Increase In Turnover Frequency Research Articles

Plasma-assisted manipulation of vanadia nanoclusters for efficient selective catalytic reduction of NOx

Supported nanoclusters (SNCs) with distinct geometric and electronic structures have garnered significant attention in the field of heterogeneous catalysis. However, their directed synthesis remains a challenge due to limited efficient approaches. This study presents a plasma-assisted treatment strategy to achieve supported metal oxide nanoclusters from a rapid transformation of monomeric dispersed metal oxides. As a case study, oligomeric vanadia-dominated surface sites were derived from the classic supported V2O5-WO3/TiO2 (VWT) catalyst and showed nearly an order of magnitude increase in turnover frequency (TOF) value via an H2-plasma treatment for selective catalytic reduction of NO with NH3. Such oligomeric surface VOx sites were not only successfully observed and firstly distinguished from WOx and TiO2 by advanced electron microscopy, but also facilitated the generation of surface amide and nitrates intermediates that enable barrier-less steps in the SCR reaction as observed by modulation excitation spectroscopy technologies and predicted DFT calculations.

Amendment of palladium nanocubes with iron oxide nanowires for boosted formic acid electro−oxidation

To quickly move the formic acid (FA) fuel cells closer to a real commercialization, an inexpensive, efficient, and durable electrocatalyst for the direct FA electro-oxidation (FAEO) was developed. This involved a sequential modification of a glassy carbon (GC) substrate with palladium nanocubes (ca. 70 nm, nano-Pd) and iron oxide nanowires (nano-FeOx, ca. 40 nm and 150 nm in average diameter and length, respectively). The deposition sequence and loading level of nano-FeOx in the catalyst were optimized to minimize the catalyst's poisoning with CO that might probably release from a parallel dehydration of FA or from CO2 reduction. Surprisingly, the FeOx/Pd/GC catalyst exhibited a high (21.6 mA cm−2) specific activity for FAEO, which denoted ca. 7 times that of the “pristine” Pd/GC catalyst. This was synchronized with a better (up to fivefold increase in turnover frequency) “long-termed” stability that extended for 90 min of continuous electrolysis at room temperature. A successful effort was dedicated to improving more the catalyst's stability by activating the catalyst electrochemically at –0.5 V vs Ag/AgCl/KCl (sat.) in 0.2 mol L−1 NaOH. The CO stripping agreed perfectly with the impedance analysis in appending the observed enhancement in the catalytic efficiency of FAEO to a favorable electronic modulation at the Pd surface that boosted the oxidative desorption of poisoning CO species at a lower potential.

Enhanced Catalytic Performance of a Ce/V Oxo Cluster through Confinement in Mesoporous SBA-15.

To increase catalytic efficiency, mesoporous supports have been widely applied to immobilize well-defined metal oxide clusters due to their ability to stabilize highly dispersed clusters. Herein, a redox-active heterometallic Ce12V6-oxo cluster (CeV) was first presynthesized and then incorporated into mesoporous silica, SBA-15, via a straightforward impregnation method. Scanning transmission electron microscopy (STEM) and Fourier transform infrared spectroscopy (FTIR), in concert with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), verified the successful introduction of the CeV cluster inside the pore of SBA-15. The 51V magic angle spinning solid-state nuclear magnetic resonance (51V MAS NMR) spectroscopy and differential pair distribution function (dPDF) analysis confirmed the structural integrity of the CeV cluster inside the SBA-15. The composite was then benchmarked for liquid-phase oxidation of 2-chloroethyl ethyl sulfide (CEES) under mild conditions and gas-phase oxidative dehydrogenation (ODH) of propane under high temperatures (up to 550 °C). The catalytic reactivity results demonstrated 8- and 14-fold increase in turnover frequency (TOF) values of the composite (CeV@10SBA-2) than the bulk CeV cluster under the same conditions for CEES oxidation and ODH, respectively. These results highlight the improved reactivity of the catalytically active CeV cluster as attributed to the higher dispersion of the discrete cluster upon immobilization within the SBA-15 support.

Synchronized redox pairs in metal oxide/hydroxide chemical analogues for an efficient oxygen evolution reaction.

A two-step electrodeposition of CoMn2O4 and its chemical analogue CoMn(OH)x directly over Ni-foam yields an excellent water oxidation overpotential of 260 mA cm-2 with a Tafel slope of 29 mV dec-1 and a four-fold increase in turnover frequency. The enhanced efficacy of the composite catalyst is realized through synchronized redox pairs and superior carrier transport.

Ethylene hydrogenation on supported Pd nanoparticles: Influence of support on catalyst activity and deactivation

Palladium based systems are used extensively in a wide range of catalytic hydrogenation reactions. Here, electronic modifications of two amorphous SiO2 thin films are used to influence the properties of supported Pd nanoparticles (≈1 nm). Negative charging of the metal particles leads to destabilization of both, surface and subsurface hydrogen species, whereas positive charging leads to stabilization of both hydrogen species as evidenced by temperature-programmed desorption (TPD) measurements. The impact of this finding for general hydrogenation reactions is illustrated for ethylene hydrogenation at 300 K using a pulsed molecular beam technique, where an increase in turnover frequency (TOF) over one order of magnitude is observed for positively charged particles, compared to negative ones. The difference in hydrogenation TOF can be rationalized by two reasons: (i) increased hydrogen coverage on the metal surface due to stabilization and (ii) reduction in activation barrier for hydrogenation, both caused by metal-support interactions. In addition, the active phase of the metal particles during catalysis is proposed to be influenced by two mechanisms, formation of a carbide phase and a dehydrogenated carbonaceous overlayer on the surface.

Synergizing aliovalent doping and interface in heterostructured NiV nitride@oxyhydroxide core-shell nanosheet arrays enables efficient oxygen evolution

Abstract An earth-abundant and highly efficient oxygen evolution reaction (OER) electrocatalyst has long been the holy grail in the entire energy conversion chain. Despite the considerable efforts in advancing non-precious-metal candidates by multiscale structural engineering, an adequate structural integration remains a significant challenge in achieving an efficient OER, largely bottlenecked by a low population of active sites and limited synergistic effect. Herein, we propose a synergistic strategy of effectively combining aliovalent doping and interface in the NiV nitride@oxyhydroxide (NiVN@OOH) heterostructured nanosheet arrays, successfully developed by in-situ electrochemical surface reconfiguration (ESR) from the core-shell nanostructured Ni3N@Ni3VN aiming for enabling OER kinetics. The thus-optimized NiVN@OOH with abundant core-shell interfaces, vertically aligned nanosheet arrays and purposely-chosen V-doping, demonstrates superior OER activity with an ultralow overpotential of 233 mV at the current density of 50 mA cmgeo−2, 64-fold rise in catalytic current density at 1.47 V vs. reversible hydrogen electrode (RHE) and 37-fold increase in turn-over frequency at an overpotential of 240 mV, over those of Ni3N@OOH, together with a robust long-term stability in 1 M KOH. Our DFT calculations further reveal that the synergistic effects of the aliovalent V-doping and interface engineering have boosted the intrinsic OER activity on adjacent oxygen active sites. The discovery in the present work provides a new paradigm of multiscale-controlled synergy for much enhanced electrocatalysis.

CO2 Hydrogenation to Formate by Palladium Nanoparticles Supported on N-Incorporated Periodic Mesoporous Organosilica

Development of a heterogeneous catalyst capable of selective transformation of CO2 to valuable products still remains a challenge. In this article, we account for the surfactant-directed synthesis of a new framework-incorporated nitrogen-containing periodic mesoporous organosilica nanosphere (NPMO). A thoroughly characterized N-incorporated hybrid PMO was utilized as a platform for stabilizing well-dispersed and easily accessible Pd nanoparticles (Pd-NPMO) without using any stabilizing agents or expensive dendrimers. Further, this bifunctional hybrid catalyst has been demonstrated to heterogeneously catalyze aqueous phase CO2 hydrogenation (CO2/H2 ratio 1:3) for the direct synthesis of formate under 4 MPa pressure and at 100 °C. To validate the superior performance of the Pd-NPMO catalyst, we compared the activity with Pd-SBA-15 catalysts, and the results showed a 10-fold increase in turnover frequency of 108 h–1 using Pd on NPMO which envisaged the crucial role of nitrogen sites in this catalyst to boost the CO2 valorization to formate.

SYNTHEIS OF NI IMPREGNATED HYPERCROSSLINKED POLYSTERENE FOR CATALITIC HYDROGENATION OF D-GLUCOSE

Development of efficient catalysts is in focus of modern chemical technology for production of fine chemicals. D-sorbitol is widely used as sweetener, food additive and fuel compound is typically produced by D-glucose hydrogenation over Ni-Reney catalyst. However, this catalyst is characterized by not sufficient stability and selectivity to D-sorbitol formation. The described study is devoted to synthesis of nickel containing hypercrosslinked polystyrene for D-glucose hydrogenation process. Hypercrosslinked polystyrene is one of the representatives of crosslinked polymers with rigid matrix applicable for metal nanoparticles synthesis. However, hypercrosslinked polystyrene is characterized by high surface hydrophobicity that prevent impregnation of high amount of active metal over it surface. To overcome this disadvantage hypercrosslinked polystyrene was modified by hydrogen peroxide, chlorine and ammonia. A series of nickel based catalyst theoretically containing 25wt.% of nickel were synthesized. Modification of hypercrosslinked polystyrene results in appropriate increase in surface concentration of active metal. Synthesized Ni based catalysts were characterized by comparable Ni loading varying from 20.1 wt.% to 23.2 wt.%. Some increase in Ni loading can be subscribed to presence of surface-active groups in a case of modified hypercrosslinked polystyrene application. Mesopores surface area decrease from 126 m2/g to 69 m2/g in case of catalysts on modified hypercrosslinked polystyrene. Catalytic hydrogenation of D-glucose over synthesized Ni-containing catalysts showed considerable increase in turnover frequency (TOF) for all samples compare to commonly used Reney type nickel. Increase in TOF can be subscribed to appropriate increase of active metal surface concentration. Beside, some increase in catalyst selectivity to sorbitol took place. Considerable improvement of selectivity to sorbitol can be explained by increasing of desired D-glucose hydrogenation reaction rate, while reaction rate of D-glucose isomerization process to D-fructose remain the same. The achieved TOF for most active catalyst was found to be 0.27 s-1 and catalysts selectivity to D-sorbitol 98%.

Orthogonal Supramolecular Assembly Triggered by Inclusion and Exclusion Interactions with Cucurbit[7]uril for Photocatalytic H2 Evolution.

The fabrication of efficient and convenient photocatalytic H2 evolution systems is a fascinating research topic in the field of solar energy conversion. A ternary self-assembled photocatalytic H2 evolution system was fabricated through supramolecular host-guest chemistry. The system consisted of the H2 evolution catalyst [Co(dmgH)2 (4-ppy)2 ]NO3 (1; dmgH2 =dimethylglyoxime, 4-ppy=4-phenylpyridine) and the photosensitizer Eosin Y (EY) assembled with the macrocyclic compound cucurbit[7]uril (CB[7]) to form the 1@CB[7]/EY complex through inclusion and exclusion interactions, respectively. The synchronous self-assembly drives an orthogonal arrangement of the 1@CB[7]/EY system. The inclusion complex 1@CB[7] was successfully characterized by 1 H NMR spectroscopy and single-crystal XRD. The exclusion process of CB[7] with EY was identified by NMR titration and the optimized geometry of the exclusion structure was determined by DFT calculations. The use of CB[7] resulted in a 6-fold increase in turnover number, a 3-fold increase in turnover frequency, and a 3-fold extension of lifetime for photocatalytic H2 evolution as compared with the system in the absence of CB[7]. The improvement of the light-driven H2 evolution activity was ascribed to the ability of CB[7] to link the photosensitizer and catalyst.

Understanding Structure-Property Relationships of MoO3-Promoted Rh Catalysts for Syngas Conversion to Alcohols.

Rh-based catalysts have shown promise for the direct conversion of syngas to higher oxygenates. Although improvements in higher oxygenate yield have been achieved by combining Rh with metal oxide promoters, details of the structure of the promoted catalyst and the role of the promoter in enhancing catalytic performance are not well understood. In this work, we show that MoO3-promoted Rh nanoparticles form a novel catalyst structure in which Mo substitutes into the Rh surface, leading to both a 66-fold increase in turnover frequency and an enhancement in oxygenate yield. By applying a combination of atomically controlled synthesis, in situ characterization, and theoretical calculations, we gain an understanding of the promoter-Rh interactions that govern catalytic performance for MoO3-promoted Rh. We use atomic layer deposition to modify Rh nanoparticles with monolayer-precise amounts of MoO3, with a high degree of control over the structure of the catalyst. Through in situ X-ray absorption spectroscopy, we find that the atomic structure of the catalytic surface under reaction conditions consists of Mo-OH species substituted into the surface of the Rh nanoparticles. Using density functional theory calculations, we identify two roles of MoO3: first, the presence of Mo-OH in the catalyst surface enhances CO dissociation and also stabilizes a methanol synthesis pathway not present in the unpromoted catalyst; and second, hydrogen spillover from Mo-OH sites to adsorbed species on the Rh surface enhances hydrogenation rates of reaction intermediates.

Nature of Synergy between Brønsted and Lewis Acid Sites in Sn-Beta Zeolites for Polyoxymethylene Dimethyl Ethers Synthesis.

The role of Lewis and Brønsted acid sites and their potential synergy remains ambiguous for the production of polyoxymethylene dimethyl ethers (OME), which are suitable as a Diesel substitute. Here, this synergistic effect was investigated by using a series of beta polymorph A (BEA) zeolites with various degrees of Brønsted and Lewis acidity. Lewis acidity was introduced in dealuminated zeolites by Sn grafting in dichloromethane. These sites were only active in paraformaldehyde decomposition, OME growth, and acetalization. The Brønsted acid sites arising from bridging hydroxyl groups were active for all reaction steps, and notably for trioxane ring-opening and dissociation to formaldehyde (FA), which did not occur on the Lewis acid sites. Presence of both Lewis and Brønsted acid sites led to a four-fold increase in turnover frequency and a significant decrease of byproduct formation compared with the parent zeolite during OME synthesis from dimethoxymethane and trioxane. The synergistic effect between both types of acid sites is explained by FA insertion on Lewis acid sites leading to OME growth. Interaction between tetrahedral Sn and the carbonyl group of FA resulted in an activated carbonyl bond, which was likely the initial step for insertion of FA into OME.

Stable niobia-supported nickel catalysts for the hydrogenation of carbon monoxide to hydrocarbons

Stability of metal nanoparticles under reaction conditions is crucial in many catalytic processes. Nickel-based catalysts often encounter severe particle growth in the presence of carbon monoxide due to the formation and migration of nickel carbonyl. In this research, we showed that the reduction temperature of nickel oxide supported on niobia (Nb2O5) influenced the stability of the resulting nickel catalyst during subsequent carbon monoxide hydrogenation. Low reduction temperatures resulted in high initial nickel-normalized activity towards long-chain hydrocarbons (C5+), but fast deactivation throughout the experiment. High reduction temperatures led to a shift in product distribution towards shorter hydrocarbons and a decreased initial nickel-normalized activity, while during the first hours of the experiment an increase in turnover frequency and nickel-normalized activity was observed, resulting eventually in a stable catalytic performance. Electron microscopy analysis revealed extensive particle growth after catalysis when the catalyst had been reduced at low temperatures and no significant changes in particle size when reduced at high temperatures. By use of in-situ FT-IR spectroscopy, nickel subcarbonyl species which are precursors of volatile nickel tetracarbonyl were detected on Ni/Nb2O5 after low temperature reduction and exposure to CO, but not after high temperature reduction. Hence, particle growth is explained by the formation and diffusion of nickel carbonyl and subsequent Ostwald ripening, that leads to larger nickel particles with concomitant decrease in nickel-normalized activity. The stability of the catalyst reduced at high temperature was linked to the formation of niobium suboxides and their partial coverage of the nickel particles limiting the formation of nickel carbonyl and slowing down particle growth.

Remarkable Enhancement in Hydrogenation Ability by Phosphidation of Ruthenium: Specific Surface Structure Having Unique Ru Ensembles

A series of transition metals and metal phosphides supported on silica (M/SiO2 and MxPy/SiO2, M = Pt, Pd, Ir, Rh, Ru, Ni, and Co) were prepared and tested as catalysts for hydrogenation of toluene as a hydrogen carrier. Catalytic activities were reduced greatly by phosphidation of noble metals, such as Pt, Pd, Ir, and Rh. Catalytic activity of Ru, however, showed a remarkable increase in turnover frequency—26 times higher than that of bare Ru. Ru2P/SiO2 with a much lower cost exhibited a catalytic performance comparable with those of other platinum group metals. A combination of kinetic, infrared, and computational studies revealed that (1) strong adsorption of toluene on Ru2P promoted hydrogenation and (2) a specific surface structure of Ru2P having large ensembles of electron-deficient Ru allowed the strong adsorption.

Mechanistic Role of Two-State Reactivity in a Molecular MoS2 Edge-Site Analogue for Hydrogen Evolution Electrocatalysis.

We report a first-principles quantum chemical study of the mechanistic pathways for the hydrogen evolution reaction (HER) by the molecular electrocatalyst [(PY5Me2)Mo(S2)]2+. By determining the relative thermodynamics of many possible species, we propose a mechanism fully consistent with all experimental observations. We also show the presence of two close-lying spin surfaces with the high spin state having a slightly less favorable reactivity profile than the low spin state. The energy of the high spin state is related to the ease of reduction of the S2 moiety and can be disrupted by interaction between S2 and a Lewis base. From this understanding, an explanation for the nearly 400 000-fold increase in turnover frequency on Hg drop electrode compared to glassy carbon is demonstrated. A next-generation catalyst based on the same motif has been designed to stabilize the more reactive low spin state and improve catalytic function without the need of Hg. Calculations indicate that this new species would have greatly improved HER reactivity and operate at a similar overpotential as the original system.

Propane Oxidation Over Pd/Al2O3: Kinetic and In Situ XPS Study

The series of Pd/Al2O3 supported catalysts with different sizes of palladium particles has been prepared and studied in total propane oxidation under various conditions. The preparation method provides a variation of the Pd particle sizes within the range of 3–6 nm and core–shell distribution of the palladium particles on alumina granules. It has been observed that an increase in Pd particle sizes leads to a parallel increase in turnover frequency (TOF) values in total propane oxidation. Both propane-to-oxygen ratio and temperature also affects the catalytic activity. Chemical state of palladium in the catalysts has been studied under the reaction conditions using in situ XPS. It has been shown that both the metallic and oxidized Pd species coexist on the catalyst surface during the propane oxidation reaction. Remarkably, evident linear correlation between TOF values and Pd2+/Pd0 ratios has been revealed.

Explaining the Size Dependence in Platinum‐Nanoparticle‐Catalyzed Hydrogenation Reactions

AbstractHydrogenation reactions are industrially important reactions that typically require unfavorably high H2 pressure and temperature for many functional groups. Herein we reveal surprisingly strong size‐dependent activity of Pt nanoparticles (PtNPs) in catalyzing this reaction. Based on unambiguous spectral analyses, the size effect has been rationalized by the size‐dependent d‐band electron structure of the PtNPs. This understanding enables production of a catalyst with size of 1.2 nm, which shows a sixfold increase in turnover frequency and 28‐fold increase in mass activity in the regioselective hydrogenation of quinoline, compared with PtNPs of 5.3 nm, allowing the reaction to proceed under ambient conditions with unprecedentedly high reaction rates. The size effect and the synthesis strategy developed herein may provide a general methodology in the design of metal‐nanoparticle‐based catalysts for a broad range of organic syntheses.

Explaining the Size Dependence in Platinum-Nanoparticle-Catalyzed Hydrogenation Reactions.

Hydrogenation reactions are industrially important reactions that typically require unfavorably high H2 pressure and temperature for many functional groups. Herein we reveal surprisingly strong size-dependent activity of Pt nanoparticles (PtNPs) in catalyzing this reaction. Based on unambiguous spectral analyses, the size effect has been rationalized by the size-dependent d-band electron structure of the PtNPs. This understanding enables production of a catalyst with size of 1.2 nm, which shows a sixfold increase in turnover frequency and 28-fold increase in mass activity in the regioselective hydrogenation of quinoline, compared with PtNPs of 5.3 nm, allowing the reaction to proceed under ambient conditions with unprecedentedly high reaction rates. The size effect and the synthesis strategy developed herein may provide a general methodology in the design of metal-nanoparticle-based catalysts for a broad range of organic syntheses.

High Performance Fe Porphyrin/Ionic Liquid Co-catalyst for Electrochemical CO2 Reduction.

The efficient and selective catalytic reduction of CO2 is a highly promising process for both of the storage of renewable energy as well as the production of valuable chemical feedstocks. In this work, we show that the addition of an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, in an aprotic electrolyte containing a proton source and FeTPP, promotes the in situ formation of the [Fe(0) TPP](2-) homogeneous catalyst at a less negative potential, resulting in lower overpotentials for the CO2 reduction (670 mV) and increased kinetics of electron transfer. This co-catalysis exhibits high Faradaic efficiency for CO production (93 %) and turnover number (2 740 000 after 4 hour electrolysis), with a four-fold increase in turnover frequency (TOF) when compared with the standard system without the ionic liquid.

Selective hydrogenation of furfuryl alcohol to tetrahydrofurfuryl alcohol over Ni/γ-Al2O3 catalysts

A series of nickel-based catalysts (with <5 nm Ni particle size) with γ-alumina as a support (x wt% Ni/γ-Al2O3, x represents the Ni loading amount) were synthesized by the impregnation method, which was successfully applied for the catalytic hydrogenation of furfuryl alcohol to tetrahydrofurfuryl alcohol. The effects of reaction time, reaction temperature, nickel loading amount, solvent, and hydrogen pressure on conversion of furfural alcohol as well as selectivity for tetrahydrofurfuryl alcohol were investigated systematically. The conversion of furfural alcohol over 15 wt% Ni/γ-Al2O3 was up to 99.8 % with a selectivity of 99.5 % toward tetrahydrofurfuryl alcohol, when the reaction was carried out at 353 K with an initial H2 pressure of 4.0 MPa and reaction time of 2 h. In addition, there was an increase of turnover frequency (TOF) value with the decrease of Ni particle size. The features of the Ni/γ-Al2O3 catalysts were investigated by characterization of XRD, TPR, BET, and SEM.

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of RuII polypyridyl complexes of the structural design [RuII (R-tpy)(NN)(CH3 CN)]2+ (R-tpy=2,2':6',2''-terpyridine (R=H) or 4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine (R=tBu); NN=2,2'-bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO2 to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes' reactivities. Whereas electron-donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [RuII (tpy)(6-mbpy)(CH3 CN)]2+ (trans-[3]2+ ; 6-mbpy=6-methyl-2,2'-bipyridine) and [RuII (tBu-tpy)(6-mbpy)(CH3 CN)]2+ (trans-[4]2+ ), in which the methyl group of the 6-mbpy ligand is trans to the CH3 CN ligand, show electrocatalytic CO2 reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer-chemical reaction-electron transfer), and is a direct result of steric interactions that facilitate CH3 CN ligand dissociation, CO2 coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.