The Catalyst Review June 2020 - 16

tested. Other precious metals might also be potential candidates especially Ru and Ir, because of their high production rates in
CH3OH and HCOOH, respectively. If CH3OH and HCOOH formations are integrated as a C1 oxygenates formation, the rates were in the
same magnitude of order over Ru, Rh, Pd, and Ir catalysts suggesting that a similar type of CO-assisted oxidation reaction proceeded
over precious metals.
In order to investigate the reaction pathway in this system, potential intermediates (i.e., CH3OH, HCOOH, CH3COOH and CH3CHO)
were reacted in the presence of O2 and CO. Starting from CH3OH as a reactant, the formation of HCOOH was observed along with
its consumption, and from HCOOH, no liquid phase products
were formed. In both cases, moles of the reactant consumed was Scheme 1. Overall reaction pathways. Dashed arrow shows less plausible path.
larger than that of product formed in liquid phase, suggesting
the over oxidation toward CO2. These results are in good
agreement with the assumption that CH3OH and HCOOH are
primary and secondary products, respectively, in the sequential
partial oxidation. Formation of CH3COOH was not confirmed in
both cases suggesting that carbonylation (or CO insertion) of
CH3OH to CH3COOH did not proceed in this system. Together
with the results of CH3COOH formation as a primary product, the authors propose the direct oxidative carbonylation of methane
as a plausible CH3COOH formation route (CH4+CO+1/2O2 →CH3COOH) and suggest that the overall reaction pathways in this system
can be summarized according to Scheme 1. Source: Moteki T, Tominaga N and Ogura M. (2020). ChemCatChem,
Efficient Hydrogen Oxidation Catalyzed by Strain-Engineered Nickel Nanoparticles...
The hydroxide-exchange membrane fuel cell (HEMFC) is a cost-effective alternative to the proton-exchange membrane fuel cell
(PEMFC) because of the possibilities to use platinum-group-metal-free (PGM-free) catalysts as well as more economical bipolar
plates, air loops, and membranes. However, the lack of active and PGM-free catalysts for the hydrogen oxidation reaction (HOR)
at the anode is one of the main bottlenecks to its development. Currently, most PGM-free HOR catalysts are based on nickel (Ni),
although pure nickel metal has low activity due to an overly strong hydrogen binding energy (HBE). Herein, the authors demonstrate
that strain engineering can optimize both the HBE of Ni and the number of active sites, resulting in a Ni catalyst with the highest
mass activity in HOR for a PGM-free catalyst, as well as excellent activity in the hydrogen evolution reaction (HER).
Various Ni nanoparticles supported on carbon
Figure 1. Transmission electron microscopy
(Ni/C), were synthesized through pyrolysis of a
(TEM) images of a) Ni-H2-0%, b) Ni-H2-1%,
Ni-containing metal-organic framework (MOF)
c) Ni-H2-2%, d) Ni-H2-4%.
precursor under a mixed N2/H2 gas atmosphere.
The MOF precursor Ni3(BTC)2 ((BTC = 1,3,5benzene tricarboxylate radical) was prepared
through a one-step solvothermal method.
The precursor was annealed in a tube furnace
under an N2 atmosphere doped with a low
concentration of hydrogen (0, 1, 2, 4 vol%). The
samples were labeled according to the vol% of
H2, that is, Ni-H2-x% (x = 0, 1, 2, 4). Transmission
electron microscopy (TEM) shows that Ni-H2-0%,
1%, and 2% are composed of small nanoparticles
(Figure 1), which have a mean size of 4.1 ± 1.0
nm, 4.7 ± 1.1 nm, and 6.5 ± 2.9 nm, respectively
(In contrast, Ni-H2-4% consists mainly of sintered
grains suggesting that a higher H2 concentration in the gas mixture of the pyrolysis leads to
larger particles and a more heterogeneous size distribution.

Figure 2. Electrochemical HER performance. a) LSV curves for Ni-H2 samples
and commercial 20% Pt/C. Catalyst
loading: 0.28 mgcmcat.-2, scan rate: 1 mVs-1.
b) The corresponding Tafel slopes fitted
based on the results in (a).

The authors then measured both the HOR and HER activity of Ni-H2 catalysts and found
that the activity trend in HER of these catalysts is the same as in HOR (Figure 2 a, b): NiH2-2% > Ni-H2-1% > Ni-H2-0% > Ni-H2-4%. The most active catalyst, Ni-H2-2%, catalyzed a
current density of 10 mAcm~2 at an overpotential of only 36 mV. Its activity approached
20% Pt/C, which is among the highest for state-of-the-art HER catalysts in alkaline medium16

The Catalyst Review											

June 2020 cctc.202000168 cctc.202000168

The Catalyst Review June 2020

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

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