Selectivity in Hydrocarbon Conversions and Methanol Decomposition on a Pd/Fe3O4 Model Catalyst

One of the most important objectives in chemistry is to understand and control the selectivity towards a desired reaction pathway. In catalysis, selectivity is thought to be governed by the catalyst’s structure; many practical catalysts, however, have an inherently complex structure that obstructs their detailed understanding; therefore, simplified materials are studied instead. In order to advance the understanding of catalysis, we have applied a combination of surface science experimental techniques, centered around molecular beam methods to study surface reactions under vacuum on a wellcharacterized Pd/Fe3O4 model catalyst; the reactions were followed by massspectrometric rate measurement and infrared-spectroscopic detection of adsorbates. Our experiments focussed on the origins of selectivity in two reactions, the decomposition of methanol and the conversion of 2-butene with coadsorbed deuterium; additionally, both reactants may decompose on the Pd metal to carbonaceous species or carbon. Methanol decomposition is a well-established model reaction in surface science and of high interest for application per se. We show in this work that methanol is dehydrogenated to formaldehyde and water by reaction with surface oxygen on pristine Fe3O4 with high selectivity; the surface oxygen consumed by this reaction has to be replenished in order to sustain the reactivity. In the presence of Pd nanoparticles, support-related methanol and methoxy migrate quickly to the metal particles before formaldehyde could be formed on the oxide; on the Pd metal surface methanol decomposes in two pathways, forming carbon monoxide and hydrogen in the main pathway, and carbon in the minor pathway, which accumulates slowly on the Pd surface and poisons its catalytic activity. 2-butene conversion was studied as a model for hydrocarbon conversion reactions, focussing on mechanisms that induce selectivity between the three reaction pathways (hydrogenation, H/D-exchange/isomerization and decomposition). This work demonstrates that sustained reactivity can be obtained on the Pd/Fe3O4 model catalyst at elevated temperatures (around 250 K); isomerization proceeds with sustained rates already on a clean catalyst, but sustained hydrogenation can only be induced in the presence of previously deposited, strongly dehydrogenated carbonaceous deposits (carbon); this induction of hydrogenation was explained by the ability of carbon to facilitate the formation of subsurface hydrogen under reaction conditions, which is necessary for hydrogenation. Additionally, we have compared the conversion rates with cisand trans-2-butene; selectivity occurs under conditions where the reaction rates seem to be limited by the availability of butene; cis2-butene gives higher yields in those cases, presumably because it reaches higher alkene surface coverages in adsorption-desorption equilibrium.