Kinetics and mechanism of methanol decomposition over zinc oxide

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Tawarah, Khalid Moh'd
Iowa State University; Digital Repository @ Iowa State University,
Chemistry; Physical chemistry
thesis / dissertation description
The kinetics of CH(,3)OH decomposition over ZnO was studied in the temperature ranges: 453 K - 513 K, and 563 K - 613 K. In the first range, CH(,3)OH decomposed to H(,2) and CH(,2)O, while concurrent decomposition of CH(,3)OH and CH(,2)O to H(,2), CO, and CO(,2) occurred in the second range. Production of CO(,2) was believed to be a side reaction resulting from the decomposition of a formate species. The effect of P(CH(,3)OH) on the initial rate of production of H(,2), R(H(,2)), and (CO + CO(,2)), R(,c), is given by the equation: P(,m)/R(,i)(' 1/2) = a + bP(,m). No inhibition was observed for H(,2), CO, or CO(,2). At 500 K, CH(,3)OH decomposed about 10 times faster than CH(,2)O. Comparison of the initial rates of decomposition of CH(,3)OH, CH(,3)OD, and CD(,3)OD in the low and high temperature ranges indicated that R(,i)(CH(,3)OH) = R(,i)(CH(,3)OD) > R(,i)(CD(,3)OD);The proposed mechanism assumes that the rate of the low^temperature CH(,3)OH decomposition is determined by that of the^step: CH(,3)O(,*) + H(,*) (--->) CH(,2)O(,(g)) + H(,2(g)) + 2*, while the high^temperature decomposition is governed by the step: CHO(,*) +^H(,*) (--->) H(,2(g)) + CO(,(g)) + 2*. The low temperature decomposition has^the apparent activation energies: E(,app.)(CH(,3)OH) = E(,app.)(CH(,3)OD) =^89.0 kJ mol('-1), and E(,app.)(CD(,3)OD) = 95.7 kJ mol('-1). The high^temperature decomposition gave a break in the Arrhenius plot of^1n R(,c) versus 1/T at (TURN)590 K. This was attributed to the decrease inthe equilibrium constants, appearing in the rate law, with increasingtemperature. Below 590 K, the apparent activation energies are:E(,app.)(CH(,3)OH) = E(,app.)(CH(,3)OD) = 127.1 kJ mol('-1), and E(,app.)(CD(,3)OD);= 175.4 kJ mol('-1);('1)DOE Report IS-T-1011. This work was performed under contractNo. W-7405-Eng-82 with the U.S. Department of Energy.