Ethene hydrogenation catalysis

FIGURE 40 Dependence of the Ir-lr distance of lr4 (upper) and lr6 (lower) on the partial pressure of H2 during ethene hydrogenation catalysis at 298 K (Argo et al., 2003). Reprinted with permission from (Argo et al., 2003). Copyright 2003 American Chemical Society. 

Beeck at Shell Laboratories in Emeryville, USA, had in 1940 studied chemisorption and catalysis at polycrystalline and gas-induced (110) oriented porous nickel films with ethene hydrogenation found to be 10 times more active than at polycrystalline surfaces. It was one of the first experiments to establish the existence of structural specificity of metal surfaces in catalysis. Eley suggested that good agreement with experiment could be obtained for heats of chemisorption on metals by assuming that the bonds are covalent and that Pauling s equation is applicable to the process 2M + H2 -> 2M-H. 

It is also shown that carbonaceous deposits built upon oxides via spill-over from metal sites is active in ethene hydrogenation in its own right. Thus during catalysis of hydrocarbon reactions some sites (metallic and acidic) are lost or made unavailable while simultaneously others are generated. This complex yet intriguing situation emphasises the need for in-situ characterisation of catalytic surfaces. 

Argo AM, Gates BC (2003) MgO-Supported Rh and Ir Structural characterization during the catalysis of ethene hydrogenation. J Phys Chem B 107 5519... 

Recent gas phase studies have demonstrated catalysis for ethene hydrogenation by Fe(CO)3(C2H4) produced by excimer laser pulse photolysis of a catalytically inert mixtures of Fe(CO)5, H2 and ethene. 

Quantum yields for ethene hydrogenation under these ambient temperature conditions exceeded 20. The photolysis procedure generated a reservoir of Fe(CO)3(ethene)2 which is very labile and dissociates to give the active, unsaturated species Fe(CO)3(ethene). Added CO as well as photoliberated CO inhibits catalysis due to the back reaction to form the stable Fe(CO)4(ethene) [65]. 

Cremer etal. (359, 371, 372) have also recently shown by the use of SFG that the 77-species, reactive for hydrogenation even on ethylidyne-covered Pt(lll) surfaces, is much more reactive in catalysis than the di-rr one. A turnover number of 275 s 1 per adsorbed 77-ethene species has been measured for a 35 Torr C2H4 and 100 Torr H2 mixture at 295 K in the presence of Pt(lll). This measurement was made under conditions of approximately zero order reaction in ethene and 0.8 order reaction in hydrogen. 

The mechanism of contact catalysis may vary in details, depending on the nature of the reactants. Consider the example of hydrogenation of ethene in presence of nickel. In this casse, ethene adds hydrogen in the presence of nickel as a catalyst to yield ethane. 

Coking, widely experienced in the catalysis of hydrocarbon conversion (7), can deactivate both metallic and acid catalytic sites for hydrocarbon reactions (2). Accumulation of such carbonaceous deposits affects selectivity in hydrocarbon conversion (5). Adsorbed ethene even inhibits facile o-p-Hj conversion over Ni or Pt (4 ), the surface of which it appears is very nearly covered at lower temperatures in such deposits. H spillover may enhance hydrocarbonaceous residue formation (6). Accumulated carbonaceous residues can be removed by temperature programmed oxidation, reduction and hydrogenation TPO, TPR, TPH, etc (7) as part of catalyst regeneration. 

As an example of heterogeneous catalysis, consider the reaction of ethene with hydrogen using a heated nickel catalyst ... 

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