125
Gold was found to form selectively methanol in the reaction of carbon dioxide
hydrogenation and was studied as well in the supported form over different oxides.
Sakurai and Haruta (1996) have shown that the Au/ZnO catalyst has low conversion
of carbon dioxide but very good selectivity to methanol. Another catalytic material
Au/TiO 2 demonstrates higher conversions of carbon dioxide with slightly lower
selectivities to methanol. The combination of both systems, Au/ZnO–TiO 2 , resulted
in increased yield of methanol and aligned with the performance of the classical
Cu–ZnO–Al 2 O 3 type catalyst despite the fact that it is more expensive.
The use of another noble metal—platinum—in the form of nanoparticles
revealed that the platinum nanoparticles alone as active sites are not capable of catalyzing the carbon dioxide hydrogenation reaction due to their very poor interaction
with carbon dioxide. Only the association of platinum with oxides as support, for
example, silica or titanium oxide, containing oxygen vacancies and capable to
strongly interact with carbon dioxide, could increase the catalytic activity of
platinum- based materials in carbon dioxide hydrogenation to methanol (Kattel et al.
2016). Shao and co-workers (1995) demonstrated a high selectivity to methanol
formation at low carbon dioxide conversion using PtW/SiO 2 catalysts. The good
selectivity was attributed to the perfect dispersion of metallic platinum active sites.
Other promoters, apart from tungsten, did not increase the selectivity to methanol
under similar conditions.
For comparison, Table 5.2 summarizes the catalytic results of the catalysts discussed in this section.
5.3.4 Photocatalytic Hydrogenation of Carbon Dioxide
to Methanol in the Gas Phase
Light-promoted processes entail performances on par with, or even superior to,
those of thermally induced, industrially relevant, and commercial technologies.
Thus, light activation could be a distinct possibility for the promotion of carbon
dioxide valorization into chemicals (Puga 2016). Since the advent of photocatalysis
in the 1970s, several publications focused on photocatalyst synthesis, and evaluation in various applications including carbon dioxide reduction has been reported.
Still, very few examples exist of chemical processes operating on the basis of photocatalysis technology (Kondratenko et al. 2013). Below, several examples of photocatalytic carbon dioxide hydrogenation are presented.
Copper oxide which is largely used in the thermochemical hydrogenation of carbon dioxide into methanol is usually studied in combination with titanium oxide,
one of the most important semiconductors in photocatalysis. Cu/TiO 2 or CuO/TiO 2
becomes an interesting photocatalyst due to the formation of heterojunction structures with titanium oxide and its light adsorption in visible light spectrum. This
photocatalyst was investigated in both liquid and gas phase reactions of carbon
dioxide hydrogenation into methanol. No methanol was observed in the reactions at
5 Selective Hydrogenation of Carbon Dioxide into Methanol
Gold was found to form selectively methanol in the reaction of carbon dioxide
hydrogenation and was studied as well in the supported form over different oxides.
Sakurai and Haruta (1996) have shown that the Au/ZnO catalyst has low conversion
of carbon dioxide but very good selectivity to methanol. Another catalytic material
Au/TiO 2 demonstrates higher conversions of carbon dioxide with slightly lower
selectivities to methanol. The combination of both systems, Au/ZnO–TiO 2 , resulted
in increased yield of methanol and aligned with the performance of the classical
Cu–ZnO–Al 2 O 3 type catalyst despite the fact that it is more expensive.
The use of another noble metal—platinum—in the form of nanoparticles
revealed that the platinum nanoparticles alone as active sites are not capable of catalyzing the carbon dioxide hydrogenation reaction due to their very poor interaction
with carbon dioxide. Only the association of platinum with oxides as support, for
example, silica or titanium oxide, containing oxygen vacancies and capable to
strongly interact with carbon dioxide, could increase the catalytic activity of
platinum- based materials in carbon dioxide hydrogenation to methanol (Kattel et al.
2016). Shao and co-workers (1995) demonstrated a high selectivity to methanol
formation at low carbon dioxide conversion using PtW/SiO 2 catalysts. The good
selectivity was attributed to the perfect dispersion of metallic platinum active sites.
Other promoters, apart from tungsten, did not increase the selectivity to methanol
under similar conditions.
For comparison, Table 5.2 summarizes the catalytic results of the catalysts discussed in this section.
5.3.4 Photocatalytic Hydrogenation of Carbon Dioxide
to Methanol in the Gas Phase
Light-promoted processes entail performances on par with, or even superior to,
those of thermally induced, industrially relevant, and commercial technologies.
Thus, light activation could be a distinct possibility for the promotion of carbon
dioxide valorization into chemicals (Puga 2016). Since the advent of photocatalysis
in the 1970s, several publications focused on photocatalyst synthesis, and evaluation in various applications including carbon dioxide reduction has been reported.
Still, very few examples exist of chemical processes operating on the basis of photocatalysis technology (Kondratenko et al. 2013). Below, several examples of photocatalytic carbon dioxide hydrogenation are presented.
Copper oxide which is largely used in the thermochemical hydrogenation of carbon dioxide into methanol is usually studied in combination with titanium oxide,
one of the most important semiconductors in photocatalysis. Cu/TiO 2 or CuO/TiO 2
becomes an interesting photocatalyst due to the formation of heterojunction structures with titanium oxide and its light adsorption in visible light spectrum. This
photocatalyst was investigated in both liquid and gas phase reactions of carbon
dioxide hydrogenation into methanol. No methanol was observed in the reactions at
5 Selective Hydrogenation of Carbon Dioxide into Methanol