P.I.  John Kuhn, USF

November 2015

Landfill gas (LFG) is considered for production of synthesis gas (H2 and CO) which can be used as a feedstock for Fischer Tropsch Synthesis (FTS) to obtain valuable liquid hydrocarbon fuels.  This combined process is done using tandem catalysts to convert methane from LFG to syngas under temperatures low enough to remain in the operable range for FTS (T<450°C). Many challenges arise for the combined process including finding the optimum temperature for reforming as well as FTS. Methane reforming reactions are thermodynamically favored at temperatures higher than 400°C while FTS cannot occur at temperatures higher than 450°C.  Furthermore, FTS requires a syngas (H2:CO) ratio of 2:1 for fuel production which is a challenge to obtain using dry reforming alone especially at low temperatures. The goal of this work is to convert LFG to liquid hydrocarbon fuels in a single step reactor through finding the optimum tandem catalysts and conversion temperature. This effort will address one of the major challenges that faces gas-to-liquid (GTL) technologies which is economy of scale by initially converting at least 10% of the methane in a single pass.  To accomplish these goals, low temperature (T<500°C) dry reforming of methane was studied over (0.13-0.51 wt%) Pd and (0.07-0.64 wt%) Pt doped (1.34-1.39)wt%Ni-1.0wt%Mg catalysts on a ceria-zirconia oxide support, as well as control catalysts containing Pt or Pd, but not Ni and Mg. Temperature-programmed reduction (TPR) studies showed Pd catalysts having initial reduction peaks at lower temperatures compared to Pt catalysts. Reaction studies showed the lowest 10% conversion of methane at 383°C for the 0.13%Pd-1.39%Ni-1.0%Mg/(Ce0.6Zr0.4)O2 catalyst. The same catalyst also had the lowest 10% conversion temperature (366°C) for carbon dioxide. Syngas ratios (H2:CO) for Pd catalysts (0.24-0.41) were better than Pt catalysts (0.22-0.30) at 450°C, however, still not close to the stoichiometric ratio as a result of the reverse watergas-shift (rWGS) reaction simultaneously occurring at the low operating temperatures studied.

Turnover frequencies (TOF) were calculated from steady-state reaction experiments done in the temperature range of 430-470°C. Both catalysts’ TOFs increased with increasing temperature, however overall Pt catalysts had marginally higher TOFs (2.69-4.74 s-1) compared to Pd catalysts (2.40-4.58 s-1). Although both Pd and Pt catalysts had comparable activities and rates, the 0.13%Pd-1.39%Ni-1.0%Mg/(Ce0.6Zr0.4)O2 catalyst was selected as the most promising one because of its higher H2:CO ratios, negligible changes during reaction as evidenced in postreaction characterization, more basic sites that may decrease coking, and its lower cost. A long term stability test was also done on the 0.16%Pt catalyst and resulted in minimal deactivation when left on stream for 100.5 hours.

 

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