A 3- and 2-fold increase in selectivities toward 2-ethyltoluene and n-propylbenzene, respectively, in indan ring opening (RO) was achieved by introducing palladium to the ruthenium catalyst. The product selectivities for the Ru–Pd system with the 4:1 molar ratio were the same as those for monometallic iridium, known for its outstanding single cleavage selectivity; the lights formation was suppressed as compared with the monometallic platinum catalyst. A further increase in the Pd amount did not result in the selectivity improvement and brought down the activity to the low level of Pd. The bimetallic catalysts were synthesized in the presence of poly-(vinylpyrrolidone). The bimetallic systems revealed sintering resistance up to 400 °C, as compared with their monoforms. The indan RO activity was maximized after precalcination at 200 °C. The suggested nanoparticles’ bimetallicity was consistent with the results of CO-TPD, CO–DRIFTS, thermal stability tests, and a chemical probe reaction (olefin hydrogenation, in which only Pd is active). The Pd–Ru system is envisioned as a viable alternative to monometallic Ir for RO.  © 2014 American Chemical Society, reprinted with permission.

Tungsten carbide-based electrodes under mixed hydrogen–methane and methane fuels have been investigated as potential anode materials for solid oxide fuel cell (SOFC) application. Firstly, it was shown that hydrogen is not a suitable fuel for the carbide-based materials. A conventional WC–YSZ composite and a carbide infiltrated porous YSZ support were then studied. Ac impedance spectroscopy revealed that the ohmic resistance and the charge-transfer polarization of these cells were reasonably low. The chemical reaction polarization, however, was relatively large, particularly under methane fuel. The carbide-based electrodes were then modified by incorporation of ceria and/or ruthenium. Not only did the co-existence of CeO₂ and Ru synergically enhance the cell performance, more importantly it also greatly improved the stability of the polarized cell. Although bulk phase analysis confirmed the presence of a minor amount of tungsten oxide, surface analysis showed that the oxide phase remained superficial. It was then proposed that surface oxidation of the carbide phase was essentially a part of the fuel oxidation process and, as long as the rate of carbide oxidation and that of oxide recarburization remained comparable, the cell performance was stable.  © 2012 Elsevier, reprinted with permission.

A structured catalyst support was developed based on FeCrAl alloy sintered microfibers (SMF) via multiple stage thermal oxidation in air for 1 h at 930 °C, 1 h at 960 °C and 2 h at 990 °C. The procedure resulted in the formation of a forest of predominantly α-alumina whiskers (200 nm in height and 100 nm apart). Palladium deposition and reduction yielded 0.5 wt.% Pd/SMF with 20 nm nanoparticle size. The catalyst was tested in three-phase hydrogenation of 2-methyl-3-buten-2-ol, and due to its pore structure allowed eliminating internal mass transfer limitations. The developed support can be beneficial to catalytic reactions suffering from mass transfer limitations and catalyst deactivation via pore mouth blocking. As compared to other methods of structured catalyst preparation, the thermal oxidation procedure is simple, fast and environmentally benign, and eliminates problems associated with poor adhesion of traditional washcoated layers of powdered catalysts or supports.  © 2011 Elsevier, reprinted with permission.

Pd nanoparticles catalyze Tsuji–Trost reaction for Pd fluorometric detection in the presence of PPh₃ via atomic dissolution:

Highlights

• Pd nanoparticles catalyze Tsuji–Trost reaction used for Pd fluorometric detection.
• Nanoparticle activity is correlated with the number of defect surface atoms.
• The reaction occurs via atomic dissolution and Ostwald ripening.
• The highest rate is concomitant with the formation of largest nanoparticles.
• Optimum PPh₃ concentration exists for the fastest reaction.

Abstract

Palladium nanospheres of 2.4 and 3.8 nm diameter and nanocubes of 18 nm rib length were used to catalyze a fluorometric Tsuji–Trost reaction for the transformation of a phenyl allyl ether to a fluorescent phenol in the presence of triphenylphosphine, which was pivotal to the catalytic activity. Turnover frequencies calculated per defect atoms were found similar for all nanoparticles, indicating that these atoms are the active sites. However, kinetic studies combined with Pd leaching and transmission electron microscopy analyses in the presence of various reaction components showed Pd leaching via oxidative addition of a reactant, followed by nanoparticle growth depending on the PPh₃ concentration. The formation of largest particles was found for the fastest reaction with PPh₃/Pd molar ratio of 4, in the range from 0 to 9. This study shows the validity of the atomic dissolution mechanism in the reaction of interest. © 2011 Elsevier, reproduced with permission.

The catalytic steam gasification of coke from Athabasca bitumen was investigated by thermogravimetric analysis using K₂CO₃ and Na₂CO₃ as catalysts, both of which reduced the activation energy of the reaction considerably to 1.2 × 10⁵ J mol⁻¹ and 1.3 × 10⁵ J mol⁻¹, respectively, down from 2.1 × 10⁵ J mol⁻¹ for the uncatalyzed reaction. The reaction rates varied with the partial pressure of steam between 60 kPa and 85 kPa consistent with a Langmuir–Hinshelwood model, but a first order equation was also sufficient given the low partial pressures. The initial rate of gasification of the coke particles correlated linearly with the estimated external surface area of the particles, as expected from a surface reaction involving a non-porous solid. The initial reaction rate increased with increasing the catalyst loading up to 2.4 (mol potassium)/kg. A portion of the catalyst penetrated into the coke, as confirmed by secondary ion mass spectroscopy analysis, where it could not promote the reaction with steam. This result was consistent with a small increase observed in the reaction rate at low catalyst loading. The shrinking core model was successful in predicting the rates at higher conversions from the initial rate data, despite increases in BET surface area with conversion. © 2011 Elsevier, reprinted with permission.

Monodisperse Pd nanocubes of 20 nm rib length and Pd nanospheres of 3.0 nm diameter were synthesized in the presence of cetyltrimethylammonium bromide and used to investigate the structure sensitivity of three-phase 2-methyl-3-buten-2-ol hydrogenation. Turnover frequencies per all surface atoms were found as 2.58 s⁻¹ for the cubes and 3.86 s⁻¹ for the spheres at 313 K, indicating that (100) atoms of the cubes comprising ∼98% of all surface atoms have lower activity than other surface atoms of the spheres, composed of atoms on (111), (100) terraces, edges, and vertices. Apparent activation energies of 23 kJ/mol for the cubes and 17 kJ/mol for the spheres in the verified kinetic regime confirmed the reaction structure sensitivity. Assuming that only (100) and (111) atoms are active on the sphere surface, a hypothetical most active Pd nanostructure was predicted as a tetrahedron allowing twice higher activity per Pd loading as compared to a spherical particle. © 2010 American Chemical Society, reprinted with permission.

Recent advances in the liquid‐phase synthesis of metal nanostructures of different sizes and shapes are reviewed regarding their catalytic properties. The controlled synthesis of nanostructures is based on the colloid chemistry techniques in the solution, which use organic nanoreactors and a variety of stabilizers. Their catalytic activity and selectivity depend on the particle’s shape and size, as shown for Suzuki and Heck coupling, hydrogenations, hydrogenolysis, oxidations, and electron‐transfer reactions. The knowledge of a reaction’s structure‐sensitivity relationship is important for the rational catalyst design in view of process intensification. Nanostructures can be used per se and in supported form to meet the requirements of an eventual process. © 2009 Taylor & Francis.

Palladium nanohexagons were prepared using a seed-mediated method. Their catalytic performance in 2-methyl-3-butyn-2-ol hydrogenation was compared to the one of monodispersed Pd nanospheres. Quantitative correlations between initial turnover frequencies (TOFs) and nanoparticle surface compositions showed independence of TOFs calculated per atoms on Pd(111) facets on particle size and shape. © 2009 Springer, reproduced with permission.

The solvent-free selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) was studied over a Pd/ZnO structured catalyst and compared to its behavior in water-assisted conditions. The catalytic behavior was correlated with the surface properties of the catalysts which were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The catalyst showed high selectivity and stability with the performance being superior to that of the industrial Lindlar catalyst (50%). The addition of a sulphur-containing modifier in the reaction mixture was found to affect the activity and to hinder the over-hydrogenation reaction. The MBE yield of ∼97% was attained at MBY conversion >99%. The reuse of the catalyst showed that it deactivated by a 38% and that its selectivity slightly increased (∼0.5%) over 10 runs. The reaction kinetics was modeled using a Langmuir–Hinshelwood mechanism considering competitive adsorption for the organic species and dissociative adsorption for hydrogen. The kinetic experiments were planned and the results analyzed following a design of experiments (DOE) methodology. This approach led not only to a robust model that predicts the reaction rate in a wide range of reaction conditions but also to the determination of its kinetic parameters.  © 2009 Elsevier, reprinted with permission.

Systematic studies of mass transfer interactions with intrinsic reaction kinetics were performed for the three-phase selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) over a modified Pd/CaCO₃ catalyst under solvent free conditions. Hydrogen concentration in the liquid phase (C_H2,b) was monitored in situ during the catalytic reaction by means of the “Fugatron” analyzer. Reactions were carried out in an autoclave at different stirring rates at two concentrations of hydrogen (5 and 13 mol·m⁻³). For stirring speeds higher than 1500 rpm no influence of gas−liquid mass transfer was observed. Hydrogen liquid−solid (L-S) mass transfer was found to be negligible, whereas the MBY mass L-S transfer becomes important at high MBY conversions at high hydrogen concentration. Low stirrer speed caused the reaction rate and MBE selectivity to decrease. No internal mass transfer limitations were observed, and conditions for the kinetic regime were found. The kinetics modeled followed the Langmuir−Hinshelwood mechanism and was consistent with the experimental data. © 2008 American Chemical Society, reprinted with permission.

Monodispersed Pd nanoparticles (8, 11, and 13 nm in diameter) as confirmed by high resolution transmission electron microscopy were prepared via the reverse microemulsion method and deposited on structured supports consisting of carbon nanofibers (CNF) grown on sintered metal fibers (SMF). The CNF/SMF supports were subjected to oxidative treatments to introduce O-functional groups on the CNF surface. These groups were characterized by temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy. The catalysts were used to study (a) the effect of Pd size and (b) the effect of the support nature on the selective acetylene hydrogenation. Antipathetic size dependence of TOF disappeared at particle size bigger than 11 nm. Initial selectivity to ethylene was found size-independent. The deactivation due to coke deposition was faster for smaller particles. The structure-sensitivity relations for the catalysts investigated are discussed in terms of “geometric” and “electronic nature” of the size effect and rationalized regarding Pd−C_x phase formation which is size-dependent. Supports with increased acidity diminished the formation of coke and changed the byproduct distribution toward ethane.  © 2008 American Chemical Society, reprinted with permission.

A novel method for isolation of monodispersed Pd nanoparticles from a reverse microemulsion was developed using hydrocarbon evaporation and methanol-assisted particle purification from a surfactant. Fcc Pd nanoparticles of 6, 8, 11, and 13 nm in diameter were isolated from water/AOT/isooctane mixture and used to study a size effect during solvent-free hydrogenation of 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol. The initial TOF calculated per mole of surface palladium atoms was duplicated when particle size was increased from 6 to 13 nm but remained constant when accounted per number of specific Pd atoms on Pd(111) facets. Selectivity to olefinic alcohol was not size-dependent, but an increase in particle size decreased the byproduct ratio of dimers to saturated alcohol. Acetylenic alcohol hydrogenation is shown to be a structure-sensitive but size-independent reaction for Pd particles with size of 6–13 nm. The work shows also that the Pd size controlled the reaction rate and the byproduct distribution. © 2007 Elsevier, reprinted with permission.