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Warsaw University Industrial Chemistry Research Institute
CARBON OXIDES ADSORPTION AS DIAGNOSTIC TOOL IN
STUDIES ON HYDROGEN ELECTROSORPTION
IN PALLADIUM-PLATINUM-RHODIUM ALLOYS
INTRODUCTION
M. Łukaszewski a, b, M. Grdeń a, A. Czerwiński a, b
a Warsaw University, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland
b Industrial Chemistry Research Institute, Rydygiera 8, 01-793 Warsaw, Poland
The processes of hydrogen electrosorption on noble
metals can be markedly affected by the presence of some
adsorbates such as carbon oxides. However, while CO is a
strong surface poison whose adsorption takes place on all
platinum metals at potentials from both hydrogen and double
layer regions, CO2 is adsorbed only on Pt and Rh electrodes in
a reaction with atoms of underpotentially deposited hydrogen.
Due to the different adsorption behavior of CO2 and CO
towards particular metals these compounds might be applied
as diagnostic tools in the investigations of hydrogen
sorption/desorption by noble metal alloy electrodes.
We have chosen a ternary Pd-Pt-Rh alloy as a mixture
of elements with various electrochemical properties:
Pd Pt Rh
H adsorption + + +
H absorption + – –
CO2 adsorption – + +
CO adsorption + + +
It will be demonstrated that the use of CO2 adsorbate
allows for the examination of the nature of hydrogen signals
observed in cyclic voltammetric experiments, whereas CO
adsorption experiments can reveal some facts concerning
kinetics and mechanism of hydrogen/desorption processes.
EXPERIMENTAL
Pd-Pt-Rh electrodes (thickness 0.50-0.95 mm) were
prepared by potentiostatic deposition on Au wires (diameter 0.5
mm) from a bath containing PdCl2, H2PtCl6, RhCl3 and HCl. The
roughness factor of the deposits was ca. 100-350. Bulk
compositions (expressed in atomic percentages) of the alloys were
determined using EDAX analyzer (EDR-286) coupled with a LEO
435VP scanning electron microscope. All experiments were
performed with cyclic voltammetry (CV) at room temperature in
0.5 M H2SO4 solution deoxygenated using an Ar stream. All
potentials are referred with respect to the SHE. CO2 and CO gases
with 99.9 % purity were used. After completing the adsorption, CO2
and CO were always removed from solution with Ar.
RESULTS AND DISCUSSION
SUMMARY
The voltammogram for a Pd-Pt-Rh alloy (Fig. 1)
resembles CV curves typical of pure noble metal electrodes
and their binary alloys. The hydrogen (1), double layer (2) and
oxygen region (3) are distinguishable. The multiplicity of
peaks in the hydrogen region can be attributed to the existence
of various states of hydrogen, i.e. adsorbed on Pd, Pt and Rh
surface sites and absorbed in the alloy.
On the voltammograms for Pd-Pt-Rh electrodes
recorded after CO2 adsorption (Figs. 2.1 and 2.2) practically
no changes are seen in the height and potential of peak (a),
which indicates that this signal is mainly due to the oxidation
of hydrogen absorbed in the alloy. On the other hand, a
decrease in current of signal (b) allows us to state that it
originates partly from the oxidation of adsorbed hydrogen.
In the case of a Pd-rich alloy (Fig. 3) peak (a) splitting
has occurred. The results of a CO2 adsorption experiment
enable to conclude that both (a1) and (a2) signals have main
contribution from currents of oxidative desorption of absorbed
hydrogen. Because of the high Pd content in the alloy these
peaks are significantly higher than signals connected with
adsorbed hydrogen (b) sensitive to the existence of CO2.
On a Pd-Pt-Rh electrode covered with adsorbed CO
currents in the hydrogen region are strongly diminished.
However, hydrogen insertion and removal are not totally
blocked but proceed much slower than in the absence of CO.
Hydrogen can still be absorbed in an alloy previously covered
with CO adsorption products (Fig. 4 - procedure 1). Hydrogen
trapped in an electrode subsequently poisoned by CO can be
desorbed from the bulk despite the existence of a layer of
adsorbed CO (Fig. 4 - procedure 2 and Fig. 5).
The presence of CO influences markedly the kinetics
of hydrogen sorption/desorption processes due to inhibition of
surface reactions involving atoms of adsorbed hydrogen
generated on the remaining free surface sites. However, one
cannot exclude the possibility that some amount of hydrogen
enter the alloy lattice directly, i.e. without an adsorption step.
The presence of adsorbed CO2 on Pd-Pt-Rh electrodes
causes partial blocking of hydrogen adsorption sites,
i.e. Pt and Rh surface atoms but it does not affect
hydrogen absorption proceeding via Pd atoms.
Due to differences in CO2 reactivity towards hydrogen
adsorbed on Pd and hydrogen adsorbed on Pt or Rh
atoms, the experiments with CO2 electrosorption allow
for determination of the nature of various hydrogen
peaks observed on CV curves for Pd-Pt-Rh alloys.
The presence of adsorbed CO has a substantial
influence on all hydrogen signals. Hydrogen
adsorption is strongly inhibited. The processes of
hydrogen absorption and desorption are not totally
blocked but proceed much slower than on electrodes
free from CO adsorbates.
Fig. 1. Cyclic voltammogram for a Pd-Pt-Rh alloy containing in the bulk 44 % Pd, 35
% Pt and 21 % Rh; 0.5 M H2SO4 ; scan rate 0.05 V s-1.
Fig. 4. Cyclic voltammograms for CO adsorption on a Pd-Pt-Rh alloy containing in
the bulk 44 % Pd, 35 % Pt and 21 % Rh; scan rate 0.05 V s-1. Experimental
procedures: (1) CO adsorption at 0.45 V for 900 s followed by CO removal from
solution with Ar and hydrogen sorption at - 0.05 V for 300 s, (2) hydrogen sorption at -
0.05 V for 300 s in CO-free solution followed by CO adsorption at - 0.05 V for 900 s.
Fig. 3. Cyclic voltammogram for CO2 adsorption on a Pd-Pt-Rh alloy containing in
the bulk 80 % Pd, 16 % Pt and 4 % Rh; scan rate 0.02 V s-1. CO2 adsorption at 0.10 V
for 900 s followed by 300 s hydrogen sorption at - 0.05 V.
Fig. 5. Cyclic voltammograms for CO adsorption on a Pd-Pt-Rh alloy containing in
the bulk 80 % Pd, 16 % Pt and 4 % Rh; scan rate 0.02 V s-1. CO adsorption at - 0.05 V
for 900 s followed by CO removal from solution with Ar. Experimental procedures: (1)
scan in the full potential range, (2) scan after reversing polarization at 0.75 V, (3) scan
after reversing polarization at 0.75 V and stopping at - 0.05 V for 300 s.
Figs. 2.1 and 2.2. Cyclic voltammograms for CO2 adsorption on Pd-Pt-Rh alloys. CO2
adsorption at 0.05 V for 900 s. Scan rate 0.05 V s-1. (1) – alloy rich in Rh, (2) – alloy
poor in Rh.
1 2
-0,002
-0,001
0
0,001
0,002
-0,1 0,1 0,3 0,5 0,7 0,9 1,1 1,3
potential, V
cu
rre
nt,
A
surface oxidation currents
surface oxides reduction peak
1 2 3
hydrogen adsorption/
absorption signals
hydrogen desorption signals
double layer
charging currents
If the adsorption properties of Pd, Pt and Rh atoms
towards CO2 are retained in the ternary alloy, it may be
possible using CO2 to block hydrogen bonded to Pt and Rh
surface atoms without any marked effect on hydrogen bonded
to Pd atoms. In the presence of adsorbed CO2 hydrogen
adsorption signals are expected to be diminished, while those
attributed to hydrogen absorption should be undisturbed. Such
behavior has already been observed in the case of Pd-Pt
alloys.
-0.001
0
0.001
0.002
0.003
0.004
-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
potential, V
cu
rre
nt,
A
after carbon monoxide adsorption - procedure 1
after carbon monoxide adsorption - procedure 2
after carbon monoxide adsorption - procedure 3
in background solution
strong blocking both hydrogen adsorption
and absorption in the presence of adsorbed CO
oxidation of adsorbed CO
oxidation of absorbed hydrogen
shifted into higher potentials
due to the presence of adsorbed CO
-0,002
0
0,002
0,004
0,006
-0,1 0,1 0,3 0,5 0,7 0,9 1,1 1,3
potential, V
cu
rre
nt,
A
after carbon monoxide adsorption - procedure 1
after carbon monoxide adsorption - procedure 2
in background solution
oxidation of adsorbed CO
oxidation of hydrogen
absorbed after CO adsorption
oxidation of hydrogen
absorbed before CO adsorption
2
-0,0005
-0,00025
0
0,00025
-0,1 0,1 0,3 0,5 0,7 0,9 1,1 1,3
potential, V
cu
rre
nt,
A
after carbon dioxide adsorption
in background solution
bulk composition: 28 % Pd
66 % Pt
6 % Rh
oxidation of adsorbed CO2
(a)
(b)
decrease in hydrogen oxidation current
due to the presence of adsorbed CO2
1
-0.0015
-0.001
-0.0005
0
0.0005
0.001
0.0015
-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
potential, V
cu
rre
nt,
A
after carbon dioxide adsorption
in background solution
decrease in hydrogen
oxidation current
due to the presence
of adsorbed CO2
oxidation of adsorbed CO2 (a)
(b)
bulk composition: 50 % Pd
8 % Pt
42 % Rh
-0.0005
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
-0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3
potential, V
cu
rre
nt,
A
after carbon dioxide adsorption
in background solution
oxidation of adsorbed CO2
oxidation currents
of absorbed hydrogen -
practically undisturbed
in the presence of adsorbed CO2
oxidation currents of adsorbed hydrogen -
diminished in the presence of adsorbed CO2
(a1) (a2)
(b)
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