| MXenes are a new class of layered two-dimensional(2D)graphene-like transition metal carbides and/or nitrides,and their formula is Mn+iXnTx,where M represents an early transition metal,X represents C and/or N,and T represents surface termination such as-F,-OH,-O functional groups.They exhibit large specific surface area,excellent electrical conductivity and good mechanical properties.These features ensure MXenes a wide application in energy storage,absorption,sensor and et al.In addition,the MXenes have graphene-like 2D lamellar structure and the transition group metal elements with high catalytic activity locate on the surfaces,which can provide plentiful active sites,leading to strong applied value of MXenes in catalytic field.At present,the practical applications in catalytic filed are restricted by their thermal stability,regulation of surface functional groups,preparation of derivatives and catalytic properties of MXenes,so it is urgent to make studies in these aspects.In this thesis,Ti2CTx,one of typical MXenes was used as the research object.The preparation and structure of Ti2CTx and its derivatives TiO2@C composites were investigated,and then their catalytic influences on thermal decomposition of ammonium perchlorate(AP),dehydrogenation of MgH2 and degradation of methylene blue(MB).The main work and conclusions are as follows:(1)Ti2CTx with clear layered structure was prepared by etching Ti2AlC 2.5 h in 40%HF solution and then freeze drying after removal of O2.When Ti2CT,was vacuum heated and its structure maintained up to the temperature of 1200?,it exhibited well thermal stability.Meanwhile,the-OH functional groups were removed when the temperature rose to 227?;-F functional groups were all removed when the temperature rose to 1200?,at the same time,the annealing twinning formed.(2)The effect of Ti2CTx nanosheets on the decomposition of ammonium perchlorate(AP)was studied by thermal analysis.It was demonstrated that the peak temperature of high temperature decomposition(HTD)was decreased by 90? while the heat release was increased by1897.3 J/g when the addition of Ti2CTx was 30 wt%.The catalytic mechanism could be explained as follows:i)the adsorption and little consumption of gas production by Ti2CTx;ii)the formation of super-oxygen ion O2-through electrons exited by semiconductor Ti2CTx(x?-OH and=O)and TiO2 transferring to HCIO4;iv)the thermal decomposition of AP accelerated by the presence of surface unsaturated sites and good thermal conductivity.(3)The influence of Ti2CTx nanosheets on the dehydrogenation of MgH2 was studied by DSC-TG.It was found that starting temperature for the dehydrogenation,enthalpy change(?H)and activation energy(Ea)were reduced by 37?,11%and 26.7%,respectively,through the addition of 5 wt%Ti2CTx.By comparing the changes of microstructure,surface chemical composition and chemical states of Ti2CTx before and after catalysis,we could conclude that the boosting effects result from the interaction between the Ti with multivalence in the Ti2CTx and MgH2,the adsorption of H2 through Ti2CTx,and pores created by nanosheets formed during heating.(4)TiO2@C derivative with the 2D lamellar structure of Ti2CTx was achieved by high-energy ball milling Ti2CTx nanosheets in this work.It was found that TiO2 nanoparticles with vacancies and Ti3+ were uniformly and intimately dispersed on the disordered carbon sheets by O-Ti-C bonds.These nanosheets demonstrated a 1.25 time higher photocatalytic performance than that of titanium dioxide P25 in degradation of methylene blue(MB)dye in aqueous solution under UV-irradiation.The catalytic mechanism could be explained as follows:the electrons excited by light on TiO2 could be quickly transferred by C,vacancies,and Ti3+,and thus effectively inhibiting recombinations of electrons and holes,resulting in enhancement of photocatalytic degradation of MB.In addition,the presence of O-Ti-C bonds makes the structure of TiO2@C strong enough to maintain photocatalytic stability. |
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