The Catalyst Review August 2019 - 1

INDUSTRY PERSPECTIVES

The views expressed are those of the individual author and may not reflect those of The Catalyst Review or TCGR

Solar Chemistry: An Opportunity We Cannot Miss
By Michele Aresta, PhD
In 1912 the Italian Chemist Giacomo Ciamician, Professor at the University of Bologna, in the incipit of his paper "La Fotochimica
dell'Avvenire" (Photochemistry of Future) wrote: "The modern civilization is daughter of fossil carbon; the latter gives to humanity
the solar energy in its most concentrated form; accumulated during centuries the modern man uses it with much avidity and careless
lavishness for taking control of the world." He concluded by saying, "... I believe that the industry should use all forms of energy that
Nature makes available; so far the modern civilization has mainly used fossil energy: maybe it should be better to use the other forms
of available energies?"
Over a century has passed, yet these words remain factual and relevant. The urgent need to shift from a linear economy to a
circular economy requires moving from fossil energy to perennial energies such as solar-, wind-, geothermal- and hydro-power or to
renewable energy (biomass) for powering human life.
In the paper of Ciamician one can also read, "... In desert areas where climatic conditions and the low quality of soil prevent any
vegetal culture, artificial photochemistry will add value. The arid areas will be populated by industries without smog and chimneys:
glass tubes and greenhouses - rooms made of glass - will catch solar radiations and in such transparent reactors photochemical
processes will occur that were distinctive of plants and that humans will have learnt to use: industry will be able to fasten such
processes because Nature is not in a hurry, but humanity is." However, the concept of solar chemistry is over 100 years old and is
joined to the vision of an enhanced photosynthesis! Today we are facing the urgent need to implement such concepts.
Solar chemistry is, thus, a vision of the chemical industry adherent to the need to shift to a circular economy. Solar energy can
power industrial processes in many different ways (i.e., thermal, photochemical, electrochemical, bio-electrochemical, photo-bioelectrochemical), each requiring peculiar operative conditions and producing different classes of products.
Thermal processes are quite unique as they can occur over a wide range of temperatures from low- (up to 330 K by direct heating)
to medium-high (up to ca. 700 K by the use of heating fluids such as molten salts) and to high-temperature (over 1300 K by using
Concentrators of Solar Power-CSP) (Aresta et al. 2018). A typical application of such devices is the production of "Syngas from
air." Oxides or mixed oxides are used for separated de-oxygenation of water and CO2 (Eq. 1-3) to afford H2+CO (Syngas), which is
converted into hydrocarbons (HC) by using the known Fisher-Tropsch (FT) technology.
MOx + hv à MOx-1 + 1/2O2
MOx-1 + H2O à MOx + H2
MOx-1 + CO2 à MOx + CO

1
2
3

The other processes in the list above usually occur at a temperature close to ambient. Photochemical processes can make direct
use of solar radiations, which may act directly on the substrates (Sub) rising them to an excited state (Sub*) that will generate the
products (P). Alternatively, solar photons will excite a sensitizer (S à S*) which in turn will transfer energy to the substrate (S* +
Sub à S + Sub*) that will evolve towards the P. As a third option, a homogeneous- (metal complex) or heterogeneous-catalyst
(semiconductors) can be excited (Cat à Cat*) which will drive the conversion of the substrate. A simple example of such application
is molecular isomerization that may be able to store solar energy. The incidence of a solar photon on a photo-material generates an
exciton (hole or p+ + e-), which will promote redox reactions at two different sites. It is essential that charges are kept apart otherwise
their recombination will generate heat and benefits of photoexcitation will be lost. Semiconductors (solid oxides, in general) are used
in such process: charge-separation may be secured by generating hetero-junctions between different components that avoids, thus,
recombination. Such systems can be used for diverse purposes, from water splitting (H2O à H2 + 1/2O2, often improperly labelled
artificial photosynthesis or even artificial tree or artificial leaf), to chemical bonds splitting or formation. In any case solar energy is
stored into chemical bonds. A variety of processes can be carried out that are today driven by heat by using H2 generated in this way:
from ammonia (NH3) synthesis to (energy) products, reducing their carbon footprint (CF) and overall environmental impact.
Electrochemical processes make use of PV-electrons or PV-H2 generated via water electrolysis. PV-H2 is today 2-3 times more
expensive than H2 generated via reforming (R) based on fossil-C (coal or CH4). It is foreseeable that with the increase of efficiency
of new photomaterials (from η=20% at best today to foreseen η=40% in 2040), reduction of their cost (-50% by 2040) and
longer lifetime, the cost of PV-H2 may equal that of R-H2 making available on a large scale non fossil-H2 with much benefit for the
environment. In fact, the latter hydrogen produces 0.25 to 0.5 mol of CO2 per mole of H2 depending on the fact that coal or methane
is used, respectively in the reforming process. Conversely, solar electrolysis of water would produce 0.5 mol of O2 per mol of H2, with
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The Catalyst Review

August 2019

The Catalyst Review August 2019

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