Kinetic Mechanism of Methanol Decomposition on Ni(111) Surface: A

J. Phys. Chem. B 2005, 109, 12431-12442

12431

Kinetic Mechanism of Methanol Decomposition on Ni(111) Surface: A Theoretical Study Gui-Chang Wang,*,† Yu-Hua Zhou,† Yoshitada Morikawa,‡ Junji Nakamura,§ Zun-Sheng Cai,† and Xue-Zhuang Zhao† Department of Chemistry, and the Center of Theoretical Chemistry Study, Nankai UniVersity, Tianjin, 300071, P. R. China, Research Institute for Computational Sciences (RICS), and Research Consortium for Synthetic Nano-Function Materials Project (SNAF), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan, and Institute of Materials Science, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan ReceiVed: August 10, 2004; In Final Form: April 13, 2005

The decomposition of methanol on the Ni(111) surface has been studied with the pseudopotential method of density functional theory-generalized gradient approximation (DFT-GGA) and with the repeated slab models. The adsorption energies of possible species and the activation energy barriers of the possible elementary reactions involved are obtained in the present work. The major reaction path on Ni surfaces involves the O-H bond breaking in CH3OH and the further decomposition of the resulting methoxy species to CO and H via stepwise hydrogen abstractions from CH3O. The abstraction of hydrogen from methoxy itself is the rate-limiting step. We also confirm that the C-O and C-H bond-breaking paths, which lead to the formation of surface methyl and hydroxyl and hydroxymethyl and atom hydrogen, respectively, have higher energy barriers. Therefore, the final products are the adsorbed CO and H atom.

1. Introduction The adsorption and the decomposition of methanol have been investigated on a variety of transition-metal surfaces in recent years, because of the possible usage of methanol as a new liquid energy carrier and its ease of synthesis from biomass, coal, and natural gas (all of which will be more abundant resources than crude oil).1,2 In addition, the decomposition of methanol followed by the desorption of carbon monoxide and hydrogen (i.e., the methanol reforming) can be applied in the methanolfueled vehicles in which the heat of the exhaust gas will be adsorbed during this endothermic reaction, then the generated decomposition gas will be fed to the engine.3 Besides, the advantage concerning energy transportation is also its merit: because liquid methanol is more convenient for long-distance transportation to the process sites, where it could be converted into CO and H2 and utilized for various purposes. This reaction may also be applicable to the recovery of waste heat from industries, but significant improvement of the catalysts must be achieved first. Methanol is the simplest alcohol and contains three different kinds of heteroatom bonds, that is, C-H, C-O, and O-H, and has been selected as one of the prototypes in surface science for exploration of the selective activation of chemical bonds on metal surfaces. Therefore, it is vital to understand in detail the reaction mechanism of methanol on metal surfaces, from the viewpoints of both fundamental and applied sciences. Over the last two decades, there has been considerable interest in the decomposition mechanism of methanol on transition and noble metal surfaces such as Cu,4-11 Ni(100),12-18 Ni(110),19-21 * Corresponding author. E-mail address: [email protected]. Telephone: +86-22-23505244 (O). Fax: +86-22-23502458. † Nankai University. ‡ National Institute of Advanced Industrial Science and Technology (AIST). § University of Tsukuba.

Ni(111),22,23 Pt,24-30 Pd,31-35 and Fe,33,36-38 with or without preadsorbed oxygen. For these surfaces, a number of experimental studies on this system have been performed with several techniques including temperature-programmed desorption (TPD/ TPRS), high-resolution electron energy loss spectroscopy (HREELS/EELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), near-edge X-ray adsorption fine structure (NEXAFS), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), Fourier transform infrared spectroscopy (FT-IR), and molecular beam techniques. Major results from these studies are that at low temperatures methanol is adsorbed molecularly, while at higher temperatures, it leads to the formation of an adsorbed methoxy species, followed by the successive dehydrogenation of CH3O to CO and H. Although in all these studies it has been reported that methoxy, CH3O(a), is the dominant surface intermediate species at temperatures below 350 K and could be easily formed through an O-H bond scission, the opinions on further decomposition steps of CH3O(a) toward CO formation vary: Some authors have suggested CHxO (x ) 1, 2) species as intermediates,4-12,15,16,24,38,39 and others observed C-O bond cleavage on Pd,31-34 while a direct transition from methoxy to CO without any intermediate species seems to be favored by still other researchers.13,14,17-23,25-30,35-37,40 Thus, at this stage, both the decomposition pathways for chemisorbed CH3OH as well as the general principles necessary for understanding its surface reactivity remain quite muddled. For the CH3OH adsorption and decomposition over Ni catalyst, TPR experimental results41 indicated that the CH3OH decomposition reaction occurred exclusively via a methoxy intermediate that formed via O-H bond cleavage during TPR from 140 to 240 K. Methoxy decomposes to adsorbed carbon monoxide and hydrogen at 240-290 K. No evidence for reactions involving C-O bond cleavage in either the molecularly adsorbed methanol or the methoxy was seen. By contrast, the

10.1021/jp0463969 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/03/2005

12432 J. Phys. Chem. B, Vol. 109, No. 25, 2005 C-O bond scission mechanism was proposed by Friedrich et al.,42 and others even suggested that there would be an additional quasi-stable intermediate HCO or COH.12,15-16 Because of its importance for catalysis, there is a rich literature of theoretical studies related to CH3OH decomposition. For the initial methoxy coverage less than 0.20 ML, the abstraction of the first methoxy hydrogen is determined as the rate-limiting step in CH3OH decomposition.20 In addition, the adsorption of certain intermediates (methoxy,17,23,43-51 formaldehyde,52,53 formyl54,55) in the pathway beginning with O-H scission has been studied individually with theoretical calculations, such as the manyelectron embedding theory which is at the ab initio configuration interaction level,43-45,51 and with experimental studies.17,18,46-49 However, to the best of our knowledge, there is no detailed theoretical calculation about the kinetic activation barrier analysis. The goal of this article, therefore, is to understand the methanol decomposition pathway on Ni(111). For example, we wish to explore which among the C-O, C-H, and O-H bonds in CH3OH is easier to break and also to determine which step is the rate-limiting step in CH3OH decomposition over the Ni(111) surface. The energetics of the elementary reactions involved are analyzed. The analysis is based on the calculation of the change of activation energies for all the elementary reactions involved, and the nudged elastic band (NEB) method56,57 is employed to calculate these quantities. Wherever possible, the calculated structural parameters and the energies are compared to the available experimental data. In this paper, details of the calculations are discussed in section 2. In section 3, the pathways of the reactions are introduced. In section 3.1, the characterization of species formed during the adsorption of methanol and during the following decomposition is presented and discussed. In section 3.2.1-3.2.4, the energetics of each elementary step involved are presented, and the potential energy surface of methanol decomposition is discussed in section 3.2.5. In section 4, we summarize the results and draw some conclusions. 2. Method of Calculation and Models All calculations were carried out within the DFT framework. The GGA with the Perdew, Burke, and Ernzerhof58 functional was used for the exchange and correlation energy calculation. Valence electrons were described by Vanderbilt’s ultrasoft pseudopotential,59a while Troullier and Martin’s norm-conservering pseudopotential was employed for other components.59b The cutoff energies are 25 Ry and 400 Ry for wave functions and augmentation charge density, respectively. The Ni(111) surface was modeled by a periodical array of three-layered slabs separated by ∼10 Å of vacuum region. A p(3 × 2) unit cell was chosen, which means a monolayer of adsorbate with coverage of 1/6 ML, and this coverage is corresponding to the low coverage of CH3O on the Ni surface reported in the experimental studies (smaller than 0.20 ML).17-20 In calculations, a Monkhorst-Pack mesh of 4 × 6 × 1 special k-point sampling in the surface Brillouin zone was used. The substrate atoms were held fixed in their bulk crystal configuration, while the adsorbate was allowed to relax. All these calculations were carried out using the package STATE (Simulation Tool for Atom TEchnology) which has been successfully applied to study adsorption problems in the case of semiconductor and metal surfaces.60,61 In this work, the adsorption energy (Qads) or binding energy (B.E.) of species A on Ni surface is calculated according to the formula

Wang et al.

B.E.(A) ) EA/Ni - ENi - EA where E is the calculated total energy. For a reaction like AB ) A + B, the calculated heats of reaction (or reaction energy) for those reactions including in the methanol decomposition are listed under three different definitions: ∆H1 is the heat of reaction in the gas phase, ∆H2 is calculated on the basis of the formula of ∆H2 ) EA/M + EB/M - EAB/M - EM (where EAB is the total energy for the AB/M adsorption system, etc.), and ∆H3 is the calculated heat of reaction on the surface based on the formula of ∆H3 ) E(A+B)/M - EAB/M (where E(A+B)/M is the total energy for the coadsorption system of A/B/M). The difference between ∆H2 and ∆H3 is the interaction between A and B in the coadsorption structure. The reaction paths of methanol decomposition on Ni(111) are found by the NEB method.56,57 The transition-state search is initiated by interpolating a series of images of the system between the initial and final states on the potential energy surface: On the potential energy surface, a spring force between the adjacent images is added to keep the spacing between the images constant, and the true force is applied to keep the images sliding toward the MEP (minimum-energy path), thus mimicking an elastic band. Each image is optimized with the NEB algorithm (a constrained molecular dynamics algorithm). This approach helps the images converge to the reaction path being searched, as well as locating the highest point of the MEP. The highest point of the optimized reaction coordinate along the MEP should be the transition state along the chosen reaction path, and this highest energy relative to that of the initial state gives the activation barrier of the reaction. In fact, to increase the density of images near the transition state (TS) and to locate the TS more accurately, the modified NEB method (i.e., ANEBA method57) was used. In the ANEBA method, we choose three movable images connecting two local minima on the potential energy surface and use the NEB method as a starting level. After the calculation converges to some given accuracy, we choose the two images adjacent to the one that has the highest energy as our new starting points for the next level NEB calculation. Through several such levels of NEB calculation, at the last level, the ANEBA calculation will locate three images in which the total energy of each is almost the same, and then, the one in the middle is considered the TS. Although this approach does not employ the frequency analyses, it has been shown in many cases to give excellent convergence to saddle points on the analytical potential energy surface.62,63 3. Results and Discussion The possible reaction mechanism for methanol decomposition over Ni(111) surface can be written as follows:

CH3OH(g) + / f CH3OH(a)

(M1)

CH3OH(a) + / f CH3O(a) + H(a)

(M2)

CH3OH(a) + / f CH2OH(a) + H(a)

(M3)

CH3OH(a) + / f CH3(a) + OH(a)

(M4)

H3CO(a) + / f H2CO(a) + H(a)

(M5)

H2CO(a) + / f HCO(a) + H(a)

(M6)

HCO(a) + / f CO(a) + H(a)

(M7)

2H(a) f H2(g) + 2/

(M8)

MeOH Decomposition on Ni(111)

J. Phys. Chem. B, Vol. 109, No. 25, 2005 12433

CO(a) f CO(g) + /

(M9)

CO(a) + / f C(a) + O(a)

(M10)

H2CO(a) + / f H2C(a) + O(a)

(M11)

HCO(a) + / f HC(a) + O(a)

(M12)

CO(a) + H(a) f COH(a) + /

(M13)

where g and a stand for the gas phase and the adsorbed phase, respectively. 3.1. Structures and Energetics of Adsorbed Intermediates. In general, there are four high-symmetry adsorption sites on the Ni(111) crystal surface: one atop site which resides above a surface atom, two threefold hollow sites which correspond to the fcc site and the hcp site (the hcp site resides above a subsurface atom in the second substrate layer; the fcc site does not), and one bridge site which lies halfway between and the fcc and hcp sites. Understanding the equilibrium geometry of adsorbed molecules and fragments on the surface has considerable relevance to understanding the surface reaction. It is often assumed that spin-unpolarized calculations can describe the behavior of adsorbates rather well64 and suggest that magnetism usually reduces the adsorption energy of species but has little effect on the activation barrier calculation.65 Therefore, most of the activation barrier calculations in this work do not include the magnetism effect. However, to specifically understand both the effects of spin polarization and surface relaxation on the adsorption energy and the reaction barrier, we chose some crucial adsorption species and the reaction step to test the effect (details will be introduced late). 3.1.1. Methanol. Methanol through its oxygen atom bonds weakly to the atop site of Ni(111) (Figure 1). The adsorption energy of methanol is found to be -16 kJ/mol (3.89 kcal/mol), which is consistent with the experiment study where it was adsorbed molecularly and kept intact on an Ni foil surface (5.359 kcal/mol) in the low-temperature range through the MBRS method (molecular beam relaxation spectrometry).66 The TPD experiment result13 also suggested that, at low coverage, methanol is adsorbed molecularly and desorbed intact from the Ni surface in the low-temperature range, while at high temperature, methanol was decomposed and ultimately desorbed as H2 and CO. The interaction of methanol with the surface is weak, indicated by the rather long Ni-O bond length (2.371 Å) in the optimized structure (see Table 1 for the summary of geometrical and energetic information). Hu et al. also have performed theoretical slab calculations with the DFT-GGA method to investigate the adsorption of methanol on the Pd(111) surface;67 Greeley has performed similar calculations on Cu and Pt63 and found the same geometry as our work. It is believed that the adsorption of methanol appears to occur via a donation of a lone pair of electrons from the oxygen to the metal surface.22 To estimate the effect of magnetism, a slightly lower binding energy of methanol, -15 kJ/mol, is obtained by using the spinpolarized calculations, while the geometrical properties are remarkably reproduced (as listed in Table 1), indicating that the effect of magnetization on the surface structure is rather small. 3.1.2. Methoxy. This intermediate also interacts with the surface through oxygen and has a binding energy of -249 kJ/mol (-59.66 kcal/mol) (see Table 1). This value is smaller than that determined from the cluster model (ca. 90 kcal/mol44), possibly because of the cluster size effect. For the methoxy radical, a strong energetic preference for the threefold fcc hollow

site is found from our work, as well as from other theoretical and experimental results.42,44,47,51,68a,b To ensure that the configuration considered is the most stable, we also investigate the adsorption on the bridge site and the top site. For the bridge site, the calculated adsorption energy is lower than that of the fcc site by 5 kJ/mol and with a tilting angle of 15.5°. At the same time, we find that the adsorption at the top site is so unstable it could not converge. Here, we only report the geometry of the most stable configuration in detail. The C-O axis of high symmetry (C3V) is perpendicular to the metal surface with the methyl group directed away from the surface (Figure 1). This is identical to the C-O surface angle of 180° found in the alkali methoxide.69 In addition, the angle-resolved photoemission16 and high-resolution electron loss spectra12 for the surface methoxy have been interpreted in terms of a normal or near-normal C-O axis on the Ni(111) surface, and the XPD studies42 have shown that the O-Ni and C-O distances are 1.93 ( 0.04 Å and 1.44 ( 0.05 Å, respectively, and the oxygen atom adsorbs on the fcc threefold hollow site, which are all consistent with our results. The OCH3/Ni(111) chemisorption system has been treated by an embedded cluster method based on a three-layer (28 Ni) substrate cluster and the local correlated ab initio wave functions.44 The authors found rather strong chemisorption and a tilting configuration possibly inclined about 5-10° from the surface normal at the threefold site with an adsorption energy of 90 kcal/ml, and a greater tilting angle of 20° at the bridge site with an adsorption energy of 87 kcal/mol. They suggested that Ni 3d orbitals contribute substantially and the bond of methoxy to the surface is characterized as a combination of ionic and covalent bonds. Although their adsorption energies are both higher than ours, the preferred site is the same as ours. Most recently, Remediakis et al.70 obtained an adsorption geometry on the atop site with a binding energy of 1.86 eV; however, this configuration may actually be a locally stable position during the process of CO hydrogenation to methanol, at the higher coverage of CO (1/4 ML); that is, it can be easily formed from coadsorbed formaldehyde and hydrogen atoms and then accept another hydrogen atom to form the adsorbed methanol molecule. Methoxy is stable at lower temperature according to HREELS spectra,52 then decomposes from 210 K and up.46 The optimized configuration in Figure 1 keeps the initial symmetry of C3V but with a wider H-C-H angle than its gas state, which indicates the hydrogen atoms in the methoxy radical interact with the metal surface, causing the decomposition of the adsorbed methoxy. For the adsorption of methoxy, the heat of adsorption is calculated to be 239 kJ/mol with the spin-polarization method on the unrelaxed model, and the geometric parameters are all similar to the results obtained when excluding the effect of spin polarization. In a similar way, we also check the effect of surface relaxation on the same system and obtain an adsorption energy of 257 kJ/mol for the relaxed surface layer.71 We find that the two effects of spin and relaxation are opposite to binding energy and obtain an adsorption energy of 245 kJ/mol when including both of them, which is marvelously similar to the results obtained when excluding both effects. The structures are all very similar among these three types of calculations, so we only list the brief results with or without both effects of spin polarization and surface relaxation in Table 1. Therefore, we can conclude that spin polarization has a major effect on the absolute adsorption energy but very little effect on the adsorption geometry and that the spin-unpolarized calculation combined with the unrelaxed slab can give essentially the same result.

12434 J. Phys. Chem. B, Vol. 109, No. 25, 2005

Wang et al.

Figure 1. Side view and top view of most stable binding configurations for methanol, methoxyl, formaldehyde, formayl, carbon monoxide, and atomic hydrogen on Ni(111).

TABLE 1: Properties of Decomposition Intermediates on Ni(111) species

site

R(⊥)a

R(CH)

R(CO)

∠(CONi)

CH3OH

top

CH3O

fcc top η2 brifcc

1.101 (1.099) 1.098 (1.099) 1.098 1.100 1.110

1.442 (1.442) 1.438 (1.441) 1.476 1.404 1.274 1.208 (1.207)

147.7 (147.7) 133.7 (132.5)

CH2OH H2CO HCO CO H CH3 OH

fcc fcc fcc

2.363 (2.363) 1.375 (1.363) 1.874 1.526 1.661 1.305 (1.315) 0.905 1.519 1.308

102.1

∠(OCNi)

110.6 97.0 123.2 132.2 (132.5)

R(ONi)

R(CNi)

B.E. (kJ/mol)

2.371 (2.372) 1.990 (1.999) 2.102 1.964 1.976

3.672 (3.672)

-16 (-15)b -249 (-245)b -162 -99 -232 -223 (-190)b -284 -197 -307

1.118

1.939 1.938 1.813 1.942 1.949 2.091

1.943

a

All distances in angstroms, angles in degrees, energies in kilojoules/mole. b After correction of magnetism (and combining relaxation, for methoxy only).

Then, to reduce the cost of calculation, other intermediates will be investigated with spin unpolarization and the unrelaxed slab model. The activation energy for C-H scission in methoxy will later be taken as an example for detailed evaluation of the effect of both magnetization and relaxation on barrier energy, namely, the effect of spin polarization and relaxation on the relative energy. 3.1.3. Formaldehyde. In the optimized structure, two bonds are formed here, and the O-Ni and C-Ni distances are nearly

the same (1.964 Å and 1.938 Å, respectively) (Figure 1). Because the adsorption energy depends on the orientation of the formaldehyde molecular plane with respect to the metal surface, we choose the plane parallel to the Ni(111) surface, which is the most stable geometry concluded from the results of Gomes.72a Our calculations indicate that formaldehyde bonds in a di-σ mode to Ni(111), the same as to the Cu(111) and Pt(111)72b studied by means of cluster model density functional calculations with the adsorption energy of 25 and 98 kJ/mol



Figure 2. Side view and top view of some other intermediate in the methanol decomposition.

(22.9 kcal/mol), respectively.72 The C-O bond length of 1.404 Å is substantially larger than the gas-phase value of 1.22 Å, suggesting that this bond is weakened and approaching a single bond. Because of the π electrons of the C-O double bond in the H2CO molecule, the binding energy of -99 kJ/mol is found for this state (Table 1), which is significantly stronger than methanol, although they are both molecules. 3.1.4. Formyl. HCO was proposed as one of the intermediates for methanol decomposition and the methanol oxidation reaction on various transition-metal surfaces.12,54,55,73,74 The formyl radical bonds to Ni(111) preferentially on a top-bridge-type configuration (Figure 1) with a binding energy of -232 kJ/mol (-55.49 kcal/mol). In this configuration, the formyl radical binds through carbon with a C-Ni distance of 1.813 Å, an H-C-O angle of 117°, and a C-O length of 1.274 Å suggesting a C-O double bond. The bond-order conservation-Morse potential (BOC-MP or UBI-QEP) calculations by Shustorovich predict an adsorption energy of 50 kcal/mol for HCO on the Ni(111) surface,75 which is slightly lower than our result. Yang et al. studied formyl adsorption on Ni(111),5 Cu(111), Au(111), and Pt(111)54 surfaces through DFT with the cluster approach. The results showed that formyl is preferentially adsorbed on metal surfaces through its carbon atom with the calculated adsorption energies of 59.4, 33.5, 12.4, and 60.3 kcal/mol, respectively. And because the differences in geometrical parameters are not significant, the change of adsorption energy should come from the differences of the character between the substrates that can either be charge acceptors or charge donors to the adsorbate. 3.1.5. Carbon Monoxide. The adsorption of carbon monoxide over the Ni(111) surface has been extensively investigated by various theoretical and experimental methods because of the well-known problems associated with the study of CO adsorption on transition-metal surfaces using DFT. It is widely accepted that CO binds to mental surfaces via the Blyholder mechanism,76 in which the bond to the surface is described in terms of the charge donation from the CO 5σ orbital to the metal and the back-donation from the metal to the CO 2π* orbitals, which rationalizes the increase in CO bond length upon chemisorption. However, recently, Fohlishch et al.77 found that the interaction was characterized by significant hybridization of the initial CO orbitals and the metal bands and concluded

that it was stabilized by the π interaction but destabilized by the σ interaction. In the present study, the DFT-GGA calculations indicate that CO prefers to bond in the threefold hollow site on Ni(111), because the calculated adsorption energies are 223, 225, and 172 kJ/mol at the fcc hollow, hcp hollow, and atop site, respectively. Of course, for the threefold site, the value is higher than the results obtained by using RPBE-GGA and the spin-polarized DFT method (144-193 kJ/mol) and the experimental results (108-150 kJ/mol).78a However, with spin polarization, the calculated adsorption energy of CO reduces to 190 kJ/mol at the fcc hollow site, which is close to the recent DFT-GGA result (190 kJ/mol78b), but still higher than the experiment results. Thus, the overestimated binding energy between CO and the metal surface may be due to other reasons than the spin effect;79 for example, the exchange-correlation function used in the present study, PBE, is not fully representative of these two effects (i.e., electronic exchange and correlation), and the modified PBE (RPBE) may be better to some extent.80,81 In this fcc hollow adsorption configuration, CO interacts with the metal surface through the carbon atom which coordinates with three Ni atoms only ∼1.942 Å away (C-surface distance is 1.305 Å), while the C-O bond length is 1.208 Å (Table 1), shorter than the gas-phase value of 1.22 Å, which is not consistent with the Blyholder mechanism but rather satisfies the theory of Fohlisch et al.77 It is worth noting that the calculated adsorption structures with or without spin polarization are very similar despite the fact that the discrepancy on binding energy reaches 30 kJ/mol. 3.1.6. Hydrogen. Both DFT calculations and many-electron embedding theory have found that the hydrogen atom also prefers the threefold site on Ni(111). We find that the adsorption energies of hydrogen in the fcc and hcp hollow sites are 284 and 283 kJ/mol (67.95 and 67.75 kcal/mol), respectively. This coincides with the adsorption energy of 66.64 and 66.41 kcal/mol,82,83 respectively; however, the optimized equilibrium distance of H-surface, 0.905 Å, is slightly smaller than the H-surface distance of 1.18 Å obtained in configuration interaction (CI) cluster calculations.83 3.1.7. Hydroxymethyl. This intermediate prefers to bond at the atop site on Ni(111) through its carbon atom (Figure 2), with a binding energy of -162 kJ/mol. The C-O bond is tilted slightly upward with an angle of 14.1° with respect to the surface

12436 J. Phys. Chem. B, Vol. 109, No. 25, 2005 plane so that the oxygen atom is directed away from the surface with a distance of 2.102 Å, which is 0.163 Å further than the carbon atom. In this adsorbed form, it should be noted that the C-O bond length of 1.476 Å is larger than that in the adsorbed CH3OH molecule, suggesting that the bonding through oxygen (as in the CH3OH adsorption) is weaker than through carbon (as in the CH2OH adsorption) (Table 1). 3.1.8. Methyl. Previous investigations for CH3 adsorption on Ni(111), including calculations using many-electron embedding theory, have indicated that the hollow site adsorption is energetically favored,84 and the very recent DFT study by Michaelides and Hu85 actually confirmed the fcc hollow site to be favored. In the optimized configuration, methyl is adsorbed, keeping its C3V symmetry, with the molecular symmetry axis perpendicular to the surface; the carbon atom sits in the hollow site and the hydrogen atoms in a plane parallel to the surface. We also find a preference for the fcc site, and the energies of adsorption for CH3 on the fcc and top sites of Ni(111) are 197 kJ/mol (47.08 kcal/mol) and 169 kJ/mol (40.43 kcal/mol), respectively. The preferred adsorption energy is consistent with the result (49 kcal/mol) of Siegbahn using a cluster model calculation.86 The C-H bond length of 1.12 Å is arguably longer than the gas value of 1.11 Å, which is consistent with the HREEELS experiment observed by Ceyer et al.87 and with the calculated softening of the C-H stretching vibration.88 3.1.9. Hydroxyl. The hydroxyl radical adsorbs preferentially on the fcc hollow site with the O-H axis normal to the metal surface (Figure 2), and the distance from the oxygen to the surface is 1.308 Å. The binding energy of this intermediate is -307 kJ/mol (-73.56 kcal/mol) (Table 1), which is compatible with the value of -87.0 kcal/mol obtained using an embedding model calculation.89 We also investigate the configurations when OH adsorbs on a top and bridge site, obtaining the similar binding energies of -296 and -307 kJ/mol, respectively, which indicate the radical species can form a strong bond with a metal surface despite the different adsorption sites. In view of the electronegativity of the OH radical and its partially vacant highest-lying antibonding 1π orbital, the binding of the hydroxyl radical to nickel should be expected to involve electron donation from the surface. The partially vacant antibonding π orbitals of the OH radical play a major role in chemisorptive binding on Ni(111) and form the higher binding energy. For a short summary, according to the above binding energies from our calculations, we could clearly find that radical fragments have a much stronger interaction with the metal surface than molecules, which is obviously due to the unpaired electrons of the radicals. Therefore, radicals can be expected to be highly reactive and to have the stronger charge acceptance tendency from metal surface atoms during the chemisorption. From the bonding behavior of these intermediates investigated in this work, we conclude that without a spatial block they will interact with the surface not through the O atom but through the C atom, which is in good agreement with the previous reports.90 Our reported adsorption energies are in the order OH > H > CH3O > HCO > CO > CH3 > CH2OH . H2CO . CH3OH. With regard to the CO molecule, the back-donation of electron density from the metal to the CO 2π* orbital strengthens the Ni-C bond, yielding a higher binding energy than even some radical fragments. Among these radicals, although the C atom usually has a stronger tendency than the O atom to form a bond with a metal surface, the spatial blocking effect plays such an important role that the radicals bonding through the O atom have a higher binding energy. The very high adsorption energy of H may be due to the fact that its

Wang et al. TABLE 2: DFT-GGA Energetics Data for Each Possible Elementary Step in Methanol Decomposition over the Ni(111) Surfacea step

∆H1

∆H2

∆H3

Ea

CH3OH ) CH3O + H CH3OH ) CH3 + OH CH3OH ) CH2OH + H CH3O ) H2CO + H H2CO ) HCO + H H2CO ) CH2 + O HCO ) CO + H HCO ) CH + O 2H ) H2 CO ) C + O CO + H ) COH

435 385 393 92 364

-77 -80 -22 18 -38 -106 -156 -99 135 20 -86

-54 -42 -1 7 -46 -71 -129 -60 134 142 5

39 169 120 86 46 42 17 26 141 272 161

a

71 434 1058

Energy in kJ/mol.

radius is so small that it sinks into the surface as deeply as possible, according to the distance in the Table 1. In the case of HCO, CH3, and CH2OH, which are all bonded through the C atom, HCO has the least spatial blocking among them, conducive to the highest binding energy. The C atom in the CH2OH species is less able to accept an electron from a metal surface than in CH3 because of the donor electron group, OH, and a plausible steric effect of the OH, inferring lower adsorption energy. Considering molecules binding the involved O atom (i.e., CH3OH and H2CO), we find that H2CO interacts with the metal surface through both the O and C atoms, which may be via the π electrons of the C-O double bond, resulting in higher binding energy than CH3OH. Thus, the lowest adsorption energy seen on CH3OH may come from two reasons: First, the O atom has a rather weaker tendency than the C atom to interact with a surface; second, this molecule has a larger spatial block than the molecules H2CO and CO. 3.2. Reaction Mechanism. After getting the perfected adsorption site for each species involved in methanol decomposition, we explore the detailed reaction mechanism by activation barrier calculations in the following sections. The calculated activation barriers, Ea, are listed in Table 2. At the same time, we also give the corresponding TS structure for each elementary step in Figure 3. 3.2.1. Hydrogen Abstraction from Methanol. The first decomposition step involves the activation of the C-O, C-H, or O-H bond of methanol to initiate the catalytic cycle. However, here we mainly analyze the reaction barrier for the abstraction of hydroxyl hydrogen from adsorbed methanol on Ni(111), and the C-O and C-H bonds broken will be discussed briefly later. The resulting intermediates of O-H scission are CH3O and H. It has been pointed out that, in the coadsorption structure of CH3O and H on Ni(111), the hydrogen atom and methoxy radical are placed above the two closest fcc hollow sites.67 Following the NEB method, the TS for this reaction can be approximated by interpolating a series of images between the coadsorbed CH3O/H and the adsorbed methanol under a full optimization of the coordinates of these adsorbed species. The TS structure obtained is displayed in Figure 3 (TS1), the form of which is similar to that of the adsorbed CH3OH, and the energy barrier for this reaction is 39 kJ/mol (i.e., 9.43 kcal/mol, Table 2), which is in agreement with the approximate experimental value of 10.0 kcal/mol19 by using the HREELS, TPR spectroscopy, and LEED. In the TS structure, the O-H bond does become weaker, because the O-H distance is 0.022 Å longer in the TS. And, the distance from the C atom to the metal surface is 3.30 Å, much shorter than its initial value in methanol, 3.64 Å, which



Figure 3. Geometric characterization of the transition states and the products for the hydrogenation abstraction of methanol adsorbed on Ni(111).

indicates that part of the methoxy in the TS approaches closer to the metal surface in the TS. 3.2.2. Hydrogen Abstraction from Methoxy. The methoxy intermediate can further be dehydrogenated to produce formaldehyde and a hydrogen atom. The initial state is chosen to be the most stable methoxy configuration (fcc) in Figure 1, and the final state (i.e., the coadsorption state) consists of hydrogen in an fcc site and formaldehyde in the adjacent top-bridge-top site, shown in Figure 3 (product 2), which guarantees that the repulsion between them is smaller. The transition state is formaldehyde-like with the other hydrogen atom essentially moving straight away from its initial location in the methoxy radical to its final fcc site, and the formaldehyde part is moving

up from the surface and rotating to its final state almost parallel to the surface. This process would be induced by bringing one of the methyl hydrogens closer to the surface and subjecting it to abstraction from the methoxy fragment through a proximal effect as described by Mutterties.91 To accomplish this most probable step requires tilting the axis of the metal-oxygencarbon angle from 180° to ∼100°, which turns the structure down greatly, resulting in the highest barrier encountered during the whole process. And, the activation barrier itself may also reflect several bonds broken between the O atom and the surface. The NEB calculations indicate that this activation barrier is 86 kJ/mol (20.54 kcal/mol), and the reaction is found to be endothermic by 7 kJ/mol (see the data of Ea and ∆H3 in Table

12438 J. Phys. Chem. B, Vol. 109, No. 25, 2005 2), and it is the rate-limiting step (RLS) for methanol decomposition. To test the effects of spin and relaxation on the reaction barrier, we also recalculate the rate-limiting step including the effects of the spin polarization and surface relaxation and find the energy barriers are 93 and 96 kJ/mol for the three-layered model with the first (top) layer to be relaxed and the four-layered model with the first two layers to be relaxed, respectively, slightly higher than that without spin polarization and relaxation. It may be believed that the calculated activation barrier almost converged to the four-layered model. However, the geometry parameters of the transition state in the four-layered model, with the distance of O to the surface, 1.599 Å, RCO ) 1.377 Å, and RCH ) 1.10 Å very similar to the simplest model above, and the discrepancy is no more than 0.02 Å. Other parameters, including the distance of the H atom to the surface and the broken C-H bond, are also similar and changed to 1.290 Å and 1.656 Å, respectively, which may be due to the effect of the relaxed surface on the H-atom adsorption. The activation barrier we find is comparable to the results obtained by various theoretical and experimental investigations. Theoretically, BOC-MP calculations predict a barrier of 26 kcal/mol, a little higher than ours.92 Remediakis et al.70 obtains the activation energy of 0.63 eV with spin-polarized calculations at the coverage of 1/4 ML, which is slightly lower than this work for the different coverage and geometry of the precursor state, as described above. However, experimentally, Friedrich and coworkers66 found that the reaction barrier on Ni foil was 75 kJ/mol, and Hall et al. in an SHG (second harmonic generation) experiment46 found it to be 17.0 ( 0.3 kcal/mol, which are both slightly lower than our results. The difference between experimental results and our calculation might be caused by the following reasons: first, the employed calculation model, p(3 × 2) three-layer model, is not large enough;93 second, the difference in the definitions between activation energy and the potential energy barrier94 (in our calculation, the quantum zeropoint energy and tunnel correction are not included); third, the experimental data reported were not on exactly the same Ni(111) surface and may also have some errors in the analysis of the TPD results. 3.2.3. Decomposition of Formaldehyde to Final Product. Once formaldehyde is formed at the surface, its decomposition to yield the desired final products CO and H may be determined by the competition between the reaction of formalehyde dehydrogenation to yield adsorbed formyl and hydrogen species and the formalehyde desorption. Fortunately, from our calculated value, the desorption energy of 99 kJ/mol (23.68 kcal/mol) is higher than its decomposition energy of 46 kJ/mol. Therefore, formaldehyde should decompose. In the formation of formyl, we also choose formyl at its favorable final site (i.e., the topbridge site) and hydrogen at the fcc site as the final state, show in Figure 1. Since the TS structure (see TS3 in Figure 3) has only a little change of geometry relative to the initial state, H2CO, this step shows a lower barrier. The O-C bond length of TS3 is 1.292 Å (a 0.072 Å increase from the double O-C bond in gas-phase formaldehyde), suggesting it remains approximately a divalent species. The carbon atom is always tetracoordinated, which indicates the formyl intermediate is very strongly bound to Ni. This phenomenon indicates the instability of the initial adsorbed H2CO. Indeed, much less experimental information is available about the behavior of formaldehyde molecules over clean metal surfaces, which may indicate that formaldehyde either easily decomposes to the final products of CO and H at low exposures or desorbs to the form of polymer (CH2O)x at higher exposure.20

Wang et al. In the same way, we also perform a similar calculation for the C-O bond scission in formaldehyde. The calculated activation energy is found to be 42 kJ/mol (Table 2), and this value is slightly higher than that of the C-H bond-breaking reaction, suggesting this step may be a possible pathway in methanol decomposition. To our knowledge, however, no experimental result can be found to confirm the existence of the CH2 species. So, we do not discuss it further. Once the formyl intermediate has been produced on Ni(111), it can either produce CO(a) and H(a) via the C-H bondbreaking reaction or produce CH(a) and O(a) via the C-O bondbreaking reaction. For the C-H bond scission in formyl, the calculated activation barrier is 17 kJ/mol (Table 2), which is lower than that of C-H bond-breaking in formaldehyde. Therefore, formyl is likely to decompose readily into CO and hydrogen and will not poison the surface toward the dehydrogenation of methanol. With regard to the pathway of C-O bond scission in formyl, we obtain a barrier energy of 26 kJ/mol, which is also higher than that in the scission of the C-H bond. Actually, few experiments support the existence of a formyl intermediate because of its instability12 and tendency to decompose immediately to give CO and H.72 Shustorovich has performed an empirical calculation with the BOC-MP method to identify the activation energy of formyl decomposition and reported a value of 3.0 kcal/mol,92 which is in agreement with this work (i.e., 17 kJ/mol (4.06 kcal/mol) (see Table 2 or Figure 3)). We also investigate what are the most probable final products for the methanol decomposition on the Ni(111) surface in the present work: For examples, does the CO(a) remain as a molecule or decompose to C and O atoms? Could we find molecular O2 under some fulfilled condition? Does the H atom react with an adsorbed CO molecule or combine with another H atom, or does it react with C and O atoms if the adsorbed CO could decompose? These possible reactions should result in many possible products; then, which of them might be proven experimentally? To identify what might be the products under some suitable conditions, we study the decomposition of the adsorbed CO molecule first. We calculate the barrier of the C-O bond-breaking on Ni(111) and obtain the value of 272 kJ/mol (this value is very close to the DFT-GGA results obtained by Bengaard et al.78a and Morikawa et al.78c), which is higher than the CO desorption energy of 223 kJ/mol and indicates that CO will prefer desorption without decomposition, which is consistent with the experiments that have been done for CO(a) on the Ni surface.95 As for the activation energy of the two-H-atoms combination, the activation barrier is 141 kJ/mol, which is much lower than the barrier of CO decomposition. We also investigate whether the reactions exist between adsorbed CO and H atoms to produce the COH(a) species on Ni(111). The calculated barrier energy is 161 kJ/mol (Table 2), which is much higher than that of the RLS. In addition, our result is in good agreement with the calculated results of Remediakis,70 -1.49 eV (143 kJ/mol), and the slight difference may come from the different surface coverage used on both studies (1/6 ML vs 1/4 ML). Because the activation barriers for these reactions are all much higher than that of the RLS, the final products should be the adsorbed H and CO, which is consistent with the conclusion of the TPD experiment13 and the TPR experimental results.41 3.2.4. Pathways of C-H and C-O Bond-Breaking in Methanol. The other two possible pathways involving the C-O and C-H bond-breaking in CH3OH on Ni(111) are also investigated. In the gas phase, these two reactions are found to



TABLE 3: Integral PDOS for HOMO and LUMO of CH3OH(a) at Different Conditions HOMO LUMO a

CH3OH

CH3OH ) CH3O + Ha

CH3OH ) CH3 + OH

CH3OH ) CH2OH + H

0.06336 0.20985

0.13129 0.20122

0.1752 0.11576

0.2089 0.20207

“)” symbols denote transition states.

Figure 4. Plots of projected DOS on CH3OH side for the transition states in bond-breaking steps and in CH3OH(a).

be relatively less endothermic and hence thermodynamically more favorable than the O-H bond-breaking reaction (see the data of ∆H1 listed in Table 2). However, the energy barrier of the C-H bond-breaking reaction on a metal surface to form hydroxymethyl and hydrogen intermediates is found to be 120 kJ/mol, and the C-O bond-breaking reaction is found to be 169 kJ/mol. Thus, the most favorable pathway should be the O-H bond-activation reaction, which is in agreement with the experimental result in which CH3O is the dominant species in CH3OH decomposition on Ni catalyst.7,23,43-51 The O-H bond is activated in preference to the C-O or C-H bond for several reasons. First, from the adsorbed structure of CH3OH(a) in Figure 1, we notice that to reduce the steric effect of methyl the O-H bond has to be closer to the surface than the C-O and C-H bonds, which causes the O-H bond to be more activated by the Ni metal and the bond length to change more (the bond lengths of C-H, C-O, and O-H in CH3OH(a) are 1.101 Å, 1.442 Å, and 0.986 Å, respectively, and the bond lengths for the isolated CH3OH molecule are dC-H ) 1.09 Å, dC-O ) 1.42 Å, and dO-H ) 0.95 Å96). Therefore, we predicate that the O-H bond scission reaction may be the favorable step. Second, it is easy to find that C-O or C-H scission would require the formation of a surface intermediate or transition state with a five-coordinate carbon atom.97 Third, the methoxy and hydrogen atom formed by O-H bond activation adsorbed on the metal surface have already been proven by many spectrometry studies.25,31,73,80,98,99 Fourth, the structure of the TS in the

O-H bond-breaking reaction has only one major “touch point” with the Ni surface and is very similar to the reactant CH3OH (i.e., early barrier), while the TS in the C-O or C-H reaction has two major touch points with the surface and is very similar to the product (i.e., late barrier) (see Figure 3). And, the productlike TS is carrying two dissociated fragments which are so close (i.e., dC-H ) 1.54 Å in TS5 and dC-O ) 2.33 Å in TS6) that they have a large direct repulsive interaction and bring the TS to a higher-energy state. In contrast, in the TS1 of the O-H bond scission pathway, H and OCH3 still bind together. Finally, the correlation between the distance of C-Ni at the TS of the C-H, C-O, and O-H bond-breaking pathways (2.35 Å, 3.02 Å, and 3.30 Å, respectively) and the Ea (120.6, 169, and 39 kJ/mol, respectively) confirms that the activation of CH3OH(a) is strongly affected by the steric effect and by the electron density transfer, which is more efficient between the C atom and the Ni atom than between the O atom and the Ni atom as expected. We also analyze these three scission reaction barriers using BOC-MP modeling, the principles of which have been welldescribed in the literature,100,101 and the high reliability of the model predictions has already been demonstrated by other reactions.90,92,100,101 We find that it gives the similar result regarding the order of the energy barriers of bond-breaking (i.e., C-H (53 kJ/mol) ≈ C-O (52 kJ/mol) > O-H (26 kJ/mol)), although these calculated barriers are all smaller than those determined through the DFT-GGA method.

12440 J. Phys. Chem. B, Vol. 109, No. 25, 2005

Wang et al.

Figure 5. One-dimensional potential energy surface for methanol decomposition on Ni(111) for the O-H bond scission pathway. TS5 and TS6 corresponding to the C-H and C-O bond scissions, respectively. All energies are in kilojoules/mole.

One of the best methods to understand the nature of the TSs for the above three reactions may be the electronic structure analysis. We calculate the projected density of states (DOS) on the CH3OH side for each TS, and the results are listed in Table 3 as well as in Figure 4. As shown in Table 3, the stable chemisorbed CH3OH has the largest population in its lowest unoccupied molecular orbital (LUMO) compared to other three cases, which means that more electron density in CH3OH(a) has transferred to the CH3OH side from Ni. In general, the more electrons in the LUMO, the weaker the inner binding in the CH3OH side and the stronger the interaction with the substrate, resulting in the trivial fact that the CH3OH side is more stable in the CH3OH(a) state than in those TS states. Similarly, we find that the TS for the O-H bond cleavage has the highest population in its LUMO of the CH3OH side among these three TS states, suggesting that the inner binding in CH3OH of this TS1 is weaker than that in the other two TS sates. For the case of CH3OH ) CH2OH + H, although its population in the LUMO is large (as large as 0.202 07), the population of the highest occupied molecular orbital (HOMO) is also large (0.2089), and we predict that the interaction of this TS with the substrate is still weak. 3.2.5. Potential Energy Surface. A rigorous search for the transition states of all elementary steps in methanol decomposition would be necessary to quantify the activation barriers accurately. The overall energetics for the dehydrogenation of

Figure 6. Relationship between reaction barrier and reaction energy.

methanol on Ni(111) is summarized in Figure 5. The thermochemical pathway clearly shows that methanol binds very weakly to the clean Ni(111) surface with the binding energy of -16 kJ/mol. For the adsorbed methanol molecule, the thermochemical pathway indicates that desorption is energetically preferable to decomposition on clean Ni(111). On the other hand, the formation of the methoxy intermediate is the kineti-



Figure 7. Relationship between the total energies of transition states (TS) and of the final states for (a) dehydrogenation reactions and (b) deoxygenation reactions.

cally most favorable and has the lowest activation barrier (Table 2). Furthermore, the decomposition of the methoxy intermediate into formaldehyde is the most difficult in the overall reaction (CH3OH(g) f CO(a) + 4H(a)) with the energy barrier of 86 kJ/mol. These results are in agreement with the experimental facts. Methanol is generally desorbed molecularly at 170 K, and at higher temperature, it will first decompose and lead to the formation of methoxy, then decomposition of these metastable intermediates will lead to quick production and desorption of H2, and the chemisorbed CO will be left alone as the final decomposition product on the surface.22 The barriers for hydrogen abstraction from formaldehyde and from formyl are quite small, which are in agreement with the fact that none of these intermediates is observed when methanol is dosed on a clean nickel surface.13,19,76,98 Figure 5 also demonstrates that the total energy of the final decomposition products CO(a) and H(a) is 244 kJ/mol lower than that of the initial state, but the state of CO(g) + 2H2(g) is 246 kJ/mol higher than the initial state, reflecting that CO(a) and H(a) may be too stable on the Ni surface to be conveniently released for further usage. To understand the reactions more, we also look into whether there is a clear relationship between the heat of reaction versus the activation barrier (i.e., the linear-free-energy relationships (LFER) or the Brønsted-Evans-Polanyi (BEP) behavior). Table 2 lists the calculated heats of reaction by different definitions: ∆H1, ∆H2, and ∆H3, where the difference between ∆H2 and ∆H3 indicates the difference of the coadsorption effect. For all the reactions listed in Table 2 as they are, it is hard to find a strong linear relationship between ∆H and Ea, and the reason is clearly that the elementary steps listed in Table 2 do not share a common mechanism. However, if we consider all the decomposition reactions while treating the reactions of 2H(a) ) H2(a) and CO(a) + H(a) ) COH(a) reversely as a decomposition, a clear linearly relationship is observed between activation barrier, Ea, and the heat of reaction, ∆H3 (see Figure 6; the linear correlation does not include the M4 step), but this linear relationship does not appear with either ∆H1 or ∆H2. Furthermore, when considering the series of reactions defined by the C-H bond-breaking (i.e., the reactions of M5, M6, and M7) a stronger linear correlation exists between ∆H3 and Ea (the correlation coefficient is 0.98). By the way, M3 is also the C-H bond-breaking reaction, but there are two reasons to make it an outsider: First, the binding between adsorbate and substrate is quite different from the other three as discussed already; second, the H atom in O-H pushes the methyl closer to the Ni surface than that in methoxy and generates the steric interaction. The opposite effect from these two factors on the binding

energetics makes it with weaker activation but becoming an exothermic reaction. By the way, we also checked the relationship between the absolute transition energies and the final state energies. Unfortunately, a linear relationship among these data does not exist (not listed here). However, a good linear relationship was observed if we divide these data into two groups based on the type of reactions: one belonging to the reactions of dehydrogenation, and the other deoxygenations (Figure 7). 4. Conclusions The energetics for the Ni-catalyzed dehydrogenation of methanol to form CO and hydrogen has been examined using the periodic density functional theory. The thermochemistry of the stable intermediates and the reaction barriers of the elementary steps of methanol decomposition have also been studied. The bonding energies of the methanol, methoxyl, formaldehyde, formyl, carbon monoxide, hydroxymethyl, methyl, hydroxyl, and hydrogen intermediates are found to be -16, -249, -99, -232, -223, -162, -197, -307, and -284 kJ/ mol, respectively. The results show that methanol is more likely to desorb than decompose over Ni(111). However, under higher temperature, O-H bond activation is the preferable mechanism during the dehydrogenation of methanol, which is followed by sequential hydrogen abstractions to form formaldehyde, formyl intermediate species, and the final products CO(a) and H(a). The abstraction of hydrogen from methoxy is the rate-limiting step in this pathway. Finally, CO is the most tightly adsorbed species on the surface and may poison the catalyst toward further dehydrogenation. Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant no. 20273034) and the State Key Laboratory of Coal Conversion of China. Y.M. is supported by ACT-JST. This work was also partially supported by the Nankai University ISC and the Large Scale Numerical Simulation Project of Science Information Center, University of Tsukuba in Japan. References and Notes (1) Keim, W., Ed. Catalysis in C1 Chemistry; Reidel: Dordrecht, The Netherlands, 1983. (2) Fox, J. M., III Catal. ReV.sSci. Eng. 1993, 35, 169. (3) Catalysis Looks to the Future; National Research Council; National Academy Press: Washington, DC, 1992.

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Kinetic Mechanism of Methanol Decomposition on Ni(111) Surface: A

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