The extraction of nickel from a spent primary steam reformer catalyst from an ammonia plant was carried out by chelation using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. Ni recovery was optimized by varying the particle size distribution of catalyst (pretreatment of spent catalyst), stirring speed, temperature (particularly in an autoclave, where temperatures ranging from 100 to 200 °C were used), EDTA concentration, and solid-to-liquid ratio. Approximately 95% Ni recovery was achieved in the Ni extraction carried out under hydrothermal conditions in an autoclave, at temperatures of 150 °C and higher, over a 4-h period. The resulting Ni−EDTA complex was then “dechelated” using a mineral acid (H2SO4 and HNO3), resulting in the formation of a nickel nitrate or sulfate solution and the precipitation of EDTA (about 97% of the initial weight of EDTA was recovered). However, the chelation performance of Ni was shown to decrease with every successive recovery of EDTA (in the case of dechelation using H2SO4). EDX analysis of fresh and recovered EDTA established that fresh EDTA is a disodium salt whereas recovered EDTA is protonated. EDX analysis also indicated sulfur in the recovered EDTA when sulfuric acid was used for dechelation. TGA data showed a much larger weight loss in recovered EDTA in comparison to the fresh sample, probably because of a combination of two factors: the presence of sulfur species and the protonation of EDTA after recovery. It is likely that differences in recovered EDTA as evidenced by EDX analysis and TGA are responsible for the lowering the Ni chelation efficiency. This possibility is being investigated further as part of ongoing research.
Recently, it has been demonstrated that doping of graphene by elements such as N, S, or F creates active sites for the oxygen reduction reaction (ORR). This results from bond polarization caused by the difference in electronegativity between heteroatom dopants and carbon, and/or the presence of defects within the graphene lattice. In this work, fluorine, nitrogen, and sulfur tridoped reduced graphene oxide (F,N,S‐rGO) is designed to combine these catalytically active sites. F,N,S‐rGO can be inexpensively synthesized by a facile and scalable route involving pyrolysis at 600 °C of sulfur‐doped rGO in the presence of Nafion and dimethyl formamide (DMF). The pyrolysis of Nafion and DMF provides F• and N• radicals which serve as doping agents. Rotating disk electrode investigations reveal the ORR catalytic activities of F,N,S‐rGO in both acidic and alkaline media, which are consistent with the real performances of the respective polymer electrolyte fuel cells (PEFCs). Maximum power densities of 14 and 46 mW cm−2 are obtained for the acidic and alkaline PEFCs, respectively, using F,N,S‐rGO as ORR catalysts. To the best of knowledge, this is the first report on the synthesis of F,N,S tridoped rGO and on its ORR activity in both acidic and alkaline PEFCs.
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