Applications In Heterogeneous Catalysis Research Articles

Deciphering the Ce3+to Ce4+Evolution: Insight from X-ray Raman Scattering Spectroscopy at Ce N4,5Edges.

Cerium oxide, or ceria, (CeO2) is one of the most studied materials for its wide range of applications in heterogeneous catalysis and energy conversion technologies. The key feature of ceria is the remarkable oxygen storage capacity linked to the switch between Ce4+ and Ce3+ states, in turn creating oxygen vacancies. Changes in the electronic structure occur with oxygen removal from the lattice. Accordingly, the two valence electrons can be accommodated by the reduction of support cations where the electrons can be localized in empty f states of Ce4+ ions nearby.Quantifying the different oxidation states in situ is crucial to understand and model the reaction mechanism. Beside the different techniques to quantify Ce3+ and Ce4+ states, we discuss the use of X-ray Raman Scattering (XRS) spectroscopy as an alternative method. In particular, we show that XRS can observe the oxidation state changes of cerium directly in the bulk of the materials under realistic environmental conditions. The Hilbert++ code is used to simulate the XRS spectra and quantify accurately the Ce3+ and Ce4+ content. These results are compared to those obtained from in situ X-ray Diffraction (XRD) collected in parallel and the differences arising from the two different probes are discussed.

Understanding the chemistry of mesostructured porous nanoreactors.

Porous nanoreactors mimic the structures and functions of cells, providing an adaptable material with multiple functions and effects. These reactors can be nanoscale containers and shuttles or catalytic centres, drawing in reactants for cascading reactions with multishelled designs. The detailed construction of multi-level reactors at the nanometre scale remains a great challenge, but to regulate the reaction pathways within a reactor, designs of great intricacy are required. In this Review, we define the basic structural characteristics of porous nanoreactors, while also discussing the design principles and synthetic chemistry of these structures with respect to their emerging applications in energy storage and heterogeneous catalysis. Finally, we describe the difficulties of the structural optimization of these reactors and propose possible ways to improve porous nanoreactor design for future applications.

A resorcin[4]arene-based MOF as a Lewis acid catalyst for the CO2 coupling reaction

Metal-organic frameworks (MOFs) have attracted considerable attention due to their promising applications in heterogeneous catalysis. Hence, a Co(II)-based MOF, [(CH3)2NH2]2[Co11(HL)(L)(OH)(H2O)21(DMA)2]·6DMA (NEAU-2), was synthesized using CoCl2‧6H2O and a resorcin[4]arene ligand. The synthesized NEAU-2 crystal was systematically characterized. NEAU-2 exhibits abundant Lewis acid active sites after activation treatment, making it effective in catalyzing the cycloaddition of CO2 with various epoxides at atmospheric pressure. Significantly, after 5 cycles of repeated experiments, NEAU-2 still maintained high catalytic activity, proving its stability as a heterogeneous catalyst.

Three-dimensional atomic insights into the metal-oxide interface in Zr-ZrO2 nanoparticles

Metal-oxide interfaces with poor coherency have specific properties comparing to bulk materials and offer broad applications in heterogeneous catalysis, battery, and electronics. However, current understanding of the three-dimensional (3D) atomic metal-oxide interfaces remains limited because of their inherent structural complexity and the limitations of conventional two-dimensional imaging techniques. Here, we determine the 3D atomic structure of metal-oxide interfaces in zirconium-zirconia nanoparticles using atomic-resolution electron tomography. We quantitatively analyze the atomic concentration and the degree of oxidation, and find the coherency and translational symmetry of the interfaces are broken. Atoms at the interface have low structural ordering, low coordination, and elongated bond length. Moreover, we observe porous structures such as Zr vacancies and nano-pores, and investigate their distribution. Our findings provide a clear 3D atomic picture of metal-oxide interface with direct experimental evidence. We anticipate this work could encourage future studies on fundamental problems of oxides, such as interfacial structures in semiconductor and atomic motion during oxidation process.

Infiltrated electroless deposition of gold nanoparticles on porous Prussian blue-Fe2O3 hetero-microstructures for environmental remediation applications: A novel infiltration strategy to form 3-D nano-catalytic networks

Metal and metal oxide nanocomposites are promising candidates for heterogeneous catalytic applications. In this regard, most of the metal oxide inner core volume remains as a dead volume, not participating in the catalysis, only the metal/metal oxide hetero-junctions available at the surfaces are involved in the catalysis. Herein, we report a novel strategy for the infiltration of gold nanoparticles into the inner core of 3D-porous Prussian blue/Fe2O3 hetero-microstructures (PBFHM-Au) beyond its outer surface to form an advanced heterogeneous catalyst for the environmental remediation applications. The porosity of the Prussian blue microstructures is thermally tuned for the infiltration of gold ion precursor into the inner core volume of Prussian blue/Fe2O3 hetero-microstructures to facilitate the galvanic displacement reaction between Fe and Au to enlarge the lattice shell and form Au nanoparticle 3D networks throughout the PBFHM. Notably, the resulting PBFHM-Au2 shows enhanced structural stability electron transfer kinetics and catalytic activity in comparison with the only surface-attached Au/Fe2O3 composite. The obtained PBFHM-Au2 catalyst demonstrates remarkable catalytic activity for Rhodamine-B dye degradation with rate constant (k = 3.47 × 10−2 s−1) and p-nitrophenol reduction reactions with rate constant (k = 1.199 × 10−1 s−1).

Synthesis of a Novel and Highly HOR/HER Selective Ru@Pt/TiOx/C Catalyst Exhibiting Strong Metal-Support Interactions (SMSI) for PEMFC Applications

One main goal in Proton Exchange Membrane Fuel Cell (PEMFC) research is to make Fuel Cell Electric Vehicles (FCEV) viable for long-haul travel and increase their durability. Besides many degradation modes which target different parts of a PEMFC such as the catalyst layer, the membrane, bipolar plates, and other components, one major concern is the start-up/shut-down (SUSD) of the cell, inducing degradation of the catalyst support on the cathode. Addressing this, two mitigation strategies are possible: a) the system approach, e.g., reducing number of damaging SUSD events; b) the catalyst approach, e.g., suppressing the Oxygen Reduction Reaction (ORR) activity of the anode.The latter is feasible by utilizing a highly selective HOR/HER (Hydrogen Oxidation/Evolution Reaction) catalyst on the anode side that catalyzes the HOR but has a poor ORR activity, a property that was reported for a Pt/TiOx/C catalyst by Stühmeier et al.[1,2] The high selectivity towards the HOR/HER is explained by the Strong-Metal-Support-Interaction (SMSI) formed between the Pt nanoparticles and the TiOx-support, where the support creeps over the particle during a reductive heat treatment, as observed in HR-TEM.[1] In general, the SMSI effect between a metal and a suitable metal support is found in various heterogeneous catalytic applications, altering the catalyst’s activity for various reactions.[3] In the particular case of Pt/TiOx/C, the SMSI layer forming on the Pt nanoparticle is said to be permeable for protons but impermeable for any other oxygen-containing species, explaining a still rather high HOR/HER activity compared to a Pt/C catalyst (≈ 30% vs Pt/C) and an ORR activity that is decreased by two magnitudes (≈ 2% vs Pt/C).[1] Such interaction was also observed by Stühmeier et al. between Ru and a TiOx-support, where similar trends were determined. However, since the intrinsic HOR/HER activity of a Ru catalyst is quite low compared to Pt, a Ru/TiOx/C catalyst is not feasible for PEMFC applications.[4] Ru@Pt core-shell particles, as shown by Schwämmlein et al., exhibit a 3-5x higher HOR/HER activity compared to a Pt/C catalyst, making a Ru@Pt/C catalyst viable even for alkaline fuel cell applications.[5] Due to the already successfully explored interaction of Pt and Ru with a TiOx-support, we will examine the HOR and ORR activity of Ru@Pt/TiOx/C catalysts..In this work we successfully synthesized a novel catalyst system by forming an SMSI between Ru@Pt core-shell particles and a TiOx-support. Via RDE (Rotating Disk Electrode) characterization, we elucidate if the Ru@Pt/TiOx/C catalyst gains all the benefits from the single parts of the catalyst, namely: i) a high HOR/HER activity from the Ru@Pt core-shell particles; ii) a low ORR activity due to the SMSI effect. Figure 1 shows the HOR/HER performance catalysts based on Ru@Pt particles supported on Vulcan carbon (black line) or on TiO2 nanoparticles, whereby for the latter support the catalysts were either tested as-synthesized (red line) or after a reductive heat-treatment under 5% H2/Ar to induce SMSI effects (green line). Apparently, the SMSI layer on the Ru@Pt core-shell particles enable a sustained HOR/HER activity at > 1.0 VRHE, in contrast to the other two catalysts which show the typical decrease of the HOR/HER activity at high potentials. Similar observations had been made by Stühmeier et al. for a Pt/TiOx/C and Ru/TiOx/C catalyst.[1,2,4] The findings in this particular study with the TiOx-support, as an example, should serve as proof of principle to possibly translate the gained knowledge on different types of metal oxide supports, since the stability of TiOx is known to be limited under PEMFC operation.[2,6] References [1] B. M. Stühmeier, S. Selve, M. U. M. Patel, T. N. Geppert, A. Hubert, H. A. Gasteiger, H. A. El-Sayed, ACS Appl. Energy Mater. 2019, 1–8.[2] B. M. Stühmeier, A. M. Damjanović, K. Rodewald, H. A. Gasteiger, J. Power Sources 2023, 558, DOI 10.1016/j.jpowsour.2022.232572.[3] C. J. Pan, M. C. Tsai, W. N. Su, J. Rick, N. G. Akalework, A. K. Agegnehu, S. Y. Cheng, B. J. Hwang, J. Taiwan Inst. Chem. Eng. 2017, 74, 154–186.[4] B. M. Stühmeier, R. J. Schuster, L. Hartmann, S. Selve, H. A. El-Sayed, H. A. Gasteiger, J. Electrochem. Soc. 2022, 169, 034519.[5] J. N. Schwämmlein, H. A. El-Sayed, B. M. Stühmeier, K. F. Wagenbauer, H. Dietz, H. A. Gasteiger, ECS Trans. 2016, 75, 971–982.[6] J. Zhang, F. Coms, S. Kumaraguru, J. Electrochem. Soc. 2021, 168, 024520. Figure 1

Mesoporous silica–supported metalloSalen and metalloSalphen: synthesis and application in heterogeneous catalysis

Mesoporous silica–supported metalloSalen and metalloSalphen: synthesis and application in heterogeneous catalysis

Ni2MnAl Heusler Alloy Nanoparticles for Chemoselective Hydrogenation of Nitro Compounds

AbstractDespite immense interest for years in Heusler alloys as magnetoelastic materials, their application in heterogeneous catalysis has only recently been explored. We report the development of highly active, robust and magnetically separable Ni2MnAl Heusler alloy nanoparticles as a heterogeneous catalyst for hydrogenation of aliphatic and aromatic nitro compounds to the corresponding amines. The easily synthesizable Ni2MnAl nanoparticles are thermally and chemically robust and exhibited excellent chemoselectivity in the hydrogenation of a range of aliphatic and aromatic nitroarenes, including those bearing functional groups sensitive to reduction, such as alkene, bromo, iodo, carbonyl and cyano, yielding the respective functionalized amines in good yields. Moreover, this solid catalyst is magnetically separable from the reaction mixture and could be recycled and reused at least 10 times without any loss in catalytic activity. The mechanism of the Ni2MnAl Heusler alloy nanoparticles catalyzed nitro reduction was studied by various spectroscopy, control experiments and kinetics. Our work opens a new avenue of developing heterogeneous earth‐abundant metal catalysis for economic and environment‐friendly synthesis of fine chemicals owing to straightforward synthesis and tunable composition of Heusler alloy nanoparticles from base‐metal precursors and their diverse structure and properties in conjunction with their thermal and chemical stability, excellent chemoselectivity, magnetic separability and recyclability.

Friedel Crafts Reactions Revisited: Some Applications in Heterogeneous Catalysis#

Abstract: Important organic reactions require the use of catalysts. The Friedel-Crafts reactions were discovered by Charles Friedel and James Mason Crafts in 1887. They are an essential catalytic process since they are widely applied in different areas such as fuels, cleaning, and pharmacological products. The reaction is usually carried out in the presence of Lewis acids or Brønsted acids in a homogeneous medium, with the nucleophilic aromatic substrate in excess. Although there is still work in the literature on the Friedel- Crafts reaction in a homogeneous medium using metal halides, the tendency is to replace these catalysts, which generate effluents that are harmful to the environment. Heterogeneous catalysts using solid acids show advantages over homogeneous catalysts, especially concerning separating products from the reaction medium, recycling, and reusing. This paper presents a mini-review focusing on the use of solid acids in Friedel-Crafts reactions.

URJC-1: Stable and Efficient Catalyst for O-Arylation Cross-Coupling.

The design of metal-organic frameworks (MOFs) allows the definition of properties for their final application in small-scale heterogeneous catalysis. Incorporating various catalytic centers within a single structure can produce a synergistic effect, which is particularly intriguing for cross-coupling reactions. The URJC-1 material exhibits catalytic duality: the metal centers act as Lewis acid centers, while the nitrogen atoms of the organic ligand must behave as basic centers. The impact of reaction temperature, catalyst concentration, and basic agent concentration was evaluated. Several copper-based catalysts, including homogeneous and heterogeneous MOF catalysts with and without the presence of nitrogen atoms in the organic ligand, were assessed for their catalytic effect under optimal conditions. Among the catalysts tested, URJC-1 exhibited the highest catalytic activity, achieving complete conversion of 4-nitrobenzaldehyde with only 3% mol copper concentration in one hour. Furthermore, URJC-1 maintained its crystalline structure even after five reaction cycles, demonstrating remarkable stability in the reaction medium. The study also examined the impact of various substituents of the substrate alcohol on the reaction using URJC-1. The results showed that the reaction had high activity when activating substituents were present and for most cyclic alcohols rather than linear ones.

Nanoconfinement steers nonradical pathway transition in single atom fenton-like catalysis for improving oxidant utilization

The introduction of single-atom catalysts (SACs) into Fenton-like oxidation promises ultrafast water pollutant elimination, but the limited access to pollutants and oxidant by surface catalytic sites and the intensive oxidant consumption still severely restrict the decontamination performance. While nanoconfinement of SACs allows drastically enhanced decontamination reaction kinetics, the detailed regulatory mechanisms remain elusive. Here, we unveil that, apart from local enrichment of reactants, the catalytic pathway shift is also an important cause for the reactivity enhancement of nanoconfined SACs. The surface electronic structure of cobalt site is altered by confining it within the nanopores of mesostructured silica particles, which triggers a fundamental transition from singlet oxygen to electron transfer pathway for 4-chlorophenol oxidation. The changed pathway and accelerated interfacial mass transfer render the nanoconfined system up to 34.7-fold higher pollutant degradation rate and drastically raised peroxymonosulfate utilization efficiency (from 61.8% to 96.6%) relative to the unconfined control. It also demonstrates superior reactivity for the degradation of other electron-rich phenolic compounds, good environment robustness, and high stability for treating real lake water. Our findings deepen the knowledge of nanoconfined catalysis and may inspire innovations in low-carbon water purification technologies and other heterogeneous catalytic applications.

Assessment of Hydrophilicity/Hydrophobicity in Mesoporous Silica by Combining Adsorption, Liquid Intrusion, and Solid-State NMR Spectroscopy.

We have developed a comprehensive strategy for quantitatively assessing the hydrophilicity/hydrophobicity of nanoporous materials by combining advanced adsorption studies, novel liquid intrusion techniques, and solid-state NMR spectroscopy. For this, we have chosen a well-defined system of model materials, i.e., the highly ordered mesoporous silica molecular sieve SBA-15 in its pristine state and functionalized with different amounts of trimethylsilyl (TMS) groups, allowing one to accurately tailor the surface chemistry while maintaining the well-defined pore structure. For an absolute quantification of the trimethylsilyl group density, quantitative 1H solid-state NMR spectroscopy under magic angle spinning was employed. A full textural characterization of the materials was obtained by high-resolution argon 87 K adsorption, coupled with the application of dedicated methods based on nonlocal-density functional theory (NLDFT). Based on the known texture of the model materials, we developed a novel methodology allowing one to determine the effective contact angle of water adsorbed on the pore surfaces from complete wetting to nonwetting, constituting a powerful parameter for the characterization of the surface chemistry inside porous materials. The surface chemistry was found to vary from hydrophilic to hydrophobic as the TMS functionalization content was increased. For wetting and partially wetting surfaces, pore condensation of water is observed at pressures P smaller than the bulk saturation pressure p0 (i.e., at p/p0 < 1) and the effective contact angle of water on the pore walls could be derived from the water sorption isotherms. However, for nonwetting surfaces, pore condensation occurs at pressures above the saturation pressure (i.e., at p/p0 > 1). In this case, we investigated the pore filling of water (i.e., the vapor-liquid phase transition) by the application of a novel, liquid water intrusion/extrusion methodology, allowing one to derive the effective contact angle of water on the pore walls even in the case of nonwetting. Complementary molecular simulations provide density profiles of water on pristine and TMS-grafted silica surfaces (mimicking the tailored, functionalized experimental silica surfaces), which allow for a molecular view on the water adsorbate structure. Summarizing, we present a comprehensive and reliable methodology for quantitatively assessing the hydrophilicity/hydrophobicity of siliceous nanoporous materials, which has the potential to optimize applications in heterogeneous catalysis and separation (e.g., chromatography).

Non-radical activation of peroxymonosulfate by modified activated carbon for efficient degradation of oxytetracycline: Mechanisms and applications

The application of heterogeneous metal-free catalysis for non-radical peroxymonosulfate (PMS) oxidation is of great significance because of its low environmental effects and mild oxidant dosage. The purpose of this study was to investigate the activation mechanism of PMS using acid-modified activated carbon and oxytetracycline (OTC) as a representative pollutant. Modified activated carbon (MAC)/PMS system was capable of achieving 100 % degradation efficiency of OTC within 60 min, with an observed rate constant (kobs) of 0.0414 min−1. The degradation of OTC is dominated by non-radical oxidation pathways involved in mediated singlet oxygen (1O2) and electron-transfer process through electron paramagnetic resonance (EPR) and radicals quenching studies. The activation of PMS is attributed to the structural defects of MAC, persistent free radicals, and oxygen functional groups such as C-OOH. Moreover, the MAC/PMS system consistently demonstrates high and reliable contaminant removal in various water matrices. A continuous-flow device utilizing MAC shows excellent performance in purifying micro-polluted water. Additionally, the degradation pathways of pollutants and changes in toxicity during the degradation process were identified through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS), and toxicological analysis. Finally, the cultivation of green beans, peas, and wheat was found to significantly decrease the toxicity of contaminated water through degradation. This study proposes a low-cost method to enhance the activation pathway of non-radical PMS by utilizing modified activated carbon materials for pollutant remediation.

Pd(OAc)2/P–COF: A phosphine-based COF supported palladium-salt heterogeneous catalyst for Suzuki-Miyaura coupling reactions at ambient conditions

A new type of phosphine covalent organic frameworks (P-COFs) has garnered interest because of their high specific surface area and good thermal stability. These microporous materials have potential applications in gas storage, separation, heterogeneous catalysis, and other fields. Here, we present a heterogeneous catalyst consisting of a phosphine-based COF supported by palladium salt (Pd(OAc)2/P-COF) for Suzuki-Miyaura coupling reactions. The electron-rich heteroatoms (P, N) can securely anchor the active component (Pd) within the walls of P–COF. This catalyst exhibits excellent catalytic activity and cyclic stability. The coupling yield of phenylboronic acid and chlorobenzene with low activity reaches 100 % at room temperature for 3 h. After five cycles, the coupling yield of bromobenzene and phenylboronic acid remains above 93 %.

Multistimuli‐Responsive Luminescent Porous Organic Polymers with Chiroptical Properties and Acid‐Induced Degradation

Comprehensive SummaryPorous organic polymers (POPs) have attracted great attention in past decades. Although diverse functional POPs have been developed, multistimuli‐responsive POPs with excellent aggregate‐state luminescence together with good chiroptical properties have rarely been reported. Herein, two pairs of Salen‐type enantiomeric POPs with multistimuli‐responsive luminescence and chiral features were designed and synthesized by facile polycondensation reactions between polyfunctional aggregation‐induced emission luminogen (AIEgen)‐containing salicylaldehyde derivatives and chiral diamines. With Salen units in polymer backbones as tetradentate ligands, a series of POP‐metal complexes were further prepared. The obtained POPs and metal complexes show good porosity, high thermal stability, and obvious circular dichroism signals. Moreover, benefiting from the coexistence of AIEgen and Salen units in polymer structures, these POPs exhibit excellent luminescence performance in aggregate states and tunable fluorescence behaviors in response to external stimuli of Zn2+ ion, mechanical forces, organic solvent, and acids. Due to the dynamic feature of Schiff base C=N bonds, the present POPs can efficiently undergo hydrolysis reactions under strong acidic conditions to reproduce the AIEgen‐ containing monomers, and such an acid‐induced degradation process can be directly visualized and dynamically monitored via fluorescence variation. These properties collectively make the POPs candidate materials for applications in heterogeneous asymmetric catalysis, fluorescence sensing, biomedicine, etc.

Enhancing layered perovskite ferrites with ultra-high-density nanoparticles via cobalt doping for ceramic fuel cell anode

Nanoparticles anchored on the perovskite surface have gained considerable attention for their wide-ranging applications in heterogeneous catalysis and energy conversion due to their robust and integrated structural configuration. Herein, we employ controlled Co doping to effectively enhance the nanoparticle exsolution process in layered perovskite ferrites materials. CoFe alloy nanoparticles with ultra-high-density are exsolved on the (PrBa)0.95(Fe0.8Co0.1Nb0.1)2O5+δ (PBFCN0.1) surface under reducing atmosphere, providing significant amounts of reaction sites and good durability for hydrocarbon catalysis. Under a reducing atmosphere, cobalt facilitates the reduction of iron cations within PBFCN0.1, leading to the formation of CoFe alloy nanoparticles. This formation is accompanied by a cation exchange process, wherein, with the increase in temperature, partial cobalt ions are substituted by iron. Meanwhile, Co doping significantly enhance the electrical conductivity due to the stronger covalency of the CoO bond compared with FeO bond. A single cell with the configuration of PBFCN0.1-Sm0.2Ce0.8O1.9 (SDC)|SDC|Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF)-SDC achieves an extremely low polarization resistance of 0.0163 Ω cm2 and a high peak power density of 740 mW cm−2 at 800 °C. The cell also shows stable operation for 120 h in H2 with a constant current density of 285 mA cm−2. Furthermore, employing wet C2H6 as fuel, the cell demonstrates remarkable performance, achieving peak power densities of 455 mW cm−2 at 800 °C and 320 mW cm−2 at 750 °C, marking improvements of 36% and 70% over the cell with (PrBa)0.95(Fe0.9Nb0.1)2O5+δ (PBFN)-SDC at these respective temperatures. This discovery emphasizes how temperature influences alloy nanoparticles exsolution within doped layered perovskite ferrites materials, paving the way for the development of high-performance ceramic fuel cell anodes.

Nanoscale Chemical Probing of Metal-Supported Ultrathin Ferrous Oxide via Tip-Enhanced Raman Spectroscopy and Scanning Tunneling Microscopy.

Metal-supported ultrathin ferrous oxide (FeO) has attracted immense interest in academia and industry due to its widespread applications in heterogeneous catalysis. However, chemical insight into the local structural characteristics of FeO, despite its critical importance in elucidating structure-property relationships, remains elusive. In this work, we report the nanoscale chemical probing of gold (Au)-supported ultrathin FeO via ultrahigh-vacuum tip-enhanced Raman spectroscopy (UHV-TERS) and scanning tunneling microscopy (STM). For comparative analysis, single-crystal Au(111) and Au(100) substrates are used to tune the interfacial properties of FeO. Although STM images show distinctly different moiré superstructures on FeO nanoislands on Au(111) and Au(100), TERS demonstrates the same chemical nature of FeO by comparable vibrational features. In addition, combined TERS and STM measurements identify a unique wrinkled FeO structure on Au(100), which is correlated to the reassembly of the intrinsic Au(100) surface reconstruction due to FeO deposition. Beyond revealing the morphologies of ultrathin FeO on Au substrates, our study provides a thorough understanding of the local interfacial properties and interactions of FeO on Au, which could shed light on the rational design of metal-supported FeO catalysts. Furthermore, this work demonstrates the promising utility of combined TERS and STM in chemically probing the structural properties of metal-supported ultrathin oxides on the nanoscale.

A C3‐Symmetric Amino Organocatalyst for Asymmetric Synthesis of Warfarin and Analogues: Mechanistic Insight from ESI‐MS Spectrometry and Computational Calculations

AbstractA novel C3‐symmetric multi–amino catalyst was synthesized and evaluated in the asymmetric Michael addition to produce warfarin and its analogues. The multi–amino catalyst turned out to be efficient up to 1 % molar without the use of acidic additives providing the desired product in 82 % yield and maintaining good level of enantioselectivity. ESI‐MS experiments together with data supplied by theoretical models allowed us the understand the operating mechanism of the catalyst, clarifying the role of the amine functions and confirming the catalyst's multifunctionality. The presence of three active catalytic sites makes it easy its anchoring to silica as solid support for applications in heterogeneous phase catalysis.

Cellulose/GO monolith covered with Pd–Pt bimetallic nanocrystals for continuous-flow catalytic reduction of hexavalent chromium

Cellulose monolith materials have interconnected open porous structures with very high porosity, making them attractive structures for use as support materials in heterogeneous catalysis applications. In this study, we developed a highly efficient and reusable continuous-flow reactor for Cr(VI) remediation by combining the advantageous features of cellulose monoliths with suitable reinforcement techniques. We fabricated a porous monolithic cellulose/graphene oxide (GO) composite with a continuous three-dimensional skeletal framework using the thermally induced phase separation technique. Pd nanocrystals were synthesized in situ on the surface of the composite monolith, and then converted to porous Pd–Pt bimetallic nanocrystals through a galvanic replacement reaction. This approach eliminated the need for additional reductants and stabilizers, making the process simpler and more environmentally friendly. Under carefully optimized conditions, the cellulose/GO/Pd–Pt nanocomposite monolith exhibited outstanding performance in continuous-flow reactions for Cr(VI) reduction, achieving a maximum conversion rate of 98 %. Moreover, the nanocomposite monolith-based heterogeneous catalyst exhibited remarkable long-term stability, maintaining its catalytic activity even after extended periods of storage in the dried state. These findings highlight the potential of cellulose-based composite monoliths as versatile and robust support materials for heterogeneous catalysis.

Atomistic Modeling of Spinel Oxide Particle Shapes and Reshaping under OER Conditions

The surface configurations of the low-index facets of a set of spinel oxides are investigated using DFT+U calculations to derive surface energies and predict equilibrium nanoparticle shapes via the Wulff construction. Two very different conditions are investigated, corresponding to application either in heterogeneous catalysis or in electrocatalysis. First, the bare stoichiometric surfaces of NiFe2O4, CoFe2O4, NiCo2O4, and ZnCo2O4 spinels are studied to model their use as high-temperature oxidation catalysts. Second, focusing attention on the electrochemical oxygen evolution reaction (OER) and on the CoFe2O4 inverse spinel as the most promising OER catalyst, we generate surface configurations by adsorbing OER intermediates and, in an innovative study, we recalculate surface energies taking into account adsorption and environmental conditions, i.e., applied electrode potential and O2 pressure. We predict that under OER operating conditions, (111) facets are dominant in CoFe2O4 nanoparticle shapes, in fair agreement with microscopy measurements. Importantly, in the OER case, we predict a strong dependence of nanoparticle shape upon O2 pressure. Increasing O2 pressure increases the size of the higher-index (111) and (110) facets at the expense of the (001) more catalytically active facet, whereas the opposite occurs at low O2 pressure. These predictions should be experimentally verifiable and help define the optimal OER operative conditions.