Preparation supported noble-metal catalysts

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. 

Simonov PA, Likholobov VA. 2003. Physicochemical aspects of preparation of carbon-supported noble metal catalysts. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Decker. 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

Supported noble metal catalysts (Pt, Pd, Ag, Rh, Ni, etc.) are an important class of catalysts. Depositing noble metals on high-area oxide supports (alumina, silica, zeolites) disperses the metal over the surface so that nearly every metal atom is on the surface. A critical property of supported catalysts is that they have high dispersion (fraction of atoms on the surface), and this is a strong function of support, method of preparation, and treatment conditions. Since noble metals are very expensive, this reduces the cost of catalyst. It is fairly common to have situations where the noble metals in a catalyst cost more than 100,000 in a typical reactor. Fortunately, these metals can usually be recovered and recycled when the catalyst has become deactivated and needs to be replaced. 

A comparative study of Ti02-supported noble metal catalysts, which were prepared by impregnation, was carried out for the oxidation of a low concentration of HCHO (100 ppm) by Zhang and He [102], As far as impregnation method is concerned, Au/ Ti02 is less active than Pt/Ti02, giving 90% HCHO conversion at 393 K and 100% conversion at room temperature, respectively. 

The procedure above is particularly usefiil for preparing supported noble metal (NM Pd, Pt, Rh) catalysts. T ugh obviously sensitive to the support surik e area, metal loading, and the specific experimental protocol, this procedure, at the laboratory scale, often leads to well dispersed metal systems with relatively narrow metal particle size distributions (97,117,183,235). 

ZSM-5(Si02/Al203=22.1,TOSOH)-supported noble metal catalysts were prepared by the impregnation method using aqueous noble metal chloride solutions. The metal loading was 5wt%. Catalysts were calcined at 500 C for 4 hr in air and reduced at 450"C for 1 hr prior to use. 

Physicochemical Aspects of Preparation of Carbon-Supported Noble Metal Catalysts... 

Preparation of carbon-supported noble metal catalysts (Me/C) is usually based on supporting metal precursors on carbon followed by their transformation into the metal particles. Design of an appropriate catalyst implies an optimal selection of both the support and the method of synthesis of the active component, which requires understanding the following issues ... 

Since supported gold catalysts prepared by coprecipitation were found to be active for CO oxidation even at temperatures far below room temperature, attempts are increasing to prepare other noble metal catalysts by coprecipitation, deposition-precipitation, and grafting methods, which were used for the preparation of active supported gold catalysts. Although the affinity to CO is markedly different between Pt-group metals and Au supported on selected metal oxides, the contribution of metal-support interactions to the enhancement of low-temperature catalytic activity for CO oxidation appears to be similar, namely, the enhancement of oxygen activation at the perimeter interface. This line of approach may be valid to seek for a new type of catalysts active at lower temperatures for reactions other than CO oxidation [82,83]. 

This work provides the comparison of palladium deposition-precipitation and ion exchange on functionalised or non functionalised carbon blacks or Ti02. Results show that functionalisation is mandatory for ion exchange, but that active carbon black supported noble metal catalysts can be prepared witiiout functionalisation using deposition-precipitation. 

The right choice of a carbon support greatly affects cell performance and durability. The purpose of this chapter is to analyze how structure and properties of carbon materials influence the performance of supported noble metal catalysts in the CLs of the PEMFCs. The review chapter is organized as follows. In Section 12.2 we give an overview of carbon materials utilized for the preparation of the catalytic layers of PEMFC. We describe traditional as well as novel carbon materials, in particular carbon nanotubes and nanofibers and mesoporous carbons. In Section 12.3 we analyze properties of carbon materials essential for fuel cell performance and how these are related to the structural and substructural characteristics of carbon materials. Sections 12.4 and 12.5 are devoted to the preparation and characterization of carbon-supported electrocatalysts and CLs. In Section 12.6 we analyze how carbon supports may influence fuel cell performance. Section 12.7 is devoted to the corrosion and stability of carbon materials and carbon-supported catalysts. In Section 12.8 we provide conclusions and an outlook. Due to obvious space constraints, it was not possible to give a comprehensive treatment of all published data, so rather, we present a selective review and provide references as to where an interested reader may find more detailed information. 

Balazsik, K., Torok, B., Kiricsi, 1., Dekany, I., Bartok, M. (1999) Characterization of Cinchonidine Doped Montmorillonite Supported Noble Metal Catalysts by Thermoanalytical Methods, J. Therm. Anal. 56, 337-343. Torok, B., Balazsik, K., Dekany, 1., Bartok, M. (2000) Preparation and Characterization of New Chirally Modified Laponites, Mol. Cryst. Liq. Cryst. 341, 339-344. 

The effect of some anionic modifiers has also been investigated. It is well known the structural and chemical changes induced on ceria by the incorporation of chloride ions into its lattice. This incorporation has been observed during the reduction step of the preparation of some ceria-supported noble metal catalysts, specifically, when chloride-containing noble metal precursors were used [272,289,339,400-403]. As a result, the chemisorptive [272,403] and redox [272,289,339] properties of ceria are drastically modified. Likewise, as revealed by XRD and HREM characterization studies [272,400,404], the incorporation of Cl ions into the ceria lattice leads to the formation of the corresponding oxychloride phase, CeOCl, with inherent stabilization of the Ce oxidation state. According to the TPO study reported in [289], the elimination of the trapped chloride species,... 

The injection method of DP has been also applied many times for the preparation of carbon-supported noble-metal catalysts. Jin etal. [36] have deposited 5 wt% Pd on activated carbon fibers by alkaline hydrolysis of palladium chloride and obtained metal dispersions of 55-77%. Dispersions of 40-50% have been reported by Farkas ct al. [37] who prepared Pd/C by fast addition of NaOH solution to a suspension of carbon in an aqueous solution of K2PdCl4. More highly loaded Pd and Pt catalysts (10 wt%) have been prepared by drop-wise addition of the metal salt solution to the suspension of carbon in Na2C03 solution. In this case [38] a Pt particle size of 10 nm and a Pd particle... 

The most famous papers using this type of method for the preparation of supported noble metal catalysts are those of Tsubota (31, 32) with the preparation of gold catalysts. They are famous because this is one of the few methods of preparation of gold catalysts that led to the formation of small gold particles (2-3 nm) and to the discovery of the catalytic reactivity of gold. 

Supported noble metal catalysts ate most widely used among heterogeneous catalysts because they are active for hydrogenation and dehydrogenation reactions as well as for complete and selective oxidation reactions (1). Although they ate usually prepared via impregnation, the influence of preparation method on their fine stmcture and catalytic properties has not been fully understood yet. 

Liquid-phase reductive deposition as a novel nanoparticle synthesis method and its application to supported noble metal catalyst preparation, Y. Sunagawa, K. Yamamoto, H. Takahashi, and A. Muramatsu, Catal Today, 2008,132, 81. 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

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