Periodic Reporting for period 2 - NITROGEN-LIGHT (Photo(electro)catalytic Nitrogen Fixation)
Reporting period: 2023-01-01 to 2023-12-31
From the start, the project aimed to develop an integrated photo(electro)lyzer capable of oxidizing water to oxygen and reducing nitrogen to ammonia (see image below). Often called the "holy grail" of chemistry, this approach offers a carbon-free energy pathway. Ammonia is a key fertilizer and a potential fuel cell energy carrier, though the latter is still under development. With the urgent need to reduce our carbon footprint and move beyond fossil fuels, the project’s focus on solar energy is especially relevant. Solar power offers over 1,000 times the potential of other renewables, making it a highly promising energy source for long-term innovation.
Our goal is to create a proof-of-concept device that mimics photosynthesis. Though still in early stages, developing a dual-compartment system at the microscale marks a critical step forward. As a result, we began early-stage reactor integration. The water oxidation unit was designed for easy coupling with the nitrogen fixation compartment, which is the first example of using light to overcome nitrogen’s kinetic barrier—an exciting innovation in the field.
Our goal is to create a proof-of-concept device that mimics photosynthesis. Though still in early stages, developing a dual-compartment system at the microscale marks a critical step forward. As a result, we began early-stage reactor integration. The water oxidation unit was designed for easy coupling with the nitrogen fixation compartment, which is the first example of using light to overcome nitrogen’s kinetic barrier—an exciting innovation in the field.
As mentioned above, the overall goal was to develop a combined photo(electro)lyzer able to oxidize water to oxygen and reduce nitrogen to ammonia. The oxidation part has been focused on the development of a supramolecular system which self assemble through weak interactions based on a photosensitizer able to harvest light and on a catalyst able to split water into oxygen. The ability of this system (made of perylene molecules surrounding a Ruthenium catalyst) to split water was demonstrated in 2018, with a paper published by Bonchio et al.On the other hand this structure is disorganized and self-assemble without a particular order, giving rise to potential non-radiative decay pathways that might be a limitation factor for a performative catalysis. Taking into account this argument, we decided to make a more ordered structure. In particular, we decided to lock the molecules of light harvesters in an ordered structure that can be tuned by changing the number of units efficiently. Another fair point of my research was the study of less demanding oxidative reactions that can be coupled to the reduction reaction, that is the center of our study.In particular we decided to focus on acid bromide splitting, which products are very useful storage sources to employ in fuel cells. This system is based, similarly, on a supramolecular aggregate made of several organic molecules (perylenes) packed through weak bonds that can be tuned if needed. The work has been submitted and accepted and it is going to be published soon. At the same time the reductive part was developed, taking into account the demanding kinetic associated to the process. Indeed, reduction of nitrogen to ammonia is a 6electron 6proton reaction. While is a thermodynamic favored process, from the point of view of the kinetic is problematic and requires a good catalyst. A very straightforward way to overcome the barrier is the concerted transfer of protons and electrons to nitrogen through the mean of a catalyst. This process can be very feasible if coupled with the power of sunlight. One good example of this is a paper published by Peters et al in 2022. In this particular study it was ruled in the possibility to run nitrogen reduction through the employment of a small organic molecule that in principle was only employed as electrons source for photocatalysts (namely, sacrificial electron donor). We started from that paper to study the mechanism behind the catalysis. The conclusion of the work points towards the fact that the small organic molecule is the responsible of the efficient catalysis. The paper related has been submitted and it will be published soon. At the same time, in the same directions, three different routes have been explored in parallel. All the three routes point towards the direction of using a light that is green shifted. First of all, ruthenium/Iridium metal based light harvesters were coupled with a Tungsten based catalyst for nitrogen reduction. At the same time two other routes were explored photo(electro)chemically, being the light absorbing source in the two cases respectively a Molybdenum based system which has excited state properties that favor a long lived state; and a green light absorbing organic molecule, which was firstly employed in oxygen evolution (namely perylene), with a non reducing excited state that can be tuned by the access to a dianionic state
Steps beyond the known state of the art were done in both oxidative and reductive reactions (respectively water or acid bromide oxidation and nitrogen reduction). In particular, concerning water oxidation, a new supramolecular aggregate mimicking the quantasome observed in nature was developed. For the best of my knowledge this is the only example in literature of an efficient system to copy the mechanism under which nature operates. At the same time, the light harvesters of the above mentioned proposed structure can be exploited for less demanding oxidative reactions like as acid bromide splitting. In particular, the perylene based aggregates precipitated on an optimized electrode reached efficiencies for acid splitting that are overcoming previously literature results. Thinking about the reductive process the steps beyond the state of the art are even more interesting. It is clear that the ability of a small organic molecule to drive such a demanding reaction is a breakthrough in the state of the art. Also, the mechanism involved in the reaction is of high importance as set the basis for the possibility to find substrates able to drive the same reactions but employing non-blue light and reaching a higher efficiency. In this direction lies the last part of my research point. In particular, the possibility to produce ammonia with the green component of sunlight is underexplored. Green light is known to be less harmful and is easier to manipulate on a larger scale if we think about industrial development.In the first approach ruthenium and Iridium photocatalysts were coupled with a Tungsten based catalyst. At the same time, the second and the third approaches focused on an electro-catalysed reaction coupled with light, employing both a Molybdenum photocatalyst and a perylene one. What is very interesting is the exploitation of perylene based dyes in the reductive side and in the oxidative side at the same time.This can be of great interest thinking about the possibility of engineering the final device to run the net reaction. The employment of the same species to run both the reactions is very useful if we think about the optimization of the costs and the optimization of the device, that is the final purpose of the project.The impact of the prototype from the point of view of the society, is going to be huge, being the first prototype able to exploit sunlight to run the net nitrogen and water to oxygen and ammonia reaction. Despite the prototype is going to be a proof of concept, this is the first known step towards the possibility to engineer a device on a macroscale.