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Tailoring the electron density of Pd nanoparticles through electronic metal-support interaction for accelerating electrocatalysis of formic acid
Electrochemistry Communications 107 (2019) 106540
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Tailoring the electron density of Pd nanoparticles through electronic metalsupport interaction for accelerating electrocatalysis of formic acid
T
Shuai Sua,1, Yi Shib,1, Yue Zhoub, Yue-Bo Wanga, , Feng-Bin Wangb, Xing-Hua Xiab, ⁎
a b
⁎
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255000, China State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
ARTICLE INFO
ABSTRACT
Keywords: Electronic metal–support interaction Pd nanoparticles Electron density Formic acid oxidation reaction Electrocatalysis
The electronic interaction between metal and supports can regulate the activity of heterogeneous nanomaterials. We designed four supported Pd catalysts (Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2) for the electrocatalytic activity toward the formic acid oxidation reaction (FAOR). The electrochemical results demonstrate that Pd/ZnO exhibits superior electrocatalytic activity toward FAOR, which can be attributed to the decreased electron density of ZnO-supported Pd nanoparticles, as further confirmed by X-ray photoelectron spectroscopic characterization and CO stripping electrochemical measurement. This study provides an effective method for the design of high-efficiency electrocatalysts, which could be also useful for studying the surface electronics of supported metal-nanoparticle.
1. Introduction Due to energy shortages and environmental pollution, people have invested a lot of work in the development and utilization of new energy. Direct oxidation fuel cells are power generation devices that directly convert chemical energy in fuel into electrical energy through electrochemical reactions, and thus have attracted wide attention because of high energy conversion efficiency and low environmental pollution during power generation [1,2]. Cost and performance are still two prominent problems that constrain the large-scale commercialization of direct liquid fuel cells, and the catalytic activity and stability of anode catalysts play a crucial role [3,4]. In the past studies, it was found that Pt showed excellent electrocatalytic activity for the oxidation of formic acid (FAOR) [5]. However, the strong adsorbed CO intermediate generated during FAOR can easily poison the surface of Pt, resulting in the dramatic decrease of the electrocatalytic activity [6]. Ideally, electrocatalysts for FAOR should have good toxicity resistance and high activity at low oxidation potentials, which encourages researchers to develop highly efficient non‑platinum electrocatalysts [6–8]. Among them, the Pd-based metal nanomaterials have been widely investigated as effective alternatives for FAOR [9–13]. Generally, two strategies are often used to improve the activity of electrocatalysts [14]. One is to increase the number of active sites on electrocatalysts, which usually focuses on increasing the metal loading or tailoring the electrocatalyst
structures to expose more active sites [15–19]. The other is to improve the intrinsic electrocatalytic activity of each active site, such as regulating the electron density of metals and increasing the conductivity of materials [20–22]. Based on these concepts, the reported methods to boost the electroactivity of Pd-based electrocatalysts include the regulation of particle size [23,24], the integration of support [17], the modulation of morphology [25] and the construction of polymetallic alloys or core-shell structures [26–29]. The “support effect” has been considered as one of the most important factors influencing the activity of electrocatalyst [30,31]. This “support effect” generally refers to the interaction between the heterogeneous metal nano-catalyst and the supporting substrate, which endows the supported catalyst with unique properties (e.g., electrocatalytic activity) from the original one [32]. This effect includes the adjustment of morphology (sizes and shapes) [33], dispersity [34], stability of nanoparticles [35,36] “strong metal-support interactions” (SMSI) caused by the migration of support oxide species onto metal surface [37–39]. A large number of defects and holes existing on the surface of the support provide strong binding sites for anchoring the metal nanoparticles, and the high bonding between the support and the metal enables the metal to have good dispersibility and stability on the surface of the support [40]. In recent years, the “electronic metal–support interaction” (EMSI) has been proposed to describe the effect of charge redistribution between metal and support on the catalytic
Corresponding authors. E-mail addresses: [email protected] (Y.-B. Wang), [email protected] (X.-H. Xia). 1 Shuai Su and Yi Shi contributed equally to this work. ⁎
https://doi.org/10.1016/j.elecom.2019.106540 Received 8 August 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 04 September 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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activity [41,42]. This effect will significantly affect the catalytic performance of metal nanoparticles [43]. In addition, such interactions are significant for particles adhesion, in turn for the stability of the material [44,45]. Lv et al. reported the regulation of the electronic state of noble metals by EMSI to improve its electrocatalytic activity. Electron transfer between Pd and reducible metal oxides leads to form electron-deficient Pd atoms, which could adsorb oxygenated species (e.g., *OH and *OOH), and significantly increase the OER catalytic activity [46]. Moreover, depending on the effect of EMSI there might be a large amount of electron transfer between the metal particles and the supported oxide. Libuda et al. have measured the amount of transferred electron between a supported catalyst and the support [44], and found that this effect significantly affected the activity of the catalyst. However, systematic studies of the influence of various supporting substrates on the interfacial electron density of metal and their catalytic activities have rarely been reported. Herein, we propose that the electron density and electrocatalytic activity of Pd nanoparticles toward FAOR can be modulated by various metal-oxide supports (ZnO, TiO2, Al2O3, and SiO2) through EMSI. We combine the activity analysis with XPS and CO stripping measurements to demonstrate that the electrocatalytic activity of the supported Pd nanoparticles toward FAOR is mainly affected by electron transfer from the Pd nanoparticles to the underlying oxide support. Our research on the electronic interaction between metal nanoparticles and their supports can provide further insights for the design of high-performance supported metal catalysts.
which is corresponding to the plane-centered cubic Pd (111). The XRD patterns of the four catalysts show the same diffraction peaks characteristics for the face-centric cubic structure with a band appeared at ca. 39.8°, attributed to the Pd (111) plane (Fig. 1g). Fig. 1h shows Pd (111) diffraction peak after excluding the interference of support. Pd/ ZnO, Pd/TiO2, Pd/Al2O3, Pd/SiO2 can be observed in the chronograph current curve of electrocatalytic oxidation of formic acid at a potential of 0.15 V in 0.5 M H2SO4 and 0.5 M HCOOH mixed electrolyte. The current densities of four electrocatalysts at 1000 s were 1.37, 1.11, 0.73 and 0.51 mA cm−2 respectively (Fig. S3). The cyclic voltammograms of FAOR at the four different electrocatalysts are displayed in Fig. S1. All the four catalysts show good FAOR electrocatalytic activity with low overpotential. The anodic peak currents (presented with geometric area of the electrodes) on Pd/ZnO, Pd/ TiO2, Pd/Al2O3, and Pd/SiO2 are 1.52, 0.92, 0.58, and 0.41 mA cm−2, respectively, demonstrating that the Pd/ZnO has the best FAOR activity. The specific activity of a catalyst can be used to assess the intrinsic activity. As we can see in Fig. S2, the electrochemical surface area (ECSA) of Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts are calculated as 0.176, 0.137, 0.128, and 0.103 cm2, respectively (method shown in supporting information). We find that the specific peak current density of Pd/ZnO (8.59 mA cm−2) still exhibits ca. 1.29, 1.87, and 2.17 times larger than those of Pd/TiO2, Pd/Al2O3, and Pd/SiO2, respectively (Fig. 2a). Fig. 2b shows the Tafel plots of log I vs. E for FAOR at four electrocatalysts. The Tafel slope of Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts had be fitted as 112 mV/dec, 122 mV/dec, 129 mV/dec, and 140 mV/dec respectively. As seen, the smaller slope at Pd/ZnO indicates a higher output current with respect to those of other three electrocatalysts under the same output voltage. These results demosntrate that FAOR on Pd/ZnO has a faster kinetic rate. The Pd nanoparticles with the similar particle size on the four supports show completely different electrocatalytic activities toward FAOR. We speculate that the electronic interaction between Pd and the supporting substrates might explain the observed difference in catalytic activities. In order to verify the relationship between the electronics of Pd and its electrochemical activity toward FAOR, we performed XPS measurement on the four supported Pd electrocatalysts. The binding energy of electrons reflects the binding ability of the nucleus to extra-nuclear electrons. It could be seen from Fig. 3a that the 3d5/2 binding energies of Pd on Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts vary from 340.5 to 339.0 eV, which suggests that the ZnO-supported Pd has the lowest electron density. To confirm this, CO stripping voltammetry test was also conducted to determine the electronic structure of noble metal surfaces. The electron density on the metal surface could affect the adsorption of CO on the metal surface, which is reflected in the CO stripping potential [47]. The detailed experimental method is described in the experimental section. It could be seen that the CO stripping potential of Pd/ ZnO catalyst exhibits the lowest value (0.54 V, Fig. 3b). These results indicate that the surface of Pd nanoparticles supported on ZnO has less electrons, and hence donates less electrons to the CO 2π* orbitals, which is less likely to strengthen CO adsorption and weaken the CeO bonds (Fig. 3c). The CO stripping potentials of Pd/ZnO, Pd/TiO2, Pd/ Al2O3 and Pd/SiO2 catalysts sequentially increases, which in turn proves that the electron density on the surface of Pd nanoparticles is increased. Combined with the activity of FAOR, it is further verified that the lower electron density on the surface of the oxide-supported Pd nanoparticles greatly contributes to the higher electrocatalytic activity toward FAOR. Additionally, we find that Pd/ZnO with lower electron concentration has a lower adsorption capacity for CO, which might also reduce the toxicity of CO on Pd nanoparticles in the indirect pathway of formic acid oxidation (Fig. 3d). In view of this phenomenon, we believe that more electrons transferred from Pd to the supporting substrate reduces the electron density on the surface of Pd nanoparticles. Such state of electron deficiency, could be beneficial to the FAOR.
2. Experimental ZnO, TiO2, SiO2, Al2O3, sodium borohydride (NaBH4), and potassium chloroplatinate (K2PdCl4) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion solution (5 wt% in a mixture of ethanol and water, 45% water) was obtained from Sigma-Aldrich (USA). Hexadecyl trimethyl ammonium bromide (CTAB) was purchased from Johnson Matthey. Solutions were prepared with ultrapure water with resistance of 18.2 MΩ (Milli-Q Millipore, USA). Pd/ZnO catalyst with 20 wt% Pd content was obtained as shown in supporting information. Three other catalysts were prepared in the same way except for the supporting substrates (the products named as Pd/TiO2, Pd/Al2O3 and Pd/SiO2, respectively). The morphology, element composition and structure of the catalysts were analyzed on a transmission electron microscopy (TEM, JEOL JEM2100F), X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific ESCALAB 250 spectrometer), and X-ray diffraction (XRD, D-type/maxrC X-ray diffractometer). The binding energy was calibrated by C 1s peak energy of 284.6 eV. All electrochemical experiments were carried out on a CHI 660E electrochemical workstation, using a standard three-electrode system with Pt as the auxiliary electrode, Ag/AgCl electrode as the reference, and a glass carbon electrode with a diameter of 3 mm as the working electrode, method shown in supporting information. 3. Results and discussion As a proof-of-concept, Pd nanoparticles are first modified on four different supports (named as Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2, respectively) by in-situ reduction method as illustrated in Fig. 1a. As shown in Figs. 1b~e, the Pd nanoparticles are well-dispersed on the supports, and no obvious agglomeration phenomenon is observed. The Pd catalysts has the average particle size of ca. 4–5 nm for all the catalysts as evidenced from the similar particle size distribution (Fig. 1f), which allows us to exclude the influence of particle size effect on catalytic activity for different catalysts. The high resolution TEM (HRTEM) images displayed as insets in Fig. 1b–e show that the Pd nanoparticles on the four supports exhibit obvious crystal structure with continuous ordered lattice fringes, and the lattice spacing is around 0.22–0.24 nm, 2
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Fig. 1. (a) Schematic illustration to the synthesis of electrocatalysts. TEM images of (b) Pd/ZnO, (c) Pd/TiO2, (d) Pd/Al2O3 and (e) Pd/SiO2. Inset: HRTEM images of the corresponding electrocatalysts. (f) Histograms of particle size distribution of the four electrocatalysts as indicated. (g) XRD patterns of Pd/ZnO (black), Pd/TiO2 (red), Pd/Al2O3 (green), and Pd/SiO2 (blue). (h) Pd (111) diffraction peak after excluding the interference of support. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
studied series (including XPS and CO stripping voltammetry), the electrochemical activities toward FAOR correlated with the electron density of the supported Pd nanoparticles. We believe that this is an effective and feasible method to optimize and design high-efficiency catalysts by selecting different supports to regulate the electronic structure of metal surface, and further improve its catalytic activity.
We prepared four supported Pd electrocatalysts (Pd/ZnO, Pd/TiO2, Pd/Al2O3, and Pd/SiO2) via the in-situ reduction, which were further used for FAOR. It is suggested that the interfacial electron density and hence the electrocatalytic activities of the Pd nanoparticles could be tuned by the supporting substrates. In connection with the entire 3
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Fig. 2. (a) Specific activity of the electrocatalysts as indicated. (b) Tafel plots of log I vs. E for FAOR at Pd/ZnO (black), Pd/TiO2 (red), Pd/Al2O3 (blue), and Pd/SiO2 (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 3. (a) XPS spectra of Pd/ZnO (black), Pd/ TiO2 (red), Pd/Al2O3 (blue) and Pd/SiO2 (green) in the Pd 3d region. (b) CO-stripping voltammograms of Pd/ZnO (black), Pd/TiO2 (red), Pd/ Al2O3 (blue) and Pd/SiO2 (green) in 0.5 M H2SO4 solution at 20 mV s−1. (c) Schematic illustration of the electron transfer between Pd and CO. (d) Schematic of the “triple-path way” reaction of FAOR. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments
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Tailoring the electron density of Pd nanoparticles through electronic metalsupport interaction for accelerating electrocatalysis of formic acid
T
Shuai Sua,1, Yi Shib,1, Yue Zhoub, Yue-Bo Wanga, , Feng-Bin Wangb, Xing-Hua Xiab, ⁎
a b
⁎
School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo 255000, China State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China
ARTICLE INFO
ABSTRACT
Keywords: Electronic metal–support interaction Pd nanoparticles Electron density Formic acid oxidation reaction Electrocatalysis
The electronic interaction between metal and supports can regulate the activity of heterogeneous nanomaterials. We designed four supported Pd catalysts (Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2) for the electrocatalytic activity toward the formic acid oxidation reaction (FAOR). The electrochemical results demonstrate that Pd/ZnO exhibits superior electrocatalytic activity toward FAOR, which can be attributed to the decreased electron density of ZnO-supported Pd nanoparticles, as further confirmed by X-ray photoelectron spectroscopic characterization and CO stripping electrochemical measurement. This study provides an effective method for the design of high-efficiency electrocatalysts, which could be also useful for studying the surface electronics of supported metal-nanoparticle.
1. Introduction Due to energy shortages and environmental pollution, people have invested a lot of work in the development and utilization of new energy. Direct oxidation fuel cells are power generation devices that directly convert chemical energy in fuel into electrical energy through electrochemical reactions, and thus have attracted wide attention because of high energy conversion efficiency and low environmental pollution during power generation [1,2]. Cost and performance are still two prominent problems that constrain the large-scale commercialization of direct liquid fuel cells, and the catalytic activity and stability of anode catalysts play a crucial role [3,4]. In the past studies, it was found that Pt showed excellent electrocatalytic activity for the oxidation of formic acid (FAOR) [5]. However, the strong adsorbed CO intermediate generated during FAOR can easily poison the surface of Pt, resulting in the dramatic decrease of the electrocatalytic activity [6]. Ideally, electrocatalysts for FAOR should have good toxicity resistance and high activity at low oxidation potentials, which encourages researchers to develop highly efficient non‑platinum electrocatalysts [6–8]. Among them, the Pd-based metal nanomaterials have been widely investigated as effective alternatives for FAOR [9–13]. Generally, two strategies are often used to improve the activity of electrocatalysts [14]. One is to increase the number of active sites on electrocatalysts, which usually focuses on increasing the metal loading or tailoring the electrocatalyst
structures to expose more active sites [15–19]. The other is to improve the intrinsic electrocatalytic activity of each active site, such as regulating the electron density of metals and increasing the conductivity of materials [20–22]. Based on these concepts, the reported methods to boost the electroactivity of Pd-based electrocatalysts include the regulation of particle size [23,24], the integration of support [17], the modulation of morphology [25] and the construction of polymetallic alloys or core-shell structures [26–29]. The “support effect” has been considered as one of the most important factors influencing the activity of electrocatalyst [30,31]. This “support effect” generally refers to the interaction between the heterogeneous metal nano-catalyst and the supporting substrate, which endows the supported catalyst with unique properties (e.g., electrocatalytic activity) from the original one [32]. This effect includes the adjustment of morphology (sizes and shapes) [33], dispersity [34], stability of nanoparticles [35,36] “strong metal-support interactions” (SMSI) caused by the migration of support oxide species onto metal surface [37–39]. A large number of defects and holes existing on the surface of the support provide strong binding sites for anchoring the metal nanoparticles, and the high bonding between the support and the metal enables the metal to have good dispersibility and stability on the surface of the support [40]. In recent years, the “electronic metal–support interaction” (EMSI) has been proposed to describe the effect of charge redistribution between metal and support on the catalytic
Corresponding authors. E-mail addresses: [email protected] (Y.-B. Wang), [email protected] (X.-H. Xia). 1 Shuai Su and Yi Shi contributed equally to this work. ⁎
https://doi.org/10.1016/j.elecom.2019.106540 Received 8 August 2019; Received in revised form 3 September 2019; Accepted 3 September 2019 Available online 04 September 2019 1388-2481/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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activity [41,42]. This effect will significantly affect the catalytic performance of metal nanoparticles [43]. In addition, such interactions are significant for particles adhesion, in turn for the stability of the material [44,45]. Lv et al. reported the regulation of the electronic state of noble metals by EMSI to improve its electrocatalytic activity. Electron transfer between Pd and reducible metal oxides leads to form electron-deficient Pd atoms, which could adsorb oxygenated species (e.g., *OH and *OOH), and significantly increase the OER catalytic activity [46]. Moreover, depending on the effect of EMSI there might be a large amount of electron transfer between the metal particles and the supported oxide. Libuda et al. have measured the amount of transferred electron between a supported catalyst and the support [44], and found that this effect significantly affected the activity of the catalyst. However, systematic studies of the influence of various supporting substrates on the interfacial electron density of metal and their catalytic activities have rarely been reported. Herein, we propose that the electron density and electrocatalytic activity of Pd nanoparticles toward FAOR can be modulated by various metal-oxide supports (ZnO, TiO2, Al2O3, and SiO2) through EMSI. We combine the activity analysis with XPS and CO stripping measurements to demonstrate that the electrocatalytic activity of the supported Pd nanoparticles toward FAOR is mainly affected by electron transfer from the Pd nanoparticles to the underlying oxide support. Our research on the electronic interaction between metal nanoparticles and their supports can provide further insights for the design of high-performance supported metal catalysts.
which is corresponding to the plane-centered cubic Pd (111). The XRD patterns of the four catalysts show the same diffraction peaks characteristics for the face-centric cubic structure with a band appeared at ca. 39.8°, attributed to the Pd (111) plane (Fig. 1g). Fig. 1h shows Pd (111) diffraction peak after excluding the interference of support. Pd/ ZnO, Pd/TiO2, Pd/Al2O3, Pd/SiO2 can be observed in the chronograph current curve of electrocatalytic oxidation of formic acid at a potential of 0.15 V in 0.5 M H2SO4 and 0.5 M HCOOH mixed electrolyte. The current densities of four electrocatalysts at 1000 s were 1.37, 1.11, 0.73 and 0.51 mA cm−2 respectively (Fig. S3). The cyclic voltammograms of FAOR at the four different electrocatalysts are displayed in Fig. S1. All the four catalysts show good FAOR electrocatalytic activity with low overpotential. The anodic peak currents (presented with geometric area of the electrodes) on Pd/ZnO, Pd/ TiO2, Pd/Al2O3, and Pd/SiO2 are 1.52, 0.92, 0.58, and 0.41 mA cm−2, respectively, demonstrating that the Pd/ZnO has the best FAOR activity. The specific activity of a catalyst can be used to assess the intrinsic activity. As we can see in Fig. S2, the electrochemical surface area (ECSA) of Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts are calculated as 0.176, 0.137, 0.128, and 0.103 cm2, respectively (method shown in supporting information). We find that the specific peak current density of Pd/ZnO (8.59 mA cm−2) still exhibits ca. 1.29, 1.87, and 2.17 times larger than those of Pd/TiO2, Pd/Al2O3, and Pd/SiO2, respectively (Fig. 2a). Fig. 2b shows the Tafel plots of log I vs. E for FAOR at four electrocatalysts. The Tafel slope of Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts had be fitted as 112 mV/dec, 122 mV/dec, 129 mV/dec, and 140 mV/dec respectively. As seen, the smaller slope at Pd/ZnO indicates a higher output current with respect to those of other three electrocatalysts under the same output voltage. These results demosntrate that FAOR on Pd/ZnO has a faster kinetic rate. The Pd nanoparticles with the similar particle size on the four supports show completely different electrocatalytic activities toward FAOR. We speculate that the electronic interaction between Pd and the supporting substrates might explain the observed difference in catalytic activities. In order to verify the relationship between the electronics of Pd and its electrochemical activity toward FAOR, we performed XPS measurement on the four supported Pd electrocatalysts. The binding energy of electrons reflects the binding ability of the nucleus to extra-nuclear electrons. It could be seen from Fig. 3a that the 3d5/2 binding energies of Pd on Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2 catalysts vary from 340.5 to 339.0 eV, which suggests that the ZnO-supported Pd has the lowest electron density. To confirm this, CO stripping voltammetry test was also conducted to determine the electronic structure of noble metal surfaces. The electron density on the metal surface could affect the adsorption of CO on the metal surface, which is reflected in the CO stripping potential [47]. The detailed experimental method is described in the experimental section. It could be seen that the CO stripping potential of Pd/ ZnO catalyst exhibits the lowest value (0.54 V, Fig. 3b). These results indicate that the surface of Pd nanoparticles supported on ZnO has less electrons, and hence donates less electrons to the CO 2π* orbitals, which is less likely to strengthen CO adsorption and weaken the CeO bonds (Fig. 3c). The CO stripping potentials of Pd/ZnO, Pd/TiO2, Pd/ Al2O3 and Pd/SiO2 catalysts sequentially increases, which in turn proves that the electron density on the surface of Pd nanoparticles is increased. Combined with the activity of FAOR, it is further verified that the lower electron density on the surface of the oxide-supported Pd nanoparticles greatly contributes to the higher electrocatalytic activity toward FAOR. Additionally, we find that Pd/ZnO with lower electron concentration has a lower adsorption capacity for CO, which might also reduce the toxicity of CO on Pd nanoparticles in the indirect pathway of formic acid oxidation (Fig. 3d). In view of this phenomenon, we believe that more electrons transferred from Pd to the supporting substrate reduces the electron density on the surface of Pd nanoparticles. Such state of electron deficiency, could be beneficial to the FAOR.
2. Experimental ZnO, TiO2, SiO2, Al2O3, sodium borohydride (NaBH4), and potassium chloroplatinate (K2PdCl4) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion solution (5 wt% in a mixture of ethanol and water, 45% water) was obtained from Sigma-Aldrich (USA). Hexadecyl trimethyl ammonium bromide (CTAB) was purchased from Johnson Matthey. Solutions were prepared with ultrapure water with resistance of 18.2 MΩ (Milli-Q Millipore, USA). Pd/ZnO catalyst with 20 wt% Pd content was obtained as shown in supporting information. Three other catalysts were prepared in the same way except for the supporting substrates (the products named as Pd/TiO2, Pd/Al2O3 and Pd/SiO2, respectively). The morphology, element composition and structure of the catalysts were analyzed on a transmission electron microscopy (TEM, JEOL JEM2100F), X-ray photoelectron spectroscopy (XPS, Thermo VG Scientific ESCALAB 250 spectrometer), and X-ray diffraction (XRD, D-type/maxrC X-ray diffractometer). The binding energy was calibrated by C 1s peak energy of 284.6 eV. All electrochemical experiments were carried out on a CHI 660E electrochemical workstation, using a standard three-electrode system with Pt as the auxiliary electrode, Ag/AgCl electrode as the reference, and a glass carbon electrode with a diameter of 3 mm as the working electrode, method shown in supporting information. 3. Results and discussion As a proof-of-concept, Pd nanoparticles are first modified on four different supports (named as Pd/ZnO, Pd/TiO2, Pd/Al2O3 and Pd/SiO2, respectively) by in-situ reduction method as illustrated in Fig. 1a. As shown in Figs. 1b~e, the Pd nanoparticles are well-dispersed on the supports, and no obvious agglomeration phenomenon is observed. The Pd catalysts has the average particle size of ca. 4–5 nm for all the catalysts as evidenced from the similar particle size distribution (Fig. 1f), which allows us to exclude the influence of particle size effect on catalytic activity for different catalysts. The high resolution TEM (HRTEM) images displayed as insets in Fig. 1b–e show that the Pd nanoparticles on the four supports exhibit obvious crystal structure with continuous ordered lattice fringes, and the lattice spacing is around 0.22–0.24 nm, 2
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Fig. 1. (a) Schematic illustration to the synthesis of electrocatalysts. TEM images of (b) Pd/ZnO, (c) Pd/TiO2, (d) Pd/Al2O3 and (e) Pd/SiO2. Inset: HRTEM images of the corresponding electrocatalysts. (f) Histograms of particle size distribution of the four electrocatalysts as indicated. (g) XRD patterns of Pd/ZnO (black), Pd/TiO2 (red), Pd/Al2O3 (green), and Pd/SiO2 (blue). (h) Pd (111) diffraction peak after excluding the interference of support. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
studied series (including XPS and CO stripping voltammetry), the electrochemical activities toward FAOR correlated with the electron density of the supported Pd nanoparticles. We believe that this is an effective and feasible method to optimize and design high-efficiency catalysts by selecting different supports to regulate the electronic structure of metal surface, and further improve its catalytic activity.
We prepared four supported Pd electrocatalysts (Pd/ZnO, Pd/TiO2, Pd/Al2O3, and Pd/SiO2) via the in-situ reduction, which were further used for FAOR. It is suggested that the interfacial electron density and hence the electrocatalytic activities of the Pd nanoparticles could be tuned by the supporting substrates. In connection with the entire 3
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Fig. 2. (a) Specific activity of the electrocatalysts as indicated. (b) Tafel plots of log I vs. E for FAOR at Pd/ZnO (black), Pd/TiO2 (red), Pd/Al2O3 (blue), and Pd/SiO2 (green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Fig. 3. (a) XPS spectra of Pd/ZnO (black), Pd/ TiO2 (red), Pd/Al2O3 (blue) and Pd/SiO2 (green) in the Pd 3d region. (b) CO-stripping voltammograms of Pd/ZnO (black), Pd/TiO2 (red), Pd/ Al2O3 (blue) and Pd/SiO2 (green) in 0.5 M H2SO4 solution at 20 mV s−1. (c) Schematic illustration of the electron transfer between Pd and CO. (d) Schematic of the “triple-path way” reaction of FAOR. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments
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