How to deduce catalytic activity from the state/formula of the compound?

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TheRocker
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Hi guys

I'm having trouble with this question:

Ammonia gas is formed in the combustion of biomass fuels and is a significant
pollutant when it is released directly into the atmosphere. One method of removing
this ammonia involves its oxidation using a titanium(IV) oxide catalyst.
The best explanation for the use of titanium(IV) oxide is that the titanium

(A) contains many active sites on which the reaction can occur.
(B) is readily oxidized to a higher oxidation state which can then be reduced back
to oxidation state +4.
(C) is readily reduced to a lower oxidation state which can then be oxidized back
to oxidation state +4.
(D) has partially filled d orbitals in its +4 oxidation state.

The answer is C according to the mark scheme.

I was originally under the impression that homogenous catalysts are catalysts which are chemically changed during the reaction (forming intermediates), and heterogenous catalysts are those which function via surface activity (adsorption of reactants, desorptions of products etc.). For that reason, I put A. Clearly my reasoning is flawed.

Can anyone explain why the answer is C?

Thank you .
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Pigster
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Answer A would not be unique to TiO2... surely any metal oxide would potentially have many active sites.

Ti's maximum oxidation state is like to be +4, considering the e- config of the atom. It doesn't have any d e- therefore, since they're lost.

C is the only remaining answer.
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username913907
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(Original post by Pigster)
Answer A would not be unique to TiO2... surely any metal oxide would potentially have many active sites.

Ti's maximum oxidation state is like to be +4, considering the e- config of the atom. It doesn't have any d e- therefore, since they're lost.

C is the only remaining answer.

(Original post by TheRocker)
Hi guys

I'm having trouble with this question:

Ammonia gas is formed in the combustion of biomass fuels and is a significant
pollutant when it is released directly into the atmosphere. One method of removing
this ammonia involves its oxidation using a titanium(IV) oxide catalyst.
The best explanation for the use of titanium(IV) oxide is that the titanium

(A) contains many active sites on which the reaction can occur.
(B) is readily oxidized to a higher oxidation state which can then be reduced back
to oxidation state +4.
(C) is readily reduced to a lower oxidation state which can then be oxidized back
to oxidation state +4.
(D) has partially filled d orbitals in its +4 oxidation state.

The answer is C according to the mark scheme.

I was originally under the impression that homogenous catalysts are catalysts which are chemically changed during the reaction (forming intermediates), and heterogenous catalysts are those which function via surface activity (adsorption of reactants, desorptions of products etc.). For that reason, I put A. Clearly my reasoning is flawed.

Can anyone explain why the answer is C?

Thank you .
Anything with a high surface are would have 'lots of sites'

Doesn't make it a catalyst. It's got to lower the activation energy of the process to be a catalyst
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TheRocker
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(Original post by Pigster)
Answer A would not be unique to TiO2... surely any metal oxide would potentially have many active sites.

Ti's maximum oxidation state is like to be +4, considering the e- config of the atom. It doesn't have any d e- therefore, since they're lost.

C is the only remaining answer.
Thanks for the reply.

I don't understand your logic - (A) states that TiO4 contains many active sites on which the reaction can occur.

This therefore implies that the reaction occurs on TiO4 - whether it occurs on other catalysts is of no use to answering the question.

Please explain?

Thanks in advance.
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TheRocker
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(Original post by JMaydom)
Anything with a high surface are would have 'lots of sites'

Doesn't make it a catalyst. It's got to lower the activation energy of the process to be a catalyst
(A) states that 'it has many active sites on which the reaction can occur.

So yes, TiO4 is acting as a catalyst.
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username913907
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(Original post by TheRocker)
(A) states that 'it has many active sites on which the reaction can occur.

So yes, TiO4 is acting as a catalyst.
Well, this is correct but assumes that the TiO2 has a high surface area. It is a non-specific answer compared to C. The variable ox state is what makes it act as the catalyst. The large number of surface sites makes it a better catalyst, it isn't what makes it the catalyst in the 1st place.
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