ISSN 0036�0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 1, pp. 35–40. © Pleiades Publishing, Ltd., 2011.Original Russian Text © O.V. Belousov, R.V. Borisov, S.M. Zharkov, A.S. Samoilo, 2011, published in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 1, pp. 41–46.

35

INTRODUCTION

The unique physicochemical properties of nanoc�rystalline materials very interesting from the point ofview of practical applications predetermined everincreasing attention to these objects. The appearanceof highly effective methods for examining the structureof nanocrystalline materials allowed the properties ofultradisperse colloid systems to be studied [1]. Cur�rently, substantial advances in the synthesis of nanoc�rystalline materials with the required properties andstudies of their structure and various physicochemicalproperties have been made [2–5].

A strong impetus to the development of the con�cept of nanomaterials and its main propositions camefrom Gleiter [6, 7]. Works of the authors from ourcountry can be considered pioneering. For instance,prospects for the use of fine�dispersed materials inheterogeneous systems for performing chemical trans�formations energetically forbidden in massive sampleswere analyzed in [8]. It was shown that the dispersionof a material can initiate chemical transformations.The size of particles is an active thermodynamic vari�able, which, along with other thermodynamic vari�ables, determines the state of a system. The experi�mental data that show that the equilibrium concentra�tion of vacancies in small particles was higher than inmassive samples were obtained in [9]. An increase inthe concentration of vacancies as the size of particlesdecreased explained changes in the temperature of

polymorphic transitions, a decrease in lattice parame�ters, an increase in solubility, and other special featuresof the behavior of disperse powders.

A thermodynamic approach to the synthesis andproperties of nanocrystals can be used to determinethe rules governing their formation, growth, and prop�erties and their changes during phase transitions [10,11]. Thermodynamic characteristics can be deter�mined by direct methods such as differential scanningcalorimetry (DSC) and indirectly, for instance, elec�trochemically. The DSC method was used in [12] tomeasure the specific heat capacities of nanocrystallinecopper and palladium (the mean size of particles was6 and 8 nm, respectively) over the temperature range–123–27°С. It was noted that the heat capacity ofnanocrystalline palladium increased with respect to itspolycrystalline state by 29–53% over the temperaturerange studied. The authors also found that palladiumparticles initially 8 nm in diameter grew to 20 nm afterheating to 477°С. It was shown in [13] that nanocrys�talline palladium particles increased in size at temper�atures above 250°С.

Platinum family metals are fairly stable in the fine�dispersed state and can conveniently be used to studythe influence of dispersity on thermodynamic charac�teristics. We showed in [14] that the constant of plati�num(II) disproportionation depended on the disper�sity of platinum metal. The dispersity and temperaturedependences of thermodynamic properties can conve�

The Thermodynamic Characteristics of Aggregation of Fine�Dispersed Palladium

O. V. Belousova, R. V. Borisova, S. M. Zharkovb, c, and A. S. Samoiloa

a Institute of Chemistry and Chemical Technology, Siberian Division, Russian Academy of Sciences, Krasnoyarsk, Russiab Kirenskii Institute of Physics, Siberian Division, Russian Academy of Sciences,

Akademgorodok, Krasnoyarsk, 660036 Russiac Siberian Federal University, Krasnoyarsk, Russia

e�mail: ov_bel@icct.ruReceived March 1, 2010

Abstract—The size�dependent effects of the heterogeneous reaction + 2e = Pd0 + 4Cl– were studied

in hydrochloric acid solutions of H2PdCl4 at 40–70°C. Changes in the structural characteristics of palladiumblack were analyzed by transmission electron microscopy and X�ray diffraction. The temperature depen�

dence of the redox potential of the /Pd0 pair was used to determine the thermodynamic characteristicsof aggregation of fine�dispersed palladium. The heat effect of the reaction was in satisfactory agreement withdirect differential scanning calorimetry measurements.

Keywords: palladium, nanoparticles, electron microscopy.

DOI: 10.1134/S0036024411010031

PdCl42–

PdCl42–

CHEMICAL THERMODYNAMICS AND THERMOCHEMISTRY

36

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 1 2011

BELOUSOV et al.

niently be studied using standard redox potentials,which are often measured using electrodes with depos�ited metal blacks for fast potential establishment insystems. For instance, the authors of [15] measuredredox potential shifts ΔE of nanocrystalline copper,nickel, and palladium with respect to compact materi�als at various temperatures. The results were used tocalculate changes in the Gibbs energy of nanocrystal�line materials.

The thermodynamic stability of nanocrystallinepalladium was studied in [16]. Nanocrystalline mate�rials can be treated as a new type of bodies, in whichdefects and surface boundaries constitute up to 50% oftheir volumes. It is important to determine the ther�modynamic and kinetic stability of these materials fortheir technological applications. Thermodynamically,the disordered boundaries of these grains influencetheir configurational and vibrational entropy equallyas the enthalpy of the system. The authors determinedthe Gibbs energy of grain boundaries from measuredelectromotive force (EMF) values and experimentallydisproved the earlier assumption that nanocrystallinematerials were stable. They showed that palladiumparticles increased in size from 11 and 18 nm to 20 and70 nm in 26 and 40 min, respectively, at 340°С. Theexperimental data showed that nanocrystalline palla�dium was unstable at this temperature.

The contribution of the size factor to the electrodepotential of copper�containing electron�ion exchang�ers was studied in [17, 18]. The authors not only exper�imentally determined the influence of size effects onstandard potential values but also suggested a calcula�tion model taking into account the contribution of sizeeffects to electrode potentials, which gave fairly closeagreement with the experimental data [17].

It was shown in [19] that shifts of standard oxida�tion potentials to negative values with respect to mas�sive materials were well described by the Thomsonequation. The purpose of this work was to estimate thethermodynamic characteristics of aggregation of fine�dispersed palladium by studying the temperature

dependences of the standard /Pd0 redox poten�tial and direct differential scanning calorimetry heateffect measurements.

EXPERIMENTAL

We used PdCl2 of kh. ch. (chemically pure) grade,KCl of kh. ch. grade, and HCl of os. ch. 20�4 (specialpurity 20�4) grade. The initial palladium black pow�ders were synthesized by reducing a hydrochloric acidsolution of palladium(II) chloride with hydrazinechloride or sodium formate. The precipitates formedwere separated from solutions, treated with 1 Mhydrochloric acid, washed with doubly distilled wateruntil chloride ions were removed, and dried until theirweight ceased to change in a desiccator in a vacuum ata temperature below 80°С [20].

PdCl42–

The concentration of palladium in solutions wasdetermined by atom absorption spectroscopy andspectrophotometrically. The absorption spectra wererecorded on a Shimadzu UV300 spectrophotometerover the wavelength range 200–600 nm. Quartz cellswith path lengths from 0.1 to 1 cm were used. The ref�erence solvent was 1 M hydrochloric acid.

Compact palladium was prepared by annealingfine�dispersed palladium under argon; X�ray peakbroadenings were then only caused by the instrumen�tal component. Electrochemical measurements wereperformed in a hermetic temperature�controlled celldesigned by us and described in [19]. During experi�ments, a flow of argon continuously passed throughthe cell. Potential values were measured with respectto a silver�chloride reference electrode.

The X�ray patterns of the solid phase were obtainedon a DRON�4M powder diffractometer using CuK

α

radiation and a graphite monochromator in thereflected beam. Scanning was performed in the 2θrange 30°–130°. The fine crystal structure parameterssuch as the dispersity of blocks D and lattice micros�train values Δa/a cause measurable physical broaden�ing β of X�ray lines. Line broadenings were used to cal�culate linear sizes of coherent scattering blocks Dusing the Rietveld full�profile method [21].

Studies by transmission electron microscopy(TEM) were performed using a JEOL JEM�2100microscope equipped with an Oxford InstrumentsInca TEM 250 energy dispersion spectrometer; theaccelerating voltage was 200 kV. Palladium black sam�ples were dispersed in isopropanol. Dispersions werethen deposited on a thin carbon film on an electronmicroscopic object net and dried in a vacuum.

Direct measurements of heat effects were per�formed on a NETZSCH STA 449 C/4/G Jupitersynchronous thermal analyzer in a flow of argon(30 ml/min) at a 10 K/min heating rate.

RESULTS AND DISCUSSION

The selected 40–70°С temperature interval waslimited from below by a low rate of equilibration, espe�cially in the case of compact palladium, and, fromabove, by the possibility of platinum black aggregation.It was found by transmission electron microscopy andX�ray diffraction that the structural characteristics ofpalladium(0) powder used in this work did not changenoticeably during 12 h at 80°С. According to the X�raydata, the size of initial palladium black crystallites was~12 nm, which was in close agreement with the trans�mission electron microscopy data (Fig. 1).

When electrode cups were loaded with less than10 mg of annealed palladium, the potential wasunstable and irreproducible even when divalent pal�ladium was present in solutions in a concentration of1 × 10–2 mol/l. If the amount of palladium was higherthan 10 mg, the potential was reproducible and stable.

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 1 2011

THE THERMODYNAMIC CHARACTERISTICS 37

The potential difference between two electrodes withdifferent amounts of palladium powders (10 and100 mg) having equal structural characteristics waszero (±2 mV, which was within measurement errors).When fine�dispersed palladium was used, the poten�tial became stable at sample weights of ~5 mg, whichwas related to a larger specific surface area of the fine�dispersed material.

We found that a constant potential value at 60°Сwas reached in 40–120 min for compact palladiumdepending on the concentration of palladium(II) insolution and in 20–40 min for fine�dispersed palla�dium. These differences could likely be caused by alow rate of the establishment of equilibrium for com�pact palladium because of its comparatively small sur�face area. The criterion of the reliability of our experi�mental data was the fulfillment of the Nernst equation

. (1)

The /Pd0 redox potential values measuredover the divalent palladium concentration range3.1 × 10–4–3.1 × 10–2 mol/l at 60°С are shown inFig. 2. The concentration of palladium(II) in workingsolutions was determined before and after measure�ments. Therefore, the possibility of solution of fine�dispersed palladium(0) caused by the incompleteremoval of oxygen was taken into account. When theconcentration of solutions considerably increasedduring experiments, the corresponding correction wasintroduced into the Nernst equation. With compactpalladium at a 3.1 × 10–4 mol/l concentration of palla�dium(II) in solution, equilibrium potential valuescould not be reached in reasonable time, and experi�mental points at this concentration are thereforeabsent.

The dependence of the potential on the concentra�tion of palladium(II) is described by the Nernst equa�tions

E(60°C), mV = 445 + , (2)

E(60°C), mV = 425 + (3)

for compact and fine�dispersed palladium, respec�tively. Potential values were measured with respect to aremote silver�chloride reference electrode. The Е0

values obtained were therefore normalized to thestandard hydrogen potential using corrections for thethermodiffusion potential [22]. The experimental

/Pd0 potential value with the participation ofcompact palladium normalized with respect to thestandard hydrogen electrode potential is described bythe equation

E(60°C), mV = 609 + . (4)

−

= +

PdCl24

0 lnRTE E cnF

PdCl42–

24

33l gc−PdCl

o

24

33l gc−PdCl

o

PdCl42–

24

33l gc−PdCl

o

The standard equilibrium /Pd0 value obtained

at 60°С was 609 mV, which was close to the datareported in [22].

For fine�dispersed palladium, the dependence ofthe redox potential on the concentration of palladium

PdCl42–

20 nm

Fig. 1. Initial palladium black image fragment.

400

1.5

380

360

340

320

2.0 2.5 3.0 3.5−logc

E, mV

1

2

Fig. 2. Dependences of measured /Pd0 redox

potential values on the concentration of in solution

at 60°С with respect to a silver�chloride reference elec�trode; (1) compact palladium and (2) fine�dispersed palla�dium (mean particle size 28 nm).

PdCl42–

PdCl42–

38

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 1 2011

BELOUSOV et al.

chloro complexes in solution is described by the equa�tion

E(60°C), mV = 589 + (5)

(the equation includes corrections specified above). Ateach temperature, we obtained concentration depen�dences similar to that shown in Fig. 2. These depen�dences were well described by the Nernst equation.Extrapolation was used to determine the standardredox potentials at each temperature. The tempera�ture dependence of standard redox potentials for thereactions

+ 2e = Pd + 4Cl–, (6)

+ 2e = Pd* + 4Cl–, (7)

determined at 40–70°С is shown in Fig. 3. Here, Pdand Pd* are compact and fine�dispersed palladium,respectively.

The temperature dependences of the standard

/Pd0 redox potentials are linear over the tem�

24

33l gc−PdCl

o

PdCl42–

PdCl42–

PdCl42–

perature range studied. They are described by theequations

E0, mV = 640 – 0.89(Т – 25), (8)

E0*, mV = 610 – 0.58(Т – 25) (9)

for compact and fine�dispersed palladium, respec�tively; here, Т = 40–70°С.

Temperature dependences (8) and (9) were used tocalculate the thermodynamic characteristics of elec�trode reactions (6) and (7) with the participation ofcompact and fine�dispersed metal samples. The calcu�lated ΔH, ΔS, and ΔG values are listed in Table 1. Thethermodynamic characteristics of processes at 25°Сwere determined by extrapolation on the assumptionthat the enthalpy was independent of temperature overthe temperature interval specified [22]. The thermo�dynamic values for the aggregation of fine�dispersedpalladium to the compact state,

Pd* → Pd (10)

were calculated by the Hess law; these values are listedin Table 1. The thermodynamic characteristicsobtained for reaction (6) were close to those reportedin [22]. The enthalpy of aggregation (10) was in agree�ment with the value obtained in [15].

Along with measurements described above, wedetermined direct potential shifts to negative valuesover the temperature range studied. A sample of com�pact annealed palladium metal was placed into onecurrent terminal and a fine�dispersed material, intothe second one, and the potential shift of the electrodewith fine�dispersed palladium with respect to compactmaterial was directly measured. This approach wasused in [15] for copper, nickel, and palladium. How�ever, the results of a particular electrochemical reac�tion are then difficult to interpret. To make the resultsobtained more reliable, we must measure standardpotentials too. At the same time, when the firstapproach is used (when both electrodes are loadedwith materials having identical structural characteris�tics), identical potentials of electrochemical processesthat occur on two absolutely identical electrodes arerecorded. The temperature dependence of fine�dis�persed palladium potential shifts with respect to com�pact samples were also used to calculate the Gibbsenergies of aggregation, which were identical to thoseobtained from temperature dependence equations.

The direct determination of the heat effects ofaggregation was performed by differential scanningcalorimetry over the temperature ranges 100–350 and100–700°С. The structural characteristics of palla�dium powders before measurements are presented inTable 2. The characteristics of sample no. 1 are alsoshown in Fig. 1.

The photomicrograph of the sample after recordinga DSC curve from 100 to 700°C is shown in Fig. 4. TheDSC curves of fine�dispersed powders contain twowell�defined peaks, an exothermic peak with a maxi�

630

40

610

590

50 60 70

E0, mV

T, °C

2

1

Fig. 3. Temperature dependences of /Pd0 standard

redox potentials over the temperature range 40–70°С;(1) and (2), see Fig. 2.

PdCl42–

Table 1. Thermodynamic characteristics of the processesstudied

Reaction –ΔH°,kJ/mol

–ΔS°,J/(mol K) kJ/mol

,kJ/mol

(6) 175.8 171.8 123.6 115.9

(7) 151.1 111.9 117.8 112.7

(10) 23.7 59.8 5.8 3.2

C25G−Δ �

�

C70G−Δ �

�

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 1 2011

THE THERMODYNAMIC CHARACTERISTICS 39

mum at 282°C and an endothermic peak with a maxi�mum at 665°C. The exothermic peak (200–350°C)corresponds to aggregation of fine�dispersed material,which is substantiated by the electron microscopy data(the size of particles after heating to 350°C was no lessthan 200 nm) and the results obtained in [23], where itwas shown that aggregation of fine�dispersed palla�dium in a gas medium occurred at 250°C. The heateffects were determined using KClO4 with a peak at310°C as a reference. The data obtained were used tocalculate the heat effects of palladium aggregation,which were of –9 and –3.4 kJ/mol for sample nos. 1and 2, respectively.

The endothermic effect observed at 585–685°Cwas related to the dissociation of surface palladiumoxide,

2PdO = 2Pd + O2. (11)

The temperature of the dissociation of palladiumoxide calculated from the Van’t Hoff isotherm takinginto account the content of residual oxygen in argoncorresponds to the experimental values obtained. Thearea of the endothermic peak was used to calculate theheat effect of formation of palladium oxide taking intoaccount weight changes over the temperature range ofpalladium oxide dissociation. The result was in closeagreement with the handbook data [24].

ACKNOWLEDGMENTS

This work was financially supported by the grantNSh�6580.2010.3.

REFERENCES

1. B. D. Summ and N. I. Ivanova, Vestn. Mosk. Univ., Ser.Khim. 42, 300 (2001).

2. A. A. Rempel’, Usp. Khim. 76, 474 (2007).

3. G. B. Sergeev, Usp. Khim. 70, 915 (2001).

4. R. A. Andrievskii, Usp. Khim. 71, 967 (2002).

5. M. Yu. Rusanova, G. A. Tsirlina, O. A. Petrii, et al.,Elektrokhimiya 36, 517 (2000) [Russ. J. Electrochem.36, 457 (2000)].

6. H. Gleiter, in Proc. of the 2nd Riso Int. Symp. on Metal�lurgy and Materials Science, Deformation of Polycrystals:

Table 2. Structural characteristics of palladium powders(d is the diameter of particles, nm)

Sample X�ray TEM

1 12 10

2 30 28

Note: According to the TEM data, d > 1 µm for compact palladium.

10 µm

768 nm

779 nm

653 nm

701 nm

595 nm

714 nm602 nm

1.13 nm

888 nm

Fig. 4. Photomicrograph of sample no. 1 after recording DSC curves (700°С), d ~ 1 µm.

40

RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 85 No. 1 2011

BELOUSOV et al.

Mechanisms and Microstructures, Roskilde, Denmark,Sept. 14–18, 1981, p. 15.

7. H. Gleiter, Acta Mater. 48, 1 (2000).

8. N. S. Lidorenko, S. P. Chizhik, N. T. Gladkikh, et al.,Dokl. Akad. Nauk SSSR 257, 1114 (1981) [Sov. Phys.Dokl. 26, 406 (1981)].

9. I. D. Morokhov, S. P. Chizhik, N. T. Gladkikh, et al.,Dokl. Akad. Nauk SSSR 248, 603 (1979) [Sov. Phys.Dokl. 24, 769 (1979)].

10. I. P. Suzdalev and P. I. Suzdalev, Usp. Khim. 70, 203(2001).

11. A. I. Rusanov, Ross. Khim. Zh. 50, 145 (2006).

12. R. Birringer and J. Rupp, Phys. Rev. 36, 7888 (1987).

13. R. Birringer, Mater. Sci. Eng. A 117, 33 (1989).

14. O. V. Belousov, N. L. Kovalenko, L. I. Dorokhova,et al., Zh. Neorg. Khim. 46, 684 (2001) [Russ. J. Inorg.Chem. 46, 603 (2001)].

15. R. Kirchheim and H. Gleiter, et al., Nanostruct. Mater.1, 167 (1992).

16. F. Gärtner, R. Bormann, R. Birringer, et al., ScriptaMater. 35, 805 (1996).

17. T. A. Kravchenko, V. A. Krysanov, G. A. Filatov, et al.,Elektrokhimiya 42, 272 (2006) [Russ. J. Electrochem.42, 233 (2006)].

18. T. A. Kravchenko, V. A. Krysanov, N. V. Sotskaya, et al.,Zh. Fiz. Khim. 75, 134 (2001) [Russ. J. Phys. Chem. A75, 126 (2001)].

19. O. V. Belousov, L. I. Dorokhova, L. A. Solov’ev, et al.,Zh. Fiz. Khim. 82, 1067 (2008) [Russ. J. Phys. Chem.A 82, 647 (2008)].

20. O. V. Belousov, N. L. Kovalenko, L. I. Dorokhova,et al., Zh. Neorg. Khim. 40, 678 (1995).

21. H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969).22. N. M. Nikolaeva, Chemical Equilibria in Aqueous Solu�

tions at Elevated Temperatures (Nauka, Novosibirsk,1982) [in Russian].

23. M. L. Blokhina, A. I. Blokhina, and I. I. Smirnov,Poroshk. Metall., No. 11, 23–26 (1989).

24. D. C. Sassani and E. L. Shock, Geochim. Cosmochim.Acta 62, 2643 (1998).