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)