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 engines [1]. 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 enormously. 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 European Stage IV emission limits after 100,000 km-equivalent bench engine ageing. Extending the ageing to 160,000 km, equivalent to the proposed European Stage V 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.