Number Of Surface Active Sites Research Articles

Biomass-based carbon aerogels with interconnected pores and controllable Zn–N sites for CO2 electroreduction

The rational design of renewable, highly active, and selective catalysts has always been the key to the practical application of CO2 electroreduction. In this study, biomass-based Zn/N doped carbon aerogels were fabricated for high-selectivity electrical reduction of CO2 to CO. After in situ Zn/N doping, carbon aerogels exhibited interconnected pores, high surface area, and exposed active sites, and their physicochemical properties are significantly affected by pyrolysis temperature. When the pyrolysis temperature increased to 950 °C, the number of surface active sites and pore structure parameters of carbon aerogels decreased significantly due to the escape of zinc and nitrogen. CAZN-850 exhibited the highest specific surface area of 1317.0 m2/g and abundant Zn–N sites, which results in its high activity and selectivity for electroreduction of CO2 to CO (FECO = 89 % at −1.0V vs. RHE). Moreover, the stability test results for CO2RR showed that the reduction value of FECO for Zn/N doped carbon aerogels was only 5 % under long-time operation. This work provides a new idea for the design of biomass-based catalysts for CO2 electroreduction.

Effects of Synthesis Method on Catalytic Activities of Ag/TiO2 Catalyst in CO Oxidation Reaction

Supported silver catalysts on titanium dioxide have been synthesized by three different methods to compare catalytic activity in CO oxidation. The catalysts were prepared by the wet impregnation method with 10 mole% of silver. In the first method, the silver catalysts were impregnated on the titanium dioxide nanoparticles (Ag/TNP), while in the second method, the silver catalysts were impregnated on the titanium dioxide nanotubes (Ag/TNT). The third method, the silver catalysts were impregnated on the titanium dioxide nanoparticles before the nanotube process (Ag/TNP-TNT). The Ag/TNP and Ag/TNT catalysts showed highly dispersed silver nanoparticles with an average particle size of less than 5 nm on the surface of titanium dioxide, whereas the Ag/TNP-TNT catalyst exhibited much lower dispersion of silver nanoparticles. The highly dispersed silver on the titanium dioxide resulted in a large number of surface active sites (Ag0). The catalytic activity test found that the Ag/TNT catalysts with the highest number of active sites (2.20 × 1020 atom/g-catalyst) exhibited higher CO oxidation activity than the Ag/TNP and Ag/TNT catalysts with 100% CO conversion at 125°C.

Effect of adsorption-catalytic deformation and partial deactivation on the determination of the absolute activity of a liquid phase hydrogenation catalyst

Objectives. To take into account the change in the number of active sites during the adsorptioncatalytic deformation and deactivation of a catalyst surface by means of a catalytic poison when calculating the turnover frequency (TOF) of a hydrogenation catalyst.Methods. The activity was determined by a static method, using a titanium reactor having a volume of 400 mL, an experimental temperature controlled using a liquid thermostat with an accuracy of 0.5 K, with a paddle stirrer rotation speed of 3600 rpm and system hydrogen pressure equal to atmospheric. The consumption of hydrogen used to reduce the model compound was taken into account via the volumetric method. The heats of hydrogen adsorption were determined using a reaction calorimeter with an operating mode close to that of a chemical reactor. After measuring the specific surface area using low temperature nitrogen adsorption, the results were processed using Brunauer–Emmett–Teller theory approximations. Deactivation was carried out by introducing dosed amounts of catalytic poison into the system in titration mode.Results. A kinetic experiment for the reduction of a multiple carbon bond in a sodium maleate molecule using aqueous solutions of sodium hydroxide with additions of monohydric aliphatic alcohols as solvents under conditions of partial deactivation of the catalyst was carried out. The obtained values of heats of hydrogen adsorption on skeletal nickel in the course of the experiment are given. The described approach is used to calculate TOF values taking into account changes in the number of active surface sites during the course of a catalytic reaction and upon the introduction of a deactivating agent. A refined equation for the correct calculation of TOF is proposed along with its mathematical justification. The results of TOF calculations under various assumptions for a number of catalytic systems are shown.Conclusions. When calculating absolute activity values, a change in the number of active sites has a significant effect on the obtained values. The physical meaning of a number of constants in the proposed equation relates the activity of the catalyst to the distribution of hydrogen on its surface in terms of heats of adsorption.

Ultrasound‐Triggered Surface Reconstruction with Improved Carriers Transfer Kinetics in CdIn2S4 for Solar Water Oxidation

AbstractThe surface oxygen evolution reaction (OER) is severely restricted owing to the sluggish OER kinetics and limited number of reactive sites. Although surface reconstruction can introduce defects to improve the OER performance, they often function as recombination centers and cause severe carrier recombination. With the aid of ultrasonication, the CdIn2S4 photoanode surface is reconstructed using Zn‐modified Cd defects without extreme chemical reaction environment. Owing to the surface reconstruction, the formed type‐II band structure is beneficial for separating photogenerated carriers and reduced the bulk recombination. Additionally, the Zn‐modified Cd defects are introduced in situ during the ultrasound cavitation process, which facilitated the carrier transfer, reduced the surface recombination, and increased the number of surface active sites. First‐principles calculations indicate that the modified Cd defects effectively lower the electrochemical reaction barrier and promote the surface OER kinetics by precisely regulating the four‐electron reaction. Consequently, the optimized photoanode exhibits a enhanced photocurrent of 5.30 mA cm−2 at 1.23 V versus reversible hydrogen electrode. These results demonstrate that an accurate surface reconstruction design can be achieved for introducing and modifying metal defects to improve the transfer and transport behaviors of photogenerated carriers.

Charge Concentration Limits the Hydrogen Evolution Rate in Organic Nanoparticle Photocatalysts.

Time-resolved microwave conductivity is used to compare aqueous-soluble organic nanoparticle photocatalysts and bulk thin films composed of the same mixture of semiconducting polymer and non-fullerene acceptor molecule and the relationship between composition, interfacial surface area, charge-carrier dynamics, and photocatalytic activity is examined. The rate of hydrogen evolution reaction by nanoparticles composed of various donor:acceptor blend ratio compositions is quantitatively measured, and it is found that the most active blend ratio displays a hydrogen quantum yield of 0.83% per photon. Moreover, it is found that nanoparticle photocatalytic activity corresponds directly to charge generation, and that nanoparticles have 3× more long-lived accumulated charges relative to bulk samples of the same material composition. These results suggest that, under the current reaction conditions, with ≈3× solar flux, catalytic activity by the nanoparticles is limited by the concentration of electrons and holes in operando and not a finite number of active surface sites or the catalytic rate at the interface. This provides a clear design goal for the next generation of efficient photocatalytic nanoparticles.

Double adjustment of Ni and Co in CeO2/La2Ni2-xCoxO6 double perovskite type oxygen carriers for chemical looping steam methane reforming

Chemical looping steam methane reforming (CL-SMR) is a clean and efficient technology for the co-production of syngas and hydrogen. In this work, a series of Ni and Co doped CeO2/La2Ni2-xCoxO6 double perovskite composite oxygen carriers were developed. The introduction of Co increase a large number of surface active sites, which accelerates the surface activation of CH4, while the addition of Ni provides oxygen vacancies to promote the migration of lattice oxygen in the particles. The optimal substitution ratios of cobalt and nickel are 0.6 and 1.4, respectively. The CeO2/La2Ni1.4Co0.6O6 sample exhibits excellent syngas selectivity (95%) accompanied with high methane conversion (86%) in the methane reforming stage at 850℃, and close to 100% hydrogen concentrations in the steam oxidation stage. Density functional theory calculations demonstrated that the partial oxidation of methane is a strongly exothermic process, and the double perovskite loaded with CeO2 is more conducive to the activation of CH4 and prevents the generation of carbon deposition. The Ni-Co synergistic effect formed by the appropriate doping ratio of Ni and Co can also greatly promote the partial oxidation of methane. Subsequently, the deeply reduced metals combined with oxygen vacancies provide sufficient active stable points for water vapor cracking to generate high-purity hydrogen.

Chitosan-Based Polymer Nanocomposites for Environmental Remediation of Mercury Pollution.

Mercury is a well-known heavy metal pollutant of global importance, typically found in effluents (lakes, oceans, and sewage) and released into the atmosphere. It is highly toxic to humans, animals and plants. Therefore, the current challenge is to develop efficient materials and techniques that can be used to remediate mercury pollution in water and the atmosphere, even in low concentrations. The paper aims to review the chitosan-based polymer nanocomposite materials that have been used for the environmental remediation of mercury pollution since they possess multifunctional properties, beneficial for the adsorption of various kinds of pollutants from wastewater and the atmosphere. In addition, these chitosan-based polymer nanocomposites are made of non-toxic materials that are environmentally friendly, highly porous, biocompatible, biodegradable, and recyclable; they have a high number of surface active sites, are earth-abundant, have minimal surface defects, and are metal-free. Advances in the modification of the chitosan, mainly with nanomaterials such as multi-walled carbon nanotube and nanoparticles (Ag, TiO2, S, and ZnO), and its use for mercury uptake by batch adsorption and passive sampler methods are discussed.

Nitrogen-doped Ti3C2Tx MXene prepared by thermal decomposition of ammonium salts and its application in flexible quasi-solid-state supercapacitor

Two-dimensional (2D) Ti3C2Tx MXenes show great potential for application in flexible supercapacitors, due to their good hydrophilicity, metallic conductivity and excellent flexibility. Surface modification of the MXenes by heteroatom doping is a good strategy for adjusting the layer spacing and alleviateing various shortcomings. Herein, the ammonium salt decomposition method is shown to allow rapid nitrogen doping into Ti3C2Tx nanosheets at a low temperature of 350 °C. Compared to the raw Ti3C2Tx, the nitrogen-doped Ti3C2Tx (designated N-Ti3C2Tx) shows an increased layer spacing of 1.451 nm, nitrogen doping levels of 1.42 at% and a low number of residual fluorine functional groups. For the application as an electrode of supercapacitors, the N-Ti3C2Tx electrode shows an outstanding pseudocapacitance performance and mechanical flexibility, with a high specific capacitance of up to 449 F g−1 at 2 mV s−1, which is 1.4 times that of the raw Ti3C2Tx MXene (i.e., 321 F g−1). Furthermore, a quasi-solid-state symmetric supercapacitor assembled with a H2SO4-PVA gel electrolyte is shown to deliver an energy density of 9.57 Wh kg−1 at 250 W kg−1. The outstanding pseudocapacitance of the N-Ti3C2Tx is attributed to the positive effect of nitrogen doping on MXene, such as larger layer spacing and the increased number of surface active sites. The strategy presented herein also opens up a convenient and versatile approach for preparing high performance MXene Materials for energy conversion and storage.

Structural evolution of PtCu nanoframe for efficient oxygen reduction reactions

• Synthesis of high-active PtCu nanoframes for oxygen reduction reaction. • Synergistic interaction of PtCu nanoframes has excellent electrocatalytic performance. • The etched PtCu nanoframes shows high stability in durability tests. Advances in proton exchange membrane fuel cells require Pt-based catalysts with high catalytic activity and stability. However, due to the expensiveness of Pt, scholars are mainly finding ways to improve the catalytic performance and decrease Pt usage. In this paper, the synthesis of high-active PtCu alloy nanoframes is reported. The nanoframes are prepared by the electric displacement reaction between the Cu nanoplate (Cu NP) template and the Pt precursor, and the high-active PtCu alloy nanoframes are obtained by further etching. The synergistic effect of alloy PtCu nanoframes shows excellent electrocatalytic performance for oxygen reduction reactions, with PtCu high-active nanoframes(PtCu HNF) having approximately 1.4 times the mass activity of commercial Pt/C and exhibiting high stability and tolerance in acidic ORR. Due to the increase in the number of surface active sites and the synergistic effect of bimetals, the activity and stability of the catalyst are significantly improved.

Columnar cactus-like 2D/1D BiOBr/BiVO4:Yb3+,Er3+ heterostructure photocatalyst with ultraviolet-visible-near infrared response for photocatalytic degrading dyes, endocrine disruptors and antibiotics

An innovative strategy for the integration of the upconversion (UC) effect with a Z-scheme heterojunction is proposed to develop photocatalysts to achieve high carrier separation efficiency and efficient utilization of broadband solar spectra in this work. By the above strategy, novel and columnar cactus-like BiOBr/BiVO4:Yb3+,Er3+ heterostructured nanobelts with an efficient full-spectrum light response are synthesized by electrospinning combined with a solvothermal method and are composed of two-dimensional (2D) BiOBr nanosheets modified on the surface of one-dimensional (1D) BiVO4:Yb3+,Er3+ nanobelts. The optimized BiOBr/BiVO4:Yb3+,Er3+ heterostructured nanobelts exhibit superior photocatalytic activity for degrading 89.2 % tetracycline hydrochloride (TCH, 60 min), 89.8 % bisphenol A (BPA, 180 min) and 95.9 % methylene blue (MB, 120 min) under simulated sunlight irradiation as well as 72.3 % TCH, 24.6 % BPA and 69.7 % MB under 12 h near infrared (NIR) light irradiation. The excellent photocatalytic activity can be ascribed to the synergistic effect among the porous structure of nanobelts for producing a large number of surface active sites, broadening of spectral response range by the remarkable upconversion effect, unique 2D/1D contact interface for fast carrier transfer channel and built-in electric field induced by Z-scheme heterojunction as well as Yb3+/Yb2+ redox center for spatial separation of photogenerated carriers. In addition, the photocatalyst exhibits superior reusability and recycling stability in photocatalytic applications. This work not only puts forward the innovative idea of constructing an efficient broad-spectrum photocatalyst for water purification but also provides key technical guidance for the morphology control of high-efficiency photocatalysts.

Ultrasound-promoted preparation of polyvinyl ferrocene-based electrodes for selective formate separation: Experimental design and optimization

The selective separation of ions is a major technological challenge having far-ranging impacts from product separation in electrochemical production of base chemicals from CO2 to water purification. In recent years, ion-selective electrochemical systems leveraging redox-materials emerged as an attractive platform based on their reversibility and remarkable ion selectivity. In the present study, we present an ultrasound-intensified fabrication process for polyvinyl ferrocene (PVF)–functionalized electrodes in a carbon nanotube (CNT) matrix for selective electro-adsorption of formate ions. To this end, a response surface methodology involving the Box–Behnken design with three effective independent variables, namely, PVF to CNT ratio, sonication duration, and ultrasonic amplitude was applied to reach the maximum formate adsorption efficiency. The fabricated electrodes were characterized using cyclic voltammetry, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). SEM images revealed that an optimized ultrasonic amplitude and sonication time provided remarkable improvements in electrode morphology. Through a sedimentation study, we qualitatively demonstrate that the main optimized conditions improved PVF/CNT dispersion stability, consequently providing the highest number of active surface sites for adsorption and the highest adsorption efficiency. The highest percentage of active electrode surface sites and the maximum adsorption efficiency were 97.8 and 90.7% respectively at a PVF/CNT ratio of 3, ultrasonication time of one hour, and 50% ultrasonic amplitude.

Oxygen Vacancy-Mediated Z-Scheme Charge Transfer in a 2D/1D B-Doped g-C3N4/rGO/TiO2 Heterojunction Visible Light-Driven Photocatalyst for Simultaneous/Efficient Oxygen Reduction Reaction and Alcohol Oxidation.

Hydrogen peroxide (H2O2) is a powerful oxidant that directly or indirectly oxidizes many organic and inorganic contaminants. The photocatalytic generation of H2O2 is achieved by using a semiconductor photocatalyst in the presence of alcohol as a proton source. Herein, we have synthesized oxygen vacancy (Ov)-mediated TiO2/B-doped g-C3N4/rGO (TBCN@rGO) ternary heterostructures by a simple hydrothermal technique. Several characterization techniques were employed to explore the existence of oxygen vacancies in the crystal structure and investigate their impact on the optoelectronic properties of the catalyst. Oxygen vacancies offered additional sites for adsorbing molecular oxygen, activating alcohols, and facilitating electron migration from TBCN@rGO to the surface-adsorbed O2. The defect creation (oxygen vacancy) and Z-scheme mechanistic pathways create a suitable platform for generating H2O2 by two-electron reduction processes. The optimized catalyst showed the highest photocatalytic H2O2 evolution rate of 172 μmol/h, which is 1.9 and 2.5 times greater than that of TBCN and BCN, respectively. The photocatalytic oxidation of various lignocellulose-derived alcohols (such as furfural alcohol and vanillyl alcohol) and benzyl alcohol was also achieved. Photocatalytic activity data, physicochemical and optoelectronic features, and trapping experiments were conducted to elucidate the structure-activity relationships. The TBCN@rGO acts as a multifunctional Z-scheme photocatalyst having an oxygen vacancy, modulates surface acidity-basicity required for the adsorption and activation of the reactant molecules, and displays excellent photocatalytic performance due to the formation of a large number of active surface sites, increased electrical conductivity, improved charge transfer properties, outstanding photostability, and reusability. The present study establishes a unique strategy for improving H2O2 generation and alcohol oxidation activity and also provides insights into the significance of a surface vacancy in the semiconductor photocatalyst.

Effects of temperature and time on the facile low-temperature pre-sulfurization of tube-like unsupported Co-Mo catalysts for hydrodesulfurization

A Co-Mo sulfide catalyst with tube-like structure was prepared through a low-temperature pre-sulfurization process using rod-like CoMoO4 as the precursor. The pre-sulfurization temperature greatly affected the total pore volume, the concentration of CoMoS species, the microstructure of (Co)MoS2 slabs and the catalytic desulfurization activity. The increased temperature can promote the formation of hollow structure and the sulfurization degree of the precursor. It was found that the excessive temperature induced the Co atoms to form more isolated Co9S8 species, hindering the formation of highly active CoMoS species. Therefore, appropriate temperature is more favorable to form the (Co)MoS2 slabs with shorter length and higher stacking number, leading to more coordinatively unsaturated sites, especially the corner sites. According to the results of hydrodesulfurization reaction, the reaction rate of dibenzothiophene and the yields of liquid products are highly depended on the number of surface-active sites, which are mainly originated from the Type II CoMoS species. Moreover, the extended pre-sulfurization time can further improve the catalytic desulfurization activity of the catalyst at an appropriate pre-sulfurization temperature.

Self-standing reduced graphene oxide/Nb2C MXene paper electrode with three-dimensional open structure for high-rate potassium ion storage

Potassium ion battery (PIBs) is in the primary stage of development, exploring appropriate electrode materials is the key to obtain high performance and practical application. Herein, we design a self-standing reduced graphene (rGO) and Nb2C MXene (3D-rGO/Nb2C) composite paper electrode with three-dimensional porous conductive network by electrostatic absorption self-assembly method. Compared with the compact structure of L-rGO/Nb2C paper electrode via layer-by-layer vacuum filtration approach, the structure design in which the wrinkled rGO is applied as the framework provides a large number of surface active sites and promotes the rapid transfer of K+ to improve the storage capacity and kinetics of potassium ions. Moreover, the 3D-rGO/Nb2C hybrid paper with large specific surface area can effectively accommodate the volume expansion during charging and discharging process and ensure the cycle stability. At current density of 500 mA g−1, the 3D-rGO/Nb2C hybrid paper electrode delivers an initial capacity of 207 mAh·g−1, which is maintained at 139 mAh·g−1 after 1000 cycles. The 3D-rGO/Nb2C hybrid paper combines both diffusion mechanism and pseudo-capacitance mechanism, which significantly improves its electrochemical performance in K-ion storage. These results show the good potential of the 3D-rGO/Nb2C hybrid paper as a high-performance electrode.

Fabrication of oxygen-vacancy abundant MnO2 nanowires@ NiMnxOy-δ nanosheets core-shell heterostructure for capacity supercapacitors

Structure instability and poor electrical conductivity are two major obstacles to realizing high performance of MnO2-based pseudocapacitor material. The construction of unique hierarchical core-shell nanostructures, therefore, plays an important role in the efficient enhancement of the rate capacity and the stability of this material. Herein, a stable MnO2@NiMnxOy-δ core-shell heterostructure is prepared via a simple liquid-phase reaction combined with heat-treatment method, composed of ultrathin NiMnxOy-δ nanosheets with oxygen vacancies uniformly growing on the surface of ultralong MnO2 nanowires. Electrochemical test results show that the electrode exhibits a specific capacitance of 463.5 C g−1 at 1 A g−1 and has an excellent capacitance retention as high as 94.9% after 20,000 cycles. Compared with the MnO2@NiMnxOy nanowires, the introduction of oxygen vacancies in the ultrathin nanosheets of the MnO2@NiMnxOy-δ core-shell heterostructure can provide a large number of surface active sites to accelerate electron/ions transport during electrochemical reaction. Moreover, the interfacial behavior of the electrode reaction can be also adjusted due to the potential synergistic effect between one-dimensional nanowires and two-dimensional nanosheets, further the MnO2@NiMnxOy-δ core-shell heterostructure demonstrating the superior cyclic stability.

Experimental study of the catalytic effect of iron on low-rank coal gasification

Acid-washing low-rank coal samples were loaded with different content of iron catalyst and then pyrolyzed. FT-IR, Raman spectra, and temperature-programmed experiments were used to investigate the influence of iron on the coal char. The FT-IR results revealed that iron catalyst rises the number of –OH, –CH3, and –CH2 functional groups. The Raman spectra results showed that partial large polyaromatic ring structures transform into small polyaromatic ring structures after the addition of iron. The results of temperature-programmed desorption indicated that the number of surface active sites is increased due to the addition of iron. For low-rank coal char with 3% Fe, the number of active sites increased with the increase of adsorption temperature until 800 °C and then start to decrease. At 750 °C, the adsorption capacity of CO2 increased with the increase of time and reached saturation state after 45 min. The results of the char-steam isothermal gasification experiment suggested that the iron catalyst enhances the gasification reactivity of low-rank coal. It is verified that iron catalysts can improve the gasification reactivity of low-rank coal by increasing the number of surface active sites.

A review on transition metal oxides based nanocomposites, their synthesis techniques, different morphologies and potential applications

In the field of nanotechnology and nanoscience, transition metal oxides based nanocomposites (TMONCs) are promising for various application uses such as Supercapacitors, Sensors, Bactericidal properties, Photocatalytic Degradation, Solar Cells etc. Modification of transition metal oxide nanoparticles (TMONPs) to TMONCs by doping/mixing of another transition metal and metal oxide, carbon based nanoparticles, conducting polymers etc. to achieve enhanced surface area, increasing surface activities or number of active surface sites, reducing electron-hole recombination, increasing charge transfer processes etc. have been reported in literature. These improved properties are the possible reason for the enhancement in its practical applications efficiencies. This review summarizes recent development on transition metal oxides based nanocomposites for different potential applications. Also synthesis methods of transition metal oxide based nanocomposites have obtained an increasing attractions to achieve cost effectiveness and environment friendly routes of synthesis with high rate of production, high yield of product and also less toxic waste production. Transition metal oxides nanocomposites have been fabricated by various methods such as Microwave assisted synthesis technique, Sol-Gel method, Biosynthesis method, Co-precipitation process, Simple Chemical method etc. Different morphologies of transition metal oxides based nanocomposites have been summarized in this review article.Herein, this paper discuss about several reported synthesis techniques, various characterization techniques used for structural and surface properties identifications, different morphologies and various potential applications of transition metal oxide based nanocomposites.

Radial growth kinetics of epitaxial pyrolytic carbon layers deposited on carbon nanotube surfaces by non-catalytic pyrolysis

Chemical vapor deposition of pyrolytic carbon (PyC) onto carbon nanotube (CNT) surfaces by hydrocarbon pyrolysis can thicken the nanotube diameter until the desirable size. Vapor-induced thickening of CNTs may require no metal catalysts because the CNT-templated growth can easily convert PyC deposits into epitaxial graphene shells. However, little work has been done on the kinetics of the vapor-induced CNT thickening process during the non-catalytic pyrolysis. In this study, we discuss the growth mechanism of epitaxial PyC layers on CNT surfaces achieved by the low-pressure (≤1.5 kPa) pyrolysis of ethylene, which is distinct from previous studies using much higher gas pressures (4–100 kPa) for the PyC deposition. The average thickness of deposited PyC layers is calculated by analyzing the transmission electron microscopy images of PyC-coated CNTs. The crystallinity of PyC-coated CNTs is evaluated by Raman spectroscopy and X-ray diffraction. The thickness of PyC layers on CNT surfaces can be adjusted by simply changing the growth time under typical pyrolysis parameters (800 °C-1.5 kPa of ethylene). We find a two-stage nonlinear behavior to describe the growth kinetics of PyC layers, indicating that the radial growth rate of PyC layers can be influenced by the number of surface active sites. This study reveals the self-assembly mechanism of graphitic layers on the CNT template, which is expected to promote the diameter-controlled synthesis of CNTs.

Reversible down-regulation and up-regulation of catalytic activity of poly(N-isopropylacrylamide)-anchored gold nanoparticles

Regulating catalytic activity plays an important role in further optimizing and developing multifunctional catalysts with high selectivity and high activity. Reversible dual regulation of catalytic activity has always been a challenging task. Here, we prepared poly(N-isopropylacrylamide)-anchored gold nanoparticles (AuNP@CDs-Azo-PNIPAM) through host-guest interaction of cyclodextrin capped gold nanoparticles (AuNP@CDs) and azobenzene-terminated poly(N-isopropylacrylamide) (Azo-PNIPAM). Azo-PNIPAM as thermal and light responsive ligand allows reversible dual regulation of catalytic activity. When the temperature is higher than the lowest critical solution temperature, the PNIPAM chain shrinks rapidly, increasing the steric hindrance around AuNPs and reducing the catalytic activity. Under ultraviolet light irradiation, cis-azobenzene disassembles from cyclodextrin and the number of surface active sites of AuNPs increases, which improves the catalytic activity. The reaction rate of UV irradiation is almost 1.3 times that of visible light irradiation. This work provides a simple and effective strategy for the construction of reversible catalysts.

Number of surface sites and turnover frequencies for oxide catalysts

Unlike for metal catalysts where selective chemisorption probes (H2, O2, CO and N2O) have been successfully applied to count the number of active surface sites and allow determination of TOF values, the analogous situation for oxide catalysts has developed very slowly. The reason for the sluggish development of selective chemisorption probes for oxides is that the same chemisorption probes that work well with metals aren’t compatible with oxides since these chemical probes weakly adsorb on oxides. Methanol, however, is a very reactive molecule that has been found to readily chemisorb on oxides and allows for quantitative determination of the number of active surface sites (Ns). In this mini-review, the determination of the number of active sites by methanol chemisorption is reviewed with examples for pure oxides (one-component oxides), supported oxides (submonolayer, monolayer and above monolayer) and bulk mixed oxides (surface enriched oxide present in submonolayer, monolayer and in excess of monolayer coverages).