In-Situ Vibrational Spectroscopic Studies on Model Catalyst Surfaces at Elevated Pressures

Cataloged from PDF version of article.Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O-2, H-2, CH3OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer-Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures

Direct Evidence for the Instability and Deactivation of Mixed-Oxide Systems: Influence of Surface Segregation and Subsurface Diffusion

Cataloged from PDF version of article.In the current contribution, we provide a direct demonstration of the thermally induced surface structural transformations of an alkaline-earth oxide/transition metal oxide interface that is detrimental to the essential catalytic functionality of such mixed-oxide systems toward particular reactants. The BaO(x)/TiO(2)/Pt(111) surface was chosen as a model interfacial system where the enrichment of the surface elemental composition with Ti atoms and the facile diffusion of Ba atoms into the underlying TiO(2) matrix within 523-873 K leads to the formation of perovskite type surface species (BaTiO(3)/Ba(2)TiO(4)/Ba(x)Ti(y)O(z)). At elevated temperatures (T > 973 K), excessive surface segregation of Ti atoms results in an exclusively TiO(2)/TiO(x)-terminated surface which is almost free of Ba species. Although the freshly prepared BaO(x)/TiO(2)/Pt(111) surface can strongly adsorb ubiquitous catalytic adsorbates such as NO(2) and CO(2), a thermally deactivated surface at T > 973 K practically loses all of its NO(2)/CO(2) adsorption capacity due to the deficiency of surface BaO(x) domains

Interactive Surface Chemistry of CO2 and NO2 on Metal Oxide Surfaces: Competition for Catalytic Adsorption Sites and Reactivity

Cataloged from PDF version of article.Interactive surface chemistry of CO2 and NO2 on BaOx/Pt(111) model catalyst surfaces were investigated via X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques with a particular emphasis on the competition between different adsorbates for the catalytic adsorption sites and adsorbate-induced morphological changes. After NO2 adsorption, nitrated BaO x/Pt(111) surfaces do not reveal available adsorption sites for CO2 at 323 K, irrespective of the presence/absence of exposed Pt sites on the surface. Although NO2 adsorption on thick BaO x(>10MLE)/Pt(111) overlayers at 323 K leads to the formation of predominantly nitrate species, NO2 adsorption on the corresponding carbonated surface leads to the formation of coexisting nitrates and nitrites. The presence of carbonates on BaOx/Pt(111) overlayers does not prevent NO2 uptake. Carbonated BaOx(1.5 MLE)/Pt(111) surfaces (with exposed Pt sites) obtained via CO2 adsorption can also further interact with NO2, forming surface nitrate/nitrite species, accompanied by the transformation of surface carbonates into bulk carbonate species. These results suggest that the nitrate formation process requires the presence of two adjacent unoccupied adsorption sites. It is apparent that in the presence of both NO2 and CO2, carbonate species formed on Lewis base (O2-) sites enable the formation of nitrites on Lewis acid (Ba2+) sites. Thermal aging, nitration, and carbonation have a direct impact on the morphology of the two-/three-dimensional (2D/3D) BaO x aggregates on Pt(111). While thermal aging in vacuum leads to the sintering of the BaOx domains, nitration and carbonation results in redispersion and spreading of the BaOx domains on the Pt(111) substrate. © 2013 American Chemical Society.

Role of the Exposed Pt Active Sites and BaO2 Formation in Nox Storage Reduction Systems: A Model Catalyst Study on BaOx/Pt(111)

Cataloged from PDF version of article.BaOx(0.5 MLE - 10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NOx storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaOx/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For theta(BaOx) = 2.5 MLE) were found to be amorphous. Extensive NO2 adsorption on BaOx(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaOx(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO2 formation at 650 K and (2) NO(g) + O-2(g) evolution at 700 K. O-2(g) treatment of the BaOx(10 MLE)/Pt(111) surface at 873 K facilitates the BaO2 formation and results in the agglomeration of BaOx domains leading to the generation of exposed Pt(111) surface sites. BaO2 formed on BaOx(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (theta(BaOx) = 2.5 MLE), BaO2 partially decomposes into BaOx indicating that small BaO2 clusters in close proximity of the exposed Pt(111) sites are prone to decomposition. Nitrate decomposition temperature decreases monotonically from 550 to 375 K with decreasing BaOx coverage within theta(BaOx) = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650-700 K for thicker BaOx overlayers (2.5 MLE < theta(BaOx) < 10 MLE). These two distinctly characteristic BaOx-coverage-dependent nitrate decomposition regimes are in very good agreement with the observation of the so-called "surface" and "bulk" barium nitrates previously reported for realistic NSR catalysts, clearly demonstrating the strong dependence of the nitrate thermal stability on the NOx storage domain size

Clinical-immunohistochemical characteristics of atypical endometrial hyperplasia in women of reproductive age.

The article presents the analysis of clinical-morphological and immunohistochemical features of endometrial hyperplasia. 60 patients of reproductive age with non-atypical endometrial hyperplasia (EH) were examined. The following morphological distribution of its different types was established: glandular – 56.6%, glandular-cystic – 40.0%, stromal and cystic-atrophic forms  – 1.7%, respectively. When carrying out immunohistochemical studies, the expression of CD-138, a modern reliable marker of the presence of an inflammatory process in endometrial tissue was determined. In the group with non-atypical glandular EH, signs of the inflammatory process in the endometrium were verified in 7 (20.59%) patients. In the group with signs of glandular-cystic change of EH – in 11 (45,83%) patients, as well as in the only patient with non-atypical stromal EH (100,0%). In almost one-third of patients with different morphological types of EH in 19 (31.7%) of 60 the presence of structural-morphological signs of chronic endometritis was revealed, which was manifested by signs of lymphoplasmacytic infiltration, indicating the presence of chronic inflammatory process. These data indicate the possible pathogenetic role of inflammation as one of the trigger factors for the hyperplastic transformation of endometrial structures, which allows to confirm the role of infectious factors in the occurrence of these processes in almost one third of patients with non-atypical ЕН. Thus, the conducted studies indicate that the development of EН is a process based on both the infectious factor and hormonal metabolic disorders, which dictates different personalized approaches to therapeutic tactics

Modelling the defect processes of materials for energy applications

The technological requirement for ever more efficient materials for the energy and electronics sectors has led to the consideration of numerous compositionally and structurally complicated systems. These systems include solid solutions that are difficult to model using electronic structure calculations because of the numerous possibilities in the arrangement of atoms in supercells. The plethora of such possible arrangements leads to extensive and large numbers of potential supercells, and this renders the investigation of defect properties practically intractable. We consider recent advances in oxide interfaces where studies have demonstrated that it is feasible to tune their defect processes effectively. In this review, we aim to contribute to the ongoing discussion in the community on simple, efficient and tractable ways to realise research in solid solutions and oxide interfaces. The review considers the foundations of relevant thermodynamic models to extract point defect parameters and the special quasirandom structures method to model the supercell of solid solutions. Examples of previous work are given to highlight these methodologies. The review concludes with future directions, systems to be considered and a brief assessment of the relevant methodologies

Pressure of thermal excitations in superfluid helium

We find the pressure, due to the thermal excitations of superfluid helium, at the interface with a solid. The separate contributions of phonons, $R^-$ rotons and $R^+$ rotons are derived. The pressure due to $R^-$ rotons is shown to be negative and partially compensates the positive contribution of $R^+$ rotons, so the total roton pressure is positive but several times less than the separate $R^-$ and $R^+$ roton contributions. The pressure of the quasiparticle gas is shown to account for the fountain effect in $HeI I$. An experiment is proposed to observe the negative pressure due to $R^-$ rotons.Comment: 14 pages, 4 figure