Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

Solution Reactivity and Mechanisms through Pulse Radiolysis
Sergei V. Lymar
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000
e-mail: lymar@bnl.gov
Program Scope
This program applies pulse radiolysis for investigating reactive intermediates and
inorganic reaction mechanisms. The specific systems are selected based on their fundamental
significance or importance in energy and environmental problems.
The first project investigates physical chemistry of nitrogen oxides and their congeneric
oxoacids and oxoanions. They play an essential role in environmental chemistry, particularly in
the terrestrial nitrogen cycle, pollution, bioremediation, and ozone depletion. Nitrogen-oxygen
intermediates are also central to the radiation-induced reactions that occur in nuclear fuel
processing and within attendant nuclear waste. Redox and radical chemistry of the nitrate/nitrite
system mediates the most of radiation-induced transformations in these environments. Equally
important are the biological roles of nitrogen oxides. This research program applies timeresolved
techniques for elucidation of the prospective reactions in terms of their
thermodynamics, rates and mechanisms, focusing on the positive nitrogen oxidation states,
whose chemistry is of the greatest current interest.
The goal of the second project is to gain mechanistic insight into water oxidation
catalysis through characterization of the catalyst transients involved in the catalytic cycle.
Development of catalysts to carry out the four-electron water oxidation remains the greatest
challenge in the solar energy utilization. Of all catalysts that have been examined, the dimeric
oxo-bridged ruthenium ion ([(bpy)2(H2O)Ru-O-Ru(OH2)(bpy)2] 4+ , known as the “blue dimer”)
has been the most extensively studied, but the reaction mechanisms remain to be established. The
major impediment has been the difficulties in identification and characterization of the reaction
intermediates. In this project, we use radiolysis techniques to generate and characterize the redox
states of the catalyst involved in water oxidation by the “blue dimer” and by an all-inorganic
catalyst based on cobalt hydrous oxide immobilized on silica nanoparticles that we have recently
developed.
Collaborators on these projects include M. Valiev (PNNL), 1 Shafirovich (NYU), 2 J. Hurst
(WSU), 3 and H. Schwarz (BNL, emeritus). 4
Recent Progress.
The nitrate radical ion (NO3 •2- ). This is a major species that mediates the radiationinduced
transformations in nuclear fuel processing and in
ε, 10 3 M -1 cm -1
4
2
0
ΔOD at 260 nm
.2-
NO3 expected at 1 M NaOH
observed, k = 7 x 10 4 s -1
20 40 60 80
microseconds
250 300 350 400 450
wavelength, nm
Hanford nuclear wastes. However, properties and
reactivity of NO3 •2- are not understood. With pulse
radiolysis, the radical can be generated through the rapid
e - aq + NO3 - reaction but decays rapidly via hydrolysis:
NO3 •2- + H2O ↔ NO2 • + 2OH - (data to the left). The
lowest of the literature reduction potentials E 0 (NO3 -
/NO3 •2- ) = -0.89 V 5 predicts the hydrolysis equilibrium
constant Khydr ≈ 0.02 M 2 ; that is, the equilibrium favors
NO3 •2- even at modest alkalinities. However, we do not
127