B.A. Morrow

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.

An infrared study of sulfated silica

Abstract Surface sulfates on silica can be formed via impregnation with H 2 SO 4 or (NH 4 ) 2 SO 4 followed by activation under vacuum at 100 or 250 °C, respectively. Oxygen-18 isotopic exchange has shown that the structure is (SiO) 2 SO 2 ( II ) which has two terminal SO bonds, unlike the (SiO) 3 SO species ( I ) which we previously showed to exist on sulfated Al 2 O 3 and TiO 2 [ J. Catal. 99 , 104 (1986)]. Species II is not stable and decomposes significantly upon heating beyond 250 °C, and its decomposition is total beyond 500 °C. Addition of SO 3 to silica activated at 450 or 650 °C produces, in addition to II , a surface SiOSO 3 H species ( III ), and, in the presence of excess SO 3 , a weakly held species. Unlike Al 2 O 3 or TiO 2 , silica cannot be sulfated by heating in the presence of H 2 S or SO 2 with an excess of O 2 .

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.

An infrared study of the adsorption and oxidation of ammonia on platinum

Abstract The adsorption of NH 3 on silica-supported platinum and on oxidized silica-supported platinum has been examined using infrared spectroscopy. Spectra for all H D mixed isotopic species and for adsorbed 15 NH 3 indicate that ammonia only coordinates to the bare Pt surface and is stable up to about 250 °C. There was no evidence for dissociative chemisorption. All spectral features disappeared when a large dose of O 2 was added, but with micromole doses, new spectral features were observed which were tentatively assigned to Pt:NHO as an intermediate in the oxidation. On oxidized Pt, it has been suggested that NH 3 dissociates to give PtONH 2 at 20 °C which further dissociates near 100 °C to form a bridged (PtO) 2 NH structure.

The adsorption of NO and NO2 on silica-supported nickel

The adsorption of NO and NO2 on reduced silica-supported nickel at 325 K has been studied using infrared spectroscopy and a vacuum microbalance. For all NO coverages, a single infrared band at 1864 cm− was observed which was asymmetric to low wavenumber. From the isotopic shift data using 14NO/13NO mixtures, this has been assigned to the symmetric (strong, 1864 cm−1) and antisymmetric (weak, 1820–1840 cm−1) NO stretching modes of a surface Ni(NO)2 species. The infrared and gravimetric data indicate that NO also dissociates to yield surface oxide and nitride species, as has been found using other techniques, and the Ni(NO)2 species resides on a partially oxidized surface. An identical infrared band was observed for NO adsorbed on a preoxidized surface, and for a less than saturation dose of NO2 adsorbed on bare Ni. However, with a large exposure to NO2, surface nitrate-like species were produced. Finally, the 1864 cm−1 band from either adsorbed NO or NO2 slowly disappeared when CO was added and new bands at 2201 and 2082 cm−1 appeared due to adsorbed isocyanate and CO, respectively. This reaction is discussed in detail in the paper following this.

An infrared study of the adsorption of pyridine on platinum and nickel

Infrared spectral data have indicated that pyridine dissociatively chemisorbs on silicasupported platinum forming a PtC σ bond at the “2” position and a coordinate bond with the nitrogen atom such that the molecule lies perpendicular to the surface. In the presence of HCl this is reversibly transformed into pyridinium chloride and with H2 it is instantaneously hydrogenated to adsorbed piperidine. The latter does not appreciably desorb upon evacuation but dehydrogenates to reform chemisorbed pyridine. The same phenomenon occurs when piperidine is adsorbed on PtSiO2 and confirms earlier speculation that adsorbed piperidine is mainly responsible for the poisoning of Pt during hydrogenation. Identical spectral features were observed when pyridine was adsorbed on supported Ir, Os, Rh, and Ru. On silica-supported nickel a different strongly adsorbed species is formed which has been identified as a simple nitrogen-coordinated pyridine which also lies perpendicular to the surface. As proposed by others, the coordination of pyridine on nickel can be promoted if the nickel had been pretreated with O2 or CO.

The reaction between NO and CO on silica-supported nickel

Abstract The reaction between NO and CO at 325 K on silica-supported Ni has been studied using infrared spectroscopy and a vacuum microbalance. Adsorbed NO [mainly as Ni(NO) 2 ] reacts with CO to form adsorbed isocyanate (NCO) and gaseous CO 2 . Preadsorbed CO is displaced by excess NO and no further reaction occurs but with a small dose of NO, CO is only partially displaced and isocyanate and CO 2 are generated. Data indicate that the reaction is initiated by the direct interaction of adsorbed CO and NO and that the rate of reaction is probably inhibited by strongly adsorbed isocyanate. No definite mechanism can be devised but the apparent overall reaction stoichiometry after the reaction has gone to completion (5 hr) is 4(NO) a + 3CO → (NCO) a + 2O a + 3N a + 2CO 2 (g) where the subscript a indicates an adsorbed species and at least one step involves adsorbed CO. During reaction a weakly held form of adsorbed CO is generated which has been attributed to an (NCO) a (CO) a interaction on adjacent Ni sites.

Raman spectra of pyridine adsorbed on a series of X zeolites

The adsorption of pyridine on a sodium 13X zeolite and on 10 exchanged X zeolites has been studied by Raman spectroscopy. The band position of the pyridine ν1 symmetric ring breathing mode (near 991 cm−1 for liquid pyridine) shifted linearly to higher frequency as a function of the electrostatic potential (er) induced by the charge balancing cation, and a similar linear correlation was found for this mode in the spectrum of some metal nitrate-pyridine-nitromethane solutions. We postulate that in both systems the lone pair electrons of pyridine interact directly with the positively charged cation, an interpretation which conforms with other literature results with respect to the infrared and Raman spectra of pyridine adsorbed on other zeolites. In addition, where there was residual unexchanged Na+, an aditional v1 band was observed which was assigned to a Na+-pyridine interaction and we have concluded that where ionic size is not a consideration, the exchange cations (both monovalent and bivalent) are distributed at least among sites SI and SII.

Infrared spectra of adsorbed molecules on thin silica films

Using a thin film of silica deposited on a ZnSe window we have been able to observe, after spectral subtraction, the i.r. spectra due to the SiOSi, SiOti, and SiOGa stretching modes when surface hydroxy groups react with (Me3Si)2NH, TiCl4, or GaMe3 respectively; this versatile technique permits rapid acquisition of i.r. spectral data, over a wide range of temperature or pressure, in regions where an oxide is strongly absorbing.