Sustainable Ammonia Synthesis Enabled by Plasmonic Bimetallic Catalysts

Kernkonzepte

Plasmonic bimetallic AuRu alloy catalysts can facilitate ammonia synthesis at room temperature and pressure by expediting the hydrogenation of adsorbed nitrogen species via plasmon-mediated hot electrons.

Zusammenfassung

The authors report the use of gold-ruthenium (AuRu) plasmonic bimetallic alloys for ammonia synthesis at room temperature and pressure, driven solely by visible light illumination. They synthesize AuRu alloys with different molar ratios (AuRu0.1, AuRu0.2, AuRu0.3) and disperse them on MgO supports for gas-phase ammonia synthesis.

The key findings are:

1. The AuRu0.2 sample exhibits the highest reactivity and quantum efficiency (~ 60 μmol/g/h and ~0.12% under 100 mW/cm2 visible light), optimizing both optical absorption and catalytic site density.

2. The light-driven reactions achieve higher reactivity than the thermal-driven reactions at lower temperatures, suggesting non-thermal, plasmon-mediated hot carrier effects.

3. In-situ DRIFTS measurements show the kinetics of hydrogenation of nitrogen adsorbates is accelerated under light illumination compared to thermocatalysis. The NH3* adsorbate densities increase linearly with illumination power, whereas heating alone results in a non-monotonic change.

4. Quantum mechanical calculations reveal that the hydrogen-assisted splitting of N2 in the excited state is the key mechanism for the reaction activated at ambient conditions. Light alone or H2 alone cannot achieve N2 dissociation, but their combination can accelerate this key bottleneck step.

The authors conclude that plasmons improve the kinetics of ammonia synthesis via hot-electron-controlled hydrogenation and desorption of intermediate nitrogen species, providing guidance for the design of efficient plasmonic catalysts for sustainable ammonia production.

Statistiken

"Ammonia synthesis consumes more than half of the annual industrial hydrogen and contributes up to ~3% of global greenhouse gas emissions."
"The AuRu0.2 sample exhibits a ~60 μmol/g/h reactivity and ~0.12% external quantum efficiency under 100 mW/cm2 visible light illumination."

Zitate

"Light energy can be converted into chemical energy through LSPR decay via generation of non-equilibrium hot carriers, strong electric field, and the photothermal effect."
"Fully quantum mechanical calculations reveal that the hydrogen-assisted splitting of N2 in the excited state is the key mechanism for the reaction activated at ambient conditions."

Wichtige Erkenntnisse aus

Atmospheric Pressure Ammonia Synthesis on AuRu Catalysts Enabled by Plasmon-Controlled Hydrogenation and Nitrogen-species Desorption

by Lin Yuan, Br... um arxiv.org 10-03-2024

https://arxiv.org/pdf/2410.01300.pdf

Tiefere Fragen

How can the selectivity and efficiency of the plasmonic bimetallic catalysts be further improved for practical ammonia synthesis applications?

To enhance the selectivity and efficiency of plasmonic bimetallic catalysts for ammonia synthesis, several strategies can be employed:

Optimization of Alloy Composition: The study indicates that the AuRu0.2 alloy exhibited the highest external quantum efficiency (EQE) and reactivity. Further optimization of the molar ratios of Au and Ru could lead to even better performance. Exploring a wider range of compositions and conducting systematic studies on their catalytic properties can help identify the optimal balance between light harvesting and catalytic activity.

Surface Engineering: Modifying the surface properties of the catalysts, such as increasing the surface area or creating specific active sites, can enhance the interaction between the catalyst and reactants. Techniques like nanostructuring or creating hierarchical porous structures can improve the accessibility of reactants to active sites.

Support Material Selection: The choice of support material, such as MgO, plays a crucial role in the performance of the catalyst. Investigating alternative supports that can enhance the stability and dispersion of the bimetallic nanoparticles may lead to improved catalytic performance. Supports that can also facilitate charge transfer or provide additional active sites could be particularly beneficial.

Light Management: Enhancing the light absorption properties of the catalysts through the incorporation of plasmonic materials or photonic structures can increase the generation of hot carriers. This can be achieved by optimizing the size and shape of the nanoparticles or by using photonic crystals to concentrate light at the catalyst surface.

Temperature and Pressure Optimization: While the study demonstrates ammonia synthesis at room temperature and pressure, exploring the effects of varying these parameters could lead to improved reaction rates and selectivity. Understanding the thermal and non-thermal contributions to the reaction can help in fine-tuning the operational conditions.

Integration with Other Catalytic Processes: Combining the plasmonic bimetallic catalysts with other catalytic systems, such as those that can provide additional hydrogen or facilitate nitrogen activation, could enhance overall efficiency. This integrated approach could leverage the strengths of different catalytic mechanisms.

By implementing these strategies, the selectivity and efficiency of plasmonic bimetallic catalysts for ammonia synthesis can be significantly improved, making them more viable for practical applications.

What other chemical transformations beyond ammonia synthesis could benefit from the hot-carrier-driven mechanisms enabled by plasmonic bimetallic catalysts?

The hot-carrier-driven mechanisms enabled by plasmonic bimetallic catalysts have the potential to enhance a variety of chemical transformations beyond ammonia synthesis. Some notable examples include:

Hydrogenation Reactions: The ability of hot carriers to facilitate bond activation can be applied to various hydrogenation reactions, such as the hydrogenation of alkenes, alkynes, and carbonyl compounds. This can lead to more efficient and selective production of valuable chemicals and fuels.

CO2 Reduction: Plasmonic catalysts can be utilized for the photoreduction of carbon dioxide into hydrocarbons or alcohols. The hot carriers generated from plasmonic excitation can enhance the activation of CO2, making the conversion process more efficient and potentially leading to the development of sustainable carbon capture and utilization technologies.

Water Splitting: The generation of hot carriers can also be harnessed for photocatalytic water splitting to produce hydrogen. By optimizing the plasmonic properties of the catalysts, the efficiency of water splitting can be improved, contributing to the development of clean hydrogen production methods.

Organic Synthesis: The unique properties of plasmonic catalysts can be leveraged in organic synthesis, particularly in reactions that require the activation of strong bonds, such as C–C coupling reactions. The hot carriers can facilitate these transformations under milder conditions, potentially reducing energy consumption and improving selectivity.

Nitrogen Fixation: Beyond ammonia synthesis, the hot-carrier-driven mechanisms could be explored for other nitrogen transformations, such as the production of nitrogen-containing organic compounds. This could have implications for the synthesis of pharmaceuticals and agrochemicals.

Dehydrogenation Reactions: The ability to activate hydrogen molecules can be beneficial in dehydrogenation processes, which are critical in the production of olefins and other valuable intermediates in the petrochemical industry.

By expanding the application of plasmonic bimetallic catalysts to these transformations, the potential for sustainable and efficient chemical processes can be significantly enhanced.

What insights from the proposed plasmon-mediated reaction pathways could be leveraged to understand the catalytic mechanisms of natural nitrogen-fixing enzymes like nitrogenase?

The proposed plasmon-mediated reaction pathways provide several insights that can be leveraged to understand the catalytic mechanisms of natural nitrogen-fixing enzymes like nitrogenase:

Hot Carrier Dynamics: The role of hot carriers in facilitating bond activation and reaction kinetics in plasmonic catalysis parallels the mechanisms employed by nitrogenase. Understanding how these hot carriers interact with nitrogen species can shed light on the energy landscape and transition states involved in nitrogen fixation by nitrogenase.

Hydrogen-Assisted Mechanisms: The findings that hydrogen-assisted pathways can lower activation barriers for nitrogen dissociation are relevant to nitrogenase, which also utilizes a complex mechanism involving hydrogen. Insights into how plasmonic catalysts enhance hydrogenation can inform our understanding of how nitrogenase achieves efficient nitrogen reduction under mild conditions.

Intermediate Species Stabilization: The study highlights the importance of intermediate species, such as NHx*, in the ammonia synthesis process. Similarly, nitrogenase stabilizes various nitrogen intermediates during the reduction process. Investigating how plasmonic catalysts influence the stability and reactivity of these intermediates can provide parallels to the behavior of nitrogenase.

Environmental Conditions: The ability of plasmonic catalysts to operate effectively at ambient conditions can inform the design of synthetic systems that mimic nitrogenase. Understanding how environmental factors influence the performance of plasmonic catalysts can lead to insights into optimizing conditions for nitrogenase-like activity.

Mechanistic Pathways: The detailed mechanistic pathways elucidated through quantum mechanical simulations in plasmonic catalysis can serve as a framework for exploring the reaction mechanisms of nitrogenase. By comparing the energy barriers and reaction pathways, researchers can identify similarities and differences that may reveal new aspects of nitrogenase function.

Catalyst Design Principles: The insights gained from optimizing plasmonic catalysts can inform the design of biomimetic catalysts that aim to replicate the efficiency and selectivity of nitrogenase. Understanding the balance between light harvesting and catalytic activity can guide the development of new materials for nitrogen fixation.

By leveraging these insights, researchers can deepen their understanding of nitrogenase and potentially develop new synthetic catalysts that mimic its function, contributing to advancements in sustainable nitrogen fixation technologies.