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We know that under standard conditions (room temperature and 760 mmHg air pressure) we have this reaction:

$$\ce{2H2O2(l) ->[MnO2] 2H2O(l) + O2(g)}$$

Here, manganese dioxide is behaving as a catalyst, accelerating the decomposition of hydrogen peroxide into water and oxygen.

If we want manganese dioxide to instead react with hydrogen peroxide what should the conditions be? Also, what would the products be if this reaction occurs?

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3 Answers 3

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By catalysing hydrogen peroxide decomposition, it does react with it, but this reaction cyclically regenerate it to the initial state, with manganese reversibly switching oxidation state.

One of possible models is:

$$\begin{align} \ce{H2O2(aq) + 2 MnO2(s) &-> H2O(l) + Mn2O3(s) + O2(g)}\\ \ce{H2O2(aq) + Mn2O3(s) &-> H2O(l) + 2 MnO2(s)} \end{align}$$

The point is, hydrogen peroxide has both oxidative and reductive properties, with the latter being oxidized to oxygen.

Variants ( with the $\ce{MnO2}$ reduction part ongoing in the Leclanché cell) :

$$\begin{align} \ce{H2O2(aq) + 2 MnO2(s) + H2O(l) &-> H2O(l) + 2 MnO(OH)(s) + O2(g)}\\ \ce{H2O2(aq) + MnO2(s) &-> Mn(OH)2(s) + O2(g)}\\ \ce{H2O2(aq) + 2 MnO(OH)(s) &-> 2 H2O(l) + 2 MnO2(s)}\\ \ce{H2O2(aq) + Mn(OH)2(s) &-> 3 H2O(l) + MnO2(s)} \end{align}$$

Other model may involve also $\ce{Mn^2+(aq)}$ at low enough $\mathrm{pH}$.

The way of suppress the catalysis may be catching eventual intermediates e.g. by a reagent forming stable complexes with $\ce{Mn^{II}}$ or $\ce{Mn^{III}}$. It would cause majority of $\ce{Mn}$ ending in these complexes after oxidation enough $\ce{H2O2}$ to $\ce{O2}$.

Altenative would be using acidic environment where $\ce{MnO2}$ would end as $\ce{Mn^2+(aq)}$.

Reaction of $\ce{MnO2}$ with citric acid combines both, with acid undergoing oxidation and complexation.

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    $\begingroup$ I suppose, it could stop being a catalyst if there were a set of conditions which inhibited one of the two reactions, while allowing the other one to proceed. I don't know if such conditions exist...! $\endgroup$ Mar 19, 2022 at 14:41
  • $\begingroup$ @orthocresol I suppose blocking it, e.g. by forming some stable manganese complex,would block also eventual reactions forming other manganese products. $\endgroup$
    – Poutnik
    Mar 19, 2022 at 14:45
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    $\begingroup$ @Poutnik, I highly doubt that there are redox reactions on MnO2. The above reactions imply that the decomposition is not quantitative (i.e., one molecule of H2O2 is wasted in the second equation) but as far I remember it is quantitative. $\endgroup$
    – AChem
    Mar 19, 2022 at 17:18
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    $\begingroup$ @M.Farooq I do not see your implication. Check it more carefully. The net reaction is the same as eventual direct decomposition: $\ce{2 H2O2 -> 2 H2O + O2}$ // But I do agree that in context of heterogen catalysis, it would be more complicated than the model. $\endgroup$
    – Poutnik
    Mar 19, 2022 at 17:32
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    $\begingroup$ @Poutnik, You are right. The net result is the quantitative decomposition equation. $\endgroup$
    – AChem
    Mar 19, 2022 at 17:33
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Catalyst is a substance which alters the rate of reaction without being consumed in it. It doesn't mean that catalyst doesn't participate in the reaction. It does react with reactants as Poutnik already stated in his answer. I came up with an another mechanism from [reference][1] and [reference][2]: $$ \ce{ MnO2 + H2O2 + 2H+ -> Mn^{2+} + 2H2O + O2}$$ $$ \ce{ Mn^{2+} + 2H2O <--> Mn(OH)_2 + 2H+ }$$ $$ \ce{ Mn(OH)_2 + H2O2 -> MnO2 + 2H2O }$$ giving an overall reaction: $$ \ce { 2H2O2 ->[MnO_2] 2H2O + O2}$$


References:

[1]: D. B. Broughton, and R. L. Wentworth, "Mechanism of Decomposition of Hydrogen Peroxide Solutions with Manganese Dioxide. I," J. Am. Chem. Soc. 1947, 69(4), 741–744 (DOI: https://doi.org/10.1021/ja01196a003).

[2]: D. B. Broughton, R. L. Wentworth, and M. E. Laing,"Mechanism of Decomposition of Hydrogen Peroxide Solutions with Manganese Dioxide. II," J. Am. Chem. Soc. 1947, 69(4), 744–747 (DOI: https://pubs.acs.org/doi/10.1021/ja01196a004).

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  • $\begingroup$ I think the relavent equations are given in reference 1. Reference 2 is their further studies. If you don't agree feel free to delete my addition. $\endgroup$ Mar 19, 2022 at 19:45
  • $\begingroup$ @M.Farooq: Do you think these references are good enough to backup Poutnik's answer? $\endgroup$ Mar 19, 2022 at 19:49
  • $\begingroup$ @MathewMahindaratne, I am not an expert, but the references are very outdated, not because they are not reliable, but the formation of Mn(OH)2 sounds a little dubious. In those times, chemical kinetics was not very advanced. What proof do the authors provide about the formation of Mn(OH)2 as an intermediate? $\endgroup$
    – AChem
    Mar 19, 2022 at 23:00
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    $\begingroup$ @MathewMahindaratne, I had a quick look. They have several hypotheses, but Poutnik's is not supported anywhere in those papers. They insist on the role Mn (OH)2...but keep in mind the pH effects. In short a more recent and detailed study is needed. I did not downvote this answer. $\endgroup$
    – AChem
    Mar 20, 2022 at 1:48
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    $\begingroup$ The 2nd equation is unbalanced and it is not really Redox equation..There should be rather H2O instead of H2O2. I would say formation of Mn^2+ is questionable. $\endgroup$
    – Poutnik
    Mar 20, 2022 at 1:57
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Manganese dioxide loses its catalytic activity if there is a strong acid in the reaction your mentioned in your post. Consequently, manganese dioxide simply reduces itself to colorless/light $\ce{Mn^{2+}}$ and hydrogen peroxide converts itself to water. I do not exactly recall evolution of oxygen. For the sake electron balance in a redox process, $\ce{O2}$ formation is necessary. However, the dissolution is very fast.

In fact this reaction was widely used indirectly (via $\ce{KMnO4}$ route) for cleaning glassware long time ago in classical analytical work.

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    $\begingroup$ If MnO2 is reduced to Mn(II), and H2O2 reduced to water, what gets oxidised? $\endgroup$ Mar 19, 2022 at 18:23
  • $\begingroup$ @orthocresol, MnO2 almost instantly dissolves in acidic solution in the presence of H2O2. I cannot find an equation at this moment. If you ever had a KMnO4 stain on the skin, a small amount of vinegar and medicinal H2O2 (< 1.5-3%) instantly removes it. $\endgroup$
    – AChem
    Mar 19, 2022 at 22:57

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