Exhaust gas purifying method

A method of purifying an exhaust gas, the method comprising:

• disposing a NOx trapping catalyst in an exhaust pipe of an internal combustion engine,
• the NOx trapping catalyst comprising:
• a metal substrate including cells, a corner portion of each of cells having an acute angle;
• and
• a catalyst layer supported in the metal substrate, including a noble metal, a heat-resistant inorganic oxide, an NOx trapping material, a pore formation promoting material which includes particles of said pore formation promoting material, and pores having a diameter ranging from 1 μm to 10 μm, with each pore containing therein a particle of said pore formation promoting material, wherein said pores are defined by the noble metal, the heat-resistant inorganic oxide, and the NOx trapping material in the catalyst layer,
• wherein said pores are formed by a calcination process by shrinking said pore formation promoting material from pre-calcination particle sizes with diameters ranging from 0.1 μm to 3.0 μm, and
• the NOx trapping catalyst:
• adsorbing NOx in the exhaust gas when an exhaust air-fuel ratio is in a lean state;
• and
• desorbing and reducing the adsorbed NOx when the exhaust air-fuel ratio is in a stoichiometric state or a rich state;
• and
• removing the NOx by the exhaust air-fuel ratio being shifted between the lean state and the rich state.