Catalytic Chemistry Predicted by a Charge Polarization Descriptor

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Surfaces, Interfaces, and Applications

Catalytic Chemistry Predicted by a Charge Polarization Descriptor: Synergistic O2 Activation and CO Oxidation by Au-Cu Bimetallic Clusters on TiO2(101) Chuanyi Jia, Xijun Wang, Wenhui Zhong, Zhunzhun Wang, Oleg V. Prezhdo, Yi Luo, and Jun Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00925 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Materials & Interfaces

Catalytic Chemistry Predicted by a Charge Polarization Descriptor: Synergistic O2 Activation and CO Oxidation by Au−Cu Bimetallic Clusters on TiO2(101) Chuanyi Jia a,b,‡ Xijun Wang c,‡ Wenhui Zhong a,b, Zhunzhun Wang a,b, Oleg V. Prezhdo d, Yi Luo c, Jun Jiang c,* 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) Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Mechanical Behavior and Design of Materials, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, China d) Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

‡ The first two authors contributed equally to this work.

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ABSTRACT: The versatile properties of bimetallic nanoparticles greatly expand the range of catalyzed chemical reactions. We demonstrate that surface chemistry can be understood and predicted using a simple adsorbate-surface interaction descriptor that relates charge polarization to chemical reactivity. Our density functional theory studies of O2 activation and CO oxidation catalyzed by Au7-Cu1 bimetallic nanoparticles supported on the TiO2(101) surface demonstrate that the generated oxidized Cu atom (CuOx) can efficiently inhibit the aggregation of the active Cu sites. Moreover, because of the strong dipole-dipole interaction between the surface and the adsorbate on the oxidized Cu site, the adsorption of CO+O2/CO+O can be significantly enhanced, which can decrease the CO oxidation barriers and further improve catalytic performance. The product of the two electric dipole moments provides a parameter that allows us to predict the key catalytic properties for different adsorption sites and reaction pathways. The reported findings provide important insights into the mechanism of chemical reactivity of metallic clusters and generate a valuable principle for catalyst design.

Keywords: bimetallic nanoparticles, O2 activation, CO oxidation, descriptor, dipoledipole interaction

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1. INTRODUCTION Bimetallic nanoparticles are widely applied as heterogeneous catalysts in a wide range of applications, such as fuel cell reactions1-3 and organic compounds oxidation,4,5 mainly owing to their exceptional activities and strong selectivities. It is generally believed that these excellent catalytic properties benefit from synergy between the two components of the bimetallic catalyst which can be understood in terms of assemble and ligand effects. For instance, one component of the bimetallic nanoparticles may serve as a spacer to isolate active sites or to modify the electronic environment of the other component.6,7 Comprehensive understanding of these catalytic roles at the atomic level provides useful guidance in the design and synthesis of high-efficiency bimetallic catalysts. Fundamental research into heterogeneous catalytic reactions is strongly focused on Au-based and Pt-based bimetallics due to their promising potential for application in many important industrial and environmental reactions.6,8-10 For example, using in situ techniques including XRD, EPR, XANES, and FT-IR, Liu et al.6 found that the Au-Cu bimetallic nanoparticles in the Au-Cu/SBA-15 system exhibit better catalytic performances than the monometallic counterparts. They ascribed the improvement to the formation of the CuOx islands on the reduced catalyst which can provide active oxygen for further oxidation reactions. Another systematic study of the single-step solgel Pt-Au/CeO2 catalyst by Monyanon et al.8 pointed out that the Pt-Au ratio plays an important role in the catalytic behavior. In addition, different nanostructures give rise 3



to different catalytic performances, as shown by Alayoglu et al.,9 revealing that the Ru@Pt core-shell architecture has much higher activities than the PtRu nano-alloys or mixtures of monometallic nanoparticles in preferential oxidation (PROX) reactions. Closely related to the nanostructure, surface composition is another important factor influencing the catalytic performances of bimetallic nanoparticles. A detailed investigation of Rh-Pd and Pt-Pd core-shell nanoparticles by Tao’s group10 indicates that surface composition changes with temperature and surrounding atmosphere during the pretreatment and reaction processes. Such response can alter the distribution of the active sites on bimetallic catalysts, and further influence their activities. These studies have proposed a series of factors that often have great influence on the catalytic performance. One of the most crucial issues in ration design of bimetallic catalysts is to understand how the microscopic structure and electronic properties of bimetallic nanoparticles affects catalytic properties, which is also a core subject of chemical research. To achieve this goal, various catalytic descriptors has been proposed,11 such as the d-band center,12 fermi softness13 and the recently proposed “universal descriptor” for the graphene-based single atom catalysts.14 Moreover, there also remain many issues in need of clarification, such as identification of the synergistic catalytic mechanisms involving the two components, and identification of the detailed catalytic processes on the catalyst surface. It has been recognized that aggregation of active sites can significantly reduce the catalytic activity of bimetallic nanoparticles.15-17 Therefore, 4


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isolation of active sites is another very crucial issue in need of additional insight for bimetallic catalysts. Resolving these issues will greatly improve our understanding of the synergistic catalytic mechanisms involving the two components of bimetallic catalytic materials and will provide useful guidance in the continued development and improvement of bimetallic catalytic materials. In this work, we chose to systematically study a TiO2 supported Au-Cu bimetallic nanoparticle due to its high activity and strong synergistic effect in heterogeneous catalytic reactions.18-21 The catalytic performance of the active sites is characterized via two typical heterogeneous catalytic reactions: O2 activation and CO oxidation. DFT calculations reveal that the catalytic activity of the Au-Cu nanoparticle can be improved by synergy between CO and O2. More importantly, the large dipole-dipole interaction energy between the gas adsorbates and the solid catalysts can enhance the stability of the binding state, which can further improve the CO-O2 co-catalytic activity. Based on our findings of the effects of dipole moment, we have proposed both a new way to understand chemical reactivity and a novel strategy for designing improved bimetallic catalytic materials.

2. CALCULATION DETAILS All calculations were performed using the Vienna ab initio simulation package (VASP).22 The Perdew, Burke and Ernzerhof (PBE) functional was adopted for the exchange-correlation interactions.23 The projector augmented wave (PAW) method was 5



employed to calculate the interactions between ions and electrons.24-26 The plane wave basis set with a cutoff energy of 400 eV was chosen for geometry relaxation. The convergence criterion of total energy was set to 10-5 eV. The minimum force tolerance was set to 0.01 eV/Å. To demonstrate the convergence of our simulations, we have performed test calculations with two sets of simulation parameters using 0.01 eV/Å and 0.02 eV/Å. Because these two different force tolerances have led to identical adsorption states of the clusters and CO+O2, with the energy differences less than 0.002 eV and bond length differences less than 0.002 Å, the force tolerance of 0.01 eV/Å is sufficient for our calculations. The corresponding convergence test results are shown in Figures S1 and S2 in the supporting information. Figures S3 and S4 present the results of convergence with respect to the k-point mesh. The 3 × 3 × 1, 5 × 5 × 1 and 7 × 7 × 1 κpoint grids give very similar results for the adsorption states of the clusters and CO+O2 as well. The changes of the energies are all less than 0.005 eV, and the bond lengths exhibit nearly no change. Thus, the 7 × 7 × 1 κ-point grid is sufficiently large for our calculations, and it is used throughout the paper. The minimum energy paths for the O2 dissociation and CO oxidation reactions were found via the climbing image nudged elastic band (CI-NEB) method.27,28 Due to the incomplete self-interaction cancellation for transition metals, Ti atoms in TiO2(101) were described using the DFT+U approach (U=3.5 eV29). To explore the role of the long-range van der Waals (vdW) interactions on CO adsorption, the empirical correction approach (DFT-D2) was used for comparison.30 The calculation results for CO adsorption on Au or Cu atom in (Cu-8)6


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TiO2 state are depicted in Figure S5. The vdW correction changes the binding energies by less than 0.003 eV, and the Au-C or Cu-C bond lengths by less than 0.002 Å. Since these results indicate that the vdW correction has very small effect on the binding energies and geometries, we use the DFT+U approach without the vdW correction throughout the paper. A 3 × 1 stoichiometric anatase TiO2(101) model31,32 with six atomic trilayers (108 atoms in total) was prepared to describe the TiO2 support. To confirm that the 3 × 1 TiO2(101) support is sufficiently large for the Au7-Cu1 cluster, a comparison between the 3 × 1 and 3 × 2 supercell models (as seen in Figure S6) was carried out. The test results in Figure S7 show that the larger 3 × 2 supercell model gives similar binding energies (within 0.003 eV) and binding structures (within 0.004 Å) of the (Cu-8)-TiO2, (Cu-8)-O2, (Cu-8)-CO, and (Cu-8)-CO+O2 states. The changes of the binding energies, reaction energies, and energy barriers in the potential energy profile of the CO oxidation on (Cu-8)-TiO2 are very small as well, as shown in Figure S8. Therefore, the 3 × 1 supercell model, which is selected in our paper, is sufficiently large for our purpose. We chose the stoichiometric anatase (101) surface as the support because anatase TiO2 is the dominant crystal structure in the commercial Degussa P25 catalyst (80% anatase and 20% rutile), and since (101) is the most stable anatase surface.31,33 In addition, experiments have shown that the bulk oxygen defects are more stable than the surface oxygen defects on the anatase (101) surface.32,34 This implies that the stoichiometric anatase (101) surface, used in our paper, is favored over defective 7



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surfaces in the absence of adsorbates. Thus, the stoichiometric anatase (101) surface is most appropriate among various surface models. The vacuum region separating the slabs along the perpendicular c-direction was set to 15 Å. The Au7-Cu1 cluster was created by replacing one Au atom in the two-layered Au8 cluster with a Cu atom. The initial bond length and bond angle of the two-layered Au8 cluster were based on the Au(111) surface, which is the most commonly used surface of Au.35-37 We chose this two-layered Au8 structure as the initial model for the Au-Cu bimetallic cluster because the structure is more stable and has less distortion than other single layered or three layered structure on the TiO2(101) support, as shown in Figure S9. Before placed onto TiO2(101), the two-layered Au7-Cu1 cluster was optimized in a large simulation cell of 15 Å × 15 Å × 15 Å dimensions, using the Ӷ-point. The bottom three layers of the TiO2(101) surface were frozen to mimic the bulk effects, whereas other atoms, including the three topmost layers and adsorbates (Au7-Cu1, CO and O2) were all allowed to relax. The binding energies of the adsorbates were calculated from the equation38:

Eb cluster  Esur / cluster  Esur  EF  cluster

(1)

Eb  mol  Esur / cluster / mol  Esur / cluster  EF  mol

(2)

where Esur/cluster represents the total energy of the surface and the Au7Cu1/Au7Cu1O1 cluster in their optimized reference states; Esur represents the energy of the perfect TiO2(101) surface;EF  cluster represents the energy of the free Au7Cu1 clusters in the air;

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Esur / cluster / mol represents the total energy of the surface, Au7Cu1/Au7Cu1O1 cluster, and the adsorbed molecules (including CO, O2, and CO+O2) in their optimized reference states; EF  mol represents the energy of the adsorbed free molecules in the air. The binding energy of CO2 was calculated from the equation:

Eb CO 2  Esur / cluster / mol  Eres  EF CO 2 In this equation, Esur / cluster / mol

(3)

has the same meaning as that in equation (2).

Eres represents the energy of the rest of the atoms except CO2, EF CO 2 represents the energy of the free CO2 molecule in the air. Accordingly, a more negative Eb cluster ,

Eb CO 2 or Eb  mol indicates favorable binding. Calculation of the atomic dipole moments, with consideration of periodic boundary conditions, was accomplished by application of the recently developed density derived electrostatic and chemical (DDEC6) method.39,40

3. RESULTS AND DISCUSSION 3.1 O2 Activation and CO Oxidation on the Top-layered Cu Atom in (Cu-8)-TiO2. Among a variety of possible composite configurations (seen in Figure S10), two typical models are chosen for subsequent calculations, as shown in Figure 1. (Cu-8)TiO2 is the most stable form of the Cu atom at the top-layer. (Cu-1)-TiO2 is the most stable form of the Cu atom at the interface. Because Cu is an oxo-philic element, the interfacial Cu-O interaction is much stronger than Au-O.41 Therefore, all the binding states of the Cu atom at the interface are more stable than those of the Cu atom at the 9



top-layer, as shown in Figure S10. Interface polarization between Au7-Cu1 and TiO2(101) are quantified through the Bader charge analysis.42,43 The results show that the interfacial Au and Cu atoms that bind with the surface O atoms hold more positive charges, while others are negatively charged or close to neutral. Moreover, due to the difference of the work functions of pure Cu (4.65 eV) and Au (5.10 eV), the top-layered Cu atom in (Cu-8)-TiO2 donates electrons to the nearby Au atoms and thus is more positively charged. Normally, positively charged sites can act as Lewis acids to attract the O atoms in O2 44 and thus are more favorable for O2 activation. Therefore, one can expect that the best site for anchoring O2 in (Cu-8)-TiO2 is the Cu-8 site, which holds the most positive charge and offers less steric hindrance than other sites, as confirmed by the adsorption states shown in Figure S11. Comparing the binding states in Figures 1 and S10, it can be found that the TiO2 support has two effects for the Au7-Cu1 cluster: (1) It helps to stabilize the cluster by establishing Au-O or Cu-O bonds at interface. (2) It attracts electrons from the cluster, making the cluster and the Cu atoms positively charged.

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Figure 1. Two typical and stable models of the Au7-Cu1 cluster on the TiO2(101) surface. The Bader charges carried by the Au and Cu atoms are shown at the right side. Color coding: yellow, Au atoms; pink, Cu atoms; red, O atoms; gray, Ti atoms.

In the following, the dissociation of O2 is studied on (Cu-8)-TiO2. The most stable O2 adsorption geometry ((Cu-8)-O2) is selected for further dissociation studies, as seen in Figure S11. The potential energy profile41 in Figure 2 shows that the dissociation of O2 is a two-step process. During this process, O2 initially migrates from the most stable adsorption state (Cu-8)-O2 to an intermediate (Cu-8)-O2-int state. This first step has an associated overall energy change of 0.53 eV and an activation energy barrier of 0.97 eV (TS 1). The second step involves formation of TS 2, with an activation barrier of 0.48 eV, and the subsequent energetically favorable dissociation of adsorbed O2 to form the (Cu-8)-O2-dis species (-2.64 eV). The rate-determining step of this two-step process is migration of O2 to form the (Cu-8)-O2-int species. In addition, because the absolute value of the O2 binding energy (0.69 eV) is smaller than the energy barrier of the rate-determining step (0.97 eV), the spontaneous dissociation of O2 on the Cu-8 atom in (Cu-8)-TiO2 will be very hard to realize. Therefore, this process requires relatively high temperatures, and there will be large competition between the desorption and dissociation of O2.

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Figure 2. Potential energy profile for O2 dissociation on (Cu-8)-TiO2. Color coding: blue, O atoms in adsorbed O2; other atoms are the same as in Figure 1. The minimum energy paths for these two reactions are shown in Figure S13.

Aside from the O2 activation, the synergistic effect between CO and O2 also plays an important role in the CO oxidation reaction in the composite catalysts.45-47 To examine the possibility of co-catalysis of CO and O2 on (Cu-8)-TiO2, the complete CO oxidation by co-adsorbed CO and O2 was investigated on (Cu-8)-O2. Unlike the O2 adsorption, the most stable binding site for CO was found to be the top-layered Au site (Au-6) which holds negative charges, as depicted in Figure S12. However, because the binding states of CO on (Cu-8) (-0.93 eV) and (Au-6) (-1.05 eV) are both much more stable than the most stable binding state of O2 (-0.69 eV), the active top-layered Au and Cu sites will be easily covered by CO, when CO and O2 co-adsorbed on the Au-Cu/TiO2 catalysts (CO poisoning effect). As a result, when we use the perfect Au-Cu/TiO2 catalyst for CO oxidation, an oxygen-rich condition is needed to avoid such disadvantageous effect. 48 12


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Following co-adsorption, the co-adsorbed CO and O2 can react to form a gas-phase CO2 and a residual Cu-O* on the Au7-Cu1 cluster, which is a 2.70 eV exothermic process as shown in Figure 3. Since the adsorbed CO can provide additional attraction and an additional anchoring site for the OI atom in O2, the O2 molecule will be preactivated, and the OII atom does not have to move to another side of the Cu atom (as (Cu-8)-O2-dis in Figure 2). These two responses reduce the activation barrier of this reaction to 0.48 eV (TS 3) which is much lower than the rate-determining step of the independent dissociation of O2 (TS 1, 0.97 eV). Then, the activated residual Cu-O* can easily react with another CO molecule (the subsequent CO adsorption is exothermic by 0.84 eV) with an extremely low energy barrier of 0.06 eV (TS 4). Therefore, the lifespan of Cu-O* should be very short. The above discussion indicates that the synergistic effect between CO and O2 can promote CO oxidation, which is a two-step process: (i) CO+O2→CO2+O, and (ii) CO+O→CO2. Because the binding energies of CO2 are all very small (lower than 0.10 eV), the release of CO2 does not require a high energy.

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Figure 3. Potential energy profiles for CO+O2 (up) and CO+O (down) on (Cu-8)-TiO2. Eb-CO2 and Eb-CO represents the binding energy of CO2 and CO, respectively. The Eb-CO is calculated by a similar equation as Eb-CO2. Color coding: brown, C atoms; other atoms are the same as in Figure 2. The minimum energy paths for these two reactions are shown in Figure S14. In order to gain a deeper insight into the effect of the imported Cu atom (Cu-8) on CO oxidation, the potential energy profiles for CO+O2 and CO+O on the pure Au8TiO2(101) system were further studied. From Figure S21, it can be seen that the energy barriers of CO+O2 and CO+O on the supported Au8 cluster are both higher than those on the supported Au7Cu1 cluster. Based on the calculation results in Figure 3 and Figure S21, we further calculated the free energy changes (△G) and the free energy profiles for CO+O2 and CO+O on these two catalysts, as shown in Table 1 and Figure S22 (the calculation details for △G 49 were presented in SI-Section 10). The free energy changes in Table 1 and Figure S22 show that the imported Cu-8 atom in the Au7Cu1 cluster can enhance the stability of the adsorbed CO+O2 and CO+O molecules. Making use of the Ea in Figure 22, we obtained the Sabatier activity (SA) of the rate-determining step 14


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(CO+O2 reaction) of these two catalysts, as shown in Table 1 (the detail calculation steps 50 were presented in SI-Section 10). The calculation results indicate that the SA of CO+O2 on pure Au8/TiO2(101) is -0.74, which is much lower than that on Au7Cu1/TiO2(101). The trends in the △G and SA indicate that the imported Cu atom can enhance the stability of the adsorbates and further improve the catalytic activity of the supported Au clusters on the TiO2(101) surface.

Table 1. The binding energies (Eb), Gibbs free energy changes (△G), and Sabatier activities (SA) of the CO+O2 reactions on the Au8-TiO2, (Cu-8)-TiO2, and (Cu-O)TiO2 states. Eb (eV)

△G(eV)

SA

Au8-TiO2

-1.14

-1.04

-0.74

(Cu-8)-TiO2

-1.76

-1.83

0.38

(Cu-O)-TiO2

-2.52

-2.13

0.69

3.2 O2 Activation and CO Oxidation on the Oxidized Cu Atom in (Cu-O)-TiO2. Some experimental results have shown that if the supported Au-Cu cluster is exposed to high temperature and oxygen-rich condition, the Cu atoms can be oxidized to CuOx patches.6, 48, 51 Our calculation results in Figure 2 also show that the O2 dissociation can occur on the (Cu-8)-TiO2 catalyst at the appropriate reaction conditions. Thus, a further 15



investigation of the CuOx structure is necessary for the Au-Cu/TiO2 catalyst. Compared to the Cu atom in (Cu-8)-TiO2, the adsorption energy of the additional Cu atom (Cua) on the CuOx patch is much higher as shown in Figure 4, indicating that aggregation of the active Cu sites can be efficiently inhibited by the adsorbed O atoms in the CuOx patch. Therefore, the pretreatment at high temperature and oxygen-rich condition in which the Cu atoms can be oxidized to CuOx patches is predicted to benefit achieving highly dispersed Cu active sites on Au/TiO2 catalysts.

Figure 4. Structures and energies for adsorption of an additional Cu atom (Cua) on (Cu8)-TiO2 (left) and (Cu-8)-O2-dis (right). See Figure 2 for color coding.

It is interesting to find that when the oxidized Au7-Cu1/TiO2(101) catalyst is used for CO oxidation, the oxidation process is completely changed. From Figure 5 one can see that the top-layered O atom (OI) in (Cu-8)-O2-dis is very active and it can easily react with the adsorbed CO to form a gas-phase CO2 in a 0.71 eV exothermic process with a negligible barrier of only 0.04 eV (TS 5). However, another interfacial O atom (OII) is extremely stable. To activate the interfacial OII atom, a very high temperature (much 16


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higher than that for O2 activation and CO oxidation) is necessary to overcome the large energy barrier of 2.23 eV (TS 6). As a result, the remaining OII atom is sufficiently stable to exist throughout the low-temperature CO catalytic oxidation process, and the oxidized (Cu-8)-TiO2 can thus be defined as a new catalyst (named (Cu-O)-TiO2 in this work), as shown in Figure 6.

Figure 5. Potential energy profiles for dissociated O2+CO (up) and O+CO (down) on (Cu-8)-TiO2. Eb-CO2 and Eb-CO represents the binding energy of CO2 and CO, respectively. The Eb-CO is calculated by a similar equation as Eb-CO2. See Figure 3 for color coding. The minimum energy paths for these two reactions are shown in Figure 17



S15.

Figure 6. The optimized structure of (Cu-O)-TiO2. The Bader charges carried by the Au, Cu and O atoms are shown at the right side. Color coding: green, the interfacial OII atom in (Cu-8)-O2-dis; other atoms are the same as in Figure 1.

To gain more knowledge about the catalytic performances of this newly found catalyst (Cu-O)-TiO2, detailed calculations for the O2 activation and the CO oxidation are carried out. The Bader charges analysis in Figure 6 shows that the extra attraction from the adsorbed OII atom makes the Cu-8 atom more positively charged (0.515 |e|) compared with the previous structure (Cu-8)-TiO2 (0.212 |e|), suggesting better Lewis acidity. From the reaction potential energy profile in Figure 7, one can see that the O2 dissociation becomes a one-step exothermic process, which is driven by the stronger adsorption forces generated by the larger positive charge on the Cu-8 atom in (Cu-O)TiO2. The energy barrier also reduces from 0.97 eV (the rate-determining step for the dissociation of O2 on the Cu-8 atom in (Cu-8)-TiO2) to 0.82 eV. Because the absolute value of the O2 binding energy (1.32 eV) is larger than the energy barrier (0.82 eV), the dissociation of O2 on the Cu-8 atom in (Cu-O)-TiO2 will be much easier to realize than 18


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that in (Cu-8)-TiO2. Moreover, because the binding energy of O2 on the oxidized (Cu-8) atom have reduced from -0.69 eV to -1.32 eV which is similar to CO (-1.35 eV), the CO poisoning effect on the (Cu-O)-TiO2 catalyst will become lower as well, as shown in Figures 5 and 7. Afterwards, the CO oxidation process on (Cu-O)-TiO2 is still a two-step exothermic process which can be significantly promoted by the synergistic effect between CO and O2, as shown in Figure 8. The energy barrier for the rate-determining step (CO+O2→CO2+O) is only 0.39 eV, which is lower than that on (Cu-8)-TiO2 (0.48 eV). The calculation results for △G also show that the oxidized Cu site can promote the adsorption of CO+O2, which can further increase the SA of CO oxidation from 0.38 to 0.69, as shown in Table 1. Thus, compared to the perfect (Cu-8)-TiO2 catalyst, the oxidized (Cu-O)-TiO2 catalyst shows better performance in CO oxidation.

Figure 7. Potential energy profile for O2 dissociation on (Cu-O)-TiO2. See Figures 2 and 6 for color coding. The minimum energy path for this reaction is shown in Figure S16. 19



Figure 8. Potential energy profiles for CO+O2 (up) and CO+O (down) on (Cu-O)-TiO2. Eb-CO2 and Eb-CO represents the binding energy of CO2 and CO, respectively. The Eb-CO is calculated by a similar equation as Eb-CO2. See Figures 3 and 6 for color coding. The minimum energy paths for these two reactions are shown in Figure S17.

3.3 O2 Activation and CO Oxidation on the Interfacial Cu Atom in (Cu-1)-TiO2.

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Having examined the catalytic performance of the top-layered Cu atom (Cu-8) in (Cu-8)-TiO2, we now turn to the interfacial Cu atom (Cu-1) in (Cu-1)-TiO2. The O2 adsorption state in Figure 9-(Cu-1)-O2 shows that besides the Cu-1 atom, one of the surface Ti atoms can contribute to the O2 adsorption as well. The synergistic catalytic effect of the Cu-1 and surface Ti atoms can significantly promote the dissociation of O2. The energy barrier for this process is only 0.40 eV, which is much lower than the rate-determining step on (Cu-8)-TiO2 (0.97 eV) and (Cu-O)-TiO2 (0.82 eV). However, different from the O2 dissociation, the direct CO oxidation (CO+O2→CO2+O) on (Cu-1)-TiO2 is quite disadvantageous with a much higher barrier of 1.01 eV due to the large steric-hindrance at the interface, as shown in Figure 9. Therefore, the O2 dissociation on (Cu-1)-TiO2 should be the first step for the CO oxidation. In view of this, a new three-step mechanism is proposed to account for the whole CO oxidation process: (i) O2 → OI + OII, (ii) CO + OI→ CO2, and (iii) CO + OII→ CO2.

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Figure 9. Potential energy profiles for O2 dissociation (up) and CO oxidation (down) on (Cu-1)-TiO2. See Figure 3 for color coding. The minimum energy paths for these two reactions are shown in Figure S18. The potential energy profile for the reaction between the dissociated O2 and CO on (Cu-1)-TiO2 shows that the OI atom and the adsorbed CO can easily react to form gasphase CO2 in a 1.62 eV exothermic process with a 0.41 eV energy barrier, as seen in Figure 10. However, the OII atom is inactive with a very large barrier of 2.76 eV due to both strong Ti-O interaction and steric hindrance. Thus, similar to the interfacial OII atom on (Cu-8)-O2-dis, the OII atom can exist on (Cu-1)-TiO2 throughout the lowtemperature CO catalytic oxidation process as well. Therefore, once O2 dissociates on the Cu-1 atom in (Cu-1)-TiO2, the OII atom will cover and passivate the active Cu-1 and Ti sites making them inactive for further O2 activation and CO oxidation reactions.

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Figure 10. Potential energy profiles for dissociated O2+CO (up) and O+CO (down) on (Cu-1)-TiO2. Eb-CO2 and Eb-CO represents the binding energy of CO2 and CO, respectively. The Eb-CO is calculated by a similar equation as Eb-CO2. See Figure 3 for color coding. The minimum energy paths for these two reactions are shown in Figure S19.

3.4 Exploring the relation between charge polarization and reactivity.

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Surface reactivity of a catalyst can be closely associated with charge polarization between the surface and the adsorbates.52 In the molecular science, this connection can be expressed quantitatively by the dipole-dipole interaction energy:53

[

𝐸=𝐶

𝒅𝟏 ∙ 𝒅𝟐 |𝒓|3

(𝒅𝟏 ∙ 𝒓) ∙ (𝒅𝟐 ∙ 𝒓)

-3

|𝒓|5

]

(4)

where 𝒅𝟏 and 𝒅𝟐 represent dipole moments of the two interacting molecules, r is the vector connecting the centers of mass of the two molecules, and 𝐶 is a system dependent constant. However, this formula is hard to applied to periodic systems due to the difficulty in defining the center of mass of a periodic surface model. Therefore, in order to approximately represent the dipole-dipole interaction in periodic models, we simplified this formula to the descriptor 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓, where 𝒅𝒈𝒂𝒔 and 𝒅𝒔𝒖𝒓 denote the dipole moments of the adsorbates and the surface, respectively. Moreover, recent studies suggest that the efficiency of energy transfer depends on the included angle between the two interacting species,54 which is also contained in the 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 product. In consequence, a larger 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 value represents not only stronger dipole-dipole interaction, but also higher energy transfer efficiency. In this work, we first calculate the atomic dipole moments for all the atoms involved in our system by using the DDEC6 atomic population analysis,39,40 then the atomic dipole moments that belongs to adsorbates and surface are summarized to obtain 𝒅𝒈𝒂𝒔 and 𝒅𝒔𝒖𝒓 respectively. To verify this descriptor, CO adsorbed on Au(111) (CO/Au(111)) was chosen first, as shown in Figure 11a. (The detailed dipole data for CO@Au(111) are shown in Table 24


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S1). This model allows us to explore the relation between the gas-surface dipole-dipole interaction and the adsorption energy. By adjusting the CO/Au(111) adsorption angle (θ), both CO and Au surface dipoles can be controlled. The adsorption energy shows linear relation with the 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓, indicating that a stronger dipole-dipole interaction corresponds to a stronger adsorption, which accords with our physical intuition.

Figure 11. (a) CO adsorption energy as a function of 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 in configurations with varying CO-Au(111) surface interaction angles as illustrated by the inset. (b) Reaction barrier as a function of 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 in the optimized configurations of the (Cu-8)-CO+O2, (Cu-8)-CO+O, (Cu-O)-CO+O2 and (Cu-8)-CO+O species. 𝒅𝒈𝒂𝒔: the dipole moment of the adsorbed CO, CO+O2/O; 𝒅𝒔𝒖𝒓: the dipole moment of the catalyst surface apart from the adsorbed gases.

Next, the descriptor 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 is applied to the above-mentioned CO oxidation catalysts to examine its relation with surface reactivity. It is important to note that the reactions occurring on the (Cu-1)-TiO2 surface are strongly influenced by the large 25



steric hindrance at the (Cu-1) site. Therefore, we focus on the reactions on (Cu-8)-TiO2 and (Cu-O)-TiO2, as shown in Figures 3 and 8. Because all cases represent CO and O2 co-catalytic reactions, we treated both gas molecules as a whole to calculate the 𝒅𝒈𝒂𝒔. The results in Figure 11b (The detailed dipole data for the CO oxidation systems are shown in Table S2) show an inverse proportion between the descriptor 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 and the activation barriers, indicating that the electron and energy transfer can occur more easily when 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓 is larger. The trends for △G and SA in Table 1 also show that the strong dipole-dipole interaction between CO+O2 and (Cu-O)-TiO2 can enhance the stability of the adsorbates, which can further increase the reactivity of CO oxidation.

4. CONCLUSIONS In the present work, the synergistic O2 activation and CO oxidation on TiO2(101) supported Au7-Cu1 bimetallic nanoparticles has been systematically studied. The results show that the synergistic effect between CO and O2 can significantly promote CO oxidation that occurs on the top-layered Cu atom in Au7-Cu1/TiO2(101). Further, pretreatment of Au7-Cu1/TiO2(101) in oxygen-rich conditions at high temperature is predicted to produce a new catalyst (Cu-O)-TiO2, which shows better performance with regards to CO and O2 co-catalysis. Finally, we have introduced the charge polarization descriptor 𝒅𝒈𝒂𝒔 ∙ 𝒅𝒔𝒖𝒓, which we believe is suitable for estimation of surface adsorption stability in periodic systems. These findings suggest a novel and efficient way to understand and tune the adsorption/desorption behaviors between 26


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molecules and surfaces, giving a new train of thought for improved catalyst design. We are currently employing the methodology presented in this paper to study additional solid catalytic processes in order to gain better understandings of adsorption and reactivity in these systems from the charge polarization point of view.

Supporting Information Available: Rationalization for the choice of the two-layered Au8 structure as the initial model for the Au-Cu bimetallic cluster, all possible composite configurations of the Au7-Cu1/TiO2(101) system, the stable binding structures of O2 and CO on (Cu-8)-TiO2. AUTHOR INFORMATION Corresponding Author * [email protected]. Phone: +86 551 63600029

ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (NSFC 21303027, 21603042, 21703045, 21633006), Natural Science Foundation of Guizhou Province (No. QKJ[2015]2128, QKJ[2015]2129), Natural Science Foundation of Department of Education of Guizhou Province (No. QJTD[2015]55). O.V.P. acknowledges support of the U.S. Department of Energy (Grant No. DE-SC0014429).

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Table of Contents Graphic

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Catalytic Chemistry Predicted by a Charge Polarization Descriptor

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