CeO2 Catalyst at Low

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Energy and the Environment

Total Oxidation of Propane over a Ru/CeO2 Catalyst at Low Temperature Zong Hu, Zheng Wang, Yun Guo, Li Wang, Yanglong Guo, Jinshui Zhang, and Wangcheng Zhan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03448 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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Environmental Science & Technology

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Total Oxidation of Propane over a Ru/CeO2 Catalyst at Low

2

Temperature

3

Zong Hu,† Zheng Wang,†, ‡ Yun Guo,† Li Wang,† Yanglong Guo,† Jinshui Zhang,‡ Wangcheng

4

Zhan*†

5

†Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of

6

Chemistry and Molecular Engineering, East China University of Science and Technology,

7

Shanghai, 200237, P. R. China.

8

‡State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,

9

Fuzhou University, Fuzhou, 350116, PR China

10

* Corresponding Author: Fax: +86-21-64252923, E-mail: [email protected] (W.C. Zhan)

11

Abstract: Ruthenium (Ru) nanoparticles (~3 nm) with mass loading ranging from 1.5 to 3.2 wt.%

12

are supported on a reducible substrate, cerium dioxide (CeO2, the resultant sample named as

13

Ru/CeO2), for application in the catalytic combustion of propane. Because of the unique electronic

14

configuration of CeO2, a strong metal-support interaction is generated between the Ru

15

nanoparticles and CeO2 to well stabilize Ru nanoparticles for oxidation reactions. In addition, the

16

CeO2 host with high oxygen storage capacity can provide an abundance of active oxygen for

17

redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the

18

redox steps. As a result of such advantages, a remarkably high performance in the total oxidation

19

of propane at low temperature is achieved on Ru/CeO2. This work exemplifies a promising

20

strategy for developing robust supported catalysts for short-chain VOC removal.

1 Environment ACS Paragon Plus


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Keywords: Volatile organic compounds; Ru-based catalysts; CeO2; metal-support interaction;

22

propane oxidation

23

1. Introduction

24

Volatile organic compounds (VOCs) are organic compounds with boiling points in the range of

25

50-260 °C and include alkanes, aromatics, olefins, alcohols and halogenated hydrocarbons. The

26

main sources of VOCs are emissions from industrial processes, automobile exhaust and household

27

products. The release of VOCs into the atmosphere has had strong environmental impacts and

28

severely affected human health because of the toxicity of these compounds and their involvement

29

in the formation of photochemical smog. The elimination of VOC pollutants from air is therefore

30

very important for achieving environmental sustainability and protecting human health

31

However, the removal of VOCs is rather challenging because most of these compounds are very

32

stable and often undergo incomplete destruction, producing undesirable by-products with much

33

higher toxicities than the original compounds rather than harmless decomposition products, e.g.,

34

CO2 and H2O 10-17. Thus, the development of effective removal technologies that conduct the total

35

oxidation of VOCs is highly desirable.

36

1-9

.

Catalytic oxidation is a promising technique for controlling the emission of VOCs because of its 18-23

37

high destruction efficiency, low thermal NOx emission and moderate operating temperature

38

Generally, in this technique, pollution molecules can be completely oxidized over a catalyst at

39

temperatures much lower than those required for thermal oxidation, showing significant

40

advantages related to environmental sustainability and energy utilization. Toward this end, various

41

classes of catalysts, including noble metal-based catalysts, nonnoble metal oxide catalysts and

42

perovskites, have been synthesized and examined for VOC removal, thereby greatly enhancing the 2 Environment ACS Paragon Plus

.

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43

development of the catalytic oxidation technique 24-30. Currently, a challenging issue in this area is

44

the identification of a robust catalyst for the total oxidation of light alkanes (the largest fraction of

45

hydrocarbons in automobile exhausts) at low temperature because such short carbon chain

46

molecules are both thermally and chemically stable, making them much more difficult to remove

47

than longer chain molecules. For example, propane, a typical short-chain pollutant from liquefied

48

petroleum gas, requires intermediate/high reaction temperatures to be activated 31-34. As the use of

49

propane as a fuel in transport vehicles has recently increased, a catalyst that can perform the total

50

oxidation of propane is essential for controlling this VOC and reducing its impact on the

51

environment.

52

Supported Ru catalysts are typical oxidation catalysts that have been extensively applied in 35

36-38

53

various kinds of oxidation processes, such as CO oxidation

54

long-chain VOC degradation (e.g., ethyl acetate, ortho-xylene)

55

advantages in oxidation reactions. Recently, Ru nanoparticles were supported on γ-Al2O3 and

56

applied to light alkane combustion

57

and operating conditions, a high activity with a turnover frequency (TOF) of 0.0035 s-1 was

58

achieved on Ru/γ-Al2O3 in the total oxidation of iso-butane at temperatures as low as 175 °C

59

clearly suggesting that supported Ru catalysts are promising candidates for short-chain VOC

60

removal. However, owing to the chemical inertness of nonreducible Al2O3, the interaction

61

between the metal and the support was so poor that it could be almost ignored, which significantly

62

reduced the catalytic performance and durability of Ru/γ-Al2O3 in VOC oxidation. The

63

metal-support interaction is generally known to be one of the most important factors that

64

determine the catalytic properties of supported metal catalysts, as it usually decides the oxidation

, alcohol/amine oxidation

and

39-41

, and thus show significant

42-45

. After thorough optimization of the catalyst preparation


46

,


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47, 48

65

state of the active metal and the steps involved in the redox reaction

. In this regard, the

66

generation of adequate interactions between Ru nanoparticles and the support is therefore highly

67

essential for further advancing the application of supported Ru catalysts in the total oxidation of

68

propane at relatively low temperatures.

69

Cerium dioxide (CeO2) is a reducible support that has been widely used in the preparation of

70

supported catalysts for oxidation reactions because of its high oxygen storage capacity and facile

71

Ce4+/Ce3+ redox cycle

72

oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even

73

modifies the redox steps. Herein, Ru nanoparticles were stabilized by CeO2 to create sufficient

74

metal-support interactions for achieving the total oxidation of propane. As expected, the Ru

75

nanoparticles and CeO2 worked together to catalyze the steps of propane destruction, and thus a

76

remarkably high performance in the total oxidation of propane at low temperature was achieved

77

on Ru/CeO2. We hope that this work will exemplify a promising strategy for developing robust

78

supported catalysts for short-chain VOC removal.

79

2. Materials and Methods

80

2.1. Catalyst preparation

49

. During oxidation processes, CeO2 can provide an abundance of active

81

CeO2 was obtained from the direct calcination of Ce(NO)3·6H2O (99.95% metals basis) in air at

82

500 °C for 8 h, while γ-Al2O3 (99.99% metals basis) was purchased from Aladdin Chemical Co.,

83

Ltd (Shanghai).

84

Supported Ru catalysts were prepared by a well-developed deposition-precipitation (DP)

85

method using RuCl3 as the Ru precursor. Typically, 0.5 g of support powder was suspended in 50

86

mL of H2O containing the desired amount of RuCl3, and the mixture was ultrasonicated for 15 min


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and stirred at 25 °C for 15 min to form a suspension. Then, an aqueous NaOH solution (0.01 M)

88

was added dropwise to the suspension to change the pH of the mixture to ~ 10. After this step, the

89

mixture was further stirred at 25 °C for 3 h, and then the solid was collected by centrifugation and

90

washed several times with deionized water. Finally, the obtained solid sample was dried under

91

vacuum at 60 °C overnight and then calcined in a 10% H2/Ar mixed atmosphere at 400 °C for 1 h.

92

The resultant samples were denoted Ru/CeO2-X and Ru/Al2O3-X based on the support used,

93

where X is an arbitrary sample number (X = 1, 2, 3) that represents the loading amount of Ru. The

94

actual content of Ru in the different catalysts is listed in Table S1.

95

2.2. Catalytic activity testing

96

The catalytic activity of the catalyst in the total oxidation of propane was evaluated in a

97

fixed-bed reactor containing 200 mg of catalyst at atmospheric pressure, and the feed gas

98

consisted of 0.2 vol.% C3H8 – 2 vol.% O2 – 97.8 vol.% Ar. The total gas flow rate was 100

99

mL/min, and the corresponding gas hourly space velocity (GHSV) was 30000 mL·h-1·gcat-1. The

100

temperature of the reaction bed was ramped up to 300 °C at a heating rate of 2 °C/min. The

101

conversion of C3H8 was measured after the catalytic reaction by an online gas chromatograph

102

(GC-2060) that was equipped with a flame ionization detector (FID). The C3H8 conversion (XC3H8)

103

was calculated by the equation XC3H8 =

104

are the C3H8 concentrations in the inlet and outlet gas, respectively.

105

3. Results and Discussion

106

3.1. Catalytic activity of the catalysts

[C3H8]in -[ C3H8]out × 100%, where [C3H8]in and [C3H8]out [C3H8]in

107

Figure 1 shows the propane conversions over the Ru/CeO2-X catalysts as a function of the

108

reaction temperature, where the Ru/Al2O3-X catalysts were utilized as control samples. As



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expected, CeO2 and Al2O3 alone showed rather poor activities in the total oxidation of propane at

110

temperatures below 300 °C. Once Ru was loaded on the CeO2 or Al2O3 support, the catalytic

111

activities improved drastically, which clearly suggests that Ru has intrinsic catalytic properties for

112

propane oxidation. More importantly, only CO2 and H2O were produced without any other

113

by-products, confirming that this catalytic process was a total oxidation reaction. As a result of the

114

synergic effect of Ru nanoparticles and CeO2, the Ru/CeO2-X catalysts exhibited much higher

115

catalytic activities than the Ru/Al2O3-X catalysts. Furthermore, even when reduced TiO2 oxide

116

was used as the support, the Ru/CeO2-X catalysts still showed a unique catalytic activity (Figure

117

S1). Owing to the synergic effect of the catalyst components, the effect of the Ru loading on the

118

catalytic activity of the Ru/CeO2-X catalysts was not obvious, which is different from the case of

119

the Ru/Al2O3-X catalysts, where the catalytic activity was closely related to the Ru loading.

Ru/Al2O3-1

-1

Ru/Al2O3-2

-1

80

(A)

ln (r x 10 ) (mol gRu s )

Ru/Al2O3-3

60

Al2O3 Ru/CeO2-1

40

7

C3H8 conversion (%)

100

Ru/CeO2-2 Ru/CeO2-3

20

CeO2

0

(B)

6.5 6.0 5.5

Ru/CeO2-1: Ea=58.6kJ/mol

5.0


4.5


4.0 Ru/Al2O3-1: Ea=103.7kJ/mol

3.5

Ru/Al2O3-2: Ea=99.8kJ/mol

3.0

Ru/Al2O3-3: Ea=96.3kJ/mol

2.5 100

150

200

250

300

2.05

o

120

2.10

2.15

2.20

2.25

2.30

1000/T (K)

Temperature ( C)

121

Figure 1. Catalytic activity (A) and ln r as a function of 1/T (B) of the Ru/Al2O3-X and Ru/CeO2-X catalysts in

122

the total oxidation of propane. The feed gas consisted of 0.2 vol.%C3H8 – 2 vol.%O2 – 97.8 vol.% Ar, and the

123

GHSV was 30000 mL·h-1·gcat-1 for A and 60000 mL·h-1·gcat-1 for B.

124

To further evaluate the extraordinary catalytic performance of the Ru/CeO2-X catalysts, the

125

reaction rate, TOF and apparent activation energy (Ea) were calculated, and the results are listed in


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Table S2 and Figure 1B. It was noted that the reaction rates and TOF of the Ru/CeO2-X catalysts

127

were higher than those of the Ru/Al2O3-X catalysts having the same Ru content, while the

128

corresponding Ea of the former was lower than that of the latter. These findings confirmed that the

129

Ru/CeO2-X catalysts exhibited higher catalytic activities than the Ru/Al2O3-X catalysts in the total

130

oxidation of propane. Furthermore, for both the CeO2- and Al2O3-supported catalysts, the reaction

131

rate and TOF decreased with increasing Ru loading in the catalyst, indicating a decrease in the

132

intrinsic activity. The same trend was also observed for the Ea of the Ru/CeO2-X catalysts. In

133

contrast, the Ea of the Ru/Al2O3-X catalysts decreased with increasing Ru loading, which

134

contradicted the trends in the reaction rate and TOF. This finding was due to the compensation

135

effect of the total oxidation of propane on the Ru/Al2O3-X catalysts (Figure S2).

136

To highlight the advanced catalytic performance of the Ru/CeO2 catalysts in the total oxidation

137

of propane, the activity of the Ru/CeO2-1 catalyst was compared with that of other reported

138

catalysts, and the results are listed in Table S3. Excitingly, the reaction rate and TOF of the

139

Ru/CeO2-1 catalyst were much higher than those of other supported Ru catalysts and even the

140

well-known noble metal-based catalysts (e.g., Pt/CeO2, Pt/ZSM-5, Pd/Al2O3 and Au/CoOx). In

141

addition, the Ru/CeO2-1 catalyst was rather robust that the small Ru nanoparticles have been well

142

stabilized by CeO2 without sintering after propane oxidation reaction (Figure S3 and Table S1).

143

These findings indicate that Ru/CeO2 is a promising catalyst for the total oxidation of propane.

144

3.2. Characterizations of catalysts

145

The X-ray diffraction (XRD) patterns of the Ru/CeO2-X and Ru/Al2O3-X catalysts are shown in

146

Figure 2. The Ru/CeO2-X catalysts exhibited the typical diffraction peaks of cubic fluorite CeO2

147

crystals, while all the Ru/Al2O3-X catalysts displayed the diffraction peaks of γ-Al2O3. Moreover,

148

no diffraction peaks corresponding to RuO2 or Ru phases were detected in the XRD patterns of all 7 Environment ACS Paragon Plus


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the catalysts, meaning that the Ru species were highly dispersed on the support with small particle

150

sizes or existed in the amorphous states.

A

Ru/Al2O 3-2

Ru/CeO2-2

Ru/CeO2-1

151 152

Ru/Al2O 3-1

γ− Al2 O3

CeO2

10

B

Ru/Al2O3-3

Intensity

Intensity

Ru/CeO2-3

20

30

40

50

60

70

80

10

20

2 Theta (degree)

30

40

50

60

70

80

2 Theta (degree)

Figure 2. XRD patterns of the (A) Ru/CeO2-X and (B) Ru/Al2O3-X catalysts.

153

Figures 3 and S4-S7 show the transmission electron microscopy (TEM) micrographs and

154

corresponding Ru particle size distributions of the Ru/CeO2-X and Ru/Al2O3-X catalysts. Very

155

small Ru nanoparticles were uniformly distributed on both the CeO2 and Al2O3 supports. The

156

lattice spacing of 0.21 nm observed in the magnified images was assigned to the (101) lattice

157

plane of Ru. The average Ru particle sizes on both CeO2 and Al2O3 increased with increasing Ru

158

loading. Interesting, the average Ru particle sizes on the Ru/CeO2-X catalysts were relatively

159

smaller than those on the Ru/Al2O3-X catalysts having the same Ru loading (Table S1). This

160

should be attributed to the strong metal-support interaction between Ru and CeO2. Because of the

161

specific properties of Ce with 4f orbit and the structural relaxation, the adsorption of metal atom

162

on CeO2 is accompanied by an electron charge transfer between metal atom and CeO2 surface, and

163

thus strengthens the interaction between metal and CeO2 support 50-53.

164

Figure S8 shows high-resolution TEM (HRTEM) images and fast Fourier transform (FFT)

165

patterns of a Ru nanocrystal in the Ru/CeO2-3 and Ru/Al2O3-3 catalysts. Ru metal is well known


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166

to become partially oxidized when exposed to air at ambient temperature, leading to the coverage

167

of Ru nanocrystals with a thin oxide layer

168

photoelectron spectroscopy (XPS) results. Unfortunately, as shown in Figure S8, the thin oxide

169

layer was not observed in the HRTEM images of the Ru nanocrystals of the Ru/CeO2-3 and

170

Ru/Al2O3-3 catalysts, and the FFT patterns contained spots corresponding to only the 0.21 nm

171

lattice fringes [Ru(101)]. This contradiction is due to the presence of the amorphous phase of

172

RuOx.

45,46

, which was confirmed by the following X-ray

173

174



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175

Figure 3. TEM images and corresponding Ru particle size distributions (inset) of the (A) Ru/CeO2-1, (B)

176

Ru/CeO2-2 and (C) Ru/CeO2-3 catalysts.

177

Figure 4A and B shows the Ru 3d XPS spectra of the Ru/CeO2-X and Ru/Al2O3-X catalysts. All

178

the catalysts exhibited a Ru 3d5/2 peak at ~280 eV and a Ru 3d3/2 peak at ~284.6 eV. Because the

179

Ru 3d3/2 peak and the C 1s peak at 284.6 eV overlapped, the Ru 3d5/2 peak at ~280 eV was

180

employed to calculate the atomic ratio of Ru in the different valence states. The Ru 3d5/2 peak

181

could be deconvoluted into two peaks for all the catalysts, and the resulting peaks at

182

approximately 280.2 and 281.3 eV were assigned to Ru0 and Run+ (0 < n < 4), respectively

183

Although all the catalysts were reduced during the preparation process, positively charged Ru was

184

observed in all the catalysts because Ru metal can become partially oxidized when exposed to air

185

at ambient temperature. As shown in Table S4, the atomic ratios of Run+/Ru0 in both the Ru/CeO2

186

and Ru/Al2O3 catalysts decreased with increasing Ru loading, which is due to the small size of the

187

Ru particle in the catalysts with low contents of Ru (shown in Table S1) since small metal

188

nanoparticles have a stronger affinity for oxygen than larger Ru particles. On the other hand, for

189

the same Ru loadings, the atomic ratios of Run+/Ru0 on the surfaces of the Ru/CeO2-X catalysts

190

were slightly higher than those on the surfaces of the Ru/Al2O3-X catalysts. Unlike in the series of

191

catalysts with the same support, the Ru particle size does not sufficiently explain the difference

192

between the atomic ratios of Run+/Ru0 on the surfaces of the Ru/CeO2-X and Ru/CeO2-X catalysts.

193

Although the Ru/CeO2-3 and Ru/Al2O3-1 catalysts had nearly the same Ru particle size, large

194

differences were observed in the Run+/Ru0 ratios (28.7% VS 47.7%). Therefore, the Run+/Ru0

195

ratios on the surface of the Ru-supported catalysts are strongly influenced by the nature of the

196

support.


54-56

.

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197

Based on the discussion of the particle size and the structure of the Ru active sites, the

198

Ru/CeO2-3 and Ru/Al2O3-2 catalysts had similar Ru particle sizes and Run+/Ru0 ratios on their

199

surfaces. However, the TOF of the Ru/CeO2-3 catalyst was twice that of the Ru/Al2O3-2 catalyst.

200

Therefore, the high catalytic performance of the Ru/CeO2-X catalysts in the total oxidation of

201

propane can be attributed to the role of the CeO2 support and the metal-support interaction

202

between the Ru species and CeO2.

281.3 280.2

A

Ru/CeO2-2

290

285

280

295

275

Intensity (a.u.)

Intensity (a.u.)

Ru/CeO2-2 Ru/CeO2-3

536

205

534

532

530

280

275

D

Ru/Al2O3-2

Ru/Al2O3-3 Oα

Oα

Oγ

285

Ru/Al2O3-1

Ru/CeO2-1

CeO2

290

Binding energy (eV)

C

204

Ru/Al2O3-2

Binding energy (eV)

203

281.1 280.0

Ru/Al2O3-3

Ru/CeO2-3

295

B

Ru/Al2O3-1

Intensity (a.u.)

Intensity (a.u.)

Ru/CeO2-1

Al2O3

528

Binding energy (eV)

526

536

Oβ

534

532

530

528

526

Binding energy (eV)

Figure 4. Ru 3d (A, B) and O 1s (C, D) XPS spectra of the Ru/CeO2-X and Ru/Al2O3-X catalysts.



206

3.3. Interaction between the Ru species and CeO2

207

The O 1s XPS spectra of the Ru/CeO2-X and Ru/Al2O3-X catalysts are also shown in Figure 4C

208

and D. The O 1s XPS spectra of the Ru/Al2O3-X catalysts were similar to that of Al2O3 support.

209

One strong peak (Oα) at 531.0 eV and a wide shoulder peak (Oβ) at 532.8 eV were detected, which

210

can be mainly attributed to the lattice oxygen and the surface oxygen from the adsorbed water or

211

hydroxyl species, respectively

212

XPS spectra can be deconvoluted into two kinds of oxygen species. The one at 529.2 eV (Oα) was

213

attributed to lattice oxygen, while the other one at 531.0 eV (Oγ) was attributed to

214

surface-adsorbed oxygen belonging to defect oxides, and/or the contributions from O of RuOx

215

(only in Ru/CeO2-X catalysts)

216

very low, the ratio between the area of Oα and Oγ (Oγ/Oα) for the Ru/CeO2-X catalysts was

217

analogous to the amount ratio between surface-adsorbed oxygen belonging to defect oxides and

218

the lattice oxygen. As shown in Table S4, the Oγ/Oα ratio for the Ru/CeO2-X catalysts was higher

219

than that for the CeO2 support, and decreased with increasing content of Ru in the catalysts. It was

220

reported that oxygen defects can be produced by the interaction between Ru and CeO2, leading to

221

an increase in the amount of surface oxygen species on the CeO2 surface after the introduction of

222

Ru

223

factor in the formation of oxygen defects. Thus, the total length of the perimeter of the Ru-CeO2

224

interface (I0) was calculated, and the results are shown in Table S1. I0 decreased with increasing

225

Ru loading in the Ru/CeO2-X catalysts, which is consistent with the trend observed for the Oγ/Oα

226

ratio on the surface of the Ru/CeO2-X catalysts. Since oxygen defects are good sites for oxygen

227

adsorption during oxidation reactions

57-59

. For the CeO2 support and the Ru/CeO2-X catalysts, the O 1s

40, 60,61

. Since the content of Ru in the Ru/CeO2-X catalysts was

62

. Therefore, the size of the interface between Ru and the CeO2 support should be a crucial

63-65

, the presence of oxygen defects on the surface of the


Page 12 of 29

Page 13 of 29


228

Ru/CeO2-X catalysts is beneficial for improving their catalytic activity in the total oxidation of

229

propane.

230

In Figure S9, temperature program reduction of CO (CO-TPR) experiments were carried out to

231

demonstrate that the strong metal-support interaction between Ru and CeO2 is beneficial to

232

activate the oxygen of CeO2 around the interface of Ru and CeO2 for oxidation reaction

233

expected, the area of the CO-TPR profiles for the Ru/CeO2 catalyst (7.7 × 10-8) was much bigger

234

than that of pure CeO2 (4.96 × 10-8). This observation suggested that more oxygen species

235

participated in CO oxidation on the Ru/CeO2 catalyst during TPR experiment. Thus, the oxygen in

236

CeO2 around the Ru/CeO2 interface was activate due to the interaction between RuOx and CeO2,

237

and thus resulted in the increase of CO consumption.

66, 67

. As

238

Figure 5A and B shows the O2 temperature-programmed desorption (TPD) spectra of the

239

Ru/CeO2-X and Ru/Al2O3-X catalysts. Two broad overlapped peaks were observed for the Al2O3

240

support, i.e., one desorption peak centered at approximately 400 °C and another at approximately

241

550 °C; these peaks were attributed to the desorption of oxygen species from the regular surface

242

and from imperfect or defect regions on the Al2O3 surface, respectively

243

introduced on Al2O3, no obvious changes in the O2-TPD curves were observed except for a shift in

244

the desorption peak to a lower temperature. However, for the CeO2 support, multiple overlapping

245

desorption peaks were found between 250 and 600 °C, and these peaks were assigned to the

246

chemisorbed oxygen and lattice oxygen of CeO2

247

O2-TPD curves clearly changed and exhibited a broad overlapping desorption peak at low

248

temperature, accompanying by the increase of the area under the desorption peak. As shown by

249

XPS (Figure 4C), the interactions between Ru and CeO2 remarkably increase the number of

68

. After Ru was

32, 40

. After Ru was introduced on CeO2, the



Page 14 of 29

250

oxygen defects, which can adsorb and activate more gaseous oxygen to form easily desorbed

251

oxygen species 40. Therefore, the amount of desorbed oxygen species on the Ru/CeO2-X catalysts

252

was higher than the CeO2 support, and the temperatures of desorption peaks for the Ru/CeO2-X

253

catalysts were much lower than that of the Ru/Al2O3-X catalysts. To better tell which one of the

254

Ru/CeO2-X can produce O2 fastest for oxidation reactions, the first derivatives of O2-TPD curves

255

were calculated. It was found that the first derivatives of those curves followed the sequence:

256

Ru/CeO2-1 (2.9 × 10-13) > Ru/CeO2-2 (1.41 × 10-13) > Ru/CeO2-3 (1.04 × 10-13), indicating that the

257

Ru/CeO2-1 catalyst produced O2 faster than other catalysts. Meanwhile, the area under the broad

258

desorption peak in the O2-TPD curves of the Ru/CeO2-X catalysts decreased in the order of

259

Ru/CeO2-1 (2.28×10-8) ˃ Ru/CeO2-2 (1.95×10-8) ˃ Ru/CeO2-3 (1.76×10-8) ˃ CeO2 (1.40×10-8),

260

which may be attributed to the decrease in the total length (I0) (Table S1) of the Ru-ceria interface

261

that originated from the change in the Ru particle size.

A

B

Ru/CeO2-2 Ru/CeO2-1 CeO2

100

200

300

400

500

600

MS signal (m/z=32)

MS signal (m/z=32)

Ru/CeO2-3

Ru/Al2O3-3 Ru/Al2O3-2 Ru/Al2O3-1 Al2O3

100

o

262

Temperature ( C)


200

300

400 o

Temperature ( C)

500

600

Page 15 of 29


C

2.5 x 10

-9

m/z = 43 m/z = 44

D

1 x 10

m/z = 43 m/z = 44

-9

Ru/Al 2O 3 -3

Ru/CeO 2-2 Ru/CeO 2-1

MS Signal

MS Signal

Ru/CeO 2-3

Ru/Al2O 3-2

Ru/Al2O 3-1 Al2O 3

CeO 2 100

200

300

400

500

100

o

263 264

Temperature ( C)

200

300

400

500

o

Temperature ( C)

Figure 5. O2-TPD curves (A, B) and C3H8-TPSR (C, D) of the Ru/CeO2-X and Ru/Al2O3-X catalysts.

265

Figure 5C and D shows the C3H8 temperature-programmed surface reaction (TPSR) results for

266

the Ru/CeO2-X and Ru/Al2O3-X catalysts. The mass spectrometry (MS) signals of CO2 (m/ = 44)

267

and m/z = 43 were both recorded because CO2 and C3H8 have the same mass number. No surface

268

products were detected on the CeO2 and Al2O3 supports, indicating that both of these supports

269

were nearly inactive in the total oxidation of propane at temperatures ˂ 300 °C. After Ru was

270

introduced on CeO2, a positive peak at approximately 220 °C with m/z = 44 was found, and the

271

peak intensity increased with increasing Ru loading. Since a negative peak was found for m/z = 43,

272

the peak for m/z = 44 was attributed to the total oxidation of propane instead of propane

273

desorption from the catalyst. Although a similar situation was also observed for the Ru/Al2O3-X

274

catalysts, the amount of CO2 detected on the Ru/Al2O3 catalysts was much lower than that

275

detected on the Ru/CeO2 catalysts having the same Ru loading, due to the presence of

276

surface-adsorbed oxygen on the CeO2 support and the more active surface lattice oxygen in CeO2

277

around the Ru/CeO2 interface.

278

CeO2 generally serves as a reservoir for oxygen under oxidative conditions. Based on the

279

abovementioned results, lattice oxygen and surface-adsorbed oxygen belonging to defect oxides



280

were present on the surface of the CeO2 support. On the other hand, the interaction between Ru

281

and CeO2 can remarkably increase the amount of oxygen defects, which can adsorb and activate

282

gaseous oxygen to form easily desorbed oxygen species (shown in the O2-TPD results). At the

283

same time, the metal-support interaction can activate surface lattice oxygen in CeO2 around the

284

Ru/CeO2 interface (the results of CO-TPR in Figure S9). Therefore, the CeO2 support can

285

significantly improve the catalytic activity of supported Ru catalysts in the total oxidation of

286

propane because of its oxygen supply and the metal-support interaction between the Ru species

287

and CeO2. For the Ru/CeO2-X catalysts, as the Ru loading increased, the total length (I0) of

288

Ru-ceria interface decreased, leading to decreases in the amount ratio between surface-adsorbed

289

oxygen belonging to defect oxides and the lattice oxygen on the catalyst surface and in the area

290

under the desorption peak in the O2-TPD curves. As a result, the reaction rate and TOF of the

291

Ru/CeO2-X catalysts decreased with increasing Ru loading.

292

3.4. Reaction mechanism

293

To gain deeper insight into the combined effect between the Ru species and CeO2 in the total

294

oxidation of propane, in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra were

295

collected to comprehensively study the processes of propane adsorption, oxidation and desorption

296

on the Ru/Al2O3-X and Ru/CeO2-X catalysts at 200 °C. As shown in Figure 6A, when the

297

chamber was purged with feed gas consisting of 0.5 vol% C3H8 – 99.5 vol% Ar at 200 °C for 30

298

min, absorption bands were detected at 2850 – 3000 cm−1 for the Al2O3 support; these bands were

299

attributed to the absorption of gaseous propane. The details of this experiment are shown in Table

300

S5. Subsequently, the feed gas was replaced with 0.5 vol.% C3H8 – 5 vol.% O2 – 94.5 vol.% Ar,

301

and no significant change was observed after exposure for 30 min at 200 °C. Finally, the feed gas 16 Environment ACS Paragon Plus

Page 16 of 29

Page 17 of 29


302

was replaced with Ar, and all the bands disappeared after purging the catalysts at 200 °C for 30

303

min. These results revealed that propane only minimally adsorbed to the Al2O3 support and that

304

the corresponding total oxidation of propane could not occur at 200 °C.

305

For the Ru/Al2O3-X catalysts, after the catalyst was exposed to feed gas consisting of 0.5 vol%

306

C3H8 – 99.5 vol.% Ar at 200 °C for 30 min, bands at 2850 – 3000 cm−1 were again observed

307

(Figure 6B and Figure S10). In addition, multiple bands between 1300 and 1900 cm−1 appeared.

308

The bands at 1855, 1589 and 1457 cm−1 can be attributed to ν(C=O), νas(COO) and νs(COO)

309

modes, respectively 69-72, and both the bands at 1373 and 1392 cm−1 were assigned to δs(CH3) 69, 70,

310

72

311

to form on the Ru/Al2O3-X catalysts after exposure to the feed gas. In addition, the intensity of the

312

band observed at 1855 cm−1 for the Ru/Al2O3-X catalysts clearly increased with increasing Ru

313

loading, as shown in Figure S10. When the feed gas was replaced with 0.5 vol.% C3H8 – 5 vol.%

314

O2 – 94.5 vol.% Ar at 200 °C for 30 min, the intensity of the band at 1855 cm−1 decreased, and the

315

extent of this decrease was enhanced significantly with increasing Ru loading, indicating that the

316

species containing C=O bonds was involved in the total oxidation of propane as an intermediate.

317

In contrast, the intensities of the bands at 1589, 1457, 1373 and 1392 cm−1 assigned to isopropyl

318

groups and carboxylate species significantly increased due to the occurrence of the oxidation

319

reaction. At the desorption stage, the intensities of the bands at 1589, 1457, 1373 and 1392 cm−1

320

assigned to the isopropyl groups and carboxylate species remained unchanged. However, the

321

intensity of the band at 1855 cm−1 decreased, indicating that the intermediate containing C=O

322

bonds was removed during purging.

. Since the latter two bands had the same shape and intensity, isopropyl groups were concluded



Page 18 of 29

324

only the Ru nanoparticles on the Ru/Al2O3-X catalysts to form isopropyl groups, and these

325

isopropyl groups were then converted to acetone groups. Finally, the species containing acetone

326

groups were decomposed to formate or acetate species, which then formed the products CO2 and

327

H2O. The reaction diagram for the Ru/Al2O3-X catalysts is shown in Figure 7(A), and the

328

corresponding reaction pathway is as follows 73, 74.

329

Step 1: O2 + Ru → RuOx

330

Step 2: CH3CH2CH3 + RuOx → CH3CH(OH)CH3(ads) + RuOy

331

Step 3: CH3CH(OH)CH3(ads) + RuOy → CH3COCH3(ads) + RuOz + H2O

332

Step 4: CH3COCH3(ads) + RuOz → CH3COO-(ads) + HCOO-(ads) + RuOz + H2O

333

Step 5: CH3COO-(ads) + HCOO-(ads) + RuOz → CO2 + Ru + H2O

1200

1400

1800

2800

Wavenumber (cm )

3000

B

1200

1400

1600

1800

2967

2800 -1

Wavenumber (cm )


2979

2873 2888 2902 2960

1855

1652

1457

0.01

1373 1392 1430

Kubelka-Munk

2960

1600

-1

334

2979

0.005

2873 2888 2902

Kubelka-Munk

A

1589

According to the results mentioned above, it can be concluded that propane could adsorb on

2967

323

3000


1400

-1

3000

1546 1573

1200

1843

2968

1296

1358 1385 1398 1427 1462 1504

Kubelka-Munk 1800 2800

2933 2967

2842

1600

Wavenumber (cm )

0.005

1215

1546 1371 1428 1466 1506

1358

1200

335

D

0.025

1296

Kubelka-Munk

C

1617

Page 19 of 29

1400

1600

1800 2800

3000

-1

Wavenumber (cm )

336

Figure 6. DRIFT spectra of C3H8 adsorption (0.5 vol.% C3H8 – 99.5 vol.% Ar, red line), C3H8 + O2 reaction (0.5

337

vol.% C3H8 – 5 vol.% O2 – 94.5 vol.% Ar, blue line) and desorption (Ar, green line) on Al2O3 (A), Ru/Al2O3-1

338

(B), CeO2 (C) and Ru/CeO2-1 at 200 °C. The feed gas flow rate was 25 mL/min. All spectra were obtained after

339

the catalysts were exposed to the feed gas for 30 min.

340

As shown in Figure 6C, when the CeO2 support was exposed to pure propane for 30 min, in

341

addition to the absorption band at 2967 cm−1 assigned to the absorption of gaseous propane

75, 76

342

an absorption band attributed to νas(COO) was observed at 1546 cm−1, indicating that propane

343

reacted with the oxygen of the CeO2 support to a certain degree

344

replaced with 0.5 vol.% C3H8 – 5 vol.% O2 – 94.5 vol.% Ar, multiple bands appeared at 2933,

345

2842, 1506, 1466, 1428, 1371, 1355 and 1296 cm-1. The bands at 2933 and 2842 cm-1 were

346

assigned to ν(CH2)

347

attributed to νas(COO), δas(CH3), νs(COO), δs(CH3), δs(CH3) and ν(C-O)

348

Therefore, these results revealed that unlike in the case of the Al2O3 support, propane could be

349

partially oxidized and adsorb to the surface of the CeO2 support. Furthermore, the presence of the

350

bands assigned to the vibrations of CH2 revealed that the oxidation of propane over the CeO2

351

support occurred at the terminal group of propane instead of isopropyl groups, as was the case

352

with the Ru/Al2O3-X catalysts. At the desorption stage, only the band at 2967 cm-1 assigned to

,

69-71

. When the feed gas was

69, 70, 75, 76

, and those at 1506, 1466, 1428, 1371, 1355 and 1296 cm-1 were


69-72

, respectively.


353

gaseous propane disappeared, indicating that the partially oxidized species were strongly bound to

354

the surface of the CeO2 support.

355

For the Ru/CeO2-X catalysts, after the catalyst was exposed to feed gas consisting of 0.5 vol%

356

C3H8 – 99.5 vol.% Ar at 200 °C for 30 min, many bands with strong intensities were detected

357

(Figure 6D and S11), and their assignments are listed in Table S5. The presence of bands at 1843

358

and 1573 cm-1 revealed that propane could adsorb on the Ru/CeO2-X catalyst in the same way as

359

on the Ru/Al2O3-X catalyst, i.e., via species containing isopropyl groups. Meanwhile, similar to

360

the co-adsorption of C3H8 and O2 on the CeO2 support, the presence of other strong bands showed

361

that propane could also adsorb via the terminal group. Furthermore, a band at 1617 cm-1 assigned

362

to νas(C=C) also appeared after the adsorption of C3H8 on the Ru/CeO2-X catalysts, indicating the

363

formation of a species containing acrylate groups at the Ru-Ce interface 73. In summary, propane

364

could adsorb on the Ru/CeO2-X catalysts through two different pathways: one pathway formed

365

species containing isopropyl groups, while the other formed species containing acrylate groups.

366

When the feed gas was replaced with 0.5 vol.% C3H8 – 5 vol.% O2 – 94.5 vol.% Ar, only a slight

367

increase in the intensities of the bands between 1200 and 1700 cm-1 was observed. These results

368

showed that the oxidation of propane occurred before oxygen was introduced, indicating that the

369

lattice oxygens of CeO2 took part in the reaction. At the desorption stage, the intensities of all the

370

bands decreased sharply, indicating that those species easily desorbed.

371

According to the results mentioned above, it can be concluded that for the Ru/CeO2-X catalysts,

372

in addition to the oxidation of isopropyl groups on the Ru nanoparticles, propane can also adsorb

373

and be partially oxidized at the Ru-CeO2 interface as species containing acrylate groups, as shown

374

in Figure 7 (B). Finally, CO2 and H2O were formed by the decomposition and further oxidation of


Page 20 of 29

Page 21 of 29


375

the species containing acrylate groups. The additional reaction pathway for the Ru/CeO2-X

376

catalysts is as follows 73.

377

Step 1: 2CH3CH2CH3 + 7[O-] → 2CH3CH2COO-(ads) + 3H2O

378

Step 2: CH3CH2COO-(ads) + [O-] → CH2CHCOO-(ads) + H2O

379

Step 3: 2CH2CHCOO-(ads) + 11[O-] → 6CO2 + 3H2O

380

Step 4: O2 → 2[O-](ceria)

381

382 383

Figure 7. Propane oxidation pathways on the Ru/Al2O3 catalyst (A) and the additional reaction pathway on the

384

Ru/CeO2 catalyst (B).

385 386

To conclude, a Ru-based catalyst with superior performance in the total oxidation of propane at

387

low temperature has been successfully synthesized. Based on the characterization results, this

388

advanced catalytic performance is mainly attributed to the superiority of the CeO2 support and the

389

strong interaction between the Ru species and CeO2. First, the CeO2 support can act as a reservoir

390

for oxygen and provide additional sites for propane adsorption during the reaction. Second, the

391

strong interactions between the Ru species and CeO2 can produce oxygen defects on the CeO2 21 Environment ACS Paragon Plus


392

surface that enhance the adsorption of oxygen, and activate the oxygen in CeO2 around the

393

metal-support interface for oxidation reaction. Thus, an efficient reaction pathway is then created

394

for the total oxidation of propane at low temperature. We hope that this work will exemplify a

395

promising strategy for developing robust supported catalysts for short-chain VOC removal.

396 397

ASSOCIATED CONTENT

398

Supporting information

399

The catalyst characterization information and reaction kinetics measurement are provided together

400

with the catalytic activity data, plots of lnA as a function of Ea, TEM images and DRIFT spectra.

401

The supporting information is available free of charge on the ACS Publications website.

402 403

AUTHOR INFORMATION

404

Corresponding authors

405

*E-mail: Wangcheng Zhan ([email protected])

406

ORCID

407

Wangcheng Zhan: 0000-0002-0712-4917

408

Yun Guo: 0000-0003-4778-6007

409

Yanglong Guo: 0000-0003-0021-9128

410

Jinshui Zhang: 0000-0003-4649-6526

411

Notes

412

The authors declare no competing financial interest.

413


Page 22 of 29

Page 23 of 29


414

ACKNOWLEDGMENTS

415

Z.H. and W.Z. acknowledge financial support from the National Key Research and Development

416

Program of China (2016YFC0204300) and the Fundamental Research Funds for the Central

417

Universities (222201717003). Y.G. thanks the National Natural Science Foundation of China

418

(21571061).

419 420

References

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