Dante Cordischi

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.

Structural characterization of Fe/Ti oxide photocatalysts by X-ray, ESR, and Mössbauer methods

Abstract The polycrystalline solids TiO 2 Fe 2 O 3 , with iron contents in the range 0–10 at.%, prepared by coprecipitation and by impregnation, and treated in air at temperatures in the range 500–1000°C, have been studied by X-ray, ESR, and Mossbauer methods. The TiO 2 in the samples treated at 800 and 1000°C always forms the rutile phase and the Fe 3+ has a rather low solubility in it (∼0.1 at.%). The Fe 3+ in excess forms the antiferromagnetic pseudobrookite phase (Fe 2 TiO 5 ). The samples treated at 500 and 650°C show a dependence on the preparation method. Those prepared by coprecipitation give at 500°C the pure anatase phase in which the Fe 3+ has a higher solubility (≥ 1%); those prepared by impregnation give the anatase phase accompanied by a variable amount of rutile. The treatment at 650°C provokes the partial transformation of anatase to rutile and the complete development of the Fe 2 TiO 5 phase. The relevance of these results to the photocatalytic properties shown by these solids for the photoreduction of dinitrogen to ammonia is discussed.

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.

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.

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.

The adsorption of O2 and NO on CrOx/ZrO2 catalysts, as investigated by IR and ESR spectroscopies

Abstract The adsorption of O 2 at 195 K and that of NO at 298 K on CrO x /ZrO 2 samples have been investigated by the double isotherm method. The nature of the surface species formed before and after adsorption has been studied by means of IR and ESR spectroscopies. Prior to O 2 or NO adsorption, CrO x /ZrO 2 samples were heated in O 2 at 773 K (average oxidation number of Cr, n =5.5) and reduced with CO at 423 to 623 K to controlled extents ( n =5.5 to 2.5). The results show that the adsorption of O 2 at 195 K oxidizes to Cr(VI) the Cr(II) present in the sample after the reduction. The oxidation of Cr(II) to Cr(VI) is quantitative and nearly selective. Only a small fraction of Cr(III), also present in the reduced sample, is oxidized to Cr(V) (ESR, γ-signal). The adsorption of NO on the sample heated in O 2 yields mono- and dinitrosyls of Cr(III), arising from the reduction of Cr(V). The reduction of Cr(V) to Cr(III) is evidenced by both ESR and IR data. The adsorption of NO on the reduced sample ( n =2.5) leads, on average, to 1.7 to 2.0 NO molecules adsorbed per Cr atom. In the same conditions, dinitrosyls and mononitrosyls of Cr(II) and Cr(III) are formed, in addition to N 2 O and nitrites (and/or nitrates). Upon evacuation of the latter sample at 423 K, the adsorbed NO oxidizes most of the Cr(II) to Cr(VI) ( n =4.2) and, in fact, the nitrosyls of Cr(III) are the only species detected by IR on NO re-admission after the evacuation treatment. Blank experiments on pure ZrO 2 show that the dismutation of NO leading to N 2 O and nitrites (and/or nitrates) takes place on sites of the ZrO 2 support. When the reduced sample is further reacted with H 2 O at 853 K, only the dinitrosyls and mononitrosyls of Cr(III) are detected upon exposure to NO, since the reaction with H 2 O at 853 K selectively oxidizes the Cr(II) to chromia-like species. It is suggested that coordinatively unsaturated Cr(III) ions, two vacancies at least on IR evidence, are the active sites for the hydrogenation of propene and H 2 D 2 equilibration reactions, previously investigated on the same CrO x /ZrO 2 catalysts.

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.