Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NoNoMeCat (Non-Noble Metal Catalysis - NoNoMeCat)

Teaser

Christoph SteinlechnerWhat is the problem/issue being addressed?Direct photocatalytic reduction of CO2 to CO applying Fe- and Mn-based catalysts.Why is it important for society?The massive use of fossil fuels has increased the amount of carbon dioxide, which leads to global...

Summary

Christoph Steinlechner
What is the problem/issue being addressed?
Direct photocatalytic reduction of CO2 to CO applying Fe- and Mn-based catalysts.

Why is it important for society?
The massive use of fossil fuels has increased the amount of carbon dioxide, which leads to global warming, drastically. New methods and technologies are to convert this CO2 and avoid accumulation in the atmosphere are urgent required. In this context, the use solar energy for the conversion of a waste product into highly valuable products constitutes one pathway.
A plethora of systems for photo-induced CO2 reduction were reported using precious metals like Ru, Ir or Re for both catalysts and photosensitizers. Over the last years, the trend applying non-noble metals such as Co, Ni, Mn and Fe as catalysts was set but the application of full earth abundant systems (contains nonprecious metals for catalysts and photosensitizers) are still rare.

What are the overall objectives?
The photocatalytic reduction of CO2 to carbon monoxide, formic acid or methanol would allow for an efficient technology to convert sunlight directly into fuels. For this reason, catalyst activities and stabilities were improved. In this respect, especially the possibility to substitute noble metal based by non-precious ones was proven. Further results would allow for a further step in the development of the technology as well as to evaluate the appropriateness of non-noble metal-based catalysts.

Wei Zhou
With the ever-increasing energy demand, the development of renewable and environmentally friendly sources of energy is imperative. H2 is considered as one of the most promising energy carriers for enabling a secure, clean energy future. However a “hydrogen economy” is still restricted. One problem is that hydrogen is a highly volatile gas and has very low gravimetric energy density. Hence, the development of a safe, efficient hydrogen storage and delivery system is highly demanded. Hydrogen storage techniques based on liquid-phase chemical hydrogen storage materials (especially methanol and formic acid) have become an attractive choice. Besides the well-developed dehydrogenation system based on noble metal such as Ir and Ru, catalysts based on base metal like iron, cobalt and manganese should be more preferable since these metals are cheap and less toxic. The objective of this project is to develop active dehydrogenation system based on non-noble metals.

Work performed

Christoph Steinlechner: A plethora of systems for photo-induced CO2 reduction were reported using precious metals like Ru, Ir or Re for both catalysts and photosensitizers. Over the last years, the trend applying non-noble metals such as Co, Ni, Mn and Fe as catalysts was set but the application of full earth abundant systems (contains non-precious metals for catalysts and photosensitizers) are still rare. Therefore, heteroleptic CuI complexes bearing a diimine and a diphosphine ligand were developed as light harvesting units in solar driven water reduction in our group. To simplify the system and bypass synthesis and isolation a more active in situ formed CuI PS was established which allows a rapid screening of a wide range of ligand-derivatives without high effort.
Now, we enlarged the application area of the CuI photosensitizers to the solar driven CO2 reduction process. In combination with an iron cyclopentadienone complex a new, noble-metal free photocatalytic system for CO2-to-CO conversion was introduced. A combination of both the cyclopentadienone iron complex and the in situ generated heteroleptic Cu PS formed a highly selective and active system for CO2-to-CO conversion with a TONCO of 487, a selectivity of 99% and a quantum yield of = 13.3%. To investigate alternative first row transition metals as CO2 reduction catalysts, a number of different manganese(I) complexes bearing a diimine ligand was prepared and investigated for electrocatalytic CO2 reduction during my 1st secondment in Brest in Philippe Schollhammers group, supervised by Frederic Gloaguen. A few of them showed good activities and one of them was picked out (Figure 2) and was applied for both photo- and electrocatalytic CO2 reduction. For the light driven CO2 conversion a heteroleptic CuI PS bearing xantphos and bathocuproine was chosen in combination with Mn(pyrox)(CO)3Br. A solution of MeCN/TEAO (5:1) containing Mn(pyrox)(CO)3Br as Cat, Cu(xantphos)(bathocuproine) as PS and BIH as SD was irradiated with monochromatic light (425 nm). Carbon monoxide with a TONCO up to 890 with a selectivity of >99% and a quantum yield of 9.13% was obtained accompanied by traces of formate (TONHCOO- < 1). Hydrogen was not detected. Additionally, to investigate how the reaction proceeds, quenching experiments and operando IR was performed. Due to the fact, that for solar driven CO2 reduction, a sacrificial electron donor is still essential, we investigated additionally the electrocatalytic CO2 reduction using the same catalyst. Analogous to photocatalysis, only CO was obtained as product with a faradayic efficiency up to 99% using trifluoroethanol as proton source.
During Andera Mele’s 2nd secondment at LIKAT, we tested a row of diiron hydrogenase models (figure 3), which are well-known for proton reduction, for photocatalytic CO2 reduction using an in situ formed CuI PS (see above). In this case, a mixture of CO and H2 was observed, whereby the ratio of the syngas (CO + H2) is controlled by different ligands (see table 1).

Wei Zhou: A serial of PNO and PNN ligands and some of the corresponding Mn complexes were synthesized and tested for different types of dehydrogenation reactions. All of those failed in dehydrogenation of formic acid as well as methanol (scheme 1). For the dehydrogenation of 2-propanol, only 2d and 2e with manganese as metal center showed some activity, reaching a TON of 108 and 65 respectively.
It seems that those ligands in combination with manganese are not the for dehydrogenation reactions. So, we turned our attention to another kind of ligand series, substituted oxamides, which already showed activity in the transfer hydrogenation reactions. We firstly tested the dehydrogenation of 2-propanol. It is interesting that the simple N,N\'-dimethyloxamide can promote the reaction. Subsequently, other nitrogen based ligands of similar structure were tested but N,N\'-dimethyloxamide still shows the highest activity (Scheme 2). We also tested the cata

Final results

Christoph Steinlechner
First of all, we established two of overall four fully non-noble metal based systems for photocatalytic CO2 reduction.
Currently, only half reactions of nature’s photosynthesis are studied; either CO2 reduction or water oxidation. Up to now, sacrificial electron donors, mostly amines, are used for CO2 reduction studies. The big aim in future developments will be the combination of CO2 reduction with water oxidation to avoid the use of sacrificial electron donors and introduce artificial photosynthesis for production of renewable fuels.

Wei Zhou
A series of phosphine free ligands were developed for manganese catalyzed dehydrogenation reactions, this system is low in cost and easy to prepare

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