Pd Nanoparticles Coupled to WO2.72 Nanorods for Enhanced

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Pd Nanoparticles Coupled to WO Nanorods for Enhanced Electrochemical Oxidation of Formic Acid Zheng Xi, Daniel P. Erdosy, Adriana Mendoza-Garcia, Paul N. Duchesne, Junrui Li, Michelle Muzzio, Qing Li, Peng Zhang, and Shouheng Sun Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00870 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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Pd Nanoparticles Coupled to WO2.72 Nanorods for Enhanced Electrochemical Oxidation of Formic Acid Zheng Xi,† Daniel P. Erdosy,† Adriana Mendoza-Garcia,† Paul N. Duchesne,‡ Junrui Li,† Michelle Muzzio,† Qing Li,⸹ Peng Zhang‡ and Shouheng Sun*,† † ‡

Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada



School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China Supporting Information Placeholder ABSTRACT: We synthesize a new type of hybrid Pd/WO2.72 structure with 5 nm Pd nanoparticles (NPs) anchored on 50 x 5 nm WO2.72 nanorods (NRs). The strong Pd-WO2.72 coupling results in the lattice expansion of Pd from 0.23 nm to 0.27 nm and the decrease of Pd surface electron density. As a result, the Pd/WO2.72 shows much enhanced catalysis towards electrochemical oxidation of formic acid in 0.1 M HClO4 – it has a mass activity of ~1600 mA/mgPd in a broad potential range of 0.4-0.85 V (vs. RHE) and shows no obvious activity loss after a 12 h chronoamperometry test at 0.4 V. Our work demonstrates an important strategy to enhance Pd NP catalyst efficiency for energy conversion reactions. KEYWORDS: Palladium, tungsten oxide, nano-hybrids, formic acid oxidation, electrocatalysis Among different kinds of proton exchange membrane fuel cells developed for renewable energy applications, direct formic acid fuel cells (DFAFCs) represent a unique type of energy conversion devices with fast fuel oxidation kinetics, high theoretical cell potential and less degree of fuel cross-over issues.1-3 The key to producing commercially viable DFAFCs is to develop a robust catalyst not only for the oxygen reduction reaction (ORR) at the cathode, but also for the formic acid oxidation reaction (FAOR) at the anode.4 Various forms of nanostructured Pt-based catalysts have been studied extensively for FAOR.5-7 The concerns over CO poisoning issue of these Pt catalysts also motivate the serious searches of better alternatives with higher CO tolerance.8-9 Pdbased nanoparticles (NPs) evolve as an interesting class of candidates for the FAOR due to their higher CO-tolerance in the reaction conditions.10-12 But these Pd catalysts lack the desired stability in the acidic electrochemical reaction conditions and have little value of practical uses in the electrochemical devices.13 A promising strategy that has been applied to enhance a metal catalyst activity and durability is to couple this catalyst with a metal oxide, as demonstrated in the system of Pt-TiO2,14 PdCeO2,15 Pd-SnOx,16 Pt-Ti0.7Mo0.3O217 or Pd-HoOx,18 in which the metal catalytic performance is enhanced via strong metal-support

interactions.19 Studying the roles of metal oxides played as the supports, we became interested in tungsten suboxides (WO3-x) with various degrees of oxygen deficiencies. These oxides have been widely investigated and their unique properties have been utilized to develop gas sensors,20 electrochromic devices21 and photocatalysts.22 Among different forms of suboxides studied, the monoclinic W18O49 (WO2.72) is especially an attractive system for uses in an acidic environment. It can be easily separated from other tungsten suboxides23 and is very stable against acid etching.24 Its high degree of oxygen vacancies25 allows strong interactions with reactants26 and facilitates electron transport across the oxide network.27 Here we report a strategy to enhance Pd catalytic efficiency by coupling the NP catalyst with WO2.72 nanorods (NRs). When coupled with the WO2.72 NRs, the Pd NPs can catalyze the FAOR in 0.1 M HClO4, showing a steady mass activity of ~1600 mA/mgPd in a broad potential range (0.4 - 0.85 V) and retaining its activity after the 12 h chronoamperometry test at 0.4 V.

Figure 1. (A) TEM image of the as-synthesized WO2.72 NRs; (B) TEM image of the as-synthesized Pd NPs; (C) TEM image of the hybrid Pd1.1/WO2.72; and (D) HRTEM image of the Pd1.1/WO2.72.

WO2.72 NRs were synthesized by reacting WCl4 with oleic acid (OAc) and oleylamine (OAm) in 1-octadecene (ODE) at 280 °C for 10 h (details in Supporting Information). Figure 1A shows the 1

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TEM image of the NRs with an average length of 50 nm and width of 5 nm. These WO2.72 NRs tend to align side by side, forming layers of NR aggregates due to their intrinsic oxygen vacancies and strong interactions.28 The hybrid Pd/WO2.72 was made in the condition that was same as the synthesis of WO2.72 NRs but in the presence of pre-made 5 nm Pd NPs29 (Fig. 1B). The molar ratios of the Pd/WO2.72 were controlled by the amount of Pd NPs and WCl4 added in the reaction mixture and were measured by inductively coupled plasma atomic emission spectroscopy (ICPAES). We prepared Pd1.1/WO2.72 and Pd0.6/WO2.72 as two representative hybrid structures, shown in Figure 1C & S1 respectively. The high resolution TEM (HRTEM) of the Pd1.1/WO2.72 (Fig. 1D) shows that the WO2.72 NRs and the 5 nm Pd NPs are well crystallized. The lattice fringe spacing of WO2.72 is at 0.38 nm, which is very close to the (010) inter-planar distance of the monoclinic W18O49 (0.378 nm, JCPDS 84-1516). However, the 0.27 nm fringe spacing of Pd in the hybrid is larger than the (111) interplanar distance of 0.225 nm from the face-centered cubic (fcc) Pd structure (JCPDS 46-1043). If only Pd NPs were added without WCl4 during the 10 h reaction, partial NP aggregation was observed and the Pd fringe spacing was measured to be 0.23 nm (Fig. S2), which is close to the fcc-Pd (111) spacing of 0.225 nm but smaller than 0.27 nm observed from the Pd (111) in the Pd/WO2.72 hybrids. As a control, the pre-made 5 nm Pd NPs were also mixed with WO2.72 NRs (1:1 molar ratio) via sonicating two hexane dispersions at room temperature for 1 h, giving a physical mixture of Pd and WO2.72 (Fig. S3A). The HRTEM analysis shows that the Pd NPs in the mixture also have a fringe spacing of 0.23 nm (Fig. S3B). Therefore, this notable lattice expansion of Pd in the Pd/WO2.72 structure must result from the necessary epitaxial interaction between Pd and WO2.72 during the synthesis.

Figure 2. (A) XRD patterns of the Pd NPs, WO2.72 NRs and hybrid Pd1.1/WO2.72 along with the standard peaks of W18O49 (JCPDS 84-1516) and Pd (JCPDS 46-1043); (B) Raman spectra of the Pd NPs, WO2.72 NRs and hybrid Pd1.1/WO2.72; (C) XPS spectra of the Pd1.1/WO2.72 and Pd (Pd 3d) NPs; (D) XPS spectra of the Pd1.1/WO2.72 and WO2.72 (W 4f); (E) Pd

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K-edge XANES spectra of the Pd1.1/WO2.72 and the reference Pd foil; (F) Pd K-edge EXAFS spectra of the Pd1.1/WO2.72 and the reference Pd foil.

The Pd/WO2.72 structure was further characterized by X-ray diffraction (XRD) and compared with the single component Pd NPs and WO2.72 NRs (Fig. 2A). The as-synthesized WO2.72 has the monoclinic structure of W18O49 (P2/m, JCPDS 84-1516) that is different from other common tungsten oxide phases (Fig. S4).30 The XRD of the Pd NPs shows the typical fcc structure. However, in the Pd/WO2.72, the Pd-peaks appear at smaller diffraction angles, indicating the increase of Pd lattice spacing and confirming the TEM analysis results. The Pd/WO2.72 structure was also characterized by Raman spectroscopy (Fig. 2B). In the low wavenumber region, the bands around 87 and 130 cm-1 are from the W–O–W bending modes, while the band at 267 cm-1 belongs to O–W–O bending modes that are found specifically in W18O49. In the high wavenumber region, two peaks at 712 and 805 cm-1 are from the W–O stretching modes.31 The small bump around 640 cm-1 is probably caused by the interactions between the coupled Pd with WO2.72.32 The X-ray photoelectron spectroscopy (XPS) was further applied to characterize the electronic structure change of the Pd NPs upon their coupling with WO2.72 (Fig. 2C). Compared with the non-coupled Pd NPs (3d5/2 and 3d3/2 binding energy at 335.51 eV & 340.80 eV), the coupled ones show a slightly larger 3d5/2 and 3d3/2 binding energy (335.77 eV & 341.09 eV respectively), indicating the decrease in the surface electron density on Pd NPs once they are coupled to WO2.72.27, 33 On the other hand, compared with that of the free WO2.72 NRs, the binding energy of the W 4f (doublet due to the presence of both W6+ and W5+ in WO2.72) in the Pd-coupled WO2.72 is negatively shifted (Fig. 2D), further confirming the strong electronic interactions between the two coupled species.27 The Pd/WO2.72 was also analyzed by Pd K-edge and W L3-edge X-ray absorption spectroscopy (XAS), including both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), to further elucidate the change of Pd structure upon its coupling to WO2.72. The W L3-edge XANES spectrum exhibits a sharp white line (the peak following the edge jump) corresponding to the oxidized state of W and the EXAFS spectrum shows the intense primary W–O scattering path in WO2.72, indicating that W is in a distorted WO6 octahedral center (Fig. S5).34, 35 The Pd K-edge XANES spectra for the Pd/WO2.72 and the Pd foil reference sample are very similar (Fig. 2E), suggesting that Pd in the hybrid structure has a metallic character. The absorption edge energy of Pd in the Pd/WO2.72 (24,353.17 eV) is higher than that of the Pd foil (24,352.6 eV) due to the WO2.72 electronic interactions with Pd, consistent with the XPS observation.36 From the EXAFS spectra of the Pd/WO2.72 (measured at 90 K to increase the signal to noise ratio) and the Pd foil (measured at 300 K) (Fig. 2F) and the corresponding fitting results (Fig. S6), we can obtain the Pd coordination number and Pd-Pd distance in both Pd/WO2.72 and Pd foil (Tab. S1). Compared to the Pd foil, the Pd in the hybrid structure has smaller coordination number and a slightly longer Pd-Pd distance. Considering the Pd-Pd distance is obtained from the 90 K measurement and this distance should be larger when measured at 300 K, the increase in Pd-Pd length should support the TEM analysis that Pd NPs have the lattice expansion upon their coupling to the WO2.72. Overall, these characterizations suggest that the Pd NPs are still nanosized without obvious aggregation in the hybrid Pd/WO2.72 structure, and that coupling Pd to WO2.72 induces lattice expansion of Pd NPs and the decrease of Pd surface electron density.

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Figure 3. (A) CVs and (B) FAOR CVs for the Pd1.1/WO2.72, Pd0.6/WO2.72 and Pd catalysts; (C) FAOR CVs of the 400 °C annealed Pd1.1/WO2.72, Pd and the physical mixture of Pd NPs + WO2.72 NRs; (D) FAOR CVs of the annealed Pd1.1/WO2.72 and Pd before and after 12 h chronoamperometry i-t test at 0.4 V (vs. RHE). The CVs were obtained in the N2-saturated 0.1 M HClO4 and the FAOR CVs were from the solution of 0.1 M HClO4 + 0.1 M HCOOH with a scan rate of 50 mV/s.

The as-synthesized Pd/WO2.72, as well as the Pd NPs and WO2.72 NRs, were loaded onto KetjenBlack EC-300-J carbon at a mass loading ratio of 1:1 and treated with acetic acid at 60 °C for 12 h (to remove the surfactant) followed by ethanol washing at room temperature.37 Both TEM and XRD (Fig. S7 & S8) analyses confirmed that the treatment did not cause any changes to sample morphology or structure. Cyclic voltammograms (CVs) of three carbon-supported samples (Pd, Pd0.6/WO2.72 and Pd1.1/WO2.72) in 0.1 M HClO4 were obtained (Fig. 3A). The capacitive region in the CV of each hybrid sample is broader than that of the Pd one, demonstrating the enhanced electrochemically accessible area of the hybrids. The CV behaviors of the hybrid samples are dominated by both Pd and WO2.72 - the peaks in the region of 0 - 0.28 V are related to hydrogen adsorption and desorption of Pd;38 the small bump around 0.55 V is related to the chemical valence change of mixed-valent tungsten (V,VI) in WO2.72 (Fig. S9); the single component Pd NPs have the oxidation/reduction peaks at ~0.7 V/0.60 V, while the Pd NPs in the Pd/WO2.72 show the oxidization/reduction peaks at ~0.8 V/0.59 V (WO2.72 NRs show no oxidation peak in the 0.7-0.8 V region (Fig. S9)). This Pd oxidation potential shift is illustrative of Pd stabilization by WO2.72. Pd NPs have been demonstrated to be a promising catalyst for the formic acid oxidation reaction (FAOR) in DFAFCs due to their more favorable catalysis towards dehydrogenation than dehydration of HCOOH.8, 12 However, the stability of these Pd NPs in the acidic FAOR condition remains an issue. When Pd NPs are coupled with WO2.72 NRs, their FAOR catalytic activity (in 0.1 M HClO4 + 0.1 M HCOOH, normalized to Pd mass) is improved dramatically, with the mass activity reaching ~1600 mA/mgPd (1564.7 mA/mgPd for Pd1.1/WO2.72 and 1618.3 mA/mgPd for Pd0.6/WO2.72 (Fig. 3B)), which is not obviously dependent on the composition of Pd/WO2.72. They are among the most active Pdbased FAOR catalysts ever reported (Tab. S2). For the single component Pd NP and commercial Pd catalyst, their mass activities are only at 508.3 mA/mgPd (Fig. 3B) and 355.6 mA/mgPd (Fig. S10), respectively. The mass activity of the Pd NPs from the

physical mixture of Pd + WO2.72 is slightly improved to 792.3 mA/mgPd (Fig. S11). To enhance Pd and WO2.72 interactions in the physical mixture, we annealed the carbon supported physical mixture (as well as the carbon supported Pd, WO2.72, and Pd1.1/WO2.72 as control samples) at 400 °C in Ar for 1 h. TEM, HRTEM and XRD analyses (Fig. S12A-C & Fig. S13A) confirm that these control samples show no obvious morphology and structure changes. The lattice expansion of the Pd NPs was still observed from the annealed physical mixture of Pd + WO2.72 (Fig. S12D & Fig. S13B), indicating that the annealing induces the strong Pd-WO2.72 interactions. After this annealing treatment, the mass activity of the physical mixture is increased dramatically to ~1129.6 mA/mgPd, while those of the annealed Pd (537.1 mA/mgPd) and Pd1.1/WO2.72 (1615.2 mA/mgPd) NPs are only changed slightly (Fig. 3C). As the single component WO2.72 showed no FAOR activity (either as-loaded or 400 °C annealed (Fig. S14), the enhanced Pd catalysis observed from the annealed physical mixture must originate from the increased interactions between Pd and WO2.72. One important feature of the Pd/WO2.72 catalysis for the FAOR is that the CV of the oxidation curve has a “plateau” from 0.40 V to 0.70 V and the CO poisoning peak,8 which is evident from the CV curve of the Pd NPs (Fig. 3B & C), is not observed. For the 400 °C annealed Pd1.1/WO2.72, the oxidation “plateau” extends even further to 0.85 V. The presence of the “plateau” suggests that once the oxidation reaction is initiated, the current quickly reaches to the maximum level at ~0.4 V and stay at this level (with a slow current increase) until the Pd surface is oxidized to Pd (II). Another noticeable feature of the Pd/WO2.72 catalyst is that it has much enhanced stability in the FAOR condition. For the 400 °C annealed Pd1.1/WO2.72 and Pd samples, after a 12 h chronoamperometry test at 0.4 V, the Pd1.1/WO2.72 shows no morphology change (Fig. S15A) and no obvious sign of catalyst activity degradation (Fig. 3D). As a comparison, after the same stability test, the single component Pd NP and commercial Pd catalyst show sign of aggregation/sintering (Fig. S15B & Fig. S16) and their mass activities drop to 437.0 mA/mgPd and 172.1 mA/mgPd, respectively (Fig. 3D & Fig. S17). The enhanced FAOR catalysis performance of the Pd/WO2.72 hybrid is due to the formation of the strongly coupled Pd-WO2.72 that induces the Pd lattice expansion and Pd surface electron density decrease. Previous studies have indicated that the lattice expansion of Pd surface can help to increase the formate adsorption power, promoting the dehydrogenation of FA.39-41 The decreased Pd surface electron density should help to reduce the CO adsorption, as proved by the CO stripping test (Fig. 4). A negative CO oxidation peak shift is observed, which may be attributed to the Pd coupling with WO2.72 since the pure WO2.72 has no CO adsorption. This indicates that the coupled Pd prevents the FA dehydration pathway and improves its CO-tolerance.11 On the other hand, protons adsorbed on the Pd surface during electrocatalysis may be easily transported from Pd-Hads to WO2.72 via the hydrogen spillover effect, forming tungsten hydrogen bronzes,42-44 further promoting the dehydrogenation of HCOOH on Pd. In the oxidation condition, the WO2.72 can be recovered from the tungsten hydrogen bronzes and be re-used for H-adsorption. WO2.72 may also catalyze the water dehydrogenation to produce -OHads,45 which can effectively oxidize CO from the Pd-CO that may be formed during the dehydration of HCOOH.46 Therefore, when formic acid is catalyzed by the Pd/WO2.72, the oxidation current density stays high until the Pd surface is oxidized after which the current drops steeply.

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0147 and under the Multi University Research Initiative (MURI) W911NF-11-1-0353 (S.S), as well as by NSERC Discovery Grants (P.Z). CLS@APS facilities (Sector 20-BM) at the Advanced Photon Source (APS) supported by the U.S. Department of Energy (DOE), NSERC Canada, the University of Washington, the Canadian Light Source (CLS), and the APS. Use of the APS is supported by the DOE under Contract No. DEAC02−06CH11357. The CLS is financially supported by NSERC Canada, CIHR, NRC, and the University of Saskatchewan. M. M. is supported by the National Science Foundation Graduate Research Fellowship, under Grant No. 1644760.

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In summary, we have reported a facile seed-mediated growth method to prepare the hybrid Pd/WO2.72 with Pd NPs anchored on WO2.72 NRs. The strong coupling between the Pd NPs and the WO2.72 NRs in the hybrid structure leads to evident expansion of the Pd (111) lattices from 0.23 to 0.27 nm and the decrease of Pd surface electron density. As a result, the Pd NPs are better stabilized and activated for the FAOR in 0.1 M HClO4. Its mass activity reaches ~1600 mA/mgPd in a broad potential range of 0.4-0.85 V (vs. RHE) and stays at this value even after the 12 h stability test. Our experiments have demonstrated that coupling Pd NPs with WO2.72 not only offers the desired stabilization on Pd, but also promotes dehydrogenation of formic acid, achieving the much enhanced Pd activity and stability for the FAOR. The strategy demonstrated here is not limited to Pd, but can be extended to other M NPs (M = a metal or an alloy), providing a general approach to the hybrid M/WO2.72 with much enhanced activity and stability to catalyze important electrochemical or chemical reactions for renewable energy applications.

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ASSOCIATED CONTENT

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Supporting Information Materials and experimental methods, and supplementary figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author *

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21. Cong, S.; Tian, Y. Y.; Li, Q. W.; Zhao, Z. G.; Geng, F. X. Adv. Mater. 2014, 26, 4260-4267.

[email protected]

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Notes The authors declare no competing financial interests.

23. Remskar, M.; Kovac, J.; Virsek, M.; Mrak, M.; Jesih, A.; Seabaugh, A. Adv. Funct. Mater. 2007, 17, 1974-1978.

ACKNOWLEDGMENT

24. Xi, G.; Ouyang, S.; Li, P.; Ye, J.; Ma, Q.; Su, N.; Bai, H.; Wang, C. Angew. Chem. Int. Ed. 2012, 51, 2395-2399.

The work was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under grant W911NF-15-1-

25. Zheng, H. D.; Ou, J. Z.; Strano, M. S.; Kaner, R. B.; Mitchell, A.; Kalantar-Zadeh, K. Adv. Funct. Mater. 2011, 21, 2175-2196.

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