Surfactant-Free Synthesis of Carbon-Supported Palladium Nanoparticles and Size-Dependent Hydrogen Production from Formic Acid–Formate Solution

5 years ago

Surfactant-Free Synthesis of Carbon-Supported Palladium Nanoparticles and Size-Dependent Hydrogen Production from Formic Acid–Formate Solution

Bei Jiang, Wen-Bin Cai, Kun Jiang, Shuo Zhang

Steerable hydrogen generation from the hydrogen storage chemical formic acid via heterogeneous catalysis has attracted considerable interest given the safety and efficiency concerns in handling H₂. Herein, a series of carbon-supported capping-agent-free Pd nanoparticles (NPs) with mean sizes tunable from 2.0 to 5.2 nm are developed due to the demand for more efficient dehydrogenation from a formic acid–formate solution of pH 3.5 at room temperature. The trick for the facile size-controlled synthesis of Pd/C catalysts is the selective addition of Na₂CO₃, NH₃·H₂O, or NaOH to a Pd(II) solution to attain initial pH values of 7–9.5. For comparison, cuboctahedron modeling and electrochemical CO_ads stripping methods are applied to evaluate active surface Pd sites for turnover frequency (TOF) calculation. Both mass activity and specific activity (TOF) of hydrogen production are not only time-dependent but also Pd-size-dependent. An initial H₂ production rate of 246 L·h^–1·g_Pd^–1 is achieved on 2.0 nm Pd/C at 303 K, together with a TOF of 1815 h^–1 on the basis of cuboctahedron modeling of surface-active Pd sites. The initial TOF exhibits a significant rise from 3.5 down to 2.8 nm and then levels off below 2.8 nm and even shows a maxima at ca. 2.2 nm using the electrochemical surface area for calculation. The volcano-shaped dependence of TOF on Pd NP size may be better attributed to the changing ratios of terrace sites to defect sites on Pd NPs.

Publisher URL: http://dx.doi.org/10.1021/acsami.7b08441

DOI: 10.1021/acsami.7b08441