Tunable synthesis of carbon nanotubes methane catalytic pyrolysis by adjusting Mo incorporation in Fe/MgO.

Catalytic pyrolysis of methane for the synthesis of carbon nanotubes (CNTs) was explored using an Fe-Mo/MgO catalyst. The impact of molybdenum (Mo) addition on carbon productivity and product characteristics was investigated by (S)TEM, XRD, TGA, nitrogen adsorption, and Raman spectroscopy. The Fe/MgO catalyst exhibited a carbon productivity of 0.

Experimental determination of partial charges with electron diffraction.

Atomic partial charges, integral to understanding molecular structure, interactions and reactivity, remain an ambiguous concept lacking a precise quantum-mechanical definition. The accurate determination of atomic partial charges has far-reaching implications in fields such as chemical synthesis, applied materials science and theoretical chemistry, to name a few. They play essential parts in molecular dynamics simulations, which can act as a computational microscope for chemical processes.

Quantifying electronic and geometric effects on the activity of platinum catalysts for water-gas shift.

The unique catalytic activity of small nanoparticles can be attributed to their distinctive electronic structure and/or their ability to expose sites with a unique geometry. Quantifying and distinguishing the contributions of these effects to catalytic performance presents a challenge, given the complexity arising from multiple influencing factors and the lack of a quantitative structure-activity relationship. Here, we show that the intrinsic activity of platinum atoms at the perimeter corner sites is three orders of magnitude higher as a result of an electronic structure effect, with a threshold occurring at an average nanoparticle size of 1-1.

Elucidation of the Structure-Activity Relationship for Cu-Erionite in the Direct Conversion of Methane to Methanol: An Operando XAS Study.

The partial oxidation of methane to methanol over copper-exchanged zeolites offers a promising avenue for methane valorization. Numerous zeolites have been demonstrated to be active for the selective oxidation of methane, with the methanol yield varying significantly depending on the zeolite framework, Si/Al ratio, and copper loading. Herein, we present a comprehensive study of one of the most active Cu-erionite (Cu-ERI) zeolites with different compositions for the stepwise conversion of methane to methanol, aiming to elucidate the relationship between the methanol yield and the nature of copper species in Cu-ERI zeolites.

Mn/TiO Catalysts for the High-Yield Partial Oxidation of Methane with Molecular Oxygen to a Methyl Ester.

We synthesize and evaluate a diverse range of Mn/TiO catalysts with varying physical and redox properties for the methane-to-methyl-ester reaction using molecular oxygen in a diluted acid medium (10 wt% trifluoroacetic acid). Despite initial differences in manganese distributions, nearly all catalysts are active under the reaction conditions, and the degree of activity is partially correlated to catalyst reducibility in hydrogen temperature programmed reduction and manganese K-edge X-ray absorption spectroscopy. Under methane-limited conditions, exceptionally high product yields of up to ca.

Pathways to Aromatics in the Catalytic Pyrolysis of a Polyvinylchloride Model Compound Revealed by Operando Photoelectron Photoion Coincidence Spectroscopy.

Dechlorination channels and pathways to olefins and aromatics in the catalytic pyrolysis of the polyvinylchloride (PVC) model compound 1,3-dichlorobutane are revealed using operando photoelectron photoion coincidence (PEPICO) spectroscopy. Experimental and computational results agree that the primary pathway involves double dehydrochlorination producing 1,3-butadiene and HCl. Minor radical channels are evidenced by the detection of chloromethyl, methyl, and propargyl radicals in thermal decomposition, while chlorine radicals are absent.

Aluminum distribution and active site locations in the structures of zeolite ZSM-5 catalysts.

Zeolites have exceptional catalytic performance in oil refining and chemical synthesis that can be attributed to their well-defined porous structures that host active sites. This study pinpoints the exact locations of aluminum atoms in ZSM-5 structures-a key zeolite catalyst. Aluminum siting governs catalytic efficiency in acid and redox processes.

Low-Temperature Nonoxidative Dehydrogenation of Short-Chain Alkanes over Copper(I) Mordenite via Chemical Looping.

We report selective low-temperature non-oxidative dehydrogenation of ethane and propane to ethylene and propylene via chemical looping using copper(I)-containing mordenite as active material. Combining Cu K-edge X-ray absorption spectroscopy, in situ infrared spectroscopy (FTIR), H/D kinetic isotope effect measurements, and density functional theory calculations, we show that the active sites for the dehydrogenation reaction are copper(I) cations hosted in zeolite framework, and the rate-limiting step is activation of the first C-H bond of alkane. The stoichiometric reaction between the gas-phase alkane and copper(I) cationic site results in the formation of a stable copper(I)-alkene π-complex and gaseous hydrogen.

Tracking Active Site Formation during Oxidative Activation of Copper-Exchanged Zeolites for Methane-to-Methanol Conversion.

The evolution of active sites in Cu-zeolites for the CH-to-CHOH conversion has been investigated during oxidative treatment in O. Three samples with different frameworks but comparable Cu loadings and Si/Al ratios have been prepared to assess the influence of topology on material oxidizability and the nature of the generated Cu(II) species. Complementary spectroscopic studies highlight that isomeric Cu(II) centers hosted within different topologies are characterized by distinct formation rates.

Influence of zinc oxide nanoparticles on the carbon accumulation on silver exposed to carbon dioxide hydrogenation reaction conditions.

The strong influence of surface adsorbates on the morphology of a catalyst is exemplified by studying a silver surface with and without deposited zinc oxide nanoparticles upon exposure to reaction gases used for carbon dioxide hydrogenation. Ambient pressure X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements indicate accumulation of carbon deposits on the catalyst surface at 200 °C. While oxygen-free carbon species observed on pure silver show a strong interaction and decorate the atomic steps on the catalyst surface, this decoration is not observed for the oxygen-containing species observed on the silver surface with additional zinc oxide nanoparticles.

Reaction Intermediates Involved in the Epoxidation of Ethylene Over Silver Revealed by In Situ Photoelectron Spectroscopy.

Ethylene oxide (EO) is a crucial building block in the chemical industry, and its production via ethylene epoxidation (EPO) is a pivotal process. Silver-based catalysts are known for their high selectivity and are currently largely used in the industrial process. Extensive research over the past 20 years has assumed the oxametallacycle (OMC) as a reaction intermediate, implying that ethylene reacts with adsorbed oxygen on the surface of silver.

Evidence of Preferential Aluminum Site Loss during Reaction-Induced Dealumination.

Understanding the mechanism of steam-induced dealumination of zeolite catalysts is of high relevance for tuning their performance and stability in multiple industrial processes. A combination of Al and H-H double-quantum single-quantum magic angle spinning nuclear magnetic resonance and diffuse-reflectance ultraviolet-visible spectroscopies identified a preferential dealumination of tetrahedral aluminum sites in H-ZSM-5 zeolites. Framework aluminum atoms facing channels display reactivity toward steam higher than that of those in their intersections.

Controlling the Mechanism of Nucleation and Growth Enables Synthesis of UiO-66 Metal-Organic Framework with Desired Macroscopic Properties.

By combining in situ X-ray diffraction, Zr K-edge X-ray absorption spectroscopy and H and C nuclear magnetic resonance (NMR) spectroscopy, we show that the properties of the final MOF are influenced by HO and HCl via affecting the nucleation and crystal growth at the molecular level. The nucleation implies hydrolysis of monomeric zirconium chloride complexes into zirconium-oxo species, and this process is promoted by HO and inhibited by HCl, allowing to control crystal size by adjusting HO/Zr and HCl/Zr ratios. The rate-determining step of crystal growth is represented by the condensation of monomeric and oligomeric zirconium-oxo species into clusters, or nodes, with the structure identical to that in secondary building units (SBU) of UiO-66 framework.

Quantitative localisation of titanium in the framework of titanium silicalite-1 using anomalous X-ray powder diffraction.

One of the biggest obstacles to developing better zeolite-based catalysts is the lack of methods for quantitatively locating light heteroatoms on the T-sites in zeolites. Titanium silicalite-1 (TS-1) is a Ti-bearing zeolite-type catalyst commonly used in partial oxidation reactions with HO, such as aromatic hydroxylation and olefin epoxidation. The reaction mechanism is controlled by the configuration of titanium sites replacing silicon in the zeolite framework, but these sites remain unknown, hindering a fundamental understanding of the reaction.

Catalytic pyrolysis mechanism of lignin moieties driven by aldehyde, hydroxyl, methoxy, and allyl functionalization: the role of reactive quinone methide and ketene intermediates.

The catalytic pyrolysis of guaiacol-based lignin monomers, vanillin, syringol, and eugenol over commercial HZSM-5 has been investigated using Photoelectron Photoion Coincidence (PEPICO) spectroscopy to unveil the reaction mechanism by detecting reactive intermediates, such as quinone methides and ketenes, and products. shares the decomposition mechanism with guaiacol due to prompt and efficient decarbonylation, which allows us to control this reaction leading to a phenol selectivity increase by switching to a faujasite catalyst and decreasing the Si/Al ratio. first demethylates to 3-methoxycatechol, which mainly dehydroxylates to - and -guaiacol.

Quantifying the Hydration-Dependent Dynamics of Cu Migration and Activity in Zeolite Omega for the Partial Oxidation of Methane.

Copper-exchanged zeolite omega (Cu-omega) is a potent material for the selective conversion of methane-to-methanol (MtM) via the oxygen looping approach. However, its performance exhibits substantial variation depending on the operational conditions. Under an isothermal temperature regime, Cu-omega demonstrates subdued activity below 230 °C, but experiences a remarkable increase in activity at 290 °C.

Redox and Kinetic Properties of Composition-Dependent Active Sites in Copper-Exchanged Chabazite for Direct Methane-to-Methanol Oxidation.

The CH oxidation performance of Cu-chabazite zeolites characterized by distinct Si/Al ratios and Cu loadings has been studied and the observed variations in reactivity have been correlated to the differences in the nature of the formed active centers. Plug flow reactor tests, in situ Fourier-transform infrared, and X-ray absorption spectroscopy demonstrate that a decrease in Cu loading shifts the reactivity/redox profile to higher temperatures and increases the CHOH selectivity and Cu-efficiency. In situ electron paramagnetic resonance, Raman, ultraviolet-visible, Fourier-transform infrared, and photoluminescence spectroscopies reveal that this behavior is associated with the presence of monomeric Cu active sites, including bare Cu and [CuOH] present at low Si/Al ratio and Cu loading.

Structure and Reactivity of Active Oxygen Species on Silver Surfaces for Ethylene Epoxidation.

The epoxidation of ethylene stands as one of the most important industrial catalytic reactions, and silver-based catalysts show superior activity and selectivity. Oxygen is activated on the surface of silver during the reaction and exerts a substantial impact on product selectivity. Notably, the oxygen species residing in the topmost atomic layers profoundly influence the reactivity of a catalyst.

Restructuring of Palladium Nanoparticles during Oxidation by Molecular Oxygen.

An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time-resolved X-ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided.

The Enigma of Methanol Synthesis by Cu/ZnO/AlO-Based Catalysts.

The activity and durability of the Cu/ZnO/AlO (CZA) catalyst formulation for methanol synthesis from CO/CO/H feeds far exceed the sum of its individual components. As such, this ternary catalytic system is a prime example of synergy in catalysis, one that has been employed for the large scale commercial production of methanol since its inception in the mid 1960s with precious little alteration to its original formulation. Methanol is a key building block of the chemical industry.

Water structures on acidic zeolites and their roles in catalysis.

The local reaction environment of catalytic active sites can be manipulated to modify the kinetics and thermodynamic properties of heterogeneous catalysis. Because of the unique physical-chemical nature of water, heterogeneously catalyzed reactions involving specific interactions between water molecules and active sites on catalysts exhibit distinct outcomes that are different from those performed in the absence of water. Zeolitic materials are being applied with the presence of water for heterogeneous catalytic reactions in the chemical industry and our transition to sustainable energy.

Palladium Catalysts for Methane Oxidation: Old Materials, New Challenges.

ConspectusMethane complete oxidation is an important reaction that is part of the general scheme used for removing pollutants contained in emissions from internal combustion engines and, more generally, combustion processes. It has also recently attracted interest as an option for the removal of atmospheric methane in the context of negative emission technologies. Methane, a powerful greenhouse gas, can be converted to carbon dioxide and water via its complete oxidation.

Operando Photoelectron Photoion Coincidence Spectroscopy to Detect Short-lived Intermediates in Catalysis.

Understanding the reaction mechanism is critical yet challenging in heterogeneous catalysis. Reactive intermediates, e.g.