Search for a Self-Regenerating Perovskite Catalyst Using ab Initio

Article pubs.acs.org/JPCC

Search for a Self-Regenerating Perovskite Catalyst Using ab Initio Thermodynamics Calculations Susumu Yanagisawa,*,† Akifumi Uozumi,‡ Ikutaro Hamada,§ and Yoshitada Morikawa*,∥ †

Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan ∥ Department of Precision Science and Technology, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871, Japan ‡ Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan § WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ABSTRACT: An “intelligent” automotive catalyst that self-regenerates its precious-metal nanoparticles during reduction−oxidization cycles was proposed and has been used in realistic applications (Nishihata et al. Nature 2002, 418, 164). We show that ab initio thermodynamics calculations based on density functional theory adequately describe the self-regeneration properties of catalysts consisting of precious-metal solid solutions with LaFeO3 or CaTiO3. We use these calculations to investigate catalysts containing other perovskites and the use of nonprecious metals. Based on the results, we propose some perovskites as potential materials for self-regenerating catalysts. Some discrepancies between the present theoretical results and experiments might arise from the lack of consideration of the surface structure or the oxygen vacancies of the perovskites.

■

INTRODUCTION There is growing interest in the development of automotive three-way catalysts for emissions control using minimum amounts of precious metals or low-cost common materials. For some years, precious-metal nanoparticles supported on an oxide have been the main catalytic system used to achieve the three-fold purpose of oxidizing carbon monoxide and hydrocarbons while reducing nitrogen oxide in exhaust gases. However, the growth of the metal particles and the subsequent decrease in the active surface area severely reduce the catalytic activity after continuous use under high-temperature reduction−oxidation (redox) fluctuations of the exhaust gas.1−4 In practice, this deterioration is compensated by loading an excess of precious metal in the catalytic converter, which leads to overconsumption of limited metal resources. There have been investigations and efforts to suppress metal-particle growth on an oxide support, for instance, on ceria.5−7 Recently, a self-regenerating perovskite catalyst was developed and used in practical applications; it has excellent durability because the growth of metal particles is suppressed by reversible fluctuations between metal-particle segregation on the perovskite catalyst and a solid solution of the metal with the perovskite, depending on the redox fluctuations. The first selfregenerating catalyst reported was LaFe0.57Co0.38Pd0.05O3, where the Pd occupies the B site, replacing the Fe or Co atom.1 Effectiveness of the catalyst for Suzuki cross-coupling reactions was also reported.8,9 The self-regeneration function of Pd nanoparticles in LaFe0.95Pd0.05O3 has also been clarified.10 Rh and Pt, which are used as catalysts in automobiles, have been reported to self-regenerate in CaTiO3 and are suitable for practical use.2 © 2012 American Chemical Society

Mechanisms of the self-regeneration function, based on experiments and theory, have been proposed, including rapid diffusion of the metal in response to the control frequency (1− 4 Hz) of actual engines1,4 and nanoscale spinodal decomposition.11 A common observation about the self-regeneration cycle is full exchange of the segregated metal particles and the metal solid solution in the host perovskite bulk. However, based on transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) observations of Pd nanoparticles doped in a LaFeO3 film, it was indicated that the extent to which the reversible process occurs is very limited,12 in contrast to the complete Pd precipitation/ redissolution process proposed by Nishihata et al.1 The Pd or PdO particles were found to partially sink into the LaFeO3 surface, thus implying the locality of the relevant reactions.12 Recently, based on density functional theory calculations, Hamada et al. proposed that the oxygen vacancies in the subsurface layer promote the surface segregation of preciousmetal atoms. They indicated that the Pd exists in the vicinity of the LaFeO3 surface as a LaPdO3−y layer, which facilitates the segregation of Pd particles on the surface without diffusion from deep inside the bulk.13 They also suggested that the LaPdO3−y layer is formed during sample preparation rather than during the redox cycle and that it can be enhanced by considerable amounts of oxygen vacancies during the sample preparation process. Received: June 1, 2012 Revised: December 27, 2012 Published: December 31, 2012 1278

dx.doi.org/10.1021/jp305392x | J. Phys. Chem. C 2013, 117, 1278−1286

The Journal of Physical Chemistry C

Article

Table 1. ABO3-Type Host Perovskites with A = La Investigated in This Study, Including Type of Bravais Lattice, Space Group Type, Lattice Constants, Number of Formula Units Used (FU), Magnetism, and Corresponding Reference(s) Bravais lattice LaFeO3 LaAlO3 LaCrO3 LaMnO3 LaCoO3 LaNiO3 LaCuO3

orthorhombic cubic orthorhombic orthorhombic rhombohedral rhombohedral tetragonal

space group Pnma Pm3̅m Pnma Pnma R3̅c R3c̅ P4/m

lattice constantsa a a a a a a a

= = = = = = =

d

3

0.5563 nm, b = 0.7867 nm, c = 0.5553 nm [RT ], volume = 0.0608 nm 0.381360 nm [900 K], volume = 0.0555 nm3 0.5476 nm, b = 0.7752 nm, c = 0.5512 nm [293 K], volume = 0.0585 nm3 0.5742 nm, b = 0.7668 nm, c = 0.5532 nm [4.2 K], volume = 0.0609 nm3 0.53416 nm, α = 60.966° [4 K], volume = 0.0539 nm3 0.54535 nm, c = 1.31014 nm [30 K], volume = 0.0562 nm3 0.381897 nm, c = 0.397258 nm [RTd], volume = 0.0579 nm3

FU

magnetic orderb,c

ref(s)

4 8 4 4 2 2 8

AF (G-type) NM AF (G-type) AF (A-type) NM PM [low T] PM [T > 240 K]

48, 49 50 49, 51 52 46 53, 54 55

a

For the experimental lattice constants, the temperature at which the lattice constants were observed with the neutron or X-ray diffraction technique is indicated in square brackets. Volume per one FU is also listed. bAF = antiferromagnetic, PM = paramagnetic, NM = nonmagnetic, FM = ferromagnetic cTemperature at which the paramagnetic (PM) state was experimentally observed is also indicated in the bracket. dRT = room temperature

Table 2. ABO3-Type Host Perovskites with A = Ca, Sr, or Ba Investigated in This Study, Including Type of Bravais Lattice, Space Group Type, Lattice Constants, Number of Formula Units Used (FU), Magnetism, and Corresponding Reference(s) Bravais lattice CaTiO3 CaFeO3 CaZrO3 SrTiO3 SrMnO3 SrFeO3 SrCoO3 SrZrO3 BaTiO3

orthorhombic orthorhombic orthorhombic cubic hexagonal cubic cubic orthorhombic cubic

space group Pnma Pnma Pnma Pm3̅m P63/mmc Pm3̅m Pm3̅m Pnma Pm3m ̅

lattice constantsa 3

a = 0.54458 nm, b = 0.76453 nm, c = 0.53829 nm [300 K], volume = 0.0560 nm a = 0.5352 nm, b = 0.7539 nm, c = 0.5326 nm [300 K], volume = 0.0537 nm3 a = 0.55912 nm, b = 0.80171 nm, c = 0.57616 nm [300 K], volume = 0.0646 nm3 a = 0.3901 nm [296 K], volume = 0.0594 nm3 a=b = 0.5461 nm, c = 0.9093 nm [350 K], volume = 0.0587 nm3 a = 0.3869 nm [RTd], volume = 0.0579 nm3 a = 0.385 nm [RTd], volume = 0.0571 nm3 a = 0.58151, b = 0.81960 c = 0.57862 nm [RTd], volume = 0.0689 nm3 a = 0.4000 nm [RTd], volume = 0.0640 nm3

FU

magnetic orderb,c

ref

4 4 4 8 4 8 8 4 8

NM AF NM NM PM [T > 280 K] PM [T > 130 K] FM NM NM

56 57 56 58 59 60, 61 60, 62 63 64, 65

a

For experimental lattice constants, the temperature at which the lattice constants were observed with the neutron or X-ray diffraction technique is indicated in square brackets. Volume per one FU is also listed. bAF = antiferromagnetic, PM = paramagnetic, NM = nonmagnetic, FM = ferromagnetic cTemperature at which the paramagnetic (PM) state was experimentally observed is also indicated in the bracket. dRT = room temperature

testing a body of perovskites, we performed DFT calculations of the total electronic energies of the segregated and solidsolution states of precious-metal particles in a host perovskite. For ease of screening the materials, we employed the bulk states of the constituent materials.20 The DFT calculations were performed using the STATE code,21 which uses pseudopotentials22,23 to describe electron− ion interactions. The f state of La; s, p, and d states of Fe; p state of O; and d states of Pd, Rh, Pt, and Cu were treated with Vanderbilt’s ultrasoft pseudopotential scheme,22 whereas other components were treated by the norm-conserving pseudopotential scheme.23 Wave functions and charge density were expanded by a plane-wave basis set with kinetic energy cutoffs of 25 and 225 Ry, respectively. For the exchange-correlation potential, we used the spinpolarized semilocal generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE).24 The magnetic moment or the stability of LaMO3 perovskites was reasonably well reproduced by the local density approximation or the semilocal GGA.25−29 We employed neither the on-site Coulomb (DFT + U) nor hybrid functional to correct the band-gap error in the DFT-GGA calculations because the relative stability can change depending on the value of the onsite Coulomb repulsion or the fraction of the exact exchange and the screening parameter, and these sometimes incorrectly predict the stability.30−32 To represent a solid-solution or pure-bulk state of a host ABO3-type perovskite, we used a supercell approach, with two,

Although these results highlight the roles of the surface structure and the vacancies of the perovskites in the selfregeneration cycle, in practical applications, to minimize the amount of the precious metal used, the development of a new self-regenerating catalyst is necessary. Toward this end, there have been investigations of catalytic activity of Cu or Fe hosted in perovskites.14−18 In the present study, we search for host materials with selfregeneration functions by investigating the stability of catalytic precious metals in ABO3-type perovskite oxides, using ab initio thermodynamics calculations based on density functional theory (DFT).19 The thermodynamic treatment combined with DFT was successful in describing the experimentally observed preference for segregation of Pd and Pt and for a solid solution of Rh on LaFeO3.13 First, we show that the present theoretical method can describe the observed self-regeneration properties of Pd, Rh, and Pt in the perovskites LaFeO3 and CaTiO3 reasonably well.2 Second, we apply the thermodynamics calculations to a variety of ABO3-type perovskites. Based on the results, we discuss and propose possible candidates for host perovskite oxides other than LaFeO3 and CaTiO3.

■

THEORETICAL METHODS

The purpose of the present study was to investigate perovskite oxides containing a precious metal that self-regenerate in response to oxygen partial pressures and temperatures and are appropriate for use in automobile engines. For the purpose of 1279



Article

four, and eight ABO3 formula units for rhombohedral, orthorhombic and hexagonal, and cubic and tetragonal structures, respectively. For the lattice constant, crystal structure, and magnetic order of each of the perovskites, we used experimental data (see Tables 1 and 2).33 For LaFeO3, LaCrO3, LaMnO3, and CaFeO3, we employed antiferromagnetic ordering. Brillouin zone integration was performed using the tetrahedron method34,35 with a 4 × 3 × 4 k-point mesh for an orthorhombic structure; similar meshes were adopted for the other crystal structures. In a solid solution of a precious metal and a perovskite, a Bsite atom in the supercell is replaced by a metal atom M, as observed in experiments.1−4 The compositional formulas are therefore A2BMO6 (rhombohedral), A4B3MO12 (orthorhombic), and A8B7MO24 (cubic and tetragonal). We found that higher proportions of loaded precious metal than in the experiments did not change the conclusions of the present study.36 We did not consider interstitial precious-metal atoms. It was confirmed that interstitial precious-metal atoms are energetically unfavorable by DFT-GGA calculations.13 For the solid-solution state, the lattice parameters of the corresponding host perovskites were employed. The internal atomic positions of the solids involved in the redox cycle (see eqs 1 and 2 below) were fully relaxed until the forces on them were less than 0.04 nN. To relate these calculated electronic energies to relevant working conditions, we calculated the grand potentials of the segregated and solid-solution states as a function of temperature T and oxygen chemical potential μO. In the orthorhombic structure, for instance, we assumed that the self-regeneration cycle between segregated and solid-solution states is described as A4B3MO12 −

μO(T , p0 ) = μO(0 K, p0 ) +

(5)

where G is the Gibbs free energy, H is the enthalpy, and S is the entropy. Based on eq 5, μO(T, p0) was calculated using the enthalpy and entropy of an O2 molecule at 0 K and other temperatures (see Table 3). We used SO2(T,p0) and HO2(T,p0) at various temperatures from the JANAF Thermochemical Tables.19,37 Table 3. Chemical Potential of an Oxygen Atom Calculated Using DFT Thermodynamicsa

(A = A2 +)

(3)

where F(T,{Ni}) is the Helmholtz free energy and Ni and μi are the number of atoms of type i and the chemical potential of atom i, respectively. The chemical potential of an oxygen atom μO is related to the oxygen partial pressure p as μO(T , p) = μO(T , p0 ) +

⎛ p⎞ 1 kBT ln⎜ 0 ⎟ 2 ⎝p ⎠

μO(T, p0)/eV

T (K)

μO(T, p0)/eV

100 200 300 400

−0.08 −0.17 −0.27 −0.38

500 600 700 800

−0.50 −0.61 −0.73 −0.85

900 1000 1100 1200

−0.98 −1.10 −1.23 −1.36

For the calculation method, see the text.

RESULTS AND DISCUSSION Solid-Solution and Segregated States of Pd, Rh, and Pt: Comparison with Experiments. Solid solutions of Pd, Rh, and Pt in perovskite LaFeO3 or CaTiO3 served as test cases for the present ab initio thermodynamics calculations because experimental data with realistic redox conditions are available and the materials are used in practical applications. In the case of a Pd solid solution, a perfect solid solution and segregation of Pd nanoparticles in LaFeO3, in response to the ambient redox conditions, were observed in X-ray absorption experiments, implying that this system has an excellent selfregeneration function. However, no Pd solid solution with CaTiO3 was observed.2 More than 60% of the total amount of Rh was present as a solid solution in LaFeO3, even under reducing conditions.2 In CaTiO3, Rh was found to segregate by more than 70% under reducing conditions. For Pt, the proportion of Pt inside the perovskite was in the ranges 24−83% and 20−100% in LaFeO3 and CaTiO3, respectively. These results suggest that Rh and Pt particles hosted by CaTiO3 could be used as a self-regenerating catalysts in practical applications.2 We calculated the grand potential of a perfect metal solidsolution state relative to a perfect segregated state (ΔΩ), which is defined by the difference between the grand potentials of the oxidized and reduced states. ΔΩ as a function of μO is displayed in Figure 1. The intersection point between ΔΩ and the zero horizontal line corresponds to the equilibrium between the

where the left- and right-hand sides denote oxidized and reduced states, respectively. The grand potential, Ω, is given by

∑i Niμi (T , pi )

T (K)

■

(2)

Ω(T , {μi }) = F(T , {Ni}) −

μO(T, p0) (eV)

In the X-ray measurements, the powdered AB0.95M0.05O3 catalyst was oxidized in air, then reduced in an admixture of 90% N2 and H2 or CO, and finally reoxidized in air.1−4 Under oxidizing conditions, we assumed a standard atmosphere with an oxygen mole percent of ∼20% and, thus, a partial pressure of oxygen of PO2 ≈ 0.2 atm. As a criterion to evaluate the selfregeneration property of the materials, we focused on the stability of the solid-solution and segregated states at around this oxygen partial pressure.

(1)

3 A4B4 O12 + AO + M 4

T (K)

a

or A4B3MO12 − O2 ⇔

1 [HO2(T , p0 ) − HO2(0 K, p0 )] 2 1 − T[SO2(T , p0 ) − SO2(0 K, p0 )] 2

=

3 3 1 O2 ⇔ A4B4 O12 + A 2O3 + M 4 4 2

(A = A3 +)

1 ΔGO2(T , p0 ) 2

(4)

where T is the temperature and p0 = 1 atm. μO is determined relative to the total energy of an isolated O2 molecule in the gas phase at 0 K; that is, μO(0 K, p0) = 1/2Etotal O2 is set equal to 0. 0 μO(T, p ) is obtained as 1280



Article

Figure 1. Grand potentials of solid-solution states relative to segregated states of precious metals in LaFeO3 and CaTiO3 calculated using eqs 3−5. (a) Energy diagram for the solid-solution and segregated states. ΔEelec is the difference in the grand potential at zero oxygen chemical potential, corresponding to the calculated DFT electronic energy difference. At the oxygen chemical potential with zero grand potential (μC), the segregated and solid-solution states are in equilibrium, and thus, at oxygen chemical potentials μO < μC, the segregated state is more stable, and at μO > μC, the solid-solution state is more energetically favorable. (b−d) Calculated diagrams of (b) Pd, (c) Rh, and (d) Pt in LaFeO3 and CaTiO3. The corresponding oxygen partial pressures, PO2, are shown at the top of the graphs.

10−4 atm (see Figure 1c). In CaTiO3, on the other hand, μC = −0.41 eV, corresponding to 105−107 atm. The results indicate that Rh prefers a solid solution with LaFeO3, whereas it tends to be segregated on CaTiO3. This is in qualitative agreement with experiments, in which, under reducing conditions, 37% and 80% of the Rh was segregated in LaFeO3 and CaTiO3, respectively.2 The μC values of Pt in LaFeO3 (−0.58 eV) and CaTiO3 (−0.28 eV) are similar, with corresponding oxygen partial pressures of 103−105 atm (LaFeO3) and 105−107 atm (CaTiO3). Therefore, the same self-regeneration properties of Pt were expected in LaFeO3 and CaTiO3, as confirmed by experiments.2 It is well-known that DFT-GGA significantly overestimates O2 bond energy,39 and it can be problematic in predicting heats of reaction or formation enthalpies of redox reactions.29,40,41 We found that, upon using the “experimental” oxygen total energy as proposed in the literature,29,40 the μC value shifts by −0.22 eV. Consequently, the μC value of Pd in LaFeO3 indicates PO2 closer to 100 atm at T = 1000 K, and the conditions under which Rh and Pt self-regenerate in CaTiO3 are closer to typical conditions. In the following subsection, we discuss the self-regeneration properties of a variety of perovskites by qualitative comparison with the above results. From now on, we will not employ the empirical correction to oxygen total energy. Self-Regeneration of Metal Particles in Various Perovskites. The calculated grand potentials as functions of the ambient oxygen chemical potential μO in the precious-

segregated and solid-solution states, and the corresponding oxygen chemical potential is indicated as μC. ΔEelec is the relative grand potential at zero oxygen chemical potential. The ΔΩ values of the precious metals in LaFeO3 and CaTiO3 are shown in Figure 1b−d. One can thus determine that, under reducing (oxidizing) conditions with oxygen chemical potentials below (above) μC, the metal particle is segregated on (forms a solid-solution with) the perovskite. For Pd in LaFeO3, the calculated value of μC = −0.93 eV indicates conditions with PO2 = 10−3 atm at 700 K and PO2 = 102 atm at 1000 K (see Figure 1b). The higher value of μC = −0.09 eV in CaTiO3 indicates self-regeneration at quite high oxygen partial pressures of P O 2 ≈ 10 10 atm. Given possible discrepancies of 0.2−0.3 eV between DFT-GGA and experimental reaction energies or total energy differences,38 a discrepancy in μC of the same order of magnitude is expected between the present results and experiments. Based on eq 4, the corresponding discrepancy in the ambient oxygen partial pressure PO2 is on the order of 2−3 at T = 103 K. The results are thus in semiquantitative agreement with experiments: The self-regeneration property of a Pd solid solution in LaFeO3 and the experimental result that no Pd solid solution was formed with CaTiO32 are qualitatively confirmed, and the calculated values of PO2 = 10−3 at 700 K and PO2 = 102 atm at 1000 K suggest possible self-regeneration at those temperatures with PO2 = 100 atm. For the self-regeneration of Rh in LaFeO3, μC was calculated to be −1.51 eV, corresponding to self-regeneration at 10−12− 1281



Article

under redox conditions should be 20−100%.2 We found that the following perovskites have μC values similar to that of CaTiO3: CaFeO3, SrFeO3, LaNiO3, and LaCuO3, with calculated μC values of −0.19, −0.38, −0.07, and −0.44 eV, respectively. The calculated μC values of −0.28, −0.58, −0.65, and −1.13 eV for CaTiO3, LaFeO3, CaZrO3, and SrTiO3, respectively, agree reasonably well with the observed preference for the solid-solution state of the perovskites. The observed proportions of the metal solid solution under reducing conditions were 20%, 24%, 28%, and 50%, respectively.2 For Cu and Fe solid solutions, we picked perovskites with a possible self-regeneration function based on the calculation results. At 700−1000 K, an oxygen chemical potential between −1.2 and −0.7 eV is equivalent to an oxygen partial pressure of 10−2−1.0 atm, corresponding to a realistic oxygen partial pressure. For the Cu solid solutions, the calculated μC values of LaCoO3 (−0.98 eV), LaNiO3 (−1.26 eV), LaCuO3 (−1.20 eV), and SrFeO3 (−0.72 eV) were close to the oxygen chemical potential range (see Table 4 and Figure 3). For the Fe solid solution, we found no Fe solid solution with a μC value in the range between −1.2 and −0.7 eV, except for CaTiO3. The μC values of the Fe solid solutions were generally lower than those of the other solid solutions, indicating the preference for Fe to dissolve into the host perovskites over Pd, Rh, Pt, and Cu. On the other hand, the μC values of the Cu solid solutions were generally closer to those of the Pd, Rh, and Pt solid solutions, implying greater potential as self-regenerating materials than Fe. Most of the present results can be explained qualitatively in terms of the ionic radii of the constituent metallic ions. The preference for segregation or ion solid solution of the host perovskites corresponds well to the order of the ionic radii of the B-site metal ion, rB3+ or rB4+. For instance, the descending orders of preference for dissolution of Pt ion based on the calculated μC values are For LaBO3

metal-hosting LaFeO3 and CaTiO3 were in semiquantitative agreement with the experimentally observed redox properties. We conducted a search for a self-regenerating perovskite other than LaFeO3 and CaTiO3 using the present ab initio thermodynamics calculations. Tables 1 and 2 list the host perovskites investigated in the present study, along with their Bravais lattice types, space group numbers, numbers of formula units used in the supercell model, and magnetic ordering. We investigated the self-regeneration properties of Cu and Fe in perovskites, as well as Pd, Rh, and Pt solid solutions. Table 4 lists the calculated oxygen chemical potentials μC at zero relative grand potential. The relative grand potential (ΔΩ) Table 4. Oxygen Chemical Potentials (eV) at Zero Relative Grand Potential (μC) for Metal Solid Solutions with ABO3Type Perovskites with A = A2+ (Ca, Sr, Ba) or A = La3+ host perovskite

Pd

Rh

Pt

Cu

Fe

CaTiO3 CaFeO3 CaZrO3 SrTiO3 SrMnO3 SrFeO3 SrCoO3 SrZrO3 BaTiO3 LaAlO3 LaCrO3 LaMnO3 LaFeO3 LaCoO3 LaNiO3 LaCuO3

−0.09 −0.12 −0.40 −0.95 −0.59 −0.39 −0.51 −1.00 −1.06 +0.39 +5.71 −1.24 −0.93 −0.09 −0.57 −0.75

−0.41 −0.58 −0.70 −1.26 −1.21 −0.97 −1.12 −1.33 −1.45 −0.93 +4.60 −1.69 −1.51 −0.53 −0.77 −0.73

−0.28 −0.19 −0.65 −1.13 −0.64 −0.38 −0.59 −1.26 −1.31 +1.06 +5.16 −0.73 −0.58 +0.51 −0.07 −0.44

+0.57 −0.07 +0.39 −0.35 −0.41 −0.72 −0.67 −0.25 −0.51 −0.23 +7.31 −2.08 −1.52 −0.98 −1.26 −1.20

−1.09 −1.23 −1.23 −1.80 −0.35 −1.78 −2.35 −1.90 −1.73 −1.92 +5.16 −2.42 −2.55 −1.95 −1.90 −2.00

is displayed as a function of μO in Figures 2 and 3 for each of the perovskites. For comparison, we also include the results for LaFeO3 and CaTiO3. For a Pd solid solution with perovskites, the μC value of the solid solution with LaFeO3 is an adequate reference, given its practical applications and experimentally observed excellent self-regeneration properties under redox conditions.2 The calculated μC values in SrTiO3, SrZrO3, BaTiO3, and LaCuO3 are −0.95, −1.00, −1.06, and −0.75 eV, close to that in LaFeO3 (−0.93 eV). We propose these perovskites as candidates for host materials of Pd with self-regeneration properties similar to those of LaFeO3. For the other perovskites, the absolute deviation from the calculated μC value of LaFeO3 was more than 0.3 eV larger.38 For Rh solid solutions with CaFeO3 and LaCoO3, the calculated μC values were −0.58 and −0.53 eV, respectively, and were similar to that of the solid solution with CaTiO3 (−0.41 eV), a catalyst with an excellent self-regeneration function.2 We therefore suggest the perovskites CaFeO3 and LaCoO3 as candidates for a host perovskite of Rh. As shown in Figure 2, the reference CaTiO3 has the highest μC value of all of the Rh-hosting perovskites except for LaCrO3. The very high μC value of the host LaCrO3 (+4.6 eV for Rh) is common for all of the metals (see Table 4), implying that this perovskite is unsuitable as a self-regenerating catalyst material. The reference value for the self-regeneration function of a Pt solid solution is the μC value of the solid solution with CaTiO3 (−0.28 eV), and the proportion of the solid solution observed

LaMnO3 [0.0645 (high spin, HS)] > LaFeO3 [0.0645 (HS)] > LaCuO3 [0.054 (low spin, LS)] > LaNiO3 [0.056 (LS)] > LaCoO3 [0.0545] > LaAlO3 [0.0535]

For CaBO3 CaZrO3 [0.072] > CaTiO3 [0.0605] > CaFeO3 [0.0585]

For SrBO3 SrZrO3 [0.072] > SrTiO3 [0.0605] > SrMnO3 [0.053] > SrCoO3 [0.053 (HS)] > SrFeO3 [0.0585]

where the numbers in square brackets are the Shannon’s ionic radii (in nm) of the B-site metal ions.42 The trend with respect to the radius of the A-site ion, rA2+, is similar: BaTiO3 [0.161] > SrTiO3 [0.144] > CaTiO3 [0.134]. It was found that the order of the ionic radii almost corresponds to the order of unit cell volumes with the experimental lattice constants (see Tables 1 and 2). Intuitively, a greater preference for dissolution of the precious-metal ion is expected for the host perovskites with the larger ionic radius or the larger cell volume and, thus, the more space for hosting the ions. The above trend is in agreement 1282



Article

Figure 2. Comparisons among ABO3-type perovskites of relative grand potential as a function of oxygen chemical potential μO or oxygen partial pressure PO2. As references for Pd, Rh, and Pt solid solutions, the results for LaFeO3 or CaTiO3 are highlighted. The result for LaCrO3 is not shown because the resulting grand potential and μC are outside the μO range of interest.

Finally, we discuss the accuracy of the present ab initio thermodynamics calculations. We observed that there are some inconsistencies between the present results and experimental results. For a Pt solid solution with SrZrO3, the calculation gave a μC value of −1.26 eV, implying a preference for the solidsolution state. In contrast, no Pt solid solution was observed in SrZrO3 in X-ray absorption experiments.2 In BaTiO3, the μC values of Pt and Rh were calculated to be −1.31 and −1.45 eV, respectively. Compared with the calculated μC values of −0.28 and −0.41 eV in CaTiO3, the results indicate a high preference for the solid-solution states. However, it was found by experiment that the metals form almost full solid solutions and are almost fully segregated in the perovskites, implying that they could have practical applications as automobile catalysts.2 From the above-mentioned self-regeneration trend described in terms of the ionic radii of the constituent metal ions, the calculated μC values on LaCrO3 might be rather too high for the moderate ionic radius of Cr3+ = 0.0615 nm and the volume per one formula unit of 0.0585 nm3, comparable to those of other perovskites.42

with such physicochemical insight. The same trend was found for the solid solutions of the other precious-metal ions and was applied specifically to the solid solutions of Pd, Rh, and Pt. The lower preference for forming a precious-metal solid solution on LaCoO3 than on LaMnO3, as predicted in the present study (see Figures 2 and 3 and Table 4), is in agreement with the above-mentioned trend. However, the result seems to be inconsistent with the experimental observation of Pd or Pt dissolved in LaCoO3 and LaMnO3 and their similar redox activities.43−45 We considered the lowspin ground state of LaCoO3, as confirmed in experiments at low temperature.46 The spin state of LaCoO3 has been reported theoretically and experimentally to change from a low-spin state to an intermediate- or high-spin state as the temperature is elevated.47 The Shannon’s ionic radius of Co in the high-spin state is 0.061 nm,42 suggesting a greater preference for dissolution of the precious metals than in the low-spin state (0.0545 nm). Taking into account the spin transition along with a possible transition in crystal structure from the groundstate rhombohedral structure might improve the result. 1283



Article

Figure 3. Comparisons among ABO3-type perovskites of relative grand potential as a function of oxygen chemical potential μO or oxygen partial pressure PO2. For Cu and Fe solid solutions, vertical dot-dashed lines, corresponding to μO = −0.7 and −1.2 eV, are displayed as a reference for realistic ambient oxygen chemical potential or oxygen partial pressure, and the results with μC close to the reference μO range are highlighted. The result for LaCrO3 is not displayed because the resulting grand potential and μC are outside the μO range of interest.

experimentally observed full solid solution and full segregation of the Pd particles, depending on the ambient redox conditions. No formation of a Pd solid solution with CaTiO3 was predicted, in qualitative agreement with the experimental observations. The self-regeneration properties of Rh and Pt in CaTiO3 were also confirmed, in line with experiment. The results show the suitability of the present ab initio thermodynamics calculations as a theoretical tool for investigating the self-regeneration function of metal solid solutions with perovskites. The self-regeneration properties of metal particles in a variety of perovskites were also examined using the ab initio thermodynamics method, and we identified potential perovskite oxides with a self-regeneration function. We also found that the present results can be reasonably described in terms of the ionic radii of the constituent metal atoms or the unit cell volume of the host perovskites. However, we found some discrepancies between the present results and experimental observations. These can be attributed to the absence of consideration of the perovskite surfaces or oxygen vacancies in the perovskites. The effects of the structures should be taken into account to provide more accurate predictions and screenings of oxide catalyst materials with self-regeneration functions.

As the sources of the discrepancies, the effects of segregation of precious metal on the perovskite surfaces and the roles of oxygen vacancies13 might be crucial. The perfect metal segregation/ion solid-solution cycle from/into deep inside the bulk, as proposed by Nishihata et al.,1 might still have much room for discussion, as proposed by the recent TEM and XPS work indicating the spatial locality of the process.12 Other factors, such as errors in the approximate exchange-correlation potential of DFT and the absence of entropic effects from the mixing and lattice vibrations in calculations of the grand potential, could affect the energetics. Although more experimental and theoretical studies to clarify details of the surface structure and the mechanism of the relevant processes are required, the present study can provide a guide to finding more promising self-regenerating catalysts based on intrinsic redox properties of the host perovskites.

■

CONCLUSIONS

We used ab initio thermodynamics to investigate the selfregeneration of metal solid solutions with perovskites under redox conditions. First, we used ab initio thermodynamics calculations for the solid-solution and segregated states of Pd, Rh, and Pt in the host perovskites LaFeO3 and CaTiO3. In a solid solution of Pd with LaFeO3, the oxygen partial pressure at which the solid-solution and segregated states were in equilibrium was found to correspond to realistic redox conditions, and thus, the results are in agreement with the 1284



■

Article

(23) Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43, 1993. (24) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (25) Kizaki, H.; Kusakabe, K. Surf. Sci. 2012, 606, 337−343. (26) Iglesias, M.; Rodríguez, A.; Blaha, P.; Pardo, V.; Baldomir, D.; Pereiro, M.; Botana, J.; Arias, J. E.; Schwartz, K. J. Magn. Magn. Mater. 2005, 290−291, 396−399. (27) Sarma, D. D.; Shanthi, N.; Barman, S. R.; Hamada, N.; Sawada, H.; Terakura, K. Phys. Rev. Lett. 1995, 75, 1126−1129. (28) Sawada, H.; Morikawa, Y.; Terakura, K.; Hamada, N. Phys. Rev. B 1997, 56, 12154−12160. (29) Calle-Vallejo, F.; Martínez, J. I.; García-Lastra, J. M.; Mogensen, M.; Rossmeisl, J. Angew. Chem., Int. Ed. 2010, 49, 7699−7701. (30) Barbier, A.; Stierle, A.; Kasper, N.; Guittet, M.-J.; Jupille, J. Phys. Rev. B 2007, 75, 233406. (31) Bruska, M. K.; Czekaj, I.; Delley, B.; Mantzaras, J.; Wokaun, A. Phys. Chem. Chem. Phys. 2011, 13, 15947−15954. (32) Pozun, Z. D.; Henkelman, G. J. Chem. Phys. 2011, 134, 224706. (33) By optimizing the lattice constants of the solids in the redox cycle of LaFeO3, we found that the ΔΩ−μO graphs shown in Figure 1 translate laterally by 0.1−0.2 eV, and thus, the conclusion does not change. For the magnetic order, the experimental magnetic orders reported in Tables 1 and 2 were also found to be stable by the calculations. (34) Jepsen, O.; Andersen, O. K. Solid State Commun. 1971, 9, 1763− 1767. (35) Lehmann, G.; Taut, M. Phys. Status Solidi B 1972, 54, 469−477. (36) To examine the effect of the loading proportion of precious metal on the results, we performed larger calculations employing supercells consisting of 16 formula units of LaFeO3 and LaCoO3, whose loading proportion was close to that determined experimentally.1−4 We found that the resulting ΔΩ−μO graphs were translated along the μO axis by less than 0.1 eV relative to those obtained using the smaller original supercell, and thus, the conclusions are not altered. (37) Stull, D. R., Prophet, H., Eds. JANAF Thermochemical Tables, 2nd ed.; U.S. National Bureau of Standards: Washington, D.C., 1971. (38) μC is calculated by taking the difference between the DFT-GGA energies of the segregated and solid-solution states. Taking into account a collection of previous DFT-GGA calculations, we determined that the discrepancy of a calculated μC value from experiment is 0.2−0.3 eV. (39) Kurth, S.; Perdew, J. P.; Blaha, P. Int. J. Quantum Chem. 1999, 75, 889−909. (40) Norskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. J. Phys. Chem. B 2004, 108, 17886−17892. (41) Wang, L.; Maxisch, T.; Ceder, G. Phys. Rev. B 2006, 73, 195107. (42) Shannon, R. D. Acta Crystallogr. A 1976, 32, 751−767. (43) Tan, I.; Tanaka, H.; Uenishi, M.; Kaneko, K.; Mitachi, S. J. Ceram. Soc. Jpn. 2005, 113, 71−76. (44) Ziaei-Azad, H.; Khodadadi, A.; Esmaeilnejad-Ahranjani, P.; Mortazavi, Y. Appl. Catal. B: Environ. 2011, 102, 62−70. (45) Mota, N.; Alvarez-Galván, M. C.; Navarro, R. M.; Al-Zahrani, S. M.; Goguet, A.; Daly, H.; Zhang, W.; Trunschke, A.; Schlögl, R.; Fierro, J. L. G. Appl. Catal. B: Environ. 2012, 113−114, 271−280. (46) Thornton, G.; Tofield, B. C.; Hewat, A. W. J. Solid State Chem. 1986, 61, 301−307. (47) Laref, A.; Laref, S.; Bin-Omran, S. J. Comput. Chem. 2012, 33, 673−684. (48) Marezio, M.; Dernier, P. D. Mater. Res. Bull. 1971, 6, 23−30. (49) Goodenough, J. B. Phys. Rev. 1967, 164, 785−789. (50) Hayward, S. A.; Morrison, F. D.; Redfern, S. A. T.; Salje, E. K. H.; Scott, J. F.; Knight, K. S.; Tarantino, S.; Glazer, A. M.; Shuvaeva, V.; Daniel, P.; Zhang, M.; Carpenter, M. A. Phys. Rev. B 2005, 72, 054110. (51) Khattak, C. P.; Cox, D. E. Mater. Res. Bull. 1977, 12, 463−471. (52) Elemans, J. B. A. A.; van Laar, B.; van der Veen, K. R.; Loopstra, B. O. J. Solid State Chem. 1971, 3, 238−242.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.Y.), morikawa@prec. eng.osaka-u.ac.jp (Y.M.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank H. Tanaka, M. Uenishi, M. Taniguchi, J. Mizuki, Y. Nishihata, D. Matsumura, H. Kasai, H. Nakanishi, and H. Kishi for valuable discussions. This work was partially supported by the Elements Science and Technology Project from the Japan Society for the Promotion of Science and the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, and the Japan Science and Technology Agency Program. The numerical calculations were carried out at the computer centers of Osaka University, Tohoku University, and the Institute for Solid State Physics, The University of Tokyo.

■

REFERENCES

(1) Nishihata, Y.; Mizuki, J.; Akao, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamada, N. Nature 2002, 418, 164−167. (2) Tanaka, H.; Taniguchi, M.; Uenishi, M.; Kajita, N.; Tan, I.; Nishihata, Y.; Mizuki, J.; Narita, K.; Kimura, M.; Kaneko, K. Angew. Chem., Int. Ed. 2006, 45, 5998−6002. (3) Tanaka, H.; Uenishi, M.; Taniguchi, M.; Tan, I.; Narita, K.; Kimura, M.; Kaneko, K.; Nishihata, Y. Catal. Today 2006, 117, 321− 328. (4) Uenishi, M.; Tanaka, H.; Taniguchi, M.; Tan, I.; Nishihata, Y.; Mizuki, J.; Kobayashi, T. Catal. Commun. 2008, 9, 311−314. (5) Hatanaka, M.; Takahashi, N.; Takahashi, N.; Tanabe, T.; Nagai, Y.; Suda, A.; Shinjoh, H. J. Catal. 2009, 266, 182−190. (6) Mayernick, A. D.; Janik, M. J. J. Chem. Phys. 2009, 131, 084701. (7) Li, B.; Ezekoye, O. K.; Zhang, Q.; Chen, L.; Cui, P.; Graham, G.; Pan, X. Phys. Rev. B 2010, 82, 125422. (8) Smith, M. D.; Stepan, A. F.; Ramarao, C.; Brennan, P. E.; Ley, S. V. Chem. Commun. 2003, 2652−2653. (9) Andrews, S. P.; Stepan, A. F.; Tanaka, H.; Ley, S. V.; Smith, M. D. Adv. Synth. Catal. 2005, 347, 647−654. (10) Tanaka, H.; Taniguchi, M.; Kajita, N.; Uenishi, M.; Tan, I.; Sato, N.; Narita, K.; Kimura, M. Top. Catal. 2004, 30−31, 389. (11) Kizaki, H.; Kusakabe, K.; Nogami, S.; Katayama-Yoshida, H. Appl. Phys. Express 2008, 1, 104001. (12) Katz, M. B.; Graham, G. W.; Duan, Y.; Liu, H.; Adamo, C.; Schlom, D. G.; Pan, X. J. Am. Chem. Soc. 2011, 133, 18090−18093. (13) Hamada, I.; Uozumi, A.; Morikawa, Y.; Yanase, A.; KatayamaYoshida, H. J. Am. Chem. Soc. 2011, 133, 18506−18509. (14) Mizuno, N.; Fujiwara, Y.; Misono, M. J. Chem. Soc., Chem. Commun. 1989, 316−318. (15) Peter, S. D.; Garbowski, E.; Perrichon, V.; Pommier, B.; Primet, M. Appl. Catal. A: Gen. 2001, 205, 147−158. (16) Zhang, R.; Alamdari, H.; Kaliaguine, S. J. Catal. 2006, 242, 242− 253. (17) Zhang, R.; Villanueva, A.; Alamdari, H.; Kaliaguine, S. Appl. Catal. A: Gen. 2006, 307, 85−97. (18) Liu, J.; Zhao, Z.; Xu, C.; Duan, A.; Jiang, G. J. Phys. Chem. C 2008, 112, 5930−5941. (19) Reuter, K.; Scheffler, M. Phys. Rev. B 2001, 65, 035406. (20) We have confirmed by examination of a theoretical work on Pd clusters ( Kumar, V.; Kawazoe, Y. Phys. Rev. B 2002, 66, 14413 ) and our calculation of a Pd309 icosahedral cluster that a Pd nanocluster with a size of 7.5 nm (1.1 × 104 atoms) gives an average cohesive energy per atom in agreement with that of the fcc bulk within 0.15 eV. Thus, employing the metal bulk state does not alter the conclusion. (21) Morikawa, Y. Phys. Rev. B 1995, 51, 14802−14805. (22) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892−7895. 1285



Article

(53) García-Muñ oz, J. L.; Rodríguez-Carvajal, J.; Lacorre, P.; Torrance, J. B. Phys. Rev. B 1992, 46, 4414−4425. (54) Sreedhar, K.; Honig, J. M.; Darwin, M.; McElfresh, M.; Shand, P. M.; Xu, J.; Crooker, B. C.; Spalek, J. Phys. Rev. B 1992, 46, 6382− 6386. (55) Bringley, J. F.; Scott, B. A.; Placa, S. J. L.; Mcguire, T. R.; Mehran, F.; McElfresh, M. W.; Cox, D. C. Phys. Rev. B 1993, 47, 15269−15275. (56) Koopmans, H. J. A.; van de Verlde, G. M. H.; Gellings, P. J. Acta Crystallogr. C 1983, 39, 1323−1325. (57) Woorward, P. M.; Cox, D. E.; Moshopoulou, E.; Sleirht, A. W.; Morimoto, S. Phys. Rev. B 2000, 62, 844−855. (58) Abramov, Y. A.; Tsirelson, V. G. Acta Crystallogr. B 1995, 51, 942−951. (59) Daoud-Aladine, A.; Martine, C.; Chapon, L. C.; Hervieu, M.; Knight, K. S.; Brunelli, M.; Radaelli, P. G. Phys. Rev. B 2007, 75, 104417. (60) Jaya, S. M.; Jagadish, R.; Rao, R. S.; Asokamani, R. Phys. Rev. B 1991, 43, 13274−13279. (61) Yakel, H. L., Jr. Acta Crystallogr. 1955, 8, 394−398. (62) Watanabe, H. J. Phys. Soc. Jpn. 1957, 12, 515−522. (63) Ahtee, A.; Ahtee, M.; Glazer, A. W.; Hewat, A. W. Acta Crystallogr. B 1976, 32, 3243−3246. (64) Pertosa, P.; Michel-Calendini, F. M. Phys. Rev. B 1978, 17, 2011−2020. (65) Irie, K.; Shiono, M.; Nakamura, H.; Ohnishi, N.; Okazaki, A. Solid State Commun. 1987, 62, 691−693.

1286


Search for a Self-Regenerating Perovskite Catalyst Using ab Initio

Recommend Documents