Theoretical Investigations of Pt1@CeO2 Single-Atom Catalyst for CO

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Article 2

Theoretical Investigations of Pt1@CeO Single-Atom Catalyst for CO Oxidation Yan Tang, Yang-Gang Wang, and Jun Li J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 30, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

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

The Journal of Physical Chemistry

Theoretical Investigations of Pt1@CeO2 Single-Atom Catalyst for CO Oxidation Yan Tang, 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

Abstract: In the present study we perform systematic density functional investigations on the single-atom Pt catalysts dispersed on CeO2 (111), (110) and (100) surfaces. The structural and electronic properties of these SACs and their corresponding catalytic activity for CO oxidation are discussed. It is shown that the single-atom Pt substitution of a lattice Ce ion is thermodynamically stable on each ceria surface. On (111) and (100) surface the single Pt atom is found to exist at an oxidation state of +4 upon replacement of Ce4+, while on (110) surface it exhibits a planar structure with a reduced oxidation state of +2 due to the spontaneous formation of surface peroxide O22- species. On all the surfaces CO oxidation is found to follow Mars-van Krevelen mechanism. It is the single-atom Pt, rather than the lattice Ce, that acts as the electron acceptor/donor in the redox processes during the whole catalytic cycle. This work provides insights on the significant role of single metal atom on reducible oxide of ceria and others.

1

ACS Paragon Plus Environment


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

1. Introduction CO oxidation has been extensively investigated in heterogeneous catalysis due to its importance in both industrial applications and fundamental catalytic studies. In the past decades, a number of oxide-supported noble metals were identified as active catalysts for CO oxidation, including the seminal finding of Haruta et al. on oxide-supported gold nanoparticles exhibiting significant catalytic reactivity for CO oxidation.1 Recently, significant attention in heterogeneous catalysis is called on surface single-atom catalysts (SAC) since the first practical single-atom catalyst Pt1/FeOx was reported by Qiao et al.2-3 While extensive experimental and theoretical investigations have demonstrated that noble metal SACs tend to possess high activity, selectivity and stability,4-21 a recent report shows that non-noble metal single-atom catalyst Ni1/FeOx also exhibit a high activity and stability for CO oxidation at room temperature.22 Our ab initio molecular dynamics (AIMD) simulations proposed that the single-atom catalytic process can even occur dynamically at the interfacial area on titania- and ceria-supported gold nanoparticles under reaction conditions, a phenomenon we dubbed as dynamic single-atom catalysis (DSAC).23-25 Therefore, compared to supported metal-nanoparticle catalysts, investigation of the CO oxidation on SACs is of great current interest. Among metal-oxide support materials, cerium oxide (ceria, CeO2) is one of the most common supports26-32 due to its unique redox properties originated from the quantum primogenic effect33. In the case of ceria-based SACs, gold-doped ceria single atom catalyst (Au1/CeO2) was shown to exhibit high activity, selectivity, and extreme stability for preferential oxidation of CO (PROX) at the working temperature (∼80 °C) of proton exchange membrane fuel cell.34 Doping of ceria surfaces has been considered as an efficient way to enhance oxygen storage and release capacity for promoting CO oxidation.35-38 Theoretically, among numerous recent studies, Yang et al. found that Zr dopants facilitate the reduction of Ce4+ to Ce3+ and benefit the direct formation and release of CO2.39 Fabris et al. studied the CO oxidation on Au-doped CeO2 and proposed that Au3+/Au+ reductions are the origin of the high catalytic activity.40 Liu et al. suggested the significant role of Pd4+/Pd2+ reductions in CO oxidation on Pd-doped CeO2.41 Moreover, the controllable synthesis of ceria nanocrystals has recently been developed and it is well demonstrated that ceria nanorods exhibit significant crystal plane effect in catalyzing a variety of important reactions such as CO oxidation and water-gas shift reaction.42-43 For example, Flytzani-Stephanopoulos and co-workers identified the strong shape/crystal plane effect of CeO2 2


Page 2 of 24

Page 3 of 24

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


on the gold-ceria activity for the water-gas-shift (WGS) reaction and found that the rod-like ceria enclosed by (110) and (100) planes is most active for gold stabilization/activation.43 Jones et al found that different exposed surface facets of ceria can influence the stability and activity of single-atom Pt.12 These studies inspire us to explore the single-atom metal-doped ceria catalysts to understand the nature of the crystal plane effect and their high reactivity. In this study, we have carried out systematic density functional theory (DFT) calculations corrected by on-site Coulomb interactions (DFT + U) to investigate the geometric and electronic structures of single-atom Pt doped onto CeO2 (111), (110) and (100) surfaces, and the corresponding activity for CO oxidation. The main focus is to understand the structural and electronic properties of single-atom Pt atoms doped on different ceria surfaces, as well as the crystal plane effect on CO oxidation among these single-atom catalysts. Inasmuch as the binding strengths of single Pt atom supported on the various surfaces of ceria are thermodynamically much less stable than the doped ones, we only focus on the situation with Pt1 atom doped on ceria surface in this work.

In the Pt1-doped cases, we tested all the possible CO adsorption states at O,

Ce and Pt sites on the surfaces of Pt1@CeO2 (111,110,100). The calculated adsorption energies are all lower than 0.30 eV, suggesting that CO only physisorbs on the surfaces. Therefore, we choose to consider direct CO oxidation with the surface oxygen in this paper.

2. Methods All the calculations were performed using periodic DFT methods as implemented in the Vienna ab-initio simulation package (VASP).44-45 Projector augmented wave (PAW) method was used for the interaction between the atomic cores and valence electrons.46 The valence orbitals of Ce (4f, 5s, 6s, 5p, 5d), Pt (5d, 6s), 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 via the generalized gradient approximation (GGA) with the PBE functional.47 Gaussian smearing method with a width of 0.05 eV was used. Spin-polarized DFT+U calculations of Ueff = 5.0 eV for Ce 4f state

50-51

48-49

with a value

was applied to correct the strong electron-correlation

properties of CeO2. The CeO2 (111) surface was modeled by p(3 × 3) 9 atomic layer supercells with the bottom three layers fixed; the CeO2 (110) surface was modeled by p(2 × 3) 5 atomic-layer supercells with the bottom two layers fixed, and CeO2 (100) surface was modeled by p(3 × 3) 7 atomic-layer supercells with the bottom two layers fixed. The vacuum gap was set as 3



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

∼15 Å to avoid the interaction between the periodic images. The Brillouin zone was sampled at the Γ-point. The convergence criteria for the energy and force were set to 10-5 eV and 0.02 eV/Å. The transition states (TSs) of the surface reactions were searched using the dimer method.52 Vibrational analysis was further used to confirm the transition states with only one imaginary frequency. The energy barrier (Ea) was determined as the energy difference between the corresponding transition- and initial-states. The adsorption energies was calculated according to the following equation, Eads = E(slab + adsorbate) − E(slab) − E(adsorbate), where E(slab + adsorbate), E(slab), and E(adsorbate) are the energies of species adsorbed on the surface, the bare surface, and the gas-phase molecule, respectively. Similarly, the desorption energy was estimated as Edes = -(E(slab + desorbate) − E(slab) − E(desorbate)) and the reaction energy by ΔE = E(products) − E(reactants). Atomic charges were computed using the atom-in-molecule (AIM) scheme proposed by Bader.53 The charge density differences were evaluated using the formula Δρ = ρA+B - ρA - ρB, where ρX is the electron density of X.

3. Results and Discussion 3.1 Pt1@CeO2 (111) single-atom catalysts 3.1.1 The structures of Pt1@CeO2 (111). We have investigated the structure of Pt substituted at a Ce site on the (111) surface, as shown in Figure 1a. The CeO2 (111) surface exposes three-fold O and seven-fold Ce atoms. On the (111) surface, the Pt substituent is surrounded by six oxygen anions. Upon relaxation, Pt retains its six-fold coordination, but the Pt–O bond lengths are not identical. Three Pt-O bonds with the surface O atoms are shortened to ~2.18 Å while the other three Pt–O bonds with the subsurface O atoms are at ~2.12 Å. The formation energy of Pt (Ef) substituted surface was calculated using the following equation: Pt (s) + CexO2x (surf) + O2 (g)  PtCex-1O2x (surf) + CeO2 (s). The formation energy of the Pt-substituted (111) surface is found to be -4.97 eV, indicating that the Pt substitution on ceria (111) surface is thermodynamically rather stable. The charge density difference of Pt1@CeO2 (111) in Figure 1b shows that no Ce3+ ion forms after the dopant of Pt, suggesting that the oxidation state of single-atom Pt should be similar to the replaced Ce+4.

4


Page 4 of 24

Page 5 of 24

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


Figure 1. (a) Optimized structure of Pt1@CeO2 (111). Color code: Ce (yellow), O (red/purple), Pt (blue). (b) Calculated charge density differences of Pt1@CeO2 (111). Yellow and blue areas represent charge density increase and reduction, respectively. The cutoff of the density-difference isosurfaces equals to 0.01 electrons/Å3. 3.1.2 CO oxidation on Pt1@CeO2 (111). It is well-known that the mechanism of CO oxidation at ceria or metal-doped ceria surfaces often follows the so-called Mars-van Krevelen-type (M-vK) catalytic cycle, in which CO reacts with lattice oxygen of ceria directly to form CO2 as well as an oxygen vacancy, and then the gas-phase oxygen replenishes the vacancy to complete the reaction cycle. The potential energy diagram for CO oxidation on Pt1@CeO2 (111) surface is shown in the black line of Figure 2, and the structures of the surface intermediates and the transition states are shown in Figure 3. The calculated adsorption energy of CO at Pt1@CeO2 (111) is only -0.20 eV, suggesting that CO first physisorbs on the surface. This result is similar to the previous reports that CO oxidation follows the Eley-Rideal (E-R) process on pure CeO2, Au-doped CeO2 and Pd-doped CeO2. 40-41, 54-55 The physisorbed CO can attack one lattice oxygen to produce an adsorbed CO2 species. The energy barrier with respect to the weak adsorption state is estimated to be 0.25 eV. The formed CO2 can readily desorb into the gas phase due to a small desorption energy of 0.09 eV. After CO2 desorption, one oxygen vacancy is created, which can be further filled up by O2 molecule with adsorption energy of -1.93 eV. The O–O bond distance is elongated to 1.43 Å, indicating electron transfer to the anti-bonding orbitals of O2 from the Pt1@CeO2 catalyst. Next, the second CO molecule comes to react with the adsorbed O2 species, forming CO2 and regenerating the single-atom catalyst. The barrier and the reaction energy for this process are 0.59 eV and -2.82 eV, respectively. The whole reaction pathway for 2CO(g) + O2(g) --> 2CO2(g) is highly exothermic by 6.53 eV, which is slightly overestimated when comparing with the experimental

5



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

value of -5.8 eV due to the approximate exchange-correlation functional used. The calculated barrier of the rate-determining step in the catalytic cycle is 0.59 eV, suggesting the high possibility for CO oxidation at low temperatures.

Figure 2. The potential energy diagram for CO oxidation on Pt1@CeO2 surfaces. i) two CO and O2 molecules in gas phase; ii) one CO adsorption; iii) the transition state for the adsorbed CO subtracting one lattice oxygen; iv) the formation of adsorbed CO2; v) CO2 desorbed into the gas phase; vi) O2 adsorbed at the oxygen vacancy and CO in gas phase; vii) the transition state for CO reacting with the lattice O2; viii) formed CO2 desorbed into the gas phase. The energy difference between iv and v represents the desorption energy of CO2, which is less than 0.15 eV.

Figure 3. The structures of the surface intermediates and transition states in CO oxidation process on Pt1@CeO2 (111). 3.2 Pt1@CeO2 (110) single-atom catalysts 3.2.1 The structures of Pt1@CeO2 (110). Inasmuch as CeO2 (110) surface exposes three-fold O and six-fold Ce atoms, Pt substituent is expected to be surrounded by six oxygen anions. However, after un-constraint geometry 6


Page 6 of 24

Page 7 of 24

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


optimization, the single-atom Pt disconnects with two subsurface O, as shown in Figure 4a, with the Pt-O bond length of 2.10 Å and the formation energy of -4.79 eV. In addition, we also find another stable structure with a strong distortion of the surface, where single-atom Pt, two surface O and two subsurface O atoms form a nearly planar local structure, as shown in Figure 4b. Such a structure is also reported in Ni1@CeO2 (110).56 For the sake of convenience, the Pt1@CeO2(110) structures in Figure 4a and b are defined as Pt1,s@CeO2 (110) and Pt1,p@CeO2 (110), respectively. The Pt–O distances in Pt1,p@CeO2 (110) are from 1.88 Å to 2.01 Å, shorter than that in Pt1@CeO2 (111) and Pt1,s@CeO2 (110). Compared to Pt1@CeO2 (111) and Pt1,s@CeO2 (110), Pt1,p@CeO2 (110) is expected to be formed more easily due to the lower formation energy (Ef = -5.53 eV). To further investigate the electronic structure of Pt1,p@CeO2 (110), charge density difference and spin density are calculated, as shown in Figure 4c and 4d, respectively. It is found that substituting Ce atom with Pt atom in CeO2 (110) results in a spin polarization of the oxygen ion because of the charge imbalance between Ce and Pt. One of the surface O atom connecting with Pt (denoting as O1 in Figure 4b) is chosen to investigate the details of spin polarization. Figure 5 shows the total density of states (TDOS) of Pt1,p@CeO2 (110), and projected density of states (PDOS) of the single-atom Pt and O1. It is shown that there is partially unoccupied p orbital on O1 atom, indicating the charge transfer from Pt to O1. The magnetic moment on the O1 is found to be 0.41 B, while the magnetic moment on the single-atom Pt substituent is 0.81 B, which means that the single-atom Pt does not get fully oxidized to the Pt2+ state, while the oxygen is in fact monovalent (the so-called oxygen-centered radical).57

Figure 4. Optimized structures of Pt1@CeO2 (110) without (a) or with (b) distortion. Calculated charge density differences (c) and spin density (d) of Pt1@CeO2 (110) with distortion.

7



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

Figure 5. (a) Total electronic density of state (TDOS) of Pt1@CeO2 (110) surface, and projected electronic density of state (PDOS) for O1 atom (b) and single-atom Pt (c). Energies are referred to the Fermi level. Ab initio Molecular Dynamics (AIMD) simulations have been performed to understand the distortion process. For the parameters used in the AIMD simulations, see the Supporting Information (SI) file for details. The selected snapshots for the distortion are shown in Figure 6. Interestingly, we observe that the two surface O atoms (O1 and O2) combine to form an absorbed O2 species at the Ce site (see Figure 6c and 6d). The distance and the total net charge of the two oxygen atoms have been traced during AIMD simulations, as shown in Figure 7. It is shown that at ~13 ps, the O1-O2 distance shortens to ~1.3 Å, and the total net charge reduces to ~ -0.4 |e|. At the same time, the charge of the single Pt reduces from ~0.55 |e| to ~0.15 |e| (see Fig S1). These results imply that the charge transfer from the surface atoms to the single Pt atom leads to the formation of adsorbed O2 species. To further analyze the electronic structures of the spontaneously formed O2 species, we optimized the structure (Figure 6d) using VASP. The O2 species is found to be negatively charged of -1.14 |e| with a bond length of 1.46 Å, indicating the formation of O22- peroxide species. The Bader charge of single-atom Pt is 0.97 |e|, which is similar to the Pt2+ ion in Pt1@CeO2 (111) with one oxygen atom removed. Figure S2 shows the

8


Page 8 of 24

Page 9 of 24

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


TDOS of Pt1@CeO2 (111) and Pt1@CeO2 (110) surfaces and corresponding PDOS of single-atom Pt. It is shown that Pt1@CeO2 (111) has more unoccupied d orbitals than Pt1@CeO2 (110), indicating that single-atom Pt in Pt1@CeO2 (110) is less oxidized. In addition, unlike the structure in Figure 4b, there is no spin polarization in this system. To further investigate the role of Pt in the spontaneous formation of O22- species, the structures of di-Pt substitution in CeO2 (110) have been explored, as shown in Figure S3. The preferred distribution of the second single-atom Pt is also found to form a nearly planar structure with two surface O ions and two subsurface O ions (see Figure S3a). This configuration results in the spontaneous formation of a free O2 molecule, which exists in a triplet spin ground state with a bond length of 1.24 Å. The Bader charges of two single-atom Pt atoms are both 0.93 |e|, consistent with the formation of Pt2+ ions.

Figure 6. Snapshots of the formation of adsorbed O2 on the T = 700 K AIMD simulation. The inset shows the formation energy of single-atom Pt in this structure.

Figure 7. The distances (left scale, black color) and the total Mulliken charges (right scale, red color) between O1 atom and O2 atom during AIMD simulation. O1 and O2 atoms are defined in Figure 6a.

9



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

Page 10 of 24

3.2.2 CO oxidation on Pt1@CeO2 (110).

Figure 8. The structures of the surface intermediates and transition states in CO oxidation process on Pt1@CeO2 (110). Inset shows the top view. In this section, we consider CO oxidation on the most stable Pt1@CeO2 (110) structure with formation energy of -6.10 eV. Similar to Pt1@CeO2 (111), CO oxidation at Pt1@CeO2 (110) also follows the M-vK mechanism. The potential energy diagram for CO oxidation on Pt1@CeO2 (110) surface is shown as red line in Figure 2, and the structures of the surface intermediates and the transition states are shown in Figure 8. CO molecule first attacks one oxygen atom in O22- species on the Pt1@CeO2 (111) surface, forming one CO2 and creating an O vacancy. Due to the high stability of O22- species, the energy barrier is calculated to be as high as 0.87 eV. Next, O2 molecule weakly adsorbs on the Ce site with an adsorption energy of -0.18 eV. The Bader charge of the O2 species is -0.56 |e| and the O–O bond length is 1.32 Å, suggesting the formation of a superoxide species. Simultaneously, the Bader charges of the single-atom Pt is increased to 1.12 |e|, indicating the charge transfer from single-atom Pt to the O2 species. Finally, the second CO molecule experiences a barrier of 0.74 eV to react with the adsorbed O2 species, forming CO2 and regenerating the single-atom catalyst. Overall, the rate-determining step in the whole catalytic cycle is the first CO oxidation step with a relative higher barrier of 0.87 eV, suggesting the lower activity of Pt1@CeO2 (110) than Pt1@CeO2 (111) for CO oxidation. 3.3 Pt1@CeO2 (100) single-atom catalysts 3.3.1 The structures of Pt1@CeO2 (100). CeO2 (100) surface exposes two-fold O and six-fold Ce atoms. The optimized structure and the charge density difference of Pt1@CeO2 (100) are shown in Figure 9. Single-atom Pt is surrounded by two surface oxygen anions (Os) and four subsurface oxygen anions (Oss). The distances Pt–Os and Pt-Oss are found to be 2.12 Å and ~2.05 Å (Pt-O1: 2.06 Å; Pt-O2: 2.04 Å), respectively. It is found that the distortion of the Pt1@CeO2 (100) surface is smaller than that of 10


Page 11 of 24

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


Pt1@CeO2 (110) surface. The charge density difference of Pt1@CeO2 (100) shows that the oxidation state of single-atom Pt is +4 because no Ce4+ ions have been reduced. The formation energy of the single-atom Pt in the ceria (100) surface is -5.73 eV, smaller than in (111) surface.

Figure 9. (a) Optimized structure of Pt1@CeO2 (100). The top-right inset shows the local structure of the single-atom Pt. The bottom-right inset shows the side view of Pt1@CeO2 (100). (b) Calculated charge density differences of Pt1@CeO2 (100). Inset depicts the side view. 3.3.2 CO oxidation on Pt1@CeO2 (100). Like at Pt1@CeO2 (111) and Pt1@CeO2 (110), CO oxidation at Pt1@CeO2 (100) also follows M-vK mechanism. The potential energy diagram for CO oxidation on Pt1@CeO2 (100) surface is shown in the blue line of Figure 2, and the structures of the surface intermediates and the transition states are shown in Figure 10. The energy barrier for the first CO oxidation step is only 0.08 eV, much smaller than that in Pt1@CeO2 (111) and Pt1@CeO2 (110). The desorption energy of CO2 is found to be 0.22 eV, revealing that CO2 can readily desorb to the gas phase at low temperature. The adsorption energy of O2 molecule at the vacancy site is -0.95 eV and the O–O bond distance is 1.34 Å, which is shorter than that in Pt1@CeO2 (111). The barrier and the reaction energy for the second CO oxidation are 0.51 eV and -2.57 eV, respectively. Bader charge analyses show the presence of Pt4+/Pt2+ reductions in the whole reaction pathway on Pt1@CeO2 (100) surface, validating the importance of single-atom Pt on this surface. The rate-determining step in the whole reaction pathway is the second CO oxidation with the barrier of 0.51 eV, which is lower than Pt1@CeO2 (110) and Pt1@CeO2 (111).

11



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

Page 12 of 24

Figure 10. The structures of the surface intermediates and transition states in CO oxidation process on Pt1@CeO2 (100). 3.4 Overview of the Features of Pt1/CeO2 Single-Atom Catalysts Above we have presented the mechanistic studies of CO oxidation on single atom Pt doped ceria(111), (110), and (100) catalysts. As Pt atoms strongly bind at the ceria surface oxygen atoms through covalent metal-support interaction (CMSI),15 these surface SAC active sites differ from those in metal nanoparticles and oxide surfaces. In this section we turn to discuss the catalytic features and crystal plane effect of this type of surface-doped single atom catalysts and compare them with the experimental measurements. 3.4.1 Stability of doped single Pt atoms on ceria The doped single Pt atoms show distinct thermo-dynamical stability on all three ceria surfaces with large formation energies for Pt substitution. The single-atom Pt in Pt1@CeO2(111) and (100) catalysts are substituted for Ce4+ ions in the form of Ce1-xPtxO2 and its oxidation state can be reduced from Pt4+ (d6) to Pt2+ (d8) in the presence of oxygen vacancy, which is consistent with recent experimental observations using XRD, TEM, and XPS studies by Bera et al.58 However, in Pt1@CeO2(110) catalyst, we have observed the formation of a four-fold Pt atom with nearly planar local structure and a strong distortion of the surface from AIMD simulations. The large structural distortion is attributed to the spontaneous formation of an O22- species from two surface oxygen (O2-) ions, which reduces Pt4+ to Pt2+, with the later favoring planar coordination structure as anticipated from crystal field theory. As a result, Pt substitution on ceria (110) surface is more stable than on ceria (111) and (100). Recently Dvořák et al13 also proposed a planar structure of Pt2+ on the step site of CeO2(111) surface, where the structure of ceria (111) step site is similar to the local surface structure of (110). This result is consistent with our observation here. However, similar phenomenon has not been observed for the clean ceria (111) and (100), where the planar structure is thermodynamically unstable by some 0.4-0.6 eV when compared to the regular structure of Pt4+ in the replacement of Ce4+. 3.4.2 Activity and microkinetic analysis

12


Page 13 of 24

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


Our study reveals that CO oxidation at Pt1@CeO2 (111), (110) and (100) surfaces all follows M-vK mechanism, similar to the reported mechanisms on pure CeO2, Au-doped CeO2 and Pd-doped CeO2. Based on the calculated energetics, we conclude that the order of reactivity follows Pt1@CeO2 (110) < Pt1@CeO2 (111) < Pt1@CeO2 (100). This result is in good agreement with the experimental measurements by Jones et al.12, which shows the polyhedral ceria (111/100) and the nanorods ceria (111) exhibit similar high performance for CO oxidation, while the cube ceria (110) has the lowest reactivity. To further validate the activity of SACs under realistic conditions, microkinetic analysis has been performed using the ‘Catalysis Microkinetic Analysis Package’ (CatMAP)59. The steady state reaction rate as a function of the operative pressure (P) and temperature (T) is shown in Figure S4. We find that Pt1@CeO2 (110) exhibits a rather low reaction rate (less than 10-4 s-1) for CO oxidation under all conditions (300-1000 K, 10-7 – 103 bar), and is thus the lowest-reactive plane. In contrast, on both Pt1@CeO2 (100) and Pt1@CeO2 (111), the process with observable reaction rate (> 0.1 s-1) can take place at mild condition (low temperature and relatively low pressure), suggesting that these SACs are meaningful under realistic condition. Interestingly, the reaction rate on Pt1@CeO2 (111) roughly increases with the increase of the temperature, while the reaction rate on Pt1@CeO2 (100) shows an opposite tendency. 3.4.2 Phase diagram and crystal plane effect We further show a “phase diagram” in Figure S5, which gives the highest-reactive single-atom Pt catalyst under different temperature and pressure. For the Pt1 doped ceria surfaces, CeO2 (100) is found to be the highest-reactive plane under low temperature and high pressure, while CeO2 (111) is the highest-reactive plane under high temperature and low pressure. CeO2 (110) is the lowest-reactive plane under all conditions (300-1000 K, 10-7 – 103 bar). These results point to the significant crystal plane effect on CO oxidation among these single-atom Pt catalysts. The origin of the crystal plane effect in single-atom Pt catalysts can be understood from the geometric and electronic properties. In view of the geometric structures, two surface oxygen atoms form an O22- species spontaneously on Pt1@CeO2 (110) surface, leading to a nearly planar local structure of tetra-coordinate Pt2+ ion with a strong distortion of the surface. Due to the high stability of O22- species, the energy barrier of the first CO oxidation is calculated to be as high as 0.87 eV.

13



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

Page 14 of 24

In addition, Bader charge analysis has been performed to understand the electronic nature of the different single-atom Pt catalysts, as listed in Table S1. For Pt1@CeO2 (111) surface, it is found that the charge of Pt is reduced from +1.52 |e| to +0.95 |e| after the formation of an oxygen vacancy, while the charges on Ce are nearly unchanged, implying that the single Pt atom is actually the oxidant agent for the first CO oxidation. After O2 adsorption at the O vacancy, Pt is reoxidized from +0.95 |e| to +1.57 |e|, suggesting that it is the single Pt atom that works as a reductive agent to activate the O2 molecule. The similar conclusion can also be drawn for Pt1@CeO2 (100) catalyst. It has been reported that dopants can promote the oxidation reaction by the M-vK mechanism on ceria by lowering the oxygen vacancy formation energy of CeO2.60 The calculated oxygen vacancy formation energies of Pt1@CeO2 (111) and Pt1@CeO2 (100) are 1.49 eV and 0.26 eV, respectively, which are less than that of pure CeO2 (111) and CeO2 (100) surfaces of 2.39 eV and 1.47 eV, respectively. It follows that the effective redox Pt4+/Pt2+ coupling in Pt1@CeO2(111) and Pt1@CeO2 (100) enhance the activity of surface lattice O atoms. In contrast, the reactivity of CO oxidation catalyzed by Pt1@CeO2 (110) is significantly decreased due to the formation of the stable planar structure and surface peroxide oxygen species. On the one hand, the formed (O2)2- is hard to react with CO (Ea = 0.87 eV); on the other hand, further O2 molecule is difficult to be activated (i.e. reduced) by the stable Pt atom (Eads = 0.18 eV). This result suggest that single doped Pt in CeO2(110) is not a good oxidizing agent for CO oxidation. As can be seen from the Bader charges in Table S1, the charge of Pt is only decreased from 0.97 |e| to 0.93 |e| for the first CO2 production and only increased from 0.93 |e| to 1.12 |e| for O2 activation, indicating the reluctant for redox process. 3.4.2 Single-atom catalysts Pt1/CeO2 vs. ceria One of the most cumbersome issues in heterogeneous catalysis involving CO oxidation is the formation of poisoning carbonate intermediate. Compared with the clean unmodified ceria surface catalyst, the single-atom Pt1/CeO2 catalysts exhibit significantly different reactive properties for CO oxidation. Especially noteworthy is the ability of single-atom Pt1/CeO2 catalysts to circumvent formation of the vexing carbonate species. Below we will discuss several aspects to emphasize the difference of the Pt-atom doped and undoped ceria catalysts.

14


Page 15 of 24

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


On pure CeO2 surfaces, two Ce ions are reduced to Ce3+ when one lattice oxygen is subtracted by CO via M-vK mechanism and are re-oxidized to Ce4+ by O2 filling into the oxygen vacancy.54-55 However, on all Pt1/CeO2 catalysts, we find that it is the single Pt atom, not the Ce site, that acts as the electron acceptor/donor to achieve the redox processes during CO oxidation. On CeO2(111) and CeO2(100) surface, the single Pt atom is found to significantly increase the reactivity of CO oxidation. The barriers for subtracting the lattice oxygen ion on both surface are decreased by ~ 0.30-0.40 eV, compared to the clean surfaces,54-55 whereas for CeO2(110) surface, the single Pt atom catalyst seems to increase the barrier of CO oxidation with the lattice oxygen by ~0.6 eV due to the formation of a highly stable planar structure. In addition, with undoped ceria surfaces carbonate species can be easily formed due to facile conversion of Ce3+ + CO2  Ce4+ + CO2- process, as has been observed in CO oxidation on ceria by FTIR experiments.61 Many studies suggest that the formation of carbonate species is competitive with CO2 desorption, leading to the surface poisoning and catalyst deactivation. Gong et al. investigated the CO oxidation on pure CeO2 (111) and (110) surfaces and found that bent CO2δ‐ species (i.e., adsorbed CO2- species) formed in the first CO oxidation can react with surface oxygen to generate the carbonate species.54 However, for Pt-doped CeO2 single-atom catalysts, Pt atoms are positively charged and exist in high oxidation state at ceria surface, which makes it hard to further transfer electron to formed CO2 species. As a result, the neutral linear CO2 species is directly formed without the formation of bent CO2δ‐ intermediate. Indeed, the calculated Bader charges of CO2 species are -0.02 |e|, 0.00 |e|, +0.01|e| for Pt1@CeO2(111), Pt1@CeO2(110) and Pt1@CeO2 (100) SACs, respectively. This unique feature of single-atom catalysts of Pt1/CeO2 is also seen in the cases of Au-doped CeO2(111) and Pd-doped CeO2(111).40-41 The neutral linear CO2 species can be easily released from the surface with a low desorption energy, thus reducing the possibility of forming stable intermediate carbonate. Recently a combined experimental and theoretical study on the water-mediated M-vK mechanism for CO oxidation on Pt1/CeO2 also indicates the formation of CO2 without carbonate species.62 We therefore can conclude that another role of single-atom Pt in Pt1@CeO2 SACs is to serve as an inhibitor for the formation of stable carbonate intermediate species. This characteristic feature of SACs might provide insight on elucidating the different behaviors of surface single-atom catalysts and nanocatalysts with regard to formation of

15



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

Page 16 of 24

poisoning carbonate species during CO oxidation and preferential oxidation (PROX) of CO in H2 sources.

4. Conclusions In this work, we have performed a systematical study on the geometric and electronic structures of single-atom Pt doped CeO2 (111), (110) and (100) surfaces and the activity of CO oxidation on these SACs using density functional theory calculations. The following conclusions can be drawn from this work: (1) Single-atom Pt substitution on ceria (111), (110) and (100) surfaces are all thermodynamically stable. Pt substitution on ceria (110) surface is the most stable one because the spontaneous formation of an O22- species from two surface oxygen atoms reduces Pt4+ to Pt2+. (2) CO oxidation at Pt1@CeO2 (111), (110) and (100) surfaces all follows M-vK mechanism. For Pt1@CeO2 (111) and (100) SACs, the rate-determining step is the second CO oxidation with low barrier (< 0.6 eV), indicating the high activity for CO oxidation at low temperatures. The rate-determining step for Pt1@CeO2 (110) is the first CO oxidation with the barrier of 0.87 eV. The order of reactivity is found to follow Pt1@CeO2 (110) < Pt1@CeO2 (111) < Pt1@CeO2 (100). (3) The redox Pt4+/Pt2+ couple can well correlate with the CO oxidation process and act as an electron acceptor/donor during the oxygen subtracting and O2 replenishing. This leads to the high activity of Pt1@CeO2 (111) and Pt1@CeO2 (100) SACs for CO oxidation. Our work shows that the single-Pt-atom catalyst with ceria support is a highly robust catalysis for CO oxidation. It has not only high thermal stability due to covalent metal-support interaction, but also excellent activity toward the CO2 formation. Particularly notable is the finding that in the CO oxidation on Pt1@CeO2 (111) and (100), the single Pt atom is actually the oxidant agent for the first CO oxidation and it is also the reductive agent to activate the O2 molecule in a subsequent step. This single-Pt active center averts the formation of poisoning carbonate species by facilitating direct formation of CO2 gas that is easy to desorb. As the surface adsorbed species can serve as ligands to affect the bonding and electronic structures of the active centers of surface single-atom catalysts,63 further investigation of the activity of the single-atom catalysts with dopants on water-gas-shift (WGS) and the preferential CO oxidation reactions in H2 would be interesting.

ASSOCIATED CONTENT 16


Page 17 of 24

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


Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ***. Figures of the evolution of net charge of single-atom Pt during AIMD simulation, TDOS and PDOS for Pt1@CeO2 (111) and Pt1@CeO2 (110), optimized structures of Pt2@CeO2 (110), calculated CO2 production rate and the highest-reactive single-atom Pt catalyst under different temperature and pressure, and Table of calculated Bader charges of single-atom Pt and CO2. (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +86-10-62795381. *Email: [email protected]. Tel: +86-10-62797472. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the NSFC (91426302, 21521091, 21433005). The calculations were performed by using supercomputers at Tsinghua National Laboratory for Information Science and Technology and the Supercomputing Center of Computer Network Information Center, the Chinese Academy of Sciences. The authors also acknowledge the Tsinghua Xuetang Talents Program for providing computational resources.

17



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

Page 18 of 24

REFERENCES (1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Novel gold catalysts for the oxidation of carbon monoxide at a temperature far below 0.deg.C. Chem. Lett. 1987, 16, 405‐408. (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. Nat. Chem. 2011, 3, 634‐641. (3) 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. (4) Peterson, E. J.; DeLaRiva, A. T.; Lin, S.; Johnson, R. S.; Guo, H.; Miller, J. T.; Kwak, J. H.; Peden, C. H.; Kiefer, B.; Allard, L. F. Low‐temperature carbon monoxide oxidation catalysed by regenerable atomically dispersed palladium on alumina. Nat. Commun. 2014, 5. (5) 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. (6) Liang, J.‐X.; Lin, J.; Yang, X.‐F.; Wang, A.‐Q.; Qiao, B.‐T.; Liu, J.; Zhang, T.; Li, J. Theoretical and Experimental Investigations on Single‐Atom Catalysis: Ir1/FeOx for CO Oxidation. J. Phys. Chem. C 2014, 118, 21945‐21951. (7) Zhang, X.; Shi, H.; Xu, B.‐Q. Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1, 3‐Butadiene over Au/ZrO2 Catalysts. Angew. Chem. Int. Ed. 2005, 44, 7132‐7135. (8) Long, B.; Tang, Y.; Li, J. New mechanistic pathways for CO oxidation catalyzed by single‐atom catalysts: Supported and doped Au1/ThO2. Nano Res. 2016, 9, 3868‐3880. (9) Lin, J.; Wang, A.; Qiao, B.; Liu, X.; Yang, X.; Wang, X.; Liang, J.; Li, J.; Liu, J.; Zhang, T. Remarkable Performance of Ir1/FeOx Single‐Atom Catalyst in Water Gas Shift Reaction. J. Am. Chem. Soc. 2013, 135, 15314‐15317. (10) Lucci, F. R.; Liu, J.; Marcinkowski, M. D.; Yang, M.; Allard, L. F.; Flytzani‐Stephanopoulos, M.; Sykes, E. C. H. Selective hydrogenation of 1, 3‐butadiene on platinum‐copper alloys at the single‐atom limit. Nat. Commun. 2015, 6. (11) Flytzani‐Stephanopoulos, M. Gold atoms stabilized on various supports catalyze the water–

18


Page 19 of 24

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


gas shift reaction. Acc. Chem. Res. 2013, 47, 783‐792. (12) 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‐atom platinum‐on‐ceria catalysts via atom trapping. Science 2016, 353, 150‐154. (13) Dvořák, F.; Camellone, M. F.; Tovt, A.; Tran, N.‐D.; Negreiros, F. R.; Vorokhta, M.; Skála, T.; Matolínová, I.; Mysliveček, J.; Matolín, V.; Fabris, S. Creating single‐atom Pt‐ceria catalysts by surface step decoration. Nat. Commun. 2016, 7, 10801. (14) Liang, S.; Hao, C.; Shi, Y. The Power of Single‐Atom Catalysis. ChemCatChem 2015, 7, 2559‐2567. (15) Qiao, B.; Liang, J.‐X.; Wang, A.; Xu, C.‐Q.; Li, J.; Zhang, T.; Liu, J. Ultrastable single‐atom gold catalysts with strong covalent metal‐support interaction (CMSI). Nano Res. 2015, 8, 2913‐2924. (16) Qiao, B.; Liang, J.‐X.; Wang, A.; Liu, J.; Zhang, T. Single atom gold catalysts for low‐temperature CO oxidation. Chin. J. Catal. 2016, 37, 1580‐1586. (17) Cheng, N.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B.; Li, R.; Sham, T.‐K.; Liu, L.‐M. Platinum single‐atom and cluster catalysis of the hydrogen evolution reaction. Nat. Commun. 2016, 7. (18) Ding, W.‐C.; Gu, X.‐K.; Su, H.‐Y.; Li, W.‐X. Single Pd atom embedded in CeO2 (111) for NO reduction with CO: a first‐principles study. J. Phys. Chem. C 2014, 118, 12216‐12223. (19) Neitzel, A.; Figueroba, A.; Lykhach, Y.; Skála, T. s.; Vorokhta, M.; Tsud, N.; Mehl, S.; Ševčíková, K. r.; Prince, K. C.; Neyman, K. M. Atomically Dispersed Pd, Ni, and Pt Species in Ceria‐Based Catalysts: Principal Differences in Stability and Reactivity. J. Phys. Chem. C 2016, 120, 9852‐9862. (20) Figueroba, A.; Kovács, G.; Bruix, A.; Neyman, K. M. Towards stable single‐atom catalysts: strong binding of atomically dispersed transition metals on the surface of nanostructured ceria. Catal. Sci. Technol. 2016. (21) Wannakao, S.; Maihom, T.; Probst, M.; Limtrakul, J.; Kongpatpanich, K. Porous materials as a platform for highly uniform single‐atom catalysts: tuning the electronic structure for the low‐temperature oxidation of carbon monoxide. J. Phys. Chem. C 2016, 120, 19686‐19697. (22) Liang, J.‐X.; Yang, X.‐F.; Wang, A.; Zhang, T.; Li, J. Theoretical investigations of non‐noble metal single‐atom catalysis: Ni1/FeOx for CO oxidation. Catal. Sci. Technol. 2016, 6, 6886‐6892.

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

Page 20 of 24

(23) Wang, Y.‐G.; Cantu, D. C.; Lee, M.‐S.; Li, J.; Glezakou, V.‐A.; Rousseau, R. CO Oxidation on Au/TiO2: Condition‐Dependent Active Sites and Mechanistic Pathways. J. Am. Chem. Soc. 2016, 138, 10467‐10476. (24) Wang, Y.‐G.; Mei, D.; Glezakou, V.‐A.; Li, J.; Rousseau, R. Dynamic formation of single‐atom catalytic active sites on ceria‐supported gold nanoparticles. Nat. Commun. 2015, 6. (25) Wang, Y.‐G.; Yoon, Y.; Glezakou, V. A.; Li, J.; Rousseau, R. The role of reducible oxide–metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics. J. Am. Chem. Soc. 2013, 135, 10673‐10683. (26) Ligthart, D.; van Santen, R. A.; Hensen, E. J. Supported rhodium oxide nanoparticles as highly active CO oxidation catalysts. Angew. Chem. Int. Ed. 2011, 50, 5306‐5310. (27) Guzman, J.; Carrettin, S.; Corma, A. Spectroscopic evidence for the supply of reactive oxygen during CO oxidation catalyzed by gold supported on nanocrystalline CeO2. J. Am. Chem. Soc. 2005, 127, 3286‐3287. (28) Hatanaka, M.; Takahashi, N.; Tanabe, T.; Nagai, Y.; Dohmae, K.; Aoki, Y.; Yoshida, T.; Shinjoh, H. Ideal Pt loading for a Pt/CeO2‐based catalyst stabilized by a Pt–O–Ce bond. Appl. Catal., B 2010, 99, 336‐342. (29) Lee, Y.; He, G.; Akey, A. J.; Si, R.; Flytzani‐Stephanopoulos, M.; Herman, I. P. Raman analysis of mode softening in nanoparticle CeO2−δ and Au‐CeO2−δ during CO oxidation. J. Am. Chem. Soc. 2011, 133, 12952‐12955. (30) Uchiyama, T.; Yoshida, H.; Kuwauchi, Y.; Ichikawa, S.; Shimada, S.; Haruta, M.; Takeda, S. Systematic morphology changes of gold nanoparticles supported on CeO2 during CO oxidation. Angew. Chem. Int. Ed. 2011, 50, 10157‐10160. (31) Kim, H. Y.; Lee, H. M.; Henkelman, G. CO oxidation mechanism on CeO2‐supported Au nanoparticles. J. Am. Chem. Soc. 2012, 134, 1560‐70. (32) Manzoli, M.; Boccuzzi, F.; Chiorino, A.; Vindigni, F.; Deng, W.; Flytzani‐Stephanopoulos, M. Spectroscopic features and reactivity of CO adsorbed on different Au/CeO2 catalysts. J. Catal. 2007, 245, 308‐315. (33) Tang, Y.; Zhao, S.; Long, B.; Liu, J.‐C.; Li, J. On the Nature of Support Effects of Metal Dioxides

20


Page 21 of 24

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


MO2 (M= Ti, Zr, Hf, Ce, Th) in Single‐Atom Gold Catalysts: Importance of Quantum Primogenic Effect. J. Phys. Chem. C 2016, 120, 17514‐17526. (34) Qiao, B.; Liu, J.; Wang, Y.‐G.; Lin, Q.; Liu, X.; Wang, A.; Li, J.; Zhang, T.; Liu, J. Highly Efficient Catalysis of Preferential Oxidation of Co in H2‐Rich Stream by Gold Single‐Atom Catalysts. ACS Catal. 2015, 5, 6249‐6254. (35) McFarland, E. W.; Metiu, H. Catalysis by doped oxides. Chem. Rev. 2013, 113, 4391‐4427. (36) Qin, J.; Lu, J.; Cao, M.; Hu, C. Synthesis of porous CuO–CeO2 nanospheres with an enhanced low‐temperature CO oxidation activity. Nanoscale 2010, 2, 2739‐2743. (37) Avgouropoulos, G.; Ioannides, T.; Matralis, H. Influence of the preparation method on the performance of CuO–CeO2 catalysts for the selective oxidation of CO. Appl. Catal., B 2005, 56, 87‐93. (38) Hu, Z.; Metiu, H. Effect of dopants on the energy of oxygen‐vacancy formation at the surface of ceria: Local or global? J. Phys. Chem. C 2011, 115, 17898‐17909. (39) Yang, Z.; Fu, Z.; Zhang, Y.; Wu, R. Direct CO oxidation by lattice oxygen on Zr‐doped ceria surfaces. Catal. Lett. 2011, 141, 78‐82. (40) Camellone, M. F.; Fabris, S. Reaction mechanisms for the CO oxidation on Au/CeO2 catalysts: Activity of substitutional Au3+/Au+ cations and deactivation of supported Au+ adatoms. J. Am. Chem. Soc. 2009, 131, 10473‐10483. (41) Liu, J.; Liu, B.; Fang, Y.; Zhao, Z.; Wei, Y.; Gong, X.‐Q.; Xu, C.; Duan, A.; Jiang, G. Preparation, Characterization and Origin of Highly Active and Thermally Stable Pd–Ce0. 8Zr0. 2O2 Catalysts via Sol‐Evaporation Induced Self‐Assembly Method. Environ. Sci. Technol. 2014, 48, 12403‐12410. (42) Mai, H.‐X.; Sun, L.‐D.; Zhang, Y.‐W.; Si, R.; Feng, W.; Zhang, H.‐P.; Liu, H. C.; Yan, C.‐H. Shape‐selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes. J. Phys. Chem. B 2005, 109, 24380‐24385. (43) Si, R.; Flytzani‐Stephanopoulos, M. Shape and Crystal‐Plane Effects of Nanoscale Ceria on the Activity of Au‐CeO2 Catalysts for the Water‐Gas Shift Reaction. Angew. Chem. Int. Ed. 2008, 120, 2926‐2929. (44) Kresse, G.; Furthmüller, J. Efficiency of ab‐initio total energy calculations for metals and semiconductors using a plane‐wave basis set. Comput. Mater. Sci 1996, 6, 15‐50.

21



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

Page 22 of 24

(45) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total‐energy calculations using a plane‐wave basis set. Phys. Rev. B 1996, 54, 11169. (46) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented‐wave method. Phys. Rev. B 1999, 59, 1758. (47) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (48) 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. (49) Anisimov, V. I.; Aryasetiawan, F.; Lichtenstein, A. First‐principles calculations of the electronic structure and spectra of strongly correlated systems: the LDA+U method. J. Phys.: Condens. Matter. 1997, 9, 767. (50) Nolan, M.; Grigoleit, S.; Sayle, D. C.; Parker, S. C.; Watson, G. W. Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria. Surf. Sci. 2005, 576, 217‐229. (51) Nolan, M.; Parker, S. C.; Watson, G. W. The electronic structure of oxygen vacancy defects at the low index surfaces of ceria. Surf. Sci. 2005, 595, 223‐232. (52) Henkelman, G.; Jónsson, H. A dimer method for finding saddle points on high dimensional potential surfaces using only first derivatives. J. Chem. Phys. 1999, 111, 7010‐7022. (53) Bader, R. F. A quantum theory of molecular structure and its applications. Chem. Rev. 1991, 91, 893‐928. (54) Chen, F.; Liu, D.; Zhang, J.; Hu, P.; Gong, X.‐Q.; Lu, G. A DFT+ U study of the lattice oxygen reactivity toward direct CO oxidation on the CeO2 (111) and (110) surfaces. Phys. Chem. Chem. Phys. 2012, 14, 16573‐16580. (55) Wang, Y.‐G.; Mei, D.; Li, J.; Rousseau, R. DFT+ U study on the localized electronic states and their potential role during H2O dissociation and CO oxidation processes on CeO2 (111) Surface. J. Phys. Chem. C 2013, 117, 23082‐23089. (56) Li, W.; Srinivasan, S. G.; Salahub, D.; Heine, T. Ni on the CeO2 (110) and (100) surfaces: adsorption vs. substitution effects on the electronic and geometric structures and oxygen vacancies. Phys. Chem. Chem. Phys. 2016, 18, 11139‐11149.

22


Page 23 of 24

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


(57) Ding, X.‐L.; Wu, X.‐N.; Zhao, Y.‐X.; He, S.‐G. C–H bond activation by oxygen‐centered radicals over atomic clusters. Acc. Chem. Res. 2011, 45, 382‐390. (58) Bera, P.; Gayen, A.; Hegde, M.; Lalla, N.; Spadaro, L.; Frusteri, F.; Arena, F. Promoting effect of CeO2 in combustion synthesized Pt/CeO2 catalyst for CO oxidation. J. Phys. Chem. B 2003, 107, 6122‐6130. (59) Medford, A. J.; Shi, C.; Hoffmann, M. J.; Lausche, A. C.; Fitzgibbon, S. R.; Bligaard, T.; Nørskov, J. K. CatMAP: a software package for descriptor‐based microkinetic mapping of catalytic trends. Catal. Lett. 2015, 145, 794‐807. (60) Kim, H. Y.; Henkelman, G. CO oxidation at the interface between doped CeO2 and supported Au nanoclusters. J. Phys. Chem. Lett 2012, 3, 2194‐2199. (61) Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.‐i.; Onishi, T. Carbon monoxide and carbon dioxide adsorption on cerium oxide studied by Fourier‐transform infrared spectroscopy. Part 1.—Formation of carbonate species on dehydroxylated CeO2, at room temperature. J. Chem. Soc., Faraday Trans. 1 1989, 85, 929‐943. (62) 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 Pt1 catalyst , ACS Catal. 2017, 7, 887−891. (63) Xu, C.‐Q.; Xing, D.‐H.; Xiao, H.; Li, J. Manipulating stabilities and catalytic properties of trinuclear metal clusters through tuning the chemical bonding: H2 adsorption and activation", J. Phys. Chem. C, 2017, DOI: 10.1021/acs.jpcc.7b00081.

23



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

TOC Graphics

24


Page 24 of 24

Theoretical Investigations of Pt1@CeO2 Single-Atom Catalyst for CO

Recommend Documents