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Origin of Enhanced Activities for CO Oxidation and O Reaction over Composition-Optimized Pd Cu Nanoalloy Catalysts 50

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Wei Zhang, Shiyao Shan, Jin Luo, Adrian C. Fisher, Chuan-Jian Zhong, Jiqin Zhu, and Daojian Cheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10814 • Publication Date (Web): 25 Nov 2016 Downloaded from http://pubs.acs.org on November 25, 2016

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The Journal of Physical Chemistry

Origin of Enhanced Activities for CO Oxidation and O2 Reaction over Composition-Optimized Pd50Cu50 Nanoalloy Catalysts Wei Zhang1,2, Shiyao Shan3, Jin Luo3, Adrian Fisher1, Chuan-Jian Zhong3*, Jiqin Zhu2*, and Daojian Cheng1* 1

International Research Center for Soft Matter, State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

2

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, China

3

Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902, USA

ABSTRACT It has been shown experimentally that Pd50Cu50 nanoalloy achieves the maximum activity for CO oxidation (COox) and oxygen reduction reaction (ORR) on composition-tuned PdCu bimetallic catalysts, but the origin of this catalytic synergy remains unclear. In this work, results of our density functional theory (DFT) calculations show that the weakest adsorption strength of O2 in terms of the most pronounced charge transfer between Pd and Cu is responsible for the experimentally-observed highest catalytic activity of Pd50Cu50 catalyst for both COox and ORR over a series of composition-tuned PdCu nanoalloys. For COox, the lowest barrier energy is attributed to the weakest adsorption strength of O2 on Pd50Cu50 catalyst. In ORR, the lowest barrier energy for O2 dissociation and also the weakest adsorption strength of O, OH, and OOH species are related to the weakest adsorption strength of O2 over the catalyst with a 50:50 ratio of Pd:Cu. Our work represents the first attempt to address an in-depth correlation between the theoretical and experimental data on the highly-active PdCu catalysts, the results of which has significant implications for the design of advanced nanoalloy catalysts with superior catalytic synergy in terms of the alloy compositions.

*

Authors to whom correspondence should be addressed. Electronic addresses: [email protected];

[email protected]; [email protected]; Fax: +8610-64427616

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INTRODUCTION Nanoscale catalysts containing non-platinum (non-Pt) metals such as palladium (Pd) offer intriguing opportunities for the design and preparation of stable, active, and low-cost catalysts. Unfortunately, the catalytic activity of Pd is much lower than that of Pt and Pt-based alloys. Recently, many studies have shown that the catalytic activity of Pd-based nanoalloy catalysts can be improved significantly by adding the second or third transition metal such as Fe, Co, Ni, Ir, Cu, Au, etc. into the alloy1-17 The investigation of the unique catalytic properties of Pd-based nanoalloys has been an important focus of recent experimental studies. For example, Pd-Cu nanoalloys with different bimetallic compositions are being increasingly explored as catalysts for several catalytic reactions, such as oxygen reduction reaction (ORR), CO oxidation (COox), oxidation of small organic molecules, methane combustion, and hydrogenation reactions.3-4,

7, 10, 13, 18-25

It is very interesting to note that PdCu

nanoalloys with Pd:Cu ratio close to 50:50 is shown to exhibit the maximum activity for COox.26 More interestingly, a similar phenomenon has also been reported for ORR on PdCu nanoalloys,27 where Pd50Cu50 nanoalloy is shown to display the maximum activity. Because of the complexity resulting from the adsorption characteristic of these reactants and the reaction mechanism, it is difficult to determine the exact reaction pathways and the active sites for COox and ORR on PdCu nanoalloys by experimental methods. Thus, the origin of the enhanced COox and ORR activities for composition-optimized Pd50Cu50 nanoalloys remains unclear, which presents a major challenge for the rational design of nanoalloy catalysts in terms of the alloy compositions. An important pathway to address the challenge is computational modeling, which offers an effective means for aiding the understanding of the catalytic mechanism on alloy catalysts, as demonstrated for many different bimetallic alloy systems, including limited examples on PdCu system. For example, computation of Cu-rich Pd-Cu bulk and surface alloys showed that CO adsorption strength was significantly enhanced due to a downshift of d band center of Pd.28 For CO bonding on PdCu surface alloys, the electronic structure of the nearby Pd sites was found to be less perturbed by the addition of Cu atoms.29 For ORR over a bimetallic PdCu system, Pd was found increase the adsorption strength of O atom on Cu whereas Cu was found to reduce that on Pd atom, which were a result of charge transfer between Pd 2


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and Cu.30 For PdCu alloy, the L11 surface structure in which the Cu and Pd atoms are ordered along the fcc [111] direction exposing a close-packed Pd surface to the reactants, was found to exhibit significantly-improved ORR kinetics.31 In addition, lattice strain effect on the surface reactivity of Pd on PdCu nanoalloys was also revealed by density functional theory (DFT) calculations.32 Despite these theoretical studies in terms of d band center shifts, molecule adsorption energies of CO and O, and/or lattice strains on PdCu alloy catalysts, there have been no theoretical studies of the reaction mechanism of COox and ORR on PdCu nanoalloys of different compositions in correlation with the experimental results. In particular, none of the prior studies has provided an insight for understanding the origin of the catalytic synergy on Pd50Cu50 nanoalloys.26-27 This understanding is essential for the design of new nanoalloy catalysts with superior catalytic synergy in terms of the alloy compositions. The work presented herein represents the first attempt to address an in-depth correlation between the theoretical and the experimental data of PdCu catalysts for COox and ORR, focusing on unraveling the origin of the high catalytic activities of Pd50Cu50 nanoalloy for both COox and ORR over composition-tuned PdCu nanoalloys. One of the common reaction characteristics for the two reactions (COox and ORR) is the importance of catalytic activation of O2. To unravel the detailed mechanisms, both the binging energies of O2, O, OH, and OOH oxygen intermediates and the barrier energies for COox and O2 dissociation reaction were calculated. It is found that the barrier energies for COox and O2 dissociation are closely related to the adsorption strength of O2, indicating that lower adsorption strength of O2 promotes the catalytic activity for COox and O2 dissociation reaction in ORR. In addition, the lower adsorption strength of O2 is shown to correlate with the lower adsorption strength of O, OH, and OOH intermediates, an activity descriptor for the ORR. Our results show that the weakest adsorption strength of O2 in terms of the most pronounced charge transfer between Pd and Cu is responsible for the maximum COox and ORR activities at 50:50 ratio of Pd:Cu among these nanoclusters. These theoretical results provide not only new insights for understanding the latest experimental data on CO oxidation and O2 reaction over the nanoalloy catalysts, but also useful guidelines for designing optimal nanoalloys with superior functionalities by controlling compositions. 3



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COMPUTATIONAL METHOD In order to identify the optimal PdCu nanoalloys from various compositions for COox and ORR, we calculated their reaction mechanisms on PdCu nanoalloys with different compositions. The barrier energies for COox are determinderd by a complex interplay of reactants and catalyst, which can be used to describe the COox activity.33-37 In general, two well-established reaction mechanisms, namely LangmuiHinshelwood (LH) and Eley-Rideal (ER), are considered for COox.36-40 Considering the weaker adsorption strength for O2 compared with the CO on PdCu nanoalloy surfaces, the LH mechanism for COox is considered in the process of producing the first CO2. the In contrast, there are three feasible mechanisms41-42 and seven elementary steps43-44 for the ORR, which make the calculation generally unaffordable if all the mechanisms and elementary steps are to be calculated. It is generally accepted that the trends for the ORR activity can be obtained by the analysis of barrier energies of O2 dissociation and the adsorption energies of the main reaction intermediates of adsorbed O, OH, and OOH, where low barrier energies of O2 dissociation and weak adsorption energies of O, OH, and OOH species would favor to the ORR.30, 45 DFT calculations were performed using the Plane-Wave Self-Consistent Field (PWSCF) code in the Quantum ESPRESSO package.46 All the calculations were carried out by the spin-polarized generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE)47 xc-functionals and ultrasoft pseudopotentials.48 Spin-polarized calculations were performed here by using values of 40 and 400 Ry as the energy cut-off for the selection of the plane waves for the description of the wave function and the electronic density, respectively. A cube supercell of size 30 × 30 × 30 bhor3 was employed in the calculations. The geometry of the cluster upon or without adsorption is optimized until the total energy is converged to 10-6 eV, and the forces on atoms in the obtained structures are 10-4 a.u. The first Brillouin zone was sampled at the Gamma-point and the electronic levels were broadened through a Gaussian smearing technique with a smearing parameter of 0.002 Ry.

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The adsorption energies of the O, OH, OOH, O2, and CO adsorbates were calculated according to the equation:

Eads = Etotal − Ecluster − Eadsorbate

(1)

Furthermore, the coadsorption energies of CO with the molecular or atomic oxygen were calculated as: Ecoads = Etotal − Ecluster − Eco − Eo2 ( or O )

(2)

In the above equations, Etotal , Ecluster , Eadsorbate , Eco and Eo2 ( or O) correspond to the optimized energies of cluster-adsorbate complexes, the bare cluster, the isolated adsorbate, gaseous CO and gaseous O2 (or O), respectively. The climbing-image nudged elastic band (CI-NEB) method49-50 was used to map out the minimum energy path (MEP) and obtain the transition state for COox and O2 dissociation. At least seven intermediate images were interpolated between reactant and product states for each elementary step. The transition state of the minimum energy pathway for each elementary step was confirmed by vibrational frequency calculations yielding a single imaginary frequency along the reaction coordinate

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.

The energies along the pathways were referred to the sum of energies of the noninteracting cluster and reactant(s), and the barrier heights Eb along them representing the activation energies were calculated as E b = E TS − E RS ,

(3)

where E TS and E RS are the total energies of the transition state and the connected to it equilibrated reactant cluster-adsorbate(s) complex, respectively.

RESULTS AND DISCUSSION Structure and Stability As stated, a major motivation for the computational work presented in this report was the latest experimental findings of the catalytic synergy of COox and ORR over PdCu nanoalloy catalysts with different compositions.26-27 The PdCu nanoalloys with 5



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different compositions were synthesized by wet chemical reduction of the two metal precursors in ethylene glycol. The average size of PdCu nanoparticles is ~3 nm. As shown by bulk ICP-AES and surface-sensitive XPS data, the surface and bulk chemical compositions of PdCu nanoalloys are rather similar, proving that the chemical composition of nanoalloys does not change significantly close to/at the NP surface. To simulate the nanoalloys properly and to avoid the unaffordable computational time, quasi-spherical nanoparticle models of the icosahedron (ICO) structure with 55 atoms were constructed, which is commonly found as the low energy and high internal strain configuration of free clusters.45, 52 The lowest-energy atomic ordering of PdCu nanoalloys with different compositions are obtained at the empirical potential level,53-56 and are then subjected to DFT local relaxation. Fig. 1a shows the configurations of Pd55, Pd43Cu12 (Pd77Cu23), Pd27Cu28 (Pd58Cu42), Pd12Cu43 (Pd25Cu75), and Cu55 clusters used in this work. Note that the compositions in parentheses correspond to the actual composition in the experimental work,26 which facilitate the verification of the activity of these clusters by DFT calculations in comparison with the activity of nanoalloy catalysts with similar composition measured in experiments. In the models, the Pd43Cu12 cluster possesses a three-shell onion-like structure,57 where Pd atoms are located on the surface, the Cu atoms are in the subsurface shell, and a Pd atom is located in the center; Pd27Cu28 cluster consist of 13 core Cu atoms and the a random PdCu alloy shell; For the Pd12Cu43 cluster, 12 Pd atoms occupy the vertex sites on the surface layer, in the so-called “crown-jewel” structure,58 as shown in Fig. 1a. The stability of the catalyst is a critical requirement for the catalytic reactions. To investigate the energetics and compare the relative stabilities of PdCu nanoalloys with different compositions, the mixing energy ( Emix ) is used for assessing the relative stabilities of nanoalloys with different compositions at a fixed size. The Emix at the DFT level is defined as59

n Pd55 m Cu55 Etot − Etot (4) 55 55 Cu55 is the total energy for a certain cluster, EtotPd55 and Etot are the Emix = EtotPd n Cum −

Pd n Cu m where, E tot

energies of the pure Pd55 and Cu55 clusters, respectively, and, n and m are the number of atoms of Pd and Cu, respectively, where n + m = 55 . Negative values of the 6


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mixing energies imply that the alloying is thermodynamically favorable.59 Figure S1 shows the mixing energy curves of 55-atom PdCu nanoalloys as a function of Cu concentration. It is found that negative values are observed for all selected PdCu nanoalloys, indicating it is favorable for alloying Pd with Cu. It can be noted that the Pd27Cu28 cluster in our study practically possesses the absolute minimum mixing energy, suggesting that the Pd27Cu28 cluster at 50:50 ratio of Pd:Cu has the best stability among these PdCu nanoalloys.

Adsorption of O2 Figure 1 shows all the possible adsorption sites for the adsorption of O2 on these PdCu nanoalloy clusters. For Pd55, Pd43Cu12, Pd12Cu43, and Pd55 clusters, the chemical ordering of the surface atoms features a certain regularity in terms of simple adsorption sites: T1 (on top of vertex), T2 (on top of edge), B1 (between one edge and one vertex), B2 (between two edges), H1 (on the hollow site among two edges and one vertex), H2 (on the hollow site among three edges), T-H (on the bridge site between T and H sites) including T1-H1, T2-H1, and T2-H2 three different sites, and B-B (on the bridge site between two B sites) including B1-B1, B1-B2,and B2-B2 three different sites, as shown in Fig. 1b. However, for the Pd27Cu28 cluster, the surface sites are random, including different T, B, and H sites. The facets of Pd27Cu28 cluster expose either 1 or 2 Cu atoms on the edge sites which are shared by the two kinds of facets. One Cu atom has no significant effect on the adsorption of reactant, while the adsorption sites are more complex in the facet which contains two Cu atoms. So we choose the facet which has relatively complicated adsorption sites as the reacting facet, see Fig. 1c. For O2 adsorption on B, T-H, and B-B sites, the molecule prefers a “side-on” configuration with both O atoms binding to Pd or Cu atom; on the T and H sites, O2 prefers an “end-on” configuration with one O atom binding to Pd or Cu atom,60 see Fig. 1d-e. The adsorption sites and the corresponding adsorption energies for the adsorption of O2 on these PdCu nanoalloy clusters are listed in Tables S1-2. For the adsorption of O2 on these PdCu nanoalloy clusters, the adsorption 7



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strength of the most preferred O2 adsorption configurations follows the order of Pd27Cu28 < Pd43Cu12 < Pd12Cu43 < Cu55 < Pd55. There appears to be no clear trend in this order as a function of the corresponding composition ratio. Pd27Cu28 cluster with a 50% Pd/Cu composition has the weakest adsorption strength among these PdCu nanoalloys. It is well known that the adsorption strength of O2 to the cluster is strongly impacted by the charge transfer from the 5d-metal to the 2π*-O2. Therefore, in order to get the origin of the unusual adsorption of O2 on the PdCu nanoalloys, we calculated spin polarized LDOS projected onto O2 and the partial density of electronic states (PDOS) projected on Pd atom for O2−metal system, as shown in Figure 2. Compared with the LDOS of the isolated O2 in the gas (see Fig. 2a), the adsorption of O2 on the surface of PdCu nanoalloys resulted in more filled 2π* components in the LDOS of O2 (see Fig. 2b-f). The adsorption strength of O2 to the metal atom is attributed to overlapping of the 2π* orbitals of O2 with the d orbitals of Pd (Cu) atom. Thus, the smallest orbital overlapping on the Pd27Cu28 indicates the weakest adsorption strength among these PdCu nanoalloys. In general, the number of the d-electron of the Pd and Cu atoms in PdCu nanoalloy clusters can also be used to understand the adsorption trends of O2 on these PdCu nanoalloys. Figure S2 shows the number of d-electron of the surface Pd and Cu atom in the PdCu nanoalloys, indicated by different colors. The exact numbers are also listed in Table S3. In general, gaining d-electron can promote the adsorption of the O2, while losing d-electron can hinder the adsorption of the O2. In order to use the d-electron model to evaluate the changes in the adsorption energy of O2 on these PdCu nanoalloys, we investigate the d-electron of the Pd and Cu atoms in the different position of PdCu nanoalloy clusters. It is well-known that Cu has a low electronegativity, so Pd can get electron from the Cu in PdCu nanoalloys. In addition, for pure Pd and Cu clusters, the Pd or Cu atoms located in the vertex position can get electron from the Pd or Cu atoms in the edge position. As shown in Figure S2, the d-electron of vertex Pd atom in pure Pd cluster is more than that in Pd43Cu12 cluster, so the adsorption strength of O2 on Pd43Cu12 is lower than that on pure Pd cluster. For Pd12Cu43 cluster, the surface Pd atoms reduce the d-electron of the Cu atom, so hinder the adsorption of O2 on the Pd12Cu43 cluster 8


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compared with the pure Cu cluster. For Pd27Cu28 nanoalloy, the d-electron of surface Pd atoms shows a slightly raised level by the introduction of Cu atoms, but the d-electron of surface Cu atoms achieves a significantly lowered level, which finally leads to the weaker adsorption strength of O2 on Pd27Cu28 nanoalloy compared with both Pd43Cu12 and Pd12Cu43 clusters. Notably, the origin of the different chemical reactivity on these PdCu nanoalloys with different composition could be due to the different degrees of charge transfer between Pd and Cu.61 In this work, we investigate the amount of the charge transfer between Pd and Cu by DFT calculations. Figure 3 shows the amount of the charge transfer between Pd and Cu and also the total number of Pd-Cu bonds as a function of Cu fraction. We can see from Figure 3 that the amount of the charge transfer between Pd and Cu on Pd27Cu28 (4.461 e) is bigger than that on Pd12Cu43 (4.109 e) and Pd43Cu12 (2.440 e) clusters. Accordingly, the total number of Pd-Cu bonds on Pd27Cu28 cluster is much more that on both Pd12Cu43 and Pd43Cu12. It can be concluded that the origin of the adsorption trends of O2 on these PdCu nanoalloys can be explained by the different amount of charge transfer between Pd and Cu. In general, the adsorption strength of O2 can be reduced with the increasing of the total amount of charge transfer between Pd and Cu among these PdCu nanoalloys. Thus, the Pd27Cu28 at 50:50 ratio of Pd:Cu possesses the weakest adsorption energy of O2 in terms of the most pronounced charge transfer between Pd and Cu among these PdCu nanoalloys.

CO oxidation In our latest experimental study,26 the catalysts of the PdCu/C nanoalloys (PdCu nanoalloy supported on carbon black) were activated by thermochemical treatment, i.e., heating first at 260℃under O2 for 30 mins and then either at 200 ℃ or 600 ℃ under H2 for another 30 mins. By determining the T10 values, i.e., the temperature at which 10 % CO conversion was achieved (light-off temperature) and the activation energy (Ea, kJ/mol) from Arrhenius plot, the catalytic activity were assessed as a function of the composition of Cu.26 A highly active catalyst for COox possesses the lowest T10 values and Ea. In this study26, the highest catalytic activity 9



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for COox over PdCu/C catalysts was found for the catalyst with a Pd:Cu ratio of ~50:50, an intriguing finding but its fundamental origin was unknown. In order to understand the origin of the catalytic synergy of PdCu nanoalloys for COox, we investigated the reaction mechanism of COox on PdCu nanoalloy clusters with different composition. For COox reaction, understanding the adsorption properties of CO is the first step. Also shown in Figure 1, the CO molecule prefers the “end-on” configurations (see Fig. 1f). Three kinds of surface site (T, B, and H) were calculated. The results are summarized in Tables S4-5. It is found that the optimal adsorption sites of CO varied with the PdCu composition. Cu-enriched cluster was in favor of T site, while pure Cu and Pd cluster, and Pd-enriched cluster were dominated by H site. For Pd:Cu~50:50, the favorable adsorption site was calculated to be B site. The prediction of optimal CO adsorption sites in the above DFT calculation on the Pd27Cu28 cluster was further evaluated over TiO2 supported Pd58Cu42 nanoalloy by Diffuse Reflectance Infrared Fourier Transform spectroscopy (DRIFTS).26 Although a quantitative interpretation is complicated in terms of CO coverage, dipole-dipole interaction, and alloy degree, bridge sites are believed to be more favorable in Pd:Cu ~ 50:50 ratio than other sites.26 This conclusion is supported by the DRIFTS data showing a greatly-enhanced peak intensity for CO adsorption on bridge sites at 1930 cm-1 than that on atop sites at 2079 cm-1. Table 1 lists the adsorption sites and the corresponding energies for CO on PdCu nanoalloy clusters at their optimal adsorption sites. From the calculation results we can know that the Pd55 cluster possesses the strongest ability for the adsorption of CO (-2.459 eV), while the Cu55 cluster binds CO the weakest (-1.154 eV). This difference was likely correlated with a strong Pd-CO adsorption energy as Pd% increases, in which CO adsorption energy is found to be linearly dependent on PdCu composition.28 This finding is consistent with the predominated role of Pd atom other than Cu in the adsorption of CO. The interaction between the d-metal and the p (or s)-adsorbate can used to describe the adsorption strength of adsorbate on metal surfaces, which proposed by Hammer and Nøskov.62-64 To use the interaction to evaluate the changes in CO molecule binding due to alloying, we investigated the 10


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hybridization between the d orbitals of Pd or Cu and the p orbitals of the C atom in the adsorbed CO molecule, as shown in Fig. S3b-f the hybridization is more obvious with the increasing of the content of Pd. So the adsorption strength of CO increases linearly with the increasing of the content of Pd. The PDOS of p-C of the isolated CO molecule (Fig. S3a) serve as a contrast to these CO-cluster systems. The influence of pressure and coverage on the reaction can be ignored if CO and O2 molecules both exit in the gas. When CO molecules occupy the catalytic active site on the catalyst surface, the adsorption and dissociation of O2 are not preferred steps. For CO oxidation, the first step is CO adsorption, which is followed by co-adsorption of CO and O2 with the bimolecular Langmuir–Hinshelwood (LH) mechanism.To search for the minimum energy paths for COox, we select the most stable co-adsorption configuration as an initial state, where CO and O2 molecules are co-adsorbed onto their respectively most preferable sites. Following the adsorption, CO and O2 move closer to form an intermediate state OCOO* with a C-O single bond and a peroxide O-O bond. This step is followed by crossing of the first energy barrier (Eb1). In the third step, the peroxide O-O bond dissociates into two O atoms by crossing the second energy barrier (Eb2). Table 2 shows the adsorption properties of the intermediate states and reaction barrier energies on these PdCu nanoalloy cluaters. For clarity, the potential energy profile and configurations for COox on Pd27Cu28 cluster are shown in Figure 4. Same reaction path for Pd55, Pd43Cu12, Pd12Cu43, and Cu55 clusters, see Fig. S4a-d, respectively. The calculated results indicate that the reaction barrier energies for the COox are determined by differences between the adsorption energy of intermediates and the initial energy, revealing a strong dependence on the composition of PdCu nanoalloys. Among the five selected models, Pd27Cu28 is shown to exhibit the minimal adsorption energy of reactants (e.g. CO+O2), intermediates (e.g. OOCO) and products (e.g. CO2) as well as the lowest reaction barrier (Eb1 = 0.563 eV). To fully restore the catalytic system, the O adatom left by the initial part of the reaction needs to react with another CO molecule to produce another CO2 molecule. If CO molecule comes from the gas phase, the reaction occurs with an Eley-Rideal (ER) 11



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mechanism to produce the second CO2 molecule. This reaction is generally barrier-less, and is usually not the rate-limiting step. If the O adatom reacts with an adsorbed CO molecule, the reaction maintains the LH mechanism. From our calculations, we know that the adsorption strength of O atom is strong, in addition to the strong adsorption of CO. The energy barriers for the LH mechanism are too high with the values of 2.580, 1.580, 1.526, 1.806, and 0.685 eV for Pd55, Pd43Cu12, Pd27Cu28, Pd12Cu43, and Cu55 clusters, respectively, indicating that this step is unfavorable. So, this step often occurs as CO comes from the gas phase to form the second CO2 according to the ER mechanism. Considering the whole reaction process, the Pd27Cu28 cluster possesses the lowest barrier energy for COox. To understand the origin of this catalytic synergy for COox on PdCu catalysts, calculations of the adsorption energies of reactants (e.g. O2 and CO) as a function of the composition of Cu were performed. The results are shown in Figure 5. In general, CO adsorption energy is found to be linearly dependent on PdCu composition,28 while the adsorption energy of O2 was revealed to reach a minimal at Pd:Cu ~50:50. In terms of the COox reaction mechanism, CO and O2 move closer to form an intermediate state OCOO*, which is the rate-determining step. In this step, the adsorpted O2 dissociates on the catalyst surface, forming a peroxide O-O bond with the adsorpted CO. The lower adsorption energy of O2 on PdCu nanoalloy is greatly favored for activation of the reactants, which is responsible for the enhancement of catalytic activity for COox over these PdCu nanoalloys. As shown in Fig. 6a, the barrier energy for COox is closely related to the adsorption energy of O2. The lower barrier energy can be attributed to the weaker adsorption strength of O2 for COox on these PdCu nanoalloys. In this case, the highest catalytic activity for COox is achieved over the Pd27Cu28 cluster which features the weakest adsorption energy of O2. Remarkably, this theoretical finding agrees quite well with the experimental finding26 which is shown in Fig. 6b, exhibiting a maximum of catalytic activity for the PdCu/C catalyst with a Pd:Cu ratio of ~50:50.26 The agreement between the theoretical results (Fig. 6a) and the experimental data (Fig. 6b) shines thus a light into the origin of the catalytic synergy for COox over PdCu catalysts. 12


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Oxygen Reduction Reaction The electrocatalytic activity of OOR in 0.1 M HClO4 over PdCu/C catalysts was investigated in our recent experimental study27 using cyclic voltammetry and rotating disk electrode techniques.

The catalytic activity was assessed by the mass activity

(MA) and specific activity (SA), which were obtained at 0.65 V(vs. Ag/AgCl) for PdCu catalysts (Pd77Cu23/C, Pd58Cu42/C, and Pd25Cu75/C) thermally treated under H2 at 400 oC. In this study, Pd58Cu42/C catalyst was found to exhibit a maximum MA or SA value as a function of the bimetallic composition. This experimental finding is significant for the design of better ORR catalysts, but the question why the catalytic activity is maximized at a Pd:Cu ratio close to 50:50 remains to be answered. In order to understand the origin of the highest catalytic activity for ORR on Pd50Cu50 among these PdCu nanoalloys, we calculated the reaction mechanism of the ORR on the PdCu nanoalloys. As discussed in earlier reports, there are three feasible mechanisms for ORR: (a) oxygen (O2) dissociation mechanism, (b) peroxyl (OOH) dissociation mechanism, and (c) hydrogen peroxide (H2O2) dissociation mechanism, which have been proposed to describe how an oxygen molecule (O2) is reduced to form a water molecule (H2O). The reaction mechanism is composed of 7 elementary steps: (1) O2 dissociation, (2) OH formation, (3) OOH formation, (4) OOH dissociation, (5) H2O2 formation, (6) H2O2 dissociation and (7) H2O formation.31, 43-44 O2 dissociation reaction is a key step in the ORR, for which the reaction rate was evaluated in terms of the barrier energy (Eb) of O2 dissociation. The stationary points along the minimum energy pathway for O2 dissociation on each of the PdCu clusters were mapped out using the corresponding most preferred O2 adsorption arrangement as the initial state. The final state is the adsorption of two separated O atoms on the clusters. Table 3 lists the most favorable O2 adsorption energy , the O-O bond distance, and the barrier energy for O2 dissociation on these PdCu clusters with different compositions. The corresponding barrier energies are found to be 0.711, 0.661, 0.292, 0.505 and 0.131 eV for the Pd55, Pd43Cu12, Pd27Cu28, Pd12Cu43 and Cu55 clusters, respectively. Even though the barrier energy for O2 dissociation on Cu55 cluster is the lowest in these PdCu nanoalloys, the adsorption of the oxygen intermediates to Cu is too strong (see disscussion later), which limited the reaction rate by the removal of products from the catalysts. For PdCu nanoalloys, the barrier 13



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energies for O2 dissociation reation is closely related with the adsorption strength of O2. Figure 7 shows the energy profile of the entire pathway from noninteracting Pd27Cu28 cluster and O2 to the state of dissociative adsorption of O atom on the cluster is depicted for the case of 50:50 ratio of Pd:Cu, and the barrier energy (Eb) is found to be only 0.292 eV. The profiles of the pathways for the other four clusters are qualitatively similar (see Figure S5). Not only the O2 dissociation reaction is the key step, but the adsorption energies of O, OH, and OOH can affect the overall reaction rate. The adsorption properties of the intermediates including O, OH, and OOH were investigated as a function of the composition of Cu. We focused on studying the adsorption of O, and OH on the T, B and H sites, and OOH on the B and T-H sites. It should be noted that atomic O and OH species prefer binding to the surface with “end-on” configuration (like the adsorption of CO), while the OOH prefers binding to the surface with “side-on” configuration (like the adsorption of O2). Tables S6-7 list all the adsorption sites and the corresponding adsorption energies. In order to facilitate the comparison and analysis, we calculated the average adsorption energy of O, OH, and OOH. Table 4 lists the average adsorption energy of O, OH, and OOH on these PdCu nanoalloy clusters. From the results, we can know that the adsorption strength of O firstly decreases then increases with the increasing of the Cu fraction in the PdCu nanoalloys. For O atom, Pd27Cu28 has the weakest adsorption strength, and Cu55 has the strongest adsorption strength. Similar trends are also observed for the adsorption of OH and OOH, as shown in Figure 8. For ORR, the adsorption strength of oxygen intermediates too strongly can hinder the removal of oxygen intermediates from the catalysts. Pure Cu catalysts is not a good catalyst for ORR, although it has the lowest barrier energy for O2 dissociation. Thelowest adsorption energy for intermediates including O, OH, and OOH was revealed in terms of PdCu composition, showing a minimal adsorption energy also at ~50:50 ratio of PdCu. The theoretical finding is further compared with the experimental data in terms of the mass activity and specific activity as a function of Cu % in PdCu nanoalloys (see Fig. 9a),27 which reveals a maximum MA or SA at a Pd:Cu ratio of ~50:50. As 14


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shown in Fig. 9 b, a volcano feature for the adsorption strength of O2 is obtained by DFT calculations, showing a minimal adsorption strength at a Pd:Cu ratio of ~50:50. In addition, the adsorption strength of O2 is closely related to the barrier energies for O2 dissociation reaction in ORR, as shown in Fig. 9b. It means that a lower adsorption strength of O2 can promote the catalytic activity for O2 dissociation reaction. As shown in Fig. 9c, the adsorption strength of O2 displays a linear correlation with adsorption strength of oxygenated intermediates such as O, OH, and OOH, which are generally considered as the reactivity descriptor for ORR. Taken together, there is a good agreement between theoretical and the experimental data, indicating that the combination of the lowest barrier energy for O2 dissociation and the weakest adsorption strength of O, OH, and OOH contributes to the weakest adsorption strength of O2 at a Pd:Cu of 50:50 ratio. Importantly, this agreement shines a light into the use of the adsorption strength of O2 as an appropriate “descriptor” for ORR activity in assessing the electrocatalytic activity data observed for the single-phase PdCu nanoalloy catalysts.27.

CONCLUSIONS In conclusion, the results from our DFT calculations have provided new insights for addressing an in-depth correlation between the theoretical and the experimental data on the synergistic enhancement of the catalytic activity of PdCu catalysts of different compositions for both COox and ORR. The experimental composition-tuned PdCu nanoalloy catalysts were modeled by using 55-atom icosahedral PdCu nanoalloys with five compositions of Pd55, Pd43Cu12, Pd27Cu28, Pd12Cu43, and Cu55. For COox, the PdCu catalyst with the highest catalytic activity is identified at a Pd:Cu ratio of 50:50 ratio. This composition features the lowest barrier energy among the PdCu clusters of different compositions, which reflects the weakest adsorption strength of O2.

For ORR, the PdCu catalyst with the maximum activity is also

identified at a Pd:Cu ratio of 50:50 ratio, which corresponds to the lowest barrier energy for O2 dissociation and the weakest adsorption strength of O, OH, and OOH. The result is indicative of the weakest adsorption strength of O2 at a Pd:Cu ratio of 15



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50:50. On the basis of the findings from our theoretical modeling, the origin of the experimentally-observed highest catalytic activity at a Pd:Cu ratio of ~50:50 for both COox and ORR26-27 can be attributed to the weakest adsorption strength of O2 in terms of the most pronounced charge transfer between Pd and Cu on these composition-tuned PdCu nanoalloys. This finding is also reminiscent of the maximum activity for formic acid oxidation on PdCu catalysts with a similar Pd:Cu ratio in previous experiments.25. It is important to emphasize that the new insights obtained in this work for an in-depth correlation between the theoretical and experimental data on high-active Pd50Cu50 catalysts represent a significant advance in understanding the composition–activity relationships, which is critical for the design of new nanoalloy catalysts with tunable catalytic properties by controlling compositions. Future investigations are planned in order to expand this understanding and to explore the highly-active Pd50Cu50 catalysts for catalytic oxidation reactions of small hydrocarbon molecules.

ASSOCIATED CONTENT

Supporting Information Detailed description of the adsorption properties (Tables S1-4 and S6-7), the mixing energy (Figure S1), electronic properties (TableS5 and Figure S2-3) and reaction pathways (Figure S4-5) of PdCu nanolloys.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] Tel: +86-010-64453523-605 16


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21576008, 91334203), BUCT Fund for Disciplines Construction and Development (Project No. XK1501), Fundamental Research Funds for the Central Universities (Project

No.

buctrc201530),

“Chemical

Grid

Project”

of

BUCT,

DOE

(DE-SC0006877), and NSF (CHE 1566283).

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20


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Table 1. The adsorption and coadsorption of sites and energies (in eV) for CO, O2, and CO-O2 on their optimal adsorption sites on PdCu nanoalloys.

cluster

CO Adsorption Ead (eV) Site

O2 Adsorption Ead (eV) Site

CO+O2 Adsorption Ead (eV) Site

Pd55

H1

-2.459

T1

-1.566

H1+T1

-3.614

Pd43Cu12

H2

-2.340

T1-H1

-1.125

H2+T1

-2.807

Pd27Cu28

B5

-1.728

B3

-0.935

B3+B4

-2.131

Pd12Cu43

T1

-1.288

B1-B1

-1.137

T1+B1-B1

-2.359

H2

-1.154

B1-B2

-1.331

T1+B1-B2

-2.085

Cu55

Table 2. Computed relative energies (eV) of coadsorption (CO+O2), intermediate (OCOO), and the final (CO2+O) states and energy barriers (eV) of the first (TS1) and second (TS2) transition states for LH reaction mechanism on PdCu nanoalloys. cluste

CO+O2

TS1

OOCO

TS2

O+CO2

Eb1 (->OCOO)

Eb2 (->CO2+O)

Pd55

-3.614

-2.137

-2.421

U

-4.922

1.477

0

Pd43Cu12

-2.807

-1.912

-2.224

U

-4.450

0.895

0

Pd27Cu28

-2.131

-1.568

-1.738

U

-4.313

0.563

0

Pd12Cu43

-2.359

-1.577

-1.977

U

-4.963

0.782

0

-2.085

-1.329

-1.566

-1.237

-4.903

0.756

0.329

r

Cu55

Table 3. Most favorable O2 adsorption energy (Ead, eV) , the bond distance of O-O (dO-O, in Å), and the barrier energy (Eb, in eV) for O2 dissociation on PdCu nanoalloys. Pd55 Pd43Cu12 Pd27Cu28 Pd12Cu43 Cu55 Ead (eV) -1.566 -1.125 -0.935 -1.137 -1.331 dO-O (Å) 1.295 1.392 1.339 1.449 1.497 Eb (eV) 0.711 0.661 0.292 0.505 0.131

21



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Table 4．The average adsorption energies (in eV) for O, OH, and OOH on the surface of PdCu nanoalloys. cluster Pd55 Pd43Cu12 Pd27Cu28 Pd12Cu43 Cu55

Ead (O) -4.446 -4.247 -4.099 -4.719 -4.933

Ead (OH) -3.101 -2.904 -2.862 -3.262 -3.376

Figure Captions 22


Ead (OOH) -1.601 -1.315 -1.272 -1.315 -1.691

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Figure 1. (a) Schematic pictures of the Pd55, Pd43Cu12, Pd27Cu28, Pd12Cu43, and Cu55 clusters investigated in the present work. Adsorption sites for one of the 20 equivalent triangular facets of the Pd12Cu43 (b), and Pd27Cu28 (c) clusters ( Pd55, Pd43Cu12, and Cu55 clusters have same adsorption sites with the Pd12Cu43 cluster), Adsorption mode of O2 with “end-on” (d) and “side-on” (e) configurations, adsorption mode of CO with “end-on” configuration (f). In addition, the adsorption configuration of OOH is same with O2, and the adsorption configurations of O and OH are same with CO. For the nomenclature of adsorption sites, please see the text. Figure 2. The spin polarized local density of states (LDOS) projected on isolated O2 (a), the LDOS projected of O2 and the partial density of electronic states (PDOS) projected on metal atom (Pd, Cu) in the systems for the adsorption of O2 on (b) Pd55, (c) Pd43Cu12, (d) Pd27Cu28, (e) Pd12Cu43, and (f) Cu55 nanoclusters at the most preferred O2 adsorption configuration. Figure 3. Polts of the total number of Pd-Cu bonds (red) and charge transfer from Cu to Pd (blue) as a function of Cu % in PdCu nanoalloys. Figure 4. Potential energy profile and configurations for CO oxidation with the Langmuir–Hinshelwood (LH) mechanism on the Pd27Cu28 cluster. Figure 5. The adsorption energies of the CO (black) and O2 (red) as a function of Cu % on PdCu nanoalloys. Figure 6. (a) Plots of adsorption energy of O2 and barrier energy of CO oxidation on Pd43Cu12, Pd27Cu28, and Pd12Cu43 clusters: CO (black) and O2 (red), as a function of Cu % on PdCu nanoalloys. (b) Plots of T10 values and Ea as a function of the bimetallic composition (Cu %) for CO oxidation over PdCu/C catalysts.26 Figure 7. Structures and energies of the stationary configurations along the reaction pathway of O2 dissociation on the Pd27Cu28 cluster. TS denotes the transition state. Figure 8. The average adsorption energies of the O, OH, and OOH as a function of Cu % on PdCu nanoalloys. Figure 9. (a) Plots of mass activity (MA) and specific activity (SA) at 0.55 V (vs. Ag/AgCl (KCl sat’d.)) for ORR in 0.1 M HClO4 over PdCu catalysts thermally treated at 400 oC (Pd77Cu23/C, Pd58Cu42/C, and Pd25Cu75/C) as a function of Cu % in the PdCu nanoalloys.27 (b) Plot of adsorption energy and the dissociation barrier of O2 on PdCu nanoalloys (Pd43Cu12, Pd27Cu28, Pd12Cu43). (c) The relations between adsorption energies of O (black), OH (red), or OOH (blue) and that of O2 on the surfaces of PdCu nanoalloy catalysts.

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Figure 1. (a) Schematic pictures of the Pd55, Pd43Cu12, Pd27Cu28, Pd12Cu43, and Cu55 clusters investigated in the present work. Adsorption sites for one of the 20 equivalent triangular facets of the Pd12Cu43 (b), and Pd27Cu28 (c) clusters ( Pd55, Pd43Cu12, and Cu55 clusters have same adsorption sites with the Pd12Cu43 cluster), Adsorption mode of O2 with “end-on” (d) and “side-on” (e) configurations, adsorption mode of CO with “end-on” configuration (f). In addition, the adsorption configuration of OOH is same with O2, and the adsorption configurations of O and OH are same with CO. For the nomenclature of adsorption sites, please see the text.

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Figure 2. The spin polarized local density of states (LDOS) projected on isolated O2 (a), the LDOS projected of O2 and the partial density of electronic states (PDOS) projected on metal atom (Pd, Cu) in the systems for the adsorption of O2 on (b) Pd55, (c) Pd43Cu12, (d) Pd27Cu28, (e) Pd12Cu43, and (f) Cu55 nanoclusters at the most preferred O2 adsorption configuration.

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Figure 3. Polts of the total number of Pd-Cu bonds (red) and charge transfer from Cu to Pd (blue) as a function of Cu % in PdCu nanoalloys.

Figure 4. Potential energy profile and configurations for CO oxidation with the Langmuir–Hinshelwood (LH) mechanism on the Pd27Cu28 cluster.

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Figure 5. The adsorption energies of the CO (black) and O2 (red) as a function of Cu % on PdCu nanoalloys.

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Figure 6. (a) Plots of adsorption energy of O2 and barrier energy of CO oxidation on Pd43Cu12, Pd27Cu28, and Pd12Cu43 clusters: CO (black) and O2 (red), as a function of Cu % on PdCu nanoalloys.

(b) Plots of T10 values and Ea as a function of the

bimetallic composition (Cu %) for CO oxidation over PdCu/C catalysts.26

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Figure 7. Structures and energies of the stationary configurations along the reaction pathway of O2 dissociation on the Pd27Cu28 cluster. TS denotes the transition state.

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Figure 8. The average adsorption energies of the O, OH, and OOH as a function of Cu % on PdCu nanoalloys.

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Figure 9. (a) Plots of mass activity (MA) and specific activity (SA) at 0.55 V (vs. Ag/AgCl (KCl sat’d.)) for ORR in 0.1 M HClO4 over PdCu catalysts thermally treated at 400 oC (Pd77Cu23/C, Pd58Cu42/C, and Pd25Cu75/C) as a function of Cu % in the PdCu nanoalloys.27 (b) Plot of adsorption energy and the dissociation barrier of O2 on PdCu nanoalloys (Pd43Cu12, Pd27Cu28, Pd12Cu43). (c) The relations between adsorption energies of O (black), OH (red), or OOH (blue) and that of O2 on the surfaces of PdCu nanoalloy catalysts. 31



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Origin of Enhanced Activities for CO Oxidation and ... - ACS Publications

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