Welcome DoubleHee!
Nice to see that you have been married for over 20 years! I have been married for 3 and am striving to be like you
Anyways, I can understand your concern about sexual relations! After me and my wife had children sex went waaaaaay down! No fun doing all of the work and getting no play!
I do not know about your relationship, or how you interact and the whatnot. I would recommend doing something romantic first off. Suprise her! Not just take her to dinner either. Be original, or at least do a throwback date, where you go where your first date was or something. Be a gentleman. All of that stuff. Have eveything planned in advance. Women eat that up. However, it will get you some that night/day or whatever. LOL
Also talk about it. I know it sounds stupid, but really communication is key. Especially when it comes to sex. You have to work for what you want. Women are just as sexual as men, they just need to be steered sometimes. 
Now as far a pheromones go, first off, you are talking about a wife of 20 years. She knows your distinct pheromone scent. When you add a pheromone to your body, she WILL notice. She may not say anything, or act different, but she will notice. It will take time for her to adjust to anything like A314 or IG. So keep that in consideration.
IO or IH would help in the communication department. Funny story-- my wife decided she was going to make our dog smell good. She had no idea what pheromones were at the time. She put IH all over the dog, because she liked the scent. Damn, she called me crying and all emotional did not know why. She was spilling her guts about so many feelings. It was great for our relationship in the end. So funny to me though.
TUTH would be my recommendation. However it is a timing thing. You would want to put it on when you are getting her in the mood. Do not overdo this stuff though or it will backfire! Start with one spray. The next time try 2 sprays. If you overdo, you will not get anything. I have been there on this one!
Also if you are wanting to really use pheromones outside of the bedroom do a search for Bosshank. In his signature, he has a few links to using pheromones and being married. It was a great place for me to start.
If you have any other questions feel free to ask!