Pt Nanoparticles Supported on Mesoporous CeO2 Nanostructures

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Pt nanoparticles supported on Mesoporous CeO2 Nanostructures obtained through green approach for Efficient Catalytic Performance towards Ethanol Electrooxidation Paskalis Sahaya Murphin Kumar, Sivakumar Thiripuranthagan, Tsubasa Imai, Gopalakrishnan Kumar, Arivalagan Pugazhendhi, Sriram Kumar Vijayan, Rodrigo Esparza, Hideki Abe, and Siva Kumar Krishnan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02019 • Publication Date (Web): 26 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Sustainable Chemistry & Engineering

Pt nanoparticles supported on Mesoporous CeO2 Nanostructures obtained through green approach for Efficient Catalytic Performance towards Ethanol Electrooxidation Paskalis Sahaya Murphin Kumar, †, ‡ Thiripuranthagan Sivakumar, ‡ Tsubasa Imai, †

Gopalakrishnan Kumar,‫ ׀׀‬Arivalagan Pugazhendhi,⊥ Sriram Kumar Vijayan,# Rodrigo Esparza, § Hideki Abe,† and Siva Kumar Krishnan §,∇ *

†

National Institute for Materials Science (NIMS), 1-1 Namiki, Ibaraki Tsukuba, 305-0044 Japan.

‡

Department of Applied Science and Technology, Anna University, Chennai, Tamil Nadu 600-

025, India. ‫׀׀‬

Department of Environmental Engineering, Daegu University, Gyeongsan, Gyeongbuk, 38453,

Republic of Korea. ⊥Faculty

of Environment and Labour safety, Ton Duc Thang University, Ho Chi Minh City,

Vietnam. #

Department of Environmental Biotechnology, School of Environmental Sciences, Bharathidasan

University, Tiruchirappalli, 620 024, Tamil Nadu, India. §

Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México,

Boulevard Juriquilla 3001, Santiago de Querétaro, Qro., 76230, México. ∇CONACYT-

Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apdo. Postal J-

48, Puebla 72570, México.

*Corresponding author: S. K. K ([email protected], [email protected]) 1

ACS Paragon Plus Environment


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ABSTRACT. In this report, an easy and green approach to the synthesis of mesoporous cerium oxide (CeO2) nanostructures and followed by supporting platinum nanoparticles (NPs) on CeO2 nanostructures (Pt/CeO2) and their application as versatile electrocatalysts for ethanol electrooxidation has been established. The synthesis of mesoporous Pt/CeO2 nanostructures involves two steps. First, mesoporous CeO2 nanostructures were synthesized via macroalgae polymer mediated approach and followed by supporting of PtNPs of ca.5-10 nm over the mesoporous CeO2 nanostructures using seed-mediated chemical reduction process. The structural and spectroscopic characterization techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), and small angle X-ray scattering (SAXS) studies confirm the strong coupling between PtNPs and the mesoporous CeO2 support resulting in the generation of more oxygen vacancies, which can facilitate the enhanced charge transport at their functional interface. Significantly, the synthesized mesoporous Pt/CeO2 nanostructures were found to show enhanced electrocatalytic activity for ethanol electrooxidation reaction. The enhanced performance is attributed to the synergistic effect of both mesoporous structure and the formation of more oxygen vacancies in the resultant Pt/CeO2 nanostructures. Our facile and eco-friendly synthetic approach to the synthesis of mesoporous CeO2 nanostructures that supports PtNPs with an excellent catalytic activity validates as a promising strategy for potential applications in fuel cells.

KEYWORDS: Platinum nanoparticles, Cerium oxide, Red algae, ethanol electrooxidation, electrocatalysis.

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Introduction Developing a mesoporous nanostructure with distinct structural features have attracted great attention as a promising heterogeneous catalyst because of their fascinating properties such as higher specific surface area, large pore volume, capabilities of strong coupling with metal nanoparticles, and reduction in the catalytic activity loss.1–4 Platinum nanoparticles (PtNPs) have been ubiquitously employed in fuel cells owing to their excellent catalytic properties arising from their distinct electronic structure that enables the formation of strong bonding with different adsorbates.2,5,6 However, two main challenges of using PtNPs catalysts has become a major concern, causing significant limitation to their practical applications and sustainability. i) The limited abundance of Platinum, and CO poisoning issues.7 ii) The chemically inert PtNPs becomes unstable under alkaline reactive conditions, in which the surface Pt atoms dissolve and migrate, leading to the agglomeration of NPs, and consequently cuts down the catalytic activity due to the loss of large surface area.8 Substantial research efforts are dedicated toward increasing the utilization efficiency and reducing Pt content without compromising the activity of Pt-based catalysts.7 This includes alloying Pt with other transition metals (e.g., Cu, Ni, Co),9 depositions of Pt-skin shells on the less expensive substrates,10 and integration of PtNPs with the other metal oxides as support.11 Among all, one promising strategy that has been widely applied to improve catalytic activity and durability is the integration of Pt catalysts onto strongly interacting metal oxide, such as CeO2,1 TiO2,12 CO3O4,13 SnO2,14,15 and MnO213 nanostructures as supporting materials, that significantly enhance the oxygen transfer to catalytically active PtNPs.16–18 Among various metal oxide support materials studied, cerium oxide (CeO2) has proven to be an ideal support material due to their high degree 3



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of oxygen vacancies and stabilized Ce3+ active site on the surface, that can promote the electron transport across the supported metal oxide network.19–21 Moreover, CeO2 exhibits higher oxygen storage capacity and stability in an acidic environment. Also, it is used as low-cost electrocatalysts in solid oxide fuel cells,22,23 and as oxidizing cocatalysts in automobile catalytic converters along with Pt-based nanoparticles.24 Numerous groups have extensively investigated the supported NPs on oxide nanostructures for various catalytic reactions, in which the catalytic activity and selectivity are largely governed by creating a strong metal-support electronic interaction, size, and dispersion of the metal component.25–32 Specifically, the strong coupling of NPs with the CeO2 support boosts the generation of Ce3+ active sites at their interface.33 As a result, a slight lattice expansion either in NPs34 or in the oxide support35 that primarily associated with the creation of more oxygen vacancies originated from the NPs-mediated reduction of Ce4+ to Ce3+, which critically influence the catalytic activity. Furthermore, previous studies have implied that the supporting PtNPs onto the mesoporous CeO2 nanostructures, with significant amount of strain/defect, can play an indispensable role in boosting the charge transfer rate between PtNPs and the CeO2 support for accomplishing enhancement in the catalytic activity.3,13,18 With the advancement in the tailored methods, numerous synthetic strategies have been reported for preparing mesoporous CeO2 and Pt/CeO2 nanostructures with functional interface that improves electrocatalytic properties.36–38 Most of these syntheses involve chemical routes for synthesizing CeO2 and Pt/CeO2 nanostructures with controlled size and shapes.39 The growth of mesoporous structures and supporting metal NPs in large-scale, and control over their uniformity has largely remained challenging, primarily, due to the formation of inhomogeneous structures and weak coupling; these composite nanostructures results in detachment or migration under harsh reactive conditions affecting the overall catalytic activity.40 Hence, the demand for efficient 4


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synthetic methods based on “green and eco-friendly approach” is not only on the rise but also a critical concern in developing high-performance heterogeneous catalysts toward energy sustainability.41 Recently, macroalgae polymer extracted from seaweeds exploited widely as an alternative to the conventional approach for preparing a broad range of nanostructures comprising of noble metal NPs and metal oxide nanostructures.42,43 For instance, Dutta et al.,44 described the green approach to synthesize CeO2 nanoparticles using Aloe vera extracts. Wang et.al.18 demonstrated the green route in the synthesis of Pt/CeO2 nanostructures. However, to the best of our knowledge, the synthesis of mesoporous CeO2 nanostructures mediated by macroalgae polymer along with supporting PtNPs to form a novel Pt/CeO2 nanostructure catalyst with improved catalytic activity toward ethanol electrooxidation reaction has not yet been reported. Herein, we present a facile, green strategy for fabricating mesoporous CeO2 nanostructures and supporting of PtNPs onto the mesoporous CeO2 (Pt/CeO2) hybrid nanostructures for improved catalytic activity toward ethanol electrooxidation reaction. Mesoporous CeO2 was prepared through galactose-mediated (red macroalgae polymer) reduction and followed by supporting the PtNPs using facile chemical reduction process. Owing to the unique mesoporous structure and strong electronic coupling between the supported PtNPs and the mesoporous CeO2 nanostructures, the resultant Pt/CeO2 nanostructures exhibited superior catalyst activity for the ethanol electrooxidation compared to PtNPs supported on commercial CeO2 (Pt/Comm CeO2), and commercial PtNPs (Pt/C) catalyst. EXPERIMENTAL SECTION Chemicals: Cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O), potassium tetrachloroplatinate (II) (K2PtCl4.6H2O, 99.98%), Sodium borohydride (NaBH4) were purchased from Sigma-Aldrich and 5



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used as received. Commercial cerium (IV) oxide nanopowder (< 25 nm particle size) was purchased from Sigma Aldrich. All glassware was cleaned thoroughly with de-ionized (DI) water and air-dried before using in the experiments. Ultrapure Milli-Q water was used throughout the study. Preparation of seaweed extract: The brown seaweed, Turbinaria conoides (J. Agardh, Kutzing, 1860), was obtained from Mandapam coastal region (78°8'E, 9°17'N), Gulf of Mannar, southeast coast of India. To separate the adhering salts, and associated biota and impurities, collected samples were thoroughly washed, first with tap water, and then several times with DI water. Subsequently, the samples were dried at ambient conditions for one week. The dried-out samples were then ground into fine powder using mixer grinder. Then, the resultant seaweed powder (2.5 g) was boiled in 100 mL of DI water for 10–15 min and filtered using Whatman number 1 filter paper. The resultant aqueous filtrate was used in the synthesis of mesoporous CeO2 nanostructures. Green synthesis of mesoporous CeO2 nanostructures: In a typical synthesis procedure of porous CeO2 NPs, 8.68 g of Ce(NO3)3.6H2O was dissolved in 180 mL Milli-Q water. This solution was magnetically stirred until a homogeneous solution was formed. To this was added 20 mL of seaweed extract and was heated at 60°C under magnetic stirring at 1,500 rpm on a magnetic hotplate stirrer for 3 hr. The reaction mixture is turned into a coffee brown color, which is separated by centrifugation at 10,000 rpm for 10 minutes. The resultant CeO2 nanostructures were purified through washing cycles with Milli-Q water and were dried, in a hot air oven, at 80°C for 4 hr. Subsequently, the material was annealed in a muffle furnace at 600°C for 2 hr. The light yellowcolored mesoporous CeO2 samples obtained were stored in an air-tight jar at room temperature.

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Synthesis of mesoporous Pt/CeO2 nanostructures: Pt/CeO2 nanostructures have been achieved by chemical reduction process similar to the previously reported protocol.15 In a typical synthesis, 5 mM of aqueous Pt precursor solution (K2PtCl4· 6H2O, was dissolved in 50 mL of Milli-Q water containing 50 mg of pre-synthesized CeO2 nanostructures. This mixture was magnetically stirred for 2 hr and followed by dropwise addition of, freshly prepared, 50 mL of 0.1 M NaBH4 solution while being stirred until the completion of the reduction process. The final product was washed several times with Milli-Q water and dried at 60oC in an oven. The PtNPs supported on commercial CeO2 nanoparticles ( denoted as Pt/Comm CeO2) were synthesized using the similar protocol used for preparing Pt/CeO2 nanostructure, except we used 50 mg of commercial CeO2 nanoparticles. Electrochemical measurements: The catalytic activity toward ethanol electrooxidation of the assynthesized porous Pt/CeO2 nanostructures were carried out using a standard three-electrode electrochemical cell (cell volume: 100 mL, PINE Co.,), that was connected to a computercontrolled automatic polarization system HSV-110 (HOKUTO DENKO Co.) and a rotating disk five electrode system. We employed Pt wire as the counter electrode, and Ag/AgCl (4 M KCl, PINE Co.,) and Pt/CeO2 catalysts deposited on glassy carbon (GC) electrode (5 mm in diameter, PINE Co.,) were used as a reference and working electrodes, respectively. Prior to use, the GCE was polished well with Gamma Micropolish Alumina (Baikalox, Type 0.05 μm), and rinsed well with DI water and dried in air. For preparing the catalysts-loaded working electrode, 4 mg of catalyst and 5 mg of carbon black (Vulcan XC 72) were suspended in a mixture of 1.75 ml MilliQ water, 0.44 ml of isopropyl alcohol, 20 μl of a 5% w/w aqueous solution of Nafion (EW: 1100, Aldrich), and the mixture was sonicated for 60 min. After that, an aliquot of 45 μl of the obtained suspension was drop-casted onto the GCE and thoroughly dried at ambient condition. The 7



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electrocatalytic activity toward ethanol oxidation for the Pt/CeO2 catalysts, PtNPs supported on commercial CeO2 (Pt/Comm CeO2), and a carbon-supported commercial PtNPs (Pt/C, 20 wt %) as control, were studied using cyclic voltammetry (CV) in 0.5 M sulphuric acid (H2SO4) and a scan rate of 20 mV s-1. Prior to each measurement, the solution was saturated by purging with pure Ar gas to avoid the interference of atmospheric oxygen. Materials Characterization: Transmission electron microscopy (TEM) images were obtained using JEOL JEM 2100F with an operating voltage of 200 kV. For TEM analysis, one drop of welldispersed as-prepared nanostructure solution in DI water was drop-casted on a carbon-coated Cu grid and allowed to dry at room temperature. The high anglular dark field scaning transmission microscope (HAADF-STEM) and STEM- energy dispersive spectroscopy (EDX)- elemental mapping analyses were carried out using JED-2300T, JEOL, NIMS, Japan. The UV-vis absorption spectra were recorded on a UV-Vis spectrophotometer (V-7200, JASCO, NIMS, Japan) in the wavelength ranging from 250 to 800 nm in 1 nm steps. The X-ray Diffraction (XRD) patterns were acquired by using X’Pert

PRO Analytical X-ray diffractometer (PANalytical, NIMS, Japan).

Infrared transmittance spectra were recorded using Nicolet Thermo spectrophotometer (4700, NIMS, Japan) equipped with ATR accessory in the range 400 cm-1 to 4000 cm-1. The X-ray photoelectron spectra (XPS) were obtained using nonmonochromatic Al Kα X-ray source (1.4×0.1 mm, 100 W, 20 kV, 5 mA) and a hemispherical electron analyzer (ECALAB MKIV). A JascoNRS-3100 Raman spectrometer equipped with an excitation light source of 633 nm He: Ne laser was used to obtain the Raman spectra of the samples. The small-angle X-ray scattering (SAXS) spectra were obtained from Rigaku MicroMax-007HF with high-intensity microfocus rotating anode X-Ray generator and a camera length of 700 mm. N2-adsorption/desorption isotherm was carried out using Micrometrics ASAP 2020 (NIMS, Japan) after degassing at 150ºC for 3 hr. 8


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Results and Discussion Characterization Pt/CeO2 nanostructures The morphology of the as-prepared CeO2 and Pt/CeO2 samples were analyzed by transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) imaging. Figure 1a shows the low-magnification TEM image of the synthesized CeO2 nanostructure with numerous mesopores and nanocavities. On closer examination, the CeO2 particles were found to have an average size of ca. 55 nm (Figure 1b). The HR-TEM image showed that the lattice spacing of the particles were 0.268 and 0.30 nm, which were close to the values of the inter-planar distance of the (100) and (111) planes of the CeO2 nanostructure (Figure 1c).35,39,45 In addition, fast fourier transform (FFT) pattern (Figure 1 d) obtained from the HR-TEM image is further confirms the similar lattice distance.

Figure 1. TEM and HR-TEM images of as-obtained mesoporous CeO2 nanostructures. (a) Low, (b) high-magnification TEM images, (c) HR-TEM image, and (d) FFT pattern extracted from the HR-TEM image. 9



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Figure 2 a, b shows the low-magnification TEM images of the Pt/CeO2 nanostructures, indicated that the mesoporous features in which the PtNPs are embedded in the porous CeO2 nanostructure. The HR-TEM images (Figure 2 c, d) show PtNPs with an average size of ca. 5-10 nm that are strongly coupled with the mesoporous CeO2 nanostructures was clearly evidenced. The PtNPs were straightforwardly distinguished by their dark colored tiny particles which are highlighted in yellow color circles, while the larger ones are the mesoporous CeO2. The estimated lattice spacing value of 0.278 nm is assigned to the (100) plane of CeO2, whereas the lattice spacing value of 0.227 nm is correspond well to the (111) plane of metallic Pt (Figure 2e).20 Moreover, the corresponding FFT patterns are also extracted from the regions of CeO2 and PtNPs marked as white color box in Figure 2d, which revealed that the distributions of diffraction spots in the pattern are associated to the typical FFT pattern of CeO2 and crystalline PtNPs, respectively(Figure 2e). Notably, the lattice spacing (0.278 nm) for hybrid Pt/CeO2 nanostructures is slightly higher when compared to the bare CeO2 (0.268 nm) nanostructures. This slight increment in the lattice expansion for CeO2 in the hybrid Pt/CeO2 nanostructures may result from the strong coupling between Pt and CeO2 and creation of more defects during the chemical reduction process.35 The composition and elemental distribution of Pt/CeO2 nanostructures were obtained using STEM energy dispersive X-ray spectroscopy (EDX)-elemental mapping analysis. As appeared in Figure 3, the STEM image exhibits the clear contrast difference between CeO2 and PtNPs. The EDX-elemental maps further confirm the distribution of PtNPs (yellow) all over the CeO2 nanostructure (green and red). In addition, we observed few PtNPs aggregates were formed upon chemical reduction using optimized concentration of NaBH4. Such effect could be due to the mesoporous feature of CeO2 nanostructure seeds, the nanocavities with surface defects or oxygen vecancies serve as active sites for nucleation and growth of PtNPs, resulting formation 10


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few agglomerated PtNPs. It should be noted, due to the strong reduction power of NaBH4 most of the PtNPs were uniformly dispersed onto the CeO2 nanostructures. The amount of Pt content is estimated to be 14.87 wt% as accessed by EDX-elemental quantification analysis (Figure S1).

Figure 2. TEM and HRTEM images of Pt/CeO2 nanostructures. (a, b) Low magnification TEM images, (c) HR-TEM image of Pt/CeO2 nanostructures, (d) HR-TEM image of the marked regions of CeO2 and PtNPs and corresponding FFT patterns. The possible mechanism of the formation of mesoporous Pt/CeO2 nanostructures involves two stages. Firstly, the mesoporous CeO2 nanostructures with well-controlled size were obtained through macroalgae polymer mediated reduction process. Previous studies have shown that the macroalgae extracted from seaweed, which was used to synthesize metal and metal oxide nanoparticles with controlled size and shapes, serves as a mild reducing agent for the reduction of metal ions due to their diversified functional groups.42,46 Additionally, the final size and shape of the porous nanostructures are relay on the nucleation and self-assembly process, which is directly associated with the reduction rate.18 The formation of mesoporous CeO2 nanostructures could arise 11



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from the mild reducing power of macroalgae extracts, and the reduction of Ce3+ ions occurs relatively slow. As a result, the nucleation and self-assembly process rate is low, resulting in the formation of mesoporous structures with controlled pores. Secondly, the incorporation of PtNPs on the CeO2 nanostructures was carried out by seed-mediated chemical reduction of Pt precursor solution by addition of an aqueous NaBH4 solution (See experimental section). Subsequently, tiny PtNPs are formed that are uniformly dispersed over the mesoporous CeO2 nanostructures, in turn resulting in strong coupling of PtNPs with mesoporous CeO2 nanostructures via electrostatic interaction.18

a)

b)

50 nm

50 nm

Ce

d)

c)

50 nm

O

50 nm

Pt

Figure 3. (a) STEM image and (b-d) corresponding EDX-elemental mapping of Pt/CeO2 nanostructures showing Ce (green), Oxygen (red), and Pt (yellow) elements.

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The structures of CeO2 and Pt/CeO2 nanomaterials were further characterized by UV-vis spectrophotometry and X-ray diffraction (XRD) analyses. The UV-vis absorption spectra of bare CeO2 (Figure S2) showed the absorption peak at 435 nm that is associated with the CeO2 nanostructures with a slightly higher particle size.47 Previous reports have demonstrated that the CeO2 nanoparticles with smaller size (< 8 nm) exhibit an absorption peak at 313 nm, whereas for bulk CeO2, it is at 470 nm.47 The appearance of extinction peak at 435 nm (Figure S2) indicates the larger CeO2 nanostructures (ca. 55 nm) as evidenced by TEM analysis (Figure 1). In comparison with CeO2, the Pt/CeO2 nanostructures exhibited a narrow surface plasmon resonance (SPR) peak at 318 nm that is attributed to the characteristic SPR peak of PtNPs.48 Figure 4a shows the powder X-ray diffractogram (pXRD) pattern of the as-synthesized CeO2 and Pt/CeO2 nanostructures. The mesoporous CeO2 nanostructures displayed peaks at 2θ = 28.6°, 33.1°, 47.6°, 56.5°, 59.2°, 69.4°, 76.7°, and 79.1° corresponding to the planes (111), (200), (220), (311), (222), (400), (331), and (420), respectively of cubic fluorite structure of CeO2 (JCPDS no.43-1002).49 The XRD pattern of Pt/CeO2 nanostructures shows four additional peaks at 2θ = 40.1°, 67.7°, 81.6°, and 86.4° that are assigned to the characteristic diffraction peaks of typical face-centered cubic (fcc) structure of metallic Pt (JCPDS card no. 00-001-1194),20 confirming the formation of PtNPs over the mesoporous CeO2 nanostructure. Notably, there is a small increment in the intensity of XRD pattern, indicates a change in the crystalline structure of CeO2 after PtNPs was supported, which is in good line with the previous report by Alayoglu et.al.35

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a)

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b)

Figure 4. Characterization of Pt/CeO2 microstructures. (a) X-ray diffraction (XRD) pattern, (b) Normalized Raman spectra of the bare CeO2, and Pt/CeO2 nanostructures. The effective electronic interaction of PtNPs with the porous CeO2 nanostructure and the subsequent formation of oxygen vacancies is examined by Raman spectroscopy. Figure 4b compares the Raman spectra of commercial CeO2 (CeO2/C), as-prepared mesoporous CeO2, and Pt/CeO2 nanostructures, respectively. Raman spectra were normalized with respect to the Raman signal at 460 cm-1. As displayed in Figure 4b, the CeO2/C, exhibits only one prominent peak at 445 cm-1, while the as-prepared porous CeO2 exhibits two peaks. A prominent peak at 445 cm-1 and a weak peak at 583 cm-1 are associated with the triply degenerate F2g mode of fluorite structured CeO2, and the defect induced (D) mode of CeO2, respectively.50,51 For Pt/CeO2 nanostructure, the intensity of Raman spectra was enlarged, and the additional peak appeared at 1037 cm-1, can be assigned to the Pt-O vibration of Pt-O-Ce bond, which is in agreement with the earlier report by Lee et al,50 suggesting a stong electroning interaction between PtNPs and CeO2 nanostructures. The interaction of PtNPs with the CeO2 leads to the formation of more oxygen vacancies.51 It should be accounted that the emergence of oxygen vacancies higher than 4.2% can tremendously favor the electrocatalytic activity.52 The chemical interactions of CeO2 and Pt/CeO2 14


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nanostructures were studied using Fourier transform infrared (FT-IR) spectroscopy. To better understand the interaction between PtNPs and CeO2, we have prepared the Pt/CeO2 sample using mechanical mixing and compared with the as-synthesized CeO2, and Pt/CeO2 samples. The FT-IR spectra (Figure S3) of as-synthesized porous CeO2 nanostructures showed bands at 804.1, 877, 968, 1105, 1143, 1346, 1446, 1538, and 1629 cm-1, and are that of CeO2 nanostructures.53 Compared with the bare CeO2, the significant decrease in the intensity at 877, 968 and 1105 cm-1 for Pt/CeO2 nanostructures. However, the Pt/Comm CeO2 nanostrutures obtained through mechanical grinding showed a little change in their intensity. This results suggest that the existence of strong interaction between supported PtNPs and mesoporous CeO2 in the Pt/CeO2 nanostructures.53 X-ray photoelectron spectroscopy (XPS) analysis was carried out to further demonstrate the oxidation states of Pt/CeO2 nanostructures. The XPS spectra of Ce 3d, O 1s, and Pt4f of Pt/CeO2 are displayed in Figure 5. The Ce 3d XPS spectra of Pt/CeO2 in Figure 5a shows the binding energies of Ce3+ components (marked as u, v, u11, v11), whereas Ce4+ components (marked as u1,v1, u111, and v111), where

u and v are associated with the Ce 3d3/2 and 3d5/2 core levels,

respectively.18,33,54 The high-resolution spectra of Ce 3d and O1s region for the as-prepared mesoporous CeO2 nanostructures are shown in Figure S4. In correspondence with the bare mesoporous CeO2 nanostructures, the binding energy peaks of Pt/CeO2 are negatively shifted, suggesting a strong electronic interaction between reduced PtNPs and porous CeO2.55 Significantly, after supporting PtNPs with the CeO2 nanostructures, the atomic ratio of Ce3+ species were increased from 36.35 % to 45.31% (Table S1 in the supporting information), indicating the Ptmediated reduction of Pt4+ into Pt

3+ .35

.

Figure 5b exhibit O 1s XPS spectra of the Pt/CeO2

nanostructure. The O 1s spectra depict two distinct peaks such as relatively lower binding energy 15



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at 529.28 eV is assigned to the lattice oxygen (Oα) on the surface of CeO2, whereas the other peaks at higher binding energy at 531.6 eV is assigned to the C-O (oxygen vacancy (Oβ)) are in line with the previous reports.56 Figure 5c shows the high resolution XPS spectrum of Pt4f region convey two spin-orbital separations of 3.66 eV, and each Pt 4f spectrum is deconvoluted into additional two peaks. The Pt4f deconvoluted peaks of 4f5/2 at 74.32 eV, and 4f7/2 at 71.02 eV are assigned to the binding energies positions of metallic Pt0 state, whereas another two peaks of 4f5/2 at 74.82 eV, and 4f7/2 peak at 71.37 eV are associated with the Pt2+ state, respectively.11

Figure 5. Core-level XPS spectra of (a) Ce 3d, (b) O 1s, and (c) Pt 4f spectra of the mesoporous Pt/CeO2 hybrid nanostructures. The small-angle X-ray scattering (SAXS) pattern of the CeO2 and Pt/CeO2 nanostructures are presented in Figure 6a. In the low q-range (up to q= 0.6 nm-1) of the SAXS profile for the bare CeO2 nanostructures exhibits three distinct peaks. A quasi-Bragg diffraction peak in the low qregion such as at ~0.13 nm-1 and its second order peak at 0.16 nm-1 is due to the scattering from the ordered mesoporous structures.57 It is noteworthy that after coupling with PtNPs, a significant increment in the intensity of these peaks was observed, implying the formation of more pores/defects in the Pt/CeO2 nanostructure. N2-adsorption/desorption isotherms were carried out to examine the porosity and to calculate the specific surface area (SSA) of the as-synthesized CeO2, and Pt/CeO2 nanostructures (Figure 6b). The evolution of typical hysteresis loops for the CeO2 16


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and Pt/CeO2 nanostructures with a sudden increase in the adsorption at higher relative pressure (P/P0) region like H3-type, is an indication of their ordered mesoporous structures.58,59 The SSA was estimated using Brunauer-Emmett-Teller (BET) method,60 and the particle parameters are summarized in Table 1. The calculated BET surface area for CeO2 and Pt/CeO2 nanostructures were 45.32 m2/g and 52.54 m2/g, respectively. The higher SSA of Pt/CeO2 nanostructures might be due to the well-distribution of the active metal nanoparticles and their strong coupling with the oxide support which in turn improves the catalytic activity.59 The inset of Figure 6b shows the pore-size distribution for CeO2 and Pt/CeO2 nanostructures, which disclose narrow distribution of pore size with an average pore size of 16.9 and 16.1 nm, respectively. The slight decrement in the pore diameter from 16.9 to 16.1 nm upon incorporation of PtNPs into CeO2 nanostructures could result from the collapse of some pores in the CeO2 nanostructures during the chemical reduction process and subsequent integration of PtNPs with mesoporous CeO2. These results provide unambiguous corroboration for the highly mesoporous structures of Pt/CeO2 nanostructure as well as the existence of a strong electronic interaction between supported PtNPs with the CeO2 nanostructure support. a)

b)

Figure 6. (a) Small-angle X-ray scattering pattern for the mesoporous CeO2, and Pt/CeO2 nanostructures. (b) N2-adsorption/desorption isotherms of the as-synthesized mesoporous CeO2 17



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and Pt/CeO2 nanostructures. The inset in (b) compares the pore-size distribution of CeO2 and Pt/CeO2 nanostructures. Table 1. Particle Parameters for obtained CeO2 and Pt@CeO2 nanostructures sample CeO2 Pt/CeO2

BET surface area (m2/g) 45.32 52.54

BJH pore diameter (nm ) 16.9 16.1

The demonstration of superior electrocatalytic activity for ethanol electrooxidation reactions of PtNPs based catalysts is well documented.61 Also, an incorporation of PtNPs over porous CeO2 nanostructures may not only enhance the active surface area of PtNPs but also further boost the catalytic activity by synergizing both counterparts by the formation of a strong interaction between PtNPs and CeO2 support at their functional interface and subsequent Pt-mediated creation of Ce3+ in their interface.13,18,50 Thus, the electrocatalytic activity of as-obtained Pt/CeO2 catalysts was performed for ethanol electrooxidation reaction. For electrochemical analysis, Pt/CeO2 catalyst was deposited onto the glassy carbon electrodes (GCE), and the cyclic voltammetry (CV) curves were collected between the potential cycling from -0.1 to 1.0 Vat a sweep rate of 20 mVs-1 in alkaline media (sulphuric acid, H2SO4), and the results were compared with Pt/ Comm CeO2, and commercial PtNPs supported carbon (Pt/C). Figure 7a compares the CV curves of Pt/CeO2, Pt/Comm CeO2, and Pt/C catalysts. The onset potential of ethanol electrooxidation on Pt/CeO2 catalyst is slightly negatively shifted about 20 mV than that of Pt/C catalyst. The electrochemically active surface areas (EASA) of the mesoporous Pt/CeO2, Pt/CeO2/C, and Pt/C catalyst were estimated to be 0.49 cm2, 0.38 cm2, and 0.27 cm2, respectively. After normalization to these values, the current density values were taken into account to compare the electrocatalytic activity toward ethanol electrooxidation of as-synthesized Pt/CeO2 catalysts. To acquire the specific 18


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electrocatalytic activity, the measured current densities were normalized according to the ECSA, which was estimated from the CV curves in the hydrogen adsorption region. Figure 7b shows the CV curves obtained from the Pt/CeO2, Pt/CeO2/C, and Pt/C catalysts toward electrooxidation of ethanol in 0.5 M of H2SO4+1 M of the ethanol solution. As can be seen from the Figure 7b, it is clear from the forward-potential scan, the peak current density values toward ethanol electrooxidation for Pt/CeO2, Pt/Comm CeO2, and Pt/C catalysts were 4.4, 3.8 and 3.1 mA cm-2, respectively. The calculated mass activities in ethanol electrooxidation reaction for Pt/CeO2, Pt/Comm CeO2, and Pt/C catalysts were found to be 3.12 mA, 2.2 mA, and 1.59 mA, respectively. These results demonstrate that the Pt/CeO2 catalyst show two-fold improvement in the activity than that of Pt/C catalyst, and approximately one-fold greater than that of Pt/Comm CeO2 catalysts. Thus, our observations substantiate the excessive electrocatalytic activity of the mesoporous Pt/CeO2 catalyst than that of the Pt/C catalyst. These results prove that the mesoporous structural features of CeO2 nanostructures significantly influence the overall electrocatalytic efficiency. The current density value of Pt/CeO2 catalyst was higher than that of most of the previous reports of PtNPs-supported CeO2 catalysts as shown in Table 2.

a)

b)

Figure 7. (a) Cyclic voltammograms (CVs) of Pt/CeO2 (blue), Pt/ Comm CeO2 (black), and Pt/C catalysts (red) on GCE in 0.5 M H2SO4 solution. (b) The current densities of the Pt/CeO2 (blue) 19



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Pt/ Comm CeO2 (black), and Pt/C (red) in 0.5 M H2SO4+1 M of ethanol. The CVs were recorded at room-temperature at a scan rate of 20 mV s-1. Table 2. Comparison of the electrocatalytic activity of the as-prepared Pt/CeO2 catalysts with previously reported Pt/CeO2 based electocatalysts.

Electrocatalysts

Electrolyte

ECSA (m2 g-1)

Mesoporous Pt/CeO2 nanostructures

0.5 M H2SO4+1 M of ethanol

Pt/CeO2 microstructure

0.5 M H2SO4 + 2 M 35.6 CH3OH 0.5 M H2SO4 + 1 M 51 CH3OH 1 MClO4 + 68.7 ethanol 0.5 M H2SO4 + 2 M -CH3OH 0.5 M H2SO4+1 M 68.6 of ethanol

Shuttle shape Pt/CeO2 nanostructures PtNPs/CeO2 (20% of CeO2) Helical shaped Pt/CeO2 nanostructures Pt/CeO2/graphene sheets

49

Current densities (mA cm-2)

ref.

3.12

this work

2.80

62

14.6

63

0.98

64

0.99

65

1.37

66

According to the results of structural analysis, it is evident that the presence of strong interaction of PtNPs with the porous CeO2 nanostructure leads to the creation of oxygen vacancies arising from the Pt-mediated reduction of Ce4+ to Ce3+, which agrees with the previous study by the Somorjai and co-workers.35 The improvement in the electrocatalytic performance of obtained mesoporous Pt/CeO2 nanostructure can be ascribed to the formation of the strong electronic interaction of PtNPs with the mesoporous CeO2 via Pt-O-Ce linkages.67 Apart from the strong coupling of PtNPs with the mesoporous surface, the contact areas between the porous surface and the active cataliytic-sites of metal PtNPs could be reduced and elevate the surface reactivity to ethanol, which advances the electron transfer rate at their interface and prevents the catalytic 20


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activity loss.3 Based on our findings, we ascribe that the both PtNPs induced formation of more oxygen vacancies and Ce3+ active sites at the Pt-CeO2 interface from their strong coupling between PtNPs and mesoporous CeO2, could provide electrically favorable functional interface in achieving superior catalytic activity toward ethanol electrooxidation reaction of the mesoporous Pt/CeO2 catalyst.

a)

b)

c)

d)

Figure 8. CO stripping voltammetry curves of (a) Pt/CeO2 hybrid nanostructures, (b) Pt/C catalysts, and (c) Comparison of the obtained mesoporous Pt/CeO2, and Pt/C catalysts, obtained in 0.5 M H2SO4 aqueous solution at a sweep rate of 20 mV s−1. The solid line is from the first scanning cycle and dotted line is from the second scanning cycle. (d) Chronoamperometric curves of Pt/CeO2, Pt/Comm CeO2, Pt/C catalysts in 0.5 M H2SO4+1 M ethanol solution at 0.6 V. It is well accepted that many carbonaceous species are generated as intermediates during the process of electrooxidation of alcohols and get adsorbed on the Pt active sites in alkaline medium, 21



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which identified as major poison species for Pt-based catalysts.68 Specifically, the high adsorption ability of Pt toward CO because of donation of electrons from the 5σ orbital of CO to Pt and followed by the back-donation from the Pt d-band to the 2π* molecular orbital of CO. This orbital mixing process is very strong, which poisons the Pt-sites, thus the catalytic activity becomes completely reduced.63,69 In addition, a recent study has evidenced that the strong coupling of the metal component to the oxide support resulted in the significant decrease in surface electron density of supported metallic nanoparticles, which helps to suppress the CO adsorption.34 To understand the surface-poisoning effect and evaluate the CO tolerance of the obtained Pt/CeO2 catalyst, CO stripping experiments was performed and the CO stripping curves are presented in Figure 8 a-c. From the Figure 8 a, b it is evident that the electrooxidation of COads occurs in the first cycle and during the second cycle, the appearance of characteristic peaks in the Hupd region. These results essentially suggest that the complete oxidation of adsorbed CO and subsequent formation of pure Pt-active sites. Figure 8c compares the CO-stripping profiles of Pt/C, and Pt/CeO2 catalysts indicated that the negative shift in the CO oxidation peak potential is observed. Such negative shift can be attributed because of the strong coupling of PtNPs with mesoporous CeO2 and further demonstrates that the Pt/CeO2 catalyst is tolerant to CO poisoning.34 Furthermore, the higher electrocatalytic activity of the Pt/CeO2 nanostructures was examined by the chronoamperometry (CA) analysis at a fixed potential of 0.6V, and the results are compared with the Pt/Comm CeO2, and Pt/C catalysts (Figure 8d). The typical CA profile of Pt/CeO2, Pt/Comm CeO2, and Pt/C samples measured in 0.5 M H2SO4+1M ethanol solution at fixed potential of 0.6 V. It is very clear from Figure 8d, the CV response of the Pt/CeO2 exhibit a higher steady state current and less current decay with polarization time toward ethanol oxidation reaction. The obtained specific current responses of the Pt/CeO2, Pt/Comm CeO2, and Pt/C after 120 was 22


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estimated to be 0.24, 0.17, and 0.06 mA cm-2, indicating approximately four-times higher oxidation current of Pt/CeO2 catalysts than that of Pt/C, and 1.4 times greater than that of Pt/Comm CeO2 catalysts. These results conclusively demonstrate that the as-obtained mesoporous Pt/CeO2 systems exhibit greatly enhanced eletocatalytic performance than that of Pt/C.

Conclusions In conclusion, we have demonstrated a facile approach to synthesize mesoporous CeO2 nanostructures that integrates 5-10 nm PtNPs to obtain mesoporous Pt/CeO2 nanostructures. The results proved that the strong electronic coupling between the PtNPs and the highly porous CeO2 nanostructures resulted in increase in the oxygen vacancies. Finally, the catalytic properties of the as-prepared Pt/CeO2 catalyst toward ethanol electrooxidation reaction have been evaluated. The results manifested that the resultant Pt/CeO2 catalysts exhibit significantly improved catalytic activity toward ethanol electrooxidation compared with the Pt/Comm CeO2, and Pt/C catalysts. The enhanced catalytic activity could result from the formation of large oxygen vacancies in the mesoporous CeO2, which further increased upon the integration of PtNPs through strong electronic interaction between them. As an outcome, the charge transfer between Pt/CeO2 nanostructure interface was improved, which significantly strengthens the overall catalytic activity. This green approach is propitious towards the preparation of other metal-oxide nanostructures with excellent porous features toward the development of efficient heterogeneous electrocatalysts for potential applications in energy conversion and fuel cells. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] 23



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Phone: +01-442381150 Fax: +01- 4422381165 Author Contributions P.S.M. Kumar design the synthesis and characterization part and T. Imai for TEM measurements and characterization. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS All authors are acknowledging Dr. Ya Xu National Institute for Materials Science Research for his help in N2-adsorptions/desorption isotherm measurements. Supporting Information Available EDX-analysis, UV-vis spectra, FT-IR results, XPS spectra for CeO2 nanostructures, This material is available free of charge via the Internet at http://pubs.acs.org.

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Synopsis: Green synthesis of mesoporous CeO2 nanostructures mediated by macroalgae polymer and supporting Pt nanoparticles on CeO2 with strong Metal-Oxide interactions for efficient electrochemical oxidation of ethanol.


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Pt Nanoparticles Supported on Mesoporous CeO2 Nanostructures

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