Oxidation catalyst for a diesel engine exhaust

An oxidation catalyst is described for treating an exhaust gas produced by a diesel engine comprising a catalytic region and a substrate, wherein the catalytic region comprises a catalytic material comprising: a copper (Cu) component; a platinum group metal (PGM) selected from the group consisting of (i) platinum (Pt), (ii) palladium (Pd) and (iii) platinum (Pt) and palladium (Pd); and a support material, which is a refractory oxide comprising alumina; wherein the platinum group metal (PGM) and the copper (Cu) component is each supported on the support material.