A catalyst article for oxidising ammonia, the catalyst article comprising a composite heterogeneous extruded honeycomb which comprises at least a first phase which is an inorganic matrix component and a second phase which is an active interface layer, the heterogeneous extruded honeycomb having longitudinally extending parallel channels, which channels being defined in part by channel walls having a total longitudinal length, wherein the channel walls comprise a pore structure including a periodic arrangement of pores embedded in the inorganic matrix component, the pores defined by internal surfaces with a mean internal surface coverage in a population of 100 pores of 80% or more, preferably 90% or more of a catalytically active material which forms the active interface layer, wherein the catalytically active material comprises a precious metal supported on particles of a support material, wherein the precious metal is a platinum group metal, gold or silver; wherein the catalyst article comprises a selective catalytic reduction (SCR) catalyst which is either: (i) provided in a washcoat coated on at least part of a total length of the channel walls; and/or (ii) wherein the catalytically active material in at least some of the pores comprises a mixture of SCR catalyst and the precious metal supported on the support material, wherein the precious metal is present in the catalyst in an active component loading from 0.01 to 0.3 wt.% inclusive, wherein active component loading refers to the weight of the precious metal support + the weight of the precious metal + the weight of the SCR catalyst in the active interface layer of a population of pores embedded in the inorganic matrix component. wherein the channel walls comprise a pore structure including a periodic arrangement of pores embedded in the inorganic matrix component, the pores defined by internal surfaces with a mean internal surface coverage in a population of 100 pores of 80% or more, preferably 90% or more of a catalytically active material which forms the active interface layer, wherein the catalytically active material comprises a precious metal supported on particles of a support material, wherein the precious metal is a platinum group metal, gold or silver; wherein the catalyst article comprises a selective catalytic reduction (SCR) catalyst which is either: (i) provided in a washcoat coated on at least part of a total length of the channel walls; and/or (ii) wherein the catalytically active material in at least some of the pores comprises a mixture of SCR catalyst and the precious metal supported on the support material, wherein the catalytically active material comprises a precious metal supported on particles of a support material, wherein the precious metal is a platinum group metal, gold or silver; wherein the catalyst article comprises a selective catalytic reduction (SCR) catalyst which is either: (i) provided in a washcoat coated on at least part of a total length of the channel walls; and/or (ii) wherein the catalytically active material in at least some of the pores comprises a mixture of SCR catalyst and the precious metal supported on the support material, (i) provided in a washcoat coated on at least part of a total length of the channel walls; and/or (ii) wherein the catalytically active material in at least some of the pores comprises a mixture of SCR catalyst and the precious metal supported on the support material, wherein the precious metal is present in the catalyst in an active component loading from 0.01 to 0.3 wt.% inclusive, wherein active component loading refers to the weight of the precious metal support + the weight of the precious metal + the weight of the SCR catalyst in the active interface layer of a population of pores embedded in the inorganic matrix component. <EOS>