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

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 H-2/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/H-2 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% O-2 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 N-2-sorption, XRD, ICP-AES, XPS, in situ DRIFT, and H-2- 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 O-2 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.