Water decomposition modeling

Photopolymerizable coatings relief-image-forming systems, 6,125 Photoreactivity environmental effects, 1, 394 Photoredox properties bipyridyl metal complexes, 2, 90 Photoresist systems, 6,125 Photosensitive materials, 6, 113 Photosynthesis anoxygenic, 6, 589 magnesium and manganese, 6, 588 water decomposition models, 6, 498... 

In order to reproduce the temporal behavior of water decomposition products, two theoretical approaches based on spur diffusion model and Monte Carlo calculations have been developed. 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

In this review article, the functions of polymers and molecular assemblies for solar energy conversion will be described including photochemical conversion models, elemental processes for the conversion such as charge separation, electron transfer, and catalysis for water decomposition, as well as solar cells. 

Fig. 15.11 A. model of photocatalytic active sites of Ru02-loading BaTi409 and M2Ti6013(M = Na,K,Rb) titanates for water decomposition.

Finally it is worthwhile noting that MV2+ (also known as paraquat) is an effective herbicide which operates by intercepting an electron from photoexcited chlorophyll in photosystem I, preventing the plant from utilizing solar energy. There then exists an obvious parallel between the behaviour of MV2+ as a herbicide and its behaviour as an electron-transfer reagent in model systems for photochemical water decomposition. 

PHREEQC always refers to one liter or one kg of water. The model describes a batch reaction with 1 liter water. 10 mmol calcite as well as 1 mol pyrite and 1 mol organic matter shall be present in the respective sediment/rock. To describe the kinetics of calcite and pyrite, the BASIC program given at the end of the data set PHREEQC.dat is used. For the degradation of organic matter the PHREEQC.dat notation is used, too. However, the lines 50 and 60 have to be changed as follows to accelerate the decomposition of the organic matter. Nitrate is not taken into account in this example. 

Another possible correlation between coal structure and pyrolysis behavior is indicated by the temperature dependence of the evolution of pyrolytic water being strikingly different for the two coals. Figure 5 shows pyrolytic water evolution data for experiments in which the sample was heated at 1000°C/sec to the peak temperature indicated on the abscissa and then immediately allowed to cool at around 200°C/sec. The smooth curves are based on a single reaction, first-order decomposition model (7,8) and on the stated temperature-time history. Parameters used for the lignite have been published (8) while for the bituminous coal the Arrhenius frequency factor and activation energy were taken as 1013 sec"1 and 35 kcal/mol, respectively, with the yield of pyrolytic water ultimately attainable estimated from experimental measurements as 4.6 wt % of the coal (as-received). 

The explanation of this behavior, which has paramount importance to the nuclear industry, was given by Allen [3]. After World War II, Allen depicted a model of water decomposition under radiation that considers the production and consumption of Hj. The key role of OH, H2 and O2 involved in the chain as a carrier or a breaker is clear (see Inset). Within this chain reaction, the reaction between H2 and O2 (which is thermodynamically favorable) takes place in water at high temperature only in the presence ofa catalyst such as copper or silver cations. In the radiolysis of water, the reaction can take place at room temperature in the presence of free radicals which form the molecular products H2, H2O2 and O2 at the first step inside the tracks or the spurs. Subsequently in the bulk of the solution, the free radicals which have escaped recombination in the tracks recombine as molecular products into water. The molecular products are formed in the nanosecond range and their recombination takes place in the millisecond range. 

Fig. 15.5 Model of the decomposition of organic substance by dissolved oxygen and dissolved nitrate in a diffusion controlled pore water profile. Modeling was performed according to the Press-F9-method with the spreadsheet software Excel . Details pertaining to this model are explained in Table 15.5 and in the text.

Relevant to water radiolysis in nuclear reactor, G-values of the water decomposition by fast neutrons have been determined by using a fast reactor at elevated temperatures [59]. Since fast neutron radiolysis is equivalent to proton radiolysis because of the recoil proton formation through the elastic collision of fast neutrons with H2O molecules [60], an alternative approach as a model experiment is the ion beam radiolysis with different LET particles from accelerators at elevated temperatures [61]. 

For the modeling of the decomposition of glycerol in near- and supercritical water on the basis of elementary reactions, we assumed a combination of a free-radical thermal decomposition model and an acid-catalyzed ionic decomposition model [128]. This is supported by some studies in the hterature, which also include the... 

Fig. 3-3. The reactions of formation and decomposition of water shown with molecular models.

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

Figure 5. Cartoon models of the reaction of methanol with oxygen on Cu(llO). 1 A methanol molecule arrives from the gas phase onto the surface with islands of p(2xl) CuO (the open circles represent oxygen, cross-hatched are Cu). 2,3 Methanol diffuses on the surface in a weakly bound molecular state and reacts with a terminal oxygen atom, which deprotonates the molecule in 4 to form a terminal hydroxy group and a methoxy group. Another molecule can react with this to produce water, which desorbs (5-7). Panel 8 shows decomposition of the methoxy to produce a hydrogen atom (small filled circle) and formaldehyde (large filled circle), which desorbs in panel 9. The active site lost in panel 6 is proposed to be regenerated by the diffusion of the terminal Cu atom away from the island in panel 7.

Combinations of hydrogen peroxide, sulfuric acid, and urea have been proposed [1]. The temperature influences the urea decomposition into ammonia and carbon dioxide that provokes pressure buildup in a formation model and a 19% increase of oil-displacement efficiency in comparison with water. 

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