Supported Nanoparticle Catalysts Research Articles

Heterogeneous Bimetallic Cu–Ni Nanoparticle-Supported Catalysts in the Selective Oxidation of Benzyl Alcohol to Benzaldehyde

Three bimetallic Cu–Ni nanoparticle-supported catalysts were synthesized by co-immobilization followed by H2 reduction. A chromium(III) terephthalate metal organic framework (MIL-101), titanium dioxide (TiO2), and carbon (C) with different properties (acidity and Brunauer–Emmett–Teller surface area) were selected as supports for studying the effect of the support nature on the catalytic activity and selectivity in the oxidation of benzyl alcohol. The physicochemical properties of the Cu–Ni-supported catalysts were characterized by XRD, NH3-TPD, nitrogen adsorption/desorption, TEM, EDS, XPS, and ICP-OES. Bimetallic Cu–Ni nanoparticles were highly dispersed on the support. The catalytic activities of CuNi/MIL-101, CuNi/TiO2, and CuNi/C were tested in the selective oxidation of benzyl alcohol to benzaldehyde in the presence of molecular oxygen under mild reaction conditions. The highest benzaldehyde yields were achieved with CuNi/TiO2, CuNi/MIL-101, and CuNi/C catalysts at 100 °C within 4 h under 5, 3, and 3 bar of O2, respectively. The bimetallic Cu–Ni-supported catalysts possessed two types of catalytic active sites: acid sites and bimetallic Cu–Ni nanoparticles. The CuNi/MIL-101 catalyst possessed a high number of acid sites and exhibited high yield during selective benzyl alcohol oxidation to benzaldehyde. Importantly, the catalysts exhibited a high functional group (electron-donating and electron-withdrawing groups) tolerance. Cu–Ni-supported catalysts with an Cu:Ni mole ratio of 1:1 exhibited the highest yield of 47% for the selective oxidation of benzyl alcohol to benzaldehyde. Reusability and leaching experiment results exhibited that CuNi/MIL-101 showed better stability than CuNi/TiO2 and CuNi/C catalysts due to the large porous cavities of MIL-101 support; these cavities can be used to trap bimetallic Cu–Ni nanoparticles and inhibit nanoparticle leaching.

Impacts of calcination on surface-clean supported nanoparticle catalysts

Traditional methods of preparing size-controlled supported nanoparticle catalysts typically require the use of stabilizing ligands that passivate the nanoparticle surface to prevent overgrowth and aggregation. The presence of these ligands on catalytic surfaces can be detrimental to activity and are typically removed via a high-temperature annealing step, which ultimately results in changes to nanoparticle morphology and thus, reduced activity. We previously demonstrated that silylamine switchable surfactants can be used to synthesize and deposit highly active nanoparticles on to a support surface while preserving monodispersity. Here, we demonstrate using XPS that supported nanoparticles prepared in this manner are surface-clean after deposition, eliminating the need for traditional activation steps. Further, it is shown that even low-temperature calcinations have detrimental effects on the catalyst properties including changes in nanoparticle morphology that result in a significant decrease in activity, a change in surface hydrophilicity, a change in activation energy, and results in the formation of an induction time when utilized in the hydrogenation of 4-nitrophenol. While calcination remains a widely used method of catalyst activation, the detrimental effects of high temperatures on catalyst properties are often overlooked.

2D and 3D Characterization of PtNi Nanowire Electrode Composition and Structure

Extended surface Pt or Pt–M (M = Ni or Co) catalysts are a viable alternative to supported nanoparticle catalysts for the oxygen reduction reaction (ORR) at the cathode in proton exchange membrane ...

Pd0-PyPPh2@porous organic polymer: Efficient heterogeneous nanoparticle catalyst for dehydrogenation of 3-methyl-2-cyclohexen-1-one without extra oxidants and hydrogen acceptors

In this contribution, we have developed an efficient and recyclable porous organic polymer (POP) supported Pd nanoparticle catalyst (Pd°-PyPPh2@POP) for dehydrogenation of 3-methyl-2-cyclohexen-1-one. This heterogeneous catalytic system represents a totally clean process without using any extra oxidant and hydrogen acceptors. The SEM-EDS mapping images of the Pd°-PyPPh2@POP catalyst reveal the highly uniformly dispersed character of C, Pd, P and N elements. The coordination bonds between Pd nanoparticle and exposed P atom as well as N atom on the surface of PyPPh2@POP polymer are confirmed by means of solid-state 31P NMR and XPS. Importantly, both P atom and pyridyl ring on the PyPPh2@POP polymer are themselves used as solid base over the Pd°-PyPPh2@POP catalyst, leading to a catalytic conversion of 88.2% even without the employment of inorganic base additives (K2CO3). Our results have provided a strategy for designing highly active bifunctional POP supported nanoparticle catalysts.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd.

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

Performance of Supported Au-Pd Alloy Nano Particles Catalyst for Base-free Synthesis of Imines by Self-coupling of Amine

Abstract A series of supported nano particles catalysts, Au/γ-Al2O3, Pd/γ-Al2O3 and Au-Pd/γ-Al2O3, were prepared by the impregnation reduction method. The catalytic performance of these catalysts for direct base-free synthesis of imines by self-coupling of benzylamine was investigated. The results show that the yield of imine could reaches 93% over bimetallic alloy Au-Pd/γ-Al2O3 catalyst, much higher than monometallic Au or Pd nano particle catalyst. The used catalyst is recovered and 44% yield of imine could be obtained after 5 recycling use of Au-Pd/γ-Al2O3.

Manufacture of highly loaded silica-supported cobalt Fischer\u2013Tropsch catalysts from a metal organic framework

The development of synthetic protocols for the preparation of highly loaded metal nanoparticle-supported catalysts has received a great deal of attention over the last few decades. Independently controlling metal loading, nanoparticle size, distribution, and accessibility has proven challenging because of the clear interdependence between these crucial performance parameters. Here we present a stepwise methodology that, making use of a cobalt-containing metal organic framework as hard template (ZIF-67), allows addressing this long-standing challenge. Condensation of silica in the Co-metal organic framework pore space followed by pyrolysis and subsequent calcination of these composites renders highly loaded cobalt nanocomposites (~ 50 wt.% Co), with cobalt oxide reducibility in the order of 80% and a good particle dispersion, that exhibit high activity, C5 + selectivity and stability in Fischer–Tropsch synthesis.

Switchable Surfactants for the Preparation of Monodisperse, Supported Nanoparticle Catalysts.

Synthesis methods for the preparation of monodisperse, supported nanoparticles remain problematic. Traditional synthesis methods require calcination following nanoparticle deposition to remove bound ligands and expose catalytic active sites. Calcination leads to significant and unpredictable growth of the nanoparticles resulting in polydisperse size populations. This undesired increase in nanoparticle size leads to a decrease in catalytic activity due to a loss of total surface area. In this work, we present the use of silylamines, a class of switchable solvents, for the preparation of monodisperse, supported nanoparticles. Silylamines are switchable molecules that convert between molecular and ionic forms by reaction with CO2. Upon addition of an alkane, the switchable solvent behaves as a switchable surfactant (SwiS). The SwiS is used to template nanoparticles to aid in synthesis and subsequently used to release nanoparticles for deposition onto a support material. The use of SwiS allowed for the preservation of nanoparticle diameter throughout the deposition process. Finally, it is demonstrated that supported gold nanoparticle catalysts prepared using SwiS are up to 300% more active in the hydrogenation of 4-nitrophenol than their traditionally prepared analogues.

Clay Minerals—Ionic Liquids, Nanoarchitectures, and Applications

AbstractClay minerals, whose world resources are extremely large, have the potential to be more exploited as the basis for functional materials. Of interest are their interactions with ionic liquids (ILs). These compounds have found a large number of applications in the last few decades due to unique properties, such as low vapor pressure, high thermal stability, and remarkable solvation abilities. In the case of the swelling smectites, the organic cation of ILs can replace interlayer cations and find applications in the preparation of nanocomposites. This feature article is mainly focused on kaolinite, a nonswelling 1:1 phyllosilicate, whose layers are essentially neutral. Consequently, the intercalation of ILs involves both cation and anion. The organic cations can be designed to bear hydroxyl groups that will react with the aluminol internal surface of kaolinite, resulting in ionic liquids not only intercalated but also grafted. The resulting nanohybrid materials are characterized by a fixed, rigid, constrained 2D structure, whose dimension can be tuned by the size of the organic cation, whereas the anion is exchangeable. These materials are used for sensing applications such as the specific detection of anions as well as their quantitative analysis. They are also used as catalyst support for nanoparticles.

Catalyst stabilization by stoichiometrically limited layer-by-layer overcoating in liquid media

The use of metal oxide overcoats over supported nanoparticle catalysts has recently led to impressive improvements in catalyst stability and selectivity. The deposition of alumina is especially important for renewable catalysis due to its robustness in liquid-phase conditions. However, there are limited reports of work on alumina deposition and stabilization that goes beyond atomic layer deposition (ALD). Here, we present a layer-by-layer deposition technique for the controlled formation of conformal alumina overcoats in the liquid phase. This technique is easy to perform in common wet chemistry conditions. Alternated exposure of the substrate to stoichiometric amounts of aluminum alkoxide and water in liquid-phase conditions leads to the formation of a porous overcoat that was easily tunable by varying synthesis parameters. The deposition of 60 Al2O3 layers onto Al2O3-supported copper nanoparticles suppressed irreversible deactivation during the liquid-phase hydrogenation of furfural – a key biomass-derived platform molecule. The porous overcoat leads to highly accessible metal sites, which significantly reduces the partial site blocking observed in equivalent overcoats formed by ALD. We suggest that the ease of scalability and the high degree of control over the overcoat’s properties during liquid-phase synthesis could facilitate the development of new catalyst overcoating applications.

Nanoparticle Based on Poly(Ionic Liquid) as an Efficient Solid Immobilization Catalyst for Aldol Reaction and Multicomponent Reaction in Water.

An environmentally friendly nanoparticle-supported catalyst was successfully prepared via in situ ionic complexation between imidazolium-based polymer ionic liquid (PIL) and poly(l-prolinamide-co-MAA). The physical and chemical properties of the obtained nanoparticles were characterized by TEM, FTIR, XPS, and static water contact angle experiments. The surface properties of the nanoparticle were found to significantly affect the catalytic performance. The nanoparticle with PIL outer facilitated the adsorption of reaction substrate in it. As a result, the catalytic system catalyzed the asymmetric Aldol reaction and multicomponent reaction in pure water efficiently. The catalytic system was able to be reused and recycled five times, and with no discernible loss in catalytic activity and enantioselectivity. These findings suggest that nanoparticles based on PIL may provide a new approach for preparing high performance supported catalysts for organic reactions in water. This technology also addresses issues associated with mass transfer in pure water reactions.

Monodispersed Pd Nanoparticles Supported on Mg-Al Mixed Metal Oxides: A Green and Controllable Synthesis.

Aimed at the green synthesis of supported noble metal nanoparticles without any additional reducing agents or stabilizers at room temperature, a new strategy in which monodispersed Pd nanoparticles were successfully immobilized on Mg–Al mixed metal oxides derived from layered double hydroxides was developed, and the reduction mechanism was also proposed. It allowed the in situ immobilization of monodispersed Pd nanoparticles with a face-centered cubic structure, using only alcohols with α-hydrogen atom as the solvent and reducing agent. Control over the mean particle size and content of Pd nanoparticles was realized by adjusting the temperature, initial concentration, and type of alcohol. With the content of Pd varied from 6.4 to 42.3 mg/g, the mean particle size of Pd increased from 1.1 to 4.9 nm. The as-prepared catalyst showed high catalytic activity for oxidation of benzyl alcohol: when the Pd content was 6.4 mg/g, conversion of 60.3% and selectivity of 99.2% were obtained with a turnover frequency of 4807 h–1. Similarly, on the basis of the high stand electrode potentials, immobilized Ag (5.6 nm) and Au (4.7 nm) nanoparticles were also prepared through this methodology. Moreover, the choice of supports was extended to other powder supports that would react with the byproduced acid. Overall, this facile method opens up new opportunities for controllable synthesis of supported nanoparticle catalysts in a green way.

Revisiting alternative pathways in the Fischer–Tropsch process: Accurate density functional theory calculations on “magic” Ru12 clusters

AbstractModels of the Fischer–Tropsch reaction typically focus on two proposed mechanisms for the initial carbon monoxide dissociation: unassisted dissociation (carbide mechanism), and hydrogen‐assisted dissociation via an adsorbed oxymethylidene (HCO*) intermediate. Much evidence for hydrogen‐assisted dissociation comes from density functional theory calculations modeling ruthenium nanoparticle catalysts as infinite, periodic metal slabs. However, the generalized gradient approximations (GGAs) used in these calculations can make significant errors in reaction barrier heights. How these errors affect the predicted selectivity to unassisted vs. hydrogen‐assisted dissociation is not well understood. We address the problem by considering a different regime, applying GGA and beyond‐GGA approximations to CO dissociation on a “magic” nonmagnetic Ru12 cluster modeling supported nanoparticle catalysts. Both approximations concur that hydrogen‐assisted dissociation is facile on this cluster, providing additional support for its potential role in real catalysts.

Rapid extraction of quantitative kinetic information from variable-temperature reaction profiles

Analysis of variable-temperature reaction profiles, measured in an isothermal packed-bed reactor (PBR) whose temperature increases during the experiment, has the potential to yield accurate and precise kinetic parameters quickly for some heterogeneous catalysts. The method is demonstrated here for a typical supported nanoparticle catalyst, 2wt% Pd/Al2O3, in the oxidation of H2, C3H8 and CO by O2. These reactions do not exhibit major changes in activation energy as a function of conversion over the range of reaction conditions analyzed. Reliable and quantitative information about rate laws was extracted readily from the shapes and positions of the profiles, as an alternative to more laborious conventional kinetic analyses. Temperature and pressure gradients were minimized by the use of sieved catalyst particles and large amounts of inert diluent for both the catalyst and feed gas. Curve-fitting of analytical expressions with as few as two adjustable parameters results in remarkable agreement between models and data. First-order profiles are kinetically-limited, without mass and heat transfer effects, while inverse-first-order profiles deviate from kinetically-controlled behavior at intermediate-to-high conversions. The activation energy and reaction order with respect to the limiting reactant obtained from a single reaction profile (with appropriate data truncation for non-kinetic phenomena, as necessary) are at least as accurate and precise as those obtained from a conventional Arrhenius analysis conducted with data obtained under differential conditions, and can be measured in a fraction of the experimental time. Information about more elaborate rate laws can be obtained by global curve-fitting of a family of such profiles recorded with different volumetric flow rates.

Synthesis of a self-assembling gold nanoparticle-supported organocatalyst for enamine-based asymmetric aldol reactions

The self-assembling gold nanoparticle (GNP)-supported l-proline derivative, which was readily synthesized in four steps from 4-hydroxy-l-proline and chloroauric acid, was used for enamine-based aldol reactions. The modified Brust-Schiffrin (BS) synthesis was favored over the ligand exchange reaction in order to develop a new and simple synthetic pathway for nanoparticle-supported catalyst. Use of immobilized organocatalyst on nanoscale solid carrier led to excellent selectivities (up to 94:6 dr and 94% ee) in asymmetric reactions of ketones and benzaldehydes. GNPs operate under pseudo-homogeneous conditions which are easily recycled and reused at least five times without significant loss of weight, activity, diastereo- and enantioselectivities.

Identification of step-edge sites on Rh nanoparticles for facile CO dissociation

Understanding the dependence of the rate of catalytic reactions on metal nanoparticle size remains one of the great challenges in heterogeneous catalysis. Especially, methods to probe step-edge sites on technical supported nanoparticle catalysts are needed to put structure–activity relations on a surer footing. Herein, we demonstrate that N2 is a useful IR probe for the semi-quantitative identification of step-edge sites on zirconia-supported metallic Rh nanoparticles. The intensity of the strongly perturbed band at 2205cm−1 correlates with the CO bond dissociation rate under conditions relevant to the Fischer–Tropsch reaction. Due to the intermediate reactivity of Rh, step-edge sites are required to dissociate the strong CO bond. DFT calculations show that N2 prefers to adsorb on top of low-coordinated surface atoms such as steps, corners and edges. The occurrence of the intensity maximum at intermediate particle size is explained by the presence of surface overlayers on terraces that give rise to step-edges. These step-edge sites are important in the dissociation of di-atomic molecules such as CO, NO and N2.

Revealing local variations in nanoparticle size distributions in supported catalysts: a generic TEM specimen preparation method.

The specimen preparation method is crucial for how much information can be gained from transmission electron microscopy (TEM) studies of supported nanoparticle catalysts. The aim of this work is to develop a method that allows for observation of size and location of nanoparticles deposited on a porous oxide support material. A bimetallic Pt-Pd/Al(2)O(3) catalyst in powder form was embedded in acrylic resin and lift-out specimens were extracted using combined focused ion beam/scanning electron microscopy (FIB/SEM). These specimens allow for a cross-section view across individual oxide support particles, including the unaltered near surface region of these particles. A site-dependent size distribution of Pt-Pd nanoparticles was revealed along the radial direction of the support particles by scanning transmission electron microscopy (STEM) imaging. The developed specimen preparation method enables obtaining information about the spatial distribution of nanoparticles in complex support structures which commonly is a challenge in heterogeneous catalysis.

Redox-induced in situ formation of Pd nanoparticles on surfaces of Fe3O4/PANI core/shell hybrids as high-performance catalysts for Suzuki cross-coupling reactions

Fe3O4/polyaniline (PANI)/Pd nanoparticles as supported catalysts with high recyclability and durability have been developed. Fe3O4/PANI as reactive magnetic supporters have been firstly synthesized, followed by the addition of PdCl2 to initial redox reaction between PANI and Pd ions to form Pd nanoparticles on surfaces of Fe3O4/PANI cores, leading to the formation of Fe3O4/PANI/Pd nanoparticle-supported catalysts. The structures of Fe3O4/PANI/Pd nanoparticle-supported catalysts have been confirmed by TEM, energy dispersive X-ray spectroscopic (EDS) elemental mapping analysis, and XPS, and the formation mechanisms have been revealed by FTIR spectra. The catalytic activity of Fe3O4/PANI/Pd nanoparticle-supported catalysts involved in liquid phase reactions have been established through the model catalysis reaction involving reduction of nitroaromatic compounds by NaBH4. Furthermore, their high stability and reactivity for Suzuki cross-coupling reactions of aryl bromides with phenylboronic acids have also been investigated.

Theoretical analysis of electrochemical surface-area loss in supported nanoparticle catalysts.

In polymer electrolyte fuel cells a decrease in catalytic surface-area within the cathode catalyst layer is a critical barrier to commercialization. This loss in catalytic surface-area manifests as a loss in cell voltage and thus power density of the cell. It has been established that potential cycling accelerates the loss in catalytic surface-area yet isolating the contributing mechanisms as well as relating mechanisms to operating conditions is not as straightforward. We approach the issue of surface-area loss deconvolution with a combined experimental, modelling and theoretical framework. The methodology is based on the Lifshitz-Slyozov-Wagner and Smoluchowski theories of particle size distribution evolution. Electrochemical surface-area loss experiments probing upper potential limits of 0.9 and 1.2 V as well as temperatures from 298 to 343 K were analyzed with the model. A dissolution and redeposition mechanism was correlated with the measurements for both upper potential limits; however, at the upper potential limit of 1.2 V, ambiguity between the coagulation and the dissolution and redeposition mechanisms was found. Notwithstanding, the extracted dissolution and redeposition parameters aligned with independent studies on Pt dissolution whereas similar positive comparisons with independent results were unable to be made for the coagulation mechanism.

In Situ Fourier Transform Infrared Spectroscopy (FTIR) Studies on Pt-Rh-Sn Nanoparticle Electrocatalysts for Ethanol Oxidation in Direct Liquid Fuel Cells

Alcohol fuel cells are of great interest in academic and industrial fuel cell research due to their high volumetric energy density compared to hydrogen/air fuel cells, and have high potential as an environmental friendly power source.1 As a liquid fuel, ethanol has higher energy densities (8 kWh kg-1), better efficiency, low toxicity, biocompatibility and abundant availability. However, ethanol oxidation suffers from slow kinetics, sluggish adsorption and C-C bond cleavage as well as poisoning of the active sites on Pt catalysts by CO-intermediates. The desired reaction involves the cleavage of the C-C bond, i.e. the total ethanol oxidation to CO2, while partial oxidation leads to by-products like acetic acid or acetaldehyde, which reduce the faradaic efficiency of the anodic reaction of DEFC:C2H5OH + 3 H2O --> 2 CO2 + 12 H+ + 12 e- To improve the performance for the ethanol oxidation reaction (EOR) the development of novel efficient electrocatalysts is necessary. Research has been widely focused on the addition of co-catalysts to platinum. Recently, ternary metal systems like Pt-Rh-Sn have shown enhanced EOR activity.2 However, the elucidation of the mechanism and the role of single metal atoms is still not completely understood and therefore a pressing subject of current research.3 Our aim is to increase the understanding of the relation between catalyst structure and composition and the EOR activity. Therefore, we tested different carbon supported nanoparticle catalysts, prepared by simultaneous reduction or consecutive reduction of Pt, Rh and Sn precursor salts, using oleic acid and oleylamine as surfactants and tetradecanediol as the reducing agent in dioctyl ether. A uniquely comprehensive set of experimental techniques was employed to determine the composition, morphology and atomic-scale structural coherence of the synthesized ternary nanoalloy electrocatalysts, including inductively coupled plasma optical emission spectroscopy (ICP-OES), Cu Kα based and synchrotron-based X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-Ray photoelectron spectroscopy, etc. Cu Kα and synchrotron XRD studies coupled to atomic pair distribution function analysis showed that PtRhSn/C nanoalloy had PtSn niggliite type structure with Rh occupying random Pt sites. Electrochemical experiments showed that the catalytic activity towards EOR of chemically homogeneous PtRhSn/C is the highest when compared to those of the other catalysts. The results showed that while Sn is more important than Rh as a component in nanoalloys used for oxidation of ethanol, the role of Rh is definitely not negligible since PtRhSn/C nanoalloy showed best catalytic property as compared to the corresponding catalyst without Rh/Sn. Achieving a very good degree of chemical homogeneity and the underlying atomic arrangement in the PtRhSn/C ternary alloy is a crucial condition in producing a catalyst with improved properties for oxidation of ethanol.Here, we present results of an in situ FTIR study of Pt-Rh-Sn based nanoparticle for the ethanol oxidation reaction (EOR). While the investigation of the different prepared catalysts revealed structural changes in morphology and chemical composition and electrochemical behaviour and the products /intermediates distributions on the electrode surface could provide valuable conclusions, we are also able to give an insight into significant results for the different materials and their part in the electrocatalysis of ethanol to CO2. In-situ FTIR data showing formation of CO2during the EOR as a result of C-C bond splitting in ethanol at an early potential is will help in further understanding of the mechanistic aspects of these electrocatalysts towards EOR and has the potential to help in the future design of DLFC catalysts.