Preparation and Comparison of Supported Gold Nanocatalysts on Anatase, Brookite, Rutile, and P25 Polymorphs of TiO2 for Catalytic Oxidation of CO

Comparative study between supported bimetallic catalysts for nitrate remediation in water

As the population grows and the demand for water rises, the development of efficient and sustainable water purification techniques is becoming increasingly important to ensure access to clean and safe water in the future. The pollution of surface and groundwater by nitrate ( NO 3 − {\text{NO}}_{3}^{-} ) is a growing global concern due to the rise in nitrogen-rich waste released from agriculture and industry. The removal of nitrate ions from aqueous media using bimetallic catalysts loaded on several supports was studied. Multiwalled carbon nanotubes, activated carbon, titanium dioxide, titanium dioxide/multiwalled carbon nanotubes, and Santa Barbara Amorphous-15 were used as supports to synthesize these bimetallic catalysts. The effects of the support type, supported metal, and catalyst reduction method on the nitrate reduction activity in water were investigated. The catalysts were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, Brunauer-Emmett-Teller isotherm, inductively coupled plasma spectroscopy, and field emission gun scanning transmission electron microscope. In terms of nitrate conversion, high-temperature hydrogen reduction of the catalysts was a more effective method of catalyst preparation than NaBH4 reduction. Except for the carbon nanotube-TiO2 composite, pH fixation using CO2 flow improved the efficiency of supported catalysts. The catalysts 1Pd–1Cu/TiO2 and 1Pd–Cu/SBA-15 presented the highest catalytic activity, but the latter was the most selective to nitrogen.

Green Route Synthesis and Characterization Techniques of Silver Nanoparticles and Their Biological Adeptness

The development of the most reliable and green techniques for nanoparticle synthesis is an emerging step in the area of green nanotechnology. Many conventional approaches used for nanoparticle (NP) synthesis are expensive, deadly, and nonenvironmental. In this new era of nanotechnology, to overcome such concerns, natural sources which work as capping and reducing agents, including bacteria, fungi, biopolymers, and plants, are suitable candidates for synthesizing AgNPs. The surface morphology and applications of AgNPs are significantly pretentious to the experimental conditions by which they are synthesized. Available scattered information on the synthesis of AgNPs comprises the influence of altered constraints and characterization methods such as FTIR, UV-vis, DLS, SEM, TEM, XRD, EDX, etc. and their properties and applications. This review focuses on all the above-mentioned natural sources that have been used for AgNP synthesis recently. The green routes to synthesize AgNPs have established effective applications in various areas, including biosensors, magnetic resonance imaging (MRI), cancer treatment, surface-enhanced Raman spectroscopy (SERS), antimicrobial agents, drug delivery, gene therapy, DNA analysis, etc. The existing boundaries and prospects for metal nanoparticle synthesis by the green route are also discussed herein.

Fabrication of More Oxygen Vacancies and Depression of Encapsulation for Superior Catalysis in the Water-Gas Shift Reaction

Fabrication of sufficient oxygen vacancies and exposure of active sites to reactants are two key factors to obtain high catalytic activity in the water-gas shift (WGS) reaction. However, these two factors are hard to satisfy spontaneously, since the formation of oxygen vacancies and encapsulation of metal nanoparticles are two inherent properties in reducible metal oxide supported catalysts due to the strong metal-support interaction (SMSI) effect. In this work, we find that addition of alkali to an anatase supported Ni catalyst (Ni/TiO₂(A)) could well regulate the SMSI to achieve both more oxygen vacancies and depression of encapsulation; therefore, more than 20-fold enhancement in activity is obtained. It is found that the in situ formed titanate species on the catalyst surface is crucial to the formation of oxygen vacancies and depression of encapsulation. Furthermore, the methanation, a common side reaction of the WGS reaction, is successfully suppressed in the whole catalytic process.

Diabat method for polymorph free energies: Extension to molecular crystals

Lattice-switch Monte Carlo and the related diabat methods have emerged as efficient and accurate ways to compute free energy differences between polymorphs. In this work, we introduce a one-to-one mapping from the reference positions and displacements in one molecular crystal to the positions and displacements in another. Two features of the mapping facilitate lattice-switch Monte Carlo and related diabat methods for computing polymorph free energy differences. First, the mapping is unitary so that its Jacobian does not complicate the free energy calculations. Second, the mapping is easily implemented for molecular crystals of arbitrary complexity. We demonstrate the mapping by computing free energy differences between polymorphs of benzene and carbamazepine. Free energy calculations for thermodynamic cycles, each involving three independently computed polymorph free energy differences, all return to the starting free energy with a high degree of precision. The calculations thus provide a force field independent validation of the method and allow us to estimate the precision of the individual free energy differences.

Recovery of waste gold for the synthesis of gold nanoparticles supported on radially aligned nanorutile: the growth of carbon nanomaterials

Precious and expensive metals are lost each year through the discarding of old jewellery pieces and mine tailings. In this work, small amounts of gold were recovered by digestion with aqua regia from waste tailings. The recovered gold in the form of HAuCl₄ was then used to deposit Au⁰ onto radially aligned nanorutile (RANR) to form a supported catalyst material. The support material, RANR, was synthesized using the hydrothermal technique whereas the deposition of gold was achieved using the deposition–precipitation with urea method at various loadings. Electron microscopy was used to show that the structure of the support is a sphere formed by multiple nanorods aligned in a radial structure. The Au nanoparticles were observed at the tips of the nanorods. It was confirmed by XRD that the support was indeed a rutile phase of TiO₂ and that the Au nanoparticles had a face-centred cubic structure. The various catalysts were then used to synthesize carbon nanomaterials (CNMs) using the chemical vapour deposition technique. A parametric study varying the reaction temperature, duration and carbon source gas flow rate was conducted to study the effects these conditions have on the structural properties of the resulting CNMs. Here, it was found that mainly carbon nanofibers were formed and that the different reaction conditions influenced their graphicity, width, structure and thermal properties.

A hydrothermal method was used to prepare rutile TiO₂ dandelions. A deposition–precipitation method using urea (DPU) was used to load Au metal nanoparticles in calculated weight percentages and the Au/RANR catalysts where used to synthesise CNFs in a CVD reaction.

Highly Crystalline Mesoporous Titania Loaded with Monodispersed Gold Nanoparticles: Controllable Metal-Support Interaction in Porous Materials

We report the syntheses of mesoporous Au/TiO₂ hybrid photocatalysts with ordered and crystalline frameworks using co-assembly of organosilane-containing colloidal amphiphile micelles (CAMs) and poly(ethylene oxide)-modified gold nanoparticles (AuNPs) as templates. The assembled CAMs can convert to inorganic silica during calcination at elevated temperatures, providing extraordinary thermal stability to preserve the porosity of TiO₂ and the nanostructures of AuNPs. Well-defined AuNPs supported within mesoporous TiO₂ (Au@mTiO₂) can be prepared using thermal annealing at temperatures up to 800 °C. High-temperature treatment (≥500 °C) under air is found to not only improve the crystallinity of TiO₂ but also induce oxidative strong metal-support interactions (SMSIs) at Au/TiO₂ interfaces. For oxidative SMSIs, the surface oxidation of AuNPs can generate positively charged Au^δ+ species, while TiO₂ gets reduced simultaneously. Using photocatalytic oxidation of benzyl alcohol as a model reaction, Au@mTiO₂ calcined at 600 °C for 12 h exhibited the best activity under UV irradiation, while Au@mTiO₂ calcined at 600 °C for 2 h showed the best activity under visible light. The delicate balance between the crystallinity and porosity of TiO₂ and the SMSIs at Au-TiO₂ interfaces is found to impact the photocatalytic activity of these hybrid materials.

Solvent-Free Catalytic Oxidation of Benzyl Alcohol over Au-Pd Bimetal Deposited on TiO2: Comparison of Rutile, Brookite, and Anatase

TiO₂ (P25)-supported Au-Pd bimetal nanoparticles displayed excellent performance in the solvent-free benzyl alcohol catalytic oxidation. However, little research attention has been paid to investigate the effects of TiO₂ form on the catalytic activity of Au-Pd/TiO₂. In the present research, rutile, brookite, and anatase TiO₂ were successfully synthesized and subsequently applied as the carrier to load Au-Pd nanoparticles by the deposition-precipitation method. The experimental results indicated that the benzyl alcohol conversion employing the rutile TiO₂-supported Au-Pd catalyst is higher than the conversion of anatase and brookite TiO₂-loaded Au-Pd catalysts. However, the Au-Pd/TiO₂-rutile displayed the lowest and highest selectivity toward benzaldehyde and toluene, respectively. ICP-AES, XRD, XPS, and TEM were conducted to characterize these catalysts. The corresponding experimental results revealed that the excellent performance of Au-Pd/TiO₂-rutile catalyst was attributed to both the smaller Au-Pd nanoparticle size distribution and the higher concentrations of O_α and Pd²⁺ species on the catalyst surface. In the recycle experiments, the Au-Pd/TiO₂-rutile catalyst displayed lower reaction stability compared with the Au-Pd/TiO₂-anatase and Au-Pd/TiO₂-brookite, which might be due to the coverage of larger amount of aldehyde products on the surface.

Bacteria-Assisted Synthesis of Nanosheet-Assembled TiO2 Hierarchical Architectures for Constructing TiO2-Based Composites for Photocatalytic and Electrocatalytic Applications

Synthesis and application of three-dimensional TiO₂ hierarchical architectures are one of the major priorities in the research and development of TiO₂ catalysts. Using bacteria as a template and a reactor, a bioinspired strategy was developed in the present study to synthesize nanosheet-assembled TiO₂ hierarchical architectures (N-TiO₂-HA) and relative composites for photocatalytic and electrocatalytic applications. In the first part of this work, three kinds of bacteria were used for the synthesis of N-TiO₂-HA with satisfactory monodispersity, and the growth mechanism was investigated. In the second part, porous TiO₂ hollow spheres (P-TiO₂-HS), which were obtained by calcining N-TiO₂-HA at 750 °C in air, were incorporated with MIL-101(Fe) to improve the visible-light photocatalytic efficiency. The results of the photo-Fenton-assisted degradation of rhodamine B and ciprofloxacin indicate that the synthesized composites have excellent visible-light photocatalytic activity. In the third part, the nanosheet-assembled TiO₂-carbon hollow spheres (N-TiO₂-C-HS), which were obtained by calcining N-TiO₂-HA at 750 °C in argon atmosphere, were electrodeposited with Pt for electrocatalytic oxidation of methanol. The electrochemical measurements show that Pt-deposited N-TiO₂-C-HS have better electrocatalytic activity, stability, and tolerance to CO poisoning than commercial Pt/C catalysts.

Visible light driven photo-degradation of Congo red by TiO2ZnO/Ag: DFT approach on synergetic effect on band gap energy

In this paper, we report the combination of two metal oxides (TiO₂ZnO) that allows mixed density of states to reduce band gap energy, facilitating the photo-oxidation of Congo red dye under visible light. For the oxidation, a possible mechanism is proposed after analyzing the intermediates by GC-MS, and it is consistent with Density Functional Theory (DFT). The nanohybrids were characterized comprehensibly by several analytical techniques such as X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and X-ray Photoelectron Spectroscopy (XPS). For the addition of ZnO to TiO₂, a dominance of anatase phase was found rather than other phases (rutile or brookite). A broad band (∼550 nm) is observed in UV-Visible spectra for TiO₂ZnO/Ag NPs nm because of Surface Plasmon properties of Ag NPs. The band gap energy was calculated for TiO₂ZnO/Ag system, and then it has been further studied by DFT in order to show why the convergence of two semiconductors allows a mixed density of states, facilitating the reduction of the energy gap between occupied and unoccupied bands; ultimately, it improves the performance of catalysts under visible light. Significantly, the interaction of crystal planes (0 0 Ī) of TiO₂ anatase and (0 0 1) of ZnO crucially plays as an important role for the reduction of energy band-gap. Additionally, TiO₂ZnOAg NPs were used recognize Saccharomyces cerevisiae cells by con-focal fluorescence microscope, showing that it develops bright bio-images for the cells; while for TiO₂ or ZnO or TiO₂ZnO NPs, no fluorescent response was seen within the cells.

Safety protocol for the gold nanoparticles administration in rats

Gold nanoparticles (GNPs) have antioxidant and anti-inflammatory effects. However, toxicity is still a concern; therefore, it is critical to study both the therapeutic and toxic properties of GNPs. In this study, we evaluated the effects of the intraperitoneal administration of GNPs (20nm, at a concentration of 2.5mg/L for 21days) every 24 or 48h on oxidative stress, antioxidant status, and electron chain transport (ETC) in the brain. Liver histology and blood marker analyses were conducted to establish a time routine of GNP administration. The concentrations of GNP in the brain and liver were similar. Hepatic and serum levels of cholesterol, triglycerides, and transaminases were not altered after the administration of GNP every 24 or 48h. The superoxide and nitric oxide levels were unchanged after administration of GNP. Dichlorodihydrofluorescein (DCFH) levels decreased after the administration of GNP every 48h compared with that in the saline group. Sulfhydryl and carbonyl levels, as well as superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT), and glutathione (GSH) activities were not altered in the brain after administration of GNP in the two time periods studied. The GNP 48h group showed increased brain ETC activity. Compared to that in the saline group, the GNP 24h group showed marked parenchyma changes with cell necrosis and leukocyte infiltration. We therefore suggest that a concentration of 2.5mg/L of GNP administered every 48h has potential therapeutic benefits without toxicity.

Diphosphine-Protected Au22 Nanoclusters on Oxide Supports Are Active for Gas-Phase Catalysis without Ligand Removal

Investigation of atomically precise Au nanoclusters provides a route to understand the roles of coordination, size, and ligand effects on Au catalysis. Herein, we explored the catalytic behavior of a newly synthesized Au₂₂(L⁸)₆ nanocluster (L = 1,8-bis(diphenylphosphino) octane) with in situ uncoordinated Au sites supported on TiO₂, CeO₂, and Al₂O₃. Stability of the supported Au₂₂ nanoclusters was probed structurally by in situ extended X-ray absorption fine structure (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and their ability to adsorb and oxidize CO was investigated by IR absorption spectroscopy and a temperature-programmed flow reaction. Low-temperature CO oxidation activity was observed for the supported pristine Au₂₂(L⁸)₆ nanoclusters without ligand removal. Density functional theory (DFT) calculations confirmed that the eight uncoordinated Au sites in the intact Au₂₂(L⁸)₆ nanoclusters can chemisorb both CO and O₂. Use of isotopically labeled O₂ demonstrated that the reaction pathway occurs mainly through a redox mechanism, consistent with the observed support-dependent activity trend of CeO₂ > TiO₂ > Al₂O₃. We conclude that the uncoordinated Au sites in the intact Au₂₂(L⁸)₆ nanoclusters are capable of adsorbing CO, activating O₂, and catalyzing CO oxidation reaction. This work is the first clear demonstration of a ligand-protected intact Au nanocluster that is active for gas-phase catalysis without the need of ligand removal.

Enhanced CO catalytic oxidation of flower-like Co3O4composed of small nanoparticles

The enhanced catalytic performance of flower-like Co₃O₄composed of nanoparticles originates from the redistribution of surface ions induced by the calcining process.

Magnetic iron oxide nanoparticles encapsulating horseradish peroxidase (HRP): synthesis, characterization and carrier for the generation of free radicals for potential applications in cancer therapy

We report synthesis of iron oxide nanoparticles encapsulating HRP. The average diameter of the particles was around 20 nm. HRP has been used to convert IAA to a toxic oxidized product and its toxic effect has been seen on cancerous cell lines.

Titanium dioxide as a catalyst support in heterogeneous catalysis

The lack of stability is a challenge for most heterogeneous catalysts. During operations, the agglomeration of particles may block the active sites of the catalyst, which is believed to contribute to its instability. Recently, titanium oxide (TiO₂) was introduced as an alternative support material for heterogeneous catalyst due to the effect of its high surface area stabilizing the catalysts in its mesoporous structure. TiO₂ supported metal catalysts have attracted interest due to TiO₂ nanoparticles high activity for various reduction and oxidation reactions at low pressures and temperatures. Furthermore, TiO₂ was found to be a good metal oxide catalyst support due to the strong metal support interaction, chemical stability, and acid-base property. The aforementioned properties make heterogeneous TiO₂ supported catalysts show a high potential in photocatalyst-related applications, electrodes for wet solar cells, synthesis of fine chemicals, and others. This review focuses on TiO₂ as a support material for heterogeneous catalysts and its potential applications.

A simple and generic approach for synthesizing colloidal metal and metal oxide nanocrystals

A simple and generic approach--alternating voltage induced electrochemical synthesis (AVIES)--has been reported for synthesizing highly dispersed colloidal metal (Au, Pt, Sn, and Pt-Pd) and metal oxide (ZnO and TiO2) nanocrystals. The respective nanocrystals are produced when a zero-offset alternating voltage at 60 Hz is applied to a pair of identical metal wires, which are inserted in an electrolyte solution containing capping ligands. In the case of Au, the obtained nanocrystals are highly crystalline nano-icosahedra of 14 ± 2 nm in diameter, the smallest Au icosahedra synthesized in aqueous solutions via green chemistry. Their catalytic activity has been demonstrated through facilitating the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride. This AVIES approach is an environmentally benign process and can be adopted by any research lab.

Surface coating effects on the assembly of gold nanospheres

Optical spectra and atomic force microscopy (AFM) images of individually selected spheres and mechanically assembled silica-coated gold nanosphere pairs were recorded. The shell served as a means of rigid control of the minimum spacing between the metal cores. The spectra of the assembled spheres were simulated using classical electrodynamics. The observed spectra resulted in superior characterization of the particle assembly geometry, relative to the AFM data. Experimental investigations regarding less-rigid polyvinylpyrrolidone (PVP) sphere coatings were also performed and some comparisons were made.

Preparation and properties of gold nanoparticle-electrodeposited titanium substrates with Arg-Gly-Asp-Cys peptides

Titanium metal has good biocompatibility, superior mechanical properties and excellent corrosion resistance. Like most metals, however, it exhibits poor bioactive properties and fails to bond to bone tissue. To improve its bioactivity, bioactive molecules, such as peptides, can be grafted onto titanium surfaces. In order to do this, the first step may be to establish a stable and compatible linking layer on the titanium surface. In this study, we used electrochemical methods to deposit gold (Au) nanoparticles onto titanium substrates, to which we then grafted arginine-glycine-asparagine-cysteine (RGDC) peptides by thiolate covalent coupling. Properties of electrodeposited Au nanoparticles were evaluated using a variety of techniques, including microstructural, chemical and electrochemical measurements. The biological responses of the RGDC-grafted Ti substrates were evaluated using MG3 human osteoblast-like cells. The results of thin-film X-ray diffraction (TFXRD) and scanning electron microscopy (SEM) indicated the polycrystalline orientation of Au nanoparticles deposited on the titanium surfaces with high density and controllable particle size. The RGDC peptide could be covalently bonded to Au-deposited Ti substrates via Au-thiolate species, as expected. Cell morphology showed that, on RGDC-immobilized titanium with Au particles, MG63 cells attached and spread more rapidly than on Ti substrates either without peptide or with direct loading of the peptide. Immunostaining for focal adhesion kinase (FAK) demonstrated that RGDC enhanced cell attachment. The present method for the formation of Au nanoparticles may serve as an alternative route for bioactive molecule immobilization on Ti implants.