problems, such as density and exposure of the catalyst active sites, conductivity,
mass transport and stability of the catalyst-derived material or electrode, all make
the catalyst intrinsic activity and efficiency quite different from the device performance numbers). In addition, scale-up feasibility and whole device long-term stability (also associated with the costs of using expensive high-purity semiconductors
to achieve high efficiency) have to be attained. Nevertheless, artificial photosynthesis devices might become economically viable sooner than many anticipate:
considering the energy consumption forecasted for 2050, the future solar energy
devices only need a % 10% solar-to-fuel efficiency (already achievable in proof of
concept devices!) if 1% of the Earth’s surface is covered [44].
Formic acid can also be produced chemically from CO 2 and dihydrogen. This
CO 2 hydrogenation is just the thermal overall CO 2 reduction by dihydrogen using
molecular catalysts (Eq. 3), as an alternative to direct electrochemical or photoelectrochemical reduction of CO 2 [5, 10, 12, 13, 45–71]. Hence, here, it is the
rational design of the catalytic systems (efficiency and selectivity) that must be
attained and the systems that have been developed to date exhibit selectivity and
yield lower than desirable, besides requiring a high temperature and/or high pressure. Dihydrogen is a “clean” fuel (potentially zero contribution to the global carbon
cycle), but its real environmental impact depends on how it is produce. The industrial
production of dihydrogen (primarily from methane) requires harsh temperatures and
emits as much CO 2 into the atmosphere as natural gas burning [72]. To be environmentally friendly, dihydrogen must be produced by electrolysis of water using a
RES and selected heterogeneous or homogeneous catalysts or biological systems
[73–93]. As noted above, besides producing formic acid itself, CO 2 hydrogenation is
thought as a relevant way to storage dihydrogen. Therefore, also the reversible
interconversion of formic acid to CO 2 and dihydrogen must be carefully considered.
4 How to Convert Carbon Dioxide to Formic
Acid/Formate?—Exploiting the Power of Formate
Dehydrogenases (Enzymes for Solving Humankind’s
Problems)
4.1 The Biochemical Way
In contrast to purely physicochemical, biological processes are substrate and
product-specific (life requires a well-defined metabolism) and occur under truly
“green”, sustainable conditions, at ambient temperature and pressure and close to
neutral pH. Biological catalysts—enzymes—offer selectivity and specificity, coupled with high specific activity (in terms of active sites) and maximal rate (under the
respective cellular context). Enzymes have evolved to become perfect catalysts
2 ,
2
It should be kept in mind that enzymes did not evolve to maximise “our” VC production.
Enzymes and all cellular components evolved to achieve sustained life. The statement of “perfect
catalysts” must be taken within the respective context.
Carbon Dioxide Utilisation—The Formate Route
35
mass transport and stability of the catalyst-derived material or electrode, all make
the catalyst intrinsic activity and efficiency quite different from the device performance numbers). In addition, scale-up feasibility and whole device long-term stability (also associated with the costs of using expensive high-purity semiconductors
to achieve high efficiency) have to be attained. Nevertheless, artificial photosynthesis devices might become economically viable sooner than many anticipate:
considering the energy consumption forecasted for 2050, the future solar energy
devices only need a % 10% solar-to-fuel efficiency (already achievable in proof of
concept devices!) if 1% of the Earth’s surface is covered [44].
Formic acid can also be produced chemically from CO 2 and dihydrogen. This
CO 2 hydrogenation is just the thermal overall CO 2 reduction by dihydrogen using
molecular catalysts (Eq. 3), as an alternative to direct electrochemical or photoelectrochemical reduction of CO 2 [5, 10, 12, 13, 45–71]. Hence, here, it is the
rational design of the catalytic systems (efficiency and selectivity) that must be
attained and the systems that have been developed to date exhibit selectivity and
yield lower than desirable, besides requiring a high temperature and/or high pressure. Dihydrogen is a “clean” fuel (potentially zero contribution to the global carbon
cycle), but its real environmental impact depends on how it is produce. The industrial
production of dihydrogen (primarily from methane) requires harsh temperatures and
emits as much CO 2 into the atmosphere as natural gas burning [72]. To be environmentally friendly, dihydrogen must be produced by electrolysis of water using a
RES and selected heterogeneous or homogeneous catalysts or biological systems
[73–93]. As noted above, besides producing formic acid itself, CO 2 hydrogenation is
thought as a relevant way to storage dihydrogen. Therefore, also the reversible
interconversion of formic acid to CO 2 and dihydrogen must be carefully considered.
4 How to Convert Carbon Dioxide to Formic
Acid/Formate?—Exploiting the Power of Formate
Dehydrogenases (Enzymes for Solving Humankind’s
Problems)
4.1 The Biochemical Way
In contrast to purely physicochemical, biological processes are substrate and
product-specific (life requires a well-defined metabolism) and occur under truly
“green”, sustainable conditions, at ambient temperature and pressure and close to
neutral pH. Biological catalysts—enzymes—offer selectivity and specificity, coupled with high specific activity (in terms of active sites) and maximal rate (under the
respective cellular context). Enzymes have evolved to become perfect catalysts
2 ,
2
It should be kept in mind that enzymes did not evolve to maximise “our” VC production.
Enzymes and all cellular components evolved to achieve sustained life. The statement of “perfect
catalysts” must be taken within the respective context.
Carbon Dioxide Utilisation—The Formate Route
35