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Synthesis gas conversion to short olefins and aromatics using bifunctional catalysts has gained much attention in recent years. Here, we study the interaction between the components of bifunctional catalysts to design a more stable catalyst system. Mixing α-alumina supported iron (-carbide) promoted with sodium and sulfur with an H-ZSM-5 zeolite to convert synthesis gas to aromatics and short olefins we observed selectivity loss of the iron (-carbide) catalyst as well as the acid function. This was displayed by increasing methane and decreasing aromatics selectivity when the two individual catalysts were mixed in close proximity. We introduced different approaches to understand this selectivity related deactivation. Larger spatial separation of the iron and zeolite allowed a more stable system with constant methane and aromatics selectivity. Alternatively, iron supported on carbon nano tubes mixed with the zeolite in close proximity did not display selectivity related deactivation. We conclude that the selectivity loss was caused by migration of sodium ions that were used next to sulfur as promoters on the iron catalyst over the α-alumina support to the zeolite, which was supported by XPS model experiments. This migration seems hindered on carbon supported iron catalysts.

In the epoxidation of ethylene, catalyzed by supported Ag particles, the support plays not only a beneficial role, but can also negatively impact the selectivity to ethylene oxide. Especially high surface area supports, and supports containing acid groups, seem detrimental for the selectivity, which is attributed to secondary reactions on the support surface. We prepared Ag nanoparticles on supports with a wide range of specific surface areas and different chemical nature, but with similar Ag metal loading and particle size. Indeed, a strong dependence of selectivity on both the specific surface area, and the chemical nature of the oxide support was found, with α-Al 2 O 3 giving much more selective catalysts than SiO 2 . Furthermore, the conversion was an important factor in determining the selectivity, while on the other hand the support had no influence at all on the ethylene conversion. We described our experimental selectivities with a reaction model that used the rate constant for the secondary reaction of oxidation of ethylene oxide to carbon dioxide and water as the only variable parameter. The model was able to adequately describe the experimental results. It gave insight into the dependence of selectivity on conversion, how the secondary reaction depended on the chemical nature of the support, and how its rate scaled linearly with the support's specific surface area. Building on this insight we modified a SiO 2 support to passivate its OH-groups, which indeed yielded a 94% decrease in the rate of the secondary reaction, performing even better than α-Al 2 O 3 with similar specific surface area would. We have shown that the selectivity towards ethylene oxide is dependent on a metal specific, intrinsic selectivity, and the decrease in selectivity with conversion to be support dependent. This decrease is caused by the isomerization of ethylene oxide, which is found to be first order in ethyle