MoW synergetic effect supported by HAADF for alumina based catalysts prepared from mixed SiMonW12-n heteropolyacids

Abstract MoW catalysts supported on Al 2 O 3 with equal surface content of metals (Mo + W = 3.9 at/nm 2 ) were synthesized by using mixed Keggin type heteropolyacids ( HPAs ) H 4 [SiMo 1 W 11 O 40 ] and H 4 [SiMo 3 W 9 O 40 ] and corresponding mixture of monometallic H 4 [SiMo 12 O 40 ] and H 4 [SiW 12 O 40 ] HPAs. After liquid phase sulfidation, catalysts were characterized by high-resolution transmission electron microscopy ( HRTEM ) and X-ray photoelectron spectroscopy ( XPS ). For the first time, High Angle Annular Dark Field microscopy ( HAADF ) was used to evidence the morphology and composition of the active sulfide phase. The catalysts were tested in hydrotreating of model feed that contained dibenzothiophene ( DBT ) and naphthalene. Using mixed SiMo n W 12-n HPAs as starting precursors had a beneficial effect on the catalytic activity. Incorporation of molybdenum in the structure of SiW Keggin-type HPA led to a decrease of the size of active phase crystallites, to an increase of the metal sulfidation degree and of the number of active sites. Substitution of a quarter of tungsten atoms by molybdenum allowed to achieve DBT conversion comparable to value obtained on pure SiMo 12 HPA based catalyst and higher conversion of naphthalene. Both SiMo n W 12-n HPAs based catalysts had higher rate constants in studied reactions compared to their corresponding references prepared from two separate monometallic HPAs. Moreover, Mo n W 12-n /Al 2 O 3 catalysts had the highest selectivity in respect of hydrogenation ( HYD ) pathway of DBT hydrodesulfurization ( HDS ). The presence of mixed Mo x W 1-x S 2 slabs was evidenced by HAADF analysis. On Mo 3 W 9 /Al 2 O 3 , small randomly distributed islands of Mo were present in the WS 2 slabs, while on Mo 3  + W 9 /Al 2 O 3 , Mo islands appeared larger and in the core of the WS 2 slabs. Moreover, on this last sample, some monometallic slabs were also present. It was concluded that using mixed HPA precursors resulted in the formation of mixed Mo x W 1-x S 2 active phase possessing higher synergetic effect between the two metals and consequently higher catalytic activity.