PAMAM metal colloid

In this specific case, the colloid stabilizers are dendrimers, for instance, polyamidoamine (PAMAM), which are hyperbranched polymers that ramify from a single core and form a porous sphere [103, 104] (Scheme 17.1). Dendrimer-encapsulated nanoparticles (DENs) are synthesized by sequestering metal ions within appropriate dendrimers, and then by chemically reducing the resulting composite. They can be synthesized in various media, such as water or ethanol. The size of the nanoparticles is usually nearly monodisperse, and can be tuned by varying the metal-to-dendrimer ratio prior to reduction. Supported catalysts can then he prepared by immobilizing DENs onto a sohd support. As in the case of colloids, the last step, which consists in the removal of dendrimers hy thermal treatment, may lead to an increase in both the metal-particle size and particle-size distribution. 

The first two stages of the synthesis of catalysts prepared by dendrimers are inextricably linked. Proper incorporation of the metal precursor within the PAMAM dendrimer is essential for the formation of dendrimer-metal nanocomposites and, eventually, nanoparticles with controlled particle sizes. Complications in the complexation stage, such as incomplete or inadequate incorporation of the metal precursor, will leave free metal cations or colloidal particles in the impregnating solution, resulting in the formation of supported catalysts that exhibit wide particle size distributions. In the case of bimetallic catalysts, the loss is twofold In addition to an array of metal particle sizes, there will also be a significant loss of compositional control in the active phase. In short, if the complexation step is not tightly controlled, the dendrimer-prepared catalyst will not differ substantially from a catalyst prepared by wet impregnation. 

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