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Phase chemistry of the system Nb–Rh–O
Journal of Solid State Chemistry 202 (2013) 234–240
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Phase chemistry of the system Nb–Rh–O K.T. Jacob a,n, Preeti Gupta a, M. Vinay a, Y. Waseda b a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 21 March 2013 Accepted 26 March 2013 Available online 2 April 2013
Phase relations in the system Nb–Rh–O at 1223 K were investigated by isothermal equilibration of eleven compositions and analysis of quenched samples using OM, XRD, SEM and EDS. The oxide phase in equilibrium with the alloy changes progressively from NbO to NbO2, NbO2.422 and Nb2O5−x with increasing Rh. Only one ternary oxide NbRhO4 with tetragonal structure (a¼ 0.4708 nm and c¼ 0.3017 nm) was detected. It coexists with Rh and Nb2O5. The standard Gibbs energy of formation of NbRhO4 from its component binary oxides measured using a solid-state electrochemical cell can be represented by the equation;
Keywords: NbRhO4 Gibbs free energy Enthalpy Entropy Oxygen potential System Nb–Rh
ΔGof ;ox ðJ=molÞ ¼ −38; 350 þ 5:818 Tð 7 96Þ Constructed on the basis of thermodynamic information of the various alloy and oxide phases are oxygen potential diagram for the system Nb–Rh–O at 1223 K and temperature–composition diagrams at constant partial pressures of oxygen. & 2013 Elsevier Inc. All rights reserved.
1. Introduction Because of the importance of rhodites and rhodates in catalysis, systematic investigations on phase equilibria and thermodynamic properties of the compounds in several ternary systems M–Rh–O have been reported in recent times [1–10]. Niobia supported metal catalysts form an interesting class of materials because of strong metal-support interaction, which can provide the active sites for reactions [11–13]. Metal Rh also has a unique property of chemisorbing CO, producing a wide variety of CO hydrogenation products such as methane, methanol and higher hydrocarbons. It has been reported that Nb2O5 supported Rh particles have high efficiency for CO hydrogenation with high selectivity to hydrocarbons [14]. Kunimori et al. [15] reported that the Nb2O5 promoted Rh/SiO2 is an effective catalyst for the ethane hydrogenolysis reaction. It was found that NbRhO4 formed as a new phase during calcination of the Nb2O5 promoted Rh/SiO2 catalyst [16,17]. NbRhO4 decomposes into Rh and Nb2O5 in H2 at 773 K. This leads the suppression of H2 chemisorption capacity and ethane hydrogenolysis activity. Recently Ito et al. [18] studied the Rh–niobia interaction in niobia-supported Rh (Rh–Nb2O5), niobia
n
Corresponding author. Fax: +91 80 2360 0472. E-mail addresses: [email protected], [email protected] (K.T. Jacob). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.03.056
promoted Rh/SiO2 (Nb2O5–Rh/SiO2) and RhNbO4/SiO2 catalyst. These catalyst have been applied to selective oxidation of CO in H2 (CO+H2+O2) and hydrogenation of CO (CO+H2). It has been found that niobia increases the activity and selectivity of catalysts in binary CO oxidation in H2 and CO hydrogenation. Phase diagram for the Nb–Rh system has been compiled by Massalski et al. [19]. The diagram shows the existence of nine solid phases, two terminal solid solutions (α-Nb and α-Rh), one intermetallic phase of almost constant composition (α′) and six intermetallics (s, γ, δ, ζ, η and κ) of variable composition at 1473 K. The Nb–O binary system has been extensively studied by number of researchers [20–24] and several oxides have been reported. Among all the reported phases, NbO, NbO2, NbO2.417 and Nb2O5 are stable at 1223 K [22–24]. Recently, the Gibbs energies of formation of NbO, NbO2, and NbO2.422 have been measured and the full range of thermodynamic properties for the four stable oxides reassessed by Jacob et al. [25]. Kleykamp [26] investigated phase relations in a limited range of compositions in the system Nb–Rh–O at 1273 K. The intermetallic phase “NbRh3” (κ) and the Rh-solid solution (α-Rh) were found to coexist with Nb2O5−x. The lower oxides of niobium were incompatible with metal Rh. The Gibbs energies of formation for Nb0.13Rh0.87 (solid solution) and NbRh3.55 (κ) were obtained by emf measurements on galvanic cells based on an oxygen-ion conducting solid electrolyte in temperature range from 1100 to 1300 K. In the binary system Rh–O, Rh2O3 is the only stable phase at high
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temperature and its standard Gibbs energy of formation is known accurately [10]. Survey of literature revealed the absence of information on the phase relations in the full range of compositions in the system Nb– Rh–O and thermodynamic properties of NbRhO4. In view of this, the isothermal section of ternary phase diagram for Nb–Rh–O system at 1223 K was established. Based on the phase relations, a solid-state electrochemical cell was designed to measure the standard Gibbs energy of formation of NbRhO4 as a function of temperature. Oxygen potential diagram for the system Nb–Rh–O at 1223 K and temperature–composition diagrams at fixed oxygen partial pressures were then computed from the results.
2. Materials and methods 2.1. Materials Powders of Nb2O5 and Rh2O3 of purity greater than 99.9% were dried under flowing Ar gas at 850 K and then mixed in equimolar proportion using an agate mortar and pestle. The homogeneous mixture was pelletized at 150 MPa using a steel die, placed in an alumina crucible and heated at 1473 K for a total duration of 270 ks under dry flowing oxygen gas for synthesizing NbRhO4. NbO was produced by heating of a mixture of Nb and Nb2O5 in the equimolar ratio under vacuum. The binary oxide NbO2 was also prepared from powders of metal Nb and Nb2O5 in the molar ratio 0.5:1. The pelletized mixtures contained in an alumina crucible were heated at 1473 K for a total duration of 180 ks in an evacuated quartz ampule. To avoid contamination by the crucible material, the samples pellets were supported on sacrificial pellets of the same composition. At intervals of 90 ks, the samples were removed from the furnace, cooled to room temperature, ground thoroughly and repelletized for further heat treatment. The formation of single phase NbRhO4, Nb and NbO2 was confirmed by XRD. A binary Nb-Rh alloy having the composition XRh ¼ 0.51 was prepared by arc melting on a water-cooled copper hearth under flowing high-purity deoxidized Ar gas. The alloy pellet was remelted three times for homogenization. Alloy powder was prepared by filing. Iron particles in the powder were removed using a powerful magnet. Powders of the alloy, NbO2 and NbRhO4 were used in phase diagram studies. Other materials used were powders of Nb and Rh of purity greater than 99.9%. Samples containing metal Nb, alloy and NbO2 were handled in an inert atmosphere glove box to prevent oxidation. The oxygen partial pressure in the inert gas was less than (P O2 =P o ¼ 9:7 10−18 ), where Po is the standard atmospheric pressure.
Fig. 1. Isothermal section of the phase diagram for the system Nb–Rh–O at 1223 K. The average compositions of the samples equilibrated at high temperature are shown by X marks.
were identified by optical and scanning electron microscopy (OM/ SEM), powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Pure Nb, Rh, Nb2O5 and Rh2O3 were used as standards for EDS. The phase composition of the samples was found to be unaltered by heat treatment beyond 1000 ks. 2.3. Measurement of emf The reversible emf of the solid-state electrochemical cell, Pt−13% Rh, Rh+NbRhO4+Nb2O5//(CaO) ZrO2//Rh2O3+Rh, Pt−13% Rh
2.2. Determination of phase diagram of the system Nb–Rh–O at 1223 K
was measured as a function of temperature in the range from 900 to 1300 K. The cell is written such that the reference electrode on the right-hand side is positive. Calcia-stabilized zirconia functioned as the solid electrolyte with predominant oxygen ion conduction (tion 40.999) under the experimental conditions. Wires of Pt–13% Rh alloy served as electrical leads. The cell is designed such that the emf is directly related to the Gibbs energy of formation of NbRhO4 from component binary oxides Nb2O5 and Rh2O3.
An isothermal section of the phase diagram of the system Nb–Rh–O at 1223 K was established by equilibrating eleven samples representing different compositions inside the ternary triangle. The average composition of the samples used is shown by cross marks in Fig. 1. The starting materials used to make the samples and their overall compositions are listed in Table 1. The components of each sample were mixed thoroughly and pelletized at 100 MPa. The samples were equilibrated for a total period of 1000 ks at 1223 K in evacuated quartz ampules to prevent oxidation. The samples were quenched, ground to −325 mesh and repelletized thrice during this period. The samples, contained in yttria-stabilized zirconia crucibles and supported on sacrificial pellets of the same composition, were placed inside the quartz ampules. After equilibrating at 1223 K, the quartz ampules were quenched in chilled mercury. Phases present in quenched samples
The measuring electrode was prepared by compaction of a mixture of Nb2O5 (α), NbRhO4 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 support tungsten electrode connection sealed into glass as shown in Fig. 2. The alumina sheath was pressed down by means of the metal spring between the Pyrex 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 Rh2O3 in the molar ratio of 1:1.5 against a closed-end calcia-stabilized zirconia crucible, with a Pt–13% Rh lead embedded
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Table 1 Starting materials used for making samples, overall composition and equilibrium phases at 1223 K identified in quenched samples by XRD and EDS. Sample number
1 2 3 4 5 6 7 8 9 10 11
Starting materials
Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 NbRh2O4+γ (XRh ¼0.51) γ (XRh ¼0.51)+NbO2+NbRhO4 γ (XRh ¼0.51)+NbO+Nb2O5 Nb+γ (XRh ¼ 0.51)+Nb2O5 Nb+NbRhO4 Nb+NbRhO4+Nb2O5
Overall composition of sample
Phases identified after equilibration
XNb
XRh
XO
0.05 0.135 0.2 0.24 0.27 0.295 0.34 0.37 0.42 0.5 0.56
0.55 0.465 0.4 0.36 0.33 0.305 0.26 0.23 0.18 0.1 0.04
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
NbRhO4+Rh2O3+Rh NbRhO4+Nb2O5+Rh Nb2O5−x+α-Rh (XRh ¼ 0.914) Nb2O5−x+κ+α-Rh NbO2.422+κ(XRh ¼ 0.759) NbO2.422+NbO2+κ NbO2+ζ+η NbO2+δ+ζ NbO2+s+ζ NbO+NbO2+s NbO+α-Nb+α′
displaced from equilibrium by essentially an infinitesimal amount. Since the emf returned to same value after successive displacements in opposite directions, reversibility was confirmed. The emf was also reproducible on temperature cycling of the cell.
3. Results and discussions 3.1. Phase relations
Fig. 2. Schematic diagram of the apparatus used for the measurement of emf of the solid state cell.
in the mixture. The calcia-stabilized zirconia crucible containing the reference electrode was placed inside a fused quartz tube, closed at one end. The measuring half-cell assembly contained in the calciastabilized zirconia tube was placed over the reference electrode and was pressed down by means of metal spring between the bell and the top Pyrex cover. The joint between the top Pyrex cover and the quartz tube enclosing the cell was sealed with De-Khotinsky cement. The cement was allowed to solidify while pressing the top 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 (71 K) zone. The upper part of the assembly, where cement seal was 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 induced emf on the cell leads. The temperature of the furnace was controlled to 71 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 (41012 Ω) digital voltmeter with the sensitivity of 70.01 mV. The reversibility of the cell emf was established by micro-coulometric titration in both directions. A small direct current ( 50 μA) was passed through the cell, using an external potential source for 300 s. The open circuit emf was subsequently monitored as a function of time. The emf was found to return to the steady state value before the titration. During the titration, the chemical potential of oxygen at each electrode was
Results of phase analysis of eleven compositions in the ternary system Nb–Rh–O equilibrated at 1223 K are summarized in Table 1. These results are combined with information on the three binary systems available in the literature [19] to develop the isothermal section of the phase diagram displayed in Fig. 1. There is only one stable oxide, Rh2O3, along the binary Rh–O edge of the ternary triangle. The Rh2O3 has orthorhombic structure, space group Pbca (61), with a¼ 0.5148, b¼ 0.5438 and c¼1.4693 nm. Along the Nb–O binary, there are four stable oxides, NbO, NbO2, NbO2.422 and Nb2O5−x. NbO has cubic structure in the space group Pm3m, with lattice parameter a ¼0.42101 nm. The solubility of oxygen in metal Nb in equilibrium with NbO at 1223 K is 2 at%. NbO2 occurs in two different forms. A phase transition occurs from a low-temperature distorted-rutile structure (I41/a, a¼ 1.3702 nm, c¼0.5985 nm) to the high-temperature rutile (P42/mnm, a¼ 0.4846 nm, c¼ 0.3032 nm) having a body-centered tetragonal cell at Ttr ¼ 1080 K. In the low-temperature form, NbO6 octahedra are joined at edges and corners. Nb–Nb distances along the c-axis vary alternatively as short and long. The high-temperature rutile form adopts a four long, two short Nb–O bond patterns. NbO2.422 (Nb12O29) crystallizes as monoclinic (A2/a, a ¼3.132 nm, b¼0.3832 nm, c¼ 2.072 nm; β¼112.931). It transforms to an orthorhombic form at temperatures above 1473 K. Although Nb2O5 exists in several forms, the α-form, also some times designated as H, (with monoclinic structure, space group P2 and lattice parameters a ¼2.115 nm, b¼0.3831 nm, and c¼1.937 nm) is the thermodynamically stable modification [25]. Along the Nb–Rh edge, seven intermetallic phases α′, s, γ, δ, ζ, η, κ (“NbRh3”) are identified, six of which have variable composition. The Nb-rich terminal solid solution (α-Nb) extends up to XRh ¼0.199, whereas Rh-rich terminal solid solution (α-Rh) exists between 0.875≤XRh≤1.0 at 1223 K. The intermetallic α′ is almost stoichiometric. Since the alloy phases and their compositions detected in this study are compatible with the binary phase diagram for the system Nb-Rh given in the compilation of Massalski et al. [19], their nomenclature is used in this study for designating alloy phases. In the Nb–Rh–O system at 1223 K, only one stable ternary oxide, NbRhO4, with tetragonal rutile-type structure (space group
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P43/mnm) was found with lattice parameters a ¼0.4692 nm c ¼0.2995 nm. The lattice parameters measured in this study are in fair agreement with values reported in the literature [17,27]. The α-Rh terminal solid solution and phase κ (“NbRh3”) for XRh≥0.785 are in equilibrium with Nb2O5−x. The two oxide phases Nb2O5−x and NbO2.422 coexist with κ (“NbRh3”) phase characterized by XRh ¼0.785. The intermetallic phase κ is in equilibrium with NbO2.422 for 0.785≥XRh≥0.724. The oxide phases NbO2.422 and NbO2 coexist with κ phase of composition XRh ¼0.724. The intermetallic η, ζ, δ γ and s (XRh≥0.384) are in equilibrium with NbO2. The intermetallic phase s, α′ and α-Nb are in equilibrium with NbO. The oxide NbO and NbO2 coexist with s phase defined by XRh ¼0.384. There are thirteen three-phase region involving condensed phases, α-Nb+α′+NbO, α′+ s+NbO, s+NbO+NbO2, s+γ +NbO2, γ+δ+NbO2, δ+ζ+NbO2, ζ+η+NbO2, η+κ+NbO2, κ+NbO2+ NbO2.422, κ+NbO2.422+Nb2O5−x, κ+α−Rh+Nb2O5−x, Rh+Nb2O5−x+ NbRhO4, and Rh+NbRhO4+Rh2O3. The measurement of oxygen chemical potential corresponding to the three-phase equilibrium involving Rh, Nb2O5−x and NbRhO4 would allow determination of the standard Gibbs free energy of formation of NbRhO4. The phase relations delineated in this study refine and expand the data provided by Kleykamp [26] for the restricted region bounded by NbO2, Nb2O5, Rh and NbRh3 at 1273 K. Kleykamp [26] assumed that NbO2 is in equilibrium with Nb2O5−x (x¼0.2), following the earlier suggestion of Worrell [28]. The results of this study and others [22–25] indicate an additional oxide phase of composition NbO2.422 between NbO2 and Nb2O5−x at 1223 K. The limit of nonstoichiometry of Nb2O5−x is characterized by x≤0.0458 at 1223 K [25]. 3.2. Thermodynamic properties of NbRhO4 The reversible emf of solid-state electrochemical cell is shown in Fig. 3 as a function of temperature in the range from 900 to 1300 K. The emf decreases slightly with increasing temperature. Least squares regression analysis yields the following expression: EðmVÞ ¼ 132:5−0:0201 T ð 70:33Þ
ð1Þ
The uncertainty limit is based on twice the standard deviation and the estimated error in emf and temperature measurement. The oxygen chemical potential at the reference electrode on the right side of the cell is determined by decomposition of Rh2O3: 2/3 Rh2O3 (ortho) ¼4/3 Rh+O2
(2)
237
The oxygen potential corresponding to this decomposition reaction was determined earlier [10] and can be represented as: ΔμrO2 ðJ=molÞ ¼ ΔGo2 ¼ −264; 243 þ 188:0 T ð 7 150Þ
ð3Þ
At the left-hand side measuring electrode, the oxygen potential is fixed by the equilibrium between the three condensed phases: 4/3 Rh+2/3Nb2O5 (α)+O2 ¼4/3 NbRhO4
(4)
(Δμm O2 )
of the measuring elecThe oxygen chemical potential trode, computed from the emf using the Nernst equation, ðΔμrO2 −Δμm O2 ¼ 4FEÞ is given by: Δμm O2 ðJ=molÞ ¼ −315; 376 þ 195:757 Tð 7 196Þ
ð5Þ
The overall cell reaction, obtained by combining the two halfcell reactions, can be written as: 1/2Nb2O5 (α)+1/2 Rh2O3 (ortho) ¼NbRhO4 The standard Gibbs free energy of formation from the component binary oxides is given by:
(6) (ΔGo6 )
ΔGo6 ðJ=molÞ ¼ −ηFE ¼ −38; 350 þ 5:818 T ð 796Þ
of NbRhO4 ð7Þ
where η¼ 3 is number of electrons involved in the electrode reactions corresponding to formation of NbRhO4. F¼96,485 J/V is the Faraday constant and E/V is the reversible emf of the cell. The temperature independent term in Eq. (7) gives the enthalpy of formation of NbRhO4 from its component binary oxides according to reaction (6) at a mean temperature of 1100 K. The enthalpy of formation of NbRhO4 from its components Rh2O3 (ortho) and Nb2O5 (α) is −38.3570.13 kJ/mol. The entropy of formation of NbRhO4 from its component binary oxides is −5.82 70.12 J/K mol at 1100 K. The negative value is a little surprising since the high temperature entropy of NbRhO4 has a configurational contribution from the mixing of Nb5+ and Rh3+ on the cation sublattice of the rutile structure. If the ions are randomly distributed, the entropy contribution would be 11.53 K/mol. The results of this study would suggest that the distribution is perhaps not completely random. Thermodynamic properties of NbRhO4 at 298.15 K can be calculated by invoking the Neumann–Koop rule for the estimation of heat capacity of NbRhO4. The standard enthalpy of formation of NbRhO4 from elements in their normal states is evaluated as ΔH of ¼ −1190:27 7 1:14 kJ=mol (T ¼298.15 K). The standard entropy of NbRhO4 at 298.15 K is 100.6770.5 J/K/mol. Auxiliary data for Nb2O5 (α), ΔH of ¼ −1898:31 72:1 kJ=mol (T ¼298.15 K) [25] and So ¼137.30 7 1.3 J/K/mol (T ¼298.15 K) [25,29], and for Rh2O3 (ortho) ΔH of ¼ −405:53 7 0:26 kJ=mol, So ¼ 75.6970.5 J/K/ mol [30] are used in making the estimates for NbRhO4 at 298.15 K. There is no thermodynamic information on NbRhO4 in the literature for comparison with the values obtained in this study. It would be useful to measure the enthalpy of formation and heat capacity of NbRhO4 as a function of temperature using calorimetric methods in order to confirm the results obtained. 3.3. Decomposition temperature On lowering the oxygen potential, the compound NbRhO4 will decompose according to Eq. (4). The oxygen potential corresponding to this decomposition is given by Eq. (5). The decomposition temperature for NbRhO4 is 1611 716 K in pure oxygen at standard pressure (P O2 =P o ¼ 1). The decomposition in air at standard atmospheric pressure would occur at 1512 715 K under equilibrium conditions. 3.4. Computation of phase diagrams
Fig. 3. Temperature electrochemical cell.
dependence
of
reversible
emf
of
the
solid-state
Information on activity of Nb in the alloy phases and thermodynamic data on rhodium and niobium oxides are required to
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compute oxygen chemical potentials for various three-phase regions shown in Fig. 1. The Gibbs free energies of mixing for two Rh-rich binary compositions have been measured by Kleykamp [26]. The measured values are in fair agreement with Miedema's model [31] as shown in Fig. 4. Gibbs free energy of mixing for all the phases in the Nb–Rh system have been estimated based on the available information, consistent with the binary Nb–Rh and ternary Nb–Rh–O phase relations observed in this study. The results are shown in Fig. 4. The Gibbs energy of mixing of terminal solid solutions (α-Nb and α-Rh) in Nb–Rh binary system can be approximated by regular solution model assuming random distribution of atoms; ΔGM ¼RT (XNb ln XNb+XRh ln XRh)+Ω XNb XRh. The regular solution parameters (Ω) for Nb-rich and Rh-rich terminal solid solutions are −178 and −216 kJ/mol, respectively. Using the tangent intercept method, chemical potentials and activities of Nb and Rh in different phase fields of the binary Nb–Rh system have been calculated. The results are shown in Fig. 5. Oxygen potential diagram for the Nb–Rh–O system at 1223 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, ηRh =ðηRh þ ηNb Þ, where ηi 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. When three condensed phases and a gas phase coexist at equilibrium in a ternary system such as Nb–Rh–O, the system is monovariant. At a fixed temperature, three condensed phases coexist only at unique partial
Fig. 6. Oxygen potential–composition diagram for the system Nb–Rh–O at 1223 K.
Fig. 4. Gibbs free energy of mixing in the system Nb–Rh at 1223 K. Phase boundaries are indicated by vertical lines. Common tangent of free energy of mixing curves of adjacent phases define chemical potentials in the corresponding two phase field.
Fig. 5. Composition dependence of the calculated activities (ai) of Rh and Nb in the system Nb–Rh at 1223 K.
Fig. 7. Temperature–composition phase diagram for the system Nb–Rh–O in pure oxygen. Oxygen partial pressure (P O2 =P o ¼ 1), P o ¼ 0.1 MPa.
K.T. Jacob et al. / Journal of Solid State Chemistry 202 (2013) 234–240
pressure of oxygen. The three-phase equilibria are therefore represented by horizontal lines in the oxygen potential diagram at constant temperature. Phase relations in the ternary system Nb–Rh–O can also be computed as the function of temperature at constant oxygen partial pressure. The computed phase diagrams at three oxygen partial pressures,P O2 =P o ¼ 1, P O2 =P o ¼ 0:212 and P O2 =P o ¼ 10−6 are shown in Figs. 7–9, respectively. It is clear that the temperature– composition diagrams are very sensitive to oxygen partial pressure in the ambient atmosphere. The decomposition temperature of both NbRhO4 and Rh2O3 decreases significantly with decreasing oxygen partial pressure. However, the decomposition temperature
239
of NbRhO4 is always higher than that of Rh2O3 at all partial pressures of oxygen. The decomposition temperature of NbRhO4 in air computed in this study (1512 715 K) is in fair agreement with the value of 1503 720 K reported by Shaplygin et al. [27] and 1523 710 K reported by Prosychev et al. [32].
4. Conclusions Presented in this article are studies on the phase relations in the system Nb–Rh–O at 1223 K and thermodynamic properties of the ternary oxide NbRhO4 as a function of temperature. NbRhO4 is the only stable ternary compound. The oxygen potential corresponding to the decomposition of NbRhO4 to Rh, Nb2O5 and O2 has been measured as a function of temperature. The oxygen potential is given by the expression, ΔμO2 ðJ=molÞ ¼ −315; 376 þ 195:757 Tð 7 127Þ. The standard Gibbs free energy of formation of NbRhO4 from its binary component oxides was directly obtained from the emf of the solid-state electrochemical cell. For the reaction, 1/2 Nb2O5 (α)+1/2 Rh2O3 (ortho) ¼NbRhO4 ΔGof ;ox ðJ=molÞ ¼ −38; 350 þ 5:818 Tð 7 96Þ The standard enthalpy of formation of NbRhO4 at 298.15 K from elements in their normal standard states has been evaluated as ΔH of ¼ −1190:27 7 1:14 kJ=mol and the standard entropy of NbRhO4 at 298.15 K is So ¼100.67 7 0.5 J/K/mol. With the help of available thermodynamic information for various phases, phase relation in the system Nb–Rh–O has been computed as a function of oxygen potential at constant temperature and as a function of temperature at constant oxygen partial pressures.
Acknowledgments
Fig. 8. Temperature–composition phase diagram for the system Nb–Rh–O in air. Oxygen partial pressure (P O2 =P o ¼ 0:212), P o ¼ 0.1 MPa.
K. T. Jacob is grateful to the Indian National Academy of Engineering for support as INAE Distinguished Professor. Preeti Gupta wishes to thanks the University Grants Commission, India, for the award of Dr. D.S. Kothari Postdoctoral Fellowship. References
Fig. 9. Temperature–composition phase diagram for the system Nb–Rh–O at oxygen partial pressure (P O2 =P o ¼ 10−6 ), P o ¼0.1 MPa.
[1] K.T. Jacob, T.H. Okabe, T. Uda, Y. Waseda, Bull. Mater. Sci. 22 (1999) 741–749. [2] K.T. Jacob, Y. Waseda, J. Am. Ceram. Soc. 78 (1995) 440–414. [3] K.T. Jacob, T. Uda, T.H. Okabe, Y. Waseda, J. Chem. Thermodyn. 32 (2000) 1399–1408. [4] K.T. Jacob, C. Shekhar, Y. Waseda, Mater. Chem. Phys. 116 (2009) 289–293. [5] K.T. Jacob, C. Shekhar, Y. Waseda, J. Chem. Thermodyn. 41 (2009) 56–61. [6] K.T. Jacob, D. Prusty, G.M. Kale, J. Alloys Compd. 513 (2012) 365–372. [7] K.T. Jacob, Y. Waseda, J. Solid State Chem. 150 (2000) 213–220. [8] K.T. Jacob, S. Priya, Y. Waseda, Bull. Mater. Sci. 21 (1998) 99–103. [9] K.T. Jacob, S. Priya, Y. Waseda, J. Phase Equilib. 19 (1998) 340–350. [10] K.T. Jacob, M.V. Sriram, Metall. Mater. Trans. A 25A (1994) 1347–1357. [11] F.M.T. Mendes, A. Uhl, D.E. Starr, S. Guimond, M. Schmal, H. Kuhlenbeck, S. K. Shaikhutdinov, H.J. Freund, Catal. Lett. 111 (2006) 35–41. [12] F.B. Noronha, D.A.G. Aranda, A.P. Ordine, M. Schmal, Catal. Today 57 (2000) 275–282. [13] E.B. Pereira, M.M. Pereira, Y.L. Lam, C.A.C. Perez, M. Schmal, Appl. Catal., A 197 (2000) 99–106. [14] T. Uchijima, Catal. Today 28 (1996) 105–117. [15] K. Kunimori, Y. Doi, K. Ito, T. Uchijima, J. Chem. Soc. Chem. Commun. (1986) 965–966. [16] Z. Hu, H. Nakamura, K. Kunimori, Y. Yakayama, H. Asano, M. Soma, T. Uchijima, J. Catal. 119 (1989) 33–46. [17] Z. Hu, H. Nakamura, K. Kunimori, H. Asano, T. Uchijima, J. Catal. 112 (1988) 478–488. [18] S.I. Ito, T. Fujimori, K. Nagashima, K. Yuzaki, K. Kunimori, Catal. Today 57 (2000) 247–254. [19] T.B. Massalski, P.R. Subramanian, H. Okamoto, L. Kacprzak, second ed.,Binary Alloy Phase Diagrams, Vol 3, ASM, Ohio, 1990. [20] S. Kimura, J. Solid State Chem. 6 (1973) 438–449. [21] J.F. Marucco, J. Solid State Chem. 10 (1974) 211–218. [22] K. Naito, N. Kamegashira, N. Sasaki, J. Solid State Chem. 35 (1980) 305–311.
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[23] K. Naito, T. Matsui, Solid State Ionics 12 (1984) 125–134. [24] T. Matsui, K. Naito, J. Solid State Chem. 59 (1985) 228–236. [25] K.T. Jacob, C. Shekhar, M. Vinay, Y. Waseda, J. Chem. Eng. Data 55 (2010) 4854–4863. [26] H. Kleykamp, J. Less Comm. Met. 83 (1982) 105–113. [27] I.S. Shaplygin, I.I. Prosychev, V.B. Lazarev, Russ. J. Inorg. Chem. 23 (1978) 773–775. [28] W.L. Worrell, Measurement of the Thermodynamic Stabilities of Niobium and Tantalum Oxides Using a High Temperature Galvanic Cell: In Thermodynamics, Vol. I, International Atomic Energy Agency, Vienna131.
[29] M.W. Chase Jr. NIST-JANAF thermochemical tables, fourth ed., Part I, Journal of Physics and Chemistry Reference Data Monograph 9, ACS, New York, 1998. [30] K.T. Jacob, T. Uda, H. Okabe, Y. Waseda, High Temp. Mater. Proc. 19 (2000) 11–16. [31] F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen, in: F.R. de Boer, D.G. Pettifor (Eds.), Cohesion in Metals: Transition Metal Alloys, Elsevier, New York, 1988, p. 544. [32] I.I. Prosychev, V.B. Lazarev, I.S. Shaplygin, Russ. J. Inorg. Chem. 26 (1981) 768–773.
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Phase chemistry of the system Nb–Rh–O K.T. Jacob a,n, Preeti Gupta a, M. Vinay a, Y. Waseda b a b
Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 18 February 2013 Received in revised form 21 March 2013 Accepted 26 March 2013 Available online 2 April 2013
Phase relations in the system Nb–Rh–O at 1223 K were investigated by isothermal equilibration of eleven compositions and analysis of quenched samples using OM, XRD, SEM and EDS. The oxide phase in equilibrium with the alloy changes progressively from NbO to NbO2, NbO2.422 and Nb2O5−x with increasing Rh. Only one ternary oxide NbRhO4 with tetragonal structure (a¼ 0.4708 nm and c¼ 0.3017 nm) was detected. It coexists with Rh and Nb2O5. The standard Gibbs energy of formation of NbRhO4 from its component binary oxides measured using a solid-state electrochemical cell can be represented by the equation;
Keywords: NbRhO4 Gibbs free energy Enthalpy Entropy Oxygen potential System Nb–Rh
ΔGof ;ox ðJ=molÞ ¼ −38; 350 þ 5:818 Tð 7 96Þ Constructed on the basis of thermodynamic information of the various alloy and oxide phases are oxygen potential diagram for the system Nb–Rh–O at 1223 K and temperature–composition diagrams at constant partial pressures of oxygen. & 2013 Elsevier Inc. All rights reserved.
1. Introduction Because of the importance of rhodites and rhodates in catalysis, systematic investigations on phase equilibria and thermodynamic properties of the compounds in several ternary systems M–Rh–O have been reported in recent times [1–10]. Niobia supported metal catalysts form an interesting class of materials because of strong metal-support interaction, which can provide the active sites for reactions [11–13]. Metal Rh also has a unique property of chemisorbing CO, producing a wide variety of CO hydrogenation products such as methane, methanol and higher hydrocarbons. It has been reported that Nb2O5 supported Rh particles have high efficiency for CO hydrogenation with high selectivity to hydrocarbons [14]. Kunimori et al. [15] reported that the Nb2O5 promoted Rh/SiO2 is an effective catalyst for the ethane hydrogenolysis reaction. It was found that NbRhO4 formed as a new phase during calcination of the Nb2O5 promoted Rh/SiO2 catalyst [16,17]. NbRhO4 decomposes into Rh and Nb2O5 in H2 at 773 K. This leads the suppression of H2 chemisorption capacity and ethane hydrogenolysis activity. Recently Ito et al. [18] studied the Rh–niobia interaction in niobia-supported Rh (Rh–Nb2O5), niobia
n
Corresponding author. Fax: +91 80 2360 0472. E-mail addresses: [email protected], [email protected] (K.T. Jacob). 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.03.056
promoted Rh/SiO2 (Nb2O5–Rh/SiO2) and RhNbO4/SiO2 catalyst. These catalyst have been applied to selective oxidation of CO in H2 (CO+H2+O2) and hydrogenation of CO (CO+H2). It has been found that niobia increases the activity and selectivity of catalysts in binary CO oxidation in H2 and CO hydrogenation. Phase diagram for the Nb–Rh system has been compiled by Massalski et al. [19]. The diagram shows the existence of nine solid phases, two terminal solid solutions (α-Nb and α-Rh), one intermetallic phase of almost constant composition (α′) and six intermetallics (s, γ, δ, ζ, η and κ) of variable composition at 1473 K. The Nb–O binary system has been extensively studied by number of researchers [20–24] and several oxides have been reported. Among all the reported phases, NbO, NbO2, NbO2.417 and Nb2O5 are stable at 1223 K [22–24]. Recently, the Gibbs energies of formation of NbO, NbO2, and NbO2.422 have been measured and the full range of thermodynamic properties for the four stable oxides reassessed by Jacob et al. [25]. Kleykamp [26] investigated phase relations in a limited range of compositions in the system Nb–Rh–O at 1273 K. The intermetallic phase “NbRh3” (κ) and the Rh-solid solution (α-Rh) were found to coexist with Nb2O5−x. The lower oxides of niobium were incompatible with metal Rh. The Gibbs energies of formation for Nb0.13Rh0.87 (solid solution) and NbRh3.55 (κ) were obtained by emf measurements on galvanic cells based on an oxygen-ion conducting solid electrolyte in temperature range from 1100 to 1300 K. In the binary system Rh–O, Rh2O3 is the only stable phase at high
K.T. Jacob et al. / Journal of Solid State Chemistry 202 (2013) 234–240
235
temperature and its standard Gibbs energy of formation is known accurately [10]. Survey of literature revealed the absence of information on the phase relations in the full range of compositions in the system Nb– Rh–O and thermodynamic properties of NbRhO4. In view of this, the isothermal section of ternary phase diagram for Nb–Rh–O system at 1223 K was established. Based on the phase relations, a solid-state electrochemical cell was designed to measure the standard Gibbs energy of formation of NbRhO4 as a function of temperature. Oxygen potential diagram for the system Nb–Rh–O at 1223 K and temperature–composition diagrams at fixed oxygen partial pressures were then computed from the results.
2. Materials and methods 2.1. Materials Powders of Nb2O5 and Rh2O3 of purity greater than 99.9% were dried under flowing Ar gas at 850 K and then mixed in equimolar proportion using an agate mortar and pestle. The homogeneous mixture was pelletized at 150 MPa using a steel die, placed in an alumina crucible and heated at 1473 K for a total duration of 270 ks under dry flowing oxygen gas for synthesizing NbRhO4. NbO was produced by heating of a mixture of Nb and Nb2O5 in the equimolar ratio under vacuum. The binary oxide NbO2 was also prepared from powders of metal Nb and Nb2O5 in the molar ratio 0.5:1. The pelletized mixtures contained in an alumina crucible were heated at 1473 K for a total duration of 180 ks in an evacuated quartz ampule. To avoid contamination by the crucible material, the samples pellets were supported on sacrificial pellets of the same composition. At intervals of 90 ks, the samples were removed from the furnace, cooled to room temperature, ground thoroughly and repelletized for further heat treatment. The formation of single phase NbRhO4, Nb and NbO2 was confirmed by XRD. A binary Nb-Rh alloy having the composition XRh ¼ 0.51 was prepared by arc melting on a water-cooled copper hearth under flowing high-purity deoxidized Ar gas. The alloy pellet was remelted three times for homogenization. Alloy powder was prepared by filing. Iron particles in the powder were removed using a powerful magnet. Powders of the alloy, NbO2 and NbRhO4 were used in phase diagram studies. Other materials used were powders of Nb and Rh of purity greater than 99.9%. Samples containing metal Nb, alloy and NbO2 were handled in an inert atmosphere glove box to prevent oxidation. The oxygen partial pressure in the inert gas was less than (P O2 =P o ¼ 9:7 10−18 ), where Po is the standard atmospheric pressure.
Fig. 1. Isothermal section of the phase diagram for the system Nb–Rh–O at 1223 K. The average compositions of the samples equilibrated at high temperature are shown by X marks.
were identified by optical and scanning electron microscopy (OM/ SEM), powder X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). Pure Nb, Rh, Nb2O5 and Rh2O3 were used as standards for EDS. The phase composition of the samples was found to be unaltered by heat treatment beyond 1000 ks. 2.3. Measurement of emf The reversible emf of the solid-state electrochemical cell, Pt−13% Rh, Rh+NbRhO4+Nb2O5//(CaO) ZrO2//Rh2O3+Rh, Pt−13% Rh
2.2. Determination of phase diagram of the system Nb–Rh–O at 1223 K
was measured as a function of temperature in the range from 900 to 1300 K. The cell is written such that the reference electrode on the right-hand side is positive. Calcia-stabilized zirconia functioned as the solid electrolyte with predominant oxygen ion conduction (tion 40.999) under the experimental conditions. Wires of Pt–13% Rh alloy served as electrical leads. The cell is designed such that the emf is directly related to the Gibbs energy of formation of NbRhO4 from component binary oxides Nb2O5 and Rh2O3.
An isothermal section of the phase diagram of the system Nb–Rh–O at 1223 K was established by equilibrating eleven samples representing different compositions inside the ternary triangle. The average composition of the samples used is shown by cross marks in Fig. 1. The starting materials used to make the samples and their overall compositions are listed in Table 1. The components of each sample were mixed thoroughly and pelletized at 100 MPa. The samples were equilibrated for a total period of 1000 ks at 1223 K in evacuated quartz ampules to prevent oxidation. The samples were quenched, ground to −325 mesh and repelletized thrice during this period. The samples, contained in yttria-stabilized zirconia crucibles and supported on sacrificial pellets of the same composition, were placed inside the quartz ampules. After equilibrating at 1223 K, the quartz ampules were quenched in chilled mercury. Phases present in quenched samples
The measuring electrode was prepared by compaction of a mixture of Nb2O5 (α), NbRhO4 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 support tungsten electrode connection sealed into glass as shown in Fig. 2. The alumina sheath was pressed down by means of the metal spring between the Pyrex 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 Rh2O3 in the molar ratio of 1:1.5 against a closed-end calcia-stabilized zirconia crucible, with a Pt–13% Rh lead embedded
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Table 1 Starting materials used for making samples, overall composition and equilibrium phases at 1223 K identified in quenched samples by XRD and EDS. Sample number
1 2 3 4 5 6 7 8 9 10 11
Starting materials
Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 Nb+Rh+Rh2O3 NbRh2O4+γ (XRh ¼0.51) γ (XRh ¼0.51)+NbO2+NbRhO4 γ (XRh ¼0.51)+NbO+Nb2O5 Nb+γ (XRh ¼ 0.51)+Nb2O5 Nb+NbRhO4 Nb+NbRhO4+Nb2O5
Overall composition of sample
Phases identified after equilibration
XNb
XRh
XO
0.05 0.135 0.2 0.24 0.27 0.295 0.34 0.37 0.42 0.5 0.56
0.55 0.465 0.4 0.36 0.33 0.305 0.26 0.23 0.18 0.1 0.04
0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4
NbRhO4+Rh2O3+Rh NbRhO4+Nb2O5+Rh Nb2O5−x+α-Rh (XRh ¼ 0.914) Nb2O5−x+κ+α-Rh NbO2.422+κ(XRh ¼ 0.759) NbO2.422+NbO2+κ NbO2+ζ+η NbO2+δ+ζ NbO2+s+ζ NbO+NbO2+s NbO+α-Nb+α′
displaced from equilibrium by essentially an infinitesimal amount. Since the emf returned to same value after successive displacements in opposite directions, reversibility was confirmed. The emf was also reproducible on temperature cycling of the cell.
3. Results and discussions 3.1. Phase relations
Fig. 2. Schematic diagram of the apparatus used for the measurement of emf of the solid state cell.
in the mixture. The calcia-stabilized zirconia crucible containing the reference electrode was placed inside a fused quartz tube, closed at one end. The measuring half-cell assembly contained in the calciastabilized zirconia tube was placed over the reference electrode and was pressed down by means of metal spring between the bell and the top Pyrex cover. The joint between the top Pyrex cover and the quartz tube enclosing the cell was sealed with De-Khotinsky cement. The cement was allowed to solidify while pressing the top 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 (71 K) zone. The upper part of the assembly, where cement seal was 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 induced emf on the cell leads. The temperature of the furnace was controlled to 71 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 (41012 Ω) digital voltmeter with the sensitivity of 70.01 mV. The reversibility of the cell emf was established by micro-coulometric titration in both directions. A small direct current ( 50 μA) was passed through the cell, using an external potential source for 300 s. The open circuit emf was subsequently monitored as a function of time. The emf was found to return to the steady state value before the titration. During the titration, the chemical potential of oxygen at each electrode was
Results of phase analysis of eleven compositions in the ternary system Nb–Rh–O equilibrated at 1223 K are summarized in Table 1. These results are combined with information on the three binary systems available in the literature [19] to develop the isothermal section of the phase diagram displayed in Fig. 1. There is only one stable oxide, Rh2O3, along the binary Rh–O edge of the ternary triangle. The Rh2O3 has orthorhombic structure, space group Pbca (61), with a¼ 0.5148, b¼ 0.5438 and c¼1.4693 nm. Along the Nb–O binary, there are four stable oxides, NbO, NbO2, NbO2.422 and Nb2O5−x. NbO has cubic structure in the space group Pm3m, with lattice parameter a ¼0.42101 nm. The solubility of oxygen in metal Nb in equilibrium with NbO at 1223 K is 2 at%. NbO2 occurs in two different forms. A phase transition occurs from a low-temperature distorted-rutile structure (I41/a, a¼ 1.3702 nm, c¼0.5985 nm) to the high-temperature rutile (P42/mnm, a¼ 0.4846 nm, c¼ 0.3032 nm) having a body-centered tetragonal cell at Ttr ¼ 1080 K. In the low-temperature form, NbO6 octahedra are joined at edges and corners. Nb–Nb distances along the c-axis vary alternatively as short and long. The high-temperature rutile form adopts a four long, two short Nb–O bond patterns. NbO2.422 (Nb12O29) crystallizes as monoclinic (A2/a, a ¼3.132 nm, b¼0.3832 nm, c¼ 2.072 nm; β¼112.931). It transforms to an orthorhombic form at temperatures above 1473 K. Although Nb2O5 exists in several forms, the α-form, also some times designated as H, (with monoclinic structure, space group P2 and lattice parameters a ¼2.115 nm, b¼0.3831 nm, and c¼1.937 nm) is the thermodynamically stable modification [25]. Along the Nb–Rh edge, seven intermetallic phases α′, s, γ, δ, ζ, η, κ (“NbRh3”) are identified, six of which have variable composition. The Nb-rich terminal solid solution (α-Nb) extends up to XRh ¼0.199, whereas Rh-rich terminal solid solution (α-Rh) exists between 0.875≤XRh≤1.0 at 1223 K. The intermetallic α′ is almost stoichiometric. Since the alloy phases and their compositions detected in this study are compatible with the binary phase diagram for the system Nb-Rh given in the compilation of Massalski et al. [19], their nomenclature is used in this study for designating alloy phases. In the Nb–Rh–O system at 1223 K, only one stable ternary oxide, NbRhO4, with tetragonal rutile-type structure (space group
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P43/mnm) was found with lattice parameters a ¼0.4692 nm c ¼0.2995 nm. The lattice parameters measured in this study are in fair agreement with values reported in the literature [17,27]. The α-Rh terminal solid solution and phase κ (“NbRh3”) for XRh≥0.785 are in equilibrium with Nb2O5−x. The two oxide phases Nb2O5−x and NbO2.422 coexist with κ (“NbRh3”) phase characterized by XRh ¼0.785. The intermetallic phase κ is in equilibrium with NbO2.422 for 0.785≥XRh≥0.724. The oxide phases NbO2.422 and NbO2 coexist with κ phase of composition XRh ¼0.724. The intermetallic η, ζ, δ γ and s (XRh≥0.384) are in equilibrium with NbO2. The intermetallic phase s, α′ and α-Nb are in equilibrium with NbO. The oxide NbO and NbO2 coexist with s phase defined by XRh ¼0.384. There are thirteen three-phase region involving condensed phases, α-Nb+α′+NbO, α′+ s+NbO, s+NbO+NbO2, s+γ +NbO2, γ+δ+NbO2, δ+ζ+NbO2, ζ+η+NbO2, η+κ+NbO2, κ+NbO2+ NbO2.422, κ+NbO2.422+Nb2O5−x, κ+α−Rh+Nb2O5−x, Rh+Nb2O5−x+ NbRhO4, and Rh+NbRhO4+Rh2O3. The measurement of oxygen chemical potential corresponding to the three-phase equilibrium involving Rh, Nb2O5−x and NbRhO4 would allow determination of the standard Gibbs free energy of formation of NbRhO4. The phase relations delineated in this study refine and expand the data provided by Kleykamp [26] for the restricted region bounded by NbO2, Nb2O5, Rh and NbRh3 at 1273 K. Kleykamp [26] assumed that NbO2 is in equilibrium with Nb2O5−x (x¼0.2), following the earlier suggestion of Worrell [28]. The results of this study and others [22–25] indicate an additional oxide phase of composition NbO2.422 between NbO2 and Nb2O5−x at 1223 K. The limit of nonstoichiometry of Nb2O5−x is characterized by x≤0.0458 at 1223 K [25]. 3.2. Thermodynamic properties of NbRhO4 The reversible emf of solid-state electrochemical cell is shown in Fig. 3 as a function of temperature in the range from 900 to 1300 K. The emf decreases slightly with increasing temperature. Least squares regression analysis yields the following expression: EðmVÞ ¼ 132:5−0:0201 T ð 70:33Þ
ð1Þ
The uncertainty limit is based on twice the standard deviation and the estimated error in emf and temperature measurement. The oxygen chemical potential at the reference electrode on the right side of the cell is determined by decomposition of Rh2O3: 2/3 Rh2O3 (ortho) ¼4/3 Rh+O2
(2)
237
The oxygen potential corresponding to this decomposition reaction was determined earlier [10] and can be represented as: ΔμrO2 ðJ=molÞ ¼ ΔGo2 ¼ −264; 243 þ 188:0 T ð 7 150Þ
ð3Þ
At the left-hand side measuring electrode, the oxygen potential is fixed by the equilibrium between the three condensed phases: 4/3 Rh+2/3Nb2O5 (α)+O2 ¼4/3 NbRhO4
(4)
(Δμm O2 )
of the measuring elecThe oxygen chemical potential trode, computed from the emf using the Nernst equation, ðΔμrO2 −Δμm O2 ¼ 4FEÞ is given by: Δμm O2 ðJ=molÞ ¼ −315; 376 þ 195:757 Tð 7 196Þ
ð5Þ
The overall cell reaction, obtained by combining the two halfcell reactions, can be written as: 1/2Nb2O5 (α)+1/2 Rh2O3 (ortho) ¼NbRhO4 The standard Gibbs free energy of formation from the component binary oxides is given by:
(6) (ΔGo6 )
ΔGo6 ðJ=molÞ ¼ −ηFE ¼ −38; 350 þ 5:818 T ð 796Þ
of NbRhO4 ð7Þ
where η¼ 3 is number of electrons involved in the electrode reactions corresponding to formation of NbRhO4. F¼96,485 J/V is the Faraday constant and E/V is the reversible emf of the cell. The temperature independent term in Eq. (7) gives the enthalpy of formation of NbRhO4 from its component binary oxides according to reaction (6) at a mean temperature of 1100 K. The enthalpy of formation of NbRhO4 from its components Rh2O3 (ortho) and Nb2O5 (α) is −38.3570.13 kJ/mol. The entropy of formation of NbRhO4 from its component binary oxides is −5.82 70.12 J/K mol at 1100 K. The negative value is a little surprising since the high temperature entropy of NbRhO4 has a configurational contribution from the mixing of Nb5+ and Rh3+ on the cation sublattice of the rutile structure. If the ions are randomly distributed, the entropy contribution would be 11.53 K/mol. The results of this study would suggest that the distribution is perhaps not completely random. Thermodynamic properties of NbRhO4 at 298.15 K can be calculated by invoking the Neumann–Koop rule for the estimation of heat capacity of NbRhO4. The standard enthalpy of formation of NbRhO4 from elements in their normal states is evaluated as ΔH of ¼ −1190:27 7 1:14 kJ=mol (T ¼298.15 K). The standard entropy of NbRhO4 at 298.15 K is 100.6770.5 J/K/mol. Auxiliary data for Nb2O5 (α), ΔH of ¼ −1898:31 72:1 kJ=mol (T ¼298.15 K) [25] and So ¼137.30 7 1.3 J/K/mol (T ¼298.15 K) [25,29], and for Rh2O3 (ortho) ΔH of ¼ −405:53 7 0:26 kJ=mol, So ¼ 75.6970.5 J/K/ mol [30] are used in making the estimates for NbRhO4 at 298.15 K. There is no thermodynamic information on NbRhO4 in the literature for comparison with the values obtained in this study. It would be useful to measure the enthalpy of formation and heat capacity of NbRhO4 as a function of temperature using calorimetric methods in order to confirm the results obtained. 3.3. Decomposition temperature On lowering the oxygen potential, the compound NbRhO4 will decompose according to Eq. (4). The oxygen potential corresponding to this decomposition is given by Eq. (5). The decomposition temperature for NbRhO4 is 1611 716 K in pure oxygen at standard pressure (P O2 =P o ¼ 1). The decomposition in air at standard atmospheric pressure would occur at 1512 715 K under equilibrium conditions. 3.4. Computation of phase diagrams
Fig. 3. Temperature electrochemical cell.
dependence
of
reversible
emf
of
the
solid-state
Information on activity of Nb in the alloy phases and thermodynamic data on rhodium and niobium oxides are required to
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K.T. Jacob et al. / Journal of Solid State Chemistry 202 (2013) 234–240
compute oxygen chemical potentials for various three-phase regions shown in Fig. 1. The Gibbs free energies of mixing for two Rh-rich binary compositions have been measured by Kleykamp [26]. The measured values are in fair agreement with Miedema's model [31] as shown in Fig. 4. Gibbs free energy of mixing for all the phases in the Nb–Rh system have been estimated based on the available information, consistent with the binary Nb–Rh and ternary Nb–Rh–O phase relations observed in this study. The results are shown in Fig. 4. The Gibbs energy of mixing of terminal solid solutions (α-Nb and α-Rh) in Nb–Rh binary system can be approximated by regular solution model assuming random distribution of atoms; ΔGM ¼RT (XNb ln XNb+XRh ln XRh)+Ω XNb XRh. The regular solution parameters (Ω) for Nb-rich and Rh-rich terminal solid solutions are −178 and −216 kJ/mol, respectively. Using the tangent intercept method, chemical potentials and activities of Nb and Rh in different phase fields of the binary Nb–Rh system have been calculated. The results are shown in Fig. 5. Oxygen potential diagram for the Nb–Rh–O system at 1223 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, ηRh =ðηRh þ ηNb Þ, where ηi 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. When three condensed phases and a gas phase coexist at equilibrium in a ternary system such as Nb–Rh–O, the system is monovariant. At a fixed temperature, three condensed phases coexist only at unique partial
Fig. 6. Oxygen potential–composition diagram for the system Nb–Rh–O at 1223 K.
Fig. 4. Gibbs free energy of mixing in the system Nb–Rh at 1223 K. Phase boundaries are indicated by vertical lines. Common tangent of free energy of mixing curves of adjacent phases define chemical potentials in the corresponding two phase field.
Fig. 5. Composition dependence of the calculated activities (ai) of Rh and Nb in the system Nb–Rh at 1223 K.
Fig. 7. Temperature–composition phase diagram for the system Nb–Rh–O in pure oxygen. Oxygen partial pressure (P O2 =P o ¼ 1), P o ¼ 0.1 MPa.
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pressure of oxygen. The three-phase equilibria are therefore represented by horizontal lines in the oxygen potential diagram at constant temperature. Phase relations in the ternary system Nb–Rh–O can also be computed as the function of temperature at constant oxygen partial pressure. The computed phase diagrams at three oxygen partial pressures,P O2 =P o ¼ 1, P O2 =P o ¼ 0:212 and P O2 =P o ¼ 10−6 are shown in Figs. 7–9, respectively. It is clear that the temperature– composition diagrams are very sensitive to oxygen partial pressure in the ambient atmosphere. The decomposition temperature of both NbRhO4 and Rh2O3 decreases significantly with decreasing oxygen partial pressure. However, the decomposition temperature
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of NbRhO4 is always higher than that of Rh2O3 at all partial pressures of oxygen. The decomposition temperature of NbRhO4 in air computed in this study (1512 715 K) is in fair agreement with the value of 1503 720 K reported by Shaplygin et al. [27] and 1523 710 K reported by Prosychev et al. [32].
4. Conclusions Presented in this article are studies on the phase relations in the system Nb–Rh–O at 1223 K and thermodynamic properties of the ternary oxide NbRhO4 as a function of temperature. NbRhO4 is the only stable ternary compound. The oxygen potential corresponding to the decomposition of NbRhO4 to Rh, Nb2O5 and O2 has been measured as a function of temperature. The oxygen potential is given by the expression, ΔμO2 ðJ=molÞ ¼ −315; 376 þ 195:757 Tð 7 127Þ. The standard Gibbs free energy of formation of NbRhO4 from its binary component oxides was directly obtained from the emf of the solid-state electrochemical cell. For the reaction, 1/2 Nb2O5 (α)+1/2 Rh2O3 (ortho) ¼NbRhO4 ΔGof ;ox ðJ=molÞ ¼ −38; 350 þ 5:818 Tð 7 96Þ The standard enthalpy of formation of NbRhO4 at 298.15 K from elements in their normal standard states has been evaluated as ΔH of ¼ −1190:27 7 1:14 kJ=mol and the standard entropy of NbRhO4 at 298.15 K is So ¼100.67 7 0.5 J/K/mol. With the help of available thermodynamic information for various phases, phase relation in the system Nb–Rh–O has been computed as a function of oxygen potential at constant temperature and as a function of temperature at constant oxygen partial pressures.
Acknowledgments
Fig. 8. Temperature–composition phase diagram for the system Nb–Rh–O in air. Oxygen partial pressure (P O2 =P o ¼ 0:212), P o ¼ 0.1 MPa.
K. T. Jacob is grateful to the Indian National Academy of Engineering for support as INAE Distinguished Professor. Preeti Gupta wishes to thanks the University Grants Commission, India, for the award of Dr. D.S. Kothari Postdoctoral Fellowship. References
Fig. 9. Temperature–composition phase diagram for the system Nb–Rh–O at oxygen partial pressure (P O2 =P o ¼ 10−6 ), P o ¼0.1 MPa.
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