Adsorbate Electronic Structure Research Articles

Atomic-level insights into the activation of nitrogen via hydrogen-bond interaction toward nitrogen photofixation

Summary As the activation of N2 molecules is generally regarded as the bottleneck in N2 reduction, it is of significant importance to develop promising strategies to efficiently activate N2 molecules. Herein, we discovered an innovative approach to activate N2 molecules via hydrogen-bond interaction in N2 photofixation. Porous Cu with surface modification of S atoms (3%-S/Cu) exhibited remarkable catalytic activity and stability in N2 photofixation. Further mechanistic studies revealed that surface S atoms directly participated in the catalytic process by accepting and donating H atoms. In addition, the forming S–H bond significantly promoted the activation of N2 molecules via hydrogen-bond interaction, contributing to the enhanced catalytic activity.

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.

Stability of Pt-Modified Cu(111) in the Presence of Oxygen and Its Implication on the Overall Electronic Structure

The electronic structure and stability of Cu(111)-hosted Pt overlayers with and without the presence of atomic oxygen have been studied by means of core-level spectroscopy and density functional theory (DFT). Because of lattice mismatch, Pt(111) overlayers grown on Cu(111) are compressively strained, and hard X-ray photoelectron spectroscopy together with Pt L3-edge X-ray absorption spectroscopy (XAS) reveals a pronounced downshift of the Pt d-band owing to the increased overlap of the d-orbitals, an effect also reproduced theoretically. Exposure to oxygen severely alters the surface composition; the O–Cu binding energy largely exceeds that of O–Pt, and DFT calculations predict surface segregation of Cu atoms. Comparing the adsorbate electronic structure for O on unstrained Pt(111) with that of O on Pt-modified Cu(111) using O K-edge XAS and X-ray emission spectroscopy salient differences are observed and calculations show that Cu-segregation to the topmost layer is required to reproduce the measured spectra. It is proposed that O is binding in a hollow site constituted by at least two Cu atoms and that up to 75% of the Pt atoms migrate below the surface.

The growth and electronic structure of azobenzene-based functional molecules on layered crystals

In situ ultraviolet photoelectron spectroscopy is used to study the growth of ultrathin films of azobenzene-based functional molecules (azobenzene, Disperse Orange 3 and a triazatriangulenium platform with an attached functional azo-group) on the layered metal TiTe2 and on the layered semiconductor HfS2 at liquid nitrogen temperatures. Effects of intermolecular interactions, of the substrate electronic structure, and of the thermal energy of the sublimated molecules on the growth process and on the adsorbate electronic structure are identified and discussed. A weak adsorbate–substrate interaction is particularly observed for the layered semiconducting substrate, holding the promise of efficient molecular photoswitching.

Electronic Structure of TiO2 Surfaces and Effect of Molecular Adsorbates Using Different DFT Implementations

We model TiO2 rutile (110) and anatase (101) surfaces and investigate the effect of adsorption of benzoic acid on the electronic structure of these systems. The surface−adsorbate electronic structure determines the rates of electron-transfer processes, which are important for dye-sensitized solar cells, in particular. We use density functional theory (DFT) and test the accuracy and efficiency of several DFT implementations (plane waves vs localized basis sets, all-electron vs pseudopotential calculations) as applied to the TiO2−adsorbate system. We explore the variation in the band gap, surface energies, and benzoic acid adsorption energies as a function of slab thickness. Our results show that a two-layer slab is sufficient to model the adsorption of benzoic acid on anatase, whereas for rutile, convergence is much more slow and not yet achieved in five-layer slabs. The effect of the adsorbate on the electronic structure is small in the case of anatase, but noticeable for rutile, where adsorbate states ap...

X-ray emission spectroscopy of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001): comparison to c(2x2)CO/Ni(100) and c(2x2)CO/Cu(100).

The atom specific electronic structure of (2 square root of 3 x 2 square root of 3)R30 degrees CO on hcp Ru(0001) has been determined with resonantly excited x-ray emission spectroscopy. We find that the general features of the local adsorbate electronic structure are similar to the situation of CO adsorbed on the fcc metals Ni(100) and Cu(100). The interpretation of the surface chemical bond of (2 square root of 3 x 2 square root of 3)R30 degrees CO/Ru(0001) based on the direct application of the local, allylic model from on-top adsorption on the fcc(100) surfaces Ni(100) and Cu(100) explains many aspects of the surface chemical bond. However, also nonlocal contributions like adsorbate-adsorbate interaction and the deviation from upright on-top adsorption on the Ru(0001) surface influence observables like the heat of adsorption and the Me-CO bond strength.

Controlled modification of individual adsorbate electronic structure

Modification of the electronic structure of a single Mn adsorbate placed within a geometrical array of adatoms on Ag(111) is observed using local spectroscopy with the scanning tunneling microscope. The changes result from coupling between the adsorbate level and surface electronic states of the substrate. These surface states are scattered coherently within the adatom array, mediating the presence and shape of the array to the adsorbate within. The dimension and geometry of the adatom array thus provide a degree of control over the induced changes.

Franck–Condon breakdown in core-level photoelectron spectroscopy of chemisorbed CO

The photon energy dependence of the vibrational fine structure in the C1s and O1s X-ray photoelectron main lines of chemisorbed CO on Ni(100) and Ru(0001) has been measured from 6 to 150 eV above the core-level thresholds. Significant deviations from the behavior in gas-phase CO are found. A strong dominance of the adiabatic peak towards threshold is found for the C1s, but not the O1s, lines. In the C1s lines, we observe a broad maximum of vibrational excitation 5 eV above the shape resonance. At high photon energies, Franck–Condon behavior is observed in both the C1s and O1s lines. This behavior is discussed in terms of the adsorbate electronic structure and the dynamic metallic screening upon core ionization.

Surface-induced alteration of adsorbate electronic structure and intramolecular vibrational coupling: The vibrational spectrum of 2-propoxide on Mo(110) as determined by ab initio calculations and experiments.

Interactions with the molybdenum surface have a strong influence on the vibrational spectrum of 2-propoxide on Mo(110). Using ab initio electronic-structure calculations the vibrational spectrum of the adsorbed 2-propoxide is determined. All major effects, experimentally observed by electron-energy-loss spectroscopy, are well reproduced by the calculation. Kinematic effects do not explain the observed changes. Calculations indicate that the changes in vibrational spectra are due to alterations of the intramolecular potential function and charge redistribution upon binding to the Mo.

An extension of the tersoff-hamann model for a molecule on a graphite support: STM-image construction

A method is proposed for the theoretical construction of STM images of adsorbates on a graphite support. Tunnel current is taken to be proportional to the local density of graphite electronic states perturbed by the interaction with the adsorbed molecule. The coefficients of mixing of the graphite states with the adsorbate MOs are found by first-order perturbation theory, using the CNDO-S2 method to calculate the adsorbate electronic structure together with analytical expressions for the graphite states. The long-distance behavior of adsorbate MOs is taken into account by means of energy-dependent MO asymptotics. As an example of the implications of this technique, the theoretical STM images of benzene and butane molecules on a graphite surface are presented and discussed.

Core-level shake-up spectra from ordered C, N and O overlayers on Ni(100)

The XPS core-level shake-up spectra reveal significant differences between C, N and O adsorbed on Ni (100). It is shown that the satellites are due to local excitations on the adsorbate and not to excitations within the substrate. This makes shake-up spectroscopy an important tool for investigating the adsorbate electronic structure. The screening of the O1s core hole leads to a complete filling of the 2p shell whereas the main satellite has a 2p 5 configuration. The observation of an atomic-like 1s—2p exchange splitting of the satellite indicates a strong localization tendency for the 2p shell. The observed differences between the satellite spectra for the p(2×2) and c(2×2) phases of adsorbed oxygen can be related to different 2p—2p overlap strengths.

Perturbation of the adsorbate electronic structure by local fields at surface defects

PAX (photoemission of adsorbed xenon) measurements provide direct evidence for rather strong localized surface potential variations near surface step sites. In the present study the influence of the associated localized electric fields parallel to the surface on the electronic structure of nearby adsorbed CO molecules is examined within a simple theoretical model: a point charge in the vicinity of a Cu5CO cluster. The results demonstrate a downwards shift and a splitting of the CO2π∗ derived cluster orbital leading to an increased CO2π∗ occupation as a contribution to C-O bond weaking near step defects.

Perturbation of the adsorbate electronic structure by local fields at surface defects

Perturbation of the adsorbate electronic structure by local fields at surface defects

Gas-phase rhodium cluster chemistry: Influence of adsorbate electronic structure on reaction rate

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