We report the systematic investigation of the effects of oxygen on the synthesis of 3p sub-family armchair graphene nanoribbons (3p-AGNRs),which revealed a strong catalytic effect with a reduction in the reaction temperature by approximately 180 K without degradation of the AGNRs.Poly(para-phenylene) (3-AGNR) was generated through Ullmann-type coupling of 4,4"-dibromo-p-terphenyl on Cu(111),which was then converted into wider 3p-AGNRs via lateral fusion.Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy demonstrated the formation of different ribbons up to 12-AGNR,which contained regions exhibiting increased STM contrast that we attribute to the intercalation of Br atoms during lateral fusion.