Synthesis of palladium phosphides for aqueous phase hydrodechlorination: Kinetic study and deactivation resistance

We report the synthesis of noble metal phosphides by a temperature-programmed reduction method for aqueous phase hydrodechlorination (HDC) of 4-chlorophenol (4-CP). Pd3P0.95/SiO2, Rh2P/SiO2 and PtP2/SiO2 catalysts were prepared and showed higher 4-CP HDC rates than their corresponding noble metal catalysts. Pd3P0.95/SiO2 sample exhibited a higher HDC rate than the other phosphide catalysts and Pd/SiO2 catalysts. The XPS spectra show the surface interaction reactions of the Pd and P species that occur via the electron transfer from Pd to P. Pd3P0.95/SiO2 was synthesized with different phosphide particle size (1.1–13.5 nm based on hydrogen chemisorptions) via changing the metal loading from 1 to 10 wt%. The 4-CP HDC reaction is sensitive to the size of the Pd and Pd3P0.95 particles, and the optimum sizes for Pd/SiO2 and Pd3P0.95/SiO2 were ∼8 and ∼5 nm, respectively. The effects of the initial 4-CP and Cl− ion concentration and the S to Pdsurf (surface active Pd metal sites determined by hydrogen chemisorptions) ratio on the 4-CP HDC activity were examined for both catalysts. The Langmuir-Hinshelwood kinetic model that was developed considering the surface reaction as the rate-determining step suggests P has an enhancing effect on sorption in the 4-CP HDC reaction and an inhibiting effect on chloride sorption for Pd3P0.95/SiO2. Moreover, Pd3P0.95/SiO2 possesses better sulfide resistance than Pd/SiO2 because P can prevent the conversion of Pd metallic sites into inactive Pd4S compounds with a sulfide treatment. As a result, Pd3P0.95/SiO2 shows better HDC stability than those of Pd/SiO2 catalysts.