Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm

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Energy, Environmental, and Catalysis Applications

Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm Hollow Titania Nanospheres for Low-Temperature Water-Gas Shift Reaction Hongyu Zhao, Siyu Yao, Mengtao Zhang, Fei Huang, Qikui Fan, Shumeng Zhang, Hongyang Liu, Ding Ma, and Chuanbo Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12192 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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ACS Applied Materials & Interfaces

Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm Hollow Titania Nanospheres for LowTemperature Water-Gas Shift Reaction Hongyu Zhao,†# Siyu Yao,§# Mengtao Zhang,§# Fei Huang,‡ Qikui Fan,† Shumeng Zhang,† Hongyang Liu,‡* Ding Ma,§* and Chuanbo Gao†* †

Frontier Institute of Science and Technology and State Key Laboratory of Multiphase Flow in

Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China. ‡

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Shenyang, Liaoning 110016, China. §

College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.

ACS Paragon Plus Environment

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Abstract: Ultra-small platinum nanoparticles loaded over titania is a promising catalyst for the low-temperature water-gas shift (WGS) reaction and shows the potential to work in a mobile hydrogen fuel cell system. Their precise size engineering (< 3 nm) and reliable stabilization remain challenging. To address these issues, we report a reverse-micelle synthesis approach, which affords uniform ultra-small platinum nanoparticles (tunable in ~1.0–2.6 nm) encapsulated in hollow titania nanospheres with a shell thickness of only ~3–5 nm and an overall diameter of only ~32 nm. The Pt@TiO2 yolk/shell nanostructured catalysts display extraordinary stability and monotonically increasing activity with decreasing size of the Pt nanoparticles in the WGS. The size-dependent variation in the electronic property of the Pt nanoparticles and the reducible oxide encapsulation that prevents the Pt nanoparticles from sintering are ascribed as the main reasons for the excellent catalytic performance.

Keywords: Ultra-small platinum nanoparticles; hollow titania nanospheres; encapsulation; water-gas shift; size-dependent catalysis


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1. Introduction Noble metal nanoparticles promise excellent catalytic activity in many reactions, especially when they form heterostructures with a reducible metal oxide.1-9 Among these, supported platinum-group nanoparticles have been recognized as an excellent catalyst for the lowtemperature water-gas shift (WGS) reaction, which may be potentially applied in mobile hydrogen fuel cell systems to remove poisonous CO in H2 feedstock.2, 7-9 It has been recognized that the size of the noble metal nanoparticles is one of the most critical parameters that determine their catalytic performance in many reactions. Nanoparticles with ultra-small sizes, typically from subnanometers to a few nanometers (< 3 nm), show distinctive catalytic activity and selectivity.10-20 Conventionally, supported noble metal nanoparticles are usually synthesized by simple deposition, precipitation, or impregnation methods that cannot effectively control the size distribution of the metal nanoparticles.7, 9, 21 The surface-stabilized metal nanoparticles synthesized in these ways may be susceptible to sintering under the reaction condition and/or in “start-up, shut-down” cycles of the hydrogen fuel cells, leading to a loss of active sites and causing considerable deactivation. Furthermore, the polydispersity of the metal nanoparticles makes it difficult to identify the optimal size of the metal nanoparticles for the catalytic reactions. Although uniform ultra-small noble metal nanoparticles (< 3 nm) can be synthesized by employing dendrimers14, 17, 22-24 or reverse micelles (as indicated in our previous work25-26) as nanoreactors, it remains a challenge to reliably stabilize (or encapsulate) these nanoparticles by a reducible metal oxide (TiO2 for example) for the synergistic low-temperature WGS catalysis. To address this issue, we report that uniform ultra-small platinum nanoparticles (tunable in ~1.0– 2.6 nm) can be synthesized and encapsulated in anatase TiO2 hollow spheres27-34 with an unprecedentedly small diameter of ~32 nm and a shell thickness of only ~3–5 nm by employing


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reverse micelles as nanoreactors. Reverse-micelle nanoreactors ensure size engineering of the ultra-small Pt nanoparticles, controllable sol-gel chemistry of silica and titania for encapsulation, and convenient scale up for large-scale production of the catalyst (Figure S1). The effect of the size-dependent oxidation state of the Pt nanoparticles on their performance has been unraveled in the low-temperature WGS reaction. We found that Pt nanoparticles of smaller sizes demonstrate drastically increased activity. Based on in situ characterizations, this strong size effect has been reasonably ascribed to the size-dependent binding energy of CO on these Pt nanoparticles. In addition, with the protection of the TiO2 hollow spheres, the sintering of the Pt nanoparticles under a highly reducing atmosphere was successfully suppressed, rendering extraordinary stability of the Pt@TiO2 catalysts over the traditional heterogeneous counterparts. Our strategy enables systematic tunability and rational stabilization of ultra-small Pt nanoparticles in TiO2 hollow spheres for optimal catalytic activity and stability for the WGS and a much broader range of catalytic reactions.

2. Experimental Section 2.1 Materials. Chemicals including tetrabutyl titanate (TBOT), acetylacetone (acac), polyoxyethylene (10) cetyl ether (Brij C10), tetraethyl orthosilicate (TEOS), chloroplatinic acid hexahydrate (H2PtCl6·6H2O), diethylamine (DEA), ethanol (HPLC grade), and cyclohexane were purchased from Sigma-Aldrich. Ammonium hydroxide (NH3·H2O, 25 wt%) was purchased from Tianjin Kemiou. 2.2 Synthesis of (NH4)2PtCl6@SiO2@TiO2@SiO2 nanospheres in reverse micelles. A reverse micelle system was first prepared by dissolving 4.25 g of Brij C10 in 15 mL of cyclohexane, and the temperature was maintained at 50 °C throughout the synthesis. Then, 50 µL of H2PtCl6·6H2O


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(0.25 M) was added to the reverse micelle system dropwise under stirring. After a transparent solution was formed, 0.8 mL of NH3·H2O (25 wt%) and 0.5 mL of TEOS were added in order with an interval of 2 min. After stirring for 2 h, a solution of TBOT/acac (prepared by mixing 1 mL of TBOT with 1 mL of acac, which was stirred at 75 °C for 15 h before use) was injected into the reaction system at a rate of 66.7 µL min–1 using a syringe pump. After the injection was completed, the solution was stirred for 12 h. At last, 8 mL of TEOS and 0.5 mL of DEA was added into the reaction solution and stirred for 12 h. The resulting (NH4)2PtCl6@SiO2@TiO2@SiO2 nanospheres were collected by centrifugation, washed three times with ethanol, and dried at 60 °C overnight. 2.3 Synthesis of Pt@TiO2 yolk/shell nanospheres. The (NH4)2PtCl6@SiO2@TiO2@SiO2 nanospheres were calcined at 600 °C in N2 for 3 h to allow the thermal reduction of (NH4)2PtCl6 into Pt nanoparticles and crystallization of titania. The resulting Pt@SiO2@TiO2@SiO2 nanospheres were then dispersed in deionized water and etched with NaOH (0.02 M) at room temperature for 84 h to remove the SiO2. The resulting Pt@TiO2 yolk/shell nanospheres were collected by centrifugation, washed with water until the pH was neutral, and finally freeze-dried. The size of the Pt nanoparticles were tuned by the amount of the Pt precursor, H2PtCl6, which were 5, 10, 12.5, 15 and 25 µmol, in a typical synthesis, to yield Pt@TiO2 yolk/shell nanospheres with Pt nanoparticles of 1.0, 1.2, 1.7, 2.2 and 2.6 nm, respectively. 2.4 Catalytic WGS reaction. The catalytic activities of the Pt@TiO2 yolk/shell nanospheres in the WGS reaction were examined in a fixed-bed quartz tubular reactor. First, the catalysts (72, 66, 90.4, 96 and 167.5 µg Pt for Pt1.0@TiO2, Pt1.2@TiO2, Pt1.7@TiO2, Pt2.2@TiO2, and Pt2.6@TiO2, respectively) were pretreated in H2 (5 % in N2) at 200 °C for 1 h and cooled to the room temperature. Then, the raw gases with a volume ratio of He/CO/H2O = 88/2/10 flowed over the catalyst at a rate of 15 mL min–1. The products were finally analyzed by the gas chromatography (Agilent 7890).


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2.5 Characterizations. Low-magnification transmission electron microscopy (TEM) imaging was conducted on a Hitachi HT-7700. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high-resolution TEM (HRTEM) were performed on an FEI Tecnai G2 F20 FEG-TEM. X-ray diffraction (XRD) was analyzed on a Rigaku SmartLab Powder X-ray diffractometer with Cu Kα radiation. The porous properties of the samples were analyzed by the N2 physisorption at 77 K on a Quantachrome Quadrasorb SI-3. The pore size distribution was calculated by the nonlocal density function theory (NLDFT) based on the adsorption branch of the isotherm. The surface area and microporosity were estimated by the Brunauer–Emmett–Teller (BET) and t-plot methods, respectively. Quantitative elemental measurements were conducted by the inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7500CE. The core-level X-ray photoelectron spectroscopy (XPS) was obtained on a Kratos Axis Ultra spectrometer, with the binding energy calibrated using the C 1s peak (284.6 eV). The Pt 4f peaks were fitted by Pt 4f7/2 and 4f5/2 doublets with a binding energy difference of 3.33 eV and an area ratio of 4/3. For each set of the doublet peaks, the full-width at half maximum (FWHM) was set to be the same. The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was collected on a Thermo Fisher Nicolet iS10 instrument. CO chemisorption experiments were carried out on a Quantachrome CHEMBET-3000 instrument in order to determine the dispersion of the Pt nanoparticles. The reduction properties of the samples were measured by the hydrogen temperature-programmed reduction (H2-TPR). The Pt/TiO2 catalyst was placed in a quartz reactor and heated from the room temperature to 600 °C at a rate of 10 °C min−1 in a H2/He gas (5 vol% H2, 20 mL min−1).

3. Results and Discussion


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3.1 Synthesis and characterization of the Pt@TiO2 yolk/shell nanospheres. In order to synthesize size-tunable ultra-small Pt nanoparticles (Figure 1a), a (NH4)2PtCl6 precursor enriched in reverse micelles was used to achieve the size engineering and well-controlled silicate and titanate coating of the Pt nanoparticles. Different from the traditional procedures that employed large-size templates (> 100 nm),27-34 the well-controlled sol-gel process of titania in the reverse micelles makes it possible to synthesize sub-50 nm hollow titania nanospheres for encapsulating ultra-small noble metal nanoparticles. In a typical synthesis, nanoparticles of insoluble (NH4)2PtCl6 were first synthesized by successively introducing H2PtCl6 and NH3·H2O into the reverse micelle system. The silica coating in this system gave rise to (NH4)2PtCl6@SiO2 multi-core/shell nanospheres (~22 nm), which can be observed by the HAADF-STEM imaging (Figure 1b). Then, a thin layer of amorphous TiO2 was coated on these SiO2 nanospheres by applying a sol-gel process of TiO2, with its hydrolysis and condensation rates slowed down by the introduction of acetylacetonate (Figure 1c).35-37 After a protective layer of silica was coated on these nanospheres, the resulting (NH4)2PtCl6@SiO2@TiO2@SiO2 core/shell nanospheres were calcined at 600 °C in N2, which transformed the ultra-small (NH4)2PtCl6 nanoparticles to Pt nanoparticles, producing Pt@SiO2@TiO2@SiO2 core/shell nanospheres (Figure S2). A single Pt nanoparticle was eventually formed in each hollow titania nanosphere as a result of the nanoparticle agglomeration in the calcination. The calcination process also allowed the crystallization of the amorphous TiO2 shells to form an anatase phase, which was confirmed by the XRD patterns (Figure S3). Finally, Pt@TiO2 yolk/shell nanospheres were obtained after etching of silica (Figure 1 d, e, EDS see Figure S4). The TEM image shows that the nanospheres are highly uniform with an average size of ~32 nm and a shell thickness of only ~3–5 nm. Although the Pt nanoparticles are difficult to observe by the TEM due to the ultra-small size (Figure 1d),


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they can be clearly identified by the HAADF-STEM as bright nanodots (Figure 1e). It is inferred that only a single Pt nanoparticle is present in most of the hollow titania nanospheres. More than one Pt nanoparticles in hollow titania nanosphere can be occasionally observed, which may arise from the incomplete nanoparticle agglomeration during the calcination process. The Pt nanoparticles showed a narrow size distribution at ~1.7 nm with a standard deviation of ~0.3 nm (Figure 1e, inset), which indicates excellent uniformity of the Pt nanoparticles.


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Figure 1. Synthesis of the Pt@TiO2 yolk/shell nanospheres. (a) A cartoon illustrating the synthesis route. (b) HAADF-STEM image of the (NH4)2PtCl6@SiO2 intermediate. Multiple (NH4)2PtCl6 nanoparticles were observed in each silica nanosphere. (c) HAADF-STEM image of the (NH4)2PtCl6@SiO2@TiO2 intermediate. (d, e) TEM and HAADF-STEM images of the Pt@TiO2 yolk/shell nanospheres. Insets of (e), the size distribution of the Pt nanoparticles in the hollow TiO2 nanospheres. To further examine the crystallinity of the Pt nanoparticles and the titania shells, two individual Pt@TiO2 yolk/shell nanospheres with Pt nanoparticles of ~2.6 (synthesis discussed later) and ~1.7 nm were investigated by the HRTEM imaging (Figure 2a–c). For both yolk/shell nanospheres, a single Pt nanoparticle can be clearly distinguished in the hollow titania nanospheres (Figure 2a, b). While no obvious lattice fringes can be imaged from the Pt nanoparticle of ~1.7 nm, the Pt nanoparticle of ~2.6 nm shows clear lattices with a spacing of ~2.27 Å, corresponding to the (111) planes of a Pt lattice (Figure 2c). The different crystallinity of the Pt nanoparticles represents a typical size effect of the metal nanoparticles, in good agreement with previous observations.17-19, 23, 38-39

In addition, the lattices from both anatase nanoshells could be clearly observed (Figure 2a–

c), confirming the high crystallinity induced by the thermal treatment. Both the size-dependent crystallinity of the Pt nanoparticles and the high crystallinity of the titania nanoshells are in good agreement with the XRD results (Figure S3, S5, S6). During the thermal process, small anatase crystallites are formed at the expense of amorphous titania, which makes it possible to create porosity in the titania nanoshells as inter-crystallite nanogaps.40 The nitrogen physisorption isotherm of the Pt@TiO2 yolk/shell nanospheres displays a rapid desorption of nitrogen at a relative pressure (P/P0) of 0.4–0.5, which is typical of a bottleneck structure, and thus confirms the presence of porosity in the titania nanoshells (Figure


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2d). The surface area is ~204 m2 g–1 measured by the BET method, which contains ~35 m2 g–1 of the microporous surface area according to the t-plot analysis. The average size of the micropores was estimated to be ~1.5 nm according to an NLDFT analysis (Figure 2d, inset). The microporosity in the titania shells ensures convenient mass transfer for the catalytic WGS reaction.

Figure 2. Characterizations of the Pt@TiO2 yolk/shell nanospheres. (a, b) HRTEM images of the Pt@TiO2 yolk/shell nanospheres with different average sizes of the Pt nanoparticles: a, ~2.6 nm; b: ~1.7 nm. The arrow indicates an ultra-small Pt nanoparticle in the hollow TiO2 shell. (c) HRTEM images of the Pt nanoparticle and the anatase phase TiO2, respectively, as marked in (a).


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Right, corresponding Fourier diffractions. (d) Nitrogen physisorption isotherm of the Pt@TiO2 yolk/shell nanospheres at 77 K. Inset: pore size distribution. 3.2 Size engineering of the TiO2-encapsulated ultras-small Pt nanoparticles. The precise size engineering of the ultra-small Pt nanoparticles is a prerequisite for unambiguously disclosing their size-dependent catalytic properties in the WGS reaction. To this end, the fine size tuning of the Pt nanoparticles has been conveniently achieved by adjusting the amount of the feeding precursor (Figure 3a–f). With decreasing amount of H2PtCl6 in a typical synthesis, the average size of the Pt nanoparticles decreases monotonically from ~2.6 to ~1.0 nm, as evidenced by the HAADF-STEM (Figure 3a–e) and HRTEM images (Figure S7). The change in the size of the Pt nanoparticles can be further confirmed by the XRD patterns, which show the broadening and eventual disappearance of the X-ray diffraction peaks with decreasing size of the Pt nanoparticles (Figure S6). For all samples investigated, the ultra-small Pt nanoparticles show a narrow size distribution and are encapsulated in hollow titania nanospheres for effective stabilization. These catalysts are denoted as Ptx@TiO2, with x representing the average size of the Pt nanoparticles with a unit of nanometer. The Pt nanoparticles in the Pt@TiO2 yolk/shell nanospheres show size-dependent variation in the electronic property, which is observed by the Pt 4f XPS (Figure 3g). In the spectra, the 2.6 nm Pt nanoparticles in the hollow titania nanospheres show two sets of the XPS peaks with the 4f7/2 components appearing at ~71.1 (strong) and ~72.1 eV (weak) of the binding energy, respectively, corresponding to the Pt0 and PtII species. When the size of the Pt nanoparticles decreases to 2.2 nm, besides Pt0 and PtII, a PtIV species emerges with the 4f7/2 peak appearing at ~75.0 eV. When the size of the Pt nanoparticles decreases further (1.7, 1.2 and 1.0 nm), the PtII and PtIV species become dominant over Pt0. Therefore, in general, the oxidation state of the Pt nanoparticles


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increases with decreasing size. It also indicates that the Pt nanoparticles are not literally “pure metallic nanoparticles” and they inevitably contain a portion of the Pt oxide. The partial oxidation of the ultra-small Pt nanoparticles may be related to their strong metal-support interaction (SMSI) with the TiO2 substrate.41 The size-dependent variation in the electronic property of the Pt nanoparticles may exert a significant influence on their catalytic properties, which will be confirmed later by in situ characterizations.

Figure 3. Pt@TiO2 yolk/shell nanospheres with different sizes of the Pt nanoparticles (Ptx@TiO2, x: diameter of the Pt nanoparticles with a unit of nm). (a–e) HAADF-STEM images of the


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Ptx@TiO2 (x = 1.0, 1.2, 1.7, 2.2 and 2.6) yolk/shell nanospheres. Inset: size distributions of the Pt nanoparticles, unit: nm. The average sizes are denoted as “mean ± standard deviation”. (f) A plot of the average size of the Pt nanoparticles versus the amount of H2PtCl6 in the feeding precursor. The error bar represents the standard deviation for each size. (g) Core-level Pt 4f XPS of the Ptx@TiO2 yolk/shell nanospheres. Dashed lines indicate appropriate positions of the 4f7/2 peaks for the Pt0, PtII and PtIV species, from the right to the left. 3.3 Size-dependent catalytic properties of the Pt@TiO2 catalysts in the WGS reaction. The ultra-small Pt nanoparticles reside on the inner surface of the titania shells with strong metalsupport interaction, which forms Pt/TiO2 interfaces as the active sites for the WGS reaction. The yolk/shell nanostructure allows sufficient exposure of the active sites to the reactants due to the presence of a void space, and maintains the protective effect of the shell against sintering of the Pt nanoparticles.42-43 Therefore, these Pt@TiO2 yolk/shell nanospheres represent a promising catalyst for the low-temperature WGS with high catalytic stability and an excellent platform for the size effect investigation for further optimization of the catalytic activities. After being activated by H2 at 200 °C, the size-dependent catalytic property of the Pt@TiO2 catalysts was examined in the WGS reaction (Figure 4a). Significant differences in the catalytic activity can be observed with Pt nanoparticles of varying sizes. The light-off temperature increases monotonically with increasing size of the Pt nanoparticles. It is difficult to compare the activities of the Pt@TiO2 catalysts solely by the light-off plots, because the exposed surface area of the Pt nanoparticles varies in different catalysts (Table S1). To obtain a better evaluation on the intrinsic catalytic activity as a function of the nanoparticle size, the turnover frequencies (TOFs) at 160 °C (with low conversions) were estimated by normalizing the activity to the surface Pt atoms (Figure 4a, inset, Table S1; calculation see Supporting Information). The TOF values of the Ptx@TiO2


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catalysts were 771, 719, 293, 174, and 67.8 h–1, for x = 1.0, 1.2, 1.7, 2.2, and 2.6 nm, respectively. It is clear that the TOF increases with decreasing size of the Pt nanoparticles. The best catalyst (Pt1.0@TiO2) exhibits a ~11.4-fold increase in the TOF, compared with the Pt2.6@TiO2 catalyst. Moreover, these Pt@TiO2 nanospheres show superior catalytic stability thanks to the yolk/shell nanostructure, in clear contrast to the P25-stabilized Pt nanoparticles prepared by a depositionprecipitation (DP) method, which indicates that the physical encapsulation of the Pt nanoparticles in the reducible oxide shells is effective to prevent the sintering of the metal nanoparticles (Figure 4b, more data see Figure S8–S10).

Figure 4. Catalytic activity and stability of the Ptx@TiO2 yolk/shell nanospheres (x = 1.0, 1.2, 1.7, 2.2, and 2.6) in the WGS reaction. (a) Light-off plots of the CO conversion versus the reaction


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temperature. Inset: plot of the TOF at 160 °C as a function of the size of the Pt nanoparticles. (b) Plots of CO conversion versus the reaction time with the Ptx@TiO2 catalysts. P25-supported Pt nanoparticles were also examined for comparison. The reactions were carried out at 200 and 300 °C, respectively. The size-dependent catalytic property of the Pt nanoparticles is related to their electronic structure,1-2, 7-9 which was investigated by the in situ XPS (Figure 5a). Taking Pt1.7@TiO2 for example, after the activation (reduction in H2), while all the Pt0, PtII and PtIV species were retained in the Pt nanoparticles due to the strong metal-support interaction, the fraction of the Pt0 species increased from ~16% (as made) to ~22%, estimated by the areas of the XPS peaks. Interestingly, when the catalytic condition was applied, the PtII species disappeared, accompanying an increase in the fractions of the PtIV (~76%, in comparison with ~54% in the activated sample) and Pt0 (~24%) species, which can be attributed to the oxidation of the PtII species by the hydroxyl (or activated water) at the Pt/TiO2 interface, and the reduction of the PtII species by CO at the same time. The effect of the size of the Pt nanoparticles on their oxidation states was further investigated under the catalytic condition. Compared with the Pt1.7@TiO2, the Pt1.0@TiO2 catalyst showed a much lower fraction of the metallic Pt0 component (~16%), confirming an increased average oxidation valence and hence more electron-deficient metal centers. The H2-temperature programmed reduction (H2-TPD) of the Ptx@TiO2 catalysts provides another hint for the size-dependent electronic property of the Pt nanoparticles (Figure 5b). Two H2 consumption peaks were observed in the profiles at ~100–150 °C and ~300–400 °C, which can be attributed to the reduction of the ultra-small Pt nanoparticles and the reduction of the adjacent TiO2 by hydrogen spillover from the Pt nanoparticles, respectively.44-45 With decreasing size of the Pt nanoparticles, the reduction occurs at continuously elevated temperatures. This trend clearly confirms that the metal-support


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interaction becomes stronger with decreasing size of the Pt nanoparticles, which is in line with, and may account for, the size-dependent oxidation valence of the Pt nanoparticles.

Figure 5. (a) In situ Pt 4f XPS of the Ptx@TiO2 (x = 1.0 and 1.7) yolk/shell nanospheres (as made, after activation, and in the WGS catalysis). (b) H2-TPR of the Ptx@TiO2 yolk/shell nanospheres (x = 1.0, 1.2, 1.7, 2.2, and 2.6). (c) CO-TPD of the Ptx@TiO2 yolk/shell nanospheres (x = 1.0, 1.2, 1.7, 2.2, and 2.6). (d) DRIFTS of the CO adsorbed on the Ptx@TiO2 nanospheres (x = 1.0 and 1.7) at 200 °C. The variation in the electronic property tends to influence the adsorption and activation of CO, one of the reactants in the low-temperature WGS reaction. The CO temperature-programmed desorption (CO-TPD) was used to investigate the adsorption energy of CO on the Pt nanoparticles


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(Figure 5c). It is inferred that the desorption temperature continuously drops with decreasing size of the Pt nanoparticles, indicating that the coordination of CO on the Pt nanoparticles is significantly weakened, which benefits the reaction of CO with the hydroxyl species at the interface, leading to improved catalytic activity. In situ DRIFTS was applied to observe the backdonation of the metal d-electrons to the CO 2π* antibonding orbital (Figure 5d).46-48 The Pt1.7@TiO2 catalyst showed CO chemisorption mainly at the Pt0 sites (~2059 cm−1). In clear contrast, the Pt1.0@TiO2 catalyst showed substantial CO chemisorption at the Ptδ+ site (~2110 cm−1) besides the Pt0 sites (~2067 cm−1). The blueshift of the band positions (2059 cm−1 → 2067 & 2110 cm−1) indicates substantially weakened electron back-donation and hence the lowered Pt–CO binding energy with decreasing size of the Pt nanoparticles. Based on these analyses, the size-dependent catalytic activity can be justified as follows. Usually, the Pt–CO bonding is too strong for a favorable kinetics of the WGS reaction. When the size of the Pt nanoparticles is decreasing, the Pt nanoparticles become continuously oxidized with decreasing electron density. It leads to a weakening of the Pt–CO bonds due to the decreased electron back-donation, which accounts for the ready oxidation of the adsorbed CO and thus the significantly increased catalytic activity. As a result, the Pt1.0@TiO2 catalyst (Pt: 0.32%) showed the highest activity in the WGS reaction.

4. Conclusions In summary, we have demonstrated a robust synthesis strategy to afford uniform Pt@TiO2 yolk/shell nanostructures composed of Pt nanoparticle cores with tunable size from ~1.0 to ~2.6 nm and anatase-phase titania nanoshells of only ~32 nm in diameter and ~3–5 nm in thickness. These Pt@TiO2 yolk/shell nanospheres are excellent catalysts for the low-temperature WGS


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reaction, showing superior catalytic stability and size-dependent activity. The size dependence has been ascribed to the size-dependent oxidation states of the Pt nanoparticles and thus the binding energy of CO on these Pt nanoparticles. We believe this synthesis strategy and the strong size effect of the Pt nanoparticles revealed in this work may fundamentally advance the catalyst design for the WGS and a broader range of catalytic reactions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. The following files are available free of charge. Additional EDS spectra, XRD patterns, electron microscopy images, elemental analysis, and TOF calculations (PDF)

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (C.G.) *Email: [email protected] (H.L.) *Email: [email protected] (D.M.) Author Contributions #These authors contributed equally to this work. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT C.G. acknowledges the support from the National Natural Science Foundation of China (21671156, 21301138) and the Tang Scholar Program from the Cyrus Tang Foundation. H. L. acknowledges the support from the Ministry of Science and Technology (MOST) (2016YFA0204100), National Natural Science Foundation of China (91545110, 21573254) and Youth Innovation Promotion Association Chinese Academy of Sciences.

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SYNOPSIS:


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Ultra-Small Platinum Nanoparticles Encapsulated in Sub-50 nm

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