Download Study of graphite electrode surface with In and Pt deposits E.M.

Study of graphite electrode surface with In and Pt deposits
E.M. Ustinova1,a, A.Ya. Pshenichkin1,b, N.A. Kolpakova1,c
1
634050, Russia, Tomsk, Lenina Avenue, 30
a
[email protected], [email protected], [email protected]
Key words: indium, platinum, intermetallic compound, binary alloy, stripping voltammetry, graphite
electrode, electron microscopy.
Abstract: Electrolytic deposits of indium, platinum and platinum-indium alloy on the surface of
graphite electrodes have been studied by scanning electron microscopy and stripping voltammetry.
The scanning electron microscopy method has discovered that the platinum-indium alloy on the
surface of the graphite electrode contains indium, platinum and chlorine. But it was unable to
determine platinum on the surface of the graphite electrode, because without a metal activator it is
precipitated in very small quantities. Under anodic oxidation of the indium-platinum precipitation
the anodic peaks being caused by the selective electro-oxidation of indium from intermetallic
compounds with platinum are observed on the current-voltage curves. Platinum remains on the
electrode surface, and can be oxidized from the electrode at the potentials of electrode purification
(more than 1 V).
Introduction
It has recently been established [1,2] that it was impossible to fix the platinum ion reduction (II, IV)
on the graphite electrode (GE) and the electrochemical oxidation of platinum on the current-voltage
curves. This is due to the fact that catalytic hydrogen release always occurs with the platinum ion
reduction (IV). This leads to the process of platinum ions electroreduction overlapping and the
simultaneous oxygen release from water, as it occurs at the potentials which are greater than 1 V. In
addition, electro-oxidation of platinum occurs before its oxide [3].
Our research showed [4,5] that determination of platinum by the stripping voltammetry can be
used in case of platinum precipitation into the base metals alloy: copper, lead, mercury (metalactivators), etc. Platinum with metal-activators is supposed to form one or more intermetallic
compounds (IMC) with these metals on the surface of the electrode.
During the process of the deposit oxidizing, several additional peaks on the current-voltage
curves are observed, except the electrochemical oxidation peak of pure metal-activator with the
potentials related to the selective electrochemical oxidation of an electronegative component from
different compositions of IMC.
The authors of the papers [4,5] proposed to use indium as a metal activator for the platinum
determination by the stripping voltammetry method.
Indium is of interest because its electro-oxidation peak is observed at the more electronegative
potentials than that of spontaneous electro-oxidation of indium from the IMC with platinum, which
provides to introduce high concentrations of indium (III) ions and low content of platinum ions (II,
IV) into solutions.
When there is high content of indium (III) ions in the sample solution at the preliminary
electrolysis stage, a modifying indium layer with the highly-developed active surface is formed;
with the platinum (II, IV) ions being precipitated with less tension. Selective electro-oxidation of
the electronegative component (of the different composition of IMC) allows to determine platinum
according to the anodic peaks.
However, there is no clear answer whether platinum is deposited on the graphite electrode
surface from the dilute solutions without a metal-activator, and whether platinum is kept after
electro-oxidation of all IMCs, or whether it is oxidized with indium (uniform alloy electrooxidation).
The goal of this paper is to study the electrode surface by the scanning electron microscopy
method at various shooting points of the voltage curve and to determine the anodic peak nature
using the voltammetry method.
Experimental section
In our work we used a ТА-4-type two-electrode-cell coulometric analyzer manufactured by a
Research and Production Enterprise “Tom’analit”, Tomsk, and quartz glasses of V=20 ml as
electrolysers. A polyethylene-impregnated graphite electrode as a work electrode was produced
according to the methodology [6]. A saturated chloride- silver electrode served as a reference one.
Platinum (IV) and indium (III) were deposited from solutions containing chlorides of theses metals
at the potential of 1.2 V. The graphite electrode was electrochemically cleaned by keeping it at the
potential 1.05 V for 60 s.
Surface shooting of the graphite electrode after anodic processes was carried out by the
scanning electron microscope S-3400 N Hitachi (Japan), with the resolution of 3 nm, using the
energy dispersive spectrometer Bruker xflash mark 4010.
Result and discussion
Indium ions (III) in 1 M HCl solutions are precipitated on the impregnated graphite electrode
surface at the electrolysis potential E = -1.2 V.
The peak of electric oxidation of In from the graphite electrode surface occurs at the potential
of -0.7 V. The indium deposit on the graphite electrode surface was investigated by means of the
scanning electron microscopy method. For this process, after electric precipitation of In from the 1
M HCl solution at the electrolysis potential E = -1.2 V for 200 seconds, GE was taken out from the
device, dried at room temperature, and then, a surface research on the scanning electron microscope
was carried out.
The GE surface spectrum with the indium deposit is shown in Fig. 1. It can be seen that there
is a significant amount of chlorine and indium on the GE surface, and it is presented by several
characteristic peaks.
Then, electric precipitation (on the GE surface) of Pt from chloride solutions was carried out. After
that, the electrode surface was investigated by the scanning electron microscope.
Fig.1.
The
graphite
electrode
surface
spectrum
with
In
deposit.
Fig. 2. The graphite electrode surface spectrum with Pt deposit.
Fig. 2 shows the GE surface spectrum with Pt precipitation. It is clear the electron microscope
didn’t fix the Pt precipitation on the electrode, although Pt is fixed by the stripping voltammetry
method. This may be due to the fact that the deposit has insufficient amount of Pt since the
microscope resolution is more than 1 volume percent.
There are maxima of elements that are contained in the GE as impurity. This fact indicates
that Pt is not precipitated on the electrode without metal -activators. We have studied the surface of
the GE after platinum precipitation by the stripping voltammetry method. Indium precipitation was
carried out on the graphite electrode surface with Pt deposit. The obtained deposit was oxidized in
the potential region from -0.45 V. In the voltage-current curve the anodic peak of selective electric
oxidation of In from IMC with Pt at the potential of -0.13 V (Fig. 3) was observed, but the peak was
very small.
Fig. 3. Voltammogram of the selective electric oxidation of In from IMC with Pt from the graphite
electrode surface: a – background 1 M HCl; b - concentration of Pt (IV) 0.04 mgl-1 and the
concentration of In (III) ions in the solution of 0.5 gl-1
These experiments showed that Pt is precipitated on the graphite electrode surface, but the
current efficiency is very small ~ 24%. The current efficiency was estimated as the maximum
current of In from the IMC with Pt in the described experiment, and in the experiment, when the InPt deposit occurred «in situ». This phenomenon can be explained by the small active GE surface for
the electric precipitation of Pt and a large current efficiency of the water decomposition process
with hydrogen release in the reduction reaction of Pt.
Then, electric precipitation of In-Pt alloy was carried at the potential -1.2 V. As in the
previous experiment after electrolysis for 200 seconds the electrode was removed, dried at room
temperature and the study of the surface was provided by the scanning electron microscopy method.
The results are shown in Fig. 4.
In the papers [4, 5] the anodic peak at the potential of -0.1 V is established to be due to the selective
electric oxidation of In from IMC with Pt.
Fig. 4. The spectrum of the graphite electrode surface with In-Pt deposit
Fig. 4 shows, there are platinum, indium and chlorine on the GE surface.
The obtained deposits were found to be formed on the active centers of the graphite electrode
surface; these represent different scratches, surface asperity and roughness. The more them, the
greater the observed currents of the precipitate electro-oxidation in the stripping voltammetry
method. Fig. 5 shows the photograph of the graphite electrode surface with the deposit containing
In and Pt, obtained by electric precipitation method. .
Fig. 5. The graphite electrode surface with In-Pt deposit
The study of the electric oxidation current of In from the GE surface precipitated together
with Pt has revealed the following: the electric oxidation current of pure In observed at the
potential of -0.7 V while adding the platinum ions into the solution, first increases and then
decreases exponentially (Fig. 6 ). This can be explained by the fact that when platinum
electroprecipitating, on the GE surface active centers are formed, the active surface of the electrode
increases and the amount of the electroprecipitated indium increases resulting in oxidation current
increase. By increasing the Pt content in the precipitation, there is such a ratio of Pt and In on the
electrode surface, that it becomes possible to form IMC.
Electro-oxidation current of pure In decreases. The current-voltage curves have additional
peaks caused by the selective electro-oxidation of In from IMC. These peaks are adequately
described [4, 5].
Fig. 6. Relationship between the electro-oxidation current of In from the In-Pt deposit on the
surface of the graphite electrode and the concentration of In (III) ions in the solution of 0.5 gl-1
The research to identify the behavior of platinum at the electrochemical potentials of indium
from the IMC has been conducted. This Pt-In deposit was oxidized to the potential +0.1 V when
total indium was removed away from the electrode surface. Then the electrode was washed with the
distilled water and added to the solution with In ions (III). Electric concentration of In was carried
out on the existing centers of Pt. Then the deposit was oxidized. On the current-voltage curve the
peaks of the electro-oxidation of In from the IMC with Pt were observed. The platinum of the GE
surface is not considered to be oxidized at the potentials of the selective electro-oxidation of In.
Conclusion
The electrolytic deposit of In and Pt as well as the Pt-In alloy on the surface of the graphite
electrode were studied by the scanning electron microscopy, and the stripping voltammetry
methods.
By the scanning electron microscopy method it was discovered that the Pt-In alloy on the
surface of the graphite electrode contains In, Pt and Cl. It was impossible to fix Pt on the surface of
the graphite electrode by means of the scanning electron microscopy method, because without a
metal activator Pt is deposited in very small quantities on the surface of the graphite electrode.
The anodic peaks caused by the selective electro-oxidation of In from intermetallic
compounds with Pt are observed at anodic oxidation of the In-Pt deposit on the current-voltage
curves.
Platinum remains on the surface of the electrode and can be oxidized from the electrode at the
electrode potentials of purification more than 1 V.
References:
[1] N. A. Kolpakova, N.V. Borisova, V.A. Nevostruev, Nature positive anodic peak current on the
current-voltage curve in the stripping voltammetry binary systems platinum metal,
J of Anal Chem. 8 (2001) 835-838.
[2] R.R. Barefoot, J.C. Van Loon, Recent advances in the determination of the platinum group
elements and gold, Talanta. 49 (1999). 1-14.
[3] Chemodanov AN, Kolotyrkin JaM, Dembrovsky MA (1970) Elektrochem 4:460-465
[4] E.M. Ustinova, E.V. Gorchakov, N.A. Kolpakova, Izvestiya Tomskogo politehnicheskogo
universiteta. 3 (2012) 56-58.
[5] E.M. Ustinova, E.V. Gorchakov, N.A. Kolpakova, J of Solid State Electrochem, 7 (2012)
2455–2458.
[6] X.Z. Braynina, E.Ya. Neyman, Solid - state reactions in electroanalytical chemistry, Chemistry,
Moscow, 1982.