In the hydrothermal synthesis of TiO2, [Ti(OH)h(H2O)6−h]4−h (h is the hydrolysis ratio) monomers are generated by the dissolution of the precursor containing titanium ions and then, the monomers form TiO2 polymorphs via a condensation reaction.

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In this paper, the Langmuir-Hinshelwood (L-H) model has been used to investigate the kinetics of photodegradation of gaseous benzene by nitrogen-doped TiO 2 (N-TiO 2 ) at 25 • C under visible light irradiation. Experimental results show that the photoreaction coefficient k pm increased from 3.992 × 10 −6 mol·kg −1 ·s −1 to 11.55 × 10 −6 mol·kg −1 ·s −1 along with increasing illumination intensity. However, the adsorption equilibrium constant K L decreased from 1139 to 597 m 3 ·mol −1 when the illumination intensity increased from 36.7 × 10 4 lx to 75.1 × 10 4 lx, whereas it was 2761 m 3 ·mol −1 in the absence of light. This is contrary to the fact that K L should be a constant if the temperature was fixed. This phenomenon can be attributed to the breaking of the adsorption-desorption equilibrium by photocatalytically decomposition. To compensate for the disequilibrium of the adsorption-desorption process, photoreaction coefficient k pm was introduced to the expression of K L and the compensation form was denoted as K m . K L is an indicator of the adsorption capacity of TiO 2 while K m is only an indicator of the coverage ratio of TiO 2 surface. The modified L-H model has been experimentally verified so it is expected to be used to predict the kinetics of the photocatalytic degradation of gaseous benzene.

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