Surface Chemistry Experiments Research Articles

Reactive Interactions between the Ionic Liquid BMP‐TFSI and a Na Surface

AbstractIn order to obtain atomistic insights into the initial stages of the formation of the solid electrolyte interphase (SEI) in Na ion or Na metal batteries, we employ surface chemistry experiments and DFT calculations to study the interactions and reactions between a Na surface and the ionic liquid (IL) 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP‐TFSI), a candidate to be used as electrolyte in batteries. Oxygen‐free Na thin films, which were grown on Ru(0001) and characterized by X‐ray and ultraviolet photoelectron spectroscopy (XPS, UPS), can be understood as model of a Na‐rich electrode. After deposition of submonolayer to multilayer BMP‐TFSI films on the Na thin films at room temperature, XPS measurements revealed partial decomposition and the formation of a ‘contact layer’ at the Na surface, consisting of mainly TFSI‐based decomposition products. By comparison to core level binding energies obtained from density functional theory calculations for energetically feasible reaction products, the constituents of the ‘contact layer’ were identified both as atomic fragments of TFSI (F, O, S) and as larger fragments of TFSI (NSO2CF3, NSO2CF3SO2), presumably remaining at the surface due to kinetic barriers. Increasing the temperature results in cumulative decomposition towards the stable atomic species at or within the Na surface.

Revising the pathways of the Li reaction with organic carbonates.

The metallic lithium electrode has major concerns such as extremely high reactivity and nonuniform needle-like electrodeposition, limiting its wide application as a negative electrode in secondary batteries. Its reactions with the electrolyte leading to solid electrolyte interphase (SEI) formation play an important role, and controlling its composition and properties can help to overcome both difficulties. Even though solid electrolyte interphase chemistry and properties seem to be well known, many surface chemistry experiments reported are not perfect with respect to the purity needed for Li studies and can be interpreted alternatively. Here, we studied reactions between lithium and propylene carbonate and ethylene carbonate in model reactions realized in an ultra-high vacuum. In addition to the already reported reaction pathway yielding lithium carbonate and semicarbonate, our theoretical (DFT) modeling confirms the preference of alternative routes. Along with the most beneficial final lithium carbonates, dilithium 1,2-dialkoxide (DD) can form barrierlessly as a final product by two-electron transfer. Experimental XPS/NEXAFS studies of gas phase and solid-gas model reactions revealed that in both cases DD is the main reaction product. Understanding of the discovered reaction pathway can also be essential for reactions in liquid electrolytes, although the low electric conductivity of the SEI makes it less probable.

Tyrosinase enzyme Langmuir monolayer: Surface chemistry and spectroscopic study

This study investigates the surface chemistry properties of the tyrosinase enzyme Langmuir monolayer at air-aqueous interface using sodium chloride in the subphase to induce the surface activity of the enzyme. Investigation of surface packing and stability of the tyrosinase Langmuir monolayer were performed using surface chemistry experiments while spectroscopic analysis was done to study enzyme conformation. It was found that the tyrosinase enzyme forms a fluid film at air-aqueous interface with good stability as shown by compression-decompression cycles experiments and stability measurements at various surface pressures. UV-vis absorption and fluorescence measurements at different surface pressures revealed that the Langmuir monolayer has good homogeneity with no evidence of aggregates during compression. To gain insight on the conformation of tyrosinase Langmuir monolayer p-polarized infrared-reflection absorption spectroscopy was used. It was found that at high surface pressures the predominant secondary structures were β-sheets while at lower surface pressure both α -helices and β-sheets were present. The circular dichroism spectra were obtained by transferring the Langmuir monolayer at 10 mN.m−1 to a solid quartz support (Langmuir-Blodgett film, LB film), which showed that the major conformation present were α-helices. Images from the immobilized LB films were obtained using atomic force microscopy which showed homogenous and regular deposition with a mean thickness ranging from 3 to 4 nm.

Surface chemistry and spectroscopic studies of the native phenylalanine dehydrogenase Langmuir monolayer at the air/aqueous NaCl interface

This study investigates the main aspects of the surface behavior of the native phenylalanine dehydrogenase (PheDH) enzyme at the air/aqueous interface employing a saline subphase to induce the enzyme surface activity. Surface chemistry experiments were performed in order to determine the surface packing and stability of the formed layer, while spectroscopic experiments provided information regarding its secondary structure conformation. It was found that the PheDH enzyme forms a fluid film, which is quite homogeneous throughout its entire compression, being stable for long periods of time with no significant evidence of aggregates or irreversible domains during interfacial compression/decompression processes. The main secondary structures of the interfacial PheDH film were accessed via in situ reflectance-absorbance infrared spectroscopy, indicating a majority presence of α-helices, which were maintained after the film transfer to solid muscovite supports. The immobilized films presented a homogeneous and regular deposition, with controlled roughness and a mean thickness in the range of 8–10 nm.

Surface chemistry experiments speed up

A new camera makes it possible to collect angular information about all products of a reaction at once.

Surface chemistry experiments speed up

Surface chemistry experiments speed up

Surface chemistry experiments speed up

Surface chemistry experiments speed up

ON THE FORMATION OF INTERSTELLAR WATER ICE: CONSTRAINTS FROM A SEARCH FOR HYDROGEN PEROXIDE ICE IN MOLECULAR CLOUDS

Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H2O2), for the production of water (H2O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H2O2 ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H2O2 should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H2O2/H2O ice films between 2.5 and 200 micron, from 10 to 180 K, containing 3%, 30%, and 97% H2O2 ice. Integrated absorbances for all the absorption features in low-temperature H2O2 ice have been derived from these spectra. For identifying H2O2 ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3 micron. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H2O ice absorption bands, no absorption features are found that can definitely be identified with H2O2 ice. In the absence of definite H2O2 features, the H2O2 abundance is constrained by its possible contribution to the weak absorption feature near 3.47 micron found on the long-wavelength wing of the 3 micron H2O ice band. This gives an average upper limit for H2O2, as a percentage of H2O, of 9% +/- 4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

A surface-science-based model for the selectivity of platinum–gold alloy electrodes in zirconia-based NO x sensors

Recent surface chemistry experiments help explain the operation of an exhaust gas NO x sensor that uses a platinum–gold alloy electrode to selectively remove oxygen but not NO from an interior chamber. The electrode surface must dissociate O 2 for removal by electrochemical pumping, but must be at least 100 times less active for dissociating NO. In ultrahigh vacuum studies on stepped platinum single crystals, NO dissociation is strongly suppressed by surface gold coverages as low as 0.4 monolayer (ML), as well as by co-adsorbed oxygen and NO, probably because the dissociation process requires a special ensemble of several unoccupied Pt atoms. Oxygen dissociation is much less affected by surface modifications. In general, the surface composition of an alloy is very different from its bulk composition and depends on gas composition. By considering the energetics of the Pt–Au system we argue that under all conditions of interest for sensor operation the ensembles of free Pt atoms required for NO dissociation are strongly disfavored. A surface chemistry-based explanation is also suggested for the ammonia interference reported for this NO x sensor.

Erratum: Requiem for the Reaction-Rate Equation?

In a Perspectives in Catalysis article that appeared recently in Catalysis Letters, Professor Michel Boudart hailed a coming revolution in catalytic kinetics [1]. He predicts that the twenty-first century will be dominated by the rate constant, which will be determined by a combination of microkinetic analysis, computational chemistry, and surface science techniques. By contrast, the twentieth century belonged to the rate equation, elucidated using a mechanistic approach to kinetic modeling and supported by discrete experimental data sets arising from a large number of reactions carried out over a range of reaction conditions. Boudart argues that by perfecting the methodology of microkinetic analysis over the next few decades, kinetics-assisted design of catalysts will become predictive, obviating the need for rate equations. Since catalysis is a kinetic phenomenon [2], an approach advocating a prominent role for kinetics in the future development of protocols for discovery, design, and rapid screening of catalysts holds great promise. However, it is perhaps premature to hold a requiem for the rate equation in catalytic reaction kinetics. Indeed, I would argue that the role of the rate equation is currently in ascendance in one area of growing importance, that of research involving the complex organic catalytic reactions common to pharmaceutical applications. In this field, kinetic analysis and the determination of rate constants via the rate equation presents exciting opportunities for future research, aimed at increasing fundamental mechanistic understanding, aiding future catalyst design, and streamlining pharmaceutical process research and development in an ever more competitive environment. The key to this ‘‘back-to-the-future’’ approach to reaction kinetics in liquid-phase organic reactions lies in the recent development of accurate in situ experimental methods for the continuous monitoring of reaction rate as a function of reaction progress. These tools now insure that the fastest way to obtain information about these reactions is simply to run them—and by that I mean to run them just as one would in the commercial application, without resorting to experiments that measure initial rates using distorted reactant concentrations. Monitoring reactions using such in situ methods, we may obtain the equivalent of thousands of separate ‘‘initial-rate’’ measurements at different reactant concentrations in one reaction—typically over the course of an hour or less. What would Langmuir, Rideal, or Michaelis and Menten have given for access to such plentiful data for use in their rate equations? The attraction in pharmaceuticals of an approach based on the rate equation may be stronger than in petrochemical applications. The reasons for this become more clear when we compare key characteristics of typical pharmaceutical reactions with those of the petrochemical or basic chemicals industry, where the methodology of microkinetic analysis [3] described by Boudart has been most successfully applied (table 1). Petrochemical reactions are conventionally carried out at relatively high temperatures over fixed beds of heterogeneous catalysts that maintain a constant level of conversion of the flowing gaseous reactants. The reactants are small molecules, and the products may be many. Networks describing the global reactions can be extremely complex, with parallel and consecutive elementary steps often numbering in the dozens. Incorporating all of these into an explicit rate equation may prove to be intractable. The aim of the microkinetic treatment is to identify the unior bimolecular rate constants for all of these steps, culminating in the presentation of a mechanism-based macroscopic picture of the reaction. For such an approach to be successful, the assembling of the data required entails extensive experimental work, including various ex situ adsorption and surface chemistry experiments in addition to the The publisher regrets that this article first published in Vol. 83, Nos. 3–4 November 2002, pp. 133–136, appeared without any of the equations. Since the author originally supplied the complete article, the publisher accepts full responsibility for this grievous error. This version of the article, now published correctly and in its entirety, is the true representation of the author’s intent. Please contact the author directly regarding reprints. Catalysis Letters Vol. 89, Nos. 3–4, September 2003 (# 2003) 281

Development of a multitask and multiinstrument sample transfer system

A multiinstrument, multitask UHV sample transfer system being developed for integration with a wide range of synthesis and analysis instruments is described. The specimen transfer capability allows a sample to be synthesized, processed, and characterized by several surface science techniques without exposing the sample to air. Although several types of transfer systems now exist, no existing system has the range of operation desired for a user facility which will be equipped with a wide range of vacuum and controlled atmosphere-based techniques. Three different kinds of sample platens, which can be used in ambient, high-temperature, and surface chemistry experiments, have been designed and tested. The temperature range of the specimen can be as high as 2000 K during heating and as low as 150 K during cooling.

Fracture mechanics and surface chemistry studies of subcritical crack growth in AISI 4340 steel

Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel (tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined. A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5 ± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation of alloying and residual impurity elements on crack growth was also considered.