Number Of Catalytic Active Sites Research Articles

Surface and Interface Engineering in Ag2S@MoS2 Core–Shell Nanowire Heterojunctions for Enhanced Visible Photocatalytic Hydrogen Production

AbstractAn Ag2S@MoS2 core–shell nanowire heterojunction, facilely synthesized by simultaneous sulfuration of Ag nanowires (NMs) and growth of MoS2, is used as a model system to disclose how the surface and interface structures influence the photocatalytic activity. The Ag2S@MoS2 NWs with different loading amounts of MoS2 are used as photocatalysts for H2 production. It is found that the highest photocatalytic activity is realized by a moderate loading amount of MoS2. A lower loading amount of MoS2 not only reduces the interfacial contact for insufficient electron–hole separation but also decreases the number of catalytic active sites for H2 production, while a higher loading amount of MoS2 increases the light‐shielding effect of Ag2S and extends the distance of electron transfer to the catalytic active sites for H2 production. Furthermore, with the same loading amount of MoS2, Ag2S@MoS2 NWs also exhibit superior H2 production activity in comparison with pre‐Ag2S@MoS2 NWs with MoS2 grown on pre‐synthesized Ag2S nanowires. The proposed reason is that the simultaneous sulfuration of Ag nanowires and growth of MoS2 results in an intimate contact between Ag2S and MoS2 for smooth interfacial charge transfer, while the two‐step synthetic method leads to a lower quality of the MoS2‐Ag2S interface and thus poor interelectron transfer. This work highlights the importance of a rational surface and interface design of semiconductor heterojunctions for realizing high‐performance photocatalytic applications.

Efficient Charges Separation Using Advanced BiOI-Based Hollow Spheres Decorated with Palladium and Manganese Dioxide Nanoparticles

Multishell hollow structures have attracted tremendous attention due to their outstanding properties for application on photocatalysis. In this work, we demonstrated a straightforward and general method to construct BiOI-based core-double shell hollow spheres for the first time. The core-double shell hollow spheres consist of Pd particles and MnOx loaded onto the inner and outer surfaces of BiOI hollow spheres, respectively (Pd/BiOI/MnOx), and utilized them as an advanced photocatalyst for photocatalytic oxidation of formaldehyde gases and methyl blue. The hollow spheres structure could provide a large specific surface area, exposing a large number of catalytic active sites. Additionally, the Pd particles and MnOx serve the function of separating the reduction and oxidation reactive sites. The unique morphology combined with enhanced light-absorption provided the Pd/BiOI/MnOx core-double shell hollow spheres with high efficiency for the photocatalytic oxidation of formaldehyde gases and methyl blue. In si...

A Novel and Template‐Free Synthesis of Multifunctional Double‐Shelled Fe3O4‐C Nanoreactor as an Ideal Support for Confined Catalytic Reactions

AbstractA novel, facile and straightforward approach is developed to fabricate magnetic carbon nanoreactor consisting of multi‐functional double‐layered shell, large void space, tailored dimensions, and high surface area. The presented strategy involves unexampled one‐pot combination of gas‐bubble assisted Ostwald ripening method and polymerization of resorcinol‐formaldehyde. The inner Fe3O4 layer imparts the nanoreactor with superparamagnetic character and thereby, simplifies its separation from the reaction media. The outermost carbon shell protects the inner layer from harsh external environment and also provides additional sites for metal decoration. The internal cavity of the nanoreactor was easily filled with copper metal. Thus, due to the presence of large number of catalytic active sites and the advantages of hollow architecture, Cu@Fe3O4‐C exhibited excellent catalytic activity with perfect reusability for the oxidative bromination reaction. This work indicates that magnetic porous carbon nanoreactor could provide an ideal support for the immobilization of metals in the urge to develop highly efficient catalysts.

Performance of MOF‐Derived Spinel Type NixCo3‐xO4‐y Nanocages in Efficient Methanol Electro‐Oxidation

AbstractThe present study reports electrocatalytic activity towards methanol oxidation of mesoporous NixCo3‐xO4‐y fabricated through a metal‐organic framework (MOF) template route. MOF‐derived NixCo3‐xO4‐y nanocages exhibit a methanol oxidation reaction (MOR) current of approximately 200 Ag−1 at 0.55 V versus Hg‐HgO, which is around 12 times higher than that from Co3O4. Improved activity for the catalyst is ascribed to its higher number of catalytic active sites and non‐stoichiometry in the composition. The improved charge‐transfer kinetics for the MOR is evident from the electrochemical impedance measurements, where a lower charge‐transfer resistance is observed for NixCo3‐xO4‐y nanocages and the trend is consistent with the MOR results. The potential‐dependent impedance measurements suggest that direct electrochemical MOR is facilitated through a surface intermediate, where the surface intermediates (CH3OH or CO) are adsorbed on low coordinated Ni and Co sites, respectively.

Top-down synthesis strategies: Maximum noble-metal atom efficiency in catalytic materials

Top-down synthesis has been used to prepare catalytic materials with nanometer sizes, but fabricating atomically dispersed metal catalysts remains a challenge because surface single metal atoms are prone to aggregation or coalescence. A top-down strategy is used to synthesize atomically dispersed metal catalysts, based on supported Ag nanoparticles. The changes of the geometric and electronic structures of the Ag atoms during the top-down process are studied using the in situ synchrotron X-ray diffraction technique, ex situ X-ray absorption spectroscopy, and transmission electron microscopy. The experimental results, coupled with the density functional theory calculations, demonstrate that the electronic perturbation of the Ag frontier orbitals, induced by the Ag-O interactions at the perimeter of the metal-support interface, is the driving force of the top-down process. The top-down synthesis has two important functions: to increase the number of catalytic active sites and to facilitate the study of complex reaction mechanisms (e.g., formaldehyde oxidation) by developing single-site model catalysts.

Preparing Co3O4 urchin-like hollow microspheres self-supporting architecture for improved glucose biosensing performance

A new Co3O4 electrode material with self-supporting urchin-like hollow microspheres structure was fabricated via a self-assembly method. Incorporating the adsorption and entrapment, the high porosity and large surface area were favorable for the highly effective immobilization of glucose oxidases. Multiple responses, consisting of enzymatic and material catalysis, were subsequently employed and achieved on the microsphere surface. As the radially oriented nanowires array provided a vast number of catalytic active sites, which lowered the energy barrier of glucose redox and acted as a media to boost the electron transfer, the detection potential was largely decreased. Furthermore, numerous electron transport paths in a short distance were formed among the interconnected bridge-like nanowires, resulting in improved electron transfer efficiency. The prepared glucose biosensor exhibited extra high sensitivity (102.77μAmM−1cm−2), fast response (3s) and outstanding selectivity and stability at a lower applied potential (0.35V).

Determining Number of Active Sites and TOF for the High-Temperature Water Gas Shift Reaction by Iron Oxide-Based Catalysts

This study demonstrates, with C16O2/C18O2 isotope switch and H2-TPR experiments, for the first time that (i) the high-temperature water–gas shift (HT-WGS) reaction by copper–chromium-iron oxide catalysts follows a redox mechanism dominated by the surface layer, (ii) the number of catalytic active sites can be quantified by the isotopic switch, and (iii) the turnover frequency (TOF) can be determined from knowledge of the number of sites. The quantitative TOF values reveal that chromium is only a textural promoter, whereas copper is a chemical promoter.

Highly Ordered Mesoporous Fe3O4@Carbon Embedded Composite: High Catalytic Activity, Wide pH Range and Stability for Heterogeneous Electro‐Fenton

AbstractHighly ordered mesoporous Fe3O4@carbon embedded composite was prepared and grafted onto monolithic carbon aerogel to form a composite cathode (Fe3O4@OMC/CA). The Fe3O4@OMC/CA was used as heterogeneous electro‐Fenton cathode for the degradation of dimethyl phthalate (DMP). 95 % removal and 65 % mineralization of DMP was achieved within 120 min. Highly dispersed Fe3O4 nanoparticles increased the number of catalytic active sites, and the OMC layer provided accessible pathway to the catalytic active sites and reduced the resistance of mass transport. The main oxidants were ⋅OH and the surface‐controlled heterogeneous catalysis was the critical step. The composite cathode remained electrocatalytic activity after five consecutive runs.

Conductive Polymer/Graphene Supported Platinum Nanoparticles as Anode Catalysts for the Extended Power Generation of Microbial Fuel Cells

Platinum (Pt) nanoparticles anchored over reduced graphene oxide (rGO) and rGO/conductive polyaniline (PANI) composites were synthesized and exploited as anode catalysts in microbial fuel cells (MFC). PANI bridges rGO and Pt nanoparticles through the electrostatic interaction/π–π stacking force/hydrogen bonding and Pt–N bond, respectively, and increased the intrinsic stability of rGO/PANI/Pt composite. The electrocatalytic performances of rGO/PANI/Pt exhibited the better oxidation current and lower internal resistance over the prepared rGO/PANI and rGO/Pt composites as evidenced from the cyclic voltammetric and electrochemical impedance techniques, respectively. By the combined efforts of active support, high electrical conductivity, and number of catalytic active sites, the prepared rGO/PANI/Pt nanocomposite exhibited a maximum MFC power density of 2059 mW/m2 with the concrete life durability. Thus, the proposed approach has paved new dimensions in not only the preparation of rGO-supported conductive pol...

A double-shelled yolk-like structure as an ideal magnetic support of tiny gold nanoparticles for nitrophenol reduction

A facile method is proposed to fabricate yolk–shell microspheres consisting of a movable silica core, a multifunctional double-layered shell, and plenty of tiny gold nanoparticles (Au NPs, ∼2 nm) confined within the interior cavity and the mesoporous shell. The presented strategy involves the one-step coating of a Fe3O4/carbon double-layered shell, the partial etching of the silica cores and the in situ immobilization of Au NPs. The inner Fe3O4 layer of the double-layered shell endows the composites with superparamagnetism and thereby simplifies the introduction procedure of a magnetic component. The outer carbon layer not only protects the Fe3O4 layer from outside harsh conditions but also provides additional adsorption sites for Au NPs besides the interior space. The large number of catalytic active sites together with the advantages of the yolk–shell architecture make the nanocomposite a perfect catalyst for the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4 (TOF value is 17.4 min−1). Moreover, the synthesized catalyst can be easily recovered and reused for at least nine cycles due to its magnetically separable feature and good stability. These confirm that the as-prepared yolk-like nanocomposites are promising candidates for catalytic application.

ChemInform Abstract: Catalysis Science of Bulk Mixed Oxides

AbstractReview: 148 refs.

Catalysis Science of Bulk Mixed Oxides

Bulk mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, electrocatalysts for solid oxide fuel cells, and solid oxide electrolyzers for the production of hydrogen), but fundamental understanding of their structure–performance relationships have lagged in the literature. The absence of suitable surface composition and surface structural characterization techniques and methods to determine the number of catalytic active sites, with the latter needed for determination of specific reaction rates (e.g., turnover frequency (1/s)), have hampered the development of sound fundamental concepts in this area of heterogeneous catalysis. This Perspective reviews the traditional concepts that have been employed to explain catalysis by bulk mixed oxides (molybdates, vanadates, spinels, perovskites, and several other specific mixed oxide systems) and introduces a modern perspective to the fundamental surface structure–activity/selectivity relationships for bulk mixed oxide catalysts. The new insights have recently been made available by advances in surface characterization techniques (low-energy ion scattering, energy-resolved XPS, and CH3OH-IR) that allow for direct analysis of the outermost surface layer of bulk mixed metal oxide catalysts. The new findings sound a note of caution for the accepted hypotheses and concepts, and new catalysis models need to be developed that are based on the actual surface features of bulk mixed oxide catalysts.

Effect of Microstructure of Nitrogen-Doped Graphene on Oxygen Reduction Activity in Fuel Cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Effects of Lanthanum on the Structure and the Catalytic Performance of Ni/Al2O3 Catalysts for the Hydrotreating of Crude 2-Ethylhexanol

To study the effects of La modifier on Ni/γ-Al2O3 catalysts for the hydrotreating of crude 2-ethylhexanol, four samples with various La contents were prepared by the hydrothermal deposition method and characterized by ICP–AES, XRD, XPS, TPR, and H2 chemisorption. It was found that the number of catalytic active sites of the four samples with a similar Ni loading follows the order: Ni/La0/Al (13.2μmol gcat-1) < Ni/La0.5/Al (15.1μmol gcat-1) < Ni/La2.0/Al (15.8μmol gcat-1) < Ni/La1.0/Al (17.1μmol gcat-1). The results indicate that the catalyst modified by an appropriate La can obtain more active centers and higher hydrogenation performance because it possesses a higher surface Ni content and an appropriate Ni dispersion degree.

NaBr Poisoning of Au/TiO2 Catalysts: Effects on Kinetics, Poisoning Mechanism, and Estimation of the Number of Catalytic Active Sites

Sodium bromide was used to intentionally poison a commercial Au/TiO2 catalyst with the goals of understanding the nature of halide poisoning and evaluating the number and nature of the catalytic active sites. A series of eight poisoned catalysts were prepared by impregnating the parent catalyst with methanolic solutions of NaBr. Each catalyst was tested with CO oxidation catalysis under differential reactor conditions; O2 reaction orders and Arrhenius activation energies were determined for each material. All of the kinetic data, including a Michaelis–Menten analysis, indicated that the primary effect of adding NaBr was to reduce the number of catalytically active sites. Density functional theory calculations, employed to evaluate likely binding sites for NaBr, showed that NaBr binds more strongly to Au corner and edge atoms than it does to the titania support or to exposed Au face atoms. Infrared spectroscopy of adsorbed CO, along with a Temkin analysis of the data, was also used to evaluate changes to the catalyst upon NaBr deposition. These studies suggested that NaBr addition induces some subtle changes in the coverage dependent properties of CO adsorption, but that these did not substantially impact the CO coverage of the CO binding sites. The experimental and computational results are discussed in terms of possible poisoning mechanisms (site-blocking vs off-site binding and modification); the nature and number of active sites are also discussed in the context of the results.

Detailed kinetics of the Fischer–Tropsch synthesis over Co-based catalysts containing sulphur

The effect of sulphur poisoning on the activity and selectivity in the Fischer–Tropsch synthesis of a Co/γ-Al 2O 3 catalyst, investigated in a previous work under conditions relevant to industrial operation, has been described in this work by means of a complete kinetic model, able to predict at the same time the CO conversion and the product distribution (paraffins and olefins from C 1 to C 50) on the basis of the process conditions and the amount of sulphur on the catalyst. To reach this goal, a mechanistic kinetic model, developed in a previous work for an unpoisoned catalyst, has been modified introducing the dependence on sulphur of the total number of catalytic active sites (to describe the progressive decrease of the catalyst activity with increasing sulphur loading on the catalyst), of the rate of the chain growth step (to describe the decrease of the selectivity to heavy products observed with sulphur-poisoned catalysts), and of the rates of paraffins and olefins formation from the growing adsorbed intermediate (to describe the variation of the olefin/paraffin ratio in the reaction products collected with the poisoned catalysts).

Is there a relationship between the M [dbnd]O bond length (strength) of bulk mixed metal oxides and their catalytic activity?

It is widely accepted in the catalysis literature that the bulk M O bond of bulk mixed metal oxides controls catalytic activity. In the present study, for the first time, the bulk M O bond lengths (strengths) and the surface catalytic activity of bulk metal vanadates and molybdates were quantitatively compared to allow examination of this long-standing hypothesis. The bulk M O bond lengths were obtained from crystallographic studies and also determined by Raman spectroscopy. The surface catalytic activity was determined by CH 3OH-temperature programmed surface reaction (TPSR) spectroscopy and steady-state methanol oxidation. The CH 3OH-TPSR experiments provided the first-order rate constants for breaking of the C H bond for the decomposition of the surface CH 3O * intermediate to H 2CO and the number of catalytic active sites ( N s ). The corresponding steady-state methanol oxidation studies provided the equilibrium adsorption constant ( K ads ) for breaking the methanol O H bond and the specific reaction rate (TOF). The findings clearly demonstrate the lack of correlations among k rds , K ads , TOF, and the bulk M O bond length (strength). This finding is not so surprising when one considers that the adsorption step involves breaking the methanol O H bond and the rate-determining step involves breaking the surface methoxy C H bond on surface MO x sites, not bulk M O bond-breaking.

Molecular/electronic structure–surface acidity relationships of model-supported tungsten oxide catalysts

A series of model-supported WO 3 catalysts were synthesized on preformed Al 2O 3, Nb 2O 5, TiO 2, and ZrO 2 supports by impregnation of aqueous ammonium metatungstate, (NH 4) 10W 12O 41⋅5H 2O. The molecular and electronic structures of the supported tungsten oxide phases were determined with in situ Raman and UV–vis spectroscopy, respectively. The supported tungsten oxide structures are the same on all oxide supports as a function of tungsten oxide surface density (W/nm 2). Below monolayer coverage (<5 W/nm 2), both monotungstate and polytungstate surface WO x species are present under dehydrated conditions and the polytungstate/monotungstate ratio increases with increasing surface coverage. Above monolayer coverage (>5 W/nm 2), crystalline WO 3 nanoparticles are present on top of the surface WO x monolayer. Above ∼10 W/nm 2, bulk-like WO 3 crystallites become dominant. The number of catalytic active sites and surface chemistry of the supported tungsten oxide phases were chemically probed with CH 3OH dehydration to CH 3OCH 3. The specific oxide support was found to significantly affect the relative catalytic acidity of the surface WO x species (Al 2O 3 ≫ TiO 2 > Nb 2O 5 > ZrO 2) to that of the supported WO 3 nanoparticles. Consequently, no general relationship exists between the molecular/electronic structures or domain size and the specific catalytic acidity of the supported tungsten oxide phases present in the model-supported WO 3 catalysts.

Assessment of the CO 2-carbon gasification catalyzed by calcium. A transient isotopic study

Transient isotopic experiments ( i.e., step response and temperature-programmed desorption (TPD)) have been applied on calcium-carbon samples to obtain a consistent explanation of the carbon gasification reaction catalyzed by calcium. The use of 13CO 2 shows the complexity of the TPD experiments resulting after 13CO 2 contact at 573 K. The analysis of a set of TPD experiments obtained from calcium-carbon samples with different metal content and sintering degree has allowed us to assign the different features observed in the TPD spectra. In this sense, the use of this labelled gas permits us to distinguish the two steps involved in the CO 2-carbon gasification mechanism. The quantification of the 12C evolved in the TPD experiments gives a direct measure of the number of catalytic active sites (CAS). Using this CAS number, a value of the turnover for the calcium-catalyzed CO 2-carbon gasification is determined that is in good agreement with that previously obtained from TPD experiments after 12CO 2 chemisorption.

CO 2-Carbon gasification catalyzed by alkaline-earths: Comparative study of the metal-carbon interaction and of the specific activity

The present work extends previous investigations about the calcium-catalyzed carbon gasification to the case of strontium and barium with the purpose of performing a comparative study of their catalytic activity. This study also analyzes their intrinsic activity in order to understand the working mechanism of these three catalysts. The results of isothermal reaction in CO 2 at different temperatures and TPD after CO 2 contact at 573 K (or after a gasification experiment) have been used to interpret the carbon gasification mechanism. The active species is the carbonate and its decomposition at the interface produces the carbon gasification. The analysis of TPD spectra after CO 2 contact at 573 K allows us to determine the number of catalytic active sites and to compare, quantitatively (i.e., independently of the catalyst dispersion), the specific activity of the three catalysts.