CO 2 reduction is typically performed at neutral pH. Under these conditions CO 2 is in equilibrium with H 2 CO 3 , HCO 3 À and CO 3 2À . However, despite their presence so far most studies solely focus on the contribution of CO 2 while carbonate species as alternative reactants are generally neglected. Using density functional theory (DFT) modelling we explore the possible contribution of these carbonate species to the overall CO 2 reduction activity for a Fe porphyrin model catalyst. Considering only reaction Gibbs free energies, we find the reduction of carbonic acid (H 2 CO 3 ), bicarbonate (HCO 3 À ) and CO 2 to be equally likely. However, owing to a very high activation barrier for the initial adsorption of CO 2 onto the catalyst, bicarbonate and carbonic acid reduction are found to be several orders of magnitude faster. These data are used to model the pH dependence of the reaction rates of the different reactants. These results confirm that carbonic acid and bicarbonate are the most likely reactants independent of the pH and reactor setup.

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The chemistry of hypochlorous acid in the presence of metal impurities is of high importance for many areas of chemistry ranging from water purification and disinfection to sea water splitting and the electrosynthesis of chlorate. The production of chlorate comprises one of the most important electrochemical processes and relies on the Cr­(VI) catalyzed transformation of HOCl to chlorate. Since the use of Cr­(VI) has been restricted in the European Union due to its toxicity, mutagenicity, and carcinogenicity, alternative catalysts need to be developed. Building on the recently identified mechanism for the chromate catalyzed oxidation of HOCl to chlorate, we performed a screening for potential alternative catalysts using density functional theory modeling. Based on these results, a volcano plot, which combines all previously reported reaction paths, was constructed and the thermodynamic limitations were identified. Our results indicate that, in principle, many materials are able to catalyze the chlorate formation at much higher rates. However, in practice, the reaction is limited by the strong correlation between the pKa of the active site and the activity and the limitation to pH neutral conditions. This renders chromate, despite its overall poor performance, effectively the most active material. Furthermore, we show that none of the remaining considered molecules is able to replace Cr­(VI) as the homogeneous catalyst due to side reactions or process limitations. Thus, these results will have a significant impact in the political decision of banning Cr­(VI) in the chlorate process and also clearly show that alternative approaches for chlorate synthesis need to be developed if Cr­(VI) is to be removed from the process.