The influence of Ni 2 P loading on the properties of Ni 2 P/SiO 2 catalysts was investigated by means of N 2 adsorption-desorption, X-ray diffraction, H 2 and NH 3 temperature-programmed desorption, CO chemisorption, inductively coupled plasma atomic emission spectroscopy and activity evaluation for the hydrodechlorization of chlorobenzene. It was found that increasing Ni 2 P loading results in the decrease of Ni 2 P dispersion. Higher P/Ni ratio occurs in the catalysts with lower Ni 2 P loadings, indicating that smaller Ni 2 P crystallites may more strongly interact with surplus P. We suggest that the acid amount of the Ni 2 P/SiO 2 catalyst is related to the exposed Ni sites and P-OH groups, while Brönsted acidity of P-OH groups facilitates hydrogen spillover. Increasing Ni 2 P loading is favorable to the catalyst activity, while this effect is not obvious when the Ni 2 P loading is greater than 9.6 wt%. Apart from the metallic property of Ni 2 P phase and spiltover hydrogen species, we propose that the acidity and the role of P in activating reactants should be considered for the catalyst performance.
An effective approach to enhancing the activity of the Ni 2 P/SiO 2 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, Ni 2 P/SiO 2 gave a chlorobenzene conversion of 5.6%. Surprisingly, after Ni 2 P/SiO 2 was pretreated with a 0.8-3.0% H 2 O/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 Ni 2 P/SiO 2 was also improved when 0.8% H 2 O or 0.5% O 2 was introduced into the reaction system. In addition, the passivation of Ni 2 P/SiO 2 followed by the reduction led to a conversion of 33%. The Ni 2 P/SiO 2 catalysts before and after the pretreatment were characterized by N 2 -sorption, XRD, ICP-AES, XPS, in situ DRIFT, and H 2 -and NH 3 -TPD. The results show that the pretreatment did not obviously influence specific surface area, pore structure, Ni 2 P crystallite size, the electron density of Ni in Ni 2 P, 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 Ni 2 P/SiO 2 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 H 2 O and O 2 on the activity of Ni 2 P/SiO 2 , and the synergism mainly took place on the Ni 2 P particles and at the interface between the Ni 2 P particles and SiO 2 .
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