Joaquim L. Faria

Design of graphene-based TiO2 photocatalysts—a review

There is a recent increase in the interest of designing high-performance photocatalysts using graphene-based materials. This review gathers some important aspects of graphene–TiO2, graphene oxide–TiO2, and reduced graphene oxide–TiO2 composites, which are of especial relevance as next generation photocatalysts. The methods used for the preparation of these materials, the associated mechanistic fundamentals, and the application of graphene-based composites on the photocatalytic degradation of pollutants are reviewed. Some structural, textural, and chemical properties of these materials and other photo-assisted applications, such as hydrogen production from water splitting and dye-sensitized solar cells, are also briefly included.

Catalytic properties of carbon materials for wet oxidation of aniline

A mesoporous carbon xerogel with a significant amount of oxygen functional groups and a commercial activated carbon, were tested in the catalytic wet air oxidation of aniline at 200 degrees C and 6.9 bar of oxygen partial pressure. Both carbon materials showed high activity in aniline and total organic carbon removal, a clear increase in the removal efficiency relatively to non-catalytic wet air oxidation being observed. The best results in terms of aniline removal were obtained with carbon xerogel, an almost complete aniline conversion after 1h oxidation with high selectivity to non-organic compounds being achieved. The materials were characterized by thermogravimetric analysis, temperature programmed desorption, N(2) adsorption and scanning electron microscopy, in order to relate their performances to the chemical and textural characteristics. It was concluded that the removal efficiency, attributed to both adsorption and catalytic activity, is related to the mesoporous character of the materials and to the presence of specific oxygen containing functional groups at their surface. The effect of catalytic activity was found to be more important in the removal of aniline than the effect of adsorption at the materials surface. The results obtained indicate that mesoporous carbon xerogels are promising catalysts for CWAO processes.

Laccase immobilization over multi-walled carbon nanotubes: Kinetic, thermodynamic and stability studies.

The biocatalytic performance of immobilized enzyme systems depends mostly on the intrinsic properties of both biomolecule and support, immobilization technique and immobilization conditions. Multi-walled carbon nanotubes (MWCNTs) possess unique features for enzyme immobilization by adsorption. Enhanced catalytic activity and stability can be achieved by optimization of the immobilization conditions and by investigating the effect of operational parameters. Laccase was immobilized over MWCNTs by adsorption. The hybrid material was characterized by Fourier transformed infrared (FTIR) spectroscopy, scanning and transmission electron microscopy (SEM and TEM, respectively). The effect of different operational conditions (contact time, enzyme concentration and pH) on laccase immobilization was investigated. Optimized conditions were used for thermal stability, kinetic, and storage and operational stability studies. The optimal immobilization conditions for a laccase concentration of 3.75μL/mL were a pH of 9.0 and a contact time of 30min (522 Ulac/gcarrier). A decrease in the thermal stability of laccase was observed after immobilization. Changes in ΔS and ΔH of deactivation were found for the immobilized enzyme. The Michaelis-Menten kinetic constant was higher for laccase/MWCNT system than for free laccase. Immobilized laccase maintained (or even increased) its catalytic performance up to nine cycles of utilization and revealed long-term storage stability.

Photocatalytic Oxidation of Phenolic Compounds by Using a Carbon Nanotube‐Titanium Dioxide Composite Catalyst

A nanostructured multiwalled carbon nanotube (CNT) and titanium dioxide composite catalyst is prepared by a modified acid-catalyzed sol-gel method. Pure anatase TiO(2) and the CNT-TiO(2) composite are tested in the photocatalytic degradation of four para-substituted phenols: 4-chlorophenol, 4-aminophenol, 4-hydroxybenzoic acid and 4-nitrophenol. The effect of several operational parameters on the photoefficiency of the composite catalyst is studied by using 4-chlorophenol as model compound, namely catalyst loading, pH of the medium, hydrogen peroxide concentration, substrate concentration. A relationship between the Hammett constant of each para-substituted phenolic compound and its degradability by the photocatalysts is found. The effect of the carbon phase in the catalyst is ascribed to its photosensitizer action. A clear beneficial effect is observed for the degradation of 4-aminophenol and 4-chlorophenol. For the molecules with stronger electron-withdrawing (deactivating) groups, such as 4-hydroxybenzoic acid and 4-nitrophenol, no synergy effect is observed.

Reaction mechanism of aerobic oxidation of alcohols conducted on activated-carbon-supported cobalt oxide catalysts.

Catalytic performances and the reaction mechanism of Co(3)O(4)/AC (AC=activated carbon) for aerobic oxidation of alcohols carried out in the liquid phase were investigated. Co(3)O(4)/AC shows a high activity for aerobic oxidation of benzyl alcohol, comparable to noble metal catalysts (e.g., Au/AC) even in the absence of additives or promoters (e.g., NaOH). Changing preparation conditions, such as treatment temperature and/or time, can affect the catalytic performances of Co(3)O(4)/AC, due to decomposition of surface groups of the carbon support. Careful studies show that low alcohol conversions are obtained with either Co(3)O(4) or AC alone, which indicates that the high conversion observed over the Co(3)O(4)/AC is due to a synergistic effect between Co(3)O(4) and AC. Parallel experiments using a high-surface-area covalent triazine framework or oxygen-inert carbon nitride as support for the Co(3)O(4) catalyst also show lower conversions, which suggest that the ability of AC (in Co(3)O(4)/AC) to activate molecular oxygen is essential for the reaction. FTIR and XPS spectra taken from catalysts before and after the reaction confirm that oxygen activation proceeds mainly on the carbon support. As a result, it can be assumed that the alcohol dehydrogenation step proceeds on the metal oxide, whereas the oxygen activation step occurs mainly on the carbon support.

Fast mineralization and detoxification of amoxicillin and diclofenac by photocatalytic ozonation and application to an urban wastewater.

The degradation of two organic pollutants (amoxicillin and diclofenac) in 0.1 mM aqueous solutions was studied by using advanced oxidation processes, namely ozonation, photolysis, photolytic ozonation, photocatalysis and photocatalytic ozonation. Diclofenac was degraded quickly under direct photolysis by artificial light (medium-pressure vapor arc, λ(exc) > 300 nm), while amoxicillin remained very stable. In the presence of ozone, regardless of the type of process, complete degradation of both organic pollutants was observed in less than 20 min. Photolysis or ozonation on their own led to modest values of total organic carbon (TOC) removal (<6% or 41%, respectively in 180 min), while for photocatalysis (no ozone present) a significant fraction of nonoxidizable compounds remained in the treated water (∼15% after 180 min). In the case of photolytic ozonation, the kinetics of TOC removal was slow. In contrast, a relatively fast and complete mineralization of amoxicillin and diclofenac (30 and 120 min, respectively) was achieved when applying the photocatalytic ozonation process. The absence of toxicity of the treated waters was confirmed by growth inhibition assays using two different microorganisms, Escherichia coli and Staphylococcus aureus. Photocatalytic ozonation was also applied to an urban wastewater spiked with both amoxicillin and diclofenac. The parent pollutants were easily oxidized, but the TOC removal was only as much as 68%, mainly due to the persistent presence of oxamic acid in the treated sample. The same treatment allowed the effective degradation of a wide group of micropollutants (pesticides, pharmaceuticals, hormones and an industrial compound) detected in non-spiked urban wastewater.

Graphene oxide based ultrafiltration membranes for photocatalytic degradation of organic pollutants in salty water

Flat sheet ultrafiltration (UF) membranes with photocatalytic properties were prepared with lab-made TiO2 and graphene oxide-TiO2 (GOT), and also with a reference TiO2 photocatalyst from Evonik (P25). These membranes were tested in continuous operation mode for the degradation and mineralization of a pharmaceutical compound, diphenhydramine (DP), and an organic dye, methyl orange (MO), under both near-UV/Vis and visible light irradiation. The effect of NaCl was investigated considering simulated brackish water (NaCl 0.5 g L(-1)) and simulated seawater (NaCl 35 g L(-1)). The results indicated that the membranes prepared with the GOT composite (M-GOT) exhibited the highest photocatalytic activity, outperforming those prepared with bare TiO2 (M-TiO2) and P25 (M-P25), both inactive under visible light illumination. The best performance of M-GOT may be due to the lower band-gap energy (2.9 eV) of GOT. In general, the permeate flux was also higher for M-GOT probably due to a combined effect of its highest photocatalytic activity, highest hydrophilicity (contact angles of 11°, 17° and 18° for M-GOT, M-TiO2 and M-P25, respectively) and higher porosity (71%). The presence of NaCl had a detrimental effect on the efficiency of the membranes, since chloride anions can act as hole and hydroxyl radical scavengers, but it did not affect the catalytic stability of these membranes. A hierarchically ordered membrane was also prepared by intercalating a freestanding GO membrane in the structure of the M-GOT membrane (M-GO/GOT). The results showed considerably higher pollutant removal in darkness and good photocatalytic activity under near-UV/Vis and visible light irradiation in continuous mode experiments.

Hydrogenation of p‐Chloronitrobenzene over Nanostructured‐Carbon‐Supported Ruthenium Catalysts

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been used for the first time to support ruthenium nanoparticles for the hydrogenation of p-chloronitrobenzene (p-CNB) to produce selectively p-chloroaniline. The preparation of well-dispersed ruthenium catalysts from the [Ru(3)(CO)(12)] precursor required activation of the purified supports by nitric acid oxidation. The supports, purified and functionalized, and the supported catalysts have been characterized by a range of techniques. The catalytic activity of these materials for the hydrogenation of p-CNB at 35 bar and 60 °C is shown to reach as high as 18 mol(p-CNB)g(Ru)(-1) h(-1), which is one order of magnitude higher than a commercial Ru/Al(2)O(3) catalyst. Selectivities between 92 and 94 % are systematically obtained, the major byproduct being aniline.

Enhancing the photocatalytic properties of TiO2 by coupling with carbon nanotubes and supporting gold.

The photodegradation of methylene blue in aqueous solutions is studied using various photocatalysts, including neat TiO(2), CNT-TiO(2), Au-TiO(2), and Au-CNT-TiO(2) composites MB. Materials were synthesized and extensively characterized by XRD, TEM, DRFIT spectroscopy, N(2) adsorption-desorption isotherms, as well as diffuse reflectance UV-vis spectroscopy. By using CNT-TiO(2) composite as catalysts, it was found that CNT act as adsorbent and photosensitizer to improve the photoactivity of neat TiO(2). Among the CNT-TiO(2) composites with different CNT weight ratio (0.2-20%), the 2%CNT-TiO(2) shows the best photoactivity. When CNT content is larger than 2%, the surplus CNT may absorb and scatter light photons. Combined with the decrease of TiO(2) amount in composite, the photoactivity is reduced. To further improve the photoactivity of 2%CNT-TiO(2), different Au loads varying from 0.25% to 1% were introduced by the deposition-precipitation method. The 0.25%Au-2%CNT-TiO(2) composite had the highest photoactivity. The increase in activity was explained by the surface plasmon resonance of Au that makes the composite to absorb more photons than the 2%CNT-TiO(2), thus overcoming the disadvantages of surplus CNT addition. On the other hand, 0.25%Au-2%CNT-TiO(2) composite also presents higher activity than 0.25%Au-TiO(2) due to higher adsorption capacity provided by CNT introduction. The addition of CNT and Au simultaneously has a much stronger synergic role than when each of them is introduced individually.

Liquid‐Phase Hydrogenation of Unsaturated Aldehydes: Enhancing Selectivity of Multiwalled Carbon Nanotube‐Supported Catalysts by Thermal Activation

Platinum and iridium organometallic precursors are used to prepare nanosized, thermally stable multiwalled carbon nanotube‐supported catalysts. The materials are characterized by N2 adsorption at 77 K, temperature‐programmed desorption coupled with mass spectrometry, H2 chemisorption, transmission electron microscopy and thermogravimetric analysis; they are tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol under mild conditions (363 K and 1 MPa). A thermal activation at 973 K is found to have a very positive effect over both activity and selectivity, leading to selectivities of approximately 70 %, at 50 % conversion, regardless of the active metal phase (Pt or Ir). Since no noticeable differences in the metal particle sizes are detected, the results are interpreted in light of an enhanced metal/support interaction. This effect, induced by the removal of oxygenated surface groups, is thought to change the adsorption mechanism of the cinnamaldehyde molecule.