Preparation, Characterization, and Performance of Alumina-Supported Nanostructured Mo−Phosphide Systems

A series of γ-Al 2 O 3 -supported molybdenum phosphide catalysts containing fixed Mo-loading (9.9 wt %) and variable P amounts (1.5−9.6 wt %) were prepared by temperature-programmed reduction (TPR) from their oxide precursors in hydrogen. The effect of the P/Mo atomic ratio on the structure of the molybdenum phosphide materials was studied by S BET, XRD, HRTEM, TPD−NH 3, H 2 chemisorption, XPS, and 31 P- and 27 Al-MAS NMR techniques. The reactions involved in the transformation of oxide precursor to Mo−phosphide were followed by TPR. Specific surface areas of samples subjected to reduction at 1123 K were rather high (210−263 m 2 g -1 ). These catalysts were tested in the gas-phase hydrodesulfurization (HDS) of dibenzothiophene (DBT) at 553 K and 3.4 MPa. All Mo−phosphide catalysts were more active than a molybdenum sulfide sample with the same Mo-loading. The Mo−phosphide catalysts with P/Mo ratios of 1 and 1.1 were found to be the most active among the catalysts studied. The activity drop observed for higher P-loadings runs in parallel with the drop in specific areas. Moreover, the XPS and electrons diffraction analyses revealed the formation of MoP phase in all samples and the appearance of polyphosphates and AlPO 4 phases in P-rich ones. For the P-rich samples, the EFTEM study indicated that phosphate is accumulated around Mo atoms. It was found that the H 2 chemisorption capacity of the samples reduced at 1123 K correlates with the HDS performance of the catalysts.