Abstract
The constant increase in the CO2 concentration in the atmosphere requires us to look for opportunities to convert CO2 into more valuable compounds. In this review, the activity and selectivity of different supported metal catalysts were compared in the hydrogenation of carbon dioxide, and found that Rh is one of the best samples. The possibility of the CO2 dissociation on clean metal and on supported Rh was discussed separately. The hydrogenation of CO2 produces mainly CH4 and CO, but the selectivity of the reaction is affected by the support, in some cases the reduction of the support, the particle size of Rh, and the different additives. At higher pressure methanol, ethanol, and acetic acid could be also formed. The activity of the various supported Rh catalysts was compared and the results obtained for TiO2-, SiO2-, and Al2O3-supported catalysts were discussed in a separate chapter. The compounds formed on the surface of the catalysts during the reaction are shown in detail; mostly, different CO species, adsorbed formate groups, and different carbonates were detected. In a separate chapter the mechanism of the reaction was also discussed.
Highlights
The carbon dioxide concentration in the atmosphere increased dramatically since the industrial revolution
If we want to prevent the further increase of CO2 concentration in the atmosphere we have to hinder the enhancement of the carbon dioxide emission and we need to increase the conversion of CO2 to other more valuable compounds
In the sample which was doped with Li, as adsorbed CO is accumulated on the surface of stable Rh particles, mainly CO production occurs. Contrary to these observations on titanate nanotube supported Rh catalyst it was found that the intensity of the XPS peak characteristic of highly dispersed Rh gradually decreased and nearly completely vanished after 60 min of the H2 + CO2 (4:1) reaction at 498 K [43] indicating the agglomeration of Rh sites during the CO2 + H2 reaction
Summary
The carbon dioxide concentration in the atmosphere increased dramatically since the industrial revolution. It means that conventional products and technologies for CO2 utilization do not influence the CO2 concentration in the atmosphere This means that new procedures should be applied especially where CO2 emissions are significant. Products that can be produced in the hydrogenation of carbon dioxide, methane, and methanol could be used directly either in the energy industry or in the transportation. Another important product is CO which is the component of synthesis gas and from it plenty of products can be produced including gasoline. These processes could help the storage of electricity, too, whereas the greatest problem with the renewable energy (wind and solar) production is their fluctuation and the energy consumption is uneven. It is not surprising that the hydrogenation of carbon dioxide is in the focus of academic and industrial research again
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