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This work, for the first time, reports visible-light active bare graphitic carbon nitride nanotubes (CN NTs) for photocatalytic hydrogen generation, even in the absence of any cocatalyst. Upon uniform dispersion of the cocatalysts, Ag-Cu nanoparticles, on the well-ordered bare CN NTs, they exhibit twice the H evolution rate of the bare CN NTs. The improved activity is attributed to their unique tubular nanostructure, strong metal-support interaction, and efficient photoinduced electron-hole separation compared to their bare and monometallic counterparts, evidenced by complementary characterization techniques. This work reveals that the H production rates correlate well with the oxidation potentials of the sacrificial reagents used. Triethylamine (TEA) outperforms other sacrificial reagents, including triethanolamine (TEOA) and methanol. Mechanistic studies on the role of various sacrificial reagents in photocatalytic H generation demonstrate that irreversible photodegradation of TEA into diethylamine and acetaldehyde via monoelectronic oxidation contributes to the improved H yield. Similarly, TEOA is oxidized to diethanolamine and glycolaldehyde, whereas methanol is unable to quickly capture the photoinduced holes and remains intact due to the low oxidation potential.

Carbon-supported first-row transition metal complexes drive electroreduction of CO 2 to CO in aqueous medium with remarkable activity and selectivity. However, their durability under negative potentials is quite low and the deactivation mechanisms are still not clear. Herein, we present an in-depth mechanistic study on the stability of Co porphyrin-based catalysts during CO 2 reduction in an aqueous electrolyte. The mechanisms of the degradation reactions were evaluated for Co tetraphenylporphyrin (CoTPP) using a combination of spectral, electrochemical, and theoretical methods. Our evidence shows that two major pathways contribute to the gradual activity loss. The first route is oxidative and yields the catalytically inactive complex [Co III TPP]OH. The second pathway is based on reductive carboxylation of the porphyrin ring via transient formation of [Co 0 TPP] 2− and [Co 0 TPP-CO] 2− . The latter reaction disrupts the π-system of the porphyrin structure and leads to the complete disintegration of the macrocyclic core. In contrast to the earlier reports, we found that the direct poisoning by CO, demetallation, and reduction to chlorins play no significant role in the deactivation process. It was further determined that the bulky donating functional groups disfavor the formation of dianionic species and restrict access of CO 2 to the vulnerable meso-position of the porphyrin ligand, thus improving the catalyst stability. The effect was found to be especially strong for the −OMe-substituted complex CoTPP-(OMe) 8 that shows excellent reusability under overpotentials below 500 mV. In turn, electronegative substituents such as fluorine suppress the activity of the catalyst and provide no advantages in terms of durability.

Complexes of first row transition metals are a promising class of tunable and inexpensive catalysts for electrochemical energy applications. Although considerable efforts have been devoted to the activity studies, little attention has been paid to the effects of different immobilization modes on reaction mechanisms. In this work, we studied the influence of covalent immobilization on the performance of Mn tetraphenylporphyrin in oxygen evolution (OER) and oxygen reduction (ORR) reactions. Ligation of the complex to carbon surface was attained via potentiostatic electroreduction of porphyrin diazonium salt with the following metalation and electrodeposition time was found to be a convenient tool to control the amount of electrochemically active catalyst on the electrode. Cyclic voltammetry suggests that the increase of porphyrin surface concentration upon prolonged electrodeposition shortens average Mn–Mn distance and proportionally enhances probability of at least two metal atoms simultaneously participating in a catalytic process. Optimization of organic layer density has profound effect on the catalyst performance in ORR in alkaline medium. 5 min electrodeposition furnishes the best catalyst, which features the 4-electron pathway being predominant at low overpotentials where the noncovalent counterpart shows selectivity to H2O of ∼50%. What is more, overall catalytic current at −0.79 V vs NHE was 2.4 times higher for covalently immobilized porphyrinate. Electrokinetic measurements and impedance spectroscopy suggest that the reaction proceeds via formation of MnII intermediate with stepwise O2 reduction to H2O2 and then to H2O. Similar effects were observed in acidic electrolyte. The OER rate is less sensitive to immobilization mode and mainly depends on the amount of accessible catalyst.

The knowledge of nonequilibrium electron transfer rates is paramount for the design of modern hybrid electrocatalysts. Herein, we propose a general simulation-based approach to interpret variable-frequency square wave voltammetry (VF-SWV) for heterogeneous materials featuring reversible redox behavior. The resistive and capacitive corrections, inclusion of the frequency domain, and statistical treatment of the surface redox kinetics are used to account for the non-ideal nature of electrodes. This approach has been validated in our study of Co II /Co I redox transformation for Co tetraphenylporphyrin (CoTPP) immobilized on carbon cloth and multiwalled carbon nanotubes (CNTs) -one of the most active heterogeneous molecular catalysts in carbon dioxide (CO 2 ) electroreduction. It is demonstrated that the modeling of experimental data furnishes the capacitance of the surface double layer C, uncompensated resistance R u , symmetry coefficients α, kinetic constants k 0 , and equilibrium redox potentials E 0 in one experiment. Moreover, the proposed method yields a stochastic map of the redox kinetics rather than a single value, thus exposing the inhomogeneous nature of the electrochemically active layer. The computed parameters are in excellent agreement with the results of the classic methods such as cyclic voltammetry and fall in line with the reported CoTPP catalytic activity. Thus, VF-SWV is suitable for the study of high-level composites such as covalent organic frameworks and organometallic-CNT mixtures. The resulting insights into the electron transfer mechanisms are especially useful for the rational development of the catalyst−support interfaces and immobilization methods.