Understanding the detailed mechanism responsible for cis-trans-selectivity of alkene formation reactions is an important goal of catalysis research and significant for many industrial processes. As a model reaction, we study the interconversion between cis- and trans-2-butene in presence of coadsorbed deuterium. According to the generally accepted Horiuti-Polanyi mechanism, the reaction proceeds by addition of a deuterium atom to one of the central C atoms in the molecule followed by internal group rotation in the 2-butyl group and detachment of the original H-atom from the same central carbon. This sequence of events leads to a combination of H-D exchange, which can be detected by mass spectrometry, and cis-trans-isomerization. This reaction pathway competes with full deuteration of the double bond and decomposition of the reactant on the surface.
Experiments have been performed on well-defined model catalysts consisting of Pd nanoparticles supported on an Fe3O4/Pt(111) thin film. The correlation between the reactivity and the structural properties of the catalyst has been addressed in a systematical way by varying particle size in a broad range from 3 to 65 nm. To study reactivity, we performed fully remote-controlled kinetic experiments using molecular beam methods, time-resolved IRAS and TPD.
Specifically, we have been able to detect all possible reaction pathways (exchange-isomerization, deuteration and decomposition) and to show that sustained catalytic H-D exchange accompanied by isomerization and both exchange and deuteration reactions could be maintained under ultrahigh vacuum conditions on surfaces previously treated with hydrocarbons and covered by carbonaceous deposits.
In exchange-isomerization, the cis-2-butene was found to be more reactive than the trans-2-butene. Deuteration, on the other hand, shows similar reaction rates for both molecules. The change of relative reactivity with particle size was found to be small, but absolute reactivity showed a maximum at intermediate particle size.
All reactions were shown to start only above a minimum coverage. The adsorption behaviour and different possible adsorption states of butene will be discussed. |