G. Ghiotti

The chemisorption of carbon monoxide on various transition aluminas

Carbon monoxide chemisorption was investigated at 36 °C on four transition aluminas (η, γ, θ, Alon C) by means of microcalorimetry and infrared spectroscopy. On all the aluminas coverages are fairly low, and the interaction only involves a coordination to surface cations. There is evidence for two families of admolecules, characterized by different stretching frequencies (Δ¯ v ∼ 30 cm −1 ) and adsorption heats (14 and 2 kcal/mole, respectively). The same species are found on all the aluminas although in different amounts. In particular the remarkable differences observed between η-alumina and γ-alumina seem to parallel their different catalytic activity.

An infrared spectroscopic investigation of the surface properties of magnesium aluminate spinel

Abstract Magnesium aluminate (spinel) samples were prepared by decomposition of Mg, Al mixed oxalate and by thermal decomposition of a Mg(NO 3 ) 2 -impregnated transition alumina. In the latter preparation, which yields samples more suitable for ir work, transformation to the spinel phase occurs at ~400 °C, as monitored by the XRD spectrum, and by a sudden shift of the surface OH band to ~3740 cm −1 . Pyridine (Py), CO, and CO 2 were adsorbed in order to investigate the surface acidity and basicity. Py is most effective in revealing Lewis acidity, which is mostly ascribed to coordinatively unsaturated (cus) Mg IV ions, whereas CO and CO 2 σ-coordinate to only a few such sites. (The Roman numeral in Mg IV symbolizes tetrahedral coordination). CO 2 also adsorbs to give bicarbonates as well as monodentate and bidentate carbonates. Evidence is presented to show the importance of the octahedral coordination (Al IV ) and of the degree of surface hydration with respect to surface oxygen basicity. The tetrahedral coordination is shown to be important for the formation of surface bicarbonates, which probably have a bridged structure. The partly inverted cation distribution of some samples was monitored by spectral studies in the surface OH group region and by the adsorption behavior of Py, CO, and CO 2 .

IR investigation of polymerization centres of the phillips catalyst

The CO-reduced Phillips catalyst (Cr(II) ions supported on silica) has been studied by IR spectroscopy. The following points have been examined: (i) CO/C2H4 interaction; (ii) vibrational features of the polymer; (iii) its reactivity towards oxygen; (iv) the effect of CO on the spectrum of the polymer and vice versa. The number of active sites is shown to increase during the polymerization and to reach some 10% of the total loading. The spectral features of the Cr ions involved in the active sites are consequently visible in IR: before polymerization they chemisorb CO more strongly and form stable CO-ethylene complexes; after polymerization they can still adsorb CO in a distinguishable form (stretching frequency at 2173 cm−1). The polymer chain does not show any terminal group such as methyl or vinyl: a cyclic structure is proposed. The chain may exist in either alkyl or alkylidenic form, in equilibrium with each other and differing by a proton reversibly transferred to a nearby oxygen. A model for the polymerization mechanism is advanced involving a metallacyclobutane intermediate, and accounting for the observed absence of scrambling among H atoms.

The chemistry of silica-supported Cr ions: A characterization of the reduced and oxidized forms of chromia/silica catalyst by calorimetry and ultraviolet-visible spectroscopy

CrO3SiO2 samples with low Cr loading (0.5% Cr by weight) have been prepared by impregnation. Formation of the oxidized surface phase, reduction, thermal deactivation and reoxidation have been studied. During the initial thermal treatment of the samples, two processes occur. These are anchorage of CrO3 onto the surface, yielding surface chromates and dichromates, stable toward temperature and second, degradation of nonanchored CrO3 to lower valence states, easily reoxidized at 823 K. Reduction with CO at 623 K yields a 98% population of CrII. Four species of such ions (with different electronic transitions) are present at the surface: two are reactive (CrA and CrB) and two less reactive (Crc1 and Crc2). Thermal deactivation converts CrA into Crc and does not affect CrB. A fraction of Crc (at least some 30%) is unavoidably present at the surface. In the most active samples CrA is about 45% and CrB about 25%. Reoxidation of CrA and CrB occurs with comparable reaction heats (480 and 425 kJ, respectively) via a nonactivated process. Crc oxidation is strongly activated and occurs under O2 pressure.

Chemical and Physical Properties of Copper-Based Catalysts for CO Shift Reaction and Methanol Synthesis

Abstract The modern low-pressure methanol synthesis catalysts are based on copper-containing systems such as Cu/ZnO/Al2O3 and Cu/ZnO/Cr2O3 with various compositions. These catalysts are also highly active for the low-temperature CO shift reaction. For both reactions the nature of the active sites is still an open question.

Preparation and characterization of SnO2 and MoOx–SnO2 nanosized powders for thick film gas sensors

Abstract This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

Cr2+ ions grafted to silica and silicalite surfaces: FTIR characterization and ethylene polymerization activity

Abstract The coordinative situation of Cr 2+ ions grafted to the external surface of amorphous silica and to the internal surface of silicalite has been studied by adsorption (at room temperature and 77 K) of different probe molecules (CO, NO and CO/NO mixtures) by means of FTIR spectroscopy. On amorphous silica three main families (A, B, C) of Cr 2+ centres are singled out. The cations with highest coordinative unsaturation are able to form Cr 2+ (CO) n . (where 1 ⩽ n ⩽ 4) polycarbonylic complexes with CO and Cr 2+ (NO) n (with 2 ⩽ n ⩽ 4) polynitrosylic complexes with NO. Mixed species formation is observed with CO/NO mixtures. Cr 2+ centres grafted to the internal surface of silicalite are in an average higher coordination state and hence their tendency to form polycarbonylic species is depressed. The Cr 2+ ions on both SiO 2 and silicalite polymerize ethylene giving rise to living (CH 2 ) n , polymer. The more active sites on Cr 2+ /silica belong to the Cr A family. Due to the very fast polymerization rate the initiation mechanism on such catalyst cannot be fully investigated even by means of time resolved FTIR spectroscopy. On Cr 2+ /silicalite the spatial limitations imposed by the porous character of the solid depress the growth of the polymeric chains, so allowing a more accurate study of the initiation steps. It is so confirmed that during the chain initiation no methyl groups are formed.

The chemistry of silica-supported chromium ions: Calorimetric and spectroscopic study of nitric oxide adsorption

At the surface of CO-reduced, low-loaded (0.5% by weight) chromia/silica samples, four kinds of divalent Cr ions are present. All react with NO eventually yielding linear dinitrosyl species. Differences are found as to the energetic and kinetic behaviour. The most exposed ions (A and B) readily form dinitrosyls, with NO infrared stretching vibrations at 1747–1865 and 1755–1880 cm−1, respectively. Less reactive Cr ions (designated C1 and C2) form at low coverage stable mononitrosyls (NO stretch at 1810–1815 cm−1), which change at high NO pressure into A and B dinitrosyls (C1 ions) or to new dinitrosyls (C2 ions: NO stretches at 1765–1987 cm−1). The different behaviour is explained assuming that A, B, and C ions are, before reaction, respectively 2-, 3-, and 4-coordinate to the surface.

In situ FT-IR and reactivity study of NOx storage over Pt-Ba/Al2O3 catalysts

The mechanism of NOx storage on Pt–Ba/Al2O3 and, for comparison, on Ba/Al2O3 catalysts was investigated. In situ FT-IR spectroscopy was used to follow temporal changes in the types and relative amounts of surface species formed at three different temperatures (423, 523, 623 K) during the reaction with NO, NO/O2, NO2 or NO2/O2 atmospheres. At 623 K the NOx storage capability was also investigated by a mass-spectrometry transient response method (TRM) as a complementary technique. The results show that in absence of oxygen appreciable amounts of NO are adsorbed only on the Pt–Ba/Al2O3 system mainly as ionic nitrites. When oxygen is present, NO is stored according to two different mechanisms, both promoted by Pt. The first occurs through the formation of surface nitrites that evolve to nitrates; the second occurs through the NO oxidation to NO2 over Pt, followed by NO2 migration to the Ba phase and nitrate formation. When NO2 is used with or without O2, an extensive storage in the form of ionic nitrates occurs both on Ba/Al2O3 and Pt–Ba/Al2O3 catalysts at all the temperatures studied, without the formation of nitrites. NO2 storage occurs initially through a very fast mechanism, giving nitrates without NO release. In parallel, it proceeds via a disproportionation reaction of NO2 to nitrate and NO. Both mechanisms are not promoted by Pt.

Surface acidity of η-alumina. Part 1.—Pyridine chemisorption at room temperature

The room temperature adsorption of pyridine on η-Al2O3 dehydrated at several temperatures in the 25–700°C range has been studied by i.r. spectroscopy, u.v.-vis reflectance spectroscopy and microgravimetry.Several species are formed that can be identified on the basis of the spectroscopic features of the 8a ring mode, whereas the 8b mode, which is common to all species, yields the total amount adsorbed. Quantitative interpretation of the spectral data is carried out using a computer program.The adsorbed species are interpreted as follows: a liquid-like physisorbed species, a species H bonded to surface OH groups through the nitrogen lone pair and three Lewis coordinated species. The latter are assigned to purely octahedral, tetrahedral–octahedral and purely tetrahedral cationic sites, respectively.The reactions occurring upon Py desorption above 400°C are discussed and a different interpretation is given to that in the literature.