AJKOER
Radically Dubious
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Aqueous FeCl3 - Photosensitive Or a Photocatalyst?!
First, some background, per a source https://www.lewisu.edu/academics/biology/pdf/Ferric_Chloride...) on a FeCl3 solution to quote:
"Stable, but light sensitive. Incompatible with many common metals,..."
Also, there is some history and a noted published work from 1928: "The Influence of Light on the Color of Ferric Chloride Solutions" at https://pubs.acs.org/doi/pdf/10.1021/j150290a015 in the ACS no less which is worth reading (but do not expect a discussion on band width
separation in this old source).
Another 2007 source https://pubs.acs.org/doi/10.1021/es0625778 , to quote:
"FeCl3/NaNO2 is an efficient photocatalyst which is active on natural light irradiation."
where here it is likely that Ferric chloride is supplying its coloration for the capturing of light to induce a reaction in natural light where my
recollection is nitrite is performed in both UV and sunlight.
But could Ferric chloride be considered as a good photocatalyst?
Per this 2009 source: "The comparison of photocatalytic activity of Fe-salts, TiO2 and TiO2/FeCl3 during the sulfanilamide degradation process" at https://www.infona.pl/resource/bwmeta1.element.elsevier-df6d... , to quote:
"In this work, the activities of selected photocatalysts during the degradation process of sulfanilamide were compared. Experiments were carried out
in the presence of homogeneous Fe-salts (FeCl3 , Fe(NO3)3 , Fe2(SO4)3 , FeSO4 ), heterogeneous compounds (TiO2 ) and a mixture of catalysts (TiO2
/FeCl3 ) under UV-A and solar irradiation. Based on the comparison of several factors such as initial degradation rate and quantum yields, as well as
economic and ecological aspects, **FeCl3 is proposed as the optimal catalyst**.
where FeCl3 is even 'optimal' and defeats widely hailed TiO2 and even a TiO2/FeCl3 mix as a preferred photocatalyst in that system!
Related confirming work, here is a 2017 paper: "Effect of FeCl3 on the photocatalytic processes initiated by UVa and vis light in the presence of
TiO2–P25" at https://www.sciencedirect.com/science/article/abs/pii/S09263... where interestingly, to quote the cited mechanism:
"This result indicates that the photocatalytic degradation rate of the sulfonamides depends on the possibility for direct oxidation of the cationic
form of the substrate and its interaction with the catalyst particles via labile electrostatic interactions."
Here is a work "The influence of FeCl3 on the photocatalytic degradation of dissolved azo dyes in aqueous TiO2 suspensions", at https://www.researchgate.net/publication/10633772_The_influe... reflecting a divergence effect, to quote:
"The process of decolouration during illumination of the solutions studied containing FeCl3 underwent significant intensification in the case of
anionic dyes and unfavourable inhibition in case of cationic dyes. It was also observed that FeCl3 had a diverse influence on the adsorption of the
dyes studied on TiO2. The adsorption of anionic dyes and decolouration of solutions before the illumination was observed only in the presence of
FeCl3. In case of cationic dyes the addition of FeCl3 caused elimination of these phenomena. An additional cause of decolouration of anionic dyes
solutions before illumination was the precipitation of their poorly soluble compounds from Fe3+."
Now, seemingly have been called a photocatalyst, that implies it can produce electrons and electron holes which are tools for photo reactions of
interest.
Generally speaking, cited properties of a good photocatalyst at https://www.researchgate.net/figure/Desirable-properties-of-... , to quote:
"After the formation of photoexcited charge carriers, most of the photoexcited electrons and holes tend to be consumed via recombination before the
completion of surface catalytic reactions ( Figure 4). [116][117][118][119][120][121] Because the recombination processes significantly decrease the
quantum efficiency of CO2 photoreduction, the researchers have been trying their best to relieve the recombination rate and degree of electrons and
holes. [42,122] "
Also, from same source:
"The general consensus is that a good photocatalytic material should encompass a good charge separation, a fast charge transfer, absorption in the
visible part of the spectrum, high stability, low cost, and nontoxicity. 56,57 Finding a photocatalyst that fulfils all of the above requirements is a
remarkably difficult endeavor. Although the trial-and-error approach has historically revealed several well performing catalysts, such as TiO2..."
Unfortunately, I have not be able to find much explanatory detail on how FeCl3 actually works, although it cited use in combinations does suggest
adding a unique quality.
My guess is its coloration and associated light absorption abilities especially, for example, in the aqueous duo FeCl3/NaNO2.
Any comments or interesting related aspects on especially mechanics, or suggested experiment to perform with it would be appreciated.
[Edited on 9-1-2022 by AJKOER]
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AJKOER
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Did find a 2014 reference "Voltage Effect in Holograms of Polyvinyl Alcohol with FeCl3" which can be freely downloaded.
From the abstract, to quote:
"We show experimentally that the metallic salt, FeCl3, at different concentrations, provides photosensitivity and conductivity characteristics with
poly(vinyl alcohol) material....In addition, we describe the photo-mechanism, physicochemical processes,..."
My take per Page 919 (please correct me as you read the material yourself) is that there is a photo-active reduction (or photo reduction ) of Fe3+ to
Fe2+ from an electron associated with a photo radicalized species .Cl- to .Cl + e- . Not clear if the latter electron is caged with Fe3+. However, if
not available as solvated electron, the subsequent oxidation of ferrous to ferric (as with oxygen) would create a solvated electron in an acidic
medium.
Importantly, no electron hole (h+) activity, so while FeCl3 is photo active, it does NOT appear to be a classic photo-catalyst developing a offsetting
h+ and e- with an associated bandgap value. This actually makes sense as it explains the difference in FeCl3 behavior between anionic dyes and
cationic dyes, as having electron-hole activity would be reflected to some extent in both cases.
So, technically I would classify FeCl3 NOT as a photo-catalyst, but just photosensitive (or photo-active). The study I cited above basically compared
FeCl3 to salts some of which are noted as photo-catalyst.
This result is actually not too surprising as I alluded to in the opening comments, as there appears to be, at times, a diverging impact and
something relatively unique about aqueous FeCl3 in light.
[Edited on 10-1-2022 by AJKOER]
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AJKOER
Radically Dubious
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CLAIM: Light assisted FeCl3 presence may constitute one of the mechanism responsible for the significant decomposition of HOCl and household
hypochlorite (like NaOCl) upon exposure to transition metal dust, CO2 from air and light exposure over time leading to the induced creation of
chlorate, when the FeCl3 presence is in even very small amounts due to its stable photocatalytic properties.
I start with the known creation of HOCl from the action of CO2 on aqueous hypochlorite (source, see for example, Page 425 here https://ajph.aphapublications.org/doi/pdf/10.2105/AJPH.4.5.4... ):
OCl- + CO2 + H2O = HCO3- + HOCl
Per above we know that photo-sensitive aqueous FeCl3 in light can be a source of e- in solutions. Further, the slow action of solvated electrons on
Hypochlorous acid, per a source (see https://www.bnl.gov/isd/documents/92710.pdf Page 13):
e- + HOCl = •OHCl-
If pH > 5: •OHCl- --> •OH + Cl- k = 6.1x10^9 (Source: Supplementary Material at: http://www.mdpi.com/1420-3049/22/10/1684 and click on Supplement File 1 to download)
(Note: If pH < 5: •OHCl- + H+ --> •Cl + H2O k = 2.1×10^10 Source: Supplement Table S1)
Now, a series of radical based reactions leading to the formation of ClO3- (or HClO3):
OCl- + •OH = •OCl + OH-
•OH + HOCl = H2O + •ClO
•ClO + •ClO = Cl2O2
Cl2O2 + HOCl = HClO3 + HCl (See Eq 8 "Effect of Chloride Ion on the Kinetics and Mechanism of the Reaction between Chlorite Ion and Hypochlorous
Acid" by Balazs Kormanyos, et al., 2008, at https://www.researchgate.net/publication/23141635_Effect_of_... ).
Supporting background is the reported decomposition of NaOCl per this comment, "STABILITY OF SODIUM HYPOCHLORITE SOLUTIONS" here https://www.forceflowscales.com/downloads/chemical-safety/hy... , as an example. To quote relating to concentration of associated metal impurites:
"Copper, iron, nickel and cobalt are powerful catalysts for decomposition. Metals predominantly catalyze the O2 producing reaction (2). Copper and
iron are the ones most likely to be present and should be kept under a concentration of 0.5 and 1 part per million respectively in the finished
bleach. In this connection, it is important to observe every precaution to avoid copper and iron contamination from pipelines and valves, which may be
used in handling the chlorine. All lines and valves must be kept internally free of moisture to prevent rapid corrosion by the chlorine. "
On Photo-assisted decomposition per the same source:
"Light speeds up the decomposition of sodium hypochlorite solutions....In summary, the most stable solutions are those of low hypochlorite
concentration, with a pH of 10 or higher, low copper, iron and nickel content, and stored in the dark at low temperatures. "
[Edited on 11-1-2022 by AJKOER]
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