First-principles investigation on the segregation of Pd at LaFe1-xPd x O3-y surfaces
© Tian et al.; licensee Springer. 2013
Received: 31 December 2012
Accepted: 21 March 2013
Published: 1 May 2013
First-principles calculations were performed to investigate the effect of Pd concentration and oxygen vacancies on the stability of Pd at LaFeO3 surfaces. We found a much stronger tendency of Pd to segregate by taking the aggregation of Pd at LaFe1-xPd x O3-y surfaces into consideration, resulting in a pair of Pd-Pd around a vacancy. Moreover, we predicted that one oxygen-vacancy-containing FeO2-terminated surfaces would be stable at high temperatures by comparing the stability of LaFe1-xPd x O3-y surfaces, which further supports our previous conclusion that a Pd-containing perovskite catalyst should be calcined at 1,073 K or higher temperatures in air to enhance the segregation of Pd in the vicinity of surfaces to rapidly transform the Pd catalyst from oxidized to reduced states on the perovskite support.
A three-way catalyst simultaneously transforms toxic exhaust emissions from motor vehicles into harmless gases. However, the sintering problem, i.e., the growth and agglomeration of precious metal particles on conventional catalysts during vehicle use dramatically degrades catalytic activity, and large amounts of precious metals are required to retain the activity of catalysts after long periods of use. Thus, intelligent catalysts have attracted worldwide attention due to their greatly improved durability as a result of the self-regenerative function of precious metal nanoparticles[1–3]. It has been confirmed that the activity of catalysts can be preserved, and the amount of precious metals that are required can be reduced by 70% to 90%[4, 5]. The self-regenerative function, which can be explained as resulting from the transformation of the state of precious metals (Pd, Pt, and Rh) that reversibly move into and out of the LaFe1-xM x O3 perovskite lattice, significantly suppresses the growth of precious metals during the use of catalysts.
Thus far, many experiments have been devoted to research on the state of Pd in perovskite in redox processes. Uenishi et al. investigated the superior start-up activity of LaFePdO x at low temperatures (from 100°C to 400°C) using X-ray spectroscopic techniques under the practical conditions where they controlled automotive emissions. They found the Pd0 phase partially segregated outside the surface even at low temperatures; thus, the segregation of Pd0 under a reductive atmosphere induced the start-up activity of LaFePdO x . Eyssler et al. found a high concentration of Pd distributed on the LaFeO3 (LFO) surface that contributed to high methane combustion due to the formation of PdO in which Pd2+ was in square planar coordination. Additionally, two Pd species (Pd2+ at the surface and Pd3+ in a solid solution) were found to be generated in further calcination. Pd2+ and Pd3+ could be transformed in equilibrium under thermal treatment conditions[7, 8]. More recently, Eyssler et al. studied the state of Pd in different B-site substitutions and compared the effect of catalytic activities on methane combustion. A well-dispersed octahedral Pd-O species was found for Fe- and Co B- site cations, and PdO particles were on the LaMnO3 surface. Above all, related investigations have become more important as the activity of catalysts strongly depends on the state of the precipitated Pd.
Hamada et al. more recently found in a density functional theory (DFT) investigation that oxygen vacancies (VOs) that formed near the LFO surface promoted the segregation of Pd. They also proposed a scenario that perovskite containing precious metal is calcined during the catalyst preparation step at 1,073 K for 2 h in air, and then VOs are produced that enhance Pd segregation, resulting in a LaPdO3-y layer that eventually forms close to the surface. The LaPdO3-y layer in the vicinity of the surface promotes efficient switching between Pd metal particles under reductive conditions and the dissolved state of Pd in the LaFe1-xPd x O3 perovskite lattice under oxidative conditions. Therefore, the LaPdO3-y layers formed in the vicinity of the oxide surface play a key role in the self-regenerative function. Almost simultaneously, transmission electron microscopy observations of atomic-scale processes in Pd-LFO catalysts have demonstrated that redox reactions between the formation of Pd particles on the Pd-LFO surface under reducing conditions and the dissolution of Pd particles into LFO under oxidizing conditions take place in spatially-limited areas, especially in the proximity of oxide surfaces, indicating strong interactions between Pd and oxide surfaces. Katz's results also provided strong support for the mechanism proposed by Hamada et al. However, the stability of the LaPdO3-y layer and the mechanism for Pd leaving the LaPdO3-y layer have not been discussed in detail. The interaction between Pd atoms in the perovskite host is especially important considering the possibility of nanoscale spinodal decomposition as pointed out by Kizaki et al.. Therefore, we systemically studied the relative stability of the Pd m VOn-containing surfaces (m =1 and 2 and n =0, 1, and 2) in our present work to investigate possible phases appearing in steps to prepare catalysts at high temperature in air.
Model and computation
We have calculated the lattice constants of LFO and the segregation tendency of Pd at two terminations of the perovskite surfaces with and without VO by using state[14, 15] and quantum ESPRESSO (QE) codes. We found that both state and QE codes yielded the similar bulk lattice constants and caused the segregation behavior of Pd, which was a strong indication that both codes could admirably describe the properties of Pd incorporated in the LaFe1-xPd x O3-y surfaces. Here, we employed the state code to do the first-principles calculations. The ion-electron interactions were described using ultrasoft pseudopotentials, and the exchange and correlation potential was represented by a generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof formula. DFT calculations with Hubbard correction (DFT+U) are known to correct the bandgap and magnetic moment in local-density approximation and generalized gradient approximation calculations. This method can yield reasonable agreement with the experimental results. We omitted DFT+U from this work because Hamada et al. verified that electronic structures with DFT+U are qualitatively the same as those in GGA calculations, and they have not changed their conclusions. However, since the relative energies that are used to determine the stability of perovskite surfaces might be influenced by the exchange and correlation potential, even though DFT+U fails to give better results than GGA calculations to predict the phase stability of hematite surfaces, we still intend to investigate the effect of DFT+U in later work. The original unit cell used to construct the LFO perovskite surface was a GdFeO3-type orthorhombic unit cell (adapted from Figure one in), in which the local magnetic moments of Fe are aligned in G-type anti-ferromagnetic order. The relaxed lattice constants for a, b, and c in bulk LFO correspond to 0.575, 0.559, and 0.792 nm, respectively, which are in reasonable agreement with the experimental values of 0.558, 0.556, and 0.785 nm. The cutoff energies for the wave function and augmentation charge density are 25 Ry for the former and 225 Ry for the latter.
Results and discussion
Since VOs are more likely to form at the subsurface (LaO layer) than the surface in the Pd-containing FeO2-terminated surface, we placed another VO in the same LaO layer (Figure 2 group III (a) to (c)). If two VOs are both located at the subsurface, the second Pd atom tends to substitute the Fe atom either at the second FeO2 layer forming a pair of Pd atoms (Figure 2 group III (b)) or on the surface forming the PdO2 layer (Figure 2 group III (c)). The difference in energy between these two configurations is less than 0.05 eV. Thus, the additional VO stabilizes the PdO2-layer exposed to the vacuum.
We can find from Figure 4 that when ΔμO is greater than -1.17 eV (point A), no VOs form on the surface. The Pd-segregated surface (Figure 2 group I (b)) is slightly more stable than the surface with Pd inside the bulk of the perovskite (Figure 2 group I (a)). This indicates that Pd preferentially stays at the first layer of the LFO surface than the bulk position to some extent. One VO in the surface appears at the subsurface (LaO layer) when ΔμO is lower than -1.17 eV. The surface containing Pd2VO is predicted to be stable between points A and B, indicating conditions with standard pressure at temperatures between 1,000 and 1,500 K. Two Pd atoms attract each other in such a surface by sharing one VO in the first LaO layer (Figure 2 group II (b)). The Pd1VO1-containing surface (Figure 2 group II (n)) becomes dominant at ΔμO below -1.67 eV (point B) under standard pressure at temperatures over 1,500 K. Two VOs-containing surfaces are predicted to be dramatically unstable compared with the other three surfaces due to the greater formation energy of two VOs under the conditions given in Figure 4. The Pd1VO2-containing surface (Figure 2 group III (d)) will appear under standard pressure at temperatures far above 1,500 K (the pink line: the critical point is beyond the scale of Figure 4). The surface containing Pd2VO2 (Figure 2 group III (b)) for the blue line is predicted to be unstable under any conditions as presented in Figure 4. From what we have mentioned above, one VO can be produced at the first LaO layer of the FeO2-terminated surfaces with segregated Pd m (m =1 and 2) under reasonable working conditions, and such surfaces are predicted to be dominantly stable over a wide range of ΔμO.
We investigated what effect oxygen vacancies had on the tendency of additional Pd atoms to segregate at the LaFe1-xPd x O3-y surface, as well as compared the relative stability of FeO2-terminated surfaces that contained Pd m VOn versus the oxygen chemical potential, by using first-principles theoretical calculations. We pointed out that Pd atoms repulse one another without VOs. However, if there are VOs at the subsurface layer, Pd atoms become attractive, forming a pair of Pd atoms while sharing one VO. Furthermore, we clarified that the FeO2-terminated surface containing Pd m VO could be predicted to become stable over a wide range of oxygen chemical potentials below -1.17 eV. Therefore, the present results provide support that the calcination of precious metals containing catalysts at 1,073 K or high temperatures in air during the catalyst preparation step leads to the formation of oxygen vacancies near the surface and then enhances the formation of a LaPdO3-y layer in the vicinity of the LaFeO3 oxide surface.
The present work was partly supported by a Ministry of Education, Culture, Sports, Science and Technology (MEXT) program called the “Elements Strategy Initiative to Form Core Research Center” (since 2012). The Advanced Institute for Materials Research (AIMR) was established by the World Premier Research Center Initiative (WPI), MEXT, Japan. The calculations were done at the supercomputer centers of Osaka University, the Institute for Solid State Physics, the University of Tokyo, and Tohoku University.
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