In the decomposition of hydrogen peroxide, phosphoric acid acts as a-

$2 \mathrm{H}_{2} \mathrm{O}_{2} \stackrel{\mathrm{H}_{3} \mathrm{PO}_{4}}{\longrightarrow} 2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}$

The presence of $\mathrm{H}_{3} \mathrm{PO}_{4}$ slows down the decomposition of $\mathrm{H}_{2} \mathrm{O}_{2}$. Hence it is a negative catalyst.