Dft study of co2 activation on precious metal single atom anchored graphene

Herein, the CO, activation on precious metal single atom Au,, Pt, and Ir, decorated graphene M/G has been investigated by means of periodic density functional theory.. Analysis of geomet

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Tail naavill

DFT STUDY OF CO, ACTIVATION ON PRECIOUS METAL SINGLE ATOM ANCHORED GRAPHENE

HO VIET THANG, DUONG THE HY, PHAN THE ANH, NGUYEN THI MINH XUAN -

The University of Danang, University of Science and Technology;

THONG LE MINH PHAM - Institute of Research and Development, Duy Tan University, Da Nang City; Faculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang; MAI VAN BAY, TRAN DUC MANH - Department of Chemistry, The University of Danang, University of Science and Education, Danang SUMMARY:

The CO, activation is the key step in converting CO, into useful chemicals Herein, the CO, activation on precious metal single atom (Au,, Pt, and Ir,) decorated graphene (M/G) has been investigated by means of periodic density functional theory Analysis of geometrical properties and electronic structures of M/G and CO, adsorbed M/G revealed that while graphene is unable to activate CO,, the precious metal single atom anchored graphene can activate CO, The CO, activation on adsorbed metal single atoms is due to the charge transfer from the metal center to the antibonding x* orbital of CO, resulting in bending CO, molecule with respect to linear CO, gas phase While graphene supported Au, weakly adsorbed CO,, graphene supported Pt, and Ir, is strongly bound to CO, The nature of CO, activation on precious metal single atom anchored graphene is demonstrated by analyzing Bader charge, magnetic moment, density of states and charge density difference of adsorbed CO,,

Key words: CO2 activation; graphene; precious metal single atom; DFT; single atom catalyst

I INTRODUCTION CO, is mainly exhausted gas from many different

resources including industrial activities, transportation and human activities which causes a big problem in

global warming, the greenhouse effect[1] Therefore, the

conversion of CO, into value-added chemicals or fuels (methanol, polycarbonate, methane) is an urgent task to

reduce the harmful impact of 0, on the environment{2]

Various approaches have been dedicated to dealing

with this problem including electrochemical, thermal, biochemical, and photocatalytic methods[3] However, the strong bonding of C=O with the linear geometry of CO, (O=C=0) and the high activation barrier make it becomes a big challenge for CO, conversion[4] Furthermore, the

activation of adsorbed éo, in which the linear CO, is bent

is a key step for CO, reduction[5] Thus, exploring a new

catalyst that can speed up this step is an interesting topic for scientists in the world In the past decades, different catalysts such as Zeolites, metal oxides, metal-organic frameworks, and graphene have been experimentally and

theoretically studied for CO, transformation[3] Among

these materials, graphene exhibits as a potential material due to zero band gap semiconductor, large surface

Hóa học & Ứng dụng cỐ#1B (60B)/5-2022

Ban co the xoa dona chu navi!!!

area, high electron mobility[6] Furthermore, graphene can be straightforwardly decorated by adsorbing or doping metals or precious metals which can improve the

adsorption and catalytic activity For example, decoration

of transition metals on graphene significantly enhances

the H, reduction or CO, activation[6, 7] In recent years,

precious metals both cluster and single atoms (Ru, Pt, Au) deposited on graphene have been experimentally synthesized and exhibited potential applications including

CO, conversion[8-11] Its electrical properties require

further improvement for industrial application This study reports a pathway of doping graphene by selective atomic layer deposition (ALD However, the activation of CO, on precious metal single atom deposited on graphene has not been reported yet at the atomic level

To this end, we investigated the nature of precious metal single atom (Au,, Pt,, Ir,) decorated graphene for CO, activation by means of density functional theory The adsorption energy, Bader charge, density of state and charge density difference are characterized and analyzed to figure out the insight into the precious metal single atoms (Au,, Pt,, Ir.) anchored on graphene for CO, activation

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H METHOD AND MODELS

All calculations were carried out by spin-polarized density functional theory using Vienna Ab initio Simulation Package (VASP)[12] The exchange-correlation of electrons is described within the generalized gradient approach with Perdew-Burke-Ernzerhof (PBE)[13], functional The interaction of nuclei and core electrons are treated with projector augmented wave (PAW)([14], while the valence electrons are explicitly considered C(2s’ 2p’), Au(5d" 6s"), Pt(5d° 6s"), Ir(5d’ 6s’) and O (2s? 2p’) Grimme’s scheme (DFT-D3)[15] was used to account for the long-range dispersion forces The plane wave basis set with cut off energy of 400eV was adopted The 2-2-1 k-point mesh was used for geometrical optimization and a denser k-points mesh of 4-4-1 was applied for density of state (DOS) calculations[7] The optimized structures were obtained when the ionic forces on each atom are less than 0.01eV/A

7x7x1 supercell containing 98 atoms[7] has been adopted to model the electronic properties of precious metal single atom decorated graphene and the CO, activations The binding energy (E, in eV) of precious metal single atom on graphene (G) was computed by the following equation

E, = E(M,/G) - E(6) - E(M,)

where E(M,/G), E(G), E(M,) are the total energy of precious metal single atom M, (Au,, Pt,, Ir,) on graphene, of pristine graphene and of isolated metal, respectively

The adsorption energy of CO, (E,,, in eV) on graphene or precious metal single atom anchored graphene was calculated by

E,„ = E(00,@S) - E(S) - E(CO,)

Where E(C0,@S), E(S), E(00,) are the total energy of

CO, adsorption complex on M,/G or G; of M/G or G; of

free CO, molecule, respectively

The charge density difference (p,,,) [16] was obtained by following equation

Popo = p(00,@M/6) iz p(M,/G) = p(0,)

where p(C0,@M.,/6), p(M,/G) and p(C0,) are charge density of CO, adsorption complex on M./G, of M,/G and of CO, molecule taken from adsorption complex structure, respectively

The effective charge of atoms was calculated by applying the Bader method[17, 18)

Ill RESULTS AND DISCUSSION 1 Electronic properties of precious metal

single atom anchored graphene

To obtain the optimized geometry of precious metal single atom anchored on graphene, all possible sites including hollow, top C and C-C bridge sites, (Figure 1) of metal adsorbed on graphene were investigated It is

shown that the considered single atoms (Au,, Pt, and Ir,)

prefer to locate at C-C bridge sites The characteristics

and optimized structures of these structures are reported

in Table 1 and illustrated in Figure 2 It can be seen from Table 1 that while Au, is physisorbed on graphene with an adsorption energy of -0.40eV, much stronger adsorption was found for Pt, and Ir, on graphene with adsorption energies of -1.65 and -1.39eV, respectively This is in

good agreement with previous DFT studies[19, 20] The

strong binding of Pt, and Ir, on graphene compared to

Au, on graphene is demonstrated by a shortened distance

between metal and graphene (1.954A and 1.938A vs 2.967A), Table 1 and Figure 2 Among these metals, the small charge transfer from graphene to Au, was found with the Bader charge of Au, of -0.16 |e|, while the neutral

charge was observed for Pt, and Ir, with Bader charge of

0.02 and 0.08 |e|, respectively The strong adsorption

of Pt, and Ir, on graphene was further confirmed by the

large overlap of the valence band and conduction band of graphene with metal, while less overlap was observed for Au, on graphene (see DOS profile in Figure 2)

The valence electron configuration of precious metal single atom anchored on graphene remains as its gas phase, Au, (5d"° 6s"), Pt, (5d° 6s’) and Ir, (5d’ 6s*) This is evidenced by spin density and a mid gap in DOS profile (Figure 2) Figure 1 Side view (left) and top view (right) of the optimized

structure of graphene The different adsorption sites on graphene are shown including top C (1),

C-C bridge (2) and hollow site (3)

Table 1: Characteristics of precious metal single atom M (M=Au, Pt, Ir) anchored on graphene ~—s energy, E,, magnetic moment, Mag.(M), Bader charge, Q(M) and distance of precious single atom and graphene, d(M-G)

(eV) (u,) (lel) (A)

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ree GofAu¿/@ — Au; of Au;/G — o _ ° Density of State (states/eV) aon ADS Ryn My enn aL) oe E - Eg (eV) - n ores — Pl; of Auy/G em = o T = © Density of State (states/eV) a o œ a2 4 -2 0 2 4 6 E - Ez (eV) = « o A = ° a oa - -4 > -2 0 2 6 E- Er (eV)

Figure 2 Side view (left), top view (middle) and DOS (right) of a) Au,/G, b) Pt,/G and c) lr,/6 The transparent yellow is spin density of adsorbed metal single atoms with isosurtace level of 0.003 |e|.borh® C, Au, Pt, Ir and O are brown, gold, grey, green and red spheres, respectively

2 CO, adsorption on precious metal single atom anchored graphene

For comparison, we firstly consider the adsorption of CO, on pristine graphene It is obvious that CO, is weakly adsorbed on pristine graphene with physisorption energy of -0.13eV (Table 2 and Figure 3a) This is mainly

contributed by van der Waals forces The weak binding of CO, is illustrated by a large distance (3.116A) between

CO, and pristine graphene, unchange in bond angle of OCO (180°) and C=O bond lengths (1.176A) of CO, as in the gas phase Furthermore, the CO, bonding and antibonding

m* molecule orbital are degenerate as in free CO, molecule

(DOS profile in Figure 3a)

Table 2: Characteristics of CO, adsorption on precious single atom M (M=Au, Pt, Ir) anchored on Graphene Adsorption energy, E 4: Magnetic moment, Mag.(M), Bader charge, Q(M) of adsorbed single atoms, Bader charge of CO,, Q(CO,), distance precious single atom and C of C0,, d(C-M), bond angle of C0,, ⁄(000) and C-0 bond lengths of 0, r(C0;C0) of System ĐÀ Mag.(M) Q(M) 0(00,) R(C-M) ⁄(000) r(C0:00) (eV) (u,) (lel) (Ie[) (Ä) (°) (A) G -0.13 - - 0.00 - 180 1.176;1.176 Au,/G -0.04 0.00 -0.01 -0.25 2.245 151 1.211:1.211 Pt/@ -1.25 0.00 0.25 -0.33 2.005 145 1.277:1.200 Ir/G -1.88 0.00 0.42 -0.50 1.958 139 1.323:1.206

For CŨ, adsorption on M/@, very weak adsorption

was observed for CO, on Au/G with adsorption energy of -0.04eV, however different from adsorbed CO, on pristine

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from Au, to antibonding x* orbital of CO,, confirmed by the Bader charge (-0.25 |e|) of CO, (Table 2) This gives rise to prolonging C=O bond length (1.211A) by 0.04A with respect to C=O bond length of free CO, (0.176A) The

charge transfer from Au, to CO, was also demonstrated by the disappearance of spin density and a mid gap of adsorbed Au, in DOS profile (Figure 3b) and the small amount of charge

accumulation on adsorbed CO, (Figure 4a)

Compared to CO, on Au,/G, much stronger adsorption was found for CO, on Pt,/G and Ir,/G with adsorption energy

of -1.25 and -1.88 eV, respectively While CO, on Au,/G

was bent symmetrically with two identical é =0 bond

lengths of 1.211A (Figure 3b), on both Pt,/G and Ir,/G, the

CO, molecule was bent with asymmetric geometry with two different C=O bond lengths (Figure 3c and Figure 3d) As can be seen from Table 2 that the stronger adsorption the more charge transfer from metal single atom to adsorbed CO, In particular, the CO, adsorption energy

(-1.88eV) on Ir,/G was 0.63 eV (in magnitude) stronger

than CO, on Pt/G (-1 25 eV) This is associated with the 180° c-o-o | 3.116 A BOD 151° 2.245 A 145° 2.005 A 139° 1.958 A d

Figure 3 Side view (left), top view (middle) and DOS profile (right) of CO

and d) Ir,/G C, Au, Pt, Ir and O are brown, gold, grey, green and red spheres, respectively o c Density of State (states/eV) + Density of State (states/eV) Density of State (states/eV) 2 3 2 Samoa dB hf at ht 4 =r ˆ a ®

-0.17 |e| more charge transfer from Ir, to CO, (-0.50 |e|)

than from Pt, to C0, (0.33 |e|) (Table 2) As a result

OCO bond angle of CO, on Ir,/G (139°) is smaller than

that on Pt,/G (145) and the C=O bond length, where they bound to the metal single atom of CO, on Ir,/G (1.323 A)

is 0.05A longer than that on Pt/G (1 DITA), while the rest C=0 bond lengths (1.200 and 1.2006A) are mostly the

same, Table 2 and Figure 3

Furthermore, the much stronger binding of CO, on Pt/G

and Ir,/G compared to CO, on Au/G was demonstrated by the large overlap of bonding electrons between metal and CO, which is illustrated in the DOS profile in Figure 3 and further confirmed by a large amount of charge accumulation on CO, and the partial area between the metal atom and CO, (Figure

4) Clearly, the charge transfer from adsorbed Pt, (50° 6s”)

and Ir, (5d’ 6s*) which is involved 5d electrons is much more

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.Y

2S Be Figure 4 Side view (left) and top view (right) of charge

density difference of CO, adsorbed on a) Au ue , b) Pt/G

and c) Ir,/G with an isosurtace level of 0.003 | e|.bohr® Transparent yellow and blue are charge accumulation

and charge depletion, respectively

IV CONCLUSIONS

We have investigated the electronic properties of precious metal single atoms (Au,, Pt, and Ir,) anchored on graphene and the characteristics ‘of 0, activation on these supported metal single atoms compared to that on pristine graphene by means of density functional theory While pristine graphene is unable to activate CO,, the precious metal single atom anchored graphene can enhance the CO, adsorption and associated with its structure bending

On Au /G, the CO, is physisorbed and symmetrically bent

its structure, on the other hand, strong adsorption was observed for CO, on Pt,/G and Ir,/G with the asymmetrical bending CO, molecule The driven force for CO, activation on the supported single atoms is the charge transfer from the metal center to the antibonding x* orbital of CO, The charge transfer from 5d electrons is much more efficient than from 6s electrons This study shed some light on designing new and efficient materials for CO, conversion applications which is an urgent issue nowadays

Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.06-2020.50 Hoa hoc & Ung dung 0 1B (60B)/5-2022 REFERENCES

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