Adsorption and Photooxidation of CH3CN on TiO2

Comparative visible-light driven selective oxidation to aldehydes of phenylmethanol (benzyl alcohol) and 4-pyridinylmethanol (4-pyridinecarbinol) on N-TiO2 and some commercial TiO2 samples

Visible light (λ > 420 nm) selective photooxidation of phenylmethanol and 4-pyridinylmethanol in CH₃CN to the corresponding aldehydes on N-TiO₂ is compared with homemade undoped TiO₂ (U-TiO₂) and commercial undoped anatase specimens (such as PC105, PC500). Significant differences observed between N-TiO₂ and undoped TiO₂ are neither directly related to the surface area nor to the adsorbed amount of alcohol in the dark by surface area unit. FTIR and EPR spectroscopies are used to study the surface of TiO₂ samples and to deeply understand the phenomena intervening in the visible-light photocatalytic activation of the doped vs the undoped oxides. In particular, it is shown that on N-TiO₂ (and also on undoped PC105) strong Lewis acid sites (LAS) exist. The favorable role of LAS on the photocatalytic activity is illustrated by the higher photooxidation of 4-pyridinylmethanol vs phenylmethanol over N-TiO₂ and PC105 in contrast to the other undoped samples, whose visible light sensitivity originates from a charge transfer between the alcohol and the solid. EPR spectra of N-TiO₂ point out the presence of paramagnetic centers related to nitrogen that disappear when the photocatalyst is irradiated with visible light in the presence of alcohol, which starts its oxidative process. On the basis of presented results, we propose that doping with N introduces new intraband gap states that not only contribute to LAS and adsorption of alcohol but also are directly involved in the photochemical process occurring under visible light irradiation.

Study of the surface species during thermal and plasma-enhanced atomic layer deposition of titanium oxide films using in situ IR-spectroscopy and in vacuo X-ray photoelectron spectroscopy

By the powerful combination of in situ FTIR and in vacuo XPS, the surface species during ALD of TDMAT with different reactants could be identified.

Synthesis of Nanoporous TiO2 with the Use of Diluted Hydrogen Peroxide Solution and Its Application in Gas Sensing

The chemical routes of metal oxidation in presence of hydrogen peroxide solutions are tailor-made for the synthesis of biocompatible metal oxide surfaces with clean intermediate and end products, such as oxides, hydroxides, hydrogen and water. The hydrolysis of titanium in hydrogen peroxide solutions is particularly interesting for medical applications, forming micro- and nanoscale titania surfaces. In this paper, the content of the hydrolysis solution is revised, allowing the fabrication of gas sensor devices based on nanoporous titania. Nanopore and microcrack formations were discussed in detail by monitoring the structural changes on the thin film surface with field-emission scanning electron microscopy (FE-SEM). A stable rutile crystalline phase was detected by glancing incidence X-ray diffraction (GI-XRD) measurement after repetitive hydrothermal processes. Electrical conductance measurements were carried out at high temperatures (400–600 °C) under humid airflow (40% RH@20 °C) with the injection of various concentrations of a wide set of test compounds (C2H3N, CO, H2, NO2, C2H6O), to observe the sensing capabilities of the material. Furthermore, the humidity effects on the sensing properties toward H2, CO, and C2H6O have been discussed.

Flexible high-resolution broadband sum-frequency generation vibrational spectroscopy for intrinsic spectral line widths

The difficulty in achieving high spectral resolution and accurate line shape in sum-frequency generation vibrational spectroscopy (SFG-VS) has restricted its use in applications requiring precise detection and quantitative analysis. Recently, the development of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution generated by synchronizing two independent amplifier lasers have opened new opportunities for probing an intrinsic SFG response. Here, we present a new flexible approach to achieve HR-BB-SFG-VS. In this system, two regeneration amplifiers shared the same oscillator laser as the seed, and a time-asymmetric visible pulse with a nearly Lorentzian line shape filtered by an etalon was used to overlap with a femtosecond broadband infrared pulse. This Lorentzian line shape of the visible pulse can greatly simplify the spectral fitting and analysis. We also demonstrated that the single-sided long visible pulse provided both high spectral resolution (1.4 cm^-1) and effective suppression of the non-resonant background by detuning the time delay between visible and infrared pulses in SFG-VS measurements. With this new SFG setup, a pair of spectral splittings by 3.1 ± 0.7 and 3 ± 0.2 cm^-1 for the symmetric and antisymmetric stretching of the CH₃ group was resolved at the CH₃CN/TiO₂(110) surface, which are tentatively attributed to two different orientational methyl groups. These technological advancements can help broaden the applications of HR-BB-SFG-VS and provide solid ground for a better understanding of complex molecular structures and dynamics at interfaces.

A DFT study on the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde in different solvents employing an OH-defected TiO2 cluster model

Density functional theory calculations are employed to study the mechanism of photoselective catalytic reduction of 4-bromobenzaldehyde (4-BBA) in acetonitrile and in ethanol solvents. A totally relaxed Ti₃O₉H₆ cluster model is proposed to represent titanium dioxide (TiO₂) surfaces. The reduction selectivity of an adsorbed 4-BBA molecule on Ti₃O₉H₆ has been investigated. Owing to the difference in the proton and H atom donating capabilities between explicit CH₃CN and C₂H₅OH solvent molecules, the photocatalytic reduction of 4-BBA is the debromination process in acetonitrile, whereas in ethanol it is the carbonyl reduction process. Therefore 4-BBA can be selectively reduced to benzaldehyde in acetonitrile and 4-bromobenzyl alcohol in ethanol, respectively. Our computational results have verified the reaction mechanism proposed by experiments and show that the debromination of 4-BBA would be efficient if both 4-BBA and Ti₃O₉H₆ have an extra photoelectron. The Ti₃O₉H₆ cluster, playing a role as a hydrogen source and a bridge to transfer photoelectrons from bulk TiO₂, would have potential to be an ideal molecular model for understanding photocatalytic reactions on the TiO₂ surface.

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NO(x)), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as well as mechanism studies based on spectroscopic and kinetic approaches and molecular simulations. Finally, concluding remarks and perspectives are given. In the present work, emphasis is placed on the structure-performance relationship with an aim to design an ideal zeolite-based catalyst for the effective elimination of harmful N-containing compounds.

Photochemical grafting of n-alkenes onto carbon surfaces: the role of photoelectron ejection

The grafting of molecular layers to carbon-based materials provides a way to combine the high chemical and thermal stability of these materials with surface properties such as chemical recognition or reactivity. The functionalization of surfaces with ultraviolet light has emerged as a way to modify difficult-to-functionalize materials, such as diamond. We have performed a combined experimental and computational investigation of the photochemical reaction of terminal alkenes with hydrogen-terminated carbon surfaces. 1-Alkenes carrying various terminal functional groups (-NHCOCF3, -NHCOO(tert-butyl), -COOCH3, -CH3) were grafted from the neat liquids using 254 nm light. These layers were characterized using X-ray Photoelectron Spectroscopy and Infrared Reflectance Absorption Spectroscopy. Pronounced differences in reactivity were observed between the molecules: trifluoroacetamide-terminated alkenes grafted the fastest and yielded self-terminating layers after approximately 4 h. Ultraviolet photoelectron spectroscopy and photocurrent measurements show that the grafting reaction involves photoemission of electrons into the liquid. Density functional calculations show that the reactivities of the four molecules are correlated with their electron affinities, with the trifluoroacetamide group acting as the best electron acceptor and having the highest reactivity. Our results demonstrate that photoejection of electrons from the solid into the acceptor levels of the alkenes initiates the functionalization reaction and controls the overall rate. Finally, marginally reactive n-alkenes were induced to react and form dense monolayers by seeding the carbon surface with small amounts of a good electron acceptor, such as the trifluoroacetamide moiety. This study provides important new mechanistic insights into the use of ultraviolet light to initiate grafting of alkenes onto surfaces.

Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2

New nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2 having only the homogeneous anatase crystalline phase and high surface area were successfully synthesized by a modified sol-gel route. The catalysts were characterized by EDX, XRD, BET, UV-vis, IR, and Raman spectroscopy. The effects of the experimental parameters on the visible light reactivity of the catalysts were evaluated for the photodegradation of gaseous acetaldehyde as a model indoor pollutant. The activity results show that the silver(I) ion, Ag(+), doping significantly promotes the visible light reactivities of carbon and sulfur-doped TiO2 catalysts without any phase transformation from anatase to rutile. Moreover, Ag/(C, S)-TiO2 photocatalysts degrade acetaldehyde 10 times faster in visible light and 3 times faster in UV light illuminations than the accredited photocatalyst P25-TiO2. The commendable visible photoactivities of Ag/(C, S)-TiO2 new nanoparticle photocatalysts are predominantly attributable to an improvement in anatase crystallinity, high surface area, low band gap and nature of precursor materials used.

Photochemical functionalization of hydrogen-terminated diamond surfaces: a structural and mechanistic study

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.

FTIR study of adsorption and surface reactions of N(CH3)3 on TiO2

Fourier transform infrared spectroscopy has been employed to investigate the N(CH3)3 adsorption, thermal stability, and photochemical reactions on powdered TiO2. N(CH3)3 molecules are adsorbed on TiO2 without dissociation at 35 degrees C and are completely desorbed from the surface at 300 degrees C in a vacuum. The CH3 rocking frequencies of N(CH3)3 on TiO2 are affected via the interaction between N(CH3)3 and TiO2 surface OH groups. In the presence of O2, adsorbed N(CH3)3 decomposes thermally at 230 degrees C and photochemically under UV irradiation. In the latter case with comparative (16)O2 and (18)O2 studies, CO2(g), NCO(a), HCOO(a), and surface species containing C=N or NH(x) functional groups are identified to be the photoreaction products or intermediates. In the presence of (18)O2, the main formate species formed is HC(16)O(18)O(a). As H2O is added to the photoreaction system, a larger percentage of adsorbed N(CH3)3 is consumed. However, in the presence of (18)O2 and H2O, the amount of HC(16)O(18)O(a) becomes relatively small, compared to HC(16)O(16)O(a). A mechanism is invoked to explain these results. Furthermore, based on the comparison of isotopic oxygens in the formate products obtained from CH3O(a) photooxidation in (16)O2 and (18)O2, it is concluded that the N(CH3)3 photooxidation does not generate CH3O(a) in which the oxygen belongs to TiO2.

Acetonitrile-assisted highly selective photocatalytic epoxidation of olefins on Ti-containing silica with molecular oxygen

Highly selective photocatalytic epoxidation of olefins proceeds on Ti-containing silica with tetrahedrally coordinated Ti-oxide species with molecular oxygen and a simple addition of MeCN.