Understanding the partial oxidation of propylene over silver catalysts: The effect of water and surface structure.

Propylene oxidation over silver surfaces was investigated to develop an understanding of this important catalytic oxidation process. To determine the effects of surface reconstruction, water coadsorption, and support interactions, silver single crystal surfaces and supported/promoted silver materials were characterized using a combination of surface spectroscopies. On oxygen covered Ag(110), propylene combustion is observed. Disordered oxygen atoms are 10 times more active for combustion than oxygen embedded into the reconstructed O-Ag(110) surface. Oxygen adsorption on Ag(110) results in a 30% increase in the propylene and propylene oxide desorption activation energy. Coadsorption of water on O-Ag(110), forming OH groups, completely passivates propylene combustion and lowers the desorption activation energy of both propylene and propylene oxide. These results are consistent with the increased catalytic selectivity for propylene oxide production observed in the presence of water. Multilayer water coverages displace non-polar propylene resulting in an even lower desorption energy, while more polar propylene oxide and 2-propanol are not displaced. 2-propanol reacts with oxygen on Ag(110) forming acetone via an alkoxide intermediate, even in the presence of water multilayers. On silver supported on high surface area TiO$\sb2,$ a form of rehybridized adsorbed propylene not previously identified, was characterized using x-ray absorption spectroscopy. Propylene rehybridization may be facilitated by interactions between silver and the oxide support which would play a significant role in oxidation reactions. Silver magnesium alloys were studied to understand the surface chemistry of magnesium as a promoter. Magnesium is rapidly oxidized resulting in a surface dominated by MgO, modifying propylene adsorption and possibly passivating acid sites on the support. A new understanding of several important factors in propylene oxidation has been developed. The effect of surface reconstruction, the critical role of water on propylene oxidation, and support/promoter interactions were investigated providing significant new insight into oxidation catalysis. This enhanced understanding of propylene oxidation helps establish a foundation for developing a direct selective process for propylene partial oxidation.Ph.D.Applied SciencesChemical engineeringPhysical chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131077/2/9825327.pd