Supported Metal Catalysts Research Articles

Hierarchically ordered macro-mesoporous electrocatalyst with hydrophilic surface for efficient oxygen reduction reaction.

Metal-organic frameworks (MOFs) offer versatile templates/precursors to prepare supported metal catalysts. However, the afforded catalysts usually exhibit microporous structures and unsuitable wettability, which would restrict the accessibility of active sites in liquid-phase reactions. Herein, we develop an etching-functionalization strategy for the construction of a tannic acid-functionalized MOF with a unique hollow-wall and three-dimensional ordered macroporous (H-3DOM) structure. The functional MOF can be further employed as an ideal precursor for the synthesis of cobalt supported on oxygen, nitrogen co-doped carbon composites with H-3DOM structures and hydrophilic surface. The H-3DOM structure can improve the external surface area to maximize the exposure of active sites. Moreover, the oxygen-containing functional groups can enhance the surface wettability to guarantee the external active sites to be more electrochemically accessible in aqueous electrolyte. Benefitting from these outstanding characteristics, H-3DOM-Co/ONC exhibits high electrocatalytic activity in the oxygen reduction reaction, superior to its counterparts without the hierarchically ordered structure and surface functionalization. This article is protected by copyright. All rights reserved.

Facet-Controlled Cu2O Support Enhances Catalytic Activity of Pt Nanoparticles for CO Oxidation.

The metal-support interaction plays a crucial role in determining the catalytic activity of supported metal catalysts. Changing the facet of the support is a promising strategy for catalytic control via constructing a well-defined metal-support nanostructure. Herein, we developed cubic and octahedral Cu2O supports with (100) and (111) facets terminated, respectively, and Pt nanoparticles (NPs) were introduced. The in situ characterizations revealed the facet-dependent encapsulation of the Pt NPs by a CuO layer due to the oxidation of the Cu2O support during the CO oxidation reaction. The CuO layer on Pt at cubic Cu2O (Pt/c-Cu2O) significantly enhanced catalytic performance, while the thicker CuO layer on Pt at octahedral Cu2O suppressed CO conversion. The formation of a thin CuO layer is attributed to the dominant Pt-O-Cu bond at the Pt/c-Cu2O interface, which suppresses the adsorption of oxygen molecules. This investigation provides insight into designing high-performance catalysts via engineering the interface interaction.

Reforming of methane: Effects of active metals, supports, and promoters

ABSTRACT The dry reforming reaction (DRM) had capacity to mitigate the greenhouse gases from environment as well as to produce syngas as a chemical feedstock. Various doped metal oxides, supported metal catalysts and promoted catalyst system over different supported metal catalysts had profound activity toward this specific reaction. Herein, a sequential review on DRM is presented which covers most of the excellent and conceptual work of this field from the last two decades. The review covers 8 active metals i.e. Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt dispersed over 12 support system as activated carbon or Char, MgO, CeO2, La2O3, Y2O3, TiO2, Si2O3, Al2O3, ZrO2, SiC BN, Clay for DRM reaction. Further under each support, a deep literature review of a long range of promotors (total 35) belonging to different groups of periodic tables is elaborated. At the end, a brief about the state-of-the-art of possible industrial potent catalyst and the future research portfolio for catalytic advancement are presented.

Efficient and Stable Ni/SBA-15 Catalyst for Dry Reforming of Methane: Effect of Citric Acid Concentration

Citric acid, one of the representative chelate compounds, has been widely used as an additive to achieve the highly dispersed metal-supported catalysts. This study aimed to investigate the effect of citric acid concentration on the preparation of the highly dispersed Ni catalysts on mesoporous silica (SBA-15) for the dry reforming of methane. A series of Ni/SBA-15 catalysts with citric acid were prepared using the acid-assisted incipient wetness impregnation method, and the Ni/SBA-15 catalyst as a reference was synthesized via the impregnation method. First of all, the citric acid addition during the catalyst synthesis step regardless of its concentration resulted in highly dispersed Ni particles of ~4–7 nm in size in Ni/SBA-15 catalysts, which had a superior and stable catalytic performance in the dry reforming of methane (93% of CO2 conversion and 86% of CH4 conversion). In addition, the amount of coke formation was much lower in a series of Ni/SBA-15 catalysts with citric acid (~2–5 mgcoke gcat−1 h−1) compared to pristine Ni/SBA-15 catalysts (~22 mgcoke gcat−1 h−1). However, when the concentration of citric acid became higher, the more free NiO species that formed on the SBA-15 support, leading to large Ni particles after the stability test. The addition of citric acid is a very clear strategy for making highly dispersed catalysts, but its concentration needs to be carefully controlled.

Low-temperature selective synthesis of metastable α-MoC with electrochemical properties: Electrochemical co-reduction of CO2 and MoO3 in molten salts

Metastable molybdenum carbide (α-MoC), as a catalyst and an excellent support for metal catalysts, has been widely used in thermo/electro-catalytic reactions. However, the selective synthesis of α-MoC remains a great challenge. Herein, a simple one-pot synthetic strategy for the selective preparation of metastable α-MoC is proposed by electrochemical co-reduction of CO2 and MoO3 in a low-temperature eutectic molten carbonate. The synthesized α-MoC shows a reed flower-like morphology. By controlling the electrolysis time and monitoring the phase and morphology of the obtained products, the growth process of α-MoC is revealed, where the carbon matrix is deposited first followed by the growth of α-MoC from the carbon matrix. Moreover, by analyzing the composition of the electrolytic products, the formation mechanism for α-MoC is proposed. In addition, through this one-pot synthetic strategy, S-doped α-MoC is successfully synthesized. Density functional theory (DFT) calculations reveal that S doping enhanced the HER performance of α-MoC by facilitating water absorption and dissociation and weakening the bond energy of Mo-H to accelerate H desorption. The present work not only highlights the valuable utilization of CO2 but also offers a new perspective on the design and controllable synthesis of metal carbides and their derivatives.

Directing Reaction Pathways on Supported Metal Catalysts with Low-Density Self-Assembled Monolayers

Controlling reactant adsorption on catalyst surfaces is crucial to reaction activity and selectivity. One method for improving selectivity is by imposing steric constraints to bias the reactant binding orientation. In this study, thiol self-assembled monolayers (SAMs) were deposited onto Pt/Al2O3 catalysts as a method for controlling activity and selectivity via steric effects. In addition to a full monolayer, a low-density SAM-coated catalyst was employed. A number of characterization techniques demonstrated the successful deposition of homogeneous low-density SAMs on the metal surface with reduced site-blocking compared to a full high-density monolayer. Reaction kinetic studies showed increased benzyl alcohol hydrodeoxygenation (HDO) selectivity for both SAM-modified catalysts. This was attributed to the inability of the reactant to adsorb on the catalyst with the aromatic ring parallel to the surface, thus preventing decarbonylation and ring hydrogenation reaction pathways. Additionally, SAM density influenced reaction activity significantly, with the low-density-modified SAM catalyst being more active than the catalyst coated with a full monolayer. Moreover, liquid-phase hydrogenation reactions were used to investigate the relationship between SAM density and reactivity for reactant molecules of various sizes. In all cases, the low-density SAM improved reaction rates relative to dense SAMs. The effect of controlling ligand density depended on the type of reaction: high ligand densities greatly diminished ring hydrogenation, while HDO was largely unaffected, suggesting a potential strategy for size-selective reaction rate and selectivity control.

Observations of Ethylene-for-CO Ligand Exchanges on a Zeolite-Supported Single-Site Rh Catalyst by X-ray Absorption Spectroscopy

Quick-scanning X-ray absorption fine structure (QXAFS) measurements were used to characterize the exchanges of ethylene and CO ligands in a zeolite HY-supported single-site Rh complex at a sampling rate of 1.0 Hz. The two ligands were reversibly exchanged on the rhodium, with quantitative results determined for the C2H4-for-CO exchange that are consistent with a first-order process. The apparent rate constant for the exchange decreased with increasing temperature. Fourier-transform infrared spectra characterizing the C2H4 sorbed in the zeolite showed that the amount decreased with increasing temperature, consistent with the decrease in the exchange rate with increasing temperature. The results, illustrating the dynamics of ligand exchanges on a single-site supported metal catalyst, demonstrate the broad emerging applicability of the QXAFS technique for characterizing the dynamics of reactive intermediates on catalysts.

Conversion of Glycerol to Lactic Acid by Using Platinum-supported Catalyst Combined with Phosphatidylcholine Vesicles

A combination of metal-supported catalysts with vesicles successfully achieved a conversion of glycerol to lactic acid (lactate) under the mild heated and alkaline condition. The high reaction yield (reaction temperature: 333 K; yield: 90% for 24 h) was achieved, which was an environmentally-friendly reaction process.

Hydrogenation of CO2 over Mn-Substituted SrTiO3 Based on the Reverse Mars–van Krevelen Mechanism

Oxygen storage materials (OSMs) such as CeO2 have attracted great attention as catalysts for the reverse water gas shift (RWGS) reaction, replacing metal-supported catalysts from the perspectives of thermal stability and CO selectivity. Because perovskite-type oxides are also known to be powerful OSM candidates, here we employed perovskite-type Mn-substituted SrTiO3 as a catalyst for RWGS and investigated the effects of oxygen vacancies on the activity toward CO generation. Among the solid solutions with different Mn ratios, SrTiO3 with 20% Mn substitution exhibited the highest activity. It is suggested that oxygen vacancies generated by the substitution of Mn ions for Ti ions in SrTiO3 exhibit selective activity toward CO2 splitting, whereas the redox of bulk Mn oxides does not have the capacity for CO generation in the control experiments. Quantitative H2-temperature programmed reduction and CO2-temperature programmed oxidation experiments revealed that half of the oxygen vacancies in Mn-substituted SrTiO3, which were generated by reduction with H2, were used as active sites for CO2 splitting. We conclude that this generated active oxygen vacancy, which is attributed to the valence change of the Mn ions, contributes to efficient CO2 splitting to form CO through the reverse Mars–van Krevelen mechanism.

Accelerated Deactivation of Mesoporous Co3O4-Supported Au–Pd Catalyst through Gas Sensor Operation

High activity of a catalyst and its thermal stability over a lifetime are essential for catalytic applications, including catalytic gas sensors. Highly porous materials are attractive to support metal catalysts because they can carry a large quantity of well-dispersed metal nanoparticles, which are well-accessible for reactants. The present work investigates the long-term stability of mesoporous Co3O4-supported Au–Pd catalyst (Au–Pd@meso-Co3O4), with a metal loading of 7.5 wt% and catalytically active mesoporous Co3O4 (meso-Co3O4) for use in catalytic gas sensors. Both catalysts were characterized concerning their sensor response towards different concentrations of methane and propane (0.05–1%) at operating temperatures ranging from 200 °C to 400 °C for a duration of 400 h. The initially high sensor response of Au–Pd@meso-Co3O4 to methane and propane decreased significantly after a long-term operation, while the sensor response of meso-Co3O4 without metallic catalyst was less affected. Electron microscopy studies revealed that the hollow mesoporous structure of the Co3O4 support is lost in the presence of Au–Pd particles. Additionally, Ostwald ripening of Au–Pd nanoparticles was observed. The morphology of pure meso-Co3O4 was less altered. The low thermodynamical stability of mesoporous structure and low phase transformation temperature of Co3O4, as well as high metal loading, are parameters influencing the accelerated sintering and deactivation of Au–Pd@meso-Co3O4 catalyst. Despite its high catalytic activity, Au–Pd@meso-Co3O4 is not long-term stable at increased operating temperatures and is thus not well-suited for gas sensors.

Toward a Realistic Surface State of Ru in Aqueous and Gaseous Environments

Identifying the surface species is critical in developing a realistic understanding of supported metal catalysts working in water. To this end, we have characterized the surface species present at a Ru/water interface by employing a hybrid computational approach involving an explicit description of the liquid water and a possible pressure of H2. On the close-packed, most stable Ru(0001) facet, the solvation tends to favor the full dissociation of water into atomic O and H in contrast with the partially dissociated water layer reported for ultra-high-vacuum conditions. The solvation stabilization was found to reach -0.279 J m2, which results in stable O and H species on Ru(0001) in the presence of liquid water even at room temperature. Conversely, introducing even a small H2 pressure (10-2 bar) results in a monolayer of chemisorbed H at the interface, a general trend found on the three most exposed facets of Ru nanoparticles. While hydroxyls were often hypothesized as possible surface species at the Ru/water interface, this computational study clearly demonstrates that they are not stabilized by liquid water and are not found under realistic reductive catalytic conditions.

In-situ adaptive evolution of rhodium oxide clusters into single atoms via mobile rhodium-adsorbate intermediates

It is a common phenomenon for supported metal catalysts to undergo thermally-induced or adsorbate-induced reconstruction. Great efforts have been devoted to making these reconstruction adaptive to the reaction environment instead of deactivation. Herein, we reported the evolution of initially inactive RhOx clusters on Al2O3 into the formation of catalytically active oxygen vacancies and Rh single atoms via mobile Rh-CO intermediates during hydroformylation of propene. The activated catalyst exhibited a high specific activity of 3.0 × 104 mol molRh–1 h–1 towards hydroformylation reaction. Mechanistic studies revealed the evolution paths. Specially, RhOx clusters were reduced by CO to form oxygen vacancy where the surrounding unsaturated Rh atoms enabled the chemisorption of CO*. Rh atoms that were ejected from RhOx clusters diffused on Al2O3 supports to generate Rh single atom via the formation of carbonyl or geminal dicarbonyl species. Meanwhile, the Rh atoms on clusters were also leached to the solution by the adsorbed CO molecules, followed by partial re-adsorption on the support. This work not only offers an efficient catalyst for propene hydroformylation, but also advances the understandings of dynamic evolution of catalysts.

Development of Silicon Nanowire Array–Metal Hybrid Catalysts for Batch and Flow Organic Reactions

AbstractThe development of highly efficient and reusable supported metal catalysts is important for academic and industrial synthetic organic chemistry; however, their widespread application remains a challenge because supported Pd, Rh, and Pt catalysts are expensive. To overcome these problems, we have developed novel, highly stable, reusable, and selective heterogeneous catalysts consisting of silicon nanowire arrays (SiNAs) and metal nanoparticle composites. Metal nanoparticles on SiNA have been applied as heterogeneous catalysts in the Mizoroki–Heck reaction, C–H arylation, hydrosilylation, hydrogenation, reductive alkylation of amines, and hydrogenative decarboxylation of fatty acids. The catalysts used in this study showed high catalytic activity in batch and microflow conditions. Their structural investigation using X-ray Photoelectron Spectroscopy (XPS) suggests that strong metallic bonding (alloy/agglomeration) between the metal and silicon (metal silicide bond formation) is key to the high catalyst stability.1 Introduction2 Development of Silicon Nanowire Array (SiNA) Hybrid Catalysts and Silicon Nanostructure (SiNS) Hybrid Catalysts3 Application of SiNA-Pd to Organic Synthesis4 SINA-Supported Mono- and Bimetallic Nanoparticles for Hydrogenation Reactions5 Application of SiNA-Pd to Microflow Reductive Alkylation Reactions6 Application of SiNA-Rh to Hydrogenative Decarboxylation Reactions using Microwave Irradiation7 Conclusions

The Effect of Metal-Support Interactions on Formaldehyde Oxidation and By-Product Selectivities in TiO2 Supported Metal Catalysts (Co, Cr, Mo, and Ag).

The Effect of Metal-Support Interactions on Formaldehyde Oxidation and By-Product Selectivities in TiO2 Supported Metal Catalysts (Co, Cr, Mo, and Ag).

Electronic regulation over Pd nanoparticles: One-pot synthesis of B, N Co-doping MXene-supported Pd catalyst for efficient ethanol oxidation

Multi-element doping support is considered an effective method to improve the electrocatalytic performance of catalysts, but there is a lack of simple and mild methods to prepare catalysts. In this article, dimethylamine borane is used as a heteroatom dopant and a metal-reducing agent at the same time, realizing the co-doping of B and N atoms and the deposition of metal Pd on Ti3C2 through a rapid, one-step hydrothermal method. By exploring the electrocatalytic properties of ethanol oxidation, the Pd/DMAB-Ti3C2 exhibits much higher electrochemically active surface areas, ethanol-oxidated current densities, and electrochemical stability than those of the single-doped and undoped catalysts. And the current densities after 350 redox cycles of Pd/DMAB-Ti3C2 can retain 74.6 % of the maximum capacity. Such different properties can be attributed to the synergy of B and N species in the Ti3C2 introduced by dimethylamine borane, which significantly affects the electron transfer and d band center of supported metal Pd, thus facilitating the progress of the EOR along a favorable pathway. This work provides a new insight into synthesizing the high-performance B and N co-doped supported metal catalyst through a rapid one-step synthesis.

Identifying the Structure of Supported Metal Catalysts Using Vibrational Fingerprints from Ab Initio Nanoscale Models.

Identifying active sites of supported noble metal nanocatalysts remains challenging, since their size and shape undergo changes depending on the support, temperature, and gas mixture composition. Herein, the anharmonic infrared spectrum of adsorbed CO is simulated using density functional theory (DFT) to gain insight into the nature of Pd nanoparticles (NPs) supported on ceria. The authors systematically determine how the simulated infrared spectra are affected by CO coverage, NP size (0.5-1.5nm), NP morphology (octahedral, icosahedral), and metal-support contact angle, by exploring a diversity of realistic models inspired by ab initio molecular dynamics. The simulated spectra are then used as a spectroscopic fingerprint to characterize nanoparticles in a real catalyst, by comparison with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. Truncated octahedral NPs with an acute Pd-ceria angle reproduce most of the measurements. In particular, the authors isolate features characteristic of CO adsorbed at the metal-support interface appearing at low frequencies, both seen in simulation and experiment. This work illustrates the strong need for realistic models to provide a robust description of the active sites, especially at the interface of supported metal nanocatalysts, which can be highly dynamic and evolve considerably during reaction.

Selective recovery of caprolactam from the thermo-catalytic conversion of textile waste over γ-Al2O3 supported metal catalysts

The massive generation of synthetic textile waste has drawn considerable attention. Landfilling/incineration of textile waste has been widely made. To abate the environmental burdensome from the conventional management processes, a thermo-catalytic conversion was used for rapid volume reduction of textile waste and simultaneous valorization by recovering textile monomer in this study. Stockings were chosen as a model feedstock. Because stockings consisted of nylon with other contents, different products (caprolactam (nylon monomer), imines, cyclic dimers, and azepines) were recovered. The yield of caprolactam from the thermal conversion at 500 °C was 53.6 wt%. To selectively enhance the caprolactam yield, catalytic pyrolysis was done using γ-Al2O3 supported metal catalysts (Ni, Cu, Fe, or Co). γ-Al2O3 itself increased the caprolactam yield up to 69.0 wt% via a based-catalyzed reaction of nylon depolymerization and intramolecular cyclization. Under the presence of metal catalysts, the caprolactam yield increased up to 73.3 wt%. To offer desired feature of green chemistry, CO2 was adopted as reactive gas. Under the CO2-mediated catalytic pyrolysis, caprolactam yield was enhanced up to 77.1 wt% over Cu/Al2O3 (basis: stocking mass). Based on the net content of nylon in the stockings, the yield of caprolactam was deemed 95.3 wt%. This study proves that textile waste (stocking) and CO2 are useful resources for recovery of nylon monomer, which can reduce the waste generation with simultaneous recovery of value-added product.

Strong Metal‐Support Interactions through Sulfur‐Anchoring of Metal Catalysts on Carbon Supports

AbstractIn supported metal catalysts, the supports would strongly interact with the metal components instead of just acting as a carrier, which greatly affects both of their synthesis and catalytic activity, selectivity, and stability. Carbon is considered as very important but inert support and thus hard to induce strong metal‐support interaction (SMSI). This mini‐review highlights that sulfur—a documented poison reagent for metal catalysts—when doped in a carbon supports can induce diverse SMSI phenomenon, including electronic metal‐support interaction (EMSI), classic SMSI, and reactive metal‐support interaction (RMSI). These SMSI between metal and sulfur‐doped carbon (S−C) supports enables the catalysts with extraordinary resistance to sintering at high temperatures of up to 1100 °C, which allows the general synthesis of single‐atom, alloy cluster, and intermetallic compound catalysts with high dispersion and metal loading for a variety of applications.

Strong Metal-Support Interactions through Sulfur-Anchoring of Metal Catalysts on Carbon Supports.

In supported metal catalysts, the supports would strongly interact with the metal components instead of just acting as a carrier, which greatly affects both of their synthesis and catalytic activity, selectivity, and stability. Carbon is considered as very important but inert support and thus hard to induce strong metal-support interaction (SMSI). This mini-review highlights that sulfur-a documented poison reagent for metal catalysts-when doped in a carbon supports can induce diverse SMSI phenomenon, including electronic metal-support interaction (EMSI), classic SMSI, and reactive metal-support interaction (RMSI). These SMSI between metal and sulfur-doped carbon (S-C) supports enables the catalysts with extraordinary resistance to sintering at high temperatures of up to 1100 °C, which allows the general synthesis of single-atom, alloy cluster, and intermetallic compound catalysts with high dispersion and metal loading for a variety of applications.

Systematic Investigation on Supported Gold Catalysts Prepared by Fluorine-Free Basic Etching Ti3AlC2 in Selective Oxidation of Aromatic Alcohols to Aldehydes.

Conventional methods to prepare supported metal catalysts are chemical reduction and wet impregnation. This study developed and systematically investigated a novel reduction method based on simultaneous Ti3AlC2 fluorine-free etching and metal deposition to prepare gold catalysts. The new series of Aupre/Ti3AlxC2Ty catalysts were characterized by XRD, XPS, TEM, and SEM and were tested in the selective oxidation of representative aromatic alcohols to aldehydes. The catalytic results demonstrate the effectiveness of the preparation method and better catalytic performances of Aupre/Ti3AlxC2Ty, compared with those of catalysts prepared by traditional methods. Moreover, this work presents a comprehensive study on the influence of calcination in air, H2, and Ar, and we found that the catalyst of Aupre/Ti3AlxC2Ty-Air600 obtained by calcination in air at 600 °C performed the best, owing to the synergistic effect between tiny surface TiO2 species and Au NPs. The tests of reusability and hot filtration confirmed the catalyst stability.