certain of/about

[Foreigner]

They felt quite certain …. their failure.

a) of
b) about

I think, the two answers are possible here.
My questions:
1) Are the two prepositions always interchangeable?
2) Which one is more often used?

Thank you..

[Bazza]

I think, the two answers are possible here.

These days, you are probably right. Trying to piece together the history of all this is not helped by the meaning of your sentence, which would be more likely expressed:
They felt quite certain they had failed.


Originally, it would have been, for example: "They felt quite certain about (their facts)."

If you are happy for me to rephrase the sentence, then I could proceed.