Studies Of Catalytic Reactions Research Articles

In situ surface-enhanced Raman spectroscopy study of interfacial catalytic reaction of bifunctional metal nanoparticles

In situ surface-enhanced Raman spectroscopy study of interfacial catalytic reaction of bifunctional metal nanoparticles

Revealing the Mechanism of Combining Best Properties of Homogeneous and Heterogeneous Catalysis in Hybrid Pd/NHC Systems

AbstractThe formation of transient hybrid nanoscale metal species from homogeneous molecular precatalysts has been demonstrated by in situ NMR studies of catalytic reactions involving transition metals with N‐heterocyclic carbene ligands (M/NHC). These hybrid structures provide benefits of both molecular complexes and nanoparticles, enhancing the activity, selectivity, flexibility, and regulation of active species. However, they are challenging to identify experimentally due to the unsuitability of standard methods used for homogeneous or heterogeneous catalysis. Utilizing a sophisticated solid‐state NMR technique, we provide evidence for the formation of NHC‐ligated catalytically active Pd nanoparticles (PdNPs) from Pd/NHC complexes during catalysis. The coordination of NHCs via C(NHC)−Pd bonding to the metal surface was first confirmed by observing the Knight shift in the 13C NMR spectrum of the frozen reaction mixture. Computational modeling revealed that as little as few NHC ligands are sufficient for complete ligation of the surface of the formed PdNPs. Catalytic experiments combined with in situ NMR studies confirmed the significant effect of surface covalently bound NHC ligands on the catalytic properties of the PdNPs formed by decomposition of the Pd/NHC complexes. This observation shows the crucial influence of NHC ligands on the activity and stability of nanoparticulate catalytic systems.

Revealing the Mechanism of Combining Best Properties of Homogeneous and Heterogeneous Catalysis in Hybrid Pd/NHC Systems.

The formation of transient hybrid nanoscale metal species from homogeneous molecular precatalysts has been demonstrated by in situ NMR studies of catalytic reactions involving transition metals with N-heterocyclic carbene ligands (M/NHC). These hybrid structures provide benefits of both molecular complexes and nanoparticles, enhancing the activity, selectivity, flexibility, and regulation of active species. However, they are challenging to identify experimentally due to the unsuitability of standard methods used for homogeneous or heterogeneous catalysis. Utilizing a sophisticated solid-state NMR technique, we provide evidence for the formation of NHC-ligated catalytically active Pd nanoparticles (PdNPs) from Pd/NHC complexes during catalysis. The coordination of NHCs via C(NHC)-Pd bonding to the metal surface was first confirmed by observing the Knight shift in the 13C NMR spectrum of the frozen reaction mixture. Computational modeling revealed that as little as few NHC ligands are sufficient for complete ligation of the surface of the formed PdNPs. Catalytic experiments combined with in situ NMR studies confirmed the significant effect of surface covalently bound NHC ligands on the catalytic properties of the PdNPs formed by decomposition of the Pd/NHC complexes. This observation shows the crucial influence of NHC ligands on the activity and stability of nanoparticulate catalytic systems.

Heats of Adsorption in Molecules

The number of chemical reactions proceed by the phenomenon of adsorption of organic molecules the heat is liberated which is called the heat of adsorption and this can be calculated by using the mathematical chemistry and gauss Bonett Theory differential geometry. When the molecules is absorbed on the interacting surfaces of organic molecules the phenomenon is called adsorptionThe physico-chemical properties of organic molecules depends on the nature of catalytic surfaces of organic molecules. The importance of this methods used in the study of bio-chemical reaction and catalytic reaction proceed on the surfaces of organic molecules.

Cluster Preface: Modern Boron Chemistry: 60 Years of the ­Matteson Reaction

James P. Morken (left) was born in Concord, California (USA) and obtained his B.S. in chemistry in 1989 from UC Santa Barbara (USA) working with Prof. Bruce Rickborn. He obtained his Ph.D. from Boston ­College (USA) in 1995 with Prof. Amir Hoveyda and was an NSF ­Postdoctoral Fellow with Prof. Stuart Schreiber at Harvard University (USA). In 1997, he became an Assistant Professor of Chemistry at the University of North Carolina at Chapel Hill (USA). He was promoted to Associate Professor in 2002 and in 2006 joined the faculty of Boston College (USA). In 2014, he was named the Louise and James Vanderslice and Family Chaired Professor of Chemistry at Boston College. Professor Morken’s research focuses on the development of transition-metal-­catalyzed asymmetric processes and their use in complex molecule synthesis. He is particularly interested in the study and development of catalytic enantioselective reactions involving organoboron reagents. Varinder K. Aggarwal (right) studied chemistry at Cambridge University and received his Ph.D. in 1986 under the guidance of Dr. Stuart ­Warren. After postdoctoral studies under Prof. Gilbert Stork at Columbia University (1986–1988), he returned to the UK as a lecturer at Bath University. In 1991, he moved to Sheffield University, where he was promoted to professor in 1997. In 2000, he moved to Bristol University, where he holds the Chair in Synthetic Chemistry. His research interests include the development of new catalytic processes for asymmetric synthesis, the development of chiral carbenoids and their use and subsequent applications in catalysis and synthesis, and the development of new methodology and its applications in synthesis and the total synthesis of biologically important targets.

Kinetic Study of Catalytic Surface Reaction in Methane Pyrolysis in Molten Metal Bubble Column Reactors

Kinetic Study of Catalytic Surface Reaction in Methane Pyrolysis in Molten Metal Bubble Column Reactors

Theoretical Insights into Single-Atom Catalysts Supported on N-Doped Defective Graphene for Fast Reaction Redox Kinetics in Lithium-Sulfur Batteries.

Single-atom catalysts are proven to be an effective strategy for suppressing shuttle effect at the source by accelerating the redox kinetics of intermediate polysulfides in lithium-sulfur (Li-S) batteries. However, only a few 3d transition metal single-atom catalysts (Ti, Fe, Co, Ni) are currently applied for sulfur reduction/oxidation reactions (SRR/SOR), which remains challenging for screening new efficient catalysts and understanding the relationship between structure-activity of catalysts. Herein, N-doped defective graphene (NG) supported 3d, 4d, and 5d transition metals are used as single-atom catalyst models to explore electrocatalytic SRR/SOR in Li-S batteries by using density functional theory calculations. The results show that M1 /NG (M1 = Ru, Rh, Ir, Os) exhibits lower free energy change of rate-determining step and Li2 S decomposition energy barrier, which significantly enhance the SRR and SOR activity compared to other single-atom catalysts. Furthermore, the study accurately predicts the by machine learning based on various descriptors and reveals the origin of the catalyst activity by analyzing the importance of the descriptors. This work provides great significance for understanding the relationships between the structure-activity of catalysts, and manifests that the employed machine learning approach is instructive for theoretical studies of single-atom catalytic reactions.

Rhodium-Catalyzed Asymmetric Dehydrocoupling: Enantioselective Synthesis of a P-Stereogenic Diphospholane with Mistake-Correcting Diastereoselectivity

Catalytic asymmetric dehydrocoupling of secondary phosphines is a potentially valuable route to enantiomerically enriched P-stereogenic diphosphines for use as ligands or building blocks for chiral bis(phosphines). Rh(diphos*) catalyst precursors converted a rac/meso mixture of PhHP(CH2)3PHPh (1) to the C2-symmetric P-stereogenic anti-diphospholane PhP(CH2)3PPh (2) in up to a 58:42 enantiomeric ratio (er) with complete diastereoselectivity via catalyst-mediated isomerization of the intermediate syn-diphospholane 3 to 2 (mistake correction by conversion of the diastereomer meso-3 to chiral C2-2). NMR studies of catalytic reactions identified the resting state Rh((R,R)-i-Pr-DuPhos)(PhHP(CH2)3PPh) (4) and suggested a proposed mechanism for stereocontrolled P–P bond formation via oxidative addition and reductive elimination steps.

Methanol Adsorption and Reaction on TiO2(110) at Near Ambient Pressure

Although methanol on rutile TiO2(110) has been widely investigated under ultrahigh vacuum (UHV) conditions as a prototypical model, the adsorption structure and reaction of this system under or near real reaction conditions are still unclear. Surface-sensitive sum frequency generation vibrational spectroscopy (SFG-VS) has been utilized to in situ study the adsorption and reaction of methanol on TiO2(110) at near ambient pressure and 400 K. The SFG results indicate that the bridge-bonded methoxy (CH3Ob) is the dominant adsorbed species on TiO2(110) at an elevated pressure of methanol, and its density rises with increasing the methanol pressure. By evacuating the methanol gas, only CH3Ob species leave behind on the TiO2(110) surface, which is also confirmed by ultraviolet (UV) irradiation experiments. Furthermore, by a comparative SFG study of methanol adsorption on TiO2(110) as a function of methanol pressure at 250 K, we concluded that the recombinative desorption of bridging hydroxyl (OHb) groups in the form of water can facilitate the continuous dissociative adsorption of methanol on bridging oxygen vacancy (Ov) sites and the continuous generation of CH3Ob. Our results not only shed light on the adsorption and reaction of methanol on TiO2(110) at near ambient pressure but also demonstrate the studies of catalytic reaction under or near real reaction conditions at the molecular level.

Intercepting Elusive Intermediates in Cu-Mediated CO Electrochemical Reduction with Alkyl Species.

Understanding of the reaction network of Cu-catalyzed CO2/CO electroreduction reaction [CO(2)RR] remains incomplete despite intense research efforts. This is in part because the rate-determining step occurs early in the reaction network, leading to short lifetimes of subsequent surface-bound intermediates, the knowledge of which is key to selectivity control. In this work, we demonstrate that alkyl groups can effectively couple with surface intermediates in the Cu-catalyzed CORR and, for the first time, intercept elusive C1 and C2 intermediates. Combined reactivity data and in situ spectroscopic results demonstrated that surface-bound alkyl groups derived from the corresponding alkyl iodides are able to couple with adsorbed CO to form carboxylates and ketones via one and two successive nucleophilic attacks, respectively. Leveraging this new chemistry, CHx (x ≤ 3) and C2Hx (x ≤ 4) are intercepted and identified as precursors for methane and n-propanol in the CORR, respectively. Importantly, reaction pathways leading to methane and C2+ products are not intrinsically orthogonal, but their connection is mainly impeded by low coverages of energetic intermediates. This study shows that perturbing the reaction of interest by introducing a slightly interacting probe reaction network could be an effective and general strategy in mechanistic studies of catalytic reactions.

A density functional theory study of catalytic oxygen reduction reaction on Co-CoO(111)

The demand for clean energy has greatly increased given the current situation of environmental pollution and energy shortages. Fuel cells have attracted much attention because of their high energy density and low pollutant emission. However, the oxygen-reduction reaction (ORR) kinetics of the cathode are slow, and many platinum-based catalysts are needed to drive the reaction. The high platinum price and scarce reserves limit the application of fuel cells. It is therefore urgent to develop cheaper and more efficient non-noble-metal catalysts to drive the ORR. One promising approach is to develop transition-metal-oxide catalysts with high ORR catalytic performance. Here, the structure of CoO was modified, and the surface of partially reduced CoO(111) catalyst [Co-CoO(111)] was simulated via density functional theory (DFT). The catalytic ORR on two kinds of Co (Co-2, Co-3) on the Co-CoO(111) surface were simulated and free-energy diagrams were created. When Co-2 was used as the active site, the Co-CoO(111) catalyst had a low overpotential of 0.34 eV. Besides, when Co-3 site was covered with O and Co-2(*)-Co-3(O*) is used as ORR reaction site, it still had good catalytic performance of ORR. The electronic structure of Co-CoO(111) was used to explain the high level of the ORR.

Recent advances in solid-state NMR of zeolite catalysts.

Zeolites are important inorganic crystalline microporous materials with a broad range of applications in the areas of catalysis, ion exchange, and adsorption/separations. Solid-state nuclear magnetic resonance (NMR) spectroscopy has proven to be a powerful tool in the study of zeolites and relevant catalytic reactions because of its advantage in providing atomic-level insights into molecular structure and dynamic behavior. In this review, we provide a brief discussion on the recent progress in exploring framework structures, catalytically active sites and intermolecular interactions in zeolites and metal-containing ones by using various solid-state NMR methods. Advances in the mechanistic understanding of zeolite-catalysed reactions including methanol and ethanol conversions are presented as selected examples. Finally, we discuss the prospect of the solid-state NMR technique for its application in zeolites.

Role of Surface Oxygen Vacancies and Oxygen Species on CuO Nanostructured Surfaces in Model Catalytic Oxidation and Reductions: Insight into the Structure-Activity Relationship Toward the Performance.

Defect engineering, such as modification of oxygen vacancy density, has been considered as an effective approach to tailor the catalytic performance on transition-metal oxide nanostructured surfaces. The role of oxygen vacancies (OV) on the surface of the as-prepared, zinnia-shaped morphology of CuO nanostructures and their marigold forms on calcination at 800 °C has been investigated through the study of model catalytic reactions of reduction of 4-nitrophenol and aerobic oxidation of benzyl alcohol. The OV on the surfaces of different morphologies of CuO have been identified and quantified through Rietveld analysis and HRTEM, EPR, and XPS studies. The structure-activity relationships between surface oxygen vacancies (OV) and catalytic performance have been systematically investigated. The enhanced catalytic performance of the cubic CuO nanostructures compared to their as-prepared forms has been attributed to the formation of surface oxygen species on the reactive and dominant (110) surface that has low oxygen vacancy formation energy. The mechanistic role of surface oxygen species in the studied reactions has been quantitatively correlated with the catalytic activity of the different morphological forms of the CuO nanostructures.

<italic>In-situ</italic> characterization technique in electrocatalytic nitrogen reduction to ammonia

<p indent="0mm">As a basic chemical raw material, ammonia is one of the most important chemicals and noncarbon-based energy carriers. Industrial ammonia synthesis mainly relies on the Haber-Bosch process, which requires Fe (or Ru)-based catalysts, high temperature, and high pressure. High-purity H₂ gas is produced by steam reforming of methane from natural gas or coal, resulting in 3%–5% of global natural gas consumption and 1%–2% of global CO₂ emissions. In recent years, under the background of energy strategy and “double carbon”, using renewable energy to develop low energy consumption, low carbon emission, and environmentally friendly technology for ammonia synthesis has become important. Electrochemical nitrogen reduction technology has received extensive attention in the field of synthetic ammonia. This method can potentially provide a new pathway to an alternate Haber-Bosch process, using nitrogen gas and water as raw materials to realize the conversion of nitrogen to ammonia under ambient temperature and pressure. In this process, the nitrogen gas is adsorbed on the surface of a catalyst and then activated by electrons, and the activated nitrogen species are further hydrogenated with the protons supplied by water. Although this electrochemical nitrogen reduction technology has rapidly developed in recent years, the efficiency of ammonia synthesis is still very low and many problems are to be solved: (1) N₂ is a stable, nonpolar molecule with a high bond energy of <sc>941 kJ mol⁻¹,</sc> and hence, N ≡ N is difficult to activate; (2) most catalysts have slow kinetics for nitrogen adsorption and decomposition; (3) most metal atoms are more easily bonded to hydrogen atoms, leading to a competitive hydrogen evolution reaction; (4) the nitrogen reduction reaction is a multielectron, multiproton reaction process, which involves complex proton-coupled electron transfer processes and multiple reaction intermediates, so the relevant reaction mechanisms are very unclear. Importantly, ammonia synthesis catalysts with high activity and stability have not yet been developed. Therefore, to improve the selectivity and efficiency of ammonia synthesis, <italic>in-situ</italic> characterization techniques must be developed to monitor the changes in the catalyst structure and the dynamic evolution of the reaction species under the real conditions of catalytic reactions. This paper summarizes the application of various <italic>in-situ</italic> characterization techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy, particularly in the study of catalysts and catalytic reactions. More importantly, it summarizes the existing problems and challenges in combining multiple <italic>in-situ</italic> technologies and enhancing the spatial and temporal resolution of<italic> in-situ</italic> characterization technologies for the nitrogen-water system.

A near-ambient pressure flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy for operando studies of heterogeneous catalytic reactions over model catalysts.

The model catalyst approach is often used for fundamental investigations of complex heterogeneous catalysis, in which operando characterizations are critical. A flow reactor is usually adopted for gas-solid heterogeneous catalytic reactions. Herein, we report a home-designed near-ambient pressure (NAP) flow reactor coupled with polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) and an online quadrupole mass spectrometer for operando studies of heterogeneous catalytic reactions over model catalysts. A unique gas supply system is designed and manufactured to enable a stable gas inlet to the NAP flow reactor at pressures up to ∼100mbar. An ultrahigh vacuum chamber equipped with the facilities for x-ray photoelectron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy, E-beam evaporation source, and ion sputtering gun is connected to the NAP flow reactor via a gate valve for preparations and routine characterizations of model catalysts. The functions of the system are demonstrated by in situ PM-IRAS characterization of CO adsorption on Pt(111) and operando characterizations of CO oxidation on Pt(111) under NAP conditions.

Mixed Valence (μ-Phenoxido) FeII FeIII and FeIII FeIV Compounds: Electron and Proton Transfers.

Mixed-valence non-heme diiron centers are present at the active sites of a few enzymes and confer them interesting reactivities with the two ions acting in concert. Related (μ-phenoxido)diiron complexes have been developed as enzyme mimics. They exhibit very rich spectroscopic properties enabling independent monitoring of each individual ion, which proved useful for mechanistic studies of catalytic hydrolysis and oxidation reactions. In our studies of such complexes, we observed that these compounds give rise to a wide variety of electron transfers (intervalence charge transfer), proton transfers (tautomerism), coupled electron and proton transfers (H. abstraction and PCET). In this minireview, we present and analyze the main results illustrating the latter aspects.

Infrared surface spectroscopy and surface optical reflectance for operando catalyst surface characterization

Using a new sample environment, the two operando surface diagnostic techniques two-dimensional surface optical reflection (2D-SOR) and polarization-modulated infrared reflection–absorption spectroscopy (PM-IRRAS) have been combined with mass spectrometry (MS) to characterize a catalytic reaction. With both techniques integrated in a custom-built setup, we can correlate molecular information of the adsorbed surface species from PM-IRRAS with information about oxide formation on the sample from 2D-SOR. The new setup was evaluated by performing CO oxidation over a Palladium single crystal Pd(100) surface. The results reveal that the macroscopic and the microscopic molecular behavior correlate well. When the CO adsorption peak disappears in the PM-IRRAS spectrum, the formation of a well-defined ultra-thin surface oxide is observed in the 2D-SOR trend. We discuss the benefits and limitations of the two techniques as well as their potential for further studies of catalytic reactions at both gas–solid and liquid–solid interfaces.

In situ/operando soft x-ray spectroscopy of chemical interfaces in gas and liquid environments

Many energy storage and energy conversion systems are based on the complexity of material architecture, chemistry, and interfacial interactions. To understand and thus ultimately control the energy applications calls for in situ/operando characterization tools. Over the years, in situ/operando soft x-ray spectroscopy has been developed for the studies of gas molecules, molecular liquids, catalytic, and electrochemical reactions. Soft x-ray spectroscopy offers electronic structure characterization of materials in energy conversion, energy storage, and catalysis regarding functionality, complexity of material architecture, and chemical interactions. It has been shown how to use the powerful in situ/operando soft x-ray spectroscopy characterization techniques of interfacial phenomena and how to reveal the mechanism of charge transfer and chemical transformation of solid/gas and solid/liquid interfaces of energy storage and catalytic materials in a realistic environment. It has been demonstrated how to overcome the challenge that soft x-rays cannot easily peek into the high-pressure catalytic or liquid electrochemical reactions. The unique design of in situ/operando soft x-ray spectroscopy instrumentation and fabrication principle and examples of experiments are presented.

Cover Picture: A Versatile Ambient‐to‐High‐Pressure Reaction Transmission Cell for in situ/operando Infrared Spectroscopic Investigations (Chemistry ‐ Methods 7/2021)

The Front Cover shows a self-developed stainless-steel reaction cell for operando transmission FTIR studies of heterogeneous catalytic gas-phase reactions at pressures up to 30 bar and temperatures up to 300°C, which is connected to a mass spectrometer for online product analysis. The performance of the cell has been tested in three types of CO2 hydrogenation reactions (CO2 Fischer-Tropsch synthesis, CO2 methanation, CO2 conversion to dimethyl ether) on various solid catalysts implemented as self-supporting wafers. Catalytic data obtained with this cell showed the same trends as those measured in a fixed-bed plug-flow reactor, thus confirming their reliability. More information can be found in the Full Paper by Jane Weiß et al.

Bridging the Gap: From Homogeneous to Heterogeneous Parahydrogen-induced Hyperpolarization and Beyond.

Demonstration of parahydrogen-induced polarization effects in hydrogenations catalyzed by heterogeneous catalysts instead of metal complexes in a homogeneous solution has opened an entirely new dimension for parahydrogen-based research, demonstrating its applicability not only for the production of catalyst-free hyperpolarized liquids and gases and long-lived non-equilibrium spin states for potential biomedical applications, but also for addressing challenges of modern fundamental and industrial catalysis including advanced mechanistic studies of catalytic reactions and operando NMR and MRI of reactors. This essay summarizes the progress achieved in this field by highlighting the research contributed to it by our colleague and friend Kirill V. Kovtunov whose scientific career ended unexpectedly and tragically at the age of 37. His role in this research was certainly crucial, further enhanced by a vast network of his contacts and collaborations at the national and international level.