jp5b12380_si_liveslides.zip (6.86 MB)
Photocatalytic Reduction of Fumarate to Succinate on ZnS Mineral Surfaces
media
posted on 2016-04-28, 20:23 authored by Ruixin Zhou, Marcelo I. GuzmanThe reductive tricarboxylic acid
(rTCA) cycle is an important central
biosynthetic pathway that fixes CO2 into carboxylic acids.
Among the five reductive steps in the rTCA cycle, the two-electron
reduction of fumarate to succinate proceeds nonenzymatically on the
surface of photoexcited sphalerite (ZnS) colloids suspended in water.
This model reaction is chosen to systematically study the surface
photoprocess occurring on ZnS in the presence of [Na2S]
(1–10 mM) hole scavenger at 15 °C. Experiments at variable
pH (5–10) indicate that monodissociated fumaric acid is the
primary electron acceptor forming the monoprotic form of succinic
acid. The following reaction scheme is proposed: (1) photoexcitation
of ZnS generates conduction band electrons and valence band holes,
(2) the hole scavenger donates electrons while producing sulfur-containing
intermediates en route to sulfate formation, (3) a first electron
transfer occurs at the conduction band converting chemisorbed monoprotic
fumaric acid at surface zinc sites into an adsorb radical anion, and
(4) the radical anion accepts a second electron and forms an adsorbed
carbanion, which (5) abstracts two protons consecutively from either
hydronium ion (acidic condition) or water (neutral and basic condition)
to be desorbed as monodissociated succinic acid. The apparent quantum
yield measurement of succinate production (Φs) under
periodic irradiation at λ ≥ 305 nm shows that the time
scale of electron transfer on the conduction band (t1) and valence band hole loss (t2) are in the order of hundred microseconds and a few milliseconds,
respectively. These transitions (t1 and t2) become undistinguishable at 520 μs
for a zeta potential ζ = −22.09 mV corresponding to [Na2S] = 0.57 mM. Overall, this work provides new insights to
model heterogeneous processes such as the reduction of CO2 occurring on the surface of photocatalysts and advance present understanding
of photocatalytic reactions.
History
Usage metrics
Categories
Keywords
surface zinc sitesvalence band holest 2t 1conduction bandmonodissociated succinic acidvalence band hole lossreductive tricarboxylic acidchemisorbed monoprotic fumaric acidelectron transfermonodissociated fumaric acidsuccinate proceeds nonenzymaticallyZnS Mineral Surfaceshole scavengerNa 2 Sfixes CO 2conduction band electrons
Licence
Exports
RefWorks
BibTeX
Ref. manager
Endnote
DataCite
NLM
DC