Density Functional Theory (DFT) and Kinetic Monte Carlo (KMC) Study

Nov 15, 2016 - Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan ...
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Density Functional Theory (DFT) and Kinetic Monte Carlo (KMC) Study of the Reaction Mechanism of Hydrogen Production from Methanol on ZnCu(111) Zhi-Jun Zuo,† Xiao-Yu Gao,† Pei-De Han,‡ Shi-Zhong Liu,*,§ and Wei Huang*,† †

Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China ‡ College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China § Department of Chemistry, Stony Brook University, Stony Brook, New York 11794, United States S Supporting Information *

ABSTRACT: Cu/ZnO-based catalysts have been widely used for methanol decomposition (MD), partial oxidation of methanol (POM), steam reforming of methanol (SRM), and oxidative steam reforming of methanol (OSRM). In this work, we systematically studied all possible reaction paths involved in MD, POM, SRM, and OSRM on ZnCu alloys (111) using density functional theory (DFT). On the basis of these results, Kinetic Monte Carlo (KMC) simulations show that the ratelimiting step of the four reactions is CH2O formation from CH3O dehydrogenation. The reaction pathway of MD occurs via the direct decomposition of CH3OH, and the main reaction pathways of POM and SRM occur via CH2OO and CH2OOH, respectively. There are two main reaction pathways of OSRM as follows: one occurs via CH2OO, whereas the other occurs via CH2OOH. Finally, according to the results of sensitivity analysis, some possible modifications to improve the CO2 selectivity and turnover frequency (TOF) of H2 for OSRM on Cu/ZnO-based catalysts are also presented. The results may be useful for designing and optimizing Cu-based catalysts for MD, POM, SRM, and OSRM.

1. INTRODUCTION Concerns regarding global warming and the inevitable exhaustion of oil reserves make the development of an alternative energy source for combustion engines highly desirable.1−4 Currently, the catalytic reforming and decomposition of oxygenates have attracted much attention for the production of high-purity hydrogen.3,5−8 Among various oxygenates, methanol is considered to be a potential oxygenate for hydrogen production because of its low conversion temperature, high H/C ratio, and absence of C−C bonds.7,9,10 Therefore, the production of hydrogen from methanol has attracted great interest. In general, there are four methods to produce hydrogen from methanol, namely, methanol decomposition (MD; CH3OH → CO + 2H2),11−13 partial oxidation of methanol (POM; CH3OH + (1/2)O2 → CO2 + 2H2),14−16 steam reforming of methanol (SRM; CH3OH + H2O → CO2 + 3H2),9,17 and oxidative steam reforming of methanol (OSRM; CH3OH + (1 − n)H2O + 0.5nO2 → (3 − n)H2 + CO2).3,18−20 The OSRM reaction has attracted tremendous attention as it possesses many advantages over the POM and SRM reactions because of its autothermal process with ideal reaction stoichiometry. Moreover, OSRM can produce hydrogen with very low CO © XXXX American Chemical Society

concentrations and a high hydrogen gas concentration compared with the MD, POM, and SRM reactions.19,20 It is generally accepted that Group VIII and Cu-based catalysts are the best catalysts for the MD, POM, SRM, and OSRM reactions. 3,7,9,12,14−21 Notably, these catalysts favor the production of syngas from methanol and the production of H2 with high selectivity toward CO23. Given the importance of H2, many theoretical investigations have focused on the production of hydrogen from methanol using Group VIII and Cu-based catalysts.22−33 Recently, we studied the main reaction paths of the MD, SRM, and POM reactions on the Cu(111) surface.4,33 The results showed that the highest activation energy occurs for CH2O formation from methoxy dehydrogenation in the following order: Ea(MD, 1.85 eV) > Ea(SRM, 1.65 eV) > Ea(POM, 1.57 eV). However, these studies focused only on the active site and did not consider the role of the carrier and coverage. Nevertheless, research on carriers has been steadily increasing as the important role of carriers in catalysts has been recognized.34−37 Received: October 11, 2016 Revised: November 14, 2016 Published: November 15, 2016 A

DOI: 10.1021/acs.jpcc.6b10261 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C To the best of our knowledge, no report has systemically examined the OSRM reaction, presumably because of its complexity. To this end, to better understand the mechanism of OSRM and provide a guide for the rational design of Cu-based catalysts for the production of hydrogen from methanol, we have investigated the reaction process of methanol on clean, O precovered, and OH precovered ZnCu(111) surfaces using DFT. Subsequently, the reaction mechanisms of the MD, POM, SRM, and OSRM reactions are compared using Kinetic Monte Carlo (KMC) simulations. Finally, the key factors of these reactions are discussed.

Figure 1. Side and top views of the ZnCu(111) surface morphology and its adsorption sites.

2. THEORETICAL MODELS AND METHODS 2.1. DFT. A similar protocol to that used in our previous paper examining the MD, SRM, and POM reactions on Cu(111) surfaces was used in the present work.4 Specifically, all calculations were performed using the Dmol3 package in the Material Studio software package.38,39 The calculations used the generalized gradient approximation with the Perdew−Wang exchange−correlation functional (GGA-PW91).40 The electronic structures were obtained by solving the Kohn−Sham equations self-consistently under spin-unrestricted conditions41,42 with double-numeric quality basis sets. The transition states (TSs) were identified using the complete linear/quadratic synchronous transit (L/QST) method.43 The LST maximization was performed followed by an energy minimization in the forward and reverse directions of the reaction pathway. In this way, the obtained TS was used to perform a QST maximization, which was then followed by another conjugate gradient minimization. The cycle was repeated until a stationary point was located or the number of allowed QST steps was reached. Further details are as follows: First, some coadsorption structures were optimized. Second, the bonding strengths of the adsorbates on the surface were obtained. Third, the coadsorption structures were then defined as the initial state (IS)/final state (FS). Fourth, the TS was identified using the complete LST/QST method and was characterized by vibrational frequency analysis. In this study, the ZnCu(111) surface was modeled based on the Cu(111) surface. Experiments have shown that the best molar ratio of Cu/ZnO for methanol decomposition is ∼1:1.20,44 Therefore, 50% of Cu atoms were doped by Zn atoms, which is similar to that in the model employed by Chen and co-workers.34,35 The surface was modeled using a fourlayered mode p(4 × 4) supercell with 16 atoms in each layer (8 Cu and 8 Zn atoms). During the calculations, the top two layers with the adsorbates were allowed to relax, while the other layers remained fixed. The volume was kept constant throughout all calculations. A 15 Å vacuum slab was used to separate the periodically repeated slabs. A 2 × 2 × 1 k-point mesh was used to sample the Brillouin zone. According to the side and top views of the ZnCu(111) surface morphology, as shown in Figure 1, there were nine different adsorption sites as follows: TopCu, TopZn, BriCu, BriZn, BriCuZn, fccZn, fccCu, hcpCu, and hcpZn. 2.2. KMC. KMC was performed with the Kinetix module implemented in the Material Studio software package.45,46 We built a simplified model to describe the CuZn(111) surface, in which the surface was cleaved from the face-centered cubic crystal structure and all sites were set identical. The surface was modeled using a p(256 × 256) supercell for the KMC simulations. In the KMC simulation, all possible elementary steps i and their rates ri were evaluated. One particular process

q was executed if it fulfilled the condition ∑qi=1 ri ≥ ρ × R ≥ n ∑q−1 i=1 ri, where R and ρ are the total rate (R = ∑i ri) and a random number (ρ ∈ (0,1)). When the steady condition was achieved, the data were collected. In order to improve the efficiency of simulation, a binary tree search was used.47−49 The activation energies were obtained from the DFT results. A prefactor of 1.0 × 1013 s−1 was used for all surface reactions.45,50 The rate coefficient for adsorption was calculated from kads = PAsiteσ/(2πmkBT)1/2; here P is the pressure of adsorbate, Asite is the area of a single site, σ is the sticking coefficient, m is the mass of the adsorbate, kB is the Boltzmann constant, and T is the temperature.51 Additionally, contributions from entropy were included in the KMC simulations for adsorption and desorption.45,52

3. RESULTS AND DISCUSSION 3.1. Adsorption of the Reactants and Possible Intermediates. The configurations of the possible intermediates involved in the adsorption process for the MD, POM, SRM, and OSRM reactions on the ZnCu(111) surface are shown in Figure 2, and the corresponding adsorption energies and key geometrical parameters are listed in Table 1.

Figure 2. Most stable adsorption configurations of the possible intermediates. C, O, H, and Cu atoms are shown in gray, red, white, and orange, respectively.

3.2. Possible Reaction Steps of the MD, POM, SRM, and OSRM Reactions. 3.2.1. O2 and H2O Dissociation. The IS, TS, and FS of O2 and H2O dissociation are shown in Figure S1. For O2 dissociation, the O−O bond breaks after optimization when O2 is adsorbed in a parallel configuration on the ZnCu(111) surface. When O2 is adsorbed vertically on the ZnCu(111) surface, the activation energy of O2 dissociation is 0.14 eV. In fact, the spontaneous O−O bond cleavage and small activation energy for O2 dissociation on the ZnCu(111) surface indicate that the O−O bond is broken at reaction temperatures ranging from 473 to 573 K. This result is similar to previous results in which the activation energies of O2 dissociation on Cu(hkl) surfaces are approximately 0.1−0.3 eV.22,28,53,54 Regarding H2O dissociation, the activation energy is 1.33 eV. The OH group then dehydrogenates and forms O and H (direct dissociation) or the OH group reacts with another OH group to yield O and H2O (proportionation reaction).55,56 O is B

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and 1.58 eV, respectively. For CH3 formation, the activation energy on a clean surface is 2.15 eV. Therefore, the process of CH 2O formation is energetically comparable with CH3 formation. Comparing the activation energies of CH3O dehydrogenation on clean and OH and O precovered Cu(111) surfaces,4 the activation energies on the ZnCu(111) surface are slightly smaller than those on the Cu(111) surface. This result demonstrates that ZnO will decrease the activation energy of CH3O dehydrogenation, indicating that ZnO is beneficial to CH2O formation. 3.2.3. CH2O Dehydrogenation, Oxidation, and Hydroxylation. As mentioned above, CH2O is the main product of CH3O dehydrogenation. The potential energy diagram for the CH2O reaction on clean and O and OH precovered ZnCu(111) surfaces is shown in Figure 3 together with the structures of IS, TS, and FS. The following two possible species are formed from the reaction of CH2O on a clean ZnCu(111) surface: CHO is formed from CH2O dehydrogenation, and CH2 is formed following C−O bond scission in CH2O. The activation energies of CHO and CH2 formation on the clean ZnCu(111) surface are 1.29 and 2.01 eV, respectively. Therefore, the adsorbed CH2O species on clean ZnCu(111) undergo dehydrogenation to form HCO, which is identical to the results obtained when Cu(hkl) surfaces were investigated.4,26,27,30,33 In the presence of nearby O adsorbates, the activation energy of CH2OO formation (0.08 eV) is clearly lower than that of CHO formation (0.63 eV). Further, the activation energies of CHO and CH2OOH formation are 1.05 and 0.47 eV, respectively. Therefore, the formation of CH 2 OO or CH2OOH with O or OH assistance is preferred. As shown in Figure 3, the presence of nearby O and OH adsorbates not only decreases the activation energies of additional reactions involving CH2O but also changes the pathway. This result is similar to those obtained with Cu(111) and Pd(111) surfaces.4,27 3.2.4. CH2OOH Dissociation to CO2. CH2OO and CHOOH are two possible products from the reaction of CH2OOH. The resulting IS, TS, and FS are shown in Figure S3. On a clean surface, the activation energies to generate CH2OO and CHOOH are 1.03 and 0.85 eV, respectively. With assistance from O or OH, the activation energies to form CH2OO and CHOOH are 0.41 and 0.29 eV, respectively, or 0.47 and 0.07 eV, respectively. The activation energy of CH2OO formation is slightly larger than that of CHOOH formation on clean and O precovered surfaces, indicating that the formation of CH2OO and CHOOH is possible. Although the activation energy of CHOOH formation is clearly smaller than that of CH2OO formation on a OH precovered surface, the low activation energy suggests that CH2OO formation is possible at 573 K. Given the feasibility of these outcomes, the reactions of CH2OO and CHOOH are examined further. There is only one possible product for CH2OO dissociation, that is, CHOO (see Figure S3). On a clean ZnCu(111) surface, the process must overcome an activation energy of 1.11 eV. With O and OH assistance, the activation energies are 0.73 and 0.86 eV, respectively, which are smaller than those calculated on clean ZnCu(111) surfaces. The following three possible pathways exist for the subsequent reaction of CHOOH: the first is O−H bond scission to form CHOO; the second is C−H bond scission to form COOH; and the third is C−O bond scission to form CHO (see Figure S3). On a clean surface, the activation

Table 1. Adsorption Energy (Eads, eV) and Key Geometrical Parameters (d, Å) of the Possible Intermediates Involved in the MD, POM, SRM, and OSRM Reactions on a ZnCu(111) Surface at Their Most Favorable Sites d intermediates

Eads

dCu−C/dZn−C

dCu−O/dZn−O

CH3OH CH3O CH2O

−0.38 −2.26 −0.18

CHO CO H CH2OO

−1.23 −0.78 −2.46 −3.22

CH2OOH

−1.78

2.071, 2.516/2.058

CHOO CO2 H2O OH O O2 CHOOH H2

−2.90 −0.13 −0.12 −2.64 −4.51 −0.22 −0.27 −0.11

2.036/2.086

2.068 1.988 1.997 dCu−H: 1.707

2.683 2.084/2.062 2.136/2.122

dZn−H: 1.957 2.085/2.010

2.088/2.055 1.944/1.927 2.104/2.135

configuration TopCu via O hcpZn via O fccCu via CTopCu/ O-BriCuZn TopCu via C BriCu via C fccCu hollow via OBriCuZn hcpCu viaOBriCuZn/O(H)TopCu BriCuZn via O no bond no bond hcpZn via O hcpZn via O hcpZn via O no bond no bond

preferentially produced via a proportionation reaction (Ea = 0.74 eV) rather than direct dissociation (Ea = 2.10 eV), which is in agreement with previous theoretical studies of H2O dissociation on Cu(hkl) surfaces.55,57,58 When the ZnCu(111) surface is covered by O species, the activation energy of H2O dissociation is 0.30 eV. These results show that O will accelerate H2O dissociation, which is similar to the previous experimental and theoretical results of H2O adsorption on oxygen covered Cu(111) surfaces.59,60 3.2.2. CH2O Formation. The IS, TS, and FS of CH2O formation from CH3OH dissociation are shown in Figure S2. Methanol dissociation has the following two possible intermediates: one is generated from O−H bond scission to give CH3O, and the other is generated via MeOH dissociation to give CH2OH and H. The activation energies to form CH3O and CH2OH on the clean ZnCu(111) surface are 1.03 and 2.75 eV, respectively. On an O precovered surface, the activation energies to form CH3O and CH2OH are 0.72 and 1.96 eV, respectively. Sakong and Gross proposed that the activation energy of CH3O formation on clean Cu(110) surfaces is approximately 0.7 eV; however, the O−H bond breaks during geometry optimization on the oxygen covered Cu(110) surface.31,32 These results also show that O is beneficial to CH3O formation. Finally, on a OH precovered surface, the activation energies to form CH3O and CH2OH are 0.67 and 1.85 eV, respectively. Therefore, the results show that CH3O prefers to be dehydrogenated from CH3OH rather than from CH2OH. Previous theoretical results found that methanol dissociation proceeds through cleavage of the O−H bond rather than the C−H bond on Cu(hkl)surfaces.4,26,27,30,33 Additionally, the following two possibilities exist for the reaction of CH3O: one is dehydrogenation to form CH2O; the other is dissociation into CH3. The activation energy of CH2O formation on a clean surface is 1.66 eV. With O and OH assistance, the activation energies of CH2O formation are 1.45 C

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Figure 3. Potential energy diagram of (a) the CH2O reaction on clean (b−f), O (g−k), and OH (l−p) precovered ZnCu(111) surfaces with depictions of the IS, TS, and FS structures. Bond lengths are in Å. C, O, H, and Cu atoms are shown in gray, red, white, and orange, respectively.

energies to generate CHOO (2.02 eV), COOH (2.19 eV), and CHO (2.01 eV) are similar. The high activation energies indicate that CHOOH scission on a clean surface is very difficult. On an O precovered surface, when the OH group of CHOOH points to the precovered O atoms, OH enables barrierless O−H bond cleavage of CHOOH to form bi-CHOO. When the H−C group points to the precovered O atoms, the activation energy to form COOH is 1.31 eV. On a OH precovered surface, the activation energies to form bi-CHOO and COOH are 0.19 and 1.58 eV, respectively. In the presence of nearby O and OH adsorbates, the activation energies to generate bi-CHOO and COOH decrease, but the activation energies to generate bi-CHOO are far smaller than those to generate COOH. The results show that bi-CHOO formation is preferred on O and OH precovered surfaces, which is consistent with previous theoretical results of CHOOH dissociation on Cu(111) surfaces.4,27 In fact, CHOOH has been frequently used to deliver CHOO species on Cu surfaces covered by O, where it has been found that HCOOH readily decomposes into CHOO rather than COOH.61−64

Previous studies show that CHOO has the following two types of adsorption configurations: bi-CHOO and monoCHOO.65,66 The binding strength of bi-CHOO is larger than that of mono-CHOO by 0.67 eV, which is consistent with previous results.65 The IS, TS, and FS of the reactions of biCHOO and mono-CHOO are shown in Figure S4. The reaction of bi-CHOO can lead to the formation of CHO or CO2. The activation energy of CO2 formation (0.49 eV) is far lower than that of CHO formation (2.58 eV), suggesting that the dominant pathway is CO2 formation. With O and OH assistance, the activation energies of bi-CHOO hydrogenation are 1.35 and 1.56 eV, respectively. The activation energy to form mono-CHOO from bi-CHOO is 0.94 eV, and the activation energies of CO2 and CHO formation from mono-CHOO are 0.52 and 2.14 eV, respectively. The results indicate that the difference in the configuration for CHOO adsorption does not affect the C−H bond scission, but it will decrease the activation energy of C−O bond scission compared with that on a clean surface. With O and OH assistance, the activation energies of CO2 formation D

DOI: 10.1021/acs.jpcc.6b10261 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C are 0.53 and 0.68 eV, respectively. Comparing the activation energies of the reactions of bi-CHOO and mono-CHOO with O and OH assistance, it is clear that CO2 formation from mono-CHOO is easier than that from bi-CHOO. However, the activation energy to generate mono-CHOO from bi-CHOO is higher than that of the direct hydrogenation of bi-CHOO, indicating that O and OH prevent CHOO from dehydrogenating. 3.2.5. CHO Dissociation to CO. As above, CHO is formed from the reaction of CH2O scission without O or OH assistance. Therefore, we examined the C−H and C−O bond scissions of CHO on a clean ZnCu(111) surface (see Figure S5). Clearly, the activation energy of CH formation (2.26 eV) is larger than that of CO formation (0.51 eV), indicating that CO formation from CHO dehydrogenation is the primary reaction pathway. 3.3. Microkinetic Modeling. According to the above results, it is clear that O and OH not only influence the activation energies during methanol decomposition but also alter the reaction pathway. Therefore, OH or O coverage on ZnCu(111) surfaces is an important factor in POM, SRM, and OSRM. In this section, the reaction mechanisms of MD, POM, SRM, and OSRM are studied using KMC at P = 10 atm and T = 573 K. Additionally, the selectivity of CO (SCO = CO/(CO + CO2)) and the turnover frequency (TOF) of H2 from CH3OH are investigated. Table 2 summarizes the optimal reaction pathways for MD, POM, SRM, and OSRM on ZnCu(111) together with the corresponding activation energies and reaction energies that were used in the KMC simulations. First, we studied the reaction pathway of MD with only methanol feed. The KMC results show that the MD reaction pathway is CH3OH (g) → CH3OH* → CH3O* → CH2O* → CHO* → CO* → CO(g) (see Figure 4), in which the ratelimiting step is CH2O formation from CH3O dehydrogenation. The surface is mainly covered with 0.103 ML of CH3O* and 0.001 ML of H* for this reaction. The TOF of H2 and selectivity of CO are 6.10 × 10−5 s−1 and 100%, respectively. In the case of OSRM, the H2O/CH3OH/O2 molar ratio is 1.1/1/0.12.20,67,68 The surface is mainly covered with 0.246 ML of CH3O*, 0.018 ML of OH*, 0.011 ML of O*, and 0.004 ML of H*, and the coverage of other species is smaller than 1 × 10−3 ML. The TOF of H2 and selectivity of CO are 8.76 × 10−3 s−1 and 0%, respectively. The main pathway of CH3OH bond scission is shown in Figure 4. As shown in Figure 4, there are two main pathways for H2 production as follows: one is CH3 OH (g) → CH 3OH* → CH3O* → CH2 O* → CH2OOH* → CHOOH* → CHOO* → CO2* → CO2(g), and the other is CH3OH (g) → CH3OH* → CH3O* → CH2O* → CH2OO* → CHOO* → CO2* → CO2(g). The rate-limiting step is CH2O formation from CH3O dehydrogenation. For the SRM reaction, the H2O/CH3OH molar ratio is 1.1/ 1.69,70 The surface is primarily covered with 0.152 ML of CH3O*, 0.016 ML of OH*, 0.002 ML of O*, and 0.002 ML of H*. The TOF of H2 and selectivity of CO are 1.25 × 10−3 s−1 and 9.75%, respectively. The water dissociation pathway is 2H2O (g) → 2H2O* → 2OH* + 2H* → H2O* + O* + 2H*; thus, the surface is mainly covered by OH* and a few O*. The principal role of the adsorbed O is to accelerate H2O dissociation, which is also the central pathway of H2O dissociation. The main pathway of SRM is CH3OH (g) → CH3OH* → CH3O* → CH2O* → CH2OOH* → CHOOH* → CHOO* → CO2* → CO2(g). We note that a few methanol

Table 2. Activation Energy (Ea, eV) and Reaction Energy (ΔE, eV) of H2 Production from CH3OH Hydrogenation on a ZnCu(111) Surface no.

reaction

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

O2(g)+ 2* → 2O* H2O(g) + * → H2O* CH3OH(g) + * → CH3OH* H2O* + * → H* + OH* OH* + * → H* + O* OH* + OH* → H2O* + O* CH3OH* + * → CH3O* + H* CH3OH* + O* → CH3O* + OH* CH3OH* + OH* → CH3O* + H2O* CH3OH* + * → CH2OH* + H* CH3OH* + O* → CH2OH* + OH* CH3OH* + OH* → CH2OH* + H2O* CH3O* + * → CH2O* + H* CH3O* + O* → CH2O* + OH* CH3O* + OH* → CH2O* + H2O* CH3O* + * → CH3* + O* CH2O* + * → CHO* + H* CH2O* + * → CH2* + O* CH2O* + O* → CHO* + OH* CH2O* + O* → CH2OO* + * CH2O* + OH* → CHO* + H2O* CH2O* + OH* → CH2OOH* CH2OOH* + * → CH2OO* + H* CH2OOH* + * → CHOOH* + H* CH2OOH* + O* → CH2OO* + OH* CH2OOH* + O* → CHOOH* + OH* CH2OOH* + OH* → CH2OO* + H2O* CH2OOH* + OH* → CHOOH* + H2O* CHOOH* + * → bi-CHOO* + H* CHOOH* + * → COOH* + H* CHOOH* + * → CHO* + OH* CHOOH* + O* → bi-CHOO* + OH* CHOOH* + O* → COOH* + OH* CHOOH* + OH* → bi-CHOO* + H2O* CHOOH* + OH* → COOH* + H2O* CH2OO* + * → CHOO* + H* CH2OO* + O* → CHOO* + OH* CH2OO* + OH* → CHOO* + H2O* bi-CHOO* + * → CO2* + H* bi-CHOO* + * → CHO* + O* bi-CHOO* + O* → CO2* + OH* bi-CHOO* + OH* → CO2* + H2O* bi-CHOO* → mono-CHOO* mono-CHOO* + * → CO2* + H* mono-CHOO* + * → CHO* + O* mono-CHOO* + O* → CO2* + OH* mono-CHOO* + OH* → CO2* + H2O* CHO* + * → CH* + O* CHO* + * → CO* + H* CO* → CO + * 2H* → H2(g) + * + * CO2 * → CO2 (g) + *

Ea

ΔE

1.33 2.10 0.74 1.03 0.72 0.67 2.71 1.96 1.85 1.66 1.45 1.58 2.15 1.29 2.01 0.63 0.08 1.05 0.47 1. 03 0.85 0.41 0.29 0.47 0.07 2.02 2.19 2.01 − 1.31 0.19 1.58 1.11 0.73 0.86 0.49 2.58 1.35 1.56 0.94 0.52 2.14 0.53 0.68 2.26 0.51

−2.65 −0.12 −0.38 0.35 0.54 0.19 0.19 −0.35 −0.16 1.27 0.73 0.92 1.11 0.57 0.76 0.70 0.32 0.59 −0.22 −0.95 −0.03 −0.65 0.24 0.08 −0.30 −0.46 −0.11 −0.27 −0.27 1.62 0.89 −0.81 1.08 −0.62 1.27 −0.43 −0.97 −0.78 0.11 1.70 −0.43 −0.24 0.67 −0.56 1.03 −1.10 −0.91 0.92 −0.54 0.98 −0.26 0.13

a

decompositions also follow the MD pathway (see Figure 4). The rate-limiting step of all of the pathways is CH2O formation from CH3O dehydrogenation. For POM, the CH3OH/O2 molar ratio is 1/0.12. The surface is mainly covered with 0.798 ML of CH3O*, 0.003 ML of E

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Figure 4. MD, SRM, POM, and OSRM reaction pathways. The red arrows indicate the rate-limiting steps.

and R9)); therefore, the activation energies of R7, R8, and R9 are simultaneously shifted by the same value. According to the results of the sensitivity analysis, the reaction CH3O* → CH2O* (R13, R14, and R15) influences both SCO and the TOF of H2 (Figure 5). As shown in Figure 5,

OH*, 0.002 ML of H*, and 0.022 ML of O*. The TOF of H2 and CO selectivity are 7.19 × 10−4 s−1 and 20.00%, respectively. The main pathway of the POM reaction is CH3OH (g) → CH3OH* → CH3O* → CH2O* → CH2OO* → CHOO* → CO2* → CO2(g). A few methanol decompositions occur via CH3 OH (g) → CH 3OH* → CH3O* → CH2 O* → CH2OOH* → CHOOH* → CHOO* → CO2* → CO2(g), whereas other CH3OH follow the MD pathway (see Figure 4). The rate-limiting steps of all of the pathways are CH2O formation from CH3O dehydrogenation. Table 3 reports the coverage (ML) of the main adsorption species, SCO, and the TOF of H2 on the surface of these four Table 3. Coverage (ML) of the Main Species, SCO (%), and the TOF of H2 (s−1) coverage MD OSRM POM SRM

CH3O

H

0.103 0.246 0.798 0.152

0.001 0.004 0.002 0.002

OH 0.018 0.003 0.016

O

SCO

0.011 0.022 0.002

100 0 20.00 9.75

TOF of H2 6.10 8.76 7.19 1.25

× × × ×

10−5 10−3 10−4 10−3

Figure 5. Sensitivity of SCO and the TOF of H2 to variations in ΔEa (CH3O* → CH2O*).

the TOF of H2 increases as the activation energy of CH3O* → CH2O* decreases. The activation energy of CH3O* → CH2O* decreases by 0.3 eV, with a SCO value equal to 0. Then, SCO increases as the activation energy of CH3O* → CH2O* decreases further. The KMC results show that CO is obtained from the CH2O* → CHO* → CO* reaction. The reason for this is that there are many CH2O* species on the surface because the activation energy of the CH3O* → CH2O* reaction decreases. However, some CH2O* species do not react with OH* or O* and directly hydrogenate to CHO*, which leads to the formation (and detection) of CO. We note that the amount of H2O is higher than that of H2 when the activation energies of CH3O* → CH2O* decrease by 0.4 and 0.5 eV. The results indicate that the TOF of H2 will be improved by reducing the activation energy of the CH3O* → CH2O* reaction, but SCO and the amount of H2O byproduct will increase. Therefore, an appropriate activation energy of the CH3O* → CH2O* reaction is necessary for OSRM. Recently, the SCO of SRM on NiZn, PdZn, and PtZn (111) surfaces has been studied.36 It was found that the activation

reactions. As shown in Table 3, the main adsorption species on the surface is CH3O for the four reactions. Because of the dissociative adsorption of O2 under the reaction conditions, O will accelerate CH3OH dissociation; thus, the coverage of CH3O for POM is the highest. Although OH species can also accelerate CH3OH dissociation, the coverage of OH is low because of the high activation energy of H2O dissociation. Therefore, the coverage of CH3O for SRM is lower than that of POM. For ORSM, O will accelerate H2O dissociation, yielding moderate O and OH coverage. As a result, no CO is detected for OSRM. This result is in agreement with the previous result, in which the SCO is approximately 0%.68 In addition, the TOF of H2 is lowest for MD, and the TOF of H2 for OSRM is slightly larger than that for POM and SRM. To better understand the OSRM reaction, a sensitivity analysis at P = 10 atm, T = 573 K, and CH3OH/H2O/O2 = 1.1/1/0.12 is examined in which each parameter in the KMC model is shifted by a small amount from its original value while the other parameters are kept constant.45,71 We note that there are three parallel reactions (i.e., H3OH* → CH3O* (R7, R8, F

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The Journal of Physical Chemistry C energy of CH2O* + OH* → CHO* + H2O* is larger than that of CH2O* + OH* → CH2OOH* varied from 0.26 to 0.49 eV (0.49 versus 0.26 eV, respectively), which favors CO 2 selectivity. The energy difference of R19 and R20 on the CuZn(111) surface is 0.55 eV, whereas it is 0.58 eV for R21 and R22 on the same surface, indicating strong selectively for CO2 in the OSRM reaction. Additionally, previous results have shown that the behavior of PdZn-based catalysts is similar to that of Cu-based catalysts.27,34,72,73 In this work, sensitivity analysis for R17 and R19−R22 has been examined. During the process, the activation energies of R20 and R22 are constant, and the activation energies of R17, R19, and R21 are simultaneously shifted by the same value. As shown in Figure 6, the TOF of H2 is not affected as the activation energies of

Previous experimental and DFT results show that Cu(111) is the most stable surface among Cu surfaces and shows the worst activity for CH3OH decomposition or synthesis.33,74,75 Therefore, the best way to improve the TOF of H2 is to improve the percentage of Cu(hkl) surfaces, excluding Cu(111). In addition, the benefit of nanoparticle Cu to reduce the activation energies for methanol decomposition has been reported.22,50 There are two ways to decrease SCO for OSRM as follows: the first is to reduce the activation energy of CH2O* + OH* → CH2OOH* + *, and the second is to improve the activation energy of CH2O* + OH* → CHO* + H2O*. Improving H2O pressure and adding additives are two strategies that could be used to obtain a low SCO for OSRM.36

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b10261. IS, TS, and FS for O2 and H2O dissociation, CH2O formation from CH3OH dissociation, bi-CHOO formation from CH2OOH dissociation, CO2 formation from bi-CHOO and mono-CHOO dissociation, and CO formation from CHO dissociation involved MD, POM, SRM, and OSRM on ZnCu(111) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Z.L.). *E-mail: [email protected] (W.H.).

Figure 6. Sensitivity of SCO and the TOF of H2 to variations in ΔEa (CH2O* → CHO*).

ORCID

Wei Huang: 0000-0002-7914-2719 Notes

The authors declare no competing financial interest.

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R17, R19, and R21 decrease. However, SCO increases as the activation energies of R17, R19, and R21 decrease. The KMC results also show that the CH2O* + OH* → CHO* + H2O* (R21) and CH2O* + O* → CHO* + OH* (R19) reactions occur on the CuZn(111) surface as the activation energies of R17, R19, and R21 decrease. However, CO is obtained from the CH2O* + OH* → CHO* + H2O* reaction. Thus, SCO primarily depends on the energy difference between CH2O* + OH* → CHO* + H2O* and CH2O* + O* → CH2OOH* + *, which is similar to the results obtained for SRM on PdZn surfaces.67

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this study by the key project of the National Natural Science Foundation of China (21336006), the National Natural Science Foundation of China (21176167 and 21306125), the key project of Basic Industrial Research of ShanXi(201603D121014), and the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi.

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4. CONCLUSIONS In this paper, the MD, POM, SRM, and OSRM reactions on ZnCu(111) surfaces are systemically studied using DFT and KMC. The results show that the Zn species is beneficial to CH2O formation, which is the rate-limiting step of the four reactions. O or OH groups on the ZnCu(111) surface not only influence the activation energies but also alter the reaction pathways. According to the results of sensitivity analyses for OSRM, the TOF of H2 largely depends on the activation energy of CH3O* → CH2O*. A low activation energy of CH3O* → CH2O* is beneficial to a high TOF of H2, but the H2O byproduct and SCO increase as the activation energy of CH3O* → CH2O* decreases. SCO was found to mainly depend on the energy difference between the CH2O* + OH* → CHO* + H2O* and CH2O* + O* → CH2OOH* + * reactions. In general, smaller energy differences between these reactions leads to higher SCO values for OSRM.

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