Closing the carbon cycle – from carbon dioxide capture to solar fuels





1. Background

Utilization of CO2 as a feedstock to produce chemicals is an exciting area of the Carbon Capture Utilization and Storage (CCUS) scheme. Although it will not eliminate the need for long-term CO2 storage, CO2 utilization/conversion would diversify the range of carbon management options and potentially improve the overall economics, encouraging early adoption of carbon capture.

We have developed composites to not only capture CO2 but also catalyse its conversion into fuels (CO) using sunlight (i.e. act as solar fuel photocatalysts). These composites include a porous matrix (to capture large amounts of CO2) and a reactive part (to catalyse CO2 conversion).

2. Methods

1) Materials selection, synthesis and characterisation

Metal Organic Frameworks (MOFs) were selected as the porous matrix. MOFs are coordination polymers comprised of metal clusters connected by organic linker molecules. The MOF structure called NH2-UiO-66 was used since its high surface area provides sorption sites for CO2 and it is robust especially in the presence of moisture. Titania (TiO2) was selected as the photocatalyst component since it has been demonstrated as a photocatalyst for CO2 reduction.

Composites of different compositions were prepared. A key aspect of this work is that the synthesis method developed allowed for a close contact between TiO2 and the MOF hence proximity of the sorption and catalytic sites.

The materials were all characterized using a battery of analytical tools and imaging techniques to understand their physical, optical and chemical properties.

2) Materials testing

To evaluate the performance of the nanomaterials, the CO2 adsorption capacity and photocatalytic activity were measured. The photocatalyst tests were carried out in a custom built photoreactor, using hydrogen as a sacrificial agent and > 325 nm radiation. A unique feature of this work is the fact that the materials were tested in a gas(CO2)/solid(composite) reactor to mimic real conditions of CO2 capture.

3. Results

1) The analytical techniques confirmed that the materials were made successfully, with homogeneous mixing and composite formation at the nanoscale of the two components. A key attribute for the photocatalytic activity so that the CO2 molecules adsorbed on the materials are close to the sites where they are converted.

2) The materials had high surface areas and absorbed a large amount of CO2 even in the presence of the lower adsorbing TiO2 phase.

3) The composites synthesised produced CO from CO2 and performed around twice as better than the MOF and TiO2 alone. This highlighted a synergetic effect. The reason for that effect was found using transient absorption spectroscopy which investigated the charge generation and recombination taking place when the materials were illuminated. It revealed charge transfer between the MOF and TiO2. Charge transfer was highly desirable as it suppressed charge recombination of the oppositely charge electrons and holes. This resulted in a higher abundance of electrons available for the catalysis of CO2.

4. Conclusions

This study demonstrated:

·        The development of a nanocomposite synthesis of TiO2/MOF materials of various ratios

·        The close contact between these components was confirmed by various analytical techniques and successfully exploited during CO2 conversion

·        The enhanced photocatalytic activity for the nanocomposite materials owing to charge transfer between the two components

·        The photocatalytic reduction of CO2 using TiO2 can be improved by using a MOF to aid charge separation and improved CO2 adsorption capacity

5. Future ideas/collaborators needed to further research?

Future ideas include:

·        Development of photocatalysts with a narrower band gap so they are active under solar illumination

·        Investigations into the selectivity of CO2 conversion, in order to diversify the products formed

·        Further investigations into the photocatalytic CO2 reduction process under alternate environments closer to those present for industrial CO2 capture processes


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Angus Crake is a PhD student in the Chemical Engineering department at Imperial College London. His research, which he started in 2014, focuses on multifunctional nanomaterials for photocatalytic c...

Round: Open Peer Vote
Category: Student Prize






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