C. Otero Areán

Low-temperature Fourier-transform infrared investigation of the interaction of CO with nanosized ZSM5 and silicalite

Nanosized ZSM5 zeolites with microcrystal dimensions in the 20–120 nm range have been characterized by means of IR spectroscopy and HRTEM microscopy. The vibrational spectrum of the OH groups on the external and internal surfaces of H-ZSM5 and Na-ZSM5 samples of different crystallite dimensions has been investigated. For the sake of comparison the spectra of silicalite samples containing different concentrations of sodium and aluminium are also shown. For this purpose high-purity silicalite samples were prepared following a novel synthesis route.Carbon monoxide (a very weak Lewis base) was used to probe the acidity present on the external and internal surfaces of the zeolites through formation of 1 : 1 adducts with silanols (both internal and external), Bronsted-acid groups (both framework and extraframework), Na+ ions, and Lewis Al3+ centres (in extraframework and framework positions). The IR-active CO stretching modes of the complexes are shifted to higher wavenumber with respect to the free molecule; the positive shift can be used to estimate the acid strength. CO that was physically adsorbed in the zeolite channels has also been investigated.

Sol-gel method for preparing high surface area CoAl2O4 and Al2O3-CoAl2O4 spinels

Abstract Cobalt aluminate, CoAl 2 O 4 , and Al 2 O 3 –CoAl 2 O 4 solid solutions (at Co:Al=1:4, and Co:Al=1:8) were prepared by hydrolysis of mixed metal alkoxides, followed by mild calcination of the resulting gels. Powder X-ray diffraction showed that all samples prepared were single phase materials having the spinel-type structure. The cubic lattice parameter, a 0 , was found to change gradually with aluminium content of the mixed metal oxides. Nitrogen adsorption–desorption isotherms, at 77 K, were used to determine surface area and pore texture. The BET surface area was found to be 235 m 2 g −1 for CoAl 2 O 4 , and higher values (up to 365 m 2 g −1 ) were found for the Al 2 O 3 –CoAl 2 O 4 solid solutions. Samples having the same composition were also prepared by the classical ceramic method (starting from mechanical mixtures of the parent metal oxides); for these materials the corresponding surface areas were found to be in the range 1.7–3.3 m 2 g −1 . The sol–gel method, starting from mixed metal alkoxides, was thus shown to be a convenient route to prepare cobalt aluminate spinels in a very high surface area form.

STRETCHING FREQUENCIES OF CATION-CO ADDUCTS IN ALKALI-METAL EXCHANGED ZEOLITES : AN ELEMENTARY ELECTROSTATIC APPROACH

The C–O stretching frequencies, obtained by a 77 K Fourier transform infrared study of the interaction of CO with the extraframework cations in alkali–metal exchanged M‐ZSM‐5 and M‐mordenites (M=Na,K,Rb,Cs) have been used to evaluate the local electric field strength tested by the probe molecule. For all the investigated zeolites, the dipolar CO molecule was found to interact specifically with extra‐framework metal ions, and its stretching frequency was found to increase continuously from Cs+ to Na+ samples. The observed frequency shifts, with respect to that of the free CO molecule (νCO=2143 cm−1), were used to evaluate the electrostatic field created by the cation sites. Electric fields in the range of 2 to 6 V nm−1 were obtained; experimental values are inferior to those expected if generated in vacuo, at the distance of the probe molecule, from a point charge of +‖e‖. The observed fields can be reproduced by the sum of a positive contribution of the cation itself, and by two negative contributions, t...

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.

XANES, EXAFS and FTIR characterization of copper-exchanged mordenite

Over-exchanged CuI-mordenite has been prepared by gas phase reaction between the protonic form of the zeolite and CuICl. No evidence was found for the presence of CuII ions or copper species in microaggregates. EXAFS analysis revealed that CuI ions in the outgassed zeolite are surrounded by 2.5 ± 0.35 oxygen atoms of the zeolite framework, the average CuI–O distance being 1.96 (±0.02) A. Both X-ray absorption and FTIR spectroscopy showed that the zeolite adsorbs CO at room temperature with formation of CuI(CO)n adducts (n = 1, 2). At liquid nitrogen temperature, tricarbonyl species were also formed. The IR spectra of adsorbed CO showed broad and complex bands, which is consistent with the fact that the zeolite contains different types of cation sites accessible to carbon monoxide. This was further demonstrated by comparison with corresponding IR and EXAFS spectra of CO adsorbed on CuI-ZSM-5.

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.

IR-Spectroscopic Study of the Binding Isomerism of Adsorbed Molecules

This paper discusses the use of IR spectroscopy in the studies of isomerism in the binding of adsorbed molecules with a surface when a molecule may form several different surface species at the same site. Species whose geometry does not provide minimal adsorption energy can be considered as adsorption complexes in an excited state. The spectral manifestations of such a “steric excitation” are compared with the electronic and vibrational excitations of surface species. The “sterically excited” isomeric states existing in thermodynamic equilibrium with ordinary adsorption species are found and studied in detail. Examples are CO molecules bound through C and O atoms with metal cations in zeolites or with surface hydroxyl groups, the thiophene molecule via hydrogen bonding with silanol groups, and HD molecules dissociatively adsorbed on ZnO. A possible role of “sterically activated” isomeric states in catalysis is discussed.

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.

Variable‐temperature IR spectroscopic studies of CO adsorbed on Na‐ZSM‐5 and Na‐Y zeolites

CO interacts with extra‐framework alkali metal cations (M+=) of zeolites to form both M+⋯CO and M+⋯OC species. By using variable‐temperature FTIR spectroscopy, these C‐bonded and O‐bonded species were found to be in a temperature‐dependent equilibrium. For the same cation, the difference in interaction energy depends upon the zeolite framework. Thus, for the equilibrium process ZNa+⋯=CO ⇌ ZNa+⋯OC, where Z represents the zeolite framework, ΔH0 was found to take the values 3.8 and 2.4 kJ mol− for CO/Na‐ZSM‐5 and CO/Na‐Y, respectively. The C‐bonded species show always the highest cation–CO interaction energy.