hmm..don't know about a single format...could you do something like this?
column A contains your values 1.5, 2.25, etc (formatted as numbers)
format Column B like h:mm, and enter this formula:
=TIME(INT(A1),(A1-INT(A1))*60,0)
that would convert whatever decimal number is in A1 to a time in B1
if your values are over 24 hours, you need to add that in there too somehow.......
what are you trying to do? like a timecard?