Niobia-Supported Metal Catalysts for Synthesis Gas Conversion

Abstract

Niobium-based catalysts have shown great potential in the conversion of synthesis gas to hydrocarbons, either as support material or promoter. In this research, we studied different synthetic strategies to enhance the catalytic performance, either in terms of activity, selectivity and/or stability, of niobium-based metal catalysts employed in the conversion of synthesis gas. We have focused primarily on cobalt supported on niobia (Nb2O5) for Fischer-Tropsch synthesis, due to its notoriously high selectivity towards long-chain hydrocarbons and great activity per unit weight of cobalt. First, we explored a simple carbon-templated crystallization method to overcome the limited porosity of niobia, allowing then for higher cobalt loadings. The resulting catalyst displayed increased catalyst-weight normalized activity while maintaining a high cobalt-based activity and selectivity for long-chain hydrocarbons. In the next chapter, a method and related mechanism to tune the Strong Metal-Support Interaction (SMSI), that is coverage of metal nanoparticles from suboxide species from the support, was studied for cobalt catalysts supported on reducible metal oxides, i.e. Nb2O5, TiO2. This was achieved by reduction-oxidation-reduction pre-treatments, resulting in a twofold increase in cobalt surface area and a proportional enhance of the cobalt-based FT catalytic activity. Formation of hollow cobalt oxide particles during oxidation of metallic cobalt particles at elevated temperatures was pointed out to be instrumental in tuning the SMSI effects. Then, we studied the catalytic effect of combining cobalt with nickel on reducible supports for FT synthesis. Co-Ni supported on Nb2O5 and TiO2 showed a stable catalytic performance, high activities and remarkably high selectivities for long-chain hydrocarbons. Characterization of the catalysts indicated a cobalt enrichment of the nanoparticle’s surface and a weaker adsorption of CO, modifying the rate determining step and the catalytic performance. Thereafter, we expanded the research towards nickel supported on niobia for the conversion of synthesis gas and the effect of different reduction temperatures. It was observed that an increase in the reduction temperature led to a decrease in catalytic activity, however stable catalytic performance was gained in return with high selectivity for long-chain hydrocarbons. Ex-situ electron microscopy analysis and in-situ FT-IR were used to find that after low reduction temperatures, particle sintering was the main cause of decreased activity. For high reduction temperatures, formation of niobium suboxides and their partial coverage of the nickel particles limited the formation of nickel carbonyl and slow down particle growth. In the final experimental chapter, the acidic and water resistance characteristics of hydrated niobium pentoxide and niobium phosphate were exploited in the production of DME from synthesis gas in combination with a copper-based methanol synthesis catalyst. The mixtures of the niobium-based solid acids showed comparable activity and DME selectivity than the reference solid acid (gamma-alumina). The niobium-based catalysts showed improved DME productivity per unit volume due to their high density and acid site concentration. Overall, the work in this thesis aims to reflect the potential and versatility of niobium-based catalysts in synthesis gas conversion, understand the underlaying mechanisms for improved catalytic performance and expand the available synthetic tools for catalyst design.