To improve the activity for glycerol oxidation reaction (GOR) of Pt, PtAg (mole ratio of Pt/Ag = 3 and 1) alloy nanoparticle-loaded carbon black (Pt/CB, PtAg(3:1)/CB, PtAg(1:1)/CB) catalysts were prepared by a wet method. The resultant catalysts, moreover, were heat-treated in a N2 atmosphere at 200 °C. The alloying of Pt with Ag for each PtAg/CB was confirmed by X-ray diffractometry and electron dispersive X-ray spectrometry. The heat-treatment did not change the crystal structure of the PtAg alloys and increased their particle size. X-ray photoelectron spectroscopy exhibited that stabilizers were completely removed from the PtAg alloy surface, and the Pt4f and Ag3d doublets due to metallic Pt and Ag, respectively, shifted to lower binding energies, supporting the alloying of Pt with Ag. Both PtAg/CB electrodes had two oxidation waves of glycerol irrespective of heat-treatment, which was different from the Pt/CB electrode. The onset potential of the first oxidation wave was -0.60 V, which was 0.20 V less positive than that for the Pt/CB electrode, indicating the alloying of Pt with Ag greatly improved the GOR activity of Pt. The heat-treated PtAg(3:1)/ CB electrode improved the GOR current density of the second oxidation peak. In the potentiostatic electrolysis at -0.1 and 0 V for both PtAg/CB electrodes, the ratio of oxidation current density at 60 min to that at 5 min ( j 60/ j 5), an indicator of the catalyst deterioration, at 0 V was higher than that at -0.1 V, because the adsorbed oxidation intermediates were greatly consumed at the larger overpotential. The heat-treatment of the PtAg(3:1)/CB electrode increased the j 60 / j 5 value at -0.1 V but decreased that at 0 V. This could be attributed to the formation of high-order oxidation intermediates which might have stronger poisoning effect.
Biodiesel fuel (BDF) is known as a carbon-neutral fuel which is produced with glycerol byproduct through the transesterification of vegetable fats and oils or long-chain alkyl esters with methanol. The production of BDF is on the increase, so new application of the increasing glycerol byproduct is an important and urgent issue. Direct alcohol fuel cells (DAFCs) have attracted considerable interest because the alcohols such as methanol and ethanol as a fuel have a high energy density and are easy to handle, store, and transport compared with pure hydrogen [
Palladium is less expensive than gold and platinum, and less active for GOR in alkaline media than platinum. So the alloying of Pd with various foreign metals has been attempted for improving its GOR activity [
A solution of KBH4 (4 mmol) as a reducing agent in H2O (1.5 mL) was added dropwise to a solution containing PtCl2 (0.5 or 0.75 mmol) and AgCl (0.5 or 0.25 mmol) and N(Oct)4Br (4 mmol) in THF (150 mL) with stirring at 1800 rpm at 30˚C in an Ar atmosphere. Each reaction mixture was still stirred at the same rate for 30 min, resulting in a black colloidal suspension. The suspended solid was isolated by suction filtration in air and washed with excess H2O, acetone, and ethanol at room temperature, yielding a black waxy solid. The black solid was redispersed in ethanol. Ketjen black (67 mg) was added in the dispersion and sonicated for 10 min. After suction filtration, the residue was redispersed in 300 mL of 0.1 M HClO4 aqueous solution and then sonicated for 10 min. After suction filtration, the residue was washed with excess H2O, acetone, and ethanol at room temperature. The resultant powder is called PtAg(1:1)/CB or PtAg(3:1)/CB hereafter. The Pt/CB, Ag/CB powders were also prepared by the same procedure. The heat-treatment of PtAg(3:1)/CB and PtAg(1:1)/CB was performed in a N2 atmosphere at 200˚C for 15 min.
To evaluate the metal loading in each catalyst, thermogravimetry was performed in air by heating from room temperature to 700˚C at a rate of 1 K∙min−1. The metal or alloy loading on CB was 50, 55, 55 and 50 wt% for Pt/CB, heat-treated PtAg(3:1)/CB, heat-treated PtAg(1:1)/CB and Ag/CB, respectively.
Structure and average size of metal or alloy nanoparticles for each catalyst were performed using an X-ray diffractometry (XRD) and electron dispersive X-ray (EDX) spectrometry. Chemical state of Pt and Ag in each catalyst was measured by X-ray photoelectron spectroscopy (XPS). The X-ray source was Mg Kα at 1253.6 eV operating at 10 kV and 20 mA. The base pressure of the system was 1.3 × 10−7 Pa.
The modification of each catalyst on a glassy carbon (GC) substrate (5 mmf) was performed according to Ref. [
XPS analysis for all catalysts did not show any peaks in the N1s and the Br3d core level spectra, suggesting that the N(Oct)4Br stabilizer was completely removed by washing with H2O, ethanol, acetone, and HClO4 solution.
The Pt4f and Ag3d core level spectra for each catalyst are shown in
For the PtAg(3:1)/CB and PdAg(1:1)/CB electrodes, irrespective of the Ag content and heat-treatment, two couples of butterfly waves for hydrogen adsorption/desorption changed to broad irreversible waves, and the onset potential of the oxidation current due to the surface Pt-OHad formation greatly shifted to higher potentials, which seemed to indicate that the corner and edge sites on the PtAg nanoparticle surfaces were occupied by Ag atoms. The ECSAs of the surface Pt site for the PtAg(3:1)/CB and PdAg(1:1)/CB electrodes were estimated as 0.14 and 0.10 cm2, respectively, regardless of heat-treatment. Since the sizes of the heat-treated PtAg(3:1) and PtAg(1:1) particles are similar to the size (3.8 nm) of the Pt particles, the ECSA of the surface Pt site is estimated as three quarter (0.27 cm2) for the PtAg(3:1)/CB and a half (0.18 cm2) for the PtAg(1:1)/CB compared to the ECSA of Pt/CB if the surface composition is equivalent to the bulk composition. The experimental ECSAs are smaller than the estimated ones, suggesting that the PtAg particle surfaces are Ag-enriched compared to the bulk compositions.
For the PtAg(3:1)/CB and PtAg(1:1)/CB electrodes, two oxidation waves for GOR were observed irrespective of heat-treatment. In both electrodes the first oxidation wave with the lower potential had the onset potential around ca. −0.6 V, which was 0.20 V less positive than that for the Pt/CB electrode. The appearance of the first oxidation wave, however, was more negative than the onset potential of the Pt-OHad formation in contrast to the Pt/CB electrode. The electronic modification of Pt based on its alloying with Ag may activate adsorbed glycerol. Also, Ag is known to promote the removal of adsorbed CO or similar poisoning intermediates out of the PdAg surface [
The GOR current density for the PtAg(3:1)/CB electrode was increased after the heat-treatment, whereas that for the PtAg(1:1)/CB electrode was not influenced by the heat-treatment. In the CV of the heat-treated PtAg(3:1)/CB electrode in a 1 M KOH solution, the onset potential for the formation of Pt-OHad was observed at more negative potential than that before the heat-treatment, as shown in
Two PtAg (mole ratio of Pt/Ag = 3 and 1) alloy nanoparticle-loaded CB catalysts for GOR in alkaline medium
were prepared by a wet method at room temperature. Also, the resultant catalysts were heat-treated in N2 atmosphere at 200˚C. The alloying of Pt with Ag for each PtAg/CB was confirmed by XRD and EDX spectrometry. The heat-treatment did not change the crystal structure of the PtAg alloys and increased their particle size. XPS data exhibited that stabilizers were completely removed from the PtAg alloy surface, and the Pt4f and Ag3d doublets due to metallic Pt and Ag, respectively, shifted to lower binding energies, supporting the alloying of Pt with Ag.
Both PtAg/CB electrodes had two oxidation waves of glycerol irrespective of heat-treatment, which was different from the Pt/CB electrode. The onset potential of the first oxidation wave was −0.60 V, which was 0.20 V less positive than that for the Pt/CB electrode, indicating the alloying of Pt with Ag greatly improved the GOR activity of Pt. The heat-treated PtAg(3:1)/CB electrode could increase the GOR current density of the second oxidation peak. In the potentiostatic electrolysis at −0.1 and 0 V for the PtAg(3:1)/CB and PtAg(1:1)/CB electrodes, the j60/j5 value at 0 V as an indicator of the catalyst deterioration was higher than that at −0.1 V, because the adsorbed oxidation intermediates were greatly consumed at the larger overpotential. The heat-treatment of the PtAg(3:1)/CB electrode increased the j60/j5 value at −0.1 V but decreased that at 0 V. This could be attributed to the formation of high-order oxidation intermediates which might have stronger poisoning effect.
This work was partially supported by JSPS KAKENHI Grant Number 15H04162.
Binh Thi Xuan Lam,Masanobu Chiku,Eiji Higuchi,Hiroshi Inoue, (2016) PtAg Nanoparticle Electrocatalysts for the Glycerol Oxidation Reaction in Alkaline Medium. Advances in Nanoparticles,05,167-175. doi: 10.4236/anp.2016.53018