Jr. John T. Yates’s research while affiliated with University of Virginia and other places

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.Keywords: Au/TiO2; catalysis; oxidative-dehydrogenation; carboxylic acid oxidation; decarboxylation; ketenylidene

Infrared studies of Au/TiO2 catalyst particles indicate that charge transfer from van der Waals-bound donor or acceptor molecules on TiO2 to Au occurs via transport of charge carriers in the semiconductor TiO2 support. The ∆νCO on Au is shown to be proportional to the polarizability of the TiO2 support fully covered with donor or acceptor molecules, producing a proportional frequency shift in νCO. Charge transfer through TiO2 is associated with the population of electron trap sites in the bandgap of TiO2 and can be independently followed by changes in photoluminescence (PL) intensity and by shifts in the broad infrared (IR) absorbance region for electron trap sites, which is also proportional to the polarizability of donors by IR excitation. Density functional theory calculations show that electron transfer from the donor molecules to TiO2 and to supported Au particles produces a negative charge on the Au, whereas the transfer from the Au particles to the TiO2 support into acceptor molecules results in a positive charge on the Au. These changes along with the magnitudes of the shifts are consistent with the Stark effect. A number of experiments show that the ~3 nm Au particles act as "Molecular Voltmeters" in influencing ∆νCO. Insulator particles, such as SiO2 do not display electron transfer effects to Au particles on their surface. These studies are preliminary to doping studies of semiconductor-oxide particles by metal ions which modify Lewis acid/base oxide properties and possibly strongly modify the electron transfer and catalytic activity of supported metal catalyst particles.

CO2 is one of the most abundant components of ices in the interstellar medium; however, its formation mechanism has not been clearly identified. Here we report an experimental observation of an Eley-Rideal-type reaction on a water ice surface, where CO gas molecules react by direct collisions with surface OH radicals, made by photodissociation of H2O molecules, to produce CO2 ice on the surface. The discovery of this source of CO2 provides a new mechanism to explain the high relative abundance of CO2 ice in space.

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 and butyric acid, respectively, on a ~3 nm Au diameter Au/TiO2 catalyst at 400 K. Desorption experiments also show gas phase acrylic acid is produced. Isotopically labeled (13)C and (12)C 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-Ti(4+) sites at the nanometer Au particles' perimeter. The DFT calculated energy barriers between 0.3-0.5 eV for the reaction pathway are consistent with the reaction occurring at 400 K on Au/TiO2.

Photoluminescence spectroscopy was employed to observe electron transport between TiO2 particles. Ultraviolet (UV) irradiation (3.88 eV) was shown to positively enhance the photovoltage of TiO2 particles at the powder surface, causing an enhancement of their photoluminescence (PL) at 530 nm. The charging of the TiO2 particles on the powder surface by UV irradiation partially discharges in the dark, where the displaced bulk negative charge diffuses back toward the TiO2 surface. This charge flow partially restores upward band bending, causing the PL intensity to decrease. The rate of the discharging process was used to estimate the electron migration mobility (10–10 m2 V–1 s–1 at 300 K) between TiO2 particles in the TiO2 matrix. Electron migration between TiO2 particles is temperature-dependent with an activation energy of 0.015 ± 0.008 eV. In addition, it was found that the adsorption of an immobile electron-donor molecule (NH3), attracts negative charge on the TiO2 surface which does not exhibit mobility behavior, in contrast to mobile electrons produced by UV. These measurements were carried out in high vacuum in the absence of oxygen and surface impurities detectable by IR spectroscopy.

The unique interfacial sites of Au nanoparticles supported on TiO2 are known to catalyze the activation of oxygen and it's addition to small molecules including H2, CO, NO and propylene. Herein we extend these ideas and show that the unique Au-Ti dual perimeter sites that form at the Au/TiO2 interface can also catalyze more demanding C-H and C-O bond activation reactions involved in the deoxygenation organic acids such as acetic acid. We have shown previously that acetic acid can be partially oxidized on a Au/TiO2 catalyst to form a novel gold ketenylidene (Au2==C==C==O) intermediate. In the present work we use in situ infrared spectroscopy and first-principle density functional theory (DFT) to examine the mechanism and the kinetics by which this reaction proceeds. The reaction was found to be localized at the dual perimeter sites of the Au/TiO2 catalyst, where 02 was activated. In contrast to Au/TiO2, no ketenylidene formation was observed on a similar Au/SiO2 catalyst or a TiO2 blank sample. The reaction involves the activation of multiple C-H bonds as well as the C-O bond in the adsorbed CH3COO species. C-O bond scission is postulated to occur at the TiO2 sites, while C-H bond scission occurs on Au sites, both near the active Au-Ti4+ dual perimeter sites. 18O2 isotopic labeling indicated that the O moiety of the ketenylidene species originates from the acetic acid during the oxidation process involving molecular O2. The rate-limiting step was found to be the C--O bond scission resulting in an apparent overall activation energy of 1.72 eV as determined from DFT calculations. This is in very good agreement with the experimentally measured apparent activation energy of 1.7 +/- 0.2 eV. A deuterium kinetic isotope effect of approximately 4 indicates that C-H bond activation is kinetically involved in the overall acetate oxidation reaction.

Single walled carbon nanotubes (SWNTs) exhibit high surface areas and precisely defined pores, making them potentially useful materials for gas adsorption and purification. A thorough understanding of the interactions between adsorbates and SWNTs is therefore critical to predicting adsorption isotherms and selectivities. Metallic (M-) and Semiconducting (S-) SWNTs have extremely different polarizabilities that might be expected to significantly affect the adsorption energies of molecules. We experimentally and theoretically show that this expectation is contradicted, for both a long chain molecule (n-heptane) and atoms (Ar, Kr, and Xe). Temperature programmed desorption experiments are combined with van der Waals corrected density functional theory, examining adsorption on interior and exterior sites of the SWNTs. Our calculations show a clear dependence of the adsorption energy on nanotube diameter, but not on whether the tubes are conducting or insulating. We find no significant experimental or theoretical difference in adsorption energies for molecules adsorbed on M- and S-SWNTs having the same diameter. Hence we conclude that the differences in polarizabilities between M- and S-SWNTs have a negligible influence on gas adsorption for spherical molecules as well as for highly anisotropic molecules such as n-heptane. We expect this conclusion to apply to all types of adsorbed molecules where van der Waals interactions govern the molecular interaction with the SWNT.

The differences in the polarizabilities of metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) might give rise to differences in adsorption potentials. We show from experiments and van der Waals-corrected density functional theory that the binding energies of Xe adsorbed on M- and S-SWNTs are nearly identical. Temperature programed desorption experiments of Xe on purified M- and S-SWNTs give similar peak temperatures, indicating that desorption kinetics and binding energies are independent of the type of SWNT. Binding energies computed from vdW-corrected density functional theory are in good agreement with experiments.

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 O2- 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.