Renewable conversion of CO2 to green fuels

Abstract

The excessive release of greenhouse gases has significantly endangered the Earth's ecosystem and resulted in major environmental impacts. The increasing concentrations of carbon dioxide (CO2) from fossil fuel usage disturb natural cycles, increase global warming, cause glacial melting, and lead to climate change. Therefore, prompt actions are required to capture CO2 and transform it into renewable energy fuels to mitigate this problem. For decades, researchers and scientists have pursued the development of systems that can capture CO2 and transform it into valuable chemical molecules. Several techniques exist for the conversion of CO2 into valuable products, including biochemical, thermochemical, and photochemical processes; however, electrochemical approaches are particularly advantageous as they can utilize renewable energy, hence reducing the degradation of the environment. Electrocatalysis is considered a key and efficient technique for the sustainable generation of fuels and chemicals from renewable energy sources. These technologies not only diminish atmospheric CO2 levels but also serve as effective methods to meet the rapidly increasing energy demands by converting the captured CO2 into a fuel such as methane, methanol, ethanol, etc. in a renewable manner. This thesis employed the state-of-the-art Density Functional Theory (DFT) calculations to model more than 30 different surfaces in the rock-salt structured transition metal carbides (TMC) and in three dominant facets of (100), (110), and (111) as catalyst material for CO2 reduction reactions (CO2RR) and carbon monoxide reduction reactions (CORR). In addition to the conventional mechanism, the Mars van Krevelen (MvK) mechanism was investigated, a mechanism that is unique for these TMCs due to their crystallographic compositions. For CO2 reduction via a conventional mechanism, the (100) facet of vanadium carbide (VC) was found to enable formic acid production at an onset potential of 0.0 V. The (100) facet of wolfram carbide (WC) was identified as showing high activity for methanol synthesis at -0.36 V. From the (110) facets, tantalum carbide (TaC) demonstrated outstanding activity for C1 products, enabling formic acid formation at 0.0 V and methane formation at -0.21 V. The VC (110) facets showed relatively high activity for formic acid (-0.36 V) and methanol (-0.50 V) formation. The WC (110) was found to be a better option for generating C2 products, such as ethanol, ethane, and ethylene, with a comparatively low onset potential of -0.65 V. For CORR, our study demonstrates that CO is exergonically adsorbed on the surface of carbides, where TaC (100) and TaC (110) were predicted to have catalytic activity for methane formation at -0.32 V and -0.26 V, respectively. Overall, the analysis of CO2 and CO adsorption indicates that CO adsorption is more exergonic than CO2, hence making CORR more interesting than CO2RR on TMC catalysts. In addition, the (111) facets were found less interesting for CO2 reduction when compared with (100) and (110) facets.