The Local and Observed Photochemical Reaction Rates Revisited

O.M. ALFANO; H.A. IRAZOQUI; A.E. CASSANO

Photochemical and Photobiological Sciences

ROYAL SOC CHEMISTRY

Año: 2009 vol. 8 p. 1047 - 1058

1474-905X

  In a broad sense, photochemical reactions proceed through pathways involving several reaction steps.The initiation step is the absorption of energy both by the reactant or sensitizer molecules and in somecases, by the catalyst, leading to intermediate products that ultimately give rise to stable end products.Preferably, the reaction rate expression is derived from a proposed mechanism together with soundsimplifying assumptions; otherwise, it may be adopted on an empirical basis. Under a kinetic controlregime, the rate expression thus obtained depends on the local rate of photon absorption according to apower law whose exponent very often ranges from one half to unity. The kinetic expression should bevalid at every point of the reactor volume. However, due to radiation attenuation in an absorbingand/or scattering medium, the value of the photon absorption rate is always a function of the spatialposition. Therefore, the overall photochemical reaction rate will not be uniform throughout the entirereaction zone, and the distinction between local and volume average photochemical reaction rates becomesmandatory. Experimental values of reaction rates obtained from concentration measurementsperformed in well-mixed reaction cells are, necessarily, average values. Consequently, for validationpurposes, experimental results from these cells must be compared with volume averages of themechanistically or empirically derived local reaction rate expressions. In this work it is shown thatunless the rate is first order with respect to the photon absorption rate or the attenuation in theabsorbing and/or scattering medium is kept very low, when the averaging operation is not performed,significant errors may be expected.