טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentChristine Warwar Damouny
SubjectDevelopment of Ni-Based Catalysts through a Polymer Assisted
Non-Hydrolytic Sol-Gel Route for the Dry
Reforming of Methane
DepartmentDepartment of Energy
Supervisor Dr. Gazit Oz


Abstract

Methane dry reforming reaction (MDR) is with high environmental and industrial aspects, since it utilizes the two of the major greenhouse gases, methane and carbon dioxide, to produce valuable syngas of CO and H2. This process however is not commercially viable due to the lack of finding a suitable economic catalyst, with high resistance to coke formation and metal sintering.

Highly dispersed nickel catalysts were identified as extremely active for MDR exemplifying a strong resistance to coking. The extreme high driving force for catalyst sintering leads to fast activity loss, due to loss of surface area which also promotes coking. Supporting Ni sites on a tetragonal-phase ZrO2 (t-ZrO2) has shown stable activity in MDR due to slower CH4 decomposition, and the lack of a significant concentration of strong Lewis acid sites. However, zirconia suffers from low surface areas due to sintering and structure collapse at high temperatures, and for t-ZrO2 this structure collapse is further promoted by the phase transformation to the thermodynamically stable less-active monoclinic phase.

The main research goal is to develop catalysts for MDR with high activity and stability, where Ni particles are encapsulated by a porous t-zirconia support. For that we develop a new methodology, by combining chitosan (CS), a natural polymer, for dispersing the Ni precursor followed by its encapsulation using a non-hydrolytic sol-gel route (NSG). In order to keep the CS polymeric framework, open and to maintain accessibility for zirconium NSG solution impregnation, SiCl4 was added to the Ni@CS composite. Through NSG process, the Ni@Si-CS composite is embedded onto zirconium-based gel, and through polymer elimination by thermal treatment a porous zirconia is obtained with NiO particles on its surface.

Our results show that Ni@Si-CS composites prepared from acid dissolution-based system can be successfully synthesized with 4% Ni and 11%Si homogeneously distributed within the framework of CS, with high surface areas, up to 178 m2/g. The properties of the composites and the obtained encapsulated samples were evaluated as function of composite porosities and composite content. The Ni@Si-CS porosity was found to significantly affect the Ni particle size obtained after encapsulation into zirconia, and thus the sample catalytic performance. The encapsulated samples show higher activity and stability compared to the conventional impregnated samples. In addition, the H2/CO ratio was found to be a function of the zirconia polymorph, the Ni dispersion and particle size.