Supported Transition Metal Catalysts Research Articles

Support Effect of Boron Nitride on the First N-H Bond Activation of NH3 on Ru Clusters.

Support effect is an important issue in heterogeneous catalysis, while the explicit role of a catalytic support is often unclear for catalytic reactions. A systematic density functional theory computational study is reported in this paper to elucidate the effect of a model boron nitride (BN) support on the first N-H bond activation step of NH3 on Run (n = 1, 2, 3) metal clusters. Geometry optimizations and energy calculations were carried out using density functional theory (DFT) calculation for intermediates and transition states from the starting materials undergoing the N-H activation process. The primary findings are summarized as follows. The involvement of the model BN support does not significantly alter the equilibrium structure of intermediates and transition states in the most favorable pathway (MFP). Moreover, the involvement of BN support decreases the free energy of activation, ΔG≠, thus improving the reaction rate constant. This improvement is more obvious at high temperatures like 673 K than low temperatures like 298 K. The BN support effect leading to the ΔG≠ decrease is most significant for the single Ru atom case among all three cases studied. Finally, the involvement of the model BN may change the spin transition behavior of the reaction system during the N-H bond activation process. All these findings provide a deeper insight into the support effect on the N-H bond activation of NH3 for the supported Ru catalyst in particular and for supported transition metal catalysts in general.

Evaluating the catalytic performances of supported transition metal catalysts by positron annihilation lifetime spectroscopy

Supported transition metal catalysts have played a critical role in the modern synthetic chemical industry. Herein, MCM-41 was chemically modified with amino groups and then used as the support of six transition metal cations. The chemical structures, microstructures and dispersions of these supported transition metal cations were carefully characterized by XRD, TEM and BET. The chelations of modified MCM-41 support with these transition metal cations were confirmed by FT-IR and XPS. The reduction reaction of 4-nitrophenol into 4-aminophenol was employed to evaluate the catalytic activities and recyclabilities of these supported transition metal catalysts. Meanwhile, their reactivities with o-Ps species were assessed by positron annihilation lifetime spectroscopy (PALS). It was found that the reduced o-Ps annihilation intensities (ΔI3) were well associated with their catalytic activities while the o-Ps annihilation lifetime values (τ3) were related to their catalytic recyclabilities. In all, the catalytic activities and recyclabilities of supported transition metal cations can be conveniently evaluated by their o-Ps annihilation properties. This method holds great potential in the design and preparation of highly active and stable supported transition metal catalysts.

Acetophenone hydrogenation and consecutive hydrogenolysis with Pd/CNT catalysts: Highlighting the synergy between single atoms and nanoparticles by kinetic modeling

Understanding the cooperative action of palladium nanoparticles (NPs) and palladium single atoms (SAs) supported on Nis essential for developing more efficient supported transition metal catalysts. In this work, the comparison of a series of Pd-catalysts on CNTs having the same palladium loading (∼1.4 wt%) but involving different and controlled SA/NP ratios (between 2 and 200) is described. The hydrogenation of acetophenone into 1-phenylethanol and its consecutive hydrogenolysis to ethylbenzene are the chosen test reactions. Appropriate kinetic modeling of these two consecutive reactions was performed and allowed a quantitative assessment of the activity differences between catalysts as well as the demonstration of a synergy between the two palladium species (SAs and NPs). The maximum activity was clearly dependent on the distribution of the surface palladium between SAs and NPs, and interestingly, different optima for the two reactions are evidenced. This work may offer a new perspective for the design and synthesis of supported transition metal catalysts with targeted catalytic performance.

The oxygen reduction reaction activity and selectivity of porous-carbon supported transition metals (M-C: M Mn, Fe, Co, Ni, Cu) electrocatalysts

In general, both transition metal addition and nitrogen doping are important to the four-electron (4e−) oxygen reduction reaction (ORR) activity of carbon-based catalysts, in which nitrogen-containing species, such as M-Nx, pyridinic N and graphitic N, are believed to be the active sites. On the other hand, nitrogen-free carbon supported transition metals often show their excellent electroactivity toward two-electron (2e−) ORR. Herein, carbon supported transition metal catalysts (M-C: MMn, Fe, Co, Ni, Cu) with high specific surface areas were prepared by a simple zinc-mediated pyrolysis method and their ORR activity and selectivity were systematically studied. It is found that various transition metals result in a large difference in the content and type of oxygen-containing functional groups of the M-C catalysts. The results show that CuC and CoC exhibit promising ORR activity close to the 4e− transfer mechanism in 0.1 M KOH electrolyte. FeC and MnC show a mixture of 2e− and 4e− transfer pathways. NiC is an efficient catalyst for efficient H2O2 generation through a primary 2e− ORR. It is found that the ORR selectivity of M-C is correlated to the content and type of oxygen-containing functional groups in the catalysts, and C-O-C is likely an effective active site for 4e− ORR based on both experimental results and theoretical calculations.

Hollow hemispherical Si-doped anatase for efficient carbamazepine degradation via photocatalytic activation of peroxymonosulfate

Photocatalytic persulfate activation on semiconductor (such as TiO2) supported transition metal catalysts exhibits great potential in removing refractory organic pollutants, but it often encounters the issues of unsatisfactory activity and undesirable metal leaching. Herein, novel three-dimensional (3D) Si-doped TiO2 materials without transition metal doping are proposed for efficient photocatalytic activation of peroxymonosulfate (PMS). Unlike creating an inverse opal structure using polymer sphere templates in prior research, the SiO2 opal template enables the creation of hollow hemispherical structured Si-doped TiO2 (denoted as HHS-Si/TiO2) with hierarchical macro-mesopores. The SiO2 template also endows the HHS-Si/TiO2 materials excellent sintering-resistant properties and completely inhibits the crystalline phase transition from anatase to rutile at high temperatures, benefiting from the confinement effect and formation of Si-O-Ti bonds. The HHS-Si/TiO2 materials calcined at 400–900 °C show similar and excellent activities and stability in PMS activation under solar light irradiation to fast degrade carbamazepine (CBZ) and various other pollutants. The influences of PMS dosage, pH value and water matrix on CBZ degradation are illustrated. The HHS-Si/TiO2 materials are advantageous (especially at high calcination temperatures) over the commercial, the template-free synthesized and the inverse opal structured TiO2 materials for photocatalytic activation of PMS, because of their 3D hollow hemispherical structure, interconnected small anatase nanocrystals and Si-doping. PMS is mainly activated by the photo-generated electrons. HO• (dominant), SO4•− and holes are the reactive species responsible for CBZ degradation. This work provides enhanced understanding on the synthesis of 3D Si-doped TiO2 materials and it would provide useful information on developing efficient catalysts for photocatalytic PMS activation.

Recent Insight in Transition Metal Anchored on Nitrogen-Doped Carbon Catalysts: Preparation and Catalysis Application

The design and preparation of novel, high-efficiency, and low-cost heterogeneous catalysts are important topics in academic and industry research. In the past, inorganic materials, metal oxide, and carbon materials were used as supports for the development of heterogeneous catalysts due to their excellent properties, such as high specific surface areas and tunable porous structures. However, the properties of traditional pristine carbon materials cannot keep up with the sustained growth and requirements of industry and scientific research, since the introduction of nitrogen atoms into carbon materials may significantly enhance a variety of their physicochemical characteristics, which gradually become appropriate support for synthesizing supported transition metal catalysts. In the past several decades, the transition metal anchored on nitrogen-doped carbon catalysts has attracted a tremendous amount of interest as potentially useful catalysts for diverse chemical reactions. Compared with original carbon support, the doping of nitrogen atoms can significantly regulate the physicochemical properties of carbon materials and allow active metal species uniformly dispersed on the support. The various N species in support also play a critical role in accelerating the catalytic performance in some reactions. Besides, the interaction between support and transition metal active sites can offer an anchor site to stabilize metal species during the preparation process and then improve reaction performance, atomic utilization, and stability. In this review, we highlight the recent advances and the remaining challenges in the preparation and application of transition metal anchored on nitrogen-doped carbon catalysts.

Preparation of a transition metal-supported TiO2 catalyst and the catalytic oxidative degradation of toluene

The synthesis of supported transition metal oxides with satisfactory low-temperature catalytic performance and application potential is still a challenge for the complete degradation of VOCs. Ni-Cu-loaded TiO2 catalysts were prepared by an impregnation method. The lattice defect and oxygen vacancy concentrations of the prepared materials could be tuned by controlling the Ni/Cu molar ratio and the mass proportion of the TiO2 support. 5Ni-Cu/TiO2 exhibited the best catalytic activity and showed superior catalytic stability during complete toluene oxidation. The obtained catalysts were fully characterized by XRD, FT-IR, TEM, N2 adsorption analysis, XPS, H2-TPR, O2-TPD, TPD analysis, and XPS. The 5Ni-Cu/TiO2 catalyst had improved pore sizes and specific surface area, had more oxygen vacancies on its surface, had increased concentrations of adsorbed and lattice oxygen species, and had good redox properties; these characteristics were favorable for low-temperature catalytic performance and catalytic stability. Cyclic transformations of metal valence states between Cu+ and Cu and between Ni and Ni2+ were observed in the catalytic system. A potential catalytic reaction process based on the M-vK mechanism was proposed. This work serves as a reference for the application of supported transition metal catalysts for the degradation of VOCs.

Surface hydroxyl dependent adsorption of ruthenium on SiO2(0 0 1) – Understanding metal–support interaction

To understand the metal–support interaction of oxide supported transition metal catalysts, we computed the adsorption structures and energies of ruthenium on the idealized SiO2(001) surfaces with full (OH, 100%), partial (OH, 50%) surface hydroxyl and without surface hydroxyl (OH, 0%) at different ruthenium loading. On the fully (up to 9Ru) and partially (up to 6Ru) hydroxylated surfaces, ruthenium adsorption prefers surface O-H oxidative addition and forms regularly distributed Ru(H)2 species. After the reductive and re-combinative desorption of H2, the adsorbed ruthenium atoms prefer structures with aggregated 2Ru dimeric units on the fully hydroxylated surface, while structures of remote and dispersed singly adsorbed Ru atoms on the partially hydroxylated surface. On the hydroxyl free surface (up to 6Ru), the adsorbed ruthenium atoms prefer three-dimensional aggregated structures, which become more stable with the increase of ruthenium loading. This indicates the surface hydroxyl dependent ruthenium adsorption on SiO2 (001), and this finding is important for understanding the structure, stability and activity of silica supported ruthenium catalysts and catalysis.

Hydrodeoxygenation of phenolics over uniformly dispersed Pt–Ni alloys supported by self–pillared ZSM–5 nanosheets

Zeolite supported transition metal catalysts have been proved highly efficient for hydrodeoxygenation (HDO) of bio–oil, but always faced decreased catalytic activity due to the aggregations of metal cluster and the restricted diffusion by only microporous zeolite. In this work, Pt–Ni bimetals supported on a self–pillared nanosheet ZSM–5 zeolite catalyst (Pt–Ni/SP) was prepared by wetness impregnation. The Pt–Ni/SP catalyst with highly exposed surface areas improved the dispersion of Pt–Ni alloy on the carrier surface with the establishment of strong metal–support interaction, after sequenced calcination and reduction. The promoted adsorption and spillover of active hydrogen were realized by the Pt–Ni/SP catalyst, thus giving an enhanced HDO of phenolics, than the monometallic Ni/SP and Pt/SP catalysts. Moreover, the adsorption of phenolics was promoted, due to the hierarchical structure. Pt–Ni/SP catalyst show significant higher catalytic conversion of anisole and guaiacol than the bimetallic catalyst using commercial microporous ZSM–5 as support (Pt–Ni/CZ), through sequenced hydrogenation and deoxygenation procedures.

Boosting CO2 methanation on ceria supported transition metal catalysts via chelation coupled wetness impregnation

The incipient wetness impregnation (IWI) method is widely used in the preparation of supported transition metal catalysts for its high throughput and cost-effective synthesis, yet suffers from poor metal-support interaction, restricting its further application at an industrial scale. Herein, a universal strategy of chelation coupled impregnation (CCI) is presented. The as-prepared Ni/CeO2(CCI) showed superior catalytic performance for CO2 conversion (84.3%) and CH4 selectivity (100%) under the experimental conditions (WGHSV = 24,000 mL g−1 h−1 and H2/CO2 = 4:1) even at low temperatures (T = 275 °C). The surface characterization results confirmed that the agglomeration of metal active sites in Ni/CeO2(CCI) was restricted and more surface oxygen vacancies were generated on CeO2. Further, the in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) analysis suggested that the surface oxygen vacancies that served as active sites could facilitate the direct dissociation of CO2 more favorably than the associative route, thus significantly promoting CO2 methanation activity.

Automated Image Analysis for Single-Atom Detection in Catalytic Materials by Transmission Electron Microscopy

Single-atom catalytic sites may have existed in all supported transition metal catalysts since their first application. Yet, interest in the design of single-atom heterogeneous catalysts (SACs) only really grew when advances in transmission electron microscopy (TEM) permitted direct confirmation of metal site isolation. While atomic-resolution imaging remains a central characterization tool, poor statistical significance, reproducibility, and interoperability limit its scope for deriving robust characteristics about these frontier catalytic materials. Here, we introduce a customized deep-learning method for automated atom detection in image analysis, a rate-limiting step toward high-throughput TEM. Platinum atoms stabilized on a functionalized carbon support with a challenging irregular three-dimensional morphology serve as a practically relevant test system with promising scope in thermo- and electrochemical applications. The model detects over 20,000 atomic positions for the statistical analysis of important properties for establishing structure-performance relations over nanostructured catalysts, like the surface density, proximity, clustering extent, and dispersion uniformity of supported metal species. Good performance obtained on direct application of the model to an iron SAC based on carbon nitride demonstrates its generalizability for single-atom detection on carbon-related materials. The approach establishes a route to integrate artificial intelligence into routine TEM workflows. It accelerates image processing times by orders of magnitude and reduces human bias by providing an uncertainty analysis that is not readily quantifiable in manual atom identification, improving standardization and scalability.

Self-regeneration of supported transition metals by a high entropy-driven principle

The sintering of Supported Transition Metal Catalysts (STMCs) is a core issue during high temperature catalysis. Perovskite oxides as host matrix for STMCs are proven to be sintering-resistance, leading to a family of self-regenerative materials. However, none other design principles for self-regenerative catalysts were put forward since 2002, which cannot satisfy diverse catalytic processes. Herein, inspired by the principle of high entropy-stabilized structure, a concept whether entropy driving force could promote the self-regeneration process is proposed. To verify it, a high entropy cubic Zr0.5(NiFeCuMnCo)0.5Ox is constructed as a host model, and interestingly in situ reversible exsolution-dissolution of supported metallic species are observed in multi redox cycles. Notably, in situ exsolved transition metals from high entropy Zr0.5(NiFeCuMnCo)0.5Ox support, whose entropic contribution (TΔSconfig = T⋆12.7 J mol−1 K−1) is predominant in ∆G, affording ultrahigh thermal stability in long-term CO2 hydrogenation (400 °C, >500 h). Current theory may inspire more STWCs with excellent sintering-resistance performance.

Supported Transition Metal Catalysts for Organic Fine Chemical Synthesis: A Review

Transition metal catalysts play an important role for synthesis of industrially and laboratory important organic fine chemicals to control the selectivity, activity and stability. In this review, we focus on mainly transition metal based supported catalyst, mainly oxide supported catalyst for heterogeneous catalytic hydrogenation and oxidation of some synthetically important organic molecules. First we discuss the industrially important catalytic organic synthetic reactions. This is followed by the role of supported metal catalysts in the heterogeneous synthetic catalytic reactions with specific attention to hydrogenation and oxidation of organic molecules. The role of base metals and noble metals in monometallic and bimetallic catalysts are then discussed. Some synthetic routes for preparation of oxide supported metal catalysts are also discussed. Finally, a general discussion of the metal-support interaction (MSI) in oxide supported metal catalysts is made.

Simple mechanisms of CH4 reforming with CO2 and H2O on a supported Ni/ZrO2 catalyst.

To understand the metal-support interaction of oxide supported transition metal catalysts, we computed the reaction mechanisms of dry and steam reforming of methane on a tetragonal ZrO2(101) supported Ni catalyst. Based on the limited number of active sites on the surface, an irregular and non-ideal Ni13 cluster on ZrO2(101) is identified as a catalyst. A simple reaction mechanism is proposed, and the first direct dissociation step of CO2, CH4 and H2O is the most difficult based on the computed Gibbs free energies and no surface CHXO and CHXOH intermediates are involved, different from that on the flat Ni(111) surface. Analysis of other supported nickel catalysts shows that not only the support but also the size and shape of the metal clusters play an important role in the reaction mechanisms and kinetics.

Spherical supported transition metal catalyst.

Spherical supported transition metal catalyst.

A promising plasma-catalytic approach towards single-step methane conversion to oxygenates at room temperature

Direct conversion of methane into chemicals and fuels under mild conditions has been considered as a ‘holy grail’ of chemistry and catalysis in the 21st century. Plasma-catalytic partial oxidation of methane (POM) to higher-value liquid fuels and chemicals over supported transition metal catalysts (Ni/γ-Al2O3, Cu/γ-Al2O3 and Fe/γ-Al2O3) has been investigated in a co-axial dielectric barrier discharge (DBD) reactor at room temperature and atmospheric pressure. The selectivity of oxygenates was 58.3% in the plasma POM reaction without a catalyst, while the combination of DBD with the catalysts enhanced the selectivity of oxygenates up to 71.5%. Of the three catalysts, Fe/γ-Al2O3 showed the highest methanol selectivity of 36.0% and a significant methanol yield of 4.7%, while the use of Cu/γ-Al2O3 improved the selectivity of C2 oxygenates to 9.4%, which can be attributed to the presence of more acid sites on the surfaces of the Cu catalyst. The possible reaction pathways in the plasma-catalytic POM reaction have been explored by combined means of plasma electrical and optical diagnostics, analysis of gas and liquid products, as well as comprehensive catalyst characterization. The plausible reaction routes for the production of major oxygenate (methanol) on the Fe/γ-Al2O3 surfaces have been proposed. The surface CHx species are found to be critical for methanol synthesis; they can be formed through the direct adsorption of CHx radicals generated in the plasma gas-phase reactions or through the dissociation of adsorbed CH4 on the catalyst surface.

Cost-effective mechanochemical synthesis of highly dispersed supported transition metal catalysts for hydrogen storage

Here, we demonstrate a quasi-solid-state template strategy to synthesis highly dispersed supported metal catalysts on nitrogen-doped carbon (M-N-C) with a large fraction of single atom Ni species, in which dispersion of metal precursor, evaporation of solvent and downsizing of templates can be simultaneously achieved during the one-step ball-milling process. The incorporation of such nickel-based catalysts into MgH 2 greatly improved the kinetics with the reduced activation energy of 87.2 ± 5.4 kJ mol −1 (156.5 ± 3.2 kJ mol −1 for blank). The versatility of this method is confirmed by the successful synthesis of the whole series of 3d transition elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) and La, Ce. The kinetic enhancement of their MgH 2 integrated composites lies on these elements’ electronegativity which determines the dispersity of metal atoms and the strength of metal-hydrogen interaction. A quasi-solid-state template method is developed to synthesize highly dispersed transition metal catalysts for solid-state de/rehydrogenation reactions. • Highly dispersed catalysts are synthesized via ball-milling process. • Recyclable and easily removable KCl templates are used in the process. • This method is versatile, applicable to the whole period of 3d transition metals. • Incorporating catalyst into MgH 2 greatly accelerate its hydrogen storage kinetics. • Catalytic activities depend on the electronegativity of center elements.

Heterogeneous catalysis under flow for the 21st century fine chemical industry

Due to metal leaching and poor catalyst stability, the chemical industry's fine chemical and pharmaceutical sectors have been historically reluctant to use supported transition metal catalysts to manufacture fine chemicals and active pharmaceutical ingredients. With the advent of new generation supported metal catalysts and flow chemistry, we argue in this study, this situation is poised to quickly change. Alongside heterogenized metal nanoparticles, both single-site molecular and single-atom catalyst will become ubiquitous. This study offers a critical outlook taking into account both technical and economic aspects.

Sodium-treated sepiolite-supported transition metal (Cu, Fe, Ni, Mn, or Co) catalysts for HCHO oxidation

Sodium-treated sepiolite (NaSep)-supported transition metal catalysts (TM/NaSep; TM = Cu, Fe, Ni, Mn, and Co) were synthesized via a rotary evaporation method. Physicochemical properties of the as-synthesized samples were characterized by means of various techniques, and their catalytic activities for HCHO (0.2%) oxidation were evaluated. Among the samples, Cu/NaSep exhibited superior performance, and complete HCHO conversion was achieved at 100 °C (GHSV = 240000 mL/(g·h)). Additionally, the sample retained good catalytic activity during a 42 h stability test. A number of factors, including elevated acidity, the abundance of oxygen species, and favorable low-temperature reducibility, were responsible for the excellent catalytic activity of Cu/NaSep. According to the results of the in-situ DRIFTS characterization, the HCHO oxidation mechanism was as follows: (i) HCHO was rapidly decomposed into dioxymethylene (DOM) species on the Cu/NaSep surface; (ii) DOM was then immediately converted to formate species; (iii) the resultant formate species were further oxidized to carbonates; (iv) the carbonate species were eventually converted to CO2 and H2O.

Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

AbstractSupported transition metal catalysts have been extensively applied to oxidative and reductive processes. The understanding of surface speciation and active site‐support interactions in these materials play a substantial role in developing improved heterogeneous catalysts. Herein, a series of impregnated 3D ferrierite and 2D ITQ‐6 siliceous supports with variable loading of vanadium oxide was prepared. Chemical and structural properties of the materials were studied by X‐ray diffraction, N2 physisorption, inductively coupled plasma – optical emission spectrometry, X‐ray absorption, Fourier transform infrared and diffuse reflectance UV‐vis spectroscopies, and temperature‐programmed reduction with H2. Reactivity of the catalyst surface, associated with the incidence of isolated silanol groups, was found to be more effective when vanadium oxides were better dispersed and stabilized than increases in surface area. Differences in activation and the oxidation state dynamic behavior of active sites were then probed by methanol oxidation as a model reaction monitored by in situ FTIR spectroscopy and XANES/MS. By applying isothermal periods of reaction under non‐oxidizing atmosphere and regeneration of catalysts by O2, it was found that, even at distinct rates, all types of sites are accessible during reaction, since a complete reduction to V4+ was observed. However, reoxidation of sites to V5+ is limited and sensitive to the different vanadium species on the surface, and probably, the determinant factor of the distinct V5+/V4+ equilibrium reached for the catalysts when the reaction is carried out under constant oxidizing atmosphere.