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Highly Efficient CeO2 Decorated Nano-ZSM-5 Catalyst for Electrochemical Oxidation of Methanol Balwinder Kaur, Rajendra Srivastava, and Biswarup Satpati ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00525 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 11, 2016
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ACS Catalysis
Highly Efficient CeO2 Decorated Nano-ZSM-5 Catalyst for Electrochemical Oxidation of Methanol Balwinder Kaura, Rajendra Srivastava*a, and Biswarup Satpatib a
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, India
b
Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF,
Bidhannagar, Kolkata 700 064, India ______________________________________________________________________________ E-mail:
[email protected] Phone: +91-1881-242175; Fax: +91-1881-223395
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_____________________________________________________________________________ Abstract Cerium oxide (CeO2) decorated nanocrystalline zeolite (Nano-ZSM-5) nanocomposites with different weight ratios were prepared by the calcination of a physical mixture of nanocrystalline CeO2 and Nano-ZSM-5. Materials were characterized by the complementary combination of Xray diffraction, N2-adsorption, transmission electron microscopic, and X-ray photoelectron spectroscopic techniques. The material was investigated as precious metal free electrode catalyst for methanol oxidation. The electrochemical oxidation of methanol was investigated at CeO2/Nano-ZSM-5 modified glassy carbon electrode in the alkaline medium using electrochemical impedance spectroscopy, cyclic voltammetry and chronoamperometry. Comparative investigations were made with commercial Pt(20%)/C catalyst with respect to current density, stability, and CO tolerance capacity. CeO2/Nano-ZSM-5 with a weight ratio of 30% exhibited remarkably high electrocatalytic activity in the methanol oxidation when compared to nanocrystalline CeO2 and commercial Pt (20%)/C catalyst. The material was found to exhibit stable electrocatalytic activity even after 1000 cycles. High electrocatalytic activity in the methanol oxidation can be attributed to the synergistic contribution provided by CeO2 nanocrystals and Brönsted acidity of the high surface area Nano-ZSM-5. Results demonstrate that the excellent current density and high stability of CeO2/Nano-ZSM-5 would be interesting for its commercial application in direct methanol fuel cells.
Keywords: Cerium oxide; Nanocrystalline zeolite; Electrocatalytic oxidation; Methanol, Fuel cell, Bi-functional catalyst. __________________________________________________________________________
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1. Introduction Methanol is commercially prepared from synthesis gas.1 Methanol is used as a reactant to produce a large numbers of fine chemicals.2 Methanol can be converted to olefins and gasoline using zeolite catalysts in vapor phase condition. 3 Therefore, methanol is an economical and readily available raw material that can be used in the fuel cell application. Direct methanol fuel cells (DMFCs) are considered to be a promising candidate to fulfill future energy demand due to its high energy conversion efficiency, low operating temperature, and low pollutant emission.4 Among various fuel cells, proton exchange membrane fuel cells (PEMFCs) are highly efficient.5 However, fabrication of PEMFCs is complicated and expensive because of the management of H2 & water and costly electrode catalysts & nafion membranes.5b, 6 Therefore, efforts are being made for the development of DMFCs, especially for portable applications. The overall efficiency of DMFC is dependent on the oxidation of methanol at the anode. High overpotential is required to oxidize methanol at the anode of DMFCs, therefore it requires efficient anode catalyst. Platinum (Pt) based anode catalysts are used in the DMFCs.7 However, Pt based anode is associated with several inherent drawbacks such as high cost, limited reserve, and poor CO tolerance.8 CO is an oxidation intermediate formed during the electrocatalytic oxidation of methanol at Pt anode in acid medium.8 Intermediate CO acts as poison for Pt anode electrocatalyst. In order to overcome these problems, other precious and transition metals have been incorporated with Pt to form binary, ternary and quaternary alloys.9 With these alloys, CO tolerance of the Pt based catalyst was improved but the cost of the electrode is still high which limits its commercial applications. Therefore, efforts have been made to develop alternative economical transition metal oxides/hydroxides such as NiO, Co3O4, NiCo2O4, Ni(OH)2, MnO2 as electrocatalysts for methanol oxidation.10 Very recently, SnO2 supported mesoporous zeolite is reported for the electrochemical oxidation of methanol.11 These electrode catalysts are known for the electocatalytic oxidation of methanol to CO2 in basic medium.
CeO2 is important with respect to fuel cell applications.12 CeO2 is widely used in solid oxide fuel cells due to high oxide ion conductivity. Fluorite structure of CeO2 facilitates the storage and release of oxygen, which is important for complete oxidation of CO. 13 This is the reason CeO2 is also used in catalytic convertor. CeO2 can effectively reduce NOx emissions as well as convert harmful CO to the benign CO2.14 CeO2 was found to be an efficient support material for the various forms of Pt towards the effective oxidation of methanol in the acidic 3 ACS Paragon Plus Environment
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medium.13a, 15 It has been proposed that the anodic performance for Pt-CeOx is improved because of the high oxygen storage capacity of CeOx surface and capability of CeO2 to oxidize CO to CO2 on Pt.13a,
15
Hence, CeO2 plays an important role in the electrocatalytic oxidation of
methanol. Efficiency of this electrode process is limited because of the low surface area of CeO2. Therefore, efforts are being made to develop strategies to prepare CeO2 having high surface area.16 To the best of our knowledge there is no report in the literature where CeO2 itself acts as electrode catalyst (and not as support material) for methanol oxidation. Our group is continuously making effort to develop economical electrode catalysts for methanol oxidation.17 We are committed to develop economical anode catalysts for methanol oxidation with high electrochemical activity and low over-potential. Recently, our group has demonstrated the application of NiCo2O4 and β-Ni(OH)2-NiCo2O4 hybrid nanostructures as effective electrocatalyts for the oxidation of methanol.17a, 17c We have also shown that Cu2+/Ni2+ ion-exchange nanocrystalline ZSM-5 can also be used in basic medium for the methanol oxidation.17b It is known in the literature that metal oxide can be finely dispersed on the high surface area Nano-ZSM-5 for their possible application in electroanalysis.18 Therefore, we anticipate that if CeO2 is supported on Nano-ZSM-5, it may impart high electrocatalytic activity in methanol oxidation. Development of electrode materials based on nanocrystalline zeolites is one of the current research interests of our group.19 A novel synthesis strategy is reported here for the preparation of CeO2 nanoparticles decorated on high surface area Nano-ZSM-5 (here after represented as CeO2/Nano-ZSM-5). Pt free, economical and robust CeO2/Nano-ZSM-5 nanocomposite based electrode was fabricated for the electrocatalytic oxidation of methanol for DMFCs application. Impressively high electrocatalytic
activity
was
observed
at
CeO2/Nano-ZSM-5
nanocomposite
in
the
electrochemical oxidation of methanol when compared with nanocrystalline CeO2, Nano-ZSM-5, and commercial Pt(20%)/C. To the best of our knowledge, this is the first report, which deals with the electrocatalytic oxidation of methanol using CeO2/Nano-ZSM-5 nanocomposite as an electrode material in the alkaline medium.
2. Experimental Section 2.1. Materials. All chemicals were of analytical reagent grade and used as received without further purification. Tetraethylorthosilicate (TEOS, 98%), tetrapropylammonium hydroxide 4 ACS Paragon Plus Environment
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(TPAOH), propyltriethoxy silane (PrTES, 97%), platinum (20 wt%) on graphitized carbon (Pt(20%)/C), and Pluronic F-127 (HO(CH2CH2O)106(CH2CH(CH3)O)70 (CH2CH2O)106 H, designated as EO106PO70EO106) were purchased from Sigma Aldrich, India. Cerium nitrate hexahydrate (Ce(NO3)3.6H2O) and phloroglucinol were obtained from Spectrochem Pvt. Ltd., India. Formaldehyde was purchased from SD Fine Chemical Ltd., India. Deionized water from Millipore Milli-Q system (Resistivity 18 Mcm) was used in the electrochemical studies. Electrochemical measurements were performed in 0.5 M NaOH solution.
2.2. Synthesis of CeO2/Nano-ZSM-5 nanocomposite. Nanocrystalline ZSM-5 zeolite (NanoZSM-5) was prepared using molar composition 90 TEOS/10 PrTES/2.5 Al2O3/3.3 Na2O/25 TPAOH/2500 H2O by following the reported procedure.17b Nano-ZSM-5 was then converted into H+ form by ion-exchange (three times) using 1 M NH4Cl aqueous solution. In a typical ionexchange, 1 g of zeolite was added to 50 mL of 1 M NH4Cl aqueous solution and stirred at 343 K for 6 h. The resultant product was filtered, washed with distilled water several times, and dried in oven at 373 K for 4 h. Final product was then calcined at 773 K for 6 h. CeO2 was synthesized by following our reported procedure.20 In a typical synthesis, 1.62 g of phloroglucinol and 4.86 g of F-127 were dissolved in 25 ml of absolute ethanol. After dissolving the solid, 4.34 g of cerium nitrate hexahydrate was added and stirred at ambient temperature for half an hour to ensure the complete dissolution. 0.15 g of HNO3 (65%) was added to the above reaction mixture. The resultant solution was stirred at ambient temperature for half an hour. Subsequently, 1.6 g of formaldehyde (37%) was added to the above solution. The reaction mixture was stirred for further 4 h at ambient temperature and then transferred to a Petri dish and ethanol was evaporated at ambient temperature. It was then cured at 373 K for 24 h before carbonization. Material was carbonized at 1073 K under nitrogen atmosphere via heating ramp of 1 deg/min and kept at 1073 K for 3 h. The resultant carbon-metal composite was further calcined at 773 K for 4 h. For the synthesis of CeO2/Nano-ZSM-5 nanocomposite materials, CeO2 and Nano-ZSM-5 with different weight ratios (20, 30, and 40, denoted as CeO2(20%)/Nano-ZSM-5, CeO2(30%)/Nano-ZSM-5 and CeO2(40%)/Nano-ZSM-5, respectively) were grounded uniformly with ethanol using mortar and pastel. Ethanol was slowly removed during the mixing process.
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The mixture was heated at 473 K in air for 45 min to remove the residual ethanol, followed by calcination at 773 K for 12 h in air to obtain CeO2/Nano-ZSM-5 nanocomposite. 2.3. Instrumentation. X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5-80° with a scan speed of 2°/min on a PANalytical X’PERT PRO diffractometer, using Cu Kα radiation (λ=0.1542 nm, 40 kV, 40 mA) and a proportional counter detector. Nitrogen adsorption measurements were performed at 77 K by Quantachrome Instruments, Autosorb-IQ volumetric adsorption analyzer. Sample was out-gassed at 573 K for 3 h in the degas port of the adsorption apparatus. The specific surface area of zeolites was calculated from the adsorption data points obtained at P/P0 between 0.05-0.3 using the Brunauer-Emmett-Teller (BET) equation. The pore diameter was estimated using the Barret–Joyner–Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) using a PHI Quantum 2000 XPS system with a monochromatic Al Ka source and a charge neutralizer. All binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. TEM investigations were carried out using a FEI, Tecnai G2 F30-ST microscope operating at 300 kV. The microscope is equipped with a scanning unit and a HAADF detector from Fischione (model 3000). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS, EDAX Inc.) was performed using the same microscope. The sample was dispersed in ethanol using ultrasonic bath, and dispersed sample was mounted on a carbon coated Cu grid, dried, and used for TEM measurement.
2.4. Electrode fabrication. Cyclic voltammetry (CV) and chronoamperometry studies were performed
using
Potentiostat-Galvanostat
BASi
EPSILON,
USA.
A
three-electrode
electrochemical cell was employed with Ag/AgCl as the reference electrode (3M KCl), CeO2/Nano-ZSM-5 mounted glassy carbon (3 mm diameter) as the working electrode and Pt foil as the counter electrode. Before modification, the glassy carbon electrode (GCE) was first polished to a mirror like surface with alumina slurry and then ultrasonicated in ethanol and deionized water for 5 min, respectively. 10 µL aliquot of CeO2/Nano-ZSM-5 suspension (a homogenous sonicated solution of 2 mg of CeO2/Nano-ZSM-5 nanocomposite, 10 µL of Nafion and 1 mL of deionized water) was placed onto the GCE surface. The electrode was dried in air leaving the material mounted onto the GC surface. For comparison, CeO2 modified GCE, 6 ACS Paragon Plus Environment
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Pt(20%)/C modified GCE, and Nano-ZSM-5 modified GCE were also fabricated in a similar way. Electrochemical impedance spectroscopy (EIS) was performed using Autolab PGSTAT302N. For the CO stripping analysis, the electrochemical cell was purged with CO into 0.5 M NaOH for 60 min to ensure the sufficient adsorption of CO at the modified electrode surface. It was then purged with pure N2 for 30 minutes to remove the CO from supporting electrolyte solution before recording the voltammogram. Two subsequent sweeps were then performed at CeO2(30%)/Nano-ZSM-5/GCE at a scan rate 50 mV/s.
3. Results and Discussion A direct, one step synthetic protocol was adopted to prepare nanocrystalline CeO2 using F127 triblock copolymer (as structure-directing agent), a mixture of phloroglucinol and formaldehyde (as carbon source) and metal nitrate (as metal source) under mild acidic condition.20 In this method, the obtained ethanolic synthesis mixture was dried under ambient condition in flat Petri disc, cured at 373 K, which was then carbonized at 1073 K and then calcined at 773 K to obtain nanocrystalline CeO2. The importance of this method is the direct use of the self-assembly of block copolymers as templates for the generation of porous metal-carbon structures, without the extra step of generating templating silica structures. The phloroglucinol and block co-polymers based synthetic route promote the cooperative assembly of inorganic/co-polymer mesostructures with the help of hydrogen bonding at the inorganic/organic interface, yielding nanostructured thermally stable nanocrystalline CeO2. Nano-ZSM-5 was synthesized in the presence of propyltriethoxysilane as an additive. 17b CeO2/Nano-ZSM-5 nanocomposite materials with different weight ratios were prepared by the calcination of CeO2 and Nano-ZSM-5 at 773 K. Calcination process was important for the uniform dispersion of CeO2 nanoparticles on to the surface of Nano-ZSM-5. Surface silanol groups of Nano-ZSM-5 are required to create interface between catalytic active CeO2 and NanoZSM-5 during the calcinations process. The detail of physico-chemical characterization is provided in the following section. The electrocatalytic activity of the synthesized material was investigated in the electrochemical oxidation of methanol. In-depth investigation was made using CeO2(30%)/Nano-ZSM-5 because of its higher electrocatalytic activity as described in the later part of the manuscript.
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3.1. Physico-chemical characterization. Nano-ZSM-5 exhibited XRD pattern corresponding to a highly crystalline MFI framework structure with high phase purity (Figure 1a). XRD pattern of Nano-ZSM-5 is broad, confirming the nanocrystalline nature of the material. The diffraction peaks for CeO2 can be indexed to the cubic fluorite-type structure of crystalline CeO2 (Figure 1a). Intense peaks at 2θ = 28.6°, 33.2°, 47.7°, 56.6°, 59.2°, 69.5°, 76.9°, and 79.1° can be assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), and (4 2 0) reflections of crystalline CeO2, respectively (Figure 1a).20 The XRD pattern of CeO2(30%)/Nano-ZSM-5 shows the diffraction peaks corresponding to both, CeO2 and Nano-ZSM-5 phases (Figure 1a). Calcination was favorable in the dispersion process, which appears to cause the dispersion of CeO2 on the surface of Nano-ZSM-5. CeO2(20%)/Nano-ZSM-5 and CeO2(40%)/Nano-ZSM-5 also exhibited the similar XRD pattern as that of CeO2(30%)/Nano-ZSM-5 (Figure S1 in the Supporting Information). The textural properties of the synthesized materials were investigated by N2-sorption measurement. CeO2 exhibited type IV isotherm with H2 hysteresis loop, which is the characteristics of mesoporous material (Figure 1b). The adsorption jump in the isotherm appears between partial pressures P/Po of 0.4-0.70, followed by overlapping adsorption and the desorption branch with negligible increase in the adsorption volume. The increase in slope at ca. 0.4 corresponds to capillary condensation, typical of mesoporous materials with uniform pore systems. Pore size distribution was calculated from the adsorption branches. BJH pore size distribution for CeO2 shows a pore size distribution in the range of 1.5-7 nm (inset of Figure 1b). The N2-adsorption isotherms for Nano-ZSM-5 and CeO2(30%)/Nano-ZSM-5 also exhibited a typical type-IV isotherm similar to that of mesoporous materials (Figure 1b). The distinct increase of N2 adsorption for Nano-ZSM-5 in the region 0.4
