Plasma-catalytic Ammonia Reforming Of Methane For The Production Of HCN And CH₃CN:Study On Reaction Mechanism

Co-conversion of CH₄ and NH₃to produce HCN in industry must be carried out at a high temperature above 1000 ^oC,and the precious metal Pt catalysts must be used.Our group’s previous studies found that the coupling of CH₄/NH₃plasma and cheap metal catalysts can initiate the ammonia reforming of CH₄to produce HCN and CH₃CN at lower temperature.However,the reaction mechanism is still unknown.Therefore,in this thesis,the reaction mechanism of ammonia reforming of CH₄on Cu based catalysts to produce HCN and H₂has been systematically studied,and the reaction principles of CH₃CN synthesis through coupling Ni based catalysts with CH₄/NH₃plasma has been explored.The main results and conclusions are shown as follows:In ammonia reforming reaction of CH₄for production of HCN and H₂,Cu/S-1 catalyst shows a synergistic effect with plasma,which not only improves CH₄conversion,HCN selectivity and energy efficiency,but also reduces energy consumption.Based on catalysts characterization results（XRD,XPS,H₂-TPR）and in-situ tracking of reaction products by mass spectrometry,metallic Cu phase has been proved to be the real active sites of CH₄ammonia reforming reaction for production of HCN and H₂.CH₄/NH₃plasma has been diagnosed by in-situ optical emission spectroscopy,showing abundant CH_xand NH_xspecies in the plasma,and Cu/S-1 catalyst adsorbs these highly active species.The energy barriers of both E-R and L-H reactions between N,NH,NH₂and CH species on Cu surface have been calculated by DFT（CP2K）,indicating that E-R reactions are barrierless,while the L-H reactions have high energy barriers.Therefore,the highly active species（N,NH,NH₂and CH radicals）tends to react with each other through the barrierless E-R mechanism to generate HCN,which is the main reason why plasma-catalyzed ammonia reforming of CH₄can produce HCN and H₂at lower temperature.By investigating the reaction conditions and the Cu-based catalysts,it is found that both the CH₄conversion and HCN selectivity increase with the increase of the reaction temperature,the performance of the Cu/S-1 catalyst is much better than those of other Cu-based catalysts,and both CH₄conversion and HCN selectivity increased with the increase of Cu loading.Under the optimized conditions（400 ^oC and 30%Cu loading）,the CH₄conversion reached 32.4%with 86.2%HCN selectivity.The experimental results show that there is no internal diffusion limitation in CH₄ammonia reforming reaction using Cu/S-1catalyst,and the increase of Cu loading does not change the exposed crystal plane of Cu particles.The dispersion of Cu/S-1 catalysts with different loadings were measured by N₂O titration experiments,and the TOF values of HCN production were calculated.It was found that as the loading of Cu increased,the dispersion of Cu decreased,but the TOF values of HCN production increased,indicating that the plasma-catalyzed CH₄ammonia reforming to produce HCN and H₂is a structure-sensitive reaction.Ni/HZSM-5 catalyst shows much better performance（14.1%CH₄conversion with93.0%CH₃CN selectivity）than those of other Ni based catalysts in CH₄ammonia reforming to produce CH₃CN.Characterization of Ni-based catalysts show that metallic Ni,Ni O and Ni₃N species exist in the spent sample,Ni-based catalysts with larger internal specific surface area are beneficial for CH₄conversion,and the CH₃CN selectivity is positively correlated with the acid content of the catalysts.It can be seen that Ni/HZSM-5 acts as a bifunctional catalyst:on the one hand,the coupling of metal Ni with plasma plays a role of dehydrogenation,which can dehydrogenate CH₄and NH₃to form CH_xand NH_xspecies;on the other hand,HZSM-5 plays the role of acid catalysis,allowing CN or HCN to further react with CH₃through C-C coupling reaction to generate CH₃CN molecules.