Catalytic converters made of palladium (pd) reduce automobile pollution by catalyzing the reaction between unburned hydrocarbons and oxygen. how does pd increase the rate of this reaction

Unburned hydrocarbon on reacting with oxygen undergoes combustion reaction. However, the activation energy of this reaction is significantly high. When a catalyst like Pd is added to the reaction system, it provides active sites for the reaction to occur. It acts are a heterogeneous catalyst. It is pertinent of note that catalyst is refereed as heterogeneous, when it exist in different phase as compared to reactant and products. In present case, reactants and products are in gas phase, while catalyst is in solid phase. Due to availability of larger surface area at active site of Pd, activation energy of reaction decreases and decrease in activation energy favors higher reaction rates. 

The gas would begin to expand, and eventually the can would burst or explode.