Concentration Of Active Sites Research Articles

Atomically Isolated Nickel–Nitrogen–Carbon Electrocatalysts Derived by the Utilization of Mg2+ ions as Spacers in Bimetallic Ni/Mg–Metal–Organic Framework Precursors for Boosting the Electroreduction of CO2

Electrochemical CO2 reduction (CO2R) is a promising avenue for the conversion of CO2 into fuels and beneficial chemicals. Significant efforts have been devoted to the development of active and selective electrocatalysts for CO2R. Atomically dispersed transition metal electrocatalysts have recently attracted consideration for CO2R due to their unique electronic and structural properties that can impart good catalytic performance and high active metal utilization. Among different precursors used for preparing these catalysts, metal–organic frameworks (MOFs) have been utilized as templates for generating atomically dispersed transition metal active sites as they provide well-defined structures that contain transition metals isolated from each other by organic ligands, along with high surface areas. However, maintaining the isolation of the transition metal species, achieving high surface concentrations of active sites, and imparting porosity during the high-temperature heat treatment of MOFs used during synthesis remain a challenge. In this report, Mg2+ ions have been employed as spacers in a bimetallic metal–organic framework (NiMg-MOF-74) to assist in preventing the coalescence of the Ni atoms into Ni-based particles during heat treatment, which combined with the use of urea as a nitrogen source resulted in the formation of isolated Ni–Nx/C active sites. Our findings demonstrated that Mg2+ ions play a crucial role in extending the distance between the adjacent Ni sites in the precursor structure and generating atomically isolated Ni sites. On the contrary, the utilization of Mg-free Ni-MOF-74 led to the formation of metallic Ni particles embedded in a carbon nanotube-based structure. Furthermore, we investigated the impact of pyrolysis temperature on the produced catalyst morphology. The generation of isolated Ni–Nx/C sites was promoted at higher pyrolysis temperatures (900 °C), while Ni-based particles were predominantly formed at a lower temperature (700 °C). The optimized atomically dispersed Ni–N–C catalyst exhibited excellent selectivity toward CO with a Faradaic efficiency of ∼90% and a current density of −4.2 mA/cm2 at −0.76 V vs a reversible hydrogen electrode.

Single and multisite detailed kinetic models for the adsorption and desorption of NO2 over Cu based NH3-SCR catalyst

Kinetic modeling of NH3 Selective Catalytic Reduction (NH3-SCR) of NOx in Cu-chabazite washcoated monolithic reactors has recently become an important task for design, control and calibration of heavy-duty engine aftertreatment systems. Development of detailed and accurate kinetic models rely on the correct simulation of the NO2 and NH3 storage at different conditions. Here, different kinetic schemes for NO2 adsorption and desorption were developed and compared to experimental data. For this purpose, firstly, realistic values of the active Cu sites in the Cu-zeolite were obtained using the temperature programmed desorption (TPD) of NH3 and NO2 which showed fractional coverages of 0.04 and 0.17 for the so-called ZCuOH and Z2Cu species which reside in the 8 and 6 membered rings (MR) of the zeolitic framework, respectively. Active site concentrations were used in the kinetic models which included simultaneous formation of nitrate/nitrite species or the formation of HNO3 intermediate which in turn resulted in the formation of nitrates or nitrites over the ZCuOH. Models also included or excluded the NO2 storage over the so called secondary Z2Cu sites. It was shown that models taking into account HNO3 intermediate formation along with two NO2 storage sites were better fits to the experimental data.

Fluorescent Probes Design Strategies for Imaging Mitochondria and Lysosomes.

Modern cellular biology faces several major obstacles, such as the determination of the concentration of active sites corresponding to chemical substances. In recent years, the popular small-molecule fluorescent probes have completely changed the understanding of cellular biology through their high sensitivity toward specific substances in various organisms. Mitochondria and lysosomes are significant organelles in various organisms, and their interaction is closely related to the development of various diseases. The investigation of their structure and function has gathered tremendous attention from biologists. The advanced nanoscopic technologies have replaced the diffraction-limited conventional imaging techniques and have been developed to explore the unknown aspects of mitochondria and lysosomes with a sub-diffraction resolution. Recent progress in this field has yielded several excellent mitochondria- and lysosome-targeted fluorescent probes, some of which have demonstrated significant biological applications. Herein, we review studies that have been carried out to date and suggest future research directions that will harness the considerable potential of mitochondria- and lysosome-targeted fluorescent probes.

In Situ Imaging of Catalytic Reactions on Tungsten Oxide Nanowires Connects Surface-Ligand Redox Chemistry with Photocatalytic Activity.

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis. Understanding the role of ligands used to prepare colloidal nanocrystals in catalysis is challenging due to the complexity and heterogeneity of nanocrystal surfaces. We use in situ single-molecule fluorescence imaging to map the spatial distribution of active regions along individual tungsten oxide nanowires before and after functionalizing them with ascorbic acid. Rather than blocking active sites, we observed a significant enhancement in activity for photocatalytic water oxidation after treatment with ascorbic acid. While the initial nanowires contain inactive regions dispersed along their length, the functionalized nanowires show high uniformity in their photocatalytic activity. Spatial colocalization of the active regions with their surface chemical properties shows that oxidation of ascorbic acid during photocatalysis generates new oxygen vacancies along the nanowire surface. We demonstrate that controlling surface-ligand redox chemistry during photocatalysis can enhance the active site concentration on nanocrystal catalysts.

Sulfur-modified copper synergy with nitrogen-defect sites for the electroreduction of CO2 to formate at low overpotentials

Reducing carbon dioxide to formate is one of the most promising methods to achieve peak carbon dioxide emissions. Nevertheless, the low overpotential in energy conversion remains a major challenge. Herein, a new composite catalyst consisting of cuprous sulfide and graphite-phase carbon nitride with nitrogen defects (Cu2S/NDg-C3N4) is synthesized, which simultaneously achieves a high Faradaic efficiency of 82.3% to formate at a low overpotential of 57 mV. Our studies suggest that electron-rich N defects serve as Lewis basic sites to activate CO2 and increase the local CO2 concentration of active sites, which improves the catalytic activity. The catalyst can spontaneously form a heterojunction by in situ Raman spectroscopy detected during the CO2 reduction process, which improves the electrolysis durability. The *OCHO* species detected by in situ ATR-IR spectroscopy verified the O-end adsorption pathway of CO2 reduction to formate on Cu2S/NDg-C3N4. DFT calculations show that sulfur promotes the activation of interface water, forming hydrogen species that can promote intermediate hydrogenation to *OCHO*, which reduces the overpotential. This work shows lower overpotentials than other reported copper-based catalysts and provides a new strategy for designing a simple and energy-saving CO2 reduction catalyst.

The fabrication strategies and enhanced performances of metal-organic frameworks and carbon dots composites: State of the art review

Metal-organic frameworks (MOFs) with large specific surface area, considerable pore volume, controllable structure, and high concentration of active metal sites have been applied widely in researches like catalysis and sensing. However, potential applications of MOFs in both photocatalysis and luminescence sensors are facing major challenges arising from their severe charge recombination, low utilization of solar energy, low quantum yield, limited charge transfer between the metal ions/clusters and the ligand. Recent studies revealed that rational introduction of carbon dots (CDs) with excellent optical properties, unique quantum confinement and high conductivity can greatly enhance the functions of MOFs. In this paper, typical synthesis methods of these CD-MOF composites as well as their potential applications in photocatalysis and sensing are reviewed with emphasis. Representative examples of these CD-MOF composites are discussed, and key features and advantages of CD-MOF composites that will facilitate future applications are highlighted. This review summarizes recent advances on the rational design of CD-MOF composites towards photocatalytic, sensing and other applications.

Hydrogen production from COx-Free thermocatalytic decomposition of methane over the mesoporous iron aluminate spinel (FeAl2O4) nanopowder supported nickel catalysts

In the present study, the thermocatalytic decomposition of methane (TDM) was performed over the NiO(x)/FeAl2O4 catalysts with various contents of nickel oxide. The FeAl2O4 catalyst support with mesoporous structure and high SBET (80.26 m2 g−1) was synthesized according to the carbonate-based mechanochemical method. The calcined catalysts were characterized by the XRD, BET, H2-TPR, CO2-TPD, TPO, and FESEM analyses. The obtained results demonstrated that the SBET of the synthesized catalysts reduced from 62 to 26 m2 g−1 by increasing the nickel loading from 20 to 60 wt.%, which is ascribed to the blockage of FeAl2O4 support pores. Furthermore, the activity results showed that the catalytic activity improved by increasing the nickel content from 20 to 50 wt.% due to the rise of active site concentration. However, the methane conversion was reached to 40% at 600 °C over the NiO(50)/FeAl2O4 catalyst. The more increment of nickel content decreased the catalytic efficiency due to the decline in active phase dispersion. Moreover, the increment amount of deposited carbon was seen by increasing the weight percentage of NiO. Therefore, the catalyst with 50 wt.% of NiO possessed excellent catalytic potential in the TDM process under the hard operating conditions (GHSV = 50,000 ml.h−1.g−1cat). The influence of GHSV, feed ratio (CH4:N2), calcination temperature, and reduction temperature on the textural properties and catalytic activity of the NiO(50)/FeAl2O4 catalyst were also evaluated in detail.

Propane dry reforming over highly active NiO-MgO solid solution catalyst for synthesis gas production

In the present study, the textural characteristics and catalytic efficiency of the NiO(x)-MgO solid-solution catalysts (x = 10,15,20, and 25 wt.%) were determined in CO2 reforming of propane. All samples were fabricated by the simple mechanochemical preparation route. The chemicophysical characteristics of the synthesized catalysts were determined using XRD, BET, H2-TPR, TPO, and SEM exams. The obtained results demonstrated that the physicochemical properties and catalytic activity of the samples were affected with the increment in NiO content, which is assigned to the change in the concentration of active sites and improvement of the reduction features. The NiO(25)-MgO catalyst was selected as the sample with the highest activity, and the effect of the operating factors and calcination temperature were studied over this catalyst. Also, the TPO analysis showed that the whisker carbon was formed on the spent catalyst and the sample with 25 wt.% Ni possessed the highest amount of deposited carbon. Moreover, the initial catalytic activity of all catalysts was not changed during 300 min time on stream.

Fabrication of high sensitivity 2-PEA sensor based on Aldehyde-functionalized mesoporous carbon

2-Phenylethylamine (2-PEA) is one of the main ingredients for stimulants. Therefore, it is necessary to limit its use and illegal trade by conveniently detecting 2-PEA vapour. Here, a QCM based 2-PEA gas sensor was constructed by using aldehyde functionalized mesoporous carbon (FDU-15-CHO) as sensing materials designed according to Schiff base adsorption role. The 2D hexagonal mesoporous structures of the sensing material have been confirmed by small-angle X-ray diffraction (SXRD), transmission electron microscopy (TEM), and N2 adsorption-desorption isotherms. The covalent grafting of aldehyde group onto the FDU-15 was confirmed by Fourier transform infrared spectroscopy (FT-IR). FDU-15-CHO based Quartz Crystal Microbalance (QCM) sensor shows better sensitivity to 2-PEA than its parent FDU-15. Besides, the detection limit of FDU-15-CHO based sensor can reach down to 1 ppm, and its selectivity and reproducibility are satisfactory. The high concentrations of active sites in the mesopores of FDU-15 are believed to facilitate 2-PEA adsorption, and thus the presence of the -CHO group leading to sensitive and selective response, which is verified by Gaussian simulation

A Facile Synthesis of Noble-Metal-Free Catalyst Based on Nitrogen Doped Graphene Oxide for Oxygen Reduction Reaction.

A simple method for the mechanochemical synthesis of an effective metal-free electrocatalyst for the oxygen reduction reaction was demonstrated. A nitrogen-doped carbon material was obtained by grinding a mixture of graphene oxide and melamine in a planetary ball mill. The resulting material was characterized by XPS, EPR, and Raman and IR spectroscopy. The nitrogen concentration on the N-bmGO surface was 5.5 at.%. The nitrogen-enriched graphene material (NbmGO has half-wave potential of −0.175/−0.09 V and was shown to possess high activity as an electrocatalyst for oxygen reduction reaction. The electrocatalytic activity of NbmGO can be associated with a high concentration of active sites for the adsorption of oxygen molecules on its surface. The high current retention (93% for 12 h) after continuous polarization demonstrates the excellent long-term stability of NbmGO.

SrxTi0.6Fe0.4O3−δ (x = 1.0, 0.9) catalysts for ammonia synthesis via proton-conducting solid oxide electrolysis cells (PCECs)

The high concentration of OVs and active sites of S0.9TF promote its high NRR activity. The proton-conducting solid oxide electrolysis cells (PCECs) with S0.9TF material as a cathode showed a high output (bar) and durability (line).

Hierarchical Ni- and Co-based oxynitride nanoarrays with superior lithiophilicity for high-performance lithium metal anodes

Lithium metal has emerged as the most prospective candidate for the realization of improved battery systems. However, notorious Li dendrite formation and the huge volume effect during cycling critically impair the further practical deployment of Li metal batteries. Herein, we propose hierarchical Ni- and Co-based oxynitride (NiCoO2/CoO/Ni3N) nanoarrays with superior lithiophilicity on a three-dimensional nickel foam (NiCoON/NF) as a host for highly stable Li metal anodes. The uniform nitrogen-infused nanorod-on-nanosheet arrays present improved electrical conductivity and an increased concentration of active sites with oxygen vacancies to enhance the surface lithiophilicity, which effectively facilitates homogeneous Li nucleation/growth. Moreover, the hyperbranched structure can induce a homogeneous distribution of Li-ion flux, owing to the enlarged surface area, thereby providing sufficient space to store deposited lithium and relieve the volume expansion. Consequently, the NiCoON/NF host delivers a high Coulombic efficiency (98.4% over 600 cycles) at 1 mA cm-2 and an ultralong lifespan (> 2000 h) under a high capacity of 3 mAh cm-2. Remarkably, a Li@NiCoON/NF-LiFePO4 full battery also reveals impressive electrochemical performance. This work demonstrates new insights into safe rechargeable Li metal batteries.

Boosting the ORR performance of Fe-N/C catalyst via increasing the density and modifying the electronic structure of Fe-NX active sites

Due to the existence of highly active Fe-NX coordination moieties, iron-nitrogen/carbon (Fe-N/C) materials have been considered as potential electrocatalysts toward oxygen reduction reaction (ORR). Increasing the density of Fe-NX sites is supposed to be a useful strategy to enhance the ORR performance of Fe-N/C catalysts. Herein, we report a stepwise synthesis strategy to construct Fe-N/C catalyst possessing a high ratio of mesopores and a high concentration of accessible Fe-NX active sites. Integrated with the properties of composition and morphology, the synthesized catalyst, Fe-N/C‑meso-evap, achieves remarkable ORR activity with a half potential of 0.835 V and a kinetic current density of 31.02 mA cm−2 at 0.8 V in 0.1 M HClO4. This work provides new insight for designing single-atom catalysts towards ORR.

Near-Ambient Pressure XPS and MS Study of CO Oxidation over Model Pd-Au/HOPG Catalysts: The Effect of the Metal Ratio.

In this study, the dependence of the catalytic activity of highly oriented pyrolytic graphite (HOPG)-supported bimetallic Pd-Au catalysts towards the CO oxidation based on the Pd/Au atomic ratio was investigated. The activities of two model catalysts differing from each other in the initial Pd/Au atomic ratios appeared as distinctly different in terms of their ignition temperatures. More specifically, the PdAu-2 sample with a lower Pd/Au surface ratio (~0.75) was already active at temperatures less than 150 °C, while the PdAu-1 sample with a higher Pd/Au surface ratio (~1.0) became active only at temperatures above 200 °C. NAP XPS revealed that the exposure of the catalysts to a reaction mixture at RT induces the palladium surface segregation accompanied by an enrichment of the near-surface regions of the two-component Pd-Au alloy nanoparticles with Pd due to adsorption of CO on palladium atoms. The segregation extent depends on the initial Pd/Au surface ratio. The difference in activity between these two catalysts is determined by the presence or higher concentration of specific active Pd sites on the surface of bimetallic particles, i.e., by the ensemble effect. Upon cooling the sample down to room temperature, the reverse redistribution of the atomic composition within near-surface regions occurs, which switches the catalyst back into inactive state. This observation strongly suggests that the optimum active sites emerge under reaction conditions exclusively, involving both high temperature and a reactive atmosphere.

Quantifying and Understanding PGM-Free Oxygen Reduction Reaction Active Sites by in Situ Molecular Probes

Gaining a better understanding of PGM-free oxygen reduction reaction (ORR) catalyst active site structure and activity relationships is key to developing more active catalysts. Active sites in a variety of Fe-N-C catalysts including commercial Pajarito Powder catalyst, with a range of activity and durability, were studied via in situ catalyst poisoning using thiol-molecular probes. The selectivity and extent of catalyst de-activation by the probes was monitored by comparing changes in ORR activity of the catalysts and N-containing carbon materials. Electrochemical stripping of the molecular probe was used to calculate the concentration of available active sites. These experiments were coupled with catalyst degradation and Fe-leaching experiments to better understand the specificity of the molecular probe binding. Probe-treated and untreated catalysts were also characterized using XPS. We plan to use these methods to better understand structural factors of the Fe-N-C active site, as well as the surrounding NC structure that may influence catalyst activity or durability.This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office (HFTO) through Electrocatalysis Consortium (ElectroCat).

Oxidative kinetic characteristics of dried soaked coal and its related spontaneous combustion mechanism

Spontaneous combustion of the lone-term water soaked coal has been frequently taken place during air drying after water discharge, however the reason for the easier spontaneous combustion of water-immersed coal is still not clear. In this study, raw and coal sample soaked for nearly seven years was selected, low-temperature oxidation and crossing point temperature tests were first analyzed. The increased gaseous products concentration and the decreased crossing point temperature indicated that water immersed coal was more prone to oxidation and spontaneous combustion. Besides, the species and concentration of cations, pore structure, functional groups and thermal kinetic parameters were measured by ICP-MS, N2 adsorption, IR and TG analysis. The concentration of alkali cations that had the inhibitory effect on spontaneous combustion was found to decrease in the water-immersed coal, the pore size and specific surface area were enlarged, thus providing suitable internal conditions for the occurrence of coal spontaneous combustion. Moreover, the variation of functional groups and activation energies during the oxidation process showed that numbers of active sites were accumulated in the process of long-term water immersion and the water immersed coal can be oxidized at low temperatures, leading to the shorting of the side chain and the generation of oxygen-containing functional groups. Therefore, it is proposed that the water immersion process enhances the concentration of active sites in coal and provide physical channels for its oxidation, which led to rapid oxidation and even uncontrollable spontaneous combustion of water-immersed coal. The results help with our understanding of the mechanism of long-term water immersion on spontaneous combustion of coal, in addition to prevent the coal fires of water immersed coal by reducing the concentration of primary active sites.

Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance.

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl2-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene and enables us to propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations.

Effect of moisture and molecular weight of polyaniline on H2S sensing characteristics

Chemical behavior of some gases, such as H2S, changes with presence of moisture. Sensing of H2S by polyaniline (PANI) has been studied; the sensing characteristics depend upon the specific surface area, concentration of active surface sites, pore size, and surface heterogeneity controlled by the chain configuration, which is further governed by the molecular weight. However, effects of moisture in the environment and molecular weight of PANI on H2S sensing characteristics have not been studied, and this is the motivation of the present work. Here we have studied the H2S sensing by PANI of different molecular weights in the same environment (N2) with different moisture (dry and humid) conditions, and in different environments (N2 and compressed air) at the same humidity level. It was observed that regardless of the environment, (i) with increasing concentration of H2S gas, the resistance of the sensors increased in dry atmosphere and decreased in presence of moisture, and (ii) the response time was lower in moist conditions. The sensitivity and response were non-monotonous with respect to the molecular weight. However, response time decreased monotonically with increase in band gap of PANI. New correlations were developed to get surface site kinetics from the response data.

Biomass-derived bifunctional electrocatalysts for oxygen reduction and evolution reaction: A review

Oxygen electrode catalysts are important as inter-conversion of O2 and H2O is crucial for energy technologies. However, the sluggish kinetics of oxygen reduction and evolution reactions (ORR and OER) are a hindrance to their scalable production, whereas scarce and costly Pt and Ir/Ru-based catalysts with the highest electrocatalytic activity are commercially unviable. Since good ORR catalysts are not always efficient for OER and vice versa, so bifunctional catalysts on which OER and ORR occurs on the same electrode are very desirable. Alternative catalysts based on heteroatom-doped carbon nanomaterials, though showed good electrocatalytic activity yet their high cost and complex synthesis is not viable for scalable production. To overcome these drawbacks, biomass-derived heteroatom-doped porous carbons have recently emerged as low-cost, earth-abundant, renewable and sustainable environment-friendly materials for bifunctional oxygen catalysts. The tunable morphology, mesoporous structure and high concentration of catalytic active sites of these materials due to heteroatom (N)-doping could further enhance their ORR and OER activity, along with tolerance to methanol crossover and good durability. Thus, biomass-derived heteroatom-doped porous carbons with large surface area, rich edge defects, numerous micropores and thin 2D nanoarchitecture could be suitable as efficient bifunctional oxygen catalysts. In the present article, synthesis, N-doping, ORR/OER mechanism and electrocatalytic performance of biomass-derived bifunctional catalysts has been discussed. The selected biomass (chitin, eggs, euonymus japonicas, tobacco, lysine and plant residue) except wood, act as both C and N precursor, resulting in N self-doping of porous carbons that avoids the use of toxic chemicals, thus making the synthesis a facile and environment-friendly green process. The synthetic strategy could be further optimized to develop future biomass-based N self-doped porous carbons as metal-free high performance bifunctional oxygen catalysts for commercial energy applications. Recent advances and the importance of biomass-based bifunctional oxygen catalysts in metal-air batteries and fuel cells has been highlighted. The material design, perspectives and future directions in this field are also provided.

Construction of 2D layered TiO2@MoS2 heterostructure for efficient adsorption and photodegradation of organic dyes

In this work, heterostructures of coupled TiO2@MoS2 with different phases of MoS2 were synthesized via hydrothermal technique. The prepared materials were thoroughly characterized using various techniques, including XRD, SEM, transmission electron microscopy, Brunauer–Emmet–Teller, XPS, Zeta potential and UV–vis spectroscopy. The optimized nanocomposites were tested for the photocatalytic degradation of methyl Orange (MO) under visible light as well as the adsorption of Rhodamine b (RhB) and methelene blue (MB) dyes. The TiO2@1T/2H-MoS2 heterostructures exhibited a narrow bandgap compared to the other studied nanomaterials. A remarkable photodegradation efficiency of TiO2@1T/2H-MoS2 was observed, which completely degraded 20 ppm of MO after 60 min with high stability over four successive cycles. This can be assigned to the formation of unique heterostructures with aligned energy bands between MoS2 nanosheets and TiO2 nanobelts. The formation of these novel interfaces promoted the electron transfer and increased the separation efficiency of carriers, resulting in high photocatalytic degradation. Furthermore, the adsorption efficiency of TiO2@1T/2H-MoS2 was unique, 20 ppm solutions of RhB and MB were removed after 1 and 2 min, respectively. The superior adsorption performance of the TiO2@1T/2H-MoS2 can be attributed to its high surface area (279.9 m2 g−1) and the rich concentration of active sites. The kinetics and the isothermal analysis revealed that the TiO2@1T/2H MoS2 heterstructures have maximum adsorption capacity of 1200 and 970 mg g−1 for RhB and MB, respectively. This study provides a powerful way for designing an effective photocatalyst and adsorbent TiO2-based nanocomposites for water remediation.