The photocatalytic reduction of NO3- to N2 with ilmenite (FeTiO3): Effects of groundwater matrix.

This work analyzes the role of natural groundwater, as well as the effect of HCO3-, Ca2+, Mg2+, K+, SO42- and Cl- concentrations, upon the photocatalytic nitrate reduction using ilmenite as catalyst and oxalic acid as hole scavenger. The nitrate removal and the selectivity towards N2 are significantly limited compared to previous experiments using ultrapure water matrix. Calcium (Ca2+), bicarbonate (HCO3-) as well as pH are claimed as the major controlling factors related to the process yield. Thus, Ca2+ promotes the formation of insoluble oxalate microcrystals, reducing the amount of hole scavenger available. The presence of HCO3- leads to a steeply increase in the pH value, favoring the adsorption onto the ilmenite surface of ions OH-instead of NO3-, NO2- and C2O42. The aforementioned issues are overcome by working with C2O42-/NO3- ratio well above the stoichiometric one, that also maintains the pH value in an acid range. A completed depletion of the starting NO3-, the no detection of either NO2- or NH4+ in the aqueous phase, and a selectivity towards N2 above 95% were achieved using two times the stoichiometric dose.

Diclofenac photodegradation with the Perovskites BaFeyTi1-yO3 as catalysts.

Perovskite oxides BaFeyTi1-yO3, with y = 0, 0.6, 0.8 and 1, were prepared by ceramic (CM) and complex polymerization methods (CPM) and utilized in UV-LED (365 nm) photocatalytic degradation assays of 25 mg L-1 diclofenac (DIC) model solutions. BaTiO3-CM was also used in the photocatalytic degradation test of a real mineral water for human consumption spiked with 2 mg L-1 DIC. The XRD patterns of the synthesized perovskites showed cubic structure for those prepared by CPM, with distortions of the cubic lattice to hexagonal or tetragonal when prepared by CM, except for BaTiO3. All the perovskites showed good catalytic activity, higher than photolysis, except BaFeO3-CM that presented similar results. BaTiO3-CM and CPM and BaFeO3-CPM were also utilized in UV-LED photocatalytic DIC degradation assays with peroxydisulfate addition. BaFeO3-CPM and BaTiO3-CPM showed better ability to persulfate activation, but the highest mineralization degree was obtained with BaTiO3-CM. This last perovskite was also able to perform DIC degradation in a real matrix. The studied oxides show potentiality for photocatalytic degradation of organic compounds, with or without persulfate addition. A degradation mechanism is proposed.

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.

Photogenerated defects in shape-controlled TiO2 anatase nanocrystals: a probe to evaluate the role of crystal facets in photocatalytic processes.

The promising properties of anatase TiO(2) nanocrystals exposing specific surfaces have been investigated in depth both theoretically and experimentally. However, a clear assessment of the role of the crystal faces in photocatalytic processes is still under debate. In order to clarify this issue, we have comprehensively explored the properties of the photogenerated defects and in particular their dependence on the exposed crystal faces in shape-controlled anatase. Nanocrystals were synthesized by solvothermal reaction of titanium butoxide in the presence of oleic acid and oleylamine as morphology-directing agents, and their photocatalytic performances were evaluated in the phenol mineralization in aqueous media, using O(2) as the oxidizing agent. The charge-trapping centers, Ti(3+), O(-), and O(2)(-), formed by UV irradiation of the catalyst were detected by electron spin resonance, and their abundance and reactivity were related to the exposed crystal faces and to the photoefficiency of the nanocrystals. In vacuum conditions, the concentration of trapped holes (O(-) centers) increases with increasing {001} surface area and photoactivity, while the amount of Ti(3+) centers increases with the specific surface area of {101} facets, and the highest value occurs for the sample with the worst photooxidative efficacy. These results suggest that {001} surfaces can be considered essentially as oxidation sites with a key role in the photoxidation, while {101} surfaces provide reductive sites which do not directly assist the oxidative processes. Photoexcitation experiments in O(2) atmosphere led to the formation of Ti(4+)-O(2)(-) oxidant species mainly located on {101} faces, confirming the indirect contribution of these surfaces to the photooxidative processes. Although this work focuses on the properties of TiO(2), we expect that the presented quantitative investigation may provide a new methodological tool for a more effective evaluation of the role of metal oxide crystal faces in photocatalytic processes.