Metalloporphyrins as Catalytic Models for Studying Hydrogen and Oxygen Evolution and Oxygen Reduction Reactions

Abstract

Metrics & More Article Recommendations * sı Supporting Information CONSPECTUS: The hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are involved in biological and artificial energy conversions. H−H and O−O bond formation/cleavage are essential steps in these reactions. In nature, intermediates involved in the H−H and O−O bond formation/cleavage are highly reactive and short-lived, making their identification and investigation difficult. In artificial catalysis, the realization of these reactions at considerable rates and close to their thermodynamic reaction equilibria remains a challenge. Therefore, the elucidation of the reaction mechanisms and structure−function relationships is of fundamental significance to understand these reactions and to develop catalysts. This Account describes our recent investigations on catalytic HER, OER, and ORR with metalloporphyrins and derivatives. Metalloporphyrins are used in nature for light harvesting, energy conversion, electron transfer, O 2 activation, and peroxide degradation. Synthetic metal porphyrin complexes are shown to be active for these reactions. We focused on exploring metalloporphyrins to study reaction mechanisms and structure−function relationships because they have stable and tunable structures and characteristic spectroscopic properties. For HER, we identified three H−H bond formation mechanisms and established the correlation between these processes and metal hydride electronic structures. Importantly, we provided direct experimental evidence for the bimetallic homolytic H−H bond formation mechanism by using sterically bulky porphyrins. Homolytic HER has been long proposed but rarely verified because the coupling of active hydride intermediates occurs spontaneously and quickly, making their detection challenging. By blocking the bimolecular mechanism through steric effects, we stabilized and characterized the Ni III −H intermediate and verified homolytic HER by comparing the reaction behaviors of Ni porphyrins with and without steric effects. We therefore provided an unprecedented example to control homolytic versus heterolytic HER mechanisms through tuning steric effects of molecular catalysts.For the OER, the water nucleophilic attack (WNA) on high-valent terminal Mn-oxo has been proposed for the O−O bond formation in natural and artificial water oxidation. By using Mn tris(pentafluorophenyl)corrole, we identified Mn V (O) and Mn IVperoxo intermediates in chemical and electrochemical OER and provided direct experimental evidence for the Mn-based WNA mechanism. Moreover, we demonstrated several catalyst design strategies to enhance the WNA rate, including the pioneering use of protective axial ligands. By studying Cu porphyrins, we proposed a bimolecular coupling mechanism between two metal-hydroxide radicals to form O−O bonds. Note that late-transition metals do not likely form terminal metal-oxo/oxyl. For the ORR, we presented several strategies to improve activity and selectivity, including providi...

N/O‐co‐doped Carbon Shell Structures Loaded with Iron Phthalocyanine for Oxygen Reduction Catalysis

“…Therefore, the identification of new cathode catalysts that are inexpensive, methanol resistant and easy to prepare as replacements for Pt‐based materials is a crucial task related to fuel cell development. Good progress has been made in the study of the ORR mechanism by porphyrins and their derivative bioenzyme catalysts, as reported by Cao and his colleagues [5–7] . Carbon‐based monoatomic metal cathode catalysts are an excellent alternative to Pt‐based catalyst materials because they are less expensive, resistant to methanol, stable and can be produced on a large scale [8,9] .…”

“…Good progress has been made in the study of the ORR mechanism by porphyrins and their derivative bioenzyme catalysts, as reported by Cao and his colleagues. [5][6][7] Carbonbased monoatomic metal cathode catalysts are an excellent alternative to Pt-based catalyst materials because they are less expensive, resistant to methanol, stable and can be produced on a large scale. [8,9] Many current studies on transition metal single-atom catalysts, such as the single-atom Cu-S 1 N 3 reported by Yu and his colleagues, have shown good catalytic activity.…”

Long-lived excited states of platinum(ii)-porphyrins for highly efficient photocatalytic hydrogen evolution

“…Recently, the research on photocatalytic hydrogen evolution (PHE) has been developed very fast for the production of H 2 as a clean energy and renewable fuel that would reduce the consumption of fossil fuels and alleviate the global warming crisis. 6 To date, various photocatalysts based on transition metal complexes, [7][8][9][10][11][12] porous materials, 3,[13][14][15][16] metal suldes/ oxides, [17][18][19] organic and inorganic hybrids, 11,[20][21][22][23][24][25] organic molecules, [26][27][28][29] and porphyrins 5,[30][31][32][33][34][35][36] have been demonstrated for efficient PHE. Among them porphyrins received much more attention for PHE due to their strong light absorption in the UVvisible region and easily tailorable photophysical and redox properties by integrating different aromatic chromophores to the porphyrin ring and inserting different metals in the porphyrin ring.…”

Charge-Transfer Mechanism in Oxygen Reduction over Co Porphyrins: Single-Site Molecular Electrocatalysts to Macromolecular Frameworks

“… − As such, a protonated 2-pyridyl substituent at the Co porphyrin core improves the formal selectivity to 4e – reduction to 70% compared to 30% for tetraphenylporphyrin (CoTPP) in the 1 M H 2 SO 4 electrolyte at an overpotential of 830 mV . Similarly, an atropoisomer bearing four amido groups favoring the stabilization of the O 2 adduct via hydrogen bonding was also demonstrated to give an electron transfer number (ETN) of 3.75 in the alkaline electrolyte. , An alternative approach utilizes inter- or intramolecular binding of two metal atoms to the O 2 molecule with the formation of a μ-superoxo intermediate [Co–O–O–Co], which facilitates the cleavage of the O–O fragment. , The intramolecular binding could be illustrated on the example of Co porphyrin-porphyrin dyad, which places Co atoms close enough for the 4e – mechanism to become dominant (selectivity to H 2 O ≈ 95% at η = 510 mV, electrolyte 0.5 M H 2 SO 4 ) . In turn, the involvement of intermolecular binding is possible only for small, sterically unloaded molecules such as unsubstituted Co porphyrin (Figure S1b) which shows selectivity to 4e – reduction of more than 90% at 0.77 V vs NHE .…”