Photocatalytic Conversion of Nitrobenzene to Aniline through Sequential Proton-Coupled One-Electron Transfers from a Cadmium Sulfide Quantum Dot

Abstract

This paper describes the use of cadmium sulfide quantum dots (CdS QDs) as visible-light photocatalysts for the reduction of nitrobenzene to aniline through six sequential photoinduced, proton-coupled electron transfers. At pH 3.6-4.3, the internal quantum yield of photons-to-reducing electrons is 37.1% over 54 h of illumination, with no apparent decrease in catalyst activity. Monitoring of the QD exciton by transient absorption reveals that, for each step in the catalytic cycle, the sacrificial reductant, 3-mercaptopropionic acid, scavenges the excitonic hole in ∼5 ps to form QD(•-); electron transfer to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine then occurs on the nanosecond time scale. The rate constants for the single-electron transfer reactions are correlated with the driving forces for the corresponding proton-coupled electron transfers. This result suggests, but does not prove, that electron transfer, not proton transfer, is rate-limiting for these reactions. Nuclear magnetic resonance analysis of the QD-molecule systems shows that the photoproduct aniline, left unprotonated, serves as a poison for the QD catalyst by adsorbing to its surface. Performing the reaction at an acidic pH not only encourages aniline to desorb but also increases the probability of protonated intermediates; the latter effect probably ensures that recruitment of protons is not rate-limiting.

Control of the Redox Activity of PbS Quantum Dots by Tuning Electrostatic Interactions at the Quantum Dot/Solvent Interface

“…The concentration of AQ decays with decreasing distance from the QD core due to increasing Coulomb repulsion (i.e., increasing ( )). Equation 6 can be rationalized further by the concept of electrochemical equilibrium, which is explained in detail in the Supporting Information, eqs S4-S8. We then define the Gibbs free energy change upon translation of an AQ from bulk solution to the surface of a QD (located at = the radius of the QD core, 1.6 nm), ∆ , using eq 7.…”

Electrostatic assembly of gold nanorods on a glass substrate for sustainable photocatalytic reduction via sodium borohydride

“…This reaction has been previously studied using a variety of gold nanostructures as catalysts with observed rate constants ranging from ~10 -2 -10 -3 min -1 . Under photolytic conditions, the electrons in the Au NR surface plasmon increase the rates of charge injection, [59][60] which promote the overall To compare the efficiencies of Au NR-functionalized coverslip catalysts to unadhered Au NRs, the number of nanorods on a coverslip was first calculated so that the solution concentration of unadhered nanorods could be calibrated. 58 The first step in the catalysis is thought to occur through the adsorption of the reactive species onto the surface of the gold nanoparticle where the nitroarene is then reduced to a nitroso compound (Scheme 2, eqn.…”

Improving Photocatalytic Performance from Bi2WO6@MoS2/graphene Hybrids via Gradual Charge Transferred Pathway

“…At present, various types of semiconductor photocatalysts have been reported such as TiO 2 3–5 and ZnO 6–8 with broad bandgap which absorbs only ultraviolet light which accounts for only about 3~4% of sunlight. Others like CdS 9–11 and CdSe 12–14 , with narrow band gap which absorbs visible light, whereas contain cadmium element which is a widespread environmental pollutant with high toxicity. Thus, it is extremely necessary to look for a narrow band gap, non-toxic and environmentally-friendly visible-light photocatalysts.…”