M. Valigi

Studies on chromia/zirconia catalysts I. Preparation and characterization of the system

Abstract The preparation and characterization (chemical, textural, DTA, XRD, XPS) of chromium oxide/zirconia, of interest as hydrogenation catalysts, are reported. The support (obtained via ZrOCl2 hydrolysis) can be tailored in surface area from high (360 m2g−1) to low (about 20 m2g−1) values, and in texture from microporous to mesoporous and macroporous according to the treatment temperature (from 383 to 923 K) of the starting “hydrous zirconia.” By contacting the support with Cr(VI) solution, chromium-loaded specimens, ZC, are prepared. The Cr uptake is roughly constant (1.5 to 1.9 Cr atoms nm−2) for zirconia previously heated at T ≥ 573 to 923 K. Higher loadings can be reached on the hydrous zirconia. Supported Cr oxide is an effective antisintering agent for zirconia, and it also opposes the tetragonal → monoclinic transition. By subjecting the ZC specimens to various heat and redox treatments, the average oxidation number of Cr, n changes. From an initial value of +6, n decreases to +5.5 after oxygen treatment at 773 K, to +2.5 after CO treatment at 623 K, and can be restored to +5.5 if the sample is reoxidized in oxygen at 773 K. Treatment in H2O vapor at 723 K of a reduced ( n = 2.5 ) specimen brings n to 3.0. The existence of Cr(VI), Cr(V), Cr(III), and Cr(II) is inferred and is supported by XPS analysis. A separate paper presents an ESR investigation and discusses the nature of the surface Cr species.

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 QUANTITATIVE ANALYSIS AND MODELS OF SUPPORTED OXIDE CATALYSTS

Abstract Application of XPS to the quantitative analysis of supported oxide catalysts is reviewed. Starting from the basic quantitative equations of XPS, the development of different models is summarized, including those for flat surfaces and rough profile surfaces, as well as models treating high surface area and porous materials. The significance of the approximations used in the various models is discussed, and the different approaches are compared. Examples from the literature are reported and critically evaluated to illustrate practical examples of application.

A structural, thermogravimetric, magnetic, electron spin resonance, and optical reflectance study of the NbOxTiO2 system

Abstract Titanium dioxide samples containing niobium (up to 10%), heated at 1223 K in vacuo and subsequently in air, have been investigated by X-ray diffraction, thermogravimetry, magnetic susceptibility measurements, electron spin resonance, and optical reflectance spectroscopy. The results show that for the samples prepared in vacuo, niobium is incorporated as Nb(V) compensated by an equivalent amount of Ti(III). The analysis of the magnetic data reveals that some Ti(III) are paired giving a metal-metal interaction. On subsequent heating in air the Ti(III) ions are oxidized to Ti(IV), with the Nb(V) in solid solution being compensated by cation vacancies. The maximum amount of Nb(V) that may be accommodated in the rutile structure is 6.6 Nb atoms/100 Ti atoms. The niobium in excess reacts with TiO2 giving TiNb2O7. The effect of the various incorporated species on the TiO2 unit-cell volume is discussed.

Studies on chromia/zirconia catalysts. II, ESR of chromium species

Abstract The characterization of CrO x /ZrO 2 samples (Cr content 0.05 to 6 wt%) by means of ESR spectroscopy is reported. On samples heated in O 2 at increasing temperatures up to 1173 K, the presence of Cr(V) (γ-signal, g ‖ = 1.960 and g ‖ = 1.979) is detected by ESR. Its concentration (Cr(V) ions nm −2 ) is found to increase with temperature, remaining about constant above 773 K. Experiments with the 53 Cr isotope allow assignment of the species to a surface mononuclear chromyl-complex in a square pyramidal configuration. At higher temperatures (generally at T ⩾ 973 K, depending also on textural features of the ZrO 2 support and Cr content) the ESR signals of (i) a chromia-like phase (β′-signal, g = 1.98 and Δ H pp = 1500–1800 G) and (ii) α-Cr 2 O 3 (g = 1.98, Δ H pp = 480–500 G, spectra recorded at T ⩾ 308 K) are observed in addition to Cr(V). The particle size of the β− is too small (⩽7 nm) to show strong antiferromagnetic interactions. On samples reduced with CO, the γ-signal sharply decreases with increasing temperature of the reduction, and disappears at 623 K. In the more concentrated samples and after extensive reduction only, an ESR signal from Cr(III) is observed (δ-species, σ ≈ 2.2 with a broad maximum at g in the range 3.8 to 5.0), and assigned to weakly interacting Cr(III) ions exposed on the surface of ZrO 2 . If reduced samples are treated with H2O at increasing temperatures up to 1073 K, the selective oxidation of Cr(II) to Cr(III)β species (g = 1.98, ΔH pp = 1500–1600 G) is observed. Species Cr(III)-β and Cr(IIl)-β′ differ from each other by cluster size only, as indicated by their different redox behavior. Reoxidation with O 2 at room temperature only minimally restores the γ-signal, and hardly affects the 8-signal. Full reversibility is achieved upon heating in 0, at 773 K. ESR results and average oxidation numbers from redox cycles allow the identification of two distinct redox couples on the ZrO 2 surface: Cr(III)/Cr(V) and Cr(II)/Cr(VI). The stabilization effect of the ZrO 2 matrix on the various chromium species is discussed.

Preparation, Characterisation, and Photocatalytic Behaviour of Co-TiO2 with Visible Light Response

The preparation of cobalt-modified T i O 2 (Co- T i O 2 ) was carried out by the incipient impregnation method starting from commercial T i O 2 (Degussa, P-25) and cobalt acetate. XPS data show that cobalt is incorporated as divalent ion, and it is likely present within few subsurface layers. No appreciable change in structural-morphologic properties, such as surface area and anatase/rutile phase ratio, was observed. Conversely, Co addition brings about conspicuous changes in the point of zero charge and in surface polarity. Diffuse reflectance spectra feature a red shift in light absorption that is dependent on the amount of cobalt. The influence of cobalt addition on the performance of T i O 2 as a photocatalyst in the degradation of 4-chlorophenol and Bisphenol A is investigated. The results show that the modified oxide presents a higher photoactivity both for illumination with UV-visible ( 𝜆 > 3 6 0  nm) and visible light ( 𝜆 > 4 2 0  nm; 𝜆 > 4 5 0  nm), and that this enhancement depends on the amount of the added species and on the final thermal treatment in the preparation step. We also show that Co- T i O 2 is a more active catalyst than pure T i O 2 for the reduction of O 2 in the dark, which is an important reaction in the overall photocatalytic processes.

ESR and reflectance spectra of manganese in polycrystalline TiO2 (rutile)

Abstract The ESR and reflectance spectra of polycrystalline TiO 2 containing manganese oxide (up to 8% atomic ratio) have been investigated. The results show that there is only a limited solubility of substitutional Mn 4+ in TiO 2 . The manganese ions are isolated, since the solubility limit prevents clustering of Mn 4+ in TiO 2 matrix. Manganese in excess (with respect to the solubility value) is present as MnTiO 3 . The electronic spectrum of Mn 4+ in the rutile lattice is discussed on the basis of the behavior of isoelectronic Cr 3+ in solid solution in TiO 2 .

Structural, surface, and catalytic properties of bismuth molybdovanadates containing foreign atoms: I. X-ray characterization of iron-containing bismuth molybdovanadate catalysts☆

Abstract The selective oxidation of propene to acrolein has been studied on iron-containing bismuth molybdovanadates with the scheelite structure and general formula Bi 1 − x 3 x 3 − y Me y ( V 1 − x Mo x − y Fe y ) O 4 , where Me = Fe or Bi , = cation vacancy, with x = 0.45 and 0.60 and 0 ⩽ y ⩽ x 3 , using the pulse technique in the temperature range 573–673 K. Total conversion, rate constant, and selectivity were found to depend on catalyst composition, in particular cation vacancy concentration, content, and coordination symmetry of the iron ions. The results, discussed in terms of compositional parameter y and surface geometry of BiO and MoO species, confirm the importance of the cation vacancies in the selective oxidation process and suggest a role of the eight-coordinated cations in the allyl formation.

Manganese oxide–zirconium oxide solid solutions. An X-ray diffraction, Raman spectroscopy, thermogravimetry and magnetic study

Manganese oxide-doped zirconium oxide samples, prepared by heating mixtures of coprecipitated hydroxides at 1073 K in a hydrogen stream (water content 0.2% by volume), were analysed to obtain information on the solid solution formation. The state and the thermal stability of the incorporated species were also investigated. The samples (manganese content up to 14.74 mass%), were studied ‘as-prepared’ and after subsequent thermal treatments in oxygen up to 753 K. The results of several techniques [X-ray diffraction (XRD), Raman spectroscopy, thermogravimetry (TG) and magnetic susceptibility measurements] show that in the ‘as-prepared’ samples (1073 K, H2) a high fraction of manganese is incorporated in the zirconia structure, only a small fraction being present as an MnO separate phase. Most of the manganese in solid solution is present in the +2 oxidation state, the remainder as + 3 and +4. TG experiments and magnetic susceptibility measurements reveal that the Mn3+ and/or Mn4+ are formed both during the cooling in hydrogen by reaction with water present as an impurity in the gas phase, and during the exposure to the atmosphere. As the amount of manganese in solid solution increases, the volume of the zirconia unit cell slightly decreases. The solid-solution formation favours the tetragonal and the cubic modifications at the expense of the thermodynamically stable monoclinic phase. When the samples are heated up to 753 K in oxygen, the Mn2+ in solid solution is partially oxidized to Mn3+ and/or Mn4+. TG and XRD experiments show that the oxidation starts at low temperature and takes place in solid solution without appreciable manganese oxide segregation.

XPS quantitative evaluation of the overlayer/support intensity ratio in particulate systems

Abstract The XPS intensity ratios M M / I Zr were measured for supported MO x /ZrO 2 systems (where M is Cr, Mo, Na, K, Re, designated as the “promoter”) and compared with the intensity values calculated for different models. When the support is the high surface area (about 300 m 2 g −1 ) hydrous zirconia (HZ) treated at 383 K, the Kerkhof and Moulijn model fits the data very well. If the zirconia support is heated at T between 773–923 K before promoter adsorption, the observed intensity ratios are roughly twice as high as those predicted by a non-attenuating overlayer on a semi-infinite support (referred to as “planar”) model. To achieve a correct fit for the experimental data, the contribution of ejected electrons from low angles must be taken into account. A model has accordingly been tested, which can fit the observed intensity ratios independently of the nature of the promoter.