Support Morphology-Dependent Catalytic Activity ... - ACS Publications

Jun 29, 2015 - (24) The morphology of an oxide support, including crystal planes exposed on ... Hence, it is worth applying the Pd/CeO2 catalyst to th...
0 downloads 3 Views 3MB Size
Subscriber access provided by UNIV OF MISSISSIPPI

Article

Support Morphology-dependent Catalytic Activity of Pd/CeO2 for Formaldehyde Oxidation Hongyi Tan, Jin Wang, Shuzhen Yu, and Kebin Zhou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b01264 • Publication Date (Web): 29 Jun 2015 Downloaded from http://pubs.acs.org on July 6, 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.

Environmental Science & Technology 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.

Page 1 of 24

Environmental Science & Technology

1

Support Morphology-dependent Catalytic Activity

2

of Pd/CeO2 for Formaldehyde Oxidation

3

Hongyi Tan, Jin Wang, Shuzhen Yu, and Kebin Zhou*

4

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

5

Beijing 100049, P.R. China

6

Keywords: Formaldehyde (HCHO), Catalytic oxidation, Pd/CeO2, Morphology-dependent,

7

Metal-support interaction

8

9

Abstract: To eliminate indoor formaldehyde (HCHO) pollution, Pd/CeO2 catalysts with

10

different morphologies of ceria support were employed. The palladium nanoparticles loaded on

11

{100}-faceted CeO2 nanocubes exhibited much higher activity than those loaded on {111}-

12

faceted ceria nanooctahedrons and nanorods (enclosed by {100} and {111} facets). The HCHO

13

could be fully converted into CO2 over the Pd/CeO2 nanocubes at a GHSV of 10 000 h-1 and a

14

HCHO inlet concentration of 600 ppm at ambient temperature. The prepared catalysts were

15

characterized by a series of techniques. The HRTEM, ICP-MS and XRD results confirmed the

16

exposed facets of the ceria and the sizes (1-2 nm) of the palladium nanoparticles with loading

17

amounts close to 1%. According to the Pd 3d XPS and H2-TPR results, the status of the Pd-

18

species was dependent on the morphologies of the supports. The {100} facets of ceria could

ACS Paragon Plus Environment

1

Environmental Science & Technology

Page 2 of 24

19

maintain the metallic Pd species rather than the {111} facets, which promoted HCHO catalytic

20

combustion. The Raman and O 1 s XPS results revealed that the nanorods with more defect sites

21

and oxygen vacancies were responsible for the easy oxidation of the Pd-species and low catalytic

22

activity.

23 24 25 26 27 28 29 30 31 32 33 34 35 36

ACS Paragon Plus Environment

2

Page 3 of 24

37

Environmental Science & Technology

Introduction:

38

Formaldehyde (HCHO) is considered to be a major toxic indoor pollutant that can be

39

sourced from various wood-based materials, insulation materials, adhesives and coatings.1 Long-

40

term exposure to formaldehyde may induce eye, nose, and skin irritation, pulmonary diseases or

41

even nasopharyngeal cancer.2 For the sake of improving human health, significant efforts have

42

been made to eliminate indoor HCHO pollution. Among the numerous HCHO purity

43

techniques,3-6 catalytic oxidation has proven to be an efficient and promising method.7-9 To

44

achieve HCHO purification at ambient temperature, the supported noble catalysts (Pt, Au, Pd, Rh

45

and Ag) are more favorable.

46

received much attention in recent years due to their high efficiency.15-18 However, the high prices

47

of these precious metals limit their widespread application, and there is still a large demand for

48

the development of low cost catalysts with good activity at room temperature.

10-14

Particularly, the Pt- and Au- based supported catalysts have

49

Among the noble metals, palladium is less expensive and more abundant compared with

50

gold and platinum.19 The palladium-based catalysts have been demonstrated to be very

51

competent at catalyzing the oxidation of various compounds, including methane, benzene, ethyl

52

acetate and other volatile organic compounds.20-22 In the early research, however, the activity of

53

Pd-based catalysts toward the catalytic oxidation of HCHO does not stand out in comparison

54

with the other noble metals.13 Most recently, He et al. found that the addition of sodium ions to

55

Pd/TiO2 led to completely catalytic oxidation of HCHO at ambient temperature, and the

56

negatively charged Pd-species played a key role in the significant activity promotion.23

57

Therefore, to improve the performance of palladium catalysts in the catalytic oxidation of

58

HCHO, it seems crucial to understand the relationship between the structure of the catalysts and

59

the chemical state of the Pd-species and to stabilize the Pd-species in its metallic form.

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 24

60

Metal-oxide interactions are of growing importance with respect to supported noble metal

61

catalysts. The charge redistribution and mass transport processes at the interface may happen

62

because of the contact between a metal nanoparticle and the oxide.24 The morphology of an

63

oxide support, including crystal planes exposed on the surface and crystallinity, is an important

64

structural aspect in the process. Therefore, morphology-controlling strategies have become a new

65

method for tuning the catalytic performance of oxide-incorporating catalysts.25, 26 Ceria (CeO2) is

66

one of most important oxides in heterogeneous catalysis and is widely used as the support

67

because of its redox performance.27-30 Various uniform and well-defined morphologies of ceria

68

have been synthesized to explore the relationship between morphology and catalytic

69

performance in a series of catalytic reactions.31, 32 In 2008, when applying Au/CeO2 catalysts to

70

the water-gas shift reaction, Si et al. found that the valence of gold varied with the shape of the

71

ceria supports. The chemical state of the active component could be affected when being loaded

72

on different crystal planes.33 Similar phenomena were observed when platinum or ruthenium was

73

supported on ceria with different morphologies.34-36 The morphological effect of the ceria on the

74

supported Pd catalysts, to the best of our knowledge, has seldom been studied. Hence, it is worth

75

applying the Pd/CeO2 catalyst to the catalytic oxidation of HCHO and trying to tune its catalytic

76

activity with different morphologies of ceria.

77

In this work, we prepared different Pd/CeO2 catalysts by employing different morphologies

78

of ceria (nanocubes, nanooctahedrons, and nanorods). The catalytic performances for HCHO

79

oxidation were found to be quite different with these three catalysts. The palladium nanoparticles

80

showed the best redox ability with the ceria nanocubes, and the activity was much higher than

81

for the other two catalysts, achieving full combustion of 600 ppm of HCHO at a gas hourly space

82

velocity (GHSV) of 10 000 h-1 at ambient temperature. The relationship between the catalytic

ACS Paragon Plus Environment

4

Page 5 of 24

Environmental Science & Technology

83

performance and the morphology of the ceria support was elucidated by analyzing the

84

characterization results.

85 86 87 88

Experimental Section All the materials are analytically pure and were used as received without further purification in this study.

89

Preparation of the Ceria Supports. All of the ceria materials were prepared via the

90

hydrothermal method. The preparation of CeO2 nanocrystals exposing different facets followed

91

the method used in our previous studies.32, 37 For ceria nanorods, 1.5 g of Ce(NO)3·6H2O was

92

dissolved in 20 ml deionized water, and a proper amount of 10% NaOH solution was added.

93

Then, all of the slurry was rapidly transferred into a 50 ml autoclave, which was filled with water

94

up to 80% of its total volume to give a final NaOH concentration of approximately 2 M. The

95

autoclave was heated at 120 °C in the oven for 12 h. The final product was collected by filtration,

96

washed with deionized water to remove any possible ionic remnants, and then dried and calcined

97

at 350 °C for 4 h. To obtain CeO2 nanooctahedrons, 6 g of Ce(NO)3·6H2O was dissolved in 30

98

ml water, and 10 ml solution containing 0.01 g NaOH was added under vigorous stirring. The

99

stock solution was stirred vigorously for approximately 10 min and then sealed in a 50 ml

100

autoclave. Hydrothermal treatment was conducted at 180 °C for 12 h. The final product was

101

collected by filtration, washed with deionized water and then dried and calcined at 350 °C for 4

102

h. The preparation of CeO2 nanocubes followed the same procedure as nanooctahedrons except

103

that the dosage of Ce(NO)3·6H2O was 1 g, 8 g of NaOH was used, and the reaction temperature

104

was set to 200 °C for 24 h.

ACS Paragon Plus Environment

5

Environmental Science & Technology

Page 6 of 24

105

Loading of the Palladium Nanoparticles. CeO2 powder (0.3 g of nano-rods, octahedrons,

106

and cubes) was dispersed in 100 ml deionized water and sonicated for 10 min, then 0.005 g of

107

PdCl2 was added and stirred for an hour. After that, the pH of the slurry was adjusted to neutral

108

with NaOH. After that, 2 ml of an aqueous solution containing 0.01 g NaBH4 was added into the

109

suspension as reducing agent under stirring after aging. The obtained precipitates were washed

110

with deionized water and ethanol 3–4 times and dried at 60 °C overnight. The obtained

111

palladium supported CeO2 nanorods, octahedrons, and cubes are denoted as Pd/Rod, Pd/Oct, and

112

Pd/Cube, respectively.

113

Catalytic Activity Test. The catalytic activity test for the oxidation of formaldehyde over

114

the catalysts was conducted in a continuous flow fixed-bed quartz tubular reactor (4 mm internal

115

diameter) at atmospheric pressure. 100 mg of catalyst was sandwiched between two quartz wool

116

layers in the micro-reactor. Before the test, the catalyst was reduced under a hydrogen

117

atmosphere at 200 °C for an hour and stored in the air after cooling in case the catalysts were

118

oxidized during drying process. The standard feed gas contained 600 ppm of HCHO, 20% O2,

119

and N2 comprised the balance. Gaseous HCHO was generated by flowing N2 over

120

paraformaldehyde, which was placed in a thermostatic water bath. The concentration of HCHO

121

was determined by the flow rate of N2 and the temperature of the water bath. The total flow rate

122

was 45 ml min-1, corresponding to a gas hourly space velocity (GHSV) of 10 000 h-1. Addition of

123

moisture was achieved by another stream of air flowed through a water bubbler in a thermostatic

124

water bath, and the relative humidity was tuned by the temperature of the water bath ranged of

125

10 to 40 oC. The streams of moisture and HCHO were rapidly mixed before catalytic reaction

126

and the total humidity was calculated with saturated vapor pressure of water. The total flow rate

127

was raised to 60 ml min-1, corresponding to a gas hourly space velocity (GHSV) of 15 000 h-1.

ACS Paragon Plus Environment

6

Page 7 of 24

Environmental Science & Technology

128

The temperature of the reactor stayed at the room temperature under different moisture

129

conditions proved that the temperature of water bath has little effect on the reaction temperature.

130

The generated CO2 resulting from HCHO oxidation was monitored by an on-line SP-2100 gas

131

chromatograph equipped with a FID detector. The conversion of formaldehyde is expressed as

132

[CO2]/[CO2]*, where [CO2]* is the concentration of CO2 in the effluent when HCHO is oxidized

133

completely and [CO2] is the concentration of CO2 at various temperatures. The initial

134

concentration of HCHO is determined by external standard method. A CO2 standard curve was

135

created using the different CO2 concentrations. And the concentration of CO2 when HCHO is

136

oxidized completely is 600 ppm according to the standard curve.

137

Catalysts Characterization. The size and morphology of the catalysts were characterized

138

with a FEI Tecnai F20 high-resolution transmission electron microscope (HRTEM). The powder

139

X-ray diffraction (XRD) patterns were obtained on an MSAL-XD2 X-ray diffractometer with

140

Ni-filtered Cu Kα radiation (λ=0.1541 nm), and the data were recorded at a scan rate of 4

141

degrees min-1. The elemental analysis was obtained with a Varian Vista MPX Inductively

142

Coupled Plasma Optical Emission Spectrometer (ICP-OES). The Brunauer−Emmett−Teller

143

(BET) surface area was evaluated from nitrogen adsorption data recorded using a Gemini V

144

Micromeritics (U.S.A). X-ray photoelectron spectroscopy (XPS) measurements were performed

145

with an ESCALAB 250 Xi photoelectron spectroscope. The samples were reduced under a

146

hydrogen atmosphere at 200 °C for an hour and stored in the air after cooling before XPS test. H2

147

temperature-programmed reduction (TPR) was conducted with a ChemiSorb 2720 apparatus

148

equipped with a TCD detector. Before the H2-TPR analysis, the samples were treated in pure

149

oxygen at 200 °C for an hour. TPR was performed by heating the catalysts (approximately 50

150

mg) from approximately 0 to 850 °C in a 5 vol% H2-Ar mixture with a flow rate of 25 mL min-1.

ACS Paragon Plus Environment

7

Environmental Science & Technology

151

Page 8 of 24

Results and Discussion

152

Crystal and structural properties. Figure 1a shows the typical low-resolution HRTEM

153

image of the as-obtained Pd/Cube, with the average sizes of the ceria nanocubes approximately

154

50 nm. The high-resolution image in Figure 1b shows the clear (200) lattice fringe with an

155

interplanar spacing of 0.27 nm, revealing that the CeO2 nanocubes are mainly enclosed by the

156

{100} planes.38 Figure 1c shows that sizes of the CeO2 nanooctahedrons are between 40–60 nm,

157

and the high-resolution image (Figure 1d) shows a clear (111) lattice fringe with an interplanar

158

spacing of 0.31 nm, implying the nanooctahedrons are mainly enclosed by {111} facets. Figure

159

1e shows the low-resolution image of the Pd/Rod, and the ceria nanorods have a less uniform

160

diameter between 10–20 nm and lengths between 100–300 nm. Based on the interplanar spacing,

161

the high-resolution image results in Figure 1f suggests that the predominantly exposed planes are

162

both {100} and {111} planes, which is consisted with our previous results.32 The lattice fringe is

163

less clear in the nanorods, which may be due to a poorer crystallinity and crystal defects on its

164

surface. In addition, the palladium nanoparticles are noted with white arrows in the HRTEM

165

images of the three samples. The hemispheric palladium nanoparticles are homogeneously

166

dispersed on the surface of the CeO2 supports. The sizes of the nanoparticles are similar, all

167

approximately 2 nm.

168

ACS Paragon Plus Environment

8

Page 9 of 24

Environmental Science & Technology

169 170

Figure 1. HRTEM images of (a, b) Pd/Cube, (c, d) Pd/Oct, (e, f) Pd/Rod with low and high

171

resolution, respectively.

172

Figure 2 shows the XRD patterns of the Pd/CeO2 catalysts. The feature peaks are observed

173

at 2θ= 28.6, 33.1, 47.5, 56.3, 59.1, 69.4 and 76.7, which correspond to the (111), (200), (220),

174

(311), (222), (400), (331) plane diffraction patterns of the fluorite structure of CeO2 (JCPDS 34-

175

0394). However, no feature peaks corresponding to the palladium species can be found in the

176

XRD patterns, confirming that Pd nanoparticles should be small in size and highly dispersed on

177

the surface of all three CeO2 supports.39 The broadening of the diffraction peak ascribed to the

178

nanorods distinctly indicates its poorly crystalline nature, with agrees with the results shown in

179

the HRTEM image.

180

ACS Paragon Plus Environment

9

Environmental Science & Technology

Page 10 of 24

181 182

Figure 2. XRD patterns of Pd/Cube, Pd/Oct and Pd/Rod.

183

BET surface areas of the catalysts are listed in Table 1. The specific area of Pd/Rod is 83.1

184

m2/g, which is 3–4 times that of Pd/Cube and Pd/Oct (18.4 and 29.9 m2/g, respectively). The

185

palladium loading content was measured with ICP-OES. As shown in Table 1, the palladium

186

contents in these three samples are all similar to our desired 1 wt% palladium loading. Combined

187

with the HRTEM results, it can be concluded that the physical states of Pd nanoparticles in the

188

three catalysts are similar.

189 190

Table 1. Specific area and the palladium content of different Pd/CeO2 catalysts Sample

Pd/Cube

Pd/Oct.

Pd/Rod

BET area (m2/g)

18.4

29.9

83.1

Pd content (wt%)

0.91

0.94

0.96

191

ACS Paragon Plus Environment

10

Page 11 of 24

Environmental Science & Technology

192

Activity Test. The catalytic activities of formaldehyde oxidation on the three Pd/CeO2 catalysts

193

were evaluated with an air stream containing 600 ppm HCHO at a GHSV of 10 000 h-1, and the

194

temperature dependence of formaldehyde conversion is shown in Figure 3. The oxidation of

195

HCHO starts slightly higher than the room temperature over the Pd/Rod catalyst, it accelerates

196

quickly after 50 °C, and a complete conversion is achieved at 90 °C. The Pd/Oct catalyst

197

performs much more efficiently than Pd/Rod. The “light-off” temperature of Pd/Oct is much

198

lower than that of Pd/Rod, which is approximately 5 °C. The conversion of HCHO reached 45%

199

at room temperature and was completed at 60 °C. When compared to the Pd/Oct, the Pd/Cube

200

shows an even higher catalytic activity. The HCHO is fully converted into water and carbon

201

dioxide at 20 °C. A 35% conversion of HCHO can be achieved even at 0 °C.

202

203 204

Figure 3. HCHO conversion over Pd/Cube, Pd/Oct and Pd/Rod samples at different

205

temperatures. Reaction conditions: 600 ppm of HCHO, 20% O2, N2 balance, GHSV 10 000 h-1.

ACS Paragon Plus Environment

11

Environmental Science & Technology

Page 12 of 24

206

The specific reaction rate of formaldehyde oxidation in terms of consumed amount of

207

HCHO per gram of palladium per second was measured. The effects of internal diffusion and

208

external diffusion were eliminated by tuning the amount of catalysts and diluting with quartz

209

sand. At room temperature, the specific reaction rate for Pd/Oct and Pd/Cube is 6.0 and 21.6

210

µmol g(Pd)-1 s-1, respectively, and the Pd/Rod showed no activity at this temperature. This result

211

indicates that the morphology of the CeO2 supports has a significant effect on the rate of

212

formaldehyde oxidation. For comparison, we calculated the apparent reaction rate of some

213

catalysts that can realize the total removal of formaldehyde at room temperature in the literature.

214

The apparent reaction rate was 11.2 µmol g(Pt)-1 s-1 for Pt/f-SiO2,40 10.78 µmol g(Au)-1 s-1 for

215

Au/FeOx,41 and 1.5 µmol g(Pd)-1 s-1 for Pd/TiO2,42 respectively. And under our experiment

216

condition, the apparent reaction rate of Pd/Cube is 13.6 µmol g(Pd)-1 s-1. These results suggest the

217

Pd/Cube can be an inexpensive and efficient catalyst for indoor formaldehyde removal.

218 219

Figure 4. Pd 3d XPS spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.

ACS Paragon Plus Environment

12

Page 13 of 24

Environmental Science & Technology

220

XPS Analysis of Pd Species. Within the different substrates, the active chemical state of the Pd

221

species may vary.42, 43 To investigate the chemical states of the Pd elements on the prepared

222

catalysts surfaces and determine the relationship with their catalytic activities, XPS analyses of

223

Pd 3d were performed. The obtained spectra are summarized in Figure 4, and the binding energy

224

peaks were deconvoluted according to the literature.44 It can be found that the Pd/Cube displays

225

two Pd 3d5/2 peaks at 335.89 and 336.98 eV, the typical characteristics of metallic Pd and Pd

226

oxide, respectively. In the case of Pd/Oct, the two peaks of Pd 3d5/2 shift to higher binding

227

energy, at 336.10 and 337.00 eV, respectively. For Pd/Rod, the binding energy of Pd 3d5/2 is

228

337.39 eV, which can be attributed to Pd oxide. The calculation results shows that approximately

229

54% of Pd species on the Pd/Cube are in the metallic state, and on the Pd/Oct, this value

230

decreases to 27%, whereas all the Pd species in Pd/Rod are in oxide form. These results reveal

231

that the palladium of Pd/Cube is easier to keep metallic state in air and more electron rich than

232

that in other two samples, which may be due to the metal-support interaction in Pd/CeO2.45 It is

233

well known that the formation of metallic Pd species can promote the activation of oxygen

234

species because the O2 adsorption is enhanced by the donation of electrons from the metal to the

235

antibonding п* orbital of O2.46, 47 Hence, the percentage of metallic Pd species depending on the

236

morphology of CeO2 is vital to the low temperature catalytic oxidation of HCHO. High

237

percentages of metallic Pd species lead to more efficient catalytic activity at ambient temperature.

238 239

Reducibility of Catalysts. To explore the probable reason for the different chemical states of Pd

240

species, H2-TPR was employed to further investigate the redox ability of Pd species on the

241

Pd/CeO2 catalysts. Figure 5 shows the H2-TPR profiles of the three Pd/CeO2 catalysts. All of

242

these profiles show two reduction peaks in the temperature region of 0–850 °C. The peaks at

ACS Paragon Plus Environment

13

Environmental Science & Technology

Page 14 of 24

243

high temperature regions (> 600 °C) can be attributed to the reduction of lattice oxygen of bulk

244

CeO2 (not shown), while the peaks at low temperatures can be attributed to the reduction of Pd

245

species in the oxide state. As shown in Figure 5, the intense reduction peaks of the Pd species

246

center at 31, 40 and 75 °C for the Pd/Cube, Pd/Oct and Pd/Rod, respectively. This result

247

indicates the reducibility of the Pd species of different catalysts follows the trend: Pd/Cube >

248

Pd/Oct > Pd/Rod. The Pd species on the CeO2 nanocubes are easier to reduce, which benefits the

249

maintenance of its metallic state and its HCHO catalytic oxidation activity. The hydrogen

250

consumption of the reduction peaks at low temperatures was calculated, while the theoretical

251

value of hydrogen consumption on Pd/CeO2 was also calculated based on the assumption of PdO

252

formation from palladium concentration in ICP-OES results (Table 1). The actual hydrogen

253

consumption amounts of Pd/Cube and Pd/Oct are 8.19 and 13.61 µmol, respectively, which is

254

close to the theoretical value 4.29 and 4.62 µmol. However, the actual hydrogen consumption of

255

Pd/Rod is 72.40 µmol, much higher than the theoretical value of 4.62 µmol, which implies

256

significant surface oxygen species reduction together with the reduction of Pd species. This

257

phenomenon may be due to the strong palladium and ceria nanorods interaction, oxygen transfer

258

and reduction enhancement allowed by the defects in the ceria rods crystallites.48

259

Considering most of the characteristics of Pd/Cube and Pd/Oct are similar, the differences

260

in redox ability of Pd species should be attributed to the different facets to which they are

261

exposed. That is to say, the Pd species have more favorable redox properties on the {100} facets

262

of ceria than on the {111} facets, and are able to maintain the metallic state on the {100} facets,

263

which leads to more efficient oxidation activity.

ACS Paragon Plus Environment

14

Page 15 of 24

Environmental Science & Technology

264 265

Figure 5. H2-TPR profiles of Pd/Cube, Pd/Oct and Pd/Rod samples.

266

Effect of Ceria Surface Defects. If one solely ascribes the different chemical states of the Pd

267

species to the exposed facets, it is difficult to explain the fact that the Pd/Rod catalyst which

268

exposes both the (100) and (111) facets showed the poorest activity. To further clarify how the

269

morphology of CeO2 affects the redox property of the supported palladium species, Raman

270

spectra were collected to study the ceria surface defects. Figure 6 displays the visible Raman

271

spectra (514 nm) of the catalysts. To facilitate observation, the data of Pd/Rod were 4 times

272

magnified. A distinct F2g symmetry mode of the CeO2 phase centers at approximately 462 cm-1,

273

which can be attributed to symmetrical stretching of the Ce-O vibrational unit in 8-fold

274

coordination.49 The peaks at 598 and 1179 cm-1 are attributed to the defect induced mode (D) and

275

the second-order longitudinal optical mode (2 LO), respectively. The peak intensity of defect

276

induced mode depends on the presence of some defects and is enhanced when oxygen vacancies

277

are present in the ceria lattice.50, 51 The degree of defect sites on CeO2 can be determined from

278

the relative intensity of D/F2g.52 The intensity ratios of Pd/Cube and Pd/Oct are quite close,

279

which are 0.024 and 0.026, respectively. However, the intensity ratio of Pd/Rod is 0.188, much

ACS Paragon Plus Environment

15

Environmental Science & Technology

Page 16 of 24

280

larger than the other two catalysts, revealing that the Pd/Rod has the most intrinsic defect sites

281

and the oxygen vacancies are abundant.

282 283

Figure 6. Raman spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.

284

It is well known that the oxygen vacancies are conducive to absorbing oxygen species.53, 54

285

The surface oxygen species were further characterized by the O 1 s XPS analysis. Figure 7

286

shows the deconvoluted O 1 s XPS spectra of the Pd/CeO2 catalysts. The O 1 s XPS data exhibit

287

a main peak at the binding energy range of 529.30–529.44 eV and a smaller shoulder peak at the

288

binding energy range of 531.03–531.30 eV. The common main peak can be ascribed to the

289

lattice oxygen of bulk CeO2 (Olat), and the shoulder peak can be attributed to surface

290

chemisorbed oxygen (Osur), which contains O-, O2-, O22-, etc. The ratios of Osur/(Olat + Osur) are

291

approximately 34%, 31% and 43% for Pd/Cube, Pd/Oct and Pd/Rod, respectively. The

292

percentage of Osur on the Pd/Rod catalyst is higher than that of Pd/Cube and Pd/Oct. This

293

phenomenon implies that the Pd/Rod exhibited a higher concentration of chemisorbed oxygen,

294

which may due to the higher proportion of Ce3+ in the ceria nanorods generated more oxygen

295

vacancies (Figure S1). According to the literature, the oxygen vacancies on the ceria can

296

generate adsorbed atomic oxygen.55 The adsorbed oxygen can greatly influence the oxidation

ACS Paragon Plus Environment

16

Page 17 of 24

Environmental Science & Technology

297

and reduction properties of the supported Pd-species; the metallic palladium can be oxidized and

298

subsequently stabilized by the oxygen vacancies,27, 28 resulting in deactivation of the Pd/Rod

299

catalyst.

300 301

Figure 7. O 1 s XPS spectra of Pd/Cube, Pd/Oct and Pd/Rod samples.

302

Stability of Pd/Cube Catalyst. The durability and humidity resistance of the catalysts are very

303

important properties in their practical applications. The catalytic performance of the Pd/Cube

304

was measured by a long isothermal test at the room temperature. With the same reaction

305

conditions, no obvious activity loss is observed and approximately 100% HCHO conversion is

306

maintained during the 12 h test as shown in the Figure 8a. For the different catalysts of HCHO

307

catalytic oxidation, the effect of moisture on catalytic activity may be positive or negative which

308

is attributed to desorption of products, competitive adsorption of water and reactant56 or catalytic

309

effect of surface hydroxide radical.57 Figure 8b shows the effect of humidity on catalyst

310

performance of Pd/Cube. An additional stream of moisture was added to the system. With the

311

elevated GHSV (15000 h-1), the HCHO supply was also increased to maintain the concentration

ACS Paragon Plus Environment

17

Environmental Science & Technology

Page 18 of 24

312

of HCHO at 600 ppm. As seen, the presence of water has little effect on the activity of the

313

catalyst. The Pd/Cube catalyst is robust within a relative humidity range of 0–70 %.

314 315

Figure 8. (a) Durability test of Pd/Cube. Reaction conditions: 600 ppm of HCHO, 20% O2, N2

316

balance, GHSV 10 000 h-1, at room temperature; (b) Catalytic activity of Pd/Cube under different

317

humidity conditions. Reaction conditions: 600 ppm of HCHO, 20% O2, N2 balance, GHSV 15

318

000 h-1 at room temperature.

319

In summary, this work demonstrates that palladium nanoparticles supported on ceria

320

nanocubes can be promising catalysts for practical application to the ambient catalytic oxidation

321

of formaldehyde. This catalyst can reliably convert formaldehyde into water and carbon dioxide

322

with high efficiency and has great resistance against moisture. The palladium in the metallic state

323

is crucial for the low temperature catalytic oxidation of formaldehyde. Compared to the {111}

ACS Paragon Plus Environment

18

Page 19 of 24

Environmental Science & Technology

324

facets, which is exposed on ceria nanooctahedrons, exposing the {100} facets on the ceria

325

nanocubes increases the reducibility of the palladium species. Meanwhile, the degree of

326

crystallinity also has a significant effect on the catalytic activity. The defects and oxygen

327

vacancies on ceria nanorods are favored by the palladium oxide thus hindering the process of

328

catalytic oxidation. This work confirms the importance of support morphology on the catalytic

329

oxidation of formaldehyde with noble metal based catalysts.

330 331

ASSOCIATED CONTENT

332

Supporting Information.

333 334

XPS Analysis of Cerium Species. This material is available free of charge via the Internet at http://pubs.acs.org.

335 336

AUTHOR INFORMATION

337

Corresponding Author

338

*Address: School of Chemistry and Chemical Engineering, University of Chinese Academy of

339

Sciences, Beijing 100049, P.R. China. Phone and Fax number: 86-10-88256940. E-mail:

340

[email protected]

341

Author Contributions

342

All authors have given approval to the final version of the manuscript.

343

Notes

344

The authors declare no competing financial interests.

ACS Paragon Plus Environment

19

Environmental Science & Technology

Page 20 of 24

345

ACKNOWLEDGMENT

346

This work was supported by the National Natural Science Foundation of China (21473199 and

347

U1162113) and the Beijing Municipal Science & Technology Commission.

348 349

REFERENCES

350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382

1. Salthammer, T.; Mentese, S.; Marutzky, R., Formaldehyde in the Indoor Environment. Chem. Rev. 2010, 110 (4), 2536-2572. 2. Jones, A. P., Indoor air quality and health. Atmos. Environ. 1999, 33 (28), 4535-4564. 3. Lee, K. J.; Shiratori, N.; Lee, G. H.; Miyawaki, J.; Mochida, I.; Yoon, S.-H.; Jang, J., Activated carbon nanofiber produced from electrospun polyacrylonitrile nanofiber as a highly efficient formaldehyde adsorbent. Carbon 2010, 48 (15), 4248-4255. 4. Rong, H.; Liu, Z.; Wu, Q.; Pan, D.; Zheng, J., Formaldehyde removal by Rayon-based activated carbon fibers modified by P-aminobenzoic acid. Cellulose 2010, 17 (1), 205-214. 5. Lu, Y.; Wang, D.; Ma, C.; Yang, H., The effect of activated carbon adsorption on the photocatalytic removal of formaldehyde. Build. Environ. 2010, 45 (3), 615-621. 6. Shie, J. L.; Lee, C. H.; Chiou, C. S.; Chang, C. T.; Chang, C. C.; Chang, C. Y., Photodegradation kinetics of formaldehyde using light sources of UVA, UVC and UVLED in the presence of composed silver titanium oxide photocatalyst. J. Hazard. Mater. 2008, 155 (1–2), 164-172. 7. Pei, J.; Zhang, J. S., Critical review of catalytic oxidization and chemisorption methods for indoor formaldehyde removal. Hvac&r. Res. 2011, 17 (4), 476-503. 8. Bai, B.; Arandiyan, H.; Li, J., Comparison of the performance for oxidation of formaldehyde on nano-Co3O4, 2D-Co3O4, and 3D-Co3O4 catalysts. Appl. Catal. B 2013, 142–143 (0), 677-683. 9. Sekine, Y., Oxidative decomposition of formaldehyde by metal oxides at room temperature. Atmos. Environ. 2002, 36 (35), 5543-5547. 10. Liu, B.; Li, C.; Zhang, Y.; Liu, Y.; Hu, W.; Wang, Q.; Han, L.; Zhang, J., Investigation of catalytic mechanism of formaldehyde oxidation over three-dimensionally ordered macroporous Au/CeO2 catalyst. Appl. Catal. B 2012, 111–112 (0), 467-475. 11. Ma, L.; Wang, D.; Li, J.; Bai, B.; Fu, L.; Li, Y., Ag/CeO2 nanospheres: Efficient catalysts for formaldehyde oxidation. Appl. Catal. B 2014, 148–149 (0), 36-43. 12. Zhang, C.; He, H.; Tanaka, K. i., Catalytic performance and mechanism of a Pt/TiO2 catalyst for the oxidation of formaldehyde at room temperature. Appl. Catal. B 2006, 65 (1–2), 37-43. 13. Zhang, C.; He, H., A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature. Catal. Today 2007, 126 (3–4), 345-350. 14. Huang, Z.; Gu, X.; Cao, Q.; Hu, P.; Hao, J.; Li, J.; Tang, X., Catalytically Active SingleAtom Sites Fabricated from Silver Particles. Angew. Chem. Int. Ed. 2012, 51 (17), 4198-4203.

ACS Paragon Plus Environment

20

Page 21 of 24

383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427

Environmental Science & Technology

15. Zhang, C.; Liu, F.; Zhai, Y.; Ariga, H.; Yi, N.; Liu, Y.; Asakura, K.; FlytzaniStephanopoulos, M.; He, H., Alkali-Metal-Promoted Pt/TiO2 Opens a More Efficient Pathway to Formaldehyde Oxidation at Ambient Temperatures. Angew. Chem. Int. Ed. 2012, 51 (38), 96289632. 16. Chen, B.-B.; Shi, C.; Crocker, M.; Wang, Y.; Zhu, A. M., Catalytic removal of formaldehyde at room temperature over supported gold catalysts. Appl. Catal. B 2013, 132–133 (0), 245-255. 17. Yu, J.; Li, X.; Xu, Z.; Xiao, W., NaOH-Modified Ceramic Honeycomb with Enhanced Formaldehyde Adsorption and Removal Performance. Environ. Sci. Technol. 2013, 47 (17), 9928-9933. 18. Xu, Q.; Lei, W.; Li, X.; Qi, X.; Yu, J.; Liu, G.; Wang, J.; Zhang, P., Efficient Removal of Formaldehyde by Nanosized Gold on Well-Defined CeO2 Nanorods at Room Temperature. Environ. Sci. Technol. 2014, 48 (16), 9702-9708. 19. González-Velasco, J. R.; Aranzabal, A.; Gutiérrez-Ortiz, J. I.; López-Fonseca, R.; Gutiérrez-Ortiz, M. A., Activity and product distribution of alumina supported platinum and palladium catalysts in the gas-phase oxidative decomposition of chlorinated hydrocarbons. Appl. Catal. B-environ 1998, 19 (3–4), 189-197. 20. Centi, G., Supported palladium catalysts in environmental catalytic technologies for gaseous emissions. J. Mol. Catal. A 2001, 173 (1–2), 287-312. 21. Kim, S. C.; Shim, W. G., Properties and performance of Pd based catalysts for catalytic oxidation of volatile organic compounds. Appl. Catal. B 2009, 92 (3–4), 429-436. 22. He, C.; Li, P.; Cheng, J.; Hao, Z. P.; Xu, Z. P., A Comprehensive Study of Deep Catalytic Oxidation of Benzene, Toluene, Ethyl Acetate, and their Mixtures over Pd/ZSM-5 Catalyst: Mutual Effects and Kinetics. Water, Air, Soil Pollut. 2010, 209 (1-4), 365-376. 23. Zhang, C.; Li, Y.; Wang, Y.; He, H., Sodium-Promoted Pd/TiO2 for Catalytic Oxidation of Formaldehyde at Ambient Temperature. Environ. Sci. Technol. 2014, 48 (10), 5816-5822. 24. Fu, Q.; Wagner, T., Interaction of nanostructured metal overlayers with oxide surfaces. Surf. Sci. Rep. 2007, 62 (11), 431-498. 25. Zhou, K.; Li, Y., Catalysis Based on Nanocrystals with Well-Defined Facets. Angew. Chem. Int. Ed. 2012, 51 (3), 602-613. 26. Xie, X.; Li, Y.; Liu, Z. Q.; Haruta, M.; Shen, W., Low-temperature oxidation of CO catalysed by Co3O4 nanorods. Nature 2009, 458 (7239), 746-749. 27. Colussi, S.; Gayen, A.; Farnesi Camellone, M.; Boaro, M.; Llorca, J.; Fabris, S.; Trovarelli, A., Nanofaceted Pd-O Sites in Pd-Ce Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane. Angew. Chem. Int. Ed. 2009, 48 (45), 8481-8484. 28. Xiao, L. h.; Sun, K. p.; Xu, X. l.; Li, X. n., Low-temperature catalytic combustion of methane over Pd/CeO2 prepared by deposition–precipitation method. Catal. Commun. 2005, 6 (12), 796-801. 29. Trovarelli, A., Catalytic Properties of Ceria and CeO2-Containing Materials. Catal. Rev. 1996, 38 (4), 439-520. 30. Guo, H.; He, Y.; Wang, Y.; Liu, L.; Yang, X.; Wang, S.; Huang, Z.; Wei, Q., Morphology-controlled synthesis of cage-bell Pd@CeO2 structured nanoparticle aggregates as catalysts for the low-temperature oxidation of CO. J. Mater. Chem. A 2013, 1 (25), 7494-7499. 31. Zhou, K.; Yang, Z.; Yang, S., Highly Reducible CeO2 Nanotubes. Chem. Mater. 2007, 19 (6), 1215-1217.

ACS Paragon Plus Environment

21

Environmental Science & Technology

428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471

Page 22 of 24

32. Liu, X.; Zhou, K.; Wang, L.; Wang, B.; Li, Y., Oxygen Vacancy Clusters Promoting Reducibility and Activity of Ceria Nanorods. J. Am. Chem. Soc. 2009, 131 (9), 3140-3141. 33. Si, R.; Flytzani-Stephanopoulos, M., Shape and Crystal-Plane Effects of Nanoscale Ceria on the Activity of Au-CeO2 Catalysts for the Water–Gas Shift Reaction. Angew. Chem. Int. Ed. 2008, 47 (15), 2884-2887. 34. Huang, H.; Dai, Q.; Wang, X., Morphology effect of Ru/CeO2 catalysts for the catalytic combustion of chlorobenzene. Appl. Catal. B 2014, 158–159 (0), 96-105. 35. Gao, Y.; Wang, W.; Chang, S.; Huang, W., Morphology Effect of CeO2 Support in the Preparation, Metal–Support Interaction, and Catalytic Performance of Pt/CeO2 Catalysts. ChemCatChem 2013, 5 (12), 3610-3620. 36. Singhania, N.; Anumol, E. A.; Ravishankar, N.; Madras, G., Influence of CeO2 morphology on the catalytic activity of CeO2-Pt hybrids for CO oxidation. Dalton. Trans. 2013, 42 (43), 15343-15354. 37. Wang, L.; Lu, G.; Yang, D.; Wang, J.; Zhu, Z.; Wang, Z.; Zhou, K., Manipulation of the Reducibility of Ceria-Supported Au Catalysts by Interface Engineering. ChemCatChem 2013, 5 (6), 1308-1312. 38. Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C.-H., Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes. J. Phys. Chem. B 2005, 109 (51), 24380-24385. 39. Luo, Y.; Xiao, Y.; Cai, G.; Zheng, Y.; Wei, K., Complete methanol oxidation in carbon monoxide streams over Pd/CeO2 catalysts: Correlation between activity and properties. Appl. Catal. B 2013, 136–137 (0), 317-324. 40. An, N.; Zhang, W.; Yuan, X.; Pan, B.; Liu, G.; Jia, M.; Yan, W.; Zhang, W., Catalytic oxidation of formaldehyde over different silica supported platinum catalysts. Chem. Eng. J. 2013, 215–216 (0), 1-6. 41. Chen, B.-b.; Zhu, X.-b.; Crocker, M.; Wang, Y.; Shi, C., FeOx-supported gold catalysts for catalytic removal of formaldehyde at room temperature. Appl. Catal. B 2014, 154–155 (0), 73-81. 42. Huang, H.; Leung, D. Y. C., Complete Oxidation of Formaldehyde at Room Temperature Using TiO2 Supported Metallic Pd Nanoparticles. ACS Catal. 2011, 1 (4), 348-354. 43. Faticanti, M.; Cioffi, N.; Rossi, S. D.; Ditaranto, N.; Porta, P.; Sabbatini, L.; BleveZacheo, T., Pd supported on tetragonal zirconia: Electrosynthesis, characterization and catalytic activity toward CO oxidation and CH4 combustion. Appl. Catal. B 2005, 60 (1–2), 73-82. 44. Zhu, Z.; Tan, H.; Wang, J.; Yu, S.; Zhou, K., Hydrodeoxygenation of vanillin as a bio-oil model over carbonaceous microspheres-supported Pd catalysts in the aqueous phase and Pickering emulsions. Green Chem. 2014, 16 (5), 2636-2643. 45. Kim, M. S.; Chung, S. H.; Yoo, C. J.; Lee, M. S.; Cho, I. H.; Lee, D. W.; Lee, K. Y., Catalytic reduction of nitrate in water over Pd–Cu/TiO2 catalyst: Effect of the strong metalsupport interaction (SMSI) on the catalytic activity. Appl. Catal. B 2013, 142–143 (0), 354-361. 46. Hutchings, G. J., Nanocrystalline gold and gold-palladium alloy oxidation catalysts: a personal reflection on the nature of the active sites. Dalton. Trans. 2008, (41), 5523-5536. 47. Arrii, S.; Morfin, F.; Renouprez, A. J.; Rousset, J. L., Oxidation of CO on Gold Supported Catalysts Prepared by Laser Vaporization: Direct Evidence of Support Contribution. J. Am. Chem. Soc. 2004, 126 (4), 1199-1205.

ACS Paragon Plus Environment

22

Page 23 of 24

472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498

Environmental Science & Technology

48. Zhu, H.; Qin, Z.; Shan, W.; Shen, W.; Wang, J., Pd/CeO2–TiO2 catalyst for CO oxidation at low temperature: a TPR study with H2 and CO as reducing agents. J. Catal. 2004, 225 (2), 267-277. 49. Kosacki, I.; Suzuki, T.; Anderson, H. U.; Colomban, P., Raman scattering and lattice defects in nanocrystalline CeO2 thin films. Solid State Ionics 2002, 149 (1–2), 99-105. 50. Taniguchi, T.; Watanabe, T.; Sugiyama, N.; Subramani, A. K.; Wagata, H.; Matsushita, N.; Yoshimura, M., Identifying Defects in Ceria-Based Nanocrystals by UV Resonance Raman Spectroscopy. J. Phys. Chem. C 2009, 113 (46), 19789-19793. 51. Spanier, J. E.; Robinson, R. D.; Zhang, F.; Chan, S. W.; Herman, I. P., Size-dependent properties of CeO2-y nanoparticles as studied by Raman scattering. Phys. Rev. B 2001, 64 (24), 245407. 52. Lin, J.; Li, L.; Huang, Y.; Zhang, W.; Wang, X.; Wang, A.; Zhang, T., In Situ Calorimetric Study: Structural Effects on Adsorption and Catalytic Performances for CO Oxidation over Ir-in-CeO2 and Ir-on-CeO2 Catalysts. J. Phys. Chem. C 2011, 115 (33), 1650916517. 53. Wang, N.; Shen, K.; Huang, L.; Yu, X.; Qian, W.; Chu, W., Facile Route for Synthesizing Ordered Mesoporous Ni-Ce-Al Oxide Materials and Their Catalytic Performance for Methane Dry Reforming to Hydrogen and Syngas. ACS Catal. 2013, 3 (7), 1638-1651. 54. Palmqvist, A. E. C.; Wirde, M.; Gelius, U.; Muhammed, M., Surfaces of doped nanophase cerium oxide catalysts. Nanostruct. Mater. 1999, 11 (8), 995-1007. 55. Zhou, K.; Wang, X.; Sun, X.; Peng, Q.; Li, Y., Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes. J. Catal. 2005, 229 (1), 206-212. 56. Wang, Z.; Pei, J.; Zhang, J., Catalytic oxidization of indoor formaldehyde at room temperature - Effect of operation conditions. Build. Environ. 2013, 65 (0), 49-57. 57. Huang, H.; Ye, X.; Huang, H.; Zhang, L.; Leung, D. Y. C., Mechanistic study on formaldehyde removal over Pd/TiO2 catalysts: Oxygen transfer and role of water vapor. Chem. Eng. J. 2013, 230 (0), 73-79.

499

ACS Paragon Plus Environment

23

Environmental Science & Technology

500

Page 24 of 24

Table of Contents

501 502

ACS Paragon Plus Environment

24