Role of Ni in Bimetallic PdNi Catalysts for Ethanol Oxidation Reaction

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Role of Ni in Bimetallic PdNi Catalysts for Ethanol Oxidation Reaction Bei Miao, Zhi-Peng Wu, Minhua Zhang, Yifei Chen, and Lichang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05812 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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The Journal of Physical Chemistry

Role of Ni in Bimetallic PdNi Catalysts for Ethanol Oxidation Reaction

Bei Miao,†,‡ Zhi-Peng Wu,†,‡ Minhua Zhang,†,‡ Yifei Chen,†,‡ and Lichang Wang†,‡,$,* †

Key Laboratory of Ministry of Education for Green Chemical Technology, R & D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China. ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China. $ Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, Illinois 62901, United States.

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Abstract Bimetallic PdNi catalysts have garnered great interest in the study of ethanol oxidation reactions (EORs), though mechanistic insights into their catalytic performances are lacking, which hinders further improvement and rational design of the next generation of PdNi catalysts. As such, DFT calculations were performed for six key elementary reactions using four model catalysts, one with pure Pd and three for PdNi. DFT results indicate that the reduced catalytic activities observed experimentally when Ni atoms were placed under Pd layers is the result of an increase in the reaction barrier for CH3COOH formation. Further analysis illustrated that this is largely owed to the charge transfer from the Ni to the Pd atoms. On the other hand, the enhanced activities of the PdNi catalysts with respect to pure Pd catalysts in EORs when Ni atoms are exposed at the catalyst surfaces are due to the lowering of the reaction barrier toward C-C bond cleavage and increasing of that toward C-O bond coupling. Therefore, surface Ni atoms are responsible for the superior activity of the PdNi catalysts in EORs. Further analysis of DFT results suggests that the reaction barriers of the C-C bond cleavage and the C-O bond coupling approach similar values when the composition of surface Ni atoms in a PdNi catalyst reaches about 44%. To achieve a complete EOR, the estimated surface Ni atoms should be as high as 77%. However, stability may become a concern for catalysts with such a high exposure of Ni atoms at the catalyst surface.

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1. INTRODUCTION Bimetallic Pd-based catalysts have been studied extensively due to their promising performances towards ethanol oxidation reactions (EORs).1-6 The often used second component of bimetallic Pd-based catalysts includes Ag,2, 7-9 Au,3, 10-13 Cu,5, 14 as well as Ni.15-27 Among these secondary elements, Ni has attracted much attention because of its combined outstanding advantages in cost and performance. For instance, Maiyalagan et al.19 examined the catalytic activity of Pd3Ni for EORs and found that the onset potential is 200 mV lower than that of pure Pd catalysts and the peak current density is 4 times higher. In the work by Zhang et al., the onset potential using Pd4Ni5 catalysts was found to be 180 mV lower than Pd, in which the Pd4Ni5 catalysts were prepared via a solution phase-based method.20 The favorable effect of Ni within PdNi catalysts toward EORs was also confirmed by others.21-25 Furthermore, when Shen et al.26 changed the anode catalysts of the direct ethanol fuel cells (DEFCs) from Pd/C to Pd2Ni3/C, they found that the Pd2Ni3/C catalysts led to an obvious increase in the cell performance, however, the selectivity of CO2 did not improve. For the role of Ni in PdNi catalysts towards EORs, there were two explanations. The first is that Ni provides more OHad species on the catalyst surfaces since Ni is an oxyphilic element and therefore has the capacity to provide an oxygen source such as OHad at low potential. The generated OHad would facilitate the oxidative desorption of the intermediates and thus accelerate EORs.16, 28-29 Therefore, the sufficient number of OHad species on a catalyst’s surface is responsible for the improved activities of PdNi catalysts for EORs. The second 3



explanation is that the addition of Ni reduces the CO poisoning of Pd catalysts. For instance, Rostami et al.30 concluded that Ni improved the poison tolerance of Pd/C catalyst by removing the adsorbed CO-like intermediate species during ethanol oxidation, which was also supported by others.31-32 Interesting observations were made by del Rosario et al.16 that the catalytic activities of PdNi with respect to pure Pd depend strongly on the positions of Ni. When Ni was placed beneath Pd, the activities of such PdNi catalysts decreased with respect to the pure Pd catalysts. In contrast, when Ni was placed above Pd, the catalytic activities increased. Despite the extensive experimental studies of PdNi catalysts towards EORs, mechanistic insights into their catalytic activities is still lacking but is in great demand for further improvement and better design of future generations of PdNi catalysts. As such, we performed Density Functional Theory (DFT) calculations for a better understanding of the activities of PdNi catalysts toward EORs. Specifically, monometallic Pd and three bimetallic model PdNi catalysts were investigated to determine their activities towards the scissions of C-C and C-H bonds and the coupling of the C-O bond to form CH3COOH using DFT calculations. There are more than 128 C2 and 21 C1 potential intermediates in the reaction network of EOR.6, 33-34 Therefore, it is necessary to select the most important reactions in this work. Yang et al.35 studied EOR on Pd surfaces in alkaline media using in situ Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS). Their results indicate that ethanol follows successive dehydrogenation at Cα to form adsorbed

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acetyl (CH3CO*), which is a pivotal intermediate that can be oxidized to CH3COOH by the adsorbed OHad on the surface. The elementary step of CH3COOH formation is: CH3CO* + OH* → CH3COOH* + *

(R1)

This reaction (R1) was also confirmed by Monyoncho et al.6, 36 In addition, Hibbitts et al.37 calculated the reaction barrier of this elementary step on Pd(111) surface to be 0.18 eV. The mechanism of CH3COOH formation via R1 is also widely accepted in other studies.15, 38-39 Therefore, the intermediate CH3CO* was also regarded here as the precursor to CH3COOH formation and, as such, the R1 reaction on both Pd and three PdNi model catalysts was studied. The scission of the C-C bond of ethanol on a Pd electrode was also observed in the study by Yang et al.35 They found that the surface ethanol molecules would first dehydrogenate into the adsorbed CH3CO* species, which were detected earlier than the COad species. More importantly, the band intensity of the adsorbed CH3CO* gradually diminished, which was accompanied by increasing COad coverage. As such, in addition to being oxidized into CH3COOH, the intermediate of CH3CO* is also likely the starting intermediate leading to COad and CHx through C-C bond cleavage. The pivotal role of CH3CO* was highlighted again in the work by Monyoncho et al.,6, 36 in which the CH3CO* was thought to either undergo C-C bond cleavage or be oxidized by OHad into CH3COOH at low potential. The latter will present a distinct advantage at higher potential. The cleavage of C-C bond in CH3CO* can be described as: CH3CO* + * → CH3* + CO*

5


(R2)


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The calculated barrier of R2 on Pd(110) by Guo et al.s40 is 0.63 eV. Li et al.34 systematically investigated the reaction network of EOR over Pd(111) through DFT calculations. Based on their study, the most likely decomposition pathway of EOR from CH3CO* is via CH3CO* → CH2CO* → CHCO* followed by the C-C bond cleavage of CHCO* with an activation barrier of 0.91 eV. As such, the C-C bond cleavage of both the CH2CO* and CHCO* were also studied in this work, which are depicted below as R3 and R4, respectively. CH2CO* + * → CH2* + CO* CHCO* + * → CH* + CO*

(R3) (R4)

Based on the calculated reaction network for the EOR,34 energy barriers for the C-C bond scission decreased along the decomposition pathway, while the barriers insignificantly changed for Cβ-H bond cleavages. Hence, there is a competitiveness between the C-C and Cβ-H bond cleavages. Therefore, the Cβ-H bond cleavage of CH3CO* and CH2CO* were also considered in our work. These elementary reactions are described as R5 and R6, respectively. CH3CO* + * → CH2CO* + H* CH2CO* + * → CHCO* + H*

(R5) (R6)

Hence, six key elementary steps that involve the CH3COOH formation, three C-C bond scissions, and two Cβ-H bond cleavages were studied on one Pd and three PdNi catalysts using DFT calculations in this work to illustrate the role of Ni in PdNi catalysts toward more complete EORs. 2. COMPUTATIONAL DETAILS 2.1. Model Catalysts In this work, four model catalysts –one for pure Pd and three for PdNi –were designed. For the pure Pd catalyst, Pd(111) was chosen and represented by using a periodic 4×4 unit 6


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cell with four layers, in which the bottom two layers were fixed in their bulk positions, while the uppermost two layers and the adsorbates were fully relaxed without any constraints (Figure 1 (a)). This work also allowed us to compare with reported DFT studies on the EOR on Pd catalysts.37 More detailed descriptions on simulation details are given in section 2.2. Our optimized lattice parameter of Pd bulk was found to be 3.891 Å, which is close to the values reported in literatures.41-43 The bond length between two Pd atoms in the same layer is 2.751 Å.

Figure 1. The top (left) and side (right) views of the calculated structures of Pd(111) (a) and PdNi(111) (b), (c), (d). Pd and Ni atoms are shown in cyan and pink, respectively.

For the bimetallic PtNi catalysts, it is well known that the chemical composition at the surface may differ from that in the bulk. In other words, one of the components may be 7



enriched on the surface of a catalyst.5,

44-46

Therefore, three PdNi(111) models were

constructed in this work (Figure 1 (b), (c), and (d)). The surface of PdNi(111)-A is completely covered by Pd atoms while all of the Ni atoms were placed in the second layer (Figure 1 (b)). For PdNi(111)-B model, the Pd and Ni atoms are uniformly distributed in the uppermost two layers (Figure 1 (c)). In the PdNi(111)-C, the surface is completely covered by Ni atoms (Figure 1 (d)). We note that models PdNi(111)-A and PdNi(111)-C are similar to the layered catalysts being studied experimentally by del Rosario et al.16 A periodic structure of a 4×4 unit cell and four layers were used for all three models. Furthermore, the bottom two layers of Pd atoms were frozen, while the atoms on the top two layers and adsorbates were fully relaxed with any constraints. In addition, spin unrestricted DFT calculations were performed and the direction of up high spin state was included for the Ni atoms in all three PdNi models. The total energies of these three model PdNi catalysts were obtained. The DFT results show that the most stable model is A, followed by model B, which is 2.75 eV higher in energy than model A. The least stable structure is model C, which is 7.29 eV higher than model A. These results are consistent with the reported work. For PdNi bimetallic catalysts, it was found that the higher cohesive energy of Ni and the lower surface energy of Pd, together with the smaller size of Ni, caused Pd to separate out to the surface.24, 47-48 Wei et al.28 prepared PdNi catalysts for EORs in alkaline electrolytes through a deposition-precipitation route. Then, the Pd and Ni coexistent surface species were found to gradually convert to only Pd, i.e. a formation of Pd skin. In theoretical studies, Zhang et al.49 compared the total 8


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energies between PdshellNicore and PdcoreNishell obtained from the spin-polarized DFT calculations. Their results revealed that the total energy of the latter is 40.6 eV higher, which further validated the enrichment of Pd on the surface. These results, as well as our own, all address the concern for the stability of PdNi(111)-C, should it be synthesized and used in EORs. Finally, we mention that the similar sandwich structure of PdNi(111)-A was also employed in the work by Zhang et al.41 and Hou et al.50 in their studies on other reactions.

2.2. Simulation Details The PBE functional was used in our spin-polarized DFT calculations, which were performed using the DMol3 package in the Materials Studio.51-53 The global orbital cut-off energy of 4.0 Å was used for both Pd(111) and the three PdNi(111) models. To improve the computational efficiency, the DFT Semi-core Pseudopots (DSPP) was employed, together with a double-numerical basis set with polarization functions (DNP). The K-point sampling was carried out using the method developd by Monkhorst and Pack54 with a 3×3×1 grid. 0.109 eV/Å, 5.44×10-4 eV, and 0.005 Å were the convergence criteria for the force, energy, and displacement, respectively. The Self-Consistent-Field (SCF) density convergence threshold was set to 2.72×10-4 eV. To test the accuracy of the results using the above medium criteria, we performed geometry optimization calculations using fine conditions, such as the force was set to be 0.05 eV/ Å, and the results are provided in Table S1 of the supporting information. The comparison of results obtained using two criteria shows that the reaction energy or barrier differs by 0.01-0.02 eV. Furthermore, the coordinates and total energy of the optimized systems are also provided in Tables S2-S49 as part of the supporting information. 9



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Transition states searches were performed using a generalization of the linear synchronous transit (LST) method for periodic systems combined with a quadratic synchronous transit (QST) method. Only one imaginary frequency was permitted in each TS after the searching process, which was characterized by vibrational analysis. We also mention that the convergence conditions and transition state search used here were also employed in our previous studies of EORs on Cu55 and CuO56 surfaces. Based on these DFT results, we calculated the adsorption energies of CH3COOH*, CH3CO*, CH2CO*, and CHCO*, and the co-adsorption energies of CH3CO* + OH*, CH3* + CO*, CH2* + CO*, CH* + CO*, CH2CO* + H*, and CHCO* + H*. These adsorption and co-adsorption energies, () , were calculated by:

() = (/ ) + ∑

− () + ∑

− ( ) + ∑

,

(1)

where (/ ) is the total energy of X adsorbed on the model M catalyst (M = Pd, PdNi-A, PdNi-B, and PdNi-C), () is the total energy of X in gas phase, and ( ) is the total energy of the clean surface without any adsorbed species. is the frequency of the jth vibrational modes of each adsorption configuration, and ℎ is Planck’s constant. By this definition, a positive value () means an endothermic adsorption process while a negative value means exothermic, and a more negative value suggests a more stable adsorption. The reaction barrier (∆ ) and reaction energy (∆ ) of a studied elementary step were also obtained based on the following:

∆ = + ∑

- + ∑

,

10


(2)

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∆ = + ∑

- + ∑

,

(3)

where , , and denote the energies of the transition state, the initial state, and the final state, respectively. is the frequency of the jth vibrational modes of the transition state, initial state, and final state, respectively. The d-band center ( ) of the transition metals was also evaluated by:

= where

(, ") # , (, ")#

(4)

(, ") is the density of the d-states at energy E and position r.

3. RESULTS AND DISCUSSION In this section, we will first present the DFT results on the studies of six reactions, R1R6, on the model catalyst PdNi(111)-A. Secondly, we will explore the role of Ni in PdNi-A catalyst toward EORs, which is accomplished through the comparison of DFT results with Pd(111). Furthermore, the role of Ni will be explained according to electronic analysis. Thirdly, we will present the DFT results on the scission of the C-C bond cleavage and the generation of CH3COOH on two PdNi models, PdNi(111)-B and PdNi(111)-C to further explore the correlation between the properties of a catalyst and its activities toward EORs. Finally, we will briefly describe the effect of potential on the studied reactions.

3.1. Mechanistic Studies of Six Key Elementary Reactions of EORs on PdNi(111)-A Six elementary reactions (R1-R6) that are key to the EOR on the model PdNi catalyst, i.e. PdNi(111)-A (Fig. 1(b) were investigated using DFT calculations. The structures and key geometric parameters obtained from these calculations are shown in Figure 2. 11



PdNi-A-R1-IS

PdNi-A-R1-TS

PdNi-A-R1-FS

PdNi-A-R2-IS

PdNi-A-R2-TS

PdNi-A-R2-FS

PdNi-A-R3-IS

PdNi-A-R3-TS

PdNi-A-R3-FS

PdNi-A-R4-IS

PdNi-A-R4-TS

PdNi-A-R4-FS

PdNi-A-R5-IS

PdNi-A-R5-TS

PdNi-A-R5-FS

12


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PdNi-A-R6-IS

PdNi-A-R6-TS

PdNi-A-R6-FS

Figure 2. The initial state (IS), transition state (TS), and final state (FS) of R1 - R6 on PdNi(111)-A surface. The key bond parameters are given in Å. Color Code: H atom (white ball); O atom (red ball); C atom (gray ball); Pd atom (cyan ball); Ni atom (pink ball).

For the formation of CH3COOH (R1), as seen from Figure 2, the CH3CO* is initially adsorbed at the atop site of one Pd atom, while OH* at the bridge site of two Pd atoms. At this co-adsorption state, the O atom of the OH* inclined toward the carbonyl C atom of CH3CO* with a distance of 2.850Å, which allows coupling between the O and C atoms to easily occur. Then, the adsorbed CH3CO* and OH* species moved closer to each other with the distance between the C and O atoms reducing to 1.723Å at the transition state. Finally, the generated CH3COOH* adsorbed vertically at the surface of PdNi(111)-A via a O-Pd bond. Accordingly, the newly formed O-C bond was found to be 1.340Å. For the C-C bond cleavage of CHxCO* (x=3,2,1) through R2, R3 and R4 on PdNi(111)-A, Figure 2 shows that at the initial state, all of the carbonyl C atoms of CHxCO* (x=3,2,1) preferably adsorbed at the atop site with only one C-Pd bond. However, the Cβ atom of the CHxCO* (x=3,2,1) had different configurations that depended on the number of H atoms in CHxCO*. More specifically, there was no bond between the Cβ atom of CH3CO* and the surface Pd atom, while one bond for CH2CO* and two bonds for CHCO*. Li et al. studied the decomposition of ethanol on a Pd(111) surface.34

The adsorption of the involved

intermediates in their work tends to follow the gas-phase bond order rules, wherein the C atom is tetravalent with missing H atoms replaced by metal atoms. The most stable adsorption structures of CHxCO* (x=3,2,1) on the PdNi(111)-A surface in this work verified these gas-phase bond order rules. The initial distances of the C-C bond in CHxCO* (x=3,2,1) 13



were 1.511, 1.473, and 1.385Å, respectively. At the transition state, the CHx* (x=3,2,1) and CO* fragments moved farther away from each other, stretching the C-C bonds that would then be broken. Then, in the final state, the CH3* fragment adsorbed at the atop site and the CH2* fragment at the bridge site, while the CH* fragment adsorbed at the hollow site. For the Cβ-H bond cleavage in CHxCO* with x=3 and 2 (R5 and R6), Figure 2 depicts the dehydrogenation processes on the PdNi(111)-A catalyst. We can see that at the initial state, the most stable configurations of CH3CO* and CH2CO* were the same as those of the C-C bond breaking. At the transition state, one of the H atom dissociated from the CHxCO* (x=3,2) and moved to the catalyst surface. Simultaneously, the newly generated CHx-1CO* (x=3,2) species adjusted to a more stable adsorption configuration. Lastly, the CHx-1CO* (x=3,2) stably adsorbed on the surface with the same configuration as if they were adsorbed alone, while both the H atoms in R5 and R6 adsorbed at the hollow site. Furthermore, the corresponding reaction barriers and reaction energies of R1- R6 on PdNi(111)-A are provided in Table 1, together with the adsorption/co-adsorption energies of reactants and products of the studied reactions. To compare the catalytic properties of PdNi(111)-A toward EORs, we plotted the energy diagrams of R1-R6 in Figure 3. For convenience, these six reactions were divided into three stages. The first was the CH3CO* stage, which includes the C-O bond coupling between CH3CO* and OH* (R1), the C-C bond cleavage of CH3CO* (R2), and the Cβ-H bond cleavage of CH3CO* (R5). The second was the CH2CO* stage that is composed of the C-C bond scission of CH2CO* (R3) as well as the

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Cβ-H bond cleavage of CH2CO* (R6). The last CHCO*stage included the C-C bond breaking of CHCO* (R4) alone.

Table 1 The reaction barriers ( ∆ ) and reaction energies ( ∆ ) of R1 ~ R6, and the adsorption/co-adsorption energies of reactants ($% ) and products ($& ) on the PdNi(111)-A surface

PdNi(111)-A Reaction

Eads-R

Eads-P

△Ea

△E

(eV)

(eV)

(eV)

(eV)

R1

CH3CO*+ OH* → CH3COOH* + *

-7.38

-3.80

0.43

-1.34

R2

CH3CO* → CH3* + CO*

-4.69

-5.32

1.65

0.28

R3


-3.15

-6.71

1.59

0.31

R4

CHCO* → CH* + CO*

-4.56

-8.40

1.43

0.17

R5

CH3CO* → CH2CO* + H*

-4.69

-5.58

1.37

0.80

R6

CH2CO* → CHCO* + H*

-3.15

-6.97

1.42

0.55

Figure 3 The energy diagrams of R1 - R6 on the PdNi(111)-A catalyst. Pink: C-O bond coupling (R1); Blue: C-C bond cleavage (R2, R3, R4); Red: Cβ-H bond cleavage (R5, R6);) 15



The DFT results shown in Table 1 illustrated that, on the surface of PdNi(111)-A, the reaction energies of both the C-C bond cleavage (R2, R3, R4) and Cβ-H bond cleavage (R5, R6) were positive, meaning these C-H and C-C bond breaking reactions are endothermic processes. However, the formation of CH3COOH (R1) has a negative reaction energy value of -1.34 eV, revealing its thermodynamic advantage. Furthermore, for the reaction barriers, we can see more clearly in Figure 3 that, the three barrier values at the CH3CO* stage were quite different (R1, R2, R5), while two barriers are similar to each other at the CH2CO* stage (R3, R6). At the CH3CO* stage, the reaction barrier of CH3COOH formation was much lower than the C-C and Cβ-H bond cleavages, signifying the thermodynamics advantage of R1. Therefore, for the intermediate of CH3CO*, it was much more easily oxidized to CH3COOH on the PdNi(111)-A catalyst, which is the dominant incomplete oxidation products of EOR. In other words, for EOR on PdNi(111)-A catalyst, both the C-C and Cβ-H bonds were more difficult to break with higher reaction barriers and positive reaction energies as compared to the CH3COOH formation. Shen et al.26 analyzed the products of EOR on a Pd2Ni3/C catalyst and found that the selectivity of CH3COOH was still much higher compared to pure Pd/C catalyst. The DFT results in this work confirmed that the high selectivity of CH3COOH on the PdNi catalyst in the work reported by Shen et al.26 may still be due to both the thermodynamics and dynamic advantages of R1 and that their catalyst surface is Pd-rich.

2.2. Impact of a Pd Skin in PdNi Catalysts on EORs 16


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To explore the relative catalytic activities of the bimetallic PdNi-A catalyst with respect to the pure Pd catalysts, we studied the same reactions as those on PdNi-A, namely, the catalytic activity of pure Pd(111) towards R1 - R6. The Pd(111) model is shown in Figure 1 (a). The DFT results of the studied reactions on Pd(111) are depicted in Figure 4 with the key parameters provided. In addition, Table 2 also summaries the reaction barriers and reaction energies of R1- R6 on Pd(111),as well as the adsorption/co-adsorption energies of reactants and products of the studied reactions.

Pd(111)-R1-IS

Pd(111)-R1-TS

Pd(111)-R1-FS

Pd(111)-R2-IS

Pd(111)-R2-TS

Pd(111)-R2-FS

Pd(111)-R3-IS

Pd(111)-R3-TS

Pd(111)-R3-FS

Pd(111)-R4-IS

Pd(111)-R4-TS

Pd(111)-R4-FS

17



Pd(111)-R5-IS

Pd(111)-R5-TS

Pd(111)-R5-FS

Pd(111)-R6-IS

Pd(111)-R6-TS

Pd(111)-R6-FS

Figure 4. The initial state (IS), transition state (TS), and final state (FS) of R1 - R6 on Pd(111) surface. The key bond parameters are given in Å. Color Code: H atom (white ball); O atom (red ball); C atom (gray ball); Pd atom (cyan ball).

First, not surprisingly, the optimized structures on the Pd(111) surface are very similar to those obtained by Li et al.34 Furthermore, by comparing Figure 4 with Figure 2, one can see that, in all of the studied reactions the same configurations were present on Pd(111) and PdNi(111)-A catalysts. This is also not surprising due to the existence of the Pd skin, which means the presence of Ni in PdNi-A bimetallic catalyst does not affect the adsorption configurations significantly in this model bimetallic PdNi catalyst. Second, for the energies, the comparison between the reactions on PdNi(111)-A and Pd(111) shown in Table 1 and Table 2 illustrated that almost all of the adsorption/co-adsorption energies change to more negative values on Pd(111), although they have the same adsorbed configurations. In other words, the involved species have more stable adsorption on the Pd(111) surface than on the PdNi(111)-A. The Ni atoms beneath the Pd layer have weakened the adsorption of the intermediates on the catalyst surface. Goda et al.57 calculated the adsorption energies on other

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reactions using DFT and found that the adsorption energy values of the hydrogen, ethylene, and acetylene on PdNi(111) alloy also become smaller than those on Pd(111).

Table 2 The reaction barriers ( ∆ ) and reaction energies ( ∆ ) of R1 - R6, and the adsorption/co-adsorption energies of reactants (Eads-R) and products (Eads-P) on the Pd(111) surface

Pd(111) Reaction

Eads-R

Eads-P

△Ea

△E

(eV)

(eV)

(eV)

(eV)

R1

CH3CO*+ OH* → CH3COOH* + *

-7.70

-3.77

0.19

-1.00

R2


-4.93

-5.96

1.63

-0.15

R3


-3.68

-7.47

1.57

0.08

R4

CHCO* → CH* + CO*

-5.23

-9.49

1.14

-0.24

R5

CH3CO* → CH2CO* + H*

-4.93

-6.34

1.43

0.28

R6

CH2CO* → CHCO* + H*

-3.68

-7.85

1.42

0.21

Furthermore, comparing the reaction barriers in Table 2 with the corresponding barriers in Table 1, the most obvious distinction should be the C-C bond cleavage of CHCO* (R4) and the CH3COOH formation (R1). For the C-C bond cleavage of CHCO*, the reaction barrier of R4 was increased to 1.43 eV on PdNi(111)-A surface from the value of 1.14 eV on Pd(111) surface. Besides, this C-C breaking reaction was an endothermic process on PdNi(111)-A while exothermic on Pd(111). As a result, for CHCO*, it more difficult to break the C-C bond on PdNi(111)-A catalyst than on Pd(111). Hence, the addition of Ni atoms 19



Page 20 of 38

beneath a Pd skin does not promote catalytic activity toward the C-C bond cleavage. On the other hand, for CH3COOH formation, the reaction barrier of R1 was as low as 0.19 eV on Pd(111), while increased to 0.43 eV on PdNi(111)-A. At the CH3CO* stage, the reaction barriers of both the C-C (R2) and Cβ-H (R5) bond scissions changed just slightly from Pd(111) to PdNi(111)-A catalysts. Therefore, the dynamics advantage of CH3COOH formation at the CH3CO* stage was reduced on the PdNi(111)-A catalyst. In summary, the direct roles of Ni in PdNi-A catalyst were mainly reflected in two aspects.

On

one

hand,

the

presence

of

Ni

atoms

underneath

weakens

the

adsorption/co-adsorption of the involved species on the catalyst surface; On the other hand, addition of the Ni atoms underneath a layer of Pd atoms inhibits the CH3COOH formation in comparison to pure Pd catalysts. Therefore, the current DFT results suggest that the reduced activities observed by del Rosario et al.16 are mostly due to the reduced catalytic activities towards CH3COOH formation when the Ni atoms were covered by Pd layers in bimetallic PdNi catalysts. As observed in catalysts containing more than one kind of transition metal, a large number of literature results have demonstrated that there is always an electronic effect between the metal components.15, 58-61 In other words, the electronic charge distributions of each metal in bimetallic or trimetallic catalysts differ from those when the metal is presented in the catalysts alone. To study the electronic effect in the PdNi(111)-A catalyst, we firstly investigated the charge distribution and the Mülliken charges. The results are depicted in Figure 5. The charge distribution in Figure 5 indicated charge decreases on the Ni layer while 20


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increases on the Pd layers, implying charge transfer from the Ni to the Pd atoms. The Mülliken charge analysis verified the positive charge of Ni atoms and negative of Pd atoms, especially the Pd atoms on the top layer. Therefore, our DFT results illustrated that there is indeed an electronic effect in the PdNi(111)-A catalyst and the charge is transferred from the Ni to Pd atoms, which was also concluded by Dutta et al.15

21



Figure 5. The calculated charge distribution (left) and the Mülliken charge (right) of the PdNi(111)-A (top); PdNi(111)-B(middle), and PdNi(111)-C (bottom) catalysts.

To further investigate whether this electronic effect contributes to the role of Ni atoms in PdNi(111)-A catalyst toward EOR, we plotted the d-orbital partial density-of-states (d-PDOS) of Pd and Ni atoms in PdNi(111)-A and that of Pd in pure Pd(111) and that Ni in pure Ni(111) in Figure 6. It is shown that both the curves of Pd (blue) and Ni atoms (red) in PdNi(111)-A catalysts are significantly changed from the pure metal catalysts. We point out that the electronic effect between the Pd and Ni mainly affect the valence electrons of these two elements, i.e. the d-orbital electrons. The d-band centers ( ) of Pd and Ni in different catalysts were also calculated using eq (4) and the results were also marked in Figure 6. The

of Ni in the pure Ni catalyst was -1.95 eV, while the value changed to -1.79 eV in the PdNi catalyst, which is closer to the Fermi level (EF=0). However, the tendency of Pd is opposite. The value of moved to -2.29 eV in the PdNi catalyst from -1.96 eV in the pure Pd catalyst. Further distance from the Fermi level (EF=0) of the valence electrons of the Pd element resulted in decreased activity and further implied a weaker interaction between the surface Pd atoms and the adsorbates. As such, weaker interaction between the Pd and the adsorbed intermediates resulted in the weaker adsorption on the PdNi catalyst surface compared with the pure Pd surface.

22


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Figure 6. The d-PDOSs and d-band centers, the vertical lines, ( ) of the Pd in the pure Pd catalyst, the Ni in the pure Ni catalyst, as well as the Pd and Ni in the bimetallic PdNi(111)-A (top); PdNi(111)-B (middle), and PdNi(111)-C(bottom) catalysts.

23



3.3 Role of the Surface Ni in the Enhanced Activities of PdNi Catalysts Although the electronic effect in the PdNi(111)-A catalyst influenced almost all of the involved intermediates of EOR studied here, the reaction barriers of R1 - R6 on the PdNi(111)-A and pure Pd catalysts did not show any major differences, except for the CH3COOH formation (R1) and the C-C bond cleavage from CHCO* (R4). This indicates that both reactions are most likely to be influenced by the Ni atoms. Therefore, we investigated the R1 and R4 reactions on the other two model PdNi(111) catalysts, PdNi(111)-B and PdNi(111)-C, whose surface Ni compositions are different from the PdNi(111)-A. The optimized reaction configurations of R1 and R4 on the PdNi(111)-B and PdNi(111)-C are depicted in Figure 7.

PdNi-B-R1-IS

PdNi-B-R1-TS

PdNi-B-R1-FS

PdNi-C-R1-IS

PdNi-C-R1-TS

PdNi-C-R1-FS

24


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PdNi-B-R4-IS

PdNi-B-R4-TS

PdNi-B-R4-FS

PdNi-C-R4-IS

PdNi-C-R4-TS

PdNi-C-R4-FS

Figure 7 The initial state (IS), transition state (TS), and final state (FS) of R1 and R4 on the PdNi(111)-B and PdNi(111)-C surfaces. The key bond parameters are given in Å. Color Code: H atom (white ball); O atom (red ball); C atom (gray ball); Pd atom (cyan ball); Ni atom (pink ball).

As shown in Figure 7, the adsorption configurations of both R1 and R4 are very different on the PdNi(111)-B and PdNi(111)-C, and they are also different from the PdNi(111)-A. For CH3COOH formation, at the initial state, the co-adsorption configurations of CH3CO*+OH* on the PdNi(111)-B is similar to that on the PdNi(111)-A, but both the CH3CO* and OH* preferred the adsorption site of Ni rather than Pd. At the finial state, CH3COOH* also favored the Ni site. When the surface was changed to PdNi(111)-C, the co-adsorption configuration of CH3CO*+OH* changed. On this surface, the OH* moved to the hollow site among the three Ni atoms and was adsorbed vertically with the O atom close to the surface. As for the adsorption of the CH3CO* species, the O atom of CH3CO* remained at the atop site, while the carbonyl C atom moved to the bridge site between two Ni atoms on the PdNi(111)-C from the atop site on the PdNi(111)-A and PdNi(111)-B. Next, for the C-C bond cleavage in CHCO*, at the initial state, both two C atoms of CHCO* adsorbed steadily at the bridge site but the orientation of the O atom differed. Furthermore, on the PdNi(111)-C, the most stable adsorption configuration of CHCO* was that both of the two C atoms bonded with three Ni atoms with two Ni atoms in common. The C-C bond was more significantly stretched at the 25



Page 26 of 38

transition state on the PdNi(111)-C. Both the CH* and CO* fragments adsorbed at the hollow sites on PdBi(111)-B. The newly formed CO* adsorbed at the atop site while the CH* species at the hollow site. For the reaction barriers and reaction energies of the C-C bond cleavage and C-O bond coupling to CH3COOH formation, Table 3 lists the energy values of R1 and R4. For PdNi(111)-A, there were no Ni atoms on the catalyst surface. On such surface, the barrier of CH3COOH formation via R1 was only 0.43 eV, while the value of the C-C bond cleavage via R4 was as high as 1.43 eV. When the catalytic surface was changed to PdNi(111)-B, the exposure of Ni atoms increased to 50%. In this case, the barrier of R1 increased to 0.82 eV, about two times of that on the PdNi(111)-A (0.43 eV). The barrier of R4 on the PdNi(111)-B is reduced to 0.62 eV, which is about half of that on the PdNi(111)-A (1.43 eV). More interestingly, when the exposure of Ni atoms on the catalyst surface was further increased to 100%, i.e. PdNi(111)-C, the reaction barrier of R1 more than doubled to 2.21 eV in comparison to that on the PdNi(111)-B (0.82 eV), while the barrier of R4 reduced by half again to only 0.38 eV.

Table 3 The adsorption/co-adsorption energies (Eads-R， Eads-P), reaction barriers (△Ea), and reaction energies (△E ) of CH3COOH formation (R1) and C-C bond cleavage (R4) on different model PdNi catalysts

Model Pd(111) PdNi(111) A

CH3CO*+ OH* → CH3COOH* + * (R1) Eads-R Eads-P △Ea △E (eV) (eV) (eV) (eV) -7.70 -3.77 0.19 -1.00 -7.38

-3.80

0.43

-1.34

CHCO* + * → CH* + CO* (R4) Eads-R Eads-P △Ea △E (eV) (eV) (eV) (eV) -5.23 -9.49 1.14 -0.24 -4.56

26


-8.40

1.43

0.17

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PdNi(111) B PdNi(111) C

-6.67

-2.28

0.82

-0.53

-3.62

-8.19

0.62

-0.55

-12.76

-6.56

2.21

1.28

-4.87

-9.26

0.38

-0.37

We used the data in Table 3 plotted the Brønsted–Evans–Polanyi (BEP) correlations for these two reactions, R1 and R4, in Figure S1. Unlike the results in EOR on Cu55 or other pure metals,62 no nice linear relationship was found. Furthermore, the reaction barriers in Table 3 showed that the barrier tendencies of R1 and R4 are opposite of one another. To make a more clear comparison between these two trends, the reaction barriers of CH3COOH formation and C-C bond cleavage were plotted in Figure 8 as a function of surface Ni composition. There is a rather nice relationship between the surface Ni composition on the catalyst surface and the catalytic activities of PdNi. For the C-O coupling to CH3COOH formation, the fitting line is y (eV) = 0.0002 x2 (%) -0.0022 x (%) + 0.43 (R2=1), while the C-C bond cleavage has a fitting as y (eV) = 0.0001 x2 (%) -0.0219 x (%) + 1.43 (R2=1), where y=△Ea and x=surface atomic Ni composition.

Figure 8 Reaction barrier for the C-O coupling to CH3COOH formation (R1) and the C-C bond cleavage from CHCO (R4) as a function of surface Ni composition on the PdNi catalysts.

27



The intersection of these two fitted lines in Figure 8 corresponds to a Ni surface composition of 44%, implying that the exposure amount of surface Ni atoms on the PdNi catalyst should be at least, if not more than 44% to induce significant C-C bond cleavage. In this case, both the reaction barriers of the C-C bond cleavage and the C-O bond coupling are 0.90 eV. Furthermore, taking the scission of Cβ-H bond into consideration, the barrier of the C-O coupling to CH3COOH formation on a PdNi catalyst should be higher than 1.4 eV, which was the value of the Cβ-H bond cleavage on the PdNi-A and pure Pd catalysts and did not significantly change on both catalysts. As such, the surface Ni composition should be more than 65% to achieve complete EORs. The more Ni atoms exposed on the catalysts surface, the better activities of the PdNi catalysts were toward complete EORs. Indeed, Carvalho et al.23 studied the catalytic activities of PdNi, and revealed that, with increasing Ni content, the electrochemical oxidation current increased while the onset potential was reduced. del Rosario et al.16 also constructed the PdNi bimetallic catalysts using a bilayer structure and investigated the activities of these catalysts toward EOR in alkaline media. They found that the NiPd catalyst, whose surface was covered by the Ni component, provided the best activity in comparison to the monometallic catalysts. The current density produced using NiPd at an applied voltage of -0.2 V (vs. Hg/HgO) was found to be about 18 times higher than that of the PdNi electrodes. The catalytic activities in these experimental studies were in agreement with our results. The reduced onset potential and the increased oxidation current, as well as the dramatic difference between the NiPd and PdNi performances should be due to the surface Ni atoms that promoted the cleavage of the C-C 28


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bond and thus released many more electrons while also inhibiting the C-O coupling to form CH3COOH. However, as our DFT results in section 2.1 indicated, when surface composition of Ni increases on the PdNi catalyst to around 77% ~ 100%, the stability of such catalysts decreases. Although stability has been a long-standing issue, mechanisms of causing the catalytic stability are complicated and are interesting work. Meanwhile, a suitable Ni composition should also be investigated to balance the selectivity and stability of such PdNi catalysts toward complete EOR. 3.4 Effect of Potential on the Reactions Being Studied The widely accepted mechanism for CH3CO* formation in EOR was via reactions R7 and R8, while the OH* species was produced by R9.16, 20, 27, 63 Therefore, these reactions are the precursors of the C-C and Cβ-H bond cleavages and the C-O bond coupling to CH3COOH formation listed as R1-R6. To further investigate the effect of potential applied in electrochemical environments on the studied reactions (R1-R6), we calculated the reaction energetics of R1 - R6 on the PdNi(111)-A under a potential of 0.20 V (vs. Hg/HgO), which was also used in the experiments, for instance by del Rosario et al.16 The calculated results are depicted in Figure 9. CH3CH2OH + * ↔ CH3CH2OH* -

(R7)

CH3CH2OH* + 3OH → CH3CO* + 3H2O + 3e -

OH + * → OH* + e

-

-

(R8) (R9)

29



Figure 9 The energy diagrams of R1- R6 from ethanol on the PdNi(111)-A under a potential of 0.20 V (vs. Hg/HgO). Pink: C-O bond coupling (R1); Blue: C-C bond cleavage (R2, R3, R4); Red: Cβ-H bond cleavage (R5, R6).

Figure 9 shows that after CH3CH2OH* is continuously dehydrogenated to CH3CO* via R8 and the energy of the generated CH3CO* was decreased to -0.60 eV, that of CH3CO*+OH* was -0.80 eV due to the potential applied. Under the potential of 0.20 V (vs. Hg/HgO), the reaction barrier of R1 on the PdNi(111)-A was 0.43 eV, which remains the same as when there was no potential. Thus the potential mainly influences the energy values of the species involved in R7, R8, and R9 and the relative engines of various species, but not the reaction energy and spontaneity of the reaction being studied here. Moreover, the comparison between Figure 9 and Figure 3 for the barriers of the C-C and Cβ-H bond cleavages demonstrated that, for bond cleavage reactions, the potential does not affect the activation energies since there was no electron transfer involved. The inclusion of a potential does not change the conclusions drawn in the previous sections. We note that the current inclusion of potential is a simplified treatment that has been used in other studies of EORs.36 The energy, which is 0.2 eV in this work, was added or subtracted to the step if a change of an electron was involved. More extensive studies, including solvent molecules, will be interesting and worthwhile for future work. We also 30


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point out that the goal of our DFT studies of EOR is to predict catalysts that can effectively eliminate the formation of CH3COOH. However, another important related research is the study of whether CH3COOH will be oxidized further to oxalic acid (HOOC-COOH), which may lead to different outcomes.

4. CONCLUSIONS Six reactions involving the C-C and Cβ-H bond cleavages in the EOR network as well as the C-O bond coupling in CH3COOH formation on three PdNi model catalysts and a pure Pd catalyst were investigated using DFT calculations to understand the role of Ni in bimetallic PdNi catalysts for EORs. The comparison between the DFT results from the pure Pd catalyst and those from the model catalyst PdNi-A, in which the Ni atoms were placed beneath the Pd atoms indicated that the presence of Ni atoms does not influence the configuration of the involved reaction intermediates, although their presence reduces the adsorption/co-adsorption energies of the adsorbates. More importantly, model catalyst PdNi-A prevents the formation of CH3COOH due to an increased reaction barrier (0.49 eV) with respect to the pure Pd catalyst (0.19 eV). Further analysis on charge distributions revealed that the charge is transferred from the Ni to the Pd atoms in the PdNi-A model catalyst. The charge density of the Pd atoms increases while the activity decreases, leading to a weaker interaction between Pd and the adsorbates on the catalyst surface.

31



The activities of bimetallic PdNi catalysts toward EOR correlate strongly with the exposure of Ni atoms on the catalyst surfaces. As the exposure of Ni atoms increases, the reaction barrier of the C-C bond scission decreases. More interestingly, the tendency toward CH3COOH formation is also reduced, which further benefits complete EORs. Furthermore, in relation to the composition of the surface Ni atoms, the C-C bond cleavage and C-O coupling exhibit a linear decrease and increase, respectively. When the surface Ni composition of the PdNi model catalyst is 44%, C-O coupling in CH3COOH formation and C-C bond cleavage have similar reaction barriers. To prevent CH3COOH formation, DFT results suggest that the composition of surface Ni atoms on a PdNi catalyst should be at least 77%. However, stability of such a PdNi catalyst may become an issue.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The coordinates and total energy of the optimized systems being reported (in PDF).

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] Notes 32


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The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the Tianjin 1000 talent program. LW acknowledges the sabbatical program at Southern Illinois University.

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Role of Ni in Bimetallic PdNi Catalysts for Ethanol Oxidation Reaction

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