Single-site Catalysts Research Articles

MAO- and Borate-Free Activating Supports for Group 4 Metallocene and Post-Metallocene Catalysts of α-Olefin Polymerization and Oligomerization.

Modern industry of advanced polyolefins extensively uses Group 4 metallocene and post-metallocene catalysts. High-throughput polyolefin technologies demand the use of heterogeneous catalysts with a given particle size and morphology, high thermal stability, and controlled productivity. Conventional Group 4 metal single-site heterogeneous catalysts require the use of high-cost methylalumoxane (MAO) or perfluoroaryl borate activators. However, a number of inorganic phases, containing highly acidic Lewis and Brønsted sites, are able to activate Group 4 metal pre-catalysts using low-cost and affordable alkylaluminums. In the present review, we gathered comprehensive information on MAO- and borate-free activating supports of different types and discussed the surface nature and chemistry of these phases, examples of their use in the polymerization of ethylene and α-olefins, and prospects of the further development for applications in the polyolefin industry.

Palladium and Ruthenium Dual-Single-Atom Sites on Porous Ionic Polymers for Acetylene Dialkoxycarbonylation: Synergetic Effects Stabilize the Active Site and Increase CO Adsorption.

Heterogeneous single-metal-site catalysts usually suffer from the poor stability, limiting its industrial applications. Herein, the dual-sites Pd1-Ru1 supported on porous ionic polymers (Pd1-Ru1/PIPs) was constructed using wetnessimpregnation method. The two isolated metal species in the form of binuclear complex was immobilized on the cationic framework of PIPs through ionic bonds. Compared to the single-Pd or Ru-site catalyst, it exhibits higher activity with 98% acetylene conversion and near 100% selectivity of dialkoxycarbonylation products as well as better cycling stability for ten times cycles without obvious decay. Based on DFT calculations, it was found that the single-Ru-site exhibited a strong CO adsorption energy of -1.6 eV, leading to an increase in the local CO concentration of the catalyst. Notably, the Pd1-Ru1/PIPs catalyst had a much lower energy barrier of 2.49 eV compared to 3.87 eV of Pd1/PIPs for the rate-determining step. The synergetic effect between neighbouring single-sites Pd1 and Ru1 not only enhanced the overall activity, but also stabilized Pd(II) active sites. The discovery of synergetic effect between single-sites can deepen our understanding of single-site catalysts at the molecular level.

Single Mo Atoms Stabilized on High-Entropy Perovskite Oxide: A Frontier for Aerobic Oxidative Desulfurization.

The design and preparation of catalysts with both excellent stability and maximum exposure of catalytic active sites is highly desirable; however, it remains challenging in heterogeneous catalysis. Herein, a entropy-stabilized single-site Mo catalyst via a high-entropy perovskite oxide LaMn0.2Fe0.2Co0.2Ni0.2Cu0.2O3 (HEPO) with abundant mesoporous structures was initiated by a sacrificial-template strategy. The presence of electrostatic interaction between graphene oxide and metal precursors effectively inhibits the agglomeration of precursor nanoparticles in a high-temperature calcination process, thereby endowing the atomically dispersed Mo6+ coordinated with four O atoms on the defective sites of HEPO. The unique structure of single-site Mo atoms' random distribution with an atomic scale greatly enriches the oxygen vacancy and increases surface exposure of the catalytic active sites on the Mo/HEPO-SAC catalyst. As a result, the obtained Mo/HEPO-SAC exhibits robust recycling stability and ultra-high oxidation activity (turnover frequency = 3.28 × 10-2) for the catalytic removal of dibenzothiophene (DBT) with air as the oxidant, which represents the top level and is strikingly higher than the state-of-the-art oxidation desulfurization catalysts reported previously under the same or similar reaction conditions. Therefore, the finding here for the first time expands the application of single-atom Mo-supported HEPO materials into the field of ultra-deep oxidative desulfurization.

Structure-Property Relationship of Defect-Trapped Pt Single-Site Electrocatalysts for the Hydrogen Evolution Reaction.

Single-site catalysts (SSCs) have attracted significant research interest due to their high metal atom utilization. Platinum single sites trapped in the defects of carbon substrates (trapped Pt-SSCs) have been proposed as efficient and stable electrocatalysts for the hydrogen evolution reaction (HER). However, the correlation between Pt bonding environment, its evolution during operation, and catalytic activity is still unclear. Here, a trapped Pt-SSC is synthesized by pyrolysis of H2PtCl6 chemisorbed on a polyaniline substrate. In situ heated scanning transmission electron microscopy and temperature-dependent X-ray photoelectron spectroscopy clarify the thermally induced structural evolution of Pt during pyrolysis. The results show that the nitrogen in polyaniline coordinates with Pt ions and atomically disperses them before pyrolysis and traps Pt sites at pyridinic N defects generated during the substrate graphitization. Operando X-ray absorption spectroscopy confirms that the trapped Pt-SSC is stable at the HER working potentials but with inferior electrocatalytic activity compared with metallic Pt nanoparticles. First principle calculations suggest that the inferior activity of trapped Pt-SSCs is due to their unfavorable hydrogen chemisorption energy relative to metallic Pt(111) surfaces. These results further the understanding of the structure-property relationship in trapped Pt-SSCs and motivate a detailed techno-economic analysis to evaluate their commercial applicability.

Synergic Effect of Isolated Ce3+ and Ptδ+ Species in UiO-66(Ce) for Heterogeneous Catalysis.

In this work, we have synthesized through an efficient electrostatic deposition a Pt single-atom catalyst (SAC) supported on a Ce-MOF. The basic solution employed in the impregnation process favors the deprotonation of the hydroxyl groups allocated on the clusters that can easily interact with the cationic Pt species. The resulting material, denoted as Pt/UiO-66(Ce), shows an increment of Ce3+ content, as demonstrated by UV-vis and Ce L3-edge XANES spectroscopy. These Ce3+ species and their corresponding oxygen vacancies are able to accommodate very disperse Pt single sites. Moreover, Pt L3-edge XANES and CO-FTIR spectroscopy confirm the cationic nature of the supported Ptδ+ (2+ < δ < 4+). For comparison purpose, we have synthesized and characterized a well-known Pt single-site catalyst supported on nanocrystalline ceria, denoted as Pt/nCeO2. Since the simultaneous presence of Ce3+ and Ptδ+ on the MOF clusters were able to activate the oxygen molecules and the CO molecule, respectively, we tested Pt/UiO-66(Ce) for the CO oxidation reaction. Interestingly, this catalyst showed ∼six-fold increment in activity in comparison with the traditional Pt/nCeO2 material. Finally, the characterization after catalysis reveals that the Pt nature is preserved and that the activity is maintained during 14 h at 100 °C without any evidence of deactivation.

Molecular-level design of isolated molybdenum oxide anchored on carbon nitride for photocatalytic H2 production and environmental remediation

Polymeric carbon nitride typically suffers from sluggish intrinsic charge separation and low available active sites. This paper reports that isolated non-crystalline molybdenum oxide species anchored to N-coordinating cavities (MoCN) have abundant surface-active sites for solid–liquid two-phase reactions, whether for photocatalytic H2 evolution or organic pollutant degradation. Molecular dynamic simulations and density functional theory (DFT) revealed six-fold cavities to stabilize the MoO3 species with non-crystalline features, endowing high dispersion and less aggregation. As proven in single-site heterogeneous catalysts, the photocatalyst benefits from size reduction and accelerated interfacial charge transfer because of its mutual contact between two semiconductors. The MoCN shows a high visible-light H2 evolution of 1265 µmol g−1h−1 under visible light (λ ≥ 400 nm) illumination. The photocatalyst degraded more than 95% tetracycline within 30 min and rhodamine B in 10 min. The MoO3 species confined within π-conjugated systems increase the catalytic contact sites, extending visible light harvesting ability to a longer wavelength. Each single catalytic site facilitates the separation and transfer of charge carriers while interfacial charge still occurs between MoO3 and CN. This molecular-level design and strategy provide a new opportunity and a universal way to extend the boundaries of liquid–solid phase catalysts.

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure, Reactivity, and Mechanism.

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates "catalyst by design" principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.

CoSe2 supported single Pt site catalysts for hydrogen peroxide generation via two‐electron oxygen reduction

AbstractElectrocatalytic oxygen reduction reaction (ORR) to prepare H2O2 in acidic medium has the advantages of green, safety, and portability, which shows broad development prospects. However, it still suffers from low catalyst activity, insufficient selectivity, and high cost. Herein, Pt1/CoSe2 with ultralow 0.01 wt.% Pt atomic distribution was synthesized by a simple hydrothermal method. The Pt1/CoSe2 with ultralow Pt content exhibits high activity, high selectivity, and long‐term stability for ORR to H2O2 in O2‐saturated 0.1 M HClO4. The onset potential is as low as 0.75 V versus reversible hydrogen electrode (RHE), H2O2 selectivity is as high as 84% (0.4 V vs. RHE), and the electron transfer number is 2.3 (0.4 V vs. RHE). Moreover, the hydrogen peroxide yield using the flow cell testing is 110.02 mmol gcat.−1 h−1 with high Faradaic efficiency of 78% (0 V vs. RHE) at 0.1 M HClO4, and the catalyst did not deactivate significantly after 60 h stability testing. Mechanistic studies and in situ X‐ray photoelectron spectroscopy characterization confirm that the ultralow Pt content on CoSe2 can effectively regulate the electronic structure of Co as the real active site around the Pt site, which gives a suitable ∆dp value (the difference between the d‐band center of the active metal site and the p‐band center of the terminal oxygen in *OOH), provides an ideal *OOH binding energy, and inhibits the O–O bond breakage. This work successfully improves the intrinsic activity of the Co active sites around Pt in Pt1/CoSe2 for acidic ORR to H2O2 by constructing ultralow‐content Pt single atom.

Design of NiNC single atom catalyst layers and AEM electrolyzers for stable and efficient CO2-to-CO electrolysis: Correlating ionomer and cell performance

Deploying single-site NiNC catalysts in cathode catalyst layers of bipolar electrolyzer cells enables catalytic CO2 valorization to e-CO at industrially relevant yields and efficiencies. The performance of the cathode layers is controlled by the turnover frequency of the active sites as well as mass transfer to and from the active sites. While the atomic scale structure-reactivity relations of single-site NiNC catalysts have been extensively studied, the mass transfer characteristics of single atom catalyst layers were poorly discussed. In this work, we design, build, and test NiNC catalyst layers using a novel set of distinct ion exchange ionomer materials and correlate the performance of cathode catalyst layers with their reactivity and stability in full single MEA electrolyzer cells. The Sustainion anion exchange ionomer delivered optimal performance, yielding about 90% CO faradaic efficiency up to 300 mA cm−2 and 15 h stable performance at 200 mA cm−2. Our analysis attributes its favorable electrolyzer performance to its balanced conductivity and hydrophobicity, which mitigates electrode flooding while ensuring excellent ion and CO2 transfer rates even at high current densities.

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.

Rationally Designed Water Enriched Nano Reactor for Stable CO2 Hydrogenation with Near 100% Ethanol Selectivity over Diatomic Palladium Active Sites

CO2 (CO) hydrogenation presents the widest route to synthesis of various valuable organic molecules, but precise carbon–carbon coupling control to targeted products along with the elimination of byproducts remains a challenge. We overcome these limitations by synthesizing a CeO2-supported dual Pd site catalyst that could actively catalyze CO2 conversion into single-product ethanol almost without C1 byproducts in a continuous-flow fixed-bed reactor. This surprising finding is derived from the observation that the synergistic catalysis between dual Pd atoms leads to extraordinary ability for the cleavage of C–O bond in *CHxOH species and the carbon–carbon coupling between *CHx and *CO species. Furthermore, the dual Pd sites could be stabilized through enriching in situ formed water in the nano reactor with a hydrophobic shell layer, thus leading to remarkably improved catalytic stability for ethanol production. As a result, the as-constructed dual Pd site catalyst exhibited superior selectivity to ethanol at 98.7%, corresponding to a productivity up to 11.6 g per gram of Pd per hour and excellent stability during the continuous test for 60 h. Our results demonstrate that multifunctional synergistic catalysis of dual active sites can break through the restriction of a reaction involving a single active site catalyst.

Ammonia Tolerance of Atomically Dispersed Single Metal Site Catalysts: Mechanistic Understanding and High‐Performance Oxygen Reduction Electrocatalysis

AbstractThe anion‐exchange membrane direct ammonia fuel cell, as a carbon‐free fuel cell type, has recently received increasing attention albeit suffering from high cost of using the platinum‐group metal oxygen reduction reaction (ORR) catalysts. To pave the development of this promising power source, the atomically dispersed transition metal‐nitrogen‐carbon (M‐N‐C) materials with low cost and high ORR performance have allured to investigate their ammonia tolerance during the ORR. Herein, it is initially deconvoluted how compositional and structural elements of FeN4 sites modulate catalyst's performance. Furthermore, ORR catalytic activities of the M‐N‐C (M = Fe, Co or Mn) and Pt/C catalysts are investigated in ammonia‐containing electrolytes, showing that M‐N‐C catalysts have better ammonia tolerance than Pt/C. Among others, the Fe‐N‐C exhibits the best ammonia tolerance with only 4 mV negative shifts of half‐wave potential, 2.7% decrease of current, and negligibly irreversible activity loss. The superior ammonia tolerance of MN4 sites to Pt (111) surface is further confirmed by density functional theory calculations. The adsorption capacity of MN4 for O2 is higher than NH3 and the bonding force between MN4 and O2 is stronger than NH3, whereas opposite adsorption capacity and bonding force trends are observed on Pt (111) surface.

Polymerization of Allyltrimethylisilane and 4-Methyl-1-Pentene by Using Metallocene Catalysts.

Polymers of higher olefin, obtained by Ziegler-type polymerization, have been used in some critical fields, e.g., as the membrane for extracorporeal membrane oxygenation (ECMO), which plays an important role in the treatment of patients with severe COVID-19. The polymer obtained by a single-site catalyst, e.g., metallocene catalysts, demonstrated a higher performance. The homo- and co-polymerization of allyltrimethylisilane (ATMS) and 4-methyl-1-pentene (4M1P) were conducted using syndiospecific (cat 1) and isospecific (cat 2) metallocene catalysts. Cat 1 showed low conversions and provided a polymer with a higher molecular weight, while cat 2 behaved oppositely. 13C-NMR spectra certified the stereotacticity of the resultant polymer, and the resonance of the carbon atom of CH2 (αα') between the two tertiary carbon atoms of the ATMS and 4M1P units were observed. This could be the evidence of the formation of a true copolymer. The crystallization of the polymer was explored using a differential scanning calorimeter (DSC) and wide angle X-ray diffraction (WAXD). All homopolymers and some of the copolymers showed high melting temperatures and low melting enthalpies. The WAXD patterns of the syndiotactic polymer and isotactic homopolymer or the ATMS-rich copolymer were consistent with the reported literature, but the isotactic 4M1P-rich copolymer provided the crystal form I, which is unusual for a 4M1P polymer without any pretreatment.

Highly efficient electrocatalytic CO2 reduction over pyrolysis–free conjugated metallophthalocyanine networks in full pH range

Herein, we report the first work on pyrolysis–free synthesis of robust conjugated metallophthalocyanine–based covalent triazine frameworks (MPc–CTF, M= Co, Ni) for pH-universal electrocatalytic CO2 reduction reaction (CO2RR). CoPc–CTF shows high CO Faradaic efficiency over 94%, and long-term stability over 10 h in full pH range, and industry level CO partial current densities of 378.8 mA cm−2, 165.9 mA cm−2, 172.5 mA cm−2 in alkaline, neutral and acidic electrolytes, respectively. Particularly, the CO2 utilization efficiency reaches 51.3% in acidic electrolytes. Moreover, operando characterizations and density functional theory calculations indicate the promoting effect of triazine moiety in CoPc–CTF to accelerate the formation of *COOH intermediates during CO2RR. This work provides an encouraging direction for designing robust conjugated network supported single site catalysts to access high product selectivity and high carbon utilization efficiency in electrocatalytic CO2 reduction reaction.

Degree-of-isolation shapes the catalytic performance of single-site alloy catalysts

Degree-of-isolation shapes the catalytic performance of single-site alloy catalysts

Pd(2‐pymo)2]n/Al2O3 as MOF Single Site Catalyst for the Selective Hydrogenation of Acetylene**

AbstractDespite the great potential of metal‐organic frameworks (MOFs) in catalysis, industrial applications are still scarce. This is mainly due to a lack of performance when changing from idealized lab conditions towards realistic conditions of the actual application. In this work, we demonstrate the applicability and outstanding catalytic performance of an alumina‐supported [Pd(2‐pymo)2]n MOF catalyst in the selective hydrogenation of acetylene to ethylene under industrial front‐end conditions. It shows a competitive performance to an industrial benchmark catalyst and even exceeds it in terms of ethane selectivity due to the combination of well‐defined isolated Pd active sites and synergies due to MOF‐support‐interactions. The high stability was proven for up to 60 h time‐on‐stream and supported by XPS and XRD structural analysis.

Assessing factors that determine adatom migration and clustering on a thin film oxide; Pt1 and Rh1 on the “29” CuxO/Cu(1 1 1) surface

The dynamical nature of single-site catalysts under non-equilibrium conditions poses significant challenges in the full characterization of their active sites. The “29” oxide is a thin CuxO film grown on Cu(111) which provides a well-defined surface on which to study the structure and chemistry of atomically dispersed precious metals. A combination of experimental and first-principles approaches is used to investigate the factors that influence the mobility of metal adatoms and their clustering tendencies. Our work elucidates how a homogenous coordination environment of neutral single-atoms can be obtained when its tendency to break oxide bonds within its vicinity is low. We demonstrate that this in turn affects the chemical activity of atomically dispersed species on thin-film oxides, as defects on the oxide would in fact allow such species to be more accessible to CO. We thus highlight the importance of understanding the relationship between defects and atomically dispersed active sites upon designing single-site catalysts.

Designing single-site alloy catalysts using a degree-of-isolation descriptor.

Geometrically isolated metal atoms in alloy catalysts can target efficient and selective catalysis. However, the geometric and electronic disturbance between the active atom and its neighbouring atoms, that is, diverse microenvironments, makes the active site ambiguous. Herein, we demonstrate a methodology to describe the microenvironment and determine the effectiveness of active sites in single-site alloys. A simple descriptor, degree-of-isolation, is proposed, considering both electronic regulation and geometric modulation within a PtM ensemble (M = transition metal). The catalytic performance of PtM single-site alloy is examined thoroughly using this descriptor for an industrially important reaction, propane dehydrogenation. The volcano-shaped isolation-selectivity plot reveals a Sabatier-type principle for designing selective single-site alloys. Specifically, for a single-site alloy with a high degree-of-isolation, alternation of the active centre has a great impact on tuning selectivity, validated by the outstanding consistency between experimental propylene selectivity and the computational descriptor.

Atomically Dispersed NiNx Site with High Oxygen Electrocatalysis Performance Facilely Produced via a Surface Immobilization Strategy

Nonprecious-metal heterogeneous catalysts with atomically dispersed active sites demonstrated high activity and selectivity in different reactions, and the rational design and large-scale preparation of such catalysts are of great interest but remain a huge challenge. Current approaches usually involve extremely high-temperature and tedious procedures. Here, we demonstrated a straightforward and scalable preparation strategy. In two simple steps, the atomically dispersed Ni electrocatalyst can be synthesized in a tens grams scale with quantitative yield under mild conditions, and the active Ni sites were produced by immobilizing preorganized NiNx complex on the substrate surface via organic thermal reactions. This catalyst exhibits excellent catalysis performances in both oxygen evolution and reduction reactions. It also exhibited tunable catalysis activity, high catalysis reproducibility, and high stability. The atomically dispersed NiNx sites are tolerant at high Ni concentration, as the random reactions and metal nanoparticle formation that generally occurred at high temperatures were avoided. This strategy illustrated a practical and green method for the industrial manufacture of nonprecious-metal single-site catalysts with a predictable structure.

Tandem coordinative chain transfer polymerization for long chain branched Polyethylene: The role of chain displacement

The controlled introduction of long chain branches (LCBs) in the commonly linear structure of polyolefins has been frequently pursued by researchers to improve their melt processability. To account for that, we introduce a new tandem dual catalytic system for the formation of LCB in ethylene polymerization. Coordinative chain transfer polymerization in the first step produces Al-alkyls via frequent transfer of growing polymer chains from the single-site α-diimine nickel polymerization catalyst to AlEt2Cl chain transfer agent (CTA). Next, the displacement catalyst, Ni(acac)2, converts the dormant alkyl chains into macromers through a β-hydride elimination and retrieves the Al-based CTA. Due to the high comonomer affinity of the polymerization catalyst, the released macromers are incorporated in growing polymer chains. By repeating this reaction cycles, a branch-on-branch structure is formed. Interestingly, a separate cocatalyst is not required, as AlEt2Cl serves as both the cocatalyst and the CTA for the CCTP system. This reaction scheme employs the “nickel-effect” mechanism for the formation of LCB in coordinative polymerization for the first time. The efficiency of the proposed mechanism in the formation of LCBs is confirmed by comprehensive rheological studies and NMR spectroscopy. Moreover, density functional theory (DFT) simulations rationalize the formation of LCBs through the proposed Ni-based displacement reaction.