Porcelain tiles, which are heated at higher temperatures than ceramic and made from a sand-based formula, are stronger and more dense than ceramic tiles. They are also less water absorbent, which means it is much harder to find an adhesive that will hold these tiles in place.
The Tile Council of North America recommends using the highest quality adhesive for porcelain tiles. Ceramic tile, for example, which absorbs more water in everyday use, will grab hold of the adhesive, so it's easier to use. But since porcelain does not, they recommend an epoxy or latex modified thinset.
Ready Mixed Adhesives
Ready mixed adhesives can sometimes be problematic with porcelain tiles. These type of adhesives depend on evaporation to dry them out. Since ceramic tile absorbs water, it would help speed up the evaporation process, allowing the adhesive to dry evenly. But since porcelain tile absorbs so little water, the ready mixed adhesive can dry unevenly and sometimes not all. The tiles can fall and break, even after months of use, if the adhesive is not completely dry.
Hold in Place
Whatever adhesive you choose, be aware that porcelain needs extra help if you're laying it on a wall. Be prepared to hold it in place while you're waiting for the adhesive to dry. Plastic spacers can do the trick.