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

Abstract

Plasma‐catalytic ammonia synthesis has been known since the early 1900s, but only until now efforts to optimize catalysts for this purpose are emerging. Here, we investigate various transition metals, low‐melting‐point metals, and gallium‐rich alloy catalysts for their activity towards ammonia production under a plasma environment. The best three pure metal catalysts were Ni, Sn and Au, which are not traditional catalysts for the current industrial ammonia synthesis. The ammonia yields for these catalysts were 34 %, 29 %, and 19 %, respectively. Synergistic effects were detected when employing alloys, as some alloys presented ∼25–50 % higher yields than their constituent metals. The employed metals were classified into two categories. Category I metals (Cu, Ag, Au and Fe), which are nitrophobic (excluding Fe, the Haber‐Bosch catalyst) and poor hydrogen sinks. For these metals, the measured concentration of Hα in the gas phase tended to correlate inversely with ammonia yield and directly with the H binding strength on the catalyst surface. Category II metals (Ga, In, Sn and Ni), which are good hydrogen sinks, tend to have a lower concentration of Hα in the gas phase than that of category I metals, which is consistent with their expected sink behavior. For these metals, the concentration of Hα correlates with ammonia yield. Plasma characterization experiments and DFT calculations suggest that the higher performance of Ni and Sn is related to the benefit of dissolving hydrogen to slow down H recombination, which is a feature that could be potentially optimized in future studies by rationally altering the catalyst composition.

Al2O3-Supported Transition Metals for Plasma-Catalytic NH3 Synthesis in a DBD Plasma: Metal Activity and Insights into Mechanisms

“…In any case, the difference between the results obtained with the 3:1, 1:1, and 1:3 H 2 :N 2 ratios in our work is very small when put on a logarithmic scale. Besides the two works by Wang et al [38] and Mehta et al [42] describing the performance of different catalytic metals in DBD reactors, we also added a dataset obtained by Shah et al in a low-pressure radiofrequency plasma [63], which interestingly exhibited the same properties (i.e., the absence of the "volcano" trend), thus indirectly suggesting that the implications of our model may reach beyond atmospheric pressure plasma conditions.…”

Plasma-Catalytic Ammonia Synthesis in a DBD Plasma: Role of Microdischarges and Their Afterglows

“…H atom recombination is also reported by Shah et al to be more significant due to the surface in comparison with gas-phase reactions. 34 Looking further to the NH 3 formation, the NH 3 precursors are formed according to the same reactions during both the microdischarges and afterglows, that is, a combination of ER and LH reactions (cf. Figures 7 and 8).…”

Energetics of pathways enabled by experimentally detected radicals during catalytic, plasma-assisted NH3 synthesis

“…Again, these calculations were motivated by the hydrogen-sink effect postulated to aid catalyst performance by removing H* from the surface, hence hindering recombination to H2(g) and boosting H* availability for reaction pathways that lead to NH3. 20 We denoted subsurface Figure 7. BEP relationship between activation and reaction energies for H dissolution reaction (r1).…”

“…ER activation barriers. In our previous work 20 , we assumed a scaling relationship proposed by Bird et al 46 to estimate barriers for ER reactions involving molecular species to hold for ER reactions involving radicals. On the other hand, Bogaerts and coworkers have recently assumed energy barriers for ER reactions involving radicals to be zero.…”

“…a) Current experiments: DBD reactor, T =125 C, P = 1 atm, N2:H2 ratio= 1, flow rate = 25 sccm, plasma power = 15W, b) DBD reactor, T = not reported (no heat exchange), P = 1 atm, N2:H2 ratio= 1, flow rate = 100 sccm, applied voltage = 5 kV, taken from ref. 26 , c) RF reactor, T = 400 C, P = 3.5 x 10 -4 atm, N2:H2 ratio= 0.25, flow rate = 20 sccm, plasma power = 300 W, taken from ref 20 . of NH3(g), N2H2(g) and N2H4(g), respectively.…”