Propylene oxide oxidation— surface coverage

The surface coverage 8 of the i-mer is defined as the fraction of propylene oxide adsorption sites covered by any of the i units of the i-mer. 

A simple dynamic model is discussed as a first attempt to explain the experimentally observed oscillations in the rate of propylene oxide oxidation on porous silver films in a CSTR. The model assumes that the periodic phenomena originate from formation and fast combustion of surface polymeric structures of propylene oxide. The numerical simulations are generally in qualitative agreement with the experimental results. However, this is a zeroth order model and further experimental and theoretical work is required to improve the understanding of this complex system. The in situ use of IR Spectroscopy could elucidate some of the underlying chemistry on the catalyst surface and provide useful information about surface coverages. This information could then be used to either extract some of the surface kinetic parameters of... 

Under the differential reaction conditions used in this study [12], the concentrations of all products in the gas phase are small and therefore their respective surface coverages are small. Under these constraints, the following kinetic expressions apply for the partial oxidation of propane to propylene, and propylene to acrolein, respectively ... 

Figure 9 Adsorbed amount versus fraction of anchor segment for poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) ABA triblock copolymers adsorbed onto poly(styrene latex). Inset shows the mean-field calculations of surface coverage versus fraction of anchor segment Vx obtained using Scheutjens and Fleer theory. (J. A. Shar, T. Obey, and T. Cosgrove Colloids and Surfaces. In press.)... 

C. Zhao and l.E. Wachs, Selective oxidation of propylene over model supported V2O5 catalysts Influence of surface vanadia coverage and oxide support, J. Catal, 257, 181-189 (2008). 

In this catalytic system, as schematically shown in Fig. 10.9 [48], a most probable pathway is that over the Au surfaces O2 and H2 react with each other to form H2O2, which then move to isolated sites of Ti cations to form Ti-OOH species [49]. This oxidic species react with propylene adsorbed on the support surfaces to form PO. It has recently been verified that Ti-OOH species is a true reaction intermediate and that a bidentate propoxy species is probably a spectator on the surface [50]. The coverage, 6, of the Ti-hydroperoxo species was determined from the area of the pre-edge peak in the Ti K-edge XANES spectra at reaction conditions. Measurement of the changes in Ti-hydroperoxo coverage, dO/dt, under transient experiments at reaction conditions with H2/02/Ar and C3H6/H2/O2 gas mixtures, allowed the estimation of initial net rate of propylene epoxidation (3.4 x 10 s ), which closely matched the TOP (2.5 x 10 s ) obtained for the same catalyst at steady-state conditions. 

Metal oxide catalysts are extensively employed in the chemical, petroleum and pollution control industries as oxidation catalysts (e.g., oxidation of methanol to formaldehyde, oxidation of o-xylene to phthalic anhydride, ammoxidation of propylene/propane to acrylonitrile, selective oxidation of HjS to elemental sulfur (SuperClaus) or SO2/SO3, selective catalytic reduction (SCR) of NO, with NHj, catalytic combustion of VOCs, etc.)- A special class of metal oxide catalysts consists of supported metal oxide catalysts, where an active phase (e.g., vanadium oxide) is deposited on a high surface area oxide support (e.g., alumina, titania, ziiconia, niobia, ceria, etc.). Supported metal oxide catalysts provide several advantages over bulk mixed metal oxide catalysts for fundamental studies since (1) the number of surface active sites can be controlled because the active metal oxide is 100% dispersed on the oxide support below monolayer coverage,... 

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