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Twinned Enzymatic and Metal CATalysis - Overcoming bottlenecks in the electrocatalysis of the carbon and nitrogen cycles

Final Report Summary - TEAMCAT (Twinned Enzymatic and Metal CATalysis - Overcoming bottlenecks in the electrocatalysis of the carbon and nitrogen cycles)

Electrocatalysis examines electrochemical reactions and studies their molecular underpinnings with an eye to developing more efficient catalysts, as a first step towards their application in chemical analysis or industrial chemistry.
In this respect, the research project TEAMCAT addressed electrochemical reactions of the nitrogen and carbon cycles having a considerable significance to environmental issues such as pollution and the accumulation of carbon dioxide.
In a multi-step reaction, the individual steps can be catalysed by single catalysts which, once combined together, give rise to “twinned” or “cascade” catalysis with synergistic effects. Although cascades have been implemented in (bio)catalysis for a long time, there was no example of an electrochemical cascade. Therefore, TEAMCAT aimed to achieve cascade electrocatalysis of two selected electrochemical reactions, the reduction of nitrate (NO3−) and the reduction of carbon dioxide (CO2), both of which are limited by kinetically sluggish steps. At the same time, reaction products analytical techniques were implemented to determine the reaction selectivity, while an attenuated total reflection infrared (ATR-IR) setup, previously developed by the host research group, was modified and applied to the study of reaction intermediates and mechanisms.

Description of objectives
Three main objectives were identified. The first two addressed the reduction of NO3−, and the catalysts selected for this preliminary phase were platinum (Pt) nanoparticles and the E. coli enzyme nitrate reductase (NarGHI):
1. Establishing a procedure to assemble a composite catalyst to obtain a proof of concept of the cascade catalysis of reduction of NO3− in neutral media.
2. Performing a detailed study of the cascade catalysis with a view to its optimisation, while also aiming to obtain insight into the mechanistic details and to identify and quantify the reaction products.

The third objective focussed on another reaction, the reduction of carbon dioxide (CO2). Once more, two catalysts were identified as promising candidates for the cascade catalysis, the C. hydrogenoformans enzyme carbon monoxide dehydrogenase (CODH) and copper (Cu) particles.

3. Extending objectives 1 and 2 to achieve cascade catalysis of the reduction of CO2 by combining CODH and Cu.

Description of work performed
Research towards objective 1 was focussed on practical aspects. For NarGHI to be useful as electrocatalyst, it must be immobilised on a conductive support, in our case carbon black particles, while also retaining its catalytic activity (i.e. it must not denature). Therefore, deposition procedures were screened and tested, eventually leading to a standardized sequence of steps ensuring successful and reproducible immobilisation of NarGHI. Secondly, electrochemical techniques were used to carry out tests aiming to that NarGHI and Pt can give rise to the twinned catalysis of the reduction of NO3−. Similar experiments were performed with the individual catalysts, and their catalytic activity was used as a benchmark.

Objective 2 involved gaining more detailed insight into the reduction of NO3−. At the same time, other transition metals (rhodium and palladium) were combined to NarGHI, showing the differences and the similarities with Pt. Several analytical techniques were developed or applied to determine the product selectivity of the reaction: spectrophotometric methods were particularly useful. On the other hand, nitrite (NO2−) is a key reaction intermediate and its reduction at metal particles was investigated with a specific technique combining ATR-IR spectroscopy and electrochemistry. In this way, molecules adsorbed at the metal surface can be identified in real time as a function of the potential applied to drive the electrochemical reaction.

Objective 3 was based on the approach adopted for objective 1. First, procedures for the immobilisation of CODH and Cu particles onto carbon black were tested, and the performance of the individual catalysts in terms of catalytic activity towards the reduction of CO2 was evaluated with electrochemical methods. This phase also addressed different types of Cu particles available commercially, which differ in terms of the presence of surface oxides. Adsorption of CODH onto carbon particles pre-modified with Cu was also attempted. The effect of electrolyte composition and pH was also investigated. Finally, gas chromatography (GC) was used to analyse the reaction products.

Description of results

Cascade catalysis of the reduction of NO3− was demonstrated. NH4+ was shown to be the final product of this reaction, and its concentration at the end of an experimental run was quantified. The amount of NO2−, produced as intermediate, was also measured. The ratio between metal particles and NarGHI on the catalytic properties was changed and its influence on the catalysis of the reaction was established.
Infrared spectroscopy combined with electrochemistry highlighted the surface species formed onto Rh and Pt nanoparticles during the electrochemical reduction of NO2−. Nitric oxide was the main species, also acting as a poison. Pt is the only metal seemingly capable of adsorbing NO2−.

Part of these findings resulted in a peer-reviewed research paper in ChemElectroChem (doi: 10.1002/celc.201500166) while other results concerning objective 2 will be the subject of a second publication focussed on the mechanistic aspects of the reduction of NO2− studied with ATR-IR spectroelectrochemistry (manuscript in preparation). All of these results secured a travel grant by the Faraday Division of the Royal Society of Chemistry, which financed the fellow’s participation to the 66th annual meeting of the International Society of Electrochemistry.

Research on objective 3 (cascade catalysis of the reduction of CO2) confirmed that this reaction poses significant challenges. However, we were able to immobilise Cu particles and CODH successfully on carbon black, and co-deposit these two catalysts on a graphite electrode. CODH showed the highest catalytic activity in a pH 6 organic buffer. The formation of CO by the enzyme was confirmed by product analysis with GC.

Finally, a brief mention of the main achievement concerning outreach activities which were an integral part of the TEAMCAT project. The fellow was selected as one of the five finalists of the 2016 Science Communication competition of the Royal Society of Chemistry.

Results and their potential use/impact
The main impact of TEAMCAT is conceptual: the demonstration of the feasibility of an alternative approach to tackle sluggish electrochemical reaction. Two catalysts, despite being as different as enzymes and metal particles, can be paired and work in tandem. This idea could be extended to several other reactions.
From a more practical point of view, NarGHI is a relatively robust enzyme that does not deactivate in the presence of oxygen, and therefore it seems particularly suited for applications. Small-scale pilot reactors could be developed, testing process conditions for the reduction of NO3−.

The stage reached by the research activities at the end of the project lays the foundations for future fundamental studies. For instance, these could focus on the kinetics of cascade catalysis: NarGHI produces NO2− much faster than metals can further reduce it, and this calls for a detailed kinetic analysis of the two processes.