Low and High Index Faceted Pd Nanocrystals Embedded in Various

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Energy, Environmental, and Catalysis Applications

Low and High Index Faceted Pd Nanocrystals Embedded in Various Oxygen-deficient WOx Nanostructures for Electrocatalytic Oxidation of Alcohol (EOA) and Carbon Monoxide (CO) Karuppasamy Lakshmanan, Chin-Yi Chen, Sambandam Anandan, and Jerry J. Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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ACS Applied Materials & Interfaces

Low and High Index Faceted Pd Nanocrystals Embedded in Various Oxygendeficient WOx Nanostructures for Electrocatalytic Oxidation of Alcohol (EOA) and Carbon Monoxide (CO) Karuppasamy Lakshmanana,b, Chin-Yi Chena, Sambandam Anandanc, Jerry J. Wub,* aDepartment

of Material Science and Engineering, Feng Chia University, Taichung 407, Taiwan

bDepartment

of Environmental Engineering and Science, Feng Chia University, Taichung 407, Taiwan

cDepartment

of Chemistry, National Institute of Technology, Trichy, India

* Corresponding author: [email protected], +886-4-24517250 #5206

ABSTRACT This work suggests a modest hydrothermal method applied for the synthesis of oxygen-deficient WOx (x=2.75, 2.83, and 2.94) nanomaterial with various morphology, such as bundled nanorods (NR), nanobelts (NB), and nanosheets (NS), by changing the inorganic additives, such as HCl, NaHSO4, and HNO3. In addition, the WOx supported high and low index faceted Pd nanoparticles (Pd-WO2.75 NB, Pd-WO2.83 NR, and PdWO2.94 NS) have been successfully synthesized by the facile sonochemical method to function

as

the

high-electrocatalytic

activity

of

electrocatalysts

for

alcohol

electrooxidation, including ethanol, ethylene glycol, and glycerol. Among the three different electrocatalysts, the versatile high index {520} faceted Pd nanoparticles on 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

WO2.75 NB (Pd-WO2.75 NB) shows the better electrocatalytic performance compared with low index {100} faceted Pd-WO2.83 NR and Pd-WO2.94 NS nanocomposites. This work has identified that the high density of low coordinated surface atom of Pd strongly interacts alcohol, which facilitates of C-C bonds cleavage and may prevent the CO poisoning of nanoparticles. Furthermore, the high concentration of oxygen deficient provided additional benefit for the generation of OH species and boosted the electrocatalytic performance of alcohols as well.

Keywords: Pd-WOX nanocomposites, low versus high index facet, oxygen-deficient, alcohol oxidation, CO poisoning

1. Introduction The modern life and increases in environmental concerns facing one of the major challenges are needed for new resources for the replacement of traditional fossil fuel energy. In this regard, the electrochemical energy conversion of fuel cells is prospective alternative energy systems owing to their high power density, environmentally friendly, and easily renewable feature.1 In terms of energy conversion, alkaline direct alcohol fuel cells (ADAFC) have involved much attention over hydrogen fuel cells because of several favorable features, including high volumetric and gravimetric energy power densities, low cost, high stability in alkaline electrolytes, facile transportation, and storage. The widely studied ADAFC is the direct methanol fuel cell.2-5 Nevertheless, its industrial and commercialization are deeply hindered by the small energy power density and highly hazardous to the earth environment. Alternatively, ethanol offerings a larger energy 2 ACS Paragon Plus Environment

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storage density (8.0 kWh kg-1) than methanol or formic acid, and can be obtained directly as a biofuel. Also, the liquid fuels of polyols, such as ethylene glycol (EG) and glycerol (Gly), were considered as an alternative source of ADAFC.6 These volatile liquids with lower boiling point fuels are easily oxidizable by electrochemical reactions and they are of less catalyst poisoning, high energy density (5.2 kWh kg-1 for EG, and 5.0 kWh kg-1 for Gly), and less pronounced crossover, thus making them more efficient and simple to work7-10. Furthermore, the liquid fuels of ethylene glycol and glycerol of each carbon carrying alcohol (OH-Hydroxyl) group can be partially oxidized to the formation of oxalate and mesoxalate intermediates in alkaline electrolyte. Fortunately, lacking the C-C bond cleavages and CO32- formation, it enhances to exchange 8 and 10 electrons against 10 and 14 electrons for the entire electrooxidation of CO32-. Therefore, the possibility for oxidized alcohol groups without cleavages of C-C bond can be enhanced to increase the faradic efficiency of 71.5 % (EG) and 80 % (Gly), respectively.11, 12 There are many research articles on fuel cell electrocatalysts focusing on carbonsupported nanomaterials. In addition to the carbon nanomaterials, the metal oxides supported nanocomposites afford new opportunities to develop the more efficient ADAFC. The metals in metal oxides have higher oxidation state, signifying that they can readily lose and gain oxygen.13-16 Besides, the metal oxides have gained interest for their synergistic effect with bi-functional mechanism and easier conductivity of the electron and proton to the formation of metal-hydrogen bronzes (HxMO3, M=W, Mo, and Ti, etc.)33. The most common metal support for fuel cell electrocatalyst used carbon or carbon-based materials, however, which have a low corrosion resistance in alkaline or acidic medium. For this reason, the excellent catalytic stability and high corrosion 3 ACS Paragon Plus Environment


resistance of metal oxides (WO3, MoO3, and TiO2, etc.) have been considered as alternative catalysts support for alcohol oxidation reaction.17-21 Furthermore, introducing oxygen vacancies on metal oxide supports have been recognized to be beneficial for the generation of OHads species. Whereas the further assistance can appear from strong metal-support interaction (SMSI), which handles bonding between the transition metal and poisonous intermediates.22, 23 Selectively enhancing the electrooxidation of alcohol and better introducing high index facets into noble metal nanoparticles (Pd, Pt, Au, and Ag) have been focused. Over the past decades, the synthesis of shape-controlled noble metal nanoparticles enclosed by open surface structures is a potential route to improving the catalytic performances of the nanoparticles. Inspired by this, we acknowledged that the enhanced catalytic activity of palladium nanoparticles is mainly dependent on their exposed crystal facets, such as high index facets (which enclose a high density of atomic steps and kinks)24. In addition, these faceted structures could interact more strongly with alcohol and break C-C bonds easier, as well they prevent the CO poisoning of noble metal nanoparticles25-29. However, the rational design and preparation of well-defined Pd nanoparticles with open surface structures is one of the great challenges for the electrocatalyst community. Recently, Tian and co-workers reported an electrochemical method for the synthesis of Pt-Pd nanocrystals containing high index facets30. However, the large-scale synthesis of electrocatalyst by an electrochemical method is not suitable. Recently our research article reported a wet chemical method for the synthesis of Au nanoparticles containing high index {720} surfaces33. Following this method and synthesis of several Pd nanocrystals with various high index facets. To date, very few reports were recognized regarding the 4 ACS Paragon Plus Environment

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preparation of Pd nanocrystals containing high index facets by conventional wetchemical synthesis31. Motivated by this, we currently report a novel noble metal Pd with low and high index faceted surfaces and compare their enhanced catalytic activity towards alcohol electrooxidation. Also, we discover the Pd-embedded on various oxygendeficient perovskite-type WOx (X= 2.75, 2.83 and 2.94) support (bundled nanorods (NR), nanosheets (NS), and nanobelts (NB)). Among the various oxygen-deficient support, the more oxygen-deficient concentration of Pd-WO2.75 NB is especially an attractive electrocatalyst for alcohol fuel cells, which shows an enhanced electrocatalytic performance than the other two suboxides. Furthermore, the high concentration of oxygen deficiencies is allowed for strong interaction with reactants and facilitates faster electron transfer, which would help to develop the ADAFC.32 Moreover, we have found that the Pd nanocrystals enclosed high index {520} surfaces exposed an excellent catalytic performance than a commercial Pd/C or Pd nanocrystals containing a low index {110} facets and the electrocatalytic performance of Pd nanocrystals facets followed the decreasing order of {520}>{420}>{320}>{110}, respectively.

2. Experimental section 2.1. Hydrothermal synthesis of bundled-WO2.83 Nanorods All the chemicals were of analytical reagent grades and without further purification. The pristine one-dimensional nanorods (1D-WO2.83) were successfully prepared by a hydrothermal method. Briefly, the 0.99 g of Na2WO4.2H2O and 0.3 g of NaCl was dissolved in 20 ml of deionized water. Subsequently, the pH value of 2-4 obtained by slowly adding 3 M HCl with vigorous magnetic stirring. The mixed solution was moved 5 ACS Paragon Plus Environment


to a 45 ml Teflon-lined autoclave and maintaining the constant temperature of 180 oC for 24 h in an oven. After completion of the reaction, the final white precipitate of WO2.83 nanorods was obtained by continually washing the precipitate with ultrapure water. 2.2. Synthesis of WO2.75-nanobelts and WO2.94 nanosheets Tungsten oxide (WO3) nanocrystals (NCs) with nanobelts and nanosheets morphologies were successfully synthesized with sodium tungstate (Na2WO4.2H2O) as the tungsten source. Firstly, 0.99 g of Na2WO4 was slowly added to 40 ml of DI water with vigorous magnetic stirring for 20 min. In this work, under the condition of various solvents, such as 1.19 g of NaHSO4.H2O and 10 ml of 65% HNO3, the morphologies of WO3 NCs were changed and the detailed synthetic procedure is revealed in scheme 1. Subsequently, the solution mixture was moved to a 45 ml Teflon-lined stainless steel autoclave and maintaining the constant temperature of 180 oC for 24 h in an oven. After completion of the reaction, the resulting precipitates of WO2.75 nanobelts and WO2.94 nanosheets were obtained by continually washing the precipitate with ultrapure water to remove the unreacted impurities and dried in vacuum air at 80 oC for overnight. 2.3. Synthesis of Pd embedded WOx (Nanorods, Nanobelts, and Nanosheets) hybrid nanomaterials For catalyst preparation, Pd nanocrystals were deposited on WOx (nanorods, nanobelts, and nanosheets) by the ultrasonic probe irradiation method. The detailed procedure was as follows: (NH4)2PdCl4 (3 ml, 0.01 M) and 120 mg of WO2.83 bundled nanorods were taken together with 10 ml deionized water and 3 ml Lysine (0.01 M). After vigorous magnetic stirring for 10 min, the mixture solution pH value of 11-12 obtained by adding 0.10 M of NaOH. Then drop by drop 0.1 M NaBH4 reducing agent

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was added to the above reaction solution followed sonic probe irradiation under Ar atmosphere for 5 mins (0.50 s On, 0.10 s Off). The solution color was transformed from yellow to dark brown, resulting colloidal precipitate was collected by centrifugation and continually washing the precipitate with ultrapure water. Likewise, the same procedure was as followed to prepare the remaining electrocatalyst, such as Pd-WO2.75 nanobelts and Pd-WO2.94 nanosheets. The entire synthesis process of all the catalysts completed within 1 hr. 2.4. Physical characterization of materials Structural features and elemental composition of the nano electrocatalysts were studied using high-resolution transmission electron microscopy (HRTEM; JEOL, JEM2010) system furnished with energy dispersive X-ray spectroscopy (EDS) and field emission scanning electron microscopy (FE-SEM; JEOL, JSM-7610F) operated at 200 kV. To determine the phase purity and crystal structures of the products, powder X-ray diffraction patterns (XRD) were recorded with a Philips XPertPro X-ray diffractometer using the Cu Kα radiation. The X-ray photoelectron spectroscopy (PHI 5600 XPS spectrophotometer with a monochromatic Al Kα (1486.6 eV)) instruments were accomplished to investigate the oxidation states (Pd 3d and W 4f regions) of the electrocatalyst. The surface area of the catalysts was measured using Brunauer-EmmettTeller (BET) method at 77.35 K on an automatic N2 adsorption/desorption tool (Quanta Chrome Instruments, version 3.01) and the catalysts were outgassed in vacuum at 150 °C for 24 h before each adsorption. 2.5. Electrochemical characterization of materials



All the electrochemical measurements were conducted by the conventional threeelectrode cell. In this, the working electrode is glassy carbon electrode (GCE, 3mm), the Ag/AgCl and Pt-wire as a reference and counter electrode. The GCE was prepared by dispersion of 5 mg electrocatalyst powder in 1 ml ethanol containing 0.05 wt% Nafion solution and ultrasonicated to form a homogeneous ink. Then, 10 μl of the catalyst ink was placed on a polished 3 mm GCE and then dried in air for 30 min. Prior to all the electrochemical testings, the high purity of N2 gas was purged for 20 min. The electrochemically active surface area (ECSA) and poisonous effect of the catalyst surface were found by CO-stripping voltammogram as follows: First, the catalyst surface was saturated by bubbling CO gas through 1 M KOH solution for 15 min, the excess of CO was replaced by N2 gas for 20 min. Then, the CO-stripping voltammograms were tested by oxidizing pre-adsorbed CO in 1 M KOH at a scan rate of 50 mVs-1.

3. Results and discussion 3.1. Structural characterization For XRD analysis. We account the shape-controlled preparations of tungsten oxide (WOx) with various nanostructured materials (Nanorods, Nanosheets, and Nanobelts). Likewise, this procedure supports to switch the phase purity of WOx, such as hexagonal phase of WO2.83 NR, monoclinic phase of WO2.94 NS, and hexagonal phase of WO2.75 NB, by altering the inorganic solvent. The different W to O molar ratios of WOx have various colors, such as brown WO2.83 NR phase, white WO2.75 NB, and yellow WO2.94 NS. As a result, we confirmed that the phase purity and crystalline structures of WOx had altered with the composition of various inorganic solvents. From Figure 1a, b, and c, all the


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diffraction peaks at WO2.83 NR and WO2.75 NB can be exclusively indicated to a crystalline hexagonal phase of WO3 (JCPDS 75-2187) and WO2.94 NS matches well with the monoclinic phase of WO3 (JCPDS 43-1035). The hexagonal WO2.83 NR and WO2.75 NB were obtained from the 3 M HCl and NaHSO4, and finally, the monoclinic WO2.94 NS phase was obtained from the HNO3. As a result, we concluded the various inorganic additives strongly affect the crystalline phase and the morphologies of the nanostructured WOx substrates system. Intense diffraction peak along with the (2 0 0) plane presents a prior growth orientation of the WOx nanostructures. The smooth, narrow, and strong diffraction peaks have indicated that the as-synthesized nanostructured WOx possess good crystallinity, as well as after the sonochemical-deposition of Pd nanoparticles (Figure 1a, b, and c,). The XRD peaks intensity and pattern of Pd embedded WOx were noticed to be unchanged compared with that of pristine WOx, which might have been a result of the smaller size Pd than WOx supporting materials. The average crystal size of Pd embedded WOx with various nanostructures materials was shown in Figure S1-S3 (Supporting information), which was calculated by the Scherrer equation.33,34 The calculated crystal size was smaller than that of pristine WOx, indicates that Pd nanoparticles strongly prohibited the grain growth of WOx supporting materials. This result consistent with the previous article of Pd embedded WO3 NFs by electrospinning.35 In recent time, the growth of nanomaterials and nanotechnologies was encouraged by numerous oxygen deficient perovskite type WO3-based nanomaterials.36 The WO3based one dimensional (1D nanorods and nanobelts) and even two dimensional (2D, nanosheets) networks have paid attention to their outstanding physicochemical properties 9 ACS Paragon Plus Environment


for energy storage and conversion applications. Commonly, these multidimensional WO3 nanomaterials are known with less risk to suspension and accumulation than zerodimensional (0D) WO3 nanomaterials.37-39 Actually, oxygen deficient perovskite type WOx supported Pd-based nanomaterials can be responsible for alternative electrocatalysts of alcohol oxidation because they show enhanced catalytic activity, compared with the use of carbon supports, WOx supported materials do not have any undesirable problems (like corrosion, and less interaction between Pd/carbon supports).40 In this perspective, the three different shaped WOx nanomaterials were prepared via a simple hydrothermal method by varying the inorganic additives, such as HCl, NaHSO4, and HNO3, in growth solution and the detailed growth of nanocomposites is shown in Scheme 1. Pd-WO2.83 Bundled Nanorods. The structure of the as-prepared nanocomposite was studied by FE-SEM and HR-TEM technique. The FE-SEM image of WO2.83 (Figure 2a) reveals the bundled nanorods (NR) structure. The bundled nanorods with their typical facet and good uniformity in size with a mean diameter of 68 nm and a few micrometers in length were obtained by the selective composition (HCl/NaCl) and pH of the solution. Then the Pd embedded WO2.83 NR was fabricated by the reduction of PdCl4- in the presence of NaBH4 and Lysine. Figure 2b shows that the TEM image of Pd-WO2.83 NR hybrid nanostructures and the ultrafine Pd nanoparticles are strongly dispersed on the surface of bundled nanorods (as shown in inset Figure 2b). The average sizes of Pd nanoparticles are 4.8 ± 1.5 nm and the calculated histogram is revealed in Figure 2c. The characteristic selected area electron diffraction (SAED) pattern (inset Figure 2b) illustrate a series of well-defined rings, which can be associated with various crystallographic


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planes structure of bundled WO2.83 nanorods. No additional pattern related to Pd were observed, which has the same result as discussed in XRD. Meanwhile, the lattice-resolved HR-TEM image (shown in Figure 2d, e, and f) taken from individual Pd-WO2.83 NR appeared to have high crystallinity and its lattice spacing are measured as 0.24 (Pd) and 0.38 nm (WO2.83), verifying the existence of Pd nanoparticles on the surface of WO2.83 bundled nanorods. Additionally, the steps and terraces of low coordinated {110} facets were observed (Figure 2e), which is important for further boosting the electrocatalyst. Also, the Fast Fourier transform (FFT) from HRTEM image (shown in Figure 2g, and h) has given an additional direct proof of Pd and WO2.83 zone axis along direction. Furthermore, Figure 2i and j exhibited the intensity profile of line scan across the lattice spacing, it evidently shows the lattice spacing of Pd and WO2.83 NR, respectively. Pd-WO2.75 Nanobelts. The strongly dispersed Pd on WO2.75 nanobelts (NB) was synthesized for comparative evaluation of alcohol oxidation. The morphology of WO2.75 nanobelts was initially characterized by FE-SEM (Figure 3a). The unique nanobelts structure with a mean diameter of 17.2 nm (Figure 3b) displays a large surface area that will favor the deposition of more number of small-sized Pd nanoparticles. Figure 3b shows the magnified TEM images of individual Pd-WO2.75 NB structures, which implies that the Pd nanoparticles were homogeneously dispersed on the surfaces of WO2.75 nanobelts. The mean diameter of Pd nanoparticles was calculated by randomly choosing 100 nanoparticles and their corresponding histogram graph is displayed in Figure 3c. The sizes of Pd nanoparticles are 3.8 ± 1.5 nm. The SAED of Figure 3b (inset) only illustrates



the crystallographic planes of WO2.75 and the signal of Pd was not seen, the same result reliable with the above XRD result. The HRTEM performance was supported to investigate the surface atomic structure of as-prepared Pd-WO2.75 NB. The well-defined lattice fringes are shown in Figure 3d, the two clear lattice fringes with the interplanar distance of 0.24 and 0.41nm, corresponding to the {111} plane of Pd and {200} plane of WO2.75, which also match the Fast Fourier transform pattern (FFT, Figure 3f and g). In addition, the line scale of the lattice distance shown in Figure 3h and I further indicates the lattice spacing of Pd and WO2.75, respectively. More importantly, the high density of open surface structures, such as steps and terrace atoms of (210) and (310) facets, were often observed around the surface of Pd nanoparticles, as shown in Figure 3e (which was obtained from yellow circle marked HRTEM image of Figure 3d. Pd-WO2.94 Nanosheets. Figure 4a displays the top-view FE-SEM image of twodimensional (2D) WO2.94 nanosheets (NS) and almost all of them revealed the same structures. The typical TEM image and their corresponding SAED pattern were recorded after the sonochemical deposition of Pd on WO2.94 NS shown in Figure 4b. It can well define the formation of hybrid Pd-WO2.94 NS structures. Meanwhile, the average diameter of WO2.94 and sizes of Pd nanoparticles were measured to be 352 nm and 5.2 nm ± 0.5nm (Figure 4c). The HR-TEM image (Figure 4d) of the individual WO2.94 nanosheets and their equivalent Fast Fourier transform (FFT, inset in Figure 4d) pattern show a single crystalline structure without any grain boundaries. The continuous lattice fringes are calculated to be 0.38 nm and 0.19 nm with an angle of 900, conforming to the lattice planes of {200} and {002} monoclinic WO2.94. The HR-TEM image (Figure 4e) of the 12 ACS Paragon Plus Environment

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selective area of Pd nanoparticles and their corresponding FFT pattern (inset, Figure 4e) indicated a good crystallinity, which also clearly confirmed the majority of lattice fringes with lattice spacing value of 0.22 nm equivalent to {111} plane of fcc Pd nanoparticles. Moreover, some low-coordinated atomic steps, such as {110} low index facets, were frequently observed around the surface of Pd nanoparticles. In addition, the intensity profile of line scan across the lattice spacing shown in Figure 4f and g, further endorsing the lattice spacing values of Pd and WO2.94 NS. As a result, the lattice spacing values of Pd on WO2.83 NR, and WO2.75 NB different than lattice spacing value of Pd on WO2.94 NS, which further confirmed that the various oxygen deficient WOx strongly affected the crystalline phase and morphology of Pd nanocrystals. The same results are reliable with the XRD analysis. The TEM with EDS-elemental analysis of all the three catalysts (shown in Figure S4, Supplementary information) exhibits strong Pd, W, and O peaks (inclusive of Cu peak which belongs to the grid sample). Among the three catalysts, the atomic and weight percentage (%) of Pd embedded on WO2.75 nanobelts exhibit higher values, which further indicates the structure of nanobelts with larger surface available for deposition of more number Pd nanoparticles. It has been well-recognized that the catalytic activity of metal-metal oxide nanocrystals is strongly dependent on their size, shape, and arrangements of the surface atoms. Among them, the unique structures of Pd-WO2.75 NB were considered as a highly capable electrocatalyst due to the smaller size of Pd (3.8 nm) with the highest deposition, smaller diameter WO2.75 (17.2 nm) with a larger surface, and high concentration of oxygen vacancies support. Another important approach to developing the catalysis is to 13 ACS Paragon Plus Environment


modify the noble metal Pd nanocrystals with high indexed crystallographic facets. Inspired by this, the synthesis of Pd nanocrystals contain high indexed surface is a challenging target because the kinetic growth rate of forming high index facets is larger than low index facets.41 Therefore, the use of foreign molecules can act as surfaceregulating agents to the synthesis of Pd nanocrystals with high energy facets. Here we introduce foreign molecules of WOx with various nanostructures, thus the different surface nanostructures can significantly modify the surface energies of different facets. In this respect, the larger surface area of WO2.75 NB is favored for high energy Pd surface structures, which is turned a faster growth rate of these Pd surfaces, important to the exposure of high index facets. The HR-TEM image of Pd on WO2.75 NB is shown in Figure 3e, where the corner atoms are clearly resolved. More importantly, these nanoparticles contain a high density of open surface structures such as steps, terrace, and kinks atom than Pd embedded on bundled nanorods and nanosheets (Figure 2e and Figure 4e). Structurally, the surface of Pd on WO2.75 NB are enclosed by high index {520} surface, which is periodically composed of {210} and {310} of sub-facets steps and kinks atom. All these exposed crystal facets are of high surface energies than low index {110} facets of Pd on WO2.83 NR and WO2.94 NS. Owing to the presence of surface regulating agent of foreign molecules (WOx support), the reduction rate of Pd is very fast and beneficial for the growth of high index faceted nanoparticles42. Until now, very few articles have recommended the kinetic parameters, like the crystal growth rate which can affect the surface structure of the precious metal.43 A previous article in our work has reported high


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and low index faceted Au-MoO3 nanomaterials were prepared by changing the kinetic parameter.33 For comparative studies, the various morphology and surface structures of the Pd nanocrystals on WO2.75 NB substrate can be easily tailored by a controlled growth mechanism. In this controlled reaction mechanism, the L-lysine functions as a shaperegulating reagent and the amino acid molecule of L-lysine contains two positive (-NH2+) and one negative (-COO-) end groups. This unique feature of L-lysine selectively coordinates the metal cations (Pd2+). which is boosting the growth of Pd nanoparticles with numerous high index facets owing to their controlled reduction and slower growth kinetics44. Consequently, the synthesized Pd nanocrystals on WO2.75 NB substrate at a 0.1 M L-lysine concentration were found to be enclosed by {520} facets. With 0.05 M Llysine, the Pd nanocrystals were enclosed by {420} facets, and with 0.01 M L-lysine, the nanocrystals were enclosed by {320} facets. It is notable that the size and shape of Pd nanoparticles with various facets structures are not perfectively arranged, which implies that altering the concentration of L-lysine plays a key role to control the size and structures of nanoparticles. The {320}surface contains of two atomic {210} terraces and {110} steps (as exposed in Figure S8 (a-c)), while the {520} surface is made of {310} terraces and monoatomic {210} steps (Figure S8 (d-f)), and the {420} surface comprises of {310} terraces and {110} steps (Figure S8 (g-i)). All the results confirmed that the Miller indices of {hk0} facets of Pd nanocrystals on WO2.75 NB are adjustable. These three types of Pd nanocrystals with high index facets surface showed various electrocatalytic performance, where the catalytic performance for alcohol electrooxidation monitored the 15 ACS Paragon Plus Environment


decreasing order of Pd (520) > Pd (420) > Pd (320) and the complete electrocatalytic activity is discussed at the supporting information (Figure S9). For XPS analysis. The XPS was applied to investigate the oxidation state and stoichiometry of the tungsten oxides (Figure 5). The survey XPS spectrum endorses the existence of Pd, W, and O in as-synthesized electrocatalyst (Figure 5a). The core-level Pd 3d spectra from various Pd-WOx hybrids nanostructures are fitted to two pairs of doublet peaks (Figure 5b, c, and d) and the predominant peaks at 335.0 and 341.0 eV binding energies are corresponding to the Pd 3d5/2 and Pd 3d3/2 spin-orbital of zerovalent state Pd(0), respectively. In addition, the pronounced shoulder peaks at around 338.4 and 333.2 eV resemble the Pd 3d3/2 and Pd 3d5/2 levels of Pd2+ in the form of PdO on the surface of Pd-WOx hybrids. The deconvolution of the XPS spectrum showed that of the integrated intensities of the Pd0 and Pd2+ peaks in Figure b, c, and d. Among the three catalysts, the Pd2+ peak intensities of Pd-WO2.75 NB are very weaker because more Pd exists as Pd0 in the Pd-WO2.75 NB, which is confirmed that we have successfully synthesized Pd-WO2.75 NB with a little amount of Pd2+. The percentage of Pd2+ is estimated to be 48 %, 34%, and 18 % of the corresponding Pd-WO2.94 NS, Pd-WO2.84 NR, and Pd-WO2.75 NB, respectively (Figure S10). This observation is well consistent with the literature45, 46. In comparison with those of Pd-WO2.83 NR and Pd-WO2.94 NS nanocomposites, the Pd 3d5/2 peak of Pd-WO2.75 NB has a positive shift (~0.34 eV, Figure 5c). This result suggests that strong electronic interaction should occur between Pd and WO2.75 NB support to enhance the electrocatalytic performance. Meanwhile, the broad W 4f spectra (Figure 5e, f, and g) are deconvoluted into three doublet peaks that are assigned to three different oxidation states, including W6+, W5+, 16 ACS Paragon Plus Environment

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and W4+. The first doublet peaks have binding energies of 35.5 and 37.8 eV, which appropriate to W 4f7/2 and W 4f5/2 of the W6+ formal oxidation state. Another doublet peaks, observed at the binding energies of 33.6 and 36.3 eV, corresponds to the lower W5+ oxidation state. In addition, the predominant doublet peaks centered at 32.9 and 34.5 eV correspond to the valence of W4+. For all the three samples, the peak intensity of W4+ is larger than those of W5+ and W6+. This result suggests the surface of used WOx develops a higher concentration of oxygen vacancies. Furthermore, comparing the detailed peak positions of all catalysts, we see a slight negative shifted binding energy on PdWO2.75 NB, which further specifies the consistency of more oxygen vacancies. Several important conclusions can be drawn from the analysis of XPS spectra, so the calculated surface atomic O/W ratios of 2.83, 2.75, and 2.94 correspond to the Pd-W17O48 NR (WO2.83), Pd-W12O33 NB (WO2.75), and Pd-W17O50 NS (WO2.94). This is suggestive of the non-stoichiometric features of the as-synthesized materials. Moreover, the composites of Pd-WOx, which has varied valence oxides on the surface, may consider as another type of perovskite (ABOx) in which A is Pd and B is W. In addition, the Pd-W-O system is that both PdO and WOx are potential catalysts for the electrocatalytic oxidation of alcohol and CO.47 The O1s spectrum is shown in Figure 5h, I, and j, the peak observed at 524.9 eV is related the Pd-O bonds. The other three oxygen signals located at the binding energies of 527.9, 530.2 and 532.9 eV are related to the lattice O2- (W-O bonds), hydroxyl (W-OH, tungsten bronze) species, and adsorbed water molecule (H2O), respectively. For BET analysis. The nitrogen sorption isotherms of all the catalysts have been investigated and shown in Figure S5 (Supporting information). All the catalysts display the similar isotherm peaks with a weak jump at 0.2-0.4 and another jump at P/P0= 0.7-



0.9, which indicates the characteristic mesoporous structures of supporting WOx materials. The BET of the WO2.83 bundled nanorods, WO2.75 nanobelts, and WO2.94 nanosheets enclosed by 49.3 m2g-1, 158.7 m2g-1, and 18.4 m2g-1, respectively, after the deposition of Pd nanoparticles. The BET surface area of various Pd-WOx with NR, NB, and NS slightly increased, such as 58.2 m2g-1, 172.6 m2g-1, and 22.9 m2g-1, which have clearly observed the existence of Pd on WOx supporting materials. Furthermore, the pore size distribution of WO2.75 NB is larger than those of WO2.83 NR and WO2.94 NS (showed in inset Figure S5a, b, and c), which mean as prepared WO2.75 NB support has more porous structures, which is advantageous for catalytic application of fuel cells. In addition, after the deposition of Pd nanoparticles, the pore size of all the samples decreased, which is due to more number of Pd nanoparticles deposited and blocking the pore channel. Both observations indicate that WOx is good supporting materials for noble metal nanoparticles growth and catalytic application. 3.2. The electrocatalytic oxidation reaction of an alcohol Cyclic voltammetry (CV) analysis is one of the most significant electrochemical methods for screening the catalysts for the alcohol oxidation reaction. The intrinsic electrocatalytic performances are established in terms of (i) the Eonset (more negative value at alkaline electrolyte), indicating the easier electrooxidation of alcohol, (ii) the Δep, related to the removal of poisonous intermediates from the catalyst surface (lesser the value indicating highly capable catalyst), (iii) forward peak current density (jf), indicating the generation of maximum current by catalysts, and (iv) the ratio of jf/jb, which imitates the tolerance of poisonous species accumulated (a larger value means more tolerant of poisoning the catalyst). Furthermore, the electrocatalysis in alkaline


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solution offerings numerous unique advantages, such as enrichment in reaction kinetics at the anode and cathode.48,49 We first studied the catalytic properties of Pd-WOx nanocomposites toward ethanol electrooxidation. Figure 6 display the CV curves (up to 1000 cycle) of the three different Pd-WOx catalysts in 1 M ethanol + 1 M KOH with a scan rate of 50 mVs-1. The peak current density values were normalized by the loading mass of Pd (Note: in the electrocatalytic tests, the loading mass (mPd) of all the three catalysts are the same, which is 5 μg cm-2). In accordance with the reported results (Figure 6a, b, and c), two separated oxidation peaks were observed at the range of -0.4 to 0.1 V (Forward scan, which is derived from ethanol oxidation) and -0.6 to -0.35 V (Reversed scan, which is associated with the removal of unoxidized species, such as small organic molecules (aldehyde, and ketone group molecules) produced in the forward scan electrooxidation).33,50 Among these electrocatalysts, the more oxygen-deficient perovskite type of high index faceted {520} of Pd-WO2.75 NB (1968.3 mA cm-2 mgPd) exhibits highest mass current density than those of low index faceted {100} of Pd-WO2.83 NR (1172 mA cm-2 mgPd) and PdWO2.94 NS (702.6 mA cm-2 mgPd). The stability of various catalysts was studied by repeating 1000 cycle numbers. After 1000 cycles, the Pd-WO2.75 NB showed the final mass current density of 84.8 %, which was better than those of Pd-WO2.83 NR (80.3 %) and Pd-WO2.94 NS (78.9 %), respectively. Besides, the earlier ethanol oxidation or onset potential of -0.568 V associated with Pd-WO2.75 NB is shifted more negatively than those of the Pd-WO2.83 NR (-0.474 V) and Pd-WO2.94 NS (-0.462 V). For comparison, the commercially available Pd/C material was chosen as the reference electrocatalyst as shown in Figure 6d. The peak current density and onset potential of ethanol 19 ACS Paragon Plus Environment


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electrooxidation measured on the commercial Pd/C catalyst is three times less current density and more positively shifted onset potential than that of Pd-WO2.75 NB. Figure 6e shows the mass activity (jf normalized by loading mass of Pd) and specific activity (jf normalized by ECSA) of all the four different catalysts, highlighting a much better catalytic activity of high indexed Pd-WO2.75 NB. Meanwhile, the interfacial properties of ethanol oxidation were studied by EIS. The Nyquist plots (Figure 6f) of Pd-WO2.75 NB (12.8 kΩ) revealed the smaller charge transfer resistance than those of Pd-WO2.83 NR (16.7 kΩ), Pd-WO2.94 NS (17.3 kΩ) and Commercial Pd/C (21.4 kΩ). The charge transfer resistance (Rct) of the catalyst is more related to their diameter of semicircle arc. Smaller semicircle arc of electrocatalyst demonstrates rapid electron transfer during the ethanol oxidation.34 The durability and anti-poisoning

of

the

Pd-WOx

nanocomposites

had

also

been

studied

by

chronoamperometry. The current-time curves are shown in Figure S11 (a). It can be observed, after 5000s, the steady-state current density value of the Pd-WO2.75 NB (5.85 mAcm-2) was much higher than that of Pd-WO2.83 NR (2.49 mAcm-2), Pd-WO2.94 NS (1.37 mAcm-2), and Commercial Pd/C(0.66 mAcm-2). As a consequence of all above, the Pd-WO2.75 NB obviously exists with different catalytic properties by changing their structures and composition and these phenomena confirm that the enhanced catalytic activity of Pd-WO2.75 NB is strongly dependent on their unique structures and composition. For ethylene glycol (EG) oxidation, there are two separated anodic peaks were seen in the forward and reversed scans (Figure 7a-d), which are associated to the EG oxidation and the removal of intermediate species. Obviously, the forward peak mass current 20 ACS Paragon Plus Environment

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density of Pd-WO2.75 NB (4313.7 mA cm-2 mgPd) is much higher than those of Pd-WO2.83 NR (3189 mA cm-2 mgPd), Pd-WO2.94 NS (2478.2 mA cm-2 mgPd), and Commercial Pd/C (1190 mA cm-2 mgPd) catalyst-modified electrodes. Meanwhile, after 1000 cycle numbers, the retention of mass current density for Pd-WO2.75 NB (75.1 %) is better than those of

Pd-WO2.83 NR (72.6 %) and Pd-WO2.94 NS (71.6 %) catalyst-modified

electrodes. Besides, the EG oxidation onset potential for Pd-WO2.75 NB, Pd-WO2.83 NR, Pd-WO2.94 NS, and Commercial Pd/C are -0.419, -0.218, -0.205 V, and -0.197 V, respectively. These negatively shifted onset potentials and enhanced peak current density have highlighted the improved electrocatalytic activity of Pd-WO2.75 NB. Furthermore, the order of mass and specific activity of catalysts (Figure 7e, the values in Table 1) are following the order of Pd-WO2.75 NB > Pd-WO2.83 NR > Pd-WO2.94 NS > Commercial Pd/C, respectively. The EIS measurements were conducted in the 1 M EG + 1 M KOH at 0.1 V for different catalysts. The Nyquist plots with smaller semicircle arc (Figure 7f) of PdWO2.75 NB (7.9 kΩ) exhibited the smaller charge transfer resistance during EG oxidation as compared to Pd-WO2.83 NR (11.2 kΩ), Pd-WO2.94 NS (13.6 kΩ), and Commercial Pd/C (15.3 kΩ). Furthermore, among the four catalysts, the Pd-WO2.75 NB had much higher stability and anti-poisoning properties than those of other two catalysts-modified electrodes in this paper (Figure S11 (b)). After 5000s, the current-time measurements showed that the retention of current density values for Pd-WO2.75 NB (20.2 mA cm-2) was 2.7 times for Pd-WO2.83 NR (7.3 mA cm-2), 7.4 times for Pd-WO2.94 NS (2.7 mA cm-2), and 11.8 times for Commercial Pd/C (1.87 mA cm-2), respectively.



The prepared Pd-WOx nanocomposites were also studied for glycerol (Gly) electrooxidation reactions. The CV, EIS, and CA methods were applied to estimate the catalytic activity of the different catalysts. Figure 8 a-f exhibited that the oxygendeficient perovskite with high index faceted Pd-WO2.75 NB had highest mass current density, lowest electron charge transfer resistance, better stability and anti-poisoning of CO than that of low index faceted Pd-WO2.83 NR, Pd-WO2.94 NS, and Commercial Pd/C. As can be seen, this similar trend can be observed in ethanol and ethylene glycol oxidation. The catalytic activity followed the order Pd-WO2.75 NB > Pd-WO2.83 NR > PdWO2.94 NS > Commercial Pd/C and the detailed electrochemical characterization values are shown in Table 1. Among the alcohols, the Gly exhibited higher electrocatalytic current densities which highlight that the electrocatalysts are more tolerant of the poisoning produced by glycerol than those of ethylene glycol and ethanol. Moreover, the Pd-WOx nanocomposites exhibit the highest jb/jr ratio in all the investigated electrocatalysts during Gly oxidation (Table 1), indicating their more tolerant toward CO poisoning50,51. Furthermore, the smaller reversed scan current density may recommend that lower quantity of small organic intermediate molecules produced in the forward scan. Inspired by this, the glycerol and ethylene glycol showed smaller reversed scan current density than ethanol (Figure 6a-c (ethanol), Figure 7a-c (EG), and Figure 8a-c (Gly) ), which means as prepared electrocatalysts have high ability to remove the poison species (e.g. CO) at EG and Gly electrooxidation. All the above-mentioned studies explained that the unique oxygen-deficient perovskite type of high index faceted Pd-WO2.75 NB displays high catalytic performance for ethanol, ethylene glycol, and glycerol oxidation. A final 22 ACS Paragon Plus Environment

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important factor for the high catalytic activity, the high index {520} faceted Pd surfaces are known to be active for the cleavage of C-H and C-C bond to generate more CO2 for ethanol, ethylene glycol, and glycerol electrooxidation than low index {110} faceted Pd surfaces. The high density of atomic steps and kinks atoms that we observed on high index {520} planes have a large number of dangling bonds, which can strongly interact with ethanol, ethylene glycol, and glycerol to weaken the C-H, and C-C bond, while this selectivity of C-H and C-C bond breaking may prevent the poisoning of Pd surfaces and enhance the complete electrooxidation of alcohol. 3.3. CO-electrooxidation The high CO tolerance of the Pd-WOX nanocomposites was also studied by COstripping voltammetry. Here the electrolyte solution of 1 M KOH was purged by N2 for 20 min and then bubbled CO for 15 min. Figure 9a-c exhibit a typical oxidation peak of CO between -0.2 to 0.3 V at first CO-stripping CV scan, a much higher COelectrooxidation peak on Pd-WO2.75 NB than those of Pd-WO2.83 NR and Pd-WO2.94 NS catalysts modified electrodes. On the second CV scan, only the redox peak of PdO and the smaller CO oxidation peak appeared because of CO removal on the Pd active sites, In addition, Pd-WO2.75 NB exhibits a more negative onset potential (-0.367 V) of COelectrooxidation than those of Pd-WO2.83 NR (-0.208 V) and Pd-WO2.94 NS (-0.088 V). In the meantime, the ECSA of as various catalysts were calculated48,52 and shown in Figure 9d, which clearly showed that Pd-WO2.75 NB (79.5 m2g-1) had much higher ECSA as compared with those of Pd-WO2.83 NR (26.04 m2g-1) and Pd-WO2.94 NS (19.7 m2g-1). This result has explained that the unique Pd-WO2.75 NB can enable the elimination of adsorbed CO on the Pd active sites. 23 ACS Paragon Plus Environment


4. Conclusion In this article, we have successfully synthesized a new type of electrocatalyst, such as various oxygen deficient perovskite type WOx supported on low and high index faceted Pd nanoparticles. Among the different WOx supports (WO2.94 NS, WO2.83 NR, and WO2.75 NB), highly oxygen deficient WO2.75 NB exhibits a smaller diameter of the larger surface area, which is an additional benefit for the deposition of more Pd nanoparticles on their surface. Furthermore, the oxygen-deficient WOx support and low coordinated surface atom of Pd nanoparticles were confirmed by EDS, XPS, and HRTEM measurements. These versatile nanocomposites can be applied to electrocatalytic oxidation of various alcohol. Among the various Pd-WO2.75 NB, Pd-WO2.83 NR, and PdWO2.94 NS electrocatalysts studied in this paper, the unique WO2.75 supported on high index faceted Pd nanoparticles (Pd-WO2.75 NB) shows a better electrocatalytic performance than those of low index faceted nanocomposites. This work has identified the high density of steps and kinks atoms of Pd surface strongly interact with the alcohol, which facilitates of C-C bonds cleavage and may prevent the CO poisoning particles. Among the alcohol (ethanol, EG, and Gly), the electrocatalytic oxidation of glycerol generated higher current density, as well as better CO tolerance. We expect our unique nanomaterials will inspire much catalytic application in the future, not only as fuel cell catalysts.

Supporting information


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The supporting information contains detailed data for electrocatalysts as prepared, including XRD, XPS, BET, HRTEM, EDS-elemental analysis, Cyclic voltammetry analysis (CV), and Chronoamperometry analysis (CA).

Acknowledgment The research was financially supported by the Ministry of Science and Technology (MOST), Taiwan (NSC-102-2923-035-001-MY3 and MOST-107-2221-035-001-MY3) and Department of Science and Technology, India (GITA/DST/TWN/P-50/2013) under the Taiwan-India collaborative research grant. The support in providing the fabrication and measurement facilities from the Precision Instrument Support Center of Feng Chia University is also acknowledged.

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FIGURE CAPTION Figure 1. XRD of (a) WO2.83 NR and Pd-WO2.83 NR, (b) WO2.75 NB and Pd-WO2.75 NB, and (c) WO2.94 NS and Pd-WO2.94 NS Scheme 1. Schematic illustration of the fabrication of Pd-WOx with various morphological electrocatalysts Figure 2. (a) FE-SEM image of bundled WO2.83 NR, (b) TEM (inset SAED) images of bundled Pd-WO2.83 NR, (c) Corresponding histogram for calculated size of Pd nanoparticles, (d) HR-TEM image of bundled Pd-WO2.83 NR, (e) lattice image of low index faceted Pd, (f) lattice image of Pd and WO2.83 NR, (g) and (h) FFT pattern of individual Pd and WO2.83 NR, (i) and (j) IFFT profile of lattice distance analysis for individual Pd and WO2.83 NR Figure 3. (a) FE-SEM image of WO2.75 NB, (b) TEM (inset SAED) images of Pd-WO2.75 NB, (c) Corresponding histogram for calculated size of Pd nanoparticles, (d) HR-TEM image of Pd-WO2.75 NB, (e) lattice image of high index faceted Pd, (f) and (g) FFT pattern of individual Pd and WO2.75 NB, (h) and (i) IFFT profile of lattice distance analysis for individual Pd and WO2.75 NB Figure 4. (a) FE-SEM image of WO2.94 NS, (b) TEM (inset SAED) images of Pd-WO2.94 NS, (c) Corresponding histogram for calculated size of Pd nanoparticles, (d) HR-TEM image of bundled Pd-WO2.94 NS (inset corresponding FFT of WO2.94 NS), (e) lattice image of low index Pd (inset corresponding FFT of Pd nanoparticles), (f) and (g) IFFT profile of lattice distance analysis for individual Pd and WO2.94 NS



Figure 5. XPS of (a) Survey spectra, (b-d) Pd 3d spectra, (e-g) W4f spectra, (h-j) O1s spectra of Pd-WO2.83 NR, Pd-WO2.75 NB, and Pd-WO2.94 NS, respectively. Figure 6. Ethanol electrocatalytic oxidation via Cyclic voltammetry analysis with various cycle number (50, 500, 1000) curves at 1 M ethanol +1 M KOH at a scan rate of 50 mV s-1 for (a) Pd-WO2.94 NS, (b) Pd-WO2.83 NR, (c) Pd-WO2.75 NB and comparison of as prepared electrocatalysts with commercial Pd/C of (d) Cyclic voltammetry (CV) (e) mass and specific activity, and (f) electrochemical impedance spectroscopy (EIS), respectively. Figure 7. Ethylene glycol (EG) electrocatalytic oxidation via Cyclic voltammetry analysis with various cycle number (50, 500, 1000) curves at 1 M EG +1 M KOH at a scan rate of 50 mV s-1 for (a) Pd-WO2.94 NS, (b) Pd-WO2.83 NR, (c) Pd-WO2.75 NB. Comparison of as prepared electrocatalysts with commercial Pd/C of (d) Cyclic voltammetry (CV) (e) mass and specific activity, and (f) electrochemical impedance spectroscopy (EIS), respectively. Figure 8. Glycerol (Gly) electrocatalytic oxidation via Cyclic voltammetry analysis with various cycle number (50, 500, 1000) at 1 M Gly + 1 M KOH at a scan rate of 50 mV s-1 for (a) Pd-WO2.94 NS, (b) Pd-WO2.83 NR, (c) Pd-WO2.75 NB. Comparison of as prepared electrocatalysts with commercial Pd/C of (d) Cyclic voltammetry (CV) (e) mass and specific activity, and (f) electrochemical impedance spectroscopy (EIS), respectively.


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Figure 9. CO-electrooxidation of 1M KOH containing bubbled CO with a scan rate of 50 mV s-1 curves for (a) Pd-WO2.75 NB, (b) Pd-WO2.83 NR, and (c) Pd-WO2.94 NS. (d) Calculated electrochemical active surface (ECSA) area plots



Page 36 of 47

Table 1. Comparison of electrocatalytic oxidation of alcohols for as-prepared Pd-WOx electrocatalysts and commercial Pd/C

Catalyst

Eonset

Jf

Mass activity

Specific activity

Jf/Jb

(V)

(mA mg-1Pd)

(A mg-1Pd)

(A cm-2)

Pd-WO2.94 NS

-0.474

702.6

0.70

0.93

1.53

Pd-WO2.83 NR

-0.482

1172.0

1.18

1.51

3.67

Pd-WO2.75 NB

-0.568

1968.3

1.98

2.16

1.45

Commercial Pd/C

-0.3429

398.6

0.39

0.42

2.55

Pd-WO2.94 NS

-0.205

2478.5

2.21

3.28

2.95

Pd-WO2.83 NR

-0.218

3189.7

2.56

4.03

3.86

Pd-WO2.75 NB

-0.419

4313.7

4.58

4.97

9.08

Commercial Pd/C

-0.197

1190

1.19

1.59

2.8

Pd-WO2.94 NS

-0.402

2597.1

2.52

4.15

7.28

Pd-WO2.83 NR

-0.414

3320

3.21

4.60

7.71

Pd-WO2.75 NB

-0.427

5047.8

4.9

5.39

8.83

Commercial Pd/C

-0.29

1385.3

1.44

2.13

6.04

Ethanol

Ethylene glycol

Glycerol


Page 37 of 47

200

Figure 1.

WO2.83NR Pd-WO2.83NR

111 30

40 50 Degree(2)

70

60

70

WO2.94NS Pd-WO2.94NS

10

30

50


422

222 40 Degree(2)

240

022 202 120 112 20

004 014 114

200

020 002

(c)

002 220

201

001 20

60

WO2.75NB Pd-WO2.75NB

110 101

100

Intensity(a.u) 10

401

300 211 002

210

40 50 Degree(2)

200

(b)

220 310 112 221 400

201 30

401

20

221 400

10

111

001 110 101

Intensity(a.u)

100

(a)

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

70


Scheme 1.


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Figure 2.



Figure 3.


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Figure 4.



Figure 5.

Pd-WO2.83NR Pd-WO2.75NB

(a)

W4f 5/2

W4d

5/2

3/2

5/2 W4d

3/2 Pd3d

Pd3d

Intensity (a.u)

W4f 7/2

O1s

Pd-WO2.94NS

1200

1000 Pd-WO2.84NR

(b)

800 600 400 Binding Energy(eV) 0

(e)

Pd

(h) 4+ W

0 2+

Intensity (a.u)

342

340

338

336

334

Intensity (a.u)

Pd

2+

Pd

38

332

Binding energy (eV) (c)

Pd-WO2.75NB

6+ W

(f)

0

Pd

5+ W

37

36 35 34 33 Binding energy (eV)

Pd-WO2.75 NB

W 4f

Pd-WO2.83 NR

0

2-

lattice-O

O1S

Intensity (a.u)

Pd

344

200

Pd-WO2.83 NR

W 4f

Pd 3d

32

31

adsorbed H2O

536

4+ W

534

W-OH

532 530 528 Binding energy (eV)

Pd-WO2.75 NB

(i)

Pd-O

526

524

2-

lattice-O

Intensity (a.u)

O1S

Intensity (a.u)

2+

2+

Pd

344

342

(d)

Pd


Pd-WO2.94 NS

Pd 3d

332

6+ W 5+ W

38

(g) 0

Pd

W4f

37

Intensity (a.u)

0

Pd


Pd-WO2.94 NS

32

adsorbed-H2O

Pd-O

31

534

(j)

4+

W

W-OH

532 530 528 Binding energy (eV)

Pd-WO2.94 NS

526

524

2-

lattice-O

0

Pd

Intensity (a.u)

2+

Pd

Intensity (a.u)

2+

Pd

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 47

5+

W 6+

W

W-OH adsorbed H2O Pd-O

344

342


332

38

37

36

35

34

33

32

31

Binding energy (eV)


536

534

532 530 528 Binding Energy(eV)

526

524

Page 43 of 47

Figure 6.

1000 (a)

1500

Pd-WO2.94 NS

(b)

Pd-WO2.83 NR

0

th

50 cycles th 500 cycles th 1000 cycles

-500

-1000

j/ (mAcm-2mgPd)

2000

-0.8 -0.6 -0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

(c)

Pd-WO2.75 NB

1500

0.4

500 0

-500

0.6

-0.8 -0.6 -0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

th


th


-1000

2000

0.4

0.6

Commercial Pd/C Pd-WO2.94NS

(d)

Pd-WO2.83NR

1500 j/(mAcm-2mgPd)

j/ (mAcm-2mgPd)

500

j/ (mAcm-2mgPd)

1000

Pd-WO2.75NB

1000

1000 500

500 0 -500

0

-1000 -0.8 -0.6 -0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl) 2.0

(e)

0.4

0.6

Mass activity Specific activity

-0.8

14 2.0

1.5

1.5

1.0

1.0

0.5

0.5

0.0

Pd/C Commercial

Pd-WO2.94 NS

Pd-WO2.83 NR

Pd-WO2.75 NB

0.0

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6

Pd-WO2.75NB

(f)

Pd-WO2.94NR

12

Current density (Acm-2) Z'' (k 

Current density (Acm-2mgPd)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pd-WO2.83NS Commercial Pd/C

10 8 6 4 2 0 0

4

8

12 Z' (k  )


16

20

24


Figure 7.

5000 (a)

Pd-WO 2.94 NS

th

3000 2000 1000

2000

0

-0.8

(c)

-0.6

-0.4

-0.2 0.0 0.2 E/V (vsAg/AgCl)

Pd-WO2.75 NB

0.4

-0.8

0.6

8000

th


-0.4

-0.2 0.0 0.2 E/V (vsAg/AgCl)

0.4

0.6


(d)

Pd-WO2.83NR Pd-WO2.75NB

6000 4000

3000

2000

1500

0

0

-2000

-0.6

(e)

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6


5 4

3

3

2

2

1

1

0

0

Current density (Acm-2)

4

Pd/C Commercial

Pd-WO2.94 NS

Pd-WO2.83 NR

Pd-WO2.75 NB

10

-0.8

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6

Pd-WO2.75NB

(f)

Pd-WO2.83NR

8


Z'' (k 

-0.8


-0.6

j/mAcm-2mgPd

4500

5

th


4000

0

6000

Pd-WO 2.83 NR

6000 j/ (mAcm -2 mgPd )

4000 j/ (mAcm -2 mgPd )

(b)


j/ (mAcm-2mgPd)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 47

6 4 2 0 0


2

4

6

8 10 Z' (k  )

12

14

16

Page 45 of 47

Figure 8.

(a)

Pd-WO2.75 NS

th


3000 j/ (mAcm-2mgPd)

j/ (mAcm-2mgPd)

3000

2000

1000

th


1000

0

-0.8

5000 (c)

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6

-0.8

6000

Pd-WO2.75 NB

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)


0.6

Pd-WO2.83NR

4500

Pd-WO2.75NB

j/(mAcm-2mgPd)

3000

0.4


(d)

th

4000 j/ (mAcm-2mgPd)

(b) Pd-WO2.83 NR

2000

0

3000

2000

1500

1000 0

0 -0.8

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

-0.8

0.6 24

5

(e)


5

4

4

3

3

2

2

1

1

0

Pd/C Commercial

Pd-WO2.94 NS

Pd-WO2.83 NR

Pd-WO2.75 NB

0

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6

Pd-WO2.75NB

(f)

Pd-WO2.83NR

20

Current density (Acm-2) Z'' (k 


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



16 12 8 4 0

2

4

6

8

10

Z' (k  )


12

14

16

18


Figure 9.

(a)

30

nd

2 Cycle

0

4

-0.208V

nd

2 Cycle

2 0

-10 CO-oxidation After CO-oxidation

-20 -0.8

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

6

CO-oxidation After CO-oxidation

-2 -4

0.6

Pd-WO2.94 NS

(c)

-0.8

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

80 (d) 70

4

st

2

1 Cycle

ECSA /m2g-1

-0.088V

nd

2 Cycle

0

60 50 40 30 20

-2 -4

1 Cycle

6

-0.283 V

10

-30

Pd-WO2.83 NR

(b)

st

st

1 Cycle

20 j/ mAcm-2

8

Pd-WO2.75 NB

j/ mAcm-2

40

j/ mAcm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 47

CO-oxidation After CO-oxidation -0.8

-0.6

-0.4 -0.2 0.0 0.2 E/V(vsAg/AgCl)

0.4

0.6

10 0

Pd-WO2.94 NS


Pd-WO2.83 NR

Pd-WO2.75 NB

0.6

Page 47 of 47 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Low and High Index Faceted Pd Nanocrystals Embedded in Various

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