Vicente Sanchez Escribano

An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

Abstract The adsorption and the oxidation of ammonia over sub-monolayer TiO2-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NOx by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SCO process). For comparison, the adsorption of ammonia and hydrazine over the TiO2-anatase support has also been studied. CrOx TiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe2O3 TiO2, CrOx TiO2, CoOx TiO2 and CuO TiO2, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions,N−2. NiOx TiO2 and MnOx TiO2 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SCO processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SCO activity.

Characterization of manganese and iron oxides as combustion catalysts for propane and propene

Abstract Mn2O3–Fe2O3 powders have been prepared by a coprecipitation method. The pure compounds have been characterized as constituted of α-Fe2O3 (haematite) and α-Mn2O3 (bixbyite) while the mixed oxides are constituted of a mixture of haematite- and bixbyite-structure solid solutions. The observed reciprocal solubilities calculated from the XRD patterns approach the thermodynamic ones. α-Mn2O3 is more active than α-Fe2O3 as catalyst for both propane and propene oxidation. However, α-Mn2O3 is less active than Mn3O4 powder. Propene oxidation is in all cases very selective to CO2 while propane oxidation gives rise to significant amounts of propene on α-Fe2O3. Mn2O3–Fe2O3 powders are slightly more active than α-Mn2O3 as combustion catalysts. The selectivities to propene upon propane oxidation decrease with increasing Mn content.

FT Raman and FTIR studies of titanias and metatitanate powders

FT Raman and FTIR/FTFIR skeletal spectra of different TiO2 powders, of both synthetic and commercial origin, and of Sr, Ba, Co and Ni metatitanates are reported and discussed in relation to the predictions of factor group analyses. The ability of vibrational techniques to show the presence of brookite impurities in both anatase and rutile, and of rutile in anatase, as well as to give morphological and surface information is emphasized. The spectra of SrTiO3, BaTiO3, NiTiO3 and CoTiO3 are also discussed in relation to their different structures (pervoskite and ilmenite-type).

FT-113 study of the surface properties of the spinels NiAl2O4 and CoAl2O4 in relation to those of transitional aluminas

Abstract The surface chemistry of δ-Al2O3, NiAl2O4, and CoAl2O4 has been investigated comparatively by FT-IR spectroscopy using adsorbed carbon monoxide and pyridine as probe molecules. The surface OHs of the inverse spinel NiAl2O4 are very similar to those of δ-Al2O3 and other transitional aluminas while those of CoAl2O4 are different and resemble those of other normal spinels. Both carbon monoxide and pyridine adsorption provide evidence for the presence of tetrahedral Al3+ ions exposed on the surface of the inverse spinel NiAl2O4. On the normal spinel CoAl2O4 octahedral Al3+ ions predominate with respect to the tetrahedral ones, also present because of partial inversion. Ni2+ and Co2+ ions are also detected on the respective surfaces. Slight modifications to the literature criteria are suggested for the assignments of the IR bands to surface OHs and of adsorbed pyridine on aluminas.

FTIR studies on the selective oxidation and combustion of light hydrocarbons at metal oxide surfaces. Part 2.—Propane and propene oxidation on Co3O4

The interaction of propane and propene and of C3, C2 and C1 oxygenates (propane-1-ol, propan-2-ol, allyl alcohol, acetone, propanal, acrolein, acrylic, propionoic, acetic and formic acids and CO2) on oxidized Co3O4 +x has been studied by FTIR spectroscopy, with the aim to collect data on the mechanism of hydrocarbon catalytic combustion. Propene is already oxidized to acrylate species at RT and burns completely starting from 473 K. Propane already gives rise to acetate, propanoate and acrylate species below 373 K. A reaction scheme for catalytic propane oxidation is proposed. The behaviour of Co3O4 +x combustion catalyst surfaces are compared with those of MgCr2O4 +x, previously investigated. In both cases nucleophilic oxygen species (lattice O2– anions at the oxidized surface) are thought to be involved in catalytic combustion, which occurs via overoxidation of adsorbed partially oxidized compounds.

Thermal stability of vanadia-titania catalysts

The thermal behaviour of seven different ‘pure’ TiO2(anatase) preparations, as well as of materials obtained by doping them with potassium sulfate and carbonate, silica, and the oxides of tungsten, molybdenum and vanadium has been investigated by TG-DTA, XRD, FTIR and surface-area measurements. Vanadia–titania catalysts prepared by impregnation of these supports have also been investigated by the same techniques. The temperature at which anatase sintering and phase transformation to rutile occur strongly depends on the morphology of the TiO2 preparation. The anatase phase is much less stable in high-area, highly porous materials than in low-area powders. Vanadium oxide speeds up the anatase-to-rutile transition. However, common catalyst additives like silica, tungsten oxide and alkali-metal carbonates and sulfates strongly slow down both anatase sintering and its transformation to rutile. These phenomena influence the formulation of additives for vanadia–titania (anatase) catalysts for both selective oxidation and reduction of NOx.

Anatase crystal growth and phase transformation to rutile in high-area TiO2, MoO3–TiO2 and other TiO2-supported oxide catalytic systems

Anatase-to-rutile phase transition has been studied on high-area TiO2-anatase and bicomponent systems containing molybdena and, for comparison, cobalt oxide, copper oxide, vanadia and corresponding TiO2-rutile based systems, using TG–DTA, XRD and surface-area measurements. These materials were prepared by impregnation and (co)precipitation methods. Conversion to rutile and crystal size growth strongly depend on the minority phase oxide. It is found that, while Cu and V oxides speed up both phase transition and particle sintering, Mo and Co oxides inhibit them. Moreover, the rutile particles obtained by phase transition are always much larger than both the starting and residual anatase particles. A mechanism of rutile formation by coalescence and phase transformation of anatase particles is proposed.

Solid-state and surface chemistry of CuO–TiO2(anatase) powders

CuO–TiO2 samples have been prepared by impregnation of two different preformed TiO2 supports with copper nitrate, and have been characterized by X-ray diffraction (XRD), thermogravimetry–differential thermal analysis (TG–DTA), Fourier-transform infrared (FTIR), Fourier-transform far-infrared (FT-FIR) and ultraviolet–visible (UV-VIS) spectroscopies. The surface properties have been investigated through surface area and porosity measurements, and FTIR spectra of surface OH groups and of adsorbed carbon monoxide at low temperature. The surface is thought to consist of complexes of Cu2+ and Cu+, involving surface Ti–O groups of the support as the ligands. These complexes can be reduced by CO under very mild conditions with the appearance of small Cu metal clusters. Ti4+ centres are also in part exposed on the surface, but their electron-withdrawing effect is weakened by the surface copper complexes. Surface copper ions act as electron-donor centres, giving rise to absorption over in the entire visible range, with a parallel weakening of the TiO2, absorption edge. Surface copper complexes have a pronounced accelerating effect on the anatase-to-rutile phase transition. A parallelism between the CuO–TiO2 and the V2O5–TiO2 systems is proposed.

An FT-IR and flow reactor study of the selective catalytic oxy-dehydrogenation of C3 alcohols on Mn3O4

Abstract The C3 alcohols 1-propanol, 2-propanol and prop-2-en-1-ol (allyl alcohol) are catalytically oxidized in He/oxygen to the corresponding carbonyl compounds propanal, acetone and acrolein with high selectivities and yields over Mn 3 O 4 . The reaction occurs in the temperature range 400–550 K and yields far over 85% are obtained in the synthesis of acrolein and acetone. The reaction rate is faster for the secondary alcohol 2-propanol, than for the primary ones. The oxidation of the unsaturated allyl alcohol is faster than for the saturated one, 1-propanol. Selectivity is limited by overoxidation of the carbonyl compounds. IR studies show that the high yields obtained in carbonyl compounds production over this catalyst are mainly due to their very weak adsorption. The main factor limiting the selectivity to acetone and, mainly, propanal is their tendency to give enolate anions that further convert into acetaldehyde in both cases. Overoxidation of aldehydes to the corresponding car☐ylate species is a less efficient mechanism limiting selectivity. The selectivity/activity behavior is definitely different than that observed for stoichiometric alcohols oxidations with Mn(III) compounds and also than that observed on other catalysts. This difference is associated to the low surface acidity of the Mn 3 O 4 catalyst.

Preparation and characterization of Fe2–xCrxO3 mixed oxide powders

Mixed oxides of the general formula Fe2–xCrxO3 have been prepared by a coprecipitation–calcination procedure. The precursors contain a crystalline goethite-like phase and an amorphous trihydroxide phase if Cr is present. Calcnnation for 5 h at 673 K resulted in crystalline powders whose surface areas decrease from 102 to near 20 m2 g–1 with increasing Cr content. IR spectra suggest that the structure of these materials consists of a superstructure of the corundum-type lattice of haematite. In parallel with the decrease of surface area, the smallest slit-shaped pores arising from goethite-to-haematite topotactic decomposition progressively disappear. Cr is observed to form chromate species, some of which are exposed on the surface of the calcined materials. These species are thought to have a role in the catalytic combustion activity of these materials. The optical behaviour of the powders in the IR, visible and UV regions is discussed.