2206
J. Phys. Chem. 1996, 100, 2206-2212
Cyclohexylamine Adsorption and Hydrogenolysis on the Ni(100) Surface Sean X. Huang† and John L. Gland* Chemistry Department, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: July 3, 1995; In Final Form: October 26, 1995X
Adsorption and hydrogenolysis of cyclohexylamine (CHA) on the Ni(100) surface have been characterized using temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). Cyclohexylamine interacts with the surface primarily through both the amino group and one carbon in the cyclohexyl group below 240 K. Substantial hydrogenolysis of adsorbed CHA is observed even in the absence of external hydrogen as the result of disproportionation to form ammonia (365 K) and benzene (465-480 K) during a TPD cycle. In the absence of external hydrogen, dehydrogenated residues are also observed above 500 K. External hydrogen pressures rapidly enhance the hydrogenolysis of CHA resulting in complete hydrogenolysis of a monolayer of adsorbed CHA for hydrogen pressures as low as 10-5 Torr with no residual carbon or nitrogen remaining on the surface.
Introduction Substituted cyclohexylamines are generally believed to be an important class of intermediates during catalytic hydrodenitrogenation (HDN).1,2 Cyclohexylamine (CHA) derivatives are often observed during HDN of large aromatic heterocyclic nitrogen compounds because hydrogenation and ring opening often precedes C-N bond activation.3,4 For example, during HDN of quinoline on Ni-Mo catalysts, o-propylcyclohexylamine is found to be the main intermediate leading to nitrogen removal.5 Compared to aniline derivatives, these substituted CHAs are much more susceptible to C-N bond cleavage via β-elimination (to form unsaturated hydrocarbons) or hydrogen displacement (to form saturated hydrocarbons), both removing the -NH2 group as NH3(g). We recently characterized aniline HDN on the Ni(100) surface and found that the C-N bond cleavage in aniline is preceded and facilitated by partial ring saturation.6 Since complete saturation of aniline yields CHA, we have undertaken this study of CHA HDN to explore the mechanism of HDN is more detail on the Ni(100) surface. The surface intermediates, reaction temperatures, and reaction products observed during CHA HDN are similar to those observed during aniline HDN, indicating that CHA may be a key reaction intermediate during aniline HDN on the Ni(100) surface. Very little information is available regarding the adsorption and surface reactions of CHA on transition metal surfaces. However, comparisons with other aliphatic amines suggest that the primary interaction mode will involve electron transfer from the amino group into the nickel as observed for methylamine.7,8 Comparisons with bonding and reactions of cyclohexane and cyclohexene on the Ni(100) surface suggest that dehydrogenation to form benzene may occur with increasing temperature.9-11 On Al2O3 and Fe2O3 surfaces, thermal decomposition of CHA results in the production of NH3, cyclohexene, and aniline in the 500-600 K temperature range.12 Clearly, disproportionation results in both hydrogenolysis and dehydrogenation on these oxide surfaces. The surface reactions and adsorbed state of cyclohexylamine on the Ni(100) surface have been characterized using multiplexed mass spectrometry during TPR experiments both in the † Current address: Automotive Technical Center, The Sherwin-Williams Co. 10909 South Cottage Grove Av., Chicago, IL 60628. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2206$12.00/0
presence and absence of external H2 pressures up to ∼10-6 Torr. The bonding and chemical states of adsorbed CHA and CHAderived species have been characterized using XPS. Experimental Section The surface characterization experiments were carried out in two stainless steel ultrahigh vacuum (UHV) chambers both with base pressures lower than 2 × 10-10 Torr. The TPRS experiments were performed at the University of Michigan. The XPS experiments were performed in the Chemistry Department of Brookhaven National Laboratory. The Ni(100) sample could be resistively heated to 1200 K, and cooled with liquid nitrogen to ∼100 K. Molecular CHA is adsorbed at 100 K by background dosing. An exposure of 1.0 langmuir (1 × 10-6 Torr for 1 s) of CHA produces a surface coverage of ∼1.0 ML (ML ) monolayer) on the Ni(100) surface, based on the appearance of the multilayer ice peak. During temperatureprogrammed reactions, linear heating rates of 5 K/s were generated using a programmed power supply. The sample temperatures were measured either by a K-type thermocouple (chromel/alumel) or by a W-5% Re/W-26% Re thermocouple. The Ni(100) crystal was cleaned by a combination of Ar+ sputtering (750 eV, 2µA) and annealing (1175 K). The XPS experiments were performed with AlKR radiation (1484.6eV) and an 150 mm hemispherical analyzer equipped with a multichannel detector. Results and Discussion Temperature-Programmed Reaction Studies. TPRS Results. A temperature-programmed reaction spectrum for the primary desorbing products of a saturated cyclohexylamine monolayer adsorbed at 100 K reveals that substantial hydrodenitrogenation occurs even in the absence of external hydrogen (Figure 1). The primary desorbing species are benzene (C6H6, m/e ) 78) at 470 K, ammonia (NH3, m/e ) 17) at 365 K, molecular cyclohexylamine (C6H5NH2, m/e ) 43) at 200 and 335 K, and hydrogen (H2, m/e ) 2) in the 320-700 K temperature range. No cyclohexane or cyclohexene resulting from surface reactions is detected. The masses presented were chosen to represent the primary products with minimum interference. Mass 43 is not the largest CHA fragment; however, it is relative free from interfering features which complicate the other major fragmentation peaks for CHA. © 1996 American Chemical Society
CHA Adsorption and Hydrogenolysis on the Ni(100) Surface
J. Phys. Chem., Vol. 100, No. 6, 1996 2207
Figure 1. TPR spectra of surface reaction products for 1.0 langmuir exposure of cyclohexylamine on the Ni(100) surface. The heating rate is 5 K/s. A 1 langmuir CHA dosage results in ∼1 ML CHA coverage.
Molecular CHA desorption is observed at 200 K. A second molecular C6H11NH2 peak can be observed at 335 K for CHA exposures above 0.5 langmuir. Multiple overlapping H2 desorption peaks are observed over a broad temperature range from 320 to 700 K, with the largest peak at 350 K. Other than undissociated cyclocexylamine at 200 K, the only desorbing species that contain carbon and nitrogen are ammonia and benzene. Benzene (m/e ) 78) desorption occurs in the 465480 K temperature range, depending on th initial surface coverage of CHA. Ammonia desorbs from the surface between 350 and 365 K. The background and structure below 300 K in the mass 17 spectra results from fragmentation of background water and is not associated with CHA reactions on the Ni(100) surface. The background water is substantial because of extensive use of hydrogen flow and background CHA dosing. Temperature-programmed spectra taken at m/e ) 16 where water fragmentation intensity is much smaller confirm that the desorption peak at ∼365 K is ammonia. The 200 K peaks in m/e ) 84 and m/e ) 17 remain fairly constant in size (∼0.1 equivalent monolayers) for a wide range of CHA exposures and appear to be associated with a small amount of cyclohexane and ammonia contamination from the background associated with background dosing of CHA. Molecular cyclohexane desorbs molecularly without dehydrogenation at 200 and does not interfere with these experiments. Ammonia also desorbs moleculary without reaction at 200 K. C-N bond cleavage in chemisorbed CHA clearly occurs at 365 K as indicated by the reaction limited desorption of ammonia (Figure 1). Hydrogen addition to the C-N bonds must produce NH3(g) and C6H11-x (x ) 0-4)(ad) in this temperature range. The only carbon-containing product in this reaction, benzene, does not desorb until ∼470 K, 110 K higher than the ammonia desorption temperature. Benzene’s desorption temperature is similar to that observed for benzene desorption following benzene adsorption on the Ni(100) surface.13 This desorption-limited benzene appears to be formed by the dehydrogenation of an originally hydrogen-rich carbonaceous inter-
Figure 2. TPR coverage sets of benzene, ammonia, hydrogen, and molecular desorption from the reaction of adsorbed cyclohexylamine on the Ni(100) surface. A 1.0 langmuir dosage of CHA correlates to ∼1 ML CHA coverage on the surface. Water background at m/e ) 17 has been substrated to give the ammonia spectra shown.
mediate which has undergone hydrodenitrogenation. Apparently, these cyclic C6H11-x species themselves cannot desorb from the surface when formed, since neither cyclohexane nor cyclohexene is observed. With increasing temperature the surface becomes more and more hydrogen deficient as indicated by the evolution of reaction products from CHA disproportionation (Figure 1). Initially, substantial hydrogen desorption (starting at 320 K with a peak at 350 K) reduces the hydrogen content of the surface. Next, hydrogen-rich ammonia is formed and desorbs at 365 K. The desorbing benzene at 470 K clearly indicates reduced hydrogen availability since dehydrogenation of CHA derived C6H11-x species is clearly occurring. With further increases in temperature above 500 K, complete dehydrogenation and decomposition of CHA-derived adsorbed species produce residual C and N on the surface. Carbide is observed with AES after heating chemisorbed CHA to 900 K. A quantitative analysis of the nonvolatile CHA-derived C- and N-containing surface species based on XPS will be presented in a later section. A detailed coverage set for molecular CHA, benzene, ammonia, and hydrogen is shown in Figure 2. For an initial CHA exposure of 0.1 langmuir, complete dehydrogenation dominates. No molecular CHA, benzene, or ammonia desorption is observed; instead, dehydrogenation results in H2 formation. With increasing coverage, both benzene and ammonia formation increase for exposures up to 1.0 langmuir. The benzene desorption temperature decreases with increasing CHA exposures, from 480 K (0.2 langmuir) to 465 K (1.0 langmuir) and remains at 465 K for CHA exposures larger than 1.0 langmuir. For exposures below 0.5 langmuir of CHA, all the CHA in the adsorbed monolayer undergoes bond-breaking
2208 J. Phys. Chem., Vol. 100, No. 6, 1996 processes to form ammonia, benzene, and adsorbed C and N residues. The molecular CHA peak at 335 K grows rapidly with increasing coverage for exposures above 0.5 langmuir, while the increase in the ammonia and benzene peaks is quite limited (Figure 2). Increasing the CHA coverage by a factor of 2 from 0.5 to 1.0 ML increases the yield of benzene only by a factor of ∼1.4. The limited increase in benzene formation for CHA coverages above 0.5 ML is correlated with increased molecular desorption at 335 K and increased surface crowding. The chemisorbed CHA peak at 335 K saturates for an exposure of 1.0 langmuir, indicating completion of the first monolayer. Above 1.0 langmuir exposure, the CHA multilayer peak at 200 K also begins to increase rapidly, confirming saturation of the monolayer. Hydrogen desorption (lower panel, Figure 2) is observed at 350, 450, and 495 K for low CHA coverages, and also at 600 K for high CHA coverages. The H2 peak at 350 K becomes increasingly dominant with increasing CHA coverage. TPRS Discussion. Adsorption and reactions of cyclohexane (C6H12),10,11 cyclohexene (C6H10),9-11 cyclohexadiene (C6H8),10 and benzene (C6H6)10,11,13 have been studied on the Ni(100) surface. The behavior of these potential intermediates suggests that benzene is formed by hydrogen loss of a cyclic denitrogenated intermediate after C-N bond cleavage. Cyclohexane is not dehydrogenated but desorbs molecularly at 205 K on the Ni(100) surface.11 Both cyclohexene and cyclohexdiene are dehydrogenated to form benzene between 435 and 495 K. We believe that similar dehydrogenation mechanisms are involved in forming benzene from a CHA derived cyclic C6H11-x intermediate. Noticing that H2 desorption starts at 320 K and that cyclohexane would molecularly desorb well below the hydrogenolysis temperature, we suggest that the cyclic C6H11-x formed at 365 K is dehydrogenated. Cyclohexene and cyclohexadiene appear to model the CHA-derived surface intermediate after denitrogenation at 365 K. Above 365 K, chemisorbed cyclohexene can neither desorb molecularly9 nor disproportionate to form cyclohexadiene and cyclohexane.11 Tsai et al. has proposed that adsorbed cyclohexene readily forms benzene because the observed parallel configuration of the olefinic bond positions several C-H bonds close to the substrate.10 The reactivity patterns observed for adsorbed cyclohexene are very similar to those observed for CHA derived hydrocarbons. Effect of External Hydrogen. Surface hydrogen plays a key role in CHA hydrogenolysis since competition between dehydrogenation, hydrogenation, and hydrogenolysis controls both the structure of surface intermediates and the selectivity of surface reactions. In particular, hydrogen desorption at 350 K (Figure 2), just below the proposed hydrogenolysis temperature, limits the hydrogen supply for ensuing hydrogenation and hydrogenolysis processes. In the following series of experiments, external hydrogen is used to increase the concentration of surface hydrogen. Results of TPRS in the Presence of External Hydrogen/ Deuterium. External hydrogen significantly increases the yield of benzene, the primary carbon-containing product from CHA hydrogenolysis. Figure 3 presents the TPRS data taken while adsorbed CHA (0.2 ML) is heated in H2 flux from a directed doser ∼10 mm from the surface. The pressure readings reported below are uncorrected ion gauge background pressures which do not reflect the enhanced flux incident on the surface due to directional H2 flow. The directional dosing provides a H2 pressure approximately 10-100 times the background pressure. For 0.2 ML CHA in 2 × 10-7 Torr of H2, the benzene yield increases by a factor of 2.2 relative to the vacuum result as indicated in Figure 3. With increasing pressure above 2 × 10-7 Torr of H2, the benzene yield does not increase further because
Huang and Gland
Figure 3. Temperature-programmed reaction of 0.2 langmuir cyclohexylamine on the Ni(100) surface at various H2 pressures. The upper panel presents benzene production, the lower panel CHA parent molecule desorption.
complete hydrogenolysis removes all the CHA from the surface. For 0.1 ML CHA (0.1 langmuir exposure), no benzene is produced in vacuum (Figure 2); however, benzene production is clearly observed at 470 K in hydrogen flow (data not shown). For larger CHA coverage above 1/2 ML (data not shown), increased benzene formation is also observed in 2 × 10-7 Torr of H2 flow; however, the increase is smaller. Even for a saturated monolayer of CHA, complete hydrogenolysis occurs in ∼10-5 Torr of H2, as observed with XPS discussed below. External hydrogen also decreases the ammonia formation temperature by approximately 20 K. Similar ammonia temperature decreases are obtained for both hydrogen and deuterium coadsorption. Compare the results in Figure 4 for the deuterated and hydrogenated isotopes of ammonia with the ammonia formation peaks in Figures 1 and 2. External H2 also enhances the CHA parent molecule desorption at 335 K (lower panel, Figure 3). For CHA coverages below 1/2 ML, no molecular CHA desorbs from the Ni(100) surface without external H2. In ∼10-7 Torr of H2 flow, a molecular CHA peak is observed at 335 K which increases with increasing H2 pressure. Deuterium coadsorption studies reveal that external hydrogen species participate in both CHA hydrogenolysis and exchange with CHA. The TPR results for 0.2 ML CHA in the presence of 20 langmuirs preadsorbed D2 are presented in Figure 4. CHA hydrodenitrogenation with preadsorbed deuterium produces C6H6-d1 (m/e ) 79) and C6H6-d2 (m/e ) 80), both at 480 K, and NH3-d1 (m/e ) 18), NH3-d2 (m/e ) 19), and NH3-d3 (m/e ) 20), all at 342 K. Discussion of Hydrogen Effects in TPRS. The ammonia formation temperature decreases with increasing hydrogen coverage as expected for a rate-limiting step involving hydrogen.
CHA Adsorption and Hydrogenolysis on the Ni(100) Surface
Figure 4. Temperature-programmed reaction of 0.2 ML cyclohexylamine on the Ni(100) surface with 20 langmuirs of preadsorbed deuterium.
The desorption temperature of all the ammonia isotopes shift uniformly from 362 to 342 K for 0.2 ML CHA in the presence of 20 langmuirs preadsorbed D2. Similar shifts were observed in the presence of preadsorbed hydrogen, clearly indicating that the temperature shift is a hydrogen coverage effect instead of a kinetic isotope effect. Deuterium incorporation into ammonia and benzene provides substantial insight into the mechanism of CHA hydrogenolysis. Addition of a single deuterium atom appears to be dominant during ammonia formation since NH3-d1 makes up about 65% of the total ammonia produced (including all four isotopic ammonia species). This result clearly suggests that C-N bond activation may involve addition of a single hydrogen atom. Multiply deuterated ammonia makes up 19% of the desorbing ammonia. We propose that this fraction of the adsorbed CHA undergoes deuterium exchange prior to hydrogenolysis. The extent of isotope incorporation in benzene is also quite large since approximately 40% of the benzene produced with 20 langmuirs of preadsorbed deuterium is singly deuterated C6H6-d1. We believe that this isotope content indicates hydrogen addition to the C6 ring as part of the HDN process. The desorption temperature for deuterated C6H6 remains at 470 K, the same temperature observed for non-deuterated benzene formed in the presence of coadsorbed hydrogen. Since the carbonaceous intermediate derived from CHA is hydrogen rich after the C-N bond cleavage, the 470 K benzene must be formed by the subsequent dehydrogenation of a C6H11-x intermediate. The extent of deuterium incorporation in this hydrocarbon intermediate has not been established directly. However, 0-2 deuterium atoms are found to be incorporated in the final product, benzene (Figure 4). Since desorption of free hydrogen on the surface is complete by about 390 K, the ratios of hydrogen isotope desorption above 390 K may reflect primarily the stoichiometry of the CHA derived hydrocarbon intermediates. In the 390-470 K temperature range, the ratio of desorbing H2/HD/D2 is (73.5):(7.9):1. The dehydrogenation of a C6H10-d1 intermediate and statistical mixing of surface
J. Phys. Chem., Vol. 100, No. 6, 1996 2209 hydrogen species should produce a mixture of H2, HD, and D2 with the ratio of 92:9:1(assuming the rates of C-H and C-D bond cleavage on the Ni(100) surface are similar). This ratio is consistent with our proposal that dehydrogenation of an intermediate similar to cyclohexene (C6H10) results in benzene formation. However, the overall scenario may be even more complicated since (1) the hydrocarbon intermediates could have a range of hydrogen stoichiometries, and (2) the hydrocarbon intermediate could have a range of deuterium levels ranging from d0 to d8. To summarize, we believe that two primary mechanisms may contribute to the enhanced benzene yield in external H2: (1) increasing the concentration of surface H(ad) directly enhances H addition to C-N bonds, and (2) increasing the concentration of surface H(ad) inhibits CHA dehydrogenation and stabilizes favorable intermediates for C-N bond activation. These proposals are also consistent with the increased molecular CHA desorption at 335 K in the presence of hydrogen. External hydrogen is believed to enhance the 335 K CHA desorption either by inhibiting dehydrogenation or by hydrogenating a partially dehydrogenated CHA intermediate. Characterization of the CHA-Derived Adsorbed Species. The bonding in the adsorbate, the bonding to the nickel surface, and the stoichiometry of cyclohexylamine and its derivatives on the Ni(100) surface have been characterized with X-ray photoelectron spectroscopy (XPS). Adsorbed cyclohexylamine is bonded to the surface primarily through the amino group as indicated by nitrogen core level shifts. Interaction between the Ni surface and approximately 1/6 of the carbon in a monolayer of CHA is indicated by XPS even at low temperature. Increased hydrogen availability decreases high-temperature C and N residues on the surface as indicated by the decrease of C and N peak areas at 500 K in the presence of hydrogen. XPS Results. Adsorbed surface intermediates formed both in the presence and absence of hydrogen have been characterized by heating known initial coverages of adsorbed CHA to a specified temperatures then quenching rapidly to 100 K. Quenching requires approximately 100 s from 500 to 300 K. Hydrogen atmospheres are evacuated after cooling to 100 K to minimize dehydrogenation during quenching. Using this procedure we have characterized the concentration, composition and chemical states of surface species after heating to a series of key reaction temperatures. XPS characterization requires approximately 0.5-1.5 h so that low temperatures are clearly desirable for stabilizing potential intermediates. The C 1s and N 1s peak positions and integrated intensities obtained after annealing successively to 240, 500, and 650 K are summarized in Table 1. The assignment of the XPS C 1s peaks for a multilayers of cyclohexylamine (bottom spectrum, Figure 5) are discussed in the following paragraph. For condensed CHA, the major C 1s peak is located at 285.6 eV (fwhm 1.9 eV). The multilayer C 1s peak is accompanied by a high BE peak with an integrated intensity of about 20% of the main peak, which is assigned to the carbon (CR) that is directly bonded to the nitrogen atom.14 The inductive effect of the neighboring electronegative N atom increases the binding energy (BE) by 1.0 eV relative to the other five carbon atoms in the ring (Cβ, Cγ). Similar separations have been observed for aniline, where the C 1s peak attributed to CR is shifted up by 1.0 eV in BE.14 Also observed in the multilayer spectrum is a small peak shifted down in binding energy to 283.0 eV. This low-energy C 1s peak retains the same intensity (within (2% error range) after annealed to 240 K, while the higher BE C 1s peaks diminish significantly during multilayer desorption. This 283.0 eV peak is attributed to the CHA carbon
2210 J. Phys. Chem., Vol. 100, No. 6, 1996
Huang and Gland
TABLE 1: XPS C 1s and N 1s Peak Positions for Cyclohexylamine Annealed to Successively Higher Temperaturesa C 1s temperature in vacuum 100 K (multilayer) 220 K (monolayer) 500 K 650 K carbide in 1 × 10-5 Torr of H2 500 K
C1 283.0 283.0 283.0 282.7 ∼283
C2 285.6 285.2 ∼283.8 284.6
N 1s C3 286.6 286.4
∼287
N1 (arb) (1.00) (0.36) (0.28) (trace)
397.3
N2
N3
398.1
399.6 399.8 400.2
(arb) (1.00) (0.18) (0.00) (0.00)
a
BE unit: eV. The numbers in parentheses give the total integrated area under each XPS peaks. The dominant peak, when there is one, is marked in bold type.
Figure 5. XPS C 1s spectra for cyclohexylamine on the Ni(100) surface annealed to successively higher temperatures. The top curve presents data taken after annealed to 500 K in 1 × 10-5 Torr of H2. The data are fitted to Gaussian function as shown by the dashed curves.
in the first layer that interacts with the nickel substrate as discussed below. The C 1s peaks for the chemisorbed monolayer at 240 K clearly indicate that two forms of carbon are present (Figure 5). The primary C 1s peak for chemisorbed CHA appears at 285.15 eV (fwhm 1.9 eV) and is down-shifted from the multilayer peak due to increased metal shielding. The second C 1s peak is located at 283.0 eV and accounts for ∼1/6 of the total C 1s area for monolayer CHA at 240 K. A similar feature due to direct C-Ni bonding has been found at 283.5 eV for chemisorbed aniline on evaporated Ni films; its BE shifts down due to increasing metal screening.15-17 This result suggests that approximately one carbon in the ring is interacting with the surface even at 240 K. A comparison of XPS C 1s spectra taken in the presence and absence of external hydrogen clearly demonstrates the role of hydrogen in removing carbon-containing species from the surface below 500 K. All the carbon-containing species are removed from the surface after annealing a CHA monolayer to 500 K in 1 × 10-5 Torr of H2 (top spectrum, Figure 5). However, the C 1s XPS spectrum of CHA annealed to 500 K in vacuum indicates only a 64% reduction in the C 1s integrated
intensity relative to that at 240 K (Figure 5). These contrasting results clearly confirm increased reactivity observed in increased hydrogen pressures. The broad C 1s peak centered around 284 eV after heating to 500 K in vacuum appears to be quite complex. We have not attempted to separate contributions to this spectrum because of the lack of detailed reference spectra for potential surface species. Further temperature increase to 650 K in vacuum yields carbidic species from further dehydrogenation of CHA-derived C-containing species (Figure 5). This C 1s peaks can be fit with a dominant peak located at 282.9 eV (fwhm ∼1.6 eV), and a secondary peak is at 284.3 eV. In this case, the dominant peak at 282.9 eV is clearly a carbidic layer derived from CHA as indicated by comparison with carbidic carbon spectra on the Ni(100) surface.18 Little or no graphitic carbon is present since graphite has a much higher C 1s BE on nickel.19,20 The second XPS peak (∼30% in area) at 284.3 eV appears at a binding energy which is consistent with a partially hydrogenated form of residual carbon remaining at this temperature. This suggestion is supported by TPD in Figure 2 which clearly show that hydrogen desorption continues above this temperature. The absolute coverage of remaining carbon atoms is estimated to be 7.4 × 1014 atoms/cm2 at 650 K, indicating that approximately three-quarters of the carbon is removed in the absence of external hydrogen. The XPS N 1s spectra of adsorbed CHA on the Ni(100) surface are shown in Figure 6. For the undissociated amine group in multilayer CHA, the N 1s peak is located at 399.6 eV (fwhm 1.9 eV), accompanied by a small satellite feature in the high BE shoulder. The CHA monolayer formed after heating to 240 K has a N 1s binding energy of 399.8 eV (fwhm ∼1.9 eV), indicating that the amino group remains undissociated. Dehydrogenation of the amino group would result in a shift in N 1s levels from 400.0 to 400.2 eV for the undissociated amine, to ∼398.2 eV for the singly dehydrogenated form and ∼397.4 eV for the doubly dehydrogenated form.6,16,21-23 The N 1s position is almost independent of the type of hydrocarbon groups attached within a (0.2 eV range.16 For nitrogen-containing species as diverse as adsorbed aniline,6,21,22 alkylamines,16 and ammonia23 on Ni, their amino groups (-NH2) exhibit similar XPS N 1s binding energies: 400.0-400.2 eV when undissociated. No nitrogen-containing species remain on the surface after annealing a CHA monolayer to 500 K in 1 × 10-5 Torr of H2. However, about 20% of a nitrogen monolayer remains after annealing in vacuum to 500 K. The residual nitrogen remaining after vacuum annealing has an unusually broad N 1s peak suggesting contributions from N in various chemical states. The 500 K spectrum vacuum annealed spectrum can be fit quite well (small χ2) with three peaks of equal widths (2.2 eV) for the three most likely surface species(400.2 eV for -NH2, 398.1 eV for -NH, and 397.3 eV for -N). The curve fitting suggests
CHA Adsorption and Hydrogenolysis on the Ni(100) Surface
Figure 6. XPS N 1s spectra for cyclohexylamine on the Ni(100) surface annealed to successively higher temperatures. The top curve presents XPS taken after annealed in 1 × 10-5 Torr of H2. The data are fitted to Gaussian function as shown by the dashed curves.
Figure 7. Schematic diagram showing dominant surface reaction pathways and the effect of external hydrogen on the surface reaction selectivity of adsorbed cyclohexylamine on the Ni(100) surface. The diagram does not show the exact bond angles and bond lengths.
that 60% of the remaining amine groups are in dehydrogenated form (-NH, and -N) when annealed in vacuum to 500 K. Discussion of Adsorbate Structure and Hydrogenolysis Mechanism. Chemisorbed CHA clearly adsorbs primarily through the amino group based on the N 1s XPS results (Figure 6). N bonding is demonstrated by the 0.2 eV decrease in binding energy for the 240 K XPS N 1s relative to the condensed layer.
J. Phys. Chem., Vol. 100, No. 6, 1996 2211 The relatively small decrease in nitrogen BE on the Ni(100) surface results from partial compensation for N’s lone pair electron donation to Ni by increased shielding due to proximity to the Ni surface.22 A tilted configuration is expected because of bonding through the amine nitrogen. A tilted adsorption geometry is also indicated by the interaction between the ring carbons and the Ni surface evidenced even at 100K by the C 1s peak at 282.9-283.0 eV which is shifted down from the primary carbon peak at 285 eV (Figure 5). With increasing temperature above 240 K in the absence of external hydrogen, more extensive interactions between carbon and nickel are indicated by the increase in absolute intensity of the 283 eV peak and the decrease in the 285 eV peak. This increased interaction at elevated temperature appears to correlate with dehydrogenation of the cyclohexyl group observed during temperature-programmed reaction experiments. Further support for a tilted adsorption geometry is provided by estimated coverages and simple geometric arguments. The CHA monolayer formed at 240 K is estimated to have an absolute surface coverage of 1.8 × 1015 C atoms/cm2 or 2.9 × 1014 CHA molecules/cm2. These figures are based on a comparison between CHA’s C 1s XPS peak area and the C 1s intensity from a saturated CO monolayer on the Ni(100) surface which is known to contain 1.1 × 1015 C atoms/cm2 (0.68 relative to the nickel density). The area of each surface nickel (100) unit cell is 6.3 Å2.24 Therefore, at 4.3 × 1014 molecules/cm2 coverage, each CHA molecule can occupy ∼34.1 Å2 in the monolayer. Since the plane of a CHA molecule is estimated to have an area of ∼40 Å2, the adsorbed CHA molecules must tilt relative to the surface in order to accommodate the measured molecular density. When CHA is heated in the absence of hydrogen, nitrogen removal appears to be more facile than carbon removal since more C remains than N at 500 K. As indicated in Table 1, annealing a CHA monolayer to 500 K in vacuum causes an ∼80 % loss in total N 1s peak areas and only an ∼64% loss in the C 1s peak area. Preferential removal of nitrogen is not surprising since desorption of ammonia occurs at 365 K, a temperature where substantial free hydrogen still remains on the surface. Benzene, the primary gas-phase carbon product, begins to desorb only above 420 K, after the surface is depleted of free hydrogen. Approximately 36% of the carbon in the carbon from the original CHA remains on the surface at 500 K because dehydrogenation results in formation of nonvolatile surface species. These carbon-containing decomposition products are the precursor for the carbidic species at higher temperatures. Ammonia is formed and desorbs at 365 K leaving a hydrocarbon intermediate on the Ni(100) surface. In the absence of external hydrogen, the hydrogen-rich hydrocarbon intermediate does not desorb from the surface but remains on the surface to form benzene at 465 K and other dehydrogenated surface species which remain on the surface with increasing temperature. Apparently, significant bonding between the hydrogen-rich hydrocarbon intermediate and the surface inhibits desorption at 365 K the temperature where the C-N bond is broken. Recently, we have characterized aniline hydrogenolysis on this same Ni(100) surface6 and find ammonia formation at 365 K and benzene formation at 465 K. The similarity between the reactivity patterns for CHA and aniline suggests that similar mechanisms may be dominant in both cases. Spectroscopic examination of the aniline derived surface intermediates formed after C-N bond activation indicate C-Ni σ-bonding characteristic of a cyclohexyl type intermediate. We propose that a similar σ-bonded intermediate of the cyclohexyl type is likely
2212 J. Phys. Chem., Vol. 100, No. 6, 1996 to be the primary hydrocarbon intermediate during CHA hydrogenolysis. During CHA thermal activation in the presence of ∼10-5 Torr of hydrogen, a full monolayer of adsorbed CHA can be removed from the surface, leaving no C and N on the surface at 500 K (Table 1). Contributions to C and N removal by both enhanced CHA hydrogenolysis and increased molecular desorption are indicated in Figure 3. External hydrogen clearly enriches the surface H(ad) concentration below hydrogen’s desorption temperature and increases the C-N bond activation. The increase in the concentration of hydrogen-rich, denitrogenated intermediates results in enhanced benzene formation at 465 K. Thus, the formtion of surface intermediates in the 300-400 K region clearly determines the selectivity observed during a temperatureprogrammed cycle. Conclusions Adsorbed cyclohexylamine interacts with the Ni(100) surface through both the amino group and one carbon in the cyclohexyl group at 240 K, indicating a tilted adsorption geometry. Substantial hydrogenolysis of adsorbed CHA is observed even in the absence of external hydrogen as the result of disproportionation to form ammonia (365 K) and benzene (465-480 K). In the absence of external hydrogen, dehydrogenated surface residues containing carbon and nitrogen are observed above 500 K. External hydrogen pressures rapidly enhance the hydrogenolysis of CHA resulting in complete hydrogenolysis of a monolayer of adsorbed CHA for hydrogen pressures as low as 10-5 Torr with no residual carbon or nitrogen remaining on the surface. C-N bond activation occurs at 365 K, producing NH3 and a hydrogen-rich surface intermediate. Dehydrogenation of this denitrogenated intermediate results in benzene formation in the 465-480 K temperature range even in the presence of 10-5 Torr of external hydrogen. Acknowledgment. We thank the Office of Basic Energy Sciences, U.S. Department of Energy for supporting this research (Grant no. DE-FG02-91ER14-190).
Huang and Gland References and Notes (1) Ho, T. C. Catal. ReV.sSci. Eng. 1988, 30 (1) 117. (2) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed. McGraw-Hill: New York, 1990. (3) Satterfield, C. N.; Cochetto, J. F. AIChE J. 1975, 21, 1107. (4) Nelson, N.; Levy, R. B. J. Catal. 1979, 58, 485. (5) Laine, R. M. Catal. ReV.sSci. Eng. 1993, 25 (3), 459. (6) Huang, S. X.; Fischer, D. A.; Gland, J. L. J. Phys. Chem., submitted for publication. (7) Baca, A. G.; Schulz, M. A.; Shirley, D. A. J. Chem. Phys. 1995, 83, 6001. (8) Chang, C.-C.; Khong, C.; Saiki, R. J. Vac. Sci. Technol. 1993, 11 (4), 2122. (9) Son, K.-A.; Mavrikakis, M.; Gland, J. L. J. Phys. Chem. 1995, 99, 6270. (10) Tsai, M.-C.; Friend, C. M.; Muetterties, E. L. J. Am. Chem. Soc. 1982, 104, 2539. (11) Schoofs, G. R.; Benziger, J. B. Langmuir 1988, 4, 526. (12) Sokoll, R.; Hobert, H.; Schmuck, I. J. Catal. 1990, 125, 276. (13) Blass, P. M.; Akhter, S.; Seymour, C. M.; Lagowski, J. J.; White, J. M. Surf. Sci. 1989, 217, 85. (14) Solomon, J. L.; Madix, R. J.; Stohr, J. Surf. Sci. 1991, 255, 12. (15) Grunze, M.; Brundle, C. R.; Tomanek, D. Surf. Sci. 1982, 119, 133. (16) Inamura, K.; Inoue, Y.; Ikeda, S. Surf. Sci. 1985, 155, 173. (17) Benndorf, C.; No¨bl, C.; Thieme, F. Surf. Sci. 1982, 121, 249. (18) Based on separate experiments of our own, as well as ref 13. (19) Nagashima, A.; Tejima, N.; Oshima, C. Phys. ReV. B 1994, 50, 17487. (20) Fujita, D.; Yoshihara, K. J. Vac. Sci. Technol., A 1994, 12, 2134. (21) Keane, M. P.; Naves de Rtito, A.; Correia, N.; Svenson, S.; Lunell, S. Chem. Phys. 1991, 155, 379. (22) Kishi, K.; Chinomi, K.; Inoue, Y.; Ikeda, S. J. Catal. 1979, 60, 228. (23) Grunze, M.; Dowben, P. A.; Brundle, C. R. Surf. Sci. 1983, 128, 311. (24) Weast, R. C.; Astle, M. J. Eds. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Press: Boca Raton, FL, 1983.
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