PtIr/C Catalysts Synthesized by Electrochemical Dispersion Method for Proton Exchange Membrane Fuel Cells

PtIr/C electrocatalyst with the metal phase uniformly distributed over the carbon support surface and the average size of PtIr nanoparticles of 5.9 nm is synthesized by electrochemical dispersion of Pt₉₀Ir₁₀ alloy under the action of alternating pulse current. It is shown that the presence of iridium within the composition of a Pt/C catalyst lowers down the overpotential of CO oxidation and increases catalyst’s specific activity with respect to electrochemical oxidation of ethanol.     

Promotion effect of adsorbed water/OH on the catalytic performance of Ag/activated carbon catalysts for CO preferential oxidation in excess H2

Activated carbon (AC) supported silver catalysts were prepared by incipient wetness impregnation method and their catalytic performance for CO preferential oxidation (PROX) in excess H₂ was evaluated. Ag/AC catalysts, after reduction in H₂ at low temperatures (≤200 °C) following heat treatment in He at 200 °C (He200H200), exhibited the best catalytic properties. Temperature-programmed desorption (TPD), X-ray diffraction (XRD) and temperature-programmed reduction (TPR) results indicated that silver oxides were produced during heat treatment in He at 200 °C which were reduced to metal silver nanoparticles in H₂ at low temperatures (≤200 °C), simultaneously generating the adsorbed water/OH. CO conversion was enhanced 40% after water treatment following heat treatment in He at 600 °C. These results imply that the metal silver nanoparticles are the active species and the adsorbed water/OH has noticeable promotion effects on CO oxidation. However, the promotion effect is still limited compared to gold catalysts under the similar conditions, which may be the reason of low selectivity to CO oxidation in PROX over silver catalysts. The reported Ag/AC-S-He catalyst after He200H₂00 treatment displayed similar PROX of CO reaction properties to Ag/SiO₂. This means that Ag/AC catalyst is also an efficient low-temperature CO oxidation catalyst.     

Brominated Methanes as Photoresponsive Molecular Storage of Elemental Br2

The photochemical generation of elemental Br₂ from brominated methanes is reported. Br₂ was generated by the vaporization of carbon oxides and HBr through oxidative photodecomposition of brominated methanes under a 20 W low‐pressure mercury lamp, wherein the amount and situations of Br₂ generation were photochemically controllable. Liquid CH₂Br₂ can be used not only as an organic solvent but also for the photoresponsive molecular storage of Br₂, which is of great technical benefit in a variety of organic syntheses and in materials science. By taking advantage of the in situ generation of Br₂ from the organic solvent itself, many organobromine compounds were synthesized in high practical yields with or without the addition of a catalyst. Herein, Br₂ that was generated by the photodecomposition of CH₂Br₂ retained its reactivity in solution to undergo essentially the same reactions as those that were carried out with solutions of Br₂ dissolved in CH₂Br₂ that were prepared without photoirradiation. Furthermore, HBr, which was generated during the course of the photodecomposition of CH₂Br₂, was also available for the substitution of the OH group for the Br group and for the preparation of the HBr salts of amines. Furthermore, the photochemical generation of Br₂ from CH₂Br₂ was available for the area‐selective photochemical bleaching of natural colored plants, such as red rose petals, wherein Br₂ that was generated photochemically from CH₂Br₂ was painted onto the petal to cause radical oxidations of the chromophoric anthocyanin molecules.     

Investigation on the mechanism and applications of the reaction Cl2+2HBr=2HCl+Br2

The mechanism of reaction Cl₂+2HBr=2HCl+Br₂ has been carefully investigated with density functional theory (DFT) at B3LYP/6-311G^** level. A series of three-centred and four-centred transition states have been obtained. The activation energy (138.96 and 147.24 kJ/mol, respectively) of two bimolecular elementary reactions Cl₂+HBr→HCl+BrCl and BrCl+HBr→HCl+Br₂ is smaller than the dissociation energy of Cl₂, HBr and BrCl, indicating that it is favorable for the title reaction occurring in the bimolecular form. The reaction has been applied to the chemical engineering process of recycling Br₂ from HBr. Gaseous Cl₂ directly reacts with HBr gas, which produces gaseous mixtures containing Br₂, and liquid Br₂ and HCl are obtained by cooling the mixtures and further separated by absorption with CCl₄. The recovery percentage of Br₂ is more than 96%, and the Cl₂ remaining in liquid Br₂ is less than 3.0%. The paper provides a good example of solving the difficult problem in chemical engineering with basic theory.     

Catalysis by HBr in the bromination of o-xylene in acetic acid

Hydrogen bromide is known to inhibit the bromination of aromatic substrates (ArH), either by fixing up bromine as HBr₃ or ArH as ArH · HBr. However, there is catalysis by HBr in the bromination of mesitylene in acetic acid. The bromination of o-xylene in acetic acid in the dark is found to be autocatalytic, and the reaction is overall third order, first order in o-xylene with the orders in Br₂ and HBr depending on the concentrations. A composite rate expression involving Br₂ and HBr as electrophiles has been proposed and verified using iodine bromide as a catalyst where the orders are one in each of the reactants, irrespective of the concentrations used.     

Thiol‐Assisted Synthesis of Carbon‐Supported Metal Nanoparticles for Efficient Electrocatalytic CO2 Reduction

The typical preparation route of carbon‐supported metallic catalyst is complex and uneconomical. Herein, we reported a thiol‐assisted one‐pot method by using 3‐mercaptopropionic acid (MPA) to synthesize carbon‐supported metal nanoparticles catalysts for efficient electrocatalytic reduction of carbon dioxide (CO₂RR). We found that the synthesized Au?MPA/C catalyst achieves a maximum CO faradaic efficiency (FE) of 96.2% with its partial current density of ?11.4 mA/cm², which is much higher than that over Au foil or MPA‐free carbon‐supported Au (Au/C). The performance improvement in CO₂RR over the catalyst is probably derived from the good dispersion of Au nanoparticles and the surface modification of the catalyst caused by the specific interaction between Au nanoparticles and MPA. This thiol‐assisted method can be also extended to synthesize Ag?MPA/C with enhanced CO₂RR performance.     

Decomposition of nitrous oxide on AlFe-PILC catalyst

Summary Two AlFe-PILC catalysts were prepared with different OH/metal ratio and applied in nitrous oxide (N₂O) decomposition reactions. The 100% conversion of N₂O with NH₃into N₂and H₂O was achieved below 500^oC with both applied catalysts. However, the activity of catalysts in direct conversion of N₂O into N₂and O₂did not exceed 40 % below 500^oC. In this reaction the activity of AlFe-PILC catalyst synthesized at higher OH/metal ratio (4) is higher compared to the activity of AlFe-PILC catalyst with OH/metal ratio (2). Free FeO·Fe₂O₃particles were registered in the AlFe-PILC catalyst with higher OH/metal ratio (4).     

Investigation on the mechanism and applications of the reaction Cl2+2HBr=2HCl+Br2

The mechanism of reaction Cl₂+2HBr=2HCl+Br₂ has been carefully investigated with density functional theory (DFT) at B3LYP/6-311G^** level. A series of three-centred and four-centred transition states have been obtained. The activation energy (138.96 and 147.24 kJ/mol, respectively) of two bimolecular elementary reactions Cl₂+HBr→HCl+BrCl and BrCl+HBr→HCl+Br₂ is smaller than the dissociation energy of Cl₂, HBr and BrCl, indicating that it is favorable for the title reaction occurring in the bimolecular form. The reaction has been applied to the chemical engineering process of recycling Br₂ from HBr. Gaseous Cl₂ directly reacts with HBr gas, which produces gaseous mixtures containing Br₂, and liquid Br₂ and HCl are obtained by cooling the mixtures and further separated by absorption with CCl₄. The recovery percentage of Br₂ is more than 96%, and the Cl₂ remaining in liquid Br₂ is less than 3.0%. The paper provides a good example of solving the difficult problem in chemical engineering with basic theory.     

Der chemische Transport von Lanthanphosphat LaPO4 mit Br-haltigen Systemen

The chemical transport of LaPO₄ (1400 → 1200°K) is discussed on a thermodynamical basis and experimentally checked. Br₂ + PBr₃ used as transport agents give good transportrates. Br₂ + CO and Br₂ + C are also suitable transportsystems. On the other hand Br₂ without additions, caused by an unfavourable equilibrium position gives no measurable LaPO₄-transport. Using HBr as transport agent, the transport rate is small. In addition there are difficulties, caused by the partial decomposition of HBr into the elements and the diffusion of H₂ through the wall of the quartz ampoule. LaPO₄-crystals prepared by chemical transport have the well known monoclinic monazite structure.     

Preparation of Pd/C catalyst for formic acid oxidation using a novel colloid method

A novel colloid method using (WO₃)_n·xH₂O as colloidal source was developed to prepare Pd/C catalyst for formic acid oxidation. Transmission electron microscopy image shows that the Pd/C nanoparticles have an average size of 3.3 nm and a narrow size distribution. Electrochemical measurements indicate that the Pd/C catalyst exhibits significantly high electrochemical active surface area and high catalytic activity with good stability for formic acid oxidation compared with that prepared by common method. The colloid method is very simple and has great potentials for mass-producing Pd/C and others noble metal catalysts.     

Catalytic Reduction of NOx by CO and Hydrocarbons Over Different Catalysts in the Presence or Absence of O2

The performances of a non-noble metal (catalyst A), a non-noble metal catalyst containing a smaller amount of a noble metal (catalyst B) and noble metal (catalyst C) for NOx reduction at 400 - 600°C and space velocity of 16×10^-1h^-1 have been studied by means of a fixed-bed continuous flow system. In both the absence and presence of oxygen, the following activity orders of catalysts for the reduction of NOx to N₂ by CO and C₃H₆(HC) can be given: C > B > A and B > A C, respectively. Meanwhile, there was CO formation in the reduction reactions of NOx by HC on the three catalysts. The amount of CO produced on catalyst C was the largest of the three catalysts.     

Mechanistic investigation inspired “on water” reaction for hydrobromic acid‐catalyzed Friedel–Crafts–type reaction of β‐naphthol and formaldehyde

The mechanism of the HBr‐catalyzed Friedel‐Crafts‐type reaction between β‐naphthol and HCHO was investigated by DFT to improve this reaction. The HBr‐H₂O co‐catalyzed the preferential pathway undergoes the concerted nucleophilic addition and hydrogen shift, stepwise followed by H₂O elimination and the C C bond formation. The origin of the high catalytic activity of HBr is ascribed to C H···Br⁻ and O H···Br⁻ interactions, which suggest that the active species is Br⁻. Moreover, water molecules efficiently assist in improving the activity of Br⁻. The computational results show that solvent polarity profoundly affects the activation barriers. To our delight, the activation barrier of the rate‐determining step for the favored pathway in water is comparable (0.6 kcal/mol difference) with that in acetonitrile. The experimental observation further confirmed our results and demonstrated that the title reaction can be successfully achieved “on water.” Therefore, we open a new efficient and green strategy for the synthesis of biphenol derivatives. © 2017 Wiley Periodicals, Inc.     

Nb-TiO2 supported Pt cathode catalyst for polymer electrolyte membrane fuel cells

The Nb-doped TiO₂ nanostructure (Nb-TiO₂) was prepared as a support of metal catalyst in polymer electrolyte membrane fuel cells. Using the Nb-TiO₂ nanostructure support, we prepared the Nb-TiO₂ supported catalyst. The Nb-TiO₂ supported Pt catalyst (Pt/Nb-TiO₂) showed the well dispersion of Pt catalysts (∼3 nm) on the Nb-TiO₂ nanostructure supports (∼10 nm). The Pt/Nb-TiO₂ showed an excellent catalytic activity for oxygen reduction compared with carbon supported Pt cathode catalyst. The enhanced catalytic activity of Pt/Nb-TiO₂ in electrochemical half cell measurement may be mainly due to well dispersion of Pt nanoparticles on Nb-TiO₂ nanosized supports. In addition, from XANES spectra of Pt L edge obtained with the supported catalysts, the improved catalytic activity of Pt/Nb-TiO₂ for oxygen reduction may be caused by an interaction between oxide support and metal catalyst.     

Development of magnetic,ferrite supported palladium catalysts for 2,4-dinitrotoluene hydrogenation

Cobalt, copper, and nickel ferrite spinel nanoparticles have been synthesized by using a combination of sonochemical treatment and combustion. The magnetic nanoparticles have been used as supports to prepare ~4 wt% palladium catalysts. The ferrites were dispersed in an ethanolic solution of Pd(II) nitrate by ultrasonication. The palladium ions were reduced to metallic Pd nanoparticles, which were then attached to the surface of the different metal oxide supports. Thus, three different catalysts (Pd/CoFe₂O₄, Pd/CuFe₂O₄, Pd/NiFe₂O₄) were made and tested in the hydrogenation of 2,4-dinitrotoluene (DNT). A possible reaction mechanism, including the detected species, has been envisaged based on the results. The highest 2,4-diaminotoluene (TDA) yield (99 n/n%) has been achieved by using the Pd/NiFe₂O₄ catalyst. Furthermore, the TDA yield was also reasonable (84.2 n/n%) when the Pd/CoFe₂O₄ catalyst was used. In this case, complete and easy recovery of the catalyst from the reaction medium is ensured, as the ferrite support is fully magnetic. Thus, the catalyst is very well suited for applicationy in the hydrogenation of DNT or other aromatic nitro compounds.     

Regulation of Strong Metal-Support Interaction by Alkaline Earth Metal Salts

Classical strong metal-support interaction (SMSI) is of significant importance to heterogeneous catalysis, where electronic promotion and encapsulation of noble metal by reducible support are two main intrinsic properties of SMSI. However, the excessive encapsulation will inevitably hamper the contact between active sites and reactant, leading to reduced activity in catalysis. Herein, alkaline earth metal salts are employed to depress the encapsulation of Ru nanoparticles in Ru/TiO₂ catalyst in the present study. Thermodynamic calculation, transmission electron microscopy (TEM) and chemisorption results show that the alkaline earth metal salts could successfully prevent the migration of TiO_2-x overlayer to Ru nanoparticles in Ru/TiO₂ catalyst via in situ formation of titanates, resulting in high exposure of active metal. Meanwhile, X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H₂-TPR) results reveal that an even stronger electron donation from the reduced support to Ru nanoparticles is achieved. As a result, the alkaline earth metal salts-doped Ru/TiO₂ catalysts exhibit superior activity in catalytic hydrogenation of aromatics, which is in contrast to the pristine Ru/TiO₂ catalyst that shows negligible activity under the same conditions due to the excess encapsulation of Ru nanoparticles in Ru/TiO₂ catalyst.     

A highly stable and active mesoporous ruthenium catalyst for ammonia synthesis prepared by a RuCl3/SiO2-templated approach

Molecular nitrogen is relatively inert and the activation of its triple bond is full of challenges and of significance. Hence, searching for an efficiently heterogeneous catalyst with high stability and dispersion is one of the important targets of chemical technology. Here, we report a Ba-K/Ru-MC catalyst with Ru particle size of 1.5–2.5 nm semi-embedded in a mesoporous C matrix and with dual promoters of Ba and K that exhibits a higher activity than the supported Ba-Ru-K/MC catalyst, although both have similar metal particle sizes for ammonia synthesis. Further, the Ba-K/Ru-MC catalyst is more active than commercial fused Fe catalysts and supported Ru catalysts. Characterization techniques such as high-resolution transmission electron microscopy, N₂ physisorption, CO chemisorption, and temperature-programmed reduction suggest that the Ru nanoparticles have strong interactions with the C matrix in Ba-K/Ru-MC, which may facilitate electron transport better than supported nanoparticles.     

A Copper‐Based Metal–Organic Framework Acts as a Bifunctional Catalyst for the Homocoupling of Arylboronic Acids and Epoxidation of Olefins

A copper(I)‐based metal–organic framework ({[Cu₂Br₂(pypz)]_n?nH₂O} (Cu—Br–MOF) [pypz=bis[3,5‐dimethyl‐4‐(4’‐pyridyl)pyrazol‐1‐yl] methane] has been synthesized by using an elongated and flexible bridging ligand. The structure analysis reveals that each pypz ligand acts as a tritopic ligand connected to two Cu₂Br₂ dimeric units, forming a one‐dimensional zig–zag chain, and these chains further connected by a Cu₂Br₂ unit, give a two‐dimensional framework on the bc‐plane. In the Cu₂Br₂ dimeric unit, the copper ions are four coordinated, thereby possessing a tetrahedral geometry; this proves to be an excellent heterogeneous catalyst for the aerobic homocoupling of arylboronic acids under mild reaction conditions. This method requires only 3 mol % of catalyst and it does not require any base or oxidant—compared to other conventional (Cu, Pd, Fe, and Au) catalysts—for the transformation of arylboronic acids in very good yields (98 %). The shape and size selectivity of the catalyst in the homocoupling was investigated. The use of the catalyst was further extended to the epoxidation of olefins. Moreover, the catalyst can be easily separated by simple filtration and reused efficiently up to 5 cycles without major loss of reactivity.     

Synthesis,characterization and catalytic application of a new organometallic oligomer based on polyhedral oligomeric silsesquioxane

Although homogeneous catalysts provide high performance and selectivity, the difficulty of separation and recycling of these catalysts has bothered the scientific community worldwide. Therefore, the demand for heterogeneous catalysts that possess the advantages of homogeneous ones, with ease of separation and recyclability remains a topic of major impact. The oligomeric catalyst synthesized in this work was characterized using elemental analysis, Fourier transform infrared, ¹³C NMR, ²⁹Si NMR and energy‐dispersive X‐ray spectroscopies, X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy and Brunauer–Emmett–Teller analysis and compared to its homogeneous counterpart [W(CO)₃Br₂(ATC)] in the epoxidation of 1‐octene, cyclooctene, (S )‐limonene, cis ‐3‐hexen‐1‐ol, trans ‐3‐hexen‐1‐ol and styrene. The results showed that the percentage conversion for the homogeneous species [W(CO)₃Br₂(ATC)] was slightly higher than for the oligomeric catalyst (POSS‐ATC‐[W(CO)₃Br₂]). Furthermore, the selectivity for epoxide of the oligomeric catalyst was greater than that of the homogeneous catalyst by about 25% when (S )‐limonene was used. Great conversions (yields) of products were obtained with a wide range of substrates and the catalyst was recycled many times without any substantial loss of its catalytic activity.     

Development of Ceria-Supported Ruthenium Catalysts Effective for Various Synthetic Reactions

Our recent results on organic transformations such as C–C bond formation via the activation of stable C–C or C–H bonds and aerobic oxidation of alcohols catalyzed by CeO₂-supported ruthenium are reviewed. A simple, recyclable heterogeneous Ru/CeO₂ catalyst showed excellent activity for sequential transfer-allylation/isomerization of homoallyl alcohols with aldehydes to saturated ketones via the C–C bond activation. While homogeneous ruthenium and rhodium complex catalysts require additives and/or pressurized CO, the reaction with Ru/CeO₂ smoothly proceeded in the absence of any additives. The Ru/CeO₂ catalyst also showed excellent activity for the addition of sp² C–H bonds of aromatic ketones to vinylsilanes. The Ru/CeO₂ catalyst realized the chelation-assisted arylation of stable aromatic C–H bonds with aryl chlorides. The activity of the catalyst was greatly improved by the PPh₃-modification under hydrogen atmosphere prior to the reactions. The catalyst acts heterogeneously without a significant leaching of ruthenium species, indicating that the Ru/CeO₂ catalyst has an advantage over homogeneous catalysts from practical and environmental points of view. The effects of chemical and physical properties of CeO₂ on the activity of CeO₂-supported noble metal catalysts were examined. Porous CeO₂ powders were prepared by the coagulation of solvothermally synthesized colloidal ceria nanoparticles, and the thus-prepared CeO₂ powders showed an oxygen migration ability far superior to the CeO₂ samples prepared by the usual precipitation method. The ruthenium catalysts supported on the former CeO₂ powders showed a high activity for the aerobic oxidation of benzyl alcohol. The effects of the pore structure of CeO₂ powders on the activity of the Ru/CeO₂ catalysts are also discussed.     

Low Temperature WGS Catalysts for Hydrogen Station and Fuel Processor Applications

Generally, water gas shift (WGS) reaction is a very important step in the industrial production of hydrogen, ammonia and other bulk chemicals utilizing synthesis gases. In this paper, we are reporting WGS reaction carried out in our research group for the application of hydrogen station and fuel processor. We prepared various Mo₂C, Pt–Ni-based and Cu-based catalysts for low temperature WGS reaction. The characteristics of the prepared catalyst were analyzed by N₂ physisorption, CO chemisorptions, XRD, SEM and TEM technologies, and compared with that of commercial Cu-Zn/Al₂O₃ catalyst. It was found that prepared catalysts displayed reasonably good activity and thermal cycling stability than commercial LTS (Cu–Zn/Al₂O₃) catalyst. It was found that the deactivation of commercial LTS catalyst during the thermal cycling run at 250 °C was caused by the sintering of active metal even though it shows high activity at less than 250 °C. The deactivation of Mo₂C catalyst during the thermal cycling run was caused by the transition of Mo^δ+, Mo^IV and Mo₂C on the surface of Mo₂C catalyst to Mo^VI(MoO₃) with the reaction of H₂O in reactants. However, they showed higher stability than the commercial LTS catalyst during thermal cycling test. The Pt–Ni/CeO₂ catalyst after the thermal cycling shows slightly deactivation due to the sintering of Ni metal. Among Cu-based catalysts, it was found that Cu–Mo/Ce_0.5Zr_0.5O₂ catalyst has higher WGS activity and stability over commercial LTS catalyst. The results suggested that Pt–Ni/CeO₂ and Cu–Mo/Ce_0.5Zr_0.5O₂ catalysts are desirable candidates for application in hydrogen station and fuel processor system even though all other catalysts deactivated slowly during the thermal cycling run.