Page 125 - Principles of Catalyst Development
P. 125
CATALYST PREPARATION 113
TABLE 6.3. Adsorption of Catalytic Ions"
Adsorption of element as
Group Cation cr- NO)" SO~-
IB Cu H Cationic < 4 M Cationic Anionic
Anionic> 4 M
Ag+ Insoluble Cationic (weak) Anionic> 0.05 M
Au H Anionic> 0.1 M Anionic (strong) Anionic (weak)
VIII3 NiH Cationic Cationic Cationic
Pd H Anionic> 0.1 M Anionic Anionic
Pt4+ Anionic> 0.1 M Anionic Anionic
VIIJ 2 CO H Cationic < 6 M Cationic Cationic
Anionic> 6 M
RhH Anionic Anionic Cationic/ anionic
Ir4 + Anionic> 0.1 M Anionic Anionic
VIII, Fe H Cationic < 1 M Cationic Anionic> 0.1 M
Anionic> 1 M
Ru 4 + Anionic> I M Anionic Anionic
OS4+ Anionic> 0.1 M Anionic Anionic
a Reference 157.
Unfortunately, saturation amounts are generally small. With nickel
solutions and alumina, for example, only loadings up to 2%-3% are
possible. Multiple adsorptions with intermediate calcination give higher
loadings, but this is time consuming. Other methods are generally preferred.
With platinum and other expensive noble metals, however, amounts of less
than 1 % are often needed. Low loadings with high dispersion give satisfac-
tory results. Supports are soaked in solutions of chloroplatinic acid, H 2PtCI 6 ,
to yield desired levels of adsorbed (PtCI4)2-. Washing is not necessary, nor
desirable, since it induces desorption. Drying and calcination are carried
out as usual, with the chloroplatinic ion decomposing to platinum oxide
or platinum.
Transport effects are encountered when using large particles. Adsorp-
tion of chloropiatinic acid is so rapid that diffusion of the solute into the
pores controls the rate. Deposition takes place in an outer shell, as shown
in Fig. 6.15.
In some cases, such profiles are desirable. For example, with fast
reactions and external diffusion resistance, the reaction occurs only on
the outside of the pellet. Platinum deposited deep inside the particle is
wasted. Shell deposition is not satisfactory, however, in r,eactions that are
not diffusion controlled, such as catalytic reforming. It can be avoided by
adding hydrochloric acid to the solution. The HCl competes with chloro-
platinic acid for adsorption sites, driving platinum deeper into the particle.