What is the mechanism of platinum as catalyst poisons on alumina?

I'm wondering if anyone knows by what mechanism is platinum (or other noble metals) catalyst on alumina substrate poisoned?


Presently, I'm using platinum on alumina to decompose a hydrogen peroxide / $
\ce{KNO3}$ solution and have found the catalyst material poisoned. I'm wondering if the poisoning is due to the $\ce{KNO3}$ or if my peroxide is contaminated. 


I can imagine $\ce{KNO3}$ crystallizing and thereby occluding the active sites on the alumina but I'm not a chemist, so I've come here looking for better insight.


