Characterization of AlPO4(110 ... - ACS Publications

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Characterization of AlPO4(110) Surface in Adsorption of Rh Dimer and Its Comparison with γ‑Al2O3(100) Surface: A Theoretical Study Masafuyu Matsui,‡ Masato Machida,‡,† and Shigeyoshi Sakaki*,‡,§ ‡

Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto 615-8520, Japan † Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1, Kurokami, Chuo, Kumamoto 860-8555, Japan § Fukui Institute for Fundamental Chesmitry, Kyoto Univesity, Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan S Supporting Information *

ABSTRACT: Adsorption of Rh dimer on AlPO4(110) and γAl2O3(100) surfaces was theoretically investigated by periodic DFT calculation with a slab model to elucidate characteristic features of the AlPO4 surface in comparison with the γ-Al2O3 surface. The adsorption at the PO site is the most favorable in both nonhydrated and hydrated AlPO4 surfaces, which is consistent with the experimental finding. The adsorption at the AlO site is the least favorable. The adsorption energy at the PO site of the AlPO4 surface is considerably larger than that at the γ-Al2O3 surface. One important reason is that the deformation energy of the γ-Al2O3 surface is much larger than that of the AlPO4 surface. Bader charge analysis, difference electron density map, and projected density of states (p-DOS) clearly disclose that the charge transfer (CT) occurs from the Rh dimer to the AlPO4 surface. This CT is stronger than in the adsorption on the γ-Al2O3 surface. The lowest unoccupied band (LU band in conduction band) plays a crucial role as an electron-acceptor orbital in this CT interaction. The LU band of the AlPO4 exists at a lower energy than that of γ-Al2O3. Therefore, the CT from the Rh dimer to the AlPO4 surface is considerably larger than that to the γ-Al2O3 surface. These results show that the presence of the isolated LU band at a low energy and the flexible AlPO4 structure are important factors for the anchoring effect, which achieves outstanding thermal stability of the supported Rh nanoparticles on the AlPO4 surface and therefore enables a reduction in quantity of Rh in the three-way catalyst using AlPO4.

1. INTRODUCTION Metal elements of groups 9 and 10 such as Pt, Pd, and Rh are used now in many three-way catalysts for automobiles. Particularly, these metals play crucial roles in NO−CO and NO−hydrocarbon reactions. Considering that these metals are not abundant on the earth, it is of considerable importance to reduce the content of these metals in catalysts. In general, these metals are used as dispersed nanoparticles combined with the surface of the support to suppress sintering and maintain dispersion of small metal particles.1,2 Actually, almost all excellent state-of-the art catalysts consist of metal nanoparticles and appropriate supports.3−17 Pd-substituted LaFeO3 perovskite is a good example.3,4 Another good example is Pt nanoparticle supported on CeO2.6−11 In this catalyst, the Pt metal particle is produced under reducing conditions, but it is again redistributed to the CeO2 surface under oxidizing conditions because the Pt−O−Ce bonding interaction is formed in the oxidizing conditions. It plays a crucial role in the Pt redistribution.6−11 Recently, Machida and co-workers found that the use of tridymite-type aluminum orthophosphate (AlPO4) as a support can markedly decrease the quantity of Rh in the three-way catalyst because thermally stable and highly © XXXX American Chemical Society

dispersed Rh nanoparticles are anchored well on the phosphate surface.13−17 Considering that AlPO4 is a robust and cheap material, their finding is of considerable importance in constructing a low-cost but effective catalyst. To elucidate the reasons this AlPO4 exhibits such an excellent anchoring effect, it is necessary to ascertain where the Rh adsorption occurs, how strong the interaction between the Rh particle and the surface is, and to clarify the origin of the interaction. Additionally, it is indispensable to elucidate the characteristic features of the AlPO4 surface in comparison with the usual support materials. In this regard, Machida and co-workers experimentally and theoretically investigated the Rh2O3−AlPO4 system and found that the Rh atom of Rh2O3 binds with the O atom of AlPO4 to form an Rh−O−P linkage, which plays an important role in the anchoring effect to suppress sintering, where first-principles DFT calculations were conducted with the slab model of Rh2O3−AlPO4.17 Received: March 19, 2015 Revised: July 31, 2015

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DOI: 10.1021/acs.jpcc.5b02691 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C The adsorption of Pt18−22 and Ir clusters23,24 on γ-Al2O3 surface and that of Cu, Ag, and Au clusters on α-Al2O3 surface25−30 were investigated theoretically using first-principles DFT calculations. In those studies, the nucleation, growth, and adsorption positions have been discussed. However, the adsorption of Rh cluster on the support material has not been investigated theoretically except for one pioneering work of the Rhn cluster (n = 1−5) on the γ-Al2O3 surface.31 In this work, the first-principles DFT calculations disclosed that the Rhn adsorption occurs at the terminal O or Al atom and that Rhn binds more strongly on the nonhydrated surface than on the hydrated one. However, the adsorption of Rh clusters on the AlPO4 surface has not been investigated theoretically at all. Only the interaction between Rh2O3 and the AlPO4 surface has been investigated experimentally and theoretically by Machida and co-workers,17 as described above. To suppress the sintering, this linkage must be preserved in both oxidizing and reducing atmospheres. Therefore, it is indispensable to provide theoretical knowledge of the interaction of the Rh cluster with AlPO4 and the differences in the adsorption of Rh metal particle between AlPO4 and a usual support material such as γ-Al2O3. Additionally, characterization of the AlPO4 surface in comparison with the γ-Al2O3 surface is helpful for finding a new and better support material for the Rh cluster. This study theoretically investigated the adsorption of Rh dimer on the AlPO4(110) surface using first-principles DFT calculations. We employed the Rh dimer here as a minimum model of a Rh cluster because this is the first step in theoretical study of the AlPO4 surface.32,33 Our purposes here are to elucidate the adsorption position, to compare the adsorption between purely nonhydrated and hydrated surfaces of AlPO4, and to elucidate the characteristic features of the AlPO4 surface in comparison with the γ-Al2O3 surface. This paper is structured as follows: Section 2 describes the models, methods, and computational details used in this study. Results and Discussion are presented in Section 3. The summary is presented in Section 4.

Figure 1. Structures of AlPO4: (A) Polyhedral representation of βtridymite AlPO4 structure, (B) nonhydrated surface, and (C) hydrated surface. *The second and lower layers in the side and top views are represented by pale colors.

nonhydrated surface. For the hydrated surface, it is likely that the partial dehydration occurs to afford dehydrated sites which can interact with the Rh dimer. This is true because the fully hydrated surface is covered by H atoms. It is difficult for metal particles to interact with the surface covered by the H atom. Actually, the theoretical study of the Rh cluster on a hydrated γAl2O3 surface showed that the Rh cluster interacts with O atoms bound with Al atom preferably to the OH groups.31 In addition, Machida’s group reported that the partial dehydration occurs by increasing the temperature.17 In the partially dehydrated surface, three types of adsorption structure are possible, depending on the manner of partial dehydration. In one structure, the Rh dimer interacts with the dehydrated PO site, which is near the dehydrated Aluns atom, as shown in Figure 2B. This is named PO-IIhy, where the superscript “hy” signifies a hydrated surface. In the second one, the Rh dimer interacts with the PO site, too, which is distant from the Aluns, as shown in Figure 2C. This is named a PO-IIIhy site. The difference between PO-IIhy and PO-IIIhy arises from the dehydration position: in the PO-IIhy, the dehydration of two water molecules occurs at the POH and AlOH moieties which are close to the Rh dimer, as shown in Figure 2B. In PO-IIIhy, deprotonation occurs at the same POH sites as those of the PO-IIhy site, but the dissociation of the OH group occurs at different AlOH sites that are somewhat distant from the Rh dimer but neighboring the deprotonated PO site, as shown in Figure 2C. We also investigated the adsorption of the Rh dimer with the AlO site of the partially dehydrated surface. In the partially dehydrated surface, the AlO site is covered by the H atom before Rh dimer adsorption. However, the H atom moves

2. MODELS, METHODS, AND COMPUTATIONAL DETAILS 2.1. Models. We investigated here the interaction between Rh dimer and the β-tridymite AlPO4(110) surface. The structure of the bulk β-tridymite AlPO4 (P63mc)34 and its (110) plane are shown in Figure 1A. In this bulk structure, corner-sharing tetrahedral sites are occupied by either aluminum or phosphorus atom. In this work, we investigated both a nonhydrated surface and a hydrated one as shown in Figure 1B and C, respectively. Three-coordinate unsaturated Al (Aluns) atom and one-coordinate O atom of phosphate are exposed to the nonhydrated surface (Figure 1B), where the superscript “uns” denotes the three-coordinate unsaturated site. The hydrated surface is constructed from dissociative hydration of the nonhydrated surface (Figure 1C). We examined four kinds of adsorption structure of the Rh dimer on the nonhydrated and hydrated AlPO4 surfaces. In the adsorption on the nonhydrated surface, it is likely that the Rh dimer bridges two PO groups, which are mutually neighbors, as shown in Figure 2A. This is reasonable because the O−O distance (longer than 5 Å) between these two PO groups fits the Rh dimer well. It is noteworthy that the Rh−Rh and Rh−O distances are, respectively, 2.7 Å35 and 2.0 Å.36 This PO site is near the Aluns, as shown in Figure 2A. Hereafter, this adsorption site is named PO-Inh, where the superscript “nh” stands for B


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The Journal of Physical Chemistry C E int = Eeq [Rh 2/AlPO4 ] − Edis[Rh 2] − Edis[AlPO4 ]

(4)

Equation 5 shows that the adsorption energy is represented by the interaction energy and the deformation energy as Ead = E int + Edf [Rh 2] + Edf [AlPO4 ]

(5)

2.3. Projection of the Band Structure of the Total System onto That of the Fragment System. Density of states (DOS) is defined by eq 6 as ρ (ε) =

∑ ω k ∑ δ(ε − εi , k) i

k

=

∑ ω k ∑ ⟨ψi ,k|ψi ,k⟩δ(ε − εi ,k) i

k

(6)

where ψi,k and εi,k, respectively, denote the one-electron orbital and the orbital energy of ith orbital at wave-vector k. The projected density of states (p-DOS) onto the nth orbital ϕAn,k of a fragment system A is defined by eq 7 as ρAn (ε) =

∑ ω k ∑ |⟨ϕAn,k |ψi ,k⟩|2 δ(ε − εi ,k) i

k

(7)

Using plane−wave expansion, the projection term is expressed by eq 8 as ⟨ϕAn , k |ψi , k ⟩ =

* , k + Gci , k + G ∑ cAn G

where ci,k+G and cAn,k+G are plane−wave coefficients of the total system and the fragment system A, respectively, and G is a reciprocal lattice vector, as described in an earlier report in the literature.37 In the analysis, the Rh2/AlPO4 system is separated into the Rh2 and AlPO4 surface. The orbital of the total system is represented by the linear combination of the orbitals of such fragment systems as Rh2 and AlPO4. Therefore, the projection of the band structure of the total system onto that of the fragment system represents the contribution of the band structure of the fragment system to that of the total system. When p-DOS of the conduction band (the unoccupied orbital) of the fragment system appears in the valence band of the total system, it is concluded that the conduction band of the fragment system participates in charge transfer (CT) to receive electron density from another fragment. Therefore, this analysis is useful to investigate whether CT occurs or not. 2.4. Computational Details. We used a spin-polarized density functional theory. Plane−wave basis sets were used with a cutoff of 415 eV for the kinetic energy. The effects of the inner cores on the valence states were considered by the projector-augmented-wave (PAW) method,38 according to Kresse and Hafner.39 The valence density was defined by the 3s, 3p, and 3d of Al and P; the 2s, 2p, and 3d of O; and the 5s, 5p, and 4d of Rh; where the d component was involved in P and Al considering its importance.40 A 1 × 2 × 1 Monkhorst− Pack grid41 of special k-points was used to calculate the numerical integration on the reciprocal space. A generalized gradient approximation (GGA) PBE functional42,43 was used in the calculation. Several theoretical studies44−46 reported that the PBE functional provides a good agreement with experimental results by magnetic deflection studies of Rh clusters, but such hybrid-GGA functionals as PBE047 and HSE48−53 provide vibrational spectra and geometries more accurately. We calculated Rh2 with PBE, PBE0, and HSE functionals and found that the differences in geometry and frequency between the PBE and other functionals are not large;

Figure 2. Rh2 adsorption structure on the AlPO4(110) surface: (A) PO-Inh site of the nonhydrated surface; (B) PO-IIhy site, which is near the unsaturated three-coordinate Al (Aluns) of the hydrated surface; (C) PO-IIIhy site, which is distant from the Aluns of the hydrated surface; and (D) AlOhy site on the hydrated surface. *The second and lower layers in the side and top views are represented by pale colors. Red and green arrows indicate deprotonated O atom and dehydroxylated Al atom, respectively.

to the neighboring deprotonated PO moiety by Rh dimer adsorption (Figure 2D). This is named an AlOhy site. 2.2. Adsorption, Deformation, and Interaction Energies. The adsorption energy Ead is defined in eq 1 as Ead = Eeq [Rh 2/AlPO4 ] − Eeq [Rh 2] − Eeq [AlPO4 ]

(1)

where Eeq[Rh2/AlPO4] is the total energy of the adsorption system, Eeq[AlPO4] is the total energy of the equilibrium AlPO4 surface without Rh2, and Eeq[Rh2] is the total energy of the equilibrium Rh2. The adsorption energy can be decomposed into deformation and interaction energies. The deformation energy (Edf) of the Rh dimer is defined as the difference in energy between the equilibrium structure of the isolated Rh2 and the distorted one with the structure taken to be the same as that in the Rh2/AlPO4 system as Edf [Rh 2] = Edis[Rh 2] − Eeq [Rh 2]

(2)

where Edis[Rh2] is the energy of the Rh2 with the distorted geometry. The deformation energy of the AlPO4 surface is defined similarly as Edf [AlPO4 ] = Edis[AlPO4 ] − Eeq [AlPO4 ]

(8)

(3)

The interaction energy is defined as the difference in energy between the total system and the sum of Rh2 and AlPO4 with deformed structures taken in the Rh2/AlPO4 system, as C


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The Journal of Physical Chemistry C

adsorption, deformation, and interaction energies of the Rh2 adsorption on the PO and AlO sites together with several important geometrical parameters. The most stable adsorption site is the PO-IIhy, which is the PO site near the Aluns of the hydrated surface. Its adsorption energy is −4.54 eV. The PO-Inh is the second most stable adsorption position, which is the PO site near the Aluns of the nonhydrated surface. The PO-IIIhy is the next stable adsorption position, which is the PO site distant from the Aluns on the hydrated surface. These adsorption energies are, respectively, − 3.63 and −2.46 eV. The adsorption energy at the AlOhy site (−0.46 eV) is much smaller, indicating that Rh2 adsorption does not occur at the AlOhy site. This result is consistent with the experimental suggestion that Rh particles adsorb at the PO site.15,17 Table 1 also shows that the deformation energy of the surface is considerably high in all adsorption structures, whereas the deformation energy of the Rh2 is extremely small in all cases. Although the interaction energy between the Rh2 and the AlOhy site is considerably large, the deformation energy of the surface is also extremely large at this site, which is the reason for the small binding energy of the Rh2 adsorption at the AlOhy site. The large deformation energy of the surface arises from the inherent instability of the AlO group. One must recall that the surface consists of the Al−OH and P−O moieties before the Rh2 adsorption but the H+(proton) moves to the neighbor PO site from the Al−OH site upon Rh2 adsorption. Such H+(proton) transfer induces a large destabilization energy of 2.7 eV.67 This is true because the Al−O moiety is much more polarized than the P−O moiety, because the Al atom is much more electropositive than the P atom. In other words, the Al− O moiety interacts much more strongly with H+(proton) than the P−O moiety. As a result, the AlOhy site is much less favorable for Rh2 adsorption than the PO−Inh, PO−IIhy, and PO−IIIhy sites. Hereinafter, we will skip the discussion of the adsorption at the AlOhy site. In the best PO−IIhy site, the interaction energy is the largest and the deformation energy of the surface is moderate. In the worst PO−IIIhy, the interaction energy is the smallest and the deformation energy of the surface is the largest. However, the adsorption energy decreases in parallel to the interaction energy and the difference in the deformation energy of the surface is not very large. These results strongly suggest that the analysis of the interaction energy provides the reason the adsorption energy decreases in the order of PO−IIhy > PO−Inh > PO− IIIhy. Changes in Bader charge induced by the Rh2 adsorption are presented in Table 2, where a positive value indicates an increase in electron population around the atom and vice versa.

the results and related discussion are presented in page S2 in Supporting Information. The ground state of Rh2 is calculated to have a quintet spin multiplicity, which agrees with the spin multiplicity of Rh2 calculated using CASSCF/CI54 and various DFT functionals.55−57 The calculated structural properties of AlPO417 and γ-Al2O358 surfaces using the PBE functional were in good agreement with experimental results. Also, it is currently difficult to use the hybrid functional in the firstprinciples calculation of large system consisting of several hundred atoms in the slab model because such a calculation needs very long computational time. All these results suggest that the use of the PBE functional is a reasonable choice here. Geometries of the first and second surface layers of AlPO4 were optimized, whereas the geometry of the other moiety was fixed to be the same as that of the bulk structure. Spin multiplicity is estimated from the difference in number between up-spin electrons Nup and down-spin ones Ndown; e.g., Nup − Ndown = 0, 2, and 4, respectively, correspond to singlet, triplet, and quintet. For these calculations, we used the VASP program package.59−61 The slab model used in this work was constructed using a 2 × 2 surface unit cell with 7 AlPO4 layers, corresponding to 15 atomic layers, where each slab was separated from another by a vacuum width of about 20 Å. To prevent artificial electrostatic interaction between the repeated surfaces, Rh dimers were adsorbed onto both sides of the slab. The Rh coverage of this model is 25% with respect to the number of surface O atoms in the supercell. Bader charge analysis was performed using the program developed by Henkelman group.62−64 Structure, charge density, and wave function were displayed using XCRYSDEN65 and VESTA66 visualization software.

3. RESULTS AND DISCUSSION 3.1. Relative Stability of Rh2 Adsorption on AlPO4 Surface and Bader Charge Analysis. Table 1 shows Table 1. Adsorption Energy, Interaction Energy, Deformation Energy, and Several Important Geometrical Parameters of Rh2/AlPO4 and Rh2/γ-Al2O3 Systems surface-type

nonhydrated

site

PO-Inh

PO-IIhy

POIIIhy

AlOhy

γ-Al2O3

spin state

quintet

quintet

quintet

triplet

triplet

−0.46 0.00 +6.74 −7.20

−2.41 +0.17 +2.85 −5.26

2.17 1.85 1.88 -

2.33 2.01 2.98 2.39 2.68

Ead Edf(Rh2) Edf(AlPO4) Eint Rh(1)−Rh(2)b Rh(1)-O Rh(2)-O P(1)−O(1)c P(2)−O(2)c Rh(1)−Al(1)d Rh(2)−Al(1)d

hydrated

Energy (eV) −3.63 −4.54 −2.46 +0.06 +0.05 +0.06 +1.61 +1.85 +2.32 −5.29 −6.43 −4.84 Interatomic Distance (Å)a 2.26 2.26 2.26 2.16 2.08 2.09 2.14 2.13 2.03 1.50 1.53 1.53 1.50 1.51 1.51 2.53 2.58 4.74 2.73 2.66 5.98

nonhydrated

Table 2. Changes in the Bader Charge Induced by Rh2 Adsorption.a AlPO4 Rh2 O(1) O(2) P(1) P(2) Al(1)

a

The index of each atom is represented in Figures 2 and 5. bIn the gas phase, the Rh−Rh distance is 2.18 Å. cIn the equilibrium structures of the PO-Inh, PO-IIhy, and PO-IIIhy surfaces, the P−O distance is 1.47, 1.48, and 1.50 Å, respectively. dAl(1) corresponds to Aluns.

PO-Inh

PO-IIhy

PO-IIIhy

γ-Al2O3

−0.08 −0.02 −0.01 −0.07 −0.10 +0.15

−0.05 −0.09 −0.05 −0.06 −0.08 +0.21

−0.56 −0.05 −0.06 −0.06 −0.08 +0.68

+0.25 −0.22 −0.19 +0.10

a

A positive value indicates an increase in electron population of an atom and vice versa.

D


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The Journal of Physical Chemistry C In the adsorption at the PO−Inh, PO−IIhy, and PO−IIIhy sites, the electron population of the Rh2 decreases and that of the AlPO4 surface increases, which is consistent with the experimental suggestion that the CT from the Rhn to the AlPO4 surface contributes to the interaction between the Rhn and the AlPO4 surface.17 Although the Rh2 interacts with the O atom of the PO group, the electron population considerably increases around the Aluns but moderately changes around the P and O atoms. These results indicate that the CT from the Rh2 to the Aluns is induced by the Rh2 adsorption. However, the change in the Bader charge is not parallel to the interaction energy. To clarify the interaction between the Rh2 and the AlPO4 surface, more detailed analyses are necessary. 3.2. Interaction between Rh2 and AlPO4 Surface at the PO−Inh Site. This site is near the Aluns of the nondehydrated surface. The Rh2 adsorption system at this site is named Rh2/ AlPO4(PO−Inh). The total spin state is quintet. Table 1 shows that the Rh(1)−O(1) and the Rh(2)−O(2) distances are mutually similar (2.16 and 2.14 Å, respectively), and that the Rh(1)−Rh(2) distance is somewhat elongated upon the adsorption by 0.08 Å. The P(1)−O(1) distance is moderately elongated by the Rh2 adsorption by about 0.02 Å. The difference density map by the Rh2 adsorption is depicted in Figure 3(A-1). Apparently, the electron density increases in the area between the Rh2 and the Aluns near the PO site, suggesting that the CT occurs from the Rh2 to the Aluns. This increase is not surprising because the Rh(1)−Aluns and Rh(2)− Aluns distances (2.53 and 2.73 Å, respectively) are not very long, as shown in Table 1, where Al(1) is Aluns. This result agrees

with the Bader charge analysis described above. The average O−Aluns−O angle remarkably decreases from 119.9° to 105.4° by the Rh2 adsorption, indicating that the hybridization of the Aluns changes from sp2 to sp3. The average P−O−Al angle was previously discussed in relation to the acidity change by water adsorption.68,69 Therefore, the angle change is discussed briefly here. The average P−O−Aluns angle (151.3°) does not change by the Rh2 adsorption. However, the P(1)−O−Aluns and P(2)− O−Aluns angles increase considerably from 135.6° to 151.0° and from 145.8° to 161.9°, respectively, but the P(3)−O−Aluns decreases considerably from 172.7° to 141.1°. These changes suggest that the Aluns is pulled by the Rh2 from the AlPO4 surface. Although the acidity is related to the average P−O−Al angle,68,69 it is not easy to discuss here the acidity change by Rh2 adsorption based on the average P−O−Al angle.70 Figure 4(A-1) shows DOSs of the equilibrium AlPO4 and the deformed AlPO4(PO−Inh) surfaces, where the geometry of the deformed AlPO4(PO−Inh) surface is taken to be the same as that in the Rh2/AlPO4(PO−Inh) system. The band gap of the deformed AlPO4(PO−Inh) surface decreases considerably to 0.85 eV compared to that (3.93 eV) of the equilibrium AlPO4 surface because the isolated lowest unoccupied band, hereinafter called the isolated LU band, appears just above the Fermi level in the deformed AlPO4(PO−Inh) surface; see Figure S1 in Supporting Information (SI) which presents detailed band structure around HO and LU bands. The energy level of the isolated LU band is lower than that of the highest occupied molecular orbital (HOMO) of Rh2. Supporting Information page S3 shows the evaluation procedure for the orbital energy. Figure 3(A-2) and (A-3) respectively show orbital plots of the bottom of the conduction band of the equilibrium AlPO4 surface and the isolated LU band of the deformed AlPO4(PO− Inh) surface. In the equilibrium AlPO4, the bottom of the conduction band mainly consists of 3p orbitals of Al and it is delocalized over the whole surface. In the deformed AlPO4(PO−Inh) surface, the isolated LU band is localized on the Aluns near to the adsorption site. This orbital localization on the Aluns corresponds to the hybridization change of the Aluns from sp2 to sp3 which is mentioned above. This feature suggests that the isolated LU band plays the role of an acceptor orbital; in other words, the AlPO4(PO−Inh) surface has the character of a Lewis acid. Figure 4(A-2) shows the DOS of the Rh2/AlPO4(PO−Inh) system and the p-DOS of the isolated LU band localized on the Aluns of the deformed AlPO4(PO−Inh) surface. Figure S3 in SI shows the p-DOS of the isolated Rh2. The p-DOS of the isolated LU band dominantly appears in the valence band of the Rh2/AlPO4(PO−Inh) system, clearly indicating that the CT occurs from the Rh2 to the isolated LU band to increase the electron population around the Aluns. This CT is an important factor supporting the strong interaction between the Rh cluster and the AlPO4 surface. In conclusion, the surface deformation, which is induced by the Rh2 adsorption at the PO−Inh site near to the Aluns, produces the isolated LU band localized on the Aluns, which plays a role of an acceptor orbital. The CT from the Rh2 to the isolated LU band is crucial for providing a stabilizing interaction between Rh2 and AlPO4. 3.3. Interaction between Rh2 and AlPO4 Surface at the PO−IIhy Site. This site is near Aluns of the partially dehydrated surface. The Rh2 adsorption system at the PO−IIhy site is named Rh2/AlPO4(PO−IIhy) and the partially dehydrated AlPO4 in this system is designated as AlPO4(PO−IIhy)

Figure 3. Difference electron density map in the Rh2/AlPO4 system, orbital plots of the bottom of conduction band of the equilibrium AlPO4 surface, and that of the isolated LU band of the deformed AlPO4 surface: (A) the PO-Inh site of the nonhydrated surface, (B) the PO-IIhy site, which is near the unsaturated three-coordinate Al (Aluns) of the hydrated surface, and (C) the PO-IIIhy site, which is distant from the Aluns of the hydrated surface. *The cyan area represents an increase in electron density and the orange area represents its decrease. † The red/blue colors indicate the different sign of the wave function. E


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Figure 4. DOSs of the equilibrium AlPO4, the deformed AlPO4 surfaces, and Rh2/AlPO4, and p-DOSs of the isolated LU band and Rh2: (A) the POInh site of the nonhydrated surface; (B) the PO-IIhy site, which is near the unsaturated three-coordinate Al (Aluns) of the hydrated surface; and (C) the PO-IIIhy site, which is distant from the Aluns of the hydrated surface. Vertical red solid line indicates Fermi level. Red arrow indicates the isolated LU band. See Figure S1 for the detailed representation of HO and LU bands and their band gap of the distorted AlPO4.

164.8°, and the P(3)−O−Aluns angle decreases considerably from 170.7° to 151.5°. This is true because the Aluns is pulled by Rh2 like in the Rh2/AlPO4(PO−Inh) system, as described above. Hence, the very small decrease in the average P−O−Al angle does not always indicate the slight decrease in acidity. All these features are similar to those of the Rh2 adsorption at the PO−Inh, as mentioned in ref 70. Figure 4(B-1) shows DOSs of the equilibrium AlPO4(PO− IIhy) and the deformed AlPO4(PO−IIhy) surfaces, where the geometry of the deformed AlPO4(PO−IIhy) is taken to be the same as that of the Rh2/AlPO4(PO−IIhy) system. The band gap of the deformed AlPO4(PO−IIhy) surface decreases extremely to 0.17 eV compared to that (2.89 eV) of the equilibrium AlPO4 surface (inset in Figure 4(B-1)); also, Figure S1 in SI shows the detailed band structure. This is true because the isolated LU band appears just above the Fermi level in the deformed AlPO4(PO−IIhy) surface like that of the AlPO4(PO− Inh) surface. Figure 3(B-2) and (B-3) shows that the isolated LU band of the deformed AlPO4(PO−IIhy) surface is localized on the Aluns

hereafter. The Rh2 adsorption at this site occurs similarly to that at the PO−Inh site of the nonhydrated surface. The total spin state is quintet, as reported in Table 1. The Rh(1)−Rh(2) distance (2.26 Å) is almost identical to that of the Rh2/ AlPO4(PO−Inh) system. The Rh(1)−O(1) and Rh(2)−O(2) distances (2.08 and 2.13 Å, respectively) are moderately shorter and the P(1)−O(1) and P(2)−O(2) distances (1.53 and 1.51 Å, respectively) are slightly longer than in the Rh2/AlPO4(PO− Inh) in which the Rh(1)−O(1) and Rh(2)−O(2) distances are 2.16 and 2.14 Å and both of the P(1)−O(1) and P(2)−O(2) distances are 1.50 Å. As shown in the difference density map of Figure 3(B-1), the electron density increases in the area between the Rh2 and the Aluns near the PO−IIhy site. The averaged O−Aluns−O angle changes from 119.9° to 102.0° by the Rh2 adsorption, indicating that the hybridization of Aluns changes from sp2 to sp3. The averaged P−O−Aluns angle slightly decreases from 153.1° to 151.5° by the adsorption of Rh2. However, the P(1)− O−Aluns and P(2)−O−Aluns angles somewhat increase, respectively, from 139.0° to 151.6° and from 149.7° to F


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The Journal of Physical Chemistry C near the PO−IIhy site like that of the deformed AlPO4(PO− Inh). Figure 4(B-2) shows that the p-DOS of the isolated LU band dominantly appears in the valence band of the Rh2/ AlPO4(PO−IIhy), indicating that the CT occurs from the Rh2 to the isolated LU band to increase the electron population around the Aluns. It is concluded that the CT from the Rh2 to the isolated LU band localized on the Aluns contributes to the bonding interaction between the Rh2 and the AlPO4(PO−IIhy) surface. 3.4. Interaction between Rh2 and AlPO4 Surface at the PO−IIIhy Site. This adsorption occurs at the distant PO site from the Aluns of the partially dehydrated surface. The adsorption system is named Rh2/AlPO4(PO−IIIhy) and the partially dehydrated AlPO4 moiety in this system is named AlPO4(PO−IIIhy) hereafter. The adsorption energy (−2.46 eV) at this PO−IIIhy site is considerably smaller than at the PO−Inh and the PO−IIhy sites (−3.36 and −4.54 eV, respectively). The spin state is quintet, as shown in Table 1. The Rh2 adsorption structure is similar to that of the Rh2/AlPO4(PO−IIhy) except for the moderately shorter Rh(2)−O(2) distance (1.88 Å) and the much longer Rh(1)−Al and Rh(2)−Al distances (4.74 and 5.98 Å, respectively) than in the Rh2/AlPO4(PO−IIny) (2.53 and 2.73 Å, respectively) and the Rh2/AlPO4(PO−IIhy) (2.58 and 2.66 Å, respectively). The averaged O−Aluns−O angle changes from 119.8° to 107.5°, indicating that the hybridization of the Aluns changes from the sp2 to sp3 like those at the PO−Inh and PO−IIhy sites. The averaged P−O−Aluns angle decreases somewhat from 148.8° to 137.3° by the adsorption of Rh2. The P(1)−O−Aluns and P(2)−O−Aluns angles decrease somewhat from 142.6° to 129.4° and from 134.1° to 128.8°, respectively, and the P(3)−O−Aluns considerably decreases from 169.7° to 153.8°. In this case, the position of the Aluns is not influenced directly by Rh2 because Rh2 is distant from the Aluns unlike in the Rh2/AlPO4(PO−Inh) and Rh2/AlPO4(PO−IIhy) systems. Therefore, the decrease in the average P−O−Aluns angle indicates the decrease in acidity.68,69 Figure 3(C-1) shows that the electron density around the Aluns considerably increases by Rh2 adsorption, despite the fact that the Aluns is distant from the Rh(1) and Rh(2) by 4.74 and 5.98 Å, respectively. Figure 4(C-1) shows that the band gap of the deformed AlPO4(PO−IIIhy) surface decreases considerably to 0.32 eV by the deformation; Figure S1 in Supporting Information also shows the detailed band structure. The isolated LU band of the deformed AlPO4(PO−IIIhy) surface is localized on the Aluns, as shown in Figure 3(C-2) and (C-3). The band gap narrowing and the localization of the isolated LU band are similar to those of the deformed AlPO4(PO−Inh) and AlPO4(PO−IIhy) surfaces. In contrast to the adsorptions at the PO−Inh and the PO− hy II sites, the p-DOS of the isolated LU band appears in both the valence and conduction bands in the Rh2/AlPO4(PO−IIIhy) system, as shown in Figure 4(C-2). Therefore, the isolated LU band of the deformed AlPO4(PO−IIIhy) becomes partially occupied in the Rh2/AlPO4(PO−IIIhy), suggesting that the CT is weaker in the Rh2 adsorption at the PO−IIIhy site than those at the PO−Inh and the PO−IIhy sites.71 This inference is not surprising because the Aluns is somewhat distant from the Rh2 adsorption site. 3.5. Comparison with Rh2/Al2O3. It is interesting to make a comparison between the γ-Al2O3(100) and AlPO4(110) surfaces in the Rh2 adsorption to characterize the AlPO4(110) surface. We made a comparison with the nonhydrated surface

because the hydration and partial dehydration would be different between them. We used the γ-Al2O3(100) surface model proposed by Digne et al.58 and the Rh2 adsorption model used by Sholl et al.31 Table 1 shows that this adsorption system has a triplet ground state. The Rh(1) is bound with the O atom of the surface, but the Rh(2) is somewhat distant from the surface, where the Rh(1)−O(1) and Rh(2)−O(1) distances are 2.01 and 2.98 Å, respectively, and the Rh(1)−Rh(2) distance (2.33 Å) is considerably longer than that (2.18 Å) of free Rh2 (Figure 5 and Table 1). These features are essentially

Figure 5. Rh2 adsorption structure on the γ-Al2O3(100) surface. Atoms are represented by the same colors as in Figure 2.

the same as those of previous work.19 The Rh(1)−Al distance (2.39 Å) is shorter than those of the Rh2/AlPO4(PO−Inh) and Rh2/AlPO4(PO−IIhy) systems (2.53 and 2.58 Å, respectively) but the Rh(2)−Al distance (2.68 Å) is similar to them (2.73 and 2.66 Å, respectively). The adsorption energy of the Rh2/γ-Al2O3 is −2.41 eV, which is considerably smaller than that of the Rh2 adsorption at the PO−Inh (−3.63 eV). The interaction energy (−5.26 eV) is similar to that of the Rh2/AlPO4(PO−Inh) system. The respective deformation energies of the Rh2 and the γ-Al2O3 surface are 0.17 and 2.85 eV. Although the deformation energy of Rh2 is moderately larger than in the adsorption at the PO− Inh, that of the γ-Al2O3 is much larger, by about 1.24 eV, than that of AlPO4(PO−Inh). This large deformation energy is the reason for the small adsorption energy. The Bader charge analysis indicates that the electron population of the Rh2 moiety increases by 0.25 e by the adsorption. The difference density map also shows that the electron density increases around the Rh2, as shown in Figure 6A. These results differ completely from the results found for the Rh2 adsorption at the AlPO4(PO−Inh) site, in which the electron population of the Rh2 decreases by the adsorption.

Figure 6. (A) Difference density map in the Rh2/γ-Al2O3, (B) orbital plots of the bottom of conduction band of the equilibrium γ-Al2O3 surface, and (C) the isolated LU band of the deformed γ-Al2O3 surface. *The cyan area represents an increase in electron density and the orange area represents its decrease. †The red and blue colors indicate the different sign of the wave function. G


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Figure 7. (A) DOS of the equilibrium γ-Al2O3 and the deformed γ-Al2O3 surfaces. (B) DOS of Rh2/γ-Al2O3 and p-DOS of the isolated LU band together with p-DOS of Rh2. Vertical red line indicates Fermi level.

which plays a crucial role in CT interaction with metal particles. It is noteworthy that the CT from Rh2 to the γ-Al2O3 surface is weak, but the reverse CT from the γ-Al2O3 surface to Rh2 is formed, suggesting that the electronic structures of Rh clusters of AlPO4 and γ-Al2O3 surfaces differ.

These results suggest that the interaction energy is similar but the bonding nature differs between Rh2/γ-Al2O3 and Rh2/ AlPO4 systems. Elucidating the reason the CT interaction is completely different between γ-Al2O3 and AlPO4 is of considerable interest. Figure 6B,C shows orbital plots of the bottom of the conduction band of the equilibrium γ-Al2O3 surface and the isolated LU band of the deformed γ-Al2O3 surface, respectively. The bottom of the conduction band is delocalized over the whole surface, in a similar manner to that of the equilibrium AlPO4(PO-Inh); on the other hand, the isolated LU band is localized on the Al atoms at the adsorption site, similarly to that of the deformed AlPO4(PO-Inh) which is localized on the Aluns. As shown in Figure 7A, the band gap of the deformed γ-Al2O3 surface is 1.60 eV, which is larger than that of the deformed AlPO4(PO−Inh) surface. A much more important result is that the isolated LU band is formed at a higher energy (−10.44 eV) in the deformed γ-Al2O3 surface than that (−12.67 eV) in the deformed AlPO4(PO−Inh) surface; Supporting Information page S3 shows the evaluation procedure of orbital energies. However, the top of the valence band is calculated at −12.04 eV, which is higher than that of AlPO4 (−13.52 eV). Figure 7B shows that the p-DOS of the isolated LU band appears in both the valence band and the conduction band. Therefore, the isolated LU band of the deformed γ-Al2O3 is partially occupied, indicating that the occupation of the isolated LU band is smaller than in the Rh2/AlPO4(PO−Inh) system. Another unfavorable factor is the absence of the Aluns on the γ-Al2O3 surface because it plays a crucial role as an electron-acceptor orbital to form the CT interaction in the AlPO4 surface; note that the γ-Al2O3(100) surface consists of five-coordinate Al atoms which participate only slightly in the CT interaction because of the saturated structure. All these results indicate that the CT from Rh2 to γ-Al2O3 occurs to a lesser extent than in the Rh2/AlPO4(PO−Inh) system, but the reverse CT from γAl2O3 to Rh2 occurs in the Rh2/γ-Al2O3. This CT to Rh2 contributes to the elongation of the Rh−Rh distance because the antibonding Rh−Rh MO receives electron populations through this CT. In summary, the interaction energy between Rh2 and γ-Al2O3 is similar to that between Rh2 and AlPO4(PO−Inh). The small adsorption energy derives mainly from the large deformation energy of the γ-Al2O3 surface. Actually, AlPO4 has a flexible structure that easily undergoes deformation to fit to Rh cluster without large energy loss and provides the isolated LU band at a low energy level. This LU band is localized on the Aluns site,

4. SUMMARY The adsorption of Rh dimer on the AlPO4 surface was studied with the DFT method, employing the slab model. Four types of adsorption structures were examined: Rh2 adsorption at the PO−Inh site of the nonhydrated surface, Rh2 adsorption at the PO−IIhy site near to the three-coordinate unsaturated Aluns atom of the hydrated surface, Rh2 adsorption at the PO−IIIhy site a little bit distant from the Aluns of the hydrated surface, and Rh2 adsorption at the AlOhy site of the hydrated surface. The most stable adsorption occurs at the PO−IIhy site, the adsorption energy Ead of which is −4.54 eV. The second and the third most stable ones occur at the PO−Inh (Ead = −3.63 eV) and the PO−IIIhy (Ead = −2.46 eV), respectively. The adsorption at the AlOhy site (Ead = −0.46 eV) is considerably less stable than those at the PO−Inh, PO−IIhy, and PO−IIIhy sites because the bare AlOhy site is inherently unstable. In other words, the AlO site preferably interacts with H+(proton) strongly. The interaction between the Rh dimer and the AlPO4 surface at these PO sites is analyzed based on the Bader charge and the band structure of the Rh2/AlPO4 system. In the adsorptions at the PO−Inh and the PO−IIhy sites, the isolated LU band localized on the Aluns is formed by the surface deformation, which is induced by the Rh2 adsorption. The pDOS of this isolated LU band is found predominantly in the valence band of the Rh2/AlPO4 system, indicating that the LU band plays a crucial role as an acceptor orbital in the CT from the Rh2 to the Aluns. In the Rh2 adsorption at the PO−IIIhy site, the isolated LU band is observed in both the valence and the conduction bands of the Rh2/AlPO4 system, suggesting that the CT from Rh2 to AlPO4 is weaker at the PO−IIIhy site than at the PO−Inh and the PO−IIhy sites. This is true because the Aluns is somewhat distant from Rh2. The stabilization energy by the Rh2 adsorption on the γAl2O3 surface is −2.41 eV, which is much smaller than that of the Rh2/AlPO4 at the PO−Inh and PO−IIhy sites. The small adsorption energy arises from the large deformation energy of the γ-Al2O3 surface. Analysis of the Bader charge and the pDOS of the Rh2/γ-Al2O3 indicates that the CT from the Rh2 to the γ-Al2O3 surface is weaker than those of the Rh2/ AlPO4(PO−Inh) and Rh2/AlPO4(PO−IIhy) systems. One H



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important reason is that the isolated LU band of the deformed γ-Al2O3 surface exists at a higher energy than that of the deformed AlPO4 surface. Another important reason is that the three-coordinate unsaturated Aluns site is present on the AlPO4(PO−Inh) and AlPO4(PO−IIhy) surfaces but absent on the γ-Al2O3 surface. Only five-coordinate saturated Al sites are exposed on the γ-Al2O3 surface which cannot participate in the CT from Rh2. It is noteworthy that the CT from the γ-Al2O3 surface to Rh2 occurs in this surface, suggesting that the electronic structure of Rh2 differs between the AlPO4 and γAl2O3 surfaces. Present results show that the AlPO4 surface readily undergoes geometric deformation and provides the LU band at a low energy to form strong CT from the Rh2 to the AlPO4 surface. In addition, this surface has a three-coordinate unsaturated Aluns site. For all of these reasons, AlPO4 is a good support for Rh cluster.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02691. Procedure used in comparing energy levels among different surfaces is presented in Section S1. DOSs of the isolated Rh dimer are shown in Figure S3 (PDF) Geometrical details of nonhydrated γ-Al2O3 (CIF) Geometrical details of partially dehydrated tridymite AlPO4 (CIF) Geometrical details of partially dehydrated tridymite AlPO4 (CIF) Geometrical details of nonhydrated tridymite AlPO4 (CIF) Geometrical details of Rh2 adsorption on nonhydrated γAl2O3 (CIF) Geometrical details of Rh2 adsorption on AlO site of partially dehydrated tridymite AlPO4 (CIF) Geometrical details of Rh2 adsorption on PO site of partially dehydrated tridymite AlPO4 (CIF) Geometrical details of Rh2 adsorption on PO site of partially dehydrated tridymite AlPO4 (CIF) Geometrical details of Rh2 adsorption on PO site of nonhydrated tridymite AlPO4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +8175-711-7907. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is carried out in ‘Element Strategy Initiative for Catalysts and Batteries (ESICB)’ which is financially supported by Ministry of Education, Culture, Science, Sports, and Technology. S.S. wishes to acknowledge the support from the Ministry of Education, Culture, Science, Sport and Technology through Grants-in-Aid of Specially Promoted Science and Technology (No. 22000009), Grants-in-Aid for Scientific Research (No. 15H03770). We wish to thank the computational center at the Institute of Molecular Science, Okazaki, Japan for use of their computers. I


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Characterization of AlPO4(110 ... - ACS Publications

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