Carbon Dioxide Reduction Mediated by Iron Catalysts: Mechanism and Intermediates That Guide Selectivity

Abstract

The reduction of carbon dioxide represents an ambitious target, with potential impact on several of the United Nations’ sustainable development goals including climate action, renewable energy, sustainable cities, and communities. This process shares a common issue with other redox reactions involved in energy-related schemes (i.e., proton reduction to hydrogen and water oxidation to oxygen), that is, the need for a catalyst in order to proceed at sustainable rates. Moreover, the reduction of CO 2 faces an additional selectivity complication, since several products can be formed, including carbon monoxide, formic acid/formate, methanol, and methane. In this Mini-Review, we will discuss iron-based molecular catalysts that catalyze the reduction of CO 2 , focusing in particular on the selectivity of the processes, which is rationalized and guided on the basis of the reaction mechanism. Inspired by the active sites of carbon monoxide dehydrogenases, several synthetic systems have been proposed for the reduction of CO 2 ; these are discussed in terms of key intermediates such as iron hydrides or Fe-CO 2 adducts, where the ligand coordination motif, together with the presence of co-additives such as Brønsted acids, nucleophiles, or CO 2 trapping moieties, can guide the selectivity of the reaction. A mechanistic comparison is traced with heterogeneous iron single-atom catalysts. Perspectives on the use of molecular catalysts in devices for sustainable reduction of CO 2 are finally given.

Water coordinated on Cu(I)-based catalysts is the oxygen source in CO2 reduction to CO

“…25 and 26 ). These results indicated that supplementary H + was indeed important for boosting the reduction efficiency of CO 2 to CO in the presence of [Cu(H 2 O)] + catalyst, in which H + from the coordinated H 2 O participated into the elimination of O atoms in CO 2 46 , 48 , 70 .…”

“…16 ), 107 Ag, and 104 Pd (Supplementary Fig. 17 )-based reactions, allowed us to draw a definite conclusion that in the CO 2 RR process, the two O atoms in CO 2 were eliminated completely by losses of two molecules of H 2 O 46 , 48 , 70 , and the O atom in CO originated from the coordinated H 2 O. Moreover, H 2 18 O-labeling experiments in tandem with off-line GC-MS analysis were carried out to confirm the source of O atom in resulting CO from the reversed WGSR.…”

“…Thus, the two O atoms in CO 2 needed to be eliminated prior to the formation of CO. According to the previous CO 2 RR mechanisms 46 , 48 , 70 , the generation of CO was achieved by first proton-electron (H + /e − ) transfer processes to CO 2 , followed by the elimination of H 2 O. To promote the formation of CO, an efficient step to proton generation and capture was necessary 46 .…”

First Phenol Carboxylation with CO2 on Carbon Nanostructured C@Fe-Al2O3 Hybrids in Aqueous Media under Mild Conditions

“…In this sense, the hybrids wettability properties showed, as their structural formulation, that they could be necessary in order to observe phenol conversion, as can be evidenced by the fact that no reaction was observed under the same reaction conditions with γ-alumina spheres or FeAl spheres. Although values were far from full conversion, it is in the same range of those reported in the most common reaction, converting CO 2 to methanol in heterogeneous catalysis [ 52 , 53 ]. The porous distribution of the samples allowed for the phenol and CO 2 spread in the hybrids to the active sites by diffusion, while the aqueous solution was maintained outside the spheres due to their hydrophobicity properties created by the nanofibers and nanowhiskers-like growth, resulting in a biphasic media.…”

“…Neither double carboxylation nor meta-carboxylation product was detected. The formation of benzoic acid (0–6%), p-hydroxybenzoic (<1%) acid, and salicylic acid (0–10%), which presented an additional carbon in the molecular structure, should be related with the formation of C–C bonds by carboxylation [ 53 , 54 , 55 ]. The C@Al-923 and C@Al-1023 hybrids showed the highest relative amount of salicylic acid among carboxylation products (>90% of C7 compounds).…”