O. Saur

FT-IR characterization of the surface acidity of different titanium dioxide anatase preparations

The surface acidity of four different anatase preparations has been studied by FT-IR spectroscopy of adsorbed benzene, ammonia, pyridine, carbon monoxide and dioxide. Our results indicate that surface chemistry of anatase, in particular acidity, greatly depends on the preparation of the sample, both through the resulting morphology and the presence of surface impurities. Different morphologies may result in different types of Lewis acid sites. If pure, anatase does not show Bronsted acidity. The presence of silica impurities is responsible for an enhancement of Bronsted acidity on several samples, and may be evaluated by the intensity of an IR band situated near 3730 cm−1. Very strong Bronsted acidity is generated by the presence of sulfate impurities.

The structure and stability of sulfated alumina and titania

The oxidation of H2S or SO2 in excess O2 over Al2O3 or TiO2 (anatase) yields, for either catalyst under anhydrous conditions, an infrared spectrum which is characterized by an intense sharp band near 1380 cm−1 and a broad band or doublet near 1040 cm−1. The same spectrum arises from the impregnation of Al2O3 with either (NH4)2SO4 or Al2(SO4)3 · 18H2O or of TiO2 with (NH4)2SO4 or TiOSO4 and heating the dried mixture at 450 °C under vacuum. The sulfated surface does not exchange with 18O2 but does with H218O and only one new shifted high-wavenumber band is produced for partial or complete oxygen-18 exchange. The infrared spectrum changes in the presence of H2O at 20 °C and resembles that of a more traditional bidentate type sulfate species and we postulate that, in the absence of OH groups or water the sulfate has a structure resembling (M3O3)SO [M = Al or Ti], whereas in the presence of H2O or excess surface OH groups this is converted to type groups, thus accounting for the increased Bronsted acidity. Finally, the sulfated Al2O3 surface is more thermally stable and more resistant to reduction in H2 than is the TiO2 surface and the ramifications are discussed in terms of the use of these oxides for SOx transfer catalysts or in air pollution abatement.

An infrared study of sulfated zirconia

Abstract Fourier transform infrared spectroscopy and vacuum microbalance methods have been used to study the sulfation of zirconium oxide. Sulfation could be carried out by oxidizing either H2S or SO2 in excess O2 at 450°C and the infrared spectrum showed an intense three component band near 1375 cm−1 accompanied by a complex profile having at least six components between 1100 and 800 cm−1. Two surface sulfate species were formed after oxidation of between 10 and about 250 μmol g−1, and although the extent of formation of each was dependent on the quantity oxidized, the total band area between 1400–1350 cm−1 was a linear function of the quantity of H2S or SO2 introduced into the infrared cell. Both species contain a single S=O oscillator and their structures are possibly of the type (ZrO)3S=O resident on different crystal faces of ZrO2. A third species is formed at higher coverages and its suggested structure is a S2O7 type species in which four of the oxygen atoms are bonded to surface Zr atoms, there being one SOS bridge and two uncoupled S=O oscillators. All three sulfate species have similar thermal stabilities (decomposition between 600 and 800°C) and are reduced at the same rate in hydrogen (the IR bands disappear rapidly between 450 and 475°C). Finally, sulfation could also be carried out via impregnation of ZrO2 with H2SO4, (NH4)2SO4 or Zr(S04)2 followed by heating under vacuum at 450°C.

Acidity of zirconium oxide and sulfated ZrO2 samples

Abstract Lewis acidities of pure zirconium oxide and of two sulfated samples containing 200 or 280 μmol g −1 of sulfate, activated at 720K, have been compared by FT-IR using probe molecules such as CO, CO 2 and pyridine. The sulfated samples show an increase of the strength of some Lewis acid sites. However, their number does not seem to be affected. CH 3 SH adsorption is very sensitive to the presence of sulfate : on pure ZrO 2 , it dissociates while on sulfated samples it is mainly coordinated. It is therefore deduced that sulfate formation involves the basic oxygen atoms which also facilitate the dissociation of CH 3 SH. The increase of the Lewis acidity observed, in the activation conditions used, is not sufficient to explain alone the superacidic properties of sulfated zirconia.

Claus Catalysis and H2S Selective Oxidation

Abstract This review article deals with the development of sulfur recovery from the Claus process to H2S selective oxidation. Governments are constantly tightening regulations to limit the emission of sulfur compounds into the air. This makes it necessary to constantly enhance the level of sulfur recovery from natural, refinery, or coal gasification geses, and many improvements in the Claus process have been introduced to this end. In this review, emphasis has been put on the mechanism of reactions occurring in most of the sulfur recovery units, reactions between H2S and SO2 or O2 and side reactions such as hydrolysis of COS and CS2 or sulfation of the catalyst.

Effects of crystallinity and morphology on the surface properties ofalumina

Abstract Five samples of alumina (one amorphous and four γ-aluminas prepared from different boehmites) were compared using physical methods (X-ray diffraction, transmission electron microscopy, 27 Al NMR and Fourier transform infrared spectrometry) and the butene isomerization test. All the γ-alumina samples preferentially present the (110) face, whereas the ratio of the (001) and (111) faces depends on the precursor. Isomerization tests and IR experiments with different probe molecules show very similar acidities for the crystallized samples, which is discussed taking account the models of Kno¨zinger and Ratnasamy and Tsyganenko et al. The lower Lewis acidity of the amorphous sample is related to the smaller number of Al IV sites shown by NMR experiments. The crystallinity effect is also evidenced by an IR study of the hydroxyl groups, the amorphous sample showing a specific band at 3787 cm −1 .

Infrared study of alcohols adsorption on zirconium oxide: reactivity of alkoxy species towards CO2

Abstract The i.r. spectrum of a ZrO 2 sample activated at 723 K mainly presents two bands at 3775 and 3670 cm -1 characterizing two types of OH groups (type I and type II respectively). The latter is not affected by CO 2 introduction while the former is involved in the formation of hydrogenocarbonate species. CH 3 OH adsorption on activated ZrO 2 leads to the appearance of two strong bands at t160 and 1054 cm -1 . Use of CH 1 3 8 OH shifts both towards lower wavenumbers (1122 and 1023 cm -1 respectively). This clearly shows that they correspond to the v (C-O) mode o f two types of methoxy species (I and II respectively) resulting from the dissociation of the methanol O-H bond. Their reactivity towards CO 2 is quite different: type I species react and lead to the formation of a methyl carbonate like species while type II are almost unreaetive. Extension of the study to ethanol provides also evidence of two types of ethoxy species. The assignment of species I to monodentate groups and species II to bridged ones allows us to explain their different reactivity towards CO 2 considering that the first step of the reaction is an electron donor-acceptor interaction.

Comparative study of SO2 adsorption on metal oxides

SO2 adsorption on different metal oxides (MgO, CeO2, ZrO2, MgAl2O4, TiO2-anatase, TiO2-rutile, Al2O3 and Na–Al2O3) has been studied using various techniques (thermogravimetry, temperature-programmed desorption, IR spectroscopy). Several types of species are formed and the results are discussed in terms of their thermal stability. Weakly adsorbed species can result from the coordination of SO2 on Lewis acid sites such as Al2O3 or TiO2. However, such interactions are quite weak. On more basic oxides, such as Na–Al2O3, SO2 acts as an electron acceptor and adsorbs on either weakly basic O2– sites or on the basic OH– groups. In this latter case, the formation of hydrogen sulfite species is suggested. More strongly adsorbed species occur on all the oxides. These are characterized by strong absorption bands between 1100 and 800 cm–1, and are due to sulfite species. Several types generally occur. Gravimetric measurements allow us to compare the thermal stability and the amount of the different types of adsorbed SO2 species. The quantity of SO2 irreversibly adsorbed after evacuation at T < 370 K gives rise to an evaluation of the number of basic sites and therefore to a scale of basicity of the different oxides.The results are compared with those obtained using other probe molecules such as carbon dioxide or hexafluoroisopropanol. A special case is that of ceria for which it is observed that heating under vacuum transforms sulfite species into sulfates, preventing the use of TPD for the determination of its relative basicity.

EVALUATION OF MAGNESIUM ALUMINATE SPINEL AS A SULFUR DIOXIDE TRANSFER CATALYST

Abstract Infrared spectroscopy and vacuum microbalance techniques have been used to study the sulfation of magnesium aluminate spinel, MgAl 2 O 4 , by oxidation of sulfur dioxide in excess oxygen at temperatures from 300 to 550°C. The results have been compared with those previously obtained for sulfated Al 2 O 3 , TiO 2 , ZrO 2 and MgO with a view to evaluating which material would be a superior SO x transfer catalyst. For low doses of sulfur dioxide, surface sulfates on spinel are mainly formed which are covalently bonded to the surface but, at higher doses, ionic sulfates are incorporated into the bulk of the spinel. At least two types of surface sulfate species have been identified and that formed preferentially at low coverages has the probable structure (−0) 3 SO, as was found previously for SO 2 oxidation on Al 2 O 3 and TiO 2 . All sulfate species are thermally stable to evacuation up to 800°C but are removed between about 800–900°C. They are also readily removed following reduction in excess hydrogen in a relatively narrow temperature range from about 550 to 640°C. On a very low surface area spinel, only bulk sulfate is formed. Relative to the other oxides studied, sulfated spinel has a high thermal stability, the sulfates are easily reduced in hydrogen and it has a high sorption capacity by virtue of its ability to form bulk sulfate species (not observed under the same conditions on Al 2 O 3 , TiO 2 , or ZrO 2 ). These favorable characteristics account for the use of spinel as a transfer catalyst for reducing SO x emissions.

Comparative adsorption of H2S, CH3SH and (CH3)2S on alumina. Structure of species and adsorption sites

The adsorption of H2S, CH3SH and (CH3)2S has been studied by gravimetry and infrared spectroscopy. Some experiments have been carried out with preadsorbed pyridine as a poison. The results show that (CH3)2S is bonded to Lewis acid sites and that 60 µmol g–1 are irreversibly coordinated to stronger sites of this type. Dissociative chemisorption has been observed for H2S and CH3SH. Poisoning experiments lead us to conclude that the first adsorption steps involve either a coordinative adsorption (ca. 30 µmol g–1) or a hydrogen bond between a surface basic oxygen atom and the H atom of the SH bond (ca. 50 µmol g–1). The reversible adsorption of CH3SH is thought to occur by these two processes besides another one in which the S atom is H bonded to a hydroxyl surface group. These results clarify the nature of the adsorption sites of alumina.