The article information
- Sha-sha Wang, Min-zhen Jian, Hai-yan Su, Wei-xue Li
- 王莎莎, 简敏珍, 苏海燕, 李微雪
- First-Principles Microkinetic Study of Methanol Synthesis on Cu(221) and ZnCu(221) Surfaces
- Cu(221)和CuZn(221)表面甲醇合成的基于第一性原理微观动力学的理论研究
- Chinese Journal of Chemical Physics, 2018, 31(3): 284-290
- 化学物理学报, 2018, 31(3): 284-290
- http://dx.doi.org/10.1063/1674-0068/31/cjcp1803038
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Article history
- Received on: March 16, 2018
- Accepted on: April 1, 2018
b. School of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at Microscales, University of Science and Technology of China, Hefei 230026, China;
c. University of Chinese Academy of Sciences, Beijing 100049, China
Methanol synthesis has attracted great interest owing to its significance in the chemical industry, where methanol can be used as liquid fuel and raw material to synthesize valuable chemical feedstock [1-3]. Additionally, the CO
Because of the broad range of applications and the importance of this reaction, copper-based methanol synthesis catalysts have been widely studied, but the reaction mechanism and the interplay between the catalysts' surface properties and the feed gases is still uncertain [5-10]. Several important open questions include the nature of the preferred carbon source for methanol-CO [11] or CO
The enormous advances have been also achieved with the understanding toward active site in methanol synthesis [16-19]. Jong et al. have studied the influence of the Cu particle size smaller than 10 nm where variations in surface structures occur, under industrially relevant condition [18]. They found a dramatic decrease of specific activity when Cu particles are smaller than 8 nm, and together with DFT studies, they propose that the reaction occurs at Cu surface sites with a unique atomic structure such as step-edge sites. Additionally, it was believed that the addition of Zn can largely increase the activity of Cu catalysts. Depending on the preparation method and pretreatment conditions, different structures such as metallic CuZn alloy and Cu/ZnO interface have been detected, and controversy exists about which structure is active site and the role of Zn [17, 20, 21]. For instance, it has been implied that the turnover frequency (TOF) for methanol depends on the coverage of the coper surface with metallic Zn atoms, and the reducibility of ZnO component of the catalyst under reaction conditions prefers to decorate the low-coordinated coper sites (such as the step sites), and the terrace coordinated sites as Zn coverage increases [17, 21].
Theoretically, most mechanistic studies concentrate on the direct understanding of DFT-derived energy profiles, which gives a qualitative description of elementary pathways and relative energetics [16, 22]. However, it has been argued recently it is not sufficient to reliably assess the relative activities to methanol synthesis, and systematic kinetic study of DFT energy profile is called for [23, 24]. Herein, using DFT calculations and microkinetic simulations, we investigate CO and CO
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FIG. 1 The surface configurations of (a) Cu(221), (b) CuZn(221), and (c) the side view of CuZn(221). |
Self-consistent DFT calculations were performed via Vienna ab initio Simulation Package (VASP) [25] code. The exchange-correlation interaction were described within the generalized gradient approximation (GGA) using van der Waals interaction reversed Perdew-Burke-Ernzerhof [26] with optPBE-vdW [27]. The plane wave pseudopotential within the projected augmented wave (PAW) [28] method has a kinetic cutoff energy of 400 eV. Twelve-layer slab with (3
The adsorption energies,
$ \begin{eqnarray} \Delta {E_{{\rm{ads}}}} = {E_{{\rm{ad/sub}}}} - {E_{{\rm{ad}}}} - {E_{{\rm{sub}}}} \end{eqnarray} $ | (1) |
where E
$ \begin{eqnarray} {E_{\rm{r}}} = \sum\limits_{}^{} ( {E_{{\rm{ads}}}}{)_{\rm{p}}} - \sum\limits_{}^{} {{{({E_{{\rm{ads}}}})}_{\rm{r}}}} + \Delta {E_{{\rm{gas}}}} \end{eqnarray} $ | (2) |
where
The E
$ \begin{eqnarray} k = \frac{{{k_{\rm{B}}}T}}{{\hbar}}\frac{{{Q^{\rm{TS}}}}}{Q}{{\rm{e}}^{ - \left( {{{{E_{\rm{a}}}} / {{k_{\rm{B}}}T}}} \right)}} \end{eqnarray} $ | (3) |
where k is the reaction rate constant in s
The molecular adsorption rate constant is expressed as:
$ \begin{eqnarray} {k_{{\rm{ads}}}} = - \frac{{PA'}}{{\sqrt {2πm{k_{\rm{B}}}T} }}S \end{eqnarray} $ | (4) |
where P, S refer to the partial pressure and the sticking coefficient (S=1 in this work).
The rate constant for desorption is calculated by:
$ \begin{eqnarray} {k_{{\rm{des}}}} = \frac{{{k_{\rm{B}}}{T^3}}}{{{{\rm{\hbar}}^3}}}\frac{{A'(2π{k_{\rm{B}}})}}{{\sigma {\theta _{\rm{rot}}}}}{{\rm{e}}^{ - \left( {{{{E_{{\rm{des}}}}} / {{k_{\rm{B}}}T}}} \right)}} \end{eqnarray} $ | (5) |
where
$ \begin{eqnarray} {r_i} = \sum\nolimits_{j = 1}^N {\left( {{k_j}v_i^j\prod\nolimits_{k = 1}^M {c_k^{v_k^j}} } \right)} \end{eqnarray} $ | (6) |
in which, k
The reaction rate is calculated by MKMCXX program [34, 35]. The rates of the individual elementary reactions were calculated based on the steady-state coverages. The limited rate step can be analyzed by the degree of rate control (DRC) [36-38]. For elementary step i, the degree of rate control X
$ \begin{eqnarray} {X_{{\rm{RC}}, i}} &=& \frac{{{k_i}}}{r}{\left( {\frac{{\partial r}}{{\partial {k_i}}}} \right)_{{k_j} \ne i, {K_i}}} \nonumber\\ &=& {\left( {\frac{{\partial \ln r}}{{\partial \ln {k_i}}}} \right)_{{k_j} \ne i, {K_i}}} \end{eqnarray} $ | (7) |
where k
$ \begin{eqnarray} \sum\nolimits_i {{X_{{\rm{RC}}, i}}} = 1 \end{eqnarray} $ | (8) |
A positive DRC for reaction step i indicates that corresponding step limits the rate of reaction, whereas negative values point to rate-inhibiting reaction steps.
Ⅲ. RESULTS AND DISCUSSION A. DFT calculationsWe first perform DFT calculations for CO and CO
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FIG. 2 Optimized configurations of intermediates on Cu(221) (Ⅰ) and CuZn(221) (Ⅱ). (a) H, (b) CO, (c) HCO, (d) CH |
The structure insensitive intermediates adsorption leads to slight variation in reaction heat (E
For CO
The addition of Zn mildly lowers the E
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FIG. 3 Optimized configurations of transition states of elementary reactions involved in methanol synthesis on Cu(221) (Ⅰ) and CuZn(221) (Ⅱ):
(a) CO (b) HCO (c) CH (d) CH (e) CO (f) HCOO (g) HCOOH (h) HCOO (i) H (j) H (k) OH |
Having obtained the energetic for CH
Microkinetic simulations predict the formation rate of methanol as a function of the reaction temperature. As shown in FIG. 4(a), the composition of feed gas has a dramatic influence on the methanol formation rate, which follows the order of CO hydrogenation>CO/CO
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FIG. 4 Activity of methanol synthesis on (a) Cu(221) and (b) CuZn(221). (c) The carbon source in the feed gas of CO/CO |
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FIG. 5 Coverage of main surface species for methanol synthesis as a function of temperatures on (a) Cu(221) and (b) CuZn(221) in CO/CO |
To provide insight into the carbon source in methanol synthesis, we separate the total conversion rate of CO/CO
The reaction steps controlling carbon consumption can be decided by DRC for each elementary step considered (see Method Section for a more detailed description). As shown in FIG. 6(a), on Cu(221) formate hydrogenation (HCOO
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FIG. 6 Degree of rate control (DRC) of methanol synthesis as a function of temperatures on (a) Cu(221) and (b) CuZn(221) in CO/CO |
According to the DFT and microkinetic simulation results, the rate of CO hydrogenation is higher than that of CO
The effect of alloying and feed gas composition on methanol synthesis is investigated by optPBE-vdW DFT and microkinetic simulation. The results show that both Cu(221) and CuZn(221) have higher carbon consumption rate for CO hydrogenation, followed by CO/CO
This work was supported by the National Key R & D Program of China (No.2017YFB0602205, No.2017YFA0204800), the National Natural Science Foundation of China (No.91645202, No.91421315), the Chinese Academy of Sciences (No.QYZDJ-SSW-SLH054, No.XDA09030101).
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b. 中国科学技术大学化学与材料科学学院, 合肥微尺度物质科学国家研究中心, 合肥 230026;
c. 中国科学院大学, 北京 100049