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TiO2-based nanosheets (TNS) modified with Fe2O3 and Y2O3 particles (Fe2O3/Y2O3@TNS), possessing a laminar structure with large specific surface area of 382 m2 g−1, were synthesized via a one-pot hydrothermal method. The optimized Fe2O3/Y2O3@TNS (Fe/Y/Ti = 0.1 at%: 0.75 at%: 1) exhibited high photocatalytic efficiency due to the synergistic effect, and the photodegradation rate towards RhB and K2Cr2O7 solutions were 5.82-fold and 2.35-fold higher, respectively, as compared to TNS under visible light. Simultaneous photocatalytic degradation in the Cr(VI)–phenol coexisting system revealed that there was synergism between Cr(VI) reduction and phenol degradation for promoting photocatalytic efficiency. Besides, Fe2O3/Y2O3@TNS demonstrated strong adsorption and excellent photocatalytic regeneration performance for high-chroma crystal violet (CV), showing potential application in the removal of organic or inorganic pollutants. Based on photoluminescence (PL) spectra and photocurrent (SPC) methodology, the separation efficiency of photogenerated e−–h+ pairs could be significantly strengthened. A possible mechanism for enhanced photocatalytic efficiency under visible light was proposed. [ABSTRACT FROM AUTHOR]

Selective growth of cocatalyst on the surface of photocatalyst has been attracted considerable attention due to their efficient charges transfer property. In this study, the robust NiS modified graphitic carbon nitride (g-C3N4) hybrids were successfully synthesized by a facile surface photochemical deposition process. The structure and composition characterization results revealed that the NiS is highly dispersed loading on the surface of g-C3N4 nanosheets, and the NiS/g-C3N4 hybrids possess large surface areas and excellent optical properties. Under the visible light illumination, the NiS/g-C3N4 hybrids with 1.0% weight content of NiS cocatalyst exhibits the highest hydrogen evolution rate of 1346.1 μmol h−1 g−1 with an apparent quantum efficiency (AQE) of 7.67%. On the basis of photoluminescence (PL) spectra and photoelectrochemical methodology, the photocatalytic hydrogen evolution mechanism was proposed. The results demonstrated that the excellent activity arises from the strong electronic coupling, highly efficient charges separation and migration. This work demonstrates a facile photochemical deposition method to consciously construct the robust two-dimensional (2D) hybrids, so as to realize accurate deposition of cocatalyst and efficient migration of photo-generated carriers. [ABSTRACT FROM AUTHOR]

Two-dimensional TiO 2 -based nanosheets (TNSs) co-modified by surface-enriched carbon dots (CDs) and Gd 2 O 3 nanoparticles: (Gd-C-TNSs), capable of exhibiting visible-light-driven photo catalysis were synthesized using a two-pot hydrothermal route. The samples had a sheet-like structure, thickness of approximately 3.6 nm, large specific surface area of 240–350 cm 2 /g. The CDs (2–3 nm) and Gd 2 O 3 nanoparticles (1–2 nm) were highly dispersed over the surface of the nanosheets. The co-modification by Gd 2 O 3 nanoparticles and CDs influenced the crystallinity, crystal structure, and surface area of the TNSs, and improved the visible-light absorption. Surface photocurrent and fluorescence spectral studies revealed that the photo-generated charge carrier separation efficiency could be improved by an appropriate amount of modification. A very high efficiency was obtained using 0.5 at% Gd/Ti and 3.0 g/L of CDs. The visible-light-induced photocatalytic activity is enhanced under the isolated Cr(VI) system, isolated Rhodamin B (RhB) system, and the synergism between RhB degradation and Cr(VI) reduction for the Gd-C-TNSs photocatalysts. Initially, the photocatalytic activity gradually increased with an increase in the amount of CDs, and then decreased after attaining a maximum, in the case where 0.5 at% Gd/Ti and 3.0 g/L of CDs were used. The enhancement in the photocatalytic activity was attributed to the synergetic effect of the Gd 2 O 3 nanoparticles, TNSs, and CDs in the Gd-C-TNSs composites. The effect led to a fast separation and slow recombination of photo-induced electron–hole pairs. An alternate mechanism for enhanced visible-light photocatalytic activity was also considered. [ABSTRACT FROM AUTHOR]

Highlights: [•] The titanate nanosheets (TNS) were synthesized by a short hydrothermal reaction time. [•] CdS decorated TNS were prepared via ion-exchange and in situ sulfuration process. [•] The composites show enhanced photocatalytic efficiency and stability. [•] Their catalytic activity is higher than that of the CdS/titanate nanotubes. [•] The quick migration of charge carriers in CdS/TNS increased the photoactivity. [Copyright &y& Elsevier]

N-doped carbon (NC)/CoSe 2 /ZnIn 2 S 4 Z-scheme heterojunction was constructed by growing ZnIn 2 S 4 on N-doped carbon-coated CoSe 2 derived from the metal-organic framework (ZIF-67) via the self-sacrificing template method and hydrothermal method. X-ray photoelectron spectroscopy, electron paramagnetic resonance, and density-functional theory (DFT) calculations demonstrate that NC/CoSe 2 /ZnIn 2 S 4 (NSZ) Z-scheme heterojunction was constructed with CoSe 2 as an electron mediator. The optimal 5NSZ composite achieved 97.3% in photocatalytic degradation efficiency of tetracycline hydrochloride (TCH) within 30 min, and H 2 O 2 production of 645.3 μM under visible light for 60 min. Moreover, the top-flight photocatalytic hydrogen evolution (PHE) rate of 6772.9 μmol g−1 h−1 is 19.1 folds and 2.2 folds higher than pure ZnIn 2 S 4 and Pt–ZnIn 2 S 4 , respectively. The photoreduction CO 2 gas produces rates of CO (53.7 μmol g−1 h−1), C 2 H 4 (7.0 μmol g−1 h−1), and C 2 H 6 (6.4 μmol g−1 h−1) products were obtained within 10 h under visible light. This work provides an effective strategy for designing unique noble-metal-free Z-scheme heterojunction photocatalysts to purify the environment and produce clean energy. [Display omitted] • CoSe 2 as a solid-state electron mediator was designed in Z-scheme heterojunction. • The optimal TCH degradation efficiency is 97.3% for 30 min under visible light. • The prime PHE rate is 19.1 and 2.2 times higher than that of ZnIn 2 S 4 and Pt–ZnIn 2 S 4 , respectively. • Z-scheme photocatalytic mechanism was verified by XPS, EPR and DFT analysis. [ABSTRACT FROM AUTHOR]

[Display omitted] The exploration of highly efficient sunlight-assisted photocatalyst for photodegradation of organic contaminants or energy conversion is strongly encouraged. In this work, we designed a novel three-dimensional spindle-like Sv-ZIS@NMFe heterojunction made of amino functionalized NH 2 -MIL-88B(Fe) (NMFe) and ZnIn 2 S 4 nanosheets with abundant sulfur vacancies (Sv-ZIS). The structural properties of NMFe materials, such as a clearly defined system of pores and cavities, were retained by the Sv-ZIS@NMFe composites. Additionally, the incorporation of sulfur vacancies, –NH 2 functional groups, and well-matched energy level positions led to various synergistic effects that considerably enhanced internal electron transformation and migration, as well as improved adsorption performance. Consequently, under visible light irradiation, the optimized sample exhibited superior hydrogen production activity and tetracycline hydrochloride photodegradation performance. At last, density functional theory calculations was used to further elucidated the possible photoreactivity mechanism. This study demonstrates that the Sv-ZIS@NMFe heterojunction materials formed by ZnIn 2 S 4 with suitable sulfur vacancies and amino functionalized Fe-MOFs have promising applications in photocatalysis. [ABSTRACT FROM AUTHOR]

A visible-light-driven Tb x O y loaded TiO 2 -oriented nanosheets (Tb-TNSs) of 3.6 nm thick and specific surface area of 240–350 m 2 /g provided with highly dispersed Tb x O y nanoparticles of 1–2 nm over the surfaces were synthesized using a one-pot hydrothermal route. Loaded Tb x O y nanoparticles influenced morphology, structure, and optical properties of the TNSs. The XPS results showed that Tb 3+ and Tb 4+ co-exist in Tb-TNSs, and loaded Tb x O y resulted in changes in binding energies of Ti and O. The amount of Ti 3+ increased gradually with the amount of loaded Tb x O y nanoparticles. FL and surface photocurrent spectra results indicated that an appropriate amount of Tb x O y (≤1.0 at.%) loading can effectively improve the separation efficiency of charge carriers. The visible-light-driven photocatalysis could be measured by the removal of phenol or dichromate separately along with synergistic degradation in phenol-Cr(VI) coexistence system. The optimum photocatalytic activity was obtained at Tb/Ti = 1.0 at.% under degradation of aqueous phenol or Cr(VI) solution. Besides, the desorbed Tb-TNSs could be easily regenerated by remedying the damaged structure and reused with excellent performance. High regeneration and stability of photocatalysts were confirmed by seven cyclic tests. An alternant mechanism for the enhanced visible-light-driven photocatalytic activity was also considered. [ABSTRACT FROM AUTHOR]

Vanadium doped TiO 2 -based nanosheets (V-TNSs) with different V/Ti ratios were prepared by a facile one-pot hydrothermal method using vanadium nitrate and P25 as the vanadium precursor and titanium precursor, respectively. The results indicated that as-synthesized photocatalysts exhibited sheet-like structure with large specific surface area (270–340 cm 2 /g) and small thickness (4–5 nm). XPS results revealed that vanadium exists in the form of V 4+ and V 5+ , and the binding energies of Ti O bonds have been changed with the concentration of vanadium. Vanadium doping resulted in considerable enhancement of visible light absorption, red-shift, and the band-gap of photocatalysts reduced from 3.18 eV to 2.91 eV. The density functional theory (DFT) calculations for band structure and total energy also provided a good explanation and further confirmation for the experimental results. It has been found that the photo-activity increased gradually with the concentration of vanadium, and then decreased after attaining a maximum with an optimal content of vanadium at 1.0 at.% for RhB or Cr(VI). The reaction rate K app of 1.0%-V-TNSs are 9.27-fold and 3.26-fold as compared to undoped TNSs under UV–vis and visible light irradiation, respectively. The cyclic tests that performed six times demonstrated high stability and reusability of the photocatalysts. A possible alternate mechanism for the enhancement of the photocatalytic activity under visible-light irradiation was also proposed. [ABSTRACT FROM AUTHOR]

The Z-scheme heterojunction photocatalysts possess excellent photocatalytic activity benefitting from their properly matched edge potentials. In this work, ZnCo 2 S 4 nanoparticles are anchored on Zn 0.2 Cd 0.8 S solid solution nanowires and assembled into binary ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S nanocomposites. The as-synthesized ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S nanocomposites act as bifunctional photocatalysts, which can be used for high-performance H 2 production coupling synthesis of high-value-added benzaldehyde in low concentration benzyl alcohol solution. Under visible light irradiation, 20%-ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S nanocomposite shows the highest H 2 evolution rate of 23.02 mmol g−1 h−1 in the first photocatalytic cycle, which is 395.0 and 526.7 times higher than that of Zn 0.2 Cd 0.8 S and ZnCo 2 S 4 under the same conditions. Eventually, after six-time cycles, the conversion rate and selectivity of benzyl alcohol oxidation to benzaldehyde are 54.3% and 92.2%, respectively. The enhancement of photocatalytic performance is mainly attributed to the Z-scheme heterojunction between ZnCo 2 S 4 and Zn 0.2 Cd 0.8 S, which promote the separation and transfer of photogenerated charge carriers. This work provides strong support for the rational design of Z-scheme nano-heterojunction of highly efficient photocatalytic application in H 2 evolution and fine chemicals production. The bifunctional ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S Z-scheme heterojunction photocatalysts show efficient H 2 evolution rate and high-value-added benzaldehyde synthesis activity under visible light irradiation. [Display omitted] • An efficient bifunctional ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S heterojunction is prepared. • ZnCo 2 S 4 /Zn 0.2 Cd 0.8 S shows excellent H 2 evolution and benzaldehyde synthesis activity. • The photocatalytic mechanism between ZnCo 2 S 4 and Zn 0.2 Cd 0.8 S is proposed. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ni-TNSs with high stability and recyclable usability were prepared by one-pot method. • Ni x O y loading results in the changes in the structure of the TNSs and red shift. • Ni x O y loading leads to obviously increase of the Ti3+ ions content. • Ni x O y loading can effectively trap electrons to enhance the separation of charges. • Ni-TNSs exhibit excellent visible-light photocatalytic activities for organic pollution. Abstract Two-dimensional (2D) TiO 2 -based nanosheets (TNSs) decorated with surface-enriched Ni x O y (Ni = Ni2+, Ni3+) nanoparticles (Ni-TNSs) were synthesized and found to exhibit a high visible light photocatalytic activity. The thickness and high specific surface area of the Ni-TNSs were measured to be ∼5 nm and 280–350 cm2/g, respectively. Properties, such as the crystal structure, crystallinity, surface area and visible light absorption of the TNSs, were closely associated with the distributed nanoparticles of Ni x O y (The particle size is less than 2 nm) on the surface. Surface photocurrent, fluorescence spectroscopy, linear sweep voltammetry and Tafel measurements revealed that the separation efficiency of the photogenerated charge carriers can be improved by optimizing the concentration of Ni x O y (the optimized ratio of Ni/Ti = 3.0 at.%). The photocatalytic degradation of organic pollutants (RhB and MB) under visible light was measured, and the efficiency also strongly depended on the amount of Ni x O y nanoparticles with an optimal percentage of 3.0 at.%. The photoreactivity of the sample was further studied by determining the formation of photoinduced hydroxyl radicals (OH) by applying coumarin (COU) as a probe material and by obtaining ESR spectra of the superoxide radical adducts (O 2 −) trapped by DMPO (5,5-dimethyl-1-pyrroline n-oxide). Ten cyclic tests were used to confirm the high stability. A feasible mechanism is described to interpret the enhanced photocatalysis. [ABSTRACT FROM AUTHOR]

Ternary nanocomposites comprised of one-dimensional (1D) WO 3 &WS 2 nanorods (NRs) and CdS nanoparticles (NPs) fabricated through a two-step process were characterized using a combination of X-ray, microscopy, and spectroscopy techniques. The amount of CdS strongly influences the properties of the nanocomposites, with 40 wt.% of CdS loading showing optimal optical properties and photoelectrochemical (PEC) as well as photocatalytic activities for hydrogen evolution reaction (HER). Compared with WO 3 &WS 2 NRs, the optimized nanocomposite shows 11.4 times enhancement in photocurrent and 6 times increase for HER with lactic acid as a sacrificial agent. The enhanced PEC and HER activities can be attributed to CdS absorption of visible light as well as the intimate contact between WO 3 &WS 2 and CdS that efficiently enhances charge carrier separation. This work demonstrated a promising strategy in the design and fabrication of high-performance photocatalysts for photocatalytic HER applications. [ABSTRACT FROM AUTHOR]

High recombination rate of photogenerated electron-hole pairs and relatively narrow photoresponsive range of TiO 2 -based photocatalysts are the remaining challenges for their practical applications. To address such challenges, photocatalysts consisting of AgCl covered Ag nanoparticles (AgCl@Ag), titanate nanotubes (TiNT), and nitrogen-doped reduced graphite oxide (rGO N ) are fabricated through alkaline hydrothermal process, followed by deposition and in situ surface-oxidation of silver nanoparticles. In the synthesized photocatalysts, the titanate nanotubes have average length of about 100 nm with inner diameters of about 5 nm and the size of the formed silver nanoparticles is in the range of 50–100 nm. The synthesized photocatalyst degrades almost all the model organic pollutant Rhodamine B in 35 min and remains 90% of photocatalytic efficiency after 5 degradation cycles under visible light irradiation. Since the oxidant FeCl 3 applied for oxidation of surface Ag to AgCl is difficult to be completely removed due to the high adsorption capacity of TiNT and rGO N , the effect of reside Fe atoms on photocatalytic activity is evaluated and the results reveal that the residue Fe atom only affect the initial photodegradation performance. Nevertheless, the results demonstrate that the formed composite catalyst is a promising candidate for antibiosis and remediation in aquatic environmental contamination. [ABSTRACT FROM AUTHOR]

A binary heterostructured CdS/MoS 2 flowerlike composite photocatalysts was synthesized via a simple one-pot hydrothermal method. This photocatalyst demonstrated higher photocatalytic hydrogen production activity than pure MoS 2 . The heterojunction formed between MoS 2 and CdS seems to promote interfacial charge transfer (IFCT), suppress the recombination of photogenerated electron–hole pairs, and enhance the hydrogen generation. Based on the good match between the conduction band (CB) edge of CdS and that of MoS 2 , electrons in the CB of CdS can be transferred to MoS 2 easily through the heterojunction between them, which prevents the accumulation of electrons in the CB of CdS, inhibiting photocorrosion itself and greatly enhancing stability of catalyst. Hydrogen evolution reaction (HER) using Na 2 S/Na 2 SO 3 or glucose as sacrificial agents in aqueous solution was investigated. The ratio between CdS and MoS 2 plays an important role in the photocatalytic hydrogen generation. When the ratio between CdS and MoS 2 reaches 40 wt%, the photocatalyst showed a superior H 2 evolution rate of 55.0 mmol g −1 h −1 with glucose as sacrificial agent under visible light, which is 1.2 times higher than using Na 2 S/Na 2 SO 3 as sacrificial agent. Our experimental results demonstrate that MoS 2 -based binary heterostructured composites are promising for photocorrosion inhibition and highly efficient H 2 generation. [ABSTRACT FROM AUTHOR]

Photocatalysts containing different ratios of anatase and rutile are prepared via heat treatment of Degussa P-25 titania. X-ray diffraction (XRD), Bruuauer-Emmett-Teller (BET), ultraviolet–visible light diffuse reflectance spectra (DRS), Raman spectra (Raman), positron annihilation lifetime spectra (PAL) and temperature-programmed desorption (TPD) are applied to investigate the phase composition of the synthesized catalysts. Using crystal violet (CV) as the target pollutant, the unexpected visible light decolorization of rutile is observed. Despite the decreased specific surface area, the as-synthesized rutile samples exhibit much higher adsorption capability of CV than P-25 does, which in turn leads to improved photoreaction efficiency. Since the rutile samples can't absorb the visible light, the degradation under visible light irradiation is attributed to self-sensitization of CV on the surface of rutile. [ABSTRACT FROM AUTHOR]

TiO 2 -nanosheet (TNS) was firstly fabricated using a one-pot hydrothermal route followed with photo depositing various dosages of Pt nanoparticles over its surface. The hydrogen production performance of these as-prepared samples was assessed in triehanolamine (TEOA) aqueous solutions under visible-light irradiation. The results showed the optimal ratio of Pt: TNS was 0.5 wt %, with apparent quantum efficiency (QE) climbing 15.44%. And on this basis, it was used to H 2 production simultaneous photodegradation with individual or both kinds of dyes (namely, methyl orange (MO), eosine Y (EY), methyl blue (MB) and rhedamine B (RhB)), respectively. As the figures indicated, although the rate of hydrogen production saw a steady increase in the single dye in the solution, it was outmatched by any two kinds of dyes group, which played significant positive synergism effects. In particular, when MB/EY group was added intoTEOA solution, the QE reached a peaking of 25.42% dramatically. In addition, real textile industrial wastewater was applied in this system, and the QE was 20.45%, which was 1.32 fold that of clean water. The crucial factor in improving the photocatalytic activity was that appropriate amount of functional 0D-2D Schottky barrier heterostructures were constructed. Such a novel architecture provided a large and intimate contact interface for fast interfacial photo-charges separation. The effective charge transfer from TNS to Pt was demonstrated by the significant enhancement of photocurrent responses in 0D-2D Pt-TNS composite. This work created new opportunities for designing and constructing highly efficient photocatalysts by interfacial engineering. [ABSTRACT FROM AUTHOR]

Visible-light-driven (VLD) CoO x loaded TiO 2 -based nanosheets (Co-TNSs) with surface-enrich CoO x nanoparticles were successfully synthesized by using a facile one-pot hydrothermal method. The photocatalysts were characterized by SEM, AFM, HRTEM, XRD, Raman, XPS, FL and the surface photocurrent (SPC) technique, etc. The as-synthesized samples exhibit sheet-like structure with large specific surface area (260–350 cm 2 /g) and small thickness (3–4 nm). The CoO x nanoparticles (1–2 nm) are highly dispersed on the surface of TNSs. Loaded CoO x nanoparticles not only have influenced the crystal structure, crystallinity and surface area of the TNSs, but also resulted in considerable enhancement of visible-light (VL) absorption and a red-shift of the band gap of the TNSs. XPS results reveal that cobalt mainly exists in the form of Co 2+ and Co 3+ , and the binding energies of Ti O bonds are changing with the loaded amount of CoO x nanoparticles. FL and SPC results indicate appropriate amount of CoO x (2.5 at.%) loading can effectively inhibit the recombination of photo-generated electron-hole pairs, thus improve the separation efficiency of charge carriers. The photocatalytic activity of the samples was evaluated by the dichromate (Cr(VI)) solution under VL irradiation. It can be found that the Co-TNSs photocatalysts showed excellent VL photocatalytic activity for the removal of Cr(VI). Initially the photocatalytic activity increased with the content of CoO x , and then decreased after attaining a maximum value at an optimal content (2.5 at.%) for degradation of aqueous Cr(VI) solution. The addition of Ca 2+ promotes photocatalysis owing to its ionic bridging function in the form of TiOH + Cr(VI)–Ca 2+ Cr(VI) linkages, while SO 4 2− slightly inhibits the photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at higher ionic strength of electrolyte. The photocatalytic reduction of Cr(VI) is also significantly promoted by phenol and synergism between Cr(VI) reduction and degradation of phenol which is demonstrated by measuring the effect of multiple usage of Co-TNSs on its photocatalytic efficiency. Desorbed Co-TNSs (Co-TNSs-Des) are easily regenerated and reused for Cr(VI) removal with excellent performance. A possible alternate mechanism for the enhancement of photocatalytic activity under visible light irradiation is also proposed. [ABSTRACT FROM AUTHOR]

Abstract: Novel magnetic Bi2WO6/TiO2 hierarchical microspheres with grape-like morphology were successfully synthesized by a reliable and effective approach. The multifunctional composite microspheres compose of a magnetic core of Fe3O4@SiO2, and an outer shell of Bi2WO6/TiO2 heterostructure. The photocatalytic activities of the samples were evaluated by photodegradation of Rhodamine B (RhB) under visible light irradiation. It is discovered that the Fe3O4@SiO2@Bi2WO6/TiO2 (FSBT) composite microspheres exhibit superior photocatalytic activity, and the photocatalytic reaction rate reaches 0.0640min−1, which is 1.5 and 49.2 times greater than that of the Fe3O4@SiO2@Bi2WO6 (FSB) composites and the TiO2 (Degussa P25) nanoparticles, respectively. Besides, the FSBT composite microspheres also present excellent magnetic recoverability. [Copyright &y& Elsevier]

• NiCo 2 S 4 /CdS showed excellent HER with lactic acid under visible light irradiation. • Ultrafast dynamics studies showed that NiCo 2 S 4 /CdS had more efficient charge separation. • The mechanism enhanced performance of the NiCo 2 S 4 /CdS nanocomposites were revealed. One-dimensional NiCo 2 S 4 /CdS nanocomposites are constructed using a two-step hydrothermal method and used for photocatalytic hydrogen generation with visible light. The nanocomposites are characterized using a combination of techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), positron annihilation lifetime measurement, spectroscopy techniques, and femtosecond transient absorption (TA) spectroscopy. In comparison to bare CdS NWs, the NiCo 2 S 4 /CdS nanocomposites exhibit enhanced visible light hydrogen production, with an optimal rate reaching 20.0 mmol·h−1·g−1. The experimental results, in conjunction with theoretical electronic structure calculations, reveal that the enhanced hydrogen evolution reaction activities can be attributed to the formation of Schottky junction between NiCo 2 S 4 NPs and CdS NWs that facilitates photogenerated electron transfer from CdS to NiCo 2 S 4. Meanwhile, the NiCo 2 S 4 , as a metallic cocatalyst, increases the electron density and active sites desired for H 2 evolution. [ABSTRACT FROM AUTHOR]