The effect of precursors on the active phase formation and hydrodesulfurization activities of noble metal phosphide catalysts was examined. The maximum activities of PdP and RhP catalysts prepared from acetylacetonato (acac) complex were observed at lower reduction temperatures than those for catalysts prepared from chlorides (Cl). The PdP catalyst prepared from acac also showed higher activity than that obtained from Cl. However, this trend was not observed in the RhP catalyst. The characterization results revealed that the higher activity of the PdP catalyst prepared from the acac is due to the highly dispersed Pd 4.8 P.The combustion of organic sulfur compounds in fuels used for boilers and engines results in the formation of sulfur oxides (SO X ). It is well known that SO X causes air pollution, acid rain, and the deactivation of automotive catalysts. Clean fuels are produced by the petroleum industry using hydrodesulfurization (HDS) processes. Recently, the development of new HDS catalysts that exhibit higher activity than commercial CoMo/ Al 2 O 3 catalysts have been claimed to produce cleaner fuels with low sulfur content. We reported that Rh 2 P and Pd 4.8 P showed higher catalytic activity than Ni 2 P and other NM X P Y (Ru and Pt).10 On the other hand, supported Pt catalysts prepared from acetylacetonato (acac) complexes have higher HDS activity than supported Pt catalysts prepared from other precursors.15 Thus, it may be possible to prepare highly active NM X P Y catalysts from acac complexes. In this study, the effect of precursors on the formation of NM X P Y (Pd 4.8 P and Rh 2 P) and its catalytic activity was examined in order to develop a new highly active HDS catalyst.Silica (SiO 2 , 295 m 2 g ¹1 ) was used as the catalyst support. P-added NM (NMP) catalysts were prepared using a previously described impregnation method. 10,12 The chloride precursors, RhCl 3 ¢3H 2 O and PdCl 2 , were dissolved in water and a 1.0 mol L ¹1 HCl solution, respectively. Ammonium dihydrogenphosphate (NH 4 H 2 PO 4 , AHP) was used for the preparation of NMP(Cl) catalysts. After impregnation, the catalysts were dried at 110°C for 24 h followed by heat treatment at 450°C in N 2 for 1 h to decompose the salts. The sieved catalysts (3042 mesh granules) were calcined in air at 500°C for 4 h (10°C min ¹1 ). The NMP(acac) catalysts were prepared using the same procedure but with acac complexes ([Rh(C 5 H 8 O 2 ) 3 ] and [Pd(C 5 H 7 O 2 ) 2 ]) dissolved in acetylacetone. In addition, triphenylphosphine ((C 6 H 5 ) 3 P, TPP) was used as the P precursor for the preparation of the NMP(acac) catalysts, because AHP does not dissolve in acetylacetone.The HDS of thiophene was performed using a fixed bed flow reactor under atmospheric pressure. The sieved catalyst (0.1 g) was treated in He at 500°C for 1 h, followed by reduction with H 2 at 350700°C for 1 h prior to the reaction. The HDS reaction was carried out using a mixture of H 2 and vaporized thiophene at 0°C, which was introduced into the quartz reactor at 350°C. The W/F of the...