Catalyst pellet preparation

The primary consideration for all AEM analysis is that the specimen be thin (generally carbon coated electron microscope grid either dry or in a suitable liquid. If a liquid suspension is used in preparing the specimen, it is important that all elements of interest are insoluble in that liquid. Only particles thin enough to meet AEM thin-film criteria (15) should be analyzed quantitatively. Scraping surface particles from a catalyst pellet for specimen preparation may be more useful than grinding the entire pellet. 

Although a carrier is often the major constituent of a catalyst pellet, one often finds that there are materials that are added in small amounts during catalyst preparation in order to impart improved characteristics to the finished catalyst. These materials are referred to as promoters. They may lead to better activity, selectivity, or stability. The manner in which promoters act is not well understood, although a number of plausible explanations have been set forth. They remain one of the reasons for the black magic aura of catalysis. 

Although this reaction network has been studied extensively, its mechanism is still under debate (10). In this study, a single-pellet reactor was used, and the pellet was prepared mechanically by pressing the active catalyst layer between two alumina layers. In this way a step-type catalyst pellet was produced, which approximated a Dirac-type catalyst distribution. 

The size of the catalyst pellets. For a given shape the size influences only the flow and diffusion phenomena, but small pellets are often much easier to prepare. 

Catalysts are prepared by impregnation by spraying a solution of a metal salt onto pellets of a porous support until incipient wetness. The pellets are then dried and calcined to transform the metal into insoluble form. 

Catalysts, prepared by impregnation of the porous support, in many cases exhibit intrapellet activity gradients, which are traditionally thought to be detrimental to catalyst performance. The effect of a deliberate nonuniform distribution of the catalytic material in the support, on the performance of a catalyst pellet received attention as early as the late 1960s [18,19]. These, as well as later studies, both experimental and theoretical, demonstrated that nonuniformly distributed catalysts can offer superior conversion, selectivity, durability, and thermal sensibility characteristics over those wherein the activity is uniform. 

Cylindrical pellets of four industrial and laboratory prepared catalysts with mono- and bidisperse pore structure were tested. Selected pellets have different pore-size distribution with most frequent pore radii (rmax) in the range 8 - 2500 nm. Their textural properties were determined by mercury porosimetry and helium pycnometry (AutoPore III, AccuPyc 1330, Micromeritics, USA). Description, textural properties of catalysts pellets, diameters of (equivalent) spheres, 2R, (with the same volume to geometric surface ratio) and column void fractions, a, (calculated from the column volume and volume of packed pellets) are summarized in Table 1. Cylindrical brass pellets with the same height and diameter as porous catalysts were used as nonporous packing. 

For those cases where the catalyst is not attached to the membrane, catalyst pellets are typically used. The preparation of catalyst pellets has been extensively documented and will not be reviewed here. Only the preparation of the catalyst inside the membrane porous structure is discussed in this chapter. 

The narrow region Pt/alumina catalyst pellets have been prepared by the impregnation procedure The initial activity distributions (Fig. 1) have been estimated from the kinetic measurements by the method described in (ref 11). 

For the vanadium deposition experiments a molybdenum on silica catalyst is prepared. A molybdenum catalyst is used since it is active for the hydrodemetallisation of VO-TPP and it can be used as a reference element for determining the radial vanadium deposition in catalyst pellets. 

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