KV Rybalka

Journal of New Materials for Electrochemical Systems 10, 81-89 (2007) 

© J. New Mat. Electrochem. Systems 

Binary Pt– and Pd–based Electrocatalysts for Oxidation 

of Hydrogen with Co Admixtures 

* K. V. Rybalka 1 , M. R. Tarasevich 1 , B. M. Grafov 1 , V. A. Bogdanovskaya 1 , L. A. Beketaeva 1 , 

E. N. Loubnin 1 and Yu. A. Kolbanovskii 2 

1 A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, RAS, Moscow 119071, Leninsky pr.31, Russia 

2 A.I. Topchiev Institute of Petrochemical Synthesis, RAS, Moscow 119991 Leninsky pr.29, Russia 

Received: April 15, 2005, Accepted: December 20, 2006 

Abstract: The results are presented of studying the electrochemical and electrocatalytic properties of binary hydrogen electrooxidation 

catalysts including Pt or Pd and some other metal (Au, Mo, Ru) and tolerant towards CO admixtures. The methods of catalyst synthesis at 

the dispersed carbon carrier (XC72) are suggested, the binary system structure is studied. According to the results of the studies by the 

electrochemical impedance method, the quantitative estimation is performed for the developed catalysts of their tolerance towards CO 

admixtures. PdAu catalyst features the highest tolerance, which can result from the decoration of palladium surface by gold. The decoration 

of Pd nanoparticles by gold hinders CO adsorption while seemingly preserving enough accessible sites for hydrogen adsorption. In 

PdRu binary system, the tolerance is probably provided by the formation of an alloy, at which the oxidative desorption of CO is considerably 

facilitated in comparison with pure Pd. 

Keywords: : electrooxydation,catalyst,tolerance,impedance spectra 

1. INTRODUCTION 

Gas diffusion hydrogen electrodes with dispersed platinum content 

of 0.1–0.2 mg/cm 2 demonstrate excellent characteristics when 

pure hydrogen is used, but are poisoned already with traces of CO. 

The problem of employing gaseous hydrogen with CO admixtures 

in fuel cells (FC) was first studied in the 80-s [1,2] when it was 

shown that CO admixture of 10 ppm caused the voltage decrease 

of 0.2 – 0.3 V at the load of 0.8 A/cm 2 . At the load of 0.2 A/cm 2 at 

80 ºC the presence of CO in hydrogen, its amount being 250 ppm, 

leads to a drastic voltage drop of 0.4 V [3]. CO concentration in 

hydrogen obtained in the reforming process after its additional 

purification by means of the shift method usually constitutes from 

20 to 100 ppm. The inhibiting effect of CO on the hydrogen electrooxidation 

process results from the fact that the bonding strength 

of Pt–CO is higher than that of Pt–H [4], which complicates the 

dissociative hydrogen adsorption on platinum and its further ionization. 

At present, two possibilities of eliminating or decreasing the 

poisoning effect of CO presence in hydrogen are considered. The 

*To whom correspondence should be addressed: 

first of them is the increase of FC working temperature. It is 

shown experimentally in work [5] that at the temperature of hydrogen–oxygen 

FC being close to 200 ºC the specific parameters do 

not change even at CO content of 20000-30000 ppm. The second 

way is based on the development of new anodic catalysts tolerant 

towards CO admixtures in hydrogen, while retaining the construction 

and functioning conditions of FC. 

Platinum modification by adatoms [6] and, later, the decoration 

of its surface by metal submonolayers [7,8] seemed to be a prospective 

way of purposeful optimization of platinum properties in 

respect to CO oxidation. However, such systems are rather unstable 

in the conditions of FC functioning. Therefore, despite the 

importance of these studies for understanding the mechanism of 

electrocatalysis, they can hardly possess any practical value. 

At present it seems most prospective to use as electrocatalysts 

the binary systems based on platinum group metals which results 

in lower surface coverage by adsorbed CO. Such systems can be 

obtained by employing the methods of fusing, codeposition, thermochemical 

and mechanochemical synthesis, cathodic deposition 

etc. The most trustworthy and reproducible effects of increasing 

the tolerance towards CO admixtures in gaseous hydrogen were 

obtained by introducing Ru admixtures into Pt. As shown in [8, 9] 

81

82 K. V. Rybalka et al. / J. New Mat. Electrochem. Systems 

by means of STM and IR spectroscopy, the minimum oxidation 

overpotential of linearly adsorbed CO ads is achieved for the system 

of Pt 0.5 Ru 0.5 . According to [8], the promoting effect of Ru in the 

system of Pt–Ru also manifests itself in the absence of the alloy 

formation. The optimum ratio of Pt 0.5 Ru 0.5 was confirmed in a 

number of works [10–13], in particular, at the example of commercial 

E–TEK catalyst. The system of PtMo also proved to be of 

interest [7,12,14–16]. In papers [17,18] it was shown at well– 

characterized smooth PtMo surfaces that the presence of 20–25 at. 

% Mo accelerates the oxidative removal of CO and facilitates the 

adsorption of hydrogen. These data were confirmed by using 

PtMo system with the ratio of 4:1 on carbon black XC72 [14]. 

Later in [15], the authors specified the optimum composition by 

increasing Mo content in PtMo system up to 3:1. 

Pd–based electrocatalytic systems are of real interest alongside 

with Pt catalysts. In [19], a method of PdAu black synthesis on 

carbon black XC72 is offered. In the case of these catalysts, hydrogen 

adsorption is observed at Pd atoms in the range of potentials 

of 0.05–0.3 V, same as the well–expressed limiting diffusion 

current of hydrogen oxidation at the rotating disc electrode [20]. 

In respect to CO oxidation, the properties of PdAu are close to 

those of PtRu, but CO oxidation rate is somewhat lower. The advantages 

of PdAu system as compared to PtRu are better manifested 

at the conditions of 60 ºC and CO concentration in the 

range of 250–1000 ppm. It was demonstrated [21] that at the content 

of Au > 70% in PdAu alloys hydrogen adsorption is not 

observed. A conclusion was made as to its occurring only at Pd 

atoms. Herewith, CO adsorption on gold is also negligible [22]. 

This system can probably turn out to be tolerant towards CO and 

at the same time to be active towards hydrogen electrooxidation. 

In the present work, the possibility is considered of developing 

Pt– and Pd–based binary catalysts of hydrogen oxidation that are 

tolerant towards CO admixtures. The electrocatalytic properties of 

such systems were compared both as to pure hydrogen oxidation 

and the oxidation of hydrogen with CO admixtures, their content 

being from 50 to 500 ppm. Herewith, Pt and PtMo supported on 

carbon black XC72 were used in this work for comparison with 

the properties of binary PdM system (М=Au, Mo,Ru). 

2. EXPERIMENTAL 

2.1. Method of catalyst preparation and performing 

electrochemical and structural studies 

All Pt– and Pd–based binary catalysts studied in this work were 

synthesized on carbon black XC72. The commercial E–TEK catalyst 

(XC72 + 20% Pt) that we used for comparison was heated 

prior to experiments in an inert atmosphere at 300 ºC [23]. 

The catalyst synthesis was performed by simultaneous and successive 

deposition of the components onto carbon black. Various 

methods of reducing the initial compounds were employed, catalyst 

synthesis temperature and the duration of its reduction process 

were optimized. Several methods of catalyst preparation were 

developed. 

To obtain XC72 + 20 % PtMo (75:25) catalyst, successive application 

of Pt and Mo onto carbon black from the aqueous solutions 

of H 2 PtCl 6 and (NH 4 ) 6 Mo 7 O 24 was used. The reducing agent 

was formic acid or sodium borate. The mixture was evaporated, 

washed, and subjected to thermal treatment in an inert atmosphere 

and then in hydrogen. 

The most effective XC72 + PdAu catalysts for hydrogen oxidation 

are prepared as follows. In the process of catalyst synthesis, 

the diluted solutions of components (0.033 M) were used [24]. 

The reducing agent was ethanol. The reduction was carried out in 

a synthesizer with vigorous stirring for four hours in the atmosphere 

of an inert gas at 100 ºC. The obtained catalyst was baked 

for three hours in the atmosphere of hydrogen at 530 ºC and then 

cooled in the atmosphere of an inert gas. After that, the catalyst 

was many times washed by water and dried in a vacuum compartment. 

The synthesis of binary catalysts of PdMo and PdRu was carried 

out on carbon black XC72 using metal salts (PdCl 2 , 

RuOHCl 3 , (NH 4 ) 6 Mo 7 O 24 ). The total amount of promoting metals 

constituted 15–30 wt.%, the metal ratio was changed from 1:1 to 

4:1 atomic parts. The optimum method of PdMo catalyst preparation 

is the successive deposition of metals. PdRu catalyst features 

better characteristics at its preparation by means of the codeposition 

method. 

The catalyst activity was estimated from the run of the polarization 

curves of hydrogen electrooxidation reaction obtained at the 

rotating disc electrode and from the electrochemical impedance 

spectra. All the electrochemical measurements were performed at 

the disc pyrographite electrode sealed into Teflon with the diameter 

of 5 mm. 100 µg/cm 2 of the catalyst with 0.7 wt.% of Nafion 

were applied to the disc electrode according to the technique developed 

in [25]. 500 µl of ethanol and the necessary amount of 5% 

Nafion solution were added to the catalyst sample (2mg). The 

mixture was subjected to ultrasonic homogenization for 30 min. A 

sample of the obtained mixture was applied to the previously purified 

and polished electrode and dried in air for 12 hours. 

The potential sweep rate while registering the polarization 

curves at the rotating disc electrode (from the steady–state potential 

to more positive values) constituted 1 mV/s. 

The curves of hydrogen oxidation close to those obtained in 

[26] were observed at the same electrode with E–TEK catalyst of 

20wt.% Pt in 0.5 M H 2 SO 4 . Herewith, the value of 0.62 n Fc H2 D 

H2 2/3 ν -1/6 product in the equation of Levich [27] i lim = 0.62 n Fc H2 

D H2 2/3 ν -1/6 ω 0.5 constituted 7.65 10 –2 mA cm -2 rpm -0.5 , which 

somewhat differs from the value of 7.80 10 -2 mA cm -2 rpm -0.5 

specified by the authors of the above work. This evidences the 

satisfactory hydrodynamic conditions that are realized at the disc 

electrode with a thin catalyst layer. 

The measurements of the electrochemical impedance components 

were performed with the help of numeric impedance meter 

X-206 in the frequency range of 20Hz – 100 kHz. All the measurements 

were performed using a two–electrode scheme, i.e. the 

impedance included the impedance of both the studied and the 

auxiliary electrode, which was a platinated platinum disc with the 

diameter of 20 mm. The auxiliary electrode impedance is negligible 

as its surface considerably exceeded the studied electrode surface. 

All the impedance measurements were performed at the electrode 

steady–state potential. 

The tolerance of the catalysts towards CO was estimated from 

the variation of the impedance spectrum as dependent in the time 

of electrode conditioning in the electrolyte saturated by hydrogen 

containing CO admixtures. The ratio of the exchange current of 

the hydrogen reaction determined after a two–hour exposure of the 

electrode in 0.5 M H 2 SO 4 solution saturated by the gas mixture to

Binary Pt– and Pd–based Electrocatalysts for Oxidation of Hydrogen with Co Admixtures / J. New Mat. Electrochem. Systems 

83 

Table 1. Structural characteristics of the catalysts based on binary metal systems of Pd + M (M- Au, Ru, Mo). 

Catalyst Phases Particle size (nm) Lattice parameter (nm) 

XC72 +20% PdAu (70:30 at.%) Alloy, separate phases of gold and palladium particles 6 (±30%) Alloy - 0.3956 

XC72 +20% PdMo (70:30 at.%) Palladium 18-20 (±20%) 0.3897 

XC72 +20% PdRu (50:50 at.%) Alloy, separate phases of ruthenium and palladium particles 3-5 (±40%) Alloy - 0.3871 

Table 2. Electrochemical parameters of Pd–based binary catalysts. 

Catalyst: XC72+ S, m 2 /g met E st , V 

i, mА/см 2 

at E=0.05V 

I lim , mA/cm 2 

m=2000 rpm 

30 wt.% PdAu 

(50:50 at. %) 

10.7 0.001 1.79 2.33 

30 wt.% PdAu 

(70:30 at. %) 

- 0.000 0.94 1.45 

30 wt.% PdAu 

(80:20 at. %) 

- 0.000 0.70 1.30 

20 wt.%PdRu 

(50:50 at. %) 

97 0.010 1.88 2.53 

20 wt.%PdRu 

(70:30 at. %) 

40.5 0.000 2.17 3.32 

20 wt.%PdRu 

(80:20 at. %) 

146.7 0.003 0.94 2.42 

20 wt.%PdMo 

(50:50 at. %) 

37 0.005 1.15 2.29 

20 wt.%PdMo 

(70:30 at. %) 

110 0.004 1.78 1.96 

20 wt.%PdMo 

(80:20 at. %) 

111 0.001 2.10 3.28 

the exchange current value obtained in the absence of CO was chosen 

to be the quantitative tolerance parameter. Alongside with that, 

the electrocatalytic activity of different catalysts was compared in 

the reaction of CO oxidation from the solution bulk. The higher the 

rate of this reaction, the higher tolerance towards CO can be expected 

from the binary catalytic system. 

The specific surface area values for the metal component of the 

binary catalyst were determined from the amount of electricity 

consumed in hydrogen and CO desorption [28]. The cyclic potentiodynamic 

curves were registered at the potential sweep rate of 40 

mV/s, in the range of potentials of 0.03÷0.07 V, in order to reduce 

hydrogen evolution or its dissolution in palladium. Firstly, the solution 

was deaerated for 40 min by helium, after which a potentiodynamic 

curve was registered that allowed to make conclusions about 

the character of hydrogen adsorption at the studied catalyst. Then 

the solution was saturated by gaseous CO (30 min). After that, CO 

was removed from the solution by bubbling helium (40 min), and 

more potentiodynamic curves were registered (2 cycles). When 

calculating the surface area value, it was assumed that in the case of 

the monolayer coverage of the catalyst metal component by hydrogen 

its desorption consumes 210 mC/cm 2 , while in the case of CO 

the value is 420 mC/cm 2 . 

The catalyst compositions were analyzed by energy dispersive 

X-ray spectroscopy (EDX) and compared with the nominal ones. 

According to the EDX measurements atomic compositions were 

very close to the nominal composition of catalysts pointed in tables 

1-3. 

The physical and chemical characteristics of the catalysts were 

studied using the methods of X–ray photoelectron spectroscopy 

(XPS) at electron spectrometer VG ESCA 3МКII and X–ray diffraction 

analysis (XRD) at diffractometer JDX – 10 PA (JEOL, 

Table 3. Kinetic parameters of hydrogen oxidation catalysts in the 

mixture of H 2 + CO. 0.5 M H 2 SO 4 , the temperature is 60 ºC. 

Catalysts 

XC72 + 20wt.% 

PdMo(50:50 at.%) 

XC72 + 20wt.% 

PdMo(70:30at.%) 

XC72 + 20wt.% 

PdMo(80:20at.%) 

XC 72 + 30wt.% 

Pd 

XC 72 + 20wt.% 

PdRu(50:50at.%) 

XC 72 + 20wt.% 

PdRu(70:30at.%) 

XC 72 + 20wt.% 

PdRu(80:20at.%) 

XC 72 + 30wt.% 

PdAu(50:50at.%) 

XC72 + 20 wt.% 

Pt 

i 0 (0), 

A/g met 

i 0 (0), 

mA/cm 2 met 

i 0 (120)/i 0 (0) 

in H 2 +500 ppm CO 

1570 4.24 0.55 

700 0.64 

2800 2.52 

846 1.29 

476 0.49 0.50 

54.5 0.14 

1290 0.88 

641 6.0 0.60 

4400 8.55 0.24 (for 226 ppm CO) 

where i 0 (120) is the exchange current of the hydrogen reaction after the electrode was 

conditioned for 120 min in the solution saturated by H 2 +CO gas mixture; i 0 (0) is the 

exchange current in the absence of CO. 

Japan) using the filtered CuKα - radiation. The samples of dispersed 

catalysts for XPS analysis were stored without protecting 

the sample surface from the contact with the air. The studied catalysts 

were pressed onto a copper gauze with the cell size of 1 mm 

fixed on a gold support. The analyzed sample size constituted 5×10 

mm. The measurement of the spectra was performed in a chamber 

evacuated up to the vacuum of 3×10 -8 Pa. MgK α source was used 

for electron excitation. The treatment of the spectral lines was performed 

using a computer and service software package VG X 

DATA 1000. The nature of the chemical bonds was characterized 

basing on the variation of the core level bonding energies, i.e., the 

chemical shifts. All the bonding energies of the spectral lines of the 

core levels were calibrated vs. Au4f 7/2 level with the bonding energy 

of 83.8 eV. The accuracy of determining the chemical shifts in 

the spectra corresponded to the values of the bonding energy shifts 

from the average values measured in six spectra ±0.2 eV. 

The relative atomic abundances of the elements at catalyst surfaces 

were estimated by XPS. The atomic concentrations were 

calculated by integration of the signals recorded included the core 

level spectra and the atomic sensitivity factors. Similar procedures 

were used to calculate the surface compositions of the metallic 

nano-size particles in inorganic support [29-33]. In present investigation 

the errors of XPS method resulting from the effect of change 

of particle sizes and surface roughness were taken into account. 

The surface atomic ratios of catalysts obtained by XPS were calculated 

with the top error 20%. Thus, these XPS experiments could 

give indications about the surface atomic ratio within this error.


Figure 2. . Cyclic potentiodynamic curves on PdRu catalysts, 60 

ºC, the potential sweep rate is 40 mV/s. 

The working electrode potential was measured vs. the saturated 

calomel reference electrode connected with the cell through an 

electrolytic bridge. All the values of the potentials given in the 

paper were re-calculated vs. the potential of the reversible hydrogen 

electrode in the same solution. 

3. RESULTS AND DISCUSSION 

Figure 1. Diffractogram fragments for the catalysts of (a) XC72 + 

20 wt.% PdAu (70:30 at.%) and (b) XC72 + 20 wt.% PdRu (50:50 

at.%). 

The diffraction spectra were registered at the room temperature 

using the mode of the continuous recording of spectra and the 

mode of scanning with the step of 0.1 0 with pulse accumulation per 

point for 20÷100 s. The appearance of alloys in the binary systems 

was observed from the angle shifts of the position of the diffraction 

reflex maximums in the range of high diffraction angles 2θ 0 of 64 0 

– 70 0 corresponding to reflexes from (220) planes of the cubic palladium 

phase. In the case of a very low intensity of this reflex, the 

structural data were obtained after analyzing the form of the maximums 

of lines (111) and (200) of the cubic palladium phase in the 

diffraction angle range of 2θ 0 of 39 0 – 48 0 . In this case, the line 

profiles for the expansion of complex lines were approximated by a 

combination of Voigt and Lorentz functions that provide the best fit 

for the line profiles of the dispersed palladium particles [34]. 

All the electrochemical measurements were carried out in 0.5 M 

solution of twice distilled H 2 SO 4 in a thermostated cell with separate 

cathodic and anodic compartments. The compounds used for 

the synthesis of the catalysts produced by company “AURAT” 

were of chemical grade. The temperature was maintained by thermostat 

U-3. The polarization curves were registered with the help 

of numeric potentiostat IPC-20. 

3.1. Catalyst structure according to the data of 

XPS, XRD, and CO adsorption 

PdAu catalyst synthesized by the above method is characterized 

according to XRD method by a rather homogeneous particle size 

distribution, of which the average value constitutes 6 nm (table 1, 

fig. 1a). Herewith, the system of [XC72 + 20wt.% PdAu (70:30 

at.%)] is in fact heterophaseous and consists of the particles of 

metallic gold, palladium, and the alloy. The enrichment of the surface 

layer by gold is found from XPS spectra. At the nominal 

Au/Pd ratio of 0.43 the experimental value constituted 0.62. 

The system of PdRu synthesized on XC72 is the most highly 

dispersed one. For this sample, the intensity of reflex (220) proved 

to be extremely low due to the high dispersion degree of the metal 

particles. Therefore, all the data on the particle size, the particle 

atomic lattice parameters were obtained by analyzing the form of 

the complex line in the diffraction angle range of 2θ 0 of 39 0 – 48 0 

after the profile approximation and extension to the component 

lines corresponding to the reflexes of planes (111) and (200) of 

palladium phase. The corresponding result of the treatment of the 

profile of this line is presented at fig. 1b. This sample includes the 

phases of metallic ruthenium, metallic palladium, and their alloy 

containing up to 10% ruthenium in palladium. The alloy formation 

is evidenced by the decrease of Pd lattice parameter at the introduction 

of Ru, as compared to pure Pd. The concentration ratio at the 

surface is Pd:Ru=1:1. Two lines were observed in the spectra of the 

both metal core levels Pd3d5 and Ru3p that corresponded to the 

metals, ruthenium particles oxidized to RuO 2 and palladium particles 

oxidized to PdO. The concentration ratio of M:MO=4:1 was


85 

Figure 3. Polarization dependences for the hydrogen oxidation reaction at the rotating disc electrode coated with Pt–based catalysts at 

the rotation rate, rpm: (1) 680; (2) 1080; (3) 1480; (4) 2040; 0.5 M H 2 SO 4 , 60 ºC. 

characteristic of both types of particles. 

At the diffractograms of the samples of {XC72 + 20 % PdMo}, 

only the diffraction lines corresponding to the position of maximums 

of the cubic palladium lattice were expressed, which pointed 

to the absence of a separate metallic molybdenum phase. The lattice 

parameter determined for palladium particles proved to be 

somewhat higher than in the case of crystalline palladium 

(a=0.38902 nm). This means that the changes of the cubic dispersed 

palladium phase observed for this sample are negligible 

owing to the intercalation into the molybdenum atomic lattice. 

However, according to the data of the studies using XPS method, 

most of molybdenum in this sample was in the form of oxide. The 

structural characteristics of the synthesized catalysts is presented in 

table 1. 

The surface area of the metallic catalyst component was determined 

from the results of potentiodynamic measurements. These 

data, together with the other studies performed by means of XRD 

and XPS methods, also allow to estimate the effect of the binary 

system components on their catalytic properties. The comparison of 

the specific surface area of PdRu (50:50 at.%) determined from CO 

desorption (97 m 2 /g) and calculated from the particle size according 

to XRD data using relationship S=6 * 10 4 /ρ * D (160 m 2 /g, where ρ is 

the metal density, D is the particle diameter in Е) demonstrates that 

palladium particles are decorated by ruthenium oxides. This is evidenced 

by the decrease of the metallic component surface area 

accessible for CO adsorption. 

Fig. 2 contains, as an example, the cyclic potentiodynamic 

curves obtained at the catalysts of XC72+20 wt.%Pd, XC72+20 

wt.%PdRu (50:50 at.%) and XC72+20 wt.%PdRu(80:20 at.%). 

From the data presented at fig. 2, it is clearly seen that the potential 

of the maximum of oxidative CO desorption essentially depends on 

the catalyst composition. At the increase of Ru content in the catalyst, 

the potential of CO peak shifts towards the negative values. At 

the transition from Pd to PdRu (50:50 at.%), the shift of the peak 

potential constitutes more than 300 mV. The presented data unambiguously 

point to the fact that the introduction of Ru into the catalyst 

results in the decrease of the over–potential of the oxidative 

CO desorption adsorbed at the catalyst surface. This probably is the 

main cause of the decrease of the shown below inhibiting effect of 

CO on the process of hydrogen electrooxidation at the introduction 

of Ru into the catalyst. 

3.2. Electrochemical characteristics of PtM and 

PdM catalysts in the reaction of hydrogen oxidation 

and their tolerance towards CO 

At fig. 3, the polarization curves of hydrogen electrooxidation at 

the catalysts of XC72 + 20 % Pt and XC72 +20 % PtMo (75:25 

at.%) are presented for the different electrode rotation rates. In both 

cases, the process rate is limited by the step of hydrogen molecule 

diffusion towards the rotating electrode surface. 

The polarization curves of hydrogen electrooxidation at PdM 

catalysts at different ratios of the binary system components are 

presented at fig. 4. The limiting current value for these catalysts 

grows essentially at the increase of the electrode rotation rate, 

which evidences the considerable effect of the mass transport step 

on the hydrogen reaction rate. As seen from the data presented at 

fig. 4 and in table 2, PdM compositions synthesized on carbon 

black XC72 are sufficiently effective catalysts of hydrogen oxidation. 

However, one should note the more slow raise of the hydrogen 

oxidation current at the overpotential growth at these catalysts 

as compared to platinum (fig. 3). 

In table 2, some electrochemical parameters are presented that 

characterize the electrocatalytic properties of the obtained catalysts: 

catalyst specific surface areas s, electrode steady–state potential 

values in the hydrogen–saturated solution E st , and current densities 

at the electrode polarization being 0.05 V.


Figure 4. Polarization dependences for the hydrogen oxidation reaction at the rotating disc electrode coated with Pd–based catalysts at the 

rotation rate, rpm: (1) 680; (2) 1080; (3) 1480; (4) 2040; 0.5 M H 2 SO 4 , 60 0 C. 

Figure 5. Dependence of the hydrogen oxidation limiting current 

on the square root of the electrode rotation rate: (1) calculated 

from the formula of Levich; (2) PdAu (50:50 at.%); (3) PdRu 

(50:50 at.%); (4) PdMo (50:50 at.%); 0.5 M H 2 SO 4 , 60 ºC. 

Figure 6. Polarization curves of CO oxidation at the catalysts of 

(1) XC72 + 30 wt.% Pd; (2) XC72 + 30 wt.% PdAu (50:50 at.%); 

(3) XC72 + 20 wt.% PdMo (50:50 at.%); (4) XC72 + 20 wt.% 

PdRu (50:50 at.%). 

At fig. 5, the dependences of the hydrogen electrooxidation limiting 

current on the square root of the electrode rotation rate are 

presented for three catalysts: PdAu, PdRu, PdMo at the component 

ratio being 50:50 at.%. These dependences are compared with the 

calculated data for the rotating disc electrode. For all the three catalysts, 

the limiting current changes linearly with the square root of 

the electrode rotation rate; however, unlike the calculated dependence, 

the experimental curves do not pass the origin of coordinates 

at the electrode rotation rate tending to zero. This evidences that the 

rate of the hydrogen electrooxidation reaction at these catalysts is 

limited not only by mass transport but also by kinetic limitations. 

As already noted above, negligible admixtures of CO in hydrogen 

already result in the poisoning of platinum catalysts used at 

present and to the drastic decrease of their activity in the hydrogen 

reaction. One of the methods that can be used to predict the catalyst 

tolerance towards CO is the comparison of the relative activity of 

different systems in CO oxidation reaction in the electrolyte solution 

saturated by this gas. At fig. 6, the dependences of CO electrooxidation 

current at various catalysts are presented. As CO 

molecules adsorbed at the catalyst surface block the hydrogen oxidation 

process, so the start of CO oxidation probably corresponds 

to the appearance of CO–free sites at the catalyst surface that can


87 

Figure 7. Impedance plots of the hydrogen reaction at the equilibrium potential at various Pd–based catalysts in the frequency range of 20– 

100000 Hz. (1) pure H 2 ; (2–5) gas mixture of H 2 +500 ppm CO after conditioning the electrode in the solution (min): (2) 30, (3) 50, (4) 90, 

(5) 120. 0.5 M H 2 SO 4 , at the temperature of 60 ºC. 

be accessible for hydrogen adsorption and electrooxidation. As can 

be seen (fig. 6), the degree of the catalyst surface unblocking can 

grow in the series of Pd < PdMo < PdAu < PdRu. Herewith, the 

catalyst tolerance in the end is obviously characterized both by the 

potential of CO oxidation start and by the ratio of the rates of this 

reaction and that of oxygen electrooxidation. 

As was shown above, for all the considered catalysts the rate of 

hydrogen supply from the solution bulk produces a considerable 

effect on the hydrogen reaction flow. To eliminate the effect of the 

mass transport step and allow measuring directly the rate of the 

electrochemical step of the hydrogen reaction and the influence of 

CO admixtures on it, the electrochemical impedance method was 

used. At fig. 7, the impedance plots of the electrooxidation process 

are presented for hydrogen containing 500 ppm CO at three Pd– 

based catalysts. Similar dependences were obtained in H 2 SO 4 solution 

saturated by the gas mixture of H 2 + 150 ppm CO. However, in 

the latter case the growth of the impedance components in time is 

less expressed than in the case of the electrolyte saturation by the 

mixture of H 2 + 500 ppm CO. 

As follows from the above data, the radius of impedance plots 

grows with the increase of the time of electrode conditioning in the 

solution, which evidences the hydrogen reaction deceleration at the 

adsorption of CO at the catalyst surface. The comparison of the 

data presented at fig. 7 with the results of impedance measurements 

obtained in similar conditions at the catalyst of XC72 + 20 % Pt 

demonstrates already at lower CO content (fig. 8) that CO adsorption 

at the studied PdM catalysts produces a less inhibiting effect 

on the hydrogen reaction flow, as compared to platinum. In order to 

estimate quantitatively the effect of CO admixtures on the hydrogen 

reaction rate, the exchange current values were calculated for 

this reaction using the impedance spectra. The calculation was 

based on Randles model [35,36], which includes the double electric 

layer capacitance connected in parallel to the electrochemical reaction 

resistance. In [37], where the impedance method is used for 

treating the kinetics of hydrogen discharge/ionization on smooth 

palladium, a more complicated equivalent scheme offered in paper 

[38] was used; however, as shown in [37], in the range of high 

enough frequencies the impedance of Pd electrode is sufficiently 

well described by Randles model. 

At fig. 9, the dependences are presented of the ratios of the exchange 

current in pure hydrogen to that determined in the electrolyte 

saturated by H 2 + CO gas mixture on the time of electrode 

conditioning in the solution. As follows from the curves presented 

at the figure, the poisoning of the catalysts by CO molecules adsorbed 

at their surface results in the essential decrease of the hydrogen 

reaction rate. Simultaneously, these data illustrate the dynamics 

of the catalyst surface poisoning by CO traces in hydrogen. The 

data on the hydrogen reaction kinetics at various catalysts obtained 

by analyzing the impedance spectra are summarized in table 3. As 

can be seen, all the studied catalysts are much more tolerant towards 

CO admixtures in hydrogen than Pt. PdAu alloys feature the 

highest tolerance. This can be caused by the decoration of palladium 

surface by gold. As CO is much less adsorbed on gold, as 

compared to palladium (the maximum value of 0.2 vs. 1.0, correspondingly), 

thence palladium surface decoration with gold must 

be the cause of its tolerance towards CO. Although the activity of


nanoparticles by gold hinders CO adsorption, while seemingly 

preserving enough sites that are accessible for hydrogen adsorption. 

In the binary system of PdRu, the main role is probably played by 

the alloy formation that facilitates greatly the oxidative desorption 

of CO, as compared to pure Pd. 

5. ACNOWLEGMENT 

The work was financially supported by the Presidium of the 

Academy of Sciences, Russian Academy of Sciences. 

Figure 8. Impedance plots of the hydrogen reaction at the equilibrium 

potential at Pt catalyst in the frequency range of 20–100000 

Hz. (1) pure H 2 ; (2–5) gas mixture of H 2 +500 ppm CO after conditioning 

the electrode in the solution (min): (2) 30, (3) 50, (4) 90, 

(5) 120. 0.5 M H 2 SO 4 , at the temperature of 60 ºC. 

Figure 9. Dependence of relative exchange current i 0 (t)/i 0 (0) on the 

electrode conditioning time in 0.5 M H 2 SO 4 solution saturated by 

the gas mixture of H 2 +500 ppm CO. 60 0 C. (1) XC72 + 30 wt.% 

PdAu (50:50 at.%); (2) XC72 + 20 wt.% PdMo (50:50 at.%); (3) 

XC72 + 20 wt.% PdRu (50:50 at.%); (4) XC72 + 20 wt.% Pt, CO 

content is 200 ppm. 

this catalyst in pure hydrogen is much lower than that of platinum, 

in time, due to the slower poisoning of the electrode by CO adsorption 

on its surface as compared to platinum, it proves to be more 

effective in the electrooxidation reaction of hydrogen with CO 

admixtures. 

4. CONCLUSION 

As follows from the comparison of the presented data, the binary 

systems of PdM are characterized by different mechanisms of tolerance 

towards CO. The decoration of Pd nanoparticles by Mo oxides, 

similarly to PtMo system, results in the earlier oxidation of 

adsorbed CO, which is probably due to the redox process with the 

participation of Mo 4+ /Mo 6+ pair [39]. The decoration of Pd 

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