Giovanni Ferraris

CATALYTIC ACTIVITY OF CO-ZSM-5 FOR THE ABATEMENT OF NOX WITH METHANE IN THE PRESENCE OF OXYGEN

Abstract The abatement of NO with CH4 in the presence of oxygen ([NO] = [CH4] = 1000 or 4000 ppm, [O2] = 0 to 2%, by volume) was studied on Co-ZSM-5 catalysts (Co content 0.29 to 4.1 wt.-%), prepared from H-ZSM-5 or Na-ZSM-5 by the ion-exchange method. On all samples, the amount of CO and NO adsorbed at room temperature was proportional to the cobalt content (CO/Co⋍ 0.5 and NO/Co⋍ 1.6), with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the adsorption was only slightly higher than that on Co-ZSM-5 with Co 2.0 wt.-%. Infrared spectroscopy (FTIR) showed the formation of carbonyls (one type only, on cobalt equivalent sites), cobalt mononitrosyls (two types) and dinitrosyls (two types). The intensity of bands from carbonyls and nitrosyls was about proportional to the cobalt content, with the exception of the Co-ZSM-5 sample with Co 4.1 wt.-%, on which the bands were roughly as intense as in the sample with Co 2.0 wt.-%. In the Co-ZSM-5 sample with Co 4.1 wt.-%, after heating with O2 at 773 K, or after its use in catalysis, diffuse reflectance spectroscopy (DRS) showed the presence of Co3O4, not detected by X-ray diffraction. In the presence of O2, the NO reduction rate was proportional to the Co content, except for the sample containing Co 4.1 wt.-%. The CH4 oxidation rate was proportional to the Co content, in the entire range of Co concentrations. The selectivity of catalysts for NO abatement (selective catalytic reduction, SCR), was nearly independent of Co content but was markedly lower on the sample with Co 4.1 wt.-%. The results suggest that only CoII ions exchanged in the framework of the ZSM-5 matrix are active in CO and NO adsorption and in the SCR reaction, whereas also the cobalt of the dispersed Co3O4 phase contributes to CH4 oxidation with O2.

Epoxidation on titanium-containing silicates: do structural features really affect the catalytic performance?

Five titanium-containing silicates, with different structural features, were compared: Ti–MCM-41, ordered titanium-grafted mesoporous silica, Ti–SiO2 Davison, nonordered titanium-grafted porous silica, Ti–SiO2 Aerosil, nonporous pyrogenic titanium-grafted silica, MST, nonordered in-framework mesoporous material, and TiO2–SiO2 Grace, commercial amorphous porous mixed oxide. They were tested in the liquid-phase epoxidation reaction on six unsaturated cyclic terpenes. Good performances were obtained on the commercial mixed oxide and also on the three grafted silicates. The in-framework MST showed the worst activity results. Under these conditions, the porosity features do not affect the catalytic performances noticeably and the use of an ordered mesoporous material is not strictly required. Likewise, a very high surface area is not mandatory in order to have an efficient titanium-grafted catalyst in the epoxidation of these substrates.

Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts

Copper and zinc hydroxycarbonates with Cu/Zn atomic ratios of 100/0, 92/8, 85/15, 77/23, 67/33, and 50/50 were prepared by the coprecipitation method. The samples with the ratios 100/0, 92/8 and 85/15 were monophasic with the monoclinic P2/sub 1//a malachite structure, whereas the richer zinc samples contained in addition to the malachite phase the orthorhombic B22/sub 1/2 aurichalcite, whose concentration was very low in the 77/23 specimen but increased continuously with zinc content. The amount of aurichalcite in the biphasic powders has been estimated by means of quantitative X-ray diffraction and differential thermal analysis. With only those X-ray reflections belonging to the malachite structure taken into account, the unit cell constants for all the solid solution hydroxycarbonates have been determined, showing that as a whole the cell volume of malachite decreases continuously with increasing zinc content. This result is related, as shown also by DTA measurements and thermochemical literature data, to an increasing stability of compounds richer in zinc, to increasing covalent effects, and/or to less octahedral site distortion around the metal atoms when zinc substitutes for Cu/sup 2 +/ ions in the malachite lattice. Magnetic susceptibility and thermogravimetric measurements together with diffuse reflectance spectra are also presented and discussed.

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 selective catalytic reduction of NOx with CH4 on Mn-ZSM5: A comparison with Co-ZSM5 and Cu-ZSM5

The abatement of NO with CH4 in the presence of O2 was studied on Mn-ZSM5 (Mn percent exchange 2 to 75%), prepared from Na-ZSM5 by the ion-exchange method. Samples were characterised by FTIR, ESR and magnetic measurements. Both ESR and magnetic measurements showed that all manganese was present as MnII. FTIR showed the formation of carbonyls (one type only, on manganese equivalent sites). The intensity of bands from carbonyls was almost proportional to the manganese content. The selectivity for NO abatement, reaction orders in O2, NO or CH4, and apparent activation energy were independent of the manganese content, suggesting that the same surface complexes were formed on all Mn-ZSM5 catalysts. On Mn-ZSM5, turnover frequencies (molecules s−1 Mn-atom−1) for both NO abatement and CH4 reaction were nearly independent of the manganese content and close to the relevant values on Co-ZSM5 (previously investigated in our laboratory), but substantially higher than those on Cu-ZSM5 (Cu percent exchange 98%, studied for a comparison). On Mn-ZSM5, Co-ZSM5 and Cu-ZSM5, the adsorption at RT of NO+O2 or NO2 caused the formation of nitrates. The normalised intensity (per atom) of monodentate nitrate bands (∼1520 and ∼1320 cm−1) was (i) independent of the metal content, (ii) the same in Mn-ZSM5 and Co-ZSM5, and (iii) much higher in Mn-ZSM5 and Co-ZSM5 than in Cu-ZSM5. On all these catalysts, NO abatement rates were proportional to the concentration of these monodentate nitrates. On FTIR evidence, these species reacted with methane at the same temperature at which the various catalysts were active. These findings suggest a role of monodentate nitrates in the SCR reaction.

Preparation and characterisation of mesoporous silica–alumina and silica–titania with a narrow pore size distribution

Abstract Amorphous mesoporous materials with a different degree of order in the arrangement of pores are outlined. Particularly, the synthesis of a class of mesoporous silica–alumina (MSA) materials with narrow pore size distribution and a disordered arrangement of pores is reported and discussed. Likewise, the preparation of titanium-containing ordered mesoporous silicates (Ti-MCM-41) and disordered mesoporous silica–titania (MST) are also described in detail. The structural properties of the solids are compared by means of X-ray diffraction and UV-Vis diffuse reflectance spectroscopy. The nitrogen adsorption–desorption measurements were performed and the textural properties are evaluated by the BET, DFT, BJH and t -plot methods. The high specific surface area and pore volume, as well as the acidity, make MSA solids interesting catalysts in several petrochemical transformations, i.e. oligomerisation, alkylation, hydroisomerisation, rearrangement reactions. Besides, thanks to the width of the mesopores of such solids, the catalytic activity of titanium-containing silicates may have a potential application in the epoxidation of bulky unsaturated fine chemical substrates.

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.

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.

Characterization of well dispersed copper species on the surface of ZnO by x-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy was employed to characterize Cu(II) species on the surface of calcined CuO-ZnO systems with Cu/Zn atomic ratios ⪯ 0.10. Such solids are of interest as precursors of Cu-ZnO catalysts. It is shown that well dispersed Cu(II) species on the surface of ZnO have unique spectroscopic properties. For the Cu(2p3/2) transition the intensity of the satellite structure, typical of bulk Cu(II) compounds, is strongly reduced or even absent in the more dilute samples. To explain this result, it is suggested that a stronger Cu-O covalent bond than in bulk CuO is formed at the interface between the surface oxygen ions of ZnO and the supported copper species. The strong interaction between Cu(II) species and the support leads on reduction to the stabilization of Cu(I) species at the periphery of metallic copper particles which could play a role in the reaction mechanisms of CO hydrogenation at the surface of Cu-ZnO catalysts.

Comparison of carbon monoxide and nitrous oxide adsorption on copper/zinc oxide catalysts

Abstract The determination of metal specific surface areas by selective chemisorption on a supported metal offers the advantage of monitoring the surface exposed to the catalytic partners, and it is therefore directly related to the catalytic processes. Among supported metals, particular attention has been devoted to copper containing systems such as Cu/ZnO and Cu/ZnO/Al2O3 methanol synthesis catalysts [1–4]. In recent years many papers have appeared concerning both the physico-chemical and technical aspects of methods based mainly on the great affinity of copper towards oxygen, derived either from dioxygen species or from nitrous oxide. One inherent difficulty was the stringent requirement that bulk diffusion be excluded in order to avoid apparently large O/Cu ratios. By comparison with irreversible oxygen adsorption at 78 K. Parris and Klier [3] showed that reversible carbon monoxide adsorption at 293 K can also be used to titrate copper surface areas in Cu/ZnO catalysts, as demonstrated by the good linearity found between the two capacities. On the other hand, nitrous oxide dissociative adsorption was considered not to be reliable because of bulk oxidation and metal particle size dependence, as previously reported by Scholten and Konvalinka [5]. Therefore, it seemed of interest to compare the room temperature dissociative adsorption of nitrous oxide, which leaves on the surface oxygen atoms that possibly penetrate into the bulk, with the room temperature carbon monoxide adsorption, which correlates only with surface sites. In this paper we present the results of a systematic study of the correlation between carbon monoxide and nitrous oxide room temperature chemisorption capacities of several Cu/ZnO catalysts of wide compositional range and prepared by different procedures.