Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis

Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis

Applied Catalysis A: General 291 (2005) 6–12 www.elsevier.com/locate/apcata Simulation of pathways for CO oxidation over Au nano-clusters by paired i...

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Applied Catalysis A: General 291 (2005) 6–12 www.elsevier.com/locate/apcata

Simulation of pathways for CO oxidation over Au nano-clusters by paired interacting orbitals (PIO) analysis Akinobu Shiga a,*, Masatake Haruta b b

a LUMMOX Laboratory, Takezono 2-18-4-302, Tsukuba 305-0032, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan

Received 14 September 2004; received in revised form 18 February 2005; accepted 18 February 2005 Available online 10 May 2005

Abstract CO oxidation pathways over Au nano-clusters, such as face centered cubic Au14, icosahedron Au13, cubo-octahedron Au13 and tetrahedron Au20 were investigated by using PIO analysis proposed by Fujimoto et al. Four types of oxidation pathways were examined: reaction between (a) adsorbed CO and gaseous O2 (E–R mechanism), (b) adsorbed O2 and gaseous CO (E–R mechanism), (c) adsorbed CO and vicinally adsorbed O2 (L–H mechanism), and (d) adsorbed CO and geminally adsorbed O2 (L–H mechanism). Only in the case of L–H mechanism, when CO molecule and O2 molecule are adsorbed on adjacent two surface Au atoms in vicinal manner (c), CO oxidation occurs favorably. In comparison of the vicinal CO and O2 co-adsorption on icosahedron Au13 with that on cubo-octahedron Au13, CO oxidation is favorable in the former, whereas it is unfavorable in the latter because of considerably longer distance between CO and O2. It is also predicted that CO oxidation is also favorable in the case of vicinally co-adsorbed state of CO and O2 on tetrahedron Au20. The present work has shown that the PIO analysis based on extended Hu¨ckel MO is an effective way to investigate reaction pathways, especially in large catalytic systems. # 2005 Elsevier B.V. All rights reserved. Keywords: CO; Oxidation mechanism; Au clusters; Orbital interaction

1. Introduction Twenty years have passed since the discovery of surprisingly high catalytic activity of ultra fine Au particles supported on metal oxides for CO oxidation at a temperature as low as 200 K [1]. Since then Au catalysts have attracted growing interests [2–5]. The effects of the size and shape of Au particles are crucial in CO oxidation. A typical example is Au supported on Mg(OH)2, where the active Au species are indicated to be 13-atom Au clusters. Interestingly among the two structures of Au13 clusters, it is suggested that the icosahedron is active, whereas the cubo-octahedron is * Corresponding author at: Materials Chemistry Course, Faculty of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 MinamiOsawa, Hachioji 192-0397, Tokyo, Japan. E-mail addresses: [email protected] (A. Shiga), [email protected] (M. Haruta). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.02.046

inactive [6]. In addition, the existence of tetrahedron Au20 cluster has recently been reported [7]. In view of above experimental results, it appears to be instructive to predict why Au13 clusters are active and whether tetrahedron Au20 is catalytically active for CO oxidation or not. Recently, Molina and Hammer [8] have reported precise theoretical study of CO oxidation on Au nano-particles supported by MgO (1 0 0). They have shown that the key COO2 intermediate in CO oxidation is formed from adsorbed CO and gaseous O2 molecule or weakly adsorbed O2 on the Au nano-particles surface. In this paper we study CO oxidation over Au nano-clusters by using paired interacting orbitals (PIO) analysis proposed by Fujimoto et al. to clarify the difference in catalytic activity between icosahedron and cubo-octahedron [9]. PIO analysis is a method for unequivocally determining the orbitals which should play dominant roles in chemical interactions between two systems, [A] and [B], where [A] is Au clusters and [B] is

A. Shiga, M. Haruta / Applied Catalysis A: General 291 (2005) 6–12

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Fig. 2. Three types of fragmentation of CO and O2 co-adsorption on corner Au.

Fig. 1. Schematic illustrations of reaction pathways (a) and (c); - - - : fragmentation line for PIO analysis.

CO. This paper consists of three parts: (1) selection of reaction pathways, (2) investigation of CO oxidation step and (3) CO oxidation on Au nano-clusters: icosahedron Au13, cubooctahedron Au13 and tetrahedron Au20.

2. Models and calculation methods 2.1. Models 2.1.1. Reaction pathways Four types of reaction pathways can be assumed for CO oxidation over Au2 cluster: (a) reaction between adsorbed CO and gaseous O2, (b) reaction between adsorbed O2 and gaseous CO, (c) reaction between adsorbed CO and vicinally adsorbed O2 and (d) reaction between adsorbed CO and geminally adsorbed O2, where (a) and (b) are Eley–Redeal (E–R) mechanism and (c) and (d) are Langmuir–Hishellwood (L–H) mechanism.

The structural parameters used for adsorbed species of CO and O2 were those reported by Okumura et al. [10] and ˚ , R(Au–C): Ha¨ kkinen et al. [11], R(Au–Au): 2.884 A ˚ ˚ ˚ , R(O–O): 2.040 A, R(C O): 1.190 A, R(Au–O): 2.280 A ˚ 1.300 A, angle(Au–C–O): 132.38, angle(Au–O–O): 119.78. Schematic illustrations of reaction pathways are shown in Fig. 1. The distances R1 and R2, shown in Fig. 1, are ˚ , respectively. calculated to be 2.000 and 2.323 A 2.1.2. Investigaton of CO oxidation pathways The Au clusters investigated are face centered cubic Au14, icosahedron Au13, cubo-octahedron Au13, and tetrahedron Au20. There are two types of CO and O2 co-adsorption on Au: a geminal one and a vicinal one. We examined three types of fragmentation of each co-adsorption state, OC/Au cut, O*CO/ Au cut and P OO*CO/Au cut as shown in Fig. 2. Total overlap population OP was calculated to estimate the strength P of interaction between the two fragments. The larger the OP of OC/Au cut is,P the more favorably the oxidation takes place. The smaller the OP of O*CO/Au cut is, Pthe more favorable the oxidation is whereas the smaller the OP of OO*CO/Au cut is, the more favorable the desorption of CO and O2, that is more unfavorable the oxidation. We used the structural parameters shown below as a starting state and investigated the oxidation paths by changing the values of R3, R4 and

Fig. 3. Schematic illustrations of O2 attack on CO adsorbed on fcc Au14 cluster. (c) Vicinal co-adsorption, (d) geminal co-adsorption.

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˚, angle(Au–C–O) in reaction plane; R3(*O–O): 1.300 A ˚ R4(Au–C): 2.040 A, angle(Au–C–O): 132.38. A schematic illustration of O2 attack on vicinal and geminal co-adsorbed CO on fcc Au14 cluster is shown in Fig. 3.

in which pi;k ¼ nt;u

mþn X ci; f dk; f f ¼1 mþn X

pi;nþl ¼ nt;u

2.2. MO calculation For PIO analysis extended Hu¨ ckel MO calculation was carried out. The extended Hu¨ ckel parameters are given in Appendix A and the REX-extended Hu¨ ckel parameters determined by Pyyko were used for Au [12].

i ¼ 1em; k ¼ 1en

; ;

i ¼ 1em; l ¼ 1eN  n

cmþ j; f dk; f ; f ¼1 mþn X

j ¼ 1eM  m; k ¼ 1en

ci; f dnþl; f

f ¼1 mþn X

pmþ j;k ¼ nt;u

pmþ j;nþl ¼ nt;u

cmþ j; f dnþl; f ;

f ¼1

j ¼ 1eM  m; l ¼ 1eN  n 2.3. PIO calculation A model complex (combined system C) is divided into a catalyst portion (fragment A) and an attacking molecule (O2 or CO) portion (fragment B). The geometries of [A] and [B] are the same as those in the original complex ([A–B][C]). The MOs of [A], [B] and [C] are calculated and PIOs are obtained by applying the procedure that was proposed by Fujimoto et al. [9]. (1) The MOs of a complex are expanded in terms of the MOs of two fragment species, to determine the expansion coefficients ci,f , cm+j,f , dk,f , and dn+l,f in Eq. (1) m Mm n X X X ci; f fi þ cmþ j; f fmþ j þ dk; f ck Ff ¼ i¼1

j¼1

N n X dnþl; f cnþl ;

(3) Transformation matrices UA (for A) and UB (for B) are obtained by A ˜ PPU ¼ UAG (3) N X B A ¼ ðg v Þ1=2 pr;s Ur;v v ¼ 1; 2; . . . ; N (4) Us;v r

(4) and finally the PIOs are obtained by Eqs. (5) and (6)(for A) f0v ¼

(5)

r

(for B) c0v ¼

k¼1

N X A Ur;v fr

N X

B Us;v cs

(6)

s

(1)

The N M (N < M) orbital interactions in the complex C can thus be reduced to the interactions of N PIOs, N

(2) An interaction matrix P is constructed which represents the interaction between the MOs of the fragment [A] and the MOs of the fragment [B]   pi;k pi;nþ1 P¼ (2) pmþ j;k pmþ j;nþ1

indicating smaller numbers of MOs of the two fragments, A and B. Although this analysis was proposed originally for ab initio calculations, we reported that this approach was also useful in analyzing the results of extended Hu¨ ckel MO calculations [13]. PIO calculations were carried out on LUMMOXTM system [14].

þ

f ¼ 1; 2; :::; m þ n;

l¼1

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Table 1 The eigenvalues of PIOs of the four types of CO oxidation: (a)–(d) Model

PIO-1

PIO-2

PIO-3

PIO-4

PIO-5

PIO-6

PIO-7

PIO-8

Eigenvalue (a) Au2(CO)4_(O2)gas (b) Au2(CO)2(O2)2_(CO)gas (c) Au2(CO)2(O2)_(CO)vicinal (d) Au2(CO)2(O2)_(CO)geminal

0.224 0.313 0.295 0.527

0.078 0.100 0.288 0.239

0.034 0.019 0.245 0.201

0.018 0.008 0.041 0.009

0.003 0.004 0.017 0.003

0.001 0.001 0.002 0.001

0.000 0.000 0.001 0.000

0.000 0.000 0.001 0.000

Table 2 P The overlap population and total overlap population ( OP) of PIOs of the four types of CO oxidation: (a)–(d) Model

PIO-1

PIO-2

PIO-3

PIO-4

PIO-5

PIO-6

PIO-7

PIO-8

SOP

Overlap population (a) Au2(CO)4_(O2)gas (b) Au2(CO)2(O2)2_(CO)gas (c) Au2(CO)2(O2)_(CO)vicinal (d) Au2(CO)2(O2)_(CO)geminal

0.039 0.159 0.262 0.096

0.002 0.016 0.291 0.070

0.036 0.009 0.076 0.293

0.007 0.011 0.111 0.047

0.003 0.002 0.023 0.002

0.000 0.001 0.006 0.001

0.000 0.000 0.000 0.001

0.000 0.000 0.001 0.002

0.009 0.146 0.490 0.414

3. Results and discussion 3.1. Selection of reaction pathways: E–R mechanism or L–H mechanism? The eigenvalues and overlap population of PIOs of the four types of CO oxidation, (a)–(d), are shown in Table 1, and Table 2, respectively. Since the eigenvalue of PIO represents the contribution of the PIO to the interaction, Table 1 indicates that the interaction between fragment [A] and fragment [B] can be expressed by one major interaction (PIO-1) in the case of (a) and (b), whereas it needs three major interactions in the case of (c) and (d). PThe overlap population of each PIO and the sum of them ( OP) in Table 2 show that the strength of the interaction

decreases in the following order: (c) > (d) >> (a) > (b). Fig. 4 shows the contour maps of PIO-1 for four reaction pathways. When a gaseous oxygen molecule attacks on the adsorbed CO a clear out-of-phase overlap can be seen between the adsorbed CO and the Au in the contour map of PIO-1 of (a). The CO desorption will take place in this case. A large out-of-phase overlap can be seen between the attacking gaseous CO and the adsorbed oxygen molecule in the contour map of PIO-1 of (b), indicating that the attack of gaseous CO on the adsorbed oxygen molecule will be difficult. A good in-phase overlap exists between the vicinally co-adsorbed CO and oxygen molecule and also between the adsorbed CO and the surface Au atom in the case of (c). Although a good in-phase overlap exists between the adsorbed CO and the surface Au atom in the case of (d),

Fig. 4. Contour maps of PIO-1 for reaction paths: (a), (b), (c) and (d).

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Table 3 P CO P OOC P OO CO Total energies, OP , OP and OP of CO coordinated state of Au14(CO)28(O2)_(CO) complexes P OOC P CO OPOO*CO Model Total energy (eV) OP OP 7829.74

(c-10 ) 0.4734

7829.30

0.6848

7834.52

0.9867

7831.11

0.3964

7829.74

0.6769

7828.91

0.7707

(c-1) (c-2) (c-3) (d-1) (d-2) (d-3)

(c-100 ) ... (c-20 ) 0.5759 (c-30 ) 0.0388 (d-10 ) ... (d-20 ) 0.5100 (d-30 ) 0.3036

... (c-200 ) ... (c-300 ) 0.3684 (d-100 ) ... (d-200 ) ... (d-300 ) 0.1680

no overlap can be observed between the geminally coadsorbed CO and oxygen molecule. Based on these results, it can be concluded that CO oxidation takes place over Au clusters through the L–H mechanism rather than the E–R mechanism. It appears that the attack of oxygen molecule on CO will be favored in the case that they are vicinally co-adsorbed than that they are geminally co-adsorbed. 3.2. CO oxidation pathway on a fcc Au14 cluster in L–H mechanism: vicinal co-adsorption or geminal co-adsorption? There are two types of CO and O2 co-adsorption on Au clusters, a geminal one and a vicinal one. As shown in Fig. 2, we examined three types of fragmentation P of the each coadsorption, P for whichPthe following P OP values can be CO

OOC calculated; OP and OPOO CO (Table 3). P , COOP The larger the OP value is, the P more favorably the reaction takes place. The smaller the OP OOC value is, the more readily the desorption of produced CO2 occurs to favor CO reaction to proceed. The smaller the P oxidation OPOO CO value is, both CO and O2 molecules tend to desorb causing the reaction to be less probable. When the vicinally adsorbed O2 molecule approaches the CO Padjacent CO molecule (c-3), total energies decrease, the OP value P markedly the OP OOC also markedly decreases, Pincreases, OO CO and the OP value is still large. All these shows that CO oxidation takes place very readily in the vicinally coadsorbed state of the CO and O2. In contrast, when the geminally adsorbed O2 approaches the carbon atom of CO adsorbed on the same Au, total energy increases in the P order (d-1) < (d-2) < (d-3), while the increase in the OPCO value from (d-2) to (d-3) is not large compared with that P from (c-2) to (c-3). The decrease between OP OOC of (d2) that of (d-3) is not large, and interestingly P and OO CO OP value of (d-3) is very small. This small value P OO CO of OP means that the co-adsorbed CO and O2 are simultaneously desorbed during reaction. The above results are schematically illustrated the P in OOC energy diagram of Fig. 5. Here the ratio of OP /

Fig. 5. An energy diagram: the ratio of of CO oxidation on fcc Au14 clusters.

P

P OP(Au//O*CO)/ OP(Au//CO)

P

OPCO can be used as a parameter for the degree of oxidation, the smaller the ratio is, the more rapid the oxidation is. The ratio also clearly shows that CO oxidation is easy in the vicinally co-adsorbed state of the CO and O2, however, difficult in the geminally co-adsorbed state. Recently, Molina and Hammer [8] have reported precise theoretical study of CO oxidation on Au nano-particles supported by MgO (1 0 0). They have shown that the key COO2 intermediate of CO oxidation is formed from adsorbed CO and gaseous O2 molecule on the Au nanoparticles surface. The vicinally co-adsorbed state of CO and O2 on the adjacent surface Au atoms has been observed in the structure of this key intermediate complex. Our above findings coincide with their result that a vicinally coadsorbed state of CO and O2 is necessary for CO oxidation. 3.3. CO oxidation on Au13 clusters: icosahedron or cubo-octahedron? Schematic illustrations of vicinally co-adsorbed CO and O2 on icosahedron Au13 and cubo-octahedron Au13 are shown in Fig. 6. The oxidation path was investigated by keeping the distance 0R (the distance between C atom of CO

Fig. 6. Schematic illustrations of vicinally co-adsorbed CO and O2 on Au clusters. (e) Icosahedron Au13 (CO)11(O2) and (f) cubo-octahedron Au13 (CO)11(O2), other 10 adsorbed CO molecules are deleted for clarity.

A. Shiga, M. Haruta / Applied Catalysis A: General 291 (2005) 6–12

P OP OOC P OPCO Fig. 7. An energy diagram: DE and the ratio OP / OP of CO oxidation paths on icosahedron Au13 and on cubo-octahedron Au13.

and O atom of O2) and changing the distance 1R (the O–O* distance) and 2R (the distance between C atom of CO and *O atom of O2). The energy diagram is shown in Fig. 7. In the initial stages of the oxidation, the total energies of the both cases tend to decrease. In the middle stages of the cubooctahedron cluster total P the P energy largely increases while the ratio of OP OOC / OPCO still remains large. In contrast, in the middle stages of the icosahedron cluster the increase of the total energy is not so large as that of Pthe cubooctahedron one and moreover the ratio of OP OOC / P CO OP is significantly small. These results indicate that CO oxidation is facile on the icosahedron whereas difficult on the cubo-octahedron and the reason of this reactivity difference mainly depends on the difference of the distance between C atom of CO and O atom of O2.

Fig. 9. An energy diagram and the ratio of CO) of CO oxidation on tetrahedron Au20.

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P

P OP(Au//O*CO)/ OP(Au//

P CO value molecule, total energies the OP P decrease,

OOC markedly increases, the OP also markedly decreases P OOC P CO and the ratio of OP / OP is very small (g-2).

3.4. CO oxidation on tetrahedron Au20: vicinal co-adsorption or geminal one? Schematic illustrations of vicinally and geminally coadsorbed CO and O2 on tetrahedron Au20 are shown in Fig. 8. The oxidation paths were simulated by approaching the position of O* toward the C of CO, the P similarly P to OOC CO manner shown in Fig. 3. Total energies, OP , OP P ðO ÞCO and OP 2 of those models are calculated and the energy diagram is shown in Fig. 9. When the O* atom of vicinally adsorbed O2 molecule approaches the adjacent CO

Fig. 8. Schematic illustrations of (g) vicinally and (h) geminally coadsorbed CO and O2 on tetrahedron Au20.

Fig. 10. Contour maps of PIO-3 and PIO-4 of model states: (g-2) and (h-2).

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In addition, as shown in Fig. 10, very large in-phase overlap can be seen between the O* atom and the C atom of CO. These results indicate that CO oxidation is very facile in the vicinally co-adsorbed state of the CO and O2. On the other hand, in theP case of geminally co-adsorbed state of the CO CO and O , the 2 P OOC P OPCO value does not change and the ratio of OP / OP is fairly large compared with the ratio of the vicinally co-adsorbed state (h-2). Besides, large outof-phase overlap can be seen between the O* atom and the C atom of CO in contour map of PIO-4 of Fig. 10, accordingly, CO oxidation is difficult in the geminally co-adsorbed state of the CO and O2.

Appendix A Coulomb integrals and orbital exponents are listed in Table 4. Table 4 Extended Hu¨ ckel parameters Orbital

Hii(eV)

z1

C2s C2p O2s O2p Au6s Au6p Au5d

21.40 11.40 32.30 14.80 7.937 3.351 11.667

1.625 1.625 2.275 2.275 2.124 1.428 3.398

4. Conclusion CO oxidation pathways over Au nano-clusters, such as fcc Au14, icosahedron Au13, cubo-octahedron Au13 and tetrahedron Au20 were investigated by using PIO analysis. Formation of vicinally co-adsorbed state of CO and O2 is crucial for CO oxidation. In this context, CO oxidation takes place via; (1) L–H mechanism rather than E–R mechanism, (2) vicinal co-adsorption rather than geminal co-adsorption, (3) icosahedron Au13 rather than cubo-octahedron Au13, and (4) tetrahedron Au20: a good catalyst for CO oxidation. The PIO analysis based on EHMO is effective way for investigating reaction pathways and understanding reaction mechanisms, especially in large catalytic systems.

Acknowledgement We thank Professor P. Pyykko¨ of University of Helsinki for giving us REX parameters of gold and helpful advices.

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[10] [11] [12] [13] [14]

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