Ab initio atomistic thermodynamics modeling of adsorption of oxygen on gold and gold-silver surfaces

A theoretical study of oxygen adsorption on gold and gold-silver surfaces by means of density functional theory (DFT) calculations with an atomistic thermodynamic model is performed. The (111) and (211) facets of gold and gold-silver alloy surfaces are considered, and their stabilization is discussed upon adsorption of oxygen depending on O and Ag coverage. The details of how the DFT-based atomistic thermodynamic model can apply to the transition metal surface are also presented in this work

Katalitik ve aygıt uygulamaları için malzeme özelliklerinin ilk prensip modellenmesi.

The first-principles computations based on density functional theory is used to study the adsorption properties and the activation of CO, CO2 and H2O on gamma-Al2O3(100) surface. A systematic study has been conducted to identify the most stable adsorption sites for both mono- and di-atomic Pt clusters. Several stable adsorption geometries have been identified for the species as well as introduces their interaction with Pt clusters and the support. In this context, analysis of the adsorption properties allows us to establish the most stable configuration in a reaction mechanism. Another important factor in the reactivity of catalyst is the support material which has a great influence on the efficiency and activity of a catalytic reaction. Transition metal carbides (TMCs) are a good alternative as a support material in a catalytic reaction. Therefore, studying on the surfaces of platinum carbide (PtC) as a support material is being important. While the bulk structure of zincblende (ZB) PtC has been investigated several times, a detailed understanding of the electronic and structural properties of its low-index surfaces is lacking. Within this study, we present an ab-initio investigation of the properties of five crystallographic ZB PtC surfaces, Pt/C-terminated PtC(100), PtC(110) and Pt/C-terminated PtC(111). Adsorption geometries have been identified for the atomic oxygen, and its interaction with surface atoms is characterized in terms of adsorption energies and the nature of bonding between the adsorbed and surface atoms. Calculated vacancy formation energies indicate facile C removal (exothermic) on the ZB PtC(111) surface, and Pt-vacancy formation is endothermic with respect to the vacancy formation energy. An ab-initio thermodynamical analysis shows that the most stable surfaces are the Pt-terminated PtC(100) and PtC(111) surfaces, and the higher oxygen coverages of PtC(100) surface are stable even at temperature as high as 3000 K. In addition to the catalytic properties of an oxide surface and TMCs, we have also studied the interaction of chlorine (Cl) atom with graphene sheet and H-terminated graphene nanoribbons (GNRs) based on density functional theory (DFT). We have discussed the electronic and structural properties of adsorbed Cl atom on pristine and defective graphene under applied electric field. We have found that the most stable adsorption configurations are the on-site geometry and hollow site aligned parallel to the graphene plane for single and molecular Cl atom (Cl2), respectively. The energy band structures are also performed to understand the nature of size effect including the effects on the magnetization, adsorption behavior of single Cl atom and charge transfer in graphene nanoribbons with zig-zag and armchair edges. Ph.D. - Doctoral Progra

Investigation of Magnetic Properties of Spin 5/2 Ising Chain by Using Tranfer Matrix Method

A magnetic property of the one dimensional spin 5/2 Ising model under the magnetic field has been investigated by means of transfer matrix method. Thermodynamic response functions are also obtained for varying values of temperature (in K) and scaling magnetic field. Entropy and heat capacity of the system were calculated by benefiting from the temperature dependencies of Helmholtz free energy. We observed that the heat capacity tends to shift to the relatively higher temperature regions as the strength of the magnetic field is increased, and these findings are consistent with previous results for the low spin values in one dimensional Ising systems

Investigation of Magnetic Properties of Spin 5/2 Ising Chain by Using Transfer Matrix Method

A magnetic property of the one dimensional spin 5/2 Ising model under the magnetic field has been investigated by means of transfer matrix method. Thermodynamic response functions are also obtained for varying values of temperature (in K) and scaling magnetic field. Entropy and heat capacity of the system were calculated by benefiting from the temperature dependencies of Helmholtz free energy. We observed that the heat capacity tends to shift to the relatively higher temperature regions as the strength of the magnetic field is increased, and these findings are consistent with previous results for the low spin values in one dimensional Ising systems

Magnetic properties of the spin-5/2 Blume-Capel chain in a magnetic field

The magnetization plateaus and thermodynamic properties of the antiferromagnetic spin-5/2 chain system were studied by using Blume-Capel model. The system with crystal field interaction in the presence of an external magnetic field has been investigated by means of transfer matrix method. We have seen that the crystal field strongly affects magnetic plateau mechanism. Our simulation results demonstrate that the plateau becomes smoother when the value of the temperature is increased. We also found that the increase of the crystal field parameter leads to wider plateaus for each spin gap. From the analysis of the magnetic heat capacity of the spin-5/2 chain system, the value of heat capacity becomes constant at higher temperatures with the increase in the crystal field

SU-GAZ DEĞİŞİMİ REAKSİYONUNDA ALTTAŞ MALZEMENİN ETKİSİNİN YÜK YOĞUNLUĞU FONKSİYONELİ KULLANILARAK İNCELENMESİ

SU-GAZ DEĞİŞİMİ REAKSİYONUNDA ALTTAŞ MALZEMENİN ETKİSİNİN YÜK YOĞUNLUĞU FONKSİYONELİ KULLANILARAK İNCELENMES

First-principles study of coupled effect of ripplocations and S-vacancies in MoS 2

Sensoy, Mehmet Gokhan/0000-0003-4815-8061; Shirodkar, Sharmila N/0000-0002-9040-5858; Tritsaris, Georgios/0000-0002-5738-4493WOS: 000483884600020Recent experiments have revealed ripplocations, atomic-scale ripplelike defects on samples of MoS2 flakes. We use quantum mechanical calculations based on density functional theory to study the effect of ripplocations on the structural and electronic properties of single-layer MoS2, and, in particular, the coupling between these extended defects and the most common defects in this material, S-vacancies. We find that the formation of neutral S-vacancies is energetically more favorable in the ripplocation. in addition, we demonstrate that ripplocations alone do not introduce electronic states into the intrinsic bandgap, in contrast to S-vacancies. We study the dependence of the induced gap states on the position of the defects in the ripplocation, which has implications for the experimental characterization of MoS2 flakes and the engineering of quantum emitters in this material. Our specific findings collectively aim to provide insights into the electronic structure of experimentally relevant defects in MoS2 and to establish structure-property relationships for the design of MoS2-based quantum devices. Published under license by AIP Publishing.ARO MURIMURI [W911NF14-0247]; DOE BES AwardUnited States Department of Energy (DOE) [DE-SC0019300]; National Science Foundation (NSF)National Science Foundation (NSF) [ACI-1053575]The authors would like to thank Venkataraman Swaminathan and Daniel Larson for helpful discussions. S.S. acknowledges support by the ARO MURI (Award No. W911NF14-0247). This work was supported by the DOE BES Award No. DE-SC0019300. For calculations, computational resources were used on the Odyssey cluster, which is maintained by the FAS Research Computing Group at Harvard University, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the National Science Foundation (NSF) under Grant No. ACI-1053575

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

© 2021 American Chemical Society.Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of μ4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity