A First-Principle Study of Synergized O2 Activation ... - ACS Publications

Subscriber access provided by University of Pennsylvania Libraries

Article 2

A First Principle Study of Synergized O Activation and CO Oxidation by Ag Nanoparticles on TiO(101) Support 2

Chuanyi Jia, Guozhen Zhang, Wenhui Zhong, and Jun Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01369 • Publication Date (Web): 06 Apr 2016 Downloaded from http://pubs.acs.org on April 13, 2016

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.

ACS Applied Materials & Interfaces 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 32

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

ACS Applied Materials & Interfaces

A First Principle Study of Synergized O2 Activation and CO Oxidation by Ag Nanoparticles on TiO2(101) Support Chuanyi Jia a,b, Guozhen Zhang c,Wenhui Zhong a,b, Jun Jiang *b,c,d

a) Guizhou Provincial Key Laboratory of Computational Nano-material Science, Institute of Applied Physics, Guizhou Education University, Guiyang, 550018, China

b) Guizhou Synergetic Innovation Center of Scientific Big Data for Advance Manufacturing Technology, Guizhou Education University, Guiyang, 550018, China

c) School of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China (USTC), Hefei, 230026, China

d) CAS Key Laboratory of Mechanical Behavior and Design of Materials (LMBD), University of Science and Technology of China (USTC), Hefei, 230026, China

Keywords: O2 activation, CO oxidation, First principle study, Synergistic effect, Polarized charge

1

ACS Paragon Plus Environment


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

ABSTRACT: We performed density functional theory (DFT) calculations to investigate the synergized O2 activation and CO oxidation by Ag8-cluster on TiO2(101) support. The excellent catalytic activity of the interfacial Ag atoms in O2 dissociation is ascribed to the positive polarized charges, up-shift of Ag d-band center, and assistance of surface Ti5c atoms. CO oxidation then takes place via a two-step mechanism coupled with O2 dissociation: (i) CO+O2→CO2+O, and (ii) CO+O→CO2. The synergistic effect of CO and O2 activations reduces the oxidation energy barrier (Ea) of reaction (i), especially for the up-layered Ag atoms not in contact with support. It is found that the co-adsorbed CO and O2 on the up-layered Ag atoms form a metal-stable four-center O-O-CO structure motif substantially promoting CO oxidation. On the oxygen defective Ag8/TiO2(101) surface, because of the decreased positive charges and the down-shift of d-band centers in Ag, the metal cluster exhibits low O2 adsorption and activation abilities. Although the dissociation of O2 is facilitated by the TiO2(101) defect sites, the dissociated O atoms would cover the defects so strongly that prohibit further CO oxidation unless much extra energy is introduced to re-create oxygen defects.

2


Page 2 of 32

Page 3 of 32

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


INTRODUCTION Noble metal nanoparticles dispersed on metal oxide semiconductor support have received considerable attentions in recent years because of their high activity in many important catalytic oxidation processes, including CO oxidation,1-6 selective oxidation of organic compounds,7-10 and photocatalytic oxidation reactions.11,12 The high activity were ascribed to the large fraction of under-coordinated surface atoms,1,8 dynamic structural fluxionality,6 and strong interactions between deposited cluster and support.13-15 Comprehensive control of these variables requires a quantitative understanding of the structural and electronic properties and catalytic oxidation processes at atomic level. These knowledge will provide useful guidance towards targeted design of metal-semiconductor hybrid catalysts.16 Among many noble-metal/support hybrid catalysts,17-27 Ag-based nanocatalysts are of particular interest because of their unique catalytic, electric, and optical properties.17-20 In recent years, a large amount of experimental and theoretical studies have been carried out on the catalytic effect of oxide surface supported Ag nanoparticles.28-35 Those researches proposed a variety of oxidation mechanisms to understand the reaction processes, and revealed many influencing factors for catalysts performance optimization. On Ag/SiO2, Zhang et al. suggested that Ag nanoparticle deposited SiO2 is an efficient catalyst for low-temperature CO oxidation and the particle size plays a very important role in the catalytic activity.28 On Ag/Al2O3, Lei et al., based on their experimental results, pointed out that the nanometer sized Ag 3



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

clusters are very active for direct propylene epoxidation and the binding mechanism of metal on support matters.29 Among many types of supports, the anatase TiO2 support is perhaps the most commonly used one because of its wide applications for catalytic oxidation reactions. By conducting laser pulse excitations on core-shell composite clusters of Ag/TiO2, Hirakawa et al. revealed that the charge separation, charge storage, and interfacial charge transfer are all beneficial for O2 activation.30 Another research on a series of silver-deposited anatase TiO2 nanoparticles was carried out by Yang et al. 31 Their results showed that the Ag on TiO2 could inject additional electrons into molecules adsorbed on the Ag/TiO2 composite catalyst, which would further promoting catalytic activities. For CO oxidation reactions, Frey’s study32 showed that the best conversion performance is obtained in a CO/O2 = 1:1 mixture over Ag/TiO2(anatase) catalyst with 10% Ag loading. These exciting findings from different aspects thus require systematic theoretical studies to shed light on the actual mechanisms, and more importantly to examine the possibility of synergizing various processes toward high catalytic performance. Although metal oxide-supported Ag nanoparticles have been extensively studied, there are still many important issues remaining unclear, such as the oxygen activation mechanisms on Ag clusters affected by substrate, the synergistic effect between adjacent reactants (e.g. CO and O2), and the influence of semiconductor surface oxygen defects. Resolving these issues will greatly improve our understandings of the catalytic oxidation processes on nanometer sized noble-metal-based catalysts. 4


Page 4 of 32

Page 5 of 32

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


In this contribution, we have chosen a model system of Ag8-cluster on the anatase TiO2(101) surface which had been reported as an active and reasonable model for various catalytic reactions,36,37 and performed density functional theory (DFT) calculations to examine the synergized catalytic mechanisms of O2 activation and CO oxidation, as well as the influence of metal-support interactions. Our calculation results show that the catalytic activity can be improved by the positive polarized charges and up-shift of d-band centers of Ag atoms due to support interfacing, as well as the synergistic reaction of O2 dissociation and CO oxidation. The generation of oxygen defects on the support surface could change the reaction mechanism of CO oxidation. However, because the re-activation of oxygen defects after O2 dissociation needs to overcome high energy barrier, their synergized promoting effect can only work at high temperature. CALCULATION DETAILS The DFT calculations were performed with the Vienna ab initio simulation package (VASP)38 and made use of the Perdew-Burke-Ernzerhof (PBE)

39

functional and

periodic boundary condition(PBC). A plane-wave basis set using the projector augmented wave (PAW) scheme40-42 was adopted with a kinetic energy cutoff of 400 eV. For structural optimizations, the convergence criterion of total energy was set to be 10-5 eV and that of force on each atom was set to be 0.02 eV/Å, and a Monkhorst–Pack grid of 2 × 2 × 1 k-points was used (convergence test in Table S1 in the Supporting Information). For further density of states (DOS) calculations the 5



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

k-points was increased to 15 × 15 × 1. The climbing image nudged elastic band (CI-NEB) method was used to search the minimum energy paths of O2 dissociation and CO oxidation reactions.43,44 It is known that because of the incomplete self-interaction cancellation for transition metal, the DFT methods can not accurately model Ti atoms which have strongly correlated localized 3d electrons. To mitigate such localization deficiency, the DFT+U method was employed. The value of U = 3.5 eV reported in the work of Sorescu et al. was chosen in our calculations.45 The bare anatase TiO2(101) surface was modeled using a 3 × 1 supercell. 108 atoms in each slab were distributed in six trilayers. The self-interaction effects of the periodic boundary conditions were screened by a 15 Å vacuum gap above surface. The structure of Ag8-clusters were optimized in a unit cell of 15 Å × 15 Å × 15 Å and only the Ӷ-point was used. Because the Ag(111) surface was a common exposed surface for Ag clusters on TiO2 support, 46,47 the initial bond lengths and bond angles of Ag8-clusters were all referred to this surface. For geometry optimization, the adsorbates (Ag8, CO and O2) and the three topmost trilayers of TiO2(101) surface were allowed to relax, whereas the bottom three trilayers were frozen to simulate the bulk effects. The binding energies of the adsorbates were defined as: Eb = Esur + ad − Esur − Ead , where Esur + ad represents the total energy of the surface with the adsorbates, Esur represents the energy of the clean surface, and Ead represents the total energy of the 6


Page 6 of 32

Page 7 of 32

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


adsorbates. According to the above definition, a negative value of Eb corresponds to an exothermic process, whereas a positive value corresponds to an endothermic process. RESULTS AND DISCUSSION O2 Activation on the Stoichiometric Ag8/TiO2(101) System. To find a stable structure of Ag8-cluster, we optimized various Ag8-clusters in the gas phase and then again on the stoichiometric TiO2(101) surface. The optimized binding structures for Ag8-clusters (in both planar and non-planar form) on the stoichiometric TiO2(101) surface are illustrated in Fig. S1 in the Supporting Information, among which the Ag8/TiO2-1 structure presented in Fig. 1 is the most stable one. The most stable state in Fig. 1 shows that charge polarization takes place at the interface when Ag8-cluster adsorbs on the support surface. Because the computed work function of pure silver (4.26 eV, in Fig. S2) is smaller than bare stoichiometric TiO2(101) surface (6.60 eV, in Fig. S3), the Ag8-cluster donates electrons (effectively 0.531e) to TiO2(101) and itself become positive-charged. Normally, the positive-charged sites can act as Lewis acids to attract the O atoms in O2,48 which is favorable for O2 activation. However, these eight Ag atoms are unevenly charged according to the Bader charges49,50 analysis. It is found that the Ag-1, Ag-2 and Ag-3 atoms which are in direct contact with surface O atoms hold more positive charges, while the charges carried by non-contacting Ag atoms (Ag-4, Ag-5 and the second layered Ag atoms) are close to neutral. Therefore, the charge transfer mostly involves the Ag atoms that are in direct 7



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

contact with the substrate. Moreover, the size effect of Ag cluster has been tested by comparing Ag8 with a smaller cluster of Ag7 on the TiO2(101) surface (Fig. S4, S5, S6). The computational results of Ag7/TiO2(101) system are consistent with those of Ag8-cluster.

Fig. 1. The most stable optimized structure of Ag8-cluster on the stoichiometric TiO2(101) surface. The Bader charges carried by Ag atoms are shown at the right side. Color coding: light blue, Ag atoms; red, O atoms; gray, Ti atoms.

Among various possible binding structures of O2, six representative conformers were identified to be stable in simulations, as labeled with Per-O2-1~6 in Fig. 2. The spin polarization effect was tested and proved to be negligible in the systems under investigation (Fig. S7). As expected, the interfacial Ag-1 (or Ag-3) and Ag-2 sites with significant positive polarized charges are active for O2 adsorption. In addition, by comparing the conformer Per-O2-1 with other 5 configurations in Fig. 2 and the configuration of O2 on the Ti5c site of perfect TiO2(101) surface (TiO2-O2 state in Fig. S8), we found that the participation of five-coordinated surface Ti atom (Ti5c) can significantly promote the adsorption of O2. The Ti5c-O2 interactions help stabilize the 8


Page 8 of 32

Page 9 of 32

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


Per-O2-1, making its energy to be >1.6 eV lower than those in Per-O2-2 to Per-O2-6 (where O2 couples weakly with Ti atoms). Thus, the interfacial Ag sites near the surface Ti5c atoms will be the best sites for anchoring O2.

Fig. 2. Representative binding structures and binding energies of O2 adsorbed by the Ag8-cluster supported on the stoichiometric TiO2(101) surface. Color coding: blue, O atoms in adsorbed O2; others are the same as in Fig. 1. It is interesting to note that conformer Per-O2-2 where O2 is attached to Ag-6, Ag-7, and Ag-8 is more stable than conformer Per-O2-4 where O2 is attached to Ag-2, Ag-6, and Ag-7. This is the indicative of other factor(s) than polarized charges on Ag atoms that may play a non-trivial role in the O2 adsorption. It is known that, besides polarized charge, the d-band electronic states in transition metal cluster are also closely related to O2 adsorption. Projected d-band partial density of states (pDOS) of 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

the initial eight Ag atoms on TiO2(101) surface are shown in Fig. 3. It is apparent that the d-band centers of all Ag atoms experience positive shift towards the Fermi level, where the value of the 8-th Ag atom (Ag-8) is the most positive one among all atoms. As a result, the antibonding states of the Ag-8 atom will be pushed above the Fermi level, and the Pauli repulsion will be decreased.51 Such responses can increase the interactions between Ag and O atoms, making the adsorption state Per-O2-2 more stable.

Fig. 3. Projected d-band states of the initial Ag atoms on the stoichiometric TiO2(101)-O surface. The d-band center is marked by black arrow. The Fermi level is specified to 0 eV for all Ag atoms. (a) pDOS of the Ag atom on the pure Ag(111) surface; (b) pDOS of the Ag-1 or Ag-3 atom; (c) pDOS of the Ag-2 atom; (d) pDOS of the Ag-4 or Ag-5 atom; (e) pDOS of the Ag-6 or Ag-7 atom; (f) pDOS of the Ag-8 atom. 10


Page 10 of 32

Page 11 of 32

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


After elucidating the molecular adsorption, we now turn to explore the dissociative process of O2. As depicted in Fig. 4, both dissociation processes at the interfacial site and on the up-layered Ag atoms are thermodynamically favorable, evidenced by their exothermal characteristics (more than 0.98 eV). Moreover, because the energy barrier of the dissociation process at the interfacial site (TS1, Ea=1.29 eV) is much lower than that on the up-layered Ag atoms (TS2, Ea=1.93 eV), the O2 activation at the interfacial site will be kinetically preferable.

Fig. 4. Potential energy profiles for the dissociation of O2 by Ag8-cluster on stoichiometric TiO2(101) surface.

11



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

CO

oxidation

coupled

with

O2

dissociation

on

Page 12 of 32

the

Stoichiometric

Ag8/TiO2(101) System. It is well accepted that the synergistic effect between the reactants usually play important roles in the catalytic oxidation reactions on noble-metal-based catalysts.52,53 In order to have a comprehensive understanding of Ag8/TiO2(101) catalyst, a further investigation of coupled O2 dissociation and CO oxidation has been carried out. Different from the adsorption of O2, the main binding forces of CO are generated by Ag-C bonds, as seen in Fig. S9. Since the C atom in CO holds positive charges, the positive polarized charges on the interfacial Ag atom (Ag-1, 2, 3 in Fig. 1) thus disfavor CO adsorption. The computed binding energies in Fig. S9 demonstrated that the CO adsorption is more inclined to occur on the up-layered Ag atoms. Fig. 5 shows the most stable conformers of CO and O2 co-adsorption at the interfacial site (binding structure Per-CO+O2-1), as well as on the up-layered Ag atoms (binding structure Per-CO+O2-2). When CO and O2 are co-adsorbed on the up-layered Ag atoms (Per-CO+O2-2), a stable four-center O-O-CO complex is formed14 due to strong interactions between OI and C atoms. Apparently this synergistic adsorption is energetically more favorable than the simple adds-up of separated adsorption of CO (Per-CO-1 in Fig. S9) and O2 (Per-O2-2 in Fig. 2) (the binding energy of Per-CO+O2-2 is much higher than the summation of Per-CO-1 and Per-O2-2). In contrast, such a four-center complex has not been found in the interfacial co-adsorption. 12


Page 13 of 32

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


Fig. 5. The side (left) and top (right) views of the most stable co-adsorption states of CO and O2 at the interfacial site of Per-CO+O2-1 and on the up-layered Ag atoms of Per-CO+O2-2. Color coding: brown, C atoms; others are the same as in Fig. 2. The reaction coordinate profile in Fig. 6 shows that CO oxidation occurs at the interfacial site via a two-step mechanism: (i) CO+O2→CO2+O, and (ii) CO+O→CO2. The co-adsorbed CO and O2 can react to form a gas-phase CO2 and a residual Ag-O* on Ag8-cluster via an exothermic process with -3.49 eV. The energy barrier (TS3) is 0.95 eV which is lower than that of the direct dissociation of O2 at the interfacial site (TS1, Ea=1.29 eV). Then, the residual Ag-O* directly reacts with an additional CO molecule, through a small energy barrier of 0.02 eV (TS4). In this reaction path, the

13



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

rate-determining step is the reaction between adsorbed CO and O2. For another reaction path where the CO oxidation takes place on the up-layered Ag atoms, it involves a similar step-wise mechanism to that of the interfacial oxidation process. It is interesting to note that the energy barrier of this CO+O2 step (TS5) is merely 0.05 eV, much lower than that at the interfacial site (TS3, Ea=0.95 eV). This is due to the synergetic effect between the new C-O bond formation and O-O bond dissociation, in which the positive charges on C atom promote the dissociation of neighboring O2 and the co-adsorbed CO provides an adjacent site to anchor the produced O atom. For the CO+O step, the energy barrier (TS6) is 0.16 eV higher than the first step. As a result, the reaction between CO and Ag-O* will be the rate-determining step for CO oxidation.

Comparison of the reaction coordinates of the above two paths indicate that the coupled CO oxidation and O2 dissociation on the up-layered Ag atoms is kinetically more favorable than separated events at the interfacial site. In this sense, the up-layered Ag atoms in the Ag8-cluster is preferable for CO oxidation, albeit interfacial Ag atoms is favorable for producing O atom required for CO oxidation. This suggests that the thin and flat structures of Ag clusters on TiO2(101) surface containing more interfacial Ag atoms will favor O2 activation, while synergized O2 and CO activations prefer stereo structures.

14


Page 14 of 32

Page 15 of 32

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


Fig. 6. Potential energy profiles for CO oxidation by Ag8-cluster on stoichiometric TiO2(101) surface.

O2 Activation on the Oxygen-Defective Ag8/TiO2(101) System. Besides interface interaction, oxygen defects on metal oxide support may provide another knob to engineer the atomic structure, polarized charge distribution and d-band of metal cluster, so as to synergize O2 and CO activations toward high catalytic activity.52,54 A three-step mechanism is proposed to account for the process. (i) O2 → OI + OII, (ii) CO + OII→ CO2, and (iii) CO + OI → CO2. To describe the formation of O atoms promoted by oxygen defects, two stable and typical oxygen defects (two-coordinated oxygen defect Vo2c and three-coordinated 15



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

oxygen defect Vo3c) are investigated, as depicted in Fig. 7. It is found that the generation of oxygen defect decreases the positive polarized charges transferred to Ag8-cluster. For example, both Ag-5 on the Vo2c defective surface and Ag-1 on the Vo3c defective surface hold more negative charges by 0.2e relative to their counterparts on the ideal surface. The pDOS analysis in Fig. 8 shows that compared to the corresponding Ag atoms on the stoichiometric surface, their d-band centers have been down-shifted to lower energies. Such changes often disfavor the adsorption of O2. For the 8-th Ag atom on Vo3c defective surface, the appreciable down-shift of its d-band center leads to the decrease of O2 adsorption stability (Fig. S11), as compared with the counterpart Ag atom on the Vo2c defective (Fig. S10) and stoichiometric surface (Fig. 2). For other Ag atoms, because their polarized charges and d-band centers are not effectively changed, the adsorption energies of O2 on these sites are not significantly altered (as seen in Fig. S10 and Fig. S11).

16


Page 16 of 32

Page 17 of 32

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


Fig. 7. Optimized structures of Ag8-cluster on the Vo2c (left) and Vo3c (right) defective TiO2(101) surface. The Bader charges carried by Ag atoms are shown at the right side of the corresponding structure. Color coding: black, oxygen defects; others are the same as in Fig. 1.

Fig. 8. Projected d-band states of Ag atoms on the Vo2c (left) and Vo3c (right) defective TiO2(101) surface. The d-band center is marked by black or red arrow. The Fermi level is set to 0 eV for all Ag atoms. Left: (a) pDOS of the Ag atom on the pure Ag(111) surface; (b) pDOS of the Ag-1 and Ag-3 atoms on the Vo2c defective TiO2(101) surface; (c) pDOS of the Ag-2 atom on the Vo2c defective TiO2(101) surface; (d) pDOS of the Ag-4 and Ag-5 atoms on the Vo2c defective TiO2(101) surface; (e) pDOS of the Ag-6 and Ag-7 atoms on the Vo2c defective TiO2(101) surface; (f) pDOS of the Ag-8 atom on the Vo2c defective TiO2(101) surface. Right: (a)

17



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

pDOS of the Ag atom on the pure Ag(111) surface; (b)-(f) the pDOS of the corresponding Ag atoms on the Vo3c defective TiO2(101) surface. In sharp contrast, the oxygen defect itself is active for O2 adsorption due to positive polarized charge accumulation. Fig 9 shows that O2 molecules tend to be adsorbed on the vacancy sites, with higher adsorption energy on Vo2c-O2 and Vo3c-O2 in comparing to the corresponding interfacial adsorption configurations of Vo2c-O2-1 (Fig. S10) and Vo3c-O2-1 (Fig. S11). Then, the adsorbed O2 molecules will barrierlessly dissociate to produce two adsorbed O atom (Vo2c-O2-dis and Vo3c-O2-dis in Fig. 9). However, we need to point out that because of the negative charges accumulated on Ag atoms neighboring to the vacancy sites (Ag-1 and Ag-5), the repulsive forces between Ag and O2 make the adsorption configurations of Vo2c-O2 and Vo3c-O2 less stable than the states of O2 on the Vo2c/Vo3c sites in the absence of the Ag8 cluster (TiO2-Vo2c-O2 and TiO2-Vo3c-O2 in Fig. S11).

18


Page 18 of 32

Page 19 of 32

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


Fig. 9. The optimized structures and binding energies of O2 molecular and dissociative adsorptions on the defect sites of the Vo2c and Vo3c defective TiO2(101) surface. See Fig. 2 for color coding.

CO oxidation on the Oxygen-Defective Ag8/TiO2(101) System. The above analysis has shown that the activations of O2 on the oxygen-defective Ag8/TiO2(101) surfaces are both kinetically and thermodynamically favorable. We then move further to investigate the participation of dissociated O atoms on the catalytic CO oxidation. As shown in Fig. 9, the dissociated O2 on Vo2c and Vo3c defective Ag8/TiO2(101) surface can form two kinds of O atoms: OI covering the defect site and OII adsorbed on the Ag8-cluster generating an Ag-OII* state. The potential energy profiles in Fig. 10 show that the reactions between adsorbed CO and Ag-OII* in both model systems are exothermic, with energy barriers (TS7 and TS9) significantly lower than the counterpart (TS3) on flawless Ag8/TiO2(101) support. While for the OI atoms, their occupying of the Vo2c and Vo3c defects are reminiscent of flawless stoichiometric surface. Thus, the CO oxidation by the OI atoms are equivalent to removing an oxygen atom from TiO2(101) surface and making it bond with CO to form CO2. This reaction is known as Mars-van Krevelen (M-vK) mechanism, in which the metal oxide directly oxidizes CO.52 In Fig. 10, the associated energy barriers (TS8 and TS10) are significantly high with 2.30 and 3.52 eV, suggesting that the second CO oxidation via the M-vK mechanism is infeasible at normal temperature. Furthermore, 19



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 products of Vo2c-CO2 and Vo3c-CO2 are higher in energy than the reactants in Vo2c-CO+O and Vo3c-CO+O, meaning it is thermodynamically unfavorable. In short, although oxygen vacancies on the surface can promote O2 dissociation, the O atoms occupying the Vo2c and Vo3c defect sites are bound so tightly that they are reluctant to be desorbed and to react with CO. Therefore, reactivation of catalytic activity of oxygen defects on the surface will consume extra energy such as heat, and the synergistic effect is not available for oxygen-defective systems.

Fig. 10. Potential energy profiles for CO oxidation by Ag8-cluster on Vo2c and Vo3c defective TiO2(101) surface.

20


Page 20 of 32

Page 21 of 32

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


CONCLUSIONS This work studied synergized O2 and CO activations on Ag/TiO2(101) surface using DFT+U calculations and explored several key factors that can significantly affect the reaction kinetics and thermodynamics. It is found that the interfacial Ag atoms offer ideal sites for O2 adsorption and activation, by integrating advantages of positive polarized charges, the up-shifted d-band electronic states, and the assistance of neighboring surface Ti5c atoms. The up-layered Ag atom whose d-band center is also significantly up-shifted toward the Fermi level is a good site for anchoring O2 as well. When CO oxidation occurs on the Ag8/TiO2(101) surface, the dissociation of O2 will be promoted by the synergistic effect of CO and O2 activations. The generation of oxygen defects on the support surface decreases the positive polarized charges and down-shift the d-band centers of the neighboring Ag atoms, making metal sites unfavorable for O2 adsorption and activation. On the other hand, O2 can be easily dissociated on the semiconductor defect sites. Nevertheless, the dissociated O atoms would cover the defect sites through tight bonds, and therefore deactivate the oxygen defects on Ag/TiO2(101) catalyst. In summary, our results demonstrate that the atomic and electronic structures of metal cluster, synergistic effect of adjacent reactants, and oxygen defects on semiconductor support all play key roles in catalytic O2 activation and CO oxidation. The up-layered Ag sites a bit away from TiO2 support, adsorbing both O2 and CO to form a metal-stable four-center O-O-CO structure motif, synergize efficient O2 21



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

adsorptions and dissociations with easy CO oxidation driven by dissociated O. These findings have important implications for the synergized processing of various reactions on Ag/TiO2 surface, and provide useful guidance for future rational designing of the widely utilized metal-semiconductor hybrid catalysts.

Supporting Information Available: Geometry optimizations for various binding structures and binding sites of Ag8-cluster on the stoichiometric TiO2(101) surface, the work functions of the pure silver and the bare stoichiometric TiO2(101) surface, the adsorption states of CO on the stoichiometric Ag8/TiO2(101) surface, the adsorption states of O2 on the Vo2c and Vo3c defective Ag8/TiO2(101) surface. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]. Phone: +86 551 63600029

ACKNOWLEDGMENT

This work is supported by National Natural Science Foundation of China (NSFC 21303027, 21473166), Hefei Science Center CAS (2015HSC-UP011), the Natural

22


Page 22 of 32

Page 23 of 32

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


Science Foundation of Guizhou Province (no. QKJ[2015]2129), the GZNC startup package (no. 14BS022).

References

1 Bonanni, S.; Ait-Mansour, K.; Harbich, W.; Brune, H. Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Ptn/TiO2(101)-(1×1). J. Am. Chem. Soc. 2014, 136, 8702-8707. 2 Duan, Z.; Henkelman, G. CO Oxidation at the Au/TiO2 Boundary: The Role of the Au/Ti5c Site. ACS Catal. 2015, 5, 1589-1595. 3 Sasirekha, N.; Sangeetha, P.; Chen, Y. W. Bimetallic Au-Ag/CeO2 Catalysts for Preferential Oxidation of CO in Hydrogen-Rich Stream: Effects of Calcination Temperature. J. Phys. Chem. C 2014, 118, 15226-15233.

4 Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem. 2011, 3, 634− 641. 5 Bonanni, S.; Aït-Mansour, K.; Harbich, W.; Brune, H. Effect of the TiO2 Reduction State on the Catalytic CO Oxidation on Deposited Size-Selected Pt Clusters. J. Am. Chem. Soc. 2012, 134, 3445-3450.

23



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

6 Du, M.; Sun, D.; Yang, H.; Huang, J.; Jing, X.; Odoom-Wubah, T.; Wang, H.; Jia, L.; Li, Q. Influence of Au Particle Size on Au/TiO2 Catalysts for CO Oxidation. J. Phys. Chem. C 2014, 118, 19150-19157.

7 Mateos-Pedrero, C.; González-Carrazán, S. R.; Soria, M. A.; Ruiz, P. Effect of the Nature of TiO2 Support over the Performances of Rh/TiO2 Catalysts in the Partial Oxidation of Methane. Catal. Today 2013, 203, 158-162.

8 Nie, L.; Yu, J.; Li, X.; Cheng, B.; Liu, G.; Jaroniec, M. Enhanced Performance of NaOH-Modified Pt/TiO2 toward Room Temperature Selective Oxidation of Formaldehyde. Environ. Sci. Technol. 2013, 47, 2777-2783.

9 Kesavan, L.; Tiruvalam, R.; Rahim, M. H.; Saiman, M. I.; Enache, D. I.; Jenkins, R. L.; Dimitratos, N.; LopezSanchez, J. A.; Taylor, S. H.; Knight, D. W.; Kiely, C. J.; Hutchings, G. J. Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles. Science 2011, 331, 195-199.

10 Fiorenza, R.; Crisafulli, C.; Condorelli, G. G.; Lupo, F.; Scirè, S. Au-Ag/CeO2 and Au-Cu/CeO2 Catalysts for Volatile Organic Compounds Oxidation and CO Preferential Oxidation. Catal. Lett. 2015, 145, 1691-1702.

11 Choi, Y.; Kim, H.; Moon, G.; Jo, S.; Choi, W. Boosting up the Low Catalytic Activity of Silver for H2 Production on Ag/TiO2 Photocatalyst: Thiocyanate as a Selective Modifier. ACS Catal. 2016, 6, 821–828.

24


Page 24 of 32

Page 25 of 32

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


12 Hou, W.; Liu, Z.; Pavaskar, P.; Hung, W. H.; Cronin, S. B. Plasmonic Enhancement of Photocatalytic Decomposition of Methyl Orange under Visible Light. J. Catal. 2011, 277, 149− 153. 13 Janssens, T. V. W.; Carlsson, A.; Puig-Molina, A.; Clausen, B. S. Relation between Nanoscale Au Particle Structure and Activity for CO Oxidation on Supported Gold Catalysts. J. Catal. 2006, 240, 108-113.

14 Liu, Z. P.; Hu, P.; Alavi, A. Catalytic Role of Gold in Gold-Based Catalysts: A Density Functional Theory Study on the CO Oxidation on Gold. J. Am. Chem. Soc. 2002, 124, 14770-14779.

15 Wu, T. B.; Zhang, P.; Jiang, T.; Yang, D. X.; Han, B. X. Enhancing the Selective Hydrogenation of Benzene to Cyclohexene over Ru/TiO2 Catalyst in the Presence of a Very Small Amount of ZnO. Sci China Chem. 2015, 58, 93-100.

16 Ferguson, G. A.; Mehmood, F.; Rankin, R. B.; Greeley, J. P.; Vajda, S.; Curtiss, L. A. Exploring Computational Design of Size-Specific Subnanometer Clusters Catalysts. Top. Catal. 2012, 55, 353-365.

17 Yun, J.; Hwang, S. H.; Jang, J. Fabrication of Au@Ag Core/Shell Nanoparticles Decorated

TiO2

Hollow

Structure

for

Efficient

Light-Harvesting

Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 2055−2063.

25


in


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 He, Y.; Basnet, P.; Murph, S. E. H.; Zhao, Y. Ag Nanoparticle Embedded TiO2 Composite Nanorod Arrays Fabricated by Oblique Angle Deposition: Toward Plasmonic Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 11818−11827. 19 Qin, C.; Sremaniak, L. S.; Whitten, J. L. CO Adsorption on Ag(100) and Ag/MgO(100). J. Phys. Chem. B 2006, 110, 11272-11276.

20 Lu, Q.; Lu, Z.; Lu, Y.; Lv, L.; Ning, Y.; Yu, H.; Hou, Y.; Yin, Y. Photocatalytic Synthesis and Photovoltaic Application of Ag-TiO2 Nanorod Composites. Nano Lett. 2013, 13, 5698−5702. 21 Green, I. X.; Tang, W.; Neurock, M.; Yates, J. T. Spectroscopic Observation of Dual Catalytic Sites during Oxidation of CO on a Au/TiO2 Catalyst. Science 2011, 333, 736− 739. 22 Arandiyan, H.; Dai, H.; Ji, K.; Sun, H.; Li, J. Pt Nanoparticles Embedded in Colloidal Crystal Template Derived 3D Ordered Macroporous Ce0.6Zr0.3Y0.1O2: Highly Efficient Catalysts for Methane Combustion. ACS Catal. 2015, 5, 1781–1793.

23 Song, Y. J.; Jesús, Y. M. L.; Fanson, P. T.; Williams, C. T. Preparation and Characterization of Dendrimer-Derived Bimetallic Ir-Au/Al2O3 Catalysts for CO Oxidation. J. Phys. Chem. C 2013, 117, 10999–11007.

26


Page 26 of 32

Page 27 of 32

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


24 Watanabe, Y.; Wu, X.; Hirata, H.; Isomura, N. Size-Dependent Catalytic Activity and Geometries of Size-Selected Pt Clusters on TiO2(110) Surfaces. Catal. Sci. Technol. 2011, 1, 1490−1495. 25 Kaden, W. E.; Wu, T. P.; Kunkel, W. A.; Anderson, S. L. Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces. Science 2009, 326, 826−829. 26 Zhang, Q.-H.; Han, W.-D.; Hong, Y.-J.; Yu, J.-G. Photocatalytic Reduction of CO2 with H2O on Pt-Loaded TiO2 Catalyst. Catal. Today 2009, 148, 335− 340. 27 Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911-921.

28 Zhang, X.; Qu, Z.; Li, X.; Wen, M.; Quan, X.; Ma, D.; Wu, J. Studies of Silver Species for Low-Temperature CO Oxidation on Ag/SiO2 Catalysts. Sep. Purif. Technol. 2010, 72, 395-400.

29 Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Meyer, R. J.; Redfern, P. C.; Teschner, D.; Schlögl, R.; Pellin, M. J.; Curtiss, L. A.; Vajda, S. Increased Silver Activity for Direct Propylene Epoxidation via Subnanometer Size Effects. Science 2010, 328, 224−228.

27



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

30 Hirakawa, T.; Kamat, P. V. Charge Separation and Catalytic Activity of Ag@TiO2 Core-Shell Composite Clusters under UV-Irradiation. J. Am. Chem. Soc. 1999, 127, 3928-3934.

31 Yang, L. B.; Jiang, X.; Ruan, W. D.; Yang, J. X.; Zhao, B.; Xu, W. Q.; Lombardi, J. R. Charge-Transfer-Induced Surface-Enhanced Raman Scattering on Ag-TiO2 Nanocomposites. J. Phys. Chem. C 2009, 118, 16226-16231.

32 Frey, K.; Iablokov, V.; Melaet, G.; Guczi, L.; Kruse, N. CO Oxidation Activity of Ag/TiO2 Catalysts Prepared via Oxalate Co-precipitation. Catal. Lett. 2008, 124, 74-79.

33 Yu, Y.; Li, Y.; Zhang, X.; Deng, H.; He, H.; Li, Y. Promotion Effect of H2 on Ethanol Oxidation and NOx Reduction with Ethanol over Ag/Al2O3 Catalyst. Environ. Sci. Technol., 2015, 49, 481–488.

34 Chen, Y.; Huang, W. He, D.; Situ, Y.; Huang, H. Construction of Heterostructured g-C3N4/Ag/TiO2 Microspheres with Enhanced Photocatalysis Performance under Visible-Light Irradiation. ACS Appl. Mater. Interfaces 2014, 6, 14405–14414.

35 Han, J.; Han, F.; Ouyang, J.; He, L.; Zhang, Y.; Na, N. Low Temperature CO Sensor Based on Cataluminescence from Plasma-Assisted Catalytic Oxidation on Ag Doped Alkaline-Earth Nanomaterials. Nanoscale, 2014, 6, 3069-3072.

28


Page 28 of 32

Page 29 of 32

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


36 Yang, C. T.; Balakrishnan, N.; Bhethanabotla, V. R.; Joseph, B. Interplay between Subnanometer Ag and Pt Clusters and Anatase TiO2(101) Surface: Implications for Catalysis and Photocatalysis. J. Phys. Chem. C 2014, 118, 4702-4714.

37 Wu, T. S.; Wang, K. X.; Li, G. D.; Sun, S. Y.; Sun, J.; Chen, J. S. Montmorillonite-Supported Ag/TiO2 Nanoparticles: An Efficient Visible-Light Bacteria Photodegradation Material. ACS Appl. Mater. Interfaces 2010, 2, 544-550.

38 Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169–11186.

39 Perdew, J. P. Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868.

40 Blöchl, P. E.; Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979.

41 Kresse, G.; Jouber, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758–1775.

29



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

42 Bengone, O.; Alouani, M.; Blöchl, P. E.; Hugel, J. Implementation of the Projector Augmented Wave LDA+U Method: Application to the Electronic Structure of NiO. Phys. Rev. B 2000, 62, 16392–16401.

43 Mills, G.; Jonsson, H.; Schenter, G. K. Reversible Work Transition State Theory: Application to Dissociative Adsorption of Hydrogen. Surf. Sci. 1995, 324, 305337.

44 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.

45 Sorescu, D. C.; Al-Saidi, W. A.; Jordan, K. D. CO2 Adsorption on TiO2(101) Anatase: A Dispersion-Corrected Density Functional Theory Study. J. Chem. Phys. 2011, 135, 124701.

46 Mazheika, A. S.; Bredow, T.; Ivashkevich, O. A.; Matulis, V. E. Theoretical Study of NO Conversion on Ag/TiO2 Systems. I. Anatase (100) Surface. J. Phys. Chem. C 2012, 116, 25262−25273. 47 Xin, B.; Jing, L.; Ren, Z.; Wang, B.; Fu, H. Effects of Simultaneously Doped and Deposited Ag on the Photocatalytic Activity and Surface States of TiO2. J. Phys. Chem. B 2005, 109, 2805-2809.

30


Page 30 of 32

Page 31 of 32

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


48 Zhao, Z. Y.; Li, Z. S.; Zou, Z. G. A Theoretical Study of Water Adsorption and Decomposition on the Low-Index Stoichiometric Anatase TiO2 Surfaces. J. Phys. Chem. C 2012, 116, 7430 −7441.

49 Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18, 9-15.

50 Henkelman, G.; Arnaldsson, A.; Jonsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354-360.

51 Xiao, J. P.; Frauenheim, T. Theoretical Insights into CO2 Activation and Reduction on the Ag(111) Monolayer Supported on ZnO(0001) Substrate. J. Phys. Chem. C 2013, 117, 1804−1808.

52 Kim, H. Y.; Lee, H. M.; Henkelman, G. CO Oxidation Mechanism on CeO2-Supported Au Nanoparticles. J. Am. Chem.Soc. 2012, 134, 1560−1570. 53 Wang, P.; Tang, X.; Tang, J.; Pei, Y. Density Functional Theory (DFT) Studies of CO Oxidation over Nanoporous Gold: Effects of Residual Ag and CO Self-Promoting Oxidation. J. Phys. Chem. C 2015, 119, 10345−10354. 54 Lee, S.; Fan C.; Wu, T.; Anderson, S. L. Cluster Size Effects on CO Oxidation Activity, Adsorbate Affinity, and Temporal Behavior of Model Aun/TiO2 Catalysts. J. Chem. Phys. 2005, 123, 124710-124722.

31



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 of Contents

32


Page 32 of 32

A First-Principle Study of Synergized O2 Activation ... - ACS Publications

Recommend Documents