The photocatalytic conversion in aqueous media of phenol and guaiacol as a lignin model compound using Nb2O5 with different crystal phases was studied. Nb2O5 particles were synthesized using hydrothermal methods, where it was observed that changes in the solvent control their morphology and crystal phase. Different photocatalytic behavior of Nb2O5 was observed with the selected model compounds, indicating that its selection directly impacts the resulting conversion and selectivity rates as well as the reaction pathway, highlighting the relevance of model molecule selection. Photocatalytic conversion of phenol showed conversion rate (C%) up to 25 % after 2 h irradiation and high selectivity (S%) to pyrogallol (up to 50 %). Orthorhombic Nb2O5 spheres favored conversion through free hydroxyl radicals while monoclinic rods did not convert phenol. Guaiacol photocatalytic oxidation showed high conversion rate but lower selectivity. Orthorhombic and monoclinic Nb2O5 favored the formation of resorcinol with S % ~0.43 % (C % ~33 %) and ~13 % (C % ~27 %) respectively. The mixture of both phases enhanced the guaiacol conversion rate to ~55 % with ~17 % of selectivity to salicylaldehyde. The use of radical scavengers provided information to elucidate the reaction pathway for these model compounds, showing that different reaction pathways may be obtained for the same photocatalyst if the model compound is changed.

Valorization of lignin into high valuable chemical is a critical challenge. Its availability is a key factor for the development of viable lignocellulosic processes to replace fossil derived compounds. In this work, new insights on the high photocatalytic conversion of guaiacol (82%) as a lignin model compound was achieved, also, high selectivity to p-benzoquinone (59%), catechol (27%), and pyrogallol (6%) was obtained using metal-free pyrolyzed g-C3N4 under visible light irradiation. To highlight the new insights, experimental parameters were modified to control the reaction mechanism to increase selectivity and photo-conversion. g-C3N4 photocatalyst was synthesized through urea calcination at 550 degrees C and the photocatalytic performance was assessed in terms of pyrolysis time, where higher time resulted in better photocatalytic activity. This effect was attributed to smaller structures and therefore better quantum confinement of the charges. The oxidation was promoted by OH radicals, which were detected through EPR operando mode and the addition of radical scavengers. A reaction pathway was proposed, in which the OH attacks guaiacol through a methoxy group. The photocatalytic reaction can be tuned using external oxidant agents such as O-2 and/or H2O2 to promote certain radical formation, enhancing conversion rates and promoting selectivity for a specific product, where yield shifting from p-benzoquinone to pyrogallol was experimentally observed.

Metal oxide ceramics find widespread use as catalytic/photo-catalytic materials for a vast range of chemical reactions. For water-based reactions, hydroxylization, a process connected to the wettability of the surface, can greatly influence catalytic activity of a metal oxide. We present an investigation of the wetting properties of CuFeO2 delafossite oxide synthesized using hydrothermal methods. The material displayed significant variance in wetting properties, from highly hydrophobic for the as-grown powder, to complete wetting for porous cold-pressed pellets. A series of annealing treatments was performed to remove adsorbed functional groups. After annealing treatment at relatively low temperatures T > 300 degrees C (T-melt similar to 1100 degrees C), the CuFeO2 surface displayed superhydrophilicity and a rapid absorption of H2O droplets via its intergranular porosity. (C) 2019 Elsevier B.V. All rights reserved.

The ever more diverse applications of carbon materials require new fabrication methods to satisfy the demand of functionalization. This work seeks extending functionality of carbon foams known to be determined by their porous properties, which in turn are mainly produced during synthesis. In this work, we report a simple and versatile method to produce hierarchical mesoporous carbon with bottom-up grown crystalline ZnO coating obtained in one pot. The synthesis is based on foaming xylene-epoxy-zinc suspensions, which are further subjected to carbonizing treatments. Two carbon foams carbonized at high (1000 degrees C) and low (500 degrees C) temperatures are compared. During carbonization, zinc particles were found to diffuse towards the foam's surface and evaporate in case of the high temperature process, whereas reaction to form the crystalline ZnO coating was found in the low temperature process. The surface area and pore volume of the carbon foams depend on the Carbonizing temperature due to the direct relation with the diffusion rate of particles in the material. This also explains the formation of the oxide layer at the foam external surface observed in the low temperature treatment. This work highlights the possibilities of improving the functionality of porous carbon materials by adding ceramic coatings.

Delafossite CuFeO2 oxides were grown inside a hydrothermal reactor using Cu2O and FeOOH as precursors and NaOH as mineralizer. During this work the effect of the NaOH mineralizer and the reaction atmosphere was studied by varying the amount of NaOH used in the hydrothermal synthesis and by changing the reactor atmosphere from room air to high purity nitrogen. The oxides obtained were analyzed with Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS), in order to obtain their morphological, chemical and structural characteristics. It was found that increasing the amount of mineralizer from 0.4 g up to 1.02 g improves considerably the hydrothermal reaction efficiency obtaining a resulting oxide with 93% of 3R-CuFeO2 phase and a subsequent decrease of the 2H-CuFeO2 phase. Moreover, using the same hydrothermal route it was shown possible to obtain high purity CuFeO2 compounds using small amounts of NaOH (0.4 g) if the reaction is performed under a non-oxidative atmosphere injecting pure nitrogen gas to the hydrothermal reactor where an increase of 3R-CuFeO2 phase from 36% to 94% was obtained. Finally, direct band gap of the semiconducting oxides were estimated using Tauc method from UV-vis spectra obtained by Diffuse Reflectance spectroscopy.

The photoconductivity within a wavelength range of 450–1100 nm was determined for a sample of epitaxial delafossite CuFeO 2 film grown by pulsed laser deposition. The film thickness was estimated to be 75 nm. The resistance of the films was determined with four-contact van der Pauw’s method and using monochromatic illumination of the film. The most significant change in resistance resulted in three rapid lineal conductivity increases at photon energies of ~ 1.5 eV (gap-1), ~ 2.1 eV (gap-2) and ~ 2.5 eV (gap-3). The conductivity properties are well correlated with prior optical absorption results obtained in the NIR-VIS region using transmittance spectroscopy.

Hydrogen storage capacities were investigated for two forms of MoO3 nanomaterial, amorphous of low crystallinity, and other highly crystalline, using the quartz crystal microbalance technique. Effect of a catalytic Pd capping on the nanomaterials was evaluated. MoO3 materials were grown using the gas condensation method, and both the amorphous and crystalline samples were composed of orthorhombic phases with Mo6+ oxidation state. For 4-min measurements, uncapped amorphous MoO3 achieved a higher storage capacity than its crystalline counterpart, while Pd-capped samples exhibited lower values due to slower kinetics. Then, Pd-capped samples were measured using longer H2 exposure times of 30 min, finding that Pd-capped crystalline MoO3 sample exhibited higher hydrogen storage capacity than its amorphous counterpart. Pd capping was found to affect the hydrogenation of the underlying oxide layer, mainly due to differences in long-range order and layered structure between crystalline and amorphous MoO3 samples.

The H2 uptake performance at room temperature of porous tungsten oxide nanomaterials with and without a catalytic Pd coating was studied by the quartz crystal microbalance technique. Tungsten oxide composed mainly of WO3 was synthesized by inert gas condensation method using He. The samples consisted of semi-amorphous nanomaterials of low crystallinity and high porosity, as revealed by SAED, TEM, XRD, Raman spectroscopy and N2 adsorption–desorption isotherms. The H2 uptake capacity of the porous oxide was studied under increasing H2 exposure pressures (1000–7000[Formula: see text]Pa), with and without Pd coating. After reaching a maximum value of 1.2 H2[Formula: see text]wt.%, at 1160[Formula: see text]Pa, the H2 uptake capacity of the Pd-coated oxide consistently decreased. Successive hydrogenation cycles were carried out on the Pd-coated oxide at 3300 Pa and 6000 Pa to evaluate the H2 uptake performance of the sample under this H2 loading and unloading process. It was found that the H2 uptake capacity decreased from around 1 to values below 0.53[Formula: see text]H2[Formula: see text]wt.%, which is the reference H2 storage capacity achieved by a 15[Formula: see text]nm-thick Pd film. We argued that water molecule formation in the Pd/oxide interface and sublayers negatively affects the H2 uptake capacity of the oxide under successive hydrogenation cycles at room temperature.

Films of CuFeO2 and CuFe0.75Ga0.25O2 were grown over sapphire substrates in high vacuum using a pulsed laser deposition technique. The films grew with rhombohedral delafossite structure and highly epitaxial in the c-direction. Samples were characterized by X-ray diffraction, Raman spectroscopy and atomic force microscopy. Surface of the films were inspected with X-ray and UV photoelectron spectroscopy. Adsorption of CO2 and H2O was studied by a thermal program desorption technique. In both films Cu and Fe were exposed at the surface–gas interface. X-ray photoelectron data indicated that CO2 adsorbs preferentially at Cu sites forming a similar coordination to CuCO3. The energy for desorption of CO2 and H2O was estimated to be 30 kcal mol−1 (1.3 eV atom−1) for CuFeO2 and 36 kcal mol−1 (1.6 eV mol−1) for CuFe0.75Ga0.25O2. UV photoelectron spectroscopy showed that the valence band of the CuFeO2 delafossite oxides is modified with the substitution of Fe by Ga in the crystal lattice. The semiconductor band gap of CuFeO2 delafossite oxides also increased from 1.2 to 1.5 eV due to the substitution of Fe by Ga in the crystal lattice.

The present work describes oligomeric poly(amide-imide)s (PAIs) containing several L-aminoacidic residues and two silicon atoms in their repetitive unit, whose carboxylate terminal group was chemisorbed onto metallic particles (Cu, Ag or Au) previously deposited in controlled conditions via Physical Vapor Deposition (PVD). Thus, for each prepared polymer-metallic hybrid, the surface morphology, particles size distribution, percentage of organic material, silicon and metal were studied by Scanning Electron Microscope and Energy Dispersive X-ray Spectroscopy. The results show that the hybrids were formed probably via the electrostatic interaction between the carboxylate anions of the PAIs and nanoparticle cations. This bridging ligand is visualized by Raman spectroscopy and corroborated by X-ray diffraction. Optical studies and resistivity measurements (conductivity) of each hybrid were developed by UV/Vis and the four-point probe method; respectively. X-ray photoelectron spectroscopy (XPS) was used to study the oxidation states of the metallic particles at surface level. Thus, a simple and spontaneous protocol is proposed to prepare metallic particles stabilized “in-situ” by the oligomer, procedure that takes place from seconds to a few minutes. Finally, particle diameter was measured by Atomic Force Microscopy in order to study possible agglomeration of the metallic particles with the time.

Three samples of epitaxial delafossite CuFeO2 and CuFe1−xGaxO2 films were grown using Pulsed Laser Deposition techniques in high vacuum. The sample thicknesses were estimated to be 21nm, 75nm for the CuFeO2 films and ~37nm for the composite sample containing gallium. The estimated gallium fraction of substituted ferric atoms was x=0.25 for the composite sample. We present the study of the fundamental band gap(s) for each sample via observation of their respective optical absorption properties in the NIR-VIS region using transmittance and diffuse reflection spectroscopy. Predominant absorption edges measured at 1.1eV and 2.1eV from transmittance spectra were observed for the CuFeO2 samples. The sample of CuFe1−xGaxO2 showed a measurable shift to 1.5eV of the lower band-gap and a strong absorption edge located at 2.3eV attributed to direct band to band transitions. This study also found evidence of changes between apparent absorption edges between transmittance and diffuse reflectance spectroscopies of each sample and it may be resultant from absorption channels via surface states.

A highly epitaxial delafossite CuFe1-xGaxO2 film was made with pulse laser deposition in high vacuum. The sample thickness was around 48nm and it was terminated with CuFeO2. This delafossite sample was exposed to a CO2 atmosphere under controlled conditions and chemisorption of CO2 on the surface was observed. Transmittance and reflectance spectroscopies were recorded for the sample post surface heat treatment held in a vacuum chamber (0.05Kpa) and after exposure to CO2. Both spectra were recorded over a wavelength range of 350–1100nm. Chemisorption of CO2 was correlated with an increase in transmittance and decrease in reflectance in the pressure range 0–60kPa. These observations were confirmed with X-ray photoelectron spectroscopy and thermal programmed desorption data obtained in an independent experiment. The CO2 is bound to the surface forming a carboxylate structure via coordination of a bent CO2−δ molecule to a Cu center.

Delafossite CuFeO2 oxide was synthesized by a hydrothermal technique using Cu2O and FeOOH as precursors with the addition of fused NaOH as mineralizer. The amount of rhombohedral and hexagonal delafossite phase formed depends on the synthesis time lapses between 2 and 5 days and on the NaOH concentration. The compounds obtained were analyzed with Raman Spectroscopy, X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) in order to obtain their morphological and structural properties. Optical behavior was studied by UV–vis Spectroscopy and gas adsorption measured with a Quartz-Crystal Microbalance (QCM). Our results show that this type of hydrothermal synthesis is capable of recreating the delafossite-type structure of copper-iron oxide and produces a high yield of material with the right stoichiometry. The highest uptake of carbon dioxide is observed on the sample with the highest ratio between rhombohedral and hexagonal delafossite phase.