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

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

Direct evidence for the instability and deactivation of mixed-oxide systems: Influence of surface segregation and subsurface diffusion

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 2TiO 4/Ba xTi yO z). At elevated temperatures (T &gt; 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 &gt; 973 K practically loses all of its NO 2/CO 2 adsorption capacity due to the deficiency of surface BaO x domains. © 2011 American Chemical Society

Electron stimulated hydroxylation of a metal supported silicate film

Water adsorption on a double-layer silicate film was studied by using infrared reflection–absorption spectroscopy, thermal desorption spectroscopy and scanning tunneling microscopy. Under vacuum conditions, small amounts of silanols (Si–OH) could only be formed upon deposition of an ice-like (amorphous solid water, ASW) film and subsequent heating to room temperature. Silanol coverage is considerably enhanced by low-energy electron irradiation of an ASW pre-covered silicate film. The degree of hydroxylation can be tuned by the irradiation parameters (beam energy, exposure) and the ASW film thickness. The results are consistent with a generally accepted picture that hydroxylation occurs through hydrolysis of siloxane (Si–O–Si) bonds in the silica network. Calculations using density functional theory show that this may happen on Si–O–Si bonds, which are either parallel (i.e., in the topmost silicate layer) or vertical to the film surface (i.e., connecting two silicate layers). In the latter case, the mechanism may additionally involve the reaction with a metal support underneath. The observed vibrational spectra are dominated by terminal silanol groups (ν(OD) band at 2763 cm−1) formed by hydrolysis of vertical Si–O–Si linkages. Film dehydroxylation fully occurs only upon heating to very high temperatures (∼1200 K) and is accompanied by substantial film restructuring, and even film dewetting upon cycling hydroxylation/dehydroxylation treatment

Postdural puncture subdural hematoma or postdural puncture headache? -two cases report

Spinal anesthesia is widely used for many obstetric, gynecological, orthopedic, and urological operations. Subdural hematomasamay occur after trauma and are associated with high morbidity and mortality rates. Postdural puncture headachea(PDPH) is a benign condition and the most frequent complication of spinal anesthesia. The high rate of headacheaafter spinal anesthesia may mask or delay the diagnosis of subdural hematoma. The true incidence of postdural punctureasubdural hematoma (PDPSH) is unknown because most affected patients are probably managed without investigatioa. Therefore, the true incidence of PDPSH may be greater than suggested by previous reports. The differentiation of headache associated with subdural hematoma from PDPH is crucial. We herein report two cases of bilateral subdural hematoma after epidural anesthesia and emphasize the importance of suspicion for PDPSH and careful evaluation of patients with headache after spinal anesthesia. © the Korean Society of Anesthesiologists, 2015

Role of the exposed Pt active sites and BaO 2 formation in NO x storage reduction systems: A model catalyst study on BaO x/Pt(111)

BaO x(0.5 MLE-10 MLE)/Pt(111) (MLE: monolayer equivalent) surfaces were synthesized as model NO x storage reduction (NSR) catalysts. Chemical structure, surface morphology, and the nature of the adsorbed species on BaO x/Pt(111) surfaces were studied via X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). For θ BaOx &lt; 1 MLE, (2 × 2) or (1 × 2) ordered overlayer structures were observed on Pt(111), whereas BaO(110) surface termination was detected for θ BaOx = 1.5 MLE. Thicker films (θ BaOx ≥ 2.5 MLE) were found to be amorphous. Extensive NO 2 adsorption on BaO x(10 MLE)/Pt(111) yields predominantly nitrate species that decompose at higher temperatures through the formation of nitrites. Nitrate decomposition occurs on BaO x(10 MLE)/Pt(111) in two successive steps: (1) NO(g) evolution and BaO 2 formation at 650 K and (2) NO(g) + O 2(g) evolution at 700 K. O 2(g) treatment of the BaO x(10 MLE)/Pt(111) surface at 873 K facilitates the BaO 2 formation and results in the agglomeration of BaO x domains leading to the generation of exposed Pt(111) surface sites. BaO 2 formed on BaO x(10 MLE)/Pt(111) is stable even after annealing at 1073 K, whereas on thinner films (θ BaOx = 2.5 MLE), BaO 2 partially decomposes into BaO, indicating that small BaO 2 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 BaO x coverage within θ BaOx = 0.5 to 1.0 MLE. Nitrate decomposition occurs at a rather constant temperature range of 650-700 K for thicker BaO x overlayers (2.5 MLE &lt; θ BaOx &lt; 10 MLE). These two distinctly characteristic BaO x-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 NO x storage domain size. © 2011 American Chemical Society