This is the most popular type of diesel particulate filter element, and is made from a type of porous ceramic, either cordierite or silicon carbide. The element consists of lots of parallel channels, which are blocked at alternate ends. This forces the exhaust gas to pass through the wall from one channel into the next in order to be able to exit the filter, and in doing so, diesel particulate matter is left behind in the pores. A wall-flow filter can remove roughly 99% of diesel particulate matter by mass and a very high proportion of particles by number.
Wall-flow filters can be catalytically coated, which is one way of achieving passive regeneration. Since the pores in a wall-flow filter are very small, it is impossible to coat them with a slurry without blocking them up, therefore it is best to use a sol-gel process to apply the washcoat. The washcoat contains nanoparticles of alumina, ceria and other rare-earth elements which perform an important role in the overall activity of the catalytic coating. After the washcoat has been fired, it is impregnated with platinum group metals from acid solutions as separate stage. Beware – some coaters which lack an understanding of nanotechnology don’t bother with the washcoat, with the result that the coating is much less effective, even if they use a lot of expensive precious metals.