ABSTRACT
Several H-H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho- or para-positions of the meso-phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e-reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2 . Understanding HER processes, especially controllable switching between homolytic and heterolytic H-H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis.
ABSTRACT
FeIII chloride tetrakis(pentafluorophenyl)porphyrin (1) is known to have poor electrocatalytic activity for the CO2 -to-CO conversion in dimethylformamide. In this work, we re-examined Fe porphyrin 1 as a CO2 reduction catalyst in various solvents. Our results show that 1 displays fairly high electrocatalytic CO2 -to-CO activity in acetonitrile with a turnover frequency (TOF) up to 4.2×104 â s-1 . On the other hand, 1 shows a modest activity in propylene carbonate, and is inefficient to catalyze CO2 reduction in benzonitrile, dimethylformamide, and tetrahydrofuran. Several solvent effects were considered, but none of these effects alone can explain the observed large activity difference of 1 for CO2 reduction in these solvents. Based on the results, it is suggested that more care should be paid when comparing different CO2 reduction catalysts because solvent effects are significant and are underevaluated.
ABSTRACT
A cobalt(ii) tetrakis(2,3,5,6-tetrafluoro-4-ethynylphenyl)porphyrin (FCoP-H) was designed and synthesized. With carbon-nanotube-templated polymerization, covalent porphyrin frameworks using FCoP-H can be synthesized via the Hay-coupling. The resulting FCoP@CNT is efficient to catalyze the hydrogen evolution reaction (HER) in aqueous media, and its performance is superior, in terms of both lower overpotentials and larger catalytic currents, to covalent porphyrin frameworks using non-fluorinated cobalt porphyrin analogues and also monomeric fluorinated cobalt porphyrins simply adsorbed on CNTs. This work combines the merits of CNT-templated covalent frameworks and fluorinated cobalt porphyrins to significantly improve the HER performance. This strategy is valuable to be explored in other electrocatalytic systems using molecule-engineered carbon materials.
