Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The aim of the study was to study the performance of nickel-based catalysts in the production of synthetic natural gas from synthesis gas (CO and H,). Nickel-based catalysts are preeminent catalysts in this reaction due to their high activity and selectivity to methane. Nevertheless, these are threatened by severe intrinsic deactivation phenomena. In more detail, the goal of this work is to study the influence of different catalyst properties on the three most severe deactivation mechanisms (nickel carbonyl formation, carbon formation and thermal sintering). For that purpose, alumina-supported nickel catalysts containing 30 wt% Ni and promoted with Zr02, MgO, CaO and BaO were prepared and tested in a high pressure catalytic reactor. The experimental work was divided in 3 parts according to specific study goals.
The first part of the work consisted in the study of the catalyst pellet size on deactivation due to nickel carbonyl formation. For that purpose, the non-promoted alumina-supported nickel catalyst
was tested at 20 bar and 310 oc using three different pellet sizes. It was found and proved that the
use of large catalyst pellet sizes significantly suppresses this kind of deactivation. The results were scientifically explained by the influence of internal mass transfer phenomena. In order to support this conclusion, COMSOL multihpysics computations were performed to estimate internal temperature and concentration profiles for the different pellet sizes.
The second part of the work was focused on the effect of the selected promoters on the resistance towards carbon formation. For that purpose, the different catalysts were tested at 310 oc, 1 bar and
a H,/C0=3; conditions at which the carbon formation is favorable. The spent samples were analyzed by means of temperature-programmed hydrogenation analyses in order to quantify the amount of carbon formed. It was found that the use of Zr02, in particular, significantly reduces the rate of carbon formation.
The third part of the work consisted in studying the effect of these promoters on the catalyst resistance towards thermal sintering. For that purpose, the different catalysts were exposed to an
accelerated aging procedure (690 oc and H20/H2=2). The intrinsic catalytic activity of the catalysts
was then determined for both the fresh and aged samples by testing these at non-deactivating conditions (300 oc, 1 bar and H,jC0=9). No clear improvement neither decline in catalyst stability
was observed for any of the promoted catalyst samples.
2015. , 54 p.