Effect of gold loading and TiO2 support composition on the activity of Au/TiO2 photocatalysts for H2 production from ethanol–water mixtures

Oxygen vacancy-rich high-pressure rocksalt phase of zinc oxide for enhanced photocatalytic hydrogen evolution

The generation of hydrogen as a clean energy carrier by photocatalysis, as a zero-emission technology, is of significant scientific and industrial interest. However, the main drawback of photocatalytic hydrogen generation from water splitting is its low efficiency compared to traditional chemical or electrochemical methods. Zinc oxide (ZnO) with the wurtzite phase is one of the most investigated photocatalysts for hydrogen production, but its activity still needs to be improved. In this study, an oxygen-deficient high-pressure ZnO rocksalt phase is stabilized using a high-pressure torsion (HPT) method, and the product is used for photocatalysis under ambient pressure. The simultaneous introduction of oxygen vacancies and the rocksalt phase effectively improved photocatalytic hydrogen production to levels comparable to benchmark P25 TiO2, due to improving light absorbance and providing active sites for photocatalysis without any negative effect on electron-hole recombination. These results confirm the high potential of high-pressure phases for photocatalytic hydrogen generation.

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

Gamma Radiation Synthesis of Ag/P25 Nanocomposites for Efficient Photocatalytic Degradation of Organic Contaminant

Titanium dioxide (TiO₂) has garnered significant attention among various photocatalysts, whereas its photocatalytic activity is limited by its wide bandgap and inefficient charge separation, making the exploration of new strategies to improve its photocatalytic performance increasingly important. Here, we report the synthesis of Ag/P25 nanocomposites through a one-step gamma-ray radiation method using AgNO₃ and commercial TiO₂ (Degussa P25). The resulting products were characterized by powder X-ray diffraction, UV-Vis diffused reflectance spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The effect of free radical scavengers, feed ratios of Ag/P25, and dose rates on the photocatalytic activity of the Ag/P25 nanocomposites were systematically investigated using rhodamine B under Xenon light irradiation. The results showed that the Ag/P25 photocatalyst synthesized with a feed ratio of 2.5 wt% and isopropyl alcohol as the free radical scavenger at a dose rate of 130 Gy/min exhibited outstanding photocatalytic activity, with a reaction rate constant of 0.0674 min^-1, much higher than that of P25. Additionally, we found that the particle size of Ag could be effectively controlled by changing the dose rate, and the Ag/P25 nanocomposites doped with smaller size of Ag nanoparticles performed higher photocatalytic activities. The synthesis strategy presented in this study offers new insight into the future development of highly efficient photocatalysts using radiation techniques.

Photosynthesis of Au/TiO2 nanoparticles for photocatalytic gold recovery from industrial gold-cyanide plating wastewater

A series of Au_x/TiO₂ nanoparticles (NPs) with different gold loadings (x = 0.1-1.0 wt%) was synthesized by the photodeposition and then employed as photocatalysts to recover precious component from the industrial gold-cyanide plating wastewater. Effects of Au loading, catalyst dosage and types of hole scavenger on the photocatalytic gold recovery were investigated under ultraviolet-visible (UV-Vis) light irradiation at room temperature. It was found that different Au loadings tuned the light absorption capacity of the synthesized photocatalysts and enhanced the photocatalytic activity in comparison with the bare TiO₂ NPs. The addition of CH₃OH, C₂H₅OH, C₃H₈O, and Na₂S₂O₃ as a hole scavenger significantly promoted the photocatalytic activity of the gold recovery, while the H₂O₂ did not. Among different hole scavengers employed in this work, the CH₃OH exhibited the highest capability to promote the photocatalytic gold recovery. In summary, the Au_0.5/TiO₂ NPs exhibited the best photocatalytic activity to completely recover gold ions within 30 min at the catalyst dosage of 0.5 g/L, light intensity of 3.20 mW/cm² in the presence of 20 vol% CH₃OH as hole scavenger. The photocatalytic activity slightly decreased after the 5th cycle of recovery process, indicating its high reusability.

Development of efficient SALDI substrate based on Au-TiO2 nanohybrids for environmental and forensic detection of dyes and NSAIDs

Herein, a matrix-free approach is presented for comprehensive environmental and forensic analysis of dyes and nonsteroidal anti-inflammatory drugs (NSAIDs) using Au-TiO₂ nanohybrids coupled with surface-assisted pulsed laser desorption ionization-mass spectrometry (SALDI-MS). The Au-TiO₂ nanohybrids was prepared and characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), surface area measurements, ultraviolet-visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy-energy dispersive spectrometry (SEM-EDS). Initially, the optimal Au content was assessed using the survival yield (SY) method, confirming that 7.5% Au content on the TiO₂ surface offered the highest ionization efficiency. Subsequently, environmental analyses of dyes and NSAIDs in water samples were performed, and sensitive detection of all analytes was achieved with limits of detection (LODs) ranging from 10.0 ng mL^-1 to 10.0 fg mL^-1 and good spot-to-spot reproducibility. Additionally, the effect of potential contaminants commonly found in environmental samples, such as salts, surfactants and pesticides was also considered. Despite signal intensity reduction at high concentrations of some salts, the target analytes were detected, while the presence of surfactants and pesticides did not cause significant signal intensity reduction. Additionally, dyed and undyed Tetoron fibers and the effect of adhesive tape were evaluated. Direct analysis of the dyed Tetoron fibers on the target plate, using the nanohybrids, enabled higher detection sensitivity of the dyes, in addition to adducts of polystyrene and cellulose, the main components of the fiber. Finally, NSAIDs in oral fluid were analyzed and sensitive detection of the analytes was observed using the nanohybrids with LODs and LOQs in the range of 0.1-10 ng mL^-1 and 1-20 ng mL^-1, respectively. The trueness of the exact mass was in the range of 0.64-6.2 ppm while the recovery of the spiked samples was in the range of 82.90-107.54%% indicating the efficiency of the Au-TiO₂ nanohybrids as SALDI substrate. Thus, the Au-TiO₂ nanohybrids hold considerable promise in terms of sensitivity, reproducibility, and LOD, and may significantly contribute to environmental and forensic identification.

Unidirectional/Bidirectional Electron Transfer at the Au/TiO2 Interface Operando Tracked by SERS Spectra from Au and TiO2

Although it is well-known that the size can influence the surface plasmon resonance property of coinage metals and the electronic state of the Mott-Schottky junction formed at the metal/semiconductor interface, insights into how the size can be exploited to optimize the photocatalytic activity and selectivity of metal/semiconductor composites are lacking. Here we utilize operando SERS spectroscopy to identify the size effect on the electron-transfer dynamics and the direction at the Au/TiO₂ interface. This effect was characterized by the photocatalytic reduction sites of p-nitrothiophenol, which were self-tracked with the SERS spectra from Au nanoparticle and inverse-opal structured TiO₂, respectively. The size-dependent unidirectional/bidirectional transfer of photoinduced electrons at the Au/TiO₂ interface was revealed by operando SERS spectroscopy, which enables the rational tuning of the reduction selectivity.

Highly efficient visible-light photocatalytic ethane oxidation into ethyl hydroperoxide as a radical reservoir

Photocatalytic ethane conversion into value-added chemicals is a great challenge especially under visible light irradiation. The production of ethyl hydroperoxide (CH₃CH₂OOH), which is a promising radical reservoir for regulating the oxidative stress in cells, is even more challenging due to its facile decomposition. Here, we demonstrated a design of a highly efficient visible-light-responsive photocatalyst, Au/WO₃, for ethane oxidation into CH₃CH₂OOH, achieving an impressive yield of 1887 μmol g_cat ^-1 in two hours under visible light irradiation at room temperature for the first time. Furthermore, thermal energy was introduced into the photocatalytic system to increase the driving force for ethane oxidation, enhancing CH₃CH₂OOH production by six times to 11 233 μmol g_cat ^-1 at 100 °C and achieving a significant apparent quantum efficiency of 17.9% at 450 nm. In addition, trapping active species and isotope-labeling reactants revealed the reaction pathway. These findings pave the way for scalable ethane conversion into CH₃CH₂OOH as a potential anticancer drug.

Plasmonic Au–Pd Bimetallic Nanocatalysts for Hot-Carrier-Enhanced Photocatalytic and Electrochemical Ethanol Oxidation

Gold–palladium (Au–Pd) bimetallic nanostructures with engineered plasmon-enhanced activity sustainably drive energy-intensive chemical reactions at low temperatures with solar simulated light. A series of alloy and core–shell Au–Pd nanoparticles (NPs) were prepared to synergistically couple plasmonic (Au) and catalytic (Pd) metals to tailor their optical and catalytic properties. Metal-based catalysts supporting a localized surface plasmon resonance (SPR) can enhance energy-intensive chemical reactions via augmented carrier generation/separation and photothermal conversion. Titania-supported Au–Pd bimetallic (i) alloys and (ii) core–shell NPs initiated the ethanol (EtOH) oxidation reaction under solar-simulated irradiation, with emphasis toward driving carbon–carbon (C–C) bond cleavage at low temperatures. Plasmon-assisted complete oxidation of EtOH to CO2, as well as intermediary acetaldehyde, was examined by monitoring the yield of gaseous products from suspended particle photocatalysis. Photocatalytic, electrochemical, and photoelectrochemical (PEC) results are correlated with Au–Pd composition and homogeneity to maintain SPR-induced charge separation and mitigate the carbon monoxide poisoning effects on Pd. Photogenerated holes drive the photo-oxidation of EtOH primarily on the Au-Pd bimetallic nanocatalysts and photothermal effects improve intermediate desorption from the catalyst surface, providing a method to selectively cleave C–C bonds.

Hierarchical TiO2 Nanoflower Photocatalysts with Remarkable Activity for Aqueous Methylene Blue Photo-Oxidation

This study systematically evaluates the performance of a series of TiO₂ nanoflower (TNF) photocatalysts for aqueous methylene blue photo-oxidation under UV irradiation. TNF nanoflowers were synthesized from Ti(IV) butoxide by a hydrothermal method and then calcined at different temperatures (T = 400-800 °C) for specific periods of time (t = 1-5 h). By varying the calcination conditions, TNF-T-t photocatalysts with diverse physicochemical properties and anatase/rutile ratios were obtained. Many of the TNF-T-1 photocatalysts demonstrated remarkable activity for aqueous methylene blue photo-oxidation at pH 6 under UV excitation (365 nm), with activities following the order TNF-700-1 > TNF-600-1 > TNF-500-1 > TNF-400-1 ∼ P25 TiO₂ ≫ TNF-800-1. The activity of the TNF-700-1 photocatalyst (99% anatase, 1% rutile) was 2.3 times that of P25 TiO₂ at pH 6 and 14.4 times that of P25 TiO₂ at pH 4. Prolonged calcination of the TNFs at 700 °C proved detrimental to dye degradation performance due to excessive rutile formation, which reduced the photocatalyst surface area and suppressed OH^• generation. The outstanding activities of TNF-700-1 and TNF-600-1 are attributed to their hierarchical nanoflower morphology which benefitted UV absorption, a near-ideal anatase crystallite size for efficient charge separation, and their unusually low isoelectric point (IEP = 4.3-4.5).

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.

Room-Temperature Laser Synthesis in Liquid of Oxide, Metal-Oxide Core-Shells, and Doped Oxide Nanoparticles

Although oxide nanoparticles are ubiquitous in science and technology, a multitude of compositions, phases, structures, and doping levels exist, each one requiring a variety of conditions for their synthesis and modification. Besides, experimental procedures are frequently dominated by high temperatures or pressures and by chemical contaminants or waste. In recent years, laser synthesis of colloids emerged as a versatile approach to access a library of clean oxide nanoparticles relying on only four main strategies running at room temperature and ambient pressure: laser ablation in liquid, laser fragmentation in liquid, laser melting in liquid and laser defect-engineering in liquid. Here, established laser-based methodologies are reviewed through the presentation of a panorama of oxide nanoparticles which include pure oxidic phases, as well as unconventional structures like defective or doped oxides, non-equilibrium compounds, metal-oxide core-shells and other anisotropic morphologies. So far, these materials showed several useful properties that are discussed with special emphasis on catalytic, biomedical and optical application. Yet, given the endless number of mixed compounds accessible by the laser-assisted methodologies, there is still a lot of room to expand the library of nano-crystals and to refine the control over products as well as to improve the understanding of the whole process of nanoparticle formation. To that end, this review aims to identify the perspectives and unique opportunities of laser-based synthesis and processing of colloids for future studies of oxide nanomaterial-oriented sciences.

Black TiO2 nanoparticles with efficient photocatalytic activity under visible light at low temperature: regioselective C–N bond cleavage toward the synthesis of thioureas, sulfonamides, and propargylamines

Several different colored forms of TiO₂ were prepared through the easy treatment of white TiO₂ and NaBH₄ as a safe hydrogen source. Then, tertiary amines were harnessed toward the regioselective synthesis of three prominent scaffolds.

Au/MoS2/Ti3C2 composite catalyst for efficient photocatalytic hydrogen evolution

The Au/MoS₂/Ti₃C₂ composite catalyst has efficient photocatalytic hydrogen evolution performance.

Hydrogen Photo-Production from Glycerol Using Nickel-Doped TiO2 Catalysts: Effect of Catalyst Pre-Treatment

In the present piece of research, hydrogen production via the photo-reforming of glycerol (a byproduct from biodiesel generation) is studied. Catalysts consisted of titania modified by Ni (0.5% by weight) obtained through deposition–precipitation or impregnation synthetic methods (labelled as Ni-0.5-DP and Ni-0.5-IMP, respectively). Reactions were performed both under UV and solar irradiation. Activity significantly improved in the presence of Ni, especially under solar irradiation. Moreover, pre-reduced solids exhibited higher catalytic activities than untreated solids, despite the “in-situ” reduction of nickel species and the elimination of surface chlorides under reaction conditions (as evidenced by XPS). It is possible that the catalyst pretreatment at 400 °C under hydrogen resulted in some strong metal–support interactions. In summary, the highest hydrogen production value (ca. 2600 micromole H2·g−1) was achieved with pre-reduced Ni-0.5-DP solid using UV light for an irradiation time of 6 h. This value represents a 15.7-fold increase as compared to Evonik P25.

Synergistic Effect of Dual Particle-Size AuNPs on TiO₂ for Efficient Photocatalytic Hydrogen Evolution

Design of efficient catalysts for photocatalytic water-splitting hydrogen evolution is of fundamental importance for the production of sustainable clean energy. In this study, a dual particle-size AuNPs/TiO₂ composite containing both small (16.9 ± 5.5 nm) and large (45.0 ± 9.8 nm) AuNPs was synthesized by annealing two different sized AuNPs onto TiO₂ nanosheets. Dual particle-size AuNPs/TiO₂ composites of 2.1 wt% catalyze photocatalytic H₂ evolution 281 times faster than pure TiO₂. Control experiments indicate the observed rate increase for the 2.1 wt% dual particle-size AuNPs/TiO₂ composites is larger than 2.1 wt% small AuNPs/TiO₂ composites, or 2.1 wt% large AuNPs/TiO₂ composites in isolation. The observed photocatalytic enhancement can be attributed to the synergistic effect of dual particle-size AuNPs on TiO₂. Specifically, small-sized AuNPs can act as an electron sink to generate more electron-hole pairs, while the surface plasmon resonance (SPR) effect of large-sized AuNPs concurrently injects hot electrons into the TiO₂ conduction band to enhance charge transfer. In addition, a gold-dicyanodiamine composite (GDC)-directed synthesis of 2.1 wt% dual particle-size AuNPs/TiO₂ composites was also completed. Notably, a photocatalytic efficiency enhancement was observed that was comparable to the previously prepared 2.1 wt% dual particle-size AuNPs/TiO₂ composites. Taken together, the synergistic effects of dual particle-size AuNPs on TiO₂ can be potentially used as a foundation to develop semiconductor photocatalyst heterojunction with enhanced photocatalytic activity.

In situ Synthesis of Au-Induced Hierarchical Nanofibers/Nanoflakes Structured BiFeO3 Homojunction Photocatalyst With Enhanced Photocatalytic Activity

In order to further improve the photocatalytic performance of BiFeO₃ (BFO), novel Au-induced hierarchical nanofibers/nanoflakes structured BiFeO₃ homojunctions (Au_x-BFO, x = 0, 0. 6, 1.2, 1.8, 2.4 wt%) were in situ synthesized through a simple reduction method with assist of sodium citrate under the analogous hydrothermal environment. The effect of loading amount of Au nanoparticles (NPs) on the physicochemical properties and photocatalytic activity was investigated in detail. The Au_1.2-BFO NFs sample show the best photocatalytic activity (85.76%), much higher than that for pure BFO samples (49.49%), mainly due to the hierarchical nanofibers/nanoflakes structured homojunction, the surface plasmon resonance (SPR) effect of Au NPs, as well as the presence of defects (Fe²⁺/Fe³⁺ pairs and oxygen vacancy). Furthermore, the possible formation mechanism of the unique homojunction and the enhanced photocatalytic mechanism for the degradation of methylene blue (MB) dye are proposed. It is proven that holes (h⁺) play the decisive role in the photocatalytic process. The present work provides a fascinating way to synthesize efficient homojunctions for the degradation of organic pollutes.

Photo-catalytic hydrogen production over Au/g-C3N4: effect of gold particle dispersion and morphology

Metal/semiconductor interactions affect electron transfer rates and this is central to photocatalytic hydrogen ion reduction. While this interaction has been studied in great detail on metal oxide semiconductors, not much is known of Au particles on top of polymeric semiconductors. The effects of gold nanoparticle size and dispersion on top of g-C3N4 were studied by core and valence level spectroscopy and transmission electron microscopy in addition to catalytic tests. The as-prepared, non-calcined catalysts displayed Au particles with uniform dimension (mean particle size = 1.8 nm) and multiple electronic states: XPS Au 4f7/2 lines at 84.9 and 87.1 eV (each with a spin-orbit splitting of 3.6-3.7 eV). These particles, which did not show localized surface plasmon resonance (LSPR), before the reaction, doubled in size after the reaction giving a pronounced LSPR at about 550 nm. The effect of the heating environment on these particles (in air or in H2) was further investigated. While heating in H2 gave Au nanoparticles of different shapes, heating under O2 gave exclusively spherical particles. Similar activity towards photocatalytic hydrogen ion reduction under UV excitation was seen in both cases, however. XPS Au 4f analyses indicated that an increase in deposition time, during catalyst preparation, resulted in an increase in the initial fraction of oxidized gold particles, which were easily reduced under hydrogen. The valence band region for Au/gC3N4 was further studied in an effort to compare it to what is already known for Au/metal oxide semiconductors. A shift of over 2 eV for the Au 5d doublets was noticed between reduced and oxidized gold particles with mean particle sizes between 2 and 6 nm, which is consistent with the final state effect. A narrow range of gold loading for optimal catalytic performance was seen, where it seems that a density of one Au particle per 10 × 10 nm2 is the most suitable. Particle size and shape had a minor effect on performance, which may indicate the absence of a plasmonic effect on the reaction rate.

Predicting Monolayer Oxide Stability over Low-Index Surfaces of TiO2 Polymorphs Using ab Initio Thermodynamics

Monolayer metal oxide coatings on metal oxide supports have the possibility of tuning the surface chemical properties of the coated systems. However, the (meta)stability of these structures makes experimental discovery challenging. A computational approach can help to determine properties that make a coating/substrate system stable and evaluate the stability of a variety of combinations. Herein, we use density functional theory (DFT) to study the stability of monolayer transitional metal oxides over different facets of anatase, brookite, and rutile phase of TiO₂. We find that coatings that have a stable polymorph matching that of the support, as well as substrates with higher surface energies, are more likely to form monolayer-coated systems. DFT calculations recommend a number of coating/TiO₂ surface facet combinations that may be stable. Despite these predictive observations, we did not find a significant correlation between monolayer stability and a single atomic, surface, or structural property of the coating/support metal/metal oxide and coating oxide monolayer stability. More complex predictive relationships need future study.

Fast and Simple Microwave Synthesis of TiO2/Au Nanoparticles for Gas-Phase Photocatalytic Hydrogen Generation

The fabrication of small anatase titanium dioxide (TiO₂) nanoparticles (NPs) attached to larger anisotropic gold (Au) morphologies by a very fast and simple two-step microwave-assisted synthesis is presented. The TiO₂/Au NPs are synthesized using polyvinylpyrrolidone (PVP) as reducing, capping and stabilizing agent through a polyol approach. To optimize the contact between the titania and the gold and facilitate electron transfer, the PVP is removed by calcination at mild temperatures. The nanocatalysts activity is then evaluated in the photocatalytic production of hydrogen from water/ethanol mixtures in gas-phase at ambient temperature. A maximum value of 5.3 mmol·[Formula: see text]h^-1 (7.4 mmol·[Formula: see text]h^-1) of hydrogen is recorded for the system with larger gold particles at an optimum calcination temperature of 450°C. Herein we demonstrate that TiO₂-based photocatalysts with high Au loading and large Au particle size (≈50 nm) NPs have photocatalytic activity.

Recent advances in membrane reactors for hydrogen production by steam reforming of ethanol as a renewable resource

During the last decade, hydrogen has attracted lots of interest due to its potential as an energy carrier. Ethanol is one of the renewable resources that can be considered as a sustainable candidate for hydrogen generation. In this regard, producing hydrogen from ethanol steam reforming (ESR) would be an environmentally friendly process. Commonly, ESR is performed in packed bed reactors; however, this process needs several stages for hydrogen separation with desired purity. Recently, the concept of a membrane reactor, an attractive device integrating catalytic reactions and separation processes in a single unit, has allowed obtaining a smaller reactor volume, higher conversion degrees, and higher hydrogen yield in comparison to conventional reactors. This paper deals with recent advances in ESR in terms of catalyst utilization and the fundamental of membranes. The main part of this paper discusses the performance of different membrane reactor configurations, mainly packed bed membrane reactors, fluidized bed membrane reactors, and micro-membrane reactors. In addition, a short overview is given about the impact of ESR via different catalysts such as noble metal, non-noble metal, and bi-metallic catalysts.

Oxidation and Reduction of TiOx Thin Films on Pd(111) and Pd(100)

Thin films of TiO_x on Pd(100) and Pd(111) have been investigated with respect to their properties after oxidation and reduction cycles. High-resolution photoemission spectroscopy (HRPES) and low energy electron diffraction (LEED) have been applied to characterize the thin film oxidation states and structure before and after oxidation and reduction under ultrahigh vacuum conditions. Fully oxidized TiO₂ films were formed on both surfaces. These structures display Moiré patterns in LEED, in one dimension for Pd(100) and in two dimensions for Pd(111), and they have previously not been reported for TiO₂/Pd. The oxidation process causes strong reduction in the interaction between the oxide thin film and the Pd substrate, most significantly for Pd(111). Reversible oxidation/reduction cycling of TiO_x thin films on Pd(111) and Pd(100) was possible.

Novel hetero-bimetallic coordination polymer as a single source of highly dispersed Cu/Ni nanoparticles for efficient photocatalytic water splitting

Monodispersed Cu and Ni nanoparticles are deposited over TiO₂ using a hetero-bimetallic coordination polymer for efficient photocatalytic water splitting.

Visible light thiocyanation of N-bearing aromatic and heteroaromatic compounds using Ag/TiO2 nanotube photocatalyst

In this study, Ag/TiO₂ nanotubes (Ag/TNT) were synthesized via simple hydrothermal process, and this photocatalyst was successfully exploited in thiocyanation reactions at room temperature under visible light irradiation.

Electrospun metal and metal alloy decorated TiO2 nanofiber photocatalysts for hydrogen generation

Photocatalytic hydrogen generation by electrospun TiO₂ nanofibers decorated with various co-catalysts (Pt₂Pd, PtCu, Cu, Pt, Pd) was explored.

Role of Support Oxygen Vacancies in the Gas Phase Hydrogenation of Furfural over Gold

Abstract

We have examined the role of support oxygen vacancies in the gas phase hydrogenation of furfural over Au/TiO₂ and Au/CeO₂ prepared by deposition–precipitation. Both catalysts exhibited a similar Au particle size distribution (1–6 nm) and mean (2.8–3.2 nm). Excess H₂ consumption during TPR is indicative of partial support reduction, which was confirmed by O₂ titration. Gold on CeO₂ with a higher redox potential exhibited a greater oxygen vacancy density. A lower furfural turnover frequency (TOF) was recorded over Au/CeO₂ than Au/TiO₂ and is linked to suppressed H₂ chemisorption capacity and strong –C=O interaction at oxygen vacancies that inhibited activity. Gold on non-reducible Al₂O₃ as benchmark exhibited greater H₂ uptake and delivered the highest furfural TOF. Full selectivity to the target furfuryl alcohol was achieved over Au/TiO₂ and Au/Al₂O₃ at 413 K and over Au/CeO₂ at 473 K with hydrogenolysis to 2-methylfuran at higher reaction temperature (523 K). A surface reaction mechanism is proposed to account for the activity/selectivity response.

Graphical Abstract

Parametric Studies of Titania-Supported Gold-Catalyzed Oxidation of Carbon Monoxide

This paper remarks the general correlations of the shape and crystallinity of titanium dioxide (TiO₂) support on gold deposition and carbon monoxide (CO) oxidation. It was found that due to the larger rutile TiO₂ particles and thus the pore volume, the deposited gold particles tended to agglomerate, resulting in smaller catalyst surface area and limited gold loading, whilst anatase TiO₂ enabled better gold deposition. Those properties directly related to gold particle size and thus the number of low coordinated atoms play dominant roles in enhancing CO oxidation activity. Gold deposited on anatase spheroidal TiO₂ at photo-deposition wavelength of 410 nm for 5 min resulted in the highest CO oxidation activity of 0.0617 mmol CO/s.g_Au (89.5% conversion) due to the comparatively highest catalyst surface area (114.4 m²/g), smallest gold particle size (2.8 nm), highest gold loading (7.2%), and highest Au⁰ content (68 mg/g catalyst). CO oxidation activity was also found to be directly proportional to the Au⁰ content. Based on diffuse reflectance infrared Fourier transform spectroscopy, we postulate that anatase TiO₂-supported Au undergoes rapid direct oxidation whilst CO oxidation on rutile TiO₂-supported Au could be inhibited by co-adsorption of oxygen.

Quantum tunneling injection of hot electrons in Au/TiO2 plasmonic photocatalysts

Visible light absorption of plasmonic Au nanoparticles supported on semiconductor TiO₂ leads to injection of their photoactivated "hot electrons (e_hot^-)" into the TiO₂ conduction band. This charge separation facilitates several oxidation and reduction reactions. These plasmonic systems, however, suffer from low quantum yields because the Schottky barrier created at the Au-TiO₂ interface suppresses e_hot^- injection. Here we report that Au nanoparticles supported on the anatase particles isolated from Degussa (Evonik) P25 TiO₂ promote e_hot^- injection with much higher efficiency than those supported on other commercially-available TiO₂ and catalyze aerobic oxidation with very high quantum yield (7.7% at 550 nm). Photoelectrochemical and spectroscopic analysis revealed that the number of Ti⁴⁺ atoms located at the Au-TiO₂ interface is the crucial factor. These Ti⁴⁺ atoms neutralize the negative charge of the Au particles and create a Schottky barrier with narrower depletion layer. This facilitates efficient e_hot^- injection by "quantum tunneling" through the Schottky barrier without overbarrier energy. The e_hot^- injection depends on several factors, and loading of 2 wt% Au particles with 3.5-4 nm diameters at around room temperature exhibits the highest activity of plasmonic photocatalysis.

Titania supported MOF-199 derived Cu–Cu2O nanoparticles: highly efficient non-noble metal photocatalysts for hydrogen production from alcohol–water mixtures

Water splitting over Cu–Cu₂O/TiO₂ photocatalysts.