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Maps of reaction rate constants across all surface orientations vicinal to Cu(111) reveal that d-tartaric acid decomposes preferentially on Cu(hkl)S surfaces while l-tartaric acid decomposes preferentially on Cu(hkl)R orientations.

Enantioselective chemistry on intrinsically chiral surfaces is the quintessential form of structure-sensitive surface chemistry, arising purely from the dissymmetry of the surface structure. Identification or design of chiral surface structures that maximize enantioselectivity for a given processes is extremely challenging because of the limited magnitude of the enantiospecific interaction energetics of chiral molecules with chiral surfaces. Using spherical Cu single crystals exposing surfaces with a continuous two-dimensional distribution of crystallographic orientations, we mapped the enantiospecific surface reaction kinetics of tartaric acid decomposition across the surface orientation space. These measurements reveal both the mechanistic origin of enantioselectivity and identify the structural features of the most enantiospecific surface orientation.

Surface explosion reactions have highly nonlinear reaction kinetics that exhibit autoacceleration under isothermal conditions. These can lead to phenomena such as oscillatory surface reaction rates...

Decomposition of tartaric acid (TA, HO2CCH(OH)CH(OH)CO2H) and aspartic acid (Asp, HO2CCH(NH2)CH2CO2H) on Cu surfaces occurs via a vacancy-mediated surface explosion mechanism with nonlinear kinetics: re = keθ(1 – θ)2, where θ is the adsorbate coverage, and (1 – θ) is the vacancy coverage. During temperature-programmed reaction experiments on naturally chiral Cu(hkl)R&S surfaces, these kinetics, coupled with the chirality of the adsorbates, lead to highly enantiospecific decomposition rates. Herein, it is demonstrated that, in the presence of a thermal molecular beam with a constant Asp flux, FAsp, the decomposition of Asp on Cu(111) occurs catalytically and in steady-state to turnover numbers >40 without contamination or deactivation of the surface. Moreover, the decomposition rates at a given (T, FAsp) manifest the two different steady-states predicted by the nonlinearity of the decomposition kinetics. The unusual ability to catalytically decompose Asp on Cu, coupled with the ability to distinguish betwe...

The relationship between alloy catalyst activity and valence band electronic structure has been investigated experimentally across a broad, continuous span of CuxPd1–x composition space. CuxPd1–x composition spread alloy films (CSAFs) were used as catalyst libraries with a 100 channel microreactor to measure the H2–D2 exchange kinetics over a temperature range of 333–593 K at 100 discrete CuxPd1–x compositions spanning the range x = 0.30–0.97. The H2–D2 exchange activity exhibits a monotonic decrease over the composition range x = 0.30–0.97. A steady state, microkinetic model was used to estimate the energy barriers to dissociative H2 adsorption, ΔEads‡, and recombinative H2 desorption, ΔEdes‡, as functions of alloy composition, x. Their values fall in the ranges ΔEads‡(x) = 0.15 to 0.45 eV and ΔEdes‡(x) = 0.55–0.65 eV. Spatially resolved UV photoemission spectra were obtained from the CuxPd1–x CSAF and used to estimate the average energy of the filled states of the valence band as a function of alloy com...

A high throughput study has been conducted of the Cu 2p3/2, Au 4f7/2, and Pd 3d3/2 X-ray photoemission spectra obtained from a continuous distribution of CuxAuyPd1–x–y alloy samples prepared as a single composition spread alloy film (CSAF). All three elements exhibit shifts of their core level binding energies with respect to their pure states when diluted into the alloy. The Cu 2p3/2 core level shift (CLS) exhibits additional shifts over the composition ranges at which the CuxAuyPd1–x–y alloy transitions between FCC and B2 phases. This discontinuous CLS has been used to map the extent of the B2 phase across the ternary CuxAuyPd1–x–y alloy composition space. The sensitivity of core level binding energies to the alloy phase offers an opportunity to use XPS to study phases in alloy nanoparticles, ultrathin films, and other morphologies that are not amenable to structure determination by diffraction based methods.

We describe a 100-channel microreactor array capable of spatially resolved measurement of catalytic activity across the surface of a flat substrate. When used in conjunction with a composition spread alloy film (CSAF, e.g., PdxCuyAu1−x−y) across which component concentrations vary smoothly, such measurements permit high-throughput analysis of catalytic activity and selectivity as a function of catalyst composition. In the reported implementation, the system achieves spatial resolution of 1 mm2 over a 10 × 10 mm2 area. During operation, the reactant gases are delivered at constant flow rate to 100 points of differing composition on the CSAF surface by means of a 100-channel microfluidic device. After coming into contact with the CSAF catalyst surface, the product gas mixture from each of the 100 points is withdrawn separately through a set of 100 isolated channels for analysis using a mass spectrometer. We demonstrate the operation of the device on a PdxCuyAu1−x−y CSAF catalyzing the H2–D2 exchange reaction at 333 K. In essentially a single experiment, we measured the catalytic activity over a broad swathe of concentrations from the ternary composition space of the PdxCuyAu1−x−y alloy.

PdAgAu alloy films were prepared on porous stainless steel supports by sequential electroless deposition. Two specific compositions, Pd83Ag2Au15 and Pd74Ag14Au12, were studied for their sulfur tolerance. The alloys and a reference Pd foil were exposed to 1000H2S/H2 at 623 K for periods of 3 and 30 h. The microstructure, morphology and bulk composition of both non-exposed and H2S-exposed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). XRD and SEM analysis revealed time-dependent growth of a bulk Pd4S phase on the Pd foil during H2S exposure. In contrast, the PdAgAu ternary alloys displayed the same FCC structure before and after H2S exposure. In agreement with the XRD and SEM results, sulfur was not detected in the bulk of either ternary alloy samples by EDS, even after 30 h of H2S exposure. X-ray photoelectron spectroscopy (XPS) depth profiles were acquired for both PdAgAu alloys after 3 and 30 h of exposure to characterize sulfur contamination near their surfaces. Very low S 2p and S 2s XPS signals were observed at the top-surfaces of the PdAgAu alloys, and those signals disappeared before the etch depth reached ∼10 nm, even for samples exposed to H2S for 30 h. The depth profile analyses also revealed silver and gold segregation to the surface of the alloys; preferential location of Au on the alloys surface may be related to their resistance to bulk sulfide formation. In preliminary tests, a PdAgAu alloy membrane displayed higher initial H2 permeability than a similarly prepared pure Pd sample and, consistent with resistance to bulk sulfide formation, lower permeability loss in H2S than pure Pd. Fil: Braun, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera". Universidad Nacional del Litoral. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera"; Argentina Fil: Miller, James. University of Carnegie Mellon; Estados Unidos Fil: Gellman, Andrew J. University of Carnegie Mellon; Estados Unidos Fil: Tarditi, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera". Universidad Nacional del Litoral. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera"; Argentina Fil: Fleutot, Benoit. University of Carnegie Mellon; Estados Unidos Fil: Kondratyuk, Petro. University of Carnegie Mellon; Estados Unidos Fil: Cornaglia, Laura Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera". Universidad Nacional del Litoral. Instituto de Investigaciones en Catálisis y Petroquímica "Ing. José Miguel Parera"; Argentina

A high throughput methodology for the study of surface segregation in alloys has been developed and applied to the Cu x Pd 1―x system. A novel offset-filament deposition tool was used to prepare Cu x Pd 1―x composition spread alloy films (CSAFs), high throughput sample libraries with continuous lateral composition variation spanning the range x = 0.05―0.95. Spatially resolved low energy ion scattering spectroscopy (LEISS) and X-ray photoelectron spectroscopy (XPS) were used to characterize the films' top-surface and near-surface compositions, respectively, as functions of alloy composition, x, and temperature. Electron backscatter diffraction (EBSD) was used to identify the bulk phases in the CSAF as a function of alloy composition, x. Films equilibrated by annealing at temperatures ≥ 700 K displayed preferential segregation of Cu to their top-surfaces at all bulk compositions; segregation patterns did not, however, depend on local structure. The Langmuir―McLean thermodynamic model was applied to segregation measurements made in the temperature range 700―900 K in order to estimate the enthalpy (ΔH seg ) and entropy (ΔS seg ) of segregation as a function of bulk Cu x Pd 1―x composition. Segregation measurements at x = 0.30 on the CSAF compare well with results previously reported for a bulk, polycrystalline Cu 0.30 Pd 0.70 alloy, demonstrating the utility of the CSAF as a high throughput library for study of segregation.

We discuss our own studies of molecular adsorption on and inside of single-wall carbon nanotubes in the broader context of important theoretical and experimental developments in the field. We show that adsorption in the nanotube interior sites as well as in the groove and exterior sites may be resolved by various experimental methods. In addition, the changes that the adsorbate phases undergo due to confinement in the nanotube interior are discussed, particularly focusing on confined molecules of water, alkanes, and an alkene. Attention is also devoted to the use of oxidizing agents such as ozone to open the ends and walls of nanotubes for interior adsorption.

We have experimentally measured long-range self-diffusivity of normal heptane adsorbed on single-walled carbon nanotubes at 275 K. This was done by observing the displacement kinetics of heptane by a deuterium-labeled molecule, 1-deuteroheptane. We also performed MD simulations of heptane self-diffusion inside an individual nanotube under full loading. The experimentally measured diffusion coefficient (D = 7.7 × 10-12 cm2 s-1) was 8 orders of magnitude lower than the diffusion coefficient inside an individual nanotube from the simulation (D = 8.2 × 10-4 cm2 s-1). Because heptane only adsorbs in the nanotube interior at 275 K, we conclude that the experimentally observed long-range transport is rate-limited by a very slow exchange of heptane between different nanotubes in the bulk SWNT sample. At the same time, the mixing of heptane inside individual nanotubes is fast. The reasons for slow diffusion between different tubes could be the potential energy barriers at the entry points into the nanotubes, as we...

We show that thermal desorption kinetics clearly resolve adsorbates bound in different sites on single walled carbon nanotube bundles. The molecules n-C9H20 and CCl4 were compared and it was found that the nanotube internal sites exhibited the highest desorption temperature, whereas external groove sites exhibited the next highest desorption temperature for both molecules. When n-C9H20 and CCl4 coadsorb, the more strongly bound n-C9H20 quantitatively displaces CCl4 from internal sites to groove sites. Molecular shape governs the capacity of the different sites for the two molecules.

Fermi resonance interaction between a distributed fundamental vibrational level and a distributed overtone is considered. The overtone is assumed to be spectrally inactive. Simple analytical expressions are derived for the resulting spectral profile in terms of the distribution functions for the fundamental and overtone. The formulas enable straightforward modeling of spectra with Fermi resonances.

Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O(3) at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF(4) adsorbs on closed SWNTs, exhibiting its nu(3) asymmetric stretching mode at 1267 cm(-1) (red shift relative to the gas phase, 15 cm(-1)). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF(4) exhibits its nu(3) mode at 1247 cm(-1) (red shift relative to the gas phase, 35 cm(-1)). It was shown that, at 133 K, Xe preferentially displaces internally bound CF(4) species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF(4) inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF(4).

A computer-controlled system that allows acquisition of large volumes of temperature programmed desorption data under ultrahigh vacuum conditions has been designed and tested. The vacuum part of the system consists of a sample mounted on a rotation–translation manipulator, a shielded quadrupole mass spectrometer for the desorption flux measurement, and a collimated molecular beam doser for adsorbate delivery. The experiment involves a series of tasks: (1) dosing of the adsorbate onto the surface; (2) calibration of the mass spectrometer gain; (3) translating the dosed sample to the mass spectrometer position; (4) linearly ramping the temperature of the sample to acquire the desorption spectrum; and (5) cooling the sample. In the system described here, these tasks are accomplished automatically. As a result, the time needed for the experiment and the potential for operator errors are substantially reduced. The performance of the system is demonstrated for CCl4 adsorbed on gold-supported single wall carbon nanotubes.A computer-controlled system that allows acquisition of large volumes of temperature programmed desorption data under ultrahigh vacuum conditions has been designed and tested. The vacuum part of the system consists of a sample mounted on a rotation–translation manipulator, a shielded quadrupole mass spectrometer for the desorption flux measurement, and a collimated molecular beam doser for adsorbate delivery. The experiment involves a series of tasks: (1) dosing of the adsorbate onto the surface; (2) calibration of the mass spectrometer gain; (3) translating the dosed sample to the mass spectrometer position; (4) linearly ramping the temperature of the sample to acquire the desorption spectrum; and (5) cooling the sample. In the system described here, these tasks are accomplished automatically. As a result, the time needed for the experiment and the potential for operator errors are substantially reduced. The performance of the system is demonstrated for CCl4 adsorbed on gold-supported single wall carbon ...

The physical adsorption of NO on purified single-walled carbon nanotubes was investigated at low temperatures by means of transmission infrared spectroscopy. Ozone treatment followed by annealing was used as the method of opening the entry ports for adsorption into the interior of the nanotubes. The IR spectra show that the NO that is adsorbed inside nanotubes exists exclusively in its dimer form, with no infrared bands of the monomer visible at a sensitivity level of 5 mol % NO. The internally adsorbed NO dimer species observed is cis-(NO)2, with ν1(symmetric) = 1853 cm-1 and ν5(asymmetric) = 1754 cm-1. Introduction of Xe into the system leads to selective displacement of the internally bound cis-(NO)2 dimer. A large spectroscopic red shift of the symmetric (−0.80%) and the asymmetric (−1.96%) modes for cis-(NO)2 adsorbed inside nanotubes [relative to gas-phase cis-(NO)2] points to the strong interaction of the dimer with the nanotube interior. The enthalpy of dissociation of the adsorbed dimer molecule ...

Thin diamond-like carbon films were subjected to fluorination with gaseous XeF2 under ultrahigh vacuum conditions in order to increase the work function of the diamond-like carbon surface. Changes in the work function and surface composition were monitored with UV photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS), respectively. Successive XeF2 exposures raised the work function by as much as 1.55 eV. Surprisingly, approximately half of the increase in the work function occurred while the coverage of fluorine remained below 0.02 monolayers (ML). This suggests that initial doses of XeF2 remove extrinsic adsorbates from the diamond-like carbon film and that fluorine desorbs with the reaction products. Increasing the exposure of the diamond-like carbon to XeF2 leads to the expected covalent fluorination of the surface, which saturates at fluorine coverages of 6 F atoms/nm2 (~0.3 ML). Annealing of the diamond-like carbon to temperatures above 850 K was required to reduce the surface fluorine concentration to undetectable levels. This did not, however, cause the work function to return to its original, pre-fluorination value. Fil: Tarditi, Ana Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Investigaciones en Catálisis y Petroquímica ; Argentina Fil: Kondratyuk, Petro. University of Carnegie Mellon; Estados Unidos Fil: Wong, Pak Kin. Sae Technology Centre; China Fil: Gellman, Andrew J.. University of Carnegie Mellon; Estados Unidos. Institute For Advanced Energy Studies

Three well-defined adsorption sites have been found on opened single-wall carbon nanotubes by temperature-programmed desorption measurements for several alkanes. A series of linear chain alkanes from pentane to nonane, as well as a branched alkane molecule, 2,2,4-trimethylpentane, were used to elucidate the effect of molecular length on the capacity of the adsorption sites. The two highest-energy adsorption sites were assigned as the nanotube interior sites and groove sites on the outside of the nanotube bundles. Hybrid Monte Carlo simulations were performed to probe the molecular-level details of adsorption. Both in experiments and in the simulation, the groove sites were seen to behave as one-dimensional adsorption space, demonstrating an inverse dependence of capacity on the length of the adsorbed molecule. In contrast, the capacity of the internal sites was found to depend inversely on the volume occupied by the molecule.

The photooxidation of 2-chloroethyl ethyl sulfide (2-CEES), a simulant for mustard gas, was studied using transmission IR spectroscopy on a mixed-oxide TiO2-SiO2 photocatalyst. Ultraviolet irradiation in the photon energy range from 2.1 to 5 eV was employed at a catalyst temperature of 200 K. Rapid photooxidation was observed by the loss of infrared intensity in the v(CHx) stretching region, and concomitant infrared features of adsorbed oxidation products were observed to develop. The oxidation products, captured on the photocatalyst at 200 K, were found to block 2-CEES readsorption. Upon heating the poisoned photocatalyst to about 300 K, infrared measurements indicate that the adsorbed CO2 oxidation product was desorbed. The capability for full readsorption of 2-CEES was achieved upon heating the poisoned photocatalyst to 397 K, and continued rapid photooxidation of the 2-CEES was then possible at about 1/3 the rate found for the fresh catalyst. Thus thermal treatment at 397 K of oxidation-product-poisoned TiO2-SiO2 material is able to partially restore the TiO2-SiO2 photooxidation activity.

Composition spread alloy films (CSAFs) are combinatorial materials libraries that contain broad, continuous composition ranges of binary or higher-order alloys on a single, compact substrate. When characterized for composition and functional properties using spatially resolved methods, CSAF libraries enable rapid determination of composition-property relationships across broad continuous regions of alloy composition space. In this report, we describe the design and operation of a novel offset filament deposition tool for preparation of CSAFs. The spatial distribution of individual alloy component fluxes to the substrate surface, and thus the film composition across the substrate, is controlled by the location and temperature of chemically distinct evaporative line sources. The tool can be used for quantitative deposition of thin (≤100 nm) CSAFs with up to four components. The authors demonstrate the performance of the tool by applying it to preparation of 100 nm thick Pd-Cu CSAFs, with lateral composition gradients that span the range Cu0.05Pd0.95 to Cu0.95Pd0.05, on a 12 mm diameter Mo(110) substrate.