Is liquid nitrogen used in refrigerators for cooling?

No. Refrigerators use a substance called Freon that can readily change from liquid to gas to liquid, moving heat from inside the fridge to the outside. Liquid nitrogen is FAR too cold, and requires too much energy to make the change from gas to liquid.

Yes, liquid nitrogen is used in refrigerators. Nitrogen is a diatomic gas. The two nitrogen atoms are bond with a triple bond.