6456
J. Phys. Chem. B 2002, 106, 6456-6464
Reaction of Methanol with TiC and VC (100) Surfaces Peter Frantz* and Stephen V. Didziulis Materials Science Department, Space Materials Laboratory, The Aerospace Corporation, El Segundo, California 90245
Luis C. Fernandez-Torres, Rebecca L. Guenard, and Scott S. Perry Department of Chemistry, UniVersity of Houston, Houston, Texas 77240-5641 ReceiVed: December 13, 2001; In Final Form: March 19, 2002
The reaction of methanol on the (100) surfaces of single crystal vanadium carbide (VC) and titanium carbide (TiC) has been studied using high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Methanol forms a mixed monolayer of molecular methanol and a methoxy intermediate upon adsorption at 153 K on both VC(100) and TiC(100). With increasing temperature, methanol is evolved from both surfaces through molecular and recombinative desorption. Approximately half of the methoxy intermediate reacts with the VC surface to produce formaldehyde and hydrogen, with a small amount of methane and persistent oxygen surface species. By contrast, very little of the methoxy intermediate reacts with the TiC surface, producing methane and hydrogen. A model of the surface reactions has been constructed based upon differences in the electronic structures of the carbide substrates.
Introduction The group IV and V transition metal carbides (TMC) have found relevance in numerous applications that require tailored chemical and mechanical properties1. Typical attributes of these materials include extreme hardness, high melting point, catalytic activity,2,3 and conductivity, magnetic susceptibility, and heat capacity similar to the parent metal. As a result, they have been applied as antiwear coatings on cutting tools4 and advanced bearing systems,5 field electron emitters,6 and nuclear reaction chamber coatings,7 and considered as replacements for noble metal catalysts.8,9,10 The interaction of methanol with TMCs is of importance to many of these applications as a model for the interaction of carbide coatings with lubricants and additives found in a tribological environment, and because of the significance of surface catalyzed reactions in the synthesis and breakdown of methanol. A large body of literature exists to describe the catalyzed reactions of methanol on transition metal surfaces and on transition metal oxide surfaces. The motivation of many early studies was to define the detailed mechanism of the industrially important Fischer-Tropsch reaction in which methanol and other organic molecules are synthesized by reaction of CO and H2 over Fe, Co, Ni, or Ru at high pressure.11 A more recent motivator is for the development of electrocatalysts for direct methanol fuel cells.12 The results of these studies show that methanol often dissociatively adsorbs to bare transition metal surfaces to form a methoxy intermediate at temperatures above 110 K.12 Higher temperatures produce further decomposition of methanol, leading to a number of products that vary from one surface to the next.12-16 The mechanism and extent of these reactions is often influenced by the presence of oxygen or sulfur.17-20 Previous work specifically addressing catalytic * To whom correspondence should be addressed. E-mail: peter.p.frantz@ aero.org. Fax: (310) 336-5624.
activity suggests that TMCs may serve as poison resistant and kinetically favorable alternatives to group VIII metal catalysts.8,9,10 In this paper, we present a comparison of the chemical reactivity of methanol with the nonpolar (100) faces of single crystal VC and TiC. Both of these carbide materials exist in the rock salt structure; a (100) termination of this structure ideally produces a nonpolar surface with equal numbers of metal and carbon sites with 4-fold symmetry within the surface plane. Although structurally similar, the (100) surfaces of VC and TiC exhibit different chemical interactions with small molecules such as O2, H2O, and CO. Both surfaces dissociatively adsorb O2, but VC forms a stable vanadyl surface species while TiC loses surface carbon to the emission of CO and forms TiOx. TiC dissociates water to a much greater extent at low temperature than does VC, whereas CO bonds more strongly to the terrace metal atoms on the VC surface. We have linked these chemical differences to a significant difference in the electronic structure of these materials due to the one additional 3d valence electron per formula unit of VC, with respect to TiC. The significance of this extra valence electron is seen when considering a molecular orbital description of these materials possessing a polar covalent bond character.21 Such a treatment reveals that the highest lying occupied orbital is associated predominantly with carbon sites for the TiC. The additional valence electron of V, with respect to Ti, populates an orbital that is mostly 3d in character, and is therefore associated with metal sites for the VC. Johansson has observed these differences in electronic structure using band structure calculations.22 This electronic structure difference results in electron accepting adsorbates favoring V atoms of VC and C atoms of TiC, consistent with the chemistry observed with O2 and CO.23 This model is extended in the present paper to describe the reactivity of methanol with these two carbide materials.
10.1021/jp0145311 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/30/2002
Methanol with TiC and VC (100) Surfaces Two previous publications have addressed methanol chemistry on TiC (100), whereas its interaction with VC has not been previously explored. The sole experimental paper used UPS to investigate the room temperature adsorption of methanol on TiC, with the conclusion that methanol adsorbs molecularly under the conditions studied.24 One theoretical work presumed that methanol or methoxy bonded with the surface carbon atom of the TiC (100) surface. This work concluded that a slight preference exists for bonding a methoxy species and that the interaction of the hydrogen with the surface plays an influential yet fully undetermined role.25 We aim to clarify the nature of these interactions on both the VC and TiC surfaces. In this work, the adsorption and reaction of methanol at cryogenic temperatures and the subsequent reaction at higher temperatures have been investigated with an array of surface analytical techniques. High-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) have been used to probe the vibrational modes and chemical nature of adsorbed species and to follow their evolution as a function of surface temperature. Temperature programmed desorption (TPD) has been used to document the desorption characteristics of methanol and volatile reaction products from these carbide surfaces. Density functional theory (DFT) calculations have been performed to predict trends in the relative bond strengths between various surface/adsorbate pairs. Together, the results demonstrate that methanol undergoes O-H bond scission on both of these carbide surfaces at cryogenic temperatures to form a methoxy intermediate. However, at elevated temperatures, we find further evidence of decomposition for VC, but not for TiC. This difference in reactivity is discussed in terms of the relative electronic structure of the two carbides. Experimental Section The vanadium carbide sample used in these experiments was obtained from Linfield Research Institute, Linfield College, McMinnville, OR. Titanium carbide was received as a gift from UBE Industries of Japan. XPS measurements indicate these carbides had a 1:1 surface stoichiometry; however, the crystal growers estimated a bulk carbon-to-metal ratio of slightly less than one. The growth techniques used to produce these sample have been described elsewhere.26 Experiments performed at the University of Houston and The Aerospace Corporation employed separate (100) oriented single crystals of TiC and VC prepared from the same boules. Although, the experiments were not performed on the same exact samples, independent XPS measurements carried out at both locations verified identical surface stoichiometries and cleanliness. Preparation procedures, prior to analysis, of the carbide surfaces have been described in previous papers.27,28 In experiments where VC(100) was exposed to alcohol adsorbates, a sputtering and annealing cycle was required between adsorbate exposures to fully recover the level of reactivity between different exposures. After the substrate was satisfactorily cleaned, the sample temperature was lowered for alcohol dosing. Cooling to ∼150 K in the HREELS system (Aerospace Corporation) was provided through a copper braid attached to a liquid nitrogen cooled heat sink. In the TPD system (University of Houston), the samples were mounted directly on a liquid nitrogen filled annular dewar that allowed cooling to temperatures approximately 50 Κ colder than in the HREELS system. The temperature was monitored with a type K thermocouple mounted on the sample face for TPD measurements and on the sample stage for HREELS measurements. The alcohols used in this study included methanol (CH3OH), methanol-d (CH3OD), methanol-d3(CD3-
J. Phys. Chem. B, Vol. 106, No. 25, 2002 6457 OH), and methanol-d4 (CD3OD). Each was obtained from Aldrich Chemical Co., Milwaukee, WI. All of the methanol samples were transferred into dosing vials under an atmosphere of nitrogen to avoid water contamination from the ambient environment. The samples were then degassed by multiple freeze-pump-thaw cycles and their water content was determined by mass spectrometry. Gas exposures in the HREELS experiment were performed by placing the sample in the path of a gas-dosing system that consisted of a turbo-pumped gas supply, a leak valve, and a 24.8 cm long dosing tube terminated by a channel plate. This system provided an enhancement in the mass adsorbed of approximately 25 times the mass deposited through back-filling the chamber. Methanol vapor exposures in the TPD system were performed using a pinhole doser consisting of a 4 in. long, 5 mm I. D. drift tube, mounted on the vacuum side of a 2 µm pinhole. The substrate was placed approximately 0.5 mm from the end of the drift tube to ensure that all emerging gases encountered the substrate surface. Because both substrates are small (2 mm diameter VC and 3.5 mm diameter TiC) and the dosed area was confined to an even smaller area to avoid exposure of the sample holder, the signal-to-noise is low in the data presented below. All TPD figures show a series of sequentially increasing methanol dosage, representing dosing times of 20, 30, 45, and 60 s. The relative amounts of molecules effused from the surface were determined from the TPD spectra by the following standard method. For each desorbed molecule, integrated intensities were determined at the mass of the most abundant fragment. Because the intensity of the most abundant mass peak is some fraction (known from literature mass spectra) of the sum of all cracking fragments, the intensity was normalized by this fraction to give a quantity that could be compared with other desorbed species. The quantities were not adjusted for differences in ionization potential because these were approximately equal for each of the products. The HREELS data collection procedure has been described previously.27,28 Spectra were collected with a double-pass spectrometer (LK 2000, LK Technologies, Inc.) with a sample current of 10-10 A and pass energy of 1.5 eV. TPD experiments were achieved with a computer controlled Hiden Analytical Laboratories, Model 301 mass spectrometer. In all temperature programmed reaction experiments, a preexposed sample was placed within 2 cm of the quadrupole ionizer and heated at a rate of 4.7 K/s. Reactions of methanol-d3 on the carbide surfaces were monitored by mass-to-charge ratios of 36, 35, 34, 32, 20, 18, and 3. Photoelectron spectra were collected at the University of Houston with an Omicron EA 125 energy analyzer and a VG Mg KR radiation source (1253.6 eV), housed in the same vacuum system as the TPD system. Spectra were collected from a 1.5 mm spot on the sample surface with a 25 eV pass energy and the analyzer situated 30° from the surface normal. DFT calculations were performed using Titan version 1.0.1 software from Wavefunction, Inc., Irvine, CA. This program uses the computational program Jaguar 3.5 (Schro¨dinger, Inc. Portland, OR 1998) through a graphical interface developed by Wavefunction. DFT calculations were performed using the B3LYP Model and the LACVP** pseudopotential basis set. Calculations to determine the adsorbate bond strengths were performed on the methoxy fragment, hydrogen atoms, M9C9 carbide clusters, and the adsorbate/carbide cluster. Details of the computational protocol were presented elsewhere.29 Results Methanol on VC(100). HREELS data for CH3OH adsorbed on VC(100) at low temperature (153 K) is presented in Figure
6458 J. Phys. Chem. B, Vol. 106, No. 25, 2002
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Figure 1. HREELS data obtained from two separate experiments: (A) clean VC(100), (B) 10 L of CH3OH on VC(100) at 153 K.
1. The spectrum of clean VC prior to dosing (spectrum A) consists of a single loss feature due to the surface phonon modes of the VC substrate surface (515 cm-1).30 After exposure to 10 L of CH3OH at 153 K, spectrum B shows the following loss features: a surface-methoxy stretching vibration at 360 cm-1,14,19,31 the substrate vanadium-carbon vibration at 515 cm-1,30 an oxygen-hydrogen bending mode at 780 cm-1,18 the adsorbate carbon-oxygen stretch at 1050 cm-1,14,18,20 the carbonhydrogen deformation and stretching modes at 1440 and 2950 cm-1,14,18,20,32 and an oxygen-hydrogen stretch at 3430 cm-1.18,20 Sequential experiments with lower exposure have shown that the peak heights of spectrum B approach saturation with this 10 L exposure. Temperature programmed desorption experiments (TPD) shown below indicate that this experiment was performed above the multilayer desorption temperature, yet below the first monolayer desorption state. Therefore we believe that this spectrum contains all of the features that are present in the spectrum of a complete single monolayer on VC. A series of HREELS measurements, shown in Figure 2, was performed to study the evolution of the adsorbate with increasing temperature. CH3OD was used in these measurements in order to observe potential surface hydrogen species with loss features near 1010 cm-1 (750 cm-1 for deuterated species), which would be obscured by the C-O stretch at 1050 cm-1.33 A submonolayer exposure of 2.5 L was chosen for this experiment to mitigate the systematic loss of HREELS signal that we observed during heating cycles after larger doses. The initial low temperature (150 K) dose of CH3OD on VC (100) is shown at the bottom of Figure 2. We find the same energy loss features as shown in Figure 1, with the following exceptions: the O-D bending mode has shifted to 550 cm-1 with isotopic substitution14 (this feature is distinct from the metal-carbon lattice vibration at 515 cm-1), a surface carbondeuterium stretching feature has appeared in the range from 2000 to 2080 cm-1,32 and no O-D stretch (expected14,34 to be found at 2360 cm-1) can be resolved. The low temperature measurement was followed by sequential flashes to higher temperature, maintaining the peak temperature for approximately 1 min before cooling to 150 K for collection of the HREELS spectrum. After 175 K, the spectrum showed slight intensity reductions in all of the vibrational features, with a greater loss of the O-D bend at 550 cm-1. The 360 cm-1 peak (surface-methoxy stretch) appears to have increased in intensity. Growth in this region could be due to additional decomposition of methanol to methoxy caused by the increase
Figure 2. HREELS data obtained after dosing clean VC (100) with 2.5 L of CH3OD at 150 K, and after subsequent heating cycles to the indicated temperatures and cooling back down to 153 K.
in temperature, or it could be due to enhancement of the total HREELS background signal resulting from changes in the background tail of the elastic peak. At 245 K, we find that the 360 cm-1 peak has shifted to higher frequency and the substrate V-C surface phonon peak (515 cm-1) has become prominent, indicative of a loss of surface species. This result is substantiated by the further loss of the O-D bending mode, and reductions in the intensity of the adsorbate C-H bending and stretching modes. Finally, the C-O stretching vibration (1050 cm-1) has broadened to higher frequency, indicative of a surface reaction or change in local environment. After flashing to 300 K, the surface-methoxy stretch (360 cm-1) can no longer be resolved and the C-O stretch (1050 cm-1) has shifted or been completely replaced by a peak centered at 1120 cm-1. The adsorbate C-H modes have lost a large fraction of their intensity, and a new peak has appeared at 1360 cm-1. Little change is noted after flashing to 475 K, although an improvement of signal quality makes the existing peaks clearer. These peaks include the substrate V-C vibration at 515 cm-1 and well-defined peaks at 1120 and 1360 cm-1, which were not among the features of the low-temperature spectrum. Finally, flashing to 675 K results in a reduction in intensity of all peaks except for the substrate V-C vibration (515 cm-1) and the 1360 cm-1 peak. The VC/methanol vibrational peak assignments are listed in Table 1. To determine if the 1120 cm-1 peak is related to a bending mode of C-H that was not observed at lower temperature, we repeated the experiment with a fully deuterated sample, CD3OD. Figure 3 shows the HREELS spectrum of VC (100) following separate exposures to 2.5 L of CH3OH, CD3OD, and CH3OD. In each case, the exposure occurred at 150 K, the substrate was flashed to approximately 300 K, and then returned to 150 K for data collection. We find that the position of the 1120 cm-1 feature remains unchanged with isotopic substitution of the methyl group or the hydroxyl group. Therefore, this peak is not related to a hydrogen or deuterium vibration. The same
Methanol with TiC and VC (100) Surfaces
J. Phys. Chem. B, Vol. 106, No. 25, 2002 6459
TABLE 1: HREELS Loss Features (in cm-1) Obtained after Cryogenic CH3OH (CH3OD) Adsorption on VC (100) and TiC (100) and Subsequent Annealing VC (100) 153 K VC (100) 300 K. TiC (100) 153 K TiC (100) 300 K
CH3O-surface
O-H bend
C-O stretch
C-H deform
H-surface stretch
C-H stretch
O-H stretch
360 (360)
780 (550)
1050 (1050) 1120 (1120) 1060 (1050) 1060 (1050)
1450 (1450) 1450 (1450) 1450 (1440) 1425 (1440)
(2070)
2950 (2950)
3430 (2950)
(2050)
2870 (2870) 2870 (2870)
360 (350)
(MC)-O 1360 (1360)
Figure 3. Three separate experiments in which HREELS data was obtained after dosing the clean VC (100) surface with 2.5 L of one of three different methanol isotopes at 153 K: CH3OH, CD3OD, and CH3OD.
is true for the peak at 1360 cm-1. Assignments for these will be further discussed below. To detect the volatile products of the reaction of methanol on VC(100), the CD3OH/VC (100) system was investigated with TPD. Figure 4A shows the desorption spectra of the parent mass (m/q ) 35) of CD3OH after exposures ranging from submonolayer to multilayer. The narrow, unsaturated methanol multilayer feature with a peak desorption temperature of 143 K resides in the low temperature region of the highest exposure data. A series of peaks, all saturating as coverage is increased, cover a large temperature range beyond the multilayer feature. These peaks are assigned to monolayer desorption states arising from the desorption of molecularly adsorbed methanol or the recombinative desorption of surface species. The peak desorption temperatures of these features are listed in Table 2. The various peaks appear to approach saturation at the same rate of exposure, except the peak centered at 268 K. The integrated intensity of this peak reaches saturation at a larger exposure than other features. Several other masses were also monitored to detect the desorption of reaction products. The series of spectra shown in Figure 4B represent m/q ) 34, or CD3O. This mass, due to some combination of the cracking fragments of both CD3OH and CD3OD, shows many of the same desorption features of Figure 4A. However, in comparison with Figure 4A, the high temperature peaks of 4B are enhanced with respect to the low temperature peaks, indicating that this feature is not due solely to fragmented CD3OH (as in Figure 4A). This observation, coupled with the detection of a 491 K peak in the m/q ) 36 channel (Figure 4C) reveals that CD3OD is produced at high temperature. The ratio of intensities of mass 36 to its cracking fraction at mass 34 is consistent with this conclusion. The lowtemperature peaks in the mass 36 spectrum, at 268 and 305 K,
Figure 4. Thermally induced desorption of methanol (CD3OH) from the VC (100) surface after a variety of exposure quantities. Products shown here are different isotopes of molecular ethanol which result from isotopic mixing on the surface, and cracking fragments of these isotopes: (A) m/q ) 35, CD3OH; (B) m/q ) 34, CD3O; and (C) m/q ) 36, CD3OD.
TABLE 2: TPD Peak Desorption Temperatures for the Various Reaction Products (K)
VC (100) TiC (100)
molecular methanol
recom. methanol
268 266
305, 420, 491 300, 565
formaldehyde methane hydrogen 480
495 592
450, 568 552
are of appropriate intensity relative to the CD3OH parent mass (less than 3%) to be accounted for as an isotopic contaminant of CD3OD in the CD3OH sample. The m/q ) 32 channel, due to CD2O, is shown in Figure 5A. The features in these data are located at approximately the same temperatures as the parent mass in Figure 4A. However, the high temperature peak appears shifted to slightly lower temperature (480 K), and the absolute intensity is much larger than in any of the molecular species in this temperature range. Thus, a significant fraction of this peak must be due to the production of formaldehyde from the surface, as cracking fragments of CD3OH and CD3OD would have lower intensity than found for masses 34 through 36. We note that there are no other potential sources of CD2O and that production of ethylene (C2D4 in this case, also at mass 32) was not observed in experiments with other isotopes. Figure 5B shows the desorption spectra of m/q)20. For the reaction of CD3OH, intensity in this channel potentially arises from both deuterated methane and water (CD4 and D2O). However, we find a peak of nearly equal intensity at 500 K in
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Figure 6. XPS difference spectra of the C(1s) region of the VC surface after exposure to methanol. Data shown are the signal from the clean VC surface subtracted from (A) the surface dosed with 1 ML methanol at 100 K, (B) after heating to 244 K, and (C) after heating to 304 K. The inset shows the entire C(1s) region after dosing at 100 K.
TABLE 4: XPS C (1s) Peak Positions at Various Approximate Temperatures (eV) Figure 5. Thermally induced desorption of methanol (CD3OH) from the VC (100) surface after a variety of exposure quantities. Products shown here are the result of chemical reaction on the VC (100) surface: (A) m/q ) 32, CD2O; (B) m/q ) 20, D2O, CD4; (C) m/q ) 3, HD.
VC (100) TiC (100)
carbide
adsorbate (100 K)
adsorbate (240 K)
adsorbate (300 K)
282.3 281.4
286.7 286.5
285.9 286.3
286.3
TABLE 3: TPD Relative Reaction Product Quantities (percent of the sum of all carbon containing products)
VC (100) TiC (100)
molecular methanol
recom methanol
35 23
30 75
formaldehyde
methane
30
5 2
the mass 18 channel (not shown). Since the cracking fraction of D2O at m/q ) 18 is 21% of the parent peak at m/q)20, and the cracking fraction of CD4 at m/q ) 18 is 85%, we conclude that this peak is due primarily to methane production. Finally, Figure 5C shows the desorption of hydrogen in the form of DH at m/q ) 3. We observe two features centered at 450 and 568 K, bracketing the evolution of CD2O and CD4. The peak desorption temperatures for each product are listed in Table 2, and the relative quantities are listed in Table 3. Carbon 1s XPS difference spectra of the dosed VC (100) sample surface after subtraction of the clean surface spectrum are shown in Figure 6. At low temperature (spectrum A, 100 K), we find a peak due to emission of electrons from the methoxy and methanol carbon at 286.7 eV. After flash heating the sample to 240 K (spectrum B), this peak has diminished in intensity and shifted to 285.9 eV. After heating again to 300 K (spectrum C), we were unable to resolve a signal from the adsorbate carbon. To demonstrate the quality of the raw data from which these difference spectra originate, the entire C(1s) region after dosing at 100 K is shown in the inset of Figure 6. Peak positions are listed in Table 4. Methanol on TiC(100). HREELS data of 2.5 L of methanol (CH3OH) adsorbed on TiC(100) at 153 K and heated to a series of higher temperatures are presented in Figure 7. Referring to the lowest temperature spectrum at the bottom of the figure, there are many similarities with the analogous experiment on VC (100), shown in Figure 2. We find a surface-methoxy stretch at 360 cm-1,14,19,31 the methoxy carbon-oxygen stretch at 1050
Figure 7. HREELS data obtained after dosing clean TiC (100) with CH3OH at 153 K, and after subsequent heating cycles to the indicated temperatures and cooling back down to 153 K.
cm-1,14,18,20 and the methoxy carbon-hydrogen deformation modes and stretching modes at 1440 and 2850 cm-1,18,20,32 respectively. However, there was no evidence of the O-H vibrations due to molecular methanol at 780 or 3400 cm-1. Very little change was found in the spectrum after heating to 190 K as shown in Figure 7. In fact, after heating to 275, 300, and 475 K, the only observed changes are the concomitant
Methanol with TiC and VC (100) Surfaces
Figure 8. Thermally induced desorption of methanol (CD3OH) from the TiC (100) surface after a variety of exposure quantities. Products shown here are different isotopes of molecular ethanol which result from isotopic mixing on the surface, and cracking fragments of these isotopes: (A) m/q ) 35, CD3OH; (B) m/q ) 34, CD3O; (C) m/q ) 36, CD3OD.
reduction of all of the adsorbate peaks, accompanied by the growth of the two substrate Ti-C surface phonon features at 520 and 660 cm-1, respectively.35 No shift in the 1050 cm-1 C-O stretching feature was detected. The TiC/methanol vibrational peak assignments are listed in Table 1. The m/q ) 35 desorption spectrum of the parent molecule, CD3OH, is shown in Figure 8A. The multilayer desorption peak appears at 143 K, equal to the position of the multilayer desorption peak on VC. In addition, a single monolayer desorption feature was found, centered at 320 K, at low coverage. With increasing coverage the peak desorption temperature decreases to 300 K and a shoulder grows in on the low temperature side at 266 K. This stands in contrast to the methanol on VC (Figure 4A) where a prominent second peak at 268 K was ascribed to molecularly adsorbed methanol. A peak at 560 K in the m/q ) 36 channel (Figure 8C) indicates that CD3OD is produced on the TiC surface. This conclusion is supported by the observation of a similar peak in the m/q ) 34 channel, consistent with the CD3O cracking fragment of this molecule (Figure 7B). Note that this high temperature peak was not observed in the spectrum of the parent molecule (Figure 8A); thus we conclude that the high temperature feature in Figure 8B arises solely from the desorption of CD3OD, without a significant contribution from CD3OH. As discussed with VC, the 300 K peak in Figure 8C is consistent with an approximately 3% contamination of CD3OD in the CD3OH sample. The absence of a high-temperature peak in the m/q ) 32 spectrum (Figure 9A) indicates that little or no formaldehyde is produced, in contrast to the reaction on VC (100). We also find that methane (CD4, Figure 9B), and hydrogen (HD, Figure 9C) are produced, though to a lesser extent and at higher temperatures than on the VC (100) surface. Relative quantities
J. Phys. Chem. B, Vol. 106, No. 25, 2002 6461
Figure 9. Thermally induced desorption of methanol (CD3OH) from the TiC (100) surface after a variety of exposure quantities. Products shown here are the result of chemical reaction on the VC (100) surface: (A) m/q ) 32, CD2O; (B) m/q ) 20, D2O, CD4; and (C) m/q ) 3, HD.
Figure 10. XPS difference spectra of the C(1s) region of the TiC surface after exposure to methanol. Data shown are the signal from the clean TiC surface subtracted from (A) the surface dosed with 1 ML methanol at 105 K, (B) after heating to 240 K, and (C) after heating to 280 K. The inset shows the entire C(1s) region after dosing at 105 K.
of these products are listed in Table 3. The relative intensities of the mass 35 and mass 32 peaks in the 300 K region match the expected intensities of parent and cracking fractions. XPS difference spectra of the dosed TiC (100) sample surface after subtraction of the clean surface spectrum are shown in Figure 10. At low temperature (spectrum A, 100 K), we find a peak due to emission of electrons from the 1s methyl carbon at 286.5 eV. After flash heating the sample to 244 K (spectrum B), this peak has diminished in intensity and shifted only slightly to 286.3 eV. After heating again to 304 K (spectrum C), the peak intensity has again decreased, but the position remains approximately the same. To demonstrate the quality of the raw
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Frantz et al. TABLE 6: Adsorption Energy Predicted by DFT for Methoxy Adsorbed on TiC and VC Clusters atom-OCH3 cluster energy adsorption energy surface atom-OCH3 bond length (H) (kcal/mole) VC VC TiC TiC
Figure 11. Representative image of methoxy bonded to the M9C9 cluster.
TABLE 5: Adsorption Energy Predicted by DFT for H Adsorbed on TiC and VC Clusters material atom-H VC VC TiC TiC
V C Ti C
bond length cluster energy adsorption energy (Å) (H) (kcal/mole) 1.65 1.15 1.80 1.10
-984.9371 -984.9213 -866.0291 -866.07388
-47 -37 -14 -42
data from which these difference spectra originate, the entire C(1s) region after dosing at 100 K is shown in the inset of Figure 6. Peak positions are listed in Table 4. DFT Results. DFT calculations were performed on M9C9 clusters having C4V symmetry to represent the (100) surface of both VC and TiC. A representative image of methoxy bonded to the M9C9 cluster is shown in Figure 11. As we believe methoxy and hydrogen species to be the predominant surface product from the dissociative adsorption of CH3OH at low temperatures, we have endeavored to determine whether the surface metal or carbon is the preferred bonding site for these species. For hydrogen, this calculation is straightforward for atop sites. We bonded a hydrogen atom to either the central metal or carbon atom on opposite sides of the M9C9 cluster, and then systematically varied the surface-H bond length until a minimum energy was obtained. We then compared the absolute energies for hydrogen bonded to the metal and carbon for a given material, and compared the relative stabilities of H on the two different materials by subtracting energies of the bare cluster and the H atom as calculated by the Titan/Jaguar program. The results for hydrogen are presented in Table 5, along with the optimized bond length, (0.05 Å. These calculations indicate the bonded hydrogen atom is slightly more stable on the V atom, as compared to the C site on VC. Alternatively, hydrogen is significantly more stable on the C site of TiC relative to Ti. In addition, the C-H bond on TiC appears to be slightly stronger than the C-H bond on VC, consistent with HREELS data. The methoxy surface interactions are more difficult to model because of the numerous possible geometries that the adsorbate could have on the surface. For example, the theoretical work by Jansen and Hoffman25 assumed a surface-O-C bond angle of 180°, instead of being bent as the H-O-C bond of methanol. We believe the bent configuration more accurately reflects the
V C Ti C
1.95 1.50 2.00 1.50
-1099.4955 -1099.3956 -980.6205 -980.5768
-48 +14 -38 -11
actual oxygen bond angle and have adopted this geometry for all sites on both surfaces for the sake of comparison, yet acknowledge that the geometry has not been fully characterized. For comparison, we performed one calculation with the 180° geometry cited above, finding it to be less stable by approximately 13 kcal/mol than the bent configuration with a similar V-O bond length. A summary of the results of these calculations is presented in Table 6. While realizing that the geometries chosen are probably not optimized, we believe that a trend is evident from these results. The methoxy group is much more stable bonded to the metal atom on either surface than to the carbon. It is also evident that the methoxy is more stable on vanadium, which may enable enhanced reactivity. These results also say nothing regarding the mechanism (deprotonation versus H-atom transfer) that would truly determine why dissociative adsorption is more likely on TiC. Discussion Methanol on VC (100). At cryogenic temperatures, the primary surface reaction is dissociative adsorption, however this reaction does not proceed to completion. Based upon the lowest temperature HREELS results shown in Figures 1 and 2, we assert that the monolayer contains a mix of molecularly and dissociatively adsorbed methanol. The peak at 360 cm-1,assigned to the stretching vibration of the methoxy bound to a surface vanadium atom,14,19,31 demonstrates the dissociation of methanol upon adsorption at 150 K. This frequency is consistent with methoxy species formed on metal surfaces and metal sites of transition metal oxides. We conjecture that a methoxy bonded to a surface carbon atom would have a different, likely significantly higher, stretching frequency. The existence of O-H stretching and bending modes at 3430 and 780 cm-1 in Figure 1, and at 550 cm-1 in Figure 2, indicates that some methanol is molecularly adsorbed at high exposure levels. In the context of these experiments, we describe reactivity at higher temperatures in terms of the adsorbed methanol and the products of its dissociative adsorption, surface hydrogen and surface methoxy. The m/q ) 35 TPD series shown in Figure 4 reveals two large monolayer peaks, located at 250 and 310 K. We assign the 250 K peak to the desorption of molecularly adsorbed methanol and the 310 K peak to recombinative desorption. Molecularly adsorbed methanol makes up approximately 35% of the monolayer, whereas 30% is effused as recombinative methanol at 310 K. A similar assignment of methanol desorption peaks has been made previously19 for an oxygen modified Fe(100) surface, where methanol dissociation was shown to be limited by preadsorption of the oxygen overlayer, shifting desorption peaks from approximately 300 K (recombinative) to 200 K (molecular) as oxygen coverage was increased. Our assignment is also consistent with our previous work with water on VC and TiC (100).33 In that work, desorption of water below 220 K arose from molecularly adsorbed water, while emission above 220 K was due to recombinative desorption. These assignments are further supported by the loss of intensity in the HREELS O-D bending
Methanol with TiC and VC (100) Surfaces mode at 550 cm-1 (Figure 2) at a temperature (245 K), corresponding with the first monolayer desorption feature in the TPD spectrum (Figure 4A). Beyond 350 K, the reaction of the surface methoxy species yields a number of products over a range of temperatures. These reactions produce formaldehyde, methane (approximately 5%), hydrogen and the additional recombinative desorption of methanol. All of these products proceed through an intermediate formed through the surface reaction of the methoxy species. The HREELS data show the appearance of a 1120 cm-1 peak at approximately 300 K concurrent with a shift in the position of the methoxy peak from 360 to 400 cm-1 and a shift of the adsorbate C 1s XPS spectrum to lower binding energy. We assign the 1120 cm-1 peak to a coupled vibration of a V-CHx-O-V species, bonded to the surface through both the adsorbate carbon and oxygen. A similar feature has been identified at 1130 cm-1 in studies of methanol adsorbed to ZrO2.36 Similarly, the 820 cm-1 peak may be assigned to another vibrational mode of the same complex.36 We also note that a small fraction of chemisorbed CO molecules on VC (100) were assigned as a bridging species with a C-O stretching frequency of 1170 cm-1.29,37 The changes observed in the C 1s XPS data cited above are consistent with the adsorbate C bonding to a less electronegative surface atom. From this surface intermediate, formaldehyde is evolved as the temperature increases. Figure 5A (m/q ) 32) shows that this reaction product is evolved with a peak desorption temperature of 480 K. The intensity of this peak indicates that formaldehyde precursors constitute 30% of the monolayer, or approximately half of the adsorbed methoxy. The HREELS peaks due to the formaldehyde precursor, at 1120 and 820 cm-1, decline significantly in intensity between 300 and 675 K. Other products of the reaction of the surface intermediate include methanol and hydrogen, observed in the mass channels m/q ) 36, 35, 34 and 3. The evolution of CD3OH follows from the recombination of the intermediate with surface hydrogen at 420 K and deuterium at 490 K. The HREELS data show that hydrogen (deuterium) is present on the surface at 153 K, due to dehydrogenation or deprotonation of the alcohol upon adsorption. Some of this hydrogen (deuterium) is consumed during the emission of methanol at 300 K but is reintroduced to the surface population with the formation of the surface intermediate at 245 K. The emission of HD appears bimodal, peaking at approximately 450 K and 568 K. (It is possible that emission of high-temperature methane and methanol was stimulated by the availability of hydrogen.) Finally, methane is evolved from the surface with a peak desorption temperature of approximately 500 K. The intensity of the m/q ) 20 methane peak indicates that 5% of the monolayer is evolved as methane, possibly due to chemistry at defect sites. The presence of residual oxygen observed at 1360 cm-1 in the HREELS spectra is consistent with the evolution of methane at higher temperatures. It has previously been shown that oxygen left on the VC (100) surface from the decomposition of water exhibits a vibrational mode at 1375 cm-1, and that this residual oxygen is stable to very high temperature.33 Methanol on TiC (100). At cryogenic temperatures, dissociative adsorption occurs producing adsorbed methoxy and hydrogen much as is the case on VC; note the similarity of the low-temperature spectra of Figures 1 and 7. Although the HREELS data suggest that this reaction proceeds to completion in the monolayer (no O-H vibrations at 780 cm-1 and in the vicinity of 3400 cm-1), the TPD suggest that ∼23% of the adsorbed methanol does not dissociate. This discrepancy is likely
J. Phys. Chem. B, Vol. 106, No. 25, 2002 6463 due to the formation of an incomplete monolayer in the HREELS experiment in which the sample was dosed at a temperature higher than in the TPD experiment. Thus, it appears that a mixed monolayer of molecular methanol and methoxy was formed, although the reaction to form methoxy proceeded slightly further on TiC (77% methoxy) than on VC (65% methoxy). As the surface temperature increases, the predominant reaction is the recombinative desorption of methanol. The methoxy vibrational features in the HREELS spectrum of Figure 7 are stable from 153 to 190 K, but then drop significantly in intensity from 190 to 275 K, and again between 275 and 300 K. This is consistent with the significant evolution of methanol over this temperature range in the TPD spectrum of the parent mass m/q)35, Figure 8A, where the peaks remaining above 300 K are smaller than on VC. In contrast to the results on VC, the C 1s XPS feature (Figure 10) does not exhibit a binding energy shift in this temperature range. Furthermore, the C-O stretching frequency of the surface methoxy species (1050 cm-1) does not shift to higher frequency, nor does the 1120 cm-1 peak indicative of the surface intermediate appear in HREELS spectrum. All of these observations indicate that there has been no change of the methoxy configuration and the only reaction occurring is recombinative desorption. At temperatures above 300 K, the TPD data provide no evidence of formaldehyde production, as the m/q ) 32 series of spectra show only the cracking fragments of the parent molecule at 300 K. However, hydrogen and a small amount of methane and CD3OD were produced at higher temperatures than they were on VC. These minority reaction products potentially arise from the methanol or surface methoxy adsorbed at defect sites on the TiC surface. Surface Bonding and Activation. For both carbide surfaces, we conjecture that the initial adsorption of a methanol molecule must take place by donation from an oxygen lone pair with an empty surface orbital. This interaction is more likely to occur on a surface metal atom based on the availability of low lying, predominantly empty 3d orbitals. Either concurrent with or after this initial adsorption, a hydrogen atom is transferred from the molecule to the surface, resulting in a methoxy species bound to a metal site. Our experiments strongly suggest that a surface carbon atom accepts the hydrogen. Our DFT calculations provide some support for this picture, as the surface methoxy species are shown to be significantly more stable on surface metal sites than surface C sites on both VC and TiC. On TiC, a surface hydrogen atom is energetically more stable on a carbon site, whereas on VC, it is more stable on a vanadium site. Neither our experiments nor our calculations probe the nature of the hydrogen transfer from the methanol to the surface. One intriguing possibility is that the reaction is actually a proton transfer, akin to the reactions often envisioned on metal oxides where the basic oxide ions abstract the proton. We suggest that the carbon atoms on the carbide (100) surfaces possess some basic character, and that the extent of deprotonation is slightly greater on TiC because the carbon is a stronger base than on VC. Support for this idea is observed in the lower binding energy of the C 1s XPS feature on TiC, and the fact that a similar trend was observed with our water adsorption experiments. Specifically, approximately 40% of monolayer water adsorbed dissociatively on TiC (100), whereas only 20% did so on VC. As the temperature was increased, a greater tendency for water decomposition was found on TiC, with TPD showing approximately 75% of the evolved water resulting from surface recombination, equal to the fraction of methanol that
6464 J. Phys. Chem. B, Vol. 106, No. 25, 2002 resulted from surface recombination. We note, however, that much more methanol (which has greater gas phase acidity) than water adsorbs dissociatively at low temperature on VC. It appears, therefore, that the first step of the reaction of methanol on the carbides may be an acid-base reaction to form the methoxy (perhaps methoxide) and a surface hydrogen (proton). The assertion that the monolayer is primarily composed of methoxy bound to the surface metal atoms on TiC (100) conflicts with conclusions drawn previously in the literature. In one UPS study24 performed after exposure of TiC (100) to methanol at room temperature, valence band peaks due to the occupation of specific molecular orbitals that are characteristic of molecular methanol were reported. This result, however, is ambiguous because occupation of these orbitals is dependent upon the assumption of a specific molecular geometry, which was not demonstrated and could be confounded by mixed molecular and dissociative adsorption. In a theoretical study,25 it was assumed that methanol/methoxy would bond to the surface carbon, based upon the behavior of oxygen on transition metal carbides. As we have demonstrated with water,33 the oxygen of methanol will bond to the surface through σ-donation of a lone pair to the lowest unoccupied molecular orbital. On both surfaces described in this work, this orbital is located on the surface metal atom.25,29 The next step in the reaction of methanol on VC results in the cleavage of a carbon hydrogen bond, forming a stable surface intermediate. The vibrational signature and photoemission spectrum of the intermediate are consistent with the surface intermediate bound through both the carbon and the oxygen atoms. The reaction to form this intermediate and the subsequent reaction producing formaldehyde are only observed on VC. Although they may be viewed as having some acid-base character, these carbide surfaces have much less charge separation than an oxide, and will be much less basic than an analogous oxide. More importantly, the added electron population of VC lends some metallic character to the surface, and likely promotes the homolytic C-H bond cleavage leading to the steps described above. This ability to cleave the C-H bond could have negative implications for hydrocarbon lubricant stability. Finally, further decomposition of the remaining adsorbates on both surfaces results in emission of methane and hydrogen. The amount of methane produced is consistent with defect chemistry on both TiC and VC. We have consistently observed variations in surface reactivity on the order of
