A self-adjusting platinum surface for acetone hydrogenation.

We show that platinum displays a self-adjusting surface that is active for the hydrogenation of acetone over a wide range of reaction conditions. Reaction kinetics measurements under steady-state and transient conditions at temperatures near 350 K, electronic structure calculations employing density-functional theory, and microkinetic modeling were employed to study this behavior over supported platinum catalysts. The importance of surface coverage effects was highlighted by evaluating the transient response of isopropanol formation following either removal of the reactant ketone from the feed, or its substitution with a similarly structured species. The extent to which adsorbed intermediates that lead to the formation of isopropanol were removed from the catalytic surface was observed to be higher following ketone substitution in comparison to its removal, indicating that surface species leading to isopropanol become more strongly adsorbed on the surface as the coverage decreases during the desorption experiment. This phenomenon occurs as a result of adsorbate-adsorbate repulsive interactions on the catalyst surface which adjust with respect to the reaction conditions. Reaction kinetics parameters obtained experimentally were in agreement with those predicted by microkinetic modeling when the binding energies, activation energies, and entropies of adsorbed species and transition states were expressed as a function of surface coverage of the most abundant surface intermediate (MASI, C3H6OH*). It is important that these effects of surface coverage be incorporated dynamically in the microkinetic model (e.g., using the Bragg-Williams approximation) to describe the experimental data over a wide range of acetone partial pressures.

Toward an Understanding of Selective Alkyne Hydrogenation on Ceria: On the Impact of O Vacancies on H2 Interaction with CeO2(111).

Ceria (CeO2) has recently been found to be a promising catalyst in the selective hydrogenation of alkynes to alkenes. This reaction occurs primarily on highly dispersed metal catalysts, but rarely on oxide surfaces. The origin of the outstanding activity and selectivity observed on CeO2 remains unclear. In this work, we show that one key aspect of the hydrogenation reaction-the interaction of hydrogen with the oxide-depends strongly on the presence of O vacancies within CeO2. Through infrared reflection absorption spectroscopy on well-ordered CeO2(111) thin films and density functional theory (DFT) calculations, we show that the preferred heterolytic dissociation of molecular hydrogen on CeO2(111) requires H2 pressures in the mbar regime. Hydrogen depth profiling with nuclear reaction analysis indicates that H species stay on the surface of stoichiometric CeO2(111) films, whereas H incorporates as a volatile species into the volume of partially reduced CeO2-x(111) thin films (x ∼ 1.8-1.9). Complementary DFT calculations demonstrate that oxygen vacancies facilitate H incorporation below the surface and that they are the key to the stabilization of hydridic H species in the volume of reduced ceria.

O2 Activation on Ceria Catalysts-The Importance of Substrate Crystallographic Orientation.

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O2 on ceria powders.

Support effect in oxide catalysis: methanol oxidation on vanadia/ceria.

Density functional theory is used for periodic models of monomeric vanadia species deposited on the CeO2(111) surface to study dissociative adsorption of methanol and its subsequent dehydrogenation to formaldehyde. Dispersion-corrected PBE+U calculations are performed and compared with HSE and B3LYP results. Dissociative adsorption of methanol at different sites on VO2·CeO2(111) is highly exothermic with adsorption energies of 1.8 to 1.9 eV (HSE+D). Two relevant pathways for desorption of formaldehyde are found with intrinsic barriers for the redox step of 1.0 and 1.4 eV (HSE+D). The calculated desorption temperatures (370 and 495 K) explain the peaks observed in temperature-programmed desorption experiments. Different sites of the supported catalyst system are involved in the two pathways: (i) methanol can chemisorb on the CeO2 surface filling a so-called pseudovacancy and the H atom is transferred to an V-O-Ce interphase bond or (ii) CH3OH may chemisorb at the V-O-Ce interphase bond and form a V-OCH3 species from which H is transferred to the ceria surface, providing evidence for true cooperativity. In both cases, ceria is directly involved in the redox process, as two electrons are accommodated in Ce f states forming two Ce(3+) ions whereas vanadium remains fully oxidized (V(5+)).

Palpebro-orbital apocrine cystadenoma: immunohistochemical verification of a unique variant with a critical differential diagnosis.

PURPOSE: To describe a unique apocrine cystadenoma of the superonasal eyelid and anterior orbit. METHODS: Clinical evaluation with axial and coronal CT; histopathologic and immunohistochemical studies including sections stained with hematoxylin-eosin, periodic acid Schiff, alcian blue, mucicarmine, and Prussian blue for iron; and monoclonal antibodies against cytokeratin-7, epithelial membrane antigen, smooth muscle actin for myoepithelial cells, gross cystic disease fluid protein-15 for apocrine differentiation, and CD-68 and lysozyme for histiocytes. RESULTS: The cyst possessed a multilaminar lining composed of polygonal to low cuboidal cells. A stalk of solid tumor ingrowth from the wall was composed of adenomatous units of eosinophilic cells with apical snouts ("decapitation secretion"). No goblet cells were discovered. Both the cyst's lining cells and the adenoma expressed gross cystic disease fluid protein-15 indicative of apocrine differentiation. The adenoma displayed inner adlumenal cells that were CK-7 and epithelial membrane antigen positive, and outer myoepithelial cells that were smooth muscle actin positive. Simple local excision was curative. CONCLUSION: This unique lesion is the first example in the ophthalmic and dermatopathologic literatures of a solid adenoma encompassed by an apocrine cyst, which more typically features short or blunt papillae. It must be distinguished from other eyelid and/or anterior orbital cystic lesions including eccrine hydrocystomas; classical and extratarsal dermoid cysts; congenital and acquired conjunctival cysts and dermoids; dacryocystocoeles/mucoceles; canaliculops and lacrimal gland dacryops; and congenital cystic odontogenic choristomas.

Stability and migration barriers of small vanadium oxide clusters on the CeO2(111) surface studied by density functional theory.

By virtue of periodic density functional theory, we investigate structure and thermodynamic stability of (VO)k and (VO2)k (k = 1, 2, 3) clusters deposited on the CeO2(111) surface, which serve as models for the very active sub-monolayer vanadia catalyst on a ceria support. We find V always completely oxidized (oxidation state +5) and coordinated to four O atoms. As a consequence, Ce4+ is (partially) reduced to Ce3+. Thus, localized Ce-4f states are populated, which requires an onsite U-term (PBE+U) to avoid over-delocalization off-electrons. Importantly, trimers of VO2 were found to be extraordinarily stable (agglomeration energy: -1.68 eV), whereas aggregation of VO species on CeO2(111) is thermodynamically clearly unfavourable (agglomeration energy: 3.45 eV). As a consequence a large area of the VnOm phase diagram (for relevant temperatures) is dominated by the VO2 trimer. The latter is less active towards reduction/oxidation than the active monomer and dimer of VO2, which are not present in the phase diagram at all, although directly observed by recent STM measurements. This suggests that kinetic effects hinder VO2 to grow into larger oligomers. The lowest migration energy barrier we found is as high as 1.95 eV, which indicates that adsorbed monomeric VO2 is "kinetically locked" at low temperatures and explains why monomers are stabilized on the ceria surface.