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Role of Heteronuclear Interactions in Selective H Formation from HCOOH Decomposition on Bimetallic Pd/M (M=Late Transition FCC Metals) Catalysts Jinwon Cho, Sangheon Lee, Sung Pil Yoon, Jong Hee Han, Suk Woo Nam, Kwan-Young Lee, and Hyung Chul Ham ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02825 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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Role of Heteronuclear Interactions in Selective H2 Formation from HCOOH Decomposition on Bimetallic Pd/M (M=Late Transition FCC Metals) Catalysts Jinwon Cho1,# , Sangheon Lee1,4,#, Sung Pil Yoon1, Jonghee Han1,3, Suk Woo Nam1,3, Kwan Young Lee3,*, and Hyung Chul Ham1,2,*

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Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea

Clean Energy and Chemical Engineering, Korea University of Science and Technology, 217 Gajungro, Yuseong-gu, Daejeon, 305-333, Republic of Korea

Green School (Graduate School of Energy and Environment), Korea University, 145, Anamro, Seongbuk-gu, Seoul, 136-701, Republic of Korea

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Department of Chemical Engineering and Materials Science, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul, 03760, Republic of Korea

*

Corresponding Authors:

Dr. Hyung Chul Ham: [email protected], Phone: +82-2-958-5889, Fax: +82-2-958-5199 Prof. Kwan-Young Lee: [email protected], Phone: +82-2-3290-3299, Fax: +82-2-926-6102

[#]

These authors equally contributed as the first authors.

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Abstract In this study, by using spin-polarized density functional theory calculations, we have elucidated the role of the heteronuclear interactions in determining the selective H2 formation from HCOOH decomposition on bimetallic Pdshell/Mcore [M=late transition FCC metals (Ag, Au, Cu, Rh, Ir, Pt)] catalysts. We found that the catalysis of HCOOH decomposition strongly depends on the variation of surface charge polarization (ligand effect) and lattice distance (strain effect), which are caused by the heteronuclear interactions between surface Pd and core M atoms. In particular, the contraction of surface Pd—Pd bond distance and the increase of electron density in surface Pd atoms compared to the pure Pd case are responsible for the enhancement of the selectivity to H2 formation via HCOOH decomposition. Our calculations also unraveled that the d-band center location and the density of states for d-band (particularly, dz2, dyz, dxz) near the Fermi level are the important indicators that understand the impact of strain and ligand effect in catalysis, respectively. That is, the surface lattice contraction (expansion) leads to the down-shift (up-shift) of d-band centers compared to the pure Pd case, while the electronic charge increase (decrease) of surface Pd atoms results in the depletion (augmentation) of the density of states for dz2, dyz, and dxz orbitals. Our study highlights the importance of properly tailoring the surface lattice distance (d-band center) and surface charge polarization (the density of states for dz2, dyz, dxz orbitals near the Fermi level) by tuning the heteronuclear interactions in bimetallic Pdshell/Mcore catalysts for enhancing the catalysis of HCOOH decomposition toward H2 production, as well as other chemical reactions.

Key words: H2 production, lattice distance, surface charge polarization, core-shell, HCOOH, bimetallic catalysts

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1. Introduction Hydrogen (H2) is found in the gaseous molecular H2 form under ambient conditions, and its conversion into a liquid or solid form is a key to the easy storage and transport for mobile fuel cells.1-4 Conventional liquefaction processes of H2 via refrigeration and pressurization require tremendous amount of energy cost due to the extremely low critical temperature of H2 (Tc = 33.145K).5 Formic acid (HCOOH) is a nontoxic liquid at room temperature, and during the last few years there has been a revitalization of effort in developing economically viable HCOOH-based in situ H2 production systems.6-14 In the presence of a suitable catalyst, HCOOH decomposes primarily into H2 and CO2 and secondarily into H2O and CO.2,

15

Considering the notorious CO-poisoning effect of

platinum-based fuel cell electrodes, catalysts that are highly active for the production of H2 but selective enough to suppress the formation of CO (< 10 ppm) are indispensable to enabling HCOOH-based mobile fuel cells.3, 6, 16 Exhibiting unique Pd-H interactions, palladium (Pd) has been known as the most active catalytic element for the H2 production via HCOOH decomposition.2, 6, 17 However, reported H2 production rates of monometallic Pd catalysts are not sufficient for extensive applications of HCOOH-based fuel cells.6, 18-19 In addition, monometallic Pd catalysts are prone to be deactivated by CO, as noticeable amounts of CO are produced during the HCOOH decomposition.2 Incorporation of a secondary metal is a proven method of promoting the Pd catalysts6, 20. In particular, Pd/Ag core-shell catalysts exhibit noticeably enhanced selective H2 production capabilities while uniform alloying of Pd with Au or Ag is effective in mitigating the notorious CO poisoning effect.2,

6, 15, 18, 21

An ensuing first-

principles study elucidated that the charge-transfer from the Ag core to the Pd shell and the subsequent modification of the surface reactivity for key intermediates are responsible for the enhanced selective H2 production capability.2 These results suggest the likelihood of further improvement in the selective H2 production capabilities by the rational design of Pd alloy catalysts. Theoretical understandings derived from first-principles calculations can be an effective guide to the rational design of new alloy catalysts prior to the real experimental synthesis and characterization.2,

22-23

For bimetallic core-shell catalysts, it has been well

established that the combined effects of strain subjected to the surface (the so-called strain -3-


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effect) and interatomic electronic interactions between the heterogeneous surface and subsurface layers (the so-called ligand effect) lead to unique catalytic reaction properties.24-25 However, general inclination explaining the effect of core metal incorporation on the Pd surface catalytic activity has yet to be established, hampering further improvement in the selective H2 production capabilities. In this work, we establish an integrated view on the underlying mechanisms behind the Pd surface promotion induced by heterogeneous metal core. To this end, we employ multiple bimetallic Pd/M (M = Rh, Pt, Ir, Cu, Pd, Au, and Ag) core-shell models and perform a series of density functional theory (DFT) calculations to evaluate catalytic activities of the core-shell systems. Then, we analyze the variations of calculated catalytic activities in terms of the strain and ligand contributions. Finally, we integrate the identified strain and ligand effects with the Pd surface electronic structures. The integrated approach leads us to establish a useful correlation that can provide catalytic activity estimates for Pd/M core-shell catalysts in the absence of experimental data, ultimately enabling computation based ration design of H2 production catalytic systems.

2. Calculation Details The calculations reported herein were performed on the basis of spin polarized density functional theory (DFT) within the PW91 functional as implemented in the Vienna Ab-initio Simulation Package (VASP).26-27 The projector augmented wave (PAW) method with a planewave basis set was employed to describe the interaction between ion cores and valence electrons.28 An energy cutoff of 350 eV was applied for expansion of the electronic eigenfunctions. For the Brillouin zone integration, we used a (5×5×1) Monkhorst-Pack mesh of k points to determine the optimal geometries and total energies of systems.29 For the electronic structure and d-band center calculation, we increase the k point mesh to (12×12×1).28 Reaction pathways and barriers were determined using the climbing-image nudged elastic band method (c-NEBM) with six intermediate images for each elementary step.30 For model surfaces, we used a supercell slab that consists of a 2×2 hexagonal (111) surface unit cell with five atomic layers of which contains 4 atoms (see Figure 1). Here, we checked the validity of a 2×2 supercell model (corresponding to 1/4 monolayer coverage) in the selective H2 production via HCOOH decomposition by performing the additional -4-


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calculations over the 3×3 (corresponding to 1/9 monolayer coverage) periodic Pd/Ag(111) and Pd(111) slabs. Note that the small supercell size may lead to the wrong adsorption energy and structure of intermediates since the calculated surface energies are affected by the coverage effects. As depicted in Table S-1, two cases were found to exhibit similar trends and therefore we used a 2×2 hexagonal (111) surface unit cell for the effective computational cost. The bottom four layers are late transition FCC metals, such as Cu, Rh, Pd, Ir, Pt, Au, and Ag (listed in increasing order of lattice constant) and the top surface layer is consisted of Pd atoms. The slab is separated from its periodic images in the vertical direction by a vacuum space corresponding to seven atomic layers (16.73, 17.71, 18.36, 17.90, 18.44, 19.53, and 19.49 Å for Pd/Cu, Pd/Rh, Pd/Pd, Pd/Ir, Pd/Pt, Pd/Au, and Pd/Ag models, respectively). While the bottom two layers of the five-layered slab are fixed at corresponding bulk positions, the upper three layers are fully relaxed using the conjugate gradient method until residual forces on all the constituent atoms become smaller than 5×10-2 eV/Å.28 The lattice constant for bulk Cu, Rh, Ir, Pd, Pt, Au, and Ag is predicted to be 3.61, 3.80, 3.84, 3.89, 3.92, 4.08 and 4.09 Å respectively, which are virtually identical to the previous calculations.2 The surface strain (ε) is calculated as in percentage via Strain% = (Pdlattice – Mlattice) x 100 / Pdlattice where Pdlattice is lattice parameter of Pd and Mlattice is lattice parameter of M. In addition, the adsorption energy (Eads) of a species M on a surface are determined via Eads = EM* – (Esurf + EM), where EM* and Esurf are the total energies of the surface with and without the adsorbed M, respectively, and EM is the total energy of the species M in the vacuum. These Pd/M models are expected to be kinetically stabilized at relatively lower temperatures (< 100 ˚C) in HCOOH-based low temperature fuel cells, as there is an energy barrier required for the defect-mediated surface segregation as a minimum of 0.29eV to 1.89eV at maximum (See Figure S-1 of supporting information).

3. Results and Discussion 3.1 Catalytic activity of Pd/M system toward HCOOH decomposition Prior to discussing the reaction properties of the Pd/M surfaces, let us examine the HCOOH decomposition behavior over the Pd surfaces. We accept a dual path mechanism, which consists of dehydrogenation and dehydration paths as depicted in Figure 2.2, 15, 21 -5-


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In the dehydrogenation (H2 production) path, HCOOH is oxidized to form H2 and CO2 through two elementary steps in series: dehydrogenation from HCOOH via the O—H cleavage [HCOOH → HCOO + H, denoted as DH-I] and subsequent dehydrogenation from a bidentate HCOO (which is linked to a bridge site by its two oxygen atoms on the catalytic surface) via the rotation of a bidentate HCOO into a monodentate HCOO and the C—H cleavage from a monodentate HCOO [HCOO → CO2 + H, defined as DH-II].2, 15, 21-22, 31-32 In the dehydration (CO production) path, HCOOH is oxidized to form CO and H2O through two elementary steps in series: decomposition of HCOOH via the C—O cleavage [HCOOH → HCO + OH, denoted as DCO-I] and subsequent formation of CO and H2O [HCO + OH → CO + H2O, denoted as DCO-II].2, 22 Here, in the dual path mechanism of our study, we excluded the COOH formation via WGS (RWGS) reaction and carboxyl pathway for H2 production since those pathways are kinetically slow in HCOOH decomposition at low temperature conditions (see the reaction energetics and barriers for WGS (RWGS) reaction and carboxyl (COOH) pathway for H2 production in Figure S-2, S-3 and Table S-3 of supporting information section). The previous experimental and theoretical studies also exhibit that the formate is believed to be a viable intermediate in dehydrogenation pathway2, 6, 15,

17,

22,

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and CO and H2O are produced from HCOOH decomposition

(HCOOH→CO+H2O) through the formation of formyl (HCO) or carboxyl (COOH) intermediates and the dehydration reaction via HCO is kinetically more favorable than that via COOH species17, 35-36. Table 1 summarizes the calculated total energy changes (∆E), and activation energy barriers (Ea) for the aforementioned elementary reaction steps over the seven Pd/M models. Considering that both dehydrogenation and dehydration take place in parallel at low temperatures and their rates are kinetically determined, catalytic activity of a given model surface should be evaluated on the basis of the calculated activation energy barriers2, 6, 15, 21-22, 37

. Here, we chose the PW91 functional that can be widely and reliably used in the calculation

of geometrical and electronic properties of materials. Note that although the PW91 functional can be reliably used for a lot of systems, the prediction of chemical and physical properties using PW91 functional should be carefully compared to the result calculated by RPBE functional (which can resolve the overbidding issue of the chemisorption energetics of atoms and molecules on the transition metal surfaces when DFT calculation is based on PW91 -6-


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functional).38-39 Table S-4 compares the calculated reaction energetics and activation barriers of dehydrogenation and dehydration on the Pd(111) and Pd/Ag(111) surfaces using the two different PW91 and RPBE functional. As shown in Table S-4, DH-II and DCO-I have the highest barrier of each dehydrogenation and dehydration pathway for both PW91 and RPBE functional. Thus, we concluded functional does little to the changes in the calculated energy barrier of the dehydrogenation and dehydration. For the dehydrogenation path, step DH-II exhibit noticeably greater activation energy barriers than step DH-I in all the Pd/M cases except the Pd/Cu case in which the calculated activation energy barriers for step DH-I and DH-II are comparable. Therefore, we may take step DH-II as the key step for understanding the dehydrogenation path in all Pd/M cases, which is also well supported by a number of experimental or theoretical reports.2, 6, 15, 17-18, 2122, 40-41

For the dehydration path in all the Pd/M cases, step DCO-I yields noticeable activation energy barriers, while step DCO-II yields negligible activation energy barriers. Therefore, we may take step DCO-I as the important step, which can limit the CO formation rate for the dehydration path in all the Pd/M cases.2,

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It is also worth noting that the

activation energy barrier for step DCO-I is noticeably greater than the corresponding value for step DH-II, suggesting that the dehydrogenation path is the primary reaction path for the HCOOH decomposition. These calculation results suggest that for the selective H2 production, the catalytic system should be designed to (i) lower the activation energy barrier for step DHII, thereby enhancing the dehydrogenation reaction rate and (ii) raise the activation energy barrier for step DCO-I, thereby suppressing the dehydration reaction rate.2, 6, 22 To understand the nature of the kinetic and rate determining step (RDS) for the H2 production via HCOOH decomposition, we first developed a microkinetic model by calculating Gibbs free energy, entropy, zero point energy, and rate constant for each elementary reaction42 (See Supporting Information for a microkinetic model, calculated Gibbs free energy, entropy, and zero point energy for the elementary reactions of HCOOH decomposition). Table 1 exhibits the turnover frequency (TOF) of the H2 production for all Pd/M surfaces at T=323K. We see that the H2 TOF (hr -1) shows the highest production rate for the Pd/Cu case, whereas the Pd/Pt case yields the lowest production rate. Tedsree et. al. have experimentally reported the H2 TOF values6 for the several M(M=Ag, Rh, Au, Pt)@Pd -7-


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core shell catalysts where the order of H2 TOF value is Ag@Pd>Rh@Pd>Au@Pd> Pd@Pd>Pt@Pd. This is in good agreement with our calculated trend in the variation of H2 TOF [4.48×10-2 (Pd/Ag), 4.24×10-2 (Pd/Rh), 1.53×10-3 (Pd/Au), 8.44×10-5 (pure Pd), 2.74×109

(Pd/Pt)]. Next, based on a developed microkinetic model, we attempted to choose a RDS in

both dehydrogenation and dehydration steps by calculating the degree of rate control (denoted as XRC), which is defined below43-44. Here, the larger the value of XRC is for a given step, the bigger is the impact of its rate constant on the overall reaction rate (See the supporting information for the calculated XRC values). =

,

where, TOF, ki, and Kj are the turnover frequency of the overall H2 or CO production, the rate constant of step i, and the equilibrium constant of step j, respectively. As displayed in Table S-10 (here, XRC is calculated by using the TOF of the overall H2 production), for all the Pd/M catalysts, we find that the HCOO → H2+CO2 (DH-II) step exhibits the largest numeric value of XRC (1.69 ~2.00) compared to other reaction steps, suggesting that HCOO → H2+CO2 (DH-II) step limits the overall reaction rate and is considered to be the RDS for the H2 production via HCOOH decomposition. In this study, we additionally selected the HCOOH → HCO + OH (DC-I) as the key reaction step for representing the CO production via dehydration steps (DC-I and DC-II), which will be used for understanding the relative role of strain and ligand effects in dehydration reactions in the next section. Note that HCOOH → HCO + OH (DC-I) step exhibits the largest XRC value (0.97~1.00) among the dehydration reaction steps considered [see Table S-11 (XRC is determined by using the TOF of the overall CO production)]. Here, we want to point out that there is no switching of the RDS (HCOO → H2+CO2) for the overall H2 production via HCOOH decomposition and of the key step (HCOOH → HCO + OH) for representing the CO formation through dehydration steps when varying the core metals in Pd/M model surfaces, indicating that the following analysis of the relationship between reaction barrier and surface electronic state, based on HCOO → H2+CO2 (DH-II) and HCOOH → HCO + OH (DC-I) steps could extend to understand the electronic origin of controlling the H2 selectivity in Pd/M -8-


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catalysts.

3.2 Impact of surface charge polarization and lattice variation in catalysis Next, we discuss how the selected core metals boost or suppress the selective H2 production capability of the Pd overlayer surface. For bimetallic core-shell catalysts, it has been well established that the combined effects of strain subjected to the surface (the socalled lattice strain effect) and heteroatomic electronic interactions between the heterogeneous surface and subsurface layers (the so-called ligand effect) lead to unique catalytic reaction properties.24-25, 45 In fact, the predicted equilibrium lattice constants for bulk Rh (3.80 Å), Pt (3.92 Å), Ir (3.84 Å), Cu (3.61 Å), Au (4.08 Å), and Ag (4.09 Å) exhibit significant deviations from the corresponding bulk Pd value of 3.891 Å. As a consequence, the pseudomorphic Pd overlayer is under biaxial strain (ε) by ε = −7.2% over Cu, ε = −2.3% over Rh, ε = −1.3% over Ir, ε = +0.77% over Pt, ε = +4.9% over Au, and ε = +5.1% over Ag, where the minus (plus) signs in ε refers that the Pd overlayer is under compressive (tensile) strain. Although the strain and ligand effects usually cannot be varied independently, it is very useful to consider the two independent effects on the reactivity of a surface. To decouple the contributions of strain and ligand, we obtain the strain contribution (∆Eastrain) by calculating the activation energy barrier change (with respect to the strain-free reference Pd case) over the strained monometallic Pd surface [denoted as PdStr-M] with the same lattice constant as the corresponding Pd/M case.2 Then, the ligand contribution (∆Ealigand) is subsequently obtained by subtracting the corresponding strain contribution (∆Eastrain) from the calculated activation energy barrier changes (∆Ea) relative to the reference Pd case.24 To assess the relative contribution from each factor, we made separate plots in Figure 3 for (a) the variation of ∆Eastrain as a function of the strain (ε) subjected to the Pd overlayer and (b) the variation of ∆Ealigand as a function of the surface charge polarization (∆σ). Here, ∆σ, calculated on the basis of the Bader charge analysis, represents the surface charge gain (∆σ > 0) or loss (∆σ < 0) relative to the reference Pd case. For step DH-II [dehydrogenation (H2 formation)], both ∆Eastrain (as a function of ε) and ∆Ealigand (as a function of ∆σ) exhibit monotonically varying trends: ∆Eastrain for step DH-II tends to gradually increase from −0.06 eV (at ε = −7.2%) to 0.03 eV (at ε = 5.1%) as shown in Figure 3(a), while ∆Ealigand for step DH-II tends to gradually decrease from +0.06 eV (at ∆σ = −0.2 e per Pd atom) to −0.12 eV -9-


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(at ∆σ = +0.39 e per Pd atom) as shown in Figure 3(b). For step DCO-I[dehydration(CO formation)], the ∆Eastrain and ∆Ealigand values exhibit significantly greater variations in comparison with the corresponding step DH-II counterparts: as shown in Figure 3(a), ∆Eastrain for step DCO-I ascends steeply up to 0.64 eV (at ε = −7.2%) as the Pd overlayer becomes more compressed, while descending rather modestly up to −0.1 eV (at ε = 5.1%) as the Pd overlayer becomes under tensile strain; as shown in Figure 3(b), ∆Ealigand for step DCO-I is significantly boosted up to 0.3 eV (at ∆σ = +0.35e per Pd atom) and 0.19 eV (at ∆σ = +0.39e per Pd atom) with huge fluctuations as the magnitude of surface charge gain increases, while descending rather modestly up to −0.05 eV (at ∆σ = −0.2 e per Pd atom) as the magnitude of surface charge loss increases. This analysis clearly demonstrates that the selective H2 production capability of the bimetallic Pd/M core-shell catalysts can be enhanced by two effective routes: (i) in-plane lattice contraction of the Pd overlayer, leading to the decrease in ∆Eastrain for step DH-II and the increase in ∆Eastrain for step DCO-I and (ii) charge transfer from core M to Pd overlayer, leading to the decrease in ∆Ealigand for step DH-II and the increase in ∆Ealigand for step DCO-I. Such an effective control of the four independent contributions (∆Eastrain and ∆Ealigand for step DH-II, and ∆Eastrain and ∆Ealigand for step DCO-I) via the two independent variables (surface lattice strain and charge polarization) becomes feasible because step DH-II and DCO-I cases exhibit oppositely varying trends of ∆Eastrain and ∆Ealigand in their response to the variations of lattice strain and charge transfer, respectively. Based on the improved understandings, the most selective H2 production capability of the Pd/Cu model (∆Es = 1.24 eV, see Table 1) can be attributed to the combined effects of the significant compressive strain (ε = −7.2%) and charge accumulation (∆σ = +0.35 e per Pd atom) in the Pd overlayer, whereas the least selective H2 production capability of the Pd/Pt model (∆Es = 0.05 eV, see Fig. 3) can be attributed to the combined effect of the tensile strain (ε = +0.77%) and charge depletion (∆σ = −0.2 e) in the Pd overlayer. Next, in order to further understand the underlying reasons (electronic origin) for the identified strain and ligand effects, we investigated how the surface lattice variation and charge polarization can be connected with the Pd surface electronic structure in terms of the shape of d-band in the density of states (DOS).

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3.3 Electronic origin of lattice variation (strain) in catalysis Figure 4 shows the calculated DOS plot for the d-band of surface Pd atoms for the PdStr-M models, together with the d-band center locations (red vertical arrows). We observe the upshift and downshift of the d-band centers compared to the pure Pd case under tensile (lattice expansion) and compressive (lattice contraction) strain conditions, respectively. This variation of the d-band centers under strained environments is closely related to the change of the local surface reactivity toward reaction intermediates, which in turn affect the dehydrogenation and dehydration catalysis. For the better understanding of the trends between the d-band centers and catalysis, the variation of the strain contribution to the binding energy of key intermediates with respect to the pure Pd case (indicated by ∆Eadsstrain) and the d-band centers as a function of imposed strain is displayed in Figure 5. Here, we choose HCOO and HCO+OH as representative intermediates for understanding the catalysis in the rate-determining steps of dehydrogenation (DH-II) and dehydration (DCO-I). First, for the HCOO case in dehydrogenation, we predict that the decreased d-band centers by lattice contraction [such as the Cu(ε = −7.2%), Rh(ε = −2.3%), Ir(ε = −1.3%) cases] reduce the ∆Eadsstrain of HCOO by 0.24eV, 0.07eV and 0.05eV, respectively, compared to the pure Pd case, while for the increased d-band centers by lattice expansion [such as Ag(ε = +5.1%), Au(ε = +4.9%), Pt(ε = +0.77%) cases], the ∆Eadsstrain is rather increased by 0.17eV, 0.16eV, and 0.02eV, respectively. As a result, the compressive (expansive) strain enhances (suppresses) the kinetics of HCOO→CO2+H reaction (see Figure 3). This strain effect in catalysis is in good agreement with the d-band theory24(anti-bonding orbitals are more likely to be empty from electron occupation as a result of upshift of the d-band center in a clean surface, and this can lead to a stronger interaction with an adsorbate). Looking at the HCO+OH case in dehydration, we see the similar trend to the HCOO case. That is, the down-shift of d-band center by lattice contraction compared to the pure Pd case tends to reduce the ∆Eadsstrain of HCO+OH by 0.09~0.46eV, while for the up-shift of dband center by lattice expansion, the opposite is true, resulting in the suppression of HCOOH→HCO+OH reaction on the compressively-strained surfaces, and vice versa. Another important factor that can influence the catalysis of dehydration (a ratedetermining DCO-I step) as well as the d-band center location is the atomic rearrangements at the transition states under strained environments. Figure 6 depicts the variations in the - 11 -


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transition state configurations and the corresponding activation energy barriers for the DCO-I step as a function of decoupled strain (surface lattice distance) and ligand (surface charge states) effects; here, strain variations are realized by employing the PdStr-M models, while ligand effects are examined by using the PdLig-M models (the lattice of core metal M is fixed to the equilibrium Pd lattice, thus the strain contribution in catalysis is decoupled from ligand contribution). We find that only the compressively strained PdStr-Cu and PdStr-Rh cases exhibit the noticeable changes in the site preference for the OH adsorption from the Pd-Pd bridge site to the Pd top site compared to the equilibrium Pd case, while the HCO adsorbs on the Pd top site. Considering that the OH adsorbs on an energetically favorable Pd-Pd bridge site on the equilibrium Pd surface, the switching move indicates that much greater energy is required to break HCOOH into HCO and OH (consequently, raise the activation energy barrier of DCO-I step), when the surface Pd atoms are under compressive strain. In contrast, for all the PdLig-M models, the charge state variations do not induce such OH adsorption site changes despite the upshift in the activation energy barrier. This analysis clearly demonstrates that the site preference change for the OH adsorption at transition states by the compressive strain is one of key factors in determining catalysis of dehydration, in addition to the d-band center location.

3.4 Electronic origin of surface charge polarization (ligand) in catalysis Figure 7 indicates the orbital resolved density of states (ORDOS) for the d-band of surface Pd atoms for the ligand PdLig-M models (in this model, surface charge states are only affected by associated core metals). First, we see that the density of the dz2 and dyz+dxz states is more significantly affected by the variation of surface charge states (by the charge transfer between surface Pd atoms and core metals), rather than the dxy + dx2-y2 case. In particular, the ORDOS for the dz2 and dyz+dxz near the Fermi level (–0.25 eV < E – Ef < 0 eV) for the PdLigCu,

PdLig-Ag PdLig-Au, and PdLig-Rh cases is greatly reduced by the charge gain of surface Pd

atoms (notice the nearly-zero ORDOS at the Fermi level for Cu, Ag, and Au cores), while for the PdLig-Ir and PdLig-Pt cases, the density of the dz2 and dyz+dxz states near the Fermi level is increased by the charge loss of surface Pd atoms. The modified ORDOS for the dz2 and dyz+dxz bands near the Fermi level can be largely related to the magnitude of change in surface charge polarization, considering that the empty ORDOS above the Fermi level are - 12 -


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ACS Catalysis

likely to be occupied from electron transfer. This change of the ORDOS for the dz2 and dyz+dxz band near the Fermi level is closely related to the catalysis in dehydrogenation and dehydration. Figure 8 displays the variation of the ORDOS for the dz2 + dyz + dxz bands near the Fermi level (–0.25 eV < E – Ef < 0 eV) and ligand contribution to the binding energy of HCOO and HCO+OH with respect to the pure Pd case (indicated by ∆Eadsligand) as a function of surface charge polarization (∆σ). For comparison, we also present the ORDOS for the dxy + dx2- y2 orbitals. For the HCOO case in dehydrogenation, the change in the ∆Eadsligand of HCOO tends to show an ascending trend as the density of the dz2 + dyz+ dxz states near the Fermi level decreases (notice little variation of ORDOS of the dxy + dx2- y2 orbitals). That is, for the PdLig-Cu, PdLig-Ag PdLig-Au, and PdLig-Rh cases, the strikingly decrease of the ORDOS for the dz2 + dyz + dxz bands near the Fermi level by the increase of electronic charge states (induced by the heteronuclear interaction between Pd and subsurface core atoms) reduces the ∆Eadsligand of HCOO by 0.29eV, 0.34eV, 0.15eV, 0.03eV, respectively, compared to the pure Pd case, leading to the enhanced kinetic of HCOO dehydrogenation. Note that surfaces having a lower density of state near the Fermi level tend to be less reactive in adsorption at the condition of the strong ligand interaction in the bimetallic system2, 22, 46. On the other hand, such a tendency is opposite for the PdLig-Ir and PdLig-Pt cases, resulting in the decreased activity of catalysts toward H2 selectivity.

Note that the ∆Eadsligand of HCOO is increased by

0.18eV (PdLig-Ir) and 0.11eV (PdLig-Pt) via the increase of the ORDOS for the dz2 + dyz + dxz bands near the Fermi level. For the HCO+OH case in dehydration, our calculation also predicts that the density of the dz2 + dyz + dxz states near the Fermi level play an important role in determining HCOOH→HCO+OH reaction (DCO-I step). That is, the reduced level in the density of the dz2 + dyz + dxz states (caused by the increase of surface charge states) decreases the ligandrelated binding strength (∆Eadsligand) of HCO+OH, leading to the suppression of CO formation, while for the augmented density of the dz2 + dyz + dxz states, the opposite is true. Our examination of the Pd surface electronic structures provides an important foundation for understanding the relationship between the surface charge polarization and the d-band shape in the DOS near the Fermi level, which ultimately governs the identified ligand contributions.

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4. Conclusion Using spin-polarized DFT calculations, we have investigated the role of the surface charge polarization and surface lattice variation (which are associated by the heteronuclear interactions between surface Pd and core M atoms) in determining the catalysis of H2 production from HCOOH decomposition on the Pd/M(M=Ag, Au, Cu, Pt, Ir, Rh) core-shell catalysts. By decoupling surface lattice variation effect (strain effect) from surface charge polarization effect (ligand effect) in the Pd/M systems, we revealed that the in-plane lattice contraction and charge accumulation of the surface Pd atoms significantly boost the kinetic of rate-determining HCOO→CO2+H reaction and suppress the dehydration rate of HCOOH→HCO+OH step, leading to the enhanced selectivity to H2 formation from HCOOH decomposition on the Pd/Cu, Pd/Ag, Pd/Au and Pd/Rh catalysts compared to the pure Pd case. The electronic structure calculation of Pd/M catalysts suggests that the trend of variation in the lattice distance and electronic charge states of surface Pd atoms is closely correlated to the change of the d-band center position and ORDOS for the dz2 + dyz+ dxz bands near the Fermi level, respectively. That is, the lattice contraction(expansion) and electronic charge increase (decrease) of surface Pd atoms give rise to the down-shift (up-shift) of d-band center and the depletion (augmentation) of density of the dz2 + dyz+ dxz states near the Fermi level, which results in improved (reduced) H2 productivity and selectivity from HCOOH decomposition. Based on the improved understandings of heteronuclear interactions, we found that the Pd/Cu catalyst showed the highest H2 selectivity due to the combined effects of the significant compressive strain and charge gain in the Pd surface, while the Pd/Pt catalyst displayed the lowest H2 selectivity owing to the combined effect of the tensile strain and charge depletion in the Pd surface. Our study highlights the importance of properly tailoring the lattice strain (d-band center) and surface charge polarization (the density of states for the dz2, dyz, dxz orbitals near the Fermi level) in Pdshell/Mcore alloys for enhancing the catalysis of HCOOH decomposition toward H2 production and further extend the other catalytic reactions to rationally design the core shell catalysts.

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Corresponding Author *E-mail: [email protected], Phone: +82-2-958-5889, Fax: +82-2-958-5199 [email protected], Phone: +82-2-3290-3299, Fax: +82-2-926-6102 Acknowledgement The current work was financially supported by the New & Renewable Energy Core Technology Program (No. 20133030011320, No. 20153030041170) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea, and also supported by KIST institutional program for Korea Institute of Science and Technology (2E27302) and also supported from the Basic Science Research Program through the National Research Foundation

(NRF)

of

Korea

funded

by

the

Ministry

of

Education

(NRF-

2013R1A6A3A04059268).

Supporting Information The stability (surface segregation) of Pd/M catalysts under HCOO environment, projected density of d states for the PdLig-M case, developed microkinetic model, reaction energetics/barriers for WGS (RWGS) and carboxyl pathway for HCOOH decomposition are shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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References

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1730. 45. Biligaard, T.; Norskov, J. K. Electrochim. Acta 2007, 52, 5512-5516. 46. Tong, Y. Y.; Yonezawa, T.; Toshima, N.; vanderKlink, J. J. J. Phys. Chem. 1996, 100, 730-733.

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Table 1. Calculated Energy Change in Reaction (∆E) and Activation energies (Ea) for Each Dehydrogenation and Dehydration Pathway on Pd/M with respect to Fully Separated Adsorbed Species (here, separation state was evaluated by individually placing each adsorbed species on the 2×2 surface unit cell). The Calculated H2 TOF (Turnover frequency) Using a Microkinetic Model is given in hr-1.

Metals used for M substrates ∆E/(Ea) (eV)

Rh

Pt

Ir

Cu

Pd

Au

Ag

(DH-I) HCOOH → HCOO + H

0.06 /(0.73)

−0.33 /(0.51)

−0.07 /(0.71)

0.21 /(0.78)

−0.14 /(0.68)

−0.18 /(0.69)

−0.04 /(0.72)

−0.30 /(0.87)

−0.25 /(0.94)

−0.18 /(0.92)

−0.94 /(0.76)

−0.41 /(0.90)

−0.44 /(0.85)

−0.58 /(0.81)

1.15 /(1.22)

0.77 /(0.99)

1.07 /(1.09)

1.92 /(2.02)

0.90 /(1.08)

0.91 /(1.06)

0.97 /(1.19)

−1.60 /(0.00)

−1.44 /(0.00)

−1.67 /(0.00)

−2.56 /(0.00)

−1.79 /(0.00)

−1.67 /(0.00)

−2.02 /(0.00)

4.24×10-2

2.74×10-9

3.35×10-4

2.60×102

8.44×10-5

1.53×10-3

4.48×10-2

(DH-II) HCOO → CO2 + H (DCO-I) HCOOH → HCO + OH (DCO-II) HCO + OH → CO + H2O H2 TOF (hr-1)

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Figure 1. Top (the upper row) and side view (the lower row) of Pd/M models used in this study. From left, Pure Pd(111), monolayer Pd supported on metal substrates, Pd/M (111), lattice parameter modified Pd, PdStr-M (111), and Pd/M with fixed lattice parameter 2.80 Å, PdLig-M (111) where M is Rh, Pt, Ir, Cu, Au, and Ag. Pd atoms are presented in green, and M is shown in gray.

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Figure 2. Two possible decomposition pathways of HCOOH oxidation on Pd/M. One is dehydrogenation, presented in blue line, proceeds to HCOO + H (DH-I) and then to CO2 to H (DH-II). The other, presented in red line, dehydration proceeds to HCO + OH (DCO-I) and followed by CO and H2O (DCO-II).

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Figure 3. a) Strain effect on the change in activation energy of dehydrogenation (purple square), dehydration (red triangle) with respect to Pure Pd. b) Effect of surface charge polarization on the change in activation energy of dehydrogenation and dehydration with respect to Pure Pd.

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Figure 4. Projected density of states plots on the d-band of PdStr-M. Red arrow denotes the position and value of d-band center respectively. The dotted line at 0 eV denotes Fermi level position

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Figure 5. Effect of lattice strain (ε) on the change of d-band center (red cross) and the strain contribution to the binding energy of HCOO (EHCOO) and HCO+OH (EHCO+EOH) with respect to Pd(111), denoted in blue square and circle respectively. Here, EHCO+EOH is calculated with respect to fully separated adsorbed species (separation state was evaluated by individually placing each adsorbed species on the 2×2 surface unit cell).

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Figure 6. The change in the transition state configurations and the corresponding activation energy barriers for the DCO-I step as a function of decoupled strain (surface lattice distance) and ligand (surface charge states) effects. The top row is the structure of transient state (hereby termed TS) of dehydration process DCO-I by strain effect and the bottom is by ligand effect.

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Figure 7. Orbital resolved density of states of PdLig-M for d-band; a) dz2; b) dxy + dx2- y2; c) dyz + dxz. The dotted line at 0 eV denotes Fermi level position.

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Figure 8. Effect of surface charge polarization on the d orbitals near the Fermi level (−0.25 < E−Ef < 0). Red circles and × represent dz2 + dyz + dxz and dxy+ dx2-y2 , respectively and blue square and triangles represent binding energy of HCOO (EHCOO) and HCO + OH(EHCO+EOH), respectively. Here, EHCO+EOH is calculated with respect to fully separated adsorbed species (separation state was evaluated by individually placing each adsorbed species on the 2×2 surface unit cell).

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Table of Content (graphical abstract)

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Role of Heteronuclear Interactions in Selective H2 ... - ACS Publications

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