A facile approach to enhancing activity of Ni2P/SiO2 catalyst for hydrodechlorination of chlorobenzene: promoting effect of water and oxygen

Abstract

An effective approach to enhancing the activity of the Ni₂P/SiO₂ catalyst for the hydrodechlorination of chlorobenzene (CB) was reported. At atmospheric pressure, 513 K, a CB WHSV of 4.1 h⁻¹ and a H₂/CB molar ratio of 9.0, Ni₂P/SiO₂ gave a chlorobenzene conversion of 5.6%. Surprisingly, after Ni₂P/SiO₂ was pretreated with a 0.8–3.0% H₂O/H₂ flow below 543 K, the conversion reached as high as 99%. As the pretreatment temperature increased, the conversion tended to decrease; however, a conversion of 42.6% was still obtained even at the pretreatment temperature of 673 K. The activity of Ni₂P/SiO₂ was also improved when 0.8% H₂O or 0.5% O₂ was introduced into the reaction system. In addition, the passivation of Ni₂P/SiO₂ followed by the reduction led to a conversion of 33%. The Ni₂P/SiO₂ catalysts before and after the pretreatment were characterized by N₂-sorption, XRD, ICP-AES, XPS, in situ DRIFT, and H₂- and NH₃-TPD. The results show that the pretreatment did not obviously influence specific surface area, pore structure, Ni₂P crystallite size, the electron density of Ni in Ni₂P, and the Ni and P contents, while it created new P–OH groups and reduced the amount of surface Ni sites. We propose a surface model of Ni₂P/SiO₂ containing the Ni sites and the P–OH groups and consider that the synergism between the Ni site and the P–OH group can explain the promoting effect due to H₂O and O₂ on the activity of Ni₂P/SiO₂, and the synergism mainly took place on the Ni₂P particles and at the interface between the Ni₂P particles and SiO₂.