Domenica Scarano

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.

FOURIER-TRANSFORM INFRARED AND RAMAN SPECTRA OF PURE AND AL-, B-, TI- AND FE-SUBSTITUTED SILICALITES : STRETCHING-MODE REGION

A systematic investigation of the IR and Raman spectra of pure and Al-, B-, Ti- and Fe-substituted silicalites in the SiO stretching region (1500–700 cm–1) is presented. As well as the characteristic stretching modes of the skeleton, silicalites containing hydroxylated nests show also broad bands at ca. 960 cm–1(IR) and at ca. 976 cm–1(Raman), associated with O3Si—OH group modes, with prevailing Si—OH stretching character. The replacement of Si with heavier elements (like Ti or Fe) causes the appearance of new IR- and Raman-active modes (i) at 960 cm–1(IR and Raman) and at 1127 cm–1(Raman) in Ti silicalite; (ii) at 1015 cm–1(IR) and at 1020 cm–1(Raman) in Fe silicalite. Neither the Raman nor the IR spectra of the skeletal modes are substantially modified by the introduction of Al (ZSM5). The presence of boron induces the appearance in the IR spectra of a complex absorption at 1380 cm–1 and at 960–930 cm–1, which corresponds to the absorptions at 1417 and 976 cm–1 in the Raman spectra. The assignment of the absorptions associated with heteroatoms and hydroxylated nests is discussed in detail.

Well defined CuI(NO), CuI(NO)2 and CuII(NO)X (X = O− and/or NO 2 − ) complexes in CuI-ZSMS prepared by interaction of H-ZSM5 with gaseous CuCl

In this note an exchange procedure of the acidic protons of H-ZSM5 by CuI ions through reaction with CuCl in the gas phase is described. In the so obtained CuI-ZSM5 exchanged zeolite the CuI ions are in well defined configuration and form with NO mono and di-nitrosyl complexes of high structural and spectroscopic quality. The CuI(NO)2 species are transformed at RT into CuII(NO)X (X=O− and/or NO2−) species which could represent an intermediate in NO decomposition.

Structural characterization of Ti centres in Ti-silicalite and reaction mechanisms in cyclohexanone ammoximation

Abstract The main results obtained by means of many physical methods (IR, Raman, UV-Vis and XAFS spectroscopies) concerning the structure of the Ti centre in titanium silicalite and the reaction intermediates in the ammoximation of cyclohexanone are concisely reviewed. The Ti is in tetrahedral coordination in vacuo and expands its coordination sphere upon interaction with adsorbates. In the presence of H2O and H2O/H2O2 solutions one of the SiOTi bridges is hydrolyzed with formation of (SiO)3L2TiOH (LH2O) and (SiO)3L2TiOOH species, respectively. When NH3 is dosed on (SiO)3L2TiOOH structures (SiO)3L2TiOO−NH4+ species are formed. These species are thought to play an important role in the ammoximation reaction. The geometries of the peroxidic species (open or bridged) are discussed also on the basis of ab initio calculations.

IR investigations of CO2 adsorption on chromia surfaces: Cr2O3 (0001)/Cr(110) versus polycrystalline α-Cr2O3

Abstract The adsorption of carbon dioxide has been studied on a single-crystalline Cr 2 O 3 (0001) film as well as on samples of polycrystalline α -Cr 2 O 3 . The Cr 2 O 3 (0001) film has been grown on the (110) surface of a chromium single crystal. Upon CO 2 dosage, two chemisorbed and two weakly bound adsorption states are identified. We associate the strongly bound carbon dioxide with carboxylates, bent CO δ− 2 species adsorbed on top of the chromium ions of the ‘polar’ (0001) surface. An assignment to surface carbonate formed upon adsorption on surface oxygen ions is not compatible with the vibrational data. The IR spectra of CO 2 chemisorbed on polycrystalline α-Cr 2 O 3 differ substantially from the IRAS spectra of the CO 2 /Cr 2 O 3 (0001)/Cr(110) system. As the faces of the microcrystals show mainly non-(0001) termination, we consider different modes of coordination of the CO 2 chemisorbates. In particular, bidentate carbonate species are discussed.

IR studies of CO and NO adsorbed on well characterized oxide single microcrystals

Abstract A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, α-Cr 2 O 3 , α-Al 2 O 3 , MgAl 2 O 4 and other spinels, TiO 2 , ZrO 2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions. In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Me x+ ··· CO/NO bond (Me x+ = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-π overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different. On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, α-Cr 2 O 3 , etc.) FWHM values comprised in the 1.5–3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations. Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, steps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions.

Lateral interactions in CO adlayers on prismatic ZnO faces: a FTIR and HRTEM study

Abstract In this paper the CO adsorption at 77 K on the ZnO (Kadox) surface is studied. CO adsorption occurs mainly on the (1010) prismatic faces of ZnO microcrystals (one CO per Zn 2+ ion), giving a complex IR spectrum in the 2168–2190 cm −1 interval. The CO frequency shifts with the coverage from 2190 cm −1 (θ ≈ 0, singleton) to 2168 cm −1 ( θ = 1), as a consequence of through-space interactions (either dynamic and static) among the oscillators. At least five discrete components appear and disappear in the 0 ⩽ θ ⩽ 1 interval. These features arise from the progressive stepwise filling of the surface Zn 2+ sites, and correspond to different variants of the surrounding of the CO molecules. The first overtones of the CO stretching mode are also observed in the 4335–4372 cm −1 interval, while the combination mode between the CO and the M-C stretching vibrations are found in the 2375–2415 cm −1 interval. The erosion of localized lattice modes in the 600–750 cm −1 region, indicates that CO adsorption is accompanied by surface relaxation. Finally some considerations concerning the perturbation of the OH groups, upon CO adsorption, are included.

Surface Structures of Oxides and Halides and Their Relationships to Catalytic Properties

In this review, the relationships between structure, morphology, and surface reac-tivity of microcrystals of oxides and halides are assessed. The investigated systems we discuss include alkali halides, alkaline earth oxides, NiO, CoO, NiOMgO, CoOMgO solid solutions, ZnO, spinels, cuprous oxide, chromia, ferric oxide, alumina, lanthana, perovskites, anatase, rutile, and chromia/silica. A combination of high-resolution transmission electron microscopy with vibrational spectroscopy of adsorbed probes and of reaction intermediates and calorimetric methods was used to characterize the surface properties. A few examples of reactions catalyzed by oxides are also reported.

Dipole coupling and chemical shifts of CO and NO adsorbed on oxides and halides with rock-salt structure

The infrared spectra of CO and NO adsorbed at 77 K on sintered alkali-metal halide films (LiF, NaCl, KCl and NaI) and on sintered polycrystalline oxides (MgO and NiO) are reported. The most intense bands have been assigned to monomeric (CO) and dimeric (N2O2) species adsorbed on (100) faces. The CO molecules are adsorbed in a localized form on surface cations with orientation perpendicular to the (100) planes and form a regular array of parallel oscillators which interact via dynamic and static effects. The dynamic polarizability (αv) of CO adsorbed on alkali-metal halides and magnesium oxide is very small (αv≈ 0.02–0.03), while that of CO on NiO is one order of magnitude larger. The molecules of CO adsorbed on (100) faces also interact through the solid via inductive effects. NO is adsorbed on (100) planes of alkali-metal halides and MgO in the dimeric cis form; on NiO monomeric nitrosylic species predominate.

IR study of ethene and propene oligomerization on H-ZSM-5: hydrogen-bonded precursor formation, initiation and propagation mechanisms and structure of the entrapped oligomers

The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through: (i) formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Bronsted sites, (ii) a protonation step and (iii) a chain-growth step. The relative strength of the hydrogen bonds in the ethene–OH and propene–OH π-complexes (precursors) is estimated on the basis of the downward shift of both the ν(OH) and ν(CC) frequencies (–389 and –11 cm–1 for ethene and –539 and –19 cm –1 for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear of low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework.