Patent ID: 7361320

Claim:
A method for purifying exhaust gas from an internal combustion engine, having an exhaust gas system comprising a particulate filter, an SCR catalytic converter downstream of the particulate filter and a nitrogen oxide storage catalytic converter upstream of the particulate filter, the method comprising: enriching the exhaust gas with hydrogen upstream of the nitrogen oxide storage catalytic converter in a first range of exhaust gas temperatures such that at least one of the nitrogen oxide storage catalytic converter, the particulate filter or the SCR catalytic converter is able to reduce nitrogen oxides contained in the exhaust gas with hydrogen in a pure lean operation mode; switching from the pure lean operation mode to an alternating lean/rich operating mode when the temperature of the nitrogen oxide storage catalytic converter exceeds a given upper threshold; and reducing nitrogen oxides contained in the exhaust gas in the nitrogen oxide storage catalytic converter in a second temperature range for the nitrogen oxide storage catalytic converter above said temperature threshold by means of the alternating lean/rich operating mode, wherein nitrogen oxides passed through the nitrogen oxide storage catalytic converter as nitrogen oxide slippage are converted to harmless components in the SCR catalytic converter.