Which of the following statements are correct:

Options

1. AThe flocculating value of $\mathrm{PO}_4^{3-}$ is less than that of $\mathrm{SO}_4^{2-}$.
2. BCharcoal adsorbs gases with higher critical temperature more than gases with lower critical temperature.
3. CChemisorption is multilayered.
4. DIn the manufacture of $\mathrm{NH}_3$ by Haber's process, molybdenum is used as promoter for the catalyst iron.