Acetone Chemistry on Oxidized and Reduced TiO2(110)

Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO₂(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO₂, Au₉/s-TiO₂, and reduced (r) Au₉/r-TiO₂. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au₉ clusters on stoichiometric TiO₂ are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor.

Breath Acetone Sensing Based on Single-Walled Carbon Nanotube-Titanium Dioxide Hybrids Enabled by a Custom-Built Dehumidifier

Acetone is a metabolic byproduct found in the exhaled breath and can be measured to monitor the metabolic degree of ketosis. In this state, the body uses free fatty acids as its main source of fuel because there is limited access to glucose. Monitoring ketosis is important for type I diabetes patients to prevent ketoacidosis, a potentially fatal condition, and individuals adjusting to a low-carbohydrate diet. Here, we demonstrate that a chemiresistor fabricated from oxidized single-walled carbon nanotubes functionalized with titanium dioxide (SWCNT@TiO₂) can be used to detect acetone in dried breath samples. Initially, due to the high cross sensitivity of the acetone sensor to water vapor, the acetone sensor was unable to detect acetone in humid gas samples. To resolve this cross-sensitivity issue, a dehumidifier was designed and fabricated to dehydrate the breath samples. Sensor response to the acetone in dried breath samples from three volunteers was shown to be linearly correlated with the two other ketone bodies, acetoacetic acid in urine and β-hydroxybutyric acid in the blood. The breath sampling and analysis methodology had a calculated acetone detection limit of 1.6 ppm and capable of detecting up to at least 100 ppm of acetone, which is the dynamic range of breath acetone for someone with ketosis. Finally, the application of the sensor as a breath acetone detector was studied by incorporating the sensor into a handheld prototype breathalyzer.

Real-time monitoring of surface acetone enolization and aldolization

Real-time DRIFTS reveals the formation of acetone enolate and its subsequent aldolization via an Eley–Rideal type mechanism on Zn₁Zr₁₀O_z.

Photocatalytic selectivity switch to C–C scission: α-methyl ejection oftert-butanol on TiO2(110)

The thermal and photochemical mechanistic pathways for tertiary alcohols on the rutile TiO₂(110)-surface are studied with the example oftert-butanol.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Photoinduced decomposition of acetaldehyde on a reduced TiO2(110) surface: involvement of lattice oxygen

A simple model reveals that BBO atoms are involved in the photo-induced decomposition of CH₃CHO on TiO₂(110).

Elementary photocatalytic chemistry on TiO2surfaces

In this article, we review the recent advances in the photoreactions of small molecules with model TiO₂surfaces, and propose a photocatalytical model based on nonadiabatic dynamics and ground state surface reactions.

On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO2

Partial insight into the reaction mechanism of the photooxidation of 2-propanol over Au/TiO₂. 2-Propanol is activated by hole capture and converted to acetone requiring the presence of O₂ as an electron acceptor. The deposited Au nanoparticles are assumed to facilitate the electron transfer from the TiO₂ conduction band to adsorbed O₂.

Defects in surface chemistry--reductive coupling of benzaldehyde on rutile TiO₂(110)

The surface chemistry of oxygen and oxygenates on Rutile TiO2(110) is of great interest for various applications such as heterogeneous catalysis and photo catalysis. Though it is generally accepted that surface defects are active sites, the role of subsurface defects is under debate. We have therefore investigated the influence of the bulk defect density on the reductive coupling of benzaldehyde to stilbene as a model system. Using IRRAS we identify stilbene diolate as a reduction intermediate. The concentration of this intermediate is proportional to the bulk defect density, whereas adsorption of benzaldehyde at lower temperatures is not affected, which indicates a dominant role of Ti interstitials at temperatures above 400 K.

Photocatalytic removal of soot: unravelling of the reaction mechanism by EPR and in situ FTIR spectroscopy

Photocatalytic soot oxidation is studied on P25 TiO(2) as an important model reaction for self-cleaning processes by means of electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Contacting of carbon black with P25 leads on the one hand to a reduction of the local dioxygen concentration in the powder. On the other hand, the weakly adsorbed radicals on the carbon particles are likely to act as alternative traps for the photogenerated conduction-band electrons. We find furthermore that the presence of dioxygen and oxygen-related radicals is vital for the photocatalytic soot degradation. The complete oxidation of soot to CO(2) is evidenced by in situ FTIR spectroscopy, no intermediate CO is detected during the photocatalytic process.

Acetone-Assisted Oxygen Vacancy Diffusion on TiO2(110)

We have studied the dynamic relationship between acetone and bridge-bonded oxygen (Ob) vacancy (VO) defect sites on the TiO2(110)-1 × 1 surface using scanning tunneling microscopy (STM) and density function theory (DFT) calculations. We report an adsorbate-assisted VO diffusion mechanism. The STM images taken at 300 K show that acetone preferably adsorbs on the VO site and is mobile. The sequential isothermal STM images directly show that the mobile acetone effectively migrates the position of VO by a combination of two acetone diffusion channels: one is the diffusion along the Ob row and moving as an alkyl group, which heals the initial VO; another is the diffusion from the Ob row to the five-coordinated Ti(4+) row and then moving along the Ti(4+) row as an acetone, which leaves a VO behind. The calculated acetone diffusion barriers for the two channels are comparable and agree with experimental results.

Interaction of petroleum-relevant organosulfur compounds with TiO2(110)

The interaction of two sets of structurally related molecules, thiophenol/thioanisole, and thiophene/tetrahydrothiophene, with vacuum-annealed and ion-bombarded TiO(2)(110) surfaces has been studied using a combination of temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). All thioethers studied were observed to adsorb and desorb from both surfaces without producing reaction products, while thiophenol, the only species studied containing a S-H bond, reacted with both surfaces. Approximately 25% of surface bound thiophenol decomposed over the vacuum-annealed surface. On the bombarded surface, thiophenol both decomposed into surface-bound C(x)H(y)/S fragments, and reacted to form benzene, which desorbed from the surface at 400 K. We propose that phenylthiolate formation on the bombarded surface leads to the observed production of benzene. These results highlight the importance of defects in the reactivity of titania, and lay the foundation for the study of larger, refractory sulfur compounds present in fuel.

Butyrophenone on O-TiO2(110): one-dimensional motion in a weakly confined potential well

We demonstrate the one-dimensional confinement of weakly bound butyrophenone molecules between strongly bound complexes formed via reaction with oxygen on TiO(2)(110). Butyrophenone weakly bound to Ti rows through the carbonyl oxygen diffuses freely in one dimension along the rows even at 55 K, persisting for many minutes before hopping out of the 1-D well. Quantitative analysis yields an estimate of the migration barrier of 0.11 eV and a frequency factor of 6.5 × 10(9) Hz. These studies demonstrate that weakly bound organic molecules can be confined on a surface by creating molecular barriers, potentially altering their assembly.

Site competition during coadsorption of acetone with methanol and water on TiO2(110)

The competitive interaction between acetone and two solvent molecules (methanol and water) for surface sites on rutile TiO(2)(110) was studied using temperature-programmed desorption (TPD). On a vacuum-annealed TiO(2)(110) surface, which possessed ~5% oxygen vacancy sites, excess methanol displaced preadsorbed acetone molecules to weakly bound and physisorbed desorption states below 200 K. In contrast, acetone molecules were stabilized on an oxidized surface against displacement by methanol through formation of acetone diolate species. The behavior of acetone with methanol differs from the interactions between acetone and water which are less competitive. Examination of acetone + methanol and acetone + water multilayer combinations shows that acetone is more compatible in water-ice films than in methanol-ice films, presumably because water has greater potential as a hydrogen-bond donor than does methanol. Acetone molecules displaced from the TiO(2)(110) surface by water are more likely to be retained in the near-surface region, in turn having a greater opportunity to revisit the surface, than when methanol is used as a coadsorbate.

Molecular imaging of reductive coupling reactions: interstitial-mediated coupling of benzaldehyde on reduced TiO2(110)

We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface-a diolate formed from benzaldehyde coupling on TiO(2)(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to ∼400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.

Acetone reactions over the surfaces of polycrystalline UO2: a kinetic and spectroscopic study

The reaction of acetone is studied on the surfaces of polycrystalline UO2, prepared by hydrogen reduction of U3O8 at 770 K. The study is conducted by in situ Fourier transform infrared (FTIR) and temperature-programmed desorption (TPD). Acetone adsorption does not fit the simple Langmuir model, and adsorbate-adsorbate interactions are found to be significant. Acetone adsorbs molecularly on UO2 as evidenced by the nuCO of the eta1(O) mode at 1686 cm(-1). Part of acetone is reduced to the isopropoxide species ((CH3)2HC-O-U4+) upon heating (nu(CC), rho(CH3) at 1167 cm(-1) and nu(CO), rho(CH3) at 980 cm(-1)), and upon further heating, acetates (CH3COO(a), (a) for adsorbed) are observed. Detailed TPD studies indicated that the main reaction of acetone on UO2 is the deoxygenation to propene, driven by the oxophilic nature of UO2. Other reactions were also observed to a lesser extent, and these included reductive coupling to 2,3-dimethylbutene and condensation to mesityl oxide. An attempt to extract kinetic parameters from TPD data was conducted. Three models were studied: variation of heating rate, leading edge analysis (Habenschaden-Kuppers method), and complete analysis. The complete analysis provided the most plausible results, in particular, at low coverage. With this method, at nearly zero coverage the activation energy, Ed, for desorption was found to be close to 140 kJ/mol with a prefactor of 10(13) s(-1). Ed dropped sharply with increasing coverage, theta, to ca. 35 kJ/mol at theta=0.15 with a prefactor of 10(11) s(-1). The activation energy for the desorption of acetone on UO2(111) single crystals, at saturation coverage, was previously found to be equal to 65 kJ/mol using the leading edge analysis.

Two pathways for water interaction with oxygen adatoms on TiO2(110)

Scanning tunneling microscopy and density functional theory studies show that oxygen adatoms (Oa), produced during O2 exposure of reduced TiO2(110) surfaces, alter the water dissociation and recombination chemistry through two distinctive pathways. Depending on whether H2O and Oa are on the same or adjacent Ti4+ rows, Oa facilitates H2O dissociation and proton transfer to form a terminal hydroxyl pair, positioned along or across the Ti4+ row, respectively. The latter process has not been reported previously, and it starts from a "pseudodissociated" state of water. In both pathways, the reverse H transfer results in H2O reformation and O scrambling, as manifested by an apparent along- or across-row motion of Oa's.

Computational study of the reaction of atomic oxygen with acetone in the gas phase

Mechanisms and kinetics of the reaction of atomic oxygen with acetone have been investigated using ab initio quantum chemistry methods and transition state theory. The structures of the stationary points along the possible reaction pathways were obtained using the second-order Møller-Plesset theory and the coupled-cluster theory with single and double excitations with the triple-zeta quality basis sets. The energetics of the reaction pathways were calculated at the reduced second-order Gaussian-3 level and the extrapolated full coupled-cluster/complete basis set limit. The rate coefficients were calculated in the temperature range 200-3000 K, with the detailed consideration of the hindered internal rotation and the tunneling effect using Eckart and the semiclassical WKB approximations. It is shown that the predominant mechanism is the direct hydrogen abstraction producing hydroxyl and acetonyl radicals. Although the nucleophilic OC addition/elimination channel leading to CH3 and CO2 involves comparable barrier with the direct hydrogen abstraction channel, kinetically it cannot play any role in the overall reaction. It is predicted that the rate coefficients show positive temperature dependence in the range 200-3000 K and strong non-Arrhenius behavior. The tunneling effect plays a significant role. Moreover, the reaction has strong kinetic isotope effect. The calculated results are in good agreement with the available experimental data. The present rigorous theoretical work is helpful for the understanding of the characteristics of the reaction of atomic oxygen with acetone.

Interaction of the indoor air pollutant acetone with Degussa P25 TiO2 studied by chemical ionization mass spectrometry

Preventing a build-up of indoor pollutant concentrations has emerged as a major goal in environmental chemistry. Here, we have applied chemical ionization mass spectrometry to study the interaction of acetone, a common indoor air pollutant, with Degussa P25 TiO2, an inexpensive catalyst that is widely used for the degradation of volatile organic compounds into CO2 and water. To better understand the adsorption of acetone onto Degussa P25, the necessary first step for its degradation, the experiments were carried out at room temperature in the absence of UV light. This allowed for the deconvolution of the nonreactive and reactive thermal binding processes on Degussa P25 at acetone partial pressures (10(-7)-10(-4) Torr) commonly found in indoor environments. On average, 30% of the adsorbed acetone is bound irreversibly, resulting in a surface coverage of irreversibly bound acetone of approximately 1 x 10(12) molecules/cm2 at 3-4 x 10(-5) Torr. Equilibrium and dynamic experiments yield a sticking coefficient of approximately 1 x 10(-4) that is independent of the acetone partial pressures examined here. Equilibrium binding constants and free energies of adsorption are reported.

Adsorption and Solar Light Decomposition of Acetone on Anatase TiO2 and Niobium Doped TiO2 Thin Films

Adsorption and solar light decomposition of acetone was studied on nanostructured anatase TiO2 and Nb-doped TiO2 films made by sol-gel methods (10 and 20 mol % NbO2.5). A detailed characterization of the film materials show that films contain only nanoparticles with the anatase modification with pentavalent Nb oxide dissolved into the anatase structure, which is interpreted as formation of substituted Nb=O clusters in the anatase lattice. The Nb-doped films displayed a slight yellow color and an enhanced the visible light absorption with a red-shift of the optical absorption edge from 394 nm for the pure TiO2 film to 411 nm for 20 mol % NbO2.5. In-situ Fourier transform infrared (FTIR) transmission spectroscopy shows that acetone adsorbs associatively with eta1-coordination to the surface cations on all films. On Nb-doped TiO2 films, the carbonyl bonding to the surface is stabilized, which is evidenced by a lowering of the nu(C=O) frequency by about 20 cm(-1) to 1672 cm(-1). Upon solar light illumination acetone is readily decomposed on TiO2, and stable surface coordinated intermediates are formed. The decomposition rate is an order of magnitude smaller on the Nb-doped films despite an enhanced visible light absorption in these materials. The quantum yield is determined to be 0.053, 0.004 and 0.002 for the pure, 10% Nb:TiO2, and 20%Nb:TiO2, respectively. Using an interplay between FTIR and DFT calculations we show that the key surface intermediates are bidentate bridged formate and carbonate, and H-bonded bicarbonate, respectively, whose concentration on the surface can be correlated with their heats of formation and bond strength to coordinatively unsaturated surface Ti and Nb atoms at the surface. The oxidation rate of these intermediates is substantially slower than the initial acetone decomposition rate, and limits the total oxidation rate at t>7 min on TiO2, while no decrease of the rate is observed on the Nb-doped films. The rate of degradation of key surface intermediates is different on pure TiO2 and Nb-doped TiO2, but cannot explain the overall lower total oxidation rate for the Nb-doped films. Instead the inferior photocatalytic activity in Nb-doped TiO2 is attributed to an enhanced electron-hole pair recombination rate due to Nb=O cluster and cation vacancy formation.