TiO2-supported Au Research Articles

Novel TiO2-Supported Gold Nanoflowers for Efficient Photocatalytic NOx Abatement.

In this study, we pioneered the synthesis of nanoflower-shaped TiO2-supported Au photocatalysts and investigated their properties. Au nanoflowers (Au NFs) were prepared by a Na-citrate and hydroquinone-based preparation method, followed by wet impregnation of the derived Au NFs on the surface of TiO2 nanorods (TNR). A uniform and homogeneous distribution of Au NFs was observed in the TNR + NF(0.7) sample (lower Na-citrate concentration), while their distribution was heterogeneous in the TNR + NF(1.4) sample (higher Na-citrate concentration). The UV-Vis DR spectra revealed the size- and shape-dependent optical properties of the Au NFs, with the LSPR effect observed in the visible region. The solid-state EPR spectra showed the presence of Ti3+, oxygen vacancies and electron interactions with organic compounds on the catalyst surface. In the case of the TNR + NF(0.7) sample, high photocatalytic activity was observed in the H2-assisted reduction of NO2 to N2 at room temperature under visible-light illumination. In contrast, the TNR + NF(1.4) catalyst as well as the heat-treated samples showed no ability to reduce NO2 under visible light, indicating the presence of deformed Au NFs limiting the LSPR effect. These results emphasized the importance of the choice of synthesis method, as this could strongly influence the photocatalytic activity of the Au NFs.

Interface-enhanced catalytic performance of TiO2-supported Cu and Au for dimethyl oxalate hydrogenation: A comparative microkinetic analysis

The electronic structures of Cu-TiO2 and Au-TiO2 and their catalytic behaviors in the hydrogenation of dimethyl oxalate (DMO) have been studied by the stochastic surface walking-global neural network potential method, density functional theory calculation, and microkinetic analysis. Calculated results indicate that metal clusters on the pristine and oxygen-deficient TiO2 are mostly coordinated to O2c ions, and the defective surface may provide additional oxygen vacancies for Cu6 and Au6 cluster anchoring. It turns out that the interfacial Ti5c and Cuδ+/Auδ- ions provide adsorption and hydrogenation sites for the reaction intermediates with unsaturated O and C atoms, respectively. The turnover frequency for DMO consumption is much higher on Cu-TiO2 than on Au-TiO2, Cu(111), and Cu(211) at 443.15 K and 20 bar of total pressure. Given the satisfactory methyl glycolate (MG) selectivity, the Cu-TiO2 catalyst has the potential to act as an excellent catalyst for the selective MG production.

Sunlight-Powered Reverse Water Gas Shift Reaction Catalysed by Plasmonic Au/TiO2 Nanocatalysts: Effects of Au Particle Size on the Activity and Selectivity.

This study reports the low temperature and low pressure conversion (up to 160 °C, p = 3.5 bar) of CO2 and H2 to CO using plasmonic Au/TiO2 nanocatalysts and mildly concentrated artificial sunlight as the sole energy source (up to 13.9 kW·m-2 = 13.9 suns). To distinguish between photothermal and non-thermal contributors, we investigated the impact of the Au nanoparticle size and light intensity on the activity and selectivity of the catalyst. A comparative study between P25 TiO2-supported Au nanocatalysts of a size of 6 nm and 16 nm displayed a 15 times higher activity for the smaller particles, which can only partially be attributed to the higher Au surface area. Other factors that may play a role are e.g., the electronic contact between Au and TiO2 and the ratio between plasmonic absorption and scattering. Both catalysts displayed ≥84% selectivity for CO (side product is CH4). Furthermore, we demonstrated that the catalytic activity of Au/TiO2 increases exponentially with increasing light intensity, which indicated the presence of a photothermal contributor. In dark, however, both Au/TiO2 catalysts solely produced CH4 at the same catalyst bed temperature (160 °C). We propose that the difference in selectivity is caused by the promotion of CO desorption through charge transfer of plasmon generated charges (as a non-thermal contributor).

TiO2-supported Au144 nanoclusters for enhanced sonocatalytic performance.

The production of reactive oxygen species (ROS), such as hydroxyl radicals, by ultrasonic activation of semiconductor nanoparticles (NPs), including TiO2, has excellent potential for use in sonodynamic therapy and for the sonocatalytic degradation of pollutants. However, TiO2 NPs have limitations including low yields of generated ROS that result from fast electron-hole recombination. In this study, we first investigated the sonocatalytic activity of TiO2-supported Au nanoclusters (NCs) (Au NCs/TiO2) by monitoring the production of hydroxyl radicals (•OH) under ultrasonication conditions. The deposition of Au144 NCs on TiO2 NPs was found to enhance sonocatalytic activity for •OH production by approximately a factor of 2. Electron-hole recombination in ultrasonically excited TiO2 NPs is suppressed by Au144 NCs acting as an electron trap; this charge separation resulted in enhanced •OH production. In contrast, the deposition of Au25 NCs on TiO2 NPs resulted in lower sonocatalytic activity due to less charge separation, which highlights the effectiveness of combining Au144 NCs with TiO2 NPs for enhancing sonocatalytic activity. The sonocatalytic action that forms electron-hole pairs on the Au144/TiO2 catalyst is due to both heat and sonoluminescence from the implosive collapse of cavitation bubbles. Consequently, the ultrasonically excited Au144 (3 wt.%)/TiO2 catalyst exhibited higher catalytic activity for the production of •OH because of less light shadowing effect, in contrast to the lower catalytic activity when irradiated with only external light.

Compensation Effect Exhibited by Gold Bimetallic Nanoparticles during CO Oxidation.

While CO oxidation catalyzed by gold nanoparticles has been practiced academically for several decades, there are still important discoveries to be made. One area of current interest is to pair Au with another alloying metal and observe the catalytic consequences of the presence of the other metal. In this work, TiO2-supported bimetallic Au nanoparticles are alloyed with Cu, Co, Ni, Pd, and Ru and used as catalysts for CO oxidation. Two synthetic methods for the alloys are presented: a strong electrostatic adsorption (SEA) method and a sterically demanding ligand synthesis (SDLS) method which uses triphenylphosphine (TPP) as the ligand. The catalytic performance of the materials synthesized with the SEA and SDLS methods is compared in CO oxidation. The results indicate that the materials tested present an enthalpy–entropy compensation effect. Interestingly, both the enthalpy of activation, ΔH‡, and the entropy of activation, ΔS‡, generally decrease with particle size. AuCo and AuRu materials exhibit a decrease in the overall activity as compared to Au and the other Au alloys when synthesized via SEA. Au face-centered-cubic alloys AuCu, AuNi, and AuPd prepared via SEA show an improvement in activity compared to monometallic Au in our reaction conditions. In situ diffuse reflectance infrared Fourier transform spectroscopy presents two distinct regions for Au bimetallics where AuCo and AuRu show peak positions in the region of 2070–2050 cm–1, indicating a weaker interaction for AuCo and AuRu with CO when compared to that of the other alloys. For the SDLS method samples, the hypothesis is that TPP would enhance the CO oxidation rate by enhancing the charge transfer to the metallic surface. The results indicate that SDLS samples have lower CO oxidation rates and if any charge transfer occurs, it is masked by the lateral interactions of the CO π bonds and the phenyl groups of TPP.

Strong Activity Enhancement of the Photocatalytic Degradation of an Azo Dye on Au/TiO2 Doped with FeOx

The doping of Au/TiO2 with FeOx is shown to result in a strong enhancement of its photocatalytic activity in the degradation of the azo dye Orange II. In order to examine the source of this enhancement, Au-FeOx/TiO2 nanocomposites containing different molar ratios of Au:Fe were synthesized, and X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM) analyses indicated that the TiO2-supported Au nanoparticles were partially covered with an amorphous layer of FeOx species, in which the iron was present as Fe2+ and Fe3+. The metal-semiconductor system, i.e., Au/TiO2, showed only a moderate degradation rate, whereas doping with FeOx strongly enhanced the degradation activity. The bandgap energy decreased gradually from Au/TiO2 (3.13 eV) to the catalyst with the highest FeOx loading Au-FeOx (1:2)/TiO2 (2.23 eV), and this decrease was accompanied by a steady increase in the degradation activity of the catalysts. XPS analyses revealed that compared to Au/TiO2, on Au-FeOx/TiO2 a much higher population density of chemisorbed and/or dissociated oxygen species was generated, which together with the decreased bandgap resulted in the highest photocatalytic activity observed with Au-FeOx (1:2)/TiO2. The processes occurring during reaction on the catalyst surface and in the bulk liquid phase were investigated using operando attenuated total reflection IR spectroscopy (ATR-IR) combined with modulation excitation spectroscopy (MES), which showed that the doping of Au/TiO2 with FeOx weakens the interaction of the dye with the catalyst surface and strongly enhances the cleavage of the azo bond.

Structured Light by Rotating Au Nanoparticles in a Dynamic Distribution

Herein is reported the potential impact of the optical Kerr effect exhibited by rotating plasmonic nanoparticles on the generation of structured light. The third-order nonlinear optical properties exhibited by a round continuously variable distribution of metal nanoparticles incorporated in a dielectric substrate were analysed. The nanosystems were studied by using the finite difference method and TiO2-supported Au nanoparticles explored by a vectorial two-wave mixing technique. Attractive applications for developing quantum functions assisted by mechano-optical effects can be considered.

Enhancing Au/TiO2 Catalyst Thermostability and Coking Resistance with Alkyl Phosphonic-Acid Self-Assembled Monolayers.

Metal oxide-supported Au catalysts, particularly those with small Au nanoparticles, catalyze a variety of reactions including low-temperature CO oxidation and selective hydrogenation of alkynes. However, the facile nature of Au particle growth at even moderate temperatures poses significant challenges to maintaining catalyst activity under reaction conditions. Here, we present a method to reduce the rate of sintering and coke formation in TiO2-supported Au catalysts via the deposition of alkyl-phosphonic acid (PA) self-assembled monolayers. After surface modification with PAs, the resultant catalysts exhibited significantly improved resistance to Au sintering. PA deposition strongly suppressed CO oxidation rates, consistent with poisoning of active sites. In contrast, modification with PAs significantly improved the rate of C2H2 hydrogenation on Au/TiO2. The enhanced activity was accompanied by a dramatically improved resistance to accumulation of surface carbonaceous species.

Increasing the Activity and Selectivity of TiO2-Supported Au Catalysts for Renewable Hydrogen Generation from Ethanol Photoreforming by Engineering Ti3+ Defects

We examine in detail the roles of Ti3+ defects and the associated oxygen vacancies in the activity and selectivity of TiO2-supported Au catalysts (Au@TiO2) for renewable hydrogen production by phot...

Application of TiO2-supported Au for ozone molecule removal from environment: a van der Waals-corrected DFT study

In this paper, we examined the interaction of ozone molecule with TiO2-supported Au nanoparticles by performing first-principles calculations within density functional theory. The structural properties including bond lengths, bond angles and adsorption energies were calculated. The electronic properties were analyzed in view of the Mulliken charges and projected density of states. The results show that the adsorption of O3 molecule on the N-doped TiO2-supported Au nanoparticle is more energetically favorable than the adsorption on the pristine one, suggesting that the N-doped TiO2-supported Au nanoparticle can react with O3 molecule more strongly. It was found that the O3 molecule tends to be strongly adsorbed on the surface of Au nanoparticles through its side oxygen atoms, providing a bridge geometry. On the TiO2 side of TiO2-supported Au overlayer, there is also a strong binding between the fivefold coordinated titanium atoms and the side oxygen atoms of O3 molecule, where the titanium atoms represent a double contacting point with oxygen atoms. Therefore, the obtained results also propose a theoretical basis for the potential applications of TiO2-supported Au nanoparticles in gas sensor devices.

Photoelectrochemical Methanol Oxidation by TiN Nanoparticles Supported on TiO2

Photocatalytic activity of TiO2-supported TiN and Au nanoparticles towards methanol oxidation is investigated under both visible and UV light excitation. We demonstrate that the TiN/TiO2 photocatalyst performs better than the Au/TiO2 under visible excitation at potentials more positive than 0.1 V vs a Ag/AgCl reference electrode. Under UV excitation, photocurrent enhancement is observed for both the TiN/TiO2 and the Au/TiO2 photocatalysts in the entire potential region from -0.6 to 0.5 V, as compared to pure TiO2. The photocurrent enhancement under UV excitation was assigned to (1) better charge separation in the Au/TiO2 photocatalyst, and (2) generation of highly reactive holes due to interband transitions in TiN and Au nanoparticles.

Hydrogenation of p-Chloronitrobenzene on Au Nano-Clusters: Effects of Support.

Gold nanocluster has been reported to have high activity and selectivity for various liquid phase hydrogenation reactions. In this study, gold clusters were loaded on various supports, i.e., TiO2, ZnO, Fe2O3, ZrO2, and CeO2. The Au-cluster catalysts were prepared by deposition-precipitation method using HAuCl4 as the precursor. These catalysts were characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The catalytic properties of these catalysts were studied in a batch slurry reactor for hydrogenation of p-chloronitrobenzene (p-CNB) at 1.1 MPa H2 pressure, 373 K and 300 rpm. The activity decreased in the following order: Au/TiO2 ≧ Au/ZnO ≧ Au/CeO2 (Nikki) ≧ Au/CeO2 (Evonik) ≧ Au/Fe2O3 ≧ Au/ZrO2. TiO2-supported Au clusters had a higher activity than others, due to its hydrophilicity.

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.gAu (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.

A novel nitrogen dioxide gas sensor based on TiO2-supported Au nanoparticles: a van der Waals corrected DFT study

The interactions of nitrogen dioxide molecule with TiO2-supported Au nanoparticles were investigated using density functional theory. Surface Au atoms on the TiO2-supported Au overlayer were found to be the most favorable binding sites, thus making the adsorption process very strong. Both oxygen and nitrogen atoms of the NO2 molecule can bind to the Au surface by forming strong chemical bonds. The adsorption of NO2 molecule on the considered structures gives rise to significant changes in the bond lengths, bond angles, and adsorption energies of the complex systems. The results indicate that NO2 adsorption on the TiO2-supported Au nanoparticle by its oxygen atoms is energetically more favorable than the NO2 adsorption by its nitrogen atom, indicating the strong binding of NO2 to the TiO2-supported Au through its oxygen atoms. Thus, the bridge configuration of TiO2/Au + NO2 is found to be the most stable configuration. Both oxygen and nitrogen atoms of NO2 move favorably towards the Au surface, as confirmed by significant overlaps in the PDOSs of the atoms that forming chemical bonds. This study not only suggests a theoretical basis for gas-sensing properties of the TiO2-supported Au nanoparticles, but also offers a rational approach to develop nanostructure-based chemical sensors with improved performance.

Highly active and stable TiO2-supported Au nanoparticles for CO2 reduction

Au nanoparticles (2.2nm) deposited on TiO2 by the deposition-precipitation method were found to be extremely active and stable for CO2 hydrogenation to CO. A CO2 to CO yield up to 50% was achieved at temperatures as low as 400°C for at least 60h. Under identical reaction conditions, both bare TiO2 and Au/Al2O3 were practically inactive for CO2 reduction. The unique combination of the Au nanoparticles and TiO2 support – linked to a peculiar synergistic effect – is essential to the enhanced CO2 reduction activity.

Adsorption and dissociation of SO3 on N-doped TiO2 supported Au overlayers investigated by van der Waals corrected DFT

We present a density functional theory (DFT) study of the reactivity towards SO3 adsorption and dissociation over the TiO2 supported Au overlayers. We model different adsorption configurations of SO3 on the considered structures. For all adsorption systems, we find that the interaction of SO3 molecule with the surfaces of Au atoms is strongly favored. On the TiO2 supported Au overlayer, SO3 also dissociated into SO2 and adsorbed atomic oxygen. The atomic oxygen binds directly to the Au atoms. The structural properties are explained in terms of the bond lengths, bond angles and adsorption energies. The electronic structure of the system and Mulliken population analysis were also studied for the discussion of results. The results indicate that the SO3 interaction with N-doped TiO2-supported Au overlayer is more favorable in energy than the interaction with undoped one, suggesting the increased sensing capability of N-doped TiO2-supported Au towards detection of SO3 molecule. The oxygen atoms of the SO3 molecule bind to the Au and Ti atoms of the TiO2 supported Au overlayer in a bridge geometry. The significant overlaps in the PDOSs of the Au and oxygen atoms of the SO3 molecule, as well as titanium and oxygen atoms indicate the formation of chemical bonds between these atoms. Our DFT study therefore provides a theoretical basis for why the reactivity of SO3 molecule with N-doped TiO2 supported Au overlayer may increase, thus being a helpful procedure in the development of efficient sensor devices for SO3 detection.

Highly selective photocatalytic oxidation of biomass-derived chemicals to carboxyl compounds over Au/TiO2

Highly selective photooxidation of various biomass-derived chemicals to the corresponding carboxyl compounds is achieved over TiO2-supported Au nanoparticles using atmospheric air as the oxidant.

Elucidating the electronic structure of supported gold nanoparticles and its relevance to catalysis by means of hard X-ray photoelectron spectroscopy

We report on the electronic structure of Au (gold) nanoparticles supported onto TiO2 with a goal of elucidating the most important effects that contribute to their high catalytic activity. We synthesize and characterize with high resolution transmission electron microscopy (HRTEM) 3.4, 5.3, and 9.5nm diameter TiO2-supported Au nanoparticles with nearly spherical shape and measure their valence band using Au 5d subshell sensitive hard X-ray photoelectron spectroscopy (HAXPES) conducted at Spring-8. Based on density functional theory (DFT) calculations of various Au surface structures, we interpret the observed changes in the Au 5d valence band structure as a function of size in terms of an increasing percentage of Au atoms at corners/edges for decreasing particle size. This work elucidates how Au coordination number impacts the electronic structure of Au nanoparticles, ultimately giving rise to their well-known catalytic activity.

Base-free aerobic oxidation of glycerol on TiO2-supported bimetallic Au–Pt catalysts

The aerobic oxidation of glycerol provides an economically viable route to glyceraldehyde, dihydroxyacetone and glyceric acid with versatile applications, for which monometallic Pt, Au and Pd and bimetallic Au–Pt, Au–Pd and Pt–Pd catalysts on TiO2 were examined under base-free conditions. Pt exhibited a superior activity relative to Pd, and Au–Pd and Pt–Pd while Au was essentially inactive. The presence of Au on the Au–Pt/TiO2 catalysts led to their higher activities (normalized per Pt atom) in a wide range of Au/Pt atomic ratios (i.e. 1/3–7/1), and the one with the Au/Pt ratio of 3/1 exhibited the highest activity. Such promoting effect is ascribed to the increased electron density on Pt via the electron transfer from Au to Pt, as characterized by the temperature-programmed desorption of CO and infra-red spectroscopy for CO adsorption. Meanwhile, the presence of Au on Au–Pt/TiO2, most like due to the observed electron transfer, changed the product selectivity, and facilitated the oxidation of the secondary hydroxyl groups in glycerol, leading to the favorable formation of dihydroxyacetone over glyceraldehyde and glyceric acid that were derived from the oxidation of the primary hydroxyl groups. The synergetic effect between Au and Pt demonstrates the feasibility in the efficient oxidation of glycerol to the targeted products, for example, by rational tuning of the electronic properties of metal catalysts.

Synthesis of 3D ordered macroporous TiO2-supported Au nanoparticle photocatalysts and their photocatalytic performances for the reduction of CO2 to methane

This work presents the design and preparation of three-dimensionally ordered macroporous (3DOM) TiO2-supported Au nanoparticles with enhanced visible-light-responsive properties for the application in carbon dioxide (CO2) photoreduction. The 3DOM TiO2 support was prepared using a colloidal crystal template (CCT) method. A series of photocatalysts of TiO2-supported Au nanoparticles with uniform sizes were successfully synthesized by the facile method of gas bubbling-assisted membrane reduction (GBMR). These photocatalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy, Brunauer–Emmet–Teller (BET) analysis of nitrogen adsorption isotherm, photoluminescence spectra (PL), and the surface photovoltage (SPV) technique. All the catalysts possess well-defined 3DOM structures, which are highly ordered and interconnected with one another by small pore windows, and the Au nanoparticles are uniformly dispersed and supported on the inner wall of the uniform macropores. The slow light effect of the 3DOM structure can enhance absorption efficiency of solar irradiation. And the introduction of Au nanoparticles can effectively extend the spectral response from UV to visible region owing to Au surface plasmon resonance (SPR), and it is favorable for enhancing the separation of photoinduced electron–hole pairs. 3DOM Au/TiO2 photocatalysts exhibit high catalytic activity for the photocatalytic reduction of CO2 with H2O to CH4 under visible illumination.