UC Santa Barbara

With the increasing awareness of human society on climate change caused by greenhouse gas emission, and the continuous discovery and exploitation of abundant low-cost natural gas resources, methane pyrolysis (CH4 →C + 2 H2) is considered as a reaction with a great potential in providing an economical way for CO2-free hydrogen production and natural gas utilization. Some of the major barriers for the commercialization of methane pyrolysis are: (1) conventional solid catalysts used to accelerate methane pyrolysis are deactivated by surface carbon deposition and no efficient non-oxidative method for activity recovery has been reported; (2) the prevention of reactor clogging and removal of carbon from the reactor are challenging; (3) the heat transfer into the reactor to maintain the high temperature and supply heat required for this endo-thermic, equilibrium-limited reaction is non-trivial. One strategy to solve these problems is to introduce non-solid phases (liquid or gas) into the reaction for catalysis, heat-transfer and/or carbon removal. In this thesis research, two new types of catalysts are investigated and found to be active for methane pyrolysis. Tellurium, an element with high electron affinity, is an active methane pyrolysis catalyst in both its liquid and vapor form. Zinc chloride, a Lewis acidic salt, is also an active vapor phase catalyst for methane pyrolysis. The activities, active phase and long-term stability of these catalysts and their performance under different hydrogen partial pressures are investigated. The morphology of the carbon produced and the contamination level of these two catalysts are also investigated. The pathway of C-C bond formation in methane pyrolysis is studied using solid Ni and Cu as two model cases. Using methane-deuterium exchange as a probe reaction and with density function theory calculations, two different C-C bond formation pathways are discussed. Similar methods are adopted to study methane pyrolysis with four different molten liquids (Ni-Bi alloy, Sn, KCl and MnCl2-KCl eutectic) as well, and the similarity and difference between these systems are identified. A three-phase reactor was designed to avoid catalyst coking and allow efficient carbon removal and heat transfer is presented in this work. Tungsten carbide as a solid packed bed is shown to be active in molten KCl and molten Sn. The removal of carbon from coked WC surface and the recovery of WC activity with molten KCl treatment is demonstrated. The performance of WC in molten Sn under high H2 partial pressures is also studied, and a high single pass CH4 conversion overcoming pseudo-equilibrium limitation is achieved.