Study On Preparation Of Nickel-Iron Layered Double Hydroxide/Graphene Composite Materials And Their Catalytic Performance

Layered double hydroxide (LDHs) and spinel derivatives have been widely applied in catalysis, energy storage and environmental restoration due to their flexibility in composition, huge specific surface area and excellent chemical stability. The potential pollution and inefficient recovery which are the bottleneck of traditional catalysts can be solved by using LDHs and spinel derivatives as catalysts. But, it is still a great challenge to enhance the catalytic activity and conductivity of LDHs and spinel derivatives to meet the application requirements. Two-dimensional graphene has attracted a great deal of attention due to its high surface area, electrical conductivity, high flexibility and mechanical strength. Therefore, effective structures for the nanocomposites that can maximize the combining catalytic performance of the LDHs and excellent conductivity of carbon nanomaterials. These nanocomposites can be applied as catalyst for Advanced Oxidation Process and Electro catalytic system. Out of the above-mentioned situation, the main contents of the thesis are described as follows:(1) Combing high specific surface area of reduced graphene oxide (rGO) and magnetic Ni2+-Fe3+ layered double hydroxide (Ni-Fe-LDH), a hybrid Ni2+-Fe3+ layered double hydroxide/reduced graphene oxide (Ni-Fe-LDH-rGO) material was prepared and used in the electrochemical (EC) enhanced heterogeneous activation of PDS process for Rhodamine B (RhB) removal. Special attention was given to understanding the influence of the initial dye concentration, the dosages of LDH-rGO catalysts and PDS, current density and initial pH. Results demonstrated that the decolorization efficiency is not significantly affected by the initial pH but increases with the current density, PDS and catalysts dosage in a certain range. We also investigated the decolorization efficiency of RhB with different catalysts in different systems. The results indicated that the combination of EC technology and LDH-rGO/PDS process has a synergetic effect on the decolorization of RhB, which can be completely removed in 25 min under the conditions of 20 mg L-1 RhB,1 g L-1 catalyst,1 g L"1 PDS, and the current density of 19.53 mA cm-2. Both the sulfate radical (SO4--) and the hydroxyl radical (HO·) are the primary reactive oxidants for RhB decolorization. The LDH-rGO catalysts still maintain high activity after three-time cycle use. Moreover, the catalyst can be easily recovered from solution by a magnet due to its good magnetism.(2) NiO-NiFe2O.4-rGO nanohybrids were synthesized by the pyrolyzation of a single-source Ni-Fe-LDH-rGO precursor. The catalytic performance of NiO-NiFe2O4-rGO nanohybrids in heterogeneous activation of PMS was also investigated for RhB removal. Special attention was given to understanding the influence of the dosages of catalysts and PMS, initial pH, reaction temperature, different ion species (NO3-, H2PO4-, HCO3- and Cl-), different systerm, catalytic stability as well as degradation mechanism. The lower activation energy on NiO-NiFe2O4-rGO (29.1 kJ mol-1) confirms the higher chemical performance. Both SO4- and HO·are the primary reactive oxidants for RhB decolorization. The NiO-NiFe2O4-rGO catalysts maintain high activity after three-time cycle use. Moreover, the catalyst can be easily recovered from solution by a magnet due to its good magnetism.(3) The electrocatalytic activity of the as-prepared NiO-NiFe2O4-rGO nanocomposites towards ORR and OER was studied in detail. It presents the highest electrocatalytic activity for ORR in 0.1 M KOH. Two successive two-electron electrochemical reductions of dissolved O2 were observed. In addition, the same hybrid also exhibit highly active for OER in alkaline medium. Results demonstrated that in contrast to the other highly nanostructured catalysts, NiO-NiFe2O4-rGO nanohybrid exhibits a lower onset potential (Eonset= 1.46 V vs. RHE) and affords a smaller overpotential of 0.298 V achieving a current density of 10 mA cm-2 and a smaller Tafel slope of 48.3 mV dec-1 during OER, which also presents superior electroactivity to the commercial IrO2 catalyst. Additionally, the NiO-NiFe2O4-rGO nanohybrid possesses outstanding durability under rigorous OER cycling conditions.