CO₂ reduction via photo- and photo-thermo coupled heterogeneous catalytic reaction
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Date
27/11/2021Item status
Restricted AccessEmbargo end date
27/11/2022Author
Wang, Ke
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Abstract
CO₂ has been seen as a green-house gas mainly responsible for the global
climate change. According to National Oceanic and Atmospheric
Administration (NOAA), CO₂ emission since 1980 contributes to half of the CO₂
increases in the last 300 years and 25% of the CO₂ concentration rise is
attributed to the emission since 2000. The CO₂ concentration is increasing
faster than ever now. Instant and effective actions are urgently required to curb
the trend in order to constrain the global temperature increase lower than
1.5 °C. Besides CO₂ capture and storage, CO₂ catalytic conversion to value-added chemicals is another important strategy to reduce its concentration in
the atmosphere.
Solar light is one of the massive and widespread sustainable energy sources
on the Earth. Using solar energy to drive CO₂ reduction is a promising and
green way to reduce CO₂ concentration in atmosphere. The main challenges
in practical implementation of the CO₂ conversion in large-scale are the
relatively slow reaction rates and poor selectivity to more valuable products.
The key to address these issues relies on better understanding of the reaction
and photoexcitation mechanisms. This thesis focuses on building the
correlation between the microstructure of catalysts, light induced excitation,
reaction pathway, product selectivity and reaction rate.
The research work presented in this thesis starts from investigating the CO₂
photocatalytic reduction with H₂O over g-C₃N₄ (Chapter 3). A comprehensive
optimisation of molecular and electronic structure of pristine g-C₃N₄₄ is
conducted. The designed modification of co-doped K, B and N Vacancy (NV)
enhances light absorption, excited-electron/hole separation, CO₂ adsorption,
therefore the overall significantly increases the photocatalytic CO and CH₄
production rate by 527% and 161% comparing with the pristine g-C₃N₄. The
reaction mechanism has been investigated via in-situ DRIFTS. An important
correlation is found between activated CO₂* concentration on catalyst surface
and reaction rate. The enhanced capability to activate the adsorbed CO₂* is
the key guidance for future photocatalysts design.
Most of the current CO₂ photocatalytic reduction with H₂O can only produce
CH₄ and CO. Therefore, Chapter 4 focuses on how to promote higher
hydrocarbon production via CO₂ photocatalytic reduction with H₂O over
Au/TiO₂-x with abundant oxygen vacancies (VO). The reaction performances
are compared under UV and green light irradiation conditions. The UV light-driven CO₂ reduction shows primary product of CO; by comparison, the green
light-driven CO₂ reduction produces 20% C₂H₆ and 80% CH₄. The electronic
structure and microstructure of Au/TiO₂₂-x are investigated by a series of in-situ
spectroscopy methods. The involvement of VO is found indispensable for this
reaction. The different product selectivity under ultraviolet (UV) or green light
irradiation is derived from the different excitation mechanism. The UV light
bandgap excitation induces negatively charged Au that retards CO* adsorption;
on the contrary, the green light plasmonic excitation causes positively charged
Au that stabilises CO* adsorption. The stability of CO* key reaction
intermediate is essential for higher hydrocarbon production.
The two chapters mentioned above have demonstrated the feasibility to
significantly improve the photocatalytic CO₂ reduction rate by optimising the
catalysts. However, the photocatalytic reaction rate is still far from comparable
with traditional thermocatalytic CO₂ reduction. The H₂O has been proven as a
less effective proton donor in Chapter 4 and the role of H₂O needs further
investigations. Therefore, in the Chapter 5 & 6, CO₂ hydrogenation to CO,
known as the reversed water gas shift reaction (RWGS reaction), is
investigated under photo-thermo coupled catalytic reaction condition. The
external light promoted thermocatalytic RWGS reaction performances are
compared over two typical catalysts: Au/TiO₂ and Au/γ-Al₂O₃. The reducible
TiO₂ and non-reducible γ-Al₂O₃ supports are fundamentally different on
whether the plasmonic hot electrons transfer to the support. The CO₂ reduction
rates are found significantly higher over Au/TiO₂ than Au/Al₂O₃ on either pure
thermocatalytic or photo-thermo coupled catalytic reaction conditions.
Moreover, the plasmonic enhancement efficiency is also superior on Au/TiO₂
over Au/A₂O₃.
In Chapter 5 & 6, the RWGS reaction mechanism in dark and photo-thermo
coupled conditions are investigated via a series of in-situ spectroscopy and
isotope kinetic analyses. The plasmonic enhancement mechanisms are also
investigated via both experimental and theoretical approaches. The reaction
mechanisms are proven to be quite different over Au/TiO₂ and Au/Al₂O₃.
RWGS reaction follows a redox pathway with CO₂ self-dissociation at the VO
site of Au/TiO₂. By comparison, the RWGS reaction mechanism over Au/Al₂O₃
is a mixture of formate and carboxyl reaction pathways. Accordingly, the
plasmonic enhancement mechanisms are different too. In the case of Au/TiO₂,
plasmonic hot electrons mainly facilitate the VO generation; while on Au/Al₂O₃,
the formate pathway, especially the formate in bridged configuration, is
preferentially promoted by plasmonic excitations.
Overall, this thesis has expanded and deepened the understandings of the
photocatalytic and photo-thermo coupled catalytic CO₂ reductions. The
mechanisms elucidated in this thesis can inspire future works on designing
and developing more efficient and economically-feasible CO₂ utilisation to
address climate change challenge.