ZnO Catalysts

Subscriber access provided by MEMORIAL UNIV

Article

Selective Methanol Oxidation to Hydrogen over Ag/ZnO Catalysts Doped with Mono- and Bi-Rare Earth Oxides Hany Mohamed Abdeldayem, Shar Saad Al-shihry, and Salah Abdou HASSAN Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 26 Nov 2014 Downloaded from http://pubs.acs.org on November 27, 2014

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.

Industrial & Engineering Chemistry Research 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 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

1

Selective Methanol Oxidation to Hydrogen over Ag/ZnO Catalysts

2

Doped with Mono- and Bi-Rare Earth Oxides

3

Hany M. AbdelDayem,†, ‡* Shar S. Al-Shihry,† and Salah A. Hassan‡

4

†

5

‡

6

ABSTRACT: The hydrogen production by the catalytic partial oxidation of methanol (POM)

7

over rare earth oxide (RE = La, Dy, Gd and Ce)-modified ZnO-supported silver catalysts, as well

8

as silver supported on Ce1-XGdXOy-modified ZnO catalysts, was investigated. The effect of the

9

temperature on the activity was studied in the range between 150°C and 400°C, and the catalyst

10

stability was monitored with time-on-stream (TOS). The addition of rare earth metal oxide

11

promoters resulted in a significant improvement in the catalytic performance. The optimal

12

performance in methanol oxidation was achieved using AgCe20Zn, which exhibited a hydrogen

13

selectivity of 90.8% with 95.2% methanol conversion at 350°C; however, the catalyst suffered

14

from marked deactivation with TOS. The good stability of the Ag/ZnO catalyst was verified

15

using a gadolinium promoter. Doping of the AgCe20Zn catalyst by Gd greatly enhanced its life

16

span over at least 24 h on-stream and markedly reduced the CO content (down to 97% at methanol conversion ca.

67

92%) than Ag/ZnO.[9,10] The physical properties of the promoted catalyst have been subsequently

68

characterized, and it was suggested that the Ag+ ions were reduced by Ce3+. In the same catalytic

69

reaction, La and Sm have been introduced to promote the activity and stability of Au and Cu

70

based catalysts, respectively.[14,15]

71

The use of CeO2-Gd2O3 in the composition of solid oxide fuel-cell (SOFC) anodes has been

72

proposed as an approach for suppressing the carbon deposition of SOFC running on ethanol.[16]

73

Recently, Huang and Chen have demonstrated that the methanol steam reforming activity of

74

Cu/CeO2 can be enhanced by the addition of Gd2O3 to ceria and that the CO content in reformate

75

is approximately 1% at 240°C.[17]

76

The objective of this work focused on the study of the influence of trivalent rare earth oxide

77

(RE = La, Gd, and Dy) doping on the activity and stability of an Ag/ZnO catalyst for the partial

78

oxidation of methanol to hydrogen, compared with Ag/CeO2-ZnO. Ag supported on CexGd1-xOy-

79

ZnO (x = 0.1, 0.2, 0.3, and 0.5) catalysts were also studied for methanol oxidation to explore the

80

effect of promotion by both Ce and Gd on the catalyst stability and the reduction of CO

81

formation.

82

4 ACS Paragon Plus Environment

Page 4 of 43

Page 5 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


83

2. EXPRIMENTAL SECTION

84

2.1. Catalysts preparation, ZnO particles were synthesized by the direct-precipitation

85

method.[18] Analytical grade Zn(NO3)2 and (NH4)2CO3 (Sigma-Aldrich, 99.5%) were first

86

dissolved in high-purity water to form solutions with 1.5 and 2.25 mol/L, respectively. The

87

Zn(NO3)2 solution was slowly poured into the (NH4)2CO3 solution with vigorous stirring, and

88

then the precipitate derived from the reaction was collected by filtration and rinsed three times

89

each with high-purity water and ethanol. The product was dried at 80°C to form the ZnO

90

precursor. Finally, the precursor was calcined at 550°C for 2 h in a muffle furnace to obtain

91

nanoscale ZnO particles.

92

Rare earth metal (20 wt %) (RE = Ce, La, Dy, and Gd) doped-ZnO supports were prepared

93

by the dry-impregnation technique. The ZnO particles were impregnated with a salt solution

94

containing the appropriate amounts of the rare earth metal nitrates. The resulting slurry was

95

dried by continuous stirring and heating at 70°C. The solid obtained was then kept in an oven

96

overnight at 110°C, crushed in an agate mortar and calcined for 4 h at 550°C. The products

97

were denoted as RE20Zn, namely, Ce20Zn, La20Zn, Gd20Zn, and Dy20Zn.

98

Ce1-XGdXOy (x= 0.1, 0.2. 0.3 and 0.5, where x is the atom fraction of gadolinium)-doped

99

ZnO with (20 wt % Ce1-XGdXO(4-X)/2) supports were prepared using the carbonate coprecipitation

100

method.[19] Each Ce1-XGdXOy solid was prepared following the same procedure below in the

101

absence of ZnO to identify its yield (wt). The high-purity (>99.5%) reagents Ce(NO3)3·6H2O and

102

Gd(NO3)3·6H2O were used as the starting materials, and ammonium carbonate as the

103

precipitation medium. The typical procedure involved the following steps: (1) Ce(NO3)3·6H2O

104

and Gd(NO3)3·6H2O were dissolved separate in high-purity water. A 150-mL mixture of Gd3+

105

and Ce3+ ions was prepared to be 0.3 M by blending the two solutions. (2) The mixture was 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

106

slowly added to 150 mL of an ammonium carbonate solution (0.75 M) containing the appropriate

107

amount of ZnO, with constant stirring at room temperature. (3) After the precipitation was

108

completed, the stirring was continued for 1 h at ca. 70°C. (4) The precipitate was filtered and

109

then washed several times with high-purity water and ethanol. Finally, the precursor produced

110

was dried at room temperature with flowing nitrogen gas for 20 h and calcined at 800°C for 2 h.

111

The solids produced were denoted as Ce0.9Gd0.1Zn, Ce0.8Gd0.2Zn, Ce0.7Gd0.3Zn and

112

Ce0.5Gd0.5Zn.

113

Two catalyst series were prepared via the dry-impregnation technique as follows: the first

114

series was silver supported on REZnO (RE= Ce, La, Dy, Gd) with 5 wt% loading, and the

115

second series was silver supported on CexGd1-xZn supports with 5 wt % loading. Each support

116

was impregnated with an aqueous solution containing the appropriate amount of silver nitrate

117

under continuous stirring for 24 h at 25°C. The silver-loaded samples were washed with

118

methanol several times, filtered, dried at 110°C for 12 h and then calcined at 350°C for 4 h. The

119

catalysts in the first series were labeled as AgCe20Zn, AgLa20Zn, AgDy20Zn and AgGd20Zn

120

and in the second series as Ag/ Ce1-xGdxZn; where x= 0.1, 0.2, 0.3, and 0.5. Silver supported on

121

undoped ZnO was denoted as AgZn.

122

2.2. Characterization, The silver content in these catalysts was determined by atomic

123

absorption spectroscopy (AAS) on a Perkin Elmer model 3100. X-ray diffraction (XRD)

124

measurements were performed employing a Philips X’Pert MPD (multipurpose X-ray

125

diffractometer) employing Cu Kα1,2 radiation (λ = 1.5405 Å) for 2θ angles varying from 10° to

126

80°. H2-TGA analysis was performed using a Shimadzu - Japan Thermal Analyzer model 50

127

using a heating rate of 10.0 °C min-1 from 50 to 1000°C with a hydrogen flow rate of 20 mL min-

128

1

. Temperature-programmed reduction (H2-TPR) was performed using a ChemBET 300 6 ACS Paragon Plus Environment

Page 6 of 43

Page 7 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


129

Quantachrome. A 100-mg sample of the freshly calcined catalyst was subjected to a heat

130

treatment (20°C min-1 up to 1000°C) in a gas flow (85 mL min-1) composed of a mixture of

131

hydrogen/nitrogen (5/95 vol.%). Prior to the TPR experiments, the samples were heated for 3 h

132

under an inert atmosphere (nitrogen) at 200°C. The surface areas (SBET) of the various samples

133

were determined from the adsorption of nitrogen gas at liquid-nitrogen temperature (-195.8 °C)

134

using a NOVA3200e (Quantachrome-USA). Prior to the measurements, all samples were

135

perfectly degassed at 150°C and 10–4 Torr overnight. N2O pulse chemisorption was applied to

136

determine the Ag degree of dispersion using ChemBET 3000 and the TPR-Win V. 1.50 software.

137

Before chemisorption a pretreatment procedure has been applied: the sample (0.1g) was oxidized

138

in flow of oxygen at 300°C for 30 min; purged in He flow at the same temperature for 15 min,

139

followed by reduction in 5%H2/He flow (20 ml min-1) at 300°C for 1 h, and then purged in He

140

flow (30 ml min-1) at the same temperature for 30 min. The sample was cooled down to 100°C in

141

He flow for 15 min at the same temperature for 0.5%N2O/He flow (20 ml min-1) pulse

142

chemisorption. N2O pulses were injected in a series of 3-min intervals. The surface area of silver

143

was determined considering the stoichiometry of the reaction (2). Scanning electron microscopy

144

(SEM) images were obtained with a JEOL JSM-7600F microscope operated at 20 kV.

145

Transmission electron micrographs were obtained using a JEOL 1200 EX II transmission

146

electron microscope (TEM) operated with an acceleration voltage of 50 kV.

147

2.3. Catalytic test, Catalytic tests were performed at atmospheric pressure in a tubular

148

quartz reactor (‘6 mm i.d.), which was placed inside a programmable furnace in which the

149

temperature was measured by a type K thermocouple located in a pocket in the center of the

150

catalyst bed. Pure methanol was fed to a purpose-vaporizer by a Master Flex C/L variable-speed

151

tubing pump (1 to 6 rpm) EW-95990-18. The carrier gas, containing oxygen for the partial 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

152

oxidation experiments, was also fed to the vaporizer. The gas flow was adjusted by a Cole-

153

Parmer compact mass flowmeter. All chemical pipes were heated to avoid condensation in the

154

system. The feed and the product gas compositions were determined by online gas

155

chromatography (GC), using a Bruker 450 GC equipped with three channels. The first is for

156

hydrogen analysis using TCD. The gas separation was performed by HayeSep Q and 5 Å

157

molecular sieves. Channel two is for analyzing nonflammable gases (viz., O2, N2, CO, CO2)

158

using TCD, and separation was accomplished by HayeSep Q and MolSieve 13X columns

159

connected in series. The third channel is for analyzing oxygenates (methanol, formic acid and

160

formaldehyde), and separation was accomplished by HayeSep Q and Varian select

161

formaldehyde. The product composition was measured at 15-min intervals, as determined by the

162

duration of the GC analysis, with intermittent increments of the furnace temperature.

163

Calibration enabled the quantitative analysis of methanol, hydrogen, oxygen, nitrogen, carbon

164

monoxide and carbon dioxide. The rapid ignition of the partial-oxidation reaction complicates

165

the time-consuming GC analysis. For all data reported, the reactant and product concentrations

166

at the inlet and outlet of the reactor were estimated using a multi-channel computer program.

167

The initial concentrations of methanol and oxygen were calculated before each catalytic run to

168

verify their constant values with a 99% degree of confidence. The carbon and hydrogen mass

169

balances with error were under 2%. For each run, three to five trials were performed. These

170

multiple trials were used to estimate the 98% confidence intervals. Typically, 100 mg of the

171

catalyst, ground and sieved to 212- 500 µm was used in all experiments. The catalyst was

172

diluted with SiO2 to 10 wt% to prevent hot-spot formation in the bed. The catalyst activation

173

was performed in situ by exposing the catalyst to 100 mL min-1 of 10% H2/N2, increasing the

174

temperature to 250°C at 10°C min-1. This temperature was maintained for 1 h. Subsequently, the


Page 8 of 43

Page 9 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


175

furnace temperature was lowered to ~100°C. The partial-oxidation experiments were performed

176

under a total flow rate of 220 mL min-1 with an O2/methanol molar ratio of 0.5, balanced with

177

nitrogen and the weight hourly space velocity; WHSV = 13.2 x 104 mL h-1 g-1 {WHSV= flow

178

rate of feed gas (mL h-1) per weigh of catalyst (g)}. The catalytic reaction was performed from

179

150-400°C to with seven evenly spaced temperature points, and each point was maintained for

180

at least 1.5 h. The product selectivity (%) was defined as the product concentration x 100

181

divided by the methanol conversion. The CH3OH conversion (%) = concentration of methanol

182

consumed x100/ concentration of methanol in the feed. The product concentrations were

183

calculated according to the following equation (1):

184

185

CH3OH (g) + ½ O2

2 H2 + CO2.

(1)

3. RESULTS AND DISCUSSION

186

3.1. Textural and structural characterization of the catalysts, The silver contents of the

187

RE-doped ZnO-supported catalysts measured by atomic absorption spectroscopy (AAS) are

188

summarized in the Table 1. It is clear that the rare earth oxide (viz, Gd, Dy)-doped catalysts

189

had nearly the same silver content as the undoped AgZn catalyst. However, the AgLa20Zn and

190

AgCe20Zn catalysts had relatively lower silver contents than that of the undoped AgZn

191

catalyst. Furthermore, the AAS measurements showed that the measured rare earth metal

192

content in each Ag-RE-Zn catalyst is approximately 20 wt %. On the other hand, the BET

193

surface areas of the AgREZn samples are smaller than those of ZnO and AgZn.

194

The XRD patterns of the nanoscale ZnO and RE-modified ZnO supports are shown in Figure

195

1. All of the diffraction peaks of ZnO could be indexed to the hexagonal phase reported in the

196

JCPDS file (36-1451). The obtained diffraction peaks of ZnO are intensive and narrow, 9 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 43

197

indicating the well-crystallized nature of the support components. The calculated average particle

198

size of ZnO was ca. 35.2 nm, calculated by the Debye-Scherrer formula.[20] The SEM images of

199

ZnO (Supporting Information, Figure S1) showed that the sample is composed of discrete

200

spherical particles with an average diameter of ca. 28.0 nm. The H2-TGA analysis revealed that

201

the synthesized ZnO was inert to hydrogen reduction, as it did not show any reduction peak up to

202

700°C (Supporting Information, Figure S2). Furthermore, the 2theta angle (degree) of the CeO2

203

phase, i.e., 28.568°, 33.278° and 47.532° (JCPDS file, 81-0792), and of the Dy2O3 phase, i.e.,

204

28.667° and 47.803° (JCPDS file, 19-0436), were detected in the diffraction patterns of Ce20Zn

205

and Dy20Zn, respectively (Figure 1). However, for the Gd20Zn support, new peaks at the 2theta

206

angle (degree) = 28.554°, 33.278°, and 47.526° were detected, which are most likely

207

characteristic of a Gd2O3 phase (JCPDS file, 86-2477). It is of special interest to record that, in

208

the case of the La20Zn support, only one weak, broad peak characteristic of a La2O3 phase was

209

observed (Supporting Information, Figure S3). The absence of the other peaks characteristic of

210

La2O3 may be linked to its amorphous nature and/or its high degree of dispersion within the

211

solid-support composite system.

212

In all XRD patterns of the Ag catalysts, except for those of the ZnO and rare earth oxide

213

phases, a peak characteristic of the cubic Ag2O phase was detected at the 2theta angle (degree) =

214

38.072° (JCPDS file, 65-3289), and a second peak that is characteristic of the hexagonal Ag2O

215

phase (JCPDS file, 42-0874) was detected at the 2theat angle (degree) = 44.275° (Figure 2).

216

The typical XRD peaks characteristic of the ZnO and Ag2O phases were observed in all

217

patterns of AgCe1-xGdxZn (Figure 3). The fluorite CeO2 phase was also observed at the 2theta

218

angle (degree) = 28.582°, 33.087°, 47.529°, 56.603° and 59.152°. We note that the development


Page 11 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


219

of C-type Gd2O3 in the CeO2 matrix cannot be clearly identified from the patterns due to the

220

overlapping of their diffraction lines. Finally, the introduction of gadolinium into the cerium

221

oxide lattice resulted in a shift in the diffraction peaks of CeO2 to higher 2theta values as the

222

gadolinium content increased. This is indicative of the dissolution of Gd3+ into the cubic fluorite

223

ceria lattice. Additional evidence for the formation of a Ce-Gd solid solution is that, with Gd2O3

224

doping, the cell parameter “a” gradually increased from 5.40 (AgCe20Zn) to 5.44

225

(Ag/Ce0.5Gd0.5Zn) (Table 1). Considering that the ionic radius of Gd3+ (0.105 nm) is larger than

226

that of Ce4+ (0.097 nm), the incorporation of Gd3+ into the CeO2 lattice will result in lattice

227

expansion. [21-23]

228

As shown in Figure 4a, a reduction peak appeared at Tr = 205°C in the TPR curve of the

229

AgZn catalyst. This peak indicated the presence of Ag2O in the catalyst.[24] No ZnO reduction

230

peaks were observed in this study (up to 600°C). Reduction of the bulk ZnO to metallic Zn is

231

thermodynamically feasible, but a temperature as high as 650°C is required.[25] Three reduction

232

peaks were observed for the AgCe20Zn catalyst (Figure 4 b), in contrast to the results for AgZn.

233

The first peak at 182°C is most likely attributable to the reduction of Ag2O but is shifted to a

234

lower temperature by 23 degrees, which indicates that the addition of Ce to AgZn facilitates the

235

catalyst reducibility. The second and third peaks at 450°C and 482°C can be ascribed to the

236

reduction of the CeO2 surface-capping oxygen.[26] For the AgGd20Zn catalyst, the TPR profile

237

(Figure 4 c) shows that the reduction peak corresponding to Ag2O is located at the same

238

temperature as that of AgZn. In the reduction profiles of the AgLa20Zn and AgDy20Zn catalysts

239

(Figure 4, d and e), only one peak is shown at 251°C. Compared to the results for AgZn, this

240

peak can be attributed to the reduction of the Ag+ species because lanthanum oxide and

241

dysprosium oxide are nonreducible. [27] The increase in the reduction temperature appears thus to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 43

242

be linked with the probable strong interaction involving Ag2O and the RE oxide (viz., La and

243

Dy). On the other hand, the presence of Ag splits the reduction peak of the CeO2 species into two

244

peaks located at 452°C and 485°C, respectively (Figure 4b), which are much lower than the peak

245

observed at 556°C for the neat Ce20Zn support (Supporting Information, Figure S4). In fact,

246

many researchers have shown that the presence of noble metals can improve the cerium oxide

247

reducibility at lower temperatures, as assisted by hydrogen spillover.

248

that, in the case of the AgCe20Zn system, Ag may catalyze H2-spillover on the surface, thereby

249

accelerating the reduction of CeO2 surface oxygen.

250

Figure 5 shows the H2-TPR profiles of Gd-doped AgCe20Zn catalysts with various atomic

251

fractions of Gd. The profile of the unmodified AgCe20Zn catalyst is also included for

252

comparison. In the case of the AgCe0.8Gd0.2Zn catalyst, the H2-TPR curve (Figure 5b) exhibited

253

only one peak at 189°C. Referring to the H2-TPR profiles of AgZn and AgCe20Zn (Figure 4, a

254

and b), this peak should be ascribed to the reduction of surface Ag2O, as facilitated by the

255

presence of Ce; however, it became much weaker by Gd doping. Furthermore, the main peak at

256

Tr = 189°C disappeared completely when the Gd atomic fraction increased from 0.2 to 0.5

257

(Figure 5c). The absence of an Ag2O reduction peak can be attributed to the possibility that the

258

Ag+ species in fresh AgCeGdZn catalysts had been completely reduced to metallic Ag by the Ce-

259

Gd combination in the preparation, i.e., doping AgCe20Zn with Gd severely facilitates the

260

reducibility of the Ag2O species.[10] In view of the TEM results (see Figure 6), the significant

261

high reducibility of Ag2O in this catalyst can be attributed to the fact that the addition of Gd to

262

AgCeZn decreased the size of Ag particles. Additionally, the presence of Gd (atom fraction=

263

0.5) appears to have an effect on the location of the reduction peaks of the CeO2 and their

264

intensity (Figure 5c). The two reduction peaks of CeO2 were shifted to higher temperatures, viz., 12 ACS Paragon Plus Environment

[28-30]

We can thus suggest

Page 13 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


265

523°C and 575°C. This may confirm the strong interaction between CeO2 and Gd2O3. The new

266

reduction peak at 315°C appears to be most likely to be assigned to the reduction of the surface

267

oxygen of the Ce-Gd solid solution (cf., XRD results).

268

TEM micrographs of the AgZn, AgCe20Zn and AgCeGdZn catalysts are shown in Figure 6.

269

In the TEM micrograph of the AgZn catalyst both silver and zinc particles gather into larger

270

particles, with diameter larger than 10 nm (Figure 6a). However, in the TEM micrograph of

271

AgCe20Zn catalyst (Figure 6b), the sintering of Ag particles is somewhat inhibited. Furthermore,

272

the micrograph of AgCeGdZn catalyst (Figure 6c) shows smaller particles in highly dispersed

273

state, without significant aggregations, and these particles are dispersed on the bulk of CeGdZn

274

support with diameter smaller than 10 nm.

275

3.2. Catalytic activity: partial oxidation of methanol (POM), The effect of La, Ce, Dy and

276

Gd as promoters on the performance of the AgZn catalyst is shown in Table 2. It is evident that

277

the activities of the samples under study decrease in the following order: AgCe20Zn>

278

AgLa20Zn≈ AgDy20Zn> AgGd20Zn> AgZn. AgLa20Zn has the highest hydrogen selectivity

279

among the studied catalysts, i.e., the selectivity is 97.8% but with only 32% methanol

280

conversion. The optimal performance in methanol oxidation was achieved by AgCe20Zn, which

281

exhibited a hydrogen selectivity of 90.8% with 95.2% methanol conversion. Furthermore, as

282

shown in Table 2 most of supports are inactive however, Ce20Zn and Ce0.5Gd0.Zn supports

283

exhibited very low activity with a high selectivity to hydrogen.

284

Figure 7 shows the effect of the reaction temperature on the methanol conversion over RE-

285

doped AgZn. The study was undertaken in the temperature range of 150 and 400°C using the

286

O2/CH3OH ratio 0.5. In general, methanol conversion over most of catalysts increased with

287

increasing reaction temperature up to 400°C. Obviously, the selectivity toward hydrogen (SH2) 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

288

increased with increasing temperature and reached a maximum at 350°C followed by a

289

significant decrease, especially in the case of the AgDy20Zn catalyst, as shown in Figure 8.

290

Generally, a large variation occurred in the catalytic behavior for POM with temperature,

291

depending on the type of promoting rare earth oxide used (Figure 8).

Page 14 of 43

292

It is well known that a short lifespan limits the practical applications of catalysts. Many

293

catalysts have high initial activity but deactivate with reaction time.[31-33] The changes in the

294

methanol oxidation performance over RE-doped catalysts with time on stream (TOS) are shown

295

in Figures 9 and 10. From these figures, it is clear that AgGd20Zn is the most stable among the

296

studied catalysts from the point of view of methanol conversion and H2-yield with TOS.

297

One can thus conclude that because Ce is considered an excellent promoter of AgZn catalysts

298

for the production of H2, Gd can be considered an effective stabilizing dopant for methanol

299

conversion and the H2 yield with TOS. Based on these findings, further study was undertaken to

300

investigate the effect of various Gd/Ce atomic ratios on the catalytic behavior of the AgZn

301

catalyst system. Table 2 summarizes the results of methanol oxidation on Gd-doped AgCeZn

302

with various atomic fractions at 350°C. The presence of Gd in the catalyst resulted in a

303

significant decrease in the methanol conversion, not only at 350°C but also at various reaction

304

temperatures (Figure 11), without a significant effect on the selectivity to H2 (Figure 12). In this

305

catalyst series, the maximum performance was achieved by the Ag/Ce0.8Gd0.2Zn catalyst,

306

(selectivity to H2 of 90% at a methanol conversion of 62.2%). More importantly, the stability

307

study presented in Figure S5 (Supporting Information) and Figure 13 indicated that the addition

308

of Gd to the AgCeZn catalyst can greatly enhance its lifespan: no further decrease was observed

309

in the catalytic performance with TOS up to 20 h.


Page 15 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


310

It is clear that the addition of RE oxides (viz., Ce, Gd, La and Dy) to the Ag/ZnO catalyst has

311

a large beneficial effect on the selectivity to H2 in the partial oxidation of methanol. Mo et al.

312

reported for an Ag/ZnO catalyst that methanol undergoes dissociative adsorption to form

313

methoxy (CH3O) species over silver and that the C-H bond in the methoxy group is cleaved to

314

form a hydrogen atom.[10] Consequently, the hydrogen dissociated on silver crystallites is

315

oxidized to water by oxygen adsorbed onto the crystallites (Equation 2), with a loss of hydrogen

316

selectivity:

317

2Hads

+ Oads

H2O.

(2)

318

They claimed that, in the case of a Ce-doped catalyst, CeO2 served as a spillover sink to

319

collect hydrogen dissociated on silver crystallites, and ZnO acted as a porthole for the fast

320

desorption of hydrogen from the catalyst. In the present study, the H2-TPR results confirmed the

321

suggestion of hydrogen spillover from the Ag crystallites to the CeO2 particles where an

322

enhancement in the reducibility of the CeO2 species in the AgCe20Zn catalysts occurred.

323

Moreover, the good catalytic performance of AgCe20Zn may also result because the reducibility

324

of the Ag species on the surface of the AgZn catalyst was greatly enhanced by Ce doping, as

325

revealed by the H2-TPR results.

326

Although the observed enhancement in the activity of the AgCe20Zn catalysts can be

327

ascribed to a hydrogen-spillover phenomenon, this phenomenon still cannot interpret the superior

328

performance of the AgGd20Zn, AgLa20Zn and AgDy20Zn catalysts, as they are nonreducible

329

and/or they have a low affinity for hydrogen. Recently, Hereijgers et al. discussed the effect of

330

promoting an Au catalyst with La2O3 in POM to hydrogen. It was proposed that the acid/base

331

properties of the La2O3 play a crucial role in directing the reaction to hydrogen and suppressing



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[14]

Page 16 of 43

332

CO formation.

Considering the analogy between Au and Ag, one can apply similar logic to

333

Au/La2O3 as that in the discussion by Hereijgers et al.

334

Lewis-acid sites of La2O3 in the AgLa20Zn system play a crucial role for stabilizing the methoxy

335

intermediate. It was also proposed that La2O3 as an amphoteric oxide adsorbed CO onto its basic

336

sites and activated it for hydrogenation to methanol on the neighboring acidic sites. Furthermore,

337

Hereijgers et al. [14] suggested that La2O3 facilitated the formation of O surface species, which

338

resulted in an increased activity of the CO oxidation.

[14]

; it can be proposed that the strong

339

The above results demonstrated that AgCeGdZn catalysts have a higher stability than

340

AgCeZn. Recently, it was also found that the addition of Gd to ceria had a positive effect on the

341

stability of Cu/CexGd1-xOy and Pt/CexGd1-xO2 catalysts during the steam reforming of methanol

342

and the catalytic decomposition of methane, respectively.[17,34] It was reported that a Ce-Gd solid

343

solution was created by the substitution of Ce4+ cations with Gd3+cations, and hence, more

344

oxygen vacancies were generated. The oxygen vacancies in the solid then provide a route to

345

increase the metal (Pt,Cu)-support interaction and inhibit metal sintering. Here, the XRD and

346

TEM results confirmed the formation of a Ce-Gd solid solution in the AgCeGdZn catalysts and

347

the presence of both Ce and Gd as modifiers increased the well dispersed silver species over

348

ZnO. Thus, the higher stability of Ag in the Ce-Gd-containing support catalysts may be due to

349

the formation of an Ag-O-Ce bond because this bond may act as an anchor, inhibiting Ag

350

sintering under oxidizing conditions at high temperature.

351

According, to the suggestions of Salazar-Villapando et al. and Trovarelli, .[34,35] it appears

352

that oxygen vacancies at the Ag-O-CeGd interface may also be responsible for improving the

353

mobility of oxygen in the bulk of Gd-doped AgCeZn catalysts, and the high resistance to carbon


Page 17 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


354

deposition might be caused by the transfer of active O from the carrier to an Ag atom, resulting

355

in carbon oxidation and improved accessibility of CH3OH to the active sites.

356

From the dispersion and activities (yield of H2) it is possible to evaluate relative turn-over

357

frequencies (TOF) for the silver catalysts studied here. TOF defined as mole of hydrogen per

358

exposed silver surface site (mole Ag in catalyst X dispersion) and second. Herein, TOF depends

359

on estimates of dispersion, which are calculated using experimentally N2O chemisorption (Table

360

3). It is clear that the selective POM to hydrogen is a structure-insensitive reaction on RE oxides

361

doped AgZn catalysts where, in the case of AgLa20Zn and AgDy20Zn catalysts TOF increased

362

upon doping AgZn with La and Dy; however, dispersion did not change. In addition, the higher

363

TOF of AgCe20Zn catalyst compared to AgZn catalyst cannot attributed only to increase of Ag

364

dispersion where dispersion did not increase significantly. Furthermore, addition of Gd to AgZn

365

leads to a significant increase in Ag dispersion; however, TOF decreased. It is not surprising that

366

there is no clear correlation between particle diameter and TOF. We plot TOF vs. particle size

367

for Ag particles for catalysts under study (Figure 14) and see no trend between TOF and the size

368

of the Ag particles. The TOFs results discussed above indicated clearly the low structure-

369

sensitivity of POM to hydrogen reaction on rare earths modified-Ag/ZnO catalysts.

370

conclusion confirmed our previous explanations for the higher selectivity of AgREZnO catalysts

371

for POM to hydrogen, which may be results of additional effects (viz., enhancement of hydrogen

372

spillover, increasing reducibility of Ag particles, acid/base properties of the rare earth oxide).

373

Interestingly, chemisorption results (Table 3) shows that the Ag dispersion increased with doping

374

AgCeZn catalyst with Gd which indicating that Ag particle dispersion must play a role in the

375

observed higher stability of AgCeGdZn catalysts. Table 3 shows that the average particle

376

diameter in most of AgCeGdZn catalysts is less than 1.5 nm. This particle size is very important; 17 ACS Paragon Plus Environment

This


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 43

377

Farmer and Campbell[36] reported that silver atoms in nanoparticles less than 1.5 nm bind more

378

strongly to vacancy sites on unsaturated CeO1.8 than to stoichiometric sites and the sintering

379

rates of these particles on CeO1.8 will much slower than on CeO2.[36,37] Again, our finding that the

380

addition of Gd to AgCeZn catalyst decreased Ag average particles size less than 1.5 nm and

381

consequently increased its stability during POM is consistent with finding of Wang et al. and

382

Farmer et al.[36,37]

383

384

4. CONCLUSIONS

385

The addition of rare earth oxides (RE= La, Ce, and Gd) to ZnO-supported Ag catalyst

386

materials has a large positive effect on the selectivity toward hydrogen, and toward CO in the

387

partial oxidation of methanol. The trend in the H2 selectivity depends on both the degree of

388

reduction of Ag2O and the Ag/RE-ZnO interaction. The introduction of ceria into the ZnO-

389

supported catalysts is most beneficial for methanol oxidation to hydrogen over Ag/RE-ZnO

390

catalysts: 90.8% hydrogen selectivity with 95.2% methanol conversion was achieved over an

391

AgCe20Zn catalyst at 350°C with a weight hourly space velocity of = 13.2 x 104 mL h-1 g-1.

392

However, the AgCe20Zn catalyst showed a low stability during the reaction time on stream. The

393

addition of a slight amount of Gd to AgCeZn improved the catalyst stability for at least 24 h on

394

stream. Here, it is suggested that the formation of an Ag-O-CeGd bond caused the high stability

395

of Ag in the Ag/CeGdZn catalysts because this bond may act as an anchor, inhibiting the

396

sintering of Ag. The Ag/Ce0.9Gd0.1Zn-based catalyst can be regarded as a promising material for

397

H2 generation by the partial oxidation of methanol under severe reaction conditions because it

398

generated a lower amount of CO (~0.2%). 18 ACS Paragon Plus Environment

Page 19 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ACKNOWLEDGMENTS

399

400 401 402 403

The financial support via research grant (PE-29-126) by King Abdul-Aziz City for Science and Technology is gratefully acknowledged. The principal investigator thanks Dr. Tiancun Xiao, inorganic chemistry laboratory, Oxford University for reviewing the language of a portion of this work.

404

405

Supporting Information Available

406 407 408

SEM Images, H2-TGA curve of the ZnO and XRD patterns of RE-ZnO supports in the 2-theta values range from 10-35°.This information is available free of charge via the Internet at http://pubs.acs.org/.

409 410

AUTHOR INFORMATION Corresponding Author

411 412

31982 Al-Hasa. Al-Hofof; P.O. Box 380, Saudi Arabia. Tel.: +966 (3) 5800000(1854), +96654788700, Fax: +966-(3)-5801243E-mail: [email protected]

413 414

Permanent address: 11566 Cairo, Egypt;Tel.: +202-4831836- 108, Fax: +202 – 5255657. E-mail:

415

416 417

ASSOCIATED CONTENT

[email protected]

REFRENCES

(1) Barbaro, P.; Bianchini., C. Catalysis for Sustainable Energy Production; Wiley, VCH: Weinheim, 2008.

418

(2) Lubitz, W.; Tumas, W. Hydrogen: An Overview. Chem. Rev. 2007, 107, 3900.

419

(3) Navarro, R. M.; Pena, M. A.; Fierro, J. L. G. Hydrogen Production Reactions from Carbon

420

421 422

Feedstocks: Fossil Fuels and Biomass. Chem. Rev. 2007, 107, 3952. (4) Michael, Q. C.-G.; M. Nash, M.; N, Yusof, N. 31 Annual World Methanol Conferences: Singapor; 11-12 September, 2013, Methanol Institute 2013, available on the web:



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

423

http://www.methanol.org/Methanol-Basics.aspx; last checked May 2014. GulFBase, 2014,

424

available on the web: http://www.gulfbase.com/ar/newarticles/specialarticledetail/4791, last

425

checked May 2014.

426 427

(5) Larminie, J. E.; Dicks, A. Fuel Cell Systems Explained; John Wiley & Sons: Chichester; England, 2000; p 308.

428

(6) ICIS Pricing, Chemical Price Review, 2008; vol. 2009.

429

(7) Houseman, J.; Cerini, D.J. On board Hydrogen Generation for Automobiles; Proceedings

430

of the 11th Intersociety Energy Conversion Engineering Conference: New York; SAE Paper

431

769001; 1976, 5th edn.; p.6

432

(8) Houseman, J.; Cerini, D. J. Hydrogen-Rich Gas Genrator. U. S. Patent. 1976, 3,982,910.

433

(9) Mo, L.; Zheng, X.; Yeh, C.-T. Selective Production of Hydrogen from Partial Oxidation of

434

435

Methanol over Silver Catalysts at Low Temperatures. Chem. Commun. 2004,12,1426. (10) Mo, L.; Wan A. H.; Zheng, X.; Yeh, C.-T. Selective Production of Hydrogen from Partial

436

Oxidation of Methanol Over Supported Silver Catalysts Prepared by Method of Redox Co-

437

precipitation. Catal. Today. 2009, 148, 124.

438 439

440 441

(11) Mo, L.; Zheng, X.; Yeh, C.-T. A Novel CeO2/ZnO Catalyst for Hydrogen Production from the Partial Oxidation of Methanol. ChemPhysChem. 2005, 6, 1470. (12) Yeh, C.-T.; Chen, Y. J.; Hung, H. S. A Process of Producing Hydrogen under low Temperature. Taiwan Patent. 2005, 1226308.


Page 20 of 43

Page 21 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


442

(13) Qayyum, E.; Castillo, V. A.; Warrington, K.; Barakat, M. A.; Kuhn, J. N. Methanol

443

Oxidation over Silica-Supported Pt and Ag Nanoparticles: Toward Selective Production of

444

Hydrogen and Carbon Dioxide. Catal. Commun. 2012, 28, 128.

445

(14) Hereijgers, B. P. C.; Weckhuysen, B. M. Selective Oxidation of Methanol to Hydrogen

446

over Gold Catalysts Promoted by Alkaline-Earth-Metal and Lanthanum Oxides. ChemSusChem.

447

2009, 2, 743.

448 449

450

(15) Wang, J. B.; Li, C.-H.; Huang, T.-J. Study of Partial Oxidative Steam Reforming of Methanol over Cu–ZnO/Samaria-Doped Ceria Catalyst. Catal. Lett. 2005, 103: 239. (16) da Costa, L. O. O A.; da Silva, M.; Noronha, F. B.; Mattos, L. V. The Study of the

451

Performance of Ni Supported on Gadolinium Doped Ceria SOFC Anode on the Steam

452

Reforming of Ethanol. Int. J. Hydrogen Energy. 2012, 37, 5930.

453 454

455 456

457

(17) Huang, T.-J.; Chen, H.-M. Hydrogen Production via Steam Reforming of Methanol over Cu/(Ce,Gd)O2−x Catalysts. Int. J. Hydrogen Energy. 2010, 35, 6218. (18) Chen, C. C.; Liu, P.; Lu, C. H. Synthesis and Characterization of Nano-Sized ZnO Powders by Direct Precipitation Method. Chem. Eng. J. 2008, 144, 509. (19) Zhang, T. S.; Ma, J.; LuO, L. H.; Chan, S. H. Preparation and Properties of Dense

458

Ce0.9Gd0.1O2−δ Ceramics for Use as Electrolytes in IT-SOFCs. J. Alloys Comp. 2006, 422, 46.

459

(20) Patterson, A. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev.

460

1939, 56, 978.

461 462

(21) Balazs, G.B.; Glass, R. S. ac Impedance Studies of Rare Earth Oxide Doped Ceria Solid State Ionics. 1995, 76, 155. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

463 464

465 466

467 468

469 470

Page 22 of 43

(22) Peng, C.; Zhang, Z. Nitrate–Citrate Combustion Synthesis of Ce1−xGdxO2−x/2 Powder and Its Characterization. Ceram. Int. 2007, 33, 1133. (23) Huang, T. J.; Huang, M. C. Effect of Ni Content on Hydrogen Production via Steam Reforming of Methane Over Ni/GDC Catalysts. Chem. Eng. J. 2008, 145, 149. (24) Smeltzer, W.W.; Tollefson, E. L.; Cambron, A. Adsorption of Oxygen by a Silver Catalyst. Can. J. Chem. 1956, 34. 1046. (25) Imoto, T.; Harano, Y.; Nishi, Y.; Masuda, S. The Reduction of Zinc Oxide by Hydrogen III: The Effect of Nitrogen on the Reduction. Bull. Chem. Soc. Jpn. 1964, 37, 441.

471

(26) Rumruangwong, M.; Wongkasemjit, S. Synthesis of Ceria-Zirconia Mixed Oxide from

472

Cerium and Zirconium Glycolated via Sol-Gel Process and its Reduction Property. Appl.

473

Organometal. Chem. 2006, 20, 615.

474

(28) Alayoglu, S.; An, K.; Melaet, G.; Chen, S.; Bernardi, F.; Wang, L. W.; Lindeman, A. E.;

475

Musselwhite, N.; Guo, J.; Liu Z.; Marcus, M. A.; Somorjai, G. A. Pt-Mediated Reversible

476

Reduction and Expansion of CeO2 in Pt Nanoparticle/Mesoporous CeO2 Catalyst: In Situ X-ray

477

Spectroscopy and Diffraction Studies under Redox (H2 and O2) Atmospheres. J. Phys. Chem. C.

478

2013, 117, 26608.

479

(29) Luo, M.; Shan, W.; Ying, P.; Lu, J. L.; Li, C, TPR Study of PdO Catalysts Supported on

480

CexTi1−xO2 and CexY1−xO1.5+0.5x: Effects of Hydrogen Spillover; Guerrero-Ruiz, A.; Rodriguez-

481

Ramos, I. Eds.; Stud. Surf. Sci. Catal. 2001, 138, 61.

482 483

(30) Dutta, G.; Waghmare, U. V.; Baidya, T.; Hegde, M. S. Hydrogen Spillover on CeO2/Pt: Enhanced Storage of Active Hydrogen. Chem. Mater. 2007, 19, 6430. 22 ACS Paragon Plus Environment

Page 23 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

484 485


(31) Murata, K.; Kiyozumi, Y. Oxidation of Propene by Molecular Oxygen over Ti-Modified Silicalite Catalysts. Chem. Commun. 2001, 1356-1357.

486

(32) Murata, K.; Liu, Y.; Mimura, N.; Inaba, M. Direct Vapor Phase Oxidation of Propylene

487

by Molecular Oxygen over MCM-41 or MCM-22 Based Catalysts. Catal. Commun. 2003, 4,

488

385.

489 490

(33) Haruta, M.; Daté, M. Advances in the Catalysis of Au Nanoparticles. Appl. Catal. A. 2001, 222, 427.

491

(34) Salazar-Villalpando, M. D.; Miller, A. C. Hydrogen Production by Methane

492

Decomposition and Catalytic Partial Oxidation of Methane Over Pt/CexGd1 − xO2 and

493

Pt/CexZr1 − xO2. Chem. Eng. J. 2011, 166, 738.

494 495

496 497

498 499

(35) Trovarelli, A. Catalytic Properties of Ceria and CeO2-Containing Materials. Catal. Rev.Sci. and Eng. 1996, 38, 439. (36) Farmer, J. A.; Campbell C. T. Ceria maintains smaller metal catalyst particles by strong metal-support bonding. Science, 2010, 329, 923. (37) Wang, J. H.; Liu, M . L.; Lin, M. C. Oxygen reduction reactions in the SOFC cathode of. Ag/CeO2. Solid State ion. 2006, 177, 939.

500

501

502

503 23 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 43

504 505

Table 1.

506 507

Overview of the structural properties of the investigated rare earth oxide-modified Ag-based catalyst materials, including Ag loading, surface area of the catalysts, Ag and cell parameters. catalyst

Ag[a]

SBET /m2g-1[b]

RE2O3[c]

(w/w%)

508 509

Cell parameter a/nm

ZnO

-

38.8

-

AgZn

4.44

33.3

-

AgLa20Zn

3.95

24.3

-

AgDy20Zn

4.41

24.9

-

AgCe20Zn

3.68

26.6

5.4031

AgGd20Zn

4.37

27.7

5.4506

Ag/Ce0.9Gd0.1Zn

4.12

16.7

5.4047

Ag/Ce0.8Gd0.2Zn

4.01

17.1

5.4193

Ag/Ce0.7Gd0.3Zn

4.07

17.4

5.4257

Ag/Ce0.5Gd0.5Zn

3.93

14.3

5.4362

[a] From AAS measurements, [b] from N2-adsorption data, and [c] from XRD data after Rietveld analysis.

510 511 512 513 514 515 516 517 518 519


Page 25 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


520

Table 2.

521 522

Performance of the rare earth metal-doped Ag/RE-ZnO catalysts at 350°C and after ca. 6 h on stream. Selectivity (%) Methanol conversion (%)

Hydrogen

CO

CO2

AgZn

6.4

87.8

6.8

5.4

AgLa20Zn

32.5

97.8

0.1

2.1

AgDy20Zn

33.2

88.1

3.3

8.6

AgCe20Zn

95.3

90.9

3.1

5.0

AgGd20Zn

31.9

85.2

5.3

8.5

Ag/Ce0.9Gd0.1Zn

32.9

96.1

0.2

3.7

Ag/Ce0.8Gd0.2Zn

62.2

90.0

1.2

8.9

Ag/ Ce0.7Gd0.3Zn

32.1

82.6

3.2

14.4

Ag/ Ce0.5Gd0.5Zn

41.2

92.7

4.3

3.2

0

-

-

-

Ce20Zn

4.1

85.9

2.8

11.3

Ce0.5Gd0.5Zn

2.7

87.5

2.7

9.8

Catalyst

Supports†

523

†

Supports = ZnO, La20Zn, Dy20Zn, Gd20Zn

524

525

526

527

528 529 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 43

530 Table 3 531 Chemisorption data and turn over frequency calculated for catalysts under study.

Catalyst

Dispersion (%)

Particle size (nm)

Metal surface area/gmetal

TOF(sec-1) x 102

(m2/g) AgZn

37.7

3.12

183.2

8.4

AgLa20Zn

36.9

3.19

179.3

54.5

AgDy20Zn

36.1

3.26

175.61

45.9

AgCe20Zn

42.2

2.79

204.9

139.3

AgGd20Zn

50.1

2.36

243.39

31.0

Ag/Ce0.9Gd0.1Zn

88.5

1.33

429.57

21.7

Ag/Ce0.8Gd0.2Zn

91.5

1.29

444.4

38.1

Ag/ Ce0.7Gd0.3Zn

84.4

1.40

409.66

19.3

Ag/ Ce0.5Gd0.5Zn

76.1

1.55

369.35

31.9

532

533

534

535

536

537

538

539


Page 27 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


540 541 542

Figure 1. Powder X-ray diffraction patterns of ZnO and of La, Ce, Dy, and Gd-doped ZnO supports. Peaks marked by the symbols "o", "*", "□" and "+" indicate the peaks assigned to ZnO, CeO2, Dy2O3 and Gd2O3, respectively.

543 544

Figure 2. Powder X-ray diffraction patterns of AgZn and of La, Ce, Dy, Gd-doped ZnO supported silver catalysts.

545 546 547

Figure 3. Powder X-ray diffraction patterns of Ag/Ce1-xGdxZn catalysts. Peaks marked by the symbols "o", "*", "■" and "+" indicate the peaks assigned to ZnO, CeO2, Gd2O3 and Ag2O, respectively.

548 549

Figure 4. Temperature-programmed reduction profiles of AgZn and of La, Ce, Dy, Gd-doped ZnO-supported silver oxide catalysts.

550

Figure 5. Temperature-programmed reduction profiles of the Ag/Ce1-xGdxZn catalysts.

551

Figure 6. TEM micrographs of the catalysts: (a) AgZn, (b) AgCe20Zn and (d) AgCeGdZn.

552

Figure 7. Methanol conversion over an undoped AgZn catalyst and a doped Ag/(RE-ZnO)

553 554 555

catalyst as function of the temperature. Figure 8. Hydrogen selectivity over an undoped AgZn catalyst and a doped Ag/(RE/ZnO) catalyst as function of the temperature.

556 557

Figure 9. Time course of the methanol conversion over an undoped AgZn catalyst and a doped Ag/(REZnO) at 400°C.

558 559

Figure 10. Time course of the hydrogen yield over an undoped AgZn catalyst and a doped Ag/(RE-ZnO) catalyst at 400°C.

560

Figure 11. Methanol conversion over Gd-doped AgCeZn catalysts as function of the temperature.

561

Figure 12. Hydrogen selectivity over Gd-doped AgCeZn catalysts as a function of the temperature.

562

Figure 13. Time course of the hydrogen yield over Gd-doped AgCeZn catalysts at 400°C.

563

Figure 14. TOF vs. Ag particle size for catalysts under study.

564 565 566 567 27 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

568 569

Figure 1

570 571 572 573 574 575 576 577 578 579 580 581 28 ACS Paragon Plus Environment

Page 28 of 43

Page 29 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


582 583

Figure 2

584

585 586 587 588 589 590 591 592 593 594 595



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

596 597

Figure 3

598


Page 30 of 43

Page 31 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


611 612

Figure 4

613

614 615 616 617 618 619 620 621 622 623 624 625 626 31 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

627 628

Figure 5

629 630


Page 32 of 43

Page 33 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


643 644

Figure 6

645

646

647 648



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

649

Figure 7

650 651

652 653 654 655 656 657 658 659 660 661 662 663 664 34 ACS Paragon Plus Environment

Page 34 of 43

Page 35 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

665


Figure 8

666

667 668 669 670 671 672 673 674 675 676 677 678 679 680



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

681

Figure 9

682 683

684 685 686 687 688 689 690 691 692 693 694 695


Page 36 of 43

Page 37 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


696 697

Figure 10

698

699 700 701 702 703 704 705 706 707 708 709 37 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

710 711

Figure 11

712

713 714 715 716 717 718 719 720 721 722 723 38 ACS Paragon Plus Environment

Page 38 of 43

Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


724 725

Figure 12

726

727 728 729 730 731 732 733 734 735 736 737 738



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

739 740

Figure 13

741

742

743 744 745 746 747 748 749 750 751 752


Page 40 of 43

Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


753 754

Figure 14

755 756

757 758 759 760 761 762 763 764 765



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

766


Page 42 of 43

Page 43 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract Graphic

AgCe0.1Gd0.9Zn

0.2%

AgCeZn

3.1%

AgZn

6.8%

H2 selectivities of both Ag/ZnO and Ag/CeO2-ZnO catalysts deactivate with increased time on stream at 350°C. The addition of Gd to AgCeZn improves the H2 selectivity and the catalyst stability under severe reaction conditions. An AgCeGdZnO catalyst also exhibits low CO selectivity.

ACS Paragon Plus Environment

ZnO Catalysts

Recommend Documents