Benzyl alcohol photoadsorption

The fitting of the Langmuir, Freundlich, and R-P models to the data has been firstly applied to the photoreactivity results obtained from runs carried out at equal mass of cafalysf and lamp power. For the Langmuir model, the following procedure has been followed. In order to have an estimate of parameters values, the data at high initial concentration of benzyl alcohol have been fitted to Equation (A6) (see Appendix Al) and those at low initial concentration to Equation (A13). The parameters obtained by these fitting procedures have been used to determine Ng by means of Equation 

The slopes and linear coefficients of lines allow to determine the Ng and values corresponding to these runs these values are almost the same as those determined by means of Equation (19) (error percentage, +4%). Eigure 8 also reports the adsorption results obtained in the absence of irradiation, that is, in the dark. A least-squares best fitting procedure allows to determine the values (R 0.99) of the Langmuir equilibrium constant and the maximum adsorption capacity in the absence of irradiation, that is, fCL = 350 and Ns = 4.57 x 10 mol/g of catalyst. 

This finding may be explained by considering that an increase of catalyst amoimt determines an increase of photons absorbed by the suspension but the photon flow absorbed by the unit mass of catalyst decreases. As photoadsorption is likely to be strongly dependent on absorbed photon flow, the increase of catalyst amount is eventually detrimental for specific photoadsorption. 

Absorbed photon flow per gram of catalyst [Einstein s g ] 

---

Figure 3 Experimental values of benzyl alcohol concentration vs. irradiation time. Home-prepared catalyst amount 0.4 g L lamp power, 125 W. The empty symbols indicate the concentration of starting solution. The solid lines represent the Langmuir photoadsorption model [Equation (19)] (Yurdakal et at, 2008b).

The effect of catalyst amount on photoadsorption capacity is shown in Figure 9. This figure reports the benzyl alcohol moles photoadsorbed per unit mass of catalyst vs. the catalyst amount the reported data refer to runs carried out at equal initial benzyl alcohol concentration and lamp power. From the observation of data of Figure 9, a decrease of specific photoadsorption capacity by increasing the catalyst amount, differently from that expected on thermodynamic basis for which an increase of catalyst amount determines a corresponding increase of adsorbed substrate, may be noted. 

The values of kinetic and Freundlich parameters (k, Kp, and Ng), obtained from runs carried out with different amounts of catalyst, are reported in Figure 12 vs. the absorbed photon flow per unit mass of catalyst. These values show the same feature of Langmuir parameters obtained for benzyl alcohol oxidation, that is, they decrease by decreasing the photon flow absorbed by the unit mass of catalyst. As in the case of benzyl alcohol, the consideration that the photon flow absorbed by the unit mass of catalyst is the parameter mainly affecting the photoadsorption phenomenon is strengthened by the results reported in Figure 12. 

---