Lata Pasupulety

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.

Thermochemistry of manganese oxides in reactive gas atmospheres : Probing redox compositions in the decomposition course MnO2 → MnO

Abstract The thermal decomposition course MnO2 → MnO was examined in various gas atmospheres (O2, air, N2 and H2) by temperature-programmed studies employing thermogravimetry and differential thermal analysis. Weight-invariant thermal events encountered were subjected to non-isothermal and isothermal kinetic analysis. Product analysis was carried out using infrared spectroscopy and X-ray diffractometry. Cyclic TG experiments carried out in air have revealed that, of the intermediate decomposition products characterized, viz. Mn5O8, Mn2O3 Mn3O4, the mixed-valence Mn3O4 (= Mn(II)Mn2(III)O4) can tolerate reversible oxygenation-deoxygenation processes at (500–1050°C). Moreover, the presence of Mn(II) in the mixed-valence Mn5O8 (= Mn2(II)Mn3(IV)O8) is seen to sustain a synproportionation of Mn(II) Mn(IV) during the oxide deoxygenation, giving rise to Mn(III) species (= Mn2O3). The electron-mobile environment thus established in such mixed-valence oxides is seen to promise a catalytic potential in oxidation/reduction reactions.

Low-temperature Synthesis of Hausmannite Mn3O4

[15, 16] resulted in a major product thatcontained varied amounts of â-MnOOH (Feitknech-tite), depending in whether the oxidant was merelyair or pure oxygen atmosphere and on the exposureperiod to the oxidizing atmosphere [15, 16]. Employ-ing X-ray diffractometry, the major product wassometimes identified as being Mn

Bulk and surface characteristics of pure and alkalized Mn2O3: TG, IR, XRD, XPS, specific adsorption and redox catalytic studies

α-Mn2O3 (containing a minor proportion of Mn5O8) was obtained by calcination of pure MnO2 at 700°C for 2 h. It was alkalized by impregnation of the parent dioxide with potassium and barium nitrate solutions prior to the calcination. K-Mn2O3 (α-Mn2O3+KMn8O16) and Ba-Mn2O3 (α-Mn2O3) thus respectively produced were subjected, together with the unmodified Mn2O3, to the title bulk and surface characterization techniques. It has been implied that the alkalization improves the electron density and the mobility of lattice and surface oxygen species. As a result, the bulk thermochemical stability is reduced on heating in a CO atmosphere, and a capacity towards CO2 uptake is developed. Moreover, the surface catalytic behaviour towards CO oxidation in the gas phase is maintained, and the behaviour towards H2O2 decomposition in the liquid phase is considerably promoted.

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.

Thermochemistry of manganese oxides in reactive gas atmospheres: Probing catalytic MnOx compositions in the atmosphere of CO+O2

Abstract A series of Mn-oxides in the range MnO 2 –MnO were subjected to thermogravimetry on heating up to 1100°C in the reactive gas atmosphere of O 2 , CO and CO+O 2 . Some selected solid residues were analyzed for the MnO x composition by infrared spectroscopy, and for carbon deposits by CHNS-analyzer. The CO+O 2 gas stream was chromatographically analyzed, following heating the gas/solid interface to some selected temperatures. The results indicate that Mn-oxides in the range MnO 2 –Mn 1.3 exhibit catalytic properties towards the CO oxidation in the gas phase. The higher thermal stability (up to ∼1000°C) of compositions in the range MnO 1.5–1.3 , i.e. in the Mn 2 O 3 –Mn 3 O 4 system, may render them particularly interesting in the field of applied catalysis. The experimental approach adopted in the present investigation may help accounting for the interfacial chemistry of solid-state reactions involving the surrounding gas atmosphere.