Particle size molybdate catalysts

Analytical electron microscopy of individual catalyst particles provides much more information than just particle size and shape. The scanning transmission electron microscope (STEM) with analytical facilities allows chemical analysis and electron diffraction patterns to be obtained from areas on the order of lOnm in diameter. In this paper, examples of high spatial resolution chemical analysis by x-ray emission spectroscopy are drawn from supported Pd, bismuth and ferric molybdates, and ZSM-5 zeolite. 

One further difference exists between HDS and HDM. Bridge [37] has shown, very clearly, that HDS is not limited by diffusion while HDM is. Using a nickel-molybdate based catalyst with a unimodal microporous size distribution, the demetalation of Arabian heavy atmospheric residuum was found to be affected by catalyst particle size, while HDS was not. As the diameter of the pore was decreased, the maximum in the metals deposition profile moved closer to the external surface of the pellet, agmn indicating difiusional limitations for FIDM. 

The Ni-Co-molybdate samples were prepared by co-precipitation of solutions containing Ni and Co nitrates (Merck) and ammonia heptamolybdate (Fluka) at pH between 6.0 and 8.5 followed by steps of filtering, washing, drying (110°C, 12 h) and calcination (500°C, 3 h in air). The amounts of metals used for all preparations correspond to the following stoichiometry Nio.sCoo,5Mo04. The mixed Co and Ni nitrate solution was added stepwise to the ammonium heptamolybdate solution, both heated at 60°C and then the co-precipitation was realized for 8 h. Ammonium hydroxide solution was used to reach and maintain a constant pH at 6.0, 7.5 and 8.5 in successive runs. The catalyst was crushed and sieved in particle sizes of 200-250 pm for catalytic tests. 

Catalytic results for a structure-sensitive reaction show that this control of Pd dispersion may be used to tune the activity of Pd-supported systems. However, they also show that the presence of surface molybdates has other effects on Pd particles than simple size control. They also point at the need of a still finer characterisation of M0/7AI2O3 precursors, especially concerning the presence of Mo-O-Al bonds. Altogether, this work presents encouraging prospects for fine-tuning the properties of supported metal catalysts by prior surface engineering of the support. 

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