Pt/Al2O3 catalyst

Moreover, the comparison of the respective contributions of bond shift and cyclic mechanism in the isomerization of several labeled and C, hydrocarbons on 10% Pt/Al2O3 catalysts (Table III) shows that the contribution of bond shift to the overall isomerization process decreases with an increase in the number of carbon atoms, but not to the same extent for methyl shift and chain lengthening. Thus, on going from methylpentanes to... 

A more careful study of the hydrogenolysis of methylcyclopentane on two catalysts of extreme dispersion (0.2 and 10% Pt) showed that, in the temperature range 250°-310°C, the product distributions were temperature insensitive on the 0.2% Pt/Al2O3 catalyst, but temperature sensitive on the 10% Pt/AljOj catalyst (86). On the latter, all the observed distributions appeared as combinations of two limiting distributions, one of which includes only methylpentanes and therefore corresponds to a completely selective hydrogenolysis of—CH2—CH2— bonds the other one contains n-hexane, but is different from the one obtained on the 0.2% Pt/Al2O3 catalyst. Platinum films are intermediate between the two types of supported catalysts (86,87. ... 

The initial formation, from isopropylcyclopentane, of ethylcyclopentane, 1-methyl-l-ethylcyclopentane, and ethylbenzene on platinum films or Pt/Al2O3 catalysts 68, 131), illustrates these points concerning the metallocyclobutane mechanism and carbene-olefin addition (Scheme 64). 

More recently, it was found that the hydroxylation state of the alumina has a determining influence upon the particle size of platinum crystallites. Controlled dehydroxylation of the alumina proved to be the most reliable method for preparing stable catalysts with gradual variations of the dispersion, and a continuous series of Pt/Al2O3 catalysts with dispersions (characterized by the H/Pt ratio) ranging from 0.04 to 1.0 was prepared by... 

The most striking result obtained with the series of Pt/Al2O3 catalysts with various dispersions was the constancy (18 + 3%) of the contribution of the cyclic mechanism on all the catalysts of low and medium dispersion (H/Pt... 

Figure 3. IR spectra of CO+H2 adsorption over 10%Co-Pt/Al2O3 catalysts...

The catalytic tests show that, over the Pt(l 0 0)/Al2O3 catalyst, the formation of CO and NH3 is largely prevented, whereas the yield of N2O increases compared with the Pt(polycrystalline)/Al203 catalyst. These main differences observed should be ascribed to the morphological differences between two catalysts, i.e., the dominant orientation of the crystallographic facets and the average size... 

In order to set up the ATR reactor, a commercial pelletized catalyst, 0.3%Pt/Al2O3 (Engelhard), was used. 

The capability of NO to reduce nitrates, providing a pathway for the production of ammonium nitrite and thus of nitrogen, has also been demonstrated recently by Weitz and co-workers, mainly on the basis of IR data collected over BaNa-Y zeolite [75] however, according to a parallel additional route NO would also react with NO2 to form N2O3 and then nitrogen [reactions (13.25) and (13.26)], as already discussed. The oxidation of NO by surface nitrates over a Pt-Ba/Al2O3 catalyst has also been reported by Olsson et al. [76], whereas the formation of surface nitrates from NO2 on bare AI2O3 has been reported by Apostolescu et al. [77] and previously observed in our laboratories also [78]. 

The equipment depicted in Fig. 17 also allows monitoring of species adsorbed on a solid catalyst. For this application, the ZnSe IRE is coated with a layer of the catalyst before assembly of the cell and the start of the reaction. This approach was chosen for investigation, for example, of the interaction of the reactant with the catalyst during the asymmetric hydrogenation of ethyl pyruvate catalyzed by cinc-honidine (CD)-modified Pt/Al2O3 in the presence of supercritical ethane (79). 

The beneficial effects of sulphur (a few p.p.m. in the feed) on Pt reforming catalysts (lower initial activity but enhanced lifetime and stability, less coking) were reviewed in the earlier Report and further examples have appeared. A selectively sulphided Pt(0.25)-Re(0.25)-Q(1.0)/Al2O3 catalyst with S/Re atomic ratio 0.93 was more active and longer-lived in the reforming... 

Since electrical conductivity reflects the mobility of electrons in the bulk solid (14), the data in Table 2 can be used to compare the amount of mobile electrons in each sample. Table 3 shows the amount of excess mobile electrons (in conductivity unit) of the catalysts shown in table 2. The value (B-A) is the electrical conductivity of 0.3wt%Sn added to Y-AI2O3 support. The value (D-C) is the electrical conductivity of 0.3wt%Sn added to 0.3%Pt/y-Al2O3 catalyst. If Sn does not have any electronic effect on the Pt site, the value (B-A) should be equal to the value (D-C). The calculation, however, clearly indicates that 0.3wt%Sn loaded on 0.3%Pt/y-AI2O3 catalyst does provide more mobile electrons to the catalyst than its presence on y-Al203 support. The addition of alkali metals also shows an interesting result. The value (E-D) is the increase in electrical conductivity of 0.3%Pt-0.3%Sn/y-Al203 after 0.6wt% of the alkali metals was added. The result demonstrates that the alkali metals greatly increase the amount of the excess mobile electrons in the bulk catalysts. 

Figure 1 shows the influence of velocity of alkane feed on the aromatic hydrocarbons yield over 0.35%Pt-0.35%Re/Al2O3 catalyst. Maximum yield of benzene was observed at 2.8-4 h velocity of n-hexane feed. The yield of toluene in this range is 20-23%. Total yield of aromatic hydrocarbons and benzene sharply declined at 5.5 h velocity of n-hexane feed. Optimum volume velocity of hexane feed is in the range 3-4 h. The n-hexane conversion under these conditions reached 87-95%. 

The occurrence of mechanism A on 0.2% Pt/Al2O3 was confirmed for several substituted cyclopentanes and cyclobutanes (Table IV). On the other hand, while Mechanism C, which seems to occur on 10% platinum catalysts at high temperatures, could not be isolated on account of the fast consecutive isomerization of acyclic products that occurs above 320°C, Mechanism B was associated with the distribution of methylpentanes obtained at 220°C... 

The occurrence of a third mechanism, C, was suggested by a number of results that could be explained by none of the above mechanisms A and B, especially those involving the rupture of methyl-substituted C-C bonds, that is, the formation of n-hexane from methylcyclopentane on 10% Pt/AI2O3 catalysts at high temperature (86), n-hexane from trans-1,2-dimethylcyclobutane on metal films (89), n-heptane and 3-methylhexane from 1,2-dimethylcyclopentane, and methylhexanes from 1,3-dimethylcyclopentane on 10% Pt/Al2O3 at relatively low temperature (94). 

The results for the 12.4%Co/Si02 catalyst, the 0.2%Ru-10%TiO2 catalyst, the 0.5%Pt-15%Co/Al2O3 catalyst, and the 25%Co/Al203 catalyst are summarized in Tables 6 - 9 for comparative purposes among the different catalyst systems, and for comparison with the four case studies previously summarized. 

Figure 13.16 TPD in He and TPSR in H2 (2000ppm) + He after NO-O2 adsorption at 35O C over Pt-Ba/Al2O3 (1 20 100 w/w) catalyst. Adapted from ref. [119].

---