Palladium-based Catalysts Research Articles

Deposition of N-doped carbon layers inside acidic ZrSBA-15: significant enhancement of catalytic performance of Pd NPs toward benzyl alcohol aerobic oxidation

Palladium-based catalysts with equably distributions of surface functionalities are synthesized by anchoring palladium nanoparticles onto N-doped carbon layers stabilized by mildly acidic platelet ZrSBA-15 framework with short mesochannels. The catalysts were fully characterized by X-ray diffraction, N2 adsorption-desorption isotherms, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. Simultaneously, the Pd–NC–ZS catalysts exhibited the expectable activity in aerobic oxidation of benzyl alcohol with 82% conversion and absolute selectivity to benzaldehyde. Inappreciable loss in catalytic performance under solvent- and base-free conditions after ten times recycle demonstrated that there was strong interaction between palladium nanoparticles and the modified framework. Namely, a synergistic effect can be certified due to the surface acidic sites of ZrSBA-15 and N-doped carbon layers, which could strengthen the interaction between palladium nanoparticles and support, thus facilitating the anchoring and stabilization of palladium nanoparticles onto the pore walls of the NC–ZS.

Zeolite supported palladium catalysts for hydroalkylation of phenolic model compounds

Bifunctional palladium-based catalysts with zeolite supports were investigated in phenol hydroalkylation, representing high-potential model reaction for transformation of lignocellulose-derived compounds to automotive and jet fuels. Conversion of phenol correlated with the amount of accessible acid sites and textural properties of the solid acid support: under the same reaction conditions and at the same Pd loading. The highest phenol conversions (99–100%) were obtained for large-pore zeolite Beta and mesopore-containing MCM-36/56 materials. The selectivity towards the targeted cyclohexylcyclohexane on different supports decreased in the following order Beta (57%) > MCM-36 (42%) ≈ MCM-56 (37%) > MCM-22 (20%), while the contribution of undesired reaction termination through formation of non-reactive cyclohexane was more pronounced for the expanded MCM-36/56 based catalysts. Substrate consumption after 60 min over the least active MCM-22-based catalysts increased with increasing Pd content as follows 27% (0.3 wt % Pd), 80% (0.5 wt % Pd) and 98% (0.7 wt % Pd). High activity of Pd/Beta catalysts was not vitiated even by sintering effects being the most pronounced for Beta as a support.

Review on Copper and Palladium Based Catalysts for Methanol Steam Reforming to Produce Hydrogen

Methanol steam reforming is a promising technology for producing hydrogen for onboard fuel cell applications. The methanol conversion rate and the contents of hydrogen, carbon monoxide and carbon dioxide in the reformate, significantly depend on the reforming catalyst. Copper-based catalysts and palladium-based catalysts can effectively convert methanol into hydrogen and carbon dioxide. Copper and palladium-based catalysts with different formulations and compositions have been thoroughly investigated in the literature. This work summarized the development of the two groups of catalysts for methanol steam reforming. Interactions between the activity components and the supports as well as the effects of different promoters were discussed. Compositional and morphological characteristics, along with the methanol steam reforming performances of different Cu/ZnO and Pd/ZnO catalysts promoted by Al2O3, CeO2, ZrO2 or other metal oxides, were reviewed and compared. Moreover, the reaction mechanism of methanol steam reforming over the copper based and palladium based catalysts were discussed.

Linear Selective Isomerization/Hydroformylation of Unsaturated Fatty Acid Methyl Esters: A Bimetallic Approach

Herein, we report about the development of an isomerization/hydroformylation tandem reaction to selectively convert fatty acid methyl esters into asymmetric α,ω-functionalized aldehyde esters. An orthogonal tandem catalytic system consisting of a palladium-based isomerization catalyst and a rhodium-based hydroformylation catalyst was developed, using methyl 3-hexenoate as a model substrate. Using this catalyst, high yields (81% at 99% conversion) and regioselectivities (l/b-ratio of 98/2) toward the desired terminal hydroformylation product are obtained in the conversion of methyl 3-hexenoate under mild conditions. Ethyl 4-decenoate was subsequently applied as a second model substrate to identify challenges associated with the longer chain length of the unsaturated ester. Finally, methyl oleate was converted using the developed catalyst system. High aldehyde yields of 74% (at 99% conversion) with an l/b-ratio of 91/9 are obtained.

Advances in carbon nanotubes as efficacious supports for palladium-catalysed carbon–carbon cross-coupling reactions

Since the 1970s, palladium-catalysed carbon–carbon (C–C) bond formation has made a critical impact in organic synthesis. In early studies, homogeneous palladium catalysts were extensively used for this reaction with limitations such as difficulty in separation and recycling ability. Lately, heterogeneous palladium-based catalysts have shown promise as surrogates for conventional homogeneous catalysts in C–C coupling reactions, since the product is easy to isolate, while the catalyst is reusable and hence sustainable. Recently, a better part of these heterogeneous palladium catalysts are supported on carbon nanotubes (Pd/CNTs), that have shown superior catalytic performance and better recyclability since the CNT support imparts stability to the palladium catalyst. This review discusses the wide variety of surface functionalization techniques for CNTs that improve their properties as catalyst supports, as well as the methods available for loading the catalyst nanoparticles onto the CNTs. It will survey the literature where Pd/CNTs catalysts have been utilized for C–C coupling reactions, with particular emphasis on Suzuki–Miyaura and Mizoroki–Heck coupling reactions. It will also highlight some of the important parameters that affect these reactions.

Investigation of Electrochemical Performance of Li-O2 Batteries Using Catalyst Containing Carbon Nanotube Cathodes

Batteries with high specific energy densities have attracted a lot of attention due to their demand in electric vehicles (EVs). Due to the increase in demand for energy storage devices for applications such as load-levelling of electrical energy supply and demand propulsion for electric vehicles and microelectro-mechanical devices, the research for new classes of electrode materials that provide high energy, high power and cycle life longer than lithium-ion batteries is being motivated. Lithium air batteries have high specific energy density and they can achieve four times higher energy density than the current lithium ion batteries. The search for high energy density batteries has motivated the research in lithium-air batteries. Catalysts have been shown to improve both the battery capacity and the recyclability of these batteries when used in cathodes. Several catalysts have been discovered for lithium air batteries. Debart et al. used several catalysts for lithium oxygen batteries with non-aqueous electrolyte. The electro-catalysts used by them included Pt, La0.8Sr0.2MnO3, Fe2O3, NiO, CuO, CoFe2O4. Co3O4 exhibited highest discharge capacity and best cycling performance. Later Debart et al. examined manganese oxides as catalysts for lithium oxygen batteries. Mn3O4, bulk Mn2O3, bulk α, β, λ, γ-MnO2, α-MnO2 nanowires and β-MnO2 nanowires were used as catalysts. The air cathode with α-MnO2 as the catalyst showed the highest capacity of about 3000 mAh/g (carbon) Lu et al. reported ORR studies on polycrystalline metal catalysts of palladium, platinum, ruthenium, gold and glassy carbon surfaces in non-aqueous electrolyte. In our study, we investigate various modes of loading palladium-based catalysts and their effect on the capacity and round-trip efficiency of lithium-air batteries. Our aim is to increase the first discharge capacity, improve the recyclability, increase capacity retention and improve the recyclability of the battery. The loading and morphology the catalyst-CNT cathode is studied using transmission electron microscopy. Electrochemical testing using electrochemical impedance spectroscopy and charge-discharge cycling is also investigated. Voltammetry, Raman spectroscopy, FTIR, XRD, UV/Vis spectroscopy and visual inspection of the discharge products using scanning electron microscopy confirms the increased stability of the electrolyte due to this encapsulation and suggest that this approach could lead the increasing Li-O2 battery capacity and cyclability performance.

Effect of transition metal doping on the catalytic performance of Au–Pd/3DOM Mn2O3 for the oxidation of methane and o-xylene

Palladium-based catalysts are highly active for eliminating volatile organic compounds. Reducing the use of noble metals and enhancing performance of a catalyst are always desirable. The three-dimensionally ordered macroporous (3DOM) Mn2O3-supported transition metal M (M=Mn, Cr, Fe, and Co)-doped Au–Pd nanoparticles (NPs) with an Au–Pd–xM loading of 1.86–1.97wt% were prepared using the modified polyvinyl alcohol-protected reduction method. It is found that the Au–Pd–xM NPs with a size of 3.6–4.4nm were highly dispersed on the surface of 3DOM Mn2O3. The 1.94wt% Au–Pd–0.21Co/3DOM Mn2O3 and 1.94wt% Au–Pd–0.22Fe/3DOM Mn2O3 samples performed the best for the oxidation of methane and o-xylene, respectively. The methane oxidation rate at 340°C (339.0×10−6mol/(gPds)) over 1.94wt% Au–Pd–0.21Co/3DOM Mn2O3 was three times higher than that (93.8×10−6mol/(gPds)) over 1.97wt% Au–Pd/3DOM Mn2O3, and the o-xylene reaction rate at 140°C (2.59μmol/(gNs) over 1.94wt% Au–Pd–0.22Fe/3DOM Mn2O3 was two times higher than that (0.93μmol/(gNs) over 1.97wt% Au–Pd/3DOM Mn2O3. It is concluded that doping a certain amount of the transition metal to Au–Pd/3DOM Mn2O3 could modify the microstructure of the alloy NPs, thus improving the oxygen activation and methane adsorption ability. We are sure that the M-doped Au–Pd/3DOM Mn2O3 materials are promising catalysts for the efficient removal of volatile organic compounds.

The Reduction of Alkynes Over Pd-Based Catalyst Materials - A Pathway to Chemical Synthesis

Many reactions, including selective hydrogenation of alkynes, take place on solid surfaces. These reactions are vital in many areas of industry including the manufacture of polymers and fine chemicals such as vitamins, fragrances, and drugs. The choice of a catalyst is a trade-off between activity, selectivity and costs. Palladium-based heterogeneous catalysts are traditionally used for these processes as they provide the activation of hydrogen at room temperatures and offers reasonable selectivity, but these catalysts have a number of practical drawbacks. This review discusses recent research work in the selective hydrogenation of alkynes on palladium-based catalysts, emphasises the mechanism and catalytic materials and important applications including alkyne removal from gasphase alkene precursors for polymer synthesis and liquid phase selective hydrogenation for the synthesis of fine chemicals. Langmuir-Hinshelwood reaction kinetic models, reaction intermediates, formation of carbonaceous layer, the nature of active sites and the effects of reversible and irreversible adsorbates over Pd surface are discussed as well as the factors affecting catalyst activity and selectivity and how these can be optimised in synthetic protocols for these reactions.

Synthesis and Study of Chemical, Thermal, Mesomorphic, and Optical Properties of Terphenyls Modified with Nitrile Groups

We report the synthesis, characterization, and the thermotropic and photoluminescence properties of dialkoxyterphenyls with (T12-CN and T12-2CN) and without (T12) nitrile groups. These terphenyls were prepared through the Suzuki-Miyaura cross-coupling reaction using a palladium-based catalyst. The products obtained were analyzed as powders or after being drop-casted or spin-coated on glass. Nuclear Magnetic Resonance (1H NMR) and Fourier Transform Infrared (FTIR) spectroscopy techniques confirmed the structure and purity of the synthesized terphenyls. The mesomorphic behavior was studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). T12 developed various mesophases, whereas T12-CN and T12-2CN displayed one single mesophase of low order over a wide temperature range. The films topology was studied by AFM and the optical properties were determined by ultraviolet-visible (UV-Vis) spectroscopy and spectrofluorometry. Higher roughness was found for the films prepared with the asymmetric terphenyl (T12-CN). The photoluminescence (PL) spectrum obtained for the asymmetric terphenyl (T12-CN) exhibited the expected characteristics with an emission band centered at 381 nm and an overtone around 760 nm.

Matchmaking in Catalyst-Transfer Polycondensation: Optimizing Catalysts based on Mechanistic Insight.

Catalyst-transfer polycondensation (CTP) has emerged as a useful living, chain-growth polymerization method for synthesizing conjugated (hetero)arene-based polymers with targetable molecular weights, narrow dispersities, and controllable copolymer sequences-all properties that significantly influence their performance in devices. Over the past decade, several phosphine- and carbene-ligated Ni- and Pd-based precatalysts have been shown to be effective in CTP. One current limitation is that these traditional CTP catalysts lead to nonliving, non-chain-growth behavior when complex monomer scaffolds are utilized. Because these monomers are often found in the highest-performing materials, there is a significant need to identify alternative CTP catalysts. Recent mechanistic insight into CTP has laid the foundation for designing new catalysts to expand the CTP monomer scope. Building off this insight, we have designed and implemented model systems to identify effective catalysts by understanding their underlying mechanistic behaviors and systematically modifying catalyst structures to improve their chain-growth behavior. In this Account, we describe how each catalyst parameter-the ancillary ligand(s), reactive ligand(s), and transition metal-influences CTP. As an example, ancillary ligands often dictate the turnover-limiting step of the catalytic cycle, and perhaps more importantly, they can be used to promote the formation of the key intermediate (a metal-arene associative complex) and its subsequent reactivity. The fidelity of this intermediate is central to the mechanism for the living, chain-growth polymerization. Reactive ligands, on the other hand, can be used to improve catalyst solubility and accelerate initiation. Additional advantages of the reactive ligand include providing access points for postpolymerization modification and synthesizing polymers directly off surfaces. While the most frequently used CTP catalysts contain nickel, palladium-based catalysts exhibit a higher functional group tolerance and broader substrate scope (e.g., monomers with boron, magnesium, tin, and gold transmetalating agents). Overall, we anticipate that applying the tools and lessons detailed in this Account to other monomers should facilitate a better "matchmaking" process that will lead to new catalyst-transfer polycondensations.

Palladium-Catalyzed Aerobic Dehydrogenation of Cyclic Hydrocarbons for the Synthesis of Substituted Aromatics and Other Unsaturated Products.

Catalytic dehydrogenation of saturated or partially saturated six-membered carbocycles into aromatic rings represents an appealing strategy for the synthesis of substituted arenes. Particularly effective methods have been developed for the dehydrogenation of cyclohexanones and cyclohexenes into substituted phenol, aniline, and benzene derivatives, respectively. In this Perspective, we present the contributions of our research group to the discovery and development of palladium-based catalysts for aerobic oxidative dehydrogenation methods, including general methods for conversion of cyclohexanones and cyclohexenones into substituted phenols and a complementary method for partial dehydrogenation cyclohexanones to cyclohexenones. The mechanistic basis for chemoselective conversion of cyclohexanones to phenols or enones is presented. These results are presented within the context of recent methods developed by others for the synthesis of aryl ethers, anilines and other substituted arenes. Overall, Pd-catalyzed dehydrogenation methods provide a compelling strategy for selective synthesis of aromatic and related unsaturated molecules.

Investigation of methane oxidation by palladium-based catalyst via ReaxFF Molecular Dynamics simulation

Catalytic oxidations of methane over palladium-based nanoparticles, with and without oxygen coating, are investigated using ReaxFF Molecular Dynamics simulations. The simulation results show the complete dynamic process of the above catalytic reactions at the atomic level and help to reveal the underlying mechanisms both qualitatively and quantitatively. It is found that oxygen molecules are significantly easier to be adsorbed on both bare and oxygen-coated Pd surfaces compared with CH4. The presence of adsorbed O2 molecules on the surface blocks the active sites for CH4 adsorption on the oxygen-coated Pd surfaces. By comparing the adsorptive dissociation of CH4 over Pd nanoparticles with different levels of oxygen coverage, we find that it is much easier for the adsorptive dissociation of CH4 on oxygen-coated Pd nanoparticles than that on bare Pd nanoparticles at low temperatures. In contrast to the rapid dissociation of CH4 after adsorption, the dissociation of O2 requires much higher temperature than adsorption. Moreover, the CH4 dissociation rate increases with the rising temperature and is sensitive to the level of oxygen coverage on the surface. In addition, the activation energies for the adsorptive dissociation of CH4 are determined by fixed-temperature simulations from 400to 1000K through the changes of CH4 concentration and are found to be 3.27 and 2.28kcal mol−1 on 0.3 and 0.7 ML oxygen-coated Pd nanoparticles, respectively, which are consistent with density functional theory calculations and experiments.

Flame synthesis of novel ternary nanocatalysts Pd/CeO2[sbnd]TiO2 with promotional low-temperature catalytic oxidation properties

One-step flame-assisted spray pyrolysis with highly-quenched premixed stagnation flame is developed to synthesize the supported palladium catalysts on the mixed ceria-titania oxides. Promotional oxygen reducibility and inherent high activity of the catalysts are discovered in contrast to the same loading palladium catalyts on either single support, and the complete methane oxidation is achieved at as low as 450 °C. Palladium valence electronic structure analysis from X-ray photoemission spectroscopy (XPS) indicates that the metal palladium is highly positively charged and electrons transfer from palladium clusters to support oxides during the flame synthesis process. Similar electron transfer can even occur during the cyclic catalytic process for the initially non-charged clusters. From the cyclic experiments, theelectrostatic interaction built up at the metal–support interface plays a major role in the superior sintering resistance for palladium-based catalysts during the combustion process. Moreover, the introduction of water vapor to the feedstock induces a negative effect on the methane oxidation at low temperatures. XPS spectra reveals that the palladium 3d binding energy varies and the interfacial electron transfer flow changes over heating–cooling cycles when exposed to water vapor. More importantly, the excellent thermal stability performances remain even under the water vapor environment.

Palladium-Based Catalysts as Electrodes for Direct Methanol Fuel Cells: A Last Ten Years Review

Platinum-based materials are accepted as the suitable electrocatalysts for anodes and cathodes in direct methanol fuel cells (DMFCs). Nonetheless, the increased demand and scarce world reserves of Pt, as well as some technical problems associated with its use, have motivated a wide research focused to design Pd-based catalysts, considering the similar properties between this metal and Pt. In this review, we present the most recent advancements about Pd-based catalysts, considering Pd, Pd alloys with different transition metals and non-carbon supported nanoparticles, as possible electrodes in DMFCs. In the case of the anode, different reported works have highlighted the capacity of these new materials for overcoming the CO poisoning and promote the oxidation of other intermediates generated during the methanol oxidation. Regarding the cathode, the studies have showed more positive onset potentials, as fundamental parameter for determining the mechanism of the oxygen reduction reaction (ORR) and thus, making them able for achieving high efficiencies, with less production of hydrogen peroxide as collateral product. This revision suggests that it is possible to replace the conventional Pt catalysts by Pd-based materials, although several efforts must be made in order to improve their performance in DMFCs.

IMMOBILIZATION OF HIGHLY EFFECTIVE PALLADIUM CATALYST ONTO POLY(4-VINYLPYRIDINE): SYNTHESIS AND CHARACTERIZATION

A commonly known weakness of homogeneous catalysts is the difficulty to recover the active catalyst from the product. Due to the disadvantage, the designing of supported catalyst has been approached to overcome the separation difficulty of the palladium-based homogeneous catalyst. New polymer supported N2O2 metal complex was successfully immobilized by mixing of poly(4-vinylpyridine) with palladium(II) complex in the presence of ethyl acetate as solvent. Then, the reaction was stirred for 72 hours at room temperature to form corresponding P4VP-Pd catalyst. The properties of immobilized catalyst were characterized by various techniques such as fourier transform infrared (FTIR), thermogravimetric (TGA), X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray (SEM/EDX) and inductively coupled plasma-optical emission (ICP-OES) spectroscopy.

Virus-Based Metal Nanofoams As Scalable Three-Dimensional Current Collectors

In this work, we demonstrate the use of M13 virus to produce metal nanofoams that can be used as current collectors in lithium ion battery electrodes. Metal foam current collectors have been demonstrated to promote rapid electronic conduction in battery electrodes while maintaining a high active material mass fraction. An ideal three dimensional metal current collector comprises a nanofoam, with pore size on the order of one lithium diffusion distance through the active material on the timescale of one (dis)charge. In such a structure, the high specific surface area of a metal nanofoam allows deposition of large quantities of active material, while a continuous metal network embedded within the electrode provides an electronically conductive pathway. However, unlike macroscopic metal foams, metal nanofoams are not widely available because robust and scalable synthetic routes for their production are difficult to achieve. Our lab has taken steps toward this by producing metal nanofoams using M13 virus as a biological template, and showing that these materials can be used as 3D current collectors. M13 is a filamentous virus that has been used as a template for nanomaterials for both electronic and non-electronic applications. Its utility arises from its nanoscale dimensions, as well as the fact that its surface chemistry and morphological properties can be controlled genetically. To fabricate metal nanofoams, we treat the M13 virus capsid as a high aspect ratio nanoparticle and use it to assemble a template. The process uses amino acid residues at three positions on the pVIII major capsid protein: the lysine residue located at position 14 (L14), a glutamic acid fusion to the protein’s N terminus (EEAE), and a tyrosine to methionine point mutation at site 21 (Y21M). The first step in nanofoam fabrication uses glutaraldehyde to crosslink a concentrated virus solution, making use of the lysine residues exposed on the virus capsid. This facile crosslinking process results in a hydrogel. These hydrogels can be formed on a wide range of substrates, and freestanding hydrogels can be produced. The virus used for this process is genetically modified with an EEAE amino acid fusion that increases its negative surface charge. That surface charge is used to bind a palladium-based catalyst, which is then used for electroless deposition of nickel onto the hydrogel. The resulting open-cell nanofoam comprises struts that are templated onto the filamentous virus. As such, modifying the morphology of the virus particles was expected to affect the foam morphology. Because virus genes encode the amino acid sequence of the proteins forming the virus capsid, this can be done genetically. We demonstrate this control using the Y21M mutation, a point mutation shown to increase the persistence length of the virus. Using a Y21M variant as a genetically programmable control, we are able to lower the relative density of our foams. We also demonstrate control over foam morphology using virus concentration, and by controlling immersion time in the electroless bath. Lithium ion battery active materials were deposited on the nanofoam surface using electrodeposition. This technique is employed to deposit a controllable thickness of manganese oxide onto the surface of our biologically derived metal nanofoams. The resulting foams are then assembled into half cells. Using pulsed electrodeposition, the active material thickness was controlled by varying the number of pulses. By depositing thin active materials, we demonstrate that it is possible to produce ‘capacitor-like’ biotemplated electrodes with excellent rate capability. Thicker active material layers are used to produce more energy-dense electrodes with lower rate capability. Beyond the use of virus to synthesize three-dimensional current collectors for coin cells, we demonstrate that the biological templating method allows scale-up of the nanofoams to produce larger batteries. As the foams are based on self-assembling virus nanoparticles, it is possible to obtain large quantities of the template through biological amplification. Unlike a process employing extensive top-down fabrication techniques, genetically programmable templates provide a facile route toward large-format electrodes with control over nanoscale morphology. Through amplification of the virus and scale-up of the nanofoam production process, we demonstrate 4”x3” pouch cells. These electrodes are shown to be flexible, allowing compatibility with a wide range of systems. This demonstrates our biological templating method as an advance toward practical metal nanofoam engineering materials. Figure: an M13-templated nickel nanofoam before active material deposition. Figure 1

New magnetically recoverable palladium-based catalysts active in the alkoxycarbonylation of iodobenzene

Abstract New magnetically recoverable catalysts have been synthesized by deposition of 1% palladium (Pd)-metal on the polymer poly(1-oxo-trimethylene), containing 15% of magnetite. The magnetite allows the complete recovery of the catalyst with the simple application of an external magnetic field. The activity of such a catalyst has been studied under phosphine-free conditions in the alkoxycarbonylation of iodobenzene to the corresponding benzoic acid esters.

Performance of carbon-supported palladium and palladium[sbnd]ruthenium catalysts for alkaline membrane direct ethanol fuel cells

The activity of carbon supported palladium-based catalysts towards ethanol oxidation reaction in alkaline medium is investigated. To this end, Pd and PdxRu (x = 1, 6, 12) materials are synthesized supported on Vulcan XC72R by a modified impregnation method. These materials are characterized physicochemically and their composition varies between 40 and 50 wt. % of metal content. The mean size of nanoparticles is below 4 nm, and the size of the majority of particles ranges from 2 to 6 nm. The catalytic activity of these samples is also tested in half-cell and alkaline membrane direct ethanol fuel cell (AMDEFC). Lower onset potential and higher oxidation current densities is observed in palladium-rich bimetallic catalysts. By using the prepared samples as anode of AMDEFC and a commercial membrane of polybenzimidazole as solid electrolyte, the maximum power density is observed for Pd12Ru/C, 97 mW cm−2, almost doubling the value achieved by the catalyst Pd/C (50 mW cm−2), and above the obtained by commercial PtRu/C catalyst (89 mW cm−2).

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction.

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.

Enhancing low-temperature activity and durability of Pd-based diesel oxidation catalysts using ZrO2 supports

We investigated the impact of ZrO2 on the performance of palladium-based oxidation catalysts with respect to low-temperature activity, hydrothermal stability, and sulfur tolerance. Pd supported on ZrO2 and SiO2 were synthesized for a comparative study. Additionally, in an attempt to maximize the ZrO2 surface area and improve sulfur tolerance, a Pd support with ZrO2-dispersed onto SiO2 was studied. The physicochemical properties of the catalysts were examined using ICP, N2 sorption, XRD, SEM, TEM, and NH3-, CO2-, and NOx-TPD. The activity of the Pd catalysts were measured from 60 to 600°C in a flow of 4000ppm CO, 500ppm NO, 1000ppmC3H6, 4% O2, 5% H2O, and Ar balance. The Pd catalysts were evaluated in fresh, sulfated, and hydrothermally aged states. Overall, the ZrO2-containing catalysts showed considerably higher CO and C3H6 oxidation activity than Pd/SiO2 under the reaction conditions studied. The good performance of ZrO2-containing catalysts appeared to be due in part to high Pd dispersion resulting from strong Pd and support interaction. Another beneficial effect of strong interaction between Pd and ZrO2 was manifested as a greater hydrothermal stability with good oxidation activity even after aging at 800 and 900°C for 16h. In contrast, Pd/SiO2 suffered significant performance loss due to Pd particle coarsening. Although the Pd/ZrO2-SiO2 catalyst was not more active than Pd/ZrO2, improved tolerance to sulfur was realized. Unlike the bulk ZrO2 support, the ZrO2-incorporated SiO2 presented only weak basicity leading to a superior sulfur tolerance of Pd/ZrO2-SiO2. These results confirmed the potential of developing Pd-based oxidation catalysts with enhanced low-temperature activity and durability using ZrO2-SiO2 supports. Controlling morphology and accessible area of the dispersed ZrO2 layer appeared critical to further maximize the catalytic performance.