New trends on photoelectrocatalysis (PEC):nanomaterials, wastewater treatment and hydrogen generation

The need for novel water treatment technologies has been recently recognised as concerning contaminants (organics and pathogens) are resilient to standard technologies. Advanced oxidation processes degrade organics and inactivate microorganisms via generated reactive oxygen species (ROS). Among them, heterogeneous photocatalysis may have reduced efficiency due to, fast electron-hole pair recombination in the photoexcited semiconductor and reduced effective surface area of immobilised photocatalysts. To overcome these, the process can be electrically assisted by using an external bias, an electrically conductive support for the photocatalyst connected to a counter electrode, this is known as photoelectrocatalysis (PEC). Compared to photocatalysis, PEC increases the efficiency of the generation of ROS due to the prevention of charge recombination between photogenerated electron-hole pairs thanks the electrical bias applied. This review presents recent trends, challenges, nanomaterials and different water applications of PEC (degradation of organic pollutants, disinfection and generation of hydrogen from wastewater)

Metal-doped imine frameworks for the oxygen reduction reaction in acidic media

11 pags., 7 figs., 2 tabs.The overall performance of proton exchange membrane fuel cells is limited by the sluggish kinetics of the oxygen-reduction reaction (ORR). Among the most active PGM-free ORR electrocatalysts are metal-nitrogen-carbon (M-N-C), such as Fe–N–C. The Fe–N4 ensembles in these PGM-free catalysts, present in different configurations, are proposed to be the active sites for the ORR in acid. In this work, we have synthesized a Fe/N/C catalyst via thermal treatment of a polymeric CxNy precursor obtained by the wet-polymerization of melamine (a nitrogen rich molecule) and terephthaldehyde. The materials obtained (Im-FeNC-1HT and Im-FeNC-2HT) display high ORR activity in acid electrolyte compared to other Fe–N–C catalysts prepared with precursors different than 2-methylimidazole or ZIF-8. Characterization data indicate the formation of high- and low-spin Fe-Nx ensembles, with a site density of 4.4·1019 sitesFe·g−1 estimated by electrochemical stripping of NO. The ORR activity was evaluated in a RRDE configuration in 0.1 M HClO4 and in MEA configuration in a single cell.Financial support from PID2020-116712RB-C21 funded by MCIN/AEI/10.13039/501100011033 is acknowledged. The Deputyship for Research & Innovation, Ministry of Education of Saudi Arabia is acknowledged for funding this research work through the project number 341. We also thank the Consejería de Educación, Juventud y Deporte of the Comunidad de Madrid for the Ayuda Destinada a la Atracción de Talento Investigador (2020-T2/AMB-19927) granted to Álvaro Tolosana Moranchel. The single-cell testing was possible thanks to the Margarita Salas grant (REGAGE21e00017738309) given to Álvaro García Corral, distributed by the Spanish Ministry of Universities in the frame of the NextGenerationEU program. This work was carried out with the support of Diamond Light Source, Beamline B07 B branch (proposal SI30338).Peer reviewe

Elucidation of the photocatalytic-mechanism of phenolic compounds

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after the 24 months of embargo periodIn the present work, phenol and other phenolic compounds (4-chlorophenol, 4-nitrophenol and methyl-p-hydroxybenzoate) as typical aromatic pollutants were selected to investigate and elucidate their photocatalytic-mechanism over two commercial TiO2 catalysts: Hombikat UV-100 and Aeroxide® P25, with different physico-chemical properties. To try to clarify which pathway is more relevant in the corresponding photodegradation mechanism, the following scavengers were employed: formic acid (scavenger of photogenerated holes), methanol (strong hydroxyl radical scavenger) and cupper (II) nitrate (scavenger of electrons from conduction band). Higher photodegradation rates were always reached with P25, regardless of the studied parent compound. The radiation absorbed by these TiO2 catalysts were calculated by means of Monte Carlo simulations, and volumetric rate of photon absorption (VRPA) values were always higher for P25 because of its greater scattering and absorption coefficients. At the same time, higher values of the quantum efficiency were found in the photocatalytic degradation of phenol, methyl 4-hydroxybenzoate and 4-chlorophenol with P25, whereas in the case of 4-nitrophenol Hombikat catalyst was more photo-efficient despite the drop of the VRPA caused by the 4-nitrophenol light absorption. Regardless of the used photocatalyst, phenol, methyl 4-hydroxybenzoate and 4-chlorophenol were mostly degraded by HO• radicals generated from photo-generated holes, whereas an electron mediated mechanism was the most relevant in the case of 4-nitrophenol. Finally, it was demonstrated that the photodegradation rate of this parent compound could be appreciably increased by adding a hole acceptor, such as formic or oxalic acid, which promotes electrons from titania valence band (VB) to the conduction band (CB).This work has been supported by the Spanish Plan Nacional I+D+i through the project CTM2015-64895-R and CTM2016-76454-R. Alvaro Tolosana-Moranchel thanks to Ministerio de Educación, Cultura y Deporte for his FPU grant (FPU14/01605

An approach on the comparative behavior of chloro / nitro substituted phenols photocatalytic degradation in water

This Accepted Manuscript will be available for reuse under a CC BY-NC-ND licence after 24 months of embargo periodThe study of position and number of substituents on the photocatalytic removal of some mono-, di- and tri-, chloro- and nitrophenols, as well as more known initial TOC concentration effect, has revealed the noteworthy impact on the process efficiency. Despite the complex effect of multiple substituents directing the HO attack to their preferential positions, Hammett constant could be used to predict photocatalytic degradation performance. TiO2 P25 was able to mineralize initial TOC concentrations up to 25-50 mg·L-1. Higher TOC concentrations constitute a drawback and drive to residual parent pollutants and organic by-products, which become more important when raising the starting TOC loading. Increasing the number of chloro- or nitro- groups in the aromatic ring does not imply higher ecotoxiticy values; contrarily, the position of the substituent can lead to significant differences. TOC conversion values are hardly affected by the substituent position, but by the number of groups in the organic molecule, probably due to steric hindrance. The formation of chloride and nitrogen inorganic ions, inherent to photocatalytic degradation, fulfills the Cl and N mass balances. Finally, the effect of number, position and electronic nature of substituents on contaminant initial photocatalytic degradation rates was studied by corresponding Hammett constant correlationsThis work has been supported by the Spanish Plan Nacional de I+D+i through the project CTM2015-64895-R. Alvaro Tolosana-Moranchel thanks to Ministerio de Educación, Cultura y Deporte for his FPU grant (FPU14/01605). The authors are also grateful to Evonik Company for TiO2 sampl

Solar Assisted Photodegradation of Isoproturon: Recovery and Reuse of Titania Catalysts

Trabajo presentado en el 8th European Meeting on Solar Chemistry and Photocatalysis, celebrado en Salónica (Grecia) del 25 al 28 de Junio de 2014.Heterogeneous Photocatalysis, using TiO2 is one of the most successful applications to degrade moderate charges of recalcitrant pollutants, such as pesticides, in ambient conditions. However, catalyst efficiency and recovery after the photocatalytic processes are key challenges that have limited the practical deployment of TiO2 catalysts in wastewater applications. In this context, the present work analyzes the photo-efficiency of two titania catalysts in the solar light assisted photodegradation of isoproturon, and their reuse in two consecutive cycles, where titania catalysts were easily recovery by sedimentation and no catalyst deactivation was observed.This work have been supported by the Spanish Plan Nacional I +D+I throught the projet CTM2010-14883/TECNO.Peer Reviewe

Analysis of photoefficiency in TiO2 aqueous suspensions: Effect of titania hydrodynamic particle size and catalyst loading on their optical properties

Effect of TiO hydrodynamic particle sizes and catalyst loading on the optical properties of three commercial photocatalysts has been analyzed (P25 Aeroxide, P25/20 VP Aeroperl and P90 Aeroxide). These catalysts, characterized by similar structural and electronic properties, but with singular differences in morphology and aggregation particle sizes, have been studied to understand the corresponding crossed effects on their final photo-efficiencies throughout organic matter removal in aqueous suspensions, with a pollutant such as phenol, where photo-oxidation is regularly described as mediated by an indirect photo-mechanism via HO radicals. Reflectance measurements in the range of visible wavelength, close to TiO absorption edge, could be comparable to extinction coefficient in the UV-A range and may well be suitable to optimize catalyst loadings. Phenol photocatalytic efficiency followed P25 > P90 > P25/20 order, emphasizing that increases in TiO hydrodynamic particle sizes are detrimental to phenol photo-efficiency, and highlighting that radiation–photocatalyst interactions are essential but not enough to guarantee an improved photodegradation rate. Finally, the higher values of HO found in sonicated P25/20 catalyst could corroborates its better performance in phenol photodegradation, as a consequence of lowest hydrodynamic particle sizes in reaction media, which take advantage of light as a result of a significant increase in exposed surface area.This work has been supported by the Spanish Plan Nacional de I+D+i through the projects CTM2015-64895-R and CTM2016-76454-R. Jaime Carbajo thanks to the Spanish Ministerio de Economía y Competitividad (MINECO) for his FPI grant. Alvaro Tolosana-Moranchel thanks to Ministerio de Educación, Cultura y Deporte for his FPU grant (FPU1016054/). The authors are also grateful Evonik company for TiO2 sample

Effect of water composition on the photocatalytic removal of pesticides with different TiO2 catalysts

The objective of this work is double—firstly to explore the photocatalytic efficiency of five different commercial TiO2 catalysts in the photodegradation of a mixture of pesticides classified by the EU as priority pollutants and secondly to analyze the correlation between their physicochemical properties and the inhibition of the studied photocatalytic process when natural water was employed. Photocatalytic efficiencies when ultrapure water was used seem to point out that surface area was not a prerequisite for the photodegradation of the selected mixture of pesticides. On the other hand, significant differences in total organic carbon (TOC) conversions were obtained with the two studied water compositions. On one side, Evonik materials appear to be mostly inhibited when natural water was employed, whereas on the other, it should be remarked that anatase Sigma-Aldrich (SA) and, particularly, Hombikat UV100 (HBK) materials presented a very limited photo-efficiency inhibition or even a higher initial rate of TOC removal when a natural water matrix was used, probably due to their specific surface properties (PZC, S BET). Therefore, heterogeneous photocatalysis has proved to be a promising technology for the degradation of the selected mixture of pesticides where the final photo-efficiency of the five commercial titania catalysts studied here responds to a complex balance between its surface and structural properties.This work has been supported by the Spanish Plan Nacional de I + D + i through the project CTM2010-14883/TECNO.Peer Reviewe

Fotodegradacion de una mezcla de pesticidas: evaluacion de la recuperacion del catalizador

Trabajo presentado en el Congreso de la Sociedad Española de Catálisis SECAT 13, celebrado en Sevilla (España) del 26 al 28 de junio de 2013.La contaminación de las aguas superficiales con pesticidas es uno de los principales peligros para el medio ambiente y (la salud humana debido a la toxicidad y naturaleza refractaria de estos contaminantes a los tratamientos convencionales. Una de las alternativas más prometedoras para el tratamiento de este típo de efluentes es la fotocatálísis heterogénea caracterizada por su alta efectividad y bajo coste. Sin embargo, la eficiencía final del proceso fotocatalítico depende de distintos factores, entre los que destacan tanto las propiedades intrínsecas del propio semiconductor (TiOz) como las condiciones de operación del proceso, donde la recuperación final del catalizador determinará la viabilidad de su implementación real. Por tanto, el objetivo de este trabajo ha consistido en estudiar la fotodegradación de una mezcla de pesticidas con distintos catalizadores basados en TiO 2 y en distintas matrices acuosas (con iones inorgánicos que limitan la degradación de estos contaminantes) para comparar tanto su fotoeficiencia corno el efecto de la matriz acuosa sobre el proceso. Finalmente para analizar la aplicación final de este tratamiento se estudió la recuperación mediante sedimentación de los distintos catalizadores estudiados.Patricia García-Muñoz agradece la concesión Investigación de la SECAT.Peer Reviewe