Noble catalysts, catalytic properties

This study presents catalytic properties of Pd and Pt for hydrogen adsorption. Metals were deposited by easy to perform procedure on powder materials. This method does not required use of hydrogen as reactant gas and can be easy scale-up. Presence of metal particles was verified by SEM experiment. Average size of noble metals was approximately 1pm. From experiment conditions amount of catalyst was found and it was revealed that mass of metal (Pt or Pd) do not exceed l %wt. 

A typical probe reaction for estimating catalytic properties in selective hydrogenations is the hydrogenation of cinnamaldehyde. This molecule contains both a C=C and a C=0 double bond, thus the formation of hydrocinnamaldehyde and/or cinnamyl alcohol by reduction of the one or the other, or the formation of phenyl propanol in the case of complete reduction may indicate the potential of the catalyst for other fine chemical transformations. Indeed, this reaction was one of the first to be tested by CNT-supported catalysts [120]. Noble metals show a high activity in this reaction and... 

The commercial applications of osmium are limited and considerably fewer than other platinum group metals. Its aUoys are very hard and are used to make tips of fountain-pen nibs, phonograph needles, and pivots. The metal also exhibits effective catalytic properties in hydrogenation and other organic reactions. Such catalytic apphcations, however, are limited and osmium fads to replace other noble metals, particularly paUadium and platinum, which are more effective as catalysts and cost less. 

The carbides and nitrides of the early transition metals have attracted considerable attention. Much of this can be attributed to the catalytic properties of these materials. Levy and Boudart1 were the first to note the Pt-like catalytic behavior of WC, which for many reactions is more catalytically active than metallic tungsten.2 Others have noted that the early transition metal carbides and nitrides, in general, are excellent catalysts for reactions characteristic of the more expensive noble metals.3,4 For example, WC has been found to catalyze the isomerization of neopentane to isopentane,1 a reaction that previously had been known to be catalyzed only by iridium and platinum.5... 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

These results and the fact that Pd-hybrid catalyst showed very similar catalytic properties to Pt-hybrid catalyst suggest that isomerization occurs on zeolite surface and supported noble metal catalyst promotes selective isomerization on zeolite in the hybrid catalyst. 

The carbides of the early transition metals exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals [1], Typically, early transition metals are very reactive elements that bond adsorbates too strongly to be useful as catalysts. These systems are not stable under a reactive chemical environment and exhibit a tendency to form compounds (oxides, nitrides, sulfides, carbides, phosphides). The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability [2]. Furthermore, in a metal carbide, the carbon atoms moderate the chemical reactivity through ensemble and ligand effects [1-3]. On one hand, the presence of the carbon atoms usually limits the number of metal atoms that can be exposed in a surface of a metal carbide (ensemble effect). On the other hand, the formation of metal-carbon bonds modifies the electronic properties of the metal (decrease in its density of states near the Fermi level metal—>carbon charge transfer) [1-3], making it less chemically active... 

With the advent of synthetic methods to produce more advanced model systems (cluster- or nanoparticle-based systems either in the gas phase or on planar surfaces), we come to the modern age of surface chemistry and heterogeneous catalysis. Castleman and coworkers demonstrate the large influence that charge, size, and composition of metal oxide clusters generated in the gas phase can have on the mechanism of a catalytic reaction. Rupprechter (Chap. 15) reports on the stmctural and catalytic properties of planar noble metal nanocrystals on thin oxide support films in vacuum and under high-pressure conditions. The theme of model systems of nanoparticles supported on planar metal oxide substrates is continued with a chapter on the formation of planar catalyst based on size-selected cluster deposition methods. In a second contribution from Rupprecther (Chap. 17), the complexities of surface chemistry and heterogeneous catalysis on metal oxide films and nanostructures, where the extension of the bulk structure to the surface often does not occur and the surface chemistry is often dominated by surface defects, are discussed. 

Lambert reviews the role of alkali additives on metal films and nanoparticles in electrochemical and chemical behavior modihcations. Metal-support interactions is the subject of the chapter by Arico and coauthors for applications in low temperature fuel cell electrocatalysts, and Haruta and Tsubota look at the structure and size effect of supported noble metal catalysts in low temperature CO oxidation. Promotion of catalytic activity and the importance of spillover are discussed by Vayenas and coworkers in a very interesting chapter, followed by Verykios s examination of support effects and catalytic performance of nanoparticles. In situ infrared spectroscopy studies of platinum group metals at the electrode-electrolyte interface are reviewed by Sun. Watanabe discusses the design of electrocatalysts for fuel cells, and Coq and Figueras address the question of particle size and support effects on catalytic properties of metallic and bimetallic catalysts. 

From a catalytic point of view, bimetallic nanoparticles composed of 3d-transition metal and noble metal with specific structures will provide a great number of new candidates of catalysts for various chemical reactions, since the catalytic properties of bimetallic nanoparticles can be potentially tailored by both the ligand effect and... 

Support Effect on Chemisorption and Catalytic Properties of Noble Catalysts... 

Heterogenous hydrogenation catalysts can be combined with materials possessing other types of catalytic properties, resulting in bi- or multifunctional catalysts. For example, the catalyst used in the hydrocracking process is a combination of a noble metal, an acidic zeolite, and some other components. It leads, under the process conditions, to very effective hydrogenolysis of the long-chain components in the crude oil. ... 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Physico-chemical and catalytic properties of zirconia supported ruthenium and ruthenium-platinum catalysts were investigated. In order to improve the ruthenium stability a second noble metal, namely platinum, was introduced into the catalyst. Ten catalysts, consisting of zirconia supported Ru and/or Pt were prepared. 

It is well known that addition of base metal oxides can enhance the catalytic properties of noble metals. Addition of CeOa in three-way catalysts has improved the performance by the ability to store oxygen, promote the water-gas-shift reaction, stabilise the alumina support, suppress strong Rh-ALOa interactions and promote noble metal dispersion [4-5]. Beside the precious metals, oxides of the first row of transition metals are generally active as oxidation catalysts. By promoting Pt and Pd with cobalt oxide it has recently been shown that the activity for oxidation of CO and propene is significantly increased [6-10]. 

Another degree of modification of the catalysts can be achieved by introduction of components which on one hand affect the dispersion of the noble metal similarly to the ceria discussed earlier, but also possess catalytic activities of their own. One example of such an additive explored in depth at Ford Research is molybdenum oxide. Molybdena, similar to ceria, forms a two-dimensional phase on 7-AI2O3 and thereby also affects the Pt dispersion and its catalytic properties. Platinum, in turn, affects strongly the reducibility of molybdena, as shown in Fig. 4, using ESCA to characterize the oxidation state after reduction in the absence and presence of Pt [7]. 

Highly dispersed active noble metal or metal oxide particles on high surface area supports are frequently used in environmental, industrial and consumer applications due to their important catalytic properties. Supported catalysts properties are based on high active phase dispersion and also an adequate stability to reduce sintering and metal loss during use. 

The results (as compiled in Table 2) show that the catalytic properties of a platinum catalyst with respect to hydrogenation of the double bonds of croton-aldehyde can be subtly modified by (i) the addition of non noble metal atoms or... 

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