Characterization of zirconized silica supports for HPLC

The preparation and characterization of zirconized silica has been investigated. The material was prepared via the reaction of silica with zirconium tetrabutoxide, optimized by a central composite design and response surface methodology. The new material was characterized by nitrogen adsorption-desorption investigations (BET/BJH) showing specific surface areas adequate for use as a chromatographic support. DRUVS, FTIR, XPS, XAS, XRF and SEM methods also were used to characterize the new material. It was shown that silica networks were not significantly modified with the introduction of zirconium. Surface analyses show that there is appreciable element enrichment at the surface, while significant changes in binding energies of Zr 3d, Si 2p, and O 1s have been detected. The above observations indicate that SiOZr bonds were formed, with zirconium grafted onto the silica surface, yielding a support suitable for HPLC.     

A more general method for ranking the enrichment of alloying elements in passivation films

The corrosion of alloys is often characterized by an enrichment of one or other of the alloying elements within the surface oxide or even within the underlying metallic phase. For some three decades the measurement of such surface enrichment has been one of the most important contributions made by XPS to corrosion science. X‐ray photoelectron spectroscopy permits the composition of the oxide film to be distinguished from that of the metallic surface underlying the oxide so that both can be determined independently. In the mid‐1970s these measurements were crucial in showing that the selective enrichment of chromium in the electrochemical passivation of stainless steel occurred by selective dissolution of iron, and not by selective oxidation of chromium. Measurements of this type are important also in active corrosion, where there is no stable oxide film, as in studies of dealloying phenomena. The literature now contains numerous cases, many published by one of us (K.A.) based on studies of novel alloys produced in the Institute for Materials Research at Sendai. Typically, measurements are made for a series of binary compositions A_xB_y ranging from A‐rich to B‐rich alloys, in media that reflect the intended use of the alloy. The results are normally produced in the form of plots of cation composition Aⁿ⁺/(Aⁿ⁺ + B^m+) against bulk composition A/(A + B) or, in the case of dealloying, as A_surface/(A_surface + B_surface) against the bulk composition. Although graphical representation is useful in giving the full picture of the alloy's behaviour, it is not so useful in cataloguing or indexing performance. In this paper we suggest a means to give a rank or performance index as a single number that will characterize the behaviour of the alloy over a wide range of composition for a given medium or exposure condition. The rank number does not imply that any particular mechanism of enrichment is in operation; in various cases of corrosion it might occur by selective precipitation (FeOOH on steels, CuO on cupronickels), by selective dissolution (dezincification of brass) or by preferential incorporation of ions in a passivating oxide or oxyhydroxide (Cr³⁺ on stainless steels). Although the rank has no mechanistic implications, it is useful within a known class of mechanistic behaviour for indicating the magnitude of the effect and thus for enabling an XPS measurement to be indicative of wanted, or unwanted, behaviour. For example, the rank number could be useful in indicating, perhaps from a single alloy composition, whether a degree of passivation is likely to be achieved for that mix of metallic elements. Copyright © 2004 John Wiley & Sons, Ltd.     

X-Ray Spectroscopic Investigation of the Zr-Site in Thin Film Sol-Gel Surface Preparations

X-ray photoelectron spectroscopy (XPS) and x-ray absorption spectroscopy (XAS) measurements were made on thin film (1000 Å) sol-gel adhesion promoting surface treatments. These silicon/zirconium-containing sol-gel coatings are possible replacement processes for traditional surface preparations that use environmentally undesirable and potentially toxic materials. The sol-gels were waterborne mixtures formulated with tetra-n-propoxyzirconium (TPOZ) and a silane, either 3-glycidoxypropyl-trimethoxysilane (GTMS) or 3-aminopropyl-triethoxysilane (APS). Results show that dried sol-gel powders formulated with TPOZ or TPOZ + GTMS have longer Zr—O bond lengths (2.18 Å, CN 7 or 8) than the TPOZ + silane + metal substrate samples (2.10 Å, CN 6). The fraction (+/– 0.10) of Zr in a short bonded 6-fold site is highest (0.80) for TPOZ + (APS or GTMS) on (Ti or Al), at an intermediate value (0.40) for TPOZ on Ti and low (0.10) for the powders. For sol-gels deposited on a metal substrate, there are indications of a chemical bond between the thin film sol-gel and the metal substrate. The TPOZ + APS coatings on Ti data suggest that this Zr—O—Ti bond is present with a Zr—Ti separation of 3.5 Å. Only subtle differences were observed in the near-neighbor bonding due to curing treatment variations from room temperature to 125°C, alloy substrate variations (Ti-6Al-4V/Al 2024), and age of the deposited sol-gel coating (up to 1 year).     

Thermodynamic studies of the surface and transport properties in Ag–In and Ag–Sb liquid alloys

The surface tension, surface concentration, viscosity and mutual diffusion co-efficients of the Ag–In and Ag–Sb liquid alloys have been calculated using energetics and derivables from a statistical mechanical framework which recognises the formation of atom clusters of self associates. Our calculations suggest the existence of some form of local order in the systems. Ag–In showed higher tendencies to heterocoordination in the bulk-manifested higher values of mutual diffusion coefficient throughout the concentration range. The viscosity values of Ag–In and Ag–Sb were calculated using the expression reported by Kucharsky which relates the viscosity of a liquid binary alloy to the activity coefficients of the liquid alloy components that are raised to some power m. This exponent m is a fitted parameter. The calculated viscosity values for Ag–Sb had some reasonable agreement with experiment above 0.5 atomic fraction of Sb, using a fitted parameter value of m = 4.5. The fitted parameter value for the viscosity of Ag–In is expected to be in the range 1.5 ≤ m ≤ 3.5.     

Structural Investigation of Mixed Nitrided Galloaluminophosphates “AlGaPON” by EELS, XAS, and XPS Spectroscopies

Amorphous nitrided galloaluminophosphates “AlGaPON” catalysts with nitrogen contents varying from 0 to 23.3 wt% N were obtained by nitriding an Al_0.5Ga_0.5PO₄ precursor under ammonia flow at 750°C in a tubular furnace. The structural changes induced by this treatment were investigated by electron energy loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), and X-ray photoelecton spectroscopy (XPS). XANES and XPS results indicate that the first-coordination spheres of P, Ga, and Al atoms are modified by nitridation. In particular, the comparison of the P XANES spectra recorded on “AlGaPON” and on a PON phosphorus oxynitride (reference of mixed PO₂N₂ tetrahedra) reveals that mainly PO₂N₂ tetrahedra are present in highly nitrided samples. Moreover, the replacement of oxygen by nitrogen probably concerned P-O-Ga bonds rather than P-O-Al. EELS investigation reveals that the precursor is homogeneous at the used probe scale, but indicates that nitridation is accompanied by a loss of homogeneity of the material.     

Local order structure and surface acidity properties of a Nb2O5/SiO2 mixed oxide prepared by the sol-gel processing method

The sol-gel processing method was used as an alternative route to obtain Nb₂O₅ phase homogenously dispersed in the SiO₂ matrix, improving the thermal stability of the Brønsted acid sites, Nb-OH and Nb-OH-Si groups. The local niobium structure and the influence of the amount of niobia on the surface of the Nb₂O₅/SiO₂ system were studied by XAS and XPS, respectively. For the samples calcined at 423 and 873 K, the 3d_5/2 BE values are at ca. 208.2 eV, indicating an ionic character for Nb(V) species in the SiO₂ matrix, probably associated to Si-O-Nb linkages. The features of Nb K-edge XANES spectra of samples show the absence of NbO species. The Nb K-edge EXAFS oscillations exhibit a shoulder at ca. 5.6 Å⁻¹, which probably arises from Nb-O-Si. This fact corroborates the EXAFS simulation data of the second coordination shell, whose best fitting is achieved with three distances, two Nb-Nb lengths and one Nb-Si.     

Sorption Kinetics and Intracrystalline Diffusion of Methanol in Ferrierite: An Example of Disguised Kinetics

Sorption kinetics of methanol in large crystals of ferrierite have been studied in detail by interference microscopy (IFM) and infra-red microscopy (IRM). The IFM measurements yield the transient concentration profiles, thus providing a direct measurement of both the surface resistance to mass transfer and the internal diffusion resistance. It is shown that, for this system, the uptake rate is controlled by the combined effects of surface resistance and diffusion through the 8-ring channels (in the y-direction). Transport through the 10-ring channels (in the z-direction) appears to be blocked by surface resistance. Although the overall uptake curves conform well to the “root t law” the diffusivity values derived from the uptake curves vary widely depending on the assumed direction of diffusion. Even if the correct direction of diffusion is assumed, the diffusivity values derived from the uptake curves are seriously in error as a result of the intrusion of surface resistance. The existence of transport resistances at the crystal surface is clearly apparent from the transient concentration profiles but is not obvious from the uptake curves.     

Characterization of Ni?B amorphous alloys with x‐ray photoelectron and secondary ion mass spectroscopy

Amorphous Ni? B alloys with nominal compositions 30 at.% B and 50 at.% B were produced via electrodeposition on pure Ni polycrystalline substrates. The surfaces of the alloys were characterized with x‐ray photoelectron spectroscopy (XPS) and dynamic secondary ion mass spectrometry (DSIMS). Information on the compositional variation with depth was acquired with XPS both non‐destructively, in angle‐resolved mode (ARXPS), and destructively with argon ion etching, as well as with DSIMS. Boron oxide dominates the outermost surface of the alloys. Its presence also in the bulk of the alloys is attributed to oxidation during processing, whereas the presence of hydrogen detected with SIMS is attributed to adsorption occurring during processing. The Auger parameter concept and information from the primary and secondary structure of the XPS spectrum were employed to probe the electronic changes occurring upon alloying. It is suggested that the main electronic changes occurring are hybridization of the Ni spd states with the B sp states and an apparent increase of the electron density around the Ni sites. Copyright © 2005 John Wiley & Sons, Ltd.     

Fabrication of a High‐Quality,Porous, Surface‐Confined Covalent Organic Framework on a Reactive Metal Surface

A major goal of heterogeneous catalysis is to optimize catalytic selectivity. Selectivity is often limited by the fact that most heterogeneous catalysts possess sites with a range of reactivities, resulting in the formation of unwanted by‐products. The construction of surface‐confined covalent organic frameworks (sCOFs) on catalytically active surfaces is a desirable strategy, as pores can be tailored to operate as catalytic nanoreactors. Direct modification of reactive surfaces is impractical, because the strong molecule–surface interaction precludes monomer diffusion and formation of extended architectures. Herein, we describe a protocol for the formation of a high‐quality sCOF on a Pd‐rich surface by first fabricating a porous sCOF through Ullmann coupling on a Au‐rich bimetallic surface on Pd(111). Once the sCOF has formed, thermal processing induces a Pd‐rich surface while preserving the integrity of the sCOF architecture, as evidenced by scanning tunneling microscopy and titration of Pd sites through CO adsorption.     

An X‐ray photoemission study of interfacial reaction during annealing of Mg/GaAs(100) interface

In this paper, we present an in situ annealing study on Mg(15 Å)/GaAs(100) interface using X‐ray photoelectron spectroscopy (XPS). The core level results indicate that an interfacial reaction starts between Mg and Ga atoms at room temperature and enhances with annealing. However, at a higher temperature of annealing, a mixed layer of ternary Mg–Ga–As phase is formed at the interface. Corresponding Auger spectra of Mg, Ga and As also show considerable modifications in their nature and therefore support the above reaction. The observed results are correlated and interpreted in terms of continuous changes in electronic and chemical environment at the Mg/GaAs interface. Copyright © 2005 John Wiley & Sons, Ltd.