Unexpected Ultrafast Silver Ion Reduction: Dynamics Driven by the Solvent Structure

Role of Oxide-Derived Cu on the Initial Elementary Reaction Intermediate During Catalytic CO2 Reduction

The catalytic role of oxide-derived Cu (OD-Cu) in promoting CO2 reduction (CO2R) to C2+ products has been appreciated for decades. However, the dynamic evolution of the surface oxidation states, together with their real correlation to the binding of reaction intermediates, remains unclear due to technical challenges. Here, we show the time-resolved spectroscopic signatures of key OD-Cu-CO2•- intermediates during catalytic CO2 reduction through one electron transfer from nanoseconds to seconds time scale. We generated the initial intermediate CO2•- radicals in the bulk solution and monitored the interfacial reaction kinetics with well-defined OD-Cu (Cu(0), Cu(I), and Cu(II)) nanoparticles. Combined with molecular simulations, transient absorption profiles analysis reveals that Cu(I) induced a faster CO2•- radical coupling reaction than Cu(0), whereas Cu(II) is only reduced to Cu(I) by the CO2•- radical. Furthermore, the newly developed multistep cumulative pulse methodology uncovered the transition in chemical states of mixed OD-Cu during radical coupling reactions. This pulse radiolysis study provides compelling evidence for the beneficial role of subsurface oxides in early time catalytic CO2 transformation.

Reactivity of quasi-free electrons toward N3- and its impact on H2 formation mechanism in water radiolysis

Picosecond pulse radiolysis measurements were employed to assess the effectiveness of N3- in scavenging quasi-free electrons in aqueous solutions. The absorption spectra of hydrated electrons were recorded within a 100 ps timeframe across four distinct solutions with N3- concentrations of 0.5, 1, 2, and 5 M in water. The results revealed a concentration-dependent shift in the maximum absorption spectra of fully solvated electrons. Notably, at 5 M concentration, the maximum absorption occurred at 670 nm, in contrast to 715 nm observed for water. Intriguingly, the formation yield of hydrated electrons within the initial 5 ps electron pulse remained unaffected, showing that, even at a concentration of 5 M, N3- does not effectively scavenge quasi-free electrons. This is in disagreement with conclusions from stochastic models found in the literature. This observation has an important impact on understanding the mechanism of H2 formation in water radiolysis, which we discuss briefly here.

Excited State Nonadiabatic Molecular Dynamics of Hot Electron Addition to Water Clusters in the Ultrafast Femtosecond Regime

Dissociative electron attachment (DEA) reactions of water result in the production of hydrogen atoms and hydroxide anions. This has been studied for a long time and is relatively slow in liquid water for thermalized hydrated electrons but much faster with a higher-energy electron. Here, we probe the nonadiabatic molecular dynamics after the addition of a hot electron (6-7 eV) to a neutral water cluster (H₂O)_n, where n = 2-12, considering the 0-100 fs time scale using the fewest switches surface hopping method, in conjunction with ab initio molecular dynamics and the Tamm-Dancoff approximation density functional theory method. The nonadiabatic DEA occurs within 10-60 fs, and with high probability, giving H + OH^- above an energy threshold. This is faster than time scales estimated previously for autoionization or adiabatic DEA. The change in threshold energy with cluster size is modest, ranging from 6.6 to 6.9 eV. Dissociation on a femtosecond time scale is consistent with pulsed radiolysis experiments.

Colloidal assembly manipulated by light-responsive Ag3PO4 nanoparticles

We report controllable assembly of polystyrene (PS) microspheres via a photocatalytically driven electroosmotic flow deriving from UV irradiation of Ag₃PO₄ nanoparticles in water. A series of assembly phases, including crystallites, chains and gels, are programmed by systematically modulating the UV intensity, the packing density of the PS microspheres and the concentration of the Ag₃PO₄ nanoparticles. Our findings demonstrate an important ability of light-responsive nanoparticles for colloidal assembly, which offers a new pathway toward effective manipulation of assembly at the microscale.

Presolvated electron reactivity towards CO2 and N2O in water

The reactivity of presolvated electrons with CO₂ and N₂O was studied in the gas pressure range of 1 to 52 bar.

Direct Observation of Solvated Electrons in Deep Eutectic Solvents: Efficient Capture of Presolvated Electrons by DNA Base

The solvated electron being the simplest form of an extremely reactive intermediate is of great fundamental interest in chemistry, physics, and biology since its discovery. Recently, deep eutectic solvents (DESs) have been in focus as biodegradable and cost-effective alternative to ionic liquids (ILs) for different applications. These include areas where electron transport and transfer processes are involved. Herein, we present the first report on the existence, yield, and properties of solvated electrons in three deep eutectic solvents, reline, ethaline, and glyceline, composed of choline chloride as a hydrogen bond acceptor and urea, ethylene glycol, and glycerol (Gly) as hydrogen bond donors, respectively, at a molar ratio of 1:2. The varied transient absorption spectra of solvated electrons in these DESs have been explained on the basis of polarity, hydrogen-bonding effect, and the moieties responsible for creating the environment for solvation. The yield and average lifetime follow the trends in viscosity as well as the reactivity of electrons with the components. The C₃₇ value, a measure of the efficiency of scavenging presolvated electrons, is the highest in ethaline in the case of nitrate ions, which indicates the slowest solvation process in this DES. The presolvated electron capture by a DNA base, an aspect considered to be important in cancer radiotherapy, could be monitored conveniently in these liquids at a much longer time scale compared to that reported in aqueous solutions. Bimolecular rate constants for the reaction of solvated electrons with nitrate and the DNA base have been calculated and compared in the three DESs. Unlike in ILs, these experimentally obtained values are comparable to the diffusion-controlled rate constants in DESs.

Effect of the solvation state of electron in dissociative electron attachment reaction in aqueous solutions

It is generally considered that the pre-solvated electron and the solvated electron reacting with a solute yield the same product. Silver cyanide complex, Ag(CN)₂^-, is used as a simple probe to demonstrate unambiguously the existence of a different reduction mechanism for pre-hydrated electrons. Using systematic multichannel transient absorption measurements at different solute concentrations from millimolar to decimolar, global data analysis and theoretical calculations, we present the dissociative electron attachment on Ag(CN)₂^-. The short-lived silver complex, Ag⁰(CN)₂^2-, formed by hydrated electron with nanosecond pulse radiolysis, can be observed at room temperature. However, at higher temperatures only the free silver atom, Ag⁰, is detected, suggesting that Ag⁰(CN)₂^2- dissociation is fast. Surprisingly, pulse radiolysis measurements on Ag(CN)₂^- reduction, performed by a 7 ps electron pulse at room temperature, show clearly that a new reduced form of silver complex, AgCN^-, is produced within the pulse. This species, absorbing at 560 nm, is not formed by the hydrated electron but exclusively by its precursor. DFT calculations show that the different reactivity of the hydrated and pre-hydrated electrons can be due to the formation of different electronic states of Ag⁰(CN)₂^2-: the prehydrated electron can form an excited state of this complex, which mainly dissociates into Ag⁰CN^- + CN^-.

Inhibition of Radiolytic Molecular Hydrogen Formation by Quenching of Excited State Water

Comparison of experimental measurements of the yield of molecular hydrogen produced in the gamma radiolysis of water and aqueous nitrate solutions with predictions of a Monte Carlo track chemistry model shows that the nitrate anion scavenging of the hydrated electron, its precursor, and hydrogen atom cannot account for the observed decrease in the yield at high nitrate anion concentrations. Inclusion of the quenching of excited states of water (formed by either direct excitation or reaction of the water radical cation with the precursor to the hydrated electron) by the nitrate anion into the reaction scheme provides excellent agreement between the stochastic calculations and experiment demonstrating the existence of this short-lived species and its importance in water radiolysis. Energy transfer from the excited states of water to the nitrate anion producing an excited state provides an additional pathway for the production of nitrogen containing products not accounted for in traditional radiation chemistry scenarios. Such reactions are of central importance in predicting the behavior of liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.

Ultrafast Scavenging of the Precursor of H(•) Atom, (e(-), H3O(+)), in Aqueous Solutions

Picosecond pulse radiolysis measurements have been performed in several highly concentrated HClO4 and H3PO4 aqueous solutions containing silver ions at different concentrations. Silver ion reduction is used to unravel the ultrafast reduction reactions observed at the end of a 7 ps electron pulse. Solvated electrons and silver atoms are observed by the pulse (electron beam)-probe (supercontinuum light) method. In highly acidic solutions, ultrafast reduction of silver ions is observed, a finding that is not compatible with a reaction between the H(•) atom and silver ions, which is known to be thermally activated. In addition, silver ion reduction is found to be even more efficient in phosphoric acid solution than that in neutral solution. In the acidic solutions investigated here, the species responsible for the reduction of silver atoms is considered to be the precursor of the H(•) atom. This precursor, denoted (e(-), H3O(+)), is a pair constituting an electron (not fully solvated) and H3O(+). Its structure differs from that of the pair of a solvated electron and a hydronium ion (es(-), H3O(+)), which absorbs in the visible region. The (e(-), H3O(+)) pair , called the pre-H(•) atom here, undergoes ultrafast electron transfer and can, like the presolvated electron, reduce silver ions much faster than the H(•) atom. Moreover, it is found that with the same concentration of H3O(+) the reduction reaction is favored in the phosphoric acid solution compared to that in the perchloric acid solution because of the less-efficient electron solvation process. The kinetics show that among the three reducing species, (e(-), H3O(+)), (es(-), H3O(+)), and H(•) atom, the first one is the most efficient.

Ultrafast Decay of the Solvated Electron in a Neat Polar Solvent: The Unusual Case of Propylene Carbonate

The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC(-•). The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates.

Transient electrochemistry: beyond simply temporal resolution

Transient electrochemistry is a powerful method to solve many physicochemical issues.

Proton Transfer of Guanine Radical Cations Studied by Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis

The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis. The transient absorption spectral change demonstrates that the neutral radical of G (G(•)(-H(+))), generated by the deprotonation of G radical cation (G(•+)), is rapidly converted to other G radical species. The formation of this species shows the pH dependence, suggesting that it is the G radical cation (G(•+))' formed from the protonation at the N7 of G(•)(-H(+)). On one hand, most Raman bands of (G(•+))' are up-shifted relative to those of G, indicating the increase in the bonding order of pyrimidine (Pyr) and imidazole rings. The (G(•+))' exhibits the characteristic CO stretching mode at ∼1266 cm(-1) corresponding to a C-O single bond, indicating that the unpaired electron in (G(•+))' is localized on the oxygen of the Pyr ring.

Scavenging the Water Cation in Concentrated Acidic Solutions

Picosecond pulse radiolysis techniques were used to observe the kinetics of the SO4(•-), H2PO4(•), Cl2(•-), and Br2(•-) species formed in the fast oxidation of concentrated and highly acidic solutions of SO4(2-), PO4(3-), Cl(-), and Br(-). Experimental results were compared with model predictions to gain insight into the possible mechanisms occurring on the fast time scales. Simple kinetics involving the oxidizing OH(•) radical formed by radiolytic water decomposition could not account for the observed yields at the very short times (within the electron pulse ∼7 ps). Diffusion-kinetic simulations of the spur reactions induced by the incident electrons show that additional oxidation of the solutes must occur at very short times and involves their direct ionization along with scavenging of the highly oxidizing H2O(•+) radical formed in the initial ionization of the water medium. The fraction of H2O(•+) radicals scavenged varies as 0.26, 0.68, 0.92, and 0.97 for PO4(3-), SO4(2-), Cl(-), and Br(-) solutions, respectively. These studies represent the first semiquantitative estimation of the H2O(•+) radicals scavenging fractions for such a wide range of solutes.