Atomic and Electronic Structures of M (=Ni, Fe, NiFe, or FeNi) Adlayer

ARTICLE pubs.acs.org/JPCC

Atomic and Electronic Structures of M (=Ni, Fe, NiFe, or FeNi) Adlayer-Modified r-Al2O3(0001) Catalyst Interface Kenneth Wong,† Qinghua Zeng,†,‡ and Aibing Yu*,† † ‡

School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia School of Engineering, University of Western Sydney, Penrith, NSW 2751, Australia ABSTRACT: In this study, density functional theory calculation is applied to examine the interfacial electronic and physical structures between the monometallic (Fe, Ni) or bimetallic (NiFe, FeNi) adlayer-modified RAl2O3(0001) support and its connection with catalytic activity, for example, methane cracking. It is shown that bimetallic interfaces display the key factors for highly catalytic activity as a result of a balance of system stability, favorable d orbital directionality for molecular adsorption, spin quenching, and electron accumulation at the interface. The most stable interfaces promoting the strong metal
support interaction are the monometallic Fe or Ni and bimetallic NiFe interfaces formed with R-Al2O3(0001). Such interfaces are composed of polar/ionic bonds in which bimetallic modification experiences the most significant interfacial M
M (M = Fe or Ni) and Al
O
M bond expansion. In addition to the spin quenching of the metal adlayer, it is identified that interface lattice expansion/distortion upon metal modification can induce two different molecular adsorption environments where the diffusion and strong adsorption of molecules at the interface and top metal adlayer can occur, respectively.

1. INTRODUCTION The interface and the resulting electronic interactions between nanostructured metals and ceramics are features important for technological applications, such as electronics, heterogeneous catalysis, fuel cells, sensors, and thermal barrier coatings. Ultrathin R-Al2O3 ceramic films are by far one of the most commonly used metal oxides to support metal nanoparticles for industrial catalytic reactions, such as hydrogenation, steam reforming of hydrocarbons, and Fischer
Tropsch processes. Extensively used metal nanoparticles for these reactions include Pt and Pd; however, other metals, such as Ni, Fe, Rh, and Au, have also been explored for other reactions.1
5 Traditional metal particles used as monometallic catalysts are Fe or Ni, but bimetallic Ni/Fe particles may also be promising.6,7 In fact, bimetallic Ni/Fe catalysts have been reported to be useful for the methanation of CO to prevent the deactivation of proton-exchange membrane fuel cell systems.6,8 Furthermore, other bimetallic particles, such as Ni
M (where M = Fe, Pd, or Mo), supported on Al2O3 or MgO are known to promote the catalytic decomposition of methane, ethane, or propane into H2 and carbon nanotubes, which are free of CO2 and CO.9
12 With these types of Al2O3-supported catalysts, the activity of the supported metal particles are dependent on their interfacial contact structure. The issue of concern is the interface between the bimetallic modification and metal oxide support because the reactions are very much dependent on the electronic and physical properties of this region. As a result, it is vital to identify key electronic effects at the interfacial region for the design of robust, selective, and active catalysts. It is very difficult to experimentally investigate the strong metal
support effect, activity, and electronic environment at the atomic level.13,14 To r 2011 American Chemical Society

overcome this difficulty, in recent years, computational techniques, such as density functional theory (DFT), have been used to provide useful insights into the interfacial electronic interactions that are crucial to catalyst activity. In fact, the capability of DFT calculation has been demonstrated in the study of methane adsorption, dissociation, and activation energies on the surfaces of Ru,15 Ni,16 Pt,17 Pd,18 Fe
Co alloy,19 Rh,20 and MgO.21 Currently, DFT calculations focus on Ni systems for methane activation and graphite/graphene formation.22
24 However, DFT calculations of the bimetallic modified R-Al2O3 system and the interfacial effects that can have major implications on catalytic activity are not available in the literature. Furthermore, there are still disagreements in regards to the type of bonding and structure at interfaces of monometallic modifications. For instance, it is reported that the bonds of ionic or covalent type can be formed between Pt or Pd with the O atom of an R-Al2O3 surface.25 However, metallic bonds between Pd, Pt, or Ni modification and the Al-terminated R-Al2O3 surface may exist,26 as well as intraceramic bonds between Ni modification on the O/Al-terminated R-Al2O3 surface.27 DFT studies mainly focus on the initial stage of interface formation through the adsorption of a single metal atom on the metal oxide support. As such, little is known about the physical and electronic structures of the monometallic or bimetallic adlayer modification on the R-Al2O3(0001) support. Such an understanding should be very useful for the fabrication of catalysts with favorable Received: April 6, 2011 Revised: June 2, 2011 Published: June 08, 2011 13796

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Figure 1. Schematic diagram of the Fe-, FeNi-, Ni-, and NiFe-modified R-Al2O3(0001) model catalyst showing the M1 and M2 adlayers and RAl2O3(0001) support where black represents the metal adlayer and red and pink colors represent O and Al atoms, respectively.

properties pertinent to synergistic28 and strong metal
support effects.13,14 To our knowledge, however, the direct comparison of the electronic interfacial effects for Ni- and Fe-type monometallic or bimetallic adlayers on R-Al2O3(0001) has not been addressed previously. In this work, DFT calculation is utilized to examine the electronic and physical characteristics at the interface of the M (=Fe, Ni, NiFe, or FeNi)-modified R-Al2O3(0001) system and their connection with catalytic activity. The results will be used to address the following issues: (i) What are the differences in the electronic/physical structure at the interface for monometallic and bimetallic modifications? (ii) Are there spin-quenching effects at the metal adlayer upon interface formation with RAl2O3(0001)? (iii) How do these findings relate to the catalytic properties? (iv) What are the key factors that give rise to the enhanced catalyst activity in bimetallic systems?

2. METHODOLOGY In this work, DFT calculations are performed using Dmol329,30 with the GGA (generalized gradient approximation) scheme and PBE (Perdew, Burke, and Ernzerhof) exchange-correlation functional.31 Spin-unrestricted geometry optimization and single-point energy calculations are performed, with the double numerical plus polarization basis set to expand the wave functions with a real-space cutoff of 5.8 b. DFT semicore pseudopotentials (DSPP) core treatment is used to consider the relativistic effects in heavier elements. The M/R-Al2O3 model (unit cell: a = b = 4.76 Å, c = 30.00 Å) is created under three-dimensional periodic boundary conditions incorporating a 15 Å vacuum gap. To test the approach used, DFT/GGA calculations are performed on the bulk R-Al2O3 model to compare the lattice parameter and band gap with literature values. It must be noted that using the GGA approximation leads to a slight lattice expansion26 and lower band-gap energy,32 but it is also recognized that it can produce reliable results at the least for qualitative analysis. This is evident in our calculated bulk R-Al2O3 lattice parameters of a = b = 4.83 Å and c = 13.11 Å, which are close to the experimental values of a = b = 4.76 Å and c = 12.99 Å.33 Thus, such DFT calculation would be able to give some meaningful predictions for metal-modified R-Al2O3 systems. To investigate electronic properties at the interface, the RAl2O3 layer is modeled as a thin three-layer slab terminated with O atoms. DFT calculations indicate that oxygen-terminated Al2O3 surfaces with less than three layers are more stable than Al-terminated surfaces.27 The R-Al2O3 slab used in this study is thus modeled as an oxygen-terminated surface exposed to water that has OH groups occupying positions consistent with the

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Figure 2. Geometry-optimized (a) Ni-, (b) Fe-, (c) NiFe-, and (d) FeNi-modified R-Al2O3(0001) catalyst models, where the colors purple and blue represent Fe and Ni atoms, respectively.

perfect O-terminated configurations.34 The R-Al2O3(0001) surface is selected to be the crystallographic plane because it is the most stable face in ultra-high-vacuum (UHV) conditions.27,35,36 All atoms in the unit cell are unconstrained and allowed to relax and reach their ground state during geometry optimization. The convergence tolerance is set as follows: energy = 1  10
5 Ha, force = 0.002 Ha/Å, and displacement = 0.005 Å. A thermal broadening scheme is applied at 0.1 eV, and Brillouin zone integration for the unit cell is performed through a 6  6  1 Monkhorst
Pack grid with 18 irreducible k-points. The geometry-optimized models for the Fe-, Ni-, NiFe-, or FeNi-modified R-Al2O3(0001) catalysts are constructed through single-atom deposition until the adlayer is formed. The single “one-by-one” atomic deposition of metal atoms for adlayer formation is similar to the UHV low kinetic energy “soft landing” deposition technique on substrates to form stable metal adlayer/ clusters.37,38 The metal adlayer away from the R-Al2O3 support is labeled as M1, whereas the layer closest to the support is labeled as M2 (Figure 1). To quantitatively compare the strength of adsorption for different metal adlayers on R-Al2O3, the adsorption strength is defined as26 Eads ¼ EM=Al2 O3
ðEAl2 O3 + Em Þ=n

ð1Þ

where EM/Al2O3 is the energy of the total system, EAl2O3 is the energy of the R-Al2O3 support, Em is the energy of the isolated metal layer, and n is the number of atoms in a single plane. The deformation of the bulk R-Al2O3 caused by the adsorption of a metal layer can affect the stability of the interface. The assessment of this bulk R-Al2O3 structural deformation is described as39 Edef ¼ ðErelaxed=Al2 O3
Edef =Al2 O3 Þ=n

ð2Þ

where Edef/Al2O3 is the isolated R-Al2O3 support after metal adlayer modification, Erelaxed/Al2O3 is the clean R-Al2O3 support with no surface modification, and n is the number of Al and O layers. Electronic difference maps are obtained according to the following equation FΔ ðrÞ ¼ Ftotal ðrÞ
Fmetal ðrÞ
FAl2 O3 ðrÞ

ð3Þ

where Ftotal(r) is the electron density of the total system and Fmetal(r) and FAl2O3(r) are the unperturbed electron densities of the metal adlayer and the R-Al2O3 surface, respectively. 13797

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Figure 3. Differences between the interfacial contact structure of the (a) NiFe and (b) FeNi modification on R-Al2O3(0001).

3. RESULTS AND DISCUSSION 3.1. Atomic Structure. To compare the differences in interface formation, geometry-optimized models of the Fe, Ni, NiFe, or FeNi adlayer modifications on R-Al2O3 from side and top views are examined (Figure 2a
d). The results show that Ni arranges into a (111) surface through the adsorption on hollow 3-fold O sites at the R-Al2O3 surface (Figure 2a). Similarly, inspection of the Fe/R-Al2O3 system (Figure 2b) shows that interfacial Fe atoms adsorb on the 3-fold hollow O site of the RAl2O3 surface but pack in a (110) arrangement. In contrast, the bimetallic NiFe case (Figure 2c) shows that the Ni adlayer adsorbs at the O atop site of R-Al2O3 and conforms to a bodycentered cubic packing similar to the monometallic Ni case. For the NiFe case (Figure 2d), the Fe atom adsorbs between two oxygen atoms, forming a bridge site. These differences are highlighted in Figure 3a,b, where the arrangement of Fe atoms is markedly different from that of the Ni atoms. It is also noted that, in the NiFe case, an Fe atom is 44.97° (Fe
O
O °) to the surface of R-Al2O3, whereas in the FeNi case, a Ni atom has is 89.38° (Ni
O
O °) to the surface. The bond lengths of the selected atoms are presented in Table 1 to highlight the structural differences between monometallic and bimetallic modifications on the R-Al2O3 support. For the monometallic cases, the Ni
Ni or Fe
Fe bond length is 2.63 or 2.69 Å, respectively, comparable to 2.95 and 2.53 Å as reported in the literature.7,40,41 The interfacial M1
O bond lengths for the Ni and Fe adlayers are both 2.11 Å, comparable to 2.09 and 1.98 Å for Ni
O and Fe
O, respectively.7,40,41 After adlayer modification, the interfacial Al
O bond of R-Al2O3 is 1.84 and 1.87 Å for the Ni and Fe cases, respectively, similar to the bulk Al
O bond length of R-Al2O3.42 This implies that a majority of the structural changes occur at the metal adlayer where the average M
M or M1
O bond length contracts. For the case of the bimetallic NiFe adlayer, the Ni
Ni and Fe
Fe bond lengths are 2.75 and 2.66 Å, respectively, while the intermetallic Ni
Fe length is 2.84 Å. In contrast, the Fe
Fe and Ni
Ni bond lengths for the FeNi adlayer are both 2.75 Å and the intermetallic Fe
Ni bond length is 2.65 Å. This shows that the bonds of the bimetallic NiFe modification are larger than those of the FeNi modification. At the interface, the M1
O bond for the NiFe adlayer is 2.13 Å, larger than the FeNi adlayer interfacial bond length of 1.82 Å. The interfacial surface Al
O bond of RAl2O3 after NiFe or FeNi modification is 1.87 or 1.95 Å, respectively. It is noteworthy to mention that the FeNi modification induces the largest Al
O bond compared with the other modifications. On average, the intermetallic bonding in bimetallic modified R-Al2O3 systems is larger than that in the

Table 1. Bond Lengths of the M/r-Al2O3(0001) System monometallic adlayer Ni

Fe

bimetallic

bond

length (Å)

adlayer

bond

length (Å)

Ni
Ni

2.63

NiFe

Ni
Ni

2.75

Ni
O Al
O

2.11 1.84

Fe
Fe Ni
Fe

2.66 2.84

Fe
Fe

2.69

Fe
O

2.11

Al
O

1.87

FeNi

Fe
O

2.13

Al
O

1.87

Fe
Fe

2.75

Ni
Ni

2.75

Fe
Ni

2.65

Ni
O

1.82

Al
O

1.95

monometallic modified R-Al2O3 systems. This also shows that a majority of the structural changes upon interface formation occur at the metal adlayer and interface consisting of M1
O and Al
O bonds. Similar DFT studies of a single atomic Ni, Pd, Pt, Ag, or Au adsorption on Al- or O-terminated R-Al2O3 by Xiao et al.43 and Briquet et al.44 also demonstrate that metal adsorption can occur on hollow O sites of R-Al2O3. Our calculations show that this characteristic can extend to monometallic adlayer adsorption. Our calculated monometallic adlayer interfacial M1
O bond lengths are also similar to the results from their atomic adsorption model. The difference is that bimetallic modifications prefer O atop/bridge sites for interface formation. Furthermore, the interfacial M1
O bond for the FeNi modification is much smaller compared with the other modifications. The FeNi adlayer interface is formed on the O atop position of the RAl2O3 surface. This suggests that it could be less stable compared with the other modifications as the FeNi adlayer is coordinated with fewer surface O atoms. The results also demonstrate that the average intermetallic bonds for bimetallic modifications are longer than that of the monometallic type. Such enlarged bonds in metallic modifications leading to lattice strain have been attributed to enhanced catalytic activity in other systems, such as Ru, Pt, and Au.45
48 Thus, the bimetallic systems in our study may also display enhanced catalytic activity. 3.2. Adsorption and Deformation Energies. To determine how the R-Al2O3 support structure compensates after adlayer modification, adsorption and deformation energies are calculated. First, we assess the adsorption strength on the monometallic and monometallic medications on the R-Al2O3 support. The monometallic Ni and Fe modifications have adsorption 13798

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Table 2. Adsorption and Deformation Energies of the Ni-, Fe-, NiFe-, and FeNi-Modified r-Al2O3(0001) Surface Eads (eV)

Edef (eV)

Ni


1.68


0.39

Fe NiFe


1.51
1.71


0.4
0.39

FeNi


1.13


0.17

energies of
1.68 and
1.51 eV, respectively, indicating that the Ni modification forms a more stable interface in comparison to the Fe modification. For the adsorption of the bimetallic adlayers, the NiFe and FeNi modifications have adsorption energies of
1.71 and
1.13 eV, respectively. Our calculation shows that bimetallic interface formation with the Fe atom, that is, NiFe modification, is the most stable. It is noteworthy to mention that experimental studies suggest that bimetallic modifications exist as an Fe
Ni alloy.6,49 In light of this, because bimetallic modifications can exist as a mixed alloy experimentally, the average adsorption energy of
1.42 eV for the above-mentioned bimetallic modifications (in comparison to
1.71 and
1.13 eV for NiFe and FeNi modifications, respectively) gives a more meaningful measure of its adsorption strength. The comparison of this value with the monometallic modification adsorption energies (i.e.,
1.68 and
1.51 eV for Ni and Fe modifications, respectively) shows that it is comparable in strength, though it is weaker. To investigate the extent of R-Al2O3 deformation induced by metal adlayer modification, deformation energy calculations are performed. The results (Table 2) show that the Ni, Fe, and NiFe cases experience similar energies of
0.39,
0.40, and
0.39 eV, respectively. On the other hand, FeNi induces the greatest instability to the R-Al2O3 bulk with a deformation energy of
0.17 eV. These findings indicate that the deformation of bulk R-Al2O3 contributes to strain related expansion, leading to the formation of a weak FeNi interface. The present analysis also shows that both the support and the metal adlayer are crucial for stable interface formation as the mutual relaxation of these components is required to achieve system equilibration. It has been suggested that atomic interaction of metal structures with metal oxide supports has a significant role toward catalyst activity.43 In particular, metal modification stability is crucial for catalyst activity as it can reduce particle sintering and enhance the metal
support interaction and reaction activity enhancement.13,14 Our energy calculations show that the deformation of the R-Al2O3 support leading to surface Al
O bond perturbation can influence the atomic adsorption/desorption of O for reactions related to the Eley
Ridel or Langmuir
Hinshelwood mechanisms.50 In light of these facts, adsorption and deformation energies show that stable and catalytically active systems are composed of strong adlayer adsorption coupled with minor lattice deformation, that is, Ni, Fe, and NiFe modifications. From this understanding, the bimetallic NiFe system has the most favorable structural and energetic properties to enhance the catalytic activity. 3.3. Electronic Structure at the Metal Adlayer/r-Al2O3(0001) Interface. The understanding of the electronic structure at the interface, for example, the orientation of the molecular orbitals and its effect on electron transfer, is important for the rational design of catalysts.51,52 For this reason, electron density difference analysis for the Ni-, Fe-, NiFe-, and FeNi-modified RAl2O3(0001) systems is performed (Figure 4a
d). The density

Figure 4. Electron density differences for (a) Ni-, (b) Fe-, (c) NiFe-, and (d) FeNi-modified R-Al2O3(0001) (isosurface contours =
0.05e/ Å3 (yellow) and 0.05e/Å3 (blue)).

differences for each system are similar in electron transport characteristic. However, the arrangement of the orbital interactions is dependent on the atomic metal adlayer packing. Isosurface values for electron density difference plots are
0.05 and 0.05 e/Å3, where the accumulation and depletion of electrons are represented in blue and yellow, respectively (Figure 4). At the interface, the metal dz2 orbitals from the M2 adlayer redistribute electrons vertically to interact with the O surface atoms. There is a considerable amount of electron delocalization at the O px and py orbitals, which tilt toward the 3dz2 orbital of the metal adlayer. In addition to this, because the O2
ion has a closed-shell structure in R-Al2O3, the type of bonding at the interface is of a polar/ionic character. Electron activity is the most predominant at the interface, but not at the R-Al2O3 bulk. Across all cases, both the RAl2O3 and the M2 adlayer display localized electron distribution and activity at the core of the metal atom. On the other hand, the M1 adlayer does not interact significantly with the M2 adlayer or R-Al2O3 surface. The electron orbital configurations here allow for the sharing of electrons from the electronegative O to the positively charged metal atoms. The strong metal
support effect is known to enhance the robustness and activity of some heterogeneous catalysts, but the underlying reasons are not clear.53,54 The present results suggest that this effect is due to electron accumulation confined between the M2 metal adlayer and the topmost surface of R-Al2O3. The vertical dz2 orbitals contribute to the majority of the orbital interactions at the interface to facilitate electron activity; this leaves the t2g(dzx, dyz, or dxy) orbitals free to stabilize the metal modification. As a result, the periphery of metal modifications is free for adsorbate interaction and reactions to occur. The dependence of orbital directionality has been demonstrated for other isolated systems involving CO or H2 adsorption on Fe55 or Pt56
58 surfaces; however, the influence of metal oxide supports was not considered for these cases. Our work displays how the metal oxide can distort the d orbitals of the metal layer, leading to unfavorable reaction environments. The results show that the 13799

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Table 3. Mulliken Charge and Spin Analysis of the MModified r-Al2O3(0001) System monometallic adlayer Ni

atom

charge |q|

spin μ

adlayer

atom

charge |q|

spin μ

NiFe

0.79

Ni1


0.03

0.75

Ni


0.03

Ni2

0.28

1.18

Fe

0.3

3.11

O1 O2


0.86
0.88

0.17

O1 O2


0.88
0.88

0.15

Al

1.49

Ala

1.22

Oa Fe

a

bimetallic

Al

1.49

0.66

Fe1


0.01

2.89

Fe

0.02

Fe2

0.27

3.03

Ni

0.14

0.83

O1


0.89

0.48

O1


0.78

0.12

O2 Al


0.88 1.5

O2 Al


0.76 1.37

FeNi

3.01

Calculated clean R-Al2O3(0001) surface values.

horizontal and titled t2g orbitals of the metal layer are the most important as it composes the majority of the interfacial perimeter. The local distortion of the vertically aligned d orbitals at the metal layer induces a less favorable adsorption environment for molecules as the alignment of the horizontal d orbitals are affected. Therefore, systems with minimal distorted vertical d orbitals, such as the FeNi modification, are better for molecular adsorbate stability and the environment for reaction. Additionally, in relation to the aforementioned energetic analysis, it is shown that bimetallic modifications can offer a balance of system stability and favorable electronic interactions to promote a better reaction environment compared with monometallic catalysts. To further probe the influence of electron activity at the interfacial perimeter and its relationship with catalytic activity, electron spin and charge characteristic (Table 3) at the metal adlayer-modified R-Al2O3 interface is investigated with the socalled Mulliken population analysis.59 This analysis produces additional information about the nature of bonding at the interface between the metal adlayer and the R-Al2O3 surface. The trend from charge analysis shows that the M1 adlayer exhibits a neutral charge, whereas the M2 adlayer has positive charges of 0.28, 0.27, 0.30, and 0.17 |q| for Ni, Fe, NiFe, and FeNi modifications, respectively. Electron donation is the most efficient for the Ni modification, followed by the Fe and NiFe modifications toward the negatively charged O atoms at the RAl2O3 surface. In comparison, the neutral M1 adlayer is less influenced by the electronic perturbations at the interface. The intrinsic charge accumulation within the interface is attributed to the electron back-bonding through charge transfer between the Ni or Fe atoms and 3-fold O atoms on the R-Al2O3 surface. The interfacial O atom of R-Al2O3 increases in electronegativity from an initial bulk value of
0.61 |q| to
0.86,
0.89,
0.88, and
0.78 |q| after Ni, Fe, NiFe, and FeNi modification, respectively. Furthermore, the interfacial Al atom of the R-Al2O3 support experiences a positive charge increase from an initial bulk value of 1.22 |q| to 1.49, 1.50, 1.49, and 1.37 |q| after Ni, Fe, NiFe, and FeNi modifications, respectively. Therefore, the electronegativity enhancement of the O interfacial atom after metal adlayer modification is evidence of increased charge density that is promoted by metal cations. On the other hand,

the increase of cationic Al charge indicates that there is charge donation to the surface O atoms. Many reports speculate that the perimeter of the metal catalyst and its stability on the metal oxide support are extremely important for catalytic reactions.60
62 However, there are few reports that attempt to describe these features, especially for Feor Ni-based catalysts. We identify that these features emerge from charge-transfer effects at R-Al2O3, as the localized charge in this area can cause electron polarization at the interface. Such effects are clear for the Ni, Fe, and NiFe modifications and also have impacts on their energetic stabilities on the R-Al2O3 support. Such electrostatic polarization effects at the interface have been demonstrated for the single atomic metal deposition of Pd, Pt, Au, Cu, or Ag on R-Al2O3.63
65 Interestingly, these electronic effects also extend to monometallic or bimetallic adlayer deposition. However, there are differences for the bimetallic modifications as the combination of Fe and Ni leads to unfavorable metal lattice stability and interface formation. For instance, in relation to the aforementioned structural analysis section, the FeNi interface forms at the atop O site of R-Al2O3. Such a decrease of metal coordination with the R-Al2O3 surface leads to reduced electrostatic interactions, resulting in weaker interface stability, as predicted by energy analysis. Magnetic moments and spin density analyses are also performed to probe the spin-dependent properties of the metal adlayer modification. The average magnetic moment for the monometallic Ni adlayer is 0.97 μB and is consistent with the average literature value of 0.8 ( 0.4 μB,66
68 but higher than the bulk Ni value of 0.61 μB per atom.69 On the other hand, the average magnetic moment for the monometallic Fe adlayer is 2.96 μB per atom. This is consistent with the literature value of 3.4 ( 0.7 μB for an Fe cluster of six atoms, but higher than the bulk Fe value of 2.2 μB.70 Similarly, the lower magnetic moment displayed by the M2 monometallic metal adlayer (Table 3) is due to its increased coordination at the interface. When considered separately, this effect is more pronounced for Ni compared with Fe due to the higher atomic coordination of Ni(111) packing compared to that of Fe(110) packing. On the contrary, for the bimetallic NiFe or FeNi adlayers, the Fe and Ni atoms retain similar magnetic moments to their monometallic types of ∼3 and 1 μB per atom, respectively, with no significant spin quenching. Upon metal modification, the O1 atoms at the interface experience spin polarization in which the magnetic moments for the Ni, Fe, NiFe, and FeNi systems are 0.17, 0.48, 0.15, and 0.12 μB, respectively. The spin polarization and increased magnetic moment of the M2 adlayer at the interface contrasts with similar experimental reports of atomic O adsorption on the Fe or Ni surface.71
74 However, our results show that this effect may occur on oxygen-terminated metal oxides. The enhanced magnetic moment at the interface can be attributed to the geometric environment of the metal adlayer related to decreased coordination, increased symmetry, and reduced dimensionality.53,75 To illustrate the aforementioned magnetic moment features in the system, spin density illustrations are presented in Figure 5a
d, in which the electron density isosurfaces are plotted at 0.2 e/Å3. In parallel with the aforementioned spin analysis, the locations and magnitude of spin are plotted graphically to understand the electronic environment. The results indicate that the volume of the spin density for the Fe modification is larger than that of the Ni case and corresponds to the higher magnetic moments. This trend is also consistent with the bimetallic cases. The minor spin density leakage from the M2 13800

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bimetallic modifications experience significant interfacial M
M and Al
O
M bond expansion. Such bond expansion and lattice strain in metallic modifications are attributed to enhanced catalytic activity. Bimetallic Ni and Fe modifications display the key factors that signify catalytic activity as a balance of energetic stability, favorable d orbital directionality, spin quenching, and electron accumulation characteristics at the interface compared to the monometallic modifications. As a result, the metal modification has two different reaction regions, that is, at the interface and the metal surface. It is found that the perimeter size effect is related to the spin quenching of the metal modification, leading to lattice distortion at the interface. This can weaken adsorbate stability, promoting molecular diffusion and reaction at the interface, and promote the stable adsorption of molecules at the M1 upper surface adlayer. Figure 5. Spin densities of (a) Ni-, (b) Fe-, (c) NiFe-, and (d) FeNimodified R-Al2O3(0001) (isosurface contours = 0.2e/Å3).

adlayer toward the O1 atoms of R-Al2O3 illustrates the aforementioned spin polarization and charge redistribution of the M2
O bond at the interface. Our analysis also shows that spin quenching can affect catalyst activity as it can induce localized metal adlayer lattice expansion, leading to interfacial strain. The reason for this notion is because there is a connection between magnetic moment quenching and catalyst activity as metal adlayer lattice expansion can reduce metal f adsorbate π* back-donation, leading to metal
adsorbate bond weakening.76 Our results show that metal adlayers have two different spin environments where the magnetic moments of the M2 and M1 adlayers are enhanced and quenched, respectively. This implies that there are two different reaction environments where the M2 layer promotes diffusion as adsorbates are weakly bound, and the M1 layer promotes adsorbate stability, increasing its probability of reaction. This finding is pertinent for the explanation of the perimeter size effect14,77,78 for catalyst enhancement as the reason for this effect is still unclear. Our results suggest that spin quenching of the metal layer is influential to this effect as the M2 layer promotes the stable adsorption of molecules, whereas adsorbates are weakly bound to the M1 layer such that it can diffuse down to the perimeter sites for reaction. Additionally, in connection with the Mulliken charge and electron density difference analysis, the charge accumulation at the interfacial perimeter and d orbital directionality may also contribute to the perimeter size effect. Such electron accumulation can promote the transfer of electrons from the metal perimeter to the π* antibonding orbital of the molecular adsorbate, leading to internal bond weakening and a higher chance of reaction.

4. CONCLUSIONS DFT calculations have been utilized to investigate the electronic and structural characteristics of the interface between a metal adlayer and the R-Al2O3(0001) support and its connection with catalyst activity. It is shown that the interface of monometallic systems is formed on the hollow 3-fold O sites at the R-Al2O3 surface, whereas bimetallic modifications prefer O atop or bridge sites for interface formation. The strong metal
support interaction is most dominant for the monometallic Fe or Ni or bimetallic NiFe interface formed with R-Al2O3(0001) as these systems are most stable. Electronic and structural analyses show that interfaces are composed of polar/ionic bonds in which

’ AUTHOR INFORMATION Corresponding Author

*Tel: +61-2-9385-4429. Fax: +61-2-9385-5956. E-mail: a.yu@ unsw.edu.au.

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