Zhao, Huilei (2017-08). Innovative Nanomaterials for Efficient CO2 Photocatalytic Conversion to Fuels. Doctoral Dissertation. Thesis uri icon

abstract

  • Photocatalytic reduction of CO2 with water by photocatalysts such as TiO2 to produce solar fuels is an attractive approach to alleviate the environmental influences of greenhouse gasses and in the meantime produce valuable solar fuels. However, TiO2 has several limitations for CO2 photoreduction. First, TiO2 has a large band gap, leading to the low solar energy usage efficiency. Second, the fast charge recombination rate in TiO2 causes the low photocatalytic performance. Third, relatively insufficient CO2 adsorption capability results in the low production rate of solar fuels. This work addressed the above limitations by designing innovative TiO2 based photocatalysts for CO2 photoreduction with H2O from four different aspects: (1) crystal structures, (2) CO2 gas adsorption capability, (3) exposed facets and (4) defect property. First, this study demonstrated the high performance of mixed anatase/brookite phase TiO2 for CO2 photoreduction with H2O. Single-phase anatase and brookite, and mixed-phase anatase/brookite TiO2 nanomaterials were synthesized and evaluated for CO2 photoreduction in the presence of water vapor for the production of solar fuels (CO and CH4). The results showed that bicrystalline anatase/brookite was more active than single-phase anatase, brookite, and P25. The bicrystalline mixture with a composition of 75% anatase and 25% brookite showed the highest photocatalytic activity, likely due to the enhanced interfacial charge transfer between anatase and brookite nanocrystals. Second, this study showed improved CO2 adsorption and conversion by combing CO2 adsorbents LDHs materials with photocatalyst TiO2. A novel material of crystallized titanium-embedded magnesium-aluminum layered double hydroxides (MgAlTi-LDHs) is developed and tested for CO2 photoreduction with water. Three different materials synthesis methods were explored: (1) coprecipitation, (2) coprecipitation + hydrothermal, and (3) coprecipitation + calcination + reconstruction. The material hydrothermally treated at 150 to 200 ?C demonstrated the highest CO production due to a well-balanced TiO2 crystallinity and specific surface area. Third, high-performance TiO2 photocatalysts by combining the effect of oxygen deficiency and exposing facets were synthesized. And atomic layer deposition (ALD) technique was applied to deposit Al2O3 thin film on the surface of TiO2 with thickness of Al2O3 controlled by the number of ALD cycles. All the synthesized photocatalysts (ANR, ReANR, and Al2O3 coated ReANR) were tested for CO2 photocatalytic reduction with water. Compared with ANR, ReANR had more than 50% higher CO production and more than ten times higher CH4 production due to the oxygen vacancies that possibly enhanced CO2 adsorption and activation. By applying less than 5 cycles of ALD, the Al2O3 coated ReANR had enhanced overall production of CO and CH4 than uncoated ReANR, with 2 cycles being the optimum. Whereas, both CO and CH4 production decreased with increasing number of ALD cycles when more than 5 cycles were applied. Photoluminescence (PL) analysis showed both 2 and 200 cycles of Al2O3 ALD coating on the ReANR were able to reduce the charge carrier recombination rate, likely because of the passivation of surface states. On the other hand, a relatively thick layer (e.g. 200 cycles) of Al2O3 may act as an insulation layer to prohibit electron migration to the catalyst surface.
  • Photocatalytic reduction of CO2 with water by photocatalysts such as TiO2 to produce solar fuels is an attractive approach to alleviate the environmental influences of greenhouse gasses and in the meantime produce valuable solar fuels. However, TiO2 has several limitations for CO2 photoreduction. First, TiO2 has a large band gap, leading to the low solar energy usage efficiency. Second, the fast charge recombination rate in TiO2 causes the low photocatalytic performance. Third, relatively insufficient CO2 adsorption capability results in the low production rate of solar fuels. This work addressed the above limitations by designing innovative TiO2 based photocatalysts for CO2 photoreduction with H2O from four different aspects: (1) crystal structures, (2) CO2 gas adsorption capability, (3) exposed facets and (4) defect property.

    First, this study demonstrated the high performance of mixed anatase/brookite phase TiO2 for CO2 photoreduction with H2O. Single-phase anatase and brookite, and mixed-phase anatase/brookite TiO2 nanomaterials were synthesized and evaluated for CO2 photoreduction in the presence of water vapor for the production of solar fuels (CO and CH4). The results showed that bicrystalline anatase/brookite was more active than single-phase anatase, brookite, and P25. The bicrystalline mixture with a composition of 75% anatase and 25% brookite showed the highest photocatalytic activity, likely due to the enhanced interfacial charge transfer between anatase and brookite nanocrystals.

    Second, this study showed improved CO2 adsorption and conversion by combing CO2 adsorbents LDHs materials with photocatalyst TiO2. A novel material of crystallized titanium-embedded magnesium-aluminum layered double hydroxides (MgAlTi-LDHs) is developed and tested for CO2 photoreduction with water. Three different materials synthesis methods were explored: (1) coprecipitation, (2) coprecipitation + hydrothermal, and (3) coprecipitation + calcination + reconstruction. The material hydrothermally treated at 150 to 200 ?C demonstrated the highest CO production due to a well-balanced TiO2 crystallinity and specific surface area.

    Third, high-performance TiO2 photocatalysts by combining the effect of oxygen deficiency and exposing facets were synthesized. And atomic layer deposition (ALD) technique was applied to deposit Al2O3 thin film on the surface of TiO2 with thickness of Al2O3 controlled by the number of ALD cycles. All the synthesized photocatalysts (ANR, ReANR, and Al2O3 coated ReANR) were tested for CO2 photocatalytic reduction with water. Compared with ANR, ReANR had more than 50% higher CO production and more than ten times higher CH4 production due to the oxygen vacancies that possibly enhanced CO2 adsorption and activation. By applying less than 5 cycles of ALD, the Al2O3 coated ReANR had enhanced overall production of CO and CH4 than uncoated ReANR, with 2 cycles being the optimum. Whereas, both CO and CH4 production decreased with increasing number of ALD cycles when more than 5 cycles were applied. Photoluminescence (PL) analysis showed both 2 and 200 cycles of Al2O3 ALD coating on the ReANR were able to reduce the charge carrier recombination rate, likely because of the passivation of surface states. On the other hand, a relatively thick layer (e.g. 200 cycles) of Al2O3 may act as an insulation layer to prohibit electron migration to the catalyst surface.

publication date

  • August 2017