Download FORMATION, COMPOSITION AND SOME PROPERTIES OF

FORMATION, COMPOSITION AND SOME PROPERTIES OF
Ni-, Cu- AND Pt-CONTAINING OXIDE LAYERS
V.S. Rudnev, L.M. Tyrina, I.V. Lukiyanchuk, A.Y. Ustinov
Institute of Chemistry FEB RAS, Vladivostok
Abstract. At the present report the results of aluminium alloy composition effect on
catalytic properties of coatings formed by micro arc oxidation (or plasma
electrochemical oxidation, PEO) technique and containing nickel and copper and the
data of platinum building-up into oxide layers on aluminium and titanium under PEO
will be presented.
The most activity in CO oxidation have been showed by the Ni-,Cu-containing
oxide compositions based on aluminium alloys containing Cu, Mg and Mn
simultaneously.
The possibility of Pt introducing into PEO layers on aluminium and titanium has
been established. The films have been investigated with EPXMA and XPS. The films
obtained have been included up to 1 at. % Pt. It should be noted that the oxidation
level of Pt is 0 (zero) in surface layers on aluminium and +2 in ones on titanium.
The results show the possibility of PEO-deposition of catalytic activity coatings
on different aluminium alloys. The obtaining films with Pt are promising for catalytic
testing in different processes.
INTRODUCTION
The micro arc oxidation (or plasma electrochemical oxidation, PEO) is one of the
methods of obtaining oxide layers of certain composition on the supports from valve
metals (aluminium, titanium, magnesium etc.). The corrosion and wear protection of
metals became the conventional field of PEO-layers application. In recent years field
of potential use of PEO film/Al(Ti) compositions is expanding. For example such
compositions can be used in catalysis as support (1) and as catalytic active system (2,
3). Previously we showed that deposited on aluminium alloy the coatings containing
compounds of copper and nickel were active in oxidation of CO by O2 at 300-500С
(4). Such catalytic active surface structures, being perspective for an environmental
application, can be deposited on different parts of internal-combustion engines. So it
is important to know the aluminum alloy nature affect on catalytic properties of such
compositions.
Another problem is related to the investigation of platinum building-up into oxide
layers on aluminium and titanium under PEO, because it is of interest for catalytic
application of PEO-structures.
EXPERIMENTAL
The Ni-, Cu- containing PEO layers on samples from aluminum (99.99 Al) and its
alloys (AMg2 (1.8-2.8 Mg, 0.2-0.6 Mn), AMg5 (4.8-5.8 Mg, 0.5-0.8 Mn), AMts (1.01.6 Mn), D16 (3.8-4.9 Cu, 1.2-1.8 Mg, 0.3-0.9 Mn) were formed galvanostatically for
10 min at anodic polarization (effective current density iA = 0.1 A/cm2). Electrolytic
cell, PBWNiCu-electrolyte preparation features and current power supply were
described in (4). Pt-containing PEO layers on aluminum alloy AMts and titanium
(99,9%Ti) samples (dimensions 2550.5 mm) were formed at iA = 0.1 A/cm2 during
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6 and 10 min respectively in aqueous electrolytes included 0.049 mol/l Na6P6O18 and
1.9∙10-3 (I) or 2.9∙10-3 mol/l H2PtCl6 (II). Furthermore on aluminum alloys coatings
were formed by alternating current of power frequency (50 Hz) at an effective current
density of 0.2 A/cm2.
The pretreatment of aluminum alloys samples included chemical polishing in
mixture of concentrated acids H3PO4/H2SO4/HNO3=4:2:1 (by volume) at 110-120C.
And the titanium samples were chemically polished in mixture HF/HNO3=1:3 at
70C. The polished samples were washed with distilled water and dried in air at 70C.
The thickness of layers was determined using an eddy-current thickness gauge.
Data on elemental composition and surface morphology were obtained on a JXA 8100
Electron Probe Microanalyzer (Japan) with INCA energy spectrum analyzer (the
United Kingdom). X-ray diffraction patterns were measured on D8 Advance X-ray
diffractometer (Germany) using CuK radiation. X-ray diffraction analysis was
performed with the use of EVA retrieval program with PDF-2 database.
X-ray photoelectron spectroscopy (XPS) was used for surface analysis. The XPS
spectra were measured on Specs ultrahigh-vacuum system using a 150-mm
electrostatic hemispherical analyzer. Ionization was performed using MgK radiation.
The working vacuum was 210-7 Pa. The spectra were calibrated using the C1s lines
of hydrocarbons, whose energy was taken equal to 285.0 eV. The bombardment by
argon ions with 5000 eV energy has been used for etching of surface layers.
Catalytic tests were performed using a BI-CAT flow 4.2(A) multipurpose flowtype system (Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of
Sciences). Finely cut wire samples of aluminum alloy or titanium (the geometric
surface area of the coating on aluminum alloy was 20 cm2 and that on titanium alloy
was 10 cm2) were placed in active zone (0.9 cm in diameter and 3 cm in height) of
quartz tube reactor. The initial reaction mixture contained 5% CO and air. The gas
flow rate was 50 ml/min. The outlet concentrations of CO and CO2 were determined
using a PEM-2 IR gas analyzer. The test temperature range was 20-500C.
RESULTS
Ni-, Cu-containing PEO coatings on aluminum
The elemental composition and thickness of films formed on samples from
different aluminum alloys in PBWNiCu-electrolyte are given in Table 1. The
thickness of coatings is 8-14 m. The concentration of Ni, Cu, Al, P, O and W in
coatings weakly depends on the alloy composition. From alloying elements Mg
(AMg2 and AMg5) and Mn (AMtsM) are detected in films. In spite of lack of relation
between alloying additions and elemental composition of coatings, the catalytic
activities of coatings are different (Fig. 1).
Coatings, formed on aluminum alloy D16, simultaneously containing Cu, Mg and
Mn as alloying elements, exhibited highest activity in CO oxidation. Compositions
supported on AMg5 and AMg2, containing Mg and Mn, have less activity. And PEO
layers on aluminum and aluminum alloy AMts are lowest active. In repeated cycles
the catalytic activity of all samples increased. A hysteresis loop observed in curves
indicates that all samples are activates in high-temperature interaction with the gas
mixture. The activator is likely CO.
It is known that manganese oxides are one of the most active in CO oxidation. In
spite of manganese is detected only in PEO layers supported on AMts alloy they are
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the least active. So the presence of Mn in lack of Mg and Cu is not improving the
catalytic properties of compositions.
Table 1. Effect of aluminum alloy on elemental composition (according to X-ray
spectroscopic analysis data) and thickness of films formed in PBWNiCu-electrolyte
Aluminum
alloy
Al
AMts
AMg2
AMg5
D16
h, m
11±2
13±2
14±1
8±1
Ni
3.3
3.2
4.6
4.8
3.2
Cu
0.4
0.4
0.6
0.6
0.5
Elemental composion, at %
Al
P
W
Mg
39.2
0.7
0.1
38.8
0.7
0.1
38.1
1.0
0.1
0.8
36.8
1.0
0.1
1.8
39.5
0.8
0.1
Mn
0.2
O
56.3
56.6
54.8
54.9
55.9
Fig. 1. The temperature dependence of the conversion of CO () for PEO
layer/aluminum alloys compositions (data of first cycle of catalytic tests), formed in
PBWNiCu-electrolyte: 1- aluminum, 2 – AMts, 3 – AMg2, 4 – AMg5, 5 – D16.
Thus the nature of aluminum alloy effects on catalytic properties of compositions
Ni-, Cu-containing PEO film/ aluminum alloy. In the case of CO oxidation into CO2
aluminum alloys on base of systems Al-Mg (AMg) and Al-Cu-Mg are more
perspective for plasma electrolytic deposition for preparation of such compositions.
Pt-containing PEO layers on aluminum and titanium
The elemental and phase compositions of films on aluminum and titanium formed
by PEO technique are given in Table 2. In addition to anodizing metal and oxygen
obtained layers includes elements from electrolyte: P, Na and Pt. The platinum
concentration is 0.04-0.08. % for coatings on aluminum alloy AMts and 0.2-0.5 at at
% for those on titanium. The concentration of Pt in films slightly depends on current
mode but increases when concentration of H2PtCl6 in electrolyte is rose. When the
concentration of hexachloroplatinate-ions is more then using in experiments the
electrolyte becomes unstable on application of current and flocks probable of metallic
platinum appear in solution. In this case sparking on electrode surface is absent
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because of the change of solution composition and state and the increase of
concentration of aggressive chloride ions. Under using concentrations of H2PtCl6 the
solution of electrolyte turn into colloid after long time plasma electrochemical
treatment of metals. When alternative current is applied the electrolyte is more stable
than under direct current application.
It should be noted that Pt, P and Na content in films on titanium exceeds that on
aluminum. It is related with various titanium phosphates formation in layers under
plasma electrochemical treatment of titanium in phosphate electrolytes. For example,
when Na6P6O18 solution is used, NaTi2(PO4)3 and TiP2O7 are received in the
composition of PEO films on titanium, while minor quantity of AlPO4 is obtained in
PEO layers on aluminum (5).
Table 2. Elemental composition (according to X-ray spectroscopic analysis data) and
phase composition of PEO coatings formed in Pt-containing electrolyte.
Metal,
electrolyte
Al, I
Al, II
Al, I
Ti, I
Ti, II
regime
AC
AC
DC
DC
DC
Pt
0.04
0.08
0.06
0.2
0.5
Elemental composition, at %
P
Na
Al
Ti
1.1 0.04 34.6
1.3 0.06 37.9
1.8
0.1 34.4
12.3 0.1
17.2
13.6 0.1
14.9
O
64.2
60.7
63.6
70.2
70.9
Phase
composition
γ- Al2O3
γ- Al2O3
γ- Al2O3
TiO2 (anatase), Pt
TiO2 (anatase), Pt
Notes: AC – alternating current of power frequency (50 Hz), DC - direct current.
Crystal phases of oxides of metals under treatment such as -Al2O3 on aluminum
alloy and TiO2 (anatase) on titanium are formed in PEO films. In addition in films on
titanium metallic platinum is detected by XPD, Fig 2.
Fig. 2. X-ray diffraction pattern of coating on titanium.
Investigations of aluminum and titanium films surface have been carried out using
XPS. The example of surface and near-surface (30Å deep, after etching) layers
compositions for coating on aluminum is given in Table 3. Considerable quantity of
carbon is contained in film surface. The carbon content decrease, when film surface is
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etched by argon ions beam. So it is possible that carbon located on the surface is
adsorbed from air or appeared as contamination. In comparison with elemental
composition determined by XSA (Table 2), the content of phosphorus and platinum is
higher in surface and near-surface layers. According to binding energy (71.6 eV for
Pt4f7/2) platinum present on the aluminum film surface has oxidation level 0. At the
same time on titanium film surface platinum appears as Pt2+ (Fig. 3), its binding
energy is about 73 eV. It should be noted that under films etching the oxidation level
of platinum in PEO films on aluminum is not changed, but Pt0 is present in PEO films
on titanium along with Pt2+. The last is agreed with the presence of metallic platinum
on X-ray diffraction patterns (Fig. 2). However it is impossible to except the some
platinum reduction in near-surface layer under etching by argon ions beam.
Table 3. Concentrations of elements in the surface and near-surface layers of films
formed in Pt-containing electrolyte on aluminum (Al, II) and binding energy (E)
according to XPS data.
Element
Elemental composition, at %
O
C
P
Al
Surfacec layer
52.8
14.7
9.1
21.7
Near-surface layer
53.6
2.0
7.1
34.5
Pt
1.7
2.7
E, eV
532.3 (O1s)
285.0 (C1s)
134.7 (P2p)
75.0 (Al2p)
71.6
(Pt4f7/2)
Notes: Binding energy is given for initial surface.
Fig. 3. X-ray electron spectra of films surface on aluminum alloy (a) and titanium (b).
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Figure 4 depicts scanning electron micrographs of PEO layers in phase
representation. On the SEM-images surface sites with heavy elements appear clearer
than others. The films on aluminum and titanium differ in morphology. The films on
titanium have bigger surface fragments including pores. Grain-placer mines of clear
particles in characteristic dimensions of 1m can be seen on the surface of films on
titanium (Fig. 4 c).
Fig. 4. Scanning electron micrographs (a phase representation) of Pt-containing PEO
layers formed on aluminum alloy (a) and on titanium (b). The surface fragment of
film with dispersed particles (c) formed on titanium.
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According to XSA the film site with dispersed particles includes next elements, at
%: 1.4 Pt, 22.4 Ti, 11.8 P, 64.4 O, i.e. the concentrations of platinum and titanium in
dispersed particles (micrograines) are higher that those in the film. According to (6)
conditions for formation of amorphous or crystal dispersed particles appear in the
field of oversaturated solutions located near anode/electrolyte interface under electric
discharges. Colloidal particles of oxidizing titanium may serve as nucleation centers.
Colloidal particles are formed as result of interaction of titanium ejected from the base
material and hydroxide ions in near-anode layer. Electrophoretic deposition of
micrograines with the next their conversion by discharges is one of the possible
mechanism of formation of multiphase and multicomponent anodic oxide films by
PEO technique.
The temperature dependences of CO conversion for Pt-containing PEO
layer/AMts-alloy compositions formed under DC regime (Al, I and Al, II, table 2) are
shown in Fig. 5. PEO coatings formed in Na6P6O18 electrolyte in lack of H2PtCl6 were
not active in CO oxidation (curve 1). The catalytic activity of PEO coatings
containing 00.4 and 0.08 at % Pt was weakly different. Sharp leap in CO conversion
was observed in the temperature range 300-350C. This leap is typical of Ptcontaining catalysts. The activity of the coatings increased upon cooling (hysteresis).
Fig. 5. The temperature dependence of the conversion of CO () for Pt-containing
PEO layer/aluminum alloys compositions (data of first cycle of catalytic tests),
formed in electrolyte with Na6P6O18. The content of Pt in coatings, at %: 1 – 0, 2 –
0.04, 3 – 0.08.
The catalysts containing noble metals are traditionally obtained in several stages
using different variants of penetration method or filling with noble metal compounds
of support, for example of oxide systems or films. Then operations of drying,
calcination and reducing of noble metals should be carried out. Data of this work
show that plasma electrolytic oxidation technique allows forming surface structures
containing noble metals in addition to support metals oxides in one stage. The regime
variation and addition of noble metal compounds, not enough aggressive as H2PtCl6,
for example platinum nitrate, sulfate or acetate will allow to change the composition
and characteristics of surface structures.
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CONCLUSIONS
Along with published data (2, 3), the results obtained in this work demonstrated
that the PEO process is promising for the manufacture of catalysts supported on metal
substrates, which are active in various reactions. In the case of CO oxidation
aluminum alloys on base of the system Al-Mg (AMg) and Al-Cu-Mg (AMts) are
more perspective for obtaining the catalytic activity compositions Ni, Cu-containing
film/ aluminum alloy. Using the Pt-containing electrolyte allows build-up about 0.08
at % Pt into PEO-layers on aluminum and about 0.5 at % into those on titanium.
Platinum is concentrated in surface and near-surface layers of films and affect the
catalytic activity of films in CO oxidation.
ACKNOWLEDGMENTS
We are grateful to Cand. Sci. (Chem.) P.M. Nedozorov for her assistance in
performing elemental analysis.
This work was supported in part by the Russian Foundation for Basic Research
(project no. 06-03-32184) and the Presidium of the Far East Division of the Russian
Academy of Sciences (grant nos. 06-I-PV-012 and 06-01-OKhNM-137).
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