Unpaired Electrons as Probes of Catalytic Systems

Abstract

An overview is provided of the importance of molecular species containing unpaired electrons in catalytic systems, as revealed using ESR spectroscopy. The review aims to demonstrate the considerable extent of scientific progress that has been made in this broad topic during the past few decades. Studies of catalytically active surfaces, including zeolites, are surveyed, and the detection of radical species, formed as intermediates in their reactions, using matrix isolation and spintrapping techniques. Radical cation formation in zeolites is discussed, and the employment of muon spin rotation and relaxation techniques to study the mobility of labelled radicals in various porous and catalytic media. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, degradation of environmental pollutants, hydrocarbon conversions, fuel cells and sensor devices employing graphene. The review concludes with recent developments in the study of enzymes and their reactions, using ESR-based methods.

Alcohol imination catalyzed by carbon nanostructures synthesized by C(sp2)-C(sp3) free radical coupling

“…56 Thus, the adsorbed O 2 can be reduced to O 2 ·− by unpaired electrons from the PCS-1, which serves to leave the holes free to react. 57 Thermal excitation to generate e-h pairs does not require any other type of initiation, which can be caused by defects. 58 Note that the positive “hole” is created in the valence band (VB) whereas an electron is transferred to the conduction band (CB) after thermal excitation.…”

“… 59 Primary benzylamine is firmly anchored to the catalytic system to form an electron-donor complex that is subsequently oxidized to form the radical cation by the positive charge of the hole. 57 We, therefore, carried out first-principles density functional theory (DFT) calculations on the reaction pathway free-energy profiles for further validation of the reaction mechanism hypothesis ( Figure 5 C).

Figure 5 High spin catalyzed dehydrogenative coupling of alcohols and amines analysis (A) Theoretical calculations of the HOMO and LUMO distributions of PCS-1 in a defect-free state.

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“…The initial species (hole in VB and electron in CB) can either react directly with the organic reagents when adsorbed on the surface of the particle, or they may be converted into reactive intermediates which react with the organic pollutants either directly on the surface or close to it. 57 Therefore, the benzyl alcohol undergoes complete oxidation under the influence of O 2 ·− , transforms to an aldehyde (TS1), 59 and then reacts with the benzylamine radical complex (TS2) to generate N-benzylidene-1-phenylmethanamine. It then passes through different transition states (TS3-5) to finally form the corresponding imine and amide, whereas this process appears to be concomitant with the elimination of one molecule of water in the case of pathways 2 and 3, 17 , 48 pathway 1 to generate amide is less kinetic favorable.…”