De or het stofvrij?

The correct Dutch article is:

de stofvrij

Rule Words ending in -ij and -erij: usually de. See the rule

How to use stofvrij

the stofvrijde stofvrij
this stofvrijdeze stofvrij
that stofvrijdie stofvrij
our stofvrijonze stofvrij
your stofvrijjouw stofvrij
every stofvrijelke stofvrij
which stofvrij?welke stofvrij?
a big stofvrij / the big stofvrijeen grote stofvrij, de grote stofvrij

Dutch words ending in stofvrij

zuurstofvrij

In compound words, the last part decides the article. See the rule