Microwave-assisted Preparation Of Transition Metal Carbides And Their Catalytic Properties

Transition metal carbides as catalytic materials have received considerable attention for their exceptionally high activity,which are similar to those of noble metal catalysts in hydrogen-involving reactions,hydrazine decomposition and isomerization.The preparation of interstitial carbides with nanostructured particles and/or high surface area,however,is very difficult by the conventional methods which have been inherited from the metallurgical industry at high temperature and are energy-intensive,extremely tedious and result in large grains of low surface area that need to be passivated before exposured to the air.Here,evenly distributed tungsten and molybdenum carbides nanoparticles supported on carbon nanotubes (CNTs)/active carbon(AC) have been prepared via microwave-assisted metallo-organic chemical vapor deposition(MOCVD)/thermolysis.The catalytic hydrazine decomposition over the carbides supported on CNTs was performed in a U-shaped quartz microreator.The Pt@WC_x/CNTs electrocatalysts were then prepared by in situ reduction of the metal salt on the uniform WC_x/CNTs hybrid support and used as electrocatalyst for oxygen reduction reaction(ORR).The main results are as follows:The uniform distributed tungsten carbides nanoparticles supportd on CNTs were perared via microwave-assisted MOCVD with tungsten,hexacarbonyl as precursor.The great advantages of microwave heating appear to be the very short time scales involved in the preparation and the selectivity in energy transfer from the microwave field.The results form XRD parttens revealed that the tungsten and molybdenum carbides were successfully synthesized.The TEM images of the tungsten and molybdenum carbides showed that uniform nanostructured tungsten and molybdenum carbides with the particle sizes of 2-5 nm were evenly dispersed on the outer surface of CNTs.The tungsten carbides coated CNTs can be obtained with increasing the tungsten loading to 50%.In contrast to other methods such as direct-thermal decomposition,carbothermal hydrogen reduction and temperature programmed reaction,microwave-assisted MOCVD is a rapid,energy efficient and environmental friendly processing.The as prepared WC_x/CNTs nanocomposites were active catalysts for hydrazine decomposition,and exhibited high selectivity to hydrogen,indicating that nanostructured tungsten carbides on CNTs were an inexpensive and promising alternative to the noble metal catalysts. Pt nanoparticles were then introduced on the WCx/CNTs hybrid support via ethylene glycol reduction method.The results from XRD partterus and TEM images showed that the nanostructured Pt with particle sizes of 2-5nm were evenly deposited on the WC_x/CNTs.The obtained Pt@WC_x/CNTs showed much higher electrocatalytic activity for ORR in sulfuric acid solution than Pt/CNTs catalyst,while the WC_x/CNTs nanocomposite was also active in acid solution for ORR.Meantime.the unique immunity to methanol of the novel electrocatalysts was also observed.Moreover,we have combined the microwave irradiation and thermolvsis processing to prepare molybdenum carbide supported on active carbon with a mixed salt complex containing ammonium heptamolybdate and hexamethylenetetramine(HMT) as precursor.The results revealed that the formation of Mo-HMT complex precursor with abundant HMT in an aqueous solution was a necessary process for the synthesis of molybdenum carbide.TEM images indicated that the uniform molybdenum carbide nanoparticles were obtained in 10 min and evenly distributed on the active carbon.The resultant molybdenum carbide and nitride supported on active carbon exhibited high catalytic activity and selectivity for the hydrogenation of naphthalene.The main product was tetrahydronaphthalene.