Toward Catalyst Design for Sulfur Resistant Hydrocarbon Reforming Nickel Catalysts

Rangan, Meghana

Graduate Thesis Or Dissertation

Toward Catalyst Design for Sulfur Resistant Hydrocarbon Reforming Nickel Catalysts Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/cz30ps83k

Abstract

A combination of density functional theory (DFT) calculations and experimental studies of supported catalysts was used to identify H2S-resistant biomass gasification product reforming catalysts. DFT calculations were used to rationally design nickel-based (111) surfaces with lower sulfur adsorption energies and enhanced ethylene adsorption energies. These metrics were used as predictors for H2S resistance and activity toward steam reforming of ethylene, respectively. This approach suggested that the Ni/Sn surface is highly resistant to sulfur poisoning and the Ni/Ru system is the most favorable for ethylene binding with a small increase in sulfur binding energy. The Ni/W system was also investigated as a potential sulfur resistant catalyst due to its effectiveness as a hydrodesulfurization catalyst. A series of supported bimetallic nickel catalysts (Ni/Sn, Ni/Ru and Ni/W) were prepared and screened under model ethylene reforming conditions and simulated biomass tar reforming conditions. Ni/Ru showed higher reforming activity than pure Ni in the absence of H2S. In the presence of H2S, the Ni/W and Ni/Ru catalyst showed higher activity than pure Ni. The Ni/Sn catalyst lost less activity in the presence of sulfur than the other Ni bimetallic catalysts examined for this study. All three bimetallic catalysts recovered activity, on the removal of H2S, at a much faster rate than Ni. These trends in activity can be explained by density functional theory and characterization techniques including H2 chemisorption, temperature programmed reduction (TPR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and extended X-ray absorption fine structure spectroscopy (EXAFS). Density functional theory studies reveal that S and C2H4 adsorption are dependent on several factors including the adsorbate coverage, Ru concentration in the bimetallic alloy, the adsorption site and the homogeneity of the bimetallic alloy. Sulfur adsorption was found to be dependent on the density of occupied states near the Fermi level while C2H4 adsorption is dependent on the number of unoccupied states in the d-band. Experimental characterization techniques reveal that the two metals in the bimetallic alloys (Ni/Ru, Ni/Sn and Ni/W) are in intimate contact with each other and the addition of the second metal to the Ni catalyst significantly changes its properties. Ru improved the reducibility of the Ni catalyst while W and Sn had the reverse effect. The W in the Ni/W catalyst was found to oxidize in the presence of steam much more than either Ni/Sn or Ni/Ru. The catalysts were found to sinter under reaction conditions.

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