Computational Insights into Morphology and Interface of Zeolite

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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

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The Journal of Physical Chemistry

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Computation Insights into Morphology and Interface of

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

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Different Si/Al Ratios

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

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

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1State

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

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2National

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

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3University

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

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4Department

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

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Abstract:

We have developed a computational framework to simulate the external

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

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

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

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

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

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ACS Paragon Plus Environment

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

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

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

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

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morphology modifications and the interfacial interactions between metals and zeolites.

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1. Introduction

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

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

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

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

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

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progress in understanding the numerous factors controlling the zeolite morphology has

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been made9,

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

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

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mesoporous zeolite catalysts present higher activity than the traditional microporous

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

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

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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


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

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

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interfacial area.

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Theoretical studies on zeolite external surface and morphology promote the

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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possible to accomplish sufficiently accurate results without significant computational cost

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using DFT packages like SIESTA25 and FIREBALL26 etc. 3



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

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

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inherent morphology control and corresponding morphology effect on catalytic

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

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

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firstly described the simulation framework and computational methods. Through this

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

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

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

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

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2. Models and methods

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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enough to ensure convergence of surface energy.

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

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

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

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

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

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

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

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

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

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

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

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

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



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

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

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

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

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

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

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

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take the dispersion interaction into account.

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

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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)

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

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

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


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123 124 125


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

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

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3. Results and Discussion

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3.1 Al and K+ distributions

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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

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

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The agreement between experiments and computations suggests that the distribution of

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

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

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

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

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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


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

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

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

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distribution in all the Al distribution.

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3.2 Surface structures and morphology for purely siliceous LTL

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

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

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

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

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

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

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

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

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shapes of purely siliceous LTL. 11



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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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3.3 Surface structures and morphology for K-LTL

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


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

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

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

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

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atoms, respectively.

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

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

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

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

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

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

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

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

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



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

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

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inserted along [100] direction.

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

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

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

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

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

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

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LTL.

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

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


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

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

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

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

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

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

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

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surfaces, thereby improving the surface free energies.

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Figure

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

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

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

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

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



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

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

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

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

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

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effective factor to control the morphology of K-LTL.

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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

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

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

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

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

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

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supported.

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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)

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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



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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


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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.

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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



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

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

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

357

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

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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


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

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

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

363

References

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Computational Insights into Morphology and Interface of Zeolite

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