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Sunlight driven carbon-dioxide reduction: Hybrid catalytic systems consisting of molecular catalysts and light-harvesting Quantum-dots and semiconductors

Periodic Reporting for period 1 - CO2RED (Sunlight driven carbon-dioxide reduction: Hybrid catalytic systems consisting of molecular catalysts and light-harvesting Quantum-dots and semiconductors)

Reporting period: 2018-03-01 to 2020-02-29

Artificial photosynthesis, in which sunlight is directly used to generate fuels and value-added chemicals from carbon dioxide and water, is an attractive solution to both global energy challenges and environmental issues caused by rising levels of greenhouse gases. However, recycling the carbon dioxide in such a manner requires efficient catalysts and light-harvesters to power this energetically uphill reaction. In this context, CO2RED project is aimed towards developing new hybrid photocatalysts and photocathodes for light-induced carbon dioxide reduction using molecular catalysts and semiconductor light-harvesters. Molecular catalysts are typically transition metal complexes that display good selectivity towards CO2 reduction and offer synthetic tunability. However, their application in practical devices require mounting those molecules on a solid support to enable heterogeneous catalysis. This project focuses on integrating molecular catalysts into semiconductor materials and explores different immobilization techniques for specific surfaces. The molecule/semiconductor assembly, termed as hybrid material, is used as photocatalysts in colloidal/suspension type photoreactors for CO2 reduction. The second part of the project aims to incorporate this hybrid assemblies in transparent electrodes to build photoelectrochemical devices. Such device uses both sunlight and electricity to drive the catalytic reactions at the photoelectrodes and it offers scope for scaling up.
Overall, the project aims to deliver new molecular catalysts, methods for immobilizing catalyst on surfaces, and hybrid materials capable of mediating carbon dioxide reduction driven by solar energy and electricity.
The work performed in this project has covered several aspects related to the main scope, which is the development of molecular catalyst/semiconductor assemblies for carbon dioxide reduction.

Visible light driven carbon dioxide reduction (publication 1): A new hybrid photocatalyst was developed by polymerizing cobalt phthalocyanine on carbon nitride. While phthalocyanines are traditionally used as colorants in green and blue pigments, metal phthalocyanines are known for their interesting redox properties. We used cobalt phthalocyanine polymer as a the 'molecular catalyst' component in the hybrids. Porous carbon nitride, a highly stable solid semiconductor composed of carbon and nitrogen, served as the light harvester in the hybrid system. Upon visible light illumination, an electron/hole pair is generated in carbon nitride. The excited electron gets transferred to the cobalt phthalocyanine anchored on the surface to form reduced metal centre that subsequently catalyses conversion of carbon dioxide to carbon monoxide. The holes on carbon nitride is quenched by a sacrificial electron donor to complete the redox cycle. One milligram of hybrid material generated approximately 0.5 micromol of CO after 24 h visible light irradiation.

Coupled electrolyzer (publication 2): The same polymeric cobalt phthalocyanine catalyst was for electricity driven carbon dioxide reduction in an electrolyzer device. The prototype device consists of a novel anode that oxidizes alcohol to aldehyde and a cobalt phthalocyanine cathode that reduces carbon dioxide and water to syngas (mixture of CO and hydrogen). Electricity provides the energy for the coupled electrolyzer and the electron generated from alcohol oxidation (anode) is utilized for syngas production at cathode. The energy efficiency of the cell, defined as the ratio of the chemical energy stored in the products and the energy input, is estimated to be approximately 17%.

Photoelectrode for carbon dioxide reduction (manuscript 4): p-doped silicon is widely used photovoltaic industry and is considered as one of the most promising materials for fabrication of photoelectrodes. We therefore employed a silicon wafer as photoelectrode which contained a mesoporous titanium dioxide protection layer. A newly designed cobalt phthalocyanine catalyst with four phosphonic acid anchoring groups was immobilized on the titania surface. The photocathode assembly displayed good activity towards syngas generation from CO2 saturated aqueous solution under visible light illumination.

1) Souvik Roy, Erwin Reisner, Light driven CO2 reduction by mesoporous carbon nitride modified with polymeric cobalt phthalocyanine, Angew. Chem. Int. Ed., 2019, 58, 12180–12184
2) Mark A. Bajada†, Souvik Roy†, Julien Warnan†, Kaltum Abdiaziz, Andreas Wagner, Maxie M. Roessler, Erwin Reisner, A Precious-metal-free Hybrid Electrolyzer for Alcohol Oxidation Coupled to CO2-to-syngas Conversion, Angew. Chem. Int. Ed., 2020, doi: 10.1002/anie.202002680
3) Stefan Vanicek, Markus Jochriem, Christopher Hassenrück, Souvik Roy, Holger Kopacka, Klaus Wurst, Thomas Müller, Rainer F. Winter, Erwin Reisner, Benno Bildstein, Redox-Rich Metallocene Tetrazene Complexes: Synthesis, Structure, Electrochemistry and Catalysis, Organometallics, 2019, 38, 1360–1371
4) Souvik Roy, Melanie Miller, Julien Warnan, Jane Leung, Erwin Reisner, Electrocatalytic and Solar-Driven Reduction of Aqueous CO2 with Immobilized Molecular Cobalt-Phthalocyanine Catalyst, in preparation
5) Arjun Vijeta, Souvik Roy, Erwin Reisner, Nickel modified mesoporous carbon nitride for organic transformations, in preparation

Conference Talks:
1) Visible-light driven carbon dioxide reduction by metal-organic polymer integrated carbon nitride (ISF-3), Electrocatalytic hydrogen evolution from a cobaloxime-Based metal–organic framework thin film (ISF-3 Young); 3rd International Solar Fuels Conference (ISF-3/ICARP2019); Hiroshima, Japan; November 19-24, 2019
a) Development of a new method (in situ polymerization) for integrating molecular catalysts with semiconductors
b) First report on a precious-metal-free prototype electrolyzer for alcohol oxidation coupled to CO2 reduction which employs fully molecular cathode and anode
c) Development of a silicon/titania photoelectrode with an immobilized cobalt phthalocyanine catalyst which is the first example of molecular photocathode capable of selectively converting CO2 to CO in water
d) Anchoring the phosphonic acid containing cobalt phthalocyanine catalyst on a transparent electrode allowed spectroelectrochemical studies which shed new light on the mechanistic details of the catalytic pathway
hybrid photocatalyst containing carbon nitride and cobalt phthalocyanine