Oscillatory Electro-oxidation of Methanol on Nanoarchitectured Pt pc

Subscriber access provided by CMU Libraries - http://library.cmich.edu

Article

Oscillatory Electro-oxidation of Methanol on Nanoarchitectured Pt pc /Rh/Pt Metallic Multilayer Ernesto Chaves Pereira ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs501652u • Publication Date (Web): 29 Dec 2014 Downloaded from http://pubs.acs.org on January 2, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

ACS Catalysis

This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies.

Oscillatory Electro-oxidation of Methanol on Nanoarchitectured Pt pc /Rh/Pt Metallic Multilayer

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

ACS Catalysis cs-2014-01652u.R1 Article 22-Dec-2014 Nagao, Raphael; IQSC/USP, Physical Chemistry Freitas, Renato; UFSCar, Chemistry; University of Mato Grosso, Chemistry Silva, Camila; UFSCar, Chemistry Varela, Hamilton; Instituto de Quimica de Sao Carlos, USP, Physical Chemistry; Fritz Haber Institute of the Max Planck Society, Physical Chemistry Pereira, Ernesto; Universidade Federal de Sï¿½o Carlos, Chemistry

ACS Paragon Plus Environment

Page 1 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1 2 3 4 5 6 7 8

Oscillatory Electro-oxidation of Methanol on Nanoarchitectured Ptpc/Rh/Pt Metallic Multilayer

9 10 11 12 13

Raphael Nagao,1,2,† Renato Freitas,1,3 Camila Silva,1 Hamilton Varela,2,4 Ernesto Chaves

14

Pereira1,* 1

15

Department of Chemistry, Federal University of São Carlos, POBox 676, 13565-905, São Carlos, SP, Brazil.

16 2

17

Institute of Chemistry of São Carlos, University of São Paulo, POBox 780, 13560-970, São Carlos, SP, Brazil.

18 3

19

Department of Chemistry, Federal Univeristy of Mato Grosso, POBox 78060-900, Cuiaba, MT, Brazil.

20 4

21

Ertl Center for Electrochemistry and Ctalysis, GIST,

Cheomdan-gwagiro 261, Buk-gu, Gwangju 500-712, South Korea

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

*corresponding authors: [email protected] (ECP). †

current address: Department of Chemistry, Saint Louis University, 3501 Laclede Ave., St. Louis, MO 63103, USA.

1 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68

Page 2 of 26

Abstract The oscillatory electro-oxidation of methanol was studied on polycrystalline Ptpc and Ptpc/Rh2.0/Pt1.0 metallic multilayers. The surfaces investigated consisted of 1.0 Pt outlayer surface deposited onto 2.0 Rh intralayers beneath a Pt outlayer and over the polycrystalline Ptpc substrate. Besides experimental studies, numerical simulations were performed using a dimensionless kinetic model for the electro-oxidation of methanol in order to provide a better understanding of the role played by the nanostructured metallic multilayer electrode in the electrocatalytic activity. A comparable electrochemical behavior found for the cyclic voltammetries in blank acidic media was observed in both electrodes. Remarkably, an increase of 90% in the peak current density around 0.88 V vs. RHE in the electro-oxidation of methanol appeared when Ptpc/Rh2.0/Pt1.0 was utilized. The numerical simulations suggested that this increase in the electrocatalytical activity for the metallic multilayers electrode is due to the prevention of carbon monoxide adsorption on the surface, and a consequent increase in the production of carbon dioxide from the direct pathway. Indeed, a decrease in the reaction rate constant of carbon monoxide formation resulted in an increase of the current density associated to CO2 formation in the potenciodynamic sweep, besides the decrease in amplitude and frequency of the oscillatory time-series. As the rate of carbon monoxide adsorption is suppressed by the presence of the metallic multilayers, the intrinsic drift usually found in the oscillatory electro-oxidation of methanol was enhanced and oscillations ceased earlier. Overall, the combination of electrochemical experiments and numerical simulations suggests that carbon monoxide acts as a poisoning species instead of a reaction intermediate in the electro-oxidation of methanol. keywords: oscillations, metallic multilayer, methanol, electrocatalysis.

69 70 71 72 73 74 75 76 77 78 79 80 81 82 83


Page 3 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

84

1. Introduction

85

The search for more selective and efficient catalysts for complete electro-

86

oxidation of methanol has been an active field of research in past decades due the

87

possibility of use fuel cells as a green and renewable energy source.1,2 Several

88

methodologies were employed in order to enhance the catalytic activity by

89

modifications in their structural properties, and among them, the use of alloys,

90

electrodeposits and nanoparticles have shown to be quite promising in this

91

direction.3,4,5,6 Metallic multilayers are a new class of advanced engineered materials

92

that are known to exhibited high structural stability, mechanical strength, high ductility,

93

toughness and resistance to fracture and fatigue.7 Moreover, it has been widely explored

94

mainly due its magnetic properties.8,9 Under the perspective of the electrocatalysis,

95

Pereira and co-workers10,11,12,13,14,15,16 have extensively studied the preparation and

96

characterization of metallic multilayers deposited onto polycrystalline platinum (Ptpc) as

97

catalysts for the electro-oxidation of small organic molecules.

98

For instance, the authors observed that, although Ptpc/Metal/Pt (Metal = Rh, Ru,

99

Bi and Ir) electrode has a Pt outlayer surface, the electrocatalytic activity for such

100

metallic multilayers is higher than that observed for Ptpc electrode. Oliveira et al.10

101

experimentally observed increases in the peak current density between about two and

102

four times for the electro-oxidation of methanol, ethanol, formaldehyde and formic acid

103

on Ptpc/Rh/Pt with respect to that on Ptpc. Interestingly, a high selective oxidation of

104

ethanol to carbon dioxide was obtained when the reaction was carried out on Ptpc/Ir/Pt

105

nanostructured electrode,14 as reflected by the absence of the infrared bands relative to

106

acetic acid and acetaldehyde molecules.

107

In despite of much effort devoted to this topic, the role played by the metallic

108

multilayer electrodes in the improvement of the electrocatalytic activity is not

109

completely understood. Herein we provide some mechanistic evidences based on a

110

combination of experiments, modeling and numerical simulations, under both

111

conventional and oscillatory regimes, that the increase in the current density observed in

112

the electro-oxidation of methanol over the metallic multilayer electrode is mainly

113

caused by the production of carbon dioxide from the oxidative decomposition of the

114

active intermediate, i.e. direct pathway. This argument is supported by the highly

115

selective electronic effect caused by nanostructured metallic multilayer in preventing

116

the adsorption of carbon monoxide. Potential oscillations registered under galvanostatic


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

117

control were utilized as a mechanistic probe, corroborating the scenario of suppression

118

of adsorbed carbon monoxide.

119 120

2. Experimental Procedure

121

Solutions were prepared using ultrapure water (Millipore, 18.2 MΩ cm), HClO4

122

(Sigma-Aldrich, 70%), H2SO4 (Mallinckrodt, 99.8%), RhCl3.3H2O (Alpha-Aesar,

123

99.9%), H2PtCl6.6H2O (Sigma-Aldrich, 99.9%) and H3COH (J. T. Baker, 99.9%).The

124

electrochemical cell, all glassware and electrodes were cleaned in a solution of KMnO4

125

and NaOH for 12 h. Subsequently, all electrochemical apparatus were left soaked for 1

126

h in a bath of H2O2 and H2SO4 concentrated solution. Finally, the system were carefully

127

and exhaustively rinsed with ultrapure water and boiled in it for at least three times.

128

This cleaning procedure resulted in voltammograms free from any impurities.

129

In order to prepare the metallic multilayer electrode, a polycrystalline Ptpc (0.3

130

cm2, Sigma-Aldrich, 99.99%) substrate was utilized. It was mechanically polished with

131

diamond paste down to 1.0 µm, washed with acetone and a large amount of ultrapure

132

water. The potential of deposition was 0.05 V (vs. a reversible hydrogen electrode,

133

RHE) for 1800 and 120 s leading to about 2.0 and 1.0 Rh and Pt monolayers (ML),

134

respectively. The procedure was as follows: Rh intra-layer was electrodeposited from

135

the precursor solution 2.0 10-5 mol L-1 of RhCl3.3H2O in 0.1 mol L-1 of HClO4 and the

136

charge related to electrodeposition process was used to obtain the number of Rh ML.

137

After that, the electrode was washed kindly in ultrapure water and transferred to another

138

electrochemical cell with 5.0 10-4 mol L-1 of H2PtCl6.6H2O solution in 0.1 mol L-1 of

139

HClO4 where Pt outlayer could be electrodeposited leading to Ptpc/Rh2.0/Pt1.0

140

nanostructured electrodes.

141

The electrochemical characterization of the metallic multilayer electrodes was

142

carried out using a potentiostat/galvanostat Autolab PGSTAT 302N equipped with

143

SCANGEN module. Voltammetric curves were measured in 0.5 mol L-1 of H2SO4

144

solution in the potential range between 0.05 and 1.40 V. A platinized-Pt plate (Sigma-

145

Aldrich, 99.99%) was used as auxiliary electrode. Prior to the experiments, the solutions

146

were sparged with N2 (White Martins, 5N) for 30 min. The electrolyte was kept free of

147

atmospheric pollutant-gas by continuously introducing N2 into the region of the

148

electrochemical cell above the electrolyte. Prior each galvanostatic time-series, the

149

working electrode was cycled between 0.05 to 1.40 V in 0.5 mol L-1 of H3COH and 0.5


Page 5 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

150

mol L-1 of H2SO4 along 30 cycles. Soon after, the galvanostatic time-series for several

151

current values were recorded.

152

3. Modeling the electro-oxidation of methanol

153

Electrochemical experiments were compared to numerical simulations in order

154

to shed light in some mechanistic aspects in the electro-oxidation of methanol on the

155

modified surfaces. The modeling procedure was based on ten elementary steps, r1 to

156

r10, in a dimensionless model previously proposed by Nagao et al.17 As widely

157

accepted, the electro-oxidation of methanol proceeds through the so-called dual

158

pathway mechanism.18 The indirect pathway is described by the adsorption of the

159

methanol molecule with consecutive dehydration steps which results in the formation of

160

adsorbed carbon monoxide,19,20 i.e. COad. See steps r1 and r2, respectively,

161 k1

162

H3 COH + 3Pt → HCOad + 3H+ + 3e-

163

HCOad → COad + H+ + e- + 2Pt

k2

(r1) (r2)

164 165

In our model we considered only the formation of OHad as the oxygenated

166

species by the oxidation of water. In this case, step r3 represents the oxidation reaction

167

while r4 is related to the reduction one. This simplification has been used successfully

168

in different modeling procedures for the electro-oxidation of small organic

169

molecules.17,21,22 In addition, COad reacts with adsorbed oxygenated species in a

170

Langmuir-Hinshelwood mechanism, step r5, resulting in CO2.23

171 k3

172

H2 O + Pt → OHad + H+ + e-

173

OHad + H+ + e- → H2 O + Pt (r4)

174

COad + OHad → CO2 + H+ + e- + 2Pt (r5)

(r3)

k4

k5

175 176

Occurring in parallel, the direct pathway takes place by the formation of soluble

177

products, such as formic acid and formaldehyde which can also form CO2 from the

178

oxidative decomposition of the active intermediates,24,25,26 and methylformate that is

179

easily detected by means of differential electrochemical mass spectrometry

180

(DEMS).27,28,29 The nature of the active intermediates is still under debate, but it is

181

currently believed to proceed in a very fast path.30 In this context, we adopted the

182

production of adsorbed formate, i.e. HCOOad in step r7 as the active intermediate 5 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

183

formed only from formic acid dehydration, and ignored the formation of HCOOCH3 by

184

the homogeneous reaction between HCOOH and H3COH in solution27 and the

185

esterification reaction between HCOad and H3COH.29 Nevertheless, methylformate can

186

be followed by HCOad time-evolution as it is a precursor to its formation.17

187 k6

188

HCOad + Pt + H2 O → HCOOHad + H+ + e- + 2Pt

189

HCOOHad → HCOOad + H+ + e-

k7

k8

190

HCOOad + H+ + e- → HCOOHad

191

HCOOad + Pt → CO2 + H+ + e- + 3Pt

k9

(r6) (r7) (r8) (r9)

192 193

We attributed the steps r6 to r9, as also the responsible for the production of

194

CO2 derived of re-adsorption process of soluble species. In this case, only formic acid is

195

incorporated in the model and all molecules form adsorbed formate. An excellent

196

agreement between experiments and simulations based on these assumptions was

197

recently observed.31 Finally, formic acid might also result in COad from a reduction step

198

of HCOOad, as proposed by Cuesta et al.32

199 200

k10

HCOOad + Pt + H+ + e- COad + H2 O + 2Pt (r10)

201 202

Each coverage variable was converted in a generic representation xn according

203

to: x2 = θHCO , x3 = θOH , x4 = θCO , x5 = θHCOOH , and x6 = θHCOO ; xe = eωϕ as the electrical

204

component for faradaic reactions, and ω as the transfer coefficient. The surface free

205

sites are denoted as x1 and can be written as,

206

x1 = 1 - 3x2 - x3 - x4 - 2x5 - 2x6 207 208

applying the kinetic laws in each elementary step, it is obtained,

209 210

v1 = k1 x21 xe (e1)

211

v2 = k2 x2 xe (e2)

212

v3 = k3 x1 xe (e3)

213

v4 = k4 x3 xe (e4)

-1


Page 7 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

214

v5 = k5 x3 x4 xe (e5)

215

v6 = k6 x1 x2 xe (e6)

216

v7 = k7 x5 xe (e7)

217

v8 = k8 x6 xe (e8)

218

v9 = k9 x1 x6 xe (e9)

-1

-1

v10 = k10 x1 x6 xe

219

(e10)

220 221

Additional simplifications as the assumption of two free active sites33 in

222

Equation e1 and the independency of the number of electrons produced in xe were

223

adopted, and as previously observed, do not result in significant changes compared to

224

experimental observations.17,34 Hence, the electrochemical model can be written as a set

225

of six autonomous ordinary differential equations, e11 to e16,

226 227

x 2 = v1 - v2 - v6 (e11)

228

x 3 = v3 - v4 - v5 (e12)

229

x 4 = v2 - v5 + v10 (e13)

230

x 5 = v6 - v7 + v8 (e14)

231

x 6 = v7 - v8 - v9 - v10 (e15)

232

ϕ = i - vF (e16)

233 234

with,

235 236

vF = 3v1 + v2 + v3 - v4 + v5 + v6 + v7 - v8 + v9 - v10 (e17) i =

237

u - ϕ r

(e18)

238 239

In Equation e18, i might be interpreted as the total current, u as the applied

240

potential and r as the ohmic drop between the working and reference electrodes. The six

241

ordinary differential equations were numerically integrated using the tool ode15s in the

242

Matlab software. When not specified, the initial conditions were considered as zero.

243 244

4. Results and Discussion


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

245

Figure 1 depicts the electrochemical characterization carried out by linear

246

potentiodynamic sweeps at dU/dt = 0.10 V s-1 on a polycrystalline platinum electrode,

247

Ptpc as the blue curves, and metallic multilayer nanostructured electrode, Ptpc/Rh2.0/Pt1.0

248

as the red ones; experiments were performed in sulfuric acid 0.1 mol L-1 in the (a)

249

absence and (b) presence of methanol 0.5 mol L-1. Numerical simulations presented in

250

(c) were performed at du/dt = 0.01 with changes in k2 and k3 in order to qualitatively

251

reproduce the experimental observations. In this case, the set of reaction rate constants,

252

k2 = 5.0 and k3 = 1.0 represents the blue curves, while k2 = 0.7 and k3 = 0.9 the red ones.

253


Page 9 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

254 255 256 257 258 259 260 261 262 263 264

Figure 1. (a) cyclic voltammetry at dU/dt = 0.10 V s-1 under Ptpc (blue curves) and Ptpc/Rh2.0/Pt1.0 (red curves) in sulfuric acidic media; (b) potential linear sweep at same rate measured in electrochemical experiments and (c) numerically calculated in the electro-oxidation of methanol. k2 = 5.0 and k3 = 1.0 (blue curve); k2 = 0.7 and k3 = 0.9 (red curve) in numerical simulations at du/dt = 0.01, u(0) = -0.3 and x4(0) = 0.9. Additional parameters are: k1 = 6.0, k4 = 4.0, k5 = 0.079, k6 = 50, k7 = 600, k8 = 30, k9 = 300, k10 = 0.1, ω = 15, r = 0.05.

265

A typical voltammetric behavior can be observed in Figure 1(a) for Ptpc and

266

Ptpc/Rh2.0/Pt1.0 electrodes in sulfuric acid solution,35 where the electrochemical process 9 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

267

on the cyclic voltammograms can be associated to: i) hydrogen adsorption and

268

desorption between 0.05 to 0.40 V, ii) double layer region between 0.40 to 0.80 V, iii)

269

formation and reduction of platinum oxides, between 0.80 to 1.40 V and 1.4 to 0.40 V,

270

respectively. As evident in Figure 1(a), the voltammetric profile for polycrystalline

271

platinum is thoroughly recovered when Ptpc/Rh2.0/Pt1.0 structure is formed by the

272

deposition of a Pt layer over the Rh surface, which implies in a surface composed

273

mostly by platinum atoms. XPS measurements have confirmed this experimental

274

evidence.10,12 The electrochemical surface areas for Ptpc and Ptpc/Rh2.0/Pt1.0 electrodes

275

were calculated using the procedure well established in the literature36 which considers

276

a charge density of 210 µC cm-2 as being equivalent to desorption of one hydrogen

277

monolayer. The electrochemical surface area for Ptpc and Ptpc/Rh2.0/Pt1.0 electrodes

278

differ about 5% and then all current densities were normalized by its values. These

279

results are in agreement with in situ AFM experiments which revealed the same

280

roughness mean square values for both electrodes.11,12,13

281

Analyzing the electro-oxidation of methanol over Ptpc and Ptpc/Rh2.0/Pt1.0

282

electrodes in Figures 1(b), an increase of c.a. 90% was observed in the peak current

283

density for Ptpc/Rh2.0/Pt1.0 along the linear potentiodynamic sweep. Similar behavior of

284

the increased catalytic activity in the electro-oxidation of different small organic

285

molecules have been showed by Pereira and co-workers10,11,12,13,14,15,16. As commonly

286

accepted, the electro-oxidation of methanol proceeds through parallel pathways,18 as

287

already presented in Section 3. One controversial point to be stressed is the role played

288

by carbon monoxide during the electro-oxidation process. Due to its strong influence in

289

fuel cells efficiency, it has triggered a continuous study of its removal in low potentials

290

by the presence of different metals such as Ru, Rh, Sn, etc.37,38,39 In the literature, a

291

great effort has been done in the development of more selective catalysts based on the

292

bi-functional mechanism and electronic effects.40,41,42,43

293

Considering the absence of Rh ad atoms in the surface of metallic multiyear

294

electrode, justified by the no observation of the characteristic bands of the second metal

295

in XPS measurements10,12 and the similarity between the profiles for the hydrogen

296

underpotential deposition in the cyclic voltammetries in Figure 1(a), the mechanism that

297

would explain the experimental results in Figure 1(b) might be attributed to: (a) the

298

electronic interaction of the exposed Pt outlayer and the Rh interlayer beneath,

299

weakening the back-donation between the CO anti-ligand orbitals 2π*, with energies

300

above the Fermi level, and d band from the Pt outlayer;42,43 (b) the compressive strain 10 ACS Paragon Plus Environment

Page 11 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

301

due to the electrodeposition of a monolayer between two distinct substrates and

302

interatomic distances, resulting in an enlargement of d band, and consequently, lower

303

energies of adsorption.44,45 Both electronic effects can, indeed, prevent the formation of

304

COad on the surface. Experimental results have showed the catalytic effect of Rh for the

305

electro-oxidation of methanol,10,38,46 but as previously discussed, in this set of results we

306

can discard the contribution from the bi-functional mechanism and the formation of

307

rhodium oxides.

308

Numerical simulations were also carried out and a good correspondence with the

309

electrochemical measurements was obtained when the reaction rate constants of

310

formation of adsorbed COad and OHad were changed, as it can be seen in Figure 1(c).

311

Although a suppression for carbon monoxide layer formation is enhanced in the

312

presence of metallic multilayer electrode, interestingly, just a slight shift in the current

313

density peak for more negative potentials has been observed. According with our

314

simulations, this behavior is mostly attributed to the decrease of k2 values which results

315

in the increase of catalytic activity while k3 affected drastically the potential range

316

where the maximum current occurs (see Figure S1 in Supporting Information for

317

additional details). Hence, the enhancement of the catalytic activity in the electro-

318

oxidation of methanol might be understood in terms of the removal of carbon monoxide

319

from the surface.

320

Galvanostatic experiments were carried out in different applied currents and

321

concomitantly compared with numerical simulations. In Figure 2, potential time-series

322

measured at j = 0.32 mA cm-2 on (a) Ptpc (blue curves) and (b) Ptpc/Rh2.0/Pt1.0 (red

323

curves) working electrodes were compared with numerical simulations at i = 0.20 with

324

(c) k2 = 5.0, k3 = 1.0, k6 = 50 (blue curves) and (d) k2 = 0.5, k3 = 0.8, k6 = 23 (red

325

curves).


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

326 327 328 329 330 331 332 333 334 335

Page 12 of 26

Figure 2. Experimental (item (a) and (b)) and calculated (item (c) and (d)) potential time-series in the electro-oxidation of methanol at j = 0.32 mA cm-2 and i = 0.20, respectively. The blue curves represent Ptpc and k2 = 5.0, k3 = 1.0, k6 = 50; while red ones represent Ptpc/Rh2.0/Pt1.0 and k2 = 0.5, k3 = 0.8, k6 = 23; working electrodes in electrochemical experiments and numerical simulations. Additional parameters are: k1 = 6.0, k4 = 4.0, k5 = 0.079, k7 = 600, k8 = 30, k9 = 300, k10 = 0.1, ω = 15.

336

Electrochemical oscillations presented an usual profile described in the

337

literature.47,48 As observed in Figure 2, a good adjustment between experiments and

338

simulations was obtained when the formation of COad and OHad were disfavored. It

339

reinforces the evidence that the metallic multilayer electrode might prevent more

340

selectively the formation of carbon monoxide on the surface. Additionally, qualitative

341

agreement was even improved when k6 values were decreased from 50 to 23. In spite of

342

a significant increase of the current density in voltammetric experiments, the

343

electrochemical variables do not reveal explicitly the presence of the second metal,

344

nevertheless some hints become clearly evident in oscillatory regime. As already

345

pointed out, a rich variety of dynamic behavior found during the electrochemical

346

oscillations might be used to extract mechanistic information, such as reaction rate 12 ACS Paragon Plus Environment

Page 13 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

347

constants.49 In this specific case, the amplitude shift for lower potential values followed

348

by the decrease of the period is a good indicative of a surface modification by a second

349

metal.50,51

350

The experimental and numerical results support the proposition that the

351

oxidation of adsorbed carbon monoxide by oxygenated species in a Langmuir-

352

Hinshelwood mechanism is not the predominant reaction step for the CO2 formation,

353

and consequently, the faradaic current. In other words, our results suggest that adsorbed

354

carbon monoxide acts as a poisoning species instead of a reaction intermediate in the

355

electro-oxidation of methanol in a wide potential window. This hypothesis has already

356

been investigated52,53,54 and it is in agreement with our observations. This controversial

357

behavior played by carbon monoxide have been discussed with the aid of spectroscopic

358

and spectrometric techniques coupled in the electrochemical cell.52,53,54,55,56,57,58,59 The

359

classification as a reaction intermediate was attributed to the high sensitiveness of CO2

360

production when different anions were present in the electrolyte. As the formation of

361

adsorbed carbon monoxide by the surface dehydration of methanol requires a larger

362

number of active sites, CO oxidation was considered a non-negligible elementary step

363

in the overall carbon dioxide production,56,57,58 e.g. above 0.6 V vs. RHE,

364

approximately. In contrast, recent results have showed a strong impact of soluble

365

intermediates in the total production of CO2 and the faradaic current.31,60,61 In fact,

366

Nagao et al.34 have demonstrated that sulfate anions in electrolyte impede more

367

effectively the CO2 production from the direct than the indirect pathway during the

368

oscillatory electro-oxidation of methanol.

369

Based on this considerations, we utilized modeling and numerical simulations to

370

have some insights of the relative weight of production of CO2 from the direct and

371

indirect pathways. Figure 3 shows the production of CO2 from the elementary steps r5

372

and r9 in the (a) linear sweeps described in Figure 1(c); and (b) potential time-series in

373

Figure 2(c) and 2(d). Table 1 depicts a more quantitative relationship of the Figure 3 in

374

terms of the maximum current found in the linear potential sweep, and the mean

375

production during the oscillations.

376


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

377 378 379 380 381 382 383 384

Page 14 of 26

Figure 3. (a) simulated linear sweeps: k2 = 5.0 and k3 = 1.0 (blue curves); k2 = 0.7and k3 = 0.9 (red curves), both at k6 = 50, du/dt = 0.01, u(0) = 0.3 and x4(0) = 0.9; (b) potential time-series with k2 = 5.0, k3 = 1.0 and k6 = 50 (blue curves); and k2 = 0.5, k3 = 0.8 and k6 = 23 (red curves), both at i = 0.20. Additional parameters are: k1 = 6.0, k4 = 4.0, k5 = 0.079, k7 = 600, k8 = 30, k9 = 300, k10 = 0.1, ω = 15, r = 0.05.


Page 15 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

385

Under potentiodynamic control, vide Figure 3(a1) to (a3), the decrease of the

386

reaction rates k2 and k3 resulted in a substantial increase of the formation of CO2,

387

represented by v5 + v9, from 0.230 to 0.335, which is in agreement with the Figures 1(b)

388

and 1(c). However, the enhancement in the electro-catalytic activity is clearly related to

389

the higher contribution from the direct pathway. It is mainly occasioned by the

390

consumption of COad, releasing free sites to the oxidative decomposition of the active

391

intermediate. Otherwise, in oscillatory conditions, Figure 3 (b1) to (b3), the decrease of

392

k2 and k3 also resulted in an inversion for the global contribution of CO2 formation:

393

decrease of v5 from 0.020 to 0.007 and a consequent increase of v9 from 0.012 to 0.032,

394

respectively. Nevertheless, it does not exhibit dramatic changes in the mean production,

395

see Table 1 for a more detailed description. Differently from observed in

396

electrochemical experiments coupled with a mass spectrometer,17,31,34 these oscillations

397

do not end up with CO2 peak splitting. Harmonic oscillations, favored in lower applied

398

currents, tend to generate period-1 carbon dioxide time-series, instead of relaxation ones

399

where period-3 and 4 have been observed.17,31,34

400 401

Table 1. Relative production of carbon dioxide extracted from Figure 3. v5 blue maximum current in linear potential sweep mean production during oscillations

v9 red

blue

v 5 + v9 red

blue

red

0.190 0.151 0.042 0.240 0.230 0.335 0.020 0.007 0.012 0.025 0.032 0.032

402 403

The removal of carbon monoxide from the surface seems to be the key

404

parameter to increase the activity. Clearly, the metallic multilayer is very efficient in

405

this purpose. This idea was also tested using the electrochemical oscillations in long

406

term dynamics as a probe to infer about the reaction mechanism. A general overview of

407

the effect of Ptpc/Rh2.0/Pt1.0 working electrode in the oscillatory electro-oxidation of

408

methanol is better visualized in Figure 4.

409 410 411


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

412 413 414 415 416 417 418 419 420

Page 16 of 26

Figure 4. Period (a) and amplitude (b) of the galvanostatic oscillations experimentally measured as a function of number of cycles in the electro-oxidation of methanol over Ptpc (blue curves) and Ptpc/Rh2.0/Pt1.0 (red curves). The dashed arrow indicates the direction of the applied current density from the lowest to the highest values: j = 0.32, 0.48, 0.64 and 0.81 mA cm-2.

421

The oscillatory period and amplitude were accounted as function of the number

422

of cycles over Ptpc and Ptpc/Rh2.0/Pt1.0 surfaces and under different applied current

423

densities, e.g. j = 0.32, 0.48, 0.64 and 0.81 mA cm-2. Overall, the period and amplitude

424

were not significantly influenced neither by the variation in applied current and the use

425

of the metallic multilayer electrode. As observed in Figure 4(a) and (b), the decrease of j

426

resulted in slower oscillations with smaller amplitudes, while the use of Ptpc/Rh2.0/Pt1.0

427

electrode enhanced this behavior in all range of current densities studied. In contrast to

428

previous experimental findings,62 the relation between period and amplitude in the

429

oscillatory electro-oxidation of methanol does not seem to be straightforward. This 16 ACS Paragon Plus Environment

Page 17 of 26

430

aspect is very important and will be discussed later with the aid of numerical

431

simulations.

432

Another interesting effect of the nanostructured metallic multilayer electrode in

433

the oscillations is the temporal stability of time-series. Perini et al.51 have demonstrated

434

the highly efficient way to attenuate the intrinsic drift during potential oscillations in the

435

electro-oxidation of formic acid using Sn as a second metal on the surface. The authors

436

were able to keep a robust time-series for a very long period of time, e.g. 40 h. Based on

437

mechanistic considerations, the rationalized method was performed aiming the

438

prevention of OsubPt (sub means subsurface oxygen35) formation, in its turn, leads Sn as

439

the main responsible to form oxides that might be used in the oxidation with

440

carbonaceous species. The efficiency of the Ptpc/Rh2.0/Pt1.0 electrode towards the

441

stabilization of time-series in the electro-oxidation of methanol was analyzed as well.

442

Table 2 summarizes some experimental data collected in Figure 4 as the measured drift

443

in terms of ∆Um.∆cycles-1 and the total number of cycles. Both were reproduced by

444

numerical simulations. In this set of results the drift was simulated by a linear increase

445

of the applied current from initial i of 0.05, rate of 2.00 10-3 and 2.24 10-3. This

446

characteristic has been proved experimentally.48,63

447

Table 2. Effect of the metallic multilayer Ptpc/Rh2.0/Pt1.0 and reaction rate constant of COad formation on the total number of oscillatory cycles. Simulations consider: k1 = 6.0, k3 = 1.0, k4 = 4.0, k5 = 0.079, k6 = 50, k7 = 600, k8 = 30, k9 = 300, k10 = 0.1, ω = 15.

experiments

448 449 450

simulations

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

electrode

drift

ratio in drift

Ptpc

0.130 mV cycle-1

Ptpc/Rh2.0/Pt1.0

0.117 mV cycle-1

Ptpc

0.190 mV cycle-1

# of cycles 669

1.11

Ptpc/Rh2.0/Pt1.0

0.169 mV cycle

k2 = 5.0

2.24 10-3

-1

1.64 406 502

1.12

2.16 232 132

1.12 k2 = 1.0

2.00 10

-3

ratio in # of cycles

2.16 61

451 452

As observed in all experimental results with constant applied current, the drift

453

evolves faster in the Ptpc related to Ptpc/Rh2.0/Pt1.0 electrode (see Table S1 in Supporting

454

Information for additional values for ∆Um.∆cycles-1 and the total number of cycles). 17 ACS Paragon Plus Environment

ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

455

However, instead of compare the drift at same applied current, where the available area

456

of the electrode plays a pivotal role, we conducted the analysis normalizing the drift in

457

terms of ∆Um.∆cycles-1. With a ratio of approximately 1.1 in the drift, the number of

458

cycles was roughly twice higher in the presence of Ptpc. Remarkably, the same behavior

459

was obtained when the reaction rate of COad formation was increased from 1.0 to 5.0.

460

This is a clear evidence that the Ptpc/Rh2.0/Pt1.0 electrode does not have a stabilizing

461

effect as observed by Perini et al.51 even with the presence of a second metal. The major

462

difference by their experiments and ours is that in our case the second metal is located

463

beneath the platinum outlayer in the Ptpc/Rh2.0/Pt1.0 electrode. As the bi-functional

464

mechanism can be discarded in this electrode configuration, the surface oxidation still

465

occurs on platinum active sites, but in this particular situation, the formation of carbon

466

monoxide is much slower. Considering that COad is the main specie responsible to

467

consume platinum oxides on the surface, formed in high potential values, it is expected

468

a lesser number of cycles when the metallic multilayer is used as working electrode.

469

The intrinsic drift might also induce morphological changes in period and

470

amplitude simultaneously.48,63 Figure 5 depicts an amplitude vs. period diagram for

471

experiments (filled circles in bold axes) and numerical simulations (open circles in italic

472

axes). The blue curves represents the Ptpc electrode and k2 = 5.0; while the red ones is

473

associated to Ptpc/Rh2.0/Pt1.0 electrode and k2 = 1.0, for electrochemical experiments and

474

numerical simulations, respectively. The surface drift was implemented by slow

475

changes in the applied current with initial i of 0.05 and a linear sweep of 0.002.

476 477 478


Page 19 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

479 480 481 482 483 484 485 486 487 488 489 490 491 492

Figures 5(a) and (b) show a typical characteristic of quadratic relationship

493

between the oscillatory amplitude and bifurcation parameter, along the period drifting,

494

i.e. applied current. As it is changing slowly during the time, we can state the

495

proportional relation: amplitude ∝ applied current which is predicted theoretically.64

496

However, differently from item (b), Figure 5(a) shows a distinct behavior in the onset of

497

the time-series. At first, there is an inverse tendency of increase between the period and

Figure 5. Amplitude vs. Period diagram represented as filled circles for the experiments (amplitude / V vs. period / s in bold axes) at j = 0.64 mA cm-2 and open circles for numerical simulations (a vs. p in italic axes). (a) Electrochemical reaction on Ptpc and k2 = 5.0 for numerical simulations (blue curves); (b) Ptpc/Rh2.0/Pt1.0 and k2 = 1.0 (red curves). The black arrows indicate the direction of increasing the number of cycles from initial i of 0.05 and drift of 0.002. Both simulations consider: k1 = 6.0, k3 = 1.0, k4 = 4.0, k5 = 0.079, k6 = 50, k7 = 600, k8 = 30, k9 = 300, k10 = 0.1, ω = 15.


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

498

amplitude, followed by concomitant increment after a critical point. In Figure 4, the

499

increase of the applied current resulted in faster oscillations with larger amplitudes. As

500

verified experimentally,62 larger amplitudes are usually connected with larger periods. A

501

suitable explanation for this dynamic behavior is that higher potential values must be

502

reached in order to promote an effective surface reactivation when the trajectories of the

503

limit cycle have taken a slow excursion in low potentials. Experimental and numerical

504

results65,66 converge to the idea that poisoning species might block the surface, slowing

505

down the reaction rates. Considering the electro-oxidation of methanol, the

506

consumption of carbon monoxide by oxygenated species is the elementary reaction

507

which mostly influence the oscillatory period.65 Based on this statement, we can clearly

508

make a distinction between Figure 5(a) and (b) by the amount of adsorbed carbon

509

monoxide formed on the surface. The former initially speed-up the formation of

510

carbonaceous intermediates which can be latter used in the oxidation of oxygenated

511

species, prolonging the time-series, and consequently, the overall number of cycles.

512

Nevertheless, Figure 5(b) presents a monotonic and gradual decrease by the slow

513

platinum oxidation (see Figure S2 in Supporting Information for the time evolution of

514

x4). Similar behavior has been reported in the literature.48,63

515

In summary, the experimental and numerical results described here allow us to

516

propose that a metallic multilayer is indeed a new electrocatalytic material. In the

517

present case, it has improved properties for the electro-oxidation of methanol in acidic

518

solution, leading important changes in the mechanistic reaction compared to bulk

519

polycrystalline platinum.

520 521

5. Conclusions

522

Metallic multilayer electrodes have been utilized as a new kind of

523

nanostructured materials with the aim of increasing the catalytic activity in the electro-

524

oxidation of small organic molecules. In this work we provided additional mechanistic

525

evidences of the role played by Ptpc/Rh2.0/Pt1.0 surface in the electro-oxidation of

526

methanol by a combination of electrochemical experiments, modeling and numerical

527

simulations. A similar voltammetric profile in acidic solutions was observed in both

528

electrodes, i.e. Ptpc and Ptpc/Rh2.0/Pt1.0. Remarkably, an increase of about 90% in the

529

peak current density around 0.88 V vs. RHE has appeared in an anodic linear

530

potentiodynamic sweeps for the Ptpc/Rh2.0/Pt1.0 electrode, with respect to that for Ptpc.

531

Based on spectroscopic measurements previously reported10,12 and similar profiles for 20 ACS Paragon Plus Environment

Page 21 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

532

the hydrogen underpotential deposition in the cyclic voltammetries, the bi-functional

533

mechanism and the formation of rhodium oxides on the surface can be discarded as

534

responsible for increasing the catalytic activity. Consequently, it might be attributed

535

solely to the highly selective electronic effect in the prevention of adsorbed carbon

536

monoxide.

537

Additionally, a considerable increase of period and decrease of amplitude of the

538

oscillations were observed in a wide range of applied current densities, and in contrast

539

to previous studies, the presence of a second metal enhanced the intrinsic drift

540

embedded in the time-series. A good qualitative agreement between experiments and

541

simulations was obtained only when lower values of reaction rate constant of carbon

542

monoxide adsorption were considered in the numerical model. These evidences

543

highlight the effect of metallic multilayer electrodes on the prevention of carbon

544

monoxide layer formation. In other words, carbon monoxide seems to act as a poisoning

545

specie instead of a reaction intermediate that contribute to the faradaic current and total

546

CO2 production.

547 548

6. Associated Content

549

Supporting Information

550

Simulated voltammetric profiles for i and CO2 besides the time-series for COad and CO2

551

in the electro-oxidation of methanol. Number of cycles and drift in terms of

552

∆Um.∆cycles-1 measured experimentally under different j. This material is available free

553

of charge via the Internet at http://pubs.acs.org.

554 555

7. Acknowledgments

556

The authors would like to thank the Brazilian research funding agencies: National

557

Council of Scientific and Technological Development (CNPq, grants: 229171/2013-3

558

and 304458/2013-9) and São Paulo Research Foundation (FAPESP, grants:

559

2010/05555-2, #2013/16930-7 and 2013/16930-7) for financial support.

560 561

8. References

562

(1)

563 564

(2) Liu, H. S.; Song, C. J.; Zhang, L.; Zhang, J. J.; Wang, H. J.; Wilkinson, D. P., J. Power Sources 2006, 155, 95-110.

Wasmus, S.; Kuver, A., J. Electroanal. Chem. 1999, 461, 14-31.


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

565 566 567

(3) Gurau, B.; Viswanathan, R.; Liu, R. X.; Lafrenz, T. J.; Ley, K. L.; Smotkin, E. S.; Reddington, E.; Sapienza, A.; Chan, B. C.; Mallouk, T. E.; Sarangapani, S., J. Phys. Chem. B 1998, 102, 9997-10003.

568 569

(4) Iwasita, T.; Hoster, H.; John-Anacker, A.; Lin, W. F.; Vielstich, W., Langmuir 2000, 16, 522-529.

570

(5)

571 572

(6) Sun, X.; Li, D.; Ding, Y.; Zhu, W.; Guo, S.; Wang, Z. L.; Sun, S., J. Am. Chem. Soc. 2014, 136, 5745-5749.

573

(7)

574 575

(8) Johnson, M. T.; Bloemen, P. J. H.; denBroeder, F. J. A.; deVries, J. J., Rep. Prog. Phys. 1996, 59, 1409-1458.

576

(9)

577 578

(10) Oliveira, R. T. S.; Santos, M. C.; Marcussi, B. G.; Nascente, P. A. P.; Bulhoes, L. O. S.; Pereira, E. C., J. Electroanal. Chem. 2005, 575, 177-182.

579 580

(11) Oliveira, R. T. S.; Santos, M. C.; Marcussi, B. G.; Tanimoto, S. T.; Bulhoes, L. O. S.; Pereira, E. C., J. Power Sources 2006, 157, 212-216.

581 582

(12) Lemos, S. G.; Oliveira, R. T. S.; Santos, M. C.; Nascente, P. A. P.; Bulhoes, L. O. S.; Pereira, E. C., J. Power Sources 2007, 163, 695-701.

583

(13)

584 585

(14) Freitas, R. G.; Pereira, E. C.; Christensen, P. A., Electrochem. Commun. 2011, 13, 1147-1150.

586 587

(15) Freitas, R. G.; Antunes, E. P.; Christensen, P. A.; Pereira, E. C., J. Power Sources 2012, 214, 351-357.

588

(16)

589 590

(17) Nagao, R.; Cantane, D. A.; Lima, F. H. B.; Varela, H., Phys. Chem. Chem. Phys. 2012, 14, 8294-8298.

591

(18)

Parsons, R.; Vandernoot, T., J. Electroanal. Chem. 1988, 257, 9-45.

592

(19)

Cuesta, A., J. Am. Chem. Soc. 2006, 128, 13332-13333.

593

(20)

Neurock, M.; Janik, M.; Wieckowski, A., Faraday Discuss. 2008, 140, 363-378.

594

(21)

Strasser, P.; Eiswirth, M.; Ertl, G., J. Chem. Phys. 1997, 107, 991-1003.

Radmilovic, V.; Gasteiger, H. A.; Ross, P. N., J. Catal. 1995, 154, 98-106.

Zebarjadi, M.; Shakouri, A.; Esfarjani, K., Phys. Rev. B 2006, 74, 6.

Bass, J.; Pratt, W. P., J. Magn. Magn. Mater. 1999, 200, 274-289.

Freitas, R. G.; Pereira, E. C., Electrochim. Acta 2010, 55, 7622-7627.

Zulke, A. A.; Matos, R.; Pereira, E. C., Electrochim. Acta 2013, 105, 578-583.


Page 23 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

595 596

(22) 82.

Karantonis, A.; Koutsaftis, D.; Kouloumbi, N., Chem. Phys. Lett. 2006, 422, 78-

597 598

(23) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A., J. Phys. Chem. B 2002, 106, 12938-12947.

599

(24)

600 601

(25) Osawa, M.; Komatsu, K.; Samjeske, G.; Uchida, T.; Ikeshoji, T.; Cuesta, A.; Gutierrez, C., Angew. Chem.-Int. Edit. 2011, 50, 1159-1163.

602 603

(26) Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T. M.; Osawa, M., J. Am. Chem. Soc. 2013, 135, 9991-9994.

604

(27)

Wang, H.; Loffler, T.; Baltruschat, H., J. Appl. Electrochem. 2001, 31, 759-765.

605

(28)

Wang, H. S.; Baltruschat, H., J. Phys. Chem. C 2007, 111, 7038-7048.

606

(29)

Abd-El-Latif, A. A.; Baltruschat, H., J. Electroanal. Chem. 2011, 662, 204-212.

607 608

(30) Brimaud, S.; Solla-Gullon, J.; Weber, I.; Feliu, J. M.; Behm, R. J., ChemElectroChem 2014, 1, 1075-1083.

609 610

(31) Delmonde, M. V. F.; Nascimento, M. A.; Nagao, R.; Cantane, D. A.; Lima, F. H. B.; Varela, H., J. Phys. Chem. C 2014, 118, 17699-17709.

611 612

(32) Cuesta, A.; Cabello, G.; Gutierrez, C.; Osawa, M., Phys. Chem. Chem. Phys. 2011, 13, 20091-20095.

613 614

(33) Sauerbrei, S.; Nascimento, M. A.; Eiswirth, M.; Varela, H., J. Chem. Phys. 2010, 132, 154901.

615 616

(34) Nagao, R.; Cantane, D. A.; Lima, F. H. B.; Varela, H., J. Phys. Chem. C 2013, 117, 15098-15105.

617 618

(35) Angerste, H.; Conway, B., E.; Sharp, W. B. A., J. Electroanal. Chem. 1973, 43, 9-36.

619

(36)

Trassati, S.; Petrii, O. A., Pure & Appl. Chem. 1991, 63, 711-734.

620

(37)

Liu, P.; Norskov, J. K., Fuel Cells 2001, 1, 192-201.

621 622

(38) Choi, J. H.; Park, K. W.; Park, I. S.; Nam, W. H.; Sung, Y. E., Electrochim. Acta 2004, 50, 787-790.

623 624

(39) Lima, A.; Coutanceau, C.; Leger, J. M.; Lamy, C., J. Appl. Electrochem. 2001, 31, 379-386.

Samjeske, G.; Osawa, M., Angew. Chem.-Int. Edit. 2005, 44, 5694-5698.


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

625 626

(40) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., J. Phys. Chem. 1995, 99, 82908301.

627

(41)

Shubina, T. E.; Koper, M. T. M., Electrochim. Acta 2002, 47, 3621-3628.

628

(42)

Hammer, B.; Norskov, J. K., Surf. Sci. 1995, 343, 211-220.

629 630

(43) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K., J. Mol. Catal. A-Chem. 1997, 115, 421-429.

631 632

(44) Kibler, L. A.; El-Aziz, A. M.; Hoyer, R.; Kolb, D. M., Angew. Chem.-Int. Edit. 2005, 44, 2080-2084.

633 634

(45) Rauscher, H.; Hager, T.; Diemant, T.; Hoster, H.; Buatier de Mongeot, F.; Behm, R. J., Surf. Sci. 2007, 601, 4608-4619.

635 636

(46) Narayanamoorthy, B.; Datta, K. K. R.; Eswaramoorthy, M.; Balaji, S., ACS Catalysis 2014, 4, 3621-3629.

637 638

(47) Carbonio, E. A.; Nagao, R.; Gonzalez, E. R.; Varela, H., Phys. Chem. Chem. Phys. 2009, 11, 665-670.

639

(48)

640 641

(49) Lopes, P. P.; Batista, B. C.; Saglietti, G. A.; Varela, H.; Ticianelli, E. A., J. Solid State Electrochem. 2013, 17, 1851-1859.

642 643

(50) 26.

644 645

(51) Perini, N.; Batista, B. C.; Angelo, A. C. D.; Epstein, I. R.; Varela, H., ChemPhysChem 2014, 15, 1753-1760.

646 647

(52) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K., J. Electroanal. Chem. 1981, 121, 343-347.

648 649

(53) Beden, B.; Hahn, F.; Juanto, S.; Lamy, C.; Leger, J. M., J. Electroanal. Chem. 1987, 225, 215-225.

650

(54)

Kunimatsu, K.; Kita, H., J. Electroanal. Chem. 1987, 218, 155-172.

651

(55)

Iwasita, T., Electrochim. Acta 2002, 47, 3663-3674.

652 653

(56) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T., J. Electroanal. Chem. 2004, 571, 273-282.

654 655

(57) Batista, E. A.; Malpass, G. R. P.; Motheo, A. J.; Iwasita, T., Electrochem. Commun. 2003, 5, 843-846.

Nagao, R.; Sitta, E.; Varela, H., J. Phys. Chem. C 2010, 114, 22262-22268.

Perini, N.; Sitta, E.; Angelo, A. C. D.; Varela, H., Catal. Commun. 2013, 30, 23-


Page 25 of 26

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

656 657

(58) Housmans, T. H. M.; Wonders, A. H.; Koper, M. T. M., J. Phys. Chem. B 2006, 110, 10021-10031.

658 659

(59) Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M., J. Am. Chem. Soc. 2003, 125, 3680-3681.

660 661

(60) Reichert, R.; Schnaidt, J.; Jusys, Z.; Behm, R. J., ChemPhysChem 2013, 14, 3678-3681.

662 663

(61) Reichert, R.; Schnaidt, J.; Jusys, Z.; Behm, R. J., Phys. Chem. Chem. Phys. 2014, 16, 13780-13799.

664 665

(62) Mota, A.; Lopes, P. P.; Ticianelli, E. A.; Gonzalez, E. R.; Varela, H., J. Electrochem. Soc. 2010, 157, B1301-B1304.

666 667

(63) Cabral, M. F.; Nagao, R.; Sitta, E.; Eiswirth, M.; Varela, H., Phys. Chem. Chem. Phys. 2013, 15, 1437-1442.

668 669

(64) Strogatz, S. H., Nonlinear Dynamics and Chaos: With Applications to Physics, Biology, Chemistry, and Engineering. Addison-Wesley: Cambridge, 1994.

670 671

(65) Batista, B. C.; Ferreira, G. C. A.; Varela, H., J. Phys. Conf. Ser. 2011, 285, 012003.

672

(66)

Ferreira, G. C. A.; Batista, B. C.; Varela, H., Plos One 2012, 7, e50145.

673 674 675 676 677 678 679 680 681 682 683 684


ACS Catalysis

1 2 3 4 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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

685

Page 26 of 26

Graphical Abstract

686 687


Oscillatory Electro-oxidation of Methanol on Nanoarchitectured Pt pc

Recommend Documents