Methanol-Selective Oxidation Pathways on Au Surfaces: A First- Principles Study

pubs.acs.org/jpcc Methanol-Selective Oxidation Pathways on Au Surfaces: A First- Principles Study Lei Wang,, Chaozheng He, Wenhua Zhang,*,, Zhenyu Li,, and Jinlong Yang*,,, Key Lab of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China Hefei National Lab for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China College of Physics and Electronic Engineering, Nanyang Normal University, Nanyang 473061, P. R. China *S Supporting Information ABSTRACT: With density functional theory, all elementary steps of methanol (CH 3 OH) dehydrogenation and oxidation on atomicoxygen-covered or OH-covered Au (111) surfaces are systematically studied. Our results suggest that on low oxygen coverage Au (111) surface the production of CH 2 O 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 CH 2 O for the cooperative interaction of two nearby atomic oxygen. The missing observation of CH 2 O may come from the fact that the newly formed CH 2 O is ready to react with surface atomic oxygen and hydroxyl to form CH 2 OO(H) rather than desorption from the surface. The rate-limiting step of the oxidation of CH 2 OO(H) is the dehydrogenation of CHO 2 with an energy barrier of 0.95 ev. Also, the newly formed CH 2 O can be dehydrogenated by surface atomic oxygen to form CO and then to CO 2 with low energy barrier. Our results give good explanation for experimental observations and make up the discrepancy between experimental observation and previous theoretical work. 1. INTRODUCTION Since Haruta 1 found that gold nanoparticles have a catalytic effect on the oxidation of carbon monoxide at low temperature in 1987, the catalytic properties of gold (and its alloys) have become a hot topic. It is found that gold nanoparticles have high catalytic activity, mild reaction condition requirement, good selectivity 2 for carbon monoxide (CO) hydrogenation, 3 and low-temperature oxidation reaction, water vapor change reaction (WGS), 4 selective oxidation, 5 7 and nucleophilic addition reactions. 8 10 Atomic-oxygen-covered Au singlecrystal surfaces are prepared to make effort to understand the complicated catalytic oxidation or partial oxidation processes, such as oxidation of CO 11 13 and NO, 14 16 selective partial oxidation of alcohols, 2,5,7,17 20 and so on. Because of the potential application in fine chemical engineering, selective oxidation of methanol on atomicoxygen-covered Au (111) surface has been intensively studied. Different oxidation products including CH 2 O, HCOOCH 3, CO, CO 2 are observed at low temperature with low atomic oxygen coverage, 18,20 and on high oxygen coverage Au (111) surface, the main product of CO 2 can be observed at both low and high temperature. 20 The mechanisms of the formation of CH 2 O and HCOOCH 3 at low oxygen coverage have been investigated by theoretical calculations, 26,28 and it is suggested that the oxidation processes start from the elimination of α-h (hydrogen in OH group) in methanol. On high oxygen coverage surface, it is suggested that the decomposition of CH 2 O 2 and combination of CH 2 O and atomic oxygen are responsible for the production of CO 2 at high temperature. Unfortunately, the experimental production of CO and CO 2 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 CH 3 group) elimination is also an important reaction pathway, which results in different products. 21 Thus, in this work, possible reaction Received: February 14, 2014 Revised: June 21, 2014 Published: July 11, 2014 2014 American Chemical Society 17511

Figure 1. Side views of the adsorption structures of possible surface species involved in the dehydrogenation and oxidation of methanol on Au (111) surface: (a) for CH 3 OH (a), (b) for CH 3 O (a), (c) for CH 2 OH (a), (d) for CHOH (a), (e) for CHO (a), (f) for COH (a), (g) for CH 2 O 2(a), and (h) for CHO 2(a). Yellow, red, gray, and white spheres represent gold, oxygen, carbon, and hydrogen atom, respectively. The distances (angstroms) of important geometric parameters are labeled in the figures. 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. On the basis of 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 CH 2 O was responsible for CO 2 produced at low and high temperature, respectively. 2. COMPUTATION DETAILS Electronic structure calculations are performed with the DMol 3 implementation 22,23 of density functional theory (DFT) using the PBE exchange-correlation functional. 24 Double-numeric quality basis set with polarization functions (DNP) is used, whose size is comparable to Gaussian-type 6-31G* basis set. DFT semicore 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 calculations are performed within the spinpolarized frame. The transition-state search is performed with the synchronous transit methods. 25 All atoms are used to calculate the vibrational frequencies to verify the transition state and to calculate the zero-point energy (ZPE). The ZPE correction is used only 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 in the Supporting Information; the adsorption sites, adsorption energies, and key parameters are listed in Table 1. Table 1. Adsorption Sites, Adsorption Energies, and Structural Parameters for Intermediates Involved in Methanol Dehydrogenation over Au (111) species sites E ads (ev) bond lengths (Å) CH 3 OH top 0.17 d O - Au = 2.70 CH 3 O fcc 1.18 d O - Au = 2.33 d C - O = 1.15 CH 2 OH top 1.26 d C - Au = 2.15 CHOH bridge 1.94 d C - Au = 2.15 CHO top 1.32 d C - Au = 2.11 COH fcc 2.39 d C - Au = 2.06 d C - O = 1.31 CH 2 O 2 bridge 2.27 d O - Au = 2.41 d C - O = 1.39 CHO 2 bridge 1.81 d O - Au = 2.34 d C - O = 1.25 CH 3 OH weakly binds Au atom through oxygen atom with adsorption energy of 0.17 ev, as shown in Figure 1a, which agrees well with 0.15 ev in previous theoretical work. 26 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 (CH 3 O), the intermediate by α-h elimination of methanol, binds through oxygen on the threefold fcc site with an adsorption energy of 1.18 ev. The O Au distance is 2.33 Å, and the O C axis is almost perpendicular to the surface, as shown in Figure 1b. Hydroxymethyl (CH 2 OH), 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 1c. Different from the previous mentioned methoxy (CH 3 O), this configuration is featured by both the methylene ( CH 2 ) 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 CH 2 OH is 0.39 ev lower than that of CH 3 O, which indicates that β-h elimination is energetically preferred. 17512

Table 2. Calculated Energy Barrier (E a ), Reaction Energies (ΔH), and Key Geometric Parameters of Transition States of Each Elementary Step of Methanol Oxidation by Atomic Oxygen or Hydroxyl Starting from α-h Elimination and β-h Elimination on Au (111) Surface with Low Oxygen Coverage reactions E a (ev) E ZPE (ev) ΔH (ev) bond lengths (Å) CH 3 OH (a) +O (a) CH 3 O (a) +OH (a) 0.27 0.11 0.19 d CH3O - H = 1.20 d H O = 1.21 CH 3 O (a) +O (a) CH 2 O+OH (a) 0.46 0.28 1.70 d OC H2 - H = 1.19 d H O = 1.48 CH 3 OH (a) +OH (a) CH 3 O (a) +H 2 O (a) 0.11 0.06 0.04 d CH3O - H = 1.02 d H OH = 1.44 CH 3 O (a) +OH (a) CH 2 O+H 2 O 0.48 0.27 1.62 d OCH2 - H = 1.25 d H OH = 1.38 CH 3 OH (a) +O (a) CH 2 OH (a) +OH (a) 0.68 0.50 0.26 d HOCH2 - H = 1.42 d H O = 1.14 CH 2 OH (a) +O (a) CHOH (a) +OH (a) 0.51 0.35 0.57 d HOCH - H = 1.25 d H O = 1.32 CH 2 OH (a) +O (a) CH 2 O (a) +OH (a) 0.15 0.11 0.99 d CH2O - H = 0.98 d H O = 2.15 CHOH (a) +O (a) CHO (a) +OH (a) 0.07 0.02 1.44 d CHO - H = 1.01 d H O = 1.74 CHOH (a) +O (a) COH (a) +OH (a) 0.81 0.62 0.40 d HOC - H = 1.26 d H O = 1.33 CHO (a) +O (a) CO (a) +OH (a) 0.00 COH (a) +O (a) CO (a) +OH (a) 0.00 CH 3 OH (a) +OH (a) CH 2 OH (a) +H 2 O (a) 0.96 0.78 0.54 d HOCH2 - H = 1.33 d H OH = 1.20 CH 2 OH (a) +OH (a) CHOH (a) +H 2 O (a) 0.57 0.39 0.33 d HOCH - H = 1.29 d H OH = 1.28 CHOH (a) +OH (a) CHO (a) +H 2 O (a) 0.25 0.16 1.73 d CHO - H = 0.98 d H OH = 2.34 CHOH (a) +OH (a) COH (a) +H 2 O (a) 0.43 0.33 0.78 d HOC - H = 1.26 d H OH = 1.31 CHO (a) +OH (a) CO (a) +H 2 O (a) 0.00 COH (a) +OH (a) CO (a) +H 2 O (a) 0.00 By further β-elimination of CH 3 O or α-elimination of CH 2 OH, formaldehyde (CH 2 O) can be formed on Au (111) surface. CH 2 O weakly binds to the top site of Au atom through oxygen atom with an adsorption energy of 0.11 ev, which indicates that if CH 2 O is formed during the oxidation process it should be easily observed. By β-elimination of CH 2 OH, 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 1d. Formyl radical (CHO) formed by H abstraction of CH 2 Oor 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 1e. 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 d C O is 1.31 Å and C O H is 111, as shown in Figure 1f. Finally, by hydrogen abstraction of CHO or COH, CO is generated, which prefers to bind at the top 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, and COH on clean Au (111) surface are calculated as 0.28, 0.37, and 0.20 ev, respectively. The diffusion paths are shown in Figure S2 in the Supporting Information. The adsorption of oxygen atom on clean Au (111) surface has been intensively previously studied. 11,18,26 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 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 and 0.19 ev, respectively. With high oxygen coverage, more species such as CH 2 O 2 and CHO 2 can be formed on metal surfaces. The most stable configuration of CH 2 O 2 that 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 1g. The length of C O and Au O are 1.39 and 2.41 Å. The adsorption energy of formate (CHO 2 )onau (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 1h. The lengths of C O and Au O are 1.25 and 2.34 Å. The C H distance is 1.10 Å. Dehydrogenation of CHO 2 generates CO 2 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 (CH 3 OH (a) CH 3 O (a) +H (a) ), the energy barrier is calculated as 2.00 ev with the reaction energy of 1.29 ev. For β-h elimination (CH 3 OH (a) CH 2 OH (a) +H (a) ), the energy barrier is calculated as 2.20 ev with the reaction energy of 1.15 ev. Then, all dehydrogenation processes to CO starting from both α-h elimination and β-h elimination are investigated. The energy and geometric informations of the whole dehydrogenation process are listed in Table S1 in the Supporting Information, which demonstrate that elimination of the first H atom is the rate-limiting step of the dehydrogenation process. The structures of initial states, transition states, and final states of all elementary steps are shown in Figures S3 and S4 in the Supporting Information. Because of the relatively high barriers, dissociation of methanol on Au (111) is not energetically possible at low temperature. Therefore, oxygen groups 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 preadsorption 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 17513

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 coadsorption configurations are used to investigate the oxidation of methanol on oxygen precovered Au (111) surface starting from α-h and β-h elimination, respectively. A. α-h Elimination: Formation of CH 2 O. For the elimination of α-h reaction CH 3 OH (a) +O (a) CH 3 O (a) + OH (a), an energy barrier 0.27 ev is found. At transition state, the bond lengths d O H and d O1 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, CH 3 O (a) can dissociate with the help of atomic oxygen with an energy barrier of 0.46 ev. At transition state, the bond lengths d O H and d O1 H are 1.19 and 1.48 Å, respectively. Considering its low adsorption energy ( 0.1 ev), CH 2 O, 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 CH 2 O and HCOCH 3 are observed. 19 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. length of 1.42 Å. At transition state, the bond length of O Hin 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 CH 3 O (a) dehydrogenation is 0.48 ev. At transition state the C H distance in CH 3 O (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 2d. B. 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 d O H and d C H are 1.14 and 1.42 Å (Figure 3), respectively. For Figure 2. Geometric structures of initial states, transition states, and final states of the elementary steps of the oxidation of methanol by atomic oxygen or OH starting from α-h elimination. Panels a d correspond to the reaction of CH 3 OH (a) +O (a) CH 3 O (a) +OH (a), CH 3 O (a) +O (a) CH 2 O+OH (a),ch 3 OH (a) +OH (a) CH 3 O (a) + H 2 O (a), and CH 3 O (a) +OH (a) CH 2 O (a) +H 2 O (a), respectively. The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses shows the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). Then, the reaction between hydroxyl and methanol are investigated. First, α-h elimination with the help of hydroxyl is considered. α-h of methanol is hydrogen-bonded with the oxygen of hydroxyl with a bond length of 1.66 Å, as shown in Figure 2c. In the final state, the generated water is also strong hydrogen-bonded with the oxygen in CH 3 O (a) with a bond Figure 3. Geometric structures of initial states, transition states, and final states of the elementary steps of the oxidation of methanol by atomic oxygen starting from β-h elimination. Panels a e correspond to the reaction of CH 3 OH (a) +O (a) CH 2 OH (a) +OH (a),ch 2 OH (a) +O (a) CHOH (a) +OH (a),ch 2 OH (a) +O (a) CH 2 O+OH (a), CHOH (a) +O (a) CHO (a) +OH (a), and CHOH (a) +O (a) COH (a) +OH (a), respectively. The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses shows the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). elimination of the second hydrogen, there are two possible ways: one is the α-h elimination to form CH 2 O 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 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 17514

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 CH 2 OH (a) and H 2 O, the energy barrier is 0.96 ev. At transition state, the distances between C and H are elongated to 1.33 Å, and d O(H) H is shortened to 1.20 Å, as shown in Figure 4a. For the elimination of second surface hydroxyl or being dehydrogenated. The energy barrier of the combination between CH 2 O and atomic oxygen to form CH 2 O 2(a), as shown in Figure 5a, is 0.32 ev. At transition state, Figure 4. Geometric structures of initial states, transition states, and final states of the elementary steps of the oxidation of methanol by hydroxyl starting from β-h elimination. Panels a d correspond to the reaction of CH 3 OH (a) +OH (a) CH 2 OH (a) +H 2 O (a),ch 2 OH (a) + OH (a) CHOH (a) +H 2 O (a), CHOH (a) +OH (a) CHO (a) +H 2 O (a), and CHOH (a) +OH (a) COH (a) +H 2 O (a), respectively. The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses show the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). hydrogen with the help of hydroxyl, there are two possible ways: one is α-h elimination to form CH 2 O and the other is β- H elimination to CHOH (a). The former process is almost barrierless when OH (a) and CH 2 OH (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 4b. 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 CO 2 directly with a reaction barrier of 0.39 ev or by OH (a) to form COOH (a) and then CO 2 by the help of atomic oxygen (or hydroxyl or water) at low temperature. 13 3.4. Evolution of CH 2 O on Atomic Oxygen- or Hydroxyl-Covered Au (111) Surface. When CH 2 O is formed, it can combine with surface atomic oxygen 27,28 or Figure 5. Reaction between CH 2 O with surface atomic oxygen and surface hydroxyl and the evolution of formed H 2 COOH with surface atomic oxygen and hydroxyl: (a) CH 2 O+O (a) CH 2 O 2(a), (b) CH 2 O +OH (a) H 2 COOH (a), (c) H 2 COOH (a) +O (a) CH 2 O 2(a) +OH (a), (d) H 2 COOH (a) +O (a) HCOOH (a) +OH (a), (e) H 2 COOH (a) + OH (a) HCOOH (a) + H 2 O (a), (f) H 2 COOH (a) + OH (a) HCOOH (a) +H 2 O (a), and (g) CH 2 O+O (a) CHO (a) +OH (a). The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses shows the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). the distance between C and atomic oxygen is 2.25 Å. CH 2 O also easily reacts with surface hydroxyl to form CH 2 OOH (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 5b. With the help of atomic oxygen or hydroxyl, the α-h elimination of CH 2 OOH (a) is an easy process with an energy barrier of 0.21 or 0.19 ev to produce CH 2 O 2(a) on Au (111) surface. For the β-h elimination of CH 2 OOH (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 CH 2 OOH 17515

Table 3. Calculated Energy Barriers (E a ), Reaction Energies (ΔH), and Key Geometric Parameters of Transition States of Each Possible Elementary Step of Methanol Oxidation on Au (111) Surface with High Oxygen Coverage reactions E a (ev) E ZPE (ev) ΔH (ev) bond lengths (Å) CH 2 O (a) +O (a) CH 2 O 2(a) 0.32 0.35 0.42 d CHO O = 2.25 CH 2 O (a) +OH (a) H 2 COOH (a) 0.06 0.04 0.54 d CH2O - OH = 3.17 H 2 COOH (a) +O (a) H 2 CO 2(a) +OH (a) 0.21 0.06 0.07 d H2COO - H = 1.51 d H O = 1.28 H 2 COOH (a) +O (a) HCOOH (a) +OH (a) 0.39 0.26 2.30 d HOOHC - H = 1.17 d H O = 1.57 H 2 COOH (a) +OH (a) H 2 CO 2(a) +H 2 O (a) 0.19 0.01 0.14 d H2COO - H = 1.18 d H OH = 1.23 H 2 COOH (a) +OH (a) HCOOH (a) +H 2 O (a) 0.62 0.45 2.49 d HOOHC - H = 1.18 d H OH = 1.59 HCOOH (a) +OH (a) COOH (a) +H 2 O (a) 0.47 0.03 0.58 d HCOO - H = 1.37 d H O = 1.09 HCOOH (a) +OH (a) HCOO (a) +H 2 O (a) 0.01 0.08 0.60 d H2COO - H = 1.12 d H OH = 1.29 CH 2 O 2(a) CHO 2(a) +H (a) 0.30 0.26 1.25 d O2CH - H = 1.09 CHO 2(a) CO 2(g) +H (a) 0.95 0.84 0.25 d O2C - H = 1.17 CH 2 O 2(a) +O (a) CHO 2(a) +OH (a) 0.12 0.02 3.35 d O2CH - H = 1.15 d H O = 1.90 CHO 2(a) +O (a) CO 2(g) +OH (a) 1.16 1.01 1.96 d O2C - H = 1.18 d H O = 1.60 CH 2 O 2(a) +OH (a) CHO 2(a) +H 2 O (a) 0.19 0.08 3.27 d O2CH - H = 1.18 d H OH = 1.66 CHO 2(a) +OH (a) CO 2(g) +H 2 O (a) 1.01 0.84 2.07 d O2C - H = 1.45 d H OH = 1.23 only weakly adsorbs on clean Au (111) surface with an adsorption energy of 0.03 ev. All of the previous steps are exothermic as shown in Table 3. The evolution of CH 2 O 2 and HCOOH will be discussed later. Except combination with atomic oxygen or hydroxyl, CH 2 O 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 5g, but unfortunately we cannot determine the transition state of the dehydrogenation of CH 2 O by hydroxyl. 3.5. Evolution of HCOOH on Atomic-Oxygen- or Hydroxyl-Covered Au (111) Surface. Although 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 precovered 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 distances between α-h and oxygen in HCOOH and adsorbed hydroxyl are 1.12 and 1.29 Å, respectively. The strong hydrogen bond makes Figure 6. Evolution of generated HCOOH (a) on high oxygen coverage surface: (a) hydrogen bonded with atomic oxygen and (b) hydrogen bonded with surface hydroxyl. The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses shows the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). HCOOH easy to convert to CHO 2(a). Then, the further oxidation of CHO 2(a) forms CO 2. The important geometric parameters at transition states are listed in Table 3. 3.6. Evolution of CH 2 O 2 on Atomic Oxygen or Hydroxyl-Covered Au (111) Surface. The decomposition processes of CH 2 O 2(a) by itself and with the help of atomic oxygen and hydroxyl are investigated. It is found that the energy barriers of CH 2 O 2(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 CH 2 O 2(a) is also easy with an energy barrier of 0.19 ev to form CHO 2(a). Therefore, CH 2 O 2(a) is ready to be oxidized to CHO 2(a) with the present of surface atomic oxygen, hydroxyl, and even only gold surface. For the self-dehydrogenation of CHO 2(a), the dissociation barrier is calculated as 0.95 ev. The presence of atomic oxygen and hydroxyl even prohibits this process with a higher energy barrier of 1.16 and 1.01 ev. This is similar to the case on Ag(111) surface where the presence of atomic oxygen or hydroxyl increases the reaction barrier of CHO 2(a) dehydrogenation. 27 Thus, the dissociation of CHO 2(a) is the time-limiting step of the formation of CO 2 at high oxygen coverage. The key parameters of all 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 CH 2 O, HCOOCH 3, CO, CO 2, and so on. It is suggested, on high atomic oxygen coverage surface, that the main product is CO 2. However, on low atomic oxygen coverage surface, with different atomic oxygen preparation methods, different partial oxidation products are observed. 19 On the basis of 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 CH 2 O and HCOOCH 3(a) formation have been carefully investigated in previous theoretical work. 26,28 It is suggested the production of CH 3 O is the critical step of the whole reaction and α-h can be abstracted by atomic oxygen, hydroxyl, and also surface CH 3 O (a) species with low-energy barriers. The second step is the dehydrogenation of β-h of CH 3 O (a).inxu swork, the energy barriers of self-dehydrogenation, with the help of atomic oxygen, hydroxyl, and another CH 3 O (a), are calculated as 0.64, 17516

methanol on Au (111) surface with low atomic oxygen coverage may proceed starting from β-h elimination. The generated hydroxyl group is close to β-h (as shown in Figure 4) rather than α-h, and the elimination of second β-h with hydroxyl to form CHOH (a) is expected, and thus for the low oxygen coverage we can expect the formation of CHOH (a) rather than CH 2 O. If CHOH (a) is formed, two cases are needed to be considered: one is the elimination of α-h and the other is the third β-h. On the basis of our calculations, CHOH (a) is easier to be oxidized to CHO (a) with an energy barrier of 0.07 ev. Further oxidation of CHO (a) by atomic oxygen and hydroxyl group is almost barrierless, and the processes may be controlled by the diffusion of surface species such as atomic oxygen, hydroxyl, or CHO (a) itself, and CO (a) is formed on Au (111) surface. The most possible paths to produce CO are shown in Figure 8, and all possible elementary steps are listed in Table S2 in the Supporting Information. Figure 7. Dehydrogenation processes of CH 2 O 2(a) by itself or by surface atomic oxygen and surface hydroxyl: (a) CH 2 O 2(a) CHO 2(a) +H (a), (b) CHO 2(a) CO 2(g) +H (a), (c) CH 2 O 2(a) +O (a) CHO 2(a) +OH (a), (d) CHO 2(a) +O (a) CO 2(g) +OH (a), (e) CH 2 O 2(a) + OH (a) CHO 2(a) +H 2 O (a), and (f) CHO 2(a) +OH (a) CO 2(g) + H 2 O (a). The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses show the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). 0.49, 0.63, and 0.66 ev, respectively, 26 which suggests that if CH 3 O (a) is present on Au(111) surface, CH 2 O is ready to form. However, Gong et al. only observed CO and CO 2 desorption peaks at low temperature (<180 K) in temperatureprogrammed reaction spectra (TPRS) 20 with low oxygen coverage and no other partial oxidation products are observed. Thus, there should be a reaction path avoiding the formation of CH 2 O, which corresponds to the formation of CO. On the basis of our calculations (as listed in Table 2), the energy barrier of the dehydrogenation of α-h with the help of coadsorbed atomic oxygen is 0.27 ev with reaction energy of 0.19 ev (endothermic), which agree well with 0.41 for energy barrier and 0.27 for reaction energy in previous theoretical work. 26 It is noticed that an extra 0.10 ev is needed to break the hydrogen bond between CH 3 O (a) and OH (a) in the final state. Thus, the formation of CH 3 O (a) is not energy favorable. However, it is found that the process of β-h elimination by atomic oxygen to form CH 2 OH (a) and OH (a) is 0.26 ev exothermic and the energy barrier of this process is 0.68 ev and with ZPE correction it is reduced to 0.50 ev (0.45 ev in previous work 28 ), which indicates that this process can proceed at low temperature. Thus, at low temperature, the oxidation of Figure 8. Schematic diagram of the production of CO by methanol oxidation on Au(111) surface at low atomic oxygen coverage. The selectivity of the formation CH 2 O (path-α) or CHOH (path-β) by the reactions between CH 3 OH and one atomic oxygen is listed in Table S3 in the Supporting Information and is analyzed by simple microkinetic analysis to give a further explanation on the selectivity of production of CH 2 OorCOat low oxygen coverage on Au (111) surface. 29,30 (More details can be found in the Supporting Information.) The potential energy surfaces and the relative selectivity of two paths are shown in Figure 9. The overall energy barriers for path-α and path-β are 0.45 ev (E a α ) and 0.39 ev (E a β ) corresponding to gas-phase CH 3 OH molecule, respectively. The rate constants between 100 and 500 K are calculated and listed in Table S4 in the Supporting Information. It is suggested that at experimental temperature (100 200 K), the selectivity of path-β is about three orders higher than that of path α, as shown in Figure 9, which indicates the selectivity of the formation of CHOH, and thus CO and agrees well with the observation of CO in experiment. 20 It is noticed that the highest energy barrier of elementary step in path α (0.48 ev) is lower than that in path β (0.68 ev). Why is the overall energy barrier of path α higher than that of path β? From energy potential surface shown in Figure 10, it is found that the first step of path α is endothermic, which increases the energy barrier of second step higher corresponding to CH 3 OH. Thus, the formation of CO is actually controlled by thermodynamics. However, if the elementary of formation of CH 3 O (a) converts to an exothermic process, for 17517

Figure 9. Potential energy surfaces and the relative selectivity of the reaction between CH 3 OH and one surface atomic oxygen via path α to CH 2 O (g) and path β to CHOH (a). Figure 10. Formation of CH 2 O on high oxygen coverage surface: CH 3 OH (a) +3O (a) CH 2 O (a) + 2OH (a) +O (a). The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses show the energy barriers (electronvolts) of the elementary steps with and without zero-point energy corrections (ZPEs). the low-energy barrier of each elementary step, path α will dominantly exceed path β, which indicates that path α is kinetic controlled. Actually, the thermodynamics property of the formation of CH 3 O (a) and surface hydroxyl is sensitive to the local structure or metal species of the catalyst. For example, on Au(110) surface, the energy of the formed CH 3 O (a) and OH (a) is 0.73 ev more stable than the coadsorption of CH 3 OH (a) and O (a), which indicates the possibility of the formation of CH 3 O (a) on Au (110) surface, which is consistent with the experimental observation. 2 Also, on Cu (111) or Ag (111) surface, the formation of CH 3 O (a) by the exothermic α-h elimination of methanol is dominated, which are reported by the experimental observation. 18 20 So, if the surface structure changes for the different preparation method of atomic oxygen to make the formation of CH 3 O (a) energy favorable, products from CH 3 O (a) via α-h elimination should be observed. The formed CO (a) can be oxidized by atomic oxygen or hydroxyl to CO 2(g) 13 at low temperature. The energy profiles with lowest energy barrier for each step are described in Figure 8, and more detailed descriptions are listed in Table S2 in the Supporting Information. B. Oxidation Paths at High Oxygen Coverage. Three atomic oxygen atoms are put on Au (111) surface to investigate the oxidation process of methanol at high oxygen coverage. Interestingly, it is found that a concerted reaction of two hydrogen abstraction proceeds simultaneously and the energy barrier is calculated as 0.47 ev, as shown in Figure 10. CH 2 O (a) is formed and hydrogen-bonded with the formed hydroxyl groups with adsorption energy of 0.40 ev, and the energy barrier for forming H 2 COOH from CH 2 O and hydroxyl group is calculated as 0.06 ev with 0.54 ev energy release. Similarly, the energy barrier of the reaction between hydrogen-bonded CH 2 O and neighboring atomic oxygen is calculated as 0.32 ev with 0.42 ev energy released. The increasing interaction and fast reaction between CH 2 O and atomic oxygen or hydroxyl on surface may result in the absence of CH 2 O in the gas phase, which has been suggested in a previous experimental observation on Au(110) surface where no gas-phase CH 2 Ois detected. 2 Then, the further dehydrogenation of formed CH 2 OOH or CH 2 O 2(a) leads to the formation of CO 2. According to previous results, CH 2 O 2(a) can dehydrogenate by self-elimination or with the help of atomic oxygen and hydroxyl, and the barriers are 0.30, 0.12, and 0.19 ev, respectively, which indicates that this elementary step is ready to happen on Au (111) surface. As for the dehydrogenation processes of CH 2 OOH (a), α-h can be eliminated by surface atomic oxygen or hydroxyl with energy barrier of 0.21 or 0.19 ev, respectively. The energy barrier for β-h elimination by atomic oxygen and hydroxyl is calculated as 0.39 and 0.62 ev, respectively. For the relative low energy barriers, CH 2 O 2(a) is expected as the main intermediate for the dehydrogenation of CH 2 OOH (a). After CH 2 O 2(a) is formed on the surface, it is ready to be dehydrogenated to CHO 2(a) with and without surface species such as atomic oxygen or hydroxyl. The last step of the formation of CO 2 is the dehydrogenation of CHO 2(a), and the energy barriers on clean Au (111), with atomic oxygen and hydroxyl are calculated as 0.95, 1.16, and 1.01 ev, which indicates that the dehydrogenation prefers to proceed on clean Au (111) surface. This step is the rate-limiting step of the formation of CO 2, which is responsible for the higher CO 2 desorption peak in experiment. With ZPE correction included, the energy barriers are lowered to 0.84 17518

ev, which corresponds to the CO 2 peak at high temperature on high atomic oxygen coverage Au (111) surface. The whole processes with lowest energy barrier for each step are described in Figure 11, and more detailed descriptions are listed in Table 3 and Table S5 in the Supporting Information. Figure 11. Schematic diagram of CO 2 formation by methanol oxidation on Au (111) surface at high atomic oxygen coverage. Except for the formation of CO 2 from CH 2 O combination with atomic oxygen or hydroxyl, CO also can be produced from the hydrogen abstraction by atomic oxygen with energy barrier of 0.31 ev, and the dehydrogenation of CHO to form CO by atomic oxygen or hydroxyl is almost barrierless, as previously discussed. Then, CO can be oxidized by O or OH at low temperature, as previously discussed. Thus, the dehydrogenation of CH 2 O by atomic oxygen responds for the CO and CO 2 observed at low temperature on Au (111) surface with high oxygen coverage. 20 4. CONCLUSIONS In conclusion, this work represents an attempt to gain insight into the mechanisms of methanol dehydrogenation and oxidation on oxygen-covered Au (111) surface. On the basis of our theoretical results, it is suggested that on Au (111) surface with low oxygen coverage the formation of CO and CO 2 is initiated from the elimination of β-h. The reaction routine to form CO is suggested as CH 3 OH (a) + O (a) CH 2 OH (a) +OH (a) CHOH (a) +H 2 O and then CHOH (a) +O (a) CHO (a) +OH (a) CO (a) +H 2 O. Although the energy barrier of elementary step of path-α is lower than path-β, the endothermic process of the first step of the α-h elimination makes the overall energy barrier of higher than that of path-β. The overall energy barrier of path-β is 0.06 ev lower than that of path-α, which makes the selectivity to form CHOH rather than CH 2 O at low temperature. It is proposed that if the first step of path-α is exothermic such as on Au(110), Cu(111), or Ag(111) surface path-α will dominantly proceeds for the lowenergy barrier. The discrepancy of experimental observations may come from the different surface configuration stimulated by different atomic oxygen preparation methods. On surface with high oxygen coverage, CH 2 O can be formed. However, it is ready to combine with surface atomic oxygen or hydroxyl with low-energy barrier to form CH 2 O 2(a) or CH 2 OOH. The rate-limiting step is the dehydrogenation of CHO 2(a) with an energy barrier of 0.95 ev, which agrees well with the desorption peak of CO 2 at high temperature 300 K in experiment. CH 2 O can also be dehydrogenated by surface oxygen with low-energy 17519 barrier, and thus CO and CO 2 can be formed at low temperature. ASSOCIATED CONTENT *S Supporting Information Top view of the adsorption configuration of possible species, the diffusion paths of O, OH, COH, CHO, CHOH on Au (111) surface, the self-dehydrogenation processes of CH 3 OH starting from α-h elimination and β-h elimination on clean Au(111) surface, all possible elementary steps of CH 3 OH oxidation to CO and CO 2 on atomic oxygen precovered Au (111) surface at low or high coverage, as well as the details of microkinetics analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Fax: + 86-551-3606408. E-mail: whhzhang@ustc.edu.cn. (W.Z.) *E-mail: jlyang@ustc.edu.cn. (J.Y.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21103156, 21233007), National Basic Research Program of China (2013CB933104, 2010CB923301), MOE Fundamental Research Funds for the Central Universities, General Financial Grant from the China Postdoctoral Science Foundation (2012M510159), the USTC- HP HPC project, the USTC-Lenovo 1800 project, and Shanghai Supercomputer Center. REFERENCES (1) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. Gold Catalysts Prepared by Coprecipitation for Low-temperature Oxidation of Hydrogen and of Carbon Monoxide. J. Catal. 1989, 115, 301 309. (2) Outka, D. A.; Madix, R. J. Broensted Basicity of Atomic Oxygen on the Gold(110) Surface: Reactions with Methanol, Acetylene, Water, and Ethylene. J. Am. Chem. Soc. 1987, 109, 1708 1714. (3) Sakurai, H.; Haruta, M. Carbon Dioxide and Carbon Monoxide Hydrogenation Over Gold Supported on Titanium, Iron, and Zinc Oxides. Appl. Catal., A 1995, 127, 93 105. 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