Sergio Navalon

Carbocatalysis by graphene-based materials

Financial support by the Spanish Ministry of Economy and Competitiveness (MINECO, Severo Ochoa program, CTQ 2012-32315 and CTQ2010-18671) and Generalitat Valenciana (GV/2013/040) is gratefully acknowledged. A.D.M. thanks University Grants Commission, New Delhi for the award of Assistant Professorship under its Faculty Recharge Programme.

Photocatalytic CO2 reduction by TiO2 and related titanium containing solids

The shortage of fossil fuels and the need to find alternative renewable and sustainable fuels for transportation are triggering an increasing interest in the photocatalytic reduction of CO2. In this review, we have focused on titanium containing photocatalysts that effect the reduction of CO2 to fuels. The various products that are more generally formed are CH4, CH3OH, CO as well as HCOOH. This review has been organized primarily depending on the type of titanium material used as the photocatalyst. The list includes pure TiO2 as well as metal- and non metal-doped titania, noble metals supported on titania and micro-/mesoporous titanosilicates or porous matrices containing titania clusters. In a general introduction we comment on the limitations of the current approaches and the various possibilities and conditions for performing the irradiation. In a final section, we also give our view on future developments and open issues to be addressed in this field.

Photocatalytic CO2 Reduction using Non‐Titanium Metal Oxides and Sulfides

Titanium dioxide (TiO2 ) is by far the most widely used photocatalyst both for the degradation of pollutants and in the field of renewable energies for the production of solar fuels. However, TiO2 has strong limitations in CO2 reduction, particularly under visible light irradiation. The flat-band potential of electrons in the conduction band of TiO2 is lower than that required for CO2 reduction and, therefore, it seems appropriate to develop and validate materials other than TiO2 . In addition, the photoresponse of TiO2 requires photons of wavelengths in the UV range shorter than 380 nm and strategies to implement a visible-light photoresponse on TiO2 by doping have not been completely satisfactory particularly because of problems in reproducibility and stability of the materials. For these reasons, we focus in this Review on semiconductors other than TiO2 that show photocatalytic activity in CO2 reduction. Attention has been paid to the irradiation conditions to put the productivity data into context. The role of co-catalyst and heterojunctions to increase the efficiency of charge separation is also discussed. Our aim is to describe the state of the art in the field of photocatalytic CO2 reduction using materials other than TiO2 , trying to trigger further research in this area.

Metal nanoparticles as heterogeneous Fenton catalysts.

The Fenton reaction (the generation of hydroxyl radicals from hydrogen peroxide) is the most useful method for degradation of organic pollutants in aqueous solution at moderate concentrations. In this Review we summarize the use of metal nanoparticles, either unsupported or deposited on large-surface-area solids, as Fenton catalysts. The Review complements two other reviews in the field of heterogeneous Fenton catalysis using aluminosilicates and carbonaceous materials. Herein, particular emphasis is given to the reaction conditions in which these catalysts are used, highlighting the operating mechanism and the relative efficiency of the materials. Aspects such as leaching of the metal to the solution, reusability, and the concentration of hydrogen peroxide used are analyzed in detail. Besides a critical description of the present status of the field, future trends and the need to establishing valid comparisons to assess the relative efficiencies of the materials are commented on.

Enhancement of the Catalytic Activity of Supported Gold Nanoparticles for the Fenton Reaction by Light

Laser flash photolysis of supported gold nanoparticles exciting at the surface plasmon band (532 nm) has allowed in the case of Au/CeO(2) and Au/OH-npD (OH-npD: Fenton-treated diamond nanoparticles) detection of transients decaying in the microsecond time scale that have been attributed as indicating photoinduced electron ejection from gold based on N(2)O quenching and the observation of the generation of methyl viologen radical cations. This photochemical behavior has led us to hypothesize that there could be assistance to the catalytic activity of these materials by irradiation in those cases wherein the mechanism involves electron transfer to or from a substrate to the gold. This hypothesis has been confirmed by observing that the catalytic activity of Au/OH-npD for the Fenton degradation of phenol with hydrogen peroxide can be increased over 1 order of magnitude by irradiation at 532 nm. Moreover, there is a linear relationship between the initial reaction rate and the incident photon flux. This photoenhancement allows promoting Fenton activity at pH 8 in which the catalytic activity of Au/OH-npD is negligible. The same photo enhancement activity for the Fenton degradation of phenol was observed for other supported gold catalysts including those that do not exhibit microsecond transients in the nanosecond laser flash photolysis (Au/TiO(2) and Au/SiO(2)) due to their lifetime shorter than microseconds. It is proposed that the photo enhancement should be a general phenomenon in gold catalysis for those reaction mechanisms involving positive and/or negative gold species.

Gold on Diamond Nanoparticles as a Highly Efficient Fenton Catalyst

The Fenton reaction consists of the generation of highly aggressive hydroxyl radicals from hydrogen peroxide and is widely used to degrade organic pollutants. Due to its general applicability, the Fenton reaction is employed in water and soil disinfection/remediation and for removal of non-biodegradable chemicals. The main limitation of the Fenton reaction is the consumption of stoichiometric amounts of transition metals, mostly iron. There is considerable incentive in developing a catalytic Fenton process using exclusively hydrogen peroxide and a catalyst. Herein we report that gold nanoparticles grafted on nanoparticulate diamond catalyze the formation of hydroxyl radicals from hydrogen peroxide with at least 79 % efficiency and reach a turnover number of 321 000, many orders of magnitude higher than any currently available catalysts. This extraordinary activity is derived directly from the nanometric diameters of gold and diamond (“nanojewels”) and from the remarkable inertness of the diamond surface. The Fenton reaction, in which highly aggressive hydroxyl radicals (HOC) are generated from H2O2 by reduction with Fe II ,C u II , or other transition metal salts, is a general process that can be used for the degradation/mineralization of recalcitrant organic pollutants as well as for disinfection. [1–4] In spite of the wide applicability of the Fenton reaction for decomposing almost any organic compound, its widespread use for pollution abatement and disinfection is limited by the need for stoichiometric amounts of Fe II or other transition metals. Most of the efforts to transform the Fenton reaction from a stoichiometric to a catalytic process have met with failure or at best can produce HOC with remarkably low efficiency. [5] For instance, the photo-Fenton process requires transparency of the solution (a prerequisite not frequently fulfilled in polluted waters or soils) and consumes “expensive” photons as stoichiometric reagents. A large number of iron-containing solids such as iron-exchanged zeolites and montmorillonites have also been reported as heterogeneous

Heterogeneous fenton catalysts based on activated carbon and related materials.

The Fenton reaction is widely used for remediation of waste water and for the degradation of organic pollutants in water. Currently, there is considerable interest to convert the classical Fenton reaction, which consumes stoichiometric amounts of iron(II) salts, into a catalytic process that is promoted by a solid. This review describes the work that has used carbonaceous materials either directly as catalysts or, more frequently, as a large-area support for catalytically activated transition metals or metal-oxide nanoparticles. The interest in this type of catalyst derives from the wide use of carbon in conventional water treatments and the wide applicability of the Fenton reaction. After two general sections that illustrate the scope and background of Fenton chemistry, the review describes the activity of activated carbon in the absence or presence of metal-containing particles. The last sections of the review focus on different types of carbonaceous materials, such as carbon nanotubes and diamond nanoparticles. The review concludes with a section that anticipates future developments in this area, which are aimed at overcoming the current limitations of low activity and occurrence of metal leaching.

Carbohydrates as trihalomethanes precursors. Influence of pH and the presence of Cl-and Br-on trihalomethane formation potential

Upon chlorination carbohydrates can give trihalomethanes (THMs). In the present work, we have studied the influence of pH, chloride or bromide concentration on the formation of THMs from carbohydrates. We have observed that THMs are not formed at acidic pH, while basic pH values only increase slightly the THM content, although the consumption of chlorine increases up to 100% with respect to pH 8. The presence of chloride in ppm increases the THM formation from carbohydrates without influence of the chlorine consumption. In the same manner, the presence of bromide ions in ppb also increases remarkably the THMs formed upon chlorination of saccharides. Even more, we have observed that at bromide concentrations below 100ppb, complete incorporation of bromide in THMs occurs. Overall, the results obtained show that saccharides widely present in natural waters can give rise to significant THM concentrations in the disinfection process by chlorine.

MIL-101 promotes the efficient aerobic oxidative desulfurization of dibenzothiophenes

MIL-101 promotes aerobic oxidation in n-dodecane of dibenzothiophene (DBT) and its methyl-substituted derivatives to their corresponding sulfones with complete selectivity, without observation of the sulfoxide. DBT sulfones can be completely separated from n-dodecane by water extraction. MIL-101(Cr) without the need of pre-activation was found to be more convenient than the also-active MIL-101(Fe) analog. The reaction exhibits an induction period due to the diffusion inside the pore system of the solvent or oxygen and it is not observed if the MIL-101 sample is first in contact with the solvent at the reaction temperature for a sufficiently long time. MIL-101 is reusable for at least five times without any sign of deactivation according to the time-conversion plots. Evidence by electron paramagnetic resonance spectroscopy detecting the hydroperoxide radical adduct with a spin trapping agent and Raman spectroscopy detection of superoxide supports that the process is an auto-oxidation reaction initiated by MIL-101 following the expected radical chain mechanism inside the MIL-101 cages.

Chlorine dioxide reaction with selected amino acids in water.

Chlorine dioxide is a hypochlorite alternative disinfectant agent. In this context, we have determined the products formed in the reaction of ClO(2) with selected amino acids as model compounds that can be present in natural waters. The reaction of tryptophane, histidine and tyrosine (10 ppm each) with ClO(2) were studied at molar ratios ranging from 0.25 to 4 in the presence or absence of oxygen. It was found that in the absence of oxygen adding substoichiometric amounts of ClO(2) creates products that are structurally similar to the starting amino acids. Through a series of cascade reactions the initial product distribution gradually evolves toward simple, small carbon chain products that are far from the starting amino acid. The reaction product distribution revealed that chlorine dioxide can attack the electron-rich aromatic moieties as well as the nitrogen atom lone electron pair. Our study is relevant to gain knowledge on the reaction mechanism of ClO(2) with ubiquitous amino acids present in natural waters.