Reduction Of CrVI Research Articles

Conversion of clay minerals to photocatalysts for CrVI reduction and salicylic acid decomposition

This study investigates the conversion of biotite, a subgroup of clay minerals, into photocatalysts through heat treatment with CaCl2. The resulting reaction products were analyzed in terms of their composition, structure, optical properties, and photocatalytic activity against CrVI and salicylic acid (SA). Biotite and CaCl2 mixtures could be heated to 600 °C while retaining the biotite crystal structure, whereas heating to 700 °C resulted in transformation to octahedral wadalite crystals. The band gap of the wadalite obtained after heat treatment was approximately 3.10 eV. The photocatalytic reduction rate per unit surface area increased markedly with increasing heat-treatment temperature, and the CrVI reduction and SA degradation rates of the sample calcined at 800 °C were approximately 18 and 9 times greater, respectively, than those of the sample calcined at 500 °C. Leaching tests of the reaction products revealed that the elution of Ca2+ and Cl− was particularly significant. Even the samples that retained the biotite structure after heat treatment displayed some photocatalytic activity, indicating that this method may be applicable to preparing photocatalysts from mica and related minerals with similar structures.

PH-Triggered Recovery of Organic Polymer Photocatalytic Particles for the Production of High Value Compounds and Enhanced Recyclability.

Pseudo-homogeneous polymeric photocatalysts are an emerging class of highly efficient and tunable photocatalytic materials, where the photocatalytic centers are easily accessible. The creation of highly efficient photocatalytic materials that can be rapidly separated and recovered is one of the critical challenges in photocatalytic chemistry. Here, we describe pH-responsive photocatalytic nanoparticles that are active and well-dispersed under acidic conditions but aggregate instantly upon elevation of pH, enabling easy recovery. These responsive photocatalytic polymers can be used in various photocatalytic transformations, including CrVI reduction and photoredox alkylation of indole derivative. Notably, the cationic nature of the photocatalyst accelerates reaction rate of an anionic substrate compared to uncharged species. These photocatalytic particles could be readily recycled allowing multiple successive photocatalytic reactions with no clear loss in activity.

Photocatalytic Reduction of Cr VI by TiO2/GO Nanocomposite

AbstractGraphene Oxide (GO) was successfully deposited on TiO2 NPs through sol‐gel process. The fabricated TiO2/GO (T/G) nanocomposite was used for reduction of CrVI under simulated solar light illumination. 10 wt. % GO was used for the preparation of T/ G nanocomposite. Several analytical methods such as powder X‐ray diffraction (PXRD), scanning electron microscope (SEM), transmission electron microscopy (TEM) and UV‐visible diffuse reflectance spectra (DRS) spectrophotometer was used to characterize the samples. The PXRD revealed the formation of the anatase phase of TiO2 NPs and T/G nanocomposites, as per literature more favourable form of TiO2 for photocatalytic activity. The average crystallite size was found 5.1 nm and 23 nm for TiO2 NPs and T/G nanocomposite respectively. Under the simulated sunlight illumination, the TiO2 NPs was excited to generate the electron‐hole pairs and then the photo‐generated electron transferred by the GO to the CrVI which further converted into CrIII. The band gap for T/G nanocomposite was calculated 2.92 eV. Under visible light,27.3 % photoreduction of (10 mg/L) CrVI solution at λmax=347 nm was achieved within 90 min on the T/G sample at the pH 5–6 while overall reduction was 48 % including adsorption. The rate of reaction was found 0.00305 min−1 and the order of reaction was pseudo first order.

Effect of dissimilatory iron reduction and

Environmental context Dissimilatory iron reduction and Carex-produced dissolved organic matter (DOM) have an important influence on Cr(VI) reduction by the Fe(III)-reducing bacterium Enterobacter. The role of Carex DOM and ferrihydrite in the biotransformation of Cr(VI) by the bacterial isolate was investigated. The findings should help underpin the remediation and detoxification of chromium in anaerobic environments, and provide promising insights into the quaternary system of bacterium/Fe(III)/Cr(VI)/DOM. Rationale FeIII oxides and organic matter are important factors influencing CrVI degradation in wetland soils. However, it remains unclear how they interact in anaerobic systems. Methodology In this study, a strain of iron-reducing bacterium was isolated from Poyang Lake Wetland and identified as Enterobacter sp. PY16 (PY16) by PCR-16S-rDNA sequence analysis. Moreover, microbial reduction of FeIII/CrVI by PY16 and their mutual transformation in the quaternionic system of PY16/ferrihydrite/CrVI/dissolved organic matter (DOM, extracted from Carex cinerascens) were investigated. Results The results showed that PY16 could directly participate in the reduction of ferrihydrite and CrVI. The rate of CrVI reduction decreased with the increase of initial CrVI concentration, while it was enhanced by 2.78–42.99% in the presence of ferrihydrite. Moreover, 15 mg L−1 CrVI was almost eliminated after 72 h and the content of FeII increased by 78.21 mg L−1 in the presence of DOM. Discussion The fastest CrVI reduction rate occurred when ferrihydrite and DOM coexisted in the system, mainly because the promoting effect of DOM on ferrihydrite synergistically promoted CrVI reduction. DOM and FeII produced during the ferrihydrite reduction process served as electron shuttles that promoted CrVI reduction by a biochemical redox pathway. However, the electron transfer and donation capacity of DOMox/DOMred and FeIII/FeII in the reaction process still need to be further studied. Implications for future research The results underscored the importance of FeIII oxides and DOM on microbial CrVI reduction, thus providing a valuable technique to remove and detoxify chromium in wetland soils.

Fabrication of Wide-Spectra-Responsive Na /Nh 2 -Mil-125(Ti) with Boosted Activity for Cr (Vi) Reduction and Antibacterial Effects

Fabrication of Wide-Spectra-Responsive Na /Nh 2 -Mil-125(Ti) with Boosted Activity for Cr (Vi) Reduction and Antibacterial Effects

In-Situ Synthesis of Α-Fe2o3/Lafeo3 Modified with G-C3n4 and Ti3c2 for Construction of Multiple Z-Scheme/Schottky Heterojunctions as an Efficient Visible Light Photocatalyst for Cr (Vi) Reduction and Organic Pollutants Removal

In-Situ Synthesis of Α-Fe2o3/Lafeo3 Modified with G-C3n4 and Ti3c2 for Construction of Multiple Z-Scheme/Schottky Heterojunctions as an Efficient Visible Light Photocatalyst for Cr (Vi) Reduction and Organic Pollutants Removal

Ag Nanoparticles Interlayered Fe3o4/Ag/M(Tio2-Zro2) Magnetic Photocatalysts with Enhanced Stability and Photocatalytic Performance for Cr(Vi) Reduction

Ag Nanoparticles Interlayered Fe3o4/Ag/M(Tio2-Zro2) Magnetic Photocatalysts with Enhanced Stability and Photocatalytic Performance for Cr(Vi) Reduction

Novel Znfe2o4/Bi2s3 High-Low Junctions for Boosting Tetracycline Degradation and Cr(Vi) Reduction

Novel Znfe2o4/Bi2s3 High-Low Junctions for Boosting Tetracycline Degradation and Cr(Vi) Reduction

Synthesis and characterization of iron-nitrogen-doped biochar catalysts for organic pollutant removal and hexavalent chromium reduction

Fe, N atoms deposited on porous biochar (Fe-N@BC) composites were synthesized and employed as an efficient catalyst for organic pollutant removal and CrVI reduction. Naturally abundant, renewable and N-rich pomelo peel as a carbon and nitrogen source and unsubstituted phthalocyanine/iron phthalocyanine complexes as a Fe and nitrogen resource were used to develop the Fe-N@BC via a carbonization process. It is found that Fe-N@BC hybrids have homogeneous dispersion of Fe and N atoms on 3D hierarchically porous biochar, which significantly improves the performance toward the detoxification of organic pollutants using peroxymonosulfate as an oxidant, as well as the reduction of hexavalent chromium by formic acid as a reductant. Furthermore, the effects of Fe loading and pyrolytic temperature on catalysis were comprehensively analyzed and optimized. The excellent activity of Fe-N@BC in acid media can be attributed to the high dispersion of Fe species, high content of doped nitrogen as well as hierarchical micro-mesopores, which induce to expose more active sites for catalysis. Owing to the structure-enabled acidic stability, Fe-N@BC efficiently retains its activity as well as its structural stability after several cycles of reactions. This work provides a new approach to construct Fe, N-doped biochar as an effective catalyst for the detoxification of organic and inorganic pollutants.

Rational synthesis of SrTiO3 nanodots anchored mesocrystalline anatase TiO2 submicrospheres for photocatalytic reduction of CrVI

A novel spherical anatase TiO 2 mesocrystal/SrTiO 3 heterojunction with favored Cr VI adsorption, bolstered charge carrier separation and well-preserved reduction capability was successfully produced, which exhibited enhanced activity toward photocatalytic reduction of Cr VI . • Spherical anatase mesocrystal/SrTiO 3 nanodot type-II heterojunction was prepared. • Favored Cr VI adsorption and bolstered charge carrier separation were realized. • The heterojunction showed remarkably enhanced photocatalytic reduction of Cr VI . Mesoporous anatase TiO 2 sphere based materials has long been the focus of intensive research dealing with environmental-related issues due to their significant advantages in terms of structural isotropy, high surface area, structural stability, and low cost. However, in most cases, the employed mesoporous anatase spheres were comprised of nanocrystalline subunits with random crystalline orientations, which means that numerous grain boundaries exist in the sphere-shaped particles, suppressing their performance in various applications. Herein, by using mesocrystalline anatase submicrospheres that were synthesized via utilizing SO 4 2- ions to modulate the growth dynamics of anatase crystals in the green mixed solvents of PEG-400 and H 2 O as both template and reactant, novel sphere-shaped anatase mesocrystal/SrTiO 3 nanodot heterostructures were successfully produced. These spherical anatase mesocrystal/SrTiO 3 heterostructures exhibited noticeably enhanced activity toward reduction of hexavalent chromium (Cr VI ) under stimulated solar light irradiation, which was largely attributed to their favored adsorption of Cr VI , bolstered photogenerated charge carrier separation and well-preserved reduction capability due to the construction of a well-defined type-II heterojunction with tight interfacial contacts and strong interactions. This work will be useful for the designed fabrication of micro/nanostructured TiO 2 based photocatalysts with complex spherical morphologies for photocatalytic applications in environmental purification.

Hollow Core-Shell potassium Phosphomolybdate@Cadmium Sulfide@Bismuth sulfide Z-Scheme tandem heterojunctions toward optimized Photothermal-Photocatalytic performance

A hollow core–shell potassium phosphomolybdate (KMoP)@cadmium sulfide (CdS)@bismuth sulfide (Bi2S3) Z-scheme tandem heterojunction is fabricated by a simple hydrothermal strategy and kept in a water bath to continue the reaction. At the same time, the ternary structure combined Keggin-type polyoxometalate with two photosensitive sulfide semiconductors to form a stable hollow core–shell heterojunction. KMoP@CdS@Bi2S3 with a narrow band gap of ∼ 1.2 eV also has excellent photothermal performance, which may further promote photocatalytic efficiency. The hollow core–shell KMoP@CdS@Bi2S3 tandem heterojunction shows excellent H2 production performance, CrVI reduction ability and photocatalytic degradation performance of highly toxic tetracycline (TC). Under visible light irradiation, the photocatalytic H2 generation rate of the KMoP@CdS@Bi2S3 tandem heterojunction reaches 831 μmol h−1, which is 103 times higher than that of pristine KMoP. The photocatalytic reduction efficiency of CrVI and degradation efficiency of TC are as high as 95.5 and 97.51%, ∼4 times higher than that of KMoP. The boosted photocatalytic performance can be ascribed to the formation of core–shell Z-scheme tandem heterojunctions favoring spatial charge separation and the narrow band gap, which extends the photoresponse to visible light/NIR regions. When TC and CrVI exist at the same time, the reduction efficiency of CrVI can be as high as 99.64% because the intermediate of TC degradation can promote the reduction of CrVI. In addition, the photocatalytic performance of the KMoP@CdS@Bi2S3 heterojunction remains nearly constant after 4 recycles, which indicates high stability. The design strategy may provide new insights for preparing other high-performance core–shell tandem heterojunction photocatalysts for solar energy conversion.

Fe, Cu-coordinated ZIF-derived bimetal encapsulated N‑doped carbon nanotube for efficient remediation of various aqueous pollutants

Exploring highly stable and efficient non-precious metal catalysts for decontamination of various non-biodegradable contaminants is a major challenge. Herein, Fe, Cu-coordinated ZIF-derived bimetal encapsulated N-doped carbon nanotubes (Fe-Cu@N-C) were developed and employed as novel bifunctional catalysts for organic oxidation and CrVI reduction. Pyrolysis of ZIF-8 containing Fe2+ and Cu2+ has successfully produced Cu nanoparticles and atom-dispersed Fe bimetals embedded in N-doped carbon nanotube catalysts. It was observed that the obtained optimal Fe-Cu@N-C possessed outstanding activity and stability for efficient degradation of organic contaminants via PMS activation, and toxic CrⅥ reduction with Formic acid (FA) as the reductant. Singlet oxygen (1O2·) was verified as the dominant active species for organic oxidation, while surface-adsorbed atomic hydrogen (H*) was responsible for effective reduction of CrVI to CrIII in the Fe-Cu@N-C/FA system. Compared with other ZIF derived systems such as Fe@N-C, Cu@N-C, and metal-free N-C, Fe-Cu@N-C exhibited prominent catalytic activities, which were attributed to bimetallic active sites (Fe-Nx and Cu-Nx), high N-doped level, large surface area and conductivity of porous carbon structure for fast mass and electron transport. This report provides a new approach to design highly efficient multifunctional catalysts for the purification of water to remove various types of pollutants.

A microwave irradiation-persulfate-formate system for achieving the detoxification and alkali-activated composite geopolymerization of the chromate-contaminated soil

A microwave (MA) irradiation-persulfate-formate system was constructed to detoxify Cr contamination and solidify the geopolymerization of the alkali-activated composite material. Three series of experiments were correspondingly conducted to evaluate the treatment for the chromate-contaminated soil. The changes in the molar ratios of formate to persulfate and the mass rates of fortifier to soil led to a significantly greater reduction of CrVI in the detoxification experiments. The increase of blast furnace slag from 50% to 80% in the composite cementitious materials (CCM) intensified the immobilization efficiencies of chromate and the compressive strengths of geopolymer blocks. MA irradiation potentially enhanced the binding of Ca cations to the aluminosilicate compounds. The degree of reaction in the phenomenological kinetics model mathematically verified the geopolymerization process. Ettringite was formed within the structure of the geopolymer in the coupling system. Sulfate radicals released from persulfate not only contributed to the detoxification process but also strengthened the immobilization process.

Microwave-Assisted Synthesis of Water-Stable Zirconium Metal-Organic Polyhedron: Serving as Efficient Visible-Light Photocatalyst for Cr(Vi) Reduction

Microwave-Assisted Synthesis of Water-Stable Zirconium Metal-Organic Polyhedron: Serving as Efficient Visible-Light Photocatalyst for Cr(Vi) Reduction

Modulation of the Bifunctional CrVI to CrIII Photoreduction and Adsorption Capacity in ZrIV and TiIV Benchmark Metal-Organic Frameworks

The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.

Encapsulated chitosan-modified magnetic carbon nanotubes for aqueous-phase CrVI uptake

This work demonstrates the performance of a composite adsorbent encapsulated with chitosan (CS) and maghemite-doped multiwall carbon nanotubes (MWCNTs). Hexavalent chromium (CrVI) was used as the representative hazardous heavy metal to characterize the adsorption behavior of the composite beads. The chromium speciation chemistry combined with the chitosan surface chemistry rendered pH at 4 to be the optimized condition in which the uptake of CrVI was quantitatively most effective. The Langmuir isotherm with a maximum adsorption capacity of 119 mg g−1 at 298 K was observed. The CrVI adsorption studies using partial compositions of the beads further demonstrated the synergistic role of combining the surface properties of CS and MWCNTs. The CrVI removal efficiency increased from 82 % to 96 % as the CS contents doubled in the CS/MWCNTs/Fe beads. Kinetically, the pseudo-second-order model showed the best regression fit (r2 > 0.99) with the experimental data. The intra-particle diffusion model further elucidated that the multiphasic rate-limiting steps during CrVI adsorption included external mass transfer resistance and intra-particle diffusion. The thermodynamic parameters (ΔG, ΔH, ΔS) indicated the adsorption process was spontaneous and endothermic in the tested temperature range (298–313 K). The weak negative Gibbs free energy (ΔG < −20 kJ mol−1) suggested a spontaneous adsorption process governed by electrostatic adsorption. The evolution of CrIII indicates a possible charge transfer effect of Fe2+/Fe3+ mediated through conductive carbon nanotubes, leading to the reduction of CrVI. In five adsorption-desorption cycles, the CrVI removal efficiency of CS/MWCNTs/Fe beads remained about 80 % in adsorption capacity.

Gamma (60Co) irradiation and thermal effect on redox behavior of interlayer iron in montmorillonite

Fe2+-montmorillonite (Mt) is expected to be one of the major altered clay minerals formed in the engineered barrier system proposed for deep geological disposal of high-level waste (HLW). The high energy radiations imparted by the radionuclides present in HLW as well as the temperature can impact the redox behavior of interlayer iron in clay. Therefore, we have studied the effect of γ- radiation (60Co) on the stability of Fe2+-Mt dispersed in water. Irradiation at a dose rate of 0.6 kGy/h for 18 h resulted in the oxidation of about 14% interlayer Fe2+ as indicated by Fe2+/Fetotal ratio. On further irradiation, these oxidized Fe3+ are reduced back to Fe2+. Such behavior was observed in inert, aerated, and N2O-saturated conditions.The effect of temperature was found to be similar to that of γ-irradiation, particularly on the reduction of interlayer Fe3+ from Fe3+-Mt. When the temperature was increased from 50 to 300 °C, > 90% reduction (Fe3+ → Fe2+) was observed, which is ascribed to the self-reduction mechanism. This sample is similar to that of Fe2+-Mt as evident from XRD, FTIR, TG and magnetic data anaysis. The increase in Fe2+/Fetotal ratio of the heat-treated samples enabled the solid-state reaction with o-phenanthroline (L) by in-situ complexation to form organo-modified Mt. (d001 = 18.5 Å) at room temperature (RT, 27 °C). Furthermore, the presence of Fe2+ in this heat-treated Mt. is evident from their reduction of aqueous CrVI. For instance, 55% of CrVI reduction occurred with Fe3+-Mt@200. On the other hand, the gradual oxidation of interlayer Fe2+ occurred when Fe2+-Mt was heated up to 300 °C. Only about 12% oxidation was observed when heated to 200 °C for 4 h. This is probably due to an equilibrium between oxidation and their self-reduction. In this heat treated sample, about 73% of CrVI reduction occurred, which is comparable to 80% reduction observed with freshly prepared Fe2+-Mt@RT. Thus, the redox behavior of interlayer iron was significantly influenced by γ-irradiation as well as thermal treatment. We believe the observations made here have significant implications on the plausible existence of clay minerals in the geothermal and extraterrestrial environment.

Zn-MoS2 nanocatalysts anchored in porous membrane for accelerated catalytic conversion of water contaminants

A series of Zn-MoS2 heterostructures were fabricated via a thermal-treatment method, and then anchored in polyvinylidene fluoride (PVDF) to yield novel catalytic membranes (Zn-MoS2@PVDF) by the phase inversion technique for enabling CrVI reduction. The SEM image showed that ZnS nanoparticles homogeneously coated on the MoS2 surface assembled in flower-like agglomerations. Zn-MoS2@PVDF exhibited unprecedented activity of CrVI reduction using formic acid (FA) as the reductant, with a high rate constant (k = 0.033 min−1) and low activation energy (Ea = 38.8 kJ mol−1). CrVI reduction rates effectively boosted with increasing FA dosages (0.234–1.170 M) and temperatures (15–55 ℃), but declined with the increase of CrVI concentrations (5–25 mg/L), solution pHs (2.03–5.11) and inorganic salts. The excellent performance of Zn-MoS2@PVDF originated from the synergistic effect of the unique composition and electronic structure of the membranes. The strong electronic interactions of ZnS and MoS2 made an insignificant contribution to generate intermediate Hads* and released molecular hydrogen. Rich porosity of PVDF membranes might not only afford a uniform dispersion of the Zn-MoS2 NPs, but also decrease mass transport resistance and provide an enlarged catalytic surface area. These outstanding characteristics indicate the potential applicability of Zn-MoS2@PVDF membranes for the remediation of heavy metal pollution.

Assessment of chromium species dynamics in root solutions using isotope tracers

BackgroundChromium (Cr) exists in the environment in two chemical forms; CrIII is an essential micronutrient for glucose and lipid metabolism, whereas CrVI is toxic and a recognised carcinogen through inhalation. Numerous studies have attempted to evaluate their transfer mechanisms from soil and solution media into plants, usually with respect to the hyperaccumulation, detoxification and tolerance of the plant to CrVI. MethodsIsotopically enriched species of Cr, added as 50CrIII and 53CrVI, were used to investigate transfer from solution into the root systems of Spinacia oleracea. In addition the effect of sulphate (SO42−), as a competitor for CrVI uptake, was investigated. Separation of 50CrIII and 53CrVI was undertaken using HPLC-ICP-QQQ following isolation of root solutions using freeze/thaw centrifugation. ResultsIrrespective of supplied CrVI concentration (250, 500 or 1000 μg L−1), the dominant species in both apoplastic (routed through cell wall and intercellular space as a passive mechanism) and symplastic (routed through cytoplasm as an active mechanism) root solutions was CrIII. There was evidence for CrVI reduction in the rhizosphere prior to uptake as an additional detoxification mechanism. Sulphate promoted uptake of CrVI through the active pathway, although increases in SO42- concentration did not yield a proportional increase in Cr symplastic solution concentration; CrIII was also the dominant species in these root solutions. ConclusionThe results indicate that Spinacia oleracea plants can effectively reduce CrVI to CrIII and that the uptake pathways for both CrIII and CrVI are more complex than previously reported. Further work is required to understand the physiological processes that result in the reduction of CrVI prior to, and during, uptake. The efficacy of sulphate to augment existing agricultural management strategies, such as liming and organic reincorporation, also requires further investigation to establish suitable application rates and applicability to other environmental contaminants.

Construction of Hierarchical Hollow Co9 S8 /ZnIn2 S4 Tubular Heterostructures for Highly Efficient Solar Energy Conversion and Environmental Remediation.

Visible-light-responsive hierarchical Co9 S8 /ZnIn2 S4 tubular heterostructures are fabricated by growing 2D ZnIn2 S4 nanosheets on 1D hollow Co9 S8 nanotubes. This design combines two photoresponsive sulfide semiconductors in a stable heterojunction with a hierarchical hollow tubular structure, improving visible-light absorption, yielding a large surface area, exposing sufficient catalytically active sites, and promoting the separation and migration of photogenerated charges. The hierarchical nanotubes exhibit excellent photocatalytic H2 evolution and CrVI reduction efficiency. Under visible-light illumination, the optimized Co9 S8 /ZnIn2 S4 heterostructure provides a remarkable H2 generation rate of 9039 μmol h-1 g-1 without the use of any co-catalysts and CrVI is completely reduced in 45 min. The Co9 S8 /ZnIn2 S4 heterostructure is stable after multiple photocatalytic cycles.