Metal Catalysts Pd Research Articles

MOFs as Multifunctional Catalysts: Synthesis of Secondary Arylamines, Quinolines, Pyrroles, and Arylpyrrolidines over Bifunctional MIL‐101

AbstractBifunctional MIL‐101 MOFs containing Lewis acid Cr3+ sites and Pd or Pt hydrogenation/reduction centers, either as isolated metal complexes or in the form of encapsulated metal nanoparticles, have shown to be highly active catalysts for the one‐pot nitroarene reduction and reductive amination of carbonyl compounds. This preparation procedure has been successfully applied to the synthesis of secondary arylamines, quinolines, pyrrols, and 3‐arylpyrrolidines. In all the cases, the MOFs have shown superior performances with respect to commercially available Pd and Pt metal catalysts under the same conditions.

Selection of oxygen reduction catalysts for rechargeable lithium–air batteries—Metal or oxide?

Carbon-supported Pd and PdO nanocatalysts were synthesised using either chemical reduction or thermal synthesis procedures and were used as model metal and oxide catalysts for oxygen reduction in rechargeable lithium–air batteries. The Pd metal catalyst showed excellent initial performance, e.g. a discharge capacity of 855mAh (gsolids)−1. However, the PdO catalyst displayed superior capacity retention to the Pd catalyst, producing a discharge capacity of 336mAh (gsolids)−1 after 10 cycles, i.e. the capacity retention was 6% per cycle. The activity and stability of Pd metal and oxide catalysts were found to be closely related to their intrinsic catalytic properties and structural changes during charge/discharge cycles in Li–air batteries. The implication of such a difference is discussed. Model Pd/C and PdO/C catalysts were compared with other widely used carbon-supported metal and oxide catalysts, including Pt/C, Ru/C, RuO2/C and MnO2/C.

Principles and Materials Aspects of Direct Alkaline Alcohol Fuel Cells

Direct alkaline alcohol fuel cells (DAAFCs) have attracted increasing interest over the past decade because of their favourable reaction kinetics in alkaline media, higher energy densities achievable and the easy handling of the liquid fuels. In this review, principles and mechanisms of DAAFCs in alcohol oxidation and oxygen reduction are discussed. Despite the high energy densities available during the oxidation of polycarbon alcohols they are difficult to oxidise. Apart from methanol, the complete oxidation of other polycarbon alcohols to CO2 has not been achieved with current catalysts. Different types of catalysts, from conventional precious metal catalyst of Pt and Pt alloys to other lower cost Pd, Au and Ag metal catalysts are compared. Non precious metal catalysts, and lanthanum, strontium oxides and perovskite-type oxides are also discussed. Membranes like the ones used as polymer electrolytes and developed for DAAFCs are reviewed. Unlike conventional proton exchange membrane fuel cells, anion exchange membranes are used in present DAAFCs. Fuel cell performance with DAAFCs using different alcohols, catalysts and membranes, as well as operating parameters are summarised. In order to improve the power output of the DAAFCs, further developments in catalysts, membrane materials and fuel cell systems are essential.

Hydrogenation of nitrile in supercritical carbon dioxide: a tunable approach to amine selectivity

The use of supercritical carbon dioxide (scCO2) on the hydrogenation of benzonitrile was investigated over Pd and other metal catalysts. Without any additive, benzonitrile was hydrogenated to benzylamine with high conversion (90.2%) and selectivity (90.9%) using the Pd/MCM-41 catalyst. A strong influence of CO2 pressure on the conversion and selectivity were observed. As the CO2 pressure increases, the conversion was increased, and after reaching the maximum at around 8–10 MPa, it decreased. Moreover, simply by tuning the CO2 pressure, it is possible to obtain benzylamine or dibenzylamine. For instance, at lower pressure CO2 acts as a protecting agent, leading to the formation of the primary amine, but at higher pressure, the yield of primary amine as well as the solubility of the imine intermediate in CO2 increases, which results high selectivity for dibenzylamine. A plausible mechanism has been proposed to show the role of CO2 on the selectivity toward primary and secondary amines. The results confirm that the presence of CO2 is mandatory for the formation of benzylamine with high selectivity. Furthermore, the other reaction parameters, such as reaction time, H2 pressure, temperature etc., also affect the conversion as well as selectivity of benzylamine. This process has been extended to the hydrogenation of a series of different nitrile compounds.

Highly durable Pd metal catalysts for the oxygen reduction reaction in fuel cells; coverage of Pd metal with silica

Pd cathode catalysts for polymer electrolyte fuel cells have been covered with silica layers a few nanometres thick. The silica-coated Pd catalysts showed high activity and excellent durability for the oxygen reduction under the severe cathode conditions of PEFCs, while Pd catalysts without silica-coating were seriously deactivated under the same conditions. The coverage of Pd metal with silica prevents the diffusion of Pd species out of the silica layers.

Interplay Between the Active Phase and Support: Preparation, Characterization and Catalytic Performance

Metal nanoparticles formed in various media and stabilized by different mechanism are the objects of intense study in contemporary catalytic research due to their unique properties, which are significantly influenced not only by the size of nanoparticles and their surfaces, but by the chemical nature of the environment surrounding such nanoparticles. While the main attention is focused on the properties of metal nanoparticles per se, the effect of the matrix on the metal properties is not very well understood, especially when comparing inorganic and organic matrices having the same metal nanoparticle sizes. At the same time the local metal environment, which can be influenced during catalyst preparation for example by catalyst pretreatment or modification of support acidity, etc., may affect both the metal particle size and shape/morphology of the catalysts. This issue contains a collection of papers describing synthesis of nanosized Ru, Pt, Au and Pd metal catalysts supported on inorganic and organic matrices (inorganic micro-mesoporous matrices, carbon nanofibers or polymer based materials) as well as their characterization and testing in industrially important reactions for producing mainly fine and specialty chemicals. We consider this collection of articles, which are written by participants in an EU funded research project NANOCAT (No. 506621), to be a valuable addition to the literature on interactions between nanosized metal and supports of varying chemical nature.

Recognizing indoor formaldehyde in binary gas mixtures with a micro gas sensor array and a neural network

Low-concentration formaldehyde (HCHO) together with ethanol/toluene/acetone/α-pinene (as an interference gas of HCHO) is detected with a micro gas sensor array, composed of eight tin oxide (SnO2) thin film gas sensors with Au, Cu, Pt or Pd metal catalysts. The characteristics of the multi-dimensional signals from the eight sensors are evaluated. A multilayer neural network with an error backpropagation (BP) learning algorithm, plus the principal component analysis (PCA) technique, is implemented to recognize these indoor volatile organic compounds (VOC). The results show that the micro gas sensor array, plus the multilayer neural network, is very effective in recognizing 0.06 ppm HCHO in single gas component and in binary gas mixtures, toluene/ethanol/α-pinene with small relative error.

Synthesize of nano-sized Pd metal catalyst on Ti-containing zeolite using a photo-assisted deposition (PAD) method

Using a photo-assisted deposition (PAD) method, the nano-sized Pd metal can be highly deposited on Ti-containing silicalite zeolite (TS-1). This nano-sized Pd metal can exhibit the high efficiency for H2O2 production under a flow of H2 and O2 and further applied to the subsequent partial oxidation of phenol on TS-1.

Catalysis of nanosized Pd metal catalyst deposited on Ti-containing zeolite by a photo-assisted deposition (PAD) method

Abstract Using a photo-assisted deposition (PAD) method, nanosized Pd metal can be highly dispersed on Ti-containing silicalite zeolite (TS-1) under UV-light irradiation (PAD-Pd/TS-1). The nanosized Pd metal was deposited directly on the photo-excited tetrahedrally coordinated titanium oxide species (tetra-Ti-oxide) of TS-1 zeolite. Under the flow of H2 and O2 in water solvent, the efficient formation of H2O2 could be observed by the PAD-Pd/TS-1 catalyst under mild reaction conditions (ambient temperature and atmospheric pressure). Furthermore, the presence of phenol in this reaction system led to the formation of the products from the partial oxidation of phenol with the formed H2O2.

A novel SiO2 supported Pd metal catalyst for the Heck reaction

A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO2 (O)) derived by calcination of Pd(acac)2/SiO2 in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd2+ is reduced to Pd0 on SiO2 by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd0/SiO2 is highly active for the Heck reaction, the remaining Pd2+/SiO2 is not responsible for the high activity. Pd/SiO2 (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO2 (H)) prepared by reduction of Pd(acac)2/SiO2 in H2 as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO2 (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO2 (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature.

Direct Synthesis of Hydrogen Peroxide from H2 and O2 Using Al2O3 Supported Au−Pd Catalysts

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of alumina supported Au, Pd, and Au−Pd metal catalysts is described and discussed in detail for both fresh catalysts and materials that have been aged in storage. The conditions previously identified as being optimal for hydrogen peroxide synthesis (i.e., low temperatures (2 °C) and short reaction residence times) are employed, and the effect of catalyst composition and preparation was investigated in detail for the fresh catalysts. The addition of Pd to the Au catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Interestingly, the addition of relatively small amounts of Pd to the Au/Al2O3 catalyst, replacing up to 20% of the Au, significantly increases the formation of hydrogen peroxide. The microstructure of the catalysts was studied using transmission electron microscopy and X-ray photoelectron spectroscopy for both the fresh and the aged catalyst, and the supported bim...

Scope and limitations of ether-directed, metal-catalysed aza-Claisen rearrangements; improved stereoselectivity using non-coordinating solvents

In an effort to understand and enhance the stereochemical outcome of the MOM-ether directed rearrangement of allylic trichloroacetimidates we have investigated various reaction conditions for this process. A range of Pd(ii) and other metal catalysts have been shown to effectively catalyse the rearrangement providing the subsequent allylic amides in high selectivity (up to 11 : 1 ratio of diastereomers). The replacement of THF as a solvent in this reaction with non-coordinating solvents such as toluene has led to an enhancement of the directing effect resulting in a significant increase in the diastereoselective outcome (15 : 1 ratio). The reaction was also carried out for the first time, using a highly coordinating ionic solvent which disrupts binding of the Pd(ii)-catalyst to the MOM-ether yielding the allylic amide in only moderate diastereoselectivity. These results provide further evidence for the ether directed aza-Claisen rearrangement of allylic trichloroacetimidates.

Synthesis of Au@Pd core–shell nanoparticles with controllable size and their application in surface-enhanced Raman spectroscopy

Au core Pd shell (Au@Pd) nanoparticles with controllable size from 35 to 100 nm were prepared by chemical deposition of Pd over pre-formed 12 nm Au seeds. Both transmission electron microscopy and UV–visible spectroscopy studies confirmed the core–shell structure of the synthesized nanoparticles. The Au@Pd nanoparticles dispersed on a polished Pt electrode surface exhibit high surface-enhanced Raman scattering (SERS) effect for the adsorbed pyridine and SCN −. With the aid of the long-range effect of the electromagnetic (EM) enhancement created by the SERS-active Au core underneath the Pd shell, the good quality SERS spectra of adsorbates on the palladium metal overlayer can be obtained. Such kind of SERS-active substrate can be used as an alternative substrate to massive metals for investigating adsorption and reactions occurring on the Pd metal catalyst.

Increase of Pd surface area by treatment in dioxygen

The surface area of Pd(111) and Pd(110) single crystals increased after oxidation in O 2 transformed them to PdO. The surface area of the oxide decreased after complete reduction in H 2. The techniques of STM, TPD, XPS, AES, and LEED were employed to study the Pd(111) and Pd(110) single-crystal surfaces after they were subjected to O 2 oxidation, methane combustion, and H 2 reduction. The surface area of the treated palladium single crystals was measured by 18O isotope exchange and by direct measurements using the STM image. These two methods showed agreement within 20%. After oxidation in O 2 (100 and 150 Torr) at 600 K, the surface area for both Pd(111) and Pd(110) single crystals increased by a factor of approximately two. The effect was more pronounced on the Pd(111) surface. The oxidized surfaces were covered with 3- to 4-nm semispherical oxide agglomerates that formed a “cauliflower-like” structure 10–20 nm in size. Similar surface structures were observed after exposure of the Pd single crystals to a lean O 2 and CH 4 reaction mixture (O 2:CH 4=10:1). Thus, the oxidized single crystal becomes amorphous. Reduction in H 2 decreased the surface area of the preoxidized Pd(111) and Pd(110) crystals. An amorphous metallic surface was produced after H 2 reduction at 373 K, whereas a smooth surface with characteristic single-crystal features was observed after reduction at 673 K. These experiments suggest that oxidizing a Pd metal catalyst or reducing an oxidized Pd catalyst, for example, before palladium metal surface area measurement, will affect the surface area of the sample. It also shows that the increase in surface area on Pd catalysts after oxidation treatment is caused by surface roughening.

Convenient and efficient reduction of 1,1'-binaphthyls to H8-1,1'-binaphthyl derivatives with Pd and Ru catalysts on solid support.

Hydrogenation of chiral 2,2'-functionalized 1,1'-binaphthyls with Pd and Ru solid-supported metal catalysts was found to be a clean and convenient pathway to 5,5',6,6',7,7',8,8'-octahydro-1,1'-dinaphthyl derivatives. In most cases no racemization was observed in the course of the reaction.

Surface Reactivity of Pd Nanoparticles Supported on Polycrystalline Substrates As Compared to Thin Film Model Catalysts: Infrared Study of CO Adsorption

To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts: (1) polycrystalline MgO and γ-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atom...

Surface reactivity of Pd nanoparticles supported on polycrystalline substrates as compared to thin film model catalysts: infrared study of CH 3OH adsorption

A detailed comparative study of CH 3OH adsorption on two categories of supported Palladium nanoparticles is reported: (1) MgO and γ-Al 2O 3-supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal-based Pd/Al 2O 3 model catalysts prepared under ultrahigh vacuum (UHV) conditions. A detailed structural characterization of the supported Pd nanoparticles allows the assignment of vibrational frequencies of CH 3OH and its decomposition products to well-defined types of sites on these systems. The decomposition of methanol on both types of catalysts is compared as an example for a reactivity study. On the model catalyst, infrared reflection–absorption spectroscopy (IRAS) experiments shed light on the role of two decomposition pathways, dehydrogenation and CO bond scission. The effect of carbon contamination in the vicinity of edge and defect sites is explored. For the Pd/MgO systems studied by transmission FTIR, a mechanism for the methanol decomposition/synthesis is proposed, which involves the simultaneous formation of carbonyl species on Pd particles (at room temperature) and methoxy on the MgO matrix. The MgO matrix is basic and hence the surface OH groups can react with CO species initially formed on Pd particles and spilled onto the matrix. This reaction channel results in the formation of intermediates with a formate-type structure, which can be easily detected by IR spectroscopy. The superior activity of Pd/MgO in the decomposition/synthesis of methanol is explained on the basis of a cooperative effect between the Pd particles and the basic matrix.

Pd–Zeolites as Heterogeneous Catalysts in Heck Chemistry

Heck reactions were performed with 4-bromoacetophenone and n-butyl acrylate, yielding the trans-arylated acrylate ester with high selectivity. Pd–zeolite catalysts were compared with supported Pd metal catalysts. In order to obtain an active and heterogeneous catalyst, the preformed or in situ-formed catalytically active Pd0 species and the PdII intermediates must be sufficiently stabilized on the zeolites against leaching and against aggregation to inactive Pd black precipitates. The different states of Pd in the zeolites, i.e., Pd(NH3)42+, lattice-coordinated Pd2+ ions, PdO particles, and Pd0 metal particles of different sizes, were studied by a combination of diffuse reflectance UV–vis, XRD, SEM, and TEM before, during, and after catalytic reactions. The heterogeneous nature of the catalysis largely depends on the catalyst pretreatment and on the solvent and base used. With tributylamine as the base and in toluene, the Heck olefinations with Pd(NH3)42+–zeolites (0.4 wt% Pd; mordenite, Y, ZSM-5) and Pd0–mordenite (0.4 and 4 wt% Pd) are truly heterogeneous. Pd leaching from the zeolites, as evaluated in a very strict filtrate activity test, is clearly related to the presence of oxidized PdII in an all-oxygen environment, i.e., ionic Pd2+ or PdO. Supported Pd catalysts (Pd/C and Pd/SiO2) give rise to Pd leaching as well. The heterogeneous reactions with the zeolites can be accelerated by the addition of a quaternary ammonium salt promoter. In addition, the heterogeneous Heck catalysts can be applied in a continuous reactor.

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H2 to H2O2 (at 295 K) in acidic medium (0.02 M H2SO4) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H2O2, N2O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H2-to-H2O2 conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO2 < Pd/ThO2 < Pd/Ga2O3. The H2 oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H2O2 decomposition activity of the catalysts. There exists a close relationship between the H2-to-H2O2 conversion activity and/or H2O2 selectivity in the oxidation process and the H2O2 decomposition activity of the catalysts; the higher the H2O2 decomposition activity, the lower the H2-to-H2O2 conversion activity and/or H2O2 selectivity.

Sintering of Pd Automotive Catalysts

Abstract The focus of this work is on processes that lead to loss of active metal surface area during high temperature operation of automotive catalysts. As under-floor catalytic converters are moved closer to the engine to achieve faster light off, the catalyst is subjected to higher operating temperatures during the normal driving cycle. Sintering of the active metal phase leading to loss of surface area represents one of the most important factors limiting the long-term durability of automotive catalysts. Despite its obvious technological importance, fundamental understanding of sintering is still lacking. In this research, we have prepared a series of Pd metal catalysts on a θ alumina support. The metal loading was varied from 1.1 wt% to 7 wt%. The catalysts were subjected to sintering at 900 °C for up to 192 hours. To ensure relevance to automotive catalysts, the sintering was performed in a gas atmosphere that contained 10 mol % H2O in flowing N2 or in air.