Interplay between Molybdenum Dopant and ... - ACS Publications

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Interplay between Molybdenum Dopant and Oxygen Vacancies in a TiO2 Support Enhances the Oxygen Reduction Reaction Meng-Che Tsai,†,∥ Trung-Thanh Nguyen,†,∥ Nibret Gebeyehu Akalework,† Chun-Jern Pan,† John Rick,† Yen-Fa Liao,§ Wei-Nien Su,*,‡ and Bing-Joe Hwang*,†,§ †

Nano Electrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan ‡ Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan § National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan S Supporting Information *

ABSTRACT: In this study, molybdenum doping of anatase TiO2, used as a Pt catalyst support, both augments resistance against the carbon corrosion that commonly occurs in oxygen reduction reaction (ORR) Pt/C catalysts and promotes the generation of oxygen vacancies that allow better electron transfer from the nanosupport to Pt, thereby facilitating the oxygen dissociation reaction. The effects of the oxygen vacancies within the Mo-doped TiO2 nanosupport on ORR activity and stability are investigated both experimentally and by density functional theory analysis. The mass activity of Ptsupported molybdenum-doped anatase TiO2 is shown to be 9.1 times higher than that of a commercial standard Pt/C catalyst after hydrogen reduction. The oxide-supported nanocatalysts also show improved stability against Pt sintering under during cycling, because of strong metal−support interactions. KEYWORDS: oxygen vacancies, molybdenum doping, doped titanium oxide, oxygen reduction reaction, strong metal−support interaction, oxide support

1. INTRODUCTION The activity and durability of the catalysts used for the oxygen reduction reaction (ORR) are crucial to the commercialization of polymer electrolyte membrane fuel cells (PEMFCs).1−7 Pt is still the most common catalytic material for the PEMFCs. Many factors, such as the catalyst’s composition, size, shape, support, and adsorbates, are related to the catalyst’s performance;7−13 such matters have been extensively studied over the past several decades in pursuit of high ORR activity, long-term stability, and reduced Pt consumption.3,8,10,11,14−19 Previous reports have suggested that the use of nanosized noble metal electrocatalysts is one approach to optimizing fuel cell performance.12,13,20 For example, Pt-based catalysts on carbon supports frequently exhibit low durability, because of carbon corrosion and weak metal−support interactions.12,21 The general requirements of support materials for a platinumbased electrocatalyst include a large surface area for the uniform dispersion of nanosized catalysts, low electrical resistance/or high electronic conductivity to facilitate electron transport during the electrochemical reactions, a porous structure that favors increased contact with the fuel or oxidant and the release of any byproduct(s), strong interactions between the catalyst nanoparticles, and a high stability under oxidative conditions © 2016 American Chemical Society

such as in the presence of oxygen, liquid water, and high potentials.22−26 Therefore, titanium oxide-based materials have been seen as promising supports for nanosized catalysts in electrochemical applications because of their low cost, nontoxicity,27 and high stability in acidic and oxidative environments.26 The cation in the vicinity of the oxygen vacancies in the metal oxide is reduced, which triggers the electronic interactions between the reduced cation and the noble metal atom, giving rise to interactions known as strong metal−support interactions (SMSI). This phenomenon has been previously reported in the literature for the Pt/TiO2 system.28,29 Additionally, noncarbon supports with TiO 2 -based materials (Ti1−xM xO y materials, where M = Nb,16 W,30 Mo,26,31 Ru,13 etc.) can enhance the performance of ORR Pt catalysts, due to SMSI and their related electronic effects, as discussed in our previous work.13,26 For metal-doped TiO2, structural defects such as oxygen vacancies and lattice defects are the most important features Received: February 27, 2016 Revised: June 13, 2016 Published: August 24, 2016 6551

DOI: 10.1021/acscatal.6b00600 ACS Catal. 2016, 6, 6551−6559

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When the reaction was completed, the sample was cooled in air and the black precipitate was collected by repeated centrifugation and washing with acetone and DI water. The resulting Pt/d-Ti0.9Mo0.1Oy was dried in a vacuum oven overnight.

because they strongly affect, in a concentration-dependent manner, the electron transfer direction between the Pt catalyst and the metal oxide support, the electronic conductivity of these support materials, and the catalytic properties of the noble metal.32 Similar lessons can be learned from various catalysis systems, such as CexTiO2 for NO reduction,33 FeOxsupported Pt or Pd for CO oxidation,34 and Ni on CeO2 nanorods for CO2 reforming of methane.35 Improved electron mobility and enriched surface oxygen vacancies are known to be important for solid oxide fuel cell cathodic materials36,37 and lithium−oxygen batteries.38,39 This study focuses on how the ORR activity is enhanced by the metal oxide support with induced oxygen vacancies, resulting from doping and thermal treatment. Various amounts of molybdenum-doped TiO2 (Ti1−xMoxOy) material were synthesized and characterized by X-ray diffraction (XRD), Raman spectroscopy, and X-ray absorption spectroscopy (XAS). As novel supports for cathodic catalysts in the PEMFC, the electrochemical properties of Pt/d-Ti0.9Mo0.1Oy were also evaluated and compared with those of a commercially available Pt/C catalyst (E-Tek). Additionally, density functional theory (DFT) calculations were used to explain the role of oxygen vacancies in the Ti0.9Mo0.1Oy nanosupport and its role in the ORR.

3. RESULTS AND DISCUSSION 3.1. Characterization of Ti1−xMoxOy. 3.1.1. XRD Measurements. Figure 1A shows typical X-ray diffraction (XRD)

Figure 1. (A and B) XRD patterns of Ti1−xMoxOy samples: (1) MoO3 (JCPDS Card No. 00-005-0506), (2) anatase TiO2 (JCPDS Card No. 21-1272), (3) Ti0.9Mo0.1Oy, (4) Ti0.8Mo0.2Oy, and (5) Ti0.7Mo0.3Oy.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Ti1−xMoxOy Nanoparticles. A simple previously reported hydrothermal method was employed to synthesize Ti1−xMoxOy.31 In a typical synthesis, a Teflon reactor containing aqueous materials, including TiCl4 and MoCl5, was placed in a sealed autoclave, heated to 200 °C, and kept at this temperature for 2 h. The precipitated Ti1−xMoxOy samples with different Mo loadings were collected, as black powders, after repeated washing with deionized (DI) water and centrifugation prior to being dried in a vacuum oven at 80 °C overnight. 2.2. Generation of Oxygen Vacancy-Rich Ti1−xMoxOy Nanosupport. To produce the nanosupports with surface oxygen vacancies, Ti1−xMoxOy powder was annealed at high temperatures under hydrogen. In a typical procedure, taking Ti0.9Mo0.1Oy as an example, a nitrogen flow was applied in a homemade U-tube, which contained around 0.07 g of Ti0.9Mo0.1Oy powder, for 30 min at room temperature to remove adsorbed gases and surface water from the Ti0.9Mo0.1Oy. After the nitrogen treatment, hydrogen gas (10 vol % H2 in argon) was passed through the Ti0.9Mo0.1Oy powder layer at different steady temperatures (100 and 200 °C) for 4 h. Next, the treated Ti0.9Mo0.1Oy samples (denoted as d-Ti0.9Mo0.1Oy) were collected after being cooled to room temperature. The H2 gas flow was maintained during cooling, and the treated samples were stored in a sealed (parafilm layer) glass bottle to avoid air contact. 2.3. Deposition of Pt Nanoparticles on d-Ti0.9Mo0.1Oy Nanosupports. Pt nanoparticles were deposited onto the surfaces of d-Ti0.9Mo0.1Oy nanosupports using a microwaveassisted polyol method. In a typical synthesis, a mixture including ethylene glycol (30 mL), d-Ti0.9Mo0.1Oy (∼90 mg) nanosupport (treated with hydrogen at 300 °C), and a H2PtCl6/ethylene glycol (50 mM) solution was ultrasonicated for 30 min. Next, the pH value of the precursor solution was adjusted to ∼11 by adding NaOH (1 M). The synthesis was conducted in a domestic microwave oven (LG MG-5021MW1, 300 W, 2450 MHz), while the suspension was exposed in the middle of the microwave oven for 1 h at 200 W (160 °C).

patterns of the dried Ti1−xMoxOy samples and XRD standards of anatase TiO2 (JCPDS Card No. 21-1272) and MoO3 (JCPDS Card No. 00-005-0506) samples. The figure shows that the Ti1−xMoxOy structure is characterized by the anatase phase TiO2. With increasing contents of Mo in TiO2, the intensity of the first peaks at 2θ ∼ 25.3° decreased and the position of the sample peaks at ∼47.9° shifts from the anatase TiO2 position to a lower degree (as Figure 1B). However, the XRD patterns for samples with 2.5 and 10 mol % Mo were not notably different. The Ti1−xMoxOy structural transition can be attributed to the inclusion of Mo in the anatase TiO2 structure, as has been observed with Nb-TiO2 in previous reports.40 Notably, the XRD patterns of the samples show only five peaks belonging to TiO2 and no MoO3 peak, implying that these asprepared materials consist of a solid solution of Mo-doped TiO2 with an anatase structure. 3.1.2. Raman and X-ray Absorption Spectroscopy (XAS) Measurements. To understand the structure of Ti1−xMoxOy, Raman measurements were taken (see the experimental section in the Supporting Information for more details). Figure 2 shows the Raman spectra of the Ti1−xMoxOy materials with anatase-TiO2 (Degussa) as a reference. For anatase TiO2, four peaks were observed at 147, 396, 514, and 633 cm−1, in agreement with the previous work.41 Additionally, the literature shows that the characteristic peaks at 290, 667, 819, and 995 cm−1 are assigned to MoO3.42−44 For doped Ti1−xMoxOy, Raman peaks were observed at the same Raman shift positions as those of anatase-TiO2, but with lower peak intensities, which may be attributed to doping of Mo into the anatase TiO2 structure. This is consistent with previous work on Nb-doped TiO2 materials.40 However, at the higher Raman shift range, for Ti0.7Mo0.3Oy, a broad asymmetric hump at ∼830 cm−1 (Mo− O−Mo band) and a peak at 995 cm−1 (MoO band) were observed (as seen in Figure 2B), which can be attributed to crystalline MoO3. These observations indicate that excess molybdenum oxide species may be present on the surface of 6552


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Ti0.9Mo0.1Oy nanoparticles are a well-mixed solid solution of Mo-doped TiO2. 3.1.4. Treatment and Characterization of Ti0.9Mo0.1Oy Materials. As shown in Figure S3, the Ti0.9Mo0.1Oy sample had the highest electronic conductivity compared to those of other oxides with different Ti:Mo ratios as determined by measuring the sheet resistance. However, it was also found that the surface of Ti0.9Mo0.1Oy was possibly covered by an oxide layer, as seen in the Ti−Mo−O mapping image in Figure 4. This would adversely affect the donation of electrons from the metal oxide support to Pt and therefore the ORR activity of Pt on this nanosupport. As a result, Ti0.9Mo0.1Oy should be reduced under H2 at an appropriate temperature to remove the oxygen layer on the sample’s surface before use. The thermally treated and gas-treated samples are denoted as d-Ti1−xMoxOy hereafter. Figure 5A shows the Raman spectra of Ti0.9Mo0.1Oy after H2 reduction at different temperatures (see Experimental Section for detail). The temperature for this reduction was below 450 °C to avoid phase transformation from anatase TiO2 into rutile TiO2 (see Figure S4). In comparison, the change in the Raman spectra of d-Ti0.9Mo0.1Oy after H2 reduction at different temperatures shows only a broad peak at 960−995 cm−1, a range typical for MoO for a molybdenum oxide stretching vibration.45 The change was noted in that the tail of these peaks shifted to lower values, indicating that the removal of oxygen from the oxide’s surface takes place more easily with Ti−O−Mo bonds than with Ti−O−Ti bonds. From the DFT calculation, the removed oxygen initially relocates to a neighboring Ti−Ti site for undoped TiO2 as shown in Figure S5 (A1), while in the case of Mo-doped anatase TiO2 in Figure S5 (A2), the formation of an oxygen vacancy reveals two defects at the Ti−Ti and Ti−M sites. Similarly, this extended peak tail, attributed to oxygen vacancies, was also observed in the reported studies of different oxide mixtures such as Ce1−xRExO2−y (RE = rare earth) solid solutions.46 The relative changes in oxygen vacancy concentration in d-Ti0.9Mo0.1Oy-100 °C and d-Ti0.9Mo0.1Oy-300 °C samples were estimated to be 3.22 and 5.10%,47 respectively. It is interesting to note in Figure 5B that the electronic conductivity of the Ti0.9Mo0.1Oy sample after H2 treatment at 300 °C showed 5.8 and 15.4% increases compared to those of a sample treated at 100 °C and an initial untreated sample. Furthermore, the oxygen vacancy formation energy was estimated by DFT calculations, and the result showed the impact of doping on the reduction of oxygen vacancy formation energy, especially for sites at the Ti−O−Mo site as shown in Figure S5B. Thus, after the reduction at 300 °C, the d-Ti0.9Mo0.1Oy sample could be a preferred catalyst support with higher electronic conductivity due to a higher content of oxygen vacancies. 3.2. Computation about the Role of Oxygen Vacancies in Pt/d-Ti0.9Mo0.1Oy Catalysts toward ORR Activity. Theoretical simulation (VASP, Vienna Ab initio Simulation Package) was conducted to investigate the adsorption behavior of atomic oxygen and the interaction between Pt catalyst and TiO2-based supports, including regular anatase TiO2(101), reduced TiO2(101), and reduced dTi1−xMoxOy. One oxygen atom on the surface was deliberately taken from the model to simulate the effect of an oxygen vacancy caused by the thermal treatment. The computational details can be found in the Supporting Information. The results for Pt and atomic oxygen adsorption energies were calculated on the basis of two possible Pt−support configurations, where the single Pt atom binds with Ti−Ti or Ti−Mo sites and

Figure 2. (A and B) Raman spectra of Ti1−xMoxOy nanocrystals with (1) an anatase TiO2 reference sample, (2) Ti0.9Mo0.1Oy, (3) Ti0.8Mo0.2Oy, and (4) Ti0.7Mo0.3Oy.

Ti0.7Mo0.3Oy, where the doping of Mo exceeded its solubility in anatase TiO2.43 For samples with lower Mo contents, including Ti0.8Mo0.2Oy, Ti0.9Mo0.1Oy, and Ti0.975Mo0.025Oy (not shown), the band at around 960 cm−1 (as seen in Figure 2B) could be also assigned to the MoO stretch. The shift from 995 to 960 cm−1 can be attributed to the decreased Mo content of Ti1−xMoxOy samples.43,44 The peaks at 960 cm−1 also show that these samples can have two kinds of molybdenum oxide, i.e., octahedrally structured (Mo7O246− or Mo8O264− cluster-like for high Mo contents) and tetrahedrally structured (MoO42−-like for low Mo contents).43 Additionally, XAS measurements (Figure 3) show similar XANES spectra for Ti0.9Mo0.1Oy and

Figure 3. XANES measurements of Ti0.9Mo0.1Oy and Ti0.8Mo0.2Oy samples with Na2MoO4, MoO2, MoO3, and MoCl5 references.

Na2MoO4, implying that Ti0.9Mo0.1Oy is largely composed of MoO42−-like tetrahedral structures. In contrast, Ti0.7Mo0.3Oy and Ti0.8Mo0.2Oy have Mo7O246− or Mo8O264− cluster-like octahedral structures. Because anatase TiO2 has an octahedral structure, it is believed that the presence of surface oxygen vacancies has caused some structural distortion of Mo-doped TiO2. 3.1.3. Transmission Electron Microscopy (TEM) and TEM Mapping Images of Ti0.9Mo0.1Oy Materials. Figure 4a shows a TEM image (consistent with the XRD results) of Ti0.9Mo0.1Oy nanoparticles with an average size of ∼8−10 nm. In Figure 4b, the crystallinity of Ti0.9Mo0.1Oy can be clearly observed from the well-defined fringes at ∼0.34 nm that correspond to the lattice spacing of the (101) plane of anatase TiO2. Furthermore, energy-dispersive X-ray spectroscopy (EDX) measurements (Figure S2) indicate a Ti:Mo atomic ratio of 88.0:12.0, which agrees well with the expected atomic ratio of 90:10 for the asprepared Ti0.9Mo0.1Oy. The mapping TEM measurements in Figure 4c−e provide strong evidence to show that the 6553


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Figure 4. (a and b) TEM images and (c−-e) elemental mapping images of d-Ti0.9Mo0.1Oy nanosupports detected from the dotted area shown in the inset of panel a.

Figure 5. (A) Raman spectra and (B) electronic conductivity values of d-Ti0.9Mo0.1Oy nanosupports after hydrogen treatment at different temperatures.

Figure 6. DFT calculations of (A) Pt adsorption energy and (B) oxygen adsorption on Pt sites with different supports containing (1) regular anatase TiO2, (2) reduced TiO2, (3) d-Ti0.95Mo0.05O2, (4) d-Ti0.9Mo0.1O2, (5) d-Ti0.8Mo0.2O2, and (6) d-Ti0.7Mo0.3O2.

atomic oxygen adsorbs on the corresponding Pt atom. The DFT results of Pt adsorption energy on supports are shown in Figure 6A and oxygen adsorption energies on Pt sites in Figure 6B. The Pt catalyst on the d-Ti0.9Mo0.1O2 support is predicted

to be a good catalyst for ORR, because of the highly stable Pt catalyst on d-Ti0.9Mo0.1O2 (the lowest energy for Pt atoms on Ti−Ti and Ti−Mo sites) and mediocre oxygen adsorption energy at the Pt site.48 6554


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Figure 7. Proposed models of selective oxygen adsorption and the different reaction pathways for oxygen dissociation on the Pt/d-Ti0.9Mo0.1O2 catalyst are shown in (A) the interface between the Pt and oxygen vacancy and (B) the supported Pt surface, where white, red, blue, and light blue spheres represent Ti, oxygen, Pt, and Mo, respectively.

Hydrogen reduction induces the generation of oxygen vacancies near the surfaces of doped titanium oxides and enhances the electronic conductivity of the oxide supports. It is also of great interest to investigate how the presence of oxygen vacancies on an oxide support enhances the ORR activity for supported Pt catalysts. The results of our DFT calculations are summarized in Figure 7. The adsorption and dissociation of oxygen molecules on oxygen vacancy and Pt sites were evaluated by considering oxygen molecules adsorbing directly onto two Pt atoms on the d-Ti0.9Mo0.1Oy support (model A) and oxygen molecules adsorbing onto surface oxygen vacancies near the Pt catalyst (model B). In model A, the oxygen molecule adsorbed on the oxygen vacancy site, the calculated oxygen adsorption energy (ΔG) was −13.172 eV, and the O−O bond splitting energy and one oxygen atom adsorbing to the Pt site (ΔG) was −14.890 eV (see Figure 7A). In model B, the oxygen molecule adsorbed to the Pt site and the oxygen adsorption and O−O bond splitting energies (ΔG) were −10.762 and −11.603 eV, respectively (see Figure 7B). These results indicate that the presence of oxygen vacancies on the dTi0.9Mo0.1Oy support surface can actually facilitate a high yield of oxygen atoms. Furthermore, the oxygen atoms can be reduced more easily to produce H2O molecules as the reported ORR mechanism on active Pt sites. Thus, the presence of oxygen vacancies on the d-Ti0.9Mo0.1Oy support surface can reduce the time needed for oxygen adsorption and O−O bond splitting on the active Pt sites. This can improve the activity of Pt sites due to the low coverage of oxygen molecules. 3.3. Characterizations of Pt/d-Ti0.9Mo0.1Oy and Pt/C (ETEK) Samples. 3.3.1. Structural and Morphological Analyses. As investigated in previous sections, d-Ti0.9Mo0.1O2 was selected as the most suitable novel support. The doped oxide was first reduced under a H2 environment at 300 °C in advance. A microwave-assisted polyol method was employed to anchor Pt nanocrystals onto the d-Ti0.9Mo0.1Oy nanosupports. A detailed description of the synthesis can be found in the Supporting Information. TEM images of 10 wt % Pt/dTi0.9Mo0.1Oy and 20 wt % Pt/d-Ti0.9Mo0.1Oy nanocrystals are shown in Figure 8A. For Pt/d-Ti0.9Mo0.1Oy samples, the average size of Pt particles was around 5−6 nm. As seen in

Figure 8. TEM images of (A−C) 10 wt % Pt/d-Ti0.9Mo0.1Oy, (D) 20 wt % Pt/d-Ti0.9Mo0.1Oy, and (E) 20 wt % Pt/C (E-TEK) nanocrystals.

Figure 8B, it is also interesting to note that the Pt particles seemed to have a flattened morphology that could be attributed to the strong metal−support interactions (SMSI) due to the high oxygen vacancy concentration on the d-Ti0.9Mo0.1Oy nanosupport surface. 49 The HR-TEM images of Pt/dTi0.9Mo0.1Oy samples show that Pt nanoparticles were enclosed by (111) planes (Figure 8C). The XRD spectra of Pt/dTi0.9Mo0.1Oy samples showed that Pt nanoparticles have a structure of face-centered cubic (FCC), as seen in Figure S6. A similar flattened morphology and distribution could be obtained for 20 wt % Pt/d-Ti0.9Mo0.1Oy, as seen in Figure 8D. By contrast, a TEM image of 20 wt % Pt nanoparticles on a 6555


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Figure 9. (A) Normalized XANES spectra at the Pt LIII edge of 10 and 20 wt % Pt/d-Ti0.9Mo0.1Oy and 20 wt % Pt/C nanocrystals with Pt foil as the reference. (B) Charge densities of supported Pt were calculated by DFT simulation in which the values of ΔρPt represent the valence electrons of Pt (5d9 6s1) on Pt/d-Ti0.9Mo0.1Oy. The results of the calculation show that the electron transfer is induced from Ti1−xMoxO2 to Pt (namely negative ΔρPt) while Pt/graphite shows a slight positive change in ΔρPt. The computational details can be found in the Supporting Information.

Figure 10. Cyclic voltammograms of (A) the bare d-Ti0.9Mo0.1Oy nanosupport at a scan rate of 100 mV s−1 and (B) 10 wt % Pt/d-Ti0.9Mo0.1Oy, 20 wt % Pt/d-Ti0.9Mo0.1Oy, and E-TEK (20 wt % Pt on carbon) catalysts at a sweep rate of 25 mV s−1 in a N2-saturated H2SO4 solution at 25 °C.

similar to results previously reported for Ti4O7.14 This indicates that the d-Ti0.9Mo0.1Oy nanosupport is more resistant to oxidation under normal PEMFC operation. Moreover, no specific oxidation and reduction current peaks emerged in the CV curve, indicating that molybdenum was perfectly doped into the TiO2 structure, which agrees with the previous results. Thus, d-Ti0.9Mo0.1Oy exhibits the high stability essential for Pt nanocatalyst supports. Figure 10B shows CV curves for the Pt catalysts in a PEMFC single cell at room temperature under bubbled N2. Detailed information can be found in the material characterization section of the Supporting Information. The usual voltammogram of Pt in acidic electrolytes was observed, and no other additional current peaks were seen. Hydrogen adsorption/ desorption peaks at a potential range between 0.05 and 0.4 V were observed due to Pt deposition on the supports. These results strongly suggest that the d-Ti0.9Mo0.1Oy has sufficient electronic conductivity and electrochemical inertness as a catalyst support for a PEMFC electrode. The features could be attributed to the doping of molybdenum into anatase TiO2 and the oxygen vacancy created by H2 treatment at high temperatures. The electrochemically active surface areas (ECSA) of Pt electrodes are usually calculated by integrating the charge associated with the H adsorption peaks. In this calculation, an assumption for a monolayer of H adsorption was 210 μC cm−2. After the normalization with Pt loading, ECSA of

Vulcan XC-72 carbon support, denoted as Pt/C (E-Tek) nanocrystals, with a size of around 4−5 nm, is shown in Figure 8E. However, the morphology of Pt nanoparticles on a carbon support is essentially spherical. Additionally, the different mass Pt loadings on the d-Ti0.9Mo0.1Oy nanosupport were examined by EDX measurements, as shown in Figure S2. 3.3.2. X-ray Absorption Fine Structure Studies. XAS measurements were performed on the 10 and 20 wt % Pt/dTi0.9Mo0.1Oy nanocrystals and Pt/C (see Figure 9). The intensities of the absorption peaks, i.e., the white line, of heterostructural 10 and 20 wt % Pt/d-Ti0.9Mo0.1Oy nanocrystals at the Pt LIII edge were lower than the corresponding peaks for the Pt/C (E-Tek) and Pt foil, indicating decreased d-band vacancy attributed to SMSI between the Pt atoms and the dTi0.9Mo0.1Oy nanosupport. As seen in the inset of Figure 9, 20 wt % Pt/d-Ti0.9Mo0.1Oy had the lowest intensity. 3.4. Electrochemical Behavior of d-Ti0.9Mo0.1Oy Nanosupports and Pt/d-Ti0.9Mo0.1Oy Nanocrystals. 3.4.1. Cyclic Voltammogram (CV) Curves. Before examining the electrochemical activity of Pt/d-Ti0.9Mo0.1Oy catalysts, we examined the stability of the d-Ti0.9Mo0.1Oy nanosupport material under a high-potential condition. The CV curve of d-Ti0.9Mo0.1Oy was recorded at room temperature (25 °C) with a sweep rate of 100 mV s−1 under atmospheric pressure and in a N2-satuarted H2SO4 solution. As shown in Figure 10A, a small anode current was observed for the bare d-Ti0.9Mo0.1Oy electrode up to 1.1 V, 6556


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different catalysts. RM when compared for the samples increased in the following order: 3.5% for 20 wt % Pt/dTi0.9Mo0.1Oy < 6.2% for 10 wt % Pt/d-Ti0.9Mo0.1Oy < 27% for 20 wt % Pt/C (E-TEK) (see Table 1). This indicates that the Pt catalysts on the d-Ti0.9Mo0.1O2 support showed performances in terms of both activity and stability much better than those of commercial Pt/C samples. This can be attributed to the role of the advanced oxide support strongly interacting with Pt, oxygen vacancies on the nanosupport’s surface, and improved conductivity. We have developed molybdenum-doped titanium oxide as a novel support material to enhance the activity and stability of Pt nanocatalysts for the oxygen reduction reaction. The experimental work was supported by DFT calculations that showed that Mo-doped anatase TiO2 supports tend to have lower formation energies for oxygen vacancies and Pt adsorption energies, compared to those of undoped materials. These features contribute to the high stability of Pt catalysts on the Mo-doped TiO2. In addition, oxygen vacancies induced by the hydrogen treatment are critically important, not only in the improved electronic conductivity but also in the oxygen adsorption during the ORR mechanism. The electronic effect originating from SMSI was evidenced by X-ray absorption near edge structure (XANES). Our results show that the 20 wt % Pt/d-Ti0.9Mo0.1Oy catalyst outperformed the 10 wt % Pt/dTi0.9Mo0.1Oy catalyst in terms of stability. It is notable that the same sized Pt nanocrystals were observed on both samples, but the coverage on the support surface was higher for the 20 wt % Pt/d-Ti0.9Mo0.1Oy catalyst than for the 10 wt % Pt/dTi0.9Mo0.1Oy catalyst, because of the higher Pt loading on the metal oxide support. Consequently, enhanced catalytic stability may result from the SMSI and oxygen vacancies induced on the oxide support’s surface. As shown in the literature and demonstrated in this work, oxygen vacancies contribute to the ORR activity due to enhanced O−O bond splitting and the better yield of oxygen atoms.50 Nevertheless, additional work is required to address the remaining issues, such as oxygen spillover onto the oxide support, and to understand the mechanistic role of oxygen vacancies with respect to the ORR, catalytic activity, and stability in d-Ti0.9Mo0.1Oy and other similar systems.

20 wt % Pt/d-Ti0.9Mo0.1Oy, 10 wt % Pt/d-Ti0.9Mo0.1Oy, and 20 wt % Pt/C (E-TEK) were calculated to be 53.6, 40.2, and 61.2 m2 gPt−1, respectively (Table 1). Table 1. Bulk Compositions, Particle Sizes, Electrochemical Surface Areas (ECSA), Decreasing Percentages of ECSA, and Mass Activities at 0.9 V of the Commercial 20 wt % Pt/ C (E-TEK) and Homemade Pt/d-Ti0.9Mo0.1Oy Catalysts

sample 10 wt % Pt/dTi0.9Mo0.1Oy 20 wt % Pt/dTi0.9Mo0.1Oy 20 wt % Pt/C (E-TEK)

particle diameter (nm)

ECSA (m2 gPt−1)

mass activity (mA mgPt−1)

5−6

40.2

3.24

5−6

53.6

5.03

4−5

61.2

0.55

percentage of decreased mass activity after 3000 cycles, RM (%) 12 3.5 27

3.4.2. Mass Activity and the Stability Test. Figure 11 shows the polarization curves for the ORR on the different Pt electrodes in an O2-saturated 0.5 M H2SO4 solution (rotating speed of 1600 rpm) at a scan rate of 10 mV s−1. The electrocatalysts’ activity is indicated by the high onset potential of O2 reduction (0.95−1.0 V), as well as by the high half-wave potentials50 and limiting current densities.51 In this work, the 20 wt % Pt/d-Ti0.9Mo0.1Oy catalyst showed the highest ORR activity with the highest half-wave potential (0.84 V). In addition, the mass activity is generally regarded as the most important factor in comparing the catalytic activity of different catalysts, because it is associated with the intrinsic activity of the catalyst.52 Catalytic mass activity is usually calculated by using the Koutecky−Levich equation, and then the kinetic current is normalized to the actual mass of Pt loading on the GC surface.7,52 Figure 11B shows that the calculated mass activity values of the different catalysts decrease in the following order: 5.03 mA mgPt−1 for 20 wt % Pt/d-Ti0.9Mo0.1Oy > 3.24 mA mgPt−1 for 10 wt % Pt/d-Ti0.9Mo0.1Oy > 0.55 mA mgPt−1 for 20 wt % Pt/C (E-TEK). Furthermore, the electrochemical stabilities of the catalysts were also investigated by accelerated durability tests, which were performed at room temperature in N2-saturated 0.5 M H2SO4 solutions by applying cyclic potential sweeps between 0.2 and 0.8 V versus RHE at a scan rate of 50 mV s−1, and the LSV curves after stability testing with 3000 cycles are shown in Figure S7. The decreased percentage of mass activity after 3000 cycles, denoted as RM, was used as an activity aging factor to evaluate the stability of

4. CONCLUSION In summary, a molybdenum-doped TiO2 (d-Ti0.9Mo0.1Oy) nanosupport was synthesized and investigated by XRD, XAS, Raman spectroscopy, DFT theories, and electrochemical

Figure 11. (A) Polarization curves and (B) mass activity before and after stability tests of 10 and 20 wt % Pt/d-Ti0.9Mo0.1Oy and E-TEK (20 wt % Pt on carbon) catalysts in a O2-saturated H2SO4 solution at 25 °C, a sweep rate of 10 mV s−1, and a rotating speed of 1400 rpm. RM (%) is the aging factor of mass activity. 6557


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measurements. The oxide support was functionalized by doping and oxygen vacancies generated through hydrogen treatment at 300 °C, so that the electronic conductivity and SMSI could be greatly enhanced. The low vacant d orbitals of the Pt nanoparticles, resulting from the transfer of an electron from the d-Ti0.9Mo0.1Oy nanosupport to the Pt surface, were responsible both for the strong binding and for the stabilization of Pt nanoparticles. Electron donation from the support to Pt accounted for the excellent catalytic activity of the Pt/dTi0.9Mo0.1Oy catalyst. Additionally, the oxygen vacancies on the oxide support can enhance the ORR by facilitating dissociative adsorption of O2 onto Pt catalyst surfaces as shown by the DFT calculations. The 20 wt % Pt/d-Ti0.9Mo0.1Oy nanocatalyst was demonstrated to have improved mass activity and stability for the ORR compared to that of 10 wt % Pt/d-Ti0.9Mo0.1Oy and commercial 20 wt % Pt/C (E-Tek). It is interesting to note that the observed behavior of oxide-supported Pt nanoparticles should not be limited to the ORR; it may be extended to different catalytic reactions such as CO oxidation and hydrogenation. The oxide-supported catalyst not only shows the capability to assist noble metal catalysts but also has a great potential to reduce the high cost of commercial noble metal catalysts while also conferring prolonged durability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00600. Additional experimental results, including morphology control and results of electrochemical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

M.-C.T. and T.-T.N. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Ministry of Science and Technology (MOST 103-2221-E-011-156-MY3, MOST 104-3113-E-011-001-ET, and 104-ET-E-011-001-ET) and the Top Universities Program from the Ministry of Education. We also thank the National Synchrotron Radiation Research Center (NSRRC), Japan Synchrotron Radiation Research Institute (JASRI)/Spring-8 (BL12B2), the National Center for High-performance Computing (NCHC), and the Department of Chemical Engineering, National Taiwan University of Science and Technology (NTUST), for providing computer time and research facilities.

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