The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

A review on g-C3N4-based photocatalysts

This Account reports the synthesis and characterization of dendrimer-encapsulated metal nanoparticles and their applications to catalysis. These materials are prepared by sequestering metal ions within dendrimers followed by chemical reduction to yield the corresponding zerovalent metal nanoparticle. The size of such particles depends on the number of metal ions initially loaded into the dendrimer. Intradendrimer hydrogenation and carbon−carbon coupling reactions in water, organic solvents, biphasic fluorous/organic solvents, and supercritical CO2 are also described.

http://rcrooks.cm.utexas.edu/research/resources/Publications/rmc113.pdf

Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis

Ligand design is becoming an increasingly important part of the synthetic activity in chemistry. This is of course because of the subtle control that ligands exert on the metal center to which they are coordinated. Ligands which contain significantly different chemical functionalities, such as hard and soft donors, are often called hybrid ligands and find increasing use in molecular chemistry. Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions remain very difficult, not only because of the considerable diversity encountered within the Periodic Table-different metal centers will behave differently towards the same ligand and different ligands can completely modify the chemistry of a given metal-but also because of the small energy differences involved. New systems may-even through serendipity-allow the emergence of useful concepts that can gain general acceptance and help design molecular structures orientated towards a given property. The concept of ligand hemilability, which finds numerous illustrations with hybrid ligands, has gained increased acceptance and been found to be very useful in explaining the properties of metal complexes and in designing new systems for molecular activation, homogeneous catalysis, functional materials, or small-molecule sensing. In the field of homogeneous enantioselective catalysis, in which steric and/or electronic control of a metal-mediated process must occur in such a way that one stereoisomer is preferentially formed, ligands containing one or more chiral oxazoline units have been found to be very valuable for a wide range of metal-catalyzed reactions. The incorporation of oxazoline moieties in multifunctional ligands of increasing complexity makes such ligands good candidates to display hemilabile properties, which until recently, had not been documented in oxazoline chemistry. Herein, we briefly recall the definition and scope of hemilabile ligands, present the main classes of ligands containing one or more oxazoline moieties, with an emphasis on hybrid ligands, and finally explain why the combination of these two facets of ligand design appears particularly promising.

Hemilability of Hybrid Ligands and the Coordination Chemistry of Oxazoline‐Based Systems

Phosphorus-doped ordered mesoporous carbons (POMCs) with different lengths were synthesized using a metal-free nanocasting method of SBA-15 mesoporous silica with different sizes as template and triphenylphosphine and phenol as phosphorus and carbon sources, respectively. The resultant POMC with a small amount of P doping is demonstrated as a metal-free electrode with excellent electrocatalytic activity for oxygen reduction reaction (ORR), coupled with much enhanced stability and alcohol tolerance compared to those of platinum via four-electron pathway in alkaline medium. Interestingly, the POMC with short channel length is found to have superior electrochemical performances compared to those with longer sizes.

Phosphorus-doped ordered mesoporous carbons with different lengths as efficient metal-free electrocatalysts for oxygen reduction reaction in alkaline media.

cis-[Pd(ONO(2))(2)(PPh(3))(2)] (1) reacts under mild conditions with CO in methanol (MeOH) in the presence of pyridine (py), yielding trans-[Pd(COOMe)(ONO(2))(PPh(3))(2)] (1a). The use of NEt(3) instead of py leads to a mixture of 1a, trans-[Pd(COOMe)(2)(PPh(3))(2)] (2), and [Pd(CO)(PPh(3))(3)]. Pure 2 was prepared by reacting cis-[Pd(OTs)(2)(PPh(3))(2)] with CO in MeOH and subsequently adding NEt(3). The nitro complex trans-[Pd(COOMe)(NO(2))(PPh(3))(2)] (3a) was prepared by reacting trans-[Pd(COOMe)Cl(PPh(3))(2)] with AgNO(2) or with AgOTs and NaNO(2). New syntheses for 1 and trans-[Pd(NO(2))(2)(PPh(3))(2)] (3) are also reported. All complexes have been characterized by IR and (1)H and (31)P{(1)H} NMR spectroscopies. Complexes 1 and 2 exchange irreversibly and quantitatively one nitrato with one carbomethoxy ligand, yielding 1a. 2 in CD(2)Cl(2) at 40 degrees C decomposes with the formation of dimethyl carbonate (DMC), whereas under 4 atm of CO, DMC and dimethyl oxalate (DMO) are formed, ca. 12% each; in the presence of PPh(3) and in the absence of CO, decomposition occurs at 60 degrees C with the formation of DMC only, suggesting that decarbonylation involves a five-coordinate intermediate or predissociation of a PPh(3) ligand. The oxidative carbonylation of MeOH does not occur when using NaNO(2) or NaNO(3) as the oxidant and 1, 1a, 3, or 3a as the catalyst precursor. On the contrary, when using benzoquinone (BQ) as the oxidant, these complexes, 2, or [Pd(COOMe)(2-n)X(n)(PPh(3))(2)] (X = Cl, OAc, OTs; n = 1, 2) promote selective catalysis to DMO. After catalysis the precursors are transformed into [Pd(BQ)(PPh(3))(2)](2).H(2)BQ, [Pd(CO)(PPh(3))](3) and [Pd(CO)(PPh(3))(3)]. Also the last with BQ gives selective catalysis to DMO. The solid-state structures of 1.CH(2)Cl(2) and 1a have been determined by means of single-crystal X-ray diffraction.

Catalytic properties of [Pd(COOMe)nX2-n(PPh3)2] (n = 0, 1, 2; X = Cl, NO2, ONO2, OAc and OTs) in the oxidative carbonylation of MeOH

The kinetics of the copolymerization of ethylene with carbon monoxide has been studied in a batch slurry reactor using the homogeneous Pd complex [(dppp)Pd(H 2 O)(TsO)](TsO) as the catalyst precursor in methanol. The effects of the catalyst loading and the partial pressures of CO and ethylene on the rate of copolymerization have been studied over a temperature range of 353-373 K. The rate of copolymerization was found to increase with increasing catalyst loading and partial pressures of CO and ethylene. The approximate orders of reaction were found to be 0.63 and 0.72 with respect to dissolved CO and ethylene, respectively. Different rate equations based on probable reaction mechanisms were evaluated for fitting the rate data. The rate data were interpreted in terms of the activities of the monomers in solution instead of their concentrations. This is particularly important because the copolymerization involves the reaction of two gaseous monomers with significant differences in their solubilities and ideal behavior. The rate models were discriminated using statistical as well as thermodynamic criteria. A model derived by assuming the insertion of ethylene into the tetracoordinated Pd-acyl species as the rate-determining step has been found to represent the kinetics in good agreement with experimental results. The apparent energy of activation for copolymerization was found to be 49 kJ/mol.

Kinetic modeling of copolymerization of ethylene with carbon monoxide using Pd complex catalyst

Fluxional behaviour of allylpalladium(II) derivatives of N,N′-diarylformamidines and 1,3-diaryltriazenes

The structure of the title compound has been determined by counter data. In this square-planar iridium complex the triazenido-ligand is unidentate and trans to the carbonyl group. The Ir–N(1) distance is 2.16(1) and the Ir–C(26)(carbonyl) 1.79(1)A. The complex crystallizes in the monoclinic system, space group C2/c, with unit-cell dimensions a= 22.121(3), b= 19.190(3), c= 10.111(2)A, and β= 92.77(5)°. The structure is refined to a conventional R index of 0.032.

Luigi Toniolo

Papers

Catalytic properties of [Pd(COOMe)nX2-n(PPh3)2] (n = 0, 1, 2; X = Cl, NO2, ONO2, OAc and OTs) in the oxidative carbonylation of MeOH

Kinetic modeling of copolymerization of ethylene with carbon monoxide using Pd complex catalyst

Fluxional behaviour of allylpalladium(II) derivatives of N,N′-diarylformamidines and 1,3-diaryltriazenes

Aryltriazenido-complexes. Crystal and molecular structure of trans-carbonyl(1,3-di-p-tolyltriazenido)bis(triphenylphosphine)iridium(I)

Stabilization of highly reactive cyclic polyolefins by coordination to iron carbonyls : Methylpentalenediiron pentacarbonyl