A New Indicator for Single Metal Dispersion on a TiO2(110) Surface

Mar 3, 2016 - Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido ... provided by the o-MBA and the O is present in the TiO2 lattice) on t...
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A New Indicator for Single Metal Dispersion on a TiO2(110) Surface Premodified with a Mercapto Compound Satoru Takakusagi,*,† Akitoshi Kunimoto,† Natee Sirisit,† Hiromitsu Uehara,†,# Tadashi Ohba,†,∇ Yohei Uemuara,‡ Takahiro Wada,§ Hiroko Ariga,† Wang-Jae Chun,∥ Yasuhiro Iwasawa,⊥ and Kiyotaka Asakura*,† †

Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido 001-0021, Japan Department of Materials Science, Institute of Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan § Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo 113-8549, Japan ∥ Graduate School of Arts and Sciences, International Christian University, Mitaka, Tokyo 181-8585, Japan ⊥ Innovation Research Center for Fuel Cells, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan ‡

S Supporting Information *

ABSTRACT: Ni and Pt structures evaporated onto a TiO2(110) surface premodified with ortho-mercaptobenzoic acid (o-MBA) were studied using polarization-dependent total reflection fluorescence X-ray absorption fine structure analysis to determine the effects of the premodification on the dispersion of the metal atoms over the TiO2(110) surface. Ni was found to be atomically dispersed with the formation of S−Ni−O bonds (where the S is provided by the o-MBA and the O is present in the TiO2 lattice) on the surface. In contrast, Pt underwent aggregation to form small clusters. The varying behavior of these metals on the o-MBA-modified TiO2(110) surface is discussed based on the energy difference between sulfur−metal−oxygen and metal−metal bond formations, and we propose a new indicator for single metal dispersion on the TiO2(110) surface. absorption fine structure (PTRF-XAFS) analysis can provide three-dimensional structural information regarding highly dispersed metal species on single crystal surfaces due to the high surface sensitivity of this technique.2 Our research group has previously determined the three-dimensional structures of Cu, Co, Pt, and Mo complexes and their derivatives grafted onto SiO2(0001), Al2O3(0001), and TiO2(110) with the aim of elucidating the metal−support interactions at the atomic level.3−8 Grafting metal species on an oxide surface typically requires preparation of the organometallic compounds in advance, even though these compounds are often unstable in contact with air or at elevated temperatures. In contrast, if the oxide support surface is premodified using an organic ligand that strongly anchors metal atoms to the surface through covalent bonds well-defined atomic metal grafting can be obtained without the need for organometallic compounds. Using this technique, we have successfully prepared an atomically dispersed Cu compound on the basis of Cu−S bonding simply by the vacuum deposition of Cu onto a TiO 2 (110) surface

1. INTRODUCTION Many heterogeneous oxide-supported metal catalysts are employed for the preparation of highly dispersed metal active phases. The metal species in such catalysts must be strongly bound to the support surface to suppress their diffusion and decomposition and also to reduce aggregation. Highly dispersed metal species can be immobilized through covalent bonding to a greater extent than by metal ion impregnation or ion exchange methods. The immobilized metal species either represent active structures or can sometimes be converted to active structures after appropriate treatment.1 The immobilization of metal species is often achieved by grafting of a metal complex onto an oxide surface through reaction with OH groups or by anchoring a metal species via a linker molecule that strongly interacts with both the metal and the support. The local structures and oxidation states of the resulting surface metal species can be determined and controlled. However, because the metal atoms are typically immobilized on an oxide powder surface it can be difficult to control their orientation and configuration definitely with respect to the support surface in order to obtain optimal synergistic effects between the metal and support. This level of control can be more easily realized by employing a single crystal oxide surface, although it can be difficult to determine the structure of highly dispersed species on single crystal oxides because of their low concentrations. Polarization-dependent total reflection fluorescence X-ray © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: November 28, 2015 Revised: March 1, 2016

A

DOI: 10.1021/acs.jpcc.5b11630 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C premodified with mercaptobenzoic acid (MBA).9 In this system, the carboxylic acid groups of the MBA act to bind the compound to the TiO2 substrate (Figure 1b). In addition,

Inc., Canada) in ethanol for more than 24 h to modify the TiO2(110) surface with an o-MBA monolayer.9 The substrate sample was then transferred to an ultrahigh vacuum (UHV) PTRF-XAFS chamber.14 Ni was evaporated onto the o-MBAmodified TiO2(110) surface via the resistive heating of a tungsten filament wrapped with Ni wire (99.999% purity, Nilaco Co., Japan). Pt was deposited by generating Pt vapor through the electron bombardment of a Pt rod (99.98% purity, Nilaco Co., Japan) by applying a high voltage (typically 800 V) between the filament and the Pt rod. The Ni and Pt coverages were estimated to be 0.27 and 0.11 ML (where 1 ML is defined as 5.2 × 1014/cm2 based on the TiO2(1 × 1) unit cell) from the X-ray photoelectron spectroscopy (XPS) Ni 2p3/2-Ti 2p3/2 and Pt 4f7/2-Ti 2p3/2 peak area ratios, respectively (Figure S1 and S2 in Supporting Information). 2.2. PTRF-XAFS Analyses. PTRF-XAFS measurements were performed following Ni or Pt deposition onto the o-MBAmodified TiO2(110) substrate without exposure to air, using the UHV PTRF-XAFS chamber at the BL9A unit of the Photon Factory at the Institute of Materials Structure Science (KEKIMSS-PF, Tsukuba, Japan).15 The storage ring energy and ring current were 2.5 GeV and 450 mA, respectively. Considering the anisotropic surface structure of TiO2(110), as shown in Figure 1a, PTRF-XAFS measurements were conducted employing three different orientations relative to the electric vector (E) of the incident X-rays: two orientations parallel to the surface, E//[001], [11̅0], and one orientation perpendicular to the surface, E//[110].5,6,8−11,16−19 The polarization dependence of the overall XAFS oscillation value, χobs(k), is given by either eq 1 or 22

Figure 1. (a) Structural model of the TiO2(110) surface. (b) Chemical structure of o-MBA absorbed on TiO2(110). Ti5c indicates a 5-fold coordinated Ti atom on the TiO2(110) surface.

even though Au is well-known to readily aggregate on oxide surfaces it was also found possible to atomically disperse this metal on an MBA-modified TiO2(110) surface.10,11 We refer to this technique as the premodified surface method, and various organic compounds may be employed for the purpose of surface premodification and for selective fine-tuning of surface metal structures. For example, in the case of the Cu atoms on an MBA-modified TiO2(110) surface the orientation of the almost linear S−Cu−O bonds (where S is provided by the MBA and the O is present in the TiO2 lattice) can be controlled by using different MBA isomers (o-MBA and pMBA), that is, 40−45° inclined from the surface normal for Cu/o-MBA/TiO2(110) and 60° from the surface normal for Cu/p-MBA/TiO2(110).9 This enables us to investigate how the orientation and conformation of the metal species affect the catalytic reactivity and electron transfer process between the metal and oxide at the atomic level. It is also possible that the premodified surface method could be used in conjunction with monomeric metal species acting as building blocks to form well-controlled metal nanoparticle structures such as dimers, trimers, and tetramers and small clusters that may show unique catalytic properties depending on the number of atoms. They are not easy to prepare on an oxide surface by metal evaporation without premodification with organic compounds because the metal atoms are easily aggregated to form larger particles there. In the present work, we applied the premodified surface method to the group 10 metals Ni and Pt, both of which are employed as catalysts more often than Cu and Au. Pt in particular is widely used in industrial catalysis systems, such as petroleum, automobile, and fuel cell catalysts. The structures of Ni and Pt evaporated onto a TiO2(110) surface premodified with o-MBA were studied using PTRF-XAFS to examine the manner in which the premodification affects the dispersion of these metal atoms. Herein, we also discuss the basic factors that govern metal surface dispersion and propose an indicator for single metal dispersion based on our present and previous studies.

χobs (k) =

∑ 3 cos2 θi·χi (k) at Ni K‐edge i

χobs (k) =

∑ (0.7 + 0.9 cos2 θi)·χi (k) at Pt L3‐edge i

(1)

(2)

Here θi and χi(k) are the angle between the ith bond direction and the electric vector of the X-rays and the partial XAFS oscillation accompanying the ith bond, respectively. Ni Kα or Pt Lα fluorescence was detected using a 19 element Ge solid state detector (SSDGL0110S, Canberra, U.S.A.) and data analysis was performed with the REX 2000 software package (Rigaku Co., Japan). The XAFS oscillations were extracted using a spline smoothing method and normalized to edge height. The preliminary analysis was carried out applying a least-squares curve fitting method, as summarized in eqs 3 and 4 2 2

χ (k ) =

∑

−2ki σi SiNF sin(2kiri + φi(ki)) i i (k i )e

i

ki =

k 2 − 2mΔEi /ℏ2

kiri 2

(3) (4)

Here Si, Ni*, σi, ri, and ΔEi are the inelastic reduction factor, effective coordination number, Debye−Waller factor, bond distance, and energy shift in the origin of the photoelectron kinetic energy of the ith bond, respectively. The backscattering amplitude, Fi(ki), and phase shift, φi(ki), were calculated using the FEFF8.04 algorithm.20 The inelastic reduction factors, Si, were estimated using reference compounds and the same fitting parameters were employed within the same coordination shells. Three-dimensional structures were determined via an iterative method using the FEFF8.04 code20 based on a real-space

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Optically polished Nb-doped (0.05 wt %) TiO2(110) substrate samples (20 × 20 × 1 mm, Shinkosya Co., Japan) were cleaned by immersion in a 10% HF solution for 10 min, followed by annealing in air at 700 °C for 1 h, which is in accordance with previously reported methods.12,13 The cleaned surface was subsequently immersed in a 2 mM solution of o-MBA (Toronto Research Chemicals B

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Figure 2. (a) Ni K-edge XAFS spectra of Ni/o-MBA/TiO2(110) obtained at three different polarization directions. (b) Ni K-edge XAFS spectra of reference samples (Ni foil, nickel sulfide (NiS), tetrabutylammonium bis(maleonitriledithiolato)nickel(III) ([N(Bu)4][Ni(mnt)2]), and nickel oxide (NiO)).

model structure. The goodness of fit between the observed XAFS oscillation values (χobs(k)) and calculated values (χcal(k)) was evaluated using eq 5 R′ =

1 ndata

∑ k

slightly shorter than those in [(N(Bu)4)][Ni(mnt)2] (0.214 nm)21 and NiS (millerite, 0.226 and 0.238 nm).22 An iterative method using the FEFF code and a real-space model structure were employed to determine the threedimensional structure of the Ni species. Figures 3 and 4

(χobs (k) − χcal (k))2 ε(k)2

(5)

Here ndata and ε(k) are the number of data points and the error, respectively. The structure model was adopted when R′ values lower than unity were obtained for all three orientations (E// [001], [11̅0], and [110]).

3. RESULTS AND DISCUSSION 3.1. Ni Deposition on a TiO2(110) Surface Premodified with o-MBA (Ni/o-MBA/TiO2(110)). Figure 2 presents the polarization-dependent EXAFS spectra obtained from Ni/oMBA/TiO2(110) in addition to the spectra of some reference compounds. It is evident that the PTRF-EXAFS oscillation envelopes of the sample are damped more rapidly in the higher k region compared to the Ni foil. This result indicates that the nearest neighboring atom of Ni in the sample is not Ni, but rather a lighter atom such as S or O. The polarization dependence was such that the amplitudes of the EXAFS oscillations in the [001] and [11̅0] directions were almost equal to one another but were both slightly lower than that in the [110] direction. A preliminary curve fitting analysis found no contribution from Ni−Ni interactions, suggesting that there was no Ni aggregation. In addition, Ni−S (0.219 ± 0.003 nm) and Ni−O (0.185 ± 0.003 nm) interactions were dominant in all three spectra. The Ni−S bond distance was comparable to or

Figure 3. A model structure for the Ni/o-MBA/TiO2(110) surface.

show the proposed model structure and the polarizationdependent FEFF simulations together with the experimental spectra, respectively. The EXAFS oscillations calculated based on the proposed model are evidently in good agreement with the observed spectra. The Ni−S bond distance was 0.219 nm, while the Ni−O interaction distance was 0.185 nm. The Ni−S and Ni−O bond angles relative to the surface normal were 57 and 40°, respectively, and the precision of the FEFF simulations was found to be better than 3°. The S−Ni−O bond angle was estimated to be 156°. C

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The oxidation state of Ni was estimated by the XANES results (Figure S3) because it is reported that the Ni 2p3/2 XPS peak binding energies of Ni compounds are rather insensitive to formal oxidation state and to the nature of the ligand to which Ni is bonded and that the peak shape often shows complex features such as multiplet splitting and satellite structures.23 Figure S3a,b shows the Ni K-edge XANES spectra of Ni/o-MBA/TiO2(110) and those of reference compounds, respectively. Figure S3c is the plot of Ni K-edge energies for the reference compounds shown in Figure S3b as a function oxidation state. The K-edge energy is sensitive to oxidation state and to ligand to which the metal is bonded. For example, a linear relation was found between the Ni K-edge energies and oxidation states in Ni oxy-compounds.24 Thus, we assumed a linear relation between the K-edge energies and oxidation states for each of the oxy- and the sulfur-containing compounds. The Ni K-edge energies for Ni/o-MBA/TiO2(110) were 8340.8 ± 0.1 eV in three different polarization directions. Considering that the Ni makes bonds with one oxygen and one sulfur as shown in Figure 3, the oxidation state can tentatively be assigned to be +2 (see the dotted line in Figure S3c) Previous studies of Ni deposited directly on a TiO2(110) surface have shown that atomically dispersed Ni is stabilized only at specific step edges (⟨11̅n⟩) through the formation of two Ni−O bonds (O−Ni−O), and that the Ni atoms are located at imaginary Ti sites when the Ni coverage is very low (∼0.02 ML).17 Aggregation of Ni was also observed with the formation of clusters on the terrace surfaces when the coverage was increased above 0.05 ML.18,25 The stable Ni adsorption sites on the terrace surfaces could be atop bridging oxygens because these oxygen atoms have a dangling bond that points

Figure 4. Comparison between the observed XAFS oscillations of the Ni/o-MBA/TiO2(110) surface (black solid line) and those calculated based on the model structure in Figure 3 (red dotted line).

Figure 5. (a) Pt L3-edge XAFS spectra of Pt/o-MBA/TiO2(110) obtained at three different polarization directions. (b) Pt L3-edge XAFS spectra of reference samples (Pt foil, platinum sulfide (PtS2), bis(tetrabutylammonium) bis(1,3-dithiole-2-thione-4,5-dithiolato)platinum(II) ([N(Bu)4]2[Pt(dmit)2]), and platinum oxide (PtO2)). D

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The Journal of Physical Chemistry C Table 1. Curve Fitting Results for Pt/o-MBA/TiO2(110) Data Obtained at Three Different Polarization Directionsa orientation

bond

E//[001]

Pt−S Pt−Pt Pt−S Pt−Pt Pt−S Pt−Pt

E//[11̅0] E//[110] a

N* 1.1 1.8 1.2 1.7 1.2 1.5

± ± ± ± ± ±

0.2 0.4 0.2 0.3 0.2 0.3

r/nm

ΔE/eV

σ/10−5 nm

R factor/%

± ± ± ± ± ±

6±2 0±2 (6) (0) (6) (0)

0.008 ± 0.002 0.007 ± 0.001 (0.008) (0.007) (0.008) (0.007)

1.4

0.230 0.267 0.229 0.268 0.229 0.266

0.002 0.002 0.002 0.002 0.003 0.003

1.1 5.1

Parentheses indicate those values fixed during the fitting procedure.

upward from the surface.26 A single Ni-bridging oxygen bond is not sufficiently strong to fix the Ni atom at the atop site so that the Ni can hop to the next bridging oxygen atom and diffuse along the [001] direction. The Ni atoms eventually find other Ni atoms and form clusters before reaching the step edges if the Ni coverage is sufficiently high (>0.05 ML). In the present study, we used a TiO2(110) surface with a very low step density (less than 4 lines every 1 μm2). The S atoms of o-MBA adsorbed on the TiO2(110) terrace surfaces were able to immediately immobilize the migrating Ni atoms in cooperation with the bridging oxygens, accompanied by rotation of the bond connecting the COO- moiety and the phenyl ring. Thus, Ni aggregation was effectively blocked on the surface even at a high Ni coverage (0.27 ML). 3.2. Pt Deposition on a TiO2(110) Surface Premodified with o-MBA (Pt/o-MBA/TiO2(110)). Figure 5a,b shows the polarization-dependent EXAFS spectra obtained from Pt/oMBA/TiO2 (110) as well as the spectra for reference compounds, respectively. We do not observe a significant degree of polarization dependence in the XAFS spectra in Figure 5a. In addition, curve fitting analysis demonstrates the presence of Pt−S bonds (0.229 nm) and Pt−Pt bonds (0.267 nm) in all orientations, as shown in Table 1. The Pt−Pt bond distance was 0.01 nm shorter than that in bulk Pt metal (0.277 nm), and it has been reported that the Pt−Pt bond distance is gradually shortened as the Pt particle size decreases.27 Considering the effective coordination number (N*) of the Pt−Pt bond, the average Pt structure might be a trimer species. However, it is also possible that a mixture of atomically dispersed Pt and small Pt clusters, such as dimers, trimers, and tetramers, were formed on the surface. The average oxidation state of Pt in Pt/o-MBA/TiO2(110) was analyzed by the XANES and XPS results. Figure S4a,b shows the Pt L3-edge XANES spectra of Pt/o-MBA/TiO2(110) and those of reference compounds, respectively. Figure S4c is the plot of white line (WL) intensities (peak areas) for the reference compounds shown in Figure S4b as a function of oxidation state. Because the WL, intense absorption peak, at the Pt L3edge is mainly assigned to electron transition from 2p3/2 to 5d3/2 and 5d5/2, it is known as an informative index for the oxidation state and also for the ligand to which platinum is bonded. The WL intensities for Pt/o-MBA/TiO2(110) were 8.5 ± 0.2 in three different polarization directions. We assumed here that the Pt species were bonded to one or more oxygen atoms since bonding only with sulfur would not be enough to stabilize them although Pt−O bond was not detected in Table 1 probably due to its small contribution compared with those of Pt−Pt and Pt−S. Then the average oxidation state of the Pt species can be expected to be +1∼+2 depending on the oxygen contribution as shown in Figure S4c. The XPS biding energy of the Pt 4f7/2 peak was 72.3 eV (Figure S2a), which was higher than that of Pt metal (71.2 eV)28 and lower than those of

PtS(II) (72.55 eV),29 PtO(II) (74.4 eV),30 which supports the XANES results. A previous scanning tunneling microscopy (STM) study of Pt deposited on a bare TiO2(110) surface found that the Pt had aggregated to form clusters with heights below 0.8 nm even at a low coverage of 0.07 ML.31 In the present study, because we detected Pt−S bonds and the formation of small clusters at a Pt coverage of 0.11 ML, we can conclude that premodification of the TiO2(110) surface with o-MBA effectively suppressed any significant aggregation of the deposited Pt atoms, although the dispersion effect of the o-MBA was not strong enough to produce perfect atomic dispersion of the Pt atoms. 3.3. Formation Mechanism of Atomically Dispersed Metal Species and Small Metal Clusters on o-MBA/ TiO2(110): Comparison of Ni, Pt, Cu, and Au Cases. Metal atoms deposited on a bare TiO2(110) surface will migrate until they find stable adsorption sites. Because the single metal-O bond on the terrace surface is too weak to stabilize the metal monomers, these atoms will continue to diffuse until finding other metal atoms, resulting in aggregation and the formation of clusters. However, when o-MBA is applied to the TiO2(110) surface, the dangling S bonds can be directed toward the diffusing metal atoms. If metal−S bond formation is energetically favorable compared with metal−metal bond formation, the metal atoms will be trapped by the S−metal−O bonding motif and stabilized, as illustrated in Figure 3. Otherwise, aggregation of the metal atoms will take place. The formation of the S−metal−O bonding motif can be described as follows, using Ni as an example. o‐MBA ad + Ni + 2O (bridging) → o‐MBA ad−Ni−O (bridging) + HO (bridging)

(6)

Deposited Ni, Cu,9 and Au11 atoms have all been found to form a stable S−metal−O bonding motif on the o-MBA/TiO2(110) surface. In Table 2, the strengths of metal−metal (M−M) and metal−S (M−S) bonds (as assessed by their bond dissociation energies at 298 K, D298 ° ) are listed for Ni, Pt, Cu, and Au.32,33 The D°298(M−M) value for Pt is the highest among the four metals, and Pt is the only metal for which D°298(M−M) is greater than D298 ° (M−S). These results indicate that metal−S bond formation is energetically more favorable compared with metal−metal bond formation in the case of Ni, Cu, and Au but not for Pt, which is in agreement with our Pt/o-MBA/ TiO2(110) results discussed in Section 3.2. The affinity of the metal atoms for oxygen will also be important in addition to the affinity for sulfur to account for the stability of the S−metal−O bond. Thus, the metal−oxygen (M−O) bond strengths (D298 ° (M−O)) for Ni, Pt, Cu, and Au are also listed in Table 2.32,34 It can be seen that the affinity for oxygen decreases in the order of Ni > Cu > Pt > Au. We can define a term, RS−M−O, that E

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possibility of obtaining an atomically dispersed species on an oxide surface.

Table 2. Strengths of M−M, M−S, and M−O bonds (Bond Dissociation Energies at 298 K, D°298) for Ni, Pt, Cu, and Au32 and the Corresponding RS−M−O and RO−M−O Values metal

° (M− D298 M)/kJ/mol

D298 ° (M− S)/kJ/mol

D298 ° (M− O)/ kJ/mol

Ni Pt Cu Au

201 307 177 226

344 233b 276 418

382 246b 269 221

RS−M−O 3.6 1.6 3.1 2.8

○c ×c ○d ○e

4. CONCLUSIONS Atomically dispersed Ni species were formed on a TiO2(110) surface premodified with o-MBA. The three-dimensional structure of the Ni species was determined using the PTRFXAFS technique. A S−Ni−O motif, which is similar to the local metal structure found in previous studies with Cu and Au, was formed on the modified TiO2(110) surface. In contrast, Pt was not atomically dispersed but instead aggregated to form clusters on the o-MBA/TiO2(110) due to the low affinity of Pt atoms for S atoms compared with that for other Pt atoms. The energetic stabilities of the S−metal−O motif when using Ni, Pt, Cu, and Au were discussed based on the difference between the S−metal−O and metal−metal bond formation energies. A new indicator, RX−M−O (X = S or O), has been proposed for the single metal dispersion on the TiO2(110) surface.

RO−M−O 3.8

○f

3.0 2.0

○g ×h

○ and × indicate atomic dispersion of the evaporated metal atoms and aggregation of them, respectively. bD°298(M−S) and D°298(M−O) for Pt are taken from refs 33 and 34, respectively. cPresent work. dRef 9. eRef 11. fRef 17. gRef 19. hRef 37. a

roughly estimates the formation preference and energetic stability of the S−metal−O bonding motif compared with metal−metal formation, as follows R S−M−O =

° (M−S) + D298 ° (M−O) D298 ° (M−M) D298

■

(7)

ASSOCIATED CONTENT

S Supporting Information *

The RS−M−O values for Ni, Pt, Cu, and Au are 3.6, 1.6, 3.1, and 2.8, respectively. Thus, the RS−M−O value for Pt is the lowest, indicating that this metal generates the least stable S− metal−O bonding motif. The aggregation of Pt is the most likely to proceed due to the fact that it has the highest D298 ° (M− M), lowest D°298(M−S) and second lowest D°298(M−O) values, indicating the preferential formation of Pt−Pt bonds, resulting in clustering on the o-MBA/TiO2(110). In the case of Ni and Cu, both D298 ° (M−S) and D298 ° (M−O) values contribute almost equally to the corresponding RS−M−O values. In contrast, in the case of Au, the D°298(M−S) value makes a much greater contribution compared with the D298 ° (M−O) value. Although the D298 ° (M−O) and D298 ° (M−M) values for Au are the lowest and second highest among the four metals, respectively, this metal has a relatively high RS−M−O of 2.8 because of its high D298 ° (M−S). These results suggest a high affinity of Au atoms for S atoms, which is in good agreement with previous reports.35,36 Similarly, if the O−metal−O bonding motif can be obtained through the formation of two metal−O bonds on the TiO2(110) surface, metal monomers can be stabilized on a TiO2(110) surface.17,19 In our previous study, atomically dispersed Ni species were formed without employing o-MBA to generate the O−Ni−O motif at the ⟨11̅n⟩ step edges under conditions in which the Ni coverage was very low (∼0.02 ML).17 Cu was also atomically dispersed on a TiO2(110) surface premodified with an acetic anhydride layer.19 This Cu formed an O−Cu−O motif in which one oxygen was a bridging O from the TiO2(110) and the other was contributed by the acetate. We also attempted to prepare the O−Au−O bonding motif on a similar surface, although this trial was unsuccessful and instead generated Au aggregates that formed clusters.37 We can also define a term RO−M−O = 2D298 ° (M−O)/D298 ° (M−M) that allows us to compare the energetic stability between O− metal−O and metal−metal bond formation in a similar manner to the procedure outlined in the above discussion of the S− metal−O bonding motif. The RO−M−O values for Ni, Cu, and Au are 3.8, 3.0, and 2.0, respectively, indicating the stability of the O−metal−O bond motif decreases in the order of Ni > Cu > Au. This result suggests the low stability of the O−Au−O motif and thus predicts Au aggregation. Therefore, the RO−M−O value might also serve as a good indicator to show the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11630. XPS and XANES results for Ni/o-MBA/TiO2(110) and Pt/o-MBA/TiO2110). (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.T.) E-mail: [email protected]. Tel/Fax: +81-11706-9114. *(K.A.) E-mail: [email protected]. Tel/Fax: +81-11-7069113. Present Addresses #

(H.U.) Corporate R&D Headquarters, Fuji Electric Co., Ltd., Tokyo 191-8502, Japan. ∇ (T.O.) Institute for Integrated Cell-Material Sciences, Kyoto University, Nishikyo-ku, Kyoto 615−8530, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to express their thanks to the staff of the Photon Factory for their helpful technical support. This work was conducted under the approval of the Photon Factory Advisory Committee (PAC Nos. 2014G552 and 2012G165). This study was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Nano Informatics” (No. 25106010) from the Japan Society for the Promotion of Science (JSPS) and was also performed as a NEDO PMFC project. In addition, two of the authors (Y.U. and T.W.) were supported by the Cooperative Research Program of Institute for Catalysis, Hokkaido University (Nos. 15A1004 and 15A1003, respectively).

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REFERENCES

(1) Averseng, F.; Vennat, M.; Che, M. Grafting and Anchoring of Transition Metal Complexes to Inorganic Oxides. In Handbook of Heterogeneous Catalysis; Wiley-VCH: New York, 2008; pp 522−539. (2) Asakura, K. Polarization-dependent Total Reflection Fluorescence Extended X-ray Absorption Fine Structure and its Application to Supported Catalysis. Catalysis; RSC Publishing: London, 2012; Vol. 24, pp 281−322. F

DOI: 10.1021/acs.jpcc.5b11630 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b11630 J. Phys. Chem. C XXXX, XXX, XXX−XXX