Platinum catalysts structure

To carry out the above reaction, the reactant gases were heated to 600 °C before being mixed and exposed to a supported platinum catalyst structure in a 70 pm diameter channel. The reaction led to an increase in the gas temperature to 1,000 °C the gas was then cooled to 120 °C in a microscale heat exchanger. 

Determination of the Atomic and Electronic Structure of Platinum Catalysts by X-ray Absorption Spectroscopy... 

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

Figure 3.23 Photograph of the catalyst structure inserted in the ceramic support. Close-up of the holes in the platinum catalyst strip [63],...

Work with ultrathin and thick fringe-free platinum films has shown that not only does the product distribution change with catalyst structure, but the specific rate of raction (per unit platinum area) changes also (30). The data in Fig. 12 for the reaction of 2-methylpentane and n-hexane show a decrease in the specific rate with increasing particle size. 

Well-defined complicated macromolecular structures require complex synthetic procedures/techniques and characterization methods. Recently, several approaches leading to hyperbranched structures have been developed and will be the focus of this section. The preparation of hyperbranched poly(siloxysilane) has been reported [198] and is based on methylvinyl-bis(dimethyl siloxysilane), an A2B type monomer, and a progressive hydrosi-lylation reaction with platinum catalysts. An appropriate hydrosilylation reaction on the peripheral - SiH groups led to the introduction of polymeric chain (PIB, PEO) or functional groups (epoxy, - NH2) [199]. 

In fuel cell development, the high cost of precious metals has led to ways to lower the platinum content. Methods include raising the activity of the catalyst, so less is needed and finding more stable catalyst structures that do not degrade over time while avoiding reactions that can... 

R Ratnasamy, A. I. Leonard, L. Rodrique, and I. I. Fripiat, Structure and surface properties of supported platinum catalysts, J. Catal 29, 374-384 (1973). 

EXAFS experiments of Pt/Si02 and tin-modified platinum catalysts demonstrate that Sn addition has strong effects on Pt structures. The results obtained from the fit of EXAFS spectra gave us important information on the structures present in... 

The Pt-Re system has been studied extensively since the 1970s because adding Re to AhOs-supported platinum catalysts increases the resistance to deactivation of the catalysts used in naphtha reforming by preventing coke deposition. By using carbonyl precursors, well-defined bimetalhc species have been prepared. A proper characterization of these species allowed a relationship to be established between their structure and their catalytic behavior. Table 8.3 shows several Pt-Re bimetaUic catalytic systems prepared using different carbonyl species in which Pt-Re interactions determine the catalytic behavior. 

The correlation between the coverage of surface platinum atoms by bismuth adatoms (Ggi) and the measured rate of 1-phenylethanol oxidation was studied on unsupported platinum catalysts. An electrochemical method (cyclic voltammetry) was applied to determine G i and a good electric conductivity of the sample was necessary for the measurements. The usual chemisorption measurements have the disadvantage of possible surface restructuring of the bimetallic system at the pretreatment temperature. Another advantage of the electrochemical polarization method is that the same aqueous alkaline solution may be applied for the study of the surface structure of the catalyst and for the liquid phase oxidation of the alcohol substrate. 

The oxygen cathode—for which platinum catalyst due to its outstanding structural and catalytic capability is the rule—is not used as an oxygen evolution anode in the electrolyzer operation mode because oxidation of Pt and fast catalyst deterioration would be the consequence. Therefore an oxygen cathode based on a platinum catalyst must operate as a -evolving cathode in the regenerative mode. 

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