164
chapter aims to address the abovementioned issues in terms of the efficient CO 2
utilization for formaldehyde (HCHO) production as a commodity chemical. The
current progress of photocatalytic CO 2 reduction to HCHO in terms of thermodynamic analysis, reaction mechanism, and catalytic performance is deeply discussed,
while other recent approaches for HCHO generation from CO 2 including CO 2
hydrogenation, selective methane oxidation, and homogeneous CO 2 reduction are
also reviewed in detail.
6.2 Fundamentals of Photocatalytic Carbon Dioxide
Reduction to Formaldehyde
6.2.1 Thermodynamics Aspects
It is well-known that C–O and C=O bond energies are of 350 and 750 kJ mol
−1
,
respectively. Thus, breaking the later bond requires a significant energy input, which
may be economically infeasible. Theoretically, CO 2 can directly react with H 2 O to
form HCHO, as shown in Eq. (6.1), with the Gibbs free energy (ΔG
o
) and the standard redox potential (ΔE
o
) of +522 kJ mol
−1
and + 1.35 V, respectively. These highly
positive values make CO 2 reduction to HCHO a thermodynamically uphill reaction
which is considerably difficult for occurring at room temperature.
CO H O HCHO O
2 +
→
+
2
2
(6.1)
An alternative and more favorable process to reduce CO 2 through the transfer of
electrons have been extensively studied. In these reactions, CO 2 can directly react
with the electrons in the presence of H 2 O or H
+
reductant to form diverse reduction
products depending on the number of electrons as shown in Table 6.1. The small and
positive—relative to the conduction band of most of semiconductors—reduction
potentials for the CO 2 reduction make it not easy to attract electrons; thus, protoncoupled electron transfer (PCET) would be beneficial due to the association of electron and proton transfer to CO 2 . Eq. (6.2) showed the ΔG values of the reduction
reactions:
∆G
zFE
= –
cell
(6.2)
where z and F are the corresponding number of transferred electrons and Faraday’s
constant, whereas E cell is the cell potential (electromotive force) at the desired
temperature.
The ΔG values of the reactions, which are listed in Table 6.1, are all positive
(non-spontaneous) but different from each other. Accordingly, the smallest and largest values are for formic acid and carbon formation, respectively. Therefore, the
T. D. Nguyen et al.
chapter aims to address the abovementioned issues in terms of the efficient CO 2
utilization for formaldehyde (HCHO) production as a commodity chemical. The
current progress of photocatalytic CO 2 reduction to HCHO in terms of thermodynamic analysis, reaction mechanism, and catalytic performance is deeply discussed,
while other recent approaches for HCHO generation from CO 2 including CO 2
hydrogenation, selective methane oxidation, and homogeneous CO 2 reduction are
also reviewed in detail.
6.2 Fundamentals of Photocatalytic Carbon Dioxide
Reduction to Formaldehyde
6.2.1 Thermodynamics Aspects
It is well-known that C–O and C=O bond energies are of 350 and 750 kJ mol
−1
,
respectively. Thus, breaking the later bond requires a significant energy input, which
may be economically infeasible. Theoretically, CO 2 can directly react with H 2 O to
form HCHO, as shown in Eq. (6.1), with the Gibbs free energy (ΔG
o
) and the standard redox potential (ΔE
o
) of +522 kJ mol
−1
and + 1.35 V, respectively. These highly
positive values make CO 2 reduction to HCHO a thermodynamically uphill reaction
which is considerably difficult for occurring at room temperature.
CO H O HCHO O
2 +
→
+
2
2
(6.1)
An alternative and more favorable process to reduce CO 2 through the transfer of
electrons have been extensively studied. In these reactions, CO 2 can directly react
with the electrons in the presence of H 2 O or H
+
reductant to form diverse reduction
products depending on the number of electrons as shown in Table 6.1. The small and
positive—relative to the conduction band of most of semiconductors—reduction
potentials for the CO 2 reduction make it not easy to attract electrons; thus, protoncoupled electron transfer (PCET) would be beneficial due to the association of electron and proton transfer to CO 2 . Eq. (6.2) showed the ΔG values of the reduction
reactions:
∆G
zFE
= –
cell
(6.2)
where z and F are the corresponding number of transferred electrons and Faraday’s
constant, whereas E cell is the cell potential (electromotive force) at the desired
temperature.
The ΔG values of the reactions, which are listed in Table 6.1, are all positive
(non-spontaneous) but different from each other. Accordingly, the smallest and largest values are for formic acid and carbon formation, respectively. Therefore, the
T. D. Nguyen et al.