Platinum catalytic activity affected

We discussed this catalysis recently (141st National Meeting of the American Chemical Society, March 1962) in terms of an olefin insertion reaction involving a Pt(II) olefin complex (3). We found that catalysis was only accomplished by platinum compounds capable of coordinating olefins. For example, substitution by tertiary phosphines blocks coordination by olefins and greatly reduces the catalytic activity of Pt(II). The substitution by phosphines does not affect the ability of the complexes to cleave the Si—H bond, however. The hindering of a catalytic reaction by blocking coordination sites is a common occurrence and is, I think, a persuasive... 

An attempt was also made to produce 0-iodo acyl iodides by the reaction of iodine, carbon monoxide and olefins in the presence of palladium or platinum chloride. This is, in effect, an attempt to make Dr. Tsuji s reaction catalytic rather than stoichiometric. No carbonyl insertion occurred at 1 atm. of carbon monoxide. However, it was found that iodination of the olefin was catalyzed by platinum olefin complexes and that an additional increase in catalytic activity accompanied the presence of carbon monoxide. There has been much speculation at this conference concerning the possibility of affecting catalytic activity by changing the ligands in the coordination sphere of the catalyst. This would appear to be such a case. 

Palladium complexes exhibit even higher catalytic activity and produce branched acids preferentially.132 133 The selectivity, however, can be shifted to the formation of linear acids by increasing the phosphine concentration.134 Temperature, catalyst concentration, and solvent may also affect the isomer ratio.135 Marked increase in selectivity was achieved by the addition of Group IVB metal halides to palladium136 and platinum complexes.137 Linear acids may be prepared with selectivities up to 99% in this way. The formic acid-Pd(OAc)2-l,4-bis(diphe-nylphosphino)butane system has been found to exhibit similar regioselectivities.138 Significant enhancements of catalytic activity of palladium complexes in car-bomethoxylation by use of perfluoroalkanesulfonic acid resin cocatalysts was reported.139,140... 

Particular interest has been shown in the catalytic oxidation of polyols with dioxygen using supported platinum-group metals as the catalysts. The most studied metals have been palladium and platinum which are, however, often affected by deactivation problems [2]. The introduction of cocatalysts such as bismuth or lead represents an enhancement in the use of these catalysts having the double effect of Increasing catalytic activity and improving catalyst life [3]. 

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]. 

The parameters that affect the degradation of supported platinum and palladium automotive exhaust catalysts are investigated. The study includes the effects of temperature, poison concentration, and hed volume on the lifetime of the catalyst. Thermal damage primarily affects noble metal surface area. Measurements of specific metal area and catalytic activity reveal that supported palladium is more thermally stable than platinum. On the other hand, platinum is more resistant to poisoning than palladium. Electron microprobe examinations of poisoned catalyst pellets reveal that the contaminants accumulate almost exclusively near the skin of the pellet as lead sulfate and lead phosphate. It is possible to regenerate these poisoned catalysts by redistributing the contaminants throughout the pellet. 

Reactions in the Gas Phase VOCs are major air pollutants, and catalytic combustion is one of the most important technologies for eliminating low concentrations of VOCs in effluent systems [64] Platinum metal is the most active catalyst for hydrocarbon combustion and is widely used supported on AI2O3 and other oxides. It is important for this combustion to take place at low temperatures. However, the water vapor produced during combustion under these conditions can be adsorbed on the oxide support due to its hydrophilicity, which can negatively affect the catalytic activity of the metal. This activity can also be affected by humidity in the feeding stream. Hydrophobicity of carbon materials could overcome this problem, and activated carbons have been proposed as supports in VOC combustion [65,66]. 

Table 1 presents the n-hexane conversion, selectivity to isomers and coke deposited after reaction for catalysts prepared by using two different platinum precursors tetraammine platinum nitrate and hexachloroplatinic acid. Both materials were calcined at different temperatures after platinum addition. For both platinum precursors, run under standard operational conditions, the optimum calcination temperature for catalytic activity was 500 °C. The amount of coke is small and the TPO profiles of the coked samples (not shown) are similar for all catalysts. Coke is completely burnt off at temperatures below that at which the catalyst was calcined after the metal addition. This is an important feature, because regeneration procedures would not affect the metal function. 

Platinum catalysts have been the subject of several studies with that perspective. Using Pt on various soUds (silicas, alumina, with various porosities etc.), Praliaud et al. [15] found a very clear correlation between dispersion and catalytic activity, whatever the support and its morphology. The lower the dispersion, and thus the bigger the particle size (in the range 1-20 nm), the more active the catalyst The selectivity (i.e. the reaction pathway itself) was not affected by particle size. It is noteworthy that the excellent correlation with metal dispersion on the solid was not observed for particle size, determined by TEM. This is probably related to the low accuracy of the measurement of size by TEM, which is a local method, whereas dispersion measurement is done by global analysis, thus providing a perfect statistical average. 

The strong catalytic activity of platinum group metals has a significant impact on the properties of the solid products obtained in above described precipitation systems after longer hydrothermal treatment (9-72 h) at I60°C. In the course of hydrothermal treatment, the products of TMAH decomposition (methanol and trimethylamine) affect the reduction of metal cations and the formation of platinum group metal nanoparticles. These nanoparticles act as the catalyst for the reduction of Fe(lll) to Fe(ll) by the products of TMAH decomposition and the formation of characteristic Fe304 octahedra, which is followed by Mossbauer spectroscopy and FE-SEM (Fig. 23.19). 

The type of anode catalyst has a strong effect on the severity of CO poisoning, since the catalyst affects the kinetics of CO adsorption (equation (2.12) and equation (2.13)) and CO oxidation (equation (2.18) and equation (2.19)). Based on these mechanisms, many CO-tolerant electrocatalysts have been developed by Pt alloying, such as PtRu (platinum/ruthenium) [24,38], PtSn (platinum/tin) [39-41], and PtMo (platinum/molybdenum) [42-44]. Generally, alloying Pt with a second element can enhance the catalytic activity of the Pt through one or more of the following effects ... 

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