J . Phys. Chem. 1988, 92, 471-476
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Scanning Kinetic Spectroscopy and Temperature-Programmed Desorption Studies of the Adsorption and Decomposition of HCN on the Ni( 111) Surface P. L. Hagans,+ I. Chorkendorff,*and J. T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: June 8, 1987)
The interaction of HCN with the Ni( 111) surface has been examined with scanning kinetic spectroscopy (SKS), temperature-programmed desorption (TPD), and Auger electron spectroscopy (AES) in the temperature range 84-1000 K. SKS was used to survey the HCN reaction channels selected as a function of temperature. Temperature regimes were identified where HCN underwent consumption, desorption, and reaction and where the Ni surface was poisoned by the adsorbed species. TPD and SKS experiments indicated that at low coverage HCN nearly completely decomposed to give N(a) + H(a) + C(a). At high coverage a second channel opens where HCN molecularly desorbs. Two HCN molecular desorption states were observed at 258 and 27 1 K which were represented by very narrow ( N 17 K fwhm) TPD peaks. HCN reactivity on Ni( 111) was observed to be intermediate between that observed by others for CH3CN which decomposes very little and CH,NC and (CN), which completely decompose. Interestingly, HCN reactivity was found to be similar to that of the saturated analogue, CH3NHz. Comparison with CH3NH2data obtained earlier in this laboratory and simple MO considerations suggest that HCN is bonded through the nitrogen and that back-donation from the metal to the unoccupied P* orbital may be occurring here.
Introduction to the surface as is the case of CH3CN on P t ( l 1 l).I' Indirect evidence substantiating the end on bonding configuration of As part of this laboratory's study of the interaction of small CN3CN is listed below: (1) molecular desorption with little molecules with transition-metal surfaces, the reactivity of H C N decomposition; (2) the presence of an ordered ~ ( 2 x 2 LEED ) has been examined in the temperature range 84-1000 K. H C N structure which is nearly impossible to rationalize with C=N (along with cyanogen, (CN),) is the least complicated molecule parallel to the s ~ r f a c e(3) ; ~ an increase in decomposition with the containing the carbon-nitrogen triple bond. Surprisingly, there presence of steps and kinks which should facilitate the approach are very few studies of this molecule on any single-crystal surface. of the CH3 group to the Ni surface at these sites.g Examples include Pt( 111)' and Pt( 110),z3Pd(100) and Pd(l1 l),One study has examined cyanogen, (CN),, adsorption on No studies on N i ( l l 1 ) or any other Ag( 1 and Cu( 11 l).' Ni( 111).8 As in the case of CH,NC, adsorption at 300 K proNi single-crystal surface have been reported to date. duced a strongly bound species which completely decomposed upon Several studies involving other WN-containing molecules on heating to N(a) and adsorbed carbon. An ordered (6x6) LEED Ni( 1 11) have been reported. Temperature-programmed destructure was observed at -400 K which could be accounted for sorption (TPD) studies exist for the isomeric pair C H 3 C N by the formation of cyanide trimers. (acetonitrile) and CH3NC (methyl isocyanide) on Ni( 111)&l0and The largest CSN-containing molecule studied on Ni( 11 1) to these molecules have been studied on a11 three low index Ni faces date has been tetracyanoethylene (TCNE), (CN)zC=C(CN)2.'2 . ~ evidence indicates and also on a stepped and a kinked s ~ r f a c e All As expected, this molecule chemisorbed at 300 K behaves very that CH3CN is weakly bound to Ni(ll1) while CH3NC is strongly much like (CN), in that only Nz is observed to desorb. No surface adsorbed. For the case of room temperature adsorption of carbon, however, was found after the completion of the TPD CH3CN, >98% desorbs as the molecular species at around -360 K while only 1-2% decomposes (as measured by H2prod~ction).~ experiments. One example exists where H C N desorption from Ni( 111) was CH3CN is also easily displaced by CO at 300 K.* Adsorption studied, except that the H C N was a product of the decomposition at 100 K produces another molecular CH3CN desorption state of CH3N02.13 Other major decomposition products include Hz at 270 K and some H2 (the percentage that decomposes is not which desorbed along with H C N at -370 K, C O and CO, at known in this case).I0 N o H C N was ever detected. Just the 675 K, and Nz at 800 K. Approximately 0.2 monolayer of adopposite behavior is observed for CH3NC on Ni(ll1). CH3NC sorbed oxygen was left on the N i surface at 1000 K. does not desorb intact; only the decomposition products H, and N, are observed to desorb at -380 and 800 K, r e s p e c t i ~ e l y . ~ ~ ~ In light of the above results a reactivity study of H C N with the Ni(ll1) surface should be quite informative in comparing the C O does not displace this molecule at 300 K.* Molecular deof the C N group in different molecules. H C N was an reactivity sorption is observed at -300, 380, and 420 K after adsorption obvious choice in order to bridge the gap between the CH3CN at 100 K.Io Also, H C N is observed as an additional decomposition product. (1) Somers, J. S.; Bridge, M. E. Surf. Sci. 1985, 159, L439. HREELS studies as a function of coverage and temperature (2) Bridge, M. E.; Lambert, R. M. J. Catal. 1977, 46, 143. were also performed for CH3CN and C H 3 N C adsorbed on Ni( 3 ) Bridge, M. E.; Marbrow, R. A,; Lambert, R. M. Surf. Sci. 1976,57, (11 l).lo Although indirect evidence suggests that, for the case 415. of CH3NC, the NEC group may be more or less parallel to the (4) Kordesch, M. E.; Stenzel, W.; Conrad, H. J . Electron. Spectrosc. Relat. Phenom. 1986, 39, 89. surface (so that the CH3 group interacts closely enough with the ( 5 ) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1986,175, L687. surface for decomposition to occur), the HREELS data cannot ( 6 ) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci., submitted for differentiate between this structure and an end-on bound species publication. through the carbon.1° On the other hand, HREELS measurements (7) Solymosi, F.;Berko, A. Surf. Sci. 1982, 122, 275. ( 8 ) Hemminger, J. C.; Muetterties, E. L.; Somorjai, G.A. J. Am. Chem. indicate that CH3CN is end-on bound through the nitrogen and SOC.1919, 101, 62. that the nitrogen is coordinated to several Ni atoms.1° This has (9) Friend, C. M.: Stein, J.; Muetterties, E. L. J . Am. Chem. SOC.1981. been disputed in a later paper where the authors believe that 103, 3256. (IO) Friend, C. M.; Muetterties, E. L.; Gland, J. L. J . Phys. Chem. 1981, reinterpretation of the results indicates the C=N group is parallel ~~~~
85, 3256.
'Present address: Dow Chemical Co., Midland, MI 48674. *Present address: Fysik Institut, Odense Universitet. 5230 Odense M, Denmark.
0022-3654/88/2092-0471$01.50/0
(11) Sexton, B. A.; Avery, N. R. Surf. Sci. 1983, 129, 21. (12) Pan, F.-M.; Hemminger, J. C.; Ushicda, S . J . Phys. Chem. 1985.89, 862. (13) Benziger, J. B. Appl. Surf. Sci. 1984, 17, 309.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
and (CN), studies where the influence of various groups attached to C N on C N surface chemistry could be assessed. The study of H C N reactivity on Ni( 111) is a useful extension of a previous study in this laboratory, where the adsorption of the saturated analogue of HCN, CH3NH, on Ni( 111) was examined.14 Experimental Section A . Apparatus and Crystal Preparation. Since the ultrahigh-vacuum (UHV) system as well as the experimental technique used has been described in detail else~here,'~-''only a brief description will be given here. A Ni(ll1) single crystal (7 mm in diameter) was cut, oriented, and polished to within 0.5' of the (1 11) direction. It was spotwelded to a tungsten wire for resistive heating and the temperature was measured by a chromel-alumel thermocouple spotwelded to the rear side of the crystal. Initially small amounts of carbon were removed by oxygen treatments as described in ref 18 after which the crystal was easily cleaned by Ar+ ion sputtering ( E P= 800 eV) and subsequent annealing to T = 950 K. The crystal was then judged to be clean by Auger electron spectroscopy. Upper limits of C, 0,and S contamination on the clean crystal are 0.14, 0.07, and 0.04 atom %, respectively, in the sampling depth of the Auger measurement. The H C N was admitted through a calibrated doser consisting of a limiting conductance and a microchannel plate collimator whereby accurately known, uniform and collimated gas fluxes could be ~ b t a i n e d . ' ~The doser was mounted 60' off the surface normal of the crystal when the crystal was positioned in front of the multiplexed quadrupole mass spectrometer (QMS). The QMS was mounted inside a differentially pumped random flux shield wiht an aperture 1.6 mm in diameter, located 1-7 mm from the crystal surface. With this arrangement the crystal could be dosed for either SKS (scanning kinetic spectroscopy) measurements16 where the crystal temperature is programmed upward in the beam of HCN, or for TPD measurements in vacuum. For the TPD experiments the crystal was moved after dosing to within 1-2 mm of the random flux shield aperture before the temperature ramp was applied. Heating rates were always 2 K/s. In most cases after each experiment the crystal was sputtered for 10 minutes (Ar', 2 PA, 800 eV) at -450 K and then annealed for 10 min at lo00 K. B. HCN Preparation and Purification. The H C N was prepared in an all-glass diffusion-pumped vacuum system with a base pressure of
