Date of Award
5-12-2024
Degree Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Biomedical and Chemical Engineering
Advisor(s)
Nancy Totah
Second Advisor
Viktor Cybulskis
Subject Categories
Chemical Engineering | Engineering
Abstract
Greenhouse gases, notably carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), are principal contributors to global warming, leading to an incremental rise in Earth's atmospheric temperature. The escalation of these gases, primarily from fossil fuel combustion, deforestation, and industrial processes, enhances the radiative forcing mechanism, thereby trapping excessive solar heat within the atmosphere. Scientific inquiries into the spectral absorption properties and life span of these gases emphasize their varying impacts on Earth's thermal equilibrium. For instance, CO2, with its extensive atmospheric residency, brings a prolonged thermal influence, whereas CH4, while being the cleanest burning fossil fuel that generates far less CO2 than petroleum due to its high hydrogen-to-carbon ratio (H/C = 4), exhibits more than 80 times the global warming potential of CO2 over a 20 year span. Industrial activities, especially those associated with the energy sector, such as natural gas extraction, heavy-duty transportation, and coal mining, are major contributors to CH4 emissions. Additionally, agricultural practices, particularly livestock farming and rice cultivation, release substantial quantities of methane through enteric fermentation and anaerobic decomposition processes. The interception and mitigation of CH4 emissions are critical for slowing the rate of global temperature rise. One of the effective mitigation strategies is to catalytically convert CH4 to CO2, since catalytic oxidation provides the added benefit of generating fewer NOx emissions compared to thermal combustion processes. Although Pd/Al2O3 and Pd/ZrO2 catalysts are active for CH4 oxidation at stoichiometric conditions, they fail to maintain adequate CH4 conversion and stability under lean-burn conditions (air/fuel equivalence ratio, > 1) at low temperatures (< 673 K) in the presence of various combustion products such as H2O, CO2, and SO2. By contrast, when utilizing zeolite-supported transition metal catalysts, for example, Pd/SSZ-13 (Pd/CHA), Pd/LTA, Pd@S-1 and PdCo@S-1, superior performance in achieving complete CH4 oxidation was observed, even in the presence of H2O. In addition, Pd-containing zeolites are able to remain hydrothermally stable due to their hydrophobicity and superior hydrothermal durability. When searching for optimal catalyst supports, our exploration included conventional zeolites like LTA, CHA, and S-1 (MFI), leading to the synthesis of other less common structures such as VET, RTH, DDR, and SSZ-42 zeolites. These zeolites were chosen due to their unique pore architectures and potential to assist distinct catalytic activities by increasing the accessibility of the reactants. Preliminary evaluation was focused on characterizing the physical properties of these zeolites, for instance, crystallinity. 1 wt.% Pd/CHA (Si/Al = 15 - 137) were synthesized in both OH and F media. 1 wt.% Pd/LTA zeolites with Si/Al molar ratios of 1, 22, 31, 39, and 52 were synthesized hydrothermally in F media. Both catalysts were evaluated for CH4 oxidation performance before and after simulated aging at 650 ºC for 1 hour under wet-lean conditions. When comparing the temperatures required to achieve 50% and 90% CH4 conversion, it was observed that CH4 oxidation performance improves with increasing hydrophobicity. The temperature that 90% of CH4 was converted (T90) over high-silica Pd-LTA (39, 52) and Pd-CHA (OH-137, F-33, F-80) were below 673K in the presence of H2O. At 250 ºC, the steady-state CH4 oxidation rates on 1 wt.% Pd/CHA-F-80 under wet-lean conditions were 9.5 × 10-9 moles CH4 gcat-1 s-1, or ~ 2.3 × greater than Pd/Al2O3. The CH4 oxidation turn-over frequency (TOF), moles of CH4 converted per mole of surface Pd per second, on 1 wt.% Pd/CHA-F-80 under the same conditions was 6.0 × 10-3 s-1, or ~ 6 × greater than Pd/Al2O3. In addition, 1 wt.% Pd/LTA-52 exhibits ~ 4.5 × steady-state CH4 oxidation rates compared to Pd/Al2O3. Stability tests for 10 h at 450 C under the reaction feed showed that Pd/LTA-52 was able to maintain 100% CH4 conversion for the duration of the experiment, while Pd/Al2O3 showed a 22% deactivation after the stability test. The study of H2O order and activation barrier indicates a weaker H2O inhibition effect and a lower activation barrier on Pd/CHA and Pd/LTA catalysts compared to Pd/Al2O3. The results demonstrate that, for both catalysts, increasing Si/Al ratios, i.e., hydrophobicity, or changing the synthesis media to F media (for Pd/CHA), can improve the catalyst stability and reduce the inhibition effect on CH4 oxidation rate in the presence of H2O.
Access
Open Access
Recommended Citation
Liu, Jingzhi, "Synthesis and Design of High-Silica Pd-Containing Zeolites for the Catalytic Combustion of Methane" (2024). Dissertations - ALL. 1936.
https://surface.syr.edu/etd/1936
