Usage notes: "Sure" and "certain" are surely and certainly emphatic enough on their own, but sometimes you just feel like giving it a bit more oomph. This kind of redundancy is informal; it’s similar to the English phrases "positively sure" and "absolutely certain."
Elle est sûre et certaine que c’est l’Europe.
She’s absolutely certain that it’s Europe.
Je suis sûr et certain qu’il vient demain.
I’m absolutely positive he’s coming tomorrow.
Ils sont sûrs et certains du bon chemin.
They’re absolutely certain of the right path.
Sûr and certain are adjectives, so as always they must agree in gender and number with the nouns they modify.
Sûr is an adjective in this expression – it has nothing to do with the French preposition sur.