Metal D-states Research Articles

Theoretical Study of Catalytic Performance of Pristine M2C and Oxygen-Functionalized M2CO2 MXenes as Cathodes for Li-N2 Batteries.

Li-N2 batteries are a promising platform for electrochemical energy storage, but their performance is limited by the low activity of the cathode catalysts. In this work, density functional theory was used to study the catalytic activity of the pristine M2C and oxygen-functionalized M2CO2 MXenes (M = Sc, Ti, and V) as cathodes for Li-N2 batteries. The calculated results suggest that the pristine M2C MXenes (M = Sc, Ti, and V) show high electrical conductivity due to the Fermi level crossing the metal 3d states. The stable adsorption of N2 occurs on M2C MXenes via a side-on model and strengthens gradually with decreasing metal atomic number. Furthermore, the kinetics of N2 dissociation can be significantly accelerated by the coadsorption of Li on M2C MXenes. However, adsorption and dissociation of N2 on the M2CO2 surfaces are too difficult to occur due to strong electrostatic repulsion. The Li-mediated nitrogen reduction reaction during discharge proceeds favorably via (N + N)* → (LiN + N)* → (LiN + LiN)* → (Li2N + LiN)* → (Li2N + Li2N)* → (Li3N + Li2N)* → (Li3N + Li3N)* to form two isolated Li3N* on M2C MXenes. The calculated charge-discharge overpotentials decrease in the order of Sc2C < Ti2C < V2C. Notably, the Sc2C MXene has great potential as a cathode catalyst for Li-N2 batteries because of its high electrical conductivity, strong N2 adsorption, favorable Li-mediated N2 dissociation, and ultralow discharging, charging, and total overpotentials (0.07, 0.06, and 0.13 V). This study offers a theoretical foundation for future research on Li-N2 batteries.

Insights into the Fe3+ Doping Effects on the Structure and Electron Distribution of Cr2O3 Nanoparticles.

Herein, we carefully investigated the Fe3+ doping effects on the structure and electron distribution of Cr2O3 nanoparticles using X-ray diffraction analysis (XRD), maximum entropy method (MEM), and density functional theory (DFT) calculations. We showed that increasing the Fe doping induces an enlargement in the axial ratio of c/a, which is associated with an anisotropic expansion of the unit cell. We found that as Fe3+ replaces Cr in the Cr2O3 lattice, it caused a higher interaction between the metal 3d states and the oxygen 2p states, which led to a slight increase in the Cr/Fe-O1 bond length followed by an opposite effect for the Cr/Fe-O2 bonds. Our results also suggest that the excitations characterize a well-localized bandgap region from occupied Cr d to unoccupied Fe d states. The Cr2O3 and Fe-doped Cr2O3 nanoparticles behave as Mott-Hubbard insulators due to their band gap being in the d-d gap, and Cr 3d orbitals dominate the conduction band. These findings suggest that the magnitude and the character of the electronic density near the O atom bonds in Cr2O3 nanoparticles are modulated by the Cr-Cr distances until its stabilization at the induced quasi-equilibrium of the Cr2O3 lattice when the Fe3+ doping values reaches the saturation level range.

High-throughput screening of dual-atom doped PC6 electrocatalysts for efficient CO2 electrochemical reduction to CH4 by breaking scaling relations

The catalytic activity of the CO2 electrochemical reduction reaction (CO2RR), which is regarded as one of the most promising approach to tackle the current excess carbon emissions problem, is significantly restricted by the scaling relationship between adsorption energy of reaction intermediates. Here, inspired by the concept of multiple active centers, we designed 36 dual-atom catalysts in PC6 monolayer, including 8 homonuclear (M2@PC6, M=Cr, Mn, Fe, Co, Ni, Cu, Pd, and Ag) and 28 heteronuclear (M1M2@PC6) catalysts, to obtain efficient CO2RR catalysts by breaking the inherent limitation of linear scaling relations. After several rounds of screening for catalysts with stability, activity, selectivity through density functional theory (DFT) study, Mn2@PC6 was identified as a promising candidate for deep reduction of CO2 to CH4 with a low limiting potential of -0.31V (vs RHE). The projected density of states analysis demonstrates that the candidates break the linear scaling relationship stemming from the apparent mixing of the O-2p states and metal-3d states. The apparent change of spin polarization and asymmetric charge distribution on the active site of Mn2@PC6 play a crucial role in its superior CO2RR catalytic performance. This work provides a potential strategy from the theoretical perspective for rational design of efficient electrocatalyst by introduction of dual active centers to break the scaling relationship of intermediates.

A comparative study on the stability of the furfural molecule on the low index Ni, Pd and Pt surfaces.

We present a comparative density functional theory investigation of the furfural (Ff) molecule on the low index Ni, Pd and Pt surfaces to understand its geometrical and electronic properties to gain mechanistic insights into the experimentally measured catalytic reactivities of these metal catalysts. We show that the number of metal d-states, which hybridize with the nearest C and O p-orbitals of the Ff molecule, can be used to explain the stability of the Ff molecule on these surfaces. We find that the hybridization between atoms with higher electronegativity and the metal d-states plays a crucial role in determining the stability of these systems. Furthermore, we also find electron transfer from metal to the Ff molecule on the Ni and Pd surfaces, with a reverse process occurring on the Pt surface.

In-Situ X-Ray Absorption Spectroscopy Study of Electrocatalytic Reduction of Carbon Dioxide with Molybdenum Disulfide

Electrocatalytic reactions offer alternative routes for the conversion and storage of energy. For instance, the electrocatalytic reduction of carbon dioxide could lead to energy-rich syngas, methanol, or even hydrocarbons. Layered transition metal dichalcogenide nanomaterials, such as MoS2, have emerged as electrocatalysts towards CO2 reduction with high efficiency and low overpotentials. However, the underlying chemical and electronic states defining the catalytic activity have not yet been completely defined. These key states can be probed for both metals and ligands in selected MX2 (M=W, Mo; X=S, Se) using synchrotron-based X-ray absorption spectroscopy (XAS). In recent work we performed in-situ XAS measurements at Mo K-edge and S K-edge for MoS2 nanosheets. Measurements at the S K-edge showed drastic changes at the pre-edge and edge indicating changes in number of available unoccupied states and Zeff respectively. On the other hand, only a small shift of the Mo K-edge was observed. Metal d-states are found to be the active site for catalysis, yet the ligand K-edge presents a better method to examine catalytic processes. Since the pre-edge reflects a transition to metal-ligand hybridized states, its intensity and position gives direct insight into the activity and changes of d-states. Given the vast compositional space of metal and ligand for MX2, the knowledge of how d-state activity will serve to pave the way to a better study of metal and chalcogenide combinations to improve the electrocatalytic performance. Figure 1

Towards controlling the reversibility of anionic redox in transition metal oxides for high-energy Li-ion positive electrodes

Transition metal d-states and oxygen p-states overlap and oxygen lattice integrity dictates the oxygen redox reversibility in metal substituted Li2RuO3 positive electrodes for Li-ion batteries.

Theory-Guided Machine Learning Finds Geometric Structure-Property Relationships for Chemisorption on Subsurface Alloys

Summary Developing physically transparent and quantitatively accurate models that relate the chemical interaction (chemisorption strength) between an adsorbate and a solid surface to the adsorption site's geometry is critical for our understanding of catalysis, corrosion, and electrochemistry. We develop a theory-guided machine-learning (ML) approach, which uses an interpretable class of ML models called generalized additive models (iGAM models), to discover predictive structure-property models that can quantify and explain the link between the geometric structure of an adsorption site and the chemisorption strength. We demonstrate the approach through several case studies, where we analyze the chemisorption strength of chemically distinct adsorbates (O, OH, S, and Cl) on subsurface metal alloy surfaces subjected to various strain- and ligand-induced changes in the local geometric structure. By comparing the ML-derived chemisorption models to previously established electronic-structure models, we clarify the critical physical parameters that control the chemisorption process on metal surfaces.

Unveiling the adsorption properties of 3d, 4d, and 5d metal adatoms on the MoS2 monolayer: A DFT-D3 investigation

The adsorption of metal (M) adatoms on two-dimensional (2D) transition-metal dichalcogenides (TMDs) monolayers have been used to tune the physical-chemical properties of 2D TMDs such as MoS2, however, our atomistic understanding of the binding mechanism, site preference, charge transfer, diffusion barriers, etc., of metal adatoms on MoS2 is far from satisfactory. Thus, we report spin-polarized density functional theory calculations within the D3 van der Waals (vdW) correction to characterize the adsorption properties of 15 metal adatoms (3d, 4d, 5d) on MoS2 at low-coverage limit. All adatoms can be separated into two groups, namely, Fe, Co, Ni, Ru, Rh, Os, Ir, and Pt bind covalently to the MoS2 monolayer on the topMo site, where the binding mechanism can be explained by the combination of two factors, namely, (i) distortions within the MoS2 monolayers, which breaks the degeneracy of the electronic states and reduces their magnitude near the valence band maximum, and (ii) strong hybridization between the partially unoccupied d- and Mo dz2-states. In contrast, Pd has a strong preference for the hollow site compared with the topMo site, however, Cu, Zn, Ag, Cd, and Hg adatoms bind to the topMo and hollow sites with nearly the same energy (differences less than 0.10eV) due to the weak hybridization of the fully occupied metal d-states, while Au binds preferentially on the topS site. Thus, the M-MoS2 interactions are dominated by covalent and van der Waals interactions, which shows strong and weak dependence on the adsorption site preference, respectively.

A Perspective on Studying Electronic Structure of Batteries through Soft X-ray Spectroscopy

Author(s): Zhuo, ZQ; Pan, F; Yang, WL | Abstract: Understanding electronic structure is crucial to enhance the battery performance. Soft X-ray spectroscopy (SXS) is one of the most effective methods to provide direct probe of electronic states. Here, spectroscopic measurements of transition metal 3d and oxygen 2p states are simply reviewed. Then, we mainly focus on the perspective of the development direction of modern SXS techniques. Although the true power of recently developed high efficiency mapping of resonant inelastic X-ray scattering (mRIXS) has been apparent for materials and chemistry studies, great challenges remain for mRIXS spectroscopic interpretation, and the understanding of the battery materials on novel redox activities remains elusive.

Direct Observation of Cr3+ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry.

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr3+ impurities in Al2O3 with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr3+ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L3 and L2 edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms.

The half-metallicity of Co2FeGe full Heusler alloy in (001) thin film: First principles study

The electronic and magnetic properties of the Co2FeGe full Heusler alloy in (001) thin film are investigated using the first-principles electronic structure calculations within the density functional theory. We employ various exchange correlation functionals including the local density approximation (LDA), the generalized gradient approximation (GGA), and the additional +U corrections for strong on-site Coulomb interaction of transition metal 3d states, aiming to examine the correlation effect on the electronic structures which determine the spin gap and thus the half-metallicity. Our results reveal that the Co2FeGe thin film is metallic in both LDA and GGA, while the +U correction opens up the spin gap for spin minority channel in GGA+U but not in LDA+U in contrast to its bulk alloy which is predicted to be half-metallic in both LDA+U and GGA+U approaches with total spin magnetic moment of 6 μ B . It is found that the surface states developed around the Fermi level and the enhanced 3d e g -t2g band splitting for the spin minority channel due to the correlation effect play critical roles to determine the emergence of the half-metallicity.

Active basal plane catalytic activity and conductivity in Zn doped MoS2 nanosheets for efficient hydrogen evolution

Transition metal dichalcogenides based electrocatalysts have been proposed as substitutes for noble metals to generate hydrogen in electrolysis of water, but these alternatives usually suffer from their inferior performance of intrinsic semiconducting property and inert basal plane. Here, Zn dopants are proved to can intrigue the catalytic activity of MoS2 nanosheets with the low overpotential of 140 mV at 100 mA/cm2 and Tafel slope of 35 mV/dec. Density functional theory calculations are performed to unravel the underlying mechanism, revealing that the inert basal plane is activated by Zn dopants with the H adsorption free energy (△GH*) of −0.07. Further, the charge distribution spreading over the basal plane induced by strong hybridizations of metal d-states and sulphurous p-states play a key role for promoting H atom adsorption and desorption kinetics simultaneously. This study opens new opportunities for the designing and understanding of MoS2-based catalysts for water splitting or other applications.

Voltage-controllable colossal magnetocrystalline anisotropy in single-layer transition metal dichalcogenides

Materials with large magnetocrystalline anisotropy and strong electric field effects are highly needed to develop new types of memory devices based on electric field control of spin orientations. Instead of using modified transition metal films, we propose that certain monolayer transition metal dichalcogenides are the ideal candidate materials for this purpose. Using density functional calculations, we show that they exhibit not only a large magnetocrystalline anisotropy (MCA), but also colossal voltage modulation under external field. Notably, in some materials like CrSe_2 and FeSe_2, where spins show a strong preference for in-plane orientation, they can be switched to out-of-plane direction. This effect is attributed to the large band character alteration that the transition metal d-states undergo around the Fermi energy due to the electric field. We further demonstrate that strain can also greatly change MCA, and can help to improve the modulation efficiency while combined with an electric field.

Superconductivity in Pd-intercalated charge-density-wave rare earth poly-tellurides RETen

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids, which are generally considered to remove electrons from the Fermi level, and thus preclude a superconducting state. However, in a variety of CDW materials, such as the prototypical transition metal chalcogenides, superconductivity has also been observed at very low temperature (Yokoya et al 2001 Science 294 2518; Morosan et al 2006 Nat. Phys. 2 544; Kiss et al 2007 Nat. Phys. 3 720), in which, although the two electronic correlated states are believed to occur in different parts of Fermi surface sheets derived mainly from chalcogen p-states and transition metal d-states, the nature of the relationship between them has not yet been unambiguously determined. Here we report the discovery of superconductivity in Pd-intercalated RETen (RE = rare earth; n = 2.5, 3) CDW systems, in which the chalcogen layers alone are responsible for both superconductivity and CDW instability. Our finding could provide an ideal model system for comprehensive study of the interplay between CDW and superconductivity due to the remarkable simplicity of the electronic structure of Te planes.

Adsorption of water and ethanol on noble and transition-metal substrates: a density functional investigation within van der Waals corrections.

We report the results of extensive computational investigation of the adsorption properties of water and ethanol on several Cu-, Pt-, and Au-based substrates, including the close-packed unreconstructed Cu(111), Pt(111), and Au(111) surfaces, defected metal substrates with on-surface low-coordinated sites generated by the intermixing of Pt-Cu and Pt-Au in the topmost surface layers and strained on-surface and sub-surface Pt-layers at Cu(111) and Au(111) substrates. The calculations are based on the density functional theory (DFT) within the van der Waals (vdW) correction. For all the substrates, we found that water and ethanol bind via the anionic O atom to the cationic one-fold coordinated on-top metal sites, which enhances the adsorbate-substrate Coulomb interactions. For water, both DFT and DFT + vdW calculations predict a flat geometry. For ethanol, the DFT and DFT + vdW results are in contrast, namely, DFT yields a perpendicular orientation of the C-C bond with respect to the surface, while we obtained a parallel orientation of the C-C bond using DFT + vdW, which maximizes the adsorption energies. Despite expected deviations due to the nature of the weak adsorbate-substrate interactions, we found that the adsorption energy of water and ethanol shows a linear dependence as a function of the position of the center of gravity of the occupied d-band, and hence, the magnitude of the adsorption energy increases as the d-band center position shifts towards the Fermi energy. Thus, it indicates hybridization between the O p- and metal d-states, which determines the magnitude of the adsorption energy of water and ethanol on clean, low-coordinated, and strained noble and transition-metal substrates.

Molecular dissociation in the presence of catalysts: interpreting bond breaking as a quantum dynamical phase transition

In this work we show that molecular chemical bond formation and dissociation in the presence of the d-band of a metal catalyst can be described as a quantum dynamical phase transition (QDPT). This agrees with DFT calculations that predict sudden jumps in some observables as the molecule breaks. According to our model this phenomenon emerges because the catalyst provides for a non-Hermitian Hamiltonian. We show that when the molecule approaches the surface, as occurs in the Heyrovsky reaction of H2, the bonding H2 orbital has a smooth crossover into a bonding molecular orbital built with the closest H orbital and the surface metal d-states. The same occurs for the antibonding state. Meanwhile, two resonances appear within the continuous spectrum of the d-band, which are associated with bonding and antibonding orbitals between the furthest H atom and the d-states at the second metallic layer. These move toward the band center, where they collapse into a pure metallic resonance and an almost isolated H orbital. This phenomenon constitutes a striking example of the non-trivial physics enabled when one deals with non-Hermitian Hamiltonian beyond the usual wide band approximation.

Probing the Electrochemical Mechanism of Li5FeO4 (LFO) Cathodes in Li-Ion Cells

The push for high capacity cathodes capable of Li storage greater than Li/TM (TM=transition metal) ratio of 1 in Li-ion batteries is extremely challenging. Considering Li2MnO3 (2Li per TM; 459 mAhg-1 theoretical), we have shown [1], as well as others [2], that two lithiums per Mn can be extracted in an initial charge yielding practical capacities >320 mAhg-1, but this process occurs only above 4.5 V, and requires a material with high-surface area. Nevertheless structural changes occur in this cathode on the first charge, and the Li2MnO3 phase is lost due to lithium and oxygen removal, wherein the resulting material (i.e. ‘MnO2’) does not possess good electrochemical reversibility. Moving away from capacity limited layered Li(TM)O2 requires exploring other crystalline structure types. The antifluorite family of L2O (Li8O4; enlarged unit cell) is an interesting option since transition metals can substitute for tetrahedral Li in the structure. Note that there are more available tetrahedral interstitials for close-packed oxygen atoms (total of two per O), than for octahedral sites (total of 1 per O) thereby providing a clue about which structure types to pursue. A representative example is Li5FeO4 (LFO) that features replacing a total of three lithium atoms in Li8O4 (enlarged unit cell) with one Fe atom (a trivalent charged cation; two vacancies are also created). This material, if electrochemically active could, in theory, provide 5 Li/Fe or ~ 867 mAhg-1, if made reversible. Certainly redox on oxygen could be necessary in order to access all of this lithium and that remains a topic for exploration in hybrid Li-ion Li-oxygen cells [3]. In the earlier publication on LFO used as a lithium source for charged cathodes in Li-ion cells, we were able to demonstrate that a total of 4 lithiums can be extracted from the cathode material up to 4.4 V vs. Li metal. This provided a charge capacity of ~ 690 mAhg-1 [4]. We revisit this material for Li-ion cells and are now focused on optimizing the synthesis process and to thoroughly characterize the electrochemical-chemical mechanism that occurs on the first charge in hopes of making the cathode reversible to supply 350 mAhg-1 on discharge above 3 V which can lead to energy densities of ca. >1000 Whkg-1. The electrochemical voltage profile of LFO is shown in Figure 1 for conventional (micron-sized Fe2O3 precursor) and a nano-Fe2O3 precursor. The over-potential is lower and the extractable capacity is higher for the latter (magenta line). The subsequent discharge to 1 V is also plotted. It is evident that the material is irreversibly converted over to another phase that only shows Fe(III/II) redox at lower voltages below 2.5 V. Reversibility, however, is fairly good over subsequent cycles within a large voltage window between 4 and 1 V yielding ~230 mAhg-1 (not shown). Figure 1. First charge and discharge voltage profile of Li/Li5FeO4cell between 4.7 and 1.0 V; C rate is C/40. A voltage window opening experiment was also conducted and better reversibility is seen up to 2 lithium cations suggesting that the following electrochemical reaction may take place: Li5FeO4 ↔ Li3FeO4 + 2 Li+ + 2 e-. In this reaction, strictly written formally with Fe(V) product likely does not exactly proceed. Earlier XANES/XAFS studies doesn’t invoke the Fe(IV) redox state in the oxidation process [5]. Certainly oxygen redox (lattice peroxide formation) must also be ascertained in this electrochemical mechanism [6] and is the subject of our ongoing experimentation. These results, and are most recent findings to date will be presented in this talk. Antiflourite Li5FeO4 presents an interesting material pathway and design model to high capacity and high energy battery systems and through careful scientific progress is expected to result in considerable advances in the knowledge of advanced Li-ion cathodes utilizing oxygen 2p orbital and metal d-states hybridization for charge storage. ACKNOWLEDGEMENT Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. This work was supported as part of the Center for Electrochemical Energy Science (CEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. REFERENCES 1. C. S. Johnson et al. Electrochem. Commun., 6, 1085 (2004) 2. A. Robertson et al, Chem. Mater., 15, 1984 (2003) 3. M. M. Thackeray et al., J. Phys. Chem. Lett., 4, 3607 (2013) 4. C. S. Johnson et al. Chem. Mater., 22, 1263-1270 (2010) 5. T. Okumura et al., J. Mater. Chem. A , 2, 11847 (2014) 6. S. Okuoka et al., Scientific Reports, 4, 5684 (2014)

Bonding characteristics and site occupancies of alloying elements in Zr3Al2 compound from first principles

The bonding characteristics and site occupancies of the intermetallic compound Zr3Al2 alloyed by Ti, V and Cr elements have been investigated using first-principles methods based on density functional theory. By calculating the impurity formation energy, the alloying elements Ti, V, and Cr prefer to occupy the Zr3 site in Zr3Al2 compound. The lattice parameter and bond length after alloying at Al site are also discussed. The results showed that alloying elements Ti, V, and Cr are beneficial to improve the internal atomic forces of Zr3Al2. The bonding characteristics and mechanism of site preference in pure and alloyed Zr3Al2 are analyzed with differential electron density as well as density of states. It can be concluded that the bonding charge accumulation at the Zr3 site is along X–Al (X=Ti, V and Cr) directions while the bonding charge depletion is along X–Zr3 directions by analyzing the differential electron density of (110) plane after replacing Al with Ti, V and Cr. Total and partial density of states analysis showed that there is a strong hybridization between Al 3s-states and transition metal d-states.

The Correlation between Structure and Color of Iron Oxide‐type Solids, Sustainable Pigments with Gentle Hues

AbstractOxidic solids with hexaccordinate iron(III) appear as colored species only, if charge‐transfer bands are present in the visible region, which originate from transitions, occurring between low‐lying mainly oxygen‐centered electronic states and antibonding, predominantly metal‐3d states. Transitions within the 3d‐shell are expected to be only very weak in intensity, because they are not only parity‐ but also spinforbidden, and hence don't usually contribute signifycantly to the coloration. It is shown by various examples, how one may steer the hue by the aimed manipulation of certain structural and chemical parameters, which effectively alter the location and, in particular, the band‐width of the charge transfer transitions. The spectral properties are compared with those of the isoelectronic hexaoxo‐coordinate manganese(II). The subsequent discussion of the color properties of 3d6‐configured iron(II) in oxidic ligand fields helps to analyze the spectral behavior of mixed‐valence FeII/FeIII solids. The herein additionally observed intervalence bands may lend a specific blue color to the solid, as in the case of Prussion Blue, and possibly also when inspecting the deeply blue coloration of iron gallic ink – the extreme case of a long‐range electronic delocalization being black Fe3O4. Iron is, in mostly oxidic solids, omnipresent on earth and lends characteristic red and yellowish hues with brownish tinges – occasionally even black colors – to the rocks, overwhelming impressions of color compositions, which – as in Armenia, for example – link nature with culture. It is furthermore known in its role in the process of respiration – as the cationic constituent of red haemoglobin – as well as to be the main component of steel, an essential in modern life. Iron and its oxidic solids are sustainable materials and become increasingly important.

A Molecular Perspective on the d-Band Model: Synergy Between Experiment and Theory

The d-band model of Hammer and Norskov (Nature 376:238, 1995 [3]) relating adsorption energies to the d-band position, and the adsorption energies to barriers in catalytic reactions, has been extremely successful in predicting reactivities and catalysts. In the present contribution we review recent combined experimental and theoretical work on chemical bond-formation at surfaces. We focus on the adsorbate and how the adsorbate electronic structure can be rehybridized through mixing with unoccupied states to generate the radical state, real or virtual, that can then form electron pairs with the metal d-states, as described by the d-band model. We discuss five important bonding situations: (i) atomic radical, (ii) diatomics with unsaturated π-systems (Blyholder model), (iii) unsaturated hydrocarbons (Dewar–Chatt–Duncanson model), (iv) lone–pair interactions, and (v) saturated hydrocarbons (physisorption). Where the d-band model predicts trends along the series of transition metals, the present work provides intuitive tools for predicting trends among different adsorbates.