9370 J. Phys. Chem. B, Vol. 108, No. 26, 2004
Fan et al.
catalyst stacks. Accordingly, the kinetic equation was simplified
into two separate terms containing chemical reaction and
diffusion contribution. This model neglects the role played by
the solution layer, where the absorption of quadricyclane is
measured. Second, the molecular mechanism for the diffusion
motion was assumed previously, but only a diffusion-related
parameter in the catalyst stacks was determined. In contrast,
explicit values of diffusion coefficients in solution and inside
the catalyst stacks are determined in this work. The packing of
catalyst turns out to decrease the diffusion flow. The quadri-
cyclane diffusion inside the catalyst layer cannot simply be
assumed to be the same as in the solution layer. Third, the
pseudo-first-order rate constant previously obtained depended
on the weight of catalyst.10 However, the pseudo-first-order rate
constant obtained in this work is independent of the catalyst
amount. The reason lies in the different design of reactors. With
previous use of a cylindrical cell aligned horizontally, the top
solid surface area may increase with the gradual deposition of
the catalyst. Therefore, the increased catalyst amount, previously
used in the range of 0.2-1.2 g,10 may lead to an increase of
the surface area contacted with quadricyclane. The pseudo-first-
order rate constant appears to increase with the catalyst amount.
In contrast, in the cuboid cell used in this work, the top surface
area remains constant no matter how much the catalyst increases.
The effective solvent volume in the catalyst particle interstices
is proportional to the amount of catalyst. Thus, the total surface
area of catalyst per unit volume of solvent remains constant.
Unless the particle size changes with consequent change in the
packing, the pseudo-first-order rate constant does not depend
on the weight of catalyst provided the same cuboid cell is used.
To confirm this point, a preliminary result for the size
dependence of the pseudo-first-order rate constant is recently
obtained, yielding a straight line with the slope indicative of
the second-order rate constant.26
V. Conclusion
By use of Fourier transform NIR absorption spectroscopy
combined with a new simulation model, we have studied the
kinetic behavior for isomerization of quadricyclane catalyzed
by anhydrous CuSO4 and SnCl2. For this heterogeneous reaction,
the reaction mixture is not agitated and the NIR radiation is
positioned to pass through the solution above the solid surface.
The kinetic model contains contributions of diffusion and
isomerization. The depletion of quadricyclane, as numerically
solved from the model, may describe the behavior of quadri-
cyclane in the catalytic system more accurately than the method
used previously. Diffusion coefficients of quadricyclane in the
solution and inside the catalyst stacks can be explicitly
determined. The results suggest that the diffusion is slower
within the catalyst, and thus the molecular mechanism is not
applicable in this system. The pseudo-first-order depletion rate
constant is independent of the weight of catalyst. Our reaction
rates are consistent with others obtained in a continuously stirred
mixture. In this surface-mediated reaction, the isomerization of
quadricyclane is primarily controlled via one-site coordination
between the reactant and the catalyst, but a contribution of a
two-site coordinated reaction cannot be excluded. The spectro-
scopic technique and simulation method developed in this work
should be a useful addition to the kinetic studies of heteroge-
neous catalysis.
Acknowledgment. This work is supported by the National
Science Council of the Republic of China under Contract No.
NSC 91-2113-M-002-033. The authors wish to thank Prof. T.
I. Ho for providing the synthesized quadricyclane.
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(
-
5
-1 -1
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(
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2
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