h i g h l i g h t s• 1% mol Ca species enhanced the activity of Fe 2 O 3 in the degradation of DecaCB.• Dominant Nona-MonoCB isomers were determined during hydrodechlorination reaction.• Thermodynamic and/or steric effects drive conversion.• Steric effect prevented the hydrodechlorination at ortho-positions. a r t i c l e i n f o b s t r a c tDegradation reaction of decachlorobiphenyl (PCB-209) was investigated over the synthesized Ca speciesdoped Fe 2 O 3 at 300°C. The 1%Ca-Fe 2 O 3 exhibited the highest activity among the four catalysts prepared with the pseudo-first order reaction at k obs = 0.103 min −1 . PCB-207, PCB-197, PCB-176, PCB-184, PCB-150, PCB-136, PCB-148, PCB-104, PCB-96, PCB-54, PCB-19, PCB-4 and PCB-1 were identified as the dominant isomers in their respective nonachlorobiphenyl (NonaCB) to monochlorobiphenyl (MonoCB) homologue groups. Analysis of the hydrodechlorination products indicated that dechlorination was much more favored on meta-and para-than on ortho-positions. The formation of significantly predominant NonaCB and octachlorobiphenyl (OctaCB) isomers was attributed to lower energy principles and to the 90°dihe-dral angles of two aromatic rings which prevented the hydrodechlorination at ortho-positions. When the number of chlorine atoms is not more than 7, the steric effect supports the formation of predominant PCB isomers having chlorines at four ortho-positions. During the dechlorination of tetrachlorobiphenyl (TetraCB) formed to generate monochlorobiphenyl (MonoCB) isomers, the chlorine atoms fully substituted at the ortho-positions have to be successively removed, with the first two dechlorinations preferentially occurring at the two different benzene rings. This is dissimilar to that of octachloronaphthalene in which the hydrodechlorination reaction happened preferentially at ortho-position due to the existence of steric effects. The opposite roles of the steric effect in ortho-position between PCB-209 and PCN-75 might be due to the difference of the π -conjugated plane caused by the dihedral angle of 90°and 0°of the two aromatic rings.

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