Study On The Mechanism And Application Of Low-temperature Graphitization Of Biomass By Iron Salts Catalytic Pyrolysis

The expansion of pore structure and the improvement of graphitization degree are the key to improve the quality of biochar and high-quality utilization of biomass.So,the idea of preparing porous graphitic carbon from low-temperature graphitization of biomass by iron salts catalytic pyrolysis was proposed in this study.The main research contents and conclusions are as follows:Firstly,the effect of iron salts on the low-temperature graphitization process of biomass was studied.The results showed that iron salts promoted the decomposition of biomass,reduced the precipitation temperature of volatile matter,and increased the yield of biochar and H₂.Ranked according to the ability to promote the precipitation of volatile:SO₄^2->Cl^->NO₃^-;Fe³⁺is better than Fe²⁺.The addition of Fe（NO₃）₃ improved the yield of pyrolytic carbon and pyrolysis gas,promoted the graphitization degree and mesoporous formation of biochar.Fe（NO₃）₃ is the best choice of graphitization catalyst.Then,the kinetic mechanism and product characteristics of low-temperature graphitization of biomass were studied.Compared with direct pyrolysis,graphitization required more energy input.Compared with lignin with benzene ring structure,cellulose and hemicellulose with sugar ring as the basic unit were more likely to be graphitized.The cellulose based graphite showed a big surface area of 185 m²/g and highly graphitization degree of 0.276.Then,the effects of temperature and the amount of Fe（NO₃）₃ added on the product characteristics were studied.It was found that Fe（NO₃）₃ played a variety of roles in the low-temperature graphitization process:as a reactant,iron salts had a complex carbothermal reduction reaction with biomass;It is also a pore enlarging agent of graphitic carbon and a catalyst for graphitization:At 400-600℃,iron salts promoted dehydration and etched the carbon structure,promoting the development of micropores;At 700～800℃,in-situ formed Fe₃C promoted the development of mesopores and catalyzed the formation of graphite microcrystals.Then the mechanism of the introduction of potassium and calcium salts on the low-temperature graphitization process and product characteristics was studied.It was found that potassium salt reduced the precipitation temperature of volatile matter,accelerated the reduction process of iron oxides,promoted the formation of biochar,and enhanced the graphitization degree of biochar;Potassium-riched biomass is more suitable as the precursor of low-temperature graphitization.Then the catalytic activities of three single metal（Fe,Co,Ni）and two bimetallic（Fe Co,Fe Ni）catalysts for low-temperature graphitization were compared.It was found that the bimetallic catalyst was superior to the single metal catalyst,among which the Fe-Co bimetallic catalyst showed the best catalytic ability,the H₂ yield was 7.51 mmol/g,and the graphitization degree（g value）was 0.43.The in-situ formed Fe-Co alloy enhanced the catalytic activity of cracking C-H,and improved carbon melting ability.Finally,the bifunctional catalytic performance of biomass based porous graphitic carbon for oxygen reduction and oxygen evolution reactions was studied,and finally a zinc-air fuel cell was assembled.The samples all showed excellent electrochemical performance,and the half-wave potential of BFe Cu N sample was 30 m V higher than that of commercial20 wt.%Pt/C.The zinc air battery showed high power density（173 m W/cm²）and could conduct 420 discharge/charge cycles stably.The excellent electrochemical performance was due to the fractal porous three-dimensional structure,core-shell structure of encapsulated metal nanoparticles（Fe,Fe Co,Fe Cu,etc.）and protective graphite shells,and the nitrogen doping of carbon matrix.