A.A. Tsyganenko

Structure of alumina surfaces

The structure of hydroxylated alumina surface is analysed by IR spectroscopy. The variety of OH groups differing in number and coordination of surrounding metal atoms, for the completely hydrated surface, either formed by ideal low-index planes, or complicated by crystal edges and corners or cation vacancies, is restricted to six, but grows dramatically upon dehydroxylation. The results account for the complex structure observed previously in the IR spectra of surface OH groups after high-temperature treatment, and enable us to explain specific properties of transition aluminas.

Two Coordination Modes of CO in Zeolites: A Temperature-Dependent Equilibrium

CO interacts with exchangeable cations M+ (gray spheres in the picture) of zeolites to form M+ ⋅⋅⋅CO and M+ ⋅⋅⋅OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M+ =Na+ ) the difference in interaction energy amounts to 3.8 kJ mol-1 , as determined by means of variable-temperature FT-IR spectroscopy.

CO interaction with the surface of thermally activated CaO and MgO

Abstract Adsorption of 12 C 16 O, 13 C 16 O, and 12 C 18 O on CaO and MgO, pretreated at 1000 K, has been studied at 40–300 K. A number of surface CO compounds have been detected, which differ in their thermal stability or sensitivity with respect to oxygen. From the measured isotopic shifts and frequencies the force constants of some compounds were calculated. The first product arising after CO admission is the “carbonite” CO 2 2− ion, which is the result of CO interaction with a coordinatively unsaturated oxygen ion. At temperatures higher than 100 K the carbonite ion reacts with excess CO to produce the dioxoketene OCCO 2 2− ion. Further reactions with CO give rise to more complex compounds, both oxidized and reduced. In the presence of oxygen, carbonite ions and reduced CO compounds are readily oxidized into carbonates; at low temperature, some intermediate products of oxidation were registered. Reversible interaction of CO with the surface O 2− ions is responsible for the catalytic reaction of low-temperature homomolecular isotopic exchange of CO on the surface of basic oxides. It is supposed that the established mechanism of CO activation by oxygen ions is typical for other reactions on the surface of basic and alkali-promoted catalysts.

Infrared study of lateral interactions between carbon monoxide molecules adsorbed on oxide catalysts

An infrared study of 12C16O and 13C16O adsorbed at 77 K on ZnO, TiO2, BeO, Al2O3, and SiO2 has provided details on two kinds of interactions between the adsorbed molecules which account for the dependence of the spectra on surface coverage. The observed vco values, which for coordinatively bonded molecules are 50–100 cm−1 higher compared with gaseous CO, are lowered significantly (up to 30 cm−1) on increasing coverage as a result of a “chemical” effect, namely the weakening of the electron-donating ability of metal ion after occupation of the adjacent sites with adsorbed molecules. The fine structure of the COZnO spectrum reflects the stepwise character of this process. The second type of interaction is dipole coupling. This causes a comparatively weak upward shift (up to 6 cm−1), partly counteracting the “chemical” effect.

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.

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.

Formation of several types of coordination complexes upon CO adsorption on the zeolite Li-ZSM-5

Carbon monoxide adsorbed at 77 K on Li-ZSM-5 gives Li+···CO and Li+(CO)2 adducts characterized by C–O stretching bands at 2195 and 2188 cm−1, respectively. Variable temperature FTIR spectroscopy has shown that, at a higher temperature, formation of Li+···OC and Li+(CO,OC) species also occurs; the corresponding C–O stretching mode of O-bonded carbon monoxide appears at 2100 and 2110 cm−1, respectively. The relative amount of O-bonded CO increases with increasing temperature, in agreement with the fact that cation–CO interaction energy is higher for C-bonded carbon monoxide.

Infrared spectroscopic evidence for the structural OH groups of spinel alumina modifications

Abstract In the structure of spinel aluminas (γ, η, δ etc.), treated at moderate temperatures, a certain part of cation vacancies are always hydrated. Protons trapped in the octahedral and tetrahedral vacancies give rise to wide bands at about 3500 and 3300 cm −1 , respectively. These bands are not affected by CO or pyridine adsorption or by D 2 O treatment at 300 K. Evacuation at temperatures higher than 750 K results in their disappearance, but after heating the sample in water vapour the structural hydroxyls can be partly restored. Treatment in D 2 Ogives rise to the corresponding OD bands at 2590 and 2480 cm −1 , respectively. Complete saturation of all the cation vacancies by protons in the spinel lattice gives an ideal “protospinel” structure with the formula HAl 5 O 8 or Al 2 O 3 ·0.2 H 2 O. The role of protonated vacancies and proton migration in the formation of alumina structure and their influence on the properties of oxides is discussed.

Brønsted acidity of silica silanol groups induced by adsorption of acids

Surface silanol groups of silica, which never revealed any Brønsted acidity, are shown to donate protons to adsorbed basic molecules, such as ammonia, pyridine or 2,6-dimethylpyridine, after addition of acids such as SO2 or NO2. The latter, when coadsorbed with bases, interact with the oxygen atoms of silanols leading to OH acidity increase and to protonation. Bases, in turn, enhance chemisorption of SO2 or NO2, and strongly held coadsorption products are formed as a result. The proposed mechanism of induced Brønsted acidity could account for the promoting effect of acidic gases in reactions catalysed by metal oxides.