The exothermic formation of $ClF_3$ is represented by the equation : <br> $Cl_2 (g) + 3F_2 (g) \leftrightharpoons 2 ClF_3 (g); \Delta Hr = -329 kJ$. <br> Which of the following will increase the quantity of $ClF_3$ in an equilibrium mixture of $Cl_2, F_2 $ and $ClF_3$?