Butylphenyl-functionalized palladium nanoparticles as effective catalysts for the electrooxidation of formic acidw

Direct formic acid fuel cells (DFAFCs) are a promising power source for portable electronic devices due to their high energy density, modest operating conditions, relatively low toxicity and the low crossover rate of formic acid through Nafion membranes. The anodic reaction of DFAFCs is formic acid electrooxidation to CO2 on Ptor Pd-based catalysts. Pt catalysts typically exhibit a high intrinsic activity. However, they are vulnerable to surface poisoning by adsorbed CO (COad), a reaction intermediate. 6,7 In contrast, Pd is free of COad poisoning in the short term, and formic acid is mainly oxidized via the direct pathway. Yet, the electrooxidation of HCOOH on Pd catalysts in general requires a substantially high overpotential (B0.3 V), which significantly impedes the large-scale commercialization of DFAFCs. To improve the catalytic activity, especially the activity per mass of precious metals, one key issue is to rationally control the size and nature of surface structures, such as crystalline planes and surface ligands, of nanoparticle catalysts. Previously, we have synthesized Pd nanoparticles through palladium–carbon covalent linkages by using diazonium derivatives as the functional ligands and controlling the particle core size at 2.4–3.6 nm by varying the ratio of Pd precursors to the diazonium salts. In this communication, we prepared butylphenyl-stabilized palladium (Pd-BP) nanoparticles (dia. 2.24 nm) by a modified procedure. The resulting particles exhibited a mass activity (3.39 A mg Pd) that was the best among the pure Pd catalysts known so far for formic acid electrooxidation. Additionally, the peak potential of formic acid electrooxidation showed a negative shift of ca. 80 mV, in comparison to that of a commercial Pd catalyst. The Pd-BP nanoparticles were synthesized by co-reduction of H2PdCl4 and butylphenyldiazonium (see ESIw for details). Briefly, the diazonium salt was synthesized from a stoichiometric amount of 4-butylaniline (1 mmol) and sodium nitrite in ice-cold 50 wt% fluoroboric acid. In comparison with the previous synthetic method, one major modification in the present study is the employment of a toluene–THF solvent instead of toluene alone. In the mixed solvent, both the diazonium salt and H2PdCl4 can be readily dissolved and co-reduced by NaBH4, without a phase transfer procedure (see ESIw for details). The morphology of the Pd-BP nanoparticles was first characterized by transmission electron microscopy (TEM, JEOL-1230 at 120 kV). Fig. 1a shows a representative TEM image of the as-prepared Pd-BP nanoparticles. A uniform and well-dispersed film of nanoparticles can be observed. The average core size of the Pd nanoparticles is 2.24 nm, with a relative standard deviation of 16% (Fig. 1b). In contrast, commercial Pd black exhibited a much larger particle size (ca. 10 nm), along with a number of particle aggregates (Figs 1c and d), consistent with previous TEM results. We then carried out FTIR spectroscopic measurements (using a Perkin-Elmer Spectrum One FTIR spectrometer) to characterize the surface ligands on the Pd-BP nanoparticles. The IR samples were prepared by dropcasting a concentrated solution of Pd-BP in toluene onto a NaCl disk. After solvent evaporation, a uniform film of nanoparticles was formed. From the FTIR spectrum of the Pd-BP nanoparticles (curve A) depicted in Fig. 2a, one can see that in the C–H stretching vibration region (2700–3100 cm ), the band at 3027 cm 1 can be assigned to the C–H stretch of the aromatic ring, and the two coupled bands to the C–H stretches of CH3 (vas= 2955 cm , and vs = 2869 cm ) and CH2 (vas = 2926 cm , and vs = 2855 cm ). The aromatic ring skeleton vibrations can be identified by the four bands at 1614 (a very weak shoulder peak, indicated by an arrow), 1584, 1497, and 1463 cm . In addition, the weak band at 827 cm 1 is the characteristic peak of a para-substituted aromatic ring (with an out-of-plane C–H deformation vibration). These infrared characteristics demonstrate that the ligands on the Pd-BP surface are indeed the butylphenyl fragments by virtue of the Pd–C interfacial covalent linkage. We note that the relative intensities of the Department of Chemistry and Biochemistry, University of California, 1156 High Street, Santa Cruz, California 95064, USA. E-mail: [email protected] b State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, CHINA w Electronic supplementary information (ESI) available: Detailed synthetic procedures. See DOI: 10.1039/c1cc11235j ChemComm Dynamic Article Links