Muhammad A. Hasan

In situ FTIR spectra of pyridine adsorbed on SiO2–Al2O3, TiO2, ZrO2 and CeO2: general considerations for the identification of acid sites on surfaces of finely divided metal oxides

Abstract Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH 3 , pyridine, CH 3 CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH 3 ; (ii) much more strongly adsorbed than CO and CH 3 CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν 8a mode of ν CCN vibrations at 1630–1600 cm −1 , where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH + ) is identifiable by (i) an ν 8a -absorption at ≥1630 cm −1 ; (ii) an ν 19b -absorption at 1550–1530 cm −1 ; as well as (iii) ν N + H and δ N + H absorptions occurring, respectively, near 2450 and 1580 cm −1 , and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH − and O 2− ) involved in formation of acid–base site pairs.

Influence of CuOx additives on CO oxidation activity and related surface and bulk behaviours of Mn2O3, Cr2O3 and WO3 catalysts

Abstract CuOx-modified and unmodified Mn2O3, Cr2O3 and WO3 catalysts were prepared by impregnation with Cu(NO3)2 solution and/or calcination at 700°C of appropriate precursor compounds. Bulk phase composition and thermochemical stability were characterized by X-ray diffractometry, infrared spectroscopy and thermogravimetry. The surface area, chemical composition, oxidation state and chemisorption capacity were determined by X-ray photoelectron spectroscopy and sorptometry of N2, O2 and CO gas molecules. The catalytic CO oxidation activity was tested, using a gas circulating system equipped with a fixed-bed microreactor and a gas chromatograph. The CuOx additives were found to promote markedly the otherwise insignificant CO oxidation activity of WO3 at 250–400°C. This was related to concomitant improvement primarily in the lattice oxygen reactivity, and in the O2 chemisorption capacity. The already high CO oxidation activities of Mn2O3 and Cr2O3 catalysts at 150–250°C were found to be, respectively, slightly and hardly improved by CuOx additives, despite a marked improvement in oxygen chemisorption capacity. This was attributed to establishment on the unmodified catalysts of a favourable electron-mobile environment. In oxygen-rich reaction atmosphere, both the modified and unmodified catalysts were thermochemically stable. However, in lean oxygen atmosphere, Cr2O3 and CuOx-modified WO3 were relatively the most stable test catalysts.

CO and CH4 total oxidation over manganese oxide supported on ZrO2, TiO2, TiO2–Al2O3 and SiO2–Al2O3 catalysts

Zirconia, titania, titania–alumina and silica–alumina supported and unsupported MnOx catalysts were prepared and characterized by X-ray diffractometry and photoelectron spectroscopy, infrared spectroscopy and nitrogen sorptometry. Their catalytic activities were tested towards total oxidation of carbon monoxide and methane. The results show the unsupported MnOx (exposed on an α-Mn2O3 bulk structure) to catalyze CO oxidation at ⩽250°C and CH4 oxidation at 250°C, and to remain chemically and structurally stable. The CO oxidation activity of MnOx is improved when dispersed on zirconia, whereas its CH4 oxidation activity is improved on silica–alumina. These results may help in concluding that (i) CO oxidation is not the rate determining step of the oxidation of CH4, (ii) availability of strong acid sites (as those exposed on silica–alumina) is important for CH4 oxidation and (iii) the difference in the catalytic activity towards the oxidation of CO and CH4 resides in the need for different catalytic functions for each reaction, which are, therefore, not related in terms of kinetic control.

Stability of surface chromate – A physicochemical investigation in relevance to environmental reservations about calcined chromia catalysts

Abstract The present investigation presents and correlates observed and reported characterization results of calcined, supported and unsupported chromias obtained via various precursor compounds. Studies performed employed a range of surface and bulk analytical techniques, in hopes of a proper assessment of the stability of surface chromate (Cr(VI)–O) species thus generated to thermal decomposition, hydrolysis and reduction. Impacts on the redox catalytic activity were probed towards the decomposition of H 2 O 2 solutions and the CO oxidation in the gas phase. The results have revealed that chromate species established on bulk and dispersed α-Cr 2 O 3 particles enjoy high stability against thermal and chemical treatments, and yet contribute to the formation of surface sites that are catalytically active in redox reactions. This should help lessening environmental reservations about the industrial application of calcined chromia catalysts.

Structures and dipole moments of non-ionic nitrite rotamers

Abstract Results related to the geometric structures and dipole moments of hydrogen nitrite, HONO, and nitritomethane, H 3 CONO, are presented. A novel procedure for constructing dipole moments for polynuclear molecules from vector sums of contributions from dinuclear fragments has been devised to include an explicit separation dependence; its validity is discussed.

Kinetic and characterization studies of the formation of barium monomolybdate in equimolar powder mixture of BaCO3 and MoO3

The formation of barium monomolybdate (BaMoO 4 ) in equimolar powder mixtures of BaCO 3 and MoO 3 was examined under isothermal and nonisothermal conditions upon heating in air at 25-1200 °C, using thermogravimetry. Concurrence of the observed mass loss (due to the release of CO 2 ) to the occurrence of the formation reaction was evident. Accordingly, the extent of reaction (x) was determined as a function of time (t) or temperature (T). The x-t and x-T data thus obtained were processed using a well-established mathematical apparatus and methods to characterize the nature of the reaction rate-determining step and derive isothermal and nonisothermal kinetic parameters (rate constant, frequency factor, reaction order, and activation energy). Moreover, the reaction mixture quenched at various temperatures (450-575 °C) in the reaction course was analyzed by various spectroscopic (x-ray diffractometry, infrared spectroscopy, and laser Raman spectroscopy) and microscopic (scanning electron microscopy and x-ray energy dispersive spectroscopy) techniques for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional diffusion of MoO 3 species through the product layer (BaMoO 4 ), which was implied to form on the barium carbonate particles. The nonisothermally determined activation energy (156 kJ/mol) was found to be close to the isothermally determined one (164-166 kJ/mol).

Theoretical Study of the Adsorption of 2-Propanol onto Silica Surfaces on the Basis of Ab Initio and Density Functional Calculations

The adsorptive interactions of 2-propanol (2-PrOH) molecules with silica surfaces were investigated using ab initio and density functional calculations. Two cluster models of silica were chosen to represent the terminal ≡SiOH groups and the siloxane bridges ≡Si–(O)2–Si≡ on the silica surface. The Hartree–Fock (HF) and Density Functional Theory (DFT) approaches, employing a 6–31G(d) basis set, were used to calculate the geometries, electronic structures, vibrational frequencies and adsorption energies of the adsorption complexes formed. The calculated adsorption energies were corrected for zero-point vibrational energies (ZPVE) and basis set superposition errors (BSSE). The results favoured a most likely surface configuration for the physisorbed species in which 2-PrOH molecules are bound to exposed silanol groups via two hydrogen bonds, with the alcoholic –OH group acting simultaneously as a proton acceptor and donor. Moreover, bonding of 2-PrOH with strained surface siloxane bridges (≡Si–O–Si≡) was shown to lead to chemisorption of the alcohol molecule. These findings have been shown to help interpreting reported infrared spectroscopic results of in-situ experimental studies.

Standard nitrogen adsorption data for manganese oxide and like (CBET = 40-60) surfaces

Calcination to 1170 K of materials obtained by mild and intensive reduction of KMnO4 and HMnO4 solutions was found to produce manganese oxides assuming the crystalline bulk structure of α-Mn2O3 Nitrogen adsorption isotherms at 77 K revealed that they also assume non-porous surfaces of low specific area (4 ± 1 m2/g) with CBET equal to 40–60; t- and αs-curves derived therefrom were found to coincide with those determined reportedly on non-porous surfaces of 11 < CBET < ∞, irrespective of the surface chemical similarity. The experimental and reported standard adsorption data were used to analyze a type-IV nitrogen adsorption isotherm determined at 77 K on a porous manganese oxide (δ-MnO2) which was obtained by sol–gel processing of a KMnO4 solution, and found to similarly reveal a mesoporous surface of SBET = 73 m2/g and CBET = 125. The close agreement between the surface area values (St and Sα) derived for the test material (δ-MnO2) from the t- and αs-plots thus obtained and its CBET value (73 m2/g) was the basis on which reference materials with 11 < CBET < ∞ were found to be appropriate. Since most of the appropriate reference materials were dissimilar chemically to the test material, the results of the present investigation may imply that the surface chemical similarity demanded by the αs-method is better manifested in terms of the surface activity (heat of adsorption monitored by the CBET value) than the chemical composition.