Efficient Transition Metal Catalysts For Electrocatalytic Ammonia And Organic Synthesis

Electrocatalysis is a powerful technique to obtain clean energy and high valueadded chemicals by controlling the applied bias,possessing the advantages of low energy consumption,environmentally friendly and mild operating conditions.The core of electrocatalytic conversion process is to design and fabricate electrocatalysts with high activity,high selectivity,and durability for specific reactions.Transition metal-based nanomaterials have the advantages of special electronic structure and relatively rich earth reserves.By means of element doping,vacancy design,interface regulation,morphology and size regulation at the atomic or molecular level,they can be applied to a variety of catalytic reactions.Ammonia is an essential nutrient for plant growth and maintains the development of all mankind.At present,industrial synthetic ammonia mainly uses high-purity nitrogen and hydrogen as raw materials and is synthesized under high temperature and high pressure by the Haber-Bosch method,arousing serious environmental problems.In addition,the conversion of small organic molecules derived from biomass and petroleum into high value-added products also depends heavily on chemical production methods with high pollution,energy consumption and risk.It is of great practical significance to develop the conversion of nitrogen into ammonia and its derivatives and realize the high value-added conversion of organic molecules under ambient conditions.In this thesis,aiming at the specific catalytic needs and characteristics of several reaction types of electrocatalytic nitrogen reduction to ammonia and the reduction or oxidation of organic platform molecules to value-added products,this thesis work designed and fabricated efficient transition metal catalysts with excellent catalytic performance.The mechanisms of high catalytic activity were revealed by combining a variety of characterization methods and theoretical calculations.The main research contents and achievements of this thesis are as follows:1.A TiOxNy phase catalyst with oxygen atom doping and the niobium oxide nanochannel film in-situ grown on niobium metal sheet(Nb2O5-NCF)are obtained by plasma etching and anodic oxidation method,respectively.The electrochemical test results show that the TiOxNy and Nb2O5-NCF catalysts have the maximum of 3.32×10-10 mol s-1 cm-2 at a potential of-0.6 V(vs.RHE)and 2.52 × 10-10 mol s-1 cm2 at-0.4 V(vs.RHE),and the Faradaic efficiency reaches 9.10%and 9.81%,respectively.In the test results using nitrogen source labeled with 15N isotope,it was found that the ammonia in the product was derived from both nitrogen gas and nitrogen atoms in the catalyst lattice,which confirmed that the mechanism of the reaction is consistent with the Mars-van Krevelen mechanism,indicating that the catalyst and the nitrogen remove nitrogen atoms to form ammonia molecules in cycle and weaken the activation energy of the N≡N bond.The ammonia synthesized via Nb2O5-NCF is from nitrogen,while it was found that electrochromism occurred at the Nb2O5-NCF electrode,and the real-time ammonia production rate decreased by about half after discoloration.With the help of various characterization methods,it was found that niobium oxide changed from pseudo hexagonal phase to cubic phase,and the oxygen vacancy concentration decreased,resulting in the decline of NRR performance.After a short time of low-temperature heat treatment or anodic oxidation,the modified electrode can restore to the initial phase and restore high activity,showing good cyclic reproducibility.2.A carbon supported Pt modified MoO3 catalyst(Pt-MoO3/C)were synthesized by a simple liquid phase synthesis method.For the electrocatalytic selective hydrogenation of cinnamaldehyde,Pt-MoO3/C can achieve about 99%conversion,78%selectivity toward cinnamyl alcohol,and 50%Faradaic efficiency at-0.4 V(vs.RHE).The theoretical calculations results revealed that on the platinum-modified molybdenum oxide surface,the electrons accumulated on the(002)surface of MoO3,which promoted the vertical adsorption of cinnamaldehyde molecules on the catalyst surface,and the C=O bond at the tail was preferentially hydrogenated,so as to improve the selectivity of cinnamyl alcohol.3.Platinum-based metal has excellent oxygen reduction activity,while nonnoble metal sulfide has high electrocatalytic oxidation activity toward aldehyde.Combined with the characteristics of these two catalysts,it can realize the bifunctional catalytic activity of oxygen reduction reaction(ORR)and biomass molecular oxidation reaction,and this concept can be further applied in biomassbased fuel cells.A kind of carbon-supported platinum and nickel sulfide nanoparticles(PtNiSx/CB)were fabricated by a combined approach of impregnation,calcination and vulcanization.The results indicated that PtNiSx/CB possesses obvious interface between platinum and nickel sulfide and exhibits bifunctional catalytic activity toward the oxygen reduction reaction(ORR)and 5hydroxymethylfurfural(HMF)oxidation to 2,5-furan dicarboxylic acid(FDCA).Based on this bifunctional PtNiSx/CB electrode,a fuel cell integrating ORR with HMF oxidation to FDCA shows the discharge efficiency of 2.12 mW cm-2 at 60℃and a current density of 6.8 mA cm-2.This constructed fuel cell can not only generate electric energy,but also produce high value-added chemicals.4.In order to solve the hydrogen and oxygen mixing problem during electrocatalytic hydrogen production from alkaline water,a system of innovatively utilization of electrocatalytic organic oxidation reaction instead of oxygen evolution reaction(OER)at the anode was proposed.For this,nickel doped cobalt hydroxide nanosheets(Co(OH)2/NF)were synthesized in-situ on commercial foam nickel substrate by a facile hydrothermal method.The results demonstrated that Co(OH)2/NF exhibits superior electrocatalytic activity toward benzyl alcohol oxidation to benzoic acid with conversion efficiency and selectivity of～99%at 1.7 V(vs.RHE)with a current density of 100 mA/cm2.The obtained benzoic acid was easily crystallined and extracted after acidizing the electrolyte.By means of coupling this organic oxidation reaction and hydrogen evolution reaction(HER),high valueadded chemicals and hydrogen can be concurrently achieved,but without oxygen generation,thus solving the issue of H2 and O2 mixing easily during electrocatalytic water splitting.