Novel CO2 capture and conversion into fuels via artificial photosynthesis by artificial inorganic leaves
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Researches on sustainable processes for efficient converting of solar energy to fuels have recently received enormous efforts. The main issue facing solar-assisted reactions is insufficient light harvesting by photocatalysts in the visible light regions. The objective of this project was photoreduction of CO2 with water (known as artificial photosynthesis) using visible light. The full architecture of the leaf photosystem was successfully replicated at both the nano and micro levels using biotemplating with TiO2. Recently, bio-inspired materials have emerged as a potential area of research for developing advanced functional systems. Our multi-step chemical replication method resulted in a unique TiO2 architecture with highly porous network. This improved biotemplating method could address the issues in controlling the morphology of final product associated with conventional procedures of synthesis of titania platforms. Scanning and transmission electron microscopy images of the final products confirmed that the detailed microscale framework and nanostructures, such as the chloroplast and the thylakoids were well replicated. In our preliminary tests of photocatalytic activity, the biotemplated artificial TiO2 leaves outperformed well-known P25 TiO2 in photocatalytic degradation of methylene blue dye under visible light. The artificial titania leaf and P25 achieved 13±1.5% and 7±0.9% of methylene blue conversion, respectively, under blue light (440 nm). Under green light irradiation (515 nm), the methylene blue conversion given by the biotemplated photocatalyst (9±1.2%) is also higher than that achieved by P25 (6.3±0.8%). The enhanced visible light activity of the bio-mimicked titania catalyst could be attributed to several key factors given by the 3-dimensional interconnected nanosheets structure (the thylakoid replicas), including enhance reactant-catalyst contact and high efficiency of the light absorption.
The novel porous TiO2 architecture was used to catalyse the photoreduction of CO2 with H2O. The artificial TiO2 leaves showed higher selectivity to methane (CH4) in CO2 photoreduction compared to non-porous commercial titania catalysts. The CO2 photoreduction reactions catalysed by artificial TiO2 leaves and P25, after 20 h under UV light (370 nm), produced 3.8±0.6 and 2.7±0.5 μmol/g-cat. of CH4, respectively. The chloroplast-like 3-D TiO2 materials also outperformed the product yields of P25 titania under visible light. The CO (0.5±0.1 μmol/g-cat.) and CH4 (2.2±0.35 μmol/g-cat.) were yielded by the biotemplated titania photocatalyst, after 30 h under green light (515 nm), compared to the CO (2±0.5 μmol/g-cat.) given by P25. We hypothesised that there is a strong correlation between the morphology of the inorganic artificial leaves made of TiO2 and their superior photocatalytic performance. Moreover, the advantages of the surface chemical modification of titania photocatalysts with the ruthenium dioxide were demonstrated. The RuO2/artificial leaf materials possessed a substantially higher efficiency of the CO2 photoreduction compared to the neat artificial TiO2 leaves in the case of visible light. The CO2 photoreduction reactions catalysed by neat and RuO2/artificial TiO2 leaves, after 30 h under green light (515 nm), produced 2.2±0.35 and 3.15±0.35 μmol/g-cat. of CH4, respectively. Finally, two kinetic models for photocatalytic reduction of CO2 with H2O, were validated with the products concentration profiles. The experimental data have obtained a very good fit to the kinetic model developed based on Eley-Rideal mechanism. The understanding of the morphological contribution of the photocatalyst provided in this study, could help to augment the efficiency and selectivity of the CO2 photoreduction.