A Kinetic Approach to Photomineralization of Methane in Air by Membranes Based on TiO2/WO3

Photomineralization of methane in air (10.0-1,000 ppm (mass/volume) of C) at 100% relative humidity (dioxygen as oxygen donor), was systematically studied at 318 \ub1 3 K, in an annular laboratory-scale reactor, by photocatalytic membranes immobilising titanium dioxide and tungsten trioxide as co-photocatalysts. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed. A kinetic model was employed, from which, by a set of differential equations, four final optimised parameters, k1 and K1, k2 and K2, were calculated, able to fit the whole kinetic profile adequately. Modelling of quantum yields, as a function of substrate concentration and irradiance, as well as of concentration of photocatalysts, was carried out very satisfactorily. Kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading to mineralization, were considered, paralleled by second competition kinetics involving superoxide radical anion. When using appropriate blends of the two photocatalysts, limiting quantum yields \uf046 1e values increase considerably and approach the maximum allowable value for the investigated molecule, in a much wider range of irradiances than that shown by the single catalysts mainly at low irradiances. This may be interpreted by strong competition kinetics of superoxide radicals generated by the catalyst defects, in the corresponding range of high irradiances. By this way, operation at high irradiance values is possible, without losing any efficiency for the mineralization process

Laboratory-scale photomineralization of n-alkanes in gaseous phase by photocatalytic membranes immobilizing titanium dioxide

Kinetics of photocatalytic oxidation of methane, ethane, and n-heptane, to yield intermediates, and photomineralization of intermediates, to yield carbon dioxide and water, was studied in the gaseous phase, at 308 +/- 2 K, by a laboratory-scale photoreactor and photocatalytic membranes immobilizing 30 +/- 3wt.% of TiO2, in the presence of aerosolized stoichiometric hydrogen peroxide as oxygen donor, and at a relative humidity close to 100%. The whole volume of irradiated solution was 4.000 +/- 0.005 L, the ratio between this volume and the geometrical apparent surface of the irradiated side of the photocatalytic membrane was 3.8 +/- 0.1 cm, and the absorbed power was 0.30 W/cm ( cylindrical geometry). The pinetic parameters of the present work substantially coincide with those of the same molecules previously studied in aqueous solution, within the limits of experimental uncertainty. Photocatalytic processes thus appear to be controlled by interface phenomena, which are ruled kinetically, and apparently also thermodynamically, by concentration gradients, independently on diffusion and other processes in the aqueous or gaseous bulk, if turbulence in these phases is adequately assured

PHOTOSYNTHETIC MEMBRANES .21. IMMOBILIZATION OF CATALASE BY PHOTOCHEMICALLY GRAFTED ULTRAFILTRATION MEMBRANES

Catalase has been immobilized in membranes prepared by photoinduced grafting onto microporous polymeric supports and its catalytic activity on hydrogen peroxide decomposition has been studied under ultrafiltration conditions by means of a recirculation apparatus. The membranes showed a very good catalytic performance and the enzyme reaction took place exclusively within the membrane structure. Initial reaction rates measured in the temperature range 5-35-degrees-C as a function of both substrate concentration and enzyme amount immobilized per unit membrane surface indicate that the mechanism of action of catalase is not altered after immobilization

Photobleaching and photomineralization of azobenzene and substituted azobenzenes in aqueous solution by photocatalytic membranes immobilizing titanium dioxide

The kinetics of photobleaching (by spectrophotometric anal.) and integral photomineralization (by total org. carbon (TOC) anal.) of azobenzene (I) and substituted azobenzenes in aq. soln. were followed in lab.-scale(coating process) runs on photocatalytic membranes immobilizing 30\ub13 wt. of semiconductor TiO2. Expts. were carried out by the technique described in preceding papers of this series, employing stoichiometric hydrogen peroxide as the oxygen donor. The following azobenzenes were examd.: (4-diethylamino)-phenylazobenzene (II), 4'-(((4-diethylamino)phenyl)azo) benzoic acid (III), 4'-(((2-amino-5-diethylamino)phenyl)azo) benzoic acid (IV), 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzoic acid (V), 4'-(((4-dimethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VI) and 4'-(((2-acetamido-4-diethylamino)phenyl)azo) benzenesulfonic acid, sodium salt (VII). From the Langmuir-Hinshelwood treatment of the initial rate data as a function of the initial concn. ((0.10-1.0) 710-3 M), the kinetic parameter k and the pseudo-thermodn. parameter K for photobleaching were obtained. With regard to photobleaching, I and II were certainly the most reactive, followed by IV. The remaining mols. showed a photo-oxidn. rate of one-third to one-quarter of that of I chosen as ref. structure. Consequently, the presence of an amino group in the 4-position (II) does not stabilize the azobenzene structure against photo-oxidn. leading to bleaching, whereas the same group in the 2-position (IV) decreases the photobleaching rate by about 40 when a carboxylic group is also present in the 1'-position. Acetylation of this amino group, such as in V, decreases the photobleaching rate more markedly. With regard to photomineralization, it was obsd. that, when photobleaching was virtually complete, a certain amt. of TOC was already mineralized. The max. amt. of TOC remaining at the end of photobleaching ranged from about 90 to about 30, varying with the dye structure and initial concn. as well as with the power and type of irradn. source. By examg. the TOC concn. profiles as a function of the substituted azobenzene structure, the following hypotheses were proposed: (1) during the photobleaching period, the ring contg. the diethylamino group breaks down (more markedly if further amino or acetamido groups are present in the structure), and photomineralization of the other ring occurs more slowly; (2) both rings break down, within certain limits, during photobleaching; however, the aliph. fragments contg. carboxyl or sulfonic groups are mineralized more slowly. The fact that a small initial plateau in the TOC profile is followed by another more evident plateau at the end of the photobleaching period, for both VI and VII, suggests that hypothesis (1) is more probable in these cases. When the second plateau is reduced to a sigmoidal curve or an inflection point, hypothesis (2) also needs to be considered

Electrolyte permeation through photo-cured acrylic polymers on steel as related to corrosion of the substrate

The effect of 1,2-diphenyl, 2,2-dimethoxy, ethanone as photoinitiator in some ultraviolet radiation-cured acrylated acrylic formulations on steel was studied. Correlation between corrosion rates, measured with ASTM D610 standardized method, and electrolyte permeation through coatings, measured with 36Cl-labelled 0.25 M NaCl solutions at 25\ub0C, was systematically investigated. The length of time necessary for the complete failure of the coatings, as observed by the extension of corrosion products over the entire surface, was evaluated by visual inspection of corrosion or by the breakthrough curve of permeation, and found to be a function of photoinitiator concentration. The slight difference between failure time obtained by the two methods may well be due to a time lag between the activation of corrosion cells over the whole surface and onset of visible corrosion products everywhere. This time lag may be correlated to the protective value of the coating, and to its permeation properties, particularly

Photosynthetic membranes IV: kinetic investigation of photoinitiated grafting and photo-cross-linking of an epoxydiacrylate resin onto cellulose

Photochemical grafting onto cellulose and successive photo-cross-linking of 2.00 - 37.00 mg cm-2 of an epoxydiacrylate prepolymer based on glycidyl ether of bisphenol A, containing 25 wt./wt.% trimethylolpropane triacrylate, were investiated kinetically at 30 \ub0C in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as a photoinitiator, with the weight concentration ratio of photoinitiator to prepolymer varied between 0.070 and 1.115. Irradiations were carried out polychromatically, in air or under a stream of nitrogen, with incident radiation of flux I ranging from 2.1 7 10-8 to 20.5 7 10-8 einsteins s-1 cm-2. Graft yields as a function of irradiation time were determined gravimetrically: two consecutive kinetic processes were observed which had rates linearly dependent on I. The quantum yields \u3a61 of the first constant-rate period showed a linear dependence on photoinitiator concentration, while the quantum yields \u3a62 for the second constant-rate process showed an inverse dependence on n/S (moles of prepolymer initially deposited per unit apparent cellulose surface). Kinetic analysis of the degree of unsaturation by multiple internal reflection IR spectroscopy was carried out in parallel and the results indicated that a structure with one pendant acrylate group per monomer unit may be assigned to the grafted polymer. At longer irradiation times, further decrease in unsaturation resulting from photo-cross-linking was observed which had a rate linearly dependent on I0.5, suggesting a bimolecular termination for this process. In the light of this information, the mechanism of photografting of poly-functional acrylate monomers onto polymeric surfaces is discussed.Photochemical grafting onto cellulose and successive photo-cross-linking of 2.00 - 37.00 mg cm-2 of an epoxydiacrylate prepolymer based on glycidyl ether of bisphenol A, containing 25 wt./wt.% trimethylolpropane triacrylate, were investiated kinetically at 30 \ub0C in the presence of 1,2-diphenyl-2,2-dimethoxyethanone as a photoinitiator, with the weight concentration ratio of photoinitiator to prepolymer varied between 0.070 and 1.115. Irradiations were carried out polychromatically, in air or under a stream of nitrogen, with incident radiation of flux I ranging from 2.1 7 10-8 to 20.5 7 10-8 einsteins s-1 cm-2. Graft yields as a function of irradiation time were determined gravimetrically: two consecutive kinetic processes were observed which had rates linearly dependent on I. The quantum yields \u3a61 of the first constant-rate period showed a linear dependence on photoinitiator concentration, while the quantum yields \u3a62 for the second constant-rate process showed an inverse dependence on n/S (moles of prepolymer initially deposited per unit apparent cellulose surface). Kinetic analysis of the degree of unsaturation by multiple internal reflection IR spectroscopy was carried out in parallel and the results indicated that a structure with one pendant acrylate group per monomer unit may be assigned to the grafted polymer. At longer irradiation times, further decrease in unsaturation resulting from photo-cross-linking was observed which had a rate linearly dependent on I0.5, suggesting a bimolecular termination for this process. In the light of this information, the mechanism of photografting of poly-functional acrylate monomers onto polymeric surfaces is discussed

EPR INVESTIGATION OF RADICAL FORMATION AND DECAY IN PHOTOPOLYMERIZATION OF DIFUNCTIONAL MONOMERS

The formation of trapped radicals under UV irradiation during the photopolymerization of butane-1,4-diyl diacrylate in the presence of different amounts of photoinitiator, as well as the radical decay after the end of irradiation, have been studied by EPR spectroscopy. Direct evidence that higher photoinitiator radical concentrations greatly favour radical-radical encounters is obtained. The formation of the same radical species in two different polymeric phases, one essentially rigid and the other more fluid, is at the origin of the two observed superimposed EPR patterns. The ratio R between them decreases with increasing photoinitiator concentration, owing to the formation of shorter and consequently more mobile polymer chains. In contrast, the more rigid phase seems to dominate in photopolymerized diallyl 3-oxapentane-1,5-dicarbonate. ENDOR measurements further support the correlation proposed by us between EPR spectral shape and the rigidity of the polymeric phase

Energy conversion efficiency of photoelectrochemical cells employing a titanium dioxide film anodic membrane and a hydrogen or oxygen cathode

The energy conversion efficiency at wavelengths (315-420 nm) was evaluated at 25\ub12\ub0 in photoelectrochem. cells, with a TiO2 film immobilized on a poly(tetraethyelene glycol diacrylate) membrane and 1N NaOH as anolyte. In one cell, Pt black in 1N H2SO4 was the counter-electrode whereas in the other cell, a Pt black cathode in 1N NaOH worked self-regeneratively. The short-circuit c.d. and open-circuit voltage were measured. The quantum efficiency was also obtained. A comparison of the exptl. values with those of conventional cells (TiO2 electrode not immobilized in a composite membrane structure) shows that no substantial difference is obsd. (outside exptl. uncertainty) in the quantum and energy conversion efficiencies and open-circuit voltages. However, the short-circuit c.d. was systematically 13-26% lower. This small effect can be attributed to a moderate increase in resistance brought about by the polymeric membrane. The performance and feasibility of photoelectrochem. membrane cells are discussed

Immobilization of reagents and catalysts by photochemical grafting onto polymers

The possibility of immobilizing reagents and catalysts by photochemical grafting of their composites with an epoxy-diacrylate resin has been investigated. Two model systems have been examined: (i) the catalytic decomposition of aqueous sodium hypochlorite by cobalt peroxide, with oxygen production; (ii) the reaction of alkali metals with alcohols. For case (i) the catalyst was embedded, in the form of a mixture with 90% of zeolite 13X, into the photografted polymer matrix; while for case (ii) the alkali metal reagent, supported onto 90% of zeolite 13X, was incorporated into the microporous membrane. Kinetics of reaction (i) were studied in standard conditions (1.0 M NaOCl; 1.0 M NaCl; 0.25 M NaOH) between 288.2 to 318.2 K, and the relation between pseudp-first order kinetic constants and amount of catalyst incorporated into the membrane determined. Isotope effects for reaction (ii), relative to H2, D2, HD production, were measured with mixtures of deuterated and non-deuterated CH3OH(D), C2H5OH(D), n-C3H7OH(D), and i-C3H7OH(D) at 25 \ub0C, as a function of volume percent of non-deuterated alkanols in the liquid phase. For reaction (i) reactivity was found to be substantially the same in the membrane reactor as in ordinary chemical conditions: loss of catalytic activity did not exceed 15%. For reaction (ii) isotope effects, the origin of which is discussed, fully coincided with those measured in the absence of the polymer matrix

PHOTOSYNTHETIC MEMBRANES .23. DEGRADATION OF SOME CHLOROALIPHATIC WATER CONTAMINANTS BY PHOTOCATALYTIC MEMBRANES IMMOBILIZING TITANIUM-DIOXIDE

The TiO2-mediated photodegradation of chloroethanoic acid, trichloroethene and tetrachloroethene, chosen as model molecules of aliphatic chloro-organics was studied at 303 +/- 2 K by employing polymeric membranes photografted on cellulose to immobilize the TiO2 (3.0-5.6 mg cm-2). The radiant flux in the absorption range and the volume-surface ratio in the photoreactor cell were kept constant at 2.5 +/- 0.2 mW cm-2 and 1.1 +/- 0.1 mL cm-2 respectively. The initial rate of photodegradation was studied as a function of the initial concentration of reactants by the linearized form of the Langmuir-Hinshelwood equation, by which rate constants k and equilibrium adsorption constants K were evaluated. Values of k (5.7 +/- 0.5-mu-mol h-1) are independent of the chemical nature of the three reactants investigated, in agreement with other literature findings. Values of K are compared with those measured in aqueous suspensions of TiO2 with the same chloro-organics and with the values predicted by a mechanism involving hydroxyl radicals as the primary oxidant adsorbed on the photocatalyst surface. Satisfactory performance and potential technological advantages of photocatalytic membranes are discussed