Thermodynamic and Kinetic Study on Carbon Dioxide Hydrogenation

Subscriber access provided by University of Florida | Smathers Libraries

Article

Thermodynamic and Kinetic Study on Carbon Dioxide Hydrogenation to Methanol Over a GaNi(111) Surface: The Effects of Step Edge 3

5

Qingli Tang, Zhemin Shen, Christopher K. Russell, and Maohong Fan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08232 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C 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 50 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

Thermodynamic and Kinetic Study on Carbon Dioxide Hydrogenation to Methanol over a Ga3Ni5(111) Surface: The Effects of Step Edge Qingli Tang,a,b Zhemin Shen,a,* Christopher K. Russellc and Maohong Fanb,d,* a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R.

China. b

Departments of Chemical and Petroleum Engineering, University of Wyoming, Laramie 82071, WY, USA.

C

Departments of Civil and Environmental Engineering, Stanford University, Stanford 94305, CA, USA.

d

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta 30332, GA, USA.

Abstract: Density functional theory (DFT) was used to study the mechanisms of carbon dioxide (CO2) hydrogenation to methanol (CH3OH) on a stepped Ga3Ni5(111) surface. Surface properties, adsorption energies of reactants, potential intermediates and products, as well as thermodynamic and kinetic parameters of elementary steps were calculated. It is found that a stepped Ga3Ni5(111) surface with low surface energy not only can highly activate CO2 but also is beneficial to dissociative H2 adsorption. Moreover, the reactants, intermediates, and products on the Ga3Ni5(111) surface prefer to adsorb to Ni sites at step edges. Accoring to calculated thermodynamic and kinetic parameters of all the elementary steps, CO2 is hydrogenated to CH3OH via trans-COOH, COHOH, COH, HCOH, and CH2OH intermediates because this pathway has the lowest activation barriers and highest rate constants. Meanwhile, water (H2O) formation is the rate-limiting step. Based on microkinetic modeling, Ga3Ni5(111) shows higher selectivity to CH3OH than CH4. In all, the stepped Ga3Ni5(111) surface is beneficial in

*

Corresponding author at: 800 Dongchuan Road, Minhang District, Shanghai 200240, China. E-mail: [email protected] 1000 E University Ave., Laramie, WY 82071, USA. E-mail: [email protected]

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

facilitating CO2 hydrogenation to CH3OH, and the presence of steps and the existence of Ga on those steps instead of step edge are required for the high activity of the Ga3Ni5 catalyst.

1. Introduction Using carbon dioxide (CO2) as a raw material for synthesis of chemicals and fuels not only mitigates its greenhouse effect1 but also provides commodity chemicals.2 Thus, CO2 hydrogenation has recently attracted great interest from many scholars. Recent studies have shown that formic acid (HCOOH),1, 3-6 methane (CH4),7-17 and methanol (CH3OH)18-27 are the three main products of CO2 hydrogenation. CH3OH, which is commercially used as fuel and as a feedstock for many chemical processes, is industrially synthesized from syngas using a Cu/ZnO/Al2O3 catalyst.28-29 Although the Cu/ZnO/Al2O3 catalyst has been commercially used, the high pressure required is still a concern. Thus, numerous theoretical and experimental studies have been conducted to modify the conventional Cu/ZnO/Al2O3 catalysts30-31 and find new catalysts32-35 to improve CH3OH selectivity at low pressure. Fiordaliso and coworkers have reported that GaPd2/SiO2 catalysts can convert CO2 and H2 to CH3OH at ambient pressure, and Ga3Ni5 is also one of the efficient catalysts that are reported to produce CH3OH efficiently at ambient pressure.34 In previous studies, the reaction mechanisms of CO2 hydrogenation to CH3OH on a flat Ga3Ni5(221) surface were probed36. However, to the best of our knowledge, many metal surfaces have a step edge effect that palyed a key role in catalysis, including Ni,37-39 Pt,40 Ag,41 Cu,42-45 and Rh.46 Ghiringhelli et al.37 found that the adsorption energies of phenol on stepped Ni(110) and Ni(221) were higher than that on a Ni(111) surface. Catapan and coworkers38 suggested that most of species involved in the water-gas shift (WGS) and coke formation reactions have


Page 2 of 50

Page 3 of 50 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


stronger binding energies on stepped (211) surfaces than those on flat (111) surfaces. They also noted that the stepped Ni(211) surface may be harder for C-O or C-H bond breaking for species with more than one O atom than mono-oxygenated species. For example, the activation barriers for CHO dissociation to CH and O, and COH dissociation to C and OH are 0.86 and 1.03 eV, respectively, while the activation barrier for COOH dissociation to COH and O is 1.33 eV on Ni(211) surface. Yang et al.40 suggested that the stepped Pt(211) surface is more favorable for propane dehydrogenation than the flat Pt(111) surface. A density functional theory (DFT) analysis of CO oxidation on Ag(111) and Ag(221) surfaces by Su et al.41 suggested that the step edge of (221) positively favors O2 dissociation, and further facilitates CO hydrogenation. Behrens et al.42 pointed out that the stepped Cu(211) surface is more active and binds intermediates stronger than the Cu(111) surface for both CO and CO2 hydrogenation. Wang and coworkers46 studied ethanol synthesis on a stepped Rh(211) surface, and they found that the Rh(211) surface can promote the formation of C2 oxygenates and CH3. This successfully improved the selectivity of ethanol compared to the flat Rh(111) surface. The above studies demonstrate that the step edge exhibits effective catalytic properties, however, up to now, the step edge effect over Ga3Ni5 stepped surfaces improves or hinders the process CO2 hydrogenation to CH3OH is still unclear. In this study, a thermodynamic and kinetic study of CO2 hydrogenation to CH3OH was carried out on a stepped Ga3Ni5(111) surface using DFT calculations. First, to choose an appropriate surface, surface energies, CO2 adsorption and dissociative H2 adsorption on five different facets were calculated. A stepped (111) surface with low surface energies was chosen, because not only can it highly activate CO2 but also H2 dissociation is spontaneous on it. Then, to verify the step edge effect and the advantanges of the Ga3Ni5(111) surface in hydrogenating



CO2 to CH3OH, the adsorption energies of twenty-three species on Ga3Ni5(111) were calculated, and a comparison was made with those on Ga3Ni5(221) and Cu(111) surfaces. Finally, the thermodynamic and kinetic parameters of thirty-two elementary steps occurring on the surface were discussed, and the most probable reaction pathway was determined to be that with the lowest activation barrier. A rate-limiting step was also obtained in the main pathway, which is assumed as the step with the highest activation barrier and, correspondingly, the lowest rate constant.

2. Computational details All the calculations were carried out using plane waves density functional theory (DFT) within a version of 5.3.5 Vienna Ab initio Simulation Package (VASP),47-49 where Projector Augmented Wave (PAW)50 was used to represent electron-ionic core interactions, and Generalized Gradient Approximation (GGA) corrections of the exchange-correlation function developed by Perdew, Burke and Ernzerhof (PBE)51 was employed for the exchange-correction energy. Geometric optimization converged on a total energy under 1.0×10-5 eV. Transition states were searched using the climbing image nudged elastic band (CI-NEB)52 method, where a linear interpolation between the initial state and final state was used as the initial guess. In this process, six orientations were considered and a maximum force of 0.05 eV/Å was used to stop the CINEB algorithm. Transition states were verified by frequency calculations. In this study, a stepped Ga3Ni5(111) surface was modeled using a slab size with a p(1×2) supercell, which corresponds to adsorbate coverages of 1/2 monolayer (ML). Convergence tests on cutoff energy (ENCUT=400, 450, 500, and 550 eV), k-points (2×1×1, 4×2×1 and 6×3×1), and sigma (SIGMA= 0.10, 0.12, 0.14, 0.16, 0.18, 0.20, 0.22, 0.24, 0.26, 0.28, and 0.30 eV) were performed. After that, a 500 eV cutoff energy was chosen to describe the number of plane waves


Page 4 of 50

Page 5 of 50 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


basis set, a 2×1×1 k-points grid was used to perform the Brillouin-zone integrations, and a 0.10 eV sigma value was used to determine the width of the smearing. The slabs were separated by a 15 Å vacuum to avoid periodic interactions. The bottom two layers were fixed in their bulk position while the top two layers together with the adsorbates are relaxed. To choose one appropriate surface to study, surface energies, CO2 adsorption energies and the activation barrier of H2 dissociation were calculated on different terminations of five facets ((001), (021), (110), (111) and (221)). Surface energies were calculated according to the following equation:

Es =（E slab − nEbulk） / 2S

(E1)

where Eslab represents the total energy of the surface, n is the multiple of the number of bulk atoms in the selected surface, Ebulk is the total energy of the bulk, and S is the area of the selected surface. Adsorption energies of adsorbates were calculated based on the following equation:

Eads = Esurface+ X − EX − Esurface

(E2)

where E ads represents the adsorption energy of adsorbate, Esurface+ X is the total energy of the system where adsorbate adsorbs on the slab. E X is the energy of the adsorbate in gas phase, and

E surface is the energy of the slab without adsorbate. For the species with the adsorption energies lower than 1.00 eV, the dispersion correction has been carried out using DFT-D3 method.53 The zero point energy (ZPE) was calculated using the following equation54-55: ZPE = ∑ hvi / 2 i

where vi is vibrational frequency, and h is Planck’s constant.


(E3)


Page 6 of 50

Reaction Gibbs free energies, reaction rate constants, activation barriers, and enthalpic contributions are important thermodynamic and kinetic parameters which are used as key factors to determine reaction mechanisms.56-62 These thermodynamic and kinetic parameters can directly reflect the difficulty and speed of reactions occurring. Therefore, using these parameters will be of great benefit to the studies. Based on previous studies,54-55 the reaction Gibbs free energy of each elementary reaction on the surface is calculated as follows:

∆G = ∆E + ∆ZPE + ∆U − T∆S hvi / k B T e hvi / k BT − 1

(E5)

hv i / k B T − ln(1 − e − hvi / k BT )] hvi / k BT e −1

(E6)

U = RT ∑ i

S = R∑ [ i

(E4)

where, in E4, ΔE is the reaction energy, ΔZPE is the ZPE correction, ΔU is the difference of standard vibrational internal energy of products and reactants, ΔS is the difference of standard vibrational entropy of products and reactants. In E5 and E6, R is the gas constant, T is the absolute temperature, h is the Plank constant, kB is the Boltzmann constant, and vi is the vibrational frequency. The rate constant (k) was calculated according to the following equations, where kB, T, h and vi are as in E5 and E6, and Ea is the activation barrier, q is vibration partition function, qTS and q R are the vibration partition functions of transition states and initial states, respectively, which are calculated by E8:

k =

q =

k BT qTS − E a / k BT e h qR

(E7)

1 vibrations

∏i

=1

(1 − e

− hv i / k B T


(E8) )

Page 7 of 50 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


3. Results and discussion 3.1 Surface selection To choose an appropriate surface to study CO2 hydrogenation to CH3OH, the surface structures and energies of low index or common surfaces of Ga3Ni5 surfaces, as well as CO2 adsorption energies and dissociative H2 adsorption were calculated, which are depicted in Table 1 for comparison for (001), (021), (110), (111) and (221) facets. The side views (top one) and top views (bottom one) of surface (221), (001), (021), (111) and (110) with different terminations are shown in Figure S1 of Section 1 in the Supporting Information. ZPE Table 1 The information of surface energies (Es), CO2 adsorption energies ( E ads and E ads −CO ) − CO 2

2

and the activation barriers (Ea and E aZPE ) and reaction energies (△E and △EZPE) of H2 dissociative adsorption on different terminations of the considered surfaces. The ones with superscript ZPE are the corresponding values with zero point energy (ZPE) correction. The surface energies are in J·m-2, the other energies are in eV.

Es

E ads −CO2

ZPE E ads − CO

Ⅰ

1.95

-0.08

Ⅱ

1.95

Ⅰ

Surface type

(001)

(021)

(110)

(111)

H2→H+H Ea

E aZPE

△E

△EZPE

-0.08

-

-

-0.74

-0.74

0.14

0.19

0.3

0.18

0.01

-0.06

1.98

0.05

0.08

0.47

0.36

-0.23

-0.23

Ⅱ

1.98

-0.57

-0.52

0.08

-

-0.73

-0.75

Ⅰ

1.88

-0.11

-0.13

0.2

0.10

0.09

0.04

Ⅱ

2.05

-0.17

-0.16

0.27

0.18

0.16

0.11

Ⅲ

2.09

-0.55

-0.56

0.07

0.003

-0.49

-0.51

Ⅳ

1.88

-0.24

-0.26

0.44

0.34

-1.16

-1.21

Ⅰ

1.98

-0.06

-0.02

0.04

-

-0.93

-0.97

Ⅱ

1.97

0.03

0.06

0.08

-

-0.39

-0.45

2



Es

E ads −CO2

ZPE E ads − CO

Ⅲ

1.97

-0.52

Ⅳ

1.98

Ⅰ

Surface type

(221)

Page 8 of 50

H2→H+H Ea

E aZPE

△E

△EZPE

-0.51

0.03

-

-0.74

-0.73

0.11

0.13

0.03

-

-1.19

-1.19

2.12

0.10

0.06

0.04

-

-1.11

-1.11

Ⅱ

2.11

0.09

0.05

0.04

-

-1.10

-1.11

Ⅲ

2.12

0.13

0.11

-

-

-1.26

-1.26

2

The “-” in this table represents that the value is under zero.

3.1.1 Surface energies According to E1, surface energies of the five facets were calculated. From Table 1, it can be seen that two terminations (I&IV) of surface (110) have the lowest surface energy of 1.88 J·m-2, while two other terminations (II&III) have relatively high surface energies of 2.05 and 2.09 J·m-2, respectively. Surface (001), (111), and (021) have similar surface energies and surface (221) has the highest surface energy with three terminations (2.12, 2.11 and 2.12 J·m-2 for terminations

I, II, and III, respectively). The surface with the lowest surface energy is the most thermodynamically stable surface, suggesting (001), (111), and (021) surfaces are more stable than surface (221).

3.1.2 CO2 adsorption As one of the reactants, CO2 activation is one of the most important factors in the process of CO2 hydrogenation to CH3OH. In this study, CO2 adsorption on all terminations of the five selected facets are considered. The adsorption energy of the most stable configuration on each facet is displayed in Table 1. From Table 1, it can be seen that the adsorption energies of CO2 on II of surface (021), III of surface (110) and III of surface (111) are -0.52, -0.56 and -0.51 eV,


Page 9 of 50 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


respectively, indicating higher activities on these surfaces compared to the others who have lower, even positive values.

3.1.3 H2 adsorption and dissociation As the source of hydrogen, dissociative H2 adsorption is also a significant factor that should be carefully considered. The activation barriers and reaction energies of dissociative H2 adsorption on different terminations of each facet are displayed in Table 1. The results show that H2 adsorption is dissociative adsorption on surface (111), (221), type I of surface (001) and type II of surface (021), while on surface (110), the activation barrier is as high as 0.34 eV. On the basis of the above analysis, the flat surface (221) with the highest surface energy has been studied in our previous work36, and (001) is also a flat surface, which is not considered in this studty since the purpose of this study is to probe into the effects of step edge. In addition, the reasons why (110) surface is not chosen are that the type I & IV of (110) surface have high H2 dissociation barriers and type II & III have relatively high surface energies. Considering the adsorption of CO2, we choose the surface with high CO2 adsorption energy as the study object. Thus, type II of (021) surface and type III of (111) surface can be as good candidates. In this work, type III of surface (111) is chosen to study, and type II of surface (021) will be studied in our next work. The stepped (111) surface with termination III is chosen in this study, as shown in Figure 1.



Figure 1 A (a) side view, and (b) top view of termination III of surface (111). Ga and Ni atoms are represented in brown and blue balls, respectively.

3.2 Adsorption energies and adsorbate structures during CO2 hydrogenation to CH3OH on the Ga3Ni5(111) surface 3.2.1 The adsorption of reactants As displayed in Figure 2, five stable configurations of CO2 on the Ga3Ni5(111) surface were obtained after optimizing all the potential initial structures. As discussed in section 3.1, the most stable structure, as shown in Figure 2(a), prefers to adsorb on the step edge bridge site in a “V” shape via the C atom with an adsorption energy of -0.51 eV without dispersion correction. Previous study by Ding et al.63 suggested that CO2 binds on Ni(110) surface with a “V” shape under ultra-high vacuum (UHV) conditions, which is very similar to the structure in this study. Numerous studies have been conducted to study CO2 adsorption on metal surfaces.1, 4, 36, 64-66 The adsorption of CO2 on the Ga3Ni5(221)36 surface is 0.10 eV, which is much lower than that on the Ga3Ni5(111) surface. Peng and coworkers have studied the adsorption energies of CO2 on Ni(111)1 and Ni(110)4 surfaces, the results of which show that the adsorption energy on flat Ni(111) is -0.06 eV, while the adsorption energy on a stepped (110) surface is -0.47 eV. Zhang et


Page 10 of 50

Page 11 of 50 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


al.66 reported that the adsorption energy of CO2 is -0.18 eV on Pd(111). All the results show that

Ga3Ni5(111) surface can active CO2 highly. After considering the dispersion correction, the binding energies of CO2 is -0.32 eV.

Figure 2 The side views (top) and top views (bottom) of the stable adsorption configurations of CO2 on the stepped Ga3Ni5(111) surface together with the corresponding adsorption energy corrected by ZPE. C and O atoms are represented by grey and red balls, respectively. Figure 3 shows four stable adsorption configurations of H on the stepped Ga3Ni5(111) surface after optimizing all the potential initial structures. From Figure 3, it can be seen that H atoms can stably adsorb at 3-fold hollow sites, bridge sites, or even atop sites, which are all composed of Ni atoms. The most stable binding site is at the step edge 3-fold hollow sites with an adsorption energy of -4.12 eV, and the second-most stable site is the step edge bridge sites, with an adsorption energy of -3.64 eV.



Figure 3 The stable structures of H atoms on the stepped Ga3Ni5(111) surface together with the corresponding adsorption energy corrected by ZPE. H atom is represented by white balls. As discussed above, both CO2 and H prefer to adsorb at Ni-containing sites, which means that the activated sites of Ga3Ni5(111) may be the sites composed of Ni atoms, which agrees with previous study36.

3.2.2 The adsorption of potential intermediates The binding parameters and adsorption energies of the most stable structures of nineteen kinds of potential intermediates are shown in Table 2, and the most stable configurations are displayed in Figure 4.

Formate (HCOO) was considered as a key intermediate in the process of the WGS by experimental and theoretical studies on Cu and Au catalysts.67-68 In the previous study, formate has two different configurations, monodentate (mono-HCOO) and bidentate (bi-HCOO),68 which can be distinguished by the number of binding bonds. Both mono-HCOO and bi-HCOO prefer to adsorb at bridge sites of step edges. Carboxyl (COOH) was experimentally detected by Miyoshi and coworker69 in gas phase by photoionization mass spectrometry. In addition, carboxyl was proposed as one of key intermediates in WGS on copper.67 As discussed before, COOH also has two isomers which are distinguished by the direction of OH corresponding to O.70 From Figure 4, we can see that the most stable structures of trans-COOH and cis-COOH both prefer to bind at the bridge sites of the step edge. Carbon monoxide (CO) is an inconvenient byproduct in the process of CO2 hydrogenation, which was experimentally observed in the process of CO2 hydrogenation reactions on a copper/zirconia catalyst by Weigel et al.71 In present study, CO was found to adsorb at the 3-fold hollow sites of the step edge, with an adsorption energy of -2.32 eV.

Oxygen (O) can be obtained by CO2 or HCOO dissociation, where the most stable site is the 3-


Page 12 of 50

Page 13 of 50 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


fold hollow site made up by three Ni atoms at the step edge, while in a previous study of the Ga3Ni5(221) surface,36 it was found that O atom prefers to adsorb at fcc sites formed by two Ni atoms and one Ga atom. Hydroxyl (OH) was found to bind at the bridge site of step edge with bond distances of 1.916 and 1.907 Å, respectively. Formic acid (HCOOH) is not only experimentally72 and theoretically1, 4 considered as a product, but also is an intermediate73 in the process of CO2 hydrogenation. Similarly to previous studies,1, 4, 73 HCOOH was found to bind at atop sites of metal surfaces, with a binding energy of -0.76 eV without dispersion correction, which is very close to its binding energy (-0.71 eV) of it on a clean Ni(110) surface as calculated by Peng et al..4 The binding energies of HCOOH corrected by DFT-D3 method is -0.49 eV.

Formyl (HCO) was experimentally identified as an intermediate of CO hydrogenation on a Ru(111) surface.74 Meanwhile, in DFT studies of the CO2 hydrogenation process, HCO was also considered as an important intermediate.31, 75-78 This study found that HCO prefers to bind at 3fold hollow sites with an O atom binding with two step edge Ni atoms. Hydroxymethylidyne

(COH), another possible intermediate during the process of CO hydrogenation,79-81 was found to prefer to bind at 3-fold hollow sites through the C atom with an adsorption energy of -5.09 eV.

Formaldehyde (H2CO) had been experimentally observed as another intermediate in the process of CO hydrogenation82 and is theoretically considered as a key intermediate in the process of CO2 hydrogenation to methanol.73, 75 H2CO was found to prefer to adsorb at 3-fold hollow sites with two Ni-O bonds and one Ni-C bond of 1.944, 1.921 and 2.009 Å, respectively.

Dioxymethylene (H2COO) has been proposed as a possible intermediate in the CH3OH synthesis from CO2 hydrogenation on ZnO(0001)83 and In2O3(110)84 surfaces. H2COO is the only species that adsorbs at a 4-fold hollow site containing three Ni atoms and one Ga atom.

Hydroxymethoxy (H2COOH) has been observed experimentally as one of the products of



methane oxidation85 and was proposed as an intermediate in the process of CO2 hydrogenation to CH3OH by Tang et al.,86 Grabow et al.,75 and Zhao et al.73 In this work, H2COOH is considered as an intermediate which prefers to adsorb at step edge bridge sites, with one O atom anchored to two Ni atoms. Dihydroxycarbene (COHOH), detected by spectroscopy in the process of oxalic acid dissociation,87 was also proposed as one key intermediate in the process of CO2 hydrogenation on YBa2Cu3O7 catalysts.88 In previous studies,36 COHOH was also considered as a key intermediate in the main reaction route. COHOH has three distinguishable isomers (t,tCOHOH, t,c-COHOH and c,c-COHOH), which are distinguished by the direction of the two OH groups. Hydroxymethylene (HCOH) was previously proposed by Kakumoto89 and Zhao et al.73 as an intermediate in CO2 hydrogenation to CH3OH. In present study, HCOH prefers to bind at the bridge sites of Ga3Ni5(111) surface, with the plane of the structure formed vertical to the step edge. Hydroxymethyl (H2COH) can be generated from HCOH or H2CO hydrogenation, which was proposed by a few scholars.73, 75, 83, 90 CH2OH was also experimentally considered as an intermediate in CH3OH decomposition.91 Methoxy (CH3O) has also been experimentally detected as an intermediate on Cu catalysts92-93 and is considered as an intermediate in CH3OH decomposition on Cu2O(111) surface.94 Previous DFT studies on CH3O adsorption suggested that the most stable site is at the hollow sites on Cu(111),73 while in this study, it was determined that CH3O prefers to adsorb at the bridge site at the step edge.

Table 2 The binding parameters and adsorption energies of reactants, potential intermediates and products in the process of CH3OH synthesis on the stepped Ga3Ni5(111) surface and the adsorption energies of species on Ga3Ni5(221) and Cu(111) surfaces. D is the distance of ZPE corresponding binding bond, which is in Å. Eads and E ads are the adsorption energies on

221 Ga3Ni5(111) surface without and with ZPE correction, respectively, E ads are the adsorption


Page 14 of 50

Page 15 of 50 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


(111 ) energies of species on Ga3Ni5(221) surface without ZPE correction and E Cu are the ads

adsorption energies of species on Cu(111) surface without ZPE correction, which are in eV. D

(111 ) 73 221 E Cu E ads ads

36

ZPE a

Eadsa E ads

Species

Sites Binding bonds

H

3-Fold Ni1-H/Ni2-H/Ni3-H

1.794/1.690/1.689

-3.58

-3.09

CO2

Bridge Ni1-C/Ni2-C

2.041/2.050

-0.05

0.10

mono-HCOO Bridge Ni1-O/Ni2-O

1.959/1.957

-2.63

-2.58

-3.64 -3.57

bi-HCOO

1.911/1.908

-3.00

-3.06

-4.02 -3.89

trans-COOH Bridge Ni1-O/Ni2-C

1.969/1.869

-1.96

-2.34

-3.26 -3.18

cis-COOH

Bridge Ni1-O/Ni2-C

1.955/1.865

-2.14

-2.42

-3.39 -3.29

CO

3-Fold Ni1-C/Ni2-C/Ni3-C

2.045/1.922/1.898

-1.06

-2.03

-2.37 -2.32

O

3-Fold Ni1-O/Ni2-O/Ni3-O

1.923/1.829/1.821

-6.55

-5.58

-7.63 -7.57

OH

Bridge Ni1-O/Ni2-O

1.916/1.907

-3.75

-3.38

-4.65 -4.51

HCOOH

Atop Ni-O

1.936

-0.24

-0.41

HCO

3-Fold Ni1-O/Ni2-O/Ni3-C

2.027/1.976/1.902

-1.73

-2.27

-3.17 -3.10

COH

3-Fold Ni1-C/Ni2-C/Ni3-C

1.894/1.855/1.851

-3.36

-4.30

-5.23 -5.09

H2CO

3-Fold Ni1-O/Ni2-O/Ni3-C

1.944/1.921/2.009

-0.12

-0.83

-1.46 -1.40

H2COO

4-Fold Ni1-O1/Ni2-O1/Ni3-O2/Ga-O2 1.956/1.9061.957/2.077

-3.76

-4.54

-4.96 -4.67

H2COOH

3-Fold Ni1-O/Ni2-O

1.932/1.908

-2.61

-2.44

-3.43 -3.27

t,t-COHOH Bridge Ni1-C/Ni2-C

1.972/1.964

-1.19

-2.04

-2.54 -2.52

t,c-COHOH Bridge Ni1-C/Ni2-C

1.939/1.938

-1.05

-1.92

-2.58 -2.52

c,c-COHOH Bridge Ni1-C/Ni2-C

1.931/1.915

-0.81

-1.81

-2.42 -2.42

HCOH

Bridge Ni1-C/Ni2-C

1.913/1.912

-1.97

-3.03

-3.58 -3.49

CH2OH

Bridge Ni1-C/Ni2-C

2.028/2.017

-1.34

-1.68

-2.73 -2.70

CH3O

Bridge Ni1-O/Ni2-O

1.914/1.901

-2.95

-2.55

-3.63 -3.52

CH3OH

Bridge Ni1-O/Ni2-O

2.338/2.063

-0.22

-0.29

H 2O

Atop Ni-O

2.079

-0.20

-0.28

a

Bridge Ni1-O1/Ni2-O2

The value in the brackets are the binding energies corrected by DFT-D3 method.


-4.28 -4.12 -0.52 -0.51 (-0.32) (-0.31)

-0.76 -0.76 (-0.49) (-0.49)

-0.63 -0.57 (-0.40) (-0.35) -0.55 -0.47 (-0.25) (-0.16)


Figure 4 The optimized structures of potential intermediates in the process of CH3OH synthesis on the stepped Ga3Ni5(111) surface.


Page 16 of 50

Page 17 of 50 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


3.2.3 The adsorption of products In this study, the most stable CH3OH configuration is found to adsorb at bridge sites composed by Ni atoms via one O atom, with two Ni-O bond distances of 2.338 and 2.063 Å, respectively. The adsorption energy of CH3OH was calculated to be -0.57 eV, the binding energies corrected by DFT-D3 method is -0.35eV. H2O prefers to bind at Ni atop sites via the O atom with a bond distance of 2.079 Å, and with an adsorption energy of -0.47 eV. After corrected by DFT-D3 method, the binding energy becomes to be -0.16 eV. Compared to other strongly adsorbed species, the weak interaction of CH3OH and H2O with the catalyst surface make them desorb easily, which is beneficial for CO2 hydrogenation, shown in the reaction, below.

CO2 + 3H 2 → CH 3OH + H 2 O

(E9)

Table 2 displays the adsorption energies of the considered species on the stepped Ga3Ni5(111) surface, and a comparison was made with those on the Cu(111)73 and a flat Ga3Ni5(221)36 surfaces. From the table, It can be seen that all species on Ga3Ni5(111) prefer to adsorb at Ni sites at the step edge and the adsorption energy is higher than that on Cu(111) and Ga3Ni5(221) surfaces for each species, which can be attributed to the step edge effect, which has been proposed by many researchers.37-42, 46, 95 On the Ga3Ni5(221) surface,36 it was found that, except for the O atom, all the other species prefer to adsorb at Ni sites, while in present studies, all the species adsorb at Ni sites. Therefore, in the Ga3Ni5 crystal, not only on the flat (221) surfaces, but also on the stepped (111) surfaces, Ni sites are the main active sites. It is remarkable that on Cu(111) surfaces,73 the adsorption energies of HCOOH and H2CO are -0.24 and -0.12 eV, respectively. The low adsorption energies show that these two species prefer to desorb than to be hydrogenated, which mean that they may be the ‘dead end’ of CO2



Page 18 of 50

hydrogenation to CH3OH, and lower the rate of CH3OH formation. Similary, on Ga3Ni5(221),36 the weak adsorption of HCOOH has the same effect. In the present study, the high adsorption energies of HCOOH and H2CO can at least avoid the ‘dead end’ problem. In addition, the adsorption energy of CO2 on the Ga3Ni5(111) surface is higher than Ga3Ni5(221) and Cu(111) by 0.61 and 0.47 eV, respectively, which mean that CO2 is highly activated on Ga3Ni5(111) surface.

3.3 Kinetics and thermodynamics of elementary steps and the optimal reaction pathway during CO2 hydrogenation to CH3OH on the Ga3Ni5(111) surface To probe the reaction mechanisms of CH3OH synthesis from CO2 and H2 on the stepped Ga3Ni5(111), thermodynamic and kinetic parameters of the elementary steps on the stepped are displayed in Table 3. The reaction scheme of CH3OH synthesis from CO2 and H2 is given in Figure 5. Studt and coworkers34 demonstrated that Ga3Ni5 facilitates CH3OH synthesis, and at temperatures above 493 K, the yield of CH3OH is significantly higher than that on Cu/ZnO/Al2O3. Therefore, reaction Gibbs free energy (∆G) and rate constants (k and kr) of the elementary steps at 500 K were calculated. In addition, the structures of initial states (ISs), transition state (TSs), and final state (FSs) of the elementary steps are displayed in Figure S2 of Section 2 in the Supporting Infromation.

Table 3 The activation barrier (Ea), reaction energy (ΔE), reaction Gibbs free energy (∆G) (500K) and reaction rate constants (k and kr are forward reaction rate constants and the corresponding reverse reaction rate constants, respectively) (500K) of the elementary steps in the CH3OH synthesis process from CO2 and H2 on the stepped Ga3Ni5(111) surface. Ea, ΔE and ∆G are in eV, k and kr are in s-1. No.

Reaction

R0

CO2(g)→CO2

R1

H2→H+H

Ea

EaZPE

∆E

∆EZPE

∆G

k

kr

-

-

-0.48

-

-

-

-

0.03

-0.03

-0.74

-0.73

-0.69

1.60×1013

1.90×106


Page 19 of 50 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


No.

Reaction

Ea

EaZPE

∆E

∆EZPE

∆G

k

kr -7

1.46×10-11

R2

CO2(g)+H→bi-HCOO

1.97

1.94

-0.49

-0.45

-0.45

5.21×10

R3

CO2+H→bi-HCOO

0.56

0.63

-0.63

-0.47

-0.55

2.59×106

7.58

R4

CO2+H→trans-COOH

0.69

0.65

-0.22

-0.06

-0.07

1.88×106

3.50×105

R5

CO2→CO+O

1.13

1.08

-0.79

-0.83

-0.83

9.67×102

3.92×10-7

R6

bi-HCOO→mono-HCOO

0.66

0.62

0.57

0.53

0.55

3.99×106

3.99×1012

R7

bi-HCOO→HCO+O

1.76

1.69

0.74

0.67

0.65

1.43×10-4

4.66×102

R8

bi-HCOO+H→H2COO

1.59

1.59

0.91

1.00

1.00

2.49×10-3

2.83×107

R9

bi-HCOO+H→HCOOH

1.06

0.94

0.83

0.91

0.89

5.41×103

5.47×1012

R10

H2COO+H→H2COOH

0.57

0.48

-0.58

-0.44

-0.49

8.40×107

1.08×103

R11

HCOOH+H→H2COOH

0.94

0.92

-0.04

0.13

0.18

9.70×103

6.76×105

R12

H2COOH→H2CO+OH

0.66

0.52

0.04

-0.07

-0.06

3.39×107

7.90×106

R13

HCOOH→HCO+OH

0.49

0.47

-0.28

-0.38

-0.37

1.75×108

3.27×104

R14

HCO+H→H2CO

0.55

0.54

0.07

0.22

0.22

4.59×108

8.01×109

R15

H2CO+H→CH2OH

1.18

1.07

0.44

0.53

0.60

6.29×102

6.44×107

R16

H2CO+H→CH3O

0.43

0.42

-0.37

-0.27

-0.30

5.46×108

4.74×105

R17

trans-COOH→cis-COOH

0.51

0.45

0.01

0.01

0.03

1.26×109

2.61×109

R18

cis-COOH→CO+OH

0.99

0.87

-0.85

-0.92

-1.01

1.56×104

9.28×10-7

R19

CO+H→COH

1.78

1.71

0.86

0.97

0.96

3.65×10-5

1.69×105

R20

CO+H→HCO

1.05

1.08

0.96

1.05

1.05

3.89×102

1.43×1013

R21

trans-COOH+H→t,t-COHOH

0.73

0.72

-0.09

0.08

0.06

4.70×105

2.05×106

R22

t,t-COHOH→t,c-COHOH

0.42

0.40

0.06

0.08

0.10

2.49×108

2.43×109

R23

t,c-COHOH→c,c-COHOH

0.41

0.39

0.40

0.33

0.37

5.25×108

3.15×1012

R24

t,t-COHOH→COH+OH

1.10

1.03

-0.34

-0.42

-0.47

1.99×102

3.47×10-3

R25

t,c-COHOH→COH+OH

0.99

0.87

-0.43

-0.48

-0.48

1.74×104

2.57×10-1

R26

c,c-COHOH→COH+OH

0.63

0.63

-0.78

-0.82

-0.89

1.30×106

1.46×10-3

R27

COH+H→HCOH

0.53

0.52

0.17

0.30

0.22

2.42×108

4.26×1010

R28

HCOH+H→CH2OH

0.43

0.44

0.15

0.22

0.22

4.00×108

6.51×1010

R29

CH2OH+H→CH3OH

0.30

0.25

-0.38

-0.26

-0.31

3.60×1010

2.55×107

R30

CH3O+H→CH3OH

1.30

1.27

0.58

0.74

0.74

5.16×10-1

1.52×107

R31

O+H→OH

1.16

1.10

-0.29

-0.19

-0.21

1.20×102

8.22×10-1

R32

OH+H→H2O

0.95

0.85

0.33

0.45

0.40

1.87×104

1.99×108

R33

CH3OH→CH3OH(g)

0.57

-

0.57

-

-

-

-

R34

H2O→H2O(g)

0.47

-

0.47

-

-

-

-



Page 20 of 50

CO2(g) R0

mono-HCOO

R6

bi-HCOO R9

H2COO

HCOOH R11

O R31

OH R32

H 2O

H2COOH

R3

CO2

*

R4

trans-COOH R21

R5 R18

CO

cis-COOH

t,t-COHOH R22

R20

t,c-COHOH

HCO

R12

R23

H2CO

COH

R16

CH3O

R26 c,c-COHOH

R27

CH2OH

R34

R29

H2O(g)

CH3OH

R28

R33

HCOH CH3OH(g)

Figure 5 The reaction scheme of CH3OH synthesis from CO2 and H2 on the stepped Ga3Ni5(111) surface.

3.3.1 H2 dissociation R1: H2→H+H. As discussed before, dissociative H2 adsorption is spontaneous on the stepped Ga3Ni5(111) surface. The reaction is highly exothermic with a reaction energy of -0.73 eV. The initial state (IS), transition state (TS), and final state (FS) of H2 dissociation are displayed in Figure 6. Molecular hydrogen first adsorbs at the atop sites of the step edge, and after the H-H bond splitting, the two bound H atoms migrate to a 3-fold hollow site and a bridge site, which are composed of Ni atoms at the step edge. The Behrens et al.42 reported that the activation barriers of molecular hydrogen decomposition are 0.84 and 0.74 eV on Cu(111) and Cu(211) surfaces, respectively. Zhang et al.96 studied H2 dissociation on the Cu2O(111) surface and the activation barrier is 0.84 eV. These results show that the Ga3Ni5(111) surface is beneficial for H2 decomposition and subsequent CO2 hydrogenation as. The spontaneous decomposition of H2 can provide a large number of H atoms for successive hydrogenation reactions.


Page 21 of 50 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


(a) IS

(b) TS

(c) FS

Figure 6 The (a) IS, (b) TS, and (c) FS of H2 dissociation on the stepped Ga3Ni5(111) surface. 3.3.2 CO2 hydrogenation and dissociation R2: CO2(g)+H→bi-HCOO. The Eley-Rideal (ER) path of CO2 hydrogenation was studied first. In this process, only R2 can directly happen, while in the trans-COOH generation and CO2 dissociation reactions, CO2 should adsorb on the surface first (R0). As for R2, the activation barrier is 1.94 eV, with a reaction energy of -0.45 eV. The rate constant of this reaction is 5.21× 10-7, with a reaction Gibbs free energy of -0.46 eV.

R3: CO2+H→bi-HCOO, R4: CO2+H→trans-COOH and R5: CO2→CO+O. In addition, the Langmuir-Hinshelwood (LH) pathway was also studied. The three reactions are all exothermic, with the reaction energies of -0.47, -0.06 and -0.83 eV, respectively. The activation barriers are 0.63, 0.65 and 1.08 eV, respectively, which means that R3 and R4 both have low barriers and are compatible. The calculated rate constants of R3 and R4 are of magnitude 106, which are 4 orders of magnitude higher than that of R5. The ISs, TSs, and FSs of the three reactions are shown in Figure 7. In all, the reaction of CO2 with H prefers the LH path due to its low activation barriers and high rate constants. From Table 3, it can be seen that, although all the reactions are exothermic and with reaction Gibbs free energies below zero, the reactions R3, R4, and R5 in the LH mechanism are 13, 13, and 9



orders of magnitude higher than that of in ER path, respectively. In previous study,73 CO2 hydrogenation was also found to prefer the LH pathway on Cu(111) surface, and the generation of bi-HCOO and trans-COOH have similar activation barriers and orders of magnitude (106) of rate constants, which is 4 orders of magnitude faster than CO2 dissociation. Hence, the first step in CO2 hydrogenation to CH3OH is the reaction of adsorbed CO2 with adsorbed H to bi-HCOO or trans-COOH, which differs from previous study,36 which found that CO2 dissociation is easier than hydrogenation on the Ga3Ni5(221) surface, and the formation of HCOO more readily occurs than the formation of COOH. Zhao et al.73 and Yang et al.31 also found that CO2 hydrogenation to HCOO is much easier than to COOH on Cu(111) surface and metal-doped Cu(111) surfaces, respectively.


Page 22 of 50

Page 23 of 50 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


Figure 7 The side views and top views of ISs, TSs and FSs involving in CO2 hydrogenation and dissociation via ER and LH pathways on the stepped Ga3Ni5(111) surface.

3.3.3 Successive reactions R6: bi-HCOO→mono-HCOO, R7: bi-HCOO→HCO+O, R8: bi-HCOO+H→H2COO and R9: bi-HCOO+H→HCOOH. The isomeric conversion reaction (R6) has an activation barrier of



0.62 eV and a reaction energy of 0.53 eV (seeing in Table 3), and its reverse reaction happens readily, with a barrier of 0.09 eV, which means that the reverse reaction is almost spontaneous. R7, the dissociation of bi-HCOO, is endothermic (0.67), with a very high energy barrier (1.69 eV), suggesting this reaction will not readily occur. R8 and R9 are the hydrogenation of biHCOO. Both of the reactions are highly endothermic (0.99 and 0.91 eV, respectively), with activation barriers of 1.59 eV and 0.94 eV for R8 and R9, respectively. All four reactions are endothermic, with reverse reactions’ rate constants more than 6 orders of magnitude higher than the corresponding forward reaction. The high barriers, the high endothermicity, as well as the high speed of the reverse reactions of the conversion, dissociation, and hydrogenation of biHCOO indicate that they are unlikely to occur on the stepped Ga3Ni5(111) surface.

R10: H2COO+H→H2COOH, R11: HCOOH+H→H2COOH and R12: H2COOH→H2CO + OH. For R10, the activation barrier is 0.48 eV, with a reaction energy of -0.44 eV. R11

reaction’s activation barrier is higher than that of the former by 0.44 eV, with a reaction energy of 0.13 eV. Therefore, H2COO can be hydrogenated to H2COOH much easier than HCOOH. The rate constants also can explain this, because R10 has a much higher reaction rate constant (~108) than R11(~104). R12, the dissociation of H2COOH, is slightly exothermic (-0.07 eV), with a low activation barrier of 0.52 eV, which means it occurs readily.

R13: HCOOH→HCO+OH. HCOOH sometimes is considered the ‘dead end’ of the CO2 hydrogenation to CH3OH,97 because of its low adsorption energy, that is, HCOOH prefers to desorb than to hydrogenate or dissociate. While in present study, it is different. Although the reaction of HCOOH hydrogenation has a higher activation barrier of 0.92 eV, HCOOH dissociation is much easier, with an activation barrier of just 0.47 eV, which is 0.29 eV lower


Page 24 of 50

Page 25 of 50 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


than the desorption barrier. The dissociation reaction has a rate constant of approximately 1.75× 108. Therefore, in this study, HCOOH prefers to dissociate than to desorb or hydrogenate.

R14: HCO+H→H2CO. R14 is one of the formation reactions of H2CO. This reaction is endothermic by 0.22 eV. For this reaction, the rate constant of reverse reaction is two orders magnitude higher than that of the forward reaction. Despite this, the forward reaction may occur due to the high concentration of H atoms. In addition, the reaction barrier is low (0.54 eV).

R15: H2CO+H→CH2OH and R16: H2CO+H→CH3O. From Table 3, we can see that the activation barrier for R15 is 1.07 eV, while for reaction R16, it is only 0.42 eV. The reaction R15 is endothermic (0.44 eV), whereas R16 is exothermic with a reaction energy of -0.27 eV. From the perspective of reaction rate constants, the K value of R16 is approximately 108, while for R15, it is only 102. In a previous study,73 the adsorption energy of H2CO is only -0.12 eV on Cu(111), which means that H2CO prefers to desorb from the surface. In present study, the adsorption energy is -1.40 eV, which is much higher than the activation barrier (0.42) of H2CO hydrogenation to CH3O. All the results show that H2CO hydrogenation to CH3O is much easier than to CH2OH.

R17: trans-COOH→cis-COOH, R18: cis-COOH→CO+OH, R19: CO+H→COH and R20: CO+H→HCO. R17 is almost thermoneutral, with a reaction energy of 0.01 eV. Thus, although

this conversion has a low barrier of 0.45 eV, its reverse conversion also has a low barrier, too. In addition, the calculated rate constant of its reverse reaction is about two times faster than this forward reaction’s. Most importantly, the dissociation of COOH was studied. In the calculation process, it was found that trans-COOH dissociation to CO and OH occurs only via cis-COOH intermediates. R18 is the dissociation of cis-COOH to CO and OH. The reaction is highly exothermic with a reaction energy of -0.92 eV. However, the activation barrier is high (0.87 eV).



The activation barrier of R19 is 1.78 eV, with a reaction energy of 0.97 eV. The activation barrier of R20 is 1.08 eV, with reaction energy of 1.05 eV, which means that the reverse reaction is almost spontaneous. The reverse reactions’ rate constants of the two reactions are ten to eleven orders of magnitude higher than their corresponding forward reactions.

R21: trans-COOH+H→t,t-COHOH, R22: t,t-COHOH→t,c-COHOH and R23: t,cCOHOH→c,c-COHOH. The activation barrier of R21 is 0.72 eV, with a reaction energy of 0.08

eV. The activation barriers of R22 and R23 are 0.40 and 0.39 eV, respectively, with reaction energies of 0.08 and 0.33 eV, respectively, indicating that the reverse reaction of R23 happens more readily. The reverse reaction rate constant of R23 is 4 orders of magnitude higher than the forward reaction.

R24: t,t-COHOH→COH+OH, R25: t,c-COHOH→COH+OH and R26: c,c-COHOH→ COH +OH. The three reactions are the dissociation of the three isomers of COHOH. All the

three reactions are exothermic, with the reaction energies of -0.42, -0.48, and -0.82 eV, respectively. As for their activation barriers, c,c-COHOH dissociation (R26) has the lowest activation barrier (0.63 eV). For the dissociation reactions, the rate constants of the forward reactions are much higher than those of the corresponding reverse reactions, suggesting that these reactions, especially R26, are the most likely reactions to occur in the reaction mechanism.

R27: COH+H→HCOH and R28: HCOH+H→CH2OH. R27 is the further hydrogenation of COH. The activation barrier of this reaction is 0.52 eV, with a reaction energy of 0.30 eV. R28 is the further hydrogenation of HCOH. For this reaction, the activation barrier is 0.44 eV, with a reaction energy of 0.22 eV. The low barriers of the two reactions show that they occur readily.

3.3.4 Products formation


Page 26 of 50

Page 27 of 50 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


R29: CH2OH +H→CH3OH and R30: CH3O+H→CH3OH. CH3OH formation is the last step in the CH3OH synthesis process from CO2 and H2. For the former reaction, the activation barrier is only 0.25 eV, with a reaction energy of -0.26 eV. Most importantly, the rate constant is 3.60 × 1010. All the parameters show that R29 happens readily. As discussed before, CH3O formation is much easier, however, its hydrogenation barrier is much higher (1.27 eV). In addition, this reaction is highly endothermic with a reaction energy of 0.74 eV. In terms of the two reactions, CH3OH is formed by CH2OH hydrogenation.

R31: O+H→OH and R32: OH+H→H2O. R31 and R32 are the formation reactions of H2O. The sources of O may be the dissociation of CO2 and bi-HCOO. The activation barrier of R31 is 1.10 eV, with a reaction energy of -0.19 eV. As for R32, the activation barrier is 0.85 eV after ZPE correction, with a reaction energy of 0.45 eV. The activation barrier is 0.99 eV on Cu(111) surface after ZPE correction, which is 0.14 eV higher than that in our present study. As discussed in section 3.3.1, CO2 hydrogenation mainly occurs via the LH pathway, genetating both bi-HCOO and trans-COOH as theirformation barriers are almost equal (0.63 and 0.65, respectively), and the corresponding reaction rate constants are in the same order of magnitude. As to the dissociation of CO2, the activation barrier is 1.08 eV. Although the formation of bi-HCOO is easy, the following reactions (R7, R8 and R9) have higher barriers of 1.69, 1.59 and 0.94 eV, respectively. While for the trans-COOH transformation, the successive reaction (R21) has a relative low barrier of 0.72 eV. Most importantly, the reactions (R22, R23, R26, R27, R28 and R29) following R21 all have barriers of no more than 0.72 eV, which is lower than any pathway according to bi-HCOO or CO. In addition, the activation barrier of OH hydrogenation to H2O is 0.85 eV. Thus, the most likely pathway for CO2 hydrogenation to CH3OH follows the reaction sequence R0→R4→R21→R22→R23→R26→R27→R28→R29→



R33→R32→R34 based on the analysis of the reaction energies and activation barriers. This sequence generates trans-COOH, t,t-COHOH, t,c-COHOH, c,c-COHOH, COH, HCOH, CH2OH and OH intermediates, as shown in Figure 8. In this pathway, the rate limiting step98 is the formation of H2O (R32), which has the highest activation barrier of 0.85 eV and the lowest rate constant of 1.87×104 in the entire pathway.

Figure 8 The most likely reaction pathway of CH3OH synthesis from CO2 and H2 on the stepped Ga3Ni5(111) surface.

3.4. Adsorption energies of intermediates and kinetic and thermodynamic of elementary steps involved in by-product (methane) formation in the process of CO2 hydrogenation to CH3OH


Page 28 of 50

Page 29 of 50 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


Methane (CH4) is one of by-products in the process of CO2 hydrogenation to CH3OH. Thus, the adsorption energies of intermediates and the activation barriers of the elementary steps involved in the process of CH4 formation are calculated.

3.4.1 The adsorption structures and adsorption energies of potential intermediates involved in CH4 formation. The binding parameters and adsorption energies of C, CH, CH2, CH3, CH4 species are shown in Table 4, the most stable configurations are displayed in Figure 9. As shown in Table 4 and Figure 9, C, CH and CH2 prefer to adsorb at 3-fold hollow sites composed by Ni atoms, with the adsorption energies of -8.47, -7.01 and -5.08 eV, respectively. However, for CH3, the most stable site is the bridge site at the step edge, and the C atoms bond with two Ni atoms. CH4 also prefers to adsorb at the bridge site, but with two H atoms binding with two Ni atoms, respectively. The adsorption energy of CH4 is -0.28 eV.

Table 4 The binding parameters and adsorption energies of reactants, potential intermediates and products in the process of CH4 formation on the stepped Ga3Ni5(111) surface. D is the ZPE distance of corresponding binding bond, which is in Å. Eads and E ads are the adsorption energies

on Ga3Ni5(111) surface without and with ZPE correction, respectively, which are in eV. D

Eadsa

ZPE E ads

Ni1-C/Ni2-C/Ni3-C

1.75/1.78/1.80

-8.47

-8.47

3-fold

Ni1-C/Ni2-C/Ni3-C

1.84/1.84/1.89

-7.17

-7.01

CH2

3-fold

Ni1-H/Ni2-C/Ni3-C

1.73/1.91/1.99

-5.22

-5.08

CH3

Bridge

Ni1-C/Ni2-C

2.03/2.01

-3.22

-3.14

CH4

Bridge

Ni1-H1/Ni2-H2

2.34/2.39

-0.08

-0.28

Species

Sites

Binding bonds

C

3-fold

CH



Figure 9 The optimized structures of potential intermediates in CH4 formation on the stepped Ga3Ni5(111) surface.

3.4.2 Kinetic and thermodynamic of elementary steps involved in CH4 formation The calculated activation barriers, reaction Gibbs free energies and reaction rate constants of the elementary steps included in the CH4 synthesis are presented in Table 5. The structures of initial states (ISs), transition state (TSs), and final state (FSs) of the elementary steps are displayed in Figure S2.

R35: CO→C+O, R36: C+H→CH, R37: CH+H→CH2, R38: CH2+H→CH3 and R39: CH3+H → CH4. CO dissociation reaction (R35) has an activation barrier of 2.77 eV, with an

activation barrier 1.70 eV. The calculated rate constant of this reaction is in a magnitude of -16. The results show that CO dissociation is difficult to happen. R36, R37, R38 and R39 are the reactions related to CHx (x=1-4) formation, and the corresponding activation barriers with ZPE correction are 0.64, 0.40, 0.38 and 0.88 eV, respectively. For R36 and R38, the reaction energies are -0.47 and -0.24 eV, respectively. R37 is almost thermoneutral, with the reaction energy of 0.06 eV, while for R39, the reaction energy is 0.43 eV.


Page 30 of 50

Page 31 of 50 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


Table 5 The activation barriers (Ea), reaction energies (ΔE), reaction Gibbs free energies (∆G) (500K) and reaction rate constants (k and kr) (500K) of the elementary steps in the CH4 synthesis. No.

Ea

Reaction

EaZPE

∆E

∆EZPE

∆G

k

kr -16

1.54×102

R35

CO→C+O

2.81

2.77

1.70

1.70

1.74

7.84×10

R36

C+H→CH

0.69

0.64

-0.54

-0.47

-0.47

4.30×106

7.76×10

R37

CH+H→CH2

0.42

0.40

0.01

0.06

0.05

8.73×108

2.55×109

R38

CH2+H→CH3

0.39

0.38

-0.34

-0.24

-0.35

1.24×109

4.07×105

R39

CH3+H→CH4

0.93

0.88

0.35

0.43

0.43

5.03×104

1.10×109

3.5 Microkinetic modeling In order to estimate the selectivity of the major production of CH4 and CH3OH on Ga3Ni5(111) surface, microkinetic modeling99-102 has been used to calculate the reaction rate of the major production and the overall reaction rate under experimental conditions (1atm, CO2:H=1:3, T=500~600 K). The detailed information about microkinetic modeling is given in Section 3 of Supporting Information. In the estimation, only the optimal reaction routes are considered for each production. The elementary steps and the corresponding reaction rate constants at the temperature of 500, 525, 550, 575 and 600 K included in the microkinetic modeling are displayed in Table S1. The reaction rates of CH3OH and CH4 are calculated using equations of rCH 4 = k 39θ CH 3 θ H and rCH 3OH = k 29θ CH 2OH θ H , respectively. The productivity of CH3OH is 3.26×10-14 s-1 pre site at 500 K, which is higher than that of CH4 (3.92×10-16 s-1 pre site). Similary, the productivity of CH3OH and CH4 at 525, 550, 575 and 600 K can be obtained. The relative selectivity of CH4 and CH3OH

are

defined

as

S CH = [100rCH /(rCH + rCH OH )]% 4

4

4

3

and

S CH OH = [100rCH OH /(rCH + rCH OH )]% , respectively, and the results are presented in Figure 10. 3

3

4

3



Form Figure 10, it can be seen that the relative reaction selectivity of CH3OH is close to 99%, which is much higher than that of CH4. The relative selectivity of CH3OH rises slowly with the increase of temperature from 500 to 600 K, while the relative selectivity of CH4 decreases slowly. All these results show that Ga3Ni5(111) surface shows much higher selectivity towards CH3OH formation than CH4 formation.

Figure 10

The relative reaction selectivity of CH4 and CH3OH in the process of CO2

hydrogenation to CH3OH at 500, 525, 550, 575 and 600 K using microkinetic modeling.

3.6 General discussion As mentioned before, the active sites on stepped Ga3Ni5(111) surface are Ni sites at the step edge and the adsorption energies are all higher than those on Ga3Ni5(221) surface. Therefore, the stepped Ga3Ni5(111) surface is more active than the flat Ga3Ni5(221) surface. Not only on the stepped (111) surface, but also on the flat (221) surface for the Ga3Ni5 catalyst, the active sites are composed of Ni atoms, that is, the reaction sites. In addition, it is also found that the adsorption energes of the adsorbates involved in CH3OH synthesis on Cu(111) surface are all lower than those on Ga3Ni5(111) surface, especially for


Page 32 of 50

Page 33 of 50 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


some key intermediates, i. e. HCOOH and H2CO. This may indicates that Ga3Ni5(111) is more active than Cu(111) surface. As shown in section 3.3, CH3OH synthesis occurs via a COOH pathway. It is noticeable that it is rarely reported that CO2 hydrogenation to CH3OH is via COOH pathway. Only our previous study on Ga3Ni5(221) surface36 and a study in the presence of H2O on Cu(111) surface by Zhao and coworkers73 reported that CO2 hydrogenation to CH3OH may via intermediate COOH. Most studies suggest that CO2 hydrogenation to CH3OH occurs via HCOO31, 42, 75, 86, 103104

or rWGS with CO hydrogenation.31, 86 . Grabow and coworkers75 held that CH3OH synthesis

from CO2 is via HCOO, HCOOH, H2COOH, H2CO and CH3O intermediates on Cu(111) surface. They particularly pointed the hydrogenation product of HCOO is HCOOH instead of H2COO. While Nakatsuji and coworker103 reported that on Cu(100) surface, HCOO prefers to hydrogenate to H2COO, which exactly is the rate-limiting step. Similarly, Hus et al.32 also reported that CO2 hydrogen proceed HCOO, then to H2COO on spinel-type Cu/ZnAl2O4 catalyst. Tang et al.86 considered that both HCOO pathway and rWGS with CO hydrogenation contributed to the yield of CH3OH on Cu/ZrO2 catalyst. Yang et al.31 have studied CO2 hydrogenation to CH3OH on Au, Pd, Rh, Pt and Ni doped Cu(111) surface. Their results show that except Au-doped Cu(111) surface prefer HCOO pathway, the others are all more likely to follow rWGS with CO hydrogenation pathway. In our previous work,36 the reaction mechanism of CO2 hydrogenation to CH3OH was studied on a flat Ga3Ni5(221) surface, and we found that the COOH pathway is the most favorable one for CH3OH synthesis and the rate-limiting step is trans-COOH formation. In addition, the previous study36 also pointed out that the presence of Ga indeed promotes the hydrogenation reactions. In present study, although we also consider that CH3OH synthesis is via



COOH pathway, the rate-limiting step is H2O formation and the activation barriers of all the elementary steps involved in the pathway are all much lower on the Ga3Ni5(111) surface than the corresponding steps on the Ga3Ni5(221) surface. Therefore, the stepped Ga3Ni5(111) surface lowers the activation barriers of the most likely pathway compared with the flat Ga3Ni5(221) surface. Previous study by Zhao et al.73 also suggested that COOH pathway is the most favorable way in the presence of H2O for synthesizing CH3OH on Cu(111). In their study, the rate-limiting step is also H2O formation, and the activation barrier is 0.99 eV, which is 0.14 eV higher than that in this study. Therefore, the Ga3Ni5(111) surface is more active than Cu(111) surface on CH3OH synthesis. Interestingly, Behrens et al.42 calculated the hydrogenation of CO2 to CH3OH on Cu(211) step surface. Their results show that CO2 is hydrogenated to CH3OH via HCOO, and the highest activation barrier in the process is 1.32 eV, which is 0.47 eV higher than the highest activation barrier in the most possible reaction route. For H2O formation, the activation barrier is 1.22 eV. All the results show that Ga3Ni5(111) surface is more active than Cu(211) surface on CO2 hydrogenation to CH3OH. The stronger adsorption for adsorbates and the lower activation barriers for the most potential pathway indicating that the stepped Ga3Ni5(111) surface has a step edge effect and is better than the traditional catalyst on CH3OH synthesis. It is worth mentioning that the studies about the active site of CO2/CO hydrogenation to CH3OH on Cu/ZnO/Al2O3 catalyst by Studt et al.42 suggested that the requirements for the high activity are the presence of steps on the surface

and defects on the steps. As for the Ga3Ni5 catalyst, based on the discussions above, the requirements for the high activity may be the presence of steps and that Ga exist on the steps but


Page 34 of 50

Page 35 of 50 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


not on the step edge. Moreover, based on the analysis of microkinetic modeling, the relectivity of CH3OH can reach to 99%, which is much higher than that of CH4.

4. Conclusions In this work, DFT was used to study the mechanisms of CH3OH synthesis from CO2 and H2 on a Ga3Ni5(111) surface. To choose an appropriate surface to study, the surface energies, CO2 adsorption energies, and dissociative H2 adsorption energies of five selected facets ((001), (021), (110), (111) and (221)) were studied. The results show that termination III of the (111) surface not only has low surface energy, but also can activate CO2 highly. Most importantly, on all four terminations of (111), dissociative H2 adsorption is spontaneous. Thus, the (111) surface with termination III was employed to study the CO2 hydrogenation process. The adsorption energies of twenty-three species including two reactants, nineteen potential intermediates, and two products were calculated. In comparison with previous results on the Ga3Ni5(221) flat surface, all the species’ adsorption energies on the (111) surface are higher than those on the (221) surface, which may be attributed to the step edge effect. In addition, a comparison was also made with the results on Cu(111), in which the Ga3Ni5(111) surface also shows stronger adsorption capacity. Futhermore, some thermodynamic and kinetic parameters including the activation barriers, reaction energies, rate constants, and reaction Gibbs free energies of thirty-five elementary steps were calculated. The results show that the most likely reaction pathway for CH3OH formation on a Ga3Ni5(111) surface is via the following reactions: R0 → R1 → R4 → R21 → R22 → R23 → R26 → R27 → R28 → R29 → R33 → R32 → R34, the same as shown by previous study on a Ga3Ni5(221) surface. Interestingly, the differenc is that, in this study, the process of CO2 hydrogenation to trans-COOH is not the rate-limiting step and has a low activation barrier. In this study, the calculated rate-limiting step is the formation of H2O



from OH and H, with the highest activation barrier (0.85 eV) and the lowest rate constant (~104). Moreover, comparing with CH4, CH3OH has higher relative selectivity on Ga3Ni5(111) surface based on the microkinetic modeling. In addition, stepped Ga3Ni5(111) surface is more active than stepped Cu(211) surface as we discussed. Results also suggest that the requirements for the high activity of Ga3Ni5 catalyst are the presence of steps and that Ga exist on the steps but not in the step edge.

Conflicts of interest There are no conflicts to declare.

Supporting Information Supporting Information available: The structures of surfaces (Part 1), the structures of all the elementary steps (Part 2), and microkinetic modeling (Part 3).

Acknowledgements This study was supported by the China Scholarship Council, and National Natural Science Foundation of China (21537002 and 21177083). The computational calculation was performed using the resources of the Advanced Research Computing Center (2012. Mount Moran: IBM System X cluster. Laramie, WY: University of Wyoming) and the USNSF-sponsored NCARWyoming Supercomputing Center (NWSC).


Page 36 of 50

Page 37 of 50 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


References (1) Peng, G. W.; Sibener, S. J.; Schatz, G. C.; Ceyer, S. T.; Mavrikakis, M. CO2 hydrogenation to formic acid on Ni(111). J. Phys. Chem. C 2012, 116, 3001-3006. (2) He, Y. M.; Zhang, L. H.; Teng, B. T.; Fan, M. H. New application of Z-scheme Ag3PO4/gC3N4 composite in converting CO2 to fuel. Environ. Sci. Technol. 2015, 49, 649-56. (3) Motokura, K.; Kashiwame, D.; Miyaji, A.; Baba, T. Copper-catalyzed formic acid synthesis from CO2 with hydrosilanes and H2O. Org. Lett. 2012, 14, 2642-2645. (4) Peng, G. W.; Sibener, S. J.; Schatz, G. C.; Mavrikakis, M. CO2 hydrogenation to formic acid on Ni(110). Surf. Sci. 2012, 606, 1050-1055. (5) Zou, F. L.; Cole, J. M.; Jones, T. G. J.; Jiang, L. Rhodium nitrosyl catalysts for CO2 hydrogenation to formic acid under mild conditions. Appl. Organomet. Chem. 2012, 26, 546-549. (6) Cao, L. L.; Sun, C. Z.; Sun, N.; Meng, L.; Chen, D. Z. Theoretical mechanism studies on the electrocatalytic reduction of CO2 to formate by water-stable iridium dihydride pincer complex. Dalton Trans. 2013, 42, 5755-5763. (7) Liu, D. P.; Wang, Y. F.; Shi, D. M.; Jia, X. L.; Wang, X.; Borgna, A.; Lau, R.; Yang, Y. H. Methane reforming with carbon dioxide over a Ni/ZiO2-SiO2 catalyst: Influence of pretreatment gas atmospheres. Int. J. Hydrogen Energy 2012, 37, 10135-10144. (8) Park, S.; Bezier, D.; Brookhart, M. An efficient iridium catalyst for reduction of carbon dioxide to methane with trialkylsilanes. J. Am. Chem. Soc. 2012, 134, 11404-11407. (9) Zhu, Y.; Zhang, S. R.; Ye, Y. C.; Zhang, X. Q.; Wang, L.; Zhu, W.; Cheng, F.; Tao, F. Catalytic conversion of carbon dioxide to methane on ruthenium-cobalt bimetallic



nanocatalysts and correlation between surface chemistry of catalysts under reaction conditions and catalytic performances. ACS Catal. 2012, 2, 2403-2408. (10) Bothra, P.; Periyasamy, G.; Pati, S. K. Methane formation from the hydrogenation of carbon dioxide on Ni(110) surface - a density functional theoretical study. Phys. Chem. Chem. Phys.

2013, 15, 5701-5706. (11) Hirunsit, P. Electroreduction of carbon dioxide to methane on copper, copper-silver, and copper-gold catalysts: A DFT study. J. Phys. Chem. C 2013, 117, 8262-8268. (12) Rodriguez, J. A.; Evans, J.; Feria, L.; Vidal, A. B.; Liu, P.; Nakamura, K.; Illas, F. CO2 hydrogenation on Au/TiC, Cu/TiC, and Ni/TiC catalysts: Production of CO, methanol, and methane. J. Catal. 2013, 307, 162-169. (13) Janke, C.; Duyar, M. S.; Hoskins, M.; Farrauto, R. Catalytic and adsorption studies for the hydrogenation of CO2 to methane. Appl. Catal. B-Environ. 2014, 152, 184-191. (14) Ahmad, W.; Al-Matar, A.; Shawabkeh, R.; Rana, A. An experimental and thermodynamic study for conversion of CO2 to CO and methane over Cu-K/Al2O3. J. Environ. Chem. Eng.

2016, 4, 2725-2735. (15) Melo, C. I.; Szczepanska, A.; Bogel-Lukasik, E.; da Ponte, M. N.; Branco, L. C. Hydrogenation of carbon dioxide to methane by ruthenium nanoparticles in ionic liquid. ChemSusChem 2016, 9, 1081-1084.

(16) Kierzkowska-Pawlak, H.; Tracz, P.; Redzynia, W.; Tyczkowski, J. Plasma deposited novel nanocatalysts for CO2 hydrogenation to methane. J. CO2 Util. 2017, 17, 312-319. (17) Martin, N. M.; Velin, P.; Skoglundh, M.; Bauer, M.; Carlsson, P. A. Catalytic hydrogenation of CO2 to methane over supported Pd, Rh and Ni catalysts. Catal. Sci. Technol. 2017, 7, 1086-1094.


Page 38 of 50

Page 39 of 50 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


(18)Natesakhawat, S.; Lekse, J. W.; Baltrus, J. P.; Ohodnicki, P. R.; Howard, B. H.; Deng, X. Y.; Matranga, C. Active sites and structure-activity relationships of copper-based catalysts for carbon dioxide hydrogenation to methanol. ACS Catal. 2012, 2, 1667-1676. (19) Samei, E.; Taghizadeh, M.; Bahmani, M. Enhancement of stability and activity of Cu/ZnO/Al2O3 catalysts by colloidal silica and metal oxides additives for methanol synthesis from a CO2-rich feed. Fuel Process. Technol. 2012, 96, 128-133. (20) Vidal, A. B.; Feria, L.; Evans, J.; Takahashi, Y.; Liu, P.; Nakamura, K.; Illas, F.; Rodriguez, J. A. CO2 activation and methanol synthesis on novel Au/TiC and Cu/TiC catalysts. J. Phys. Chem. Lett. 2012, 3, 2275-2280.

(21) Wesselbaum, S.; vom Stein, T.; Klankermayer, J.; Leitner, W. Hydrogenation of carbon dioxide to methanol by using a homogeneous ruthenium-phosphine catalyst. Angew. Chem. Int. Ed. 2012, 51, 7499-7502.

(22) Witoon, T.; Permsirivanich, T.; Donphai, W.; Jaree, A.; Chareonpanich, M. CO2 hydrogenation to methanol over Cu/ZnO nanocatalysts prepared via a chitosan-assisted coprecipitation method. Fuel Process. Technol. 2013, 116, 72-78. (23) Anicic, B.; Trop, P.; Goricanec, D. Comparison between two methods of methanol production from carbon dioxide. Energy 2014, 77, 279-289. (24) Courtemanche, M. A.; Legare, M. A.; Maron, L.; Fontaine, F. G. Reducing CO2 to methanol using frustrated Lewis pairs: On the mechanism of phosphine-borane-mediated hydroboration of CO2. J. Am. Chem. Soc. 2014, 136, 10708-10717. (25) Duan, H. M.; Yang, Y. X.; Singh, R.; Chiang, K.; Wang, S.; Xiao, P.; Patel, J.; Danaci, D.; Burke, N.; Zhai, Y. C., et al. Mesoporous carbon-supported Cu/ZnO for methanol synthesis from carbon dioxide. Aust. J. Chem. 2014, 67, 907-914.



(26) Liu, Z. J.; Tang, X. J.; Xu, S.; Wang, X. L. Synthesis and catalytic performance of graphene modified CuO-ZnO-Al2O3 for CO2 Hydrogenation to methanol. J. Nanomater. 2014. (27) Liao, F. L.; Wu, X. P.; Zheng, J. W.; Li, M. M. J.; Kroner, A.; Zeng, Z. Y.; Hong, X. L.; Yuan, Y. Z.; Gong, X. Q.; Tsang, S. C. E. A promising low pressure methanol synthesis route from CO2 hydrogenation over Pd@Zn core-shell catalysts. Green Chemistry 2017, 19, 270-280. (28) Grunwaldt, J. D.; Molenbroek, A. M.; Topsøe, N. Y.; Topsøe, H.; Clausen, B. S. In situ investigations of structural changes in Cu/ZnO catalysts. J. Catal. 2000, 194, 452-460. (29) Hansen, P. L.; Wagner, J. B.; Helveg, S.; Rostrup-Nielsen, J. R.; Clausen, B. S.; Henrik, T. Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals. Science 2002, 295, 2053-2055.

(30) An, X.; Li, J.; Zuo, Y.; Zhang, Q.; Wang, D.; Wang, J. A Cu/Zn/Al/Zr Fibrous Catalyst that is an Improved CO2 Hydrogenation to Methanol Catalyst. Catal. Lett. 2007, 118, 264-269. (31) Yang, Y. X.; White, M. G.; Liu, P. Theoretical study of methanol synthesis from CO2 hydrogenation on metal-doped Cu(111) surfaces. J. Phys. Chem. C 2012, 116, 248-256. (32) Huš, M.; Dasireddy, V. D. B. C.; Strah Štefančič, N.; Likozar, B. Mechanism, kinetics and thermodynamics of carbon dioxide hydrogenation to methanol on Cu/ZnAl2O4 spinel-type heterogeneous catalysts. Appl. Catal. B-Environ. 2017, 207, 267-278. (33) Rui, N.; Wang, Z.; Sun, K.; Ye, J.; Ge, Q.; Liu, C.-j. CO2 hydrogenation to methanol over Pd/In2 O3: effects of Pd and oxygen vacancy. Appl. Catal. B-Environ. 2017, 218, 488-497. (34) Studt, F.; Sharafutdinov, I.; Abild-Pedersen, F.; Elkjaer, C. F.; Hummelshoj, J. S.; Dahl, S.; Chorkendorff, I.; Norskov, J. K. Discovery of a Ni-Ga catalyst for carbon dioxide reduction to methanol. Nat. Chem. 2014, 6, 320-324.


Page 40 of 50

Page 41 of 50 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


(35) Fiordaliso, E. M.; Sharafutdinov, I.; Carvalho, H. W. P.; Grunwaldt, J. D.; Hansen, T. W.; Chorkendorff, I.; Wagner, J. B.; Damsgaard, C. D. Intermetallic GaPd2 nanoparticles on SiO2 for low-pressure CO2 hydrogenation to methanol: Catalytic performance and in situ characterization. ACS Catal. 2015, 5, 5827-5836. (36) Tang, Q. L.; Shen, Z. M.; Huang, L.; He, T.; Adidharma, H.; Russell, A. G.; Fan, M. H. Synthesis of methanol from CO2 hydrogenation promoted by dissociative adsorption of hydrogen on a Ga3Ni5(221) surface. Phys. Chem. Chem. Phys. 2017, 19, 18539 - 18555. (37) Ghiringhelli, L. M.; Caputo, R.; Site, L. D. Phenol near Ni(111), Ni(110), and Ni(221) surfaces in a vertical ring geometry: A density functional study of the oxygen-surface bonding and O-H cleavage. Phys. Rev. B 2007, 75, 113403. (38) Catapan, R. C.; Oliveira, A. A. M.; Chen, Y.; Vlachos, D. G. DFT Study of the water-gas shift reaction and coke formation on Ni(111) and Ni(211) surfaces. J. Phys. Chem. C 2012, 116, 20281-20291.

(39) Bengaard, H. S.; Norskov, J. K.; Sehested, J.; Clausen, B. S.; Nielsen, L. P.; Molenbroek, A. M.; Rostrup-Nielsen, J. R. Steam reforming and graphite formation on Ni catalysts. J. Catal.

2002, 209, 365-384. (40) Yang, M. L.; Zhu, Y. A.; Fan, C.; Sui, Z. J.; Chen, D.; Zhou, X. G. DFT study of propane dehydrogenation on Pt catalyst: effects of step sites. Phys. Chem. Chem. Phys. 2011, 13, 3257-67. (41) Su, H. Y.; Zeng, Z. H.; Bao, X. H.; Li, W. X. First-principles study of carbon monoxide oxidation on Ag(111) in presence of subsurface oxygen and stepped Ag(221). J. Phys. Chem. C 2009, 113, 8266–8272.



(42) Behrens, M.; Studt, F.; Kasatkin, I.; Kühl, S.; Hävecker, M.; Abild-Pedersen, F.; Zander, S.; Girgsdies, F.; Kurr, P.; Kniep, B. L., et al. The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts. Science 2012, 336, 893-896. (43) Zhang, R. G.; Wang, G. R.; Wang, B. J. Insights into the mechanism of ethanol formation from syngas on Cu and an expanded prediction of improved Cu-based catalyst. J. Catal.

2013, 305, 238-255. (44) Zhang, R. G.; Wang, G. R.; Wang, B. J.; Ling, L. X. Insight into the effect of promoter Mn on ethanol formation from syngas on a Mn-promoted MnCu(211) surface: A comparison with a Cu(211) surface. J. Phys. Chem. C 2014, 118, 5243-5254. (45) Zhang, R. G.; Liu, F.; Wang, B. J. Co-decorated Cu alloy catalyst for C2 oxygenate and ethanol formation from syngas on Cu-based catalyst: insight into the role of Co and Cu as well as the improved selectivity. Catal. Sci. Technol. 2016, 6, 8036-8054. (46) Wang, J. C.; Liu, Z. X.; Zhang, R. G.; Wang, B. J. Ethanol synthesis from syngas on the stepped Rh(211) surface: Effect of surface structure and composition. J. Phys. Chem. C

2014, 118, 22691-22701. (47)Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558-561. (48) Kresse, G.; Furthmiiller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comp. Mater. Sci. 1996, 6, 15-50. (49) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11186. (50) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979.


Page 42 of 50

Page 43 of 50 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


(51) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

(52) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901-9904. (53) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements HPu. J. Chem. Phys. 2010, 132, 154104. (54) Gokhale, A. A.; Kandoi, S.; Greeley, J. P.; Mavrikakis, M.; Dumesic, J. A. Molecular-level descriptions of surface chemistry in kinetic models using density functional theory. Chem. Eng. Sci. 2004, 59, 4679-4691.

(55) Cao, X. M.; Burch, R.; Hardacre, C.; Hu, P. An understanding of chemoselective hydrogenation on crotonaldehyde over Pt(111) in the free energy landscape: The microkinetics study based on first-principles calculations. Catal. Today 2011, 165, 71-79. (56) Darensbourg, D. J.; Kyran, S. J.; Yeung, A. D.; Bengali, A. A. Kinetic and thermodynamic investigations of CO2 insertion reactions into Ru-Me and Ru-H Bonds - an experimental and computational study. Eur. J. Inorg. Chem. 2013, 2013, 4024-4031. (57) Jeletic, M. S.; Mock, M. T.; Appel, A. M.; Linehan, J. C. A cobalt-based catalyst for the hydrogenation of CO2 under ambient conditions. J. Am. Chem. Soc. 2013, 135, 11533-6. (58) Medford, A. J.; Sehested, J.; Rossmeisl, J.; Chorkendorff, I.; Studt, F.; Norskov, J. K.; Moses, P. G. Thermochemistry and micro-kinetic analysis of methanol synthesis on ZnO (0001). J. Catal. 2014, 309, 397-407.



(59) Wolf, A.; Jess, A.; Kern, C. Syngas production via reverse water-gas shift reaction over a Ni-Al2O3 catalyst: Catalyst stability, reaction kinetics, and modeling. Chem. Eng. Technol.

2016, 39, 1040-1048. (60) Yin, H. X.; Zhang, C. H.; Yin, H. B.; Gao, D. Z.; Shen, L. Q.; Wang, A. L. Hydrothermal conversion of glycerol to lactic acid catalyzed by Cu/hydroxyapatite, Cu/MgO, and Cu/ZrO2 and reaction kinetics. Chem. Eng. J. 2016, 288, 332-343. (61) Zhang, R. G.; Song, L. Z.; Wang, B. J.; Li, Z. A density functional theory investigation on the mechanism and kinetics of dimethyl carbonate formation on Cu2O catalyst. J. Comput. Chem. 2012, 33, 1101-10.

(62) Li, Q. H.; Ma, Z. J.; Sa, R. J.; Adidharma, H.; Gasem, K. A. M.; Russell, A. G.; Fan, M. H.; Wu, K. C. Computation-predicted, stable, and inexpensive single-atom nanocatalyst Pt@Mo2C – an important advanced material for H2 production. J. Mater. Chem. A 2017, 5, 14658-14672. (63) Ding, X.; De Rogatis, L.; Vesselli, E.; Baraldi, A.; Comelli, G.; Rosei, R.; Savio, L.; Vattuone, L.; Rocca, M.; Fornasiero, P., et al. Interaction of carbon dioxide with Ni(110): A combined experimental and theoretical study. Phys. Rev. B 2007, 76. (64) Grabow, L. C.; Gokhale, A. A.; Evans, S. T.; Dumesic, J. A.; Mavrikakis, M. Mechanism of the water gas shift reaction on Pt: First principles, experiments, and microkinetic modeling. J. Phys. Chem. B 2008, 112, 4608-4617.

(65) Lin, S.; Huang, J.; Gao, X.; Ye, X.; Guo, H. Theoretical Insight into the Reaction Mechanism of Ethanol Steam Reforming on Co(0001). J. Phys. Chem. C 2015, 119, 2680−2691.


Page 44 of 50

Page 45 of 50 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


(66) Zhang, R. G.; Liu, H. Y.; Wang, B. J.; Ling, L. X. Insights into the preference of CO2 formation from HCOOH decomposition on Pd surface: A theoretical study. J. Phys. Chem. C 2012, 116, 22266-22280.

(67) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M. On the mechanism of low-temperature water gas shift reaction on copper. J. Am. Chem. Soc. 2008, 130, 1402-1414. (68) Senanayake, S. D.; Stacchiola, D.; Liu, P.; Mullins, C. B.; Hrbek, J.; Rodriguez, J. A. Interaction of CO with OH on Au(111): HCOO, CO3, and HOCO as key intermediates in the water-gas shift reaction. J. Phys. Chem. C 2009, 113, 19536-19544. (69) Miyoshi, A.; Matsui, H.; Washida, N. Detection and reactions of the hoco radical in gasphase. J. Chem. Phys. 1994, 100, 3532-3539. (70) Roldan, A.; de Leeuw, N. H. Methanol formation from CO2 catalyzed by Fe3S4{111}: formate versus hydrocarboxyl pathways. Faraday Discuss. 2016, 188, 161-80. (71) Weigel, J.; Koeppel, R. A.; Baiker, A.; Wokaun, A. Surface species in CO and CO2 hydrogenation over copper/zirconia: On the methanol synthesis mechanism. Langmuir 1996, 12, 5319-5329.

(72) Preti, D.; Resta, C.; Squarcialupi, S.; Fachinetti, G. Carbon dioxide hydrogenation to formic acid by using a heterogeneous gold catalyst. Angew. Chem. Int. Ed. 2011, 50, 12551-4. (73) Zhao, Y. F.; Yang, Y.; Mims, C.; Peden, C. H. F.; Li, J.; Mei, D. H. Insight into methanol synthesis from CO2 hydrogenation on Cu(111): Complex reaction network and the effects of H2O. J. Catal. 2011, 281, 199-211. (74) Mitchell, W. J.; Wang, Y. Q.; Xie, J.; Weinberg, W. H. Hydrogenation of CO at 100K on the Ru(001) surface - spectroscopic identification of formyl intermediates. J. Am. Chem. Soc.

1993, 115, 4381-4382.



(75) Grabow, L. C.; Mavrikakis, M. Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation. ACS Catal. 2011, 1, 365-384. (76) Lin, S.; Ma, J. Y.; Ye, X. X.; Xie, D. Q.; Guo, H. CO hydrogenation on Pd(111): Competition between Fischer-Tropsch and oxygenate synthesis pathways. J. Phys. Chem. C

2013, 117, 14667-14676. (77) Zheng, H. Y.; Zhang, R. G.; Li, Z.; Wang, B. J. Insight into the mechanism and possibility of ethanol formation from syngas on Cu(100) surface. J. Mol. Catal. A: Chem. 2015, 404405, 115-130.

(78) Zhang, R. G.; Liu, F.; Wang, Q.; Wang, B. J.; Li, D. B. Insight into CHx formation in Fischer–Tropsch synthesis on the hexahedron Co catalyst: Effect of surface structure on the preferential mechanism and existence form. Appl. Catal. A-Gen. 2016, 525, 76-84. (79) Muetterties, E. L.; Stein, J. Mechanistic features of catalytic carbon-monoxide hydrogenation reactions. Chem. Rev. 1979, 79, 479-490. (80) Sun, X. C.; Zhang, R. G.; Wang, B. J. Insights into the preference of CHx(x=1–3) formation from CO hydrogenation on Cu(111) surface. Appl. Surf. Sci. 2013, 265, 720-730. (81)Zhang, R. G.; Sun, X. C.; Wang, B. J. Insight into the preference mechanism of CHx (x=1-3) and C-C chain formation involved in C-2 oxygenate formation from syngas on the Cu(110) surface. J. Phys. Chem. C 2013, 117, 6594-6606. (82) C. Schild; Wokaun, A.; Baiker, A. On the mechanism of CO and CO2 hydrogenation reactions on zirconia-supported catalysts: a diffuse reflectance FTIR study Part I. Identification of surface species and methanation reactions on palladium/zirconia catalysts. J. Mol. Catal. 1990, 63, 223-242.


Page 46 of 50

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


(83) Zhao, Y. F.; Rousseau, R.; Li, J.; Mei, D. H. Theoretical study of syngas hydrogenation to methanol on the polar Zn-terminated ZnO(0001) surface. J. Phys. Chem. C 2012, 116, 15952-15961. (84) Ye, J. Y.; Liu, C. J.; Mei, D. H.; Ge, Q. F. Active oxygen vacancy site for methanol synthesis from CO2 hydrogenation on In2O3(110): A DFT Study. ACS Catal. 2013, 3, 12961306. (85) L. A. Khachatryan; O. M. Niazyan; A. A. Mantashyan; V. I. Vedebeev; Teitel'boim, M. A. Experimental determination of the equilibrium constant of the reaction CH3+O2-CH3O2 during the gas-phase oxidation of methane. Int. J. Chem. Kinet. 1982, 14, 1231-1241. (86) Tang, Q. L.; Hong, Q. J.; Liu, Z. P. CO2 fixation into methanol at Cu/ZrO2 interface from first principles kinetic Monte Carlo. J. Catal. 2009, 263, 114-122. (87)Schreiner, P. R.; Reisenauer, H. P. Spectroscopic identification of dihydroxycarbene. Angew. Chem. Int. Ed. 2008, 47, 7071-7074.

(88) Gao, L. Z.; Au, C. T. CO2 hydrogenation to methanol on a YBa2Cu3O7 catalyst. J. Catal.

2000, 189, 1-15. (89)Kakumoto, T. A theoretical-study for the CO2 hydrogenation mechanism on Cu/Zno catalyst. Energy Convers. Manage. 1995, 36, 661-664.

(90) Gu, X. K.; Li, W. X. First-principles study on the origin of the different selectivities for methanol steam reforming on Cu(111) and Pd(111). J. Phys. Chem. C 2010, 114, 2153921547. (91) Sato, T.; Kambe, M.; Nishiyama, H. Analysis of a methanol decomposition process by a nonthermal plasma flow. JSME Int J., Ser. B 2005, 48, 432-439.



(92) Hofmann, P.; Schindler, K. M.; Bao, S.; Fritzsche, V.; Ricken, D. E.; Bradshaw, A. M.; Woodruff, D. P. The geometric structure of the surface methoxy species on Cu(111). Surf. Sci. 1994, 304, 74-84.

(93) Johnston, S. M.; Mulligan, A.; Dhanak, V.; Kadodwala, M. The structure of methanol and methoxy on Cu(111). Surf. Sci. 2003, 530, 111-119. (94) Zhang, R. G.; Liu, H. Y.; Ling, L. X.; Li, Z.; Wang, B. J. A DFT study on the formation of CH3O on Cu2O(111) surface by CH3OH decomposition in the absence or presence of oxygen. Appl. Surf. Sci. 2011, 257, 4232-4238. (95) Saadi, S.; Abild-Pedersen, F.; Helveg, S.; Sehested, J.; Hinnemann, B.; Appel, C. C.; Norskov, J. K. On the role of metal step-edges in graphene growth. J. Phys. Chem. C 2010, 114, 11221-11227.

(96) Zhang, R. G.; Wang, B. J.; Ling, L. X.; Liu, H. Y.; Huang, W. Adsorption and dissociation of H2 on the Cu2O(111) surface: A density functional theory study. Appl. Surf. Sci. 2010, 257, 1175-1180.

(97) Vesselli, E.; Rogatis, L. D.; Ding, X. L.; Baraldi, A.; Savio, L.; Vattuone, L.; Rocca, M.; Fornasiero, P.; Peressi, M.; Baldereschi, A., et al. Carbon dioxide hydrogenation on Ni(110). J. Am. Chem. Soc. 2008, 130, 11417-11422.

(98) Ulissi, Z. W.; Medford, A. J.; Bligaard, T.; Norskov, J. K. To address surface reaction network complexity using scaling relations machine learning and DFT calculations. Nat. Commun. 2017, 8, 14621.

(99) Liu, P.; Logadottir, A.; Nørskov, J. K. Modeling the electro-oxidation of CO and H2/CO on Pt, Ru, PtRu and Pt3Sn. Electrochim. Acta 2003, 48, 3731-3742.


Page 48 of 50

Page 49 of 50 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


(100) Liu, P.; Rodriguez, J. A. Water-gas-shift reaction on metal nanoparticles and surfaces. J. Chem. Phys. 2007, 126, 164705.

(101) Zhao, X.; Zhang, R.; Wang, Q.; Li, D.; Wang, B.; Ling, L. Source and major species of CHx(x = 1–3) in acetic acid synthesis from methane–syngas on Rh catalyst: a theoretical study. RSC Adv. 2014, 4, 58631-58642. (102) Liu, H.; Zhang, R.; Ling, L.; Wang, Q.; Wang, B.; Li, D. Insight into the preferred formation mechanism of long-chain hydrocarbons in Fischer–Tropsch synthesis on Hcp Co(10−11) surfaces from DFT and microkinetic modeling. Catal. Sci. Technol. 2017, 7, 3758-3776. (103) Nakatsuji, H.; Hu, Z. M. Mechanism of methanol synthesis on Cu(100) and Zn/Cu(100) surfaces: Comparative dipped adcluster model study. Int. J. Quantum Chem 2000, 77, 341– 349. (104) Ye, J. Y.; Liu, C. J.; Ge, Q. F. DFT study of CO2 adsorption and hydrogenation on the In2O3 surface. J. Phys. Chem. C 2012, 116, 7817-7825.



Table of Contents (TOC)


Page 50 of 50

Thermodynamic and Kinetic Study on Carbon Dioxide Hydrogenation

Recommend Documents