Inorganic Chemistry
Article
1,52
5
catalytic activity with a very low limit for the identification of
Levich equation and Koutecky− Levich equations.
́
3
7
hydrazine as compared with other metal oxide species. The
birnessite-type MnO consists of edge-shared MnO octahedra
Interestingly, we found that sensitivity is also related to
rotation speed. Note that this effect has been ignored by most
previous studies. We have also found that the limit of detection
(LOD) of Li-birnessite-type MnO2 (Li-Bir) significantly
decreases from 194 to 102 nM with an increase in rotation
speed from 1000 to 6000 rpm, respectively. The selectivity and
practicality of our as-fabricated electrode in a real-world
sample (drinking water) were also assessed at an optimum
speed of electrode rotation.
2
6
with cations in its interlayer spacing providing faster ion
transportation compared to that of other MnO structures.
The reaction mechanism is based on intercalation/deinterca-
lation by exchanging protons in the solution on MnO2.
The proton (reaction 1) or alkali cations (reaction 2) are
deintercalated out of the bilayer during the oxidation process
followed by intercalation during the reduction process.
3
8
2
3
9−44
+
−
MnO + H + e ↔ MnOOH
(1)
(2)
2
EXPERIMENTAL SECTION
■
[
+
−
MnO + C + e ↔ MnOOC
Chemicals and Materials. Manganese(II) nitrate tetrahydrate
2
98% Mn(NO ) ·4H O, Loba Chemie], lithuim hydroxide (98.0%
3
2
2
The rate of kinetic exchange of cations in the layer structure
depends on a function of the hydration energy and exchange
LiOH, Sigma-Aldrich), sodium hydroxide (99.8% NaOH, Ajax
Finechem), potassium hydroxide (99.0% KOH, Sigma-Aldrich),
hydrazine hydrate (98% N H , Merck), a Nafion perfluorinated
41,45
rate between cations and protons in the solution.
One
2
4
factor is the ionic size of the intercalated cation or intercalant.
resin solution (5 wt % Nafion in a mixture of lower aliphatic alcohols
and water, containing 45% water, Sigma-Aldrich), ethanol (99.9%
2
+
For example, Ca exhibits relatively low catalytic activity due
to a higher hydration energy and a slower exchange rate
EtOH, RCI Labscan), nitric acid (65% HNO , Qrec), disodium
4
+
8
−1
9
−1
hydrogen orthophosphate dihydrate (99% Na HPO , UNILAB), and
2
4
compared to that of Na (10 s vs 10 s ), so its catalytic
activity is typically low due to the slow mobility of the proton
potassium dihydrogen phosphate (99% KH PO , Loba Chemie) were
2
4
4
5
analytical reagent grade and used without further purification. The
carbon fiber paper (CFP) was ordered from SGL Carbon. Deionized
water was collected by a Milli-Q system (DI water, 15 MΩ cm,
Millipore).
and other ions in the interlayer. To improve our under-
standing of the influence of the cation, herein, we designed the
+
+
birnessite-type MnO with different intercalants (C = Li ,
2
+
+
Na , or K ) and used them as electrocatalysts for the oxidation
of N H at pH 7.4 for potential sensor applications. The
Preparation of Birnessite Manganese Oxide Electrodes. The
birnessite materials were synthesized by following the previously
2
4
43,53,54
electrochemistry, in situ X-ray absorption spectroscopy (XAS),
and theoretical density functional theory (DFT) calculations
were carried out to explain the effect of the intercalants on the
mechanism of N H oxidation in terms of adsorption energy
described procedure.
The birnessite materials with different
intercalants were synthesized by adding 100 mL of the mixed solution
of 1 M H O and 0.6 M cation precursor in 60 mL of a 0.25 M
2
2
Mn(NO ) ·4H O solution, and the mixed solution was stirred for 1 h
3
2
2
2
4
55
under ambient conditions (25 °C). Note that the cation precursor is
LiOH, NaOH, or KOH. The product became dark brown, and then
the solid was precipitated. Then, the as-synthesized materials were
collected by vacuum filtration and dried at 50 °C overnight. The as-
synthesized birnessite materials are denoted as Li-Bir, Na-Bir, and K-
and electron transfer rate.
Hydrazine is oxidized principally through a four-electron
process (reaction 6). In the real operating system, the
significant kinetic limitations of hydrazine oxidation depend
46
+
+
+
on the solution and the nature of the electrodes. This work
presents hydrazine oxidation in the presence of 0.1 M
Bir for the intercalated Li , Na , and K , respectively, in the birnessite
structures. The ink suspension was prepared with 10 mg of birnessite
materials in 1 mL of a solution, consisting of 980 μL of ethanol and 20
μL of a Nafion perfluorinated resin solution (Sigma). Note that
47
phosphate-buffered saline (PBS) at pH 7.4 (pK = 8.1). At
a
this pH, hydrazine mostly exists in its protonated state
56,57
+
Nafion was used as an adhesive binder.
To make a uniform
(
hydrazinium, N H ). The general oxidation mechanism of
2
5
+
dispersion, the as-prepared ink was sonicated for 30 min. Two
microliters of the as-prepared ink was dropped and coated on a 3 mm
diameter glassy carbon rotating disk electrode (GC RDE) (Eco-
Chemie, Utrecht, The Netherlands) with a loading content of ∼20 μg
of finely tuned birnessite materials. Before the GC electrode was used,
it was polished with an alumina slurry and washed with DI water. The
birnessite-modified GC electrode was dried at room temperature in a
vacuum oven.
protonated hydrazine consists of three elementary steps. N H
2
5
first adsorbs on the surface of the catalyst (reaction 3). Then, it
+
is further dehydrogenated to N H (reaction 4, rate-
2
4
+
determining step). Finally, N H4 is decomposed, generating
2
4
8−50
N gas as the final product (reaction 5).
2
+
+
N H (soln) F N H (ads)
(3)
2
5
2
5
Characterization. The morphology of the as-prepared samples
was observed by using field-emission scanning electron microscopy
FE-SEM) (JEOL, JSM-7001F) at 15 kV. High-resolution trans-
mission electron microscopy (HR-TEM) images of the samples were
taken at 120 kV (Hitachi). The specimens were prepared by
dispersing the sample (∼0.05 mg/mL) in ethanol, subsequently
dropping it on the copper grids and drying the solvent at 50 °C for 3
h. X-ray diffraction (XRD) patterns of the samples were recorded with
a Bruker D8 ADVANCE instrument using Cu Kα radiation (30 kV,
+
+
+
N H (ads) → N H (ads) + H
2
5
2
4
(
+
e− (slow, rate‐determining step)
(4)
(5)
+
+
−
N H (ads) → N + 4H + 3e (fast, final step)
2
4
2
The overall reaction
+
+
−
N H → N + 5H + 4e
(6)
2
5
2
4
0 mA) with a step size of 0.01° in the 2θ range of 5−80°. The
structure of the materials was characterized by Raman spectroscopy
with a laser wavelength of 532 nm (Senterra Dispersive Raman,
Bruker Optics) and FTIR spectroscopy (PerkinElmer). The sheet
resistivity of the materials was investigated by using a a cylindrical-
type JANDEL four-point probe. To prepare the four-point probe
specimen, the ink suspension of the material was placed on an FTO
glass substrate and dried at 50 °C. The resistance of each material was
measured at a current of 10 μA.
During the electro-oxidation of N H at the working electrode,
2
4
gas is typically produced, and bubble formation usually blocks
the electrode surface. Our work has overcome this limitation
by applying the hydrodynamic diffusion effect to the system
using a rotating disk electrode (RDE). The reaction kinetics
associated with the current response depends on the angular
velocity (ω) of the electrode, which has been described in the
B
Inorg. Chem. XXXX, XXX, XXX−XXX