Chitosan -NH2 derived efficient Co3O4 catalyst for styrene catalytic oxidation: Simultaneously regulating particle size and Co valence.

In this study, a Co3O4 catalyst is synthesised using the chitosan-assisted sol-gel method, which simultaneously regulates the grain size, Co valence and surface acidity of the catalyst through a chitosan functional group. The complexation of the free -NH2 complex inhibits particle agglomeration; thus, the average particle size of the catalyst decreases from 82 to 31 nm. Concurrently, Raman spectroscopy, hydrogen temperature-programmed reduction, electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy experiments demonstrate that doping with chitosan N sources effectively modulates Co2+ to promote the formation of oxygen vacancies. In addition, water washing after catalyst preparation can considerably improve the low-temperature (below 250 °C) activity of the catalyst and eliminate the side effects of alkali metal on catalyst activity. Moreover, the presence of Brønsted and Lewis acid sites promotes the adsorption of C8H8. Consequently, CS/Co3O4-W presents the highest catalytic oxidation activity for C8H8 at low temperatures (R250 °C = 8.33 µmol g-1 s-1, WHSV = 120,000 mL hr-1∙g-1). In situ DRIFTS and 18O2 isotope experiments demonstrate that the oxidation of the C8H8 reaction is primarily dominated by the Mars-van Krevelen mechanism. Furthermore, CS/Co3O4-W exhibits superior water resistance (1- and 2- vol% H2O), which has the potential to be implemented in industrial applications.

Crystal Plane Effect of Co3O4 on Styrene Catalytic Oxidation: Insights into the Role of Co3+ and Oxygen Mobility at Diverse Temperatures.

In the oxidation reaction of volatile organic compounds catalyzed by metal oxides, distinguishing the role of active metal sites and oxygen mobility at specific preferentially exposed crystal planes and diverse temperatures is challenging. Herein, Co3O4 catalysts with four different preferentially exposed crystal planes [(220), (222), (311), and (422)] and oxygen vacancy formation energies were synthesized and evaluated in styrene complete oxidation. It is demonstrated that the Co3O4 sheet (Co3O4-I) presents the highest C8H8 catalytic oxidation activity (R250 °C = 8.26 µmol g-1 s-1 and WHSV = 120,000 mL h-1 g-1). Density functional theory studies reveal that it is difficult for the (311) and (222) crystal planes to form oxygen vacancies, but the (222) crystal plane is the most favorable for C8H8 adsorption regardless of the presence of oxygen vacancies. The combined analysis of temperature-programmed desorption and temperature-programmed surface reaction of C8H8 proves that Co3O4-I possesses the best C8H8 oxidation ability. It is proposed that specific surface area is vital at low temperature (below 250 °C) because it is related to the amount of surface-adsorbed oxygen species and low-temperature reducibility, while the ratio of surface Co3+/Co2+ plays a decisive role at higher temperature because of facile lattice oxygen mobility. In situ diffuse reflectance infrared Fourier spectroscopy and the 18O2 isotope experiment demonstrate that C8H8 oxidation over Co3O4-I, Co3O4-S, Co3O4-C, and Co3O4-F is mainly dominated by the Mars-van Krevelen mechanism. Furthermore, Co3O4-I shows superior thermal stability (57 h) and water resistance (1, 3, and 5 vol % H2O), which has the potential to be conducted in the actual industrial application.

Formation of Size-Controllable Tetragonal Nanoprisms by Crystallization-Directed Ionic Self-Assembly of Anionic Porphyrin and PEO-Containing Triblock Cationic Copolymer.

The creation of anisotropic nanostructures with precise size control is desirable for new properties and functions, but it is challenging for ionic self-assembly (ISA) because of the non-directional electrostatic interactions. Herein, the formation of size-controllable tetragonal nanoprisms is reported via crystallization-directed ionic self-assembly (CDISA) through evaporating a micellar solution on solid substrates. First, ISA is designed with a crystalline polyethylene oxide (PEO) containing cationic polymer poly(2-(2-guanidinoethoxy)ethyl methacrylate)-b-poly(ethyleneoxide)-b-poly(2-(2-guanidinoethoxy)-ethylmethacrylate) (PGn -PEO230 -PGn ) and an anionic 5,10,15,20-Tetrakis(4-sulfonatophenyl) porphyrin (TPPS) to form micelles in aqueous solution. The PG segments binds excessive TPPS with amplenet chargeto form hydrophilic corona, while the PEO segments are unprecedentedly dehydrated and tightly packed into cores. Upon naturally drying the micellar solution on a silicon wafer, PEO crystallizationdirects the micelles to aggregate into square nanoplates, which are further connected to nanoprisms. Length and width of the nanoprisms can be facilely tuned by varying the initial concentration. In this hierarchical process, the aqueous self-assembly is prerequisite and the water evaporation rate is crucial for the formation of nanostructures, which provides multiple factors for morphology regulating. Such precise size-control strategy is highly expected to provide a new vision for the design of advanced materials with size controllable anisotropic nanostructures.

N-doped CoAl oxides from hydrotalcites with enhanced oxygen vacancies for excellent low-temperature propane oxidation.

A series of nitrogen-doped CoAlO (N-CoAlO) were constructed by a hydrothermal route combined with a controllable NH3 treatment strategy. The effects of NH3 treatment on the physico-chemical properties and oxidation activities of N-CoAlO catalysts were investigated. In comparison to CoAlO, a smallest content decrease in surface Co3+ (serving as active sites) while a largest increased amount of surface Co2+ (contributing to oxygen species) are obtained over N-CoAlO/4h among the N-CoAlO catalysts. Meanwhile, a maximum N doping is found over N-CoAlO/4h. As a result, N-CoAlO/4h (under NH3 treatment at 400°C for 4 hr) with rich oxygen vacancies shows optimal catalytic activity, with a T90 (the temperature required to reach a 90% conversion of propane) at 266°C. The more oxygen vacancies are caused by the co-operative effects of N doping and suitable reduction of Co3+ for N-CoAlO/4h, leading to an enhanced oxygen mobility, which in turn promotes C3H8 total oxidation activity dominated by Langmuir-Hinshelwood mechanism. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) analysis shows that N doping facilities the decomposition of intermediate species (propylene and formate) into CO2 over the catalyst surface of N-CoAlO/4h more easily. Our reported design in this work will provide a promising way to develop abundant oxygen vacancies of Co-based catalysts derived from hydrotalcites by a simple NH3 treatment.

Amorphous Boron Dispersed in LaCoO3 with Large Oxygen Vacancies for Efficient Catalytic Propane Oxidation.

Unsatisfactory oxygen mobility is a considerable barrier to the development of perovskites for low-temperature volatile organic compounds (VOCs) oxidation. This work introduced small amounts of dispersed non-metal boron into the LaCoO3 crystal through an easy sol-gel method to create more oxygen defects, which are conducive to the catalytic performance of propane (C3 H8 ) oxidation. It reveals that moderate addition of boron successfully induces a high distortion of the LaCoO3 crystal, decreases the perovskite particle size, and produces a large proportion of bulk Co2+ species corresponding to abundant oxygen vacancies. Additionally, surface Co3+ species, as the acid sites, which are active for cleaving the C-H bonds of C3 H8 molecules, are enriched. As a result, the LCB-7 (molar ratio of Co/B=0.93:0.07) displays the best C3 H8 oxidation activity. Simultaneously, the above catalyst exhibits superior thermal stability against CO2 and H2 O, lasting 200 h. This work provides a new strategy for modifying the catalytic VOCs oxidation performance of perovskites by the regulation of amorphous boron dispersion.

Boosting total oxidation of propane over CeO2@Co3O4 nanofiber catalysts prepared by multifluidic coaxial electrospinning with continuous grain boundary and fast lattice oxygen mobility.

A one-dimensional (1D) core-shell of Co-Ce oxide has been prepared by multifluidic coaxial electrospinning method and evaluated for the total oxidation of propane (C3H8). Activity and morphological characterizations show that the CeO2@Co3O4 nanofiber catalyst, of which the core is CeO2 and the shell is Co3O4, exhibits excellent oxidation activity. The exposed Co3O4 grown on the outside of the fibers can rapidly react with C3H8 while CeO2 with high oxygen storage capacity in the inside is conductive to the enhanced oxidation rate. Besides, the continuous grain boundary provides a fast mass transfer channel for lattice oxygen, and rich oxygen vacancies favor the mobility of active oxygen species. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) confirms that the CeO2@Co3O4 catalyst have a faster rate of C3H8 adsorption and better oxidation activity with respect to the counterpart using a single-needle electrospinning method. Moreover, the CeO2@Co3O4 catalyst displays excellent thermal stability, and strong resistance against 5 vol% H2O and 5 vol% CO2 at both 300 and 400 °C. Our strategy can give some new insights into morphological engineering to promote active oxygen mobility via the construction of one-dimensional core-shell of metal oxides for catalytic oxidation of VOCs.