Electrochemical behavior of ammonia on Ni_(98)Pd_2 nano-structured catalyst

摘要

Small amounts of Pd served as a reducing agent to produce sub-100 nm polygonally-shaped Ni_(98)Pd_2 materials in ethylene glycol. As-synthesized particles were crystallized into fcc Ni with a fraction of β-Ni(OH)_2, and exhibited very low to no activity towards ammonia electrooxidation. Their catalytic activity has been significantly improved by building up a layer of Ni(OH)_2 by cyclic voltammetry between -0.95 and 1.35 V vs. HgO/Hg in NaNO_3 at pH 9. XPS analysis before and after the electrochemical treatment confirmed the transformation of Ni~0 to higher state of oxidation. Ammonia electrooxidation on Ni(OH)_2/NiPd occurred at around 1.28 V vs. HgO/Hg and was highly pH-dependent. At concentrations less than 100 mM, the direct electron transfer took place, whereas at higher ammonia concentrations it was the indirect electron transfer mechanism. A 9-h galvanostatic electrolysis at 20 mA cm~(-2) showed that 64% of the initial ammonia was degraded at 38% average current efficiency.