A cute pun on the word catalyst. The cat is representing the catalyst, quickly guiding the bonding of the blue and yellow atoms into molecules in the beaker of solution.
This design is featured on Shirtoid, a website that highlights the coolest t-shirts out there, and has been printed at the daily tee site Qwertee.
Where you can buy online
You can find the design on t-shirts in most stores, but also on other products depending on the store (e.g. hoodies, mugs, stickers, notebooks, phone cases, greeting cards, cushions, socks etc).