Photooxidation Mechanism of Methanol on Rutile TiO₂ Nanoparticles

The use of nanoparticulate TiO₂ as a photocatalyst for the conversion of organic molecules has grown tremendously in recent years; however, the roles of excited electrons, holes, and surface adsorbates in titania photochemistry remain poorly understood. In this work, detailed infrared measurements, which are sensitive to both vibrational and electronic transitions within the material, are used to uncover the mechanism of methanol oxidation on 4 nm rutile nanoparticles in both anaerobic and aerobic conditions. These experiments are performed in an ultrahigh vacuum cell where the coverage of methanol and exposure to oxygen are precisely controlled. Our measurements reveal that the primary pathway for initial methanol adsorption on TiO₂ is dissociative, leading to the production of adsorbed methoxy groups. Upon exposure of the sample to ultraviolet photons, the results show that the electron–hole pairs (e^––h⁺) generated within TiO₂ have significant lifetimes because the holes are efficiently trapped by the surface methoxy groups. The subsequent photochemistry induces a two-electron oxidative degradation process of the surface methoxy groups to formate. Formate production proceeds through the formation of a radical anion, the result of hole oxidation, followed by prompt electron injection by the radical anion into the TiO₂. Furthermore, these studies show that the role of O₂ in promoting methanol photodecomposition is to scavenge free electrons, which opens acceptor sites for the injection of new electrons during methoxy group oxidation. In this way, O₂ increases the efficiency of methoxy oxidation by a factor of 5 relative to anaerobic conditions, yet does not affect the hole-mediated oxidation mechanism that leads to final formate production

Infrared Spectroscopic Studies of Conduction Band and Trapped Electrons in UV-Photoexcited, H-Atom n-Doped, and Thermally Reduced TiO₂

Transmission FTIR spectroscopy is used to explore the electronic structure of excited TiO₂ nanoparticles. Broad infrared spectral features in UV-photoexcited, n-doped, and thermally reduced titania are found to be well-described by two theoretical models, which independently account for the creation of free conduction band electrons and trapped localized electrons that occupy states within the band gap. The infrared spectra indicate that the trapped electrons reside at shallow donor levels that exist 0.12–0.3 eV below the conduction band minimum. IR excitation of the trapped electrons is evidenced by a broad feature in the spectra, which exhibits a maximum that corresponds to the energy of the donor level. These features are well described by a hydrogenic-effective mass model. In addition, free conduction band electrons have a dramatic effect on the infrared spectra by exhibiting a broad featureless absorbance that increases exponentially across the entire mid-IR range. This absorbance is the result of intraconduction band transitions, for which free electron coupling to acoustic phonons is required to conserve momentum. Both localized (within the band gap) and delocalized (within the conduction band) electrons are found to exist in TiO₂ when excess electrons (are created by different means: UV photoexcitation in the presence of a hole scavenger (methanol), irradiation with atomic hydrogen, and thermal removal of lattice oxygen

Benzene, Toluene, and Xylene Transport through UiO-66: Diffusion Rates, Energetics, and the Role of Hydrogen Bonding

The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved nonthermal separation techniques and materials. Because of their high surface areas, tunable structures, and chemical stabilities, metal–organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10^–8 and 10^–12 cm² s^–1, where the rate follows the trend: <i>o</i>-xylene < <i>m</i>-xylene < <i>p</i>-xylene. Corresponding activation energies of diffusion (<i>E</i>_diff) were determined to be 44 kJ mol^–1 for the xylene isomers and 34 kJ mol^–1 for both benzene and toluene, with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apertures of UiO-66. Furthermore, IR spectroscopy and computational methods showed the formation of two types of hydrogen bonds between BTX molecules and the μ₃-OH groups located in the tetrahedral cavities of UiO-66, which indicates that BTX molecules are capable of fully accessing the inner pore environment of the MOF. The molecular-level insight into the diffusion mechanism and energetics of BTX transport through UiO-66 presented in this work provides rich insight for the design of next-generation MOFs for cost-effective separation processes

Oxidation of C₆₀ Aerosols by Atmospherically Relevant Levels of O₃

Atmospheric processing of carbonaceous nanoparticles (CNPs) may play an important role in determining their fate and environmental impacts. This work investigates the reaction between aerosolized C₆₀ and atmospherically relevant mixing ratios of O₃ at differing levels of humidity. Results indicate that C₆₀ is oxidized by O₃ and forms a variety of oxygen-containing functional groups on the aerosol surface, including C₆₀O, C₆₀O₂, and C₆₀O₃. The pseudo-first-order reaction rate between C₆₀ and O₃ ranges from 9 × 10^–6 to 2 × 10^–5 s^–1. The reaction is likely to be limited to the aerosol surface. Exposure to O₃ increases the oxidative stress exerted by the C₆₀ aerosols as measured by the dichlorofluorescein acellular assay but not by the uric acid, ascorbic acid, glutathione, or dithiothreitol assays. The initial prevalence of C₆₀O and C₆₀O₂ as intermediate products is enhanced at higher humidity, as is the surface oxygen content of the aerosols. These results show that C₆₀ can be oxidized when exposed to O₃ under ambient conditions, such as those found in environmental, laboratory, and industrial settings

Autocatalysis through the Generation of Water during Methanol Oxidation over a Titania-Supported Platinum Catalyst

Methanol may play a major role in a hydrogen economy by serving as one of the highest energy density compounds available; however, the precise reaction pathways for methanol oxidation catalysts have yet to be fully elucidated. Herein, a combination of packed-bed reactor studies and high-vacuum surface science techniques was used to elucidate the reaction mechanism of methanol oxidation over a Pt/TiO2 catalyst. The reactor studies highlight that methyl formate is produced under mild reaction conditions, and full combustion to CO2 is achieved at elevated catalyst temperatures. The surface science experiments show that the production of CO2 proceeds through a surface-bound formate intermediate via multiple proton-coupled electron-transfer steps. Importantly, we also find that the water produced upon initial methanol adsorption plays a key role in unlocking the oxidative chemistry of this Pt-based material. These results provide valuable insight into potential modifications that could preferentially direct catalyst activity toward partial or full oxidation, thereby unlocking methods for producing valuable commodity chemicals

High Photoreactivity of <i>o</i>‑Nitrobenzyl Ligands on Gold

We have studied the photopatterning of a gold surface functionalized with a self-assembled monolayer of an <i>o</i>-nitrobenzyl-based photouncaging ligand bound to the gold surface with a dual thiol anchor. We find that the dose of UV light required to induce the photoreaction on gold is very similar to the dose in an alcohol solution, even though many optical phenomena are strongly suppressed on metal surfaces. We attribute this finding to a combination of the large skin depth in gold at UV wavelengths, the high speed of the photoreaction, and the spatially indirect nature of the lowest excited singlet. Any photoreactive compounds where the quantum efficiency of fluorescence is sufficiently low, preferably no larger than about 10^–5 in the case of gold surfaces, will show a similarly high photoreactivity in metal-surface monolayers

Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO₂ and Au–TiO₂

Comparison of methanol photochemistry at three-dimensionally (3D) networked aerogels of TiO₂ or Au–TiO₂ reveals that incorporated Au nanoparticles strongly sensitize the oxide nanoarchitecture to visible light. Methanol dissociatively adsorbs at the surfaces of TiO₂ and Au–TiO₂ aerogels under dark, high-vacuum conditions. Upon irradiation of either ultraporous material with broadband UV light under anaerobic conditions, adsorbed methoxy groups act as hole-traps and extend conduction-band and shallow-trapped electron lifetimes. A higher excited-state electron density arises for UV-irradiated TiO₂ aerogel relative to commercial nanoparticulate TiO₂, indicating that 3D networked TiO₂ more efficiently separates electron–hole pairs. Upon excitation with narrow-band visible light centered at 550 nm, long-lived excited-state electrons are evident on CH₃OH-exposed Au–TiO₂ aerogelsbut not on identically dosed TiO₂ aerogelsverifying that incorporated Au nanoparticles sensitize the networked oxide to visible light. Under aerobic conditions (20 Torr O₂) and broadband UV illumination, surface-sited formates accumulate as adsorbed methoxy groups oxidize, at similar rates, on Au–TiO₂ and TiO₂ aerogels. Moving to excitation wavelengths longer than ∼400 nm (i.e., the low-energy range of UV light) dramatically decreases methoxy photoconversion for methanol-saturated TiO₂ aerogel, while Au–TiO₂ aerogel remains highly active for methanol photooxidation. The wavelength dependence of formate production on Au–TiO₂ tracks the absorbance spectrum for this material, which peaks at λ = 550 nm due to resonance with the surface plasmon in the Au particles. The photooxidation rate for Au–TiO₂ aerogel at 550 nm is comparable to that for TiO₂ aerogel under broadband UV illumination, indicating efficient energy transfer from Au to TiO₂ in the 3D mesoporous nanoarchitecture

Ultrathin Chitin Films for Nanocomposites and Biosensors

Chitin is the second most abundant biopolymer and insight into its natural synthesis, enzymatic degradation, and chemical interactions with other biopolymers is important for bioengineering with this renewable resource. This work is the first report of smooth, homogeneous, ultrathin chitin films, opening the door to surface studies of binding interactions, adsorption kinetics, and enzymatic degradation. The chitin films were formed by spincoating trimethylsilyl chitin onto gold or silica substrates, followed by regeneration to a chitin film. Infrared and X-ray photoelectron spectroscopy, X-ray diffraction, ellipsometry, and atomic force microscopy were used to confirm the formation of smooth, homogeneous, and amorphous chitin thin films. Quartz crystal microbalance with dissipation monitoring (QCM-D) solvent exchange experiments showed these films swelled with 49% water by mass. The utility of these chitin films as biosensors was evident from QCM-D and surface plasmon resonance studies that revealed the adsorption of a bovine serum albumin monolayer

In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methyl­phosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr₆ cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials

A New Interleukin-13 Amino-Coated Gadolinium Metallofullerene Nanoparticle for Targeted MRI Detection of Glioblastoma Tumor Cells

The development of new nanoparticles as next-generation diagnostic and therapeutic (“theranostic”) drug platforms is an active area of both chemistry and cancer research. Although numerous gadolinium (Gd) containing metallofullerenes as diagnostic magnetic resonance imaging (MRI) contrast agents have been reported, the metallofullerene cage surface, in most cases, consists of negatively charged carboxyl or hydroxyl groups that limit attractive forces with the cellular surface. It has been reported that nanoparticles with a positive charge will bind more efficiently to negatively charged phospholipid bilayer cellular surfaces, and will more readily undergo endocytosis. In this paper, we report the preparation of a new functionalized trimetallic nitride template endohedral metallofullerene (TNT EMF), Gd₃N@C₈₀(OH)_<i>x</i>(NH₂)_<i>y</i>, with a cage surface bearing positively charged amino groups (−NH₃⁺) and directly compare it with a similar carboxyl and hydroxyl functionalized derivative. This new nanoparticle was characterized by X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), and infrared spectroscopy. It exhibits excellent ¹H MR relaxivity. Previous studies have clearly demonstrated that the cytokine interleukin-13 (IL-13) effectively targets glioblastoma multiforme (GBM) cells, which are known to overexpress IL-13Rα2. We also report that this amino-coated Gd-nanoplatform, when subsequently conjugated with interleukin-13 peptide IL-13-Gd₃N@C₈₀(OH)_<i>x</i>(NH₂)_<i>y</i>, exhibits enhanced targeting of U-251 GBM cell lines and can be effectively delivered intravenously in an orthotopic GBM mouse model