Toward Rational Design of Oxide-Supported Single-Atom Catalysts

Article pubs.acs.org/JACS

Toward Rational Design of Oxide-Supported Single-Atom Catalysts: Atomic Dispersion of Gold on Ceria Jin-Cheng Liu, Yang-Gang Wang,* and Jun Li* Department of Chemistry and Key Laboratory of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: We have constructed a general thermodynamic model of chemical potentials and applied ab initio electronic structure and molecular dynamics simulations, as well as kinetic Monte Carlo analysis, to probe the dynamical, reactive, and kinetic aspects of metal single-atom catalysts (SACs) on oxide support. We choose Au single atoms (SAs) supported on ceria as a typical example to demonstrate how our model can guide the rational design of highly stable and reactive SACs. It is shown that, under realistic conditions, various factors such as temperature, pressure, particle size, and the reducibility of the support can strongly affect both the stability and the reactivity of SACs by altering the relative chemical potentials between SAs and metal nanoparticles (NPs). The Au SAs at step sites of ceria support are rather stable, even at temperatures as high as 700 K, and exhibit around 10 orders of magnitude more reactivity for CO oxidation than the terrace sites. Remarkably, under reaction conditions, Au SAs can be dynamically created at the interface of small-size Au NPs on ceria support even without step sites, which accounts for the puzzling significant size effect in gold catalysis. Our work underscores an unrecognized critical role of Au SAs in gold nanocatalysis and provides a general methodology for designing the metal SACs on oxide supports.

1. INTRODUCTION Single-atom catalysts (SACs) have attracted extensive attention as a new frontier in heterogeneous catalysis in recent years.1−4 SACs consist of isolated metal single atoms (SAs) dispersed onto the support surfaces and possess unique chemical and physical properties often different from those of the conventional supported metal nanoparticle (NP) catalysts. Due to the ubiquitous tendency of metal atoms to aggregate, the most critical component in this field is to find an effective approach to strongly anchor the metal SAs to form a highly stable and reactive catalytic active center for desired catalytic reactions.1,2 Modern theoretical chemistry and high-performance computation have played important roles in understanding the mechanistic nature of single-atom catalysis,5−11 yet it remains a grand challenge to guide the prediction and design of highly stable and reactive SACs under realistic conditions insofar as the system is often rather complex, including metal SAs, support, and reactant species and finite temperature and pressure. Nowhere is this more true than for catalytic systems such as the first practically synthesized single-atom catalyst Pt1/ FeOx as well as its analogues Ir1/FeOx and Au1/CeO2,12−15 which exhibit strong covalent metal−support interactions. In the current work, we investigate how realistic conditions affect the thermodynamic stability and the reactivity of metal SAs by using a variety of modern theoretical methodologies, including ab initio electronic structure theory, molecular dynamics simulations, and kinetic Monte Carlo analysis. We choose ceria-supported Au SAs as prototypical examples to illustrate these points, given the abundance of experimental and © 2017 American Chemical Society

theoretical studies in ceria-supported gold catalysis that could serve as validation of our model. As our model is not limited to ceria support, it has also been validated on a series of selected metal oxides. Over the past three decades, gold nanocatalysis has emerged as an exciting area in heterogeneous catalysis. Ever since Haruta16 and Hutchings17 discovered supported Au NPs for catalyzing a series of important reactions, gold nanostructures have been widely used as catalysts.18−22 While small-sized Au NPs (2−5 nm) appear to exhibit high catalytic activity,23−34 recently it was also reported that Au SAs could catalyze reactions with remarkable reactivity for, e.g., CO oxidation,15,35−37 water−gas shift,3,38 methanol reforming,39 and ethanol dehydrogenation.40 The Au SA has structural and electronic properties significantly different from those of Au clusters and NPs. Although it does not bind with any other Au atoms, it interacts substantially with the support ions, leading to the likely formation of charged Au ions, the catalytic behavior of which is still a controversial topic because of the complexity of support and environment.7,15,41−44 Among the challenges of gold catalysis, we emphasize the following. First, supported nanogold catalysts experience a severe sintering problem. Under operando conditions, supported NPs on the oxide support tend to sinter to form larger particles, either by Oswald ripening or by coalescence, leading to the deactivity.45,46 Datye and collaborators showed Received: February 15, 2017 Published: April 13, 2017 6190

DOI: 10.1021/jacs.7b01602 J. Am. Chem. Soc. 2017, 139, 6190−6199

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Journal of the American Chemical Society

minimum and transition states, and vibrational frequencies were also used to estimate the entropy contribution in the kinetic Monte Carlo simulations. Numerous computational models used in this work are list in Part I (Figures S1−S3) of the Supporting Information. 2.2. AIMD Simulations. All Born−Oppenheimer molecular dynamics (BOMD) simulations were carried out with CP2K simulation package65 for more than 30 ps at a time step of 1 fs. Canonical (NVT) ensemble and Nosé−Hoover thermostats were set to 700 K.66,67 All initial structures for AIMD simulations were optimized to local minimum, and the first 5 ps data were discarded to ensure equilibrium of the system. The spin-polarized PBE exchange− correlation functional was adopted with double-ζ Gaussian basis sets,68 which were used for the valence electrons (5d106s1 for Au, 5s25p64f15d16s2 for Ce, and 2s22p4 for O). An auxiliary plane-wave basis set with a cutoff set to 350 Ry was used for computing the electrostatic terms.69 Due to the short time scales, AIMD simulations can only sample very fast, low-energy-barrier events and may strongly depend on the initial configurations. We thus performed MD simulations at relatively high temperature to accelerate the sampling and combined the static CI-NEB calculations to confirm the observed processes.50,70 2.3. kMC Simulations. First, reaction rate constants were calculated by (harmonic) transition state theory,71

that Pt could atomically disperse on polyhedral ceria and nanorods but easily integrate on aluminum oxide or ceria cubes.47,48 Second, the reactivity of single Au atoms also seems to be elusive. Experimentally, Au SAs are found to be either inactive or much less active than Au NPs because of the observation of increased activity when SAs get aggregated.42 In contrast, Qiao et al. reported that Au SAC is remarkably stable and highly reactive for CO oxidation on FeOx and CeO2 supports.15,35 Theoretical studies also considered single Au atoms on a CeO2(111) surface but showed that Au SAs on CeO2(111) could easily drop into oxygen vacancy and block O2 and CO adsorption, leading to the poisoning of active sites.7,9 Henkleman et al. reported that the CO oxidation reaction at a metal−support interface exhibits relatively high CO2 desorption energy (Ede = 1.27 eV) on CeO2(111),49 which seems at variance with the ambient reaction temperature of CO oxidation. Recent experimental and computational evidence further showed that metal atoms could anchor at the steps of ceria after calcination at 800 °C and decrease the CO2 desorption energy. Interestingly, through ab initio molecular dynamics (AIMD), we recently found that single Au atoms can be dynamically formed on CeO2(111) from Au NPs under working conditions,50 which may contribute to the high reactivity of nanogold catalysts. This dynamic single-atom catalysis (DSAC) phenomenon is also consistent with recent scanning tunneling microscopy results of Au−CO formation in a CO atmosphere.51,52 Despite previous experimental and theoretical efforts, the existing form of gold/ceria catalysts and catalytic mechanism under working conditions remain vague in three aspects because of the complexity of oxide supports and the difficulties of in situ characterization: (i) Which ceria facets and defects are responsible for anchoring Au SAs?53 (ii) What is the environment’s influence on sintering and disintegration equilibrium between Au NPs and SAs?54 (iii) What is the difference in reaction activity between Au NPs and SA?55 Although dynamic formation of Au atoms was seen in previous work,50 it is not clear what causes the significant size effect in gold catalysis. In this work, we will show theoretical evidence of formation of trapped Au SAs by ceria step sites and investigate the mechanism of trapping, dispersion, and aggregation of Au atoms on ceria supports; we will further consider some irreducible supports such as Al2O3 and MgO for comparison. Thermodynamic analysis, DFT-based AIMD simulations, and kinetic Monte Carlo (kMC) simulations are preformed to shed light on the catalytic activity of single Au atoms and to explain the remarkable size effect in gold catalysis.

D

kHTST

IS

hv / k T k T ∏i = 1 (1 − e i B ) −ΔEa / kBT = B e h ∏ D − 1 (1 − ehvjTS/ kBT ) i=1

TS where νIS i and νj are vibrational frequencies for the initial state and transition state, respectively, ΔEa is the activation barrier, kB is the Boltzmann constant, T is temperature, and h is the Planck constant. The adsorption constants were given by molecular collision theory,72

kads = SPA / 2πmkBT where S, P, A, and m are the sticking coefficient, partial pressure of the adsorbed species, area of adsorption site, and molecular mass, respectively. To satisfy the adsorption equilibrium, the desorption rate was calculated on the basis of balance of microscopic reversibility,72

⎛ ΔG(T , P) ⎞ ⎛ μ(T , P) − Eads ⎞ kads = exp⎜ ⎟ ≈ exp⎜ ⎟ kdes k T kBT ⎝ ⎠ ⎝ ⎠ B where ΔG(T,P), Eads, and μ(T,P) are the Gibbs free energy change of the adsorption process, adsorption energy, and chemical potential of the gas molecule, respectively. The chemical potential μ(T,P) at specific temperature T and pressure P was estimated by73 ⎛ P ⎞ μ(T , P) = μ(T , P ⌀) + kBT ln⎜ ⌀ ⎟ ⎝P ⎠ = [H(T , P ⌀) − H(0 K, P ⌀)] − T[S(T , P ⌀) − S(0 K, P ⌀)] ⎛ P ⎞ + kBT ln⎜ ⌀ ⎟ ⎝P ⎠

2. COMPUTATIONAL DETAILS 2.1. DFT Parameters. All static calculations were carried out using spin-polarized density functional theory (DFT) with generalized gradient approximation of Perdew−Burke−Ernzerhof (PBE) as implemented in VASP code.56−58 DFT + U method with U = 5 eV was used to describe the localized Ce 4f states.59−61 The valence states of all atoms were expanded in a plane-wave basis set with a cutoff energy of 400 eV. Considering the large size of slabs used in this work, the single gamma-point grid sampling was used for Brillouin Zone (BZ) integration. The periodic Natural Bond Orbital (NBO) analysis developed by Dunnington and co-workers was performed to identify surface chemical bonding of Au1CO complex.62 Atomic positions were optimized by conjugate gradient algorithm until the forces were less than 0.02 eV/Å. Transition states searched by climbing image nudgedelastic-band (CI-NEB) method with convergence criterion of 0.05 eV/ Å.63,64 Vibrational analyses were further performed to ensure the local

where the enthalpy H and the entropy S were obtained from a standard thermodynamic table.74 In this work, site-dependent kinetic Monte Carlo (kMC) analysis was employed to evaluate the kinetic properties of SAs and NPs. All simulations are based on rejection-free kMC algorithm. At a given temperature (T) and partial pressure (P), the coverage of surface species (i), during the kMC simulation was given by72,75 θi =

∑n θi , nΔtn ∑n Δtn

where n is the simulation loop number; θi,n is the coverage of species i at the nth step, and Δtn is the time span of the nth loop, which could be counted as 6191


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Scheme 1. Its chemical potential μNP(R) can be expressed by the Gibbs−Thomson (G-T) relation,

Δt = (∑ ki)−1 ln(1/u) i

μ NP (R ) = 2Ωγme/R

where ∑iki is cumulative function of rate constant k for all possible elementary steps. u is a random number from 0 to 1 to make the delay associated with transition i satisfy Poisson distribution. For single-atom and interfacial catalysis, kMC simulation is an effective way to consider only the active site and avoid incorporating the adjacent site that is not relevant to the reaction. All the kMC simulations in this work are based on a one-dimensional lattice model with 108 loops. Details are shown in Part V of the Supporting Information.

(1)

where Ω is the molar volume of bulk metal atom and γme is the surface energy of the NP. Derivation of the G-T relation can be found in previous works.76,77 The chemical potential of metal SA can be approximately defined as the formation energy of metal SA with respect to the bulk metal (μbulk = 0): μSA ≈ ESA/ox − E B − Eox

3. RESULTS AND DISCUSSION 3.1. Thermodynamic Formation of Metal SAs. The stability of SAs under working conditions is one of the most critical concerns in single-atom catalysis. However, the ubiquitous atomic diffusion, aggregation, and disintegration during the catalyst preparation usually take place via Ostwald ripening, leading to the disappearance of small NPs and the growth of large ones that finally reach a thermodynamic equilibrium.42,45,46,54 In this section, we construct a chemicalpotential-based thermodynamic model to explore how to improve the stability of SAs under realistic conditions. We start from a supported metal NP in a spherical segment with the radius of curvature R and contact angle δ, as shown in

(2)

where ESA/ox is the total energy of metal SA absorbed on an oxide surface, EB the energy of bulk gold energy per atom, and Eox the energy of the oxide slab. The energy change for subtracting a metal SA from a metal NP can be estimated by the difference of the chemical potentials between metal SA and metal NP: ΔE f SA (R ) = μSA − μ NP (R )

(3)

Here, ΔEfSA(R) is an indicator that represents the stability of Au SAs on the support. A smaller value (especially negative value) generally indicates that metal SAs are easier to disintegrate from the Au NPs. The concentration of metal SAs with respect to supported NP can be estimated by54 cSA(R ) = 1/a0 2 exp( −ΔE f SA (R )/kBT )

Scheme 1. Schematic Illustration of Au NPs and SAs Supported on CeO2(111) Surfacea

(4)

where a0 is the lateral lattice constant of ceria support. According to eqs 1−4, one can conclude that the concentration of metal SAs, cSA(R), depends on temperature, particle size, and anchoring site of SAs. We now take gold on several oxide supports as examples. For Au NPs, we assume the surface is mainly composed of 86% (111) and 14% (100), based on previous estimation of Wulff construction.78,79 The surface energy of Au NPs (γme) can be calculated by the average of the surface energy: γme =

∑ fi γi

(5)

where i is surface energy of specific facet and f i is the percentage of this facet. In this work, γme is assumed as a constant, 0.045 eV/Å2. As a result, the chemical potential of Au NPs, μNP(R) is calculated to be 1.62/R eV, which is dependent

a

The inset energy graph shows the relative chemical potentials for Au NP, Au SA at terrace site, and Au SA at step site with respect to the bulk gold (μbulk = 0).

Table 1. Chemical Potential with Respect to Infinite Bulk Gold and Configuration Parameters of Au−CO Speciesa CeO2 (111) μSA/eV charge (Au)/|e|

Eads‑CO/eV charge (Au)/|e| μSA‑CO/eV LC−O/Å LAu−C/Å ∠Au−C−O νC−O/cm−1 Evac/eV

2.07 0.36

−2.35 0.53 0.42 1.15 1.88 178.4 2104 2.54

Ov 0.63 −0.60

−0.13 −0.51 1.20 1.16 2.32 125.6 2017

irreducible support

step-U

step-M

step-D

Al2O3(100)

MgO(100)

Without CO 0.15 0.55 0.31 0.33

0.64 0.32

2.06 −0.24

2.16 −0.29

With COb −1.03 0.52 −0.18 1.15 1.88 179.2 2080 1.88

−1.31 0.52 0.04 1.15 1.88 179.0 2094 1.81

−0.89 0.21 1.87 1.18 2.23 144.5 1898 6.32

−0.83 0.00 2.03 1.18 2.22 135.8 1846 5.71

−1.26 0.51 −0.01 1.15 1.88 178.8 2080 1.67

a

Eads denotes adsorption energy of CO, and Evac denotes the oxygen vacancy formation energy of oxide supports. bAt 300 K and CO partial pressure of 10−3 bar. 6192


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chemical potential of μNP(CO)(R) with NPs’ curvature, temperature, and pressure are shown in Figure 2. The chemical

on the radius of curvature R. We consider the formation of Au SAs on five different supports: (a) clean CeO2(111) surface; (b) CeO2(111) with oxygen vacancies; (c) CeO2(111) with steps sites such as U, M, and D (Figure S1); (d) clean Al2O3(100) surface; and (e) clean MgO(100) surface. The calculated μSA and Bader charges of Au SAs are listed in Table 1. Next, we compare the chemical potentials μNP(R) and μSA in Figure 1 to understand the relative stability of Au SAs on each

Figure 2. Contour plot of chemical potential of NPs, μNP(R,T,P). (a) μNP(CO) versus the temperature T and the radius of curvature R at 10−3 bar. (b) μNP(CO) of supported Au particles versus the CO partial pressure P and the radius of curvature R at 300 K. The red (blue) region indicates high (low) chemical potential.

potential of Au NPs upon CO adsorption, μNP(CO), exhibits a distinct dependence on the particle size (R), CO partial pressure (P), and the operative temperature (T). In general, the larger R, lower T, and higher P will lead to a lower μNP(CO), but the trend will become less sensitive to T and P when R becomes larger, revealing that the stability of Au NPs is mainly controlled by the particle size when it becomes larger. To enhance the possibility of the extrusion of Au SAs, one should consider decreasing the size of Au NPs. The chemical potential of metal SAs is also closely related to the pressure and temperature of the reactant species. In previous studies the formation of Au SAs was reported to be accelerated by the formation of a Au−CO complex, which is rather stable at the Au−oxide interface.50 The chemical potential of Au SAs upon CO adsorption, μSA(CO), can be estimated by

Figure 1. Chemical potential and structures of NPs and SAs in vacuum. (a) Comparison between chemical potential of NPs, μNP(R), and that of SAs, μSA, with respect to curvature of NPs. Black line, μNP(R); colored lines, μSA of Au SAs anchored on different sites. (b) Top view and side view of optimized structures of Au SAs on MgO(100), CeO2(111) surface, step-M, step-D, step-U, and oxygen vacancy on CeO2(111). CeO2-U and CeO2-D describe the (110)-type steps with upper and lower rows of oxygen atoms, respectively. CeO2M describes the (211)-type step with mixed types of edge oxygen atoms. Color code of the spheres: Au, yellow; Ce, white; Mg, green; O, red.

support. It is demonstrated that the chemical potential of Au SAs on bare CeO2(111) and MgO(100) surface is significantly higher than that of Au NPs, indicating that they tend to aggregate into NPs. In contrast, when corresponding NPs’ curvatures become smaller, Au SAs can anchor at defects of ceria, such as step-U, which could stabilize Au SAs by charge transfer from Au to Ce4+.7,61 Above we consider only the catalyst without the presence of reactants. Under reaction conditions, the reactant species would also affect chemical potential of NPs and SAs.79−82 Take CO oxidation on oxide-supported Au nanocatalyst as an example. Since CO molecules generally prefer to adsorb on Au, the surface energy of Au NPs will be reduced by83 ΔYi (T , P) =

ads θi[ECO (θi)

− μCO(T , P)]/Ai

μSA(CO) = μSA + Eads(CO) − μCO(T , P)

(7)

where μSA is the chemical potential of Au SAs without CO adsorption; Eads(CO) is CO adsorption energy on Au SAs; μCO(T, P) is the chemical potential of CO gas molecule. The Gibbs free energy change of Au disintegration (i.e., subtracting one Au−CO from Au NPs), ΔGdis, can be written as (neglecting the contribution of configurational entropy): ΔGdis = μSA(CO) − μ NP(CO)(R )

(8)

Similar to ΔE SA(R), ΔGdis is an indicator that represents the stability of Au SAs on the support. A smaller value (especially negative value) of ΔGdis generally represents that Au−CO is easier to disintegrate from the Au NPs. In Figure 3a, we consider μSA(CO) and μNP(CO)(R) as a function of temperature on different oxide supports at CO partial pressure, PCO = 10−3 bar, and the radius of NPs, R = 20 Å. It is shown that ΔGdis on MgO(100) is always higher than 1 eV at the temperature range of 0−1000 K, manifesting that the Au SAs are highly unstable and prefer clustering. μSA(CO) of Au SAs at ceria oxygen vacancy is similar to μSA due to the weak CO binding to the negatively charged Au atom at the oxygen vacancy. On the contrary, Au SAs with CO adsorption on CeO2(111) becomes more stable with the formation of Au−CO, compared to that without the presence of CO. Especially, the Au SAs on ceria steps become resistant from integration below 300 K due to a negative ΔGdis. To understand the intrinsic nature of Au−CO complex on different support, we further choose Au1(CO)/MgO and f

(6)

where θi is coverage of CO on the Au surface, and (θi) is the average adsorption energy, as shown in Figure S4. μCO(T,P) = μCO(T,P⌀) + kT ln(P/P⌀) is the chemical potential of CO molecules in gas phase, with respect to the reference state of CO at T = 0 K. The value of μCO(T,P) can be obtained from standard thermodynamic tables (Table S1).74 We further assume that the percentages of (100) and (111) facets do not change upon CO adsorption when considering the surface energy of Au NPs. Although CO adsorption may lead to the morphology change of Au NPs,79,81,82 keeping the percentage of Au surface will not cause distinct error for big NPs (R > 2 nm) based on the so-called compensation effect.54 Moreover, the G-T relation also assumes that the surface free energy of small metal NPs is independent of the size. Thus, in this work the curvature of Au NPs is kept at 2 nm. Contour plot of Eads CO

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coordination from C to Au as well as the Osurf−Au σ bond. This is also further confirmed by the Bader charge analysis. The Au SA in Au1(CO)/MgO is close to neutral (0.00e), while the charge of Au SA in Au1(CO)/CeO2(step-D) is as large as 0.52e. On the basis of these analyses, we infer that it is the reducibility of the support that alters the charge state of Au SA and promotes the stability of Au SAs. Not surprisingly, a roughly linear relation in Figure 4 is observed between the

Figure 3. Chemical potential of NPs and SAs with CO adsorption. (a) Temperature-dependent μSA(CO) and μNP(CO) on different supports, at PCO = 10−3 bar and R = 20 Å. (b) Top view and side view of optimized structures of Au−CO complex. Color code of the spheres: Au, yellow; Ce, white; Mg, green; O, red; C, gray.

Au1(CO)/CeO2(step-D) as two typical examples to compare their structural and electronic properties. As shown in Figure 1b, Au SA binds to two surface oxygen ions, forming a nearly line configuration on step-D. With the presence of CO, this Au SA is pulled out of the step, forming an Osurf−Au−CO complex. By monitoring the electron spin density, one electron is found to localize at a surface Ce site (see Figures S6 and S7). This conclusion is also confirmed by the localized Ce 4f state in the projected DOS. Bader charge analysis show that the Au SA upon CO adsorption is positively charged by ∼0.5|e|, indicating that the Au SA transfers one electron to the surface Ce site. The angle of the Au−C−O is nearly 180°, and the Au−C bond length is ∼1.88 Å. By periodic NBO analysis, (see Figures S8− S11, Tables S3 and S4), the Au−C σ bond occupancy of 1.88e (spin up + spin down) and the Osurf−Au σ bond occupancy of 1.86e (spin up + spin down) are consistent with the picture of a localized two-center bond, showing that such adsorption interactions can be effective. Chemical bond analysis using Amsterdam Density Functional (ADF) program is further performed on an Au+−CO complex to illustrate the bonding nature between CO and Au SA. It is shown that the strong interaction mainly originates from the interaction between the CO 5σ orbital and the unoccupied Au 6s orbital, forming a lowenergy molecular orbital of 8σ MO (see Figure S12 and Table S5 for details). The Au−C−O bond angle for Au1(CO)/MgO is calculated to be ∼136°. This nonlinear configuration is attributed to the Pauli repulsion between the σ orbitals of CO and the 6s orbital of Au(0) as well as the π back-donation.61 Bonding analysis on an Au(0)CO complex also confirms that the complex has to break its linear configuration to increase the interaction between the half-occupied Au 6s orbital and the CO 2π* orbital, which lowers the total energy (see Figure S13 and Table S6 for details). Open-shell periodic NBO analysis only detects the spin down of Au−C σ bond with occupation number of 0.90e, due to the electron back-donation from Au to π* orbital of CO, as shown by the spin density in Figure S6b. This geometry change results in the decrease of CO bond order and the elongation of the C−O bond from 1.14 to 1.18 Å. The distinct difference from Au1(CO)/CeO2(step-D) is that there is no localized electron on the Mg site in Au1(CO)/MgO. In Au1(CO)/CeO2(step-D), the 6s electron of Au transfers to a surface Ce site, forming a positively charged Au+ that can strongly bond with the surface O and CO simultaneously. In contrast, in Au1(CO)/MgO the 6s electron of Au partially donates to the C−O π* orbital, which weakens the donation

Figure 4. ΔGdis versus the support vacancy formation energy, Evac. The Gibbs free energy change of Au disintegration (ΔGdis) is obtained at PCO = 10−3 bar, R = 20 Å, and T = 300 K.

surface oxygen vacancy formation energy, Evac and the free energy change of disintegration. By investigating 14 types of various oxide supports, it becomes clear that the more reducible the support is (i.e., the lower Evac), the more stable the Au SA would be upon CO adsorption. Specifically, classic irreducible (inert) oxide support, such as MgO, ZrO2, HfO2, Al2O3, ThO2, and so on, all locate at the top right region of the graph with high Evac and high ΔGdis. But for reducible oxide supports with low Evac, such as CeO2, RuO2, TiO2, V2O5, and Fe3O4, Au SAs tend to be quite stable, especially for ceria support with step sites. This finding provides insight on how to screen effective materials for stabilizing the SACs on oxide surfaces. 3.2. Disintegrations/Integration Dynamics of Au SAs on Ceria. In the previous section, we discussed the thermodynamics of the formation of metal SAs on oxide supports with or without the presence of the reactant species. The dynamic behavior of metal SAs under realistic condition is also very important in determining the catalytic activity.43,50,70 For example, the diffusion of metal SAs will directly affect the speed of clustering. In addition, we recently reported a DSAC mechanism on oxide-supported Au NPs, where the integration/ disintegration behavior of SAs can well couple with the surface redox properties and promote CO oxidation reactivity.50 Here, we choose Au SAs on ceria support as an example to demonstrate how to evaluate the dynamical behavior of the metal SAs on oxide support. We first consider the static potential energy surface (PES) of a Au SA adsorbed at the step site, shown in Figure 5a. To cross the step, the Au atom should diffuse from site a to b along path I, and then from site b to a along path II, as shown in Figure 5b. The calculated barriers are 0.42 and 0.95 eV, respectively. But, at site b, a Au atom can also be trapped in a deeper potential well (site c) by overcoming a barrier of 0.41 eV. The adsorption energy Eads at the step site c is −2.33 eV, much more stable than that on a perfect (111) surface, where Eads = −0.91 eV. The adsorption of Au SAs at all the three sites leads to a formation 6194


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Figure 5. Potential energy surface (PES) and Au SA diffusion pathways. (a) Potential energy surface of a single Au atom diffusion at the step-D region of ceria. The inset shows the structure of step-D, where the step-Ce is shown as green spheres and the step-O is shown as blue spheres. Three local minima are labeled as a, b, and c. (b) Diffusion pathways from CI-NEB calculations and the corresponding configurations for Au SAs. The tetravalent Ce4+ ion is shown as a white sphere and trivalent Ce3+ as green spheres.

Figure 6. (a) Radius distribution functions (RDFs) between Au SA and surface oxygen on ceria surfaces. (b) Root-mean-square deviation (RMSD) of Au SAs.

Figure 7. AIMD simulations on the integration of Au SAs. (a,b) Initial and final snapshots of MD simulations on CeO2(111)-supported Au19 cluster and Au SA. (c,d) Initial and final snapshots of MD simulations on CeO2(111)-supported Au19(CO)7 cluster and Au1−CO complex. (e) Distances between the isolated Au SA and center-of-mass of Au19 cluster with (red curve) and without (black curve) CO adsorption. (f) Probability distribution functions P(rcm) of all Au atoms relative to the center-of-mass of the Au19 cluster.

of Ce3+ at the upper layer, originating from the charge transfer from Au SA to Ce site. We further performed DFT-based AIMD simulations to investigate the dynamic behavior of Au SAs on the step sites at finite temperature. Three perfect ceria surfaces of CeO2 (111), (331), and (110), which have 0%, 50%, and 100% step-D concentrations, respectively (see Figure S3), are considered in

the simulations. All the simulations were run for more than 30 ps at 700 K. Radius distribution functions (RDFs) between Au SA and surface oxygen on the three surfaces are shown in Figure 6a. The zero probability density between the first two peaks on (331) and (110) surface means Au−O bonds do not break during the time scale of the MD simulations, which indicates that the Au SAs on these surfaces are rather stable and 6195


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stability of SAs on the terrace sites, and thus Au SAs may be dynamically created under reaction conditions. 3.3. Reactivity of SAs: CO Oxidation on Au1/CeO2. We now turn to discuss the reactivity of SAs. Although Au SAs on perfect CeO2(111) is reported to be inactive for CO oxidation due to the formation of negatively charged Au at the oxygen vacancy,7 we have shown that the ceria support with step sites is the best candidate to stabilize the Au SA with a positively charge state of +1. This conclusion inspires us to explore whether Au SAs are reactive on the step site. Therefore, the reaction mechanism for CO oxidation on Au1/CeO2-step-D is studied. The reaction energy pathways and the intermediate configurations are shown in Figure 8. For comparison, CO oxidation mechanisms on Au1/CeO2(111), Au1/CeO2-step-U, and Au12/CeO2(111) are also shown in Figure 8 and Table S7.

are hard to diffuse. In contrast, on perfect CeO2(111) surface without step sites appreciable probability density is observed between the first two peaks, implying the ease of Au SAs diffusion. The estimated free energy barrier for the diffusion on (111) surface simulation is about 0.2 eV from potential of mean force, in good agreement with CI-NEB results (see Figures S15 and S16). By integrating the area of the first peak, the coordination numbers of Au on (111), (331), and (110) surfaces are estimated to be 1.38, 1.92, and 1.95, respectively. The coordination numbers of Au on (111) surface are significantly lower than the D-step surfaces (331) and (110), which is obviously due to the frequent diffusion between O-top and O-bridge sites.7 The root-mean-square deviation (RMSD) of Au atomic positions in Figure 6b also confirms that Au SAs are quite stable on the 50% and 100% step-D surfaces but fluctuate frequently on perfect CeO2(111). To demonstrate the dynamic integration process directly, AIMD simulations were further performed on a hypothetical model, where an isolated Au SA and a small Au NP (Au19) that was reported to be stable in the gas phase were both put on the CeO2(111) surface.84,85 The initial distance between Au SA and Au19 was set to be 8.5 Å. Inasmuch as oxygen vacancies are ubiquitous in reducible oxides, one lattice oxygen under the Au19 was removed to simulate the partially reduced ceria surface. The initial and final snapshots for the MD simulations are shown in Figure 7a,b and Movie S1. By monitoring the distance between Au SAs and the center-of-mass of Au19, it is found that during the first 8 ps, the Au SA only fluctuates between its local minima on CeO2(111) surface, but well separates from the Au19 cluster. Starting at 10 ps, Au SA is moving toward the Au19 cluster. In less than 1 ps the Au SA is integrated into the Au19 cluster, leading to formation of an Au20 nanocluster. This process significantly reduces the average potential energy by about 2 eV (see Figure S18). No SA formation is observed from the time of 10 to 30 ps. The probability distribution functions P(rcm) of the Au atoms relative to the center-of-mass of the Au19 also confirm the integration process. From 0 to 8 ps, the isolated peak of SAs is more than 3 Å from the edge of peaks for Au19, while no isolated P(rcm) peaks appear for the time of 10−30 ps (Figures 7g,h). We finally evaluate the dynamic behavior of Au SAs upon CO adsorption for comparison. Since COs were found to adsorb at the apex sites of Au clusters or NPs,86,87 the Au19(CO)7 cluster and an isolated Au1−CO are initially put on the CeO2(111) surfaces. (Figure 7c) Distinctly different from the Au SA without CO, Au1−CO is considerably stable at a surface oxygen site, and no diffusion is detected during the simulation (Figure 7d and Movie S2). The probability distribution functions P(rcm) in Figure 7f show a ∼3 Å gap between Au19(CO)7 cluster and Au1−CO. This result is consistent with the thermodynamic analysis in section 3.1, where it is shown that under working conditions the adsorbed CO species improves the disintegration of Au NPs on CeO2(111) surface. Additionally, it is also observed that CO adsorption induce significant structural changes to create more apex sites on the surface of Au NPs, which may originate from both CO adsorption and substrate effect.70,86,87 Our theoretical results illustrate a critical role of step sites in stabilizing the SAs during the catalyst preparation process when the reactant species is not present. To obtain a highly stable SA catalyst, the step site is necessary to be introduced on the ceria support. In addition, the reactant species may also improve the

Figure 8. Reaction mechanisms of CO oxidation. (a) Energy profile of CO oxidation on Au1/CeO2-step-D (black curve) and Au12/ CeO2(111) (red curve). The pathways are divided into seven steps: (i) *OAu + *O + CO → *OAuCO + *O; (ii) *OAuCO + *O → *OAuCO-O*; (iii) *OAuCO-O* → Au(Ov) + CO2; (iv) Au(Ov) + O2 → *OAu + *O2; (v) *OAu + *O2 + CO → *OAuCO + *O2; (vi) *OAuCO + *O2 → *OAuCO-O2*; and (vii) *OAuCO-O2* → *OAu + *O + CO2. (b) Optimized structures of species in CO oxidation on Au1/CeO2 step-D. Blue spheres denote oxygen from O2.

Consistent with previous reports,41,49,88 CO oxidation follows a Mars−van Krevelen (MvK) mechanism: first, the first CO on Au is oxidized by a lattice oxygen to form a CO2; next, an O2 molecule adsorbs on the oxygen vacancy; finally, another CO on Au reacts with the adsorbed O2 to form another CO2, completing the catalytic cycle. It is shown that the ratedetermining step (RDS) on Au1/CeO2-step-D is CO oxidation by the lattice oxygen, which requires a barrier of 0.83 eV and a slightly endothermic reaction energy of 0.07 eV. On the contrary, the CO oxidation by lattice oxygen on Au12/ CeO2(111) is much easier, but leads to a high CO2 desorption energy of 1.23 eV. To explore the reactivity of SAs from a kinetic view, we further perform kMC simulations on CO oxidation based on the calculated reaction potential surfaces. The partial pressures of CO, CO2, and O2 are set as 10−4, 10−4, and 10−2 bar, respectively. The steady-state coverages as a function of temperature are shown in Figure 9a, and the Arrhenius plots of the total CO2 production rate are shown in Figure 9b. On Au1/CeO2-step-D catalyst, the Au SA sites are all covered by CO at T < 300 K, and the apparent barrier based on the Arrhenius plots is estimated to be 0.81 eV, which is close to the barrier of the RDS. When the temperature is increased above 6196


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Figure 9. kMC Analysis at PCO = PCO2 = 10−4 bar and PO2 = 10−2 bar. (a) Steady-state coverage as a function of temperature. Top panel shows the coverage at the interface of Au12/CeO2(111), and bottom panel shows the coverage at Au SA site on Au1/CeO2-step-D. (b) Arrhenius plots of the total CO2 production rate on Au12/CeO2(111) and Au1/CeO2-step-D.

300 K, CO at the SA site is quickly consumed, and the bare Au SAs are exposed on surface. We note that Au SAs are rather stable on the step sites, with a large diffusion barrier of 1.31 eV, and are thus resistant to sintering under these conditions. Interestingly, the CO2 production rate almost becomes a constant (i.e., Ea = 0.0 eV) at T > 400 K. For Au12/CeO2(111), Au SAs are covered by bent-COO− species until 400 K, as shown in Figure 9. As a result, the CO2 production rate is about 10 orders of magnitude lower than that on Au SAs at 323 K. The apparent barrier is estimated to be 1.29 eV, suggesting the low reactivity for CO oxidation. Overall, both static potential energy surfaces and kMC simulations have shown that Au SAs exhibit much higher reactivity than the Au NPs. We note that in traditional heterogeneous catalysis, small metal NPs on oxide support are more or less considered to be rigid, stable, and catalytically active. However, more and more evidence51,52,80,81,89 suggests that metal NPs experience severe morphological changes or even become disintegrated under reaction conditions. In this work, we have shown that smaller Au NPs prefer to disintegrate at working conditions, whereas larger Au NPs are prone to be integrated. This finding implies that the formation of Au SAs on smaller Au NPs likely accounts for the high reactivity for CO oxidation with nanogold catalysts. This dynamic formation of active single Au atoms might be relevant to the size effect of Au catalysts that has not been completely understood in the past three decades.24,90−93 The difficulty of creating single Au atoms on large-size Au NPs is consistent with their inertness in gold catalysis.

(i) Both the stability and reactivity are strongly dependent on conditions such as reaction temperature, partial pressure, size of NPs, and reducibility of the support. By comparing chemical potentials, we have shown that Au SAs prefer to adsorb at CeO2 step sites rather than on Au NPs or the terrace surface, with or without the presence of reactant species. By fabricating ceria support with high step concentration, the dispersion and stability of SACs can be enhanced. (ii) On irreducible support, the Au SAs exhibit little redox coupling with the support and, as a result, only weakly bind CO. In contrast, on reducible support, Au SAs strongly couple with the redox properties of the support and transfer charges to the metal cation, leading to the positively charged Au+ atoms, which become vastly stable upon CO adsorption. (iii) Au SAs have much higher reactivity for CO oxidation than Au NPs on ceria support. Under working conditions, it is shown that reactant species such as CO can significantly prompt the disintegration of small Au NPs by forming Au1−CO complexes. As shown by us and others, while small Au NPs or nanoclusters can disintegrate into small clusters or even supported SAs on the surface upon CO adsorption and oxidation reaction, it is much harder to form Au SAs from larger Au NPs due to low chemical potential. It is therefore likely that the puzzling significant size effect in nanogold catalysis may originate from this dynamic phenomenon of Au NPs under reaction conditions. Our findings indicate that the observed high reactivity of Au NPs in nanocatalysis may be attributed to the dynamic formation of active sites of supported Au SAs under realistic conditions, a phenomenon we dubbed “dynamic single-atom catalysis”. As the dynamic formation of single atoms on support occurs instantly, it is challenging to identify this step ex situ. Despite significant progress in in situ characterization of catalytic processes,94 more experimental efforts are needed to validate and establish the dynamic mechanisms involved in nanocatalysis.

4. SUMMARY Our study has presented a general strategy combing a variety of modern theoretical methodologies to explore the stability and reactivity of single-atom catalysts (SACs). In the context of CO oxidation on ceria-supported gold SACs, all the thermodynamic, reactive, and kinetic aspects of the metal SACs are extensively discussed. By means of the thermodynamic model of chemical potentials, density functional theory electronic structure calculations, and ab initio molecular dynamics simulations, as well as kinetic Monte Carlo analysis, we have drawn the following conclusions: 6197


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01602. Computational models, supplementary thermodynamic analysis, electronic properties of Au SAs, supplementary dynamic analysis, and supplementary reaction pathways and kMC, including Figures S1−S24 and Tables S1−S7 (PDF) Movie S1, AIMD trajectories showing Au19 and Au1 on ceria surface (AVI) Movie S2, AIMD trajectories showing Au19(CO)7 and Au1CO on ceria surface (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yang-Gang Wang: 0000-0002-0582-0855 Jun Li: 0000-0002-8456-3980 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the NKBRSF (2013CB834603) and NSFC (21590792, 91645203, and 21521091) of China. The calculations were performed by using supercomputers at Tsinghua National Laboratory for Information Science and Technology, the Supercomputer Center of the Computer Network Information Center at the Chinese Academy of Sciences, the National Supercomputer Centre in Guangzhou (NSCC-GZ, Tianhe II), and the Computational Chemistry Laboratory of Department of Chemistry at Tsinghua University, which is supported by Tsinghua Xuetang Talents Program.

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Toward Rational Design of Oxide-Supported Single-Atom Catalysts

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