Temperature, 2-propanol decomposition surfaces

Temperature-Programmed Decomposition of 2-Propanol on the Zinc-Polar, Nonpolar, and Oxygen-Polar Surfaces of Zinc Oxide... 

The dehydrogenation reaction produces acetone and hydrogen, and is dominant over basic oxides ( ) The dehydration reaction produces propene and water, and is dominant over acidic oxides. It would be interesting to see if the competition between these two pathways depend on the exposed crystal planes of ZnO. We report here the results of such an investigation. 2-Propanol was decomposed on ZnO single crystal surfaces by the temperature programmed decomposition technique. To assist the interpretation of data, the temperature programmed desorption of propene and acetone were also studied. 

The decomposition of 2-propanol showed both similarities and differences among the surfaces. The most notable similarity is the fact that propene and acetone were produced at about the same ratio on all surfaces. Dehydrogenation to form acetone was the dominant reaction, as has been observed on ZnO powders ( ). The desorption temperatures of the reaction products, acetone, propene, and hydrogen were always higher than the temperature of desorption of the adsorbed acetone, propene, and hydrogen (hydrogen does not adsorb on ZnO under our conditions). Thus the evolution of acetone and propene are reaction limited in 2-propanol decomposition. 

From the temperature at which the decomposition products evolved, it would seem that the 0-polar surface should be the most active in 2-propanol decomposition. However, a close examination of the temperatures in Table I shows that on the 0-polar surface, the desorption temperature of the minor product water was actually rather high - higher than any of the products from the nonpolar surface. Thus in a steady state reaction at temperatures below about 100 C, the 0-polar surface could be easily poisoned by adsorbed water, leaving only the nonpolar surface active. 

The second difference among the surfaces is the fact that, except H2O, the other three decomposition products, H2, acetone, and propene, were evolved at the same temperature on the two polar surfaces, but H2 was evolved at a lower temperature on the nonpolar surface. It is interesting to compare these results with the observations by Koga et al. who studied the decomposition of 2-propanol at 100 C on ZnO powder (13) They found that if the gas phase 2-propanol was suddenly removed from the gas phase, the evolution of hydrogen continued, but the evolution of acetone stopped. The evolution of acetone resumed after readmission of 2-propanol. This behavior can be explained by the fact that the major exposed face of their ZnO powder sample was the nonpolar plane. It is only on this surface that H2 can be evolved without concurrent evolution of acetone in the absence of gaseous propanol. 

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