Comparative study of the mechanism of co and CO2 hydrogenation on Cu-based materials via heterogeneous catalysis and eletrocatalysis

Abstract

Carbon dioxide (CO2) is widely produced from combustion of fuels, mainly fossil fuels, and industrial activities. Although plants absorb part of the emitted CO2 during the photosynthesis, this gas has gradually accumulated in the atmosphere. This increase in CO2 levels has been associated with undesirable climate effects such as global warming, rising sea levels, and more erratic weather patterns. A promising strategy to reduce CO2 emissions and, thereby, curb the increase in atmospheric CO2 levels comprises the development of alternative routes to the fabrication of chemical products based on carbon, of large demand in the market, as fuels for example, in which CO2 may be used as raw material. The hydrogenation of CO2 via heterogeneous catalysis or electrocatalysis can generate a wide variety of products, as methanol, methane, hydrocarbons, ethanol, higher alcohols, etc. Among these products, ethanol and higher alcohols are particularly interesting as neat fuels, fuel additives, and chemicals. Nevertheless, efficient technologies for the synthesis of ethanol or other C2+OH from the CO2 hydrogenation via heterogeneous catalysis or electrocatalysis are still not available. Technical and commercial aspects of the C2+OH production from CO2 are scarcely investigated and, in general, catalysts employed in studies comprising the hydrogenation of CO2 present low conversion, low selectivity, low stability and/or high costs. Therefore, the development of more efficient catalysts, preferentially based on non-noble elements, is necessary. The rational design of effective materials depends on the progress of the understanding on how the catalysts properties and other experimental conditions affect the kinetic and mechanism of this reaction. Electrochemically, Cu is the unique catalyst known to convert CO2 to hydrocarbons and/or oxygenates at considerably high Faradaic efficiency, but it also produces other products such as CO, HCOO, and H2 at fairly high Faradaic efficiency. Intensive research efforts, both experimental (nanostructured Cu catalyst, oxide-derived Cu catalyst, etc.) and theoretical (key intermediates, different reaction pathways, etc.), have focused on improving the overpotential and selectivity of Cu catalysts for the electroreduction of CO2 to C2 chemicals. However, the corresponding Faradaic efficiencies are typically below 40%. In early work, CO was found to be a key intermediate in the formation of methane on copper and recently the C-C coupling to C2 species was shown to take place on copper through a reductive CO dimerization step at the first stages of the reaction mechanism. Therefore, proper electrocatalysts for the reduction of CO2 to C2 species should in principle promote efficiently the formation of CO and its reductive dimerization. On the other hand, the mechanism of the reaction corresponding to the synthesis of C2+OH from CO2 hydrogenation via heterogeneous catalysis was not completely elucidated and is still under debate. Some authors have proposed that CO is an important intermediate of the CO2 hydrogenation towards the formation of ethanol. Therefore, materials capable of generating COads may favor the formation of C2+OH. In the present project, we propose to investigate the hydrogenation of CO and CO2 on Cu-based materials via heterogeneous catalysis and electrocatalysis, aiming to elucidate the mechanism of these reactions and the role of CO in the formation of C2+OH under different chemical environments (gas vs. liquid phase). Results obtained in Brazil will be complemented by those from the fundamental part of the electrocatalytic studies to be performed by the Dutch scientific team of this Brazil-Netherlands joint project. (AU)