The palladium based {HEAD ~ three way catalyst} may be palladium or palladium with another noble metal like {TAIL ~ rhodium} 
The palladium based { HEAD ~ three way catalyst } may be palladium or palladium with another noble metal like { TAIL ~ rhodium }
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL