Journal of Physical Chemistry p. 17852 - 17861 (1995)
Update date:2022-08-12
Topics:
Yasuda, Yusuke
Iwai, Kayo
Takakura, Kazumi
"Reaction rate (or FR) spectra" of a catalytic hydrogenation of propylene over Pt or Rh at 314 K were observed in a cell reactor composed of a proton-conducting membrane.It is shown that a variety of the spectra can be reproduced well by "characteristic functions", K*H(ω) and K*C(ω), which may be derived from a three-stage model composed of five elementary steps: X(g) -->/<-- AX(a) -->/<-- BX(a) --> propane (X: hydrogen or propylene), where X denotes the gaseous molecule; AX and BX are the first and second intermediate adsorbed species.Seven rate constants concerning these five steps were evaluated by matching K*H(ω) or K*C(ω) to the spectrum; five of them, kPX, k-AX, kAX, k-BX, and kBX, are ordinary rate constants, while the other two, l-BX and lBX, are novel ones.Since all these constants except kPX are independent of the amounts of catalyst, they are characteristic of active sites and can be compared with each other.On the basis of these constants, kinetic details have been discussed; for instance, mean residence times of AX and BX, τAX and τBX, respectively, were determined by (k-AX + kAX)-1 and (k-BX + kBX)-1, resulting in (in second units) τAH ca. 0.3 and τBH ca. 3 for hydrogen and τAC ca. 3 for propylene over Pt, while over Rh they were τAH ca. 1 and τBH ca. 3; τAC ca. 102 and τBC ca. 102.The nondimensional rate constants, l-BX and lBX, were indispensable to reproduce the various FR spectra; l-BH and lBH were positive, whereas l-BC and lBC were negative over both catalysts, which suggests heat effects.
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