$1 \mathrm{mole}$ each of ${\mathrm{H}}_2 \left(\mathrm{g}\right)$ and ${\mathrm{I}}_2 \left(\mathrm{g}\right)$ are introduced in a $1 \mathrm{L}$ evacuated vessel at $523 \mathrm{K}$ and equilibrium ${\mathrm{H}}_2 \left(\mathrm{g}\right)+{\mathrm{I}}_2 \left(\mathrm{g}\right)\rightleftharpoons 2\mathrm{HI} \left(\mathrm{g}\right)$ is established. The concentration of $\mathrm{Hl} \left(\mathrm{g}\right)$ at equilibrium

The dissociation of phosgene, which occurs according to the reaction

${\mathrm{COCl}}_2\left(\mathrm{g}\right)\rightleftharpoons \mathrm{CO}\left(\mathrm{g}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)$
Is an endothermic process. Which of the following will increase the degree of dissociation of ${\mathrm{COCl}}_2?$