Minna Cao

Recent Advances in the Stabilization of Platinum Electrocatalysts for Fuel‐Cell Reactions

Polymer electrolyte membrane fuel cells (PEMFCs) feature high energy densities, low operating temperatures, and low environmental impact, which make them a promising technology for power applications. As a key component of PEMFCs, Pt‐based catalysts are still under widespread investigation and have shown exciting performance; however, to move towards their successful commercialization, focusing solely on their catalytic activity is not sufficient. Instead, more effort is required to improve their stability and to decrease costs. Herein, we provide a comprehensive review of current research activities that have concentrated on how to stabilize the Pt‐based catalysts. We devote the most attention to the structure‐optimization of the Pt‐based catalysts and the development of advanced supports. The feasible strategies for structure optimization are subdivided into three groups: 1) dimension effects; 2) electronic and bifunctional effects; and 3) steric effects. Then, we discuss the techniques that have been developed for improving carbon black and for generating various types of carbon‐free supports and composites supports (e.g., graphite, carbon nanotubes, new‐type oxides and nitrides, and macromolecules). An outlook on the future trends and developments in this area is also provided at the end of the review.

Porous Anionic, Cationic, and Neutral Metal-Carboxylate Frameworks Constructed from Flexible Tetrapodal Ligands: Syntheses, Structures, Ion-Exchanges, and Magnetic Properties

A series of coordination polymers with anionic, cationic, and neutral metal-carboxylate frameworks have been synthesized by using a flexible tetrapodal ligand tetrakis[4-(carboxyphenyl)oxamethyl] methane acid (H(4)X). The reactions between divalent transition-metal ions and H(4)X ligands gave [M(3)X(2)]·[NH(2)(CH(3))(2)](2)·8DMA (M = Co (1), Mn (2), Cd(3)) which have anionic metal-carboxylate frameworks with NH(2)(CH(3))(2)(+) cations filled in channels. The reactions of trivalent metal ions Y(III), Dy(III), and In(III) with H(4)X ligands afforded cationic metal-carboxylate frameworks [M(3)X(2)·(NO(3))·(DMA)(2)·(H(2)O)]·5DMA·2H(2)O (M = Y(4), Dy(5)) and [In(2)X·(OH)(2)]·3DMA·6H(2)O (6) with the NO(3)(-) and OH(-) serving as counterions, respectively. Moreover, a neutral metal-carboxylate framework [Pb(2)X·(DMA)(2)]·2DMA (7) can also be isolated from reaction of Pb(II) and H(4)X ligands. The charged metal-carboxylate frameworks 1-5 have selectivity for specific counterions in the reaction system, and compounds 1 and 2 display ion-exchange behavior. Moreover, magnetic property measurements on compounds 1, 2, and 5 indicate that there exists weak antiferromagnetic interactions between magnetic centers in the three compounds.

Development of a polyoxometallate-based photocatalyst assembled with cucurbit 6 uril via hydrogen bonds for azo dyes degradation

A water insoluble cucurbit[6]uril-polyoxometallates (CB[6]-POMs) composite assembled from α-Keggin type polysilicontungstate anions and macrocycle cucurbit[6]uril (CB[6]) via hydrogen bonding has been synthesized as visible light active photocatalyst. The physical and photocatalytic properties of such photocatalyst have been fully characterized by PXRD, FTIR, TG, XPS, and UV/vis diffuse reflectance spectra. The catalyst shows a good photocatalytic activity towards the degradation of methyl orange (MO) under visible light irradiation and displays good reproducibility of photocatalytic degradation by a simple recycled procedure without obvious loss in catalytic activity, which is of great significance for practical use of the photocatalyst. In the photodegradation process, the {Ni-CB[6]}(n) chain of the photocatalyst acts as sensitizer and can be induced by visible light, meanwhile the POMs chain of the photocatalyst acts as electron acceptor and deposits the electron in its LUMO. The effects of various experimental parameters and the proposed mechanisms are discussed in detail.

Platinum Nanoparticles Stabilized by Cucurbit[6]uril with Enhanced Catalytic Activity and Excellent Poisoning Tolerance for Methanol Electrooxidation

Three sub-10 nm platinum nanoparticles (PtNPs) with distinctive morphologies were developed by using cucurbit[6]uril (CB[6]) as stabilizing agent and support. Both the size and shape of the PtNPs were simultaneously controlled by tuning the reducing agents. The prepared NPs have been comprehensively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and cyclic voltammetry. On account of the presence of CB[6] and its unique structural features, the as-prepared PtNPs are homogeneous in morphologies and exhibit higher activities toward methanol electrooxidation than commercial Pt/C. CB[6] has the ability to bind small molecules that can promote CO oxidation, therefore, all the three PtNPs showed enhanced poisoning tolerance. Such unique abilities of CB[6] can even promote the poisoning tolerance of commercial Pt/C through simple physical mixing.

Synthesis of palladium nanocatalysts with cucurbit[n]uril as both a protecting agent and a support for Suzuki and Heck reactions

Pd nanocatalysts based on cucurbit[n]uril were successfully synthesized by an operationally simple one-pot liquid-phase method using different reducing agents. Through changing the reducing agents and the ratio of feedstock, well-defined Pd nanostructures with various shapes were obtained. The results of morphological analysis demonstrate that cucurbit[6]uril plays a key role in guiding the formation of Pd nanostructures, and is responsible for the Suzuki and Heck coupling reactions with high efficiency.

Sub-5 nm Pd–Ru Nanoparticle Alloys as Efficient Catalysts for Formic Acid Electrooxidation

Both Pd and Ru are important elements in electrochemistry. However, Pd–Ru nanoparticle (NP) alloys are rarely reported so far, owing to the limitations described by Hume‐Rothery. Herein, we successfully synthesized Pd–Ru NP alloys over the whole composition range using a facile atomic‐diffusion‐based strategy, and we carefully investigated the corresponding formation mechanism. For the first time, we confined the size of all alloy NPs at 3–5 nm, which is favorable in direct formic acid fuel cells. Experimental data indicate that the core‐level binding energy of Pd positively shifts as a result of electron transfer from Pd to Ru. Such an electronic effect can effectively tune the adsorption intensity of reactive molecules during electrocatalysis. Driven by all these effects, these Pd–Ru alloys are highly active and stable towards formic acid oxidation. Composition–activity study further indicates that Pd60Ru40 exhibits the best mass activity, which is 4.1 times higher than that of commercial Pd/C.

Facile synthesis of polyoxometalate–thionine composite via direct precipitation method and its photocatalytic activity for degradation of rhodamine B under visible light

(TH)(3)PW(12) (TH=thionine, PW(12)=PW(12)O(40)(3-)) composite was prepared by direct precipitation of TH and PW(12). The (TH)(3)PW(12) was characterized via UV-vis spectrum, FT-IR, SEM, and BET surface area. PW(12) was intact during the precipitation process. The composite has a bar-like shape and relatively large surface area (Langmuir surface of (TH)(3)PW(12) was 31.59 m(2)g(-1), BET surface was 20.26 m(2)g(-1)). Using the material as the photocatalyst, rhodamine B (RhB) was efficiently bleached and mineralized under visible light irradiation (λ>420 nm). The kinetics of the photodecomposition follow the first-order reaction. The (TH)(3)PW(12) catalyst can be easily separated from the reaction system and has good stability for reuse.

Self-assembly of polyoxometalate–thionine multilayer films on magnetic microspheres as photocatalyst for methyl orange degradation under visible light irradiation

(PW(12)-TH)(n) multilayer films (PW(12)=PW(12)O(40)(3-), TH=thionine) were deposited successfully on core-shell structured Fe(3)O(4)@SiO(2) magnetic microspheres through layer-by-layer (LbL) self-assembly method. The physical and photocatalytic properties of such magnetic microspheres coated with (PW(12)-TH)(n) films have been characterized by SEM, FTIR, and UV-vis spectra. The microspheres exhibit better photocatalytic activity toward the degradation of methyl orange (MO) under visible light irradiation than the quartz slides support. In addition, the use of magnetic support guarantees facile, clean, fast, and efficient separation of the photocatalyst after the degradation of MO. Such catalysts can be reused several times and display good reproducibility by magnetic separation.

Palladium nanoparticles in situ generated in metal–organic films for catalytic applications

Palladium nanoparticles were first in situ generated in metal–organic films for catalytic applications. Layer-by-layer assembly of metal–organic films consisting of rigid-rod chromophores connected by terminal pyridine moieties to palladium centers on solid substrates was presented. Bipyridyl and polypyridyl ligands were used as building blocks to explore the influence of different ligand structures on catalytic properties. Metal–organic films were characterized by UV-Vis spectra, atomic force microscopy (AFM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results show that the deposition mechanism of metal–organic films is perfect layer-by-layer self-assembling with complete surface coverage and regular growth. Moreover, the catalytic activity toward the hydrogenation of olefin was investigated. Based on XPS and TEM, the catalytic activity toward the hydrogenation of olefin was ascribed to the in situ formation of Pd nanoparticles from Pd ions in metal–organic films. This film material is an active catalyst for the hydrogenation of olefin under mild conditions. Furthermore, catalytic results indicated that monodentate bipyridyl ligands exhibited superior catalytic activity than tridentate polypyridyl ligands. Catalytic activity is related to the loading amount of catalysts and permeability. More importantly, this study points toward the potential application of metal–organic films as heterogeneous catalysts with easy separation and good recyclability.

Porous hollow MoS2 microspheres derived from core–shell sulfonated polystyrene microspheres@MoS2 nanosheets for efficient electrocatalytic hydrogen evolution

Porous hollow MoS2 microspheres were fabricated from core–shell monodisperse sulfonated polystyrene (SPS) microspheres@MoS2 nanosheets by pyrolysis at 800 °C under a N2 atmosphere. SPS microspheres were employed as the template and carbon precursor. The composite was characterized by powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), N2 adsorption and X-ray photoelectron spectroscopy (XPS). The unique hollow structure not only enlarges the specific surface area and increases Mo-edge active sites, but also facilitates the diffusion of the electrolyte. Furthermore, the residual porous carbons in the hollow MoS2 microspheres were produced due to the incomplete decomposition of SPS, which will enhance the conductivity of the composite, thus promoting the electrocatalytic activity of hydrogen evolution. The as-prepared porous hollow MoS2 exhibits a low overpotential of 90 mV associated with a low Tafel slope of 51 mV per decade and excellent stability in the hydrogen evolution reaction (HER).