Photocatalytic N2 reduction utilizing the upconverted hot electron

Ammonia (NH3) is among the most important molecules produced at an industrial scale due to its critical role for agriculture and other chemical industries. The well-known Haber-Bosch process currently used to manufacture NH3 requires high pressure and operating temperatures leaving a very large carbon footprint consuming over 1% of the total energy produced globally.

The project explores a photocatalytic alternative to the fossil fuel driven thermal Haber-Bosch process, potentially achieving drastic reductions in the carbon footprint and energy consumption. Although the photocatalytic approach derives energy from the sun, the solar utilization efficiency at the current level of technology is too low for commercial application.

The project thus investigates a novel catalyst design that potentially can boost the photocatalytic NH3 manufacturing efficiency significantly beyond the current state-of-the-art. The project is bolstered by educational and outreach activities targeting K-12 students, teachers, and undergraduate students.

Photocatalytic and electrocatalytic approaches are being explored for the conversion of N2 into NH3 to resolve the issues of the Harbor-Bosch process. However, because of the high reduction potential of N2, its highly stable triple bond, and weak surface adsorption affinity, the reduction of N2 to NH3 remains one of the most challenging photocatalytic reactions.

The project will develop a new photocatalytic approach to convert N2 to NH3 by utilizing hot electrons that are produced via an exciton-to-hot electron upconversion process in Mn-doped semiconductor quantum dots (QDs). This allows for the use of visible light to generate hot electrons that possess very high excess energy above the conduction band and exhibit long-range transfer capability.

These hot electrons have recently been shown to enhance photocatalytic H2 production as well as CO2 reduction, and are expected to (i) be of sufficiently high reduction potential for N2 to NH3 conversion and (ii) produce solvated electrons that can additionally participate in N2 to NH3 conversion. Specifically, the research will explore three different approaches with the goal of increasing the overall quantum efficiency of N2 to NH3 reduction significantly beyond the current state-of-the-art (~1%).

The first approach aims at enhancing the kinetics of the reduction of N2 and intermediate species by hot electrons and solvated electrons. This will be accomplished by employing binary solvent systems that greatly increase the concentration and stability of N2 and intermediate species. The second approach uses QD/molecular catalyst hybrid systems in which the long-range hot electron sensitization will be exploited to enable the use of molecular N2 reduction catalysts without requiring covalent attachments to the QDs.

The third approach aims at enhancing the rate of hot electron generation and the redox balance simultaneously by using indium tin oxide photonic crystals imbedded with QD photocatalysts leading to dual functionality of enhancing light absorption as well as hole transfer. In sum, the project aims to establish hot electron-driven visible light photocatalytic N2 reduction as a new approach that can bring much needed improvement in the photocatalytic N2 reduction efficiency.

Beyond the research focus, the project will integrate undergraduate education with research via the Texas A&M Innovation [X] program designed to foster interdisciplinary education through research activities solving real-world problems. In addition, the investigators will continue to be involved in the university-wide Chemistry Open House and nation-wide US Crystal Growing Competition outreach activities that bring K-12 students, teachers and the general public to lectures, tours and hands-on activities on STEM subjects.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.