Pt/Sn catalysts

At this stage, it should be pointed out that modihcation of a Pt-Sn catalyst by Ru atoms increases cell performance (and hence catalytic activity with regard to ethanol electro-oxidation), but has no effect on the OCV or on product distribution [Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be produced when the anode potential is increased and noncatalytically active tin oxides are formed. 

Stoichiometric model reactions in alkene hydroformylation by platinum-tin systems have been studied for the independent steps involved in the hydroformylation process, insertion of the alkene, insertion of CO, and hydrogenolysis, with use of Pt-Sn catalysts and 1-pentene as alkene at low pressure and temperature.92... 

C10-C14 long paraffin dehydrogenation is a key-step for linear alkyl benzene (LAB) production. However, this reaction, which requires monofunctional catalysis, is implemented on Pt-Sn catalysts deposited on controlled acidity alumina. It is generally associated with several secondary reactions, among which aromatic formation is extremely problematic it is catalyzed by a metallic phase (M) or by residual support (A) activity. Indeed, on the one hand, these arylaromatics are very good coke precursors and are consequently responsible for a large part of the... 

Industrial Pt-Sn catalyst studied. Sn is dissolved in Pt forming a solid solution at low Sn concentrations. Sn crystallities observed at higher concentrations.50 ... 

Burch and coworkers (50-53) have reported on the use of Pt-Sn catalysts for hydrocarbon conversions. It was concluded that no proper alloys of Pt and Sn were formed so that this could not account for the changes in the catalytic properties imparted by the presence of Sn (50). Burch and Garla concluded for their catalysts that (i) n-hexane is isomerized by a bifunctional mechanism, (ii) benzene and methylcyclopentane are formed directly from n-hexane at metal sites, and (iii) the conversion of methylcyclopentane requires acidic sites (51). It was also concluded that the Sn (II) ions modified the Pt electronically with the result that self-poisoning by hydrocarbon residues is reduced. However, these later observations were based upon conversions at one bar. 

For the most part, the data for alkane and cycloalkane conversion with Pt-Sn catalysts have been at atmospheric pressure. For commercial reforming operations, a much higher pressure is utilized. 

Davis (82) compared the conversion of n-octane at 1 atmosphere and 200 psig and found that the metal catalyzed dehycrocyclization selectivity, based upon the distribution of C8-aromatic isomers, was the same at both pressures. Thus, for the Pt or Pt-Sn catalyst the dominant metal catalyzed cyclization pathway to produce aromatics was a 1,6-ring closure provided the support did not have acidity to effect isomerizations. 

For PtSn supported on a nonacidic alumina the addition of Sn causes an increase in activity up to Sn/Pt = 4, and then a decline in activity for low pressure operation (42). The increase in activity is much less at 400 psig operation than the two-fold increase observed at atmospheric pressure. However, there is a change in the selectivity of aromatic isomers produced from n-octane at both 15 and 400 psig as Sn is incorporated into the catalyst. Thus, both Pt and Pt/Sn catalysts produce only (> 90-95%) ethylbenzene and o-xylene as the dehydrocyclization products from n-octane. However, Pt produces ethylbenzene o-xylene = 1 1 whereas a catalyst with Sn/Pt = 4 produces ethylbenzene o-xylene = 1 2. This change in aromatic isomerization leads to two postulates ... 

The TPR and chemisorption experiments were carried out in an apparatus described elsewhere [3] equipped with a thermal conductivity detector. The experiments were performed using a gas mixture of 7 vol% H2 in Ar and a heating rate of 10 G/min. Separate TPR experiments (not shown here) indicate that the degree of reduction of tin, based on the reaction Sn02 + 2 - Sn + 2H20, was about 50 % for the Pt-Sn catalyst. This indicates that most of the tin is in the Sn2+ state. 

The results from the hydrogen chemisorption experiments were for Pt/Al203 13% dispersion and for Pt-Sn/AljO 29% dispersion. Recently Sfinchez et al. [5] showed that hydrogen chemisorption may give an overestimation of the dispersion of a Pt-Sn catalysts using the same procedure as here. Therefore caution should be taken in the interpretation of data based on hydrogen chemisorption (e.g. dispersion and turnover frequency). 

After the Pt-Sn catalyst was exposed to the reaction mixture for two minutes, the HD formation rate fell to about a third compared to the activity for the fresh catalyst. For the platinum catalyst, the activity fell to l/30th after only 15 seconds dehydrogenation reaction,... 

These results are consistent, with those reported by Lin et al. [7] who used hydrogen chemisorption to measure the free metal area on Pt/Al203 and Pt-Sn/Al20- catalysts after deactivation. They found that the free metal surface decreased rapidly in the beginning when hydrocarbons were deposited on the catalyst, but reached a minimum level of coverage. At this level 10% and 30% of the metal remained uncovered, for the Pt and Pt-Sn catalyst... 

A large effect on the deactivation profile, due to the H2-D2 experiments was found. During the first hours of a run, the deactivation was accelerated after each stop for an H2-D2 experiment. A step down in activity directly after a stop, and then a slight increase in activity, was found. This increased deactivation is probably due to dehydrogenation of species on the surface in the absence of hydrogen. After more than 15 h on stream the conversion of propane increased immediately after each H2-D2 experiment followed by a slow return to the initial deactivation profile. This activity increase was much higher for the Pt-Sn catalyst compared to the Pt catalyst... 

The fresh Pt catalyst was found to be much more active than the bimetallic one, for the HD formation. After the catalysts were exposed to the dehydrogenation reaction mixture, the activity fell to only a fraction of the original activity. The decrease was much higher for the Pt than the Pt-Sn catalyst. 

Results from gas chemisorption, catalytic testing, and TPR and TPD experiments demonstrate that after H2 reduction at 773 K a part of tin (germanium) cations reduces and forms a metallic Pt-Sn(Ge) alloy. Alloying does not increase the thiotolerance of Pt-Sn catalysts compared to monometallic Pt catalyst. On the contrary, alloying enhances the thiotolerance of Pt in Pt-Ge catalysts because the formation of Pt-Ge clusters changes the electronic properties of Pt, via electron withdrawal by the reduced Ge ions. 

Numerous other bimetallic situations may occur in catalysis, perhaps related to the addition of alkali cations. For example, Burch (3) has examined cases where Group 4A species such as Sn are added to the Pt/Cl/Al203 reforming combination in order to enhance the stability, and also favorably affect related features. Sexton et al. (7) have examined similar systems with ESCA, and, as noted above, have found that in the common catalytic doping range, e.g., Pt(0.25-0.75 wt%) and Sn(0.4-0.75 wt%), they were able to monitor the behavior of the tin and found that in the oxidized version Sn(+4) species were present, whereas, following reduction, surface Sn(+2) entities persisted (7). We have also examined simulated Pt/Sn catalysts of similar composition and found that reduction yields Sn(II). This was consistent with the arguments of Burch (3), and seemed to be contrary to the alloy formation concept of Sachtler et al. (4b) and Clarke (4a). 

In conclusion by using rhenium as adsorbent instead of platinum, it is possible to achieve the ensemble control by sulfur passivation, at sulfur levels comparable to those applied for nickel and much lower than that which would have been required on a non-alloyed Pt-catalyst. A similar ensemble effect is achieved by alloying alone on Pt-Sn catalysts ... 

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