Surface Science最新文献

We present the detailed derivation of the equations describing the evolution of the hydrodynamic fluctuations of the coverage of particles adsorbed on homogeneous lattices. Using the method of the non-equilibrium statistical operator, we reduce the balance equation governing the behavior of the individual particles to the diffusion equation. On a macroscopic level, this equation describes the approach to equilibrium of the hydrodynamic coverage fluctuations. We obtain the analytical expressions for the Fickian diffusivity and Onsager phenomenological coefficient. These expressions are derived with account of the lateral interaction between the particles. They are simple functions of the thermodynamic quantities — derivatives of the thermodynamic potential over its arguments. The transport coefficients accurately describe the development of fluctuations in the entire coverage region and in the wide range of lateral interactions. We presented an elementary introduction to the theory of fluctuations in the lattice gas systems. For calculations of the correlation function and spectral density of fluctuations, we use the Langevin approach and the method of moments. The exact coincidence of the analytical expressions for the diffusion coefficients obtained by the two independent calculations is the direct proof of the accuracy of the approach developed in Chumak and Tarasenko (1980).

The use of CO₂ as a source of carbon for energy-storage through their transformation into methane represents an attractive approach in modern days. The development of stable and active catalysts as well as a deeper understanding of reaction mechanisms is essential in that field. Herein, the CO₂ methanation reaction over zirconia was thoroughly investigated via DFT calculations and microkinetic modeling. The catalytic surface was represented by a monoclinic ZrO₂ surface where CO₂ adsorption and elementary hydrogenation steps have been systematically explored. It was demonstrated that the participation of an active oxygen vacancy (reduced surface) is crucial to activate the stable bonds of CO₂ for further hydrogenation steps; the stoichiometric surface has a minor contribution to CH₄ production. Two main reaction pathways were investigated: i) CO₂ dissociation with later hydrogenations; (ii) H-assisted (early hydrogenation) mechanisms. It was demonstrated that the latter is kinetically favorable passing through a formate intermediate. Microkinetic simulations indicate that the initial adsorption configuration plays a central role to methane formation: more weakly adsorbed geometry happens to be more important. Thus, the present findings provide new insights and critical information on the role of oxygen vacancies to methanation mechanisms.

With performed DFT calculations it is shown that AgO as well as PdO monolayer is bound to the Mo(112) surface with Ag or Pd atoms lying in surface furrows and O atoms situated in sites on Mo surface rows. The intrinsic corrugation of the surface facilitates the placement of oxide monolayers on the surface by effectively compensating for the difference in the sizes of O and Ag or Pd atoms, resulting in the adsorbed AgO and PdO layers becoming almost flat, which is typical of free monolayers. The surface states associated with the oxide layers on Mo(112) are either significantly above or below the E_F, which does not lead to any noticeable changes in the band structures and densities of states in the vicinity of the E_F of these layered systems. Nevertheless, it can be expected that adsorbed oxide monolayers can exhibit useful catalytic properties due to the adsorption-modified structure of the layers.

The adsorption and degradation of the Soman molecule (Pinacolyl methylphosphonofluoridate, C₇H₁₆FO₂P) was investigated using the pure and Nb-doped δ-FeOOH (001) surfaces with density functional theory (DFT) calculations. We verified the Soman molecule adsorb on pure and doped surface through interaction preferably via interaction between phosphoryl oxygen (P = O) and hydroxyl groups from surface. The degradation of the Soman molecule on the δ-FeOOH and Nb-δ-FeOOH (001) surfaces was evaluated by the study of the reaction path, were found one transition state for both surfaces, corresponding to a maximum stretch of F-P = O group from Soman molecule and the bond breaking of hydroxyl group bonded to Fe/Nb. The activation energies found are 16.58 and 8.80 kcal/mol to pure and doped surface, respectively. The obtained products consisted of a negatively charged pinacolyl methylphosphonate molecule and HF molecule adsorbed on the positively charged surface. Both δ-FeOOH and Nb-δ-FeOOH (001) surfaces show great potential to adsorb and degrade the Soman neurotoxic agent, however the presence of Nb further favors the process.

We have utilized scanning tunneling microscopy and spectroscopy (STM/STS) as well as x-ray linear dichroism (XLD) along with DFT calculations to determine the structural and electronic changes of the gold-covered nonstoichiometric Bi₂Te₃ single crystal surface. XLD spectra supported by a numerical simulation of several configurations show that at low coverage (< 1.0 ML) gold forms insoluble islands, while for higher coverage (> 1.0 ML) mixing at the Au/Bi₂Te₃ interface may occur. For local electronic characterization around stand-alone Au islands, small coverages were selected for which the inter-island distance is large enough to avoid overlapping island-wide effects. STS spectra reveal that for small coverages the topological surface states (TSS) are preserved. However, at a distance of about 10 nm from an Au island, by approaching the islands, the electronic structure gradually changes. This is manifested by a gradual shift of the STS spectra to more negative energies with respect to the pristine surface. The results are discussed in view of possible Au-Bi₂Te₃ mixing, a local charge transfer and the presence of in-plane downward band bending.

Bimetallic Co/Ni catalysts have emerged as promising candidates for efficient H₂ production via ammonia decomposition. However, due to the diverse range of surface configurations and atomic ratios observed in various Co/Ni catalysts, it is necessary to systematically understand their heightened activity. In this study, a comparative theoretical investigation employing density functional theory was presented to explore the mechanisms of ammonia decomposition on the Co-Ni(1 1 1), Ni-Co(1 1 1), Co₂Ni(1 1 1), and CoNi₂(1 1 1) surfaces. Our findings highlighted the outstanding catalytic activity presented by Co/Ni catalysts can be attributed to the simplified NN recombination process, with Ni-Co(1 1 1) displaying the lowest energy barrier. We verified that the exceptional performance of Co/Ni surfaces is due to the exclusive synergistic and alloying effects that arise from combining Co and Ni metals. Electronic structure analysis had proved the combined effect of electron transfer from Cobalt to Nickel, resulting in moderate N binding energy, aid in the desorption and transfer of Nitrogen atoms. In addition, we showed that the appropriate Co/Ni catalyst ratio and catalyst surface configuration can improve the catalytic activity of hydrogen production, which provided another strategy for catalyst design.

Adsorption and reaction properties of environmental gases including O₂, H₂, and H₂O on the PuO₂(111) surface were studied via density functional theory simulations along with thermodynamic and kinetic analysis. Simulation results show that the stoichiometric PuO₂(111) remains intact under O₂ atmosphere and extremely low or high O₂ pressure is required to form oxygen vacancy or adsorbed-O on the surface. The H₂O prefers to stay as molecular state when adsorbing on PuO₂(111) and a relatively high humidity is required for H₂O to be stably binding on the surface. For H₂ interaction with PuO₂, the dissociative adsorption of H₂ molecule induces reduction of Pu(IV) ions to Pu(III), and remains thermodynamically stable at H₂ pressure as low as ∼10⁻³⁵ bar under room temperature. Kinetic modeling shows that at temperature below 350 K, the PuO₂(111) surface is mainly covered by OH species when exposing to H₂ environment while bare metal sites appear with increased temperature and reaction time.

After a brief recapitulation of the historic development of photoelectron spectroscopy, we review the attempts to interpret, in particular, x-ray photoelectron spectra to extract information on electronic structure and constitution, i.e. stoichiometry of materials systems. We focus on materials systems, specifically in the area of heterogeneous catalysis, and modelling their complexity by designing single crystal-based model systems with well-defined structures. This allows us to relate measured chemical shifts observed in X-ray photoelectron spectra, obtained with both laboratory and synchrotron radiation, to structures and also allows us to test those against theory. We discuss a number of systems by combing experimental observations and theory and demonstrate the usefulness of combined theory/experiment approach. We also point out a number of issues, that are often ignored in previous and present studies published in the field of heterogeneous catalysis.

With the increasing attention in environmental issues caused by CO₂ emissions, methanol conversion by CO₂ hydrogenation is an effective strategy to solve this existing energy dilemma. The rationale behind hydrogen spillover on methanol synthesis is unraveled via density functional theory (DFT) calculations in this work, furthermore, the activation energy of hydrogen transfer process as affected by spillover is also summarized in a general paradigm for facilitating the understanding of hydrogenation characteristics. The results demonstrate that the spillover strategy significantly facilitates the hydrogenation reaction by supplying available hydrogen adatoms. This effect is particularly pronounced during the stage when OH is formed directly at the substrate site and combines with H to produce H₂O, leading to a substantial reduction in activation energy from the initial 3.74 eV to 0.78 eV. In addition, a comprehensive predictive model for the kinetic characteristics of hydrogen spillover process is established based on the machine learning algorithm and SISSO guidance. By employing the combined approach of SISSO and neural network, we have achieved a stable prediction performance for activation energy with R² = 0.99 and RMSE = 0.07 eV. The variable of $Ch{g}_{FS}^{Au}$ is identified as the most representative factor in describing the activation energy, demonstrating a correlation coefficient of -0.60. The extended multidimensional expression of Dist_Au further highlights its close connection to activation energy, achieving an RMSE value of 0.41 eV. To sum up, this work elucidates the possible thoughts of catalyst design with spillover effect and gives reference for the description screening towards the chemical reactions similar to hydrogen spillover.

It is well known that the atomic structure of the Ir(001)-(5 × 1) surface consists of a quasi-hexagonal topmost layer on top of quadratic substrate layers. In the present work, we investigate how the electronic structure of Ir(001)-(1 × 1) is modified by the (5 × 1) reconstruction. For this purpose, we perform a first-principles density functional theory (DFT) calculation for semi-infinite Ir(001)-(1 × 1) and -(5 × 1) surfaces by employing the surface embedded Green’s function technique. It will be shown that surface bands on the (1 × 1) surface are strongly modified upon reconstruction. At the same time, we will demonstrate that one-dimensional (1D) surface bands localized in atomic rows in the $\left[110\right]$ direction characterizing the buckled hexagonal layer emerge on the reconstructed surface.