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A method of reducing nitrogen oxides (NOx) in a flowing combustion exhaust gas to N2, which method comprising oxidizing nitrogen monoxide to nitrogen dioxide on a transition metal/elite catalyst at catalyst bed temperatures below 50° C. and reducing NOx with the catalyst using an hydrocarbon (HC) re

A method of reducing nitrogen oxides (NOx) in a flowing combustion exhaust gas to N2, which method comprising oxidizing nitrogen monoxide to nitrogen dioxide on a transition metal/elite catalyst at catalyst bed temperatures below 50° C. and reducing NOx with the catalyst using an hydrocarbon (HC) reductant at catalyst bed temperatures below 150° C.

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1. A method of reducing nitrogen oxides (NOx) in a flowing combustion exhaust gas to N2, which method comprising oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2) on a transition metal/zeolite catalyst at temperatures below 50° C. and reducing NOx with the transition metal/zeolite catalyst

1. A method of reducing nitrogen oxides (NOx) in a flowing combustion exhaust gas to N2, which method comprising oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2) on a transition metal/zeolite catalyst at temperatures below 50° C. and reducing NOx with the transition metal/zeolite catalyst using a hydrocarbon (HC) reductant introduced actively into the exhaust gas at temperatures below 150° C., wherein the transition metal/zeolite catalyst comprises a transition metal selected from the group consisting of cobalt, manganese, cerium, copper, iron, chromium and mixtures of any two or more thereof and a zeolite selected from the group consisting of ZSM-5, A, beta, X, Y, Linde type L and faujasite. 2. A method according to claim 1, wherein HC reductant is adsorbed on the catalyst prior to contacting the flowing combustion gas. 3. A method according to claim 1, wherein HC reductant is present in the exhaust gas at >50 ppm C1 HC. 4. A method according to claim 1, wherein the C1 HC:NOx molar ratio is from 30:1 to 1:1. 5. A method according to claim 1, further comprising adsorbing NO on the catalyst prior to the step of oxidising the NO to NO2. 6. A method according to claim 1, wherein the transition metal is iron. 7. A method according to claim 1, wherein the zeolite is beta zeolite. 8. A method according to claim 1, wherein the combustion exhaust gas is derived from combustion of a hydrocarbon fuel in a lean-burn internal combustion engine. 9. A method according to claim 1, wherein the hydrocarbon reductant is diesel fuel. 10. A method according to claim 1 further comprising reducing NOx in the combustion exhaust gas to N2 by contacting the transition metal/zeolite catalyst with a nitrogenous reductant at a temperature of ≧150° C. 11. A method according to claim 10, wherein the nitrogenous reductant is ammonia. 12. A method according to claim 10, comprising oxidising NO in the exhaust gas to NO2 to produce a gas mixture comprising NO and NO2 before contacting the resulting exhaust gas with the transition metal/zeolite catalyst. 13. A method according to claim 1, wherein the transition metal/zeolite catalyst comprises an Fe/beta zeolite catalyst. 14. The method according to claim 1, wherein the oxidising nitrogen monoxide (NO) to nitrogen dioxide (NO2) step generates a mixture of NO:NO2 of 2:1 to 1:2. 15. A method for reducing nitrogen oxides (NOx) in a flowing combustion exhaust gas to N2, comprising: oxidizing nitrogen monoxide (NO) to nitrogen dioxide (NO2) on a transition metal/zeolite catalyst at temperatures below 50° C. and reducing NOx with the transition metal/zeolite catalyst using hydrocarbon (HC) reductant at temperatures below 150° C., andswitching the source of reductant from HC reductant to a nitrogenous reductant in response to a detected catalyst bed temperature of ≧150° C.,wherein the transition metal/zeolite catalyst comprises a transition metal selected from the group consisting of cobalt, manganese, cerium, copper, iron, chromium, and mixtures of any two or more thereof and a zeolite selected from the group consisting of ZSM-5, A, beta, X, Y, Linde type L, and faujasite.