Catalytic Properties of Surface Sites on Pd Clusters for Direct H2O2

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Article

Catalytic Properties of Surface Sites on Pd Clusters for Direct HO Synthesis from H and O: A DFT Study 2

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Masakazu Iwamoto, and Takashi Deguchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4056297 • Publication Date (Web): 07 Aug 2013 Downloaded from http://pubs.acs.org on August 13, 2013

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The Journal of Physical Chemistry

Catalytic Properties of Surface Sites on Pd Clusters for Direct H2O2 Synthesis from H2 and O2: A DFT Study Takashi Deguchi and Masakazu Iwamoto* Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-5 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan * To whom correspondence should be addressed. Tel.: +81-45-924-5225. Fax.: +81-45-924-5228. E-mail: [email protected]

Density functional theory (DFT) calculations were applied for the H2-O2 reaction on Pd catalysts to distinguish the reactivity of coordinatively more unsaturated sites such as corners and edges (Site A) and of more saturated sites such as a (111) face (Site B).

The effect of the cluster size of Pd on the

reactivity was examined first and a Pd12+12+7 cluster was employed as a typical model catalyst. adsorption energies of H2, O2, H2O2, and HBr were greater on Site A than on Site B.

The

The energy

profiles of the H2+O2 reaction suggested that H2O2 would be smoothly produced on Site B, whereas the formation of H2O and the decomposition of H2O2 would be preferred on Site A.

The H+ and

Br- ions adsorbed in pairs more strongly on Site A than H2 and O2, which would suppress the undesired side reactions on Site A to result in improvement of the H2O2 yield.

The direct

participation of H2O in the reaction through hydrogen bonding was also proposed to explain the high H2O2 selectivity observed on Pd catalysts. surface was studied.

Furthermore, sulfur poisoning effect on a (111)

The findings are all consistent with the reaction mechanism suggested based

on the kinetic studies in the previous papers.

Keywords: Palladium catalyst; Reaction mechanism; Adsorption energy; Hydrogen halide; Water -1ACS Paragon Plus Environment


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participation; Coordinative unsaturation. 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

The direct synthesis of H2O2 from H2 and O2 in the liquid phase has attracted much interest from both industry and academia, although it has not yet been realized industrially.1-3

Pd, Pt, Au,

and their alloys are active as the catalysts with or without proton and halide ions such as Cl– or Br–. Four reactions, direct H2O2 formation (Rf), direct H2O formation (Rc), decomposition of H2O2 to H2O + 1/2O2 (Rd1), and hydrogenation of H2O2 to 2H2O (Rd2) are involved in the synthesis.

It is

necessary to determine how to accelerate the main reaction Rf while depressing the concurrent side reaction Rc and the consequent side reactions Rd1 and Rd2.

Therefore, many efforts have been

devoted to reveal the reaction mechanism, the active sites, and the roles of additives including H+ and Br-.1-21

In our recent reports using kinetics including the mass transfer rate of H2,19-21 surface

sites on Pd particles were categorized into 3 types, which have respective coordinative unsaturation and catalytic properties.

It is very difficult, even using modern in-situ analysis techniques, to

clarify the individual properties of the catalytic sites on metal nano-particles in the solid-liquid-gas phases.

Some elaborately designed apparatuses, the preparation of well-designed Pd nano-particles,

and very careful experiments would be able to reveal them.

At this stage, theoretical calculation

should be another effective approach. Several research groups have indeed investigated the reaction mechanism by the density functional theory (DFT) calculation based on the slab/supercell approach using a plane-wave basis set.22-28

They calculated the adsorption energies of the related molecules on metal surfaces as well

as the activation energies of the individual reaction steps to search the actual reaction courses. Although various types of adsorbed species, surface reactions, and surface structures were suggested and calculated, the attention was mostly focused on the elucidation of the superior performance of the Au-Pd bimetallic alloy catalyst.

The results are very informative for explaining the effect of

alloying Pd with Au, but the following basic questions19-21 remain unsolved: the difference among the catalytic activity of Pd atoms located at planes, edges, and corners and the roles of H+ and Br- in the reaction.

One more significant unresolved subject in the catalysis of Pd clusters is the property -2ACS Paragon Plus Environment

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or activity of the Pd(111) plane. 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

Some reports suggested a low H2O2 selectivity on Pd(111), while

we proposed excellent H2O2 formation activity of plane sites on the Pd/C catalyst in the presence of H+ and Br-.19-21

Moreover, it was suggested that the direct hydrogenation of H2O2 hardly proceeds

on Pd(111) because of the high activation barrier,27 but our studies suggested that the activated hydrogen readily reacted with H2O2 on the plane site of the Pd/C catalyst in the presence of H+ and Br-.

The subjects described above should be disclosed using a cluster model approach. DFT calculation has been extensively applied to study the mechanism of the cathode reaction in

a polymer electrolyte fuel cell.29-37

The oxygen reduction reaction on the Pt catalyst was studied

using the slab/supercell method based on a plane-wave basis and also using cluster models. effects of water solvation were studied as well by various approaches.

The

For example, Jacob and

Goddard III reported the results of theoretical calculations of water formation on Pt and Pt-based alloys by a cluster model approach.33

They showed that the size of the cluster has to be large to

some extent to obtain stable results and that the clusters with at least 3 layers and 28 atoms gave plausible results.

It was also shown that calculations under gas-phase conditions were quite

different from those under solvated conditions, indicating that using the gas-phase system to interpret a solvated system might be misleading. In the present theoretical studies, the cluster model approach was applied to the H2O2 synthesis over Pd catalysts because it was considered more favorable for distinguishing the surface sites than the slab/supercell models using a plane-wave basis set.

The cluster size effect was examined first,

and consequently the Pd12+12+7 cluster, composed of 12 Pd atoms in the first layer, 12 Pd atoms in the second layer, and 7 Pd atoms in the third layer, was employed as the standard model.

There are

3 types of Pd atoms on the first layer of the cluster, representing corner, edge, and plane atoms. Next, the adsorption energies of the various molecules on the cluster were calculated, and the observed results were employed to discuss the reaction paths and the effects of the halide additives. Finally, the participation of H2O in the mechanism was studied under a solvation condition to explain the kinetic observations more consistently.

In these studies, however, the water solvation

treatment was limited because the calculation in solution frequently caused convergence troubles -3ACS Paragon Plus Environment


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due to the unstable nature. 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

2.

Methods

DFT calculations were mainly performed using Gaussian 09 with B3LYP as the Hamiltonian and LANL2DZ as the basis, and with the self-consistent field (SCF) convergence criterion of 10-5.

The

study on the sulfur poisoning were calculated using the DMol3 software as will be described in §3.5. The multiplicity was set to singlet in all cases except for free O2 molecule, whose multiplicity was triplet.

The three-layered Pd12+12+7 cluster exhibited in Figure 1a was mainly applied for the

calculation with the first layer Pd atoms relaxed and the others frozen (Pd-Pd bond length of 2.7511 Å and Pd-Pd-Pd bond angle of 60º), and several other clusters were additionally used for supplementary analyses.

Figure 1b shows the specification of the sites in the first layer of the

Pd12+12+7 cluster, in which the fcc site composed of 3 Pd atoms with the coordination number of 9 was classified as Site B and the other sites as Site A.

There are 4 types of hollow sites in the first

layer of the Pd12+12+7 cluster; 2 fcc sites (f1 and f2) and 2 hcp sites (h1 and h2).

Two types of bridge

sites on the corner are shown in the figure, omitting the other bridge sites in the plane. b1

b2

h2

f2

h1 Site B f1 Site A

a) Top view

Figure 1.

b) Site specification of the 1st layer

A Pd12+12+7 cluster mainly employed in the DFT calculation.

The standard enthalpy was calculated in most cases by computing the vibrational frequencies. The adsorption energy without zero-point energy correction, ∆Ead, and the adsorption enthalpy at the standard conditions, ∆Had, were calculated according to Equations 1 and 2, respectively, in which E and H represent the SCF energy and the enthalpy of the molecule shown in the bracket, respectively. ∆Ead = E[Complex] – E[Cluster] – E[Adsorbate] -4ACS Paragon Plus Environment

(1)

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∆Had = H[Complex] – H[Cluster] – H[Adsorbate] 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

(2)

Hereafter, H, O, Cl, Br and I atoms will be represented by yellow, red, green, brown, and purple colored balls, respectively, in molecular structure figures.

3. 3.1.

Results and Discussion Influence of Cluster Size on Adsorption Energy

As described in the introduction, the calculated adsorption energy of O2 depended on the Pt cluster size, and at least 28 atoms were necessary to obtain stable results.33

Thus the cluster size

effect on the O2 adsorption energy was first studied using 2-layer Pd cluster models, in which the number of Pd atoms in the first layer was varied from 3 to 18 taking the symmetry into account. The second layers were attached such that all of the Pd atoms in the first layers were bonded to at least one of the Pd atoms in the second layers.

O2 was placed on the bridge position of the

terminal site representing Site A and on the 3-fold position of the central fcc site representing Site B. The Pd atoms bonded or to be bonded to O2 were relaxed and the others frozen in the calculation. The optimized structures of the adsorbed O2 on the clusters are exhibited in Figure 2, and the cluster size dependency of the O2 adsorption energy in Figure 3.

Pd3+5 Pd7+10

Pd9+10 Pd12+12

Pd14+12

a) O2/Site A models

Pd18+12

Pd5+5

Pd3+6

Pd5+10 Pd12+12

Pd16+12 Pd14+12

Pd7+10

Figure 2.

Pd18+12

b) O2/Site B models Pd10+12

Optimized structures of O2 adsorbed on various 2-layer Pd cluster models, Pdm+n,

representing Sites A and B.

Pd atoms bonded to O2 were relaxed and the others frozen. -5ACS Paragon Plus Environment


According to Figure 3, the ∆Ead values on the Site A models varied with the cluster size but showed only limited fluctuation, whereas those on the Site B models were significantly dependent on the cluster size, presumably reflecting the degree of unsaturation of the adsorption site.

The

latter results indicated that a certain number of Pd atoms should be used for the sufficient calculation, as suggested on the Pt clusters.33 Figure 3 showed that the difference between the adsorption energies on the Sites A and B models became sufficiently large at 12 Pd atoms in the first layer, although the ∆Ead value on Site B somewhat decreased for the larger clusters.

Because the

required computing time increased very rapidly with the number of surface atoms, the Pd12+12+7 cluster, added the third layer to the Pd12+12, was hereafter employed as the standard cluster. 35

-∆Ead (kcal mol-1)

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

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30

Site A

25 Site B

20

15

0

5

10

15

20

Number of Pd in the 1st layer

Figure 3.

Cluster size dependency of the O2 adsorption energy.

The clusters used are shown in

Figure 2. Recently, Staykov et al. reported that the Mulliken atomic charges of the surface Au atoms correlated to the O-O bond elongation and the charge of the O atoms, and concluded that the activation of O2 occurred by partially negatively charged surface Au atoms.38

The same

calculations were thus examined here on the Pd clusters of Figure 1, but the difference between the ∆Ead values on Sites A and B was not induced by the Mulliken charges.

The higher adsorption

energy on Site A would be connected to the d-band narrowing on the coordinatively more unsaturated Pd atoms as presented by Hammer et al.39, 40

The calculation results of Mulliken

charges and so on are summarized in the Supporting Information. -6ACS Paragon Plus Environment

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It should be noted that only 3 atoms in the Pd12+12+7 cluster can represent the fcc site with the 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

coordination number of 9.

The other 3 and 6 Pd atoms in the first layer have the coordination

numbers of 7 and 5, corresponding to edge and corner atoms, respectively.

These were categorized

as Sites A1 and A2 in the previous paper21 but are discussed without distinction in this study because of the small size of the cluster.

As the Pd12+12+7 cluster was too small in size to directly simulate

the H2-O2 reaction on the specific sites, some data processing devices were adopted to discuss the energetics of the reaction as will be described later.

3.2. 3.2.1.

Adsorption of H2, O2, and H2O2 on a Pd12+12+7 Cluster Dissociative Adsorption of H2

The adsorption energies, ∆Had (theor.), of a couple of H atoms on 14 combinations of the hollow sites of the first layer of the Pd12+12+7 cluster were calculated and are summarized in Table 1a. clear that the values changed with the combinations.

It is

Data treatment will become very convenient

hereafter if “specific adsorption energies”, ∆E†ad and ∆H†ad, of a hydrogen atom on each adsorption site can be determined and if additivity applies.

The values of ∆E†ad and ∆H†ad for f1 to h2 were

indeed determined by applying the least square method to the values of ∆Ead and ∆Had obtained for No. 1 to 10 adsorption sites with standard deviations of 3.9 % and 3.0 %, respectively, while those for b1 and s1 were obtained by dividing the values of ∆Ead and ∆Had obtained for No. 13 and 14 adsorption sites by 2. The results are shown in Table 1b.

In Table 1a, the ∆Had values obtained

by adding the corresponding two ∆H†ad values in Table 1b are also shown as ∆Had (calc.).

The

values were in good agreement with the ∆Had (theor.) values within the errors of 1 kcal mol-1, indicating the establishment of the additivity. the sites.

The values in Table 1b would reflect the natures of

Coordinatively more saturated f1 and h1 sites gave negatively smaller adsorption energies

than more unsaturated f2 and h2 sites. as those on the f2 and h2 sites.

The adsorption energies on the b1 site were almost the same

The H atom adsorbed on the side site (s1) shown by the No. 14

model in Table 1a gave larger ∆E†ad and ∆H†ad values, which will be used for convenience later. The dissociative adsorption energy of H2 on Pd(111) has been experimentally and theoretically -7ACS Paragon Plus Environment


Conrad et al. reported the value of 20.8 kcal mole-H2-1 for Pd(111) from the

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

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adsorption isotherms41, and Dong et al. showed by DFT calculation using the vienna ab-initio simulation package that the adsorption energy of H on the fcc threefold hollow position was 11.5 kcal g-atom-H-1, compatible with the experimental result.42

The ∆H†ad value of the f1 site in Table

1b was approximately two-thirds of the values in the literature, which is most likely due to the limited size of the calculated cluster.

It was assumed here that the relative magnitude correlations

of the adsorption energies on the respective sites will not be changed with the cluster size. Table 1.

Dissociative adsorption of H2 on various sites of the Pd12+12+7 cluster.

a) Adsorption enthalpy of H2 (in kcal mol-1). Model No.

1

2

H

Adsorption sites of 2H*1

H

3

4

H

H

H

H

5

6

H

H

7 H

H

H

H

H

H

∆Had (theor.) ∆Had (calc.) *2 Diff.

-18.8 -19.1 0.2

Model No.

-23.4 -23.6 0.1

8

9

H

Adsorption sites of 2H*1

-24.3 -23.6 -0.7 10

H H

-22.8 -23.6 0.7

H

-23.6 -23.6 -0.1

11

12

H

H

H

-18.0 -17.4 -0.6

H

-17.0 -17.4 0.3

13

14 H

H

H

H

∆Had (theor.) -23.7 -19.4 -17.6 -11.1 -17.1 -22.3 -25.1 ∆Had (calc.) *2 -23.7 -19.2 -17.5 -11.1 -17.5 Diff. 0.0 -0.2 -0.2 0.1 0.4 1 2 * Corresponding to Figure 1b, * Estimated value by adding corresponding 2 specific adsorption enthalpies in Table 1b. b) Specific adsorption energy of a H atom at each site (in kcal g-atom-H-1). Site*1 ∆E†ad ∆H†ad

f1 -6.8 -7.3

*1 Exhibited in Figure 1b.

3.2.2.

f2 -11.8 -11.8

h1 -4.7 -5.6

h2

b1

-11.3 -11.9

-11.1 -11.1

s1*2 -12.3 -12.5

*2 s1 represents the side position shown in model no. 14 in Table 1a.

Molecular Adsorption of H2

Dong et al. reported a DFT study where the dissociative adsorption of H2 on Pd(111) passed through a precursor state of a H2 molecule adsorbing on a Pd atom.42

On the present Pd12+12+7

cluster, ∆Ead and ∆Had (kcal mol-1) of the molecular adsorption of H2 were -8.8 and -7.8 on Site A (corner), -4.8 and -5.2 on Site A (edge), and 0.3 and -0.9 on Site B.

The values on Site B indicated

a metastable state of the adsorbate and thus the pair of H atoms migrated far apart from the surface -8ACS Paragon Plus Environment

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in the calculation. 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

H2 was evidently able to adsorb molecularly much more stably on the corner Pd

and the edge Pd than on Site B.

The H-H distances of the H2 adsorbed on the corner and the edge,

0.82 and 0.84 Å, were longer than that on Site B, 0.76 Å, suggesting that H2 was more activated on the corner and the edge.

All of the calculated results suggested that the rate of dissociative

adsorption of H2 would be more rapid on the corner and the edge.

In other words, it is reliable that

the corner and the edge Pd atoms have high capabilities for supplying adsorbed H atoms.

This

concept is in good agreement with the high H2-O2 reaction rates in the absence of Br- ion reported in previous papers.20, 21

3.2.3.

Adsorption of O2

The adsorption energies of O2 on the f1, b1 and b2 sites are exhibited together with the optimized structures of the complexes in Table 2.

It was confirmed separately that the O2 molecule placed as

an end-on adsorbate on the surface was transformed to a side-on adsorbate during the optimization. The adsorption energies of O2 were largely changed with the adsorption sites, obviously reflecting the order of the unsaturation of the Pd atoms.

The ∆Had value at the f1 site in Table 2 was

somewhat smaller than 20.5-23.3 kcal mol-1 calculated on Pd(111) using ab initio local-spin-density calculations with a plane-wave basis.43 Table 2.

Adsorption of O2 on the Pd12+12+7 cluster (in kcal mol-1).

site

Site B

model no.

15

16

Site A 17

∆Ead

-16.6

-25.1

-28.4

∆Had

-16.6

-23.8

-28.1

optimized structure

3.2.4.

Adsorption of H2O2

Two types of adsorption, molecular and dissociative adsorption (splitting to HOO and H), were studied.

Table 3 summarizes the numerical values and the respective optimized structures.

In the

dissociative adsorption, the adsorbed H atom was placed at the common fcc site on the side face (s1 -9ACS Paragon Plus Environment


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site shown in Table 1) in the calculation, and then the observed values were corrected by subtracting 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 values of ∆E†ad and ∆H†ad at the s1 site and by adding those at the f1 (Site B) or f2 (Site A) site to simulate the proper adsorption of a H atom on Site B or Site A with wider space.

The calculated

energies in the gas phase were corrected to the energies in the liquid phase by subtracting the dissolution enthalpy of H2O2 (sum of the enthalpies of condensation44 and mixing with water45) according to Equation 4 to discuss the H2-O2 reaction in an aqueous phase. H2O2 (g) → H2O2 (aq.) Table 3.

∆H = -13.83 kcal mol-1

(4)

Molecular and dissociative adsorption of H2O2 on a Pd12+12+7 cluster (in kcal mol-1).

type site model No.

molecular adsorption Site B Site A 18 19

dissociative adsorption Site B Site A 20 21

optimized structure

∆Ead ∆Had ∆Ead ∆Had ∆Ead ∆Had gas phase -9.3 -7.8 -15.4 -12.7 -10.7 -11.5 corrected*1 -5.1 -6.3 4.6 6.0 -1.6 1.1 8.7 7.5 liq. phase*2 *1 Ha at s1 site was transferred to f1 (Site B) and f2 (Site A) sites. energy of H2O2 in water was subtracted.

∆Ead -22.0 -21.5 -7.7

∆Had -24.0 -23.2 -9.4

*2 Dissolution

Table 3 demonstrates that the molecular adsorption of H2O2 on Site B was more advantageous than the dissociative one, although the adsorption was endothermic in both cases.

In contrast, the

dissociative adsorption on Site A was exothermic, while the molecular one was thermodynamically almost neutral.

It follows that H2O2 adsorbed molecularly on Site A would be easily transformed

to the dissociative adsorbate, whereas H2O2 could adsorb only molecularly on Site B though its quantity would be very small.

3.3. 3.3.1.

Reaction Paths of H2O2 Synthesis and Effect of Halide Additives Adsorption Energy Profile along the H2+O2 Reaction

Processing the adsorption data calculated above could result in adsorption enthalpy profiles along the H2+O2 reaction to form H2O2 (Figure 4) in which four combinations of the adsorption sites of H2 and O2 were treated on the assumption that (OOH)a as well as (H2O2)a remained on the - 10 ACS Paragon Plus Environment

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same site as (O2)a.

When both H2 and O2 adsorbed on Site B (Case 1), the reaction to form H2O2

was distinctly downhill, suggesting that the H2+O2 reaction on Site B would proceed smoothly to form H2O2.

On the contrary, when both H2 and O2 adsorbed on Site A (Case 4), the energy level of

2Ha + (O2)a was lower than that of H2O2 (liq.), and the reaction step Ha + (OOH)a → (H2O2)a was considerably uphill, suggesting that H2O2 would be hardly formed from H2 and O2 and that H2O would be formed on this site.

When the adsorption site of H2 was different than that of O2 (Cases

2 and 3), H2O2 might be formed with some uphill reaction steps. consistent with the suggestions in the previous paper20,

21

These features are very

that Site A with a high degree of

coordinative unsaturation is active for direct H2O formation and H2O2 decomposition, whereas Site B with a low degree of unsaturation catalyzes H2O2 formation.

The energy profiles of Cases 1 and

4 are basically identical to those shown in the previous paper.20

However, Cases 2 and 3 may

correspond to the reaction on Site A2 on which the reaction proceeded following the Langmuir-Hinshelwood mechanism.21

Although there must exist various types of sites on actual

Pd catalysts, the H2+O2 reactions on them would be relatively characterized by some of the energy profiles in Figure 4. 10

Adsorption site

0 Case 1 Case 2 Case 3 Case 4

-10 ∆Hf (kcal mol-1)

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

-20

2H

O2

B A B A

B B A A

-30 -40 -50 2H a +(O 2)a -60

Figure 4.

H 2+O 1 2 (g)

2

(H 2O2)a

H a +(OOH) a 4 3

H 2O 5 2 (l)

Energy profiles along the reaction steps.

were assumed to be the same as those of (O2)a.

The adsorption sites of (OOH)a and (H2O2)a

∆Hf of H2O2 (l) was determined by adding ∆H in

Equation 4 to the calculated value of the formation enthalpy of H2O2 in gas phase.

The high activity of Site A for the decomposition of H2O2 in the absence of Br- 11 ACS Paragon Plus Environment

20

can be well


explained by the energy profile of Case 4 in Figure 4. 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 25

Namely the downhill pathway from (H2O2)a

to the surface intermediates in Case 4 would lead to easy decomposition as in Equations 4 to 6.

In

addition, the energy barrier of the O-O splitting of (H2O2)a is reported to be very low25, 26, 28, and the splitting to form 2(OH)a would also be involved in the reaction.

The resulting (OH)a would be

reduced with Ha to form H2O (Equation 6). (H2O2)a → Ha + (OOH)a

2Ha + (O2)a

(4)

(H2O2)a + Ha → H2O + (OH)a

(5)

Ha + (OH)a → H2O

(6)

3.3.2.

Adsorption of Hydrogen Halides

Table 4 exhibits the adsorption energies of HX (X = Br, Cl and I) on the Pd12+12+7 cluster together with the optimized structures.

The structure of Br adsorbed on Site B was metastable; in

the course of the structure optimization, the Br atom was transferred from the fcc hollow site (f1) to the bridged site, at which the SCF energy was almost flat, and subsequently transferred to the neighboring hollow site (h1).

The adsorption site of the H atom was optimized by the same

protocol as that applied in the dissociative adsorption of H2O2 and the calculated adsorption energies were corrected as such.

To discuss the adsorption energies in the aqueous phase, the values in the

gas phase were further corrected by subtracting the standard dissolution enthalpy of HX in water according to Equations 7 to 9. Table 4.

Adsorption of HX on the Pd12+12+7 cluster (in kcal mol-1).

adsorbate site model no.

HCl Site A 22

HBr Site B 23*1

HI Site A 24

Site B 25

Site A 26

optimized structure

gas phase corrected*2 liq. phase *3

∆Ead

∆Had

∆Ead

∆Had

∆Ead

∆Had

∆Ead

∆Had

∆Ead

∆Had

-44.1 -43.5 -25.6

-43.9 -43.1 -25.2

-37.2 -31.7 -11.3

-37.3 -32.0 -11.7

-49.2 -48.7 -28.3

-48.6 -47.8 -27.5

-43.4 -37.8 -18.3

-43.8 -38.4 -18.9

-54.1 -53.4 -33.9

-52.6 -51.7 -32.2

*1 Metastable. *2 Ha at s1 site was transferred to f1 (Site B) and f2 (Site A) sites. in water were subtracted from the corrected gas phase adsorption energies.

- 12 ACS Paragon Plus Environment

*3 The dissolution enthalpies

Page 13 of 25

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


HCl (g) → H+ (aq.) + Cl- (aq.)

∆H = -17.89 kcal mol-1

(7)44

HBr (g) → H+ (aq.) + Br- (aq.)

∆H = -20.35 kcal mol-1

(8)44

HI (g) → H+ (aq.) + I- (aq.)

∆H = -19.52 kcal mol-1

(9)44

Table 4 indicates that H+ and X- adsorbed in pairs on the Pd surface in the aqueous phase and that the adsorption energies were much higher on Site A than on Site B.

It should be noted that the

∆Had value of HBr on Site A was greater than each of the ∆Had values of H2, O2, and H2O2 on Site A shown in Tables 2-4, except for the comparable ∆Had of O2 on the b1 site, suggesting that the reactions of H2, O2, and H2O2 at the site are easily blocked by HBr adsorption.

However, the ∆Had

value of HBr on Site B was smaller than the ∆Had values of H2 and O2 on the site, suggesting little interference in the H2 or O2 adsorption with HBr.

These results are very consistent with the

suggested role of H+ and Br- in the previous papers.20, 21 As shown in Table 4, HCl could also block Site A, although the ∆Had value was negatively smaller than that of HBr.

However, HI adsorbed not only on Site A but also on Site B more

strongly than H2 and O2, indicating that HI would inhibit all of the reactions of H2 and O2.

It is

widely reported that Br- is the most effective promoter for the H2-O2 reaction to produce H2O2 in an acidic medium, Cl- is also effective, and I- strongly decreases the catalytic activity for H2 conversion.3

It is also reported that addition of Cl-, Br- or I- in acidic conditions depresses the

decomposition of H2O2 on Pd catalysts in the order Cl-< Br-< I-.6

The results shown in Table 4 are

consistent with these experimental findings.

3.4.

Participation of Water in the H2-O2 Reaction

The activation energies of the reactions included in the H2O2 synthesis on Pd(111) were calculated by several research groups based on a plane wave basis.22-28, 33 some of the results.

Table 5 summarizes

Unfortunately, the activation energies for the formation of (OOH)a from Ha +

(O2)a as well as of (H2O2)a from Ha + (OOH)a are considerable in dispersed conditions.

In addition,

the energy barriers to O-O splitting of (OOH)a and (H2O2)a are much lower than the above. However, the kinetics of the H2+O2 reaction on the Pd/C catalyst indicated that H2O2 could be - 13 ACS Paragon Plus Environment


Page 14 of 25

formed in high selectivity and that the rate determining step was the activation of H2.19, 20 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

Thus,

there are lower energy barriers to the formation of both (OOH)a and (H2O2)a than those to the side reactions such as the O-O splitting of (OOH)a and (H2O2)a.

The apparent discrepancy between the

theoretical and the experimental results should be solved: thus, the possibility of the participation of water in the reaction was investigated. reaction over Pt and Pd catalysts.33

Jacob et al. noted the importance of solvation in the H2+O2

Furthermore, it is reasonable to consider that water must take

part in the reaction more directly because not only H2O2 but also the intermediate species adsorbed on the catalyst surface must be connected to H2O molecules by hydrogen bonding and the bond rearrangement of the molecules would be possible. Table 5. mol-1).

Calculated activation energy for each step of the H2 + O2 reaction on Pd(111) (in kcal

literature phase 2Ha + (O2)a → Ha + (OOH)a Ha + (OOH)a → (H2O2)a Ha + (OOH)a → Ha + Oa + (OH)a (H2O2)a → 2(OH)a

[26] gaseous

[28] gaseous

[25] gaseous

gaseous

[33] solvated

17.3 26.1 8.1 0.9

21.2 14.6 6.3 0.6

20.5

12.7

17.1

6.0

2.3

* 0.2

* The OOH hydrogenation results in almost spontaneous HO-OH dissociation with no sizable barrier, that is, Ha + (OOH)a + Ha → 2(OH)a.

Several attempts to determine the transition states directly associated with H2O were conducted using the Pd12+12 cluster under the conditions of solvation (SCRF with CPCM) in which all Pd atoms were fixed.

All of the calculations except for one trial were disappointingly unsuccessful

because of the failure in the optimization of the precursor or the product states or in the TS determination by the QST2 method, which was caused by the instability of the SCRF calculation. Only one model computed the participation of a H2O molecule in the reaction of Ha + (O2)a to form (OOH)a.

Figure 5 demonstrates the precursor, the transition, and the product states of the reaction.

It is evident in the figure that the O-H bond was formed not by direct transfer of the adsorbed H atom but by indirect transfer through the hydrogen bond rearrangement.

The value of the

activation energy, 13.6 kcal mol-1, was smaller than the value of 17.1 reported by Jacob et al. for the solvation conditions (Table 5) and was furthermore reduced to 8.7 by the correction of the adsorption site of Ha (from Site A to Site B), assuming that the values of ∆E†ad on the Pd12+12+7 cluster could be applied.

This result definitely indicated the direct involvement of H2O in the - 14 ACS Paragon Plus Environment

Page 15 of 25


H2+O2 reaction. 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

Although the reliability of the activation energy could not be confirmed by additional calculations, the results of Figure 5 lead one to deduce the extended hypotheses, as schematically illustrated in Scheme 1.

First, the participation of H2O would be generalized to the reactions of Ha

with several O-containing adsorbed species such as (OOH)a, (H2O2)a, (OH)a, and Oa (Scheme 1b to 1h), reducing the activation energies.

Schemes 1e and 1f are suggestions for the reaction of H2O2

in the second coordination sphere on the Pd surface.

Second, two or more H2O molecules can be

involved through hydrogen bonds, as illustrated in Scheme 1i, suggesting that the reaction of Ha and the O-containing species may take place within wider ranges of the Pd surface to further reduce the activation energies, which may well account for the finding that the H2 activation was rate-determining in the H2-O2 reaction over the Pd/C catalyst in the presence of H+ + Br- assuming that the reaction took place on Site B as illustrated by Case 1 in Figure 4.

In Case 2 in Figure 4,

however, the activation energy will become higher because of the lower energy level of the adsorbed H2, most likely causing the Langmuir-Hinshelwood mechanism.21

a) initial state model no. 27 (0.0)

Figure 5.

b) transition state model no. 28 (13.6)

c) product state model no. 29 (-3.2)

Transition state of the Ha + (O2)a reaction, in which H2O is directly involved under

solvation conditions (SCRF with CPCM).

The values in parentheses exhibit the relative formation

energies in kcal mol-1.

DFT calculations on the hydrogenation of H2O2 by Li et al.27 showed that the activation energy of the reaction between (H2O2)a and Ha on the Pd(111) surface was as high as 30.8 kcal mol-1 because H2O2 was situated in a high position over the surface demanding more energy to lift the H - 15 ACS Paragon Plus Environment


atom up from the metal surface. 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 25

The participation of H2O may reduce the activation energy to a

sufficiently low level to explain why the H2 activation was rate determining in the kinetics of H2 hydrogenation over the Pd/C in the presence of H+ and Br- and in the absence of O2.20

In the

presence of O2, however, the adsorptions of H2O2 and H2O compete with that of O2, and the latter would be much more favorable.

H2O2 can barely be adsorbed on Site B due to the inconvenient

∆Had value compared to that of O2, as shown in Tables 3 and 4.

The results can explain why little

H2O2 was lost despite the low activation energies for the O-O splitting of (H2O2)a shown in Table 5. H

H

O

O

H

H

O

O

H

H H O O H H H H O H O O H O H H

H H H

O

O

O

H

H H

O

O

H H H O O H H O H O H

metal surface

metal surface

metal surface

metal surface

metal surface

metal surface

a)

b)

c)

d)

e)

f)

O H

H H

O O H

H

H

H H

O H O

H O H

n

O

H

H H

H O H

+

H O

O

n

O

H

metal surface

metal surface

metal surface

metal surface

g)

h)

i)

j)

Scheme 1.

H H

O

O

Possible mechanism of direct water participation in the reaction of dissociatively

adsorbed hydrogen and oxygen-containing adsorbed species. a) Ha + (O2)a → (OOH)a, b) Ha + (OOH)a → (H2O2), c) Ha + (OOH)a → H2O + Oa, d) Ha + (H2O2)a → H2O + (OH)a, e, f) Ha + H2O2 → H2O + (OH)a, g) Ha + (OH)a → (H2O)a, h) Ha + Oa → (OH)a, i) involvement of (H2O)n network, j) involvement of proton.

It is suggested in the previous paper20 that the proton accelerates the reaction between Ha and (OOH)a.

The acceleration of the H2–O2 reaction with acids in the absence of Br- was also

reported.20, 21

The effects of the proton could be explained by the participation of water with

hydrogen bonding, which are analogous to the reactions of the O-containing adsorbed species as exemplified in Scheme 1j.

3.5.

Poisoning Effect of Sulfur

Our previous study20 reported that one S atom adsorbed on the Pd/C catalyst deactivated 5-8 surface Pd atoms to depress the H2-O2 reaction rate exponentially and increase the H2O2 destruction - 16 ACS Paragon Plus Environment

Page 17 of 25


rate. 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

Since the S atom is suggested to be adsorbed most stably at the hollow site (fcc and hcp) with

3-fold bonds46, 47 and the present calculation suggested the model 15 (Table 2) to be the active intermediate of O2 for the H2O2 formation, the above Pd12+12+7 model is not appropriate for investigating the effect of the S adsorption on the reaction owing to the small space for the simultaneous adsorption of S and O2.

DFT study was here carried out using the DMol3 software

(Materials Studio 5.0; accelrys®) with a 4×4×3 supercell model having gga(p91) as Hamiltonian and DND as basis.

The Pd atoms in the third layer were frozen (Pd-Pd bond length of 2.751 Å and

Pd-Pd-Pd bond angle of 60º) and the other atoms were relaxed. The individual adsorption states of S, a pair of H, and O2 on the fcc site were first optimized independently and then a pair of H or O2 was placed on the S-adsorbed surface.

That is, the S

atom was put on the position optimized in the single adsorption of the S atom and the pair of H atoms or the O2 was also set on the position optimized in the solo adsorption (Figure 6, initial states).

Then geometrical optimization was performed and the results are summarized in Figure

6 as “optimized”.

The figure clearly shows the migrations of the H atom or the O2 initially

located at the fcc site adjacent to the S atom to the neighboring hollow site apart from the S atom (Figures 6a, b, and c).

In Figure 6d, both of the S and the O2 migrated to leave from each other.

The results indicated that both Ha and (O2)a shared no Pd atoms with Sa.

That is, H2 and O2

could not adsorb on the adjacent 13 hollow sites including the 3 Pd atoms bonded to the S atom (see Figure 1b).

In other words, the number of active hollow sites is reduced by 13 per S atom

adsorbed as the first approximation at the low S coverage, although the adsorption energies and the reactivity of H2 and O2 might change with coverage of Sa, Ha and (O2)a.

The coverage of H

on the Pd surface under the reaction conditions of the H2O2 synthesis on the Pd/C is regarded as very low due to the activation of H2 very slow compared to the subsequent reactions,19, 20 and consequently H2 will be activated in proportion to the number of the active hollow sites.

On a

sufficiently wide (111) surface, the number of Pd atoms is half of that of hollow sites and, therefore, one S atom inactivates 6.5 Pd atoms on the average, which is consistent with the previous experimental results.20 - 17 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

initial

optimized

a) Sa and 2Ha (model no. 30)

Figure 6.

initial

optimized

b) Sa and (O2)a (1) (model no. 31)

Page 18 of 25

initial optimized d) Sa and (O2)a (3) (model no. 33)

initial optimized c) Sa and (O2)a (2) (model no. 32)

Migration of H and O2 adsorbed on the sites adjacent to the S atom.

Since the adsorption of O2 and the molecular adsorption of H2O2 take place on the 2- or 3-fold sites and on the top of Pd, respectively, as described in the sections 3.2.3 and 3.2.4, the latter will become relatively preferential with the increasing S/Pd ratio.

Thus the destruction of H2O2, mainly

caused by the hydrogenation,20 will be accelerated, which will increase the H2O2 destruction rate constant.

The involvement of H2O in the hydrogenation in Schemes 1d to 1f will not change the

situation.

4.

Conclusions

DFT studies were performed using cluster models to simulate the H2-O2 reaction on Pd catalysts. Site A (coordinatively more unsaturated such as a corner or an edge) and Site B (more saturated such as a (111) face) were distinguished in the calculation, and the adsorption behaviors of the related molecules on the respective sites were calculated.

The results were very consistent with the

concepts suggested in the previous papers or explained the results of the kinetic studies well, although there were some limitations due to the restricted cluster size.

The H2-O2 reaction

mechanism over Pd catalysts can be summarized as follows. 1) H2 adsorbed dissociatively more strongly on Site A than on Site B.

H2 also adsorbed

molecularly as a precursor more easily on Site A than on Site B, causing more efficient H2 activation on Site A.

O2 also adsorbed more strongly on Site A than on Site B.

molecularly and easily transformed to the dissociative form.

H2O2 adsorbed on Site A

On Site B, however, the dissociative

adsorption of H2O2 was more disadvantageous than the molecular adsorption. - 18 ACS Paragon Plus Environment

Page 19 of 25


2) Energy profiles along the reaction steps, H2 + O2 (g) → 2Ha + (O2)a → Ha + (OOH)a → 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

(H2O2)a → H2O2 (l), demonstrated that the reactions on Site B were always downhill indicating a smooth production of H2O2, whereas the reaction 2Ha + (O2)a → (H2O2)a was uphill on Site A. The latter suggested that the side reactions of the formation of H2O from H2 and O2 and of the decomposition of H2O2 to form H2O + 1/2O2 would be progressive. 3) HBr also adsorbed more strongly on Site A than on Site B. advantageous than those of H2, O2, and H2O2 on Site A. and O2 on Site B.

Its adsorption was more

However, it was weaker than those of H2

Thus, the combined use of H+ and Br- inhibited the undesirable side reactions on

Site A and kept the main reaction on Site B.

HCl adsorbed on Site A more strongly than H2 and

less strongly than HBr, which resulted in the weaker blocking effect. In contrast, HI adsorbed more strongly than H2 and O2 even on Site B, which caused the non-selective inhibition of the reactions. 4) Water, the solvent, participated in the H2+O2 reaction directly.

The H atom in the water

molecule could be bonded and transferred to (O2)a, and the lack of hydrogen was filled up by the surface hydrogen atom, suggesting that the hydrogen bonding with water would lead to easier reactions than those in the gas phase.

The same concept was extended to other reactions of Ha

with O species such as (OOH)a, (H2O2)a, (OH)a and Oa, which could be the reason why the H2 activation is rate-determining in the H2O2 synthesis on the Pd/C despite the rather high activation energies for the hydrogenation of the adsorbed O-O moiety, which were reported in the gas phase. In addition, it would explain the exellent H2O2 selectivity on the Pd/C despite the very low energy barrier of O-O splitting of (OOH)a in the gas phase.

The effect of proton addition was also

recognized as an acceleration of the reactions of Ha with the O-containing adsorbed species. 5) The S atom adsorbed on a 3-fold site of (111) surface and inhibited the adsorption of H2 and O2 on the surface Pd atoms adjacent to the S atom.

One S atom inactivated 13 hollow sites or 6.5

Pd atoms, in agreement with the experimental result.

Acknowledgement - 19 ACS Paragon Plus Environment


Page 20 of 25

We are grateful to Drs. Masashi Tanaka and Akinobu Shiga, Chemical Resources Laboratory, 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

Tokyo Institute of Technology, for their support and discussions about this study.

This work was

financially supported by three Grants-in-Aid (JSPS, NEDO, and ALCA) from the ministries MEXT and METI of Japan. Supporting Information Available: Atomic Cartesian coordinates and Mulliken charges for the optimized geometries of the complexes listed in the tables and the figures.

This material is

available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents (TOC) Image 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

Pd12+12+7 Cluster

A B

on Sitie A HBr > H2, O2 on Site B H2, O2 > HBr


Catalytic Properties of Surface Sites on Pd Clusters for Direct H2O2

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