Surface structure elucidation, methods

The surface chemistry of silica was a subject of intensive study in the period between 1960 and 1970 as a consequence of the widespread industrial use of colloidal, pyrogenic, and precipitated silicas, as well as silica hydrogels and xerogels. Chemical surface reactions and IR spectroscopy were the most-applied methods in surface structure elucidation. Significant contributions to the understanding of the silica surface were made by Fripiat (I), Kiselev and co-workers (2), Hair (3), Little (4), Peri... 

Although both XPS and UPS techniques can be adapted for the investigation of either condensed or gas-phase samples, the former technique is almost exclusively applied for surface analysis while UPS is considered mainly as a gas-phase electron structure elucidation method. 

A number of methods that provide information about the structure of a solid surface, its composition, and the oxidation states present have come into use. The recent explosion of activity in scanning probe microscopy has resulted in investigation of a wide variety of surface structures under a range of conditions. In addition, spectroscopic interrogation of the solid-high-vacuum interface elucidates structure and other atomic processes. 

Since the main topic of this review is STM imaging, growth properties, surface morphology, and atomic structures of oxide nanosystems are the central themes. Oxide nanolayers on noble metal surfaces often display very complex structural arrangements, as illustrated in the following sections. The determination of the surface structure of a complex oxide nanophase by STM methods is, however, by no means trivial resolution at the atomic scale in STM is a necessary but not sufficient condition for elucidating the atomic structure of an oxide nanophase. The problem... 

Feltz, A. Martin, A. (1987) Solid-state reactivity and mechanisms in oxide systems. 11 Inhibition of zinc ferrite formation in zinc oxide - a-iron(lll) oxide mixtures with a large excess of a-iron(lll) oxide. In Schwab, G.M. (ed.) Reactivity of solids. Elsevier, 2 307—313 Fendorf, S. Fendorf, M. (1996) Sorption mechanisms of lanthanum on oxide minerals. Clays Clay Miner. 44 220-227 Fendorf, S.E. Sparks, D.L. (1996) X-ray absorption fine structure spectroscopy. In Methods of Soil Analysis. Part 3 Chemical Methods. Soil Sd. Soc. Am., 377-416 Fendorf, S.E. Eick, M.J. Grossl, P. Sparks, D.L. (1997) Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Techn. 31 315-320 Fendorf, S.E. Li,V. Gunter, M.E. (1996) Micromorphologies and stabilities of chromiu-m(III) surface precipitates elucidated by scanning force microscopy. Soil Sci. Soc. Am. J. 60 99-106... 

The exact structural and spatial characterization of the lipids is still very limited, particularly in routine determination of lipids. As an example, routine determination of snl/sn2 acyl positions in PLs is not possible, nor is the determination of positions of double bonds or acyl chain branching. Such information would be important to understand the data in the biochemical context (e.g., affinity for specific lipid enzymes). While promising efforts are under way, it may still take time before the emerging tools for lipid structural elucidation are introduced in routine lipidomic analyses. Big advances have also been made over the past years in the developments of methods for surface analysis of lipids. These approaches will be crucial for in-depth elucidation of the spatial complexity of cellular and subcellular lipidomes. 

All available methods of diffraction, microscopy, and spectroscopy are used for structure elucidation in present-day materials chemistry.1,2 For detailed structure determination, even powders suffice for the most part because of the advances in diffraction profile analysis. These advances in structural tools enable more meaningful correlations of structure with properties and phenomena. Catalysis is becoming more of a science partly because of our ability to unravel the structures and surfaces of catalysts. Phase transitions of all varieties5 are being investigated more and more by chemists. 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

It is commonly recognized that a comprehensive understanding of the properties of a new material is an essential prerequisite to finding its new applications. In this respect, the study of ultrafine diamond is incomplete and its properties remain to be fully elucidated. For example, the nature of the surface functional groups and the method of their modification the nature of the agglomeration of ultrafine crystallites and effective methods of de-agglomeration to prepare mono-dispersed suspension the crystalline and surface structures of the nano-scaled diamond, etc., are appropriate subjects of research An efficient method for the determination of particle size distributions and structures of nano-sized particles in suspension is very important, and is worth developing in the near future. 

Substantial progress in the elucidation of the surface structure of crystalline and amorphous silicas has been achieved by means of high-resolution spectroscopic techniques, for example, Si cross-polarization magic-angle spinning NMR spectroscopy and Fourier transform IR spectroscopy. The results lead to a better understanding of the acidity, dehydration properties, and adsorption behavior of the surface. These properties are key features in the design of novel advanced silica materials. The current methods of characterization are briefly reviewed and summarized. 

Third, many pretreatment methods have been developed for activating carbon electrodes for electron transfer. Pretreatments often affect more than one electrode property (e.g. surface chemistry and microstructure) making it difficult to elucidate structure-function relationships. As a consequence of the variable nature of pretreatments, carbon electrode surfaces tend to vary greatly from laboratory to laboratory and from day to day, complicating one s ability to relate the electrode surface structure to the electrochemical response [1,5,12-16],... 

Mass spectrometry (MS) is a physical method for analysis introduced more than 100 years ago. During that period, MS applications have successfully proliferated in almost all areas of science and technology— from early studies of the structure of atoms and molecules culminating with the discovery of isotopes to characterization of planetary atmospheres and surfaces and search for extraterrestrial life. MS is an indispensable tool in organic chemistry and biochemistry for structural elucidation of various classes of natural products and synthetic compoimds. In the last quarter century, advances in MS methods and instrumentation have been at the forefront of efforts to map complex biological systems, including the human metabolome, proteome, and microbiome. 

Chemists rely on diffraction methods for the structural determination of molecular solids (i.e. solids composed of discrete molecules), non-molecular solids (e.g. ionic materials) and, to a lesser extent, gaseous molecules. As the technique has been developed, its range of applications has expanded to include polymers, proteins and other macromolecules. The most commonly applied techniques are single crystal and powder X-ray diffraction. Electron diffraction is important for the structural elucidation of molecules in the gas phase and for the study of solid surfaces. Neutron diffraction is used for the accurate location of light atoms (e.g. H, D or Li), or if one needs to distinguish between atoms of similar atomic numbers, e.g. C and N, or Ni and Cu. 

It is clear that a detailed knowledge of the structure of the catalyst under reaction conditions, in particular FTS reaction in harsh conditions (230-300 °C, 2-5 MPa) is required for any theoretical study to he successful described. Recent studies are combining DFT with simple thermodynamics to obtain phase diagrams and predict the thermodynamically favored catalyst structure under different conditions. This approach would be appreciable for the elucidation of surface structure of Co-based catalysts for FTS reaction. In addition. Kinetic Monte Carlo methods allow bridging the pressure gap from the vacuum conditions of DFT to real conditions, leading to the understanding, optimization and design of new catalysts. 

The preferential CO oxidation in H2-rich streams is a reaction of great relevance due to its application in the purification of feeds for hydrogen fuel cells, and because of the scientific interest. This reaction is known to be very sensitive to catalytic surface structures and to the pretreatments. Au catalysts supported on metal oxides with high metal dispersions have been demonstrated as very effective in this PROX reaction [1]. However, the studies carried out over these systems have allowed us to conclude that several factors affect the performance of the catalyst, such as particle sizes, preparation method, supports, etc. Nevertheless, there is still some controversy with respect to the nature of the active site or about the mechanism of reactioa In the present conununication and with the aim to obtain further information to elucidate these unresolved points a study on the role of the support in the CO PROX mechanism, particularly emphasizing in the aspects related to the preparation of Au-supported catalysts is presented. The use of a TAP (Temporal Analysis of Products) reactor is also applied to reveal elementary processes that are taking place on the surface under reaction conditions, since this reactor system allows the detection of reactants and products with a submilhsecond time resolution [2]. 

Kennedy et al. reported [79,80] that the amphiphilic materials consisting of hydrophic polyisobutylene (PIB) and hydrophilic poly(, V-dimethylac-rylamide) showed lower human blood monocyte adhesion than that of PIB and hydrophilic poly(2-hydroxyethyl methacrylate), for which the weight fractions between hydrophobic and hydrophilic components were identical. However, the amounts of various adsorbed proteins onto these samples were similar [79,80]. The relation between protein adsorption and cell adhesion has not yet been elucidated. The surface characterization methods that we employ were contact angle measurements, XPS, and SEM. There must be some structural differences among these samples of PAS that we could not detect using the aforementioned apparatus. Further studies on the surface structure of PAS by TEM and atomic force microscopy (AFM) in dry and wet conditions are now in progress. 

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