Apparent quantum efficiency

Semiconductor (such as Ti02) + hv(energyofphoton)=e (electron)+h+ (hole) In this case, the apparent quantum efficiency (QE) of the felt material prepared from the titania/silica fiber is calculated by the following equation ... 

Fig. 31 The apparent quantum efficiency of the titania/silica fiber. The wavelength and the intensity of the light at the surface of the felt material were 352 nm and 1 mW/cm2, respectively. Actual irradiation area perpendicular to the ray of light was 8.33xlO-4 m2...

Yu et al. [105] reported instead visible light photocatalytic H2 production from water on CNT modified by Cdo Zn S quantum dots. Under visible light, an H2-production rate of 78.2 pmol h 1 with apparent quantum efficiency (QE) of 7.9% at 420 nm was shown. These are interesting performances, but it is known that these quantum dots do not show enough long term stability. 

Strictly speaking, when apparent quantum efficiency is discussed, the light intensity should be adjusted to be the same in number of photons, not in energy. 

Mitchell (41) treated theoretically the absorption of light by a mono-layer, showing that the apparent quantum efficiency could be directly proportional to the absolute temperature for certain condensed films. The molecular extinction coefficient cannot be deduced from that in solution it is, however, proportional to sin- d (where 6 is the angle... 

Fio. 13. Apparent quantum efficiency of the photochemical dccomiiOHition of films of stearanilide on 5 Af HjS04. The points show the experimental results the full line is calculated from the surface dipole moment, mb, of the molecules in the film. As the film is compressed, the orientation of the anilide chromophore group changes. This causes both an increase in and an alteration in mo (Uideal and Mitchell, 42). 

Ibrahim, H., and de Lasa, H., 2003, Photo-catalytic degradation of air bonre pollutants apparent quantum efficiencies in a novel photo-CREC-air reactor, Chem. Eng. Sci., 58(3-6) 943-949. 

Peill and Hoffmann, (1995) also define an average apparent quantum, efficiency for photooxidation based on the time required to achieve a 90% conversion of pollutant molecules. 

One additional problem with the quantum yield is in the evaluation of the absorbed radiation inside the photocatalytic reactor (Cabrera et al., 1996). This is due to the fact that semiconductor particles are highly reflective (Fox and Dulay, 1993), favoring photon scattering and reflection (Valladares and Bolton, 1993) reducing, as a result, the photons absorbed. An apparent quantum yield can be considered when the absorbed photon rate is not available, providing a minimum value that bounds the actual quantum yields (refer to Chapter VI). While, the quantum yield was meant for monochromatic radiation, both quantum efficiencies and apparent quantum efficiencies should be considered for polychromatic radiation (Cabrera et al., 1994). As well quantum yields and quantum efficiencies are affected by the light intensity reaching the photocatalyst as reported by Pelizzetti et al (1993). 

The debate still remains regarding which configurations are the most adequate for the photocatalytic reactors to photoconvert air borne pollutants (Ollis and Al-Ekabi, 1993). Several options have been described (Chapter 11) fluidized bed (Brucato et al., 1992 Dibble and Raupp, 1992 Yue et al., 1983 ), annular packed bed (Raupp et al., 1997), coated honeycomb (Sauer and Ollis, 1994 Suzuki et al., 1991 Suzuki, 1993), fixed powder layer (Formenti et al., 1971 Peral and Ollis, 1992) and fiber optic reactor (Peill and Hoffmann, 1995). The Photo-CREC-Air reactor (Ibrahim and de Lasa, 2002) optimizes TiOa-mesh iiradiation and air contacting the supported TiOa. As reported in the upcoming section, this configuration yields model pollutant conversions with high apparent quantum efficiencies. 

APPARENT QUANTUM EFFICIENCY IN PHOTO-CREC-AIR REACTORS... 

Regarding the number of pollutant molecules converted per unit time involved in the apparent quantum efficiency, (papp can be evaluated using the initial reaction rate (Vapp.in) (Chapter VI) ... 

Using the data reported in Figure 8.2b and equation (8-19) the average apparent quantum efficiencies for acetone, (papp,av are evaluated in the 28-36% range (Table 8.5). 

The calculated (papp.av are slightly affected by the initial concentration as well as by the type of catalyst, with Hombikat UV-100 displaying slightly higher average apparent quantum efficiencies. These oscillations are in the range of the experimental error. 

In addition, considering the initial photoconversion rates, apparent quantum efficiencies, (papp.in were assessed to be in the 50-55% range. These apparent quantum efficiencies are approximately 1.5 times higher than the (papp.av obtained using equation (8-19). 

On the basis of the acetaldehyde conversion data reported in Figures 8.4 and 8.5b, the average apparent quantum efficiencies can be calculated and they surpass the 100% level for both Degussa P25 and Hombikat UV-100 (Table 8.6). 

This (Papp.av in excess to 100% is consistent with apparent quantum efficiencies reported in the literature with the high apparent quantum yields being justified on the basis of the potential role assigned in the oxygenate photoconversion to a free radical chain mechanism (Cassano etal., 1995 Jacoby etal., 1995 Luo and Ollis, 1996 Nimlos et ai, 1996 Raupp and Junio, 1993 Sauer and Ollis, 1994). Furthemiore, the (Papp.tn apparent quantum efficiencies for acetaldehyde were in the 345-450% range as reported in Table 8.6. These (Pappjn values are about 3-3.5 times larger than the (papp.av obtained using equation (8-19). 

TABLE 8.6. Apparent Quantum Efficiencies of Acetaldehyde Photodegradation on Degussa P25 and Hombikat UV-100 Using the 90% Conversion (Papp.av) and the Initial Photoconversion Rates (Pappjn)... 

Neither temperature of leaf expansion or the measurement temperatures (3 and 16C1 affected the apparent quantum efficiency or light saturated rates of whole leaf OU2 uptake measured as umol OO2 m s (Table 1). Apparent quantum efficiencies ranged from 0.037 to 0.043 + 0.004 umol 002/uB m s " PPFD and light saturated rates of about 23 umol OO2... 

---