Electrocatalytic Reduction of Nitrate Using MagnÃ©li Phase TiO2

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Remediation and Control Technologies 2

Electrocatalytic Reduction of Nitrate using Magnéli Phase TiO Reactive Electrochemical Membranes Doped with Pd-based Catalysts Pralay Gayen, Jason Spataro, Sumant M Avasarala, Abdul-Mehdi S. Ali, Jose M Cerrato, and Brian P. Chaplin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03038 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Environmental Science & Technology

Electrocatalytic Reduction of Nitrate using Magnéli Phase TiO2 Reactive Electrochemical Membranes Doped with Pd-based Catalysts

1 2 3 4

Pralay Gayen§, Jason Spataro§, Sumant Avasarala†, Abdul-Mehdi Ali ‡, José M. Cerrato† and

5

Brian P. Chaplin§*

6 7

§

Department of Chemical Engineering, University of Illinois at Chicago, 810 S. Clinton St., Chicago, IL 60607

8 9

†

Department of Civil Engineering, MSC01 1070, University of New Mexico, Albuquerque, NM 87131

10 11

‡

Department of Earth and Planetary Sciences, MSC03 2040, University of New Mexico, Albuquerque, New Mexico 87131

12 13 14 15 16 17

*Corresponding

author at: Department of Chemical Engineering, University of Illinois at Chicago, Chicago, IL 60607, USA E-mail address: [email protected] (Brian P. Chaplin) Phone No.: +13129960288

ACS Paragon Plus Environment

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Abstract

19

This research focused on synthesis, characterization, and application of point-of-use catalytic

20

reactive electrochemical membranes (REMs) for electrocatalytic NO3- reduction. Deposition of

21

Pd-Cu and Pd-In catalysts to the REMs produced catalytic REMs (i.e., Pd-Cu/REM and Pd-

22

In/REM) that were active for NO3- reduction. Optimal performance was achieved with a Pd-

23

Cu/REM and upstream counter electrode, which reduced NO3- from 1.0 mM to below the EPAs

24

regulatory MCL (700 µM) in a single pass through the REM (residence time ~ 2 s), obtaining

25

product selectivity of < 2% towards NO2-/NH3. Nitrate reduction was not affected by dissolved

26

oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca2+

27

and Mg2+ scaling. Energy consumption to treat surface water was 1.1 to 1.3 kWh mol-1 for 1 mM

28

NO3- concentrations, and decreased to 0.19 and 0.12 kWh mol-1 for 10 and 100 mM NaNO3

29

solutions, respectively. Electrocatalytic reduction kinetics were shown to be an order of

30

magnitude higher than catalytic NO3- reduction kinetics. Conversion of up to 67% of NO3-, with

31

low NO2- (0.7—11 µM) and NH3 formation (< 10 µM), and low energy consumption obtained in

32

this study suggest that Pd-Cu/REMs are a promising technology for distributed water treatment.

33

Keywords: Electrocatalyst, Reactive Electrochemical Membrane, Magnéli Phase TiO2


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1.0 Introduction

41

Distributed water treatment systems are common in developing countries, rural areas, and in

42

emergency situations.1-5 They have also been proposed as sustainable solutions for decentralized

43

water treatment in urban areas, as they can utilize alternative water sources and avoid the

44

expense of large water distribution systems.6 Developing point-of-use (POU) technologies for

45

distributed water treatment becomes challenging when the water contains poorly adsorbable

46

organic and inorganic contaminants, such as trace organics and oxyanions (e.g., NO3-),

47

respectively. POU reverse osmosis (RO) systems are often necessary to achieve the desired water

48

quality, but suffer from low water recovery (~50%), short membrane life due to fouling, and

49

concentrate management issues.4 When NO3- is of primary concern,7 POU ion exchange is often

50

used, but results in a concentrated brine that is difficult or unsustainable to discharge.2,8

51

Ideally, a single POU treatment technology should provide the desired water quality by

52

removal of various classes of water contaminants. However, only RO treatment can accomplish

53

this task. Due to the short-comings of RO treatment mentioned above, other approaches have

54

been investigated, and electrochemical technologies are being researched as POU treatment

55

devices.9-11 Electrochemical technologies can efficiently oxidize various organic contaminants12

56

and reduce NO3-.13 However, until recently mass transport limitations in electrochemical cells

57

have prevented the possibility of single pass electrochemical treatment.14 Recent work has shown

58

that porous flow-through electrodes can enhance mass transport rates by over an order of

59

magnitude relative to traditional parallel plate electrochemical cells, which allows

60

electrochemical POU water treatment devices to achieve the desired water quality in a single

61

pass through the electrode, with residence times on the order of seconds.14


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Recent work on porous flow-through electrodes for water treatment has primarily utilized

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carbon nanotube (CNT) electrochemical filters and substoichiometric TiO2 reactive

64

electrochemical membranes (REMs) for the oxidation of organic contaminants15-22 and filtration

65

and inactivation of pathogens.23,24 Though select reduction reactions have been investigated (e.g.,

66

nitrobenzene, Cr(VI)),25,26 these flow-through electrochemical cells have not been investigated

67

for NO3- reduction, nor is there a clear understanding of the effects of solution conditions on

68

electrocatalytic NO3- reduction. Successful electrochemical NO3- reduction coupled with

69

electrochemical oxidation would allow development of POU devices that are capable of

70

removing major classes of water contaminants present in rural and developing areas (i.e.,

71

pathogens, organic contaminants, and nitrate).

72

The purpose of this study was to determine the feasibility of REMs for nitrate removal via

73

electrochemical reduction. Known catalysts for nitrate reduction (i.e., Pd-Cu and Pd-In)27 were

74

deposited on sub-stoichiometric TiO2 microfiltration REMs. For the first time, these catalytic

75

REMs were characterized and studied for NO3- reduction and product selectivity in flow-through

76

mode. To provide insight into system performance, various operating conditions were

77

investigated including electrode placement, flow rate, electrode potential, NO3- concentration,

78

and reactor number. Different solution conditions were tested to determine the effects of

79

dissolved oxygen, carbonate species, and natural water constituents present in surface water on

80

NO3- reduction and product selectivity. The REM performance was evaluated in terms of nitrate

81

conversion, product selectivity, and energy consumption. A kinetic model was used to interpret

82

the experimental data and determine rate constants for electrochemical NO3- reduction and

83

compare them to those for catalytic reduction.

84


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2.0 Materials and Methods

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2.1 Reagents. Chemicals were reagent grade and purchased from Sigma-Aldrich and Fisher

88

Scientific. Gases (purity of 99.999%) were obtained from Praxair. Solutions were prepared using

89

Type I water (> 18.2 MΩ.cm at 25°C) obtained from a Barnstead NANOpure system (Thermo

90

Scientific).

91

2.2 Powder Catalyst Preparation. The TiO2 powder was reduced to a Magnéli phase titanium

92

suboxide (TinO2n−1) (n = 4, 6) at 1050 °C under flowing H2 gas in a tube furnace (OTF-1200X,

93

MTI) for 6 hours. A 2 wt% Pd catalyst with a 2:1 molar ratio of Pd to promoter metal (M = Cu or

94

In) were deposited on 2.5 g of TinO2n−1 powder using the incipient wetness method. Details are

95

provided in the Supporting Information (SI).

96

2.3 Catalyst Loaded REM Preparation. Magnéli phase REMs were synthesized from

97

Magnéli phase powder, according to a recently described method.21 The prepared pellet was

98

defined as the “REM”. Catalysts were deposited on the REMs using the incipient wetness

99

method, with the same catalyst loadings as the powders. Details are provided in the SI.

100

2.4 Batch Nitrate Reduction Experiments. Nitrate reduction experiments were performed in a

101

batch system using Pd-Cu/TinO2n-1, Pd-Cu/TiO2, and Pd-In/TinO2n-1 powders (0.67 g/L). The

102

solution was purged with H2 and CO2 at flow rates of 85 and 15 mL min-1, respectively, for the

103

duration of the experiment. Samples were taken at consistent time intervals, filtered, and

104

analyzed by ion chromatography (IC, Dionex ICS-2100).

105

2.5 Flow-through Nitrate Reduction Experiments. Electrocatalytic NO3- reduction was

106

performed with the REM, Pd-Cu/REM, and Pd-In/REM in a flow-through reactor, which is

107

described in the SI (Figure S-1). Two flow modes were investigated, referred to as cathode-


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anode and anode-cathode flow modes (SI, Figure S-1). The cathode-anode flow mode had a

109

downstream counter electrode, and solution flowed from REM cathode to anode. The anode-

110

cathode flow mode had an upstream counter electrode, and the flow was reversed.

111

Electrocatalytic NO3- reduction experiments were performed under different solution conditions,

112

including Ar- and air-saturated water, 10 mM NaHCO3 buffer (pH = 8.2), and in a surface water

113

sample (pH = 8.1). Reduction experiments were conducted using either a NaNO3 (1 to 100 mM)

114

or NaNO2 (1 mM) feed solution. Most experiments used a feed NaNO3 concentration of 1 mM,

115

as this is a typical concentration found in NO3- contaminated drinking water sources, which

116

ranges from 0.7 to 1.6 mM.28 Flow rates between 0.2 and 1.8 mL min-1 were used, which

117

corresponded to surface area-normalized membrane fluxes (J) of 240 to 2160 L m-2 h-1 (LMH).

118

Catalytic NO3- and NO2- reduction experiments were performed with Pd-Cu/REM in anode-

119

cathode flow mode with 1.0 atm H2 purging and with J between 240 and 2160 LMH, in order to

120

assess catalytic reduction in the absence of an applied electrode potential. More details of

121

reduction experiments are provided in the SI.

122

2.6 Membrane Characterization. The REM, Pd-Cu/REM, Pd-In/REM, and TinO2n-1 powder

123

were characterized by X-ray diffraction (XRD, Siemens D-5000) with a Cu X-ray tube (40 kV

124

and 25 mA). Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM/EDS)

125

were used for imaging and elemental analyses, and X-ray mapping was performed using electron

126

microprobe at the University of New Mexico (UNM). Bulk elemental concentrations of the

127

metals were determined by an acid digestion method and subsequent elemental analyses using

128

inductively coupled plasma –optical emission spectrometry (ICP-OES) at UNM. Additional

129

details are provided in the supporting information (SI).


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130

Through plane electrical conductivity (σ) was determined using electrochemical impedance

131

spectroscopy with an amplitude of ±7 mV about the open circuit potential (OCP) and a

132

frequency range of 0.5 to 100 kHz using a Gamry Reference 600 potentiostat/galvanostat

133

(Warminster, PA). Values for σ were calculated according equation (1).

135

x (1) ARREM RREM is the measured resistance (ohm) of the pellet, A is the cross-sectional area (1 cm2) and x is

136

the thickness (0.25 cm).

134

s=

137

2.7 Analytical Methods. Concentrations of NO3-, NO2-, NH3OH+, and NH4+ were determined

138

using ion chromatography (IC, Dionex ICS-2100). The pH was measured using a meter and

139

probe (PC2700, Oakton). Chemical oxygen demand (COD) was measured by Hach method

140

8000. Total nitrogen (N) was determined through oxidative digestion of dissolved nitrogen

141

species, excluding dissolved N2, to NO3- followed by IC detection.29

142 143 144

2.8 Product Selectivity, Current Efficiency, and Energy Calculations. The selectivity of NO3reduction (Si) was calculated by equation (2): !" (%) = '" (

(),+

0 1(),-.0 ,,-./ /

∗ 100

(2)

145

where species i is NO2-, NH3, N2O, or N2, vi is the stoichiometric coefficient (e.g., moles of i per

146

mole of NO3- degraded), and C f , NO - and C p , NO- are feed and permeate concentrations (mol m-3) of

147

NO3-, respectively. The apparent N2 selectivity was calculated assuming that it represented the

148

gap in the N-mass balance, as other soluble products were not detected (e.g., NH3OH+). Total N

149

analysis of gas-tight, acidified permeate samples was performed immediately after collection,

150

both with and without a CO2 pre-purge, and the difference between these measurements was used

151

to estimate N2O production. This approach was deemed reasonable since electrochemical NO3-

152

reduction results in mainly NO2-, NH3, N2, N2O, and NH3OH+.13

3

3


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153 154

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Current efficiency (CE) was calculated using the following equation:

CE (%) =

JFz (C f , NO - - C p , NO - ) 3

3

j

´100

(3)

155

where z is the moles of electrons transferred per moles of reactant, F is the Faraday constant

156

(96485 C mol-1), and j is current density (A cm-2). Experiments were performed at room

157

temperature (21 ± 1 °C). Values for z were determined based on the observed product

158

distributions.

159

Energy consumption (EC) (kWh mol-1) was calculated according to the following equation:

160

EC = 10−3 *

Vcell I Q(C f ,NO− − C p,NO− ) 3

(4)

3

161

where, Vcell is cell potential (V), I is the current (A), and Q is the volumetric permeate flow rate

162

(m3 h-1). The electrical energy per order (EEO) metric was also calculated, which is a measure of

163

the electric energy (kWh m−3) needed to reduce the NO3- concentration by 1 order of

164

magnitude.30

165

166

EEO = 10−3 *

Vcell I ⎡C − ⎤ f ,NO3 ⎥ Q *log ⎢ ⎢C − ⎥ ⎣ p,NO3 ⎦

(5)

3.0 Results and Discussion

167

3.1 Materials Characterization. Bulk elemental compositions of the Pd-Cu/REM and Pd-

168

In/REM samples provided molar ratios of Pd:Cu = 2.0 ± 0.2 and Pd:In = 2.2 ± 0.1, respectively,

169

which were similar to the expected molar ratio (2:1). The pore radius of the REMs was

170

determined as 0.46 ± 0.04, 0.40 ± 0.02, and 0.44 ± 0.03 µm for the REM, Pd-Cu/REM, and Pd-

171

In/REM, respectively, indicating that the pore size did not change significantly as a result of

172

catalyst deposition (SI, Figure S-2).


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The XRD data for the TinO2n-1 powder, REM, Pd-Cu/REM, and Pd-In/REM are shown in

174

Figure 1 along with the XRD characteristic peaks for Ti4O7, Ti5O9, Ti6O11, Pd, Cu, and In. The

175

XRD data for the TinO2n-1 powder contained characteristic peaks representative of Ti4O7 (10-2),

176

(104), (120), (2-13) and (1-20) crystal faces, with minor amounts of Ti5O9 (10-2) and Ti6O11 ((1-

177

21) and (101)). The TinO2n-1 REM contained additional peaks for Ti4O7 (1-22), (10-4), and (200)

178

crystal faces. The XRD data for Pd-Cu/REM and Pd-In/REM were similar to the TinO2n-1

179

powder, with additional peaks for Pd(111), Pd(200), Cu(111), and Cu(200) for Pd-Cu/REM and

180

Pd(111), Pd(200), In(111), In(200), and In(002) for Pd-In/REM. Significant peaks were not

181

detected for CuO, Cu2O, In2O3, PdO, or PdO2, which indicated that the catalyst precursors were

182

predominately reduced to their metallic oxidation states (Figure 1). Peaks suggested that Pd, Cu,

183

and In were present as separate phases and alloys were not formed, which was consistent with

184

the low temperature synthesis methods.

185

The through plane conductivity was measured as s = 785 ± 15 S m-1 for REM, s = 835 ± 21

186

for Pd-Cu/REM, and s = 824 ± 12 S m-1 for Pd-In/REM. The increases in s values were

187

attributed to the highly conductive Pd, Cu, and In metals. The measured s values were similar to

188

that reported for a Ti4O7 ultrafiltration REM.19

189

SEM analysis of the REM showed a porous structure with particle size of ~2 µm (SI, Figure

190

S-3). The SEM/EDS results of the Pd-M/REMs showed fairly uniform distribution of the catalyst

191

metals on the planar surface and a high spatial correlation between Pd and M (SI, Figure S-4).

192

Analyses using SEM/EDS on vertical cross sections of the Pd-M/REM samples determined that

193

the catalyst metals penetrated to approximate depths of 420 µm for Pd and 400 µm for Cu for the

194

Pd-Cu/REM, and 900 µm for Pd and 800 µm for In for the Pd-In/REM (Figure 2). The

195

differences in penetration depths were likely affected by the deposition of Cu0 onto the REM at


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196

OCP conditions (reaction (6)), followed by a galvanic replacement reaction that deposited Pd0

197

(reaction (7)). Once the Pd is deposited on the REM, it will further enhance Cu0 deposition via a

198

electroless deposition method,31,32 and this process will inhibit the transport of both metals. By

199

contrast, the redox potential for the deposition of In3+ is much more negative (reaction (8)) and

200

not feasible at the measured OCP (Eocp = 0.3 V/SHE). Therefore, deposition onto the REM

201

support was likely controlled by weaker physisorption interactions and thus the metals were able

202

to penetrate to a further depth into the pellet.

203

Cu2+ + 2e- à Cu0

E0 = 0.34 V/SHE

(6)

204

Cu0 + Pd2+ à Cu2+ + Pd0

Erxn = 0.61 V/SHE

(7)

205

In3+ + 3e- à In0

E0 = -0.34 V/SHE

(8)

206

3.2 Catalytic Nitrate Reduction. Batch experiments for catalytic NO3- reduction were

207

conducted with Pd-Cu/TinO2n-1, Pd-Cu/TiO2, and Pd-In/TinO2n-1 powders in the presence of H2 as

208

a reductant. The relevant half reactions during NO3- reduction are as follows:13,27

209

>?1@

6781 + :; 7 + 2= 1 A⎯⎯C 67;1 + 27: 1 >?1@

210

6781 + 2.5:; 7 + 4= 1 A⎯⎯C 0.56; 7 + 57: 1

211

6781 + 3:; 7 + 5= 1 A⎯⎯C 0.56; + 67: 1

212

6781 + 6:; 7 + 8= 1 A⎯⎯C 6:8 + 97: 1

>?1@

>?1@

(9) (10) (11) (12)

213

Pseudo first-order rate constants for up to 50% conversion were fit to the NO3- concentration

214

versus time profiles (SI, Figure S-5). The catalyst metal normalized NO3- reduction kinetics were

215

higher for Pd-Cu/TinO2n-1 (3.6 ± 0.2 L gmetal-1 min-1) than for Pd-In/TinO2n-1 (1.4 ± 0.2 L gmetal-1

216

min-1) and Pd-Cu/TiO2 (0.9 ± 0.1 L gmetal-1 min-1) (Figure S-5). The 2.7-fold increase in rate

217

constant for Pd-Cu/TinO2n-1 over Pd-Cu/TiO2 was unexpected since the surface area of the

218

TinO2n-1 catalyst was 7.0 times less (e.g., 1.2 m2 g-1 (TinO2n-1) versus 8.4 m2 g-1 (TiO2)). The


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219

increase in reactivity was attributed to oxygen vacancies in the TinO2n-1 support, which created

220

Ti3+ sites that reduced Pd and Cu to their metallic states during reaction and thus increased

221

catalytic turnover.33

222

The measured rate constants for Pd-Cu/TinO2n-1 (3.6 ± 0.2 L gmetal-1 min-1) and Pd-In/TinO2n-1

223

(1.4 ± 0.2 L gmetal-1 min-1) were comparable to literature values (Pd-Cu: 0.1 – 5.1 L gmetal-1 min-1

224

and Pd-In: 1.7 – 7.6 L gmetal-1 min-1).27,34 However, due to the low specific surface area of the

225

TinO2n-1 powder support (1.2 m2 g-1) compared to that of typical catalyst supports (~100-200 m2

226

g-1), these rate constants would be much higher if compared on a surface area normalized basis.

227

However, catalytic metal dispersion is rarely reported and therefore a comparison on a surface

228

area basis was not made.

229

The maximum NO2- concentrations during the batch experiments were 25 ± 4 µM and 47 ± 2

230

µM for the Pd-Cu/TinO2n-1 and Pd-In/TinO2n-1 catalyst, respectively (SI, Figure S-5b). The NO2-

231

concentration remained below the U.S. EPA’s maximum contaminant level (MCL) for drinking

232

water of 70 µM for both catalysts.7 The final !KLM0 values were 4.2 ± 0.2% for Pd-In/TinO2n-1 and

233

< 5.2 x 10-3 % for Pd-Cu/TinO2n-1. The final !KN/ were 19 ± 1% for Pd-In/TinO2n-1 and 22 ± 2%

234

for Pd-Cu/TinO2n-1. These results indicated that the catalysts exhibited high catalytic NO3-

235

activity but substantial NH3 production.

236

3.3 Electrocatalytic Nitrate Reduction. Electrochemical NO3- reduction has advantages over

237

catalytic reduction, as it does not require the storage and delivery of an external electron donor

238

(e.g., H2) and thus the development of compact POU treatment technologies are possible. The

239

REM, Pd-Cu/REM, and Pd-In/REM were tested for electrocatalytic NO3- reduction in flow-

240

through mode. Results for all experiments are summarized in Table S-1 and experiments

241

conducted at a cathodic potential of -2.5 V/SHE are summarized in Table 2.


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3.3.1 Effect of Flow Direction. Experiments conducted as a function of flow direction were

243

performed with Ar purging to remove dissolved oxygen. Reduction experiments for the cathode-

244

anode flow mode were performed at the OCP to -3.6 V/SHE (J = 600 LMH), and experiments

245

for the anode-cathode flow mode were performed at the OCP to -2.5 V/SHE (J = 600 LMH). The

246

-3.6 V/SHE potential was not used for the anode-cathode flow mode due to H2 bubble formation

247

at the REM surface, which resulted in increased pressure drop across the REM and large

248

fluctuations in current. The NO3- and NO2- concentration versus time profiles at different

249

potentials are summarized in Figure 3 for the anode-cathode flow mode and Figure S-6 for the

250

cathode-anode flow mode.

251

Nitrate conversion for both flow modes was highest for Pd-Cu/REM, followed by Pd-

252

In/REM and REM (Figures 3 and S-6; Table S-1). Both catalysts showed significant

253

enhancements in NO3- conversion compared to the REM, which is consistent with LSV scans

254

that showed increased current for Pd-M/REM compared to REM in the presence of 5 mM

255

NaNO3 (Figure S-7a). LSV scans also indicated that NO3- reduction became mass transport

256

limited in the anode-cathode flow mode at potentials more negative than ~ -2.0 V/SHE (Figure

257

S-7b). For both flow modes, the product selectivity was more favorable for the Pd-Cu/REM

258

relative to the Pd-In/REM, where the later consistently had higher selectivity towards NO2- and

259

NH3. Results for electrocatalytic NO3- reduction were similar to catalytic batch experiments that

260

showed Pd-Cu was more active and selective then Pd-In.

261

There were several differences in performance observed for the two flow modes. Discernable

262

NO3- reduction for all REMs was observed at lower potentials for the anode-cathode flow mode

263

(-0.2 V/SHE) compared to the cathode-anode flow mode (-1.2 V/SHE). Although the total

264

current was comparable at a given potential, the extent of NO3- conversion was always greater


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265

for the anode-cathode flow mode (Figures 3 and S-6). For example, at a potential of -2.5 V/SHE

266

NO3- conversion for the Pd-Cu/REM was approximately 20% in cathode-anode flow mode and

267

43% in anode-cathode flow mode (Table 2). The cathode-anode flow mode had high NO2- and

268

NH3 selectivity, while the anode-cathode flow mode had primarily N2 and N2O selectivity (Table

269

S-1, Table S-2, and Table 2). At a potential of -2.5 V/SHE, NO3- selectivity for the Pd-Cu/REM

270

in the cathode-anode flow mode was !KLM0 = 35%, !KN/ = 31%, !KM L = 13%, and !KM = 23%; and

271

the selectivity in the anode-cathode flow mode was !KLM0 = 0.07%, !KN/ < 2.3%, !KM L = 71%,

272

and !KM > 27% (Table 2).

273

The higher NO3- conversion and lower NO2- formation for the anode-cathode flow mode

274

relative to the cathode-anode flow mode were attributed to the placement of the counter

275

electrode. These results were consistent with prior studies that concluded a downstream counter

276

electrode (cathode-anode) resulted in lower conversion compared to an upstream counter

277

electrode (anode-cathode) in the presence of a more favorable side reaction (i.e., H2 evolution).35

278

In our experiments H2 production was more significant in the cathode-anode flow mode and

279

inhibited electrocatalytic NO3- reduction. Another potential advantage of the anode-cathode flow

280

mode is a lowering of the local pH, which could limit mineral scaling on the cathode surface.

281

The NH3 selectivity in the anode-cathode flow mode (NH3 not detected) was among the lowest

282

reported selectivity for electrochemical NO3- reduction,13 which is an important requirement for

283

a POU treatment technology.

284

These experiments indicated that the Pd-Cu/REM operated in the anode-cathode flow mode

285

was the most active for NO3- reduction and had the highest product selectivity for N atom

286

coupling (100% of detected products). At a potential of -2.5 V/SHE the permeate concentrations

287

were below the EPA MCLs of 0.7 mM for NO3- and 70 µM for NO2-, and NH3 concentrations


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288

were always below the IC detection limit (< 10 µM), which was less than the World Health

289

Organization (WHO) limit of 30 µM. Therefore, the anode-cathode flow mode was utilized for

290

all additional experiments.

291

3.3.2 Effect of Solution Conditions. Solution conditions can have an adverse effect on both

292

catalytic NO3- reduction activity and product selectivity,27,36-39 but few studies have investigated

293

the effects of solution conditions on electrocatalytic NO3- reduction. The O2 reduction reaction

294

(ORR) occurs at similar cathodic potentials as NO3- reduction,40 and dissolved organic matter can

295

adsorb and block catalytic sites.41,42 Therefore, the effects of dissolved oxygen, bicarbonate, and

296

a surface water sample (Table 1) on NO3- reduction were investigated.

297

The results for NO3- conversion and product selectivity for both Pd-Cu/REM and Pd-In/REM

298

in the air-saturated solution were similar to the Ar-saturated solution over the same applied

299

potentials (Figure 3, Table 2, Figure S-8, and Table S-1), indicating that the ORR did not

300

compete for NO3- reduction sites. LSV scans support this result, as measured currents were

301

nearly identical for Ar-saturated and air-saturated electrolytes (SI, Figure S-9). These results are

302

attributed to the low ORR activity of the TinO2n-1 support,43 promoter metals,44,45 and the Pd

303

crystal faces observed by XRD (i.e., Pd(111) and Pd(200)).46 The lack of competition from the

304

ORR during electrocatalytic NO3- reduction with the Pd-M/REMs is a significant advantage over

305

other electrodes and catalytic systems that require deoxygenated solutions.27,38,47

306

Nitrate conversion was not significantly affected by any of the other solution conditions

307

tested compared to the Ar-saturated solution, with the exception of the surface water sample that

308

showed NO3- conversion was inhibited at -2.5 V/SHE. The NO3- conversion in the surface water

309

decreased by 1.4-fold compared to the 10 mM NaHCO3 buffer solution of similar pH at -2.5

310

V/SHE (Figure 3a and Table 2). These results were most likely caused by scaling of CaCO3(s)


14

Page 15 of 31


311

and Mg(OH)2(s) onto the catalyst surface, as the alkalinity decreased from 124 mg L-1 to 67 mg L-

312

1

313

decreased from 0.26 mM to 0.19 mM at a cathodic potential of -2.5 V/SHE. In a long-term

314

treatment application mineral scaling can be overcome by periodic reverse polarity treatments to

315

dissolve the scale. This approach has been demonstrated in other electrochemical studies.48

, Ca2+ concentration decreased from 0.88 mM to 0.66 mM, and the Mg2+ concentration

316

Product selectivities were not greatly affected by the different solution conditions (Figure 3b,

317

Table 2, and Table S-1). The NO2- and NH3 concentrations were always below the MCL and IC

318

detection limit, respectively. The Pd-Cu/REM was able to reduce NO3- from a 1 mM feed

319

concentration to below the regulatory MCL in a single pass through the REM in the anode-

320

cathode flow mode, without negative effects from dissolved oxygen and carbonate species and

321

only minor effects from constituents found in surface water (e.g., Ca2+, Mg2+). In addition, the

322

oxidation of Cl- was not observed in the surface water sample, as the anode potential was < 2.0

323

V/SHE, and Cl2, ClO3-, and ClO4- were all below the detection limits of the analytical methods of

324

0.02 mg/L, 0.9 µg L-1, and 1.0 µg L-1, respectively.

325

3.3.3 REMs in Series. Though NO3- concentrations at the most cathodic potentials tested

326

were lower than the MCL (0.7 mM), two Pd-Cu/REMs in series were used to further decrease

327

the NO3- concentration. Experiments were performed in air-saturated solutions containing 1 mM

328

NaNO3 and in either the 10 mM NaHCO3 buffer or surface water sample. Results are

329

summarized in Figure 3, Table 2, and Table S-1. The results indicated that the electrochemical

330

cells operated as approximately two first-order reactors in series, where NO3- conversion (R)

331

followed equation (13):

332

V

()

O = 1 − QR( S U ,

(13)

T


15


333

Page 16 of 31

(

where R() S is the ratio of NO3- permeate and feed concentrations for a single reactor and n is ,

T

334

the number of reactors in series. The measured NO3- conversion in the 10 mM NaHCO3 buffer

335

for two Pd-Cu/REMs in series were 20% (17%), 29% (31%), 44% (49%), and 65% (69%) at

336

cathodic potentials of -0.2, -1.2, -1.9, and -2.5 V/SHE, respectively, where values in parentheses

337

were those predicted by equation (13). In the presence of the surface water sample, the NO3-

338

conversion for two reactors in series were 14% (7.5%), 23% (25%), 36% (43%), and 51% (54%)

339

at cathodic potentials of -0.2, -1.2, -1.9, and -2.5 V/SHE, respectively (Figure 3c and Table S-1).

340

The lower NO3- conversion in the surface water sample again was most likely caused by

341

CaCO3(s) and Mg(OH)2(s) scaling, as the alkalinity decreased from 124 mg L-1 to 32 mg L-1, Ca2+

342

concentration decreased from 0.88 mM to 0.49 mM, and the Mg2+ concentration decreased from

343

0.26 mM to 0.11 mM at a cathodic potential of -2.5 V/SHE. The product selectivities were

344

similar for the one and two reactors in series operations. The effluent pH values were as high as

345

8.9 in the surface water sample (SI, Table S-1). Therefore some adjustment of pH or

346

optimization in reactor design or operation may be necessary.

347

3.4 Reactivity and Mass Transport Characterization. Electrocatalytic and catalytic NO3-

348

reduction were both tested in the Pd-Cu/REM flow-through system as a function of J (240 to

349

2160 LMH (6.7 x 10-5 to 5.4 x 10-4 m s-1)) and results are shown in Figure 4. The NO2-

350

concentrations were low for both catalytic (0.10 to 0.13 µM) and electrocatalytic (0.70 to 0.52

351

µM) NO3- reduction, and the NH3 concentrations were always below the detection limit (< 10

352

µM) (Table S-3).

353

Based on LSV data, it was determined that electrocatalytic NO3- removal was mass transport

354

limited at potentials more negative than -2.0 V/SHE (Figure S-7). Therefore, at -2.5 V/SHE


16

Page 17 of 31


355

equation (14) was used to fit the electrocatlytic NO3- reduction data in the Pd-Cu/REM flow-

356

through reactor.

357

(

R() S = exp Z− ,

[\ ]^ _

`

(14)

358

In equation (14), km is the mass transport rate constant (m s-1), a is the specific surface area (m-1),

359

and l is the reactive length (m). Values for km in packed bed electrochemical reactors have been

360

shown to follow equation (15).

361

ab = c de.88

(15)

362

where b is a proportionality constant. Values for a and l were combined with b, and equation

363

(14) was rewritten as follows,

364

(

R() S = exp Z− ,

b_ f.// _

` = exp Z−

[ghi _

`

(16)

365

where m is a proportionality constant that was determined to equal 0.0017 and yielded observed

366

rate constant (kobs) values between 7.0 x 10-5 and 1.5 x 10-4 m s-1. The model fit of equation (16)

367

to experimental data is shown in Figure 4.

368

Since the conversion of NO3- during catalytic experiments was small (< 10%) relative to the

369

stoichiometric conversion with H2 (i.e., up to 32% via reaction 11), pseudo first-order kinetics

370

were assumed and equation (16) was also used to fit the catalytic NO3- reduction data. However,

371

since catalytic NO3- removal was much lower than the mass transport limit, kobs was represented

372

by a resistance in series model that accounts for the resistances from mass transport and reaction

373

kinetics (equation 17).

374

[

ajkl = am / Z1 + [ o `

(17)

\

375

In equation (17), kr is the heterogeneous pseudo first-order reaction rate constant (m s-1). Values

376

determined for km from electrocatalytic experiments were used in equation (17) and a best fit


17


Page 18 of 31

377

value of kr = 1.1 x 10-5 m s-1 was determined by fitting equation (17) to the experimental data

378

(Figure 4a). Results indicated that the kobs values for catalytic NO3- reduction were

379

approximately an order of magnitude less than those for electrocatalytic NO3- reduction, which

380

suggested that electrocatalysis was the predominant mechanism for NO3- removal in all

381

electrochemical REM flow-through experiments, as the retention times (0.2 to 2.2 s) in the REM

382

were too short to allow for significant catalytic NO3- reduction. Identical experiments were

383

performed for NO2- reduction, and the extent of removal with respect to J were similar to those

384

for catalytic NO3- reduction (Figure S-10), which explains why only trace NO2- concentrations

385

were detected in all experiments. These results suggest that electrocatalyic reduction is a

386

promising process to achieve significant NO3- removal without significant NO2- production

387

under single pass operation. By contrast, catalytic NO3- reduction in flow-through reactors

388

require residence times that are > 40 times longer than were reported in Figure 4 for

389

electrocatalytic NO3- reduction.37

390

3.5 Technological and Environmental Implications. Sustainable POU water treatment

391

technologies need to operate with minimal energy consumption. The energy requirements were

392

calculated according to equations 4 and 5 and are reported in Table 2 and Table S-4. The EEO and

393

EC values followed the order of: REM > Pd-In/REM > Pd-Cu/REM (Table 2 and Table S-4).

394

The minimal EC and EEO values for Pd-Cu/REM (EC = 0.57 kWh mol-1; EEO = 1.1 kWh m-3) and

395

Pd-In/REM (EC = 0.89 kWh mol-1; EEO = 1.7 kWh m-3) occurred for the Ar-saturated, 1 mM

396

NaNO3 solutions operated in anode-cathode flow mode. Values approximately doubled in the air-

397

saturated surface water solution with the Pd-Cu/REM (EC = 1.1 kWh mol-1; EEO = 2.3 kWh m-3).

398

The energy consumption values were EC = 0.62 ± 0.02 kWh mol-1 and EEO = 1.1 ± 0.12 kWh m-3

399

at 1 mM NO3- concentration, EC = 0.19 ± 0.07 kWh mol-1 and EEO = 3.5 ± 0.73 kWh m-3 at 10


18

Page 19 of 31


400

mM NO3- concentration, and 0.12 ± 0.06 kWh mol-1 and EEO = 26 ± 0.73 kWh m-3 at 100 mM

401

NO3- concentration, using the Pd-Cu/REM in anode-cathode flow mode (Figure S-4).

402

Concentrated NO3- solutions showed more favorable EC values, due to increased solution

403

conductivity and a 2.3RT/F positive Nernstian shift in the redox potential for every 10-fold

404

increase in NO3- concentration. Therefore, coupling the REM system to technologies that

405

separate and concentrate NO3-, without the accumulation of chloride, may be another useful

406

application for this technology (e.g., ion exchange with NaOH regeneration). Competitive

407

technologies, such as RO treatment and ion exchange, can achieve near complete removal of

408

NO3-, with energy requirements of 0.46 kWh m-3 for RO49 and 0.06 kWh m-3 for ion exchange.50

409

However, both RO and ion exchange produce brines that require further treatment or disposal.

410

Further improvements in REM reactor design and electrode preparation will lead to reduced

411

energy consumption and more efficient utilization of the catalytic metals. For example,

412

narrowing the inter-electrode gap will reduce solution resistance and lower the overall cell

413

potential, and more efficient dispersion of the catalyst metals will allow for lower loadings of

414

precious metals.

415

Current efficiencies were also calculated based on the observed product distributions (Table

416

S-1 and Table 2). The current efficiencies were between 1.9 to 12% for the REM, 2.1 to 105%

417

for the Pd-Cu/REM, and 2.3 to 71% for the Pd-In/REM. The maximum current efficiency for Pd-

418

Cu/REM (105 ± 5.4%) was in the presence of air purging and 1 mM NaNO3. The current

419

efficiency decreased to between 51 and 52% in the presence of the 10 mM NaHCO3 or surface

420

water solutions (Table S-1 and Table 2). These results were attributed to the higher ionic

421

conductivity and proton donor ability of the HCO3- electrolyte that gave rise to increased H2


19


Page 20 of 31

422

evolution. The range of current efficiencies observed in this study were comparable to values

423

reported in the literature for different electrocatalysts.13

424

The selectivity towards N2O was significant for all electrocatalytic NO3- reduction

425

experiments. The formation of N2O has environmental implications as it acts as a greenhouse

426

gas.51 The reduction of N2O to N2 has been shown to occur on Pd electrocatalysts with 100%

427

selectivity.52 However, in our studies the residence time (< 2 s) in the REM reactor was too short

428

for significant conversion of N2O to N2. For the treatment of dilute NO3- solutions (mM levels)

429

the total N2O flux is small compared to natural sources. For example, assuming 100% reduction

430

of 106 L (typical of a small drinking water plant) of 1 mM NO3- solution per day to N2O would

431

account for < 0.0003% of the total estimated N2O production rate per year (3.3 x 1011 kg yr-1).53

432

However, more selective electrocatalysts or passing the permeate through an additional Pd

433

catalyst bed with a longer residence time would eliminate N2O outgassing to the atmosphere. For

434

the treatment of concentrated NO3- solutions, the produced N2O could be recovered and used as

435

an energy source, which would improve the energy efficiency of the treatment process. Such a

436

strategy has been proposed for the biological treatment of nitrogen in wastewater.54

437

Supporting Information

438

Additional experimental details, SEM/EDS analyses, catalytic batch nitrate reduction results,

439

additional REM experimental results, LSV scans, and tabulated experimental results.

440

Corresponding Author

441

*E-mail: [email protected]

442

Acknowledgements

443

We thank Dr. Yin Wang from the University of Wisconsin-Milwaukee for BET analysis.


20

Page 21 of 31


444

Funding for this work was provided by a National Science Foundation CAREER award to BPC

445

(CBET-1453081).


21

b)

37

448 449

450

451

42

447

47

2ϴ Ti4 O7 (30-2) Ti4 O7 (1-2-6) Ti6 O11 (1-2-11) Ti4 O7 (1-4-2) Ti4 O7 (2-42) Ti6 O11 (2-40)

25

Cu (200)

In (200)

20 Pd (200)

30 2ϴ

52

57


Ti4O7 (02-4)

Ti4O7 (200) Ti4O7 (2 0 10) Ti4O7 (104) Ti4O7 (120)

Ti4O7 (10-4) Ti4O7 (022)

Ti4O7 (1-22) Ti6O11 (10-1) Ti4O7 (1-2-2)

Ti6O11 (1-21)

Ti4O7 (1-20)

Ti6O11 (101)

Ti5O9 (10-2)

Ti4O7 (10-2)

a)

In (111) Pd (111) Ti4 O7 (2-13) Ti4 O7 (031)

Normalized Intensity

446

Cu (111) In (002)

Normailzed Intensity

Environmental Science & Technology Page 22 of 31

Figures and Tables.

4

3

2 1

35

4

3

2

1

62

Figure 1. XRD data for 1) TinO2n-1 powder, 2) REM, 3) Pd-Cu/REM and 4) Pd-In/REM. The locations of the characteristic peaks for Ti4O7, Ti6O11, In, Pd, and Cu are represented by the vertical dashed lines. a) 2ϴ = 20-38 deg and b) 2ϴ = 38-52 deg.

22

Environmental Science & Technology Page 23 of 31

452

456

455

454

are estimated penetration depths of the catalyst metals.

g) Ti, h) O, i) Pd, and j) In in Pd-In/REM. Top surface of the pellet is the left side of the image. Solid black lines on panels d, e, i, and j

Scanning electron microscope and microprobe mapping of the vertical cross-section: b) Ti, c) O, d) Pd, and e) Cu in Pd-Cu/REM; and

Figure 2. Scanning electron microscope and microprobe image of the vertical cross-section of a) Pd-Cu/REM and f) Pd-In/REM.

453

457

23



= REM-Ar = Pd-Cu/REM-Buffer-Air = Pd-Cu/REM-Air = Pd-Cu/REM-Ar

458

Page 24 of 31

= Pd-Cu/REM-Surface Water = Pd-Cu/REM-Series-Surface Water = Pd-Cu/REM-Series-Buffer

459

Figure 3. a) Nitrate and b) nitrite concentration profiles for REM and Pd-Cu/REM at different

460

potentials in the anode-cathode flow mode under different solution conditions. c) Nitrate and

461

d) nitrite concentration profiles for REM and two Pd-Cu/REM in series at different potentials in

462

anode-cathode mode.

463


24

Page 25 of 31


464

kobs (m s-1)

a)

1.60E-04 1.20E-04 8.00E-05

465

Nitrate (C/Co)

Catalytic

4.00E-05 0.00E+00 0.00E+00

b)

Electrocatalytic

4.00E-04 J (m s-1)

8.00E-04

1.2 1.0 0.8

MCL

0.6 0.4

Catalytic

0.2

Electrocatalytic

0.0 0.00E+00 466

4.00E-04 J (m s-1)

8.00E-04

467

Figure 4. Electrocatalytic and catalytic NO3- reduction in the Pd-Cu/REM flow-through reactor.

468

a) kobs values versus J, b) NO3- concentration versus J. The solid lines are the model fits using

469

equations 16 and 17 and the dashed horizontal line represents the MCL for NO3-.

470


25


471

Page 26 of 31

Table 1. Water quality data of the surface water sample. Sodium Potassium Calcium Magnesium Chloride Sulfate Bicarbonate Alkalinity COD pH

Filtered Surface Water 0.56 mM 0.17 mM 0.90 mM 0.25 mM 0.63 mM 0.015 mM 2.42 mM 124 mg L-1 as CaCO3 39 mg L-1 8.1

472


26

473

Anode-Cathode/Air Purging/Surface Water

Anode-Cathode/Air Purging/Bicarbonate Buffer

Anode-Cathode/Air Purging

Anode-Cathode/Ar Purging

Cathode-Anode/Ar Purging

65 ± 1.3

32 ± 1.09

44 ± 1.4

42 ± 1.8

43 ± 2.2

20 ± 1.02

NO3Conversion (%)

0.07 ± 0.14

0.40 ± 0.06

0.07 ± 0.01

0.56 ± 0.02

0.78 ± 0.04

0.07 ± 0.02

35 ± 2.4

S(NO2-) (%)

< 1.9 ± 0.08

< 1.5 ± 0.04

< 3.1 ± 0.11

< 2.2 ± 0.07

< 2.3 ± 0.1

< 2.3 ± 0.11

31 ± 2.3

S(NH3) (%)

52 ± 0.84

43 ± 0.36

43 ± 0.32

42 ± 0.36

55 ± 0.29

70 ± 0.38

12 ± 0.09

S(N2O) (%)

47 ± 0.93

56 ± 0.94

56 ± 0.49

57 ± 0.03

43 ± 1.01

29 ± 0.72

23 ± 0.91

S(N2) (%)

8.9 ± 0.3

8.0 ± 0.2

8.8 ± 0.2

7.8 ± 0.4

9.4 ± 0.2

9.2 ± 0.1

9.8 ± 0.2

Effluent pH

35 ± 5.01

60 ± 4.2

51 ± 4.3

51 ± 2.5

106 ± 5.4

102 ± 5.7

60 ± 2.4

CE (%)

6.9 ± 0.13

3.5 ± 0.27

2.7 ± 0.17

2.3 ± 0.34

1.1 ± 0.12

1.1 ± 0.23

2.6 ± 0.06

EEO (kWh m-3)

3.7 ± 0.04

2.4 ± 0.06

1.4 ± 0.03

1.3 ± 0.08

1.06 ± 0.03

0.90 ± 0.07

1.9 ± 0.02

EC (kWh mol-1)

Table 2. Summary of results for 1 mM nitrate reduction with a Pd-Cu/REM at a cathodic potential of -2.5 V/SHE

Anode-Cathode/Air Purging/Bicarbonate Buffer/Series 51 ± 2.07

Reaction Conditions

Anode-Cathode/Air Purging/Surface Water/Series

27


474

475


Page 27 of 31


Page 28 of 31

476

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614 615

Table of Contents/Abstract Graphic

N2

H H

H2

Pd

NO2NO3-

H

H H

REM Cathode (+ne-)

Cu

H2 O

REM (+ne-)

616


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Electrocatalytic Reduction of Nitrate Using MagnÃ©li Phase TiO2

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