Photocatalytic conversion of CO2 using perovskites stabilized with UiO-67

Abstract

The effects of CO2 in our air have been widely researched and the way we are generating energy and producing materials at this moment is unsustainable. A way to mitigate the CO2 that is pumped into our atmosphere is to find a way to commercially convert itinto fuels or valuable chemicals, such as CO and CH4, that can be used in our economy. Lately solar fuels have been getting some attention.. They can be formed by using artificial photosynthesis which is based on the reaction that plants have been performing and perfecting for millennia.

Since CO2 is a very stable molecule, catalysts are used to enhance its conversion by minimizing the energy needed in order for the reaction to take place . One of the promising catalysts that has been researched are perovskites. Due to their ionic nature and small bandgap they are ideal for photocatalytic conversion. In this research, CsPbBr3 was used as a catalyst to reduce CO2 into CO and CH4. However because CsPbBr3 contains lead which is toxic and harmful for the environment, lead free Cs2AgBiBr6 double perovskite was explored as well.

One of the serious drawbacks of perovskites is their instability in ambient atmosphere and aqueous solutions. To make them more stable the molecular organic framework UiO-67 was used to cover the perovskite crystals. UiO-67 is a very stable molecule in aqueous environments and also has good CO2 adsorption capabilities which will help to increase the reduction activity. The synthesis of this UiO-67 crystal was optimised by changing reaction conditions and ratios of starting products. This resulted in a repeatable synthesis to make mesoporous structures with a high BET surface area.

To test the perovskites they were spin coated on top of a borosilicate wafer and placed in a photocatalytic cell. For the combined sample the UiO-67 layer was coated over the perovskite layer forming a heterojunction. H2 and CO2 gas were lead over the sample and the products were analysed with a continuous GC.

To analyse the catalysts XRD, IR, Ar-physisorption, TEM and SEM were used Pure CsPbBr3 was not unambiguously proven to be photo catalytically active but when UiO-67 was added it did catalyse the reduction of CO2 to CO. Pure Cs2AgBiBr6 was not active. However when adding H2O to the reaction mixture Cs2AgBiBr6 did become active.