Natures Miracle products are popular. Here's one link:

http://www.natures-miracle.com/products/pet-odor-stain-removers/cat-urine-destroyer-stain-odor-remover-removing-cat-urine-odor.aspx

I've had luck with Natures-Miracle for Cats but you can also use some of the enzyme products made for removing odor from kennels. You're going to need to saturate the area so don't just lightly spray it around the porch. I would buy the large containers then use a garden sprayer to apply it. You may have to do this several times......letting it dry in between each treatment. Also, don't forget to treat the walls at least 12 inches up from the floor.

Here's info on why you NEED to use an enzyme cleaner.

The cheapest best thing to use is white vinegar. It is a neutralizer so use it full strength and sop it everywhere and let dry. Then cut it 50/50 with water and sop it again and let dry. Your house will smell like pickles for a couple days but ANYTHING is better than cat pee. It could take a few more applications if there are really bad spots but overall i have had success with carpets and wood floors. After get a good paint or poly and you should be good.

I just bought a home. I have a female cat that's fixed. I let her out for a little while a few nights ago. Now, there is a very Strong male cat piss odor coming from my back patio- porch. It's concrete but I have some plants and trees around also. Can't even open the window that faces the back patio . Smells up the whole house when window is open. Making me sick !!

I tried Natures Miracle. I tried vinegar, Neither worked for long. The smell always came back. Oxyclean neutralized as well as anything else. Then I painted the wood with two coats of oil based primer. Get the best you can afford.