Fcc Transition Metal Surfaces Research Articles

Hydrogen adsorption on fcc metal surfaces towards the rational design of electrode materials

The successful large-scale implementation of hydrogen as an energy vector requires high performance electrodes and catalysts made of abundant materials. Rational materials design strategies are the most efficient means of reaching this goal. Here we present a study on the adsorption of H-atoms onto fcc transition metal surfaces and propose descriptors for the rational design of electrodes and catalysts by means of correlations between fundamental properties of the materials and among other properties, their experimentally measured performance as hydrogen evolution electrodes (HEE). A large set of quantum mechanical modelling data at the DFT level was produced, covering the adsorption of H-atoms onto the most stable surfaces (100), (110) and (111) of: Ag, Au, Co, Cu, Ir, Ni, Pd, Pt and Rh. For each material and surface, a coverage dependent set of minimum energy structures was produced and chemical potentials for adsorption of H-atoms were obtained. Averaging procedures are here proposed to approach modelling to the experiments. Several correlations between the computed data and experimentally measured quantities are done to validate our methodology: surface plane dependent adsorption energies, chemical potentials and experimentally determined surface energies and work functions. We search for descriptors of catalytic activity by testing correlations between the DFT data obtained from our averaging procedures and experimental data on HEE performance. Our methodology allows us to obtain linear correlations between the adsorption energy of H-atoms and the exchange current density (i0) in a HEE, avoiding the volcano-like plots. We show that the chemical potential has limitations as a descriptor of i0 because it reaches an early plateau in terms of i0. Simple quantities obtained from database data such as the first stage electronegativity (χ) as devised by Mulliken has a strong linear correlation i0. With a quantity we denominate modified second-stage electronegativity (χ2m) we can reproduce the typical volcano plot in a correlation with i0. A theoretical and conceptual framework is presented. It shows that both χ and χ2m, that depend on the first ionization potential, second ionization potential and electron affinity of the elements can be used as descriptors in rational design of electrodes or of catalysts for hydrogen systems.

Structure sensitivity in adsorbate-induced adatom formation on FCC transition-metal surfaces

Recent surface science discoveries reveal that adsorbates may induce in situ sub-nanometer cluster formation on transition-metal surfaces. To elucidate the structure sensitivity behind this phenomenon, we performed density functional theory calculations to construct an adatom formation energy database for eight fcc metals and 26 adsorbates commonly involved in catalytic reactions. We show that the adatom formation on (100) surfaces is generally easier than that on (111) surfaces. Many adsorbate/metal pairs exist, mostly on Pd, Ni, Rh, Pt, and Ir, for which adsorbates might induce adatom formation under near-ambient conditions on the (100) facet, but not on (111), highlighting the role of more open facets in adatom formation on metals intrinsically harder than Ag, Au, and Cu. Our study offers a new perspective towards understanding structure sensitivity in heterogeneous thermal- and electro-catalytic systems such as methane steam reforming, Fischer-Tropsch synthesis, and ammonia decomposition.

Looking beyond Adsorption Energies to Understand Interactions at Surface using Machine Learning

AbstractIdentifying factors that influence interactions at the surface is still an active area of research. In this work, the importance of analyzing bond length activations (BLact) along with adsorption energies (Ea) while interpreting Density Functional Theory (DFT) results is emphasized. Investigating adsorption of different small molecules, such as O2, N2, CO, and CO2, on commonly studied facets ((100), (110), and (111)) of seven fcc transition metal surfaces (M=Ag, Au, Cu, Ir, Rh, Pt, and Pd) demonstrates the missing linear correlation between Ea and BLact. Further, tree based Machine Learning (ML) models reinforce the missing linear correlation between the two parameters and also highlight the importance of analyzing both to develop a better understanding of adsorption at surfaces. The best performing Random Forest models have a mean absolute error (MAE) of 0.19 eV for Ea prediction, and even lower MAE of 0.012 Å for BLact prediction. While often d‐band center is correlated with Ea, our observations show that infact the d‐band center has a better correlation with BLact. These observations emphasizes the role of BLact in gaining a fuller picture for catalysis. The fact that the factors responsible for BLact is a lesser‐explored subject adds to the novelty of the findings.

Multilayer relaxations of (3 1 1), (3 3 1) and (2 1 0) fcc transition metal surfaces studied by pseudopotential DFT calculations

The multilayer relaxations of (3 1 1), (3 3 1) and (2 1 0) surfaces of Ni, Cu, Rh, Pd, Ag, Ir and Pt have been investigated by pseudopotential DFT calculations. Our calculations show a relaxation sequence of − +⋯ for the interlayer spacings of all (3 1 1) surfaces and − − + ⋯ for all (3 3 1) and (2 1 0) surfaces. We also found that the surfaces of Ir and Pt had more significant relaxations than other surfaces. Comparisons with LEED results, where possible, are also made in this paper.

Relation between surface stress and (1×2) reconstruction for (1 1 0) fcc transition metal surfaces

In this work we question the validity of surface stress relief as a driving force for surface reconstruction in the particular case of the (110) face of fcc transition metals. To this aim, we calculate within tight-binding second moment potential, both surface energy and surface stress before and after the (1×2) missing row reconstruction. Doing that, one indeed checks that reconstruction is energetically favourable for Au and Pt and not for Ni and Cu as experimentally seen, but we also find that it enhances surface stress along the dense direction for all the elements and along the less dense direction for those which reconstruct. Therefore, surface stress relief is definitely not the driving force for this reconstruction.

Surface phonon dispersion along [formula omitted] on Pd(100)

High resolution electron energy loss spectroscopy was used to measure the dispersion of the Rayleigh wave along the 〈110〉 direction on the Pd(100) surface. It was found that the phonon frequency (2.7 THz) at zone boundary was lower than that calculated by the slab method using bulk force constants (3.0 THz). The result is anomalous from that investigated at X on other fcc transition metal surfaces. The experimental data can be fit by a considerable softening (40%) of the surface interlayer force constant. The phonon softening is qualitatively in agreement with the LEED analysis which revealed a surface expansion by 3% instead of the inward relaxation found on most fcc (100) transition metal surfaces.

Reconstruction mechanism of fcc transition metal (001) surfaces.

Ab initio studies of the reconstruction mechanism of (001) fcc transition metal surfaces are presented, showing that a close-packed quasihexagonal reconstruction is possible for the late 5d metals Ir, Pt, and Au, while it is disfavored in the isovalent 4d metals Rh, Pd, and Ag. The driving mechanism is identified in the tensile excess stress of the unreconstructed surfaces. The stress overcomes the substrate resistance to reconstruction only in 5d metals, where it is much larger than in 4d metals because of the stronger relativistic effects. The origin of surface stress is a d charge depletion from the surface layer.

Relaxation and force constants near fcc transition metal surfaces

Within the tight-binding approximation used together with the moment method, we get a contraction of the surface layer of transition metals. This result is discussed in view of recent experimental measurements. We notably show that this top layer contraction strongly enhances force constants between atoms close to the surface giving rise to a localized step phonon at a frequency above the bulk phonon spectrum. The relaxation energy is also calculated. This term is shown to favour monoatomic steps but in any case it remains quite small.