The impact of metal dopants on the properties of nZVI: a theoretical study

The substitution of Fe with metal dopants shows potential for enhancing the wastewater remediation performance of nanoscale zero-valent iron (nZVI). However, the specific roles and impacts of these dopants remain unclear. To address this knowledge gap, we employed density functional theory (DFT) to investigate metal-doped nZVI on stepped surfaces. Four widely used metal dopants (Ag, Cu, Ni, and Pd) were investigated by replacing Fe atoms at the edge of the stepped surface. Previous research has indicated that these Fe atoms exhibit chemical reactivity and are vulnerable to water oxidation. Our DFT calculations revealed that the replacement of Fe atoms on the edge of the stepped surface is energetically more favorable than that on the flat Fe(110) surface. Our results shed light on the effects of metal dopants on the surface properties of nZVI. Notably, the replacement of Fe atoms with a metal dopant generally led to weaker molecular and dissociated water adsorption across all systems. The results from this study enhance our understanding of the complex interplay between dopants and the surface properties of nZVI, offering theoretical guidance for the development and optimization of metal-doped nZVI for efficient and sustainable wastewater remediation applications.


Introduction
Nanoscale zero-valent iron (nZVI) has been widely used to remediate contaminated groundwater because of its powerful reductive properties.However, nZVI tends to be oxidized quickly upon contact with water, leading to passivation and reduced performance towards the degradation of targeted contaminants degradation [1][2][3][4][5].This water oxidation process ultimately limits the lifespan of nZVI for remediation applications [6][7][8][9][10].Therefore, bimetallic nZVI through second metal doping is gaining popularity, especially as it has been shown to produce more complete, less toxic products of chloroethenes [11].The metal dopant is usually Cu, Ni, Ag, or Pd [12,13].The doping of the second metal can form a galvanic cell where the Fe acts as an anode and the second metal acts as a cathode, therefore becoming more reactive [11,14,15].Additionally, the metal dopant helps facilitate hydrogen to be used for the reduction of contaminants [11,15].
Out of all the metals, Pd, Ni and Cu-doped nZVI have been shown to have extremely high removal efficiency [9,13,16].Valiyeva et al found that the doping of Ni to nZVI enhanced its reactivity and enabled the conversion of nitrates into gaseous nitrogen, which is ecologically friendly [7].Huang et al found that Ni is a very effective electron transfer medium and has a beneficial effect on the adsorption of hydrogen molecules, which causes the formation of hydrides on the surface of nZVI [17].The produced hydrogen can then be used to reduce contaminants.Liu et al found that the Ni/Fe bimetallic system was four times more efficient in remediating chlorinated contaminants than plain nZVI [13].Venkateshaiah et al studied Pd/nZVI, Ni/nZVI, Ag/nZVI and Cu/nZVI [9].They found that both the Pd/nZVI and Ni/nZVI bimetallic nanoparticles exhibited better degradation efficiencies of cis-dichloroethene (DCE), trichloroethene (TCE), and tetrachloroethene (PCE) compared to Ag/nZVI and Cu/nZVI.However, when Cwiertny et al investigated Au/nZVI, Co/nZVI, Cu/nZVI, Ni/nZVI, Pd/nZVI, and Pt/nZVI bimetallic nanoparticles, they found that not all metal dopants enhanced the reaction rate of TCE and no clear trend was observed [18].
To reveal the role of metal dopants at the atomic level, theoretical studies have become imperative.However, such investigations are very few.Reddy et al used the density functional theory (DFT) to investigate the Fe 3 Ni(111) surface to understand the role of Ni dopants in the adsorption of TCE [19].Their theoretical results suggest that hydrogen preferentially adsorbs on the hollow-Ni site with hydrogen interacting with two Fe atoms and one Ni atom.From the Bader charge analysis, H adsorption led to a charge transfer of 0.31 e − to hydrogen.This indicates that Ni atoms play an important role in the hydrogenation of TCE.Additionally, different bimetallic alloys have been investigated [20][21][22][23][24][25].The theoretical results reveal that the replacement processes of Fe atoms by a metal dopant are highly favorable.Additionally, introducing a metal dopant has a notable impact on the electronic properties, the specific outcome depending on the identity of the metal dopant incorporated.Notably, the formation of bimetallic nZVI involves dissolution, which further supports the idea that Fe atoms are more likely to be replaced.The dissolution process suggests that Fe atoms are involved in the replacement, while the metal dopant is less actively engaged in the reaction [26,27].Yan also suggested that Pd prefers to migrate into Fe because it is thermodynamically allowed and can stabilize Pd based on their experimental results [28].Understanding these dynamics is crucial for optimizing the synthesis and performance of bimetallic nanoparticles in various applications [6,16,[29][30][31][32].
To our knowledge, no theoretical research has been conducted on metal-doped stepped Fe surfaces and their interaction with water.The stepped surfaces can be used to replicate the properties of nZVI because Fe atoms with low coordination numbers (CNs) at the edge of the stepped surface are more reactive than those on the flat Fe (110) surface [33][34][35].These reactive Fe atoms on the stepped surface easily adsorb water molecules and dissociate them with a low-energy barrier.To this end, understanding the role of metal dopants in nZVI and the replacement of reactive Fe atoms at the edge of the stepped surface is essential.
In this study, we examined the Ag, Cu, Ni, and Pd doped Fe(210), Fe(211), and Fe(110) surfaces to represent bimetallic nZVI.Both molecular and dissociation water adsorption were investigated to determine the impact of the metal dopant.The electronic properties were also analyzed to gain a better understanding of the role of metal dopants.This study is a fundamental step in understanding bimetallic nZVI nanoparticles to further enhance this technology for the degradation of contaminants.

Computational details
All DFT computations were performed using the Vienna Ab initio Simulation Package (VASP) based on the projector augmented wave (PAW) method with consideration of spin polarization [36,37].Vaspkit was used to build atomic models and post-process the data [38].The optPBE exchange-correlation energy was employed considering the vdW interaction correction [39][40][41][42].The OptPBE vdW correction method was chosen because it has been shown in our previous studies to accurately represent the electronic and magnetic properties of Fe [35].The electron-ion interactions were described using the PAW potentials, with the 3s 2 3p 6 3d 7 4s 1 , 2s 2 2p 4 , 1s 1 , 4s 2 3d 9 , 4s 2 3d 8 , 4d 10 and 4d 10 5s 1 treated as valence electrons of Fe, O, H, Cu, Ni, Pd, and Ag, respectively [43].A plane-wave basis set with a cutoff kinetic energy of 520 eV was used.Gamma-centered k-point meshes with a reciprocal space resolution of 2π × 0.04 Å -1 and 2π × 0.02 Å -1 were utilized for structural optimization and static self-consistent calculations, respectively.The convergence criteria for the self-consistent electronic and structural optimization loops were set to 1 × 10 −5 eV and 1 × 10 −3 eV/Å, respectively.
The (110), (211), and (210) surfaces were modeled using slab models separated by a vacuum region greater than 15 Å to avoid interactions between the surfaces of neighboring slabs along the z-direction.The interlayer distance determines the number of atomic layers.The (210) surface had the smallest interlayer distance of a 0 / √ 20, where a 0 is the lattice constant.Thus, 12 atomic layers were used in this study.The (211) surface has a slightly larger interlayer distance of a 0 / √ 6, hence seven atomic layers were used.For comparison, the interlayer distance of the (110) surface was a 0 / √ 2. Thus, only four atomic layers were used.When the surface structures were optimized, the bottom two, three, and six layers were fixed at the bulk position for the (110), (211), and (210) surfaces, respectively.The positions of the atoms in the other topmost layers and adsorbates were allowed to relax.
The replacement energy (E rpm ) was calculated by: where N is the number of Fe atoms replaced by the metal dopant per unit cell, and E bimetallic , E surf , E FeBulk and E Metal Bulk are the energies of the bimetallic surface, clean Fe surface, Fe bulk, and metal dopant bulk, respectively.The water adsorption energy (∆E H2O ) was calculated using the formulas: where N is the number of water molecules per unit cell, and E ad , E surf and E H2O are the energies of the surface with adsorbed water, surface, and water molecules, respectively.Bader charge analysis was conducted to understand the electron transfer between Fe and the metal dopant [44].

Replacement energies
To assess the energetic feasibility of the formation of the bimetallic nZVI, we first calculated the replacement energy of Fe atoms with the smallest CN value by a metal dopant.We investigated the replacement of one, two and four Fe atoms in each surface cell, which structures are shown in figure S1.The calculated replace energies are listed in table 1.Our findings revealed that all the systems were energetically favorable, except for the replacement of Fe atoms by 1, 2, and 4 Ag atoms on the Fe(110) surface and by 4 Cu atoms on the Fe(110) surface.Notably, Fe atoms with lower CN values are easier to be replaced.The only exception is that of Ni.This indicates that in the preparation of bimetallic nZVI, replacement is likely to occur at lower CN Fe atoms.Moreover, based on the definition of the replacement energy shown in equation ( 1), the cohesive energy of the metal also plays a crucial role in determining the overall trend of the replacement energies.The complexity of these considerations explains why the trends in bimetallic nanoparticles are intricate and require careful examination.Thus, our study highlights the significance of various factors in determining the favorability of energy replacements.Understanding the interplay between these factors is vital for predicting and optimizing the behavior of bimetallic nanoparticles.Our findings also indicate that this average multiple-atom replacement is similar to the replacement of only one Fe atom on the surface.On Fe(110), the replacement of Pd and Ni are much easier than that of Ag and Cu.On the stepped surface, the replacement of Pd is most energetically preferred, followed by Ni.On Fe(210), the surface Fe atoms at the edge of the step are more easily replaced by Pd and Ag.Given the addition of trace amounts of a metal dopant to create bimetallic nZVI, with increasing dosage leading to declining reactivity [13,28,[45][46][47][48], we focused on surfaces with one replaced Fe atom in each surface cell.The optimized atomic structures of the bimetallic Fe(110), ( 211) and (210) surfaces with the corresponding replacement energies are shown in figure 1.

Charge and partial density of states (PDOS) analysis
The electronic properties of nZVI have been proposed to play a crucial role in wastewater remediation since its reduction capability is one of the most important factors affecting its overall performance.The replacement of the metal dopant introduces distinct properties, altering the reducing power of the system.Instead of Fe donating electrons to the pollutant, it is now the metal dopant that takes on this role.Consequently, a galvanic cell scenario arises, with the more reactive metal losing electrons to the more noble metals [13,28].Based on the metal reactivity trend, we anticipate that Ag, Cu, Ni, and Pd will gain electrons.To understand the charge transfer within the bimetallic nZVI after the replacement, the Bader charges of the metal dopants were calculated and shown in figure 1. Bader charge was calculated based on the charge density of each atom.The atom was divided according to the zero-flux surface, on which the charge density is a minimum perpendicular to the surface.The Bader charge analysis can offer valuable information to infer and understand reactivity in a qualitative manner.On the Fe(210) surface, the metal dopant gained electrons for all systems, except for Ni.In contrast, on the Fe(110) surface, Ni and Pd gained electrons, while Ag and Cu lost electrons.Similarly, on the Fe(211) surface, Cu and Ni lost electrons, whereas Ag and Pd gained electrons.
To gain further insights into the properties of the metal dopant and its interaction with the Fe atoms, the partial density of states (PDOS) was calculated.Figure 2 shows the PDOS of the metal dopant and the surface Fe atoms bonded with the dopant on Fe(210).The corresponding PDOS images on Fe(110) and Fe(211) are shown in figures S2 and S3 since they are similar to that on Fe(210).The distinct PDOS images of metal dopants may explain why they exhibit varying attributes when interacting with certain pollutants, as reported in experimental studies [9,13,18].All the PDOS images on three Fe surfaces suggest that there is a strong bonding between Fe and Pd or Ni dopants because their d states are well overlapped.As a comparison, the PDOS images indicate that the Ag has the weakest bonding strength with Fe atoms.Our analysis of the Bader charge and PDOS suggested a strong interaction between Fe and Pd atoms.The overlap of the states of Interestingly, this trend from the PDOS analysis is different from that either from the replacement energies or from the Bader charge analysis.On the Fe(210) surface, the replacement energy shows a different trend of Pd > Ag > Ni > Cu.Accordingly, Pd, Ag and Cu gain a similar amount of charge from the neighboring Fe atoms.And Ni donates electrons to the neighboring Fe atoms.Such trend discrepancies imply that other factors also play a vital role in determining charge transfer between the metal dopant and Fe atoms.One most possible factor can be the different coordination environments of the metal dopants caused by the surface structures.On the Fe(110) surface, the CN value of the dopant is 6.As a comparison, the CN values of the dopants on Fe(211) and (210) are 5 and 4, respectively.Another possible factor is the size of the dopant.The Ag replacement becoming increasingly more energetically favorable could be due to its large size and open Fe(210) surface [49].Therefore, more surface Fe atoms can effectively interact with the Ag atoms.This was also observed for Pd on Fe(210), which has a replacement energy more favorable at ∼0.20 eV compared to Pd on Fe(211) and Fe(110).

Molecular water adsorption
Previous studies have revealed that the introduction of a metal dopant can significantly affect the lifetime of nZVI [11,14,28].To determine the impact of bimetallic nZVI on corrosion, molecular water adsorption, which is the first step in water corrosion on nZVI, was investigated.Our previous DFT results demonstrated that the top site is the most favorable adsorption site for molecular water [34].Thus, the water molecule was placed on the top site of both the Fe atom and metal dopant atom (figure 3).Our results revealed that even with the doping of a metal dopant, the water molecule exhibited a preference for adsorption onto the top site of the Fe atom (table S1).In comparison, the adsorption energies of molecular water on the pristine Fe(110), (211) and (210) surfaces are −0.43 eV, −0.60 eV and −0.63 eV, respectively [34].Across all systems, the adsorption of molecular water was weaker than that of the clean Fe(211) and Fe(210) surfaces, except for the Fe atoms on the Ni/Fe(211) and Cu/Fe(211) surfaces, which displayed adsorption energies of −0.65 eV and −0.63 eV, respectively.On Fe(110), it was found that the molecular water prefers to adsorb on the Fe atom with the same adsorption energy for all bimetallic (110) surfaces.The only exception is Ag-doped Fe(110) surface, which has a slightly weaker adsorption strength of −0.41 eV, indicating that the type of metal dopants on the flat Fe(110) surface had a small impact on molecular water adsorption properties in comparison to that on the stepped Fe surfaces.
The distances between the oxygen of the water and the Fe atom on the Fe(110) are similar.On the Fe(211) bimetallic surface, the corresponding O-Fe bond lengths follow the trend of Ag (2.29 Å) > Pd (2.21 Å) > Cu (2.18 Å) > Ni (2.17 Å) = Fe (2.17 Å).This trend is in agreement with the adsorption energy, with the water molecule closer to the Fe atom having stronger adsorption.Interestingly on the Fe(210) surface, the distance between the water molecule and the Fe atom did not correspond to adsorption energy, the adsorption energy followed the trend Fe (−0.63 eV) > Cu (−0.58 eV) > Pd (−0.54 eV) > Ag (−0.50 eV) = Ni (−0.50 eV), whereas the distance followed the trend Pd (2.22 Å) > Ag (2.21 Å) > Fe (2.20 Å) > Cu (2.19 Å) = Ni (2.19 Å), indicating that the distance between water and the metal dopant on the Fe(210) surface is also considerably affected by the size of the metal dopant.
While molecular water preferred the adsorption on the Fe atom, we also considered the hydrophobicity of molecular water adsorption on the metal dopant (see table S2 and figure S4).The weakest adsorption was found to occur on the Cu/Fe(110) with −0.20 eV, followed by Ni/Fe(110) with −0.22 eV, Ag/Fe(110) with  that Fe atoms with the smallest CN value on the stepped surface are responsible for the fast oxidation of nZVI by water [34].Since the undoped surface have a higher adsorption strength with the molecular water, the replacement of Fe atoms with Ag, Cu, Ni, or Pd can significantly blocks water adsorption on the edge of the stepped surface.This further underlines the influence of the presence of a metal dopant on adsorption properties.

Dissociative water adsorption
The dissociative water adsorption on the Fe (110), ( 211) and (210) surfaces was then theoretically investigated.All the detailed adsorption properties at different adsorption sites are provided in the supplementary document (tables S3-S5, figures S5-S16).To understand the role of the metal dopant, the adsorbed OH directly interacted with the metal dopant (figure 4).As a comparison, the adsorption of OH at the three-fold hollow site only with Fe atoms was also considered (figure 5).On the Fe(110) surface, OH had weaker adsorption when interacting with the metal dopant, following a trend of Cu/Fe(110) with −1.44 eV, Ni/Fe(110) with −1.37 eV, and Ag/Fe(110) with −1.27 eV.The OH adsorption on Pd/Fe(110) is unfavorable when interacting with Pd, leading to migration to the three-fold hollow site involving three Fe atoms.On the three fold hollow site only with Fe atoms, more favorable adsorption was observed on Ni and Cu doped Fe(110) surfaces, with energy values of −1.67 eV and −1.70 eV, respectively.Conversely, adsorption on Fe atom sites in Pd/Fe(110) and Ag/Fe(110) exhibited weaker interactions, with energy values of −1.50 eV and −1.49eV, in contrast to the adsorption energy of −1.66 eV on pristine Fe(110).
On Fe(210), the OH adsorption strength on the nearby three-fold Fe sites follows the trend of Ni < Pd < Ag < Cu.The most favorable H adsorption site was on the three-fold Fe site adjacent to the metal dopant.Additionally, OH predominantly interacts with the two Fe atoms when the metal dopant is Pd or Ag.On the other hand, H showed the most favorable adsorption on the three-fold Fe hollow site adjacent to the metal dopant for Cu.However, the three-fold hollow sites were found to be slightly more favorable for Ag, Ni, and Pd.Interestingly, the Cu doped Fe(210) surface exhibited very similar energies to the OH adsorbed on the three-fold hollow Fe site, whereas the Ag and Pd Fe(210) surfaces showed significantly more favorable OH adsorption on the three-fold Fe atom site.This is in contrast to Ni/Fe(210), where OH adsorption onto the Fe and Ni three-fold hollow sites was the most favorable.On Fe(211), the adsorption on the bridge site, where OH interacts with Fe and the metal dopant, was found to be less favorable.The trend for OH adsorption followed Cu < Pd < Ni < Ag, whereas OH adsorption on the Fe bridge site demonstrated a trend similar to that of Cu < Pd < Ag < Ni.However, for all bimetallic surfaces except Ni/Fe(211), H prefers the three-fold hollow Fe adsorption site.As expected, Pd/Fe(210) and Ag/Fe(210) exhibited the weakest adsorptions, followed by Cu/Fe(210) and Ni/Fe(210).Notably, the three-fold Fe hollow site next to the metal dopant and the (446) Fe atom site are the most preferred site for H adsorption.The OH adsorption strength on bimetallic Fe(210) follows the trend of Cu > Ag > Pd > Ni.
On bimetallic Fe(211), The sites interacting with the metal dopant and those interacting with nearby Fe atoms were considered to gauge the impact of the metal dopant on the adjacent Fe atoms.Notably, adsorption on the bridge site, where OH interacts with both Fe and the metal dopant, was found to be less favorable.The trend for OH adsorption followed Cu/Fe(211) with the weakest adsorption, followed by Pd/Fe(211), Ag/Fe(211), and Ni/Fe(211) with the strongest adsorption.The OH group was also adsorbed on two Fe atom sites on the Fe(211) surface.Ni/Fe(211), Cu/Fe(211), and Ag/Fe(211) were found to have similar adsorption energies of −1.73 eV.−1.72 eV and −1.72 eV, respectively.In comparison, Pd/Fe(211) was less favorable, with an adsorption energy of −1.63 eV.The hydrogen preferred to adsorb at the three-fold hollow This H adsorption is in agreement with the theoretical results reported by Reddy et al [19].
These findings shed significant light on the interplay between bimetallic surfaces and water adsorption behaviors, providing valuable insights into the implications of second-metal inclusions on Fe(210) and Fe(211) surfaces.Among all the metal dopants investigated, the Pd dopant offers the best protection to the Fe (211) and (210) surfaces since the weakest dissociative water adsorption was found there.
Based on our Bader charge analysis results, Pd gain electrons in three surfaces, which suggests an increased reduction capability of Pd due to the charge transfer.This may explain the experimental results that Pd/nZVI has the outstanding performance in remediating contaminants [9,14,54,56].In comparison, Ag only gains electrons on the stepped surfaces of (211) and (210).Ni only gains electrons in the flat (110) surface and Cu only gains electrons in the stepped (210) surface.This shows that the morphology of surface can significantly impact the electron transfer of the metal dopant.As a result, not all the metal dopants can have the enhanced the remediation capability, as reported by Cwiertny et al [18].
Additionally, using the dissociative water adsorption of bimetallic Fe(211) as an example, the adsorption configuration and energies with different H adsorption sites (figure 6).A higher adsorption energy on all bimetallic Fe(211) surfaces indicates a weaker hydrogen adsorption strength on site when H is associated with the metal dopant except for the Ni/Fe(211) surface.On the bimetallic Fe(210) surface, H exhibited a pronounced preference for either the three-fold hollow Fe site in close proximity to the metal dopant or the stepped hollow site.This weaker H adsorption on the metal dopant agrees with the experimental conclusion that the adsorbed H species can easily be desorbed to reduce contaminants.The strong H adsorption on Ni/Fe(211) also agrees with previous studies that H adsorbs more strongly on the Ni bimetallic surface [57].On the flat Fe(110) surface it was found that the hydrogen would not favorably adsorb on the bimetallic surfaces for all but Ni/Fe(110), with the hydrogen having weaker adsorption when interacting with the Ni.The great match, therefore, suggests that stepped surface plays an extremely important role in nZVI for their practical applications.
The outcomes of our study reveal a consistent preference of water molecules to adsorb onto the Fe atom rather than the metal dopant.A divergence was observed solely in the case of Ni and Cu doped Fe(211), resulting in slightly favorable adsorption of molecular water compared to the undoped surface.Therefore, the introduction of the metal dopant renders the surface more resistant to corrosion due to the comparatively weaker adsorption of water molecules on this metal, consequently offering protection to the stepped site.
Our findings illustrate the impressive reactivity of bimetallic nZVI, attributed to the highly reactive uncovered Fe atoms with strong reducing capabilities, facilitating the generation of adsorbed H through iron corrosion.However, the introduction of a metal dopant resulted in relatively weaker adsorption capacities for both molecular and dissociative water adsorption.This characteristic could potentially enhance the selectivity of the material towards contaminants rather than water, with the exception of molecular adsorption on the Ni and Cu Fe(211) surfaces.This observation sheds light on the reason behind the decrease in efficiency observed at higher loads of the second bimetallic nZVI [11,46,58].

Conclusion
In this study, we applied DFT to understand the properties of bimetallic stepped Fe(210) and Fe(211) surfaces in comparison with the flat Fe(110) surface.The surface atoms at the edge of the stepped Fe(210) and (211) surface with low CN can represent the active site of nZVI, which are easy to be oxidized by water.Our DFT results reveal that these active sites of nZVI can be thermodynamically replaced by the metal dopants including Ni, Cu, Pd and Ag as evidenced by the negative replacement energies.Moreover, the introduction of the metal dopants can weaken the molecular adsorption strength of water on the stepped surface, except the Cu and Ni doped Fe(211) surface.For dissociative water adsorption on both Fe surfaces, the doping of a metal dopant led to a reduction in adsorption strength.However, the adsorption configurations vary in terms of the metal dopants.The Bader charge analysis demonstrates that the Pd dopants can gain electrons from Fe surface to enhance its reduction capability.However, the charge transfer between other metal dopants and the surface is greatly affected by the morphology of surfaces.The introduction of the metal dopants except Ni/Fe(211) can also facilitate the desorption of H from the dissociative adsorption of water, which may benefit the reduction of the contaminants.
In sum, our DFT results suggest that (1) the Pd-doped bimetallic surface has emerged as the most promising candidate among considered systems with improved reduction capabilities since Pd dopants effectively extract electrons from the surface Fe atom; (2) the Pd dopant can effectively protect nZVI from water corrosion due to its weaker adsorption of both molecular and dissociative water; and (3) the Ni-doped nZVI can be the best candidate when the hydrogen is used for reducing pollutants since Ni dopants can facilitate hydrogen desorption.Our DFT findings shed light on the complex interplay between metal dopants and Fe surfaces, therefore, providing valuable implications for understanding surface properties of bimetallic nZVI for wastewater remediation.

Figure 2 .
Figure 2. Partial density of states of the d states of metal dopant and the Fe atom bonded with the dopant on Fe(210)-(black = Fe, blue = metal dopant).

Table 1 .
Calculated replacement energies (Erpm) in eV of the metal dopant on the Fe(110), Fe(211), and Fe(210) surfaces with different numbers (No.) of replaced Fe atoms.Fe and Pd d electrons indicates strong coupling between their d states, leading to a more favorable replacement energy.This is shown by the 0.3 e − gained on the Pd-doped Fe(110) bimetallic surface and the 0.2 e − gained on both the Pd-doped Fe(211) and Fe(210) bimetallic surfaces.