Mechanism of Catalytic and Electrocatalytic CO2 Reduction to Fuels and Chemicals

6.1 INTRODUCTION

High carbon dioxide (CO₂) emission levels in the environment have necessitated efforts toward the development of technologies for CO₂ mitigation. Recent research work has highlighted the potential of CO₂ as a renewable resource for the production of fuels and chemicals which may help in reducing CO₂ concentration in the environment [1,2]. However, the thermodynamics of direct CO₂ reduction to a hydrocarbon product is not favorable [3]. CO₂ molecule is a stable linear structure (O=C=O), where the Gibbs free energy of its formation at standard condition is estimated to be –396 kJ/mol [2] (Figure 6.1). Therefore, the direct conversion of CO₂ to any other product molecule such as carbon monoxide, methane, or methanol will involve a substantial positive change in Gibbs free energy (Figure 6.1) at standard conditions and is unlikely to proceed [2,4]. In hydrogen-assisted conditions, the thermodynamics may become favorable. For example, the change in Gibbs energy (ΔG°) for CO₂ conversions to methanol and methane at 298 K is estimated to be –9.2 and –130.8 kJ/mol [5] (Table 6.1), respectively. Interestingly, ΔG° as given by the Gibbs-Helmholtz equation (ΔG° = ΔH° – TΔS°) is dominated by the enthalpy change at standard conditions (ΔH°) and remains less or not affected by the entropy contribution (–TΔS°) (Table 6.1). It should, however, be noted for reactions, where ΔG° is positive at standard conditions, such as conversion to carbon monoxide and formic acid, may become favorable with improved kinetics at higher temperatures. Endogenic reactions can further be tuned for obtaining desired conversions by utilizing an electrocatalyst and thus changing the ΔG° of the reaction via an applied electrical potential, given by ΔG° = -nFE°. Considering the overall energy cycle, the electrocatalytic conversions need to be implemented with one condition, that the required electricity has to be produced from a renewable source.