The Catalyst Review December 2019 - 17

EXPERIMENTAL
NC (Figure 2e). However, for defective NiN4-C, the
reaction activity is significantly improved. NiN4-r6-1
and NiN4-r5 possess the lower free energy with
-1.52 and -1.21 eV, indicating the outstanding
methane oxidation ability (Figure 2e). Density
of state (DOS) calculations shows that defective
NiN4-C combining with O* intermediate has
more transfer electrons around the Fermi level in
comparison with the pure NiN4 construction (Figure
2d). This result demonstrates that the surface
digging effect can activate the substrate effectively.
At the same time, the product of CH3OOH can also
be reduced with defects engineering. These results
reinforce that carbon defects adjacent to the NiN4
center induced by the digging effect can alter the
electron density distribution of active metal sites
and greatly improve the catalytic activity, agreeing
well with the experimental results. Source: Zhou H,
Liu T, Zhao X, et al. Angew. Chem. Int. Ed. 10.1002/
anie.201912785.

Figure 2. (a, b) The yield and the selectively of methane oxidation for the prepared catalysts.
The sample labeled as B1 and B2 is blank comparison. 1 to 5 refers to CNT@NC, CNT@NC@Ni
NPs, CNT@PNC/Ni SAs (0.31 wt% Ni), CNT@PNC/Ni SAs (0.68 wt% Ni), and CNT@PNC@Ni NPs/
SAs (1.07 wt% Ni), respectively. (c) Six possible atomic configurations. NC and V refer to N-doped
carbon matrix and C atoms defect; r5 and r6 refer to the five- or sixmembered Ni−N heterocyclic
ring; 1 and 2 refer to the different locations of the C atoms defect, respectively. (d) DOS of NiN4,
NiN4+O, NiN4(V) and NiN4(V+O). NiN4-r5 was adopted as the defective NiN4-C model. (e) The free
energy diagram of different configurations reaction pathway. Plat-O means that the active center
(such as NC, NC(V), NiN4, NiN4-r5) combines with one O*.

Plasma-Enhanced Catalytic Synthesis of Ammonia over a Ni/Al2O3 Catalyst at Near-Room
Temperature: Insights into the Importance of the Catalyst Surface on the Reaction Mechanism...
Efforts to find alternatives to the thermal catalytic production of ammonia (Haber-Bosch process) have focused on biochemical,
electrochemical, and plasma-assisted pathways. Of these, nonthermal plasma (NTP) is particularly attractive because it generates
highly energetic electrons and reactive species (e.g., radicals, excited atoms, molecules, and ions) that can significantly enhance
reaction kinetics and enable thermodynamically unfavorable reactions to proceed under ambient conditions (e.g., dissociation of
N2). Combining NTP's with heterogeneous catalysis (plasma-catalysis) has been shown to demonstrably improve the performance
of many plasma-activated reactions, including ammonia synthesis resulting from the synergy generated by the interaction between
the plasma and the catalyst. Herein, the authors describe a plasma-enhanced catalytic process for the synthesis of ammonia directly
from N2 and H2 at near-room temperature (-35 °C) and ambient pressure using transition metal catalysts (M/Al2O3, M= Fe, Ni, Cu) in a
specially designed temperature-controlled dielectric barrier discharge (DBD) reactor that uses water as a ground electrode (Scheme
1).
Compared to plasma synthesis of
Scheme 1. Schematic diagram of the experimental setup.
Scheme 2. Proposed mechanism for the enhancement of ammonia synthesis by Ni/Al2O3 catalyst.
NH3 without a catalyst, plasmacatalysis significantly enhanced
the NH3 synthesis rate and energy
efficiency, which increased with
different catalysts in the order: Ni/
Al2O3 > Cu/Al2O3 > Fe/ Al2O3 > Al2O3.
All the catalysts provided stable
performances for at least 6 h., and
Ni/Al2O3 maintained an efficient
performance when recycled five
times. The highest NH3 synthesis
rate of 471 μmol g−1 h−1 was achieved with Ni/+Al2O3, which was 100% higher than that of plasma only. The authors went on to
investigate the synergetic effects of plasma-catalytic synthesis of ammonia at near-room temperature and atmospheric pressure
via a series of characterization methods. They found that the weak acid sites of the Al2O3 support can influence the performance
of M/Al2O3 catalysts in the plasma-catalyzed synthesis of NH3. Ni/Al2O3 produced a more uniform plasma discharge than Al2O3 or
plasma only that enhanced both the gas-phase radical reactions of N, H, and NH in the plasma and the reactions on the surface of
the catalyst. The surface acidity of Ni/Al2O3 was also altered during the reaction, reducing the number of medium and strong acid
sites, which may also improve the rate of ammonia synthesis in situ. Furthermore, the synergy is also reflected in the order of the
M-N bond strength (Ni-N < Cu-N < Fe-N); the weakest M-N bonds are expected to inhibit ammonia decomposition and enhance
the turnover frequency for ammonia synthesis by reducing the energy required to dissociate ammonia from the metal sites on the
catalyst surface (Scheme 2). Source: Wang Y, Craven M, Yu X, et al. (2019). ACS Catal., 9: 10780−10793.
The Catalyst Review

December 2019

The Catalyst Review December 2019

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