The_Catalyst_Review_December_2023 - 11

SPECIAL FEATURE
By modulating the
catalyst composition,
structure, and active
sites, researchers
can tune the reaction
pathways and product
selectivity, enabling
the synthesis of
products ranging
from hydrocarbons to
oxygenates.
Electrochemical synthesis, utilizing the principles of electrocatalysis, has emerged
as a promising alternative, offering the potential to mitigate the environmental
impact associated with traditional methods. This method presents several
superior properties compared to the conventional Haber-Bosch method, which is
the predominant industrial process for ammonia production (Figure 3).
Considering the energy efficiency, the electrochemical synthesis of ammonia
allows for the direct conversion of nitrogen and hydrogen into ammonia using
electricity. The Haber-Bosch process operates at high temperatures (400-500°C)
and pressures (150-300 atmospheres), demanding substantial energy inputs for
maintaining these extreme conditions. In contrast, electrochemical methods
can occur under milder conditions, potentially reducing energy consumption.
Moreover, The electrochemical synthesis of ammonia can be integrated with
renewable energy sources, such as solar or wind power. This offers the potential
for sustainable and environmentally friendly ammonia production, whereas HaberBosch
process relies heavily on fossil fuels, contributing to carbon emissions.
Additionally, electrochemical methods offer better selectivity and control over
the reaction. By adjusting the applied voltage and tuning the electrocatalysts, it
is possible to enhance the selectivity of ammonia production and minimize the
formation of undesired byproducts. The Haber-Bosch process tends to produce a
mixture of gases, and additional separation and purification steps are required to
obtain pure ammonia. This can lead to increased energy consumption and costs.
Electrochemical ammonia synthesis can be adapted for decentralized production, allowing for smaller-scale and modular units. This
flexibility is advantageous for distributed ammonia production, especially in regions where centralized large-scale facilities are not
feasible. The Haber-Bosch process is typically conducted in large, centralized facilities, making it less adaptable to smaller-scale or
distributed production models.
Numerous catalysts have been investigated for electrochemical ammonia synthesis via Nitrogen Reduction Reaction (NRR), ranging
from transition metals and metal alloys to molecular complexes. Recent advancements in catalyst design have focused on tailoring
active sites, nanostructuring, and incorporating novel materials to improve catalytic performance. Exploring the catalytic landscape
is crucial for optimizing electrochemical cells and achieving sustainable ammonia synthesis.
NRR and HER are the cathodic reaction taking place during the electrochemical synthesis of NH3
reactions in acidic and basic medium are as follows:
In acidic electrolytes:
and they compete each other. The
In basic electrolytes:
Although both reactions are thermodynamically possible, HER is much more kinetically preferred compared to NRR. The reason is
the complex kinetic of electrochemical NRR which is a multi-step process that involves six electrons and six protons. Therefore, NRR
selectivity is usually very low resulting in high energy expenditure and lack of industrial competitiveness. The catalyst design plays an
important role for improving the selectivity for NRR.
The catalysts employed in electrochemical nitrogen reduction reaction (NRR) can be categorized into three groups: (1) precious metals
like Au, Ru, Rh, Pd, Pt, Ag, and Ir; (2) base metals encompassing Y, Sc, Ti, Zr, V, Cr, Nb, Mo, Fe, Co, Mn, Ni, Cu, W, Re, Sn, Sb, Bi, La,
Ce, and Dy; (3) non-metal elements, which include B, C, N, O, F, P, S, Se, and Te. These elements give rise to diverse catalysts for
electrochemical NRR, spanning metals, metal alloys, transition metal carbides, nitrides, oxides, and sulfides, as well as carbon-based
materials, black and red phosphorous, Mxene, covalent organic frameworks (COF), C3
N4
, among others. This classification delves
into noble metal catalysts, non-noble metal catalysts, and metal-free catalysts, offering insights into the evolving trends in catalyst
development for electrochemical NRR (Zhao et al., 2019, Wu et al., 2021).
The Catalyst Review
December 2023
11

The_Catalyst_Review_December_2023

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