Gas-phase water photolysis

Gas-phase Water Photolysis by NaOH-coated Photocatalysts... 

Water Photolysis by Metallized Semiconductor Powders 13.3.1 Gas-phase Water Photolysis by NaOH-coating... 

The above results indicate that a requirement for water photolysis by Pt/Ti02 is to prevent the reverse reaction on Pt sites. Wagner and Somoijai8) successfully carried out gas-phase water photolysis by Pt/SrTi03-crystal coated with deliquescent basic materials. Their method is reasonable to suppress the reverse reaction, because a deliquescent material coated on a substrate absorbs a large amount of water to form a thin film of its aqueous solution. The film inhibits the reaction products to readsorb directly on the catalyst, while the products on the catalyst can escape to the gas phase by diffusion, it is very important that H2 and 02 can desorb from the catalyst surface to the gas phase without making bubbles, because if they desorb as bubbles then they would inevitably mix with each other in the growing process of bubbles and recombine on Pt sites. In addition, an aqueous basic solution would work as an electrolyte which enhances ion transfer in photoelectrochemical reactions. 

When Pt/Ti02 powder is coated with NaOH and illuminated in the presence of gas-phase water, H2 and 02 are produced in a stoichiometric ratio of 2 1 even in the dry state.9,13) Rh and Pd loaded Ti02 powders also show photocatalytic activity for gas-phase water photolysis when coated with NaOH.14) The product formation rates decline with time due to the reverse reaction. The yield of gas-phase water photolysis depends on the pressure of gas-phase water, as shown in Fig. 13.4,14) Since the yield is also dependent upon the amount of NaOH coated,... 

Fig. 13.4 Dependence of the yield of gas-phase water photolysis on water vapor pressure over 1.3 wt% and 10 wt% NaOH-coated Rh/Ti02. Psat is the saturated vapor pressure at ambient temperature. H2/02 ratio is stoichiometric in all runs.

The yield of gas-phase water photolysis over NaOH-coated Pt/Ti02 increases with elevating temperature as shown in Fig. 13.8.16>33) At temperatures below or... 

To examine the occurrence of gas-phase water pnotolysis in the dry state, CO was added to the system. H2 and COz are formed ir a ratio of 2 1, indicating that /the reaction, H20 -r CO — H2 + C02, takes place.n) Since this reaction does not occur on Ti02 alone, H2 should be formed at Pt.sites and C02 at TiOz sites t>y the reaction of CO with the oxidation products of water such as oxygen moiecules or OH radicals. Interestingly, a small amount of 02 is also formed when CO pressure is low, as shown m Fig. 13.3.n) After the complete consumption of CO, 02 as well as H2 decreases. This result implies that gas-phase water photolysis can occur on Pt/Ti02 in the dry state if the reverse reaction is inhibited by, for example, CO adsorption on Pt. It was also demonstrated that the reaction of QH4 with gas-phase... 

To estimate the optimum thickness of the solution, the yield was measured as a function of tne amount of water on the catalyst. In this experiment, Pt/Ti02 powder was immersed in the measured amount of NaOH solution and the photoyield determined After the yield was measured, the solution volume was reduced by pumping for an appropriate time through an outside cold trap to measure the amount of water removed from the NaOH solution. As shown in Fig. 13.6, the yield sharply increased when the solution was reduced to a certain amount and then decreased upon further removal of water. 15-16) This result indicates that the yield is mainly influenced by the thickness of the solution on the catalyst and a concentrated NaOH solution appears to enhance the reaction. Wagner and Somoqai8) also reported that the yield of gas-phase water photolysis by NaOH-coated SrTi03 increases with increased NaOH loading. The thickness of NaOH solution at the optimum condition in Fig. 13.6 is estimated to be less than 0.1 mm. The rate constant for the reaction of H2 with 02 in the dark was measured as a function of the amount of NaOH solution. As shown m Fig. 13.7, the rate constant decreases linearly with increase in the amount of solution and drops to almost zero at 0.2 ml of the solution.165... 

KOH is a deliquescent basic electrolyte like NaOH, and its coating on Pt/Ti02 makes possible gas-phase water photolysis as described earlier for Sri i03 crystal.8) In experiments similar to that shown in Fig. 13.6, the yield is maximized at ca 0.12 ml of KOH solution.16) The maximum yield is, however, less than one half the maximum yield in NaOH solution.,6) In addition, the photocatalytic activity of Pt/Ti02 declines with illumination time in KOH solution by unknown mechanisms. 

LiOH is a nondeliquescent basic electrolyte so that a very thin film of LiOH solution is not so likely to be formed on the catalyst to achieve gas-phase water photolysis, but it may work as an electrolyte in wet-state water photolysis. When Pt/Ti02 is immersed in LiOH solution, no 02 is evolved and a small amount of H2 is produced irrespective of the amount of LiOH solution.16) Similar deactivation has been observed for Pt/Ti02 in H2S04 solution (0.1 N).16> The mechanisms of these deactivations of Pt/Ti02 are not clear. 

Furthermore, because these reactions result in the effective removal of NOx from ozone production, by removing N02, the model also predicts that O, concentrations will decrease. Figure 7.17, for example, shows the model results for the ratio of O, (R0i) with the heterogeneous removal of N03 and N2Os included to that without these aerosol reactions. In some locations, the 03 concentrations are predicted to be as much as 30% lower than they would have been in the absence of the heterogeneous reactions. Because 03 is also the major OH source on a global scale, via its photolysis to electronically excited oxygen atoms, O( D), which react in part with gas-phase water, this also decreases the predicted OH levels. 

Hashem, T.M., Zirlewagen, M., and Braun, A.M., Simultaneous photochemical generation of ozone in the gas phase and photolysis of aqueous reaction systems using one UV light source, Water Sci. Technol., 35, 41—48, 1997. 

Gas-phase reactions have been carried out in 160 mL quartz vessels, and the products analyzed online by mass spectrometry (Brubaker and Hites 1998). Hydroxyl radicals were produced by photolysis of ozone in the presence of water ... 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

Air t,/2 = 2.02-20.2 h, based on estimated sunlight photolysis half-life in water (Howard et al. 1991) calculated atmospheric lifetime of 26 h based on gas-phase OH reactions (Brubaker Hites 1998). 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

The early work on the photolysis of water was in the gas phase employing one photon. The branching ratio of the photodissociation into H + OH and H2 + O was reported by McNesby et al. [28] as 3 1 at a photon energy of 10.03 eV. Ever since, that ratio has been consistently revised in favor of the H + OH reaction with the final result of Stief et al. [29] giving 0.99 0.01 for 6.70-8.54 eV photon energy and 0.89 0.11 for the interval 8.54-11.80 eV. In the absence of direct determination these ratios often are assumed valid in the liquid phase. In the early work of Sokolev and Stein [30], mainly the photodissociation quantum yield in liquid water was measured, but a small photoionization yield of -0.05 was attributed to the process... 

First a fundamental photochemical catalysis by Ti02 powder suspension in water should be mentioned as a trial to cleave water by UV light. Platinized Ti02 powdets (Pt/Ti02) suspended in water have been expected to photolyze water to produce H2 and 02 by (JV light, and some reports appeared as described in other chapters of this book. However, established true water photolysis by UV light takes place only under special conditions, e.g., with high concentration carbonates anions in water, or with the addition of NaOH in a gas phase. 

Figure 6. Hydrogen production at 25°C in deaerated solutions as a function of catalyst (rhodium) concentration during the first two hours of irradiation using 350 nm cut-off and water filters. Plotted are the amount of hydrogen produced in 25 ml DHP vesicle solution.and measured in the gas phase (16 ml) by GC s 2 x 10- J M DHP, 2 x 10 M CdS symmetrically distributed on both sides of the vesicles, and 10 J M PhSH as electron donor pH approximately 7 at sonication and during photolysis. Concentrations of the catalyst, reduced by uv irradiation prior to visible light photolysis ( - Dl 1 ° ...

Photolysis. Photolysis of a chemical can proceed either by direct absorption of light (direct photolysis) or by reaction with another chemical species that has been produced or excited by light (indirect photolysis). In either case photochemical transformations such as bond cleavage, isomerization, intramolecular rearrangement, and various inter-molecular reactions can result. Photolysis can take place wherever sufficient light energy exists, including the atmosphere (in the gas phase and in aerosols and fog/cloud droplets), surface waters (in the dissolved phase or at the particle-water interface), and in the terrestrial environment (on plant and soil/mineral surfaces). 

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