Benzyl Alcohol Oxidation Using Gold Catalysts Derived from Au8 Clusters on TiO2

Structure control and evolution of atomically precise gold clusters as heterogeneous precatalysts

Metal clusters have distinct features from single atom and nanoparticle (>1 nm) catalysts, making them effective catalysts for various heterogeneous reactions. Nevertheless, the ambiguity and complexity of the catalyst structure preclude in-depth mechanistic studies. The evolution of metal species during synthesis and reaction processes represents another challenge. One effective solution is to precisely control the structure of the metal cluster, thus offering a well-defined pre-catalyst. The well-defined chemical formula and configurations make atomically precise metal nanoclusters optimal choices. To fabricate an atomically precise metal nanocluster-based heterogeneous catalyst with enhanced performance, careful structural design of both the nanocluster and support material, an effective assembling technique, and a pre-treatment method for these hybrids need to be developed. In this review, we summarize recent advances in in the development of heterogeneous catalysts using atomically precise gold and alloy gold nanoclusters as precursors. We will begin with a brief introduction to the structural properties of atomically precise nanoclusters and structure determination of cluster/support hybrids. We will then introduce heterogeneous catalysts prepared from medium size (tens to hundreds of metal atoms) and low nuclearity nanoclusters. We will illustrate how ligand modification, support-cluster interaction, hybrid fabrication, and heteroatom (Pt, Pd Ag, Cu, Cd, Fe) introduction affect the structural properties and pretreatment/reaction-induced structural evolution of gold nanocluster pre-catalysts. Lastly, we will highlight the synthetic method of NCs@MOF hybrids and their effectiveness in circumventing the adverse cluster structural evolution. These findings are expected to shed light on the structure-activity relationship studies and future catalyst design strategies using atomically precise metal nanocluster pre-catalysts.

Promotion of Au nanoparticles on carbon frameworks for alkali-free aerobic oxidation of benzyl alcohol

We synthesized a series of modified Co-ZIF-67 materials with tunable morphology to support fine Au nanoparticles for the alkali-free aerobic oxidation of benzyl alcohol. Structure promotion was performed using Stöber silica as a hard template, which was subsequently removed by NaOH etching before gold immobilization. The texture structure of Au/(Si)C was greatly improved with increasing surface area and volume. CoOx was simultaneously introduced into the carbon shell from the Co-ZIF-67 precursor, which consequently facilitated the specific Au-support interaction via bimetallic synergy. XRD, XPS, and TEM images demonstrated the redispersion of both Au and CoOx as well as the electronic delivery between metals. Analysis of the chemical and surface composition suggested a surface rich in Auδ+ with abundant lattice oxygen contributed by CoOx in the final Au/(Si)C, which improved the transformation rate of benzyl alcohol even in an alkali-free condition. Au/(Si)C with finely dispersed Au particles showed excellent catalytic performance in the alkali-free environment, with 89.3% benzyl conversion and 74.5% benzaldehyde yield under very mild conditions.

Electronic Tuning of Gold Nanoparticle Active Sites for Reduction Catalysis

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Boosting the catalytic behavior and stability of a gold catalyst with structure regulated by ceria

In this work, a series of colloidal gold nanoparticles with controllable sizes were anchored on carbon nanotubes (CNT) for the aerobic oxidation of benzyl alcohol. The intrinsic influence of Au particles on the catalytic behavior was unraveled based on different nanoscale-gold systems. The Au/CNT-A sample with smaller Au sizes deserved a faster reaction rate, mainly resulting from the higher dispersion degree (23.5%) of Au with the available exposed sites contributed by small gold particles. However, monometallic Au/CNT samples lacked long-term stability. CeO₂ was herein decorated to regulate the chemical and surface structure of the Au/CNT. An appropriate CeO₂ content tuned the sizes and chemical states of Au by electron delivery with better metal dispersion. Small CeO₂ crystals that were preferentially neighboring the Au particles facilitated the generation of Au–CeO₂ interfaces, and benefited the continuous supplementation of oxygen species. The collaborative functions between the size effect and surface chemistry accounted for the higher benzaldehyde yield and sustainably stepped-up reaction rates by Au-Ce₅/CNT with 5 wt% CeO₂.

In this work, a series of colloidal gold nanoparticles with controllable sizes and CeO₂ promotion were anchored on carbon nanotubes (CNT) for the aerobic oxidation of benzyl alcohol.

Support, composition, and ligand effects in partial oxidation of benzyl alcohol using gold–copper clusters

The effect of metallic composition, support, and ligands on catalytic performance using AuCu clusters in benzyl alcohol oxidation is investigated.

Thermal stability of crown-motif [Au9(PPh3)8]3+ and [MAu8(PPh3)8]2+ (M = Pd, Pt) clusters: Effects of gas composition, single-atom doping, and counter anions

The thermal behaviors of ligand-protected metal clusters, [Au₉(PPh₃)₈]³⁺ and [MAu₈(PPh₃)₈]²⁺ (M = Pd, Pt) with a crown-motif structure, were investigated to determine the effects of the gas composition, single-atom doping, and counter anions on the thermal stability of these clusters. We successfully synthesized crown-motif [PdAu₈(PPh₃)₈][HPMo₁₂O₄₀] (PdAu8-PMo12) and [PtAu₈(PPh₃)₈][HPMo₁₂O₄₀] (PtAu8-PMo12) salts with a cesium-chloride-type structure, which is the same as the [Au₉(PPh₃)₈][PMo₁₂O₄₀] (Au9-PMo12) structure. Thermogravimetry-differential thermal analysis/mass spectrometry analysis revealed that the crown-motif structure of Au9-PMo12 was decomposed at ∼475 K without weight loss to form Au nanoparticles. After structural decomposition, the ligands were desorbed from the sample. The ligand desorption temperature of Au9-PMo12 increased under 20% O₂ conditions because of the formation of Au nanoparticles and stronger interaction of the formed O=PPh₃ than PPh₃. The Pd and Pt single-atom doping improved the thermal stability of the clusters. This improvement was due to the formation of a large bonding index of M-Au and a change in Au-PPh₃ bonding energy by heteroatom doping. Moreover, we found that the ligand desorption temperatures were also affected by the type of counter anions, whose charge and size influence the localized Coulomb interaction and cluster packing between the cationic ligand-protected metal clusters and counter anions.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Monodisperse-porous cerium oxide microspheres as a new support with appreciable catalytic activity for a composite catalyst in benzyl alcohol oxidation

Monodisperse porous ceria microspheres as a support with individual catalytic activity, facile post-functionalization and high surface area for heterogeneous catalysis.

Surface functionalization: an efficient alternative for promoting the catalytic activity of closed shell gold clusters

Surface functionalization through adsorption of ligands or non-metal atoms is considered to be an interesting and viable approach for tuning the physicochemical properties of gold clusters. Highly stable and magic numbered electronic configurations of thiolate protected gold clusters such as Au25(SR)18, Au38(SR)24etc. with intriguing properties are the direct manifestation of the rich chemistry of the Au-S interface. The present investigation discerns the CO oxidation activity of structurally well characterized sulphur functionalized gold cluster anions AumS4-, m = 6-10. To establish an in-depth understanding, their activities are analyzed and compared with the corresponding pristine gold clusters. It is seen that sulphur functionalization irrespective of a closed or open shell nature leads to a significant decrease in the O2 adsorption energies on the anionic gold clusters. However, in sharp contrast to O2 adsorption, surface functionalization gives rise to multifarious catalytic behavior in AumS4- clusters with catalytic activity ranging from low (for Au6S4-, Au8S4-) to moderate (for Au9S4-, Au10S4-) to very high (for Au7S4-) for CO oxidation. It is interesting to note that the closed shell Au7S4- and Au9S4- clusters with poor O2 adsorption show remarkably low activation barriers and enhanced catalytic activity as compared to the open shell AumS4- clusters with an odd number of electrons. In particular, in the case of Au7S4- the lowest activation energy barriers of 0.01 and 0.21 eV are obtained, making the CO oxidation reaction facile. Moreover, ab initio molecular dynamics are performed to confirm the enhanced catalytic behaviour of Au7S4- and its dynamical stability during the desorption of CO2 molecule from its surface.

Molecular and Dissociative Adsorption of Oxygen on Au-Pd Bimetallic Clusters: Role of Composition and Spin State of the Cluster

Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O-O dissociation barrier in O2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O2 adsorption and dissociation on Au8, Pd8, and Au8-n Pd n (n = 1-7) nanoclusters in different spin states. The density functional calculations point out that the O2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au5Pd3 and Au1Pd7 show very large O2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O2 dissociation. Furthermore, the computed O2 dissociation barriers are significantly low as compared to the already reported barriers (1.95-3.65 eV) on monometallic and bimetallic Au-Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au-Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.

Heterogeneous Gold Catalysis: From Discovery to Applications

In the present review, the important and current developments of gold catalysts for a wide range of applications are comprehensively summarized. This review also provides a detailed study of the literature data concerning the preparation, characterization, and catalytic applications of gold catalysts. Additionally, the main aspects of using supported gold nanoparticles (AuNPs) as catalysts for oxidation reactions are considered. In particular, the oxidation of benzyl alcohol to benzaldehyde and the production of adipic acid from cyclohexane are discussed in detail. Lastly, the key properties of gold catalysts are described, and an outlook on the application of gold catalysts is presented.