Photocatalytic Reaction Engineering-Hugo de Lasa, Benito Serrano, Miguel Salaices

Photocatalytic Reaction Engineering Photocatalytic Reaction Engineering Hugo de Lasa Benito Serrano Miguel Salaices iX.lillil fr %mJ' § 400 600 Time (min.) 800 1000 FIGURE 9.2 Phenol and TOC degradation in Photo-CREC-Water II at 12, 22 and 30 ppm-C Open symbols: TOC Filled symbols: phenol Thus, considering the definition of the PTEF for the photoconversion of an organic pollutant, case of phenol, it results ^ = PTEF : v\ r^j„iAHoH) V (9-12) Qa dCi with r Given that AH2 = 94600 Jmole-^ and Qa = ?,93^Jmin-\ the PTEF values reported in Table 9.2 are obtained This table shows that the PTEFs for phenol are smaller than those for silver and this is consistent with the following statements: a) The rates for silver photoreduction are higher than those for phenol photoconversion b) The heat of formation of OH' radicals is 20% more endothermic than that of the reduction of Ag^ cations TABLE 9.2 PTEF for phenol photooxidation Concentration of Phenol (ppm C) 10 20 30 46 55 PTEF 0.005434 0.005903 0.0063 0.0053 0.00571 176 CHAPTER' TABLE 9.3 Calculated PTEFs using equation (9-8) PTEF evaluated with data from experiments using silver (alone) and phenol (alone) Initial Phenol Concentration (ppm) PTEF calculated with equation (9-12) (phenol alone) Initial silver concentration (ppm) PTEF calculated with equation (9-11) (silver alone) PTEF Total 0.00.543 0.00543 0.00.544 0.00590 23 44 98 98 0.0572 0.0459 0.0388 0.0388 0.0627 0.0512 0.0443 0.0447 10.33 10.33 10.33 19.22 As a result, if PTEFs are calculated for water solutions containing phenol or silver and if one considers that oxidation-reduction processes proceed as the direct addition of individual processes, one should expect, following equation (9-8), the results reported in the last column of Table 9.3 9.2.6 PTEF Evaluation for Oxidation-Reduction Experiments can also be developed to have both phenol and silver cations simultaneously present in a water solution that requires treatment Photocatalytic conversion under these conditions shows that the concurrent conversion of organic compounds and inorganic species enhances the overall process efficiency Figure 9.3 reports silver cation photoreduction in the presence of phenol Phenol is added to the water solution with a 20-ppm-C initial concentration It can be observed that in all cases there is a consistent improvement of the rate of silver photoreduction versus the photoreduction rates of silver alone For instance, if one compares (Figure 9.4) the time for silver cation depletion (initial Ag+ concentration of 100 ppm) a reduction of the photoconversion time from 140 to 80 is observed Thus, as a result of the simultaneous reduction-oxidation process, there is an increase in Ag+ photoreduction with mobile electrons having a much lower probability of recombining with electron holes This leads to an enhanced silver reduction with the Lig ht turn "Off ^ Light turn "On' • 100 — • i> 80 S 601 km • < 40 O ! kx X 20 tea X 0 • I • > • • • > * • M Ic 10 20 30 40 50 60 ft( M 70 80 90 100 110 120 Time (min.) FIGURE 9.3 Effect of Phenol 20 ppm C on Silver photoreduction (•) 100 ppm, tmu = 173.67, (•) 68 ppm C, tutii = 141.84), (A) 50 ppm, tmii = 209.31 h, (x) 35 ppm, tuni = 196.32 h and (*) 22 ppm, Um = 137.1 h ADVANCES AND PERSPECTIVES FOR PHOTOCATALYSIS 177 Light turn "On 20 40 60 80 100 120 140 160 180 200 Time (min.) FIGURE 9.4 Effect of 20 ppm-C of phenol on silver reduction (•) 100 ppm of silver, ppm-C of phenol,