Improved photocatalytic performance of TiO2/carbon photocatalysts: Role of carbon additive.

A series of TiO2 - based photocatalysts have been prepared by the incorporation of 10 wt% of various carbon-based nanomaterials as modifying agents to titania. More specifically, commercial TiO2 P25 was modified through a wet impregnation approach with methanol with four different carbon nanostructures: single-walled carbon nanotubes (SWCNTs), partially reduced graphene oxide (prGO), graphite (GI), and graphitic carbon nitride (gCN). Characterization results (XPS and Raman) anticipate the occurrence of important interfacial phenomena, preferentially for samples TiO2/SWCNT and TiO2/prGO, with a binding energy displacement in the Ti 2p contribution of 1.35 eV and 1.54 eV, respectively. These findings could be associated with an improved electron-hole mobility at the carbon/oxide interface. Importantly, these two samples constitute the most promising photocatalysts for Rhodamine B (RhB) photodegradation, with nearly 100% conversion in less than 2 h. These promising results must be associated with intrinsic physicochemical changes at the formed heterojunction structure and the potential dual-role of the composites able to adsorb and degrade RhB simultaneously. Cyclability tests confirm the improved performance of the composites (e.g., TiO2/SWCNT, 100% degradation in 1 h) due to the combined adsorption/degradation ability, although the regeneration after several cycles is not complete due to partial blocking of the inner cavities in the carbon nanotubes by non-reacted RhB. Under these reaction conditions, Rhodamine-B xanthene dye degrades via the de-ethylation route.

Conducting Polymer-TiO2 Hybrid Materials: Application in the Removal of Nitrates from Water.

Materials able to produce the reduction of nitrate from water without the need of a metal catalyst and with avoiding the use of gaseous hydrogen have been developed by combining the synergistic properties of titania and two conducting polymers. Polymerization of aniline and pyrrol on titanium dioxide in the presence of two different oxidants/dopants (iron trichloride or potassium persulfate) has been evaluated. The resulting hybrid materials have good thermal stability imparted by the titania counterpart, and a considerable conductivity provided by the conducting polymers. The capability of the hybrid materials of reducing aqueous nitrate has been assessed and compared to the catalytic hydrogenation of nitrate using a platinum catalyst supported on these hybrid synthesized materials. The mechanism of nitrate abatement implies adsorption of nitrate on the polymer by ion exchange with the dopant anion, followed by the reduction of nitrate. The electron transfer from titania to the conducting polymer in the hybrid material favors the reductive ability of the polymer, in such a way that nitrate is selectively reduced with a very low production of undesirable side products. The obtained results show that the activity and selectivity of the catalytic reduction of nitrate with dihydrogen in the presence of a platinum catalyst supported on the hybrid materials are considerably lower than those of the metal-free nanocomposites.

Metal-organic and covalent organic frameworks as single-site catalysts.

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

Ethanol steam reforming on Ni/Al2O3 catalysts: effect of the addition of Zn and Pt.

Ni-based catalysts supported on Zn-modified alumina were investigated in the ethanol steam reforming reaction. A commercial γ-alumina was impregnated with different amounts of zinc nitrate (0, 2, 5, 10, 15, 20 wt.% on Zn basis), calcined, and then impregnated with nickel nitrate aqueous solutions. The samples were characterized by a number of techniques: N(2) adsorption at 77 K, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray fluorescence (XRF), and temperature-programmed reduction (TPR). Their catalytic behavior in the ethanol steam reforming reaction was studied at 873 K, with a H(2)O/ethanol ratio of 5:1. Two effects of the presence of Zn were detected. On the one hand, zinc modifies the surface structure and the surface chemistry of the catalysts by formation of zinc aluminates, and on the other hand, zinc oxide can be reduced to metallic zinc under reaction conditions, thus modifying the catalytic properties of the active phase. The presence of Zn increases the ethanol conversion to gaseous compounds as compared with the catalyst supported on the Zn-free commercial alumina. The addition of a small amount of Pt (1 wt.%) causes a beneficial effect in the reaction. When Ni catalysts were used without a previous reduction treatment, ethylene was formed in high amounts; however, the Pt-Ni catalysts need no reduction pre-treatment to achieve high H(2) yields (close to 70%) and showed a high stability versus time on stream because of the control of the production of ethylene, a coke precursor.

Spectroscopic and microcalorimetric study of a TiO(2)-supported platinum catalyst.

This paper reports a CO adsorption study on a Pt/TiO(2) catalyst reduced at two different temperatures, 473 and 773 K, followed by in situ infrared spectroscopy and adsorption microcalorimetry. The study is complemented with XPS characterization of the reduced catalysts.

Is there any microporosity in ordered mesoporous silicas?

The porous structure of nanostructured silicas MCM-41 and SBA-15 has been characterized using N2 adsorption at 77 K, before and after n-nonane preadsorption, together with immersion calorimetry into liquids of different molecular dimensions. Selective blocking of the microporosity with n-nonane proves experimentally that MCM-41 is exclusively mesoporous while SBA-15 exhibits both micro- and mesopores. Additionally, N2 adsorption experiments on the preadsorbed samples show that the microporosity on SBA-15 is located in intrawall positions, the micropore volume accounting for only approximately 7-8 % of the total pore volume. Calorimetric measurements into n-hexane (0.43 nm), 2-methylpentane (0.49 nm), and 2,2-dimethylbutane (0.56 nm) estimate the size of these micropores to be < or = 0.56 nm.