The electrochemical reduction of CO
2 has the capacity to address climate change by utilizing
CO
2 to produce valuable compounds that can be used as fuels or as chemical feedstocks.
Unfortunately, current electrocatalysts exhibit limited activities, efficiencies, and stabilities.
Furthermore, the complex multi-electron reduction mechanism is poorly understood. This
thesis aims to design electrocatalysts and provide detailed mechanistic studies on the
electrochemical reduction of CO
2 towards advanced products (2e
–), such as hydrocarbons and
alcohols.
In the first project, metal-organic framework/ionic liquid hybrid catalysts are synthesized for
the selective production of CH
4 from CO
2 reduction, reaching a maximum faradaic efficiency
(FE) of 65.5% at −1.13 V vs. RHE. Hydrogen evolution is...[
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The electrochemical reduction of CO
2 has the capacity to address climate change by utilizing
CO
2 to produce valuable compounds that can be used as fuels or as chemical feedstocks.
Unfortunately, current electrocatalysts exhibit limited activities, efficiencies, and stabilities.
Furthermore, the complex multi-electron reduction mechanism is poorly understood. This
thesis aims to design electrocatalysts and provide detailed mechanistic studies on the
electrochemical reduction of CO
2 towards advanced products (>2e
–), such as hydrocarbons and
alcohols.
In the first project, metal-organic framework/ionic liquid hybrid catalysts are synthesized for
the selective production of CH
4 from CO
2 reduction, reaching a maximum faradaic efficiency
(FE) of 65.5% at −1.13 V vs. RHE. Hydrogen evolution is also significantly inhibited in the
presence of the embedded ionic liquid, reaching a minimum FE of 6.8%. The ionic liquid
molecules are proposed to enhance local CO
2 concentrations. Furthermore, DFT calculations
show that the presence of ionic liquid molecules on the metallic Cu active sites can improve
the thermodynamics of the CO
2-to-CH
4 electrochemical pathway. In the second project, 4-(dimethylamino)pyridine (4-DMAP) was embedded within a dealloyed, porous Pd substrate to
selectively produce methanol. The resulting electrocatalyst achieved a maximum FE of 22% at
−0.7 V vs. RHE. In-situ FTIR experiments show that in the presence of 4-DMAP, the formation
of strongly adsorbed bridge-bonded *CO on palladium is promoted over weakly-adsorbed
linearly-bonded *CO, thereby enhancing its further reduction over CO desorption. The
transient formation of a suspected intermediate for methanol formation in the in-situ IR spectra
is observed, indicating that methanol formation may be governed by a transient process.
The production of C
2 compounds from CO
2 was also extensively studied via the reduction of
glyoxal, a suspected CO
2RR intermediate, on a copper cathode. Three main reaction pathways
were observed – (1) reduction to aldehydes (glycolaldehyde, acetaldehyde) and alcohols
(ethanol, ethylene glycol), (2) disproportionation to glycolate and formate, and (3) carboncarbon
coupling via aldol reactions. The local pH was shown to significantly affect the
preferred reaction pathway due to the pH-dependence of carbonyl hydration, Cannizzaro
disproportionation, and aldol addition. The results indicate that glyoxal is not the main
intermediate for ethanol production on copper due to the low CO
2RR FE towards other glyoxal
reduction products (such as ethylene glycol) commonly observed on polycrystalline copper.
In-situ FTIR results show that the anodization of copper enhances the *CO coverage at
intermediate potentials, promoting its further reduction. A suspected C
2 intermediate was also
observed at intermediate potentials, indicating that a low-overpotential pathway towards C-C
coupling exists on oxide-derived copper. To selectively produce alcohols and hydrocarbons,
MOF-derived porous bimetallic CuO/Ag and CuO/ZnO electrocatalysts were synthesized by
annealing HKUST-1 in air. The resulting porous CuO exhibited a maximum C
2+ product FE of
61.1%. The presence of Ag significantly improved the alcohol over hydrocarbon selectivity,
reaching a maximum FE ratio of 1.7 at −0.92 V vs. RHE. The high observed selectivity can be
attributed to the initial oxidation of copper, which can promote both CO production (for further
reduction) and C-C coupling, as shown in the FTIR results. Furthermore, the CO spillover from
Ag sites and further entrapment of CO in the porous structures are also hypothesized to
contribute to the large FEs achieved.
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