Global warming and climate change are among the most critical issues in the 21st century. Emission control of CO2 and CH4, the top two greenhouse gases, is the key to dealing with those issues. As one of the most promising greenhouse gas control techniques, direct greenhouse gas conversion/utilization can be an economically-friendly option to mitigate greenhouse gas emissions.
In this dissertation, innovative catalyst designs in two greenhouse gas conversion/utilization processes, namely, CO2 photoreduction on TiO2-based catalysts and photo-thermal-chemical dry reforming of methane (PTC-DRM) on Pt/CeO2-based catalysts, for performance enhancements are demonstrated and discussed.
In the CO2 photoreduction process, the low catalyst surface-CO2 affinity is one of the major factors that limit the CO2 photoreduction performance on TiO2 catalysts. In this dissertation, a highly porous TiO2 material derived from MOF material MIL-125 was prepared, which showed a CO2 photoreduction performance that is 4.2 times as high as commercialized P25 material under a 400 W Xe-lamp irradiation. To further enhance the CO2 photoreduction performance, three types of alkali surface modifications with MgO, namely, (1) MgO ALD coating, (2) MgO ALD coating/Ag co-modifications, and (3) MgO doping, were applied on the porous TiO2. All of the modifications were found to substantially enhance the CO2 photoreduction performance up to ~60 times performance improvements compared with P25 materials.
In the PTC-DRM process, the occurrence of side reaction reverse water-gas shift reaction has long been an issue that affects the performance, especially the low H2/CO production ratio, of DRM catalysts. This dissertation applied Pt/CeO2 catalyst as an example and demonstrated a simple approach to improve both H2/CO production ratio and PTC-DRM reactivity by applying acidic metal oxide Al2O3 in catalyst preparation. Thanks to the favorable Al2O3-CeO2 synergetic effects, under a reaction temperature of 700 ℃ with 30-sun equivalent solar irradiation, the Pt/Al2O3-CeO2 catalyst exhibits a near-unity H2/CO production ratio and 39.6% and 80.0% improvements in CO and H2 generation efficiencies, respectively, compared with Pt/CeO2 catalyst.
The demonstrated innovations should be directly transferable to advance catalytic greenhouse conversions/utilizations in mitigating greenhouse gas emissions and provide guidelines in catalyst design in other photo-driven and/or thermal-driven catalytic processes
