Theoretical Approach To Predict the Stability of Supported Single

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A Theoretical Approach to Predict Stability of Supported Single-Atom Catalysts Ya-Qiong Su, Yifan Wang, Jin-Xun Liu, Ivo A. W. Filot, Konstantinos Alexopoulos, Long Zhang, Valerii Muravev, Bart Zijlstra, Dionisios G. Vlachos, and Emiel J.M. Hensen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00252 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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A Theoretical Approach to Predict Stability of Supported Single-Atom Catalysts Ya-Qiong Su,†,# Yifan Wang,‡,# Jin-Xun Liu,†,# Ivo A.W. Filot,† Konstantinos Alexopoulos,‡ Long Zhang,† Valerii Muravev,† Bart Zijlstra,† Dionisios G. Vlachos*,‡ and Emiel J.M. Hensen*,† †Laboratory

of Inorganic Materials & Catalysis, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, 221 Academy St., Newark, Delaware 19716, United States Supporting Information Placeholder ABSTRACT: Heterogeneous single-atom catalysts involve isolated metal atoms anchored to a support, displaying high catalytic performance and stability in many important chemical reactions. We present a general theoretical framework to establish the thermodynamic stability of metal single atoms and metal nanoparticles on a support in the presence of adsorbates. As a case study, we establish for Pt-CeO2 the CO partial pressure and temperature range within which Pt single atoms are more stable than Pt nanoparticles. Density functional theory and kinetic Monte-Carlo simulations demonstrate that Pt atoms doped into the CeO2 surface exhibit a very high CO oxidation activity and thermodynamic stability in comparison to models involving Pt single atoms on terraces and steps of CeO2. An intermediate CO adsorption strength is important to explain a high activity. Our work provides a systematic strategy to evaluate the stability and reactivity of single-atoms on a support. KEYWORDS: theoretical chemistry, single-atom catalysis, stability, Pt/CeO2, CO oxidation

INTRODUCTION Supported metal catalysts are extensively used in exhaust gas clean-up because of their high catalytic activity for CO and hydrocarbon oxidation and NOx reduction.1-3 The size of the supported metal nanoparticles (NPs) is often decisive for the catalytic performance.4-6 Due to their highly undercoordinated nature, supported metal single atom catalysts (SACs) can display a very high catalytic activity. SACs are rapidly becoming a new frontier in heterogeneous catalysis.7-9 Besides the low coordination number of the catalytic metal atom, the support usually plays an important role in the catalytic cycle, akin to non-innocent ligands in homogeneous catalysts.10, 11 ACS Paragon Plus Environment

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In environmental catalysis, ceria is one of the most used supports, because it can reversibly store oxygen atoms and also because it strongly interacts with transition metals, leading to a large metal-support interface.12, 13 Generally, single atoms (SAs) are more mobile on a support than NPs and, thus, prone to agglomeration.14-16 SA stabilization on oxide supports is therefore of critical importance for the design of SACs. The preparation and performance of Pt SAs on ceria have recently been extensively studied.1, 8, 17-19

Experimental and theoretical data also emphasize the role of surface defects in ceria, such as steps

and cation vacancies, for trapping of Pt SAs.1, 8, 18 Alternatively, Pt SAs can also strongly interact with the less stable CeO2(100) surface by coordinating to four surface oxygen atoms in a square-planar configuration.8, 19 Another aspect of highly dispersed supported metal systems is the strong adsorption of reactants that can lead to the break-up and dispersion of metal clusters and nanoparticles into SAs.2025

For instance, Berko et al. observed by scanning tunneling microscopy a very rapid disintegration of

TiO2-supported Rh NPs of 1-2 nm to atomically dispersed Rh at 300 K and a pressure of 10-1 mbar CO.22 Based on binding energies computed by density functional theory (DFT), we develop a theoretical framework that forms the basis for constructing condition-dependent stability diagrams of supported metal SAs with respect to supported metal NPs in the presence of CO. The influence of CO coverage and lateral interactions are also taken into account. This leads to predictions about the stability of NPs and a critical size below which the supported NPs will disperse into SAs in the presence of CO at given conditions. We employ this methodology to predict the stability of Pt SAs on stoichiometric CeO2(111), stepped CeO2(111), defective CeO2(111) and CeO2(100) surfaces. Finally, we compute potential energy diagrams for CO oxidation by Pt SAs for several of these cases. CO oxidation rates are predicted by graph-theoretical kinetic Monte-Carlo (GT-kMC) simulations.26

COMPUTATIONAL METHODS Density functional theory (DFT). We carried out spin-polarized DFT calculations as implemented in the Vienna ab initio simulation package (VASP).27 The ion-electron interactions are represented by the projector-augmented wave (PAW) method28 and the electron exchange-correlation by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.29 The Kohn-Sham valence states were expanded in a plane-wave basis set with a cut-off energy of 400 eV. The Ce(5s,5p,6s,4f,5d), O(2s,2p), Pd(4d5s) and C(2s,2p) electrons were treated as valence states. The DFT+U approach was used, in which U is a Hubbard-like term describing the onsite Coulombic interactions.30 This approach improves the description of localized states in ceria, where standard LDA and GGA functionals fail. For Ce, a value of U = 4.5 eV was adopted, which was ACS Paragon Plus Environment

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calculated self-consistently by Fabris et al.31 using the linear response approach of Cococcioni and de Gironcoli32 and which is within the 3.0-6.0 eV range that results in the localization in Ce 4f orbitals of the electrons left upon oxygen removal from ceria.33 For Pt/CeO2(111), we use a periodic ceria slab with a (4×4) surface unit cell. For Brillouin zone integration, a 1×1×1 Monkhorst-Pack mesh was used. The bulk equilibrium lattice constant (5.49 Å) previously calculated at the PBE+U level (U = 4.5 eV) was used.34 The CeO2(111) slab model consists of three Ce-O-Ce layers (48 Ce and 96 O atoms) and a vacuum gap of 15 Å. The atoms in the bottom layer were frozen to their bulk positions and only the top two Ce-O-Ce layers were relaxed. The climbing image nudged-elastic band (CI-NEB) algorithm35, 36 was used to identify the transition states in the elementary reaction steps of CO oxidation. A frequency analysis was performed to confirm that each transition state has only a single imaginary frequency in the direction of the reaction coordinate. The total energy difference was less than 10-4 eV and the relaxation convergence criterion was set at 0.05 eV/Å. Graph-theoretical kinetic Monte-Carlo (GT-kMC). The DFT calculated energetics and vibrational frequencies were input in the GT-kMC framework as implemented in the software package Zacros.37 In this approach, participating species and elementary steps are represented as graphs. For non-activated adsorption as employed in this work, we assume an early 2D gas-like transition state. The rate constants of adsorption and desorption are calculated as 26, 38, 39 Px Ast 2 mx k BT

kads 

(1),

qvib , X ( gas )  qrot , X ( gas )  qtrans 2 D , X ( gas ) k BT E exp( ads ) qvib , X h k BT

kdes 

(2).

Here h is Planck’s constant, kB Boltzmann’s constant, T the temperature, Px the partial pressure of a species x, Ast is the area of an adsorption site, m the molecular mass of the adsorbent, Eads the gas adsorption energy, and q partition functions for gas-phase species and adsorbates (computed using statistical mechanical expressions). For each surface reaction, the rate constants were calculated by transition state theory in an Arrhenius form,40 yielding E ( ) k BT Q ‡ exp( fwd ) h QR k BT ‡

k fwd 

(3)

and krev 

E ‡ ( ) k BT Q ‡ exp( rev ) h QP k BT

(4),

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where kfwd (krev) is the rate constant of the forward (reverse) elementary reaction step, kB the Boltzmann constant, T the temperature and h Planck’s constant. Q‡, QR and QP are the quasi-partition functions of the transition, reactant and product states, respectively. Finally, E‡fwd (σ) ( E‡rev (σ) ) is the activation energy for configuration  in the forward (backward) direction. Other inputs to the software include the operating conditions (temperature, pressure and gas-phase composition), a lattice structure, energetics of each surface species and the reaction mechanism parameterized by equations (1)-(4).26, 41 We did not take into account lateral interactions, because we only consider dispersed single Pt atoms and their neighboring sites; as such interactions between adsorbates are irrelevant. The GT-kMC supports sitedependent chemistry. Three types of sites were considered in each unit cell including a Pt atom, to which CO can adsorb, and two oxygen vacancy sites, where O2 can adsorb and dissociate (see supporting information). The chemical master equation describes the time evolution of the probability pi(t) of finding the system in a particular state i42 dpi (t )    wi j p j (t )  w j i pi (t )  j i dt

(5).

wi  j denotes the propensity of the system going from state i to j, which is related to the rate constants of

the microscopic events.43, 44 Solving the chemical master equation numerically is often impractical due to the large number of possible states. Instead, the kMC framework solves the chemical master equation stochastically. At each instant, the possible next events on the lattice are identified by solving subgraph isomorphism problems given the current lattice configurations. The probability of a queue of events is generated and one event is chosen proportional to its probability26 

pi ( i )  ki  exp[  ki d ' ]

(6),

0

Where t is the current time and τi is the time increment between the current time t and the next event i. The time increment is generated stochastically for all possible events at time t following an exponential distribution, 1 ki

 i   ln(1  u )

(7).

Here u is a random number sampled from the uniform distribution between 0 and 1. These times are stored in a priority queue in the form of a binary heap structure. The event i with the smallest occurrence time occurs next. Next, the lattice and simulation time are updated. The procedure is repeated until the stopping criteria are reached. In practice, to reduce stochastic noise, multiple independent trajectories


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generated using different random seeds are run in parallel and averaged. Given long enough simulation time, steady state properties such as average surface species coverages, turnover frequency (TOF), and reaction orders can be obtained.

RESULTS AND DISCUSSION Stability of supported NPs and SAs. The driving force for sintering of supported metal catalysts is the reduction in surface free energy of the active metal phase. Metals usually cannot be kept in atomic dispersion on a support, because a metal adatom usually does not bind strong enough to the support.14 The cohesive energy is typically used to account for the strength of metallic bonds in NPs. Its absolute value decreases when particles become smaller, indicative of the weaker metallic bonds in NPs compared to bulk metal. When a nanoparticle is placed on a support, the total energy is lowered by the interactions with a part of the metal atoms that form the interface. However, the influence of the support on the total energy is minor for particles with more than 20 atoms and we can use the cohesive energy of the corresponding free NP in vacuum as a first approximation as demonstrated for Pd NPs on CeO2(111).14 In order to compare stabilities, this cohesive (or binding) energy per metal atom for the supported NP system can be compared to the binding energy of a single metal adatom placed on a support. We develop our approach for Pt on the most stable CeO2(111) surface. The resulting expressions for the binding energies of ceria-supported Pt NPs and a ceria-supported single Pt atom, Pt1/CeO2(111), are bind EPt  n

(nEPt1  EPtn )

(8)

n

and (9),

EPtbind  EPt1  ECeO2 (111)  EPt1 / CeO2 (111) 1 / CeO2 (111)

where EPt , EPtn , 1

ECeO2 (111)

and EPt /CeO (111) are the energies of a free single Pt atom, a free Pt NP, the CeO2(111) 1

2

slab and the Pt1/CeO2(111) model, respectively, and n is the number of Pt atoms in a NP. The binding energy for a free metal NP of n atoms scales with n-1/3 (Figure S1).45, 46 We compute a binding energy of 3.21 eV for a single Pt atom on the most stable CeO2(111) termination, which is close to the earlier reported value of 3.30 eV.8


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Figure 1. Schematic illustration of the relative stability of Pt SAs and NPs on CeO2 in the presence of adsorbed CO molecules. Adsorbates like CO stabilize a supported single Pt adatom more than a Pt NP because of the stronger adsorption of CO to a single metal atom and also because only a fraction of the Pt atoms in a NP are covered by CO, shown in Figure 1. The binding energy of a single Pt atom with adsorbed CO as a function of temperature (T) and pressure (P) is given by ads EPtbind1CO /CeO2 (111) (T , P)  EPtbind1 /CeO2 (111)  [ EPtCO1 /CeO  CO (T , P)] 2 (111)

(10),

in which  is the chemical potential of CO in the gas phase at temperature T and a CO partial pressure P with respect to a reference state of CO in the gas phase at standard conditions. Here, we ignored the contribution of vibrational entropy of the adsorbed CO to the binding energy of a single Pt atom or Pt NPs. The vibrational entropy contribution to the free energy is less than 0.03 eV for CO adsorption on ads metal surface.47-50 The computed adsorption energy of CO on Pt1/CeO2(111), EPtCO/CeO , is -3.45 eV. (111) 1

2

Accordingly, we can rewrite Eq. 10 as EPtbind1CO /CeO2 (111) (T , P)  6.66  CO (T , P)

(11).

CO adsorption also stabilizes a supported Pt NP. The stabilization will depend on the number of available surface Pt atoms and the CO coverage at a certain T and P. Therefore, we evaluate the binding energy per Pt atom for such a system by taking into account that m CO adsorb to a NP containing n Pt atoms at a given temperature and pressure EPtbind  n ( CO ) m

(nEPt1  mECO  EPtn (CO )m ) n



(nEPt1  EPtn )  mEPtCOn  ads n

 EPtbind  n

mEPtCOn  ads n

(12).

CO  ads Here, EPtn is the average CO adsorption energy obtained from the DFT-computed data

EPtCOn  ads 

EPtn (CO )m  ( EPtn  mECO )

(13).

m


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Thus, the binding energy of a CO-covered Pt NP can be expressed in terms of the binding energy of the free Pt NP and a correction for the adsorption of CO. This correction term can be computed in the following manner by taking the chemical potential of CO molecules into account E (n, P, T ) 

m[ EPtCOn  ads (T , P)  CO (T , P)] n



N m CO  ads [ EPtn (T , P)  CO (T , P)]   (n) CO [ EPtCOn  ads (T , P)  CO (T , P)] n N

(14),

where ω(n) = N/n, i.e. the ratio between the number of surface Pt sites to which CO can adsorb (N) and the total number of Pt atoms (n) and  = m/N, i.e. the surface coverage at given temperature and pressure. As we will show below, (n) depends strongly on the particle size

 ( n) 

bind bind Ebulk  EPt n

2  Ptn Aatom



5.529n 1/3 2  Ptn Aatom

(15),

bind in which Ebulk is the binding energy of Pt bulk (6.40 eV) extrapolated from Figure S1,  Ptn is the surface

energy of Pt NPs and Aatom = A/N is the area of a surface atom of Pt. The detailed derivation of equation (15) is in the Supporting Information. To compute the surface energy of Pt NPs, we use the Wulff construction to estimate that sufficiently large NPs consist of 65% (111) and 35% (100) facets. The explicitly calculated binding energy of Pt bulk is 6.37 eV, in accordance with the extrapolated value of 6.40 eV. Accordingly, we neglect the more reactive atoms at the edges and corners. The surface free energy is then the weighted surface energy based on DFT-computed values. Equation (15) can be generally used for the prediction of the dispersion of free metal NPs. For the supported NPs considered in our study, we assume that one of the surface facets of the cuboctahedra NPs (predicted by the Wulff theorem) forms an interface with the CeO2(111) support. In this way, we can determine (n) as a function of n for the non-wetting case. The deviation between (n) for free and supported NPs is relatively small. We also considered the case of partial wetting in which half of the Wulff shape is visible in a Winterbottom approach.51 This partial wetting case results in a lower (n) for given n (Figure S3). Nevertheless, (n) depends much less on the wetting degree than on n. Based on these assumptions for the surface energy and (n) we can write the correction term as E (n, T , P)   (n) fi i ,CO [ EPtCOn  ads (i, T , P)  CO (T , P)]

(16),

i

in which f1 and f2 are the fractions of the NP surface occupied by (111) and (100) facets. The binding energy per Pt atom for our Ptn/CeO2(111) system at a given temperature and pressure is then given by EPtbind (n, T , P )  EPtbind   (n) f i i ,CO [ EPtCOn  ads (i, T , P )  CO (T , P )] n ( CO ) m n i


(17).

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The coverage dependence of the CO adsorption energy for the Pt(111) and Pt(100) surface facets was evaluated by DFT calculations of CO adsorption in a (3×3) surface unit cell and fitted with polynomial functions (Figure 2a). At a given T and P, the corresponding coverage of CO can be determined by21 CO dEPt (i ) 

CO  ads d [i  EPt (i )] i

d i

(18),

 CO (T , P )

where dEPtCO (i ) is the differential adsorption energy of CO molecules on a particular facet (here, either the (111) or (100) facet). As an example, Figure 2b shows the CO coverage on Pt(111) facets as function of T and P. These predictions compare well with available experimental data for single crystal Pt(111) data.52-54

Figure 2. (a) Computed CO adsorption energy on Pt(111) and Pt(100) facets as a function of the CO coverage and (b) CO coverage at given temperature and CO partial pressure for Pt(111). We can now determine n, T and P for which a Pt NP with n Pt atoms covered with m CO is equally or less stable than a CO-covered single Pt atom located on CeO2(111) (19).

EPtbind (T , P)  EPtbind (n, T , P) 1CO / CeO2 (111) n ( CO ) m

We depict the results in Figure 3a in which the isolines represent this condition for a given n. Figure 3a predicts that, at a typical condition of low-temperature CO oxidation (PCO = 10-5 atm, T = 100 ℃), Pt(CO) can be dislodged from Pt NPs of up to ~150 atoms on the CeO2(111) terrace. Particles larger than this critical size will not break up to form Pt SAs. With increasing CO partial pressure, Pt SAs can compete with larger Pt NP. The reason is that the higher coverage of CO on the Pt NPs means that the larger number of Pt-CO bonds can increasingly outweigh the Pt-Pt interactions in the Pt NP. If, on the other hand, the temperature is raised, the CO coverage will decrease and Pt NP will become increasingly favored over single Pt(CO) species (e.g. T > 300 ℃). It can explain some experimental observations that CO induces sintering of metal adatoms on oxide supports at extremely low CO pressure and/or high


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temperature.55 Figure 3b and Figure S4 show how these predictions are affected by the wetting of the CeO2(111) surface by the Pt NPs. Clearly, the main trends remain very similar, while increasing wetting leads to a lower dispersion and a lower stability of Pt NPs.

Figure 3. Critical size diagrams of the supported Pt NPs with respect to thermodynamic stability as a function of temperature and CO partial pressure relative to isolated Pt SAs (a) on the terrace of CeO2(111) with non-wetting and (b) wetting, (c) on the step-edges of CeO2(111), and (d) on the terrace of CeO2(100). The grey area relates to Pt NPs with less than 38 atoms, which were not considered in this study.

It is important to emphasize that these predictions refer to thermodynamic stability of an isolated Ptcarbonyl species in comparison to CO-covered Pt NPs. In experiment, the size of the NPs is determined by many factors including the preparation and activation (reduction) method.56 The dislodgement of a Pt-carbonyl species from a supported Pt NP will involve overcoming a kinetic barrier. For instance, the activation barrier for dislodging a Pt atom from CeO2-supported Pt4 is 2.46 eV. This barrier decreases to 0.45 eV when a CO is adsorbed to the dislodging Pt atom. When the Pt4 cluster is covered by 4 CO, the cluster will spontaneously break up into 4 Pt(CO) SA species (Figure S5). The activation barrier for dislodging a Pt atom from a Pt8 cluster amounts to 2.14 eV and 0.61 eV in the absence and presence of CO, respectively (Figure S6). Therefore, these results demonstrate that dislodgement of Pt atoms in the presence of CO is feasible at typical reaction conditions. ACS Paragon Plus Environment

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The computed frequency of the CO stretching vibration for the Pt(CO) SA on CeO2(111) is 2047 cm-1. Notably, this value is significantly different from the typical frequencies of linearly adsorbed CO on Pt(111) and Pt(100), which are 2090 cm-1 and 2080 cm-1, respectively.57-60 Bazin et al. observed a CO feature at 2046 cm-1 and attributed this to linear CO adsorption on a Pt atom at the Pt-CeO2 interface in Pt/CeO2.61 Teschner and co-workers also reported that linear CO adsorption leads to the strongest adsorption at the Pt-CeO2 boundary and a vibrational frequency of 2054 cm-1.62 Therefore, we speculate that these signals should be attributed to dislodged Pt(CO). We also refer to related IR studies of CO adsorption on oxide-supported Pt catalysts.63,64 The binding energy of a Pt atom at the ceria step-edge is much higher (5.54 eV) than on CeO2(111) (3.21 eV). This leads to a higher propensity of ceria step-edge sites to break up Pt NP, as shown in Figure 3c. These results show that atomically dispersed Pt can be stabilized on a ceria step edge over a much wider range of conditions than on the CeO2(111) surface.18 The finding that this stability extends to high temperature and low CO coverage can explain why ceria-supported small Pt NPs are atomically dispersed at high temperatures, where kinetic barriers for atom dislodgement from Pt NPs can be easily overcome.1, 18 Based on the similar stability of Pt(CO) complexes on the CeO2(111) step and CeO2(100), we predict that Pt SAs can also be stabilized over a wide range of conditions on the more open CeO2(100) surface (Figure 3d), in agreement with experimental observations.65

Figure 4. Pt SAs located at a CeO2(111) step before (a) and after (b) O2 dissociation, and (c-f) doped into the CeO2(111) surface (color code: white, Ce; grey, irrelevant O; red, relevant O; black, C; blue, Pt). (g) Ab initio-determined phase diagram of the Pt-doped CeO2(111) surface. Pd doping in the CeO2(111) surface has recently been explored in the context of stable CO and CH4 oxidation catalysis.66 Figure 4c-f shows similar structures for a Pt atom doped in the CeO2(111) surface. Pt-dop-I contains an octahedral Pt atom obtained by placing a Pt atom on a Ce vacancy (ΔE = -4.04 eV with respect to bulk Pt).67 Pt-dop-II represents a slightly less stable structure (ΔE = +0.36 eV) in which


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Pt is located in a square-planar coordination between 4 lattice oxygens. Whereas removal of a surface oxygen from Pt-dop-I is endothermic (ΔE = 1.75 eV), forming an O vacancy (VO) on Pt-dop-II is exothermic (Pt-dop-II-VO, ΔE = -0.35 eV). Removing a further oxygen from Pt-dop-II-VO is unfavorable (ΔE = 1.23 eV), however. Notably, Figure 4d-f shows that the local geometry around Pt in Pt-dop-II is very similar to that of a Pt atom at step-edge to which two excess O atoms are bound (Figure 4b).8 A phase diagram of these doped structures depicted in Figure 4g predicts that, under typical CO oxidation conditions, Pt-dop-II-VO is a thermodynamically stable structure.

Figure 5. CO oxidation pathways by Pt SAs on CeO2(111), on stepped CeO2(111) and doped in the CeO2(111) surface.

CO oxidation. Having established the conditions under which isolated Pt1(CO) complexes are stable on the CeO2(111) terrace and step-edges, we will now compare the rate of CO oxidation on different isolated Pt species. We also explored alternative locations of a Pt SA doped into the CeO2(111) surface. Co-adsorption of O2 on isolated Pt(CO) is unfavorable (Eads = 0.08 eV). This implies that CO oxidation via a conventional Langmuir−Hinshelwood (LH) mechanism is unlikely. Therefore, we explored a Mars-van Krevelen (MvK) mechanism involving lattice O atoms of the ceria support.1, 17, 68 Figure 5 shows the potential energy surface for CO oxidation. It is shown that the rate-limiting step on Pt1/CeO2(111) and Pt1O2/step is CO2 desorption, which costs energy of 2.38 and 1.31 eV to produce an oxygen vacancy, respectively. While for CO oxidation by Pt-dop-II-VO, the CO2 desorption is much


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easier, and requires an energy barrier of 0.97 eV for CO2 formation. Notably, CO adsorption can cause dynamic hopping of Pt dopants from subsurface to surface as well as Pd.66 Next, we employed the GT-kMC simulations to determine the catalytic performance of these candidate sites in CO oxidation. The lattice for these kinetic Monte Carlo simulations consisted of 100 Pt SA on ceria lattice consisting of 1010 unit cells. Each cell contains one Pt atom and 2 empty sites accommodating O atoms. The steady-state turnover frequencies (TOF) and occupation of the states along the potential energy surface expressed in terms of fractional coverages are reported in Figure 6. The total pressure is set to 0.1 atm and the CO-to-O2 ratio to 2. Figure 6a shows the CO oxidation rates of the different Pt SAs supported on ceria as a function of the temperature. The Pt-dop-II-VO site is much more reactive than the Pt1O2/step model, while the Pt1/CeO2(111) is nearly inactive for CO oxidation under practical conditions. Recently, Datye and coworkers also suggested that atomically dispersed Pt on step-edges of CeO2 exhibits low low-temperature CO oxidation activity.18 Our study alternatively suggests that Pt atoms doped into the surface can be very active for CO oxidation. Further insight can be obtained by inspecting the dominant surface states (also known as MARI, most abundant reaction intermediate) during CO oxidation in Figure 6b-d. For Pt1O2/step, the dominant state is CO adsorbed on Pt1O/step at all temperature tested indicating strong CO adsorption. In Pt1/CeO2(111) model, strong CO adsorption results in the CO adsorbed state on Pt1O/CeO2(111) being the MARI at high temperatures and adsorbed CO2 at low temperatures. On Pt-dop-II-VO, at high temperatures (T>600 K), the system is in the initial state Int0 (Pt-dop-II-VO), where CO oxidation becomes ratelimiting. Reaction orders were also computed and emphasize that CO reaction orders are zero when the MARI is a CO-adsorbed state and unity for an empty surface (Figure S16). This discussion is further supported by a degree of rate control analysis (Figure S18). Overall, we find that strong CO adsorption leads to higher barriers for CO oxidation. Moderate CO bonding of a Pt atom embedded in the CeO2(111) surface is therefore preferred to achieve high CO oxidation activity.


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Figure 6. (a) Arrhenius plot of the rate of CO oxidation on three types of Pt SA on ceria and (b-d) Steady-state coverages of dominant surface states of Pt SAs on CeO2(111) during CO oxidation (p = 0.1 atm, CO/O2 ratio = 2).

CONCLUSION We developed a novel methodology to determine the thermodynamic stability difference between supported metal SAs and NPs in the presence of adsorbates at a given pressure and temperature. As an example, we investigated under which conditions Pt(CO) complexes may be more stable than (partially) CO-covered Pt NPs on CeO2 surface. The constructed stability diagrams show that isolated Pt(CO) may be expected to form on terrace and step and doping sites of CeO2(111). In general, the thermodynamic propensity for dislodgement increases with lower temperature and higher CO coverage. Kinetically, CO coverage facilitates the disruption of Pt-Pt bonds. Further investigation of CO oxidation reactivity demonstrates that doping of Pt in the CeO2(111) surface presents the highest CO oxidation turnover rate among the SA models. This is mainly because CO strongly binds to a Pt SA located on the stoichiometric CeO2 surface or Pt1O on a ceria step-edge site, resulting in low activity. The moderate CO binding suggests the importance of a Sabatier optimum for CO oxidation for Pt SAs on CeO2. The herein developed single-atom thermodynamic concept can be generally applied for supported metal catalysts. This study provides deeper insight into the stability and activity of SACs, and establishes a useful concept facilitating the rational design of SACs with high stability and catalytic activity.

AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Authors * E-mail: [email protected] (Emiel, J.M. Hensen) * E-mail: [email protected] (Dionisios G. Vlachos) Author Contributions # (Y. Su, Y. Wang and J. Liu) These authors contributed equally. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting information Supporting information available (1) derivation of dispersion formula; (2) critical size diagrams of supported Pt NPs and frequency analyses of CO bound to Pt facets; (3) kinetic data of Pt atom dislodging from Pt4 and Pt8 supported on CeO2(111); (4) DFT-determined catalytic cycles of CO oxidation and GT-kMC data. ACKNOWLEDGMENTS The authors acknowledge financial support for the research from The Netherlands Organization for Scientific Research (NWO) through a Vici grant and Nuffic funding. Supercomputing facilities were provided by NWO. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant No 686086 (Partial-PGMs). K.A., Y.W., and D.G.V. acknowledge support for the GT-KMC studies from the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001004. Y.W. acknowledges also support from the Ferguson fellowship.

REFERENCES (1) Nie, L.; Mei, D.; Xiong, H.; Peng, B.; Ren, Z.; Hernandez, X. I. P.; DeLaRiva, A.; Wang, M.; Engelhard, M. H.; Kovarik, L. Activation of Surface Lattice Oxygen in Single-Atom Pt/CeO2 for LowTemperature CO Oxidation. Science 2017, 358, 1419-1423. (2) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nature Chem. 2011, 3, 634-641. (3) Nolte, P.; Stierle, A.; Jin-Phillipp, N.; Kasper, N.; Schulli, T.; Dosch, H. Shape Changes of Supported Rh Nanoparticles During Oxidation and Reduction Cycles. Science 2008, 321, 1654-1658. (4) Liu, L.; Corma, A. Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles. Chem. Rev. 2018, 118, 4981-5079.


Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(5) Nesselberger, M.; Roefzaad, M.; Hamou, R. F.; Biedermann, P. U.; Schweinberger, F. F.; Kunz, S.; Schloegl, K.; Wiberg, G. K.; Ashton, S.; Heiz, U. The Effect of Particle Proximity on the Oxygen Reduction Rate of Size-Selected Platinum Clusters. Nature Mater. 2013, 12, 919-924. (6) Turner, M.; Golovko, V. B.; Vaughan, O. P.; Abdulkin, P.; Berenguer-Murcia, A.; Tikhov, M. S.; Johnson, B. F.; Lambert, R. M. Selective Oxidation with Dioxygen by Gold Nanoparticle Catalysts Derived from 55-Atom Clusters. Nature 2008, 454, 981-983. (7) Wang, A.; Li, J.; Zhang, T. Heterogeneous Single-Atom Catalysis. Nature Rev. Chem. 2018, 2, 6581. (8) Dvořák, F.; Camellone, M. F.; Tovt, A.; Tran, N. D.; Negreiros, F. R.; Vorokhta, M.; Skála, T.; Matolínová, I.; Mysliveček, J.; Matolín, V. Creating Single-Atom Pt-Ceria Catalysts by Surface Step Decoration. Nature Comm. 2016, 7, 10801. (9) Yang, X. F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740-1748. (10) Grützmacher, H., Cooperating Ligands in Catalysis. Angew. Chem. Int.l Ed. 2008, 47, 1814-1818. (11) Boyer, J. L.; Rochford, J.; Tsai, M. K.; Muckerman, J. T.; Fujita, E. Ruthenium Complexes with Non-Innocent Ligands: Electron Distribution and Implications for Catalysis. Coordin. Chem. Rev. 2010, 254, 309-330. (12) Cargnello, M.; Jaén, J. D.; Garrido, J. H.; Bakhmutsky, K.; Montini, T.; Gámez, J. C.; Gorte, R.; Fornasiero, P. Exceptional Activity for Methane Combustion over Modular Pd@CeO2 Subunits on Functionalized Al2O3. Science 2012, 337, 713-717. (13) Bruix, A.; Rodriguez, J. A.; Ramírez, P. J.; Senanayake, S. D.; Evans, J.; Park, J. B.; Stacchiola, D.; Liu, P.; Hrbek, J.; Illas, F. A New Type of Strong Metal–Support Interaction and the Production of H2 through the Transformation of Water on Pt/CeO2(111) and Pt/CeOx/TiO2(110) Catalysts. J. Am. Chem. Soc. 2012, 134, 8968-8974. (14) Su, Y. Q.; Liu, J. X.; Filot, I. A. W.; Hensen, E. J. M. Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria. Chem. Mater. 2017, 29, 9456-9462. (15) Negreiros, F. R.; Fabris, S. Role of Cluster Morphology in the Dynamics and Reactivity of Subnanometer Pt Clusters Supported on Ceria Surfaces. J. Phys. Chem. C 2014, 118, 21014-21020. (16) Hansen, T. W.; DeLaRiva, A. T.; Challa, S. R.; Datye, A. K. Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening? Acc. Chem. Res. 2013, 46, 1720-1730. (17) Wang, C.; Gu, X. K.; Yan, H.; Lin, Y.; Li, J.; Liu, D.; Li, W. X.; Lu, J. Water-Mediated Mars–Van Krevelen Mechanism for CO Oxidation on Ceria-Supported Single-Atom Pt Catalyst. ACS Catal. 2016, 7, 887-891. (18) Jones, J.; Xiong, H.; DeLaRiva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G.; Oh, S.; Wiebenga, M. H.; Hernández, X. I. P. Thermally Stable Single-AtomPlatinum-on-Ceria Catalysts via Atom Trapping. Science 2016, 353, 150-154. (19) Bruix, A.; Lykhach, Y.; Matolínová, I.; Neitzel, A.; Skála, T.; Tsud, N.; Vorokhta, M.; Stetsovych, V.; Ševčíková, K.; Mysliveček, J. Maximum Noble Metal Efficiency in Catalytic Materials: Atomically Dispersed Surface Platinum. Angew. Chem. Int. Ed. 2014, 53, 10525-10530. (20) Liu, J. C.; Wang, Y. G.; Li, J. Toward Rational Design of Oxide-Supported Single-Atom Catalysts: Atomic Dispersion of Gold on Ceria. J. Am. Chem. Soc. 2017, 139, 6190-6199. (21) Ouyang, R.; Liu, J. X.; Li, W. X. Atomistic Theory of Ostwald Ripening and Disintegration of Supported Metal Particles under Reaction Conditions. J. Am. Chem. Soc. 2013, 135, 1760-1771. (22) Berkó, A.; Solymosi, F. Adsorption-Induced Structural Changes of Rh Supported by TiO2(110)-(1× 2): an STM Study. J. Catal. 1999, 183, 91-101. (23) Goldsmith, B. R.; Sanderson, E. D.; Ouyang, R.; Li, W. X. CO-and NO-Induced Disintegration and Redispersion of Three-Way Catalysts Rhodium, Palladium, and Platinum: An ab Initio Thermodynamics Study. J. Phys. Chem. C 2014, 118, 9588-9597. (24) Suzuki, A.; Inada, Y.; Yamaguchi, A.; Chihara, T.; Yuasa, M.; Nomura, M.; Iwasawa, Y. Time Scale and Elementary Steps of CO‐Induced Disintegration of Surface Rhodium Clusters. Angew. Chem. Int. Ed. 2003, 42, 4795-4799. ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 19

(25) Berko, A.; Szökő, J.; Solymosi, F. Effect of CO on the Morphology of Pt Nanoparticles Supported on TiO2(110)-(1× n). Surf. Sci. 2004, 566, 337-342. (26) Stamatakis, M.; Vlachos, D. G. A Graph-Theoretical Kinetic Monte Carlo Framework for onLattice Chemical Kinetics. J. Chem. Phys. 2011, 134, 214115. (27) Kresse, G.; Hafner, J. Ab initio Molecular-Dynamics Simulation of the Liquid-Metal–AmorphousSemiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251. (28) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (29) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation mMade Simple. Phys. Rev. Lett. 1996, 77, 3865. (30) Dudarev, S.; Botton, G.; Savrasov, S.; Humphreys, C.; Sutton, A. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+ U study. Phys. Rev. B 1998, 57, 1505. (31) Fabris, S.; de Gironcoli, S.; Baroni, S.; Vicario, G.; Balducci, G. Reply to “Comment on ‘Taming Multiple Valency with Density Functionals: A Case Study of Defective Ceria'”. Phys. Rev. B 2005, 72, 237102. (32) Cococcioni, M.; De Gironcoli, S. Linear Response Approach to the Calculation of the Effective Interaction Parameters in the LDA+ U Method. Phys. Rev. B 2005, 71, 035105. (33) Castleton, C.; Kullgren, J.; Hermansson, K. Tuning LDA+ U for Electron Localization and Structure at Oxygen Vacancies in Ceria. J. Chem. Phys. 2007, 127, 244704. (34) Da Silva, J. L.; Ganduglia-Pirovano, M. V.; Sauer, J.; Bayer, V.; Kresse, G. Hybrid Functionals Ppplied to Rare-Earth Oxides: The Example of Ceria. Phys. Rev. B 2007, 75, 045121. (35) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978-9985. (36) Sheppard, D.; Terrell, R.; Henkelman, G. Optimization Methods for Finding Minimum Energy Paths. J. Chem. Phys. 2008, 128, 134106. (37) Stamatakis, M. Zacros: Advanced Lattice-KMC Simulation Made Easy. http://www.elucid.com/i/software/material_modelling/Zacros.html. (38) Reuter, K.; Scheffler, M. First-Principles Kinetic Monte Carlo Simulations for Heterogeneous Catalysis: Application to the CO Oxidation at RuO2(110). Phys. Rev. B 2006, 73, 045433. (39) Pineda, M.; Stamatakis, M. Beyond Mean-Field Approximations for Accurate and Computationally Efficient Models of on-Lattice Chemical Kinetics. J. Chem. Phys. 2017, 147, 024105. (40) Eyring, H. The Activated Complex in Chemical Reactions. J. Chem. Phys. 1935, 3, 107-115. (41) Piccinin, S.; Stamatakis, M. CO Oxidation on Pd (111): a First-Principles-Based Kinetic Monte Carlo Study. ACS Catal. 2014, 4, 2143-2152. (42) Peleš, S.; Munsky, B.; Khammash, M. Reduction and Solution of the Chemical Master Equation using Time Scale Separation and Finite State Projection. J. Chem. Phys. 2006, 125, 204104. (43) Stamatakis, M.; Vlachos, D. G. Unraveling the Complexity of Catalytic Reactions via Kinetic Monte Carlo simulation: Current Status and Frontiers. ACS Catal. 2012, 2, 2648-2663. (44) Stamatakis, M.; Vlachos, D. G. Equivalence of on-Lattice Stochastic Chemical Kinetics with the Well-Mixed Chemical Master Equation in the Limit of Fast Diffusion. Comput. Chem. Eng. 2011, 35, 2602-2610. (45) Boyer, L.; Broughton, J. Statics and Dynamics of Icosahedrally Twinned and Single-Crystal FCC Clusters. Phys. Rev. B 1990, 42, 11461. (46) Vlachos, D. G.; Schmidt, L. D.; Aris, R. Structures of Small Metal Clusters. i. Low Temperature Behavior. J. Chem. Phys. 1992, 96, 6880-6890. (47) Beniya, A.; Isomura, N.; Hirata, H.; Watanabe, Y. Low Temperature Adsorption and SiteConversion Process of CO on the Ni(111) Surface. Surf. Sci. 2012, 606, 1830-1836. (48) Ge, Q.; King, D. Surface Diffusion Potential Energy Surfaces from First Principles: CO Chemisorbed on Pt{110}. J. Chem. Phys. 1999, 111, 9461-9464. (49) Abild-Pedersen, F.; Andersson, M. CO Adsorption Energies on Metals with Correction for High Coordination Adsorption Sites–A Density Functional Study. Surf. Sci. 2007, 601, 1747-1753. ACS Paragon Plus Environment

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(50) Feibelman, P. J.; Hammer, B.; Nørskov, J. K.; Wagner, F.; Scheffler, M.; Stumpf, R.; Watwe, R.; Dumesic, J. The CO/Pt (111) Puzzle. J. Phys. Chem. B 2001, 105, 4018-4025. (51) Winterbottom, W. Equilibrium Shape of a Small Particle in Contact with a Foreign Substrate. Acta Metallurgica 1967, 15, 303-310. (52) Longwitz, S. R.; Schnadt, J.; Vestergaard, E. K.; Vang, R. T.; Lægsgaard, E.; Stensgaard, I.; Brune, H.; Besenbacher, F. High-Coverage Structures of Carbon Monoxide Adsorbed on Pt(111) Studied by High-Pressure Scanning Tunneling Microscopy. J. Phys. Chem. B 2004, 108, 14497-14502. (53) Ertl, G.; Neumann, M.; Streit, K. Chemisorption of CO on the Pt(111) Surface. Surf. Sci. 1977, 64, 393-410. (54) Vestergaard, E. K.; Thostrup, P.; An, T.; Lægsgaard, E.; Stensgaard, I.; Hammer, B.; Besenbacher, F. Comment on “High Pressure Adsorbate Structures Studied by Scanning Tunneling Microscopy: CO on Pt(111) in Equilibrium with the Gas Phase”. Phys. Rev. Lett. 2002, 88, 259601. (55) Parkinson, G. S.; Novotny, Z.; Argentero, G.; Schmid, M.; Pavelec, J.; Kosak, R.; Blaha, P.; Diebold, U. Carbon Monoxide-Induced Adatom Sintering in a Pd–Fe3O4 Model Catalyst. Nature Mater. 2013, 12, 724-728. (56) Teschner, D.; Wootsch, A.; Pozdnyakova-Tellinger, O.; Kröhnert, J.; Vass, E.; Hävecker, M.; Zafeiratos, S.; Schnörch, P.; Jentoft, P.; Knop-Gericke, A. Partial Pressure Dependent in Situ Spectroscopic Study on the Preferential CO Oxidation in Hydrogen (PROX) over Pt/ceria Catalysts. J. Catal. 2007, 249, 318-327. (57) Wadayama, T.; Todoroki, N.; Yamada, Y.; Sugawara, T.; Miyamoto, K.; Iijama, Y. Oxygen Reduction Reaction Activities of Ni/Pt(111) Model Catalysts Fabricated by Molecular Beam Epitaxy. Electrochem. Comm. 2010, 12, 1112-1115. (58) Hayden, B. E.; Bradshaw, A. M. The Adsorption of CO on Pt(111) Studied by Infrared-ReflectionAdsorption Spectroscopy. Surf. Sci. 1983, 125, 787-802. (59) McCrea, K.; Parker, J. S.; Chen, P.; Somorjai, G. Surface Structure Sensitivity of High-Pressure CO Dissociation on Pt(557), Pt(100) and Pt(111) Using Sum Frequency Generation Surface Vibrational Spectroscopy. Surf. Sci. 2001, 494, 238-250. (60) Gajdoš, M.; Eichler, A.; Hafner, J. CO Adsorption on Close-Packed Transition and Noble Metal Surfaces: Trends from ab initio Calculations. J. Phys. Conden. Matt. 2004, 16, 1141. (61) Bazin, P.; Saur, O.; Lavalley, J.; Daturi, M.; Blanchard, G. FT-IR Study of CO Adsorption on Pt/CeO2: Characterisation and Structural Rearrangement of Small Pt Particles. Phys. Chem. Chem. Phys. 2005, 7, 187-194. (62) Pozdnyakova-Tellinger, O.; Teschner, D.; Kröhnert, J.; Jentoft, F. C.; Knop-Gericke, A.; Schlögl, R.; Wootsch, A. Surface Water-Assisted Preferential CO Oxidation on Pt/CeO2 Catalyst. J.Phys. Chem. C 2007, 111, 5426-5431. (63) Ding, K.; Gulec, A.; Johnson, A. M.; Schweitzer, N. M.; Stucky, G. D.; Marks, L. D.; Stair, P. C. Identification of Active Sites in CO Oxidation and Water-Gas Shift over Supported Pt Catalysts. Science 2015, 350, 189-192. (64) Moses-DeBusk, M.; Yoon, M.; Allard, L. F.; Mullins, D. R.; Wu, Z.; Yang, X.; Veith, G.; Stocks, G. M.; Narula, C. K. CO Oxidation on Supported Single Pt Atoms: Experimental and ab initio Density Functional Studies of CO Interaction with Pt atom on θ-Al2O3(010) Surface. J. Am. Chem. Soc. 2013, 135, 12634-12645. (65) Wu, T.; Pan, X.; Zhang, Y.; Miao, Z.; Zhang, B.; Li, J.; Yang, X. Investigation of the Redispersion of Pt Nanoparticles on Polyhedral Ceria Nanoparticles. J. Phys. Chem. Lett. 2014, 5, 2479-2483. (66) Su, Y. Q.; Filot, I. A. W.; Liu, J. X.; Hensen, E. J. M. Stable Pd-Doped Ceria Structures for CH4 Activation and CO Oxidation. ACS Catal. 2017, 8, 75-80. (67) Su, Y. Q.; Liu, J. X.; Filot, I. A. W.; Zhang, L.; Hensen, E. J. M. Highly Active and Stable CH4 Oxidation by Substitution of Ce4+ by two Pd2+ Ions in CeO2(111). ACS Catal. 2018, 8, 6552-6559. (68) Kopelent, R.; van Bokhoven, J. A.; Szlachetko, J.; Edebeli, J.; Paun, C.; Nachtegaal, M.; Safonova, O. V. Catalytically Active and Spectator Ce3+ in Ceria-Supported Metal Catalysts. Angew. Chem. 2015, 127, 8852-8855. ACS Paragon Plus Environment

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Theoretical Approach To Predict the Stability of Supported Single

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