Single Au Atom Research Articles

Single-atom Au catalyst loaded on CeO2: A novel single-atom nanozyme electrochemical H2O2 sensor

Owing to the maximum utilization efficiency of the metal atoms, single-atom catalysts (SACs), which have higher catalytic activity and selectivity than traditional nanocatalysts, have been used as sensing materials for signal amplification and sensitive detection of biomolecules. Recently, single-atom Au catalysts have attracted attention. Au nanoparticles have high catalytic activity, and the loading efficiency on the support can be further increased by downsizing Au nanoparticles to single atoms. Compared with other oxide supports, the higher density of vacancies on the CeO2 surface can accommodate more Au atoms, and thereby CeO2 can stabilize more metal atoms. In this paper, we describe an electrochemical sensor based on Au–CeO2 nanocomposites for ultrasensitive and highly selective detection of H2O2 released from A549 cells by immobilizing single-atom Au on specific facets of CeO2. Due to the abundant oxygen vacancies on the surface and the strong interactive effect between Ce ions and Au atoms, this sensor displayed excellent electrochemical performance, with a detection limit of 1.4 nM. The synergistic effect of CeO2 and Au atoms also resulted in high catalytic activity and stability. This study shows that the single-atom nanozyme sensing strategy can be applied to the sensitive detection of reactive oxygen species. Thus, single-atom sensors have broad application prospects in clinical analysis and physiology.

Ostwald ripening microkinetic simulation of Au clusters on MgO(0 0 1)

Sintering is one of the most common processes responsible for the loss of supported metal nanoparticle catalysts' activity. We have combined ab-initio calculations with microkinetic simulations to investigate the digestion and growth mechanism on Au clusters supported on MgO(001) following a bottom-up approach. The energy barrier for diffusing a single gold atom on the clean MgO surface was found to be 0.29 eV in full agreement with previous reports. Additionally, and as an extension to the entire energy profile related to Ostwald mechanisms, we found all of the activation energies to be below 1.05 eV in the cases investigated. An odd–even cluster trend was observed during ripening, attributed to the stability of pairing the unpaired electrons associated with the single gold atoms. Microkinetic analyses showed that Au single atoms are present on the surface of magnesia up to a temperature of 160 K. At higher temperatures, the system has enough energy for the single atom to diffuse across the surface and attach to other atoms or clusters. At temperatures akin to room temperature, the cluster undergoes ripening to form larger particles in order to achieve a more stable equilibrium.

Interactions of noxious molecules on a single gold atom decorated an ultra-small carbon nanotube: A density functional theory study

We investigated the interaction of five noxious molecules such as CO, CO2, H2S, NO2, and SO2 on a single gold (Au) atom decorated an ultra-small (5, 5) CNT using the density functional theory (DFT) method. We also calculated and observed the changes in geometry as a result of each molecule’s interaction on a single Au atom decorated an ultra-small (5, 5) CNT, as well as the changes in their respective adsorption energy, charge transfer, and even the changes in their electronic structure. The results of this investigation indicated that the five molecules have good and strong interactions on a single Au atom decorated an ultra-small (5, 5) CNT. Thus, we believe that a configuration of a single Au atom decorated an ultra-small (5, 5) CNT is very likely to be developed into a sensor material for detecting molecules of CO, CO2, H2S, NO2 and SO2 in the near future.

Revealing the Phase Separation Behavior of Thermodynamically Immiscible Elements in a Nanoparticle.

Phase-separation is commonly observed in multimetallic nanomaterials, yet it is not well understood how immiscible elements distribute in a thermodynamically stable nanoparticle. Herein, we studied the phase-separation of Au and Rh in nanoparticles using electron microscopy and tomography techniques. The nanoparticles were thermally annealed to form thermodynamically stable structures. HAADF-STEM and EDS characterizations reveal that Au and Rh segregate into two domains while their miscibility is increased. Using aberration-corrected HAADF-STEM and atomic electron tomography, we show that the increased solubility of Au in Rh is achieved by forming Au clusters and single atoms inside the Rh domains and on the Rh surface. Furthermore, based on the three-dimensional reconstruction of a AuRh nanoparticle, we can visualize the uneven interface that is embedded in the nanoparticle. The results advance our understanding on the nanoscale thermodynamic behavior of metal mixtures, which is crucial for the optimization of multimetallic nanostructures for many applications.

Single Au atom supported defect mediated boron nitride monolayer as an efficient catalyst for acetylene hydrochlorination: A first principles study

• Monovacancy defect (nitrogen and boron) in boron nitride effectively adjust the properties of single gold and create the different charge state of Au. • Defective sites are stable active site for Au adsorption compare to pristine one. Single gold anion shows good catalytic performance in reaction which is comparable with single gold cation. • An E-R like reaction mechanism is revealed which begins with C 2 H 2 adsorption followed by HCl activation. • A novel LH mechanism with comparatively lower energy barrier than ER mechanism observed for Au/hBN • Defective boron nitride supported single Au atom catalyst show excellent stability and good catalytic performance towards acetylene hydrochlorination reaction. Hexagonal boron nitride (hBN) with defects shows remarkable performance as catalyst support for single-atom stabilization in several catalytic reactions, such as CO oxidation, N 2 fixation, and ORR. However, the anchoring sites for single-atoms and their catalytic performance towards acetylene hydrochlorination still needs a comprehensive investigation. The boron and nitrogen-vacancy in hBN not only stabilizes Au but can also effectively tune its electronic structure. The single-atom Au has charged negatively and positively on hBN with the formation nitrogen (hBNv) and boron (hBvN) vacancy, respectively. The electronic structure analysis, MD simulations, phonon band structure, and migration barrier uncovered the stability of Au supported on hBN. The reaction proceeded either with Eley–Rideal (E-R) mechanism, starting with C 2 H 2 adsorption after that HCl activation, product formation, or with Langmuir–Hinshelwood (L-H) method, starting with co-adsorption of C 2 H 2 and HCl. The L-H mechanism results in a smaller reaction barrier of 0.90 and 1.15 eV compared to the counterpart E-R mechanism with a barrier of 1.14 and 1.25 eV for Au/hBNv and Au/hBvN respectively. Moreover, both Au in cation and anion state can efficiently catalyze acetylene hydrochlorination with a competing energy barrier. Overall, current research gives a comprehensive explanation of the catalytic properties of hBN-based single-atom Au catalyst. Defected boron nitride can tune the electronic structure of Au and both Au cation and anion shows comparable performance towards acetylene hydrochlorination.

Bi2Se3/Bi2Se3 and TlSe/TlSe junctions: enhanced coupling of topological interface states by intercalation

Based on first-principles calculations, we studied the coupling of topological interface states at the interface of Bi2Se3/Bi2Se3 and TlSe/TlSe junctions. From our calculations, the coupling leads to an energy gap at the Dirac point of topological interface states and the energy gap is an exponential decay function of interface spacing when the interface spacing is large enough. The law of exponential decay may be used to identify the formation of interface states from interface-bulk mixed states. It also suggests that the coupling is caused by quantum tunneling. The coupling of the interface states in TlSe/TlSe can be greatly enhanced by intercalating hexagonal BN, although hexagonal BN is often used as a 2D substrate or insulator. The intercalation of bilayer BN or single atomic Au layer can further enhance the interfacial coupling. Our calculations may be helpful for the application of junctions composed of topological insulators.

Electric-Field Control of a Single-Atom Polar Bond.

We expose the polar covalent bond between a single Au atom terminating the apex of an atomic force microscope tip and a C atom of graphene on SiC(0001) to an external electric field. For one field orientation, the Au─C bond is strong enough to sustain the mechanical load of partially detached graphene, while for the opposite orientation, the bond breaks easily. Calculations based on density-functional theory and nonequilibrium Green's function methods support the experimental observations by unveiling bond forces that reflect the polar character of the bond. Field-induced charge transfer between the atomic orbitals modifies the polarity of the different electronegative reaction partners and the Au─C bond strength.

Atomically dispersed Au catalysts for preferential oxidation of CO in H2-rich stream

Carbon monoxide preferential oxidation (CO-PROX) is regarded as a promising strategy to remove trace amount of CO in H2-rich stream. Herein, atomically dispersed Au catalysts with different Au contents supported on cerium-zirconium solid solution supports were obtained by a deposition-precipitation method. The optimal catalyst, with 0.02 wt% Au, exhibits nearly 100 % CO conversion and 50 % CO2 selectivity for CO-PROX reaction under PEMFC working temperature range (80–120 ℃) and remains catalytically stable after 72 h testing at 80 ℃. The superior catalytic performances are attributed to the strong adsorption of CO and the low energy barrier of CO oxidation reaction, as well as the weak H2 adsorption and dissociation over highly dispersed Au single atoms, as supported by density functional theory calculations. Furthermore, combining characterization and experimental results, it is concluded that although catalysts are atomically dispersed, their performances can be adjusted by the synergetic interaction between neighboring Au single atoms.

Modulating electron density of vacancy site by single Au atom for effective CO2 photoreduction

The surface electron density significantly affects the photocatalytic efficiency, especially the photocatalytic CO2 reduction reaction, which involves multi-electron participation in the conversion process. Herein, we propose a conceptually different mechanism for surface electron density modulation based on the model of Au anchored CdS. We firstly manipulate the direction of electron transfer by regulating the vacancy types of CdS. When electrons accumulate on vacancies instead of single Au atoms, the adsorption types of CO2 change from physical adsorption to chemical adsorption. More importantly, the surface electron density is manipulated by controlling the size of Au nanostructures. When Au nanoclusters downsize to single Au atoms, the strong hybridization of Au 5d and S 2p orbits accelerates the photo-electrons transfer onto the surface, resulting in more electrons available for CO2 reduction. As a result, the product generation rate of AuSA/Cd1−xS manifests a remarkable at least 113-fold enhancement compared with pristine Cd1−xS.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

Water enables mild oxidation of methane to methanol on gold single-atom catalysts

As a 100% atom-economy process, direct oxidation of methane into methanol remains as a grand challenge due to the dilemma between activation of methane and over-oxidation of methanol. Here, we report that water enabled mild oxidation of methane into methanol with >99% selectivity over Au single atoms on black phosphorus (Au1/BP) nanosheets under light irradiation. The mass activity of Au1/BP nanosheets reached 113.5 μmol gcatal−1 in water pressured with 33 bar of mixed gas (CH4:O2 = 10:1) at 90 °C under light irradiation (1.2 W), while the activation energy was 43.4 kJ mol−1. Mechanistic studies revealed that water assisted the activation of O2 to generate reactive hydroxyl groups and •OH radicals under light irradiation. Hydroxyl groups reacted with methane at Au single atoms to form water and CH3* species, followed by oxidation of CH3* via •OH radicals into methanol. Considering the recycling of water during the whole process, we can also regard water as a catalyst.

Double-shell microcapsules with spatially arranged Au nanoparticles and single Zn atoms for tandem synthesis of cyclic carbonates.

Tandem catalysts with multifunctional sites can achieve high-efficiency catalytic transformations for quickly converting simple raw materials into complex value-added products. The integration of highly active species of metal nanoparticles (NPs) and single-atom catalytic sites (SACs) into one tandem system promises to synthesize an ideal bifunctional catalyst on account of the synergistic effect between NPs and SACs. However, such ideas face some challenges as deactivation or loss of active species, and low efficiency or side reactions caused by the disorder of different active species. Herein, a double-shell microencapsulated nanoreactor was fabricated as a bifunctional catalyst for the one-pot synthesis of cyclic carbonates from olefins. The microcapsules consist of an inner shell of nitrogen-doped porous carbon rich in Zn SACs, an outer shell of mesoporous SiO2, and Au NPs confined between the outer and inner shells, noted as Zn-N-C/Au@mSiO2. Particularly, two active species are spatially compartmented within microcapsules. Furthermore, the catalyst was applied in the one-pot synthesis of styrene carbonate from styrene with CO2 under normal pressure and showed admirable performance. The yield of cyclic carbonate reached 92.9% at 93.2% olefins conversion. Furthermore, the catalyst shows good reusability with little loss of catalytic performance (4.0%) even after using it 15 consecutive times. The unique structure used in this work can rationally integrate diverse catalytic species into one system and offering adequate protection, which provides an effective strategy for the development of multi-site catalysts.

Spontaneous chemical functionalization via coordination of Au single atoms on monolayer MoS2.

Surface functionalization of metallic and semiconducting 2D transition metal dichalcogenides (TMDs) have mostly relied on physi- and chemi-sorption at defect sites, which can diminish the potential applications of the decorated 2D materials, as structural defects can have substantial drawbacks on the electronic and optoelectronic characteristics. Here, we demonstrate a spontaneous defect-free functionalization method consisting of attaching Au single atoms to monolayers of semiconducting MoS2(1H) via S-Au-Cl coordination complexes. This strategy offers an effective and controllable approach for tuning the Fermi level and excitation spectra of MoS2 via p-type doping and enhancing the thermal boundary conductance of monolayer MoS2, thus promoting heat dissipation. The coordination-based method offers an effective and damage-free route of functionalizing TMDs and can be applied to other metals and used in single-atom catalysis, quantum information devices, optoelectronics, and enhanced sensing.

Theoretical study of CO oxidation on Au1/Co3O4 (110) single atom catalyst using density functional theory calculations

Single-atom catalyst (SAC) using Au1/Co3O4 has been under investigation experimentally for CO oxidation in recent years. However, the reaction mechanisms for CO oxidation on Au1/Co3O4 are still poorly understood. In this paper, theoretical investigations have been conducted to elucidate the catalytic mechanism of CO oxidation on the Au1/Co3O4 (110) and the pristine Co3O4 (110) by means of the spin-polarized density functional theory puls Hubbard U (DFT + U) calculations, respectively. The calculation results show that 1) the top site of Co3+ (site 1) is the most favorable anchored site for single Au atom on the Co3O4 (110); 2) CO molecule always anchored on the top of the Au atom by a C–Au bond; 3) O2 molecules can be only physisorbed on the single Au atom in whatever way; 4) CO2 molecule can be easily escaped from Au1/Co3O4 (110) at low temperature; 5) the catalytic mechanisms for CO oxidation by extracting the twofold-coordinate oxygen and threefold-coordinate oxygen on the pristine Co3O4 (110) are follow the Mars-van Krevelen mechanism, and the catalytic activity of CO oxidation on the pristine Co3O4 (110) O2f is better than that on O3f; 6) although the pristine Co3O4 (110) has some catalytic activities for CO oxidation, the catalytic activities are far below those of the Au1/Co3O4 (110) via the Eley-Rideal mechanism. In other words, the introducing of single atom Au into the pristine Co3O4 (110) can greatly enhance the catalytic activity of the pristine Co3O4 (110) for CO oxidation. The above results will help in understanding the theoretical mechanisms and the previous experimental results at the molecular lever, and provide a theory base for designing and fabrication of highly active and stable Co3O4 supported SACs.

Methanol oxidation over rutile Au1@TiO2 catalyst: Importance of facets and oxygen vacancy

Single-atom catalysts have attracted exceptional interest of researchers. However, the arguing whether tuning the interactions between the dispersed single metals and the support can enhance the activity, selectivity and stability of single atom catalysts is still open. Herein, single atom Au catalyst (rutile Au1@TiO2) is presented via density functional theory calculations, which exhibits high efficiency for methanol oxidation to formate, especially the exposed (110) facet. Rutile Au1@TiO2 (110) facet prefers to localize at 6-fold Ti site under rich-oxygen conditions; methanol goes through adsorption, OH dissociation, CH dissociation, O2 molecule adsorption, and withdraws H2O and formate to gas phase separately. In particular, a significant step on the way to methanol oxidation is the oxygen vacancy formation at the reduced rutile Au1@TiO2 (110) surface after CH scission and H2O desorption. The subsequent adsorbed O2 molecule does only provide oxygen species for methanol oxidation, but not fill surface oxygen vacancy, which promotes formate formation and desorption to develop very well.

Water‐Based Synthesis of Gold Single Atoms‐Embedded, Metal–Organic Frameworks‐Derived Nanoporous Carbon Nanoparticles with Enhanced Reduction Ability

AbstractSingle‐atom catalysts (SACs) have emerged to be a new research frontier in catalysis due to their distinctive physical and chemical properties, including better catalytic performance, stability, and recyclability. However, the widely used organic synthesis methods for SACs limit the ability to uniformly distribute single atoms’ water‐soluble precursors on the support of the catalysts. A water‐based synthesis of single gold (Au) atom catalyst is used in this work. Through this method, water‐soluble Au precursors are successfully turned into Au single atoms embedded in metal–organic framework‐derived nitrogen‐containing nanoporous carbon. The resulting catalyst is applied on the reduction of 4‐nitropenol. The catalytic performance of the reaction reaches 100% conversion in 30 min, while utilizing Au nanoparticles as active sites on the same support can only get 70% conversion under the same reaction conditions.

Understanding the Activity of Single-Atom Catalysis from Frontier Orbitals.

The d-band center and charge states are often used to analyze the catalytic activity of noble or transition metal surfaces and clusters, but their applicability for single-atom catalysts (SACs) is unsure. This work suggests that the spatial structure and orientation of frontier orbitals which are closest to the Fermi level of SACs play a vital role. Taking adsorption of several molecules and CO oxidization on C_{3}N-supported single-atom Au as examples, we demonstrate that adsorption and catalytic activities are well correlated with the characteristics of frontier orbitals. This work provides an effective guidance for understanding the performance of single-atom catalysts.

Electrochemical N2 Reduction to Ammonia Using Single Au/Fe Atoms Supported on Nitrogen-Doped Porous Carbon.

The electrochemical nitrogen reduction reaction (NRR) to ammonia (NH3) is a promising alternative route for an NH3 synthesis at ambient conditions to the conventional high temperature and pressure Haber–Bosch process without the need for hydrogen gas. Single metal ions or atoms are attractive candidates for the catalytic activation of non-reactive nitrogen (N2), and for future targeted improvement of NRR catalysts, it is of utmost importance to get detailed insights into structure-performance relationships and mechanisms of N2 activation in such structures. Here, we report density functional theory studies on the NRR catalyzed by single Au and Fe atoms supported in graphitic C2N materials. Our results show that the metal atoms present in the structure of C2N are the reactive sites, which catalyze the aforesaid reaction by strong adsorption and activation of N2. We further demonstrate that a lower onset electrode potential is required for Fe–C2N than for Au–C2N. Thus, Fe–C2N is theoretically predicted to be a potentially better NRR catalyst at ambient conditions than Au–C2N owing to the larger adsorption energy of N2 molecules. Furthermore, we have experimentally shown that single sites of Au and Fe supported on nitrogen-doped porous carbon are indeed active NRR catalysts. However, in contrast to our theoretical results, the Au-based catalyst performed slightly better with a Faradaic efficiency (FE) of 10.1% than the Fe-based catalyst with an FE of 8.4% at −0.2 V vs. RHE. The DFT calculations suggest that this difference is due to the competitive hydrogen evolution reaction and higher desorption energy of ammonia.

Substitution of O with a Single Au Atom as an Electron Acceptor in Al Oxide Clusters.

Al atoms generally adopt the +3 oxidation state and form stoichiometric oxides such as Al2O3 in the bulk phase. Among small cationic gas-phase clusters, near-stoichiometric clusters such as Al3O4+, Al3O5+, Al4O6+, Al4O7+, Al5O7+, and Al5O8+ have been readily generated in experimental studies. However, when a single Au atom was included in the clusters, oxygen-deficient clusters such as AuAl4O5+ were formed in high abundance; in these clusters, the Au atom accepted electron density from the Al atoms. The geometrical structures and atomic charges in the clusters suggest that a single Au atom can substitute for O atoms in Al oxide clusters. This propensity originates from the high electron and low oxygen affinities, which, together, constitute an unusual property of Au.

Hollow mesoporous CeO2 microspheres for efficient loading of Au single-atoms to catalyze the water-gas shift reaction

Abstract The atomically dispersed catalysts have aroused great attention in the recent decade. However, the preparation of atomically dispersed Au catalysts presents more challenge than other atomically dispersed catalysts such as Pt, Pd, etc. From the aspect of support, this work contributes a novel hollow mesoporous CeO2 microspheres (CeO2-hm) for efficient dispersion of single Au atoms (Au1). Contributed by the high specific surface area and mesoporous structure, CeO2-hm possesses much higher capacity for undertaking Au1, than commercial CeO2 particles (CeO2-c). Typically, with Au loading of 0.2 at%, CeO2-hm supported Au (Au1/CeO2-hm) presents atomic dispersion while CeO2-c supported Au (Au/CeO2-c) contains large amount of Au nanoparticles. Confirmed by catalyzing the water-gas shift (WGS) reaction, the obtained Au1/CeO2 catalysts exhibit notably higher performance than the Au-based catalysts supported by CeO2-c and CeO2 synthesized through co-precipitation and sol-gel strategies. For example, 0.2 at% Au1/CeO2-hm exhibits 9.1 times reactivity of equal Au loaded Au/CeO2-c catalyst. Besides, the CeO2-hm supported single-atom Au catalyst presents good tolerance under long-time WGS running.