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

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

Cathodic Corrosion at the Bismuth–Ionic Liquid Electrolyte Interface under Conditions for CO₂ Reduction

Bismuth electrodes undergo distinctive electrochemically induced structural changes in nonaqueous imidazolium ([Im]⁺)-based ionic liquid solutions under cathodic polarization. In situ X-ray reflectivity (XR) studies have been undertaken to probe well-ordered Bi (001) films which originally contain a native Bi₂O₃ layer. This oxide layer gets reduced to Bi⁰ during the first cyclic voltammetry (CV) scan in acetonitrile solutions containing 1-butyl-3-methylimidazolium ([BMIM]⁺) electrolytes. Approximately 60% of the Bi (001) Bragg peak reflectivity is lost during a potential sweep between −1.5 and −1.9 V vs Ag/AgCl due to a ∼ 4–10% thinning and a ∼40% decrease in lateral size of Bi (001) domains, which are mostly reversed during the anodic scan. Repeated potential cycling enhances the thinning and roughening of the films, suggesting that partial dissolution of Bi ensues during negative polarization. The mechanism of this behavior is understood through molecular dynamics simulations using ReaxFF and density functional theory (DFT) calculations. Both approaches indicate that [Im]⁺ cations bind to the metal surface more strongly than tetrabutylammonium (TBA⁺) as the potential and the charge on the Bi surface become more negative. ReaxFF simulations predict a higher degree of disorder for a negatively charged Bi (001) slab in the presence of the [Im]⁺ cations and substantial migration of Bi atoms from the surface. DFT simulations show the formation of Bi···[Im]⁺ complexes that lead to the dissolution of Bi atoms from step edges on the Bi (001) surface at potentials between −1.65 and −1.95 V. Bi desorption from a flat terrace requires a potential of approximately −2.25 V. Together, these results suggest the formation of a Bi···[Im]⁺ complex through partial cathodic corrosion of the Bi film under conditions (potential and electrolyte composition) that favor the catalytic reduction of CO₂