Research Article pubs.acs.org/journal/ascecg
Pt Nanoparticles Supported on Mesoporous CeO2 Nanostructures Obtained through Green Approach for Efficient Catalytic Performance toward Ethanol Electro-oxidation Paskalis Sahaya Murphin Kumar,†,‡ Sivakumar Thiripuranthagan,‡ Tsubasa Imai,† Gopalakrishnan Kumar,§ Arivalagan Pugazhendhi,∥ Sri Ramkumar 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 756600, 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, Querétaro 76230, México ∇ CONACYT- Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apdo Postal J-48, Puebla 72570, México ‡
S Supporting Information *
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 ecofriendly approach to the synthesis of mesoporous CeO2 nanostructures that supports PtNPs with an excellent catalytic activity is validated as a promising strategy for potential applications in fuel cells. KEYWORDS: Platinum nanoparticles, Cerium oxide, Red algae, Ethanol electrooxidation, Electrocatalysis
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bonding with different adsorbates.2,5,6 However, two main challenges of using PtNPs catalysts are of major concern, causing significant limitation to their practical applications and sustainability: (i) The limited abundance of Pt and CO poisoning issues.7 (ii) The chemically inert PtNPs becomes unstable under alkaline reactive conditions, under which the
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 © 2017 American Chemical Society
Received: June 20, 2017 Revised: October 18, 2017 Published: October 26, 2017 11290
DOI: 10.1021/acssuschemeng.7b02019 ACS Sustainable Chem. Eng. 2017, 5, 11290−11299
Research Article
ACS Sustainable Chemistry & Engineering surface Pt atoms dissolve and migrate, leading to the agglomeration of NPs, and consequently cutting 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, and 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 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 cells22,23 and as oxidizing cocatalysts in automobile catalytic converters along with Pt-based nanoparticles.24 Numerous groups have extensively investigated 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 primarily associated with the creation of more oxygen vacancies originated from the NPs-mediated reduction of Ce4+ to Ce3+, which critically influences the catalytic activity. Furthermore, previous studies have implied that the supporting PtNPs on 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 synthesis 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 sizes and shapes.39 The growth of mesoporous structures and supporting metal NPs on a 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 result in detachment or migration under harsh reactive conditions affecting the overall catalytic activity.40 Hence, the demand for efficient synthesis methods based on a “green and eco-friendly approach” is not only on the rise but also of 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 galactosemediated (red macroalgae polymer) reduction, 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.
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EXPERIMENTAL SECTION
Chemicals. Cerium(III) nitrate hexahydrate (Ce(NO3)3.6H2O), potassium tetrachloroplatinate (II) (K2PtCl4.6H2O, 99.98%), and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich and used as received. Commercial cerium(IV) oxide nanopowder (
