Aspects of the invention relate to a method of treating a gas stream generated by a motorcycle, the method comprising: contacting a gas stream containing hydrocarbons, carbon monoxide and nitrogen oxides and generated by a motorcycle under both rich and lean engine operating conditions with a base metal catalyst composition, thereby removing at least a part of the hydrocarbons, carbon monoxide and nitrogen oxides in gas stream. The base metal catalyst composition comprises a support including at least 10% by weight of reducible ceria, and about 3 to about 7 wt % MnO and about 8 to about 22 wt % CuO on the reducible ceria support. The base metal catalyst composition is effective to promote a steam reforming reaction of hydrocarbons and a water gas shift reaction to provide H2 as a reductant to abate NOx.