An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO2, ranging from molecular systems to single-atom and nanostructured catalysts.

Abstract:The electrochemical behavior of fac- [Mn(pdbpy) (CO)3Br] (pdbpy = 4-phenyl-6-(phenyl-2,6-diol)-2,2'-bipyridine), 1, in acetonitrile under Ar and its catalytic performances for CO2 reduction with added water, 2,2',2''-trifluoroethanol (TFE) and phenol are discussed in detail. Preparative-scale electrolysis experiments, carried out at -1.5 V vs. SCE in CO2-saturated acetonitrile solutions, reveal that the process selectivity is extremely sensitive to the acid strength, providing CO and formate in different faradaic yields. A detailed spectroelectrochemical (IR and UV-Vis) study under Ar and CO2 atmospheres shows that 1 undergoes fast solvolysis; however dimer formation in acetonitrile is suppressed, providing an atypical reduction mechanism in comparison with other reported Mn I catalysts. Spectroscopic evidence of Mn hydride formation supports the existence of different electrocatalytic CO2 reduction pathways. Furthermore, a comparative investigation performed on the new fac-[Mn(ptbpy)(CO)3Br] (ptbpy = 4-phenyl-6-(phenyl-3,4,5-triol)-2,2'-bipyridine) catalyst, 2, bearing a bipyridyl derivative with OH groups in different positions to those in 1, provides complementary information about the role that the local proton source plays during the electrochemical reduction of CO2.

We report here the first purely organometallic fac‐[MnI(CO)3(bis‐MeNHC)Br] complex with unprecedented activity for the selective electrocatalytic reduction of CO2 to CO, exceeding 100 turnovers with excellent faradaic yields (η CO≈95 %) in anhydrous CH3CN. Under the same conditions, a maximum turnover frequency (TOFmax) of 2100 s−1 was measured by cyclic voltammetry, which clearly exceeds the values reported for other manganese‐based catalysts. Moreover, the addition of water leads to the highest TOFmax value (ca. 320 000 s−1) ever reported for a manganese‐based catalyst. A MnI tetracarbonyl intermediate was detected under catalytic conditions for the first time.

The effect of a local proton source on the activity of a bromotricarbonyl Mn redox catalyst for CO2 reduction has been investigated. The electrochemical behaviour of the novel complex [fac-Mn(dhbpy)(CO)3Br] (dhbpy = 4-phenyl-6-(1,3-dihydroxybenzen-2-yl) 2,2'-bipyridine), containing two acidic OH groups in the proximity of the metal centre, under a CO2 atmosphere showed a sustained catalysis in homogeneous solution even in the absence of Brønsted acids.

Mechanistic understanding of electro-and photocatalytic CO2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, herein it is presented for a new CO2-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and in-situ spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [L N4 Co(OTf)2] (1) (L N4 =1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7triazacyclononane) such as a highly reactive cobalt (I) (1 (I)) and cobalt (I) carbonyl (1 (I)-CO) species. The combination of spectroelectrochemical studies under CO2, 13 CO2 and CO with DFT disclosed that 1 (I) reacts with CO2 to form the pivotal 1 (I)-CO intermediate at the 1 (II/I) redox potential. However, at this reduction potential, the formation of 1 (I)-CO restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO2-to-CO electrocatalysis at the Co I/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the 1 (II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer to form 1 (I)-CO from 1 (II)-CO is under diffusion control. Then, the CO release from 1 (II)-CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible-light irradiation has a positive impact under electrocatalytic conditions. We envision that light-irradiation can serve as an effective strategy to circumvent the CO poisoning and improve the performance of CO2 reduction molecular catalysts. 1750 1950 2050 2150 Wavenumber (cm-1) Co I-CO Co 0-CO Co II-CO Theoretical 43 cm-1 Experimental under CO 2 Experimental under CO 1910 cm-1 44 cm-1 [L N4 Co I-CO] +

The development of CO2 electroreduction (CO2RR) catalysts based on covalent organic frameworks (COFs) is an emerging strategy to produce synthetic fuels. However, our understanding on catalytic mechanisms and structure–activity relationships for COFs is still limited but essential to the rational design of these catalysts. Herein, we report a newly devised CO2 reduction catalyst by loading single-atom centers, {fac-Mn­(CO)3S}, (S = Br, CH3CN, H2O), within a bipyridyl-based COF (COFbpyMn ). COFbpyMn shows a low CO2RR onset potential (η = 190 mV) and high current densities (>12 mA·cm–2, at 550 mV overpotential) in water. TOFCO and TONCO values are as high as 1100 h–1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. Furthermore, we accessed key catalytic intermediates within a COF matrix by combining experimental and computational (DFT) techniques. The COF imposes mechanical constraints on the {fac-Mn­(CO)3S} centers, offering a strategy to avoid forming the detrimental dimeric Mn0–Mn0, which is a resting state typically observed for the homologous molecular complex. The absence of dimeric species correlates to the catalytic enhancement. These findings can guide the rational development of isolated single-atom sites and the improvement of the catalytic performance of reticular materials.

What inspired you for the cover design?The image shows aP ortuguese caravel as as ymbol of the Portuguesed iscoveries in the Renaissance,h ighlighting the fact that this Special Issue is dedicated to the "Portuguese Conference on Catalysis". The unexpected flourishing of manganese as ac atalyst reminded us of the tale of the black swan. We have also included an image of the catalytic transformation of ak etone into the corresponding alcoholm ediated by am anganese-basedc atalyst, represented by the black swan.

Electronic effects provide a general mechanistic scenario for rationalizing photocatalytic water reduction activity with aminopyridine cobalt complexes.