Single-crystal surfaces 1,3-butadiene hydrogenation

Oudar et al. (112) have studied the influence of sulfur on the hydrogenation of 1,3-butadiene and H2-D2 equilibration over Pt(110). The rates decayed linearly with sulfur coverage, so that each sulfur atom poisons one dissociation site for hydrogen without influencing the activation energies or mechanism. The authors established the first isotherm for sulfur adsorption under actual catalytic reaction conditions. The adsorbed hydrocarbons influenced the equilibrium coverage of sulfur on the Pt surface. The thermodynamics of adsorbed sulfur on several metal single crystal surfaces have been presented by Bernard et al. (114). 

DHT does not eliminate 1,3-butadiene under these conditions. The 2,5-DHT also releases 1,3-butadiene when adsorbed on single crystal Mo(llO) surfaces and when passed over a 5% Re/Al203 catalyst at 300 °C under HDS conditions. The more saturated C4 products (equation 3) observed in thiophene HDS are presumably formed by hydrogenation of the 1,3-butadiene. In step (e), the active metal site is regenerated by removal of the sulfide ligand as H2 S upon reaction with H2. The key steps (a-d) in this HDS mechanism (Figure 3) have all been observed either in reactions of thiophene complexes or on catalyst surfaces. 

Butadiene hydrogenation was studied on single crystals exposing different crystal planes. The (111) plane of MgO is active but the (100) plane is not. The (111) plane is composed of surface ions of coordination number 3. This supports the hypothesis that the active sites for hydrogenation are ion pairs of low coordination numbers. 

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