Particulate filters

A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of:

• a) containing a dry powder in a reservoir;
• b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure;
• c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter;
• d) transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter;
• and
• e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure;
• wherein the dry powder comprises or consists of a metal compound for forming by thermal decomposition a metal oxide.