Facile and Eco-Friendly Approach To Produce Confined Metal Cluster Catalysts

Dynamic evolution of metal structures on/in zeolites for catalysis

Dynamic changes of metal species always occur during catalysis, and primarily rely on forming mobile metal species initiated by thermal or chemical conditions. During these processes, a support is important in affecting the catalyst stability and dynamic change pathways. Among several supports, zeolites provide ideal features for regulating the migration of metal species due to their unique pore structures and specific defect sites. This review provides a comprehensive summary of typical cases about dynamic migration of metal species on/in metal-zeolite catalysts, analyzing the mechanisms and driving factors of metal migration under different reaction conditions. We discuss the roles of zeolite supports in the migration process of metal species, particularly their crucial contributions to the stability of metal species and the optimization of active sites. In addition, the potential mechanism of the dynamic migration of metal species, theoretical studies, and practical guidance for designing highly efficient catalysts are also included in this review.

Directed hydrogenolysis of "cellulose-to-ethylene glycol" using a Ni-WO x based catalyst

Biomass is an important renewable resource in nature, and cellulose is a crucial component within it. The chemically directed conversion of cellulose into ethylene glycol offers a green alternative to traditional petroleum-based production methods. In this study, a multifunctional Ni-WO x /SAPO-11 catalyst was designed. By optimizing the processing parameters of catalysts and the reaction conditions of them, it was demonstrated that this catalyst could efficiently catalyze cellulose into alcohol products through a series of tandem reactions such as hydrolysis, retro-aldol condensation, and hydrogenation under relatively mild conditions. The yield of ethylene glycol climbed from 4% (at 180 °C) to 66.6% (at 240 °C) with the increase of reaction temperature. Characterization (XPS, TEM, TPD/TPR) revealed that a reduction temperature of 500 °C maximized Brønsted acidity and W5+/W6+ ratios, enhancing C-C cleavage efficiency. Further increases in the reduction temperature would weaken the Brønsted acid on the surface of SAPO-11, but its surface area would also increase (mainly in the form of mesopores). The uniformly dispersed elemental tungsten could form new acidic sites on the catalyst surface; in combination with active Ni0, this high-temperature reduced catalyst could achieve the direct hydrogenolysis of cellulose to produce ethylene glycol, benefiting the efficient utilization of lignocellulosic biomass in the future.

Highly efficient Ni/Ac-Al2O3 catalysts in the dry reforming of methane: influence of acetic acid treatment and Ni loading

The presence of abundant hydroxyl groups on the surface of Al2O3 can promote the dispersion of Ni species but produce an inactive NiAl2O4 phase at high temperatures. Moreover, the catalysts prepared by the conventional incipient wetness impregnation method lack the sites for the activation of CO2, which leads to coke deposition and thus affects the catalyst activity. The above restricts the utilization of Ni in conventional Ni/Al2O3 catalysts. In this paper, Al2O3 support was pre-treated by acetic acid to selectively remove hydroxyl groups without affecting the coordination environment of Al. Results revealed that the Al2O3 support after hydroxyl removal not only showed moderate metal-support interaction but also produced more sites for the adsorption and activation of the reactant, which significantly improves the utilization of nickel species and the stability of the catalyst. The conversion of CH4 and CO2 at 700 °C was as high as 88% and 90%, respectively, and has an excellent stability of 50 h. This study provides a feasible strategy for the design of highly active methane dry-reforming catalysts.

Confining platinum clusters in indium-modified ZSM-5 zeolite to promote propane dehydrogenation

Designing highly active and stable catalytic sites is often challenging due to the complex synthesis procedure and the agglomeration of active sites during high-temperature reactions. Here, we report a facile two-step method to synthesize Pt clusters confined by In-modified ZSM-5 zeolite. In-situ characterization confirms that In is located at the extra-framework position of ZSM-5 as In+, and the Pt clusters are stabilized by the In-ZSM-5 zeolite. The resulting Pt clusters confined in In-ZSM-5 show excellent propane conversion, propylene selectivity, and catalytic stability, outperforming monometallic Pt, In, and bimetallic PtIn alloys. The incorporation of In+ in ZSM-5 neutralizes Brønsted acid sites to inhibit side reactions, as well as tunes the electronic properties of Pt clusters to facilitate propane activation and propylene desorption. The strategy of combining precious metal clusters with metal cation-exchanged zeolites opens the avenue to develop stable heterogeneous catalysts for other reaction systems.

Identification of coke species on Fe/USY catalysts used for recycling polyethylene into fuels

The Fe/USY catalyst used for converting plastic waste into fuels faces coking problems. A comprehensive understanding of coke distribution and structure is crucial for catalyst design, enabling resistance to coke deposition and facilitating regeneration. In this study, we analyze the coke deposition on Fe/USY catalysts after catalytic pyrolysis of polyethylene for fuel oil, and present insights into the coke distribution over the metal and acid sites, as well as its specific molecular structure. The coke distributes over both the metal and acid sites, exhibiting distinct TPO peaks corresponding to metal-site coke (370 °C) and acid-site coke (520 °C). The total coke yields range from 2.0% to 2.4%, with distribution on metal and acid sites dependent on Fe loading and acidity. Structurally, the coke is highly-condensed, containing more than four aromatic rings with limited alkyl groups. The acid-site coke is more condensed than the metal-site coke, showing lower H/C ratios (0.5-0.75) relative to the acid-site coke (0.75-0.9). Identified by MALDI-TOF mass analysis, the predominant molecular structures of the coke located on metal and acid sites are illustrated. The metal-site cokes typically exhibit 4-7 aromatic rings, while the acid-site cokes display even greater condensation with 5-12 aromatic rings.

Highly dispersed noble metal nanoparticle composites on biomass-derived carbon-based carriers: synthesis, characterization, and catalytic applications

Precious metal nanoparticles have been widely investigated due to their excellent activity shown in catalysis and sensing. However, how to prepare highly dispersed noble metal nanoparticles to improve the lifetime of catalysts and reduce the cost is still an urgent problem to be solved. In this study, a carbon-based carrier material was prepared by an expansion method and loaded with Pd or Ag nanoparticles on this carbon material to synthesize precious metal nanoparticle composites, which were characterized in detail. The results show that the nanoparticles prepared using this method exhibit superior dispersion. Under the synergistic effect of noble metal nanoparticles and porous carbon carriers, the composites exhibited excellent catalytic degradation of p-nitrophenol and showed excellent sensing performance in the modified hydrogen peroxide sensor electrode. This approach is highly informative for the preparation of nanocomposites in medical and environmental fields.

Niobic acid as a support for microheterogeneous nanocatalysis of sodium borohydride hydrolysis under mild conditions

This study explores the stabilization by niobic acid, of Pt, Ni, Pd, and Au nanoparticles (NPs) for the efficient microheterogeneous catalysis of NaBH4 hydrolysis for hydrogen production. Niobic acid is the most widely studied Nb2O5 polymorph, and it is employed here for the first time for this key reaction relevant to green energy. Structural insights from XRD, Raman, and FTIR spectroscopies, combined with hydrogen production data, reveal the role of niobic acid's Brønsted acidity in its catalytic activity. The supported NPs showed significantly higher efficiency than the non-supported counterparts regarding turnover frequency, average hydrogen production rate, and cost. Among the tested NPs, PtNPs and NiNPs demonstrate the most favorable results. The data imply mechanism changes during the reaction, and the kinetic isotope assay indicates a primary isotope effect. Reusability assays demonstrate consistent yields over five cycles for PtNPs, although catalytic efficiency decreases, likely due to the formation of reaction byproducts.

Toward High-Performance Hydrogenation at Room Temperature Through Tailoring Nickel Catalysts Stable in Aqueous Solution

The development of highly active, reusable catalysts for aqueous-phase reactions is challenging. Herein, metallic nickel is encapsulated in a nitrogen-doped carbon-silica composite (SiO2@Ni@NC) as a catalyst for the selective hydrogenation of vanillin in aqueous media. The constructed catalyst achieved 99.8% vanillin conversion and 100% 4-hydroxymethyl-2-methoxyphenol selectivity at room temperature. Based on combined scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman analyses, the satisfactory catalytic performance is attributed to the composite structure consisting of an active metal, carbon, and silica. The hydrophilic silica core promoted dispersion of the catalyst in aqueous media. Moreover, the external hydrophobic NC layer has multiple functions, including preventing oxidation or leaching of the internal metal, acting as a reducing agent to reduce the internal metal, regulating the active-site microenvironment by enriching the concentrations of H2 and organic reactants, and modifying the electronic structure of the active metal via metal-support interactions. Density functional theory calculations indicated that NC facilitates vanillin adsorption and hydrogen dissociation to promote aqueous-phase hydrogenation. This study provides an efficient strategy for constructing encapsulated Ni-based amphiphilic catalysts to upgrade biomass-derived compounds.

Sustainable biofuel synthesis from non-edible oils: a mesoporous ZSM-5/Ni/Pt catalyst approach

This work examines the hydrodeoxygenation (HDO) activity of non-edible oils using a high surface area catalyst. The HDO activity was thoroughly examined and contrasted using the high surface area catalyst Ni/Pt-ZSM-5 as well as other supports like MCM-48 and H-beta. Ni/Pt bimetals supported on mesoporous ZSM-5 were created via reverse order impregnation to facilitate HDO of non-edible oils. Techniques such as XRD, FT-IR, BET, HR-TEM, HR-SEM, TPD, and TGA were used to characterize the produced catalysts. The synthesized catalysts considerably influenced the hydrodeoxygenation activities for the synthesis of lengthy chain hydrocarbons in a stainless-steel reactor with a high-pressure fixed bed between 300 and 375 °C under 10-40 bar hydrogen pressure. High levels of Ni/Pt-ZSM-5 acidity, textural, and H2 consumption qualities were discovered. Distributions of the products were also reviewed, along with comparisons of the structure-activity connections.

Constructing Symmetry-Mismatched RuxFe3-xO4 Heterointerface-Supported Ru Clusters for Efficient Hydrogen Evolution and Oxidation Reactions

Ru-related catalysts have shown excellent performance for the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR); however, a deep understanding of Ru-active sites on a nanoscale heterogeneous support for hydrogen catalysis is still lacking. Herein, a click chemistry strategy is proposed to design Ru cluster-decorated nanometer RuxFe3-xO4 heterointerfaces (Ru/RuxFe3-xO4) as highly effective bifunctional hydrogen catalysts. It is found that introducing Ru into nanometric Fe3O4 species breaks the symmetry configuration and optimizes the active site in Ru/RuxFe3-xO4 for HER and HOR. As expected, the catalyst displays prominent alkaline HER and HOR performance with mass activity much higher than that of commercial Pt/C as well as robust stability during catalysis because of the strong interaction between the Ru cluster and the RuxFe3-xO4 support, and the optimized adsorption intermediate (Had and OHad). This work sheds light on a promsing approach to improving the electrocatalysis performance of catalysts by the breaking of atomic dimension symmetry.