Pt/alumina catalysts

Pt-Re-alumina catalysts were prepared, using alumina containing potassium to eliminate the support acidity, in order to carry out alkane dehydrocyclization studies that paralleled earlier work with nonacidic Pt-alumina catalysts. The potassium containing Pt-Re catalyst was much less active than a similar Pt catalyst. It was speculated that the alkali metal formed salts of rhenic acid to produce a catalyst that was more difficult to reduce. However, the present ESCA results indicate that the poisoning effect of alkali in Pt-Re catalysts is not primarily due to an alteration in the rhenium reduction characteristics. 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Such a strong analogy between the electronic behavior of the metallic core of large molecular clusters and small metal particles was already suspected by Basset, Primet et al., who had discovered already in 1975 [21] that the extent of back donation from the core of a metal particle to [NO] (a ligand isoelectronic to CO) adsorbed Pt/alumina catalyst was particle size dependent as if the small particles were behaving as molecular clusters, since the extent of back donation on coordinated CO was shown clearly to be dependent on the size of the cluster in the series of molecular [PtsCCOJ j]" (n = 1-5, etc.) clusters made by Chini s group [22]. 

As another variation, the production of alkanes can be accomplished by modifying the support with a mineral acid (such as HCl) that is co-fed with the aqueous sorbitol reactant. In general, the selectivities to heavier alkanes increase as more acid sites are added to a non-acidic Pt/alumina catalyst by making physical mixtures of Pt/alumina and silica-alumina. The alkane selectivities are similar for an acidic Pt/silica-alumina catalyst and a physical mixture of Pt/alumina and silica-alumina components, both having the same ratio of Pt to acid sites, indicating that the acid and metal sites need not be mixed at the atomic level. The alkane distribution also shifts to heavier alkanes for the non-addic Pt/alumina catalyst when the pH of the aqueous sorbitol feed is lowered by addition of HCl. The advantages of using a solid acid are... 

Catalyst particle nucleation in the initial stages and their subsequent growth play an important role in catalytic mechanisms. In a model Pt/alumina catalyst, the general view is that the formation of particles is a stepwise process incorporating the following steps (Wynblatt and Gjostein 1975, Cottrell 1971) individual metal atoms (called monomers) transform to two-dimensional islands, which subsequently transform to three-dimensional clusters. These clusters eventually transform into finite-sized particles. 

Weiland et al observed that a small amount of Pt metal present in the Rh-based catalyst could significantly improve the catalyst activity for ATR of gasoline range fuels. They claimed that the role of Pt is to enhance oxidation activity, whereas Rh provides high SR activity. The Rh-Pt/alumina catalyst used in the study was supported on monolithic honeycombs and had a Rh to Pt ratio of 3-10 by weight. The geometry (metal monolith, ceramic monolith, or ceramic foam) of the support did not affect the product composition. ... 

Gault et al. (79) converted 13C labeled hydrocarbons over catalysts containing 10% Pt and varying amounts of tin (0.2 to 5 wt.%) on alumina catalysts at low (5.5 torr) or atmospheric pressure. The support was an inert alumina and, even though the catalysts contained 1 to 1.2% Cl, the authors did not consider the catalyst to have acidic character. They reported that the 10% Pt loaded catalyst, when loaded with small amounts of Sn, has the same catalytic properties as very highly dispersed Pt-alumina catalysts. 

A theoretical and experimental study of multiplicity and transient axial profiles in adiabatic and non-adiabatic fixed bed tubular reactors has been performed. A classification of possible adiabatic operation is presented and is extended to the nonadiabatic case. The catalytic oxidation of CO occurring on a Pt/alumina catalyst has been used as a model reaction. Unlike the adiabatic operation the speed of the propagating temperature wave in a nonadiabatic bed depends on its axial position. For certain inlet CO concentration multiplicity of temperature fronts have been observed. For a downstream moving wave large fluctuation of the wave velocity, hot spot temperature and exit conversion have been measured. For certain operating conditions erratic behavior of temperature profiles in the reactor has been observed. 

The catalytic CO oxidation by pure oxygen was selected as a model reaction. The Pt/alumina catalyst In the form of 3.4 mm spherical pellets was used. The CO used In this study was obtained by a thermal decomposition of formic acid In a hot sulphuric acid. The reactor was constructed by three coaxial glass tubes. Through the outer jacket silicon oil was pumped, while air was blown through the inner jacket as a cooling medium. The catalyst was placed in the central part of the tube. The axial temperature profiles were measured by a thermocouple moving axially in a thermowell. Gas analysis was performed by an infrared analyzer or by a thermal conductivity cell. [7]. 

Figure 4. Effects of hydrogen and oxygen atmospheres and of metal loading on sintering rates of 0.6% and 5% Pt/alumina catalysts [28,331.

On the other hand, rate constants for 0.6 and 5% Pt/alumina catalysts sintered in H2 at 973 K (see Table 1) of 0.53 and 0.84 h 1 are not substantially different. This result is not altogether unreasonable, as the number of crystallites per unit area of support surface and the metal surface area would be about the same in both 0.6 and 5% catalysts because of the much lower dispersion of the 5% catalyst. Nevertheless, it is fascinating that these two catalysts sinter at much different relative rates in air (see discussion above), a fact suggesting that different mechanisms (i.e., atomic migration vs. crystallite migration) may be involved in air versus H2 atmospheres as proposed by Wynblatt and Ahn [5J. 

An isotope labelling study of the deactivation of a Pt/alumina catalyst used for propane dehydrogenation. 

The activity and selectivity of the Pt/alumina catalyst for propane dehydrogenation is critically dependent on the extent of formation of a carbonaceous deposit, which contains both carbon and hydrogen, on the catalyst. 

Olsbye U, Wendelbo R, Akporiaye D. Stucy of Pt/alumina catalyst preparation. Appl Catal A. 1997 152 127 11. 

Liu, X., Korotkikh, O., and Farrauto, R. Selective Catalytic Oxidation of Co in H2 Structural Study of Fe-oxide promoted Pt/alumina catalyst, in Applied Catalysis A General 226, 293, 2002. 

Here we focus on the hydrogenation of trifluoromethyl ketones over a Pt/alumina catalyst modified by cinchonidine (CD) and some of its simple derivatives. Our recent research has revealed interesting similarities and dissimilarities in the enantioselective hydrogenation of trifluoromethyl ketones and other activated ketones, which observations may be useful for a future mechanistic study. 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

Figure 1.15 shows the temperature programmed oxidation profile for a coked Pt/alumina catalyst. The volume of oxygen consumed is used to determine the quantity of carbon initially... 

Figure M5 Temperature programmed oxidation profile for a coked Pt/alumina catalyst.

Sinfelt, Hurwitz, and Rohrer (1960) proposed essentially the following reaction scheme for the isomerization of n-pentane (nCs) to z-pentane (zCs) on a Pt/alumina catalyst ... 

Procedure for solid samples. All crop and soil samples were analysed by combustion in a Raddec pyrolyser. The pyrolyser first removes water from the sample by slowly raising the temperature to 180 C and maintaining it at that temperature for 45 minutes. A stream of air carries the water vapour over a Pt-alumina catalyst at 800 C and then into a trap solution of dilute nitric acid. At the end of this cycle the trap solution is replaced with fresh nitric acid solution and the sample temperature slowly raised to 550 C. The air is then replaced by oxygen and the sample is maintained at that temperature in the oxygen atmosphere for at least 12 hours. In all cases, approximately 10 g aliquots of crop and 5 g aliquots of soil were analysed. 

The narrow region Pt/alumina catalyst pellets have been prepared by the impregnation procedure The initial activity distributions (Fig. 1) have been estimated from the kinetic measurements by the method described in (ref 11). 

As in the case of conventional Pt/alumina catalysts, model Pt/alumina catalysts are observed to undergo redispersion in oxygen at ten ieratures in the range of 773 to 873 25.40,42,4338 Thnge previous studies have reported redispersion of model Pt/alumina during treatment in O2 at 773 These observations were explained by either crystallite splitting... 

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