Answer is: increasing the temperature will reduce the amount of phosgene (COCl₂) formed.
Balanced chemical reaction: CO + Cl₂ → COCl₂ ΔHrxn = −107.6 kJ/mol.
This reaction is exothermic, which means that heat is released. According to Le Chatelier's principle when the reaction is exothermic heat is included as a product and when the temperature increased, the heat of the system increase, so the system consume some of that heat by shifting the equilibrium to the left, producing less phosgene.
Above 200°C, phosgene reverts to carbon monoxide (CO) and chlorine(Cl₂).