CO oxidation mechanism on the γ-Al2O3 supported single Pt atom: First principle study

Accepted Manuscript Title: CO Oxidation Mechanism on the ␥-Al2O3 Supported Single Pt Atom: First Principle Study Author: Hongwei Gao PII: DOI: Reference:

S0169-4332(16)30749-8 http://dx.doi.org/doi:10.1016/j.apsusc.2016.04.009 APSUSC 33010

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Received date: Revised date: Accepted date:

11-2-2016 28-3-2016 3-4-2016

Please cite this article as: Hongwei Gao, CO Oxidation Mechanism on the rmgammaAl2O3 Supported Single Pt Atom: First Principle Study, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.04.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

CO Oxidation Mechanism on the γ-Al2O 3 Supported Single Pt Atom: First Principle Study

Hongwei Gao

Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, China



Corresponding author: Tel.: +86-991-3858319; Fax : +86-991-3858319; E-mail: [email protected]

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Graphical abstract

Highlights 

DFT studied on CO Oxidation Mechanism on Pt/γ-Al2O3 Catalyst.



DFT studied on the adsorption properties of single Pt on Pt/γ-Al2O3 Catalyst.

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Pt adsorptions on the Al-terminated surface are more favorable than the ones on the O-terminated surface.



The reactive O*-O-C*=O intermediate mechanism is the dominant reaction pathway for CO oxidation on Pt/γ-Al2O3.

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ABSTRACT: Understanding the role of metal-support interaction for the supported single-atom catalysts is very important in heterogeneous catalysis. Here, Three different CO oxidation mechanisms on Pt/γ-Al2O3 catalyst were probed by periodic density functional theory (DFT) calculations in detail, namely the reactive O*-O-C*=O intermediate mechanism, the reactive CO3 intermediate mechanism and the Pt-Al3+ double sites mechanism. According to the calculated results analysis, we concluded that the dominant reaction pathway at the low temperatures is the reactive O*-O-C*=O intermediate mechanism. Our results are in very good agreement with the experimental evidence for O*-O-C*=O coverage on Pt/γ-Al2O3 at room temperature by an in situ diffuse reflectance infrared detector.

KEYWORDS: CO oxidation, Single Pt, γ-Al2O3, DFT, Mechanism.

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1. Introduction CO in engine emissions can cause environmental and health problems. Therefore, the noble metal catalysts oxidizing CO has been widely investigated by both theoretical1–3 and experimental methods4–6 due to its importance in pollution control. The conventional Langmuir-Hinshelwood scheme (L-H scheme)7 has been widely accepted as the reaction mechanism of CO oxidation on transition metals. CO oxidation on supported platinum nanoclusters has been proposed as L-H scheme or CO-assisted O2 dissociation pathways mechanism. CO-assisted O2 dissociation pathways have the lower barriers than unassisted O2 dissociation pathways. O2* (or O2) reacts with CO* to form reactive O*-O-C*=O intermediate, and then decomposes to produce free CO2 and chemisorbed O*.8 The proposed mechanism of CO oxidation on Pt/θ-Al2O3 (010) is a variation on the conventional L-H scheme where a single supported Pt atom bonds with O2 first, and then bonds with CO. CO3 intermediate forms before producing free CO2 and chemisorbed O*.9 A modified L-H scheme is suggested as the mechanism of CO oxidation on Pt/Fe2O3 where oxygen for CO oxidation is provide by the Fe2O3 support.10 To the best of our knowledge, there have not been any comparative study on the CO oxidation mechanism on Pt/γ-Al2O3. The mechanism of CO oxidation on Pt/γ-Al2O3 still remains an open question. Pt/γ-Al2O3 is a promising candidate for CO oxidation catalyst.11

– 19

Pt/γ-Al2O3 is

active for low temperature CO oxidation.20 High metal dispersion in Pt/γ-Al2O3 improved the reducibility of the surface PtOx species and heightened the CO removal 4

reactivity.21

Structure-performance

relationships

for

Pt/Al2O3

catalysts

was

investigated for CO oxidation under lean-burning diesel exhaust conditions by in situ XAFS. The experimental results show that Pt particles of 2-3 nm exhibit the most active for CO oxidation.22 TiO2-supported PtFe-FeOx nanowires demonstrated the superior performance for room temperature CO oxidation as a prototype reaction and achieve 100% CO conversion.23 CO oxidation on Pt/graphene was investigated using the first-principles method based on density functional theory, the calculated results show that the CO oxidation reaction on Pt/SV-graphene (SV for defective-graphene) has a low energy barrier of 0.58 eV by the Langmuir-Hinshelwood (LH) mechanism.24 Doped h-BN monolayer are also the efficient noble metal-free catalysts for CO oxidation.25 Monodisperse Pt atoms anchored on N-doped graphene as efficient catalysts for CO oxidation was investigated by a first-principle.26 The supported single-atom catalysts are very important to understand the structure-activity relationships of catalysts due to they bridge the gap between homogeneous and heterogeneous catalysis. Single atoms supported on metal oxides have attracted much attention.27–29 The goal of this work is to provide an atomic-scale insight into the nature of the active sites and interfacial effects that determine CO oxidation on Pt/γ-Al2O3 catalyst. This study investigates the mechanism of CO oxidation on Pt/γ-Al2O3 using DFT calculations. Three possible pathways are proposed in this part. To our knowledge, the direct comparison of the two reactive intermediates (O*-O-C*=O and CO3) has not 5

been cleanly addressed previously. This comparison will confirm the most possible pathway for CO oxidation on Pt/γ-Al2O3 catalyst. This study will also provide a useful information for further investigation on Pt/γ-Al2O3 catalyst.

2. Computational Details Due to the higher computational efficiency of Dmol3 code30,31 for periodic structure, the initial structures were optimized using Dmol3 code, then we withdrew the results of Dmol3 code and re-optimized these structures using CASTEP code. All properties calculations were carried out using CASTEP code. Three different reaction mechanism for CO oxidation on γ-Al2O3 (110)-supported single Pt atom catalyst were also carried out using the CASTEP module32 implemented in the Material Studio (MS) v8.0 environment. The Perdew, Burke and Ernzerhof (PBE) functional in the generalized gradient approximation (GGA)33,34 is used for electron exchange and correlation. The calculations were performed with the ultra-fine quality. Ultrasoft pseudopotentials35 were used along with the 400 eV plane-wave kinetic energy cutoff. k-point grid was kept to maintain as 4×4×1. The convergence criteria for the energy, maximum force, maximum stress, maximum displacement and SCF tolerance were set as 5.0×10-7 eV/atom, 0.01 eV/Å, 0.02 GPa, 5.0×10-4 Å and 5.0×10-7 eV/atom, respectively. We built a slab model of γ-Al2O3 (110) surface in this paper, consisting of 5 atomic layers with a 4×4 surface supercell. This computational model contains 184 atoms and has a vacuum gap of 15 Å. During geometry optimization, the top two layers and 6

the adsorbents were relaxed and the bottom three layers were fixed at their fractional position. Single-point energy calculations were also performed in this paper. In order to calculate the energies of single Pt atom, free CO, O2 and CO2 molecules, we used a cubic cell with a = 10 Å, b =10 Å and c =10 Å.

3. Results

3.1. Adsorption of single Pt on the O-terminated γ-Al2O3 (110) surface The experiments demonstrated that the γ-Al2O3(110) is the most stable and the most common facet exposed at aluminum oxide among the three low index surfaces, namely γ-Al2O3 (111), (110) and (100).36–38 Accordingly, we established a slab model of γ-Al2O3(110) using MS CASTEP interface in this work. To explore the effect on adsorption energy for exposed atoms on the surface, we built O-terminated and Al-terminated surfaces. In order to compare the structure stability, the adsorption energy (Eads, eV) of single Pt on the γ-Al2O3 (110) surface was calculated as follows:39 Eads = Ecluster/adsorbate - Ecluster - Eadsorbate where Ecluster/adsorbate is the total energy of the surface of γ-Al2O3 (110) with adsorbate, Ecluster is the total energy of the surface of γ-Al2O3 (110), and Eadsorbate is the total energy of adsorbate. The total energy of the surface of γ-Al2O3 (110) represents the overall energy of the slab which was performed by the single point energy calculation. The Pt adsorption energies at the O-top, O-bridge, Al-top and Al-bridge positions 7

on the O-terminated γ-Al2O3 (110) surface are -0.81, -2.31, -1.76 and -1.66 eV/atom, respectively (see Table 1). The most favorable site for Pt adsorption on the O-terminated γ-Al2O3 (110) surface is O-bridge position. We performed a geometry optimization with a higher accuracy criterion for the Pt adsorption structures on the O-terminated γ-Al2O3 (110) surface. The optimized adsorption structures are show in Figure 1. The optimized bond distance of Pt-O in the O-top site is 2.55 Å, which is in good agreement with the experimental value of 2.50 Å.40 The calculated distance between the Pt atom and the Al atom in the Al-top site is 2.74 Å, whereas the calculated Pt-Al bond distance reported by Zhou is 2.415 Å.41

3.2. Adsorption of single Pt on the Al-terminated γ-Al2O3 (110) surface. The Pt adsorption energies at the O-top, O-bridge, Al-top and Al-bridge positions on the Al-terminated γ-Al2O3 (110) surface are -3.59, -3.73, -4.00 and -0.13 eV/atom, respectively (see Table 1). The most favorable site on the Al-terminated γ-Al2O3 (110) surface is Al-top position. The Pt adsorption structures on the Al-terminated γ-Al2O3 (110) surface are show in Figure 2. The optimized Pt-O bond distance in the Al-top site is 2.64 Å, which is little larger than the experimental value of 2.50 Å. Adsorption energies in Table 1 reveals the following trends: The adsorption energies on the Al-terminated γ-Al2O3 (110) surface are much larger than the ones on the O-terminal surface. Pt adsorptions on the Al-terminated surface are more favorable than the ones on the O-terminated surface. 8

3.3. Population analysis for Pt absorbed on the O-terminated (110) surface and the Al-terminated (110) surface. To gain further insight into the nature of bonding, we report a comparison of Mulliken charges for Pt absorbed on the O-terminated (110) surface and the Al-terminated (110) surface in Table 1. For the O-terminated (110) surface, Pt atoms have the positive charge of 0.17 (O-top), 0.05 (O-bridge), 0.08 (Al-top) and 0.03 e (Al-bridge), respectively. For the Al-terminated (110) surface, the calculated mulliken charge of Pt atoms have the negative values of 0.25, 0.08, 0.08 and 0.06 e, respectively. These results indicate that there is small charge transfer between the Pt atom and the γ-Al2O3 support. Population analysis shows the following trend: the total Mulliken charges of Pt atoms are positive, indicating that Pt has been oxidized as the result of interaction with the γ-Al2O3 support. The most stable positions on O-terminated (110) surface and Al-terminated (110) surface have more charge transfer. Electronic configurations are considered in this part: For Pt[Xe]4f145d96s1, Al[Ne]3s23p1 and O1s22s22p4, the 4f145d96s1, 3s23p1 and 2s22p 4 electrons are considered as the true valence, respectively. O 2p state is hybridized with the Pt-5d to form the chemical bonding. The coupling of Pt with the surface O have the appreciable d character. There is a charge transferred from the empty d orbital of Pt to the surface O for O-terminated (110) surface. For Al-terminated (110) surface, electron transfers from the 3p orbital of Al to the 5d orbital of Pt. More charge transfer 9

show more covalent contribution, therefore, the position with more charge transfer performs more stable.

3.4. CO Oxidation by the reactive O*-O-C*=O intermediate mechanism. The reactive O*-O-C*=O intermediate mechanism in Figure 3 can be illustrated as follows: Pt* + CO → *Pt(CO)(IM1: CO adsorption) -1.2677 eV *Pt(CO)(IM1: CO adsorption) + O2 → *Pt(O2)(CO)(IM2: O2 adsorption) -1.2362 eV *Pt(O2)(CO)(IM2: CO adsorption) → *Pt(O*-O-C*=O) (TS1) 0.8451 eV *Pt(O*-O-C*=O)(TS1)→*Pt(O*-O-C*=O)(IM3:O*-O-C*=O formation) -1.3201 eV *Pt (O*-O-C*=O) (IM3: O*-O-C*=O formation) →*Pt (CO2)(O) (TS2) 0.6401 eV *Pt (CO2)(O) (TS2) → *Pt (CO2)(O) (IM4: CO2 formation)

-2.2681 eV

*Pt (CO2)(O) (IM4) → *Pt(O) (IM5: CO2 desorption) + CO2 -4.2788 eV *Pt(O) (IM5: CO2 desorption) + CO → *Pt(O)(CO)(IM6:CO adsorption) -4.0662 eV *Pt(O)(CO)(IM6:CO adsorption) → *Pt(CO2)(TS3) 0.2547 eV *Pt(CO2)(TS3) → *Pt(CO2)( IM7: CO2 formation) -3.3429 eV *Pt(CO2)( IM7: CO2 formation) → Pt* + CO2 0.0942 eV The asterisk (*) in the equations represents the support, and the reaction energies based on the calculated energy profile in Figure 4. Most of the reactions are energetically favorable except the formation of transition states which is an endothermic step. The adsorption energies of CO and O2 are -1.26 eV and -1.23 eV, respectively. The negative values illustrate that CO and O2 easily adsorbed on the 10

single supported Pt atom. Although DFT calculations suggest that O2 replacement by CO is energetically feasible, Moses et al.9 found that once O2 is adsorbed on Pt, CO can replace O2 only at low O2 pressure and very high temperatures. In the reactive O*-O-C*=O intermediate mechanism, the catalytic process starts with the CO adsorption, CO can bind to Pt at the interface with an adsorption energy of -1.26 eV (see IM1 in Figure 3), which indicates that the strong interaction exists between CO and Pt. Single supported Pt atom are active species, which prefers to bond to CO over O2. O2 adsorption (IM2) follows the adsorption CO (IM1). The reaction between the adsorbed CO and the adsorbed O2 on the single supported Pt atom has a very low energy barrier of 0.84 eV (TS1), leading to the formation of O*-O-C*=O intermediate (IM3). The low energy barrier of 0.84 eV indicates that this reaction step is easy to occur. The formation of CO2 from the O*-O-C*=O intermediate also has a very low energy barrier of 0.64 eV (TS2) with the cleavage of O-O bond. This process is highly exothermic by -4.27 eV (IM5), which indicates that the adsorbed CO2 can be readily desorbed into the free CO2. In the next step, a second CO adsorbed on the single supported Pt atom (IM6) and then reacts with the remaining the adsorbed O atom to form CO2 formation (IM7) with an energy barrier of 0.25 eV (TS3). The CO2 formation in this step is exothermic by -3.34 (IM7) eV, and its desorption is endothermic by 0.09 eV (FS). Ioanna et al.

42

investigated the CO oxidation reaction on graphene-supported Pt13

nanoclusters using first-principles density functional theory calculations. They found that the reaction proceeds via a CO*-assisted activation of the O2 molecule and the 11

formation of an O*-O-C*=O transition state, and eventually forms a CO2 molecule and a chemisorbed O* species. The CO oxidation reaction barrier is approximately 0.5 eV, which is close to our calculation results.

3.5. CO Oxidation by the reactive CO3 intermediate mechanism. The reactive CO3 intermediate mechanism in Figure 5 can be illustrated as follows: Pt* + CO → *Pt(CO)(IM1: CO adsorption) -1.2677 eV *Pt(CO)(IM1: CO adsorption) + O2→ *Pt(O2)(CO)(IM2: O2 adsorption) -1.2362 eV *Pt(O2)(CO)(IM2: CO adsorption) → *Pt(CO3) (TS1) 0.8451 eV *Pt(CO3)(TS1)→*Pt(CO3)(IM3: CO3 formation) -5.2331 eV *Pt (CO3) (IM3: CO3 formation) →*Pt (CO2)(O) (TS2) 2.6633 eV *Pt (CO2)(O) (TS2) → *Pt (CO2)(O) (IM4: CO2 formation)

-0.3783 eV

*Pt (CO2)(O) (IM4) → *Pt(O) (IM5: CO2 desorption) + CO2 -4.2788 eV *Pt(O) (IM5: CO2 desorption) + CO → *Pt(O)(CO)(IM6:CO adsorption) -4.0662 eV *Pt(O)(CO)(IM6:CO adsorption) → *Pt(CO2)(TS3) 0.2547 eV *Pt(CO2)(TS3) → *Pt(CO2)( IM7: CO2 formation) -3.3429 eV *Pt(CO2)( IM7: CO2 formation) → Pt* + CO2 0.0942 eV The asterisk (*) in the equations represents the support, and the reaction energies calculations based on the calculated energy profile in Figure 6. In the reactive CO3 intermediate mechanism, the energy barrier for the formation of reactive CO3 intermediate (TS1) is 0.84 eV, which is equal to that for the formation of reactive O*-O-C*=O intermediate. However, the energy barrier for the CO2 formation from 12

the reactive CO3 intermediate is 2.66 eV, which is much larger than that for the CO2 formation from the reactive O*-O-C*=O intermediate. This result indicates that the reaction pathway by the reactive CO3 intermediate is dynamically much less favorable. Although the formation energy of CO3 (-5.23 eV) is bigger than that of O*-O-C*=O (-1.32 eV), the kinetics of CO oxidation reaction on Pt/γ-Al2O3 catalyst dominate the reaction pathways instead of thermodynamics. The DFT calculated barrier for the desorption of O*-O-C*=O intermediate is much lower than that of CO3 intermediate. Allian et al.8 also found that O*-O-C*=O intermediate pathway has the lower barrier than other pathways by kinetic, isotopic and infrared studies combined with first-principle theoretical methods.

3.6. CO Oxidation by the Pt-Al3+ double sites mechanism. The Pt-Al3+ double sites mechanism in Figure 7 can be illustrated as follows: Pt* Al* + CO → CO *Pt Al* (IM1: CO adsorption) -1.2677 eV CO *Pt Al* (IM1)+ O2→(CO)*PtAl*O2 (IM2: O2 adsorption) -3.2934 eV (CO)*PtAl*O2 (IM2: CO adsorption)→*Pt(O*-O-C*=O)Al* (TS1) 1.4196 eV *Pt(O*-O-C*=O)Al*(TS1)→*Pt(O*-O-C*=O)Al*(IM3) -2.2862 eV *Pt(O*-O-C*=O)Al*(IM3:O*-O-C*=O formation)→*Pt(CO2)(O)Al*(TS2) 0.3588 eV *Pt (CO2)(O) Al* (TS2) → *Pt (CO2)(O)Al* (IM4: CO2 formation)

-4.2447 eV

*Pt (CO2)(O) (IM4) → *Pt + Al* (O) + CO2 (IM5: CO2 desorption) 1.5047 eV *Pt+ Al* (O) (IM5) + CO → (CO)*Pt Al* O (IM6:CO adsorption)

-2.2387 eV

(CO)*Pt Al* O (IM6:CO adsorption) → *Pt Al*(CO2)(TS3) 2.1839 eV *Pt Al*(CO2) (TS3) → *Pt Al*(CO2) ( IM7: CO2 formation) -0.9059 eV *Pt Al*(CO2) ( IM7: CO2 formation) → Pt* + Al*+ CO2 -2.3426 eV 13

The asterisk (*) in the equations represents the support, and the reaction energies calculations based on the calculated energy profile in Figure 8. On the basis of our calculation result, the adsorption O2 at the interface can bind to the Pt-Al3+ dual sites to form the intermediate (IM1) with an adsorption energy of -3.29 eV. This result indicate that the strong interaction and charge transfer exist between Pt and the Al2O3 support. The similar dual site mechanism of the CO oxidation by O2 adsorbed on Pd/CeO2 was investigated using DFT+U.21 Subsequently, the adsorbed CO reacts with the adsorbed O2 to form an O*-O-C*=O complex with an energy barrier of 1.42 eV (TS1), which is much higher than the corresponding value of 0.84 eV (TS1) in the reactive O*-O-C*=O intermediate mechanism. Moreover, the energy barrier for CO2 formation is 2.18 eV (TS3), which is also much higher than the corresponding value of 0.25 eV (TS3) in the reactive O*-O-C*=O intermediate mechanism. This calculation result indicates that the reaction pathway by the Pt-Al3+ double sites mechanism is also dynamically much less favorable.

4. Discussion 4.1. The electron density difference for CO* adsorption at Pt-bridge site of Pt/γ-Al2O3. In order to obtain insights into the electronic character of the interaction between Pt and γ-Al2O3 support or the interaction between CO and Pt/γ-Al2O3 (110), we carried out the electron density difference studies (Figure 9). The electron density difference is defined as follows: Δρ = ρset/substrate -ρset- ρsubstrate where ρ set/substrate is the electron density of Pt/γ-Al2O3 (110) or CO and Pt/γ-Al2O3 (110) system; ρset is the electron densities of Pt or CO; ρsubstrate is the electron densities of γ-Al2O3 (110) support. The electron density difference of Pt/γ-Al2O3 (110) (Figure 9A) shows that there is

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small charge transfer between Pt and γ-Al2O3 (110) support. Mulliken charge analysis indicates that the electronic interaction is largely restricted within Pt and the surface Al or O atoms. These results provide direct evidence that charge transfer occurs at the interface between Pt and γ-Al2O3 (110) support. The electron density difference of CO adsorption at the Pt-bridge site of Pt/γ-Al2O3 (110) (Figure 9B) shows a obvious charge accumulation between Pt and CO and the redistribution around Pt and CO. 4.2. The projected density of states (PDOS) for CO* adsorption at Pt-bridge site of Pt/γ-Al2O3. The importance of d valence electrons in Pt for chemical binding and orbital hybridization with CO molecules can be visualized by projected density of states (PDOS) for CO adsorption at the Pt-bridge site of Pt/γ-Al2O3 (110). For CO/Pt/γ-Al2O3 (110) (Figure 10a and 10b), the contribution to the HOMO and LUMO in CO/Pt/γ-Al2O3 (110) is mainly due to the Pt atom and its nearest neighboring atoms of CO, the contributions from the γ-Al2O3 (110) support are negligible. The HOMO that consists of Pt 5d reacts with the LUMO that consists of CO p orbital. The 5d orbital of Pt hybridizes with the p orbital of CO to form the bonding and antibonding states below and above the Fermi level. The hybridization between the 5d orbital of Pt and the p orbital of CO is weak near the Fermi level (EF) which is consistent with the computational findings for CO adsorption on Pt/γ-Al2O3 (110): the adsorption energy of CO on Pt/γ-Al2O3 (110) is small (-1.26 eV). 5. Conclusions In this work, we performed DFT calculations on the adsorption of single Pt on the O-terminated and Al-terminated γ-Al2O3 (110) surface. We found that Pt adsorptions on the Al-terminated surface are more favorable than the ones on the O-terminated surface. We also carried out DFT study of three different reaction mechanism for CO oxidation on γ-Al2O3 (110)-supported single Pt atom catalyst: the reactive

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O*-O-C*=O intermediate mechanism, the reactive CO3 intermediate mechanism and the Pt-Al3+ double sites mechanism. From energetic analysis, it can be concluded that the reactive O*-O-C*=O intermediate mechanism is the dominant reaction pathway for CO oxidation on Pt/γ-Al2O3 catalyst. Our study will be of significance for all studies related to the nature of active sites and interfacial effects that determine CO oxidation on Pt/γ-Al2O3 catalyst.

Acknowledgements This work was supported by Recruitment Program of Global Experts and the Director Foundation of XTIPC, CAS, Grant No. 2015RC011.

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Figure captions

(A)

(B)

(C)

(D)

Figure 1. Top view of optimal Pt adsorption structure on the O-terminated γ-Al2O3 (110) surface: (A) O-top, (B) O-bridge, (C) Al-top, (D) Al-bridge. Pt, O and Al atoms are represented as blue, red and magenta spheres, respectively.

20

(A)

(B)

(C)

(D)

Figure 2. Top view of optimal Pt adsorption structure on the Al-terminated γ-Al2O3 (110) surface: (A) O-top, (B) O-bridge, (C) Al-top, (D) Al-bridge. Pt, O and Al atoms are represented as blue, red and magenta spheres, respectively.

21

Figure 3. Calculated structures of the reactants, intermediates, transition states and products in the reactive O*-O-C*=O intermediate mechanism. Red, blue, magenta and gray spheres represent O, Pt, Al and C atoms, respectively.

22

Figure 4. Calculated energy profile of CO oxidation via the reactive O*-O-C*=O intermediate mechanism.

23

Figure 5. Calculated structures of the reactants, intermediates, transition states and products in the reactive CO3 intermediate mechanism. Red, blue, magenta and gray spheres represent O, Pt, Al and C atoms, respectively.

24

Figure 6. Calculated energy profile of CO oxidation via the reactive CO3 intermediate mechanism.

25

Figure 7. Calculated structures of the reactants, intermediates, transition states and products in the Pt-Al3+ due sites mechanism. Red, blue, magenta and gray spheres represent O, Pt, Al and C atoms, respectively.

26

Figure 8. Calculated energy profile of CO oxidation via the Pt-Al3+ due sites mechanism.

27

(A)

(B)

Figure 9. The electron density difference for (A) Pt on the γ-Al2O3 support and (B) CO* adsorption at Pt-bridge site of Pt/γ-Al2O3.

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Figure 10. The projected density of states (PDOS) of (a): CO adsorption on Pt/γ-Al2O3 (110) and (b): Pt in Pt/γ-Al2O3 (110).

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Table 1: Adsorption Energy Eads (eV), Pt-(Al/O) Distances and Mulliken charge (e) for Pt at the different sits on γ-Al2O3 (110) Surface. Site

Eads (eV)

d(Pt-site)/Å

Mulliken charge (e)

O-top

-0.81

2.55

0.17

O-bridge

-2.31

2.84

0.05

Al-top

-1.76

2.74

0.08

Al-bridge

-1.66

2.98

0.03

O-top

-3.59

2.64

-0.25

O-bridge

-3.73

2.92

-0.08

Al-top

-4.00

2.64

-0.08

Al-bridge

-0.13

2.84

-0.06

O-terminated surface

Al-terminated surface

30