Active Catalyst Species Research Articles

Alkylamine-Grafted Organic Semiconductors with Plasma-Induced Defects as Electron Promoters of CO-Resistant Pd-Based Nanoparticles for Efficient Light-Driven On-Demand H2 Generation

Tailoring the adsorption characteristics of catalytic substrates and the electronic behavior of catalytically active species in heterogeneous catalysts is beneficial for regulating their catalytic performance. Herein, we report a highly efficient room-temperature photocatalytic formic acid (HCOOH) dehydrogenation system, where plasma-induced carbon nitrides (CNs) grafted by aminosilanes are used as electron promoters of catalytically active AuPdM (M = Ni and Co) nanoparticles (NPs). The various characterizations verify that the plasma-induced CNs have rich nitrogen defects and the efficient separation of photogenerated charge carriers. The visible-light-driven synergism of alkylamine and defective CNs with tunable band structures in the catalysts accounts for their enhanced HCOOH adsorption and the enriched electron density of metal NPs. This indicates that the catalysts have 100% H2 selectivity and higher activities under visible light irradiation than in the dark. The optimal CO-resistant AuPdNi catalyst possesses 66.9% of apparent quantum yield at 420 nm and exhibits the record room-temperature H2 generation activity with the total turnover frequencies of 1075 and 1278 h–1 in the absence and presence of scavenger of photogenerated holes, respectively.

(Invited) Power-to-Value: Metal Foam Catalysts for the Electrochemical Conversion of CO2

The electrochemical conversion of CO2 (denoted as CO2RR) has attained significant interest in the catalysis research worldwide over the recent years as it provides means of transforming an environmentally harmful molecule back into products of higher value (e.g. high energy density fuels, chemical feedstock and useful commodity chemicals such as carbon monoxide). This process becomes particularly sustainable when using the surplus of electric power produced from renewable hydro, wind or solar sources (power-to-value concept). Substantial progresses have already been made during the last decade in the tailored design of novel catalyst materials needed to increase the energy efficiency of the CO2/water co-electrolysis and to guide the product distribution reaction into the desired direction. These progresses remained, however, restricted to the fundamental research whereas no technologically and economically feasible process could be established on an industrial scale so far. One key challenge left is related to the particular stability of the catalyst materials, a crucial pre-requisite for any industrial application of catalytic processes.In this contribution, concepts for the design of monometallic and bimetallic CO2RR catalysts will be discussed that are based on the additive-assisted electroplating of high-surface area metal foams. Main target products of the CO2RR addressed herein are formate (Bi foams), syngas (Ag foams; see Figure below) as valuable chemical feedstock for a subsequent non-electrochemical conversion process (Fischer-Tropsch, Rheticus process/Siemens-Evonik), and high energy density fuels (e.g. ethanol, n-propanol; bimetallic AgCu foams and co-alloed PdCu foams). Advanced operando Raman spectroscopy will be demonstrated to be highly useful for the identification of active catalyst species under CO2 electrolysis conditions. Figure 1

Active oxygen species and oxidation mechanism over Ce-doped LaMn0.8Ni0.2O3/hierarchical ZSM-5 in pentanal oxidation

Hierarchical ZSM-5 (HZ) molecular sieves based on fly ash were synthesized using a method combining water heat treatment with step-by-step calcination. The coupling catalysts between La1−xCexMn0.8Ni0.2O3 (x ≤ 0.5) perovskites and HZ were prepared through the impregnation method, which were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), N2 adsorption, X-ray photoelectron spectroscopy (XPS), NH3-temperature programmed desoprtion (NH3-TPD), H2-temperature programmed reduction (H2-TPR) and O2-TPD techniques and investigated regarding pentanal oxidation at 120−390 °C to explore the effects of Ce doping on the catalytic activity and the active oxygen species of the coupling catalysts, meanwhile, the reaction mechanism and pathway of pentanal oxidation were also studied. The results reveal that Ce substitution at La sites can change the electronic interactions between all the elements and promote the electronic transfer among La, Ce, Ni, Mn and HZ, influencing directly the physicochemical characteristics of the catalysts. Moreover, the amount and transfer ability of surface adsorbed oxygen (O2– and O–) regarded as the reactive oxygen species and the low temperature reducibility are the main influence factors in pentanal oxidation. Additionally, La0.8Ce0.2Mn0.8Ni0.2O3/HZ exhibits the best catalytic activity and deep oxidation capacity as well as a better water resistance due to its larger amount of surface adsorbed oxygen species and higher low temperature reducibility. What’s more, appropriate Ce substitution can significantly enhance the amount of O2– ions, which can distinctly enhance the catalytic activity of the catalyst, and moderate acid strength and appropriate acid amount can also facilitate the improvement of the pentanal oxidation activity. It is found that there is a synergic catalytic effect between surface acidity and redox ability of the catalyst. According to the in situ DRIFTS and GC/MS analyses, pentanal can be oxidized gradually to CO2 and H2O by the surface oxygen species with the form of adsorption in air following the Langmuir–Hinshelwood (L-H) reaction mechanism. Two reaction pathways for the pentanal oxidation process are proposed, and the conversion of the formates to carbonates may be one of the main rate-determining steps.

Active copper species of co-precipitated copper-ceria catalysts in the CO-PROX reaction: An in situ XANES and DRIFTS study

Properties of co-precipitated CuO/CeO2 catalysts with nominal content of 1–3 wt.% copper were investigated by EDS, XRD, BET, STEM, EDS map, H2-TPR, in situ DRIFTS and in situ XANES. By comparing results of DRIFTS, XANES and catalytic performance in the CO-PROX reaction, it was evidenced a relationship between the catalytic behavior and the copper oxidation state of the materials. In situ DRIFTS and XANES analyses shed light on the oxidation state of active copper species involved in the CO-PROX reaction between 120 °C and 200 °C. The investigated catalysts present a balance between Cu° and Cu+ species under PROX conditions and the Cu+ is the main species liable to adsorb CO. Furthermore, our studies indicated that the PROX reaction on CuO/CeO2 is impacted by the nature of Cu species, which in turn may vary with reaction temperature, depending on the Cu content. From these results, we propose a new mechanism of PROX reaction on CuCe catalysts, including Cu°/Cu+ and Ce3+/Ce4+ redox pairs and direct CO oxidation on Cu°.

Controllable construction of Ce‐Mn‐Ox with tunable oxygen vacancies and active species for toluene catalytic combustion

A series of Ce‐Mn‐Ox catalysts synthesized under different hydrothermal conditions were evaluated by catalytic removal of toluene. The results of characterization showed that the contents of oxygen vacancies and active species in catalysts were crucial for the catalytic oxidation process. The concentration of Ce3+, Mn3+, and adsorbed oxygen associated with structural defects in Ce‐Mn‐Ox catalysts could be controlled by hydrothermal conditions, which were considered to promote redox capacity and improve catalytic oxidation performance. In addition, suitable synthetic conditions could increase the SBET and Vp of catalysts. Among the prepared catalysts, CM‐100 showed the best catalytic performance due to the generation of more defective oxygen and active species (Ce3+, Mn3+, and surface‐adsorbed oxygen). In addition, the CM‐100 catalyst showed satisfactory water resistance and stability.

Sodium Periodate as a Selective Oxidant for Diclofenac Sodium in Alkaline Medium: A Quantum Chemical Approach

Diclofenac sodium is a well known anti-inflammatory drug. It has also been proclaimed to exhibit adverse effects on aquatic animals through sewage and waste water treatment plants. Kinetic and mechanistic studies of the novel oxidation of diclofenac sodium (DFS) by sodium periodate were discussed with an emphasis on structure and reactivity by using kinetic and computational approach. The proposed work had been studied in alkaline medium at 303 K and at a constant ionic strength of 0.60 mol.dm−3. Formation of [2-(2,6-dicloro-phynylamino)-phenyl]-methanol as the oxidation product of DFS is confirmed with the help of structure elucidation. The active species of catalyst, oxidant and oxidation products were recognized by UV and IR spectral studies. Proton inventory studies in H2O−D2O mixtures had been shown the involvement of a single exchangeable proton of OH− ion in the transition state. All quantum chemical calculations were executed at level of density functional theory (DFT) with B3LYP function using 6-31G (d,p) basis atomic set for the validation of structure, reaction and mechanism. Molecular orbital energies, nonlinear optical properties, bond length, bond angles, reactivity, electrophilic and nucleophilic regions were delineated. Influence of various reactants on rate of chemical reaction were also ascertained and elucidated spectro-photometrically. Activation parameters have been assessed using Arrhenius-Eyring plots. A suitable mechanism consistent with observed kinetic results had been implicated and rate law deduced. Copyright © 2020 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Mesoporous Silica Encapsulated Metal-Organic Frameworks for Heterogeneous Catalysis

Encapsulation of metal-organic frameworks (MOFs) with a shell of mesoporous silica can boost overall performance of MOFs, offering versatility for synthetic architecture of integrated nanocatalysts with reactor-like features. Recently, Yu et al. report a green approach to make yolk-shell nanoreactors.

The study of industrializable ionic liquid catalysts for long‐chain alkenes Friedel–Crafts alkylation

Catalysts based on different halo‐alkanes structures with durable catalytic performance were synthesized and applied to the Friedel–Crafts alkylation of long‐chain alkenes (mixed C16–24 olefins) with toluene. Surprisingly, compared with the usual industrial catalysts (~10 runs), the cyclic times of the ionic liquid (IL) catalysts reached up to 24 runs, which greatly promotes the industrialization process. Then, Lewis acids of catalysts with different precursor/AlCl3 molar ratios were investigated and a close relation was discovered between the Lewis acid and catalytic activity. In addition, a comparison of the different halo‐alkanes structures about those catalysts was made. The results showed that the [C6Et3N]Cl–AlCl3 had the strongest Lewis acid, corresponding to the highest catalytic performance. Also, the structures of precursors and the specific gravity and active site species of catalysts were investigated by Fourier transform infrared and Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Meanwhile, the various parameters (catalyst dosage, toluene/olefin molar ratio, reaction temperature and reaction time) of long‐chain alkenes alkylation with toluene were studied. Finally, under the optimized reaction conditions, the conversion and selectivity of long‐chain alkenes alkylation reached 99.92 and 32.99%, respectively.

Selective distribution and contribution of nickel based pre-catalyst in the multisite catalyst for the synthesis of desirable bimodal polyethylene

We report a selective immobilization strategy to control the catalytic performance of the target active species in hybrid multisite catalysts. The sterically bulky and electronically donated polyhedral oligomeric silsesquioxane (POSS) is introduced onto the support and offers extra immobilization sites for the target pre-catalyst. DFT and CO low-temperature adsorption facilitate to understand the distribution of active sites. The nickel pre-catalyst can interact with the POSS, endowing the scattered distribution of nickel sites. The large steric and electronic donated iron pre-catalyst exhibits the inertness to POSS, confining iron species on the POSS-free space. The selective dispersion of nickel species enhances its contribution in the synthesis of UHMWPE fraction with an exceptional activity, a much higher molecular weight and branching degree. This promoted contribution synchronously enhances the impact resistance (+27%), tensile strength (+53%), Young’s modulus (+111%) and elongation ratio (+37%) of the synthesized bimodal PE compared to those of commercial UHMWPE.

Boosting the on-demand hydrogen generation from aqueous ammonia borane by the visible-light-driven synergistic electron effect in antenna-reactor-type catalysts with plasmonic copper spheres and noble-metal-free nanoparticles

To tailor the electronic behavior of catalytically active species in heterogeneous catalysts is beneficial for regulating their catalytic performance in various reaction including hydrogen generation. Herein, we rationally synthesize a series of antenna-reactor-type catalysts with the combination of plasmonic Cu spheres (55, 130, 190, 270 and 440 nm) with catalytically active Co or Ni nanoparticles (NPs), which are used for hydrogen generation from aqueous ammonia borane (NH3BH3) at 298 K. All the catalysts show the higher activities under visible light irradiation than in the dark and the optimal Co-based catalyst has the highest room-temperature photocatalytic activity with the total turnover frequency (TOF) value of 164.8 min−1, which outperforms the values of all the reported non-noble metal catalysts. The capture of photogenerated electrons, the monochromatic light-dependent activity and hydrogen generation of different catalysts verify that there exists the visible-light-driven synergistic electron effect bewteen plasmonic Cu spheres with energetic electrons and Co NPs with excited electrons generated by interband transitions. This leads to the electron density enrichment of Co NPs and the resultantly enhanced hydrogen generation activity under visible light irradiation.

Trimesic acid assisted synthesis of cerium-manganese oxide for catalytic diesel soot elimination: Enhancement of thermal aging resistibility

In this work, trimesic acid was introduced in the preparation of cerium-manganese oxide, and its effect on thermal aging resistibility for catalytic diesel soot elimination was investigated systematically. The prepared catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption–desorption, X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR). The experimental results show that trimesic acid addition has no significant effect on the morphologies of cerium-manganese oxide catalyst, and also contribute litter to the suppression of catalyst particles sintering and textual features destroying. In addition, the crystal structure and high-temperature (800 °C) phase separation of CeO2-MnOx solid solution are not improved by the trimesic acid modification. But, noteworthily, trimesic acid assisted synthesis can obviously promote Mn4+ generation and keep up oxygen vacancy concentration and hence improve the product of highly active oxygen species (especially O2-) of CeO2-MnOx catalyst during the thermal aging process. Furthermore, trimesic acid assistance can significantly alleviate the loss of CeO2-MnOx metal-support interaction caused by thermal aging, and maintain the ability to supply active oxygen at low-temperature (approximately 200–400 °C). Finally, after thermal aging at 800 °C for 12 h, the trimesic acid assisted synthesized CeO2-MnOx catalyst shows significantly better catalytic soot oxidation activity. Thus this work reveals that besides stabilizing textual and structural properties, stabilizing and optimizing electronic structure is another feasible pathway to enhance the thermal aging resistibility of CeO2-based soot oxidation catalysts.

Identification of Active Species and Mechanisms in Non-Precious Metal Oxygen Reduction Catalysts By Poisoning and Magnetic Measurements

This talk addresses the Oxygen Reduction Reaction (ORR) utilizing Fe- and Co-based non precious metal (NPM) catalysts. There has long been uncertainty as to the active site for ORR in these materials due to the heterogeneity of the as-prepared catalyst. We have developed several methods with which to interrogate these materials. First, by using a purification scheme which removes and then remetallates the active site we substantially reduce catalyst heterogeneity. Second, we have developed a method by which to poison some of these active sites. Third, we use magnetic measurements to probe those sites affected by poisoning. We show as well that there is a minimum metal requirement for high ORR activity and that the metal must be arranged in certain critical configurations.

Mesostructured carbon-based nanocages: an advanced platform for energy chemistry

The electrochemistry in energy conversion and storage (ECS) not only relies on the active species in catalysts or energy-storage materials, but also involves mass/ion transport around the active species and electron transfer to the external circuit. To realize high-rate ECS process, new architectures for catalysts or energy-storage electrodes are required to ensure more efficient mass/charge transport. 3D porous mesostructured materials constructed by nanoscale functional units can form a continuous conductive network for electron transfer and an interconnected multiscale pores for mass/ion transport while maintaining the high surface area, showing great promise in boosting the ECS process. In this review, we summarize the recent progress on the design, construction and applications of 3D mesostructured carbon-based nanocages for ECS. The role of the hierarchical architectures to the high rate performance is discussed to highlight the merits of the mesostructured materials. The perspective on future opportunities and challenges is also outlined for deepening and extending the related studies and applications.

Constructing FeCoSe2/Co0.85Se heterostructure catalysts for efficient oxygen evolution

The slow kinetics and high cost of noble metal catalysts greatly hinder the oxygen evolution reaction (OER) in the application of large-scale oxygen production, as well as water splitting. In order to overcome this burgeoning issue, worldwide research efforts have been carried out to find alternative, and efficient electrocatalysts. Due to its cheap and abundant sources, FeCo-based materials are considered as better candidate electrocatalysts for OER. Herein, a nanocomposite catalyst consisting of FeCoSe2 nanoparticles and Co0.85Se nanosheets was synthesized via a one-pot hydrothermal method. The resultant FeCoSe2/Co0.85Se heterostructure catalysts can further used as a self-supported catalytic electrode to drive the OER, which exhibits the overpotential of 0.33 V at a current density of 10 mA cm−2 and Tafel slope of 50.8 mV dec−1. Raman spectroscopy was employed to explore the intrinsic active species of as-prepared hybrid catalyst during the OER process, and test result shows that metal selenides was acting as the precursor of the real reaction species. This work provides a possibility to develop cheap and effective OER electrocatalysts to replace the costly noble metal catalysts for OER in electrochemical devices.

Oxygen-Reconstituted Active Species of Single-Atom Cu Catalysts for Oxygen Reduction Reaction

Identification of an active center of catalysts under realistic working conditions of oxygen reduction reaction (ORR) still remains a great challenge and unclear. Herein, we synthesize the Cu single atom embedded on nitrogen-doped graphene-like matrix electrocatalyst (abbreviated as SA-Cu/NG). The results show that SA-Cu/NG possesses a higher ORR capability than 20% Pt/C at alkaline solution while the inferior activity to 20% Pt/C at acidic medium. Based on the experiment and simulation calculation, we identify the atomic structure of Cu-N2C2 in SA-Cu/NG and for the first time unravels that the oxygen-reconstituted Cu-N2C2-O structure is really the active species of alkaline ORR, while the oxygen reconstitution does not happen at acidic medium. The finding of oxygen-reconstituted active species of SA-Cu/NG at alkaline media successfully unveils the bottleneck puzzle of why the performance of ORR catalysts at alkaline solution is better than that at acidic media, which provides new physical insight into the development of new ORR catalysts.

Revealing the active species of Cu-based catalysts for heterogeneous Fenton reaction

Cu-based heterogeneous Fenton catalysts are promising for industrial wastewater treatment, nevertheless it remains a great challenge to understand the catalytic mechanism due to the difficulty in identifying the critical roles of coexisting Cu0, Cu+ and Cu2+ species in the technical catalysts. In order to identify the catalytic roles of various copper species, core-shell Cu@SiO2 catalysts reduced at different reduction temperatures were studied for rhodamine B degradation. The Cu+ dominating catalyst, Cu@SiO2-R200, exhibited the highest degradation rate with more than 95% RhB (10 ppm) removal within 10 min. Combined with multiple techniques, including in situ spectroscopy, and density functional theory calculations, it can be concluded that Cu+ acts as the primary active species with the highest efficiency in activating the H2O2 to produce ·OH and subsequently degrading the contaminants. In this work, the structure-performance relationship of Cu-based heterogeneous Fenton reaction was elucidated to inspire the rational design of high-performance heterogeneous catalysts.

Cobalt-Catalyzed Hydrogenations via Olefin Cobaltate and Hydride Intermediates

Redox noninnocent ligands are a promising tool to moderate electron transfer processes within base-metal catalysts. This report introduces bis(imino)acenaphthene (BIAN) cobaltate complexes as hydrogenation catalysts. Sterically hindered trisubstituted alkenes, imines, and quinolines underwent clean hydrogenation under mild conditions (2–10 bar, 20–80 °C) by use of the stable catalyst precursor [(DippBIAN)CoBr2] and the cocatalyst LiEt3BH. Mechanistic studies support a homogeneous catalysis pathway involving alkene and hydrido cobaltates as active catalyst species. Furthermore, considerable reaction acceleration by alkali cations and Lewis acids was observed. The dinuclear hydridocobaltate anion with bridging hydride ligands was isolated and fully characterized.

Hydrochlorination of acetylene over the Ru-based catalysts treated by plasma under different atmospheres

Ru-based catalysts modified in different atmospheres by plasma technology were prepared to catalyze the acetylene hydrochlorination reaction. The (Ru/AC)-N2 (AC = activated carbon) catalyst yielded by the plasma modification of Ru/AC catalyst in N2 atmosphere exhibits the best catalytic performance with a stable C2H2 conversion of 87.2%; a relative increase of 27.1% in C2H2 conversion was achieved compared with that of the untreated Ru/AC catalyst. The results of the analysis revealed that the modification produced a mutual effect between the generated function groups on carrier AC and the active components, which can disperse and yield more active species in the fresh catalysts. These are benefits of enhancing the activity of the catalysts. Moreover, the modification can restrain coke formation and inhibit the loss of active species in the reaction, as well as strengthen the adsorption ability of reactants on the catalysts. These are benefits of improving the catalysts’ performance.

Uncharted Pathways for CrCl3 Catalyzed Glucose Conversion in Aqueous Solution

Innovative processes for converting carbohydrates to materials and fuels form the basis for profitable bio-based economies of the future. For such innovation, approaches that provide detailed structural information would be desirable to detect uncharted chemistries of carbohydrate conversion. Among the most important value-added products accessible though conversion of biomass is 5-hydroxymethylfurfural (HMF), whose market values strongly depends on the quality of the product. Here, we use high-field in situ NMR spectroscopy to shed light on obscure competing reactions of CrCl3-catalyzed carbohydrate conversion to HMF in water. High-field NMR spectroscopy has enormous prowess in distinguishing isomeric and dehydrated products formed from glucose. Previously unidentified compounds were identified with this approach and include anhydrosugars and a variety of branched and linear mono- and disaccharides. Other identified compounds leverage the understanding of the active catalyst species. In addition to providing mechanistic insight, the approaches and findings described herein help to measure and manage carbon balances and product quality in HMF production.

Are Brønsted Acids the True Promoter of Metal-Triflate-Catalyzed Glycosylations? A Mechanistic Probe into 1,2-cis-Aminoglycoside Formation by Nickel Triflate.

Metal triflates have been utilized to catalytically facilitate numerous glycosylation reactions under mild conditions. In some methods, the metal triflate system provides stereocontrol during the glycosylation, rather than the nature of protecting groups on the substrate. Despite these advances, the true activating nature of metal triflates remains unclear. Our findings indicated that the in situ generation of trace amounts of triflic acid from metal triflates can be the active catalyst species in the glycosylation. This fact has been mentioned previously in metal triflate-catalyzed glycosylation reactions; however, a thorough study on the subject and its implications on stereoselectivity has yet to be performed. Experimental evidence from control reactions and 19F NMR spectroscopy have been obtained to confirm and quantify the triflic acid released from nickel triflate, for which it is of paramount importance in achieving a stereoselective 1,2-cis-2-amino glycosidic bond formation via a transient anomeric triflate. A putative intermediate resembling that of a glycosyl triflate has been detected using variable temperature NMR (1H and 13C) experiments. These observations, together with density functional theory calculations and a kinetic study, corroborate a mechanism involving triflic acid-catalyzed stereoselective glycosylation with N-substituted trifluoromethylbenzylideneamino protected electrophiles. Specifically, triflic acid facilitates formation of a glycosyl triflate intermediate which then undergoes isomerization from the stable α-anomer to the more reactive β-anomer. Subsequent SN2-like displacement of the reactive anomer by a nucleophile is highly favorable for the production of 1,2-cis-2-aminoglycosides. Although there is a previously reported work regarding glycosyl triflates, none of these reports have been confirmed to come from the counter ion of the metal center. Our work provides supporting evidence for the induction of a glycosyl triflate through the role of triflic acid in metal triflate-catalyzed glycosylation reactions.