Incorporation of a Metal Catalyst for the Ammonia Synthesis in a Ferroelectric Packed-Bed Plasma Reactor: Does It Really Matter?

Abstract

Plasma-catalysis has been proposed as a potential alternative for the synthesis of ammonia. Studies in this area focus on the reaction mechanisms and the apparent synergy existing between processes occurring in the plasma phase and on the surface of the catalytic material. In the present study, we approach this problem using a parallel-plate packed-bed reactor with the gap between the electrodes filled with pellets of lead zirconate titanate (PZT), with this ferroelectric material modified with a coating layer of alumina (i.e., Al₂O₃/PZT) and the same alumina layer incorporating ruthenium nanoparticles (i.e., Ru-Al₂O₃/PZT). At ambient temperature, the electrical behavior of the ferroelectric packed-bed reactor differed for these three types of barriers, with the plasma current reaching a maximum when using Ru-Al₂O₃/PZT pellets. A systematic analysis of the reaction yield and energy efficiency for the ammonia synthesis reaction, at ambient temperature and at 190 °C and various electrical operating conditions, has demonstrated that the yield and the energy efficiency for the ammonia synthesis do not significantly improve when including ruthenium particles, even at temperatures at which an incipient catalytic activity could be inferred. Besides disregarding a net plasma-catalysis effect, reaction results highlight the positive role of the ferroelectric PZT as moderator of the discharge, that of Ru particles as plasma hot points, and that of the Al₂O₃ coating as a plasma cooling dielectric layer.

Figure 1

(a) Schematic of PZT, Al₂O₃/PZT, and Ru-Al₂O₃/PZT pellets. (b) Arrangements of the sets of pellets for the used barrier configurations. Ground and HV (high voltage) symbols refer to the electrical connections of bottom and top electrodes.

Figure 2

Analysis of the electric field distribution in the packed-bed barrier by means of Comsol Multiphysics simulations. (a) Distribution of the electric field between two equally separated pellets of PZT (top) and a PZT and a Al₂O₃/PZT pellet (bottom). The “r” line indicates the straight line selected to evaluate the electric field. (b) Electric field distribution along the line “r” for the PZT and Al₂O₃/PZT configurations.

Figure 3

(a) Sketch and (b) SEM micrographs of a Ru-Al₂O₃ coated PZT pellet. The dots in sketch (a) refer to Ru aggregates and the fact that it is distributed in the interior of the alumina coating layer. (c) SEM micrograph and (d) EDX analysis in the form of Al (red) and Ru (green) maps of the Al₂O₃-supported Ru catalyst powder.

Figure 4

Electrical characterization of the reactor for the three barrier configurations (PZT, Al₂O₃/PZT, and Ru-Al₂O₃/PZT). I(t) curves during plasma ignition at (a) ambient temperature (2.5 kV, 5 kHz) and (b) 190 °C (2.5 kV, 2 kHz). (c) - (d) Lissajous plots determined from the curves in (a) and (b), respectively.

Figure 5

Evolution at ambient temperature of (a) the reaction yield and (b) the energy efficiency for the NH₃ synthesis reaction as a function of the applied voltage. Data corresponding to low reaction yields are plotted with empty dots and dash lines. Experiments were carried out at a frequency of 5 kHz. The star dot corresponds to a 7% nitrogen conversion in experiments reported in ref (31).

Figure 6

Evolution of (a) reaction yield and (b) energy efficiency for the ammonia synthesis reaction as a function of frequency. For the experiments at 1 kHz, results are plotted with empty dots and dash lines, indicating a possible inaccuracy in the determination of these values. Experiments were carried out at ambient temperature, a voltage amplitude of 2.5 kV, and a variable frequency between 1 and 5 kHz.

Figure 7

Evolution of (a) reaction yield and (b) energy efficiency for the ammonia synthesis reaction as a function of frequency. Experiments were carried out at 190 °C for a voltage amplitude of 2.5 kV and variable frequencies between 1 and 3 kHz. For comparison, data obtained under the same operating conditions at ambient temperature are included in the plot as dashed lines and faint colors.

Figure 8

OES spectra recorded at (a) ambient temperature (2.5 kV, 5 kHz) and (b) 190 °C (2.5 kV, 2 kHz).