Valerio Indovina

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.

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 catalytic activity of Cu-ZSM-5 and Cu-Y zeolites in NO decomposition: dependence on copper concentration

NO decomposition was studied on Cu-ZSM-5 (Cu exchange extent from 23 to 210%) and Cu-Y (Cu exchange extent from 5 to 105%) catalysts at 773 K. The results show that the activity (NO molecules decomposed per gram of catalyst per second) increases by roughly 100-fold when the extent of exchange with copper in the ZSM-5 framework increases from 80 to 100%. This behaviour shows that not all Cu sites are equivalent in their decomposition activity. Cu-ZSM-5 samples prepared with either H-ZSM-5 or Na-ZSM-5 show the same activity pattern.

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.

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.

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.

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.

Supported oxides: Preparation, characterization and catalytic activity of CrOx/ZrO2, MoOx/ZrO2 and VOx/ZrO2

Abstract In the paper, we review the preparation and the characterization of CrOx/ZrO2, MoOx/ZrO2 and VOx/ZrO2. Catalysts were prepared by various methods (equilibrium adsorption, impregnation or mechanical mixing) and characterized by XRD, XPS, ESR and FTIR techniques. In the paper, we also review the catalytic activity of the ZrO2 supported systems for propene hydrogenation (CrOx/ZrO2 and MoOx/ZrO2), propane or isobutane dehydrogenation (CrOx/ZrO2), abatement of NO with H2, propane, or propene (CrOx/ZrO2), abatement of NO with NH3 (VOx/ZrO2). In selected cases, in order to assess the influence of the support on the catalytic activity, we compare the activity of the MeOx/ZrO2 samples with that of the same MeOx on other supports.