I add baking soda to plain cat litter to keep it smelling fresher. I usually sprinkle a little on the bottom of the litter box, add the clay litter, then sprinkle a little more baking soda on top of that. Baking soda is relatively inexpensive at Aldi and a small box lasts for several litter box changes.
Baking soda is fine to use in a liter box. Generally if your cat is eating healthy food and enough water and the box is kept clean there should not be any odor. I find that people who use the clay liter, and only feed low quality dry food, the liter smells real bad. If your liter box has a strong odor a vet check may be in order. Urinary tract infections are very common in cats.