First-Principles Theoretical Study and Scanning Tunneling

Research Institute for Computational Sciences (RICS), National Institute of AdVanced .... Y. Morikawa, Present address: The Institute of Scientific an...
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14446

J. Phys. Chem. B 2004, 108, 14446-14451

First-Principles Theoretical Study and Scanning Tunneling Microscopic Observation of Dehydration Process of Formic Acid on a TiO2(110) Surface† Yoshitada Morikawa,‡,* Ittetsu Takahashi,‡,§ Masaki Aizawa,§ Yoshimichi Namai,§ Takehiko Sasaki,| and Yashuhiro Iwasawa§,* Research Institute for Computational Sciences (RICS), National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan, Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ReceiVed: January 18, 2004; In Final Form: April 12, 2004

We have studied the dehydration process of formic acid on a TiO2(110) surface by using first-principles theoretical calculations. Formic acid dissociatively adsorbs to form formate and hydroxyl. It turns out that simple decomposition processes of the formate on the stoichiometric surface are energetically unfavorable. The formation of H2O and O vacancies from two neighboring bridging hydroxyls is relatively easy and the activation barrier is calculated to be 114 kJ/mol. On the TiO2(110) surface with oxygen defect sites, formate adsorbs with one O at a defect site and with the other O on a five-fold Ti, forming a bridging configuration. Further decomposition of the formate occurs through a monodentate configuration with an activation barrier of 129 kJ/mol. We have also performed STM observation corresponding to the theoretical results. It was imaged that some formates were located along the oxygen row and at an intermediate position between the oxygen row and the Ti row at elevated temperatures at which reaction takes place, indicative of the interaction between oxygen vacancy and formate. The catalytic dehydration cycle is discussed based on these results.

I. Introduction The adsorption and decomposition of formic acid (HCOOH) on metal oxides have been intensively studied as a prototypical system of catalytic reactions at metal oxide surfaces.1-42 It has been well established that HCOOH decomposes to a formate anion (HCOO-) and an acidic proton that forms a surface hydroxyl group. HCOOH further decomposes to H2O and CO (dehydration) or H2 and CO2 (dehydrogenation) and the selectivity depends strongly on the nature of substrates and reaction conditions. For example, under continuous flow reaction conditions, the dehydration mainly takes place on acidic oxides such as Al2O3, while the dehydrogenation is a dominant route on basic oxides such as MgO.1 On the other hand, under ultrahigh vacuum conditions, the dehydration is the only reaction observed in a temperature-programmed desorption (TPD) experiment over MgO.6 The TiO2 rutile (110) surface is the most well studied surface as the substrate for HCOOH adsorption and decomposition processes.17-42 It is widely accepted that HCOOH dissociatively adsorbs at as low as 110 K to form a formate anion HCOObound to two five-fold coordinated Ti4+ ions (denoted by Ti(5), †

Part of the special issue “Gerhard Ertl Festschrift”. * Y. Morikawa, Present address: The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki Osaka, 567-0047 Japan, Tel: +81-6-6879-8535, fax: +81-6-6879-8539, e-mail address: [email protected]. Y. Iwasawa Tel: +81-3-5841-4363, fax: +81-3-5800-6892, e-mail address: [email protected] ‡ Research Institute for Computational Sciences (RICS), National Institute of Advanced Industrial Science and Technology (AIST) § Department of Chemistry, Graduate School of Science, The University of Tokyo | Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo

hereafter) in a bridging form and an acidic proton bound to a bridging oxygen (OB). Onishi et al.18,21 reported that both dehydration and dehydrogenation processes occur on TiO2(110) and they can be switched over to each other by changing the reaction temperature and gas-phase pressure. The dehydration reaction dominates at high temperature (above 600 K) and low HCOOH pressure with an activation energy of 120 ( 10 kJ/ mol and the reaction rate is proportional only to the coverage of HCOO-. Therefore, its rate-determining step was proposed to be a unimolecular decomposition process of the formate. On the other hand, the dehydrogenation process dominates at low temperature (below 500 K) and high HCOOH pressure with an activation energy of 15 ( 10 kJ/mol and the reaction rate is proportional to both the coverage of HCOO- and the pressure of HCOOH. Therefore, the rate-determining step in the dehydrogenation process was proposed to be a bimolecular process between a surface HCOO- and a HCOOH molecule. Henderson argued that the dehydration reaction did not proceed simply because CO and H2O were formed from two independent reaction steps.29 He demonstrated that two bridging hydroxyl groups are combined to form an H2O and a surface O vacancy below 500 K, while CO is formed from HCOO- decomposition above 500 K. He also reported significant inclusion of substrate O into desorbed CO, suggesting facile exchange of O between HCOO- and TiO2(110) substrates. Oxygen defects cause further complication of the chemistry of HCOOH on the TiO2(110) surface. As mentioned above, on a stoichiometric TiO2(110) surface, HCOO- adsorbs on 2 Ti(5) in a bridging configuration (denoted by bridging A, hereafter). On a reduced TiO2(110) surface with O vacancies, Wang et al.31 suggested that HCOO- adsorbs with its one O at the

10.1021/jp0497460 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004

First-Principles Theoretical Study

Figure 1. Structure of the TiO2 rutile (110) surface. Formates (HCOO-) in the bridging A and the bridging B configurations, bridging oxygen (OB), bridging hydroxyl group (HOB), in-plane oxygen (OI), five-fold Ti (Ti(5)), and oxygen vacancy are shown in the figure.

vacancy site and with the other O at Ti(5) (shown in Figure 1 and denoted by bridging B, hereafter). In the bridging A configuration, the O-C-O plane is aligned along the [001] direction, while in the bridging B configuration, it is aligned along the [11h0] direction. Formates in the bridging B structure were observed by Hayden et al.35 using Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS) and also by Bowker et al.41 using scanning tunneling microscopy (STM). Oxygen defects also alter the selectivity of HCOOH decomposition. Barteau and co-workers7,10 observed formaldehyde (H2CO) formation from HCOOH on TiO2(001) surfaces. They proposed that reduced Ti3+ cations reduce HCOOH to H2CO, while 4 coordinated Ti4+ cations promote bimolecular coupling to form H2CO. Formation of H2CO is also observed on a TiO2(110) surface and is ascribed to the existence of O defects.29,42 Despite detailed experimental study of HCOOH adsorption and decomposition processes on the TiO2(110) surface, there are only three theoretical studies using density functional theoretical (DFT) calculations and all of them studied only the adsorption state and diffusion process of formate on stoichiometric TiO2(110).34,38,39 In this study, we have investigated dehydration processes of HCOOH on the TiO2(110) surface including the effect of defect by using DFT calculations and STM observations.

J. Phys. Chem. B, Vol. 108, No. 38, 2004 14447 optimization, atoms of adsorbates and the two top O-Ti-O layers were allowed to relax, while the bottom O-Ti-O layer was fixed to its ideal bulk position. The structure optimization was continued until the maximum force becomes less than 50 kJ/mol/nm. To obtain minimum energy paths and activation barriers of reaction steps, we have adopted the nudged elastic band (NEB) method.56 Eight images were generated between the initial and the final states of each reaction process. II.B STM Experiment. The experiments were performed in an ultrahigh vacuum (UHV) STM (JEOL JSTM4500VT) equipped with an Ar+ ion gun, quadrupole mass spectrometer, and LEED optics. The base pressure was 1 × 10-8 Pa. A polished TiO2(110) wafer of 6.5 × 1 × 0.25 mm3 (Earth Chemical) was used after deposition of Ni film on the rear side of the sample to resistively heat the sample. The heating rate and cooling rate were controlled to be 7-10 K s-1. The TiO2(110) surface was cleaned with cycles of Ar+ ion sputtering (3 keV for 3 min) and annealing under UHV at ca. 900 K for 30 s. Constant current topographies (CCT) were obtained at room temperature and at 370 K with electrochemically etched W tips. Deuterated formic acid (DCOOD, Wako, 98% purity, most of the contaminant is water) was purified by repeated freeze-pump-thaw cycles and introduced into the chamber by backfilling. The surface temperature of the crystal was monitored by an infrared radiation thermometer. III. Results and Discussion Molecular Adsorption on TiO2(110). First, we investigated the adsorption states and adsorption energies of HCOOH, HCOO-, H+, H2O, and CO on the TiO2(110) surface. Experimentally, p(2 × 1) structure with 2 × periodicity parallel to the [001] direction was observed on an HCOOH-adsorbed TiO2(110) surface by low energy electron diffraction (LEED). Therefore, we adopted a p(2 × 1) unit cell. Table 1 shows the adsorption energies of HCOOH, HCOO-, H+, H2O, and CO on TiO2(110). Dependence of adsorption energies on NO-Ti-O is also shown. The adsorption energies are defined by

II. Methods

Ead(Molecule) ) [E(Molecule) + E(TiO2(110))] [E(Molecule/TiO2(110))] (1)

II.A Computational Method. All calculations were carried out using a program package STATE (Simulation Tool for Atom TEchnology), which has been successfully applied for molecular adsorption and reactions over semiconductor, metal, and metal oxide surfaces.43-50 We adopted a generalized gradient approximation (GGA) in the density functional theory (DFT)51,52 and the Perdew, Burke, and Ernzerhof formula53 as the exchange-correlation energy functional. We constructed pseudopotentials of H 1s, C 2p, O 2p, Ti 3p, and 3d states by Vanderbilt’s scheme,54 while other components were constructed by an optimized norm-conserving scheme of Troullier and Martins.55 The cutoff energy for the wave function is 25 Ry and that for the augmentation charge is 225 Ry. The lattice constants of rutile TiO2 are optimized to be a ) 0.4665 nm and c/a ) 0.642, being in good agreement with the experimental values of a ) 0.4594 nm and c/a ) 0.644. We used a repeated slab model and the number of O-Ti-O triple layers (NO-Ti-O) is changed from 3 to 7 (see below). The vacuum region of more than 1 nm is inserted between two neighboring slabs. Adsorbates are introduced only on one side of each slab. The integration within the surface Brillouin zone was done using 2 × 4 uniform mesh of k-points for the 1 × 1 unit cell and similar mesh points were used for other surface unit cells. For the structural

where, E(Molecule), E(TiO2(110)), and E(Molecule/TiO2(110)) are the total energies of the isolated molecule, clean TiO2(110) substrate, and the combined system, respectively. It is obvious that calculations with NO-Ti-O ) 3 overestimate the adsorption energies significantly, in agreement with results by Bates et al.34 It is necessary to use at least NO-Ti-O ) 4. However, the energy differences among different adsorption systems are quite reasonable even with NO-Ti-O ) 3. In Sections III B and III C, we used a p(2 × 1) unit cell with NO-Ti-O ) 3 to roughly calculate stable adsorption states, many possible intermediate states, and reactant states. Then in Section III D, we use a p(2 × 2) unit cell with NO-Ti-O ) 5 to get more accurate adsorption energies and activation barriers. Table 2 shows the structural parameters for dissociatively adsorbed HCOOH on the stoichiometric TiO2(110) surface. Results are in good agreement with previous calculation38 as well as experimental results.33 Table 1 also shows that O vacancies destabilize the adsorption of HCOO- in the bridging A configuration. It turned out that HCOO- in the bridging B configuration is more stable than that in the bridging A configuration, supporting the existence of the previously suggested bridging B configuration.31,35,41 The dissociation energy of HCOOH into HCOO and H in the gas phase is calculated to be 443 kJ/mol. If HCOO and H

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Morikawa et al.

TABLE 1: The Adsorption Energies (Ead) of HCOOH, HCOO, H, H2O, and CO on the TiO2(110) Surfacea adsorbates

NO-Ti-O/layers

Ead/kJ/mol

3 4 5 6 7 3 3 3 4 5 6 7 3 3 3 3 4 5 6 7 3 3 3 3 3

172 130 137 135 135 97 178 180 129 130 129 128 -32 51 190 288 236 247 238 239 75 93 34 8 -66

HCOO (bridging A) + HOB

HCOO (bridging A) + HOB + V HCOO (bridging B) + HOB HCOO (bridging A)

H (ontop Ti(5)) H (bridge Ti(5)) H-OI H-OB

H2O-Ti HO-Ti + HOB OC-Ti CO-Ti CO-OB a

Dependence of E on the number of O-Ti-O layers (NO-Ti-O) is also shown. The definition of the adsorption energies is explained in the text. Ti(5), OB, and OI indicate five-fold Ti, bridging O, and inplane O, respectively. V denotes oxygen vacancy. All calculations were done using a p(2 × 1) unit cell with 2 × periodicity parallel to the [001] direction.

Figure 2. Schematic of formate decomposition paths starting from HCOO- in the bridging A configuration and HOB. Bonds that will be cleaved during the reactions are shown in thick lines. Arrows indicate movement of atoms during the reactions.

TABLE 3: Energy Difference ∆E Before and After the Decomposition of Formate on the Stoichiometric TiO2(110) Surfacea path

products

∆E/kJ/mol

1 2 3 4 5

CO-Ti(5) + HO-Ti(5) + HOB OC-Ti(5) + O-Ti(5) + 2HOB OC-Ti(5) + H2O-Ti(5) + 2OB OC-Ti(5) + H2O + 2OB OC-Ti(5) + O-Ti(5) + H2OB + OB

195 179 221 224 269

TABLE 2: Structural Parameters of Dissociatively Adsorbed HCOOH on the Stoichiometric TiO2(110) Surfacea a

this work r(OF-Ti(5))/nm r(C-OF)/nm r(C-H)/nm r(OB-H)/nm z(OF-Ti(5))/nm ∠OF-C-OF/degree

0.209 0.128 0.111 0.098 0.206 127

Kaeckell et al.38 0.208 0.132 0.111 0.107 127

experiment33

0.21 ( 0.01 126 ( 4

a r(A-B) and z(A-B) indicate the bond length and the vertical height difference between A and B atoms, respectively, while ∠ OF-C-OF indicates the angle netween two C-O bonds of HCOO. OF and OB are O atoms of HCOO and bridging O, respectively.

are separately adsorbed from each other on the TiO2(110) surface, the dissociative adsorption energy of HCOOH becomes -443 + 130 + 247 ) -66 kJ/mol (here, we used the adsorption energies with NO-Ti-O ) 5), being less stable than the case in which HCOO and H are adsorbed close to each other by 203 kJ/mol. This was previously pointed out by Kaeckel and Terakura.39 Formate Decomposition on the Stoichiometric TiO2(110) Surface. Next, we investigated decomposition paths of the formate on the stoichiometric TiO2(110) surface. We examined all possible reaction paths starting from HCOO- in the bridging A configuration and HOB, and ending with CO and two OH or H2O. Figure 2 shows five decomposition paths, producing CO above Ti(5). Thick lines indicate bonds that are cleaved in the product states and the arrows indicate movement of atoms during the reaction. In path 1, the formate decomposes to CO and OH on the Ti(5) row. In path 2, H of HCOO- moves onto a neighboring OB, forming HOB and one of two CO bonds of HCOO- is cleaved, forming CO and atomic O above Ti(5). In path 3, both H of HCOO- and H of HOB move onto one of two O atoms of HCOO-, forming H2O and CO. In path 4,

Each reaction path is described in Figure 2 and in the text.

HCOO- decomposes to CO on Ti(5) and H2O hydrogen-bonded to OB. In path 5, H of HCOO- moves to HOB making H2O, and one of two O atoms of HCOO- remains above Ti(5). We did not consider the possibility of H transfer to a surface Ti(5) site because H adsorption on the Ti(5) site is quite unstable compared to H on OB as shown in Table I. Table 3 shows the energy difference between the initial and the final states of each path. The energetically most stable final state is produced by path 2, but the energy difference of +179 kJ/mol is significantly larger than the experimental activation energy of +120 kJ/mol. Therefore, we conclude that the simple unimolecular decomposition of formates on the stoichiometric TiO2(110) surface is energetically unfavorable and is quite improbable. Formate Decomposition on TiO2(110) Defect Surfaces. In this section, we describe the effect of vacancy on the decomposition of formates. We considered 4 unimolecular decomposition paths denoted by paths 1, 2, 3, and 5 starting from HCOOin the bridging A configuration. These paths are the same as those examined in Section III B except for the existence of O vacancies next to HOB. Table 4 shows the energy difference before and after the decomposition processes. Compared with the cases for the stoichiometric surface, the energy difference is reduced in paths 1, 2, and 3. Thus, it is quite probable that the O vacancy promotes HCOO- decomposition. As seen in Table 1, the bridging B configuration is more stable than the bridging A configuration near an O vacancy. Therefore, we next investigated formate decomposition paths starting from the bridging B configuration. Figure 3 shows possible decomposition paths starting from HCOO- in the bridging B configuration. In path 1, HCOO- decomposes to CO on Ti(5) and HOB, which fills the vacancy. In path 2, H of HCOO- transfers to nearby

First-Principles Theoretical Study

J. Phys. Chem. B, Vol. 108, No. 38, 2004 14449

TABLE 4: Energy Difference ∆E Before and After the Decomposition of Formate on the Reduced TiO2(110) Surface with O Vacancya path

products

∆E/kJ/mol

1 2 3 5

OC-Ti(5) + HO-Ti(5) + HOB + V OC-Ti(5) + HOB + H/V OC-Ti(5) + H2O-Ti(5) + OB + V OC-Ti(5) + O-Ti(5) + H2OB + V

167 128 160 289

a

Reaction paths are the same as those in Table 3 except the existence of O vacancies next to HOB. V denotes the O vacancy.

Figure 4. Calculated energy diagram for dehydration process of HCOOH on the TiO2(110) surface.

Figure 3. Schematic of formate decomposition paths starting from HCOO- in the bridging B configuration and HOB. Bonds that will be cleaved during the reactions are shown in thick lines. Arrows indicate movement of atoms during the reactions.

TABLE 5: Energy Difference ∆E Before and After the Decomposition of Formate in the Bridging B Configurationa

a

path

products

∆E/kJ/mol

1 2 3

CO-Ti + 2HOB CO-Ti(5) + OB + H2OB HO-Ti(5) + COB + HOB

106 193 261

(5)

Each reaction oath is described in Figure 3 and in the text.

HOB, forming CO and H2O. In path 3, HCOO- decomposes to CO at the vacancy and OH at Ti(5). The energy difference before and after the decomposition processes is listed in Table 5. The smallest energy difference we have ever examined appears in path 1. From the above results, we conclude that the unimolecular decomposition of formate on the stoichiometric TiO2(110) surface is quite improbable and that the O vacancy promotes the decomposition process. The most probable decomposition path is path 1 in Figure 3, that is, the decomposition from HCOO- in the bridging B configuration into CO and HOB. In the final product of the path, OH adsorbs at the vacancy and the vacancy is healed. To promote another formate decomposition and maintain the catalytic dehydration reaction, the O vacancy should be regenerated. As suggested previously, the O vacancy can be generated through condensation of two HOB and the formation of H2O. In the following sections, we consider these catalytic reactions in more detail. Catalytic Dehydration Process. In this Section, we describe the catalytic dehydration cycle, that is, dissociative adsorption of HCOOH, condensation of HOB to form H2O and the creation of an O vacancy, and decomposition of HCOO- at the O vacancy and healing of the vacancy to complete the catalytic cycle. Two bridging hydroxyl groups are necessary to form one H2O and one vacancy. As discussed in Section III A, one HCOO- should always accompany one HOB and therefore we need two dissociatively adsorbed HCOOH to form one H2O and one O vacancy. Thus we used a p(2 × 2) unit cell and adsorbed two HCOOH in a unit cell. In Figure 4, the calculated

Figure 5. Formation of H2O from two HOB. (a) Initial configuration, (b) transition state, (c) final state, (d) HCOO- in the monodentate A configuration forms hydrogen bond with H2O.

energy diagram of the whole catalytic cycle is shown. In the following, each step is described. HCOOH Adsorption. The dissociative adsorption energy of one HCOOH on the TiO2(110) p(2 × 2) surface unit cell (Θ ) 0.5) is calculated to be 134 kJ/mol, while that of the second HCOOH (Θ ) 1.0) is 140 kJ/mol. Therefore, the coverage dependence of the adsorption energy is rather small. However, the adsorption energy depends somewhat on the position of the second H. In the most stable adsorption structure, two H atoms are separated onto two different rows of OB. If two H atoms are adsorbed on the same OB row as shown in Figure 5 (a), the dissociative adsorption energy of the second HCOOH is reduced to 102 kJ/mol, being less stable than the most stable case by 38 kJ/mol. Therefore, two HOB have repulsive interaction with each other and in the equilibrium structure, the H’s are separated from each other. But at high temperatures, they occasionally come close to each other and form H2O. Formation of H2O and O Vacancy. Figures 5 (a), (b), and (c) show the initial state, the transition state, and the final state of the H2O formation process. In this process, one H moves from one HOB to neighboring HOB and form H2O. The activation barrier is calculated to be 114 kJ/mol as shown in Figure 4. The final state is less stable than the initial state by 87 kJ/mol. A more-stable intermediate state can be formed as shown in Figure 5 (d). In this state, HCOO- forms a monodentate structure (denoted by monodentate A, hereafter) and forms a hydrogen bond with H2O. The monodentate A configuration is more stable than the bridging A configuration by 72 kJ/mol. From the monodentate A configuration, it is easy to form the bridging B configuration by removal of H2O from the vacancy site (Figure 6(a)). The energy cost is 85 kJ/mol. We also confirmed that the bridging B configuration is more stable than the bridging A configuration using a p(2 × 2) unit cell and NO-Ti-O ) 5 by 68 kJ/mol.

14450 J. Phys. Chem. B, Vol. 108, No. 38, 2004

Figure 6. The decomposition process of HCOO- from the bridging B configuration into CO and HOB. Formate H is moved to one of two formate O atoms. The activation barrier is 258 kJ/mol.

Figure 7. The decomposition process of HCOO- from the monodentate B configuration into CO and HOB. Formate H is moved to neighboring OB. The activation barrier is 129 kJ/mol.

Formation of CO and HOB from Formate in the Bridging B Configuration. To complete the catalytic cycle, HCOO- should be decomposed into CO and HOB. First, we investigated the direct decomposition path by moving H from C to OF bound to the O vacancy. The initial state, the transition state, and the final state are shown in Figures 6 (a), (b), and (c), respectively.

Morikawa et al. The activation barrier is calculated to be 258 kJ/mol, being extremely high compared with the experimentally observed activation energy of 120 kJ/mol. Miura and co-workers12,16 suggested that HCOO- takes a monodentate configuration as a reactive intermediate state of the HCOOH decomposition reaction on an NiO(111) surface. We also investigated the possibility of the monodentate configuration as an intermediate state. It turns out that the monodentate state (shown in Figure 7 (a) and denoted by monodentate B, hereafter) is only 25 kJ/ mol less stable than the bridging B configuration. A further decomposition process was investigated and the transition state and the final state are shown in Figures 7 (b) and (c), respectively. The calculated activation energy is 129 kJ/mol. In the final state, one HCOO- in the bridging A configuration and one HOB are left in the p(2 × 2) unit cell, returning back to the 0.5 ML HCOOH dissociatively adsorbed state. STM Observation of Formates on TiO2(110) at Elevated Temperature. The present theoretical results clearly show that the oxygen vacancy plays an essential role to reduce the activation barrier in the dehydration reactions. To corroborate these results STM observation on a TiO2(110) surface was performed, paying attention to the effects of oxygen vacancy and temperature. When an STM image was obtained after exposure to 6 L formic acid at room temperature, bright spots with a height of 0.14-0.22 nm were observed along the Ti rows in agreement with our previous reports (not shown).19,20,22,23 The desorption temperature of H2O from the TiO2 (110) surface exposed to formic acid was reported to be 370 K.21 STM images obtained after annealing to 370 K are shown in Figure 8. At this temperature, bright spots assignable to formates were classified into three types; formates along the Ti rows (denoted a in Figure 8), formates along the oxygen rows (b) and formates at an intermediate position between the Ti row and the oxygen

Figure 8. (A) STM image (CCT, 40 × 40 nm2, Vs: +0.998 V, It: 0.20 nA) of formates on TiO2(110) exposed to 6 L formic acid at room temperature after annealing to 370 K. Magnified images (4.4 × 4.4 nm2) for formates: a, bridging A configuration: b, monodentate B configuration; c, bridging B configuration. White lines represent Ti-rows. Each adsorbed formate ion was imaged as a bright spot. (B) A schematic view of the TiO2(110) surface with three types of formates.

First-Principles Theoretical Study row (c). The number of formates in Figure 8 was counted; a: 37, b: 5, c: 7. The formate a is assigned to the one in the bridging A configuration. The formate b and the formate c can be assigned to the one in the monodentate B configuration and the one in the bridging B configuration, respectively, based on the present theoretical results. After cooling to room temperature, the formates on the oxygen rows were still observed. The similar STM image was also reported by Bowker et al.41 These observations support theoretical results indicating that dehydration process involves an oxygen vacancy site. Detailed STM observations will be published elsewhere. IV. Conclusion We have extensively examined the dehydration process of formic acid on the TiO2 rutile (110) surface using first-principles theoretical calculations. We found that simple decomposition processes of formates on the stoichiometric TiO2(110) surface are energetically unfavorable. The formation of H2O and O vacancies from two neighboring bridging hydroxyl groups is relatively easy and the activation barrier is calculated to be 114 kJ/mol. On the TiO2(110) surface with an O vacancy, formate adsorbs with its one O at the vacancy site and with the other O at the 5-fold Ti site (bridging B configuration). To further decompose into CO and bridging OH, the formate takes a monodentate configuration, which is only 25 kJ/mol less stable than the bridging B configuration. The activation barrier of the formate decomposition process is calculated to be 129 kJ/mol. Because the activation barrier of the OH disproportionation reaction is lower than that of HCOO decomposition reaction, the former process should be faster than the latter process. Therefore, HCOO decomposition always takes place in the presence of vacancies produced by the OH disproportionation reaction. Thus it is not necessary for the surface to contain oxygen vacancies prior to exposure to HCOOH. From STM observation at elevated temperature, formates along bridging oxygen rows and those at an intermediate position between the oxygen row and the Ti row are found, which indicates the interaction between formates and oxygen-vacancy sites and supports the theoretical results. Acknowledgment. Y.M is supported by Japan Science and Technology Corporation through ACT-JST. The numerical calculations were carried out at the computer centers of Tsukuba Advanced Computing Center (TACC), Institute for Solid State Physics, The University of Tokyo, Kyoto University, Kyushu University, and Nagoya University. This study was supported by a Grant-in-aid for The 21st Century COE Program for Frontiers in Fundamental Chemistry from the Ministry of Education, Culture, Sports, Science and Technology. References and Notes (1) Mars, P.; Scholten, J. J. F.; Zwietering, P. AdV. Catal. 1963, 14, 35. (2) Fukuda, K.; Noto, Y.; Onishi, T.; Tamaru, K. Trans. Faraday Soc. 1967, 63, 3072. (3) Noto, Y.; Fukuda, K.; Onishi, T.; Taramu, K. Trans. Faraday Soc. 1967, 63, 3081. (4) Vohs, J. M.; Barteau, M. A. Surf. Sci. 1986, 176, 91. (5) Onishi, H.; Egawa, C.; Aruga, T.; Iwasawa, Y. Surf. Sci. 1987, 191, 479. (6) Peng, X. D.; Barteau, M. A. Catal. Lett. 1990, 7, 395. (7) Kim. K. S.; Barteau, M. A. Langmuir 1990, 6, 1485. (8) Nakatsuji, H.; Yoshimoto, M.; Hada, M.; Domen, K.; Hirose, C. Surf. Sci. 1995, 336, 232.

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