Anions as Intermediates in Plasmon Enhanced Photocatalytic Reactions

Abstract

Hot electron transfer to unoccupied molecular orbitals is the most popularized mechanism for plasmon enhanced photocatalytic reactions, while a few reactions have been attributed to near-field enhanced intramolecular adsorbate electronic photoexcitation. In this Perspective, it is argued that the role of hot electrons may be limited to preparing anionic complexes that can undergo further chemical transformation via photoexcitation to dissociative potential energy surfaces. The proposed mechanism is based on careful analysis of two different examples of plasmon mediated reactions: N-demethylation of methylene blue (MB) and dissociation of CO2. In the case of MB N-demethylation, experimental evidence obtained from in situ surface enhanced Raman scattering (SERS) spectroscopy shows that MB converts selectively to thionine on gold nanoparticles (NPs) at optimal surface–molecule separation at an excitation wavelength that matches the electronic transition of the molecule. When MB is directly adsorbed on the metal NPs, the SERS spectra recorded at near-infrared wavelength indicate the formation of an anionic complex that can undergo nonselective N-demethylation when visible light is used. Similarly, comparison of the wavelength dependent plasmon mediated photocatalytic dissociation and hydrogenation of CO2 on metal NPs to the related gas phase photochemistry suggests that the mechanism likely involves photoexcitation of surface bound CO2 – anions prepared via metal to molecule electron transfer. In general, it is proposed that near-field enhanced direct electronic excitation of adsorbate or adsorbate–metal complex plays a critical role in most, if not all, plasmon mediated photochemical reactions, while hot electrons prepare reactive intermediate species.

Theoretical Investigations on the Plasmon-Mediated Dissociation of Small Molecules in the Presence of Silver Atomic Wires

“…The terminal N2 initially picks up more negative charge than N1. This charge transfer profile provides support for the idea that transient negative ions play an important role in plasmon-mediated molecular activations . For this system (parallel adsorbate, longitudinal plasmon excitation, 0.03 a.u.…”

“…This charge transfer profile provides support for the idea that transient negative ions play an important role in plasmon-mediated molecular activations. 55 For this system (parallel adsorbate, longitudinal plasmon excitation, 0.03 a.u. field strength), a transient negative ion forms within the first 5 fs of the simulation, then dissipates around 42 fs into the simulation.…”

Mechanistic Study of Plasmon-Assisted In Situ Photoelectrochemical CO2 Reduction to Acetate with a Ag/Cu2O Nanodendrite Electrode

“…The *CO 2 to *CO pathway has been studied extensively, and various stabilization mechanisms have been proposed, for example, some works have shown that the *CO 2 – is stabilized by a single proton transfer from water and then, through a proton-electron transfer, reduced to *CO. , Another pathway proposed by Sheng and collaborators is that the *CO 2 – is reduced to *CO through a single electron transfer of a *CO 2 dimer radical anion to convert it to *CO and CO 3 2– . Other works indicate that the pathway from CO 2 toward the intermediate *CO strongly depends on the excitation wavelength and the nature of the catalyst, where recent research suggests that the activation and division of the CO 2 molecule depends on hot electrons produced from the decay of and that the strong reduction of this molecule from these hot electrons is accompanied by its stabilization through the surface (S) and solvent molecules (Ln) as shown in the following equation: false( normalS * CO 2 false) − Ln + h v → false( normalS false* normalO false) − Ln * CO where the *CO 2 – dissociation takes place on the excited state potential energy surface, which may yield an adsorbed *CO molecule. The formation of adsorbed *CO agrees with our in situ time-resolved experiments, where we observe a rapid increase in *CO during the first minutes of electrolysis that then persists during the entire time the experiment is carried out.…”

Plasmon Modifies Formic Acid Dehydrogenation Kinetics

“…It has been well demonstrated field-enhanced direct excitation of adsorbate or adsorbate-metal complex plays a critical role in plasmon induced chemical reactions. 29,30 An in situ monitoring of FA dehydrogenation by a single-particle spectroscopy also indicated FA molecules could be activated through direct energy transfer or indirect energy transfer over Au−Ag@Pd or Au@Pd nanorods, 13 and the strong EF at the tips of the nanorods particularly contributed to FA adsorption and activation. 11 It is thus believed that the distinct near-field enhancement of the PDA@AgPd-400 greatly contributes to its highest R plas .…”