The Mg-catalyzed dehydrogenation of ethanol to yield acetaldehyde is an important step in the Lebedev reaction. In this work, we prepared a model MgO−SiO 2 catalyst by impregnation of MgO onto an SBA-15 support and used this material to study the reaction kinetics of ethanol dehydrogenation to acetaldehyde. The rates of acetaldehyde and ethylene production were measured for ethanol partial pressures ranging from 0.92 to 5.25 kPa. Both rates are fractional order at 723 K, decreasing to nearly zero-order at 648 K. Consistent with the literature for MgO−SiO 2 Lebedev catalysts, both basic sites and Lewis acidic sites were observed on this catalyst. The rates of both acetaldehyde and ethylene were inhibited by pyridine but not by 2,6-ditertbutylpyridine, suggesting that both reactions involve not only basic but also Lewis acidic sites. To elucidate the origin of this cooperativity, a microkinetic model was constructed using a recently published mechanism for the Lebedev reaction catalyzed by MgO. The model was fit to our data using four fitting parameters. The fitting suggests that adsorbed ethanol and hydrogen atoms have a weaker bond with mixed-oxide MgO−SiO 2 catalysts than with bulk MgO catalysts, which we attribute experimentally to an increase in the number of moderate-strength Mg 2+ − O 2− site pairs formed at the expense of strongly basic MgO sites.

Au nanoparticles (NP) on TiO 2 have been shown to be effective catalysts for selective oxidation reactions by using molecular oxygen. In this work, we have studied the influence of support morphology on the catalytic activity of Au/TiO 2 catalysts. Two TiO 2 anatase supports, a nanoplatelet-shaped material with predominantly the {001} facet exposed and a truncated bipyramidal-shaped nanoparticle with predominantly the {101} facet exposed, were prepared by using a nonaqueous solvothermal method and characterized by using DRIFTS, XPS, and TEM. Au nanoparticles were deposited on the supports by using the deposition−precipitation method, and particle sizes were determined by using STEM. Au nanoparticles were smaller on the support with the majority of the {101} facet exposed. The resulting materials were used to catalyze the aerobic oxidation of benzyl alcohol and trifluoromethylbenzyl alcohol. Support morphology impacts the catalytic activity of Au/TiO 2 ; reaction rates for reactions catalyzed by the predominantly {101} material were higher. Much of the increased reactivity can be explained by the presence of smaller Au particles on the predominantly {101} material, providing more Au/TiO 2 interface area, which is where catalysis occurs. The remaining modest differences between the two catalysts are likely due to geometric effects as Hammett slopes show no evidence for electronic differences between the Au particles on the different materials.

Numerous studies show that sulfating titania narrows its band gap, facilitating longer-wavelength photochemistry, but there are contradictory reports on whether this improves or degrades UV photocatalysis and whether reactions proceed via electron-or hole-mediated pathways. The widely proposed role of sulfur is that it induces deep electron traps, which increase hole lifetimes. There is, however, no direct evidence for unoccupied states deep in the band gap. By contrast, transient absorption spectroscopy indicates that sulfur induces hole traps. We present experiments on sulfur-free and sulfated titania in which dissociation of hydrogen generates electrons that fill the lowest unoccupied states. The energy of these electrons relative to the conduction band minimum (CBM) was measured with diffuse reflectance spectroscopy in the infrared and UV−vis ranges. For all commercial sulfur-containing anatase materials, conversion of tridentate sulfate species into sulfur substituted on lattice sites occurred under highly oxidizing conditions above 400 °C and led to partially unoccupied states ∼2.8 eV below the CBM. We assign this deep trap state to sulfur atoms substituted on a titanium lattice site with a formal charge of S 5+ in non-stoichiometric TiO 2+x , based on agreement between the experiment and the predicted UV− vis spectrum of Harb, Sautet, and Raybaud, using HSE06 density functional perturbation theory. Our band structure calculations demonstrate that titanium vacancies (or excess oxygen) are necessary to create partially unoccupied states, and X-ray diffraction Rietveld analysis confirms the existence of these vacancies. The partial occupancy of these states, along with sulfur's ability to switch oxidation states, explains their role as both electron (S 5+ + e − → S 4+ ) and hole (S 5+ + h + → S 6+ ) traps, reconciling previous work. We discuss how relative rates of electron vs hole trapping can enhance or degrade activity depending on the pathway and the TiO 2+x non-stoichiometry. We consider how increasing the dopant concentration can induce band bending or pin the Fermi level and shift the redox reactions that are thermodynamically accessible.