Delia Gazzoli

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.

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.

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.

Preparation and Characterization of Nanocomposite Polymer Membranes Containing Functionalized SnO2 Additives

In the research of new nanocomposite proton-conducting membranes, SnO2 ceramic powders with surface functionalization have been synthesized and adopted as additives in Nafion-based polymer systems. Different synthetic routes have been explored to obtain suitable, nanometer-sized sulphated tin oxide particles. Structural and morphological characteristics, as well as surface and bulk properties of the obtained oxide powders, have been determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) and Raman spectroscopies, N2 adsorption, and thermal gravimetric analysis (TGA). In addition, dynamic mechanical analysis (DMA), atomic force microscopy (AFM), thermal investigations, water uptake (WU) measurements, and ionic exchange capacity (IEC) tests have been used as characterization tools for the nanocomposite membranes. The nature of the tin oxide precursor, as well as the synthesis procedure, were found to play an important role in determining the morphology and the particle size distribution of the ceramic powder, this affecting the effective functionalization of the oxides. The incorporation of such particles, having sulphate groups on their surface, altered some peculiar properties of the resulting composite membrane, such as water content, thermo-mechanical, and morphological characteristics.

Preparation and properties of new acid catalysts obtained by grafting alkoxides and derivatives on the most common supports. Part III : grafting titanium alkoxides and sulphate derivatives on silica

Abstract Some catalysts have been prepared by grafting different amounts of titanium isopropoxide dissolved in toluene on a silica support and by steaming and calcinating the products obtained. A catalyst has been prepared also by first treating with pure sulphuric acid titanium isopropoxide, grafting the titanium alkoxide sulphate on silica and again by steaming and calcinating the product obtained. All the catalysts prepared have been characterized using different techniques such as: electrochemical potentiometric titrations to evaluate the PZC (point of zero charge), the hydroxyl densities and the surface dissociation equilibrium constants; TGA (thermal gravimetric analyses) to evaluate the hydroxyl densities; TPD (thermal programmed desorption) of organic bases such as pyridine and lutidine; and XPS (X-ray photoemission spectroscopy). All catalysts have also been tested by determining the performances in three different reactions: dehydration of methanol, epoxidation of cyclooctene with cumene hydroperoxide and isomerization of 1-butene to 2-butene. The behaviour of the catalysts prepared shows that grafting titanium on silica strongly modifies the original surface and the catalysts obtained with the mentioned grafting techniques are different from those obtained by impregnation and coprecipitation.

Preparation and properties of new acid catalysts obtained by grafting alkoxides and derivatives on the most common supports. Part II: Grafting zirconium and silicon alkoxides on γ-alumina

Abstract In this work, the preparation and properties of the acid catalysts obtained by grafting zirconium and silicon alkoxides and zirconium-sulfated alkoxide on γ-alumina were studied. The catalysts prepared were examined with electrochemical techniques to evaluate the hydroxyl density, the ZPC (zero point charge) and the intrinsic surface ionization constants. The catalysts were also submitted to TPD (temperature-programmed desorption) analysis using different organic bases to determine the distribution of acid sites. Some samples were also submitted to microcalorimetric analysis using pyridine as probe molecule, and to XPS measurements. Finally, the catalysts prepared were tested in three different reactions: methanol dehydration, n -hexane isomerization and cracking and skeletal isomerization of 1-butene to isobutene. The most relevant results obtained were: (i) grafted zirconia strongly promoted methanol dehydration up to an upper limit of grafting; (ii) grafted zirconium sulfated alkoxide showed a moderate activity, but good selectivity in n -hexane isomerization; (iii) silicated aluminas (prepared in the absence of solvents) showed high activity, selectivity and stability in the 1-butene skeletal isomerization reaction.

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 .