NixPd1−x (x = 0.98, 0.93, and 0.58) nanostructured catalysts for ammonia electrooxidation in alkaline media

The effect of Pd atomic content in NixPd1−x (x = 0.98, 0.93 and 0.58) nanometer-sized catalysts synthesized with modified polyol method is investigated for the electrooxidation of ammonia in alkaline media. Physicochemical characterization by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and X-ray diffraction (XRD) are carried out to identify the sizes, shapes, chemical and structural features of the catalysts. Both synthesized Ni98Pd2 and Ni93Pd7 are composed of large 50-100 nm Ni particles coated with finer Pd ones in the 5-20 nm range. The Ni58Pd42 catalyst has smaller and narrower size distribution in the 20 nm size range, indicating that Pd stabilizes the particle size of Ni. According to XRD, Ni and Pd form two separate face-centred cubic phases with traces of β-Ni(OH)2. Before ammonia electrooxidation reaction, the catalysts were subjected to an electrochemical treatment that consisted of 200 potential cycles in NaNO3 at pH 10.5 to build a layer of Ni(OH)2. Ammonia electrooxidation proceeds on NiOOH phase and follows the direct pathway at low concentrations and indirect pathway at higher concentrations where NiOOH is reducing and NH3 is oxidizing. It was found that the Ni-rich compositions have higher activity towards the degradation of ammonia.