EUROPT

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

Fig. 7. Infrared spectra from ethene on Pt near room temperature (A) Pt/SiO> (32) (B) Pt/AliOi (47) (C) Pt/Al203 (171) (D) Pt/Si02 (EUROPT-I) (63) (E) Pt/Si02 (167) (F) Pt/ALO,. reprinted with permission from (50), copyright 1987 American Chemical Society (G) Pt/Si02 (96) (II) Pt/Al2Oj, reprinted with permission from (3 ), copyright 1991 American Chemical Society (I) Pt/Si02 (EUROPT-I), DRIFT spectrum (62) (J) Pt/Si02 (6 ).

Morrow and Sheppard (32) studied the absorption of ethene on their large-particle catalysts at several temperatures up to 473 K. The absorptions at ca. 2885 and 2800 cm , nowadays attributed to ethylidyne, slowly decreased in intensity while an absorption near 2920 cm 1 increased and broadened. A weaker band at ca. 2960 cm 1 was also strengthened. The increased absorptions can probably be attributed to VCH3 and vCH2 modes of a dimerized surface species (see below), a conclusion also supported by an analysis of the spectrum on EUROPT-1 (Fig. 7D) (63). 

No systematic differences appear to exist between spectra taken with pressed-disk or deposited-film catalysts samples. As yet, measurements based on diffuse reflection from powders have been few. Such spectra on EUROPT-1 in Fig. 71 and Ni/Si02 in Fig. 13E exhibit the same absorptions as those of pressed-disk samples, but with somewhat different relative intensities. Systematic DRIFTS experiments would be of interest. 

The apparatus used in the experiments is shown in figure 1 It is a slight modification of an apparatus designed for transient kinetic experiments [6]. The reactor was made by an U-shaped quartz tube (inner diameter 1,9 mm), In the H2-D2 experiments the oven was removed and the reactor was placed in a thermos with frozen C02 mixed with acetone to obtain -78 C An additional reactor, the equilibrium reactor", filled with 5.6 g commercial Pt/Al203 catalyst (EUROPT-3, CK 303), was used to achieve the equilibrium concentrations of H2, D2 and HD,... 

This can in pan be answered because Pt/alumina, e,g. EUROPT-3 (and Pt/Re/alumina) has also been studied [7]. In n-butane hydrogenolysis on Pt/alumina the accumulation of carbonaceous deposits on the catalyst surface suppressed ethane formation (i.e. relative to that of propane formation (i.e. S3), Thus for Pt/alutnina sites responsible for central C-C bond scission in n-butane may be selectively deactivated, e.g. at 603K sample S2 S3... 

As the work on these first catalysts progressed, it became clear that there was a demand for standardized platinum catalysts more akin to those used in industry. EUROCAT therefore selected two typical petroleum-reforming catalysts manufactured by AKZO, namely, 0.3% Pt/Al203 and 0.3% Pt-0.3% Re/Al203. These were called EUROPT-3 and -4, respectively (Section 11.1.2.2). More recently still, EUROCAT decided to expand its range of standard catalysts to cover oxides... 

EUROPT-1 is a silica-supported platinum catalyst containing 6.3 %Pt [1—8a]. This unusually high metal loading was chosen in order to facilitate the use of as many techniques of characterization as possible (see discussion following Ref. 8a) in particular, it is difficult to apply electron microscopy if the metal content is less than 1%. Samples were distributed to some 20 members of EUROCAT, and detailed results on its physical characterization were reported in five papers [1-5] and summaries presented in two conference proceedings [8a, 9a]. Results concerning its catalytic behavior were published later [6], and most recently a comprehensive review of work performed by other users of the catalyst has appeared [7], Publications describing its use continue to appear. 

Figure 1. Particle size distributions of EUROPT-1 by transmission electron microscopy [3] (A-E) material as received examined in five different laboratories (F) after rereduction at 623 K. Number of particles counted A, B, D, F 500 C, E 1000.

Figure 2. H2 chemisorption isotherms on EUROPT-1 [14] (a) after reduction at 758 K ( >) oxidized before reduction at 758 K...

Figure 3 Hi chemisorption isotherms on EUROPT 1 plotted according to the Langmuir equation [14] (see also Table 1) Symbols as in Fig 2...

Tablet. Monolayer capacities (/imol g ) for H2 on EUROPT 1 denved in various ways...

Figure 4. Infrared spectra of carbon monoxide adsorbed on H2-depleted EUROPT-l surface [5]. Spectra A-F represent 3 min exposures at progressively increasing pressures of from 1.3 x 10"5 to 15 mbar.

Turning now to the catalytic properties of EUROPT-1, very extensive measurements of all kinds of hydrocarbon transformations have been undertaken, and an impressive body of information is now available [6, 7, 12, 15, 16], Only the briefest of surveys is possible here the available reviews [6, 7] and the references cited therein give additional information. 

Extensive studies have been described of the reactions of linear alkanes with H2 over EUROPT-l [6, 7, 12, 15, 16]. Table 2 summarizes the kinetic parameters reported for ethane, propane, and -butanc. Skeletal isomerization is of course possible with this last molc-... 

Table 2. Kinetic parameters for the hydrogenolysis of linear alkanes on EUROPT-1 TOF and selectivities at 573 K [6, 15].

In so far as it is possible to summarize the vast body of information available, it appears in the main EUROPT-1 behaves as expected for a Pt catalyst having particles of about 2 nm in size. Compared to other catalysts, however, its hydrogenolysis selectivity is low, as is its tendency to form carbonacous residues [7] these characteristics may be connected. An attempt has been made [17] to show how the various reactive intermediates might be accommodated on the faces of a 55-atom cubooctahedron (Fig. 5). 

EUROPT-1 has also proved to be of exceptional value as a model catalyst to which systematic modifications may be made. The effects on its catalytic properties of the addition of silver, titania and alumina [16], chlorine, oxygen, sulfur, ammonia, and carbon monoxide have all been examined [7]. Of particular interest is its ability to be modified by alkaloids of the cinchona family, becoming in consequence enantio-selective for the hydrogenation of methyl pyruvate... 

For information on the availability of samples of EUROPT-1, please contact the author of this section. 

Figure 5. The 55-atom cubooctahedron as a model for the most abundant structure in EUROPT-1. (1) cyclopentane adsorbed on a comer atom, (2) a Cs cyclic species on the (100) surface (3) a C6 species en route to benzene on the (111) surface [7]...

As noted above, these are typical petroleum-reforming catalysts [9a, b, 18, 19]. EUROPT-3 is 0.3% Pt AI2O3, and EUROPT-4 contains in addition 0.3% Re. Their surface areas are about 185m2g I, their N2 pore volumes about 0.5 cm3 g-1, and both contain about 1% Cl [18]. 

It was appreciated at the outset that the characterization of these materials would be an altogether more difficult problem than that of EUROPT-1, by reason of the lower metal content, and the bimetallic nature of EUROPT-4. It was hoped that study of the latter would help resolve some of the questions concerning the intimacy of mixing of the two metals in the functioning catalyst, but these queries have been only partially answered. Temperature programmed reduction (TPR) studies have confirmed the sensitivity of the reduction profile to experimental conditions and sample pretreatment but, notwithstanding reports in the liter-... 

Figure 6. Apparent activation energy for n-butane hydro-genolysis on EUROPT-3 as a function of pressure [24].

The kinetics of alkane hydrogenolysis, that is, the dependence of rate on reactant concentration, have been the subject of numerous studies, and much effort has been devoted to devising rate expressions based on the Langmuir-Hinshelwood formalism to interpret them. Reactions of ethane, propane, and n-butane with H2 on EUROPT-3 and -4 have been carefully studied, with the surfaces in either as clean a state as possible, or deliberately carbided [21, 22] kinetic measurements at different temperature permitted adsorption heats and true activation energies to be obtained. There were two surprises (but like all surprises they were obvious afterwards) ... 

The amount of work so far performed with these catalysts is less than with EUROPT-1, and it is more difficult to achieve consistent and reproducible results with them. What has been done does, however, serve to emphasize the extreme care that is needed to eliminate spurious effects thus despite all possible care it seems almost impossible to avoid totally the formation of some carbonaceous deposit, so quickly is it formed. Perhaps, although undesirable, it will have to be tolerated. 

Table 4. Mean nickel particle sizes (nm) in EUROPT-1 determined by various methods [27],...

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