Defect-Electron Spreading on the TiO₂(110) Semiconductor Surface by Water Adsorption

The dissociative adsorption of water at oxygen-vacancy defect sites on the TiO₂(110) surface spatially redistributes the defect electron density originally present at subsurface sites near the defect sites. This redistribution of defect-electrons makes them more accessible to Ti⁴⁺ ions surrounding the defects. The redistribution of electron density decreases the O⁺ desorption yield from surface lattice O^2‑ ions in TiO₂, as excited by electron-stimulated desorption (ESD). A model in which OH formation on defect sites redistributes defect electrons to neighboring Ti⁴⁺ sites is proposed. This switches off the Knotek–Feibelman mechanism for ESD of O⁺ ions from lattice sites. Conversely, enhanced O⁺ reneutralization could also be induced by redistribution of defect electrons. The redistribution of surface electrons by adsorption is further verified by the use of donor and acceptor molecules that add or remove electron density

Selective Catalytic Oxidative-Dehydrogenation of Carboxylic AcidsAcrylate and Crotonate Formation at the Au/TiO₂ Interface

The oxidative-dehydrogenation of carboxylic acids to selectively produce unsaturated acids at the second and third carbons regardless of alkyl chain length was found to occur on a Au/TiO₂ catalyst. Using transmission infrared spectroscopy (IR) and density functional theory (DFT), unsaturated acrylate (H₂CCHCOO) and crotonate (CH₃CHCHCOO) were observed to form from propionic acid (H₃CCH₂COOH) and butyric acid (H₃CCH₂CH₂COOH), respectively, on a catalyst with ∼3 nm diameter Au particles on TiO₂ at 400 K. Desorption experiments also show gas phase acrylic acid is produced. Isotopically labeled ¹³C and ¹²C propionic acid experiments along with DFT calculated frequency shifts confirm the formation of acrylate and crotonate. Experiments on pure TiO₂ confirmed that the unsaturated acids were not produced on the TiO₂ support alone, providing evidence that the sites for catalytic activity are at the dual Au–Ti⁴⁺ sites at the nanometer Au particles’ perimeter. The DFT calculated energy barriers between 0.3 and 0.5 eV for the reaction pathway are consistent with the reaction occurring at 400 K on Au/TiO₂

Mechanistic Insights into the Catalytic Oxidation of Carboxylic Acids on Au/TiO₂: Partial Oxidation of Propionic and Butyric Acid to Gold Ketenylidene through Unsaturated Acids

The partial oxidation of model C₂–C₄ (acetic, propionic, and butyric) carboxylic acids on Au/TiO₂ catalysts consisting of Au particles ∼3 nm in size was investigated using transmission infrared spectroscopy and density functional theory. All three acids readily undergo oxidative dehydrogenation on Au/TiO₂. Propionic and butyric acid dehydrogenate at the C2–C3 positions, whereas acetic acid dehydrogenates at the C1–C2 position. The resulting acrylate and crotonate intermediates are subsequently oxidized to form β-keto acids that decarboxylate. All three acids form a gold ketenylidene intermediate, Au₂CCO, along the way to their full oxidation to form CO₂. Infrared measurements of Au₂CCO formation as a function of time provides a surface spectroscopic probe of the kinetics for the activation and oxidative dehydrogenation of the alkyl groups in the carboxylate intermediates that form. The reaction proceeds via the dissociative adsorption of the acid onto TiO₂, the adsorption and activation of O₂ at the dual perimeter sites on the Au particles (Au–O–O-Ti), and the subsequent activation of the C2–H and C3–H bonds of the bound propionate and butyrate intermediates by the weakly bound and basic oxygen species on Au to form acrylate and crotonate intermediates, respectively. The CC bond of the unsaturated acrylate and crotonate intermediates is readily oxidized to form an acid at the beta (C3) position, which subsequently decarboxylates. This occurs with an overall activation energy of 1.5–1.7 ± 0.2 eV, ultimately producing the Au₂CCO species for all three carboxylates. The results suggest that the decrease in the rate in moving from acetic to propionic to butyric acid is due to an increase in the free energy of activation for the formation of the Au₂CCO species on Au/TiO₂ with an increasing size of the alkyl substituent. The formation of Au₂CCO proceeds for carboxylic acids that are longer than C₂ without a deuterium kinetic isotope effect, demonstrating that C–H bond scission is not involved in the rate-determining step; the rate instead appears to be controlled by C–O bond scission. The adsorbed Au₂CCO intermediate species can be hydrogenated to produce ketene, H₂CCO­(g), with an activation energy of 0.21 ± 0.05 eV. These studies show that selective oxidative dehydrogenation of the alkyl side chains of fatty acids can be catalyzed by nanoparticle Au/TiO₂ at temperatures near 400 K

Hybridization of Phenylthiolate- and Methylthiolate-Adatom Species at Low Coverage on the Au(111) Surface

Using scanning tunneling microscopy we observed reaction products of two chemisorbed thiolate species, methylthiolate and phenylthiolate, on the Au(111) surface. Despite the apparent stability, organometallic complexes of methyl- and phenylthiolate with the gold-adatom (RS‑Au‑SR, with R as the hydrocarbon group) undergo a stoichiometric exchange reaction, forming hybridized CH₃S‑Au‑SPh complexes. Complementary density functional theory calculations suggest that the reaction is most likely mediated by a monothiolate RS‑Au complex bonded to the gold surface, which forms a trithiolate RS‑Au‑(SR)‑Au‑SR complex as a key intermediate. This work therefore reveals the novel chemical reactivity of the low-coverage “striped” phase of alkanethiols on gold and strongly points to the involvement of monoadatom thiolate intermediates in this reaction. By extension, such intermediates may be involved in the self-assembly process itself, shedding new light on this long-standing problem

Methyl Radical Reactivity on the Basal Plane of Graphite

The reaction of submonolayer Li atoms with CH₃Cl at 100 K on a highly oriented pyrolytic graphite (HOPG) surface has been studied under ultrahigh vacuum. We exploit the low defect density of the high quality HOPG used here (∼10⁹ defects cm^–2) to eliminate the effects of step edges and defects on the graphite surface chemistry. Li causes C–Cl bond scission in CH₃Cl, liberating CH₃ radicals below 130 K. Ordinarily, two CH₃ species would couple to form products such as C₂H₆, but in the presence of graphite, CH₃ preferentially adsorbs on the flat basal plane of Li-treated graphite. A C–CH₃ bond of 1.2 eV is formed, which is enhanced relative to CH₃ binding to clean graphite (0.52 eV) due to donation of electrons from Li into the graphite and back-donation from graphite to CH₃. A low yield of C₁, C₂, and C₃ hydrocarbon products above 330 K is found along with a low yield of H₂. The low yield of these products indicates that the majority of the CH₃ groups are irreversibly bound to the basal plane of graphite, and only a small fraction participate in the production of C₁–C₃ volatile products or in extensive dehydrogenation. Spin-polarized density functional theory calculations indicate that CH₃ binds to the Li-treated surface with an activation energy of 0.3 eV to form a C–CH₃ adsorbed surface species with sp³ hybridization of the graphite, and the methyl carbon atoms is involved in bond formation. Bound CH₃ radicals become mobile with 0.7 eV activation energy and can participate in combination reactions for the production of small yields of C₁–C₃ hydrocarbon products. We show that alkyl radical attachment to the graphite surface is kinetically preferred over hydrocarbon product desorption

Localized Partial Oxidation of Acetic Acid at the Dual Perimeter Sites of the Au/TiO₂ CatalystFormation of Gold Ketenylidene

Chemisorbed acetate species derived from the adsorption of acetic acid have been oxidized on a nano-Au/TiO₂ (∼3 nm diameter Au) catalyst at 400 K in the presence of O₂(g). It was found that partial oxidation occurs to produce gold ketenylidene species, Au₂CCO. The reactive acetate intermediates are bound at the TiO₂ perimeter sites of the supported Au/TiO₂ catalyst. The ketenylidene species is identified by its measured characteristic stretching frequency ν­(CO) = 2040 cm^–1 and by ¹³C and ¹⁸O isotopic substitution comparing to calculated frequencies found from density functional theory. The involvement of dual catalytic Ti⁴⁺ and Au perimeter sites is postulated on the basis of the absence of reaction on a similar nano-Au/SiO₂ catalyst. This observation excludes low coordination number Au sites as being active alone in the reaction. Upon raising the temperature to 473 K, the production of CO₂ and H₂O is observed as both acetate and ketenylidene species are further oxidized by O₂(g). The results show that partial oxidation of adsorbed acetate to adsorbed ketenylidyne can be cleanly carried out over Au/TiO₂ catalysts by control of temperature

Inhibition at Perimeter Sites of Au/TiO₂ Oxidation Catalyst by Reactant Oxygen

TiO₂-supported gold nanoparticles exhibit surprising catalytic activity for oxidation reactions compared to noble bulk gold which is inactive. The catalytic activity is localized at the perimeter of the Au nanoparticles where Au atoms are atomically adjacent to the TiO₂ support. At these dual-catalytic sites an oxygen molecule is efficiently activated through chemical bonding to both Au and Ti⁴⁺ sites. A significant inhibition by a factor of 22 in the CO oxidation reaction rate is observed at 120 K when the Au is preoxidized, caused by the oxygen-induced positive charge produced on the perimeter Au atoms. Theoretical calculations indicate that induced positive charge occurs in the Au atoms which are adjacent to chemisorbed oxygen atoms, almost doubling the activation energy for CO oxidation at the dual-catalytic sites in agreement with experiments. This is an example of self-inhibition in catalysis by a reactant species