Preparation Of Transition Metal Phosphides/Biocarbon Catalysts And Their Environmental Catalytic Applications

With the rapid development of modern industry and technology,environmental problems such as organics and heavy metal ion pollution have become an important bottleneck for sustainable development.Research on the remediation of pollutants in water is the focus of ecological civilization construction and environmental protection.Catalytic technologies based on redox reaction are favored by researchers because of their advantages such as fast reaction rate,strong environmental adaptability and high removal efficiency in the treatment of environmental pollutants.The keys of using environmental catalytic technologies to treat pollutants are to develop catalysts with high activity and stability and construct AOP_S and ARPs systems according to the physical and chemical properties of different pollutants.Based on the above background,a series of transition metal phosphides/biocarbon-based materials were prepared by pyrolysis of yeast biomass preloaded transition metal elements（Fe and Ni）,and AOPs and ARPs systems were constructed to treat bisphenol organic compounds,sulfonamides antibiotic,Cr（Ⅵ）and 4-nitrophenol.The element transformation characteristic in biomass pyrolysis process,morphologies and structures of catalysts and reaction mechanisms in different catalytic systems were studied in depth.The main contents and results of this work are as follows:（1）Using Candida utilis as the sole precursor and employing NaCl and KCl as the activators,graphene-like nitrogen-doped biocarbon nanosheets（NCS-x）were prepared by pyrolysis at 700℃for 2 h.Using peroxymonosulfate（PMS）as oxidant,the NCS-x/PMS catalytic oxidation system was established to degrade bisphenol organic pollutants.The results showed that the activation of molten salt could improve the retention rate of nitrogen and promote the graphitic phase transformation of nitrogen.The main reactive oxygen species in NCS-6/PMS reaction system were ¹O₂,SO₄^-·and·OH.Due to the synergistic effect of free radical pathway and non-free radical pathway,a series of intermediates of BPA degradation were generated through chemical bond chain breaking,dehydration reaction,intermolecular coupling and hydroxyl substitution on the benzene ring.Chlorella vulgaris was used as the model organism to evaluate the ecotoxicity of bisphenol A,bisphenol F,bisphenol AF and bisphenol S before and after reaction in NCS-6/PMS system.The results showed that the ecotoxicity of the solution was significantly reduced after degradation.（2）Using Baker’s yeast preloaded FeOOH as precursor,iron phosphide/biocarbon microsphere composites（Fe_xP/BC-x）were successfully fabricated by pyrolysis at 900℃for 2 h.Using dissolved oxygen as oxidant,the Fe_xP/BC-x/O₂ catalytic oxidation system was constructed to degrade sulfonamides antibiotic pollutants.Based on thermogravimetry-mass spectrum analysis（TG-MS）analysis,it was demonstrated that reducing gases,such as H₂,CO and PH₃,produced in the pyrolysis process of biomass were the key to synthesize transition metal phosphides using biomass as phosphorus source.Fe₂P and Fe₃P can be obtained by adjusting the loading amount of Fe.After pyrolysis at high temperature,baker’s yeast maintained the morphology of hollow carbon sphere.The properties of Fe_xP are similar to nano zero-valent iron.Fe_xP can directly activate O₂ in solution to produce H₂O₂ through electron transfer,and then catalyze H₂O₂ to produce·OH、·O₂^-and ¹O₂.Due to the synergistic action of adsorption and degradation,sulfamethoxazole can be quickly and efficiently removed in Fe_xP/BC-x/O₂ reaction system.The removal efficiency of sulfamethoxazole by Fe_xP/BC-5 still reached 97%after five cycles.The liquid chromatography-mass spectrometry（LC-MS）analysis showed that the reaction of sulfamethoxazole degradation included chemical bond breaking,amino oxidation and hydroxylation of C2 and C6 on the benzene ring.Fe_xP/BC-x/O₂oxidation system can also be used to remove sulfanilamide and sulfathiazole,indicating it can be extended to the degradation treatment of other organic pollutants.（3）Using Candida utilis preloaded Ni（OH）₂ as precursor and employing NaCl and KCl as the activators,nickel phosphide/biocarbon composites（Ni₂P/BC-x）were successfully fabricated.Using solid organic acid as reducing agent,the Ni₂P/BC-x/organic acid catalytic reduction system was constructed to reduce Cr（Ⅵ）to Cr（Ⅲ）.The results showed that the reductive activities of different organic acids were as follows:oxalic acid>tartaric acid>citric acid>ethylene diamine tetraacetic acid.The highly efficient and rapid reduction of Cr（Ⅵ）in Ni₂P/BC-x/oxalic acid reduction system can be ascribed to the reasons of（1）Synergistic catalysis of Ni₂P and heteroatom doped biocarbon promoted Cr（Ⅵ）reduction;（2）The affinity and field effects of oxalic acid with Cr（Ⅵ）effectively reduced the reaction barrier of Cr（Ⅵ）to Cr（Ⅲ）conversion;（3）Ni₂P and biochar themselves can act as electron donors to reduce Cr（Ⅵ）.In addition,Ni₂P/BC-x/oxalic acid reduction system had high reduction efficiencies in a wide p H range and practical water.The Cr（Ⅵ）reduction efficiency of Ni₂P/N-BC-5 can still reach 92.7%after five cycles.（4）ATP-Fe and ATP-Ni precursors were prepared by using adenosine disodium triphosphate（ATP-2Na）chelated with iron and nickel ions.Transition metal phosphide/carbon nanosheets（TMPs/CNS）were successfully developed by pyrolysis of ATP-Fe and ATP-Ni precursors.Using sodium borohydride（NaBH₄）as reducing agent,The TMPs/CNS-NaBH₄ catalytic reduction system was constructed to reduce 4-nitrophenol（4-NP）to 4-aminophenol（4-AP）.The crystal forms of transition metal phosphides obtained from ATP-Fe and ATP-Ni precursors were FeP and Ni₂P,respectively.4-NP can be reduced by FeP/CNS in 420 s and Ni₂P/CNS in 180 s.The reduction of 4-NP in TMP/CNS-NaBH₄ catalytic reduction system is a typical Langmuir-Hinshelwood（L-H）heterogeneous reaction model.The metal-support interaction（MSI）between FeP and Ni₂P and carbon nanosheets facilitated electron transfer at the TMPs/CNS interface,which made the catalytic activity of TMPs/CNS comparable to some precious metal nanocatalysts.TMPs/CNS had good cycling stability.The 4-NP reduction efficiencies of FeP/CNS and Ni₂P/CNS were 86.0%and 95.8%after ten successive experiments,respectively.