Structural Characterization of Carbon-Supported Platinum−Ruthenium Nanoparticles from the Molecular Cluster Precursor PtRu5C(CO)16

Amphi-Luminescent MoS2 nanostructure for photocatalytic splitting of water and removal of Methylene Blue

Chemical dyes used in the textile industries are one of the major pollutants in water. Methylene blue (MB) is a commonly seen dye that creates hazardous health problems. In this article the photocatalytic degradation of MB by the nanocatalyst MoS2 (Nano-MoS2) and carbon dot (C Dots) incorporated MoS2 (Nano-CD-MoS2) is reported. The photocatalytic degradation of MB is analyzed based on the electron-hole recombination rate of the catalyst. Photoluminescence emission exhibited by the catalyst is used as a key indicator to probe the electron-hole recombination rate. Nano-MoS2 was synthesized hydrothermally at 180 0C for 8 h from ammonium tetra thiomolybdate (ATTM). C Dot was prepared following a green root from ash guard extract which later mixed with Nano-MoS2 and kept in an autoclave at a temperature 140 °C for 4 h to get Nano-CD-MoS2. The photoluminescence (PL) and photocatalytic behavior of Nano-MoS2 and Nano-CD-MoS2 and their application for water splitting and water purification are reported. The incorporation of graphene and artificial C Dot into MoS2 nanostructures are reported to increase the conductivity and active edge sites of MoS2 that enhances the photocatalytic action. Since green C Dots are eco-friendly and easily synthesizable than artificial C Dots, as a novel study, this article investigated the influence of green C Dots on the PL and photocatalytic performance of nanosized MoS2. Nano-MoS2 and Nano-CD-MoS2 exhibited both upconversion and downconversion PL; accordingly the nanostructures were termed as amphi-luminescent. The amphi-luminescence property widens the photon absorption range and hence enhances the catalytic degradation of dyes. Nano-MoS2 which exhibited lesser intensity of amphi-luminescence emission compared to Nano-CD-MoS2 showed better results in degradation of MB. C Dots may bind with the valence band electrons of MoS2, resulting in the reduction of dangling bonds. Dangling bonds can trap photo-induced excitons to hinder the rate of electron-hole recombination. So, fast electron-hole recombination occurs in Nano-CD-MoS2 than Nano-MoS2. Fast electron-hole recombination supports radiative electron-hole recombination while suppresses the non-radiative energy transfer of electrons and causes high PL intensity. However, according to the energy level diagram, Nano-MoS2 with minimal electron-hole recombination rate is more favorable for O2/O2-,.OH/ OH- and.OH/H2O reactions that facilitate MB degradation. Photocatalytic activity of catalysts were confirmed by measuring the photocurrent from a simple custom-made two-electrode water photolysis cell where the nanocatalysts were dispersed in electrolyte. Lead and steel rods were used as electrodes. Multimeter was used to measure current. Nano-MoS2 exhibited better performance with a maximum photocurrent of 141 µA. Influence of green C Dots in energy levels, PL and photocatalysis of MoS2 and mechanisms of PL and degradation of MB are thoroughly investigated in this article.

Iterative Bragg peak removal on X-ray absorption spectra with automatic intensity correction

This study introduces a novel iterative Bragg peak removal with automatic intensity correction (IBR-AIC) methodology for X-ray absorption spectroscopy (XAS), specifically addressing the challenge of Bragg peak interference in the analysis of crystalline materials. The approach integrates experimental adjustments and sophisticated post-processing, including an iterative algorithm for robust calculation of the scaling factor of the absorption coefficients and efficient elimination of the Bragg peaks, a common obstacle in accurately interpreting XAS data, particularly in crystalline samples. The method was thoroughly evaluated on dilute catalysts and thin films, with fluorescence mode and large-angle rotation. The results underscore the technique's effectiveness, adaptability and substantial potential in improving the precision of XAS data analysis. While demonstrating significant promise, the method does have limitations related to signal-to-noise ratio sensitivity and the necessity for meticulous angle selection during experimentation. Overall, IBR-AIC represents a significant advancement in XAS, offering a pragmatic solution to Bragg peak contamination challenges, thereby expanding the applications of XAS in understanding complex materials under diverse experimental conditions.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites

The development of new catalyst materials for energy-efficient chemical synthesis is critical as over 80% of industrial processes rely on catalysts, with many of the most energy-intensive processes specifically using heterogeneous catalysis. Catalytic performance is a complex interplay of phenomena involving temperature, pressure, gas composition, surface composition, and structure over multiple length and time scales. In response to this complexity, the integrated approach to heterogeneous dilute alloy catalysis reviewed here brings together materials synthesis, mechanistic surface chemistry, reaction kinetics, in situ and operando characterization, and theoretical calculations in a coordinated effort to develop design principles to predict and improve catalytic selectivity. Dilute alloy catalysts─in which isolated atoms or small ensembles of the minority metal on the host metal lead to enhanced reactivity while retaining selectivity─are particularly promising as selective catalysts. Several dilute alloy materials using Au, Ag, and Cu as the majority host element, including more recently introduced support-free nanoporous metals and oxide-supported nanoparticle "raspberry colloid templated (RCT)" materials, are reviewed for selective oxidation and hydrogenation reactions. Progress in understanding how such dilute alloy catalysts can be used to enhance selectivity of key synthetic reactions is reviewed, including quantitative scaling from model studies to catalytic conditions. The dynamic evolution of catalyst structure and composition studied in surface science and catalytic conditions and their relationship to catalytic function are also discussed, followed by advanced characterization and theoretical modeling that have been developed to determine the distribution of minority metal atoms at or near the surface. The integrated approach demonstrates the success of bridging the divide between fundamental knowledge and design of catalytic processes in complex catalytic systems, which can accelerate the development of new and efficient catalytic processes.

Latent Representation Learning for Structural Characterization of Catalysts

Supervised machine learning-enabled mapping of the X-ray absorption near edge structure (XANES) spectra to local structural descriptors offers new methods for understanding the structure and function of working nanocatalysts. We briefly summarize a status of XANES analysis approaches by supervised machine learning methods. We present an example of an autoencoder-based, unsupervised machine learning approach for latent representation learning of XANES spectra. This new approach produces a lower-dimensional latent representation, which retains a spectrum-structure relationship that can be eventually mapped to physicochemical properties. The latent space of the autoencoder also provides a pathway to interpret the information content "hidden" in the X-ray absorption coefficient. Our approach (that we named latent space analysis of spectra, or LSAS) is demonstrated for the supported Pd nanoparticle catalyst studied during the formation of Pd hydride. By employing the low-dimensional representation of Pd K-edge XANES, the LSAS method was able to isolate the key factors responsible for the observed spectral changes.

Towards Atomically Precise Supported Catalysts from Monolayer-Protected Clusters: The Critical Role of the Support

Controlling the size and uniformity of metal clusters with atomic precision is essential for fine-tuning their catalytic properties, however for clusters deposited on supports, such control is challenging. Here, by combining X-ray absorption spectroscopy and density functional theory calculations, it is shown that supports play a crucial role in the evolution of monolayer-protected clusters into catalysts. Based on the acidic nature of the support, cluster-support interactions lead either to fragmentation of the cluster into isolated Au-ligand species or ligand-free metallic Au⁰ clusters. On Lewis acidic supports that bind metals strongly, the latter transformation occurs while preserving the original size of the metal cluster, as demonstrated for various Au_n sizes. These findings underline the role of the support in the design of supported catalysts and represent an important step toward the synthesis of atomically precise supported nanomaterials with tailored physico-chemical properties.

High-throughput operando-ready X-ray absorption spectroscopy flow reactor cell for powder samples

A high-throughput, operando-ready X-ray absorption spectroscopy catalytic reaction cell consisting of 4 parallel reactors was designed to collect X-ray absorption near edge structure and extended fine structure spectra under reaction conditions. The cell is capable of operating at temperatures from ambient conditions up to 773 K and pressures from ambient to 2 MPa in a variety of gas environments. The cell design is mechanically simple, and programmable operation at beamline 8-ID (NSLS-II, Brookhaven National Laboratory) makes it straightforward to use. Reactor tube parts were available as-fabricated from commercial sources, while the heating jacket and cell mounting required custom machining.

Neural network assisted analysis of bimetallic nanocatalysts using X-ray absorption near edge structure spectroscopy

Trained neural networks are used to extract the first partial coordination numbers from XANES spectra. In bimetallic nanoparticles, the four local structure descriptors provide rich information on structural motifs.

Optimising surface d charge of AuPd nanoalloy catalysts for enhanced catalytic activity

Understanding the catalytic mechanism of bimetallic nanocatalysts remains challenging. Here, we adopt an adsorbate mediated thermal reduction approach to yield monodispersed AuPd catalysts with continuous change of the Pd-Au coordination numbers embedded in a mesoporous carbonaceous matrix. The structure of nanoalloys is well-defined, allowing for a direct determination of the structure-property relationship. The results show that the Pd single atom and dimer are the active sites for the base-free oxidation of primary alcohols. Remarkably, the d-orbital charge on the surface of Pd serves as a descriptor to the adsorbate states and hence the catalytic performance. The maximum d-charge gain occurred in a composition with 33-50 at% Pd corresponds to up to 9 times enhancement in the reaction rate compared to the neat Pd. The findings not only open an avenue towards the rational design of catalysts but also enable the identification of key steps involved in the catalytic reactions.

Correlation between the electronic/local structure and CO-oxidation activity of Pd x Ru1-x alloy nanoparticles

Pd x Ru1-x nanoparticles (NPs) were observed to display enhanced CO oxidation activity with the maximum performance obtained at the composition x = 0.5. To unveil the origin of this superior CO oxidation activity, we investigated the local structure, valence state, and electronic properties of Pd x Ru1-x NPs using synchrotron-based X-ray techniques. Site specific information obtained from X-ray absorption fine structure (XAFS) spectroscopy revealed that the local disorder around Pd and Ru atoms and their valence state can be systematically tuned by varying the Pd composition. Furthermore, the XAFS results indicated a strong correlation among the structural and valence state and the observed CO oxidation catalytic properties of Pd x Ru1-x NPs. Hard X-ray photoelectron spectroscopy (HAXPES) analysis suggested that the capability of CO oxidation requires an optimum balance between the adsorption and desorption energy for CO adsorption and eventually conversion to CO2. A comparison between the experimental valence band (VB) HAXPES spectra of Pd x Ru1-x NPs and the linear combination of VB HAXPES spectra of Pd and Ru NPs revealed that the charge transfer from Pd to Ru occurs in the Pd x Ru1-x alloy at intermediate compositions, causing electron enrichment of the Ru surface. In addition, the maximum red-shift in the edge-position relative to that of bulk Pd/Ru and high structural disorder were observed for the PdRu alloy at the intermediate composition. This coupled behavior of structure and electronic properties followed the experimental trend of CO oxidation activity in this system.

Dual reactor for in situ/operando fluorescent mode XAS studies of sample containing low-concentration 3d or 5d metal elements

Transition metal elements are the most important elements of heterogeneous catalysts used for chemical and energy transformations. Many of these catalysts are active at a temperature higher than 400 °C. For a catalyst containing a 3d or 5d metal element with a low concentration, typically their released fluorescence upon the K-edge or L-edge adsorption of X-rays is collected for the analysis of chemical and coordination environments of these elements. However, it is challenging to perform in situ/operando X-ray absorption spectroscopy (XAS) studies of elements of low-energy absorption edges at a low concentration in a catalyst during catalysis at a temperature higher than about 450 °C. Here a unique reaction system consisting two reactors, called a dual reactor system, was designed for performing in situ or operando XAS studies of these elements of low-energy absorption edges in a catalyst at a low concentration during catalysis at a temperature higher than 450 °C in a fluorescent mode. This dual-reactor system contains a quartz reactor for preforming high-temperature catalysis up to 950 °C and a Kapton reactor remaining at a temperature up to 450 °C for collecting data in the same gas of catalysis. With this dual reactor, chemical and coordination environments of low-concentration metal elements with low-energy absorption edges such as the K-edge of 3d metals including Ti, V, Cr, Mn, Fe, Co, Ni, and Cu and L edge of 5d metals including W, Re, Os, Ir, Pt, and Au can be examined through first performing catalysis at a temperature higher than 450 °C in the quartz reactor and then immediately flipping the catalyst in the same gas flow to the Kapton reactor remained up to 450 °C to collect data. The capability of this dual reactor was demonstrated by tracking the Mn K-edge of the MnO_x/Na₂WO₄ catalyst during activation in the temperature range of 300-900 °C and catalysis at 850 °C.

An in situ XAS study of the activation of precursor-dependent Pd nanoparticles

The activation of precursor-dependent Pd nanoparticles was comprehensively followed by in situ X-ray absorption spectroscopy on two inorganic supports for rationalizing the final catalytic activity.

Chemical Stabilization and Electrochemical Destabilization of the Iron Keggin Ion in Water

The iron Keggin ion is identified as a structural building block in both magnetite and ferrihydrite, two important iron oxide phases in nature and in technology. Discrete molecular forms of the iron Keggin ion that can be both manipulated in water and chemically converted to the related metal oxides are important for understanding growth mechanisms, in particular, nonclassical nucleation in which cluster building units are preserved in the aggregation and condensation processes. Here we describe two iron Keggin ion structures, formulated as [Bi₆FeO₄Fe₁₂O₁₂(OH)₁₂(CF₃COO)₁₀(H₂O)₂]³⁺ (Kegg-1) and [Bi₆FeO₄Fe₁₂O₁₂(OH)₁₂(CF₃COO)₁₂]¹⁺ (Kegg-2). Experimental and simulated X-ray scattering studies show indefinite stability of these clusters in water from pH 1-3. The tridecameric iron Keggin-ion core is protected from hydrolysis by a synergistic effect of the capping Bi³⁺ cations and the trifluoroacetate ligands that, respectively, bond to the iron and bridge to the bismuth. By introducing electrons to the aqueous solution of clusters, we achieve complete separation of bismuth from the cluster, and the iron Keggin ion rapidly converts to magnetite and/or ferrihydrite, depending on the mechanism of reduction. In this strategy, we take advantage of the easily accessible reduction potential and crystallization energy of bismuth. Reduction was executed in bulk by chemical means, by voltammetry, and by secondary effects of transmission electron microscopy imaging of solutions. Prior, we showed a less stable analogue of the iron Keggin cluster converted to ferrihydrite simply upon dissolution. The prior and currently studied clusters with a range of reactivity provide a chemical system to study molecular cluster to metal oxide conversion processes in detail.

In Situ Probing of the Active Site Geometry of Ultrathin Nanowires for the Oxygen Reduction Reaction

To create truly effective electrocatalysts for the cathodic reaction governing proton exchange membrane fuel cells (PEMFC), namely the oxygen reduction reaction (ORR), necessitates an accurate and detailed structural understanding of these electrocatalysts, especially at the nanoscale, and to precisely correlate that structure with demonstrable performance enhancement. To address this key issue, we have combined and interwoven theoretical calculations with experimental, spectroscopic observations in order to acquire useful structural insights into the active site geometry with implications for designing optimized nanoscale electrocatalysts with rationally predicted properties. Specifically, we have probed ultrathin (∼2 nm) core-shell Pt∼Pd9Au nanowires, which have been previously shown to be excellent candidates for ORR in terms of both activity and long-term stability, from the complementary perspectives of both DFT calculations and X-ray absorption spectroscopy (XAS). The combination and correlation of data from both experimental and theoretical studies has revealed for the first time that the catalytically active structure of our ternary nanowires can actually be ascribed to a PtAu∼Pd configuration, comprising a PtAu binary shell and a pure inner Pd core. Moreover, we have plausibly attributed the resulting structure to a specific synthesis step, namely the Cu underpotential deposition (UPD) followed by galvanic replacement with Pt. Hence, the fundamental insights gained into the performance of our ultrathin nanowires from our demonstrated approach will likely guide future directed efforts aimed at broadly improving upon the durability and stability of nanoscale electrocatalysts in general.

Multistep reactions of water with small Pdn clusters: A first principles study

Multistep dissociative chemisorption reactions of water with Pd 4 and Pd 7 clusters were studied using density functional theory. The adsorption energies and referred adsorption sites from water molecule ( H 2 O ) to partially dissociative ( H 2+ O and OH + H ), then to fully dissociative ( O + H + H ) configurations are carefully determined. It is found that the adsorption energies of three dissociative reactions are 5–6 times larger than that of water molecule. Atop sites of Pd 4 and Pd 7 clusters are found to be the most stable sites for the adsorbed H 2 O molecule. For the coadsorption cases of partially and fully dissociated products, H 2 and OH molecules preferably tend to bind at the low coordination (atop or bridge) sites, and O and H atoms prefer to adsorb on the high coordination (hollow) sites. It is also found that the most favorable adsorption sites for the molecular adsorbates ( H 2 O , H 2 and OH ) are adjacent to the Pd atoms with the largest site-specific polarizabilities. Therefore, site-specific polarizability is a good predictor of the favorable adsorption sites for the weakly bound molecules. The different directions of charge transfer between the Pd clusters and the adsorbate(s) is observed. Furthermore, the processes of the adsorption, dissociation, and the dissociative products diffusion of H 2 O are analyzed.

Dispersion and distribution of bimetallic oxides in SBA-15, and their enhanced activity for reverse water gas shift reaction

Bimetallic oxides could be well distributed on the SBA-15 surface, and exhibited high catalytic perforation for RWGS reaction.

Conversion of methane to methanol with a bent mono(μ-oxo)dinickel anchored on the internal surfaces of micropores

The oxidation of methane to methanol is a pathway to utilizing this relatively abundant, inexpensive energy resource. Here we report a new catalyst, bent mono(μ-oxo)dinickel anchored on an internal surface of micropores,which is active for direct oxidation. It is synthesized from the direct loading of a nickel precursor to the internal surface of micropores of ZSM5 following activation in O2. Ni 2p3/2 of this bent mono(μ-oxo)dinickel species formed on the internal surface of ZSM5 exhibits a unique photoemission feature, which distinguishes the mono(μ-oxo)dinickel from NiO nanoparticles. The formation of the mono(μ-oxo)dinickel species was confirmed with X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). This mono(μ-oxo)dinickel species is active for the direct oxidation of methane to methanol under the mild condition of a temperature as low as 150 °C in CH4 at 1 bar. In-situ studies using UV-vis, XANES, and EXAFS suggest that this bent mono(μ-oxo)dinickel species is the active site for the direct oxidation of methane to methanol. The energy barrier of this direct oxidation of methane is 83.2 kJ/mol.

Reaction engineering strategies for the production of inorganic nanomaterials

The rapid expansion of nanotechnology requires scaled-up production rates to cope with increased nanomaterials demand. However, in many cases, the final uses of nanomaterials impose strict requisites on their physical and chemical characteristics including size, shape, chemical composition and type of functional groups on their surface. Frequently, additional features such as a limited degree of agglomeration are also demanded. These requisites represent a serious challenge to present-day synthesis methods when nanomaterials must be produced in large amounts. Some of the possible solutions from the reaction engineering perspective are discussed in this work for both gas and liquid phase production processes. Special attention will be devoted to enabling technologies, which allow the production of engineered nanoparticles with limited aggregation and with a good control on their nano-scale characteristics.

WGS catalysis and in situ studies of CoO(1-x), PtCo(n)/Co3O4, and Pt(m)Co(m')/CoO(1-x) nanorod catalysts

Water-gas shift (WGS) reactions on Co3O4 nanorods and Co3O4 nanorods anchoring singly dispersed Pt atoms were explored through building correlation of catalytic performance to surface chemistry of catalysts during catalysis using X-ray absorption spectroscopy, ambient pressure X-ray photoelectron spectroscopy (AP-XPS), and environmental TEM. The active phase of pure Co3O4 during WGS is nonstoichiometric cobalt monoxide with about 20% oxygen vacancies, CoO0.80. The apparent activation energy (Ea) in the temperature range of 180-240 °C is 91.0 ± 10.5 kJ mol(-1). Co3O4 nanorods anchoring Pt atoms (Pt/Co3O4) are active for WGS with a low Ea of 50.1 ± 5.0 kJ mol(-1) in the temperature range of 150-200 °C. The active surface of this catalyst is singly dispersed Pt1Co(n) nanoclusters anchored on Co3O4 (Pt1/Co3O4), evidenced by in situ studies of extended X-ray absorption fine structure spectroscopy. In the temperature range of 200-300 °C, catalytic in situ studies suggested the formation of Pt(m)Co(m') nanoclusters along with the reduction of Co3O4 substrate to CoO(1-x). The new catalyst, Pt(m)Co(m')/CoO(1-x) is active for WGS with a very low Ea of 24.8 ± 3.1 kJ mol(-1) in the temperature range of 300-350 °C. The high activity could result from a synergy of Pt(m)Co(m') nanoclusters and surface oxygen vacancies of CoO(1-x).

Recent developments and applications of electron microscopy to heterogeneous catalysis

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are popular and powerful techniques used to characterize heterogeneous catalysts. Rapid developments in electron microscopy--especially aberration correctors and in situ methods--permit remarkable capabilities for visualizing both morphologies and atomic and electronic structures. The purpose of this review is to summarize the significant developments and achievements in this field with particular emphasis on the characterization of catalysts. We also highlight the potential and limitations of the various methods, describe the need for synergistic and complementary tools when characterizing heterogeneous catalysts, and conclude with an outlook that also envisions future needs in the field.