DFT Study On The Mechanism Of Methanol Decomposition Catalyzed By Metal Modified Carbon Nanotubes

Methanol has become a new generation of clean energy due to its wide source,large reserves and high productivity.Methanol can directly produce CO and H₂ through decomposition process,and is widely used in chemical and pharmaceutical industries.However,the decomposition mechanism of methanol on different metal surface is different,and it is easy to cause carbon deposition and CO accumulation in the reaction process,which leads to catalyst poisoning and hinders the further decomposition of methanol.Therefore,it is urgent to find a more efficient,stable and anti-CO poisoning catalyst.The decomposition mechanism of methanol on metal-modified carbon nanotubes?CNTs?was systematically studied by density functional theory using VASP software and periodic model.The aim is to find new and efficient catalysts for methanol decomposition and provide a good theoretical basis for the preparation of catalysts.Because the breakage of O-H,C-O and C-H bonds in methanol is selective on different catalyst surfaces,the following studies have been carried out:?1?The effects of modified metals?Ru and Mo?on the catalytic performance were discussed.Ru-CNTs?6,0?and Mo-CNTs?6,0?models were established respectively.The results are as follows:On Ru-CNTs?6,0?surface,the activation energies show that the fracture of C-O and C-H bond is a competitive relationship.Rate constant analysis determines that the main reaction path is CH₃OH?CH₂OH?CH₂O?CHO?CO,which is that the C-H bond is most likely to be broken in methanol decomposition,and CO is formed by continuous dehydrogenation.On Mo-CNTs?6,0?surface,the optimum reaction path,CH₃OH?CH₂OH?CHOH?COH?CO,is determined by the activation energy and reaction rate constant of each reaction.In this process,the decisive step is CH₃OH?CH₂OH+H,because its activation energy is 1.39 eV and the rate constant k is 1.84x10^-1010 s^-1.The final product of methanol decomposition on the surface of the catalyst is CO,and CO molecule is easy to desorb,so Mo-CNTs?6,0?is an effective catalyst for methanol decomposition.?2?In order to study the effect of the diameter of carbon nanotubes on methanol decomposition mechanism,Mo-CNTs?4,0?model was constructed on the basis of Mo-CNTs?6,0?.The results are as follows:On Mo-CNTs?4,0?surface,the optimum reaction path is CH₃OH?CH₃O?CH₂O?CHO?CO.The reaction path is different from that of Mo-CNTs?6,0?.The O-H bond is most easily broken during the initial decomposition of methanol,but the final product of methanol decomposition on the catalyst surface is also CO,and the CO molecule is more easily desorbed.In addition,the activation energy of each elementary reaction is slightly lower than that of Mo-CNTs?6,0?.Therefore,Mo-CNTs?4,0?has a better catalytic effect than Mo-CNTs?6,0?.?3?Carbon nanotubes?CNTs?encapsulated metals have been used in many reactions,but the mechanism of methanol decomposition has not been explored yet.Therefore,the decomposition of CH₃OH on CNTs surface of encapsulated metals has been preliminarily discussed and a Mo@CNTs?6,0?model has been constructed.Preliminary calculation of methanol decomposition on Mo@CNTs?6,0?surface shows that the activation energies of the elementary reactions are very high,and methanol molecules hardly decompose on the surface of Mo@CNTs?6,0?.The possible reason is that the selected diameter of the tube is small or the metal is not suitable.