Chemistry of Methyl Formate with TiC (100): Comparison of

J. Phys. Chem. C 2007, 111, 11275-11284

11275

Chemistry of Methyl Formate with TiC(100): Comparison of Experiment with Density Functional Calculations Stephen V. Didziulis* and Hyun I. Kim Micro/Nano Technology Department, The Aerospace Corporation, El Segundo, California 90245 ReceiVed: February 9, 2007; In Final Form: May 29, 2007

Titanium carbide is an important tribological material used as an antiwear coating on mechanical components. The TiC(100) surface has demonstrated interesting low-temperature chemistry with a variety of adsorbates, including methyl formate, the simplest ester and a model for this chemical functionality found in many aerospace lubricants. In an effort to identify and characterize surface adsorption and reaction products, we have undertaken a comparison of theoretical results employing density functional methods with experimental results using high-resolution electron energy loss spectroscopy (HREELS) obtained on clean, single-crystal TiC surfaces. The DFT results presented in this work have enabled the identification of an elusive surface reaction product with a characteristic vibration of 1120 cm-1 as a formyl (CHO) species bridging between a surface carbon and a titanium site. This result and others have demonstrated the importance of the lattice carbidic carbon atom in the adsorption of the breakdown products of methyl formate. Specifically, any atomic adsorbate and every reaction product with an unsatisfied valence on a carbon atom showed a theoretical preference for bonding with a lattice carbon rather than the titanium. Alternatively, reaction fragments containing oxygen preferred bonding to surface titanium sites through the oxygen atom, with the exception of carbon monoxide. The calculations show that the breakdown of methyl formate to methoxy and formyl groups, or to surface formate and methyl groups, is exothermic, while further decomposition reactions are endothermic. These results provide a rationale for the experimental observation of these surface reaction products at temperatures ranging from 150 to 300 K.

Introduction Titanium carbide (TiC) is an industrially important hard coating material that improves the performance of steel components in mechanical systems by resisting wear, limiting adhesive interactions with tribological counterfaces, and presumably decreasing the chemical breakdown of lubricants.1,2 In previous work, the interfacial chemical properties of TiC have been probed on a fundamental level through the adsorption of several small molecules with clean TiC(100) surfaces under ultrahigh vacuum conditions. In these studies, this surface has shown significant chemical reactivity at low temperatures. Such reactivity is consistent with the view of transition metal carbidic phases having important catalytic properties.3,4 For example, monolayer methanol,5 water,6,7 and oxygen8 are dissociatively adsorbed at 150 K, forming products that are identifiable with surface spectroscopies. Such reactivity, however, can be less desirable for tribological interfaces if the breakdown of lubricant species results. The reaction of esters with carbides is of particular interest due to the use of ester-based lubricants in aerospace applications.9 Work detailing the reaction of the simplest ester, methyl formate, on TiC at 150 K and higher has been recently published.10 That work showed predominantly dissociative adsorption of methyl formate, with subsequent molecular desorption and further chemical breakdown observed with increasing surface temperatures. In the current study, we experimentally explore methyl formate adsorption on TiC(100) at lower temperatures, and present density functional theory (DFT) results to gain a better understanding of the stable adsorbed species through their calculated surface energy and vibrational modes. * Corresponding author. E-mail: [email protected].

Our previous experimental work on the adsorption and reaction of methyl formate on TiC and VC(100) surfaces, along with studies of the adsorption of species such as methanol5 and carbon monoxide11 provide needed background for the results presented in this work. The adsorption of methyl formate at 150 K on TiC resulted in the simultaneous appearance of HREELS vibrational modes at 1050 and 1120 cm-1, while on VC, the 1050 cm-1 mode appeared with the CO stretch at 2060 cm-1. Our work on methanol adsorption had clearly defined dissociative adsorption with the surface methoxy species on both surfaces producing the HREELS mode at 1050 cm-1. In that work, the methoxy species on VC decomposed upon warming to form an intermediate with a characteristic HREELS loss feature at 1120 cm-1, and this intermediate led to the eventual evolution of gas-phase formaldehyde with increasing temperature. The methoxy species did not react on TiC in this fashion. When the methanol work was published, we hypothesized that the stable intermediate had the generic form of OCHx, with x equal to either 1 (formyl) or 2 (formaldehyde). The considerable stability of the species (present on the VC surface up to 475 K) led us to propose that it was interacting with the surface through both the oxygen and the carbon atoms of the adsorbate, but the actual sites and geometry were unknown. In the first methyl formate study on TiC,10 the simultaneous appearance of the spectroscopic features of the methoxy species and the proposed OCHx species led us to conclude that the 1120 cm-1 feature showed the presence of the surface formyl. This assignment was appealing as simply cleaving the methoxy group from the CH3O-CHO molecule would leave a surface formyl. The formation of a stable surface formaldehyde seemed unlikely from this initial reaction. Since we believe that the methoxy

10.1021/jp071131q CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007

11276 J. Phys. Chem. C, Vol. 111, No. 30, 2007 assignment is very strong and our previous work had shown that methoxy did not decompose on TiC through the 1120 cm-1 intermediate, we had confidence in the formyl assignment but could not provide absolute proof. The detected gas-phase products evolving from the methyl formate exposed TiC provide some support for these assignments. Molecular methyl formate desorbed both at low temperature (weakly adsorbed molecules) and at room temperature and above, where HREELS data showed no evidence of adsorbed molecular species present. Formaldehyde evolution coincided with a decrease in the intensity of the 1120 cm-1 HREELS peak, with methanol desorption at a slightly higher temperature. Hydrogen evolves at approximately 400 K and higher, temperatures corresponding to the loss of HREELS features related to the initial surface reaction species indicating further decomposition of the surface species. Finally, CO2 is also evolved, and its origin will be discussed in this work. Metal carbides present the potential for an array of surface adsorption and reaction sites. Focus is normally centered on the metal atom as the adsorption site and source of electron density to promote chemical reaction, but recent theoretical work suggests the potential for very strong adsorbate bonds to lattice carbons.12 Experimentally, the reaction of TiC with O2 appears to begin by the attack of the lattice carbons and the liberation of carbon oxides.13 Recent work by Rodriguez and co-workers has validated this mechanism and shown the value in using DFT calculations in understanding carbide surface chemistry.14-16 Those results reinforce the importance of the influence of the electronic structure of the carbide materials on their surface chemistry.17 The presence of carbon within the nonpolar TiC(100) surface structure can influence surface chemistry in at least three ways: by providing electron-rich surface carbon sites, by greatly altering the electronic structure of the titanium surface sites, and by providing a unique geometric structure with four carbon atoms as next nearest neighbors to any titanium site. For example, an impact of titanium bonding with carbon in forming the carbide is a redistribution of electron density relative to a metallic Ti surface, with a net transfer of charge from the titanium to the carbon atoms. Our previous DFT calculations17 have shown that the resulting charges are approximately Ti+1 and C-1, in general agreement with others on the rock-salt carbides.16 The mechanism for this charge transfer and the reason for the extreme hardness of the carbide material are the formation of very strong covalent bonds between the two atoms, resulting in virtually equal distribution of the six valence electrons predominantly in the mixed C 2p and Ti 3d σ-bonding levels. The influence of this electronic structure on surface chemistry can be observed in simple chemical interactions, such as the bonding of carbon monoxide to the TiC(100) surface. The adsorption of CO on a Ti site is very weak for a transition metal compound because there are no d-π electrons available for backbonding interactions.11 Others have used DFT in understanding TiC surface chemistry, with focus primarily on the polar, Ti terminated (111) surface.12,18 When larger molecules adsorb on the (100) surface, the potential for the interaction of multiple and chemically different surface atoms with the adsorbate must be considered. As an example of such interactions, we herein probe the preferred surface bonding sites for the methyl formate molecule and its surface reaction products on the TiC(100) surface by comparing DFT computational results with experimental surface chemical studies. Experimental Section The adsorption of methyl formate was studied using HREELS within and ultrahigh vacuum chamber with a base pressure better

Didziulis and Kim than 1 × 10-10 Torr. Briefly, the spectrometer used was an LK2000 HREELS system run with a beam energy of 7 eV, a beam current on the order of 1 × 10-10 A, and a pass energy of 1.5 eV. The TiC(100) surface used was cleaned with repeated Ar+ ion sputter/anneal cycles, with electron beam heating used to anneal the surface to 1400 K. The sample was then cooled to 107 K and exposed to methyl formate vapor through a dosing system. The sample was subsequently warmed with a resistive heater mounted under the sample holder, and the temperature was monitored by a thermocouple clamped to the sample stage. The techniques and conditions for both HREELS and sample dosing employed in this work are the same as those previously reported.10 Theoretical Techniques. The density functional theory calculations performed in this work used the DMol3 Solid State, ver. 3.2 obtained from Accelrys, Inc.19,20 The code was run under the Materials Studio version 4.0 framework, using Visualizer to generate the structures and observe the results. The calculations used the generalized gradient approximation with the Becke-Lee-Yang-Parr (BLYP) correlation functions.21,22 These functions were used on the basis of their similarity to previous work in our group performed with different codes.17,11 Select results were also examined using the RPBA functional, but overall better agreement with experiments was found with the BLYP. The all-electron core treatment was employed. The DND basis sets, a double numeric basis set employing the next highest unoccupied atomic orbital on all atoms except hydrogen, were used. The SCF convergence tolerance was set at either 2.0 × 10-5 Ha or 4.0 × 10-5 Ha, depending on the complexity of the adsorbate species. The calculations used to model the (100) surface primarily used a two layer thick Ti16C16 slab and periodic boundary conditions with bulk Ti-C bond lengths of 2.164 Å. In addition, a four layer thick Ti32C32 slab with periodic boundary conditions was used to assess the impact of having more underlying bulk atoms on the calculation results. In both cases, a 20 Å thick vacuum slab is included in the calculations above the TiC surface, and the interacting adsorbates were placed within this vacuum. Several approaches were considered and tested with respect to the geometry of the model surface. In test calculations with thicker slabs, the positions of the atoms in up to three surface layers were free to move to obtain geometries optimized by the program. We found, however, that the resulting geometries generated greater displacements from bulk lattice positions than had been observed experimentally. We then felt compelled to limit the potential for surface geometric changes to define the most stable surface products, although such changes quite likely play a role. It was decided then to fix the geometries of all substrate atoms other than those directly interacting with adsorbate molecules. This limitation should be taken into consideration in interpreting this work; we note, however, that the typical difference between a surface that was completely frozen to one in which the adsorbate interacting surface atoms were allowed to move was typically quite small, on the order of 4-8 kJ/mol. Adsorbate structures were generated from a variety of starting points, including using optimized geometries for the isolated adsorbate species and with initial structures far from the optimized geometry. The reported results are for the lowest energy structure no matter what the starting geometry. The iterative process of geometry optimization is described within the Accelrys documentation and is controlled by the DMol3 program, using steepest descent, conjugate gradient, and Newton-Raphson methods. The convergence tolerances for the

Chemistry of Methyl Formate with TiC(100)

Figure 1. Slabs used to calculate surface adsorption geometries, energies, and vibrations in this work. The top image contains the 64 atom Ti32C32 cluster within a slightly larger slab employed to demonstrate the periodic boundary conditions. The bottom image is the slab used for most of this work, the 32 atom Ti16C16 structure. In this and all subsequent images, the Ti atoms are green while the carbon atoms are gray.

minimized geometry were an energy change of less than 2.0 × 10-5 Ha, a maximum force difference of 0.004 Ha/Å, and a maximum displacement of 0.005 Å between successive cycles. Vibrational analyses were conducted using the code available within DMol3 and the resulting normal modes were observed within Visualizer. From the DMol3 documentation, the frequencies are calculated for all of the atoms in the cluster, with the Hessian elements computed by displacing each atom in the model and computing a gradient vector, resulting in a complete second derivative matrix. The resulting normal modes were then visualized within Materials Studio to identify the atoms involved. Results and Analysis Validity of DFT of the TiC(100) Surface Model. The selection of a relatively small 32 atom portion of the TiC crystal structure was made to enable calculations within a reasonable time period. As a check of the technique, some general results from the two-layer slab were compared to those of a four-layer slab containing 16 surface atoms, basically the same surface structure but two layers deeper. Each of these slabs is shown in Figure 1, with each actually extended in the drawing to Ti13C12 surface layers, although only the Ti8C8 repeating units were considered in the calculations. One straightforward comparison is through the Mulliken charges calculated on the surface atoms for each case, reflecting the distribution of electron density. For the 32 atom slab, the Mulliken charges on the surface atoms were Ti+0.98 and C-0.98. For the four-layer, 64 atom slab, the charges on the surface atoms were Ti+0.93 and C-1.00. Interestingly, the “bulk” atoms in the middle two layers of the 64 atom slab had greater charges than the surface atoms (Ti +1.30, C-1.23). The smaller slab therefore, predicts surface charges within 5%

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11277 of those calculated by the larger slab. We note that other DFT treatments of the TiC(100) surface have also predicted charges on the order of (1 e.16 An additional result of interest, although one that will not be discussed in this work, is the density of states plots that result from these calculations, which clearly show that the population of primarily metal d surface states is minimal, in agreement with experiment17 and in contrast to the metal-terminated TiC(111) surface both experimentally23 and theoretically.18 A second means of validation of the smaller slab is in the result from the adsorption of a CO molecule on the surface. The calculated energy stability of the geometrically optimized CO species on the 32 stom slab, bonding to a surface Ti site, was -0.0176 Hartree (or -46 kJ/mol), in good agreement with the experimental desorption energy result of 48 ( 8 kJ/mol. This is a very encouraging result, and when the same calculation was performed on the 64 atom slab, a very similar surface bonding geometry was adopted, with a Ti-CO bond length of 2.276 Å and a C-O bond length of 1.150 Å, compared with values of 2.256 Å and 1.152 Å on the smaller slab, both in essentially a perpendicular structure relative to the surface. The surface bonding energy of CO with the larger slab is slightly lower (-38 kJ/mol), which is still close to the experimental value within the given uncertainty limits. The CO results will also be discussed later as a possible surface reaction product. Identification of Stable Surface Species. Figure 2 presents HREELS data obtained from an initially clean TiC(100) surface exposed to 10 L of methyl formate while the surface was held at 107 K. The peaks have been labeled with their loss energies and can be reasonably assigned on the basis of literature information for the adsorbate.24,25 The assignments are listed in Table 1, along with values for the gas-phase molecule26 and data for methyl formate adsorption on Ag surfaces.27 Additionally, the peaks near 510 and 670 cm-1 are loss features of the TiC substrate, as clearly demonstrated in previous work on clean TiC(100) surfaces.8 The weak, but observable, substrate phonon peaks in the methyl formate exposed data indicate that the surface is well-covered by the adsorbate but that physisorbed layers thick enough to obscure the substrate are not present. Our previous work had provided a desorption temperature peak for physisorbed, multilayer methyl formate of 120 K, although the onset of desorption is indicated at temperatures slightly above 100 K.10 From these data, it is unclear whether we are probing multilayer species, but clearly, the major features in the HREELS data under these conditions can be identified as molecular methyl formate. Figure 3 provides a DFT optimized geometry for a methyl formate molecule near the TiC(100) surface. This geometry has a weak interaction between the carbonyl oxygen and a surface titanium atom, with a Ti-O bond distance of 2.36 Å. We attempted to optimize the geometry by promoting the interaction between other parts of the molecule and the substrate, including locating the ester linkage oxygen close to a surface site, but no structure was more stable than this geometry. As will be discussed below, this bonding interaction is weak, such that one would expect molecular desorption to occur at low temperatures. In the previously published TPD data, the next major molecular desorption feature after the multilayer peak is observed above 250 K and has been attributed to a recombinative process from surface reaction products because the molecular modes are not evident in HREELS data above 240 K. Consistent with a weakly adsorbed species, molecular methyl formate was observed desorbing between 120 and 200 K, but a well-defined maximum was not observed in the TPD data. The calculated bond lengths

11278 J. Phys. Chem. C, Vol. 111, No. 30, 2007

Didziulis and Kim

Figure 2. (bottom) HREELS data obtained from a clean TiC(100) surface that has been exposed to a relatively large dose (10 L) of methyl formate at a surface temperature of 107 K. The major loss features are labeled with the loss energy. (top) The HREELS data obtained after an exposure of clean TiC(100) to 1 L of methyl formate at 107 K.

TABLE 1: Experimental HREELS Loss Energies for Methyl Formate Adsorbed on TiC Following a 10 L Exposure at 107 K Are Assigned and Compared with Gas-Phase Values, Other Published Work, and Our DFT Results MeFor/TiC(100) HREELS

assignment

gas phase26

360 520, 670 910 1160 1245 1450 1680 2960

hindered translation TiC phonon CsOsC mode CH3 rock CsOsC mode CH3 deformation CdO stretch CsH stretch

925 1166 1207 1445, 1454 1754 2943-3045

adsorbed Ag(111)27

DFT molecule

DFT adsorbed on TiC

1240 1446 1677 (chem) 1742 (phys) 2984

948, 1049 1135 1199 1445-1484 1771 2857 to 3083

490-550, 650-680 844, 1119 1149 1189, 1376 1447-1474 1668 3002 to 3129

TABLE 2: Calculated Bond Lengths for Molecular Adsorbates, CH3OCHO, CH2O, CO, and CO2 along with the Value Calculated for the Free Molecule in Parentheses TisOHCOCH3 bond

length Å

TisO CdO OHCsOCH3 OHCOsCH3 CsH methyl CsH

2.360 1.222 (1.208) 1.346 (1.368) 1.474 (1.456) 1.104 (1.112) 1.095 (1.100)

CsCH2OsTi bond

length Å

CsC CsO TisO

1.591 1.409 (1.215) 1.935

CsH

1.108, 1.136 (1.118)

for the adsorbed methyl formate molecule are provided in Table 2, where the carbonyl bond length is shown to increase slightly upon adsorption to the surface. Also included in Table 2 are the bond lengths calculated for the free methyl formate molecule, and the bond length results of other molecular adsorbates that

Figure 3. Two views of the optimized structure of the methyl formate molecule adsorbed on the Ti16C16 slab. The oxygen atoms are red, hydrogen atoms are white, carbon atoms are gray, and titanium atoms are green in this and in all subsequent images.

TisCO bond

length Å

TisOCOsTi bond

length Å

TisC CsO

2.256 1.152 (1.143)

TisO CsO

3.51 1.178 (1.179)

will be discussed subsequently. It is interesting to note that the OHC-OCH3 bond length increases slightly (0.02 Å) in the adsorbed methyl formate molecule while the OHCO-CH3 bond length decreases by a similar amount. The normal modes of both the free methyl formate molecule and the adsorbed species shown in Figure 3 have been calculated in this work. The calculated frequencies of these modes are included in Table 1 for comparison to experimental values. The calculations have done a reasonably good job of predicting the normal-mode frequencies, particularly of the free molecule, laying the groundwork for assignments of other adsorbate vibrations. Also note that the calculated TiC substrate modes, which are numerous because of the number of atoms involved in the calculation, bracket the observed phonon peaks at 520 and 670 cm-1 very well. As for the adsorbed species, the carbonyl frequency is of particular importance as the moleculesurface bonding interaction is predicted primarily to occur through the carbonyl oxygen. The calculation has predicted a considerable red-shift of this feature relative to the free molecule, from 1771 to 1668 cm-1, consistent with the experimental

Chemistry of Methyl Formate with TiC(100) finding (1754 to 1680 cm-1). In a less stable monolayer geometry with the carbonyl oriented away from the surface, this mode was calculated to be 1710 cm-1. We note that a similar carbonyl oxygen down adsorption geometry was proposed for submonolayer adsorption of methyl formate on silver because of the absence of specific modes and the significant red-shift of the carbonyl stretch in RAIRS data (provided in Table 2). When multilayers were present in that study at a temperature of 90 K, the carbonyl stretch (1742 cm-1) was much closer to the gas-phase value.27 It is reasonable to assume that the bonding interaction between the molecule and the surface has been approximately reproduced by our DFT calculations and furthermore, because we have not seen HREELS carbonyl intensity closer to the gas-phase frequency, that we do not have appreciable physisorption of methyl formate under our experimental conditions. The various C-H modes are fairly well reproduced by the DFT calculation, while the experimental C-O-C backbone modes at 910 and 1245 cm-1 are bracketed by two sets of modes from the calculation. In addition, there is some unresolved HREELS intensity between the 910 and the 1160 cm-1 features and possibly between the 1245 and the 1450 cm-1 modes. We must note that the descriptions of the modes are approximate and typically involve movements of other parts of the molecule as well. Figure 2 also presents HREELS data obtained at a lower coverage (1 L exposure) on TiC at 107 K. Significant differences exist in these data relative to the higher coverage data, beginning with the much lower intensity of the adsorbate loss features relative to the substrate peak at 520 cm-1, implying a much lower coverage. While the two strong molecular modes at 1250 and 1660 cm-1 are present, the other molecular modes are very weak: a barely observable mode near 900 cm-1 (primarily the OHCO-CH3 stretch), weak methyl modes near 1150 and 1450 cm-1, and a weak C-H stretch near 2950 cm-1. The weakness of these molecular losses could be attributable to the orientation of the molecule on the surface or to the presence of a chemically altered adsorbate. The fact that the 1250 and 1660 cm-1 modes are similar to those at higher coverage makes it likely that the molecule is still present. The greater shift of the carbonyl mode from the gas-phase value may imply a stronger bonding interaction at the lower coverage. In the lower coverage data, new HREELS peaks are observable near 1040 and 1100 cm-1, and these species arise from a partial decomposition of the methyl formate molecule. In the previously published data, similar loss features were observed when methyl formate was adsorbed on TiC(100) at 150 K, and in fact, these losses were the strongest spectral features from 150 to 300 K when the surface was subsequently warmed. These two loss features grow simultaneously in sequential exposure experiments at 150 K. Without interference from molecular features, these peaks are located at 1050 and 1120 cm-1 and were assigned as chemisorbed methoxy (OCH3) and formyl (CHO) species on the basis of comparisons to previous HREELS work on methanol adsorption and decomposition. The breakdown of methyl formate to these components is logical, resulting from the simple cleavage of the H3CO-CHO bond. In addition, weak CO modes are present at 1985 and 2110 cm-1, very similar to results from previous CO chemisorption experiments. To confirm these assignments, methoxy and formyl species were studied on the TiC(100) model surface with DFT, and the most stable structures found are shown in Figure 4. The optimized DFT geometry for methoxy bonding in Figure 4 is as expected, with the oxygen of the adsorbate bonding to

J. Phys. Chem. C, Vol. 111, No. 30, 2007 11279

Figure 4. Optimized surface geometries of the methoxy group (top) and formyl group (bottom) calculated in this work. Note the significant distortion induced by the strong bond between the formyl group and the surface carbon atom.

TABLE 3: Calculated Bond Lengths for Methoxy (OCH3), Formyl (CHO), and Formate (OCHO) Species Ti-OCH3 bond

length Å

Ti-O C-O methyl C-H lattice C-H

1.860 1.405 1.108 3.6 to 3.7

C-CHO-Ti length Ti-O-CH-O-Ti length bond Å bond Å C-C C-O Ti-O C-H

1.421 1.298 2.148 1.111

Ti-O O-C C-H

2.133 1.271 1.108

a Ti atom site in the lattice with the Ti-O-C bond angle of 156°. The bond lengths calculated are provided in Table 3. The normal modes calculated for this structure (Table 4) correlate reasonably well with the experimental result, with a calculated C-O stretching frequency of 1090 cm-1 and methyl group modes in very good agreement with the experiment (1440, 2800 to 2900 cm-1). The short Ti-O bond length of 1.860 Å is indicative of a strong bonding interaction. The structure of the surface formyl was unknown, and in earlier work, we had speculated that it was a bidentate species bridging between two surface metal sites on VC(100).5 As shown in Figure 4, a bridging species is predicted, but the most stable structure bridges between a lattice carbon and a neighboring Ti. This structure was clearly favored over one retaining a carbonyl like C-O species. We attempted to form other structures with the formyl adsorbate, including starting geometries that would favor bridging between two lattice carbons and bridging between two lattice titanium atoms, but the structure with the formyl carbon strongly bonding to a lattice carbon atom and the formyl oxygen interacting with the neighboring Ti site was the most stable geometry. The carboncarbon bond (Table 3) was optimized at approximately 1.42 Å (shorter than a typical alkane C-C bond), while the Ti-O bond length was 2.15 Å, significantly longer than the Ti-methoxy bond, indicating that the C-C bond is dominating this interaction. The normal modes analysis for the bridging formyl also provided a key result and is summarized in Table 4. The C-O frequency of the bridge bonded formyl group was calculated as 1259 cm-1, with a C-H bending mode at 1359 cm-1. Our earlier speculation had been that the 1120 cm-1 HREELS mode was the C-O stretch, on the basis of similar frequencies


Didziulis and Kim

Figure 5. HREELS data obtained from the TiC(100) surface exposed to methyl formate at 107 K and subsequently warmed to room temperature. This result highlights the stability of the formate (1354 cm-1) and formyl (1120 cm-1) surface species.

TABLE 4: Normal Modes Calculated for the Adsorbed Methoxy, Formyl, and Formate Species TiC-methoxy mode

frequency (cm-1)

TiC-formyl mode

frequency (cm-1)

TiC-formate mode

frequency (cm-1)

Ti-methoxy C-O CH3 wags CH3 bends C-H stretches

1090 1128, 1141 1445, 1465, 1485 2880, 2927, 2944

Ti-OCH C-CHO C-O stretch C-H wag C-H bend C-H stretch

683 1127 1258 889 1359 2866

O-C-O O-C-O (sym) O-C-O (asym) C-H C-H bend C-H stretch

762 1321 1513 992 1358 2950

observed for η2-HCO adsorbed on Ru,28 but the calculation does not confirm that assignment on TiC. The calculated bond length of the formyl C-O species is approximately 1.30 Å, which is intermediate between a single and double C-O bond, consistent with the calculated stretching frequency. However, the bond between the surface carbon and the formyl carbon was calculated as having a stretching mode frequency of 1127 cm-1, and the orientation of this bond, nearly perpendicular to the surface, provides an HREELS intensity mechanism for this mode that would be much stronger than that for the C-O stretch that is nearly parallel to the surface. As would be expected, this stretching frequency is higher than that of a C-C single bond, and the calculated charges from a Mulliken population analysis of -0.938 on the lattice carbon and +0.143 on the formyl carbon generate a bond dipole moment of 3.9 D. A third prominent HREELS feature is evident when adsorption is performed at 150 K or when the methyl formate covered surface is allowed to warm from 107 to 300 K, as shown in Figure 5. This loss peak is generally located at 1320-1350 cm-1. In the initial methyl formate work, this feature had been assigned as an adsorbed oxygen species, as similar loss peaks, although typically found at higher frequencies of 1370 to 1400 cm-1, were observed after the decomposition of much smaller molecules such as H2O and O2. A surface C-O species was envisioned and is still likely for the smaller molecules. However, the significantly greater intensity of this feature when compared with that observed in those earlier studies, its presence at temperatures as low as 150 K, and its persistence to high temperatures lead us to consider other possibilities. One prominent possibility was that of a formate group (O-CH-O), a species often observed on oxide surfaces, bonded in a bidentate geometry.29,30 The DFT optimized geometry for this species on TiC is shown in Figure 6, where the two oxygen atoms bridge across two Ti sites, and the important bond lengths are given in Table 3. The predicted normal modes for this species include

the symmetric stretch of the O-C-O group at 1321 cm-1, which would clearly have a dipole moment perpendicular to the surface and agrees well with the strong mode observed in the experiment. Other modes calculated for this species are O-C-O modes at 762 and 1513 cm-1, and C-H modes at 992, 1358, and 2950 cm-1, many of which have moments that are parallel to the surface. Within the HREELS data in Figure 5, there is also a weak feature near 2000 cm-1 (perhaps a defect bound CO molecule), apparently persistent methoxy and formyl species and what appear to be two very sharp C-H modes near 2850 and 2940 cm-1. CO adsorption on the model slab has also been examined, with the most stable adsorption site being atop a Ti atom with the bond lengths provided in Table 2. Interestingly, this geometry generated a C-O stretch of 1998 cm-1, which is quite close to that cited above but much different from the assigned experimental value of 2120 cm-1 from CO adsorption experiments. This discrepancy is worth investigating but of only passing interest in the current work. The possibility for CO2 adsorption was also explored, as it evolves from the methyl formate exposed TiC surface at temperatures of 250 and 420 K. Attempts were made to optimize a CO2 adsorbate geometry directly from the stable formate species shown in Figure 6 by

Figure 6. Optimized structure of a formate group calculated in this work.



Figure 7. Two views of the optimized structure of a formaldehyde molecule on the Ti16C16 slab. As with the formyl group adsorption, a very strong C-C bond is indicated along with a Ti-O interaction.

simply removing the hydrogen and optimizing the structure. However, whatever initial geometry was used, a linear CO2 species with a very weak interaction between both oxygen atoms and Ti surface atoms resulted. It appears more likely that the high temperature form of CO2 results from decomposition of the surface formate, as this species is present at 400 K but absent at temperatures above 600 K. A final molecular adsorbate to consider is formaldehyde, a species that is observed evolving from the methyl formate exposed TiC surface in relatively small amounts. The most stable geometry adopted by a CH2O species is an interesting one, where the adsorbate bonds to a lattice carbon through the formaldehyde carbon and the oxygen interacts with a neighboring titanium site. This structure is shown in Figure 7, and the corresponding bond lengths are provided in Table 2. The most notable aspect of the bonding geometry is the significant distortion of the adsorbate from the planar molecular structure along with a lengthening of the C-O bond because of the bonds (C-C and Ti-O) formed with the surface. Previously, formaldehyde evolution from VC following methanol adsorption was conjectured to occur through a formyl intermediate. This stable adsorbed formaldehyde species would be consistent with that mechanism as the two species have very similar adsorption sites. Additionally, the calculated normal-mode frequencies for formaldehyde in its most stable surface structure do not provide a good match with experimentally observed HREELS features, with the C-O mode at 957 cm-1 and the C-C stretch at 734 cm-1, consistent with the longer bond lengths relative to those of formyl. It is logical that the formyl is a stable reaction intermediate that evolves into formaldehyde when surface hydrogen atoms are available. In summary, the most prominent HREELS features observed following methyl formate adsorption on TiC(100) at temperatures from 107 to 300 K can be accounted for by considering vibrational modes of the complete molecule and modes created by the fragmentation of the molecule into methoxy, formyl, and formate groups. In the following section, we provide the relative

energies calculated for the surface reaction products, along with other surface species that could evolve from the decomposition of the molecule. Energies of Surface Species. As stated above, the calculated adsorption energy for the methyl formate molecule (the difference in total energies for the isolated species relative to the combined cluster and molecule in the most stable structure) was only -42 kJ/mol. The energies for the isolated reaction products (that is, with no other surface species present on the slab) are given in Table 5. Within the table are the energies for the adsorbed reaction products described above, as well as those for many other potential multiple atom surface products. Where appropriate, the DFT calculated energy for a stable isolated molecule (e.g., methyl formate, CO) is also provided to enable an estimation of adsorption energies. The approximate surface bonding interaction of the most stable geometry for an adsorbate is also provided. It is interesting that every molecular fragment having a carbon atom with an unsaturated valence preferred to bond to a lattice carbon through the fragment carbon. The fragments having an oxygen atom with an unsaturated valence (OCH3, OCHO, CHO) bonded to a surface titanium through the oxygen atom. The only exception to this was CO, which bonds weakly to a Ti site through the C atom of the adsorbate. The only experimental data available for validation of the DFT energies is the adsorption energy of CO on TiC(100), which was calculated as -46 kJ/mol in good agreement with the experimentally measured desorption energy of -48 ( 8 kJ/mol.6 The formaldehyde species discussed above is calculated to have an adsorption energy 4 kJ/mol more stable than that of CO. Given that the molecular methyl formate desorbs from the surface at temperatures between 120 and 200 K and CO desorption peaks at approximately 150 K, one would expect that formaldehyde would desorb well below room temperature, implying that the observed evolution of formaldehyde at higher temperatures is reaction limited. All of the molecules in Table 5 have weak interactions with the TiC(100) surface, but it should not be concluded that all species are weakly adsorbed. For example, the adsorption energies of the methoxy and formyl groups, assuming the geometries optimized for the isolated fragments represent the “molecular” species, are calculated as -268 and -222 kJ/mol, respectively. As mentioned above, a variety of bonding geometries were probed for the formyl species, including bridging between two lattice carbons and between two lattice Ti atoms, but the species bridging between C and Ti was more stable by 25 and 63 kJ/mol, respectively. The various atomic species investigated are listed in Table 6, along with their calculated atomic energies and adsorption energies. As indicated, the atomic species had several potential bonding sites with relative energy minima, which were primarily

TABLE 5: DFT Calculated Energies (in Hartrees) for Methyl Formate and Possible Fragments Adsorbed on a TiC(100) Model Surface in Their Most Stable Geometry species bare Ti16C16 cluster CH3OCHO CH3Os sCHOs sOCHOs CO CH2O CO2 sCH3 sCH2 sCH

energy of adsorbate with substrate -14432.17814 -14318.23503 -14317.03495 -14392.35876 -14316.43320 -14317.61097 -14391.72397 -14242.93889 -14242.34118 -14241.74783

energy of isolated molecule (Ha) -14203.06712 -229.09462

-113.34849 -114.52534 -188.65555

adsorbate energy (Ha)

adsorption site

adsorption energy

-229.11102 -115.16791 -113.96784 -189.29164 -113.36608 -114.54386 -188.65685 -39.87177 -39.27407 -38.68072

CdOsTi TisO Bridging CsCsOsTi bridging TisOsCsOsTi TisC CsC, TisO Ti----O CsC CsC CsC

-42 kJ/mol

-46 kJ/mol -50 kJ/mol -4 kJ/mol


Didziulis and Kim

TABLE 6: Calculated Geometries and Energies (in Hartrees) for Potential Atomic Adsorbates Arising from the Decomposition of Methyl Formate

species

bond lengths

Ti-H C-H hollow-H Ti-O C-O hollow-O C-C

1.780 Å 1.121 Ti-H 1.976 C-H 2.246 1.689 1.368 Ti-O 2.450 C-O 1.962 1.334

energy of adsorbed adsorption atom on free atom energy substrate (Ha) (Ha) kJ/mol -14203.61873 -0.49456 -14203.65077 -14203.62662 -14278.32144 -75.08555 -14278.34427 -14278.32918 -14241.13804 -37.84930

-151 -234 -172 -444 -500 -465 -582

atop either the metal or the carbon atom or in a hollow site. Somewhat surprising, the most stable adsorption site for each atomic adsorbate involved predominantly a surface carbon interaction. It is true, however, that the atomic species are stable when bonded to Ti, such that if the reaction occurred at a relatively low temperature and the first adsorption site was a metal atom, then this species could well be detected. The hollow sites are typically intermediate between the Ti and carbon in stability, although the bond lengths to the nearest Ti or C are considerably longer than those for the atop sites. Experimentally, the presence of surface Ti-H, C-H, C-O, and Ti-O species have been assigned through HREELS data after the adsorption of variety of molecules on TiC(100). Interestingly, these atomic surface species are not clearly evident experimentally in the methyl formate decomposition process. Rodriquez and coworkers have studied the bonding of oxygen with TiC(100) both theoretically and experimentally.15 Their DFT geometries agree quite well with those that we calculate as most stable in this work, with the species cited as bonding predominantly with carbon also interacting slightly with two neighboring titanium atoms. The C-O bond length they calculate is virtually identical to our value, and indeed, our species is also not directly atop the C atom. They calculate a much more favored C-O interaction over Ti-C than do we, and this discrepancy may well be due to extent to which the surface is allowed to relax both with and without the adatom. In any case, the general finding is in agreement with this previous work. Reaction Energies. The energies of the surface species can be used to predict the energetics of specific reactions on the TiC(100) surface, assuming that surface species have minimal interaction with one another. One must recognize that these are not quantitative evaluations of these reactions at the temperatures studied, but they should provide a guide as to whether particular pathways are likely to be endothermic or exothermic. These calculations will take the relative energies of the surface species from the above-cited calculations to determine a heat of reaction. For example, we have experimentally observed that methyl formate undergoes dissociative adsorption to form adsorbed methoxy and formyl species:

CH3OCHO (g) + TiC f CH3O (ads) + CHO (ads) Using the energies from Table 5, the calculated enthalpy for this reaction is -109 kJ/mol. The reaction is therefore some 67 kJ/mol downhill from the chemisorbed form of methyl formate. A second reaction pathway involves the creation of a surface formate species. We have not been able to detect a second product for this reaction, but for the sake of this discussion, we will assume that a surface methyl group forms, bonded to a surface carbon site, making the reaction:

CH3OCHO (g) + TiC f CH3(ads) + OCHO (ads)

TABLE 7: Energy Changes Calculated for the Reaction of Stable Surface Species Generated by the Decomposition of Methyl Formate on TiC product

reactants

H3COCOHO (g) H2CO (g) CH3OH (g) CO2 (g) + H (ads) H2 (g)

CH3O (ads) + HCO (ads) HCO (ads) + H (ads) CH3O (ads) + H (ads) OCHO (ads) H (ads) + H (ads)

DFT energy change observed TPD (kJ/mol) temperature +67 +67 +75 +156 +8

275 K 250 to 400 K 360 K 250, 420 K 350 to 500 K

The enthalpy change calculated for this reaction is -180 kJ/mol. The DFT calculations predict, therefore, that both reactions are exothermic and that the surface formate pathway results in more stable surface species. The desorbed reaction products detected after heating the methyl formate exposed surface to temperatures up to 700 K include methyl formate at 275 K, methanol at approximately 360 K, formaldehyde over a broad range of temperatures from 250 to 500 K, H2 from 350 to 500 K, and CO2 at 250 and 420 K.10 The relative heats of reaction from the presumed surface intermediates that form the products can be determined in a similar fashion as the surface reactions cited above. For example, the evolution of formaldehyde from a surface formyl could result from the following reaction:

HCO (ads) + H (ads) f H2CO (g) The resulting enthalpy change for this reaction is calculated as +67 kJ/mol, assuming the surface species are in their most stable adsorption sites. Similarly, reaction enthalpies of methanol formation from the surface methoxy and hydrogen, the degradation of surface formate to evolve CO2 and an adsorbed hydrogen atom, and the reaction of two hydrogen atoms to form H2 have been calculated and are summarized in Table 7. The surface reactants postulated for each of the processes are consistent with HREELS data showing the presence of methoxy and formyl group existing at temperatures from 107 to 300-400 K and the presence of formate at temperatures from 150 K to above 400 K. The reaction of the methoxy and formyl groups to evolve molecular methyl formate is included in this table because the parent molecule is observed desorbing from the surface at temperatures well above those expected for the weakly adsorbed intact molecule. In addition, HREELS detects no evidence of molecular methyl formate at these temperatures, and any reaction to reform the molecule is simply the reverse of the two decomposition reactions detailed above. The calculations presented do not address activation barriers for the indicated surface reactions, which would presumably be related to the calculated stability of the surface adsorbates in question and also their geometries. There are some interesting trends in the predicted energy changes in the reactions relative to the observed desorption temperatures, namely, a general trend toward higher desorption temperatures with an increasing calculated endothermicity. The exception to this trend is the formation of H2 from the adsorbed hydrogen atoms, which is predicted to be only slightly endothermic. This is particularly interesting because two of the observed reactions, formation of formaldehyde and methanol, require surface hydrogen. This could imply that the surface hydrogen is more stabilized than predicted, either by potential hydrogen bonding with coadsorbates or by potentially existing in a more stable subsurface site. In addition, it would appear that the large-scale evolution of hydrogen requires further decomposition of surface species, and the experimental results would imply that this decomposition does not occur below 350 K.



TABLE 8: DFT Calculated Energy Changes for the Breakdown of Surface Species to Form Surface Hydrogen surface species

breakdown products (ads)

energy change

CH3O CH3O CH3O CHO CHO CH3 CH3 CH3 OCHO OCHO OCHO

CH2O + H CHO + 2H C + O + 3H CO + H C+O+H CH2 + H CH + 2H C + 3H CO2 + H CHO + O C + 2O + H

+ 121 kJ/mol + 102 + 180 + 46 + 96 + 38 + 67 + 130 + 134 + 121 + 218

The decomposition of the surface methoxy, formyl, methyl, and formate groups has also been examined. In this process, we can predict the most likely source of surface hydrogen and compare the results with previous work. The results for various decomposition reactions are shown in Table 8. From these results, the most likely source of surface hydrogen is from a surface methyl group, where an only +38 kJ/mol change is evident with the loss of one hydrogen and a +63 kJ/mol change is evident with the formation of two hydrogen atoms along with a CH species. This is attributed to the strong lattice carbonadsorbate carbon interaction making the fragment very stable. The next most likely source is from the decomposition of the formyl group to surface bound CO and H, requiring +46 kJ/mol. Importantly, adsorbed CO prefers to bond carbon end down on a Ti site while the stable formyl species forms a C-C bond, requiring a significant change in geometry and hence a potentially high activation barrier for this pathway to be relevant. The breakdown of methoxy is not favored as a source of hydrogen, and in fact, previous experimental work showed that this species is quite stable on the TiC(100) surface. The breakdown of the surface methyl group may also explain the presence of a unique HREELS feature observed in a narrow temperature range. A loss feature appears at 2180 cm-1 but was only observed after the methyl formate exposed surface had been warmed to 400 K, a temperature in the middle of the range of large scale hydrogen evolution. In our earlier work, we conjectured that such a species was possibly explained by a surface alkyne of some sort, with the only reasonable alternative being a bound CO. However, the stretching frequency is quite high for a stable CO species at such a temperature. We had also considered the possibility of this loss feature arising from an overtone or a combination of two loss features but could find no appropriate assignment, and the fact that the feature is only evident in a narrow temperature range where other HREELS features are disappearing led us to believe that it is a distinct chemical species. The nature of the bond between the TiC surface and the alkynyl CH group can be probed through our calculations. The energy pathway cited above indicates that the enthalpy change to produce this species is not prohibitive and that it is fairly stable with respect to further breakdown to a surface C atom. In either case, whether CH or C, the most stable adsorption site is clearly a carbon atom, and the calculated bond lengths are 1.39 and 1.33 Å respectively, values that are more consistent with an alkene structure. Conceivably, two such surface carbon species may have combined to form an adsorbed acetylene, perhaps leading to the detected vibrational mode. Further experimental work would be required to sort through these possibilities. Discussion The DFT calculations performed in this work have helped to identify surface reaction products by obtaining the optimum

geometries for potential surface species and then determining the expected normal-mode frequencies for these species. The most easily identifiable and assignable species is the methoxy group, on the basis of our previous work on methanol adsorption and numerous other reports in the literature.31 The prediction of a stable methoxy species, bonded to a surface titanium atom with a calculated C-O stretching frequency (1090 cm-1) that is in reasonable agreement with experiment (1050 cm-1), provides confidence that other surface species can be identified. In addition, the calculated weak interaction between the methyl formate molecule and the surface and the shift in the carbonyl stretching frequency indicating that the interaction occurs through this species are consistent with the experimental results and the literature.27 Along with the accurate adsorption energy predicted for CO, we have confidence that the calculations provide valuable and reasonably accurate results for the adsorption of methyl formate and the subsequent reaction products. With that, two surface species have been better defined through these experimental and theoretical results, the η2-formyl species bridging between surface C and surface Ti sites and the η2-formate species bridging between two surface Ti sites. Both of these species are predicted to be favored products, and the normal modes calculations have shown excellent agreement with experiment. All three of these fragments (including the methoxy) are observed to relatively high temperatures on the TiC surface and have been proposed to be precursors to gasphase products that are eventually evolved, including methanol, formaldehyde, and carbon dioxide. The initial breakdown of the methyl formate, therefore, appears to have two possible pathways, forming methoxy and formyl through one pathway and formate through the second pathway. We have not been able to identify the other surface reaction products from the formate pathway, but the formation of a surface methyl group or a surface CHx species along with surface hydrogen are possibilities. The possible breakdown of the methyl group has been indicated by the calculations as a likely source of surface hydrogen that is needed to evolve products such as methanol from adsorbed methoxy and formaldehyde from formyl. One result that consistently appears through this work is the greater calculated stability of certain methyl formate reaction products when they bond with one of the carbidic carbons in the TiC lattice. The participation of the lattice carbon in the deprotonation of methanol had been a conclusion of previous work, showing that such a site has some basic character by analogy to an oxygen atom in a metal oxide. What was somewhat more surprising was the prediction that the surface carbon site interacts strongly with the carbon atom from either the formyl or the methyl carbon reaction product, depending on the breakdown pathway. Key to this finding is the proposition that the 1120 cm-1 vibration has been correctly assigned as the lattice C-formyl C stretch. In previously cited work,5 the decomposition of methanol on VC(100) proceeded from a methoxy intermediate through a species with a nearly identical 1120 cm-1 vibration; the similarity of the spectroscopic features argues for a similar formyl with a similar surface bonding geometry, and the lattice carbon in a similar geometry is the common species. In addition, every atomic species was more stable when the primary bonding interaction was with a surface carbon site. The participation of the lattice carbon in bonding and reaction with oxygen has been well established. The current work strongly indicates the participation of the lattice carbon in other surface reactions as well, likely because of the electron rich nature of the species on the (100) surface of TiC. The presence of a neighboring Ti site can provide added stability to

11284 J. Phys. Chem. C, Vol. 111, No. 30, 2007 an adsorbate, as predicted for the formyl, or can act to stabilize other surface products, such as methoxy. Surface formyls have been projected reaction intermediates in many different surface reactions, including methanol synthesis32 and Fischer-Tropsch chemistry.33 However, formyls have been rarely captured on surfaces to investigate spectroscopically, usually because they are unstable relative to other surface species. In the current study, the stability of the proposed formyl surface species is due to the very strong bond formed between the lattice carbon and the adsorbate carbon, a circumstance that is clearly not possible in studies involving clean metal or oxide materials in search of catalytic mechanisms. Our DFT work predicts that any chemical change to an adsorbed formyl on TiC(100) is endothermic, with the decomposition reaction to CO and H having a slight preference over the formation of gas-phase formaldehyde. A reaction that was not specifically addressed earlier was the formation of an adsorbed formaldehyde, which the calculations predict to be endothermic by only 17 kJ/mol, and one might argue that, in the presence of free surface hydrogen, this reaction would be the predominant pathway for the surface formyl on TiC. Formaldehyde was a significant product of methanol decomposition on VC(100), and it is also detected in the present work. The fact that the initial methyl formate decomposition products form at such low temperatures but then are fairly stable as observed experimentally and predicted theoretically is interesting and may point to the potential for very interesting chemistry on the carbide materials. With regard to lubricant degradation, in particular with ester species, the ease of the initial breakdown of the methyl formate might seem somewhat troubling. Alternatively, the presence of stable reaction products could lead to the formation of a protective boundary film and possibly improve the tribological performance of such a material pairing. In related work, a VC(100) surface exposed to ethanol at room temperature formed a stable surface film that seemed to almost polymerize on the surface and acted to lower friction.34 The ethoxy species that was shown to exist through the HREELS spectrum also evolved into a species with the characteristic 1120 cm-1 vibration, perhaps showing that a similar acetyl species is participating, although a different assignment was proposed in that work. Experiments aimed at specifically probing the role of the carbidic carbon using isotopically labeled species would provide great insight into the reactivity of carbides in general and these specific materials. Conclusions A combination of experimental work and DFT calculations have provided significant insights into the reaction of methyl formate with the TiC(100) surface. Experimentally, the most stable low-temperature reaction products have been determined to be surface methoxy, formyl, and formate species, and the pathways to produce such species have been found to be exothermic by DFT. The predicted surface adsorption sites for fragments of the molecule display some interesting trends. All atomic adsorbates (C, O, H) are more stable when bonding with a surface carbon site than either directly atop a Ti site or within the hollow formed by two Ti and two C atoms. Any multiatom fragment containing O, other than CO, prefers to bond through the oxygen to a surface Ti site. Any surface fragment containing a carbon atom with an unsatisfied valence prefers to form a C-C bond with the surface. The distinctive surface species having a characteristic 1120 cm-1 vibration has been determined to be the η2-formyl group bonded to both a surface carbon and titanium. The participation of the electron rich

Didziulis and Kim surface carbidic carbon species, therefore, seems to be very clear in these interactions, showing that the role of the carbon atom in carbide chemistry should not be overlooked. Experimentally, any further degradation of these surface species requires an increase in temperatures, consistent with the DFT calculations that show all subsequent reactions to be endothermic. The production of formaldehyde, methanol, and carbon dioxide are enabled by the stability of the formyl, methoxy, and formate surface species and the presence of surface hydrogen. Acknowledgment. We gratefully acknowledge the funding support of The Aerospace Corporation Independent Research and Development Program. All trademarks, service marks, and trade names are the property of their respective owners. References and Notes (1) Boving, H. J.; Hintermann, H. E. Tribol. Int. 1990, 23, 129. (2) Jones, W. R.; Pepper, S. V.; Wheeler, D. R.; Jansen, M. J.; Quyhngiao, N.; Schro¨er, A. Tribol. Trans. 2000, 43, 685. (3) Hwu, H. H.; Chen, J. G. Chem. ReV. 2005, 105, 185. (4) Oyama, S. T., Ed. The Chemistry of Transition Metal Carbides and Nitrides; Kluwer: New York, 1996. (5) Frantz, P.; Didziulis, S. V.; Fernandez-Torres, L. C.; Guenard, R. L.; Perry S. S. J. Phys. Chem. B 2002, 106, 6456. (6) Didziulis, S. V.; Frantz, P. P.; Perry, S. S.; El-bjeirami, O.; Imaduddin, S.; Merrill, P. B. J. Phys. Chem. B 1999, 103, 11129-11140. (7) Merrill, P. B.; Perry, S. S.; Frantz, P. P.; Didziulis, S. V. J. Phys. Chem. B 1998, 102, 7607-7612. (8) Frantz, P. P.; Didziulis S. V. Surf. Sci. 1998, 412-413, 384. (9) Didziulis, S. V.; Lince, J. R.; Carre´, D. J.; Hilton, M. R. Lubrication. In NASA Space Mechanisms Handbook; Fusaro, R., Ed.; National Aeronautic and Space Administration, Glenn Research Center: Cleveland, OH, 1999; Chapter 15, NASA/TP -1999-206988. (10) Frantz, P.; Kim, H. I.; Didziulis, S. V.; Li, S.; Chen, Z.; Perry, S. S. Surf. Sci. 2005, 596, 144. (11) Didziulis, S. V.; Frantz, P.; Fernandez, L.; Guenard, R.; El-bjeirami, O.; Perry, S. S. J. Phys. Chem. B 2001, 105, 5196. (12) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Catal. Today 2005, 105, 66. (13) Souda, R.; Aizawa, T.; Otani, S.; Ishizawa, Y. Surf. Sci. 1991, 256, 19. (14) Liu, P.; Rodriguez, J. A. J. Chem. Phys. 2004, 120, 5414. (15) Rodriguez, J. A.; Liu, P.; Dvorak, J.; Jirsak, T.; Gomes, J.; Takahashi, Y.; Nakamura, K. J. Chem. Phys. 2004, 121, 465. (16) Rodriguez, J. A.; Liu, P.; Gomes, J.; Nakamura, K.; Vines, F.; Souse, C.; Illas, F. Phys. ReV. B 2005, 72, 075427. (17) Didziulis, S. V.; Butcher, K. D.; Perry, S. S. Inorg. Chem. 2003, 42, 7766. (18) Vojvodic, A.; Ruberto, C.; Lundqvist, B. I. Surf. Sci. 2006, 600, 3619, and references therein. (19) Delley, B. J. Chem. Phys. 1990, 92, 508. (20) Delley, B. J. Chem. Phys. 2000, 113, 7756. (21) Becke, A. D. J. Chem Phys. 1988, 88, 2547. (22) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 786. (23) Oshima, C.; Aono, M.; Zaima, S.; Shibata, Y.; Kawai, S. J. Less Common Met. 1981, 82, 69. (24) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (25) Zahidi, E.; Castaonguay, M.; McBreen, P. J. Am. Chem. Soc. 1994, 116, 5847. (26) Hollenstein, H.; Gunthard, Hs. H. J. Mol. Spectrosc. 1980, 84, 457. (27) Schwaner, A. L.; Fieberg, J. E.; White, J. M. J. Phys. Chem. B 1997, 101, 11112. (28) Anton, A. B.; Parmeter, J. E.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 107, 5558. (29) Liehr, M.; Thiry, P. A.; Pireaux, J. J.; Caudano, R. Phys. ReV. B 1985, 31, 42. (30) Henderson, M. A. J. Phys. Chem. B 1997, 101, 221. (31) Methoxy characterization. (32) Neurock, M. Top. Catal. 1999, 9, 135. (33) Morgan, G. A.; Sorescu, D. C.; Zubkhov, T.; Yates, J. T. J. Phys. Chem. B 2004, 108, 3614. (34) Kim, B. I.; Lee, S.; Guenard, R. L.; Fernandez-Torres, L. C.; Perry, S. S.; Frantz, P.; Didziulis, S. V. Surf. Sci. 2001, 481, 185-197.

Chemistry of Methyl Formate with TiC (100): Comparison of

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