Active Catalyst Species Research Articles

Fe-N-coordinated graphene-like honeycomb porous carbon as an extremely effective catalyst for catalytic oxidation

In this study, Fe/N co-doped graphene-like honeycomb porous carbon (Fe-NC)was produced and used for the activating PMS to degrade TC. The Fe-NC catalyst has shown high activity in the TC degradation by activated PMS, with an efficiency of up to 94 % in 30 min. while the blank catalyst without N and Fe doping exhibited the lowest performance with 47.3 % TC degradation. Moreover, other operational factors of Fe-NC catalysts were also investigated to evaluate its catalytic activity. The burst experiments and electron paramagnetic resonance (EPR) experiments demonstrated that free radical paths (O2·-) and non-free radical paths (1O2) coexist inside the Fe-NC/PMS system. The experiment results and degradation mechanism analysis revealed that N species(Pyridine N, Graphite N,Fe-Nx) and iron species (Fe0) were possible to be the active species of catalysts. Furthermore, Fe-NC/PMS system was investigated for possible pathway of TC degradation by liquid mass spectrometry (LC-MS), and the presence of intermediates was analyzed for toxicity magnitude by ECOSAR software.

(Invited) Development of Leuco Ethyl Violet as Self-Activating Prodrug Photooxygenation Catalyst of Amyloid-β

Abnormal aggregation of amyloid-β (Aβ) and tau protein (tau), called amyloid, is characteristic of Alzheimer's disease (AD). Reducing amyloid levels in AD patients is expected to be an effective approach to AD treatment. To inhibit the hydrophobic interaction of amyloid, which is the cause of toxic aggregates, we planed to generate singlet oxygen by activating oxygen in vivo through photocatalysis, thereby introducing oxygen atoms to amyloid protein and inhibiting aggregation. In accordance with this concept, we are developing photooxygenation catalysts with a switch function that generates singlet oxygen only in the presence of amyloids. We have succeeded in reducing amyloid in AD model mouse brain non-invasively with this photo-oxygenation catalyst. However, moderate catalytic activity and the side effect of scalp damage have been problematic. This time, we identified leucoethyl violet (LEV) as a highly active, amyloid-selective, blood-brain barrier (BBB)-permeable photooxygenation catalyst. LEV is a self-activating procatalyst by external redox stimuli. Autoxidation of LEV by a hydrogen atom transfer process under light irradiation produced active catalyst species Ethyl Violet (EV) only in the presence of amyloids. LEV showed higher catalytic activity than the previously developed photooxygenation catalyst and realized oxygenation of human Aβ and tau. Taking advantage of LEV's high activity and amyloid selectivity, improved BBB permeability, and low cytotoxicity, photooxygenation of Aβ was achieved in an AD model without scalp injury. Figure 1

Catalytic methane decomposition on CNT-supported Fe-catalysts

Methane, either as natural gas or as a resource obtained from various bioprocesses (e.g., digestion, landfill) can be converted to carbon and hydrogen according to.CH4(g)→C(s)+2H2(g)ΔH298K=74.8kJ/mol.Previous research has stressed the growing importance of substituting the high-temperature Steam Methane Reforming (SMR) by a moderate temperature Catalytic Methane Decomposition (CMD). The carbon formed is moreover of nanotube nature, in high industrial demand.To avoid the use of an inert support for the active catalyst species, e.g., Al2O3 for Fe, leading to a progressive contamination of the catalyst by support debris and coking of the catalyst, the present research investigates the use of carbon nanotubes (CNTs) as Fe-support.Average CH4 conversions of 75–85% are obtained at 700 °C for a continuous operation of 40 h. The produced CNT from the methane conversion can be continuously removed from the catalyst bed by carry-over due to its bulk density difference (∼120 kg/m3) with the catalyst itself (∼1500 kg/m3). CNT properties are fully specified. No thermal regeneration of the catalyst is required.A tentative process layout and economic analysis demonstrate the scalability of the process and the very competitive production costs of H2 and CNT.

Deeply revealing the mechanism of the synergistic effect of Cu and P on alleviating CaO poisoning performance of V-W-Ti catalysts in cement kilns

CaO was the inevitable cause of V2O5-WO3-TiO2 catalyst (VWTi) poisoning in cement kilns. The significant reduction in NO conversion was due to the combination of CaO and active species of the vanadium-based catalyst. In this work, the VWTi catalysts were modified with different contents of Cu and P to improve resistance to CaO, thereby extending the service life of catalysts in cement kilns. By doping Cu and P, the Ca-VWCu3P5Ti had better catalytic activity, reduced the poisoning effect of CaO to V5+, thus protecting the redox properties and increasing the surface acidic sites of the catalyst. Meanwhile, the in-situ DRIFTs results showed that the poisoning effect of CaO was weaker on the Ca-VWCu3P5Ti than that of the Ca-VWTi. In addition, the NH3 and NO species could be effectively adsorbed and activated on the surface of the Ca-VWCu3P5Ti by E-R mechanism. In this way, the harm of CaO to the vanadium-based catalysts was effectively reduced. This work provided a promising strategy to design vanadium-based catalysts with superior resistance to CaO in NH3-SCR reaction.

Unraveling the Prominent Existence of Trace Metals in Photocatalysis: Exploring Iron Impurity Effects.

Metal impurities can complicate the identification of active catalyst species in transition metal catalysis and electrocatalysis, potentially leading to misleading findings. This study investigates the influence of metal impurities on photocatalysis. Specifically, the photocatalytic reaction of inert alkanes using chlorides without the use of an external photocatalyst was studied, achieving successful C(sp3)-H functionalization. The observations reveal that Fe and Cu impurities are challenging to avoid in a typical laboratory environment and are prominently present in normal reaction systems, and iron impurities play a dominant role in the aforementioned apparent 'metal-free' reaction. Additionally, iron exhibits significantly higher catalytic activity compared to Cu, Ce, and Ni at low metal concentrations in the photocatalytic C(sp3)-H functionalization using chlorides. Considering the widespread presence of Fe and Cu impurities in typical laboratory environments, this study serves as a reminder of their involvement in reaction processes.

Catalysis using gold containing materials

This perspective article is based on a plenary lecture delivered by the author at Europacat 2023 in Prague on 29th August in recognition of the Boudart Award of EFCATS in 2021. The article charts the journey taken by the author in the discovery and application of gold catalysts over the preceding 40 years of research. Five topics are discussed. The first three parts describe three facets of monometallic gold catalysts. (1) CO oxidation and the hierarchy of activity of gold species supported on a reducible oxide Fe2O3; (2) acetylene hydrochlorination for the manufacture of vinyl chloride monomer using highly dispersed gold cations supported on carbon; (3) methane oxidation using molecular oxygen with gold nanoparticles dispersed on the zeolite H-ZSM-5. These first three parts emphasise that the active species of a gold catalyst is dependent on the reaction being catalysed and it is not a one size fits all for this catalysis. The final two parts concern gold palladium bimetallic catalysts. (4) alcohol oxidation and the new Cooperative Redox Enhancement effect (CORE); and (5) the use of in situ generation of hydrogen peroxide to enable new oxidative cascade processes. These examples demonstrate the rich tapestry that catalysis by gold can provide to the catalysis community globally, both from an academic and an industrial perspective as well as the societal impact that has been achieved by the commercialisation of gold catalysts.

Cinchona-Based Hydrogen-Bond Donor Organocatalyst Metal Complexes: Asymmetric Catalysis and Structure Determination.

In this study, we describe the synthesis of cinchona (thio)squaramide and a novel cinchona thiourea organocatalyst. These catalysts were employed in pharmaceutically relevant catalytic asymmetric reactions, such as Michael, Friedel-Crafts, and A3 coupling reactions, in combination with Ag(I), Cu(II), and Ni(II) salts. We identified several organocatalyst-metal salt combinations that led to a significant increase in both yield and enantioselectivity. To gain insight into the active catalyst species, we prepared organocatalyst-metal complexes and characterized them using HRMS, NMR spectroscopy, and quantum chemical calculations (B3LYP-D4/def2-TZVP), which allowed us to establish a structure-activity relationship.

Modulation of active metal species in MOF-derived catalysts for efficient NO reduction by CO

MOF-derived porous catalysts achieve high catalytic activity for converting NO and CO to N2 and CO2 (100% selectivity).

Two‐Dimensional BiOBr Nanosheets Coupled with FeOOH Quantum Dots as Composite Photo‐Fenton Catalysts for Organic Pollutant Degradation under Visible Light

AbstractFeOOH quantum dots (QDs) were successfully anchored on 2D BiOBr nanosheets as composite photo‐Fenton catalysts by a simple precipitation and impregnation method. Using Rhodamine B (RhB) as an organic dye model, the photo‐Fenton catalytic activity of the composite catalyst was significantly higher than that of BiOBr and FeOOH. Among them, 1.8 % FeOOH/BiOBr has the highest photo‐Fenton catalytic activity. The main reason is that the interface effect between FeOOH and BiOBr can enable photogenerated electrons to rapidly transfer from the surface of BiOBr to FeOOH to participate in the Fe3+/Fe2+ cycle. Meanwhile, the quantum size of FeOOH increases the density of active sites on the surface of the composite catalyst. The influence of external factors on the catalytic activity of the catalyst was also verified. Finally, the electron transfer direction and main active species (h+, ⋅O2−) of the composite catalyst were discussed by electrochemical analysis and radical trapping experiments, and a reasonable catalytic degradation mechanism was proposed.

Modeling the Drying Process of Porous Catalysts: Impact of the Pore Size Distribution.

The distribution of catalytically active species in heterogeneous porous catalysts strongly influences their performance and durability in industrial reactors. A drying model for investigating this redistribution was developed and implemented using the finite volume method. This model embeds an analytical approach regarding the permeability and capillary pressure from arbitrary pore size distributions. Subsequently, a set of varying pore size distributions are investigated, and their impact on the species redistribution during drying is quantified. It was found that small amounts of large pores speed up the drying process and reduce internal pressure build up significantly while having a negligible impact on the final distribution of the catalytically active species. By further increasing the amount of large pores, the accumulation of species at the drying surface is facilitated.

Octahedral Zirconium Salan Catalysts for Olefin Polymerization: Substituent and Solvent Effects on Structure and Dynamics.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast (o-anthracenyl) and a slow (o-tBu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o-anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (ΔS > 0, dissociative) and the slow catalyst (ΔS < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho-substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

Modeling the drying process of porous catalysts - impact of viscosity and surface tension

The distribution of catalytically active species in heterogeneous porous catalysts strongly influences their performance and durability within industrial reactors. A drying model for investigating this redistribution is developed and subsequently the impact of non-linear couplings of liquid viscosity and surface tension on hydrodynamics and redistribution of species during the drying process investigated. The gained insights are then used to exert control of the redistribution within the presented model by solely varying external process parameters.

Mechanistic Details of CO2 Electroreduction on Ni and Ni{Cu}-YSZ Electrodes Using Operando Spectroscopy

Electroreduction of CO2 to fuels using renewable energy can significantly help in reducing emissions and dependence of fossil fuels. Electrochemical reduction of CO2 to hydrocarbon fuels (CHx) is energy inefficient owing to multistep-multielectron transfer process which posses many kinetic limitations. The selective conversion of CO2 to CO is energy efficient. CO as product can be directly used as a fuel or converted to hydrocarbon fuels by using green hydrogen via Fischer-Tropsch reactions.Well known Ni/YSZ electrode architectures have both well-established lifetimes, performance benchmarks and optimised manufacturing protocols when it comes to use as SOFC. Unfortunately, using these electrodes as CO2 reduction electrodes requires use of H2 at the inlet. The reaction proceeds through a reverse water gas shift reaction (RWGS) (CO2 + H2 à CO + H2O) in conjunction with water electrolysis. Most of the CO originates from non-electrochemical RWGS reaction. Use of pure CO2 streams at currents exceeding 240mA/cm2 leads to catastrophic electrode failure. In the literature, it is believed that transformation of Ni metal to NiOx and carbon deposition via Bouduard reaction are the causes of electrode failure in pure CO2. We have adapted this well-known Ni/YSZ electrode by impregnation of Cu into the Ni architecture. The Ni{Cu}/YSZ electrode not only does not deactivate but also shows improved performance in every aspect compared to the pure Ni/YSZ electrode.We have developed a unique setup for operando Raman Spectroscopy and online mass spectroscopy which can be used to study electrode reactions under both steady state and transient conditions. Using this setup, we have shown that Ni-YSZ and Ni{Cu}/YSZ electrodes go through an oxide mediated mechanism of CO2 reduction. Metal oxides such as NiOx and Ni{Cu}Ox are the active catalyst species and not the metals. Upon applying strongly reducing conditions (currents > 240mA/cm2), NiOx reduces to Ni metal which can no longer catalyse the reaction, whereas the oxide on Ni{Cu}Ox is much stronger and does not reduce under even the most reducing of conditions (reducing currents >480mA/cm2). Such an electrode remains active for reducing pure CO2. Supporting studies using SEM, TEM, XPS, operando EIS, TPR and DFT modelling were also carried out. We believe that enabling CO2 reduction Ni/YSZ architecture using Cu impregnation is a game changer as all aspects of electrode manufacturing and device compatibility for Ni/YSZ have been extensively tested and market proven. This will allow for quick adaptation for this electrode in the CO2-SOEC market. Besides this, such a mechanistic study remains unique in the field of solid oxide-based research. Figure 1

Tuning of active nickel species in MOF-derived nickel catalysts for the control on acetic acid steam reforming and hydrogen production

Tuning of active nickel species in MOF-derived nickel catalysts for the control on acetic acid steam reforming and hydrogen production

A DFT study on the reaction mechanism of the gold(I)‐catalyzed cycloisomerization of alkynylhydroxyallylamides to 4‐Oxa‐6‐azatricyclo[3.3.0.02,8]octane and 3‐Acyl‐4‐alkenylpyrrolidine

AbstractIn this study, we use density functional theory calculations to investigate the discrepancy between two experimental results of Au(I)‐catalyzed cycloisomerization reactions of alkynylhydroxyallyl tosylamide under similar reaction conditions, with the only variations being reaction temperature and time. The experimental results reported by Yeh and Chung groups, respectively, showed that 3‐acyl‐4‐alkenylpyrrolidines are produced dominantly at ambient temperature, while 4‐aza‐6‐oxatricyclo[3.3.0.02,8]octanes are produced in higher yield at elevated temperature. Using (Z)‐4‐([3‐phenylprop‐2‐yn‐1‐yl]amino)but‐2‐en‐1‐ol and [Au(PPh3)]+ as the model starting material and active catalyst species, respectively, we identified two major pathways leading to 4‐aza‐6‐oxatricyclo[3.3.0.02,8]octane (pathway I) and 3‐acyl‐4‐alkenylpyrrolidine (pathway II). The overall free energy barrier (ΔG‡max) and the energetic span (ΔG‡span) of each pathway were 38.3 and 48.4 kcal/mol for pathway I and 29.0 and 37.1 kcal/mol for pathway II. Our analysis shows that the disparate outcomes observed in the experiments by two separate groups mainly originate in the reaction kinetics, with both the overall activation barrier and energetic span being the important factor.

Evidence of a Wheland Intermediate in Carboxylate-Assisted C(sp2)−H Activation by Pd(IV) Active Catalyst Species Studied via DFT Calculations

Evidence of a Wheland intermediate in carboxylate-assisted C−H activation was found using DFT calculations when the Pd(IV) catalyst species was postulated as the active catalyst species (ACS). In order to delineate the reaction mechanism of Pd-catalyzed bisarylation of 3-alkylbenzofuran, five hypothetical catalyst species, [Pd(OAc)(PMe3)(Ph)] (I), [Pd(OAc)2] (II), [Pd(OAc)2(PMe3)] (III), [Pd(OAc)2(Ph)]+ (IV) and [Pd(OAc)2(PMe3)(Ph)]+ (V) were tested as potential ACS candidates. The catalyst species I, previously reported as an ACS in the context of ambiphilic metal−ligand assistance or a concerted metalation-deprotonation mechanism, was unsuccessful, with maximum activation barriers (ΔG‡max) for the C(sp2)−H and C(sp3)−H activations of 33.3 and 51.4 kcal/mol, respectively. The ΔG‡max values for the C(sp2)−H and C(sp3)−H activations of II−V were 23.8/28.7, 32.0/49.6, 10.9/10.9, and 36.0/36.0 kcal/mol, respectively, indicating that ACS is likely IV. This catalyst species forms an intermediate state (IV_1) before proceeding to the transition state (IV_TS1,2) for C(sp2)−H activation, in which C(2) atom of 3-methylbenzofuran has a substantial σ-character. The degree of σ-character of the IV_1 state was further evaluated quantitatively in terms of geometric parameters, partial charge distribution, and activation strain analysis. The analysis results support the existence of a Wheland intermediate, which has long been recognized as the manifestation of the electrophilic aromatic substitution mechanism yet never been identified computationally.

Non-Noble FeCrOx Bimetallic Nanoparticles for Efficient NH3 Decomposition

Ammonia has the advantages of being easy to liquefy, easy to store, and having a high hydrogen content of 17.3 wt%, which can be produced without COx through an ammonia decomposition using an appropriate catalyst. In this paper, a series of FeCr bimetallic oxide nanocatalysts with a uniform morphology and regulated composition were synthesized by the urea two-step hydrolysis method, which exhibited the high-performance decomposition of ammonia. The effects of different FeCr metal ratios on the catalyst particle size, morphology, and crystal phase were investigated. The Fe0.75Cr0.25 sample exhibited the highest catalytic activity, with an ammonia conversion of nearly 100% at 650 °C. The dual metal catalysts clearly outperformed the single metal samples in terms of their catalytic performance. Besides XRD, XPS, and SEM being used as the means of the conventional characterization, the local structural changes of the FeCr metal oxide catalysts in the catalytic ammonia decomposition were investigated by XAFS. It was determined that the Fe metal and FeNx of the bcc structure were the active species of the ammonia-decomposing catalyst. The addition of Cr successfully prevented the Fe from sintering at high temperatures, which is more favorable for the formation of stable metal nitrides, promoting the continuous decomposition of ammonia and improving the decomposition activity of the ammonia. This work reveals the internal relationship between the phase and structural changes and their catalytic activity, identifies the active catalytic phase, thus guiding the design and synthesis of catalysts for ammonia decomposition, and excavates the application value of transition-metal-based nanocomposites in industrial catalysis.

Catalyst Aggregation Matters for Immobilized Molecular CO2RR Electrocatalysts

Here, we detail how the catalytic behavior of immobilized molecular electrocatalysts for the CO2 reduction reaction (CO2RR) can be impacted by catalyst aggregation. Operando Raman spectroscopy was used to study the CO2RR mediated by a layer of cobalt phthalocyanine (CoPc) immobilized on the cathode of an electrochemical flow reactor. We demonstrate that during electrolysis, the oxidation state of CoPc in the catalyst layer is dependent upon the degree of catalyst aggregation. Our data indicate that immobilized molecular catalysts must be dispersed on conductive supports to mitigate the formation of aggregates and produce meaningful performance data. We leveraged insights from this mechanistic study to engineer an improved CO-forming immobilized molecular catalyst─cobalt octaethoxyphthalocyanine (EtO8-CoPc)─that exhibited high selectivity (FECO ≥ 95%), high partial current density (JCO ≥ 300 mA/cm2), and high durability (ΔFECO < 0.1%/h at 150 mA/cm2) in a flow cell. This work demonstrates how to accurately identify CO2RR active species of molecular catalysts using operando Raman spectroscopy and how to use this information to implement improved molecular electrocatalysts into flow cells. This work also shows that the active site of CoPc during CO2RR catalysis in a flow cell is the metal center.

Phthalimide Ligand Coordination as a Critical “Key” for Constructing Chlorine–Platinum–Nitrogen Single-Site Catalysts for Effective Acetylene Hydrochlorination

Owing to the low dispersion and deficiency of active species in Pt catalysts, Pt-complexes catalysts (Pt–Lx/SAC–IPA) were synthesized using 2-propanol (IPA) solvent via ligand coordination strategy. The IPA, which exhibits a low boiling point and weak polarity, promotes the dispersion of Pt species. Further, the introduction of phthalimide ligand (L1) modulates the electronic properties of active metals, thereby constructing the single-site-dispersed Cl–Pt–N local structure bearing Pt(II) (presumably the active center of the reaction). Concurrently, the enhanced adsorption and activation performances of the catalyst toward an HCl reactant, as well as its weakened performances toward a C2H2 reactant, improve its anticoking performance and lower the reaction energy barrier. Therefore, the most active Pt–L1/SAC–IPA catalyst achieves an outstanding performance comparable with that of the standard Au/activated carbon (AC)–aqua regia (AR) catalyst, and it is reasonable to conclude that the L1 ligand functioned as a critical “key” in the Pt-based catalytic acetylene hydrochlorination.

A review on identification, quantification, and transformation of active species in SCR by EPR spectroscopy.

Electron paramagnetic resonance (EPR) is the only technique that provides direct detection of free radicals and samples that contain unpaired electrons. Thus, EPR had an important potential application in the field of selective catalytic reduction of nitrogen oxide (SCR). For the first time, this work reviewed recent developments of EPR in charactering SCR. First, qualitative analysis focused on recognizing Cu, Fe, V, Ti, Mn, and free-radical (oxygen vacancy and superoxide radical) species. Second, quantification of the active species was obtained by a double-integral and calibration method. Third, the active species evolved because of different thermal treatments and redox-thermal processes under reductants (NH3 and NO). The coordination information of the active species in catalysts and their effects on SCR performances were concluded from mechanism viewpoints. Finally, potential perspectives were put forward for EPR developments in characterizing the SCR processes in the future. After all, EPR characterization will help to have a deep understanding of structure-activity relationship in one catalyst.