The Catalyst Review May 2024 - 12

Experimental Abstracts
Mechanism-guided Realization of Selective Carbon Monoxide Electroreduction to Methanol
Electrochemical reduction of CO2
to commodity chemicals and fuels
represents a sustainable pathway
to generate desired products
while reducing concentrations of a
significant greenhouse pollutant.
Although the use of heterogeneous
metallic catalysts (including Cu,
Ag, Au, Bi) has been found to be
moderately effective in facilitating
this reaction, the presence of many
different sites on metal surfaces
inhibits mechanistic analysis. On
the other hand, molecular catalysts
represent a promising alternative
because their well-defined
structures provide a precise model
for theoretical calculations and
experimental studies to understand
the reaction mechanism
and improve the catalytic
performance. For example, cobalt
phthalocyanine can effectively
convert CO2
or CO to methanol
but exhibits low selectivity (a
methanol Faradaic efficiency, FE,
of less than 40%). In this work,
the authors conducted systematic
kinetic studies of CO electroreduction
catalyzed by aminesubstituted
cobalt phthalocyanine
molecules supported on carbon
nanotubes (CoPc-NH2/CNT)
and successfully leveraged their
mechanistic understanding to
considerably improve the FE of
methanol production to more than
80% (Figure 1).
These workers began by
introducing a microporous layer
(MPL) into the catalytic electrode
structure, which enhanced
the mass transport of CO and
increased the methanol FE
from 40 to 66%. Tafel analysis
revealed an unvarying slope close
to 118 mV dec−1
for methanol
production at electrolyte pH from
7 to 13, indicating that transfer of the first electron to CO is the rate-determining step (RDS). Furthermore, pH dependence and isotopic
labeling experiments suggested that H2
O is involved as the major proton source in the RDS, although the presence of bicarbonate
(HCO3 ) can further enhance proton transfer. A pressure dependence study showed that the methanol generation reaction is first order
−
with respect to CO partial pressure, indicating a Henry-type isotherm for CO adsorption on the catalyst surface.
12
The Catalyst Review
May 2024
Figure 1. CO-to-CH3OH conversion catalysed by CoPc-NH2/CNT, and the current challenges and
progress made in this work. The catalyst consists of CoPc-NH2
molecules highly dispersed on CNT
surfaces. This work increased methanol FE from less than 40% to more than 80% using strategies
formulated from electrochemical kinetic studies.
Figure 2. Mechanism-guided realization of high methanol selectivity.
a, pCO dependent methanol production rate from CO reduction catalysed by CoPc-NH2
KHCO3
/ CNT in 0.1 M aqueous
. b, Schematic showing that CO coverage on catalyse surface increases with CO pressure. c, Schematic of
two-compartment high-pressure electrochemical cell used in this work. WE, RE and CE stand for working, reference
and counter electrodes, respectively. AEM stands for anion exchange membrane. d, FE and partial current densities
for methanol versus electrode potential measured under 10 atm CO in 0.1 M aqueous KHCO3. Data are presented as
mean values and error bars represent standard deviations (n = 3 replicates)

The Catalyst Review May 2024

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