Vanishing of Current-Doubling Effect in Photooxidation of 2-Propanol on TiO2 in Solutions Containing Fe(III) Ions

A Light(er) Approach for the Selective Hydrogenation of 5-Hydroxymethylfurfural to 2,5-Bis(hydroxymethyl)furan without External H2

The selective conversion of 5-hydroxymethyfurfural (HMF), a biomass-derived platform molecule, to value added chemicals can ease the burden on petroleum-based fine chemical synthesis. The active contribution of renewable energy sources along with low cost, environmental friendliness, and a simple reaction system must be adopted for better sustainability. In this context, photocatalytic selective hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved over P25 titania nanoparticles without chemical squander. Simultaneously the photo-oxidation of p-methoxybenzyl alcohol (MeOBA) to p-methoxybenzaldehyde (MeOBaL), similar to biomass-derived vanillin, was carried out, abolishing the need of additional redox reagents. This system put forward the competent employment of photogenerated excitons for the valorization of lignocellulosic biomass to fine chemicals, which is an urgent requirement for sustainable chemical synthesis.

Why co-catalyst-loaded rutile facilitates photocatalytic hydrogen evolution

The photocatalytic H₂ evolution on co-catalyst loaded titania is interpreted by a new mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction.

Role of dissolved oxygen on the degradation mechanism of Reactive Green 19 and electricity generation in photocatalytic fuel cell

In this study, a membraneless photocatalytic fuel cell with zinc oxide loaded carbon photoanode and platinum loaded carbon cathode was constructed to investigate the impact of dissolved oxygen on the mechanism of dye degradation and electricity generation of photocatalytic fuel cell. The photocatalytic fuel cell with high and low aeration rate, no aeration and nitrogen purged were investigated, respectively. The degradation rate of diazo dye Reactive Green 19 and the electricity generation was enhanced in photocatalytic fuel cell with higher dissolved oxygen concentration. However, the photocatalytic fuel cell was still able to perform 37% of decolorization in a slow rate (k = 0.033 h^-1) under extremely low dissolved oxygen concentration (approximately 0.2 mg L^-1) when nitrogen gas was introduced into the fuel cell throughout the 8 h. However, the change of the UV-Vis spectrum indicates that the intermediates of the dye could not be mineralized under insufficient dissolved oxygen level. In the aspect of electricity generation, the maximum short circuit current (0.0041 mA cm^-2) and power density (0.00028 mW cm^-2) of the air purged photocatalytic fuel cell was obviously higher than that with nitrogen purging (0.0015 mA cm^-2 and 0.00008 mW cm^-2).

Renewable energy production by photoelectrochemical oxidation of organic wastes using WO3 photoanodes

The present work has studied renewable hydrogen production by photoelectrocatalytic degradation of model organic substances representing biomass derived organic wastes. Its purpose was to show that renewable energy can be produced by consuming wastes. The study has been carried out by employing nanoparticulate WO₃ photoanodes in the presence of ethanol, glycerol or sorbitol, i.e. three substances which are among typical biomass products. In these substances, the molecular weight and the number of hydroxyl groups increases from ethanol to sorbitol. The photocurrent produced by the cell was the highest in the presence of ethanol, smaller in the case of glycerol and further decreased in the presence of sorbitol. The photocurrent was roughly the double of that produced in the absence of an organic additive thus demonstrating current doubling phenomena. Hydrogen was produced only under illumination and was monitored at two forward bias, 0.8 and 1.6V vs Ag/AgCl. Hydrogen production rates followed the same order as the photocurrent thus indicating that hydrogen production by reduction of protons mainly depends on the current flowing through the external circuit connecting photoanode with cathode. The maximum solar-to-hydrogen efficiency reached by the present system was 2.35%.

Unravelling Site-Specific Photo-Reactions of Ethanol on Rutile TiO2(110)

Finding the active sites of catalysts and photo-catalysts is crucial for an improved fundamental understanding and the development of efficient catalytic systems. Here we have studied the photo-activated dehydrogenation of ethanol on reduced and oxidized rutile TiO₂(110) in ultrahigh vacuum conditions. Utilizing scanning tunnelling microscopy, various spectroscopic techniques and theoretical calculations we found that the photo-reaction proceeds most efficiently when the reactants are adsorbed on regular Ti surface sites, whereas species that are strongly adsorbed at surface defects such as O vacancies and step edges show little reaction under reducing conditions. We propose that regular Ti surface sites are the most active sites in photo-reactions on TiO₂.

Improving the photoelectrocatalytic performance of boron-modified TiO2/Ti sol–gel-based electrodes for glycerol oxidation under visible illumination

B-TiO₂ photoelectrodes showed a high photoelectrocatalytic performance under visible illumination due to efficient electron transport and charge carrier separation.

An unexpected fluctuating reactivity for odd and even carbon numbers in the TiO2-based photocatalytic decarboxylation of C2-C6 dicarboxylic acids

The degradation behaviours of five straight-chain dicarboxylic acids (from ethanedioic acid to hexanedioic acid) were compared in aqueous TiO2-based photocatalysis. When all other conditions were identical, the degradation rates were found to fluctuate regularly with the parity of the number of carbon atoms. Dicarboxylic acids with an even number of carbon atoms (e-DAs) always degraded more slowly than those acids with an odd number of carbon atoms (o-DAs). This unusual fluctuation in the reactivity for the degradation of dicarboxylic acids by TiO2-based photocatalysis is very closely related to the different pre-coordination modes of the acids with the photocatalyst. Attenuated total reflection FTIR (ATR-FTIR) of e-DAs labelled with (13)C showed that both carboxyl groups of the acid coordinate to TiO2 through bidentate chelating forms. In contrast, only one carboxyl group of the o-DAs coordinated to TiO2 in a bidentate chelating manner, whereas the other formed a monodentate binding linkage. The bidentate chelating form with bilateral symmetric coordination did not favour degradation. Isotope-labelling experiments were performed with (18)O2 to observe the different ways in which incorporated oxygen entered the initial decarboxylated products of e- and o-DAs. For the degradation of butanedioic acid, (45.9±0.5) % of the oxygen in the formed propanedioic acid came from H2O, whereas for pentanedioic acid, (97.4±0.2) % of the oxygen in the formed butanedioic acid came from H2O. Our results demonstrate that in TiO2-based photocatalysis, the reactivity of active species, such as ·OH/h(vb)(+), is far from non-selective and that the attacks of these active species on organic substrates are significantly affected by the coordination patterns of the substrates on the TiO2 surface.

Hierarchal Light-Harvesting Aggregates and Their Potential for Solar Energy Applications

The tunable optical properties of self-assembled chromophores are exploited by photosynthetic organisms to optimize their ability to harvest a broad range of the solar spectrum. Similarly, the efficiency of solar photovoltaic and photoelectrochemical devices depends strongly on the coincidence of the absorption spectrum of the photoactive components with the spectrum of the sun. While the possibility of borrowing ideas about light-harvesting aggregates from nature in order to improve the efficiency of solar energy conversion is quite attractive, progress to date is hindered by incomplete understanding of aggregate internal structure and its relation to excitonic states. In this Perspective, we describe our recent work on the hierarchal structure of self-assembled porphyrin aggregates that are similar to light-harvesting complexes of photosynthetic bacteria. We address the question of whether aggregation can be beneficial to dye-sensitized solar energy conversion and present promising results for a solar cell based on an abundant plant pigment that displays signatures of aggregation when adsorbed on TiO2.

Production of electricity and hydrogen by photocatalytic degradation of organic wastes in a photoelectrochemical cell: the concept of the Photofuelcell: a review of a re-emerging research field

The present review aims to give to a researcher who has no experience with Photofuelcells all necessary basic knowledge to join the field without much trouble and to give to an experienced researcher a handy manual of reference. The author has dealt with the principal matters related with the design of a photoelectrochemical cell and the factors that affect efficient production of electricity by photocatalytic degradation of (principally) organic and (secondarily) inorganic waste materials. A large portion of the paper is devoted to the review of materials used for making a photoanode since most of the accomplished research is on this exact matter. The paper also briefly reviews the materials used to make the rest of the components of the cell as well as the models of cell efficiency and photodegradation procedures during cell operation.

Time-resolved chemiluminescence study of the TiO2 photocatalytic reaction and its induced active oxygen species

The time-resolved chemiluminescence (CL) method has been applied to study the TiO(2) photocatalytic reaction on a micros-ms timescale. The experimental set-up for time-resolved CL was improved for confirmation of the unique luminol CL induced by the photocatalytic reaction. The third harmonic light (355 nm) from an Nd:YAG laser was used for the light source of the TiO(2) photocatalytic reaction. Luminol CL induced by this reaction was detected by a photomultiplier tube (PMT) and a preamplifier was used for amplifying the CL signal. Experimental conditions affecting the photocatalytically induced CL were discussed in detail. The involvement of active oxygen species such as .OH, O(2) (.-) and H(2)O(2) in the CL was examined by adding their scavengers. It is concluded that .OH was greatly involved in the CL on a micros-ms timescale, especially in time periods <100 micros after illumination of the pulse laser. On the other hand, CL generated by O(2) (.-) began to increase after 100 micros and became dominant after 2.5 ms. A small part of the CL might be generated by H(2)O(2) on the whole micros-ms timescale. A CL reaction mechanism related with .OH and dissolved oxygen was proposed to explain the photocatalytically induced luminol CL on a micros-ms timescale, especially in periods <100 micros.

TiO2-photocatalyzed As(II) oxidation in aqueous suspensions: reaction kinetics and effects of adsorption

Oxidation of arsenite, As(III), to arsenate, As(V), is required for the efficient removal of arsenic by many water treatment technologies. The photocatalyzed oxidation of As(III) on titanium dioxide, TiO2, offers an environmentally benign method for this unit operation. In this study, we explore the efficacy and mechanism of TiO2-photocatalyzed As(III) oxidation at circumneutral pH and over a range of As(III) concentrations approaching those typically encountered in water treatment systems. We focus on the effect of As adsorption on observed rates of photooxidation. Adsorption (in the dark) of both As(III) and As(V) on Degussa P25 TiO2 was examined at pH 6.3 over a range in dissolved arsenic concentrations, [As]diss, of 0.10-89 microM and 0.2 or 0.05 g L(-1) TiO2 for As(III) and As(V), respectively. Adsorption isotherms generally followed the Langmuir-Hinshelwood model with As(III) exhibiting an adsorption maxima of 32 micromol g(-1). As(V) adsorption did not reach a plateau under the experimental conditions examined; the maximum adsorbed concentration observed was 130 micromol g(-1). The extent of As(III) and As(V) adsorption observed at the beginning and end of the kinetic studies was consistent with that observed in the adsorption isotherms. Kinetic studies were performed in batch systems at pH 6.3 with 0.8-42 microM As(III) and 0.05 g L(-1) TiO2; complete oxidation of As(III) was observed within 10-60 min of irradiation at 365 nm. The observed effect of As(III) concentration on reaction kinetics was consistent with surface saturation at higher concentrations. Addition of phosphate at 0.5-10 microM had little effect on either As(III) sorption or its photooxidation rate but did inhibit adsorption of the product As(V). The selective use of hydroxyl radical quenchers and superoxide dismutase demonstrated that superoxide, O2-, plays a major role in the oxidation of As(III) to As(V).

Visible-light-assisted degradation of dye pollutants over Fe(III)-loaded resin in the presence of H2O2 at neutral pH values

A novel catalyst was synthesized by direct exchange of ferric ions onto a cationic resin (Amberlite IRA200). Upon visible light irradiation (lambda > 420 nm) in the presence of H2O2, this catalyst was found to be highly effective for the degradation of nonbiodegradable cationic dyes, Malachite green, Rhodamine B, and Methylene blue, even at neutral pH values. It was also easy to separate from the degraded solution. By total organic carbon, FT-IR, and GC-MS analysis, the degradation process of Malachite green was shown to proceed with demethylation and phenyl ring openings into CO2 and small molecular compounds. EPR studies revealed that *OH radicals, other than *OOH/O2*-, were involved as the active species. A possible reaction mechanism is proposed on the basis of all the information obtained under various experimental conditions.