Intermetallic Pd3X (X= Ti and Zr) nanocrystals for electro-oxidation of alcohols and formic acid in alkaline and acidic media

ABSTRACT Two highly active and stable Pd-based intermetallic nanocrystals with early d-metals Pd3Ti and Pd3Zr have been developed. The nanocrystals are synthesized by co-reduction of the respective salts of Pd and Ti/Zr. Hard X-ray photoemission Spectroscopy (HAXPES) analysis of the nanocrystals indicates that the electronic properties of Pd are modified significantly, as evident from the lowering of the d-band center of Pd. The intermetallic nanocrystals dispersed in Vulcan carbon, Pd3Ti/C and Pd3Zr/C, exhibit improved electrocatalytic activity towards methanol and ethanol oxidation in an alkaline medium (0.5 M KOH), compared to those of commercially available catalysts such as Pd/C, Pt/C, and Pt3Sn/C. In addition, Pd3Ti/C and Pd3Zr/C show significantly higher activity towards the oxidation of formic acid in an acidic medium (0.5 M H2SO4), compared to those of Pd/C and Pt/C. The modification of the d-band center of Pd as a result of the alloying of Pd with the early d-metals Ti and Zr may be responsible for the enhanced catalytic activity.


Synthesis of Pd 3 Ti/CNPs:
Intermetallic Pd 3 TiNPs were synthesized by co-reduction of metal precursors in diglyme.Palladium (II) acetate (28.86 mg, 0.13 mmol) and TiCl 4 .2THF (15 mg, 0.035 mmol) were weighed and transferred to a round bottom flask containing strong reducing agent sodium naphthalide.54 mg of Vulcan carbon is added to the reaction mixture.The reaction mixture wasthen transferred to a reaction vessel and heated the reaction mixture at 200 o Cin an oil bath for 2h under an argon pressure of 0.5 MPa. The product was then transferred to a centrifuge tube under argon atmosphere. The precipitate was separated from diglyme by centrifuging at 6000 rpm for 5 min. The product was washed several times with hexane and acetonitrile to remove the byproducts. The product was driedunder vacuum for 1h. The washing solvents were carefully chosen so that interaction with the oxophilic metal is minimized and hence leaching or dissolution of the metal resulting in non-uniform composition. The as prepared product was annealed at 1000 °C for 15h under vacuum to achiev the desired intermetallic Pd 3 Ti phase. Synthesis of Sodium naphthalide: Sodium naphthalide solution was prepared by dissolving 22.9 mg of metallic sodium and 129.4 mg of naphthalene in dry diglyme. The reaction mixture was stirred for overnight under argon atmosphere.

Synthesis of Pd 3 Zr/C NPs:
Intermetallic Pd 3 Zr NPs were synthesized by co-reduction of metal precursors in diglyme.Palladium (II) acetate (72.9 mg, 0.32 mmol) and ZrCl 4 (26.5 mg, 0.113 mmol) were weighed in a custom made vessel. 30 ml of diglyme was then added to the vessel stirred the mixture for 20 min to dissolve the reactants. 1ml of super-hydridewas then added to the reaction mixture and heated the reaction mixture at 200 o C in an oil bath for 2h under an argon pressure of 0.5 MPa. The product was then transferred to a centrifuge tube under argon atmosphere. The precipitate was separated from diglyme by centrifuging at 6000 rpm for 5 min. The product was washed several times with hexane and acetonitrile to remove the byproducts. The product was dried under vacuum for 1h. The as prepared product was then mixed with Vulcan carbon to form Pd 3 Zr/C. The as prepared product Pd 3 Zr/C was annealed at 1000 °C for 15h under vacuum to obtain intermetallic Pd 3 Zr/C.

Synthesis of bulk Pd 3 Ti and Pd 3 Zr.
Polycrystalline bulk samples of intermetallic TaPt 3 were synthesized with an arc furnace in a pure Ar atmosphere (99.9999 %). Prior to the synthesis, the arc furnace was evacuated to a vacuum level lower than 10 mPa and back-filled with pure Ar. All the starting materials were purchased from Furuya Kinzoku Co. An aliquot of 1 g of Pd powder (99.9 %) was pelletized with a stainless-steel die and melted with the arc furnace into an ingot. Ti/Zr (ingot, 99 %) was used as received. The ingots of Ti/Zr and Pd were weighed such that the molar ratio was Ti/Zr:Pt = 1:3 and melted together in the arc furnace to obtain the desired intermetallic Pd 3 Ti/Pd 3 Zr. The final product was finally annealed in vacuum at 1000 o C for 72 h.

Characterization:
Powder X-ray diffractometry (pXRD): The pXRD measurements were performed using Cu Kα radiation (λ =0.15418 nm) with an increment of 0.02 degrees in a range of diffraction angles from 20 to 100 degrees. An obliquely finished Si crystal (non-reflection Si plate) was used as a sample holder to minimize the background.
Hard X-ray photoemission spectroscopy (HX-PES): HX-PES measurements were performed using X-rays with a photon energy of 5.95 keV, at the undulator beamline BL15XU of SPring-8, Japan. Samples for HX-PES measurements were prepared by mixing the sample solution (in THF) with carbon black (Vulcan XC-72, Cabot Co. Ltd.) to avoid charging effects. 10 µl of the sample was dropped onto carbon substrates (Nilaco Co., Ltd.) and dried under vacuum. The core-level states of the samples were examined at room temperature in UHV using a hemispherical electron energy analyzer (VG SCIENTA R4000). The total energy resolution was set to 220 meV. The binding energy was referenced to the Fermi edge of an Au thin film.

Transmission electron Microscopy:
We used a 200 kV transmission electron microscope (TEM and/or STEM, JEM-2100F, JEOL) equipped with two aberration correctors (CEOS GmbH) for the image-and probe-forming lens systems and an X-ray energy-dispersive spectrometer (JED-2300T, JEOL) for compositional analysis. Both the aberration correctors were optimized to realize the point-to-point resolutions of TEM and scanning transmission electron microscopy (STEM) as 1.3 and 1.1 Å, respectively. A probe convergence angle of 29 mrad and a high-angle annular-dark-field (HAADF) detector with an inner angle greater than 100 mrad were used for HAADF-STEM observation. An ultra-high-vacuum STEM (UHV-STEM; TECNAI G 2 ) was used to perform microscopic observation of the morphology and particle size of the materials. The samples for UHV-STEM were prepared by dropping a THF suspension of the sample powder onto a commercial TEM grid coated with a collodion film. The sample was thoroughly dried in vacuum prior to observation . Electrochemical Experiment: Electrochemical measurements were carried out with a three electrode system on HSV-100 electrochemical apparatus. Ag/AgCl (4 M) and a Pt wire were used as the reference and counter electrodes respectively. Glassy carbon (GC) electrode (PINE, 5 mm diameter) was polished with Gamma Micropolish Alumina (Baikalox, Type 0.05 µm CR) and thoroughly cleaned before its use. 4 mg of the catalysts were dispersed in ultrapure water+isopropanol+5% Nafion (v/v/v =4/1/0.04) with sonication. 45 µl of the suspension wasthen drop cast on the cleaned GC electrode, and dried at 60 °C for 20 min. Prior to the electrochemical measurements, the electrolytes (0.5 M KOH/0.5 M H 2 SO 4 , Fluka) weredegassed by bubbling Ar gas for 30 min. CV measurements were performed at a sweep rate of 20 mVs -1 with 1 M methanol, ethanol and formic acid present in the electrolyte.
ECSA was obtained from the CV of each of the catalysts in 0.5 M KOH/H 2 SO 4 by measuring the columbic charge obtained from the area under the Pd-O reduction (Qo) curve assuming that the charge required for reduction of Pd-O is 405 µC cm -2 using the following equation. 1 ECSA= Q o /0.405 mC cm -2 .
For Pt/C and Pt 3 Sn/C, ECSA was calculated using the following equation