TOF Values Research Articles

Density functional theory study of Fe(CO)5 and [Fe(CO)6]2+ complexes catalyzed the water gas shift reaction extended to the aqueous phase

The WGSR with Fe(CO)5 and [Fe(CO)6]2+ complexes were investigated using DFT in both gaseous and aqueous phases. Thermodynamic analyses indicated that the high dielectric constant of water inhibits the activity of OH− and [Fe(CO)6]2+ ions, leading to a substantial decrease in the energy released during the nucleophilic attack of OH− on Fe(CO)5 and [Fe(CO)6]2+ in the aqueous phase. In addition, a potential low energy pathway has been discovered by designing new transition states for the final step of Fe(CO)5-catalyzed WGSR: 1 → 6 → 2 → 3 → 4 → 5(a) → 5(b) → 6. According to the results of the ESM, [Fe(CO)6]2+ possesses the highest gaseous phase TOF value (1.27×10−18 s–1), both Fe(CO)5 and [Fe(CO)6]2+ have higher TOF values in the gaseous phase compared to the aqueous phase, which increases with rising temperature. Therefore, these results suggest that [Fe(CO)6]2+ serves as the advantageous catalyst involved WGSR in the aqueous phase.

PtZn/ZnO intermetallic catalyst prepared by reactive metal-support interaction for the semi-hydrogenation of 1,4-butynediol

The continuous flow selective hydrogenation of 1,4-butynediol to 2-butene-1,4-diol places high demands on the activity and stability of the catalyst. Herein, an intermetallic PtZn/ZnO catalyst is designed by using the coupling strategy of strong electrostatic adsorption and subsequent reactive metal-support interaction. The TOF value of the Pt/ZnO-H500 catalyst, including PtZn intermetallic compound with electron-rich Ptδ- active sites, reaches 1421.6 h−1, which is almost 8 times higher than that of Pt/ZnO-H200 catalyst with monometallic Pt as active sites. The activation energy of Pt/ZnO-H500 catalyst (35.5 kJ mol−1) is significantly lower than that of Pt/ZnO-H200 catalyst with 49.9 kJ mol−1. In addition, the Pt/ZnO-H500 presents high stability in the continuous flow selective hydrogenation of 1,4-butynediol to 2-butene-1,4-diol for 75 h. These results suggest the intermetallic PtZn/ZnO presents as an alternative Pt-based catalyst without modification by toxic metals or organic matter and is a promising catalyst for directional hydrogenation in fine chemical synthesis.

Efficient catalytic combustion of propane at low temperature based on ignition effect caused by amorphous atomic-layer Pt clusters

Developing highly active catalysts for hydrocarbon combustion at low temperatures is paramount but yet challenging. The crystal form of carrier and the dispersion degree of active species have great influence on propane combustion catalysts, but its internal mechanism is still unclear. In this study, the amorphous atomic-layer Pt clusters (AL) and Pt single atoms (SA) on TiO2 with various crystal phases were prepared and used as propane combustion catalysts, where the as-synthesized Pt/a-TiO2 catalyst with atomic-layer Pt clusters exhibited better catalytic performance. The Pt/a-TiO2-AL could achieve a 90 % propane conversion (T90) at 230 °C, with the highest reaction rate (39.57 × 10−7 mol g−1 s−1) and TOF value (7.72 × 10−2 s−1) at 240 °C. Kinetic studies showed that oxygen had a lower inhibitory effect on Pt/a-TiO2-AL compared to that on the Pt/p-TiO2-AL and Pt/r-TiO2-AL catalysts; simultaneously, the Pt/a-TiO2-AL catalyst presented lower apparent activation energy compared with Pt/a-TiO2-SA. DFT results showed that atomic-layer Pt clusters on TiO2 had the lower formation energy of oxygen vacancy than Pt single atoms due to the synergistic effect between adjacent exposed Pt atoms in the fully exposed atomic-layer Pt clusters, which had a higher activation capacity for oxygen and thus accelerated propane combustion through the ignition effect (rapid activation of oxygen and rapid combustion of propane). Our primary results provide a promising approach for the creation of high-efficiency Pt-based catalysts, specifically tailored for the catalytic combustion of volatile organic compounds (VOCs).

Acceptorless ambient-temperature dehydrogenation and reversible hydrogenation of N-heterocycles over single-atom Co-N-C catalysts

Reversible hydrogen storage via Liquid organic hydrogen carriers (LOHCs) is one of the key technologies for efficient utilization of hydrogen energy. How to construct efficient dehydrogenation/hydrogenation catalysts that enable low-temperature catalysis still remains challenging. Herein, we present the design and construction of single-atom Co-N-C(x) catalysts allowing significantly improved acceptorless dehydrogenation of 1,2,3,4-tetrahydroquinoline (THQ). By varying the ligand/Co ratio, the optimal Co-N-C(1.5) can achieve 97.5% dehydrogenated product and 0.82 h−1 TOF value at ambient temperature, overperforming many reported non-noble metal even noble metal catalysts. This catalyst exhibits good recyclability and substrate versatility, enabling ambient temperature dehydrogenation of various N-heterocycles and efficient reverse hydrogenation of aromatic N-heterocycles, thus achieving the loop of reversible hydrogen storage. The active catalyst centers consist of specific Co-Nx moieties and nitrogen species within the carbon support. Controlled experiments reveal that the different performance of Co-N-C(x) is determined by density of the Co-Nx, and the rate-determining step of THQ dehydrogenation is correlated with the bond cleavages in surface-bound THQ. This work offers a promising pathway for energy-efficient and cost-effective hydrogen energy utilization under practical conditions.

Interfacial Ru centers coupling with oxygen vacancies in VO-RuO2@CA hybrid aerogel boosts acidic water oxidation

The exploitation of highly active, cheap cost and durable electrocatalyst for acidic water oxidation reaction still presents a major technological and scientific challenge in proton exchange membrane water electrolysers (PEMWEs). Herein, a hybrid aerogel of oxygen defect-rich RuO2 nanoparticles coupling with carbon (VO-RuO2@CA) was designed to promote their activity and stability during acid water oxidation. The optimized VO-RuO2@CA achieves an ultralow overpotential of 207 mV at 10 mA cm−2 for acidic water oxidation and sustains a prominent long-term stability during the 30 h chronopotentiometry test in 0.5 M H2SO4. Furthermore, VO-RuO2@CA exhibits high mass activities of 186.5 mA mgRu−1 and TOF values of up to 2.07 s−1 at an overpotential of 300 mV, being 17.7 and 4.8 times higher than commercial RuO2, respectively. Theory calculations and experimental results verify that the oxygen vacancies (VO) could significantly weaken the binding energy of O* with respect to OOH* for the interfacial Ru centers between RuO2 and carbon, rendering the increase of the intrinsic water oxidation activity and resistance to over-oxidation for RuO2. This research provides a novel pathway to explore remarkably efficient 3D porous hybrid aerogel electrocatalysts toward anodic electrocatalysts applications in PEMWEs.

High-electrophilic (SiO)2Nb(OH)(=O) sites confined in silanol defects over Nb-Beta zeolite for efficient cyclic alkene epoxidation reactions

Dealuminated Beta zeolite has a large amount of silanol defects on its interface, which provides an ideal place for embedding metal species and creating metal active sites in a confined microenvironment. The confined metal sites as well as their surroundings are closely related to the reactant activation and transient state achievement. Hence, unraveling the confined metal sites is of great significance for the catalytic reaction process. Herein, niobium species were incorporated into the silanol defects over dealuminated Beta zeolite with a facile dry impregnation method, co-grinding the niobium precursor with dealuminated Beta zeolite support. The successful incorporation of niobium into the silanol defects for 30Nb-Beta zeolite was verified by DRIFT, 1H MAS NMR, UV–Vis and UV-Raman characterizations. XAS characterization and DFT calculations further disclosed that the confined Nb species existed as (SiO)2Nb(OH)(=O), containing two Si−O−Nb bonds, one Nb=O bond as well as one Nb−OH bond. The synthesized 30Nb-Beta zeolite catalyst displayed a superior cyclohexene conversion of 51.1%, cyclohexene oxide selectivity of 83.1% as well as TOF value of 188.2 h−1 ascribed to the inherent electrophilicity of Nb(V) for confined (SiO)2Nb(OH)(=O) species as well as the low oxygen transfer energy barrier for NbV−OOH species. Furthermore, the prepared 30Nb-Beta zeolite can be effectively employed to other cyclic alkene epoxidation reactions.

Highly Selective Hydrogenation of Unsaturated Aldehydes in Aqueous Phase.

Chemoselective hydrogenation of carbonyl in unsaturated aldehydes is a significant process in the chemical industry, in which the development of aqueous-phase reaction systems as a substitution to organic ones is challenging. Herein, we report Ir atomic cluster catalysts anchored onto WO3-x nanorods via a reduction treatment at various temperatures (denoted as Ir/WOx-T, T = 200, 300, 400, and 500 °C), which accelerates the chemoselective hydrogenation of carbonyl groups in aqueous solutions. The optimal catalyst Ir/WOx-300 exhibits exceptional activity (TOF value: 1313.7 min-1) and chemoselectivity toward cinnamaldehyde (CAL) hydrogenation to cinnamyl alcohol (COL) (yield: ∼98.0%) in water medium, which is, to the best of our knowledge, the highest level compared with previously reported heterogeneous catalysts in liquid-phase reaction. Ac-HAADF-STEM, XAFS, and XPS verify the formation of interface structure (Irδ+-Ov-W5+ (0 ≤ δ ≤ 4); Ov denotes oxygen vacancy) induced by metal-support interaction and the largest concentration of interfacial Ir (Irδ+) in Ir/WOx-300. In situ studies (Raman, FT-IR), isotopic labeling measurements combined with DFT calculations substantiate that the hydrogenation of the C=O group consists of two pathways: water-mediated hydrogenation (predominant) and direct hydrogenation via H2 dissociation (secondary). In the former case, W5+-Ov site accelerates the activation adsorption of H2O, while Ir0 site facilitates the H-H bond cleavage of H2 and Irδ+ promotes the CAL adsorption. H2O molecule, as the source of hydrogen species, participates directly in the hydrogenation of the carbonyl group through a hydrogen-bonded network, with a largely reduced energy barrier relative to the H2 dissociation path. This work demonstrates a green catalytic route that breaks the activity-selectivity trade-off toward the selective hydrogenation of unsaturated aldehydes, which shows great potential in heterogeneous catalysis.

Ultrathin graphitic carbon nitride interface layer regulated nickel-iron oxyhydroxide stabilized on nickel foam for efficient oxygen evolution reaction

Regulating interfacial properties of electrocatalysts to enhance the performance of oxygen evolution reaction (OER) possesses practical significance for water splitting. Herein, we report a novel strategy to form a graphitic carbon nitride (g-C3N4) interface layer on nickel foam (NF), followed by the growth of nickel-iron (oxy)hydroxides (NiFe-OH). The NiFe-OH nanosheets are tightly anchored on the g-C3N4 layer, exhibiting enhanced surface and structural characteristics. Therefore, the as-prepared NF/g-C3N4/NiFe-OH demonstrates superior OER activity with low overpotentials of 272, 342, and 476 mV at the current densities of 50, 100, and 200 mA cm−2, respectively. The reduced Tafel slope and raised TOF values of the electrode compared to NF/NiFe-OH reveal the promoting roles of g-C3N4, which provides more active sites, induces charge redistribution, and facilitates electron transfer. Furthermore, the electrode presents reliable stability and durability without observable morphological damage, alteration in the surface chemical state, and significant depletion of active components.

Overview of the catalytic properties of bulk unsupported and supported group 4–5-6 oxides

The number of active sites (Ns), surface chemistry (redox, acid, base) and TOF values were compared for bulk one component oxides, bulk mixed metal oxides and supported metal oxides. The number of active sites was based on CH3OH chemisorption. The surface chemistry was based on CH3OH-TPSR and steady state CH3OH oxidation. The specific TOF values were based on the steady state methanol oxidation activity values and Ns values. The Ns values were found to depend on the catalyst morphology (isotropic > anisotropic). The surface chemistry was found to dramatically depend on the specific metal oxide element. For supported bulk mixed metal molybdates, the surfaces were found to be dominated by segregation of MoOx and, consequently, the surface chemistry and TOFredox values were dominated by the characteristics of the bulk one component MoO3. Similarly, for the supported molybdenum oxide catalysts, the surface is dominated by the surface MoOx sites that dictate the surface chemistry and TOFredox values that reflect the surface chemistry and TOFredox of the bulk one component MoO3. Thus, it is possible to predict the surface chemistry and TOF values from knowledge of the surface composition of mixed oxides and the surface chemistry and TOF values of the bulk one component oxides. This conclusion should also hold for other types of mixed oxides such as polyoxometalates (POMs) and supported MOx/silicates and MOx/zeolite catalyst systems.

Trans‐Dioxidomolybdenum(VI) Porphyrins and their Catalytic Activity Mimicking Oxidative Bromination

AbstractSynthesis of trans‐dioxidomolybdenum(VI)‐meso‐tetraphenylporphyrin (trans‐[MoVIO2(TPP)], 1) and trans‐dioxidomolybdenum(VI)‐meso‐tetra(p‐chlorophenyl)porphyrin (trans‐[MoVIO2(TCPP)], 2) by reacting the corresponding free‐base porphyrins with Mo(CO)6 in 1,2,4‐trichlorobenzene at 190 °C has been reported. These are the second examples in the literature where molybdenum‐porphyrins have trans‐[MoVIO2] group, identified by single crystal X‐ray study. Both complexes have been utilized as functional model of haloperoxidases for the oxidative bromination of various phenol derivatives using KBr, H2O2 and HClO4 as green brominating reagents in aqueous medium. These complexes displayed very high TOF values (up to 16.2 s−1 for catalyst 2) representing excellent catalytic activity. To the best of our knowledge, this is the first report presenting trans‐dioxidomolybdenum(VI) based porphyrin as haloperoxidases mimicking oxidative bromination of various phenol derivatives.

FeCoCuMnRuB Nanobox with Dual Driving of High-Entropy and Electron-Trap Effects as the Efficient Electrocatalyst for Water Oxidation.

High-entropy borides hold potential as electrocatalysts for water oxidation. However, the synthesis of the tailored nanostructures remains a challenge due to the thermodynamic immiscibility of polymetallic components. Herein, a FeCoCuMnRuB nanobox decorated with a nanosheet array was synthesized for the first time by a "coordination-etch-reduction" method. The FeCoCuMnRuB nanobox has various structural characteristics to express the catalytic performance; meanwhile, it combines the high-entropy effect of multiple components with the electron trap effect induced by electron-deficient B, synergistically regulating its electronic structure. As a result, FeCoCuMnRuB nanobox exhibits enhanced OER activity with a low overpotential (η10 = 233 mV), high TOF value (0.0539 s-1), small Tafel slope (61 mV/dec), and a satisfactory stability for 200 h, outperforming the high-entropy alloy and low-entropy borides. This work develops a high entropy and electron-deficient B-driven strategy for motivating the catalytic performance of water oxidation, which broadens the structural diversity and category of high-entropy materials.

Synthesis, Structural and Redox Properties of Vanadyl β‐Cyanoporphyrin and its Utilization as Efficient Catalyst for Epoxidation of Olefins and Oxidative Bromination of Phenols

Abstract2‐Cyano‐5,10,15,20‐tetraphenylporphyrin [H2TPPCN] (1) having cyano group at one of β‐pyrrolic positions using nucleophilic substitution reaction of tetrabutylammonium cyanide (TBACN) on free‐base 2‐nitro‐5,10,15,20‐tetraphenylporphyrin, and its oxidovanadium(IV) complex [VIVOTPPCN] (2) were synthesized in good yields. Both the porphyrins 1 and 2 have been characterized by UV‐Vis spectroscopy, mass spectrometry, and cyclic voltammetric techniques. Single crystal X‐Ray crystallography revealed quasi‐planar geometry for 1. Compounds 1 and 2 exhibited a red shift (λmax=7–9 nm) in the Soret band relative to [MTPP], where M=2H, VIVO owing to the electron‐withdrawing effect of the cyano group at the β‐position. The first oxidation (ΔE1/2=110–140 mV) and reduction potentials (ΔE1/2=220–260 mV) of 1 and 2 are anodically shifted relative to [MTPP] where M=2H, VIVO. Porphyrin 2 has been utilized as a catalyst for two reactions: epoxidation of olefins and bromination of phenols. The bromination of phenol using KBr/H2O2/HClO4 in water resulted in 100 % conversion with a TOF value as high as 19.6 s−1 in 0.5 h. Using H2O2/NaHCO3 in a CH3CN/H2O solvent mixture at 60 °C, epoxidation was carried out, and the highest conversion rate with a turnover frequency of 1.9 s−1 was achieved in the case of cyclohexene. Catalyst 2 was recovered successfully at the end of the reaction up to 5 cycles and had good thermal stability, indicating its industrial viability and applicability. Moreover, these cyano‐functionalized porphyrins could be further utilized for the generation of molecular self‐assemblies, post‐functionalization of porphyrin core and various applications. The current findings in this work present insights for a facile approach for the synthesis of β‐cyano functionalized porphyrin 1 and further shed light on the utility of its vanadyl complex 2 as an efficient catalyst for olefin epoxidation and phenol bromination reactions.

Oxygen vacancy of Pt/CeO2 enabled low-temperature hydrogen generation from methanol and water

Aqueous-phase reforming of methanol (APRM) represents an alternative approach to achieve a sustainable hydrogen generation and safe storage. Nonetheless, the limitations imposed by the high operating temperatures of heterogeneous catalysts serve as a bottleneck for the widespread adoption of this system. Herein, we demonstrated that the Pt nanoparticles supported on porous nanorods of CeO2 with abundant oxygen vacancies (Pt/PN-CeO2) enabled the efficient H2 generation through low-temperature (<60 °C) APRM reaction. The presence of oxygen vacancies on PN-CeO2 could enhance the electronic density of supported Pt nanoparticles through strong metal-support interaction, thereby promoting the activation of methanol. Meanwhile, the oxygen vacancies could further promote the H2O activation. Consequently, the interface between Pt and PN-CeO2 enabled efficient H2 generation at 60 °C with a TOF values of 173.5 h−1 and suppress CO generation. This finding provides a promising pathway towards the low-temperature APRM reaction.

Anthraquinone-Modified Silica Nanoparticles as Heterogeneous Photocatalyst for the Oxidative Hydroxylation of Arylboronic Acids.

In this work, the synthesis and characterization of a heterogeneous photocatalyst based on spherical silica nanoparticles superficially modified with anthraquinone 2-carboxylic acid (AQ-COOH) are presented. The nanomaterial was characterized by TEM, SEM, FT-IR, diffuse reflectance, fluorescence, NMR, DLS, XRD and XPS. These analyses confirm the covalent linking of AQ-COOH with the NH2 functionality in the nanomaterial and, more importantly, the photocatalyst retains its photophysical properties once bound. The heterogeneous photocatalyst was successfully employed in the aerobic hydroxylation of arylboronic acids to phenols under sustainable reaction conditions. Phenols were obtained in high yields (up to 100 %) with low catalyst loading (3.5 mol %), reaching TOF values of 3.7 h-1 . Using 2-propanol as solvent at room temperature, the visible light photocatalysis produced H2 O2 as a key intermediate to promote the aerobic hydroxylation of arylboronic acids. The heterogeneous photocatalyst was reused at least 5 times, without modification of the nanomaterial structure and morphology. This simple heterogeneous system showed great catalytic activity under sustainable reaction conditions.

Dendritic ZSM-5 zeolites as highly active catalysts for the valorization of monoterpene epoxides.

Dendritic ZSM-5 zeolites were investigated in the isomerization of monoterpene epoxides, including limonene-1,2-epoxide (LE), α-pinene epoxide, and β-pinene epoxide, which yields high-value compounds used in fragrances, cosmetics, and pharmaceuticals. The fresh catalysts were thoroughly characterized using XRD, Ar physisorption, pyridine-FTIR, TEM, FTIR/DTBPyr, and 27Al MAS NMR. In comparison with conventional and hierarchical ZSM-5 materials, the dendritic zeolite with a crystallization time of 4 days (d-ZSM-5/4d) was the most active material, with a turnover frequency value of 4.4 min-1 for LE isomerization. Likewise, remarkable yields of dihydrocarvone (DHC, 63%, 70 °C, 2 h), campholenic aldehyde (72.4%, 70 °C, 5 min), and myrtanal (47.7%, 50 °C, 5 min) were obtained with this material that exhibited the largest mesopore/external surface area (360 m2 g-1), showing also the narrowest mesopore size distribution. A direct relationship was observed between the TOF values and the concentration of external Brønsted acid sites, showing the presence of strong steric/diffusional limitations that are greatly overcome with the dendritic zeolites. The lower reactivity of trans-LE compared to cis-LE was attributed to the larger steric hindrance of the oxygen atom. Exploration of the solvent influence revealed that the reaction rate of LE was favored by non-polar solvents, while highly selective DHC formation occurred in the solvents of medium polarity. The d-ZSM-5/4d sample was shown to be robust because catalytic activity could be completely recovered by air calcination.

Heterostructure construction of covalent organic frameworks/Ti3C2-MXene for high-efficiency electrocatalytic CO2 reduction

The in situ fabrication of imine-bonded porphyrin COFs and an MXene heterostructure can accelerate the electron transfer at interfaces, exhibiting superior electrocatalytic CO2 reduction selectivity with 97.28% TOF value (CO2-to-CO) at a bias of −0.6 V vs. RHE.

A new Schiff base 2-benzoylpyridine-based copper complex on boehmite nanoparticles as a recoverable nanocatalyst for the homoselective synthesis of 5-substituted tetrazoles.

In this study, boehmite nanoparticles (B-NPs) were prepared by a simple process and then their surface was modified by (3-aminopropyl)triethoxysilane (3-APTES). The modified B-NPs (3-APTES@B-NPs) were functionalized by 2-benzoylpyridine Schiff-base ligand toward the immobilization of the Schiff-base 2-benzoylpyridine ligand on the 3-APTES@B-NPs's surface (2BP-Schiff-base@B-NPs). Finally, copper ions were coordinated with the supported Schiff-base ligand on B-NPs toward the formation of the final catalyst (Cu-2BP-Schiff-base@B-NPs). The prepared Cu-2BP-Schiff-base@B-NPs were characterized using FT-IR spectroscopy, BET analysis, XRD, SEM, AAS, TGA, EDX and elemental mapping. Further, Cu-2BP-Schiff-base@B-NPs were applied as a homoselective and recyclable catalyst for the synthesis of a diverse range of 5-substituted tetrazoles in PEG-400 as a green solvent. The main benefits of this protocol are high homoselectivity attributes, short reaction times, high product yields and TOF values, and further addition to the catalyst ability to be recycled at least four times without significantly losing catalytic efficiency.

Strategically incorporated V in rod-like Ni-MOF as an effective catalyst for the water oxidation reaction

V-incorporated Ni–BTC, exhibits good catalytic activity and stability towards OER in 1 M KOH. Moreover, TOF value was increased 20-time, due to transfer of electron from Ni2+ to V4+ results in an increase in electronic conductivity of NiV–BTC.

Carbon nanotube heterogenization improves cobalt pyridyldiimine complex CO2 reduction activity in aqueous carbonate buffer.

We present two novel cobalt pyridyldiimine complexes functionalized with pyrene. Initially modest in homogeneous acetonitrile solution, their electrocatalytic CO2 reduction performance significantly improves upon immobilization on MWCNTs in an aqueous carbonate buffer. The complexes exhibit outstanding stability, with CO selectivity exceeding 97%, and TON and TOF values reaching up to 104 and above 1.2 s-1, respectively.

The controlled engineering of surface oxygen defects on Bi2Zr2O7 compounds for catalytic soot combustion by adjusting the preparation methods.

The quantity of surface oxygen vacancies/defects is critical to promote the reactivity of metal oxide catalysts. Therefore, for the controlled engineering of Bi2Zr2O7 with rich surface defects for soot combustion, four different methods have been adopted. Bi2Zr2O7 compounds with a defective fluorite phase but with varied surface vacancy concentrations have been successfully synthesized by various methods. The best catalyst (Bi2Zr2O7-CP) was fabricated by a facile co-precipitation method. Both O2- and O22- were the active surface sites whose number positively correlated to the number of surface oxygen vacancies and determined the activity. Moreover, a sample with more surface vacancies usually had weaker Zr-O bonds, which could be the intrinsic factor to enhance the activity. In addition, a novel and simple method has been developed to accurately titrate the absolute amount of soot reactive oxygen sites and calculate the TOF values. In conclusion, by optimizing the preparation methods, Bi2Zr2O7 catalysts with rich surface defects can be tuned, which may help in designing more applicable soot oxidation catalysts.