J. Phys. Chem. 1986, 90,4341-4343
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Alteration in Surface Reaction Selectivity of Nitrlles. 1. HCN on W(100) and W(l00)-(5X1)-C K. A. Pearlstine and C. M. Friend* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received: February 14, 1986)
The adsorption and reaction of HCN has been investigated on W(100) and W(lO0)-(5x1)-C surfaces under ultrahigh vacuum conditions by temperature-programmed reaction spectroscopy in conjunction with isotopic labeling methods. Irreversible C-H bond scission is the primary reaction pathway on clean W(100) with decomposition occurring below 325 K, producing gaseous molecular hydrogen and nitrogen from atom recombination. These results establish an upper bound for the activation energy for C-H bond scission of 17 kcal/mol. The extent of irreversibleC-H bond cleavagewas limited on the W(lO0)-(5x1)-C surface resulting in desorption of a significant amount of molecular HCN in two peaks centered at 450 and 650 K, corresponding to approximate desorption energies of 27 and 39 kcal/mol, respectively. In addition, HCN decomposition occurred at temperatures greater than 450 K, indicating an increased activation energy for irreversible C-H bond cleavage of greater than 20 kcal/mol on the -(5Xl)-C vs. the clean W(100) surface.
Introduction In this work, the adsorption and reaction of H C N has been investigated on W(100) and W(lO0)-(5x1)-C surfaces. This is a companion study to investigations of alkyl nitriles and amines. An understanding of HCN reactivity on tungsten surfaces of varying composition is essential to understanding alteration of reactivity and selectivity in the more complex systems. In addition, H C N is a simple prototype for a molecule with C-H and C-N bonds, lending itself to more detailed characterization. H C N is also produced from reaction of acetonitrile on these surfaces and is a possible surface intermediate in the reaction of methylamines, making its study important for a general understanding of these reactions. Changes in reaction selectivity toward both nitriles and amines are observed on the -(5Xl)-C vs. clean W(100) ~ u r f a c e . ~ In this work, the stability of molecularly adsorbed HCN toward irreversible C-H bond scission is found to be increased by more than 20 kcal/mol on the -(5X 1)-C surface compared to clean W( 100). Additionally, the selectivity for molecular desorption vs. decomposition is also significantly greater on the -(5Xl)-C vs. clean W(100). H C N is synthesized industrially from CH4 and NH3 over a platinum-based catalyst. Polymerization of HCN is an undesirable side reaction in this synthesis.'*2 There was no evidence found for surface polymerization of H C N on clean W(100) or W(100)-(5Xl)-C in this work. The polymerization is known to be a base-catalyzed reaction. Understanding the reaction chemistry of HCN on surfaces of varied geometric and electronic structure will yield insight into which surface properties control reaction selectivity and polymerization. Experimental Section All experiments were performed in an ultrahigh vacuum system with a working base pressure of 2 X Torr, described prev i o ~ s l y . ~The W( 100) crystal was cleaned by cycles of oxygen treatment at =lo-' Torr pressure and 1400 K crystal temperature and subsequent flashing under vacuum to 2500 K. The W ( 100)-(5Xl)-C surface was prepared by dosing 60 langmuirs of ethylene on clean W(100) maintained at a temperature of 1500 K. Surface order and cleanliness were monitored by low-energy electron diffraction and retarding field Auger electron spectroscopy, respectively, for both the clean and carbide surface. The HCN was obtained from Fumico Chemical Co. (97% purity) and vacuum distilled to remove ammonia impurities. The (1) Hasenberg, D.; Schmidt, L. D. J. Caral. 1985, 91, 116. (2) Jenks, W. R. Kirk-othmer Encyclopedia of Chemical Technology; Wiley: New York, 1979; Vol. 1, pp 307-319. (3) Pearlstine, K. A.; Friend, C. M. J. Am. Chem. Soc., in press. (4) Baldwin, E. K.; Friend, C. M. J . Phys. Chem. 1985, 89, 2576.
reactants were freshly admitted to a directed dosing system with directed flux immediately prior to each experiment. The mass spectra of the gases admitted to the vacuum system were monitored in order to verify their purity. Temperature-programmed reaction data were obtained with an IBM PC. Two modes of data collection were possible: one allowing for monitoring of up to eight discrete masses in a single experiment or, alternatively, a wide mass spectral search of 100 masses in a given desorption experiment5 The broad mass range program is used as a search for reaction products and yields data with degraded temperature resolution. All data presented in the figures were obtained by monitoring a limited set of discrete masses. Radiative heating was used in all temperature-programmed reaction spectra up to ~ 7 0 K 0 presented herein with constant heating rates in the range of 12-20 K s-I. Experiments utilizing electron bombardment heating (dT/dt = 20-60 K s-I) were also performed in order to desorb molecular nitrogen via recombination and to search for residual polymerization products on the surface. The crystal was biased positively at 150 V with respect to ground in order to effect electron bombardment heating. Dosing was performed with directed beam dosers which allowed maintenance of the chamber pressure below 2 X Torr during adsorption, minimizing background reaction and pumpdown time. Temperature-programmed reaction data were obtained with a quadrupole mass spectrometer enclosed in a cryogenic shield equipped with a rotatable front with an orifice of 0.0625-in. diameter. The crystal was positioned 0.12 in. from the mass spectrometer orifice, resulting in selective detection of molecules desorbed directly from the crystal. This configuration precludes background reaction or desorption from contributing to the observed chemistry. Results and Discussion W(100). Thermal reaction of H C N on clean W(100) adsorbed a t 120 K results in desorption of gaseous HCN, HZ,and N, as depicted in Figure 1. Initial adsorption of H C N at exposures of 5 1.5 langmuirs result in irreversible dehydrogenation below 325 K, yielding molecular hydrogen from 300 to 700 K and nitrogen from 900 to 1400 K and residual carbon on the surface. Higher H C N exposures (>2.0 langmuirs) yield reversible desorption of H C N in a broad peak centered at 600 K. Exposures greater than 2.5 langmuirs result in condensation of H C N as evidenced by the 150 K desorption peak (Figure 1) and the lack of additional intensity in the desorption spectra of the decomposition products, Hz and Nz. H C N does not undergo any H-D exchange with coadsorbed atomic deuterium in either the 600 or 150 K desorption peaks, indicating that the HCN that is desorbed (5) Liu, A. C.; Friend, C. M. Reu. Sci. Insfrum., in press.
0022-3654/86/~090-4341$01.50/0 0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 18, 1986
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Figure 1. Desorption of H C N (27 amu), hydrogen (2 amu), and nitrogen (28 amu) from reaction of H C N on W(la0). The exposure of hydrogen cyanide was 3.3 langmuir at a surface temperature of 120 K. Radiative heating at a rate of 15 K/s was used to effect thermal desorption in the range of 120-700 K. Electron bombardment heating was used for desorption of nitrogen. This heating rate was approximately 20 K/s, but it was highly nonlinear.
is molecularly bound to the surface.6 Deuterium was observed in all regions of the molecular hydrogen desorption in these experiments, suggesting that the unresolved desorption involves equilibration with surface hydrogen. No new LEED features are observed subsequent to H C N adsorption. No additional desorption products were observed from H C N reaction on W ( 100) with either radiative heating or electron bombardment with all masses monitored in the range of 2-80 amuS5 No residual nitrogen was observed with Auger electron spectroscopy subsequent to heating to 1500 K. These data combined with the absence of molecular hydrogen desorption at high temperature rule out the presence of a high-temperature surface polymer of HCN. The results described indicate that hydrogen cyanide undergoes quantitative irreversible C-H bond scission below 325 K for expasures less than 1.O langmuir, accompanied by C-N bond scission and subsequent nitrogen atom recombination at higher temperature. This molecular hydrogen production is desorption-limited, setting an approximate upper bound on the activation energy for HCN decomposition of 19 kcal/mol when first-order kinetics and a preexponential factor of 1013s-I are assumed.* Some molecularly bound H C N which desorbs near 600 K is produced at exposures greater than 2.0 langmuirs. The presence of molecular HCN on the surface at temperatures lower than 600 K is likely to be dependent on the "passivation" of the surface by the initial decomposition processes. The passivation may be the result of saturation of hydrogen atom coverage, coverage-dependent adsorbate-induced reconstruction of the surface,' or modification of the surface electronic structure, and it is analogous to the reactivity associated with W( 100)-(5x1)-C, described below. W(lO0)-(5x1)-C. The W(lO0)-(5x1)-C surface has been previously ~haracterized."-'~ The proposed structure of the (6) Adsorbed atomic hydrogen (deuterium) is known to induce surface reconstr~ction;~ therefore, the surface structure may be perturbed by the D preadsorption experiments. When deuterium was preadsorbed, no high-temperature HCN desorption was observed. Adsorption of deuterium subsequent to HCN did not alter the desorption spectra. (7) Horlacher Smith, A,; Chung, J. W.; Estrup, P. J. J . VUC.Sci. Techno!. A 1984, 2, 877. (8) A preexponential factor of 1013 s-I, a typical vibrational period, is assumed as a means of crudely estimating the activation energy for a firstorder proce~s.~ This estimation may be significantly in error as demonstrated for CO adsorbed on R ~ ( 0 0 0 1 ) .Thus, ~ ~ the estimate of the activation energy for decomposition is intended only as a crude gauge of energetics. (9) King, D. A. Sur/: Sci. 1975, 47, 384. (10) Pfntir, H.; Feulner, P.; Menzel, D. J. Chem. Phys. 1983, 79, 4613. (11) Benziger, J. 9.; KO, E. I.; Madix, R. J. J. Cutul. 1978, 54, 414. (12) Rawlings, K. J.; Foulias, S. D.; Hopkins, 9. J. J . Phys. C 1981, 14, 541 1
Figure 2. A model of the W(100)-(5Xl)-C surface is depicted. The dark circles represent carbon atoms which are proposed to lie beneath the topmost tungsten layer. The structure shown is that proposed previously.'l
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Figure 3. Desorption of hydrogen cyanide (27 a m ) and reaction products, hydrogen (2 amu) and nitrogen (28 amu), from W(100)-(5Xl)-C. Thermal desorption data were generated by using radiative heating (dT/dt = 15 K/s) in the range of 120-700 K and electron and electron bombardment heating (dT/dt = 20 K/s) above 700 K. The data shown is for 2.6-langmuir exposure of HCN.
-(5X 1)-C overlayer requires surface tungsten atom reconstruction into a hexagonal array and incorporation of carbon atoms into the lattice below the surface layer, resulting in a structure similar to the basal plane of WC (Figure 2). Photon-energy-dependent ultraviolet photoemission datal3 comparing WC(OOO1) and W( 100)-(5X 1)-C demonstrate that the valence band structure of the two surfaces is virtually identical, lending support to the model. Thermal reaction of H C N adsorbed on the W(100)-(5Xl)-C surface results in competing decomposition and desorption (Figure 3). The stability of molecularly adsorbed H C N and the extent of irreversible decomposition are dramatically different particularly at low exposures in comparison to clean W(100). A limited amount of irreversible decomposition occurs above 600 K, producing gaseous H2 and N 2 a t 650 and 1290 K, respectively, for exposures C0.25 langmuir. Desorption-limited H, production from atom recombination occurs at 400 K on the -(5Xl)-C surface.I4 Higher exposures result in reversible desorption of HCN first at (13) Stefan, P.; Spicer, W. E. Surf. Sci. 1985, 149, 423. (14) Only small amounts of atomic deuterium could be adsorbed on the due W(l00)-(5x1)-C surface subsequent to prolonged exposure to gaseous D2, to a previously reported low dissociative sticking probability of molecular hydrogen and its isotopes." Attempts to atomize D2with a hot filament resulted in significant levels of CO and H20contamination as well as D atoms, precluding careful study.
H C N on W(100) and W(100)-(5Xl)-C =450 K and subsequently at 650 K (Figure 3). The 650 K H C N desorption coincides with reaction-limited molecular hydrogen production, consistent with competing H C N desorption and decompo~ition.'~Irreversible decomposition and H C N desorption compete a t exposures above 0.25 langmuir, evident in that the integrated intensities of the H2 and N2 desorptions increase up to 1.0-langmuir H C N exposure. The amount of decomposition a t saturation, as measured by the total amount of molecular hydrogen desorption, is approximately a factor of two smaller on the -(5Xl)-C compared to W(100) (Figures 1 and 3). Sublimation of condensed H C N at 150 K was also observed for exposures greater than 1.O langmuir. Exchange with preadsorbed deuterium atoms was not examined due to the low dissociative sticking probability for H2 or D2 on the -(5Xl)-C surface.I4 N o other desorption products were observed from H C N reaction on the -(5Xl)-C surface: all masses between 2 and 80 amu were searched for reaction products5 by using both radiative and electron bombardment heating. N o residual nitrogen was observed on the -(5Xl)-C surface subsequent to heating to 1500 K by Auger electron spectroscopy. Thus, high-temperature surface polymerization is not operative on the -(5XI)-C surface. The energetics and selectivity for irreversible C-H bond activation of HCN is substantially different on W( 100)-(5x1)-C than clean W(100) as evidenced by the higher decomposition temperature and the substantial fraction of HCN that molecularly desorbs from the surface. The activation energy for irreversible C-H bond activation is estimated to be approximately 40 kcal/mol, assuming first-order kinetics and a preexponential factor of lOI3 s-l.* Comparison with the upper bound for the E, for C-H bond scission on initially clean W(100) shows that adsorbed H C N is a minimum of 20 kcal/mol more stable with respect to irreversible C-H bond cleavage on W(lOO)-(5Xl)-C than on clean W(100). As on the clean surface, there is no evidence for the existence of a highly stable surface polymer on W( 100)-(5Xl)-C. Comparison of the desorption energetics of H C N and CO" on the W( 100)-(5X 1)-C surface evidences a rather counterintuitive result. Molecular carbon monoxide desorption occurs on the -(5Xl)-C surface with a peak temperature of 360 K corresponding to a desorption energy of 19.6 kcal/mol. This is considerably less than the estimated desorption energy of the HCN adsorption state having a desorption energy of 27 kcallmol. This is rather surprising in that the energy of the 2s* orbital in CO is calculated to be =4.5-eV lower in energy than the analogous orbitals in HCN.I5 Thus, the bond strength of these two homologues cannot be dictated by donation of electron density from the metal to the adsorbate 21r*. It is possible that the ability of the adsorbate to donate electron density to the metal controls the binding energy to the surface. Comparison of the energies of the 1 s and 5a (15) Jorgensen,W. L.;Salem, L. The Organic Chemists Book of Orbitals; Academic Press: New York, 1973; pp 76-78. (16) Kimura, K. et al. Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules; Halsted Press: New York, 1981.
The Journal of Physical Chemistry, Vol. 90, No. 18, 1986 4343 orbitals in CO with analogous orbitals in H C N support this assertion. The Sa orbital energy is measured to be -14.01 and -14.0 eV for CO and HCN, respectively, whereas the 11r energies are -16.91 and -13.60 eV for CO and HCN. Calculations are necessary to place this concept on a more sound basis. The alteration in the extent and energetics required for irreversible bond scission is analogous to trends observed in reactions of amine^,^ nitriles," and alkenes.ls The generality of this trend suggests that the energetics and the saturation coverage of atomic adsorbates produced from reaction may be directing the observed chemical changes. In particular, atomic hydrogen adsorption may be important in that its saturation coverage on the -(5X 1)-C surface is 3 X 1013 molecules/cm2 compared to 7.4 X lOI4 molecules/cm2 for clean W( IOO)."
Conclusions A dramatic change in the selectivity for irreversible C-H bond scission vs. desorption of molecular H C N induced by the presence of a carbide overlayer on W(100) has been identified. This selectivity change is, in part, a result of the increase in activation barrier for irreversible C-H bond cleavage on the -(5Xl)-C vs. clean W( 100) surface. Initial HCN decomposition passivates the W( 100) surface such that it exhibits reactivity more closely resembling the carbide. N o evidence for polymerization of H C N was obtained on either surface studied. Analogous alteration in the energetics of irreversible C-H bond cleavage has been identified for acetonitrile and simple alkylamines, suggesting a general trend in reactivity3 and suggesting that adsorption properties of atoms produced from reaction of the molecule on the surface control the observed chemical changes. Molecular H C N is bound more strongly to the W(100)-(5X 1)-C surface than its homologue CO. This result suggests that the ability of these molecules to donate electron density to the surface dictates bond energy to the surface rather than the ability of CO and HCN to accept electron density into their 21r* orbitals.
Acknowledgment. C.M.F. acknowledges support from an IBM faculty development award (1983-85) and an N S F Presidential Young Investigator Award (CHE-8451307). This work was supported, in part, by Cottrell Grant No. 9787 and the Harvard Materials Research Laboratory (NSF DMF-80-20247). This work also received support from the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. HCN, 74-90-8; W, 7440-33-7; C, 7440-44-0. (17) Pearlstine, K. A,; Friend, C. M. J . Phys. Chem., following article in this issue. (18) Pearlstine, K. A.; Friend, C. M. J . Vac. Sci. Technol., A 1984, 2, 1021. (19) Note added in proof: Recent vibrational studies evidence N-H bond formation to produce HC=NH(a) formation at 500 K.m Nitrogen-hydrogen bond scission is presumably operative at 650 K to regenerate HCN. (20) Serafin, J. G.; Friend, C. M., to be published.
