Theoretical Studies on the Synergetic Effects of AuâPd Bimetallic

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Theoretical Studies on the Synergetic Effects of Au-Pd Bimetallic Catalysts in the Selective Oxidation of Methanol Chun-Ran Chang, Bo Long, Xiaofeng Yang, and Jun Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03965 • Publication Date (Web): 19 Jun 2015 Downloaded from http://pubs.acs.org on June 21, 2015

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The Journal of Physical Chemistry

Theoretical Studies on the Synergetic Effects of Au-Pd Bimetallic Catalysts in the Selective Oxidation of Methanol Chun-Ran Chang,1,2 Bo Long,2,4 Xiao-Feng Yang,2,3 and Jun Li2,* 1

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China

2

Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China

3

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China

4

College of Information Engineering, Guizhou Minzu University, Guiyang 550025, China

ABSTRACT: The selective oxidation of methanol to formaldehyde with molecular O2 on Au-Pd alloy surfaces has been studied by using density functional theory (DFT). We show that the existence of Pd remarkably improves the adsorption and activation of O2 on Au-Pd surfaces. In particular, the second-neighbor Pd monomer pair (Pd-SNMP) surrounded by gold atoms can provide unique active sites for the co-adsorption of O2…CH3OH, thus facilitating the activation of O2 via a hydroperoxyl radical (*OOH). With the involvement of *

OOH and its decomposed fragments (*O and *OH), the oxidative dehydrogenation of

methanol to formaldehyde is facilely achieved on bimetallic Au-Pd surfaces, the barriers of which are calculated to be 0.02–0.45 eV on Au2Pd/(111) and AuPd/(100) surfaces. Importantly, we find that the unusual activation of O2 via an OOH-pathway instead of direct dissociation on Au-Pd catalysts is mainly responsible for the enhanced activity and selectivity in the selective oxidation of alcohols. This hydroperoxyl-based mechanism reveals the intrinsic synergy of Au-Pd bimetallic catalysts in the selective oxidation of alcohols and may provide insights for designing better bimetallic catalysts.

1

INTRODUCTION

Selective oxidation of alcohols to aldehydes is of great significance in the preparation of fine chemicals and intermediates.1,2 Utilization of O2 or air as oxidizing agents to replace conventional stoichiometric d0-metal oxides is the final goal for environmentally benign oxidation reactions. To achieve this goal, tremendous efforts have been made to develop new catalysts, including monometallic Au,3-14 Pd,15-18 and bimetallic Au-Pd,19-27 Au-Cu,28,29 Au-

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Ir30 catalysts. Among them, Au-Pd catalysts usually show enhanced activity and selectivity in catalyzing alcohols to corresponding aldehydes or ketones compared to pure Au or Pd catalysts.20-24

Moreover, alloying

Au

and Pd

improves

the

resistance

to

the

deactivation.19,21,22 Despite superior properties of Au-Pd catalysts, the knowledge of the intrinsic catalytic mechanisms and the nature of the synergetic effect between Au and Pd is still elusive. Therefore, in-depth theoretical studies are needed to provide a fundamental understanding. The activation of molecular O2 is a crucial step in the aerobic oxidation of alcohols on Au-based catalysts.31-33 Several possible routes for O2 activation have been proposed, including direct or adsorbate-assisted dissociation,34-38 the formation of superoxo (O2–) or peroxo (O22–) complexes,39-43 and activation via a hydroperoxyl (OOH) intermediate.44-48 Recently, we revealed that even on bulk Au(111) surface molecular O2 can be activated via *

OOH radical produced by abstracting a H atom from co-adsorbed methanol or water.48 The

formed *OOH species and its dissociated radicals (*O* and *OH) are highly reactive in the oxidation of methanol to formaldehyde. Nevertheless, the activation of molecular O2 remains the bottleneck of the whole catalytic cycle in gold catalysis. It is therefore interesting to explore alternative ways to activate O2 on gold catalysts. Bimetallic catalysts such as Au-Pd are promising candidates for such purpose. To elucidate the nature of bimetallic catalysts, a key issue is how to understand the inherent synergy between the constituent metals. In general, the catalytic behavior of bimetallic catalysts is governed by so-called ensemble effect and ligand effect,49 where the former represent the dilution of surface metal (e.g. Pd) by a second-type of metal (e.g. Au) and the latter accounts for the electronic structure modification due to orbital interactions and charge-transfer between constituent metals.50-53 Recent investigations find that the unusually high activity of Au-Pd catalysts towards some reactions is related to the ensemble (structural) effect.54-59 That is, the catalytic performance depends on some particular active sites that are composed of uniquely structured atoms.54,60,61 For example, individual Pd monomers isolated by gold atoms were identified as the active center for CO oxidation,54 hydrogen evolution,55 glycerol oxidation,56 and direct synthesis of H2O2.57,58,62 In particular, the second-neighbor Pd monomer pair (i.e., two Pd atoms separated by a layer of other atoms) distributed on Au(100) surface were recognized as critical reaction sites for vinyl acetate (VA) synthesis.59 Up to date, how Au-Pd catalysts function in the selective oxidation of alcohols remains an open question.


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In the present work, we report a theoretical study of methanol oxidation on Au-Pd alloy surfaces to unravel the microscopic reaction mechanisms and the synergy between Au and Pd. Our computational study demonstrates that Au-Pd bimetallic catalysts remarkably improves the adsorption and activation of O2, where the second-neighbor Pd monomer pair is found to be a critical ensemble in facilitating O2 activation along an OOH-pathway. In addition, oxidative dehydrogenation of methanol to formaldehyde can readily occur on AuPd surfaces in the presence of activated oxidative species. A mechanism of the synergetic effect between Au and Pd in this reaction is proposed.

2

COMPUTATIONAL DETAILS

All the calculations were performed using DMol3 code of the Material Studio package.63,64 The electron exchange and correlation were treated within generalized-gradient approximation (GGA) in the form of PBE functional.65 The localized double-numerical quality basis set with polarization functions (DNP) was used. The core electrons of metal atoms were described using effective core potentials (ECP) developed by Berger et al,66 in which the mass-velocity and Darwin relativistic corrections were introduced. A thermal smearing of 0.002 hartree and a real-space cutoff of 4.5 Å were applied in our calculations. The alloy Au-Pd surfaces were modeled by a four-layer slab with a vacuum region of 15 Å to avoid image interaction. We employed (3 × 3) supercells for all the (111) surfaces and (4 × 4) supercells for all the (100) surfaces. For both (111) and (100) surfaces, we used a (3 × 3 × 1) Monkhorst-Pack67 mesh of k-point to do geometric optimizations and transition states searching, and increased k-point up to (11 × 11 × 1) to re-evaluate electronic structures. The lattice parameter for bulk is 4.078 Å, consistent with the experimental value.68 During geometry optimizations, the adsorbate(s) together with the top-two layers of metal atoms was allowed to relax and the bottom-two layers were fixed. The convergences of energy, gradient, and maximum displacement were set to 10–5 hartree, 2×10–3 hartree/Å, and 5×10–3 Å, respectively. The adsorption energy Ead of an adsorbate was determined from Ead = Eads/sub – (Eads + Esub) and desorption energy Edes = – Ead, where Eads/sub is the total energy of the slab model covered with the adsorbate, Eads is the total energy of the adsorbate in the gas phase, and Esub is the total energy of the clean substrate. With these definitions, a negative value of Ead means a release of energy or a stable adsorption on the surface following the thermodynamic convention. The d-band center defined as the center of mass of the density of states projected onto the d states is calculated using equation (1),


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0

E d band center = d = Nd

∫ ∫

−∞ 0

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ρ EdE

−∞

ρ dE

(1)

where Ed is the total energy of occupied d electrons, Nd is the total number of occupied d electrons, ρ is the density of d states. The catalytic mechanisms were explored with the calculations of transition states (TS) and intermediates, where the TSs were determined by using complete LST/QST (linear synchronous transit and quadratic synchronous transit) approach69 and a mode-eigenvector following method.70 All the transition states were confirmed to possess only one imaginary frequency and the corresponding vibration mode was verified to connect the reactant and product. The activation barrier Ea is defined as the energy difference between the TS and the initial state (IS). The reaction energy ∆E is the energy difference between the final state (FS) and the IS. Therefore, a negative value of ∆E means a thermodynamically favorable process.

3

RESULTS AND DISCUSSION

3.1. Activation of O2 on Selected Au-Pd Surfaces As mentioned before, the activation of molecular O2 is critical in the methanol oxidation on pure Au(111). Experimental results point to the likely improvement of O2 activation on AuPd catalysts.26,49 To address this issue, we first construct four different Au-Pd surfaces, as shown in Figure 1(a–d), to explore the suitable surface for O2 activation on Au-Pd catalysts. The four Au-Pd surfaces are built based on the Au(111) surface, on which one, two, and three Au atoms on the topmost layer are replaced by Pd atoms. Theoretical works of Guesmi et al. have evidenced the surface segregation of Pd in Au-Pd bimetallic systems in the presence of O and O2.71,72 For the sake of simplicity, the four models are named in turn as Pd monomer, second-neighbor Pd monomer pair (Pd-SNMP), Pd dimer, and Pd trimer (Figure 1). By comparing the energies, the Pd-SNMP model is ~0.10 eV more stable than the Pd dimer model, indicating the isolation of Pd by Au atoms is energetically favorable.

Figure 1. Top views of Au-Pd alloy surfaces (a) Pd monomer, (b) Pd-SNMP, (c) Pd dimer, (d) Pd trimer, and corresponding adsorption of O2 on the four surfaces (a’– d’). Au, Pd, and O atoms are represented as yellow, blue and red spheres, respectively. ACS Paragon Plus Environment

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Our calculations indicate that on each Au-Pd surface O2 preferably adsorbs on Pd sites as shown in Figure 1(a’– d’), consistent with previous theoretical studies.57,58,73 The adsorption energies referenced to the triplet ground state of gas-phase O2 are –0.17 eV (Pd monomer), –0.17 eV (Pd-SNMP), –0.45 eV (Pd dimer), –0.55 eV (Pd trimer), respectively. On the Pd monomer and Pd-SNMP models, O2 is adsorbed at the top site of isolated Pd atoms via an end-on mode. The adsorbed O2 has an O–O bond length of 1.26 Å, close to the typical value for a superoxide (O2–: 1.25–1.30 Å).74 In contrast, on Pd dimer and Pd trimer models O2 is adsorbed at the bridge or hollow sites of Pd ensembles via two oxygen atoms. The O–O bond length of adsorbed O2 is 1.31 Å and 1.35 Å, respectively, close to the typical value for a peroxide (O22–: 1.30–1.55 Å).74 From both the adsorption energies and geometric structures, the adsorption of molecular O2 is apparently enhanced on Au-Pd alloys compared to pure Au(111), where O2 is only extremely weakly bound via physisorption at low temperature with a calculated adsorption energy of –0.03 eV.48,75 We further consider two possible pathways of O2 activation on Au-Pd surfaces: dissociative activation (O2* → 2 *O) and OOH-mediated activation (O2* + CH3OH* → *OOH + CH3O*), where an asterisk represents the adsorbed state. The calculated activation barriers and reaction energies are listed in Table 1. For the direct dissociation of O2, despite the decreased activation barrier from Pd monomer to trimer the activation barriers remain above 1 eV, indicating that dissociative activation of O2 is unfavorable on the selected Au-Pd surfaces. When it comes to the second activation pathway, we surprisingly find that the activation of O2 via OOH-pathway on Pd-SNMP is much easier than that on the other three models. The activation barrier is only 0.37 eV, 1.43 eV lower than that of direct dissociation, indicating the activation of O2 via OOH on Pd-SNMP is more feasible. It is worth mentioning that the barrier for O2* + CH3OH* → *OOH + CH3O* is calculated referring to the co-adsorption state of O2* and CH3OH*. Table 1. Calculated Activation Barriers (eV) and Reaction Energies (eV) of O2 Activation on Selected Au-Pd Models O2* → 2 *O

O2* + CH3OH* → *OOH + CH3O*

Ea

∆E

Ea

∆E

Pd monomer

1.98

0.87

1.36

1.11

Pd-SNMP

1.80

0.91

0.37

0.35

Pd dimer

1.49

0.47

1.35

1.23

Pd trimer

1.02

0.03

1.47

1.29

Models


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To understand the variation in activity of the four selected alloy surfaces for the OOHpathway, we have investigated the electronic structure of active Pd atoms on the surface. As d-band center position can approximately measure chemical activity of metallic catalysts although sometimes inaccurate,76,77 we have calculated the projected density of states (PDOS) of Pd atom. The d-band centers of Pd are determined as –1.88 eV (Pd monomer), –1.90 eV (Pd-SNMP), –1.89 eV (Pd dimer), and –1.89 eV (Pd trimer), respectively. Here the d-band centers only exhibit slight change among the four alloy surfaces (Figure 2), suggesting the position of the d-band centers does not seem to be the determining factor for the activity towards O2 activation. We note that the particular structure of the Pd-SNMP with a Pd…Pd distance of 4.95 Å provides a suitable site for the co-adsorption of CH3OH and O2 (Figure 3), thus facilitating the facile transfer of hydrogen from CH3OH to O2. On this Au-Pd bimetallic surface, Au atoms slightly gains electron from the Pd center (see Figure S1 of SI), while the Pd monomer pair help to anchor the adsorbates. This feature underscores the importance of ensemble effect in bimetallic systems. Similar phenomena was also reported in the synthesis of vinyl acetate.59

Figure 2. The projected density of d-states of the Pd atom on selected Au-Pd surfaces. The dash line indicates the Fermi level position.

Figure 3. Formation of *OOH from co-adsorbed CH3OH and O2 on Pd-SNMP.


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3.2. Au2Pd/(111) and AuPd/(100) Alloy Surfaces and O2 Activation From the above analysis, the Pd-SNMP is a critical ensemble to activate O2 via a lowbarrier pathway. We have therefore designed a Au2Pd/(111) surface (Figure 4a) by extending the active ensemble of the Pd-SNMP to the whole Au(111) surface. Considering the preferential segregation of Pd to the surface in the presence of reactive gas71,72,78-83 and to construct direct comparison with our previous results obtained on Au(111)48, we only put Pd atoms on the topmost layer of Au(111) with the bottom-three layers being retained as pure gold. On the topmost layer of Au2Pd/(111), each Pd atom is isolated by Au atoms and is located as the second-neighbor to its nearby Pd monomers (Figure 4a). Theoretically, the formation of isolated Pd atoms results from the feature that Au–Pd bonds are stronger than Pd–Pd and Au–Au bonds.84-87 Experimentally, isolated Pd monomers on gold surface can be generated by constraining Pd/Au ratio at a small value.58,88 Gotsis et al. further revealed that Pd atoms preferentially form second neighbor to each other on Au(111) and Au(100) surfaces.89 Most importantly, the unique surface configuration of Pd monomers isolated by Au atoms has a substantial effect in the selective oxidation of alcohols.23,56 Because the practical catalysts mostly expose more than one facets, we also construct the AuPd/(100) surface (Figure 4b) for a comparable study. It is worth mentioning that Au2Pd/(111) and AuPd/(100) surfaces possess the maximum proportion of isolated Pd monomers, 33% on (111) surface and 50% on (100) surface, respectively.

Figure 4. Top views of (a) Au2Pd/(111) and (b) AuPd/(100) surfaces. Although Pd prefers to segregate to the surface in reactive environments, the existence of Pd in sub-layers is still unavoidable for the practical Au-Pd catalysts. In particular under vacuum conditions Pd is prone to sink into bulk gold.84,86,90 We thus have built a test model, Au2Pd(111), with Pd occupying the second-neighboring sites of all the four layers (Figure 5) to examine the influence of sub-layer Pd atoms. It is shown that Au2Pd/(111) and Au2Pd(111) have little difference in the adsorption of O2 and CH3OH, and in catalyzing the reaction O2* + CH3OH* → *OOH + CH3O* (Table 2), indicating that the topmost Au-Pd alloy layer could approximately reflect the properties of real alloy surface. ACS Paragon Plus Environment

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Figure 5. Top view (a) and side view (b) of Au2Pd(111) Table 2. Comparison between Au2Pd/(111) and Au2Pd(111) Energies / eV

Au2Pd/(111)

Au2Pd(111)

Ead(O2)

–0.17

–0.19

Ead(CH3OH)

–0.33

–0.34

0.38

0.41

0.40

0.45

∆E (O2*+CH3OH*→*OOH+CH3O*) *

*

*

*

Ea (O2 +CH3OH → OOH+CH3O )

Table 3 summarizes the activation of O2 on Au2Pd/(111) and AuPd/(100) surfaces. On Au2Pd/(111) surface, O2 is more likely to be activated via OOH-mediate pathway in light of the much lower activation barrier (0.40 eV) than that of direct O2 dissociation (1.78 eV), which is analogous to what we have found on the model of Pd monomer pair. On AuPd/(100) surface, O2 dissociation and OOH formation have comparable activation barriers (0.58 eV and 0.47 eV), indicating the two activation pathways are competitive. The formed *OOH surface radicals can directly participate as oxidizing agent in subsequent oxidation reactions44,45,48 or dissociate into even stronger oxidizing agents of atomic oxygen (*O) and hydroxyl (*OH) radicals.91-93 The dissociation barriers of *OOH are 1.18 eV and 0.14 eV on Au2Pd/(111) and AuPd/(100), respectively. Table 3. Calculated Activation Barriers (eV) and Reaction Energies (eV) of O2 Activation on Au2Pd/(111) and AuPd/(100) Surfaces Au2Pd/(111)

AuPd/(100)

Ea

∆E

Ea

∆E

O2* → 2 *O

1.78

0.71

0.58

–0.39


0.40

0.38

0.47

0.46

*

1.18

–0.13

0.14

–0.85

elementary step

OOH → *O + *OH


8

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As described above, AuPd/(100) shows much better activity than Au2Pd/(111) in O2 dissociation and *OOH decomposition, which probably originates from the distribution patterns of active Pd atoms. On the AuPd/(100) surface, two nearest Pd atoms located within a distance of 4.08 Å are able to cleave the O–O bonds of O2 and *OOH efficiently. Whereas on Au2Pd/(111), the distance between two nearest Pd atoms is 4.95 Å, which is too far to activate the O–O bonds. In addition, the (100) and (111) surfaces have quite different periodicity along surface normal, i.e., ABAB for (100) and ABCABC for (111), which could also influence the catalytic behaviors.94 With the generation of *OOH, *O, and *OH, the oxidative dehydrogenation of methanol can proceed by reactions with these species. 3.3. Dehydrogenation of Methanol to Formaldehyde on Au2Pd/(111) and AuPd/(100) Surfaces To explore the reaction mechanisms of alcohols to aldehydes on Au-Pd catalysts, we have selected the selective oxidation of methanol (CH3OH) to formaldehyde (CH2O) as a model reaction. The adsorption energies of all reactants, intermediates and products are listed in Table S1 of supporting information (SI). According to previous studies,48,95,96 this reaction proceeds in two elementary steps: transfer of an H atom in the hydroxyl group to produce methoxy and transfer of a second H atom in the methyl group to yield formaldehyde. For simplicity, we refer to the first H atom in the hydroxyl group as α-H and the second H atom in the methyl group as β-H hereafter. 3.3.1. Dehydrogenation of Methanol by *OOH, *O, and *OH on Au2Pd/(111) Figure 6a depicts the procedure of α-H transfer to OOH, yielding adsorbed methoxy radical and H2O2. This step is slightly endothermic by 0.10 eV, with an activation barrier of 0.20 eV. However, the successive dehydrogenation of methoxy to formaldehyde (Figure 6b) is highly exothermic by –1.39 eV and the activation barrier is only 0.25 eV. Here the dehydrogenation of methoxy is rather easy because the CH3O* surface radical is electronically unsaturated in terms of the octet rule. From the favorable energetics and kinetics, the oxidation of methanol via OOH is rather straightforward. It is worth noting that a considerable concentration of H2O2 will be produced together with formaldehyde during the partial oxidation of methanol on Au-Pd alloy surface based on the much lower desorption energy (0.34 eV) of H2O2 than the dissociation barrier (0.63 eV) of H2O2, which is consistent with the scenario on Au(111) surface.48


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Figure 6. Dehydrogenation of methanol to formaldehyde by *OOH on Au2Pd/(111). (a) Transfer of α-H to *OOH to form methoxy and hydrogen peroxide. (b) Transfer of β-H to *

OOH to form formaldehyde and hydrogen peroxide. Before we address the dehydrogenation of methanol by *O and *OH, it is necessary to

remind that the production of *O and *OH on AuPd/(100) is a little difficult in light of the relatively high dissociation barrier (1.18 eV) of *OOH. However, it is still meaningful to study the dehydrogenation of methanol by all the possible oxidative species. As shown in Figure 7, the oxidation of methanol to methoxy and then to formaldehyde by *O has activation barriers of 0.17 eV and 0.45 eV, respectively. The corresponding reaction energies are 0.02 eV and –1.39 eV, respectively. Obviously, the transformation of methanol to formaldehyde in the presence of *O is very easy to occur. From the geometric structures, we find that atomic oxygen moves from a 3-fold hollow site to a 2-fold bridge site after it is hydrogenated to hydroxyl, consistent with the rules proposed by Liu et al.97

Figure 7. Dehydrogenation of methanol to formaldehyde by *O on Au2Pd/(111). (a) Transfer of α-H to *O to form methoxy and hydroxyl. (b) Transfer of β-H to *O to form formaldehyde and hydroxyl. Likewise,

*

OH can also be involved in the dehydrogenation of methanol to

formaldehyde, which is similar to the alcohol dehydrogenation by hydroxide ion (OH―) in alkaline solutions.98,99 The optimized geometric structures and energetics of the two ACS Paragon Plus Environment

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dehydrogenation steps are displayed in Figure 8. The activation barriers and reaction energies are very close to those in *O-involved dehydrogenation processes. Here as a byproduct water generated in this process can also promote the H-transfer reactions as we have found on Au(111).48 Overall, a common feature of the *OOH-, *O-, and *OH-involved dehydrogenation of methanol is that the first dehydrogenation step is almost thermoneutral or slightly endothermic, but the second dehydrogenation step is strongly exothermic. Therefore as soon as the reactions start to occur the following reactions will be easier to proceed.

Figure 8. Dehydrogenation of methanol to formaldehyde by *OH on Au2Pd/(111). (a) Transfer of α-H to *OH to form methoxy and H2O. (b) Transfer of β-H to *OH to form formaldehyde and H2O. 3.3.2. Dehydrogenation of Methanol by *OOH, *O, and *OH on AuPd/(100) In order to understand the surface effect and how different facets influence the catalytic behavior of Au-Pd alloy for methanol dehydrogenation, we have further investigated the dehydrogenation of methanol to formaldehyde by *OOH, *O, and *OH on the AuPd/(100) surface. The corresponding energetics and optimized geometric structures are displayed in Figure 9. The AuPd/(100) facets are shown to present similar catalytic performance as Au2Pd/(111) in the conversion of methanol to formaldehyde. The activation barriers of dehydrogenation steps fall between 0.02–0.35 eV on AuPd/(100) surface, slightly lower than those (0.17–0.45 eV) on the Au2Pd/(111) surface, implying AuPd/(100) is somewhat more active than Au2Pd/(111) for these reactions. Nevertheless, AuPd/(100) has drawbacks in maintaining high selectively to partial oxidants, which will be discussed in the following section.


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Figure 9. Dehydrogenation of methanol to formaldehyde by *OOH (a–b), *O (c–d), and *OH (e–f) on AuPd/(100). 3.4. The Synergetic Effect between Au and Pd Table 4 summarizes all the calculated elementary steps and corresponding energetics from methanol to formaldehyde on Au(111), Au2Pd/(111), and AuPd/(100) surfaces. There are two common features for the three surfaces: direct dehydrogenations without involving O2 are difficult to come about due to the high barriers of the initial step, whereas oxidative dehydrogenations by *OOH, *O, and *OH are all facile. However, Enache and coworkers have found that Au-Pd catalysts exhibit better activity than pure Au and higher selectivity to aldehydes than pure Pd.21 The synergetic effect between Au and Pd in Au-Pd catalysts thus holds key to manipulate the performance.


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Table 4. Calculated Activation Barriers (eV) and Reaction Energies (eV) of the Elementary Steps on Au(111), Au2Pd/(111), and AuPd/(100) Surfaces Au(111)a elementary step

Ea

Au2Pd/(111)

∆E

Ea

AuPd/(100)

∆E

Ea

∆E

direct dehydrogenation CH3OH* → CH3O* + H*

2.18

1.88

1.57

1.23

1.79

1.02

CH3O* → CH2O*b + H*

0.51

0.13

0.86

–0.08

0.44

–0.28

activation of molecular O2 O2*b → 2 *O

2.44

1.38

1.78

0.71

0.58

–0.39


0.91

0.83

0.40

0.38

0.47

0.46

*

0.79

–0.18

1.18

–0.13

0.14

–0.85

OOH → *O + *OH

dehydrogenation of methanol by *OOH CH3OH* + *OOH → CH3O* + H2O2* *

*

*

CH3O + OOH → CH2O + H2O2

*b

0.30

0.27

0.20

0.10

0.18

0.14

0.06

–2.00

0.25

–1.39

0.35

–0.91

dehydrogenation of methanol by *O CH3OH* + *O → CH3O* + *OH

0.13

0.04

0.17

0.02

0.02

–0.12

CH3O* + *O → CH2O*b + *OH

0.07

–2.06

0.45

–1.39

0.22

–1.62

dehydrogenation of methanol by *OH CH3OH* + *OH → CH3O* + H2O* *

*

*b

*

CH3O + OH → CH2O + H2O a

0.08

–0.03

0.21

–0.01

0.16

0.02

0.24

–2.20

0.45

–1.33

0.23

–1.40

Data taken from Ref [48]. b Species that does not adsorb on Au(111).

By comparing the computational results for the Au(111) and Au2Pd/(111) surfaces (Table 4), we find that the dramatic improvement on Au2Pd/(111) occurs in the process of O2 activation. As is well known, on pure Au(111) surface molecular O2 neither chemisorbs nor dissociates. One possible pathway for O2 activation is through hydrogenation to *OOH via the transfer of hydrogen from methanol. However, this activation channel on Au(111) suffers from unfavorable kinetics and energetics, as shown in Figure 10a (red). While on alloy Au2Pd/(111) surface, the existence of Pd dramatically enhances the co-adsorption of O2 and CH3OH by converting this process from endothermic (0.44 eV) on Au(111) surface to exothermic (–0.32 eV) (Figure 10a, black). Furthermore, the activation barrier to form *OOH is reduced to 0.40 eV on Au2Pd/(111) from 0.91 eV on Au(111) (Figure 10a, black). This change leads to the acceleration of the whole catalytic cycle because O2 activation is the


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rate-determining step. These results can explain why Au-Pd catalysts present higher catalytic activity than pure Au catalysts in alcohol oxidation reactions.21

Figure 10. Energy profiles for O2 activation through two possible pathways. (a) Activation of O2 via OOH-pathway on Au2Pd/(111) and Au(111) surfaces. (b) Dissociation of O2 on Au2Pd/(111) and Pd(111) surfaces. In addition, the special activation of O2 on Au2Pd/(111) surfaces may also explain the higher selectivity to aldehydes on Au-Pd than on pure Pd.21 On the Pd(111) surface O2 is predominantly activated by dissociating into atomic oxygen, which has an activation barrier of 0.67 eV and a great energy release of –1.54 eV (Figure 10b, blue). Although atomic oxygen presents high reactivity in alcohol dehydrogenation reactions, it is also the key species that causes the low selectivity to aldehydes because of its strong oxidative ability.100 As reported by Madix et al,101 partial oxidation of benzyl alcohol to benzaldehyde can only be achieved at low atomic oxygen concentrations; higher density of oxygen atom leads to deeper oxidation with the production of benzyl acid and CO2. Our results show that formaldehyde can easily combine atomic oxygen to form a CH2O…O complex on both Au2Pd/(111) and AuPd/(111) surfaces, as shown in Figure 11. Taking the reaction process on Au2Pd/(111) for example, the formed CH2O…O complex either convert to formic acid by overcoming an activation barrier of 0.90 eV or convert to CO2 and H2 with an activation barrier of 1.12 eV. Moreover, the two pathways are highly exothermic by –2.45 eV and –2.08 eV, respectively. Therefore, activation of O2 via atomic oxygen is adverse for selectively generating desired aldehydes, which is also the drawback of pure Pd(111) surface. Fortunately, the direct dissociation of O2 can be effectively suppressed on Au2Pd/(111) surface owing to the less active Au atoms on the surface. The dissociation barrier is increased to 1.78 eV on Au2Pd/(111) from 0.58 eV on Pd(111), as displayed in Figure 10b. This result suggests that the deep oxidation of aldehydes would be retarded on Au2Pd/(111).


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Figure 11. Optimized structures and energy profile for the reaction between CH2O* and *O to form HCOOH (black curve) and CO2 + H2 (blue curve) on the Au2Pd/(111) and AuPd/(100) surfaces. Considering the above two aspects, Au2Pd/(111) surface is able to overcome the drawbacks of pure Au(111) and Pd(111) surfaces, i.e., Au2Pd/(111) promotes O2 activation along the OOH-pathway and blocks O2 dissociation. Of particular significance is the fact that the OOH species is more suitable than atomic oxygen for the selective oxidation of alcohols to aldehydes because of its moderate oxidative ability.100 Even though *OOH can also dissociates into *O and *OH surface radicals, it requires to overcome a barrier of 1.18 eV, which is much higher than the barriers (0.20–0.25 eV) of direct dehydrogenation by *

OOH. From this point of view, AuPd/(100) surface is not as good as Au2Pd/(111) in

boosting the selectivity to aldehydes due to its facile production of atomic oxygen through the dissociation of O2 and *OOH (Table 4). Furthermore, Pd and Au take different functions in methanol oxidation reaction. From our optimized geometries, Pd atoms serve as the reactive centers for anchoring the species in most of the elementary steps, whereas much less-active gold atoms can assist the desorption of formaldehyde into gas phase. The more favorable desorption of CH2O rather than further dehydrogenation can be supported by the much lower desorption barrier (0.20 eV) of CH2O than its dehydrogenation barriers (higher than 0.80 eV, Table S2) on Au2Pd/(111). Such a synergetic effect between Pd and Au accelerates the reaction rate of partial oxidation of alcohols and inhibits the deep oxidation of aldehydes. This strategy forms one of the important advantages of bimetallic catalysts in tuning the catalytic performance.


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CONCLUSIONS

We have performed a mechanistic study of the partial oxidation of methanol to formaldehyde on Au-Pd bimetallic surfaces by computational modeling. It is found that alloying pure Au surface with Pd notably improves the adsorption of molecular O2. The second-neighbor Pd monomer pair is identified as a critical ensemble to facilitate the coadsorption of O2…CH3OH and O2 activation along the OOH-pathway. With the involvement of *OOH and its decomposed fragments (*O and *OH), oxidative dehydrogenation of methanol to formaldehyde can be facilely achieved on Au-Pd surfaces with low activation barriers. By analyzing the results on Au2Pd/(111), Au(111), and Pd(111) surfaces, we have elucidated the synergetic effects between Au and Pd as follows: (1) Au and Pd cooperatively promote O2 activation along the OOH-pathway and block O2 dissociation, which is vital for boosting the activity and selectivity of Au-Pd catalysts in selective oxidation reactions; (2) Au-Pd exhibits bi-functions in the methanol oxidation reaction: the more active Pd atoms serve as a reactive center and the less-active Au atoms favor the removal of product from the surface. These results provide a mechanistic understanding of the selective oxidation of alcohols to aldehydes on Au-Pd catalysts and unravel the synergetic effect between Au and Pd. Understanding of the Pd-Au synergetic effects might help rational design of better bimetallic catalysts.

ASSOCIATED CONTENT Supporting Information Supplementary data as noted in the text. This material is available free of charge via the Internet http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Key Basic Research Special Foundations (2011CB932400), the China Postdoctoral Science Foundation (2014M562391), and the ACS Paragon Plus Environment

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Fundamental Research Funds for the Central Universities. The calculations were performed by using supercomputers at the Computer Network Information Center, Chinese Academy of Sciences, Tsinghua National Laboratory for Information Science and Technology, and the Shanghai Supercomputing Center. We thank the anonymous referees for his/her valuable comments and constructive suggestions to the manuscript.

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Theoretical Studies on the Synergetic Effects of AuâPd Bimetallic

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