are relevant to the catalytic hydrogenation of nitriles to organic amines. H C N has frontier molecular orbitals similar to those of CO, the most well understood A acceptor. There are, however, important differences in the frontier orbitals of these two molecules that could cause differences in interactions with the surface. For HCN, the 2n* orbital is at 4.5 eV higher energy than that of CO,' making it a relatively poorer a acceptor for a linear, end-on bound geometry. Also, calculations have shown that most of the 2n* density is localized on the carbon atom so that adopting a bent geometry would allow more favorable n* overlap for HCN.2 W( lOO)-(5Xl)-C is of interest because it exhibits qualitatively different chemical properties than clean W(100) for a number of different reactions, including those of nitriles3p4and aminess Further, tungsten carbide has been proposed as a viable catalyst for some classes of reactions where Pt is traditionally used as a catalyst, such as alkene isomerization,6motivating our investigation of the reactions of organic molecules on a model tungsten carbide surface. The W ( 100)-(5x1)-C surface has been extensively studied previously and has been found to have 4-fold symmetry with the carbons proposed to reside in the 4-fold sites.' The W( 100)-(5X 1)-C surface has a relative deficiency of electron density near the Fermi level, shown both experimentally and through electronic structure calculations.2 The valence band electronic structure of the W( 100)-(SXl)-C surface measured with photon-energy-dependent photoelectron spectroscopy is essentially the same as that of the basal plane of bulk tungsten carbide.8 The agreement of the electronic structure of the
W( 100)-(5X 1)-C surface and the tungsten carbide basal plane make the ( 5 x l ) - C a good model for understanding the role of electronic structure in the bonding of reactant molecules to the carbide surface. Temperature-programmed reaction studies of H C N on the W( loo)-(% 1)-C surface have been carried out previ~usly.~ Figure 1 shows data taken from these studies. Multilayers of H C N desorb at 150 K for exposures greater than 1 langmuir. H C N undergoes competing desorption and decomposition with the only products of reaction being gaseous HCN, dihydrogen, dinitrogen, and atomic surface carbon. At saturation exposure, two mass 27 peaks (HCN') are observed in a temperature-programmed reaction experiment, one at 450 K, designated a,and one at 650 K, designated p. Dihydrogen is formed in a broad peak in the range 300-750 K, with a maximum of intensity also at 650 K. The rate of H2 formation is limited by hydrogen atom recombination in the leading edge of the peak and by C-H and/or N-H bond cleavage at higher temperatures, as demonstrated in this study. This work is part of a comparison of the reactivity and bonding of simple n-acceptor molecules, including CH3CN and CO, on the W(100)-(5xl)-C ~ u r f a c e . ~We J ~ have performed spectroscopic studies of acetonitrile analogous to those of H C N on the W(100)-(5Xl)-C surface9 and have shown that by 400 K, q2bound acetonitrile undergoes hydrogenation at nitrogen and dehydrogenation at the methyl carbon to form a CH2CNH species. This intermediate was characterized by using high-resolution electron energy loss spectroscopy and X-ray photoelectron spectroscopy. The objective of this study is to spectroscopicallyidentify and characterize the HCN-derived intermediate and compare the behavior of HCN to CH3CN on the (5Xl)-C surface to establish general principles governing nitrile hydrogenation.
(1) Jorgenson, W. L.; Salem, L. The Organic Chemist's Book of Orbitals; Academic: New York, 1973. (2) Jansen, S. A.; Hoffman, R.; Friend, C. M., manuscript in preparation. (3) Pearlstine, K. A.; Friend, C.M. J . Phys. Chem. 1986, 90, 4341. (4) Pearlstine, K. A,; Friend, C. M. J . Phys. Chem. 1986, 90, 4344. (5) Pearlstine, K. A,; Friend, C. M. J. Am. Chem. SOC.1986, 107, 5898. (6) Boudart, M.; Levy, R. Science (Washington, D.C.)1973, 181, 547. (7) Overbury, S. H.; Mullins, D. R. Surf. Sci. 1988, 193, 455. (8) Stefan, P.M.; Spicer, W. E. Surf. Sci. 1985, 149, 423.
Experimental Section Two ultrahigh vacuum chambers were used to perform the experiments described here. Both chambers had base pressures
0022-3654 188 12092-6694SO1.50,I O I
,
(9) Friend, C. M.; Serafin, J. G. J. Chem. Phys. 1988, 88, 4037. (10) Friend, C. M.:Serafin, J. G.; Stevens, P. A,; Madix, R. J . J . Chem. Phys. 1987, 87, 1847.
0 - 1988 American Chemical Societv - -,
Nitrile Hydrogenation on W(100)-(5Xl)-C
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6695
6000 oounta
396 9 I
396 9
398,.1
c
h
=
z
t ' 100
I
300
'
1
'
1
500 700
'
I
Ab
............ .... : I x z -r ,__ . .............. .. ... .. . .. . .. .
.
.
.
405
404
402
I
....
397 %
,-
900
Temperature ( K
1100 1300 1500
1
Temperature-programmed reaction data for HCN on W(lO0)-(5x1)-C. The exposure for the data shown is at the onset of multilayer formation and has a heating rate of approximately 17 K/s.
Figure 1.
in the IO-lO-Torr range and were equipped to perform retarding field Auger electron spectroscopy, low-energy electron diffraction, and temperature-programmed reaction spectroscopy. Directed dosing through a leak valve was used for all experiments. A Physical Electronics (PHI) 5300 X-ray photoelectron Spectroscopy system installed into one of the vacuum chambers" was used to collect X-ray photoelectron data. Details of the X-ray photoelectron spectroscopy system have been described previo~sly.~ Gaussian-Lorentzian curve fits were generated by using software provided by Physical Electronics. Binding energies are accurate to within i 0 . 2 eV and were calibrated against tungsten 4f photoemission peaks. Typical scan times for the C(1s) and N(1s) regions were 7 min each with data from both regions collected in a single experiment. Three spectra, collected separately, were added together for each region to improve the signal-to-noise ratio. For both the C( 1s) and N(1s) regions, the W(100)-(5Xl)-C clean surface spectra were subtracted from those collected after dosing HCN. Before subtraction, the C( 1s) region spectra were shifted to lower binding energy by 0.1 eV to align the maxima of the carbide peaks. Analogous shifts of the carbide binding energy have been observed when oxygen is adsorbed on the surface and may be due to a structural change in the carbide surface with adsorption.12 Preliminary ion scattering results suggest that the carbidic carbon moves below the surface plane when oxygen is adsorbed.13 Alternatively, the shift may be due to direct adsorbatecarbon interactions. Temperature-dependent spectra were obtained by dosing a multilayer of H C N onto the surface, annealing to the desired temperature, recooling the sample to 120 K, and collecting photoelectron spectroscopy data. The (5X 1)-C surface was regenerated and a fresh multilayer adsorbed prior to annealing and data collection for each temperature, precluding X-ray beam damage as a result of prolonged exposure from contributing to the observed spectra. X-ray beam damage was checked for by performing temperature-programmed reaction experiments after data collection and was minimal under the conditions of this experiment. The high-resolution electron energy loss spectroscopy was performed on a chamber described e1se~here.I~Data collection times were approximately 15 min per spectrum with all spectra presented here consisting of single scans. The resolution of the spectrometer was 10-12 meV (80-100 cm-l) full width at halfmaximum of the elastic peak. All spectra were collected at the specular angle. Annealed spectra were collected in the same manner used to collect X-ray photoelectron spectra. The W(100)-(5Xl)-C surface was prepared in the manner described previ~usly.~ Low-energy electron diffraction and Auger ( 1 1 ) Baldwin, E. K.; Friend, C. M. J . Phys. Chem. 1985, 89, 2576. (12) Baldwin. E. K.; Friend, C. M., unpublished results. (13) Mullins, D. R.;Overbury, S . H., unpublished results. (14) Stuve, E. M.; Madix, R. J.; Brundle, C. R. Surf. Sci. 1984, 146, 155.
408
400
398
396
394
Binding energy ( eV Figure 2. N(1s) X-ray photoelectron spectra for multilayers of HCN on W(100)-(5Xl)-C annealed to temperatures of (a) 200, (b) 500, and (c) 800 K.
electron spectroscopy were used to confirm the presence of the ( 5 1)-C ~ overlayer. Hydrogen cyanide was prepared by mixing potassium cyanide and 85% phosphoric acid under nitrogen and condensing the gaseous H C N evolved in a cold trap. The hydrogen cyanide was degassed and dried with calcium chloride. Purity was verified by monitoring the background mass spectrum after dosing the H C N into the ultrahigh vacuum chamber. High-purity ethylene (99.5%) used to produce the carbided surface was obtained commercially from Matheson.
Results X-ray Photoelectron Studies. N ( 1s) X-ray photoelectron data for H C N as a function of annealing temperature on the W(100)-(5Xl)-C surface are shown in Figure 2. For reference, the temperature-programmed reaction data obtained previously are shown in Figure 1. Two N(1s) photoelectron peaks with binding energies of 400.0 and 397.8 eV are observed at 200 K, the temperature where multilayer desorption is complete but below that where a-HCN is produced (Figure 2a). The binding energy of multilayers of HCN, calibrated by the position of the tungsten 4f peaks, is 401.8 eV (data not shown). The N(1s) peak with a binding energy of 400.0 eV observed after annealing to 200 K is more similar in energy to that of multilayers of HCN, indicating a smaller change in the charge density on the nitrogen atom in this adsorption state. Both N ( 1s) photoelectron peaks are obsrved after adsorption of H C N on W(lO0)-(5x1)-C at 200 K for all coverages. Further, the two N ( 1s) photoelectron peaks observed at low temperature vary independently as a function of annealing temperature. Therefore, the two peaks are assigned to two chemically distinct species on the surface at low temperature. The relative intensity of the peaks was independent of the heating rate in the range employed in this study (1-10 K/s). Annealing to 300 K (data not shown) results in three peaks in the N ( 1s) photoelectron spectrum, with binding energies of 400.0, 397.8, and 396.9 eV. There is a decrease in the absolute intensities of both the 400.0- and 397.8-eV binding energy peaks, although both are still present. The decrease in intensity is consistent with the fact that desorption of a-HCN occurs in the 2OC-500 K range. Atomic nitrogen is also formed to some extent at 300 K, as indicated by the appearance of a peak with a binding energy characteristic of atomic nitrogen at 396.9 eV. We propose that atomic nitrogen is formed from C-N bond dissociation in the state with the strongly perturbed nitrogen and that a - H C N desorption is derived primarily from the relatively unperturbed
Serafin and Friend
6696 The Journal of Physical Chemistry, Vol. 92, No. 23, 1988
TABLE I: Vibrational Assignments for HCN on W(100)-(5Xl)-C at 200 K W( 100)-(5x 1 ) - c (200 K) I
mode
descr
v(W-N) 6(C-H)
W-N str C-H bend
u(C=N) u(C=N) u(C-H)
C=N str CEN str C-H str
..... . . . . . .
,
I
283 3
i
H C N (solid)
8 20 835
HCN
CN
360 680 1336
2094 3142
2080 3280
TABLE I 1 Vibrational Modes of HCNH on W(100)-(5Xl)-C mode descr H C N H (500 K) Fe trimer u(W-C) W-C str 544 v(C=N) C=N str 1400 1353 u(C-H) C-H str 2940 2900 3360 3212 u(N-H) N-H str ~~
A
,
,
,
,
,
,
,
,
,
,
,
,
,
,
/
,
~
294 292 290 288 286 284 282 280 278 276
Binding energy ( e V 1
Figure 3. C(1s) X-ray photoelectron spectra for multilayers of H C N on W(lOO)-(5Xl)-C annealed to temperatures of (a) 200, (b) 500, and (c) 800 K. The C(1s) photoelectron peak due to the carbide overlayer has been subtracted from the data as described in the text.
state. A quantitative correlation cannot be made, however, due to the fact that three channels are available for reaction of the surface species: desorption, dissociation, or formation of a new surface species. Two N(1s) photoelectron peaks with binding energies of 398.1 and 396.9 eV are observed after annealing HCN multilayers to 500 K, as shown in Figure 2b. The photoelectron peak at 400.0 eV is absent after annealing to 500 K, consistent with the fact that a - H C N desorption is complete at this temperature. Since the 396.9-eV binding energy peak can be attributed to atomic nitrogen, there must be a single nitrogen-containing molecular species present at 500 K. Further heating to 800 K, where desorption of all hydrogencontaining species is complete, yields the X-ray photoelectron spectrum shown in Figure 2c, where a single N(1s) peak at a binding energy of 396.9 eV, due to atomic nitrogen, is observed. The integrated intensity of the atomic nitrogen peak measured after annealing to 800 K is approximately 25% greater than that measured after annealing to 500 K, demonstrating that some additional C-N bond cleavage occurs in this temperature range. Annealing to 1500 K results in a photoelectron spectrum identical with that of the original carbide surface, in which no photoelectron peaks associated with nitrogen are observed. This is consistent with the observation of dinitrogen formation in the range 1000- 1400 K. The C(1s) X-ray photoelectron spectra shown in Figure 3 correspond to the N( 1s) data shown in Figure 2. The carbon peak due to the surface carbide has been subtracted from these spectra as described in the Experimental Section. Although the C ( 1s) data are not quantitatively accurate due to the decreased signal-to-noise from background subtraction, qualitative features of the data are reliable and are generaily consistent with the corresponding N ( 1s) data shown in Figure 2. The spectrum shown in Figure 3a was obtained after annealing a multilayer dose of H C N to 200 K, producing two peaks with binding energies of 285.4 and 283.5 eV. Multilayers of H C N have a C(ls) binding energy of approximately 288.4 eV (data not shown). The two peaks observed after annealing to 200 K are assigned as two inequivalent carbons based on the fact that two types of nitrogen are present at this temperature. The 285.4-eV peak is correlated with the relatively unshifted N( 1s) peak observed at 400.0 eV. The peak with binding energy 283.5 eV is perturbed strongly from the binding energy of multilayer HCN and suggests that the charge density on the nitrile carbon atom in the species
~~~
is significantly different. The 283.5-eV C(1s) peak is associated wwith the strongly perturbed N ( Is) peak with a binding energy of 397.8 eV on the basis of their relative intensities as a function of annealing temperature. Annealing to 500 K results in the observation of two C(1s) peaks, at 285.4 and 283.3 eV. The 285.4-eV C(1s) peak is correlated with the N(1s) peak with a binding energy of 398.1 eV since the 283.3- and 396.9-eV peaks are both assigned to atomic species. The 283.3-eV peak is attributed to atomic carbon although the binding energy is not 283.0 eV since the absolute binding energies of the C( 1s) peaks are subject to inaccuracy due to the difficulty in subtraction of the carbide background, as described in the Experimental Section. The width and position of the C( Is) peak associated with the carbide carbon are dependent on the annealing temperature. Therefore, small shifts in the energy of peaks close in energy to the carbide photoelectron peak (282.1 eV) may be artifacts of the data analysis method. After annealing to 800 K, only one peak, attributed to atomic carbon that is not yet incorporated into the (5Xl)-C overlayer, is present at 283.0 eV. The absolute intensity of the 283.0-eV C( 1s) peak measured after annealing to 800 K is increased relative to the integrated intensity of the 283.3-eV peak measured after annealing to 500 K, consistent with additional C-N bond dissociation between 500 and 800 K. All hydrogen has evolved by this temperature. Further heating to 1500 K regenerated the (5Xl)-C low-energy electron diffraction pattern and shows a C( 1s) binding energy of 282.1 eV characteristic of this surface. As reported previ~usly,~ the reactivity of the regenerated (5Xl)-C surface is the same as the freshly prepared surface, indicative of incorporation of surface carbon into the carbide structure above 1500
K. Vibrational Studies. Vibrational data obtained for H C N as a function of annealing temperature are shown in Figure 4. Assignments are given in Table I for the 200 K spectrum, and in Table 11 for the 500 K spectrum. A detailed rationale of the assignments is given in the Discussion since the X-ray photoelectron data are also necessary for complete interpretation. At 200 K, a mode attributed to a triply bonded C-N group appears at 2080 crn-I, although adsorbed CO from the background certainly contributes to the intensity of this peak. A C-H stretching mode is at 3280 cm-I, and the C-H bend at 680 cm-'. The mode at 360 cm-' is assigned to a metal-nitrogen stretch on the basis of comparison to the Pt-N stretch measured for ammonia on Pt( 111) of 350 crn-'.l5 The mode at 1336 cm-' does not appear in gas-phase HCN spectra and is assigned here to the C-N stretch of a rehybridized C-N bond. By 500 K, a N-H stretch has appeared at 3360 cm-', showing that N-H bond formation has occurred. The C-H stretching mode is present at 2940 cm-' and a mode characteristic of the (5Xl)-carbide surface appears at 544 cm-'. A C-N stretch is observed at 1400 cm-' for a strongly ( 1 5 ) Sexton, B. A.; Mitchell,
G.A. Surf,
Sci. 1980, 99, 523
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6697
Nitrile Hydrogenation on W( 100)-(5Xl)-C
i L 2080
I
I
I
0
1000
2000
3
Ka
I 200 K 3000
Energy Loss (cm-') Figure 4. High-resolution electron energy loss spectra for HCN on W(lO0)-(5Xl)-Csaturation doses annealed to (a) 200 and (b) 500 K.
2
,*,
HZ
f
Cn
t
Nn
300 K
d
Stat)
---3
C,
+
NE
4
Hz
Figure 5. Reaction scheme for HCN on W(lO0)-(5x1)-C.
perturbed C-N bond attributed to adsorbed H C N H .
Discussion
On the basis of the results given above, we propose the scheme shown in Figure 5 for the reaction of H C N on W(lO0)-(5x1)-C. Some of the structural details given are derived from near-edge X-ray absorption fine structure data, described in detail elsewhere.I6 A detailed rationale for this scheme based on all measurements is given in the following discussion. Since the 500 K spectral data are the most straightforward to interpret, they will be discussed first. The vibrational data collected after heating to 500 K (Figure 4b) clearly show the presence of an N-H bond in addition to a C-H bond. The frequency of 3360 cm-' is approximately 100 cm-I higher than that observed for the C-H stretch of condensed H C N and is too high to be associated with a C-H bond, expected to be in the range 3000-3340 cm"." The formation of an N-H bond requires a source of hydrogen. Since background hydrogen is not present in significant amounts,I8 at least two species must be present on the surface, one of which must have dehydrogenated to provide a source of hydrogen. The N ( 1s) X-ray photoelectron data clearly show that there is a single molecular species with a binding energy of 398.1 eV and atomic nitrogen with a N(1s) binding energy of 396.9 eV being the second species on the surface after reaction to 500 K. Because a single nitrogen-containing molecular species is present, the interpretation of the vibrational data is simplified. As discussed below, we propose that the mo(16) Stevens, P. A,; Madix, R. J.; Friend, C. M., submitted for publication in Surf. Sci. (17) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic: New York, 1982; p 195. (18) H2has been found to have an extremely low dissociation probability on the W(lO0)-(5x1)-Csurface.
lecular species is an H C N H intermediate. Because there is only a single type of covalently bonded nitrogen present at 500 K, the species giving rise to the N ( 1s) peak with 398.1-eV binding energy must be the one associated with the N-H bond. The presence of surface N H is ruled out on the basis of the N(1s) binding energy and reaction kinetics. The binding energy of an N H group on W( 110) has been previously measured to be 397.8 eV,I9 while we observe a N( 1s) binding energy of 398.1 eV. Further, it is unlikely that C-H or N-H group on the surface would recombine with atomic nitrogen or carbon, respectively, to form 0-HCN since atomic species are not very mobile on the (5X 1)-carbide surface as evidenced by the high temperatures necessary (>900 K) to form P-N, and 0-CO from atomic recombination.20 The measured N ( 1s) binding energy for the HCN-derived intermediate is also close but not identical with the range of energies measured for a N-bound R N H group on evaporated Ni films, 398.5-398.3 eV.21 Thus, the N ( 1s) photoelectron data do not definitively identify the H C N H species. The observed binding energy of 398.1 eV is, however, reasonable for the proposed intermediate. As discussed below and elsewhere,I6 the bonding of H C N H on W(lO0)-(5Xl)-C is proposed to be significantly different than N-bound R N H . The C( 1s) X-ray photoelectron data show the presence of two carbon-containing species at 500 K, one at a binding energy nearly the same as atomic carbon (283.3 eV), and one at a higher binding energy of 285.4 eV, which we attribute to the H C N H species. The C( 1s) binding energy measured after annealing to 500 K also does not definitively identify the proposed H C N H species. The C( 1s) binding energy of an acetonitrile species proposed to be bound via both the nitrile carbon and nitrogen on Ni films is measured to be 284.6 eV.I6 We propose that the 283.3-eV peak is due to atomic carbon even though it is 0.3 eV higher in binding energy than the atomic carbon peak measured after annealing to 800 K. As noted earlier, the C( 1s) binding energies are subject to error derived from the background subtraction procedure. We cannot, however, completely rule out the presence of surface CH based on the C(1s) binding energy. We are not aware of any previously reported measurements of the C ( 1s) binding energy for C H on a metal surface. The binding energy of adsorbed C H would be expected to be relatively low, and 283.3 eV is certainly a reasonable value. As discussed below, however, the vibrational data do not show a C-H stretch at a frequency expected for adsorbed CH, rendering it unlikely that it is a major surface species. The vibrational data (Figure 4b) support the assignment of the molecular fragment as surface-bound HCNH. The presence of intact N-H and C-H bonds on the surface is clearly shown based on the observation of C-H and N-H stretch modes. The value of the C-H stretch is 2940 cm-I, in the region expected for a sp2 hybridized carbon.22 Near-edge X-ray absorption fine structure data obtained on this system at 475 K demonstrate that there is a molecular fragment with an intact C-N bond, based on the fact that electronic r * and u* resonances associated with a C-N bond are observed.I6 The C-N bond angle of the species present at 475 K is measured to be 32 f loo with respect to the surface plane and the C-N bond length to be 1.34 f 0.04 A, corresponding to a bond order of 1.8. The long C-N bond is consistent with the assignment of the vibrational loss at 1336 cm"' as a C-N stretch. Further support for our assignments are obtained from comparison with the organometallic analogue HFe3(CH3CNH)(C0)9,23which has a nitrile C-N bond length of 1.344 8, and a C-N stretch of 1353 cm-'. Further, an analogous species with a C-N stretch of 1400 cm-I derived from the reaction of CH,CN on W(100)(5Xl)-C has also been identified.9 Both the C-N bond length and C-N stretching frequency are in good agreement with that of the proposed H C N H intermediate on W(l00)-(5Xl)-C. The values of other vibrational modes of the iron trimer and the (19) (20) (21) (22) (23)
Grunze, M.; Brundle, C.R.; Tamanek, D. Surf. Sci. 1982,119, 133. Baldwin, E. K.; Friend, C. M. J . Phys. Chem. 1987, 91. 3821. Inamura, K.; Inoue, Y.;Ikeda, S . Surf. Sci. 1985, 155, 173. Lehwald, S.; Ibach, H. Surf. Sci. 1979, 89, 425. Andrews, M. A.; Kaesz, H. D. J . Am. Chem. SOC.1979,101,7238.
6698 The Journal of Physical Chemistry, Vol. 92, No. 23, 1988
adsorbed H C N H species are shown in Table 11. The possibility that a mixture of H C N and H N C species is present on the surface instead of H C N H at 500 K is deemed unlikely. First, the N(1s) X-ray photoelectron spectra evidence a single molecular species after heating to 500 K: the nitrogen centers in H C N and H N C are not likely to have identical N( Is) binding energies. Second, only one parallel-bound C-N bond is indicated by the near-edge absorption fine structure measurements. Third, the vibrational spectrum is fully consistent with a single molecular species. Furthermore, the N-H stretch of condensed H N C in an argon matrix shows a v(NH) of 3583 cm-1,24much higher than that of 3360 cm-' measured here. In summary, all of the above evidence lends overriding support to the proposal of formation of adsorbed H C N H , atomic carbon, and nitrogen at 500 K from reaction of H C N on the W(lO0)-(5Xl)-C surface. At 200 K, the X-ray photoelectron data (Figures 2a and 3a) show that there are two species present from the adsorption of H C N on the W ( 100)-(5Xl)-C surface. Unfortunately, the complexity of the 200 K spectra preclude the definitive identification of the surface species. As discussed below, the bulk of the evidence suggests that a N-bound, linear H C N and v2-CN are the species present. The higher binding energies of 400.0 eV for the N(1s) and 285.8 eV for the C(ls) electrons are attributed to a molecularly bound H C N species, which we propose to be bound through the nitrogen to the surface atop a single metal atom in a nearly perpendicular orientation. These C(ls) and N(1s) peaks are associated with a single species on the basis of their relative intensities to the other peaks present as a function of annealing temperature. We note that the C ( 1s) binding energy of 285.8 eV is shifted by -4 eV with respect to multilayer HCN, which is much more than would be expected for an end-on bound H C N species, especially since the N(Is) binding energy is not perturbed nearly as much. The weakly perturbed H C N species is present on the surface until desorption of a-HCN is complete, near 500 K. The intensity of the peak decreases with heating as a - H C N desorption commences but is present throughout the temperature range 200-500 K. Thus, the higher binding energy state in the photoelectron data is attributed to a - H C N on the surface, which desorbs in this temperature range. We also propose that competing with desorption, a-HCN is hydrogenated to form HCNH, as shown in the reaction scheme (Figure 5). These experiments do not demonstrate that this is the mechanism for H C N H formation: another path is possible. It is proposed here because only one reaction step is necessary, N-H bond formation. The vibrational modes at 3280, 2080, 680, and 360 cm-I are assigned to the a - H C N species as listed in Table I. Note that the mode at 2080 cm-' is tentatively assigned as the C-N stretch of the relatively unperturbed nitrile species and would be consistent with a linear, atop geometry. We note, however, that the peak attributed to the C-N stretch may by largely due to adsorbed background CO, known to have a u(C0) near this frequency on the W(100)-(5XI)-C surface.I0 It is possible that the C-N stretch of the species associated with the relatively unperturbed nitrile is not of sufficient intensity to be observed in the vibrational spectrum. The C-H stretch frequency of 3280 cm-l is shifted by 140 cm-' to higher frequency, and the C-H bend is shifted approximately 160 cm-' to lower energy compared to the analogous modes in solid HCN. There is better agreement between the vibrational frequencies measured for H C N in an Ar matrix24 and the a state on the W(lO0)-(5x1)-C surface. The 6(C-H) is 723 cm-', the u(C-N) = 2032 cm-I, and the u(C-H) = 3305 cm-I for matrix-isolated HCN, suggesting that the lack of correspondence between the adsorbed a-HCN and solid HCN is due, in part, to intermolecular interactions in the condensed state. We note that vibrational data obtained for H C N multilayers on W(100)-(5Xl)-C (data not shown) are in good agreement with the previously reported solid-state spectra. Also, the vibrational frequencies of HCN coordinated linearly in organometallic cluster compounds are in close agreement with those observed here. In these compounds, v(CN) is approximately 21 30 cm-', v(CH) is (24) Milligan. D E.; Jacox, M. E. J . Chem. Phys. 1963. 39, 712
Serafin and Friend about 3200 cm-', and the C-H bend at 750 ~ m - ' . ~ ~ An alternative identity of the molecular species present at 200 K with C(1s) and N(1s) binding energies of 285.8 and 400.0 eV, respectively, is H N C arising from the isomerization of HCN. This proposal seems unlikely because H N C is by far the less thermodynamically stable isomer,25 although interaction with the surface may change the relative stabilities of these isomers. Previous studies of CH,NC on Pt( 11 and Ni( 11 1)27suggest that this is unlikely, however. In both cases, isomerization of the isonitrile to the nitrile was inferred from temperature-programmed reaction and vibrational measurements, suggesting that the relative stability of the two isomers is at least qualitatively the same on the surface and in the gas phase. If HNC, bound through the carbon atom, is present on the surface the shift in the binding energy of the C( Is) peak to lower binding energy compared to HCN is potentially explained: the screening of the isonitrile carbon will be increased due to the direct interaction with the metal substrate. The N(ls) binding energy of 400.0 eV is also generally consistent with the proposal. Unfortunately, there are no reports of N( Is) or C( 1s) binding energies for isonitriles to our knowledge, so that the binding energies observed here cannot be compared to model systems. If the presence of a surface H N C species is postulated, the modes at 680 and 3280 cm-l would be associated with the N-H bend and N-H stretch of the adsorbed HNC. respectively. The agreement between the vibrational data for matrix-isolated H N C and the a state prsent at 200 K on W(100)-(5Xl)-C is poor, however. The value of the N-H stretch is significantly lower than that observed for matrix-isolated HNC (3583 cm-') and the 6(N-H) of matrix isolated HNC is 535 cm-I, significantly lower than the value of 680 cm-' measured for the a state on W(lOO)-(5Xl)-C in this work. However, since HNC is extremely unstable, there are no vibrational data for H N C coordinated to metal centers or on surfaces. Thus, the assignment of the a state as H N C is not strongly supported by the spectroscopic data and is considered less likely to form than the N-bound H C N on the basis of their relative thermodynamic stability. Hence, we assign the a state as N-bound H C N . The second molecular species present on the surface at 200 K, which we designate the y state (Figure 5), has the highly shifted N(1s) and C(1s) binding energies of 397.8 and 283.5 eV, respectively, and is tentatively assigned to a surface C-N group bound parallel to the surface. The vibrational data show a mode at 1336 cm-I, which can be attributed to the greatly weakened C-N stretch of this species. The C-N bond vector of the strongly perturbed nitrile is proposed to be oriented parallel to the surface plane on the basis of the carbon K-edge near-edge X-ray absorption fine structure data obtained at 275 K, described in detail elsewhere.16 Alternatively, the strongly perturbed species present on the surface could be a highly perturbed $-HCN. However, the spectroscopic evidence favors the assignment of the second species as surface $-CN, as discussed below. First, the C(ls) binding energy of 283.5 eV is much lower than would be expected for an H C N $-bound species, since it is significantly shifted from that of the H C N H species present at 500 K. In both cases, the nitrile carbon is covalently bonded to both hydrogen and nitrogen, is strongly perturbed, and interacts directly with the surface. Thus, it is reasonable to assert that $-HCN and HCNH would have similar C(1s) binding energies. The presence of significant amounts of atomic carbon on the surface contributing to the intensity of the 283.5-eV peak can be ruled out since no detectable atomic nitrogen is found at this temperature as shown by the N(1s) X-ray photoelectron spectrum (Figure 2a). It should be noted that the value of the C( 1s) binding energy for adsorbed C N is not known on the W(lO0)-(5x1)-C surface or other transition-metal surfaces. We have measured the C( 1s) binding energy of a surface species derived from heating C,N2 to 550 K on W(100)-(5Xl)-C in an effort to characterize surface (25) Corain, 9.Coord. Chem. Rev. 1982, 4 7 , 165. (26) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sei. 1985, 2 2 / 2 3 , 384. (27) Friend, C. M.; Muetterties, E. L.; Gland, J. L. J . Phys. Chem. 1981, 85. 3256.
Nitrile Hydrogenation on W( 100)-(5Xl)-C C N groups on this surface. The C(1s) binding energy of this species is 285.3 eV?8 significantly higher than the energy of 283.5 eV measured for the species derived from HCN. Although it is reasonable to expect cyanogen to form surface C N groups under these conditions, the surface species formed after annealing to 550 K has not been identified. Cyanogen may form an oligimer on the surface instead of isolated C N groups, making a comparison of the binding energies impossible. Thus, the C( 1s) binding energy does not definitively support, but is consistent with the y state being v2-CN. The N(1s) binding energy of 398.1 eV is equally consistent with v2-HCN and q2-CN. Further evidence that the highly perturbed nitrile is q2-CN is based on the observation of only one C-H stretch at 3280 cm-I after annealing to 200 K. This frequency is reasonable for an sp hybridized carbon center and is attributed to the a - H C N state. It is substantially higher than that expected for q2-bound sp2 rehybridized HCN: the C-H stretch for this species is expected to be near 3000 cm-’.” Therefore, the observed C-H stretch is attributed to the end-on bound a-HCN, as discussed above. It is important to note that the absence of a C-H stretch at the frequency expected for q2-HCN is not conclusive evidence for the presence of v2-CN, since the cross section for this vibration is unknown. However, the cross sections and spectrometer sensitivity for a-HCN and an v2-HCN species are expected to be comparable. The C-H stretching motion in both species will have some component perpendicular to the surface since the H C N bond angle is expected to be bent in q2-HCN on the basis of comparison with organometallic analogs of ~ ~ - n i t r i l e sMoreover, .~~ the coverages of the two different species present at 200 K is expected to be comparable, since the relative intensities of the two peaks in the X-ray photoelectron data (Figures 2a and 3a) are approximately equal. For a C N group to be present on the surface at 200 K, atomic hydrogen must be present on the surface also. The temperature-programmed reaction data (Figure 1) show that there is a small amount of H2formation commencing at 300 K. Hydrogen atom recombination commences at 300 K on this surface.30 This indicates that atomic hydrogen from dehydrogenation of some H C N is present at temperatures below 300 K and is consistent with but does not establish that C-H bond breaking has occurred below 200 K. The observation of C-H bond cleavage in CH3CN on W( 100)-(SXl)-C at low temperature has been proposed on the basis of the results of isotopic exchange experiment^,^ lending plausibility to the proposed formation of C N and hydrogen atoms through C-H bond breaking on the surface at 200 K for HCN. In summary, the weight of the evidence is consistent with the presence of v2-bound C N on the surface at 200 K, although an q2-HCN species cannot be definitively ruled out. We further propose that the y-nitrile species (either v2-CN or v2-HCN) present at 200 K with the strongly perturbed C N bond reacts to form atomic nitrogen and carbon in the range 200-500 K. The weakened C-N bond is expected to have a lower barrier to C-N bond cleavage than the CY-HCNspecies, which nominally has a C-N triple bond. If the strongly perturbed nitrile is not q2-CN but v2-HCN, it may also be hydrogenated to form the H C N H species which is stable at 500 K. The nitrogen atom of an q2-HCN species would be susceptible to hydrogenation, on the basis of model electronic structure calculations.2 Hydrogenation of v2-CN to form H C N H is deemed unlikely because formation of both C-H and N-H bonds would be necessary. Since most of the dihydrogen is formed in the temperatureprogrammed reaction experiment at 650 K, well above the temperature required for hydrogen atom recombination on the W( 100)-(5Xl)-C surface, the kinetics for formation of most of the Hz must be limited by the rate of C-H and N-H bond scission in this temperature regime. Kinetics favoring N-H bond formation over combination to form H, in the 300-500 K range are ~~
(28) Serafin, J. G.; Friend, C. M., unpublished results. (29) Andrews, M. A.; Knobler, C. B.; Kaesz, H . D. J . Am. Chem. SOC. 1979, IOI. 7260.
(30) Benziger, J. B.; KO, E. I.; Madix, R. J. J . Curd. 1978, 54, 414.
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6699 reasonable since H2 does not readily dissociate on the (5x1)carbide surface, presumably due to a kinetic barrier for the process; thus the reverse process to form H2would also be expected to have a high kinetic barrier. At 800 K, total decomposition of H C N has taken place, indicated by the atomic binding energies for nitrogen and carbon not incorporated into the (5Xl)-carbide, 396.9 and 283.0 eV, respectively. This is consistent with all H-containing species, H2 and HCN, having desorbed from the surface in a temperatureprogrammed reaction experiment. Some fraction of the H C N H presumably irreversibly decomposes, contributing to the increase in the atomic nitrogen signal in the X-ray photoelectron spectrum. The maximum amount of atomic nitrogen on the surface after temperature-programmed reaction corresponds to approximately 0.5 monolayers of nitrogen, on the basis of comparison with the integrated N ( 1s) intensity from the c(2xZ)-N surface, measured but not presented in this work. The observation of C-N, C-H, and N-H bond breaking in the decomposition of H C N H in the temperature range 500-800 K shows that the kinetics for these processes are comparable on the W( 100)-(5Xl)-C surface. The barrier for C-N bond breaking in the H C N H intermediate is estimated to be in the range 25-35 kcal/mol, assuming first-order kinetics and a preexponential factor of lOI3 s-l. It is interesting and somewhat surprising that the C-N bond is not cleaved at lower temperature given that considerable bond weakening in H C N H is evident spectroscopically and that it is strongly bound to the surface. The reactions of H C N and CH$N on the W(lO0)-(5Xl)-C surface can be compared in terms of the general reactivity of nitriles. Both nitriles undergo N-H bond formation to make an intermediate with a rehybridized C-N bond. Likewise, both molecules are proposed to undergo low-temperature C-H but not C-N bond activation on the W(100)-(5Xl)-C surface. Previous studies have shown that at the temperature where acetonitrile forms the intermediate CH2CNH, 400 K, there is no evidence of the formation of atomic carbon and nitrogen on the surface, indicating that no C-N bond cleavage has taken place.g In contrast. at 500 K, the temperature where HCN forms the hydrogenated intermediate HCNH, C-N bond cleavage has clearly occurred: in fact, some C-N bond cleavage has occurred at 275 K. In HCN, some C-H bond cleavage yielding C-N must take place to provide the hydrogens needed for N-H bond formation. The C N group formed from dehydrogenation is proposed to readily dissociate below 500 K. However, for acetonitrile, C-H bond cleavage in the methyl group may occur without affecting the stability of the C-N bond greatly. The lower temperature at which the CH2CNH is formed may be due to the relative ease with which the molecule may dehydrogenate and rehydrogenate intramolecularly. There is also a difference in the relative stabilities of the intermediates formed, CH2CNH and HCNH, although the reason for the observed difference is not defined by this study: coverage, steric, and electronic effects may all play a role in determining reactivity. The reactivity and bonding of both nitriles can be rationalized by consideration of their electronic structures. As discussed in the Introduction, the 2a* orbital of H C N is approximately 4.5 eV higher in energy than the analogous orbital in CO. Thus, a N-bound nitrile is expected to be a relatively poor a-acceptor molecule, consistent with the observation of the weakly perturbed a-HCN state, which molecularly desorbs. Another important and obvious difference in H C N is that the presence of C-H bonds in the adsorbate make reaction channels involving hydrogenation and dehydrogenation available to HCN that are not possible for CO. Finally, the 2a* orbital density in both H C N and C O is localized on the carbon atom. In HCN, the carbon center does not interact with the metal surface in a simple end-on geometry, whereas in C O coordination always occurs via the carbon. Model electronic structure calculations suggest that the localization of orbital density favors direct bonding of both the nitrile carbon and nitrogen to the surface, which in turn favors formation of an N-H bond.* The bonding of the H C N H intermediate can also be rationalized on the basis of the 27r* orbital density localization:
6700
J . Phys. Chem. 1988, 92, 6700-6705
direct carbon-tungsten interaction is favored versus nitrogentungsten bonding. We note that the C-0 bond in CO adsorbed on W(l00)-(5Xl)-C is not strongly perturbed and C O quantitatively desorbs without dissociation.I0 Nitrogen-hydrogen bond formation has been demonstrated to occur in an adsorbed nitrile (C2N2)on Pt(l1 l ) , suggesting that the observed reaction may be somewhat general for adsorbed nitriles that are strongly p e r t ~ r b e d . ~ ]The ? ~ ~fact that N-H bond formation is observed on more than one metal surface is consistent with the above argument that the electronic structure of nitriles themselves, in particular the localization of orbital density on the nitrile carbon, is important in dictating their reactivity on metal surfaces. In contrast to the observed N-H bond formation on Pt and on W(100)-(5Xl)-C, hydrogenation of C N groups formed from cyanogen is proposed to result in C-H bond formation but not N-H bond formation on Pd(l1 l).33 We note, however, that N-H bond formation does occur in the reactions of CH3CN on Pd( 111): ethylamine is a volatile reaction product.34 Further, it is interesting to note that the reactions of nitriles on W(31) Kingsley, J. R.; Dahlgren, D.; Hemminger, J. C. Surf. Sci. 1984, 139, 417. ( 3 2 ) Lloyd, K. G.; Hemminger, J. C. Surf. Sci. 1987, 179, L6. (33) Kordesch, M. E.; Stenzel, W.; Conrad, H. J . Electron Spectrosc. Relat. Phenom. 1986, 39, 89. (34) Gentle, T. M.; Grassian, V. H. ; Klarup, D. G.; Muetterties, E. L. J . Am. Chem. SOC.1983, 105, 6166.
(100)-(5X I)-C are analogous to those on the Pt-group metals.
Conclusions X-ray photoelectron and vibrational spectroscopies have been used to characterize an H C N H intermediate formed via nitrogen-hydrogen bond formation in H C N on the W(lOO)-(5Xl)-C surface. At 500 K, N-H bond formation is observed in an HCNH intermediate with a greatly weakened C-N bond. Atomic carbon and atomic nitrogen are also formed from decomposition of HCN to provide hydrogen for N-H bond formation. HCNH is proposed to re-form gaseous H C N on further heating to 650 K and decompose to atomic carbon and nitrogen. At 200 K, two species are thought to be present on the surface, one an end-bound H C N which forms a-HCN at 450 K or forms the HCNH intermediate and a C N species (y state) thought to be bound parallel to the surface which decomposes to form atomic carbon and nitrogen. Comparison to studies of acetonitrile on the same surface that also evidence N-H bond formation shows general features of the bonding of nitriles to this surface.
Acknowledgment. This work was supported by a NSF-Presidential Young Investigator Award, No. CHE-8451307, and the Harvard Materials Research Laboratory, N S F DMR-80-20247. We thank R. J. Madix for the use of the high-resolution electron energy spectrometer. Registry No. H C N , 74-90-8; W(lO0)-(5x1)-C, 12070-12-1.
Investigatlon of Sillca-Supported Vanadium Oxide Catalysts. Preparation and Characterization by 51V NMR and X-ray Photoelectron Spectroscopy B. Taouk, M. Guelton,* J. Grimblot, and J. P. Bonnelle Laboratoire de Catalyse HitProgt?ne et Homogt?ne, CIA CNRS No. 402, UniversitZ des Sciences et Techniques de Lille Flandres-Artois, 59655 Villeneuve d'Ascq CZdex, France (Received: January 1 , 1988; In Final Form: March 31, 1988)
Characterization by slV NMR of the vanadium species adsorbed on silica-supported catalysts, prepared by impregnation of silica by solutions of ammonium vanadate at various pH values, has been followed from the initial step (impregnating solutions and wet samples) to the final step of preparation (calcined solids in dry air). At first, by use of the classical high-resolution NMR, the major species both in the impregnating solutions and on the wet solids have been identified for the different initial pH, and relative quantitative evaluationshave been performed. Preparations at pH 11 give a major proportion of tetrahedral V401t-species, while at pH 4 vanadium oxide precipitates on silica. Second, the observations by solid-state NMR of these dried samples have shown that an important content of tetrahedral species was deposited on the solids prepared with the pH 11 initial solution (with a vanadium loading corresponding to the "monolayer" coverage limit, as determined further by XPS for the calcined series of samples), while only octahedral vanadium entities are detected for lower pH values. These species exhibit only few distortions; thus no support effect was evidenced in these dried samples. However, after further dehydration of the samples by calcination in dry air, the tetrahedral species (pH 11 samples) are much more affected than the octahedral entities, due probably to the formation of V-0-Si bonds, and this observation could be in correlation with the existence of the "monolayer" coverage limit observed only with these samples. The rehydration treatment breaks these bonds and restores a NMR signal of tetrahedral species.
Introduction For several years, molybdenum and vanadium oxide based catalysts have gained appreciable interest because of their activity in selective oxidation of methane into methanol or formaldehyde or in the oxidative coupling of methane to product ethane or ethylene.' Therefore these new opportunities for the production of liquid fuels and chemicals from the high resources of natural gas need research effort, in particular to control the preparation better and to characterize the catalytic material. ( I ) See, for example: (a) Mendelovici, L.; Lunsford, J. H. J . Catal. 1985, 94, 37. (b) Zhen, K. J.; Khan, M. M.; Mak, C. H.; Lewis, K. B.; Somorjai, G. A. J . Catal. 1985, 94, 501.
0022-3654/88/2092-6700$01.50/0
Recently, we have investigated a series of Md3-SiO2 catalysts with variable Mo loadings: and the nature of the polymeric Mo species deposited on silica has been thoroughly discussed. Since the temperature range to obtain comparable conversions in selective methane oxidation is significantly lower when the V,05-Si02 system is used,lb it appears important to undertake new investigations of both preparation and characterization of vanadium based catalysts. The preparation methods of such catalysts generally consist either in grafting vanadium (or molybdenum) oxohalogenides on (2) Latef, A,; Elamrani, R.; Gengembre, L.; Aissi, C. F.; Kasztelan, S.; Barbaux, Y.;Guelton, M. 2. Phys. Chem. 1987, 152, 93.
0 1988 American Chemical Society
