Carmelo Prestipino

Determination of the oxidation and coordination state of copper on different Cu-based catalysts by XANES spectroscopy in situ or in operando conditions

The use of XANES spectroscopy, both in classical and in dispersive geometries, is illustrated for the study of copper-based catalysts under in situ or in operando conditions. As case studies, copper-exchanged MFI zeolites and CuCl2/γ-Al2O3 systems are considered. In the former case, in situ XANES spectroscopy was used to characterise well defined complexes (Cu+N2, Cu+(CO)3, Cu+(NH3)(CO) and Cu+(NO)2) formed on copper ions inside the zeolite cavities under controlled conditions. From these results, useful information concerning the symmetry of the formed complexes can readily be gained. The latter case shows how the use of dispersive XANES spectroscopy allows to follow, in real time, the evolution of a system in working conditions. The simultaneous determination of the catalyst activity and of the average oxidation state of copper in the catalyst allows the evolution of a system in working conditions to be followed in real time. The criteria used for the quantification of the Cu(I) and Cu(II) fraction from XANES spectra are discussed in detail.

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 Cu2+ ions (<1%) exhibit an axial EPR spectrum, while bare Cu+ ions exhibit well resolved 1s→4pxy and 1s→4pz transitions (pxy/pz 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→4px, 1s→4py, and 1s→4pz transitions (px/py and py/pz 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.

Description of a flexible cell for in situ X-ray and far-IR characterization of the surface of powdered materials

Abstract We have designed and realized a cell where a powdered sample, in form of self-supported pellet, can be (i) in situ heated up to 900 K, either under dynamical vacuum or in reducing/oxidizing atmosphere; (ii) contacted with the desired amount of adsorbed gas; (iii) cooled down to 77 K for measurements. The cell (transmission geometry) has been employed in X-ray absorption experiments at LURE DCI (EXAFS13) and ESRF (GILDA BM8 and ID24), in far-IR experiments at LURE Super-ACO (SIRLOIN) and in conventional mid-IR experiments in our laboratory, only by changing the windows (mylar, silicon and KBr, respectively). A brief overview on the results obtained with this cell is reported to illustrate the potentialities of the instrument. Literature is quoted to allow possible in-depth study.

In Situ Characterization of Catalysts Active in Partial Oxidations: TS-1 and Fe-MFI Case Studies

A concise review of the firmly established knowledge and of the unresolved problems concerning the structure and the reactivity of Ti and Fe sites in TS-1 and Fe-MFI partial oxidation catalysts is given. Some new experimental and theoretical results are also described.

Novel high-temperature reactors for in situ studies of three-way catalysts using turbo-XAS.

Two novel high-temperature reactors for in situ X-ray absorption spectroscopy (XAS) measurements in fluorescence are presented, each of them being optimized for a particular purpose. The powerful combination of these reactors with the turbo-XAS technique used in a dispersive-XAS beamline permits the study of commercial three-way catalysts under realistic gas composition and temporal conditions.

Temperature resolved FTIR spectroscopy of Cr2+/SiO2catalysts: acetylene and methylacetylene oligomerisation

As is well known, the Cr2+/SiO2 system is an efficient catalyst for ethylene polymerisation already at RT (even if in the industrial process the running temperature is ∼380 K). For this reason it is the ideal system for in situ spectroscopic investigations on a genuinely working catalyst (Adv. Catal., 2001, 46, 265). Many questions are still uncertain as to the initiation, propagation and termination steps of the polymerisation mechanism. On a pre-reduced sample polymerisation commences very quickly and there is no way to record initial stages. When the experiments are performed at RT the phenomenon is too fast and IR spectroscopy, even in the time-resolved mode, failed up to now in the identification of the species formed during the initiation of the ethylene polymerisation step on the Cr2+/SiO2 Phillips catalyst. We present results related to time-resolved FTIR spectroscopy at variable temperature of acetylene and methylacetylene oligomerisation on a model Phillips catalyst. These experiments have been highly informative on the nature of the active sites because we have observed that acetylene and methylacetylene result in the immediate formation of benzene and 1,3,5-trimethylbenzene, respectively, without the evidence of any measurable intermediate product. This implies that the active Cr sites are able to coordinate simultaneously three monomers and thus must exhibit a high unsaturative coordination. The results of these experiments could be an insight of chromium species active in the Phillips catalyst.

Characterization of isolated Ag cations in homoionic Ag-Y zeolites: A combined anomalous XRPD and EXAFS study

Abstract In this contribution we report a structural characterization of a homoionic Ag-Y zeolite (Si/Al=2.63) on both short (EXAFS) and long range (XRD) scales. Our study shows that the zeolite is a near-100% exchanged silver faujasite, showing almost isolated Ag + counterions (EXAFS estimates that clustered species represent less than 2% of the whole silver). Synchrotron radiation XRPD measurements (ESRF, BM16), performed at the Ag-K edge ( λ =0.486103(2) A), just before ( λ =0.486093(2) A) and far away ( λ =0.491153(2) A), allowed us to locate the near totality of the expected Ag + counterions: 51.9(4) out of 52.9 per unit cell, located in five different sites. The output of the Rietveld refinement has been used successfully to simulate the Ag-K edge EXAFS spectra (ESRF, BM29).

Unprecedented Electron-Poor Octahedral Ta6 Clusters in a Solid-State Compound: Synthesis, Characterisations and Theoretical Investigations of Cs2BaTa6Br15O3

The crystal structure of Cs2BaTa6Br15O3 has been elucidated by using synchrotron X-ray powder diffraction and absorption experiments. It is built from edge-bridged octahedral [(Ta6Bri9Oi3)Bra6]4− cluster units with a singular poor metallic electron (ME) count equal to thirteen. This leads to a paramagnetic behaviour related to one unpaired electron. The arrangement of the Ta6 clusters is similar to that of Cs2LaTa6Br15O3 exhibiting 14-MEs per [(Ta6Bri9Oi3)Bra6]5− motif. The poorer electron-count cluster presents longer metal–metal distances as foreseen according to the electronic structure of edge-bridged hexanuclear cluster. Density functional theory (DFT) calculations on molecular models were used to rationalise the structural properties of 13- and 14-ME clusters. Periodic DFT calculations demonstrate that the electronic structure of these solid-state compounds is related to those of the discrete octahedral units. Oxygen–barium interactions seem to prevent the geometry of the octahedral cluster to strongly distort, allowing stabilisation of this unprecedented electron-poor Ta6 cluster in the solid state.

Zeolite characterization with spectroscopic methods

Among the different spectroscopic techniques reported in the plenary lecture presented at the FEZA 2002 conference, in this work, we will focus on IR spectroscopy only, devoted to the characterization of the acid strength of the Bronsted groups in zeolites In particular, in this brief review we will show how the systematic study of the spectroscopic manifestations observed by IR spectroscopy upon dosing to H-zeolites probe molecules with increasing proton affinity will allow to establish a spectroscopic acidity scale for the Bronsted groups hosted in different zeolites.

Cubic Lead Perovskite PbMoO3 with Anomalous Metallic Behavior

A previously unreported Pb-based perovskite PbMoO3 is obtained by high-pressure and high-temperature synthesis. This material crystallizes in the Pm3m cubic structure at room temperature, making it distinct from typical Pb-based perovskite oxides with a structural distortion. PbMoO3 exhibits a metallic behavior down to 0.1 K with an unusual T-sublinear dependence of the electrical resistivity. Moreover, a large specific heat is observed at low temperatures accompanied by a peak in CP/T3 around 10 K, in marked contrast to the isostructural metallic system SrMoO3. These transport and thermal properties for PbMoO3, taking into account anomalously large Pb atomic displacements detected through diffraction experiments, are attributed to a low-energy vibrational mode, associated with incoherent off-centering of lone-pair Pb2+ cations. We discuss the unusual behavior of the electrical resistivity in terms of a polaronlike conduction, mediated by the strong coupling between conduction electrons and optical phonons of the local low-energy vibrational mode.