CO2 is a greenhouse gas and to reduce its concentration in the atmosphere, there are three possibilities: We can minimize its production, we can store it and we can use it to make other chemical compounds. Scientists are trying realize all three possibilities to reduce the CO2 concentration in the air. There are several challenges to face, such as to make the 400ppm CO2 content of the atmosphere into usable quantities of CO2 in proper density.
Of course, it would be nice to transform CO2 into "value-added" chemicals, which are for instance methanol, fuels or methane. There are several catalysts that can transform CO2 into these chemicals and research is done to optimize these catalysts for an efficient industrial use.
In this post, we will concentrate on the production of methane. Methane is for instance used as an energy material. E.g. natural gas consists of 70-90% methane. The conversion of CO and CO2 to methane is of great industrial importance, because it also enables the synthesis of methane out of coal and biomass. This is, however, a longer process including several steps, such as steam reforming. The coal/biomass is first transformed into CO/CO2, which are then transformed into methane. The transformation of CO or CO2 into methane is called "methanation". It can be realized by reaction with hydrogen using heterogeneous catalysts. The hydrogen consumption is thereby of course a disadvantage.
The methanation of CO and CO2 is exothermic and proceeds via the following reactions:
CO + 3 H2 → CH4 + H2O
CO2 + 4 H2 → CH4 + 2 H2O
A conventional catalyst for the reactions is based on Nickel: NiO is activated under reaction conditions forming metallic Ni. The major drawback of Ni-based materials is coking, which deactivate the catalyst. Coking means that after the decomposition of CO or CO2 on the metal surface (to give atomic C and O), the carbon atoms stays on the surface and "block" the metal instead of reacting with the hydrogen to give CH4.
Quite recently, is has been discovered that Ni-Fe and Ru nano-particles supported on oxides are may be more stable under reaction conditions and even more active. [1-3] Here, the nano-particles are supported on an oxide material, e.g. silica (SiO2) or alumina (Al2O3), which are commonly used support materials in catalytic systems. The preparation method of the metal particles, the particle size and the nature of the oxide support seem to influence the catalytic activity of such systems. A high catalytic activity and 100% conversion of CO2 was reported for Ru/TiO2 with a Ru mean particle diameter of 2.5 nm.  Interestingly also the product selectivity is affected by the particle size of alumina-supported Ru particles.
Another way to transform CO2 into "value-added" compounds is electro-reduction. Therefore you can take a gold-based electrode in aqueous solution saturated with CO2, which is reduced first to CO. Then, the tricky part is the reduction of the CO into fuels. Here is a possible reaction we would like to see:
2 CO + 7 H2O + 8 e- → CH3CH2OH + 8 OH-
One important value to predict the efficiency of the process is the over-potential applied on the electrode on which the reaction should take place. The over-potential is measured against a standard reference electrode, the hydrogen electrode. When the over-potential is large, we loose energy and the process becomes very costly. So we search for an electrode on which the reaction can occur already when we apply a small over-potential. A promising electrode for the CO2 reduction is a partially reduced Cu2O electrode, because is reduces the CO instead of the water present and it has a relatively low over-potential.
Author: Philomena Schlexer
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