Methanol-Selective Oxidation Pathways on Au Surfaces: A First

Subscriber access provided by UNIV WAIKATO

Article

Methanol Selective Oxidation Pathways on Au Surfaces: A First-Principles Study Lei Wang, Chaozheng He, Wenhua Zhang, Zhenyu Li, and Jinlong Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp501620h • Publication Date (Web): 11 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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 38

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

Methanol Selective Oxidation Pathways on Au Surfaces: A First-Principles Study Lei Wang,2,3 Chaozheng He,5 Wenhua Zhang,1,4*, Zhenyu Li2,4, Jinlong Yang 2,4* 1

Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering,

University of Science and Technology of China, Hefei, Anhui 230026, China. 2 Hefei National Lab for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China., 3Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 4Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China. 5 College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, P. R. China

Corresponding author footnote: * To whom all correspondence should be addressed. Fax: + 86-551-3606408. E-mail: [email protected] , [email protected]

ACS Paragon Plus Environment 1


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

Page 2 of 38

Abstract: With density functional theory, all elementary steps of methanol (CH3OH) dehydrogenation and oxidation on atomic oxygen covered, or OH covered Au (111) surfaces are systematically studied. Our results suggest that on low oxygen coverage Au (111) surface the production of CH2O and CO start from α-H elimination and β-H elimination, respectively. The selective oxidation pathway is controlled by thermodynamics of the first step rather than kinetics. The overall energy barrier to produce CO is 0.39 eV corresponding to gas phase methanol, which indicates that the reaction can proceed at low temperature.

On high oxygen coverage Au (111) surface, the elimination of α-H and one β-H can take

place simultaneously to form CH2O for the cooperative interaction of two nearby atomic oxygen. The missing observation of CH2O may come from that the newly formed CH2O is ready to react with surface atomic oxygen and hydroxyl to form CH2OO(H) rather than desorption from the surface. The rate-limiting step of the oxidation of CH2OO(H) is the dehydrogenation of CHO2 with an energy barrier of 0.95 eV. Also the newly formed CH2O can be dehydrogenated by surface atomic oxygen to form CO and then to CO2 with low energy barrier. Our results give good explanation for experimental observations and make up the discrepancy between experimental observation and previous theoretical work.

Keywords: Density Functional Theory (DFT), Methanol Partial Oxidation, Thermodynamics or Kinetics, Overall Energy Barrier, Relative Selectivity


Page 3 of 38

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


1. Introduction Since Haruta1 found that gold nanoparticles has a catalytic effect on the oxidation of carbon monoxide at low temperature in 1987, the catalytic properties of gold (and its alloys) has become a hot topic. It is found that gold nanoparticles have high catalytic activity, mild reaction condition requirement, good selectivity2 for carbon monoxide (CO) hydrogenation3 and low temperature oxidation reaction,

water vapor change reaction (WGS)4, selective oxidation5-7, and nucleophilic addition

reactions8-10. Atomic oxygen covered Au single crystal surfaces are prepared to make effort to understand the complicated catalytic oxidation or partial oxidation processes, such as oxidation of CO11-13 and NO14-16, selective partial oxidation of alcohols2,5,7,17-20 etc. Due to the potential application in fine chemical engineering, selective oxidation of methanol on atomic oxygen covered Au (111) surface has been intensively studied. Different oxidation products including CH2O and HCOOCH3, CO, CO2 are observed at low temperature with low atomic oxygen coverage

18, 20

. And on high oxygen coverage Au (111) surface, the main product of CO2 can be

observed at both low and high temperature

20

. The mechanisms of the formation of CH2O and

HCOOCH3 at low oxygen coverage have been investigated by theoretical calculations

26, 28

and it is

suggested that the oxidation processes starting from the elimination of α-H (hydrogen in OH group) in methanol. On high oxygen coverage surface, it is suggested that the decomposition of CH2O2, combination of CH2O and atomic oxygen, is responsible for the production of CO2 at high temperature. Unfortunately, the experimental production of CO and CO2 at low oxygen coverage has not been reported in previous theoretical studies. One reason for this discrepancy may be due to the limitation of considering only α-H elimination in previous studies. Note that β-H (hydrogen atom in CH3 group) elimination is also an important reaction pathway, which results in different products21. Thus, in this



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

Page 4 of 38

work, possible reaction paths for the decomposition of methanol molecule and its reacting with atomic oxygen on Au (111) surface and also hydroxyl formed by abstraction of hydrogen starting from both α-H and β-H elimination of methanol are intensively investigated. Based on our calculations, it is suggested that at low oxygen coverage, CO is produced starting from the β-H elimination and the stability of the product of the first abstraction of hydrogen controls the reaction path of the whole reaction. The different methanol partial oxidation selectivity on Au surfaces may come from the different surface structures produced by various preparation method of atomic oxygen. The further oxidation of CO and CH2O responsible for CO2 produced at low and high temperature, respectively.

2. Computation details Electronic structure calculations are performed with the DMol3 implementation22,23 of density functional theory (DFT) using the PBE exchange-correlation functional24. Double-numeric quality basis set with polarization functions (DNP) is used, whose size is comparable to Gaussian-type 6-31G* basis set. DFT semi-core pseudopotential (DSPP) is used for all atoms. In our calculations, Au (111) surface is simulated with a four-layer-thick p(3×3) supercell slab with ~15 Å vacuum. During geometry optimization, all atoms are relaxed, except those in the two bottom layers, which are kept at their bulk positions. The tolerances of energy, gradient, and displacement convergence are 1×10−5 Ha, 2×10−3 Ha/Å, and 5×10−3 Å, respectively. For the energy calculations, the k-point grid is set as 6×6×1. Fermi smearing and a real-space cutoff of 4.5 Å are adopted. All the calculations are performed within the spin-polarized frame. The transition state search is performed with the synchronous transit methods25. All the atoms are used to calculate the vibrational frequencies to verify the transition state and to


Page 5 of 38

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


calculate the zero point energy (ZPE). The ZPE correction is only used to correct the energy barrier of each elementary steps rather than the reaction energy.

3. Results and discussion 3.1 Surface species during the reaction The possible surface species involved in methanol dehydrogenation or oxidation on Au (111) surface are investigated first. The adsorption configurations are shown in Figure 1 and Figure S1 the adsorption sites, adsorption energies and key parameters are listed in Table 1. CH3OH weakly binds Au atom through oxygen atom with adsorption energy of 0.17 eV as shown in Figure 1(a), which agrees well with 0.15 eV in previous theoretical work26. Our result is lower than the experimental data of 55.3 kJ/mol, and this discrepancy may come from the absence of hydrogen bond in our calculation. Methoxy (CH3O), the intermediate by α-H elimination of methanol, binds through oxygen on the 3-fold fcc site with an adsorption energy of 1.18 eV. The O-Au distance is 2.33Å and the O-C axis is almost perpendicular to the surface as shown in Figure 1(b). Hydroxymethyl (CH2OH), the intermediate by β-H elimination of methanol, prefers to locate at an Au top site through the carbon atom. C-Au distance is 2.15 Å, and the angle between the O-C axis and the surface normal is 32° as shown in Figure 1(c). Different from the above-mentioned methoxy (CH3O), this configuration is featured by both the methylene (-CH2) and hydroxyl-H close to the surface, which is expected to favor the scission of the corresponding bonds, possibly giving different dehydrogenation products. This conﬁguration affords an adsorption energy of 1.26 eV. The total energy of adsorbed CH2OH is 0.39 eV lower than that of CH3O, which indicates that β-H elimination is energetically preferred.



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

Page 6 of 38

By further β-elimination of CH3O or α-elimination of CH2OH, formaldehyde (CH2O) can be formed on Au (111) surface. CH2O weakly binds to the top site of Au atom through oxygen atom with an adsorption energy of 0.11 eV, which indicates that if CH2O is formed during the oxidation process it should be easily observed. By β-elimination of CH2OH, CHOH can be formed. CHOH prefers to bind at the bridge site on Au (111) through the C atom with an adsorption energy of 1.94 eV. The bond length of C-Au is 2.15 Å and the structure is shown in Figure 1(d). Formyl radical (CHO) formed by H abstraction of CH2O or α-elimination of CHOH, binds at top site of Au (111) surface through carbon atom with an adsorption energy of 1.32 eV. The C-Au distance is 2.11 Å, and the O-C axis is inclined at 35° to the surface normal as shown in Figure 1(e). By β-elimination of CHOH, COH is formed, which binds at an fcc site with an adsorption energy of 2.39 eV. The bond length dC-O is 1.31 Å and ∠C-O-H is 111° as shown in Figure 1(f). Finally, by hydrogen abstraction of CHO or COH, CO is generated, which prefers to bind at atop site of gold atom with adsorption energy of 0.28 eV in a perpendicular configuration. Carbon dioxide can be formed by the oxidation of CO. The diffusion barriers of CHOH, CHO, COH on clean Au (111) surface are calculated as 0.28 eV, 0.37 eV and 0.20 eV, respectively. The diffusion paths are shown in Figure S2. The adsorption of oxygen atom on clean Au (111) surface has been intensively studied previously11,18,26. The most stable structure comes from fcc-hollow adsorption. The adsorption energy of atomic oxygen is 3.31 eV and the O-Au bond length is 2.15 Å. As the product of combination of atomic oxygen and hydrogen, the reaction involved hydroxyl (OH) and methanol intermediate should also be considered. It is found that hydroxyl prefers to adsorb at a bridge site with an adsorption energy of 2.11


Page 7 of 38

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


eV based on our calculation. The diffusion barriers for adsorbed atomic oxygen and hydroxyl on Au(111) surface with 1/9 ML coverage are calculated as 0.41 eV and 0.19 eV, respectively. With high oxygen coverage, more species such as CH2O2 and CHO2 can be formed on metal surfaces. The most stable configuration of CH2O2 adsorbs on Au (111) has its carbon atom over a bridge site with an adsorption energy of 2.27 eV. The two C-O bonds oriented toward hollow sites such that the HCH axis is perpendicular to the OCO axis as shown in Figure 1(g). The length of C-O and Au-O are 1.39 and 2.41 Å. The adsorption energy of formate (CHO2) on Au (111) is 1.81 eV. Its carbon atom is centered on the bridge site with the C-O bonds symmetrically oriented toward adjacent atop sites. The O-CH-O molecular plane is perpendicular to the surface and parallel to the axis of the bridge site as shown in Figure 1(h). The length of C-O and Au-O are 1.25 and 2.34 Å. The C-H distance is 1.10 Å. Dehydrogenation of CHO2 generates CO2 on Au (111) surface. 3.2 Decomposition of methanol on Au (111) surface Two paths of methanol dissociation on Au (111) surface are checked. For α-H elimination (CH3OH(a) CH3O(a) +H(a)), the energy barrier is calculated as 2.00 eV with the reaction energy of 1.29 eV. For β-H elimination (CH3OH(a)→CH2OH(a) +H(a) ), the energy barrier is calculated as 2.20 eV with the reaction energy of 1.15 eV. Then all the dehydrogenation processes to CO starting from both α-H elimination and β-H elimination are investigated. The energy profiles of the whole dehydrogenation process are shown in Table S1, which demonstrate that elimination of the first H atom is the rate-limiting step the dehydrogenation process. The structures of initial states, transition states and final states of all the elementary steps are shown in Figure S3 and Figure S4. Due to the relatively high barriers, dissociation of methanol on Au (111) is not energetically possible at low temperature. Therefore, oxygen groups



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

Page 8 of 38

such as atomic oxygen or hydroxyl are expected to play important roles in methanol dehydrogenation and oxidation on Au (111) surface. 3.3 Oxidation of methanol by atomic oxygen on Au (111) surface With the pre-adsorption of atomic oxygen, the adsorption energy of methanol is increased to 0.41 eV with a 0.24 eV gained due to the formation of α-H hydrogen bond. The O-Au distance is 2.53 Å and the O-C axis is tilted at 41° from the surface normal. It is found that the formation of β-H hydrogen bond with atomic oxygen increases the adsorption energy of methanol to 0.29 eV and the distance between β-H and atomic oxygen is 2.24 Å. These co-adsorption configurations are used to investigate the oxidation of methanol on oxygen pre-covered Au (111) surface starting from α-H and β-H elimination, respectively. α-H elimination: Formation of CH2O. For the elimination of α-H reaction CH3OH(a) + O(a) → CH3O(a) + OH(a), an energy barrier 0.27 eV is found.

At transition state the bond lengths dO-H and dO1-H

are 1.20 and 1.21 Å, respectively. Hydrogen bonds are found both in initial state and final state. For the second step of dehydrogenation, CH3O(a) can dissociate with the help of atomic oxygen with an energy barrier of 0.46 eV. At transition state the bond lengths dO-H and dO1-H are 1.19 and 1.48 Å, respectively. Considering its low adsorption energy (~0.1 eV), CH2O, the important partial oxidation product for esterification, can easily desorb from the surface and thus should be detected, which agree with some experimental results where CH2O and HCOCH3 are observed19. The energy barriers, reaction energies and key parameters are shown in Table 2 and the structures of initial states, transition states and final states of these two elementary steps are shown in Figure 2. Then the reaction between hydroxyl and methanol are investigated. Firstly, α-H elimination with the help of hydroxyl is considered. α-H of methanol is hydrogen bonded with the oxygen of hydroxyl with a


Page 9 of 38

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


bond length of 1.66 Å as shown in Figure 2 (c). In final state the generated water is also strong hydrogen bonded with the oxygen in CH3O(a) with a bond length of 1.42 Å. At transition state, the bond length of O-H in methanol is elongated to 1.02 Å and the distance between α-H and hydroxyl oxygen is shortened to 1.44 Å. The energy barrier of hydroxyl induced CH3O(a) dehydrogenation is 0.48 eV. At transition state the C-H distance in CH3O(a) is elongated to 1.25 Å and the distance between oxygen in hydroxyl and H is shortened to 1.38 Å as shown in Figure 2(d).

β-H elimination: formation of CO. For the first elimination of β-H, with the help of atomic oxygen, an energy barrier of 0.68 eV is found. At transition state the bond lengths dO-H and dC-H are 1.14Å and 1.42 Å (Figure 6), respectively. For elimination of the second hydrogen, there are two possible ways: one is the α-H elimination to form CH2O with an energy barrier of 0.15 eV and the other is the second β-H to form CHOH(a) with an energy barrier of 0.51 eV. If CHOH(a) is formed, there are two ways to proceed, one is the elimination of α-H and the other is the third β-H. The energy barriers of these two processes are calculated to be 0.07 eV and 0.81 eV, respectively. Thus for the dehydrogenation of CHOH(a) by atomic oxygen, the elimination of α-H is much easier than β-H. Our calculations indicate that the dissociation of COH(a) or CHO(a) with the help of neighboring atomic oxygen is almost spontaneously. And thus the adsorbed CO is formed on the surface.

The energy barriers, reaction

energies and key parameters are shown in Table 2 and the structures of initial states, transition states and final states of these two elementary steps are shown in Figure 3. Then, the dehydrogenation starting from β-H elimination with the presence of hydroxyl is considered. For the first elimination of β-H to form CH2OH(a) and H2O, the energy barrier is 0.96 eV. At transition state, the distances between C and H is elongated to 1.33 Å and dO(H)-H is shortened to 1.20 Å as shown in Figure 4(a). For the elimination of second hydrogen with the help of hydroxyl, there are two possible ACS Paragon Plus Environment 9


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

Page 10 of 38

ways: one is α-H elimination to form CH2O and the other is β-H elimination to CHOH(a). The former process is almost barrierless when OH(a) and CH2OH(a) are set at neighboring sites. The energy barrier for the second β-H to form CHOH(a) is calculated as 0.57 eV. At transition state the bond length of C-H is elongated to 1.29 Å and O-H is shortened to 1.28 Å as shown in Figure 4(b). If CHOH(a) is formed, two cases should be considered, one is the elimination of α-H to form HCO(a) and the other is the third β-H to form COH(a). Energy barriers of these two processes are 0.25 and 0.43 eV, respectively. CHO(a) and COH(a) dehydrogenation with the help of hydroxyl group is almost barrierless and the process is controlled by the diffusion of surface species. At the end of the dehydrogenation process, CO(a) is produced. As indicated in our previous work, CO(a) can be oxidized by surface atomic oxygen to form CO2 directly with a reaction barrier of 0.39 eV or by OH(a) to form COOH(a) and then CO2 by the help of atomic oxygen (or hydroxyl or water) at low temperature13. 3.4 Evolution of CH2O on atomic oxygen or hydroxyl covered Au (111) surface When CH2O is formed, it can combine with surface atomic oxygen27, 28 or surface hydroxyl or being dehydrogenated. The energy barrier of the combination between CH2O and atomic oxygen to form CH2O2(a) as shown in Figure 5(a) is 0.32 eV. At transition state, the distance between C and atomic oxygen is 2.25 Å. CH2O also easily reacts with surface hydroxyl to form CH2OOH(a) with an energy barrier of 0.06 eV and at transition state the distance between C and hydroxyl oxygen is 3.17 Å as shown in Figure 5(b). With the help of atomic oxygen or hydroxyl, the α-H elimination of CH2OOH(a) is an easy process with an energy barrier of 0.21 or 0.19 eV to produce CH2O2(a) on Au (111) surface. For the β-H elimination of CH2OOH(a), the dehydrogenation process needs to conquer a barrier of 0.39 eV with the help of atomic oxygen or 0.62 eV with surface hydroxyl. HCOOH formed by elimination of β-H of


Page 11 of 38

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


CH2OOH only weakly adsorbs on clean Au (111) surface with an adsorption energy of 0.03 eV. All the above steps are exothermic (Table 3). The evolution of CH2O2 and HCOOH will be discussed below. Except combination with atomic oxygen or hydroxyl, CH2O can also be dehydrogenated to form CHO(a) by atomic oxygen with an energy barrier of 0.31 eV and at transition state the distance between H and C is elongated to 1.35 Å as shown in Figure 5(g). But unfortunately, we cannot determine the transition state of the dehydrogenation of CH2O by hydroxyl. 3.5 Evolution of HCOOH on atomic oxygen or hydroxyl covered Au (111) surface Though its adsorption energy is low on Au (111) surface, HCOOH is ready to form hydrogen bond with surface species such as atomic oxygen and hydroxyl with the α-H, and it thus can be fixed on the surface. The adsorption energy is increased to 0.38 or 0.55 eV with pre-covered atomic oxygen or hydroxyl. HCOOH is easy to be further oxidized. Atomic oxygen abstracts the α-H with an energy barrier of 0.47 eV and at transition state the O-H distances in HCOOH and between atomic oxygen are 1.09 and 1.37 Å, respectively (as shown in Figure 6). Also the energy barrier for the abstraction of α-H by hydroxyl is calculated as 0.01 eV. At transition state, the distance between α-H and oxygen in HCOOH and adsorbed hydroxyl are 1.12 and 1.29 Å, respectively. The strong hydrogen bond makes HCOOH easy to convert to CHO2(a). And then the further oxidation of CHO2(a) forms CO2. The important geometric parameters at transition states are listed in Table 3. 3.6 Evolution of CH2O2 on atomic oxygen or hydroxyl covered Au (111) surface The decomposition processes of CH2O2(a) by itself and with the help of atomic oxygen and hydroxyl are investigated. It is found that the energy barriers of CH2O2(a) dehydrogenation with and without atomic oxygen are 0.12 and 0.30 eV (Table 3), respectively. With the help of surface hydroxyl (OH(a)), the further dehydrogenation of CH2O2(a) is also easy with an energy barrier of 0.19 eV to form CHO2(a).



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

Page 12 of 38

Therefore, CH2O2(a) is ready to be oxidized to CHO2(a) with the present of surface atomic oxygen, hydroxyl and even only gold surface. For the self-dehydrogenation of CHO2(a), the dissociation barrier is calculated as 0.95 eV. The presence of atomic oxygen and hydroxyl even prohibit this process with a higher energy barrier of 1.16 and 1.01 eV. This is similar with the case on Ag(111) surface where the presence of atomic oxygen or hydroxyl increases the reaction barrier of CHO2(a) dehydrogenation

27

Thus the dissociation of CHO2(a)

is the time-limiting step of the formation of CO2 at high oxygen coverage. The key parameters of all the transition states are shown in Figure 7.

3.7

Discussion

On atomic oxygen covered Au (111) surface, methanol can be oxidized to various products such as CH2O, HCOOCH3, CO, CO2 and so on. It is suggested, on high atomic oxygen coverage surface, the main product is CO2. However, on low atomic oxygen coverage surface, with different atomic oxygen preparation methods, different partial oxidation products are observed19. Based on the intensive calculations of the behavior of methanol on clean Au (111) and the possible reaction with atomic oxygen and hydroxyl, we make efforts to give the reaction mechanism for the different oxidation products observed in experiments. A. Methanol Oxidation at Low Oxygen Coverage On low oxygen coverage Au (111) surface, the mechanisms of CH2O and HCOOCH3(a) formation have been carefully investigated in previous theoretical work26,28. It is suggested the production of CH3O is the critical step of the whole reaction and α-H can be abstracted by atomic oxygen, hydroxyl and also surface CH3O(a) species with low energy barriers. The second step is the dehydrogenation of


Page 13 of 38

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


β-H of CH3O(a). In Xu’s work, the energy barriers of self-dehydrogenation, with the help of atomic oxygen, hydroxyl, another CH3O(a) is calculated as 0.64, 0.49, 0.63 and 0.66 eV, respectively26, which suggests that if CH3O(a) is present on Au(111) surface, CH2O is ready to form. However, Gong et al only observed CO and CO2 desorption peaks at low temperature (

Methanol-Selective Oxidation Pathways on Au Surfaces: A First

Recommend Documents