Direct noble metals

One of the goals of this work was to prepare directly noble metals supported on BN powder in a single operation. The molecular precursor of the noble metal must react with the support... 

The more noble metals (for example copper, mercury and silver) can form oxides, and exhibit variable oxidation state in such compounds (for example CU2O, CuO), but it is not easy to prepare such oxides by direct action of oxygen on the metal, and elevated temperatures are necessary. Moreover, in the case of silver and mercury, loss of oxygen from the oxide by heating is easy. The oxidesare, however, basic (for example Ag20 - Ag, CuO - Cu in acids). 

Clark et al.n recently discovered another FS related inechanisni in CuPt, different from the above mentioned nesting. In this case, the relevant contribution to the coneentration waves suseeptibility is due to the contemporary presenee of the noble metal-like neck at the L point and the d hole pocket at X. Ifie connecting vector of these Van Hove singularities belongs to the star 1,1,1 and is commensurate with the Lli ordering. In fact, it produces a phase characterised by alt ate hexagonal Cu and Pt planes, in the direction perpendicular to (1,1,1). 

The deliberate raising of the electrical potential of titanium, either by the attachment of discrete particles of a noble metal, such as platinum or palladium, at the surface, or by the application of positive direct current to force the formation of a protective film, is dealt with at a later point. The electrochemical aspect of the corrosion of titanium is comprehensively treated in a number of papers ... 

It would be interesting to further examine the vaporization of Pu-intermetallics at higher temperatures in order to search for molecular vapor species involving Pu and the noble metals. Due to the directional nature of 5f electrons in Pu, they may not be involved in the bonding of the solid intermetallics, but could contribute to the stability of a gas phase molecule. Additional measurements of the thermodynamic stabilities of Np- and Am-noble metal intermetallics corresponding to the Pu phases considered in this work would also assist in establishing bonding trends. 

This simple concept has already found some practical applications The idea to use supported alkali-promoted noble metal catalysts for NO reduction,3,4 even under mildly oxidizing conditions,5 came as a direct consequence of electrochemical promotion studies utilizing both YSZ (Chapter 8) and p"-Al203 (Chapter 9), which showed clearly the electrophi-licity of the NO reduction reaction even in presence of coadsorbed O. This dictated the use of a judiciously chosen alkali promoter coverage to enhance both the rate and selectivity under realistic operating conditions on conventional supported catalysts. 

The ideal operating temperatures for the three-way catalyst lie between 350 and 650 °C. After a cold start it takes at least a minute to reach this temperature, implying that most CO and hydrocarbons emission takes place directly after the start. Temperatures above 800 °C should be avoided to prevent sintering of the noble metals and dissolution of rhodium in the support. 

Table 2 also lists the noble metal surface areas normalized to the total mass of the catalyst. The surface areas were calculated directly from the dispersion data taking into account the different mass of noble metal in each cataly and assuming a constant site density of 1x10 /m As with dispersion, no clear correlation exists between mass-specific noble metal surface areas and CO/NOx cross-over efficiencies. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

As was mentioned previously, photoemission has proved to be a valuable tool for measurement of the electronic structure of metal cluster particles. The information measured includes mapping the cluster DOS, ionization threshold, core-level positions, and adsorbate structure. These studies have been directed mainly toward elucidation of the convergence of these electronic properties towards their bulk analogues. Although we will explore several studies in detail, we can say that studies from different laboratories support the view that particles of 150 atoms or more are required to attain nearly bulk-like photoemission properties of transition and noble metal clusters. This result is probably one of the most firmly established findings in the area of small particles. 

Conway BE, Barnett B, Angerstein-Kozlowska H, Tilak BV. 1990. A surface-electrochemical basis for the direct logarithmic growth law for initial stages of extension of anodic oxide films formed at noble metals. J Chem Phys 93 8361-8373. 

Saalfrank JW, Maier WF. 2004. Directed evolution of noble-metal-free catalysts for the oxidation of CO at room temperature. Angew Chem Int Ed 43 2028-2031. 

X-ray photoelectron spectroscopy (XPS) of electrodes was first applied to the oxidation of noble metal electrodes. Kim and Winograd investigated in 1971 the electrochemical formation of anodic oxides on Pt [10] and later on Au electrodes [60]. The electrochemical parameters of oxide formation on these noble metal electrodes were well characterized and enabled a direct correlation between ex situ XPS and in situ electrochemical analysis. 

Another important catalytic reaction that has been most extensively studied is CO oxidation catalyzed by noble metals. In situ STM studies of CO oxidation have focused on measuring the kinetic parameters of this surface reaction. Similar to the above study of hydrogen oxidation, in situ STM studies of CO oxidation are often conducted as a titration experiment. Metal surfaces are precovered with oxygen atoms that are then removed by exposure to a constant CO pressure. In the titration experiment, the kinetics of surface reaction can be simplified and the reaction rate directly measured from STM images. 

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