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Phase relations in the system Ta–Rh–O and thermodynamic properties of TaRhO4
Materials Chemistry and Physics 116 (2009) 289–293
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Phase relations in the system Ta–Rh–O and thermodynamic properties of TaRhO4 K.T. Jacob a,∗ , Chander Shekhar a , Y. Waseda b a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 15 October 2008 Received in revised form 9 January 2009 Accepted 25 March 2009 Keywords: TaRhO4 Gibbs free energy Enthalpy Entropy Oxygen potential System Ta–Rh–O Phase diagram
a b s t r a c t Phase relations in the system Ta–Rh–O were determined by analysis of quenched samples corresponding to thirteen compositions inside the ternary triangle after equilibration at 1273 K. All the Ta–Rh alloys were found to be in equilibrium with Ta2 O5 . Only one ternary oxide TaRhO4 was detected. Based on phase relations in the ternary system, a solid-state electrochemical cell, incorporating calcia-stabilized zirconia as the electrolyte, was designed to measure the standard Gibbs energy of formation (G◦ , J mol−1 ) of TaRhO4 in the temperature range from 900 to 1300 K. For the reaction, 1 -Ta2 O5 2
+ 12 Rh2 O3 (ortho) → TaRhO4
G◦ = −42993 + 5.676T (±85) The calculated decomposition temperatures of TaRhO4 are 1644 ± 5 K in pure O2 and 1543 ± 5 K in air at a total pressure po = 0.1 MPa. Thermodynamic properties of TaRhO4 at 298.15 K have been evaluated from the results. The limited experimental thermodynamic data for Rh-rich alloys available in the literature are in fair accord with Miedema’s model. The Gibbs energies of formation of the different phases in the binary system Ta–Rh were estimated based on these inputs, consistent with the binary phase diagram. Based on the thermodynamic information on the stability of various phases, an oxygen potential diagram for the system Ta–Rh–O at 1273 K was constructed. Also presented are temperature–composition diagrams for the ternary system at constant oxygen partial pressures (pO2 /po = 0.212 and 10−6 ) calculated form the basic data. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As part of a larger program of research on systems Ln–Rh–O (Ln = lanthanide element) [1,2], M–Rh–O (M = transition metal) [3–7], phase relations and thermodynamic properties of ternary oxides in the system Ta–Rh–O have been studied. Recently, Rh supported on Ta2 O5 has been used for the partial oxidation of methane [8,9] and carbon dioxide reforming of methane [10] to synthesis gas. Rhodium supported on metal oxides is an effective catalyst for the decomposition of N2 O [11], NO [12]. Studies on Rh(1 1 1) [13] and Rh(1 0 0) [14] surfaces show that NO adsorbs on rhodium at 100 K but dissociates to N2 and O2 on heating to 300 K. Giessen et al. [15] studied Ta–Rh binary system in the temperature range from 1273 to 2673 K using metallographic, X-ray
∗ Corresponding author. Tel.: +91 80 22932494; fax: +91 80 23600472. E-mail addresses: [email protected], [email protected] (K.T. Jacob). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.03.030
techniques and thermal analysis over entire composition range. They reported the existence of six equilibrium phases of variable composition at 1273 K; two terminal solid solution phases ␣-Ta and ␣-Rh and intermetallics , ␣1 , ␣2 and ␣-TaRh3 . Kleykamp [16] and Chao [17] investigated thermodynamic properties of the Ta–Rh system using a solid-state electrochemical cell. Kleykamp [16] reported the Gibbs energies of formation of Ta0.083 Rh0.917 and TaRh3.3 in the temperature range from 1140 to 1280 K. Kleykamp [16] has evaluated Gibbs energy of formation of TaRh3 from the thermodynamic measurements of Chao [17] in the temperature range from 1173 to 1273 K. In the system Ta–O, although various intermediate oxide phases such as Ta6 O, Ta4 O and Ta2 O have been reported in the literature [18,19], Ta2 O5 is the only equilibrium phase present [20], with incommensurate structure based on an orthorhombic subcell below 1593 K and a tetragonal cell above that. Thermodynamic properties of Ta2 O5 have been recently reassessed by Jacob et al. [21] in the temperature range from 298.15 to 2200 K based on new measurements and data available in the literature. In the binary system Rh–O, Rh2 O3 is the only stable phase: the standard Gibbs energy of formation of the oxide is known accurately [3].
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K.T. Jacob et al. / Materials Chemistry and Physics 116 (2009) 289–293
Prosychev et al. [22] prepared TaRhO4 by solid-state reaction of binary oxides; the compound crystallizes in the rutile type structure (P42 /mnm, a = 0.4680, c = 0.3050 nm). The phase relations in the system Ta2 O5 –Rh2 O3 have been investigated by Prosychev et al. [22]. They showed that below 1553 K, Ta2 O5 and TaRhO4 coexist in the composition range, X from 0 to 0.5, where X represents mole fraction of Rh2 O3 . Below 1333 K, TaRhO4 and Rh2 O3 coexist in the composition range, X from 0.5 to 1. In the temperature range from 1333 to 1553 K, TaRhO4 coexists with Rh-rich alloy; the Ta content of the alloy, XTa increases from 0 to 0.1 with rising temperature. The phase diagram reported by Prosychev et al. [22] is topologically incomplete since alloy compositions in equilibrium with Ta2 O5 above 1553 K are not shown. The objective of this study was to establish an isothermal section of the ternary phase diagram for the system Ta–Rh–O by identification of phases present in samples quenched after equilibration at 1273 K, using optical and scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Based on the ternary phase relations, a solid-state electrochemical cell was designed to measure the standard Gibbs energy of formation of TaRhO4 from its component binary oxides. Thermodynamic properties of TaRhO4 at 298.15 K are evaluated. The authors are not aware of any earlier study of phase relation in the ternary system Ta–Rh–O and thermodynamic properties of TaRhO4 . The thermodynamic data are used for calculating decomposition temperature of TaRhO4 and isothermal phase diagrams at different oxygen partial pressures. The data can also be used to compute phase relations in higher order systems. 2. Experimental methods 2.1. Materials Tantalum and rhodium oxides, Ta2 O5 and Rh2 O3 , of purity higher then 99.999% were dried under flowing high purity argon gas at 873 K and mixed in the required stoichiometric ratio to make TaRhO4 . The powders were ground together in an agate mortar. The homogenous mixture was pelletized at 100 MPa using a steel die. The pellets were heated in an alumina crucible at 1523 K for a total of ∼70 h under the dry oxygen gas. The pellets used for experiments were placed on a sacrificial pellet of the same composition to prevent contamination by the crucible material. During the heat treatment the samples were removed twice from the furnace, cooled, ground thoroughly and repelletized. The formation of the single phase TaRhO4 was established by XRD. Elemental silicon was used as the internal standard for measuring the cell dimensions accurately. TaRhO4 was found to exhibit the rutile structure (P42 /mnm, with a = 0.4678 and c = 0.3047 nm). The lattice parameters of orthorhombic -Ta2 O5 obtained are a = 0.6196, b = 0.3661 and c = 0.3889 nm. Many weak superstructure reflections are present in the pattern for -Ta2 O5 , corresponding to superstructures along [10] with multiplicities (m) of 12 × b and 19 × b. The lattice parameters of the corundum-related orthorhombic Rh2 O3 are a = 0.5148, b = 0.5439 and c = 1.4693 nm. The oxygen gas used in this study (purity > 99.999%) was dried by passage through a columns containing silica gel, anhydrous Mg(ClO4 )2 and P2 O5 .
Fig. 1. Isothermal section of the phase diagram for the system Ta–Rh–O at 1273 K. Compositions of equilibrated samples used for phase analysis are shown by cross marks (+).
tion at 1273 K, the samples were quenched. The phases present in the quenched samples were identified by optical and scanning electron microscopy, powder X-ray diffraction and energy dispersive X-ray spectroscopy. Pure Ta, Rh, Ta2 O5 and Rh2 O3 were used as standards for EDS. 2.3. Electrochemical measurements Based on the phase relations in system Ta–Rh–O, a solid-state electrochemical cell (−) Pt–13% Rh, Rh + TaRhO4 + Ta2 O5 || (CaO) ZrO2 || Rh2 O3 + Rh, Pt–13% Rh (+) was designed to measure the standard Gibbs free energy of formation of TaRhO4 . Reversible e.m.f. of the cell was measured as a function of temperature in the range 900–1300 K. The cell is written such that the right-hand electrode is positive. Calciastabilized zirconia functioned as the solid electrolyte with predominant oxygen ion conduction (tion > 0.999) under the experimental conditions. The measuring electrode was prepared by compacting mixture of Ta2 O5 , TaRhO4 and Rh in the molar ratio 1:1.5:1 in a calcia-stabilized zirconia tube, with a Pt–13% Rh lead buried in it. An alumina sheath was used to insulate the Pt–13% Rh lead. The top of the zirconia tube was closed with a tight fitting bell-shaped Pyrex tube, which supported tungsten electrode connection sealed into glass as shown in Fig. 2. The alumina sheath was pressed down by means of a metal spring between the bell and the alumina sheath. The joint between the Pyrex bell and the zirconia tube was sealed with De-Khotinsky cement. The assembled half-cell containing the measuring electrode was first evacuated using side arm tube shown in the diagram, heated to ∼400 K, then flame sealed under vacuum. The reference electrode was prepared by compacting intimate mixture of the fine powders of Rh and Rh2 O3 in the molar ration
2.2. Determination of the phase diagram The phase relations in the system Ta–Rh–O were explored by equilibrating samples representing thirteen predetermined compositions at 1273 K, followed by quenching in liquid nitrogen or chilled Hg and phase identification. Different compositions were equilibrated for a total period of ∼8 days. The samples were quenched, ground to −325 mesh, and repelletized thrice during this period. The phase compositions of the samples were found to be unaltered by further heat treatments. The overall compositions of the samples used are shown by cross (+) marks in Fig. 1. In most cases samples of the same overall composition were made using different starting materials. For equilibrating the samples, two configurations were used. Mixtures containing Ta2 O5 , Rh2 O3 and TaRhO4 were equilibrated in pure oxygen at standard pressure pO2 /po = 1 using an apparatus described earlier [23]. The samples were held in recrystallized alumina crucibles. The pellets were kept on the sacrificial disk of the same composition to avoid contamination by the crucible. The mass of each pellet was determined before and after the equilibration. Mixtures containing metals or alloys were equilibrated in evacuated quartz ampules to prevent oxidation. The mixtures were prepared in an inert atmosphere glove box. The oxygen partial pressure in the inert gas was less then pO2 /po = 9.8 × 10−18 . The pellets made from each mixture were contained in a thoria crucible placed inside the quartz ampule. After equilibra-
Fig. 2. Schematic diagram of the apparatus used for e.m.f. measurement.
K.T. Jacob et al. / Materials Chemistry and Physics 116 (2009) 289–293 of 1:1.5 against the closed end calcia-stabilized zirconia crucible, with a Pt–13% Rh lead embedded in the mixture. The calcia-stabilized zirconia crucible containing the reference electrode was placed in a fused quartz tube. The measuring half-cell assembly contained in the calcia-stabilized zirconia tube was placed over the reference electrode and was pressed down by means of a metal spring between the bell and the top Pyrex cover. The joint between the top Pyrex cover and the zirconia tube was also sealed with De-Khotinsky cement. The cement was allowed to solidify while pressing the top of the cover against the spring. Then the outer quartz enclosure was also evacuated from the side arm tube and flame sealed under vacuum. The entire assembly shown in Fig. 2 was placed in the vertical resistance furnace, with the electrodes located in the even temperature (±1 K) zone. The upper part of the assembly, where cement seals were located, was at room temperature during measurement. A faraday cage made from stainless steel foil was placed between the furnace and the cell assembly. The foil was grounded to minimize the induced e.m.f. on the cell leads. The temperature of the furnace was controlled to ±1 K. The temperature was measured by a Pt/Pt–13% Rh thermocouple, checked against the melting point of gold. The cell potentials were measured by high impedance (>1012 ) digital voltmeter with a sensitivity of ±0.01 mV. The reversibility of the e.m.f. of the cell was established by micro-coulometric titration in both directions. A small direct current (∼50 A) was passed through the cell, using an external potential source for ∼300 s. The open circuit e.m.f. was subsequently monitored as a function of time. The e.m.f. was found to return to the steady state value before each titration. During the titration, the chemical potential of oxygen at each electrode was displaced from equilibrium by essentially an infinitesimal amount. Since the e.m.f. returned to same value after successive displacements in opposite directions, reversibility was demonstrated. The e.m.f. was also reproducible on temperature cycling of the cell.
3. Results and discussions
291
Fig. 3. Temperature dependence of reversible e.m.f. of the solid-state electrochemical cell. The numbers and letters identify the sequence of measurements in two separate experiments.
The oxygen potential corresponding to this dissociation reaction was determined earlier [3] and can be represented as: ◦
rO = − 23 G2 = −264243 + 188.0T (±150)
(3)
2
At the left-hand measuring electrode the oxygen chemical potential is fixed by the equilibrium between the three condensed phases:
3.1. Phase relations The isothermal section of the equilibrium phase diagram for the system Ta–Rh–O at 1273 K is shown in Fig. 1. Along the Ta–O boundary, -Ta2 O5 with orthorhombic structure was the only stable binary oxide. Similarly, along the Rh–O binary, Rh2 O3 with corundum-related orthorhombic structure was the only stable oxide. Along the Ta–Rh binary, in addition to the terminal solid solutions four intermetallics phases , ␣1 , ␣2 , and ␣-TaRh3 of variable composition were identified. The Ta-rich terminal solid solution (␣Ta) extends up to XRh = 0.081 and whereas Rh-rich terminal solid solution extends from XRh from 0.897 to 1.0 at 1273 K. There was only one ternary oxide, TaRhO4 , with rutile type structure in the Ta–Rh–O system at 1273 K. The compound appears to be stoichiometric within detection limit of EDS. The oxygen content estimated from mass loss on reduction under hydrogen at 1173 K agreed with stoichiometry. The alloys and intermetallics were in equilibrium with Ta2 O5 . There are seven three-phase regions involving condensed phases, ␣-Tass + Ta2 O5 + , + Ta2 O5 + ␣1 , ␣1 + Ta2 O5 + ␣2 , ␣2 + Ta2 O5 + ␣-TaRh3 , ␣-TaRh3 + Ta2 O5 + ␣-Rh, ␣Rh + Ta2 O5 + TaRhO4 , and ␣-Rh + TaRhO4 + Rh2 O3 . The measurement of the oxygen chemical potential corresponding to the three-phase equilibrium involving Rh + Ta2 O5 + TaRhO4 would permit the calculation of the standard Gibbs energy of formation of TaRhO4 . 3.2. Thermodynamic properties of TaRhO4 The reversible e.m.f. of the solid-state electrochemical cell, as a function of temperature in the temperature range 900–1300 K, is shown in Fig. 3. The e.m.f. decreases with increasing temperature. The least-squares regression analysis of the e.m.f. (E, mV) gives the following expression: E = 148.53 − 0.01961T (±0.21)
(1)
The uncertainty limit is based on twice the standard error estimate and the estimated error in e.m.f. and temperature measurement. The oxygen chemical potential (rO , J mol−1 ) at the right-hand 2 reference electrode is determined by the dissociation of Rh2 O3 : 2 Rh2 O3 3
→
4 Rh + O2 3
(2)
4 Rh + 23 Ta2 O5 3
+ O2 →
4 TaRhO4 3
(4) −1
, J mol ) of the measurThe oxygen chemical potential (m O2 ing electrode, computed from the e.m.f. using the Nernst equation (rO − m = 4FE), is given by: O 2
2
m O = −321567 + 195.568T (±170)
(5)
2
The virtual cell reaction, obtained by combing the two half-cell reactions, can be written as: 1 Rh2 O3 2
+ 12 Ta2 O5 → TaRhO4
(6) ◦
The standard Gibbs free energy of formation (G6 , J mol−1 ) of TaRhO4 from the component binary oxides is given by: ◦
G6 = −3FE = −42993 + 5.676T (±85)
(7)
F/(JV−1 ) = 96,485
where is the Faraday constant and E/(V) is the reversible e.m.f of the cell. The temperature independent term in Eq. (7) gives the enthalpy of formation of TaRhO4 from its component binary oxides according to reac◦ tion (6) at a mean temperature of 1100 K (Hf,ox (1100 K) =
−42.99 (±0.6) kJ mol−1 ). The entropy of formation of TaRhO4 from its component binary oxides is −5.68 (±0.5) J K−1 mol−1 at 1100 K. Thermodynamic properties of TaRhO4 at 298.15 K can be calculated by invoking the Neumann–Koop rule for the estimation of the heat capacity of TaRhO4 . The standard enthalpy of formation of TaRhO4 at 298.15 K from elements in their normal ◦ standard states is Hf (298.15 K) = −1264.78 (±5.0) kJ mol−1 . The standard entropy of TaRhO4 at 298.15 K is S ◦ (298.15 K) = 105.37 (±2) J K−1 mol−1 . Auxiliary data for Ta2 O5 = −2038.03 (±8.6) kJ mol−1 and S ◦ (298.15 K) = 146.4 (±1.4) J K−1 mol−1 ) ◦ [21] and Rh2 O3 (Hf (298.15 K) = −405.53 (±0.26) kJ mol−1 and S ◦ (298.15 K) = 75.69 (±0.5) J K−1 mol−1 ) [25] are used in making the estimates for TaRhO4 at 298.15 K. 3.3. Decomposition temperature Phase diagram in Fig. 1 indicates that TaRhO4 would decompose according to Eq. (4) when the oxygen potential is lowered. The oxy-
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K.T. Jacob et al. / Materials Chemistry and Physics 116 (2009) 289–293
Fig. 4. Gibbs energy of mixing of alloys in Ta–Rh system at 1273 K.
gen potential corresponding to this decomposition, computed from e.m.f. measured in this study and Gibbs energy of formation of high temperature orthorhombic form of Rh2 O3 [3] is given by Eq. (5). The decomposition temperature for TaRhO4 is 1644 ± 5 K in pure oxygen at standard pressure of pO2 /po = 1. The decomposition under equilibrium conditions in air at standard atmospheric pressure would occur at 1543 ± 5 K. The calculated decomposition temperature in air is 11 K lower than that reported by Prosychev et al. [22]. Fig. 6. Oxygen chemical potential diagram for the Ta–Rh–O system at 1273 K.
3.4. Computation of the phase diagrams To compute oxygen chemical potentials corresponding to the equilibrium between alloy phases and -Ta2 O5 , information on the activities of Ta in the alloy phases is required. Values of enthalpy of mixing for Ta–Rh alloys can be calculated using Miedema’s model [24]. By adding the Gibbs energy of mixing for an ideal solution to the enthalpy from Miedema’s model, Gibbs energy of mixing for the Ta–Rh alloy system can be estimated. Activities of Ta in Rh-rich alloys have been measured by solid-state electrochemical techniques [16,17]. The results are compared in Fig. 4. There is fair agreement between data from the different sources. Based on the available information, composition dependence of Gibbs energies of mixing of different phases have been estimated
Fig. 5. Composition dependence of the calculated activities (ai ) of components in the system Ta–Rh at 1273 K.
Fig. 7. Temperature–composition phase diagram for the system Ta–Rh–O in air, i.e. oxygen partial pressure (pO2 /po = 0.212); po = 0.1 MPa.
K.T. Jacob et al. / Materials Chemistry and Physics 116 (2009) 289–293
as shown in Fig. 4, consistent with the binary phase diagram. Uncertainty estimates are indicated for two compositions. The Gibbs energies of mixing of the terminal solid solutions (␣-Ta, ␣-Rh) in the Ta–Rh binary system were approximated using the regular solution model assuming random distribution of atoms. The regular solution parameter for Ta-rich terminal solid solution (␣-Ta) is −186 kJ mol−1 and for Rh-rich terminal solid solution (␣-Rh) is −250 kJ mol−1 . Using the tangent–intercept method, chemical potentials and activities of Ta and Rh in different phases were estimated. The results are shown in Fig. 5. Oxygen potential diagram for the Ta–Rh–O system at 1273 K, computed from the results obtained in this study, is shown in Fig. 6. The composition of the phases is characterized by the cationic fraction, Rh / (Rh + Ta ), where i represents moles of the component i. Since oxygen is not included in the composition parameter, information on the oxygen non-stoichiometry cannot be displayed in the diagram. Nevertheless, oxygen potential diagram provides useful information on the oxygen potential range for the stability of the various phases. The diagram is complementary to the Gibbs triangle representation of the phase relations in the ternary system (Fig. 1), where exact phase compositions can be clearly displayed. The oxygen potential diagram essentially has two distinct regions: the low oxygen potential region associated with equilibria involving the various alloy phases and -Ta2 O5 , and the high oxygen potential region involving the ternary oxide TaRhO4 . These
293
two regions are separated by a two-phase field involving essentially pure Rh and -Ta2 O5 . Phase relations can also be calculated as a function of temperature at constant oxygen partial pressures. The computed phase diagrams in air (pO2 /po = 0.212) and at an oxygen partial pres-
sure of pO2 /po = 10−6 are shown in Figs. 7 and 8, respectively. The decomposition temperature of TaRhO4 is considerably higher then that of Rh2 O3 at all partial pressures of oxygen. It is clear that the temperature–composition diagrams are very sensitive to oxygen partial pressure in the ambient atmosphere. 4. Conclusion
A study of the phase relations in the Ta–Rh–O system indicates that only one ternary compound, TaRhO4 , is present at 1273 K. The oxygen potential corresponding to the decomposition of TaRhO4 to Rh, Ta2 O5 and O2 has been obtained as a function of temperature. The oxygen potential (O2 , J mol−1 ) is given by the expression: O2 = −321567 + 195.568T (±170). The standard Gibbs energy of formation of TaRhO4 from the component ◦ ◦ binary oxides (Gf,ox , J mol−1 ) is given by: Gf,ox = −42993 + 5.676T (±85). The standard enthalpy of formation of TaRhO4 ◦ from elements at 298.15 K has been evaluated: Hf (298.15 K) = −1264.78 (±5.0) kJ mol−1 . The standard entropy of TaRhO4 at 298.15 K is S ◦ (298.15 K) = 105.37 (±2) J k−1 mol−1 . The thermodynamic properties of TaRhO4 were determined for the first time during the course of this study. Based on the thermodynamic information on the stability of various phases, phase relations in the system Ta–Rh–O have been computed as a function of oxygen potential at fixed temperature, and as a function of temperature at constant oxygen partial pressures. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Fig. 8. Temperature–composition phase diagram for the system Ta–Rh–O at oxygen. partial pressure (pO2 /po = 10−6 ); po = 0.1 MPa.
[25]
K.T. Jacob, Y. Waseda, J. Am. Ceram. Soc. 78 (1995) 440. K.T. Jacob, S. Priya, Y. Waseda, Bull. Mater. Sci. 21 (1998) 99. K.T. Jacob, M.V. Sriram, Metall. Trans. 25A (1994) 1347. K.T. Jacob, T.H. Okabe, T. Uda, Y. Waseda, Bull. Mater. Sci. 22 (1999) 741. K.T. Jacob, T.H. Okabe, T. Uda, Y. Waseda, J. Chem. Thermodyn. 32 (2000) 1399. K.T. Jacob, Y. Waseda, J. Solid State Chem. 150 (2000) 213. K.T. Jacob, C. Shekhar, Y. Waseda, J. Chem. Thermodyn. 41 (2009) 56. E. Ruckenstein, H.Y. Wang, J. Catal. 190 (2000) 32. E. Ruckenstein, H.Y. Wang, J. Catal. 187 (1999) 151. H.Y. Wang, E. Ruckenstein, Appl. Catal. A: Gen. 204 (2000) 143. V. Boissel, S. Tahir, C.A. Koh, Appl. Catal. B 64 (2006) 234. M. Haneda, K. Shinoda, A. Nagane, O. Houshito, H. Takagi, Y. Nakahara, K. Hiroe, T. Fujitani, H. Hamada, J. Catal. 259 (2008) 223. H.J. Borg, J.F.C.-J.M. Reijerse, R.A. van Santen, J.W. Niemantsverdriet, J. Chem. Phys. 101 (1994) 10052. P. Ho, J.M. White, Surf. Sci. 137 (1984) 103. B.C. Giessen, H. Ibach, N.J. Grant, Trans. Met. Soc. AIME 230 (1964) 113. H. Kleykamp, J. Less Comm. Met. 152 (1989) 15. C.-Y. Chao, MSc. Thesis, University of Wisconsin-Milwaukee, WI, 1977. D.R. Kudark, M. Sienko, J. Inorg. Chem. 6 (1967) 880. J. Niebuhr, J. Less Comm. Met. 10 (1966) 312. H. Jhen, E. Olzi, J. Less Comm. Met. 27 (1972) 297. K.T. Jacob, C. Shekhar, Y. Waseda, J. Chem. Thermodyn. 41 (6) (2009) 748. I.I. Prosychev, V.B. Lazarev, I.S. Shaplygin, J. Russ, Inorg. Chem. 26 (1981) 413. K.T. Jacob, T. Mathews, J. Mater. Chem. 1 (1991) 545. F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, F.R. de Boer, D.G. Pettifor (Eds.), Cohesion in Metals, Transition Metal Alloys, North Holland, New York, 1988, p. 544. K.T. Jacob, T. Uda, T.H. Okabe, Y. Waseda, High Temp. Mater. Proc. 19 (2000) 11.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Phase relations in the system Ta–Rh–O and thermodynamic properties of TaRhO4 K.T. Jacob a,∗ , Chander Shekhar a , Y. Waseda b a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Sendai 980-8577, Japan
a r t i c l e
i n f o
Article history: Received 15 October 2008 Received in revised form 9 January 2009 Accepted 25 March 2009 Keywords: TaRhO4 Gibbs free energy Enthalpy Entropy Oxygen potential System Ta–Rh–O Phase diagram
a b s t r a c t Phase relations in the system Ta–Rh–O were determined by analysis of quenched samples corresponding to thirteen compositions inside the ternary triangle after equilibration at 1273 K. All the Ta–Rh alloys were found to be in equilibrium with Ta2 O5 . Only one ternary oxide TaRhO4 was detected. Based on phase relations in the ternary system, a solid-state electrochemical cell, incorporating calcia-stabilized zirconia as the electrolyte, was designed to measure the standard Gibbs energy of formation (G◦ , J mol−1 ) of TaRhO4 in the temperature range from 900 to 1300 K. For the reaction, 1 -Ta2 O5 2
+ 12 Rh2 O3 (ortho) → TaRhO4
G◦ = −42993 + 5.676T (±85) The calculated decomposition temperatures of TaRhO4 are 1644 ± 5 K in pure O2 and 1543 ± 5 K in air at a total pressure po = 0.1 MPa. Thermodynamic properties of TaRhO4 at 298.15 K have been evaluated from the results. The limited experimental thermodynamic data for Rh-rich alloys available in the literature are in fair accord with Miedema’s model. The Gibbs energies of formation of the different phases in the binary system Ta–Rh were estimated based on these inputs, consistent with the binary phase diagram. Based on the thermodynamic information on the stability of various phases, an oxygen potential diagram for the system Ta–Rh–O at 1273 K was constructed. Also presented are temperature–composition diagrams for the ternary system at constant oxygen partial pressures (pO2 /po = 0.212 and 10−6 ) calculated form the basic data. © 2009 Elsevier B.V. All rights reserved.
1. Introduction As part of a larger program of research on systems Ln–Rh–O (Ln = lanthanide element) [1,2], M–Rh–O (M = transition metal) [3–7], phase relations and thermodynamic properties of ternary oxides in the system Ta–Rh–O have been studied. Recently, Rh supported on Ta2 O5 has been used for the partial oxidation of methane [8,9] and carbon dioxide reforming of methane [10] to synthesis gas. Rhodium supported on metal oxides is an effective catalyst for the decomposition of N2 O [11], NO [12]. Studies on Rh(1 1 1) [13] and Rh(1 0 0) [14] surfaces show that NO adsorbs on rhodium at 100 K but dissociates to N2 and O2 on heating to 300 K. Giessen et al. [15] studied Ta–Rh binary system in the temperature range from 1273 to 2673 K using metallographic, X-ray
∗ Corresponding author. Tel.: +91 80 22932494; fax: +91 80 23600472. E-mail addresses: [email protected], [email protected] (K.T. Jacob). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.03.030
techniques and thermal analysis over entire composition range. They reported the existence of six equilibrium phases of variable composition at 1273 K; two terminal solid solution phases ␣-Ta and ␣-Rh and intermetallics , ␣1 , ␣2 and ␣-TaRh3 . Kleykamp [16] and Chao [17] investigated thermodynamic properties of the Ta–Rh system using a solid-state electrochemical cell. Kleykamp [16] reported the Gibbs energies of formation of Ta0.083 Rh0.917 and TaRh3.3 in the temperature range from 1140 to 1280 K. Kleykamp [16] has evaluated Gibbs energy of formation of TaRh3 from the thermodynamic measurements of Chao [17] in the temperature range from 1173 to 1273 K. In the system Ta–O, although various intermediate oxide phases such as Ta6 O, Ta4 O and Ta2 O have been reported in the literature [18,19], Ta2 O5 is the only equilibrium phase present [20], with incommensurate structure based on an orthorhombic subcell below 1593 K and a tetragonal cell above that. Thermodynamic properties of Ta2 O5 have been recently reassessed by Jacob et al. [21] in the temperature range from 298.15 to 2200 K based on new measurements and data available in the literature. In the binary system Rh–O, Rh2 O3 is the only stable phase: the standard Gibbs energy of formation of the oxide is known accurately [3].
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Prosychev et al. [22] prepared TaRhO4 by solid-state reaction of binary oxides; the compound crystallizes in the rutile type structure (P42 /mnm, a = 0.4680, c = 0.3050 nm). The phase relations in the system Ta2 O5 –Rh2 O3 have been investigated by Prosychev et al. [22]. They showed that below 1553 K, Ta2 O5 and TaRhO4 coexist in the composition range, X from 0 to 0.5, where X represents mole fraction of Rh2 O3 . Below 1333 K, TaRhO4 and Rh2 O3 coexist in the composition range, X from 0.5 to 1. In the temperature range from 1333 to 1553 K, TaRhO4 coexists with Rh-rich alloy; the Ta content of the alloy, XTa increases from 0 to 0.1 with rising temperature. The phase diagram reported by Prosychev et al. [22] is topologically incomplete since alloy compositions in equilibrium with Ta2 O5 above 1553 K are not shown. The objective of this study was to establish an isothermal section of the ternary phase diagram for the system Ta–Rh–O by identification of phases present in samples quenched after equilibration at 1273 K, using optical and scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Based on the ternary phase relations, a solid-state electrochemical cell was designed to measure the standard Gibbs energy of formation of TaRhO4 from its component binary oxides. Thermodynamic properties of TaRhO4 at 298.15 K are evaluated. The authors are not aware of any earlier study of phase relation in the ternary system Ta–Rh–O and thermodynamic properties of TaRhO4 . The thermodynamic data are used for calculating decomposition temperature of TaRhO4 and isothermal phase diagrams at different oxygen partial pressures. The data can also be used to compute phase relations in higher order systems. 2. Experimental methods 2.1. Materials Tantalum and rhodium oxides, Ta2 O5 and Rh2 O3 , of purity higher then 99.999% were dried under flowing high purity argon gas at 873 K and mixed in the required stoichiometric ratio to make TaRhO4 . The powders were ground together in an agate mortar. The homogenous mixture was pelletized at 100 MPa using a steel die. The pellets were heated in an alumina crucible at 1523 K for a total of ∼70 h under the dry oxygen gas. The pellets used for experiments were placed on a sacrificial pellet of the same composition to prevent contamination by the crucible material. During the heat treatment the samples were removed twice from the furnace, cooled, ground thoroughly and repelletized. The formation of the single phase TaRhO4 was established by XRD. Elemental silicon was used as the internal standard for measuring the cell dimensions accurately. TaRhO4 was found to exhibit the rutile structure (P42 /mnm, with a = 0.4678 and c = 0.3047 nm). The lattice parameters of orthorhombic -Ta2 O5 obtained are a = 0.6196, b = 0.3661 and c = 0.3889 nm. Many weak superstructure reflections are present in the pattern for -Ta2 O5 , corresponding to superstructures along [10] with multiplicities (m) of 12 × b and 19 × b. The lattice parameters of the corundum-related orthorhombic Rh2 O3 are a = 0.5148, b = 0.5439 and c = 1.4693 nm. The oxygen gas used in this study (purity > 99.999%) was dried by passage through a columns containing silica gel, anhydrous Mg(ClO4 )2 and P2 O5 .
Fig. 1. Isothermal section of the phase diagram for the system Ta–Rh–O at 1273 K. Compositions of equilibrated samples used for phase analysis are shown by cross marks (+).
tion at 1273 K, the samples were quenched. The phases present in the quenched samples were identified by optical and scanning electron microscopy, powder X-ray diffraction and energy dispersive X-ray spectroscopy. Pure Ta, Rh, Ta2 O5 and Rh2 O3 were used as standards for EDS. 2.3. Electrochemical measurements Based on the phase relations in system Ta–Rh–O, a solid-state electrochemical cell (−) Pt–13% Rh, Rh + TaRhO4 + Ta2 O5 || (CaO) ZrO2 || Rh2 O3 + Rh, Pt–13% Rh (+) was designed to measure the standard Gibbs free energy of formation of TaRhO4 . Reversible e.m.f. of the cell was measured as a function of temperature in the range 900–1300 K. The cell is written such that the right-hand electrode is positive. Calciastabilized zirconia functioned as the solid electrolyte with predominant oxygen ion conduction (tion > 0.999) under the experimental conditions. The measuring electrode was prepared by compacting mixture of Ta2 O5 , TaRhO4 and Rh in the molar ratio 1:1.5:1 in a calcia-stabilized zirconia tube, with a Pt–13% Rh lead buried in it. An alumina sheath was used to insulate the Pt–13% Rh lead. The top of the zirconia tube was closed with a tight fitting bell-shaped Pyrex tube, which supported tungsten electrode connection sealed into glass as shown in Fig. 2. The alumina sheath was pressed down by means of a metal spring between the bell and the alumina sheath. The joint between the Pyrex bell and the zirconia tube was sealed with De-Khotinsky cement. The assembled half-cell containing the measuring electrode was first evacuated using side arm tube shown in the diagram, heated to ∼400 K, then flame sealed under vacuum. The reference electrode was prepared by compacting intimate mixture of the fine powders of Rh and Rh2 O3 in the molar ration
2.2. Determination of the phase diagram The phase relations in the system Ta–Rh–O were explored by equilibrating samples representing thirteen predetermined compositions at 1273 K, followed by quenching in liquid nitrogen or chilled Hg and phase identification. Different compositions were equilibrated for a total period of ∼8 days. The samples were quenched, ground to −325 mesh, and repelletized thrice during this period. The phase compositions of the samples were found to be unaltered by further heat treatments. The overall compositions of the samples used are shown by cross (+) marks in Fig. 1. In most cases samples of the same overall composition were made using different starting materials. For equilibrating the samples, two configurations were used. Mixtures containing Ta2 O5 , Rh2 O3 and TaRhO4 were equilibrated in pure oxygen at standard pressure pO2 /po = 1 using an apparatus described earlier [23]. The samples were held in recrystallized alumina crucibles. The pellets were kept on the sacrificial disk of the same composition to avoid contamination by the crucible. The mass of each pellet was determined before and after the equilibration. Mixtures containing metals or alloys were equilibrated in evacuated quartz ampules to prevent oxidation. The mixtures were prepared in an inert atmosphere glove box. The oxygen partial pressure in the inert gas was less then pO2 /po = 9.8 × 10−18 . The pellets made from each mixture were contained in a thoria crucible placed inside the quartz ampule. After equilibra-
Fig. 2. Schematic diagram of the apparatus used for e.m.f. measurement.
K.T. Jacob et al. / Materials Chemistry and Physics 116 (2009) 289–293 of 1:1.5 against the closed end calcia-stabilized zirconia crucible, with a Pt–13% Rh lead embedded in the mixture. The calcia-stabilized zirconia crucible containing the reference electrode was placed in a fused quartz tube. The measuring half-cell assembly contained in the calcia-stabilized zirconia tube was placed over the reference electrode and was pressed down by means of a metal spring between the bell and the top Pyrex cover. The joint between the top Pyrex cover and the zirconia tube was also sealed with De-Khotinsky cement. The cement was allowed to solidify while pressing the top of the cover against the spring. Then the outer quartz enclosure was also evacuated from the side arm tube and flame sealed under vacuum. The entire assembly shown in Fig. 2 was placed in the vertical resistance furnace, with the electrodes located in the even temperature (±1 K) zone. The upper part of the assembly, where cement seals were located, was at room temperature during measurement. A faraday cage made from stainless steel foil was placed between the furnace and the cell assembly. The foil was grounded to minimize the induced e.m.f. on the cell leads. The temperature of the furnace was controlled to ±1 K. The temperature was measured by a Pt/Pt–13% Rh thermocouple, checked against the melting point of gold. The cell potentials were measured by high impedance (>1012 ) digital voltmeter with a sensitivity of ±0.01 mV. The reversibility of the e.m.f. of the cell was established by micro-coulometric titration in both directions. A small direct current (∼50 A) was passed through the cell, using an external potential source for ∼300 s. The open circuit e.m.f. was subsequently monitored as a function of time. The e.m.f. was found to return to the steady state value before each titration. During the titration, the chemical potential of oxygen at each electrode was displaced from equilibrium by essentially an infinitesimal amount. Since the e.m.f. returned to same value after successive displacements in opposite directions, reversibility was demonstrated. The e.m.f. was also reproducible on temperature cycling of the cell.
3. Results and discussions
291
Fig. 3. Temperature dependence of reversible e.m.f. of the solid-state electrochemical cell. The numbers and letters identify the sequence of measurements in two separate experiments.
The oxygen potential corresponding to this dissociation reaction was determined earlier [3] and can be represented as: ◦
rO = − 23 G2 = −264243 + 188.0T (±150)
(3)
2
At the left-hand measuring electrode the oxygen chemical potential is fixed by the equilibrium between the three condensed phases:
3.1. Phase relations The isothermal section of the equilibrium phase diagram for the system Ta–Rh–O at 1273 K is shown in Fig. 1. Along the Ta–O boundary, -Ta2 O5 with orthorhombic structure was the only stable binary oxide. Similarly, along the Rh–O binary, Rh2 O3 with corundum-related orthorhombic structure was the only stable oxide. Along the Ta–Rh binary, in addition to the terminal solid solutions four intermetallics phases , ␣1 , ␣2 , and ␣-TaRh3 of variable composition were identified. The Ta-rich terminal solid solution (␣Ta) extends up to XRh = 0.081 and whereas Rh-rich terminal solid solution extends from XRh from 0.897 to 1.0 at 1273 K. There was only one ternary oxide, TaRhO4 , with rutile type structure in the Ta–Rh–O system at 1273 K. The compound appears to be stoichiometric within detection limit of EDS. The oxygen content estimated from mass loss on reduction under hydrogen at 1173 K agreed with stoichiometry. The alloys and intermetallics were in equilibrium with Ta2 O5 . There are seven three-phase regions involving condensed phases, ␣-Tass + Ta2 O5 + , + Ta2 O5 + ␣1 , ␣1 + Ta2 O5 + ␣2 , ␣2 + Ta2 O5 + ␣-TaRh3 , ␣-TaRh3 + Ta2 O5 + ␣-Rh, ␣Rh + Ta2 O5 + TaRhO4 , and ␣-Rh + TaRhO4 + Rh2 O3 . The measurement of the oxygen chemical potential corresponding to the three-phase equilibrium involving Rh + Ta2 O5 + TaRhO4 would permit the calculation of the standard Gibbs energy of formation of TaRhO4 . 3.2. Thermodynamic properties of TaRhO4 The reversible e.m.f. of the solid-state electrochemical cell, as a function of temperature in the temperature range 900–1300 K, is shown in Fig. 3. The e.m.f. decreases with increasing temperature. The least-squares regression analysis of the e.m.f. (E, mV) gives the following expression: E = 148.53 − 0.01961T (±0.21)
(1)
The uncertainty limit is based on twice the standard error estimate and the estimated error in e.m.f. and temperature measurement. The oxygen chemical potential (rO , J mol−1 ) at the right-hand 2 reference electrode is determined by the dissociation of Rh2 O3 : 2 Rh2 O3 3
→
4 Rh + O2 3
(2)
4 Rh + 23 Ta2 O5 3
+ O2 →
4 TaRhO4 3
(4) −1
, J mol ) of the measurThe oxygen chemical potential (m O2 ing electrode, computed from the e.m.f. using the Nernst equation (rO − m = 4FE), is given by: O 2
2
m O = −321567 + 195.568T (±170)
(5)
2
The virtual cell reaction, obtained by combing the two half-cell reactions, can be written as: 1 Rh2 O3 2
+ 12 Ta2 O5 → TaRhO4
(6) ◦
The standard Gibbs free energy of formation (G6 , J mol−1 ) of TaRhO4 from the component binary oxides is given by: ◦
G6 = −3FE = −42993 + 5.676T (±85)
(7)
F/(JV−1 ) = 96,485
where is the Faraday constant and E/(V) is the reversible e.m.f of the cell. The temperature independent term in Eq. (7) gives the enthalpy of formation of TaRhO4 from its component binary oxides according to reac◦ tion (6) at a mean temperature of 1100 K (Hf,ox (1100 K) =
−42.99 (±0.6) kJ mol−1 ). The entropy of formation of TaRhO4 from its component binary oxides is −5.68 (±0.5) J K−1 mol−1 at 1100 K. Thermodynamic properties of TaRhO4 at 298.15 K can be calculated by invoking the Neumann–Koop rule for the estimation of the heat capacity of TaRhO4 . The standard enthalpy of formation of TaRhO4 at 298.15 K from elements in their normal ◦ standard states is Hf (298.15 K) = −1264.78 (±5.0) kJ mol−1 . The standard entropy of TaRhO4 at 298.15 K is S ◦ (298.15 K) = 105.37 (±2) J K−1 mol−1 . Auxiliary data for Ta2 O5 = −2038.03 (±8.6) kJ mol−1 and S ◦ (298.15 K) = 146.4 (±1.4) J K−1 mol−1 ) ◦ [21] and Rh2 O3 (Hf (298.15 K) = −405.53 (±0.26) kJ mol−1 and S ◦ (298.15 K) = 75.69 (±0.5) J K−1 mol−1 ) [25] are used in making the estimates for TaRhO4 at 298.15 K. 3.3. Decomposition temperature Phase diagram in Fig. 1 indicates that TaRhO4 would decompose according to Eq. (4) when the oxygen potential is lowered. The oxy-
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Fig. 4. Gibbs energy of mixing of alloys in Ta–Rh system at 1273 K.
gen potential corresponding to this decomposition, computed from e.m.f. measured in this study and Gibbs energy of formation of high temperature orthorhombic form of Rh2 O3 [3] is given by Eq. (5). The decomposition temperature for TaRhO4 is 1644 ± 5 K in pure oxygen at standard pressure of pO2 /po = 1. The decomposition under equilibrium conditions in air at standard atmospheric pressure would occur at 1543 ± 5 K. The calculated decomposition temperature in air is 11 K lower than that reported by Prosychev et al. [22]. Fig. 6. Oxygen chemical potential diagram for the Ta–Rh–O system at 1273 K.
3.4. Computation of the phase diagrams To compute oxygen chemical potentials corresponding to the equilibrium between alloy phases and -Ta2 O5 , information on the activities of Ta in the alloy phases is required. Values of enthalpy of mixing for Ta–Rh alloys can be calculated using Miedema’s model [24]. By adding the Gibbs energy of mixing for an ideal solution to the enthalpy from Miedema’s model, Gibbs energy of mixing for the Ta–Rh alloy system can be estimated. Activities of Ta in Rh-rich alloys have been measured by solid-state electrochemical techniques [16,17]. The results are compared in Fig. 4. There is fair agreement between data from the different sources. Based on the available information, composition dependence of Gibbs energies of mixing of different phases have been estimated
Fig. 5. Composition dependence of the calculated activities (ai ) of components in the system Ta–Rh at 1273 K.
Fig. 7. Temperature–composition phase diagram for the system Ta–Rh–O in air, i.e. oxygen partial pressure (pO2 /po = 0.212); po = 0.1 MPa.
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as shown in Fig. 4, consistent with the binary phase diagram. Uncertainty estimates are indicated for two compositions. The Gibbs energies of mixing of the terminal solid solutions (␣-Ta, ␣-Rh) in the Ta–Rh binary system were approximated using the regular solution model assuming random distribution of atoms. The regular solution parameter for Ta-rich terminal solid solution (␣-Ta) is −186 kJ mol−1 and for Rh-rich terminal solid solution (␣-Rh) is −250 kJ mol−1 . Using the tangent–intercept method, chemical potentials and activities of Ta and Rh in different phases were estimated. The results are shown in Fig. 5. Oxygen potential diagram for the Ta–Rh–O system at 1273 K, computed from the results obtained in this study, is shown in Fig. 6. The composition of the phases is characterized by the cationic fraction, Rh / (Rh + Ta ), where i represents moles of the component i. Since oxygen is not included in the composition parameter, information on the oxygen non-stoichiometry cannot be displayed in the diagram. Nevertheless, oxygen potential diagram provides useful information on the oxygen potential range for the stability of the various phases. The diagram is complementary to the Gibbs triangle representation of the phase relations in the ternary system (Fig. 1), where exact phase compositions can be clearly displayed. The oxygen potential diagram essentially has two distinct regions: the low oxygen potential region associated with equilibria involving the various alloy phases and -Ta2 O5 , and the high oxygen potential region involving the ternary oxide TaRhO4 . These
293
two regions are separated by a two-phase field involving essentially pure Rh and -Ta2 O5 . Phase relations can also be calculated as a function of temperature at constant oxygen partial pressures. The computed phase diagrams in air (pO2 /po = 0.212) and at an oxygen partial pres-
sure of pO2 /po = 10−6 are shown in Figs. 7 and 8, respectively. The decomposition temperature of TaRhO4 is considerably higher then that of Rh2 O3 at all partial pressures of oxygen. It is clear that the temperature–composition diagrams are very sensitive to oxygen partial pressure in the ambient atmosphere. 4. Conclusion
A study of the phase relations in the Ta–Rh–O system indicates that only one ternary compound, TaRhO4 , is present at 1273 K. The oxygen potential corresponding to the decomposition of TaRhO4 to Rh, Ta2 O5 and O2 has been obtained as a function of temperature. The oxygen potential (O2 , J mol−1 ) is given by the expression: O2 = −321567 + 195.568T (±170). The standard Gibbs energy of formation of TaRhO4 from the component ◦ ◦ binary oxides (Gf,ox , J mol−1 ) is given by: Gf,ox = −42993 + 5.676T (±85). The standard enthalpy of formation of TaRhO4 ◦ from elements at 298.15 K has been evaluated: Hf (298.15 K) = −1264.78 (±5.0) kJ mol−1 . The standard entropy of TaRhO4 at 298.15 K is S ◦ (298.15 K) = 105.37 (±2) J k−1 mol−1 . The thermodynamic properties of TaRhO4 were determined for the first time during the course of this study. Based on the thermodynamic information on the stability of various phases, phase relations in the system Ta–Rh–O have been computed as a function of oxygen potential at fixed temperature, and as a function of temperature at constant oxygen partial pressures. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Fig. 8. Temperature–composition phase diagram for the system Ta–Rh–O at oxygen. partial pressure (pO2 /po = 10−6 ); po = 0.1 MPa.
[25]
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