Ethylene Hydrogenation on Pt

The adsorption of hydrogen, ethylene, and CO on Pt(l 1 1) was extensively studied. Molecular hydrogen dissodatively adsorbs on the catalytic Pt(l 1 1) surface 

These systematic studies suggest that an intrinsic connection between the adsorbate structure, mobility, and the formation of product can be established with the aid of structural information obtained from high-pressure STM. It further demonstrated the importance of STM in studies of heterogeneous catalysis at high pressure. 

This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. 

13 Somorjai, G.A. (1999) Introduction to Surface Chemistry and Catalysis, Wiley-VCH, Weinheim. 

21 Somorjai, G.A. (1981) Chemistry in Two Dimensions Surfaces, Cornell University, Ithaca, NY. 

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M.R. Prairie and J.E. Bailey, Experimental and modelling investigations of steady state and dynamic characteristics of ethylene hydrogenation on Pt/Al203, Chem. Eng. Sci. 42 2085 (1987). 

Cremer P, Su X, Shen Y, Somoijai GA (1996) Ethylene hydrogenation on Pt(lll) monitored in situ at high pressures using sum frequency generation. J Am Chem Soc 118 2942... 

Masson et al. (78) have studied ethylene hydrogenation on Pt/Al203 also prepared by vapodeposition. The shape of the TOF/FE curve is identical to that shown in Fig. 9. The maximum TOF is also at about 0.6 nm. 

Several groups have studied the reaction intermediates in the hydrogenation reaction using infrared spectroscopy [25,26]. However, they were all carried out in the absence of ethylene in the gas phase. By contrast, SFG allows us to monitor the surface in situ under catiytic conditions [27]. The conventional turnover rate (TOR) of ethylene hydrogenation on Pt(lll) at 295 K with 100 Tort Hj and 35 Torr CjH per platinum atom as determined by gas chromatography is 11 1 ethylene molecules converted to ethane per second. Three features were present on the surface under these conditions, at 2880 cm, 2910 cm and a small peak... 

A word of caution By no means do we imply that what we described above is the detailed mechanism of ethylene hydrogenation on Pt surfaces. What we suggest is that the ultimately correct mechanism will very likely contain steps (a) to (e) and versions thereof as integral parts. The reader is referred to the important work of Somorjai for the experimental facts... 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

For reactions that are the same on metal and other catalytic sites (e.g., hydrogenation or total oxidation), the reaction may seem to proceed in a similar fashion on the metallic source of spillover and on the diluent support. Some careful studies may be able to discriminate between activity on the metal and the spillover-induced sites. As an example, hydrogenation of ethylene occurs on Pt (or Ni) and on silica or alumina activated by spillover. The product (i.e., only ethane) is the same, as the kinetics often are (rate = /c[C2H4]°[H2]1), but the specific mechanism is different. Deuteration is able to discriminate between the relative rate of alkyl reversal. Deuteration of ethylene on an activated silica produces d2-ethane as the initial product (137), contrary to the results for metal-catalyzed ethylene hydrogenation (2). 

Deuterium exchange and kinetic analysis of ethylene electrogenerative hydrogenation on Pt black showed that, in the absence of pore diffusion, the rate is limited by surface addition of the second hydrogen to an adsorbed ethyl radical (25, 26) ... 

Figure 9. Suggested reaction pathways for ethylene, propylene and isobutene hydrogenation on Pt(lll).

Nevertheless, a far more important parameter in this case is the promotion index Pj (Section 4.2) which takes values up to 250 and down to -30 for the case of Na promotion and poisoning, respectively, of CO oxidation on Pt (Table 2 and Figure 18). As noted in Section 4 (Figures 16 and 17) and also shown on Table 2, p values up to infinity and down to zero have been recently obtained for the cases of NO reduction by C2H4 on Pt and benzene hydrogenation on Pt. Also the use of P"-Al203 as a Na donor in the case of ethylene epoxidation, in conjunction with the use of chlorinated hydrocarbon moderators, leads to ethylene oxide selectivity up to 88 percent (Figure 30). 

Motoo and Furuya[139, 140] demonstrated an enhancement of the ethylene reduction on Pt by Cu and Ag and inhibition by Se and Tl adlayers. Cu and Ag adatoms, which block one Pt site, desorb the ethylene molecule that blocks 2.5 Pt sites, thus leaving 1.5 sites available for H adsorption. Hydrogen adatoms at these sites facilitate a higher rate of ethylene reduction. Tl and Se adatoms cause inhibition since they block 2.5 sites as ethylene does. 

Fig. 2. Pressure fall —AP (Torr) against time t (arbitrary units) in hydrogenation of acetylene on Pt/AhOa catalyst at 110°C and Pst/Pctnt 2. In the initial slow period of the reaction the main product is ethylene, and after the acceleration, further hydrogenation of ethylene to ethane predominates. From G. C. Bond and P. B. Wells, J. CaM. 4, 211 (1965).

Both PtRu/MgO catalysts prepared from cluster precursor and organometallic mixture were active for ethylene hydrogenation. The apparent activation energy of the former catalyst obtained from the Arrhenius plot during -40 to -25°C was 5.2 kcal/mol and that of the latter catalyst obtained during -50 to -30°C was 6.0 kcal/mol. The catalytic activity in terms of turn over frequency (TOP) was calculated on the assumption that all metal particles were accessible for reactant gas. Lower TOP of catalyst prepared from cluster A at -40°C, 57.3 x lO" s" was observed probably due to Pt-Ru contribution compared to that prepared from acac precursors. 

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

Table 4. Ag/Pt molar ratio and its influence on ethylene hydrogenation rates and apparent activation energy for nanoparticle encapsulated shape-controlled Pt nanoparticles [17].

This review covers the personal view of the authors deduced from the literature starting in the middle of the Nineties with special emphasis on the very last years former examples of structure-sensitive reactions up to this date comprise, for example, the Pd-catalyzed hydrogenation of butyne, butadiene, isoprene [11], aromatic nitro compounds [12], and of acetylene to ethylene [13], In contrast, benzene hydrogenation over Pt catalysts is considered to be structure insensitive [14] the same holds true for acetonitrile hydrogenation over Fe/MgO [15], CO hydrogenation over Pd [16], and benzene hydrogenation over Ni [17]. For earlier reviews on this field we refer to Coq [18], Che and Bennett [9], Bond [7], as well as Ponec and Bond [20]. 

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