An aircraft gas turbine engine comprising: a combustor ; a turbine section downstream of the combustor ; a nozzle section downstream of the turbine section ; a heat exchanger ; and a fuel supply line for conveying fuel through the heat exchanger and to the combustor ; wherein the heat exchanger includes a fuel-cracking catalyst , the fuel-cracking catalyst comprising: a protonated solid acid support ; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section ; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support , the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites , and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites . a combustor ; a turbine section downstream of the combustor ; a nozzle section downstream of the turbine section ; a heat exchanger ; and a fuel supply line for conveying fuel through the heat exchanger and to the combustor ; wherein the heat exchanger includes a fuel-cracking catalyst , the fuel-cracking catalyst comprising: a protonated solid acid support ; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section ; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support , the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites , and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites . a protonated solid acid support ; characterised in that the heat exchanger is in the turbine section or downstream of the nozzle section ; and the catalyst further comprises 0.25-5% by weight of monodisperse metal catalyst clusters in the protonated solid acid support , the monodisperse metal catalyst clusters having a cluster size of no greater than 6 angstroms and comprising one or more transition metals including at least platinum, nickel, or copper, wherein the fuel-cracking catalyst has 0.25-5% by weight of platinum, nickel, or copper; and the protonated solid acid support is a zeolite that has a ratio of silica to alumina that is at least 11 and no greater than 25; wherein the protonated solid acid support includes acid sites and the one or more transition metals are at metal sites , and the acid sites and the metal sites are spaced more than 7 angstroms apart to prevent cracked products of fuel hydrocarbon molecules from simultaneously bonding with both the acid sites and the metal sites . <EOS>