Insights into the Role of Metal Composition, Preparation Method, Catalyst Pretreatment for Application in the Development of Catalytic Reforming Materials for Intensified Conversion of Biogas Sourced Methane
Doctor of Philosophy (Ph.D.)
Degree Granting Department
John Kuhn, Ph.D.
Babu Joseph, Ph.D.
Venkat Bhethanabotla, Ph.D.
Arash Takshi, Ph.D.
Shengqian Ma, Ph.D.
Dry Reforming, Metal Loading, Methane Conversion, Syngas Production
The combustion of fossil fuel sources, crude oil, natural gas, and coal to meet the world’s energy demand has contributed to the increased emissions of carbon dioxide and other harmful gases to the environment. These negative contributions and their resulting impact on environmental conditions and several reports of their peak production and eventual depletion have influenced the need for cleaner and alternative energy production using renewable energy sources.
From the available renewable energy sources ranging from wind, solar, hydrothermal, geothermal, and biomass, biomass is an attractive option because it is largely ubiquitous and a waste commodity that can be converted to highly valuable transportation fuels. The organic fractions of municipal solid waste, agricultural residue, and industrial waste generally originate from biomass. These organic fractions undergo anaerobic decomposition to produce biogas or landfill gas.
Biogas contains methane and carbon dioxide, two important greenhouse gases contributing to the already increasing greenhouse gas emissions if not utilized. Methane and carbon dioxide can be converted to liquid hydrocarbons such as diesel, jet fuel, and gasoline using a two-step process consisting of dry reforming followed by Fischer-Tropsch synthesis (FTS). Reforming reaction involves the adsorption of massive heat amounts and takes place at high temperatures (> 800°C). This dissertation focuses on developing catalytic materials to enable reforming reaction at significantly lower temperatures ( 400-450℃).
To study the effect of the individual metal composition, several catalyst materials consisting of ceria-zirconia (CZO) support, Pt (0.16wt%), Ni (0.67-2.68 wt%), and/or Mg (0.5-2.0 wt%) were prepared using the incipient wetness method. These samples were characterized using powder X-ray diffraction. The diffraction pattern obtained helped identify the presence of the cubic-fluorite phase of ceria-zirconia. The characterization of the basicity using TPD-CO2 desorption studies indicated that Mg's addition created additional basic sites on the catalyst samples; the CO2 desorption amount increased from 21 to 30 µmol of CO2/gcat. The CH4 conversion and reducibility of the Pt-Ni-CZO samples increased with increasing Ni content and. Increasing Mg amount resulted in a decrease in CH4 conversion but helped improve the H2:CO ratio. The increase in basicity with Mg addition was confirmed by CO2-TPD FTIR studies which showed the Pt-Mg-CZO sample having stronger carbonate bands at higher temperatures in comparison to the Pt-Ni-CZO sample. The detection of coke (5.5 X 10-4 g/gcat.h) over the 0.16wt%Pt-2.7wt%Ni-CZO and its absence with the addition of Mg reveal that both Ni and Mg need to be incorporated into the catalyst formulation to obtain a balance between catalyst activity, selectivity, and stability.
In other to study the influence of the metal impregnation strategy method on catalyst performance, a set of reforming catalysts containing Pt (0.16 wt%), Ni (2.7wt%), and/or Mg (0.50wt%) supported on ceria-zirconia (CZO) were synthesized using coimpregnation and sequential impregnation methods. These sets of catalysts samples collectively denoted as Pt-Ni-CZO and Pt-Mg-CZO samples were characterized using powder X-ray diffraction (PXRD) which indicated the presence of the cubic-fluorite structure of the mixed oxide (ceria-zirconia). The results from PXRD were corroborated by the Raman spectra, which presented a band for the F2g vibration of the fluorite lattice of ceria. Temperature-programmed reduction results indicated a difference in the reduction temperature of the metal oxides arising from different degrees of metal-support interaction. FTIR spectra presented bands of different intensities belonging to carbonates and formates. Steady-state dry reforming tests indicated differences in catalyst activities with the CH4 conversion rate at 470 ℃ being between 10.2-10.9 µmol/gcat.s for the Pt-Ni samples and between 0.9 - 1.6 µmol/gcat.s for the Pt-Mg samples. The CH4 apparent activation energies of the Pt-Mg were ranged between 30.7-38.4 kcal/mol, and that of the Pt-Ni samples ranged from 29.1-37.7 kcal/mol. This work provides information on how the metal-metal interactions and metal deposition method could impact the catalyst performance.
For the development of cost-effective low-temperature reforming catalysts, a series of supported non-noble metals were synthesized using co-precipitation and co-impregnation. These samples were a series of Pt-Fe-Mg-CZO and Ni-Co-Ng-CZO. These samples have been characterized using PXRD and temperature-programmed reduction experiments. The addition of Pt to monometallic 1.4wtFe-1.0Mg-CZO led to a reduction in the reduction temperature of the surface ceria. However, increasing the Fe content to 5wt % led to a decrease in the reactants’ conversion temperatures, this could be an indication of encapsulation of the Pt with Fe and that the catalyst surface resembles that of an unpromoted Fe-Mg-sample with an increasing no of Fe atoms.
In a continued effort to reduce material formulation cost without compromising performance, we sought a less expensive metal to replace Pt. A series of Ni (1.4wt%)-Mg (1.0wt%)-Ce0.6Zr0.4O2(CZO) samples promoted with Ru (0.16-0.32 wt%) were synthesized using the incipient wetness method. The catalysts' reducibility was found to increase with increasing Ru content, and the low reduction temperatures were between 163 and 205 ℃. N2 physisorption analysis revealed the materials were mesoporous with a specific surface area between 29.1 and 42 m2/g. In-situ CO DRIFTS measurements presented bands for linear CO adsorption on Ru metal and carbonates and formates. The activity of the materials for dry-reforming was evaluated using temperature-programmed and steady-state experiments. The reactants’ conversions increased with increasing Ru loading. Changes in conversion rates and apparent activation energies were observed by increasing the reduction temperature from 300 to 400 ℃. The 0.16Ru/CZO sample was found to be stable after a 10 h TOS study with no decrease in activity and no coke formation was detected.
Scholar Commons Citation
Sokefun, Yetunde Oluwatosin, "Insights into the Role of Metal Composition, Preparation Method, Catalyst Pretreatment for Application in the Development of Catalytic Reforming Materials for Intensified Conversion of Biogas Sourced Methane" (2021). USF Tampa Graduate Theses and Dissertations.