Activation of Oxygen by Metallic Gold in Au/TiO2 Catalysts

Gas-phase reactions driven by polarized metal-metal bonding in atomic clusters

Multimetallic catalysts exhibit great potential in the activation and catalytic transformation of small molecules. The polarized metal-metal bonds have been gradually recognized to account for the reactivity of multimetallic catalysts due to the synergistic effect of different metal centers. Gas-phase reactions on atomic clusters that compositionally resemble the active sites on related condensed-phase catalysts provide a widely accepted strategy to clarify the nature of polarized metal-metal bonds and the mechanistic details of elementary steps involved in the catalysis driven by this unique chemical bonding. This perspective review concerns the progress in the fundamental understanding of industrially and environmentally important reactions that are closely related to the polarized metal-metal bonds in clusters at a strictly molecular level. The following topics have been summarized and discussed: (1) catalytic CO oxidation with O₂, H₂O, and NO as oxidants (2) and the activation of other inert molecules (e.g., CH₄, CO₂, and N₂) mediated with clusters featuring polarized metal-metal bonding. It turns out that the findings in the gas phase parallel the catalytic behaviors of condensed-phase catalysts and the knowledge can prove to be essential in inspiring future design of promising catalysts.

Valence states of single Au atoms dictate the catalytic activity of Au1/CeO2(100)

We tune the valence state of single Au atoms anchored on CeO₂(100) by treating the catalyst in H₂ at different temperatures and obtain a series of Au₁/CeO₂(100). The transition from Au₁^+0.9 to Au₁^+0.3 leads to an enhancement of the CO oxidation activity of Au₁/CeO₂(100) by one order of magnitude. This work is of significance for an in-depth understanding of reaction mechanisms and rational design of high-performance catalysts.

High Catalytic Performance of Au/Bi2O3 for Preferential Oxidation of CO in H2

Preferential oxidation (PROX) of CO in hydrogen is of great significance for proton exchange membrane fuel cells (PEMFCs) that need a CO-free hydrogen stream as fuel. The key technical problem is developing catalysts that can efficiently remove CO from the H2-rich stream within the working temperature range of PEMFCs. Herein, we design a Au/Bi2O3 interfacial catalyst for PROX with excellent catalytic performance, which can achieve 100% CO conversion in the PROX reaction over a wide temperature window (70-200 °C) and is perfectly compatible with the operating temperature window (80-180 °C) of PEMFCs. Moreover, the catalyst also demonstrates excellent high flow performance and long-term stability. Density functional theory (DFT) calculations reveal that the electrons transferring from Bi2O3 to Au and then to adsorbed perimeter CO and O2 molecules promote the activation of CO and O2, thus enhancing the catalytic performance of PROX.

Elucidating the charge state of an Au nanocluster on the oxidized/reduced rutile TiO2 (110) surface using non-contact atomic force microscopy and Kelvin probe force microscopy

The charge state of Au nanoclusters on oxidized/reduced rutile TiO₂ (110) surfaces were investigated by a combination of non-contact atomic force microscopy and Kelvin probe force microscopy at 78 K under ultra-high vacuum. We found that the Au nanoclusters supported on oxidized/reduced surfaces had a relatively positive/negative charge state, respectively, compared with the substrate. In addition, the distance dependence of LCPD verified the contrast observed in the KPFM images. The physical background of charge transfer observation can be explained by the model of charge attachment/detachment from multiple oxygen vacancies/adatoms surrounding Au nanoclusters. These results suggest that the electronic properties of the Au nanoclusters are dramatically influenced by the condition of the support used.

Electronic Properties of Realistic Anatase TiO2 Nanoparticles from G0W0 Calculations on a Gaussian and Plane Waves Scheme

The electronic properties of realistic (TiO₂)_n nanoparticles (NPs) with cuboctahedral and bipyramidal morphologies are investigated within the many-body perturbation theory (MBPT) G₀W₀ approximation using PBE and hybrid PBEx (12.5% Fock contribution) functionals as starting points. The use of a Gaussian and plane waves (GPW) scheme reduces the usual O⁴ computational time required in this type of calculation close to O³ and thus allows considering explicitly NPs with n up to 165. The analysis of the Kohn-Sham energy orbitals and quasiparticle (QP) energies shows that the optical energy gap (O_gap), the electronic energy gap (E_gap), and the exciton binding energy (ΔE_ex) values decrease with increasing TiO₂ NP size, in agreement with previous work. However, while bipyramidal NPs appear to reach the scalable regime already for n = 84, cuboctahedral NPs reach this regime only above n = 151. Relevant correlations are found and reported that will allow one to predict these electronic properties at the G₀W₀ level in even much larger NPs where these calculations are unaffordable. The present work provides a feasible and practical way to approach the electronic properties of rather large TiO₂ NPs and thus constitutes a further step in the study of realistic nanoparticles of semiconducting oxides.

Differences in the Catalytic Behavior of Au-Metalized TiO2 Systems During Phenol Photo-Degradation and CO Oxidation

For this present work, a series of Au-metallized TiO2 catalysts were synthesized and characterized in order to compare their performance in two different catalytic environments: the phenol degradation that occurs during the liquid phase and in the CO oxidation phase, which proceeds the gas phase. The obtained materials were analyzed by different techniques such as XRF, SBET, XRD, TEM, XPS, and UV-Vis DRS. Although the metallization was not totally efficient in all cases, the amount of noble metal loaded depended strongly on the deposition time. Furthermore, the differences in the amount of loaded gold were important factors influencing the physicochemical properties of the catalysts, and consequently, their performances in the studied reactors. The addition of gold represented a considerable increase in the phenol conversion when compared with that of the TiO2, despite the small amount of noble metal loaded. However, this was not the case in the CO oxidation reaction. Beyond the differences in the phase where the reaction occurred, the loss of catalytic activity during the CO oxidation reaction was directly related to the sintering of the gold nanoparticles.

Neutral Au1-Doped Cluster Catalysts AuTi2O3-6 for CO Oxidation by O2

Oxide supported gold catalysts (e.g., Au/TiO₂) are of great significance in heterogeneous catalysis owing to their extraordinary catalytic activity. Study of heteronuclear metal oxide clusters (HMOCs, e.g., Au _xTi _yO _z ^q) is an important way to uncover the molecular-level mechanisms of gold catalysis in the related heterogeneous catalytic systems. However, the current studies of HMOCs are focused on charged clusters with little attention paid to neutral species. The reactivity study of neutral HMOCs is vital to have a comprehensive understanding of heterogeneous catalysis, but it is experimentally challenging because of the difficulty of cluster ionization and detection without fragmentation. Herein, benefiting from a homemade time-of-flight mass spectrometer coupled with a vacuum ultraviolet laser system, the reactivity of neutral Au₁-doped titanium oxide clusters AuTi₂O_3-6 in catalytic CO oxidation by O₂ has been successfully identified. The mechanistic details of the catalysis have been elucidated by quantum chemistry calculations. The crucial roles of the mobile AuCO species that can facilitate not only the process of CO oxidation but also the process of O₂ activation have been discovered in the cluster catalysis. The fascinating results are of substantial importance to understand the mechanisms of CO oxidation over Au/TiO₂, one type of the best studied gold catalysts.

Efficient base-free direct oxidation of glucose to gluconic acid over TiO2-supported gold clusters

The transformation of renewable natural resources is an appealing and sustainable protocol to minimize fossil fuel consumption. Here, a simple incipient wetness protocol is developed to prepare ultrasmall gold clusters, immobilized on TiO2 (particle size: 1.2-1.7 nm), using anthranilic acid as a stabilizing agent. The Au clusters can be reduced to metallic Au0 (Au/TH-150 and Au/TH-200) during 150 and 200 °C annealing in the presence of H2 gas, while the Au clusters are converted to Auδ+ species in air at 200 and 500 °C (Au/TA-200 and Au/TA-500), a conclusion supported by XPS and low-temperature (-150 °C) Operando-DRIFTS analysis. Au/TA-200 and Au/TA-500 showed inactivity in the base-free direct oxidation of glucose. For comparison, Au/TH-150 and Au/TH-200 exhibited salient catalytic performance (87-92% conversion and 95-97% selectivity for gluconic acid), revealing that glucose oxidation occurs preferentially on the Au0 species. The turnover frequency (TOF) of Au/TH-150 reaches 1908 molreacted glucose molAu-1 h-1, which is much higher than that of commercial Pd-Bi/C under alkaline conditions (TOF: 1298 molreacted glucose molPd-1 h-1, pH 9.5). The apparent activation energies are 37 (over Au/TH-150) and 47 kJ mol-1 (Au/TH-200), comparable to the unsupported Au colloids, indicating that the oxidation should occur at the Au surface rather than at the perimeter interface between the Au clusters and the supports.

A Rational Solid-State Synthesis of Supported Au-Ni Bimetallic Nanoparticles with Enhanced Activity for Gas-Phase Selective Oxidation of Alcohols

A facile confined solid-state seed-mediated alloying strategy is applied for the rational synthesis of supported Au-Ni bimetallic nanoparticles (BMNPs). The method sequentially deposits nickel salts and AuNP seeds into the ordered array of extra-large mesopores (EP-FDU-12 support) followed by a high-temperature annealing process. The size, structure, and composition of the AuNi BMNPs can be well tuned by varying the AuNP seeds, annealing temperature, and feeding ratio of metal precursors. Kinetic studies and DFT calculations suggest that the introduction of the Ni component can significantly prompt the O₂ activation on AuNPs, which is critical for the selective alcohol oxidation using molecular O₂ as the oxidant. The optimal Au-Ni BMNP catalyst showed the highest turnover frequency (TOF) (59 000 h^-1, 240 °C) and highest space-time yield (STY) of benzyl aldehyde (BAD) productivity (9.23 kg·g_Au^-1·h^-1) in the gas-phase oxidation of benzyl alcohol (BA), which is at least about 5-fold higher than that of other supported Au catalysts.

Contributions of distinct gold species to catalytic reactivity for carbon monoxide oxidation

Small-size (<5 nm) gold nanostructures supported on reducible metal oxides have been widely investigated because of the unique catalytic properties they exhibit in diverse redox reactions. However, arguments about the nature of the gold active site have continued for two decades, due to the lack of comparable catalyst systems with specific gold species, as well as the scarcity of direct experimental evidence for the reaction mechanism under realistic working conditions. Here we report the determination of the contribution of single atoms, clusters and particles to the oxidation of carbon monoxide at room temperature, by the aid of in situ X-ray absorption fine structure analysis and in situ diffuse reflectance infrared Fourier transform spectroscopy. We find that the metallic gold component in clusters or particles plays a much more critical role as the active site than the cationic single-atom gold species for the room-temperature carbon monoxide oxidation reaction.

TiO2 Band Restructuring by B and P Dopants

An examination of the effect of B- and P-doping and codoping on the electronic structure of anatase TiO2 by performing density functional theory calculations revealed the following: (i) B- or P-doping effects are similar to atomic undercoordination effects on local bond relaxation and core electron entrapment; (ii) the locally entrapped charge adds impurity levels within the band gap that could enhance the utilization of TiO2 to absorb visible light and prolong the carrier lifetime; (iii) the core electron entrapment polarizes nonbonding electrons in the upper edges of the valence and conduction bands, which reduces not only the work function but also the band gap; and (iv) work function reduction enhances the reactivity of the carriers and band gap reduction promotes visible-light absorption. These observations may shed light on effective catalyst design and synthesis.

Layered sphere-shaped TiO₂ capped with gold nanoparticles on structural defects and their catalysis of formaldehyde oxidation

We describe here a one-step method for the synthesis of Au/TiO2 nanosphere materials, which were formed by layered deposition of multiple anatase TiO2 nanosheets. The Au nanoparticles were stabilized by structural defects in each TiO2 nanosheet, including crystal steps and edges, thereby fixing the Au-TiO2 perimeter interface. Reactant transfer occurred along the gaps between these TiO2 nanosheet layers and in contact with catalytically active sites at the Au-TiO2 interface. The doped Au induced the formation of oxygen vacancies in the Au-TiO2 interface. Such vacancies are essential for generating active oxygen species (*O(-)) on the TiO2 surface and Ti(3+) ions in bulk TiO2. These ions can then form Ti(3+)-O(-)-Ti(4+) species, which are known to enhance the catalytic activity of formaldehyde (HCHO) oxidation. These studies on structural and oxygen vacancy defects in Au/TiO2 samples provide a theoretical foundation for the catalytic mechanism of HCHO oxidation on oxide-supported Au materials.

Characterisation of gold catalysts

Au-based catalysts have established a new important field of catalysis, revealing specific properties in terms of both high activity and selectivity for many reactions.

A heterostructured TiO2–C3N4 support for gold catalysts: a superior preferential oxidation of CO in the presence of H2 under visible light irradiation and without visible light irradiation

Introducing C₃N₄ into Au/TiO₂ promotes an increase in the electron densities of Au, resulting in the activation of CO and O₂.

Does enhanced oxygen activation always facilitate CO oxidation on gold clusters?

We investigate the catalytic activity of the subnanometer-sized bimetallic Au19Pt cluster for oxidation of CO via first-principles density functional theory calculations. For this purpose we consider two structurally similar and energetically close homotops of the Au19Pt cluster with the Pt atom occupying an edge (Td-E) or a facet (Td-S) site of a 20-atom tetrahedron. Using these homotops as catalysts we calculate the complete reaction paths and the thermodynamic functions corresponding to the oxidation of CO to CO2. It is found that the oxidation of CO on the Td-S isomer occurs through a smaller reaction barrier (0.38 eV) as compared with that on the Td-E isomer (0.70 eV), although the activation of O2 on the latter is much higher than that on the former. Therefore, a clear conclusion is that a higher O2 activation, which is generally believed to be the key factor for CO oxidation, solely cannot determine the catalytic efficiency of the Au-Pt bimetallic clusters. In addition, we find a stronger CO adsorption on the Td-E isomer (2.06 eV) as compared with that on the Td-S isomer (1.68 eV). Although stronger CO adsorption on the Td-E isomer leads to a higher O2 activation; however, high value of CO adsorption energy deteriorates the catalytic activity of the Td-E isomer towards the CO oxidation reaction.

Structure transition of Au18 from pyramidal to a hollow-cage during soft-landing onto a TiO2(110) surface

Simulation of the soft-landing process of pyramidal Au₁₈ onto a rutile TiO₂(110) surface using large-scale BOMD simulation.

Gold-Based Catalysts

The discovery of the catalytic power of gold, always regarded as inert, dates back to the early 1990s. The keystone is the nanometric scale: only when bulk gold was found to be dramatically enhanced when downsized to nanometric particles did its extraordinary catalytic activity definitely come out and it still continues to show more of this peculiarity. This represented a breakthrough in chemistry, especially in organic synthesis, allowing catalyzed selective oxidations of various substrates to be carried out to give important chemicals under green conditions. Gold, alone or alloyed with a second metal, has turned out to be particularly effective in the selective oxidation of different alcohols, which can be tuned to their carbonylic and carboxylic derivatives. In this chapter, an overview of the aerobic oxidation of alcohols carried out with supported gold-based catalysts in the liquid phase is presented, with a particular focus on substrates of interest such as glycerol and allyl alcohol. Some vapor-phase processes worthy of mention are also included, plus a section introducing the main methods of preparation of gold-based catalysts and their characterization.

Density functional theory study on the activation of molecular oxygen on a stepped gold surface in an aqueous environment: a new approach for simulating reactions in solution

The activation of oxygen molecules is an important issue in the gold-catalyzed partial oxidation of alcohols in aqueous solution. The complexity of the solution arising from a large number of solvent molecules makes it difficult to study the reaction in the system. In this work, O2 activation on an Au catalyst is investigated using an effective approach to estimate the reaction barriers in the presence of solvent. Our calculations show that O2 can be activated, undergoing OOH* in the presence of water molecules. The OOH* can readily be formed on Au(211) via four possible pathways with almost equivalent free energy barriers at the aqueous-solid interface: the direct or indirect activation of O2 by surface hydrogen or the hydrolysis of O2 following a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. Among them, the Eley-Rideal mechanism may be slightly more favorable due to the restriction of the low coverage of surface H on Au(211) in the other mechanisms. The results shed light on the importance of water molecules on the activation of oxygen in gold-catalyzed systems. Solvent is found to facilitate the oxygen activation process mainly by offering extra electrons and stabilizing the transition states. A correlation between the energy barrier and the negative charge of the reaction center is found. The activation barrier is substantially reduced by the aqueous environment, in which the first solvation shell plays the most important role in the barrier reduction. Our approach may be useful for estimating the reaction barriers in aqueous systems.

Gold Catalysis in the Complete Oxidation or Decomposition of Small Molecule Pollutants

Supported gold catalysts are useful for the elimination of small molecule pollutants at low temperature. Catalytic oxidation and decomposition are ways to eliminate these air pollutants. The complete oxidation of CO, ethylene and formaldehyde to CO2 over supported gold catalysts, which can be achieved at room temperature or lower, has been studied widely and in depth. Some research has focused on the decomposition of ozone, N2O and NO over supported gold catalysts. The mechanism of catalysis by supported gold material has been elucidated for the above mentioned reactions.

The role of reducible oxide-metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.

Theoretical tools for studying gold nanoparticles as catalysts for oxidation and hydrogenation reactions

In this contribution, the ability of small isolated gold NP to dissociate O2 and generate a reactive surface oxide layer, the nature of the new gold active sites generated, and their implication in the mechanism of alcohol oxidation to aldehydes has been analyzed from a theoretical point of view. The nature of the active sites involved in H2 dissociation and the possible ways in which Au/TiO2 catalysts can be modified in order to increase their activity toward hydrogenation of nitroaromatics without modifying their high chemoselectivity is also explored.