The development of Ni based catalysts for carbon dioxide reforming of methane

Abstract

Recently, with the rapid development of human society, meeting energy demand and controlling climate change are becoming urgent issues. Carbon dioxide (dry) reforming of methane (DRM) has been considered as a promising technology as it utilizes greenhouse gas to provide high value added liquid fuel and chemicals coupling with a Fischer-Tropsch (F-T) process. In other words, this process can both improve supply of liquid fuel and eliminate global warming issues. Since deactivation of catalysts is the major obstacle for commercialization of this approach, deep understanding of this process and development of catalysts with high activity and stability are necessary to be studied. Firstly, a comprehensive thermodynamic analysis of DRM and its side reactions was performed to get a deep understanding of this process. Low CH4/CO2 ratios improve CH4 conversion and CO selectivity, but have negative influence on CO2 conversion and H2 selectivity. While, CH4 conversion, CO2 conversion (T ≥ 630℃), H2 selectivity CO selectivity and carbon formation are all enhanced at high pressures. Severe carbon formation is found at the temperature range of 546 and 703 ℃, and carbon free regime is suggested under operating conditions of T ≥ 1000 ℃, CH4/ CO2 mole ratio = 1 and pressure = 0.1 MPa. In addition, an index about the relationship of H2/CO mole ratio and operating conditions was established in this study. It is beneficial in both process efficiency and economics in practice as the index can be used to guide the selection of appropriate operating conditions to tune H2/CO mole ratio in syngas to satisfy different requirements of different F-T processes. As drying process has big influence on structure formation of catalysts, the effects of oven drying and vacuum freeze drying on the performance of Ni/Al2O3 in DRM were investigated. The sublimation process in vacuum freeze drying increased the BET surface area but maintain small and uniform pore structure which protect NiO particle from aggregation. Besides, since the solid ice settled nickel nitrate salt in preparation stage, the aggregation of NiO after calcination is also suppressed. Comparing to oven dried catalyst (OD-Cat), anti-deactivation of vacuum freeze dried catalyst (VFD-Cat) was enhanced, which is due to small Ni particle size and high Ni dispersion. The CO2-TPD analysis shows that the amount of basicity on catalyst VFD-Cat is more than it on catalyst OD-Cat, which helps to eliminate coke formation by enhanced the adsorption and activation of CO2. Furthermore, less carbon deposition and less graphic degree coke was detected on spent catalyst VFD-Cat. Overall, vacuum freeze drying technique is suggested to synthesis catalysts for DRM to improve its stability and resistance of coke formation. In this study, the effects of calcination method (i.e. microwave and furnace) on activity and stability of catalyst Ni/Al2O3 were also studied. Microwave calcined catalyst (MC-Cat) showed a better catalytic performance than furnace calcined catalyst (FC-Cat) because of a slow deactivation rate. Because of the advantage of homogeneous volume heating in microwave calcination process, lager total surface area of catalyst and smaller Ni particle with uniform size were observed on catalyst MC-Cat than it on catalyst FC-Cat. Moreover, the amount of basic sites on catalyst MC-Cat was increased under microwave heating, which is contribution to coke formation with less amount and lower graphic degree. Therefore, microwave calcination is suggested to improve the resistance of catalytic deactivation caused by coke deposition. Additionally, the energy saving is more than 90% for microwave calcination in this case as microwave heating is a fast and energy efficiency. To improve the stability of catalyst Ni/γ-Al2O3 in carbon dioxide reforming of methane, K2CO3 was introduced as a promoter to enhance the coke resistance of catalyst. From the results, catalyst promoted with K2CO3 (K-Ni-Al) showed a relative high activity and stability in 100 h of DRM reaction. During long term test, the activity decreased at first 20h then became stable. As K2CO3 has advantages such as high specific heat, good thermal stability, strong basicity and fast heat transfer, it can increase basicity on the surface, control Ni particle size during both reduction and reaction stages, and increase the number of active metallic Ni by weaken the metal-support interactions. Moreover, it was found that that K2CO3 could react with carbon deposition, which could build a micro-cycle to eliminate coke formation.

Date of Award	8 Nov 2017
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Tao Wu (Supervisor), Stephen Adegbite (Supervisor) & Edward Lester (Supervisor)