XANES EXAFS Research Articles

Activation of Surface ReOx Sites on Al2O3 Catalysts for Olefin Metathesis

The nature of activated surface ReOx sites and surface reaction intermediates for supported ReOx/Al2O3 catalysts during propylene self-metathesis were systematically investigated for the first time using in situ spectroscopy (Raman, UV–vis, XAS (XANES/EXAFS) and IR). In situ Raman spectroscopy reveals that olefins selectively interact with the surface dioxo ReO4 sites anchored at acidic alumina hydroxyls. In situ UV–vis indicates that surface Re5+ and some Re6+ sites form, and in situ XAS indicates a reduction in the number of Re═O bond character in the propylene self-metathesis reaction environment, especially as the temperature is increased. The appearance of oxygenated products during propylene activation supports the conclusion that catalyst activation involves removal of oxygen from the surface rhenia sites (pseudo-Wittig mechanism). Isotopic CD3CD═CD2 → CH3CH═CH2 switch experiments demonstrate the presence of surface Re═CD2 and Re═CDCD3 reaction intermediates, with the surface Re═CD2 species being the most abundant reaction intermediate. In situ IR spectroscopy indicates the presence of significant surface propylene π complexes on alumina and rhenia sites of the catalyst, which complicates analysis of surface reaction intermediates during propylene self-metathesis.

A series of copper-free ternary oxide catalysts ZnAlCex used for hydrogen production via dimethyl ether steam reforming

Ce-substituted ternary oxide catalysts ZnAlCex were prepared and employed in dimethyl ether steam reforming (DME SR) to produce hydrogen. XRD, XAFS (XANES & EXAFS), H2O-TPD, CH3OH-TPD and TPSR techniques were used for catalyst characterization. It is found that the catalytic performance of these catalysts is dependent on Ce content. The catalyst containing 20 wt% CeO2 exhibits the best catalytic performance. Its calculated TOF (0.034 s−1) is nearly three times to that of ZnAlO. The kinetic results reveal that the addition of 20 wt% CeO2 to ZnAlCex greatly decreases the apparent activation energy (Ea) of DME SR, due to the formation of new reaction sites such as Ce4+–O–Zn2+ linkages. XRD and EXAFS analyses indicate that Ce addition can not only decrease the crystallite size of ZnO and ZnAl2O4, but also tune the relative contents of them. The results of H2O-TPD and CH3OH-TPD show that Ce addition can lower H2 desorption temperature, which accounts well for the better catalytic performance of ZnAlCex. It is worth noting that the Zn-based catalysts display much lower CO selectivity than the Cu-based one, especially the Ce-substituted ZnAlCex. Start-off durability tests demonstrate that this series of catalysts also possess high catalytic stability.

Structure of nanosize bimetals Fe-Co and Fe-Ni

The phase composition and structural features of nano-size powders of Fe-Co and Fe-Ni systems synthesized by reducing the metal salts from strongly alkaline aqueous solutions are investigated by XAFS (XANES/EXAFS) and X-ray diffraction methods by using the synchrotron radiation (EXAFS spectrometer and diffractometer of Siberian SR Center) in combination with ordinary X-ray diffraction methods by using DRON-3.0 and Bruker D8 Advance diffractometers.

Bimetallic Pd-M (M = Co, Ni, Zn, Ag) nanoparticles containing transition metals: Synthesis, characterization, and catalytic performance

The reductive thermolysis of Pd(OOCMe)4M(OH2) (M = NiII, CoII, ZnII) and Pd(OOCMe)4Ag2(HOOCMe)4 molecular complexes results in the generation of bimetallic Pd-based Pd-M (M = Co, Ni, Zn, Ag) nanoparticles. The composition and morphology of nanoparticles and the electron state of metal atoms were characterized using electron microscopy, elemental ICP analysis, X-ray diffraction, and XAFS (XANES/EXAFS) techniques. The catalytic performance of nanoparticles was studied using the example of reactions of catalytic hydrazine decomposition and U(VI) reduction to U(IV) by hydrazine and formic acid. The catalytic performance of Pd-Ni nanoparticles is superior to that of the standard supported Pd/SiO2 catalyst containing a similar amount of Pd atoms, while Pd-Co, Pd-Zn, and Pd-Ag nanoparticles do not catalyze the studied reactions.

Functionalization of adsorbent with different aliphatic polyamines for heavy metal ion removal: Characteristics and performance

This paper examines the characteristics of poly(glycidyl methacrylate) (PGMA) beads functionalized with different aliphatic polyamines as adsorbents and their performance in copper ion adsorption. The four aliphatic polyamines evaluated include ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and tetraethylenepentamine (TEPA). Various analytical tools, including elemental analysis, BET, and XAFS (XANES + EXAFS), were used to characterize the adsorbents in terms of their immobilized amine contents, polyamine densities, and their coordination structures for the adsorption of a typical heavy metal ion, Cu 2+. It was found that the immobilized polyamine densities followed the order of EDA > DETA > TETA > TEPA, but the immobilized amine contents followed the opposite order of TEPA > TETA > DETA > EDA. XAFS analysis for the adsorbents after copper ion adsorption revealed that the average coordination number of copper ion with the nitrogen atoms (in the range between 3 and 4) followed the order of DETA < TETA < EDA < TEPA. Hence, the molecular length and structure of the polyamines appeared to have a great effect on the adsorption performance of the prepared adsorbents. The study leads to the conclusion that among the four aliphatic polyamines, DETA-functionalized PGMA adsorbent was the most efficient one for copper ion removal because of the relatively higher amine content and lower coordination number, easier regeneration, and shorter preparation time of the adsorbent. The results provide some useful information for future studies in the selection of polyamines for adsorbent functionalization.

Unusual magnetic and transport properties of the CMR Pr1−x Ca x MnO3−y system

The substituted nonstoichiometric perovskite Pr1−x Ca x MnO3−y compounds have been synthesized by a standard combustion technique, which show uniphase solid solutions. The all samples of the Pr1−x Ca x MnO3−y system show an orthorhombic crystal system and the cell volumes are decreased with increasing the larger amounts of substituted atoms or the increasing x values. The mixed valence of Mn ions is identified by the XAS (XANES/EXAFS) spectroscopy and the amounts of Mn4+ ions are determined by an iodometric titration method. Nonstoichiometric chemical formulas of the Pr1−x Ca x Mn 1−τ 3+ Mn τ 4+ O3−y compounds have been obviously formulated. Magnetic properties are investigated by SQUID and thus the Pr1−x Ca x MnO3−y (x = 0.4, 0.6, and 0.8) compounds show the transition from antiferromagnetic state to paramagnetic state. The Pr1−x Ca x MnO3−y (x = 0.0, 0.2, and 1.0) compounds show the transition from ferromagnetic state to paramagnetic state. The facts that Mn4+ contents play important roles in the magnetic ordering have been found out. The transport properties have been studied by the DC electrical conductivity measurement under magnetic fields of 0 G and 3 kG. Maximum and minimum MR ratios are 1016% of the Pr0.6Ca0.4MnO2.846, and −77.5% of the PrMnO3.021 compound, respectively.

Selective catalytic reduction of NOx over microporous CuAPO-5: structural characterisation by XAS and XRD

The effects of copper source, copper content, Al : P ratio and silicon content for the hydrothermal synthesis of CuAPO-5 using tetraethylammonium hydroxide as template have been investigated. The copper to phosphorus ratio in the synthesis gel was varied from 0.04 to 0.2, and copper(II) oxide and copper(II) acetate were used as copper sources. The samples were characterised by XAS (XANES, EXAFS), XRD (X-ray diffraction), TGA (thermogravimetric analysis), SEM (scanning electron microscopy) and elemental analysis. Rietveld refinements of the structure of CuAPO-5 were carried out on synchrotron X-ray powder diffraction data. The catalytic properties of CuAPO-5 in selectively reducing NOx in the presence of hydrocarbons (SCR-HC) were studied. The results from EXAFS show that copper is present in a distorted octahedral environment, with 4 or 5 Cu–O distances at 1.95 A and 1 or 2 Cu–Al/P distances at about 3.14 A for the as-synthesised samples. The calcined samples still show the same four Cu–O distances at 1.95 A, but with the composite peak at 3.14 A being significantly reduced. The turquoise colour of the as-synthesised CuAPO-5 is consistent with the copper atoms being complexed by water molecules within the pores as the hexaaquacopper(II) complex. The extraframework complex cation is electrostatically bound to the anionic phosphate sites of the framework. The colour changes to olive green on calcination, indicating tetrahedrally coordinated framework copper. The selective catalytic reduction of NOx by propene is catalysed by CuAPO-5 to the extent of 18% conversion. The Rietveld analysis show that calcined CuAPO-5 belongs to the P6/mcc spacegroup in which Al and P are indistinguishable.

Characterization of the VS-1 catalyst using various spectroscopic techniques and its unique photocatalytic reactivity for the decomposition of NO in the absence and presence of C 3H 8

A Vanadium silicalite (VS-1) catalyst was prepared by hydrothermal synthesis. In situ characterizations were carried out and the photocatalytic reactivity of VS-1 were investigated using dynamic photoluminescence, XAFS (XANES, EXAFS), UV–Vis and FT-IR techniques, along with an analysis of the reaction products. It was found that VS-1 involves a highly dispersed tetrahedrally coordinated V-oxide species having a VO bond within the zeolite framework. VS-1 exhibits photocatalytic reactivity for the decomposition of NO under UV irradiation, leading to the formation of N 2, O 2, N 2O and NO 2. The photocatalytic reaction of NO was dramatically enhanced in the presence of propane, leading to the formation of N 2, propylene, and oxygen-containing compounds such as CH 3COCH 3 and CO 2, etc. with a good stoichiometry. The photocatalytic reactivity of VS-1 was found to be much higher than that of V/SiO 2. The efficiency of the dynamic quenching of the photoluminescence of the V-oxide species in the excited state of VS-1 by small molecules such as NO or propane was found to be larger than that of V/SiO 2. These results indicate that the high reactivity of the charge transfer excited triplet state of the V–O moieties, (V 4+–O −) *, of the VS-1 catalyst plays a significant role in the higher photocatalytic performance as compared with the V/SiO 2 catalyst.

Investigation of the Local Structure of Vanadium Silicalite Catalyst (VS-1) Using XAFS, FT-IR and Photoluminescence Spectroscopic Methods

The in situ characterization of the vanadium silicalite catalyst (VS-1) was carried out using XAFS (XANES, EXAFS), FTIR and dynamic photoluminescence techniques. The FT-IR spectrum of the VS-1 catalyst at around 960 cm-1 showed the incorporation of vanadium atoms into the zeolite framework. Furthermore, XAFS measurements have indicated that the VS-1 catalyst consists of highly dispersed vanadium oxide species having a tetrahedral coordination. The VS-1 catalyst exhibited an absorption band at around 300 nm and an intense photoluminescence with a vibrational fine structure at around 450-550 nm which could be attributed to the charge transfer processes on the tetrahedral vanadium oxide species involving an electron transfer from O2- to V5+ and reverse radiative decay, respectively. These results have indicated that a highly dispersed vanadium oxide species is present within the zeolite framework as a tetrahedrally coordinated species and the charge transfer excited state of this oxide species can be well-localized in the shorter V=O bond.

Design of photocatalysts encapsulated within the zeolite framework and cavities for the decomposition of NO into N 2 and O 2 at normal temperature

The design of photocatalysts encapsulated within the zeolite frameworks and cavities is the most promising approach in developing photocatalysts which will operate efficiently and effectively towards the purification of toxic agents such as NO x and SO x in the atmosphere. In the present study, the vanadium silicalite (VS-2) and Ag +/ZSM-5 catalysts were prepared by hydrothermal synthesis and ion-exchange, respectively, and the in situ characterization of these catalysts and their photocatalytic reactivities for the decomposition of NO have been investigated using dynamic photoluminescence, XAFS (XANES, EXAFS), ESR, FT—IR, LTV—VIS, solid-state NMR and XRD techniques along with an analysis of the reaction products. Results obtained with the VS-2 catalyst showed that vanadium oxide moieties are present within the zeolite framework as a 4-fold tetrahedrally coordinated vanadium oxide species having a terminal oxovanadium group (VO). UV irradiation of the VS-2 catalyst in the presence of NO led to the photocatalytic decomposition of NO to form N 2, N 2O and O 2. On the other hand, it was found that the zeolite cavities can stabilize the Ag + ions in an isolated state through their connection with two lattice oxygen anions of the zeolite (2-coordination geometry). These isolated Ag + ions exhibit high photocatalytic reactivities for NO decomposition to form N 2, N 2O and NO 2. Dynamic studies of the excited state of these catalysts showed that the charge transfer from the excited state of the vanadium oxide species or Ag + ions to NO plays a vital role in the initiation of the decomposition of NO into N and O. These findings have demonstrated that metal oxide species and metal ions included within the zeolite frameworks and cavities are strong candidates for new types of environmentally applicable photocatalysts.