Frank S. Stone

An experimental and theoretical evaluation of surface states in MgO and other alkaline earth oxides

Abstract Ultra-violet (u.v.) diffuse reflectance spectra of microcrystalline MgO, CaO, SrO and BaO reveal bands at energies below those of bulk excitonic transitions. Correlation of the spectra permit three distinctive absorptions (I, II, III) to be identified, each with similar characteristics in the respective oxides. The absorptions are ascribed to surface states, and the transitions have the characteristics of excitons. Excitons II and III obey the Mollwo–Ivey relation typical of bound excitons, but excition I with the highest energy of transition behaves similarly to a free exciton of the bulk. The four oxides are isostructural and ionic. The Levine–Mark theory of the surface states of ionic insulators, not hitherto tested for oxides, is examined. The numerical values of the parameters appropriate to the oxides are evaluated by using experimental data on bulk excitons. It is shown that the theory accounts satisfactorily for the energies of exciton I in MgO, CaO and SrO on the basis that it is an exci...

Copper–zinc oxide catalysts. Activity in relation to precursor structure and morphology

Cu–Zn hydroxycarbonates have been studied as precursors of Cu–ZnO catalysts, with particular reference to the effect on catalyst activity of ageing the precursor prior to decomposition and reduction. The precursor obtained by precipitation from mixed nitrate solution (Cu/Zn molar ratio 2:1) at 333 K and pH 7.0 consisted of zincian malachite (Cu/Zn ≈ 85:15) and aurichalcite. The precursor was aged in the mother liquor at 333 K for various times. Characterisation by XRD, i.r., DTA, electron microscopy, EDAX and XPS showed that ageing led to loss of the aurichalcite and production of a more finely divided copper-enriched (Cu/Zn = 2:1) malachite phase. The unaged precursor yielded a catalyst of low activity for both methanol synthesis (studied at 50 bar and at 1 bar) and the reverse water-gas shift reaction. The aged precursor gave catalysts of much higher activity for both reactions. Increased ageing did not change the selectivity ratio for methanol synthesis vs. reverse shift in the CO2+ H2 reaction at normal pressure.

The reactivity of oxides with water vapor

Abstract A Tian-Calvet microcalorimeter has been used to determine the heat of interaction of water vapor with a range of oxides at 303 K. New data for SiO 2 , TiO 2 , ZnAl 2 O 4 and CaO are reported which, together with already established results for Al 2 O 3 , enable an overall view to be given of the hydrophilic behavior of oxides. The change of the differential heat of adsorption with uptake for a given oxide allows the proportions of surface with high and low hydrophilicity, and with hydrophobicity if present, to be assessed. For the highly hydrophilic oxide CaO, the interaction occurs in depth, but this is suppressed when Mn ions are introduced in solid solution in CaO. The change from hydrophilic to hydrophobic behavior in oxides is correlated with the gradation from ionic to covalent bonding.

The significance for oxide catalysis of electronic properties and structure

Abstract The reasons why the application of band theory as a basis for understanding catalysis by oxide semiconductors has met with limited success are reviewed. They include the limitations of the theory itself, the complexity of crystal imperfections in oxides, and the extreme versatility of oxide surfaces to form with any given molecule a variety of chemisorbed species. It is pointed out, however, that certain important concepts survive, notably charge transfer adsorption and the control of valence and stoichiometry. The band theory approach has been superseded by developments emphasizing crystal structure, with chemisorption viewed as a process for restoring, either partially or completely, the interruption of structure brought about by the formation of a surface. Models for chemisorption and catalysis stemming from this concept of coordinative unsaturation are more easily able to incorporate chemical knowledge of bonding and molecular structure. The review concludes by referring to current researches on oxides which emphasize the role of localized centers for catalysis. Special attention is given to oxide solid solution prepared by dissolving a transition metal oxide in an insulating diamagnetic oxide as matrix. This enables catalysis to be studied with control over the electronic interaction between the active transition metal ions. Catalytic activity develops strongly even in dilute solutions, and it is possible to assess for particular reaction types the extent to which electronic interaction between transition metal ions is harmful or beneficial.

Cu–ZnO and Cu–ZnO/Al2O3 Catalysts for the Reverse Water-Gas Shift Reaction. The Effect of the Cu/Zn Ratio on Precursor Characteristics and on the Activity of the Derived Catalysts

Comparison is made between Cu–ZnO and alumina-supported Cu–ZnO as catalysts for the reverse water-gas shift (RWGS) reaction. For both types of catalyst the Cu/Zn ratio has been varied between Cu-rich and Zn-rich compositions. By applying X-ray diffractometry, X-ray line broadening, optical reflectance spectroscopy and other techniques the effects on the structural and physical properties of the hydroxycarbonate precursors, the calcined products and the ultimately derived catalysts are determined. The presence of alumina decreases the crystallite size of the CuO and ZnO particles produced on calcination and at high Cu/Zn ratios increases the dispersion of copper in the final catalyst. The activities of the catalysts for the RWGS reaction at 513K are compared and the most active are shown to be those which are Cu rich (Cu/Zn > 3) and contain alumina as support. The activities of all the catalysts can be rationalized by referring the activity to unit surface area of copper metal.

Reflectance spectra of surface states in magnesium oxide and calcium oxide

The ultraviolet and visible reflectance spectra (52 000–15 000 cm–1) of high surface area MgO (250–300 m2 g–1) and CaO (100–200 m2 g–1) have been studied in vacuo and in the presence of O2, N2O, CO2 and H2O. The effect of outgassing MgO and CaO at increasing temperatures (773–1073 K) is to develop u.v. absorption bands in the range 30 000–50 000 cm–1. The absorption can be progressively destroyed by chemisorption, and also by sintering. The bands are attributed to excitons bound in surface states. In vacuo the outgassed oxides exhibit a fluorescence which is readily quenched by exposure to a low pressure of oxygen. Absorption bands occur at 37 000 and 46 000 cm–1 for MgO and at 35 500 and 44 500 cm–1 for CaO. With each oxide the chemisorption of gases erodes the lower energy band preferentially, and a similar effect occurs on sintering. The two bands are ascribed to exciton absorption at surface oxide ions in different states of coordinative unsaturation, the lower energy exciton band corresponding to the lower coordination.The influence of gases on the exciton spectra of outgassed MgO and CaO is shown to depend on their chemical reactivity, increasing in the sequence O2, N2O, CO2 and H2O. The effects with each gas are reversible on outgassing. Exposure of CaO to O2 gives rise to an absorption band at 23 500 cm–1, tentatively attributed to an O–3 adsorbed species.The surface states observed on CaO can be produced on MgO by adsorbing Ca ions from solution and heating to 1073 K.

Thermal decomposition of managanese oxyhydroxide

Abstract Temperature-programmed thermal decomposition of γ- and α-manganese oxyhydroxide has been studied between 20 and 670°C under vacuum and under a low pressure (10 Torr) of oxygen. Solid products at various temperatures have been analyzed by X-ray diffractometry. Under vacuum γ-MnOOH decomposed below 400°C to a mixture of Mn5O8, α-Mn3O4, and water according to the reaction scheme: 8MnOOH → Mn5O8 + Mn3O4 + 4H2O. Above this temperature Mn5O8 was converted to α-Mn3O4 as a result of oxygen removal. The vacuum dehydration at 250°C of oxyhydroxide rich in α-MnOOH led to the formation of a new modification of Mn2O3 isostructural with corundum (α-Al2O3). In oxygen both oxyhydroxides decomposed to β-MnO2. γ-MnOOH transformed directly to β-MnO2 while α-MnOOH appeared to transform via corundum-phase Mn2O3 as an intermediate.

Reflectance spectra of carbon monoxide adsorbed on alkaline earth oxides

The adsorption of carbon monoxide at room temperature on well-outgassed specimens of MgO, CaO, SrO and BaO gives rise to a strong absorption spectrum between text-decoration:overlineν= 15000 and text-decoration:overlineν= 40000 cm–1. The stronger the outgassing conditions, the more intense is the absorption which can be developed.The spectrum consists of two main envelopes, one comprising absorption at text-decoration:overlineν= 29000–40000 cm–1 and the other, more composite in character, at text-decoration:overlineν= 15000–29000 cm–1. Changes in the CO pressure affect the two spectral regions differently, showing that two different kinds of adsorbed species are involved.The formation of the new spectrum as CO is dosed is accompanied by a simultaneous decrease in the absorption due to surface states (coordinatively unsaturated surface oxide ions). The effect on the oxide absorption is more pronounced with SrO and CaO than with MgO. The bulk states are unaffected, showing that the process is entirely a surface reaction.The results are interpreted as an electron donor process in which CO accepts electrons from surface oxide ions and forms anionic clusters in the adsorbed state. The high-text-decoration:overlineν absorption (ν > 29 000 cm–1) is ascribed to the dimer (CO)2–2, and the low-text-decoration:overlineν absorption to polymeric oxocarbon anions (CO)xn, where n > 2. The latter are considered to include cyclic, resonance-stabilized structures. The reactivity of the oxide towards CO increases with increase in basic character: on the MgO surface there are relatively few sites of sufficient donor potential to react with CO, but on the more strongly basic CaO and SrO the reaction is much more extensive.

Surface properties of alkaline earth oxides as studied by UV-visible diffuse reflectance spectroscopy

Abstract The merits of UV-visible diffuse reflectance spectroscopy for the study of surface phenomena on microcrystalline MgO, CaO and SrO are described and illustrated. It is shown that surface states ascribable to anions in 5-, 4- and 3-coordination can be observed and their respective interactions with adsorbed gases investigated. The behaviour of a series of XH molecules towards heterolytic dissociative chemisorption are reviewed, and shown to correlate with their acidities. With ammonia, where non-dissociative adsorption can accompany heterolytic chemisorption, the surface anions of the alkaline earth oxides exhibit strong UV charge-transfer absorption. Analogy is drawn with the behaviour of ammonia in developing charge-transfer-to-solvent spectra with anions in solution. Finally, the propensity of the alkaline earth oxides to produce anions from adsorbed molecules is treated further, and special reference is made to pyridine and to carbon monoxide, where UV-visible spectra show that adsorbed anions couple to give more complex resonance-stabilized structures.

Physico-chemical properties of MnCO3–CaCO3 and MnO–CaO solid solutions

MnCO3–CaCO3 has been shown to form a continuous phase isomorphous with calcite as the Mn concentration is varied across the whole range of composition from CaCO3 to MnCO3. Magnetic susceptibility and e.p.r. studies show that the manganese-containing carbonate is paramagnetic, and diffuse reflectance spectroscopy confirms octahedral coordination for the Mn2+ ions.The decomposition of the MnxCa1–xCO3 phase to form the oxide has been investigated at five compositions between x= 0 and x= 1 with a view to preparing MnxCa1–xO solid solutions in a finely divided state. It is shown that this can be achieved by controlled decomposition in vacuo at 925 K. X-ray powder diffraction confirms the expected variation of unit-cell parameter with Mn content in the cubic MnxCa1–xO phase. A special Gouy cell was devised to determine magnetic susceptibility without exposure to the atmosphere. The temperature range studied (77–300 K) included the Neel temperature for MnO, and attenuation of superexchange interactions as Ca2+ ions replace Mn2+ ions is manifested by a rapid decrease of the Weiss constant and by a decrease of linewidth in the e.p.r. spectrum. Mn2+ is present in the high-spin 6S5/2 state (µ= 5.9µB) in all the solid solutions.The transformation of carbonate to oxide has been followed by diffuse reflectance spectroscopy in vacuo. MnxCa1–xCO3(x= 0.65) is shown to decompose rapidly at 675–750 K, but there is evidence for an intermediate state to be passed before the MnxCa1–xO solid solution is produced.