Colloidal methods catalysts structure

Unfortunately, while catalyst components and structures in low-temperature MEAs have attracted considerable attention, optimization of high-temperature catalyst layer structures and components seems little studied. Lobato et al. [83] investigated the effect of the catalytic ink preparation method on the performance of HT-PEMFCs. They employed two methods for catalyst layer preparation the solution method and the colloid method. In the solution method, catalyst ink was prepared by mixing the catalyst (20% Pt/C) and PBl solution (5% PBl in dimethylacetamide). In the colloid method, acetone was added to the mixture of catalyst and PBI solution, which made the PBI form a colloid suspended in the solvent. They found that electrodes prepared by the solution method showed better performances at 150 °C and 175 °C, and that the electrodes prepared by die colloid method gave a better performance at 125 °C. This is probably due to differences in catalyst layer structure (see Section 18.2.7). 

The work of Lobato et al. is a typical example of high-temperature MEA preparation, as shown in Figure 18.13 [83]. The catalyst layer structure is illustrated in Figure 18.14. The solution method gave a more uniform and denser structure with lower secondary pore volume than the colloid mefliod, due to the uniform distribution of FBI in the solution. In the colloid method, FBI forms agglomerates, resulting in the enlargement of secondary pores. 

Figure 2.16. A Pt nano-catalyst on an MWCNT prepared by a colloidal method. Some CNTs seem to have straight geometries while others clearly bend, causing irregular structures.

Sarellas A., Niakolas D., Bourikas K., Vakros J., and Kordulis C. 2006. The influence of the preparation method and the Co loading on the structure and activity of cobalt oxide/y-alumina catalysts for NO reduction by propene. J. Colloid. Interf. Sci. 295 165-72. 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

Au/ZrC>2 catalysts are less active than Au/TiC>2 catalysts, whatever method of preparation is used deposition of colloidal gold,83,91 DP12 or laser vaporisation.70 Activity depends on the method used (Table 6.12), and appears to be due only to the presence of Au°. The reason for the difference between zirconia and titania is not understood Zr4+ is more difficult to reduce than Ti4+, so anion defects may be harder to form. The lattice structures also differ in monoclinic zirconia (baddleyite) the Zr4+ ion is unusually seven coordinate, and phase transitions into tetragonal and cubic structures occur at >1370 and >2570 K, respectively. However, the... 

Recently reported meso- and macroscale self-assembly approaches conducted, respectively, in the presence of surfactant mesophases [134-136] and colloidal sphere arrays [137] are highly promising for the molecular engineering of novel catalytic mixed metal oxides. These novel methods offer the possibility to control surface and bulk chemistry (e.g. the V oxidation state and P/V ratios), wall nature (i.e. amorphous or nanocrystalline), morphology, pore structures and surface areas of mixed metal oxides. Furthermore, these novel catalysts represent well-defined model systems that are expected to lead to new insights into the nature of the active and selective surface sites and the mechanism of n-butane oxidation. In this section, we describe several promising synthesis approaches to VPO catalysts, such as the self-assembly of mesostructured VPO phases, the synthesis of macroporous VPO phases, intercalation and pillaring of layered VPO phases and other methods. 

These three techniques are employed along with others not mentioned here to investigate the catalytic nature of a reaction. It is difficult to obtain positive confirmation for one catalytic nature over another because of the ability of small amounts of homogeneous catalyst (concentrations below current detection methods) to catalyze reactions [11]. Leached atoms can readsorb rapidly to heterogeneous structures, either to a substrate or to the surface of the nanoparticles [17,18], In the following sections, we review some of the major results involving colloidal nanoparticles in solution-phase catalysis. The two reaction types that will be discussed in this chapter are redox reactions and carbon-carbon bond formation reactions. 

Bimetallic particles with layered structures have opened fascinating prospects for the design of new catalysts. Schmid et al. [10m] have applied the classical seed-growth method [20] to synthesize layered bimetallic Au/Pd and Pd/Au colloids in the size range of 20-56 nm. The sequential reduction of gold salts and palladium salts with sodium citrate allows the gold core to be coated with Pd. This layered bimetallic colloid is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate. More than 90% metal can be isolated in the solid state and is redispersable in water in high concentrations. 

More recently Bouzek et al. investigated the effect of the preparation conditions of Pt-modified polypyrrole films on their electrocatalytic properties for the HOR [26]. Three methods were considered (1) cathodic deposition of Pt from H2PtCl6 in the previously synthesized film, (2) incorporation of colloidal Pt particles during the electropolymerization of polypyrrole (3) incorporation of [PtCU] as a counter-ion during the electropolymerization process and its subsequent reduction. Only the first two methods lead to active electrocatalytic films, whereas the last one gives very poor catalysts, maybe because the Pt particles are embedded in the PPy structure and therefore are not accessible to the reactant. 

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