The Catalyst Review August 2020 - 16

EXPERIMENTAL
Figure 1. XANES spectra of a) Pd-Pt/Al2O3 and b) Pd-Pt/CZ, both in fresh state and after
4 h of H2/O2 pulsing at 315°C and the references Pd and PdO.

Figure 2. Effluent CH4 conversion and temperature
for Pd-Pt/CZ in 2000 ppm CH4, bal. Ar with 30
s/180 s of 0% O2/10% O2 at 315°C.

The effects of H2 were also found to be dependent upon the gas environment of the catalyst. In the presence of constant O2 flow,
pulses of additional H2 reacted over the Pd to form H2O, resulting in a high-temperature region at the front of the catalyst. Under dry
conditions, the inhibition due to the H2O negated the improvements from the increased temperature. Under wet conditions, modest
gains were observed as the catalyst was already saturated. However, under steady-state conditions, the raised catalytic temperature
promoted activity, resulting in light-off curves significantly shifted to lower temperatures.
Experiments varying the pulse durations of H2/O2 switching revealed that the beneficial effects could still be obtained with shorter
reductive pulses and more extended oxidative periods. A test with O2 on/off pulsing showed that CH4 could stabilize catalytic
activity to a lesser degree than that accomplished by H2 pulsing (Figure 2). Source: Karinshak KA, Lott P, Harold MP, et al. (2020).
ChemCatChem, doi.org/10.1002/cctc.202000603.
Current Progress of Electrocatalysts for Ammonia Synthesis Through Electrochemical Nitrogen
Reduction Under Ambient Conditions
Although the energy-intensive and environmentally unfriendly Haber-Bosch process is still the dominant industrial route for
synthesizing NH3, electrochemical nitrogen reduction reaction (NRR) at ambient conditions represents an attractive alternative.
Significant research efforts have been devoted to NH3 synthesis via electrochemical NRR, including different designs of reactors,
electrolytes, and electrocatalysts (Figure 1). In this Review, the authors summarize various catalysts that can be used for
electrochemical
Figure 2. Reported electrocatalysts for electrocatalytic NRR at ambient
Figure 1. Scheme of electrochemical cell for NH3
NRR at ambient
production by electrocatalytic NRR.
conditions.
conditions and
classify them
according to
their chemical
composition:
metal catalysts
including pure-metal
and alloy catalysts;
metal compound
catalysts, that is,
metal sulfide, metal
oxide, and metal
nitride catalysts; metal-supported N-doped carbon catalyst; metal complex catalysts; and metal-free catalysts (Figure 2). Each
category is explored in-depth with an emphasis on the relationship between their optimal electrocatalytic conditions and NH3
production performance. Conclusions and perspectives are also provided for the future design of high-performance catalysts for
electrocatalytic NH3 production.
For example, although metal compounds as catalysts barely have an advantage regarding faradaic efficiency (FE) compared to puremetal catalysts, their higher availability is highly attractive. Taking iron oxide, its FE was higher than that of an alloy containing the
noble metal Pd. Also, based on the cross-reference of the FE pre- and post-optimization of MOS2 catalysts, it is evident that the FE
can be significantly improved by optimization. Besides, metal nitrides are currently controversial catalytic materials. Studies have
shown that some metal nitrides can decompose by themselves to produce NH3. When metal nitrides are used as NRR catalysts,
mechanism research and isotope labeling detection should be carried out to confirm further that NH3 was produced by NRR, not the
decomposition of the metal nitrides themselves.
16

The Catalyst Review

August 2020

http://www.doi.org/10.1002/cctc.202000603

The Catalyst Review August 2020

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