G. Turnes Palomino

Vibrational spectroscopy of H2, N2, CO and NO adsorbed on H, Li, Na, K-exchanged ferrierite

Abstract The vibrational spectroscopy of H 2 , N 2 , CO and NO adsorbed on the zeolite ferrierite is described. The spectral modifications induced by the adsorption process on the protonic form are compared with those observed on the alkaline-exchanged forms. Different environments of Bronsted sites and of alkaline counterions in ferrierite are evidenced, which differ in the local electric field associated with the cationic species. These different environments are associated with the cation locations in the channels and cavities. In particular, ions located in the 10-membered ring channels are more available for the interaction with CO, N 2 and H 2 , while ions in the cages form weaker adducts. On passing from lithium-, to sodium- and to potassium-exchanged samples, the local fields probed by H 2 , N 2 and CO are increasingly dependent upon the distribution of the ions in the framework. In K-ferrierite the interaction cannot be solely described in terms of 1:1 K + /B (B=N 2 , CO) adducts. As far as H-ferrierite is concerned, it is inferred from the observed shift of the ν (OH) mode upon interaction with H 2 , N 2 and CO that the bridged strong acid groups show an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta.

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.

X-ray photoelectron spectroscopy and x-ray absorption near edge structure study of copper sites hosted at the internal surface of ZSM-5 zeolite: A comparison with quantitative and energetic data on the CO and NH3 adsorption

The oxidation state of Cu species dispersed in a Cu-ZSM-5 zeolite obtained by a nonconventional gas-phase CuCl exchange, and nominally containing only Cu(I) species, was studied by x-ray photoelectron spectroscopy (XPS) and x-ray absorption near edge structure (XANES) analyses. The oxidation of Cu(I) species to Cu(II) by simple exposure to the atmosphere and subsequent reduction by thermal activation in vacuo was monitored. The quantitative and energetic aspects of the formation of carbonyl-like and amino-complexes at the metallic sites was studied by means of adsorption microcalorimetry. CO and NH3 were used as probe molecules in order to assess the coordinative unsaturation of the Cu(I) cations. Adsorption heats comprised in the 130–40 kJ mol−1 interval were obtained for the formation of both type of complexes. The perturbation induced on the Cu centers and/or on the zeolite matrix by the adsorption of the probe molecules was monitored by parallel experiments of XPS, IR, and XANES. A significant fractio...

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5

Abstract Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092 cm −1 . The standard adsorption enthalpy and entropy resulted to be Δ H 0 =−6.5(±0.5) kJ mol −1 and Δ S 0 =−90(±5) J mol −1 K −1 , respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.

Hydrogen adsorption on magnesium-exchanged zeolites

By means of variable-temperature FTIR spectroscopy, the standard adsorption enthalpy of hydrogen on the zeolite (Mg,Na)-Y was found to be ΔH° = −17.5 kJ mol−1, which suggests that magnesium-containing porous materials are good candidates in the search for suitable adsorbents for reversible hydrogen storage.

CUI-Y AND CUII-Y ZEOLITES : A XANES, EXAFS AND VISIBLE-NIR STUDY

Abstract We report a XANES, EXAFS and visible-NIR study on the local copper environment in CuI-Y prepared by an original gas-phase reaction of H-Y with CuCl and in CuII-Y prepared by conventional ion exchange with an aqueous cupric salt. XANES and visible-NIR techniques prove that the copper species in the so-prepared samples have the desired oxidation state. EXAFS reveals the presence of CuCu bonds in CuII-Y thus indicating the presence of Cu dimers. As far as CuI-Y is concerned, only CuO first shell bonds were observed. This is remarkable since it proves that even in a nearly totally exchanged zeolite with Si/Al = 3 our preparation method allows the creation of only isolated CuI species. An oxygen coordination number ranging from 2.8 to 3.0 suggests that copper cations are mainly located in sites I′, II and II′. The reduction of CuI-Y with CO, leads to the formation of small copper metal clusters well identified by EXAFS and visible-NIR spectroscopies.

Variable-temperature infrared spectroscopy: An access to adsorption thermodynamics of weakly interacting systems

Variable-temperature FTIR spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure, is shown to be a convenient method for the thermodynamic study of adsorbent–adsorbate systems. When weak interactions are concerned, the technique presents favourable features, as compared to classical microcalorimetric measurements. This recently developed spectroscopic method is demonstrated by studying the adsorption of dinitrogen on the protonic zeolite H-ZSM-5, a system for which the availability of microcalorimetric measurements affords a direct check of the new method. The relevant thermodynamic quantities determined for this system are ΔH° = −19.7(±0.5) kJ mol−1 and ΔS° = −125(±5) J mol−1 K−1; the standard adsorption enthalpy compares favourably with the microcalorimetrically determined value of about 19 kJ mol−1.

Infrared studies of the interaction of carbon monoxide and dinitrogen with ferrisilicate MFI-type zeolites

Fourier transform IR spectroscopy of CO and N2, adsorbed at liquid nitrogen temperature, was used to characterize an MFI-type H-FeZSM-5 ferrisilicate which was synthesized with a Si/Fe ratio of 50. Thermal treatment of this material at 773 and 973 K was performed in order to follow formation of extraframework species. On samples fired at 773 K Brønsted acid sites were present which gave an O-H stretching band at 3630 cm−1. These hydroxyl groups formed adducts with both probe molecules, which were monitored by the corresponding bathochromic shift, Δν. Corresponding values for adsorbed CO and N2 were Δν = −270 and Δν = − 100, respectively. Increasing the firing temperature up to 973 K led to complete removal of iron from the zeolite framework, and consequent disappearance of Brønsted acidity. In this process, extraframework iron oxide species were formed which were also characterized by IR spectroscopy of the adsorbed probe molecules. Both CO and N2 gave Lewis-type adducts with coordinatively unsaturated Fe3+ ions present in the extraframework material. A comparison is made with results of a previous study on the H-GaZSM-5 isomorph.

Preparation and characterization of spinel-type high surface area Al2O3-ZnAl2O4 mixed metal oxides by an alkoxide route

Abstract Spinel-type ZnAl2O4, γ-Al2O3, and three Al2O3-ZnAl2O4 solid solutions (at ZnO:Al2O3 = 1:2, 1:4, and 1:8) were prepared by controlled hydrolysis of mixed metal alkoxides, followed by calcination of the resulting gels at 773 K. Powder X-ray diffraction confirmed that all samples prepared were single phase cubic materials, with a lattice parameter, ao, gradually changing with composition between the limiting values ao = 0.790 nm (for γ-Al2O3) and ao = 0.809 nm (for ZnAl2O4). The mixed metal oxides were found to have specific surface areas ranging from 126 m2 g−1 for ZnAl2O4 to 288 m2 g−1 for γ-Al2O3, and pore radii of about 1.5–2 nm. IR spectroscopy of CO adsorbed at low temperature gave a main C-O stretching band at 2205–2210 cm−1, which was assigned to surface Al3+ ···CO adducts where coordinatively unsaturated Al3+ ions act as Lewis acid centers.

Stoichiometric and sodium-doped titanium silicate molecular sieve containing atomically defined –OTiOTiO– chains: Quantum ab initio calculations, spectroscopic properties, and reactivity

Ab initio calculations on a linear –OTiOTiO– chain embedded in an envelope of (SiO4) tetrahedra, mimicking the structure of Na2TiSi5O13 molecular sieve (ETS-10), confirm that the peculiar optical properties of the solid are associated with the presence of –OTiOTiO– linear chains behaving as quantum wires. The optical [in the UV-Vis (ultraviolet-range)] and the magnetic [(ESR) electron spin resonance] properties of these chains can be modified by adsorbing Na vapors. The sodium atoms diffusing into the channels undergo a ionization process with formation of Na+ (localized in the main channels) and Ti3+ (in the –OTiOTiO– chain, which so becomes a nonstoichiometric wire) characterized by Ti/Na ratios in the 2–4 range. Successive adsorption of oxygen at room temperature leads to the partial (Ti/Na∼2) or total (Ti/Na∼4) restoration of the chain stoichiometry and to the predominant formation of sodium oxide. The formation of a minor fraction of superoxide negative ions whose magnetic properties are revealed by ...