1,721,086 research outputs found
Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces
No full textPublished data on the infrared spectra and coordination of carbonate, bicarbonate and formate ions, and of CO2 in metal complexes are reported. The usual assignments of absorption bands due to products of CO and CO2 reactive adsorption on metal oxide surfaces are also critically reexamined. © 1982
Infrared study of chemisorption and reactivity of pyridine on haematite
No full textPyridine adsorption on α-Fe2O3 has been studied by IR spectroscopy. The splitting of the 8a band of chemisorbed pyridine at infrared beam temperature has been interpreted as due to the existence on the haematite surface of at least two types of Lewis acid sites, possibly tetrahedrically and octahedrically coordinated cations, with a medium electron acceptor strength. Chemisorbed pyridine undergoes chemical transformations under heating, producing at 150°C a species identified, from the comparison of its spectrum with that of bipyridyl chemisorbed on the same surface, as 2,2′-bipyridyl in cis form. The reaction is probably realized via radical-anion formation, so confirming the electron transfer ability of haematite surface. At 400°C both pyridine and bipyridyl transform into a new species, without the opening of pyridine rings. © 1981
Fourier-transform Infrared Study of the Oxidation and Oxidative Dehydrogenation of Normal-butenes On the Surface of Fecro3
No full textThe interaction of the three linear butene isomers and of butadiene with the surface of a well characterized FeCrO3 oxidative dehydrogenation catalyst has been investigated by IR spectroscopy. Both (Z)- and (E)-but-2-enes produce well characterized molecularly adsorbed species and rapidly form sec-butoxides and ethyl methyl ketone at room temperature. At higher temperatures, acetates are formed by oxidative C=C bond breaking. But-1-ene gives at least two different conformeric adsorbed species and exhibits, together with the same oxidation pathway, an allylic activation pathway, with the formation of butadiene and methyl vinyl ketone. Butadiene also forms two different adsorbed species, very weakly held, and slowly transforms into crotonaldehyde, which is further oxidized at higher temperatures. These results provide evidence for different oxidative and dehydrogenative pathways at the surface and are correlated with the known behaviour of FeCrO3 as an oxidative dehydrogenation catalyst for linear butenes. Its very weak interaction with butadiene explains the high selectivities obtained in oxidative dehydrogenation over this catalyst. In contrast, catalysts that are highly selective for the production of oxygenates like furan and maleic anhydride, have been found to interact very strongly with butadiene
Infrared study of the reactivity of acetone and hexachloroacetone adsorbed on haematite
No full textInfrared spectra of acetone adsorbed at beam temperature on α-Fe2O3 show that acetone can chemisorb on Lewis-acid sites and gives, at least partially, enolate anions; these, by aldolic condensation with molecules from the gas phase, produce a chemisorbed form of mesityl oxide. At 523 K acetate ions are formed. Two different forms of trichloroacetate ions are formed on the surface at beam temperature by the adsorption of hexachloroacetone and trichloroacetic acid. This behaviour indicates the presence of pairs of acid-base sites on the surface of haematite, and also shows that its surface hydroxy groups have a lower degree of nucleophilic character with respect to those of other oxides such as aluminas, SnO2 and alkaline-earth oxides
Infrared study of CO2 adsorption on haematite
No full textThis work has shown that CO2 molecules can interact at b.t. with high surface α-Fe2O3 obtained by thermal decomposition of goethite giving: a) physisorption, possibly via charge transfer mechanism between the carbon atom and the oxygen of a surface hydroxyl; b) formation of both monodentate and bidentate carbonate ions which can be easily decomposed by degassing even at b.t.. More severe conditions of contact allow the progressive formation of Fe2O2CO3 layers thermally stable up to 400°C which can be identified through their characteristic IR absorptions. © 1980
Infrared study of the adsorption of nitrogen dioxide, nitric oxide and nitrous oxide on hematite
No full textAn infrared spectroscopic study of the adsorption of NO2, NO, and N2O on α-Fe2O3 was carried out in order to identify chemisorbed species which could act as catalytic intermediates. Presorptions of pyridine and water were used in addition to usual pretreatments of the surface, in order to identify and discuss adsorption sites and mechanisms. Stable nitrate ions are formed on the surface, starting at beam temperature (b.t.) with NO and are predominant at 150 °C with NO2. Between b.t. and 150 °C, NO2 mainly chemisorbs as such or as its dimer N2O4. Depending on the pressure, NO chemisorbs at b.t. also as is or as NO- ions, as well as in the form of its dimer N2O2. N2O merely physisorbs on an "oxygen-rich" surface, but it strongly chemisorbs on an "evacuated" surface producing species which can be identified as N2O-. © 1981
Infrared studies of the surface of α-Fe2O3
No full textThe surface chemistry and structure of α-Fe2O3 obtained by thermal decomposition of α-FeOOH have been studied by IR spectroscopy. The spectra arising from the adsorption of some probe molecules (water, pyridine, ammonia, carbon monoxide, methanol) on samples after different pretreatments are analyzed, and the nature and behaviour of surface hydroxy groups, Lewis acid sites, acid-base couples and π-bonding adsorption sites are discussed. The results are interpreted on the basis of a disordered model of the {0001} crystal face that is predominantly exposed on such samples. © 1985
IR characterization of surface hydroxy groups on haematite
No full textThe OH stretching absorptions (3670, 3640 and 3460 cm-1) detected on haematite samples obtained by thermal decomposition of goethite, are assigned to monocoordinated and bridged free surface OH's and to hydrogen bonded OH's in surface micropores, respectively, on the basis of their behavior to successive adsorption of methanol, pyridine and D2O and surface structural models. © 1980 Akadémiai Kiadó
Surface Acidity of Solid Acids and Superacids: A Ft-Ir Study of the Behaviour of Titania Doped with Phosphoric, Sulphuric, Tungstic and Molybdic Acids
No full textThe surface structure and the acid behaviour of titanias doped with sulphuric (TS), tungstic (TW), molybdic (TMo) and phosphoric (TP) acids has been studied using FT-IR spectroscopy. The acidity scale detected using the olefin oligomerization activity in the IR cell as the discriminant is TS « TW < TMo « TP « TiO2 and seems to agree with catalytic activity data. It is deduced that the enhanced acidity of doped samples is of the Bronsted type, the Lewis sites of titania being only weakly perturbed. Moreover, doping strongly poisons basic sites. Surface mono-oxo anionic species are very evident on TS, TW and TMo, while on TP tridentate hydrogenphosphate ions seem to be predominant. © 1989
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