Computational Insights into Morphology and Interface of Zeolite

Oct 9, 2018 - We have developed a computational framework to simulate the external surface structure of zeolites and used it to examine the surface ...
0 downloads 0 Views 2MB Size
Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Computation Insights into Morphology and Interface of Zeolite Catalysts: A Case Study of K-LTL Zeolite with Different Si/Al Ratios Yunlei Chen, Xiangyu Zhang, Chunli Zhao, Yifeng Yun, Pengju Ren, Wenping Guo, James P. Lewis, Yong Yang, Yong-Wang Li, and Xiao-Dong Wen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08556 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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 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 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.

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

Page 1 of 24 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

The Journal of Physical Chemistry

1

Computation Insights into Morphology and Interface of

2

Zeolite Catalysts: a Case Study of K-LTL Zeolite with

3

Different Si/Al Ratios

4

Yunlei Chen1,2,3, Xiangyu Zhang1,2,3, Chunli Zhao1,2,3, Yifeng Yun2, Pengju Ren*1,2,

5

Wenping Guo*2, James P. Lewis1,4, Yong Yang1,2,Yongwang Li1,2, Xiao-Dong Wen*1,2

6

1State

key laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, China.

7

8

2National

Energy Center for Coal to Liquids, Synfuels China Co., Ltd, Huairou District, Beijing, 101400, China.

9

10

3University

of Chinese Academy of Sciences, No.19A Yuquan Road, Beijing, 100049, China.

11

4Department

of Physics, West Virginia University, Morgantown, WV 26506-6315, USA.

14

Abstract:

We have developed a computational framework to simulate the external

15

surface structure of zeolites and used it to examine the surface structures and morphology

16

of the K-LTL zeolite system with varying Si/Al ratio. Our calculated result shows that the

17

{100} and {110} surfaces exhibit cancrinite cages and the {001} surface terminates with

18

double six ring cages for each Si/Al ratio of the K-LTL zeolite. HRTEM images verified

19

the calculated result. The Wulff construction presents a hexagonal prism and the

12

13

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

20

length/dimeter ratio of the shape becomes smaller with gradually decreasing Si/Al ratio.

21

Furthermore, we found that Pt metals present hugely different stabilities and electronic

22

properties on the external and inner surfaces. Our computational protocol is easily

23

extended to all other zeolite systems, thereby providing a deeper understanding of

24

morphology modifications and the interfacial interactions between metals and zeolites.

25 26

1. Introduction

27 28

The control of zeolite morphology is highly desirable because the morphology and

29

corresponding external surface of zeolite play a critical role on catalytic performance of

30

zeolite catalysts1-4. Many experimentalists have studied the control of zeolite morphology

31

by designing particular organic templates5, synthesis conditions6, changing Si/Al ratio7-10

32

etc. The morphology control is difficult to achieve, although many have made a certain

33

progress in understanding the numerous factors controlling the zeolite morphology has

34

been made9,

35

affect the dispersion and location of metal particles, which further modifies the catalytic

36

performance of zeolite.1-2, 17-20 Fuentes-Ordóñez et al.20 found that Pt metals supported on

37

mesoporous zeolite catalysts present higher activity than the traditional microporous

38

catalysts, which is attributed to better dispersion of Pt on the external surface. The

39

interfacial interaction between the metal clusters and inner surfaces or external surfaces

40

may modify the electronic property of the metal cluster. However, the nature of

11-16.

For metal/zeolite catalysts, morphology and particle size of zeolites

2

ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24 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

The Journal of Physical Chemistry

41

interfacial interaction between metal clusters and zeolite surfaces is still a challenge for

42

experimental observation because only a few atoms of metal particles attach at the

43

interfacial area.

44

Theoretical studies on zeolite external surface and morphology promote the

45

understanding for morphology change of zeolites and guide the modulation of zeolite

46

morphology. Furthermore, studies on zeolite external surface help to reveal the interfacial

47

interaction between noble metals and zeolite external surfaces. The morphology of

48

materials can be determined from the Wulff-construction theory by calculating the

49

surface free energy. However, due to a large number of atoms in unit cell for zeolites, the

50

computational cost of simulating zeolite surfaces at the DFT level is expensive. In the

51

past decades, force field is one of the effective methods for simulating zeolite external

52

surface.21-24 For example, Slater et al24 used that relatively simple interatomic potential

53

calculations to get three possible terminations of {100} surfaces for BETA zeolite and

54

two of those three successfully match HRTEM imaging.24 Other terminations were

55

proposed to be the short-lived intermediates. However, the precision of force field

56

methods may not be sufficient to explain this apparent discrepancy between computation

57

and experiments. The introducing of Al and counterion to zeolite systems further increase

58

the difficulty of surface simulation. Accurate and feasible methods for simulating surface

59

of zeolite are required for a deeper understanding for zeolite systems. Nowadays, it is

60

possible to accomplish sufficiently accurate results without significant computational cost

61

using DFT packages like SIESTA25 and FIREBALL26 etc. 3

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

62

In this paper we have developed a general framework to calculate external surface

63

structures of zeolites. Zeolite-Linde-L (LTL) is a good model system because the

64

inherent morphology control and corresponding morphology effect on catalytic

65

performance as extensively explored by experiments.9-10, 27-29 The synthesis procedure of

66

LTL is relatively simple and free of structure-directing agents (SDAs)9. Herein, the work

67

firstly described the simulation framework and computational methods. Through this

68

simulation framework, the structure of K-LTL zeolite with Al and K+ was deduced for

69

various Si/Al ratios. Secondly, the surface structures and the morphology for different

70

Si/Al ratios were discussed. Finally, we compared the stability and electronic properties

71

of Ptn(n=1-4) clusters on the external surface and inner surface.

72 73

2. Models and methods

74 75

The general formula of zeolite K-LTL is K+x[(AlO2)-x(SiO2)y]·wH2O. The content of

76

K+ is in accordance with the content of Al as to compensate the negative charge of

77

framework resulting from Al doping. Given that the Si/Al ratio of K-LTL samples ranges

78

from 2.3 to 3.5 in experiments9, 30-33, the simulation involves four Si/Al ratios: 2.3, 2.6,

79

3.0 and 3.5. We depicted how to calculate the distribution of Al and K+ in Part 3.1. The

80

work takes into account the {100}, {110} and {001} surfaces of K-LTL zeolite according

81

to its crystalline symmetry and previous studies34. Based on the calculated distribution of

82

Al and K+ in unit cell of K-LTL in Part 3.1, we constructed all possible surface 4

ACS Paragon Plus Environment

Page 4 of 24

Page 5 of 24 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

The Journal of Physical Chemistry

83

terminations (Figure S1) to evaluate which yields the stable surface configuration for

84

each surface. Hydroxyl groups saturated dangling bonds on the surfaces. We ignored the

85

reconstruction surface after dehydration, which only occurred at about 800 K based on

86

calculated phase diagram as shown in Figure S2. The introducing of a 20Å vacuum layer

87

in the z direction aims to eliminate the interaction between periodic images. The surface

88

models consist of three regions. Two surface layers relaxed and the interior layer fixed

89

represent zeolite surface, bulk structure, respectively. The three regions were thick

90

enough to ensure convergence of surface energy.

91

To ensure efficiency and accuracy, we developed a multi-level computational protocol

92

as illustrating in Scheme 1. Firstly, by using GULP package35-37, the filtering among

93

many Al random distributions in unit cell will determine the stable Al distribution for

94

each Si/Al ratio. The potential model used here follows CATLOW library. We next

95

relaxed the unit cell structure with the stable aluminum distribution through SIESTA

96

package25. Herein, we described the exchange and correlation energies for all systems by

97

using the generalized gradient approximation and the Perdew-Burke-Ernzerhof functional

98

(GGA-PBE)38-39. The basis sets of all atoms are double-ζ basis sets with polarization

99

function except silicon atoms with single-ζ basis sets with polarization function. The

100

Brillouin-zone sampling was restricted to the Г-point with a mesh cutoff 300 Ry.

101

Depending on the bulk structure relaxed by SIESTA, we constructed the surface

102

structures, following the relaxation by SIESTA. The geometries of bulk and surface were

103

optimized by the conjugate gradient algorithm until the maximum force of atoms was less 5

ACS Paragon Plus Environment

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

Page 6 of 24

104

than 0.03 eV/Å. In order to ensure the accuracy of surface free energy, we carried out

105

single-point energy calculation for relaxed bulk and surface structure through VASP

106

package40-41, where projector augmented wave (PAW)42-43 potentials and GGA-PBE

107

functional will mimic the effective cores and the exchange and correlation energies,

108

respectively. In the electronic structure calculations, we adopted a cutoff energy of 500

109

eV and Г-point. The convergence criterion for electronic self-consistent field calculation

110

is 10-5. We adopted Grimme's DFT-D2 approach44 in single-point energy calculations to

111

take the dispersion interaction into account.

112 113 114

Scheme 1. The computational strategy for obtaining surface structures and energies of surface structures of zeolite

115

116 117

The surface free energy of zeolite surface with hydroxyl groups is: 45  hkl 

G(surf hkl )  G(bulk)  n H 2O A hkl

(1)

118

G(surfhkl) is the free energy of surface {hkl} with surface area of Ahkl, G(bulk) is the

119

free energy of zeolite bulk, and μH2O stands for the chemical potential of water.

120

Neglecting the variation of entropy and pV term of the condensed phase, as well as 6

ACS Paragon Plus Environment

Page 7 of 24 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

121 122

123 124 125

The Journal of Physical Chemistry

thermal variations of internal energies, the surface energy of the {hkl} facet is:  hkl 

E(surf hkl )  E(bulk)  n H 2O A hkl

(2)

E(surfhkl) and E(bulk) is the energy of surface and bulk separately.  H 2O,g  E H 2O,g  nRT  TSH 2O,g

(3)

The chemical potential of water at 373.15K, 0.1Mpa under gas-liquid balance is used

126

here, and the entropy of water refers to the NIST-JANAF Thermochemical Tables46.

127

3. Results and Discussion

128

3.1 Al and K+ distributions

129

Zeolite LTL has a hexagonal crystal structure with space group P6/mmm (a = b= 1.84

130

and c = 0.75nm). As shown in Figure 1a, this structure consists of two secondary building

131

units of cancrinite cage (CAN) and double six ring (D6R). The LTL zeolite framework

132

contains two crystallographically distinct tetrahedral T sites (T4 and T6). Al atoms will

133

locate either at T4 or T6 site as shown in Figure 1a. Figure 1b shows the six possible

134

positions of K+ reported47-48. It is required to choose a rational distribution of Al and K+

135

because the model building of surface structure depends on the bulk structure.

136

7

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

137 138 139

Figure 1. (a) Crystal hexagonal structure of zeolite LTL. Dashed lines represent a single

140

unit cell. (b) Two T sites (T4 and T6) of Al distribution and six K+ possible location: T4

141

is at the 12-ring and T6 sits on the D6R; A is inside double 6-MR, B is inside the

142

cancrinite cage, C is at the center of nonplanar 8-MR, D is at the connecting area between

143

nonplanar 8-MR and 12-MR windows, E is midway between two adjacent nonplanar

144

8-MRs, and F is at the center of 12-MR windows.

145 146

We determined the stability of each K+ site when single Al atom locates at T4 or T6 8

ACS Paragon Plus Environment

Page 8 of 24

Page 9 of 24 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

The Journal of Physical Chemistry

147

site as depicted in supporting information Table S1. We find that the stability order of

148

these K+ sites is: B > C ≈ D > F > E > A. The total number of the most three sorts of

149

stable sites reaches 11 in the unit cell of LTL zeolite (including two B sites, three C site

150

and six D site), which is enough to accommodate potassium cations for zeolite K-LTL.

151

The distribution of K+ calculated is consistent with experimental reports30-31, 49. In our

152

computations, K+ will fully occupy all the B, C sites, but partially the D sites in unit cell

153

of K-LTL zeolite with the Si/Al ratio from 2.3 to 3.5.

154

As mentioned above, Al will locate at the T4 of 12-rings or the T6 of 6-rings in the

155

LTL zeolite framework. In order to get the most stable distribution of Al at both T sites,

156

we define the number of Al atoms at T4 and T6 site as N(T4) and N(T6), respectively.

157

The amount of Al atoms in unit cell will vary from eight to eleven for the Si/Al ratio from

158

2.3 to 3.5. Limited by the Lowenstein rule50, the N(T4) at the 12 rings of zeolite LTL will

159

range from 0 to 6 theoretically. Next, we compared the relative stability of unit cell when

160

the number of N(T4) varies from 1 to 6 and the other Al locates at T6 sites (Totally six

161

N(T4)/N(T6) ratios).

162

For certain Si/Al ratios, The most stable Al distribution in unit cell is determined by

163

filtering 200 random Al distributions at each N(T4)/N(T6) ratio. Figure 2 shows the

164

relative energies of the most stable Al distribution at each N(T4)/N(T6) ratio. We find

165

that the relative energy of unit cell initially decreases and then increases when more T4

166

were occupied for each Si/Al ratio. The optimum point is at N(T4) = 5. This result

167

indicates that the aluminum atoms locate preferentially at the T4 sites rather than T6 sites. 9

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

168

In experiments, the N(T4)/N(T6) ratio of K-LTL with Si/Al ≈ 3.0 is 1.4 by detailed

169

analysis of neutron diffraction data,51 which is in good agreements with our calculations.

170

The agreement between experiments and computations suggests that the distribution of

171

aluminum atoms on the zeolite LTL may be mainly dominated by thermodynamics. In

172

the subsequent surface calculation, we use the unit cell with the most stable aluminum

173

distribution to construct the surface structures of zeolite K-LTL for each Si/Al ratio.

174

175 176

Figure 2. The relative energy Erel of the most stable unit cell at each N(T4)/N(T6) ratio

177

for Si/ Al ratio = 3.5(a), Si/Al = 3.0(b), Si/Al = 2.6(c) and Si/Al = 2.3(d). N(T4) and 10

ACS Paragon Plus Environment

Page 10 of 24

Page 11 of 24 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

The Journal of Physical Chemistry

178

N(T6) are the number of aluminum atoms at T4 and T6 sites, respectively. The relative

179

energy Erel was defined as Erel = Emin - Estable, where Emin is the minimum energy of unit

180

cells for each N(T4)/N(T6), and Estable is the unit cell energy of the most stable

181

distribution in all the Al distribution.

182 183

3.2 Surface structures and morphology for purely siliceous LTL

184 185

It is necessary and easy to investigate the surface structures and morphology for purely

186

siliceous LTL. As described in Part 2 of this work, there are four terminations for the

187

{100} and {110} surfaces, and three terminations for the {001} surface. Table S2 lists the

188

surface free energies of all these surface terminations. Figure 3a, 3b, and 3c show the

189

most stable terminations for the three surfaces {100}, {110} and {001}, respectively.

190 191

Figure 3. (a), (b) and (c) are the most stable terminations of {100}, {110}, and {001}

192

surfaces for purely siliceous zeolite LTL, respectively, where red, yellow and white sticks

193

represent silicon, oxygen and hydrogen atoms, respectively. (d) presents the equilibrium

194

shapes of purely siliceous LTL. 11

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

195 196

Cutting the nonplanar eight-rings (Figure 1a) along the {100} and {110} will generate

197

the stable terminations, which consist of CAN cages, we denote them the CAN

198

termination (Figure 3a and 3b). The stable termination of the {001} surface consists of

199

D6R rings, thereby we note it as the D6R termination. We find that the CAN termination

200

contains the lowest density of surface hydroxyls among all the surface terminations along

201

{100} and {110} directions (Figure S3(a)). Meanwhile, the surface free energies arise

202

with the increasing density of surface hydroxyls as shown in Figure S3(b).

203

The surface free energies of those stable terminations for {100}, {110} and {001} are

204

115, 127, 177mJ/m2, respectively. The morphology of zeolite K-LTL is deduced by

205

Wulff’s theorem, hi = λki, where h is the surface free energy and k is the surface normal

206

vector from the face to a point within the crystal. Figure 3d shows the equilibrium shape

207

of purely siliceous LTL zeolite, which presents a hexagonal prism shape. The percentage

208

of {100}, {110} and {001} surface on the shape is 65%, 10% and 25%, respectively.

209

Although we can’t compare pure siliceous zeolite with the aluminosilicate synthesized in

210

experiments, their morphologies are similar in general to the hexagonal prism shape. This

211

may imply that the doping of aluminum has a slight influence on morphology.

212 213

3.3 Surface structures and morphology for K-LTL

214 215

We extended the simulation strategy to zeolite K-LTL. The calculated free energies of 12

ACS Paragon Plus Environment

Page 12 of 24

Page 13 of 24 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

216

The Journal of Physical Chemistry

zeolite with four Si/Al ratios are listed in Table S2.

217 218

Figure 4. The stable termination structures of zeolite K-LTL with Si/Al = 3.0 for

219

{100}(left), {110}(middle) and {001}(right) surfaces. Blue spheres are extra-framework

220

potassium atoms. Red, yellow, and white sticks represent silicon, oxygen and hydrogen

221

atoms, respectively.

222 223

Figure 4 shows the most stable terminations of {100}, {110} and {001} surfaces,

224

which of the surface framework structure is same with siliceous zeolite LTL. We find the

225

most stable terminations of {100} and {110} surfaces contain the lowest density of

226

surface hydroxyls in the all terminations of each direction (Figure S3). Our findings

227

indicate that the density of surface hydroxyl groups dominates mainly the stability of

228

surface terminations. The CAN termination of {100} surface predicted has been proved

229

by our experimental measures. HRTEM images (Figure 5) shows CAN cages expose on

230

the {100} surface. The good agreement between calculations and experiments indicates

231

that the surface structure of zeolite may be dominated by thermodynamics. 13

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

232 233

234 235

Figure 5. HTEM images of zeolite K-LTL with the incident beam parallel to the [001]

236

directions at 300 kV. Corresponding schematic diagrams of the framework structure are

237

inserted along [100] direction.

238 239

As shown in Figure 4, the coordination environment for the extra-framework K+

240

exposed on the surface is quite different compared with that in bulk phase or inner

241

surface. For instance, K+ at D sites on surface coordinates with six framework oxygen,

242

which is half of the coordination numbers (CNs) of K+ in inner surface. The reduction of

243

CN of K+ on the surface will make the surface more unstable. As justified in Figure 6a.

244

all the surface free energies (γ) of zeolite K-LTL is higher than that of purely siliceous

245

LTL.

246

From Figure 6a, we find that the surface free energies of the CAN termination increase

247

more rapidly than that of the D6R termination with decreasing Si/Al ratio. The result 14

ACS Paragon Plus Environment

Page 14 of 24

Page 15 of 24 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

The Journal of Physical Chemistry

248

indicates the stability of {100} and {110} surfaces are much more sensitive to the

249

changing of Al content than that of {001}. The anisotropy phenomenon of the surface

250

free energies is related to the sorts of K+ site exposed on different surfaces. As shown in

251

Figure 4, the {100} and {110} surfaces expose K+ at D sites while there are only B sites

252

exposed on the {001} surfaces. As discussed above in Part 3.1, only D sites among the

253

most three stable sites (B, C, and D sites) were not completely occupied by K+. The

254

increasing Al content or decreasing Si/Al ratio will bring more K+ on the {100} and {110}

255

surfaces, thereby improving the surface free energies.

256

257

Figure

258

6 (a) the change trend of surface free energies for the stable termination of each surface.

259

(b) the change trend of morphology for each Si/Al ratio of zeolite K-LTL.

260 261

Figure 6b shows the equilibrium morphology of zeolite K-LTL with different Si/Al

262

ratios. The calculated shape presents a hexagonal prism similar to the pure siliceous case,

263

where {100} surface acts as prismatic face, {001} surface is pinacoid face and {110} 15

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

264

completely disappears. The typical morphology of zeolite K-LTL observed by SEM

265

image was reported to be hexagonal shape28, which is in a good agreement with our

266

computations. The decreasing of Si/Al ratio of the samples shortens the length/diameter

267

(L/D) ratio of the equilibrium morphology because the anisotropy character of surface

268

free energies aforementioned. Our calculations suggest that the Si/Al ratio will be an

269

effective factor to control the morphology of K-LTL.

270 271

3.4 Adsorption and electronic properties of Pt metal on the surface.

272 273

K-LTL supported platinum metal particle (Pt-KL) plays a key role in catalysis process

274

like the alkanes aromatization process for excellent catalytic performance. The dispersion

275

and location of Pt particles will significantly influence its electronic structure and its

276

catalytic performance. In this section, we investigated the structures and electronic

277

properties of Ptn (n=1-4) clusters supported on K-LTL by comparing its behavior on

278

external and inner surfaces of zeolite. We only consider {001} and {100} surfaces

279

because the calculated morphology of K-LTL zeolite only presents those two surfaces.

280

Figure 7 shows the structures, adsorption energy and charge analysis of Pt4 cluster

281

supported.

282

16

ACS Paragon Plus Environment

Page 16 of 24

Page 17 of 24 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

The Journal of Physical Chemistry

283 284

Figure 7. The structure of Pt4 cluster adsorbed on (a) the inner surface, (b) the {100}

285

surface, (c) the {001} surface. The trend of average adsorption energies (d) and charge (e)

286

of adsorbed Pt clusters on various surfaces with increasing Pt atoms numbers. (Blue, red,

287

yellow, and white spheres represent platinum, silicon, oxygen and hydrogen atoms,

288

respectively. For clear, extra-framework cations in the figure are ignored.)

289 290

We define the average adsorption energy of Pt as E(Ptads/ave) = [ (E(Ptn/surface) –

291

n×E(Pt atom) – E(surface) ]/n, where E(Ptn/surface) is the total energy of the Ptn adsorbed

292

on various surfaces, E(surface) is the energy of the surface, E(Pt atom) is the energy of

293

single Pt atom and n is the number of the Pt atoms in cluster. Figure 7d shows the

294

average adsorption energies of Ptn (n = 1-4) cluster on {001}, {100} and inner surfaces.

295

we find that the stability of Ptn clusters on those surfaces is: {001} > {100} > inner

296

surface, which means Pt on the external surface is more stable for K-LTL zeolite. 17

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

297

Furthermore, the average adsorption energies for {001} surface increase for larger Pt

298

clusters, whereas they decrease for {100} and inner surface. This result indicates that Pt

299

atoms on the {001} surface tend to single atom dispersion at the channel mouths and

300

prefer to aggregation on {100} and inner surface. Our calculation of the migration

301

barriers of Pt atom from inner surface to pore-mouth face was 1.32 eV indicating that the

302

migration may not occur at low temperatures but could happen at high temperature, such

303

as 450~500 °C for aromatic reaction (Figure S5, and S6).

304

Bader charge analysis of Figure 7e shows that the adsorbed Pt atoms at different sites

305

cause entirely different electronic properties. Pt atoms carry positive charge on the

306

external surface while they have a negative charge on inner surface. On the external

307

surface, the unsaturated oxygen at the interface directly coordinates with Pt atoms

308

because the reaction between surface hydroxyl groups and Pt metal. The significant

309

charge transfer takes place from the metallic cluster to the external surface. For the inner

310

surface, there is no direct Pt-O bonding because the framework oxygen is saturated by

311

framework Al or Si. The negative charge of Pt cluster origins from the K atoms that

312

present positive charge.

313

Figure 7c shows that Pt atoms at the interface on the {001} surface coordinates with

314

two unsaturated oxygen atoms because the close distance between vicinal hydroxyl

315

groups. But for {100} surface, the distance between vicinal surface hydroxyl groups is

316

7.5 Å so that the Pt atom at the interface only coordinates with one surface oxygen. More

317

unsaturated oxygen atoms participate in the interface interaction on the {001} surface 18

ACS Paragon Plus Environment

Page 18 of 24

Page 19 of 24 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

The Journal of Physical Chemistry

318

than the {100} surface, which lead to more significant charge transfer from Pt metals to

319

the {001} surface. It has been reported that relative negative charge on Pt cluster may be

320

the reason for the distinguishing selectivity for alkane aromatization process52.

321 322

4. Conclusion

323 324

We have developed a computational strategy to efficiently and accurately model the

325

surface structure and the morphology for K-LTL zeolite. Through our computations, the

326

{100} and {110} surfaces expose CAN cages and the {001} surface terminates with D6R

327

cages for each Si/Al ratio of K-LTL zeolite. The simulated morphology of zeolite K-LTL

328

presents a hexagonal prism similar to the purely siliceous LTL. The stability of CAN

329

terminations for {100} and {110} surfaces are more sensitive to the Si/Al ratio than D6R

330

terminations for the {001} surface, which lead to the shorter L/D ratio of K-LTL

331

morphology with increased Si/Al ratio.

332

Pt metal cluster adsorbs more strongly on the {001} surface than on the {100} and

333

inner surface indicating Pt cluster tends to aggregate on the channel mouth of zeolite

334

K-LTL. In addition, the electronic structures of Pt cluster can be significantly tuned by its

335

location. Our results demonstrates that the external surface of zeolite deserves more

336

attention for the understanding the structure-performance relation for metal/zeolite

337

supported catalysts.

338

The computational strategy developed here can be applied to the surface simulation of 19

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

339

other zeolite systems, which help to predict surface structure and assist rational design of

340

zeolite catalysts. We have recently extended the strategy describing the effect of organic

341

templates on the morphology of zeolites in another work7.

342

ASSOCIATED CONTENT

343

Supporting Information

344

The Supporting Information is available free of charge on the ACS Publications website.

345

Structure schematic diagram of LTL zeolite with 11 different surface terminations; K+

346

cations relative stability on possible sites in the LTL bulk; All the surface free energies of

347

zeolite with different Si/Al ratios; Phase Diagrams of surface dehydrogenation

348

reconstruction; The diffusion barrier of Pt single atoms along 12-rings channel.

349

AUTHOR INFORMATION

350

Corresponding Author

351

*[email protected]

352

*[email protected]

353

* [email protected]

354

ACKNOWLEDGMENT

355

The authors are grateful for the financial support from the Ministry of Science and

356

Technology of the People´s Republic of China (No. 2018YFB0604901), National Natural

357

Science Foundation of China (No. 21473229, No. 91545121, No. 21603252, No.

358

21703274, No.21703272), No. 201601D021048 from the Shanxi Province Science

359

Foundation for Youth, and funding support from Synfuels China, Co. Ltd. We also 20

ACS Paragon Plus Environment

Page 20 of 24

Page 21 of 24 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

The Journal of Physical Chemistry

360

acknowledge the innovation foundation of Institute of Coal Chemistry, Chinese Academy

361

of Sciences, Hundred-Talent Program of Chinese Academy of Sciences, Shanxi

362

Hundred-Talent Program and National Thousand Young Talents Program of China.

363

References

364 365 366

(1) Treacy, M. M. J. Pt Agglomeration and Entombment in Single Channel Zeolites: Pt/LTL. Microporous and Mesoporous Materials 1999, 28, 271-292.

367

(2) Jentoft, R. E.; Tsapatsis, M.; Davis, M. E.; Gates, B. C. Platinum Clusters Supported in Zeolite LTL:

368

Influence of Catalyst Morphology on Performance Inn-Hexane Reforming. Journal of Catalysis 1998,

369

179, 565-580.

370

(3) Kim, W.-G.; So, J.; Choi, S.-W.; Liu, Y.; Dixit, R. S.; Sievers, C.; Sholl, D. S.; Nair, S.; Jones, C. W.

371

Hierarchical Ga-MFI Catalysts for Propane Dehydrogenation. Chemistry of Materials 2017, 29,

372

7213-7222.

373

(4) Wei, F.-F.; Cui, Z.-M.; Meng, X.-J.; Cao, C.-Y.; Xiao, F.-S.; Song, W.-G. Origin of the Low Olefin

374

Production over HZSM-22 and HZSM-23 Zeolites: External Acid Sites and Pore Mouth Catalysis. ACS

375

Catalysis 2014, 4, 529-534.

376 377 378 379

(5) Lee, S.; Shantz, D. F. Zeolite Growth in Nonionic Microemulsions:  Synthesis of Hierarchically Structured Zeolite Particles. Chemistry of Materials 2005, 17, 409-417. (6) Rimer, J. D.; Kumar, M.; Li, R.; Lupulescu, A. I.; Oleksiak, M. D. Tailoring the Physicochemical Properties of Zeolite Catalysts. Catalysis Science & Technology 2014, 4, 3762-3771.

380

(7) Zhang, L.; Chen, Y.; Jiang, J.-G.; Xu, L.; Guo, W.; Xu, H.; Wen, X.-D.; Wu, P. Facile Synthesis of Ecnu-20

381

(IWR) Hollow Sphere Zeolite Composed of Aggregated Nanosheets. Dalton Transactions 2017, 46,

382

15641-15645.

383 384 385 386

(8) Shirazi, L.; Jamshidi, E.; Ghasemi, M. R. The Effect of Si/Al Ratio of ZSM-5 Zeolite on Its Morphology, Acidity and Crystal Size. Crystal Research and Technology 2008, 43, 1300-1306. (9) Larlus, O.; Valtchev, V. P. Crystal Morphology Control of LTL-Type Zeolite Crystals. Chemistry of Materials 2004, 16, 3381-3389.

387

(10) Lupulescu, A. I.; Kumar, M.; Rimer, J. D. A Facile Strategy to Design Zeolite L Crystals with Tunable

388

Morphology and Surface Architecture. Journal of the American Chemical Society 2013, 135,

389

6608-6617.

390

(11) Kumar, M.; Luo, H.; Román-Leshkov, Y.; Rimer, J. D. SSZ-13 Crystallization by Particle Attachment and

391

Deterministic Pathways to Crystal Size Control. Journal of the American Chemical Society 2015, 137,

392

13007-13017.

393

(12) Zhao, Y.; Zhang, H.; Wang, P.; Xue, F.; Ye, Z.; Zhang, Y.; Tang, Y. Tailoring the Morphology of MTW

394

Zeolite Mesocrystals: Intertwined Classical/Nonclassical Crystallization. Chemistry of Materials 2017,

395

29, 3387-3396. 21

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

396

(13) Oleksiak, M. D.; Muraoka, K.; Hsieh, M.-F.; Conato, M. T.; Shimojima, A.; Okubo, T.; Chaikittisilp, W.;

397

Rimer, J. D. Organic-Free Synthesis of a Highly Siliceous Faujasite Zeolite with Spatially Biased Q4(Nal)

398

Si Speciation. Angewandte Chemie International Edition 2017, 56, 13366-13371.

399

(14) Zhang, Q.; Chen, G.; Wang, Y.; Chen, M.; Guo, G.; Shi, J.; Luo, J.; Yu, J. High-Quality Single-Crystalline

400

MFI-Type Nanozeolites: A Facile Synthetic Strategy and MTP Catalytic Studies. Chemistry of Materials

401

2018, 30, 2750-2758.

402 403 404 405

(15) Kecht, J.; Mintova, S.; Bein, T. Nanosized Zeolites Templated by Metal−Amine Complexes. Chemistry of Materials 2007, 19, 1203-1205. (16) Lupulescu, A. I.; Rimer, J. D. Tailoring Silicalite-1 Crystal Morphology with Molecular Modifiers. Angewandte Chemie 2012, 124, 3401-3405.

406

(17) Koekkoek, A. J. J.; Kim, W.; Degirmenci, V.; Xin, H.; Ryoo, R.; Hensen, E. J. M. Catalytic Performance of

407

Sheet-Like Fe/ZSM-5 Zeolites for the Selective Oxidation of Benzene with Nitrous Oxide. Journal of

408

Catalysis 2013, 299, 81-89.

409

(18) Chen, Z.; Li, X.; Xu, Y.; Dong, Y.; Lai, W.; Fang, W.; Yi, X. Fabrication of Nano-Sized SAPO-11 Crystals

410

with Enhanced Dehydration of Methanol to Dimethyl Ether. Catalysis Communications 2018, 103,

411

1-4.

412

(19) Trakarnroek, S.; Jongpatiwut, S.; Rirksomboon, T.; Osuwan, S.; Resasco, D. E. N-Octane Aromatization

413

over Pt/KL of Varying Morphology and Channel Lengths. Applied Catalysis A: General 2006, 313,

414

189-199.

415

(20) Fuentes-Ordóñez, E. G.; Salbidegoitia, J. A.; Ayastuy, J. L.; Gutiérrez-Ortiz, M. A.; González-Marcos, M.

416

P.; González-Velasco, J. R. High External Surface Pt/Zeolite Catalysts for Improving Polystyrene

417

Hydrocracking. Catalysis Today 2014, 227, 163-170.

418 419

(21).Loades, S. D.; Carr, S. W.; Gay, D. H.; Rohl, A. L. Calculation of the Morphology of Silica Sodalite. Journal of the Chemical Society, Chemical Communications 1994, 1369-1370.

420

(22) Ohsuna, T.; Slater, B.; Gao, F.; Yu, J.; Sakamoto, Y.; Zhu, G.; Terasaki, O.; Vaughan, D. E.; Qiu, S.;

421

Catlow, C. R. A. Fine Structures of Zeolite ‐ Linde ‐ L (LTL): Surface Structures, Growth Unit and

422

Defects. Chemistry-A European Journal 2004, 10, 5031-5040.

423 424 425 426

(23) Slater, B.; Titiloye, J.; Higgins, F.; Parker, S. Atomistic Simulation of Zeolite Surfaces. Current Opinion in Solid State and Materials Science 2001, 5, 417-424. (24) Slater, B.; Catlow, C. R. A.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Camblor, M. A. Surface Structure and Crystal Growth of Zeolite Beta C. Angewandte Chemie 2002, 41, 1235-1237.

427

(25) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The Siesta

428

Method for Ab Initio Order-N Materials Simulation. Journal of Physics: Condensed Matter 2002, 14,

429

2745.

430 431 432 433 434 435 436

(26) Lewis, J. P., et al. Advances and Applications in the Fireballab Initio Tight-Binding Molecular-Dynamics Formalism. Physica Status Solidi (B) 2011, 248, 1989-2007. (27) Gaona-Gómez, A.; Cheng, C.-H. Modification of Zeolite L (Ltl) Morphology Using Diols,(Oh) 2 (Ch 2) 2 N+ 2 on (N= 0, 1, and 2). Microporous and Mesoporous Materials 2012, 153, 227-235. (28) Gomez, A. G.; de Silveira, G.; Doan, H.; Cheng, C.-H. A Facile Method to Tune Zeolite L Crystals with Low Aspect Ratio. Chemical Communications 2011, 47, 5876-5878. (29) Wortel, T. M. Zeolite L with Cylindrical Morphology. Google Patents: 1985. 22

ACS Paragon Plus Environment

Page 22 of 24

Page 23 of 24 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

The Journal of Physical Chemistry

437

(30) Ohgushi, T.; Matsuo, T.; Satoh, H.; Matsumoto, T. Cation Distribution in K, Hl Zeolite Prepared

438

through Ion-Exchange with Tma Ion. Microporous and Mesoporous Materials 2009, 117, 472-477.

439

(31) Burton, A.; Lobo, R. F. The Role of Barium Cations in the Synthesis of Low-Silica Ltl Zeolites.

440

Microporous and Mesoporous Materials 1999, 33, 97-113.

441

(32) Meeprasert, J.; Kungwan, N.; Jungsuttiwong, S.; Namuangruk, S. Location and Reactivity of

442

Extra-Framework Cation in the Alkali Exchanged LTL Zeolites: A Periodic Density Functional Study.

443

Microporous and Mesoporous Materials 2014, 195, 227-239.

444

(33) Terasaki, O.; Ohsuna, T.; Watanabe, D. HREM Study of Pt-Clusters on K-LTL Crystal Surfaces. In

445

Studies in Surface Science and Catalysis, Karge, H. G.; Weitkamp, J., Eds. Elsevier: 1995; Vol. 98, pp

446

52-53.

447 448 449 450 451 452 453 454 455 456 457 458 459 460

(34) Ohsuna, T.; Horikawa, Y.; Hiraga, K.; Terasaki, O. Surface Structure of Zeolite L Studied by High-Resolution Electron Microscopy(LTL). Chemistry of Materials 1998, 10, 688-691. (35) Gale, J. D.; Rohl, A. L. The General Utility Lattice Program (GULP). Molecular Simulation 2003, 29, 291-341. (36) Gale, J. D. GULP: A Computer Program for the Symmetry-Adapted Simulation of Solids. Journal of the Chemical Society, Faraday Transactions 1997, 93, 629-637. (37) Sanders, M.; Leslie, M.; Catlow, C. Interatomic Potentials for SiO2. Journal of the Chemical Society, Chemical Communications 1984, 1271-1273. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Physical Review Letters 1996, 77, 3865-3868. (39) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Physical Review B 1992, 45, 13244-13249. (40) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Physical review B 1996, 54, 11169.

461

(41) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and

462

Semiconductors Using a Plane-Wave Basis Set. Computational Materials Science 1996, 6, 15-50.

463

(42) Blöchl, P. E. Projector Augmented-Wave Method. Physical Review B 1994, 50, 17953-17979.

464

(43) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method.

465 466 467

Physical Review B 1999, 59, 1758-1775. (44) Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. Journal of Computational Chemistry 2006, 27, 1787-1799.

468

(45) Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. Effects of Morphology on Surface

469

Hydroxyl Concentration: A DFT Comparison of Anatase–TiO2 and Γ-Alumina Catalytic Supports.

470

Journal of Catalysis 2004, 222, 152-166.

471

(46) NIST-JANAF Thermochemical Tables. http://kinetics.nist.gov/janaf/.(accessed June 8,2018).

472

(47) Barrer, R.; Villiger, H. The Crystal Structure of the Synthetic Zeolite L. Zeitschrift für

473 474 475 476 477

Kristallographie-Crystalline Materials 1969, 128, 352-370. (48) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth Scientific Limited on behalf of the Structure Commission of the International Zeolite Association, 1982. (49) Takaishi, T. Ordered Distribution of Aluminium or Gallium Atoms in Zeolite L. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1988, 84, 2967-2977. 23

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

478 479 480 481

(50) Loewenstein, W. The Distribution of Aluminum in the Tetrahedra of Silicates and Aluminates. American Mineralogist 1954, 39, 92-96. (51) Newsam, J. Aluminium Partitioning in Zeolite L. Journal of the Chemical Society, Chemical Communications 1987, 123-124.

482

(52) Xu, D.; Wu, B.; Ren P.; Wang, S.; Huo, C.; Zhang, B.; Guo, W.; Huang L.; Wen X.; Qin, Y., et al.

483

Controllable Deposition of Pt Nanoparticles into a KL Zeolite by Atomic Layer Deposition for Highly

484

Efficient Reforming of N-Heptane to Aromatics. Catalysis Science & Technology 2017, 7, 1342-1350.

485 486

TOC Graphic

487

24

ACS Paragon Plus Environment

Page 24 of 24