Ammonia Synthesis on Wool-Like Au, Pt, Pd, Ag, or Cu Electrode Catalysts in Nonthermal Atmospheric-Pressure Plasma of N₂ and H₂

Abstract

Developing an ammonia synthesis process from N2 and H2 is of interest in the catalysis and hydrogen research community. Wool-like metal electrodes used to produce nonthermal plasma were determined to serve as efficient catalysts for ammonia synthesis under atmospheric pressure without heating. The catalytic activity of Pt, Pd, Ag, Cu, and Ni wools increased as the experiment was repeated, while that of Au, Fe, Mo, Ti, W, and Al was almost constant. The activity change was mainly due to migration of metals from the electrode to the inner wall of a silica reactor or increases in surface areas of metal catalysts. The order of the activity at each initial experiment was Au > Pt > Pd > Ag > Cu > Fe > Mo > Ni > W > Ti > Al. DFT calculations using Gaussian 09 and CASTEP were applied for energy changes in a reaction M3 + 1/2 N2 → M3N and in adsorption of a nitrogen atom on metal surface, in which M3 was a virtual minimum unit of the metal surface. The reactions were assumed to be an essential step in the ammonia production after plasma-activation of N2. The resulting values correlated with the respective initial catalytic activity, indicating that a more unstable M3N surface intermediate produced higher catalytic activity. Emission spectra in the plasma process using various electrodes were measured and showed that the efficiency of electrodes for plasma activation of nitrogen molecules was almost independent of the metals, while the reactivity of the activated species to form ammonia depended greatly on the metal used. The N2/H2 ratio dependence and formation/decomposition rate constants of ammonia were finally determined on Au and Cu, which were different from those for the conventional Haber–Bosch process. The decomposition of produced ammonia was suggested to proceed in a plasma-irradiated gas phase.

Enhancement of the Yield of Ammonia by Hydrogen‐Sink Effect during Plasma Catalysis

“…The plot is represented as Figure . The volcano plot is a function of the activation energies for these reactions whereas the one proposed for plasma catalysis at atmospheric pressure plasmas are a function of the nitrogen binding energy …”

“…Several hypotheses have been proposed in relation to ammonia formation mechanisms in a plasma‐catalyst system, suggesting key differences with thermal catalysis and providing clues on what factors could be key to optimize the catalyst under plasma conditions. For example, Schneider and coworkers postulated in their work with atmospheric DBD plasma and supported transition metals that the kinetically relevant nitrogen source was vibrationally excited N 2 .…”

“…For instance, many experiments with plasmas have focused on testing transition metals as catalysts, especially iron, (Tables S1-S3) that work well under the Langmuir-Hinshelwood (LH) mechanism characteristic of the Haber-Bosch process, but might not fully exploit other pathways relevant to plasma conditions. Several hypotheses have been proposed in relation to ammonia formation mechanisms in a plasma-catalyst system, [21][22][23][24][25][26] suggesting key differences with thermal catalysis and providing clues on what factors could be key to optimize the catalyst under plasma conditions. For example, Schneider and coworkers postulated in their work with atmospheric DBD plasma and supported transition metals that the kinetically relevant nitrogen source was vibrationally excited N 2 .…”

Atomic Modulation, Structural Design, and Systematic Optimization for Efficient Electrochemical Nitrogen Reduction

“…Both of these binding interactions are unfavorable for a smooth NRR because of the difficulty in the subsequent hydrogenation processes. The transition metals that exhibit suitable dinitrogen binding strength, including Ru, Au, Fe, Rh, Mo, Sc, Ti, Y, and Zr, have been summarized in previous reports . A summary of the adsorption properties of nitrogen as well as hydrogen on various metals is described in Figure e,f.…”

Sustainable Non‐Thermal Plasma‐Assisted Nitrogen Fixation—Synergistic Catalysis

“…[76][77][78] In addition to exploring the effects of the above-mentionedc atalysts, the authors also discussed the mechanisms of the fundamentaln onthermalp lasma-assisted ammonia synthesis based on ap roposal by Carrasco et al, [79] who stated that the plasma-assisted ammonia synthesis under vacuum occurs mainly through the ER and Langmuir-Hinshelwood (LH) mechanisms (they described the interactions between surface-adsorbed and gasphase speciesa nd the interactions betweent wo surface-adsorbed species, respectively). [77] Iwamoto et al [80] and Aihara et al [81] also compared the catalytic activities of different single-metal catalysts (Fe, Cu, Pd, Ag, etc.) [77] Iwamoto et al [80] and Aihara et al [81] also compared the catalytic activities of different single-metal catalysts (Fe, Cu, Pd, Ag, etc.)…”