137
Conversion of carbon dioxide into methanol in photoelectrochemical cells is
well studied. It involves band gap excitation of semiconductor electrode material by
ultraviolet/visible solar spectrum in an electrolyte that results in promoting the
redox reaction, i.e., electrochemical oxidation and reduction reactions.
Photoelectrolytic reduction of aqueous carbon dioxide was achieved using p-type
gallium phosphide and/or p-gallium arsenide as a photocathode yielded formic acid,
formaldehyde, and methanol (Aurian-Blajeni et al. 1983; Halmann 1978). The
study showed that photoelectrolysis of carbon dioxide in solutions using n- or
p- gallium arsenide and p-type gallium phosphide electrodes produced methanol as
the major product at pH 4–6 (Frese and Canfield 1984). Bocarsly’s team (2008,
2010; Seshadri et al. 1994) reported reduction of carbon dioxide to methanol using
a p-type gallium phosphide semiconductor with near 100% faradaic efficiency at
underpotentials greater than 300 mV below the standard potential of −52 mV versus
saturated calomel electrode at pH 5.2 and pyridine as an organic molecular catalyst.
Very recently, Rezaul and co-workers (2018) achieved the biocatalyzed reduction of
carbon dioxide to methanol using an integrated enzyme cascade system to transfer
photogenerated electrons to a multienzyme cascade in a designed tandem photoelectrochemical cell.
5.4.5 Mechanism Study
Methanol is industrially produced via the usage of syngas feedstock at high temperature, e.g., above 200 °C, and pressure, e.g., more than 50 bar, conditions over
heterogeneous copper–zinc-based oxide (Cu–ZnO–Al 2 O 3 ) catalyst (Kar et al. 2018;
Kothandaraman et al. 2016). Despite great catalytic activity, this approach could
suffer from several limitations such as the formation of undesirable and unavoidable
by-products, e.g., hydrocarbons and higher alcohols, and requirement of harsh reaction conditions (Wang et al. 2011). Alternatively, methanol can also be produced by
implementing homogeneous catalysts at milder conditions. Therefore, the selective
carbon dioxide hydrogenation to methanol on homogeneous catalysts has gained
considerable attention in recent years from both academia and industry.
Homogeneous catalysts reportedly exhibit satisfactory selectivity and outstanding activity for methanol synthesis (Kar et al. 2018). These homogeneous catalysts
can also offer numerous advantages over heterogeneous catalysts. For example,
product selectivity can be easily tuned, and most catalytic active sites in homogeneous catalysts are accessible by reactants to take part in the catalytic reaction.
Despite these aforementioned benefits, homogeneous catalysts could not be widely
commercialized at an industrial scale due to difficulty in catalyst and products separation (Cole-Hamilton 2003).
The comprehensive understanding of mechanistic pathways for homogeneous
carbon dioxide hydrogenation to methanol is crucial for the development of
advanced catalyst system with enhanced product yield and selectivity as well as
5 Selective Hydrogenation of Carbon Dioxide into Methanol
Conversion of carbon dioxide into methanol in photoelectrochemical cells is
well studied. It involves band gap excitation of semiconductor electrode material by
ultraviolet/visible solar spectrum in an electrolyte that results in promoting the
redox reaction, i.e., electrochemical oxidation and reduction reactions.
Photoelectrolytic reduction of aqueous carbon dioxide was achieved using p-type
gallium phosphide and/or p-gallium arsenide as a photocathode yielded formic acid,
formaldehyde, and methanol (Aurian-Blajeni et al. 1983; Halmann 1978). The
study showed that photoelectrolysis of carbon dioxide in solutions using n- or
p- gallium arsenide and p-type gallium phosphide electrodes produced methanol as
the major product at pH 4–6 (Frese and Canfield 1984). Bocarsly’s team (2008,
2010; Seshadri et al. 1994) reported reduction of carbon dioxide to methanol using
a p-type gallium phosphide semiconductor with near 100% faradaic efficiency at
underpotentials greater than 300 mV below the standard potential of −52 mV versus
saturated calomel electrode at pH 5.2 and pyridine as an organic molecular catalyst.
Very recently, Rezaul and co-workers (2018) achieved the biocatalyzed reduction of
carbon dioxide to methanol using an integrated enzyme cascade system to transfer
photogenerated electrons to a multienzyme cascade in a designed tandem photoelectrochemical cell.
5.4.5 Mechanism Study
Methanol is industrially produced via the usage of syngas feedstock at high temperature, e.g., above 200 °C, and pressure, e.g., more than 50 bar, conditions over
heterogeneous copper–zinc-based oxide (Cu–ZnO–Al 2 O 3 ) catalyst (Kar et al. 2018;
Kothandaraman et al. 2016). Despite great catalytic activity, this approach could
suffer from several limitations such as the formation of undesirable and unavoidable
by-products, e.g., hydrocarbons and higher alcohols, and requirement of harsh reaction conditions (Wang et al. 2011). Alternatively, methanol can also be produced by
implementing homogeneous catalysts at milder conditions. Therefore, the selective
carbon dioxide hydrogenation to methanol on homogeneous catalysts has gained
considerable attention in recent years from both academia and industry.
Homogeneous catalysts reportedly exhibit satisfactory selectivity and outstanding activity for methanol synthesis (Kar et al. 2018). These homogeneous catalysts
can also offer numerous advantages over heterogeneous catalysts. For example,
product selectivity can be easily tuned, and most catalytic active sites in homogeneous catalysts are accessible by reactants to take part in the catalytic reaction.
Despite these aforementioned benefits, homogeneous catalysts could not be widely
commercialized at an industrial scale due to difficulty in catalyst and products separation (Cole-Hamilton 2003).
The comprehensive understanding of mechanistic pathways for homogeneous
carbon dioxide hydrogenation to methanol is crucial for the development of
advanced catalyst system with enhanced product yield and selectivity as well as
5 Selective Hydrogenation of Carbon Dioxide into Methanol