Mutual influence of cupric cations and several anions in anatase and rutile TiO2 photocatalysis

Abstract

Copper ions in aqueous solution are known to promote organic oxidation in semiconductor photocatalysis, but the counter anions seem to be important as well. In this work, the performance of Cu(ClO 4 ) 2 in presence of several anions in sodium forms (F − , Cl − , ClO 4 − , NO 3 − , and SO 4 2− ) has been examined. Phenol oxidation in aqueous solution (pH 4) under UV light was used as model reaction and TiO 2 in the forms of anatase (AT) and rutile (RT) as photocatalysts. On the addition of 0.1-5 mM Cu 2+ , the reactions on AT and RT all increased. On the addition of 1 mM anions, reactions on AT increased by F − and SO 4 2− , but reactions on RT all decreased. In presence of 3 mM Cu 2+ , however, reactions on AT and RT all decreased by 1 mM anions except NO 3 − . Such anion effects were also observed for H 2 production on AT and RT in presence of Cu 2+ and 10% methanol. A possible mechanism for the positive and negative anion effects is discussed. This work indicates that the formation of a Cu(II)/Cu(I) complex with anions weakens the positive effect of copper ions on organic oxidation in TiO 2 photocatalysis.

Effective CO2 Capture and Selective Photocatalytic Conversion into CH3OH by Hierarchical Nanostructured GO–TiO2–Ag2O and GO–TiO2–Ag2O–Arg

“…The CO 2 absorbed by l -arginine on the surface of the photocatalyst is reduced to CH 3 OH in a multi-step process involving (i) photogeneration of electrons–hole pairs by light absorption, (ii) electron excitation, (iii) transfer to the catalyst surface, and ultimately (iv) water oxidation by the holes (Figure ). ,− …”

“…The photocatalytic process consists of producing electron/hole (e – /h + ) pairs, e – /h + recombination, and capture of photogenerated e – /h + by the adsorbed compound at the surface of the semiconductor photocatalyst, resulting in reduction and/or oxidation reactions. The rapid recombination of the e – /h + pair is disadvantageous to the whole process and limits the photocatalytic efficiency. − A number of semiconductor photocatalysts have been widely used for the reduction of CO 2 into fuels, such as TiO 2 , CdS, ZnO, and In 2 O 3 . Among them, TiO 2 has attracted a lot of attention as a photocatalyst due to its lower cost. − Nonetheless, TiO 2 exhibits weak performance due to the fast electron/hole (e – /h + ) pair recombination rate and large band gap energy, being active only under UV light irradiations, disadvantages that need to be overcome to become an active photocatalyst. ,− To overcome some of these limitations, different approaches can be considered such as (i) adding a sacrificial agent, (ii) depositing metallic nanoparticles, or (iii) the use of semiconductors with a narrow band gap. − Due to their higher absorption rate, lower toxicity, and higher stability, amines have been used, as a sacrificial electron donor, in the field of artificial photosynthesis by researchers from the 1970s to the present day. , The use of sacrificial amines allows the photoreduction of CO 2 to take place, when the position of the valence band (VB) of the photocatalysts is negative compared to the standard oxidation potential of H 2 O, and at the same time accelerates the separation rates of the electron–hole pairs and therefore improves the photoreduction of CO 2 . − Graphene oxide (GO) is another very favorable option in the photocatalytic field, which contributes to the effective degradation of CO 2 .…”

Effective CO2 Capture and Selective Photocatalytic Conversion into CH3OH by Hierarchical Nanostructured Photocatalysts GO-TiO2-Ag2O and GO-TiO2-Ag2O-Arg

“…The photocatalytic process consists of producing electron/hole (e -/h + ) pairs, e -/h + recombination, and capture of photogenerated e − /h + by adsorbed compound at the surface of semiconductor photocatalyst, resulting in reduction and/or oxidation reactions. The rapid recombination of the e -/h + pair is disadvantageous to the whole process and limits the photocatalytic efficiency [7][8][9] . A number of semiconductor photocatalysts have been widely used for the reduction of CO2 into fuels, such as TiO2, CdS, ZnO, and In2O3.…”