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Modulating the Electrocatalytic Performance of Palladium with the Electronic Metal-Support Interaction: A Case Study on Oxygen Evolution Reaction Hongyang He, Junxiang Chen, Dafeng Zhang, Fang Li, Xin Chen, Yumei Chen, Linyan Bian, Qiufen Wang, Peigao Duan, Zhenhai Wen, and Xiao-Jun Lv ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00460 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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Modulating the Electrocatalytic Performance of Palladium with the Electronic Metal−Support Interaction: A Case Study on Oxygen Evolution Reaction
Hongyang He,†,# Junxiang Chen,§,# Dafeng Zhang,*,†,‡ Fang Li,‡ Xin Chen,† Yumei Chen,† Linyan Bian,† Qiufen Wang,† Peigao Duan,† Zhenhai Wen,*, § and Xiaojun Lv,*,‡
†
Department of Energy and Chemical Engineering, College of Chemistry and Chemical
Engineering, Henan Polytechnic University, Jiaozuo 454003, P. R. China ‡
Key Laboratory of Photochemical Conversion and Optoelectronic Materials & CAS-HKU
Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China §
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian
Province Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P.R. China
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ABSTRACT: The present work reports a general approach to improve the electrocatalytic property of noble metal through regulating its electron status by introducing the electronic metal−support interaction (EMSI). As a case study, the catalytic activity of metallic Pd towards oxygen evolution reaction (OER) in alkaline solution has been significantly promoted by stabilizing Pdδ+ oxidic species at the interface of Pd−metal oxide support with the help of EMSI effect, suggesting an intrinsic advantage of Pdδ+ in driving OER. We further demonstrate that the chemical state of Pdδ+ can be easily modulated in the range of +2 to +3 by changing the metal oxide support, interestingly, accompanied by a clear dependence of the OER activity on the oxidation state of Pdδ+. The high Pd3+ species-containing Fe2O3/Pd catalyst has fed an impressively enhanced OER property, showing an overpotential of 383 mV at 10 mA cm−2 compared to those of >600 mV on metallic Pd and 540 mV on Fe2O3/glassy carbon. The greatly enhanced OER performance is believed to primarily derive from the distinctive improvement in the adsorption of oxygenated intermediates (e.g., *OH and *OOH) on metal-oxide/Pd catalysts. Moreover, similar EMSI induced improvements in OER activity in alkaline solution are also achieved on both of the Fe2O3/Au and Fe2O3/Pt which possess the oxidic species of Au3+, and Pt2+ and Pt4+, respectively.
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1. INTRODUCTION For noble metals, their catalytic performances have a great dependence on the interaction between noble metal and oxide support, which is well-known as the metal−support interaction (MSI).1 The strong metal−support interaction (SMSI) was initially proposed by Tauster et al. to describe the significant suppression in catalytic activity and chemisorption capacity of H2 and CO on TiO2−supported noble metal clusters after reduction at high temperatures.2-3 Since then, extensive efforts have been devoted to understand the fundamental concepts underlying the SMSI in both heterogeneous catalysis and electrocatalysis.4-6 Three major mechanisms, i.e., electronic, geometric, and bifunctional effects, have been widely accepted, corresponding to the charge redistribution between metal and support, the migration of support oxide species onto metal surface, and the dual active sites from metal and support at interface boundary, respectively.3,7-8 Among them, the electronic effect was not well recognized until recently. In 2012, Rodriguez and co-workers reported a new type of SMSI that had induced a large electronic perturbations (a charge transfer from Pt to support together with an O transfer from support to Pt) for small Pt particles in contact with the unreduced CeO2(111).9 Campbell recommended to use the term of electronic metal−support interaction (EMSI) to describe the phenomenon.8 Excitingly, the EMSI effect provides a direct way to modulate the d-electron status of supported noble metals through the electron transfer between metal and support.10-11 The related electronrich/-deficient configuration will specifically influence the critical chemisorption and activation of reactants on metal in the overall reaction, and thus the catalytic performance.1213
For instance, metal oxides (TiO2, Nb2O5, CeO2 etc.) supported electron-rich Pd, Pt, and Rh
are certainly active for selective hydrogenation of unsaturated aldehydes and ketones, owning to the weakened adsorption of products on charge donated metal surface.14-16 By contrast, electron-poor metal possesses more additional d-band vacancies.11 The extra partially vacant
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5d-orbitals of small Au nanoparticles on reducible metal oxides have shown a well-known low temperature CO oxidation property, attributed to an enhanced O2 adsorption and activation on positively charged Au atoms at interface boundary.17-18 Zhang et al. reported an extremely active single-atom catalyst of Pt1/FeOx for CO oxidation.19 The positively charged, high-valence Pt atoms were suggested to have helped reduce the CO adsorption energy and the activation barriers for CO oxidation.19 In electrocatalysis, the EMSI has been also introduced to improve the catalytic properties of noble metals.6,20-22 The electron transfer across the metal−support interface can be actuated by a general oxidation/reduction pretreatment, supplying an efficient modification on the electron configuration and catalytic property of noble metals.13 As reported, metal oxide supported Pt particles with exposed electron-rich surface atoms are more active to catalyzing the oxygen reduction reaction and methanol oxidation reaction. The suppression of the surface oxide formation and the CO-intermediates adsorption is suggested to have released more active sites for O2 and methanol adsorption in corresponding reactions.23-25 Recently, Strasser et al. published an electronic charge donation from metal oxide support (Sb-doped SnO2, ATO) to the active IrOx in Ir/IrOx/ATO catalyst, generating more stable Ir3.2+ species to catalyze oxygen evolution reaction (OER) in acidic solution. This electronic perturbation is similar to the EMSI effect, although the electron status modulation involves the oxidized noble metal (IrOx) rather than the metallic element (Ir) by its oxide support (ATO).26 Note that, among the reported studies only a few have dealt with the influence of electron-deficient configuration on the electrocatalytic property of noble metals.27-28 According to the latest research, the electrooxidation of formic acid can be significantly enhanced on Pd/WO2.72 catalyst, while the electron transfer from metal to support has made a clear reduction in the surface electron density of Pd.29
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Accordingly, in this work the EMSI effect is introduced to modulate the electron status of Pd, especially, to produce the electron-deficient Pd atoms on reducible metal oxides, based on which the influence on the electrocatalytic performance of Pd is evaluated. As a case study on OER, we suppose, the electron-deficient Pd with more additional d-band vacancies will help to improve the adsorption of oxygenated species (e.g., *OH and *OOH) on surface, which is the key step of the overall reaction.30 Ordinarily, the chemisorption of oxygen is weak on metallic Pd, however, it may be significantly improved on the electron-deficient Pdδ+ species. As a result, the moderate Pd−O bonds will favor a higher OER activity.31 As expected, when bulky Pd just presents a featureless activity toward OER, that is, a large overpotential (> 600 mV) is required to drive the reaction at a practical rate of j = 10 mA cm−2,32-33 the highvalence Pdδ+ species at Pd−metal oxide interface have contributed a distinctively enhanced OER performance. The overpotentials have been greatly decreased to as low as 383 mV on Fe2O3/Pd catalyst with a more than 200 mV reduction from >600 mV of the overpotential on metallic Pd plate. As demonstrated, the oxidized Pd at the perimeter of Pd−metal oxide interface has significantly enhanced the adsorption of oxygenated intermediates for the overall OER process, which is believed to contribute to the great improvement of catalytic property. The present work provides a worthwhile discussion on the critical role of electron status of noble metal in electrocatalysis.
2. EXPERIMENTAL SECTION 2.1. Materials. Palladium (99.99 %), platinum (99.99 %), gold (99.99 %) and glassy carbon (GC) plates were purchased from Tianjin Aida Hengsheng Co., Ltd, China. Analytical grade Fe(NO3)3·9H2O, (NH4)6Mo7O24·9H2O, (NH4)10H2(W2O7)6·xH2O, KOH, HCl, and PdCl2 were purchased from Sinopharm Chemical Reagent Co., Ltd, China, and used as received. The water used throughout all experiments was purified through a Millipore system.
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2.2. Preparation of Catalysts. Annealed Pd Bulky Plates. Pd plates were well polished with α-Al2O3 slurries in decreasing sizes (1.0 µm to 50 nm), cleaned by ultrasonication in absolute ethanol and 1 M KOH for 15 min, respectively, rinsed with copious deionized water, and dried under a stream of high-purity N2, successively. The thoroughly cleaned Pd plates were then annealed at different temperatures of 250 °C, 350 °C, and 450 °C in air for 1 h to obtain samples referred as Pd-250, Pd-350, and Pd-450, respectively. Metal-Oxide/Pd Catalysts. Ten µL precursor aqueous solution with metal cation concentration of 10 mM was pipetted onto each freshly cleaned Pd plate within a confined circle area of 0.28 cm2 (3 mm in diameter) and dried in air naturally. The generated precursor coatings were then pyrolyzed directly in air for 1 h at the temperature of 250 °C, 350 °C, and 450 °C for precursors of Fe(NO3)3, (NH4)6Mo7O24, and (NH4)10H2(W2O7)6, to fabricate Fe2O3, MoO3, and WO3 film on Pd plates, respectively. The pyrolysis temperatures are adopted according to the completely thermal decomposition of corresponding precursors.34-36 The metal-oxide/GC, metal-oxide/Au, and metal-oxide/Pt catalysts were produced in the same way of preparing the metal-oxide/Pd, but just replaced the Pd plate with GC, Au, and Pt plates, respectively. Carbon fiber paper (CFP, Toray TGP-H-060) supported Fe2O3/Pd was prepared by pyrolyzing Fe(NO3)3 on Pd film coated CFP. The Pd film on CFP was electrodeposited using potential step technique at −0.4 V (vs saturated calomel electrode, SCE) for 60 s seeding followed by a grain growing at 0.1 V (vs SCE) for 1 h in 4 mM PdCl2 and 0.1 M HCl solution. The synthesis parameters for all catalysts are further summarized in Table S1. Metal-oxide(p)/Pd Catalysts. Firstly, the powdery metal oxides were prepared by directly pyrolyzing the precursor reagent powders in air at the same temperature as producing the corresponding metal-oxide/Pd catalysts. The obtained metal oxide powder was then dispersed
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in water under ultrasonication to achieve a suspension with metal element concentration of 10 mM. Next, 10 µL of the suspension was pipetted onto a Pd plate, and then dried in air. 2.3. Physical Characterizations. Scanning electron microscopic (SEM) images were obtained from Hitachi S4800 at 5 kV. High-resolution transmission electron microscopic (HRTEM) and high-angle annular dark-field scanning TEM (HADDF-STEM) analyses were performed on JEOL 2100F at 200 kV. X-ray photoelectron spectroscopy (XPS) studies were carried out on Thermo ESCALAB 250XI using an Al Kα monochromated (150 W, hv = 1486.6 eV, 500 µm spot size) source. The hydrocarbon C 1s line at 284.8 eV from adventitious carbon was used for energy referencing. Powder X-ray diffraction (XRD) and grazing incidence XRD (GIXRD) patterns were performed on a Phillips PANalytical X’Pert Pro diffractometer operating at 40 mA and 40 kV using a curved graphite diffracted-beam monochromator with Cu Kα radiation (incident angle = 0.5° for GIXRD, λ = 1.541 Å) to remove Fe florescence. 2.4. Electrochemical Measurements. All of the electrochemical experiments herein were performed in oxygen saturated 1 M KOH solution for OER investigations on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai) using a conventional three-electrode cell at room temperature. An SCE and a platinum plate were used as reference and
counter
electrodes,
respectively.
Linear
sweep
voltammetric
(LSV)
and
chronopotentiometric (CP) measurements were carried out at a scan rate of 10 mV s−1 and a current density of 10 mA cm−2, respectively. Unless otherwise stated, all potentials were corrected for uncompensated resistance (R) and were given in the text relative to the reversible hydrogen electrode (RHE) using equation ERHE = ESCE + 0.242 V + 0.059·pH − iR, where ERHE is the potential calibrated against RHE, ESCE is the potential measured against SCE, and i is the current. Current densities were calculated using the geometric surface area (0.28 cm2).
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Figure 1. (a) Schematic illustration of the preparation of Pdδ+ species at Pd−metal oxide interface and the preferred reaction sites for OER at the interface boundary. SEM images of (b) Fe2O3, (c) WO3, and (d) MoO3 films pyrolytically deposited on Pd plates.
3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Analysis of Metal-oxide/Pd Catalysts. In order to produce stable electron-deficient Pdδ+ species in different chemical states at Pd−metal oxide interface, the widely used Fe2O3, WO3, and MoO3 active supports in studying the EMSI effect are chosen as metal oxide substrates herein.37-40 Note that, different from the regular metal−support arrangement, that is, dispersing noble metal particles on metal oxide surface in nanoscale, a simplified Pd−metal oxide electrode is adopted in this work, which contains a bulky Pd metallic plate and a metal oxide coating with thickness at sub-micrometer scale. As schematically shown in Figure 1a, this concise electrode can largely rule out the size effect in nanoscale on the Pd−metal oxide interaction and thus benefit the isolation of the intrinsic interaction between a noble metal and its support.41-43 Moreover, it can also greatly reduce the physical contact resistance of electron transferring to electrode in the overall reaction. It is 8 ACS Paragon Plus Environment
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necessary to mention that the present work focuses mainly on evaluating the influence of electron-deficient configuration on the electrocatalytic property of Pd. Any other attempting to construct large numbers of exposed active sites at interface boundary, such as to prepare conventional 3D catalysts, to achieve a much higher electrocatalytic current lies beyond the scope of our discussion. In practice, film-like metal oxide is deposited on bulky metallic Pd plate by directly pyrolysis of the precursor coating in air. Figure 1b−d illustrate the SEM images of the as-prepared Fe2O3, WO3, and MoO3 films on Pd plates. Microscale cracks are observed in Fe2O3 and MoO3 films, which become in dense in WO3. The cracks are believed to be important here, because they will supply many reaction sites exposed at the perimeter of Pd−metal oxide interface for the aggression of reactant in following catalysis. Metal oxide crystallinity is examined indirectly from XRD patterns of the corresponding powdery oxides that are produced under the same pyrolytic condition without Pd plate (Figure S1−S2), because the mass loading of film-like metal oxide in the as-prepared metal-oxide/Pd catalyst is too low to be easily detected under the strong reflection background from the underlying Pd plate even using the GIXRD method (Figure S3). The dominant phases of rhombohedral α-Fe2O3, hexagonal WO3, and orthorhombic MoO3 are confirmed from the corresponding metal oxides (Figure S2).
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Figure 2. The representative (a) HRTEM image and (b) corresponding HAADF-STEM image with elemental mapping analysis of Fe2O3/Pd. Core level XPS spectra of (c) Pd 3d in the as-produced metal-oxide/Pd catalysts and the metallic and annealed Pd plates, and (d) Fe 2p in the Fe2O3/Pd and Fe2O3/GC catalysts.
In order to examine the structure of Pd−metal oxide interface, a pyrolysis Fe2O3/Pd catalyst is prepared on and then exfoliated from CFP (Figure S4). Figure 2a shows the representative HRTEM image of the Fe2O3/Pd. Distinct lattice fringes with spacing distances of 0.23, 0.27, and 0.25 nm are observed, assigned to the lattice planes of metallic Pd (111) (JCPDS no. 461043), PdO (002) (JCPDS no. 41-1107), and α-Fe2O3 (110) (JCPDS no. 33-664), respectively. Clearly, the existence of oxidized Pd grain at the Pd−Fe2O3 interface can be identified, which has closely contacted with both of the metallic Pd and the Fe2O3. The relative spatial distributions of Pd and Fe elements in a larger aggregated particle are determined by the energy-dispersive X-ray spectroscopy (EDS) under HADDF-STEM mode. As illustrated in 10 ACS Paragon Plus Environment
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Figure 2b, each target element shows a well dispersion throughout the entire particle, indicating the Pd−Fe2O3 interface in abundance. X-ray photoelectron spectroscopy (XPS) is used to analyze the surface composition and the chemical states of Pd. Survey XPS spectra in Figure S5 confirm the existence of Pd and metals from the corresponding oxide films. Besides, no other metal is found, especially for the contaminative Fe in the as-prepared WO3 and MoO3 films (Figure S6). From the core level XPS spectra of Fe 2p, W 4f, and Mo 3d in Figure S7, dominant states of Fe3+, W6+, and Mo6+ can be recognized, according to the BE peaks at 710.9 eV (Fe 2p3/2),44 35.6 eV (W 4f7/2),45 and 232.6 eV (Mo 3d5/2),46 respectively.
Table 1. Summary of BEs of Pd 3d5/2 (3d5/2) assigned to Pdδ+ species and OER overpotentials at corresponding current densities obtained on metallic and annealed bulky Pd plates and metal-oxide/Pd catalysts. Catalysts
BEs (eV)
OER overpotentials (mV)
metallic Pd
η5a 591
Pd-250
578
η10a
Pd-350
336.8 (342.2)
526
605
Pd-450
337.1 (342.5)
462
513
Fe2O3/Pdb
337.7 (343.1)
370
383
MoO3/Pdb
337.0 (342.4)
466
524
WO3/Pdb
337.3 (342.7)
423
461
a
OER over potentials at current densities of 5 (η5) and 10 (η10) mA cm−2.
b
Catalysts were prepared under the pyrolysis temperatures of Fe2O3/Pd at 250 °C, MoO3/Pd
at 350 °C, and WO3/Pd at 450 °C in air for 1 h.
The core level XPS spectra of Pd 3d in metal-oxide/Pd catalysts are displayed in Figure 2c together with those in metallic Pd, Pd-250, Pd-350, and Pd-450. Both metallic Pd and Pd-250 11 ACS Paragon Plus Environment
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just show strong Pd 3d5/2 (Pd 3d3/2) peaks assigned to Pd0 at 335.2 eV (340.4 eV),47 all of the other catalysts show another Pd 3d5/2 (Pd 3d3/2) main peak in the range of 336.8−337.7 eV (342.1−343.1 eV) assigned to the oxidic Pdδ+ species. The Pd 3d5/2 (3d3/2) binding energies (BEs) of Pdδ+ species are summarized in Table 1. Specially, besides a general blue-shift in Pd 3d BEs caused by the annealing temperature rising, a relative slight blue-shift also exists when comparing the metal-oxide/Pd catalyst with the corresponding blank Pd plate annealed under the same temperature. Pd 3d peak BEs exhibit ca. 0.2 eV higher in WO3/Pd and MoO3/Pd catalysts than those in Pd-450 and Pd-350 annealed plates, respectively. Moreover, in the case of Fe2O3/Pd, an unusual Pd 3d5/2 (Pd 3d3/2) peak at high BE of 337.7 eV (343.1 eV) is identified, which does not exist in Pd-250 plate, which was annealed at the same temperature as Fe2O3/Pd, or in other Pd-containing catalysts prepared in this work. This is the highest BE of Pd 3d observed in all of our samples, even 0.4 eV higher than that in WO3/Pd catalyst, which is close to that of Pd4+ species (Pd 3d5/2(PdO2): ~338.0 eV).47 Ferri48 and Uenishi49 attribute the similar Pd 3d peak in Pd-doped LaFeO3 (LaFe0.95Pd0.05O3) catalyst to Pd3+ species as it falls between the BEs corresponding to Pd4+ and Pd2+. Furthermore, the asymmetry of this peak and its weak shoulder at lower energy direction indicate the coexistence of mixed oxidation states, and the deconvolution of the peak results in another weak peak at 336.9 eV (342.3 eV) (Figure S8) for Pd2+ species caused by the pyrolysis process under 250 °C in air. The unique Pd3+ species in Fe2O3/Pd suggest that the Pd electron status has been readily changed via the strong Pd−Fe2O3 interaction.50 The Fe 2p BE is also studied in both of the Fe2O3/Pd and Fe2O3/GC samples. As shown in Figure 2d, a slight redshift of ca. 0.5 eV of Fe 2p3/2 peak is observed in Fe2O3/Pd (710.9 eV) by comparing with that in Fe2O3/GC (711.4 eV). Another weak peak at 709.8 eV assigned to Fe2+ species is found after the deconvolution of the Fe 2p3/2 peak (Figure S9). Therefore, a probable transfer of a small number of electrons from Pd to Fe has taken place during the pyrolysis of
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Fe(NO3)3 coating on Pd plate. Note that, as the pyrolysis temperature (250 °C) is insufficient to produce dense PdO film on bare Pd plate, the present of Pd3+ and Fe2+ species has certainly demonstrated a strong Pd−Fe2O3 interaction.37,39 Overall, the blue-shift of Pd 3d peaks indicates the successful preparation of the positively charged electron-deficient Pd atoms at Pd−metal oxide interface, proving that the electronic structure of Pd can be tuned easily by the metal oxides attached.
Figure 3. OER electrocatalysis on metallic and annealed bulky Pd plates in 1 M KOH. iRcorrected LSV curves are recorded at a scan rate of 10 mV s−1.
3.2. OER Properties of Oxidic Pdδ+ Species. The role of oxidized Pd in OER is initially investigated on bare bulky Pd plates annealed in air. As confirmed in Figure 2c, a gradual increase in the chemical state of Pd element is clearly addressed with the annealing temperature rising. Pd oxides have become the dominant surface phase on Pd-350 and Pd-450 plates, while Pd-250 and Pd plate surfaces contain mainly metallic Pd0. The OER responses of the Pd plates are studied in 1.0 M KOH solution by using LSV technique. As shown in Figure 3, rapidly increased anodic current are found when potential sweeps to more positive side. Taking the overpotential at j = 5 mA cm−2 (η5) for comparison (Table 1), clearly, it presents a gradual decrease from 591 mV on metallic Pd plate to 462 mV on annealed Pd-450
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plate, suggesting an improvement of OER activity on Pd oxides. Interestingly, this activity enhancement is well consistent with the increase in the chemical states of Pdδ+ species, which should verify a distinctive advantage of Pdδ+ species in higher valence for OER. Actually, it is proposed that the molecular O2 is not ready to form until the Pd oxides emerge at high anodic polarization on metallic Pd,33 similar to that observed on gold anode. Koper et al. consider the formation of gold oxides under high potential acting as a precondition for the evolution of O2.51 The first O2 should probably come from a decomposition of surface oxides. Therefore, the Pd oxides have benefited the O2 evolution with improved OER activities. The OER performances of metal-oxide/Pd catalysts are shown in Figure 4a (solid lines). Steep increases in anodic current corresponding to OER process are observed on all catalysts above 1.5 V. The overpotentials at j = 5 and 10 mA cm−2 are summarized in Table 1. A smaller η10 of 383 mV is obtained on Fe2O3/Pd, while they are 461 mV on WO3/Pd and 524 mV on MoO3/Pd, indicating a better OER activity of Fe2O3/Pd catalyst than WO3/Pd and MoO3/Pd. Considering that the Fe2O3/Pd contains Pdδ+ species in much higher state than WO3/Pd and MoO3/Pd (Figure 2c and Table 1), the OER activity, in fact, also shows an improvement consistent with the increase in the chemical states of Pdδ+ species. That is, when the higher state Pd3+ containing Fe2O3/Pd shows a much better OER activity, the lower state Pd2+ containing MoO3/Pd just provides an ordinary activity. Besides, we have further compared the η5 and η10 on blank Pd-250, Pd-350, and Pd-450 with those on Fe2O3/Pd, MoO3/Pd, and WO3/Pd, respectively, and found another great decrease in the overpotential on the metal-oxide/Pd catalysts even though they are produced under the same temperatures as the corresponding blank Pd plates. For instance, there is a 200 mV reduction in the η5 on Fe2O3/Pd (370 mV) compared with that on Pd-250 (578 mV). This certainly proves a distinctive OER activity of the metal-oxide/Pd catalysts. Once again, this activity
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enhancement exhibits a clear dependence on the oxidation state of Pdδ+ species (Figure 2c, Table 1), as another solid evidence of the high-valence Pdδ+ contributed high OER reactivity.
Figure 4. OER electrocatalysis in 1 M KOH. iR-corrected LSV curves recorded at a scan rate of 10 mV s−1, (a) on metal-oxide/Pd catalysts (solid lines) and metal-oxide/GC catalysts (dash lines), (b) on metal-oxide(p)/Pd. (c) Tafel plots of metal-oxide/Pd catalysts. (d) The corresponding CP curves at j = 10 mA cm−2.
To further identify this critical role of oxidic Pdδ+ species in OER, the controlled experiments are carried out on two other kinds of catalysts (the metal-oxide/GC and the metal-oxide(p)/Pd) under the same condition. Figure 4a (dash lines) and 4b display the LSV responses of them, respectively. Among the metal-oxide/GC catalysts, only Fe2O3/GC shows an obvious η10 = 540 mV, in a good agreement with the typical OER feature of Fe2O3 anode 15 ACS Paragon Plus Environment
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reported previously,52-53 but still far inferior to the η10 = 383 mV of Fe2O3/Pd catalyst. Other metal-oxide/GC catalysts are almost inert for OER. As there is no Pd element in metaloxide/GC samples, the result proves the necessary of Pd for the distinctive improvement of OER activity in metal-oxide/Pd. In the case of metal-oxide(p)/Pd catalysts that were produced by simply drop-casting pre-synthesized metal oxide powders onto metallic Pd plates, there is only metallic Pd rather than oxidic Pdδ+ species in this kind of catalyst. In OER, as illustrated in Figure 4b, they just exhibit the activities comparable to that of metallic Pd (Figure 2b), much lower than that of metal-oxide/Pd. This confirms the key role of oxidic Pdδ+ species in enhancing the OER activity. Even after annealed, all of the Fe2O3(p)/Pd-250, MoO3(p)/Pd-350, and WO3(p)/Pd-450 samples still provide mediocre OER activities, suggesting the distinctive advantage of the pyrolysis method in producing high active Pdδ+ species. In general, the results agree that, to develop the OER property, depositing metal oxides directly on Pd using pyrolysis method is necessary, under which condition, the oxidic Pdδ+ species will inevitably be introduced at the Pd−metal oxide interface (Figure 2). Because Pd oxides possess an intrinsic advantage for OER (Figure 3), we believe, the Pdδ+ species, as the primary materials of metal-oxide/Pd catalysts different from the metal-oxide/GC and metaloxide(p)/Pd samples, have played a key role in improving the OER activity. It is suggested that the metal site serves as the center for the adsorption of oxygenated intermediates that will react with the similar species adjacent and/or the oxygen from the catalyst lattice to form O2.30,54 Accordingly, a moderate metal−adsorbate bond is favorable for the overall OER process.31 For metallic Pd, the adsorption of oxygenated intermediates is too weak to drive OER. By contrast, the positively charged Pdδ+ cations stabilized by adjacent metal oxide should possess an improved adsorption to the oxygenated anions, resulting in a moderate Pd−O bond and thus benefiting the OER process with an improved catalytic activity. In fact,
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we have certainly observed a significant enhancement of the adsorption of oxygenated species (e.g., *OH) on metal-oxide/Pd catalysts. Figure 5a illustrates the XPS spectra for O 1s region of the Fe2O3/Pd and the Fe2O3/GC for comparison. The BE intensity has been normalized to the strong main peak of O 1s at 530.3 eV. Clearly, there is no discernible difference in the location and symmetry of these main peaks between metal-oxide/Pd and metal-oxide/GC catalysts, suggesting them to be mainly assigned to the lattice oxygen of Fe−O, rather than the oxygen of Pd−O, which implies that the Pdδ+ species are in a relatively low content, well agreeing with the observations in the survey XPS spectra in Figure S5. Along with the main peaks, obviously, there are peak shoulders at high energy region for both samples. Note that, because the Pd 2p3/2 peak partially overlaps with that of O 1s, the contributions of Pd0 (~532.2 eV of 2p3/2) and Pd2+ (~534.0 eV of 2p3/2) species to the peak shoulders cannot be entirely ruled out. But as confirmed, the Pdδ+ species are relatively scarce, as well as the metallic Pd0; therefore, it is safe to attribute the peak shoulders mainly to the adsorption of oxygenated species on surface.55 More importantly, Fe2O3/Pd exhibits higher peak shoulders than Fe2O3/GC, indicating a larger amount of oxygenated adsorbates on Fe2O3/Pd surface. The increased amount of adsorbates, we believe, comes from the exposed Pd surface in the location of film cracks (Figure 1b), especially from the exposed perimeter at Pd−metal oxide interface. Furthermore, the improved adsorption of oxygenated species on metal-oxide/Pd catalysts is also investigated through their electrochemical desorption behaviors under LSV technique. As shown in Figure 5b, after an anodic polarization at 1.6 V in 1 M KOH for 3 min, the desorption of oxygenated species presents during the negative potential sweeps on all samples, beginning at about 0.75 V. Clearly, the onset potential shows a gradually negative shift as the electrode changes from metallic Pd plate to MoO3/Pd and WO3/Pd, then to Fe2O3/Pd, successively. Meanwhile, with the decrease of potential, the corresponding desorption current increases much slower on Fe2O3/Pd than that on WO3/Pd
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and MoO3/Pd, indicating the sluggishly stripping behavior of adsorbates from Fe2O3/Pd, which, in other words, means an improvement of the oxygenated species adsorption on Fe2O3/Pd catalysts. This is well consistent with the increase of chemical states of oxidic Pdδ+ cations, revealing an intrinsic advantage of high-valence Pdδ+ on promoting the oxygenated species adsorption in the overall OER process.56
Figure 5. (a) Core level XPS spectra of O 1s in the as-produced Fe2O3/Pd and Fe2O3/GC. (b) LSV curves corresponding to the electrochemical desorption of oxygenated species on metallic Pd and the pyrolytic metal-oxide/Pd catalysts.
Besides, a synergistic effect from the metal oxide attached to Pdδ+ species at the Pd−metal oxide interface is believed to have participated in the development of OER activity as well. According to the studies by Boettcher et al.,57-59 FeOOH itself possesses a noticeable inherent catalytic activity for OER, whose low apparent activity is usually hindered by its poor electrical conductivity. FeOOH well dispersed on gold substrate has exhibited a better OER activity, benefiting from the close interaction of Fe cations with electrical conductive AuOx at interface. In this work, a strong interaction between Fe2O3 and PdOx/Pd is also clearly established (Figure 2). Therefore, Fe cations interacting with the interfacial PdOx may have
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participated in the overall OER process directly, rather than just acting as a stabilizer for highly active Pd3+ species. In order to understand the synergy effect of Fe2O3 at the interface of Fe2O3/PdOx, density functional theory (DFT) calculations (the DFT details are shown in Supporting Information) are performed. The (001) surfaces are cleaved as the slab models for both Fe2O3 and PdO. The degree of surface oxidation on both surfaces during OER is accounted firstly, which turns out that the surface coordinating numbers (CN) for surfaces Pd and Fe will both be 4, as indicated in Figure 6a. Meanwhile, the Mulliken charge analysis indicates the charge per surface Pd atom under the CN = 4 is ~0.2e less than per bulk Pd of PdO, well matching the higher valence state of surface Pd indicated by the XPS results (Pd3+ in Figure 2c). Inspired by this, the slab models are built (shown in the insert of Figure 6), on which we performed DFT calculation and obtained the reaction free energy diagram (FED) of OER under the classical mechanism containing 4 proton coupled electron transfer (PCET) process (Supporting Information, Reactions S2-S5 for OER mechanism and Figure S10 for the DFT based optimized structures of intermediates). The unified electrochemical thermodynamic framework (UETF) proposed by Nørskov et al.60-61 is introduced to study the activity of OER on both surfaces, in which the theoretical activity is considered to be described by the largest Gibbs free energy differences (∆G0max) among the elementary steps. On the basis of such framework, we obtain the FEDs in Figure 6b, where it is clearly that PdO (001) (∆G0max = 0.70 eV, O* + H2O → OOH* + H+(aq) + e) is more active than Fe2O3 (∆G0max = 1.03 eV, O* + H2O → OOH* + H+(aq) + e). More interestingly, Figure 6b further suggests the origin of a better OER activity on the Fe2O3/PdOx interface to be the existence of an extra pathway (the yellow line) bridging the O* on PdO and OOH* on Fe2O3, which decreases the value of ∆G0max to as low as 0.48 eV (the step marked by the yellow line in Figure 6b). According to the UETF, the kinetic current is considered to be proportional to exp(−∆G0max/RT).62 As a 19 ACS Paragon Plus Environment
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consequence, the kinetic current on the interface would be increased by 4 and 9 orders of magnitude as compared with PdO and Fe2O3, respectively. This synergy effect may have largely contributed the exceptional OER performance on high-valence Pd containing Fe2O3/Pd catalyst.
Figure 6. (a) O* (* stands for the surface site) formation energy as a function of the O coordinating number (CN) of Fe/Pd, where the zero point of formation energy of O* (the orange dashed line) is defined as the chemical potential of O* that leads to the equilibrium of O* + 2H+ + 2e → H2O + *, which is the surface oxidation reaction on catalysts driven by water and the electron. The structures (above) below the orange dashed line are thermodynamically (un)favorable to form (more details of the calculation are shown in Supporting Information). Thus we can conclude the exact coordinating number of surface Pd and Fe will be 4 during OER, leading to the two slab models of Fe2O3 (001) and PdO (001) shown on the right of Figure 6a. (b) The reaction free energy diagram of OER on PdO (red lines) and Fe2O3 (green lines), where an extra reaction tunnel bridging O* on PdO and OOH* on Fe2O3 is marked by a yellow solid line.
The kinetics of OER is examined by Tafel plots to infer which of the four electron-/protontransfer steps are rate limiting in the OER (Figure 4c).63 Tafel slopes of 49−61 mV dec−1, comparable to those of previously reported catalysts,54,58 are obtained on metal-oxide/Pd catalysts, indicating a change of the rate-limiting step from the second electron transfer (slope 20 ACS Paragon Plus Environment
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near 40 mV dec−1) to the chemical step following the first electron transfer (slope near 60 mV dec−1) in OER.59,64 These slopes, in detail, also show a satisfied synchronous decrease with the increase of the oxidation state of Pdδ+ species. Smaller and larger slopes are detected on the higher valence Pd3+ containing Fe2O3/Pd catalyst and the lower valence Pd2+ containing MoO3/Pd catalyst, respectively, suggesting a faster kinetics of OER process on high-state Pdδ+ species. Under the durability test by chronopotentiometry at 10 mA cm−2 (Figure 4d), the Fe2O3/Pd catalyst exhibits a better stability as well, whereas others show evident increases in overpotential by over 100 mV.
Figure 7. SEM images of pyrolytic (a) Fe2O3, (b) WO3, and (c) MoO3 films on Pd plate, and (d) core level XPS spectra of Pd 3d in Fe2O3/Pd catalyst after the durability test.
Serious dissolutions of both MoO3 and WO3 films are observed and should be responsible for the decreased OER property (Figure 7a−c), due to the instability of them in alkaline media. By contrast, the film dissolution of Fe2O3 is negligible. After the durability test, moreover, the chemical state of Fe has a slightly weak blue-shift (Figure S11), and great changes in BE peaks of Pdδ+ species arise. As illustrated in Figure 7d, the main Pd 3d5/2 (Pd 3d3/2) peak of Pdδ+ species has shifted from 337.7 eV (343.1 eV) to 336.9 eV (342.2 eV), indicating a replacement of the dominate Pd3+ species by Pd2+, responsible for the slight decrease in the 21 ACS Paragon Plus Environment
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OER activity. Meanwhile, a new weak peak at 338.1 eV (343.3 eV) has emerged, suggesting the formation of Pd4+.47 However, the amount is too low to contribute an increased OER activity. In addition, the pyrolytic Fe2O3/Au and Fe2O3/Pt catalysts have also been assembled under the same condition as used for Fe2O3/Pd but just replacing Pd plate with Au and Pt plates, respectively. Smooth Fe2O3 films with microscale cracks are found to cover both plates (Figure S12). Further XPS studies verify the existence of oxidic Au3+, and Pt2+ and Pt4+ species in corresponding catalysts (Figure S13), suggesting strong Au−Fe2O3 and Pt−Fe2O3 interactions as well. Under OER investigation, impressively higher active Fe2O3/Au and Fe2O3/Pt are recognized, largely different from the mediocre features on those metallic plates (Figure S14 and Table S3). The values of η10 are also much smaller than those obtained on Fe2O3/GC and the Fe2O3(p)/Au and Fe2O3(p)/Pt catalysts, indicating the intrinsic advantages of oxidic Auδ+ and Ptδ+ species in catalyzing OER.
4. CONCLUSIONS In conclusion, with a simple pyrolysis method, oxidic Pdδ+ species in varied oxidation states have been successfully stabilized in situ at the interface of metal-oxide/Pd catalyst, induced by the strong Pd−metal oxide interaction. The influence of electron configuration of Pd on its electrocatalytic performance has been investigated. As a case study, a close relationship between OER activity and Pdδ+ oxidation state has been clearly addressed, when the high-valence Pd3+ containing Fe2O3/Pd exhibited an impressively promoted OER property. An extensively enhanced adsorption of oxygenated intermediates on metal-oxide/Pd catalyst has been recognized, which is believed to be the key role of Pdδ+ species in OER process. In general, the present work demonstrates that the electrocatalytic activities of Pd, Au, and Pt
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exhibit a large dependence on their electron status that can be efficiently modulated by introducing the electronic metal−support interaction.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:
Supporting data supplies additional computational details of DFT, SEM, XRD, and XPS analysis of all catalysts.
AUTHOR INFORMATION Corresponding author *E-mail:
[email protected] (D.F. Zhang) *E-mail:
[email protected] (X.J. Lv) *E-mail:
[email protected] (Z.H. Wen) ORCID Dafeng Zhang: 0000-0002-2124-3867 Xiaojun lv: 0000-0002-8040-881X Author Contributions #
H.H and J.C. contributed equally.
Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of this work by the Foundation of Henan Educational Committee (13B150024), Natural Science Foundation of Henan Province (182300410196), NSFC (Grant No. 21477136) and Beijing Natural Science Foundation (2182077).
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SYNOPSIS TOC
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