The Catalyst Review April 2020 - 16

EXPERIMENTAL
Figure 2. a) Schematic of the assembled fuel cell used for PEMFC. b) LSV curves of FeN4/HOPC-c-1000 and FeN4/C in O2-saturated 0.5 m H2SO4 solution with a scan
rate of 10 mVs-1. c, d) Polarization (left, filled symbols) and power density (right, open symbols) plots of PEMFC with FeN4/HOPC-c-1000 and FeN4/C as cathode
catalysts in 1 bar H2-O2 and (c) 1 bar H2-air (d). Test conditions: 800C, 100% RH. e) Fuel cell durability tests at constant potentials of 0.70 and 0.55 V, respectively.
Test conditions: 800C, 100% RH, 1 bar H2-air.

A comparison of samples pyrolyzed at different temperatures revealed that the Fe-N-C structure with high intrinsic ORR activity
could be formed only when the annealing temperature is above 8000C. A higher annealing temperature also enhances the catalytic
activity by improving the chemical stability and electron conductivity. Also, it was demonstrated that the ORR catalytic activity could
be maximized by optimizing the Fe doping level so that a maximum concentration of atomic Fe sites can be achieved in the carbon
matrix without the formation of metal clusters. Source: Qiao M, Wang Y, Wang Q, et al. et al. (2020). Angew. Chem. Int. Ed., 59:
2688-2694.
Enhancement of the Yield of Ammonia by Hydrogen-Sink Effect during Plasma Catalysis...

The iron-catalyzed Haber-Bosch
Figure 1. Proposed reaction pathway for ammonia
Table 1. Summary of results for various metals used
synthesis via both the metal catalyst categories.
process is the most widely
for plasma-catalytic ammonia synthesis. Tm=Melting
temperature.
employed method for largescale, centralized, and continuous
production of ammonia. However,
emerging alternatives such as
plasma-assisted synthesis, which
can operate at mild conditions and
through the intermittent supply
of renewable energy sources, are
currently considered promising
options for decentralized smallscale ammonia production. For
example, energy yields as high as
35 g-NH3/kWh have been using a
pulsed AC atmospheric pressure
dielectric barrier discharge (DBD)
reactor. Herein, the authors seek
to further the understanding of
catalyzed ammonia formation by
studying the reaction under a subatmospheric pressure RF plasmaconditions which focus on the role of atomic plasma species, which can be challenging to isolate under DBD conditions.
Various transition metals, low-melting-point metals, and gallium-rich alloy catalysts were evaluated with respect to their activity
towards ammonia production in a plasma environment. Table 1 summarizes the performance of the pure metals for plasma-assisted
catalysis for the current study, along with DFT calculated binding energies for N and H on the surface and in the bulk of each metal
as a measure of the tendency of each catalyst to adsorb and dissolve atomic N and H initially formed in the plasma phase. The best
three pure metal catalysts were Ni, Sn, and Au, which afforded ammonia yields of 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
used metals were classified into two categories. Category I metals (Cu, Ag, Au, and Fe), which are nitrophobic and poor hydrogen
sinks. For these metals, the measured concentration of the Hα spectral line 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.

The Catalyst Review

April 2020

The Catalyst Review April 2020

Table of Contents for the Digital Edition of The Catalyst Review April 2020