Propene desorption, decomposition

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. 

These, together with the observation that acetone and propene were always evolved at the same temperature suggest that acetone and propene are formed from a common intermediate on the different surfaces, the formation or decomposition of which is the rate limiting step. The evolution of water, however, was at the same temperature as the desorption of adsorbed water. Thus the process is desorption limited. 

In conclusion, the chemical properties of ZnO depend on the particular surface plane that is exposed. This surface specificity has now been demonstrated for the decomposition of 2-propanol, methanol, formaldehyde and formic acid, and adsorption and desorption of acetone, propene, water, CO, and CO2. These data have made possible better understanding of the results using ZnO powder. It will be intersting to se<5 how different are the catalytic properties of these surfaces. 

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