Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Synthesis and Characterization of Mo2C/MoO3 Nanohybrid as Electrocatalyst for Hydrogen Evolution Reaction Maria Sarno, Claudia Cirillo*, Eleonora Ponticorvo, Paolo Ciambelli Department of Industrial Engineering and Centre NANO_MATES, University of Salerno, Via Giovanni Paolo II ,132 - 84084 Fisciano (SA), Italy clcirillo@unisa.it The two-step preparation of core-shell Mo2C/MoO3 nanoplatelets (composed of an MoO3 core covered by 3 nm Mo2C outer layer) is reported. The process consists of the thermolysis of a solid precursor, ammonium molybdate ((NH4)6Mo7O24•·4H2O), in a reactor fed by nitrogen (200 cm3(STP)/min), heated from room temperature to 700 °C, followed by a controlled Chemical Vapor Deposition (CVD) of CH4, 10 v/v % in nitrogen, for 10 minutes, monitored on‐line by continuous gas analyzers. A Tafel slope of 48 mV/decade was measured for the Mo2C/MoO3 nanohybrid suggesting the Volmer- Heyrovsky mechanism for the nanohybrid-catalyzed HER. MoO3 deserves as a highly nanostructured conductive core, while the thin Mo2C coating serves as both a HER catalyst and a protective layer. 1. Introduction Hydrogen is being vigorously pursued as an ideal energy carrier and its production by water splitting is attracting growing attention. The electrolysis efficiency is enhanced when Pt is used as the catalyst in acidic media. In particular, the main effective catalysts are Pt-group metals but it remains challenging to look for alternative catalysts that are more abundant and at lower cost. Group VI transition metal carbides exhibit catalytic properties analogous to platinum group metals because of their unique d-band electronic structures. Tungsten carbide has been explored for the Hydrogen Evolution Reaction (HER) providing interesting results, but with a high overpotential if compared with Pt-group metals (Esposito and Chen, 2011). On the other hand, molybdenum carbide was been demonstrated able to reduce the overpotential (Chen et al., 2013). Many approaches including Chemical Vapour Deposition (CVD) (Hanif et al., 2002) and pyrolysis of metal complexes (Wolden et al., 2011) have been developed for the preparation of metal carbides. Among them the more explored is CVD using hydrocarbons (Giubileo et al., 2012) or carbonaceous gases (e.g. CO) and Mo gaseous precursors (MoF6, Mo(CO)6, MoCl5). However, the gas-phase syntheses generally required elaborate equipment, involve the use of expensive and toxic reagents, and the product is usually contaminated by chars from the pyrolysis of carbonaceous gases. Here we report the preparation of core-shell Mo2C/MoO3 nanoplatelets by a solid precursor thermolysis under nitrogen, followed by a controlled CVD of CH4. The characterization was obtained by the combined use of different physico-chemical techniques, and the Mo2C/MoO3 nanohybrid was characterized for HER. 2. Experimental Core-shell Mo2C/MoO3 nanoplatelets were prepared by a solid precursor (ammonium molybdate (NH4)6Mo7O24·4H2O) thermolysis under a nitrogen (200 cm3(STP)/min) flow, in a continuous flow reactor (Di Bartolomeo et al., 2009), from room temperature to 700°C, followed by a controlled CVD (Ciambelli et al., 2004) of CH4 (Sarno et al., 2012b) 10 v/v % in nitrogen for 10 min. DOI: 10.3303/CET1543158 Please cite this article as: Sarno M., Cirillo C., Ponticorvo E., Ciambelli P., 2015, Synthesis and characterization of mo2c/moo3 nanohybrid as electrocatalyst for hydrogen evolution reaction, Chemical Engineering Transactions, 43, 943-948 DOI: 10.3303/CET1543158 943 Methane CVD was carried out by feeding, at 700 °C (controlled by a K thermocouple), a methane–nitrogen gas mixture on the reaction bed. The reactor was heated by an electrical oven which temperature was controlled by a temperature programmer–controller (Eurotherm 2408). Cylinder gases (99.998 pure methane and 99.9990 pure nitrogen) were mixed to obtain the methane/nitrogen stream that was fed to the reactor. A constant flow rate of each gas was provided by a mass flow controller. The experimental plant for the synthesis was equipped with on-line ABB analyzers (Sarno et al., 2012a) that permit to monitor the concentrations in the effluent stream (Sarno et al., 2013b) of the following chemicals during the reaction: methane (CH4) the source of carbon, ethylene (C2H4), acetylene (C2H2) and H2. The characterization was obtained by the combined use of different techniques. Transmission electron microscopy (TEM) images were obtained using a FEI Tecnai electron microscope operated at 200 KV with a LaB6 filament as the source of electrons, equipped with an EDX probe. Scanning electron microscopy (SEM) images were obtained with a LEO 1525 microscope. Raman spectra were obtained at room temperature with a micro-Raman spectrometer Renishaw inVia with a 514 nm excitation wavelength (laser power 30 mW) in the range 100 -1100 cm-1. Optical images were collected with a Leica DMLM optical microscope connected on- line with the Raman instrument. The laser spot diameter was about 10 µm. XRD measurements were performed with a Bruker D8 X-ray diffractometer using CuKα radiation. Thermogravimetric analysis (TG-DTG) at a 10 K/min heating rate in flowing air was performed with a SDTQ 600 Analyzer (TA Instruments) coupled with a mass spectrometer. For the electrochemical measurements 4 mg of catalyst were dispersed in 80 μl of 5 wt. % nafion solution to form a homogeneous ink. Then the catalyst ink was loaded onto a glassy carbon electrode of 3 mm in diameter. Linear sweep voltammetry (using the pontentiostat from Amel Instruments) with scan rate of 2 mVs−1 was conducted in 0.5 M H2SO4, using saturated calomel electrode as the reference electrode, a graphite electrode as the counter electrode and a loadable glassy carbon electrode as the working electrode. 3. Results and discussion Figure 1a shows the diffraction pattern recorded for the Mo2C/MoO3 sample. The typical peaks of orthorhombic α-MoO3 (Stoyanova et al., 2009) and β-Mo2C phases were observed (Chen et al., 2013). The Raman spectrum of the sample excited by 514 nm line in air ambient environment is shown in Figure 1b. It is worth to notice that the spectrum has been obtained at 0.1 % of laser power and after 10 s exposition time, because under more stricter conditions the typical Raman spectrum of MoO3 appears generated by the laser, which photon energy closely resonant with the electronic absorption and the band gap energies, determines heating/photochemistry via direct absorption (Cervantes-Gaxiola et al., 2013). The typical bands of MoO3 and Mo2C can be seen in Figure 1b. The laser Raman spectrum of Mo2C (Figure 3a) is very similar to that of MoO3. In particular, the typical vibration frequencies of MoO3 at 994 cm-1 and 819 cm-1 (Frauwallner et al., 2011) and of Mo2C at 1006 cm-1 and 828 cm-1 (Xiao et al., 2001), can be seen in our spectrum. Figure 2 shows the representative SEM images for the Mo2C/MoO3 product, at increasing magnifications. The sample has a platelet elongated shape structure and thickness of few nanometers, as shown in Figure 2b. 10 20 30 40 50 60 70 (223)(231) (203) (221) (211) (020) (002) (042) (002) (211) (002) (081) (112) (210) (200) (150)(041) (101) (111) (130) (021) (040) (110) MoO 3 In te n si ty a .u . 2 Θ° Mo 2 C (020) 200 400 600 800 1000 378384 820 828 992 In te n s it y a .u . Raman shift cm -1 1006 Figure 1: XRD diffraction patterns (a) and Raman spectrum (b) of Mo2C/MoO3. a b 944 200nm __ 2μm __ Figure 2: SEM images of Mo2C/MoO3 hybrid at different magnifications TEM investigations carried out, to provide further insights into the morphology, on the resulting Mo2C/MoO3 nanostructures, are reported in Figures 3a and 3b at increasing magnifications. Figure 3a shows a typical TEM image at low magnification for the sample confirming the morphology already observed by SEM (Figure 2b). Figure 3b shows that the nanoplatelets are indeed composed of an MoO3 core and of an 3 nm Mo2C outer layer, as confirmed by the selected area diffraction pattern in the Figure 3b insert showing the typical crystallographic in plane square structure ascribable to Mo2C. During the precursor thermolysis from room temperature to 700 °C, a weight loss appears below 300 °C, arising from the decomposition of ammonium molybdate (Sarno et al., 2014b), with the formation of MoO3 (Delporte et al., 1995). At 700 °C, before sublimation that occurs at around 800 °C (Sarno et al., 2014b), the incorporation of C, coming from the decomposition of CH4, begins by filling MoO3 anionic vacancies (Frauwallner et al., 2011). In particular, the evolutions of the concentration profiles of CH4 the source of carbon, C2H2, C2H4 and H2 during the isotherm at 700 °C, are shown in Figure 4a. Figure 3: TEM images of Mo2C/MoO3 at different magnifications (a, b). SAED pattern of Mo2C obtained on the edge of the platelets (b insert). 3nm __ 20nm __ a b a b 945 0 4 8 0 5 10 15 20 time min v /v % . C2H2 CH4 C2H4 H2 80 90 100 0,0 1,2 In te n s it y a .u . m/z 44 200 400 600 800 Temperature (°C) D e ri v . W e ig h t (% ) W e ig h t (% ) Figure 4: Profiles of CH4, C2H4, C2H2 and H2 concentrations during the Mo2C/MoO3 synthesis at 700 ⁰C (a). TG-DTG profiles of the Mo2C/MoO3 as prepared and the corresponding total ion current of CO2 (b) Observing Figure 4a, three temporal phases (Sarno et al., 2014a) can be distinguished: - Pre-reaction phase (I): the reaction gas is fed to the analyzers. All of the concentration values are close to zero, except for the initial methane concentration, which is 10 v/v %. - Reaction phase (II): the gases are fed to the reactor, reaching the analyzers after passing through the catalyst bed. After the time necessary to pass the reactor and accounting for the delay time of the analyzers, the steady state methane concentration stabilizes at 9.58 v/v %, while the hydrogen concentration in the effluent stream shows the opposite trend and stabilizes at 0.69 v/v %. No significant C2H4 and C2H2 concentrations are detected during the entire running time at the chosen temperature (700°C). - Post-reaction phase (III): the reaction has ended, and the gases are sent directly back to the analyzers (bypassing the reactor). The concentrations of the four gases are the same as those in the pre-reaction phase. The amount of carbon coming from the decomposition of methane is calculated by the conversion of methane (data from Figure 4a) (Sarno et al., 2013a), considering the volumes and densities of MoO3 and Mo2C. This result is compatible with the formation of a Mo2C layer of about 3 nm around the MoO3 platelet. In Figure 4b the thermogravimetric (TG-DTG) profiles, of the synthesized Mo2C/MoO3 nanohybrid, are reported as function of temperature. As clearly indicated by the corresponding TIC of CO2 (m/z = 44), two weight losses due to carbon are visible: (i) in the range 200-400 °C, probably due to few percents of carbon impurities, coming form the methane homogeneous decomposition; and in the range 580-800 °C due to the Mo2C oxidation to form molybdenum trioxide and carbon dioxide (Chen et al., 2013), this is partially superimposed with the sublimation of MoO3 which begins at around 750 °C, as previously reported (Chen et al., 2013). The electrocatalytic HER activities of Mo2C/MoO3 were investigated. The hybrid material was deposited on a glassy carbon electrode in 0.5 M H2SO4 solution using a typical three-electrode setup. As a reference point, a commercial Pt catalyst with known high HER catalytic performances was evaluated (Sarno et al., 2014c). The results are shown in Figure 5. Polarization curve (i-V plot) recorded for Mo2C/MoO3 (Figure 5a) showed an overpotential of ~0.13 V for HER, beyond which the cathodic current rose rapidly under more negative potentials. Linear portions of the Tafel plots (Figure 5b) were fit to the Tafel equation (η = b log j + a, where η is the overpotential, j is the current density, b is the Tafel slope), yielding Tafel slopes of ~30 and ~48 mV/decade (iR-corrected) for Pt, and Mo2C/MoO3 nanohybrid, respectively. The observed Tafel slope for Mo2C/MoO3 suggests that electrochemical desorption is the rate limiting step (Li et al., 2011), the high performance is probably due to the electrocatalytic activity of Mo2C and an electronic coupling with the highly conductive MoO3 (Li et al., 2011). CH4 H2 a b 200 400 600 800 946 -0,3 -0,2 -0,1 0,0 0,1 -40 -30 -20 -10 0 -40 -30 -20 -10 0 C u rr e n t (m A /c m 2 ) E (V vs RHE) PtMo 2 C/MoO 3 1 10 0,0 0,1 0,2 Tafel linear fit Tafel linear fit O v e rp o te n ti a l (V ) Current (mA/cm 2 ) Mo 2 C/MoO 3 b=48 mV/dec Pt 30 mV/dec Figure 5: Polarization curves obtained (a), and corresponding Tafel plots (b). 4. Conclusions Core-shell Mo2C/MoO3 nanoplatelets have been prepared by a solid precursor thermolysis, avoiding the more complicated and dangerous supply of a molybdenum precursor from the gas phase, followed by a controlled CVD of CH4, monitored on-line by ABB analyzers. The characterization confirms the nature of thematerial, that shows excellent HER activity, with a Tafel slope of 48 mV/dec compatible with electrochemical desorption as rate limiting step. 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