TiO2-supported Au Nanoparticles Research Articles

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.

Real-time atomistic simulation of the Ostwald ripening of TiO2 supported Au nanoparticles.

Ostwald ripening (OR), one of the major processes of nanoparticle sintering, is critical for the rational design of functional nanomaterials. However, the atomistic mechanism of OR has not been fully understood, because the characterization of interparticle transport of atoms in real-time is challenging by either experiments or theoretical simulations. Thus, current understandings are based on ad hoc assumptions about the OR mechanism, which have never been confirmed yet at the atomic scale. Herein, we realized all-atom kinetic Monte Carlo simulation of sintering of TiO2 supported Au nanoparticles (NPs) through the OR mechanism at millisecond timescales. We demonstrated that the "semi-spherical" assumption should be removed. The OR process was a stagewise process determined by different rate-determining steps, which is in contrast to the single-stage presumption. Au dimers, rather than monomers as generally assumed, were exchanged among different NPs. Besides, we proposed a new kinetic model for describing the determining rate of OR without presumptions. This work brings deeper insights into the atomistic OR mechanism and also paves the way for real-time monitoring of catalyst sintering at the atomic scale.

Insight into surface properties of O2 plasma activated Au/TiO2 prepared by DPU in CO oxidation

Abstract Driven by excellent CO oxidation capability of gold nanoparticles at room temperature, a TiO2 supported Au nanoparticles (Au/TiO2) is synthesized by deposition-precipitation with urea (DPU). To solve the sintering issue of gold nanoparticles during activation process, atmospheric-pressure cold O2 plasma is employed to activate the Au/TiO2 catalysts and compared with the conventional calcination. The plasma activated one (S-P) shows poorer activity compared to the calcined one (S-C). However, S-P with further TPD treatment (S-PT) shows the best catalytic activity among above-mentioned samples. To elucidate the plasma effect, TEM, XPS, UV–vis spectroscopy and in situ DRIFTS techniques are used for catalyst characterization. Based on TEM results, S-P sample has smaller particle size and narrower size distribution compared to S-C sample, while the high content of low-coordinated Au atoms and the poisoning nitrate species over S-P are revealed by in situ DRIFTS. Upon further TPD treatment, the poisoning species occupying the active sites is desorbed from catalyst surface and the improvements in Au particle size and size distribution are maintained. The mechanism for plasma effect on activation of Au/TiO2 prepared by DPU is also discussed.

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.

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.

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.

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.

Promoted effect of PANI as electron transfer promoter on CO oxidation over Au/TiO2

Since a high electron density of Au surfaces can be conducive to the oxidation of CO over the supported Au nanoparticles, the conductive polymer polyaniline (PANI) was assembled into Au/TiO2 to prepare a Au/TiO2–PANI catalyst for oxidizing CO. It is found that PANI could be well wedged with Au/TiO2, which remarkably promoted the oxidation of CO over the TiO2 supported Au nanoparticles at room temperature. Moreover, the presence of a trace amount of H2O could promote the oxidation of CO over the Au/TiO2–PANI catalyst. Based on the testing results of structure characteristic, electrochemical impedance and adsorbing CO of Au/TiO2–PANI, it was proposed that the PANI could be acted as an electron transmitter to promote the electron transfer from TiO2 to Au, resulting in the increase of surface electron density of Au sites and then the promoted activation of CO adsorbed at Au sites.

Inside Cover: Effects of Au Loading and CO2 Addition on Photocatalytic Selective Phenol Oxidation over TiO2‐Supported Au Nanoparticles (ChemCatChem 3/2013)

Selective oxidation The inside cover picture shows a mystique of TiO2-supported Au nanoparticles as a photocatalyst for partial oxidations of organic substrates under sunlight irradiation and a CO2 atmosphere. In their Full Paper on p. 766 ff., Y. Ide et al. describe how the photocatalytic oxidation of aqueous phenol to hydroquinone on the catalyst under sunlight is modified to give a higher yield and selectivity if the reaction is conducted under a CO2 atmosphere. Au loading causes neither the visible light response nor the improved UV light-induced charge separation efficiency of TiO2. A unique role of Au loading in the photocatalysis is proposed; Au nanoparticle loading drastically decreases the amount of the surface titanol, promoting the desorption of the product hydroquinone from the catalyst surface, especially if CO2 is present in the system.

The promoted oxidation of CO induced by the visible-light response of Au nanoparticles over Au/TiO2

A visible light (490–780 nm) was introduced into the process of oxidizing CO over a TiO2 supported Au nanoparticle catalyst (Au/TiO2). It is found that the visible-light irradiation can promote the oxidation of CO over Au/TiO2. Based on the results of both the optical absorption of Au/TiO2 and the adsorption of CO over Au/TiO2, it is proposed that the localized surface plasmon resonance (LSPR) of Au nanoparticles induced by the visible-light would promote the adsorption of CO and its activation at the surface of Au species, resulting in the promoted oxidation of CO over Au/TiO2.

Catalytic Degradation of a Chemical Warfare Agent Simulant: Reaction Mechanisms on TiO2-Supported Au Nanoparticles

High-vacuum transmission infrared spectroscopy is used to investigate the reaction pathways of the chemical warfare agent simulant dimethyl methylphosphonate (DMMP) on nanoparticulate Au/TiO2 and pure TiO2. The infrared spectral signature of adsorbed CO, a major reaction product, is used to track the oxidation state of Au and Ti during the reaction. We find that small, <5 nm, Au particles on the surface of TiO2 activate this material toward the oxidative degradation of DMMP, in contrast to pure TiO2, which is an effective sorbant for DMMP, but little decomposition of this molecule occurs. The direct comparison of the chemistry on the samples reveals that two mechanisms may be responsible for the catalytic activity of the TiO2-supported Au nanoparticles. One mechanism occurs under anaerobic conditions, where vapor-phase DMMP reacts upon adsorption to form a variety of organic products and adsorbed CO. The active species in this initial oxidation channel is the lattice oxygen that most likely exists around ...

Role of the nanoscale in catalytic CO oxidation by supported Au and Pt nanostructures

Experiments have found that the catalytic activity of Au increases sharply for supported nanoparticles smaller than 5 nm, while Pt exhibits opposite behavior. Several authors, seeking to explain the nanoscale Au activity, reached conflicting conclusions, attributing it to different nanoscale features or to a particular bilayer structure that is independent of size. Here we report an extensive theoretical study of a large ensemble of TiO2-supported Au and Pt nanoparticles and show that several nanoscale features collectively result in the observed contrasting behavior. Low coordination is accompanied by bond weakening in Au and strengthening in Pt, while perimeter sites are active only for Au. Though there are orbital-occupancy differences for catalytically active and inactive configurations, there is insignificant variation in physical charge transfer.

Preparation of TiO 2-supported Au nanoparticle catalysts from a Au 13 cluster precursor: Ligand removal using ozone exposure versus a rapid thermal treatment

The structure of Au heterogeneous catalysts on the anatase form of TiO 2 has been characterized by high-resolution electron microscopy (HREM) and quantitative Z-contrast scanning transmission electron microscopy (STEM). These materials are prepared by deposition of highly monodisperse Au 13[PPh 3] 4[S(CH 2) 11CH 3] 4 (8-Å diameter) ligand-protected clusters on anatase, followed by reaction with ozone or a rapid oxidative thermal treatment to remove the ligands. The materials obtained differ markedly in each case. For the thermal treatment at 400 °C in air, the supported particles grow to an average size of 2.7 nm ( ± 0.6 nm ) in diameter, and the larger particles in the distribution are found to adopt a spherical geometry. Particle growth is greatly inhibited when ozone is used to remove the ligands (average diameter 1.2 ± 0.5 nm ). These particles assume a more oblate geometry, consistent with a truncated hemispherical shape. It was found that subsequent thermal treatments of the ozone-derived supported nanoparticles at 400 °C in air did not induce additional growth, indicating that sintering is strongly affected by the particle–support interactions developed by the ozone-based low-temperature ligand removal step. These materials exhibit catalytic activity and high stability for the oxidation of CO at elevated temperatures, with the level of activity dependent on catalyst preparation.

Automated electrochemical synthesis and characterization of TiO2 supported Au nanoparticle electrocatalysts

Automated systems for electrochemical synthesis and high throughput screening of catalytic materials were developed and used to prepare a library of nanoparticulate gold supported on TiO2. A two-dimensional array (library) of Au was synthesized by pulsed cathodic electrodeposition onto a thermally oxidized titanium dioxide substrate. Variations in particle size across the library were created by changing the deposition time (number of pulses). Longer deposition times led to increased Au particle sizes and greater density of Au on the surface. High throughput electrochemical screening was used to characterize the electrocatalytic activity of the supported Au clusters for: (1) photoelectrochemical water oxidation and (2) CO electro-oxidation. Au films synthesized with 5 ms pulses between 3 and 10 s of total deposition time demonstrated the greatest activity for photodecomposition of water (20–40% greater than pure TiO2). For CO electro-oxidation, it was found that the smallest Au particle (<10 nm, 1 s total deposition time) was most active, consistent with previous research in this area.