When a catalytic converter heats up and gets hotter the heat increases the rate of the reaction of the carbon monoxide as well as any fuel that is unburnt from the exhaust gases with oxygen that is in the air.
A catalyst works by an opposite chemical process that is called oxidation (which adds oxygen) and then turns carbon monoxide into carbon dioxide.
After that happens then another oxidation reaction turns the unburned hydrocarbons in the exhaust into carbon dioxide and water.