Reduced Energy Cost of Ammonia Synthesis Via RF Plasma Pulsing

Abstract

We have performed an extensive investigation of the parameters affecting the degree of nitrogen fixation and its energy cost in a low-pressure, radio frequency-driven plasma reactor. We find that the hydrogen-to-nitrogen ratio has a strong effect on both the yield and energy cost. A significant degree of nitrogen fixation (∼8%) can be achieved under hydrogen-rich mixtures, but unfortunately these conditions require a higher energy input compared to a one-to-one hydrogen-to-nitrogen ratio. The high degree of nitrogen fixation largely exceeds the degree of nitrogen dissociation in the plasma, suggesting that, while nitrogen dissociation in the plasma likely contributes to the ammonia yield, it cannot account for all of it alone. This confirms the importance of vibrational excitation in this class of processes, as it has been proposed by other research groups. In addition, our study indicates that hydrogen dissociation in the plasma is crucial for stabilizing reaction intermediates. Large-scale ab initio molecular dynamics simulations show that chemisorbed nitrogen is remarkably stable when hydrogen is present at the surface of a metal such as copper, which is otherwise a poor choice of catalyst for a heat-driven catalytic process. Finally, we have found that a pulsed-plasma regime provides a significant improvement in both yield and energy cost. The plasma not only activates surface catalytic processes but also dissociates the produced ammonia when operated in continuous mode. Under a pulsed mode, the plasma is extinguished before ammonia diffuses back into it from the catalyst surface, avoiding the waste of electrical input energy into redissociation of the desired reaction product.