Graduate Thesis Or Dissertation
 

Kinetics of thermal decomposition of tungsten hexacarbonyl

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  • The thermal decomposition of tungsten hexacarbonyl was investigated in an apparatus which allowed a solid sample of the material to be sealed into a preheated batch reactor. Because the reaction occurs with an increase in the number of moles of gas, it was possible to monitor the progress of the reaction by measuring the pressure in the reactor. A high temperature pressure transducer in the reactor wall was used as a sensing element. The reaction was found to be first order; the observed ranges in activation energy and pre-exponential factors were 39.6 to 45.8 kilocalories per gram mole and 10¹³[superscript .]⁷ to 10¹⁵[superscript .]⁸ per second, respectively. The values were found to depend upon surface-to-volume ratio, surface condition, and the character of the filler gas. The accuracy of the activation energies was calculated to be ± 8.5%. The fact that the reaction rates depended upon surface amount and condition and upon foreign gases present indicated that the reaction is of mixed homogeneous and heterogeneous nature. Approximate values for the activation energies of the homogeneous and heterogeneous portions of the reaction were found to be 70 kilocalories per gram mole and 35 kilocalories per gram mole, respectively. Energy considerations and the observed rate law indicate that the slow step in the homogeneous reaction is probably the decomposition of tungsten hexacarbonyl to tungsten pentacarbonyl. The heterogeneous portion of the reaction is believed to be controlled by a slow step from the adsorbed tungsten hexacarbonyl to an unknown intermediate. Previous workers have explained the presence of carbon in the plate on the basis of the disproportionation of carbon monoxide to give carbon and carbon dioxide. However, the fact that both carbon and oxygen were present in the plate indicates that the source of these substances is the reaction of tungsten and carbon monoxide to give carbon and tungsten dioxide. This is further indicated by the fact that an electron diffraction analysis showed tungsten dioxide to be present in the plate.
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