On the Nature of Support Effects of Metal Dioxides MO2 (M = Ti, Zr, Hf

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On the Nature of Support Effects of Metal Dioxides MO (M = Ti, Zr, Hf, Ce,Th) in Single-Atom Gold Catalysts: Importance of Quantum Primogenic Effect Yan Tang, Shu Zhao, Bo Long, Jin-Cheng Liu, and Jun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05338 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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The Journal of Physical Chemistry

On the Nature of Support Effects of Metal Dioxides MO2 (M = Ti, Zr, Hf, Ce,Th) in Single-Atom Gold Catalysts: Importance of Quantum Primogenic Effect Yan Tang, Shu Zhao, Bo Long, Jin-Cheng Liu, and Jun Li* Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

ABSTRACT: Fundamental understanding of support effects and metal-support interaction is critical in heterogeneous catalysis. In this work, theoretical investigations are carried out to study the nature of support effects of different tetravalent-metal dioxides of MO2 (M = Ti, Zr, Ce, Hf, Th) in single-atom gold catalysts using density functional theory with on-site Coulomb interactions (DFT+U). The properties of gold adatom on the stoichiometric (MO2) and reduced (MO2-x) surfaces as well as CO adsorption on Au1/MO2 and Au1/MO2-x have been investigated systematically. Our calculations indicate that the fundamental quantum primogenic effect that causes the radial contraction and low orbital energies of 3d and 4f orbitals in these MO2 oxides plays a vital role in determining the valence states and charge distribution of single-atom gold as well as the adsorption modes of CO on various MO2 supports. We find that gold atoms supported on different surfaces exhibit oxidation states from Au (-I) to Au (0) to Au (I), depending on the nature of the metal oxide supports. The support-dependent oxidation states and charge distribution of Au can further influence the adsorption mode of CO. While CO adsorbs strongly on Au (I) in Au1/MO2 (M = Ti, Ce) via Au-C -bonding, weaker adsorption occurs on Au (0) in Au1/MO2 (M = Zr, Hf, Th) with charge back-donation to CO 2π* anti-bonding orbitals. In contrast, at Au1/MO2-x of reduced support, CO adsorption is stronger for M = Zr, Hf, Th than for M = Ce. These results provide essential understanding on the nature of support effects of metal oxides in heterogeneous catalysis.

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1. INTRODUCTION Gold is the noblest metal in the Periodic Table due to significant quantum-mechanical shielding effects and relativistic effects. For a long time, gold had not been used as catalysts as few chemical compounds can chemisorb on bulk gold surfaces as a result of lack of chemical bonding between the reactants and gold. Remarkably, Haruta and Hutchings et al. found in 1980s that gold could become catalytically active under appropriate preparation and reaction conditions.1-3 Especially, small-sized gold nanoparticles (NPs) supported on metal oxides are shown to be catalytically active for CO oxidation at low temperature. Since then, gold-based materials have been widely studied as catalysts in heterogeneous catalysis for various reactions, including CO oxidation,4-8 water-gas shift reaction,9-10 propylene epoxidation,11-13 and selective hydrogenation,14-16 etc. Among the various factors that affect the catalytic performances, metal oxide supports are found to be vital for gold catalysis, especially when involving redox reactions. To design efficient gold catalysts, it is of great importance to understand the origin of the high catalytic activity and the fundamental role of the support. Despite significant experimental and theoretical efforts,17-21 the nature of the superior performance of gold nanoparticles as well as the role of the oxide support remains elusive. Several studies suggested that the under-coordinated Au atoms enhance the catalytic activity and the oxide support only played a role to stabilize the Au NPs.17, 19 However, there are also numerous evidences that the oxide support and metal-oxide interface are critical.4, 22-27 For instance, the lattice oxygen of CeO2 support is found to participate in CO oxidation via a Mars-van Krevelen (MvK) mechanism,4 and O2 can be activated by the formation of a CO-O2 complex at a so-called Ti-Au sites at the Au-TiO2 (110) interface.24-26 Moreover, many experimental and theoretical studies indicate that the activities of supported Au catalysts are rather sensitive not only to the size and morphology of gold nanoparticles but also to the surface properties and variety of oxide supports.28-32 For example, Madix et al. reported that defective TiO2 (110) surface exhibits a good selectivity in the formation of methane while the stoichiometric surface does not.29 Comotti et al. found that Au/Al2O3 and Au/TiO2 are the active catalysts for CO oxidation, whereas Au/ZnO and Au/ZrO2 catalysts are substantially


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less active.31 So far, significant efforts have been made to elucidate the origin of the oxide supports with different catalytic performances. Häkkinen et al. attributed the influence of supports to the charge transfer between the metal-oxide support and Au NPs.33 Yoon et al.34 and Yan et al.35 emphasized the importance of oxygen vacancies in the oxide support for the high rate of CO oxidation. The interactions between metals and supports, especially the strong metal-support interaction (SMSI) introduced by Tauster et al. in 1978,36 have been found to affect the activity, stability and selectivity. Recent investigations revealed that a special kind of SMSI in single-atom catalyst Au1/FeOx, i.e. the covalent metal-support interaction (CMSI), can drastically strengthen the stability and enhance the catalytic performance.8 Tauster et al. concluded that SMSI depends strongly on the nature of supports when investigating Ir NP supported on various metal oxides.37 In the case of Au, SMSI was observed when Au atoms were supported on TiO2, CeO2, ZnO, and so on, and these catalysts with SMSI showed the remarkable catalytic performance.38-39 In general, Au supported on reducible oxides (e.g. CeO2 and TiO2) tends to be more active than on irreducible oxides (e.g. ZrO2) in oxidation reactions.40-43 It is thus interesting to understand why these MO2 have drastically different properties and why these MO2 supports have different interaction with Au even though Ti, Zr, and Ce atoms all have four valence electrons, which, from the viewport of Mendeleev periodic law, should possess more or less similar properties. Therefore, a theoretical understanding on the effects of metal dioxide supports is necessary for elucidating the nature of gold catalysis. In recent years, single atom catalysts (SAC, denoted as M1/support) have aroused significant interest in heterogeneous catalysis.44 A number of experimental and theoretical investigations have shown that SACs tend to possess high selectivity, activity and stability, albeit reduced consumption of noble metals.6, 8, 45-55 Single-atoms doped on oxide surfaces are found to exhibit unique properties often different from supported nano-particles.56 Especially noteworthy is the recent finding through ab initio molecular dynamics (AIMD) simulations that single-atom catalytic active sites can be formed dynamically on titania- and ceria-supported gold nanoparticles in reaction conditions, a phenomenon we dubbed as dynamic single-atom catalysis (DSAC).57-58 Hence, compared with the Au NP model, an



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Au1/oxide model is ideal for systematic theoretical studies to elucidate the nature of support effects in gold catalysis. In this work, density functional theory corrected by on-site Coulomb interactions (DFT+U) has been performed to investigate the interaction between the gold atom and various metal dioxide (MO2, M = Ti, Zr, Ce, Hf, Th) surfaces as well as the support effects on CO adsorption at Au1/MO2 surfaces. Our calculations find that the quantum primogenic effect largely accounts for the support effects of metal dioxides in gold catalysis. Here the quantum primogenic effect refers to the fact that the first-shell atomic orbitals of each angular quantum number (e.g. 1s, 2p, 3d, 4f, 5g, …) have nodeless, rather contracted radial distribution and relatively low orbital energy.59-60 The present investigation provides new insights for understanding the support effects in heterogeneous catalysis and for designing new single-atom gold catalysts.

2. METHODS All the calculations were performed using periodic DFT methods implemented in the VASP code.61-62 Projector augmented wave (PAW)63 method was used for the interaction between the atomic cores and valence electrons. The valence orbitals of Ti (3s, 4s, 3p, 3d), Zr (4s, 5s, 4p, 4d), Ce (4f, 5s, 6s, 5p, 5d), Hf (5s, 6s, 5p, 5d), Th (5f, 6s, 7s, 6p, 6d), Au (5d, 6s, 6p), O (2s, 2p) and C (2s, 2p) were described by plane-wave basis sets with cutoff energies of 400 eV. The exchange-correlation energies were calculated by generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional.64 Spin polarization was considered using spin-unrestricted Kohn-Sham formalism. To partially correct the strong electron-correlation properties of these oxides, DFT+U calculations

65-66

were performed with U = 4 eV for Ti, Zr, Hf, Th and U = 5 eV for Ce, with the U values taken from literature.67-72 To investigate the accuracy of calculated oxygen-vacancy formation energy, the Heyd-Scuseria-Ernzerhof hybrid functional method (HSE06) is also applied.73 The exact exchange contribution of 25% was utilized with a screening length of 0.2 Å-1. CeO2 (111) surface was chosen in our work due to its high stability among the low-index ceria surfaces (i.e., (100), (110) and (111) surfaces). TiO2 (101) surface, c-ZrO2 (111), c-HfO2 (111) and ThO2 (111) surfaces with similar structures to CeO2 (111) surface were also used as


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models. Although other facets of ceria (like (110) or (100)) could sometimes be more active than (111) in real catalytic reaction,74-75 our conclusion with regard to the general trend of Au1/MO2 should be valid. The anatase TiO2 (101) surface was selected and modeled by p(2×3) 18-layer supercells with the bottom six layers fixed, while the c-ZrO2 (111), CeO2 (111), c-HfO2 (111) and ThO2 (111) surfaces were modeled by p(3×3) 9-layer supercells with the top six layers relaxed, with a vacuum gap between slabs at ~15 Å. The force threshold on each relaxed ion was set as 0.02 eV/Å. Monkhorst−Pack (1×1×1) Γ-centered k-points grid sampling within the Brillouin zones were used because of the large size of the surface models. The influences of k-points were tested, and the largest difference in binding energy and adsorption energy between (2×2×1) and (1×1×1) k-points was 1.8%, revealing that the k-points scheme used in our calculations is reasonable. The binding energy (BE) between gold adatom and supports were defined as BE = Esup + EAu - Esup+Au, where Esup+Au, Esup and EAu are the calculated energies of the Au1/support, the support, and the gold atom, respectively. The chemisorption energies (Eads) of CO molecule on the surfaces were determined according to Eads = Eslab + ECO - Eslab+CO, where Eslab+CO, Eslab and ECO are the total energies of the slab with adsorbed CO molecule on the surface, the slab of the clean surface, and the CO molecule in the gas phase, respectively. The charge density differences were evaluated using the formula Δρ = ρA+B - ρA - ρB, where ρX is the electron density of X. Atomic charges were computed using the atom-in-molecule (AIM) scheme proposed by Bader.76 The periodic NBO formalism developed by Dunnington and co-workers was also applied using VASP software to analyze surface chemical bonding via natural bond orbitals.77

3. RESULTS AND DISCUSSION 3.1. The Nature of Redox Properties of Supports. The redox property of an oxide support often plays a vital role in catalytic performance. Oxygen vacancy is one of the most important defects in metal oxides, and its formation energy is a key measure of the redox properties of metal oxides. Thus, we will begin with the electronic structures of the reduced oxides MO2-x (M = Ti, Zr, Ce, Hf, Th) and oxygen



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vacancy formation energies (Evac). 3.1.1 Electronic Structures.

The spin density differences of CeO2-x and TiO2-x (Figure

1) show that in the reduced oxides with one oxygen vacancy (Ovac) two remaining electrons localize on the Ce4+ or Ti4+ ions, leading to the formation of Ce3+ and Ti3+ ions, which is consistent with the previous results.78-79 On the contrary, for ZrO2-x, HfO2-x and ThO2-x, electron localization functions (ELF) show that two electrons are strongly localized at the vacancy site (Figure 2), and the Bader charge analyses (Table S1) also demonstrate the contributions of neighboring metals. DFT+U with different U values as well as HSE06 methods were applied to examine the likely error of the method used (Table S2) and the results show that our electronic structure data are reliable. It is noted that the type of charged oxygen vacancies in the less reducible ZrO2-x, HfO2-x and ThO2-x are similar with the so-called F-centers in MgO1-x.80 Clearly, MO2 (M = Ti, Ce) and MO2 (M = Zr, Hf, Th) represent two different cases of metal oxides with respect to the nature of the oxygen defects, which depends on the reducibility of the M4+ ions.

Figure 1. Calculated spin density for reduced (a) CeO2 (111) surface; (b) anatase TiO2 (101) surface.

Figure 2. Calculated electron localization function (ELF) of reduced (a) c-ZrO2 (111) surface; (b) c-HfO2 (111) surface; (c) ThO2 (111) surface.


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The radial distribution functions, D(r) = r2R(r)2, of M4+ (M = Ti, Zr, Ce, Hf, Th, Rf) (Figure 3) as well as the atomic orbital energies of Ti-3d, Zr-4d, Ce-4f/5d, Hf-5d and Th-5f/6d (Table S3) are calculated using ADF program to analyze the reducibility of the M4+ ions81. It turns out that Ce4+ 4f-orbitals and Ti4+ 3d-orbitals are much more contracted in the radial distribution than the nd (n = 4, 5, 6) orbitals of the other transition metals and actinides. This radial contraction of the atomic orbitals is a direct result of the quantum primogenic effect, which often makes electrons in these orbitals strongly correlated.59-60 The rather contracted nature of 3d and 4f orbitals in radial distribution enhances the ability of TiO2 and CeO2 to capture electron. Oppositely, electrons prefer to stay at the vacancy site due to the high energies of nd (n = 4, 5, 6) orbitals in ZrO2, HfO2 and ThO2. This significant difference between Ti-3d/Ce-4f and Zr-4d/Hf-5d/Th-6d5f elements accounts for the different electronic structures of MO2-x (M = Ti, Zr, Ce, Hf, Th).

Figure 3. The radial distribution function (D(r) = r2R(r)2) of M4+ (M = Ti, Zr, Ce, Hf, Th, Rf).

3.1.2 Formation energies of oxygen vacancy. Table 1 lists the calculated oxygen vacancy formation energies (Evac). The results from the Hubbard local-Coulomb-correlated and hybrid DFT methods both indicate that CeO2 has the largest reducibility with the smallest Evac of 2.25 eV (DFT+U) or 2.85 eV (HSE06). For a-TiO2, Evac is computed to be 4.07 eV using HSE06 method, suggesting that tetravalent Ti4+ can also be readily reduced. In contrast,



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ZrO2, HfO2 and ThO2 have much higher oxygen vacancy formation energies of 6.05 eV, 6.24 eV and 6.66 eV, respectively, indicating much poor reducibility of these metal dioxides. Despite the likely self-interaction error in the calculated Evac, the trends of Evac are likely to be valid due to systematic errors.

Table 1. Calculated Formation Energy of an Oxygen Vacancy (Evac) for the Studied Metal Oxide Surfaces metal oxide TiO2 ZrO2 CeO2 HfO2 ThO2 a.

Evac / eV PBE+Ua 3.59 5.94 2.25 6.09 6.46

HSE06a 4.07 6.05 2.85 6.24 6.66

other works 4.15b; 4.70c ~5.90d 2.60e; 2.13f 7.38g

This work. b. PBE, ref. 82. c. PW91, ref. 79. d. PW91, ref. 83. e. PW91+U, U = 5 eV, ref. 84. f. PW91+U,

U = 5 eV, ref. 78. g. LDA+U, U = 4 eV (this U value was determined for bulk ThO2), ref. 67.

The oxygen vacancy formation energy of an oxide depends on a variety of factors. In general, Evac consists of two parts, Evac = Eb + Er, where Eb and Er are the bonding energy and relaxation energy, respectively.83 The calculated Eb and Er are listed in Table S4. The bonding energy Eb in Evac depends upon the M-O bond strength, which consists of the ionic electrostatic interaction and the covalent bonding interaction. Periodic NBO approach is used to analyze the ionicity of M−O bonds and the results are listed in Table 2. NBO analyses show that the Ce-O bond in CeO2 is the most ionic because of the least contribution of Ce in the Ce-O bond, while Ti-O bond in TiO2 is the least ionic with the highest Ti contribution. The calculated atomic net charges from natural population analysis also reveal the same trend, i.e., the ionicity of the M-O bonds follows the trend Ce-O > Th-O > Hf-O > Zr-O > Ti-O. This conclusion is also consistent with the differences in electronegativity between O and these metal atoms. As for the covalence of M−O bonds, it depends on the overlaps of metal orbitals and oxygen orbitals. M-O bonds in


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CeO2 and TiO2 have smaller covalent interaction due to the penetrating Ti-3d and Ce-4f orbitals and the O(2p)-M(d,f) overlaps increase in the order of Ce-4f < Ti-3d

On the Nature of Support Effects of Metal Dioxides MO2 (M = Ti, Zr, Hf

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