Synthesis of PtSn nanostructured catalysts supported over TiO2 and Ce-doped TiO2 particles for the electro-oxidation of ethanol

ARTICLE IN PRESS Materials Science and Engineering B ■■ (2016) ■■–■■

Contents lists available at ScienceDirect

Materials Science and Engineering B j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e b

Synthesis of PtSn nanostructured catalysts supported over TiO2 and Ce-doped TiO2 particles for the electro-oxidation of ethanol

1

Q2

2 3 4

Q1 A.E. Alvarez a, A.N. Gravina b, J.M. Sieben a,*, P.V. Messina b, M.M.E. Duarte a

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

a b

Instituto de Ingeniería Electroquímica y Corrosión (INIEC), CONICET, Universidad Nacional del Sur. Av. Alem 1253, Bahía Blanca B8000CPB, Argentina Departamento de Química, INQUISUR, CONICET, Universidad Nacional del Sur, Av. Alem 1253, Bahía Blanca B8000CPB, Argentina

A R T I C L E

I N F O

Article history: Received 25 February 2016 Received in revised form 2 May 2016 Accepted 19 May 2016 Available online Keywords: Ce doping Titanium dioxide support PtSn nanoparticles Ethanol electro-oxidation

A B S T R A C T

PtSn/TiO2 and PtSn/Ce-doped TiO2 catalysts were synthesized and evaluated for ethanol electrooxidation in acid media. Titanium dioxide and Ce-doped TiO2 nanoparticles were prepared by hydrothermal method followed by calcination at 923 K. Bimetallic PtSn catalysts supported on the oxide materials were synthesized by microwave assisted reduction in ethylene glycol (EG). The structural properties of the resulting materials were evaluated via TEM and XRD, and the compositions were assessed by EDX and ICPAES analysis. PtSn nanoparticles of about 3–4 nm were deposited on TiO2 and Ce–TiO2 particles. It was found that the catalyst composition is scarcely influenced by the cerium content in the mixed oxides while the electrochemical surface area per unit mass decreases upon the incorporation of Ce in the anatase lattice. The electrochemical tests pointed out that the electrocatalytic activity for ethanol oxidation decreases markedly as the Ce content increases. The results indicate that the presence of cerium in the titanium dioxide crystalline network induces local structural and electronic modifications, thereby leading to a reduction of the crystallinity, surface conductivity and the amount of OH species adsorbed on the surface of the oxide support. © 2016 Elsevier B.V. All rights reserved.

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

1. Introduction Direct alcohol fuel cells (DAFCs) have emerged as one of the most versatile devices with high energy efficiency and low emissions for portable, mobile and stationary applications. However, the current bottlenecks for the widespread utilization of this technology are the elevated cost for market introduction, the low long-term stability of the stacks and the low energy efficiency [1,2]. Ethanol is a very suitable fuel for DAFC technology because it can be easily handled, stored and transported by using the existent fuel infrastructure and it provides a high energy density per unit volume. Furthermore, ethanol can be produced on an industrial scale from sunflower, corn and sugarcane plantations and it is the major renewable biofuel obtained from the fermentation of biomass, and hence the net carbon dioxide emissions can be reduced to near zero [3,4]. The low-temperature fuel cell technology available on the market today makes use of carbon powders as the catalyst support in both electrodes. The carbon supports have excellent electrical conductivity and high surface areas. However, they are susceptible to oxidization in the corrosive environment of the polymer electrolyte membrane fuel cell (PEMFC), inducing agglomeration, dissolution

59 60 61 62

* Corresponding author. Tel.: +54 291 4595100; fax: +54 291 4595182. E-mail address: [email protected] (J.M. Sieben).

and migration of platinum particles onto the membrane electrolyte and thus reducing the electrochemical surface area of the electrodes [5,6]. In the last decade, many research groups have been fully focused on the development of Pt-based nanoparticles supported over metal oxides, especially TiO2, due to their high corrosion stability in acidic media [7–10]. Several authors have discussed the role of TiO2 support on the performance of different Pt-based catalysts for methanol and ethanol oxidation in acid media [7,9,11–13] and on the photocatalytic degradation of organic compounds [14,15]. Some authors have indicated that the oxide support promotes the oxidation of the poisoning intermediaries, especially the removal of adsorbed CO, through the bifunctional mechanism [9,13], while others have explained the enhancement in catalytic activity in terms of metal reactivity variation through the change of the interaction energy between the adsorbate and the catalyst surface due to the electronic interactions between the noble metals and TiO2 [7,12]. Apart from that, carbon supported Pt-based catalysts in combination with cerium [16,17] and cerium dioxide [18–20] nanoparticles have been proposed in the last years as electrode materials for DAFCs. In general a synergistic effect between Pt and CeO2 nanoparticles was proposed. It was suggested that CeO2 nanoparticles provide labile OH ads species for the electro-oxidation of the adsorbed intermediary species on the Pt surface at a lower overpotential (bifunctional mechanism).

http://dx.doi.org/10.1016/j.mseb.2016.05.017 0921-5107/© 2016 Elsevier B.V. All rights reserved.

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87

ARTICLE IN PRESS A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

2

88 Taking into account these favorable antecedents, an investiga89 tion was undertaken to study the effect of combining TiO2 and Ce on the electrocatalytic activity of bimetallic PtSn nanoparticles sup90 ported on different cerium–titanium-oxide substrates for the ethanol 91 oxidation reaction (EOR). TiO2 and different Ce-doped TiO2 mate92 rials were synthesized by a hydrothermal route followed by 93 calcination and used as catalyst supports for PtSn nanoparticles 94 synthesized via microwave-assisted reduction in EG. The physico95 chemical characteristics of the samples were determined by TEM 96 and XRD analysis, whereas the catalyst composition was obtained 97 by EDX and ICP-AES. The electrocatalytic activity of the electrodes 98 for the EOR was evaluated via cyclic voltammetry and potentiostatic 99 experiments. Contrarily to the expected, it was found that doping 100 of TiO2 nanoparticles with Ce has a detrimental effect on the 101 102 Q3 electrocatalytic behavior of PtSn/TiO2-xCe catalysts toward ethanol oxidation reaction in acidic media. To the best of our knowledge, 103 this is the first time that a study reports the effect of doping TiO2 104 supports with Ce for ethanol oxidation on Pt-based nanoparticles. 105 106 2. Experimental 107 108 2.1. TiO2 and Ce–TiO2-synthesis 109 110 The catalyst support was obtained through experiments performed 111 on water/CTAB-ButOH/n-heptane microemulsion systems by a single 112 microemulsion method [21,22]. Briefly, a microemulsion “A” (con113 taining CTAB, ButOH, oil and aqueous phases) was mixed with a 114 solution “B” formed by titanium (IV) isopropoxide (Ti[OCH(CH3)2]4) 115 in n-heptane without stirring and left to equilibrate for 20 minutes 116 (sample CT1). For the synthesis of different Ce-doped TiO2 materi117 als different amounts of ceria valerate (Ce(Val) 3 ) were added 118 respectively to microemulsion “A” before mixing with solution “B”. 119 The as-synthesized gels were left for 24 h in an autoclave at 373 K. 120 The obtained materials were filtered, washed with bi-distilled water, 121 dried at room temperature and then calcined for 6 h at 923 K in air 122 flux to completely remove the surfactant impurities. Table 1 lists 123 the specification of all Ce-doped TiO2 materials, with varying ceria 124 loadings (samples CT2–CT4). The complete EDX analysis of the support 125 materials was reported in a previous work [22]. 126 127 2.2. Catalysts synthesis 128 129 The bimetallic PtSn nanoparticles supported on different Ce130 doped TiO2 materials were synthesized via a microwave assisted 131 reduction method in ethylene glycol. in this method ethylene glycol 132 is used as the reaction medium due to its high boiling point and 133 high dielectric loss in addition to its excellent reducing capacity for 134 noble metal salts at high temperatures. Besides, the microwave 135 heating is preferred with respect to conventional heating methods 136 because of the rapid heating of the reaction mixture. The 100.00 mg 137 of Ce–TiO2 was mixed with 50 mL of ethylene glycol (EG, Anedra) 138 and stirred for 15 min. Then, the pH value of the mixture was ad139 justed to 10 by the drop-wise addition of 0.1 M KOH (>85%, Sigma140 Aldrich)/EG and a well-dispersed slurry was obtained by sonication 141 for 45 min. After that, 1.4 mL of 20 mM SnCl2 (Merck) solution in 142 ethanol and 2.19 mL of an aqueous solution of 38.6 mM H2PtCl6·6H2O 143 (Aldrich) were added and sonicated for an hour. That time was 144

2.3. Characterization of materials The characterization of TiO2 and Ce-doped TiO2 samples was performed by field emission scanning electron microscopy (FE-SEM), high resolution transmission microscopy (HR-TEM), X-ray energydispersive (EDX) analysis and X-ray diffraction experiments as previously reported [22]. The surface acidity of the as-prepared support materials was estimated by titration following the procedure developed by Tamele [24]. TiO2 and TiO2–Ce powders (300.0 mg) were dispersed in 40 mL of benzene by sonication for 30 min and stirring for another 30 min. After that, five drops of 0.05 N methyl red (H0 ≤ 4.8) solution in benzene were added to the beaker. Then N-butylamine (0.01 N) was titrated against the powder, and the amount of titer required to effect the color change on the surface of the powder was registered. The procedure was repeated three times taken precautions to handle the powders to ensure that the results were reproducible and comparable between them. The surface acidity of the samples was expressed in milimole per gram of sample. The morphology, size and particle distribution of supported PtSn catalyst were analyzed using transmission electron microscopy (TEM, JEOL 100CX II operated at 200 keV). Whereas, the bulk composition of the electrode materials was determined by energy dispersive X-ray (EDX) coupled to a SEM microscope (JEOL 100) and the analysis was done with the incident electron beam energies ranging from 0.1 to 20 keV. The analysis was carried out in five different areas of each sample. The samples were prepared by placing 20 μL of catalyst ink onto a copper grid and over the surface of a polished glassy carbon rod (GC, 3 mm diameter) for TEM and EDX analysis, respectively. After that, the supports were dried to ensure that the catalyst was firmly adhered to the substrate surface. X-ray diffraction (XRD) patterns of as-synthesized PtSn/TiO2– Ce catalysts were recorded by means of a Rigaku Dmax III C diffractometer with monochromated Cu–Kα radiation source (λ = 0.15418 nm) operated at 40 keV and 30 mA at a scan rate of 0.05° s−1 with 2θ angles in the range of 20–80°. The peak profiles in XRD patterns of the supported catalysts were fitted with the pseudoVoigt function, using non-linear least-squares refinement procedures based on a finite difference Marquardt algorithm. The lattice parameters were estimated using Bragg’s law and the crystallite sizes estimated using Scherrer’s equation. The platinum and tin metal loadings of the different catalysts were determined by ICP-AES (Shimadzu 1000 model III). The platinum loading of the samples expressed per unit of GC geometric area was 50.0 ± 0.3 μg cm−2. 2.4. Electrochemical characterization

145 146 147

sufficient to ensure that the substrate contained the appropriate amount of precursor. Next, the suspension was introduced in the middle of a microwave oven (2450 MHz, 700 W), heated for 90 s, and then cooled naturally at room temperature. Finally, the black dispersion was filtered, washed with ethanol and dried in an oven at 333 K overnight. The nominal atomic ratio of Pt:Sn used in the synthesis was 75:25. The catalyst inks were prepared by the procedure described in our previous paper [23]. The as-prepared catalysts were named in the following way: Pt–Sn/TiO2 (PS-CT1), Pt–Sn/TiO2–Ce 0.71 wt.% (PS-CT2), Pt–Sn/TiO2–Ce 2.09 wt.% (PSCT3) and), Pt–Sn/TiO2–Ce 11.1 wt.% (PS-CT4).

Table 1 Composition of Ce-doped TiO2 materials determined by EDX analysis.

148 149

Catalyst

Ce(Val)3 wt.%

Ce wt.%

Ce at.%

Ti wt.%

Ti1 at.%

O1 wt.%

O1 at.%

150 151 152

CT2 CT3 CT4

0.17 0.86 2.13

0.71 2.09 11.1

0.13 0.39 2.14

56.63 54.31 37.99

30.95 29.41 21.41

41.31 42.84 42.08

67.58 69.47 71.00

Electrochemical experiments were performed in three-electrode glass cells with a PAR 273 potentiostat/galvanostat. A platinum spiral was used as the counter electrode, and a saturated calomel electrode (SCE, +0.241 V vs. RHE) located in a Luggin capillary was used as reference electrode. The potentials mentioned in this work are referred to the SCE reference electrode. The electrodes were prepared by placing 20 μL of catalyst ink on the surface of a mirror-like

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218

ARTICLE IN PRESS A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246

polish glassy carbon (3 mm diameter). Prior to each experience, the solutions were purged with nitrogen in order to remove oxygen gas that might be dissolved. The inert atmosphere was maintained over the solution during the tests. The electrode materials were characterized by cyclic voltammetry (30 cycles) in 0.5 M H2SO4 (96%, CarloErba) solution at a scan rate of 50 mV s−1 and room temperature. The electrocatalytic activity of PtSn/TiO2 and PtSn/TiO2–Ce electrodes was evaluated in 1 M EtOH (99.9%, J.T. Baker) +0.5 M H2SO4 aqueous solution by cyclic voltammetry (CV) at a scan rate of 50 mV s−1. Chronoamperometry experiments were performed applying pulses from an initial potential of 0 V for 15 min. Catalytic activity is displayed in terms of current per mass of Pt. The electrochemical surface area (ESA) of each electrode was estimated by copper underpotential deposition (Cu-UPD) method by following the experimental procedure described in Ref. [25]. Briefly, the Cu-UPD experiments were carried out in 0.1 M H2SO4 and 2 mM CuSO4 solution. the working electrodes were polarized at 0.059 V for 300 s to form a monolayer of copper on the catalyst surface. A linear voltammetric scan with a scan rate of 10 mV s−1 was then performed between 0.059 and 0.8 V to remove the adsorbed copper monolayer. The integration of the peak area corresponding to the Cu-UPD stripping was used to determine the electroactive surface area, with the assumption of an adsorption ratio of a single Cu atom to each surface metal atom and a monolayer charge of 420 μC cm−2. Furthermore, the electrochemical surface area per unit mass (ECSA) was calculated from Cu-UPD results and ICP-AES analysis as follows:

ECSA =

ESA wPt

(1)

3

282

Fig. 1. XRD diffraction patterns of the as-prepared materials.

283 consequence there was a clear alteration of the electronic and morphological properties of materials [22]. On the other hand, surface acidity was determined from N-butylamine titration method with the indicator methyl red. The surface acidity values calculated for CT1, CT2, CT3 and CT4 were 0.0408 mmol g−1, 0.0340 mmol g−1, 0.0314 mmol g−1 and 0.0254 mmol g−1 respectively. Thus, the surface acidity of the samples decreases with Ce content increase. This result may be associated with a diminution in the amount of surface hydroxyl groups.

247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281

3. Results and discussion 3.1. Characterization OF TiO2 and Ce–TiO2 The nanostructured materials (NMs) used in this study were created via a bottom-up microemulsion-mediated hydrothermal synthesis, which allows obtaining nanocrystalline Ce–TiO2 nanoparticles (NPs) at 373 K [22]. The complete characterization of the different materials was performed in a previous work [22]; some relevant considerations are summarized below. HR-TEM micrographies revealed that the particle sizes of the different oxide materials fall in the range of 10 to 20 nm accordingly to the Ce:Ti ratio increment. Furthermore, agglomerates of c.a. 600 nm consisting of smaller particles were also observed. It was demonstrated [20] that the defects introduced to TiO2 crystal lattice because of the Ce-atom incorporation stabilized the anatase polymorph at 650 °C and also induced, in the property synthesis conditions, a hollow, stable and closed structure similar to those observed in the inorganic fullerene-like (IF) assemblies [26]. Results from X-ray diffraction (XRD) patterns, Raman and fluorescence spectroscopy measurements confirmed that the Ce-doped materials exhibit a distorted tetragonal unit cell [22]. Moreover, a decrease in the anatase crystal organization (loss of crystallinity) was detected when the molar ratio of Ce3+:Ti4+ increased between 0 and 0.3. In despite of Ce-atom presence, detected by EDX microanalysis [22], no traces of diffraction peaks due to cerium oxide were identified in all tested samples, confirming the incorporation of Ce atoms into the anatase matrix. The anatase doping with Ce generates a Raman blueshift displacement, which is related to the lattice contraction of anatase crystal [22]. In summary through the employ of different techniques it was confirmed that Ce atoms integrated in TiO2 crystalline network stabilizing TiO 2 anatase polymorph and generating a significant distortion of the crystal lattice; interfering with vibration, photoluminescence and dimensions of TiO 2 anatase crystals. As a

3.2. Characterization of PtSn/TiO2 and PtSn/Ce-doped TiO2 materials The X-ray diffractograms of the different electrode materials presented three reflection peaks at Bragg angles of c.a. 39.8, 46.3 and 67.5° (Fig. 1). These peaks correspond to the diffraction from the face centered cubic (111), (200) and (220) planes of platinum, respectively. However, the diffraction peaks were slightly shifted toward lower Bragg angles with respect to the reflections of pure Pt. Moreover, the lattice parameters for all the as-synthesized catalysts are larger than that for Pt (3.922 Å [27]). This observation is consistent with the formation of a solid solution between Pt and Sn atoms. The presence of Sn atoms in the Pt fcc structure causes a lattice expansion, which in turn leads to an increment in the inter-planar distance. The lattice spacing (dhkl) of Pt (111) crystal plane is 2.26 Å [28], while the d-spacing in the bimetallic PtSn catalysts is between 2.28 and 2.30 Å. Furthermore, no diffraction peaks ascribed to pure Sn or its oxides were detected in all XRD patterns. The crystallite size of the PtSn alloy nanoparticles was estimated from the XRD (111) and (220) peaks by using the Scherrer’s equation

dc =

0.94 l Ka 1 B2q cosq B

284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314

(2)

where λKα1 is the wavelength of X-ray, θB is the angle of the peak, and B(2θ) is the full width at half-maximum (FWHM) of the peak broadening in radians and the value 0.94 comes from considering cubo-octahedral shape of crystallites. The crystallite size of the different bimetallic catalysts was found to be between 3.0 and 4.0 nm (Table 2). As previously reported [22], the X-ray powder diffractograms of the as-prepared catalysts also show the gradual loss of crystallinity of the TiO2 nanoparticles with the increase in Ce content. The atomic composition of the bimetallic catalysts was determined by EDX spectroscopy (see Supporting Information Fig. SI1)

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

315 316 317 318 319 320 321 322 323 324 325 326

ARTICLE IN PRESS A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

4

327 328

Table 2 Composition and characteristic parameters of the as-prepared catalysts.

329 330

Catalyst

331 332 333 334

PS-CT1 PS-CT2 PS-CT3 PS-CT4

335 336 337 338

a b c d

From From From From

aPt

aSn

aPt

aSn

bd

wt.%

wt.%

at.%

at.%

nm

nm

nm

dECSA m2g−1Pt

87.8 90.2 89.8 88.9

12.2 9.8 10.2 11.1

81.8 84.8 84.3 82.9

18.2 15.2 15.7 17.1

4.6 4.1 3.9 3.7

4.0 3.9 3.5 3.0

0.3985 0.3994 0.3949 0.3956

6.5 4.6 2.5 2.7

p

cd

c

ca

fcc

EDX. TEM. XRD (Debye–Scherrer equation). Cu-UPD and ICP-AES analysis.

339 340 341 342 343 344 345 346 347 348 349 350 351

and compiled in Table 2. The Sn atomic fraction in the as-prepared bimetallic electrodes is lower than the nominal composition. This discrepancy could be associated with the considerable difference in reduction potential between Pt4+/Pt and Sn2+/Sn redox couples. Additionally, the presence of NaOH (solution with pH 10) could inhibit, in some extent, the reduction of Sn ions. A similar discrepancy between the nominal molar ratios of Pt to Sn and the actual content of Pt and Sn in the catalysts was reported by Hsieh et al. [29]. Fig. 2 shows TEM micrographies of PtSn/TiO2 and PtSn/Cedoped TiO2 catalysts. The particle size distribution was determined by measuring the diameter of about 150 particles (only “welldefined”, not aggregated nanoparticles were taken into account)

using the ImageJ image processing and analysis software. The insets in Fig. 2 show the histograms of particle size distribution of the asprepared catalysts. The microwave-assisted EG reduction method leads to the formation of nanoparticles with spherical shape distributed on the surface of TiO2 and Ce–TiO2 mixed oxides with sizes between 3.7 and 4.6 nm (Table 2). It can be seen that the catalyst nanoparticles are regularly distributed over the supports surface. Moreover, agglomerates of about 10–20 nm in diameter are also observed in Fig. 2. PS-CT1 catalyst exhibits the presence of nanoparticles with a mean particle size of 4.6 nm and a median of 4.5 nm. Whereas, PS-CT2, PS-CT3 and PS-CT4 materials have average particle diameters of 4.1, 3.9 and 3.7 nm with median values of 4.0, 3.9 and 3.4 nm, respectively. Taking into account that in general the crystallite diameter calculated from XRD data can be smaller than the average particle size [30], TEM results are in good agreement with those estimated from XRD analysis. From Table 2 it can also be observed that the electroactive surface area per unit mass of the catalyst (ECSA) depends on the nature of the support, and it follows the order PS-CT1 > PS-CT2 > PS-CT3 ~ PSCT4. The small values of the electrochemical surface area can probably be attributed to PtSn particle agglomeration. Similar low values of ECSA were reported by other authors [31,32]. An alternative explanation for this behavior could be done in terms of crystallinity and local structure modifications in the mixed oxide materials. As indicated previously, Ce atoms integrate in TiO2

352

353 Q4 Fig. 2. TEM images and histogram of particle size distribution of the as-synthesized electrocatalysts used in this work: PS-CT1 (a), PS-CT2 (b), PS-CT3 (c) and PS-CT4 (d).

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378

ARTICLE IN PRESS A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398

crystalline network generating a significant distortion of the crystal lattice. The results published in the literature indicate that doping of TiO2 with Ce causes grain size modification, loss of crystallinity, lattice expansion and lattice strain [22,33,34]. As result of these structural modifications, the surface conductivity decreases with the increase in Ce content. Fu et al. [34] found that the doping of TiO2 with Ce causes a decrease of Ti 3d states and an increase of Ce 4f states in the bottom of conduction band, reducing the dielectric constant and conductivity of the material. On the other hand, Gafoor et al. [35] observed that the presence of Sm3+ ions in Sm-doped TiO2 reduce the number of charge carriers which are intrawell hopping, hence decreasing conductivity. A similar behavior could be expected for Ce-doped TiO2 materials. It is probable that the nanoparticles deposited on these regions of low surface conductivity do not contribute to the electroactive area determined by the Cu-UPD technique. This would explain why the electroactive area of the electrodes is low although the particle size falls between 3 and 4 nm. Fig. 3 shows the cyclic voltammetry curves of the as-prepared bimetallic PtSn/Ce–TiO2 catalysts in 0.5 M H2SO4 solution at a sweep

399

400 401

Fig. 3. Stabilized cyclic voltammograms for TiO2 and Ce-doped TiO2 supported PtSn catalysts in 0.5 M H2SO4 solution at room temperature. ν = 50 mV s−1.

5

rate of 50 mV s−1. It can be seen that the voltammetric profiles resemble Pt-based catalysts. However, the voltammetric profiles do not exhibit well-defined hydrogen peaks due to the structural changes caused by the formation of a solid solution between Pt and Sn atoms [28,36]. This behavior is usually observed in carbon supported catalysts decorated with Pt-based nanometric particles [37,38]. On the other hand, a pair of not very well defined redox peaks associated with the presence of tin oxide or hydroxy species was also observed in the CVs. The oxidation peak appears at 0.55 V while the complementary cathodic peak emerges at about 0.30 V. These peaks are attributed to the adsorption/desorption of oxygenated species from the dissociation of water onto Sn atoms [39–41].

418 3.3. Ethanol electro-oxidation 3.3.1. Potentiodynamic experiments Fig. 4 shows the cyclic voltammograms recorded for EtOH oxidation in acid media at the as-synthesized PtSn catalysts supported over TiO2 and Ce-doped TiO2 powders. The onset of ethanol oxidation takes place above 0.2 V for the catalyst PS-CT1, while the onset potential shifts to 0.4 V, 0.47 V and 0.50 V for PS-CT2, PS-CT3 and PS-CT4 respectively. The forward scan peak of ethanol electrooxidation appears at a potential of 0.64 V for PS-CT1, while the peak potential for the PtSn catalysts supported on Ce-doped TiO2 powders is displaced at about 0.71 V. This reveals that ethanol oxidation is eased and requires less overpotential with PtSn/TiO2 electrode, whether because of the formation of −OHads species on both Sn [42] and Ti [11] atoms originating in the water dissociation or because of an electronic effect, to be precise, electronic interactions between Pt and TiO2 due to the hypo d-electron character of TiO2 and the hyper d-electron character of Pt [7,11,12]. This effect facilitates the oxidation of COads and other intermediates species to CO2, limiting catalysts poisoning [43]. Some authors indicated that the bifunctional mechanism is the responsible for the synergistic effect of TiO2 support [13], while Chen et al. claimed that the bi-functional effect, in combination with geometric effects, is responsible for the improved activity of Pt catalyst supported over TiO2 [9]. Meanwhile,

402

403 404

405 406 407 408 409 410 411 412 413 414 415 416 417

Fig. 4. Stabilized cyclic voltammograms for the different electrode materials in 1 M CH3CH2OH/0.5 M H2SO4 solution at room temperature. The sweep rate was 50 mV s−1 and the arrows indicate the scan direction.

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442

ARTICLE IN PRESS 6

443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465

A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

Hasa et al. [8] indicated that the improved activity of Pt/TiO2 is mainly attributed to geometric effects, although they do not discard the electronic effects. Below 0.65 V, the CV profiles point out that the current density for ethanol oxidation is decreased in the order of PS-CT1 > PSCT2 > PS-CT3 > PS-CT4. In this potential range the activity of PSCT1 is 2, 8 and 13 times higher than those of PS-CT2, PS-CT3 and PS-CT4, respectively. However, the peak current density of PS-CT1 is only slightly higher than that of PS-CT2, but the maximum in the Ce-doped TiO2 support is reached at a more positive potential. According to the CV results, the Ce doping in TiO2 has a detrimental effect in the performance of the bimetallic PtSn systems. The diminution of the surface conductivity as a result of local structure modifications and the gradual loss of crystallinity in the mixed oxide particles are probably the main reasons for this unusual behavior. These results are clearly supported by the XRD analysis of the samples (Fig. 1), given that the crystallinity of the supports decreases from CT1 to CT4. Moreover, the reduction in the amount of −OH groups on the surface of Ce-doped TiO2 in comparison to undoped TiO2 powder [44] can contribute to this behavior. Apart from that, the reduction of the specific surface area with the increase in Ce content (geometric effect) also influence the catalytic performance of the electrodes.

466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487

3.3.2. Potentiostatic experiments The chronoamperometric response of the as-prepared catalysts is shown in Fig. 5. It can be observed that the resulting steadystate activities resemble those obtained in the potentiodynamic experiments, but with lower mass current densities due to the accumulation of adsorbed intermediaries onto the active sites of the platinum nanoparticles [28,45]. The PS-CT1 electrode gives a mass activity of 19 μA μgPt−1 (274.3 μA cmPt−2), while PS-CT2, PS-CT3 and PS-CT4 have current densities of 14, 9.2 and 4.5 μA μgPt−1 (188, 83 and 47 μA cmPt−2) at 0.5 V for 900 s, respectively. For the sake of comparison, the performance of the most active as-prepared electrode was contrasted to that of a commercial catalyst. PS-CT1 catalyst presented a higher specific activity in the EOR compared to a commercial Pt 0.51 Ru 0.49 /C material (Aldrich, 20 wt.% Pt and 10 wt.% Ru, ESA = 60.3 m2 g−1) [23]. The current density per unit of active surface area of PS-CT1 was 50% higher than that of Pt-Ru/C. Moreover, the catalytic activity of PtSn/TiO2 electrode was compared with other catalysts supported on titanium oxide materials reported in the literature (Table 3). It can be observed that the catalytic performance obtained for the Pt0.82Sn0.18/TiO2 electrode is promising when compared to catalysts, with higher Pt loading, supported over titanium

Table 3 Comparison of the activities for ethanol oxidation of Pt-based catalysts supported on titanium oxide materials as synthesized in this work and some catalysts reported in the literature.

489 490

Fig. 5. Chronoamperometry curves of the EOR recorded at 0.5 V on PS-CT1, PSCT2, PS-CT3 and PS-CT4 in 1 M CH3CH2OH/0.5 M H2SO4 at room temperature.

dp nm

Eonset V

Ep V

jp mA cm−2

495 496

Pt0.82Sn0.18/TiO2

17.5

4.6

0.20

0.65

2.00

Pt/TiO2/MWCNTs Pt3Sn/TiCN TiO2/Pt-C Pt/TiO2 nanotubes Pt/TiO2 nanorods Pt/Na2Ti3O7 nanowires

20.0 20.0 20.0 – – 35.6

3.7 4.7 3.2 6.0 3.0 8.0

0.50 0.20 0.53 0.30 0.34 0.31

0.81 0.64 0.80 0.76 0.76 0.76

1.16 0.90 0.98 1.21 1.81 0.14

497 498 499 500 501 502 503 504 505

oxide particles, nanowires, nanorods, nanotubes and TiO2 coated carbon nanotubes. The turnover number (TON) is a central parameter that can be used to compare catalysts with different compositions, structures, supports, etc. [52]. This parameter represents the number of molecules transformed per active surface site per second at a given potential and experimental condition [53]. Several authors observed that the main product of ethanol oxidation in acid media is acetic acid, instead of CO2 [54–57]. Therefore, in the TON calculations we assumed that the electro-oxidation of ethanol leads to involve the transference of four electrons

506 507 508 509 510 511 512 513 514 515 516

i NA ⎛ molecules ⎞ = TON ⎜ ⎝ site s ⎟⎠ n F 1.3 × 1015

517 (3) 518

where i is the steady-state current density per unit of real surface area (A cm−2), n is the number of electrons produced by the partial oxidation of one ethanol molecule to acetic acid (4 e−), 1.30 × 1015 is the mean surface atomic density of the Pt-based catalysts (active sites per 1 cm2 of the real surface area [53]). The values obtained for the TON were, respectively, 0.33 molecules site−1 s−1 for PS-CT1, 0.23 molecules site−1 s−1 for PS-CT2, 0.10 molecules site−1 s−1 for PSCT3 and 0.06 molecules site−1 s−1 for PS-CT4 at 0.5 V. The results indicate that the as-synthesized PtSn/TiO2 electrode can catalyze the oxidation of the alcohol more efficiently in a second per Pt active site than the other electrodes. That is, the TON value on PS-CT1 is between 1.4 and 6 times higher than on the other electrodes. In addition, the poisoning rate (δ) of the as-synthesized catalysts in the electro-oxidation of ethanol can be determined using the method reported by Jiang and Kucernak [58]

δ=

488

wt.% Pt

Catalyst This work: (PS-CT1) [46] [47] [48] [49] [50] [51]

491 492 493 494

100 ⎛ dI ⎞ ⎜ ⎟ I0 ⎝ dt ⎠ t >500 s

519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534

(4)

where (dI/dt)>500 s is the slope of the linear portion of the current decay (A s−1), and I0 is the current at the start of polarization back extrapolated from the linear current decay (A). The poisoning rate was 0.013% s−1 for PS-CT1, 0.017% s−1 for PS-CT2, 0.019% s−1 for PSCT3 and 0.027% s−1 for PS-CT4. As it can be seen, the long-term poisoning rate of the as-synthesized catalysts increases with the increase of cerium content in the titanium dioxide support. The electrochemical results illustrate the negative effect that the Ce doping in TiO2 support can have on the electrocatalytic activity of bimetallic PtSn nanoparticles for the EOR process at room temperature. In other words, the introduction of Ce in TiO2 reduces the mass activity, the turnover number and the tolerance to poisoning of the bimetallic nanoparticles. The incorporation of cerium atoms to the anatase structure induces local structure distortion and electronic modifications on anatase crystal lattice, which in turns leads to a diminution of the crystallinity and surface conductivity,

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551

ARTICLE IN PRESS A.E. Alvarez et al. / Materials Science and Engineering B ■■ (2016) ■■–■■

552 553 554 555 556 557

a higher extent of particle agglomeration and a lower amount of OH species adsorbed on the support. Current studies in course are mainly focused on the evaluation of the effect of other dopant metal ions in TiO2. Furthermore, the optimization of the method used to prepare the bimetallic PtSn catalysts is also addressed.

558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584

4. Conclusions In this work the effect of Ce doping in TiO2 support on the electrocatalytic activity of bimetallic PtSn nanoparticles for the EOR process was investigated. TiO2 and Ce-doped TiO2 nanoparticles synthesized by hydrothermal route followed by calcination were used as supports for bimetallic PtSn catalysts prepared by microwaveassisted method using EG. PtSn nanoparticles in the range of 3–4 nm were deposited over the TiO2 and Ce–TiO2 particles. It was found that the catalyst composition is scarcely affected by the cerium content in the mixed oxides, being the tin content in the different samples of about 16 at.%. Moreover, it was observed that the electroactive surface area of the electrodes decreases with the increase in Ce content. The electrochemical experiments point out that the activity for EOR is decreased in the order of PS-CT1 > PS-CT2 > PS-CT3 > PSCT4. The superior response of PtSn/TiO2 in terms of onset potential, mass activity, turnover number and poisoning tolerance can be explained by the fact that the presence of cerium in the titanium dioxide crystalline network induces local structure and electronic modifications. This results in a reduction of the crystallinity, surface conductivity and the amount of OH species adsorbed on the surface of the mixed oxides. Furthermore, the structural and electronic modifications also lead to the reduction in the number of available surface Pt sites for the electrochemical reaction due to a higher agglomeration degree.

585 586 587 588 589 590 591 592 593 594

Acknowledgments The authors acknowledge Universidad Nacional del Sur (24/ Q064), Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina (CONICET, PIP-11220130100100CO) and Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT, PICT-2370) for their financial support. ANG has doctoral a fellowship of CONICET. JMS and PVM are adjunct and independent researchers of CONICET.

595 596 597 598 599

Appendix: Supplementary material Supplementary data to this article can be found online at doi:10.1016/j.mseb.2016.05.017.

600 601 602 603 604 605 606 607 608 609 610 611 612 613 614

References [1] A. Brouzgoua, S.Q. Song, P. Tsiakaras, Appl. Catal. B 127 (2012) 371–388. [2] T. Iwasita, W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells Fundamentals, Technology and Applications, Vol. 2: Fuel Cells Electrocatalysis, John Wiley & Sons, New York, 2003, pp. 603–624. [3] L.A. Rodríguez, M.E. Toro, F. Vazquez, M.L. Correa-Daneri, S.C. Gouiric, M.D. Vallejo, Int. J. Hydrogen Energy 35 (2010) 5914–5917. [4] E.C. Petrou, C.P. Pappis, ACS Sustain. Chem. Eng. 2 (2014) 2036–2041. [5] H.-S. Oh, J.-H. Lee, H. Kim, Int. J. Hydrogen Energy 37 (2012) 10844–10849. [6] K. Schlögl, K.J.J. Mayrhofer, M. Hanzlik, M. Arenz, J. Electroanal. Chem. 662 (2011) 355–360. [7] S.V. Selvaganesh, G. Selvarani, P. Sridhar, S. Pitchumani, A.K. Shukla, J. Electrochem. Soc. 157 (2010) B1000–B1007.

7

[8] B. Hasa, E. Kalamaras, E.I. Papaioannou, L. Sygellou, A. Katsaounis, Int. J. Hydrogen Energy 38 (2013) 15395–15404. [9] C.S. Chen, F.-M. Pan, Appl. Catal. B 91 (2009) 663–669. ˇ . Lacˇnjevac, B.M. Babic´, P. Ercius, V.R. Radmilovic´, N.V. [10] M.D. Obradovic´, U.C Krstajic´, et al., Appl. Catal. B 170–171 (2015) 144–152. [11] G. Selvarani, S. Maheswari, P. Sridhar, S. Pitchumani, A.K. Shukla, J. Electrochem. Soc. 156 (2009) B1354–B1360. [12] N. Rajalakshmi, N. Lakshmi, K.S. Dhathathreyan, Int. J. Hydrogen Energy 33 (2008) 7521–7526. [13] H. Song, X. Qiu, F. Li, Electrochim. Acta 53 (2008) 3708–3713. [14] A. Manikandan, E. Hema, M. Durka, M. Amutha Selvi, T. Alagesan, S. Arul Antony, J. Inorg. Organomet. Polymer. 25 (2015) 804–815. [15] V. Umapathy, A. Manikandan, P. Ramu, S. Arul Antony, P. Neeraja, J. Nanosci. Nanotechno. 16 (2016) 987–993. [16] R.F.B. De Souza, J.C.M. Silva, M.H.M.T. Assumpção, A.O. Neto, M.C. Santos, Electrochim. Acta 117 (2014) 292–298. [17] J.M. Jacob, P.G. Corradini, E. Antolini, N. Abe Santos, J. Perez, Appl. Catal. B 165 (2015) 176–184. [18] D.R. Ou, T. Mori, K. Fugane, H. Togasaki, F. Ye, J. Drennan, J. Phys. Chem. C 115 (2011) 19239–19245. [19] S.K. Meher, G.R. Rao, ACS Catal. 2 (2012) 2795–2809. [20] L. Yu, J. Xi, Int. J. Hydrogen Energy 37 (2012) 15938–15947. [21] G. Palazzo, F. Lopez, M. Giustini, G. Colafemmina, A. Ceglie, J. Phys. Chem. B 107 (2003) 1924–1931. [22] N. Gravina, J.M. Ruso, D.A. Mbeh, L.H. Yahia, Y. Merhi, J. Sartuquia, et al., RSC Adv. 5 (2015) 8077–8087. [23] J.M. Sieben, A.E. Alvarez, V. Comignani, M.M.E. Duarte, Int. J. Hydrogen Energy 39 (2014) 11547–11556. [24] M.W. Tamele, Discuss. Faraday Soc. 8 (1950) 270–279. [25] J.M. Sieben, M.M.E. Duarte, C.E. Mayer, J. Appl. Electrochem. 38 (2008) 483–490. [26] R. Tenne, L. Margulis, M. Genut, G. Hodes, Nature 360 (1992) 444–446. [27] J.M. Sieben, M.M.E. Duarte, C.E. Mayer, ChemCatChem 2 (2010) 182–189. [28] J.M. Sieben, M.M.E. Duarte, Int. J. Hydrogen Energy 36 (2011) 3313–3321. [29] C.-T. Hsieh, Y.-S. Chang, K.-M. Yin, J. Phys. Chem. C 117 (2013) 15478–15486. [30] A. Pozio, M. De Francesco, A. Cemmi, F. Cardellini, L. Giorgi, J. Power Sources 105 (2002) 13–19. [31] B. Ruiz-Camacho, H.H. Rodríguez Santoyo, J.M. Medina-Flores, O. Álvarez-Martínez, Electrochim. Acta 120 (2014) 344–349. [32] Y.-H. Pai, C.-T. Tsai, S.-Y. Fang, J. Power Sources 223 (2013) 107–113. [33] A.M.T. Silva, C.G. Silva, G. Dražic´, J.L. Faria, Catal. Today 144 (2009) 13–18. [34] C. Fu, T. Li, J. Qi, J. Pan, S. Chen, C. Cheng, Chem. Phys. Lett. 494 (2010) 117–122. [35] A.K.A. Gafoor, J. Thomas, M.M. Musthafa, P.P. Pradyumnan, J. Electron. Mater. 40 (2011) 2152–2158. [36] J.H. Kim, S.M. Choi, S.H. Nam, M.H. Seo, S.H. Choi, W.B. Kim, Appl. Catal. B 82 (2008) 89–102. [37] E.M. Cunha, J. Ribeiro, K.B. Kokoh, A.R. de Andrade, Int. J. Hydrogen Energy 36 (2011) 11034–11042. [38] J.M. Sieben, M.M.E. Duarte, Int. J. Hydrogen Energy 37 (2012) 9941–9947. [39] D.-H. Lim, D.-H. Choi, W.-D. Lee, H.-I. Lee, Appl. Catal. B 89 (2009) 484–493. [40] E.V. Spinace, M. Linardi, A. Oliveira Neto, Electrochem. Commun. 7 (2005) 365–369. [41] M. Arenz, V. Stamenkovic, B.B. Blizanac, K.J. Mayrhofer, N.M. Markkovic, P.N. Ross, J. Catal. 232 (2005) 402–410. [42] F.C. Simões, D.M. dos Anjos, F. Vigier, J.-M. Léger, F. Hahn, C. Coutanceau, et al., J. Power Sources 167 (2007) 1–10. [43] K. Bergamaski, E.R. Gonzalez, F.C. Nart, Electrochim. Acta 53 (2008) 4396–4406. [44] K. Nagaveni, M.S. Hegde, G. Madras, J. Phys. Chem. B 108 (2004) 20204–20212. [45] A. Hamnett, A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiments, and Applications, Marcel Dekker, New York, 1999, pp. 843–883. [46] H. Song, X. Qiu, F. Li, W. Zhu, L. Chen, Electrochem. Commun. 9 (2007) 1416– 1421. [47] M. Roca-Ayats, G. García, M. Soler-Vicedo, E. Pastor, M.J. Lazaro, M.V. Martínez-Huerta, Int. J. Hydrogen Energy 40 (2015) 14519–14528. [48] H. Song, X. Qiu, X. Li, F. Li, W. Zhu, L. Chen, J. Power Sources 170 (2007) 50–54. [49] C. Cao, C. Hu, J. Tian, W. Shen, S. Wang, H. Liu, J. Electrochem. Soc. 160 (2013) H793–H799. [50] X. He, C. Hu, J. Power Sources 196 (2011) 3119–3123. [51] X. He, C. Hu, H. Liu, Catal. Commun. 12 (2010) 100–104. [52] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. 56 (2005) 9–35. [53] W. Chrzanowaki, A. Wieckowski, A. Wieckowski (Ed.), Interfacial Electrochemistry: Theory, Experiment, Applications, Marcel Dekker, New York, 1999, pp. 937–954. [54] I. Kim, O.H. Han, S.A. Chae, Y. Paik, S.H. Kwon, K.S. Lee, et al., Angew. Chem. Int. Ed. Engl. 50 (2011) 2270–2274. [55] S. Rousseau, C. Coutanceau, C. Lamy, J.M. Léger, J. Power Sources 158 (2006) 18–24. [56] F. Vigier, C. Coutanceau, F. Hahn, E.M. Belgsir, C. Lamy, J. Electroanal. Chem. 563 (2004) 81–89. [57] N. Nakagawa, Y. Kaneda, M. Wagatsuma, T. Tsujiguchi, J. Power Sources 199 (2012) 103–109. [58] J. Jiang, A. Kucernak, J. Electroanal. Chem. 543 (2003) 187–199.

Please cite this article in press as: A.E. Alvarez, A.N. Gravina, J.M. Sieben, P.V. Messina, M.M.E. Duarte, Synthesis of PtSn nanostructured catalysts supported over TiO2 and Cedoped TiO2 particles for the electro-oxidation of ethanol, Materials Science & Engineering B (2016), doi: 10.1016/j.mseb.2016.05.017

615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695