http://publicationslist.org/data/meng.chong/ref-18/sdarticle.pdf

Recent Developments in Photocatalytic Water Treatment Technology: A Review

Photoinduced reactivity of titanium dioxide

Over the past several years, cerium oxide and CeO2-containing materials have come under intense scrutiny as catalysts and as structural and electronic promoters of heterogeneous catalytic reactions. Recent developments regarding the characterization of ceria and CeO2-containing catalysts are critically reviewed with a special focus towards catalyst interaction with small molecules such as hydrogen, carbon monoxide, oxygen, and nitric oxide. Relevant catalytic and technological applications such as the use of ceria in automotive exhaust emission control and in the formulation of SO x reduction catalysts is described. A survey of the use of CeO2-containing materials as oxidation and reduction catalysts is also presented.

Catalytic Properties of Ceria and CeO2-Containing Materials

Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends

Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack

Adsorption isotherm data have been determined at 295 K for salicylic (SA), 4-aminobenzoic (ABZA) and 3-chloro-4-hydroxybenzoic (CHBZA) acids onto TiO2 particles suspended in dilute aqueous solution. Good fit is achieved between these dark-adsorption data and linearised isotherm expressions based on competitive adsorption–desorption equilibria involving competition between solute and solvent. Such linear analyses yield values for K, the equilibrium constants, and also for (ns2)max, the limiting extents of solute adsorption, attainable in competition with the solvent. The latter values do not correspond to occupancy of each surface O—Ti—O site by a single adsorbed solute, but rather indicate that, on average, each adsorbed acid molecule coexists with ca. 40 adsorbed H2O molecules. Concentration dependences predicted for rates of photodegradation of SA, ABZA and CHBZA on the basis of the experimentally determined adsorption parameters, plus an assumption that a pseudo-first-order Langmuir–Hinshelwood-type rate expression applies to TiO2-sensitised photodegradations, differ markedly from initial rates observed for the latter at 297.5 ± 2.5 K. Consideration is given to various mechanistic sources of such discrepancy.

Factors influencing efficiencies of TiO2-sensitised photodegradation. Part 1.—Substituted benzoic acids: discrepancies with dark-adsorption parameters

Continuous flows of a standard reactant mixture, featuring 0.6% nitric oxide, 0.6% ammonia and 3.3% oxygen at moderate space velocities over 4 different catalysts, have been used to compare relative activities for selective catalytic reduction of NO x at 373–773 K. The catalysts tested were: Cu-ZSM-5 featuring > 100% ion-exchange; a conventionally prepared vanadia-titania-tungstate (VTT) material and two unconventional catalysts prepared by vanadia deposition onto ex-sol-gel WO 3 TiO 2 supports. At catalytic temperature 473 K, higher conversion to N 2 was achieved over Cu-ZSM-5 than over the other three materials. Tests without NO at 473 K showed insignificant contributions to N 2 formation from ammonia oxidation over any of the catalysts, whereas tests at 573, 623, 673 and 773 K revealed larger progressive increases in such contributions over Cu-ZSM-5 than over the other catalysts. Values for SCR activities corrected for such contributions demonstrated that activity of Cu-ZSM-5 for SCR conversion of the standard NO + NH 3 + O 2 reactant mixture to N 2 at 473 K was ca. twice as great as the other three catalysts at that temperature, but that increasing the reaction temperature to 573 K caused only a slight further increase. ‘Corrected’ SCR activities in the standard reactant mixtures were rather similar for all four materials at 573 K, but with Cu-ZSM-5 marginally out-performed by one of two unconventional catalysts featuring vanadia upon an ex-sol-gel WO 3 TiO 2 support having tungsten incorporated into the TiO 2 anatase structure. Both of these unconventional catalysts outperformed a conventional ‘VTT’ catalyst. Observations upon variations in conversion to N 2 with variation in the oxygen content of the reactant gas mixture from 1 to 6% established another unique feature of the Cu-ZSM-5 catalyst at 473 K, viz. the need for ca. 4.5% O 2 to raise conversion to the maximum attainable over that catalyst at this temperature. No deactivation was observed after short-term runs at temperatures up to 823 K. Introduction of water vapour into the standard reactant mixture slightly enhanced the activity of Cu-ZSM-5 at 473 K.

Conditions in which Cu-ZSM-5 outperforms supported vanadia catalysts in SCR of NOxby NH3

Results are presented for the equilibrium extent of dark adsorption and the initial rates of TiO2(P25)-sensitized photocatalytic decomposition (PCD) for monochlorophenols (Cp) in aqueous suspension at various initial concentrations. When a high flux (2×1018 min−1 per 30 ml of suspension) of photons at 365±10 nm was used, a zero-order dependence of PCD on eq[4-Cp]aq was observed, allied to a low apparent efficiency. Apparent efficiencies were much higher under a much lower UV flux (1.7×1016 min−1) and the dependence of the initial rate on eq[4-Cp]aq then resembled that frequently associated with a Langmuir—Hinshelwood-type saturation. However, the equilibrium extent of dark adsorption of the monochlorophenols onto TiO2 determined experimentally was very small at eq[Cp]aq⩽100 ppm, above which it increased slowly in a manner consistent with an adsorption isotherm of the multilayer type. Additions of Ag+ to the suspension greatly increased the inital PCD rates and the apparent efficiency attainable with a high photon flux, such increases being consistent with decreased hole—electron recombination due to electron localization on the adsorbed Ag+. Results are also reported from the competition-type kinetic experiments at moderate intensities in which 3-chloro-4-hydroxybenzoic acid (CHBzA), isopropanol or furfuryl alcohol were added to the suspensions to compete against 4-Cp for holes and/or OH. radicals photogenerated at the TiO2—aqueous solution interface. These results cannot be rationalized solely in terms of hole scavenging by well-adsorbed CHBzA or of homogeneous scavenging of OH. radicals by alcohols in the bulk aqueous phase.

Interrelationships between pollutant concentration, extent of adsorption, TiO2-sensitized removal, photon flux and levels of electron or hole trapping additives 1. aqueous monochlorophenol—TiO2(P25) suspensions

Aerobic and anaerobic TiO2-photocatalysed purifications of waters containing organic pollutants

The n-type semiconducting oxides ZnO and TiO 2 have been used in aqueous suspensions to photosensitize the photo-oxidation of the secondary aliphatic alcohols isopropanol, cyclopentanol and cyclobutanol at 002 M High sensitivity and selectivity in detection of carbonyl photoproduct during the initial stages of photo-oxidation was achieved by derivatization and high performance liquid chromatography analysis Comparisons of the initial rates of growth of acetone photoproduct from oxygenated suspensions containing respectively (CH 3 ) 2 CHOH and (CD 3 ) 2 CDOD demonstrated the absence of any significant primary H-D isotope effect However, comparisons between initial rates of photo-oxidation of (CD 3 ) 2 CDOD at 002 M in oxygenated suspensiosn prepared respectively with H 2 O and D 2 O as the aqueous matrix did demonstrate the existence of sizeable H 2 O-D 2 O isotope effects, ie 43 for suspensions containing ZnO and 28 for suspensions containing TiO 2  An H 2 O-D 2 O effect of similar magnitude was found for cyclobutanone product from cyclobutanol On the basis of these results for H-D and H 2 O-D 2 O isotope effects (and of converse observations when (UO 2 2+ ) * was employed as photosensitizer in homogeneous aqueous solutions of the alcohols) it is concluded that photogeneration of hydroxyl radicals from water molecules is the rate-determining step for alcohol photo-oxidation in oxygenated aqueous solution

Joseph Cunningham

Papers

Factors influencing efficiencies of TiO2-sensitised photodegradation. Part 1.—Substituted benzoic acids: discrepancies with dark-adsorption parameters

Conditions in which Cu-ZSM-5 outperforms supported vanadia catalysts in SCR of NOxby NH3

Interrelationships between pollutant concentration, extent of adsorption, TiO2-sensitized removal, photon flux and levels of electron or hole trapping additives 1. aqueous monochlorophenol—TiO2(P25) suspensions

Aerobic and anaerobic TiO2-photocatalysed purifications of waters containing organic pollutants

Isotope-effect evidence for hydroxyl radical involvement in alcohol photo-oxidation sensitized by TiO2 in aqueous suspension