Manlio Occhiuzzi

The nature of surface chromium species on CrOx/ZrO2 catalysts

Abstract The nature and stability of chromium species on the surface of CrOx/ ZrO2 catalysts (Cr content, 0.05 – 5 wt.%) have been investigated by means of ESR, IR and XPS spectroscopies. On samples heated in oxygen at 773 K or at 923 K, chromium is present on the surface as CrV (ESR) and CrVI (IR and XPS). Experiments with the 53Cr isotope show that the species observed by ESR spectroscopy is a surface mononuclear chromyl complex in a square-pyramidal configuration. Surface chromates are demonstrated by IR spectroscopy. In more concentrated samples, polynuclear chromium species, and/or CrVI and CrV polychromates, are observed in addition to chromates and isolated CrV Upon reduction with CO or H2 at increasing temperatures, CrV is reduced to CrIII (ESR and IR) even at ca. 383 K. On further reduction at higher temperatures (up to 623 K), CrVI is also reduced to CrII. Specifically, samples reduced at 623 K contain CrIII and CrII in nearly equal amounts. The catalytic pattern of the CrOx /ZrO2 system, examined in the terms of the chromium species, allows us to conclude that isolated CrIII species, arising from the reduction of CrV, are the active sites for H2-D2 equilibration and propene hydrogenation. The role of CrII, either isolated or as dimers, is ruled out in the case of CrOx/ZrO2 catalysts. The zirconia matrix stabilizes the CrIII active species in surface sites of high coordinative unsaturation.

XPS and EPR study of high and low surface area CoO–MgO solid solutions: surface composition and Co2+ ion dispersion

Solid solutions CoO–MgO (MCo) of low and high surface area (LSA and HSA) have been studied by XPS and EPR analysis to evaluate the surface composition and the dispersion of cobalt ions in the systems. Different preparation routes were adopted: wet impregnation (Iw), dry impregnation (Id) and coprecipitation (C). LSA solid solutions (As= 10–20 m2 g–1) were obtained after treatment in air at 1173 K (1173 LSA) or at 1473 K (1473 LSA), HSA materials (As= 200–300 m2 g–1) after treatment under vacuum at 1173 (1173 HSA). XPS intensity ratios, ICo/IMgvs. xCo/xMg molar fraction showed no appreciable differences between the bulk and surface compositions of LSA and HSA solid solutions from Id and C precursors, calcined at 1173 and at 1473 K. MCo 1173 LSA Iw samples had evident cobalt enrichment, due to Co3O4 segregation, whereas Iw HSA did not. NO2 or O2 treatments led to the formation of CoIII. The Co2+ bulk dispersion in LSA and HSA was studied by EPR.

Manganese ions in the monoclinic, tetragonal and cubic phases of zirconia: an XRD and EPR study

Mn-doped monoclinic, tetragonal and cubic zirconia samples were characterized by XRD and EPR. The cubic modification was obtained by doping hydrous zirconia with Y2O3 (YSZ). The ZrO2 structure influences the solid solution formation and the nature of manganese species. EPR analysis revealed the following manganese paramagnetic species: isolated Mn4+ and Mn2+ in the monoclinic phase; isolated Mn2+ in the tetragonal phase; isolated Mn2+ in the cubic YSZ; and clustered Mn2+ in Mn3O4 and in MnO particles on the zirconia surface. Quantitative EPR suggested Mn3+ in all zirconia phases. After heating in air, Mn3+ and Mn4+ ions entered the monoclinic zirconia phase at 1273–1623 K and surface Mn3O4 particles formed, whereas Mn3+ and Mn2+ entered the tetragonal zirconia phase at 973–1173 K and at 1623 K into cubic YSZ. In all zirconia phases, subsequent heating in H2 at 773–973 K reduced Mnn+ ions to Mn2+ and converted Mn3O4 particles on the surface into MnO. In monoclinic zirconia heated in air at 1623 K the Mn-solubility limit was 0.2 wt.%.

Structure of Crv species on the surface of various oxides : reactivity with NH3 and H2O, as investigated by EPR spectroscopy

Samples containing chromium (both 53Cr-enriched and non-enriched) have been prepared by equilibrium adsorption or impregnation methods at low loadings (<0.5%) using ZrO2, γ-Al2O3, SnO2, TiO2(anatase) and SiO2 as supports. Heating in O2, generally at 773 K, yielded mononuclear Crv species in a square-pyramidal configuration, Crv5c(A), on all supports with the exception of SiO2 where Crv is in a tetrahedral configuration, Crv4c(A). H2O or NH3, both at room temperature (RT), yielded the Crv6v(A) species from Crv5c(A), that is, the complex changes its coordination from five to six. After H2O adsorption and evacuation at RT, Crv5c(A) is reversibly restored; however, NH3 adsorption and evacuation at increasing temperature gives a new Crv species at 473 K. In the EPR signal of this species, designated Crv5c(B), the perpendicular component is split into three lines with 14NH3(A14N= 4.0 G) and two lines with 15NH3(A15N= 5.5 G). The species is therefore assigned to a chromyl complex with an equatorial O2– ligand replaced by a nitrogen-containing NHx–3x species, possibly the NH–2 anion. With H2O or NH3 at RT, the Crv5c(B) species is transformed into the corresponding hexacoordinated species, Crv6c(B).Upon adsorption of small H2O doses on the CrOx/SiO2 sample, the Crv4c(A) species is transformed into Crv5c(A) and Crv6c(A). In the presence of excess water, the chromyl species on SiO2 becomes unstable, undergoing disproportionation to CrIII and CrVI On adsorption of NH3, Crv5c(B) is formed from Crv4c(A) at RT. With 15NH3, a small superhyperfine interaction with nitrogen is partially resolved. Computer-calculated spectra enable us to assign the Crv5c(B) species on SiO2 to a slightly distorted chromyl complex with slightly non-planar equatorial ligands.

Thermal stability and chemical reactivity of (O–2)s species adsorbed on MgO surfaces

The thermal stability and the chemical reactivity of (O–2)s species adsorbed on a MgO surface have been studied by e.s.r. spectroscopy over a wide range of experimental conditions. The (O–2)s radicals (0.4–3.3 × 1015 spins m–2, depending on the activation temperature) were reproducibly formed according to the following sequence: (i) MgO was heated in vacuo at 873–1173 K, (ii) H2(1 kN m–2) was added at 298 K, (iii) O2(1 kN m–2) was added at 298 K.The thermal stability has been studied by heating specimens containing (O–2)sin vacuo up to 550 K. At this temperature the radical was destroyed, but it could be restored to some extent by first adding H2 and then O2. The chemical reactivity of (O–2)s at different temperatures (298–550 K) with CO, H2, C2H4, NH3 and H2O-vapour was also investigated.A volumetric determination of H2 and O2 adsorption on MgO evacuated at 1173 K has also been performed. A comparison of the oxygen coverage values so obtained, with those determined by e.s.r., indicates that practically all the oxygen is adsorbed as (O–2)s.Three different, although closely related, electron donor centres at the MgO surface are proposed.

Quantitative EPR spectroscopy: Comparison between primary standards and application to MgO-MnO and α-Al2O3-Cr2O3 solid solutions

To study their reliability as primary standards in the quantitative EPR spectroscopy, a large series of pure paramagnetic compounds with known spin concentrations, whose spectra vary considerably in intensity, shape, structure and overall width are compared. The paramagnetic species examined as pure solid compounds and solutions, were free radicals (DPPH and TEMPO), vanadyl and Cu2+ ions (S = 1/2), Cr3+ (S = 3/2) and Mn2+ (5 = 5/2) ions. The quantitative EPR findings suggest that all theS = 1/2 paramagnetic compounds investigated and MnSO4 · H2O (S = 5/2) are reliable primary standards. By contrast, none of the pure Cr3+ compounds proved useful as primary standards because of their large fine-structure terms or high Néel temperature that invalidated the simple Curie law. Application of quantitative EPR in the study of dilute MgO-MnO and α-Al2O3-Cr2O3 solid solutions, focussing on the circumstances making paramagnetic species undetectable, is reported. In MgO-MnO solid solutions of high surface area, detection problems arising from the variation of local site symmetry can be circumvented and almost all Mn2+ are detected only by reducing the surface area. In concentrated α-Al2O3-Cr2O3 solid solutions, magnetic interactions lead to paramagnetic species being undetectable.

Electron spin resonance and volumetric investigations of oxygen adsorption on high surface area CoO–MgO

Oxygen adsorption on high surface area (h.s.a. 200 m2 g–1) CoO–MgO samples (cobalt content from 0.05 to 5 atoms per 100 Mg atoms) has been investigated by e.s.r. spectroscopy and volumetric techniques. After activation under vacuum at 873–1173 K and oxygen adsorption the samples show two main e.s.r. signals: the first, for which hyperfine interaction with a cobalt nucleus is observed, arises from a Co3+… O–2 species; the second arises from an O–2 species adsorbed on sites of the MgO matrix (Mg2+ or anion vacancies). A relatively weaker three g-value signal, also observed, has been attributed to an O–3 radical. Since no hyperfine structure is present the O–3 species is believed to be adsorbed on a matrix site.The relative stability of the surface species is found to decrease in the order Co3+… O–2 O–3-(matrix) O–2-(matrix). In particular, the first species is reversibly destroyed by evacuation at 298 K, the second is completely removed at 373K and the third is stable up to 400–500 K.Comparison of adsorption data (volumetric) with radical concentration values also provides evidence for the presence of diamagnetic forms of oxygen, probably O2– and O2–2; radical concentrations are found to be only a fraction of the total adsorption. Strong O2– forms appear to be favoured by increasing cobalt content.The nature of the active sites and the mechanism of oxygen adsorption are discussed. We propose that the first stage of oxygen activation involves electron transfer from Co2+ surface ions. The subsequent O—O bond cleavage is accomplished with the participation of matrix sites.

The dispersion of CrV and MoV on the surface of various oxides, as investigated by ESR spectroscopy☆

Abstract Samples containing chromium ( 53 Cr-enriched or not) or molybdenum ( 95 Mo-enriched or not) were prepared by the equilibrium adsorption or impregnation methods at two different loadings (∼ 0.05 and 0.5%-i% by weight) using as supports ZrO 2 , SiO 2 , TiO 2 , Y 2 O 3 , Al 2 O 3 , Ga 2 O 3 or La 2 O 3 . After thermal activation (heating with O 2 at 773 K for the Cr samples followed by reduction with H 2 for the Mo samples). ESR signals from surface Cr V or Mo V species are observed. The spectra are satisfactorily resolved on ZrO 2 , SiO 2 , TiO 2 , Y 2 O 3 , whereas they are broad (and always structureless in the case of chromium) on Al 2 O 3 , Ga 2 O 3 or La 2 O 3 , whether 53 Cr-enriched ( 95 Mo-enriched) was used or not for the preparation of samples. Resolved spectra are assigned to mononuclear Cr V or Mo V surface species. Notably, the ESR spectrum of 95 Mo V on Al 2 O 3 , in contrast to that of 53 Cr V on the same matrix, shows partially resolved hyperfine structure which allows for the assignment of the spectrum to mononuclear Mo V species. The finding that the ESR signals of Cr V or Mo V on the surface of Al 2 O 3 , Ga 2 O 3 or La 2 O 3 are much broader than on the other matrices is attributed to unresolved interaction with neighbouring 27 Al. 69.71 Ga or 139 La (superhyperfine interaction, shf). The shf interaction is small on Y 2 O 3 in view of the low nuclear spin and magnetic moment of the 89 Y nucleus and in fact resolved spectra are obtained on this matrix. Computer-calculated spectra assuming mononuclear Cr V or Mo V species interacting with two equivalent 27 Al nuclei (slightly anisotropic shf interaction 6 to 8.5 gauss) are in substantial agreement with experimental spectra.

LaAl1-XMnXO3 perovskite-type oxide solid solutions: structural, magnetic and electronic properties

The structural, magnetic and electronic properties were investigated for LaAl1−xMnxO3 (x = 0.0, 0.005, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0) perovskites prepared by the citrate method and calcined at 1073 K. The materials were characterised by XRD, BET surface area, diffuse reflectance spectroscopy (DRS), magnetic susceptibility measurements and electron paramagnetic resonance (EPR). XRD analysis showed that all the LaAl1−xMnxO3 samples are single phase perovskite-type oxides. A non-linear increase of the unit cell lattice parameters with x was observed. Particle sizes and surface areas were in the range of 270–1200 A and 4–33 m2 g−1, respectively. The oxidation state of manganese in the higher Mn-containing samples was checked by chemical titration, the following Mn4+/Mntot ratios being found: 0.45 for x = 0.4, 0.43 for x = 0.6, 0.49 for x = 0.8, and 0.35 for LaMnO3. Optical spectra revealed some bands attributable to both Mn4+ and Mn3+ species. A magnetic susceptibility study showed ferromagnetic behavior that decreases with the decrease of x, the high Al-containing materials exhibiting a paramagnetic behavior. Depending on the Mn-content, EPR analysis revealed the occurrence of several Mnn+ species: Mn4+ exchange-coupled by ferromagnetic interaction to nearest neighbor Mn3+, Mn4+ exchange-coupled by antiferromagnetic interaction to near-nearest neighbor Mn3+, isolated Mn4+, isolated Mn2+.

High and low surface area NiO–MgO and CoO–MgO solid solutions: a study of XPS surface composition and CO oxidation activity

Oxide solid solutions NiO–MgO of high surface area were studied by XPS. The surface Ni2+ concentration was found to be equal, within experimental errors, to the bulk concentration. The result is analogous to that found previously for the low surface area NiO–MgO system and for both the high and low surface area systems of CoO–MgO. The catalytic oxidation of CO by O2, on high and low surface area NiO–MgO and CoO–MgO materials, was investigated with the aim of relating the catalytic activity with transition metal ion nature and concentration. Turnover frequency data (CO2 molecules produced per second per surface atom) show that the activity is due primarily to the transition metal ions and is not subject to the ions being in special configurations (dimers or trimers) or in special positions (edges, corners). The activity of CoO–MgO is higher than that of NiO–MgO solid solution.