Methanol Adsorption on the Clean CeO2(111) Surface: A Density

Jun 23, 2007 - Molecular and dissociative adsorption of methanol at various sites on the stoichiometric CeO2(111) surface have been studied using dens...
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J. Phys. Chem. C 2007, 111, 10514-10522

Methanol Adsorption on the Clean CeO2(111) Surface: A Density Functional Theory Study Donghai Mei,*,† N. Aaron Deskins,† Michel Dupuis,† and Qingfeng Ge*,‡ Chemical and Materials Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352, and Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: March 19, 2007; In Final Form: May 9, 2007

Molecular and dissociative adsorption of methanol at various sites on the stoichiometric CeO2(111) surface have been studied using density functional theory periodic calculations. At 0.25 monolayer (ML) coverage, the dissociative adsorption with an adsorption energy of 0.55 eV is slightly favored. The most stable state is the dissociative adsorption of methanol via C-H bond breaking, forming a coadsorbed hydroxymethyl group and hydrogen adatom on two separate O3C surface sites. The strongest molecular adsorption occurs through an O-Ce7C connection with an adsorption energy of 0.48 eV. At methanol coverage of 0.5 ML, the dissociative adsorption and the molecular adsorption became competitive. The adsorption energy per methanol molecule for both adsorption modes falls into a narrow range of 0.46-0.55 eV. As methanol coverage increases beyond 0.5 ML, the molecular adsorption becomes more energetically favorable than the dissociative adsorption because of the attractive hydrogen bonding between coadsorbed methanol molecules. At full monolayer, the adsorption energy of molecular adsorption is 0.40 eV per molecule while the adsorption energy for total dissociative adsorption of methanol is only 0.17 eV. The results at different methanol coverages indicate that methanol can adsorb on a defect-free CeO2(111) surface, which are also consistent with experimental observations.

1. Introduction Ceria has attracted considerable attention over the past few years both as a catalyst support and as a promoter in many heterogeneous catalytic processes such as NOx reduction from automotive emissions1-3 and hydrogen production from methanol.4-12 In particular, efficient production of hydrogen from methanol would have a great impact on the future hydrogen economy. On the other hand, the catalytic role that ceria plays in methanol conversion is still not understood, and insight into the interaction between ceria surfaces and methanol molecules will help us to better understand and elucidate the reactivity and selectivity of ceria-based catalysts. In this study, we present our results of a theoretical investigation of methanol adsorption, both molecular and dissociative, on the clean CeO2(111) surface using periodic density functional theory (DFT). Several experimental investigations of methanol adsorption on the CeO2(111) surface have been reported.9,13-16 Siokou and Nix13 found that methanol dissociatively adsorbed as methoxy and hydroxyl species on a CeO2(111)/Cu(111) surface at 300 K using a combination of different experimental techniques. Upon heating, they observed that CO and H2 desorbed from the surface as the major products. The oxidation of the adsorbed methoxy to formates was believed to happen at the lower coordination sites near the perimeter of oxide islands. Further heating causes methoxy to decompose into CO, H2, and HCHO while formate also to CO2. Namai et al.14 reported multiple adsorption sites for the methoxy species on CeO2(111) surface using noncontact atomic force microscopy. Ferrizz et al.16 performed temperature-programmed desorption (TPD) on * To whom correspondence should be addressed. Phone: (509) 3756303; fax: (509) 375-4381; e-mail: [email protected] (D.M.). Phone: (618) 453-6406; fax: (618) 453-6408; e-mail: [email protected] (Q.G.). † Pacific Northwest National Laboratory. ‡ Southern Illinois University.

CeO2(111) supported on R-Al2O3(0001) and yttria-stabilized zirconia (YSZ). A small amount of methanol (0.08 monolayer) that adsorbed on their CeO2(111) surface was attributed to adsorption at the surface defect sites. The only TPD products were formaldehyde and methanol. These authors also indicated that CO production occurs only if the oxygen vacancies exist on CeO2(111) surface.16 They found that the product ratio between formaldehyde and CO depends upon the oxygen vacancy density on CeO2(111) surface. Mullins et al.15 further investigated methanol adsorption and reactivity on a CeO2(111) thin film grown on Ru(0001) using TPD, soft X-ray photoelectron spectroscopy (XPS), and nearedge X-ray absorption fine structure (NEXAFS) techniques. They concluded that methanol can adsorb on a clean defectfree CeO2(111) surface. In addition to the formation of formaldehyde and methanol, water was also formed on a fully oxidized CeO2(111) surface during their TPD experiment. No formation of CO or H2 was observed on the defect-free CeO2(111) surface. The surface coverage of methanol reported by Mullins et al.15 was about 0.25-0.5 monolayer (ML) which is much higher than the coverage of 0.08 ML as suggested by Ferrizz et al.16 although it is not clear if both groups used the same definition for a monolayer. Mullins et al. also found that methoxy is the only stable surface intermediate formed by methanol dissociation and that water formation was attributed to the recombination of surface hydroxyl groups.15 To the best of our knowledge, no DFT investigation of methanol adsorption and decomposition over ceria surfaces has been reported, although there have been many studies of methanol adsorption and decomposition over other metal oxide surfaces such as TiO2,17-20 R-Al2O3,21 MgO,22,23 SrTiO3(100),24 and β-Ga2O3.25 Bates et al. studied methanol adsorption on rutile TiO2(110) surface using first-principles total energy calculations and ab initio molecular dynamics simulations.17 There are slight

10.1021/jp072181y CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

Methanol Adsorption on the Clean CeO2(111) Surface energy differences between molecular and dissociative adsorptions of methanol at both full and half monolayer surface coverages. In addition, methanol decomposition with either O-H or C-O bond scissions was found equally favorable. Methanol adsorption and decomposition on clean and hydroxylated anatase TiO2(101)20 and TiO2(001)18,19 surfaces have also been studied using Car-Parrinello molecular dynamics simulations. On the clean (101) surface, molecular adsorption of methanol is energetically more stable. With increasing methanol surface coverage, the dissociative adsorption becomes favorable. On the hydroxylated (101) surface, however, molecular and dissociative adsorption become competitive. In the study of methanol adsorption on R-Al2O3(0001), Borck and Schroder21 found a significant surface relaxation of surface Al atoms in the surface normal direction because of methanol adsorption. These authors attributed the reduced methanol adsorption energy with increasing surface coverage to the repulsive lateral interactions between the adsorbed methanol molecules.21 In this work, we present a comprehensive theoretical study of the adsorption configurations of methanol on the defect-free CeO2(111) surface using DFT. We chose this surface because it is the most stable26 and abundant surface in ceria nanoparticles.27 Both molecular and dissociative adsorption of methanol over a variety of sites on the perfect CeO2(111) surface with coverages ranging from 0.25 to 1.0 ML were investigated. In the present work, the coverage is defined as the ratio of methanol molecule number to surface Ce atom number, and a full monolayer (1 ML) corresponds to one methanol molecule per surface Ce atom. In particular, the relative energetic stability for the dissociative adsorption of methanol via O-H bond, C-H bond, and C-O bond scissions under different coverages was investigated. Computational details are given in section 2, and results and discussion are presented in section 3. Finally, we summarize and conclude in section 4. 2. Computational Details We performed periodic DFT calculations using the Vienna ab initio simulation program (VASP).28,29 The projector augmented wave method (PAW) was used to describe core electrons.30,31 A plane wave basis set with a cutoff energy of 500 eV was used to expand the valence electronic wavefunction with valence configurations of Ce-5s25p66s24f15d1, O-2s22p4, C-2s22p2, and H-1s. The Perdew-Burke-Ernzehof (PBE) functional32 was used in the calculations. The ground-state geometries of bulk and surface were obtained by minimizing the forces on each atom to below 0.05 eV/Å. We obtained a bulk lattice parameter of 5.47 Å using a k-point grid of 12 × 12 × 12, in good agreement with the previously reported theoretical results of 5.46 Å33 and 5.38 Å34 and the experimental value of 5.41 Å.35 The CeO2(111) surface was represented by a slab in a supercell. As shown in Figure 1, the slab representing the (111) surface of CeO2 consists of neutral (O-Ce-O) repeating units. The (111) surface is terminated by 3-fold coordinated O atoms (O3C), different from the bulk O atoms which are 4-fold coordinated. The second surface layer consists of Ce atoms, which are 7-fold coordinated (Ce7C). Bulk Ce atoms are 8-fold coordinated. The third surface layer consists of the 4-fold coordinated O atoms (O4C). We performed test calculations using different sizes of the surface unit cell, number of trilayers, and k-point grid to select parameters for reliable results. On the basis of the results from our test calculations, we chose a (2 × 2) surface unit cell with three trilayers (7.73 Å × 7.73 Å × 8.05

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Figure 1. Optimized structure of clean CeO2(111) surface. Surface O atoms in red, second layer O atoms in dark red, and Ce atoms in blue.

Å) as our model of the CeO2(111) surface. Each slab consisted of three CeO2 trilayers (O-Ce-O) separated from their images in the z direction by a 15 Å vacuum. A (2 × 2 × 1) k-point grid was used for Brillouin zone sampling. In this study, the atoms in the bottom trilayer were fixed at the corresponding bulk positions while all the other atoms were allowed to relax. We obtained a surface energy of 0.67 J/m2 for this slab, which is consistent with the previously reported DFT values ranging from 0.61 to 0.68 J/m2 using different methods such as generalized gradient approximation (GGA) and DFT + U methods.36-38 Also in agreement with previous DFT results,37,38 our results show that the optimized CeO2(111) surface is nearly unperturbed from bulk positions. The Ce-O bond length of surface atoms is 2.36 Å, which is slightly shorter than the bond length in the bulk (2.37 Å). Errors due to self-interaction in describing the 4f states of the reduced ceria standard DFT may not always be negligible.26,39 The DFT + U method has been used to correct the self-interaction errors in systems containing Ce3+ ions39,40 by applying a Hubbard-like correction (the parameter U) to the system. We examined the self-interaction effect in our system by performing test calculations using the DFT + U method with two reported values of U ) 5 eV39 and U ) 7 eV.34 Our results indicate that there is a small increase in adsorption energy in the range of 0.01 and 0.13 eV when the DFT + U method was used, and the larger value involves higher degree of reduction of surface cerium atoms. However, the DFT + U method does not change the overall trends of the results and the conclusions of the present study. We therefore chose to use the standard DFT method. Adsorption energies, Ead, of methanol on the surface were calculated by the following equation:

Ead )

(Eclean slab + N‚ECH3OH) - Eadsorbed slab N

(1)

Eadsorbed slab is the total energy of the interacting system of CeO2(111) slab and methanol molecules; N is the number of molecules on the surface; Eclean slab is the total energy of bare CeO2(111) slab; ECH3OH is the energy of one methanol molecule in the vacuum. Positive Ead values indicate favorable (exothermic) adsorption. 3. Results and Discussion 3.1. Low Coverage Adsorption (θ ) 0.25 ML). First, we investigated methanol adsorption at 0.25 ML surface coverage. The system consisted of one methanol molecule in the (2 × 2) surface supercell. Methanol molecule was placed at three

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Figure 2. Structures of molecular adsorption of methanol on the clean CeO2(111) surface at 0.25 ML coverage. For each figure, the left shows the top view and the right shows the side view. All numbers in the figures show distances in Å (angstroms).

possible adsorption sites in turn (above first-layer O3C, thirdlayer O4C, and second-layer Ce7C) as shown in Figure 1. Of four possible adsorption structures such as O-Ce7C, C-O3C, O-O4C, and H-O3C (in this notation, the first atom refers to methanol, and the second atom refers to the CeO2 surface) anticipated on the basis of electrostatic matching, only two adsorption structures, those with O-Ce7C and H-O3C connections, were found to be stable. Methanol adsorbs at a surface Ce7C site through an O-Ce7C bond as shown in Figure 2a; this was found to be the most stable molecular adsorption structure. The adsorption energy is 0.48 eV. For this configuration, the hydroxyl hydrogen atom of methanol points toward a neighboring surface O3C atom and the methyl group points upward. In essence, interactions between the O atom of methanol and surface Ce7C and between methanol hydroxyl H and surface O3C contribute to stabilizing this adsorption structure. The clean CeO2(111) surface also becomes perturbed in the O-Ce7C adsorption configuration. A strong attractive interaction between the hydroxyl hydrogen of the methanol and the O3C atom makes the O3C atom move up from the CeO2(111) surface plane by 0.15 Å. The distance between the hydroxyl hydrogen and O3C is 1.72 Å. The distance between

Mei et al. the O atom of methanol and surface Ce7C atom is 2.62 Å. To make sure that the standard DFT method is valid in this work, we also calculated the adsorption energy of this configuration using the DFT + U method. The adsorption energies are 0.52 and 0.51 eV with U ) 5 and U ) 7, respectively. Methanol can also molecularly adsorb to a surface O3C atom solely through a H-O3C bond which is shown in Figure 2b. The adsorption energy of the H-O3C configuration is 0.24 eV, which indicates that the H-O3C configuration is less stable than the O-Ce7C configuration for molecular methanol adsorption. The height between the hydroxyl hydrogen and the surface O3C atom is 1.84 Å, which is slightly larger than the distance between H and O3C for the O-Ce7C configuration. The weak adsorption in the H-O3C configuration perturbs the CeO2(111) surface less than the stronger O-Ce7C adsorption. The contact surface O3C atom is only 0.05 Å higher than the other O3C atoms on the surface. We checked this adsorption structure using the DFT + U method. The adsorption energies are 0.25 and 0.25 eV with U ) 5 and U ) 7, respectively. To further understand the molecular adsorption modes, we performed projected density of states (PDOS) analysis of the two molecularly adsorbed states. Figure 3a shows that in both the O-Ce7C (designated “strong” adsorption) configuration and H-O3C configuration (designated “weak” adsorption), a broadening of the hydroxyl hydrogen 1s band occurs upon adsorption. The hydrogen atom interacts with the closest surface O3C, and the H 1s band lies in the same energy range as the O3C 2p bands (-5 to 0 eV). Figure 3b shows the interactions between the O orbitals of the methanol and the closest Ce atom. For the weak adsorption case, the O and Ce orbitals are very similar to the gas phase and clean surface orbitals. However, for the strong adsorption case, the Ce d and f orbitals between -5 and 0 eV align to overlap with the O orbitals to favor the charge transfer leading to the ionic species. These interactions serve to stabilize the structure and to increase the adsorption energy. We further calculated the vibrational frequencies of the O-H stretch for gas-phase methanol and the two adsorbed states. We see large shifts (-642 cm-1 for O-Ce7C binding and -378 cm-1 for H-O3C binding) indicative of strong interactions between the methanol O and H atoms and the ceria Ce7C and O3C atoms. Three hydrogen atoms surround the C atom of methanol. Therefore, the C atom cannot easily interact with the surface O3C atoms directly. Consequently, most of the previous theoretical DFT studies of methanol adsorption and reaction on TiO2 and other metal oxide surfaces only focused on molecular adsorption, or O-H and C-O bond breakings. To get more insight into this adsorption structure, we calculated the molecular adsorption energy of methanol with an initial bridging arrangement of one of the methyl hydrogen atoms closer to a subsurface O4C atom and the tilted hydroxymethyl group bonding to the O3C site via a C-O3C connection on the surface. An adsorption energy of 0.08 eV was obtained for this geometric configuration (see Figure 2c), indicating very weak binding. Our results are consistent with recent experimental observations by Mullins et al. They investigated methanol adsorption and reaction on a fully oxidized CeO2(111) surface using TPD and NEXAFS15 and found that methanol can adsorb on a defect-free surface. The TPD experimental results are suggestive of two adsorption modes of methanol on the CeO2(111) surface in which one is a weaker physisorbed state and the other is a stronger bound state. Both adsorption modes are bonded weakly as manifested by two desorption peaks at about 140 and 200 K, respectively.15 The two stable molecular adsorption states via O-Ce7C and H-O3C in our calculation

Methanol Adsorption on the Clean CeO2(111) Surface

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Figure 3. Projected density of states (PDOS) for the molecularly adsorbed states at 0.25 ML coverage. (a) PDOS for hydroxyl H in methanol and closest surface O. (b) PDOS for O in methanol and closest surface Ce. In all cases, the plots are shifted so that the Fermi level is at 0 eV.

may correspond to the two desorption peaks observed in the TPD experiment. The dissociative adsorption of methanol can occur by breaking of one of three bonds: C-O, O-H, or C-H. The dissociatively adsorbed methanol can form coadsorbed states of CH3,a + OHa, CH3Oa + Ha, or CH2OHa + Ha pairs on the clean surface. Most experimental and theoretical studies of methanol adsorption and reaction over metal surfaces and metal oxide surfaces have focused on O-H and C-O bond breaking. However, the C-H bond breaking during methanol decomposition was found possible on a metal oxide surface such as MoO3.41,42 A strong kinetic isotope effect was observed for the methyl C-H bond breaking of methanol. It was believed that the breaking of the C-H bond is the slowest step in methanol oxidation over MoO3 surface.41,42 We explored different adsorption combinations for dissociative adsorption of methanol. Four stable dissociative adsorption configurations are shown in Figure 4. We found that the most stable structure for the dissociative methanol adsorption was

Figure 4. Structures of dissociative adsorption of methanol at 0.25 ML coverage.

formed by breaking a C-H bond. One of the C-H bonds in the methyl group of methanol is ruptured by a surface O3C atom, resulting in a hydroxymethyl fragment adsorbed on the O3C site through a C-O3C bond and a surface hydroxyl group formed by hydrogen adsorption at the surface O3C site (see Figure 4a). The adsorption energy is 0.91 eV. Both the surface O3C atoms bonded to hydroxymethyl and hydrogen in this configuration were pulled out of the surface plane by this adsorption. The O3C atom binding with the hydroxymethyl group is about 0.52 Å higher compared to surface O3C atoms on a clean surface. As a result, the surface O3C-Ce7C bond lengths near the

10518 J. Phys. Chem. C, Vol. 111, No. 28, 2007 hydroxymethyl were elongated to 2.56 and 2.76 Å, compared to the unperturbed surface O3C-Ce7C bond length of 2.36 Å. The surface hydroxyl pulled the surface O3C atom out of the surface plane by 0.41 Å relative to clean surface O3C atoms. Similarly, the surface O3C-Ce7C bond lengths associated with this hydroxyl are elongated to 2.54 Å and 2.62 Å. We examined different initial structures and found them to converge to the same configuration as shown in Figure 4a. We also have found a slightly increased adsorption energy of the C-H bond scission configuration using DFT + U method. The adsorption energies are 0.99 and 1.04 eV with U ) 5 and U ) 7, respectively. The dissociative adsorption of methanol with O-H breaking is shown in Figure 4b. The methoxy and hydroxyl fragments bind to a Ce7C site through an O-Ce7C connection and to an O3C site through a H-O3C bond. The adsorption energy for this O-H dissociative configuration is 0.55 eV, which is less stable than the dissociative adsorption via C-H bond breakage (0.91 eV). The O atom of the methoxy and the H atom adsorbed at the O3C sites are close to each other with a distance of 1.62 Å. The formation of a surface hydroxyl pulled the occupied O3C atom out of the surface plane by 0.30 Å. Consequently, the distance between the occupied O3C atom and the occupied Ce7C atom was stretched to 2.94 Å from 2.37 Å on the clean surface. The occupied Ce7C atom is less disturbed, only relaxed by 0.1 Å outward. An alternative configuration after O-H scission adsorption is that the methoxy occupies the Ce7C site and that the stripped H atom binds at the O3C atom that is next to the Ce7C atom along the [112h] direction (see Figure 4c). The adsorption energy in this adsorption configuration is 0.40 eV, being less than that of the methoxy and hydrogen in the adjacent Ce7C and O3C sites. The last dissociative adsorption occurs via the C-O bond breaking shown in Figure 4d. The dissociation will lead to coadsorption of a methyl group at a surface O3C site and a hydroxyl group over a surface Ce7C site. The dissociative adsorption energy in this case is only 0.17 eV, being the weakest of all dissociative adsorption modes. Upon methyl binding, the O3C atom was pulled out of the surface by 0.41 Å. The resulting C-O3C bond is 1.43 Å. At 0.25 ML methanol coverage, dissociative adsorption via C-H or O-H bond breaking is energetically more favorable over the molecular adsorption. On the other hand, the dissociative adsorption via C-O bond breaking is the least favorable. This result suggests that methyl will not likely be formed from direct C-O bond breaking. We believe that the relatively more stable surface intermediates, such as hydroxymethyl and methoxy, resulting from the dissociative adsorption of methanol are likely the source of the formaldehyde. The relatively more stable surface intermediates, such as hydroxymethyl and methoxy, from the dissociative adsorption of methanol can likely be further oxidized to formaldehyde by surface oxygen atoms on the CeO2(111) surface. We analyzed the charges of the dissociatively adsorbed species using Bader’s atoms in molecules method43,44 to further understand the chemical bonding resulting from the various dissociation processes. Figure 5 shows the results of the charge distributions among the different methanol dissociation products on the surface. As can be seen, heterolytic dissociation appears to be taking place, since the products all appear charged rather than being neutral fragments. There also appears that some charge transfer takes place between adsorbed fragments and surface atoms. Experimental homolytic gas-phase dissociation energies45,46 vary between 3.97 and 4.53 eV depending on which bond is being broken. Gas-phase heterolytic dissociation energies

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Figure 5. Bader charge analysis of dissociative adsorption of methanol products. All indicated values are charges in e. The charges of O3C and Ce7C on a clean surface are -1.13 and +2.29. (a) C-H scission in HOH2C-O3C and H-O3C configuration; (b) O-H scission in CH3OCe7C and H-O3C configuration; (c) O-H scission in CH3O-Ce7C and H-O3C configuration (far); (d) C-O scission in H3C-O3C and HOCe7C configuration.

are much larger, being 11.06 eV for C-H scission, 12.02 eV for C-O scission, and 16.53 eV for O-H scission. The presence of Ce4+ and O2- ions in the surface stabilizes the charged products. The favorable mechanisms have changed from homolytic dissociation in the gas phase to heterolytic dissociation on the surface. Although the calculation results show that dissociative adsorption of methanol to form hydroxymethyl and hydroxyl via C-H bond breaking is energetically favorable at 0.25 ML methanol coverage, it does not necessarily show that the C-H bond breaking is the most favorable dissociation pathway. The experimental results indicated that both O-H and C-H bond breakings are possible on clean CeO2(111) surface at low temperatures.15 Mullins et al. attributed the small amount of D2O formed at 200 K to the breaking of C-H bond in the adsorbed methanol.15 However, both their SXPS and NEAXFS data indicated that methoxy is the only stable surface intermediate on fully oxidized CeO2(111) surface. In fact, activation barriers may determine which pathway the reaction will eventually take. This issue requires more examination. 3.2. Half-Monolayer Coverage Adsorption (θ ) 0.5 ML). We further studied methanol adsorption at 0.5 ML coverage by placing two methanol molecules in a (2 × 2) surface unit cell. For molecular adsorption, we examined three initial combinations: both methanol molecules atop Ce7C sites (A), both methanol molecules atop O3C sites (B), and one methanol atop an O3C site with the other methanol atop a Ce7C site (C). The optimized adsorption structure of two methanols atop Ce7C (A) is shown in Figure 6a. From the initial adsorption geometries of B and C, the stable configurations converge to the same structure after relaxation, which is shown in Figure 6b. The adsorption energies of this stable adsorption are 0.50 and 0.51 eV per methanol molecule, essentially the same value. When

Methanol Adsorption on the Clean CeO2(111) Surface

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Figure 6. Structures of molecular adsorption of methanol at 0.5 ML coverage.

both methanol molecules were placed initially on the O3C sites (B), one of the methanol molecules moves to a neighboring Ce7C site because of the formation of a hydrogen bond between the coadsorbed methanol molecules. This yields the only two stable arrangements for molecular adsorption at 0.5 ML coverage, that is, one methanol molecule over the O3C site and the other methanol over the Ce7C site (Figure 6b) or both methanol molecules over the Ce7C sites (Figure 6a). In both configurations, the hydroxyl hydrogen atom of adsorbed methanol at the Ce7C site points toward the oxygen atom of the other adsorbed methanol at the O3C site. The distance between this hydrogen atom and oxygen atom is 1.65 Å. A slightly longer distance (1.87 Å) was found for the O-Ce7C configuration shown in Figure 6a. The adsorption energy per methanol molecule for the most stable configuration is 0.51 eV, which is slightly more stable (0.48 eV) than a single methanol molecule adsorbed at the Ce7C site at 0.25 ML coverage (see Figure 2a). Adsorption of a second molecule at the adjacent site, however, dramatically stabilizes the methanol molecule at the O3C site as compared to 0.25 ML coverage (see Figure 2b), and the adsorption energy at the O3C sites is increased by 0.27 eV/molecule when going to 0.5 ML coverage. This adsorption strength increase is attributed to hydrogen bonding between the two methanol molecules. A more significant stabilization effect due to hydrogen bonding by adding another water molecule on anatase TiO2(101) surface was reported previously.47 On the clean anatase TiO2(101) surface, the adsorption energies per methanol molecule are 0.70 and 0.68 eV when two methanols were adsorbed, depending on the surface sites.20 When only one methanol was adsorbed on the (101) surface, the adsorption energy per methanol drops to 0.21 eV. To further quantify this stabilization effect over ceria, we calculated the adsorption energy of single methanol molecules with the same adsorption geometries as those at 0.5 ML in Figure 6b. We found the adsorption energies of single methanol molecules via the H-O3C bonding and the O-Ce7C bonding to be only 0.08 and 0.24 eV, respectively. On the other hand, there

Figure 7. Structures of dissociative adsorption of methanol at 0.5 ML coverage. (a) Partial dissociation, (b) total dissociation, (c) total dissociation.

is a substantial increase in adsorption energy (0.34 eV/molecule) when two methanol molecules were coadsorbed. This indicates a strong attractive interaction between the coadsorbed methanol molecules. The lateral interaction of two methanol molecules in gas phase with the same configuration as those in Figure 6b is attractive by 0.11 eV. The Bader charges for all the abovementioned systems (gas-phase methanols, two methanols adsorbed, or single methanols adsorbed) are all similar, indicating that the stabilization effect is not total electrostatic or chargetransfer driven. Hydrogen bonding between adsorbates could contribute to the stabilization effect. The ionic nature of the ceria surface also serves to stabilize the adsorbed methanol molecules, but some covalent bonding between the molecules and surface induced by two methanol molecules being coadsorbed could be present to further stabilize the adsorbates. For dissociative adsorption of methanol at 0.5 ML coverage, we studied three configurations with partial as well as total dissociation of two methanol molecules. In the partial dissociation configuration shown in Figure 7a, one methanol molecule binds at a Ce7C site and another methanol dissociates via O-H bond scission into an adsorbed methoxy on the Ce7C site and a hydrogen adatom on an O3C site. The average adsorption energy for this partial dissociation is 0.46 eV per methanol molecule.

10520 J. Phys. Chem. C, Vol. 111, No. 28, 2007 For total dissociative adsorption of methanol via O-H bond breaking, the structure of each dissociated methanol molecule is very similar to that of dissociative adsorption at 0.25 ML coverage, as shown in Figure 7b. The total dissociative adsorption via O-H bond breaking is slightly more stable than the partial dissociative adsorption, increasing the adsorption energy by 0.07 eV per molecule or giving an adsorption energy of 0.53 eV per methanol. We also examined total dissociative adsorption of methanol via C-H bond breaking at 0.5 ML coverage. Two configurations with different orientations of the hydroxymethyls and hydrogen atoms over the two O3C sites, as well as a third configuration with the hydroxymethyls over the O3C site and hydrogen atoms over the O4C site, were investigated. We found that only one configuration as shown in Figure 7c was weakly adsorbed on the surface. The calculated adsorption energy per methanol molecule is 0.18 eV for this configuration. These results indicate that the C-H bond breaking is no longer energetically favorable for methanol dissociative adsorption at 0.5 ML coverage. The strong repulsive interaction between methylene groups may be the reason for inhibition of the dissociative adsorption via C-H bond breaking. Our calculation results indicate that stable methanol overlayers on the clean CeO2(111) surface for either molecular adsorption or dissociative adsorption seem to occur up to 0.5 ML. This is in qualitative agreement with the experimental results of Mullins et al.15 Mullins et al. found that methanol coverage on fully oxidized CeO2(111) surface was between 0.25 and 0.5 ML, in contrast to 0.08 ML observed by Ferrizz et al.16 3.3. High Coverage Adsorption (θ g 0.75 ML). To further increase the methanol coverage to 0.75 ML, we included three methanol molecules in the supercell. We found that molecular adsorption of methanol was almost equally stable for both the H-O3C and O-Ce7C configurations. Figure 8a and 8b shows these structures. The adsorption energies for both molecular adsorption configurations are 0.43 and 0.44 eV per molecule, respectively, smaller than the molecular adsorption energies of methanol at 0.5 ML. We also examined the configuration with dissociation of all three methanol molecules via the O-H bond scission, in which the methoxy fragments bind at surface Ce7C sites and the hydrogen atoms bind at surface O3C sites (see Figure 8c). The adsorption energy is 0.30 eV per methanol molecule for the dissociative adsorption. The total dissociation of methanol via either C-H or C-O bond scission was found unstable at 0.75 ML coverage. At full monolayer coverage (θ ) 1.0 ML), three adsorption configurations, including two molecularly adsorbed states (HO3C and O-Ce7C configurations) and a total dissociative adsorbed state via O-H bond breaking, were investigated. In each configuration state, four methanol molecules were relaxed in the (2 × 2) surface supercell. Our calculations show that the molecular adsorption with all H-O3C connections on a CeO2(111) surface at full monolayer coverage was not stable. The repulsive interactions between methyl groups of the neighboring methanol molecules are likely too strong to accommodate all four methanol molecules in this configuration. The optimized structure shows that in one methanol molecule the O-H bond is broken; a hydroxyl is formed with a surface O3C atom, and the remaining methoxy moves into the vacuum. The other three methanol molecules still adsorb to surface O3C atoms via the H-O3C bonding. However, a stable molecular adsorption with all O-Ce7C connections was obtained at full monolayer coverage. The optimized structure is shown in Figure 9a. The adsorption energy

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Figure 8. Structures of adsorption of methanol at 0.75 ML coverage. (a) Molecular adsorption, (b) molecular adsorption, (c) total dissociation.

of the O-Ce7C configuration is 0.40 eV per molecule, which is slightly lower than the adsorption energy of 0.44 eV per molecule at 0.75 ML coverage. We also calculated the lateral interactions of four methanol molecules with the same arrangement as the adsorbed configuration but without the substrate. The lateral interactions were attractive with an energy of 0.15 eV for the four molecules primarily because of hydrogen bonding. In Figure 9a, an ordered pattern was formed in the methanol overlayer (see Figure 9a top view) although the initial arrangement was not ordered. The distances between O atoms of methanol molecules along one surface Ce7C row are 3.77 and 3.96 Å; the corresponding distances between O atoms of methanol molecules along another surface Ce7C row are 3.94 and 3.79 Å. This stable configuration may not be the only stable structure, and we cannot exclude other ordered bonding structures. We assume that the hydrogen-bonding network between coadsorbed methanol molecules plays the key role in stabilizing the molecularly adsorbed methanol at high coverages. An ordered arrangement may be necessary to accommodate the methanol molecules and to reduce the repulsive interactions between the methyl groups. Further investigation will be needed to explore all the possible adsorbate configurations and is beyond the scope of the present study.

Methanol Adsorption on the Clean CeO2(111) Surface

J. Phys. Chem. C, Vol. 111, No. 28, 2007 10521

Figure 10. Adsorption energies as function of methanol coverage for dissociative adsorption through O-H bond breaking and for molecular adsorption through H-O3C and O-Ce7C.

Figure 9. Structures of adsorption of methanol at 1.0 ML coverage. (a) Molecular adsorption and (b) total dissociation.

We investigated one total dissociative adsorption structure of methanol at full monolayer coverage. Each methanol molecule dissociates into a methoxy group that sits on a Ce7C atom and a hydrogen atom that forms a hydroxyl with a surface O3C atom via four O-H bond scissions. This optimized structure is shown in Figure 9b. The methoxy and newly formed hydroxyl group are tilted toward each other. The distances between the hydrogen atom of the hydroxyl and the oxygen atom of methoxy are between 1.67 and 1.85 Å. The adsorption energy is 0.17 eV/molecule. This indicates that total dissociative adsorption of methanol at full monolayer coverage is energetically favorable over the methanol in the gas phase but much weaker than the molecular adsorption. This would support the idea of the adsorption stability being driven by a balance between the formation of a hydrogen-bonding network and steric hindrance. The more stable non-dissociated state involves less steric repulsion than the dissociated state and leads to more stable adsorption configurations. Figure 10 compares the adsorption energies of methanol for two molecular configurations and for dissociative configuration (O-H bond breaking) at different coverages. Although C-H bond breaking is more favorable at 0.25 ML methanol coverage, we did not include it in the figure as it quickly became unfavorable as the coverage is increased. Two molecular adsorption configurations, O-Ce7C and H-O3C, were included in the plot. For molecular adsorption via the H-O3C connection, the adsorption energy of methanol increases from 0.24 eV per molecule at 0.25 ML to 0.50 eV per molecule at 0.5 ML. The attractive hydrogen bonding through interacting with neighboring coadsorbed methanol in O-Ce7C configuration stabilizes the methanol in H-O3C structure. As the coverage increases to 0.75 ML, the adsorption energy decreases to 0.43 eV per molecule. At full monolayer coverage, we did not find stable molecular adsorption via H-O3C interaction. For molecular adsorption via the O-Ce7C connection, the coverage effect on adsorption energy is very small. The adsorption energies of methanol with the O-Ce7C structure are in the range of 0.40-

0.51 eV per molecule in the coverage range of 0.25-1.0 ML. Unlike the H-O3C configuration, a stable molecular adsorption configuration with the O-Ce7C interaction at full monolayer coverage was found. For the dissociative adsorption via the O-H bond scission, we have found that the adsorption energy decreases with the increasing coverage. The fact that the methanol molecules that adsorbed either molecularly or dissociatively on the CeO2(111) surface over the coverage of 0.250.5 ML are stable is consistent with Mullins et al.’s recent experimental observations.15 Moreover, coverage can be used to control the states of the adsorbed methanol and further the chemistry of methanol on the surface. 4. Conclusions Density functional theory has been used to investigate methanol adsorption on an unreduced CeO2(111) surface. Our results indicate that methanol can adsorb on the surface either molecularly or dissociatively. Our observation of stable adsorption on the stoichiometric surface is consistent with recent experimental observations. At 0.25 ML coverage, the most stable structure for methanol adsorption is the dissociative adsorption forming hydroxymethyl and hydroxyl by C-H bond breaking with adsorption energy of 0.91 eV. Both dissociative adsorptions of methanol by either C-H or O-H bond scission are more favorable than the molecular adsorption. The most stable molecularly adsorbed structure occurs via O-Ce7C binding with an adsorption energy of 0.48 eV. As the methanol coverage increases to 0.5 ML, molecular adsorption and dissociative adsorption become competitive. The adsorption energies for both molecular adsorption and dissociative adsorption fall into a narrow range of 0.48-0.56 eV per methanol molecule. When the methanol coverage increases to higher than 0.5 ML, we have found that the molecular adsorption is more energetically favorable than the dissociative adsorption with the assistance of attractive hydrogen bonding between coadsorbed methanol molecules. The adsorption energy of molecular adsorption is 0.40 eV at full monolayer coverage, which is stronger than the total dissociation via the O-H scission with an adsorption energy of 0.17 eV per molecule. Acknowledgment. This research was performed using the Molecular Science Computing Facility in the William R. Wiley

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