travertine tile outdoors-----accent brick corners

Hello
My wife and I would like to accent our existing corners of our house (front only) with travertine tiles as columns
#1 is this possible
#2 what kind of mortar/thinset would we use to adhere travertine to brick?

Yes, very possible.

Is freeze/thaw an issue in your area?

I would suggest using Mapei Kerabond thinset mortar and mixing it with Mapei Keralastic additive. This will produce a better exterior thinset than most of them you can buy with the additives already in the bag.