Quantitative Surface Composition Analysis Research Articles

Oxidative Dehydrogenation on Nanocarbon: Identification and Quantification of Active Sites by Chemical Titration

Nanostructured carbon-based materials have shown high catalytic activity in several important reactions and related chemical industrial processes, such as direct or oxidative dehydrogenation of hydrocarbons and Friedel–Crafts reactions. Nanocarbon materials exhibit significant advantages over traditional metal or metal oxide based catalysts because of their tunable acidity/basicity, electron density, and convenient recycling and reusability, and they have been shown to be potential alternatives to conventional catalysts to meet the requirements of sustainable chemistry. As a result, the field of nanocarbon catalysis has been experiencing an unparalleled development of new catalyst synthesis or their applications in new reaction systems. However, there is only slow growth of mechanistic interpretation of carbon-catalyzed reactions, which is even more urgent to advance our knowledge in related fields. Present research on the mechanism of carbon catalysis suggests that oxygen containing functional groups, especially ketonic carbonyl groups on nanocarbon, which are rich in electrons, may act as the catalytic active sites for oxidative dehydrogenation (ODH) of alkanes to corresponding alkenes. The reaction process is assumed to be similar to that for transition-metal oxide catalysts. The C H bonds of alkanes dissociate at active oxygen functional groups, and the hydrogen atoms are abstracted by Lewis base sites. After the desorption of alkene products, gas-phase O2 reacts with the abstracted hydrogen to form H2O, then the active catalytic sites are regenerated to finish one catalytic cycle. The above unspecific catalytic mechanism is only based on the qualitative characterization of carbon catalysts, while the identity of the active sites or a detailed kinetic study has never been executed with direct and convincing chemical evidence. One of the most critical problems that limits the quantitative description of the catalytic mechanism is the uncertainty of the chemical structure of nanocarbon materials. The coexistence of several kinds of surface functional groups (such as hydroxyl, carbonyl, and carboxylic acid groups) is unavoidable, as the synthesis or the following surface modification procedures of nanocarbon catalysts are normally realized by a severe physical or chemical process, such as laser irradiation and oxidation by HNO3, O2, and O3. [8] There are still lack of reliable quantification methods for the surface functional groups on nanostructured carbon materials because of their complexity in type and quantity. As a result, turnover frequency (TOF), the ultimate parameter to evaluate the intrinsic activity of heterogeneous catalysts, is also rarely reported in the case of nanocarbon catalysts, making it impossible to study the detailed reaction kinetics or compare the activity of carbon catalysts bearing different structures fairly and objectively. The quantitative surface composition analysis is also desirable for the application of nanostructured carbon as a catalyst support or electrochemical devices, which takes an even larger proportion in the field of carbon materials, as the surface structure of nanocarbon materials is essential for their physical or chemical properties (for example, affinity for a certain metal or metal ion). In view of the quantification methods of oxygen functional groups, herein we propose a chemical titration method to determine the surface concentration of three kinds of typical oxygen functional groups ( C=O, C OH, and COOH) on the surface of carbon nanotubes (CNTs) (Scheme 1). Through selective deactivation of these specific oxygen functional groups and the assessment of the catalytic activity of different CNT derivatives for ethylbenzene (EB) ODH reactions, we provided chemical evidence to show that

LEIS: A reliable tool for surface composition analysis?

Different single and polycrystalline surfaces of Cu and Ag have been investigated by time-of-flight low-energy ion scattering using 4He+ ions. The fraction of ions that survived single scattering from the outermost surface layers, P+, was measured in different neutralization regimes. At low energies, a distinct difference in P+ was observed for non-equivalent Cu crystal surfaces for projectiles backscattered in a single collision. The polycrystalline surface was found to exhibit similar neutralization behaviour as the (111) single crystal surface. At higher energies, P+ shows a strong dependence on the angular orientation of the single crystal. The impact of these findings on quantitative surface composition analysis by LEIS is discussed.

A novel procedure for elimination of the peak deviations in LEIS spectra influenced by the primary ion beam profile

Quantitative surface composition analysis using low energy ion scattering (LEIS) can be obstructed by the deviations introduced by the primary beam profile and the analyzer optics. A novel procedure is presented for excluding these deviations in LEIS measurements. The LEIS spectra were calculated from the assumed energy distribution, primary beam profile and the analyzer optics. From the calculated energy spectra, the instrumental functions were determined as a deconvolution of the energy spectra and corresponding energy distributions. The obtained instrumental functions can be very well approximated by the Gaussian. It is convincingly shown that the energy distribution evaluation, by means of the deconvolution of the energy spectrum and the calculated instrumental function, significantly suppresses the problems of qualitative (the peak shift) and the quantitative (the peak intensity deviation) surface composition analysis due to the different geometric profiles of the primary ion beam. The calculation method was then successfully applied for the analysis or the real experimental data that are not too noisy.

Influence of the primary ion beam profile and the energy analyzer optics to the LEIS spectra: the analytical study

The quantitative surface composition analysis with low energy ion scattering (LEIS), which is the experimental technique with the information depth that can be restricted to the first atomic layer, is often obstructed by the experimental problems. A novel approach is presented in order to calculate the influence of the optics of the energy analyzer and the geometric primary ion beam profile to the position, the width and the intensity of peaks in LEIS spectra. The developed method is applied for the analysis of our LEIS experimental system. The obtained results are compared to simple analytical models. The results obtained for our experimental device show that the deviations are not significant, indicating that the experimental system is convenient for performing refined surface composition analysis.

Optical properties of the 127° cylindrical energy analyzer used in LEIS experiments

The optical properties of the 127° cylindrical energy analyzer used in low energy ion scattering (LEIS) experiments are studied by means of SIMION 3D version 6.0 program. The dependence of the acceptance solid angle Ω on the target plane coordinates ( x, y) and the relative particle energy ε completely describes the optical properties of an analyzer. The Ω(x,y,ε) function is calculated from the computed trajectories of ions emitted from different points of the target plane. The influence of spherical aberrations to the error in the energy measurement is determined experimentally. The experimental results agree very well with the results obtained using the numerical simulations as well as, with the results obtained by means of the second order analytical approach. The optical properties are analyzed for different electrode potential configurations i.e. for different deflection voltage modes defined according to the potentials of the inner and the outer electrode. The applied deflection voltage mode does not change Ω(x,y,ε) significantly. However, there is an important influence of the deflection voltage mode to the analyzer constant due to the acceleration of ions traversing along the optical axis of the analyzer. The knowledge of Ω(x,y,ε) can be used to determine the dependence of the energy spectra on the optical properties of the analyzer as well as, on the primary beam profile. This is of particular interest in the analysis of LEIS spectra, because deviation of spectra caused by the optics of the analyzer can be a source of significant errors in quantitative surface composition analysis.

Determination of the Inelastic Mean Free Path of Electrons in Different Polyaniline Samples

Polyaniline specimens, emeraldine sulfate, emeraldine hydrochloride, and emeraldine base, were characterized by infrared spectroscopy, X-ray photoelectron spectroscopy, elastic peak electron spectroscopy, and density measurements using helium pycnometry. This work has been stimulated by a considerable interest in potential technological applications of polyaniline. The chemical state of polyaniline has been verified with infrared spectroscopy. X-ray spectroscopy was used for the qualitative and quantitative surface composition analysis of studied samples. The inelastic mean free path of electrons has been determined from the measurements of the elastic electron backscattering probability. This is one of the most frequent applications of elastic peak electron spectroscopy. The inelastic mean free path is needed for the quantitative analysis of polyaniline surface using electron spectroscopies. The method of elastic peak electron spectroscopy utilizes the theoretical model of the electron transport, account...

Quantitative surface composition analysis using very high intensity nonresonant multiphoton ionization

The subject of this article is quantitative surface and material analysis under high sensitivity conditions. Various gases and one solid sample were studied, using high intensity (1014–1015 W/cm2), nonresonant multiphoton postionization in tandem with time-of-flight mass spectrometry. These atomic and molecular gases (Ar, Kr, NO, Xe, N2 and O2) possess high ionization potentials, requiring high photon flux density for ionization saturation. The solid sample SiO2, studied using 5 keV Ar+ sputtering, places a stringent test on quantitative analysis. Si and O have exceedingly different ionization potentials that require four and six photons for ionization, respectively. The experimental results demonstrate that it is possible not only to saturate species with high ionization potentials, but also to perform quantitative analysis by establishing a well-defined ionization volume for all species evolved from the surface.

Surface Analysis with 1014-1015 W cm2 Laser Intensities

Very high laser intensities (1014–1015 W cm-2) have been used to photoionize both sputtered and gaseous samples. The cross-sectional spatial intensity distributions of a 35 ps, 532 nm laser beam have been analyzed by direct optical imaging and show near-diffraction limited and near-Gaussian profiles at and near focus. Experimental results from sputtered atoms and gaseous atoms and molecules demonstrate that the high laser beam quality combined with the high power density leads both to the saturation of photoionization of species with very different ionization potentials and to a sharply defined ionization volume with well-characterized spatial distributions. It is possible not only to make a quantitative surface composition analysis under high sensitivity conditions using high-intensity multiphoton ionization but also to drive the relative sensitivity factors towards unity. This work demonstrates the feasibility of performing quantitative surface and material analysis under high sensitivity conditions by using very high-intensity non-resonant multiphoton ionization.

Direct comparison of ion scattering and secondary ion emission as tools for analysis of metal surfaces

In this work an experiment is described which allows in situ comparison of SIMS (Secondary Ion Mass Spectrometry) and ISS (Ion Scattering Spectrometry). Measurements on Cu and stainless stell surfaces show that in some respects qualitative agreement between both methods exists. In both cases quantitative surface composition analysis is hampered by the lack of knowledge of secondary ion yields. Especially in case of SIMS the adsorbed species like oxygen have a great influence on the ion yield.