Origin of Coverage Dependence in Photoreactivity of Carboxylate on TiO₂(110): Hindering by Charged Coadsorbed Hydroxyls

The influence of reactant coverage on photochemical activity was explored using scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS). We observed diminished reactivity of carboxylate species (trimethyl acetate, TMA) on TiO₂(110) as a function of increasing coverage. This effect was not linked to intermolecular interactions of TMA but to the accumulation of the coadsorbed bridging hydroxyls (HO_b) deposited during (thermal) dissociative adsorption of the parent, trimethylacetic acid (TMAA). Confirmation of the hindering influence of HO_b groups was obtained by the observation that HO_b species originated from H₂O dissociation at O-vacancy sites have a similar hindering effect on TMA photochemistry. Though HO_b’s are photoinactive on TiO₂(110) under ultrahigh vacuum conditions, UPS results show that these sites trap photoexcited electrons, which in turn likely (electrostatically) attract and neutralize photoexcited holes, thus suppressing the hole-mediated photoreactivity of TMA. This negative influence of surface hydroxyls on hole-mediated photochemistry is likely a major factor in other anaerobic photochemical processes on reducible oxide surfaces

Direct Observation of Site-Specific Molecular Chemisorption of O₂ on TiO₂(110)

Molecularly chemisorbed O₂ species were directly imaged on reduced TiO₂(110) at 50 K with high-resolution scanning tunneling microscopy (STM). Two different O₂ adsorption channels, one at bridging oxygen vacancies (V_O) and another at 5-fold coordinated terminal titanium atoms (Ti_5c), have been identified. While O₂ species at the Ti_5c site appears as a single protrusion centered on the Ti_5c row, the O₂ at V_O manifests itself by a disappearance of the V_O feature. It is found that the STM tip can easily dissociate O₂ species, unless extremely low magnitude of the tunneling parameters are used. The O₂ molecules chemisorbed at low temperatures at these two distinct sites are the most likely precursors for the two O₂ dissociation channels, observed at temperatures above 150 and 230 K at the V_O and Ti_5c sites, respectively

Characterization of the Active Surface Species Responsible for UV-Induced Desorption of O₂ from the Rutile TiO₂(110) Surface

We have examined the chemical and photochemical properties of molecular oxygen on the (110) surface of rutile TiO₂ at 100 K using electron energy loss spectroscopy (EELS), photon stimulated desorption (PSD), and scanning tunneling microscopy (STM). Oxygen chemisorbs on the TiO₂(110) surface at 100 K through charge transfer from surface Ti³⁺ sites. The charge-transfer process is evident in EELS by a decrease in the intensity of the Ti³⁺ d-to-d transition at ∼0.9 eV and formation of a new loss at ∼2.8 eV. On the basis of comparisons with the available homogeneous and heterogeneous literature for complexed/adsorbed O₂, the species responsible for the 2.8 eV peak can be assigned to a surface peroxo (O₂^2–) state of O₂. This species was identified as the active form of adsorbed O₂ on TiO₂(110) for PSD. The adsorption site of this peroxo species was assigned to that of a regular five-coordinated Ti⁴⁺ (Ti_5c) site based on comparisons between the UV exposure-dependent behavior of O₂ in STM, PSD, and EELS data. Assignment of the active form of adsorbed O₂ to a peroxo species at normal Ti_5c sites necessitates reevaluation of the simple mechanism in which a single valence band hole neutralizes a singly charged O₂ species (superoxo or O₂^–), leading to desorption of O₂ from a physisorbed potential energy surface

Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110)

The non-oxidative dehydrogenation of alcohols is considered as an important method to produce aldehydes for the chemical industry and hydrogen gas. However, current industrial processes are oxidative, meaning that the aldehydes are formed along with water, which, in addition to being less energy efficient, poses separation problems. Herein the production of aldehydes from methanol and ethanol on clean and dry Cu(111) and Cu(110) surfaces was investigated in order to understand the catalytic anhydrous dehydrogenation of alcohols. Both ethanol and methanol preferentially react under ultrahigh vacuum conditions at surface defects to yield acetaldehyde and formaldehyde, respectively, in the absence of surface oxygen and water. The amount of alkoxide reaction intermediates measured by scanning tunneling microscopy, and aldehyde and hydrogen products detected by temperature programmed reaction, are increased by inducing more defects in the Cu substrates with Ar ion sputtering. This work also reveals that the Cu model surfaces are not poisoned by the reaction and exhibit 100% selectivity for alcohol dehydrogenation to aldehyde and hydrogen

Bar graphs of the number of click trains, number of minutes with click trains, number of buzzes and number of minutes with buzzes per 10 minutes as a function of (A) Diel, (B) seasonal and (C) tidal phases.

<p>Results are expressed as mean ± standard error of the mean (SEM), Error bars with different lowercase letters refer to Tamhane’s T2 post hoc multiple-comparison test that yielded significant results <i>P</i>< 0.05.</p

Schematic of the assignment of (A) diel, (B) lunar, and (C) tidal phases.

<p>Diel phase was divided into night1, morning, day, evening, and night2. Lunar period was composed of new moon, first quarter, full moon, and last quarter periods. Tidal phase was composed of high, ebb, low, and flood phases. t_h and t_l represent the time point when the highest and lowest water level, respectively.</p

Bar graphs of the number of click trains, number of minutes with click trains, number of buzzes and number of minutes with buzzes per 10 minutes during night time as a function of (A) Diel, (B) lunar, (C) season and (D) tidal.

<p>Error bars (mean ± SEM) with different lowercase letters refer to Tamhane’s T2 post hoc multiple-comparison test that yielded significant results <i>P</i>< 0.05.</p

Results of four-way ANOVA (diel * lunar * season * tidal) on the number of minutes with buzzes per 10 minutes.

<p>The main effects of diel and season, and the interaction effects of diel * lunar, diel * season, diel * tidal, lunar * season, diel * lunar * season, diel * lunar * tidal, diel * season * tidal, lunar * season * tidal and diel * lunar * season * tidal were all significant sources of variability in the number of minutes with buzzes per 10 minutes. Bold numbers indicate significant effects (<i>P</i>< 0.05).</p

Results of four-way ANOVA (diel * lunar * season * tidal) on the number of minutes with click trains per 10 minutes.

<p>The main effects of diel, season and tidal, and the interaction effects of diel * lunar, diel * season, diel * tidal, lunar * season, season * tidal, diel * lunar * season, diel * lunar * tidal, diel * season * tidal, lunar * season * tidal and diel * lunar * season * tidal were all significant sources of variability in the number of minutes with click trains per 10 minutes. Bold numbers indicate significant effects (<i>P</i>< 0.05).</p

Deployment schedule and detections of humpback dolphin biosonar sound.

<p>Number of click trains and buzzes and minutes with click trains and buzzes, and echolocation encounters (including encounter numbers, encounter group size, and duration) were summarized for each trial. The duration of echolocation encounters is given as the mean ± standard error of the mean (SEM) and range. Numbers in parentheses indicate the number of echolocation encounters with group number over than two.</p