Sciencemadness Discussion Board

Literature contradictions regarding the oxidation of Cerium (III) by H2O2 in Acidic Mediums

Electra - 21-2-2014 at 00:02

Ce3+ and Ce4+ salts are very interesting with regard to organic chemistry. Many sources of literature state that under Acidic conditions Hydrogen Peroxide reduces Cerium (IV) to Cerium (III), while under alkaline conditions, Hydrogen Peroxide oxidizes Cerium (III) to Cerium (IV)

For reference.
Google Books - Application of Hydrogen Peroxide and Derivatives
Quote:

Redox chemistry with hydrogen peroxide is pH dependent; for example, cerium(IV) is reduced to cerium(III) in acid, whereas cerium(III) is oxidized to cerium(IV) in alkali.



Cerium (III) in solution is known to give a pale yellow color, while Cerium (IV) gives a orangish-red color (see wiki)

Looking at this PDF....which I don't think you will be able to see since it is on my schools database and I am unable to download it o.o
http://ac.els-cdn.com/S1381116996002555/1-s2.0-S138111699600...

It discusses the oxidation of toluene to benzaldehyde in the prescense of Cerium ions and bromide ions, in the form of Cerium Acetate and Sodium Bromate respectively, in a solution of acetic acid.

Here is a quote from the study that struck me as odd.
Quote:

Three other observations are worthy of note
before considering potential mechanisms for the
oxidation. Firstly, on addition of hydrogen per-
oxide to the reaction system the solution turns
from pale yellow to an orange colour. This
might be ascribed solely to the formation of
bromine (see below) were it not for the fact that
the omission of bromide ions lead to the same
colour change. Monitoring of the UV-visible
spectrum showed a reduction in the intensity of
a band with A,, at 253 nm, ascribed to
cerium(III), and the formation of a new absorp-
tion band of A,,, 307 nm, a similar wavelength
to the absorption shown by cerium(IV) species.
We presume that cerium(IV) is formed in our
systems.


Right there it clearly is stating that Cerium (III) is oxidized by H2O2 in an acidic medium to Cerium (IV). Not only does the color change indicate this, but so does the UV spectrum.

How is this possible? The only conclusion I can come up with is that when most literature talks about Cerium ions, they mean in the form of Cerium Ammonium Nitrate, while the study uses Cerium Acetate, though, I don't imagine that would matter seeing as they're all dissolved into Cerium Ions in the solution.

The article seems to imply that the solution is alkaline when it is not, seeing as Cerium (IV) ions in the form of Cerium Ammonium Nitrate are known to oxidize toluenes to benzaldehyde without much assistance, where as the study here says that no reaction occurs between Ce(IV) and toluene in the absense of the bromine ion.

[Edited on 21-2-2014 by Electra]

Electra - 21-2-2014 at 00:46

Here are some additional comments towards the end of the paper debating the mechanism of the reaction.

It seems clear that Cerium (III) is being oxidized by H2O2, so why does much literature state this should not happen in an acidic medium? Even the author points this contradiction out(bolded below). I don't imagine Cerium can react both ways in the same medium with Hydrogen Peroxide like that, both oxidizing and reducing it?
Quote:

One possibility is that a peroxy or a hydroxyl radical initi-
ates radical substitution. Cerium(IV) is alleged
to react with hydrogen peroxide to produce
hydroperoxy radicals (reaction 3) [15]. Hy-
droxyl radicals are also likely to be present
stemming from cerium(III) catalyzed decompo-
sition of hydrogen peroxide (reaction 4). One
other possibility is that a bromine radical anion,
formed from reaction of bromine atoms with
bromide ions at diffusion controlled rates (reac-
tion 5) is the hydrogen abstracting species [16].

Ce”‘+ H,O, -+ Ce”++ HO; + Hf (3)
Ce3’+ H,O, -+ Ce4++ HO’+ HO- (4)
Br + Br- -+ Br;- (5)

Whilst the nature of this initial step still
remains to be clarified we can nevertheless say
that Scheme 4 is the likely route by which
4-t-butyltoluene is converted to 4+butylben-
zaldehyde by the cerium(III)/H,O,/NaBr sys-
tem. The experiments already detailed demon-
strate that this catalytic process can be devel-
oped into a highly selective method of convert-
ing methyl-substituted benzene derivatives into
aromatic aldehydes providing the ring is not
strongly activated toward electrophilic substitution

AJKOER - 22-2-2014 at 08:39

Here is what Atomistry.com (link: http://cerium.atomistry.com/ceric_hydroxide.html ) says on Ceric hydroxide. To quote:

"Ceric hydroxide, Ce(OH)4. - The normal hydroxide is not known. When ammonia or an alkali hydroxide is added to a solution of a ceric salt, a yellow, gelatinous precipitate of ceric hydroxide is obtained, insoluble in excess of precipitant; it is usually contaminated with basic salt and adsorbed alkali hydroxide. A pure hydroxide may be prepared from ceric ammonium nitrate by precipitation with ammonia from a cold solution, the precipitate being washed, allowed to become nearly dry at a low temperature, and again washed with cold water to remoye ammonium nitrate. When dried over potassium hydroxide, its composition corresponds with the formula Ce2O(OH)6 or 2CeO2.3H2O.

Ceric hydroxide may be conveniently prepared by the oxidation of cerous hydroxide. For this purpose excess of alkali hypochlorite or hypobromite may be added to a cerous salt, or, what amounts to the same thing, precipitation may be effected by alkali hydroxide and the oxidation then accomplished by the use of chlorine or bromine."

I would also check out related links on Atomistry to the oxide salts. Note, the comments on Atomistry.com are actually extracts from the reputedly authoritative sources over the years. However, some of the material may be dated.

[Edited on 22-2-2014 by AJKOER]

blogfast25 - 22-2-2014 at 11:02

Quote: Originally posted by Electra  
It seems clear that Cerium (III) is being oxidized by H2O2, so why does much literature state this should not happen in an acidic medium?


It is true: peroxide can be used to oxidise Ce(III) to Ce(IV) in alkaline media, yet Ce(IV) is reduced to Ce(III) by that same peroxide in acid media. I've done both.

Cr is similar: in alkaline media Cr(III) is oxidised to Cr(VI) by peroxide but in acid media Cr(VI) is reduced to Cr(III) by that same peroxide.

You should consult the electrochemical series and you'll find confirmation. There is absolutely NO CONTRADICTION.

See:

From CRC 86th Ed.:

CeOH<sup>3+</sup> + H<sup>+</sup> + e === > Ce<sup>3+</sup> + H<sub>2</sub>O… Ered = + 1.715 V

H<sub>2</sub>O<sub>2</sub> === > 2 H<sup>+</sup> + O<sub>2</sub> + 2 e… Eox = - 0.695 V

Ered + Eox > 0, reaction can proceed.

And:

Ce<sup>3+</sup> + H<sub>2</sub>O === > CeOH<sup>3+</sup> + H<sup>+</sup> + e … Eox = -1.715 V

H<sub>2</sub>O<sub>2</sub> + 2 H<sup>+</sup> + 2 e === > 2 H<sub>2</sub>O … Ered = +1.776 V

Eox + Ered > 0, reaction can proceed.



[Edited on 22-2-2014 by blogfast25]

Electra - 22-2-2014 at 13:31

Maybe I'm confused but how is that not a contradiction?

The authors of the paper oxidzed Ce3+ to Ce4+ with Hydrogen Peroxide in Acetic Acid. That is an acidic medium, and Hydrogen Peroxide is oxidizing Ce3+... when literature states that Hydrogen Peroxide only reduces Ce4+ in a Acidic Medium and should not oxidize Ce3+...

Am I missing something here?

Edit:

I think I get it. So whether or not H2O2 oxidizes or reduces it does not depend on the PH of the medium but PH of the Ion? Right? Wrong? Still confused. Not entirely savy on electrochemical series.

[Edited on 22-2-2014 by Electra]

Metacelsus - 22-2-2014 at 19:06

Ions don't have pH.

blogfast25 - 23-2-2014 at 05:54

Electra:

The oxidation reaction of Ce(III) and reduction reaction of Ce(IV) both use hydrogen peroxide but in different ways. The oxidation only happens when the solution is alkaline. The reduction reaction only happens when the solution is acidic.

You're right that there appears to be a contradiction between the two sources you quote but there's no contradiction in the fact the H<sub>2</sub>O<sub>2</sub> can oxidise Ce(III) but also reduce Ce(IV). It simply depends on the reduction/oxidation potentials of the relevant half-reactions.

As CC said, ions don't have pH. pH is the negative logarithm of the H<sub>3</sub>O<sup>+</sup> concentration in the solution.



[Edited on 23-2-2014 by blogfast25]

Electra - 23-2-2014 at 08:10

Okay, so, I'm still trying to get this right....

The oxidation only happens when the solution is alkaline? That is, Ce3 is only oxidized to Ce4 by H2O2 when the solution is alkaline?

Then what is happening in the experimental data of the above paper that shows Ce3 is being oxidzed to Ce4 by H2O2 in an acidic solution? I understand NaBr has a promoting effect on the Cerium, preventing over oxidation of the product. Is it possible the NaBr has an effect on the interact between Hydrogen Peroxide and Cerium?

I don't think there is an actual contradiction... since many sources talk about the R/O properties of Cerium/H2O2 based on acidity, and the paper above is based from solid experimental data. There must be some unexplained mechanism at work that changes Ceriums Interaction with H2O2.

blogfast25 - 23-2-2014 at 08:23

Electra:

Maybe the bromide plays a catalytic role, of sorts. Bromide is easily oxidised by hydrogen peroxide to elemental bromine, maybe that should be taken into account.

What I do know for sure is that oxidation of Ce (III) in alkaline condition works with H2O2 and that in acidic conditions Ce (IV) oxidises the H2O2 to O2 (and Ce (III)), because I've done it, because other here have done it and because the electrochemical series confirms these reactions to be thermodynamically favourable.

BTW, not every single publication, no matter how prestigious the journal, is necessarily the Gospel truth. Chemists make plenty mistakes too. I say this without having scrutinised your sources.



[Edited on 23-2-2014 by blogfast25]

Electra - 23-2-2014 at 08:47

You may be right....! Your words made me recall that I read that one proposed pathway for this mechanism was that H2O2 oxidizes bromine into Br2 which has some catalytic effect on the Cerium. This is why the reaction only really works when the Bromine/Bromide source is used in a 7-10x molar excess to cerium, even though it is still a catalytic cycle. I don't have the link but the source that proposed this model was a book on the applications of hydrogen peroxide.

blogfast25 - 23-2-2014 at 10:02

Quote: Originally posted by Electra  
. This is why the reaction only really works when the Bromine/Bromide source is used in a 7-10x molar excess to cerium, even though it is still a catalytic cycle.


Wow. Looks more like Ce is the catalyst, at such ratios!

Electra - 24-2-2014 at 17:30

Quote: Originally posted by blogfast25  
Quote: Originally posted by Electra  
. This is why the reaction only really works when the Bromine/Bromide source is used in a 7-10x molar excess to cerium, even though it is still a catalytic cycle.


Wow. Looks more like Ce is the catalyst, at such ratios!


Yes Ce is the catalyst. Was that not clear? From what I gather H2O2 is used as the oxidation/oxygen source, sodium bromide as a synergistic promoter for CE catalyst. Basically Ce 1%, Sodium Bromide 10%, roughly.