No abstract

Microwave heated reactor which was loaded with carbon fiber supported iron and molybdenum incorporated monometallic catalysts was used to produce COx free hydrogen from ammonia. Impregnation procedure was applied to synthesize the catalysts at different metal loading (4–14 wt % according to ICP-OES) using iron nitrate or ammonium molybdate and reaction tests were carried out under the flow of pure ammonia (GHSV: 36,000 ml/hgcat). Activities of molybdenum incorporated carbon fiber catalyts are found to be lower than that of the iron loaded ones in the conventional reaction system. The effect of metal loading was clealy seen in the activity of molybdenum incorporated catalyst, unlike iron incorporated catalyts, especially under high reaction temperature conditions. Catalysts showed greatly enhanced activities in microwave heated system in comparison to conventionally heated reactor system. Complete conversion of ammonia was achieved at 500 °C by using molybdenum incorporated catalysts and at 450 °C by using iron incorporated catalysts (with GHSV of 36,000 ml/hgcat) in microwave heated reactor system. Transfer of energy directly to the active species and formation of carbide species, Fe2C or Mo2C, during the reaction under microwave application could be considered as the major factors leading to a better ammonia conversion. Structural properties of carbon support materials, such as the surface area, could be effective on the activity of the catalysts via the distribution of active species, however, this would be more pronounced effect in conventional system.

In this study, microwave-assisted ammonia decomposition reaction was investigated over molybdenum incorporated catalysts. Due to the selective, volumetric, and noncontact heating properties of the microwave system, higher conversion values could be achieved at relatively lower reaction temperatures, which is important for on-site COx-free hydrogen production. Multiwall carbon nanotube-supported molybdenum catalysts were prepared following the impregnation procedure with different metal loading (3.5%–12.5% wt%), and inductively coupled plasma, nitrogen physisorption, X-ray diffraction, and transmission electron microscopic techniques were employed to characterize the fresh and used samples. Reaction experiments were performed under the flow of pure ammonia with a gas hourly space velocity of 36,000 mL/g cat. h for both the microwave and conventionally heated reaction systems. It was found that ammonia conversion was obtained even at 400 °C, reaching 40%, and total conversion was observed even at 450 °C, while the activities of these catalysts were negligible at a reaction temperature lower than 550 °C, in the conventional heated system, which included an electrically heated furnace. Crystals of α-Mo 2 C as well as MoO 2 were observed in the structures of the synthesized catalysts and the formation of nitride species was more easily observable under microwave heating, possibly due to the nitridation of molybdenum carbide species during the reaction.