Construction Of M@CNTs And DFT Study On Its Catalytic Mechanism For Methanol Decomposition

Methanol decomposition reaction is an inevitable and important step in its efficient utilization.Catalysts play an important role in this reaction process.ZSM-5 molecular sieve and SAPO-34 molecular sieve are effective catalysts for methanol decomposition,however,there are some carbon deposition problems,such as graphite carbon covering active sites and polycyclic aromatic hydrocarbons blocking pores.Metal modification can reduce the formation of polycyclic aromatic hydrocarbons,but it is still unable to avoid the active sites covered.In addition,Metal catalysts such as platinum-based metals are also highly efficient catalysts for methanol decomposition,but there is a problem of of CO covering the active site.The carbon deposition and CO poisoning of catalyst are the important reasons that restrict the efficient utilization of methanol.Therefore,it is necessary to develop a new high-efficiency catalyst with good resistance to carbon deposition and CO tolerance.In this paper,based on density functional theory,VASP software was used to study methanol decomposition mechanism on Cu-CNT（6,0）,Cu-CNT（12,0）,Cu@CNT（12,0）,Mo@MTCNT and Mo@N-MTCNT.The details are as follows:（1）The models of Cu-CNT（6,0）and Cu-CNT（12,0）catalysts were constructed,and the methanol decomposition mechanism on the Cu-CNT（6,0）and Cu-CNT（12,0）surface were discussed.The results show that the main methanol decomposition reaction on the Cu-CNT（6,0）and Cu-CNT（12,0）surface is CH₃OH→CH₃O→CH₂O→CHO→CO,the difference is that the rate controlled reaction on the Cu-CNT（6,0）surface is formaldehyde dehydrogenation,and the rate controlled reaction on the Cu-CNT（12,0）surface is methoxy formation.The adsorption energy of CO on Cu-CNT（6,0）and Cu-CNT（12,0）surfaces is-0.30 e V and-0.52 e V,respectively,Cu-CNT（6,0）catalyst has higher CO tolerance.However,although Cu-CNT（6,0）and Cu-CNT（12,0）catalysts have good catalytic activity,but they have poor stability against carbon deposition due to the active center is exposed to the reaction medium.（2）The model of Cu@CNT（12,0）catalyst with encapsulation structure was constructed,and the methanol decomposition mechanism on the Cu@CNT（12,0）surface was discussed.The research shows that the main pathway of methanol decomposition on Cu@CNT（12,0）surface is CH₃OH→CH₃O→CH₂O→CHO→CO,and the rate controlled reaction is methanol dehydrogenation to methoxy.The adsorption energy of CO on the surface of Cu@CNT（12,0）is-0.47 e V,which is lower than the adsorption energy of CO on the Cu-CNT（12,0）surface（-0.52 e V）.Therefore,CO on the Cu@CNT（12,0）surface is more easily desorbed and has better CO tolerance.In addition,the encapsulation structure of carbon nanotubes can isolate the active center from the reaction medium,which makes Cu@CNT（12,0）have good stability against carbon deposition.（3）The models of Mo@MTCNT and Mo@N-MTCNT catalysts were constructed,and the methanol decomposition mechanism on the the two catalysts were discussed.The results show that the main reaction pathway of methanol decomposition on Mo@MTCNT and Mo@N-MTCNT catalysts are CH₃OH→CH₃O→CH₂O→CHO→CO.In addition,comparing the activation energy and rate of each elemental reaction of methanol decomposition on two catalysts surface,it was found that the catalytic activity of the Mo@N-MTCNT catalyst is better than that of the Mo@MTCNT catalyst.The adsorption energy of CO on the Mo@MTCNT and Mo@N-MTCNT surface are-0.15 e V and-0.11 e V,respectively,indicating that the Mo@N-MTCNT catalyst has stronger CO tolerance.It can be seen that the synergistic effect of the doped non-metallic N atoms and the metallic Mo atom encapsulated in the carbon nanotube channels improves the catalytic activity and stability of the catalyst.