The Pd/Al2O3 catalyst was prepared by the impregnation method to investigate the deactivation and mechanism of coke formation on metal catalysts for the selective hydrogenation of benzonitrile in a continuous fixed-bed reactor under mild conditions. The Pd/Al2O3 catalyst exhibited the initial benzonitrile conversion of 100% with a high benzylamine selectivity of similar to 86%, and continuous deactivation during the long-term reaction period of similar to 1500 h. The comparative investigation of the Pd/Al2O3 catalysts before and after the benzonitrile hydrogenation demonstrated that the deactivation of the Pd/Al2O3 was mainly attributed to carbonaceous compounds deposited on the catalyst. The carbonaceous species contained the aromatic ring conjugated with C = N double bond and tertiary amine groups, which were chemically bound onto the surface of Pd particles, leading to the irreversible loss of metallic active sites. Moreover, the carbon deposition also resulted in partial blockage of the support pores. This study is helpful to understand the deactivation and formation mechanism of carbonaceous compounds on the surface of metal catalysts for the hydrogenation reaction. (C) 2017 Elsevier B.V. All rights reserved.