In-situ/operando techniques to identify active sites for thermochemical conversion of CO2 over heterogeneous catalysts

Boosting CO2 Conversion with Terminal Alkynes by Molecular Architecture of Graphene Oxide-Supported Ag Nanoparticles

The selective hydrogenation of alkynes to their corresponding alkenes is an important type of organic transformation, which is currently accomplished by modified palladium catalysts. Herein, we rep...

Two-in-One Catalyst Turns Carbon Dioxide in Base Chemicals

Liquid fuel synthesis via CO2 hydrogenation by coupling homogeneous and heterogeneous catalysis

Catalytic conversion of carbon dioxide (CO2) into value added chemicals

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Electrochemical CO2 reduction reaction (CO2RR), which enables CO2 conversion into value-added fuels and chemical feedstocks, provides an avenue toward carbon neutrality and renewable energy storage. In the CO2RR, heterogeneous electrocatalysts are essential to activate substrate species and interact with reaction intermediates for efficient and selective CO2 conversion. Especially, Cu-based active sites in the catalysts enable C-C coupling via *CO dimerization for multi-carbon (C2+) chemicals production. Although judiciously controlled surface of catalysts is required to control the binding of CO2RR intermediates, the active sites of the catalysts undergo severe reconstruction during electrochemical reaction. This hinders the identification of active sites because it induces structure and phase evolution of electrocatalysts during CO2RR. Therefore, reconstruction is a major bottleneck in the development of CO2RR, and it has remained as a challenge to unveil the mechanism of reconstruction.Here, I will present our recent efforts to understand the reconstruction of Cu-based metal-organic framework (MOF) during electrochemical CO2RR. MOFs have porous structures where the organic linkers are bonded with metal node in secondary building unit (SBU). We verified that oxidation tendency of metal ions in SBU is a key to determine the stability of MOFs. Pourbaix diagram helps to predict the trend of metal reduction in MOFs during CO2RR. Operando X-ray absorption spectroscopy (XAS) during CO2RR enabled to track the states of metal node in MOFs. We found that Cu ions in Cu-MOFs can be reduced to pure Cu states during CO2RR. This reveals the reconstruction of Cu-MOFs under reductive potential. To unveil the reconstruction behaviors of Cu active sites in the MOFs, we investigated the CO2RR of Cu benzene-1,3,5-tricarboxylate (Cu-BTC) and Cu benzene-1,4-dicarboxylate (Cu-BDC) MOFs. Unlike the Cu-BTC which resulted in the formation of C1 chemicals as major products, Cu-BDC produced C2 chemicals such as ethylene (C2H4) in CO2RR. We elucidate how the catalytic active sites of Cu-MOFs undergo the reconstruction and evolve during CO2RR. Also, we explain the how the reconstruction of Cu active sites in MOFs affects the steering of CO2RR pathway. In the concluding remarks, the future perspective in the catalyst reconstruction during CO2RR will be discussed.

MgFe-layered double hydroxides (LDH) were widely used as catalysts for Fischer–Tropsch synthesis to produce light olefins, in which the state of Fe-species may affect the resulting catalytic active sites. Herein, the typical MgFe-LDH was hydrothermally synthesized and the obtained MgFe-LDH was pretreated with H2 at different temperatures to reveal the effects of the state of Fe-species on the catalytic performance in Fischer–Tropsch synthesis. MgFe-LDH materials were characterized by X-ray diffraction (XRD), N2 adsorption–desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS). It was found that a MgO-FeO solid solution would be formed with the increase of the reduction temperature, which made the electrons transfer from Mg atoms to Fe atoms and strengthened the adsorption of CO. The pre-reduced treatment toward Mg-Fe-LDH enabled the FeCx active sites to be easily formed in situ during the reaction process, leading to the high conversion of CO. CO2 temperature-programmed desorption (CO2-TPD) and H2 temperature-programmed desorption (H2-TPD) analysis confirmed that the surface basicity of the catalysts was increased and the hydrogenation capacity was weakened, the secondary hydrogenation of the olefins was inhibited, and therefore as were the enhancement of O/P in the product and the high selectivity of light olefins (42.7%).

Identification of active sites is one of the main obstacles to rational design of catalysts for diverse applications. Fundamental insight into the identification of the structure of active sites and structural contributions for catalytic performance are still lacking. Recently, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) provide important tools to disclose the electronic, geometric and catalytic natures of active sites. Herein, we demonstrate the structural identification of Zn-N2 active sites with both experimental/theoretical X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Further DFT calculations reveal that the oxygen species activation on Zn-N2 active sites is significantly enhanced, which can accelerate the reduction of oxygen with high selectivity, according well with the experimental results. This work highlights the identification and investigation of Zn-N2 active sites, providing a regular principle to obtain deep insight into the nature of catalysts for various catalytic applications.

Nanostructured carbon-based materials have shown high catalytic activity in several important reactions and related chemical industrial processes, such as direct or oxidative dehydrogenation of hydrocarbons and Friedel–Crafts reactions. Nanocarbon materials exhibit significant advantages over traditional metal or metal oxide based catalysts because of their tunable acidity/basicity, electron density, and convenient recycling and reusability, and they have been shown to be potential alternatives to conventional catalysts to meet the requirements of sustainable chemistry. As a result, the field of nanocarbon catalysis has been experiencing an unparalleled development of new catalyst synthesis or their applications in new reaction systems. However, there is only slow growth of mechanistic interpretation of carbon-catalyzed reactions, which is even more urgent to advance our knowledge in related fields. Present research on the mechanism of carbon catalysis suggests that oxygen containing functional groups, especially ketonic carbonyl groups on nanocarbon, which are rich in electrons, may act as the catalytic active sites for oxidative dehydrogenation (ODH) of alkanes to corresponding alkenes. The reaction process is assumed to be similar to that for transition-metal oxide catalysts. The C H bonds of alkanes dissociate at active oxygen functional groups, and the hydrogen atoms are abstracted by Lewis base sites. After the desorption of alkene products, gas-phase O2 reacts with the abstracted hydrogen to form H2O, then the active catalytic sites are regenerated to finish one catalytic cycle. The above unspecific catalytic mechanism is only based on the qualitative characterization of carbon catalysts, while the identity of the active sites or a detailed kinetic study has never been executed with direct and convincing chemical evidence. One of the most critical problems that limits the quantitative description of the catalytic mechanism is the uncertainty of the chemical structure of nanocarbon materials. The coexistence of several kinds of surface functional groups (such as hydroxyl, carbonyl, and carboxylic acid groups) is unavoidable, as the synthesis or the following surface modification procedures of nanocarbon catalysts are normally realized by a severe physical or chemical process, such as laser irradiation and oxidation by HNO3, O2, and O3. [8] There are still lack of reliable quantification methods for the surface functional groups on nanostructured carbon materials because of their complexity in type and quantity. As a result, turnover frequency (TOF), the ultimate parameter to evaluate the intrinsic activity of heterogeneous catalysts, is also rarely reported in the case of nanocarbon catalysts, making it impossible to study the detailed reaction kinetics or compare the activity of carbon catalysts bearing different structures fairly and objectively. The quantitative surface composition analysis is also desirable for the application of nanostructured carbon as a catalyst support or electrochemical devices, which takes an even larger proportion in the field of carbon materials, as the surface structure of nanocarbon materials is essential for their physical or chemical properties (for example, affinity for a certain metal or metal ion). In view of the quantification methods of oxygen functional groups, herein we propose a chemical titration method to determine the surface concentration of three kinds of typical oxygen functional groups ( C=O, C OH, and COOH) on the surface of carbon nanotubes (CNTs) (Scheme 1). Through selective deactivation of these specific oxygen functional groups and the assessment of the catalytic activity of different CNT derivatives for ethylbenzene (EB) ODH reactions, we provided chemical evidence to show that

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Direct observation of the active sites in methane dehydroaromatization by NMR

Tuning the catalytic active sites plays a crucial role in developing low cost and highly durable oxygen electrode catalysts with precious metal‐competitive activity. In an attempt to engineer the active sites in Co3O4 spinel for oxygen electrocatalysis in alkaline electrolyte, herein, controllable synthesis of surface‐tailored Co3O4 nanocrystals including nanocube (NC), nanotruncated octahedron (NTO), and nanopolyhedron (NP) anchored on nitrogen‐doped reduced graphene oxide (N‐rGO), through a facile and template‐free hydrothermal strategy, is provided. The as‐synthesized Co3O4 NC, NTO, and NP nanostructures are predominantly enclosed by {001}, {001} + {111}, and {112} crystal planes, which expose different surface atomic configurations of Co2+ and Co3+ active sites. Electrochemical results indicate that the unusual {112} plane enclosed Co3O4 NP on rGO with abundant Co3+ sites exhibit superior bifunctional activity for oxygen reduction and evolution reactions, as well as enhanced metal–air battery performance in comparison with other counterparts. Experimental and theoretical simulation studies demonstrate that the surface atomic arrangement of Co2+/Co3+ active sites, especially the existence of octahedrally coordinated Co3+ sites, optimizes the adsorption, activation, and desorption features of oxygen species. This work paves the way to obtain highly active, durable, and cost‐effective electrocatalysts for practical clean energy devices through regulating the surface atomic configuration and catalytic active sites.

A crucial consideration for supported heterogeneous catalysts is the non-uniformity of the active sites, particularly for Supported Organometallic Catalysts (SOMCs). Standard spectroscopic techniques, such as X-ray absorption spectroscopy (XAS), reflect the nature of the most populated sites, which are often intrinsically structurally distinct from the most catalytically active sites. With computational models, often only a few representative structures are used to depict catalytic active sites on a surface, even though there are numerous observable factors of surface heterogeneity that contribute to the kinetically favorable active species. A previously reported study on the mechanism of a surface organovanadium(III) catalyst [(SiO)VIII(Mes)(THF)] for styrene hydrogenation yielded two possible mechanisms: heterolytic cleavage and redox cycling. These two mechanistic scenarios are challenging to differentiate experimentally based on the kinetic readouts of the catalyst are identical. To showcase the importance of modeling surface heterogeneity and its effect on catalytic activity, density functional theory (DFT) computational models of a series of potential active sites of [(SiO)VIII(Mes)(THF)] for the reaction pathways are applied in combination with kinetic Monte Carlo (kMC) simulations. Computed results were t then compared to the previously reported experimental kinetic study.: 1) DFT free energy reaction pathways indicated the likely active site and pathway for styrene hydrogenation; a heterolytic cleavage pathway requiring a bare tripodal vanadium site. 2) From the kMC simulations, a mixture of the different bond lengths from the support oxygen to the metal center was required to qualitatively describe the experimentally observed kinetic aspects of a supported organovanadium(III) catalyst for olefin hydrogenation. This work underscores the importance of modeling surface heterogeneity in computational catalysis.

Identification of active sites and phases in heterogeneous catalysts and the understanding of the reaction mechanism remain highly challenging. In most catalysts, the existence of a multitude of surface species, which are dynamic in relation to reaction conditions, presents a challenge of distinguishing those that are involved in the catalytic cycle from those which are spectators. The emergence of the field of single-site catalysts potentially eliminates these issues, although it can be argued that these systems remain dynamic and that multiple speciation, each a candidate for the active site, often remains a consideration. A perspective on how X-ray spectroscopy and characterization tools in general, can be used to correlate the number of active sites and the rate of their formation, in single-site and redox catalyst systems, is presented. The importance of observing proportionality between spectra features and the reaction rate, to differentiate between active sites and spectator species is discussed. Performing characterisation under catalyticly relevant conditions on structures that are demonstrably representative of actual catalysts is essential.