Platinum catalysts conversion activities

Figure 4 shows the total conversion of ethanol as a function of temperature as measured by gas chromatography. Except for the silica catalysts, the platinum catalysts exhibit equal or lower light-ofif temperatures than the supported catalysts with palladium as active material (compare with Figure 7). The platinum on alumina and platinum on titania catalysts are more active than the other catalyst combinations. The conversion curves for the Pd and Pt on ceria catalysts practically coincide, which implies that ceria would be a more suitable support material for a palladium catalyst than for a platinum catalyst. The activities of the silica catalysts are low. This observation is consistent with recent results in another research project using the same type of silica sol (Zwinkels et al, 1994). According to these experiments, it is crucial to reduce the alkali content to a very low level in the support, since sodium increases the mobility of silica, which poisons the active platinum and palladium sites. Platinum is apparently more sensitive to this phenomenon than palladium.

The palladium catalysts do not exhibit the pronounced difference in ethanol oxidation activity between the different support materials as is observed for the platinum catalysts (see Figure 7). Palladium on titania is the most active catalyst below 200°C. As in the case of the platinum catalysts, the activity of the silica catalyst differs from the other three. The activity of Pd/Si02 levels off at a higher conversion, though. 

For decades, chromium/aluminium and copper/chromium/ aluminium catalysts have been used at temperatures between 250 and 320 °C to achieve a satisfactory approximation to equilibrium at space velocities between 2000 and 4000 m /m h. In recent years, pure aluminium catalysts, and very recently also platinum catalysts whose activity for COS (and CS2) conversion is good already at temperatures between 120 and 2(X) C have won favor for COS hydrolysis. Such low-temperature operation is a great advantage as the steam rate required for equilibrium decreases steeply with decreasing reaction temperature. 

In typical diesel exhaust gas, the N02/N0x fraction is only 5-10%. This fraction may be increased by passing the gas over a strong oxidation catalyst containing platinum as the active component. However, it is difficult to obtain useful fractions of N02 at temperatures below 200°C at high space velocities due to the strong temperature dependency of the NO oxidation over platinum [42], As expected the NO conversion rises exponentially with temperature, but declines at higher temperatures due to the thermodynamic limit, of the reaction (Figure 9.13). 

Homogeneous catalysts have been reported, which can oxidize methane to other functionalized products via C-H activation, involving an electrophilic substitution process. The conversion of methane into methyl bisulfate, using a platinum catalyst, in sulfuric acid, has been described. The researchers found that a bipyrimidine-based ligand could both stabilize and solubilize the cationic platinum species under the strong acidic conditions and TONs of >500 were observed (Equation (5)).13... 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

Figure 20.8 (a and b) show a comparison of the Pt// -zeolite catalyst which activated molybdenum oxycarbide catalysts, prepared from both Mo2C and Mo03, for the isomerization of n-heptane and n-octane at elevated pressure. For the platinum catalyst the chain length is important, as explained above, with the isomerization selectivity obtained for n-octane dropping more quickly with increasing conversion than that for n-heptane over this plantinum catalyst the isomerization of n-hexane can be... 

Dimethylsulfide was added to the n-heptane to give 1000 ppm S in the gas phase. Figure 22.3 shows the effect of sulfur on conversion. The platinum catalyst lost its activity in about 3 h, whereas the tungsten carbide catalyst was very slightly affected during the experiment. Sulfur tolerance is important in petroleum refineries, since it may allow the substitution of the costly noble metals with carbides in streams containing sulfur. 

Experiment 6.135 describes the preparation of a range of oc,/ -unsaturated ketones, including benzylideneacetone, furfurylideneacetone and benzylidene-acetophenone. The conversion of this latter compound into /J-phenylpropio-phenone is readily achieved by hydrogenation at atmospheric pressure over an active platinum catalyst. 

One way to clarify the role of the adsorbate is the in situ investigation of platinum catalysts with electron spectroscopy, which is capable of quantitatively determining the abundance and chemical structure of the carbon species present. This has been done for a platinum black [66] material used after various activation procedures in n-hexane conversion reactions. It was... 

The two metals that have been found to give encouraging conversions and selectivities for the hydroformylation of styrene are platinum and rhodium. The platinum-based catalytic system uses tin chloride as a promoter. It also uses triethyl orthoformate as a scavenger that reacts with the aldehyde to form the acetal. By removing it as soon as it is formed, any further degradative reactions of the aldehyde are avoided. The chirality in these reactions is induced by the use of optically active phosphorus ligands. With the best platinum catalyst, branched and linear aldehydes are produced in about equal proportion, but the former has an e.e. of >96%. 

The platinum catalyst is effective in very small amounts, and can be introduced as H2PtCl6 or as elemental platinum on an inert support. A particularly active catalyst is the soluble platinum complex of divinyltetramethyldisilox-ane, CH2=CHSiMe2-0-SiMe2CH=CH2. The hydrosilyla-tion reaction operates through the Chalk-Harrod mechanism or one of its variants. bz jn these mechanisms, the first step involves the conversion of a metal alkene complex to a metal alkene silyl hydride complex. In addition to platinum, recently ruthenium, rhodium, palladium, copper, and zinc complexes are being studied as hydrosilation catalysts. " ... 

The influence of the inlet concentration of NO was studied with four soot samples mixed with a supported platinum catalyst (I wt% Pi on ASA) at 650 K. The oxidation rate at 50% soot conversion is plotted as a function of the NO inlet concentration in Figure 12.2.a. From this figure it is clear that the influence of the NO concentration on the oxidation rate of the synthetic Printex-U and the diesel soots activated with copper or iron is comparable. There is a first order relation between the NO inlet concentration and the oxidation rate. For cerium activated soot, there is also a first order relation between the NO inlet concentration and the oxidation rate. In this case, however, the effect of NO is approximately twice as large as is the case with Printex-U, Phntex-U with a physical mixture of a cerium catalyst (not shown), and copper- or iron-activated soot. 

Figure 11 shows conversion to iso-heptanes to be negligible for (0.5 wt. %) platinum supported on activated carbon (Pt/C) as the only catalyst, and also for (0.4 wt. %) platinum on silica-gel (Pt/Si02). No detectable conversion was obtained with silica-alumina. A mechanical mixture of either of the Pt-bearing particles with silica-alumina of about 150 m.Vg-surface area, both in millimeter diameter particle size (1000m), immediately resulted in appreciable isomerization ( SiAl with Pt/C SiAl with Pt/Si02). Isomerization increases rapidly for smaller component particle sizes, of 70/i and S i diameters. It approaches the performance of a silica-alumina that has been directly impregnated with platinum, and which has... 

Although the catalysts showed high initial activity, rapid deactivation was also observed. For example, when using a Pt/t -alumina catalyst at 250 C, essentially complete TCA conversion was observed initially however, after 15 h TCA conversion had declined to < 25 percent. To understand the deactivation process, surface acidity and basicity, coke content, chlorine content, and platinum content were measured for both the fresh and the used catalysts. These measurements showed that up to 40 wt% coke formed on the supported platinum catalyst and that the acidity changed significantly during the reaction at 350°C. 

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