Chapter 1 is the introduction, which presents thestate of the art in synthesis and application of these, in fact, bi-functionalmaterials for sorption enhanced CO2 methanation.
In Chapter 2, zeolite 13X and 5A supported Ni catalysts wereutilized, which synthesized using the evaporation impregnation method. Theinfluence of using different Ni precursors (nitrate, citrate, and acetate) aswell as calcination temperatures on the catalyst properties and performancewere investigated. Using nickel citrate and acetate resulted in smaller NiOparticle sizes compared to nitrate. Methanation experiments revealed that the13X catalysts synthesized using nickel citrate displayed clearly higheractivity, compared to the catalysts synthesized using nickel nitrate or nickelacetate.
Chapter 3 describes zeolites 13X and 5A that weremodified with nickel and/or ruthenium for CO2 methanation. The results showed that Ni wasable to enter the pores of 13X, in the other cases an egg shell type structurewas formed. Methanation experimental results showed that the mono-metalliccatalysts outperformed the bi-metallic ones with Ni being the more active. Oneof the factors influencing the performance of the bi-metallic catalysts wasthat it was difficult to obtain good dispersion when both metals were present. Thecatalysts with lower weak acidity displayed higher activity. The catalyst2.5%Ru13X and 5%Ni13X showed good catalytic stability with around 97% CH4 selectivity at 360 °C, with no catalystdeactivation during a 200 h catalyst stability test.
Chapter 4 deals with sub-nanometer zeolite13X-supported Ni-ceria catalysts for CO2 methanation. Ce loading affected thecatalysts’ metal dispersion, reducibility, basicity and acidity, and hencetheir activity and selectivity. STEM-EDX elemental mappings showed that Ce andNi were predominantly highly dispersed. Ce had a positive effect on thereduction of NiO and lead to a relatively high number of medium basic siteswith a low Ce loading. Highly stable 5%Ni2.5%Ce13X displayed high activity andnearly 100% CH4 selectivity in CO2 methanation at 360 °C, which was mainlyattributed to the high dispersion of metals and relatively high amount ofmedium basic sites.
In Chapter 5, a long-term experimental study employing 5%Ni5A,5%Ni13X, 5%NiL and 5%Ni2.5%Ce13X bifunctional materials with both catalytic andwater adsorption properties was performed in a fixed bed reactor. The overallperformance of the bifunctional materials decreased going from 5%Ni2.5%Ce13X,5%Ni13X, 5%Ni5A, to 5%NiL. The highest obtained CO2 conversion and CH4 selectivity were close to 100 % duringprolonged stability testing in 100 reactive adsorption – desorption cyclesamounting to 203 hours in total with 5%Ni2.5%Ce13X.
Chapter 6 focuses on determining the kinetics of anickel on zeolite 13X catalyst in comparison with a nickel catalyst on ameso-porous γ-Al2O3 support. In this chapter, the validity ofthe obtained rate equation is discussed. The results showed that 13X zeolitesupported nickel catalyst was more active compared to the one supported on γ-Al2O3. This is mainly due to a better dispersion ofnickel on the 13X zeolite catalyst.
Finally, Chapter 7 provides the overall conclusions of thestudies reported in this thesis. Recommendations for further research are alsoprovided.