XANES Study Research Articles

In situ Ga K edge XANES study of the activation of Ga/ZSM-5 prepared by chemical vapor deposition of trimethylgallium

The activation of a dimethylgallium/ZSM-5 precursor to well-defined reduced and oxidized species is studied by in situ Ga K edge XANES. The precursor is prepared by chemical implanting of trimethylgallium on acidic HZSM-5. Subsequent reduction leads to charge-compensating Ga+ species. Direct oxidation of the trimethylgallium precursor leads to various forms of gallium oxide and regeneration of Bronsted acid protons. Oxidation of the reduced Ga+ species yields predominantly to GaO+ species. The GaO+ species exhibit a much higher H2/D2 exchange activity than reduced Ga+ species.

Structural studies in the BaO–B 2O 3–TiO 2 system by XAS and 11B-NMR

The structure of barium–titanium–metaborate xBaO– xB 2O 3– yTiO 2 ( y=0%, 4%, 8%, 16% and x=50– y/2) amorphous and crystallized powders, obtained using a polymeric precursor method, was investigated by Ti and B K-edge X-ray absorption spectroscopy (XAS) and 11B-NMR high-resolution techniques. XANES study of amorphous samples shows that Ti 4+ ions exist as [4]Ti species associated to [6]Ti and [5]Ti species in a practically equivalent amount. After crystallization, titanium environment is predominately composed by [6]Ti species. According to XANES results obtained at the B K-edge, the fraction of boron in tetrahedral sites ( [4]B) reduces as the amount of TiO 2 is increased from x=0% to 4%, with a consequent increase of boron in trigonal sites ( [3]B). By a combination of 11B-NMR spin-echo and triple quantum magic angle spinning (3Q-MAS) techniques, the detailed borate speciation was determined as consisting in [4]B and two kind of trigonal sites, [3]B A and [3]B B , corresponding, respectively, to borates sharing three and two O atoms with other boron units. NMR results reveal not only the reduction in boron coordination also seen by XANES but also the simultaneous reduction in the condensation degree of trigonal units, when the Ti content is increased in the glass. In crystallized samples, β-BaB 2O 4 and BaTi(BO 3) 2 phases were identified and quantified by 11B-NMR.

EXAFS and XANES study of two novel Pd doped Sn/SnOx nanomaterials

EXAFS and XANES at the Pd and Sn K-edges have been used to characterise a novel palladium doped Sn/SnOx nanocomposite that have found application in a new type of high performance solid-state gas sensor. The spherical nanoparticles (10 to 65 nm diameter) prepared either by co-decomposition of Sn and Pd organometallic precursors (doping in volume) or by decomposition of a palladium precursor on preformed Sn/SnOx coreshell nanoparticles (doping in surface) consist of a tin metallic core surrounded by some small platelets of a well-crystallised material embedded in an outer layer of amorphous tin oxide. In this layer tin atoms in an oxidation state close to +IV, are four-fold coordinated to oxygen with bond distances slightly larger than in bulk cassiterite SnO2. In both samples palladium was found to be always in a metallic state, only surrounded by Sn and Pd atoms located at unusually short distances. The two samples that differ mainly by their Pd coordination number present very different morphologies and electrical properties when fully oxidised. A possible location of the Pd atoms in these two materials is proposed.

XANES studies of thallium compounds and valence states and local environment of thallium in some of its superconducting cuprates

The position and shape of x-ray absorption discontinuities have been used to deduce structural and chemical bonding information on model compounds of thallium and its superconducting cuprates. The L x-ray absorption spectra were recorded on a Rigaku x-ray Spectrometer and monochromatized using a flat crystal of Si (1,1,1). The background subtraction was done using Victoreen function. The normalized absorption coefficient is plotted as a function of x-ray photon energy. It is observed that the XANES of LI edge has fewer features as compared to those of LIII edge. The LII edge spectra of thallium are similar to LIII edge spectra. The LIII absorption discontinuity of thallium splits into three components whereas no splitting is observed in LI edges. Thallium LIII edge spectra show a single absorption peak for monovalent compounds whereas in trivalent compounds a weak peak followed by a shoulder on rising absorption which culminates into a strong absorption peak. The important difference between the spectra of monovalent and trivalent compounds is the shift in the inflexion point energy of LIII edges and a shift of the main absorption maximum. The x-ray absorption spectra of superconducting cuprates show similarities with the spectra of Tl2O3 suggesting the presence of Tl3+ ions in octahedral configuration in these cuprates.

XANES study of lanthanum-doped lead titanate ceramics

Xray absorption spectroscopy was used to probe the shortrange structure in lanthanumdoped lead titanate ceramics (Pb1-xLaxTiO3) for x ranging from 0.0 to 0.30. A comparison of the XANES spectra and the x-ray diffraction (XRD) results indicates that the local structure around Ti atoms had a different compositional dependence. According to the XANES data, even for the sample containing 30% of lanthanum, for which the XRD analysis indicated the existence of a cubic structure, a local distortion around Ti atoms persisted. The intensity of the pre-edge feature of the PLT30 sample decreased by a factor of only two when compared with the lanthanum-free sample.

In situ XANES study of Mn in promoted sulfated zirconia catalysts

Mn-promoted sulfated zirconia catalysts (2 wt% Mn) were investigated in situ, during the catalyst activation, isomerization of n-butane, and subsequent re-activation, using X-ray absorption spectroscopy of the Mn K-edge. The average valence of Mn in the catalysts, as determined from the edge position, was found to change from either 2.65 or 2.77 in the calcined samples to about 2.5 during activation in He (703 K for 30 min). During the isomerization of n-butane (1% in He, 80 ml min-1, 0.5 g catalyst at 333 K), the average Mn valence did not change further. When the catalyst was activated in 50% O2 the average valence only decreased from about 2.78 to 2.72. In this case, the average valence during the isomerization reaction decreased at a nearly constant rate both during the induction of activity and deactivation of the catalyst. The data do not support a stoichiometric redox reaction involving the promoter as initiator of the isomerization. However, a higher Mn valence after activation was indicative of a higher maximum conversion. It is concluded that the promoter cations function through modification of the structure of the zirconia.

Sulfur K-edge XANES study of dihydrolipoic acid capped gold nanoparticles: dihydrolipoic acid is bound by both sulfur ends

Dihydrolipoic acid (DHLA) capped gold nanoparticles (Au@DHLA) are characterized in solid and liquid states by sulfur K-edge XANES spectroscopy; it clearly shows that DHLA is anchored to gold thanks to both sulfur ends.

In situ, Cr K-edge XAS study on the Phillips catalyst: activation and ethylene polymerization

In this in situ EXAFS and XANES study on the Phillips ethylene-polymerization Cr/SiO 2 catalyst, two polymerization routes are investigated and compared. The first mimics that adopted in industrial plants, where ethylene is dosed directly on the oxidized catalyst, while in the second the oxidized catalyst is first reduced by CO at 623 K. On this reduced catalyst C 2H 4 polymerization has been investigated at room temperature and at 373 K. To allow experiments in transmission mode, a Cr loading of 4 wt% has been adopted. At this loading a fraction of clustered Cr 2O 3 particles has been observed and quantified. The use of a third-generation synchrotron radiation source has allowed us to improve the energy resolution and signal-to-noise ratio of the XANES data, allowing us to determine the fraction of Cr sites involved in the polymerization reaction. This number represents an upper limit of the active sites. Preliminary ReflEXAFS experiments have been performed on a model catalyst prepared by impregnation of Cr on a flat Si(100) substrate covered by a thin layer of amorphous silica. Experiments have been performed ex situ on the grafted catalyst (i.e., after impregnation and thermal activation) and at the end of the polymerization stage.

Coordination and oxidation changes undergone by iron species in Fe-MCM-22 upon template removal, activation and red–ox treatments: an in situ IR, EXAFS and XANES study

We report about the characterization of an isomorphously substituted Fe-MCM-22 sample with high iron content (Si/Fe = 18) using FTIR, XANES and EXAFS spectroscopies. Template burning and subsequent activation in vacuo caused the migration of a fraction of framework Fe 3+ species to extraframework positions, accompanied by the reduction of a fraction of Fe 3+ to Fe 2+. A fraction of extraframework Fe 2+ sites was able to adsorb NO, forming Fe 2+(NO) n complexes ( n = 1 , 2 , 3 ), which indicates a high coordinative unsaturation of such sites. The parallel experiment monitored by X-ray absorption spectroscopies testified that this fraction is however small, as both EXAFS and XANES spectra were almost unperturbed by NO adsorption. The corresponding FTIR bands are highly broad and asymmetric, which does not allow to detect the presence of Fe 3+(NO) complexes. The broad character of the nitrosyl bands suggests the presence of small oxidic clusters, in agreement with XANES and EXAFS evidences. The nature and distribution of extraframework Fe species are influenced by water preadsorption, which causes the increase of the amount of the most coordinatively unsaturated Fe 2+ sites, able to form Fe 2+(NO) 3 complexes. The effect of red–ox treatments with O 2 and H 2 was also investigated. Upon oxidation, adsorbed oxygen is formed, which efficiently shields the Fe centers and does not rapidly nor efficiently react with NO. Upon reduction, the intensity of nitrosyl complexes increases, indicating the reduction of extraframework Fe 3+ (likely present on the surface of small oxidic clusters) to Fe 2+. Comparison with the most investigated Fe-MFI system is made.

On the role of aluminum in the selective oxidation of benzene to phenol by nitrous oxide over iron-containing MFI zeolites: an in situ Fe XANES study

In situ Fe K-edge XANES experiments point to a distinct difference in reducibility of iron in Fe-silicalite (without aluminum) and FeZSM-5 (with aluminum) upon dehydration. Whereas a noticeable part of Fe 3+ is reduced to ferrous ions in the iron-containing aluminosilicalite, the ferric species in the ferrosilicalite maintain their oxidation state. The ability to selectively oxidize benzene to phenol with nitrous oxide in ferroaluminosilicalite relates to these Fe 2+ centers. It is proposed that the large difference in reactivity between ferroaluminosilicalite and ferrosilicalite is due to the chemical difference of iron in Fe O Al and Fe O Fe adducts. A new preparation method is introduced which leads to more selective and stable catalysts.

A transient in situ FTIR and XANES study of CO oxidation over Pt/Al 2O 3 catalysts

We report experimental results for the oxidation of CO over supported Pt/Al 2O 3 catalysts operating in oxygen excess at atmospheric pressure. To study the reaction kinetics under transient conditions we have employed step changes of the O 2 concentration by intermittently switching off the O 2 supply at various temperatures ranging from 523 to 623 K. Detailed in situ FTIR and XANES data for CO coverage and the chemical state of Pt, respectively, are presented together with the CO conversion, which in both cases was monitored by mass spectrometry. A red-shift of the vibrational frequency of linearly bonded CO which correlates with a blue-shift of the Pt L III binding energy indicates that the Pt catalyst initially is partially oxidised and gradually reduced when the O 2 supply is switched off. Control experiments with a NO 2 oxidised Pt/Al 2O 3 catalyst support these findings. A hysteresis in the catalytic activity due to the different rates whereby Pt is oxidised and reduced as a function of gas-phase composition is observed. The activation energy for the Pt oxide reduction (decomposition) process is estimated to be about 50 kJ/mol. The results further emphasise that the conventional three-step Langmuir–Hinshelwood (LH) scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions. Moreover, FTIR data suggest that during the extinction, the partially oxidised platinum surface is reduced by chemisorbed CO which should be explicitly accounted for in the modeling of the reaction mechanism.

Grafting TiO 2 on MCM-41 as a TiO 2 support for vanadia for catalytic oxidation of ethanol—EXAFS and XANES analyses of vanadium

Titanium oxide was grafted on the surface of MCM-41 to produce a mesoporous TiO 2/MCM-41 composite to serve as a catalyst support. Vanadia was grafted on this titania support to produce V 2O 5/TiO 2/MCM-41 and catalytic oxidation of ethanol by air was used as a probe reaction to study the influence of the support on the catalytic behavior of this catalyst. Catalyst V 2O 5/TiO 2/MCM-41 has reactivity five times higher than the reactivity of vanadia on MCM-41. EXAFS analysis of vanadium indicates that vanadia on both supports have similar structures and polymeric morphologies. The enhancement of reactivity was due to the direct dispersion of vanadia on TiO 2 in V 2O 5/TiO 2/MCM-41 as indicated by the EXAFS and XANES studies. This TiO 2-grafted MCM-41 mesoporous framework thus has high potential for use as a mesoporous TiO 2 support. Such support is superior to traditional anatase TiO 2, pure mesoporous TiO 2, or TiO 2/SiO 2 supports owing to its much larger surface area and better thermal stability at elevated temperatures.

Valence of Highly Dispersed Cerium Oxide Species on Silica Quantitatively Estimated by Ce L<SUB>III</SUB>-edge XANES

A series of silica-supported cerium oxide samples having various cerium content were prepared by conventional impregnation method, followed by calcination in air. Ce L I I I -edge XANES study revealed that highly dispersedcerium oxide species on silica surface were predominantly in trivalent oxidation state and cerium oxide nano-particles on silica contain mainly tetravalent cation like as CeO 2 bulk oxide. This elucidated that UV absorption band at 265 nm is assignable to Ce(III) species dispersed on silica.

The first cycle characteristics of Li[Ni 1/3Co 1/3Mn 1/3]O 2 charged up to 4.7 V

In this research, we studied the first cycle characteristics of Li[Ni 1/3Co 1/3Mn 1/3]O 2 charged up to 4.7 V. Properties, such as valence state of the transition metals and crystallographic features, were analyzed by X-ray absorption spectroscopy and X-ray and neutron diffractions. Especially, two plateaus observed around 3.75 and 4.54 V were investigated by ex situ X-ray absorption spectroscopy. XANES studies showed that the oxidation states of transition metals in Li[Ni 1/3Co 1/3Mn 1/3]O 2 are mostly Ni 2+, Co 3+ and Mn 4+. Based on neutron diffraction Rietveld analysis, there is about 6% of all nickel divalent (Ni 2+) ions mixed with lithium ions (cation mixing). Meanwhile, it was found that the oxidation reaction of Ni 2+/Ni 4+ is related to the lower plateau around 3.75 V, but that of Co 3+/Co 4+ seems to occur entire range of x in Li 1− x [Ni 1/3Co 1/3Mn 1/3]O 2. Small volume change during cycling was attributed to the opposite variation of lattice parameter “ c” and “ a” with charging–discharging.

In situ XANES study of electrokinetic remediation of cadmium-contaminated soils.

In situ XANES study of electrokinetic remediation of cadmium-contaminated soils.

Effect of Co and Ni on benzene hydrogenation and sulfur tolerance of Pt/H-MOR

The activity and sulfur resistance of Pt promoted by Co and Ni in MOR have been studied in benzene hydrogenation. The highest benzene hydrogenation activity is observed for Pt/H-MOR and is attributed to its high acid site concentration and the higher electron deficiency of Pt. XANES studies show that the presence of Co and Ni decreases the Pt white line intensity suggesting an increase of the Pt apparent electronic density. This has been correlated with decreasing catalytic activity and sulfur tolerance of the catalysts. Brønsted acid sites seem to be involved and beneficial for benzene hydrogenation.

Depth selective XANES study of swift heavy ion irradiation effects in metal/Si systems

Swift heavy ion induced intermixing of thin metallic layers of Ti, Fe and W with Si has been studied. A single specimen containing 3.0 nm thick layers of the three metals at different depths in Si, ensures a more reliable comparison of the mixing efficiencies of the three systems. X-ray reflectivity and X-ray standing wave analysis has been used to obtain the concentration profile of the three metals before and after irradiation with 100 MeV Au ions. While Ti and Fe exhibit considerable mixing with Si after an irradiation fluence of 1 × 10 13 ions/cm 2, W profile is only slightly broadened. XANES measurements under X-ray standing wave conditions have been able to differentiate between the structure of the center of a layer from that of the interfacial region. It is found that irradiation results in only a further disordering of the central as well as interfacial regions of the W layer. In contrast, the Fe layer gets completely mixed with Si and forms an amorphous silicide.

Influence of Cr and Mo on the passivation of stainless steel 430 (18Cr) and 444 (18Cr–2Mo): In situ XANES study

We have studied the influence of metal elements on the passivation of stainless steel (SS) 430 (18Cr) and 444 (18Cr–2Mo) by real time X-ray absorption near edge spectroscopy. We developed an electro-chemical cell that is suitable for in situ electro-chemistry experiments. The behavior of Fe, Cr and Mo elements of SS 430 and 444 was studied in passive region reaction in a borate buffer solution (pH 7.0). We found that the Cr 2O 3/Fe 2O 3 ratio gradually decreased with passivation time due to the oxidation of Cr 2O 3 to CrO 4 2−, indicating weakening of passivity. In the meanwhile, the amount of Mo at passive films increased with passivation time. The improvement of passivity in the SS 444 is mainly attributed to the substitution of Cr oxide by Mo oxide in the passive film.

An in situ temperature dependent IR, EPR and high resolution XANES study on the NO/Cu +–ZSM-5 interaction

We report an in situ IR, EPR, and high resolution XANES study on the Cu +(NO) and Cu +(NO) 2 complexes formed at 80 K in Cu +–ZSM-5. Bare Cu 2+ ions (<1%) exhibit an axial EPR spectrum, while bare Cu + ions exhibit well resolved 1s→4p xy and 1s→4p z transitions (p xy /p z splitting of 3.1 eV), reflecting the same local symmetry. Cu +(NO) complex exhibits an anisotropic EPR signal, while XANES spectrum of dinitrosyl adducts presents three resolved components ascribed to the 1s→4p x , 1s→4p y , and 1s→4p z transitions (p x /p y and p y /p z splitting of 3.8 and 2.6 eV, respectively) testifying the rupture of the axial symmetry of Cu + bare cations upon NO adsorption. Upon increasing the temperature to 300 K, NO oxidizes a fraction of 75–85% of the cuprous ions.

XANES analysis of catalytic systems under reaction conditions

ABSTRACT The article gives an overview of the XANES technique contribution to the analysis of multicomponent catalysts. The theoretical basis of the technique, the interpretation of the energy position and intensity of XANES features, and the numerical methods developed to interpret XANES data on catalytic systems are described and discussed in the first part. In the second part, the most recent XANES studies of catalytic samples are reviewed, giving particular attention to the solid state chemistry information extracted under real, in-situ conditions. This concerns structural and redox properties of the systems during preparation stages, activation treatments like reduction, oxychlorination, or sulfidation, and reaction conditions. Additional effort was made to describe the use of XANES to gain insight on electronic properties of catalysts. The influence of all these redox, geometrical and electronic properties on catalytic behavior, particularly as a function of particle size and active metal-support interaction, has also been examined on the light of XANES data.