Nanoscale Catalysts Research Articles

Amphiphilic coatings on cobalt boride nanocatalysts for stability in hydrogen generation applications

While the idea of using hydrogen as a fuel has been in circulation for hundreds of years, only in recent decades have viable hydrogen generation and storage mechanism become widely appealing for a future hydrogen economy. In addition, the field of energy research continues to make great strides in the utilization of hydrogen fuel for stationary and mobile power systems. Transport of hydrogen gas itself is inherently bulky and with regards to safety concerns may be preferable in a less flammable formulation, such as stored within an advanced carbon material or more commonly as a chemical hydride. More important than ever is the exploration of an effective catalytic system for on-demand hydrogen fuel generation from such chemical hydride precursors for effective use in applications including proton exchange membrane fuel cells. Among the chemical hydrides, sodium borohydride (NaBH4) has been praised for its hydrogen storage density, stability in air and in basic solutions, and the recyclability of its sideproduct sodium metaborate (NaBO2). In prior works, cobalt has proven to be an effective and low-cost catalyst for the hydrolysis of stable NaBH4 solutions into H2 gas and NaBO2 as an alternative to high priced noble metal based catalysts. However, like many metals, cobalt is highly susceptible to surface oxide layer formation, which when in the form of a nanomaterial catalyst means a dramatic change in catalytic properties. Therefore, a system which can eliminate or slow down the passivation of cobalt to its catalytically inactive form will offer significant benefit for hydrogen generation systems. Strategies that have been applied to improve the stability of Co nanoparticles have implemented protective coatings of aluminum oxide, silica, and gold among others; however, these methods may not offer an effective catalytic surface for the hydrolytic generation of hydrogen from sodium borohydride and moreover the processing is inherently expensive. Therefore, we present in this work our analysis of a technique for generating an inexpensive Co-based catalyst for hydrolysis of sodium borohydride which makes use of stabilizing amphiphilic molecules to hinder formation of a surface oxide layer. Numerous systems have been conceived for the on-demand generation of hydrogen, taking on various forms including cobalt and nickel foam catalysts and cobalt alloy nanoparticle catalysts. Cobalt nanoclusters are often supported on poly(vinyl pyrrolidone) by reductive processes and have yielded catalytic scaffold with high hydrogen generation rates. Other recent works have shown Co2B nanoparticle catalysts to be competitive with noble metals and have revealed that particle size and dispersion characteristics could yield high catalytic rates owing to their increased accessibility to active sites. In order to continue to improve the stability and activity of such nanoscale catalysts with respect to oxidative passivation, we have explored the use of amphiphiilic components (particularly polydiacetylene and polysorbate 20) in order to protect the surface of cobalt nanoparticles. By utilizing the inherent magnetic properties of the colloidal cobalt for separation and collection of cobalt, this approach may also offer controlled magnetic removal of catalyst for on-demand catalytic generation of hydrogen gas from stable sodium borohydride precursor solution. In detail, we explore the use of amphiphilic polysorbate or amphiphilic polydiacetylene for coating of the cobalt particles in order to hinder their transformation to the non-magnetic, poorly catalytic oxidized state (Figure 1). The formation of cobalt boride (Co2B) nanoparticles was carried out by a routine technique involving the reduction of a cobalt salt solution by dropwise addition to a solution of sodium borohydride. Similar metal boride nanoparticles are commonly prepared in such a technique owing to the simple synthesis and low cost. Related reports confirm that shortly after cobalt nanoparticle particle synthesis, there is a gradual, complete loss in magnetization due to the progressing oxidation process. In this work, we proceed to collect the Co2B nanoparticles after synthesis by magnet and subsequently expose the nanoparticles to amphiphilic polysorbate 20 and 10,12-pentacosadiynoic acid (PCDA), referred to in its polymerized form as polydiacetylene (PDA). The role of the polysorbate 20 and liposomal PDA was assessed with respect to their effect on the hydrogen generation rate of the nanoparticle catalysts. In addition, we identified interesting chemisorptions effects upon mixing of Co2B nanoparticles with PDA at particular pH conditions, which was explored by spectrophotometer and surface charge measurements. In the DOI 10.1007/s13233-015-3048-7

Effective treatment of alkaline Cr(VI) contaminated leachate using a novel Pd-bionanocatalyst: Impact of electron donor and aqueous geochemistry

Palladium catalysts offer the potential for the effective treatment of a variety of priority reducible pollutants in natural waters. In this study, microbially synthesized magnetite nanoparticles were functionalized with Pd(0), creating a highly reactive, magnetically recoverable, nano-scale catalyst (Pd-BnM). This was then investigated for the treatment of model Cr(VI) contaminated solutions at a range of pH values, and also alkaline Cr(VI) contaminated leachates from chromite ore processing residue (COPR); a contaminant issue of global concern. The sample of COPR used in this study was obtained from a site in Glasgow, UK, where extensive Cr(VI) contamination has been reported. In initial experiments Pd-BnM was supplied with H2 gas or formate as electron donors, and Cr(VI) removal from model synthetic solutions was quantified at various pH values (2–12). Effective removal was noted at neutral to environmentally relevant alkaline (pH 12) pH values, while the use of formate as an electron donor resulted in loss of performance under acidic conditions (pH 2). Reaction kinetics were then assessed with increasing Pd-BnM loading in both model pH 12 Cr(VI) solutions and the COPR leachate. When formate was used as the electron donor for Pd-BnM, to treat COPR leachate, there was significant inhibition of Cr(VI) removal. In contrast, a promotion of reaction rate, was observed when H2 was employed. Upon sustained reaction with model Cr(VI) solutions, in the presence of excess electron donor (formate or H2), appreciable quantities of Cr(VI) were removed before eventual inactivation of the catalyst. Faster onset of inactivation was reported in the COPR leachates, removing 4% and 64% of Cr(VI) observed from model Cr(VI) solutions, when formate and H2 were used as electron donors, respectively. XAS, TEM-EDX and XPS analysis of the catalysts that had been inactivated in the model solution, showed that the surface had an extensive covering of reduced Cr(III), most likely as a CrOOH phase. COPR reacted catalysts recorded a lower abundance of Cr(III) alongside a high abundance of the leachate components Ca and Si, implicating these elements in the faster onset of inactivation.

Assembly of a Cost-Effective Anode Using Palladium Nanoparticles for Alkaline Fuel Cell Applications.

Nanotechnology allows the synthesis of nanoscale catalysts, which offer an efficient alternative for fuel cell applications. In this laboratory experiment, the student selects a cost-effective anode for fuel cells by comparing three different working electrodes. These are commercially available palladium (Pd) and glassy carbon (GC) electrodes, and a carbon paste (CP) electrode that is prepared by the students in the laboratory. The GC and CP were modified with palladium nanoparticles (PdNP) suspensions. The electrodes efficiencies were studied for ethanol oxidation in alkaline solution using cyclic voltammetry techniques. The ethanol oxidation currents obtained were used to determine the current density using the geometric and surface area of each electrode. Finally, students were able to choose the best electrode and relate catalytic activity to surface area for ethanol oxidation in alkaline solution by completing a critical analysis of the cyclic voltammetry results. With this activity, fundamental electrochemical concepts were reinforced.

Aqueous-phase hydrogenation of alkenes and arenes: The growing role of nanoscale catalysts

In this article we highlight some of the recent developments in the aqueous-phase hydrogenation of alkenes and arenes. The traditional types of water-soluble catalysts applied in aqueous media are outlined together with recent trends towards the development of transition metal nanoparticle and nanocomposite catalysts. In addition, we describe some palladium nanoparticle catalysts that are stabilized by ionic polymers, which demonstrate superior activities in the aqueous-phase catalytic hydrogenation of styrene, used as a model substrate, compared to related palladium nanoparticles.

CO2 Electroreduction to Hydrocarbons on Carbon-Supported Copper Nanoparticles

High-surface area catalysts are crucial for electrochemical devices that rely on kinetically-limited electrochemical reactions. Proton exchange membrane fuel cells (PEMFC) and electrolyzers are examples of such devices. Commercialization of these devices became possible due to the development of nanoscale Pt catalysts on high-surface area carbon supports.Here we discuss the electrocatalytic activities of carbon-supported Cu nanoparticles for CO2 electroreduction, a reaction that is also kinetically limited and have a potential for implemention in CO2 electrolyzers for the production of hydrocarbon fuels such as CH4 and C2H4. Cu nanoparticles supported on Carbon Black (VC), Ketjen Black (KB) and Single Wall Carbon Nanotubes (SWNT) are synthesized and their activities towards CO2 electroreduction to hydrocarbon fuels (CH4, C2H2, C2H4 and C2H6) are evaluated using online gas chromatography (GC). Faradaic efficiencies of three home-made catalysts towards CO2 electroreduction are compared to that of commercial 20 wt% Cu/VC and films of electrodeposited Cu.Supported Cu catalysts were synthesized using three different synthetic routes. X-Ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used for nanoparticle ex-situ analysis. The average Cu particle size as determined by Scherer’s equation was 12, 19, 24 and 27 nm for 40 wt% Cu/VC, 20 wt% Cu/SWNT, 50 wt% Cu/KB and commercial 20 wt% Cu/VC, respectively. Pb underpotential deposition was used for in-situ evaluation of the electrochemical surface area (ECSA) of Cu nanoparticles and Cu films [1]. Electrodeposited Cu films had a smooth surface, close to the mirror-polished surface of a RDE electrode.Activity of Cu nanoparticles towards CO2 electroreduction to hydrocarbon fuels was evaluated using a sealed RDE setup connected to GC. Thin films of 40 wt% Cu/VC, commercial 20 wt% Cu/VC, 20 wt% Cu/SWNT and 50 wt% Cu/KB were deposited on the surface of custom-made 7 mm glassy carbon disks (Pine Instruments, Inc), following a well-established protocol in the PEMFC community [2]. GC sampling from the electrochemical cell was performed at 5, 25, 45 and 65 min from the start of experiment, while holding the electrode potential at -1.2, -1.4, -1.6, -1.8, -2 and -2.2V vs. Ag/AgCl reference electrode. CH4 and C2H4 were the only gaseous hydrocarbons detected during CO2 electroreduction on Cu nanoparticles and Cu films. Figure 1 compares faradaic efficiencies vs. potential for CO, H2, CH4 and C2H4 generation on thin films of 40 wt% Cu/VC (12 nm Cu particle size) and electrodeposited Cu. In the potential region between -1.4 and -2.2V, the hydrocarbon product distribution is dominated by C2H4 for Cu nanoparticles (Fig. 1a) and CH4 for the films of electrodeposited Cu (Fig. 1b). In addition, the onset potential for C2H4 generation is shifted by 0.2 V (-1.4 vs. -1.6 V) for 40 wt% Cu/VC vs electrodeposited Cu, indicative of the higher activity of Cu nanoparticles towards C2H4 generation. Higher activity and reverse product distribution for 40 wt% Cu/VC vs electrodeposited Cu films is likely due to a large number of under-coordinated sites, such as corners, edges and defects on the surface of nanoparticles vs. a smooth Cu surface [3].This presentation will discuss the differences in electrocatalytic activities of four supported catalysts towards CO2 electroreduction and the influence of support on product distribution in more detail. Acknowledgements OAB is grateful to the Office of Naval Research for financial support of this project.

Orientation-Dependent Oxygen Evolution Activities of Rutile IrO2 and RuO2.

The activities of the oxygen evolution reaction (OER) on IrO2 and RuO2 catalysts are among the highest known to date. However, the intrinsic OER activities of surfaces with defined crystallographic orientations are not well-established experimentally. Here we report that the (100) surface of IrO2 and RuO2 is more active in alkaline environments (pH 13) than the most thermodynamically stable (110) surface. The OER activity was correlated with the density of coordinatively undersaturated metal sites of each crystallographic facet. The surface-orientation-dependent activities can guide the design of nanoscale catalysts with increased activity for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.

State of the Art First Principles Computational Evaluation of Electrochemical Stability of a Nano-Scale Catalyst

Understanding and controlling the electrochemical stability of nanometerscale catalyst is vital for securing long-term operation of fuel cells. Novel proporty of the increased surface to volume ratio achieved by particle size reduction leads to lower materials cost and higher efficiency, but there are questions as to whether the intrinsic stability of materials also decreases with particle size. In this presentation it is shown that state of the art ab initio computational electrochemistry enables us to to construct Pourbaix diagrams of nano-scale Pt catalysts which determines the stability, passivation, and dissolution behavior as a function of particle size and potential. These results identify underlying mechanism controlling the electrochemical stability of nanoparticles and thus, propose how to design durable catalysts in fuel cell architectures.

Dynamic structural disorder in supported nanoscale catalysts

We investigate the origin and physical effects of "dynamic structural disorder" (DSD) in supported nano-scale catalysts. DSD refers to the intrinsic fluctuating, inhomogeneous structure of such nano-scale systems. In contrast to bulk materials, nano-scale systems exhibit substantial fluctuations in structure, charge, temperature, and other quantities, as well as large surface effects. The DSD is driven largely by the stochastic librational motion of the center of mass and fluxional bonding at the nanoparticle surface due to thermal coupling with the substrate. Our approach for calculating and understanding DSD is based on a combination of real-time density functional theory/molecular dynamics simulations, transient coupled-oscillator models, and statistical mechanics. This approach treats thermal and dynamic effects over multiple time-scales, and includes bond-stretching and -bending vibrations, and transient tethering to the substrate at longer ps time-scales. Potential effects on the catalytic properties of these clusters are briefly explored. Model calculations of molecule-cluster interactions and molecular dissociation reaction paths are presented in which the reactant molecules are adsorbed on the surface of dynamically sampled clusters. This model suggests that DSD can affect both the prefactors and distribution of energy barriers in reaction rates, and thus can significantly affect catalytic activity at the nano-scale.

Advanced Electrocatalysts for Fuel Cells

The primary barrier to broad applications of electrocatalyst for electrolyzers, fuel cells and metal-air batteries is lack of fundamental understanding of processes that are taking place at solid-liquid interfaces. Moreover, the cost, performance and durability of the current state of the art electrocatalysts are still not at level that would warrant commercialization of these technologies. Major catalytic improvements have been accomplished for the oxygen reduction reaction in case of Pt bi/multi-metallic alloys. It has been demonstrated that tuning of surface properties such as surface and subsurface composition, surface structure, and hence, electronic properties are critical aspects toward novel materials with unique functional properties. For instance, an increase in activity of almost two orders of magnitude compared to conventional Pt/C catalyst was reported for single crystalline Pt-Skin surfaces. This finding inspired subsequent nanoscale catalyst design aiming to synthesize more active, but also more durable catalysts. A synergy between well-defined extended surfaces and practical electrocatalysts remains to be a potent approach in development of systems with advanced catalytic properties and it will be thoroughly discussed in this presentation. [1] Stamenkovic et al. Science 315 (2007) 493.[2] van der Vliet et al. Nature Mat. 11 (2012) 1051.

ChemInform Abstract: Shake‐Rattle‐and‐Roll: A Model of Dynamic Structural Disorder in Supported Nanoscale Catalysts

AbstractReview: 24 refs.

Coarse-grained molecular dynamics investigation of nanostructures and thermal properties of porous anode for solid oxide fuel cell

Incorporation of nanoscale catalysts into porous structures of SOFC has been proven highly successful in increasing active sites and catalyst utilization. In addition, electrochemical reactions as well as heat transfer process in porous anode are strongly affected by complex porous structures. It is believed that study of anode thermal properties are critical for SOFC design and operation. In this work, an AA model is developed for nickel and YSZ components via ASE, and a CG technique is further applied to represent Ni and YSZ beads by VMD, which are then self-assembled to capture the anode nanostructure via GROMACS. LAMMPS is then employed to evaluate average thermal properties of the porous anode. It is found that, at low Ni content (≤30 vol%), thermal conductivity increases with increasing temperature due to lattice vibrations. Instead, the anode exhibits metallic behavior due to rich nickel phase. Thermal expansion of the anode increases with increasing nickel content. Average thermal properties of the anode are validated by open literature data with good agreement. This approach is considered to be applied to analyze nanostructures, heat transfer and temperature distribution in the porous anode, and is also useful to capture thermal performance of SOFC and stack.

Nanostructure Electrodes for Metal-Supported Solid Oxide Fuel Cells

Metal-supported solid oxide fuel cells (MS-SOFCs) use thick porous alloy substrates as the mechanical supports for thin functional anode-electrolyte-cathode layers and have gained increasing interest due to their advantages over the traditional all-ceramic counterparts such as increased structural robustness, improved tolerance toward rapid thermal cycling, simplified stack assembling and reduced materials costs as well. However, issues such as coarsening of the active nickel anode catalysts, metalic interdiffusion between the alloy substrates and the anodes, and application of appropriate cathode layers remain to be addressed. Here we report some of recent progress in nano-scale electrode catalysts for MS-SOFCs using the liquid phase impregnation technique.

A multiscale physical model of a polymer electrolyte membrane water electrolyzer

In this paper we report a multiscale physical and transient model describing the operation of a polymer electrolyte membrane water electrolyzer single cell. This model includes a detailed description of the elementary electrode kinetics, a description of the behavior of the nanoscale catalyst–electrolyte interface, and a microstructural description of the transport of chemical species and charges at the microscale along the whole membrane electrodes assembly (MEA). We present an impact study of different catalyst materials on the performance of the PEMWEs and a sensitivity study to the operation conditions, both evaluated from numerical simulations and with results discussed in comparison with available experimental data.

Catalytic Effect of Nanoparticles on Primary and Secondary Phase Nucleation

Nanoparticles were shown to catalyze nucleation of primary and secondary phases in metal matrix nanocomposites (MMNCs). This catalysis is important as it contributes to the mechanical property enhancement in the MMNCs. Primary aluminium grain refinement was demonstrated in A356 matrix nanocomposites. Various types and sizes of nanoparticles (SiC, TiC, γ-Al2O3; 10-96 nm) were used to make these MMNCs and in all cases the MMNCs had smaller, more equiaxed grains compared to the reference A356. Using the droplet emulsion technique, undercoolings were shown to be significantly reduced. Undercoolings in the MMNCs were in good general agreement with the undercooling necessary for free growth, suggesting the applicability of this model to nucleation on nanoscale catalysts. Secondary phase nucleation catalysis was demonstrated in a zinc alloy AC43A MMNC and a binary Mg-4Zn MMNC. In AC43A, secondary phase nucleation was catalyzed with the addition of various nanoparticles (TiC, SiC, γ-Al2O3). The secondary phase nucleation catalysis in AC43A coincided with ductility enhancement. In Mg-4Zn, SiC nanoparticle addition changed the secondary phases that formed. MgZn2 was formed in the MMNC at relatively high temperatures consuming the Zn and reducing the amount of the low temperature Mg2Zn3 phase that formed in the reference alloy. The change in secondary phase formation coincided with significant enhancement in strength and ductility.

Characterization of Metal-Oxide Catalysts in Operando Conditions by Combining X-ray Absorption and Raman Spectroscopies in the Same Experiment

We have developed a new instrumental setup that combines simultaneous X-ray absorption spectroscopy, Raman spectroscopy and online mass spectrometry for operando studies of catalytic reactions. The importance of combining these techniques in the same experiment is demonstrated with the example of CO oxidation over nanoscale copper oxide catalysts supported on high surface area titanium oxide. X-ray absorption near edge structure (XANES) spectroscopy provides information on the charge state and local geometry of the catalytically active atoms. Extended X-ray absorption fine-structure (EXAFS) technique adds information about their local coordination environment. Raman spectroscopy adds sensitivity to crystallographic phase and long range order that both XANES and EXAFS are lacking. Together, these measurements enable simultaneous studies of the structural and electronic properties of all components present in metal-oxide catalysts. Coupled with online reactant and product analysis, this new setup allows one to elucidate the synergy between different components of a catalytic system and shed light on its catalytic activity and selectivity.

Mn1.5Co1.5O4−δ infiltrated yttria stabilized zirconia composite cathodes for intermediate-temperature solid oxide fuel cells

The composite cathodes of yttria stabilized zirconia (YSZ) and Mn1.5Co1.5O4 (MCO) are prepared by infiltration of the MCO oxides into porous YSZ backbones using aqueous solutions of the corresponding nitrate salts. Calcinations at 850 °C promote the formation of the MCO spinel oxide and yield nano-scale catalyst coatings on the YSZ pore walls. Impedance measurements on the symmetric MCO–YSZ cathode fuel cells show that the lowest polarization resistance in air at 800 °C is 0.43 Ω cm2 for the MCO impregnated YSZ composite at the MCO volume loading of 13.5%. Analysis of the impedance spectra suggest that the oxygen reduction kinetics is probably limited by double ionization of the adsorbed oxygen atoms or charge transfer at the triple-phase boundaries. Furthermore, introducing the oxide ion conductor of samarium-doped ceria as a second component in the coated catalysts yields much lower polarization resistances, e.g., 0.15 Ω cm2 at 800 °C.

Editorial (Hot Topic: Nanoscale Catalysts as Tools for Synthesis)

Editorial (Hot Topic: Nanoscale Catalysts as Tools for Synthesis)

Single-Molecule Catalysis Mapping Quantifies Site-Specific Activity and Uncovers Radial Activity Gradient on Single 2D Nanocrystals

Shape-controlled metal nanocrystals are a new generation of nanoscale catalysts. Depending on their shapes, these nanocrystals exhibit various surface facets, and the assignments of their surface facets have routinely been used to rationalize or predict their catalytic activity in a variety of chemical transformations. Recently we discovered that for 1-dimensional (1D) nanocrystals (Au nanorods), the catalytic activity is not constant along the same side facets of single nanorods but rather differs significantly and further shows a gradient along its length, which we attributed to an underlying gradient of surface defect density resulting from their linear decay in growth rate during synthesis (Nat. Nanotechnol.2012, 7, 237-241). Here we report that this behavior also extends to 2D nanocrystals, even for a different catalytic reaction. By using super-resolution fluorescence microscopy to map out the locations of catalytic events within individual triangular and hexagonal Au nanoplates in correlation with scanning electron microscopy, we find that the catalytic activity within the flat {111} surface facet of a Au nanoplate exhibits a 2D radial gradient from the center toward the edges. We propose that this activity gradient results from a growth-dependent surface defect distribution. We also quantify the site-specific activity at different regions within a nanoplate: The corner regions have the highest activity, followed by the edge regions and then the flat surface facets. These discoveries highlight the spatial complexity of catalytic activity at the nanoscale as well as the interplay amid nanocrystal growth, morphology, and surface defects in determining nanocatalyst properties.

Precursor Investigation in the Synthesis of PtPb Nanocatalysts

ABSTRACTIn recent years, platinum-based single crystalline nanoalloys as nanoscale catalysts, such as Pt-M (M = Ni, Co, Fe..etc.), have exhibited improved catalytic performance due to the increase in the surface-to-volume ratio. Some Pt-M nanopolyhedra such as nanocubes and nano-octahedra have been reported with enhanced activity when being used as electrocatalysts. In order to further establish a correlation between the exposed nanocrystal facets (shapes) and their corresponding activities, a pursuit of shape-controlled nanocatalyst synthesis is essential. Although PtPb nanoalloys have been prepared using solution-based methods, few studies have highlighted their catalytic activity as a function of the nanocrystal shape. This work focuses on a modified polyol synthesis technique and an adjustment of the Pb-metal precursor, which serves as a “buffer” in the nucleation stage of the shape-controlled nanoalloy development. Using this developed synthetic strategy, shape-controlled hexagonally close-packed PtPb nanoalloys can be prepared in a one-pot synthesis without additional post-treatment. The as-prepared PtPb nanocrystals demonstrate an improved anode electrocatalytic performance.

A solid–solid reaction enhanced by an inhomogeneous catalyst in the (de)hydrogenation of a lithium–hydrogen–nitrogen system

The reaction mechanism of the (de)hydrogenation of a LiNH2 + LiH mixture with a nanoscale catalyst has been investigated. In this research, the position of each solid phase was examined by transmission electron microscopy (TEM). The observation showed that LiH particles, around 100 nm in size, formed around LiNH2 in the hydrogenation process. According to in situ TEM observation of the dehydrogenation process, the LiH particles became smaller as they reacted with LiNH2. Fine particles of Li2NH with crystallites of size 30–40 nm formed on the surface of the LiNH2. It indicated that H+ from the LiNH2 moved to the interface and combined with H− from the LiH in the dehydrogenation process, consequently H2 was released. At the same time, Li+ ions diffused from the LiH to the LiNH2 through the interface and Li2NH formed. On the other hand, it was confirmed that the catalyst was located at the interface between the LiH and LiNH2. It was found that the catalyst had the effect of improving the migration of Li+ from LiH to LiNH2.