There are certain types of foods which you will want to avoid giving to your cat, though pecans are not one of them. These particular nuts are not considered to be poisonous to them so you will be able to feed it to them in moderation. The only possible reason that a cat might have an adverse reaction to eating pecans is if they are allergic to them, though it is rather unlikely.
Food allergies do still occur in animals like cats and dogs so you will want to be aware of that when you are giving them any human food. Pecans are actually very nutritious and beneficial in terms of the vitamins and minerals they contain, even for animals like cats.