We present an experimental and theoretical study of borane–aluminum surface interactions that lead to rapid production of aluminum nanoparticles when Al balls are milled in the presence of diborane or pentaborane. Mass spectrometry was used to probe reactions of the boranes with aluminum fracture surfaces produced by milling collisions, which also generate local, transient high temperatures. Density functional theory was used to examine the interactions between a model aluminum surface and diborane and pentaborane, providing insight into the energetics of the first steps in the process that ultimately enables nanoparticle production. Further insight into the surface chemistry was obtained by analyzing the nanoparticles with X-ray photoelectron spectroscopy, scanning transmission electron microscopy with both electron-energy-loss and energy-dispersive X-ray spectroscopies, and dynamic light scattering. Particles were found to have fcc aluminum cores, capped by a ∼2-nm-thick shell, rich in both boron and hydrogen. The shell partially protects the aluminum from air oxidation, and further capping of the particles with organic ligands renders the particles air-stable and confers dispersibility in hydrocarbon solvents.

Size-selected supported clusters of transition metals can be remarkable and highly tunable catalysts. A particular example is Pt clusters deposited on alumina, which have been shown to dehydrogenate hydrocarbons in a size-specific manner. Pt7, of the three sizes studied, is the most active and, therefore, like many other catalysts, deactivates by coking during reactions in hydrocarbon-rich environments. Using a combination of experiment and theory, we show that nanoalloying Pt7 with boron modifies the alkene-binding affinity to reduce coking. From a fundamental perspective, the comparison of experimental and theoretical results shows the importance of considering not simply the most stable cluster isomer, but rather the ensemble of accessible structures as it changes in response to temperature and reagent coverage.

Size-selected supported clusters of transition metals can be remarkable and highly tunable catalysts. A particular example is Pt clusters deposited on alumina, which have been shown to dehydrogenate hydrocarbons in a size-specific manner. Pt7, of the three sizes studied, is the most active and, therefore, like many other catalysts, deactivates by coking during reactions in hydrocarbon-rich environments. Using a combination of experiment and theory, we show that nanoalloying Pt7 with boron modifies the alkene-binding affinity to reduce coking. From a fundamental perspective, the comparison of experimental and theoretical results shows the importance of considering not simply the most stable cluster isomer, but rather the ensemble of accessible structures as it changes in response to temperature and reagent coverage.

X-ray absorption near-edge structure (XANES) is commonly used to probe the oxidation state of metal-containing nanomaterials; however, as the particle size in the material drops below a few nanometers, it becomes important to consider inherent size effects on the electronic structure of the materials. In this paper, we analyze a series of size-selected Ptn/SiO2 samples, using X-ray photoelectron spectroscopy (XPS), low energy ion scattering, grazing-incidence small-angle X-ray scattering, and XANES. The oxidation state and morphology are characterized both as-deposited in UHV, and after air/O2 exposure and annealing in H2. The clusters are found to be stable during deposition and upon air exposure, but sinter if heated above ∼150 °C. XANES shows shifts in the Pt L3 edge, relative to bulk Pt, that increase with decreasing cluster size, and the cluster samples show high white line intensity. Reference to bulk standards would suggest that the clusters are oxidized; however, XPS shows that they are not. Instead, the XANES effects are attributable to development of a band gap and localization of empty state wave functions in small clusters.

Mass-selected Ptn+ ion deposition in ultrahigh vacuum (UHV) was used to prepare a series of size-selected electrodes with Ptn (n ≤ 14) clusters supported on either glassy carbon (GC) or indium tin oxide (ITO). After characterization of the physical properties of the electrodes in UHV, an in situ method was used to study electrocatalytic activity for the oxygen reduction and ethanol oxidation reactions, without significant air exposure. For each reaction studied, there are similarities between the catalytic properties of Ptn-containing electrodes and those of nanoparticulate or bulk Pt electrodes, but there are also important differences that provide mechanistic insights. For all systems, strong cluster size effects were observed. For comparison, select experiments were done under identical conditions but with the Ptn electrodes exposed to air prior to electrochemical studies, resulting in strong modification/suppression of catalytic activity due to adventitious contaminants.For ethanol oxidation at Ptn/ITO, activity varies with size nonmonotonically, by more than an order of magnitude. The sharp size dependence persists during at least 30 to 40 cycles through the Pt redox potential, indicating that processes that would tend to broaden the size distribution are not efficient. All but the least active sizes are substantially more active per mass of Pt, than Pt nanoparticles under the same conditions. The oscillatory dependence of activity on size is anticorrelated with the binding energy of the Pt 4d core level, demonstrating that activity is controlled by the electronic structure of the supported clusters.For oxygen reduction at Ptn/ITO, the branching between water and hydrogen peroxide production is strongly dependent on cluster size, with small clusters selectively producing peroxide with high activity. The selectivity appears to be related to the size of the active site, with no obvious correlation to Pt electronic properties.The most unusual effect seen was for Ptn/GC, studied under acid conditions appropriate to oxygen reduction. Pt7 and a few other cluster sizes show “normal” oxygen reduction activity, similar to what is measured for Pt nanoparticles on GC under the same conditions. Many of the small clusters, however, are found to catalyze highly efficient oxidation, by water, of the glassy carbon support, with essentially no overpotential. The high activity for carbon oxidation for many Ptn/GC electrodes and the absence of significant carbon oxidation for a GC electrode with Pt nanoparticles raise the question of whether small Pt clusters may be responsible for much of the corrosion observed in Pt/carbon electrodes. This system provides another example where activity for oxidation catalysis is anticorrelated with the Pt core level binding energies, indicating that it is electronic, rather than geometric, structure that limits activity.

Milling aluminum balls together with either vapor- or liquid-phase acetonitrile (ACN) leads to production of nanoparticles by mechanical attrition; however, vapor-phase ACN is far more efficient at inducing size reduction, leading to more, smaller, and more uniform particles. The attrition process is also more efficient than traditional milling of particulate starting material and produces nanoparticles with substantially lower contamination levels. This paper is aimed at better understanding the nature of the size reduction process, the chemistry driving it, and the particles it produces. Mass spectrometry was used to probe gases generated during milling, and a combination of X-ray photoelectron spectroscopy, infrared spectroscopy, dynamic light scattering, helium ion microscopy, scanning electron microscopy, and thermogravimetric analysis/mass spectrometry was used to probe the particles and their surface layer. To provide further insight into the chemistry occurring between ACN and aluminum under milling conditions, high-level ab initio theory was used to calculate the structures and energetics for binding and reactions of ACN and its fragments at different sites on an Al80 model surface.

Ultraviolet photoelectron spectroscopy of size-selected Pdn clusters supported on TiO2 shows a decrease in density of states in the TiO2 gap after the absorption of CO, while O2 does not result in a decrease in density. Oxygen is more electron withdrawing than CO, so the Pd clusters should become more positively charged when exposed to O2 than CO. More positively charged clusters are expected to have larger electron binding energies; thus, the observed shifts in the UPS spectra are at odds with conventional wisdom. We have performed a combined experimental and theoretical investigation of the UPS spectra of Pdn clusters with adsorbed O2 and CO. Bonding of both CO molecules and O atoms to the Pd clusters results in a decrease in the density of states in the TiO2 gap; however, O atom binding on top of the clusters also results in significant final state stabilization, restoring the density of states in the TiO2 gap. We also find that 4d–5s hybridization plays a critical role in the initial state energies in X-ray photoelectron spectroscopy and evaluate two methods for determining the final state shift via periodic calculations.

Ball milling of aluminum in gaseous atmospheres of ammonia and monomethylamine (MMA) was found to produce particles in the 100 nm size range with high efficiency. A combination of mass spectrometry, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis with mass spectrometric product analysis (TGA-MS), scanning electron microscopy (SEM), infrared spectroscopy, and dynamic light scattering (DLS) was used to study the particles and the chemical interactions responsible for particle production. To help understand the nature of the surface chemistry, high level quantum chemical calculations were performed to predict the structures and energetics for binding and reactions of NH3 and MMA on aluminum surfaces. Both NH3 and MMA react with aluminum under milling conditions, producing H2 and other gaseous products, and leaving the surfaces functionalized. The surface functionalization enhances size reduction by reducing the surface free energy and the tendency toward mechanochemical welding. For both NH3 and MMA, the particle cores are metallic aluminum, but the surface chemical properties are quite different. The ammonia-milled particles are capped by an AlNxOyHz layer ∼10 nm thick, which passivates the particles. The MMA-milled particles are capped with a thinner passivating layer, such that they are pyrophoric in air and react with N2 at elevated temperatures.Keywords: aluminum; ammonia; milling; monomethylamine; nanoparticle synthesis;

The interaction of B–H-functionalized boron nanoparticles with alkenes and nitrogen-rich ionic liquids (ILs) is investigated by a combination of X-ray photoelectron spectroscopy, FTIR spectroscopy, dynamic light scattering, thermogravimetric analysis, and helium ion microscopy. Surface B–H bonds are shown to react with terminal alkenes to produce alkyl-functionalized boron particles. The interaction of nitrogen-rich ILs with the particles appears, instead, to be dominated by boron–nitrogen bonding, even for an ILs with terminal alkene functionality. This chemistry provides a convenient approach to producing and capping boron nanoparticles with a protective organic layer, which is shown to protect the particles from oxidation during air exposure. By controlling the capping group, particles with high dispersibility in nonpolar or polar liquids can be produced. For the particles capped with ILs, the effect of particle loading on hypergolic ignition of the ILs is reported.Keywords: alkylation; boron; energetic materials; ionic liquids; nanoparticles;

Understanding the factors that control electrochemical catalysis is essential to improving performance. We report a study of electrocatalytic ethanol oxidation – a process important for direct ethanol fuel cells – over size-selected Pt centers ranging from single atoms to Pt14. Model electrodes were prepared by soft-landing of mass-selected Ptn+ on indium tin oxide (ITO) supports in ultrahigh vacuum, and transferred to an in situ electrochemical cell without exposure to air. Each electrode had identical Pt coverage, and differed only in the size of Pt clusters deposited. The small Ptn have activities that vary strongly, and non-monotonically with deposited size. Activity per gram Pt ranges up to ten times higher than that of 5 to 10 nm Pt particles dispersed on ITO. Activity is anti-correlated with the Pt 4d core orbital binding energy, indicating that electron rich clusters are essential for high activity.

•CO oxidation studied over size-selected Pdn clusters supported on thin alumina films.•Highly efficient and stable Pdn clusters (∼50% CO oxidation).•Size dependent reactivity controlled by O2 activation and CO desorption kinetics.An intense, mass-selected cluster source for preparation of model supported clusters is described, and results are presented for cluster size effects on CO oxidation activity for Pdn clusters supported on alumina films grown on Re(0001) or Ta(110) single crystals. The electronic structure of the samples was probed by X-ray and UV photoelectron spectroscopy, and low energy ion scattering was used to probe binding morphology. The Pdn activity was monitored via both steady state and temperature-programmed reaction (TPR) methods. In both cases, the samples appear to be stable in repeated reaction cycles for temperatures up to 600 K. In the TPR experiments, CO oxidation activity per Pd atom varied by ∼40% between the most and least reactive clusters, while under steady state conditions, the activity varied by ∼55%, but with a different pattern of activity vs. size. The difference in the effects of cluster size is attributed to the fact that TPR experiments were done under conditions where the rate-limiting step is oxygen activation, while the steady-state reactivity conditions were most sensitive to the strength of CO binding. Two distinct types of CO binding were observed and characterized by a combination of temperature-programmed desorption (TPD), and temperature-dependent ISS (TD-ISS).

In single nanoparticle mass spectrometry, individual charged nanoparticles (NPs) are trapped in a quadrupole ion trap and detected optically, allowing their mass, charge, and optical properties to be monitored continuously. Previous experiments of this type probed NPs that were either fluorescent or large enough to detect by light scattering. Alternatively, small NPs can be heated to temperatures where thermally excited emission is strong enough to allow detection, and this approach should provide a new tool for measurements of sublimation and surface reaction kinetics of materials at high temperatures. As an initial test, we report a study of carbon NPs in the 20–50 nm range, heated by 10.6 μm, 532 nm, or 445 nm lasers. The kinetics for sublimation and oxidation of individual carbon NPs were studied, and a model is presented for the factors that control the NP temperature, including laser heating, and cooling by sublimation, buffer gas collisions, and radiation. The estimated NP temperatures were in the 1700–2000 K range, and the NP absorption cross sections ranged from ∼0.8 to 0.2% of the geometric cross sections for 532 nm and 10.6 μm excitation, respectively. Emission spectra of single NPs and small NP ensembles show a feature in the IR that appears to be the high energy tail of the thermal (blackbody-like) emission expected from hot particles but also a discrete feature peaking around 750 nm. Both the IR tail and 750 nm peak are observed for all particles and for both IR and visible laser excitation. No significant difference was observed between graphite and amorphous carbon NPs.

Photoelectron spectroscopy is a powerful tool for investigating the electronic structure of supported clusters, especially when initial state and final state contributions to the electron binding energy can be separated. We have performed a combined experimental and theoretical study of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) using atomically size-selected Pdn clusters on rutile TiO2(110). Theoretical investigations allow for the UPS and XPS shifts to be split into initial state and final state contributions. In XPS, the occupation of the 4d orbital of Pd controls the initial state shift offering information about the hybridization of the cluster, while the size and the charging of the cluster controls the final state shift. In UPS, we evaluate two methods for calculating the final state shift in periodic unit cells and find that both methods give reasonable results for pristine TiO2; however, using a p-type dopant fails when two separate donor–acceptor pairs are present. The observed UPS shifts can be described by combining the surface dipole and the final state shifts. Metallic contacts to the semiconductor surface result in band alignment between the metallic contact and the cluster, shifting the Fermi level to lie just below the conduction band of the TiO2. Information about the charge state and hybridization of the cluster are revealed by separating the initial and final state effects.

Deposition of size-selected Ptn clusters on indium tin oxide (ITO) films in ultrahigh vacuum was used to create electrodes with catalytic sites of controlled size. We report a study of the oxygen reduction reaction (ORR) in 0.1 M HClO4 at size-selected Ptn/ITO electrodes that were prepared and characterized without exposure to laboratory air. It was found that the ORR onset potential was size-dependent, varying from ∼0.66 V vs NHE for Pt1/ITO to ∼0.78 V vs NHE for Ptn (n ≥ 10). The maximum ORR currents per gram of Pt were found to be about an order of magnitude higher than that for ITO with 5 nm Pt particles. The branching ratio between the production of water and hydrogen peroxide in ORR was found to be strongly size-dependent. For 5 nm Pt particles on ITO or for polycrystalline Pt, little H2O2 was produced, but as cluster size was decreased, the H2O2 branching became large, suggesting that small Pt clusters could be useful selective catalysts for H2O2 electrosynthesis. Because there was no obvious correlation of ORR activity with Ptn electronic properties, as probed by photoemission, the effect of size on branching is tentatively attributed to size of the available oxygen binding sites.

Charged CdSe/ZnS quantum dots (QDs) trapped in the gas phase are transformed by CO2 laser heating, resulting in brightening of their photoluminescence intensities by more than 2 orders of magnitude. The transformation is shown to be thermally driven, and self-limiting, i.e., once the QDs have been fully brightened, they are unaffected by further CO2 laser irradiation at intensities up to 1 kW/cm2. The transformation clearly involves loss of the 10.6 μm chromophore, which appears to be the ligand layer. The thermally brightened QDs are tested for use as noncontact probe particles, allowing dark nanoparticles to be detected by cotrapping them with a brightened QD. We show that cotrapping has negligible effect on the secular frequency of the cotrapped particles, and that it is possible to simultaneously monitor changes in the mass and charge of up to three cotrapped nanoparticles for hours, allowing the rate of surface reactions to be measured. Application to studying the surface chemistry of free nanoparticles is discussed.

The CO oxidation activity of size-selected Pd20 clusters deposited on alumina films grown on Re(0001) is shown to depend strongly on the film thickness in the 0 to 10 nm range. For the reaction conditions of these experiments, binding and activation of O2 is shown to be the limiting process, which can be varied by a factor of 2 by tuning the alumina film thickness, due to effects on both the electronic structure of the alumina film and the morphology of the supported Pd clusters. The alumina films are shown to be doped with Re atoms diffusing from the Re(0001) substrate, leading to a strong dependence of the surface electronic properties on alumina thickness, which in turn, results in the observed thickness-dependent activity of the Pd20.

A reactant-assisted mechanochemical method was used to produce copious nanoparticles from malleable/ductile metals, demonstrated here for aluminum, iron, and copper. The milling media is intentionally degraded via a reactant-accelerated wear process, where the reactant aids particle production by binding to the metal surfaces, enhancing particle production, and reducing the tendency toward mechanochemical (cold) welding. The mechanism is explored by comparing the effects of different types of solvents and solvent mixtures on the amount and type of particles produced. Particles were functionalized with oleic acid to aid in particle size separation, enhance dispersion in hydrocarbon solvents, and protect the particles from oxidation. For aluminum and iron, the result is air-stable particles, but for copper, the suspended particles are found to dissolve when exposed to air. Characterization was performed using electron microscopy, dynamic light scattering, Fourier transform infrared spectroscopy, solid state nuclear magnetic resonance, and X-ray photoelectron spectroscopy. Density functional theory was used to examine the nature of carboxylic acid binding to the aluminum surface, confirming the dominance of bridging bidentate binding.Keywords: aluminum; copper; iron; mechanochemistry; milling; nanoparticle synthesis; wear

Ball milling of boron in an H2 atmosphere was found to result in hydrogen uptake of up to 5% by weight (36 mol %). The nature of the hydrogen binding to boron was probed by a combination of ab initio theory, IR spectroscopy, thermogravimetric analysis, and mass spectral measurements of gases evolved during sample heating. The dominant binding mode is found to be H atoms bound to B atoms in the surface layer of the particles, and the high hydrogen loading results from production of very high surface area, indicating that gaseous H2 is an effective agent promoting size reduction in milling. Hydrogen incorporated in the samples was found to be stable for at least a month under ambient conditions. Desorption is observed beginning at ∼60 °C and continuing as the temperature is increased, with broad desorption features peaking at ∼250 and ∼450 °C, and ending at ∼800 °C. Unprotected hydrogenated boron nanoparticles were found to be reactive with O2 producing a hydrated boron oxide surface layer that decomposed readily at 100 °C leading to desorption of H2O. Hydrogenated boron nanoparticles were found to promote a higher flame height in the hypergolic ignition of ionic liquids upon contact with nitric acid.Keywords: boron; energetic materials; hydrogen enrichment; mass spectroscopy;

Oxidation of CO over size-selected Ptn clusters (n = 1, 2, 4, 7, 10, 14, 18) supported on alumina thin films grown on Re(0001) was studied using temperature-programmed reaction/desorption (TPR/TPD), X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and low energy ion scattering spectroscopy (ISS). The activity of the model catalysts was found to vary by a factor of five with deposited Ptn size during the first reaction cycle (TPR) and by a factor of two during subsequent cycles, with Pt2 being the least active and Pt14 the most active. The limiting step in the reaction appears to be the binding of oxygen; however, this does not appear to be an activated process as reaction is equally efficient for 300 K and 180 K oxidation temperatures. Size-dependent shifts in the valence band onset energy correlate strongly with CO oxidation activity, and there is also an apparent correlation with the availability of a particular binding site, as probed by CO TPD. The morphology of the clusters also becomes more three dimensional over the same size range, but with a distinctly different size-dependence. The results suggest that both electronic structure and the availability of particular binding sites control activity.

Reactions of mode-selectively excited HOD+ with C2H2 and C2D2 were studied over the center-of-mass collision energy (Ecol) range from 0.15 to 2.9 eV. HOD+ was prepared in each of its fundamental vibrational states: ground state (000), bend (010), OD stretch (100), and the OH stretch (001). Charge transfer is the dominant reaction at all energies, although it is inhibited by increasing Ecol, and is accompanied by hydrogen exchange. The total charge transfer cross section is similar for C2H2 and C2D2, however, the tendency toward charge transfer with hydrogen exchange (CTHE) is significantly greater for C2D2 compared to C2H2. Charge transfer shows no significant effects of HOD+ vibrational excitation, however, CTHE is significantly enhanced by vibration at Ecol < 0.62 eV. Both H+ and D+ transfer reactions (HT, and DT, respectively) are observed for both C2H2 and C2D2, with little dependence on collision energy, but with mode- and bond-specific enhancements from excitation of the OH and OD stretches. Recoil velocity measurements show that all channels are direct, except perhaps at the lowest collision energies. Mode-specific effects on the recoil velocity distributions are also observed, revealing how vibrational excitation affects reaction at different collision impact parameters.

Gas phase spectral measurements for CdSe/ZnS core/shell nanocrystal quantum dots (QDs) before and after heating with both infrared (CO2) and visible lasers are reported. As-trapped QDs are spectrally similar to the same QDs in solution; however their photoluminescence (PL) intensities are very low, at least partly due to low absorption cross sections. After heating, the PL intensities brighten by factors ranging from ∼4 to 1800 depending on the QD size and pump laser wavelength. The emission spectra no longer resemble solution spectra and are similar, regardless of the QD diameter. Emission extends from the pump laser wavelength into the near-IR, with strong emission features above the band gap energy, between 645 and 775 nm, and in the near-infrared. Emission spectra from brightened QD ensembles, single QD aggregates, and single QD monomers are similar, showing that even single QDs support PL from a wide variety of states. The heating and cooling processes for QDs in this environment are analyzed, providing limits on the magnitudes of the absorption cross sections before and after thermal brightening. A model, based on absorption bleaching by extra electrons in the conduction band, appears to account for the changes in absorption and emission behavior induced by charging and heating.Keywords: ion trap; mass spectrometry; quantum dots; single particle;

We report measurements of fluorescence intermittency (blinking) and spectral behavior for single semiconductor nanocrystal quantum dots (QDs) isolated in the gas phase and discuss the effects on fluorescence of the QD charge state and heating to the point of sublimation. Core–shell CdSe/ZnS QDs were trapped in a quadrupole ion trap and detected by laser-induced fluorescence. The mass (M) and charge (Q) were determined nondestructively, and both were followed continuously over the course of hours or days. Emission spectra of the trapped QDs are significantly red-shifted relative to the solution-phase emission from the same particles. The temperature of the trapped QDs is determined by the balance between laser heating and collisional cooling and thermal emission, and it is possible to heat the particles to remove ligands or to the point of sublimation. QDs are observed to be emissive during sublimation, for up to 85% mass loss, with emission intensity roughly proportional to the surface area. Eventually, the fluorescence quantum yield drops suddenly, and the QDs begin to blink. The method used is versatile and will allow studies of quantum dot optical properties as a function of size, ligand removal, heating, surface oxidation, and other manipulations, where these properties are continuously correlated with the mass and charge.Keywords: blinking; ion trap; mass spectrometry; quantum dots; single particle;

Model Ptn/glassy carbon electrodes (Ptn/GCE) were prepared by deposition of mass-selected Ptn+ (n ≤ 11) on GCE substrates in ultrahigh vacuum. Electrocatalysis under conditions appropriate for the oxygen reduction reaction (ORR) was studied, for samples both in situ with no exposure to laboratory air and with air exposure prior to electrochemical measurements. Of the small clusters, only a few cluster sizes show the expected ORR activity, and in those cases, the activity per Pt atom is similar to that seen under identical conditions with a conventionally prepared electrode with Pt nanoparticles grown on a GCE. For other small Ptn on GCE, any ORR signal is overwhelmed by large oxidative currents attributed to catalysis of carbon oxidation by water. If the samples are exposed to air prior to electrochemistry, both ORR and carbon oxidation signals are absent, and instead only small capacitive currents or currents attributed to redox chemistry of adventitious organic adsorbates are observed, indicating that air exposure results in passivation of the small Pt clusters.

The structure of two different energetic ionic liquids and the nature of their binding to elemental boron surfaces were investigated by a combination of X-ray photoelectron spectroscopy, IR spectroscopy, zeta potential measurements, thermogravimetric analysis, and first-principles theory. It was found that both 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]) and 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]) ionic liquids bind to boron well enough to resist removal by ultrasonic washing and to protect the boron surface from oxidation during air exposure of the washed powder. The data suggest that both the cation and the anion of the ionic liquids interact with the boron surface; however, for [MAT][DCA], the interaction of the cation appears to dominate, while for [BMIM][DCA] the interaction with the [DCA]− anion dominates. The difference is attributed to the binding of boron to the amino group of [MAT]+, and the amino group also appears to help bind a thicker ionic liquid (IL) capping layer.

Boron nanoparticles prepared by milling in the presence of a hypergolic energetic ionic liquid (EIL) are suspendable in the EIL and the EIL retains hypergolicity leading to the ignition of the boron. This approach allows for incorporation of a variety of nanoscale additives to improve EIL properties, such as energetic density and heat of combustion, while providing stability and safe handling of the nanomaterials.

Integral cross sections and product recoil velo-city distributions were measured for the reaction of HOD+ with NO2, in which the HOD+ reactant was prepared in its ground state and with mode-selective excitation in the 001 (OH stretch), 100 (OD stretch), and 010 (bend) modes. In addition, we measured the 300 K thermal kinetics in a selected ion flow tube reactor and report product branching ratios different from previous measurements. Reaction is found to occur on both the singlet and triplet surfaces with near-unit efficiency. At 300 K, the product branching indicates that triplet → singlet transitions occur in about 60% of triplet-coupled collisions, which we attribute to long interaction times mediated by complexes on the triplet surface. Because the collision times are much shorter in the beam experiments, the product distributions show no signs of such transitions. The dominant product on the singlet surface is charge transfer. Reactions on the triplet surface lead to NO+, NO2H+, and NO2D+. There is also charge transfer, producing NO2+ (a3B2); however, this triplet NO2+ mostly predissociates. The NO2H+/NO2D+ cross sections peak at low collision energies and are insignificant above ∼1 eV due to OH/OD loss from the nascent product ions. The effects of HOD+ vibration are mode-specific. Vibration inhibits charge transfer, with the largest effect from the bend. The NO2H+/NO2D+ channels are also vibrationally inhibited, and the mode dependence reveals how energy in different reactant modes couples to the internal energy of the product ions.

The dissociative binding efficiency of oxygen over Pdn/TiO2(110) (n = 4, 7, 10, 20) has been measured using temperature programmed reaction (TPR) mass spectrometry and X-ray photoemission spectroscopy (XPS) following exposure to O2 with varying doses and dose temperatures. Experiments were carried out following two different O2 exposures at 400 K (10 L and 50 L) and for 10 L of O2 exposure at varying temperatures (Tsurf = 200, 300, and 400 K). During TPR taken after sequential O2 and CO (5 L at 180 K) exposures, unreacted CO is found to desorb in three features at Tdesorb ≈ 150, 200, and 430 K, while CO2 is observed to desorb between 170 and 450 K. We show that Pd20 has exceptionally high efficiency for oxygen activation, compared to other cluster sizes. As a consequence, its activity becomes limited by competitive CO binding at low O2 exposures, while other Pdn sizes are still limited by inefficient O2 activation. This difference in mechanism can ultimately be related back to differences in electronic properties, thus making this question one that is interesting from the theoretical perspective. We also demonstrate a correlation between one of the two CO binding sites and CO2 production, suggesting that only CO in that site is reactive.

Elemental boron has one of the highest volumetric heats of combustion known and is therefore of interest as a high energy density fuel. The fact that boron combustion is inherently a heterogeneous process makes rapid efficient combustion difficult. An obvious strategy is to increase the surface area/volume ratio by decreasing the particle size. This approach is limited by the fact that boron forms a ∼0.5 nm thick native oxide layer, which not only inhibits combustion, but also consumes an increasing fraction of the particle mass as the size is decreased. Another strategy might be to coat the boron particles with a material (e.g., catalyst) to enhance combustion of either the boron itself or of a hydrocarbon carrier fuel. We present a simple, scalable, one-step process for generating air-stable boron nanoparticles that are unoxidized, soluble in hydrocarbons, and coated with a combustion catalyst. Ball milling is used to produce ∼50 nm particles, protected against room temperature oxidation by oleic acid functionalization, and optionally coated with catalyst. Scanning and transmission electron microscopy and dynamic light scattering were used to investigate size distributions, with X-ray photoelectron spectroscopy to probe the boron surface chemistry.

Small Pd clusters Pdn (n = 1, 4, 7, 10, 13) deposited on alumina/NiAl(110) at room temperature were examined by X-ray photoelectron spectroscopy (XPS), as-deposited and after exposure to O2 at temperatures ranging from 100 to 500 K. After O2 exposure at 100 K, the Pd clusters showed XPS shifts indicative of oxidation. The exception was Pd4, which did not oxidize under any conditions. The inertness of Pd4/alumina/NiAl(110) appears to be correlated with a significantly higher-than-expected Pd 3d binding energy, which we attribute to a particularly stable valence shell. None of the clusters examined oxidized during O2 exposures at 300 K or above, but He+ scattering showed that oxygen was bound on the cluster surfaces. Upon heating, all the oxygen associated with these small clusters appeared to spill over and react with the alumina/NiAl(110) support.

Decomposition of a fuel-soluble precursor was used for in situ generation of Pd/PdO nanoparticles, which then catalyzed ignition of the methane/O2/N2 flow. To help understand the relationship between particle properties and activity, the composition, structure, and surface chemical state of the particles were determined by a combination of high-resolution transmission electron microscopy (HRTEM), electron diffraction, scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM/EDX), and X-ray photoelectron spectroscopy (XPS). The particles, collected under methane-free conditions, were found to be primarily crystalline, metallic Pd, with TEM results showing a narrow size distribution around 8 nm and scanning mobility particle sizing measurements (SMPS) indicating a median particle size of ∼10 nm. The ignition temperature was lowered ∼150 K by the catalyst, and we present evidence that ignition is correlated with formation of a subnanometer oxidized Pd surface layer.

The effects of collision energy (Ecol) and five different modes of H2CO+ vibration on the title reaction have been studied over the center-of-mass Ecol range from 0.1 to 3.2 eV, including measurements of product ion recoil velocity distributions. Electronic structure and Rice–Ramsperger–Kassel–Marcus calculations were used to examine properties of various complexes and transition states that might be important along the reaction coordinate. Two product channels are observed, corresponding to Hydrogen Transfer (HT) and Proton Transfer (PT). Both channels are endothermic with similar onset energies of ∼0.9 eV; however, HT dominates over the entire Ecol range and accounts for 70–85% of the total reaction cross section. Both HT and PT occur by direct mechanisms over the entire Ecol range, and have similar dependence on reactant vibrational and collision energy. Despite these similarities, and the fact that the two channels are nearly isoenergetic and differ only in which product moiety carries the charge, their dynamics appear quite different. PT occurs primarily in large impact parameter stripping collisions, where most of the available energy is partitioned to product recoil. HT, in contrast, results in internally hot products with little recoil energy and a more forward–backward symmetric product velocity distribution. Vibration is found to affect the reaction differently in different collision energy regimes. The appearance thresholds are found to depend only on total energy, i.e., all modes of vibration are equivalent to Ecol. With increasing Ecol, vibrational energy becomes increasingly effective, relative to Ecol, at driving reaction. For HT, this transition occurs just above threshold, while for PT it begins at roughly twice the threshold energy.

A series of planar model catalysts were prepared by deposition of size-selected Irn+ on Al2O3/NiAl(1 1 0), and hydrazine decomposition chemistry was used to probe their size-dependent chemical properties. Small Irn (n ⩽ 15) on Al2O3/NiAl(1 1 0) are able to induce hydrazine decomposition at temperatures well below room temperature, with significant activity first appearing at Ir7. Both activity and product branching are strongly dependent on deposited cluster size, with these small clusters supporting only the simplest decomposition mechanism: dehydrogenation and N2 desorption at low temperatures, followed by H2 recombinative desorption at temperatures above 300 K. For Ir15, we begin to see ammonia production, signaling the onset of a transition to clusters able to support more complex chemistry.

Oxidation of CO over size-selected Ptn clusters (n = 1, 2, 4, 7, 10, 14, 18) supported on alumina thin films grown on Re(0001) was studied using temperature-programmed reaction/desorption (TPR/TPD), X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), and low energy ion scattering spectroscopy (ISS). The activity of the model catalysts was found to vary by a factor of five with deposited Ptn size during the first reaction cycle (TPR) and by a factor of two during subsequent cycles, with Pt2 being the least active and Pt14 the most active. The limiting step in the reaction appears to be the binding of oxygen; however, this does not appear to be an activated process as reaction is equally efficient for 300 K and 180 K oxidation temperatures. Size-dependent shifts in the valence band onset energy correlate strongly with CO oxidation activity, and there is also an apparent correlation with the availability of a particular binding site, as probed by CO TPD. The morphology of the clusters also becomes more three dimensional over the same size range, but with a distinctly different size-dependence. The results suggest that both electronic structure and the availability of particular binding sites control activity.

Understanding the factors that control electrochemical catalysis is essential to improving performance. We report a study of electrocatalytic ethanol oxidation – a process important for direct ethanol fuel cells – over size-selected Pt centers ranging from single atoms to Pt14. Model electrodes were prepared by soft-landing of mass-selected Ptn+ on indium tin oxide (ITO) supports in ultrahigh vacuum, and transferred to an in situ electrochemical cell without exposure to air. Each electrode had identical Pt coverage, and differed only in the size of Pt clusters deposited. The small Ptn have activities that vary strongly, and non-monotonically with deposited size. Activity per gram Pt ranges up to ten times higher than that of 5 to 10 nm Pt particles dispersed on ITO. Activity is anti-correlated with the Pt 4d core orbital binding energy, indicating that electron rich clusters are essential for high activity.

Boron nanoparticles prepared by milling in the presence of a hypergolic energetic ionic liquid (EIL) are suspendable in the EIL and the EIL retains hypergolicity leading to the ignition of the boron. This approach allows for incorporation of a variety of nanoscale additives to improve EIL properties, such as energetic density and heat of combustion, while providing stability and safe handling of the nanomaterials.