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Specific Metal–Support Interactions between Nanoparticle Layers for Catalysts with Enhanced Methanol Oxidation Activity Sinmyung Yoon, KYUNGHWAN OH, Fudong Liu, Ji Hui Seo, Gabor A. Somorjai, Jun Hee Lee, and Kwangjin An ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00276 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Catalysis

Specific Metal–Support Interactions between Nanoparticle Layers for Catalysts with Enhanced Methanol Oxidation Activity Sinmyung Yoon,†,∥ Kyunghwan Oh,†,∥ Fudong Liu,‡,§ Ji Hui Seo,† Gabor A. Somorjai,‡,§ Jun Hee Lee*,†, and Kwangjin An*,† †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology(UNIST), Ulsan 44919, Republic of Korea. ‡ Department of Chemistry, University of California, Berkeley, CA 94720, United States § Chemical Sciences and Materials Sciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States ABSTRACT: Oxide supports often play a critical role in metal-supported catalysts owing to their charge transfer phenomena that can alter catalytic performance. Herein, in place of conventional bulk oxide supports, monodisperse oxide nanoparticles (NPs) were exploited as supports for Pt catalysts. Depending on the type of oxide NP, Pt/oxide layered catalysts exhibited dramatic changes in the catalytic activity and selectivity for methanol oxidation. While Pt NPs deposited on MnO, Fe3O4, Co3O4, Cu2O, and ZnO NPs had comparable turnover frequencies to that of the pure Pt NP catalyst, Pt deposited TiO2 NPs changed the reaction rate significantly, with preferential selectivity observed toward partial oxidation products. Facet-specific interactions between Pt and TiO2 NPs were demonstrated by density functional theory calculations and catalytic reactions using shape-controlled TiO2 NPs. When Pt NPs were attached to spherical and rhombic TiO2 NPs with abundant (001) surfaces, methanol conversion was enhanced 10-fold owing to strong charge transfer from TiO2 to Pt.

KEYWORDS Pt, TiO2, Methanol Oxidation, Selectivity, Charge Transfer. INTRODUCTION Since 1967, when Schwab first reported enhanced activity at oxide–metal interfaces for methanol oxidation over Ag supported on ZnO catalysts,1,2 considerable progress has been made in the field of heterogeneous catalysis by utilizing strong metal– support interactions (SMSIs) to enhance catalytic performance.3–5 Tauster et al. demonstrated that chemisorption properties were greatly enhanced by SMSIs for Group VIII metals, including Fe, Ni, Rh, Pt, Pd, and Ir, supported on certain oxides.6,7 In a broad sense, SMSIs encompass all changes in catalytic behavior owing to specific interactions originating from the metal–support interface. SMSIs also refer to the catalytic phenomenon in which a partially reduced oxide, such as titania or ceria, migrates onto metal nanoparticle (NP), effectively blocking the access of gas-phase molecules to the metal surface.8,9 As nanotechnology has progressed over the last few decades, many researchers have demonstrated that specific interfacial interactions between noble metals and oxides in supported NP catalysts can alter catalytic activity and selectivity.10–14 For instance, when an oxide support itself has little or no catalytic activity, deposition of active metal NPs on the oxide can significantly change the reaction pathway owing to charge transfer at the metal–oxide interface.15 Many catalytic model reactions, including alkane reforming, hydrogenation of organic molecules, oxidation of carbon monoxide, and oxidation of methanol, have been studied to investigate support-induced changes in the catalytic properties of metal NPs on various solid oxides with high surface areas or oxide NPs.10–20 However, there are few direct comparisons of the catalytic property changes of metal NPs on a series of oxide NPs with well-defined interfaces.21,22 Herein, to study NP–NP interfaces, monodisperse Pt NPs were deposited on MnO, Fe3O4, Co3O4, Cu2O, ZnO, and TiO2 NPs to obtain two-dimensional (2D) layered catalysts. Methanol oxidation was carried out over the Pt/oxide layered NP catalysts to examine the effect of the support on catalytic activity and selectivity. Among the examined oxides, TiO2 NPs were shown to enhance catalytic activity by an order of magnitude and to shift product selectivity of methanol oxidation toward

partial oxidation products (formaldehyde and methyl formate) owing to interfacial effects with Pt NPs. To extend the specific interaction between Pt and TiO2 NPs, we controlled the shape of the TiO2 NPs to investigate the facet-dependent catalytic properties for methanol oxidation. As well as catalytic reactions, theoretical calculations were performed using density functional theory (DFT) to calculate the adsorption and formation energies of the intermediate species on Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces. By correlating the catalytic reaction results with the DFT calculations, we demonstrated that TiO2(001) was the most active facet. Moreover, the enhanced catalytic reaction rate and dramatically different product selectivity observed with Pt/TiO2(001) were shown to result from charge transfer from TiO2 to the adsorbate on Pt.

EXPERIMENTAL SECTION Preparation of Various NPs and Pt/Oxide Layered Catalysts. Pt15 and oxide NPs,23 including MnO,24 Fe3O4,24 Co3O4,25 Cu2O,26 ZnO,27 TiO2,28 FePt,29 and CoPt,29 were prepared by reported methods under air-tight conditions using a Schlenk line. After loading each metal or oxide precursor with a limited amount of surfactant into a 50 or 100 mL three-neck round bottom flask, degassing was performed by heating the solution mixture to 50–80 °C and evacuating at this temperature for 30– 60 min to remove water and oxygen under vigorous magnetic stirring. After degassing, the thermal reaction was carried out via polyol reduction, thermal decomposition, or solvothermal reaction by using a programmed temperature controller connected to a heating mantle under an Ar atmosphere. After cooling the solution to room temperature, the product was purified by centrifugation in excess solvent (acetone or ethanol) and the precipitate was dispersed in ethanol or n-hexane. Detailed information on the synthesis of the various NPs is provided in Table S1. To prepare the Pt/oxide layered NP catalysts, a colloid of the oxide NPs was deposited on to a SiO2 thin film by drop casting.

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(a)

Pt

Co3O4

MnO

(c)

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(e)

Pt

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(f)

Cu2O

20 nm

(g)

20 nm

(h)

ZnO

TiO2

20 nm

20 nm

20 nm

20 nm

Fe3O4

(d)

Figure 1. TEM images of synthesized individual NPs: (a-b) Pt, (c) MnO, (d) Fe3O4, (e) Co3O4, (f) Cu2O, (g) ZnO, and (h) TiO2. (b) High resolution TEM (HR TEM) image of Pt NPs with a fcc cuboctahedral structure in the [110]-zone axis.

The oxide NPs were then exposed to UV light in air for 1 h to remove organic surfactants. Direct photodecomposition of the organic molecules encapsulating the NPs by two Hg lamps that emitted two lines (184 and 254 nm)11 combined with oxidation of the surfactants by ozone produced by the 184 nm Hg line resulted in the removal of most of the organic surfactants to expose the clean oxide surface. Subsequently, Pt NPs (10 L of 0.1 M colloid) were drop cast onto the oxide/SiO2 film, and the UV treatment was carried out again to expose the clean metal surface. Through this process, interfaces were created between the two NP arrays (Pt and oxide) to study the support-induced catalytic performances of Pt. Catalytic Methanol Oxidation Methanol oxidation in a batch mode reactor was carried out to determine the reaction rates and selectivities of the Pt/oxide NP catalysts. The reactor was composed of a 1 L vacuum chamber held at a base pressure of 1 × 10−8 Torr using rotary and turbomolecular pumps (Figure S1). The catalyst film was placed inside the reactor chamber, which was then evacuated by sequential pumping through rotary and turbomolecular pumps. Subsequently, 10 Torr of methanol and 50 Torr of O2 balanced with He (700 Torr) were introduced through a gate valve and the catalyst was heated to 60 °C on a boron nitride heater. The reactants and products were circulated continuously through a metal bellows recirculation pump at a rate of 5.5 L/min. After an equilibrium was established following recirculation for 30 min, the products were analyzed using a gas chromatograph (YL-6100) equipped with a thermal conductivity detector and a HayeSep T column (Agilent). The turnover frequencies (TOFs) of methanol oxidation were calculated based on the reaction rate, determined by fitting peak area versus time, and normalized to the number of Pt active sites. The rate was measured at methanol conversions below 20%, which corresponded to the initial reaction rate in a kinetically controlled regime.30 The number of Pt active sites in each catalyst was determined from the hydrogenation rate of ethylene to ethane normalized to a known TOF (11.7 molecules·site-1·s-1).31 The hydrogenation reaction was conducted using 100 Torr of hydrogen, 10 Torr of ethylene, and 650 Torr of He at 25 °C immediately after the methanol oxidation reaction.32 The single Pt NP layer exhibited 2.3 × 1014 sites in ethylene hydrogenation,

whereas Pt/oxide layered catalysts showed 7.0 × 1013 – 1.3 × 1014 sites owing to the mixture of Pt and oxide NPs. The product selectivity for methanol oxidation was calculated as the TOF of a given product normalized to the combined TOF for all products. Computational Methods. DFT calculations were performed using the Vienna ab initio simulation package (VASP) with the projector-augmented wave (PAW) approach.33–37 All calculations were carried out with a 450 eV kinetic energy cut-off value and the self-consistent convergence of the total energy was 1.0 × 10–7 eV/atom. The effects of the exchange-correlation were handled within the generalized gradient approximation (GGA) with the use of the Perdew–Burke–Ernzerhof (PBE) exchangecorrelation functional.36 Atoms in the lowest of the calculated slabs were fixed at their bulk positions, and the atoms in the other slabs were allowed to relax. The binding energies, EBD, of HCHO, HCOOH, and HCOOCH3 on the Pt, Pt/TiO2(101), and Pt/TiO2(001) surfaces were defined as follows: E =E – (E + E ) BD

slab + adsorbates

slab

adsorbates

where Eslab + adsorbates is the total energy of the interaction between the slab and the adsorbate, Eslab is the energy of the isolated slab, and Eadsorbates is the energy of the molecular species. The formation energies, Eform, of HCOOH and HCOOCH3 were defined as: E =E –E –E +E form

slab + HCOOCH3

slab + HCOOH

HCOOCH3

HCOOH

where Eslab + HCOOCH3 is the total energy of the interaction between the slab and HCOOCH3, Eslab + HCOOH is the energy of the interaction between the slab and HCOOH, EHCOCH3 is the energy of molecular HCOOCH3, and EHCOOH is the energy of molecular HCOOH. Brillouin-zone k-point used 2 × 2 × 1(a = 6.152, b = 9.239, c = 16.742), 2 × 4 × 1(a = 10.388, b = 7.576, c = 27.903), and 4 × 2 × 1(a = 7.612, b = 11.419, c = 23.399) MonkhorstPack mesh for calculations on Pt, Pt/TiO2(101), and Pt/TiO2(001), respectively.37 RESULTS AND DISCUSSION To study the metal–support interfacial effect, we prepared Pt NPs supported on various oxide NPs as 2D layered catalysts.

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(a)

(b)

Pt/Co3O4

(c)

Pt/Co3O4

100 nm

(e)

50 nm

Pt/Co3O4 Pt/Co3O4

Fe

Pt/Fe3O4

Pt

(d)

Pt/Fe3O4

100 nm

20 nm

(f)

Pt/TiO2

50 nm

Ti

Pt

Figure 2. (a) Schematic drawing representing the preparation of Pt/oxide layered NP catalysts. (b) SEM image of Pt/Co3O4 layered catalysts. TEM images of (c) Pt/Co3O4 and (d) Pt/Fe3O4 layered catalysts. STEM images and corresponding EDS mapping of (e) Pt/Fe3O4 and (f) Pt/TiO2 layered catalysts.

First, metal oxide NPs, including MnO, Fe3O4, Co3O4, Cu2O, ZnO, and TiO2 (Figure 1c–h) were deposited on a SiO2 substrate as oxide layers by drop casting. By utilizing versatile colloidal synthetic methods, including thermal decomposition, non-hydrolytic sol-gel, polyalcohol reduction, and hydro-/solvothermal methods, monodisperse NPs can be successfully prepared with uniform size distributions.23,24 Figure 1 shows transmission electron microscopy (TEM) images of the as-synthesized NPs, demonstrating their uniform sizes. By depositing Pt NPs on oxide layers, Pt/oxide layered catalysts were prepared as shown in Figure 2a. To remove organic capping agents, such as oleic acid and oleylamine, which were used to stabilize the oxide NPs in solution, UV/O3 treatment was performed to photothermally decompose the organic surfactants with combined 184 and 254 nm sources in air.38 After this treatment, Pt NPs encapsulated in polyvinylpyrrolidone (PVP) (Figure 1a and b) were deposited on the oxide layers by drop casting. Subsequently, PVP was removed by additional UV/O3 treatment. To confirm the photothermal decomposition of surfactants, X-ray photoelectron spectroscopy (XPS) experiments were conducted at different UV treatment times.11 The C-to-Pt and the N-to-Pt atomic ratios of the Pt/oxide layered catalysts as a function of UV treatment time were determined by XPS (Figure S2). Both the C and N signals decreased relative to the Pt signal with UV exposure, indicating that the surfactants were effectively removed and that intimate contact between Pt and oxide NPs was achieved. All the prepared catalysts contained the same amount of Pt NPs (10 μL of 0.1 M colloid), which had an average diameter of 3 nm. Figure 2b–d show scanning electron microscopy (SEM) and TEM images of various Pt/oxide catalysts. Although dark Pt NPs can be distinguished from the oxide NPs, the entire structures are not clearly defined. However, scanning TEM (STEM) images and corresponding energy dispersive spectroscopy (EDS) mapping (Figure 2e and f), clearly reveal that Pt/Fe3O4 and Pt/TiO2 layered catalysts consist of white Pt NPs on top of

oxide NPs by. Homogeneous dispersion of Pt NPs on oxide NPs is also observed for Pt NPs supported on pyramidal ZnO or wire-shaped TiO2 catalysts (Figure S3). These observations confirm that because oxide NPs layers are not deposited as a monolayer, Pt NPs are distributed randomly on the oxide NP surfaces as a mixture. Detailed information on the synthesis of the various NPs (Table S1), particle size histograms (Figure S4), and X-ray diffraction patterns (XRD) (Figure S5) are provided in the Supporting Information. 50

40

TOF(s-1)

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ACS Catalysis

30

20

10

■ CO2 ■ HCHO ■ HCOOCH3

0 Pt

Pt/MnO Pt/Fe3O4 Pt/Co3O4 Pt/Cu2O Pt/ZnO Pt/TiO2

Figure 3. TOFs and selectivities of methanol oxidation over single Pt and layered Pt/oxide catalysts, measured in 10 Torr of MeOH and 50 Torr of O2, balanced with He, at 60 oC. Inset: Schematic illustration of Pt catalyzed methanol oxidation showing reaction intermediates and possible products including carbon monoxide, formaldehyde, and methyl formate.

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ACS Catalysis (a)

TiO2

(b)

TiO2

(c)

TiO2

(d)

TiO2

(i)

■ CO2 ■ HCHO ■ HCOOCH3

50

40

(j)

TOF(s-1)

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20 nm

(e)

20 nm

20 nm

(f)

(g)

20 nm

(h) 0.34nm (101)

(101)

0.47nm

0.34nm

0.34nm 5 nm

20

0.34nm

0.47nm 5 nm

10

0 5 nm

5 nm

Pt Pt

Spheres Rods Wires Cubes Cubes Spheres Rhombuses Wires Concave

Pt/TiO2

Figure 4. (a-d) TEM and (e-h) HR TEM images of TiO2 NPs with controlled shapes: (a,e) spherical-, (b,f) rhombic-, (c,g) wire-, and (d,h) concave-cube-shaped TiO2 NPs. (i) TOFs and selectivities of methanol oxidation over Pt/TiO2 catalysts and (j) a schematic illustration of the Pt-catalyzed methanol oxidation mechanism on TiO2, with different products produced depending on the TiO2 facet.

Methanol oxidation was used as a probe reaction, as the reaction pathways were well understood. Previous studies of methanol oxidation have demonstrated that the reaction is sensitive to the size of Pt NPs as well as the type of support.32,39 When Pt NPs with sizes in the range of 1 ‒ 6 nm were used as methanol oxidation catalysts, lower TOFs were observed for smaller Pt NPs, regardless of the type of reactor (batch or flow). XPS revealed that 1 nm Pt NPs were distinctly oxidized compared with larger Pt NPs (2, 4, and 6 nm). For this reason, despite changes in relative step and edge site distributions with size, it was concluded that oxidation of Pt NPs owing to their small size was responsible for the low TOF for methanol oxidation.32 In our study, 3 nm Pt NPs were used to prepare all the catalysts to eliminate the size effect of Pt NPs. Gas-phase methanol oxidation was carried out using 10 Torr of methanol and 50 Torr of O2 balanced to 700 Torr with He at 60 °C in a batch-type reactor. The TOFs for methanol oxidation were determined from the reaction rate for methanol conversion normalized by the number of Pt active sites, as determined by ethylene hydrogenation.31 Three products were detected as main products: carbon dioxide (CO2), formaldehyde (HCHO), and methyl formate (HCOOCH3). In Pt-catalyzed methanol oxidation, CH3OH is adsorbed on the Pt surface by forming methoxy (CH 3O) intermediates (Figure 3, inset). A dissociated oxygen atom on the Pt surface is then added to the carbon center of the adsorbed methoxy intermediate, and HCHO is released by producing a water molecule. Complete oxidation occurs by C–H cleavage of the methoxy intermediates to generate CO2 and water.40,41 When formate (HCOO) species formed via oxygen addition are coupled to another methoxy intermediate, HCOOCH3 is produced. In methanol oxidation, partially oxidized HCHO and HCOOCH3 are regarded as more desirable and valuable products than fully oxidized CO2, which is a greenhouse gas. Figure 3 shows the TOFs and selectivities for methanol oxidation over Pt NPs supported on various oxide NPs. Pure oxide NPs show negligible activity, and Pt NPs on a silica substrate without oxide NP layers were used as a reference, exhibiting a TOF of 4.67 s–1 at 60 °C. The selectivity of this pure Pt NP catalyst was 36.2% CO2, 27.1% HCHO, and 36.7% HCOOCH3. Interestingly, the TOFs of the Pt/oxide layered catalysts composed of MnO, Fe3O4, Co3O4, Cu2O, and ZnO were lower than that of the pure Pt catalyst (Table S2). The number of exposed Pt active sites was determined by ethylene hydrogenation and applied to calculate the TOFs of supported Pt NPs in methanol oxidation. However, the decreased TOFs of Pt/oxides catalysts might be the reduced active sites in the NP mixture. The major product

obtained using Pt NPs supported on MnO, Fe3O4, Co3O4, Cu2O, and ZnO NPs was CO2 (>55%) by preferential full oxidation. It is known that 3d-transition metal oxides provide lattice oxygens to Pt at the interface, facilitating high oxidation conversion, with the lattice oxygens recovered from gaseous oxygen during the oxidation reaction.15 The CO2 selectivity was further confirmed by examining bimetallic FePt and CoPt NPs (Table S2, Figure S6 and Figure S7). Although direct comparison of TOFs between Pt/oxide NPs and bimetallic alloy NPs is difficult owing to poor characterization of the Pt surface sites, bimetallic FePt (Fe:Pt = 2:8) and CoPt (Co:Pt = 2:8) NPs exhibited 100 and 90.3% selectivity for CO2, respectively (Figure S6). Therefore, 3d-transition metal oxides combined with Pt facilitate full methanol oxidation, and bimetallic alloy NPs realize greater CO2 formation than layered NP catalysts. In contrast to the 3d-transition metal oxides, TiO2 NPs showed substantially different behavior as a Pt support for methanol oxidation. The Pt/TiO2 catalyst exhibited at TOF of 48.6 s–1, which is 10 times greater than that of the pure Pt NP catalyst. Furthermore, the product selectivity of the Pt/TiO2 catalyst was significantly different (15.6% CO2, 17.3% HCHO, and 67.0% HCOOCH3). The specific interfacial effect of the Pt/TiO2 catalyst yielded an enhancement of the reaction rate by an order of magnitude as well as an increase of partial oxidation products such as HCHO and HCOOCH3. Thus, methanol was predominantly oxidized to CO2 over Pt NPs supported on MnO, Fe3O4, Co3O4, Cu2O, and ZnO NPs, whereas the catalytic performance was significantly different over the Pt/TiO2 catalyst with a higher reaction rates and more partial oxidation products. To confirm the effectiveness of the layered structures, we conducted methanol oxidation over reversely ordered TiO 2/Pt catalysts. The TOF of the layered catalysts prepared by depositing an equivalent amount of TiO2 NPs on top of Pt NPs was not changed (Figure S8). However, when the amount of TiO2 NPs was increased over Pt NPs, the TOF decreased remarkably. Pt NPs as an active species were mixed with oxide NPs in the Pt/oxide catalysts and exposed to reactant molecules during the reaction. However, the active surfaces of Pt NPs were blocked above the threshold concentration of oxide layers. In the previous study conducted by the Vohs’ group, TiO 2 NPs with a truncated bi-pyramidal shape exhibited photocatalytic activity in methanol oxidation.42,43 When UV light (365 nm LED source) was focused on the TiO2 NPs, which were treated with oxygen at 430 oC in an ultra-high vacuum chamber, dimethyl ether and methane were produced at 330–430 oC. They

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ACS Catalysis found that the minority exposed TiO2‒(001) surfaces were active for the photocatalytic reaction as well as (101) surfaces, in which methoxy groups were formed via dehydrogenation of methanol, producing formaldehyde and methyl formate. Likewise, Barteau and Vohs have reported many organic reactions on well-defined oxide surfaces, including TiO2, ZnO, CeO2, and MgO, to determine surface structures and active sites.44–47 In our study, however, negligible activity was observed for not only TiO2 NPs but also other oxide NPs, because there was no UV irradiation in the closed system and the reaction temperature was low (60 oC). Considering the major activity of methanol oxidation was originated from Pt NPs, TiO2 NPs affected to the Pt NPs for the enhanced activity. As the Pt/TiO2 layered NP catalyst showed remarkable activity enhancement as well as distinct selectivity for methanol oxidation, we synthesized TiO2 NPs with different shapes to determine the most active facet of TiO2.48–50 Figure 4a–h shows TiO2 NPs with controlled shapes, including sphere, rhombus, wire, and concave cube, obtained via the solvothermal approach by exploiting the polarity of organic molecules.43,48 The relative molar ratio of Ti(nBuO)4:oleic acid:oleylamine determined the final shape of TiO2 NPs, because the organic surfactant had distinct binding affinities with specific TiO2 facets. For example, when Ti(nBuO)4:oleic acid:oleylamine ratios of 1:8:2, 1:5:5, and 1:4:6 were introduced, spherical-, concave-cube-, and truncated rhombic-shaped TiO2 NPs were produced, respectively.48 All of the obtained TiO2 NPs were shown to be anatase phase (Figure S9).48,49 The crystal growth of one-dimensional (1D) TiO2 has been well studied. TiO2 nanorods grow along the direction and for branched 1D nanostructures, small nanofragments are attached to each other by either the (101) or (001) facets.51– 53 Recently, Nunzi et al. reported shape-dependent electronic structures of TiO2 and corresponding morphology effects in compressed and elongated TiO2 nanorods with truncated bipyramidal shapes.54 Based on their calculations, the surfaces of rhombic TiO2 NPs (Figure 4b) have a 65:15 ratio of (101):(001) facets, whereas the relative ratio of (001) facets decreases to less than 5% as the aspect ratio of TiO2 nanorods increases. Thus, rhombic TiO2 NPs, which have well-defined terminal areas, have higher fractions of (001) facets than TiO2 nanowires. Similarly, spherical TiO2 NPs have a higher abundance of (001) facets than other 1D TiO2 NPs. Concave cubic TiO2 NPs (Figure 4d,h), which were mixed with other random-shaped particles, have a lower fraction of (001) surfaces. Overall, the relative fractions of (001) facets in the shape-controlled TiO2 NPs decreased in the following order: spheres > rhombuses > wires > concave cubes. TEM images show that Pt NPs were attached to either (101) or (001) facets of rhombic TiO2 NPs (Figure S10). The TOFs and selectivities of the Pt/TiO2 layered NP catalysts comprising spherical-, rhombus-, wire-, and concave-cubeshaped TiO2 NPs were compared for methanol oxidation. As shown in Figure 4i, all of these Pt/TiO2 catalysts have higher TOF values than the pure Pt NP catalyst, demonstrating the interfacial effect between Pt and TiO2. Among the shape-controlled Pt/TiO2 catalysts, spherical TiO2 NPs have the highest TOF (Figure 4i). Although the spherical TiO2 were much smaller (4.7 nm) than the other TiO2 NP shapes, it was clear that the relative fractions of (001) facets was highest in the spherical TiO2 NPs and the interfacial effect of the TiO2(001) surface was much greater than that of the (101) surface (Figure 4j). XPS analysis of the Pt/oxide catalysts revealed that there was no relationship between Pt oxidation state and the type of oxide NP

(Figure S11). It is concluded that the total TOF and product selectivity for HCOOCH3 in methanol oxidation are dependent on the shape of TiO2 NPs and that the Pt‒TiO2 interfacial effects are related to the relative fraction of TiO2(001) surfaces. DFT was used to understand the interfacial effects of the Pt/TiO2 catalysts by calculating the binding and adsorption energies for the three most likely intermediates on the catalysts: HCHO, HCOOH, and HCOOCH3. Figure 5 shows the structures of HCHO, HCOOH, and HCOOCH3 physically adsorbed on Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces and the calculated binding energies. As shown in Figure 5a, HCHO is adsorbed almost perpendicularly on the Pt(111) surface with the OH group pointing toward the top site of Pt(111) with a bond distance of 2.17 Å , whereas the C–H bond points away from the surface of Pt(111). In contrast, the distances between the OH group and the Pt(111) surface are 2.18 and 2.70 Å for HCOOH and HCOOCH3, respectively (Figure 5b and c). Thus, among these three molecules, the adsorption strength on the Pt(111) surface is smallest for HCOOCH3. The binding energies of the intermediates on the Pt(111) surface were calculated to be in the following order: HCOOH (0.505 eV) > HCHO (0.322 eV) > HCOOCH3 (0.076 eV). A higher binding energy represents a more favored intermediate, which induces predominate formation of the corresponding product. The adsorption of HCHO, HCOOH, and HCOOCH3 on Pt/TiO2(101) and Pt/TiO2(001) surfaces is also compared in Figure 5d–i. Calculation details are provided in the Supporting Information (Figure S12–S14). In both Pt/TiO2 cases, the C–H, O–H, and –CH3 bonds point away from the surface of Pt and the two oxygen atoms of HCOOH and HCOOCH3 are connected to Ti atoms, whereas the carbon atom of the adsorbates is preferentially linked to Pt.

(a)

(b)

(c)

C H

O

3.17Å

2.17Å

2.24Å

3.14Å

2.18Å

2.70Å

Pt

HCHO on Pt(111) BE = -0.322 eV H

(d)

HCOOH on Pt(111) BE = -0.505 eV

(e)

C

HCOOCH3 on Pt(111) BE = -0.076 eV

(f)

O

Pt Ti

HCHO on Pt/TiO2(101) BE = -1.299 eV H

(g)

HCOOH on Pt/TiO2(101) BE = -0.735 eV

(h)

HCOOCH3 on Pt/TiO2(101) BE = -0.828 eV

(i)

C

O

Pt Ti

HCHO on Pt/TiO2(001) BE = -2.910 eV

HCOOH on Pt/TiO2(001) BE = -2.203 eV

HCOOCH3 on Pt/TiO2(001) BE = -2.338 eV

Figure 5. Structures of adsorbed HCHO, HCOOH, and HCOOCH3 on Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces: (a) HCHO, (b) HCOOH, (c) HCOOCH3 on Pt(111) surface, (d) HCHO, (e) HCOOH, (f) HCOOCH3 on Pt/TiO2(101) surface, and (g) HCHO, (h) HCOOH, (i) HCOOCH3 on Pt/TiO2(001) surface. Calculated binding energies (BE) are provided.

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-0.5

Binding Energy (eV)

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HCHO on Pt(111)

0.448

HCOOCH3 on Pt(111)

HCOOH on Pt(111)

-1 HCHO  HCOOH

FE = -0.093eV

HCOOH on Pt/TiO2(101)

-1.5

HCOOCH3 on Pt/TiO2(101)

HCHO on Pt/TiO2(101)

-0.5845 +0.367 H2

+0.491

H1

O2

-0.558 C2 +0.462 +0.329

0.563

C1

H4

HCHO  HCOOH

FE = -0.135eV

FE = 0.707eV HCOOH on Pt/TiO2 (001)

C2 -1.269

+0.373 H1

0.570

H3

0.494 +0.039

C1

-0.266

O1 -0.153

O2

0.736 O1 -0.458

-1.124 +0.498 C2 H4 H1+0.555 +0.509 H3 +1.569 C1 -0.644 O2 O1 -0.898 -0.531

Pt

+0.092

-0.009

HCOOCH3 on Pt/TiO2(001)

Figure 6. Calculated binding energies of HCHO, HCOOH, and HCOOCH3 on Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces. Formation energies (FE) are obtained from the difference between the binding energies of reaction intermediates.

For the Pt/TiO2(101) and Pt/TiO2(001) surfaces, the calculated binding energies of the intermediates are higher than those for the Pt(111) surface, which demonstrates that Pt/TiO2 stabilizes the intermediates considerably. The Pt‒TiO2 interfacial effect, which depends on the TiO2 facet, changes the binding energies of the intermediates dramatically. All the intermediates have higher binding energies with Pt/TiO2(001) than with Pt/TiO2(101), demonstrating that the TiO2(001) surface provides stronger active sites than the TiO2(101) surface. The formation energies of HCHO, HCOOH, and HCOOCH3 were also calculated on the three catalysts (Figure S15–S17). Figure 6 shows that the formation energy of HCOOCH3 is highest on the Pt(111) surface (0.429eV). For this reason, in Pt-catalyzed methanol oxidation, HCOOCH3 is a relatively less feasible product. When TiO2 was used as a support for Pt, the reaction kinetics changed remarkably, with a higher formation energy observed for HCHO to HCOOH than for HCOOH to HCOOCH3. Moreover, lower formation energies for HCOOH to HCOOCH3 were observed with the Pt/TiO2(001) catalyst than with the Pt/TiO2(101) catalyst. To study charge transfer in the Pt/TiO2 catalysts, we carried out a topological analysis of the static electron density using the calculation developed by Bader, which is able to characterize charge transfer between a molecule and a surface (Figure S18– S20).55 As shown in Figure 7, the charge transfer value of the Pt/TiO2(001) surface to HCOOCH3 (0.838) is higher than those from the Pt/TiO2(101) and Pt(111) surfaces (0.818 and 0.394, respectively). These combined calculation results (higher binding energy and larger charge transfer between the intermediates and Pt/TiO2) are in accordance with the experimental observations for the methanol oxidation reaction, in which Pt/TiO2 layered catalysts had outstanding TOF values compared with pure Pt or other oxide-based catalysts and the selectivity toward HCOOCH3 was notably increased. It was revealed that the TiO2(001) surface, which exhibited the highest charge transfer value, stabilized HCOOCH3 as a partial oxidation product. Thus, when spherical and rhombic TiO2 NPs with relatively abundant (001) facets were used as Pt supports, HCOOCH 3 was selectively produced with higher conversion than when wire- and concave-cube-shaped TiO2 NPs were used.

Ti1

-0.054

HCOOCH3 on Pt(111) HCOOH  HCOOCH3

HCHO on Pt/TiO2 (001)

CT = 0.838

-0.476

-2

-3

(c)

CT = 0.818 H2

HCOOH  HCOOCH3

-2.5

(b)

CT = 0.394

H2

FE = -0.183eV

FE = 0.564eV

(a)

FE = 0.429eV

Ti2

-0.070

HCOOCH3 on Pt/TiO2(101)

Ti1

-0.0582

Pt

Ti2

-0.043

HCOOCH3 on Pt/TiO2(001)

Figure 7. Calculated charge transfer (CT) from catalyst surfaces to HCOOCH3: (a) Pt(111), (b) Pt/TiO2(101), and (c) Pt/TiO2(001) surfaces. O1 and O2 in the adsorbate molecules denote the carbonyl oxygen and the hydroxyl oxygen, respectively. CT is the sum of the charge transfer to each atom in the HCOOCH3 molecule.

CONCLUSION Uniform oxide NPs were synthesized and combined with Pt NPs on a substrate as layered NP catalysts to study how interfacial effects influence catalytic performance. The type of oxide NP used as a support dramatically affected the catalytic activity and selectivity of Pt-catalyzed methanol oxidation. Pt/oxide catalysts composed of Pt NPs layered on MnO, Fe 3O4, Co3O4, Cu2O, and ZnO NPs had TOFs that were comparable to that of the pure Pt NP catalyst (4.67 s–1) for methanol oxidation. However, the selectivities of the Pt/oxide layered catalysts favored formation of CO2 via full oxidation. In contrast, methanol oxidation activity was dramatically increased over Pt/TiO2 layered catalysts, and the TOF of Pt/TiO2 (48.6 s–1) was 10 times greater than that of Pt NPs. The interfacial effect between Pt and TiO2 also altered product selectivity toward HCHO and HCOOCH3 as partial oxidation products. When the shape of TiO2 was controlled, much higher TOFs and selectivities toward partial oxidation products were obtained for Pt supported on spherical and rhombic TiO2 NPs, which had higher fractions of (001) facets than wire- and concave-cube-shaped TiO2 NPs. Calculation of the adsorption energies and formation energies of HCHO, HCOOH, and HCOOCH3 on Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces using DFT revealed that adsorption of HCHO on Pt(111) was more favorable than adsorption of HCOOCH3, whereas adsorption of HCOOCH3 on Pt/TiO2 was more favorable than adsorption of HCOOH. The TiO2(001) surface, which had stronger active sites than TiO2(101), facilitated charge transfer, resulting in an enhanced TOF and high selectivity toward HCOOCH3 during methanol oxidation. We expect that our Pt‒oxide layered NP system will promote in-depth investigations into metal‒support interactions at the interfaces of nanocatalysts, and ultimately contribute to the use of nanotechnology to develop new high-performance catalysts.

ASSOCIATED CONTENT Supporting Information. Synthesis details for NPs, scheme of reactor for catalytic methanol oxidation, XPS evaluation of surfactant removal, TEM images of Pt NPs supported on pyramidal ZnO or rod-shaped TiO2 catalysts, particle size histograms and XRD patterns of synthesized NPs, TOFs and selectivities of Pt/oxide NPs, bimetallic alloy NPs, and TiO2/Pt NPs for methanol oxidation, XRD patterns, TEM images, and XPS spectra of TiO2 NPs with

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ACS Catalysis controlled shapes, DFT calculations of adsorption energies, formation energies, and charge transfer for Pt(111), Pt/TiO2(101), and Pt/TiO2(001) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected], [email protected]

Author Contributions ∥S.Y.

and K.O. contributed equally.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1C1A1A01055092) and C1 Gas Refinery Program through the NRF funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064899). G.A.S thanks for the support by the Director, Office of Basic Energy Sciences, Materials Science and Engineering Division of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.

ABBREVIATIONS NPs, nanoparticles; SMSIs, strong metal–support interactions; 2D, two-dimensional; DFT, density functional theory; TOFs, turnover frequencies; TEM, transmission electron microscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.

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