Development and Application of New Low Rhodium Three-Way Catalyst Technology

Rhodium is an important component of three-way catalysts for emissions control on gasoline engines¹. It has unparalleled activity in NO_x reduction under stoichiometric conditions, where very high conversion efficiencies are required to meet current and future legislative targets. Over 90% of world rhodium usage is in such catalysts, and in recent times the price of rhodium has increased enormously². This has lead to significant focus on reducing the quantity of rhodium required whilst still retaining the ability to meet the most stringent emissions legislation. Experiments are described where three-way catalysts employing advanced washcoat formulations and coating techniques are evaluated with a range of rhodium levels right down to 1g/ft³ which nevertheless still meet Euro 4 emissions limits after 100,000 km-equivalent bench engine ageing. Extending the ageing to 160,000 km, equivalent to the proposed Euro 5 standards, shows that subsequent deterioration is minimal despite the ultra-low loadings involved. Further experiments involving a trade-off of Pd and Rh at approximately equivalent total PGM cost show that similarly low Rh levels appear to offer the best compromise between HC and NO_x performance where these advanced formulations and coating techniques are used.