The rate of decomposition of acetaldehyde into methane and CO in the presence of I₂ at 800K follows the rate law R = $\mathrm{K}\left[{\mathrm{CH}}_3\mathrm{CHO}\right]\left[{\mathrm{I}}_2\right]\text{. }$ The decomposition is believed to go by the two step mechanism
 $$\begin{gathered} {\mathrm{CH}}_3\mathrm{CHO}+{\mathrm{I}}_2\to {\mathrm{CH}}_3\mathrm{I}+\mathrm{HI}+\mathrm{CO} \\ {\mathrm{CH}}_3\mathrm{I}+\mathrm{HI}\to {\mathrm{CH}}_4+{\mathrm{I}}_2 \end{gathered}$$
Which the following acts as catalyst for the above reaction?