Within the catalyst, the chemical reaction between the precious metals and exhaust gases contributes to the conversion of nitrogen oxides (NO_x), carbon monoxide (CO) and hydrocarbons (HC) into non-toxic compounds such as nitrogen (N₂), water (H₂O), and carbon dioxide (CO₂).

Catalytic activity of the noble metals is roughly proportional to their exposed surface area, since catalytic reactions provided by noble metals are contact reactions. To obtain the greatest extent of catalytic activity from a small amount of noble metal, manufacturers fabricate particles that are small in size, with a high specific surface area.

The Nissan engineers who developed the new catalyst note:

...in the case of minute particles in which the noble metal grain size is 1 nanometer (nm) or smaller, the surface reactivity of the noble metal particle is high and the noble metal particle has a large surface energy, so they are extremely unstable. For this reason, particles of noble metals readily cohere to each other (by sintering) when brought together at high temperature. Pt in particular undergoes marked sintering when heated, so even when supported on a carrier in a dispersed manner, sintering causes grains to coalesce and thus the average grain size increases and thus the catalytic activity decreases.

Exhaust catalysts in automobiles are routinely subjected to high temperatures in the range of 800-900° C, and even in excess of 1,000° C, making it difficult to maintain catalytic activity. To compensate, existing catalyst devices contain higher amount of precious metals in order to maintain an efficient level of cleaning. This, however, pushes costs up.

Nissan’s new material uses a wall material to prevent clustering of the precious metals. Click to enlarge.

With Nissan’s technology, the precious metal attached to the substrates are separated by a “wall material” which prevents clustering of the precious metals, leading to improved cleaning efficiency.

Masanori Nakamura, manager of the Nissan Research Center, explained the concept using the analogy of “eggs protected in a bird’s nest”.

The new under-floor catalyst to be introduced in the next generation Cube uses half the amount of precious metals, from 1.3 grams down to .65 gram. Efficiency is improved with 75% less nitrogen oxides (NO_x) and non-methane hydrocarbons (NMHC) emitted, lower than the Japan 2005 exhaust emission standards.

Nissan will produce the ultra-low precious metal catalyst in its Yokohama Plant for the domestic vehicle market. This catalyst and method for manufacturing the catalyst has been developed under the Renault-Nissan Alliance, and future applications of the approach will include Renault vehicles and diesel engines as well as potential for non-automotive application.

Resources

• Catalyst and Method for Manufacturing a Catalyst for Use in Exhaust Emission Control (WO/2006/016249)