ISS Techniques Research Articles

Tailoring acid–metal functions in molybdenum oxides: Catalytic and XPS-UPS, ISS characterization study

Formation of acid, metal and metal–acid (bifunctional) functions in titania-supported molybdena following controlled partial reduction of MoO3 using hydrogen at different temperatures are monitored by a combination of surface XPS-UPS, ISS techniques. Addition of controlled amount of sodium or potassium alkali metals to the bifunctional MoO2−x(OH)y/TiO2 (MoTi) enabled to neutralize the Brønsted acid Mo-OH functions. Sodium or potassium molybdenum bronze metallic function is formed following the addition of the alkali metal to the Mo salt following its calcination at 773K. The catalytic functions of these MoTi and Na, KMoTi systems were evaluated for the dehydration/hydrogenation, oxidation reactions of isopropanol to acetone as well as the ring shortening of cyclohexane to methylcyclopentane MCP and its oxidative dehydrogenation to benzene.

Spectroscopic characterization–catalytic activity correlation of molybdena based catalysts

Titania-supported molybdena catalyst was prepared by calcination at 773K of ammonium heptamolybdate-impregnated TiO2 pellets with the equivalent of 5 monolayers of MoO3. Then, potassium-modified versions of the catalyst were prepared at 0.5–5wt%-K. The modified and unmodified catalysts were characterized, before and after H2-reduction at 673K, by means of ISS, XPS, UPS and FT-IR spectroscopy techniques. Results obtained enabled revealing the presence of density of states (DOS) at the Fermi level and, hence, metallic properties related to the deformed rutile structure of MoO2. On the other hand, Brønsted (MoOH) acidic function was found to form on the surface of the MoO2 structure, thereby producing a metal-acid (bifunctional) MoO2−x(OH)y catalyst. The presence of Brønsted acid MoOH groups was evidenced by in situ IR spectra of adsorbed pyridine molecules. On the other hand, the K-modification was found to enhance the molybdena dispersity and reducibility, but to suppress the surface acidity by transforming the bifunctional MoO2−x(OH)y species into the monofunctional MoO2−x(OK)y species. Subsequent catalytic measurements employing hydrotreating reactions of 1-hexene and n-hexane could confirm the exposure of bifunctional or monofunctional sites on the catalysts tested. The combination of research methods adopted in the present investigation has shown competency in unravelling the nature of active sites on heterogeneous catalysts.

Preparation of highly dispersed and thermally stable nanosized cerium–hafnium solid solutions over silica surface: Structural and catalytic evaluation

A highly dispersed and thermally stable nanosized cerium–hafnium solid solution over silica support has been synthesized. The obtained catalyst exhibited a high specific surface area and superior redox as well as acid–base properties. To unveil dispersion and thermal stability, samples were thermally annealed at various temperatures from 773 to 1073K and investigated by means of XRD, TEM, XPS, ISS, Raman spectroscopy and other techniques. Catalytic activity was evaluated for selective dehydration of 4-methylpentan-2-ol to 4-methylpent-1-ene in the vapor phase. The designed catalyst exhibited better activity and selectivity, and the observed catalytic properties were correlated with structural features.

Modification of the catalytic properties of MoO2−x(OH)y dispersed on TiO2 by Pt and Cs additives

Addition of 5% Pt or alkali metals such as K or Cs each separately to the bifunctional MoO2−x(OH)y catalyst results in modification of the chemical structure of this system, especially in the case of alkali metals. A new MoO2−x(OA)y, AK, Cs, monofunctional structure having only metallic properties is formed. In the case of Cs for example, the MoOCs bond formation takes place in the course of the reduction process of MoO3 to MoO2 by hydrogen hinders the acidic BrØnsted MoOH formation, which usually is formed in this system. Characterization by surface XPS–UPS, ISS and FT-IR spectroscopic techniques as well as catalytic activity carried out at the same experimental conditions confirm the presence of this monofunctional MoO2−x(OCs)y system. On the contrary, platinum addition enhances the metallic character of the MoO2−x(OH)y bifunctional system in terms of slight improvement in the conversion of 1-heptene and n-heptane as well as dehydrogenation of methylcyclohexane to toluene.

S1 Pediculoiliac Screw Fixation in Instabilities of the Sacroiliac Complex: Biomechanical Study and Report of Two Cases

A new technique for posterior sacroiliac fixation is described and compared with conventional techniques. A patient with sacral alar fracture (zone 1) and another one with sacroiliac joint instability due to tuberculous infection underwent fixation using screws placed in the S1 pedicle and the iliac bone. Vertical stability of the new technique also was investigated using polyurethane pelvic bone analogs and compared with anterior double plating (group P) and iliosacral screw fixation (group ISS) techniques. Healing was obtained and reduction was maintained in both patients on the final follow-up examination at 2 years postoperatively. Vertical loading tests revealed that failure loads within the first 10 mm of displacement of the new pediculoiliac screw fixation technique (group PIS) was higher than plating (P = 0.03) and lower than ISS techniques (P = 0.002). Ultimate failure load of the PIS technique was slightly higher than plating (P = 0.277) and lower than ISS techniques (P = 0.003). With the addition of an iliosacral screw to the pediculoiliac screw construction (PIS+ISS), the PIS technique became more stable in early (P = 0.110) and ultimate failure loads (P = 0.003). Pediculoiliac screw fixation for sacroiliac joint disruptions and zone I sacrum fractures using iliac and S1 pedicle screws is a new and effective alternative for obtaining and maintaining anatomic reduction.

State-estimators for Chemical Reaction Networks of Feinberg-Horn-Jackson Zero Deficiency Type

This paper provides a necessary and sufficient condition for detectability for chemical reaction networks of the Feinberg-Horn-Jackson zero deficiency type. Under this condition, an explicit construction of globally convergent observers is obtained based on ISS techniques. The observers are easy to implement, and several robustness aspects are tested in numerical simulations.

Predictions of Relationships between Catalytic and Solid Phase Properties by Kinetic Models and Their Validation

The paper illustrates that realistic predictions about steady-state coverages and stability of surface species can be obtained by kinetic analysis. As an example, oxidative coupling of methane to ethane over CaO/CeO2 catalysts is considered. For this reaction, it was earlier predicted by a kinetic model that the steady-state surface coverage with OH groups, oxygen, and vacant sites depends on the CaO content of the mixed oxide catalysts. Now, the surface of solid solutions of CaO in CeO2 (5, 10, and 20 mole% CaO), in which the incorporation of Ca into the CeO2 host lattice can be demonstrated by pronounced effects on the electrical conductivity, has been characterised by XPS, UPS, ISS, and IR techniques. It was found that bulk and surface properties are interrelated but do not change proportionally. The oxygen anion conductivity increases and a change from n- to p-type conductivity occurs with increasing CaO-content. The external surface of the mixed oxides is strongly enriched in CaO, which forms CaCO3 under ambient conditions, but the degree of surface enrichment exhibits a marked minimum at 10% CaO. As predicted by former kinetic analysis in the oxidation of methane, this sample is capable of forming surface OH groups of exceptional thermal stability.

Multitechnique approach of the surface analysis of fluorinated polyethylene terephthalate

Polyethylene Terephthalate 12 μm thick films (Mylar ®) have been subjected to an industrial gaseous fluorination treatment. The chemical surface modification of the treated films have been studied with the aid of various surface sensitive techniques (ISS, SSIMS and XPS) ; thanks to the complementarity of these techniques, the chemical reaction scheme at the surface is emphasized. The ISS technique provides the elemental chemical composition of the first topmost monolayer. In the case of polymers, it is thought that the low carbon sensitivity of ISS is due to the shielding of the carbon atoms by the hydrogen atoms. ISS spectra obtained on fluorinated PET samples show that the fluorination treatment results in the incorporation of fluorine with a slight change of the O/C ratio : this has been interpreted by the substitution of hydrogen atoms from the PET by fluorine which has already been observed on other polymers subjected to the same treatment. Both XPS and SSIMS analyses confirm these results. First, the SSIMS spectra obtained on the fluorinated samples are totally different from those obtained on the untreated PET samples : these spectra exhibit new peaks characteristic of fluorine containing molecular ions similar to those observed from fluorinated polymers such as PVDF or PTFE. Since the secondary ions formation involves ruptures of the polymer chains and some recombination reactions, it is difficult to decide from SSIMS on the exact sites on the PET chains where the hydrogen substitution by fluorine takes place. This was made possible with the help of XPS : the shape of the C 1 s peak after fluorination of the PET shows that the formation of new CF x species (x⩽2) occurs on the electron rich benzenic rings of PET rather than on the aliphatic parts of the PET chains. Finally, RBS has been used in order to estimate the depth at which the fluorination treatment has proceeded : this has been estimated to a few tenths of micrometer.

3He + ISS of 11-mercaptoundecanoic acid self-assembled monolayers onto gold

ISS and XPS spectra were recorded before and after forming an organized molecular assembly (OMA) of HS(CH 2) 10COOH, 11-mercaptoundecanoic acid (MUA) on 20 nm thick gold thin films on a silicon substrate. The MUA/OMA is attached to gold via the thiol (SH) group, and the acid group (CO 2H) becomes the outer monolayer. At low current densities, l keV 3e + ISS showed no detectable amount of gold on the surface, indicating the gold film was completely covered by the OMA. XPS signals acquired from the OMA showed carbon, oxygen, sulfur, gold, and a trace of silicon. When using ISS at higher current densities to sputter the OMA, the same elements were detected. The presence of silicon at the Au/OMA interface is attributed to silicon diffusion from the substrate through 20 nm of gold at 22°C. Some potential uses of ISS for studying well-characterized OMAs are also suggested. The primary purpose of this work is to use the surface-monolayer sensitive ISS technique on a well-characterized OMA on gold to confirm conclusions reached in prior work about the coverage of gold films by the OMA. ISS of self-assembled MUA on gold has not been previously reported, and our results have provided additional interesting information.

Application of ion scattering spectroscopy to investigations of the interaction between the active component and carrier in supported catalysts

In this article we will show, however, that the electronic effect is measurable for Cu, Zn, and Na by the ISS technique, and we will use the concept of «effective mass» based upon a binary collision approach to develop a method for the determination of the chemical shift in solid-solid surface interactions in catalyst systems. A series of Cu, Na, and Zn compounds, Al foil, and supported Cu and Zn catalysts have been investigated, and the 4 He + scattering energy from the different atoms in the different compounds or catalysts has been determined. The «mass increments» in these samples are calculated from scattering peak shifts in the ion scattering spectra and the «mass increment» is used as a means to characterize interaction between surface atoms

A surface core-level shift photoemission study of the interaction of oxygen with W{100}

The interaction of oxygen with W(100) up to a coverage, θ, of ~ 2.2 monolayers has been studied using the surface W4f core-level photoemission shifts recorded using synchrotron radiation at a hν of 130 eV. Discreet chemical shifts ascribed to W bonded to overlayer oxygen; W in a 2d reconstructed oxide layer (formed by annealing above ~ 700 K); W in a WO 2-like crystallite state; and W in overlayer oxygen saturated W(110) facets (heating at high coverage) have been observed. Simultaneous Auger and LEED data were used to support the assignments by comparison to the detailed previous work that exists in the literature for this system using LEED, Auger, Δφ, ESD and ISS techniques.

ChemInform Abstract: Mechanism of the Deactivation of Hopcalite Catalysts Studied by XPS, ISS, and Other Techniques.

AbstractIt is shown that the deactivation of a commercial Hopcalite CO oxidation catalyst "CuMn2O4" is due to the segregation of potassium impurities to the surface of the catalyst.

Mechanism of the deactivation of Hopcalite catalysts studied by XPS, ISS, and other techniques

A detailed study of the thermal deactivation of a commercial Hopcalite catalyst, “CuMn 2O 4,” has been made by a multitechnique catalyst characterization approach using XPS, ISS, XRD, SEM, TA, BET, and activity testing. It is shown for the first time that the deactivation is due to segregation of the potassium component to the surface of the catalyst. The driving force of the segregation is the amorphous-to-spinel phase change at >773 K. Deactivation in N 2 or O 2 at 823 K resulted in a restructuring of the outer surface zone of the catalyst with Cu being enhanced relative to Mn. The deactivation and the concomitant phase change are accompanied by a change in the dominant oxidation state of Cu from Cu 2+ to Cu 1+ and of Mn from Mn 3+ to Mn 4+. The oxidation state change is associated with the formation of the spinel phase where the coupled redox reaction Cu 2+ + Mn 3+ Cu 1+ + Mn 4+ is experimentally supported in the XPS data by the chemical shifts and shake-up structure (Cu), and by multiplet splitting (Mn). Plasma chemical reoxidation has been used to reestablish Cu 2+ at the catalyst surface at a low temperature and nonequilibrium conditions. This study reveals the phenomenon of phase-change-induced segregation of a promoter when the ionic radius or the oxidation state precludes its accommodation in the lattice.

Assessments of surface composition and sputtering yield of Au-Cu alloys for Ar + ion bombardment

The possible effect of current density on the outermost surface-layer composition of Au-Cu alloys under keV Ar + ion bombardment at steady state was examined through the mixed-ion beam ISS technique. Measurements of the sputtering yields with a quartz-oscillator mubalance were performed simultaneously. Results of the former measurements indicate that the composition of the outermost atomic layer does not depend on the current density of the primary ion beam in the range from ≈0.5 to ≈90 μA/cm 2, resulting in the enrichment of Au atoms at the outermost atomic layer. The latter have lead to the same result for the total sputtering yield for ion beam intensity ranging from tens of a μA/cm 2 to several hundreds μA/cm 2. Hence both the results lead to the conclusion that the ion enhanced subsurface re-distribution process of the outermost surface atoms under Ar + ion bombardment is much faster in time than the sputter removal rate of atoms from the surface.

Investigation of BaTiO 3 and Gd 2(MoO 4) 3 crystal surfaces by complementary AES and ISS techniques

The chemical nature of the intrinsic surface layer in BaTiO 3 single crystals and the positive and negative surfaces of gadolinium molybdate have been determined using AES and ISS. Potassium and fluorine were detected in BaTiO 3 crystals grown by the Remeika method. The ratio of the AES 417 eV to 411 eV peaks of Ti in BaTiO 3 was stable under ion and electron beam irradiation. ISS showed that the positive and negative surface of gadolinium molybdate crystals were chemically identical, with both types exhibiting similar variations in the Gd and Mo signals with ion bombardment time. However, marked differences in the spectra of sputtered and scattered ions from each type were observed.

ISS study of the oxidation of Au on Cu (300 Å) thin films

Using ISS techniques, composition versus depth profiles were made of Au (800 Å) on Cu (300 Å) films before and after heating in vacuum or 10.6 kPa of oxygen at 163°, 202°, and 250°C for 10, 100, and 1000 minutes. Prior to heating, small amounts of Cu were found on the surface but not in the bulk of the gold. Heating the double layer film in vacuum at 250°C for 1000 minutes resulted in a homogeneous Au–Cu film. On the other hand, heating the double layer in oxygen produced an inverted structure wherein the Au was found to lie beneath a gold-free copper oxide overlayer (324 Å).

Comparative study of chemical and polarization characteristics of Pd/Si and Pd/SiOx/Si Schottky-barrier-type devices

The chemical nature of semitransparent (∼125 Å) palladium on silicon Schottky-barrier-type devices was determined by complementary AES and ISS techniques. Postdeposition analyses of metal-semiconductor (MS) and metal–thin-insulator–semiconductor (MIS) devices prepared without heat treatment showed that palladium silicide is formed in the MS structures, while the presence of an ultrathin (∼30 Å) purposefully grown semiconductor oxide film inhibits the chemical reaction between Pd and Si in the metal overlayer. Chemical bonding information extracted from the AES data was correlated with barrier-height measurements obtained from capacitance-vs-voltage (C-V) and current-vs-voltage (I-V) electrical characteristics of these devices.

イオン散乱法によるＡｕ（８００Å）／Ｃｕ（３００Å）積層薄膜酸化の研究

By means of ISS technique, in-depth profiles were made of the Au (800 A) / Cu (300 A) double layer films before and after heating in vacuum or 10.6 kPa of oxygen at 163, 202 and 250°C. Prior to heating, small amounts of copper were found on the surface but not in the bulk of gold. The films were found to be homogenious Au-Cu films after heating in vacuum at 250°C for 1000 min. On the other hand, heating the double layer films in oxygen atmosphere resulted in an inverted structure, i.e., the gold layer was found to lie beneath a copper oxide overlayer. No gold was detected in the copper oxide overlayer, but some amounts of copper oxide were found in the gold underlayer.