Catalytic reactions are ever present in applied chemistry,from laboratory synthesis to industrial scale processes.Catalysts provide the means to influence both the rate ofreactions (activity) and the product outcome (selectivity),thereby having fundamental impact on the reaction efficiency.In the ideal case, catalysts allow for 100% conversion ofreactants to the desired products with little or no energyneeded to drive the reaction. In reality this is rarely thecase.
In the work presented here, the underpinnings of chemicalselectivity in catalysis have been investigated on anatomic/molecular level. Three catalytic systems have beenstudied, representing different approaches to achieving productselectivity. Both experimental and theoretical methods havebeen used. The carbon-carbon bond forming reaction known as theHeck reaction has been studied using quantum chemical methodsbased on density functional theory. The migratory insertion ofpropene into the palladium-phenyl bond of selectedorganometallic complexes has been in focus. A cationicpalladium(II)-diimine model catalyst favored the production ofthe branched 2-phenylpropene product. This selectivity could besignificantly improved by introducing steric groups at thenitrogen positions of the ligand system. The use of neutralpalladium complexes for the same reaction shifted theselectivity towards the linear insertion product, i.e. 1-phenylpropene.
Methylamine, adsorbed on supported rhodium clustercatalysts, undergoes selfhydrogenolysis when annealed,producing ammonia and methane. When adsorbed on supportedpalladium clusters, methylamine undergoes disproportionationwith dimethylamine as the primary product. These reactions wereinvestigated using solid state nuclear magnetic resonance. Theinitial steps of the reaction involved carbonnitrogen bondscission. It is suggested that carbon fragment stability on thetwo different metals is primarily responsible for the observedselectivity. On rhodium, carbon species readily dehydrogenate,while the nitrogen species react with surface hydrogen to formammonia. On palladium, carbon fragments undergo carbon-nitrogenbond formation rather than further carbon-hydrogen bondscission.
Photoelectron spectroscopy in combination with periodicdensity functional methods were used to study the influence oftin on the selective hydrogenation of crotonaldehyde onplatinum. Catalyzed by platinum, the hydrogenation of theα,β- unsaturated aldehyde leads to the formation ofthe saturated aldehyde. With tinmodified catalysts, thereaction produces an increased amount of the unsaturatedalcohol. Crotonaldehyde was adsorbed on Pt(111) and the tinmodified surface alloys Pt(111)Sn-(2x2) andPt(111)Sn-(√3x√3)R30º. It was found that anincreased surface concentration of tin caused deactivation ofthe alkene functionality while the carbonyl bond interactedwith the surface through O-Sn coordination. Preliminary studiesof the catalytic properties of a tin modified Pt(110)-(1x2)catalyst were also performed. The surface alloy was firstcharacterized and later dosed carbon monoxide. An interestingpreference for adsorption along the Pt ridge-rows wasobserved.
Kista: Mikroelektronik och informationsteknik , 2002. , x, 60 p.