An introduction to rhodium

Rhodium is still the most effective catalytic metal to cut NO_x emissions and so it remains a key component of three-way catalysts for gasoline vehicles, where demand growth is robust. As a result, rhodium demand is set to increase, whilst a sustained weaker market for platinum could further cut primary rhodium supplies, although more recycling will help to offset some of this shortfall.

Rhodium demand has suffered losses in both gasoline and diesel powertrains. The price spike to over $10,000/oz in 2008 put the metal firmly in the substitution zone. Both platinum and rhodium lost out as lean NO_x traps (LNTs), which seemed the preferred emissions aftertreatment choice for Euro 6 prior to 2008, have been partly engineered out through a combination of price volatility and increasingly stringent CO₂ targets. Automakers have instead taken well-proven selective catalytic reduction (SCR) technology from heavy-duty vehicles and downsized it for cars. Although expensive and requiring maintenance (urea top-ups), SCRs have less of an impact on fuel efficiency (and so CO₂ emissions) compared to LNTs.

Rhodium’s price volatility has led to loadings being cut to the bare minimum, and palladium has replaced some rhodium.