PHOTOCATALYST

Thermal catalysts, especially noble metal-coated catalysts, are used to reduce the CO, HC and NOx in the exhaust gases of internal combustion engines. The noble metal-coated catalysts do indeed have a catalytic action from about 140 DEG C but to achieve particularly low emission rates the thermal catalysts must be heated on cold starting in order to be able to convert the harmful emissions on start-up as well. According to the invention, a catalyst with a photosemiconductor is used which is irradiated with u/v light. A suitable photocatalytic material is, for instance, titanium dioxide. The catalyst supports may be common monoliths (2, 3), the channels (16) of which are illuminated via reflectors (10 - 14). Thus the monoliths common especially in motor vehicles may be used. The invention may be used for diesel engines and lean-mixture spark ignition engines.